Cover-Photograph of massive sulfide mound in the NESCA area in the Escanaba Trough, 

offshore of northern California, depth 3232 m. Diffuse venting of hydrothermal fluid through 

pyrrhotite-rich massive sulfide rubble supports a hydrothermal vent community dominated by 

tube worms. The tubeworms are colonized by vent-specific anemones with red tentacles, white 

galatheid crabs and clumps of white filamentous bacteria. The white spots on the massive 

sulfide are gastropods and limpets; bluish filaments on massive sulfide may be colonies of the 

protozoan Girvanella. Photograph taken from the bow camera of the Alvin submersible; field of 

view is approximately 2 m across. 
Geologic, Hydrothermal, 

and Biologic Studies at 

Escanaba Trough, Gorda Ridge, 

Offshore Northern California 

JANET L. MORTON, ROBERT A. ZIERENBERG, and CAROL A. REISS, Editors 

U.S. GEOLOGICAL SURVEY BULLETIN 2022 
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary 

U.S. GEOLOGICAL SURVEY 

Gordon P. Eaton, Director 

For sale by 

U.S. Geological Survey, Map Distribution 

Box 25286, MS 306, Federal Center 

Denver, CO 80225 

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does 

not imply endorsement by the U.S. Government. 

Library of Congress Cataloging-in-Publication Data 

Geologic, hydrothermal, and biologic studies at Escanaba Trough, Gorda Ridge, offshore northern California I 

Janet L. Morton, Robert A. Zierenberg, and Carol A. Reiss, editors. 

p. em.- (U.S. Geological Survey bulletin ; 2022) 

Includes bibliographical references. 

1. Geology-Escanaba Trough. 2. Hydrothermal vents-Escanaba Trough. 3. Hydrothermal vent faunaEscanaba Trough. 4. Escanaba Trough. I. Morton, Janet L. II. Zierenberg, Robert A. III. Reiss, Carol A. 

IV. Series. 

QE75.B9 no. 2022 

[QE350.42.N65] 

557.3 s-dc20 

[551.46'08'09432] 

94-13924 

CIP 
CONTEN'TS 

1. Geologic., hydrothermal, and biologic studies at Escanaba 

Trough: An introduction . .. .. . .. . .. .. .. .. .. . .. . .. .. . .. .. .. . .. . . .. .. . . .. . .. .. . .. . .. . 1 

Janet L. Morton, Robert A. Zierenberg, and Carol A. Reiss 

Section 1: Geologic Processes 

2. Structural setting and interaction of volcanism and 

sedimentation at Escanaba Trough: Geophysical results ........ 21 

Janet L. Morton and Christopher G. Fox 

3. Thermal and tectonic structure of Escanaba Trough: New 

heat-flow measurements and seismic-reflection profiles......... 45 

Earl E. Davis and Keir Becker 

4. A. thermal and mechanical model for sediment hills and 

associated sulfide deposits along Escanaba Trough .. .. .. ... .. .. . .. . 65 

Roger P. Denlinger and Mark L. Holmes 

5. Crustal n1agnetization and basement structure of Escanaba 

Trough ......................................................................................... 77 

Robert E. Karlin and Janet L. Morton 

6. Sediment fill of Escanaba Trough.............................................. 91 

William R. Normark, Christina E. Gutmacher, Robert A. Zierenberg, 

Florence L. Wong, and Robert J. Rosenbauer 

7. Sedimentation and neotectonism in the SESCA area, 

Escanaba Trough, southern Gorda Ridge ................................. 131 

Robert E. Karlin and Robert A. Zieren berg 

8. Volcanic geomorphology of the SESCA and NESCA 

sites, Escanaba Trough ............................................................... 143 

Stephanie L. Ross and Robert A. Zierenberg 

9. Petrology and mineral chemistry of basalt from Escanaba 

Trough........................................................................................ 153 

Alice S. Davis, David A. Clague, and Walter B. Friesen 

III 
IV CONTENTS 

10. Geologic setting of massive sulfide mineralization in 

Escanaba Trough . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171 

Robert A. Zierenberg, Janet L. Morton, Randolph A. Koski, 

and Stephanie L. Ross 

Section II: Hydrothermal and Geochemical Studies 

11. Chemistry of hydrothermal fluids from Escanaba Trough, 

Gorda Ridge ............................................................................... 201 

Andrew C. Campbell, Chris R. German, Martin R. Palmer, 

Toshitaka Garno, and John M. Edmond 

12. Stable isotope study of hydrothermal vents at Escanaba 

Trough: Observed and calculated effects of sedimentseawater interaction ..... ... .. .... ..... .. . ....... .... .. . .. . ............ ............ .. . . 223 

J. K. Bohlke and W. C. Shanks III 

13. Evidence for hydrothermal fluid flow through surficial 

sediments, Escanaba Trough ... ..... ........ ....... .. . .. ... . .. . . ........... .. . .. . 241 

Andrew J. Magenheim and Joris M. Gieskes 

14. Sediment alteration associated with massive sulfide formation in Escanaba Trough, Gorda Ridge: The importance 

of seawater mixing and magnesium metasomatism ... . .. . .. . .. ..... 257 

Robert A. Zierenberg and Wayne C. Shanks III 

15. Hydrocarbons in sediment from Escanaba Trough .................. 279 

Keith A. Kvenvolden, John B. Rapp, and Frances D. Hostettler 

16. Composition and growth history of hydrothermal deposits 

in Escanaba Trough, southern Gorda Ridge .. . .. . .. . .. ....... ...... ... .. 293 

Randolph A. Koski, Laura M. Benninger, Robert A. Zierenberg, 

and Ian R. J onasson 

Section III: Biologic Studies 

17. Soft-sediment hydrothermal vent communities of Escanaba 

Trough ........................................................................................ 327 

J. Frederick Grassle and Rosemarie Petrecca 

18. Quaternary foraminifers from Escanaba Trough, northeast 

Pacific Ocean ............................................................................. 337 

Paula J. Quinterno 
CONTENTS v 

FIGURES 

1.1. Location map of Gorda Ridge and other mid-ocean ridge spreading centers in northeast 

Pacific Ocean .............. .................. .. ................ ............ ............ ... . ...... .......... ...... ............. .. ... . . . . ...... ...... ......... 2 

1.2. Map of Escanaba Trough showing location of volcanic edifices, approximate extent of sediment 

fill, NESCA and SESCA study areas, and DSDP Site 35 ................... .... ...... .............. ............. ...... ... . . . ...... 4 

1.3. Oblique view showing bathymetric relief of NESCA area........................................................................ 5 

1.4. Oblique view showing bathymetric relief of SESCA area ........ ............ ... . . . ...... .. . . . . ... . . . ............................ 5 

1.5-1.8. Bathymetric maps of: 

1.5. NESCA area showing location of camera tows and dredge hauls ................................................ 6 

1.6. SESCA area showing location of camera tows and dredge hauls ......................................... ........ 7 

1.7. NESCA area showing location of submersible dives .................................................................... 8 

1.8. SESCA area showing location of submersible dives ............................. ........................................ 9 

1.9--1.12. Photographs of: 

1.9. Sedimentary and volcanic features at NESCA and SESCA sites.................................................. 11 

1.10. Massive sulfide deposits and hydrothermal features at NESCA and SESCA sites...................... 12 

1.11. Massive sulfide deposits and active hydrothermal vents at NESCA and SESCA sites............... 13 

1.12. Mineralized samples collected from NESCA and SESCA sites..................................................... 14 

2.1. Bathymetric map of Escanaba Trough showing location of seismic lines................................................. 23 

2.2. Sea Beam bathymetric map of Escanaba Trough........................................................................................ 24 

2.3-2.7. Migrated single-channel (cruise L6-85-NC) and multi-channel (cruise L12-81-NP) seismicreflection profiles across Escanaba Trough along: 

2.3. Lines 16 and 20, cruise L6-85-NC .................................................................................................... 25 

2.4. Lines 9 and 12, cruise L6-85-NC ...................................................................................................... 26 

2.5. Lines 5 and 7, cruise L6-85-NC ........................................................................................................ 27 

2.6. Lines 114 and 118, cruise L12-81-NP .............................................................................................. 28 

2.7. Line 24, cruise L6-85-NC and line 111, cruise L12-81-NP............................................................ 29 

2.8. Schematic map of Escanaba Trough showing location of volcanic edifices, sediment-covered 

areas, and terraces . . . . . . . . . . . . . . .. . . . . . . . . . . .. . . . . . . . . .... . . . . . . . .. . . . . . . . .. . . . . . . . . . . .... . . . . . . . . .. .. . . .. . . . . .. . . . . .. . . . . . . . . . . . . . . . . . .. . . . . . . . . . . .. . . . . .. 30 

2.9. Sea Beam bathymetric map of SESCA area showing location of single-channel seismic reflection 

lines from cruise L 1-86-NC .................... ........... ............ ........... ........ ... . . . ............ ........ ... ...... ... . . . . .. . . . . . . . .. . . . . . . 31 

2.1 0. Sea Beam bathymetric map of NESCA area showing location of single-channel seismic reflection 

lines from cruise L1-86-NC ........................................................................................................................ 32 

2.11-2.14. Single-channel seismic-reflection profiles from cruise L 1-86-NC along: 

2.11. Line 12 in SESCA area, 3.5 kHz .................................................................................................... 33 

2.12. Lines 4, 6, 8, 10, and 12 in SESCA area ........................................................................................ 34 

2.13. Lines 14, 18, 20, 22, and 24 in NESCA area.................................................................................. 35 

2.14. Lines 16 and 25 in NESCA area .. .......... .......... ............ ............... .......... .... .. .................. ...... ...... ...... 36 

2.15. Map of SESCA area showing distribution of disturbed and undisturbed sediment and volcanic 

flows............................................................................................................................................................. 37 

2.16. Map of NESCA area showing distribution of disturbed and undisturbed sediment and volcanic 

flows............................................................................................................................................................. 38 

2.17-2.19. 3.5-kHz seismic reflection profiles at: 

2.17. NESCA area .... ... ... . . . . ........... .... . .... . . . . ... ..... .......... ............ .......... .... . . .. ............ .... . .............................. 39 

2.18. SESCA area...................................................................................................................................... 39 

2.19. NESCA area, from pinger near sea floor........................................................................................ 40 

2.20. Schematic cross section depicting inferred subsurface structures and pattern of hydrothermal flow ..... 41 

3.1. Map of Gorda Ridge region, showing Escanaba Trough study area......................................................... 47 

3.2. Bathymetric map of central part of Escanaba Trough, showing locations of heat-flow measurements and seismic-reflection profiles ................................................................ ......................................... 49 

3.3. Single-channel seismic-reflection profiles stacked along mid-valley north-south common 

reference line................................................................................................................................................. 50 

3.4. Graphs of temperature versus depth, conductivity versus depth, and temperature versus 

integrated thermal resistance........................................................................................................................ 54 

3.5. Graph of depth below sea floor of acoustically "transparent" horizon versus local sea floor depth....... 58 
VI CONTENTS 

3.6. Diagram showing heat-flow values superimposed on co-located seismic lines....................................... 59 

3.7. Schematic illustration of heat- and mass-balance model for regional hydrothermal circulation, 

fluid discharge, and mineralization associated with volcanic centers in Escanaba Trough ..................... 61 

4.1-4.5. Diagrams showing: 

4.1. Schematic view of typical sediment hill in SESCA ....................................................................... 66 

4.2. Simplified geologic model for sediment hills in NESCA and SESCA, showing circulation 

patterns ........ ............ .. . . .......... ... ....... ............ ............ ............ ............ .... .................... .. . . . . .. . . . ... . . . . ...... 67 

4.3. Sandbox model for sediment hills in NESCA and SESCA ........................................................... 68 

4.4. Idealized cooling model used to obtain rate of cooling of sediment-covered basalt laccolith .... 69 

4.5. Three stages of cooling for sediment-covered basaltic laccolith................................................... 70 

4.6. Graph of estimate of conductive heat flux from intrusion and convective flux due to flow along 

fault zone...................................................................................................................................................... 71 

5.1-5.4. Maps of: 

5.1. Bathymetry of Escanaba Trough..................................................................................................... 79 

5 .2. Sediment thickness in Escanaba Trough......................................................................................... 81 

5.3. Basement topography of Escanaba Trough.................................................................................... 81 

5.4. Magnetic anomalies of Escanaba Trough....................................................................................... 82 

5.5-5.7. Magnetic anomaly, sediment cover, and basement bathymetry profiles for: 

5.5. Lines 16 and 18, cruise L6-85-NC .................................................................................................. 82 

5.6. Lines 7 and 12, cruise L6-85-NC .................................................................................................... 83 

5.7. Line 24, cruise L6-85-NC ............................................................................................................... 83 

5.8. Map showing misfit between observed and modeled values for 1-km source layer trial ........................ 84 

5.9. Map showing distribution of crustal magnetizations computed for 1-km magnetic source layer........... 85 

5.10. Profile of magnetic modeling of line 24 ..................................................................................................... 86 

5.11. Graph of magnetic anomaly value versus sediment thickness .................................................................. 87 

5.12. Map of magnetic anomalies superimposed on basement topography....................................................... 87 

6.1. Map showing possible sources and pathways for terrigenous sediment to move into Escanaba 

Trough.......................................................................................................................................................... 92 

6.2. Map showing location of cores taken in Escanaba Trough .. .. .. .. ..... .... .. ........................ ...... ............ .......... 93 

6.3. Summary of core sections recovered at DSDP Site 35 .............................................................................. 95 

6.4. Summary core log for DSDP Site 35 .......................................................................................................... 95 

6.5-6.8. Graphs of pore-water characteristics versus depth for DSDP Site 35: 

6.5. Salinity ............................................................................................................................................. 96 

6.6. K and Ca composition ..................................................................................................................... 96 

6.7. Mg and S04 composition................................................................................................................ 97 

6.8. Minor-element composition............................................................................................................ 97 

6.9. Map of NESCA area showing core locations............................................................................................. 99 

6.10. Map of SESCA area showing core locations .............................................................................................. 100 

6.11. Photographs of three selected core intervals ................................ .................. .............. .... ... .. .... .. .. .. .. ......... 103 

6.12. X-radiographs of three selected core intervals........................................................................................... 104 

6.13. Ternary plot of quartz-feldspar-lithics of light-fraction data for DSDP Site 35 and Escanaba Trough 

sediment cores.............................................................................................................................................. 107 

6.14. Ternary plot of amphibole-epidote-pyroxene in heavy mineral suites in sediment from Escanaba 

Trough and nearby areas.............................................................................................................................. 108 

6.15. Diagram of orthopyroxene/pyroxene versus pyroxene/amphibole in sediment from Escanaba 

Trough and nearby areas.............................................................................................................................. 109 

6.16. Bar graph of pore-water chemistry for selected cores from Escanaba Trough......................................... 110 

6.17. Map showing depth to 11-ka boundary for selected cores from Escanaba Trough .... ............... ..... .. .... .... 111 

7 .1. Bathymetric map of Escanaba Trough showing location of volcanic edifices ..... .. .......... ........................ 132 

7 .2. Map of SESCA area showing core locations.............................................................................................. 134 

7.3-7.5. Graphs of split core magnetic susceptibility for sediments from SESCA area showing: 

7 .3. Holocene turbidite intervals ............................................................................................................ 136 

7 .4. Relict Pleistocene muds................................................................................................................... 137 

7.5. Hydrothermally altered sediments .................................................................................................. 138 

7 .6. Scanning electron micrograph of aggregate containing thin-bladed pyrrhotite grains ............................ 138 
CONTENTS VII 

8.1. Map of SESCA site showing volcanic geology along camera and submersible tracklines ..................... 144 

8.2. Map of NESCA site showing volcanic geology along camera and submersible tracklines ..................... 145 

8.3. Photographs of volcanic features at SESCA and NESCA sites ................................................................ 147 

8.4. Profile of basalt geomorphology on the NESCA Central Hill .................................................................. 149 

9 .1. Bathymetric map showing sample locations ...... .. ...... .. .. .. ................ .... .... .. .. .. .... .... .. .. .. .. .. .... ........ .............. 155 

9.2-9.6. Graphs of: 

9.2. K20 and Ti02 versus fractionation index for SESCA and NESCA samples ............................... 157 

9.3. Sulfur versus fractionation index and FeO showing increasing trends with differentiation ........ 157 

9 .4. Ti02 versus FeO and CaO/ Al20 3 versus fractionation index........................................................ 158 

9.5. K20IP20 5 and K20/Ti02 versus fractionation index for Escanaba Trough samples ................... 159 

9.6. Fractionation index and K20IP20 5 versus latitude showing compositional variation along 

ridge axis .......................................................................................................................................... 159 

9.7. Histogram and graphs of olivine compositions of Escanaba Trough lavas .............................................. 160 

9.8. Histogram of plagioclase content in Escanaba Trough lavas .................................................................... 161 

9.9. Graph of composition of chromian spinel in Escanaba Trough lavas ....................................................... 164 

9.1 0. Graph of calculated temperatures of olivine-glass and densities of lavas from Escanaba Trough 

versus MgO .. .. . . . ... . . . . ... . . . .. . . . . . . . . . ..... .. ............ ................ .... . . . . .. . . . .. . ......... ... . . . . . . . ...... ..... . . . . .. . . . . .. . ...... .. ... ... . .. . . . . 165 

9 .11. Ternary diagram of Escanaba Trough glasses and Galapagos Rift and experimentally determined 

compositions ................................................................................................................................................ 166 

9.12. Graphs of lead isotope compositions for southern Escanaba Trough and other Gorda Ridge lavas ....... 168 

10.1. Map of Escanaba Trough showing location of NESCA and SESCA study areas, volcanic edifices, 

and DSDP Site 35 ........................................................................................................................................ 172 

10.2. Map showing geology and bathymetry of SESCA area ............................................................................. 174 

10.3. Photographs of sedimentary features at SESCA site ................................................................................. 176 

1 0.4. Photographs of sedimentary and hydrothermal features at SESCA site ................................................... 177 

10.5. Map showing geology and bathymetry ofNESCA area ............................................................................ 180 

10.6. Map showing geology and bathymetry of Central Hill in NESCA area ................................................... 182 

10.7. Photographs of hydrothermal features at NESCA site............................................................................... 185 

10.8. Photographs of hydrothermal features at NESCA site ............................................................................... 187 

10.9. Schematic cross sections of SESCA and NESCA areas ............................................................................ 190 

11.1. Map showing submarine hydrothermal vent sites discussed in text ......................................................... 202 

11.2. Map of Central Hill at NESCA site showing active hydrothermal vents and location of fluid 

samples ......................................................................................................................................................... 204 

11.3. Schematic drawing of experimental set-up for calibration of high-temperature probes .......................... 205 

11.4--11.10. Graphsof: 

12.1. 

12.2. 

12.3. 

12.4. 

12.5. 

11.4. High-temperature probe readings versus reference thermocouple and Hg thermometer 

readings . .... .. . . . . . . . .. . . . . .. . . . . .. .. .. .. . . . . .. . . .. .. . . . . .. .. . . . . . .. .... .. . . . . .. .. . . . . . . . .. . .. . .. . .. .. .. . .. .. .. .. .. . . .. .. .. .. . . . . . . . . . . . . . . . .. . . . . . 206 

11.5. pH, total alkalinity, ammonium, and hydrogen sulfide versus magnesium in Escanaba 

Trough vent fluids............................................................................................................................ 207 

11.6. Dissolved silica, chloride, lithium, sodium, potassium, and calcium versus magnesium in 

Escanaba Trough vent fluids ........................................................................................................... 208 

11.7. Ratios of Fe/Mn in submarine hydrothermal vent fluids from sediment-hosted, back-arc 

basin, and sediment-starved mid-ocean ridge environments ......................................................... 211 

11.8. End-member chloride at different submarine hydrothermal vents versus potassium and 

lithium .............................................................................................................................................. 212 

11.9. Alkaline earths relative to chloride in submarine hydrothermal vents ....................................... .. 

11.10. Sodium and calcium systematics relative to chloride in submarine hydrothermal vents ............ . 

Map of Escanaba Trough showing location of SESCA and NESCA study areas .................................. .. 

Graph showing variation of o180H 0 with Mg concentration for mixtures of high-temperature 

vent fluids and ambient seawater .. ~ ........................................................................................................... .. 

Graph showing variation of oDH 0 with Mg concentration for mixtures of high-temperature vent 

fluids and ambient seawater ....... : .............................................................................................................. .. 

Histogram of o34S values for Escanaba Trough sulfide and sulfate minerals ........................................ .. 

Diagram of calculated mineral assemblages produced during closed system reactions of seawater 

with Escanaba Trough basalt and mud ..................................................................................................... .. 

214 

215 

224 

227 

227 

228 

233 
VIII CONTENTS 

12.6. Graph showing calculated isotope shifts for seawater reacting with Escanaba Trough basalt and 

mud in a closed system . . .... ........ ...... .. . . . . ... . . . .. .. ........ ............ ................. ... . . . . .. . . . . .. .... .. .. ........ .. . . . . .. . . . . .. . . . . . . . . . . 234 

12.7. Diagram of calculated mineral assemblages produced during open system reactions of seawater 

with Escanaba Trough basalt and mud .. .... .......................... ........................... .. . ...................................... .. . 235 

12.8. Graph showing calculated isotope shifts for seawater reacting with Escanaba Trough basalt and 

mud in an open system . . .. .. . . . . . . . . ... . . . .. . . . . .. . . . ............. ............ ............ ... . . . .. . . . . .. . . . . ...... .. .......... .. . . . . .. . . . . . . . . . . .. . . . . 236 

13.1. Bathymetric map of NESCA area showing Alvin push core locations ..................................................... 242 

13.2-13.5. Graphs of: 

13.2. Interstitial water composition of Escanaba Trough Alvin push cores ........................................... 246 

13.3. Major- and minor-element sediment constituents relative to titanium in push cores................... 250 

13.4. Organic carbon ratios for N, I, Ni, V, and Mo in push cores ......................................................... 252 

13.5. Strontium versus barium in Guaymas Basin and Escanaba Trough cores .................................... 253 

14.1. Bathymetric map of Escanaba Trough showing location of SESCA and NESCA study areas ............... 259 

14.2. Bathymetric map of SESCA study area showing locations of core and dredge samples ......................... 261 

14.3. Scanning electron micrographs of altered sediment samples .................................................................... 263 

14.4. Scanning electron micrographs of altered sediment samples .................... .................................... ............ 264 

14.5. Graphs showing chemical and isotopic composition of unaltered and altered sediment samples 

from Escanaba Trough . . .. . . . . ... . . . .. . . . . .. . . . . .. . . . . .. . . . . ... .. . . . . . . . ............ .. . . .. . . . . . . . . . . ........ .. . . . . . .. ......... .. . . . . .. . . . ... . . . . .. . . . . . 268 

14.6. Histograms showing o180 values for bulk sediment and clay fraction separates .................................... 271 

15.1. Map showing location of selected samples, submersible tracks, and survey areas of Escanaba 

Trough .......................................................................................................................................................... 280 

15.2. Gas chromatograms from six sediment core samples ................................................................................ 283 

15.3. Mass fragmentograms showing distribution of triterpanes in extracts ..................................................... 285 

15.4. Mass fragmentograms showing distribution of steranes in extracts .......................................................... 286 

15.5. Gas chromatogram and mass fragmentograms showing distributions of hydrocarbons in oil 

extracted from sediment-hosted, massive sulfide sample.......................................................................... 290 

16.1. Location map and regional bathymetry of Gorda Ridge............................................................................ 295 

16.2. Map showing bathymetry and sampling locations for NESCA and SESCA sites.................................... 296 

16.3. Color photographs of sea-floor hydrothermal deposits in Escanaba Trough ............................................ 302 

16.4. Color photographs of hydrothermal deposit types from Escanaba Trough............................................... 305 

16.5. Photomicrographs and back-scattered electron images of pyrrhotite-rich massive sulfide samples ....... 306 

16.6. Cross section of polymetallic sulfide wall .................................................................................................. 308 

16.7. Photomicrographs and back-scattered electron images of polymetallic sulfide samples ......................... 309 

16.8. Photomicrographs and back-scattered electron images of sulfate-dominant samples ............................. 310 

16.9. Ternary plots of Escanaba Trough sulfide samples .................................................................................... 315 

16.1 0. Diagram showing chemical variations across wall of polymetallic sulfide structure ...... ....... ..... ..... ....... 316 

16.11. Generalized model for development of massive sulfide mounds .. ..... ... . . . . .. . . . . .................. .. . . . . .. . . . . .. . . . . ...... 319 

16.12. Phase diagram showing arsenic species at different pH and oxygen fugacity .......................................... 320 

17 .1. Map of NESCA area showing samples identified in table 17.1 ..... .. ................. ........................................ 328 

17.2-17.5. Diagrams showing: 

17.2. NESS similarity among Gorda Ridge soft-sediment samples ....................................................... 331 

17.3. Bray-Curtis similarity among Gorda Ridge samples ..................................................................... 332 

17.4. Rarefaction diversity curves showing relationship between species and individuals of 

ambient soft-sediment, sediment adjacent to wood, and hydrothermal soft-sediment 

communities ..................................................................................................................................... 332 

17.5. Relative abundance of 15 most common species in three sediment communities ....................... 333 

18.1-18.3. Maps of: 

18.1. Escanaba Trough showing sample location of foraminifers from the central area ....................... 338 

18.2. NESCA area showing sample locations of foraminifers ............................................................... 339 

18.3. SESCA area showing sample locations of foraminifers ................................................................ 340 

18.4. Scanning electron micrographs of foraminifers from Escanaba Trough................................................... 354 

18.5. Scanning electron micrographs of foraminifers from Escanaba Trough ................................................... 355 

18.6. Scanning electron micrographs of foraminifers from Escanaba Trough................................................... 356 

18.7. Color photomicrographs of foraminifers from Escanaba Trough ................................................. ............ 357 

18.8. Scanning electron micrographs of foraminifers from Escanaba Trough ................................................... 358 

18.9. Dynaphot photomicrographs of foraminifers from Escanaba Trough ....................................................... 359 
CONTENTS IX 

TABLES 

1.1. Cruises to Escanaba Trough that contributed to this study .. ........................ ... .. .............. .. .. .......... ......... .... .. ....... 3 

3.1. Heat-flow data collected during Tully cruise 89-04, Escanaba Trough.............................................................. 56 

4.1. Physical properties of sediment and basalt and parameters of hydrothermal model ......................................... 67 

4.2. Dimensions, energy loss, and rates of energy loss for model laccolith .............................................................. 71 

6.1. Summary of Site 35, Deep Sea Drilling Program................................................................................................ 94 

6.2. Deep Sea Drilling Project Site 35 pore-water chemistry..................................................................................... 97 

6.3. Surficial cores from Escanaba Trough and adjacent ridge flanks ....................................................................... 101 

6.4. Grain-size and heavy mineral data of mineralogical samples .... .. .. ............ .......... .... ................... ........ .... .. .... .. .... 102 

6.5. Summary of light-mineral grain counts ............................................................................................................... 105 

6.6. Summary and selected ratios of heavy-mineral grain counts .............................................................................. 106 

6.7. Pore-water chemistry of surficial core samples from cruise L2-86-NC ............................................................. 110 

7.1.. Holocene sedimentation rates of SESCA cores ................................................................................................... 139 

9 .1.. Locations and compositions of basaltic glasses from Escanaba Trough ............................................................ 156 

9.2.. Modal analyses of Escanaba Trough basalt samples ........................................................................................... 158 

9.3.. Representative olivine analyses from Escanaba Trough ..................................................................................... 161 

9.4.. Representative plagioclase analyses from Escanaba Trough .............................................................................. 162 

9.5.. Representative analyses of chromian spinel from Escanaba Trough .................................................................. 163 

9.6.. Temperature, density, and viscosity of selected Escanaba Trough glasses ......................................................... 164 

11.1. Escanaba Trough and Guaymas Basin sediments: selected chemical and mineral components .. .. .. .. .. .. .... .. .. .. 203 

11.2. Calibration data for DSV Alvin high-temperature probes ................................................................................... 206 

11.3. End-member data for major and minor elements from ridge hydrothermal vents ............................................. 209 

11.4. Alkali elements, Tl, Sr, and Bin Escanaba Trough and Guaymas Basin sediments and tholeiitic basalt ........ 211 

11.5. Water-to-sediment ratios for Escanaba Trough and Guaymas Basin .................................................................. 213 

12.1. Oxygen, hydrogen, and sulfur isotope data for vent fluids and vent minerals from Escanaba Trough ............ 227 

12.2. Sulfur isotope data for sulfide minerals from Escanaba Trough ......................................................................... 229 

12.3. Compositions of heated seawater and rocks before and after calculated seawater-rock reactions ................... 232 

13 .1. Descriptions of Escanaba Trough surface cores .... .. .. .... .. ........................ ............... .......................... ...... .. .. .. ...... . 243 

13.2. Escanaba Trough interstitial fluid compositions .................................................................................................. 245 

13.3. Major-element compositions of Escanaba Trough solids .................................................................................... 247 

13.4. Minor-element compositions of Escanaba Trough solids .................................................................................... 248 

13.5. Isotope data for calcite from cores 2040 AC-1 and 2040 AC-2 .......................................................................... 252 

14.1. Major- and minor-element composition of unaltered, altered, and mineralized sediment ................................ 266 

14.2. Mineral compositions used for mass balance calculations .. ........... ..... .... .. .. .............. ........................... .... ...... .. .. . 269 

14.3. Oxygen isotope values for bulk sediment samples .............................................................................................. 270 

14.4. Oxygen isotope values for clay fraction separates .............................................................................................. 272 

14.5. Sulfur isotope values for sediment-hosted mineralization .................................................................................. 273 

15.1. Description of Escanaba Trough sediment cores from which samples were taken for organic 

geochemical analyses ........ .................. .......... ............ ..... .. . ............ ............ ............... .. . .............. ... ... ... ...... ....... ...... 281 

15.2. Organic geochemical data for sediment core samples and one oil from Escanaba Trough ............................... 282 

15.3. Identification of triterpanes and steranes ............................................................................................................. 284 

15.4. Average values of molecular ratios in sediments and an oil from Escanaba Trough ......................................... 288 

15.5. Description of sediment organic matter in four samples from Escanaba Trough .............................................. 289 

16.1. Location and description of hydrothermal samples from Escanaba Trough .. ........... ................... ........... .... ...... . 298 

16.2. Mineralogy of hydrothermal samples from Escanaba Trough ............................................................................ 304 

16.3. Bulk composition of sulfide-dominant samples .................................................................................................. 312 

16.4. Composition of polymetallic sulfide sample L2-86-14D-1 ................................................................................. 316 

16.5. Bulk composition of sulfate-dominant samples ................................................................................................... 317 

16.6. Comparison of metal contents in mid-oceanic ridge basalt samples from northeast Pacific Ocean and 

sediment samples from Escanaba Trough . ......... .......... ............ ............ ........ ...... . . . . .................. ..... ...... ............ ..... 318 

17 .1. Dives by DSV Alvin to Escanaba Trough in 1988 .............................................................................................. 329 

17.2.. Most common species in box core samples from ambient Escanaba Trough soft-sediment communities ....... 330 

17.3. Most common species in single 225-cm2 box core sample taken adjacent to a piece of wood ........................ 330 

17 .4. Abundance of species from Escanaba Trough soft-sediment hydrothermal qualitative scoop sample............. 331 
X CONTENTS 

17.5. Fauna from hard surfaces in Escanaba Trough .................................................................................................... 334 

18.1. List of samples, water depths, and summary of foraminiferal faunal characteristics, Escanaba Trough ......... 341 

18.2. Agglutinated foraminifers from Escanaba Trough .............................................................................................. 344 

18.3. Calcareous foraminifers from Escanaba Trough .................................................................................................. 346 

18.4. Ratio of sinistrally to dextrally coiled Neogloboquadrina pachyderma ............................................................ 348 
Chapter 1. Geologic, Hydrothermal, and Biologic Studies :at 

Escanaba Trough: An Introduction 

By Janet L. Morton, Robert A. Zierenberg, and Carol A. Reiss 1 

CONTENTS 

Page 

Introduction ......................................................................... . 

Acknowledgments . . ... . .. . ... . .. . . . .. .. .. .. . . . . .. . . . . . . . . . . . . . . . . . ... . . . 2 

Geologic setting of Escanaba Trough . ..... .................... ....... 2 

History of work at Escanaba Trough . ................ .... .... ..... .... 3 

Navigation methods..................................................... 9 

Contents of this volume....................................................... 9 

Significant findings ............................................................. 10 

Directions of future work.................................................... 14 

References cited................................................................... 15 

INTRODUCTION 

The discovery of sea-floor hot springs at Galapagos 

Rise in 1977 (Corliss and others, 1979) confirmed the predictions of hydrothermal venting at oceanic spreading centers, which had been inferred from several lines of 

evidence, including low heat flow near mid-oceanic ridges 

(Elder, 1965; Talwani and others, 1971; Lister, 1972; 

Sclater and Klitgord, 1973; Williams and others, 1974), 

metalliferous deep-sea sediments (Bostrom and Peterson, 

1966; Bostrom, 1973), chemical tracers in the water column (Weiss and others, 1977; Lupton and others, 1977; 

Klinkhammer and others, 1977; Bolger and others, 1978), 

near bottom temperature anomalies (Detrick and others, 

1974; Rona and others, 1975), and hydrothermal alteration 

and mineralization in ophiolites (Sillitoe, 1972; Spooner 

and Fyfe, 1973; Duke and Hutchinson, 1974; Spooner and 

Bray, 1977). The hydrothermal vents at Galapagos Rise 

were low temperature (less than about 20°C) and were surrounded by exotic biologic communities. In the following two years, massive sulfide deposits (Francheteau and 

others, 1979) and high-temperature vents (about 350°C) 

(RISE, 1980) were discovered on the East Pacific Rise near 

latitude 21 o N., and in subsequent years, additional vents 

and deposits were located at many locations in the Pacific 

and Atlantic Oceans. These hydrothermal systems have 

been the focus of intense interdisciplinary studies of fun1 U.S. Geological Survey, Menlo Park, California. 

damental problems such as the formation of mineral 

deposits, the chemical and thermal effects of vents on the 

ocean, alteration of oceanic crust, and interaction of vent 

animals and hydrothermal fluids. 

In 1979, the U.S. Geological Survey (USGS) began a 

project to study volcanic, tectonic, and hydrothermal 

processes at oceanic spreading centers. Much of the work 

has been conducted in the northeast Pacific Ocean, in 

conjunction with research programs conducted by the 

National Oceanic and Atmospheric Administration 

(NOAA), various academic institutions, and foreign government agencies. With the declaration of the U.S. Exclusive Economic Zone (EEZ) in 1983, much of the USGS 

effort became focused on Gorda Ridge (fig. 1.1 ), the only 

mid-oceanic ridge spreading center located within the U.S. 

EEZ. Two sites at Gorda Ridge became the focus of multiinstitutional studies: northern Gorda Ridge at a locale 

where water column chemical anomalies indicated re:gional 

hydrothermal venting (Baker and others, 1987) and at 

Escanaba Trough, southern Gorda Ridge, where the spreading axis is buried by several hundred meters of sediment. 

Mid-oceanic ridges can be classified as sediment 

starved or sediment dominated. Owing to their setting far 

from continental sediment sources, most mid-oceanic 

ridges, including the northern Gorda Ridge, fall into the 

sediment-starved category. Hydrothermal systems at these 

ridges reflect the interactions of the two-compone111t system basalt-seawater, and the resulting hydrothermal deposits are fairly similar in form and composition at various 

locations (Bischoff and others, 1983; Kappel and Franklin, 

1989). A blanket of sediment covering the ridge crest 

changes the thermal, hydrologic, and chemical regime, and 

can have major effects on the composition of the hydrothermal fluids and on the process of venting hydrothermal 

fluid to the sea floor. These effects are reflected in the form 

and composition of the resulting hydrothermal deposits. 

Sediment-dominated ridges are relatively uncommon, and 

hydrothermal systems at sediment-dominated ridge:s have 

been studied at only a few locations. At Guaymas Basin 

in the Gulf of California, hydrothermal products reflect the 

organic-rich composition of the sediment (Lonsdale and 

others, 1980; Simoneit and Lonsdale, 1982; Koski and others, 1985). At both Middle Valley (northern Juan de Fuca 
2 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

Ridge) and Escanaba Trough, turbiditic and hemipelagic 

sedimentary rocks host the hydrothermal deposits. The 

papers in this volume describe the results of studies made 

during 12 cruises to Escanaba Trough, mostly between 

1985 and 1989. Collectively, they represent one of the most 

comprehensive studies to date of geologic, hydrothermal, and 

biologic processes at a sedimented spreading center. 

ACKNOWLEDGMENTS 

This work represents the efforts of many people located at several institutions and was possible only as a result of close cooperation and coordination. We thank all 

Solvanco Fracture Zone 

1.' 

Juan de Fuca Ridge J Middle 

/ Valley 

Blanco Fracture Zone 

Escanaba Trough .....,. STUDY AREA 

Mendocino Fracture Zone 

PACIFIC OCEAN 

0 500 KILOMETERS I I I 

Cat 40° N) 

of the authors who contributed chapters to the volume. We 

are especially grateful to the captains and crews of the various vessels listed in table 1.1 that supported work in 

Escanaba Trough. We gratefully acknowledge the expert 

drafting assistance of Phyllis Swenson and Brigitta Fulop. 

GEOLOGIC SETTING OF ESCANABA 

TROUGH 

Gorda Ridge is located in the northeast Pacific Ocean, 

200 to 300 km west of northern California and Oregon (fig. 

1.1). It is bounded to the north by the Blanco Fracture 

Zone and to the south by the Mendocino Fracture Zone. 

NORTH AMERICA 

EXPLANATION 

Sediment-hosted 

sulfide deposits 

Figure 1.1. Location map showing Gorda Ridge and other mid-oceanic ridge spreading centers in northeast Pacific Ocean. 
GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH: AN INTRODUCTION 3 

Table 1.1. Cruises to Escanaba Trough that contributed to this study. 

Cruise 10 Year Vessel Type of studies reported in this volume 

L12-81-NC 1981 

KK2-83-NP 1983 

R/V S.P. Lee 

R/V Kana Keoki 

Multi-channel seismic reflection profiling 

Dredging 

L6-85-NC 1985 R/V S.P. Lee Single-channel seismic reflection profiling, magnetics profiling, sediment 

coring, dredging 

MMS-86-1 1986 

N1-86-NC 1986 

NOAA ship Surveyor 

USNS Narragansett 

Sea Beam bathymetric mapping 

Bathymetric profiling, camera tows 

L1-86-NC 1986 R/V S.P. Lee Bathymetric profiling, single-channel seismic reflection profiling, camera 

tows, dredging, sediment coring 

L2-86-NC 1986 R/V S.P. Lee, 

DSV Sea Cliff 

Bathymetric profiling, camera tows, dredging, sediment coring, submersible 

dives 

L3-86-NC 1986 

T1-86-NC 1986 

R/V S.P. Lee 

R/V Transquest, 

Dredging, sediment coring 

Submersible dives 

DSV Sea Cliff 

A2-88-NC 1988 R/V Atlantis II, Submersible dives 

DSV Alvin 

C1-88-NC 1988 MN Laney Chouest, Submersible dives 

DSV Sea Cliff 

TUL-89D 1989 R/V Tully Sediment coring, heat flow measurements, single-channel seismic reflection 

profiling 

Gorda Ridge and Juan de Fuca Ridge, located to its north, 

are remnants of the Pacific-Farallon spreading center and 

form the present-day accreting plate boundary between the 

Pacific and Juan de Fuca plates. The spreading rate at Gorda 

Ridge varies from medium (about 5.5 cm/yr) at the northern 

end to slow (about 2.4 cm/yr) at the southern end (Atwater 

and Mudie, 1973; Riddihough, 1980; Wilson, 1989). The 

ridge displays a Mid-Atlantic Ridge style morphology with 

a deep axial valley (2,500 to 3,300 m water depth) and flanking ridges that rise to as shallow as 1,500 m. 

Right-lateral offsets near latitudes 42°26' N. and 

41 °37' N. divide Gorda Ridge into three segments (fig. 1.1; 

Clague and Holmes, 1987). The northern segment trends 

north-northeast, normal to the Blanco Fracture Zone, and 

the southern segment trends approximately north-south, 

normal to the Mendocino Fracture Zone. The trend of the 

middle segment is less well defined but is generally between that of the northern and southern segments. 

Escanaba Trough occupies the southern, north-southtrending segment of Gorda Ridge. At the northern end of 

the trough, the axial valley is only a few kilometers wide, 

but it widens to about 18 km at the south. As the ridge 

approaches the Mendocino Fracture Zone, it curves to the 

east (Clague and Holmes, 1987; EEZ-SCAN 84 Scientific 

Staff, 1986). Immediately north of the Mendocino Fracture 

Zone, the topographic expression of the ridge disappears, 

and the zone of active extension is marked by a region of 

extensional faulting detectable on seismic-reflection 

profiles (Clague and Holmes, 1987; Morton and Fox, chap. 

2, this volume). 

The absence of a topographic ridge at the southern end 

of Escanaba Trough has allowed turbidity currents to enter 

and flow northward along the trough (Vallier and others, 

1973). Consequently, south of about latitude 41 o rl' N. 

most of the axial valley floor is sediment covered. At the 

southern end of the trough, sediment thickness may reach 

900 m (Clague and Holmes, 1987). Hemipelagic sediment 

is interbedded with the turbidites (McManus and others, 

1970; Karlin and Lyle, 1986; Normark and others, chap. 

6, this volume). Volcanic bodies pierce the sediment fill 

at several localities along the trough (fig. 1.2; Moore and 

Sharman, 1970; Clague and Holmes, 1987; Morton and 

others, 1987a). · 

HISTORY OF WORK AT 

ESCANABA TROUGH 

McManus (1964) named the median valley of southern Gorda Ridge Escanaba Trough. Wilde and others 

(1978) compiled the locations of early seismic profiles, 

underway geophysical lines, sediment samples, and heatflow stations in the Gorda Ridge area. Early work in the 

area included topographic and sedimentologic studies, summarized by McManus ( 1967), and magnetic surveying by 

Raff and Mason (1961). Vine and Matthews (1963) interpreted the axis-parallel linear magnetic anomalies in this 

region to be the result of sea-floor spreading. McManus 

( 1967) noted that the flat floor of southern Escanaba 

Trough results from a partial filling of the valley by sediment. Heat-flow measurements obtained near Gorda Ridge 

(Dehlinger, 1969) indicated that the region is characterized 

by high heat flow, and seismic-refraction work (Raitt, 

1963) indicated low mantle velocities beneath the ridge. 
4 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

41 °00' 

127°40' 

EXPLANATION 

Volcanic 

edifice 

Sediment 

fill 

Terrace 

0 10 

KILOMETERS 

Atwater and Mudie (1973) used a deep-towed survey (sidescan sonar and subbottom profiling) to describe the details 

of block faulting within one area of the axial valley. 

Escanaba Trough was chosen as Site 35 of the Deep 

Sea Drilling Program (DSDP) because the sediment cover 

permitted drilling at the axis of a mid-oceanic ridge. Sitesurvey seismic-reflection profiles indicated that the sediment cover averages about 500 m in thickness and is cut 

by several faults (Moore and Sharman, 1970; Moore, 

1970). A low ridge visible on the seismic records was interpreted by Moore and Sharman ( 1970) as a volcanic ridge 

at the axis of the trough. Drilling at DSDP Site 35 (fig. 

1.2) penetrated 395 m of predominantly Pleistocene turbidites without reaching basement rock (McManus and others, 1970). Vallier and others (1973) examined sand grains 

from turbidites in selected cores of Site 35 and concluded 

that much of the sediment is derived from the Klamath and 

Columbia River drainage basins. 

In 1981, three 24-channel seismic-reflection profiles 

were collected over Escanaba Trough. Hyperbolic reflectors within the sediment fill at the center of the trough were 

interpreted as sills (Clague and others, 1984; Clague and 

Holmes, 1987). 

The work reported in this volume was conducted primarily between 1985 and 1989. In 1985, cruise L6-85-NC, 

of the research vessel S.P. Lee, jointly sponsored by the 

USGS and the Gorda Ridge Technical Task Force, conducted a reconnaissance geophysical and sampling study 

at Escanaba Trough. Seismic-reflection profiles identified 

several discrete volcanic edifices, spaced at about 15 km 

intervals, that intrude the sediment fill (Morton and others, 1987a). High heat-flow values (up to 1,600 mW/m2) 

measured at two of the volcanic edifices were interpreted 

to indicate flow of hydrothermal fluids through the sediment (Abbott and others, 1986). Pyrrhotite-rich massive 

sulfide samples and a sediment sample impregnated with 

petroleum and interlayered with massive sulfide were recovered from one of the edifices (K venvolden and others, 

1986; Morton and others, 1987a; Koski and others, 1988). 

Two volcanic edifices were selected for more detailed 

follow-up studies on several cruises from 1986 to 1989. 

The northern Escanaba Trough study area (NESCA) (fig. 

1.3) is located near latitude 41 °00' N., longitude 127°30' 

W. (fig. 1.2), and the southern Escanaba Trough study area 

(SESCA) (fig. 1.4) is located near latitude 40°45' N., longitude 127°30' W. (fig. 1.2). The purpose of these cruises 

was to determine the location, form, and composition of 

hydrothermal deposits, the geologic setting and structural 

controls of the deposits, the nature of active venting, and 

Figure 1.2. Generalized geologic map of Escanaba Trough showing 

location of volcanic edifices, approximate extent of sediment fill, 

location of terraces, NESCA and SESCA study areas (rectangles), 

location of edifice REX, and location of DSDP Site 35. 
GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH: AN INTRODUCTION 5 

the type of biological communities associated with hydrothermal vents and deposits. Studies conducted during these 

cruises included bathymetric surveying, seismic-reflection 

profiling, sea-floor photography, dredging, sediment coring, 

and submersible dives. A total of 26 dredge hauls, 22 

camera stations (figs. 1.5, 1.6) and 22 submersible dives 

3,200 I 

3,300 

NESCA 

~ 

64,000 

METERS 

were conducted in the NESCA and SESCA areas (figs. 1.7, 

1.8). In addition, 33 sediment cores (see Normark and others, chap. 6, this volume) and approximately 1 ,350 km of 

seismic-reflection data (see Morton and Fox, chap. 2, this 

volume; Davis and Becker, chap. 3, this volume) have been 

collected at Escanaba Trough. 

50,000 

60,000 

METERS 

Figure 1.3. Bathymetric relief map of axial valley floor of Escanaba Trough at NESCA area, viewed from northwest. Notations along 

horizontal axes are coordinates of local transponder net, in meters. Axial valley floor is bounded to east and west by low terraces, most 

prominent here on the east (left). Valley floor at NESCA area is characterized by two major hills with relief of about 100 m and several 

smaller features. 

SESCA 

(/:} ~ 3,1001 ~ 

~ 3,200 

~ 

3,300 

~ 8,000 

10,000 

METERS 

14,000 

16,000 5,000 METERS 

Figure 1.4. Bathymetric relief map of axial valley floor of Escanaba Trough at SESCA area, viewed from southeast. Notations along 

horizontal axes are coordinates of local transponder net, in meters. Three prominent hills are located near east (right) side of area. High 

region on left is part of west wall that bounds axial valley floor. 
6 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

12J032' 

0 2 

KILOMETERS 

Contour interval 10 meters 

56,000 

12J030' 

58,000 60,000 

66,000 

64,000 

62,000 

60,000 

I 

58,000 

56,000 

54,000 

52,000 

62,000 50,000 

Figure 1.5. Bathymetric map of NESCA area showing location of 6 camera tows (solid lines) and 17 

dredge hauls (dotted lines). Numbers along right and bottom give coordinates of local transponder net 

(in meters). Hachures indicate area of closed low. 
GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH: AN INTRODUCTION 7 

Sea Beam swath bathymetric surveying of most of 

Escanaba Trough was conducted in 1986, but was classified until 1989 (Morton and Fox, chap. 2, this volume). 

Conventional bathymetric surveys of the SESCA and 

NESCA areas with line spacings of approximately 300 m 

were also conducted in 1986 (Morton and others, 

1987b). The bathymetric maps of the NESCA and 

SESCA areas used to show station locations in most 

chapters in this volume are based on the conventional 

bathymetric surveying, with minor modifications based 

on submersible dive profiles and Sea Beam mapping. 

6000 8000 10,000 

Although the Sea Beam maps reveal considerable more 

detail of the sea-floor morphology, the relative positions 

of major bathymetric features in the SESCA and NESCA 

areas are distorted owing to inaccuracies in the Loran C 

navigation used during the survey. The conventional 

bathymetric surveys were navigated using the same 

acoustic transponder nets used for all sampling stations, 

camera tows, and submersible dives, and thus the relative positions of sampling stations and major bathymetric features are more accurately depicted using these 

conventional bathymetric maps. 

12J032' 12J028' 

\ 

16,000 

14,000 

8000 

0 2 KILOMETERS 

Contour interval 10 meters 

12,000 14,000 

6000 

16,000 

Figure 1.6. Bathymetric map of SESCA area showing location of 16 camera tows (solid lines) and 9 dredge hauls (dotted lines). Numbers 

along right and bottom give coordinates of local transponder net (in meters). Hachures indicate area of closed low. 
8 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

12J032' 12J028' 

66,000 

64,000 

62,000 

56,000 

54,000 

52,000 

62,000 50,000 

Figure 1.7. Bathymetric map of NESCA area showing location of 17 submersible dives. Numbers 

along right and bottom give coordinates of local transponder net (in meters). Hachures indicate area of 

closed low. 
GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH: AN INTRODUCTION 9 

12r32· 12r3o· 

--~-.<:~,-·~· .. -· 6u61, u ---_·-:·.·<-.:,.)_· .. •• .. ·.~; :_ - -· _- ' • I 

- .- -_ · .. 662. ' 

12,000 

io,ooo . -,.16,000 

Figure 1.8. Bathymetric map of SESCA area showing location of 5 submersible 

dives. Numbers along right and bottom give coordinates of local transponder net (in 

meters). Hachures indicate area of closed low. 

NAVIGATION METHODS 

The positioning data for the NESCA and SESCA areas 

were obtained by sea-floor anchored acoustic transponder 

nets, continuously maintained since 1985 and 1986, respectively. This allows ongoing work to be directly correlated 

with previous data sets collected in the areas, ensuring 

internal consistency. 

Each net consists of five to seven transponders, deployed in specific locations within a 100 to 160 km2 area. 

An integrated satellite (NNSS)-Doppler sonar-Loran C 

navigation system and a satellite Global Positioning System (GPS) provided supplemental navigation during transponder deployment and provided greater accuracy for 

calibration of the net. GPS coverage was available in this 

area about 14 hours per day (Morton and others, 1987b ). 

Acoustical data received simultaneously from three or 

more sea-floor transponders provided position accuracy 

within ±5 meters relative to the net. This provides sufficient accuracy for future reoccupation of hydrothermal deposits and vent sites. 

CONTENTS OF THIS VOLUME 

This volume presents the findings of geological, geophysical, and biological investigations conducted on several cruises to Escanaba Trough between 1985 and 1989 

(table 1.1 ). Preliminary findings of some of these studies 

have been reported in various internal reports (Benninger 

and Koski, 1987; Clague and others, 1984; Karlin and 

Lyle, 1986; Morton and others, 1987b; Bibee, 1986) and 

at the Gorda Ridge Symposium in May 1987 (McMurray, 

1990; Holmes and Zierenberg, 1990; Morton and others, 

1990; Carey and others, 1990; Van Dover and others, 

1990). This volume contains the most complete synthesis to date of studies on sediment-hosted sulfide deposits 

at Escanaba Trough, and it includes much previously 

unpublished data. 

The volume is divided into three sections: (1) Geologic Processes; (2) Hydrothermal and Geochemical Studies; and (3) Biologic Studies. In the first section, chapters 

2 to 9 are topical studies, including geophysical, sedimentologic, petrologic, and theoretical, which address aspects 

of the geologic setting of Escanaba Trough and (or) the 

SESCA and NESCA study areas. Chapter 10 synthesizes 

the results of several studies to present our current understanding of the geologic processes at the SESCA and 

NESCA sites. The six chapters in the second section address sediment alteration, hydrothermal vent fluid chemistry, hydrothermal petroleum formation, and the 

mineralogy, chemistry, and isotope geochemistry of sulfide 

deposits at SESCA and NESCA. The two chapters in the 

third section present studies of soft-sediment hydrothermal 
10 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

vent communities and benthic foraminifers of the SESCA 

and NESCA areas. 

SIGNIFICANT FINDINGS 

Sea Beam mapping indicates that Escanaba Trough 

consists of two segments with a small right-lateral offset 

near latitude 41 °07' N. In most places, the depth of the 

axial valley is greater than 3,300 m; however, between 

latitudes 41°17' N. and 41°28' N., the valley floor rises 

abruptly to less than 3,250 m. This shallow region appears to act as a topographic barrier to turbidity flows, 

which enter the valley at its southern end and flow northward (Morton and Fox, chap. 2, this volume). In contrast 

to sediment-free spreading centers, the sedimented axis 

of Escanaba Trough lacks a central magnetic anomaly 

high, perhaps indicating extensive alteration of the basaltic basement (Karlin and Morton, chap. 5, this volume). 

Normark and others (chap. 6, this volume) consider probable sediment accumulation rates and constraints on the 

age of the underlying crust at Escanaba Trough and conclude that the majority of the sediment fill probably accumulated during the last two (or at most three) sea-level 

low stands during the Pleistocene. The Columbia and 

Klamath river drainages are the probable sediment source 

regions. Magnetic susceptibility of the surface sediment 

in the SESCA area records a change in sedimentation correlated with the change from glacial to interglacial conditions at the Pleistocene-Holocene transition (Karlin and 

Zierenberg, chap. 7, this volume). Locally derived 

Holocene turbidites and erosion of Holocene sediment are 

indications of recent tectonism. 

Bathymetric and seismic-reflection data (Morton and 

Fox, chap. 2, this volume; Davis and Becker, chap. 3, this 

volume) delineate six major volcanic centers along the 

sediment-covered part of the spreading axis. The volcanic edifices are characterized by an abrupt transition from 

undisturbed sediment layers to deformed, tilted, and commonly uplifted sediment layers. Sills intrude and disrupt 

the sediment layers, and basalt flows are locally extruded 

at the sea floor. These volcanic centers are areas of elevated heat flow (Abbott and others, 1986; Davis and 

Becker, chap. 3, this volume) and hydrothermal mineralization. Regionally, away from areas of venting, heat flow 

and predicted basement temperatures are relatively low. 

This result suggests that either the heat sources are localized and the upper crustal temperatures have not been significantly affected by volcanism, or that the basement 

temperatures are elevated but there has been insufficient 

time since heating for the thermal conditions to equilibrate 

in the thick sedimentary section (Davis and Becker, chap. 

3, this volume). 

Much of the work reported in this volume was conducted at two of these volcanic edifices, at the NESCA and 

SESCA sites; figures 1.9 to 1.12 illustrate some of the geologic and hydrothermal features of these two sites. Most 

of the sea floor at NESCA and SESCA is sediment covered (fig. 1.9A; Zierenberg and others, chap. 10, this volume); the sedimentary layer varies abruptly from a thin 

covering to several hundred meters thick (Morton and Fox, 

chap. 2, this volume). Several steep-sided hills, formed of 

uplifted and tilted sediment, are present at NESCA and 

SESCA (fig. 1.9B, C). Mechanical modeling suggests that 

the hills formed by the inflation of basaltic sills near the 

sediment-basement interface; permeable fault zones developed during uplift focus hydrothermal flow at the perimeters of the hills (Denlinger and Holmes, chap. 4, this 

volume). Volcanic rocks are exposed locally on the sea 

floor at both NESCA and SESCA sites (fig. 1.9D,E,F). 

The morphology of the exposed volcanic rocks in the 

NESCA area indicates a central source region for eruption; 

sea-floor relief provides the primary control on the distribution of pillow lavas and sheet flows (Ross and 

Zierenberg, chap. 8, this volume). Chemical analyses of 

several basalt samples, especially lead isotope measurements, suggest that locally the basalt assimilated small 

amounts of sediment prior to eruption (Davis and others, 

chap. 9, this volume). 

Several large massive sulfide deposits, up to several 

tens of meters high and a few hundred meters across, occur at the base of several of the small hills at NESCA and 

SESCA (fig. 1.10; Zierenberg and others, chap. 10, this 

volume). Active hydrothermal venting has been observed 

at three locations at the NESCA site (fig. 1.11). The processes and products of hydrothermal activity in Escanaba 

Trough are discussed in a series of papers in the second 

section of this volume. 

The effects of hydrothermal activity on near-surface 

sediment are described by Zierenberg and Shanks (chap. 

14, this volume). Replacement of hemipelagic sediment 

by magnesium-rich chlorite results from intense Mg-metasomatism in a mixing zone where hydrothermal fluid interacts with seawater and sediment at temperatures above 

200°C. Lower temperature reactions related to flow of 

hydrothermal fluid through sediment are recorded by 

changes in surface sediment chemistry and vertical changes 

in pore fluid composition (Magenheim and Gieskes, chap. 

13, this volume). 

Organic matter in near-surface sediments is derived 

predominantly from terrigenous sources (K venvolden and 

others, chap. 15, this volume). The organic geochemistry 

of near-surface sediment shows that organic matter in the 

Holocene sediment is more mature than the organic matter in underlying Pleistocene sediment. This inverted degree of maturation may reflect resedimentation of older 

sediment transported in locally derived Holocene turbidites. Sediment cores displaying mineralogical evidence of 

hydrothermal alteration (fig. 12D) also contain organic 

matter altered at elevated temperature. 
GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH: AN INTRODUCTION 11 

The analyses of hydrothermal fluids collected at 

active vents over a temperature range from 15°C to 217°C 

are reported by Campbell and others (chap. 11, this volume). The vent fluids are slightly alkaline and have 

A 

c 

E 

Figure 1.9. Sedimentary and volcanic features at NESCA and 

SESCA sites. Width of images is about 1.5 to 3 m. A, Typical 

sediment-covered sea floor at NESCA site showing pyramid urchin. 

Photograph taken from DSV Alvin by John Edmond. B, Lithified 

mudstone outcropping on steep face of small hill at NESCA site. C, 

chlorinity elevated by approximately 19 percent relative 

to seawater. Concentrations of dissolved transition metals 

are low, and alkali and alkaline earth elements and ammonia are high relative to hydrothermal fluids issuing from 

B 

D 

F 

Mudstone outcrops and talus at base of small hill at SESCA site 

showing effect of mass-wasting processes. D, Glassy, fresh pillow 

basalt flow at NESCA site. E, Glassy sheet flow from NESCA site. 

F, Older, fissured basalt flow exposed at SESCA site. 
12 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

vents on sediment-free spreading centers. Chemical and 

isotopic compositions of the hydrothermal fluids require 

extensive interaction with sediment within the hydroA 

c 

E 

Figure 1.10. Massive sulfide deposits and hydrothermal features at 

NESCA and SESCA sites. Width of images is about 1.5 to 3m. A, 

Coalesced sulfide and sulfate chimneys atop massive sulfide mound 

at NESCA site. In center of photograph, fresh surface has been 

exposed by removal of barite sample (fig. 1.12B). B, Older, oxidized 

sulfide deposit colonized by sponges at SESCA site. C, Sulfide 

thermal circulation system. Hydrogen and oxygen isotope 

compositions of vent fluids are close to those of starting 

seawater (Bohlke and Shanks, chap. 12, this volume). 

B 

D 

F 

mound and talus at NESCA site. Rectangular marker at center of 

photograph is 15 by 30 em. D, Sulfide talus and sediment colonized 

by clams and bacterial mats adjacent to sulfide mound at NESCA 

site. E, Small dome of massive sulfide with crude layering parallel 

to the surface at NESCA site. F, Hydrothermally lithified sediment 

located about 1 m from sulfide deposit at NESCA site. 
GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH: AN INTRODUCTION 13 

Sulfur isotope values of H2S in the fluid and in hydrothermal sulfide minerals are elevated relative to values in 

samples from sediment-free ridge crests and require incorporation of seawater sulfate that has been reduced to sulfide by high-temperature interaction with organic matter 

in the sediment. 

The composition of the hydrothermal deposits also requires extensive chemical input to the hydrothermal system from sediment (Koski and others, chap. 16, this 

volume). The sulfide deposits are pyrrhotite-rich (fig. 12A) 

and have significant contents of copper, generally in the 

mineral isocubanite. Many samples are highly enriched in 

gold. Relative to sulfide deposits on sediment-free ridge 

crests, Escanaba Trough samples are enriched in arsenic, 

antimony, bismuth, lead, and tin derived from altered sediment in the high-temperature reaction zone at depth. In 

polymetallic sulfide samples, the telescoped chemical 

zonation from high-temperature, copper-arsenic-rich 

assemblages to lower temperature silver-lead assemblages 

A 

c 

Figure 1.11. Massive sulfide deposits and active hydrothermal vents 

on sea floor at NESCA site. Width of images is about 1.5 to 3m. A, 

Side of actively venting massive sulfide chimney colonized by tube 

worms and palm worms. B, Tube worms and anemones colonize 

massive sulfide talus where diffuse hydrothermal venting is 

with barite is evidence of steep thermal gradients across 

fluid conduits at sites of sea-floor hydrothermal discharge. 

The hydrothermal vent sites in the NESCA area are 

the southernmost known occurrence of chemautotrophic 

fauna on the Juan de Fuca-Gorda Ridge system, and as 

such provide important data on the distribution, propagation, and speciation of hydrothermal vent fauna. The 

Escanaba Trough vent fauna resemble those found to the 

north on the Juan de Fuca Ridge rather than the wellstudied vent communities distributed along the East Pacific 

Rise and at hydrothermal sites in the sediment-covered 

Guaymas Basin (Van Dover and others, 1990; Grassle and 

Petrecca, chap. 17, this volume). Agglutinated foraminifers, abundant in dredge and core samples from Escanaba 

Trough, often display an unusual feature, the use of angular basaltic glass fragments in test construction (Quinterno, 

chap. 18, this volume). An agglutinated foraminifer recovered in dredge samples has not been previously described 

and may be a new genus. 

B 

D 

occurring. C, Side of sulfide mound showing cluster oftube worms, 

anemones, and bacterial mats. D, Hydrothermal deposits (primarily 

barite) adjacent to site of active venting. Shiny black material is 

possibly pyrobitumen. 
14 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

DIRECTIONS OF FUTURE WORK 

Geophysical studies have identified at least six major 

volcanic centers within the sediment-covered part of 

Escanaba Trough. Reconnaissance geologic mapping has 

been carried out only in the SESCA and NESCA study 

areas. Both of these areas contain extensive areas of outcropping massive sulfide mineralization. Exploratory submersible dives on the volcanic center at the northern edge 

of the NESCA study area (edifice REX) also discovered 

extensive areas of sulfide mineralization (Zierenberg and 

others, chap. 10, this volume). Reconnaissance sediment 

coring at the volcanic center between the SESCA and 

NESCA areas recovered hydrothermally altered and mineralized sediment (Normark and others, chap. 6, this volume). Our present understanding of the processes leading 

to sulfide mineralization at the SESCA and NESCA sites 

suggests that the same processes should have operated at 

each of the volcanic centers, and it is reasonable to speculate that the Escanaba Trough represents a major mineral 

deposit district with multiple centers of hydrothermal 

A 

c 

activity. An obvious next step is to delineate the volcanic 

centers in Escanaba Trough and to carry out reconnaissance-scale geologic and geophysical mapping to identify 

other areas of mineralization. The most efficient means 

of doing this would be a medium-resolution side-scan sonar and seismic-reflection survey of the floor of the axial 

valley in Escanaba Trough, followed by camera mapping 

and sampling cruises in areas identified as targets by the 

geophysical studies. 

Temporal evolution of the hydrothermal processes can 

be investigated in addition to the spatial distribution of deposits. The known sulfide mineralization at the volcanic 

center at the northern end of the NESCA area and the deposits in the SESCA area are inactive, mature deposits. 

Some of these deposits are being chemically degraded by 

oxidative dissolution on the sea floor or are being actively 

eroded by mass wasting (Holmes and Zierenberg, 1990; 

Zierenberg and others, chap. 10, this volume). Massive 

sulfide deposits in the NESCA area are large, mature deposits, but are still locally hydrothermally active. Seismic 

data from the volcanic center between the SESCA and 

B 

GOFlD/l.. HIDGED 

Figure 1.12. Mineralized samples collected from NESCA and SESCA sites. A, Typical pyrrhotite-rich massive sulfide sample. B, Chimney 

composed mostly of barite recovered from NESCA site (fig. I. lOA). C, Mudstone breccia cemented by sulfide. Sample is approximately 7.5 

em wide. D, Sediment core with sulfide mineralization at 70 to 80 em interval. 
GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH: AN INTRODUCTION 15 

NESCA areas suggest that the volcanic center is in the 

early stages of formation and has not affected ilhe surface 

sediments to the same extent as at the NESCA and SESCA 

centers (Davis and Becker, chap. 3, this volume; Morton 

and Fox, chap. 2, this volume). Hydrothermal activity in 

this area may also be relatively immature. Comparative 

studies between the different volcanic centers could provide the opportunity to study the volcanic-sedimentaryhydrothermal interactions at various stages of evolution. 

Although our knowledge of the SESCA and NESCA 

areas is extensive, even when compared to many wellstudied areas of the sea floor, our actual data base is small. 

Less than 0.5 percent of the sea floor in the SESCA and 

NESCA areas has actually been observed by camera tows 

or submersible dives. The areal extent of outcropping sulfide mineralization has not been determined for any of the 

deposits; only minimum figures are known. Available time 

and scientific resources preclude complete photographic or 

submersible coverage of even a small part of the sea floor, 

so that we will always be forced to extrapolate our knowledge beyond the limits of our actual observations. However, there are tools available both to assist in extrapolating 

our data base and to guide future detailed investigations 

that have not been used in Escanaba Trough. One of the 

most useful techniques to aid in interpreting the geology 

in the areas of hydrothermal activity is high-resolution sidescan sonar, especially in combination with simultaneously 

collected swath bathymetry. A detailed side-scan survey, 

coupled with the local knowledge gained by camera and 

submersible mapping, would allow delineation of the extent of outcropping sulfide deposits, provide detailed 

information on the surface exposure of structures that 

localize sites of hydrothermal discharge, and provide a 

larger scale understanding of the processes of sedimentation and sediment remobilization. 

A deeply towed side-scan-sonar package can also be 

equipped with a magnetometer and a bottom-penetrating 

sonar system. Higb-resolution magnetic data can provide 

important constraints on the locations of both exposed and 

sediment buried sulfide deposits. These data are also critical to constrain models of magma-sediment interaction including the size and depth of emplacement of igneous 

intrusions. Bottom-penetrating sonar would allow detailed 

mapping of structures by the tracing of turbidite seismic 

reflectors. In addition, the near-bottom high-resolution 

seismic data would provide our first opportunity to examine the distribution and extent of sulfide mineralization in 

the shallow subsurface. Nearly all of our data relate only 

to the superficial surface skin of the deposits, and a greater 

understanding of the subsurface processes critical to massive sulfide formation and sediment alteration cannot be 

achieved until we can address these problems in a threedimensional manner. 

The true frontier area that must be explored to expand 

our understanding of hydrothermal, geochemical, and geophysical processes operating at sediment-covered spreading centers is the region of the lowermost sediment and the 

uppermost oceanic crust. This unexplored territory holds 

the secrets to a larger scale understanding of genetic processes. The interface between the oceanic crust and overlying sedimentary section is a fundamental chemical, 

mechanical, and hydrodynamic boundary, and is the area 

of dynamic interplay between igneous and sedimentary 

processes. This is the region that controls the thermal and 

chemical fluxes from the lithosphere to the hydrosphere. 

These fluxes control the composition of the sulfide deposits, sustain the biological communities at vent sites, and 

alter the composition of the ocean, the sedimentary section, 

and the basaltic crust. Novel application of sonar imagery to steeply inclined fault surfaces may identify fortuitous exposures of this zone, amenable to study by 

submersible, on the normal faults that bound the axial valleys. A more systematic approach to the study of these fundamental problems clearly requires oceanic drilling. 

Proposals have been advanced for drilling in this area to 

study the sedimentary record of climatic, tectonic, volcanic, and hydrothermal events, to determine the physical 

properties of the sediment and upper oceanic crust that control the hydrologic circulation, and to investigate the energy and mass fluxes and the chemical reactions that affect 

the hydrosphere, the sedimentary section, and the upper 

oceanic crust. A reasonable understanding of the formation of oceanic crust at sedimented spreading centers and 

the processes that control the associated hydrothermal 

systems depends on extending our studies into the third 

dimension. 

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V.M., Francheteau, J., Guerrero, J., Hawkins, J., Haymon, 
18 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

R., Hessler, R., Juteau, T., Kastner, M., Larson, R., 

Luyendyk, B., Macdougall, J.D., Miller, S., Normark, W., 

Orcutt, J., and Rangin, C.), 1980, East Pacific Rise: Hot 

springs and geophysical experiments: Science, v. 207, p. 

1421-1432. 

Rona, P.A., McGregor, B.A., Betzer, P.R., Bolger, G.W., and 

Krause, D.C., 1975, Anomalous water temperatures over 

Mid-Atlantic Ridge crest at 26° North latitude: Deep-Sea 

Research, v. 22, p. 611-618. 

Ross, S.L., and Zierenberg, R.A., Volcanic geomorphology of the 

SESCA and NESCA sites, Escanaba Trough, in Morton, 

J.L., Zierenberg, R.A., and Reiss, C.A., eds., Geologic, hydrothermal, and biologic studies at Escanaba Trough, Gorda 

Ridge, offshore northern California: U.S. Geological Survey Bulletin 2022, chapter 8 (this volume). 

Sclater, J.G., and Klitgord, K.D., 1973, A detailed heat flow, topographic, and magnetic survey across the Galapagos 

spreading center at 86° W: Journal of Geophysical Research, v. 78, p. 6951-6975. 

Sillitoe, R.H., 1972, Formation of certain massive sulphide deposits at sites of sea-floor spreading: Institute of Mining 

and Metallurgy Transactions, v. 81, p. 141B-148B. 

Simoneit, B.R.T., and Lonsdale, P.G., 1982, Hydrothermal petroleum in mineral mounds at the seabed of Guaymas Basin: 

Nature, v. 295, p. 198-202. 

Spooner, E.T.C., and Bray, C.J., 1977, Hydrothermal fluids of seawater salinity in ophiolitic sulphide ore deposits in Cyprus: 

Nature, v. 266, p. 808-812. 

Spooner, E.T.C., and Fyfe, W.S., 1973, Sub-sea-floor metamorphism, heat and mass transfer: Contributions to Mineralogy 

and Petrology, v. 42, p. 287-304. 

Talwani, M., Windisch, C.C., and Langseth, M.G., 1971, 

Reykjanes ridge crest: A detailed geophysical study: Journal of Geophysical Research, v. 76, p. 473-517. 

Vallier, T.L., Harold, P.J., and Girdley, W.A., 1973, Provenances and 

dispersal patterns of turbidite sand in Escanaba Trough, northeastern Pacific Ocean: Marine Geology, v. 15, p. 67-87. 

Van Dover, C.L., Grassle, J.F., and Boudrias, Michel, 1990, Hydrothermal vent fauna of Escanaba Trough (Gorda Ridge), 

in McMurray, G.R., ed., Gorda Ridge: A seafloor spreading center in the United States' Exclusive Economic Zone: 

New York, Springer-Verlag, p. 285-287. 

Vine, F.J., and Matthews, D.H., 1963, Magnetic anomalies over 

oceanic ridges: Nature, v. 199, p. 947-949. 

Weiss, R.F., Lonsdale, P., Lupton, J.E., Bainbridge, A.E., and 

Craig, H., 1977, Hydrothermal plumes in the Galapagos 

Rift: Nature, v. 267, p. 600-603. 

Wilde, P., Chase, T.E., Holmes, M.L., Normark, W.R., Thomas, 

J.A., McCulloch, D.S., and Kulm, L.D., 1978, Oceanographic data off northern California-southern Oregon, 

40°-43° N including the Gorda deep-sea fan: Berkeley, 

Lawrence Berkeley Laboratories Publication 251, scale 

1:815,482. 

Williams, D.L., Von Herzen, R.P., Sclater, J.G., and Anderson, 

R.N., 1974, The Galapagos spreading centre: Lithospheric 

cooling and hydrothermal circulation: Geophysical Journal 

of the Royal Astronomical Society, v. 38, p. 587-608. 

Wilson, D.S., 1989, Deformation of the so-called Gorda plate: 

Journal of Geophysical Research, v. 94, p. 3065-3075. 

Zierenberg, R.A., Morton, J.L., Koski, R.A., and Ross, S.L., Geologic setting of massive sulfide mineralization in the 

Escanaba Trough, in Morton, J.L., Zierenberg, R.A., and 

Reiss, C.A., eds., Geologic, hydrothermal, and biologic studies at Escanaba Trough, Gorda Ridge, offshore northern 

California: U.S. Geological Survey Bulletin 2022, chapter 

10 (this volume). 

Zierenberg, R.A., and Shanks, W.C., III, Sediment alteration associated with massive sulfide formation in the Escanaba 

Trough, Gorda Ridge: The importance of seawater mixing 

and magnesium metasomatism, in Morton, J.L., Zierenberg, 

R.A., and Reiss, C.A., eds., Geologic, hydrothermal, and 

biologic studies at Escanaba Trough, Gorda Ridge, offshore 

northern California: U.S. Geological Survey Bulletin 2022, 

chapter 14 (this volume). 
Section 1: 

Geologic Processt~s 
Acoustic sidescan mosaic of the Escanaba Trough area superimposed on the bathymetric map 

(contour interval 200 m). Areas of reversed magnetic polarity are shown in red. Steep slopes 

and rough-textured volcanic rocks have strong acoustic backscatter and are shown as light areas 

on this image. Smooth sediment-covered sea floor is dark. The main trough of the Gorda Ridge 

spreading center is outlined by the red hachured lines running north-south to the right of center 

of the image. The northern one-third of the spreading center is bright in this image due to the 

presence of young volcanic rocks. The southern two-thirds of the spreading center is the Escanaba 

Trough where the spreading center is covered by turbiditic sediment. Local bright patches within 

the Escanaba Trough are volcanic edifices that have stronger acoustic backscatter due to locally 

exposed basalts flows and intrusion of sills that disrupt the sediment. These volcanic edifices are 

the sites of hydrothermal activity and massive sulfide formation. 
Chapter 2. Structural Setting and Interaction of Volcanism 

and Sedimentation at Esc:anaba Trough: (;eophysical Results 

By Janet L. Morton 1 and Christopher G. Fox2 

CONTENTS 

Abstract ........................................................ .................... .. .. 

Introduction ........................................ ............. .................... . 

Acknowledgments ...................................................... . 

Methods ............................................................................. .. . 

Results ................................................................................. . 

Morphology and structure of Escanaba Trough ........ . 

SESCA and NESCA areas ........................................ .. 

Discussion and conclusions ................................................ . 

References cited .................................................................. . 

ABSTRACT 

Page 

21 

21 

22 

22 

22 

22 

30 

37 

42 

Escanaba Trough forms the axial valley of the slowspreading southern Gorda Ridge and is charact~ zed by 

a relatively flat 5- to 15-km-wide valley floor at a depth 

of 3,300 m, flanked by steep ridges rising 900 to 1,500 m 

above the valley floor. Turbidites and hemipelagic sediment cover the southern two-thirds of the trough to an average depth of 300 to 700 m. Subbottom profiler records 

of the sedimentary fill show thin, often continuous reflective zones 2 to 10 m apart in the upper several tens of 

meters of sediment, indicative of predominantly :nud-rich 

sediment with a low percentage of thin, coars~ -grained 

beds. A widespread acoustically transparent zone from approximately 85 to 130m below the sea floor wa~ imaged 

on seismic-reflection records. This interval is interpreted 

from drilling as a homogeneous, probably rapidly deposited, coarse-grained interval. Recent igneous acth ity along 

the trough is confined to discrete locations, term,!d volcanic edifices, where sills intrude and disrupt the sediment 

layers; basalt flows are locally extruded at the ~;ea floor. 

Steep-sided hills of uplifted sediment about 1 km in diameter, probably formed by intrusion of basaltic silb near the 

sediment-basement interface, rise 50 to 100 m c:.bove the 

volcanic edifices. Massive sulfide deposits with lateral 

1 U.S. Geological Survey, Menlo Park, California. 2 National Oceanic and Atmospheric Administration, Ne:wport, 

Oregon. 

dimensions up to 200 m and thicknesses of several tens of 

meters occur near the base of these sediment hills. Smaller 

sulfide mounds and chimneys occur on the sediment-covered sea floor away from the hills; disseminated sulfide in 

a sand-rich layer was cored at one location. Two main factors control the location of sulfide mineralization: (1) faults 

that formed as the hills were uplifted provide the primary 

conduits for ascending hydrothermal fluids, localizing the 

sea-floor sulfide deposition along the perimeters of the 

hills, and (2) sand layers allow horizontal migration of 

hydrothermal fluids away from the fault zones and may 

feed the smaller deposits. Subsurface mineral deposition 

is probably occurring in these coarser sediment layers. 

lNTRODUCTION 

Most mid-oceanic ridges lack a significant sediment 

cover, owing to the slow accumulation of sediment and the 

relatively fast creation of new crust. When sediment accumulation rates greatly exceed crustal accretion rates, the 

ridge axis becomes sediment dominated. The axis of 

Escanaba Trough, on the southern Gorda Ridge, is sediment 

dominated owing to its proximity to continental sediment 

sources, the lack of a topographic barrier between the continental margin and the ridge axis (see also Normark and 

others, chap. 6, this volume, fig. 6.1), and the relatively 

slow spreading rate of 2.4 cm/yr (Atwater and Mudie, 

1973; Riddihough, 1980; Wilson, 1989). The sedimentary 

fill consists of turbidite flows and hemipelagic mud 

(McManus and others, 1970; Karlin and Lyle, 1986; 

Normark and others, chap. 6, this volume) and locally exceeds 900 min thickness (Clague and Holmes, 1987). Recent igneous activity at the ridge axis appears to have been 

confined to discrete volcanic edifices where sills intrude 

and uplift the sediment and basalt flows may be extruded 

onto the sea floor (:Morton and others, 1987a). Hydrothermal mineral deposits are spatially associated with these 

volcanic edifices (Koski and others, 1988; Morton and others, 1990; Zierenberg and others, chap. 10, this volume). 

The sedimentary fill at Escanaba Trough provides a 

record of recent deformation at the ridge axis. In this chapter we use geophysical records, including Sea Beam 

21 
22 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

bathymetry, single- and multi-channel seismic-reflection 

records, and 3.5 kHz subbottom profiles, to describe the 

morphology, structure, and sedimentation of the trough and 

the surface and subsurface expression of igneous activity. 

These geophysical results, combined with sea-floor observations and subsurface sampling, are used to construct a 

model of the structural and stratigraphic controls on the localization of hydrothermal mineral deposition. 

ACKNOWLEDGMENTS 

We thank Jon Childs and Eric Geist for assistance with 

the seismic data processing, and Carol Reiss, Maria 

Restrepo, and Gini Curl for assistance with the data reduction and figure preparation. Valuable discussions with Robert Zierenberg, William Normark, Stephanie Ross, Roger 

Denlinger, Mark Holmes, and Robert Karlin and reviews 

by Earl Davis and Tom Brocher helped clarify the interpretations presented in this paper. 

METHODS 

The data presented in this paper were collected during five different cruises between 1981 and 1986. Cruise 

L12-81-NP, in 1981, collected approximately 400 km of 

24-channel seismic- reflection data along three profiles using a 34.9 L (2,130 in3) air-gun array discharged at 13.8 

MPa (2,000 psi). Two of the profiles are oriented perpendicular to the axis of Escanaba Trough, near latitudes 

41 °00' N. and 40°30' N., respectively (fig. 2.1). A third 

profile runs approximately north-south along the axis of the 

trough. The processing of each line included velocity 

analysis, normal moveout correction, common midpoint 

stacking, predictive deconvolution, bandpass filtering, and 

automatic gain control. Sections of the data were affected 

by severe streamer noise that was greater in the higher frequency band. To reduce this noise problem, the bandpass 

filtering step used an upper frequency limit of 30 Hz. 

Cruise L6-85-NC, in 1985, conducted a regional 

single-channel seismic-reflection survey of Escanaba 

Trough (900 km of trackline) using a 3.9 L (237 in3) airgun array (fig. 2.1). The data were originally recorded in 

analog format. Subsequent processing included digitization, predictive deconvolution, bandpass filtering, and automatic gain control. We applied a constant-velocity 

finite-difference migration to selected profiles from cruises 

L12-81-NP and L6-85-NC to reduce diffraction hyperbolas. Gravity data and total field magnetic data were also 

collected on cruise L12-81-NP and magnetic data on cruise 

L6-85-NC. These geopotential data are discussed by Karlin 

and Morton (chap. 5, this volume). 

Cruise L1-86-NC, in 1986, collected 160 km of singlechannel seismic-reflection profiles at the northern 

(NESCA) and southern (SESCA) Escanaba Trough study 

areas utilizing a 1.3 L (80 in3) water gun source. These 

data were not digitally recorded; processing for these data 

included bandpass filtering and automatic gain control. 

Higher resolution images of the shallow subsurface 

were obtained with a hull-mounted 3.5 kHz subbottom profiling system using a short ping ( < 1 ms). The 3.5 kHz 

system on the R/V S.P. Lee was used during all seismicreflection profiling as well as during many sampling stations on the above-mentioned cruises and cruise L2-86-NC 

in 1986. On selected camera and heat-flow stations, a 3.5 

kHz pinger was mounted on the wire and towed 100 to 300 

m above the sea floor. 

Sea Beam bathymetric data were collected in 1986 

aboard the NOAA ship Surveyor (cruise MMS-86-1). Approximately 1,300 km of tracks were run, including cross 

lines for navigation control. The Sea Beam data were processed at the National Oceanic and Atmospheric Administration in Newport, Oregon. 

Navigation for all geophysical profiles was provided 

by an integrated satellite-Loran C-Doppler sonar system, 

or a Loran C system. Since there is a consistent offset between Loran C coordinates and true geodetic, the Loran C 

navigation has been adjusted to match transit satellite fixes. 

Additional navigation for profiles and stations in the 

NESCA and SESCA work areas (cruises L1-86-NC and 

L2-86-NC) was provided by an acoustic navigation system 

and the satellite Global Positioning System (GPS) (see 

Morton and others, chap. 1, this volume). Sea Beam data 

were collected under Loran C control only, and shifted to 

match the geophysical data controlled by transit satellite 

or GPS positioning. 

RESULTS 

MORPHOLOGYANDSTRUCTUREOF 

ESCANABA TROUGH 

Gorda Ridge exhibits a "Mid-Atlantic Ridge" morphology with a deep axial valley. A right lateral offset of the 

ridge near latitude 41° 40' N. separates Escanaba Trough, 

the southern part of Gorda Ridge, from the middle segment 

of Gorda Ridge. The east-west-striking Mendocino Fracture Zone marks the southern boundary of Escanaba Trough 

at latitude 40°25' N. The floor of Escanaba Trough is at a 

depth of 3,200 to 3,400 m, and the flanking ridges rise 900 

to 1,500 m above the valley floor (fig. 2.2). The axis of 

Escanaba Trough is divided into two segments by a 5-km 

right-lateral offset near latitude 41 °08' N. (fig. 2.2). This 

offset may represent an overlap zone between discrete ridge 

segments; however, this structure cannot be conclusively 

inferred from the bathymetry alone owing to the thick sediment cover. The lengths of the two _individual segments 

(80 to 100 km) are consistent with segment lengths on the 

northern Gorda Ridge. 
STRUCTURAL SETTING AND INTERACTION OF VOLCANISM AND SEDIMENTATION: GEOPHYSICAL RESULTS 23 

127° 30' 127° 

/ / 

I 

\ 

40° 30' 

Figure 2.1. Regional bathymetry of Escanaba Trough modified from Wilde and others ( 1978); contour interval 200m. Locations of seismic 

reflection profiles used in this study are shown: 24-channel seismic-reflection lines from cruise L12-81-NP (dashed); single-channel air-gun 

lines from cruise L6-85-NC (dot-dash); single-channel water-gun lines from cruise Ll-86-NC (dotted). Numbers along selected multi- and 

single-channel air-gun lines indicate shot number. Selected profiles are shown in figures 2.3 to 2.7. 
24 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

From north to south, the axis of the northern segment 

of Escanaba Trough is characterized by a deep basin (> 

3,500 m deep) with thin sediment cover; a narrow axial 

high (about 3,200 to 3,250 m deep) that extends from about 

latitude 41 °28' N. to 41 °17' N. (figs. 2.2, 2.3 top); and a 

flat, sediment-covered floor about 5 km wide and 3,300 m 

deep (fig. 2.3 bottom). The axial high block appears to 

have been a topographic barrier for continentally derived 

sediment that entered the trough at the southern end and 

flowed northward (Normark and others, chap. 6, this volume). Sea Beam mapping reveals numerous small volcanoes at the ridge axis on the axial high block and along 

the wide terrace bounding the western side of the valley 

(fig. 2.2). 

30' 

20' 

10' 

A 

The axis of the segment south of latitude 41 °08' N. is 

characterized by a flat floor 12 to 20 km wide and covered by as much as 900 m of sediment (figs. 2.4, 2.5, 2.6; 

see also Karlin and Morton, chap. 5, this volume, fig. 5.2). 

The walls that bound the axial valley decrease in elevation 

southward; and south of about latitude 40°35' N. the bathymetric expression of these ridges ends and they are apparent only as buried basement highs on the seismic-reflection 

records (fig. 2.6 bottom). 

In most places, the sediment fill of the southern part 

of the trough is characterized by subhorizontal, reflective 

layers (figs. 2.4, 2.5, 2.7). High-angle extensional faults 

cross-cut the sediment fill near the axis of the trough (figs. 

2.4, 2.6). In places, the floor of the trough is bounded by 

40°50' 

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in Meters 

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LINE 5 0 5 KILOMETERS 

w E 

6.0 

4.0 

- 6.0 

Figure 2.5. Migrated single-channel air-gun seismic-reflection profiles across Escanaba Trough from cruise L6-85-NC: line 7 (top) and line 5 (bottom). Vertical exaggeration is about 2:1. 

Upper part of sedimentary section within trough is characterized by flat-lying reflectors that in most places appear relatively undisturbed except for occasional high-angle faults. Remainder 

of sedimentary section is less reflective and well layered, more deformed, and characterized by discontinuous and dipping reflections. Near eastern side of axial valley along line 5, 

sedimentary section thickens to more than 1.5 s two-way travel time. Arrows mark top of igneous basement. Flat-lying, higher amplitude sections of this reflection, such as between shot 

points 3,450 to 3,490 along line 7, could represent igneous sills emplaced near sediment-basement interface. Numbers above profile indicate shot points along that line. See figure 2.1 for 

location of profiles. 

IZl 

;;3 

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w LINE 114 0 5 KILOMETERS E 

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z 

0 

0 6.0 w 6.0 (J) 

~ 

ui 

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LINE 11 8 0 5 KILOMETERS 

IE 4.0 

6.0 6.0 

Figure 2.6. Migrated multi-channel seismic-reflection profiles across Escanaba Trough from cruise L 12-81-NP: line 114 (top) and line 118 (bottom). Vertical exaggeration is about 2:1. 

Arrows mark top of igneous basement. Line 114 crosses axis of trough near volcanic edifice at latitude 41 oN. (NESCA site), and line 118 crosses axis immediately north of Mendocino 

Fracture Zone. Shot points 1 ,800 to 1 ,880 on line 114 show example of terrace of relatively undisturbed sediment bounding axial valley floor. On line 118, sedimentary section consists 

of two primary intervals: deeper interval that has been faulted, tilted, and folded, and less disturbed upper interval that is cut by high-angle faults. Region of down-dropped and 

thickened sediment layers near shot points 820 to 910 marks region of recent extension. Numbers above profile indicate shot points along that line. See figure 2.1 for location of profiles. 

N 

00 

0 

m 

0 

r 

0 

0 

0 

::I:: 

--<: 

t:l 

:::0 

0 

>-3 

::I:: 

m 

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3:: 

> r 

> z 

t:l 

t:C 

0 

r 

0 

0 

rs 

Cl'.l 

>-3 

c 

t:l 

m 

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~ m 

Cl'.l 

(j 

> z 

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> 

~ 0 

c 

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0 

0 

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0 

m 
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~~~~~i~~~~~~~~~~~~~~;~~~~~~~~~~i~~~i~~~~~~~~~i~~~~~~~~~~~~~~~~~~~~~~~~~~~i~~~~~~~~~~~~~~~~~~~j~iii~~;~~~~~~~~~~ 

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w 

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LINE 111 0 5 KILOMETERS 

$: 

I4.0 - s SESCA NESCA N 4.0 

6.0 6.0 

Figure 2. 7. Migrated single-channel seismic-reflection line 24 from cruise L6-85-NC (top) and multi-channel seismic-reflection line 111 from cruise L 12-81-NP (bottom) along axis of 

sediment-covered part of Escanaba Trough. Vertical exaggeration is about 3: l. Location of SESCA and NESCA sites is marked on each profile. Numbers above profile indicate shot 

points along that line. See figure 2.1 for location of profiles. 

CZl 

~ e 

n 

>-l 

e 

~ 

> l' 

CZl 

ti1 

~ 

z 

0 

> z 

ti 

z 

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> n 

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z 

0 

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< 

0 

l' 

n 

> z 

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~ 

> z 

ti 

CZl 

m 

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z 

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0 

m 

0 

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--< CZl 

n 

> l' 

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ti1 

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e 

t3 

CZl 

N 

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30 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

terraces where sediment has been uplifted a few tens of 

meters to about 100m (figs. 2.6 top, 2.8). The outer flanks 

of the ridges that bound the trough are commonly covered 

by several hundred meters of outward-dipping sediment 

layers (figs. 2.5, 2.6). 

The sediment fill of Escanaba Trough reaches its maximum thickness at the southern end of the trough. Multichannel seismic-reflection profile 118 crossed the axis of 

Escanaba Trough just north of the Mendocino Fracture 

Zone (fig. 2.1). Although the basement reflector cannot 

be identified along all of line 118, in places the thickness 

of the sediment fill exceeds 900 to 1,000 m (fig. 2.6 bottom). Here the sediment fill is characterized by an upper 

unit of subhorizontal reflectors, cut by normal faults, which 

overlies an interval of tilted, more highly deformed layers. 

At the south end of Gorda Ridge, near the Mendocino 

Fracture Zone, the axis of spreading, as defined by the occurrence of extensional faults, and the ridge bounding the 

eastern side of the axis curve markedly to the east (figs. 

2.2, 2.8). This eastward curvature is also apparent in structural trends outside of the ridge axis, as imaged by 

GLORIA side-scan images of the area where Gorda Ridge 

intersects the Mendocino Fracture Zone (EEZ-SCAN 84, 

1986). 

Igneous rocks intrude the sediment cover at several 

locations along Escanaba Trough. These igneous complexes are here referred to as volcanic edifices, as they are 

composed of both intrusive and extrusive components. On 

bathymetric records, the edifices are typically characterized 

by a broad uplift of a few tens of meters or less and greater 

topographic roughness than the surrounding less disturbed 

sediment fill (figs. 2.2, 2.9, 2.10). Igneous basement appears to shallow beneath these edifices (fig. 2. 7). Singlechannel reflection line 7 (fig. 2.5 top), which images a 

volcanic edifice at the SESCA area, shows a broad basement high that appears to reach the sea floor. Volcanic 

rocks have been observed at the sea floor (Morton and others, 1990; Zierenberg and others, chap. 10, this volume) 

and recovered by dredging from the SESCA edifice (Davis 

and others, chap. 9, this volume). Other edifices (fig. 2.4, 

2.5 bottom) are characterized by disrupted sediment layers and high-amplitude subhorizontal reflectors within the 

sediment section. These reflectors probably represent sills 

emplaced within the sediment section. 

SESCA AND NESCA AREAS 

The sediment fill surrounding the volcanic edifices is 

generally well layered and flat lying. In these regions, 

Figure 2.8. Generalized geologic map of Escanaba Trough showing 

location of volcanic edifices, approximate extent of sediment fill, 

location of terraces, NESCA and SESCA study areas (rectangles), 

location of edifice REX, and location of DSDP Site 35. 

41 °30' 

41 °00' 

127°40' 

EXPLANATION 

Volcanic 

edifice 

Sediment 

fill 

Terrace 

0 10 

KILOMETERS 

SESCA 
STRUCTURAL SETTING AND INTERACTION OF VOLCANISM AND SEDIMENTATION: GEOPHYSICAL RESULTS 31 

40°48'N 

46' 

Depth 

in M eters 

<3200 

3210 

3230 

44' 3250 

3270 

3290 

3310 

3330 

3350 

3370 

3390 

42' 

32' 30' 

Figure 2.9. Sea Beam bathymetric map of SESCA study area. Contour interval is 10m and is noted by color change. Locations of singlechannel water-gun reflection profiles 4 to 12, cruise Ll-86-NC, are shown. Numbers along tracklines represent time of day corresponding to 

annotations at top of seismic reflection profiles in figure 2.12. Hachures indicate area of closed low. Map area is approximately 10 km wide. 
32 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

41°06'N 

04' 

30' 28' 

Figure 2.10. Sea Beam bathymetric map of NESCA study area. Contour interval is 10m and is noted by color change. Locations of 

single-channel water-gun reflection profiles 14 to 25 and 3.5 kHz profile 31, cruise L1-86-NC, are shown. Numbers along tracklines 

represent time of day corresponding to annotations at top of seismic-reflection profiles in figures 2.13 and 2.14. Hachures indicate area of 

closed low. Map area is approximately 10 km wide. 
STRUCTURAL SETTING AND INTERACTION OF VOLCANISM AND SEDIMENTATION: GEOPHYSICAL RESULTS 33 

sub bottom penetration on 3.5 kHz records ranged from 

about 25 to 50 m. Within the upper 50 m of sediment, as 

many as eight thin, well-defined subbottom reflectors are 

identified (fig. 2.11 ). The spacing of these reflectors is 

often consistent over distances of several kilometers. In 

places, the distinct layering grades into regions of thicker, 

less continuous reflections. These echo characteristics are 

considered indicative of fine-grained sediments with small 

to moderate amounts of thin coarser grained beds (sand and 

silt) (Damuth, 1975; Damuth and Hayes, l977). Surficial 

cores and DSDP sections from Escanaba Trough confirm 

that the•sediment section imaged by the 3.5 kHz profiles 

consists predominantly of mud with thin silt-and-sand turbidites (Normark and others, chap. 6, this volume). The 

thickness of many of these turbidite beds is less than the 

resolution of the 3.5 kHz recording system; however, the 

layers on the 3.5 kHz records could represent ref1ections 

from sandy turbidites a few tens of centimeters thick or 

groups of several thinner beds. O'Connell and Normark 

(1986) carefully correlated acoustic units with lithologies 

of cores recovered by DSDP drilling on the Mississippi fan 

and suggested in several instances a causal relationship between coarse-grained beds or groups of beds and reflections of a character similar to those observed at Escanaba 

Trough. 

Single-channel water-gun reflection records reveal the 

same well-layered character of the entire sediment section 

adjacent to the SESCA and NESCA sites (figs. 2.12, 2.13, 

2.14). Steeply dipping normal faults displace the sediment 

layers in a few locations. An acoustically transparent zone 

from approximately 85 to 130 m below the sea floor is 

imaged on nearly all of the profiles near SESCA and 

NESCA. Single-channel air-gun reflection profiles from 

the region between SESCA and NESCA also imaged this 

transparent zone (Davis and Becker, chap .. 3, this volume), 

suggesting that the horizon is continuous, at least from 

SESCA through the NESCA area. Low-amplitude or 

nonreflective intervals on seismic records of sedimentary 

w~ 

~ 

i= 

.....J 

w (J) 

> 0 

<( z 

a: 0 

I- () 

~ w 

~ ~ 0 

~ Iw 

0200 

rocks generally indicate a homogeneous zone of predominantly one lithologic type, which can be either coarse 

grained or fine grained (Sangree and Widmier, 1977). 

DSDP Site 35 cores 3, 4, and 5 were sampled within the 

interval corresponding to this acoustically transparent zone. 

These three cores consist predominantly of sand, as opposed to the rest of the section which is predominantly mud 

with thin sand or silt layers (McManus and others, 1970; 

Normark and others, chap. 6, this volume). This acoustically transparent zone thus probably corresponds to a 

coarser grained unit deposited during a period of rapid sedimentation in the trough. 

At the margins of the volcanic edifices, there is an 

abrupt transition from well-layered, undisturbed sediment 

to highly disturbed (faulted and tilted) layers. This transition is apparent on both 3.5 kHz subbottom profiles (fig. 

2.11) and water-gun reflection profiles (figs. 2.12, 2.13, 

2.14) and corresponds approximately with the topographic 

expression of the edifices (figs. 2.15, 2.16). A volcanic 

edifice located between SESCA and NESCA sites has little 

sea-floor topographic expression but is readily identified 

by this abrupt transition from undisturbed to disturbed sediment layers. At both the SESCA and NESCA edifices, 3.5 

kHz profiles indicate that this disturbance of the sediment 

layering affects the youngest beds at the sea floor, indicating recent movement at the edifices (fig. 2.11 ). 

Although the single- and multi-channel seismicreflection profiles suggest that acoustic basement is present 

at or near the sea floor at volcanic edifices such as SESCA 

and NESCA (fig. 2.5), higher resolution profiling and 

visual observations indicate that much of the sea floor at 

the SESCA and NESCA sites is sediment covered. A hard 

sea-floor return on 3.5 kHz records, and camera and 

submersible observations indicate that basalt is locally 

present at or near the sea floor. At the SESCA site, the 

two primary areas of basalt exposure are west of "3170 

Hill" and at the base of "North Hill" on the south side (fig. 

2.15). The extent of the first area is inferred primarily from 

I 

0230 

E 

Figure 2.11. 3.5 kHz record along L1-86-NC line 12 at north end of SESCA area. See figure 2.9 for location of profile. Well-layered 

sediment at west side of profile illustrates typical reflectivity of sediment in trough away from volcanic edifices. Central part of profile 

shows faulted and tilted sediment typical of volcanic edifices. Numbers along bottom indicate shot points along line 12. 
34 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

geophysical profiles; several visual observations confirm 

the presence of basalt at the sea floor in the second area 

(Ross and Zierenberg, chap. 8, this volume). In the NESCA 

area, basalt is exposed on the eastern part of "Central Hill" 

and in the flat-floored basins north and south of Central 

Hill (fig. 2.16). Over most of the basin north of Central 

Hill, 3.5 kHz subbottom profiles reveal a hard sea-floor 

0 2 

KILOMETERS 

4.4 

4.8 

C/) 

0 

z 

0 

0 

w 

C/) 

z 4.4 

w 

~ 

i= 

__J 4.8 

w 

> <( 

a: 

f-

>- <( 0100 

~ 

0 

~ 4.4 

f4.8 

B 

T 

B 

return characteristic of basalt; on two profiles, however, 

layering appears to be present beneath this hard return (fig. 

2.17). It is unlikely that the 3.5 kHz system could "see 

through" thick basalt flows to image underlying structure. 

Three possible explanations are suggested for this observation: ( 1) Parts of the basin floor are covered not by 

basalt but rather by sand-rich sediment that gives rise to 

2130 

T 

B 

Figure 2.12. Single-channel water-gun profiles 

4, 6, 8, 10, and 12 from cruise L1-86-NC in 

SESCA area. See figure 2.9 for location of 

profiles. Vertical exaggeration is about 5.7. ''T' 

marks acoustically transparent zone about 85 

to 130 m below sea floor. Reflector "B" is 

interpreted as top of igneous basement. 

Numbers above profil~ indicate shot points 

along lines. 
STRUCTURAL SETTING AND INTERACTION OF VOLCANISM AND SEDIMENTATION: GEOPHYSICAL RESULTS 35 

the high-amplitude return and allows some penetration of 

3.5 kHz energy; (2) most of the basin floor is covered by 

sheet flow basalt, but isolated sediment kipukas exist, thus 

the wide beam pattern of the 3.5 kHz echo sounder could 

result in imaging both a sedimentary section and a nearby 

Figure 2.13. Single-channel water-gun profiles 

14, 18, 20, 22, and 24 from cruise L1-86-NC in 

NESCA area. See figure 2.10 for location of 

profiles. Vertical exaggeration (V.E.) varies and 

is noted on each profile. Top of igneous basement 

is marked by discontinuous reflection frequently 

consisting of coalesced hyperbolas. Thickness 

of sedimentary section varies considerably in 

NESCA area from 0 to more than 0.6 s two-way 

travel time (about 550 m). Transparent zone 

noted on SESCA profiles is also present in 

undisturbed parts of NESCA area. Highamplitude reflections within sedimentary section 

and parallel to sediment layering are observed 

along parts of lines 20 and 24 and on lines 16 

and 25 (fig. 2.14). These horizontal reflections 

probably represent buried basalt flows or sills. 

Alternatively, they could represent coarsegrained, more reflective sediment layers, or 

mineralized zones within sedimentary section. 

Numbers above profile indicate shot points along 

lines. 

(/) 

0 

z 

0 

0 

w 

(/) 

z 

w 

~ 

i= 

....J 

w 

> <( 

a: 

1-

>- <( 

~ 

0 

~ 1-

4.4 

4.8 

4.8 

4.4 

4.4 

4.8 

hard basalt sea floor; and (3) the basin floor consists of 

sediment overlain by basalt sheet flow(s) thin enough that 

the 3.5 kHz energy is able to partially image the underlying sedimentary section. The flat character of the basin 

floor (fig. 2.1 0) and the limited visual observations (Ross 

0400 0430 

s 

1230 

LINE 22 

1330 
36 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

and Zierenberg, chap. 8, this volume; Zierenberg and others, chap. 10, this volume) favor the basin floor being 

formed by basalt sheet flow(s); however, the extent of our 

visual observations is not sufficient to distinguish among 

these possible scenarios. High-amplitude reflections on 

water-gun profiles (figs. 2.13, 2.14) within the sedimentar:y section suggest the presence of shallowly buried flows 

or shallow sills at several locations in the NESCA area. 

Original glass surfaces on basalt samples dredged from 

both the SESCA and NESCA sites have no measurable 

palagonite, indicating very recent volcanic activity at both 

sites (Morton and others, 1987a). 

Detailed topographic mapping from Sea Beam (figs. 

2.9, 2.10) reveals several small hills less than 1 km in diameter and up to 100m high at several of the volcanic edifices, including the SESCA and NESCA sites. Massive 

sulfide deposits up to 200 m in length occur near the base 

of these hills (Morton and others, 1990; Zierenberg and 

others, chap. 10, this volume). The hills are relatively flat 

topped and exhibit the same well-layered sedimentary character on 3.5 kHz profiles as the surrounding sea floor (fig. 

2.18; see also Morton and others, 1990, fig. 5.5). Watergun line 10 (fig. 2.12) over North Hill at SESCA as well 

as 3.5 kHz profiles over other hills indicate that the layers 

at the top of the hills commonly dip to the west. Faulting 

is apparent at or near the base of the hills on several profiles. Visual observations from submersibles indicate that 

the slopes of the hills are steep, frequently veneered by 

0700 

LINE16 I I 

debris resulting from mass wasting, and in places expose 

bedded sediment (Zierenberg and others, chapter 10, this 

volume). A well-defined reflection interpreted as igneous 

basement is seen on line 10 immediately east and west of 

North Hill at a depth of 0.5 s and 0.4 s, respectively. The 

reflector appears to shallow slightly near the hill, then 

terminates in a series of discontinuous strong hyperbolic 

reflections beneath the hill that probably represent sills. 

To obtain better resolution in the shallow subsurface, 

and in particular to attempt to resolve areas of near-surface 

mineralization we mounted a 3.5 kHz pinger on the camera sled during selected stations. In most cases we obtained 

poor results with the 3.5 kHz pinger in the SESCA and 

NESCA sites, with little or no penetration of the subsurface. Penetration ranged from 0 to about 50 ms (about 

38 m) on the best records, and subsurface layering, where 

observed, resembles a fainter version of that imaged with 

the hull-mounted system. The pinger was rigidly attached 

to the camera sled, and it is likely that considerable energy 

was transmitted to the sled rather than downward. On one 

station in the NESCA area, a strong horizontal reflection 

was recorded about 30 ms (about 23m) below the sea floor 

(fig. 2.19). Along the camera track before and after this 

reflector, the sea floor is characterized by the typical flatlying sediment cover with no hydrothermal deposits. Directly above the bright subsurface reflection, the camera 

system revealed outcrops of probable hydrothermal deposits on the sea floor (Zierenberg and others, chap. 10, this 

0600 0500 I 

2 KILOME+ERS 

. . . ._ . . CENT_~~~ HILL L-..--v-'.-E.-4-_0-. ----1 

(/) 

0 

z 

0 

0 

w 

(/) 

z 

w 

~ 

~ 

_J 

w 

> <( 

a: 

1-

>- <( 

~ 

0 

~ 1-

4.8 

5.2 

-

1500 1600 1700 

LINE 25 O 2 KILOMETERS 

V.E. 6.9 

Figure 2.14. Single-channel water-gun profiles 16 and 25 from cruise Ll-86-NC in NESCA area. See figure 2.10 for location of profiles. 

Vertical exaggeration (V.E.) is noted on each profile. ''T" marks acoustically transparent zone. Numbers above profile indicate shot points 

along lines. 
STRUCTURAL SETTING AND INTERACTION OF VOLCANISM AND SEDIMENTATION: GEOPHYSICAL RESULTS 37 

volume). The concordance of this bright reflection with 

the local layering and the camera observations suggest one 

possible interpretation of this reflector: that it represents 

a mineralized bed underlying the sea-floor deposit, such 

as that cored at station Ll-86-27G south of NESCA 

(Normark and others, chap. 6, this volume). Other possible causes of the higher reflectivity cannot be ruled out, 

such as an along-strike change in composition or compaction of the sediments, increased amounts of volcanic detritus or a buried flow, or a change in the overlying 

sedimentary section allowing more energy to penetrate to 

EXPLANATION 

Undisturbed 

sediment 

Disturbed 

sediment 

fVTl Basalt ~ 

6000 8000 10,000 

this reflector. Explanations calling on volcanic detritus or 

flows are not, however, supported by observation of nearby 

volcanic exposures. 

DISCUSSION AND CONCLUSIONS 

Results from DSDP Site 35 indicate a Pleistocene age 

for most of the sediment fill of Escanaba Trough (Bukry 

and Bramlette, 1970; Gartner, 1970) and rapid sedimentation rates are suggested (Normark and others, chap. 6, this 

12,000 14,000 16,000 

Figure 2.15. Map of SESCA area showing regions of disturbed and undisturbed sediment and inferred extent of volcanic flows at or near sea 

floor based on geophysical records and visual observations. Numbers along right and bottom give coordinates of local transponder net (in 

meters). Hachures indicate area of closed low. 
38 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

41°06' .-----,-----------------~----------------~~------------~ 12JD30' 12JD28' 

EXPLANATION 66,000 

[=3 Undisturbed 

~ sediment 

64,000 

0 

Figure 2.16. Map ofNESCA area showing regions of disturbed and undisturbed sediment and inferred 

extent of volcanic flows at or near sea floor based on geophysical records and visual observations. 

Numbers along right and bottom give coordinates of local transponder net (in meters). Hachures indicate 

area of closed low. 
STRUCTURAL SETTING AND INTERACTION OF VOLCANISM AND SEDIMENTATION: GEOPHYSICAL RESULTS 39 

volume). Seismic profiles indicate two distinct episodes 

of sedimentation and deformation in the southern part of 

the trough. At the eastern side of the trough, along L6-

85-NC profiles 3 (not shown) and 5 (fig. 2.5 bottom) and 

Ll2-81-NP profile 118 (fig. 2.6), depth to basement and 

the thickness of the sedimentary section increase significantly. The lower sedimentary interval has been tilted to 

dips of at least 10° and is unconformably overlain by flatlying sediment layers. This older episode of deformation 

w 

appears to be associated with the eastward deflection and 

gradual disappearance of the eastern bounding ridge approaching the Mendocino Fracture Zone. We postulate an 

episode of rapid extension resulting in down-dropping of the 

basement, tilting of the overlying sediment section, and rapid 

sediment infilling of the resulting graben. Deformation of the 

upper sedimentary interval consists of normal faulting associated with extension, and localized uplift and disruption 

associated with emplacement of the volcanic edifices. 

E 

0 1 KILOMETER 

C/) 

Cl 

z 

0 

0 

w 

C/) 

w 

~ 

i= 

.....J 

w 4.45 C/) 

> 0 <( z (js.· c: 

..... 0 

>- 4. so J ___ L_~· ·_::; () 

w <( C/) ;: z 0 I I. ;: 

..... 4.55--

Figure 2.17. 3.5 kHz subbottom profile (L1-86-NC line 31) across basin north of Central Hill at NESCA site (see fig. 2.10 for 

location of profile). High-amplitude sea-floor return within basin, as well as bathymetric expression of basin, suggest presence of 

basalt flows at or near surface. Some layering is imaged beneath sea-floor return, possibly indicating that basin is floored by thin 

basalt sheet flow(s) overlying sediment. Other possible interpretations are discussed in text. 

SE NW 

3170 HILL 

0 0.2 KILOMETERS 

Z 4. 3 -'-'":i'""---~~---~---'-o'-'--~-.....J'-"'+-"-~-------"-""'~r-...o-w----"--~ .. ,.,..u~~:'·.,"··~"'·~"'!-':-·""'~,..,.,:.....,.....h-r'~ ... '\--:--~{~ 

w-

~ 

i= 

.....J 

w 

> <( 

a: 

I-

>- <( 

~ 

0 

~ IFigure 2.18. 3.5 kHz profile over 3170 Hill (fig. 2.15) at SESCA site obtained during dredging station. Horizontal scale bar is approximate 

as ship speed was varying. Sediment layers of hill have seismic-reflection character similar to that of shallow sediment of surrounding sea 

floor. 
40 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

Recent igneous activity is not evenly distributed along 

the length of the trough, but rather is confined to discrete 

centers. Models of ridge system dynamics (Francheteau 

and Ballard, 1983; Crane, 1985; Macdonald and Fox, 1988) 

would predict magma sources beneath the intrasegment topographic highs; however, no clear correlation is evident 

in this case. The trough does appear to be undergoing extension along its entire length, as indicated by the presence 

of normal faults at the axis that cut the youngest sediment 

on all seismic crossings. The restriction of volcanism to a 

few discrete centers could reflect the geometry of magma 

supply, the relatively short time interval represented by the 

volcanic edifices, and (or) possibly control by the thick 

sediment cover. The few multi-channel seismic-reflection 

profiles did not reveal the presence of an axial magma 

chamber along Escanaba Trough. However, the data do not 

preclude the presence of a narrow chamber or one with a 

small velocity contrast. As has been suggested for other 

slow-spreading ridges (see Macdonald, 1986 and references 

therein), it is possible that the surface flows are fed from 

small isolated magma bodies rather than a laterally continuous crustal magma chamber. If each edifice derives 

from a separate source, we might expect chemical differences in the eruptive products. Chemical analyses of basalt samples from several edifices do show variation among 

the edifices (Davis and others, chap. 9, this volume) that 

could be compatible with a separate source model; however, the results are also compatible with a single, poorly 

mixed magma chamber. The sediment fill may also exert 

some localizing influence on the sea-floor volcanic distribution. An ascending magma loses buoyancy when it 

reaches the sediment-basement interface and tends to form 

a sill there. Some magma might follow faults through the 

sediment to form flows at the sea floor and once a path-

.•; 

50 MILLISECONDS 

(37 METERS) L 

0 100 METERS 

~i':.:·.J~·•·::.'r')q):;:f::~r., , :. ,. 

Figure 2.19. 3.5 kHz record from NESCA site obtained with pinger 

mounted in camera sled and receiver mounted in the ship's hull. 

Camera sled was towed only a few meters above sea floor. Heavy 

black return at top of record is a combination of direct return and 

sea-floor reflection. Weak subsurface layering seen on left side of 

figure is typical of records obtained with sled-mounted pinger and, 

way has been established, future eruptions will likely 

utilize it. 

Sea Beam mapping indicates that the small hills that 

have been studied at SESCA and NESCA site are also common at other locations along the valley (see, for example, 

the southern end of fig. 2.9). Smaller hills may well occur in the area but have dimensions below the resolution 

of the Sea Beam system acoustic footprint, which is about 

2x104 m2 in these water depths. Seismic profiling and 

visual observations (Zierenberg and others, chap. 10, this 

volume) indicate that the hills are composed of sediment. 

Basalt flows are present at the perimeter of some hills (figs. 

2.15, 2.16). The hills probably form as igneous intrusions 

uplift and possibly tilt the overlying sediment. Evidence 

for sills beneath one hill is seen on L1-86-NC line 10 (fig. 

2.12). High-angle faults in the sediment were also imaged 

near the base of the hills. The location of these faults 

appears to be consistent with the predicted location of faulting from mechanical and theoretical modeling of uplift 

above a sill (Denlinger and Holmes, chap. 4, this volume). 

There is considerable evidence for mass wasting at the 

sides of the hills (Holmes and Zierenberg, 1990; Morton 

and others, 1990), and their present form is that of buttes. 

The prevalence of mass wasting suggests one other possible mechanism of formation of the hills: that they are 

erosional remnants of much more widespread uplift. This 

explanation would suggest, for example, that the several 

hills aligned in the northeast quadrant of SESCA (fig. 2.9) 

are the eroded remnants of a north-northeast-trending horst 

block. The amount of geophysical data needed to distinguish between these two models, especially high-resolution 

seismic-reflection and magnetic data directly over the hills, 

is limited. We conclude, however, that the direct observation of faulting near the base of the hills and the indirect 

although penetration is much poorer than with ship's 3.5 kHz system, 

is not unlike layering imaged with ship's system. High-amplitude 

return at about 30 ms below sea floor on right side of figure is unusual. 

Camera system photographed small hydrothermal deposits on sea 

floor at this location; thus, we speculate that high-amplitude return 

could represent bed with subsurface mineralization. 
STRUCTURAL SETIING AND INTERACTION OF VOLCANISM AND SEDIMENTATION: GEOPHYSICAL RESULTS 41 

evidence of faulting based on the distribution of large sulfide deposits (Zierenberg and others, chap. 10, this volume), as well as the presence of sills identified on seismic 

profiles strongly favor the former interpretation, that the 

hills are the result of localized uplift caused by the intrusion of basaltic sills. Basalt flows located adjacent to some 

of the hills probably erupted along the faults associated 

with this uplift. 

Numerous hydrothermal sulfide and sulfate deposits 

have been mapped at the SESCA and NESCA sites. 

(Morton and others, 1990; Zierenberg and others, chap. 10, 

this volume). Several large sulfide mounds (1 00 to 200 m 

in length and tens of meters thick) occur at or near the base 

of the small hills. Smaller isolated chimneys and mounds 

are widely distributed on the sediment- covered sea floor. 

A sand layer with disseminated pyrrhotite was cored at one 

EXPLANATION 

D Mud and silt Sulfide 

location (Morton and others, 1987b; Normark and other, 

chap. 6, this volume), indicating that subsurface sulfide 

deposition is also occurring. The geophysical results combined with the geologic mapping and sampling suggest two 

primary controls on the localization of hydrothermal flow 

and sulfide deposition (fig. 2.20). The predominantly mudrich sediment should have a low permeability, which 

restricts hydrothermal flow. Faults created during the uplift 

of the small hills provide the primary conduits for 

upwelling hydrothermal fluids, resulting in the deposition 

of large mineral deposits at the base of the hills. Sandrich sediment layers provide horizontal pathways for 

hydrothermal fluids to migrate away from these fault zones. 

Such flow might feed the smaller, isolated sulfide deposits. We also predict that considerable mineralization is occurring in the subsurface in these sand-rich beds. 

Subsurface sulfide 

---

,} Fault 

1

100 Meters 

Sand ~ 

~ Basalt / Fluid pathway 

0 1 Kilometer 

Figure 2.20. Schematic cross section summarizing basic features 

controlling formation of sulfide deposits at Escanaba Trough in 

SESCA and NESCA areas. Basaltic sills inflate near sedimentbasement interface, assimilating sediment into magma (Davis and 

others, chap. 9, this volume) and uplifting overlying sedimentary 

section to form small hills. Fault zones created by uplift are likely 

to be considerably more permeable than mud-rich sedimentary layers, 

providing primary pathways for hydrothermal fluids, which cool 

sills, to ascend to sea floor. Large massive sulfide deposits are formed 

at sea floor near base of small hills above these fault zones. Sandrich layers allow horizontal migration of hydrothermal fluids away 

from fault zones, resulting in subsurface mineral deposition along 

these beds and feeding smaller sea-floor deposits. 
42 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

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model for sediment hills and associated sulfide deposits 

along the Escanaba Trough, in Morton, J .L., Zierenberg, 

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Geological Survey Miscellaneous Investigations Series Map 

1-1792, 152 p., scale 1:500,000. 

Francheteau, Jean, and Ballard, R.D., 1983, The East Pacific Rise 

near 21° N, 13° N and 20° S: inferences for along-strike 

variability of axial processes of the mid-ocean ridge: Earth 

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Gartner, Stefan, Jr., 1970, Coccolith age determinations Leg 5, 

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eds., Initial reports of the Deep Sea Drilling Project: Washington, U.S. Government Printing Office, v. 5, p. 495-500. 

Holmes, M.L., and Zierenberg, R.A., 1990, Submersible observations in Escanaba Trough, southern Gorda Ridge, in 

McMurray, G.R., ed., Gorda Ridge: A seafloor spreading 

center in the United States' Exclusive Economic Zone: New 

York, Springer-Verlag, p. 93-115. 

Karlin, R., and Lyle, M., 1986, Sediment studies on the Gorda 

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Koski, R.A., Shanks, W.C., III, Bohrson, W.A., and Oscarson, 

R.L., 1988, The composition of massive sulfide deposits 

from the sediment-covered floor of Escanaba Trough, Gorda 

Ridge: Implications for depositional processes: Canadian 

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Macdonald, K.C., 1986, The crest of the Mid-Atlantic Ridge: 

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Vogt, P.R., and Tucholke, B.E., eds., The geology of North 

America, Volume M, The Western North Atlantic region: 

Boulder, Colorado, Geological Society of America, p. 51-68. 

Macdonald, K.C., and Fox, P.J., 1988, The axial summit graben 

and cross-sectional shape of the East Pacific Rise as indicators of axial magma chambers and recent volcanic 

eruptions: Earth and Planetary Science Letters, v. 88, p. 

119-131. 

McManus, D.A., and others, 1970, Site 35, in McManus, D.A., 

and others, eds., Initial reports of the Deep Sea Drilling 

Project: Washington, U.S. Government Printing Office, v. 

5, p. 165-172. 

Morton, J.L., Holmes, M.L., and Koski, R.A., 1987a, Volcanism 

and massive sulfide formation at a sedimented spreading 

center, Escanaba Trough, Gorda Ridge, northeast Pacific 

Ocean: Geophysical Research Letters, v. 14, p. 769-772. 

Morton, J.L., Normark, W.R., Ross, S.L., Koski, R.A., Holmes, 

M.L., Shanks, W.C., III, Zierenberg, R.A., Lyle, M., and 

Benninger, L.M., 1987b, Preliminary report, cruises Ll-86-

NC and L2-86-NC, Escanaba Trough, Gorda Ridge: U.S. 

Geological Survey Open-File Report 87-375-A, 20 p. 

Morton, J.L., Koski, R.A., Normark, W.R., and Ross, S.L., 1990, 

Distribution and composition of massive sulfide deposits at 

Escanaba Trough, southern Gorda Ridge, in McMurray, 

G.R., ed., Gorda Ridge: A seafloor spreading center in the 

United States' Exclusive Economic Zone: New York, 

Springer-Verlag, p. 77-92. 

Morton, J.L., Zierenberg, R.A., and Reiss, C.A., Geologic, hydrothermal, and biologic studies at Escanaba Trough: An 

introduction, in Morton, J.L., Zierenberg, R.A., and Reiss, 

C.A., eds., Geologic, Hydrothermal, and Biologic Studies 

at Escanaba Trough, Gorda Ridge, Offshore Northern California: U.S. Geological Survey Bulletin 2022, chapter 1 

(this volume). 

Normark, W.R., Gutmacher, C.E., Zierenberg, R.A., Wong, F.L., 

and Rosenbauer, R.J., Sediment fill of Escanaba Trough, in 

Morton, J.L., Zierenberg, R.A., and Reiss, C.A., eds., Geologic, Hydrothermal, and Biologic Studies at Escanaba 
STRUCTURAL SETTING AND INTERACTION OF VOLCANISM AND SEDIMENTATION: GEOPHYSICAL RESULTS 43 

Trough, Gorda Ridge, Offshore Northern California: U.S. 

Geological Survey Bulletin 2022, chapter 6 (this volume). 

O'Connell, S., and Normark, W.R., 1986, Acoustic facies and 

sediment composition of the Mississippi fan drill sites, Deep 

Sea Drilling Project leg 96, in Bouma, A.H., Coleman, J.M., 

Meyer, A.W., and others, eds., Initial reports of the Deep 

Sea Drilling Project: Washington, U.S. Government Printing Office, v. 96, p. 457-473. 

Riddihough, R.P., 1980, Gorda plate motions from magnetic 

anomaly analysis: Earth and Planetary Science Letters, v. 

51, p. 163-170. 

Ross, S.L., and Zierenberg, R.A., Volcanic geomorphology of the 

SESCA and NESCA sites, Escanaba Trough, in Morton, 

J.L., Zierenberg, R.A., and Reiss, C.A., eds., Geologic, Hydrothermal, and Biologic Studies at Escanaba Trough, Gorda 

Ridge, Offshore Northern California: U.S. Geological Survey Bulletin 2022, chapter 8 (this volume). 

Sangree, J.B., and Widmier, J.M., 1977, Seismic stratigraphy and 

global changes of sea level, part 9: Seismic interpretation 

of clastic depositional facies, in Payton, C.E., ed., Seismic 

stratigraphy-Applications to hydrocarbon exploration: 

Tulsa, American Association of Petroleum Geologists, p. 

165-184. 

Wilde, P., Chase, T.E., Holmes, M.L., Normark, W.R., Thomas, 

J.A., McCulloch, D.S., and Kulm, L.D., 1978, Oceanographic data off northern California-southern Oregon, 

40°-43 o N including the Gorda deep-sea fan: Berkeley, 

Lawrence Berkeley Laboratories Publication 251, scale 

1:815,482. 

Wilson, D.S., 1989, Deformation of the so-called Gorda plate: 

Journal of Geophysical Research, v. 94, p. 3065-3075. 

Zierenberg, R.A., Morton, J.L., Koski, R.A., and Ross, S.L., Geologic setting of massive sulfide mineralization in the 

Escanaba Trough, in Morton, J.L., Zierenberg, R.A., and 

Reiss, C.A., eds., Geologic, Hydrothermal, and Biologic 

Studies at Escanaba Trough, Gorda Ridge, Offshore Northem California: U.S. Geological Survey Bulletin 2022, 

chapter 10 (this volume). 

Chapter 3. Thermal and Tectonic Structure of Escanaba 

Trough: New Heat-Flow Measurements and SeismicReflection Profiles 

By Earl E. Davis 1 and Keir Becker2 

CONTENTS 

Page 

Abstract . ... ..... . ........ .... ... . ... . .. . ... . ... . ... .. .. .... . .. . .. ....... ........ .. .. . .. 45 

Introduction.......................................................................... 45 

Background.................................................................. 45 

Tectonic and geothermal setting .. .. ............ .... .... ..... ..... 46 

Acknowledgments ........................................................ 48 

New Data ... :.......................................................................... 48 

Discussion............................................................................ 51 

Implications from seismic reflection data .................. 51 

Inferred thermal structure............................................ 58 

Conclusions . . ... ... .... ..... . ... . ... ... ... . .... .. .. .. .... .. .. .. .... .. .. .. .. .... . .... . 62 

References cited . ..... .. .. .. .. . .. . ... ... . ... . ........ ... . ... .... . .... ... . ..... ... . . 63 

ABSTRACT 

Twelve closely spaced single-channel seismic profiles 

and 75 heat-flow measurements along four multi-penetration transects crossing the Escanaba Trough provide new 

information about this setting in which large sedimenthosted sulfide deposits have formed. A layer of turbidites 

roughly 600 m thick buries basement across the full width 

of the rift valley in the study area. The sediment section 

is disrupted by numerous normal faults and volcanic intrusions, although seismic reflectors remain coherent in 

most areas, which allows stratigraphic displacements to be 

quantified. A well-defined transparent acoustic layer is 

seen within the upper 200 m of the section on all profiles; 

the top and bottom of this layer, and the valley floor itself, provide excellent stratigraphic and tectonic marker 

horizons. We conclude that displacements on individual 

faults have been accumulating at rates of up to 15 mm/yr. 

The valley floor appears to be subsiding in most areas, at 

rates of up to 20 mm/yr. The sedimentation rate during 

the late Pleistocene is calculated to be in excess of 25 mm/ 

yr. Depths to the marker horizons at the centers of volcanic activity are not well resolved, but little syn-sedimen1 Geological Survey of Canada, Sidney, British Columbia, Canada. 2 University of Miami, Miami, Florida. 

tary deformation is apparent, implying that most of the volcanic activity has taken place during the Holocene. 

Heat flow in the valley is regionally low and relatively 

constant, averaging about 0.2 W/m2, as are temperatures 

predicted for the sediment-basement interface, which 

average approximately 80°C. Some variability is associated with disrupted zones in the inner rift. The highest 

heat-flow values are near the centers of intrusive volcanism. The pattern of heat flow suggests that volcanism and 

associated hydrothermal activity are both highly focused, 

that little or no intrusive volcanism has occurred in the rift 

between the observed volcanic centers, and that thermal 

conditions within the upper crust regionally beneath the 

sediment have not have been affected by the recent intrusive volcanism. A simple mass- and thermal-balance 

hydrothermal model that is consistent with the geophysical observations demonstrates that sufficient heat and 

material would be available locally to produce the large 

hydrothermal deposits at the volcanic centers. 

INTRODUCTION 

BACKGROUND 

Sediment-covered spreading centers, although relatively rare, provide unusual opportunities for quantitative 

studies of sea-floor spreading and associated submarine 

hydrothermal systems. The sediment may preserve a relatively continuous stratigraphic record of magmatic, tectonic, and thermal events associated with sea-floor 

spreading and provide clues to the spatial and temporal 

variability of these processes. A regionally continuous, 

relatively impermeable sediment cover conductively insulates the underlying young crust and limits the possible 

pathways for recharge and discharge of hydrothermal fluids. Where discharge does occur, it can be strongly 

focused, and very large sulfide deposits can be produced. 

A sediment layer also provides a benign sea-floor environment that allows the use of certain observational techniques 

that cannot be used easily at unsedimented ridge crests, 

such as sea-floor heat-flow measurements, coring, and 

45 
46 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

deep-sea drilling. In particular, a sediment cover allows 

detailed heat-flow surveys to constrain axial geothermal 

processes, although these processes are considerably modified by the sediment cover and cannot be taken to represent those at unsedimented ridge crests. 

There are three well-studied sediment-covered spreading centers in the eastern Pacific Ocean: Middle Valley 

on the Juan de Fuca Ridge, Escanaba Trough on the Gorda 

Ridge, and Guaymas Basin in the Gulf of California. The 

first two of these are the axial rift valleys of mid-ocean 

spreading centers that have migrated close enough to the 

continental margin of western North America to have received large quantities of turbidite sediment during the 

Pleistocene. The last, a youthful spreading center that has 

developed in the wake of continental rifting between Baja 

California and mainland Mexico, is covered by a mixture 

of terrigenous and pelagic sediments. High-temperature 

hydrothermal venting and sulfide deposits have been observed and sampled at all three locations (see, for example, 

Lonsdale and Becker, 1985; Davis and others, 1987; Koski 

and others, 1988; Davis and Villinger, 1992). 

Middle Valley and Guaymas Basin are medium-rate 

(about 60 mm/yr) spreading centers, with virtually no basement exposures within the axial rifts. This has allowed the 

surface heat flow to be well mapped over these "zero-age" 

valleys. Over the last 20 years, numerous heat-flow surveys have been conducted at both locations, providing 

dense distributions of several hundred well-located measurements (see, for example, Lawver and others, 1975; 

Lawver and Williams, 1979; Williams and others, 1979; 

Lonsdale and Becker, 1985; Becker and Fisher, 1991; and 

Fisher and Becker, 1992 for Guaymas Basin and Davis and 

Lister, 1977; Davis and others, 1980; Villinger and Davis, 

1984, and Davis and Villinger, 1992 for Middle Valley). 

The results have shown that reconnaissance surveys can 

easily detect the large variability and the isolated highs in 

heat flow caused by hydrothermal activity, but that much 

more detailed surveys, with spacings between measurements of hundreds of meters or less, are required to characterize properly the nature of heat-flow variability so that 

clear inferences can be drawn about the associated geothermal processes. 

At Escanaba Trough only one reconnaissance heatflow survey had been conducted prior to 1989 (Abbott and 

others, 1986); this comprised 34 measurements made in 

three transects. The heat flow was observed to be highly 

variable, and locally high (up to 1.8 W/m2) near two volcanic edifices that penetrate the sediment within the axial 

rift. These results indicated clearly that volcanic and related hydrothermal processes are active within the axial rift, 

but did not allow the nature of the heat flow variability to 

be resolved further. 

The general structure of the rift and its sediment fill 

had been documented at the time of our study with a number of single- and multi-channel seismic-reflection profiles 

(Clague and Holmes, 1987; Morton and others, 1987; 

Morton and Fox, chap. 2, this volume), although the low 

trackline density and limited resolution of the profiles limited the interpretation of the detailed tectonic and volcanic structure of the rift. 

To further elucidate the structural, volcanic, and hydrothermal history and processes currently active in this 

rift, we returned to Escanaba Trough on CSS John P. Tully 

for 10 days in May 1989 and conducted detailed singlechannel seismic and heat-flow surveys in the central part 

of the trough (figs. 3.1 and 3.2). We report here the analyses of 12 selected seismic lines, and 75 heat-flow measurements acquired in four transects along representative 

seismic lines crossing the trough. 

TECTONIC AND GEOTHERMAL SETTING 

Escanaba Trough is the most southerly of three spreading segments that form the Gorda Ridge between the 

Mendocino and Blanco fracture zones (fig. 3.1 ). This segment is also the slowest spreading of the three, opening at 

a total rate of about 23 mm/yr, whereas the two segments 

to the north open at about 60 mm/yr (Atwater and Mudie, 

1973; Riddihough, 1980). Escanaba Trough is structurally 

and morphologically typical of a slow-spreading ridge; the 

deep rift valley is bordered by steep rift mountains with 

up to 1,500 m of normal-faulted relief. The valley floor 

widens from 3-5 km at the northern end of Escanaba 

Trough to roughly 18 km near the southern junction with 

the Mendocino fracture zone. 

Terrigenous sediments enter the valley from their 

source along the North American continental margin at the 

trough's junction with the Mendocino Fracture Zone. 

Several hundred meters of sediment cover the rift valley 

south of about latitude 41° 17' N. (Moore, 1970; Vallier 

and others, 1973); a 390-m section of Pleistocene turbidites was drilled at Deep Sea Drilling Project (DSDP) Site 

35 in the southern Escanaba Trough (McManus and others, 1970). A U.S. Geological Survey single-channel seismic survey in 1985 identified six major volcanic centers 

that penetrate or disrupt the sediment of the trough 

(Morton and others, 1987). These are spaced roughly at 

12 km intervals along the trough axis. Two of these centers, at about latitude 40°45' N. and latitude 41° N. (see 

figs. 3.2 and 3.3), referred to as the Southern Escanaba 

and Northern Escanaba (SESCA and NESCA) sites, have 

been studied extensively. Exposed volcanic rocks, sulfide 

deposits, high heat flow, and hydrothermal venting were 

observed at the northern site (Abbott and others, 1986; 

Koski and others, 1988; Morton and others, 1990; Holmes 

and Zierenberg, 1990). 

Given the limitations of ship time and the need to conduct relatively detailed measurements, our seismic and 

heat-flow survey was concentrated on the axial rift from 
THERMAL AND TECTONIC STRUCTURE OF ESCANABA TROUGH 47 

the NESCA site at about latitude 41° N. southward to another center of volcanic activity at about latitude 40°53' 

N., roughly midway between the NESCA and SESCA sites. 

North of this part of the valley, the sediment cover is not 

continuous enough for a systematic distribution of heatflow measurements to be established . 

41 .....-.. ESCANABA TROUGH 

STUDY AREA 

MENDOCINO FRACTURE ZONE. 

Figure 3.1. Bathymetric map of Gorda Ridge region, showing Escanaba Trough study area and approximate area included in figure 3.2. 

Bold lines indicate approximate locations of extensional and transform plate boundaries, and the coastline. Contours define edge of continental 

shelf and base of continental slope. Contour interval is 200 m. 
48 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

ACKNOWLEDGMENTS 

We thank Wanda Bentkowski, who helped reduce the 

heat-flow data during and after the field program, Ivan 

Frydecky and Gerald Horel, who provided assistance with 

heat-flow, seismic-reflection, and navigation operations, 

Henny Groschel-Becker, Joann Bessler, and Stephanie 

Ross, who provided watchkeeping leadership, and the captain, officers, and crew of the CSS John P. Tully for capable assistance with equipment and for excellent 

stationkeeping. Chris Fox of NOAA provided the Sea 

Beam data from which figure 3.2 was derived. Figures 

were prepared by Brian Sawyer, Linda Bedard, and Gina 

L'Esperance. Financial support to Keir Becker was provided by JOI-USSSP Site Survey Augmentation contract 

JSC7-89. W. Normark and J. Morton provided critical 

reviews of the manuscript. This document is Geological 

Survey of Canada Contribution No. 51190. 

NEW DATA 

Twelve new seismic-reflection profiles and four heatflow transects across the rift valley were completed during the Tully cruise (fig. 3.2). All lines were navigated with 

a shipboard system that integrates Loran-C with Global 

Positioning System (GPS) coordinate determinations; resulting ship's positions are believed to be accurate to within 

about 100m. During heat-flow operations, the position of 

the probe was determined by correlating structures recognized on both the ship's 3.5 kHz echo-sounding system and 

the record of the probe's 3.5 kHz pinger. During heat-flow 

station 15, the probe was navigated by acoustic relay transponder. Owing to the heavy weight of the probe, the separation of the probe and ship was modest, and usually 

ranged from 100 to 500 m. The azimuth was well determined as a result of the consistent orientation of the ship's 

tracks, and the resultant determinations of probe penetration locations along heat flow stations 3, 7, and 11 are probably accurate to about ±200 m along the direction of the 

tracks and ±100m across tracks; along station 15, the errors of penetration positions are probably less than 100 m. 

The seismic lines and the 75 successful heat-flow measurements (shown superimposed on a Sea Beam-derived 

bathymetric map in figure 3.2) are distributed throughout 

the central and northern part of the rift valley where sediments are present; seismic line spacing varies according to 

the local features of interest, with the closest spacing (about 

1 km) being over an area of very recent centralized 

disruption of the sediment section. The four heat-flow 

transects are situated across parts of the valley (1) where 

the sea floor and underlying sediment section are generally undisrupted (line 6, station 7), (2) where substantial 

recent extension has occurred in the central rift (line 5, 

station 3), (3) where the central rift sediments are highly 

disrupted by small-scale faulting, minor volcanic intrusion, 

or fluid flow and associated sediment alteration (line 10, 

station 11 ), and ( 4) where a mature sulfide deposit has 

formed over a volcanic intrusion (Central Hill, NESCA 

site) and where moderately high-temperature fluids vent at 

the sea floor (line 3, station 15). 

The seismic-reflection profiles, shown in figure 3.3, 

extend the full width of the valley to the major rift-bounding normal fault scarps, and in some cases, across the 

innermost rotated fault blocks that have been lifted above 

the valley floor. The seismic source used was a single 650-

cm3 air gun fired every 10 seconds. Lines were completed 

at a speed of roughly 3.5 knots, yielding a shot spacing of 

roughly 20 m. Signals were received on a Teledyne array 

having a single active section 50 m in length. Analog 

records (shown in fig. 3.3) were made in real time; data 

were also recorded digitally for later processing and playback. The profiles generally show clearly the seismic 

stratigraphy and tectonic structure of the sediment section 

that fills the valley. Basement arrivals from beneath the 

typically 500- to 700-ms-thick sediment section are everywhere strong, except for the areas of steep topography or 

areas that have been disrupted by volcanic intrusions, 

hydrothermal induration, or small-scale deformation. 

Heat-flow data were obtained with a Lister-type probe 

4 m long that has 11 thermistors and a heating wire housed 

within an 8-mm-diameter tube held in tension beside a 65-

mm-diameter strength member (see Davis, 1988, for general description). The thermistors are spaced 30 em apart 

along the sensor string. A low-resolution acoustic link allows the state of the instrument to be monitored at all times. 

Full resolution (16 bit) data are recorded internally in solid 

state memory. This digital resolution and the nonlinear response characteristics of the thermistors allow relative temperatures of each of the 11 probe sensors, a 

water-temperature probe, and an internal water-temperature 

sensor to be determined to roughly 0.5 mK at low temperatures, while maintaining a useful range of over 50 K. Tilt 

and absolute pressure are also determined; tilt is used to 

assess the quality of questionable penetrations, and pressure is used to assist in the hindsight navigation of the 

probe as well as for automatic detection of penetrations by 

a heat-pulse firing circuit. All channels, including low temperature-coefficient reference resistors and an internal temperature sensor, are sampled and logged every 10 seconds. 

=> 

Figure 3.2. Sea Beam-derived bathymetry of central part of 

Escanaba Trough, with locations of heat-flow measurements (dots) 

and seismic-reflection profiles with times along tracklines (lines 1-

12) superimposed. Contours are shown at 10-m intervals except in 

areas of high topographic gradient. Water depths are computed with 

assumed sound velocity of I ,500 rnls. Coordinates have been 

adjusted from those of original NOAA Sea Beam data (Loran-C 

control) according to GPS-controlled bathymetric constraints 

provided by data presented in figure 3.3. Map area is approximately 

20 km wide. Hachures indicate area of closed low. 
THERMAL AND TECTONIC STRUCTURE OF ESCANABA TROUGH 49 

Typical penetrations include ( 1) a brief period during 

which a "zero-gradient" reference is obtained with the instrument held in the near-bottom water, (2) an undisturbed 

+ -~ 

136/0400 

13610130 

+ + 

Stn.3 

0800 

13812030 ~ 

1:l811soo P22 13811.0 • • •• 

140/1300 

+ + + 

+ + + 

+ 

+ 

+ 

period of 7 minutes in the bottom while the sensor string 

equilibrates, and (3) a period of roughly 7 minutes while 

the decay of a calibrated heat pulse is monitored. From 

+ 

136/070o 

+ 

+ + 

+ 

+ -+ + 
50 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

these data the thermal gradient and thermal conductivity 

are determined in the manner described in detail by 

Villinger and Davis ( 1987). Two typical measurements 

from the Escanaba Trough are shown in figure 3.4; the final 

heat-flow values are computed as the slopes of temperature versus vertically integrated thermal resistance. Thermal conductivity measurements were not made during 

many of the penetrations (see table 3.1), either inadvertA 

ently because of premature probe disturbances or deliberately as a trade-off between fewer complete measurements 

and more gradient measurements. For penetrations without conductivity measurements, an assumed conductivity 

structure, derived from the average measured conductivity-depth structure determined from all measurements, was 

used to estimate the heat flow. As can be seen in figure 

3.4, there is a substantial variation in the conductivity with 

10 KILOMETERS 

4.0 

1 

-

~ 

4.0 w 

::E 

i= 

.J 

w 

2 ~ a: 

... 

~ 

~ I 

~ ... 

3 

Figure 3.3. Single-channel seismic-reflection profiles stacked along mid-valley north-south 

common reference line (shown by tick marks above and below each set of three profiles, labeled 

A-D). Horizontal scales of all profiles are equal; vertical scales vary. Horizontal reference 

line at 4.4 seconds is shown for each profile. Vertical exaggeration is roughly 10: 1. Profile 

locations and times along tracklines are shown in figure 3.2. 
THERMAL AND TECTONIC STRUCTURE OF ESCANABA TROUGH 51 

depth; this is the largest source of uncertainty in those heatflow determinations where in situ conductivity measurements were not made. Values from all penetrations are 

given in table 3.1, along with penetration locations, water 

depths, and estimated errors. 

DISCUSSION 

IMPLICATIONS FROM SEISMIC-REFLECTION 

DATA 

The structure of the rift and its sediment fill is simple. 

Regionally, the valley floor possesses modest relief; over 

8 

10 KILOMETERS 

5 

6 

Figure 3.3. Continued. 

the 12-km width and 22-km length of the valley surveyed, 

water depths generally vary by less than 150 m (figs. 3.2 

and 3.3). This relief is associated with displacements on 

normal faults in the central part of the rift and with local 

uplift of the sediment section and the sea floor over volcanic intrusions. Where well imaged, the basement surface is relatively flat. Sediment thicknesses vary between 

about 500 and 700 ms. Depth to basement cannot be resolved in the vicinity of the volcanic centers because of 

the acoustic disruption of the sediment stratigraphy by local 

deformation and by the presence of volcanic rocks within 

the sediment and at the sea floor. 

Numerous observations can be used to constrain the 

structural and volcanic history of the rift. A crude constraint 

4.0 

4.0 

5.0 
52 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

on the maximum age of the crust within the rift is provided 

by the width of the valley (12 km at the valley floor) and 

the spreading rate (24 mm/yr), or equivalently the fraction 

of the width of the Brunhes normal-polarity Chron occupied by the valley. A 500,000-year history is implied as 

an upper limit, and this in turn provides a lower limit for 

a sedimentation rate of about 1 mm/yr. Unfortunately, no 

more precise constraints were obtained from the cores obtained from DSDP Site 35 (McManus and others, 1970; 

c 

Vallier and others, 1973); much of the section was barren 

of microfossils, and all those observed were simply Pleistocene in age, with no further resolution possible. That 

observation has been substantiated through the re-examination of DSDP Site 35 cores by Normark and others 

(chap. 6, this volume). 

Ages of the uppermost part of the sediment section 

have been determined from piston cores collected in the 

valley by McManus and others (1970), Karlin and Lyle 

10 KILOMETERS 

7 

en 

-

LLI 

:E 

~ 

..J 

LLI 8 ~ a: 

.... 

~ ;: 

I 

0 

;: 

.... 

9 

5.0 

Figure 3.3. Continued. 
THERMAL AND TECTONIC STRUCTURE OF ESCANABA TROUGH 53 

(1986), and Karlin and Zierenberg (chap. 7, this volume). 

These authors conclude that sediment accumulation during 

the Holocene has been generally less than 2m. This, and 

the high rates of supply inferred for the Pleistocene that 

would have kept developing structures buried (see discussion ~elow), imply that the tectonic and volcanic disruptions observed to disrupt the valley floor at present are 

probably of post-Pleistocene age. This allows a lower limit 

on the rates of deformation in the valley to be established. 

Assuming that the vigorous sediment supply virtually 

ended 8,000 to 10,000 years ago (Mix, 1987), the rate 

D 

inferred for the accumulation of the 150 m of throw on the 

fault bounding the eastern block in the valley at profiles 1 

to 5 is roughly 15 mrnlyr. 

The rate of subsidence of the valley floor can be estimated in the same way as the rate of relative motion across 

faults. The same argument is used that during the Pleistocene period of copious sediment supply, the valley floor 

was maintained flat (at least to the degree that the turbidite plain outside the trough is flat, with a slope of less than 

0.001 ). A reference depth of 4.27 s = 3,200 m has been 

selected from the region immediately southeast of the 

10 KILOMETERS 

4.0 

en 

-

LIJ 

::E 

~ 

..J 

1 1 LIJ 

it 

a: 

1-

~ ~ I 

5.0 

4.0 ~ 1-

12 

5.0 

Figure 3.3. Continued. 
54 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

trough that is presumably outside the influence of the rift 

tectonics. The fault block along the eastern side of the valley (fig. 3.3, lines 1-5) lies only slightly deeper than this 

level; post-Pleistocene motion of this block is arguable. 

The remainder of the valley floor lies well below the reference depth (up to 200m; figs. 3.2 and 3.3), however, and 

it can be concluded that most of the central part of the 

trough has subsided substantially. The subsidence, which 

averages 100 to 150m across the 10-km width of the valley floor, is commensurate with the amount-135m-that 

would be produced by nonisostatic stretching of a 6-kmthick crust at the full 23 mm/yr spreading rate. 

Constraints on earlier subsidence and on the sedimentation rate during the Pleistocene are provided by one of 

TEMPERATURE (K) CONDUCTIVITY(Wm-1

K-1

) TEMPERATURE (K) 

0 0.2 0.4 0.6 0.8 1.2 0.2 0.4 0.6 0.8 

0 I I I I I I I I I I I l I I I I I I I I 0 

~ .... -I 1- ~ 

~. • 

_! "' 

en 

a: 

w 

1-

w 

:::! 2 

~ 

=i' 

1-

Q. 

w 

c 

4 

-

-

-

2 I 

"\ • 

• 

., • ."' • 

., • 

., • 

.'\ • 

.\ • 

• • \. • 

TEMPERATURE <K> 

4 6 8 I I I I _l 1 

\ 

ESCANABA TROUGH STATION 11-18 

HEAT FLOW = 0.160 W;m2 

CONDUCTIVITY ( Wm - 1 K-1

) 

10 0.8 1.2 0 I I I I I _l l I 

• 

_, 

• 

0 

0 

• ~ . • 

en • 

a: 

w 

1-

w 2 

:::! 

~ 

=i' 

1-

Q. 

w 

c 

4 

-

-

-

~ • 

• . ""' .""' • 

• . ""' ., • 

• ·~ 

~ • 

. ""' • 

ESCANABA TROUGH STATION 15-10 

HEAT FLOW = 2.50W;m2 

• \ 

• '\ 

• '\ 

.\ 

.\ 

.\ 

.\ 

.\ 

• \. 

\. 

'\ 

TEMPERATURE (K) 

2 4 6 8 I I I I I I I I 

"' ."' .""" 

'." 

.""' .""" ."". 

1-2 

-

-4 

10 

0 

-

.... 

I 

E 

3: 

-

E 

J: 

1-

0. 

w 

c 

c 

a: 

< ...J 

...J 

::::) 

m 

.... 

I -

~ 

'e 

3: 

1-2:§ 

-

J: 

1-

0. 

w 

c 

c ~ a: 

""", 1-

< ...J 

...J 

4::::) 

m 

Figure 3.4. Temperature versus depth, conductivity (W/m K) versus depth, and temperature versus integrated thermal resistance (Bullard 

depth), representative of measurements of high (2.50 W/m) and low (0.160 W/m) heat flow. 
THERMAL AND TECTONIC STRUCTURE OF ESCANABA TROUGH 55 

the most striking features of the acoustic stratigraphy of 

the sediment section. A relatively transparent layer roughly 

50 ms thick is visible throughout the valley at about 100 

ms depth below the sea floor. This horizon can be traced 

through many areas where local disruptions have obscured 

the finer-scale acoustic layering (for example, profiles 3, 

6, and 8-12); it provides an easily identifiable marker 

horizon for the Escanaba Trough fill. The horizon was observed in the site-survey data collected before DSDP Site 

35 drilling (McManus and others, 1970), and a massive 

unit, grading from sand at the base to sandy silty mud at 

the top, was sampled in cores 3 to 5 (between 79 and 107 

m below the sea floor) at that site. No internal bedding 

contacts were observed, although this could reflect drilling disturbances (Normark and others, chap. 6, this volume). For the massive lithologic unit to be coincident with 

the transparent acoustic unit, very reasonable velocities of 

roughly 1,600 m/s are required for both the overlying section and the massive unit itself. Several characteristics of 

the unit suggest a unique origin. First, the unit is unusually coarse grained, with a mean grain size that generally 

decreases upwards. Second, fossils in the finer-grained part 

of the unit include neritic and freshwater forms (McManus 

and others, 1970). And third, although not unusual for the 

turbidites of Escanaba Trough, the mineralogy of the unit 

indicates source rocks for the material in the Cascade 

Range and the Columbia River Plateau. A reasonable 

speculation is that the layer contains debris produced by 

the Bretz floods, which were caused by the catastrophic 

draining of the ice-dammed Lake Missoula (Bretz, 1923, 

1925; Pardee, 1942; Bretz and others, 1956). Gravel from 

the Bretz floods has been sampled in Cascadia Channel as 

far as 750 km from the continental margin (Griggs and others, 1970), and it is likely that a considerable amount of 

finer grained material was delivered to Escanaba Trough 

via Astoria Channel. The most recent flooding events are 

believed to have occurred between about 18,000 and 

13,000 years B.P. (Baker, 1983). The more recent time, 

the assumed 10,000-year age of the present sediment surface, and the approximately 75 m thickness of the overlying section provide an estimate for the late-Pleistocene 

turbidite sedimentation rate of 25 mrnlyr. This rate is high, 

but comparable to rates estimated for other rapidly subsiding basins to which Pleistocene turbidites have had access, 

such as the Winona Basin off northern Vancouver Island 

(4 mm/yr; Davis and Riddihough, 1982), and Middle Valley of the northern Juan de Fuca Ridge (10 mm/yr; Davis 

and Lister, 1977). If this rate were characteristic for "average" Pleistocene conditions, the entire 500 to 600 m of 

valley fill could have been deposited in as little as 30,000 

years (this estimate includes the effect of consolidation at 

depth). As an average for the full section, the rate is probably high because it represents the period following the last 

glacial maximum and including the sea-level transgression 

across the continental shelf (which could also be the source 

of the material in the transparent horizon). Thus the estimate of age of the sediment fill derived using 25 mrnlyr 

as an average sedimentation rate is probably an extreme 

and somewhat unreasonable lower limit. The sedimentation rate is greater than the displacement rates across faults 

and the subsidence rates estimated above (up to 15-20 mrnl 

yr), and thus the assumptions that sedimentation maintained 

low relief of the valley floor and kept fault offsets buried 

during the late Pleistocene are supported. 

That the sedimentation rate and rates of vertical tectonism are similar is also supported by the correlation of 

stratigraphy across normal faults beneath the floor of the 

valley. If rates of syntectonic sedimentation were much 

lower than the growth of the faults, then acoustic stratigraphy would be uncorrelatable across the faults. If sedimentation rates were extremely high, then the hanging-wall 

and footwall sections would be identical, and the displacement of the Pleistocene stratigraphy would mirror precisely 

the post-Pleistocene displacement of the sea floor. The 

case in Escanaba Trough lies between these extremes. This 

is illustrated graphically in figure 3.5, where the local 

stratigraphic depths of the top and bottom of the transparent marker horizon discussed above are plotted against the 

local depth of the sea floor. Clear trends with slightly divergent slopes are defined. Sea-floor depths are shown 

relative to the 3,200-m reference depth discussed above. 

The difference between the local depth of sea floor in the 

trough and the regional reference depth (abscissa in fig. 

3.5) must reflect the local rate of subsidence averaged over 

the post-Pleistocene period of low sediment supply 

(assumed again to be 10,000 years). Stratigraphic depths 

can be taken as the product of the horizon age (relative to 

the 10,000-year sea floor) and the sum of the regional 

sedimentation rate and the local syn-sedimentary subsidence rate. The latter is assumed to be constant at any given 

point and equal to that estimated for the post-Pleistocene. 

Using the least-squares-regression fits shown on figure 3.5 

yields times for the top and bottom of the horizon of 3,250 

and 4,650 years (pre-seafloor ages), or ages of 13,250 and 

14,650 years B.P. respectively. Late-Pleistocene sedimentation rates for the intervals from the sea floor to the top 

of the marker horizon and for the horizon interval itself (in 

the central part of the rift, where the sea floor depth is 

3,350 m) are 27 and 31 mm/yr respectively. These estimates are in remarkably close agreement with one another 

and with the independently derived estimate given above. 

In addition to extensional faulting and subsidence, recent volcanic activity has disrupted the Escanaba Trough 

sediment fill. Volcanism and related sediment deformation 

are focused at several centers along the axis of the trough 

(see Morton and Fox, chap. 2, this volume). Two of these 

volcanic centers, at latitudes 41° N. (NESCA) and 40°53' 

N., and the northern edge of a third (SESCA site centered 

at latitude 40°45' N.) are included in this study. Volcanism has penetrated to the surface at the northernmost 
56 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

Table 3.1. Heat-flow data collected during Tully cruise 89-04, Escanaba Trough. 

[Errors defined by goodness of fit of linear regression through temperature versus Bullard depth. Positions of measurement locations along seismic 

profiles (last two columns) were determined by projecting heat-flow measurement locations due north or south onto nearly co-located seismic lines] 

Station Penetration Latitude Longitude Depth Penetration kl Heat flow Error Profile Time 

number number N. w. (m) (m) (mW/m2) (mW/m2) 

3 1 40°57.51' 127°25.07' 3220 4.29 1 2521 2 5 1636 

3 2 40°57.49' 127°25.3' 3220 4.11 1 693 0.2 5 1630.5 

3 3 40°57.44' 127°25.36' 3220 4.08 2 444 0.2 5 1630.5 

3 4 40°57.26' 127°26.66' 3220 4.01 1 341 0.2 5 1617 

3 5 40°57.36' 127°26.8' 3220 3.88 2 301 0.2 5 1616 

3 6 40°57.33' 127°27.13' 3220 

3 7 40°57.36' 127°27.46' 3220 3.85 2 233 0.2 5 1609 

3 8 40°57.32' 127°27.89' 3365 3.67 2 356 0.2 5 1604.5 

3 9 40°57.28' 127°28.48' 3385 3.71 1 433 0.2 5 1558 

3 10 40°57.3' 127°28.78' 3385 3.63 1 416 0.3 5 1555.5 

3 11 40°57.31' 127°29.15' 3385 3.65 2 395 0.3 5 1551.5 

3 12 40°57.35' 127°29.36' 3410 3.89 1 396 0.3 5 1548.5 

3 13 40°57.35' 127°29.83' 3365 3.86 2 212 0.2 5 1544 

3 14 40°57.37' 127°30.22' 3360 3.68 1 207 0.2 5 1538 

3 15 40°57.37' 127°30.48' 3325 3.79 2 148 0.2 5 1535 

3 16 40°57.3' 127°31.05' 3315 3.78 2 180 0.3 5 1529 

3 17 40°57.38' 127°31.26' 3320 3.83 2 160 0.3 5 1526.5 

3 18 40°57.47' 127°31.52' 3310 3.84 2 191 0.2 5 1523 

3 19 40°57.52' 127°31.93' 3305 3.87 2 219 0.2 5 1518.5 

3 20 40°57.45' 127°32.28' 3295 3.81 2 236 0.3 5 1515.5 

7 1 40°56.14' 127°27.38' 3375 3.61 2 612 0.2 6 1758 

7 2 40°56.21' 127°27.5' 3380 3.72 1 211 0.2 6 1756 

7 3 40°56.11' 127°27.83' 3380 3.73 2 174 0.2 6 1752 

7 4 40°56.02' 127°28.03' 3380 3.75 1 157 0.2 6 1749 

7 5 40°56.05' 127°28.28' 3380 3.76 1 152 0.2 6 1746.5 

7 6 40°56.08' 127°28.66' 3375 3.73 2 148 0.2 6 1742 

7 7 40°56.05' 127°29.13' 3365 3.94 1 151 0.2 6 1736 

7 8 40°55.91' 127°29.53' 3355 3.84 2 160 0.2 6 1729 

7 9 40°55.87' 127°29.97' 3365 

7 10 40°55.83' 127°30.25' 3370 3.62 1 192 0.2 6 1720 

7 11 40°56.03' 127°30.55' 3350 3.91 2 153 0.3 6 1716 

7 12 40°56.02' 127°31.04' 3340 3.89 1 149 0.2 6 1709.5 

7 13 40°56.1' 127°31.28' 3335 3.85 1 133 0.2 6 1706 

7 14 40°55.99' 127°31.55' 3335 3.94 2 153 0.4 6 1702 

7 15 40°56.06' 127°32.13' 3332 4.07 1 169 0.2 6 1654 

7 16 40°55.95' 127°32.57' 3322 3.94 1 179 0.2 6 1648.5 

7 17 40°55.92' 127°33.02' 3320 3.79 2 204 0.3 6 1642 

7 18 40°55.95' 127°33.71' 3320 3.86 2 247 0.3 6 1633.5 

7 19 40°55.93' 127°33.88' 3320 3.81 2 284 0.2 6 1631.5 

11 1 40°54.36' 127°27.07' 3360 4.23 1 217 0.2 10 1342 

11 2 40°54.41' 127°27.16' 3370 3.71 2 201 0.2 10 1343 

11 3 40°54.4' 127°27.31' 3375 3.66 1 194 0.2 10 1344 

11 4 40°54.37' 127°27.69' 3380 3.69 2 176 0.3 10 1348.5 

11 5 40°54.38' 127°28.09' 3380 3.84 1 164 0.2 10 1352 

11 6 40°54.4' 127°28.43' 3380 3.81 1 155 0.2 10 1355.5 

11 7 40°54.4' 127°28.72' 3352 3.65 2 167 0.3 10 1358 

11 8 40°54.35' 127°29.2' 3362 3.69 2 131 0.3 10 1403.5 

11 9 40°54.39' 127°29.52' 3380 3.85 1 229 0.3 10 1408 

11 10 40°54.33' 127°30.03' 3373 3.52 2 219 0.2 10 1411 

11 11 40°54.19' 127°30.26' 3363 3.68 1 83 0.2 10 1414 

11 12 40°54.22' 127°30.51' 3365 3.52 2 254 0.2 10 1416 

11 13 40°54.19' 127°30.85' 3350 3.75 1 317 0.2 10 1419 

11 14 40°54.1' 127°31.32' 3350 3.75 1 707 0.2 10 1424 

11 15 40°54.1' 127°31.69' 3354 3.68 2 298 0.2 10 1428 

11 16 40°54.04' 127°32.1' 3350 

11 17 40°54.35' 127°32.13' 3350 3.79 2 561 0.2 10 1432.5 

11 18 40°54.38' 127°32.42' 3333 3.99 1 160 0.2 10 1446 
THERMAL AND TECTONIC STRUCTURE OF ESCANABA TROUGH 57 

Table 3.1. Heat-flow data collected during Tully cruise 89-04, Escanaba Trough-Continued. 

Station Penetration Latitude Longitude Depth Penetration kl Heat flow Error Profile Time 

number number N. w. (m) (m) (mW/m2) (mW/m2) 

11 19 40°54.39' 127°32.76' 3335 3.78 2 212 0.3 10 1439.5 

11 20 40°54.43' 127°33.22' 3290 3.87 2 137 0.2 10 1443.5 

11 21 40°54.43' 127°33.58' 3295 3.78 1 409 0.2 10 1448 

11 22 40°54.44' 127°33.96' 3290 

15 1 40°59.81' 127°33.35' 3240 3.71 1 949 0.2 3 925.5 

15 2 40°59.87' 127°33.01' 3254 3.73 2 851 0.2 3 928.5 

15 3 40°59.87' 127°32.97' 3260 3.68 1 670 0.2 3 929 

15 4 40°59.89' 127°32.65' 3285 3.72 2 482 0.2 3 932 

15 5 40°59.87' 127°32.13' 3320 3.68 1 384 0.2 3 937 

15 6 40°59.93' 127°31.75' 3325 3.64 2 320 0.2 3 940.5 

15 7 40°59.95' 127°31.48' 3320 3.76 1 415 0.2 3 944 

15 8 41 °0.02' 127°31.37' 3315 3.84 2 410 0.2 3 945 

15 9 41 °0.05' 127°31.18' 3325 3.64 1 726 0.2 3 948 

15 10 41 °0.07' 127°30.85' 3310 3.55 1 2495 0.2 3 950.5 

15 11 41 °0.14' 127°30.62' 3300 3.66 1 1117 0.2 3 954.5 

15 12 41 °0.16' 127°30.46' 3295 3.71 1 1331 0.2 3 955 

15 13 41 °0.15' 127°30.06' 3270 3.75 1 3176 0.2 3 958 

15 14 41°0.15' 127°29.9' 3270 3.58 1 1675 0.3 3 1000 

1Conductivity code: 1, Conductivity measured in situ; 2, Assumed (survey average) conductivity, as listed below. 

Nominal depth k Nominal depth k 

(m) (W/mK) (m) (W/mK) 

1.0 0.84 2.2 0.94 

1.3 0.83 2.5 0.96 

1.6 0.87 2.8 0.98 

1.9 0.90 3.1 0.99 

location, where a ponded flow buries the sediment surface 

over several square kilometers (fig. 3.3, profiles 1-3; see 

also Ross and Zierenberg, chap. 8, this volume; Morton and 

Fox, chap. 2, this volume). The thickness of the flow is 

not well constrained. Much of the relief of the flow near 

its source (near profile 3) relative to the floor of the graben in which it is contained may be due to local uplift at 

the volcanic center. The flow appears to be thin or discontinuous enough in its distal parts to allow some acoustic penetration by the small acoustic source, as some faint 

layering is visible beneath the flow along profiles 1 and 4. 

Volcanism at the more southerly locations is observed to 

be limited to intrusions within and beneath the sediment 

section (profiles 7-12), although south of profile 12, 

extrusive rocks are present at the SESCA site (Morton and 

Fox, chap. 2, this volume; Ross and Zierenberg, chap. 8, 

this volume). 

Localized uplift of the sediment section is common at 

the centers of volcanic activity. Clear examples occur 

along profiles 3, 4, 8, 9, 11, and 12 (fig. 3.3). The uplifted 

structures are circular in plan (fig. 3.2), and commonly 

about 1 to 1.5 km in diameter. The structures appear to 

be domed in cross-section, although some of this appearance results from acoustic diffraction at their perimeters. 

Nominal depth k 

(m) (W/mK) 

3.4 1.00 

3.7 1.01 

4.0 1.04 

It can be seen in the profiles and it has been observed directly that the uplift is fault bounded (Zierenberg and others, chap. 10, this volume; Denlinger and Holmes, chap. 

4, this volume). 

Constraints on the timing of the uplift of the sediments 

at the volcanic centers are provided by considering the 

stratigraphic depth and thickness of the transparent marker 

horizon with respect to the local sea-floor depth, in the 

same way that subsidence and sedimentation histories were 

estimated (see discussion above). Unfortunately, where the 

uplift is large (for example, line 4, fig. 3.3), the coherence 

of the stratigraphy is nearly completely destroyed and a 

quantitative assessment cannot be made. The stratigraphy 

is well preserved at profiles 8 and 9, however, and the 

horizon depths can be determined there with confidence. 

These are plotted with respect to the local sea-floor depth 

in figure 3.5. Although the points fall within the range of 

scatter of the points derived from the "normal" sediment 

fill, they are offset systematically from the regression lines 

through the "normal"-section points in a direction (shallow) and by an amount (20 m) that is consistent with the 

local uplift of the sea floor at these volcanic centers (20-

40 m). Thus it is reasonable to conclude that unlike the 

normal faulting and subsidence of the valley floor, much 
58 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

of the volcanic activity and the associated uplift of sediment occurred during the post-Pleistocene period of low 

sediment supply (roughly the last 10,000 yr). 

The exact relationship of the pluglike uplift and underlying intrusive volcanism is not clear. In some cases 

(such as profiles 7, 8, and 9), bright reflectors are seen 

within the sediment section where there is virtually no relief at the sea floor. In other cases (profiles 8, 9, 11, and 

12) there is a correspondence between uplift and underlying bright reflectors, although the geometry is poorly defined. In general, the bright reflections within the sediment 

section are far more chaotic than the overlying deformation of the sediments. From these observations, we infer 

that if the local bright reflections are from intrusive sills 

within the sediments, they are relatively thin, and that the 

sediment uplift is not caused by these intrusions. We suggest instead that the uplift is caused by a deeper, more centralized intrusive process, possibly the construction of a 

volcanic edifice at the base of the sediment pile. Unfortunately, the deeper structure is obscured by the small but 

numerous higher level sills (see also Denlinger and 

Holmes, chap. 4, this volume). 

150 

en 

a: • 

w y.ll'!• 1- ~ I w 7. . == 

~ • • • • 

:::1: 100 ,r"'.. • /. 

tQ. . . , Po··'- y i! •• LIJ 

() 

c ........... - .......... ~ • 3: • 

Q. • 

<( 50 Ct • 

<!' 

[i 

Ct 

ten 

0~--------------.. --------------~ 

3,200 3,300 3,400 

DEPTH TO SEA FLOOR, IN METERS 

Figure 3.5. Depths below sea floor of top (circles) and bottom 

(squares) of widespread, clearly defined, acoustically "transparent" 

horizon in Escanaba Trough, plotted as a function of local sea floor 

depth. Origin for depth axis chosen to equal depth of turbidite plain 

outside rift valley at its southern end. Depths are inferred to represent 

amount of local subsidence in rift since the end of rapid late 

Pleistocene sedimentation. Trends of least-squares regression lines 

fitted through points, combined with subsidence yield estimates for 

rates of syn-tectonic sedimentation. Stratigraphic depths at volcanic 

center at seismic-reflection lines 8 and 9 are shown with stars. 

INFERRED THERMAL STRUCTURE 

In most cases, the probe penetrations were spaced 

closely enough to define the nature of heat-flow variability well (fig. 3.6). Along line 6, the heat flow is remarkably uniform. All values fall very close to 0.2 W/m2 across 

the full width of the valley, with the exception of the measurements made near the bounding normal-fault scarps. 

Toward the west end of the line, the heat flow increases 

slightly, and one value at the east end exceeds the rest by 

a factor of 3. More extreme examples of this pattern are 

seen at the west end of line 3 and the east end of line 5. 

This pattern appears to occur in measurements wherever 

the basement shoals in proximity to the bounding normal 

fault scarps. 

"Background" values elsewhere in the valley are virtually identical to those measured along line 6, suggesting 

either that hydrothermal circulation in the valley is currently weak and the heat flow is dominantly conductive and 

uniform, or that circulation within the crust beneath the 

sediment section is sufficiently vigorous to maintain 

roughly isothermal conditions near the sediment-basement 

(low permeability-high permeability) boundary. The sense 

of the variability near the bounding faults is consistent with 

either conductive or hydrothermal effects; high values near 

the valley walls could be caused either by conductive focusing of heat or by the thinning of the sediment cover over 

a hydrothermally warmed basement aquifer. However, the 

magnitude of the anomalies observed at the east end of line 

5 and the west end of line 3 is far too large to be accounted 

for by conductive effects, and the effect of hydrothermal 

heat transport in basement is very likely to be the cause . 

Some fluid leakage from beneath the sediments of the valley may occur along the valley-bounding normal faults, although there is no direct evidence for this in our 

observations . 

Three scattered high values occur along line 10 where 

the sediment is disturbed on a small scale by tectonic or 

underlying intrusive activity near the center of the valley. 

The anomalies are small and very localized (on a scale 

smaller than the measurement spacing), and probably result from very recent mechanical disturbances of the sediments at the sea floor. A piston core from this location 

(TUL 89D-7P, Normark and others, chap. 6, this volume), 

containing irregular bedding, suspended mud clasts, and a 

nearly vertical vein of sandy silt, supports this conclusion. 

Along line 5, values over the rifted central part of the valley are systematically high. These values may reflect the 

higher degree of extension in this part of the valley; they 

also may be part of the anomaly at the NESCA volcanic 

center that lies only 3 km to the north. Measurements 

along line 3 extend into the NESCA volcanic center itself; 

they show the heat flow to increase fairly systematically 

from a minimum of about 0.3 to well over 1 W/m2 toward 

the center of the valley and the center of the area disrupted 
THERMAL AND TECTONIC STRUCTURE OF ESCANABA TROUGH 59 

by volcanic activity. This pattern undoubtedly results from 

the local volcanism and hydrothermal activity at this location along the rift. The two extreme values along the eastern part of the profile are probably associated with nearby 

localized hydrothermal fluid discharge. A 108°C and a 

220°C vent are known to exist a few hundred meters from 

the easternmost two penetrations (Zierenberg and others, 

chap. 10, this volume). The systematic increase in heat 

flow at the western end of the line is discussed above. 

2.0 

C'll 

E 

~ 

~ 0 

ii 1.0 

~ 

<( 

w 

:::t: 

A 

ce 1.0 

~ 

~ 0 

...J 

LL 

~ 

<( 

w 

:::t: 

c 

Line 3, Station 15 

41°00' 

I I I 

10 KILOMETERS 

Line 6, Station 7 

40°56' 

10 KILOMETERS 

t.s 

jsECOND 

l 

------- ~ ---1-

~ 0.5 

jsECOND 

Figure 3.6. Heat-flow values superimposed on colocated seismic 

lines. Penetration locations are projected to seismic lines along 

general structural trend of valley. 

If the sediment permeability is low enough to prohibit 

thermally significant vertical fluid flux through the sediment, then a reasonable estimate of the regional hydrothermal fluid temperatures within basement can be derived by 

simply extrapolating the measured surface gradients to the 

base of the sediment section. This is probably the case, 

as the more permeable sandy interbeds in the turbidite section are probably not well interconnected, and as a result, 

the vertical component of permeability is controlled by the 

C'll 

E 

~ 

~ 0 

...J 

LL 

~ 

<( 

w 

:::t: 

8 

C'll 

E 

~ 

~ 0 

...J 

LL 

~ 

<( 

w 

:::t: 

D 

2.0 

2.0 

1.0 

Line 5, Station 3 

40°57.5' 

i I I I I I 

10 KILOMETERS 

Line 10, Station 11 

40°54.3' 

I I I I I I 

10 KILOMETERS 

~ 0.5 j SECOND 

~ 0.5 j SECOND 
60 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

fine-grained layers. Horizontal fluid transport will not disturb the thermal field significantly. Using a reasonable 

function for the porosity of the sediments at depth and an 

appropriate relationship between porosity and conductivity (see Nobes and others, 1986), an average conductivity 

of 1.46 W/m K is estimated for the roughly 600-m-thick 

sediment section. The resulting extrapolated temperatures 

calculated for the sediment-basement interface in the areas away from the NESCA volcanic site are roughly 80°C. 

This temperature is much lower than either temperatures 

measured in hydrothermal fluids that currently discharge 

through the sea floor at the NESCA site (up to 220°C; 

Zierenberg and others, chap. 10, this volume) or the even 

higher temperatures (300°C-350°C) that must have existed 

when the major sulfide mineral deposits were formed in 

this part of the valley (Morton and others, 1990). 

Two hypotheses can be developed to explain this apparent local contrast in crustal thermal conditions. One is 

that the sediment fill of the valley is in a transient thermal 

state, and that high temperatures (around 350°C) do occur 

in the crust beneath the valley. The estimated regional 

basement temperatures are based on the assumption of 

steady-state conditions, and they could be erroneously low. 

Two effects must be considered, one being that of the rapid 

sedimentation and the other the possible youth of the volcanic and hydrothermal activity. 

The sedimentation rate estimated above is very high, 

but because of the brief history of sedimentation and the 

modest total accumulation, the effect is not large. Using 

the simple correction derived from Carslaw and Jaeger 

(1959) by Von Herzen and Uyeda (1963) as a limiting 

case, we find that the surface heat flow would have been 

reduced by about 35 percent at the end of the Pleistocene; 

after 9,000 to 10,000 years of post-Pleistocene 

nondeposition, the effect of sedimentation on the present 

heat flow would have diminished and would be less than 

this. The correction for the case of sedimentation on a 

basement that is maintained at a constant temperature by 

hydrothermal circulation is even less (Wang and Davis, 

1992). Thus the effect of rapid sedimentation is probably 

not the cause of the low regional heat flow in the valley. 

The other possibility is that the high temperatures and hydrothermal activity associated with recent volcanism are 

widespread but very recent. This possibility cannot be 

eliminated with the existing data. The time for a significant thermal signal to penetrate the 600-m-thick sediment 

section that buries basement would be roughly 6,000 

years, and as concluded above, the volcanism that disrupts the sediment fill could be younger than this. Any 

associated thermal events that may have affected regional 

basement temperatures recently would not be reflected in 

the present surface heat flow. The anomalous heat flow 

seen along line 5 near the NESCA volcanic center is 

probably a result of conductive heat transfer through most 

of the sediment section, however, and this suggests that 

volcanism and hydrothermal activity in the crust is at 

least a few thousand years old. 

An alternate and preferred explanation is that all of the 

heat associated with the volcanism that has produced the 

sediment uplift and hydrothermal products has been lost 

by very efficiently focused hydrothermal discharge, with 

no regional heating having taken place. In fact, the crust 

surrounding the volcanic centers may have functioned as 

a fluid supply zone for the hydrothermal systems, and the 

crust and overlying sediment section may have been 

slightly cooled by distributed slow recharge during the activity. The cause for highly focused hydrothermal discharge may be twofold: (1) The source of heat is probably 

highly focused and limited to the intrusions and magma 

supply zones immediately beneath the volcanic centers. (2) 

The permeability of the sediments may be highly enhanced 

by the intrusion process: rapid heating of the sediments 

causes local hydrofracturing; hydrothermal induration increases the strength of the sediment and allows open fractures to be supported; and the continuing deformation 

induced by the emplacement of sub- and intra-sedimentary 

intrusions creates more new fracture-permeability as the 

volcanism continues (see discussion in Denlinger and 

Holmes, chap. 4, this volume). The resulting hydrologic 

configuration acts as a chimney, the base of which is 

underpressured with respect to the regional conductive 

thermal-profile hydrostat and overpressured with respect to 

the isothermally hot hydrostat directly overhead. 

With respect to the generation of metalliferous hydrothermal fluids, these two hypotheses concerning the nature 

of circulation beneath and within the sediment of Escanaba 

Trough differ in a fundamental way. In the first case, hightemperature fluids could be "trapped" regionally beneath 

the sediment of the valley. This would permit widespread 

high-temperature water-rock interaction within the crust 

and deeper sediment of the valley. In the second case, fluids would be in contact with sediment and crust at high 

temperatures only in the immediate vicinity of the volcanic centers. Neither case is prohibited by the data presented here. Both allow ample interaction between 

high-temperature fluids and sediment, as implied by the 

composition of hydrothermal minerals sampled at the sea 

floor (Koski and others, 1988). Both provide sufficient 

heat, although in the focused case a relatively large reservoir of heat must be tapped at depth to produce the volume of minerals that may be present. 

To support these conclusions, we have estimated mass 

and heat budgets with a very simple model that is shown 

schematically in figure 3.7. The geometry chosen is simple: 

a uniformly thick sediment fill buries a permeable igneous upper crust of the rift valley. A localized volcanic heat 

source lies beneath the center of the valley, underlying a 

zone of enhanced permeability in the sediment section created by the volcanism and the related tectonic and hydro-

~hermal effects. Fluids are supplied via distributed recharge 
THERMAL AND TECTONIC STRUCTURE OF ESCANABA TROUGH 61 

through the sediment section and basement outcrops in the 

region surrounding the volcanic center. Local porousmedium convection, which occurs wherever the permeability, thermal gradients, and permeable-layer thicknesses are 

sufficient to yield supercritical conditions, undoubtedly occurs in the upper crust but is not shown in the diagram. 

The large-scale fluid flow that is shown is driven by the 

difference between the pressure within an ascending hot 

column of fluid (taken to be 380°C) and the regional pressure beneath the impermeable sediment section, which 

probably lies close to the hydrostat defined by the density 

of a fluid column having a conductive thermal gradient. 

This is assumed to be 0.16 Kim. Using the thermal 

expansivity of seawater defined by Bischoff and 

Rosen bauer ( 1985), a maximum pressure difference of 

nearly 2 MPa at the level of the base of the sediment section (500 m below sea floor) results. Depending on the 

permeability structure, much of this pressure differential 

may be available to drive hot fluids through the sediment 

section above the volcanic centers. A small pressure differential may also exist to drive fluids from beneath the 

sediment "seal" through warm, fault-controlled conduits 

along the edges of the valley; applied over the same 500 

m depth interval, the density difference between warm 

(80°C) and cold (conductive) fluid columns would produce 

a pressure difference of about 0.1 MPa. 

If known, the quantity and composition of hydrothermal products present in the mineral deposits associated 

with the volcanic centers would set limits on the volumes 

of fluid and rock that must be involved in the system at 

depth. The quantities of hydrothermal minerals produced 

are poorly constrained; observations of hydrothermal mineralization in outcrop are numerous (see Zierenberg and 

others, chap. 10, this volume) but provide little quantitaa: 

w 

..... 

w 

:::iE 

0 

...J 

S2 

1 KILOMETER 

tive information about mineralization at depth. We used a 

deposit size of 10 million tonnes (1010 kg) for the purposes 

of the model calculation. It is reasonable to assume that 

roughly half of this mass would be iron (Koski and others, 1988; Morton and others, 1990); if this had precipitated at 100 percent efficiency from high-temperature 

hydrothermal fluid saturated with iron at a concentration 

of 500 ppm, then 1013 kg, or 1010 m3 of water would be 

required. To heat this mass of water by 350 to 400 K requires roughly 1019 J of heat. To supply this heat requires 

the solidification and cooling from 1 ,200°C to 400°C of 

roughly 4 km3 of rock, which is commensurate with the 

size that one might expect for an intrusion beneath a volcanic center like those in Escanaba Trough. Lesser volumes of rock (for example, a volume equivalent to a series 

of sills emplaced within and beneath the sediment section) 

could not provide enough heat to produce the deposit of 

the size assumed. Deposits of greater size would require 

commensurately larger heat sources or a series of intrusions 

sharing the same discharge zone. 

A simple estimate for the time required to dissipate the 

quantity of heat calculated above can be had by assuming 

an average rate of hydrothermal discharge for the system. 

At a rate of 500 MW, a value that falls at the low end of 

the range for vent fields summarized by Schultz and others (1990), 1019 J of heat would be fully dissipated in about 

600 years. If this were the case, then the rate of flow 

through the system would be roughly 0.5 m3/s. 

To maintain this rate of fluid flux would require more 

than distributed recharge through the sediment section in 

the vicinity of the volcanic centers. Much of the buoyancyderived pressure differential in the hydrothermal system is 

probably lost in the discharge path above the volcanic centers; the differential available to drive distributed recharge 

Figure 3.7. Schematic illustration of heat- and mass-balance model for regional hydrothermal circulation, fluid discharge, and mineralization 

associated with volcanic centers in Escanaba Trough. Arrows show fluid flow. See text for discussion of model. 
62 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

is probably less than a few tenths of a megapascal. This 

driving force, and the low vertical permeability of the sediment section, set a limit on the rate of distributed recharge. 

A value for the permeability of the sediment section of 

10-16 m2, and a pressure differential over the 600 m thickness of sediment of 0.1 MPa would result in a limiting velocity of roughly 1.5 x 10-11 m/s (0.5 mm/yr). To maintain 

a flux of 0.5 m3/s would require recharge to be distributed 

over an area of roughly 30,000 km2, which is clearly unreasonable given the dimensions of the valley and the distances between volcanic centers. This implies that most 

of the recharge is probably supplied along the normal-fault 

scarps that bound the valley, where the sediment cover is 

thin or basement rocks are exposed, and thus where the 

hydraulic impedance to flow is low. 

Although a large volume of rock is required to supply 

the necessary heat for the construction of a 10-milliontonne mineral deposit, a relatively small volume of rock 

is required to supply the necessary mass. This can be 

shown by considering the balance of the quantities of lead 

and sulfur (elements that are particularly mobile and likely 

to be fully stripped from the rock in the alteration zone) 

in the deposit and in the source. For this simple calculation, 10 ppm lead and 0.2 percent sulfur have been used 

for concentrations (by weight) in the source rocks, and 0.06 

percent lead and 35 percent sulfur have been used for concentrations in the sulfide deposit. Assuming the source to 

be in the sediment section, where the porosity could be as 

high as 50 percent, a source volume of between 0.4 and 

0. 7 km3 is implied. This volume is nearly 10 times smaller 

than the volume of rock required to supply the heat; it is 

small enough that it could be contained entirely within the 

sediment section in the immediate vicinity of an individual 

volcanic center. For this to be the case, the water-rock interaction would have to be very efficient, but this is not 

unreasonable. Intimate contact between sediments and 

high-temperature fluids may be permitted by the high 

porosity of the sediments, intergranular flow in highpermeability coarse-grained layers, and fine-scale fracturing produced through the deformation of the sediment 

caused by the volcanism, and by hydrofracturing due to 

rapid heating events or overpressuring in the hydrothermal 

system. Within the igneous section, less chemical interaction may take place, as the permeability there is probably dominated by relatively widely spaced, large-aperture 

fractures. 

CONCLUSIONS 

New single-channel seismic and heat-flow data across 

the central Escanaba Trough provide new insight into the 

tectonic, volcanic, and hydrothermal structure and history 

of this sediment-filled rift valley. The history of the valley contained within the sediment section may be very 

brief. The depth and inferred age of a ubiquitous, thick, 

transparent acoustic layer that may be related to the most 

recent catastrophic draining events of the Pleistocene Lake 

Missoula, and a simple stratigraphic analysis provide independent estimates of the sedimentation rate for the upper part of the sediment section. The rates agree and are 

high, on the order of 25 to 30 mm/yr. At this rate, the entire section of sediment could have been deposited in only 

30,000 years; the sediment at the base of the section would 

be only 40,000 years old. The rates estimated are derived 

from only the upper third of the section, however, and are 

representative only for the period of rapid deposition following the last glacial maximum. This age for the section 

is thus an extreme lower limit. 

The sediment fill is disrupted by normal faulting and 

by localized volcanic intrusions. Syn-sedimentary deformation across the normal faults is modest but resolvable. 

The timing of the deformation associated with the igneous 

intrusions is less well defined, but it appears that the intrusions have deformed the entire sediment section uniformly. Thus the igneous and associated hydrothermal 

activity at the volcanic centers may postdate the period of 

rapid Pleistocene sedimentation, which is assumed to have 

ended roughly 8,000 to 10,000 years ago. 

The heat flow in the valley regionally is relatively low, 

averaging roughly 0.2 W/m. Temperatures at the sedimentbasement interface estimated by extrapolating the surface 

gradients through the thickness of the underlying sediment 

section are low also, averaging about 80°C, despite the recent volcanism and indications of high-temperature hydrothermal activity at several locations along the valley axis. 

High heat-flow values occur where basement shoals near 

the valley-bounding faults, but this can be attributed to the 

thinning of the sediment section above a crustal aquifer, 

the temperature of which is maintained at a roughly constant level by efficient fluid circulation. The highest heat 

flow occurs in the vicinity of the NESCA volcanic center, 

which is known to be hydrothermally active. 

Two explanations are suggested for the localized 

nature of the thermal anomaly associated with the volcanic center and the low heat flow elsewhere. One possibility is that high-temperature crustal hydrothermal circulation 

associated with the volcanism in the valley is widespread, 

but too recent (less than a few thousand years) for the thermal effects to have reached the sea floor by conduction. 

Temperatures at depth estimated by simple extrapolation 

from sea-floor measurements are correct only for steadystate thermal conditions, and would be erroneously low if 

a transient state were present. The other, more likely, possibility is that the hydrothermal systems driven by the volcanic heat sources are highly focused by the localized 

nature of the intrusive volcanism, by the enhanced permeability of the sediment as it is modified by the induration, 

hydrofracturing, and deformation in the vicinity of the 

volcanic centers, and by the consequent geometry and 
THERMAL AND TECTONIC STRUCTURE OF ESCANABA TROUGH 63 

thermal structure of the chimney-like hydrologic system. 

If temperatures within the sediment are low regionally, then 

the interaction between high-temperature fluids and sediment that is known to occur must be limited to the region 

directly above the volcanic intrusions. A simple model of 

heat, fluid, and chemical balance demonstrates that this is 

reasonable, and provides constraints on the nature and rates 

of fluid flow through the crust and sediments of the valley. 

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Baker, V.R., 1983, Late Pleistocene fluvial systems, in Porter, S.C. 

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Becker, K., and Fisher, A.T., 1991, A brief review of heat flow 

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dispersal patterns of turbidite sand in Escanaba Trough, northeastern Pacific Ocean: Marine Geology, v. 15, p. 67-87. 

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Heat flow at the spreading centers of the Guaymas Basin, 

Gulf of California: Journal of Geophysical Research, v. 84, 

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Escanaba Trough, in Morton, J.L., Zierenberg, R.A., and 

Reiss, C.A., eds., Geologic, hydrothermal, and biologic studies at Escanaba Trough, Gorda Ridge, offshore northern 

California: U.S. Geological Survey Bulletin 2022, chapter 

10 (this volume). 
Chapter 4. A Thermal and Mechanical Model for Sediment 

Hills and Associated Sulfide Deposits along Escanaba Trough 

By Roger P. Denlinger and Mark L. Holmes 1 

CONTENTS 

Page 

Abstract................................................................................ 65 

Introduction .. .... ... ... .... .. .. .. .. . . ... . .. . ... . ... . ... .. .. . ... . ... . ... .... . . ... . . . . . 65 

Acknowledgments .... .. .. .. . .. . ... . ... . ... . ... . ... . ... . ... . ... . ... . .. .. . 66 

Geologic and geophysical constraints .......... ...................... 66 

A mechanical model for growth of the sediment hills . . .. .. . 68 

A model for hydrothermal circulation .......................... ...... 69 

The second stage of cooling........................................ 70 

The third stage of cooling .. .... .. .. .. .... .... .. .. ........ .... ...... . 72 

Conclusions ........................ ...................... .... ........................ 73 

References cited . .. . .. .. . . .. . . .. . .. . . .. . .. .. .. .. .. .. . . . . .. .. .. .. .. . . .. .. .. .. . . .. . . . 7 4 

ABSTRACT 

The flat valley floor of Escanaba Trough contains a 

series of mesalike sediment hills that are flanked in places 

by basalt flows. Geologic and geophysical observations 

show that massive sulfide deposits are common near the 

base of these sediment hills, but they have not been found 

on the gently rounded tops. Submersible observations indicate that the hydrothermal deposits consist of elongate 

mounds that occur around the flanks of the sediment hills. 

The geochemistry of hydrothermal fluids and sulfide deposits requires extensive circulation of the hydrothermal 

fluids through sediments. 

We combine these data with the physical properties of 

basalt and sediment to develop a quantitative model for 

localized hydrothermal circulation along faults associated 

with sediment hill growth. Uplifted sediment hills are 

modeled as the surface expression of a thin basalt laccolith intruded along the basalt-sediment interface about 400 

to 600 m below the sea floor. 

Laboratory experiments and theoretical analyses suggest that inflation of each laccolith uplifted and bent the 

overlying sediments, generating thrust faults that curve outward as they approach the surface. In this way each laccolith lifts and dilates the surface material, leading to the 

development of normal faults that break initially at the surface and propagate downward. With continued uplift these 

1 U.S. Geological Survey, Seattle, Washington. 

normal faults eventually supersede the thrust faults and 

form a horst structure. 

Cooling of each laccolith occurs both through conduction and through fluid convection along these fault zones. 

Using simple assumptions about the interaction of basalt 

and seawater, we calculate the rate of heat loss with time 

for the life of the hydrothermal system. If the fault zones 

described above are more permeable than the surrounding 

sediment, then convection of fluid up these fault zones 

controls the rate of heat loss from each laccolith for 90 percent of its cooling history. 

For a fault zone 2 meters wide with a permeability of 

1 o-13 m2' a laccolith forming a typical sediment hill in the 

Escanaba Trough (70 m high, 1,200 m in diameter) will 

cool completely in less than 500 years. If mineral precipitation reduces this permeability to 10-14 m2 and the effective fault zone thickness to 1 m, then the cooling time is 

extended to 23,200 years, and more heat is lost to the subsurface sediments than is delivered to the sea floor. A more 

accurate estimate of cooling will require detailed information on the permeability and thickness of each fault zone. 

INTRODUCTION 

Gorda Ridge is an actively spreading mid-ocean rift 

system that is close to the western margin of North 

America. The southern 1 00-km segment of this rift system, Escanaba Trough, receives turbidites from the continental shelf and slope (Vallier and others, 1973; Normark 

and others, chap. 6, this volume). Regional geologic and 

geophysical studies of the trough have been studied to assess the geologic setting (Zierenberg and others, chap. 10, 

this volume), petrology (Clague and Holmes, 1987; Davis 

and others, chap. 9, this volume), heat flow (Abbott and 

others, 1986; Davis and Becker, chap. 3, this volume), 

association of massive sulfides with centers of volcanic 

intrusion along axis (Morton and others, 1987), hydrothermal mineralogy (Koski and others, 1988; chap. 16, this 

volume), and sediment stratigraphy to estimate age 

relationships (Karlin and Lyle, 1986; Karlin and 

Zierenberg, chap. 7, this volume; Normark and others, 

chap. 6, this volume). 

65 
66 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

These studies show that the volcanic products of rifting are confined to discrete centers (Morton and Fox, chap. 

2, this volume). Mesalike sediment hills that are flanked 

by basalt flows are common along axis, and have been 

studied in both the northern part of the trough (NESCA 

area) and the southern part of the trough (SESCA area). 

A cross section of a typical sediment hill, from detailed Sea 

Beam bathymetry (Morton and Fox, chap. 2, this volume), 

is shown in figure 4.1 along with an interpretive cross section based on the study described below. 

The detailed Sea Beam data show that the sediment 

hills in Escanaba Trough typically have about 70 m of 

relief, and seismic profiles show that they rest on a sedimentary layer 400 to 600 m thick (Morton and others, 

1987; Morton and Fox, chap. 2, this volume). Vents that 

fed basalt flows surrounding the sediment hills penetrated 

this sediment thickness in NESCA and SESCA, and these 

lava flows locally lap up against the sediment hills to form 

a complicated surface topography (Holmes and Zierenberg, 

1990; Ross and Zierenberg, chap. 8, this volume). Submersible observations (Holmes and Zierenberg, 1990) and 

camera tows (Morton and others, 1990; Zierenberg and others, chap. 10, this volume) show that steep fault scarps on 

the flanks of the sediment hills typically expose layered 

sediments and that massive sulfide deposits occur near the 

base of these escarpments. 

Preliminary models for these sediment hills proposed 

that the hills were underlain at shallow depth by volcanic 

rock (Morton and others, 1990) in order to explain a lack 

of layering in seismic records and an association of sediment hills with sulfide deposits. Subsequent submersible 

observations of the sediment hills showed that they were 

(/) 

a: 

w 

1-

w 

~ 

~ 

I 

1-

(L 

w 

0 3,570 

METERS 

I>~ ~ r--1 Basaltic laccolith 

METERS 

Figure 4.1. Perspective view of typical sediment hill in SESCA, 

illustrating geometry and scale of these hills as well as location of 

hydrothermal deposits to the inferred structure. Arrows indicate 

relative direction of movement along fault. 

formed by uplift, and that no basalt is exposed along the 

steep flanks (Holmes and Zierenberg, 1990). Massive sulfide deposits were observed near the base of the hills, but 

do not occur on the tops of the sediment hills. This information, along with in situ measurements of permeability 

of basalt and sediment (Touloukian and others, 1981) and 

the results of chemical studies of the sulfide minerals 

(Koski and others, 1988), is used to constrain geophysical 

models of sill emplacement and cooling by hydrothermal 

circulation. 

This chapter develops and evaluates models for the 

structure and geothermal circulation associated with sediment hills in Escanaba Trough. Using geologic and geophysical information on this area as a guide, we employ 

the results of theoretical and experimental studies of fluid 

convection in porous media to eliminate several possible 

models for the sediment hills. Then we develop a simple 

mechanical and thermal model of an idealized sediment 

hill. Our mechanical model is a model for the growth of 

each hill and identifies permeable pathways for fluid flow, 

whereas our thermal model provides an estimate of the 

basic time scales in the hydrothermal flow producing the 

sulfide deposits on the flanks of each sediment hill. These 

results may be compared to cooling estimates for sills in 

the Guaymas Basin (Einsele and others, 1980; Lonsdale 

and Lawver, 1980; Becker and Fisher, 1991) for a similar 

boundary-value problem. More refined studies are possible 

with numerical methods (for example, Norton and Knight, 

1977) but are not warranted until more information is available on the physical properties of the sediment and the heat 

sources. The structure we predict may be tested by drilling, and estimates of the lifetime of each geothermal system may be refined using measurements of the permeability 

of the sediment. 

ACKNOWLEDGMENTS 

Reviews by George Bergantz, John Sass, Jan Morton, and 

Rob Zierenberg tightened and helped focus our arguments. 

GEOLOGIC AND GEOPHYSICAL 

CONSTRAINTS 

The surface morphology observed in Sea Beam studies (Morton and Fox, chap. 2, this volume) combined with 

the submersible observations (Holmes and Zierenberg, 

1990; Zierenberg and others, chap. 10, this volume) show 

that the sediment hills are uplifted sections of the sediment 

fill that predate the surface volcanic activity (Morton and 

Fox, chap. 2, this volume). Sulfides are absent from the 

gently rounded tops of these hills (Zierenberg and others, 

chap. 10, this volume) but are common on the flanks and 

in a few places on the flat valley floor. These observations 
A THERMAL AND MECHANICAL MODEL FOR SEDIMENT HILLS AND ASSOCIATED SULFIDE DEPOSITS 67 

motivated the development of a model for uplift of the sediment section that produces structural conduits for upward 

flow of hydrothermal fluids during the uplift process. 

These structural channels may intersect stratigraphic zones 

of higher permeability (Morton and Fox, chap. 2, this volume), further complicating the subsurface distribution of 

sulfides. 

Submersible observations also provide temperature 

measurements from active hydrothermal vents in NESCA 

and qualitative assessments of deposit age in both NESCA 

and SESCA. The most eroded and apparently oldest deposits are in SESCA and are confined to the flanks of the 

sediment hills (Holmes and Zierenberg, 1990; Zierenberg 

and others, chap. 10, this volume). The deposits at NESCA 

appear fresher and are also confined to the sediment hill 

flanks; active venting was found on the northwest and 

southeast flank of the Central Hill. Temperatures of vent 

fluid at this location range up to 220°C (Campbell and others, chap. 11, this volume). Assuming a fluid velocity of 

8x1o-6 m/s (appropriate for our fault zone model), diffuse 

flow produces a surface heat flow of 7,000 W/m2. This is 

much larger than the conductive heat flow of 2 to 3 W/m2 

measured near this location by Abbott and others (1986) 

and Davis and Becker (chap. 3, this volume). 

In situ measurements of the intrinsic permeability of 

fractured subaerial basalt and of various sediments (table 

4.1) show that the permeability of basalt is typically greater 

by three to five orders of magnitude. If comparable differences are realized in Escanaba Trough, then basalt is the 

preferred conduit for circulation where it is surrounded by 

sediment, particularly if it is the heat source driving the 

circulation. 

Two important constraints on our models are that 

hydrothermal fluid must circulate through a large volume 

of sediments to produce the hydrothermal fluid chemistry 

(Campbell and others, chap. 11, this volume) and the sulfide deposit chemistry (Koski and others, 1988; Koski and 

others, chap. 16, this volume), and that sulfide deposits 

only occur on the flanks of the hills. Extensive circulation of hydrothermal fluids through sediments is possible 

above an intrusion through the development of Bernard 

convection cells (Hsu and others, 1978), but this type of 

convection would concentrate surface sulfide deposits near 

the top of each hill. For these reasons we constructed a 

model in which there is strong structural control of permeability. In this model fluid circulates through a thick section of sediments along the same fault structure that formed 

with the uplift of each sediment hill (fig. 4.2). A likely 

cause for the uplift is the inflation of a basaltic sill intruded 

along the basalt-sediment interface several hundred meters 

or more below the sea floor. 

The possible mechanisms for the creation of the sediment hills can be examined with laboratory and theoretical studies of sediment deformation as well as 

hydrothermal circulation. These studies are used here to 

Table 4.1. Physical properties of sediment and basalt and parameters 

of hydrothermal model. 

Property 

Permeabilitya (m2> 

Oahu basalt-----------------------------------------

Snake River Plain basalt -------------------------

Altered pyroclastics, Wairakei, NZ ------------

Fractured graywacke -----------------------------

Fractured shale ------------------------------------

Density (kg/m3) 

Basalt -----------------------------------------------

Escanaba Trough sedimentb --------------------------

Thermal conductivity (W/m°C) 

Basalt -----------------------------------------------

Escanaba Trough sedimentc --------------------------

Thermal diffusivity (m2/s) 

Basalt -----------------------------------------------

Escanaba Trough sediment ----------------------

Specific heat at constant pressure (J/g·°C) 

Basalt -----------------------------------------------

Escanaba Trough sedimentd --------------------------

Seawater--------------------------------------------

Initial temperature of laccolith (°C) --------------

Latent heat of basalte (J/kg) ------------------------

Dynamic viscosity of seawaterf (g/m·s) ----------

aField tests; average values. 

b Assumed saturated bulk density. 

c Abbott and others, 1986. 

dEstimated from siltstone. 

e Approximate combined heat of solidification. 

fEstimated from pure water at 315°C and 137 bars. 

Value 

1o-1o 

w-w 

10-16 

4.5x10- 14 

3.5x10-13 

3,000 

1,600 

1.25 

0.88 

0.5x1o-6 

0.5x1o-6 

1.1 

1.2 

4.5 

1,200 

4.18x105 

8.7x10-2 

define a basic geologic and hydrologic model for the sediment hills in Escanaba Trough. We assume that the deformation and structure is the governing influence on 

permeability and flow, and so structure is discussed first. 

~ 

. 

. 

EXPLANATION 

Sediment Mineralized 

fault zone 

Basalt 1 ~ 1 Sulfide Mounds 

Figure 4.2. Simplified geologic model for sediment hills in NESCA 

and SESCA, showing circulation patterns (arrows). 
68 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

A MECHANICAL MODEL FOR GROWTH 

OF THE SEDIMENT HILLS 

The data and analyses described above require that 

near-surface upflow occurs along the steep flanks of the 

sediment hills. The simplest model that explains this patA 

tern isolates fluid flow along faults that result from the 

deformation creating the sediment hill. Such a model is 

supported by laboratory and theoretical studies. 

Laboratory sandbox experiments of vertical uplift of 

a block into a thick layer of sediments (fig. 4.3A; Mandl, 

1988) show that curved thrust faults form as precursors to 

Configuration of sandbox 

t 

B 

8 

CJ 

f2ZZ] 

D 

20% 

10% 

5% 

2.5% 

--2.5% 

D -5% 

Figure 4.3. A, Sandbox model (Walters and Thomas, 1982) for sediment hills in NESCA and SESCA 

where uplift is presumed to be due to inflation of a sill. B, Stereophotogrammetric determination of 

change in porosity along faults formed during uplift of model inA (Mandl, 198~, p. 74). 
A THERMAL AND MECHANICAL MODEL FOR SEDIMENT HILLS AND ASSOCIATED SULFIDE DEPOSITS 69 

the final, nearly vertical fault that accommodates uplift. 

These thrust faults start from the base and grow in imbricate fashion, each successive fault steeper than its predecessor until the final vertical fault is formed. 

Shear-induced dilation of the sediment produces highporosity (and presumably high-permeability) channels 

along the fault zones, as shown in figure 4.3B (Mandl, 

1988). 

Theoretical studies of this problem for plane strain 

conditions require incremental solutions of the highly nonlinear equations governing sediment behavior during inelastic deformation. This was first done by Walters and 

Thomas (1982) for an idealized conical yield surface and 

plane strain conditions. Extensions of this work to the 

Coulomb surface governing the behavior and stress-strain 

laws for sand have been made by Denlinger (unpublished 

work). These studies show that dilatancy of the sediment 

under shear is required for structural high-porosity channels to develop during uplift and that the pattern of deformation in figure 4.3 (Mandl, 1988) also applies to 

axisymmetric uplifts. Furthermore, the first channels to 

develop, those likely to carry the hottest effluent, curve 

away from the uplifting block of sediment and end partway into the adjacent sedimentary section. These precursor faults are the most likely candidates to pipe 

hydrothermal fluids into stratigraphic layers with high 

permeability. 

As the thrust faults approach the surface, the upward 

doming and spreading of the surface material is accommodated by internal dilation of the sediment hill. The subsequent development of normal faults along the flanks 

modifies the shape of the sediment uplifted above the sea 

floor. 

A MODEL FOR HYDROTHERMAL 

CIRCULATION 

The faults that give shape to the uplifted sediment hill 

may be more permeable pathways for hydrothermal fluids 

than the surrounding sediments, thereby providing a simple 

mechanism for the production of sediment hills with sulfide deposits on their flanks. 

Without such a permeable channel, the theoretical circulation pattern in homogeneous sediment should consist 

of convection cells within the sediment and centered over 

the heat source. This conclusion is inferred from theoretical studies of the cooling of intrusions (Norton and Knight, 

1977) and from theoretical and experimental studies of the 

convection in porous media over a heated plate (Hsu and 

others, 1978). These studies support the interpretation that 

upflow of the highest temperature fluid in an unfaulted, 

uniform layer of sediment would be centered over the cooling intrusion. Presumably such a circulation pattern would 

produce sulfide deposits near the top of the sediment hills. 

We assume here that the creation of fault zones flanking the uplift creates permeable pathways for convection 

that inhibits the development of a high-temperature plume 

centered over the intrusion. Instead, high-temperature flow 

is preferentially channeled along these flanking fault zones 

to produce sulfide deposits on the flanks of the uplifted 

sediment hills (fig. 4.4 ). 

After initial intrusion, the sill inflates to form a laccolith that is 75 to 100m thick. This is the magnitude of 

uplift required at the base of a 500-m-thick layer of sediment in order to produce a 75-m-high sediment hill on the 

sea floor, depending upon cohesion in the sediment. Dewatering of the sediment is not considered here. The water 

depth is only relevant to the amount of work done in 

inflating the sill and does not affect the mechanics of sediment deformation. 

We infer from the lack of continuity of the surface 

basalt flows that a similar distribution of basalt flows is 

interlayered in the sediment cover and prevents these flows 

from serving as primary pathways for hydrothermal cooling. Instead, we postulate that fluids circulate through a 

fault system created by bending and uplift of the sediment 

cover, and that these fluids convectively cool the basalt laccolith that caused the uplift. 

The magnitude of fluid circulation depends upon the 

mechanisms controlling the cooling of the basalt. Based 

on theoretical studies of magma-water interaction (Delaney, 

1982; Hardee, 1980), experimental studies of seawater 

(Bischoff and Rosenbauer, 1985), and field studies of 

magma intrusion into soft, water-saturated sediment 

(Kokelaar, 1982), we assume that there are three stages of 

cooling. These are (fig. 4.5): (1) an initial stage of porewater pressurization due to rapid heating; (2) a secondary 

stage during which (a) high-temperature phase separation 

of the seawater occurs at the boundaries of the basalt intrusion, (b) thermal cracking allows penetration of twophase fluid (steam and water) until solidification is 

Hot fluid 

---~~ K=16 15m

2 

'\ \\ SEDIMENTS ~~~ 

L

L \\\\ ~ ~ ~ II Kfault zone 

t/ 3 10- 13 m2 

~~-----8-A_S_A_L_T--~\.nr: 

ry --~-,,-----~-

Figure 4.4. Idealized cooling model used to obtain rate of cooling 

of sediment-covered basalt laccolith: conductive cooling occurs from 

all sides, and seawater heated by laccolith flows up fault zone that is 

one to two orders of magnitude more permeable than surrounding 

sediments. Abbreviations: L, length of fault zone; D, width of fault 

zone; K, permeability. 
70 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

Stage 1 

· ·.·· · ·.· .··.·.: .. .-::. · Pore water . .:: .. :·.· .:: .. ·:.·:·::"-:·.: ............. :·:·:pressurized by heating 

~·:.;,·~~~·~·,.·~:~~··~~~Steam+ brine 

~-- 'L .,> 'L .,> 'L .,> 'L .,> Basaltic 

"1\."'.l L-7 ",.."J. 1...,. ",.."'.l '-.,. ",..".l L...,. ",.' magma 

Stage 2 ..... · . . . . . . . .. ~- ... 

.::::.::.·.:· :·.:·:.:: ·.· :.:,... .. Sediment 

Seawater. y···~· .. ~LL'Y.o 't Basalt 

Two-phase - 1 1 \ 1 1 

boundary Steam+ brine, 

(424°C at intense convection, 

350 bars) and 

cracking of rock 

Stage 3 

Figure 4.5. Three stages of cooling for a sediment-covered basaltic 

laccolith (discussed in text). Most cooling occurs as a two-phase 

mixture of seawater invades cracks in cooling laccolith. Heat loss 

is controlled by mechanisms of heat transport away from intrusion. 

complete, and (c) fluid convection decreases the average 

rock temperature to the temperature at the two-phase 

boundary of seawater; and (3) a tertiary stage during which 

seawater convection through the hot, fractured basalt cools 

it to ambient temperatures. The heat loss from the basalt 

is ignored for the first stage, which is short lived (Delaney, 

1982), and is estimated for the next two stages. 

THE SECOND STAGE OF COOLING 

The actual mechanisms of cracking and water penetration during the second stage of cooling are complicated by 

the thermodynamic properties of seawater at 350 bars, the 

pressure at the assumed depth of the base of the sediment 

section at 3,500 m water depth. An equation of state for 

seawater is presented by Bischoff and Rosenbauer (1985), 

but a quantitative model for hot basalt-seawater interaction 

does not exist due to its thermodynamic and mechanical 

complexity. 

For the purposes of this report, we simply assume that 

a surface separating single-phase and two-phase fluid encases the basalt and is held at 424°C, the temperature of 

two-phase separation at 350 bars (Bischoff and Rosenbauer, 

1985). We assume here that the heat flux across this interface is controlled by the conductive and radiative processes that transport heat from within the basalt to this 

surface, with convection as the primary process outside the 

surface. This assumption is supported by studies of heat 

loss from basaltic lava lakes (Hardee, 1980) and active lava 

domes (Dzurisin and others, 1990). The position and 

movement of this surface depends upon the balance of this 

flux with that due to processes on the hot basalt side, as 

shown in figure 4.5, and a detailed analysis of these processes is beyond the scope of this paper. 

The heat-loss calculations assume that the basalt is 

wrapped in an envelope defined by a surface at constant 

temperature, the two-phase boundary of seawater. Hydrothermal circulation may develop above the platelike intrusion as well as along the faults flanking the uplift. 

Eventually enough heat is lost from the basalt to lower its 

temperature to the point where seawater can circulate 

through the basalt without phase separation. 

The presence of the two-phase boundary layer prevents 

convective cooling of the sill during stage 2, so that all of 

the heat loss is by conduction and radiation. Radiative 

losses are neglected below a basalt temperature of 900°C. 

Conductive heat losses during the second stage of cooling 

are calculated assuming a constant temperature boundary 

at the two-phase separation temperature of 424°C. The 

conductive component of the basalt surface heat flux decreases as the square root of time (Cars law and Jaeger, 

1959) according to 

where 

q is the heat flux per unit area, 

T5 is the surface temperature, 

k is the sediment thermal conductivity, 

a is the sediment thermal diffusivity, and 

tis time. 

(1) 

The initial heat flux is calculated assuming that the 

basalt laccolith is emplaced instantaneously, and a 2-mmthick layer of steam plus brine develops in 1 second over 

the outer surface as it contacts sediment saturated with seawater. This assumed envelope thickness and timing for 

development is comparable to the envelope observed to 

develop on cooling pillow basalts (Moore, 1975). The enthalpy change for high-pressure boiling of a synthetic 3 

percent NaCl water solution (Dave Butterfield, written 

commun., 1989) was used to estimate the enthalpy change 

for seawater as it forms a high-density brine film. The 

variation in depth over a 70-m-thick sill would produce a 

temperature difference for the phase transition of 4 °C and 

is neglected here. The volume of seawater, timing, surface area of the sill, and the enthalpy change give the initial estimate for heat flux shown in table 4.2 for a constant 

temperature interface. The conductive heat flux then decreases according to equation 1 as shown in table 4.2 and 

figure 4.6, becoming less than the convective flux in about 

5 years with the assumptions of fault-zone thickness and 

permeability used in the model. 
A THERMAL AND MECHANICAL MODEL FOR SEDIMENT HILLS AND ASSOCIATED SULFIDE DEPOSITS 71 

Table 4.2. Dimensions, energy loss, and rates of energy loss for 

model laccolith. 

Dimensions of typical disc-shaped laccolith 1: 

radius = 600 m, thickness = 75 m 

Model fault zone controlling cooling: 

2m thick; permeability 3xlo-13 m2 

Stages of cooling: 

First stage, groundwater pressurization; energy loss, 0 J; duration, seconds to minutes. 

Second stage, two-phase separation of seawater with enhanced convection and thermal cracking of basalt; energy loss, 0.8xl017J; duration, 44 years; 34 million cubic meters of seawater circulated. 

Third stage, single-phase convection within fractured laccolith; energy 

loss, 0.3xl017J; duration, 200 years; 15 million cubic meters of seawater circulated. 

Cooling history: 

Time since 

intrusion 

1 second -----------------------

5 days --------------------------

1 month ------------------------

6 months -----------------------

1 year---------------------------

2 years -------------------------

5 years -------------------------

8 years -------------------------

50 years ------------------------

60 years ------------------------

70 years ------------------------

90 years ------------------------

110 years ----------------------

130 years ----------------------

150 years ----------------------

250 years ----------------------

300 years ----------------------

Conductive heat 

flux (106W) 

380,000 

578 

236 

96 

68 

48 

30 

24 

10 

9 

8 

7 

6.5 

5.9 

5.5 

4.2 

4 

Convective fault 

heat flux (l 06W) 

0 

<33 

<33 

33 

33 

33 

33 

33 

33 

19 

12.5 

7.3 

5.0 

3.6 

2.8 

1.3 

1 Determined from mechanical modeling of sediment hill structures in 

Escanaba Trough. 

The convective heat flux due to fluid circulation has 

two components, as shown in figure 4.4. One component 

is due to fluid flow developed as hot fluid rises over the 

hot upper surface of the intrusion and draws cooler fluid 

in behind it. This problem has been analyzed by Hsu and 

others (1978). Using Hsu's model, the maximum thickness of the thermal boundary layer that develops due to 

penetrating flow is about 270m with the parameters given 

in table 4.1. If these are representative of the conditions 

in Escanaba Trough, then a convection cell of hydrothermal fluid developed over a sill along the basalt-sediment 

interface will not penetrate the 400 to 600 m of sediment 

cover to reach the sea floor. 

Development of such a convection cell of hot water 

would be subdued or prevented by fluid circulation up more 

permeable fault zones flanking the intrusion as shown in figure 4.4. In such a case, the rise of hydrothermal fluids along 

the fault zones would occur more easily than through undisturbed sediments, and would rob any centrally located hot 

plume of the fluid it needs to rise to the surface. 

Fluid flow up the fault zones is driven by a pressure 

gradient that is a result of the average density difference 

between fluid in the fault zone and cold seawater. This 

density difference can be calculated from the temperature 

difference between hot and cold seawater and the equation 

of state of seawater (Bischoff and Rosenbauer, 1985). 

Flow is calculated assuming Darcy's law, 

(2) 

where 

v is velocity, 

K is permeability, 

~ is the dynamic viscosity, and 

.1.pwg is the pressure gradient driving the flow. 

The viscosity is expected to vary exponentially with 

temperature, but this effect is neglected here. Qualitatively, 

the inclusion of the viscosity change would enhance the 

driving force for convection up the faults. If the permeability is at least an order of magnitude greater than the 

surrounding sediments, then fluid flow up the fault zone 

is a very efficient means of heat loss (table 4.2). Coupling 

of the flow and temperature fields occurs since heat losses 

from the fault zone during flow affect the temperature differences, and hence the density differences driving flow. 

We attacked this problem by first assuming an average temperature for the conduit fluid, using its density at 

this temperature to calculate a velocity (v) from equation 

100 

I 

~ 50 I 

x I ~ 1./ Conductive cooling 

!;;: r w 

I 

0 

Convective cooling 

due to flow up fault zones 

100 

YEARS SINCE INTRUSION 

200 

Figure 4.6. Estimated conductive heat flux from intrusion and 

convective flux due to flow along fault zone. Curves derived from 

idealized geometry in figure 4.4 as described in text. Model has 

one-fourth the volume of a typical intrusion, so flux expected from 

typical Escanaba Trough intrusion is about twice these values, and 

total heat loss about four times greater. 
72 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

2. From the velocity we calculated the transit time (t0 ) 

for a 300-m-long fault zone and the duration of flow using 

the heat-balance equation 

(3) 

where 

M is the total stored energy that must be removed to 

drop the temperature of the laccolith to 424°C 

Cw is specific heat of fluid, 

T w is average temperature of fluid, and 

v(2nRD) is the flux from the side of the intrusion with 

flank area in parentheses (R is the radius and D is 

the height). 

Here duration of flow for stage two is assumed equal 

to the time required to decrease the temperature of this 

body from 1,200°C to 424°C (when two-phase separation 

ceases). The energy loss required to do this is given by 

where 

L is the latent heat of solidification estimated for 

basalt, 

cr is the specific heat of basalt at constant pressure, 

Pr is the basalt density, and 

vr is the volume of basalt (see table 4.1). 

The average temperature (f) of the fluid in the conduit may be written, assuming a conductive heat loss from 

the sides, as 

where the heat flux q is 

kT 

~nat0 

(5) 

(6) 

The parameters are defined in table 4.1. Equation 5 

gives a relationship between the transit time tr, the duration of flow t

0 (through q in equation 6), and the average 

fluid temperature T in terms of the initial fluid temperature. From the duration of flow and transit time, an average temperature was determined and the procedure above 

redone until the process converged on a solution. 

The solution we obtain for a 2-m-wide fault zone that 

is 260 m long (estimated from the mechanical models in 

figure 4.3; Mandl, 1988) and has a permeability of 3x1o-13 

m2, with an initial temperature of 424°C, is an average temperature of 330°C and a surface temperature of 239°C. 

This compares well with measured vent temperatures at 

NESCA of 220°C and temperatures estimated for the copper-rich sulfide assemblages in excess of 300°C to 330°C. 

The model gives an average flow velocity of 7.5x10-6 m/s. 

For these conditions, fluid convection up the fault zone 

takes a few days to develop (table 4.2), and it takes about 

400 days for a given parcel of fluid to travel from the laccolith to the sea floor. The total convective heat loss from 

the sill is expected to be approximately constant because 

of two-phase separation of seawater within the basalt, and 

with these conditions our second stage lasts about 45 years. 

The amount of water circulated during stage two may 

be calculated either from the duration of flow or from an 

energy balance for the intrusion. Correcting for the conductive heat loss, the energy balance gives 34 million cubic 

meters of seawater. The duration of flow approach overestimates this volume because steady flow was assumed to 

start immediately, and gives 39 million cubic meters of seawater. The energy balance is more accurate and closer to 

the true value. 

THE THIRD STAGE OF COOLING 

We define the third stage of cooling as the stage when 

sill solidification is complete and seawater can circulate 

through the basalt without a phase separation. Stage three 

heat loss is calculated assuming that the basalt is much 

more permeable than the surrounding sediment or sediment 

fault zones. The three- to four-order-of-magnitude contrast 

in permeability between basalt and relatively impermeable 

sediment simplifies the calculation of heat loss. The entire laccolith is assumed to be at a temperature below 

424 °C and is cooled by circulation of seawater through 

cracks in the sill developed in stage two. 

To calculate the rate and amount of cooling, we assume 

that heat is distributed much more efficiently by flow 

within fractures in the laccolith than it can be carried away 

by flow up the fault zone. This assumption is supported 

by measurements that show that (in situ) basalt is typically 

orders of magnitude more permeable than sediments 

(Touloukian and others, 1981; see table 4.1). With this condition, 

(7) 

where the left-hand side is the change in the internal energy of the basalt and the right-hand side is the heat flux 

up the fault zone. Since by Darcy's law 

K K 

Vw = j::t g (Pooc- Pr) =j:! Llpwg (8) 

then the change in temperature of the basalt may be written 

_1_ dT = Cw _1_ 2nRD Kg dt (9) 

p.Llp T Cr Pr Vr j..l 

where PwllPw is written as p.Llp. 

An expression is needed for p.LlpT as a function of temperature. Values for density as a function of temperature 

at a mean pressure of 330 bars were obtained from Bischoff 

and Rosenbauer (1985), and p.LlpT was calculated and 
A THERMAL AND MECHANICAL MODEL FOR SEDIMENT HILLS AND ASSOCIATED SULFIDE DEPOSITS 73 

plotted versus temperature. A function was fit to the resulting curve to obtain 

(10) 

Using this expression in equation 9 and integrating, 

T= 693K 

(tit+ 1 )

0·38 (11) 

where 

D (12) 1 meter 

The rate of heat loss is 

D 

dQ = C (2nRD)_K_g_ = 

16"5 1 meter megawatts (13) 

dt w j..l ( t ~ 1.38 

'f" + 1) 

per meter of fault zone width. 

The parameters in table 4.1 were used in equations 11 

and 12 above to give the asymptotically decaying portion 

of the convection curve in figure 4.6 for a fault zone 2m 

wide. The uncertainty in both the width of the fault zone 

and the permeability of the fault zone is large, and both 

have a direct effect on these lifetime estimates. For example, a fault zone 1 m wide with a permeability of 10-14 

m2 would sustain a hydrothermal system 20 times longer 

than is estimated here, extending the lifetime from 290 

years to 5,800 years. 

The conductive heat loss continues to decline with the 

inverse square root of time since intrusion, and after a few 

decades convection of hot fluid up the fault zones is the 

primary source of heat loss and controls the rate of cooling. 

The flow of hot fluids up the fault zones could produce sulfide deposits around the steep flanks of the sediment hills, with some leakage also probable along the 

stepped normal faults that form the break in slope. Each 

parcel of fluid would be in contact with the sediment for 

at least a year in transit, at an average temperature of 

330°C, whereas the same fluid would be in contact with 

basalt less than one-fourth this time. 

The total amount of fluid required to cool our model 

laccolith (600 m across and 75 m thick) from 1,200°C to 

424°C may be calculated from either the flow/duration or 

an energy balance. The energy-balance method is more 

accurate and gives 1.1x108 m3 as the volume of seawater 

circulated during our second stage cooling. 

The mechanical models show that the surface expressions of sediment hills are a rough indicator of intrusion 

diameter. Therefore many intrusions in NESCA and 

SESCA are as much as 1,200 m across and 75 m thick, and 

would produce four times the volume of hydrothermal 

fluids during the second stage of cooling. 

The actual distribution of sub-seafloor sulfides would 

depend upon a precipitation mechanism such as cooling, 

mixing with seawater, or pH titration by wall rock alteration. It is expected that much of the deposition would be 

associated with the higher permeability fault structure produced by uplift. 

CONCLUSIONS 

A combination of geologic and geophysical observations constrain models for the genesis and structure of the 

sediment hills in Escanaba Trough, an active oceanic rift 

covered by 400 to 600 m of sediment. Mechanical, chemical, and fluid circulation analyses support the interpretation that the sediment hills are the surface expression of 

basalt laccoliths with about the same dimensions (diameters 

of 1 km and thicknesses of 70-100 m). The bending of 

the sediment overburden as uplift occurs initially generates 

a set of imbricate thrust faults that curve outward and end 

in the sedimentary section. These faults carry the hottest 

effluent and may carry these hydrothermal fluids to permeable stratigraphic horizons within the sedimentary section. With continued uplift these thrust faults are 

superseded by younger, more steeply dipping faults until 

a final, vertical fault forms that accommodates all later 

deformation. The shape of the sediment lifted above the 

seafloor is modified by normal faulting. 

According to our model, both normal and thrust fault 

structures not only localize the sediment deformation but 

also guide the upward flow of hot seawater through the 

sediment to the sea floor. This upward flow, combined 

with fracturing of the laccoliths during cooling, accelerates 

the rate of heat loss for most of the cooling history of the 

laccoliths; for the permeable fault zone in our model, hot 

fluid flow drains half of the heat energy in the system. The 

typical lifetime of hydrothermal systems surrounding the 

sediment hills in Escanaba Trough varies with the permeability within the fault zones created by sediment uplift. 

For a 2-m-wide fault zone with a permeability of 10-13 m2, 

the lifetime of hydrothermal circulation associated with a 

typical sediment hill is expected to be about 1,000 years 

(four times the 250-year estimate for our model, which is 

one-fourth the size of a typical intrusion in Escanaba 

Trough). 

The uncertainty in both the width and the permeability of the fault zones is large, and both have a direct effect 

on these time estimates. For example, if mineral precipitation reduced the effective fault zone width to 1 m, and 

reduced its permeability to 10-14 m2, then the rate of heat 

flow to the sea floor would be reduced and spread out over 

a period that is 20 times longer than is estimated here. This 

would extend the time required to cool an average laccolith from 1,000 years to 23,200 years. 

For fault permeability values in the neighborhood of 

lo-13 m2 and fault-zone thicknesses of 2m, about half of 

the heat loss occurs due to conductive heat flow from the 
74 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

surface of the intrusion and about half occurs due to convective fluid flow up the fault zones flanking the intrusion. 

With these parameters, about 34 million cubic meters of 

seawater is circulated to cool our 24-million-cubic-meter 

intrusion during the second stage of cooling, and about 15 

million cubic meters is circulated during the final stage of 

cooling. Since our model is one-fourth the size of a typical intrusion in Escanaba Trough, these numbers should be · 

multiplied by 4 for comparison with field studies. If the 

permeability-thickness product of the fault zones is much 

less than this, more energy is lost by conduction, more time 

is required to cool the intrusion, and less water is circulated during the cooling process. 

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Chapter 5. Crustal Magnetization and Basement Structure of 

Escanaba Trough 

By Robert E. Karlin 1 and Janet L. Morton 2 

CONTENTS 

Abstract ............................................................................... . 

Introduction ....................................................... · · ...... · · · · · · · · · · · 

Sedimentation and suppression of magnetic 

anomalies ................................................................ . 

Plate tectonic setting ................................................... . 

Axial volcanism and hydrothermal activity .............. . 

Acknowledgments ...................................................... . 

Methods ............................................................................... . 

Data presentation ................................................................ . 

Basement topography and structure ........................... . 

Magnetic anomalies .................................................... . 

Magnetic modeling results ................................................. . 

Inverse magnetic modeling ........................................ . 

Forward magnetic modeling ...................................... . 

Crustal magnetization and sedimentation .......................... . 

Tectonics in Escanaba Trough ........................................... .. 

Conclusions ......................................................................... . 

References cited .................................................................. . 

ABSTRACT 

Page 

77 

77 

78 

78 

79 

80 

80 

80 

80 

82 

84 

84 

85 

86 

87 

88 

88 

Sediment isopach and basement topography maps are 

derived from single-channel seismic-reflection lines run in 

1985 during the R!V S.P. Lee cruise L6-85-NC. Crustal 

magnetization and the geometry of magnetized crustal bodies are estimated using inverse three-dimensional and forward two-dimensional modeling of magnetic data taken on 

the cruise. Our results indicate that tectonic extension has 

dominated over volcanic construction in Escanaba Trough 

during the Brunhes Normal-Polarity Chron (that is, since 

730 ka). In the southernmost part of Escanaba Trough, 

very little volcanism has occurred. Mantle diapirs, as evidenced by igneous intrusions and uplifted sediments, have 

arisen along the rift valley at discrete locations. In general, magnetic anomaly amplitudes are suppressed where 

sediment cover is present, probably because of crustal 

I Mackay School of Mines, University of Nevada-Reno, Reno, 

Nevada. 2 U.S. Geological Survey, Menlo Park, California. 

alteration induced by closed-system hydrothermal activity. 

However, the volcanic edifices do show a magnetic signature, suggesting that alteration is not pervasive. The southern Escanaba Trough contains deep basins in the rift valley, 

suggesting the absence of recent volcanism. Areal changes 

in magnetic anomaly and basement topography patterns 

suggest that northwest-southeast-striking transform faults 

create discrete tilted blocks. The faulting is at a 30° angle 

to the active spreading direction, perhaps indicating that 

the deformation is caused by readjustment of the ridge to 

be perpendicular to the Mendocino Fracture Zone. 

INTRODUCTION 

Gorda Ridge is a slow-spreading mid-oceanic ridge 

located within the U.S. Exclusive Economic Zone off 

northern California and Oregon. The ridge is unusual in 

the world's rifting system, because the southern part, called 

Escanaba Trough, is covered by a thick sequence of sediments derived from glacial weathering of the American 

continent during the Pleistocene. The thermal regime and 

crustal formation processes at such a sediment-covered 

spreading center are significantly different from those 

found on normal open, unsedimented ridges. The sediments form a semipermeable, wet thermal blanket that traps 

heat associated with the creation of new oceanic crust along 

the rift zone. The sediment cover restricts hydrothermal 

circulation, leading to high upper crustal temperatures, and 

possibly pervasive crustal alteration and sediment-hosted 

mineralization. 

Hydrothermal activity associated with ridge-crest volcanism can result in baking and alteration of the host sediments and precipitation of metals leached from basaltic 

intrusions (Zierenberg and others, chap. 10, this volume). 

These processes may give rise to massive stratiform 

polymetallic ore deposits in the axial rift zone. Today, 

sedimented spreading centers are relatively uncommon 

except in such areas as Middle Valley (Juan de Fuca Ridge; 

Davis and others, 1987), Dellwood Knolls and Winona 

Basin (Explorer Ridge), Guaymas Basin (Gulf of California; Lonsdale and others, 1980), and the Eastern Chile 

Ridge as well as certain back-arc basins in the western 

77 
78 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

Pacific Ocean. However, heavily sedimented ridges may 

have been much more prevalent in the past, such as in the 

early rifting of the Atlantic Ocean during the Jurassic. An 

understanding of the relations between sedimentation, volcanism, neotectonic, and hydrothermal activity in areas 

such as the Escanaba Trough could provide valuable insight~ into hydrothermal ore formation processes that could 

be applied to ancient massive ore deposits. 

In this chapter, we examine the magnetic and basement 

structure of the Escanaba Trough region. The main goals 

of this work are to evaluate the possible causes of the suppressed magnetic anomalies observed in the heavily 

sedimented parts of Escanaba Trough and to better delineate the tectonic and volcanic framework and history of the 

area during the Brunhes Normal-Polarity Chron. Threedimensional inversions and two-dimensional forward modeling are used to evaluate the character of crustal 

magnetization and the geometry of sea-floor spreading and 

volcanism throughout the area. We also attempt to assess 

what thickness of sediment is required to suppress the magnetic anomaly amplitude and how the volcanic edifices are 

influenced by this effect. 

SEDIMENTATION AND SUPPRESSION OF 

MAGNETIC ANOMALIES 

Most of the world's spreading centers exhibit a highamplitude central magnetic anomaly that is centered in the 

middle of normally magnetized Brunhes-age crust 

(Klitgord, 1976; Tivey and Johnson, 1987, for review). The 

amplitude of the anomaly decreases with age due to progressive oxidation of titanomagnetites within the basaltic 

lavas (Irving, 1970; Marshall and Cox, 1972; Bleil and 

Petersen, 1983). 

In a deep-tow study of the Gorda Ridge axial valley 

from latitude 41°15' to 41°30' N., Atwater and Mudie 

(1973) found magnetic lows over sedimented areas. They 

attributed these features to slow cooling and consequently 

coarser grain size of magnetic minerals in the basaltic rocks 

intruded into the sediments. A second possible mechanism 

is that the rocks are thermally demagnetized under a sediment cap and that magnetization blocking would be 

delayed until the intrusions cooled. Levi and Riddihough 

(1986) argued that such thermal demagnetization is unlikely because the delay in remanence lock-in would result 

in displaced magnetic anomalies, which are not observed. 

Instead, they suggested that the upper oceanic crust was 

chemically altered by pervasive hydrothermal alteration, 

and thus the rocks are made permanently nonmagnetic. 

Another hypothesis is that the anomalies are suppressed 

simply because of the presence of an axial valley depression filled with a thick sediment cover. 

PLATE TECTONIC SETTING 

Although the Gorda/Juan de Fuca Ridge spreading system was the first to be used to develop the plate tectonics 

paradigm (Vine, 1966; Atwater and Mudie, 1968), the tectonic history of Gorda Ridge is complex and not fully 

understood. Gorda Ridge is divided into three sections, 

each with a different spreading history (Atwater and Mudie, 

1973; Riddihough, 1980; Wilson, 1986; Stoddard, 1987). 

The northern section presently has a relatively fast total 

opening rate of 5.8 cm/yr, whereas the southern and central ridge segments have total opening rates of 2.0 to 2.4 

cm/yr (Riddihough, 1980). From offsets in Pacific plate 

anomalies, Wilson (1986) and Stoddard (1987) suggested 

that since 5.3 Mathe northern segment has been propagating southward and the southern segment has been propagating northward, both at the expense of the central 

segment. Spreading rates apparently decreased at about 3 

Ma and possibly 2 Ma (Riddihough, 1980; Stoddard, 1987). 

The ridge may be spreading asymmetrically at present 

(Solano-Borrego, 1982); however, Masson and others 

(1988) argued that asymmetrical spreading cannot be differentiated from ridge jumps, based on then available magnetic and GLORIA side-scan sonar data. 

After 20 years of study, the mechanisms of deformation of the Gorda plate and the nature of its interaction with 

the Mendocino Fracture Zone remain controversial. The 

magnetic anomaly patterns on the Gorda plate are fanshaped, and the 3.8 to 4.8 Ma isochrons appear to have 

been shortened by almost 20 percent (Silver, 1971). Moreover, seismicity is broadly distributed throughout the region. Several models have been suggested to explain these 

features. Riddihough ( 1980) proposed simple rigid block 

rotation of the southern and central subplates. Silver 

(1971) favored left-lateral shear with strike-slip reactivation of isochron-parallel normal faults formed at the spreading center. In a modification of Knapp's (1982) model of 

left-lateral shear and rotation of the central Gorda Ridge 

segment, Stoddard (1987) suggested that the deformation 

occurred as elastic flexural-slip with distributed shear along 

major left-lateral faults. 

All of these models account for the fanned anomalies, 

but require oblique subduction of the Gorda plate under the 

Mendocino Fracture Zone to account for anomaly shortening. Wilson (1986, 1989) argued that strike-slip focal 

mechanisms along the Mendocino Fracture Zone and 

simple gravity modeling do not support oblique subduction. From magnetic anomaly analysis and lineation mapping of side-scan imagery, he suggested that the Gorda 

plate deformation was accommodated by distributed northwest-southeast right-lateral shear near the spreading centers that changed to northeast-southwest left-lateral faulting 

at about the 2 to 3 Ma isochron. The style of deformation 
CRUSTAL MAGNETIZATION AND BASEMENT STRUCTURE OF ESCANABA TROUGH 79 

in the zone of active rifting and its relations to the regional 

stress field have not been addressed to date, mainly because 

of the lack of closely spaced profiles along the spreading 

axis. 

AXIAL VOLCANISM AND HYDROTHERMAL 

ACTIVITY 

Several discrete volcanic centers in the sedimented part 

of the axial valley were identified during the seismic-reflection profiling of RN S.P. Lee cruise L6-85-NC in 1985 

(fig. 5.1; Morton and Fox, chap. 2, this volume). Anumber of these features had been crossed previously on a site 

survey for DSDP Site 35 (Moore, 1970; Moore and 

Sharman, 1970), but they were interpreted as one long continuous basement ridge due to the lack of adequately 

spaced seismic-reflection lines. The centers are commonly 

offset to the west of the axis of the valley. Although the 

exact nature of the volcanic-sedimentary contacts at the 

centers is difficult to decipher owing to numerous hyperbolic diffractions from sills, the volcanic edifices appear 

to consist of a combination of deep-seated plugs and thin, 

high-level sills (Davis and Becker, chap. 3, this volume; 

Morton and Fox, chap. 2, this volume). On GLORIA sidescan records, the volcanic edifices appear as almost circular, highly reflective zones within the less reflective axial 

valley sediments (EEZ-SCAN 84 Scientific Staff, 1986). 

Near some of the intrusions, the sediments have been uplifted by 50 to 100 m, forming elevated terraces and hills. 

Two of the five volcanic centers have been studied in 

more detail (Morton and others, chap. 1, this volume). Reconnaissance bottom camera and dredging work in 1985 

in the northern Escanaba Trough (NESCA) and southern 

Escanaba Trough (SESCA) sites (edifices D and B in fig. 

5.1) suggest that the volcanic centers are sites of recent and 

possibly active volcanic and hydrothermal activity (Morton 

and others, 1987a). Fresh pillow basalt outcrops are exposed on parts of the NESCA area, and fresh basaltic glass 

was dredged from both edifices. Unweathered hydrothermal sulfide samples associated with altered mudstone 

were dredged at the SESCA site. Heat-flow surveys also 

in 1985 near the two centers show values in excess of 1,200 

mW/m2 (Abbott and others, 1986), suggesting that both of 

these centers are still hydrothermally active. 

A detailed transponder-navigated bathymetric, camera, 

and seismic-reflection survey of the two sites in 1986 

showed the centers to be characterized by several small, 

uplifted sediment hills (Morton and others, 1987b; Morton 

and Fox, chap. 2, this volume). DSV Sea Cliff submersible observations (Holmes and Zierenberg, 1990) as well 

as bottom camera, dredging, and coring (Morton and others, 1990) reveal that most of the SESCA site is sediment 

covered, with only limited basaltic exposures. At the 

NESCA site, fresh basaltic flows were observed on the 

flanks of one hill, and large flat basins were observed that 

may be thinly sedimented lava lakes (Ross and Zierenberg, 

chap. 8, this volume). At both sites, large sediment-hosted 

sulfide deposits were found (Koski and others, 1988). The 

areal distribution of these deposits is not known, but constructional ledges, mounds, and chimneys of massive sulfide extend up to 200 m along steep scarps at the base of 

the sediment hills (Morton and others, 1990; Zierenberg 

41 ooo· 

40°30• L...L...-.....L..---'----1..-"---'-_..._..&..-.__._ _ __,__.:......;;;;;&._~ 

127oso• 

Figure 5.1. Bathymetric map of Escanaba Trough study area, 

compiled from available 12kHz records with additional data from 

Wilde and others (1978). Volcanic edifices in axial valley marked 

by patterns and labeled from A to H. Cruise tracks shown for L6-

85-NC (solid line), L12-81-NP (long dashed line), and F3-84-NC 

(short dashed line). Contours are at 200-m intervals and a sound 

velocity of 1500 m/s was assumed. 
80 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

and others, chap. 10, this volume). DSV Alvin submersible observations conducted in 1988 confirmed that active 

hydrothermal venting is occurring at the NESCA site 

(Zierenberg and others, 1988). 

ACKNOWLEDGMENTS 

We thank Jane Yu and Roger Burnley for help with 

data processing and reduction, Maurice Tivey for his 

assistance in the initial magnetic modeling, and Mark 

Holmes for his valiant efforts to obtain high-quality geophysical data during the L6-85-NC cruise. 

METHODS 

Bathymetric maps presented here were generated from 

12-kHz records obtained during U.S. Geological Survey 

cruises L12-81-NP, L6-85-NC, and F3-84-NC, as well as 

data filed at the NOAA National Geophysical Data Center. 

Reflection travel times to basement were digitized at 5-

minute (about 1 km) intervals from analog recordings of 

1,050 km of single-channel seismic-reflection lines run 

during cruise L6-85-NC using a 3.9 L (237 in.3) air-gun 

array. The velocity function for the deep-sea sediment column of Carlson and others ( 1986) was used to convert reflection travel times to sediment thickness in meters. These 

sediment velocity values are in good agreement with those 

derived from stacking velocities for multichannel seismicreflection records at Escanaba Trough (Morton and Fox, 

chap. 2, this volume; J.L. Morton, unpub. data, 1991). The 

minimum detection limit for sediment cover was about 25 

min smooth terrain. However, the initial pulse envelope 

and diffraction hyperbolas made it difficult to differentiate sediment-covered ridges from basaltic ridges; thus, 

sediment thicknesses should be considered a minimum 

estimate, especially in rough topography. Basement topography was estimated by subtracting the sediment cover 

from the bathymetry. 

Magnetic data on the three cruises were collected with 

a Geometries 801 proton precession magnetometer at 1-

minute intervals from a "fish" towed 350 m behind the 

ship. The observed values, corrected for towing distance, 

were merged with navigation obtained using an integrated 

satellite-Loran C-doppler sonar system. Magnetic anomalies were calculated by removing the appropriate International Geomagnetic Reference Field (IGRF) field, but 

diurnal variation corrections were not made. Crossings of 

different lines at the same location agreed to within 25 nT. 

To estimate crustal magnetization in the presence of 

topography, the magnetic anomaly data were inverted using 

the techniques of Macdonald and others (1980, 1983) and 

Miller ( 1977). To perform the inversion, grids of basement 

topography and magnetic anomalies were established at 

approximately 1 km spacing. Mirror images of the 50 by 

50 grids were added at the borders of the data set to minimize edge effects. A bandpass filter with 3.5- and 100-

km wavelengths was used to suppress high-frequency noise 

and long-wavelength trends. The inversion scheme, which 

is based on the methods developed by Parker and Huestis 

( 197 4 ), assumes that the crustal magnetization resides in 

a layer of constant thickness that follows the basement 

topography. The magnetization in each layer can vary 

areally and reverse in sign, but has a uniform direction. A 

best-fitting areal distribution of magnetization is calculated 

in an iterative manner to minimize the mean-square residual differences between the model and observed field 

data. Because of the inherent lack of uniqueness of potential field data, the solution is not unique. However, 

Parker and Huestis (1974) showed that the magnetizations 

are confined to a class of solutions that can be thought of 

as a basis solution combined with an annihilator function. 

The annihilator is the magnetization distribution that gives 

no net external magnetic field. The amount of annihilator 

added to the basis solution must be constrained by geologic 

intuition and, if possible, by magnetic intensity measurements on individual samples. 

DATA PRESENTATION 

BASEMENTTOPOGRAPHYANDSTRUCTURE 

Escanaba Trough, forming the southern part of Gorda 

Ridge, is bounded by the Mendocino Fracture Zone to the 

south and is separated from the central segment of Gorda 

Ridge by a small fracture zone at latitude 41 °40' N. The 

L6-85-NC single-channel seismic-reflection lines broadly 

define the structure of Escanaba Trough (Morton and others, 1987a; Morton and Fox, chap. 2, this volume). 

Seismic profiling (Morton and Fox, chap. 2, this volume) shows that sediment covers the axial valley south of 

about latitude 41° 17' N. and blankets the southernmost 80 

km of the rise axis. At 3,200 to 3,300 m depth, the axial 

valley is relatively flat and is flanked by a series of 

subparallel ridges 1,100 to 1,200 m high (fig 5.1). The valley is filled with several hundred meters of sediment in the 

south, but the sediment thins to the north where the axial 

valley shoals. Much of this sediment flowed off the continents as turbidites during Pleistocene low sea-level stands 

(Vallier and others, 1973), although the Holocene veneer 

is mainly hemipelagic (Karlin and Lyle, 1986). 

The reflection profiles show that the flanks of the axial 

valley are composed of uplifted and back-tilted crustal 

blocks, with the intervening valleys containing perched 

sediment ponds (Morton and Fox, chap. 2, this volume). 

At least some of the ridges appear to, have a sediment cap, 

although the fine-scale lithology and structural relations 

cannot be readily deciphered. 
CRUSTAL MAGNETIZATION AND BASEMENT STRUCTURE OF ESCANABA TROUGH 81 

The sediment fill in the axial valley is mainly composed of strongly reflecting, horizontally bedded strata (see 

Morton and Fox, chap. 2, this volume and Davis and 

Becker, chap. 3, this volume, for seismic-reflection profiles), presumably Pleistocene turbidites such as those 

found at DSDP Site 35 (McManus and others, 1970; 

Normark and others, chap. 6, this volume). High-angle 

faults cut through the strata, forming raised terraces and 

small horst and graben structures. 

As seen in the sediment isopach map in figure 5.2, the 

sediment is thickest on the eastern side of the axial valley 

where it fills a long, narrow trough. This feature may have 

been formed by a combination of uplift of the flanking 

ridges and constructional volcanism on the western side of 

the valley. In the southeastern part of the axial valley, the 

sediment is more than 1, 700 m thick, while in the central 

0 10 20 KM I I (ll> 

41 °00' 

Figure 5.2. Sediment isopach map of study area, contoured at 200-

m intervals, as estimated from L6-85-NC seismic-reflection lines. 

Areas of sediment thicknesses greater than 100 m are shaded. 

region, thickness averages about 500 m and thins to the 

north. The deep graben in the southeast contains large coherent sediment blocks that appear to have been downdropped and tilted with subsequent channel infill by 

turbidites (Morton and others, chap. 2, this volume). The 

eastern edges of the block show evidence of major drag 

folding whereas the contacts on the western edges are 

sharp. This block appears to be a downwarped extension 

of the steep sediment-capped ridge immediately to the 

north. 

In the basement topography map of figure 5.3, where 

the sediment cover has been removed where identifiable 

using the reflection profiles, a deep, sediment-filled axial 

valley in the south is clearly evident. The flanking ridges 

41°00' 

40°30' L...L--_......_-......L... _ _.w.u.u...."""'-L.~~.....I.....-'--'-'......_ _ ___._~ 

12]050' 12]030' 

Figure 5.3. Basement topography map of study area, obtained by 

removing sediment cover, using sediment thicknesses estimated from 

L6-85-NC seismic-reflection profiles. Note large hole in southeast 

part of valley, arched ridges in central area, and regularly spaced 

domal structures along axis. Contour interval is 200m. 
82 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

are highest in a central area from around latitude 40°54' 

N. to 41 °20' N. and decrease in elevation to the north and 

south. This broad regional arching is also seen in the basement of the axial valley; however, several domal structures 

occur in a regular pattern along the axis about 15 to 20 km 

apart. The arching must postdate the major Pleistocene turbidite deposition, since the flanking walls are sediment 

capped and perched sediment ponds occur on at least three 

of the tilted blocks on either side of the axial valley. 

MAGNETIC ANOMALIES 

A magnetic anomaly map of the area shows a strong, 

well-developed central anomaly in the north, which decays 

in amplitude to the south (fig. 5.4). The boundaries be41°00' 

40°30'~--~----~--~_.~--~~--~--._--~ 

12]050' 12]030' 

Figure 5.4. Magnetic anomaly map for Escanaba Trough study area 

from L6-85-NC cruise, contoured at 100-nT intervals. Positive 

anomalies are shaded. Note low anomaly amplitudes in heavily 

sedimented southern part of Escanaba Trough. 

tween the Brunhes Normal-Polarity Chron and Matuyama 

Reversed-Polarity Chron (0. 73 Ma) and between the 

Matuyama and the Olduvai Normal-Polarity Chron (1.66 

Ma) are clearly defined north of latitude 40°54' N. and 

yield a full spreading rate of 2.0 to 2.1 em/yr. Most of the 

volcanic edifices appear to have a magnetic signature, although the anomalies over the large volcanic intrusions at 

latitude 40°42' N. and 40°45' N. (edifices A and B) appear 

to be attenuated. 

Cross-axis transects of magnetic anomalies and basement structure were prepared for the lines from cruise L6-

85-NC. Line 18 (fig. 5.5), the most northerly profile shown 

here, has a well-developed anomaly pattern. Normally 

magnetized crust of Olduvai age (1.66 to 1.88 Ma) flanks 

reversely magnetized Matuyama-age (0. 73 to 1.66 Ma) 

crust, which in turn surrounds normally magnetized 

> 

...J(f) 

~:5 0Cf) 

zw 

<(t-

(.)~ 

~~ 

zz Cl_ 

<( 

~ 

(f) 

-CC IW 

t-tll.W 

~~ 

~ 

(f) 

400 

200 

0 

-rc Iw 

t-tll.w 

~ ~ 3000 ~ 

LONGITUDE 

Figure 5.5. Magnetic anomaly (top), sediment cover (solid line), 

and basement bathymetry (shaded) profiles of lines 18 and 16 from 

cruise L6-85-NC (see fig. 5.1 for location). 
CRUSTAL MAGNETIZATION AND BASEMENT STRUCTURE OF ESCANABA TROUGH 83 

Brunhes-age material (post 0.73 Ma). The structure of the 

basement and the location of the central anomaly suggest 

that the zone of neovolcanism is displaced to the west side 

of the axial rift. The amplitude of the magnetic anomaly 

over the sedimented trough is less than that over the nearby 

unsedimented intrusion. 

Line 16 (fig. 5.5), the next line to the south, shows 

relatively low amplitude negative anomalies on the sediment-capped ridge flanks. The central anomaly is more 

symmetrical than on line 18 but is still displaced slightly 

to the west. On line 12 (fig. 5.6), the central anomaly is 

almost absent in the sediment-covered valley, and the axial 

intrusion shows very little magnetic expression. Line 7 

(fig. 5.6), the southernmost of the lines shown, shows very 

low amplitude anomalies over ridges with a thick sediment 

cover. A small positive anomaly is centered over the vol-

~ 400r-------------------------------------~ 

_JCJ) LINE12 

~ :5 200 QCJ) 

ZUJ 

<( 1-- 0 

()~ i=<( 

UJZ -200 

~~ <( 

~ 

CJ) 

-a: 

IUJ 

1--1--

2000 

fl:i ~ 3000 0 

~ 

CJ) 

-a: 

IUJ 

1--1--

C..UJ 

~~ 

~ 

12J045' 12J030' 

LONGITUDE 

12J015' 

Figure 5.6. Magnetic anomaly (top), sediment cover (solid line), 

and basement bathymetry (shaded) profiles of lines 12 and 7 from 

cruise L6-85-NC (see fig. 5.1 for location). 

canic edifice on the western side of the axial valley. It is 

curious that the center of the axial rift shows a negative 

value of about -100 nT. 

Line 24 was run along the axis of the rift valley; figure 5.7 shows the basement structure, sediment cover, and 

magnetic anomaly patterns. Broad regional arching of the 

basement is evident, suggesting enhanced crustal generation in the north. Magma supply appears to decrease to 

the south, leading to diapiric intrusions. The sediment fill 

covers the valley floor and shows a gentle northward gradient to about latitude 41° N. but appears to be ponded and 

uplifted north of the NESCA edifice. The magnetic 

anomaly profile shows a clear trend toward decreasing 

amplitudes in the south. In areas with any appreciable sediment cover, magnetic anomaly amplitudes are suppressed. 

However, both edifices B (SESCA) and C (fig. 5.1) show 

small positive anomalies. These areas may have been subjected to domal uplift and associated intrusion of sills into 

thick sedimentary sequences. Volcanism has reached the 

sea floor at only a few locations in the SESCA area (Ross 

and Zierenberg, chap. 8, this volume), and edifice C may 

not contain a near-surface volcanic intrusion. The NESCA 

area shows transitional behavior. Near latitude 41 oN., the 

magnetic anomaly amplitude is low, but slightly further to 

the north, the uplifted blocks show little sediment cover 

and a strong magnetic anomaly signature of up to 200 nT. 

~ 200 <(en 

::2:<( 

0~ 

~ ~ 100 

Q~ 

t!i~ 0 zz 

~-

::2: 

·1 00 1----ot&-----------------------------------t 

en 

a: 

LlJ 

1-

LlJ 

::2: 

~ 

:I 

1-

0.. 

LlJ 

0 

3000 

4000 

SESCA 

41°00' 

LATITUDE 

Figure 5.7. Magnetic anomaly (top), sediment cover (solid line), 

and basement bathymetry (shaded) profiles of line 24 from cruise 

L6-85-NC along rift valley axis (see fig. 5.1 for location). Magnetic 

anomalies appear to be attenuated in areas with sediment cover; 

however, anomalies are observed over volcanic edifices. 
84 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

MAGNETIC MODELING RESULTS 

INVERSE MAGNETIC MODELING 

Levi and Riddihough (1986) proposed that in the 

southern area, high heat flow combined with an impermeable sediment cap caused permanent chemical alteration of 

the underlying crust. While this interpretation is plausible, 

the effects of the basement topography on the anomalies 

must be considered before it can be evaluated (Parker and 

Huestis, 1974). 

The magnetic data were inverted into magnetization 

distributions for source layer thicknesses of 0.1, 1.0, and 

3.0 km. All three cases give essentially similar magnetization patterns except that the thinner the source layer, the 

stronger its magnetization. Difference maps (observed minus modeled) for the three cases are very similar, suggest40°30' '-'--.....L...-.....L...-..u...---'-...:;.............___-'--_ 

12]050' 

A 

ing that the models are not good predictors of the thickness of the magnetized layer. The model fit generally 

agrees with the observed data within 50 nT (fig. 5.8A), except in the northwest sector where data coverage is more 

sparse and the amplitude variations are greater. The annihilator function (fig. 5.8B) is generally ridge parallel, and 

adding various amounts of the annihilator to the model results in similar patterns, but a DC shift in magnetization 

values and stronger magnetization contrasts. 

We reproduce here an inversion of the gridded magnetic field data into magnetization using a 1.0-km source 

layer (fig. 5.9). The choice of this thickness is arbitrary, 

but drilling results at DSDP Site 504B (Smith, 1985) suggest this thickness to be reasonable. The magnitudes of 

the positive and negative off-axis crustal magnetizations are 

comparable, so no additional annihilators are added to the 

solution. After inversion, the crustal magnetizations show 

B 

Figure 5.8. A, Map of misfit between observed and modeled values for 1-km source layer trial. Values are contoured at 1 00-nT intervals. 

Areas less than -100 nT are shaded. B, Magnetic annihilator distribution for 1-km source layer model. Contour interval is 0.2 Am-1. Areas 

greater than 1.0 Am-1 are shaded. 
CRUSTAL MAGNETIZATION AND BASEMENT STRUCTURE OF ESCANABA TROUGH 85 

low values in the heavily sedimented southern end of 

Escanaba Trough, consistent with Levi and Riddihough's 

(1986) earlier interpretation from the magnetic anomaly 

data. Thus, the sediment does indeed appear to suppress 

the magnetic crustal signatures. However, the various volcanic edifices show evidence of positive crustal magnetizations. 

An interesting aspect of the anomaly and crustal magnetization map is that the axial valley south of latitude 

40°42' N. shows negative values. Anomalies vary from 

-50 to -100 nT and magnetizations range from -1 to -2 

Aim. The anomaly values do not appear to be due to instrumentation miscalibration or drift, because the same behavior is observed in all three cruises. Adding sufficient 

annihilator to give zero net magnetization in the area causes 

severe distortion of the symmetrical patterns to the north; 

41 °00' 

40°30' L..L...-~.L...-....1...--'li 

127°50' 

Figure 5.9. Distribution of crustal magnetizations computed for 1-

km magnetic source layer. Areas with negative magnetizations are 

shaded. Contour interval is 2 Am-1. 

thus, the calculated magnetizations are robust in the inversion. The negative magnetic anomalies could reflect (1) 

incomplete removal of a regional field not accounted for 

by the IGRF, (2) negatively magnetized post-0.73 Ma crust 

in the valley axis, or (3) edge effects from the nearby 

1 ,500-m-high Mendocino Escarpment. 

FORWARD MAGNETIC MODELING 

To gain further insight into the nature and distribution 

of the magnetized layer, we did two-dimensional forward 

modeling of line 24 using techniques developed by Talwani 

and others (1959) and Talwani and Heirtzler (1964). The 

forward method differs from the inverse method in that the 

geometry and magnetic intensities are allowed to vary, and 

the assumption that the magnetic layer follows the topography is not required. The criterion for the modeling was 

that a satisfactory fit be achieved with the simplest possible model. The model is not designed to replicate the 

actual magnetization distribution, but represents an equivalent magnetization combination that illustrates several salient features that are required to explain the observed 

magnetic anomaly patterns. In all trials, a site Cenozoic 

field direction of 1=60°, D=0° (or l=-60°, D= 180°) was 

used; distances were calculated from latitude 40°30' N.; the 

modeled magnetic anomalies were calculated for sea level; 

and basement topography was estimated from the seismic 

reflection profiles. 

The best result of a large number of trials is shown in 

figure 5.10. The modeling revealed several interesting 

points: 

1. A minimum of three magnetic polygons are 

needed to adequately fit the data: a strongly magnetic 

block in the north, an irregularly distributed central block, 

and a positively magnetized block representing the 

Mendocino Escarpment. The Mendocino block is needed 

to produce the observed negative values in the southern 

part of the profile. This block also induces a long-wavelength positive trend extending the length of the section. 

Thus, it appears that the negative anomaly values observed in the southern Escanaba Trough are due to an 

edge effect caused by the topography associated with the 

Mendocino Fracture Zone. The three-dimensional inversion gives misleading negative magnetizations, because 

regional effects are not considered. 

2. The magnetic layer is relatively thin (less than 0.5 

km) and close to the surface. This result arises because of 

the relatively short wavelengths of the anomalies. 

3. Although the magnetization distribution follows the 

topography between 70 and 90 km distance, this is not true 

in general, regardless of whether sediment-capped ridges 

or altered zones are added to the model. This is because 

the observed anomalies are relatively smooth and are not 

always centered over the topographic highs and lows. 
86 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

400~--------------------------------------------------~ 

>--

....J 

<( Cf) 200 ~:s OCI) 

Zw 

<(I-

() 0 0 

j:::::Z 

w<( 

zz 

<!Jz <( 

~ -200 

o Observed 

Calculated 

2500r-------------------------------------------------~ 

Cf) 

a: 

w 

I-

~ 3000 

z 

I b: 3500 

w 

0 

Cf) 

a: 

w 

1-

w 

~ 

z 3000 

I 

1-

a.. 

w 

0 

5000 

-100 

.... &QQS&$\ ........... .. 

~------------------------------~ 

0 100 

DISTANCE FROM 40° 30' N, IN KILOMETERS 

Figure 5.10. Two-dimensional magnetic modeling of line 24. Observed (open circles) and modeled (solid lines) 

magnetic anomaly values at top; basement topography and magnetic polygons at bottom. Distances are given 

north of latitude 40°30' N. Three polygons were required to produce observed fit: Mendocino polygon with D=0°, 

I =60°, M=5 Am-1; central block with D=0°, I=60°, M=15 Am-1; and smaller northern block with D=0°, I =60°, 

M=30 Am-1. D, declination; I, inclination; M, magnetization. 

4. In the southern part of the profile, only a 

small amount of magnetic material is needed to 

simulate the anomaly patterns. These polygons could 

represent shallow sills, infrequent small, narrow intrusions, or alternatively, larger intrusions that have 

been partially altered. 

CRUSTAL MAGNETIZATION AND 

SEDIMENTATION 

In figure 5.11, magnetic anomaly values lie within an 

envelope that decreases in intensity with increasing sediment thickness. Over the entire area, magnetic anomalies 
CRUSTAL MAGNETIZATION AND BASEMENT STRUCTURE OF ESCANABA TROUGH 87 

400 

C/) 

:5 

C/) 

w 200 

I0 

z 

<( 

z 

~ 0 

~ 

...J 

<( • ~ 

0 -200 z 

<( 

() 

i= 

w 

z 

(!:) 

<( 

-400 ••• ~ 

-600 

0 200 400 600 800 1 000 

SEDIMENT THICKNESS IN METERS 

Figure 5.11. Scatter plot of magnetic anomaly values (in nT) and 

sediment thicknesses (in m). Magnetic anomaly values lie within an 

envelope that decreases with increasing sediment thickness. 

range from 400 nT to about -500 nT; however, at sediment 

thicknesses greater than 300 m, values seldom exceed 100 

nT, suggesting that magnetic anomalies are indeed suppressed in areas of thick sediment cover. The trend toward 

(about latitude 40°42' N. to 41 °0' N.) and appears to be 

broken up in the north. Without better imaging, it is not 

possible to determine if these features are due to the initial distribution of volcanism or later faulting. 

The basement deep at the southern end of the trough 

suggests an absence of volcanism near the intersection with 

the Mendocino Fracture Zone. This feature is similar to a 

large nodal basin at the juncture of the Kane Fracture Zone 

and the Mid-Atlantic Ridge (Macdonald, 1986). The hole 

may reflect depression of mantle isotherms because of 

cooling effects from the cold Pacific plate nearby. However, the lack of volcanism cannot be maintained indefinitely. 

slightly negative magnetic anomaly values at sediment 41 ooo· 

thicknesses greater than 300 m is probably due to the influence of the Mendocino Escarpment on the thick sediment section in the southernmost Escanaba Trough. 

TECTONICS IN ESCANABA TROUGH 

Several factors probably combine to create the present 

basement topography of Escanaba Trough: axial valley 

volcanism, sea-floor spreading, off-axis isostatic readjustment, and subsequent tectonic deformation. Joint consideration of the basement topography and magnetic data (fig. 

5.12) suggests that the area is presently subject to asymmetrical spreading and volcanism offset to the western side 

of the axial valley. The flanks of the ridge appear to be 

cut by several transform faults. These are evidenced by a 

shift in orientation of the ridge flanks and curvature of the 

magnetic anomaly patterns. The absence of matching features on the opposite side of the spreading axis suggests 

that the changes in topography and magnetic signatures are 

not due to primary volcanism or initial spreading. 

The eastern wall of the axial valley appears to be part 

of a single, coherent tilted block extending from about latitude 40°48' N. to 41 °10' N. Its counterpart on the western side of the trough has a somewhat more limited extent 

40°30' L..l.---'----'--_,,,i,j,i,i..l..l..L..~...:......I..~~~.._-""--'--~ 

12JC50' 12JC30' 12JC10' 

Figure 5.12. Magnetic anomaly patterns superimposed on basement 

topography. Note offsets of magnetic anomaly trends and basement 

structures to west and east of axial valley. Heavy lines on figure 

mark approximate location of inferred shear zones that have created 

discrete tilted blocks on flanks of ridge; arrows indicate relative 

direction of movement. Shading of magnetic anomaly intervals as 

in figure 5.4. Hachures indicate area of closed low. Contour interval 

is 200m. 
88 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

Either the spreading axis has shifted under the sediment 

blanket (probably to the west) or no spreading is occurring, 

requiring a transform fault extending to the Mendocino 

Fracture Zone. More reflection and magnetic data are 

required to distinguish between these possibilities. 

There are at least two possible interpretations for the 

north-northeast trend of the magnetic anomalies outside of 

the axial valley: either they formed initially north-south 

and have been deformed into their current orientation, or 

the axis trended slightly east of north at the time of formation and has shifted to a more north-south orientation 

(normal to the Mendocino Fracture Zone) only since about 

the start of the Brunhes. We favor the latter explanation 

for the crust west of the axis; however, crust east of the 

axis has been affected by additional deformation, especially 

further east. To the west of the axis, north-northeast trends 

are quite striking on GLORIA imagery. Older lineations 

trend north-south only right near the Mendocino Fracture 

Zone. Gorda Ridge appears to be adjusting its spreading 

to become perpendicular to the fracture zone. The possible 

westward shift of the northern part of the Escanaba Trough 

axis (as indicated by the peak magnetic anomaly on line 

18) could be part of this continuing adjustment. 

CONCLUSIONS 

Magnetic anomaly data, combined with seismic-reflection profiling, provide a record of tectonic and volcanic 

activity at the sediment-covered Escanaba Trough. Within 

Brunhes time, represented approximately by the crust 

within the axial valley, tectonic extension has dominated 

over volcanism. Recent volcanic activity is primarily confined to several volcanic edifices, discrete complexes of 

sills and surface flows. The basement topography and the 

size and distribution of the volcanic edifices suggest that 

the amount of volcanism is greater in the northern part of 

the trough. An absence of volcanism near the Mendocino 

Fracture Zone has resulted in the formation of a basement 

hole more than 1,000 m deep in the southeast part of the 

trough. This apparent lack of crustal generation coupled 

with continued horizontal extension is not a stable situation, and it suggests either that the site of crustal generation is shifting or that a transform fault must eventually 

form. 

Crustal magnetization values, obtained by three-dimensional inversion of the anomaly data, are low in the sediment-covered part of the trough, probably owing to crustal 

alteration beneath the sediment blanket. Small-amplitude 

positive anomalies are observed over the volcanic edifices, 

indicating incomplete alteration of shallow intrusions. 

Observed negative anomaly values over the center of the 

axis at the southern end probably are a result of edge effects 

of the magnetic signature of the Mendocino escarpment. 

Magnetic profiles over the northern part of Escanaba 

Trough, where there is little or no sediment cover at the 

axis, exhibit a well-developed axial magnetic high within 

the normally magnetized Brunhes-age crust. However, on 

some profiles, the axial high is not at the center of the 

Brunhes anomaly and the axial valley, but is displaced 

westward. The central high is, however, located above a 

broad terrace at the west side of the axial valley that is 

characterized on Sea Beam bathymetric maps by circular 

features resembling volcanic cones (Morton and Fox, chap. 

2, this volume). Further south, the volcanic edifices that 

penetrate the sediment cover of the trough are similarly displaced toward the west side of the axial valley. These features suggest a recent westward shift of the spreading axis. 

The ridge flanks of Escanaba Trough are being deformed along northwest-southeast-striking faults, as evidenced by offsets in magnetic anomaly and basement 

topographic trends. The axis of Escanaba Trough and the 

Brunhes magnetic anomaly trend approximately northsouth; older magnetic anomalies trend slightly east of 

north. These observations indicate that the trend of the 

southern Gorda Ridge axis is shifting slightly to become 

perpendicular to the strike of the Mendocino Fracture Zone. 

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NC and L2-86-NC, Escanaba Trough, Gorda Ridge: U.S. 

Geological Survey Open-File Report 87-375-A, 20 p. 

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90 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

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anomalies caused by two dimensional bodies of arbitrary 

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dispersal patterns of turbidite sand in Escanaba Trough, northeastern Pacific Ocean: Marine Geology, v. 15, p. 67-87. 

Vine, F.J., 1966, Spreading of the ocean floor: New evidence: 

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---1989, Deformation of the so-called Gorda plate: Journal 

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California: U.S. Geological Survey Bulletin 2022, chapter 

10 (this volume). 
Chapter 6. Sediment Fill of Escanaba Trough 

By William R. Normark, Christina E. Gutmacher, Robert A. Zierenberg, Florence L. Wong, and 

Robert J. Rosenbauer1 

CONTENTS 

Page 

Abstract................................................................................ 91 

Introduction.......................................................................... 91 

Acknowledgments....................................................... 93 

Purpose................................................................................. 93 

Site 35 ................................................................................... 94 

Methods........................................................................ 94 

Core data ..... ............... .. .... ... ................. ... . .... ... . .... .... .... 94 

Pore-water chemistry........................................... 96 

Surficial cores .................. .... .... ... ... . ....... .............................. 98 

Methods........................................................................ 98 

Light and heavy mineral separation.................... 98 

Fine sediment fraction ............ .... .. ...... .. .. ............. 98 

Pore-water chemistry........................................... 98 

Core descriptions and analyses................................... 100 

Coarse-fraction mineralogy................................. 105 

Light fraction ..... ............. ........................ ..... 105 

Heavy fraction............................................. 106 

Pleistocene(?) samples ................................ 107 

Fine-fraction mineralogy..................................... 108 

Pore water . . .. . . .. . . . . . . . . . . . . . . .. . . .. . . . .. . .. . . . . . . . . . . .. . . . . . . . . . . .. 109 

Discussion .. .............. ..... ..... ....... ............. ...................... ...... .. 110 

Sedimentation rates ...... ....... ................ .... ... . ... . ............ 111 

Sediment sources . ..... .................. ............................ ..... 112 

Conclusions . .... ..... ... ............ ....... .............. .... .... .... .... .......... .. 113 

References cited .... ..... .... .... ........... .............. .. .. ................... .. 114 

Appendix A: Lithologic summary logs for 17 cores 

recovered from Escanaba Trough at DSDP Site 35 ....... 116 

Appendix B: Lithologic summary logs for surficial 

cores from Escanaba Trough ........ .. .. .......... ..................... 120 

ABSTRACT 

The turbidite-sediment fill of Escanaba Trough distinguishes this segment from the rest of the Gorda Ridge axial 

valley and shows clear evidence for interaction with 

hydrothermal fluids that form the observed massive sulfide 

deposits at the sea floor. This conclusion is supported by 

reexamination of the sediment and chemical data from 

Deep Sea Drilling Project Site 35 and description of 41 (of 

1 U.S. Geological Survey, Menlo Park, California. 

a total of 69) core sections not examined by the shipboard 

party. In addition, descriptions and subsample data from 

34 short gravity and piston cores taken mostly in or near 

areas of known sea-floor mineral deposits document (1) 

hydrothermal alteration of turbidite mud (locally to chlorite), (2) hydrothermal mineralization within porous sandy 

turbidite units, and (3) the presence of massive-sulfide talus 

debris. The source of the turbidite-sediment fill includes 

the Columbia River drainage basin and the Klamath Mountains of southern Oregon and northern California. Conservative estimates of the age of the ocean crust underlying 

the floor of Escanaba Trough give sediment accumulation 

rates for the Pleistocene that are more than an order of 

magnitude greater than for the Holocene. 

INTRODUCTION 

The youngest or "zero-age" (meaning that under assumptions of continuous active spreading the axial zone 

includes very recent new volcanic material) crust along the 

axis of the Gorda Ridge spreading center in Escanaba 

Trough is overlain by a thick cover of dominantly terrigenous sediment, which locally exceeds a thickness of 

500 m (Moore, 1970; Moore and Sharmin, 1970; Vallier 

and others, 1973; Clague and Holmes, 1987; Morton and 

others, 1987a; Morton and Fox, chap. 2, this volume). 

Reflection-profiling and deep-tow geophysical studies 

conducted in the late 1960's provided a general knowledge 

of the structure and geometry of the sediment fill of the 

trough (Atwater and Mudie, 1968, 1973; Moore, 1970). 

Drilling at Deep Sea Drilling Project (DSDP) Site 35 in 

Escanaba Trough was the first attempt to study the environment of sediment -covered, zero-age oceanic crust, but 

coring stopped short of the basement (McManus and 

others, 1970). 

Until Site 35 was drilled, little was known about the 

nature of the sediment fill itself, except for the inference 

(based on the flat-lying sea floor and subparallel reflecting horizons) that a significant part of the fill was composed of turbidite deposits (Atwater and Mudie, 1968, 

1973; Moore, 1970). The sediment in Escanaba Trough 

could have come from source areas ranging from northern 

91 
92 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

California (Eel and Klamath rivers) to as far away as the 

Columbia River basin (fig. 6.1). Several studies have observed that there is a clear pathway, at least at present, for 

sediment to reach Escanaba Trough from Astoria fan, 

which has received material from the Columbia River 

drainage area (Nelson and others, 1968; Moore, 1970; 

100 200 kilometers L.__ ______ __j_ _____ _j 

• 

Cascadia 

Basin 

Cores with Mazama 

tuffaceous turbidites 

Tufts 

Abyssal 

Plain 

CALIFORNIA 

Figure 6.1. Location map showing possible sources and pathways for terrigenous sediment to move into Escanaba 

Trough (modified from Nelson and others, 1988). Dots denote sites where cores contain tuffaceous turbidites derived 

from Mount Mazama. Contour interval is 500 m. 
SEDIMENT FILL OF ESCANABA TROUGH 93 

Atwater and Mudie, 1973; Kulm and others, 1973; Nelson, 

1976; Clague and Holmes, 1987). Vallier and others (1973) 

concluded that there may be a substantial component of 

Columbia River sediment in Escanaba Trough based on 

petrographic analysis of a limited number of sand samples 

from the cored intervals in DSDP Site 35. Recent longrange side-looking sonar coverage of the Exclusive Economic Zone (EEZ) off the Western United States includes 

the entire Gorda Ridge and provides additional support for 

the concept that source areas could include any drainage 

basin between the Columbia River and Cape Mendocino 

(EEZ-SCAN 84 Scientific Staff, 1986; Masson and others, 

1988). 

Beginning in 1983, the sediment fill of Escanaba 

Trough was sampled as part of a program to study the hydrothermal mineralization along the Gorda Ridge spreading axis. Short (0.5 to 5.5 m long) gravity cores and a very 

limited number of piston cores are available from cruises 

in 1985, 1986, and 1989 (fig. 6.2). These samples are 

herein referred to as "surficial" cores to emphasize that 

nearly all of the gravity and piston cores are restricted to 

the upper 1 percent or less of the average thickness of sediment fill in Escanaba Trough and, therefore, are only 

representative of the most recent depositional processes in 

the area. 

ACKNOWLEDGMENTS 

The Gorda Ridge shipboard activities over the years 

have involved many participants, without whose help we 

would not have the comparative wealth of core material to 

present here. We thank the ships' crews and scientific staff 

for their support, commonly under less than ideal weather 

conditions, during cruises L6-85-NC; L1-, L2-, and L3-86-

NC; and TUL-89D. J.L. Morton and S.L. Ross assisted 

with the descriptions of cores from L1- and L2-86-NC operations. T. Nelson kindly provided core descriptions for 

the L3-86-NC cruise. G.G. Zuffa and P. Okita assisted in 

the description of the DSDP cores in August 1988. P. 

Swenson provided invaluable training to allow us to utilize Macintosh applications for core log compilations. The 

manuscript was reviewed by R. Karlin and D. E. Drake. 

PURPOSE 

This chapter presents a compilation of much of the 

available data on the composition, age, sources, and depositional processes of the sediment within Escanaba Trough. 

This compilation is done to provide a better understanding of the sediment that hosts the mineral deposits (and provides the pathways for fluids forming these deposits) 

described elsewhere in this volume. Much of the data included has not been presented elsewhere. Some of the 

surficial cores have been described in varying detail by different groups of investigators only in internal reports 

(Karlin and Lyle, 1986; Morton and others, 1987b; Karlin, 

1989). The chapter for Site 35 of the Initial Reports for 

DSDP Leg 5, as well as all investigations of the sediment 

fill based on the described drilling results, include data 

from only 28 of the 69 core sections that were recovered 

(McManus and others, 1970). We include summary logs 

in appendix A of the remaining core sections that were 

opened more than a decade after the cruise report was published. Summary logs (appendix B) as well as sediment 

and pore-water analyses are also provided for the surficial 

core samples collected during the studies of the hydrothermal-vent areas. 

41° 

10'n----.~------,---~r-------~--~~----~ 

\ I 

40° 

40' 

) il'v J I I ;tJ a 

I ./ I 7 1------*-4"- -1 I / ,-/ u I ( ~ i 

' \ :NESCA : 

I •

j 

4 

.6/ {\~ t ; i I h 1 2 1 if 

~~II ~. J 2, ,5~8,~6 I 

\ ! I 3~ 

\.,_/' j I 

//

) } ·~-~~----

2~* .11 

8* 

'\\\ )~ --------_:~--, ~ I SESCA : 

I • a 2~~o2~eJ 

/

1 l -u--• •f! 

.9 'V : \ 25,26.8 : : la : I I 

I I 

L- -----------1 

\ oDSDP35 

EXPLANATION 

o DSDP-35 (McManus and others, 1970) 

• L6-85-NC !Karlin and Lyle, 1986) • L 1-86-NC Morton and others, 1987b) 

L2-86-NC Morton and others, 1987b) 

L3-86-NC unpub. data) 6 TUL-890 (unpub. data) \J 0 5 10 15 KILOMETER 

Contour interval 500 meters 

40° 

30'~--~--------------~------------~----~ 

Figure 6.2. Location of cores taken in Escanaba Trough. Study 

areas: NESCA, northern Escanaba Trough (see fig. 6.9); SESCA, 

southern Escanaba Trough (see fig. 6.10). Bathymetry from Wilde 

and others (1978). Hachures indicate area of closed low. 
94 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

The data, including review of the methods used, are 

presented in two sections; the first deals with DSDP Site 

35 and the second with all the surficial cores. 

SITE 35 

METHODS 

When Site 35 was drilled in late April and early May 

1969, the limited size of the shipboard scientific party precluded the normal operation of splitting and describing all 

core sections immediately after recovery (T.L. Vallier, oral 

commun., 1988). As a result, the Initial Reports volume 

for Leg 5 contains core descriptions, photographs, and 

subsample data for only 28 of the 69 sections recovered 

in the 17 cores. The remaining 41 sections eventually were 

split when the entire set of samples from DSDP Leg 5 were 

rephotographed more than 10 years after they were taken. 

These recently split sections, however, were not described 

until August 1988, after our inquiries to the Ocean Drilling Program (ODP) could find no record of any earlier logs 

of these newly opened intervals. 

We publish here as table 6.1 a revised summary of the 

cored intervals and extent of recovery for Site 35, modified from table 1 of McManus and others (1970). This revision is necessary because the core sections found in the 

DSDP collection did not always match the site-chapter 

tabulations. In addition, table 6.1 shows which sections 

were described by the shipboard scientific party; it should 

be emphasized that existing reports of the deeper section 

at Site 35 data are based on physical descriptions and 

subsamples from only 7 of the 29 core sections recovered 

from the bottom 100 m of the hole. 

The source of the core descriptions used for this report is summarized in a chart (fig 6.3). Direct comparison of the original shipboard descriptions with those we 

did in 1988 is difficult because of marked changes in the 

condition of the sediment during the nearly 20 years that 

had passed. We reexamined nine of the core sections described by the shipboard scientific party (fig. 6.3) in an 

attempt to calibrate the differences in appearance and condition of the sediment between 1969 and 1988. This attempt generally was not successful because (1) the color 

change was extreme and not consistent in trend because 

the mud sequences exhibited a wide variation in degree 

of desiccation, (2) the sand-rich sequences were completely dried and handling of the cores had resulted in 

mixing the sandy intervals throughout the length of the 

liner section, and (3) the technique for description of the 

cores used in 1988 follows the current guide for sedimentologists involved in the ODP (Mazzullo and Graham, 

1987) and is not always consistent with the original descriptions. There was insufficient time for redescribing 

all of the core sections in 1988, but we did attempt to 

Table 6.1. Summary of Site 35, Deep Sea Drilling Program. 

[Modified from table 1 of McManus and others (1970) to correct errors in original 

(changes are shown in bold type). Sections described by shipboard scientific party 

also are shown in bold type, and sections described in 1988 are given in plain type 

(maximum of six sections for any given core excluding core-catcher sample). See 

figure 6.3] 

Date Depth below Core cut Recovery 

(1969) Core sea floor (m) (m) (percent) Sections 

29 April 1 0-9 9.1 100 123456 

2 39-48 9.1 100 123456 

30 April 3 79-88 9.1 100 123456 

4 88-97 9.1 33 1 2 

5 97-106 9.1 100 123456 

6 157-166 9.1 100 123456 

7 233-242 9.1 69 1234 

8 256-261 4.6 27 1 

1 May 9 280-289 9.1 43 123 

10 289-298 9.1 13 1 

11 298-307 9.1 90 123456 

12 323-332 9.1 87 123456 

2May 13 345-353 7.5 95 12345 

14 354-362 7.6 60 1234 

15 362-371 9.1 70 1234 

16 372-382 9.1 33 1 2 

17 382-390 7.6 8 1 

confirm the presence of sand beds in all sections so reported by the shipboard scientific party. 

We compiled a summary section for the entire Site 35 

sequence (fig. 6.4), presented here together with the accompanying electric logs presented in the Initial Reports 

(McManus and others, 1970). A rough estimate of the relative proportion of the sediment section in the cored intervals that is characterized by ( 1) the presence of sand even 

in minor amounts, (2) the presence of silty mud, or (3) generally clayey mud is represented graphically in figure 6.4. 

This estimate cannot be interpreted as a typical sand/silt/ 

clay ratio, but is given to indicate the general presence of 

coarser grained intervals; recovery of unconsolidated sandy 

sequences was generally very poor for all DSDP sites. Our 

newly compiled and updated lithologic summary of the 

sediment sequence (app. A) provides the basis for reexamining the observations reported by McManus and others 

( 1970) and later scientific reports based on Site 35 studies. 

CORE DATA 

Only about 100 m of the 390-m-deep hole was 

sampled by coring (fig. 6.4; table 6.1). Samples are especially sparse in the interval from 106 to 280m below the 

sea floor. On the basis of the cores recovered, only one 

interval with sand-size sediment commonly present is 

documented: cores 3 to 6, from 80 to 160 m depth (fig. 
SEDIMENT FILL OF ESCANABA TROUGH 95 

w 

a: 

0 

() 

II 

II 

SECTION 

1 2 3 4 5 

1 

2 

3 

4 

5 

6 

7 

8 

9 

10 

11 

12 

13 

14 

15 

16 

17 

EXPLANATION 

Leg 5 (April/May 1969) 

USGS (August 1988) 

Both Leg 5 and USGS 

6 cc 

No recovery 

Not described 

Figure 6.3. Summary of core sections recovered and source of 

descriptions used for Site 35, Deep Sea Drilling Project; CC, core 

catcher. 

6.4; app. A). Because only 2 of 17 sections from these four 

cores were described by the shipboard party, the extent of 

this interval with some interbedded sand was poorly documented, although Vallier and others (1973) interpreted the 

electric logs to suggest that both cores 4 and 5 are generally sandy (see their fig. 3). This interval with more common sand-sized material, like most of the other coarser 

grained units cored at Site 35, shows a fair degree of core 

disturbance; as a result, few distinct bedding contacts are 

preserved, and the presence of sand and silt beds commonly 

is based on the recognition of blebs and streaks of coarser 

sediment within a generally muddy section. Much sand is 

probably lost during the coring disturbance, so it is not possible to construct a true sand to mud ratio for this site. 

With the exception of a sequence of thin-bedded turbidite units in core 10 (with a drilled interval of 9 m but 

only 1 m of recovery) and the lowest one-third meter of 

Site 35 

Depth, 

in meters 

Core 

number 

o-._..........,....... ....................... ,............ ............ 1 EXPLANATION 

2~ 50 

3 

4 

5 

150 

6 

200 

7 

250 8 

e=~:==~1~ 300--~~~~~~ ~~~~~~11 

j.L.LI'~~~~-..:~.~12 

350 -~~~~~~ 13 

~:;:::;:~~~:;:! 14 

12== ~~~~~15 16 

1---====i 17 

• 

silt 

sand 

~mud 

D not cored 

Figure 6.4. Summary core log for Site 35, Deep Sea Drilling Project. 

Interval between core recovery shown in white. Extent of patterning 

within cored intervals is proportionally to length of core with sand 

present, silt present, or mud only. 
96 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

core 17, sand recovery is rare in the lower 200 m of the 

hole (app. A). Silty clay intervals are slightly more common (app. A), but the bulk of the cores we described in 

1988 that were not examined onboard during the drilling 

on station contain little sand. Thus, we do not see evidence 

for a generally coarser sequence in the bottom 100m of 

the hole as suggested by Vallier and others (1973; compare 

their fig. 3 with our fig. 6.4 and app. A). 

One of the more impressive coarse-grained units, however, is just below 350 m depth. The upper section of core 

14 was almost entirely a thick bed of very well sorted fine 

sand and coarse silt that probably represents the distal deposit of a large-volume turbidity current. A sample from 

40 to 43 em depth in section 1 of core 14 consists mostly 

of quartz, feldspar, and minor pyrite with the proportion 

of light grains similar to that of other samples higher in 

the hole (see later section on the mineralogic study of 

surficial cores). The pyrite occurs as free crystals or partially coating the quartz, feldspar, or occasional carbonate 

grains. Besides the pyrite, the heavy fraction consists of 

minor or trace amounts of biotite, epidote, etched garnet, 

sphene, augite, and tourmaline. There are too few 

nonopaque grains to determine this sample's relationship 

to the other heavy mineral samples. 

Examination of the previously unopened core sections 

confirmed the shipboard observation that below 260 m 

subbottom depth, the muds are generally more calcareous 

than in the upper part of the hole. 

PORE-WATER CHEMISTRY 

Manheim and others ( 1970) provided data on porefluid chemistry from Escanaba Trough sediment samples 

recovered at DSDP Site 35 (table 6.2). The strong vertical gradients observed in the pore-fluid chemistry were 

generally attributed by them to diagenetic interaction in the 

sediment, but were not further explained. 

The pore-fluid gradients are interpreted herein as an 

indication of high-temperature seawater-sediment interaction. Major-element chemistry (figs. 6.5-6.7) and minorelement chemistry of pore water (fig. 6.8) are plotted here 

as a function of depth within the sediment section. For 

comparison, the values for seawater are plotted at zero 

depth (sea floor), and the end-member values for hydrothermal-vent fluid measured at the Northern Escanaba 

Trough (NESCA) site (Campbell and others, chap. 11, this 

volume) are plotted at 500 m, the approximate depth of 

the sediment-basalt interface. The trend of increasing sodium and chloride of pore fluid with depth is consistent 

with mixing of seawater with a fluid of salinity similar to 

Escanaba Trough vent fluid (fig. 6.5). The lowermost 

pore-fluid sample also shows increased potassium and 

calcium, consistent with mixing of vent fluid with a pore 

fluid derived from seawater (fig 6.6). Pore fluid from the 

upper part of the hole generally shows an inverse correlation of potassium and calcium. The decrease in potassium between 237 m and 350 m could result from the 

100 

Cf) 

a: 

w 

1-

w 200 

~ 

z 

I 

1- 300 a.. 

w 

0 

400 

500 Escanaba vent fluid 

0 5 10 15 20 25 

CONTENT, IN GRAMS PER KILOGRAM FLUID 

Figure 6.5. Pore-water salinity as a function of depth from Deep 

Sea Drilling Project Site 35 (adapted from Manheim and others, 

1970). Escanaba Trough vent-fluid composition (Campbell and 

others, chap. 11, this volume) is plotted at 500 m depth, the 

approximate sediment-basalt interface, for comparison. 

Cf) 

a: 

w 

1-

w 

~ 

z 

I 

1-

a.. 

w 

0 

0 ~----~~----~S=e=a=w=a=te~r ______________ __, 

100 

200 

300 

400 

500 

0 

Escanaba ---- vent fluid 

0.4 0.8 1.2 1.6 

o K 

x Ca 

2 

CONTENTS, IN GRAMS PER KILOGRAM FLUID 

Figure 6.6. K and Ca composition of pore-water as a function of 

depth for samples from Deep Sea Drilling Project Site 35 (adapted 

from Manheim and others, 1970). 
SEDIMENT FILL OF ESCANABA TROUGH 97 

Table 6.2. Deep Sea Drilling Project Site 35 pore-water chemistry. 

[Values in ppm, alkalinity (Alk) in meq/kg, adapted from Manheim and others (1970)] 

Depth 

(m) Na K Ca Mg Cl so4 Alk Li B Sr Ba Si 

41 11,100 480 320 1,230 19,200 1,640 17.6 0.07 5 6.9 0.09 

160 10,900 480 310 950 19,700 

237 11,000 200 730 600 19,900 

287 11,400 150 810 460 19,900 

330 11,200 250 720 370 19,700 

350 11,500 280 740 310 20,100 

360 11,700 800 790 220 20,200 

diagen~tic conversion of smectite to illite in response to 

increasing temperature with depth (Perry and Hower, 

1972). Magnesium and sulfate contents of the pore fluid 

decrease with depth (fig. 6.7), consistent with hightemperature interaction of saline fluid with the sediment. 

Magnesium and sulfate depletion is caused by formation 

of magnesium-rich silicate alteration phases and precipitation of anhydrite (Rosenbauer and others, 1983; 

Thornton and Seyfried, 1985; Zierenberg and Shanks, 

chap. 14, this volume). Magnesium metasomatism and 

accompanying production of hydrogen ions causes 

hydrolitic alteration of silicate minerals that releases cations to the fluid phase. Minor-element concentrations are 

also elevated in the pore fluid due to hydrothermal alteration of the sediment (fig. 6.8). 

en 

a: 

w 

100 

tu 

~ 200 

z 

:i 

b: 300 

w 

Q 

400 

500 

0 

Escanaba vent fluid 

o Mg 

X 804 

0.5 1 1.5 2 2.5 3 

CONTENTS, IN GRAMS PER KILOGRAM FLUID 

Figure 6.7. Mg and S04 composition of pore-water as a function of 

depth for samples from Deep Sea Drilling Project Site 35 (adapted 

from Manheim and others, 1970). 

340 7.1 .13 3 9.2 4.6 12.0 

210 4.7 .72 3 13 5.4 5.9 

200 2.9 .44 3 17 6.2 3.4 

200 2.8 .71 7 14 6.2 5.5 

110 3.9 .79 10 19 7.7 5.6 

210 4.2 .92 9 20 8.1 9.9 

The composition of Escanaba Trough hydrothermal 

fluid is strongly influenced by sediment alteration 

(Campbell and others, chap. 11, this volume). Detrital 

minerals from the lower section of DSDP Site 35 show evidence of hydrothermal dissolution (T.L. Vallier, oral 

commun., 1988). The pore-fluid chemistry from Site 35 

appears to reflect both diagenetic reactions in the sediment 

occurring at elevated temperature and influx of higher 

salinity hydrothermal fluid derived by high-temperature alteration of sediment and (or) basalt. Site 35 was drilled 

approximately 8 km southeast of the SESCA hydrothermal 

deposits and 35 km south of the NESCA site where active 

venting of hydrothermal fluid was subsequently discovered. 

A covering of sediment with relatively low crossstratal permeability reduces the cooling rate of the oceanic 

en 

a: 

w 

100 

tu 200 

~ 

z 

I~ 

1- 300 

a.. 

w 

0 

400 

DB 

xu 

• Sr & Ba 

0 2.5 5 7.5 10 12.5 15 17.5 20 22.5 25 

CONTENTS, IN GRAMS PER KILOGRAM FLUID 

Figure 6.8. Minor-element composition as a function of depth for 

pore-water samples from Deep Sea Drilling Project Site 35 (adapted 

from Manheim and others, 1970). 
98 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

crust by hydrothermal circulation. The low thermal conductivity of sediment relative to basalt and the intrusion 

of basalt into the sediment column results in increased temperature in the upper part of the basalt relative to sedimentfree ridges. High-temperature alteration of sediment may 

be occurring throughout the sediment-covered areas within 

Escanaba Trough, rather than being limited to known sites 

of hydrothermal discharge. The lower part of the sediment 

section represents a large potential reservoir for hydrothermal fluids. The volcanic centers, which are the known 

sites of hydrothermal deposits, provide both thermal and topographic foci for hydrothermal discharge. The large volume of the hydrothermal alteration zone and of the potential 

reservoir for hydrothermal fluids, coupled with efficient 

focusing of hydrothermal discharge, results in long-lived, 

high-volume hydrothermal systems capable of forming the 

large sulfide deposits observed in Escanaba Trough. 

SURFICIAL CORES 

METHODS 

The gravity and piston cores available for this study 

were collected during a series of cruises by the U.S. Geological Survey and the Geological Survey of Canada from 

1985 to 1989 (figs. 6.2, 6.9, and 6.10). Table 6.3 lists the 

cruises, dates, core length, source of description used in 

this report, and certain ancillary observations that are available. Specific subsample intervals are given in the data 

sections that follow. The lithologic logs for cruises L1- and 

L2-86-NC (app. B) show that whole-round samples were 

taken for pore-water analysis (see below) and for heavy and 

light hydrocarbon analyses (HHC and LHC, respectively; 

see Kvenvolden and others, chap. 15, this volume). Different sets of cores were described at various times after 

the cores were collected by different personnel, thus there 

is a lack of consistency among the sets of descriptions, and 

the data are presented in separate figures (app. B). 

Data obtained from subsamples of the surficial cores 

reported herein are generally restricted to three subjects: 

(1) coarse-fraction composition, including both light and 

heavy mineral components, (2) fine-fraction (mud) composition based on X-ray diffraction measurements, and (3) 

pore-water composition for a limited number of samples. 

The techniques employed are described below. Other data 

obtained from the cores, including hydrocarbon composition and magnetic susceptibility logs, are reported in other 

chapters in this volume. 

LIGHT AND HEAVY MINERAL SEPARATION 

Twenty-four subsamples (volume 1 to 2 cm3) were 

taken from sediment layers initially described as either 

"sandy" or "silty." By decantation and sieving, three grainsize fractions were retained, 30 to 63 J..Lm, 63 to 250 J..Lm, 

and greater than 250 J..Lm (table 6.4). 

Heavy and light mineral components separated from 

the 63- to 250-J..Lm fraction in tetrabromoethane (specific 

gravity 2.96) were permanently mounted in piccolyte (refractive index 1.52) to determine mineral abundance by 

point count. Counts of more than 350 points were made 

for the heavy minerals and about 300 points for the light 

minerals. 

Not all samples yielded enough material in this fraction for heavy mineral separation, and a few samples had 

insufficient nonopaque grains to justify point counting. 

Compositions of the uncounted samples were estimated 

through low-power (about 30x) binocular microscopy. The 

>250-J..Lm fraction was also examined by binocular microscopy to determine qualitative mineral abundances. Both 

the 63- to 250-J..Lm and greater than 250-J..Lm fractions were 

examined for foraminifers (Quinterno, chap. 18, this 

volume). The 30- to 63-J..Lm fraction was not examined 

further. Other material obtained as by-products of 

subsampling for foraminifers or separated for magnetic susceptibility were also examined for mineral occurrences. 

FINE SEDIMENT FRACTION 

Sediment samples of the dominantly muddy intervals 

were examined by X-ray diffraction to determine the bulk 

and clay-fraction mineralogy. Samples for clay mineralogy were repeatedly dispersed in deionized water by 

ultrasonic probe and settled by centrifugation. Smear 

mounts on glass slides were prepared from the finest grainsize fraction (top layer of centrifuged samples) and were 

examined by X-ray diffraction three times: once dry, once 

after treatment with ethylene glycol, and again after heating to 400°C. 

PORE-WATER CHEMISTRY 

Pore-water samples were extracted by hydraulically 

squeezing the sediment in the whole round core sections 

in a refrigerated laboratory environment; the samples are 

limited to cores from cruise L2-86-NC (app. B). The fluid 

extracted was filtered sequentially through 0.45-J..Lm and 

0.2-J..Lm membrane filters. Major and minor cations were 

determined by atomic absorption spectroscopy, except for 

sodium, which was calculated by charge balance. The pH 

was determined by a Ross-type combination electrode. 

Sulfate was determined by ion-liquid chromatography. 

Silica was determined colorimetrically by the molybdate 

blue complex. Total inorganic carbon (CT=C02+H2C03-

+C03 

2-) was determined using an IR analyzer. Chloride 

was determined potentiometrically by titration. 
SEDIMENT FILL OF ESCANABA TROUGH 

12J032' 12J030' 

66,000 

64,000 

62,000 

56,000 

54,000 

52,000 

56,000 62,000 50,000 

Figure 6.9. Bathymetry of NESCA area (Morton and others, 1987b; Morton and others, chap. 1, this 

volume), showing core locations. Numbers along right and bottom give coordinates oflocal transponder 

net (in meters). Hachures indicate area of closed low. 

99 
100 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

CORE DESCRIPTIONS AND ANALYSES 

The cores taken during cruise L6-85-NC are from the 

main floor of Escanaba Trough and from the ridge areas 

both east and west of the trough (fig. 6.2). The core logs 

(app. B) are based on the core descriptions (without graphic 

logs) given in Karlin and Lyle (1986) and color photographs of the archive half taken later. These cores are 

dominantly mud with few silt and sand interbeds. The upper 27 em of core L6-85-5G repeats the interval below, possibly as a result of local slumping. 

EXPLANATION 

• 8 L6-85-NC 

e1 L 1-86-NC 

o 2 L2-86-NC 

\ 

6000 8000 10,000 

The Karlin and Lyle (1986) report provides much useful data including detailed chemical profiles, whole-core 

magnetic-susceptibility profiles, and radiocarbon dating. 

The resulting age control allowed Karlin and Lyle (1986) 

to recognize the Pleistocene-Holocene boundary, which is 

generally correlated to color and compositional changes in 

the cores, and to date several turbidity-current units that 

might be relatively local events in the Northern (NESCA) and 

Southern Escanaba Trough (SESCA) study areas. In addition, they recognized several discrete intervals of hydrothermal activity that provide a mineral or chemical signal in 

12J032' 12J030' 12J028' 

0 2 KILOMETERS 

Contour interval 10 meters 6000 

12,000 14,000 16,000 

Figure 6.10. Bathymetry of SESCA area (Morton and others, 1987b; Morton and others, chap. 1, this volume), showing core locations. 

Numbers along right and bottom give coordinates of local transponder net (in meters). Hachures indicate area of closed low. 
SEDIMENT FILL OF ESCANABA TROUGH 101 

Table 6.3. Surficial cores from Escanaba Trough and adjacent ridge flanks. 

[Cruise ID: L6-85-NC, Ll-86-NC, L2-86-NC, and L3-86-NC refer to separate cruise legs of U.S. Geological Survey RN S. P. Lee in 1985 and 1986; TUL89D, 1989 cruise leg of RN Tully used by Geological Survey of Canada. See figures 6.2, 6.9, and 6.10 for location of cores. Symbols as follows: GS1 and 

GS2, described by different groups of U.S. Geological Survey personnel; OSU, description adapted from Karlin and Lyle (1986); TN, description adapted from 

T. Nelson (written commun., 1989); na, not available. Checklists of three right-hand columns indicate that the following data exist for cores (but will not be 

presented in detail in this report): continuous flow-camera color photographs, X-radiographs, and profiles of magnetic susceptibility (see Karlin and Lyle, 1986; 

Karlin, 1989; Karlin and Zierenberg, chap. 7, this volume)] 

Cruise 10 Core number Length Description by Color photograph X-radiograph Magnetic 

(em) 

L6-85-NC lG 202 osu 

2G 241 osu 

3G 241 osu 

4G 3 na 

5G 232.5 osu 

6G 257 osu 

7G 242 osu 

8G 234 osu 

9G 211 osu 

lOG 249 osu 

llK. 31 na 

12G 154 osu 

Ll-86-NC lG 273 GSl 

7G 233 GSl 

8G 197 GSl 

19G 255 GSl 

25G 227 GSl 

26G 239 GSl 

27G 105 GSl 

33G 124 GSl 

L2-86-NC 2G 101 GSl 

4G 263 GSl 

7G 241 GSl 

9G 200 GSl 

28G 244 GSl 

L3-86-NC 4G 143 TN 

5G 62 TN 

8G 92 TN 

9G 101 TN 

TUL-89D lP 63 GS2 

2G 267 GS2 

4G 277 GS2 

5G 41 na 

6G 187 GS2 

7P 537 GS2 

8G 10 GS2 

9TR 50 GS2 

adjacent cores. The core logs (app. B) are synthesized to provide the closest possible match to core logs for samples taken 

on succeeding cruises and thus do not incorporate all these 

compositional details from Karlin and Lyle (1986). 

Cores taken during the first two legs (L 1- and L2-86-

NC) of the 1986 cruise are represented by the most detailed 

lithologic logs (app. B). This detail is possible because (1) 

the cores were described immediately after being split, thus 

susceptibility 

X 

X 

X 

X 

X 

X 

X 

X 

X X X 

X X X 

X X X 

X X X 

X X X 

X X X 

X X X 

X X X 

X X X 

X X X 

X X X 

X X X 

X X X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

giving the best range of sediment color and contrast before oxidation and desiccation began; (2) both color photographs and X-radiographs were taken at the same time 

(see examples in figs. 6.11 and 6.12). The wide variety in 

sediment units recognized, however, also reflects the effects of local tectonic and hydrothermal activity. 

Many of the cores from L1- and L2-86-NC record 

mass-wasting and turbidity-current depositional processes 
102 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

Table 6.4. Grain-size and heavy mineral data of mineralogical 

samples. 

[Silt content includes only 30- to 63-J.IIIl grains. Samples rich in clay minerals were 

not processed further. After heavy mineral separation, samples rich in opaque components were not counted] 

100 x Weight percent 

Samplea sand/ heavies Comments 

sand+silt (63-250 J.Lm) 

DSDP 35-14-1 40-43 em 56 4.8 Pyrite-rich 

L1-86 7G 124-126 em 69 Clayey 

L1-86 7G 211-212 em 16 

L1-86 8G 52-53 em 63 1.9 

L1-86 8G 150-151 em 91 1.7 

L1-86 8G 175-176 em 94 1.1 

L 1-86 25G 48-49 em 74 3.6 

L1-86 25G 119-120 em 46 1.9 

L1-86 26G 33.5-35 em 60 1.4 

L1-86 26G 40-41 em 49 2.2 

L1-86 27G 29-30 em 60 

L1-86 27G 64-65 em 58 50.0 Metallic 

L1-86 27G 72 em 95 8.3 Metallic 

L1-86 33G 47-49 em 41 Clayey 

L2-86 2G 72-74 em 66 3.1 

L2-86 7G 235-237 em 1 

L2-86 9G 106-107 em 93 Clayey 

TUL-89D 2G 94-95 em 40 2.9 

TUL-89D 6G 25-26 em 86 1.5 

TUL-89D 7P 8-9 em 82 3.1 

TUL-89D 7P 24-25 em 92 3.1 

TUL-89D 7P 42-43 em 90 3.8 

TUL-89D 7P 240-242 em 71 1.4 

TUL-89D 9T 12-14 em 28 Micaceous 

aExcept for standard DSDP designation, sample identification includes cruise 

number (Ll-86, L2-86, TUL-890), core number and type (G, gravity; P, piston; T, 

trigger for piston), and sampling interval. 

(app. B). Coarser (silty and sandy) turbidites commonly 

have erosive bases and irregular bed thicknesses. In a few 

cores, sand units are associated with mud-clast units of 

rounded mud balls in a mud or sandy-mud matrix (core L2-

86-2G, app. B and fig. 6.12). We recognize turbidite-mud 

units on the basis of a distinctive mud color that is the same 

found where turbidite mud overlies graded sand and silt 

beds that generally have sharp, erosive bases. In X-radiographs, muddy turbidites commonly appear as welllaminated units that are overlain by more bioturbated 

pelagic(?) mud. Comparison of subtle differences in the 

texture of the nonturbidite mud units (porous versus nonporous) with the magnetic-susceptibility profiles from 

Karlin (1989) and Karlin and Zierenberg (chap. 7, this volume) suggests that the textural change commonly corresponds to the 11 ka boundary based on the Karlin and Lyle 

(1986) stratigraphy. Using this age assignment, it is clear 

that turbidity currents have entered or were generated 

within Escanaba Trough several times during the Holocene. 

Samples from the thicker, coarser sand beds were examined to determine the possible sources for these turbidity 

currents (see following section). 

The cores obtained during the 1986 surveys provide 

information about both hydrothermal and tectonic activity. 

Core L2-86-9G ( app. B) penetrated metalliferous sediment 

overlying talus (sand to pebble size) of massive-sulfide 

deposits; this, in turn, overlies a completely altered mud 

interval more than 1 meter thick. In the X-radiograph (fig. 

6.12), the sulfide talus is observed to occur in two layers 

8 and 12 em thick separated by a more sandy interval of 

several centimeters. Core L1-86-27G terminates in bedded sand units containing hydrothermal mineralization 

products. In figure 6.11, the mineralized sand layers of 

core L1-86-27G show as reddish-brown intervals on the 

oxidized core surface, but in X-radiographs the mineralization is clearly observed to be confined to several thin 

intervals in this sandy section (compare core log of app. 

B with fig. 6.12; see below and Zierenberg and Shanks, 

chap. 14, this volume, for discussion). 

Core L1-86-8G is a lighter gray color and has a more 

porous texture than normal mud. This core may contain 

mineralized and altered sediment that has been redeposited 

by a local mass flow. Pyrrhotite is common in the magnetic fraction of sediment samples from this core (Karlin 

and Zierenberg, chap. 7, this volume). 

Local relative uplift is documented in core L1-86-33G, 

where an older clay unit has been exposed, probably 

through mass wasting from the edge of a sediment section 

from the trough floor uplifted during volcanic intrusion 

(Morton and Fox, chap. 2, this volume). This clay is markedly stiffer than in the other cores, and the oxidized zone 

at the top is thinner and of a different hue; both characteristics suggest different physical properties of this clay relative to the "normal." 

The L3-86-NC core logs are based on unpublished descriptions provided by T. Nelson (app. B). Two of the cores 

(L3-86-4G and L3-86-5G), from the northern edge of the 

NESCA area, are dominantly mud with little internal structure or lithologic variation. The other two cores (L3-86-

8G and L3-86-9G) are from the area between NESCA and 

SESCA (fig. 6.2) near core L1-86-27G, which terminated 

in turbidite sand beds with postdepositional sulfide-mineral precipitation; both of these cores have substantial sections of turbidite units but without signs of mineralization. 

The cores taken in 1989 from the Canadian research 

vessel Tully were split onboard during the cruise. Several 

months later, the cores were shipped to Menlo Park where 

we attempted to describe them using the same criteria used 

for the Ll- and L2-86-NC cores; X-radiographs were taken 

at this time. The core logs in appendix B show that we 

were unable to recognize the range of mud colors and textural differences documented in the detailed logs for the 

L1- and L2-86-NC cores, but the X-radiographs generally 

confirmed the details observed when the cores were first 

split and described (unpublished cruise logbook). 
0 

25 

50 

75 

A L 1-86-25G 

0-99 em 

100-

em 

SEDIMENT FILL OF ESCANABA TROUGH 

B L1-86-27G 

0-87 em 

c L2-86-9G 

0-114 em 140-200 em 

103 

Figure 6.11. Photographs of selected core intervals: A, Core Ll-86-25G depicting silty mud turbidites (darker color) overlying bioturbated 

mud. B, Core L1-86-27G showing interbedded silty turbidite units overlying a 12-cm-thick, bedded sand sequence that shows reddish 

stained horizons indicative of mineralization. C, Core L2-86-9G showing metalliferous reddish and chocolate brown mud overlying black 

sulfide talus (see fig. 6.12); lower part of core is bluish chloritized unit. See figures 6.2 and 6.9 for locations. 
104 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

L1-86-27G L2-86-9G 

55-87 em .~46-99 ~!!! .....L ~-~c=" 

Figure 6.12. X-radiographs of selected core intervals. A, Mud-clast unit in core L2-86-2G. B, Mineralization within turbidite sand beds at 

base of core Ll-86-27G. C, Sulfide talus in core L2-86-9G. See text and Zierenberg and Shanks (chap. 14, this volume) for details. 
SEDIMENT FILL OF ESCANABA TROUGH 105 

Table 6.5. Summary of light-mineral grain counts. 

Area -------------------- NESCA Other SESCA 

Cruise ------------------ TUL89D L2-86 TUL89D TUL89D Ll-86 Ll-86 Ll-86 Ll-86 Ll-86 L2-86 DSDP35 

Samplea ---------------- 60-25 270-29 7P-24 7P-240 70-211 80-52 8G-175 25G-48 260-40 2G-72 Mean StD 14-40 

Grains counted 297 393 300 298 295 403 322 356 396 349 399 

Summary percent: 

Quartz, chert, etc. 13 6 22 37 11 10 7 22 25 29 18 10 33 

Feldspar 28 12 31 34 9 35 36 30 24 28 27 9 52 

Rock fragments 42 65 26 18 23 34 42 29 23 29 33 13 6 

Mica 15 13 15 7 44 14 7 11 21 8 16 10 7 

Opaques 2 3 5 3 7 6 7 6 6 5 5 2 <.5 

Others 1 1 2 6 <.5 <.5 1 1 2 1 2 3 

Ternary ratio: 

Quartz 15 7 28 42 27 13 8 27 35 35 24 11 36 

Feldspar 34 14 39 39 23 45 42 37 33 33 34 9 57 

Lithics 51 78 33 20 50 42 49 36 32 33 42 15 6 

asample identification is abbreviated to core number and upper limit of sample interval. 

The RN Tully cores appear to consist predominantly 

of laminated turbidite mud units with silty interbeds (found 

in all cores except TUL-89-1P) and of mass-wasted sediment exhibiting steeply inclined bedding and mud-clast 

intervals (cores TUL-89-20, 4G, and 7P). 

COARSE-FRACTION MINERALOGY 

A summary of the mineralogy of the surficial cores, 

presented in tables 6.5 and 6.6, is excerpted from a more 

detailed analysis in Wong (1989). The subsample nomenclature follows the scheme YY-CC-DD, where YY is L1 

for cruise L1-86-NC, L2 for L2-86-NC, and TD for TUL89D; CC is the core number; and DD is the core depth 

(em). For example, L2-2G-72 is from 72 em depth in core 

2G from cruise L2-86-NC. 

A qualitative grain-size relation was determined for the 

30- to 63-J..Lm and greater than 63-J..Lm fraction without considering the clay fraction. The resultant dominant "grain 

size" ranged from silt to medium sand (table 6.4 ). 

The heavy-mineral yield of the 63- to 250-J..Lm fraction 

ranges from 1.9 to 3.8 percent by weight for those samples 

that do not have an abundance of oxide or sulfide particles. 

In the few samples that are enriched in opaque minerals, 

the heavy-mineral content is slightly greater. In contrast, 

the sulfide-mineralized sand unit (L1-27G-64) contains 50 

percent heavy mineral grains. 

Mica minerals, opaque grains, and volcanic rock fragments occur in both light and heavy mineral fractions. 

Chlorite is the primary mica in both fractions with lesser 

amounts of muscovite and biotite; phlogopite is a trace 

component. Except for the finer grained samples (such as 

TD-9T-12, L1-7G-211) and the greater than 250-J..Lm fraction, micas make up no more than 10 percent of the heavy 

fraction and 20 percent of the light fraction. 

Opaque grains (undifferentiated) make up as much as 

7 percent of the light fraction and, except for L2-7G-211, 

10 to 26 percent of the heavy fraction (tables 6.5 and 6.6). 

Opaque grains in the light fraction consist of thick books 

of mica, extremely fine-grained clay or oxide, or sulfide 

particles apparently buoyed by attachment to lighter grains. 

In the heavy fraction, the opaque grains include mica, oxide, and sulfide particles. 

Volcanic rock fragments consisting of plagioclase, 

clinopyroxene, and (or) opaque groundmass (oxide grains 

or opaque alteration product) occur in both the light and 

heavy fraction depending on the mineral composition; more 

plagioclase lifts the grain into the light fraction, more oxide components drop it to the heavy fraction. 

Light Fraction 

In addition to the grain types noted above, the light fraction includes feldspar minerals, quartz, other lithic fragments, 

and some nonmineralic material. Quartz generally occurs as 

angular to subangular monocrystalline and polycrystalline 

grains. Inclusion-bearing, undulose, and unstrained quartz 

grains are present but not separately tabulated. 

Plagioclase is the primary feldspar; rare microcline 

grains are present in several samples. The feldspar grains 

include both pristine fragments and altered fragments; 

some grains bear opaque inclusions and others are 

chloritized. 

Besides volcanic rock fragments, several types of lithic 

components were distinguished in the light fraction; their 

abundances are detailed elsewhere (Wong, 1989). Many 

samples contain homogeneous clay fragments that are 
106 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

Table 6.6. Summary and selected ratios of heavy-mineral grain counts. 

Area ------------------------------- NESCA Other SESCA 

Cruise ----------------------------- TUL89D TUL89D TUL89D L 1-86 TUL89D TUL89D TUL89D TUL89D Ll-86 Ll-86 Ll-86 Ll-86 Ll-86 L2-86 

Samplea -o------------------------- 2G-94 6G-25 9T-12 27G-29 7P-8 7P-24 

Wt. pet. heavy (63-250jlm) -- 2.9 1.5 3.1 3.1 

Grains counted 401 388 329 416 391 355 

Summary percent: 

Amph, nonmetamorphic 18 17 5 14 16 17 

Amph, metamorphic 15 17 3 I4 18 I2 

Orthopyroxene 4 5 1 7 6 6 

Clinopyroxene 18 23 3 20 24 25 

Epidote 7 6 3 7 4 5 

Mica 14 11 70 7 7 4 

Rock fragments 7 8 5 I2 8 11 

Opaques I2 10 10 I6 15 I8 

Others 4 2 1 4 1 2 

Ternary ratios: 

Nonopaques 76 80 51 71 75 70 

Opaques 14 II 33 I7 I7 19 

Rock fragments 10 9 16 13 9 12 

Amphibole 53 50 56 44 51 45 

Epidote 12 9 19 12 5 8 

Pyroxene 35 41 25 44 44 47 

Binary ratios: 

pyroxene/amphibole 0.66 0.83 0.44 0.98 0.87 1.06 

meta -amphibole/amphibole .46 .50 .4I .49 .53 .42 

orthopyroxene/pyroxene .16 .18 .17 .26 .19 .20 

a sample identification abbreviated to core number and upper limit of sample interval. 

subrounded (fecal?) pellets or ragged crusts (for example, 

Ll-270-29 or TD-60-25). Quartzofeldspathic sedimentary, metamorphic, and plutonic grains are scattered 

throughout. A few samples (such as Ll-250-48) host rare 

manganese(?)-coated clay nodules. 

Volcanic-glass fragments range from colorless or 

brown isotropic grains to partly devitrified or altered to 

clay minerals. In none of the samples examined (63- to 

250-J..Lm size fraction) is the glass as abundant as in the 

samples examined by Karlin and Lyle ( 1986, p. 38), who 

reported as much as 35 percent glass in one sample. Glass 

globules and fragments of glass crust were noted in several greater than 250-J..Lm samples, some with bluish opaline coating (for example, TD-7P-8). 

Nonmineralogic items in the light fraction include 

hydrocarbon blebs, sponge parts, plant and wood fragments, and tests of foraminifers, radiolarians, and diatoms. 

A few carbonate fragments include biogenic clasts and replaced feldspar(?) grains. 

A ternary plot of the quartz, feldspar, and lithic grains 

demonstrates that most of the Escanaba Trough samples 

would qualify as lithic arenites or feldspathic litharenites 

(fig. 6.13). The most quartz-rich sample is TD-7P-240, 

from a disturbed sandy lens near the bottom of this core, 

which was collected adjacent to a volcanic edifice near latitude 40°53' N. Sample Ll-270-29 is lithic rich, probably 

7P-42 7P-240 7G-211 8G-52 8G-175 25G-48 26G-40 2G-72 Mean StD 

3.8 1.4 1.9 1.1 3.6 2.2 4.7 

362 344 377 356 424 411 414 416 

16 16 5 15 19 12 13 11 14 4 

12 I6 8 18 13 15 12 9 13 4 

4 8 3 5 6 6 4 4 5 2 

22 19 22 26 13 26 22 21 20 6 

5 6 2 5 7 3 7 3 5 2 

3 4 2 3 3 3 9 4 10 17 

10 12 18 13 9 IO 11 19 11 4 

24 I7 38 13 26 23 I9 26 I9 8 

3 2 I 3 4 2 3 2 2 1 

65 69 42 73 64 66 67 53 66 10 

25 18 39 13 27 24 21 27 22 

10 13 18 14 9 10 12 20 I3 

47 49 33 48 56 43 43 42 47 

9 9 6 7 I2 5 I2 7 9 

45 42 6I 45 33 51 45 51 43 

0.96 0.87 1.88 0.94 0.58 l.I9 1.04 1.20 1.0 

.43 .50 .58 .54 .40 .54 .50 .45 .5 

.I7 .29 .11 .I6 .30 .20 .15 .16 .2 

reflecting the higher content of clay particles. Except for 

the deepest sample (V381, fig. 6.13), the sands from various depths of hole DSDP 35 are much richer in feldspar 

than the surficial Escanaba Trough sediment samples 

(Vallier, 1970). 

Heavy Fraction 

Nonopaque grains make up the bulk of the heavy fraction of the sand samples; except for Ll-70-211, opaque 

grains constitute less than 26 percent of the total, and rock 

fragments make up less than 20 percent (table 6.4). 

Clinopyroxene (mean 20 percent), ordinary green and 

brown hornblende, and metamorphic amphibole (mean 13 

percent; mainly blue-green) are the main nonopaque heavy 

minerals (table 6.6). Epidote and hypersthene each comprise about 5 percent of the grain population. Actinolitetremolite, zircon, apatite, garnet, and sphene amount to less 

than 5 percent of the grains in most samples. Trace 

amounts of basaltic hornblende, serpentinite, glaucophane, 

tourmaline, and barite are present. 

A ternary plot of the major components amphibole, 

epidote, and pyroxene confirms that these samples are epidote-poor with roughly equal amounts of total amphibole 

and total pyroxene. The pyroxene-to-amphibole ratio, 

which is a diagnostic value of different sedimentary prov8 

3 

6 

4 

8 

0.3 

.1 

.I 
SEDIMENT FILL OF ESCANABA TROUGH 107 

enances along the Oregon-California coast (Kulm and others, 1968; Whetten and others, 1969; Scheidegger and others, 1973; Wong and Klise, 1986), ranges between 0.4 and 

1.2, with most about 1.0 (fig. 6.14). Orthopyroxene minerals, mostly hypersthene, make up less than 30 percent of 

the pyroxene population (fig. 6.15). 

Altered and volcanic rock fragments are similar to 

those in the light fraction. The rare metamorphic grains 

include amphibolite and albite-epidote greenschist. 

There is rare evidence of dissolution in the heavy minerals of the surficial cores, in contrast to reports of some 

samples from DSDP Site 35 (Vallier and others, 1973). A 

few of the pyroxene and amphibole grains exhibit coxcomb 

morphology, but none of the garnets are etched to cubic 

outlines. 

Sulfide minerals dominate the heavy fraction in sample 

L1-270-64, a sand layer in a generally hydrothermally 

altered core (Morton and others, 1987b). X-ray diffraction 

and scanning electron microscope investigation identify the 

sulfide component to include pyrrhotite, sphalerite, and 

pyrite. Barite is common in this sample and was noted in 

several other cores (Wong, 1989). Sample L1-270-72 consists of yellowish to reddish-yellow oxidized grains in both 

light and heavy fractions. Tests filled with opaque minerals were noted in sample L2-20-72. 

Pleistocene(?) Samples 

The characteristics of known Pleistocene turbidite sequences in Escanaba Trough might bear on the problem of 

provenance, but unfortunately only one surficial core (L2-

86-2G) is firmly interpreted to contain a Pleistocene section (Karlin, 1989). Three other turbidite units from cores 

in which the Pleistocene-Holocene boundary cannot be 

QUARTZ 

EXPLANATION 

D DSDP-35 

¢ NESCA 

E9SESCA 

6. Other Escanaba 

Trough samples 

V102 

D 

V6 

D 

FELDSPAR 

V1 

D 

26G-40 2G-72 

7P-24 6. 

BG-52 

B~ ¢ 6G-25 

ESBG-175 6.27G-29 

LITHICS 

Figure 6.13. Ternary plot of quartz, feldspar, and lithic data (Folk, 1980) from this study and DSDP Site 35 samples (Wxxx, this study; 

Vxxx Vallier, 1970; where xxx is depth in meters below sea floor). Surficial core samples annotated by core number and core depth 

(em); see table 6.5. 
108 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

determined are of probable Pleistocene age (all are from 

greater than 150 em core depth). Sample TD-7P-240 is 

enriched in quartz and hypersthene. Sample Ll-7G-211 

from SESCA is enriched in light lithic fragments (mostly 

clay pellets), has the fewest nonopaque heavy mineral 

grains, and is richest in pyroxenes. Sample Ll-8G-175 

from SESCA is also enriched in light lithic fragments but 

has little quartz and the least amount of pyroxenes. Sample 

L2-2G-72 is unremarkable except for a slightly high pyroxene abundance. If Ll-8G-175 is excluded, the increased 

pyroxene abundance seems to be the only common thread. 

FINE-FRACTION MINERALOGY 

Bulk mineralogy was determined by X-ray diffraction 

for 67 samples of mud and sandy mud ftom both the sediment cores and additional material recovered during rock 

EXPLANATION 

D DSDP-.35 

0 NESCA 

e SESCA 

6. other samples from 

Escanaba Trough 

• other studies 

EPIDOTE 

AMPHIBOLE 

91-12 

1. 6G-25 

2. BG-52 

.3. 25G-48 

4. 26G-40 

5. 27G-29 

6. 7P-8 

7. 7P-24 

8. 7P-42 

9. 7P-240 

dredging. Sediment that has not been hydrothermally altered has very consistent bulk mineralogy regardless of 

grain size. Quartz is the dominant mineral in all samples 

and is followed in abundance by feldspar, predominantly 

plagioclase. Mica is more abundant than chlorite in most 

samples, and many samples contain trace amounts of amphibole. No carbonate or zeolite minerals were detected. 

Clay-fraction smear slides were prepared for 33 

samples. The finest clay fraction, which is taken from the 

uppermost layer of centrifuged unaltered sediment, is 

green-brown to red-brown smectite. The smectite is poorly 

crystallized and gave broad diffraction peaks at approximately 1.25 nm that expanded to 1.7 to 1.8 nm after 

glycolation and collapsed to approximately 1 nm after heating to 400°C. This smectite is easily distinguished from 

hydrothermal smectite, which was generally dark green and 

had much higher X-ray diffraction peak-to-background 

ratios and narrower diffraction peaks. 

AMPHIBOLE 

PYROXENE PYROXENE 

Figure 6.14. Ternary plot of total amphibole, epidote, and pyroxene grains in heavy mineral suites from this and previous studies. Annotation 

of samples: ET, flanks of Escanaba Trough (Fowler and Kulm, 1970); V48, V90, V301, V374 from DSDP Site 35 (Vallier and others, 1973); 

A38, A371, A754 from DSDP Site 174A on Astoria fan (Scheidegger and others, 1973); Rogue River (Kulm and others, 1968); Columbia 

River (Whetten and others, 1969); Eel and Klamath Rivers (Wong and Klise, 1986); ERBn, ERBs, northern and southern offshore Eel River 

basin (unpub. data). 
SEDIMENT FILL OF ESCANABA TROUGH 109 

The coarser clay fraction is generally brown to gray 

and contains variable amounts of chlorite, illite, and 

smectite and varying but minor amounts of quartz. Kaolinite probably occurs as a minor phase in the coarser clay 

fraction, but the presence of kaolinite is generally masked 

by chlorite, which occurs in greater abundance. Chlorite, 

illite, and quartz in the coarsest clay fraction are detrital 

grains that also occur in the coarser fractions of the sediment. The observed mineralogy is consistent with the findings of Karlin (1980), who demonstrated that fine-grained 

sediment off the southern Oregon coast is dominantly chlorite and illite derived from the California Coast Range with 

less abundant smectite derived in part from the Columbia 

River drainage. The minor amount of smectite that forms 

the finest grain size fraction of the Escanaba Trough 

samples is interpreted to include low-temperature diagenetic alteration products of basaltic glass and unstable minerals (Zierenberg and Shanks, chap. 14, this volume). 

PORE WATER 

Pore fluid was extracted from four samples to look for 

evidence of hydrothermal fluid flow through the shallow 

sediment section. Pore-fluid samples from L2-86-NC cores 

20, 40, and 280 come from sediment intervals that show 

0.5 I 

0.4 ~ 

LaJ 

z ._A38 

LLJ & ERBs X ._ERBn 0 

a:= BG-175 

>- 0.3 ~ .,Rogue a.. ED ~7P-240 

' LLJ 6,27G-29 z 

LLJ .,Klamath X A371 

0 ... 

no evidence of hydrothermal alteration, and the pore fluids are nearly identical to seawater in composition (table 

6.7). The sediment interval sampled in core L2-86-90 is 

highly altered by high-temperature magnesium metasomatism (Zierenberg and Shanks, chap. 14, this volume). Magnesium-rich chlorite has replaced the normal sediment. 

Minor amounts of talc, gypsum, and pyrite accompany the 

chlorite. Pore fluid from this interval has a lower pH, is 

depleted in magnesium, and is enriched in calcium and sulfate relative to seawater or pore fluids from the other 

samples (table 6.7; fig. 6.16). 

High-temperature interaction of seawater with basalt 

or sediment results in the depletion of magnesium in the 

fluid phase by formation of secondary magnesium silicates 

(Bischoff and Dickson, 1975). Replacement of 

hemipelagic sediment in core L2-86-90 by magnesium-rich 

smectite is evidence of the transfer of magnesium from seawater to the alteration products during high-temperature 

interaction with hydrothermal fluids. The pore waters from 

this core retain a signature of this process as recorded by 

reduced magnesium concentrations. 

Heating of seawater also leads to precipitation of anhydrite because of the retrograde solubility of this mineral 

at high temperature (Bischoff and Seyfried, 1978). Anhydrite hydrates to gypsum and redissolves in seawater at 

ambient sea-floor temperature. Minor amounts of gypsum, 

I ' I I 

0 vgo Columbia_. 

-

-

- 0 v4a 

7P-24 a:= 0.2 ~ 7p_8 ~ e25G-48 - >- a.. 9T-12 6G-250'A 7P-42 0 pV374 0 0 ~ e2G-72 :I: 

1- EXPLANATION - a:= 2G-94 / ~6G-40 0 8G-52 e7G-211 0 DSDP-35 

0.1 - 0 NESCA - e SESCA 

.,Eel 6. Other samples from - - Escanaba Trough 

... ET • ... A7~4 A Other studies 

0 . . I I I I I 

0 1 2 3 

PYROXENE/ AMPHIBOLE 

Figure 6.15. Plot of ratios orthopyroxene/pyroxene versus pyroxene/amphibole. Annotation and data sources as for fig. 6.14. 
110 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

Table 6. 7. Pore-water chemistry of surficial core samples from cruise 

L2-86-NC. 

[Values in ppm; C-r=H2C03+HCo3- +C03

2- in mmolelkg; na, not analyzed] 

2G 4G 9G 28G Seawater 

96-101 em 138-145 em 115-122 em 119-126 em 

Cl ---------- 19,300 19,300 19,300 19,200 19,350 

N a---------- 10,800 10,700 10,500 10,600 10,760 

K ----------- 542 435 423 426 399 

Mg --------- 1,220 1,260 1,150 1,270 1,290 

Ca ---------- 362 394 1,320 385 411 

504---------- 2,562 2,560 3,860 2,375 2,710 

Si02 --------- 9.6 7.7 8.9 15.2 

Fe ---------- <1 <1 <1 <1 <1 

Mn --------- 1.0 1.6 3.0 4.5 <1 

CT------------ na 3.8 1.6 6.3 2.3 

pH --------- 8.2 7.8 6.9 7.9 8.2 

which are interpreted to have formed by hydration of anhydrite, are observed in the altered sediment (Zierenberg 

and Shanks, chap. 14, this volume). Calcium and sulfate 

enrichments are not, however, stoichiometrically equivalent as would be predicted from the dissolution of anhydrite alone. Some of the excess calcium beyond that 

available by dissolution of anhydrite can be attributed to 

cation exchange during magnesium metasomatism of the 

sediment. The remaining excess calcium in the pore water 

could signal the presence of residual hydrothermal fluid, 

but this hypothesis is not supported by the observed concentrations of magnesium and potassium. Alternatively, 

sulfate could have been depleted by sulfate reduction. The 

c( 500 

w~ 

~ 2G II I-<( 9G <(a: 

~(!J 400 

<(Q W....J 

cnS2 4G []] 28G 

Oa:: 300 

1--w 

wa... 

~en 200 I-I- <(z ....Jw W....J II:<( 100 I-> 

Z:::> 

wo 

~w 0 I-o:::l 

a:~ 

Zz w_ -100 

-200 

K Mg 

altered sediment contains minor amounts of pyrite that apparently includes some sulfur formed by sulfate reduction 

(Zierenberg and Shanks, chap. 14, this volume). However, 

not only is the amount of pyrite in the core very small, but 

it is interpreted to have formed at high temperature during 

alteration of the sediment by inorganic sulfate reduction. 

Low-temperature biogenic sulfate reduction is rejected as 

a means of sulfate depletion because of the low content of 

organic matter in the altered sediment (K venvolden and 

others, chap. 15, this volume) and the absence of biogenic 

pyrite in the core. The source of the elevated calcium in 

the pore fluid, therefore, remains to be explained. 

The lower pH in the core L2-86-9G pore fluid could 

reflect the presence of residual low-pH hydrothermal fluid, 

hydrogen ions released during formation of magnesian 

chlorite, hydrogen ions released during minor oxidation of 

accessory pyrite, or a combination of these processes. 

DISCUSSION 

The composition of the core samples presented in this 

study provides good documentation of the interaction between hydrothermal fluids, volcanism, and the sediments 

filling Escanaba Trough. Major questions that we hoped 

to address from examination of these samples were ( 1) how 

long the sediment fill has been "available" to interact with 

hydrothermal fluids associated with the volcanic intrusions 

in the zones of sea-floor mineralization and (2) what are 

the products of this interaction; that is, do the mineral 

Ca so4 

CHEMICAL SPECIES 

Figure 6.16. Pore-water chemistry for selected surficial cores from cruise L2-86-NC in Escanaba Trough; see 

table 6. 7 and figure 6.10 for location. 
SEDIMENT FILL OF ESCANABA TROUGH 111 

deposits reflect the composition of the sediments, and are 

there sedimentary products from the mineralization zones 

that are now part of the sediment fill? The data presented 

in the preceding section answer the second question fairly 

clearly, but the timing aspect has not been addressed. As 

reported by McManus and others ( 1970) and Vallier and 

others (1973), the lower part of the sequence cored at Site 

35 is barren of microfossils or other components that could 

give a direct age determination. Therefore, we will first 

discuss the nature of the sedimentologic and tectonic constraints on the timing and rate of sedimentation in Escanaba 

Trough and relate these to probable changes in source areas 

of the sediment. 

SEDIMENTATION RATES 

To better understand the time frame for deposition of 

the sediment fill of Escanaba Trough, we can consider the 

amount of extension and possible rates of accumulation that 

might be reasonable for the late Pleistocene. It is unlikely 

that the trough would fill during the last lowstand of sea 

level in the Pleistocene because 500 m of sediment accumulation would correspond to a rate of about 25 m/1,000 

yr. This rate is more than double that found by DSDP drilling on the Mississippi fan, which is fed by a very large, 

nearby river (Kohl and others, 1985). On the other extreme, it would take only about 300,000 years to form the 

entire width of Escanaba Trough at a spreading rate of 2.4 

cm/yr and assuming that only 80 percent of the total separation occurs between the innermost major faults forming 

the bounding ridges of the trough. Even if the trough existed in its present form for the entire 300,000 years (an 

impossibility), the sedimentation rate implied is nearly an 

order of magnitude higher than the Holocene rate. The 

Holocene sedimentation rates derived from Karlin and Lyle 

(1986) and Karlin and Zierenberg (chap. 7, this volume) 

are generally less than 20 cm/1 ,000 yr; figure 6.17 shows 

the sediment accumulation since 11,000 years ago for the 

cores based on their magnetic-susceptibility profiles. At 

the Holocene accumulation rate, it would take more than 

2,500,000 years to fill Escanaba Trough, another unacceptable proposal following the arguments above. 

The arguments above suggest that it is reasonable to 

consider that the sediment fill of Escanaba Trough was deposited over an interval greater than 30,000 years but certainly less than 300,000 years. The character of many of 

the seismic-reflection profiles from Escanaba Trough show 

two intervals with strong parallel reflectors (Moore, 1970; 

Clague and Holmes, 1987; Morton and others, 1987a). 

These intervals might correspond to periods of very rapid 

input of turbidite fill during the lower sea-level stands. The 

interval with interbedded sand within the DSDP Site 35 

cores (fig. 6.4) roughly corresponds to the shallower of the 

two intervals with laterally persistent reflecting horizons. 

The sandy turbidite beds sampled at Site 35 are relatively 

fine grained with common mud interbeds. This lithology 

is consistent with the seismic-reflection character observed 

for overbank and distal depositional elements on submarine fans (not the channel-fill and lobe elements that are 

the general sites of coarse-sediment turbidite deposition; 

see Mutti and Normark, 1991; Normark and others, 1993). 

Assuming that both intervals of prominent reflectors 

represent periods of increased accumulation of coarser 

(nonmud) material, several possible causes can be suggested: (1) lowered sea level, (2) uplift of source area 

(relative sea-level change), (3) new sediment sources as a 

result of tectonic activity either onshore or along the 

40° 

50' 

40° 

40' 

~00? 

,._ 

r 

I' 

I' 

i 

/1 

'I 

I 

I 

I 

I 

I 

I 

L 

., 80 

~---------, i 

I I 

I I 

I I 

iNESCA i , 

•Hro : • I ; 

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I I L _________ : ; 

I 

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-----------1 

EXPLANATION 

0 

• L6-85-NC 

• L 1-86-NC o L2-86-NC * L3-86-NC 

~ TUL-89D 

10 

Contour intervol500 meters 

15 KILOMETER 

40° 

30'~--~--------------~------------~----~ 

Figure 6.17. Compilation of depth (em) to 11 ka boundary for 

surficial cores from Escanaba Trough. L1-86-NC and L2-86-NC 

values determined by Karlin ( 1989). L6-85-NC extracted by Karlin's 

1989 criteria from data in Karlin and Lyle (1986). Bathymetry from 

Wilde and others ( 1978). Hachures indicate area of closed low. 
112 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

continental rise and lower slope, and ( 4) major change in 

the depth or configuration of Escanaba Trough itself. The 

coarse fraction compositions as reported by McManus and 

others (1970), Vallier (1970), and Vallier and others (1973) 

for Site 35 and in this paper for the youngest sediment, appear to rule out any major change in sources. Given the 

short period under discussion (less than 300,000 years), tectonic changes in source rates appear to be insufficient to 

explain an abrupt change in sediment input. A non-steadystate model for spreading-ridge morphology (Kappel, 1985, 

Kappel and Ryan, 1986) could explain either or both of the 

intervals of stronger reflectors. In such a model, periods 

of ridge volcanism and crustal accretion alternating with 

periods of extension lead to a relatively rapid collapse of 

the ridge axis, forming a major graben. There are no data 

to constrain the timing of any such non-steady-state episodes in the Escanaba Trough area, so this process (if operative) provides no control on the history of filling of the 

trough. 

The simplest hypothesis of the four is to relate low 

eustatic sea-level intervals with the intervals of coarser 

sediment in the Escanaba Trough fill. In this scenario, the 

upper seismic-reflection interval would correspond to the 

last sea-level lowstand, with most of the section from approximately 150 m to the sea floor being deposited in approximately 20,000 years (from 30 to 11 ka; the timing for 

sea-level history used in this section follows Chappell and 

Shackleton (1986), who compare oxygen-isotope data in 

deep-marine sediment with dating from a sequence of uplifted marine terraces). The coarsest sediment might correspond to the period of rapid sea-level fall using the 

arguments of Mutti ( 1985), Mitchum ( 1985), and Vail 

(1987) about lowstand turbidite fans. The rate of sedimentation for the last lowstand would be approximately 7 to 8 

m/1,000 yr, and the entire Escanaba Trough fill would require only 60,000 to 70,000 years at this rate. The previous lowstand of comparable magnitude to the last was from 

170 to 130 ka; however, because a section of acoustically 

transparent (non turbidite) sediment lies below the lower interval, the implication is that the Escanaba Trough fill is 

about 200 ka old at the base. This should be considered 

the upper age limit for the axial zone. Sea-level history 

based on uplifted reef sequences suggests that the lower 

interval of stronger reflectors might relate to a 75- to 80-

m lowstand at about 70 ka (see discussion in Chappell and 

Shackleton, 1986). This would give an age of no earlier 

than 100 ka for Escanaba Trough itself and an average sedimentation rate of 5 m/1,000 yr; both values are quite reasonable within the limits discussed earlier. Also, the much 

lower sea level during the last lowering would be expected 

to give the higher accumulation rate of 7 to 8 m/1,000 yr. 

In any of the above scenarios, the deduced Pleistocene 

accumulation rate far exceeds that observed for the Holocene (fig. 6.17; Karlin and Zierenberg, chap. 7, this volume). An order-of-magnitude change in turbidite 

sedimentation rates between the Pleistocene and Holocene 

has been observed elsewhere in the northeast Pacific 

Ocean. The proximal overbank (levee) area on Monterey 

fan about 400 km south of Escanaba Trough aggraded at a 

rate of about 3 m/1,000 yr during the late Pleistocene 

(Normark, unpublished data); during the Holocene, the rate 

determined for several core locations on the same levee 

area ranges from 25 to 35 emil ,000 yr (Brunner and 

Normark, 1985). The differences in rates for both the Holocene and late Pleistocene between Escanaba Trough and 

Monterey fan are consistent with the size of the probable 

source areas for turbidity-current transport. Davis and 

Becker (1989) suggested that regional sedimentation in 

Escanaba Trough occurred during a tectonically and volcanically quiet period. This conclusion must be regarded 

with caution considering that sedimentation rates might 

have been extremely high for short periods of time during 

the latest Pleistocene. 

SEDIMENT SOURCES 

The composition of the sediment from the surficial 

cores in Escanaba Trough differs little in major light and 

heavy mineral components (in the 63- to 250-~m size 

range) among the sampling sites (figs. 6.13 to 6.15). Some 

minor differences appear in the minor and trace minerals, 

such as opaque grains, indeterminate rock fragments, and 

the greater than 250-~m fraction. Except for the highly 

altered sediment recovered in core L1-86-27G, barite is a 

trace component in samples obtained near areas with active hydrothermal venting. 

The nearest source of terrigenous debris to Escanaba 

Trough is the northern California continental margin, where 

the Eel and Klamath rivers provide substantial coarse debris (fig. 6.1 ). However, the present morphology of the 

continental rise along the eastern margin of the Juan de 

Fuca plate would permit gravity-flow deposits from as far 

north as the Oregon-Washington shelf to reach Escanaba 

Trough. The heavy-mineral contributions of the major rivers (Columbia: Whetten and others, 1969; Rogue: Kulm 

and others, 1968; Klamath, Eel: Wong and Klise, 1986) and 

of the continental shelf (Scheidegger and others, 1973; F.L. 

Wong, unpub. data) along this margin are relatively well 

known. Previous studies of sediment from Escanaba 

Trough identified sources in the Columbia River and (or) 

Klamath Mountains regions (Vallier, 1970). Fowler and 

Kulm (1970) sampled uplifted terraces on either side of 

Escanaba Trough and, based on the abundant blue-green 

amphibole and low pyroxene-to-amphibole ratios, determined that the sediments were probably from the Klamath 

Mountains region (fig. 6.14). 

Vallier (1970) and Vallier and others (1973) in their 

examination of the Site 35 core attributed an earlier source 

of Escanaba Trough sediment (more than 301 m below the 
SEDIMENT FILL OF ESCANABA TROUGH 113 

sea floor) to the Klamath region alone and the younger 

sediment to a mixed Columbia and Klamath source based 

on the depletion of pyroxene downcore. Vallier subsequently expressed some concern regarding diagenetic processes for the mineral depletion in an environment where 

a great thickness of sediment is being "cooked" (T.L. 

Vallier, oral commun., 1989). Scheidegger and others 

(1973) examined a similar mineralogic change in DSDP 

core 174A from Astoria fan, at a site well away from a 

ridge-crest heat source. About 370 m below the sea floor 

at this drill site, an amphibole-rich assemblage lower in the 

hole gives way upward to a mineralogy diagnostic of the 

modern upper Columbia River basin. On the basis of altered hypersthene grains, they concluded that dissolution, 

though present, has not persisted long enough to remove 

any substantial amount of pyroxenes in the lower part of 

the cored sequence. For Astoria fan, Scheidegger and others (1973) included the Vancouver Island and British Columbia region as an alternate source of amphibole-rich 

sediment, although this potential source is sensitive to the 

configuration of the Juan de Fuca plate at 2 Ma. 

The abundance of clinopyroxene readily identifies a 

Columbia River factor in the Escanaba Trough samples 

available for this study, but pyroxenes, specifically 

orthopyroxene, are less abundant than is typical for the 

Columbia River assemblage. The metamorphic amphibole minerals in the surficial cores require contributions 

from rivers draining the Klamath Mountains of southern 

Oregon and northern California: the Rogue, Klamath, and 

Eel. This mixed sediment source is evident in the AEP ternary diagram and the pyroxene ratios (figs. 6.14 and 6.15) 

because the Escanaba Trough samples plot between those 

of the Columbia River and of the Rogue, Klamath, and Eel 

rivers. DSDP samples similar to Escanaba Trough surficial 

sediment were recovered from the Astoria fan Site 174A 

at 38 m and 371 m depth and from the Escanaba Trough 

Site 35 at 301 m (note: all these samples lie above the 

mineralogic change mentioned above). 

If the turbidite beds sampled for this study are Holocene in age, the Columbia and Klamath regions may be 

less likely primary sources because it is generally assumed 

that submarine-canyon gradients are smaller and sediment 

supply is less than during the last low sea-level stand. Ash 

derived from the eruption of Mount Mazama (6,900 yr ago) 

has been traced in turbidite deposits south of the Blanco 

Fracture Zone along the base of the continental slope via 

Astoria Channel (fig. 6.1) and onto the Tufts Abyssal Plain 

farther west (Nelson and others, 1968, 1988). This suggests that the Columbia River, and by inference the more 

adjacent Klamath region, could provide Holocene sediment 

to Escanaba Trough. Glass from Escanaba Trough samples, 

however, has been attributed to local volcanic activity 

(Karlin and Lyle, 1986). 

Without an external source, the Holocene turbidite 

beds in Escanaba Trough sampled in the surficial cores 

might be reworked from Pleistocene deposits by slumping 

or other mass-wasting processes (Karlin and Lyle, 1986). 

The one sample from the Pleistocene(?) terrace on the flank 

of Gorda Ridge has a strong affinity with the Klamath 

sources but none with the Columbia (fig. 6.14; Fowler and 

Kulm, 1970). More samples are needed from the flanks 

of southern Gorda Ridge to identify the possible Pleistocene deposits of Columbia or mixed Columbia-Klamath 

sources that might be supplying terrigenous material to 

Escanaba Trough during the Holocene. 

CONCLUSIONS 

Under any assumptions involving steady and continuing spreading along the southern Gorda Ridge during the 

Brunhes Normal-Polarity Chron (the only time control on 

the plate motions), the sediment fill of Escanaba Trough 

accumulated in less than 200,000 years. This limit is consistent with available biostratigraphic data from Site 35 (J. 

Barron, oral commun., 1989). Sediment components 

linked to the Columbia River and Klamath Mountain rivers indicate that a large area of the northeast Pacific margin is available as a sediment source. The turbidite fill is 

dominantly fine grained as observed in both Site 35 and 

surficial cores. The parallel-reflector, flat-lying internal 

structure of the fill observed in seismic-reflection profiles 

(Clague and Holmes, 1987; Morton and others, 1987a; 

Morton and Fox, chap. 2, this volume) is consistent with 

fine-grained distal-turbidite accumulation. Channel-fill and 

mounded-lobe reflector geometries with high-amplitude, 

discontinuous reflectors characteristic of coarse-grained 

facies are not observed. 

Accumulation rates as high as 8 m/1 ,000 yr of dominantly muddy sediment seals off hydrothermal circulation 

that normally cools oceanic crust. Ridge crest extension 

and localized basaltic intrusions and associated fault systems provide the only conduits to the sea floor and therefore control hydrothermal discharge. The muddy fill of 

Escanaba Trough provides relatively few bedding intervals 

for lateral migration of the fluids (compared to turbidite 

sequences with more coarse-grained facies) so that fluid 

discharge is concentrated at the site where cross-stratal 

permeability intersects coarse-grained sediment aquifers or 

the more permeable upper oceanic crust. 

The probable high sediment accumulation rate during 

the latest Pleistocene, as much as 40 times the Holocene 

rate, should have filled most of the topographic lows in the 

southern Escanaba Trough. The irregular, fine-scale morphology of the sea floor in the study areas (figs. 6.9 and 

6.1 0) might well be Holocene in age. The hydrothermal 

deposits and the fluid conduits are associated with these 

Holocene structures. Fluids contributing to the deposits, 

however, could be substantially older if they were generally prevented from reaching the sea floor during the 
114 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

intervals of most rapid sediment accumulation. The effective sealing of the system by the rapidly deposited, finegrained sediment allows for long periods of interaction 

between interstitial fluid and sediment that overlies hot oceanic crust. When the system is sealed from hydrothermal 

circulation, the oceanic crust cools by conductive heat 

transfer through the sediment cover. Volcanic intrusion into 

the sediment provides both a thermal and topographic focus for hydrothermal fluid flow. Faulting associated with 

uplift of the sediment over the intrusions provides a pathway for the hydrothermal fluids derived from a large volume of sediment. We envision that this interplay between 

sedimentation and tectonic-volcanic activity is responsible 

for the large-volume, focused discharge of hydrothermal 

fluid required to form the extensive sulfide deposits observed in Escanaba Trough. 

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and geochemical data of surface sediments from coastal northem California rivers: U.S. Geological Survey Open-File 

Report 86-574, 13 p. 

Zierenberg, R.A., and Shanks, W.C., III, Sediment alteration associated with massive sulfide formation in the Escanaba Trough, 

Gorda Ridge: The importance of seawater mixing and magnesium metasomatism, in Morton, J.L., Zierenberg, R.A., and 

Reiss, C.A., eds., Geologic, hydrothermal, and biologic studies at Escanaba Trough, Gorda Ridge, offshore northern 

California: U.S. Geological Survey Bulletin 2022, chapter 14 

(this volume). 
Appendix A: Lithologic summary logs for 17 cores recovered from Escanaba Trough at DSDP Site 35 

This section presents lithologic summary logs for 17 cores recovered from Escanaba Trough at Site 35 of the Deep Sea Drilling Program. The lithologic characters 

selected have been simplified to allow combination of the original shipboard descriptions for 28 sections with those done in 1988 (see text). A solid vertical line to the 

right of the core means the description used is by the U.S. Geological Survey; a dashed vertical line indicates the description is from McManus and others, 1970 (see fig. 

6.3). 

EXPLANATION 

DSDP Site 35 

~ 

~ 

~ . 

. 

~ 

~ 

Mud 

Silt 

Sand 

Mudclasts 

Calcareous 

mudclasts 

Carbonate 

clasts 

Void 

(J) 

a: 

w 

~ 

w 

~ 

35.1 

0 I; ; :: :; \ ; 'i II 

1 I 

2 I 

3 

4 I 

5---1 ~ II 

6 

7 

8 

9 

35.2 35.3 

0 I : ':; ::::: n II 0 

I 

0-8.2 m 

I 

1 I silty horizons, 1 

1 if present, not 

2 I identified in 2 

I DSDP log 

I ---~ sand present 

3 I 3 in core liner 

I 

4 4 

5 l::t''t\r;; ;;''; ,, '{;?:::?:;::r::::I:i I 5 

6 6 

4.76-9.0 m 

7 7 probable turbidites 

with homogenized 

silt/mud intervals 

8 8 

9 9 

~ 

0\ 

Cl 

tTl 

0 

r 

0 

Cl 

B 

::c: 

>-<: 

0 

~ 

0 

>-3 

::c: 

tTl 

~ 

s;: 

> r 

> z 

0 

t::l:l 

0 

r 

0 

Cl 

n 

Cl'.l 

>-3 

c 

0 

s 

Cl'.l 

~ tTl 

Cl'.l 

n 

> z 

> t::l:l 

> 

;j 

0 

c 

Cl 

~::c: 

Cl 

0 

~ 

0 

> 

~ 

8 

Cl 

tTl 
Appendix A: Lithologic summary logs for 17 cores recovered from Escanaba Trough at DSDP Site 35-Continued 

35.4 35.5 35.6 35.7 

0 X 0 0 0 

sandy turbidites 

1 1 1 1 I calcareous 

2 2-1 2 H I erosive base I'i'~If' ''' ''''''''''''.. ',,,,.,,,,,,,,,.,,,,,,,,,\"! 2 

I Ill• I calcareous 

calcareous en numerous tTl 

0 mud clasts ~ 3 r-iil"'

;~--1 '''''''''.· -I."--~ ,.,.,,,,,,,,,,,.,.,.I 3-1 3 3 tTl 

z 

...., 

~ CJ) 1 silty fraction t""' 

t""' a: 

w 4 4 4 4 0 I in mud "T] 

J- tTl w I en 

~ n 

I ;:t> 

5 5 5 5 z 

;:t> 

tx:l 

;:t> 

...., 

;::a::l 

6---l I 6 X 6 1 calcareous 6 0 

c I a 

:::c 

I 

I _ .. _ ~· -~·~· I 

7----i I 7 7 I 7 

8----l I 8 8 8 

9 9 9 9 

...... 

-...J 
Appendix A: Lithologic summary logs for 17 cores recovered from Escanaba Trough at DSDP Site 35-Continued -

00 

35.11 35.12 35.13 a 

35.8 tTl 

0 

0 0 0 0 t""' 

1<:::::::::::::: ::;:::::>II IO:Z::::::: :;::::-"11 '"' /II X 0 

a 

?5 1 carbonate 

1~~1 I calcareous mud, ::c: 

noncalcareous ~ 1 1 patches? 1 1 clasts 0 

:::0 ~:-:-~·;:·;~~::~:-:;:;:\t::·:·.:.--:::-::-~ 0 

>--3 

::c: 

tTl 

2 t 'fJiJc:]\}Jix 2 2 :::0 35.9 very calcareous 3:: 

> 0 IC:::::::::::::: :::::::::::>11 r 

slightly > z 

3 

.1 mud clasts 

3 3 0 

calcareous tc 

1 I 0 

~~ 1: X in mud (up to t""' (/) 1 em across) very calcareous 0 a: a 

w 4 4 4 n 1- C/) 

w 2 >--3 

~ I carbonate c:: 

I 0 

m I patches? 5 5 5 1 carbonate C/) 

1 patches 2:3 3 I 1 common tTl 

C/) 

n 

6---1 6 > z 

> 4 ..•. .;,;.; ~ .;;;; ;;;;;;: .·. calcareous tc 

slightly > 

very >--3 

mud clasts calcareous :::0 

7 7 calcareous 7 0 

c:: 

35.10 in mud (up to interbedded a 

mudstones ~::c: 

0 1 em across) I a IO:Z::::::::: :::::::::=-'"11 0 8 8 I carbonate 8 :::0 

0 

thin-bedded patches > 

I common :::0 

1 ---~ turbidites a (0.5-1.0cm 9 different- 9 9 a 

mud interbeds) tTl 

colored muds 
Appendix A: Lithologic summary logs for 17 cores recovered from Escanaba Trough at DSDP Site 35-Continued 

35.14 35.15 35.16 35.17 

0 I 0 . I 0 0 

trace 

I 

calcareous 

I strongly 

1 ---~carbonate 1 1 1 calcareous 

sli?chtly 

. cacareous 

2 [,,,,,,,,,,,,, ..... . ,.,.,.,.,., ..... ,.·.· ....... ,,.,,,.,,,,,,j 1

x clayey silt 

silty clay 2 I 2 J slightly 2 

calcareous 

nodules 3-l I 

CZl 

3 3 3 I 

ti1 

0 

slightly silty ~ ti1 

z 

~ 

4 

I 

4 

I silty 

4 4 ~ 

r 

(fJ r 

cc 0 

w '"I'l 

ti1 I- 5 I carbonate 5 I 5 5 CZl 

w n 

~ 1 patches? 

I ).> 

z 

I ).> 

ttl 

).> 

6 6 6~ I 6----i I ~ 

:;:zj 

0 

c::: 

a 

::t I I 

7----1 I 7 7 7 

8---1 I 8 8 8 

9 9 9 9 

--\0 
Appendix B: Lithologic summary logs for surficial cores from Escanaba Trough 

This section presents lithologic summary logs for surficial cores from A, U.S. Geological Survey cruise L6-85-NC constructed from data in Karlin and Lyle (1986; no 

description available for cores 4G and llG); B, C, D, USGS cruises Ll-, L2-, and L3-86-NC; and E, Geological Survey of Canada cruise TUL-89D (no description available for 

cores 5G and 9P). Core locations are shown in figures 6.2, 6.9, and 6.10; see table 6.3. Where applicable, the lithologic patterns used in the logs-for example, the symbols for 

void spaces, oxidized zone, turbidite mud, silt, and sand-are common to all core sets. The shading patterns, however, are unique within each set of cores, as shown by the 

accompanying explanations. HHC, interval sampled for heavy hydrocarbon analyses; LHC, interval sampled for light hydrocarbon analyses; PW, interval sampled for porefluid analyses. 

EXPLANATION 

L6-85-NC 

~ Oxidized zone 

D Turbidite mud 

5Y 4/2 olive-gray mud, slightly silty 

5Y 4/1.5 slightly silty mud, in between color 

5Y4/1 dark-gray, plastic, homogeneous clay 

t _!_ ~ j Calcareous, mottled clay 

[] . 

. 

A 

~ 

~ 

Sand 

Silt 

Void 

(/) 

a: 

w 

1--

w 

~ 

1--

z 

w 

(.) 

L6-85-1 G 

202cm 

irregular, thin 

silt layers 

L6-85-2G 

0 ----r-7'"""--r--r--.--. 

25 

50 

75 

100 

125 

150 

175 

200 

225 

241 em 

layer contains 

streaks and 

mottles of 

darker mud 

82-94 em 

black streaks 

common 

layer contains 

thin, irregular 

darker beds, 

locally highly 

mottled 

-

N 

0 

a 

tr:l 

0 

l' 

0 

a 

B 

::c: 

..-< 

t:l 

::::0 

0 

.....j 

::c: 

tr:l 

::::0 

~ 

> r 

> z 

t:l 

t::C 

8 

l' 

0 

a 

n 

en 

.....j 

e 

t:l 

s 

en 

~ tr:l 

en 

n 

> z 

> t::C 

> 

;d 

0 

e 

a 

~::c: 

a 

0 

::::0 

t:l 

> 

::::0 

a 

a 

tr:l 
Appendix B: Lithologic summary logs for surficial cores from Escanaba Trough-Continued 

L6-85-3G L6-85-5G L6-85-6G L6-85-7G 

0 1 .......... ; ....................... ,.,., ..... . ··.··.·.····.···.·.,., ... ·.·.·.·,,1 0 I 7 7 7 7 I 0 X 0 ~-- -1 ._-:- .. ~ ... --:-·. numerous black 

sand fragments 

25 -i•···········;·;·;··;·;· ';'{';';' ...... ; ..... ; ....... ; ...... ...... ! 25 - prominent dark band 25 - 25.20-188cm layer contains worm holes and 

numerous small black streaks common 

black streaks . . prominent dark band ·. . . .. · . silty layers as shown 

50 50 . ·.. 50 .~·--·: , . :· ·: ·•··. 50 

75. 

c.n 

62-84 em tTl 

0 

75 ---i •• I I 75 I n1• ~ tTl 

z 

...-3 

(/) ::!l 

a: 100 100 100 100 94-104 em r 

w lighter mottled clay r 

I- 110-116 em 0 

w "T:: 

~ silty zone zone of dark streaks 110-134 em tTl 

c.n 

I- 125 mottled layer 125 125 125 lighter mottled clay n 

z > 

w z 

() > tt:l 

> 

150 150 150 150 ...-3 

:::0 

0 

layer contains c:::: 

numerous 166-174 em a 

dark and faint numerous black streaks ::c: 

175 beds 175 175 175 

200 200 200 200. grominent lighter ed, d1st1nct contact 

215-221 em several narrow bands 

225 225-- 225 of black streaks 

232.5 em 

241 em 

25oil. 

I 242em 

257em 

-

N 
Appendix B: Lithologic summary logs for surficial cores from Escanaba Trough-Continued ........ 

N 

N 

a 

tTl 

0 

r 

0 

a 

?S 

::c: 

>-<:: 

t:l 

i'O 

0 

>-3 

::c: 

tTl 

i'O 

~ 

> r 

> z 

t:l 

t:C 

0 

r 

0 

a 

n 

en 

>-3 

c:: 

t:l 

m 

en 

~ tTl 

en 

n 

> z 

> t:C 

> 

;d 

0 

c:: 

a 

~::c: 

a 

0 

i'O 

t:l 

> 

i'O 

8 

a 

tTl 
Appendix B: Lithologic summary logs for surficial cores from Escanaba Trough-Continued 

L 1-86-1 G L 1-86-?G 

0 0 

EXPLANATION 25 25 

L 1-86-NC 

~ Oxidized zone 50 50~ 

h''iii'''iid 

Porous mud 

75 75~ I 

Nonporous mud 

C/) 100 X 100 

Older mud a: 

UJ 

IUJ D Turbidite mud ~ 125 125 

~ 

z 

~~ J ~Mineralized sediment 

UJ 

(.) 

150 150 

[]sand . 175 175 

k===J Silt 

200 I I II 1'-" I 2oo -u•w\¥tThWL~J IHHCI Samples 

I I 1-

~Void 225 -fJfBttlit\£¥2W~\El 225 

233cm 

250 

273cm 

L 1-86-8G 

0 

25 

50 

75 --{_.:.·;"::-/:f//)(/.1 

HHv 

LHC 

100 X 

125- sand-size 

silty-mud 

clasts 

150-

175-

HHC 

197 em 

C/) 

tTl 

ti 

3: 

tTl 

z 

...., 

"'''l 

t= 

r 

0 

"'''l 

tTl 

C/) 

n 

> z 

> t:C 

> 

...., 

:;;tl 

0 

c 

0 

:I: 

....... 

N 

(.;.) 
Appendix B: Lithologic summary logs for surficial cores from Escanaba Trough-Continued ......... 

N 

~ 

0 

L 1-86-19G L 1-86-25G L 1-86-26G L 1-86-27G m 

0 

l' 0 17 7 7 7 I 0 C:Z:/771 0 I 7 7 7 7 I 0 X 0 

0 

{l -

::r: 

25 ----~················· ············ ········••••tf •••l 

25~ 25 25§ --< 25-55 em 0 

30-50 em heavily burrowed 50 - ~ 

0 sand stringers turbidite mud and ...., in turbidite mud sand occur in ::r: 

50 ----1 I I .I 50~.',.' .. ~ 50 porous mud m 

~ 

~ 

> r 

75 ----1 m • r 1 75 ----1 PI I I 75 -f. } I I 75 ~·: ~:miner:olization > 

black mud? -:::. · · · ·most mtense z '.~: · .. .:.: .. . :.... :_::at 72-77 em 0 

HHC to 

C/) 0 

a: 100 100 100 100 LHC l' 

UJ 100-170 em 0 

I- 105 em 0 

UJ blackish blebs, n ~ streaks and layers, Cl:l 

i= 125 125 125 L 1-86-33G ...., rarely flat-lying e z 0 UJ O.x () m 

Cl:l 

150 150 100-221 em 150 2::3 texture intermediate m 

between 'porous' 25 brownish Cl:l 

n 

and 'smooth' of clasts > 

175 175 175 -t••n•• :::::;:::;;::;;:; ·····m·•·•••m other cores black streaks z 

> as shown to 

50 > 

...., 

~ 

200 200 2oo -t:•nt:l:n:~:•:i ••:•••••;;n••••l 0 

e 

75--f 0 

~::r: 

0 

225 225 2251 HHC I 0 100- ~ 227cm ···- 0 

> 

239cm ~ 

250 -I LHC I a 

LHC 0 

255cm m 

124cm 
Appendix B: Lithologic summary logs for surficial cores from Escanaba Trough-Continued 

EXPLANATION 

L2-86-NC 

~ Oxidized zone 

Porous mud 

Nonporous mud 

~ Altered mud 

D Turbidite mud 

I_::~:_ I Metalliferous sediment 

Massive sulfide 

~~ Debris flow 

EJ Sand . 

. 

~-=-l Silt 

I H HC I Samples 

~Void 

en 

c:r: 

w 

tw 

~ 

tz 

w 

(.) 

L2-86-2G 

0 r 7 7 7 71 

25 

50 

75 

burrows filled 

with oxidized mud 

mud chips 

·:-:·· ·.~ ),J .1 bark fragments 

at 78cm 

LHC 

100--; I II 

101 em 

L2-86-4G 

0 ----.-=----.....,....., 

I"!~J!:!:-29 burrow? 

25 

50 

75 

100 

125 

150 

175 

200 

225 

250 

263cm 

C/:l 

~ 

§:: 

ti1 

z 

...., 

::!l 

t""" 

t""" 

0 

'Tl 

ti1 

C/:l 

n 

> z 

> tJ:1 

> 

...., 

~ 

0 

c 

a 

::c: 

-

N 

Ul 
Appendix B: Lithologic summary logs for surficial cores from Escanaba Trough-Continued ........ 

tv 

0'\ 

a 

tTl 

L2-86-7G L2-86-9G L2-86-28G 0 

t"""' 

0 0 0 0 I =::::: ::;::::::=-1 void from X a 

dewatering B 

of sulfides :I: .oxidized -< 

25 -; I I I 25 0 25 sulfide debris :::0 

10-120 em 0 

extensive >-3 

:I: 

subhorizontal tTl 

:::0 50 -+ I I I burrowing 50 50 ~ 

;> 

r 

;> 

75 -; I ' ., .. , .... , ... ,,,'?1 75 75 z 

0 

sulfide veining to 

(/) < 

~ < 0 (textural change) common hori- a: t----- t"""' 

w 100-- ~ ~ ~ ~ zontal burrows 0 

1- 100 100 a 

w ~- mineralized sand n :2 ~""'""""""J: ~ ~ 96-182 em en 

t= PW X-radiograph >-3 

e z 125 125- LHC 125 shows distinct 0 w laminations m (.) I II 1- en 

dark gray ~ 

150 ---Litlill£4Wi&.'¥J 150--; ~ ' v v ~ streaks 150 tTl 

en 

~ n 

~ ;> 

145-241 em z 

175 -{F"'''?:ft~ml X-radiograph o~o o~<>~ ;> 

175 common 175 to db*..;,-.,.,-=t~ : ..shows five 0 talc blebs ;> 0 0 >-3 distinct muddy 

~ ~ ~ :::0 

turbidites, each 0 

e 

200 ---&JiiY&l11 0-20 em thick 200 200 a 

200cm ~:I: 

a 

0 

:::0 

225 ----fi¥&~®;-· 225 - 220-244 em 0 .M·s;;; .. : . ... - W .. ·»& thin silty X-radiograph ;> 

interbeds shows distinct :::0 

· · ·' laminations 8 

241 em a 

244cm tTl 
Appendix B: Lithologic summary logs for surficial cores from Escanaba Trough-Continued 

L3-86-4G L3-86-8G 

0 ----.r~-r--r----r---1 0 

25 ---ftli!Ul&-&]illli] 20-26 em burrows 25 

EXPLANATION 50-- :~i,~l~ ":att~i~~ L3-86-NC 50 

subtly different color en 

~ Oxidized zone 

B 69-92 em debris unit? 

ttl 

sharp lamination and 0 

armored mudball? 

75 parting of sediment 75 mud ~ ttl 

inclined laminations z D Turbidite mud 

(]) alternating silt, clay ...., 

a: :!J 

w 92em t'""" 

I- 100 t'""" 

w 0 

~ sharp lamination "rl 

5Y 4/2 olive-gray mud ttl 

I- en 

(") 

z L3-86-9G > 

w 125 z 

5Y4/1 dark-gray mud () 0 > 1:..__>_>___2__2_1 o::l 

> 

...., 

I E :3 I Laminated silt and sand 

143 em 25-- :;zj 

0 

c 

a 

L3-86-5G ::c: 

0 

50 

25 

75 

50 

100 

62em 101 em 

...... 

N 

-.....) 
Appendix B: Lithologic summary logs for surficial cores from Escanaba Trough-Continued 

EXPLANATION 

TUL-890 

~ Oxidized zone 

D Turbidite mud 

Nonturbidite mud of 

slightly varying colors 

~~1~ Debris flow 

Massive sulfide 

[] Sand . 

. 

t====~ Silt 

~Void 

(f) 

a: 

w 

..... 

w 

~ 

i= 

z 

w 

0 

TUL-890-1 P 

0 --r--=----=-. 

25 

50 

63cm 

TUL-89D-2G 

267cm 

TUL-89D-4G 

0 -r--------, 

25 

50 

75 

100 

125 

150 

175 

200 

225 

250 

275 

277cm 

121-140 em 

faint laminations 

140-277 em 

highly disrupted 

silt beds and silt 

blebs locally 

disrupted silt bed 

-

N 

00 

a 

tTl 

0 

t"'"' 

0 

a 

B 

::c 

~ 

0 

:::0 

0 

~ 

~ 3:: 

> 

~t"'"' 

> z 

0 

tJj 

0 

t"'"' 

0 

a 

c=s 

C/) 

o--3 

c:: 

0 

m 

C/) 

~ tTl 

C/) 

n 

> z 

> tJj 

> 

~ 0 

c:: 

a 

~::c 

a 

0 

:::0 

0 

> 

:::0 

a 

a 

tTl 
Appendix B: Lithologic summary logs for surficial cores from Escanaba Trough-Continued 

TUL-89D-6G TUL-89D-7P 

0 X 0 X 

fragments of basalt 

glass and sulfides 25 --[~: ~ -~ :~j ~n':li~"J1g~~d TUL-890-BG 

-f- - - -,laminations of 

0---

25 ;--: --;- ;-- : -; sand and silt 

.. . .

1 

interbedded 7P cont. 10cm 

50~ I 50~ 

· · mud and sand 

300 Pr\n°ce.."6g 300-375 em 

highly irregular 

silt blebs locally, TUL-89D-9TR 

75--f I 75 --i<~ao~~~\)1 "vAin" nf I 325 ---t(q"C)Q~a largest one shown 0 Cl'.l 

I 7 7 7 7rx tr1 

0 

25f~j~~~b~ck ~ tr1 

en 100 100 350 z 

a: ~ 

w X :!l 1- same color, l' w 120-238 em l' 

~ friable mud 

125 125 0 widely disseminated 375 50 

~ 50 em "''l 

irregular silt blebs, ti1 z diagonal layer Cl'.l w of friable mud larger ones shown; n 

{) 150 150 isolated indurated 400 > 375-537 em z mud(?) clasts X-radiograph shows > 

all one slide block tc 

with bedding inclined > 

~ 175 

r ,,,,,,,,,,,,,,,,,,,,,,,,.,,,,,,,,,,,,,.,_,.,,,,,.,,,,,,,,,,,Lx 175 425l at 45 degrees :A' 

fragments of basalt 0 

glass and sulfides c::: 

187cm bottomed on a 

::t: massive sulfide 200 450 

225 475 

250 500 

I 

511-523 em 

visibly disrupted 

275 ~?\"OQ~~t;)] I 525L 

1 bedding 

537cm L.--="--~~~ I 

300 

-tv 

\0 

Chapter 7. Sedimentation and Neotectonism in the 

SESCA Area, Escanaba Trough, Southern Gorda Ridge 

By Robert E. Karlin 1 and Robert A. Zierenberg2 

CONTENTS 

Page 

Abstract ...... ...... .. .... ... ... ..... ........... ...... .... .. .. .. .. . . .. .. .. .. .. .. .. .. . ... 131 

Introduction ..... . ... . .. .. .... .... ... ........ ....... .... . ... . .... . .. ......... ... . . ... . 131 

Acknowledgments....................................................... 133 

Methods .. .. .. .. .. .. .. .. .. .. . .. . . .. .. .. . .. .. .. . .. .... . .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . .. . 13 3 

Geologic setting ................................................................... 133 

Stratigraphy.......................................................................... 133 

Results from the SESCA area............................................. 135 

Holocene sedimentation .............................................. 135 

Pleistocene sedimentation ...................... ..................... 135 

Hydrothermally altered sediments.............................. 137 

Discussion............................................................................ 139 

Holocene sedimentation rates ..................................... 139 

Origin of the turbidites ............................................. ... 139 

Conclusions .. .... ... ... .... .. .. .. .. .. ... . ...... . .... ............ ... . ... ...... ... . .. .. 140 

References cited .. ... ... . ... . ... . ... .. .. ...... .... ............ ... ........... ... . . .. 140 

ABSTRACT 

The lithology and stratigraphy of gravity cores recovered from the SESCA area in Escanaba Trough were evaluated by combining magnetic susceptibility profiles with 

visual observations. Hemipelagic sedimentation rates during the Holocene are variable and range up to 14 cm/kyr. 

Sediment hills are actively eroding, and mass wasting is 

common on the flanks. Erosional channels funnel sediment 

to the deeper parts of the axial valley. Holocene turbidites on the valley floor appear to be locally derived and indicate local seismotectonic activity in the last 2.5 kyr. 

Hydrothermal deposits are irregularly distributed around 

the base of the hills. The hills appear to be the source of 

mass flow deposits, and some turbidites contain hydrothermally precipitated pyrrhotite. Intensely altered sediment 

and massive sulfide mineralization also occur in relatively 

unexplored basins in the axial valley away from the sediment hills. 

1 Mackay School of Mines, University of Nevada-Reno, Reno, 

Nevada. 2 U.S. Geological Survey, Menlo Park, California. 

INTRODUCTION 

Escanaba Trough forms the southernmost segment of 

the Gorda-Juan de Fuca spreading system and is about 200 

km west of the California-Oregon coast. Slow sea-floor 

spreading combined with high terrigenous sedimentation 

has resulted in a wedge of over 500 m of Pleistocene sediment fill in the southern part of the axial valley that thins 

to the north (Atwater and Mudie, 1973; Riddihough, 1980; 

Morton and Fox, chap. 2, this volume). Volcanism along 

the rift resulted in penetration of the sediment cover by several volcanic complexes (Morton and Fox, chap. 2, this 

volume). Camera tows and sampling have revealed massive sulfide deposits associated with the volcanic centers 

(Morton and others, 1987a,b, 1990), and these deposits 

have subsequently been investigated by submersible 

(Holmes and Zierenberg, 1990). Such deposits may be the 

modern analogs of ancient sediment-hosted polymetallic 

sulfide ore bodies. Active hydrothermal vents have been 

explored and sampled at the NESCA site near latitude 

41 °00' N., longitude 127°30' W. (Zierenberg and others, 

1988). Thus, Escanaba Trough is a natural laboratory to 

study ore generation processes and the timing of volcanism, tectonism, and hydrothermal activity at an active 

spreading center. 

During two 1986 cruises of the R/V S.P. Lee, a series 

of gravity cores was collected at the Southern Escanaba site 

(SESCA) in the vicinity of the volcanic complex near latitude 40°46' N., longitude 127°31' W. (fig. 7.1). The purposes of the sampling were to determine the late 

Quaternary history of neotectonism and hydrothermal activity of the complex and assess, if possible, the areal extent of sediment alteration associated with volcanism. In 

earlier studies of sediments from the volcanic edifice in the 

Northern Escanaba (NESCA) area, Karlin and Lyle (1986) 

found that magnetic susceptibility measurements were very 

helpful in characterizing lithologic changes in the sediment 

column and in dating the sediments. Magnetic susceptibility is the response of a material to an applied field and 

is a sensitive measure of the concentration and grain size 

of magnetic mineral assemblages. Here we report magnetic susceptibility results for gravity cores taken in the 

SESCA area as well as scanning electron microscopy of 

131 
132 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

40° 30' 

Figure 7.1. Bathymetric map of Escanaba Trough, showing location of volcanic edifices (dark gray), 

SESCA and NESCA sites, DSDP Site 35, and core Ll-86-27G in axial valley of Escanaba Trough 

outside of SESCA area. Contour interval is 200 m. Hachures indicate area of closed low. 
SEDIMENTATION AND NEOTECTONISM IN THE SESCA AREA, SOUTHERN GORDA RIDGE 133 

magnetic separates extracted from selected intervals. In 

this preliminary work, our objectives are to define the sedimentary regime and establish a stratigraphic framework in 

order to address the timing of neotectonic activity. Our results provide constraints on the rates and mechanisms of 

sedimentation in the area and suggest that local 

seismotectonic activity and resulting mass-wasting processes have been prevalent in the last few thousand years. 

ACKNOWLEDGMENTS 

We thank the officers and crew of the RN S.P. Lee for 

their excellent support during the 1986 cruise. 

METHODS 

Thirteen sediment cores were taken in the Escanaba 

Trough during the RN S.P. Lee cruises L1-86-NC and L2-

86-NC. They were recovered using a conventional Benthos 

gravity corer (8.6 em diameter) with a 3-m-long barrel. Locations were established by transponder navigation (see 

Morton and others, chap. 1, this volume). All of the cores 

were cut onboard into 1.5-m lengths and selected subsections 

of some of the cores were removed for pore water and 

hydrocarbon analyses. The cores were kept in refrigerated 

storage during and after transport to the USGS Core Repository in Palo Alto, California, where they were opened and 

described (Normark and others, chap. 6, this volume). 

Whole-core magnetic susceptibility measurements 

were made on the split core sections at 1-cm intervals using a Hartington MS-2 susceptibility meter equipped with 

a 125-mm ring sensor. Susceptibility values are reported 

here in 1 o-6 cgs units ( 41t cgs units = 1 SI unit). Depths 

were determined relative to the top of the sediment rather 

than from the top of the core liner, because of compaction 

during transport and storage. Because of the sensor geometry, each value represents a center-weighted average 

over an interval of about 5 em. The stability of the instrument was about ± 1 x 1 o-6 cgs. The split-core susceptibility values reported here can be doubled to obtain 

whole-core values for a core liner 8.6 em in diameter, or 

multiplied by 3.36 for comparison with 11.4-cm ( 4.5 inch) 

diameter cores. 

Magnetic separates were extracted in selected cores 

from sandy turbiditic intervals using a Franz isodynamic 

separator. The extracts were analyzed by X-ray diffraction as well as optical and by scanning electron microscopy to establish their mineralogy. 

GEOLOGIC SETTING 

The SESCA area contains one of several volcanic complexes that locally pierce the thick sediment cover along 

the axis of Escanaba Trough. The physiography of the area 

is described by Morton and others (1987b, 1990) and 

Morton and Fox (chap. 2, this volume). The most prominent features in the area are three roughly circular sediment-covered hills that rise 60 to 100 m above the 

surrounding valley floor (fig. 7 .2). Submersible observations indicate that the hills are flat-topped and resemble 

buttes. The sides of the hills are retreating by mass wasting, and sedimentary strata are locally exposed (Holmes 

and Zierenberg, 1990; Zierenberg and others, chap. 10, this 

volume). The steep slopes are deeply incised by a complex network of erosional channels, and the lower slopes 

are covered by mudstone talus. Pillows and sheet flows 

of basalt, partially covered by sediment, are exposed to the 

southwest of North Hill (fig. 7.2) (Ross and Zierenberg, 

chap. 8, this volume). Locally, the basalt flows are cut by 

fissures and normal faults that strike parallel to the spreading axis. Sediment-hosted sulfide deposits were discovered near the base of each of the hills and may 

discontinuously rim the perimeter of all of the uplifted sediment hills (Morton and others, 1990; Holmes and 

Zierenberg, 1990; Zierenberg and others, chap. 10, this volume). Many of the deposits are old and apparently inactive, because they show signs of extensive sea-floor 

weathering and erosion. We believe that morphologic differences between deposits suggest multiple episodes of sulfide precipitation, sea-floor weathering, and erosion. 

Active hydrothermal venting has not been observed in the 

SESCA area. 

With two exceptions, the coring during the L1-86-NC 

and L2-86-NC cruises was concentrated in the vicinity of 

the 3170 Hill and the East Hill at approximately latitude 

40°46' N. (fig. 7.2). Cores L2-86-2G and L2-86-4G were 

taken near the summit of each hill, whereas cores L1-86-

19G, Ll-86-250, L1-86-26G, and L1-86-7G were collected 

along the flanks of the 3170 Hill. The relatively flat areas 

on the surrounding sediment plain were sampled by cores 

Ll-86-10, L2-86-9G, and L2-86-28G. Core L2-86-7G 

came from the top of a terrace approximately 100 m shallower than the valley floor at the western edge of the 

SESCA map area. Core L1-86-27G was taken outside of 

the SESCA map area (fig. 7.1), in a poorly studied area of 

disturbed sediment between NESCA and SESCA where the 

volcanic basement rises near the sea floor (edifice C of 

Abbott and others, 1986; Davis and Becker, chap. 3, this 

volume). 

STRATIGRAPHY 

The sediment in the study area is dominantly 

hemipelagic and typically consist of olive green to olive 

gray mottled silty clay. The cores are described in Normark 

and others (chap. 6, this volume). In an earlier study of 

Escanaba Trough sediments, Karlin and Lyle (1986) found 
134 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

that a region-wide lithologic change occurred at approximately 11 ka and was associated with a transition from 

glacial to interglacial conditions. This boundary was 

initially dated by radiocarbon analyses on cores from the 

NESCA area and the northern Gorda Ridge. More recently, 

oxygen isotope analyses on cores from either side of Gorda 

Ridge confirm this age assignment (Karlin and others, 

1992). 

In the NESCA sediments studied by Karlin and Lyle 

( 1986), the 11-ka lithologic change was recognizable as a 

subtle down core color transition from olive gray to gray 

40°42' 6000 8000 10,000 

and a fining of the sediment from a mottled silty clay to a 

slightly calcareous homogeneous stiff clay with a texture 

similar to a gley-type soil. Water contents abruptly decreased down core at the boundary from about 60 percent 

to less than 50 percent. The boundary is readily identifiable in whole-core susceptibility profiles as a doubling in 

susceptibility in the lower sediment. Holocene sediment has 

higher levels of Si, Mg, Zn, Cu, and Ba and lower levels 

of CaC03, AI, Na, K, and Ti relative to Pleistocene sediment. These abundances suggest that Pleistocene sediment 

consists primarily of terrigenous clay mixed with calcare12J030' 

8000 

0 2 KILOMETERS 

Contour interval 10 meters 6000 

12,000 14,000 16,000 

Figure 7.2. Bathymetry and core locations in SESCA area. Cores from Ll-86 and L2-86 cruises denoted by filled and open circles, 

respectively; core L6-85-8G is marked as filled square. Numbers along right and bottom give coordinates of local transponder net (in 

meters). Hachures indicate area of closed low. 
SEDIMENTATION AND NEOTECTONISM IN THE SESCA AREA, SOUTHERN GORDA RIDGE 135 

ous nannofossil ooze. The clay has a composition similar 

to illite or a mixture of illite and chlorite. In contrast, 

Holocene sediment contains a coarser grained, more felsic 

terrigenous assemblage combined with biogenic opal. 

These observations are consistent with decreased terrigenous input and relatively increased siliceous biogenic flux 

resulting from decreased continental weathering and enhanced coastal upwelling in the Holocene. The higher susceptibility values in the Pleistocene samples are probably 

due to the relatively greater proportion of fine-grained detrital minerals. Organic geochemical markers also distinguish Pleistocene sediment from Holocene sediment 

(Kvenvolden and others, chap. 15, this volume). 

The ages of the SESCA cores were established using 

the criteria developed by Karlin and Lyle (1986) to identify the lithologic change at 11 ka. The best indicator of 

this boundary is an increase in half-core susceptibility from 

about 10 in the non turbiditic Holocene intervals to 20 to 

30 in the finer grained Pleistocene sediment intervals. The 

increase generally coincides with a down-core change in 

color and texture (Normark and others, chap. 6, this volume). The location of the boundary is further constrained 

by X-radiography, because the nonturbiditic Holocene sediments are less opaque and typically more heavily 

bioturbated than the Pleistocene sediments. Although 

visually the Pleistocene sediment appears to be homogeneous dark-gray mud, the X-radiographs show that the 

Pleistocene intervals often contain very fine clay laminations typical of distal turbidites. In some of the cores, the 

lithologic change is disturbed by bioturbation. 

RESULTS FROM THE SESCA AREA 

HOLOCENE SEDIMENTATION 

Interpreted susceptibility profiles for the cores in the 

vicinity of the SESCA area (shown in figs. 7.3, 7.4, and 

7 .5) are grouped roughly by sedimentary regime, based on 

their locations and common characteristics in the profiles. 

Intervals denoted as gaps in the profiles are places where 

sections are missing due to earlier porewater and hydrocarbon sampling. 

It is evident that the susceptibility profiles share several common features. From the susceptibilities and lithologic characteristics, the 11-ka lithologic boundary can be 

identified in many of the cores. In addition, several of the 

cores (Ll-86-lG, L1-86-8G, Ll-86-25G, L1-86-26G, and 

L2-86-28G) contain large susceptibility peaks in the upper parts of the Holocene section (figs. 7.3-7.5). These 

susceptibility highs correspond closely to sand and silt layers, which, in some cases, show graded bedding and sharp 

irregular contacts. These intervals almost certainly represent turbidites. The zones of elevated susceptibility are 

broader than the discrete coarse sediment layers seen in the 

cores and define both the coarse-grained, high-energy part 

of the flow and the fine-grained tail, which is not visually 

apparent. Magnetic separates of turbidite intervals in cores 

L1-86-25G and L1-86-26G contain mainly detrital terrigenous minerals such as magnetite and ilmenite. 

In certain cores (L1-86-8G, L1-86-25G, and L1-86-

26G), the interval immediately underlying the turbidite 

zones contains relatively homogeneous, stiff gray clay. On 

visual inspection, the differences between these sediments 

and typical Holocene lutites are subtle; however, as seen 

in figure 7 .3, the susceptibilities in these intervals are elevated. Because the texture, color, and magnetic properties are similar to those of the Pleistocene sediments and 

as these intervals overlie typical Holocene mud with low 

susceptibilities, these zones may contain Pleistocene sediment that was displaced from another locality and transported to the sites, possibly as a mud flow. 

The lithologic transition from Holocene to Pleistocene 

is either missing or lies at an anomalously shallow depth 

in three of the sediment cores (fig. 7.4). The high values 

of susceptibility as well as organic geochemical markers 

(Kvenvolden and others, chap. 15, this volume) in each 

core suggest that the sections are mostly Pleistocene in age. 

Cores L2-86-2G and L2-86-4G were recovered near the 

tops of two hills in SESCA. The upper parts of the core 

section contain mud clasts in a mud matrix, which we interpret as a debris flow or slump deposit. This is consistent with submersible observations that indicate that the 

edges of the hills were subject to erosion and mass wasting (Holmes and Zierenberg, 1990; Zierenberg and others, 

chap. 10, this volume). Core L 1-86-7G was collected near 

the base of one of the uplifted hills. In this area, steepsided channels from 1 to 20 m deep were frequently observed during submersible dives (Holmes and Zierenberg, 

1990). The channels appear to be erosional and serve to 

funnel material that has been mobilized by mass wasting 

from the adjacent hill slopes to the floor of the axial valley. It is possible that this core was recovered from one 

of these channels, in which active scouring inhibited deposition of Holocene sediment. 

PLEISTOCENE SEDIMENTATION 

The record of Pleistocene sedimentation is limited by 

the shallow penetration of the gravity cores. Visual examination reveals little or no down-core variation. The magnetic susceptibility profiles are also generally smooth for 

most of the Pleistocene intervals (figs. 7.3, 7.4), but certain cores show peaks consistent with the presence of turbidites (for example, L1-86-1G, L1-86-7G, L2-86-2G). 

The X-radiographs of the cores from the valley floor show 

well-defined millimeter-scale laminations, suggesting deposition in the distal parts of turbidite flows. The preservation 

of finely laminated structures indicates that bioturbation was 
136 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

precluded, either because of the high sedimentation rates or 

disoxic conditions that discourage burrowing. 

The Pleistocene section of core L2-86-7G also has distinctive peaks in the susceptibility profiles (fig. 7.4) that 

suggest the presence of turbidites. Although this section 

was described as homogeneous gray mud (Normark and 

others, chap. 6, this volume), the X-radiographs show that 

this interval is composed of a sequence of at least four turbidite units, with sharp basal contacts for the upper three 

at 174 em, 188 em, and 204 em. The thick turbidite units 

contrast with the finely laminated distal turbidites in the 

cores taken from the valley floor. The presence of turbidites in this core is somewhat unusual in that the core was 

recovered from a terrace about 100 m above the floor of 

Escanaba Trough. The terrace appears to have been 

uplifted by normal faulting prior to deposition of the 

turbidites. Because turbidity currents flowing along the 

valley floor would not have enough volume or energy to 

MAGNETIC SUSCEPTIBILITY 

0 60 0 60 0 0 ~~~~~~~~~ 

en 

a: 

w 

1-

w 100 

:::2: 

1-

z 

w 

0 

z 

- 200 

J: 

1-

a. 

w 

Cl 

300 

L 1-86-25G 

Base of hill 

L 1-86-26G 

Base of hill 

0 60 0 600 

L2-086-28G 

Valley floor 

60 0 

60 

L 1-086-1G 

Valley floor 

60 

0 ~~~~~~~~~ ~~~~~~~~~ ~~~~~~~~~ 

en 

a: 

w 

1-

w 100 

:::2: 

1-

z 

w 

0 

z 

- 200 

J: 

1-

a. 

w 

Cl 

L 1-86-19G 

Side of hill 

- Section break 

L6-85-NC 8G 

Valley floor 

L2-86-7G 

Western edge of SESCA 

on terrace 

D 

• 

~ 

~ 

~ 

Holocene 

hemipelagic 

Pleistocene 

hemipelagic 

Turbidites 

Resedimented 

Pleistocene 

Hydrothermally 

altered sediment 

Figure 7 .3. Split-core magnetic susceptibility profiles of sediments on valley floor at SESCA. These cores contain turbidite intervals in the 

Holocene, as evidenced by elevated susceptibility values. 
SEDIMENTATION AND NEOTECTONISM IN THE SESCA AREA, SOUTHERN GORDA RIDGE 137 

reach this elevation, it is likely that the turbidites originated 

from the walls flanking the axial valley. Thus, Pleistocene 

sedimentation patterns may have been complex. Proximal 

turbidites were deposited on the terraces as a result of mass 

wasting of the sedimented axial valley walls. In contrast, 

sedimentation on the axial valley floor consisted mainly of 

hemipelagic input and distal turbidity flows. 

HYDROTHERMALLY ALTERED SEDIMENTS 

Three cores contain evidence of sulfide mineralization 

and hydrothermal alteration (fig. 7.5). The alteration 

processes are described by Zierenberg and Shanks (chap. 

14, this volume). Core L1-86-8G is located near the base 

of East Hill at SESCA. The upper part of the core contains a thin veneer about 15 em thick of typical Holocene 

hemipelagic sediment overlying a series of silty turbidites. 

The interval from 55 to 85 em may contain redeposited 

Pleistocene material. The section below 85 em appears 

bleached gray and contains mud clasts. The susceptibility 

profile and gross stratigraphy (fig. 7.5) show a close resemblance to other valley cores, especially L1-86-25G, L1-

86-26G, L1-86-1G, and L2-86-28G. However, unlike these 

cores, the sediment in L 1-86-8G is bleached gray and the 

turbidite layers contain trace amounts of sand-sized sulfide 

grains. Scanning electron micrographs of magnetic separates show abundant aggregates of thin-bladed or stubby 

hexagonal pyrrhotite with the morphology of "black 

smoke" particles (fig. 7 .6). In some cases, the pyrrhotite 

is intergrown with hydrothermal talc. These associations 

suggest that the core is in close proximity to a hydrothermal 

vent area, possibly the same sites that rim the base of the 

hill that were identified by submersible and camera observations (Zierenberg and others, chap. 10, this volume). The 

lower bleached part of the core may contain resedimented 

material that includes hydrothermally altered clay 

(Zierenberg and Shanks, chap. 14, this volume). Organic 

geochemical analyses on this core suggest maturation at 

elevated temperatures (Kvenvolden and others, chap. 15, 

this volume). This core does not record the 11-ka boundary, thus the age of the section is unknown. However, if 

the turbidites represent the same set of events as in the 

other valley cores, then the hydrothermal activity may be 

contemporaneous with the turbidity flows. 

MAGNETIC SUSCEPTIBILITY 

CJ) 

a: 

w 

1-

w 

~ 

~ 

z 

w 

() 

z 

I1-

Cl. 

w 

Q 

0 60 0 

0 ~~~~~~~~~~~~~ 

.:_·_.;_ ..... _.· 

.·:·:: >i::r:·~:;y~·~~i~:it~--::\··:··: .. ::.·.:: .. ·:.:: :.:. 

100 

200 

300 L--------------1 

L2-86-2G 

Top of hill 

L2-86-4G 

Top of hill 

60 0 

L 1-86-7G 

Channel 

60 

Figure 7.4. Split-core susceptibility profiles of cores taken in SESCA area. These sediments appear to be relict Pleistocene muds exposed 

near hill tops (L2-86-2G and L2-86-4G) and in an erosion channel(?) (Ll-86-7G). Units same as in figure 7.3. 
138 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

(/) ffi 100 

1-

w 

~ 

i= 

z 

w 

() 

~ 

II-fu 200 

0 

300 

40 

L2-86-9G 

Valley floor 

80 

MAGNETIC SUSCEPTIBILITY 

0 60 0 

Sulfide-rich turbidite 

L 1-86-27G 

Edifice between 

NESCA and SESCA 

Resedimented 

Pleistocene (?) 

Resedimented 

hydrothermally 

altered clay 

L 1-86-8G 

Base of hill 

60 

Figure 7.5. Split-core susceptibility profiles of hydrothermally altered sediments (shown by diagonal pattern) at SESCA. Units same as in 

figure 7.3. 

Core L2-86-9G, recovered from a basin in the western part of the SESCA area, contains highly altered sediment underlying an interval of massive sulfide. The core 

is unusual in that metalliferous red-brown mud overlies a 

30-cm-thick clastic zone of sand and nodules composed of 

hexagonal pyrrhotite, with less abundant isocubanite, 

sphalerite, and barite (Zierenberg and Shanks, chap. 14, this 

volume). The lower part of the section is extensively altered to monominerallic chlorite, which Zierenberg and 

Shanks (chap. 14, this volume) attribute to intensive magnesium metasomatism. The susceptibility profile for this 

core (fig. 7 .5) shows a strong peak in the metalliferous mud 

and sulfide-rich zones, but almost no signal in the lower 

chlorite-rich layer. The low magnetic susceptibility of the 

lower chloritized zone probably results from the dissolution of detrital minerals during hydrothermal alteration. 

The intense alteration precludes any age identification by 

magnetic means. 

Core L1-86-27G was taken in a relatively unexplored 

area between the SESCA and NESCA sites where volcanic basement or sills are only shallowly covered by sediment (edifice C of Abbott and others, 1986). The sediment 

lacks the continuous layering observed in seismic-reflection profiles from surrounding areas (Davis and Becker, 

0 20 MICROMETERS 

Figure 7.6. Scanning electron micrograph of aggregate containing 

thin-bladed pyrrhotite grains from magnetic separates of turbidite 

in core Ll-86-8G at 53 to 55 em. 
SEDIMENTATION AND NEOTECTONISM IN THE SESCA AREA, SOUTHERN GORDA RIDGE 139 

chap. 3, this volume). The base of the core has a sand layer 

that contains massive sulfide veins, particularly pyrrhotite, 

with lesser amounts of copper, zinc, and lead sulfide minerals and barite (Zierenberg and Shanks, chap. 14, this volume). The mineralized turbidite sand may have acted as a 

conduit for hot sulfide- and metal-rich water. Hydrothermal smectite is the dominant alteration mineral in the sediment outside of the sulfide veins. The susceptibility profile 

(fig. 7.5) shows a high associated with the red-brown oxidized surface layer. A peak also occurs in the turbiditesulfide zone near the base of the core. X-ray diffraction 

indicates that the mineralized zone contains dominantly 

hexagonal pyrrhotite, which has a relatively low specific 

mass susceptibility compared to magnetite. However, considering its abundance, the magnetic susceptibility peak for 

this interval is not strong. If this property is representative of such hydrothermally altered sediments, then magnetic surveying may not be a useful tool in exploring for 

such deposits. Given the short length of this core, it is not 

possible to estimate its age. 

DISCUSSION 

HOLOCENE SEDIMENTATION RATES 

The 11-ka lithologic boundary provides a convenient 

stratigraphic marker that is useful in establishing sedimentation rates and in making a rough estimate of the timing 

of the episodes of turbidity current activity. A necessary 

assumption that the top of the core has been preserved is 

reasonable for those cores where the surficial brown oxidized layer was recovered (Normark and others, chap. 6, 

this volume). 

Holocene hemipelagic sedimentation rates for the various cores (table 7.1 ), with the turbidite intervals removed, 

range from 5 to 14 crnlkyr. The lower rates may reflect 

some degree of erosion and scour of underlying strata during turbidity flows; however, more detailed dating is required 

to assess this possibility. The Holocene sedimentation rates 

are comparable to those found in hemipelagic sediments 

south of the Mendocino Escarpment (Karlin, 1990). However, they are considerably lower than average Pleistocene 

rates of over 50 cm/ky reported at DSDP Site 35 

(McManus and others, 1970) and probably reflect lesser 

continental erosion during interglacial intervals. 

ORIGIN OF THE TURBIDITES 

The presence of a series of turbidites in similar stratigraphic positions in several cores suggests that the cores 

are recording the same set of turbidite events. As seen in 

figures 7.3A and 7.5, the valley cores, especially cores Ll86-8G, L1-86-25G, L1-86-26G, L1-86-1G, and maybe L2-

Table 7.1. Holocene sedimentation rates of SESCA cores. 

Holocene Holocene 

Core Location Length total rate hemipelagic 

(em) (cmlkyr) rate ( cmlk:yr) 

Ll-86-1G Valley floor 273 13 11 

Ll-86-7G Erosional channel ? 233 2.7 2.7 

Ll-86-8G Base of East Hill 197 2.4? ? 

Ll-86-19G NW side of 3170 Hill 255 11 11 

Ll-86-25G Base of 3170 Hill 227 12 6.5 

Ll-86-26G Base of 3170 Hill 239 19 11 

L2-86-2G Top of 3170 Hill 101 0 0 

L2-86-4G Top of East Hill 263 5.3 0? 

L2-86-7G Terrace 241 14 14 

L2-86-9G West valley floor 200 ? 

L2-86-28G Valley floor 244 7 5 

L6-85-8G Valley floor, south 244 16 11 

86-28G, contain a series of four well-defined events that 

have a similar magnetic character. Only one valley core 

(L 1-86-7G) does not record these events, but this core 

appears to contain only Pleistocene material, suggesting 

that the core may have been recovered from one of the many 

erosional channels observed from the submersible. In addition, core L6-85-8G (fig. 7.3B), located about 1.5 km south 

of the hills, also contains a similar series of turbidites. For 

this core, Karlin and Lyle ( 1986) dated the layer immediately 

underneath the basal turbidite by 14C at 2.4 ka. 

If the turbidites do indeed record the same set of 

events, then the turbidites would appear to be proximal and 

from the nearby 3170 Hill, because the highest susceptibility values and the coarsest sand (Wong, 1989) are from 

cores L1-86-80, L1-86-250, and L1-86-26G from the side 

of the 3170 Hill. The lowest values are from core L 1-86-

10, in the basin to the east. The turbidites in cores Ll86-25G and L1-86-26G also may be underlain by relict 

Pleistocene sediment redeposited by a nearby mud flow 

associated with slumping of the sides of the hill. Core L1-

86-190 from the western flank of the hill does not show 

these events, suggesting that mass wasting may have occurred only on the eastern side. These episodes are also 

not seen in core L2-86-70 located on the teiTace at the 

western edge of the SESCA area; thus, the turbidites do 

not appear to have originated from the western valley walls. 

There are two problems in relating the turbidites in the 

different cores to a series of events from a common source. 

The turbidites in cores L1-86-25G and L1-86-26G both 

contain detrital magnetic phases with no appreciable volcanic glass or sulfide constituents. Initial petrographic 

scans of the silt layers suggest erosion and reworking of 

relict terrigenous material, such as might occur as a result 

of slumping and mass wasting of the uplifted sedimentcapped hills. In contrast, core L1-86-8G and the L6-85-

80 core described by Karlin and Lyle (1986) contain fresh 

volcanic glass and high temperature hexagonal pyrrhotite. 
140 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

Thus it appears that the turbidites may have come from different sources. 

Although the timing of the events is not well constrained, it is evident that the cores contain somewhat different thicknesses of Holocene hemipelagic sediment 

between the 11-ka boundary and the basal Holocene 

turbidite contact. Assuming the sedimentation rates discussed above, the age of the basal contact varies between 

about 1.45 ka and 2.4 ka. A major source of uncertainty 

is how much sediment was eroded as a result of flow associated with the turbidite. Without more accurate radiometric age control, it is impossible to establish whether the 

flows occurred simultaneously. For the present, the most 

that we can conclude is that the turbidites seem to have occurred at about the same time and may have been derived 

from different sources. The susceptibility profiles, as well 

as the physiographic setting of the cores, suggest that the 

source areas were local. 

Regardless of the exact ages of the turbidite events, it 

is clear that the late Holocene (since about 2.5 ka) was a 

period of intensified seismotectonic activity, possibly related to renewed uplift of the sediment hills. The presence 

of high temperature sulfide minerals such as pyrrhotite in 

some of the turbidites suggests that the tectonism was accompanied by hydrothermal activity. It is interesting that 

the turbidites found here appear to be similar in timing and 

gross structure to those found at NESCA by Karlin and 

Lyle (1986). The NESCA cores also contain significant 

amounts of volcanic glass and sulfide minerals. Further 

work is necessary to determine whether these similarities 

are a coincidence or due to coseismic disturbances associated with simultaneous uplift or volcanism at both sites. 

CONCLUSIONS 

The results from the cores collected to date show that 

the sedimentary regime in the vicinity of the volcanic edifices is active and complex. Neotectonic activity appears 

to have occurred episodically in the last 2.5 kyr following 

an indeterminate period of quiescence. The seismotectonic 

activity caused mass wasting of the sides of uplifted sediment hills. The relations between mass-wasting events, 

volcanism, and hydrothermal activity are unclear, although 

several of the turbidites contain fresh volcanic glass and 

sulfide minerals. 

The available core coverage is not adequate to resolve 

the distribution of hydrothermal deposits or the extent of 

sediment alteration due to baking and fluid migration. 

However, the recovery of intensely altered and sulfidized 

sediments in isolated cores from relatively unexplored 

areas around the base of the hills and on the valley floor 

underscores how much we have to learn about the dynamics of hydrothermal activity and the distribution and nature 

of sediment-hosted sulfide deposits. 

REFERENCES CITED 

Abbott, D.H., Morton, J.L., and Holmes, M.L., 1986, Heat flow 

measurements on a hydrothermally-active, slow-spreading 

ridge: The Escanaba Trough: Geophysical Research Letters, v. 13, p. 678-680. 

Atwater, T.M., and Mudie, J.D., 1973, Detailed near bottom geophysical study of the Gorda rise: Journal of Geophysical 

Research, v. 78, p. 8665-8686. 

Davis, E.E., and Becker, Keir, Thermal and tectonic structure of 

the Escanaba Trough: New heat-flow measurements and 

seismic-reflection profiles, in Morton, J.L., Zierenberg, 

R.A., and Reiss, C.A., eds., Geologic, hydrothermal, and 

biologic studies at Escanaba Trough, Gorda Ridge, offshore 

northern California: U.S. Geological Survey Bulletin 2022, 

chapter 3 (this volume). 

Holmes, M.L., and Zierenberg, R.A., 1990, Submersible observations in Escanaba Trough, southern Gorda Ridge, in 

McMurray, G.R., ed., Gorda Ridge: A seafloor spreading 

center in the United States' Exclusive Economic Zone: New 

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4421-4436. 

Karlin, Robert, and Lyle, Mitchell, 1986, Sediment studies on the 

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Karlin, Robert, Lyle, Mitchell, and Zahn, Rainer, 1992, Late Quaternary variations in the calcite compensation depth in the 

North East Pacific Ocean: Paleoceanography, v. 7, p.43-61. 

Kvenvolden, K.A., Rapp, J.B., and Hostettler, F.D., Hydrocarbons 

in sediment from Escanaba Trough, in Morton, J.L., 

Zierenberg, R.A., and Reiss, C.A., eds., Geologic, hydrothermal, and biologic studies at Escanaba Trough, Gorda 

Ridge, offshore northern California: U.S. Geological Survey Bulletin 2022, chapter 15 (this volume). 

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and others, eds., Initial reports of the Deep Sea Drilling 

Project: Washington, U.S. Government Printing Office, v. 

5, p. 165-172. 

Morton, J.L., Holmes, M.L., and Koski, R.A., 1987a, Volcanism 

and massive sulfide formation at a sedimented spreading 

center, Escanaba Trough, Gorda Ridge, northeast Pacific 

Ocean: Geophysical Research Letters, v. 14, p. 769-772. 

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M.L., Shanks, W.C., III, Zierenberg, R.A., Lyle, M., and 

Benninger, L.M., 1987b, Preliminary report, cruises L1-86-

NC and L2-86-NC, Escanaba Trough, Gorda Ridge: U.S. 

Geological Survey Open-File Report 87-375-A, 20 p. 

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Distribution and composition of massive sulfide deposits at 

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G.R., ed., Gorda Ridge: A seafloor spreading center in the 

United States' Exclusive Economic Zone: New York, 

Springer-Verlag, p. 77-92. 

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volcanism and sedimentation at Escanaba Trough: Geophysical results, in Morton, J.L., Zierenberg, R.A., and 

Reiss, C.A., eds., Geologic, hydrothermal, and biologic stud-
SEDIMENTATION AND NEOTECTONISM IN THE SESCA AREA, SOUTHERN GORDA RIDGE 141 

ies at Escanaba Trough, Gorda Ridge, offshore northern 

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introduction, in Morton, J.L., Zierenberg, R.A., and Reiss, 

C.A., eds., Geologic, hydrothermal, and biologic studies at 

Escanaba Trough, Gorda Ridge, offshore northern California: U.S. Geological Survey Bulletin 2022, chapter I (this 

volume). 

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and Rosenbauer, R.J., Sediment fill of Escanaba Trough, in 

Morton, J.L., Zierenberg, R.A., and Reiss, C.A., eds., Geologic, hydrothermal, and biologic studies at Escanaba 

Trough, Gorda Ridge, offshore northern California: U.S. 

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anomaly analysis: Earth and Planetary Science Letters, v. 

51, p. 163-170. 

Ross, S.L., and Zierenberg, R.A., Volcanic geomorphology of the 

SESCA and NESCA sites, Escanaba Trough, in Morton, 

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Ridge, offshore northern California: U.S. Geological Survey Bulletin 2022, chapter 8 (this volume). 

Wong, F.L., 1989, Mineralogy of sand in turbidites from the 

Escanaba Trough, Northeast Pacific Ocean: U.S. Geological Survey Open-File Report 89-661, 10 p. 

Zierenberg, R.A., Koski, R.A., Ross, S.L., Shanks, W.C. Ill, and 

Slack, J.F, 1988, Preliminary results of ALVIN dives on active sediment-hosted massive sulfide deposits in the 

Escanaba Trough, Southern Gorda Ridge [abs.]: Eos, Transactions American Geophysical Union, v. 44, p. 1488. 

Zierenberg, R.A., Morton, J.L., Koski, R.A., and Ross, S.L., Geologic setting of massive sulfide mineralization in the 

Escanaba Trough, in Morton, J.L., Zierenberg, R.A., and 

Reiss, C.A., eds., Geologic, hydrothermal, and biologic studies at Escanaba Trough, Gorda Ridge, offshore northern 

California: U.S. Geological Survey Bulletin 2022, chapter 

10 (this volume). 

Zierenberg, R.A., and Shanks, W.C., III, Sediment alteration associated with massive sulfide formation in the Escanaba 

Trough, Gorda Ridge: The importance of seawater mixing 

and magnesium metasomatism, in Morton, J.L., Zierenberg, 

R.A., and Reiss, C.A., eds., Geologic, hydrothermal, and 

biologic studies at Escanaba Trough, Gorda Ridge, offshore 

northern California: U.S. Geological Survey Bulletin 2022, 

chapter 14 (this volume). 

Chapter 8. Volcanic Geomorphology of the SESCA and 

NESCA Sites, Escanaba Trough 

By Stephanie L. Ross and Robert A. Zierenberg1 

CONTENTS 

Page 

Abstract................................................................................ 143 

Introduction.......................................................................... 143 

Acknowledgments....................................................... 143 

Data collection..................................................................... 143 

Observations . . .. . . .. . ... . . . . .. .. .. . . . . ... . .. . . .. . ... . . ... . .. . . ... . .. . . .. . . ... . .. .. .. 146 

SESCA area................................................................. 146 

NESCA areas . . .. . . ... . .. . . .. . . .. .. .. .. .. . . . ... .. .. .. . . .. . ... .. .. .. .. .... . .. 146 

Discussion and conclusions................................................. 148 

Relative age of volcanic flows.................................... 148 

Relationship of flow morphology to topographic 

slope......................................................................... 149 

Relationship of volcanic flows to formation of 

sediment hills .. .... .. . .... .. . .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .... .... .. . 150 

References cited................................................................... 150 

ABSTRACT 

Volcanic outcrops have been observed on several camera tows and submersible dives at the southern Escanaba 

Trough (SESCA) and northern Escanaba Trough (NESCA) 

sites. Pillow flows dominate the outcrops at both sites; sheet 

flows are also common but are often poorly exposed due to 

sediment cover. Basalt flows at SESCA appear older than 

at NESCA. Volcanic flow morphology correlates with bathymetric slope at the NESCA site, as sheet flow basalt is restricted to relatively flat areas. Relationships between 

volcanic flows, faults, and the base of the SESCA North Hill, 

combined with available seismic-reflection data, support the 

hypothesis that uplift of the sediment hills results from deep 

emplacement of volcanic sills. The faults that uplifted the 

hills provided a conduit for hydrothermal ·fluids and may also 

have been the path for erupted basalt. 

INTRODUCTION 

The sediment-covered Escanaba Trough comprises the 

axial valley of the southern Gorda Ridge, a slow-spreading 

1 U.S. Geological Survey, Menlo Park, California. 

mid-ocean ridge (full rate 2.4 crnlyr). Seismic-reflection 

records reveal a discontinuous chain of volcanic edifices 

along the axial zone of Escanaba Trough (Morton and others, 1987), similar to observations at other slow spreading 

centers (Macdonald, 1982). Geochemical analyses of basalt suggest that magma chambers along Escanaba Trough 

are not continuous between volcanic centers (Davis and 

others, chap. 9, this volume). 

Sediment-hosted massive sulfide deposits have been 

mapped at two sites in Escanaba Trough in areas where 

ridge volcanism forms volcanic centers that rise through 

the sediment cover: the southern Escanaba Trough 

(SESCA) site at latitude 40°45' N., longitude 127°30' W., 

and the northern Escanaba Trough (NESCA) site at latitude 41 o N., longitude 127°30' W. At these sites, several 

steep-sided circular hills, from 500 to 1,000 m in diameter, 

rise 40 to 100 m above the surrounding sea floor. These 

include the North Hill, East Hill, and 3170 Hill at SESCA 

(fig. 8.1) and the Southwest Sediment Hill at NESCA (fig. 

8.2). These hills are probably the result of localized uplift of sediment above small basaltic sills (Morton and Fox, 

chap. 2, this volume). 

Fifteen transponder-navigated camera stations and 22 

submersible dives (10 with DSV Alvin and 12 with DSV 

Sea Cliff) were conducted at SESCA and NESCA on several cruises between 1985 and 1988. The volcanic geomorphology observed on these surveys provides constraints 

on models proposed to explain the evolution of these areas. 

ACKNOWLEDGMENTS 

This work is based on several seasons of cruises. We 

greatly appreciate the efforts of the ship crews and cruise 

participants involved, especially Hank Chezar, designer of 

the U.S.G.S. camera sled. We thank Alice Davis, Robin 

Holcomb, and Janet Morton for thorough reviews and 

Roger Denlinger for stimulating scientific discussions. 

DATA COLLECTION 

Two camera tows in 1985 and six in 1986 revealed 

volcanic rocks exposed through the sediment cover at 

143 
144 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

Escanaba Trough. All stations used the U.S. Geological 

Survey's deep-towed camera sled (Chezar and Lee, 1985), 

which records 4X hours of color videotape and shoots color 

35-mm still photographs at a preset interval, generally 14 

seconds. Four of these camera tows collected fairly complete sets of both video and still images. Three of the other 

stations (L6-85-33C, L1-86-5C, and L1-86-9C) have full 

video coverage, but few or no photographs. The remain12JC36' 

ing station, L6-85-21C, obtained nearly 4 hours of still pictures, but no video record. 

The camera tows were navigated using a sea-floor net 

of acoustic transponders; the tracklines shown in figures 

8.1 and 8.2 reflect the position of the relay transponder, 

which was located on the tow cable usually 100m above 

the camera sled. Most of the camera stations were successfully transponder navigated and the position errors are 

12JC32' 12JC30' 12JC28' 

0 2 KILOMETERS 

Contour interval 10 meters 6000 

6000 8000 10,000 12,000 14,000 16,000 

Figure 8.1. Bathymetric map showing volcanic geology along 

camera and submersible tracklines at SESCA site. A and B indicate 

locations of photographs shown in figure 8.3. Numbers along right 

and bottom give coordinates of local transponder net (in meters). 

Hachures indicate area of closed low. Only stations and dives with 

observed volcanic rocks are illustrated; see Morton and others (chap. 

1, this volume), figures 1.6 and 1.8 for tracklines of all SESCA dives 

and camera tows. Bathymetry is based on surface soundings and 

shows slopes as less steep than they are; for example, divers' 

observations from dive 662 place volcanic rocks at base of North 

Hill, not on its side. 
VOLCANIC GEOMORPHOLOGY OF THE SESCA AND NESCA SITES, ESCANABA TROUGH 

Figure 8.2. Bathymetric map 

showing volcanic geology along 

camera and submersible 

tracklines at NESCA site. Letters C, D, E, and F indicate 

locations of photographs shown 

in figure 8.3. Q and Q' indicate 

start and end of segment diagrammed in figure 8.4. Numbers 

along right and bottom give coordinates of local transponder net 

(in meters). Hachures indicate 

area of closed low. Only stations 

and dives with observed volcanic rocks are illustrated; see 

Morton and others (chap. 1, this 

volume), figures 1.5 and 1.7 for 

tracklines of all NESCA dives 

and camera tows. Bathymetry is 

based on surface soundings and 

shows slopes as less steep than 

they are; for example, divers' observations place 2039 trackline 

in graben, not on valley wall, and 

easternmost basalts on dive 2034 

at foot of wall, not up on it. 

• Flat sheet flow and folded sheet flow 

0 

D 

• D 

D 

Lobate sheet flow 

Pillow flow 

Talus I breccia 

Sediment 

f-f Fissure or group of fissures 

2 

KILOMETERS 

Contour interval 10 meters 

56,000 58,000 60,000 62,000 

145 

66,000 

64,000 

62,000 

60,000 

58,000 

56,000 

54,000 

52,000 

50,000 
146 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

less than 5 m relative to the net, sufficiently accurate for 

site reoccupation. The agreement of surficial geology at 

trackline intersections confirms this accuracy. Relay navigation did not function during two of the stations. The 

tracklines for stations L6-85-21C and L2-86-19C were estimated from transponder-navigated shiptracks, except for 

the first hour of L2-86-19C, which lacked any transponder navigation; that section of trackline is extrapolated 

from the ship's integrated satellite (NNSS)-Doppler sonarLoran C navigation system. 

Volcanic rocks were observed and photographed during six submersible dives (Sea Cliff dives 659, 660, 662, 

and 765, and Alvin dives 2034 and 2039). Both 

submersibles are equipped with video and externally 

mounted 35-mm still cameras. In cases where the photography is incomplete, geologic interpretation relies heavily 

on the divers' observations (Zierenberg and others, 1991). 

The dives were navigated using the same transponder net 

as the camera stations and had position errors of generally 

less than 20 m. Dives 2039 and 765 had significantly 

greater errors; the 765 navigation has gaps of up to 1 hour 

in duration (500 min projected trackline length). 

OBSERVATIONS 

SESCAAREA 

In the SESCA area, volcanic outcrops were observed 

in four of the nine camera stations and on one of the five 

submersible dives. Most of the volcanic exposures occur 

south to southwest of the North Hill; a few are southwest 

of the 3170 Hill (fig. 8.1). These observations agree well 

with the distribution of near-surface volcanic rocks inferred 

from seismic-reflection data (Morton and Fox, chap. 2, this 

volume). 

Most of the observed volcanic outcrops are pillow lava 

(fig. 8.3A), but flat, folded, and lobate sheet flows were 

also observed in the SESCA area intermixed with the pillow lavas. The pillow lobes are generally 1 to 2 m wide, 

display inflationary "bread-crust" type fractures, and often 

have "trap doors" attached to their tops; these occur when 

one pillow sprouts through a crack in older crust and tears 

off a remnant of that crust to form a cap (Moore, 1975). 

Submersible observations at SESCA show that the trapdoor pillows commonly extruded upward from sheet flow, 

indicating that the sheet flow and pillow formed during the 

same eruptive event. This is consistent with the uniform 

chemistry of the two samples recovered from this area 

(Davis and others, chap. 9, this volume). Since the pillow 

forms have more relief than sheet flows, the pillow flows 

protrude higher above the sediment cover and are more 

readily observed; the ratio of pillow to sheet flows observed at the sea floor may thus be very different from what 

lies beneath the sediment. 

Although fresh volcanic glass was sampled in two 

SESCA dredges (Davis and others, chap. 9, this volume), 

glassy reflections from the surface of exposed basalt were 

not observed in the photographs. Also, no small projections or buds, indicative of youth, were observed, and the 

sediment generally obscures all but the largest pillows. 

The basalt flows are cut in some places by fissures up 

to 2 m wide and as deep as 3 m or more (fig. 8.3B; also 

Morton and others, chap. 1, this volume, fig. 1.9F). The 

observed fissures generally trend subperpendicular to the 

camera or dive track; however, fissures that are subparallel 

to the track may escape observation. Fissures observed on 

dive 662 apparently parallel the ridge crest (Zierenberg and 

others, 1991). The four fissures observed by camera tows 

are oblique to the ridge trend. Volcanic outcrops exposed 

in the fissures and in fault scarps confirm that sheet flows 

are more common than suggested by outcrops protruding 

through the sediment. No hydrothermal discharge or alteration was observed associated with any of the fissures. 

At SESCA, volcanic rocks were observed only at the 

base of the uplifted sediment hills and in the low-lying 

areas between them; outcrops exposed on the steep to 

near-vertical sides of the hills reveal only sediment. As 

the submersible approached the North Hill from the west 

on dive 662, the observers noticed many normal faults, 

first small ones, with offsets up to 1 m, and then, starting 

near the base of the hill, larger ones (Zierenberg and others, 1991). The last basalt exposure was observed near 

the base of the first large fault. Most of the small faults 

cut the basalt flows. However, one fault offsets some 

basalt flows and is overrun by another basalt flow; the 

basalt flows are all of similar apparent ages. This observation suggests that faulting and the extrusion of basalt 

were penecontemporaneous. 

NESCAAREA 

The largest exposed area of volcanic outcrops in the 

NESCA area occurs near the eastern peak of the Central 

Hill (fig. 8.2), extending north and south to the area inferred by seismic-reflection techniques to be underlain by 

near-surface volcanic rocks (Morton and Fox, chap. 2, this 

volume). Many flow morphologies were observed in this 

area (fig. 8.2), with no clear differences in age between the 

different flow types. Pillow basalt (fig. 8.3C; Morton and 

others, chap. 1, this volume, fig. 1.9D) accounts for approximately two-thirds of the observed basalt, while sheet 

flows, including minor amounts of lobate type, account for 

the remainder. The pillows are typically large, from 1 m 

to more than 4 m in diameter, and hemispheric in shape. 

Bread-crust fracturing formed by inflation is characteristic of the pillows observed, but evidence of local subsidence, in the form of hollow pillows and collapsed lobate 

sheet flows (fig. 8.3D) was also observed, although infre-
VOLCANIC GEOMORPHOLOGY OF THE SESCA AND NESCA SITES, ESCANABA TROUGH 147 

quently. The pillows are young enough and so little altered 

that glassy reflections and glass buds are common. Sediment cover on the observed pillows is moderate, generally 

A 

c 

E 

Figure 8.3. Representative sea-floor photographs of volcanic 

features. A, Inflationary pillow protruding through heavy sediment 

cover at SESCA site. B, Fissured basalt flow at SESCA site. C, 

Pillow basalt at NESCA site. D, Collapsed lobes of sheet flow at 

NESCA site. E, Jumbled sheet flow at NESCA site. F, Longer, 

ranging from 30 to 70 percent, but even pillows covered 

by 70 percent sediment have glassy reflections on exposed 

surfaces. 

B 

D 

F 

well-formed folds on sheet flow at NESCA site. Field of view of 

each photograph is approximately 2 to 3 m. All but B were taken by 

USGS camera sled with downward-looking camera; B is external 

still photograph from submersible Sea Cliff 
148 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

Observed sheet flows at NESCA occur mostly as 

jumbled broken short folds (fig. 8.3£; Morton and others, 

chap. 1, this volume, fig. 1.9£) with longer, more intact 

folds (fig. 8.3F) scattered throughout the jumbled sections 

and along their margins. Much of the sheet flow seemed 

to be ponded within a troughlike depression along the base 

of the eastern valley-bounding scarp. The sheet flows are 

cut in some places by 0.5-m-wide fissures that are generally parallel to the ridge. Minor amounts of jumbled, 

folded, and lobate sheet flows also occur among the pillow flows. While glassy reflections were commonly observed on the jumbled and folded sheet flows, as well as 

on the pillow flows, they were most readily discerned on 

the lobate sheet flows, probably because lobate sheet flows 

generally are smoother and better reflect the camera lights. 

At one site on dive 765 near the top of the eastern peak of 

the Central Hill, extremely glassy folds and very glassy lobate sheet flows were simultaneously observed out different portholes. 

The seismic-reflection records from the NESCA area 

suggest that sediment layers underlie thin basalt flows in 

the flat basins north of the Central Hill (Morton and Fox, 

chap. 2, this volume). The lava flows northeast of the Central Hill are interpreted to onlap a sediment hill that rises 

above the basalt-covered plain (Zierenberg and others, 

chap. 10, this volume). Brecciated basalt in sediment observed at the northernmost point of dive 2034 may have 

formed near the margin of a basalt flow where advancing 

lava might intermix with sediment. 

On a north-to-south traverse of dive 2034, across the 

lava-covered plain and up the slope of the eastern side of 

the Central Hill, observers noted an apparent correlation 

between slope and volcanic morphology. Pillow flows 

were observed on the steep sections whereas sheet flows 

were seen on the flatter parts of the traverse. No differences in sediment cover or glass surfaces were observed, 

nor were distinct flow contacts noted. Lava drapes and 

elongate lava pillows and tubes on the eastern part of the 

Central Hill provide evidence for northward flow of lava 

down toward the lava plain, indicating that the source region for at least the last eruption lay upslope to the south. 

The contact between the western sediment-covered 

part of the Central Hill and the eastern part covered by 

pillow lava was best observed on dive 765. The contact 

is a sharp transition from flat-lying basalt (pillows and lobate sheet flows) to a steep sediment-covered slope. No 

lava or basaltic talus was observed on the sediment slope; 

this is consistent with the ponding of basalt against a preeruptive sediment slope. The original contact may have 

been partly obscured by downslope movement of sediment. Sediment-covered areas on the flanks of a localized topographic high on the eastern side of the Central 

Hill were also observed on dive 765 (fig. 8.2). This hill 

and a smaller companion hill further to the east have been 

interpreted as the exposed remnants of sediment hills surrounded by later basalt flows (Zierenberg and others, 

chap. 10, this volume). 

Camera tows and submersible dives west of the lava 

outcrop examined on dive 2034 rarely imaged basalt. The 

few observed exposures (fig. 8.2, station L2-86-19 and dive 

659) occur near the break in slope that marks the western 

edge of the innermost basin (Zierenberg and others, chap. 

10, this volume; also Morton and Fox, chap. 2, this volume, fig. 2.17); all are on steep scarps covered by blocky 

basalt talus. Isolated occurrences of sediment-hosted sulfide mineralization were observed near the tops of some 

of the talus-covered scarps. 

Two dives but no camera stations were made onto edifice REX, the topographic high within the axial trough at 

the northern end of the NESCA site (latitude 41 °05.5' N., 

edifice E of Abbott and others, 1986). This hill may be 

the surface manifestation of a volcanic edifice in the sediment fill of the trough (Morton and Fox, chap. 2, this volume). Volcanic rocks were observed on one of those dives, 

dive 2039; most occur as isolated sediment-dusted talus 

blocks of sheet flow approximately 0.5 m wide, perched 

on top of sediment and heavily colonized by attached filter-feeding organisms. The talus blocks occur on the floor 

of a small graben imaged during bathymetric profiling 

using a 12 kHz echosounder. The lone volcanic outcrop 

was found on the east scarp of the graben-bounding fault 

on a bench 8 to 9 m above the graben floor. Flat to folded 

sheet flow crops out for approximately 250 m along the 

north-south-trending scarp. This flow overlies sediment and 

is overlain by at least 10 em of additional sediment; 

crinoids, anemones, tunicates, starfish, and crabs colonize 

the volcanic outcrops. The talus blocks on the graben floor 

were likely derived from the observed flow following the 

formation of the graben by normal faulting. The lack of 

glassy reflections, the sediment cover, and the displacement 

of the basalt flow by faulting suggest that it is older than 

basalt observed to the south at the NESCA site. 

DISCUSSION AND CONCLUSIONS 

RELATIVE AGE OF VOLCANIC FLOWS 

Pillow flows dominate the volcanic outcrops at 

Escanaba Trough, but exposures in fissures suggest that 

sheet flows exist below the sediment cover in greater abundance than is apparent from visual observations. Heavy 

sediment cover on very glassy pillows indicates high sedimentation rates that could quickly bury sheet flows, especially in the flatter regions where sheet flows are generally 

expected. 

Based on observed sediment cover and preservation of 

glass, the NESCA volcanic flows exposed on and adjacent 

to the east part of the Central Hill appear to be approximately the same age. This agrees with the homogeneous 
VOLCANIC GEOMORPHOLOGY OF THE SESCA AND NESCA SITES, ESCANABA TROUGH 149 

geochemistry of basalt samples collected from this area 

(Davis and others, chap. 9, this volume), which indicates 

that most of the NESCA flows may be from one eruptive 

episode. The analyzed glasses have a small range of compositions and could suggest either many short discontinuous flows or one flow large enough to produce minor 

geochemical variation. 

The single volcanic outcrop at edifice REX at the 

northern edge of the NESCA area appears older than the 

other NESCA basalt flows. The analyzed glass sample 

from this site not only is distinct from the other NESCA 

samples, but it exhibits the most distinctive composition 

of all the Gorda Ridge glasses (Davis and Clague, 1987; 

Davis and others, chap. 9, this volume). 

The volcanic outcrops at SESCA all appear to be of 

one general age, and the trap-door remnants of the parent 

sheet flows on pillows indicate that the pillows and sheets 

formed during one eruptive event. While the volcanic 

flows at SESCA appear older than those near the eastern 

part of the Central Hill at NESCA, they may still be relatively young. Palagonite measurements on original glass 

surfaces of dredged samples indicate recent volcanism at 

both SESCA and NESCA (Morton and others, 1987). Also, 

a high sedimentation rate (Karlin and Lyle, 1986; Karlin 

and Zierenberg, chap. 7, this volume; Normark and others, chap. 6, this volume) could account for the heavy sediment cover, which generally obscures all but the largest 

pillows. 

3,300 

CJ) 3,310 

a: 

w 

1-

w 

~ 

z 3,320 

I 

1-

a.. 

w 

0 3,330 

Q (SOUTH) 

- ......... ' ' 

" ' 

' \. 

RELATIONSHIP OF FLOW MORPHOLOGY TO 

TOPOGRAPHIC SLOPE 

Volcanic flow morphology on the north flank of the 

Central Hill in the NESCA area was observed to correlate 

with topographic slope. Projection of flow morphology 

observed on dive 2034 onto a straight-line path (fig. 8.4) 

illustrates the change from pillow basalt where the slope 

is steep to sheets where it is not. Observations of lava 

draping and elongate lava pillows and tubes on the hillside evince northward flow of lava from a vent area near 

the crest of the hill downhill toward the lava plain. There 

were no obvious changes in sediment cover or preservation of volcanic glass, and no sharp boundaries between 

flow types to suggest an age difference between the different flow morphologies. The composition of all the rocks 

analyzed from this area agrees with derivation from a 

single eruptive event (Davis and others, chap. 9, this volume). The simplest explanation is that lava flowing 

downslope from an eruptive vent near the top of the hill 

created pillows on the steep sections and sheets on the flat 

areas. Alternatively, rapid initial eruption of basalt could 

have flooded the inner rift floor, forming a ponded sheet 

flow, followed by growth of a pillow volcano over the vent 

area on the east side of the Central Hill as effusion rates 

subsided. This scenario seems unlikely as it requires construction of a large volcanic edifice in a single eruptive episode. The presence of sediment hills above the pillow lavas 

Q' (NORTH) --~-

EXPLANATION 

Sheet flow ------

Mixed pillow and sheet flow --

Pillow flow - - - - - - - - -

' ' ' 

3,340 '-----------'-----------'------j_ 

54,800 55,000 55,200 55,400 5~600 55,800 5q000 

DISTANCE, IN METERS 

Figure 8.4. Profile of basalt geomorphology on NESCA Central Hill made by projecting a section of dive 2034 trackline onto northsouth line and plotting it against bottom depth. Trackline segment marked on figure 8.2; distance marked along bottom of figure 

corresponds to local grid marked on right side of figure 8.2. Vertical exaggeration is 20 times. 
150 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

and the contacts between basalt flows and sediment indicate that the sediment hill topography predates the last volcanic eruption. The uplift of sediment hills in Escanaba 

Trough has been interpreted as a manifestation of the intrusion of basaltic sills into the sediment covering the 

spreading center (Morton and Fox, chap. 2, this volume; 

Denlinger and Holmes, chap. 4, this volume; Zierenberg 

and others, chap. 10, this volume). These relationships lead 

us to speculate that the surface eruption of basalt near the 

Central Hill was preceded by intrusion of basaltic sills. If 

these interpretations are correct, than the preexisting slope 

of the sediment hill may have influenced the morphology 

of the overlying basalt flow. 

Many theories have been proffered to explain the relation between the two basic types of sea-floor volcanic 

morphologies: pillow and sheet flows. Studies generalize that pillow flows dominate at slow spreading centers 

while sheet flows tend to be more prevalent at intermediate and fast ridges (Macdonald, 1982). As most of the observed volcanics at Escanaba Trough are indeed pillow 

lavas, this study concurs with that broad assertion. 

Many studies indicate a relationship between slope and 

flow morphology (Duffield, 1969; Moore and others, 1971; 

Moore and others, 1973; Lonsdale, 1977; Ballard and others, 1979); pillow lavas dominate on steep slopes and sheet 

flows cover the flattest plains. Normark (1976) and 

MacDonald (1982) suggested that pillows create relief as 

they build edifices on previously formed sheet flows. 

Ballard and others ( 1979) suggested that sheet flows form 

at the beginning of an eruptive cycle and pillow basalts 

form later as the flows become channelized and effusion 

rate decreases. This may have occurred at the NESCA site; 

however, the presence of a preexisting slope near the vent 

area atop the east side of the Central Hill precluded the formation of sheet flow until lava reached the flat inner rift 

floor where it could pond and spread into a laterally extensive sheet flow. Deep-towed camera observations show 

that the plain south of the pillow-lava-covered crest of the 

Central Hill is covered by young glassy sheet flow(s), 

which may also have been fed by eruption(s) from a vent 

area near the top of the hill. 

RELATIONSHIP OF VOLCANIC FLOWS TO 

FORMATION OF SEDIMENT HILLS 

The penecontemporaneous faulting and volcanism at 

the base of the SESCA North Hill, combined with the observation of volcanic outcrops only at the base and 

downslope of the hill, suggest a causal relationship between 

volcanism and the uplift of the hill. Uplift of sediment hills 

along circular normal faults may be driven by the same intrusions that fed volcanic flows near their bases. This is 

further support of a connection between formation of the 

sediment hills and ridge-crest volcanism. 

Lonsdale and Lawver (1980) presented a model for the 

uplift of similar sediment hills in the Guaymas Basin by 

intrusion of shallow basaltic sills, or bysmaliths, into unconsolidated sediment. The density of the basaltic melt initially prevents it from rising to the sediment surface; the 

magma flows laterally into the weak, low-density unconsolidated sediment, lifting the sediment cap. The area and 

height of uplift is controlled by the relative magnitude of 

the lateral and vertical confining pressures of the sediment 

and the volume of the intrusion. The uplifted hills at both 

Escanaba Trough and Guaymas Basin are typically 1 km 

in diameter with uplifts of approximately 100 m. 

Solidification of magma at the margins of an intrusion, 

or the presence of a preexisting intrusion, could provide 

lateral support allowing magma to build upward and feed 

surface flows provided there is a sufficient supply of 

magma. The observation of volcanic rocks up to the base 

of the North Hill at SESCA suggests surface eruption of 

basalt from the flanks of a sediment hill uplifted by 

bysmalithic intrusion; however, the seismic-reflection data 

(Morton and Fox, chap. 2, this volume) do not indicate the 

presence of a shallow solid plug of basalt underlying the 

North Hill. Denlinger and Holmes (chap. 4, this volume, 

fig. 4.4B) propose a model for uplift of sediment hills 

above sills emplaced near the sediment-basalt interface. 

The hills are uplifted along curved faults that grow outward from the top of the volcanic body to the base of the 

hill. In Denlinger and Holmes' model, the faults are the 

conduits for the hydrothermal fluids. The available visual 

and seismic observations of the SESCA and NESCA sediment hills support that model and suggest that the faults 

also provide a path for the basaltic magma that fed surface 

eruptions. 

REFERENCES CITED 

Abbott, D.H., Morton, J.L., and Holmes, M.L., 1986, Heat flow 

measurements on a hydrothermally-active, slow-spreading 

ridge: The Escanaba Trough: Geophysical Research Letters, v. 13, p. 678-680. 

Ballard, R.D., Holcomb, R.T., and van Andel, T.H., 1979, The 

Galapagos Rift at 86°W: 3. Sheet flows, collapse pits, and 

lava lakes of the rift valley: Journal of Geophysical Research, v. 84, p. 5407-5422. 

Chezar, H., and Lee, J., 1985, A new look at deep-sea video: 

Deep-Sea Research, v. 32, p. 1429-1436. 

Davis, A.S., and Clague, D.A., 1987 Geochemistry, mineralogy, 

and petrogenesis of basalt from Gorda Ridge: Journal of 

Geophysical Research, v. 92, p. 10,467-10,483. 

Davis, A.S., Clague, D.A., and Friesen, W.B., Petrology and mineral chemistry of basalt from Escanaba Trough, in Morton, 

J.L., Zierenberg, R.A., and Reiss, C.A., eds., Geologic, hydrothermal, and biologic studies at Escanaba Trough, Gorda 

Ridge, offshore northern California: U.S. Geological Survey Bulletin 2022, chapter 9 (this volume). 
VOLCANIC GEOMORPHOLOGY OF THE SESCA AND NESCA SITES, ESCANABA TROUGH 151 

Denlinger, R.P., and Holmes, M.L., A thermal and mechanical 

model for sediment hills and associated sulfide deposits 

along the Escanaba Trough, in Morton, J.L., Zierenberg, 

R.A., and Reiss, C.A., eds., Geologic, hydrothermal, and 

biologic studies at Escanaba Trough, Gorda Ridge, offshore 

northern California: U.S. Geological Survey Bulletin 2022, 

chapter 4 (this volume). 

Duffield, W.A., 1969, Concentric structure in elongate pillows, 

Amador County, California: U.S. Geological Survey Professional Paper 650-D, p. D19-25. 

Karlin, R., and Lyle, M., 1986, Sediment studies on the Gorda 

Ridge: Oregon Department of Geology and Mineral Industries Open-File Report 0-86-19, 76 p. 

Karlin, R.E., and Zierenberg, R.A., Sedimentation and 

neotectonism in the SESCA area, Escanaba Trough, southern Gorda Ridge, in Morton, J.L., Zierenberg, R.A., and 

Reiss, C.A., eds., Geologic, hydrothermal, and biologic studies at Escanaba Trough, Gorda Ridge, offshore northern 

California: U.S. Geological Survey Bulletin 2022, chapter 

7, (this volume). 

Lonsdale, P., 1977, Abyssal pahoehoe with lava coils at the 

Galapagos rift: Geology, v. 5, p. 147-152. 

Lonsdale, P., and Lawver, L.A., 1980, Immature plate boundary 

zones studied with a submersible in the Gulf of California: 

Geological Society of America Bulletin, v. 91, p. 555-569. 

Macdonald, K.C., 1982, Mid-ocean ridges: Fine scale tectonic, 

volcanic, and hydrothermal processes within the plate 

boundary zone: Annual Review of Earth and Planetary Science, v. 10, p. 155-190. 

Moore, J.G., 1975, Mechanism of formation of pillow lava: 

American Scientist, v. 63, p. 269-277. 

Moore, J.G., Phillips, R.L., Grigg, R.W., and Swanson, D.A., 

1973, Flow of lava into the sea, 1969-1971: Geological Society of America Bulletin, v. 84, p. 537-546. 

Moore, J.G., Cristofolini, R., and LoGuidice, A., 1971, Development of pillows on the submarine extension of recent lava 

flows, Mount Etna, Sicily: U.S. Geological Survey Professional Paper 750-C, p. C89-C97. 

Morton, J.L., Holmes, M.L., and Koski, R.A., 1987, Volcanism 

and massive sulfide formation at a sedimented spreading 

center, Escanaba Trough, Gorda Ridge, northeast Pacific 

Ocean: Geophysical Research Letters, v. 14, p. 769-772. 

Morton, J.L., and Fox, C.G., Structural setting and interaction of 

volcanism and sedimentation at Escanaba Trough: Geophysical results, in Morton, J.L., Zierenberg, R.A., and 

Reiss, C.A., eds., Geologic, hydrothermal, and biologic studies at Escanaba Trough, Gorda Ridge, offshore northern 

California: U.S. Geological Survey Bulletin 2022, chapter 

2 (this volume). 

Morton, J.L., Zierenberg, R.A., and Reiss, C.A., 1991, Geologic, 

hydrothermal, and biologic studies at Escanaba Trough: An 

introduction, in Morton, J.L., Zierenberg, R.A., and Reiss, 

C.A., eds., Geologic, hydrothermal, and biologic studies at 

Escanaba Trough, Gorda Ridge, offshore northern California: U.S. Geological Survey Bulletin 2022, chapter 1 (this 

volume). 

Normark, W.R., 1976, Delineation of the main extrusion zone of 

the East Pacific Rise at lat 21 °N: Geology, v. 4, p. 681-

685. 

Normark, W.R., Gutmacher, C.E., Zierenberg, R.A., Wong, F.L., 

and Rosenbauer, R.J., Sediment fill of Escanaba Trough, in 

Morton, J.L., Zierenberg, R.A., and Reiss, C.A., eds., Geologic, hydrothermal, and biologic studies at Escanaba 

Trough, Gorda Ridge, offshore northern California: U.S. 

Geological Survey Bulletin 2022, chapter 6 (this volume). 

Zierenberg, R.A., Morton, J.L., Koski, R.A., Ross, S.L., and 

Holmes, M.L., 1991, Geologic setting of massive sulfide 

mineralization in the Escanaba Trough, in Morton, J.L., 

Zierenberg, R.A., and Reiss, C.A., eds., Geologic, hydrothermal, and biologic studies at Escanaba Trough, Gorda 

Ridge, offshore northern California: U.S. Geological Survey Bulletin 2022, chapter 10 (this volume). 

Zierenberg, R.A., Morton, J.L., Reiss, C.A., and Holmes, M.L., 

1991, Preliminary results of the 1986 Sea Cliff dive program, Escanaba Trough: U.S. Geological Survey Open-File 

Report 91-137, 176 p. 

Chapter 9. Petrology and Mineral Chemistry of Basalt from 

Escanaba Trough 

By Alice S. Davis, David A. Clague, and Walter B. Friesen1 

CONTENTS 

Abstract ............................................................................... . 

Introduction ......................................................................... . 

Acknowledgments ...................................................... . 

Geologic setting .................................................................. . 

Sampling and analytical methods ...................................... . 

Compositional variation of glasses ................................... .. 

Petrography ......................................................................... . 

Mineral chemistry .............................................................. .. 

Olivine ......................................................................... . 

Plagioclase .................................................................. . 

Spinel ........................................................................... . 

Temperature, density, and viscosity of lava ....................... . 

Comparison with northern Gorda Ridge ............................ . o· . lSCUSSlOn .......................................................................... .. 

Crystal fractionation .................................................. .. 

Magma mixing ............................................................ . 

Partial melting and mantle heterogeneity .................. . 

Contamination ............................................................ .. 

Magma chambers ........................................................ . 

Conclusions ........................................................................ .. 

References cited .................................................................. . 

ABSTRACT 

Page 

153 

153 

154 

154 

154 

154 

159 

160 

160 

161 

162 

162 

163 

166 

166 

167 

167 

168 

168 

169 

169 

Basalt pillow and sheet flow fragments with glassy 

rinds were recovered by dredging, coring, and diving from 

six sites in Escanaba Trough. Of 48 glasses analyzed, all 

except one are normal, low-K20 mid-ocean ridge basalt 

(MORB). One sample from about latitude 41 °00' N. has 

a high K20 content similar to enriched (E-type) MORB. 

The basaltic glasses from Escanaba Trough range from 

relatively primitive (Mg# = 67) to moderately fractionated 

(Mg# = 56), where Mg# is 100xMg/(Mg+Fe2+). Compositional trends are similar to those observed for other 

MORB suites, with incompatible elements like potassium, 

phosphorus, and titanium generally increasing with differentiation, but a large amount of scatter indicates that the 

observed variations are not simply caused by crystal frac1 U.S. Geological Survey, Menlo Park, California. 

tionation. Textural features and compositions of mineral 

phases suggest that magma mixing may have been an important process at some sites. Variations in incompatibleelement ratios along the strike of Escanaba Trough indicate 

complex petrogenetic processes, involving heterogeneous 

source compositions, different degrees of partial melting, 

and magma mixing. Mixing with material more enriched 

in large-ion-lithophile (LIL) elements is indicated for the 

high-K20 sample. Normal MORB glasses from this location are also higher in K20 than comparably fractionated 

samples from the other sites. Spinel compositions in these 

lavas are higher in titanium and ferric iron than is normally 

observed for MORB, suggesting higher oxygen fugacity of 

the melt. Lead isotopic compositions of samples from this 

area are more radiogenic than other Gorda Ridge basalt and 

appear to lie on a mixing line with sediment from this area. 

Similar high K20 compositions have been reported from 

the Explorer Deep segment of Explorer Ridge and 

Guaymas Basin, which have similar tectonic settings as 

Escanaba Trough, suggesting that incipient rifting into a 

sediment-filled basin may produce magmas that are contaminated by small amounts of sediment. 

INTRODUCTION 

Fresh basaltic glass from the chilled margins of submarine-erupted lavas provides an indirect means to assess 

the chemical characteristics of magmatic liquids erupted 

along oceanic spreading centers. Mineral chemistry in conjunction with host glass chemical compositions provide information on fractionation and magma mixing that may 

help to evaluate chemical and physical processes in magma 

chambers. The quantity of data on oceanic ridge basalt has 

increased steadily over the last 10 years, and major chemical features of spreading centers in the North Pacific have 

become better known (Eaby and others, 1984; Cousens and 

others, 1984; Dixon and others, 1986; Davis and Clague, 

1987; 1990; White and others, 1987). 

Prior to this study, the only published glass compositions from Escanaba Trough were five glass analyses from 

three sites (Davis and Clague, 1987). In this report we 

present the compositions of 48 glasses from six sites in 

153 
154 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

Escanaba Trough, the estimated eruption temperatures, densities, and viscosities of the melts, and the compositions 

of crystalline phases. The data are used to assess petrogenetic processes beneath Escanaba Trough and to compare 

basalt from Escanaba Trough with those from the northern Gorda Ridge and other spreading centers. 

ACKNOWLEDGMENTS 

We thank J. Morton, W. Normark, M. Holmes, P. Rona, 

R. Zierenberg, and J. Edmond for providing samples from 

their cruises. 

GEOLOGIC SETTING 

Escanaba Trough comprises the sediment-filled, southernmost segment of Gorda Ridge (fig. 9.1), extending 130 

km northward from the Mendocino Fracture Zone at about 

latitude 40°25' N. to a right-lateral offset near latitude 

41 °35' N. A series of discrete volcanic edifices in 

Escanaba Trough uplift and locally pierce the sediment 

cover (Morton and others, 1987 a,b). DSDP Site 35, drilled 

slightly off axis in the southern end of Escanaba Trough 

(fig. 9.1), penetrated 390m of Pleistocene sediment but recovered no volcanic rocks (McManus, 1970). Two of the 

larger volcanic edifices were studied in detail (Morton and 

others, chap. 1, this volume) in sites SESCA and NESCA. 

Volcanic rocks were dredged from four additional sites 

north of NESCA. 

The volcanic edifices at NESCA and SESCA are broad 

uplifts that are largely sediment covered. At SESCA, exposed basaltic rocks are primarily pillow basalt, whereas 

sheet flows and pillow basalt occur at the NESCA site 

(Ross and Zierenberg, chap. 8, this volume). 

SAMPLING AND ANALYTICAL 

METHODS 

The 48 samples analyzed in this study are glassy rims 

of pillow or sheet flow fragments. Locations at start and 

end of dredge were averaged, and the resulting averaged 

sample locations are included in table 9.1 and shown in figure 9.1. Most of these samples were recovered by dredging on three U.S. Geological Survey cruises of the RN S.P. 

Lee in 1985 (L6-85-NC) and 1986 (L2-86-NC and L3-86-

NC). One sample (L1-86-33G) is glass sand recovered in 

a gravity core on a third cruise in 1986 (L1-86-NC), and 

samples labeled Alv2034, Alv2039, and SC660 were recovered by the submersibles Alvin and Sea Cliff, respectively. 

Three samples dredged on a 1983 U.S. Geological Survey 

cruise of the RN Kana Keoki, for which analyses were previously published (Davis and Clague, 1987), have been included. For brevity, dredge sample designation is limited 

to the year and dredge number (for example, 85-xx); for 

years with multiple cruises the number of the leg is shown 

before the year (2-86-xx). 

Several glass fragments were typically chosen from 

each dredge; more were selected if the dredge haul was 

large or if variation in crystal and (or) vesicle content suggested different lithologies. The polished glass mounts 

were analyzed with a nine-channel ARL-SEMQ electron 

microprobe at the U.S. Geological Survey in Menlo Park. 

The entire Escanaba Trough glass set, including previously 

analyzed samples, were reanalyzed in a single microprobe 

session to improve analytical precision. Analytical precision is 1 to 2 percent for major elements except sodium, 

and 3 to 5 percent for sodium and minor elements. Glass 

was analyzed with an accelerating voltage of 15 kV, a 

sample current of 10 nA, and a defocused beam (- 50 Jlm). 

Eleven samples were selected for microprobe analyses of mineral phases. The mineral phases were analyzed 

with the same accelerating voltage as the glass but with a 

narrowly focused beam ( < 2 J..Uil) and with sample currents 

ranging from 15 nA for plagioclase to 20 and 30 nA for 

olivine and spinel, respectively. The data were reduced 

using a modified version of the Bence and Albee method 

(1968). Details of analytical methods, the standards used, 

and precision and accuracy of microprobe analyses are 

given in Davis and Clague ( 1987). Analyses of glasses and 

mineral phases from the entire Gorda Ridge (Davis and 

Clague, 1987) were performed under the same conditions 

and hence represent an internally consistent data set in 

which small variations in glass and mineral compositions 

can be considered significant. 

COMPOSITIONAL VARIATION OF 

GLASSES 

The major- and minor-element concentrations of 48 

glasses are presented in table 9.1 and shown in figures 9.2 

to 9.6. The glasses from a single dredge haul have been 

grouped into flow units, if all elements are within analytical precision. Most frequently, samples from the same 

dredge represent the same flow unit (for example, 85-36). 

However, several dredges apparently sampled several flow 

units (for example, 3-86-10, 2-86-17, 83-16). At intensively sampled sites, such as NESCA, different dredges 

also apparently sampled the same flow unit. It is important to note that assignment to flow units is somewhat uncertain as lavas with nearly identical compositions have 

been reported from different stratigraphic horizons in 

DSDP sites (Rhodes and others, 1979; Natland and others, 

1983). Conversely, changes in glass chemistry with fractionation in large single flows are not well documented, and 

it is possible that compositions treated as different flow 

units may be variations caused by crystal fractionation 

within larger flows. 
PETROLOGY AND MINERAL CHEMISTRY OF BASALT FROM ESCANABA TROUGH 155 

The basaltic glasses from Escanaba Trough range from 

relatively primitive (Mg# = 67) to moderately fractionated 

(Mg# =56), where Mg# =lOOxMg/(Mg + Fe2+). Except 

128° 

-z, 60° 

~· ... :. ~. ' ':. -""' 

EXPLORER ~1:·: j:.~ JUAN DE FUCA--... ·.\. '% 

0 

GORDA :- Y' 

Area of _.....-: : · 

figure 

GUAYMAS 

I 50° 

30 KILOMETERS 

I 

for one Alvin dive sample (2039-2) from the northernmost 

part of the NESCA site, all glasses are low-K20 normal 

mid-ocean ridge basalt (MORB). Sample Alv2039-2 has 

DSDP 

35 

RIDGE 

127° 

\ 

Alv 2034 

2-86-14 

2-86-16 

2-86-23 

2-86-17 

{

SC660 

2-86-22 

1-86-33G 

6-85-36 

DEPTH IN METERS 

Figure 9.1. Bathymetric map of Escanaba Trough showing sample locations and DSDP Site 35. NESCA and SESCA sites are indicated. 

Inset map shows location of oceanic ridge segments in northern Pacific Ocean. Hachures indicate area of closed low. 
156 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

Table 9.1. Locations and compositions of basaltic glasses from Escanaba Trough. 

[FeOT is total Fe calculated as FeO. Mg# is 100xMg/(Mg+Fe2+); where Fe2+=0.9 FeT] 

Sample Latitude Longitude Flow Si02 Alz03 FeoT MnO MgO CaO Nap K20 PzOs TI02 s Total Mg# 

N. W. unit 

Northern sites 

83-16-1 41°31.3' 127°27.4' 1 50.0 14.8 10.8 0.21 7.08 11.2 2.76 0.24 0.21 1.89 0.12 99.3 56.5 

83-16-2 1 50.5 14.3 11.0 .20 7.12 11.2 2.35 .27 .20 1.95 .13 99.2 56.2 

83-16-3 2 50.0 15.3 9.69 .18 7.80 11.6 2.80 .13 .11 1.58 .11 99.3 61.5 

3-86-10-1a 41 °26.9' 127°25.4' 1 50.9 15.5 9.44 .17 8.22 11.8 2.53 .06 .11 1.31 .10 100.1 63.3 

3-86-10-1b 1 50.9 15.7 9.33 .17 8.18 11.6 2.52 .06 .12 1.26 .09 99.9 63.5 

3-86-10-2a 2 50.6 15.1 10.6 .20 7.40 11.3 2.77 .09 .16 1.71 .12 100.1 58.1 

3-86-10-2b 3 50.7 15.9 9.73 .20 7.30 11.7 2.83 .09 .12 1.51 .11 100.4 59.8 

3-86-10-3a 4 50.4 15.4 10.1 .20 7.55 11.5 2.74 .09 .13 1.64 .11 99.8 59.7 

3-86-10-3b 1 51.1 15.2 9.37 .18 8.12 11.9 2.48 .06 .11 1.32 .10 99.8 63.2 

3-86-10-4a 1 51.1 15.4 9.40 .18 8.05 11.8 2.50 .06 .12 1.30 .10 100.0 63.0 

3-86-10-4b 1 50.8 15.6 9.45 .17 8.16 11.7 2.56 .07 .10 1.30 .09 100.0 63.1 

3-86-10-5a 1 51.1 15.2 9.37 .17 8.10 11.8 2.49 .05 .13 1.31 .11 99.8 63.2 

3-86-10-5b 4 50.8 14.8 10.2 .18 7.58 11.6 2.66 .09 .14 1.60 .12 99.8 59.6 

3-86-2-1 41°20.1' 127°26.5' 50.0 16.4 8.81 .16 8.96 12.0 2.48 .05 .08 1.07 .09 100.1 66.9 

3-86-2-2 50.4 16.3 8.84 .16 8.88 12.0 2.44 .05 .10 1.07 .09 99.9 66.6 

3-86-3-1 41 °07.6' 127°27.6' 50.2 15.7 9.30 .17 8.11 11.6 2.62 .13 .14 1.45 .10 99.5 63.4 

3-86-3-2 50.3 15.8 9.31 .17 8.16 11.6 2.73 .13 .17 1.43 .10 99.8 63.5 

NESCA 

Alv2039-2 41 °05.3' 127°28.5' 50.4 15.5 10.4 0.19 7.52 11.3 2.19 0.43 0.27 1.74 0.12 100.1 59.0 

Alv2034-1 41 °01.0' 127°29.5' 50.8 15.3 9.98 .19 7.35 11.4 2.77 .25 .25 1.63 .11 100.0 59.4 

Alv2034-2 50.7 14.9 10.1 .20 7.21 11.4 2.79 .28 .21 1.65 .11 99.6 58.6 

2-86-14-1 41 °00.7' 127°29.7' 51.0 15.0 10.2 .18 7.20 11.3 2.76 .26 .21 1.75 .11 100.0 58.3 

2-86-14-2 50.4 14.6 10.3 .20 7.30 11.2 2.82 .25 .22 1.71 .12 99.1 58.4 

2-86-16-1 41 °00.7' 127°29.8' 50.8 14.7 10.4 .20 7.41 11.2 2.81 .25 .19 1.75 .12 99.8 58.6 

2-86-16-2 50.8 14.8 10.3 .19 7.29 11.2 2.82 .25 .22 1.70 .12 99.7 58.4 

2-86-23-1-1 41°00.5' 127°29.8' 50.7 14.8 10.4 .19 7.27 11.2 2.83 .25 .22 1.74 .12 99.7 58.1 

2-86-23-1-2 50.8 14.6 10.3 .20 7.28 11.2 2.83 .25 .21 1.72 .12 99.5 58.4 

2-86-23-4-1 51.0 14.7 10.0 .19 6.96 11.4 2.82 .25 .21 1.71 .12 99.4 58.0 

2-86-23-4-2 50.7 14.9 10.5 .20 7.10 11.3 2.87 .26 .18 1.77 .12 99.9 57.3 

2-86-17-1-1 40°59.7' 127°28.1' 1 50.8 15.0 9.99 .18 7.27 11.4 2.68 .27 .23 1.67 .11 99.6 59.1 

2-86-17-1-2 1 50.8 15.2 10.0 .18 7.36 11.2 2.76 .27 .18 1.68 .10 99.7 59.4 

2-86-17-2-1 2 51.1 15.2 10.6 .20 7.36 11.2 2.28 .28 .22 1.80 .12 100.4 57.9 

SC-660 40°59.5' 127°29.4' 51.0 15.3 9.87 .18 7.87 11.3 2.77 .26 .21 1.61 .11 100.5 61.3 

6-85-36-10 40°59.0' 127°30.2' 50.4 15.4 9.77 .19 7.58 11.3 2.72 .25 .19 1.55 .10 99.5 60.6 

6-85-36-11 50.4 15.5 9.60 .19 7.57 11.4 2.73 .24 .18 1.53 .11 99.5 61.0 

6-85-36-12 50.3 15.5 9.71 .18 7.65 11.4 2.71 .24 .17 1.53 .10 99.5 61.0 

6-85-36-13 50.3 15.3 9.76 .19 7.63 11.4 2.69 .24 .17 1.56 .11 99.4 60.8 

6-85-36-14 50.4 15.5 9.60 .18 7.50 11.4 2.74 .25 .18 1.52 .11 99.4 60.8 

6-85-36-15 50.3 15.4 9.73 .20 7.53 11.3 2.70 .25 .19 1.53 .11 99.2 60.6 

6-85-36-16 50.4 15.3 9.69 .19 7.57 11.4 2.72 .24 .20 1.57 .11 99.4 60.8 

6-85-36-17 50.1 15.4 9.73 .20 7.67 11.3 2.68 .25 .18 1.52 .11 99.1 61.0 

6-85-36-18 50.1 15.1 9.82 .19 7.56 11.3 2.72 .24 .19 1.55 .11 98.9 60.4 

1-86-33G 40°58.8' 127°30.5' 1 50.5 15.4 9.93 .18 7.57 11.1 2.76 .25 .19 1.67 .11 99.7 60.2 

2-86-22-1 40°58.5' 127°27.9' 1 • 50.8 15.4 10.1 .18 7.47 11.2 2.79 .27 .20 1.66 .11 100.2 59.5 

2-86-22-2 2 51.0 15.1 10.2 .19 7.68 11.1 2.82 .28 .21 1.73 .11 100.4 59.9 

SESCA 

6-85-31-10 40°46.4' 127°30.8' 51.7 14.8 8.66 0.18 8.32 12.6 2.44 0.11 0.10 1.05 0.10 100.1 65.6 

6-85-31-11 51.6 14.8 8.70 .17 8.35 12.5 2.45 .12 .10 1.07 .09 100.0 65.6 

2-86-12-1 40°45.9' 127°32.7' 51.3 15.6 8.43 .16 8.33 12.7 2.38 .14 .12 1.03 .09 100.2 66.2 

2-86-12-2 51.1 15.3 8.50 .17 8.40 12.6 2.37 .14 .11 1.03 .10 99.8 66.2 
PETROLOGY AND MINERAL CHEMISTRY OF BASALT FROM ESCANABA TROUGH 157 

a K20 content of 0.43 percent, much higher than is typical for normal MORB. Besides a slightly elevated P20 5 

content and lower Na20 abundance, this sample appears 

ordinary with respect to the other elements analyzed. Although the other glasses have K20 contents within the 

range typically encompassed by normal MORBs, two 

groups with distinct ~0 contents are apparent (fig. 9.2A). 

All samples at the NESCA site have higher K20 contents 

than samples from the other sites, except for the two fractionated compositions from 83-16 (Mg# = 56), from the 

northernmost site in Escanaba Trough. Unlike K20, Ti02 

1-

z 

UJ 

() 

a: 

UJ 

a.. 

1-

::c 

C!J 

w 

3: 

A 

0.5 r---------------------. 

o Alv 2039-2 

0.4 -

0.3 - -- 0 -n _ 0 North Gorda Ridge 

/ 0

0 o & ~ qsrrp-1........_ CDavis and Clague, 1987) 

02 -1 .......... , 

I ', 

\ 0 ~ ~ + 

0 0 1 - ' M I ' " 83-04 ~C\J • .......... -~-- A... ' 1 

!z 

w 

() 

a: 

UJ 

a.. 

1-

::c 

CJ 

w 

3: 

6 

C\J 

0 

f= 

-----r M ,, 

0 ~--~~-~~--~~---~~---_-_LI-_-_IL_-_-=·LIX~ 

B 

2.0 ...... 

\ 0 " 

\0 ", 

EXPLANATION 

1.8 ~ \. 0 0 " 

" ~0 0 " 

0 NESCA 

3-86-10 

1.6 -

1.4 -

\. 0 o'21._ o '-..... " cr o , 

0 83-16 

0 3-86-3 

• "~ SESCA .. c§ 0 ', 

"- <l'\_ " Q ' f:,. 3-86-2 

North Gorda Ridge " '-... ~ '\. 

(Davis and Clague, 1987)',• ', + 

1.2 ~ '-.... ' 

" " 83-04 

"• M '-. \ ..... ' ' ' 1.0 -

I I I I I , .......... - 4-x 

55 57 59 61 63 65 67 69 

Mg# = 100 Mg/(Mg+Fe2+J 

Figure 9.2. Variation diagrams of (A) K20 and (B) Ti02 versus 

Mg#, or 100xMg/(Mg+Fe2+), shows large amount of scatter in K20 

at a given Mg#. Samples from NESCA and two samples (83-16) 

from near offset at latitude 41 °35' N. have higher K20 contents than 

comparably fractionated samples from other sites. Ti02 versus Mg# 

shows better defined increasing trend with decreasing Mg# but also 

shows some scatter. Basalts from northern Gorda Ridge, shown for 

comparison (Davis and Clague, 1987), have a similar compositional 

range (outlines), but high-K20 compositions like that of A1v 2039-

2 are absent. Crosses show analytical precision. 

contents show a narrower, better defined trend, increasing 

with Mg# (fig. 9.2B). Al20 3 and CaO in general decrease 

with fractionation, whereas N~O and P20 5 increase with 

decreasing Mg#, but all show considerable scatter (figures 

not shown). Si02 ranges from 50 to 52 percent and shows 

no correlation with Mg#. 

Sulfur apparently behaved like an incompatible element, showing a general increase with differentiation (fig. 

9.3A), but variation in sulfur content at a given Mg# is 

greater than analytical precision. As expected, sulfur shows 

a positive correlation with FeO* (total iron as FeO, fig. 

9 .3B), and the trend lies below or close to the sulfur saturation line determined for MORB (Mathez, 1976). On a 

Ti02 versus FeO* plot (fig. 9.4A) the glass compositions 

show two trends, with samples of NESCA and 3-86-3 and 

83-16 higher in Ti02 at a given FeO* content, suggesting 

z 

0 

::::i 

~ 

~ 

a: 

w 

0... 

(f) 

fa: 

<( 

0... 

~ 

ci 

:::l 

LL 

A 

1, 300 r--------------------, 

0 

1, 200 - 0 co 0 

0 Cim 0 .. 

D £ITIJD 

1,100 i- D ~D fbCD 

0 DO 

1,000-

900 f.-

•• 

·.~ ~ 

800 L--~~-~~~--L---~--~-~~-~~~ 

55 57 59 61 63 65 67 69 

100 Mg/(Mg + Fe2+) 

B 

1' 300 r---------------;-------, 0 

s 1,200 

(f) 

1, 100 

1, 000 

900 

800 

8 9 

6 m o 

A 

()~ 

.. 

10 11 

FeO*, IN WEIGHT PERCENT 

+ 

12 

Figure 9.3. Sulfur versus (A) fractionation index of Mg (Mg#) and 

(B) total iron (FeO*) show generalized increasing trends with 

differentiation, but scatter at a given Mg# is larger than analytical 

precision (crosses). Compositions lie close to, or below, the sulfur 

saturation line determined for MORB (Mathez, 1976). Symbols as 

in figure 9 .2. 
158 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

that they fractionated from parental melts with higher Ti02 

contents than the other samples. 

The effects of crystallization may be qualitatively 

evaluated on a Ca0/Al20 3 versus Mg# plot (fig. 9.4B). 

Olivine fractionation lowers the Mg# without affecting the 

CaO/ Al20 3 ratio, whereas plagioclase fractionation increases the Ca0/Al20 3 ratio without changing the Mg#. 

Clinopyroxene fractionation lowers both numbers. The 

Escanaba Trough glasses show different fractionation paths 

involving different proportions of olivine and plagioclase, 

A 

2.0 

I- 1.8 z 

w 

0 

a: 

w 

0.. 1.6 

II 

(5 

w 1.4 

s 

~ ... -

N 1.2 

0 

f= & + 

1.0 

X 83-04 

8 9 10 11 12 

FeO*, IN WEIGHT PERCENT 

B 

0.90 

0.85 ~ I (") 

0 • <lJ 

N • U) 

<( 0.80 ,... ~ ... u ....... 0 0 0 t 0) co 

0 co 

cifo ...... Olivine 0.. 

0 0 0 0.75 1- A []A Do ... q'''/ t} 0~ c¥P ~ !:;,!:;, <ttr oo 0 

0.70 I I I I I i 

55 57 59 61 63 65 67 69 

Mg# = 100 Mg/(Mg+Fe2+) 

Figure 9.4. A, Ti02 versus total iron (FeO*) shows two trends 

suggesting that samples from NESCA and some northern sites are 

fractionated from parental magmas higher in Ti02. B, CaO/ Al2

0 3 

versus fractionation index ofMg (Mg#) shows different fractionation 

trends. Samples from SESCA have high CaO/ Al20 3 ratios, 

suggesting that fractionation was controlled mostly by plagioclase, 

whereas samples from the other sites show trends compatible with 

mostly olivine and minor plagioclase fractionation. Arrows indicate 

compositional changes with plagioclase, olivine, and clinopyroxene 

fractionation; ticks on arrows show effects of 5 percent fractionation 

of olivine and plagioclase. Symbols as in figure 9.2. 

Table 9.2. Modal analyses of Escanaba Trough basalt samples. 

[Data based on 1,000 point counts. Mg#, 100xMg/(Mg+Fe2+). A, in glass; B, in or 

attached to olivine; C, in or attached to plagioclase;_, absent;<, less than] 

Volume percent 

Sample Mg# Ground- Plagio- Olivine Vesicles Spinel 

mass clase 

Northern sites 

83-16-1 56.5 70 21 4 5 B,C 

83-16-3 61.5 71 26 <I 2 B 

3-86-10-2a 58.1 78 19 2 I c 

3-86-10-3b 63.2 94 4 <I <1 

3-86-10-5b 59.6 87 10 1 2 c 

3-86-2-2 66.6 97 <I I <I c 

3-86-3-1 63.4 98 0 I 1 A,C 

NESCA 

Alv2039-2 59.0 97 <I <1 <I A 

2-86-23-1-1 58.1 93 2 <I 4 B 

2-86-17-1-1 59.1 95 3 <I <1 B,C 

6-85-36-11 61.0 96 1 2 I A,C 

2-86-22-1 59.5 98 0 <I 1 c 

SESCA 

6-85-31-10 65.6 90 7 1 2 

2-86-12-2 66.2 92 5 2 <I 

assuming a parental composition similar to the most primitive composition from Escanaba Trough (3-86-2). The 

large increase in CaO/ Al20 3 ratio, without a significant 

change in Mg#, shown by SESCA glasses suggests primarily plagioclase fractionation with only minor olivine. 

Modal abundance of plagioclase in these samples is somewhat greater than for NESCA samples (table 9.2), but crystal fractionation calculations (Stormer and Nicholls, 1978) 

suggest more than 10 percent plagioclase would have to 

fractionate to increase the Ca0/Al20 3 ratio to that observed. The small variation in CaO/ Al20 3 over a large 

range of Mg#s observed for the NESCA samples is compatible with primarily olivine and minor plagioclase fractionation. Since isolated magma chambers and different 

source compositions generated by different degrees of partial melting are indicated for the northern segments of the 

Gorda Ridge (Davis and Clague, 1987), different fractionation paths are probably to be expected from geographically separated areas. However, samples from the same 

dredge haul (for example, 3-86-1 0) also show different 

trends, suggesting that variations are not the result of closed 

system fractionation. The range in Ca0/Al20 3 at a constant Mg# for samples from 3-86-10 (fig. 9.4B) suggests 

different amounts of plagioclase fractionation. 

K20IP20 5 ratios versus Mg# (fig. 9.5A) show a low 

ratio of about 0.5 for most samples from the northern sites, 

high ratios of about 1.3 or more for NESCA and samples 

of 83-16, and ratios that are between these for samples 

from SESCA and 3-86-3. K20/Ti02 ratios show a similar 
PETROLOGY AND MINERAL CHEMISTRY OF BASALT FROM ESCANABA TROUGH 159 

pattern (fig. 9.5B). Ratios of incompatible elements like 

potassium, phosphorus, and titanium should not be significantly changed by fractionation over the differentiation 

range shown by these samples. Hence, variations in these 

ratios suggest complex petrogenetic processes involving 

different percentages of partial melting and (or) different 

source compositions. 

Compositional variation along the strike of Escanaba 

Trough (fig. 9.6) shows relatively unfractionated compositions at SESCA and 3-86-2 and 3-86-3. At NESCA, all 

lavas are relatively fractionated (Mg# = 59 ± 2.0). Sites 

83-16 and 3-86-10 have relatively primitive and evolved 

compositions at the same location (fig. 9.6A). K20IP20 5 

ratios along the strike of Escanaba Trough show more enriched compositions at NESCA, with the highest ratio 

shown by the high-K20 sample Alv2039-2 (fig. 9.6B). 

Samples from SESCA and from the northernmost site close 

1.[) 

0 

N 

0.. 

....... 

0 

N 

~ 

N 

0 

1-

0 

N 

~ 

A 

2.0 

D 

1.5 - D D 

0 ornJ§ D D D 

0 i38o D~ 

1.0 - D 

J ... 0.5 -

<> 

<> .a. 

... ...... 

... 

• •• • 

83-04 

\ 

X 

+ 

0 ~---~~---~~----~~---~~--~~----~1-----l~ 

B 

0.3 ,.-----------------------------------------, 

D 

0.2 -

D ~ q9 D nffi=P 0 

0 • 

I 0.1 -

0 ~ 83-04 

... Jt ~ \ fJ.fJ. +X 

0 I I I I I I I 

55 57 59 61 63 65 67 69 

Mg# = 100Mg/(Mg+Fe2+) 

Figure 9.5. K20IP20 5 (A) and K20ffi02 (B) versus fractionation 

index of Mg (Mg#) show ratios for samples from NESCA and 83-

16, the northernmost site close to the offset, that are twice as high as 

samples from other sites. Symbols as in figure 9.2. 

to the offset (83-16) also have relatively high K20IP20 5 

ratios (> 1 ), whereas samples from the other sites have 

lower ratios. Variations in K20/P20 5 beyond analytical 

uncertainties at a given location may indicate small-scale 

heterogeneities. 

PETROGRAPHY 

Most of the analyzed samples from the NESCA site 

are sheet flow fragments, whereas samples from the other 

sites are predominantly pillow basalt. All samples are fresh 

with glass rinds a millimeter to a centimeter thick. 

Palagonite is thin or absent. Modal compositions of 

samples selected for analyses of mineral phases are given 

in table 9.2. Typically the glassy rind grades into a 

variolitic zone and then into a pilotaxitic zone toward the 

pillow interior. Except for samples from dredges 83-16 and 

3-86-10, the samples are sparsely porphyritic, with plagioclase and olivine phenocrysts. In the sparsely porphyritic 

samples, phenocrysts are most commonly small, euhedral 

single crystals or small glomerocrysts. The NESCA 

A 

70 ~-------~------------------~--------~· 

(j) 

(j) 

:5 65 (.9 

lL 

0 

'it 

([) 

c 

0 

N 

0 

c 

OJ 60 f- u 2 .g 

L() 

0 

c 

([) 

2 

B 

([) 

1.4 f- g 

N 

C\J 1.0 0.. 

-...._ 

0 

C\J 

:0:::: 

0 

c 

0.6 - g 

D 

c 

([) 

2 

() 

(j) 

w 

(j) 

• • 

) 

• 

~ 

() 

(j) 

w 

z 

a 

~8 0 

~ 

0 

u 

<) 

<) 

t 

• • 

t 

o I 

8 I 

I 

I_ 

1 .. ~ 

10 

I 

I I I I ,I O · 2 l__L_____j __ ...l...___L __ ..!..___L_L___L____.I. __ 

1 

_L_____i.._4_1.J... 0 3-0--:-"' ....____. 

40''30' 40"45' 41'00' 41"15 

NORTH LATITUDE 

Figure 9.6. Compositional variation along ridge axis of Escanaba 

Trough. A, Mg# versus latitude shows only relatively fractionated 

compositions at NESCA, only unfractionated compositions at 

SESCA, and larger range in compositions at 83-16 and 3-86-10. B, 

K20IP20 5 ratios along ridge axis are variable with high and low 

ratios in close proximity, indicating complex petrogenetic processes. 

Symbols as in figure 9.2. 
160 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

samples have a greater abundance of minute crystallites, 

which show a pronounced flow alignment. Some samples 

from dredges 83-16 and 3-86-10 are highly porphyritic 

(>30 percent), containing plagioclase phenocrysts up to a 

centimeter across with disequilibrium textures. The plagioclase phenocrysts are usually anhedral, with fritted or 

embayed margins, and show strong optical zoning. Some 

plagioclase crystals enclose other plagioclase crystals and 

(or) devitrified glass inclusions. A few larger olivine phenocrysts in the highly plagioclase-phyric samples also contain glass inclusions and have embayed margins. Some 

olivine crystals in samples from 3-86-10 subophitically enclose plagioclase or occur as rare coronate overgrowths on 

plagioclase. 

Minute euhedral to subhedral spinel crystals occur in 

many samples and may be included or attached to olivine, 

plagioclase, or in the groundmass. No clinopyroxene, either as phenocryst or as groundmass phase, was observed. 

MINERAL CHEMISTRY 

OLIVINE 

Representative olivine compositions are given in table 

9.3. Previously published analyses of olivine in samples 

from 83-16 (Davis and Clague, 1987) are not included in 

the table but are shown in figure 9.7. Olivine compositions for the Escanaba Trough set collectively range from 

Fo83 to Fo89, but Fo84 and Fo86 are the most common compositions (fig. 9.7A). Most samples show a narrow range 

of Fo contents spanning only 1 molar percent in a given 

section, but the highly plagioclase-phyric samples of 83-

16 and 3-86-10 show more variability, with the largest 

range of Fo contents in 83-16-1 (Fo83_89). However, individual crystals in these samples are typically unzoned or 

only very slightly zoned ( <1 molar percent). Maximum 

zoning of about 2 molar percent is observed in rare crystals of samples 3-86-2, 3-86-10-Sb and 3-86-10-2a. Zoning, when present, is normal with cores more magnesian 

than the rims. 

The NiO contents of olivine range from 0.14 percent 

in an embayed phenocryst of sample 83-16-1 to 0.31 percent in a euhedral microphenocryst in sample 3-86-2-2. 

NiO decreases with decreasing Fo contents but shows 

broad scatter in excess of analytical uncertainty (fig. 9.7B). 

Samples from the SESCA site are generally lower in NiO 

at comparable Fo contents than the other samples. The 

magnesian-rich core of one olivine crystal in sample 3-86-

2-2 is lower in NiO than the less magnesian rim. The highest NiO contents of 0.31 percent occurs in this sample in 

a euhedral microlite (Fo87). Fo contents versus Mg# of the 

host glasses shows that the aphyric and sparsely porphyritic glasses have some olivine compositions close to the 

calculated equilibrium Fo contents (fig. 9.7C), assuming a 

30 

(/) 

w 

(/) 

>-

~ 20 

z 

<( 

1..1.. 

0 

a: 

~ 10 

2 

::J 

z 

0 

A 

EXPLANATION 

[] With spinel 

D Cores 

• Rims 

83 84 85 86 87 88 89 90 

MOLAR PERCENT FORSTERITE 

B 

0.4 ,----------------------, 

I- Gorda Ridge olivine 

ffi (Davis and Clague, 1987) 

0 \ ---~:.-........ 

r3~ /~::ft~ ~~~7~-04 3 / UJ 0 0 4 4 4 <I!!~ ~ 0.2 r- / o 0

o li ocno 0 ••• / o o • eo ./ c5 / oo 0 • 0./ z o 0 o 4 _!....-- __..-

l __ ~------ + 

0.1 ~-~i--~'-~'-~'~-~~-~~~-~~-~~-~ 

(/) 

(/) 

<( 

66 

6 64 

I-

(/) 

0 62 I 

1..1.. 

0 

it 60 

Ol 

2 

58 

c 

+ * 56 L--~--L-~--~-~-~-~-~-~ 

81 83 85 87 89 

MOLARPERCENTFORSTERITE 

Figure 9.7. A, Olivine compositions of Escanaba Trough lavas show 

narrow range inFo contents, ranging from 83 to 89. B, NiO decreases 

with decreasing Fo content but shows large amount of scatter. Field 

of northern Gorda Ridge olivine is shown for comparison. Striped 

field shows compositional range of olivine in most primitive glass 

from northern Gorda Ridge (83-D4). C, Fo of olivine versus Mg# 

of host glass shows most evolved olivine compositions close to being 

in equilibrium with their host glass. Equilibrium Fo content, shown 

for reference (stars), was calculated assuming distribution coefficient 

(Kn) = 0.30. Many olivine crystals are too magnesian to be in 

equilibrium with their host glass. Symbols as in figure 9.2. 
PETROLOGY AND MINERAL CHEMISTRY OF BASALT FROM ESCANABA TROUGH 161 

Table 9.3. Representative olivine analyses from Escanaba Trough. 

[C, core; R, rim of phenocryst; me, center of microphenocryst; s, spinel included or attached] 

Sample no. ---------- 16-1 10-2a 10-2a 10-3b 

Position -------------- c R c R 

Si02 ------------------- 40.3 39.5 39.9 39.1 

FeO ---------------- 11.4 14.6 12.2 13.7 

MgO --------------- 47.6 45.8 47.3 45.6 

CaO ---------------- .31 .28 .29 .25 

MoO --------------- .18 .25 .20 .24 

NiO ---------------- .22 .23 .22 .25 

Total ------------ 100.0 100.7 100.2 99.1 

Fo mol pet. ------- 88.1 84.9 87.3 85.6 

NESCA 

Sample no. ---------- 2039-2 2-23-1 2-22-1 17-1 

Position -------------- me c c mc8 

Si02 ------------------- 39.0 39.3 38.9 39.1 

FeO ---------------- 15.6 15.3 14.8 14.7 

MgO --------------- 44.3 44.7 44.8 44.6 

CaO ---------------- .29 .29 .28 .30 

MoO --------------- .25 .29 .20 .26 

NiO ---------------- .18 .19 .22 .23 

Total ------------ 99.7 100.1 99.2 99.1 

Fo mol pet. ------- 83.5 83.9 84.4 84.4 

distribution coefficient (K0 ) of 0.30. The distribution of 

iron and magnesium between olivine and liquid can vary 

between K0 = 0.25 and 0.35, depending on the oxygen 

fugacity, temperature, and pressure (Walker and others, 

1979; Bender and others, 1978). More crystal-rich samples 

have a wider range of olivine compositions, containing 

magnesian-rich crystals that are not in equilibrium with 

their host glass. 

PLAGIOCLASE 

Compositions of plagioclase in Escanaba Trough 

glasses (table 9.4, fig. 9.8) range from An65 for a phenocryst core in sample 3-86-10-2a to An87 in the core of a phenocryst in 83-16-3. Except for anorthite-rich phenocrysts 

in the highly plagioclase-phyric samples from the northernmost site in Escanaba Trough (83-16), phenocrysts with 

An contents above 80 are rare, and An contents between 

72 to 75 are most common (fig. 9.8). Plagioclase phenocrysts from the SESCA site show small amounts of normal 

zoning. By contrast, all the plagioclase from the NESCA 

site show only reverse zoning with rims higher in An contents than cores by 1 to 10 molar percent. Samples from 

both flow units of 83-16 show mostly normal zoning by 2 

to 4 molar percent but can show up to 12 molar percent 

difference between core and rim. Unfractionated samples 

of 3-86-2 and 3-86-3 are almost devoid of plagioclase pheNorthern sites 

10-3b 10-5b 10-5b 2-2 2-2 3-1 3-1 

c Rs cs R c Rs cs 

39.7 39.6 39.4 39.4 39.9 39.8 39.4 

13.7 13.8 13.3 12.0 10.4 13.4 13.4 

45.8 46.1 46.4 47.0 47.9 46.3 46.4 

.26 .29 .29 .30 .31 .28 .28 

.21 .26 .25 .20 .18 .21 .23 

.25 .23 .22 .27 .23 .23 .25 

100.0 100.3 99.8 99.0 98.9 100.1 99.9 

85.6 85.6 86.2 87.6 89.1 86.0 86.1 

SESCA 

36-11 36-11 36-11 31-10 31-10 12-2 12-2 

R c me R c me me 

39.4 39.3 38.8 40.0 39.8 39.8 39.7 

13.7 13.5 14.2 12.3 12.1 11.9 12.0 

45.8 45.8 46.0 46.4 47.3 47.2 46.6 

.26 .26 .32 .31 .30 .32 .34 

.23 .21 .24 .22 .20 .24 .23 

.24 .24 .20 .16 .21 .24 .20 

99.7 99.3 99.7 99.3 99.9 99.7 99.0 

85.6 85.8 85.0 87.1 87.5 87.7 87.4 

nocrysts but contain rare micro lites with An contents of 72 

or less. K20 contents are close to or below detection limit 

for most samples, but the higher K20 host glasses from 

NESCA contain plagioclase with measurable K20 contents, 

and the high-K20 sample Alv2039-2 has 0.06 percent K20 

in plagioclase (table 9.4). 

(f) 

w 

(f) 

r 

_j 

<t: 

z 

<t: 

LL 

0 

a: 

w 

(l) 

~ 

:::> 

z 

12 ~-----------------------------------. 

10 

8 

6 

4 

2 

0 

66 

EXPLANATION 

D With spinel 

D Cores 

• Rims 

70 74 78 82 86 

MOLAR PERCENT ANORTHITE 

Figure 9.8. Histogram of plagioclase anorthite content in Escanaba 

Trough lavas shows compositions ranging from An87 to An65 , with 

An72 and An74 most frequently occurring compositions. 
162 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

Table 9.4. Representative plagioclase analyses from Escanaba Trough. 

[C, core; R, rim of phenocryst; me, center of microphenocryst; s, spinel included or attached] 

Sample no. ----------- 16-1 16-1 16-3 10-2a 10-2a 

Position -------------- c R cs c R 

Si02 ------------------- 47.1 50.8 45.9 52.1 49.9 

Al20 3 ---------------- 33.2 30.3 34.2 30.2 32.1 

FeO ---------------- .29 .47 .23 .41 .42 

CaO ---------------- 16.6 14.3 17.9 13.3 15.0 

Na20 -------------- 1.97 3.32 1.49 3.95 3.07 

K20 --------------- 0 .05 0 .02 .02 

MgO --------------- .26 .29 .21 .31 .28 

Total ------------ 99.4 99.6 99.1 100.4 100.8 

An mol pet. ------ 82.3 70.2 87.0 65.0 72.8 

NESCA 

Sample no. ---------- 2039-2 17-1 17-1 17-1 36-11 

Position -------------- me R c mc8 Rs 

Si02 ------------------- 50.5 48.6 51.0 50.3 49.8 

A120 3 ---------------- 30.8 33.1 31.1 30.7 31.2 

FeO ---------------- .56 .49 .45 .49 .45 

CaO ---------------- 14.2 16.3 14.4 14.5 15.0 

Na20 -------------- 3.35 2.20 3.25 3.30 3.02 

K20 --------------- .06 .02 .04 .05 .04 

MgO --------------- .28 .28 .29 .26 .25 

Total ------------ 99.9 101.1 100.6 99.7 99.8 

An mol pet. ------ 69.8 80.4 70.9 70.7 73.2 

SPINEL 

The majority of spinel crystals are included in or attached to olivine or plagioclase, but a few occur in the 

groundmass. Spinel crystals in most of the glasses are 

magnesiochromites (table 9.5). Mg numbers of spinel 

range from 0.70 to 0.56 and are negatively correlated with 

Cr/(Cr+Al) or Cr#, which ranges from 0.42 to 0.56 (fig. 

9.9A). Like Cr#, Ti02 generally increases with decreasing Mg#, ranging from 0.4 to 1.6 percent, but shows two 

different trends with high- and low-Ti02 spinel at comparable Mg#s. In samples from 3-86-10, high- and low-Ti02 

spinel occur in the same sample. The most magnesian 

spinels occur in olivines in the unfractionated glasses from 

the locations north of NESCA. Spinels in NESCA glasses 

are all higher in Ti02 and ferric iron than is commonly observed for MORB spinel (Sigurdsson and Schilling, 1976; 

Davis and Clague, 1987; 1990). Spinel compositions in 

the NESCA samples show a narrow range, regardless of 

host phase. No spinel was observed in the SESCA glasses, 

although they are much less fractionated than NESCA 

glasses. Mg# of the host glass versus Mg# of spinel (fig. 

9.9C) shows a positive correlation, but two distinct trends 

exist. The crystal-rich, relatively fractionated glasses from 

83-16 and 3-86-10 contain some spinel crystals that are too 

magnesian to be in equilibrium with their host glass. 

Northern sites 

10-2a 10-2a 10-3b 10-3b 10-Sb 10-Sb 2-2 3-1 

c R R c R c me me 

52.3 52.1 48.3 48.9 51.5 50.8 48.6 51.3 

30.5 30.4 33.1 32.2 30.5 30.6 32.4 30.7 

.44 .39 .39 .41 .50 .48 .45 .83 

13.4 13.6 16.4 15.9 13.7 13.8 15.8 14.3 

4.04 3.84 2.21 2.54 3.83 3.73 2.51 3.34 

.03 .03 .01 .01 .02 .01 .01 .03 

.24 .27 .26 .28 .25 .22 .29 .44 

99.5 100.6 100.6 100.3 100.3 99.6 100.1 100.7 

64.5 66.1 80.3 77.6 66.3 67.1 77.6 70.1 

SESCA 

36-11 36-11 31-10 31-10 31-10 12-2 12-2 12-2 

cs me c R me R c me 

50.1 50.0 50.0 50.8 50.5 47.7 47.3 50.1 

30.8 31.7 31.2 31.1 31.7 32.4 32.8 31.1 

.41 .58 .42 .42 .53 .43 .35 .56 

14.9 15.3 15.4 15.0 15.1 16.6 17.0 15.0 

3.22 2.99 2.79 3.03 2.94 2.ll 1.85 2.96 

.04 .04 .02 .03 .03 .02 .02 .03 

.26 .27 .33 .31 .32 .26 .28 .39 

99.6 

71.6 

100.9 100.1 100.1 101.0 99.6 99.6 100.1 

73.8 75.2 73.1 73.8 81.3 83.5 

TEMPERATURE, DENSITY, AND 

VISCOSITY OF LAVA 

73.6 

Estimated eruption temperatures, densities, and viscosities for Escanaba Trough glasses are given in table 9.6. 

Eruption temperatures have been calculated using the olivine-glass geothermometers of Roeder (1974) and Bender 

and others (1978) for samples with coexisting olivine data. 

For other samples, temperatures were determined from a 

graphic geothermometer using only the MgO of the glass 

(Davis and Clague, 1990). Density and viscosity were calculated using the method ofBottinga and others (1982) and 

Shaw (1972), respectively, using temperatures determined 

with the graphic geothermometer (fig. 9.10A). Eruption 

temperatures range from 1, 170°C to 1,220°C (fig. 9.10A). 

Densities also show a limited range from 2.654 g/cm3 for 

one of the least fractionated samples of 3-86-2, to about 

2.687 g/cm3 for the most fractionated composition of 83-

16-1. As observed for other MORB suites (Sparks and others, 1980), density increases with differentiation over the 

compositional range present, but shows considerable scatter at a given MgO content (fig. 9.10B). Viscosity is low 

in the unfractionated compositions and increases with differentiation but is not directly correlated with density (table 

9.6). 
PETROLOGY AND MINERAL CHEMISTRY OF BASALT FROM ESCANABA TROUGH 163 

Table 9.5. Representative analyses of chromian spinel from Escanaba Trough. 

[C, core; R, rim of larger crystals; me, center of microphenocryst. Fe3+ calculated from total Fe and structural formula. Spinel occurrence: 0, in 

olivine; P, in plagioclase; G, in groundmass] 

Northern sites 

Sample no. ---------- 16-1 16-1 16-l 16-3 10-2a l0-2a 10-5b 2-2 3-1 3-1 3-1 

Occurrence ---------- 0 0 0 0 0 p 0 0 G G 0 

Position -------------- me R c me me me me me R c me 

Al20 3 ---------------- 31.0 26.9 26.5 28.8 27.2 22.2 29.0 28.1 26.2 25.2 26.1 

Fe20 3 ---------------- 6.47 7.71 6.68 6.25 6.86 8.26 5.92 6.61 6.90 6.91 7.12 

FeO ---------------- 13.9 14.9 14.0 12.2 14.9 17.3 13.3 12.7 14.6 14.5 14.5 

MgO --------------- 15.0 14.1 14.5 15.9 14.5 12.6 15.7 15.8 14.6 14.4 14.8 

MnO --------------- .29 .30 .29 .26 .25 .29 .22 .25 .25 .25 .26 

Ti02 ------------------- .40 .64 .53 .49 .69 1.37 .68 .63 .98 1.06 1.01 

Cr20 3 ---------------- 32.9 35.6 37.2 35.7 37.1 38.5 36.3 36.3 37.1 37.3 37.5 

Total ------------ 99.9 100.6 99.7 99.6 101.5 100.5 101.1 100.3 100.6 99.6 101.2 

Cationic proportions 

Mg/Mg+Fe2+ ----- .658 .628 .649 .699 .634 .565 .678 .690 .641 .640 .646 

Cr/Cr+Al --------- .416 .470 .485 .454 .478 .538 .457 .464 .487 .498 .491 

NESCA 

Sample no. ---------- 2039-2 2039-2 23-1-l 22-1 22-l 17-1-1 17-1-1 17-1-1 36-11 36-11 36-11 

Occurrence ---------- G G p 0 0 p p 0 0 0 G 

Position -------------- me me me c R me me me me me me 

A120 3 ---------------- 22.3 22.0 20.7 21.9 21.4 20.5 21.1 21.5 22.2 22.2 22.5 

Fe20 3 ---------------- 9.37 9.68 9.28 8.60 9.63 9.26 8.77 8.70 8.91 8.37 8.30 

FeO ---------------- 16.9 16.6 17.1 17.3 16.6 16.8 16.8 17.0 15.7 16.1 15.6 

MgO --------------- 12.8 13.0 12.6 12.7 12.9 12.6 12.8 12.8 13.4 13.2 13.4 

MnO --------------- .29 .29 .30 .29 .29 .29 .20 .28 .27 .28 .28 

Ti02 ------------------- 1.48 1.53 1.51 1.55 1.50 1.48 1.46 1.56 1.32 1.34 1.20 

Cr20 3 ---------------- 36.8 36.9 38.8 38.3 37.7 38.6 39.1 38.5 37.7 38.2 38.2 

Total ------------ 99.9 99.9 100.2 100.6 100.1 99.5 100.3 100.3 99.5 99.7 99.5 

Cationic proportions 

Mg/Mg+Fe2+ ----- .575 .583 .569 .567 .580 .573 .576 .573 .604 .594 .604 

Cr/Cr+Al --------- .525 .529 .557 .540 

COMPARISON WITH NORTHERN 

GORDA RIDGE 

Escanaba Trough glasses have a compositional range 

similar to those from the two northern segments of the 

Gorda Ridge (Davis and Clague, 1987). However, at the 

northernmost segment of the Gorda Ridge there is one 

sample with a composition more primitive (Mg# = 69) than 

the least fractionated Escanaba Trough glass (Mg# = 67), 

and at the middle section of Gorda Ridge there is one more 

fractionated (Mg# = 55.5) than the most fractionated 

Escanaba Trough glass (Mg# = 56.6). K20 in northern 

Gorda Ridge glasses also shows considerable scatter at a 

given Mg# (Davis and Clague, 1987). However, none of 

the higher K20 samples has a K20 content as high as 

Alv2039-2 from the northern end of the NESCA site. 

Other elements in northern Gorda Ridge glasses show 

trends similar to those from Escanaba Trough. K20IP20 5 

and K20/Ti02 ratios also show two groups, one with low 

ratios ( -0.5) and one about twice as high, erupted in close 

proximity, indicating heterogeneity at a ridge segment and 

even within the area sampled by a single dredge. 

.542 .558 .554 .546 .533 .536 .533 

Mineralogically, Escanaba Trough samples are also 

similar to those from the northern Gorda Ridge, with highly 

plagioclase-phyric glasses and nearly aphyric samples in 

close proximity. Fo contents in olivine from northern 

Gorda Ridge also show a slightly larger range (Fo89_82), 

in accord with a slightly larger range of glass compositions. 

Unlike Escanaba Trough samples, some of the northern 

Gorda Ridge glasses contain olivine with significant normal or reverse zoning (> 2 molar percent; Davis and 

Clague, 1987). 

Plagioclase from northern Gorda Ridge also has a 

larger compositional range, ranging from An92 to An64. 

Most Escanaba Trough plagioclase compositions cluster 

around An contents of 72 to 74 and appear to be in equilibrium with their host glass. A similar range of An contents is also observed for microlites in northern Gorda 

Ridge glasses, but An contents of 82 to 86 are most frequent for phenocrysts. A higher frequency of more calcic 

plagioclase at northern Gorda Ridge most likely reflects an 

analytical bias of preferentially analyzing larger phenocrysts, which are more abundant than in Escanaba Trough 

lavas. Unusually calcic plagioclase (An92) occurs at one 
164 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

place on the northern Gorda Ridge that has no counterpart 

in Escanaba Trough glasses. These unusually calcic compositions are limited to large megacrysts (> 1 em) with pronounced disequilibrium textures (Davis and Clague, 1987). 

Spinel compositions from northern Gorda Ridge are 

similar to those from Escanaba Trough with respect to Cr# 

A 

0.6 r-------------------------------------~ 

%octJ 

~ 0 0 * 0 0 0 o/o 

..--

I o 

1 83-015 ~ 0.5- I 

8 

1-

z 

w 

(.) 

a: 

w 

a.. 

1-

:::c 

(!J 

w 

3: 

~ 

c\1 0 

i= 

"- goo /'o• 

North Gorda Ridge "' , 

low-Ti spinel "\,. 

- 0 

(Davis and Clague, 1987) """ 

I I O" 0.4 ~----------~------------~--~--------~ 

1.6 

1.4 

1.2 

1.0 

0.8 

0.6 

70 

B 

*\ 0°0 ~cfl 83-015""' ~ 

0:~ 

o<> 

North Gorda Ridge"' 

-~~ o '-......._ ~:::,. 

low-Ti spinel "" ~ (Davis and Clague, 1987) """ 

c 

0 0 0 0 

c 

55 ~----------~~----------~------------~ 

0.55 0.60 0.65 0.70 

Mg/(Mg+Fe2+) OF SPINEL 

Table 9.6. Temperature, density, and viscosity of selected Escanaba 

Trough glasses. 

[Mg-Gisl, olivine-glass geothermometer of Roeder (1974); Mg-Gis2, olivine-glass 

geothermometer of Bender and others (1978); MgO-Gls, graphic geothermometer (fig. 

9.IOA); -, not determined. Density and viscosity calculated crystal-free at 1 kbar, 

following methods of Bottinga and others (1982) and Shaw (1972), respectively, using 

temperatures determined with graphic geothermometer] 

Sample 

number 

83-16-1 

83-16-3 

3-86-10-2a 

3-86-10-1b 

3-86-10-5b 

3-86-2-2 

3-86-3-2 

Alv2039-2 

2-86-23-4-1 

2-86-17-1-1 

6-85-36-11 

2-86-22-1 

6-85-31-10 

2-86-12-2 

¢:= 

Temperature 

Mg-Gisl Mg-Gls2 MgO-Gls 

<oq <oq ec) 

1,182 

1,199 

1,194 

1,184 

1,193 

1,189 

1,207 

1,209 

Northern sites 

1,168 

1,187 

1,181 

1,181 

1,180 

1,175 

1,196 

1,198 

1,176 

1,193 

1,184 

1,200 

1,187 

1,217 

1,200 

NESCA 

1,186 

1,173 

1,212 

1,185 

1,184 

SESCA 

1,204 

1,206 

Density 

(g/cm3) 

2.687 

2.671 

2.683 

2.663 

2.677 

2.661 

2.666 

2.683 

2.671 

2.672 

2.668 

2.672 

2.653 

2.654 

Viscosity 

(poise) 

131 

108 

133 

122 

132 

86 

109 

135 

190 

160 

144 

145 

126 

117 

Figure 9.9. Compositions of chromian spinel in Escanaba Trough 

lavas. A, Cr/(Cr+Al) versus Mg/(Mg+Fe2+) shows Cr/(Cr+Al) 

increasing with decreasing Mg/{Mg+Fe2+). B, Ti02 versus Mg/ 

(Mg+Fe2+) shows two distinct trends with NESCA samples 

containing only high-Ti spinel, whereas some other samples contain 

high- and low-Ti spinel in the same section. Field of northern Gorda 

Ridge spinel shows compositions more magnesian and lower in TI 

than most Escanaba Trough spinel. Only one resorbed spinel (83-

D 15) from northern Gorda Ridge resembles high-Ti spinel found in 

NESCA lavas. C, Mg/{Mg+Fe2+) of spinel versus Mg# of host glass 

also shows two trends, with some spinel in equilibrium with their 

host glass and some containing spinel that is too magnesian. All 

NESCA samples appear to be in equilibrium with their host glass. 

Symbols as in figure 9.2. 

~ 

Figure 9.10. A, MgO versus experimentally determined temperatures 

of olivine-glass pairs in MORB (Walker and others, 1979; Bender 

and others, 1978) and calculated temperatures of Gorda Ridge lavas 

form well-defined trend, indicating that this plot can be used as a 

crude graphical geothermometer. Temperatures of Escanaba Trough 

lavas without coexisting olivine data were estimated from this plot. 

B, Densities of Escanaba Trough lavas (filled circles) calculated after 

the method of Bottinga and others (1982) show general increase 

with differentiation, as commonly observed for other MORB suites, 

but diffuse pattern is similar to that of type B basalt from Mid-Atlantic 

Ridge (Sparks and others, 1980). EPR, East Pacific Rise; MAR, 

Mid-Atlantic Ridge. Solid line shows calculated change in density 

with differentiation for an aphyric lava. 
PETROLOGY AND MINERAL CHEMISTRY OF BASALT FROM ESCANABA TROUGH 165 

and Mg# but include many more magnesian crystals. However, except for one resorbed spinel crystal in glass from 

one sample from the middle segment of the ridge, all northA 

1000 

(/) 

2 1050 Northern () 

~ Gorda Ridge 

w 

() Escanaba 

(/) 

w 1100 w 

0: 

CJ 

w 

0 

~ 1150 

u.r 

0: 

:::> 

1-

<( 1200 

0: 

w 

a.. 

~ 

w 

I- 1250 

1.0 2.0 3.0 4.0 5.0 

ern Gorda Ridge spinel is low in Ti02 as is typical for 

MORBs. All spinels from NESCA have more than 1 percent Ti02 and higher Fe20 3 contents than is commonly 

o Bender and others C I 978) 

• Walker and others C I 979) 

(! Roeder equation C I 9 74) 

Bender and others equation C I 9 7 8) 

{I Roeder equation C I 9 7 4) 

Bender and others equation C I 9 7 8) 

* Escanaba without olivine data 

s • ~0 0 

o........._ 

oq,..._ 

6.0 7.0 8.0 9.0 10.0 1 1.0 

MgO, IN WEIGHT PERCENT 

0: B 

w 

1-

w 

~ 2.80 i= 

z 

w 

() 

() 

co 

:::> 2.75 () 

0: 

w 

a.. (A) 

(/) 

~ 

<( 2.70 0: 

CJ 

~ 

~ 

!= 

(/) 2.65 

z 

w 

0 2 4 6 8 10 1 2 14 16 

MgO, IN WEIGHT PERCENT 
166 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

observed for MORB (Sigurdsson and Schilling, 1976; 

Davis and Clague, 1987; 1990). 

Consistent with their low degree of crystallinity and 

high calculated eruption temperatures, Escanaba Trough 

glasses have no clinopyroxene, although the imprint of 

clinopyroxene fractionation is suggested by low CaO/ 

Al20 3 ratios and displacement into the diopside field on 

the diopside-olivine-silica projection (fig. 9.11; Walker and 

others, 1979). Glasses from northern Gorda Ridge are also 

typically without clinopyroxene except for one xenolithbearing glass (Davis and Clague, 1990). The presence of 

large clinopyroxene oikocrysts in these xenoliths, which 

appear to be crystalline mush of the margins of a magma 

chamber, indicate that clinopyroxene fractionation is an 

Perfit and 

others (1983) 

Di 

important process in the magma chambers, although 

clinopyroxene phenocrysts are rarely observed in the lavas. 

DISCUSSION 

CRYSTAL FRACTIONATION 

Crystal fractionation clearly was an important process 

in the evolution of Escanaba Trough glasses, since Mg#s 

of 60 or less are quite common. Compositions converge 

toward a potential parental composition similar to the most 

primitive glass recovered from the Gorda Ridge (83-D4 

from the northern segment; Davis and Clague, 1987), but 

\ 0 83-16 

Walker and~ 

others C 1979) \ \ 

\ 

\ 

\ 

\ 

/ '~ 

• 3-86-10 

3-86-2 

0 3-86-3 

0 NESCA 

• SESCA 

/ 0 ~-~, X o ~ \ \--Gorda Ridge 

83-04' , ll 

........ •. 

~\ 0< (Davis and Clague, 1987) 

--c '-

'r 'r \c '-

15 kbar 10 kbar ' , 

Al2039 

Low Ca-pyroxene 

01 

Figure 9.11. Compositions of Escanaba Trough glasses shown on plagioclase-saturated diopside-olivine-silica projection 

of Walker and others (1979). Solid curve is experimentally determined liquidus at 1 atm. Dashed line represents natural 

phase boundary for lavas from Galapagos Rift, Inca transform, and Ecuador Rift (Perfit and others, 1983). Cotectics for 

10, 15, and 20 kbar of Stolper (1980) are shown for reference. Many Escanaba Trough glasses plot in diopside field, 

although modal clinopyroxene is not present. Displacement toward concave side of the liquidus may be result of mixing 

differentiated residue with a more primitive magma. 
PETROLOGY AND MINERAL CHEMISTRY OF BASALT FROM ESCANABA TROUGH 167 

a large amount of scatter indicates complex petrogenetic 

processes. Fractionation at the SESCA site apparently involved mostly plagioclase, whereas predominantly olivine 

with lesser plagioclase fractionation is indicated by the flat 

trend on the Ca0/Al20 3 versus Mg# plot for the NESCA 

site. Different trends shown by fractionated samples at 3-

86-10 are not compatible with a single liquid line of descent (fig. 9.4B). Compared with experimental studies of 

MORB by Walker and others (1979), the Escanaba Trough 

glasses form a cluster across the plagioclase-saturated olivine-diopside boundary (fig. 9.11), with many compositions in the diopside field. In accord with their low degree 

of crystallinity and high calculated eruption temperatures, 

clinopyroxene should not be a liquidus phase in the 

sparsely porphyritic samples, and no modal clinopyroxene 

is observed. Highly porphyritic samples also lack 

clinopyroxene, but the abundance of plagioclase suggests 

plagioclase accumulation. Walker and others (1979) suggested that displacement to the right of the curved liquidus 

could result from mixing a primitive melt with differentiated residue. If the experimentally determined liquidus of 

MORB from the Mid-Atlantic Ridge (Walker and others, 

1979) is appropriate for Escanaba Trough magmas, many 

of these lavas represent hybrids. Alternatively, the liquidus 

may not be appropriate, and a liquidus shifted toward the 

diopside-silica join, as proposed for lavas from the 

Galapagos Rift spreading center (Perfit and others, 1983), 

may be more appropriate (fig. 9.11). 

MAGMA MIXING 

Different fractionation paths at geographically widely 

separated regions is probably to be expected. Variations 

in compositions 'that are not compatible with simple crystal fractionation may have resulted from magma mixing. 

Magma mixing in continuously replenished magma chambers appears to be a common process at oceanic spreading 

centers (O'Hara, 1977; O'Hara and Mathews, 1981; 

Dungan and Rhodes, 1978; Rhodes and others, 1979; 

Dixon and others, 1986; Davis and Clague, 1987). Mineral phases that are texturally and compositionally not in 

equilibrium with their host glass and glass inclusion data 

provide evidence of magma mixing. Phenocrysts in 

samples from 83-16 and 3-86-10 show textural 

disequilibrium features such as embayed and resorbed margins of olivine and plagioclase crystals with fritted zones, 

overgrown by a layer that is compositionally different from 

the core of the crystal. A phenocryst core with An as low 

as 65, observed for plagioclase in 3-86-10 (table 9.4 ), could 

have crystallized in a highly differentiated melt. Higher 

NiO contents in rims and skeletal olivine than in phenocryst cores and prevalent reverse zoning of plagioclase suggest mixing with a more mafic, hotter magma. No such 

phenocrysts were observed in NESCA samples. All of the 

NESCA glasses contain small euhedral microphenocrysts 

that are compositionally in equilibrium with their host glass 

(fig. 9.7C), but all plagioclase shows a small amount of reverse zoning. However, lack of textural evidence does not 

preclude magma mixing. Mixing of a fractionated magma 

with a more primitive one can result in superheated magma 

that is aphyric and shows only the chemical imprint of 

magma mixing. This chemical imprint consists mostly of 

enrichment in incompatible elements beyond what would 

be expected by simple crystal fractionation. At the same 

time, compatible elements are buffered and do not show 

the depletion expected from closed system fractionation. 

Higher K20, P20 5, and Ti02 content, as observed in the 

NESCA samples, and higher NiO and Cr20 3 contents, as 

indicated by olivine and spinel compositions, is just what 

would be expected from mixing evolved with more primitive melt. 

PARTIAL MELTING AND MANTLE 

HETEROGENEITY 

Potassium, phosphorus, and titanium are incompatible 

elements, and ratios of these elements should not be significantly changed by fractionation over the compositional 

range shown by Escanaba Trough glasses, hence they may 

provide some evidence for source compositions. However, 

different degrees of partial melting can cause significant 

variations in incompatible-element ratios, especially when 

very small percentages of melt are involved. Complex 

melting processes such as dynamic melting (Langmuir and 

others, 1977) and complex magma chamber processes involving multiple mixing cycles (O'Hara, 1980; Elthon, 

1984) may also change incompatible-element ratios of 

melts. Variation in incompatible-element ratios, as observed for Escanaba Trough glasses, is also observed for 

the other segments of the mid-oceanic ridge system in the 

northern Pacific Ocean. Values of La/Sm from the northern segments of Gorda Ridge are low but variable (0.34 to 

0. 76; Davis and Clague, 1987), whereas La/Sm ratios for 

the Juan de Fuca-Explorer Ridge segments show a larger 

range and include some higher ratios (0.6 to 1.9; Schilling 

and others, 1983; Cousens and others, 1984). Yet Sr isotopic compositions suggest a relatively uniform, depleted 

mantle source, 0.70232 to 0.70267 for northern Gorda 

Ridge (Davis and Clague, 1987; White and others, 1987) 

and 0.70232 to' 0.70254 for the Juan de Fuca-Explorer 

Ridge (Eaby and others, 1984; Cousens and others, 1984). 

It is commonly accepted that radiogenic isotopic ratios are not fractionated by partial melting processes (for 

example, Hoffmann and Hart, 1978). Incompatible-element ratios at northern Gorda Ridge show no correlation 

with Sr isotopic ratios, as would be expected if variations 

were caused by source heterogeneities (Davis and Clague, 

1987). Existing isotopic data for Escanaba Trough glasses 
168 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

show 87 Sr/86Sr ratios of 0. 70261 for 83-16-1 (Davis and 

Clague, 1987) and 0.70284 for a NESCA sample (White 

and others, 1987) at comparably high K20IP20 5 values. 

These Sr isotopic values are well within the range typical 

for normal MORB, but the isotopic ratio of the NESCA 

sample is the highest 87 Sr/86Sr ratio reported for the entire spreading center in the northern Pacific Ocean. 

Decoupling between incompatible-element ratios and Sr 

isotopic compositions suggests that incompatible-element 

variation resulted from melting processes or that source 

heterogeneities are of recent origin. 

CONTAMINATION 

Trace-element abundances and isotopic ratios can be 

significantly altered if hydrothermally altered crust and (or) 

sediment are assimilated in oceanic crustal magma chambers and conduits (O'Hara, 1980). Lead isotopic ratios, 

which are particularly sensitive indicators of sediment contamination, show Escanaba Trough basalt from NESCA 

more enriched in radiogenic lead than other Gorda Ridge 

basalt (fig. 9.12). Ratios of 207Pbt2°4Pb (fig. 9.12A) and 

208Pbt2°4Pb (fig. 9.12B) versus 206Pbt2°4Pb show NESCA 

basalt displaced from the trend shown by other Gorda 

Ridge basalt toward lead compositions of sediments. However, the lead isotopic composition of a sample from northern Escanaba Trough (near 83-16) plots within the depleted 

end of Gorda Ridge basalt (White and others, 1987; fig. 

9.12). 

The samples with enriched lead isotopic composition 

from NESCA are low-K20 normal MORB. The high-K20 

sample from NESCA resembles enriched (E-type) MORB. 

E-type MORB is enriched in LIL elements, such as K, Ba, 

Rb, and Cs, but with major-element concentrations that are 

largely indistinguishable from normal MORB. Enriched 

MORB from some other spreading centers, such as Iceland, 

Galapagos, and the Azores, have been attributed to mixing with a mantle plume or hotspot component (Schilling 

and others, 1983). These MORB also have light-rare-earthelement (REE) and isotopic signatures more enriched than 

normal MORB. Samples with high-K20 compositions, 

similar to Alv2039-2, have also been reported from Explorer Ridge (Cousens and others, 1984) and from basaltic dikes and sills in Guaymas Basin (Saunders and others, 

1982; Fornari and others, 1982). However, a strontium isotopic ratio of 0. 70252 determined for one of the high-K20 

samples from the Explorer Ridge is well within the range 

of normal MORB (Cousens and others, 1984) and shows 

no plume influence. The high-K20 samples from Guaymas 

Basin are light-REE depleted, like normal MORB 

(Saunders and others, 1982). Saunders and others (1982) 

suggested that the enrichment in LIL elements without a 

corresponding enrichment in REE and isotopic composition is due to contamination by a small component of "continental" material in the source region. No lead isotopic 

data are available for these samples. If the high-K20 content of Alv2039-2 resulted from contamination with a small 

amount of sediment, the lead isotopic composition should be 

even more enriched and plot closer to that of the sediment. 

MAGMA CHAMBERS 

The presence of genetically unrelated lavas suggests 

that magma chambers along Escanaba Trough are isolated. 

A 

15.7 .----------------------, 

~ 15.6 <:t 

0 

C\J 

...... 

.D 

0... 

1'-

~ 15.5 

15.4 

39.5 

39.0 

.D 

0... 

<:t 38.5 0 

C\J ...... 

.D 

0... 

co 38.0 0 

C\J 

37.5 

37.0 

18.0 

EXPLANATION 

o N. Gorda Ridge Basalt 

• S. Escanaba Trough Basalt 

o N. Escanaba Trough Basalt 

t:>. Escanaba Trough Sediment 

B 

18.2 18.4 18.6 18.8 

206pb;204pb 

19.0 19.2 

Figure 9.12. Lead isotope compositions of (A) 207pbt204Pb and (B) 

208pb;204pb versus 206pbf204Pb for southern Escanaba Trough 

glasses show more enriched compositions than other Gorda Ridge 

lavas. One sample from northern end of Escanaba Trough (near 83-

16) plots within depleted compositions shown by northern Gorda 

Ridge basalts. Southern Escanaba Trough basalts are displaced from 

trend shown by northern Gorda Ridge basalts toward the 

compositions of sediments, suggesting contamination with small 

amount of sediment. Data for northern Gorda Ridge basalt from 

Church and Tatsumoto (1975) and White and others (1987); data for 

northern Escanaba Trough basalt and for Escanaba Trough sediments 

from LeHuray and others (1987). 
PETROLOGY AND MINERAL CHEMISTRY OF BASALT FROM ESCANABA TROUGH 169 

The magma chambers are probably small, because flow 

units appear to be small and pillow lavas predominate, but 

the magma chamber at the NESCA site may be larger since 

flow units appear more extensive and sheet flows are common. The abundance of sheet flows indicates a rapid extrusion rate and probably a larger magma supply, whereas 

pillow basalts have generally been linked to narrow conduits and small magma supply (Ballard and others, 1979). 

The gradational nature of the compositional range 

shown by NESCA glasses makes it difficult to assign them 

to specific flow units. Most of the sheet flows may stem 

from the same eruption, which may have fractionated small 

amounts of primarily olivine and minor plagioclase as they 

were rising through isolated fissures from a common 

magma chamber. Similar compositional variations and 

nearly aphyric lavas have been observed in sheeted dikes 

associated with some ophiolites (Smewing, 1981; Pallister 

and Hopson, 1981). 

CONCLUSIONS 

Basaltic glasses from Escanaba Trough consist predominantly of normal MORB. One glass composition is 

high in K20, resembling E-type MORB. Compositions 

range from relatively primitive to moderately fractionated. 

Chemical trends shown by glass and mineral phases suggest complex petrogenetic processes, involving fractionation, magma mixing, and different degrees of partial 

melting and (or) different source compositions. Mixing 

with LIL-element-enriched material is indicated for the 

high-K20 sample from the NESCA site. Other glasses 

from this site also have somewhat higher K20 contents 

than comparably fractionated samples from the other 

Escanaba Trough sites. Spinel in these lavas is higher in 

titanium and ferric iron than is typical for normal MORB, 

suggesting higher oxygen fugacity of the melt. Lead isotopic compositions of samples from Escanaba Trough 

(NESCA) are displaced from the trend shown by other 

Gorda Ridge basalt toward the isotopic compositions of 

sediment in this region, suggesting contamination by a 

small sediment-derived component. Normal MORB from 

Escanaba Trough is similar to that from the northern part 

of the Gorda Ridge, and to normal MORB from many other 

spreading centers. The unusual LIL-element-enriched composition from the northernmost part of the NESCA site has 

no counterpart on the northern Gorda Ridge, but similar 

compositions have been reported from the Explorer Deep 

segment of Explorer Ridge and Guaymas Basin, where they 

show no trace-element or isotopic enrichment compatible 

with a plume or hotspot influence. The similar tectonic 

setting of incipient spreading into a sediment-filled basin 

may suggest selective enrichment of some LIL elements, 

which may have resulted from contamination by a small 

amount of sediment. 

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Church, S.E., and Tatsumoto, M., 1975, Lead isotope relations 

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Cousens, B.L., Chase, R.L., and Schilling, J.G., 1984, Basalt geochemistry of the Explorer Ridge area, northeast Pacific Ocean: 

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Davis, A.S., and Clague, D.A., 1987, Geochemistry, mineralogy, 

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Dungan, M.A., and Rhodes, J.M., 1978, Residual glasses and melt 

inclusions in basalts from DSDP legs 45 and 46: Evidence 

for magma mixing: Contributions to Mineralogy and Petrology, v. 67, p. 417-433. 

Eaby, J., Clague, D.A., and Delaney, J.R., 1984, Sr isotopic variations along the Juan de Fuca Ridge: Journal of Geophysical 

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Fornari, D.J., Saunders, A.D., and Perfit, M.R., 1982, Major-element chemistry of basaltic glasses recovered during Deep 

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and others, eds., Initial reports of the Deep Sea Drilling 

Project: Washington, U.S. Government Printing Office, v. 

64, p. 643-647. 

Hoffmann, A.W., and Hart, S.R., 1978, An assessment of local 

and regional isotopic equilibrium in the mantle: Earth and 

Planetary Science Letters, v. 38, p. 44-62. 

Langmuir, C.H., Bender, J.F., Bence, A.E., Hanson, G.N., and 

Taylor, S.R., 1977, Petrogenesis of basalts from the FAMOUS area, Mid-Atlantic Ridge: Earth and Planetary 

Science Letters, v. 36, p. 133-156. 

LeHuray, A.P., Church, S.E., Koski, R.A., and Bouse, R.M., 1987, 

Pb isotopes in sulfides from mid-ocean ridge hydrothermal 

sites: Geology, v. 16, p. 362-365. 
170 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

Mathez, E.A., 1976, Sulfur solubility and magmatic sulfides in 

submarine basalt glass: Journal of Geophysical Research, 

v. 81, p. 4269-4276. 

McManus, D.A., 1970, Site 35, in McManus, D.A., and others, 

eds., Initial reports of the Deep Sea Drilling Project: Washington, U.S. Government Printing Office, v. 5, p. 165-172. 

Morton, J.L., Normark, W.R., Ross, S.L., Koski, R.A., Holmes, 

M.L., Shanks, W.C., III, Zierenberg, R.A., Lyle, M., and 

Benninger, L.M., 1987a, Preliminary report, cruises L1-86-

NC and L2-86-NC, Escanaba Trough, Gorda Ridge: U.S. 

Geological Survey Open-File Report 87-375-A, 13 p. 

Morton, J.L., Holmes, M.L., and Koski, R.A., 1987b, Volcanism 

and massive sulfide formation at a sedimented spreading 

center, Escanaba Trough, Gorda Ridge, northeast Pacific 

Ocean: Geophysical Research Letters, v. 14, p. 769-772. 

Morton, J.L., Zierenb~rg, R.A., and Reiss, C.A., Geologic, hydrothermal, and biologic studies at Escanaba Trough: An 

introduction, in Morton, J.L., Zierenberg, R.A., and Reiss, 

C.A., eds., Geologic, hydrothermal, and biologic studies at 

Escanaba Trough, Gorda Ridge, offshore northern California: U.S. Geological Survey Bulletin 2022, chapter 1 (this 

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Natland, J.H., Adamson, A.C., Laverne, C., Melson, W.G., and 

O'Hearn, T., 1983, A compositionally nearly steady-state 

magma chamber at the Costa Rica Rift: Evidence from basalt glass and mineral data, Deep Sea Drilling Project sites 

501, 504, and 505, in Cann, J.R., Langseth, M.G., Honnorez, 

J., von Herzen, R.P., White, S.M., and others, eds., Initial 

reports of the Deep Sea Drilling Project: Washington, U.S. 

Government Printing Office, v. 69, p. 811-858. 

O'Hara, M.J., 1977, Geochemical evolution during fractional 

crystallization of a periodically refilled magma chamber: 

Nature, v. 266, p. 503-508. 

---1980, Nonlinear nature of the unavoidable long-lived isotopic, trace and major element contamination of a 

developing magma chamber: Philosophical Transactions of 

the Royal Society of London, Series A, v. 297, p. 215-277. 

O'Hara, M.J., and Mathews, R.E., 1981, Geochemical evolution 

in an advancing, periodically replenished, periodically 

tapped, continuously fractionated magma chamber: Journal of the Geological Society of London, v. 138, p. 237-277. 

Pallister, J.S., and Hopson, C.A., 1981, Samail ophiolite plutonic 

suite: Field relations, phase variation, cryptic layering, and 

a model of a spreading ridge magma chamber: Journal of 

Geophysical Research, v. 86, p.2593-2644. 

Perfit, M.R., Fornari, D.J., Malahoff, A., and Embley, R.W., 1983, 

Geochemical studies of abyssal lavas recovered by DSRV 

Alvin from eastern Galapagos Rift, Inca Transform, and Ecuador Rift, 3, trace element abundances and petrogenesis: 

Journal of Geophysical Research, v. 88, p. 10,551-10,572. 

Rhodes, J.M., Dungan, M.A., Blanchard, D.P., and Long, P.E., 

1979, Magma mixing at mid-ocean ridges: Evidence from 

basalts drilled near 22° N on the Mid-Atlantic Ridge: 

Tectonophysics, v. 55, p. 35-62. 

Roeder, P.L., 1974, Activity of iron and olivine solubility in basaltic liquids: Earth and Planetary Science Letters, v. 23, 

p. 397-410. 

Ross, S.L., and Zierenberg, R.A., Volcanic geomorphology of the 

SESCA and NESCA sites, Escanaba Trough, in Morton, 

J.L., Zierenberg, R.A., and Reiss, C.A., eds., Geologic, hydrothermal, and biologic studies at Escanaba Trough, Gorda 

Ridge, offshore northern California: U.S. Geological Survey Bulletin 2022, chapter 8 (this volume). 

Saunders, A.D., Fornari, J.D., Joron, J.L., Tarney, J., and Treuil, 

M., 1982, Geochemistry of basic igneous rocks, Gulf of 

California, Deep Sea Drilling Project leg 64, in Curray, J.R., 

Moore, D.G., and others, eds., Initial reports of the Deep 

Sea Drilling Project: Washington, U.S. Government Printing Office, v. 64, p. 595-642. 

Schilling, J.-G., Zajac, M., Evans, R., Johnston, T., White, W., 

Devine, J.D., and Kingsley, R., 1983, Petrologic and 

geochemical variations along the Mid-Atlantic Ridge from 

29° N to 73° N: American Journal of Science, v. 283, p. 

510-586. 

Shaw, H.R., 1972, Viscosities of magmatic silicate liquids: An 

empirical method of prediction: American Journal of Science,v.272,p.870-893. 

Sigurdsson, H., and Schilling, J.G., 1976, Spinels in Mid-Atlantic Ridge basalts: Chemistry and occurrence: Earth and 

Planetary Science Letters, v. 29, p. 7-20. 

Smewing, J.D., 1981, Mixing characteristics and compositional 

differences in mantle-derived melts beneath spreading axes: 

Evidence from cyclically layered rocks in the ophiolite of 

north Oman: Journal of Geophysical Research, v. 86, p. 

2645-2660. 

Sparks, R.S.J., Meyer, P., and Sigurdsson, H., 1980, Density variation amongst mid-ocean ridge basalts: Implications for 

magma mixing and the scarcity of primitive lavas: Earth 

and Planetary Science Letters, v. 46, p. 419-430. 

Stolper, E.M., 1980, A phase diagram for mid-ocean ridge basalts: 

Preliminary results and implications for petrogenesis: Contributions to Mineralogy and Petrology, v. 74, p. 13-27. 

Stormer, J.C., and Nicholls, J., 1978, XLFRAC: A program for 

the interactive testing of magmatic differentiation models: 

Computer Geoscience, v. 4, p. 143-159. 

Walker, D., Shibata, T., and Long, S.E., 1979, Abyssal tholeiites 

from the Oceanographer Fracture Zone, II, Phase equilibria and mixing: Contributions to Mineralogy and Petrology, 

v. 70, p. 111-125. 

White, W.M., Hofman, A.W., and Puchelt, H., 1987, Isotope 

geochemistry of Pacific mid-ocean ridge basalt: Journal of 

Geophysical Research, v. 92, p. 4881-4893. 
Chapter 10. Geologic Setting of Massive Sulfide 

Mineralization in Escanaba Trough 

By Robert A. Zierenberg, Janet L. Morton, Randolph A. Koski, and Stephanie L. Ross 1 

CONTENTS 

Page 

Abstract ................................................................................ 171 

Introduction ...... .... ...... .... .... .. .... ... . ... ......... ... . .... ... ..... ... . .... .. .. 172 

Regional setting . .... .... ....... .. .. ...... .... .. .. .......... .. .. .. .. . .. . . .. 172 

Acknowledgments....................................................... 173 

SESCA study area................................................................ 173 

Morphology and structure . . ... . ... . .. .. ... . ... . ... . ... . ... . ... ... ... 173 

Volcanology ................................................................. 175 

Sedimentology............................................................. 175 

Hydrothermal deposits .... ............................................ 178 

3170 Hill.............................................................. 178 

North Hill ............................................................. 178 

East Hill . ..... ...... .. ... ........ ... . . .. . ... . ... . ... . ... . ... . .......... 179 

Core L2-86-9G location .. ... ..... .. .. .. .. .. .... .. .. .. .. ..... . 179 

Edifice C near 40°53' N. ..................................................... 179 

NESCA study area ...... ... .... ............. .. .. .. .... ............ .. .. ..... ..... . 179 

Morphology and structure .... .. .. .. .. .. .. .. .. .. .. .. .. .. .. .... ..... .. 179 

Volcanology ............................. .................................... 179 

Sedimentology .... .... .. ............... ... . ........ ........ .... .... ..... ... 181 

Hydrothermal deposits ................................................ 183 

SW Hill................................................................ 183 

Central Hill .. .. ...... ... ........ ... ......... ... . ... . ... . ... . ... .. ... . 184 

Area north of the Central Hill . ... . ... . ... . ... . ... . ..... ... 188 

Edifice REX......................................................... 188 

Discussion ............................................................................ 188 

Volcanic-sediment interaction..................................... 188 

SESCA cross sections ...... ... . .. .. .. . . ... . ... . ... . .. .... ..... 189 

NESCA cross sections . . ... ..... ... . ... . ... . ... . ... . .... .... ... 189 

Uplift of sediment hills ....................................... 192 

Sediment redistribution .. ........ ... . ........ ... . ... . ... . ........ .. ... 192 

Erosional channels............................................... 193 

Current erosion.................................................... 193 

Hydrothermal processes .............................................. 193 

Localization and timing of hydrothermal 

discharge.......................................................... 193 

Fluid-sediment interaction .................................. 194 

Sulfide deposition ..... .... ....... .... .... .... .... .... .... ...... .. 195 

Subsurface mineralization................................... 195 

References cited ......... ... ...... ........ ... ..... ... . ... . .... ... ......... ... ... .. . 196 

1 U.S. Geological Survey, Menlo Park, California. 

ABSTRACT 

Extensive massive sulfide deposits occur in the sediment-covered rift valley of Escanaba Trough. The deposits are sediment hosted, but are associated spatially and 

genetically with centers of recent volcanic activity along 

the spreading axis. This chapter focuses on geologic data 

obtained on 22 submersible dives in the area by the DSV 

Alvin and DSV Sea Cliff, supplemented by observations 

from towed camera traverses and geophysical profiles. 

The sea floor in the area of the volcanic edifices is 

characterized by irregular topography formed by uplift of 

sediment hills above igneous intrusions. The uplifted sediment hills are eroding by mass wasting and are the source 

of locally derived debris flows and turbidites. Faults 

bounding the uplifted areas provide enhanced cross-stratal 

permeability and are the primary conduits for hydrothermal discharge. The largest massive sulfide deposits occur 

near the base of the uplifted sediment hills. 

Two volcanic edifices have received the most detailed 

investigation: the southern Escanaba (SESCA) site near 

latitude 40°45' N., longitude 127°30' W. and the northern 

Escanaba (NESCA) site near latitude 41 °00' N., longitude 

127°30' W. Exposed volcanic rocks and massive sulfide 

deposits at the SESCA site are older than those at the 

NESCA site. At least three large massive sulfide deposits 

occur in the SESCA area. The deposits are not hydrothermally active and are being degraded by oxidative sea-floor 

weathering and mass wasting. 

Relatively young, glassy pillow basalts are exposed on 

the east flank of the Central Hill in the NESCA area. These 

pillow basalts formed over an eruptive vent that also fed 

volcanic sheet flows, which overlie approximately 500 m 

of sediment fill north and south of the Central Hill. Large 

massive sulfide deposits on the sediment-covered west 

flank of the Central Hill locally vent hydrothermal fluid 

at temperatures up to 220°C. Large, but apparently inactive sulfide deposits also occur around the base of the SW 

Hill at the NESCA site and are associated with a sediment 

hill astride the spreading axis at Edifice REX at the northern edge of the NESCA area. 

The enrichment of certain elements (such as As, Bi, 

Pb, Sb, and Sn) in massive sulfide indicates that sediment 

171 
172 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

alteration provides at least some of the metals to the hydrothermal systems in Escanaba Trough. The enrichment 

of the radiogenic isotopes of lead in sulfide minerals and 

strontium in hydrothermal fluid, and the presence of 

thermogenic hydrocarbon in sediment, provides additional 

evidence of contribution from the sediment. Some samples 

show impregnation and veining of sediment by sulfide. 

Sulfide crusts in sediment and the surfaces of sulfide 

mounds are buckled and cracked by growth of sulfide below the sediment interface. Deposition of sulfide in the 

shallow subsurface is interpreted to be an important process in the formation of massive sulfide mounds. Structural conduits underlying massive sulfide and 

high-permeability turbidite beds are presumed to localize 

hydrothermal fluid flow and are potential sites of extensive subsurface deposition of sulfide. 

INTRODUCTION 

Escanaba Trough is an important natural laboratory for 

investigating the processes that control the formation of 

massive sulfide deposits. In this chapter we describe the 

structure, sedimentology, volcanology, and hydrothermal 

deposits that occur at two detailed study areas within 

Escanaba Trough, the southern Escanaba (SESCA) and 

northern Escanaba (NESCA) areas, and at less well studied areas of sea-floor mineralization between the NESCA 

and SESCA areas and at the northern edge of the NESCA 

area (fig. 10.1). Our primary data sources are the observations and photographic records collected during 8 dives 

by DSV Sea Cliff in 1986, and 10 dives by DSV Alvin and 

4 by Sea Cliff in 1988. Geophysical, photo geological, and 

geochemical data supplement the submersible observations 

and help to constrain our interpretations of the structural 

controls on massive sulfide distribution and the geochemical and physical controls on the deposition of massive sulfide on and below the sea floor. 

REGIONAL SETTING 

Escanaba Trough is the southernmost part of the Gorda 

Ridge spreading center, which extends northward from the 

Mendocino Fracture Zone to a right-stepping ridge offset 

at latitude 41 °37' N. (Clague and Holmes, 1987). This segment of the Gorda Ridge spreads at a slow rate of about 

2.4 cm/yr and bas a Mid-Atlantic Ridge style morphology 

(Riddihough, 1980; Wilson, 1989). The axial valley is 

generally deeper than 3,200 m and is bounded by steep 

=> 

Figure 10.1. Generalized map of Escanaba Trough showing 

locations ofNESCA and SESCA study areas (rectangles), volcanic 

edifices (including edifice Rex), and DSDP Site 35. 

41 °30' 

41 °00' 

40°30' 

EXPLANATION 

Volcanic 

edifice 

Sediment 

fill 

Terrace 

0 10 I I I I I I 

KILOMETERS 

.. ··.·. · .. : . . ·.· ... 

>.-:·: :·MEN·DO.CINO:·FRA·CTU.RE::ZON:E··. ·. •• • •••••••••••• 0 •• •••••• 0 •• 0. • •••• 
GEOLOGIC SEITING OF MASSIVE SULFIDE MINERALIZATION IN ESCANABA TROUGH 173 

normal faults with back-tilted, sediment-covered blocks 

that rise to depths as shallow as 1,500 m (Atwater and 

Mudie, 1973; Riddihough, 1980). The axial valley is 20 

km wide at the southern end and narrows to a few kilometers wide to the north (Morton and Fox, chap. 2, this volume). The floor of the axial valley south of about latitude 

41° 17' N. is nearly flat due to burial by hemipelagic and 

turbiditic sediment that enters the valley at the southern end 

(McManus and others, 1970; Vallier and others, 1973). 

Basement topography below the sediment cover is irregular, and several volcanic complexes rise through the sediment cover (fig. 10.1; Morton and others, chap. 1, this 

volume), resulting in local exposures of basalt at the sea 

floor (Clague and Holmes, 1987; Morton and others, 1987). 

Two of these volcanic edifices, labeled B and D in Abbott 

and others ( 1986) have been investigated in more detail and 

are referred to as the SESCA and NESCA study areas, respectively. Both of these areas contain large massive sulfide deposits. Sulfide mineralization also occurs at 

volcanic edifices C and E. 

The sedimentary fill of Escanaba Trough has an average thickness of 500 to 700 m over the spreading center; 

it is thickest at the south end near the Mendocino Fracture 

Zone, where it exceeds 900 m (Clague and Holmes, 1987). 

Sediment cover thins northward, and the spreading center 

is sediment-free north of approximately latitude 41 o 17' N. 

(Morton and Fox, chap. 2, this volume). Seismic-reflection surveys of Escanaba Trough show continuous, 

subparallel sediment layering indicative of distal turbidite 

deposition (McManus and others, 1970; Clague and 

Holmes, 1987, Normark and others, chap. 6, this volume). 

Site 35 of the Deep Sea Drilling Project (DSDP) was 

drilled in the southern part of Escanaba Trough (fig. 10.1) 

and penetrated to a depth of 390 m without reaching the 

underlying basaltic basement (McManus and others, 1970). 

Terrigenous mud, silt, and sand turbidites interlayered with 

hemipelagic sediment were recovered (McManus and others, 1970; Vallier and others, 1973); the maximum age determined by microfossils contained in the sediment was 

Pleistocene. The bulk of the sediment fill in the Escanaba 

Trough was apparently deposited rapidly during the last 

two glacial lowstands of sea level within the last 100,000 

yr (Normark and others, chap. 6, this volume). 

Holocene sediment in Escanaba Trough is generally 

0.5 to 2 m thick, reflecting lower terrigenous input during 

times of higher sea level (Karlin and Lyle, 1986; Karlin 

and Zierenberg, chap. 7, this volume). The Holocene sediment is predominantly hemipelagic with locally derived 

turbidites. 

The sediment cover of Escanaba Trough precludes detailed mapping of the neovolcanic zone, structural domains, 

and along-strike variations in ridge volcanism. Ridge segmentation is recognized by the alignment of structural and 

volcanic features along the ridge axis (Morton and Fox, 

chap. 2, this volume). Several discrete volcanic centers are 

spaced approximately 12 to 15 km apart along the ridge 

axis, where a relative excess of magma supply formed volcanic edifices that rise above the base level of the ridge 

axis (fig. 10.1; Morton and Fox, chap. 2, this volume). 

These edifices are generally elongate along axis and are 

typically 5 to 7 km long and 3 to 5 km wide. Escanaba 

Trough basalts are mostly normal mid-ocean ridge basalts 

(MORB), but have lead isotope ratios more radiogenic than 

basalt from the northern Gorda Ridge, consistent with assimilation of sediment (Davis and others, chap. 9, this volume). Basalt compositional variations within each volcanic 

edifice are limited, but basalt compositional variations between centers indicate that different centers may have separate magma supplies (Davis and others, chap. 9, this 

volume). 

ACKNOWLEDGMENTS 

This work represents the efforts of many people located at several institutions and was possible only as a result of close cooperation and coordination. We gratefully 

acknowledge the support of the captains and crews of the 

various vessels listed in table 1.1 of Morton and others 

(chap. 1, this volume) that supported work in Escanaba 

Trough. The cooperation of the U.S. Navy and the dedicated efforts of the DSV Sea Cliff crew, often under difficult weather conditions, were critical to our investigations. 

We thank co-chief scientists John Edmond and J. Frederick 

Grassle for the opportunity to participate in the DSV Alvin 

dive program in 1988, and the DSV Alvin crew for executing an efficient and highly successful dive series in 

Escanaba Trough. 

SESCA STUDY AREA 

MORPHOLOGY AND STRUCTURE 

The SESCA study area is a broad domal region characterized by a general elevation of the sea floor above 

3,300 m and smaller scale variability of sea-floor morphology not typically developed on the sediment fill of the 

trough (fig. 10.2; Morton and Fox, chap. 2, this volume). 

The broad-scale shoaling occurs on the west side of the 

trough floor across an area approximately 10 by 15 km 

elongate along the direction of the ridge axis (fig. 2.2B, 

Morton and Fox, chap. 2, this volume). Seismic-reflection 

surveys over this area indicate that the volcanic basement 

occurs at shallower levels under thinner sediment cover 

compared to most areas of the trough floor (Clague and 

Holmes, 1987; Morton and Fox, chap. 2, this volume). The 

continuous seismic reflections from turbidite layers that 

characterize the fill of Escanaba Trough are disrupted by 

small-scale folding and faulting or are absent within much 
174 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

40°46' 

40°44' 

40°42' 

[·" • :1 Basalt • Sulfide H>l Disturbed sediment D Undisturbed sediment 

Figure 10.2. Geologic map of SESCA area. Sea Beam bathymetry and areas with outcropping and shallowly buried volcanic rock 

and disturbed sediment from Morton and Fox (chap. 2, this volume). Fault locations are extrapolated based on seismic-reflection 

profiles and Sea Beam bathymetric data; teeth are on downthrown side. Locations of cross-section lines presented in figure l0.9A are 

shown. Hachures indicate area of closed low. 
GEOLOGIC SETTING OF MASSIVE SULFIDE MINERALIZATION IN ESCANABA TROUGH 175 

of the SESCA area (fig. 10.2; Morton and Fox, chap. 2, 

this volume). 

Superimposed on the larger bathymetric swell are numerous steep-sided, roughly circular hills from 500 to 

1,000 m in diameter that rise 40 to 100m above the surrounding sea floor. The cluster of hills between approximately latitude 40°45' N. and latitude 40°48' N. are 

spatially associated with large deposits of massive sulfide 

(fig. 1 0.2), and have been the focus of investigation in the 

SESCA area. Other morphologically similar hills between 

latitude 40°42' N. and latitude 40°43' N. in the southern 

part of the SESCA area and near latitude 40°49' N. at the 

north edge of the SESCA map area (fig. 1 0.2) have not 

been investigated by geologic mapping techniques that 

would reveal the presence of sea-floor mineralization. 

VOLCANOLOGY 

Exposed basalt at the SESCA site is limited to the area 

southwest of the North Hill (fig. 10.2; Morton and Fox, 

chap. 2, this volume). The volcanic rocks are largely sediment covered with only bulbous pillows 1 to 2m wide exposed. Where sheet flows are observed, they are often 

overlain by large trap-door pillows formed by vertical 

break-out of lava through the top of the sheet flow crust, 

indicating inflation of the flows. The flows are locally cut 

by fissures 0.5 to 1 m wide interpreted to be related to extension parallel to the spreading direction (Ross and 

Zierenberg, chap. 8, this volume). The walls of the fissures are predominantly sheet flows. The extent of surface outcrop of sheet flows is not representative of their 

volumetric proportion, because their low relief results in 

rapid burial by sediment. Hydrothermal activity was not 

observed in the fissures. The presence of filter-feeding organisms in the fissures may indicate downward flow of seawater into the fissures, and they may serve as 

high-permeability zones that enhance recharge to the hydrothermal systems. Basalt flows extend up to the base 

of the North Hill (fig. 10.2). Normal faults near the base 

of the North Hill with 1 to 2 m offsets cut basalt sheet flows 

and are draped by unfaulted flow lobes, indicating that 

eruption and uplift were contemporaneous. No basalt is 

exposed on the steep sides of the uplifted sediment hill. 

SEDIMENTOLOGY 

The sediment hills in the SESCA area are spatially associated with massive sulfide deposits and have therefore 

been the focus of detailed investigation. Numerous camera tracks cross the hills, but the flanks of the hills are 

poorly imaged due to steep slopes. DSV Sea Cliff dives 

in 1986 (Holmes and Zierenberg, 1990; Zierenberg and others, 1991) provided detailed observations of the geologic 

setting of the 3170 Hill (dives 656 and 663) and the North 

Hill (dives 661 and 662). A typical bathymetric profile 

across one of these hills has a shallow slope at the base 

that becomes increasingly steep upwards, resulting in an 

approximately parabolic slope. From the submersible, the 

tops of the hills appear to be subhorizontal and monotonously planar. There is a lower abundance of organisms 

and more large bioturbation mounds and burrows on the 

tops of the hills than on areas of comparable slope between 

the hills. Sediment ripples, with typical wavelengths of 10 

em and amplitudes generally less than 1 em (fig. 10.3A), 

and lag deposits of coarser sediment were observed near 

the edges of the top of the 3170 Hill. Near-vertical cliffs 

exposing semiconsolidated mud interlayered with siltstone 

and sandstone beds, interpreted to be turbidites, are characteristic of the upper slopes of the hills (fig. 10.4A). The 

scarps that mark the edges of the hills can be extremely 

sharp. In the case of the North Hill at SESCA, as the submersible traversed along the scarp edge, the observer on 

one side could see a flat sediment plain a meter below 

stretching off into the distance while the observer on the 

other side could not see bottom below the vertical scarp. 

These scarps mark the headwalls of mass-wasting slumps. 

Locally, several smaller sets of slumps and flat-lying or 

back-tilted terraces are developed near the tops of the hills, 

suggesting that slumping along listric faults has occurred 

without catastrophic slope failure. However, the primary 

means of slope retreat and scarp formation is by slope failure producing mud-clast debris flows. The bases of the 

hills are surrounded by mud-clast talus aprons. Mudstone 

talus blocks in excess of 1 meter on a side are present, but 

the debris aprons are typically covered by mud with 0.5-

to-2-cm mudstone clasts. The thickness and extent of the 

debris aprons are difficult to estimate from submersible 

traverses. However, the debris aprons appear to be relatively small compared to the amount of material removed 

from hill sides to maintain the near-vertical scarps, greater 

than 50 m high, that bound the hills. Locally derived mudclast breccia debris flows and turbidites were recovered by 

piston and gravity cores collected on the sediment plain 

surrounding the hills (Normark and <?thers, chap. 6, this 

volume; Karlin and Zierenberg, chap. 7, this volume). 

Locally, canyons and channels scallop the hill slopes. 

Many areas of the hill slopes display vertical scour grooves 

cut by downslope sediment transport. An incised valley, 

estimated to be at least 10 m deep and 30 m across, on the 

side of the 3170 Hill at SESCA cut into a 5-m-thick layer 

of massive sulfide, providing a continuous outcrop of the 

sulfide body. Areas away from the steep-sided hills also 

have channels cut into sediment (fig. 1 0.3B), including areas that appear flat from submersible observation. Channels cutting the slopes south of the 3170 Hill were observed 

at several locations during Sea Cliff dive 657. Similar 

channels were observed on multiple dives in the NESCA 

area and are described in more detail in a later section. 
176 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

A 

B 

Figure 10.3. Down-looking bottom photographs taken from towed camera system deployed from USNS Narragansett. A, 

Sediment ripples near edge of top of 3170 Hill. Field of view is approximately 4 m across. B, Channel cut into surface sediment 

near base of 3170 Hill. Note curved bank of channel, consistent with down-cutting on a meander bend of an erosional channel. 

Field of view is approximately 4 m across. 
GEOLOGIC SETTING OF MASSIVE SULFIDE MINERALIZATION IN ESCANABA TROUGH 177 

A 

c 

E 

Figure 10.4. Color photographs from external 35-mm cameras of 

DSV Alvin and DSV Sea Cliff. A, Steep scarp forming side of 3170 

Hill. Scarps are typically mud covered, but locally expose 

sedimentary layering, as seen in foreground. Field of view 

approximately 4 m across (Sea Cliff dive 663). B, Steep-sided 

outcrop of weathered massive sulfide exposed near base of 3170 

Hill. Field of view approximately 3 m across (Sea Cliff dive 663). 

C, Massive sulfide mound from North Hill deposit showing crude 

layering. Outcrop is colonized by starfish, sponges, and branching 

B 

D 

F 

corals(?). Field of view approximately 1.5 m across (Sea Cliff dive 

662). D, Small inactive sulfide chimney from deposit on east side 

of North Hill. Field of view approximately 1 m across (Sea Cliff 

dive 661). E, Semiconsolidated layered sediment exposed by mass 

wasting on near-vertical side of SW Hill. Field of view 

approximately 3 m across (Alvin dive 2035). F, Brecciated massive 

sulfide and sulfide talus blocks from SW Hill colonized by yellow 

crinoid and large white starfish. Field of view approximately 3 m 

across (Alvin dive 2035). 
178 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

HYDROTHERMAL DEPOSITS 

The most extensive deposits of massive sulfide observed in the SESCA area are associated with the three circular hills referred to as the 3170, East, and North hills on 

figure 10.2. The deposits occur within a 3 by 4 km area, 

and would probably be considered separate lenses of a 

single ore deposit on land. None of the deposits were hydrothermally active, but all have formed recently enough 

to escape burial by sediment. The distribution, morphology, and apparent relative ages of the deposits suggest that 

each hill was a separate hydrothermal center, and they were 

not active contemporaneously . Numerous camera tracks 

cross the deposits, and the North Hill and 3170 Hill deposits have been investigated during DSV Sea Cliff dives; 

however, none of the deposits are well explored. 

3170 HILL 

The sulfide deposit investigated on Sea Cliff dive 663 

on the north flank of the 3170 Hill occurs near the base of 

the slope at an average depth of 3,250 m, approximately 

25 m above the surrounding sediment plain and 80 m below the flat top of the hill. The thickness of exposed massive sulfide, determined from the submersible depth 

recordings at the top and bottom of sulfide outcrop, ranges 

between 4 and 6 m. The outcrop of the deposit generally 

follows the contours of the slope, and neither the east nor 

west limits of the deposit are established. The available 

data indicate that the exposure of the sulfide body is in 

excess of 120 m parallel to the hill slope. Photographs 

from camera tows confirm that the massive sulfide is exposed to both the .east and west of the area, on trend with 

the deposit observed by submersible. Camera tows also 

imaged sulfide deposits at four other sites near the base of 

the 3170 Hill, including the south side of the hill, suggesting that the hill may be surrounded with outcropping sulfide deposits. 

The surface of the sulfide deposit on the north side 

of the 3170 Hill is rounded and smooth at the 1-to-1 0-m 

scale and lacks chimneylike projections (fig. 10.4B). At 

a finer scale, the surface is jagged and pitted due to selective dissolution of sulfide minerals during sea-floor 

oxidation. Samples collected from the exposed surface 

are predominantly porous crusts of residual amorphous 

silica, barite, and secondary iron oxyhydroxides formed 

by oxidation of pyrrhotite-rich massive sulfide (Holmes 

and Zierenberg, 1990; Zierenberg and others, 1991). The 

deposit surface is heavily colonized by stalked biota, including large and presumably mature sponge colonies. 

Areas of the deposit with relatively flat slopes are partially sediment covered, and the uphill contact of the sulfide deposit is buried by flat-lying sediment. The sulfide 

deposit appears to have formed a topographic dam 

retarding downslope movement of sediment until the 

moat behind the barrier was eventually filled, allowing 

sediment to overflow the deposit. In one area, the deposit is incised by a V-shaped erosional canyon. The 

morphology of the deposit reflects extensive sea-floor 

oxidation and erosion, indicating that this is one of the 

oldest deposits observed in Escanaba Trough. 

NORTH HILL 

A massive sulfide deposit at the base of the south side 

of the North Hill (fig. 10.2) was investigated during Sea 

Cliff dive 662. The deposit occurs between the depths of 

3,228 and 3,242 m. Sonar imaging of the deposit from the 

submersible revealed a linear ridge of outcropping sulfide 

trending N. 75° E. Transponder navigation of the submersible indicates a minimum length of 175 m with a width of 

75 m at one point. The western limit of the deposit was 

not observed. Eastward projection of the trend of the sulfide ridge intersects massive sulfide cropping out between 

3,225 and 3,231 m that was observed, but not extensively 

investigated, at the end of Sea Cliff dive 661. However, 

the submersible track for dive 662 crossed over sedimentcovered sea floor between the two areas of sulfide outcrop, 

indicating that sulfide does not crop out continuously between the two deposits. 

Small isolated chimneys, less than 1 m high, occur in 

the flat sediment-covered area around the periphery of the 

deposit. The periphery of the deposit is marked by sediment-buried sulfide mounds and sulfide crusts in the sediment that coalesce into crudely layered sulfide ledges at 

the edges of the massive sulfide mounds. The bulk of the 

deposit is composed of large, steep-sided constructional 

mounds of massive sulfide that are several tens of meters 

long and up to 11 m high, as measured from the submersible depth recorder. In the process of exploring the deposits, the submersible encountered steep-walled cui-de-sacs 

formed by coalescence of adjacent mounds in which outcropping massive sulfide extended above the field of view 

of all three observers. The surfaces of the mounds are generally smooth and rounded, with less common areas of 

brecciated or crudely layered sulfide (fig. 10.4C). The 

mounds are heavily colonized by attached filter-feeding and 

grazing biota, especially asteroids (fig. 10.4C). A few 

rounded stubby remnants of degraded chimneys were observed on the tops of the sulfide mounds. Chimney forms 

were also observed at the deposit to the east investigated 

by Sea Cliff dive 661 (fig. 10.4D), which appears to be approximately the same age. Based on the morphology, degree of weathering, and amount of sediment cover 

observed, the deposits around the base of the North Hill 

are clearly younger than the deposits of the 3170 Hill, and 

are approximately the same age as the deposits of the East 

Hill. 
GEOLOGIC SETTING OF MASSIVE SULFIDE MINERALIZATION IN ESCANABA TROUGH 179 

EAST HILL 

The deposits associated with the East Hill were not investigated by submersible, and the limited camera observations place few constraints on the extent or continuity 

of these deposits. Observations of sulfide deposits at the 

base of the hill on both the north and south flanks are consistent with the geological setting of sulfide mineralization 

at the 3170 and North Hills; by analogy, massive sulfide 

deposits may also occur around the base of the East Hill. 

In contrast to the known deposits at the 3170 and North 

Hills, outcropping massive sulfide appears to extend northward from the base of the East Hill, occurring near the 

break in slope of a narrow ridge that may mark the surface expression of an extensional fault (fig. 10.2). 

The East Hill deposits are rugged, and constructional 

mounds with stubby projections are still preserved. They 

do not appear as degraded by weathering and erosion as 

the 3170 Hill deposits. The morphology of the East Hill 

deposits indicates that they are younger than those at the 

3170 Hill; however, there are no observations that suggest 

they are still hydrothermally active. 

CORE L2-86-9G LOCATION 

Additional areas of sulfide outcrops are known only 

from camera crossings, dredge collections, and in one case 

from recovery of sulfide breccia in gravity core L2-86-

9G (fig. 10.2). The area where L2-86-9G was collected 

has not been investigated by camera or submersible mapping, but small, isolated sulfide outcrops were imaged during a camera tow to the east. In contrast to the setting of 

the explored deposits, these sulfide occurrences are in a flat 

basin that produced a hard echo return on 3.5 kHz sonic 

records interpreted as thinly sedimented basalt (Morton and 

Fox, chap. 2, this volume). The sulfide recovered in core 

L2-86-9G is overla~n by 50 em of hemipelagic sediment 

mixed with abundant iron oxide. The sulfide unit is a 25-

cm-thick mass-flow deposit of pyrrhotite-rich massive sulfide clasts in a matrix of sand- to silt-size pyrrhotite grains. 

The sulfide debris flow was deposited above magnesiummetasomatized sediment altered to chlorite (Zierenberg and 

Shanks, chap. 14, this volume). This occurrence is evidence for an earlier episode of sulfide mineralization at the 

SESCA area that is now at least partly sediment covered. 

EDIFICE C NEAR 40°53' N. 

The volcanic (?) edifice near latitude 40°53' N. (edifice C of Abbott and others, 1986) between the SESCA and 

NESCA areas (fig. 10.1) was not explored by either towed 

camera or submersible. Seismic-reflection profiles show 

that sediment in this area is disrupted, but the area does 

not show the topographic relief seen in the SESCA and 

NESCA areas (Davis and Becker, chap. 3, this volume). 

There is no indication that volcanic rocks are exposed at 

the sea floor. Of four sediment cores collected from the 

area of edifice C, one (L1-86-27G) contains turbiditic sands 

veined and mineralized by hydrothermal sulfides and barite (Normark and others, chap. 6, this volume; Zierenberg 

and Shanks, chap. 14, this volume). This core demonstrates 

that hydrothermal activity has occurred at this site, and that 

at least some sulfide mineralization is occurring within the 

accreted sediment, not just above the sediment-water 

interface. 

NESCA STUDY AREA 

MORPHOLOGY AND STRUCTURE 

The NESCA study area has a sea-floor morphology 

similar to the SESCA area and is distinguishable from areas of Escanaba Trough away from volcanic centers by the 

presence of sediment hills up to 1,500 m in diameter and 

more than 120 m in height (fig. 10.5). Massive sulfide 

mineralization is extensive in the NESCA area, and the 

largest deposits are spatially associated with the prominent 

sediment hills. 

The dominant morphologic features in the NESCA 

area are the SW Hill and the Central Hill, in the southern 

part of the area shown on figure 10.5, and the hill in the 

center of the axial trough near latitude 41 °05.5' N. at the 

northern edge of the NESCA area that corresponds with 

edifice E of Abbott and others (1986). The SW and Central hills have been explored by a series of bottom-towed 

camera traverses and several submersible dives, and are 

crossed by several geophysical lines (Morton and others, 

chap. 1, this volume; Morton and Fox, chap. 2, this volume; Davis and Becker, chap. 3, this volume). The hill 

near latitude 41 °05.5' N. was investigated on two Reconnaissance and EXploration submersible dives, and is referred to as Edifice REX. Sediment hills of similar and 

smaller size occurring both within and adjacent to the central graben (fig. 1 0.5) are generally unexplored. The central part of the trough is defined by throughgoing normal 

faults that separate an inner zone of disturbed and disrupted 

sediment from well-layered sediment typical of the turbidite fill of the trough away from volcanic centers. 

VOLCANOLOGY 

Volcanic rocks exposed at the sea floor and interpreted 

(from seismic data) to occur near the sea floor beneath a 

shallow sediment cover are more abundant at the NESCA 

site than at the SESCA site (fig. 10.5). The largest exposure of volcanic rock at the NESCA site occurs east of the 
180 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

127° 32' 127° 30' 

40° 58' 0 2 L---------1..-----' \:\ 

KILOMETERS \ 

Contour interval 10 meters 

~Basalt • Sulfide I<<J Disturbed sediment D Undisturbed sediment 
GEOLOGIC SETTING OF MASSIVE SULFIDE MINERALIZATION IN ESCANABA TROUGH 181 

crest of the Central Hill (fig. 1 0.6). The elevated area east 

of the Central Hill is covered by glassy basalt pillows 1 to 

2m in diameter. Lava tubes drape the north flank of the 

hill, indicating flow to the north, toward sheet-flow basalt 

ponded within the central depression of the spreading center (Ross and Zierenberg, chap. 8, this volume). The transition from pillow flow to sheet flow observed north of the 

hill coincides with the break in topographic slope (Ross and 

Zierenberg, chap. 8, this volume). Sediment cover and 

relative age determined from the appearance of glass are 

similar for the pillow basalts and the sheet flow; no distinct flow boundaries indicative of an age difference in lava 

emplacement were recognized. Basalts from the NESCA 

site have a limited range of major- and minor-element composition, and individual flow units are not distinguishable 

from the chemical data (Davis and others, chap. 9, this volume). Ross and Zierenberg (chap. 8, this volume) propose 

that the pillow basalt and sheet-flow lavas are of the same 

relative age and formed during a single eruptive episode 

from a vent area located somewhere on the pillow-covered 

hill top. 

Two small sediment hills protrude above the pillowbasalt-covered hill top (fig. 10.6); a third circular topographic high further east is interpreted, by analogy, also to 

be a protruding sediment hill, but could be composed of 

basalt. The small hill at approximately latitude 41°00. 8' 

N., north of the pillow-basalt-covered hill top, is also composed of sediment and is surrounded by basaltic sheet flow. 

The east margin of this sheet-flow-covered area is a sediment-covered scarp of a graben-bounding normal fault that 

rises to a sediment-covered terrace 100m above the eastern flank of the volcanic hill (fig. 10.5). A submersible 

traverse up the base of the scarp just north of the volcanic 

hill showed that this scarp was formed prior to the last basalt flow, and no basalt was exposed in the up-faulted sediment section. 

The area of the Central Hill west of the outcropping 

pillow basalt is interpreted to have been uplifted by intrusion of basalt into the sediment, but the timing of uplift 

relative to eruption of pillow basalts to the east is uncertain. The basalt-sediment contact, observed on Sea Cliff 

dive 765, occurs between relatively flat lying pillow basalt flows and a steep sediment slope, consistent with 

ponding of basalt against an uplifted sediment hill. The 

central peak of the sediment hill is surrounded by a steep 

sediment-covered scarp, and the north slope of the hill is 

a series of normal faults downdropped to the north (fig. 

<= 

Figure 10.5. Geologic map of NESCA area. Sea Beam bathymetry 

and areas with outcropping and shallowly buried volcanic rock and 

disturbed sediment from Morton and Fox (chap. 2, this volume). Fault 

locations are extrapolated based on seismic-reflection profiles and Sea 

Beam bathymetric data; fault teeth are on downthrown side. Locations 

of cross-section lines presented in figure 10.9B are shown. 

1 0.6; Holmes and Zierenberg, 1990). Basalt flows have 

not been observed on any of the sediment scarps. 

A small exposure of basaltic sheet flow was also observed near Edifice REX (Ross and Zierenberg, chap. 8, 

this volume). Sheet flow is exposed on a flat bench at the 

top of a 9-m-high sediment-covered fault scarp near the 

base of the eastern graben-bounding normal fault. Basalt 

does not crop out on the floor of the graben, but isolated 

talus blocks of basalt derived from the faulted basalt flow 

are scattered across the flat sediment surface near the base 

of the scarp. These blocks are dusted with sediment and 

extensively colonized by filter-feeding fauna. A minimum 

of 9 m of normal fault uplift has occurred after eruption 

of this flow, but the depth of sediment burial prior to faulting is not known. Rapid sedimentation has buried the assumed continuation of the flow on the graben floor, but 

talus blocks derived from the faulted part of the flow have 

not been buried by sediment. Basalt from this area has a 

high K20 content and is chemically distinct from all other 

samples analyzed from the Gorda Ridge (Davis and others, chap. 9, this volume). 

SEDIMENTOLOGY 

Observations from submersible dives in the vicinity of 

the SW and Central Hills attest to the importance of sediment redistribution, similar to observations at the SESCA 

site. Traverses up the steep walls of the SW Hill reveal 

truncated sedimentary bedding exposed by mass wasting 

of the hillside. Blocks of mudstone and siltstone talus, 

some longer than 1 m, occur at the base of the hill and on 

flatter benches on the hill slope. Massive sulfide talus associated -with mudstone talus was observed and sampled 

along the southeastern flank of the hill. The top of the SW 

Hill is flat and sparsely populated by bottom-dwelling organisms. Centimeter-scale sediment layering dipping at a 

very shallow angle into the hill top is exposed and accentuated by erosion near the fringe of the hill top. 

The sediment-covered eastern side of the Central Hill 

is bounded on the north and east by a series of stepped 

scarps each a few meters high, which contrast with the 

large mass-wasting scarps that bound most of the sediment 

hills. Sediment redistribution is also apparent in this area. 

Piston cores collected from the Central Hill recovered mudstone breccias (Normark and others, chap. 6, this volume). 

The hill slopes are cut by numerous channels, some of 

which are tens of meters deep. Large channels occur at 

all levels of the hill slope, even near the highest point of 

the hill. The eastern flank of the Central Hill is an area of 

complex bottom morphology including grabenlike depressions that are locally floored by meter-size mudstone talus. Hummocky and channeled areas with relatively low 

levels of biologic colonization and bioturbation are indicative of reduced sedimentation or sediment erosion. 
182 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

0 500 

METERS 

Contour interval 5 meters 

58,500 59,000 

54,500 

59,500 

-Sulfide f:-.::.·:::·:1 Sulfide mineralized area D Sediment D Pillow basalt 0 Lobate basalt 

Figure 10.6. Geologic map of Central Hill area of NESCA site. 

Bathymetric map base is from full-resolution Sea Beam map with 5 

m contours, adjusted in latitude and longitude so that major 

topographic features are aligned with transponder navigated map of 

NESCA area presented in Morton and others (chap. 1, this volume). 

Numbers along right and bottom give coordinates of local 

transponder net (in meters). Hachures indicate area of closed low. 

Thin solid lines show location of transponder navigated submersible 

tracks, dashed where submersible was too high above sea floor for 

visual observations during extensive traverses. Major structural 

features are shown where sufficient data exist. Teeth on faults are 

on the downthrown side. 
GEOLOGIC SETIING OF MASSIVE SULFIDE MINERALIZATION IN ESCANABA TROUGH 183 

Valleys with downslope scour grooves were also observed cutting into the planar surface of the normal fault 

that bounds the innermost graben (Alvin dive 2034, location shown on fig. 1.7, Morton and others, chap. 1, this volume). The scarp face generally dips 60° to 80° toward the 

spreading axis and is completely sediment covered. Vshaped channels up to 10 m across cut into the scarp face, 

locally exposing layered sediment. A small sediment 

slump, with a headwall scarp and a mudstone debris apron 

a few meters across, was observed on a flatter bench on 

the scarp face. 

Despite the generally low relief, the flat sediment-covered plain north of the Central Hill investigated on Sea Cliff 

dive 659 is also cut by numerous channels that locally expose sediment layering in the channel walls. The channels 

range from 1-2 m wide to at least 40 m wide. Smaller 

channels are typically about half as deep as they are wide. 

The floors of large channels often dropped below the extent of view of the observers in the submersible; some 

channels are at least 20 m deep. The walls of the channels range from near vertical to about 45°. Many channels are asymmetric, with one wall steeper than the other. 

Submersible and camera observations show that the channels locally have curved banks with the convex outer bank 

steeper than the inner bank (fig. 3B). Sediment slumping 

and scour down the walls of the channels is common, but 

there is no evidence of recent transport of sediment along 

the axis of the channels. The one-dimensional nature of 

both camera-track and submersible mapping provides little 

data on the lateral continuity of the channels. 

HYDROTHERMAL DEPOSITS 

Areas of sulfide mineralization observed either by submersible or bottom camera are also shown on figure 10.5. 

Camera observations suggest that the extent of sulfide mineralization may be larger than shown. Areas of extensive 

sulfide mineralization associated with the SW Hill, the 

Central Hill (fig. 1 0.6), and Edifice REX have been examined by submersible. Additional areas of sulfide mineralization sampled by dredging are not included on figure 

10.5; the tracks of dredge runs that recovered hydrothermal minerals are shown on figure 16.2 of Koski and others (chap. 16, this volume). The deposits associated with 

the Central Hill are the only ones in Escanaba Trough 

known to be actively venting hydrothermal fluid. 

SWHILL 

The sulfide deposits near the base of the SW Hill were 

investigated by two camera runs, Alvin dive 2035, and Sea 

Cliff dive 763, and were sampled by four dredge runs. The 

geologic setting of the SW Hill is analogous to the sediment hills at the SESCA site. The mesalike sediment hill 

has a planar top and steep sides retreating by mass wasting, which exposes semiconsolidated sediment (fig. 10.4£). 

However, the SW Hill is not circular, but is elongate parallel to a very steep eastern side bounded by a fault trending approximately N. 25° W. (fig. 1 0.5). 

The occurrence of massive sulfide deposits at the SW 

Hill is also similar to the sediment hills in the SESCA area. 

Each camera and submersible crossing of the base of the 

slope of the SW Hill encountered sulfide mineralization, 

suggesting that sulfide mineralization may at least discontinuously surround the hill. In contrast to the SESCA area, 

two areas of massive sulfide mineralization occur at different depths on the slope of the SW Hill; one is developed near the base of the slope on the flat sediment-covered 

saddle that extends southeast from the hill, and the other 

is exposed at a higher elevation on the steep slope forming the base of the hill (fig. 10.5). The lower deposits consist of large unweathered sulfide mounds topped by narrow 

spires and inactive chimneys, a morphology that shows 

these deposits formed recently and are younger than any 

of the deposits investigated at the SESCA site. Water-column temperature anomalies were recorded by a temperature sensor mounted on the camera sled towed in the area 

of the sulfide deposits (Morton and others, 1990). The 

photo crossings also indicate that these deposits are laterally extensive in the direction of the camera track, but the 

limits of mineralization are not determined. The deposits 

further up the slope consist of large talus blocks of sulfide 

below a steep exposure of massive sulfide several meters 

thick. This deposit is not topped by chimney or sulfide 

spires. The presence of extensive sulfide talus and the lack 

of easily degraded sulfide spires indicates that the upper 

deposits are older than the lower deposits. The upper deposits may have been exposed by, and are clearly modified by, either normal faulting or mass wasting. 

The youthful appearance of the lower deposit made it 

a prime target for submersible investigation in search of 

hydrothermal activity. Unfortunately, both dives in this 

area experienced problems with transponder navigation, 

and neither dive investigated the area photographed during the camera runs. Alvin dive 2035 encountered sulfide 

mineralization at a depth between 3,295 and 3,304 m in 

the area north of the deposits crossed by the camera runs. 

Much of the observed mineralization occurs as talus blocks 

of sulfide up to a few meters across (fig. 10.4F). Samples 

recovered from this site include a fragment of a concentrically zoned sulfide chimney. When the submersible 

crossed the base of the hill near its northeast side, divers 

observed massive sulfide between the depths of 3,300 and 

3,304 m. In this area, the sulfide deposit is a massive body 

exposed on the steep sediment-covered hill slope surrounded by sulfide and mudstone talus blocks. The deposit 

has been disrupted by mass wasting, similar to the upper 

sulfide zone observed in the camera runs. 
184 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

An area of massive sulfide mineralization estimated to 

be several tens of meters long was crossed on Sea Cliff dive 

763 in the area north of the camera runs. The base of the 

deposit was at 3,272 m and the top was at approximately 

3,255 m. The deposit is blocky and fractured, and large 

blocks of sulfide talus are common. Although no 

chimneylike projections were observed, the sulfide is not 

extensively weathered, and reflections off large crystal 

faces of sulfide minerals could be clearly seen. The slope 

below the deposit is cut by several normal fault scarps, 1 

to 5 m high, that expose bedded mudstone. Blocks of mudstone talus 1 to 2 m wide occur at the base of the slope. 

The base of the sulfide deposit is exposed near the top of 

one of the normal faults, and it appears that the sulfide is 

disrupted by faulting. This deposit occurs near the sulfide 

deposit observed on Alvin dive 2035, but at a shallower 

depth. It is possible that the lower deposit is a displaced 

fragment of the upper deposit. However, massive sulfide 

occurs 1 km further north along the structural boundary of 

the hill at the same depth as the lower deposit, and camera observations of the upper and lower deposits to the 

south clearly show that the lower deposit in this area is a 

constructional deposit formed in situ. Further mapping will 

be required to establish the continuity of the two levels of 

mineralization and to determine the temporal and genetic 

relationship between them. 

CENTRAL HILL 

The western, sediment-covered part of the Central Hill 

(fig. 10.6) is the area of the most extensive sulfide deposit 

observed in Escanaba Trough. It is also the only area 

known to have active hydrothermal vents. Three areas of 

outcropping massive sulfide mineralization were discovered and partly explored during 10 submersible dives (fig. 

1 0.6). The areas of outcropping massive sulfide are separated by sediment-covered sea floor, and the relationship 

between these areas is not known. The massive sulfide deposits on the west and southeast flanks of the Central Hill 

are actively venting hydrothermal fluid, and the area on the 

northern flank shows indications of very recent hydrothermal activity, suggesting that these deposits are all part of 

the same hydrothermal system. 

The geologic setting of these deposits is somewhat different than the deposits at the SW Hill or the three sediment hills at the SESCA site. Similar to the SESCA site, 

the deposits are all developed on the sediment-covered 

western part of the Central Hill; no hydrothermal deposits 

have been observed on exposed volcanic rock to the east 

(fig. 10.6). However, the deposits are not confined to the 

structural boundary around the perimeter of the hill, but 

extend to near the top of the hill. 

Uplift of the western part of the Central Hill was accommodated by a series of stepped normal faults, 

down dropped to the north (fig. 1 0.6). The western and 

eastern flank are also formed by sediment-covered scarps 

that may also represent normal faults. Uplift on the eastern flank of the sediment hill resulted in extensive disturbance of the sediment cover, including formation of small 

grabens in the overlying sediment, one of which apparently 

controls the location of the southernmost area of massive 

sulfide mineralization (fig. 10.6). The eastern flank of the 

sediment hill has irregular topography near the contact with 

the basalt. The topography is characterized by swales and 

sediment-covered scarps a few meters high. Mudstone talus formed from slumping along steep slopes is common. 

The southernmost area of massive sulfide mineralization (fig. 10.6) was investigated on Alvin dives 2040 and 

2042 and on Sea Cliff dive 766. The apparent eastern edge 

of sulfide mineralization was explored for 190 m along a 

north-south-trending scarp. Sonar images from Alvin suggest that this scarp is the west wall of a small graben. The 

floor of the graben is at a depth of 3,260 m and is sediment covered. In the area of the sulfide deposit, the graben floor near the west wall is colonized by clusters of the 

vent clam Calyptogena (fig. 10.7A; Van Dover and others, 

1990) and is locally covered by bacterial mat. Black 

sulfidic sediment and hydrothermal crusts were observed 

at the base of the graben wall (fig. 10.7B) and where the 

surface layer of brown hemipelagic sediment was disturbed 

by the submersible. No flow of hydrothermal fluid through 

:::::} 

Figure 10.7. Color photographs from hand-held and external 35-

mm cameras of DSV Alvin. A, Abundant Calyptogena in sediment 

near base of outcropping massive sulfide on east wall of small graben 

on southwest flank of Central Hill. Some crusts of sulfide and 

indurated sediment are shown; nearby areas are colonized by bacterial 

mats. Field of view approximately 3 m across (Alvin dive 2042). B, 

Sulfide crust and indurated sediment at base of graben wall. Field 

of view approximately 3m across (Alvin dive 2042). C, Indurated 

sediment on flank of massive sulfide mound discharging lowtemperature hydrothermal fluid. Field of view approximately 2 m 

across (photo by D. Foster, Alvin dive 2041). D, Dense tube worm 

cluster obscuring low-temperature vent. Tube worms are locally 

coated by white bacterial mat and are colonized by anemones with 

red tentacles. Some galatheid crabs occur on brecciated massive 

sulfide of mound. Field of view approximately 3 m across (Alvin 

dive 2036). E, White, porous, sinterlike crust of anhydrite, with 

barite, minor sulfides, and magnesium-rich smectite, capping a 

mound of pyrrhotite-rich massive sulfide. Right side of photo is out 

of focus due to shimmering from discharge of 217°C clear vent fluid. 

Yellow palm worms colonize anhydrite crust near active vents. Field 

of view approximately 2 m across (Alvin dive 2036). F, Sulfide 

talus and breccia fragments of fresh coarse-grained pyrrhotite at base 

of inactive sulfide mound. Light spots are reflections from large 

pyrrhotite crystals. Although there are many fracture surfaces that 

expose unweathered sulfide, most sulfide fragments are dusted by 

sediment. Field of view approximately 2m across (Alvin dive 2033). 
GEOLOGIC SETTING OF MASSIVE SULFIDE MINERALIZATION IN ESCANABA TROUGH 185 

the sediment was observed, even in areas of dense clam 

and bacterial colonies. The steep graben wall exposes a 

7-m-thick section of massive sulfide that is locally colonized by bacterial mats and tube worms. Diffuse flow of 

warm hydrothermal fluid was observed from the graben 

wall, but no high-temperature vents were encountered. The 

area has not been explored in detail, and it is possible that 

high-temperature hydrothermal discharge may be occurring 

A 

c 

E 

at this site. Massive sulfide mounds extend for at least 100 

m west of the graben edge, and some mounds reach up to 

at least 3,240 m depth. Some of the mounds to the west 

are also colonized by tube worms and bacterial mats, indicating that hydrothermal fluid flow is at least weakly active throughout this area. Intact but inactive sulfide and 

barite chimneys on top of some mounds suggest recent 

higher temperature focused discharge. 

B 

D 

F 
186 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

The best explored and most hydrothermally active area 

of sulfide mineralization on the Central Hill extends west 

from the northern end of the sediment-covered hill top (fig. 

1 0.6). This is not an area of continuous sulfide outcrop, 

but rather a region of abundant, closely spaced sulfide 

mounds. The mounds are typically 20 to 60 m in diameter and 5 to 10 m high. One of the larger mounds investigated lies along the steep sediment-covered hill slope and 

had continuous outcrop of massive sulfide from 3,245 m 

to 3,225 m depth. The base of the westernmost known sulfide mound is at 3,275 m. Mineralization extends from 70 

to 120m in a north-south direction and for at least 300m 

in an east-west direction, but the western edge of sulfide 

mineralization has not been determined. The distance between mounds is similar to their lateral dimensions such 

that often more than one sulfide structure was visible from 

the submersible. The mounds may be connected in the 

shallow subsurface, thus the sediment ponds between 

mounds may represent a surficial veneer over massive sulfide in the low areas. Sediment near some of the mounds 

is semilithified, presumably due to hydrothermal alteration 

(fig. 10.7C). About one-third of the mounds are colonized 

by live vent-specific biota and have diffuse discharge of 

low-temperature hydrothermal fluid. Vent fluids with temperatures of 13.5°C and l8°C were sampled from diffuse 

discharge at two mounds (fig. 1 0.6). Two mounds were 

observed to actively discharge high-temperature hydrothermal fluid; one near the eastern margin of the sulfide area 

was venting 217°C fluid, and one on the western edge of 

the explored area was venting 108°C fluid (fig. 10.6). Even 

though these mounds are 275 m apart, the major-element 

composition of the end-member fluid at each vent is identical (Campbell and others, chap. 11, this volume), a result that is consistent with the hypothesis that this large 

mineralized area is a single hydrothermal system hydrologically interconnected at depth. 

The massive sulfide mounds generally have smooth, 

rounded surface profiles. Both sulfide and barite chimneys 

occur atop some mounds, but chimneys are not common, 

even on hydrothermally active mounds. The morphology 

of the low-temperature hydrothermal vents was generally 

obscured by dense clusters of tube worms (fig. 10.7D), 

which occur in patches estimated to be up to 8 m long and 

2 m wide. High-temperature fluid flows diffusely from Ito-2-m long areas of porous anhydrite-barite sinter capping 

massive pyrrhotite-rich sulfide mounds (fig. 10.7£; Koski 

and others, chap. 16, this volume). The bases of the 

mounds, including those still active, are onlapped by 

hemipelagic sediment and are fringed by sulfide talus. The 

range from nearly sediment-buried to sediment-free blocks 

(fig. 10.7 F) indicates a long history of sulfide formation 

relative to the sedimentation rate. Angular, slab-shaped 

sulfide fragments a few centimeters thick and several tens 

of centimeters on a side are common around the base of 

the mounds. The surfaces of the mounds are also covered 

by sulfide breccia (fig. 10.8A) and are locally cut by fissures exposing fresh, coarse-grained sulfide crystals (fig. 

10.8B). Some of the sulfide blocks surrounding the 

mounds are true talus, related to the breakup of unstable 

sulfide structures and gravitational transport down steep 

slopes. However, many areas of sulfide breccia on lowrelief slopes lack chimneylike structures (fig. 10.8B). 

These breccias, and the fissures cutting the mounds, are 

forming in situ at the mound surface. Sulfide crusts forming in the sediment are also brecciated and locally buckled upward into "teepee" structures or "pressure ridges" 

(fig. 10.8C). 

The vent fauna colonizing the active mounds have affinities to faunal assemblages of Juan de Fuca Ridge and 

have been described by Van Dover and others (1990) and 

Grassle and Petrecca (chap. 17, this volume). The most 

obvious biologic features are the clusters of tube worms 

(fig. 10.7D, 10.8D), which themselves are heavily colonized by greenish-orange palm worms, white-bodied 

anemones with reddish tentacles, white bacterial mats, and 

blue filamentous mats interpreted to be the protozoan 

Girvanella (fig. 10.8D). Close inspection reveals that rock 

surfaces near vents are very heavily colonized by small 

mollusks, mostly gastropods. Galatheid crabs are common, 

but not overly abundant around the vents. Scattered shells 

of dead Calyptogena occur in the area, but no live clams 

were seen in this vent field. Inactive mounds are generally heavily colonized by tunicates, anemones lacking the 

red-tinted tentacles, and asteroids (fig. 10.8£). 

An extensive area of massive sulfide is exposed on the 

north slope of the Central Hill. Parts of this deposit were 

explored on Sea Cliff dive 658 and Alvin dives 2033, 2037, 

and 2040. The deposit formed on the steeply sloping, stepfaulted flank of the hill and is exposed between the depths 

of 3,235 and 3,276 m. The deposit extends more than 270 

m from north to south and more than 100 m from east to 

west, but the western edge has not been determined. 

Within this area there is nearly continuous outcrop of massive sulfide with few sediment-covered areas. Constructional sulfide mounds, some topped by barite chimneys 

(fig. 10.8£), occur throughout the deposit, including near 

the lowest depth. Much of the surface exposure is coarse 

breccia and talus of massive sulfide. Sulfide fragments 

exceeding 1 m in length are common, including slabs of 

sulfide 5 to 20 em thick. These fragments often expose 

fresh, coarse-grained massive sulfide with pyrrhotite crystals large enough to be visible from the submersible (fig. 

10.7F). No active venting has been observed in this area, 

but recent hydrothermal activity is indicated by the morphology of the deposits, the presence of fresh sulfide at the 

surface, and the limited occurrence of live bacterial mats, 

dead tube worms, and vent clam shells. 

The three areas of massive sulfide outcrops on the 

Central Hill are contemporaneous and are interpreted to be 

part of the same hydrothermal system related to igneous 
GEOLOGIC SETTING OF MASSIVE SULFIDE MINERALIZATION IN ESCANABA TROUGH 187 

A 

c 

E 

Figure 10.8. Color photographs from external 35-mm cameras of 

DSV Alvin and DSV Sea Cliff. A, Coarse breccia of massive sulfide, 

some showing extensive oxidation to yellow iron oxyhydroxides. 

Field of view approximately 1.5 m across (Sea Cliff dive 658). B, 

Fissures cutting flat-lying crust of massive sulfide. Field of view 

approximately 1.5 m across (Alvin dive 2040). C, Sulfide crust in 

sediment buckled upward by displacive subsurface growth. Field 

of view approximately 1.5 m across (Alvin dive 2033). D, Tube 

worms on mound of brecciated massive sulfide. Tube worms are 

colonized by anemones with red tentacles. White galatheid crabs 

B 

D 

F 

and blue filamentous mats interpreted to be the protozoan Girvanella 

can be seen on massive sulfide. Field of view approximately 1 m 

across (Alvin dive 2041). E, Inactive barite chimneys on massive 

sulfide mound colonized by tunicates and anemones lacking red 

pigment of those associated with active vents. Field of view 

approximately 0.75 m across (Alvin dive 2033). F, Isolated sulfide 

chimneys and sulfide crust in sediment exposed at top of channel 

cut into sediment-covered bottom north of Central Hill. Field of 

view approximately 1.5 m across (Sea Cliff dive 659). 
188 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

intrusion causing uplift of the Central Hill. The outcrops 

are contained within an area 600 by 1,200 m, but their extent is not constrained by present observation. Exposure 

of massive sulfide for 5 to 10m above the surrounding sea 

floor is common, and the depth range between the deepest 

and shallowest deposits exceeds 50 m. We cannot reliably 

estimate the tonnage of massive sulfide, not knowing thickness of the sulfide deposits or the extent of both sedimentburied and subsurface mineralization. However, the scale 

of these deposits, both in terms of areal distribution and 

intensity and duration of hydrothermal activity, is comparable to massive sulfide deposits that are mined on land. 

AREA NORTH OF THE CENTRAL HILL 

Isolated areas of sulfide mineralization have been observed throughout the NESCA area from both submersible 

and camera runs (fig. 1 0.5). Numerous areas with isolated 

sulfide chimneys and crusts developed in hemipelagic sediment were seen on Sea Cliff dive 659. Many of these are exposed near the tops of the channels that commonly cut 

through this area (fig. 10.8F). The sulfide crusts, as well as 

the base of a small chimney recovered in this area (Holmes 

and Zierenberg, 1990), provide evidence that mineralization 

is occurring at and below the sediment-water interface. 

Areas of older, weathered, and partly sediment-buried 

sulfide mineralization were observed during Alvin dive 

2034 on the margins of a small sediment hill onlapped by 

sheet-flow basalt flowing northward from the Central Hill 

(fig. 10.5). Based on 3.5 kHz profiles, this area is interpreted to be underlain at shallow depth by basalt (Morton 

and Fox, chap. 2, this volume). However, all of the sulfide observed in this area occurs on sediment-covered bottom, and no outcrops of basalt were observed between 

exposures of sulfide. 

Isolated sulfide occurrences were also observed from 

towed camera traverses along the topographic break that 

bounds the west side of the deepest part of the axial graben and the base of the small sediment hills on the west 

side of the axial graben near latitude 41 °02' N., longitude 

127°30' W. (fig. 10.5). The extent of outcropping sulfide 

observed in the camera runs suggests that some of these 

occurrences may be substantial sulfide deposits, but they 

have not been investigated in detail. 

EDIFICE REX 

The previously unexplored area of Edifice REX, approximately 10 km north of the Central Hill, was investigated on two reconnaissance and exploration dives during 

the 1988 Alvin dive series in hopes of finding additional 

high-temperature hydrothermal discharge. No camera mapping had been done at the site, and no bathymetric map was 

available. A bathymetric map was constructed following 

an overnight survey using the 12 kHz echo sounding system of the R/V Atlantis II. The resulting map and the 

knowledge of the ·sulfide occurrences at the SESCA and 

NESCA areas were used to target Alvin dive 2038 on the 

south slope of a large hill. This dive encountered massive 

sulfide mineralization at a depth of 3,260 m 15 minutes after reaching bottom. Alvin dive 2039 was targeted on the 

east flank of Edifice REX and encountered sulfide mineralization at a depth of 3,254 m. The area of these two dives 

was mostly beyond the range of the NESCA transponder 

grid (Morton and others, chap. 1, this volume); two additional transponders deployed in this area were used to navigate the bathymetric survey and the dives. Locations were 

justified to the NESCA net by transponder fixes at the 

southern part of the area and correlation of the global positioning system (GPS) locations. 

The only volcanic rocks observed at Edifice REX are 

sheet-flow basalt exposed at the top of an upfaulted block 

east of the hill top and basaltic talus derived from this 

scarp. The entire area of the hill top investigated by submersible is sediment covered. Extensive areas of massive 

sulfide were encountered throughout Alvin dive 2038. The 

size of the outcrops observed suggest that these are large 

deposits. The deposits are weathered and often sediment 

covered. The best exposures are along steep to vertical 

faces several meters high that exposed continuous walls of 

massive sulfide. One steep exposure appeared to show a 

massive sulfide layer a few meters thick interbedded with 

sediment. Some of the steep exposures are probably fault 

scarps that expose the sediment-buried massive sulfide. 

These faults may form the walls of a graben that controlled 

the location of hydrothermal discharge. Locally, sonar 

scans from the submersible displayed parallel reflections 

trending in the direction of the sulfide walls, consistent 

with the interpretation that the sulfides are exposed along 

small grabens a few tens of meters wide and 5 to 10 m 

deep. Neither the structures nor the deposits were systematically mapped, but at least some of the sulfide scarps are 

not parallel to the spreading axis. The deposits discovered 

on Alvin dive 2038 appear to be distributed around the 

edges of the southwesternmost of the three hills that can 

be seen on the Sea Beam map of this area (fig. 10.5). The 

deposits occur near the top of this hill between depths of 

3,250 and 3,265 m. Smaller deposits on the east flank of 

this hill, discovered on Alvin dive 2039, occur at a depth 

of3,254 m. 

DISCUSSION 

VOLCANIC-SEDIMENT INTERACTION 

Sediment cover thins over the volcanic edifices, and 

the parallel seismic reflectors that typify the Escanaba 
GEOLOGIC SETTING OF MASSIVE SULFIDE MINERALIZATION IN ESCANABA TROUGH 189 

Trough fill are often disrupted over the tops of the edifices 

(figs. 10.2, 10.5; Morton and Fox, chap. 2, this volume). 

The volcanic edifices are interpreted to have formed by a 

combination of intrusive and extrusive igneous activity 

(Morton and Fox, chap. 2, this volume). Igneous and sedimentary processes are contemporaneous and interrelated in 

this setting. Volcanic flows and sills are intercalated with 

the sedimentary fill of the trough (Morton and Fox, chap. 

2, this volume; Davis and Becker, chap. 3, this volume). 

Uplift of sediment over basalt intrusions leads to redistribution of sediment by mass wasting (Holmes and 

Zierenberg, 1990; Karlin and Zierenberg, chap. 7, this volume). The distribution of volcanic centers also provides a 

first order control on the location of hydrothermal activity. Intrusive bodies in sediment are inferred to be responsible for localization of zones of hydrothermal discharge 

by the thermal focusing of fluid flow and enhancement of 

cross-stratal permeability caused by both thermal and mechanical disturbance in the intruded sediment (Denlinger 

and Holmes, chap. 4, this volume). 

Schematic cross sections for the SESCA (fig. 10.9A) 

and NESCA (fig. 10.9B) study areas show the known and 

inferred relationships between sediment cover, igneous intrusive and extrusive rocks, faulting, and hydrothermal deposits (figs. 10.2, 10.5). The cross-section lines are drawn 

along seismic lines (lines 4, 6, 8, 10, and 12 for the SESCA 

area, shown on figure 2.9 of Morton and Fox (chap. 2, this 

volume) and lines 1 to 5 of Davis and Becker (chap. 3, this 

volume) for the NESCA area). These seismic profiles, and 

lines 14 to 25 of Morton and Fox (chap. 2, this volume) in 

the NESCA area were used to constrain subsurface geology where possible. However, these are schematic cross 

sections, not strict seismic interpretations, and as such contain information and speculative interpretations based on 

the Sea Beam bathymetry, 3.5 kHz seismic profiles, and 

sea-floor geology observed from submersible and bottomtowed camera. Fault traces shown on figures 10.9 are extrapolated between cross-section lines and (or) inferred 

from the Sea Beam bathymetry and submersible observations of scarps. 

SESCA CROSS SECTIONS 

The SESCA section lines (fig. 10.9A) run slightly oblique to the east-west spreading direction and are confined 

to the trough floor; they do not extend to the normal faults 

that bound the axial valley, which are located approximately 2 km west and 5 km east of the ends of the section 

lines. Sediment fill in the northern part of the SESCA area 

is interpreted to be approximately 400 m thick, but thins 

southward to approximately 300 m where volcanic basement is interpreted to rise closer to the sea floor. The northernmost sections show an inner set of graben-bounding 

faults of small displacement that mark the boundary 

between seismically disturbed and undisturbed sediment 

(figs. 10.2, 10.9A; Morton and Fox, chap. 2, this volume). 

The zone of disrupted sedimentary layering widens southward and extends west of the cross sections to the first large 

normal fault that bounds the west side of the axial valley. 

A seismically transparent layer that occurs throughout 

Escanaba Trough approximate! y 100 m below the sea floor 

(Morton and Fox, chap. 2, this volume; Davis and Becker, 

chap. 3, this volume) is generally not clearly imaged within 

the innermost graben, but can be traced discontinuously 

through the zone of disrupted sediment west of the inner 

graben. The eastern boundary of the inner graben is shown 

as a horst on the northern two cross sections, but the strike 

of the fault bounding the eastern side of this block is oblique to the ridge axis and extends beyond the eastern end 

of line 8 (fig. 10.2). 

Basement is poorly imaged in much of the disturbed 

central area, but is interpreted to be generally shallower 

than outside the central graben, especially on the southernmost lines. Some deeper seismic reflectors shown as sills 

and flows on the cross sections may actually be the top of 

irregular basement highs. The inferred process of oceanic 

crust formation at sediment-covered spreading centers by 

injection of sills near the base of the sediment fill (Morton 

and Fox, chap. 2, this volume; Denlinger and Holmes, 

chap. 4, this volume) will result in a complex transition 

zone between oceanic basement and sediment, rather than 

the sharp interface formed by sediment burial of an inactive rift. High-amplitude subsurface reflectors are interpreted on these sections as either basaltic sills and flows 

or volcanic basement. However, subsurface sulfide mineralization or hydrothermal lithification of sediment could 

also produce similar localized high-amplitude seismic reflections within the sediment column, and the interpretation presented on these sections is not unique. Shallow seismic 

reflectors, interpreted as sills, are especially prevalent beneath 

the small sediment hills crossed on lines 8 and 10. 

NESCA CROSS SECTIONS 

The NESCA sections show that the trough at this latitude is filled to an average depth of approximately 600 m 

with sediment (fig. 10.9B). The shoaling of igneous basement characteristic of the Escanaba Trough volcanic centers is not apparent on the NESCA cross sections. 

However, acoustic reflections from volcanic basement are 

not well developed below the central zone of disturbed 

sediment, so the depth to basement is poorly constrained. 

Sediment layering is generally well developed, and the 

transparent seismic marker bed observed throughout 

Escanaba Trough (Davis and Becker, chap. 3, this volume) 

is easily traced outside a central zone of disturbed sediment. In the center of the trough, sediment layering shows 

the disruption typical of the Escanaba Trough volcanic 
190 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

A 

3200 nn LI~E: 12 

-tC.~_·.:·.: : -· ... i: ~ 

LI~E: 10 Sulfide ~--sulfide 3200 nn 

........ ...;::=::.-..._-

· · · rn. · .: . . ~t~~. . .: . . · .• ~~ ~- ....;.... 

LI~E: 8 3200 nn 

1• ..b_·.: _: 

- ...,--..,..=_ ,""'.,..,:.,, .,. ,~ -7;'- ., ...... --~~ I..._ \ ' / ~,~c_ I ' - ' ...... ..... I ' • ::::::. 

• - • 

••• -

........_ 

. • 

_... 

·~ - - I I: 

/ ......._ / 

I 

......._ 

:. 

_.-

\ I ~ I I I 

/ 

' 

........ 

/ 

1'/ 

I 

........ 

/ 

_... 

\ I 

-

~ --I , 

I 

' ----~-- // ....._ _... ~ _-

: "5"" ,- ~ -,.. / ' ..... .,.... ~ ,-' - / / ' .,.. ....... ~ ,-' - / ' ..... ..... ....... \ ' - / ' .,.. ............ ,- ....... _ ..... _ ...... , ..... , .... _:,_, ......... _ .............. ,--:.. ...... _.,., ....... , ..... _,,_ ..... ,, / ...... -,- - ..... -- / / --,- - - -- / / ....... -,- - ..... -- / / '-,""' - - - / / 

...... 

~L~I~~~E:~------------------------------------------------------------------~3200 nn 

~ 

.... · ... 

.... '-

-· ... · .. . . .. . 

--~:... 

.. : . 

. ·:::_ 

.:-.·:·----· . 

- ~ _-: 

··. • ·-'i-1 • ,· • ...._ ~ I ' I ' ~ 

..... _ ' .... ,... - I ' ' \ 

..... _ ' .... ,. - I 

; I ·: 

......... 

LI~E: 4 

- - ..... 

. . . 

• .;__ ~7:.. ~ - \ - ' ; ~ ~~ :::= 

...:.-. ·. --::: .......... / - ....... - ., ,..... / ~ 

.. ~ ..... i - - J '~ ...... I \ I ....... \ I ' 

I ' ,. I / \ I --, I , I : / I :. \ I ~ I , ' I ' / ..... I 

I /' ·......_ / ....... , I ' I ....... / 1 ......_ _... - --I-; ......_ / ......._ _... - - I / ......_ / I......._/_... -'- ,I 

..... ...:: ,-' - / / '--., -.,.. -::- ,-' - ,. / , / / ..... - - / / ..... - I ..... ,.; ...... I,....._ :. I I ;-, ...._ - 1 ,...._ :. ' ..... I :::-, ~ ~ ~,....- :. 1' _.,... I :;-, ~ ~ -:-_ :. 1' _ ..... ...-_I :::-

/ ....... -!_ ..... -,.,..,..--,--/_ .... -,- - ......... , ....... -, .......... / ..... -!._- -,.,..-.,--, ...... -/ ...... -,- ...... ,.,..-.,--, /_ ...... 

~ Undisturbed 

~sediment D Transparent 

marker bed 

~Disturbed E2:2J sediment 

rl Basaltic 

~basement 

~ Basaltic 

sill or flow 

3200 nn 

Figure 10.9. Schematic cross sections of SESCA and NESCA areas. 

A, Schematic cross sections of SESCA area drawn along seismic 

reflection profiles 4, 6, 8, 10, and 12 from Morton and Fox (chap. 2, 

this volume). Cross sections are constrained where possible by 

seismic-reflection data but also include data from surface-towed 

camera, submersible observations, and Sea Beam bathymetry. 

Vertical exaggeration 5.3 x. B, Schematic cross sections ofNESCA 

area drawn along seismic-reflection profiles 1 to 5 from Davis and 
B 

GEOLOGIC SETIING OF MASSIVE SULFIDE MINERALIZATION IN ESCANABA TROUGH 191 

1-3200 m 

Sulfide 3200m 

\ 

Sulfide 

1 I "'3200 m 

0 ~lFO~itnft~o 0 ~so 0 °:::: 0 "1t1~~:.-~o~~.F-~~~~~~ 

LINE 3 

~---=­

r -- ~ 

f- - oiOo 0 

0.:. -t~ -~ 000 0 0 • • 0 0 

?- 0 0

:

00 

0 

~l 

I-'..._ I / ~ = -• o • • o : • o: o •• :· •, ,' o o ~o : 0 • : o o o •' • • ; • o • . : o •: • • 00 o,: 0. o_ 

--

-

~ 

--= ,. ,. ..:: 1-' - / / ' ;:-- - _--: . 

0

0 : • • 0 0 • / ' ' ----

,,. 1 "', _,. 

,- ........ ~' 1 r-~i-~~~~~-~~o~~~~~~~~o~~~·~~~o~~~~~~~~~~~~~~~~o~·~o~:~~~o~·~o~~·f . 0 • 0 ... 0 • 0 1 '~I"',~-~~' ,,. _,. .... 

( , / _ .... !_ I , ,./ .... t f-. • 0 0 ,. 0,0-, __ , ...... 0 ~ , I ,..0~· , , / .... / / _ .... !_ I ,. 

- \ ""' ...._ , I ; \ - \ ~ • • ~ '- -~·· ...... - _., \ ~ ~- \ / \ - - \ _, 

,'-/,-,_:'- ,'-,,,_,_ ... , /\1 '""'' '/\1 ",'-,,, 

_, , ........ ,._,' ,._,' ,_,' ,._,' ,._,' 

LINE4 __,=-=-~.....,.=--~==-,....1- 3200 m 

.. 0. ,..:._ 0~~.1':_ -= 

LINE5 __ -=-=-~~3200 m 

others (chap. 3, this volume). Cross sections are constrained where 

possible by seismic-reflection data from Davis and others (chap. 3, 

this volume) and Morton and Fox (chap. 2, this volume), but also 

include data from surface-towed camera, submersible observations, 

and Sea Beam bathymetry. Vertical exaggeration 3.8 x. Arrows 

show relative direction of movement along the fault. 
192 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

centers. Strong acoustic reflectors within the sediment section have been interpreted as both basaltic sills and flows 

(Morton and Fox, chap. 2, this volume; Davis and Becker, 

chap. 3, this volume). A continuous acoustic reflector at 

a depth of approximately 220 m visible at the eastern edge 

of lines 1 to 4 is interpreted to be a large basalt flow (Davis 

and Becker, chap. 3, this volume), indicating that intermittent volcanism is contemporaneous with sedimentation. 

Near-surface basalt flows shown on lines 1 to 4 are interpreted to overlie the sediment fill of the trough. Sediment 

hills protruding through the basalt flows east and northeast 

of the crest of the Central Hill (fig. 10.5, 10.6) are consistent with the interpretation that the basalt forms a thin veneer over the sediment-filled rift valley that had undergone 

previous uplift of sediment hills. 

Numerous smaller acoustic reflectors within the sedimentary section are interpreted as basaltic sills, but as with 

the SESCA sections, this interpretation is not unique, and 

we cannot rule out zones of subsurface sulfide mineralization or hydrothermally induced lithification of sediment. 

Sills are especially abundant on lines 3 and 4, which cross 

the large sediment hills. 

UPLIFT OF SEDIMENT HILLS 

The circular sediment hills spatially associated with the 

largest massive sulfide deposits have been observed from 

submersibles and are interpreted as uplifted cylindrical 

blocks of sediment bounded by curvilinear normal faults. 

The 3.5 kHz seismic-reflection profiles over some of the 

hills show slight tilting of sediment layers over the uplifted 

areas that appear to be correlatable with untilted sediment 

layers adjacent to the hills (Morton and Fox, chap. 2, this 

volume). The mechanism proposed for these localized uplifts is intrusion of large basaltic sills near the basalt-sediment interface (Morton and Fox, chap. 2, this volume; 

Denlinger and Holmes, chap. 4, this volume), but seismic 

imaging below the disturbed sediment hills provides few 

constraints on the location of basaltic basement. The surface exposure of volcanic rock in Escanaba Trough is limited in areal extent (figs. 10.5, 10.6), but appears to be 

related to the uplifted sediment hills (Ross and Zierenberg, 

chap. 8, this volume). Seismic reflectors interpreted as 

shallow sills are also common beneath the uplifted hills. 

Shallow sills and volcanic flows that occur near the perimeter of the uplifted hills may be fed by dikes following the 

faulted margins of the uplifted sediment hills. 

Surface eruption of basalt is inhibited in areas of thick 

sediment cover. Basaltic magma will rise through unconsolidated sediment until it is energetically more favorable 

to flow laterally under lower density sediment, thus forming sill complexes. Sedimentary cap rocks to the intrusions 

are uplifted in pistonlike cylindrical hills analogous to 

"punched laccoliths" (Correy, 1988). Uplifted sediment 

hills in the Guaymas Basin spreading center are also interpreted to have formed over "bysmalithic" intrusions 

(Lonsdale and Lawver, 1980). Thin sills that were intruded 

at shallow depth within the sediment cover of the Guaymas 

Basin were drilled at DSDP Sites 4 77 and 481. These thin 

shallow sills produce little uplift of the overlying sediment 

(Lonsdale and Becker, 1985); the volume for emplacement 

of these sills is apparently created by dewatering and compaction of the intruded sediment (Einsele and others, 1980). 

Magmatism at sediment-covered spreading centers can be 

anticipated to form sill complexes due to the large density 

contrast between basalt and unconsolidated sediment. 

More problematic is the occurrence of volcanic flows at 

the sediment surface. Magma can only reach the surface 

where the lateral confining pressure on an intrusion is sufficient to prevent intrusion into the sediment. Previous 

intrusives that have solidified could provide support for 

magma conduits to reach near the surface. Rapid effusion 

rates could result in surface lava flows capable of overriding the surface sediment, as observed at the SESCA and 

NESCA sites. Feeder dikes for these surface flows might 

preferentially intrude along preexisting structures, including the faults that bound the uplifted sediment hills (Ross 

and Zierenberg, chap. 8, this volume). 

Petrologic investigations of basalt recovered from surface exposures also provide limited evidence for subsurface interaction of basalt with sediment covering the ridge 

axis. Basalt samples from the sediment-covered part of 

Escanaba Trough have lead isotopic ratios that appear to 

lie along a mixing line between Gorda Ridge basalt and 

Escanaba Trough sediment and may be evidence of chemical interaction of basalt with sediment (Davis and others, 

chap. 9, this volume). 

SEDIMENT REDISTRIBUTION 

Submersible observations and gravity core sampling 

have established that sediment redistribution is an important process in the volcanically and tectonically active parts 

of the central graben of the Escanaba Trough. The Holocene sections of piston and gravity cores collected from 

the floor of the axial trough commonly contain turbidite 

layers and mass-flow deposits (Karlin and Lyle, 1986; 

Karlin and Zierenberg, chap. 7, this volume; Normark and 

others, chap. 6, this volume). Cores collected from terraces 

uplifted a few tens of meters along normal faults that bound 

the innermost floor of the axial graben (for example, core 

L2-86-7G) do not have Holocene turbidites (Normark and 

others, chap. 6, this volume). The thickness of turbidite 

units decreases with distance from uplifted hills (Karlin and 

Zierenberg, chap. 7, this volume). The distribution and 

composition of the Holocene turbidite layers and debris 

flows indicate that the deposits were derived locally from 

the uplifted hills (Karlin and Lyle, 1986; Karlin and 
GEOLOGIC SETTING OF MASSIVE SULFIDE MINERALIZATION IN ESCANABA TROUGH 193 

Zierenberg, chap. 7, this volume). Incorporation of volcanic glass and hydrothermal sulfide in some of the Holocene turbidites indicates that volcanism and hydrothermal 

activity are penecontemporaneous with the local redistribution of sediment (Karlin and Lyle, 1986; Karlin and 

Zierenberg, chap. 7, this volume). The available evidence 

indicates that tectonic and volcanic processes that formed 

the uplifted hills occurred during the Holocene, and that 

contemporaneous hydrothermal activity is related to these 

events (Karlin and Lyle, 1986; Karlin and Zierenberg, chap. 

7, this volume). 

EROSIONAL CHANNELS 

The erosive power of the mass flows generated by 

slope failure on the uplifted sediment hills is evident in the 

canyons cut into the hill slopes. Many areas of the hill 

slopes display vertical scour grooves cut by downslope 

sediment transport. Locally, channels and valleys scallop 

the hill slopes and fault scarps. 

An unexpected observation over large areas at both the 

SESCA and NESCA sites is the abundance of channels cut 

into sediment in areas with very gentle slopes, including 

areas that appear flat from submersible observation. Some 

channels could also be small grabens related to ridge extension. This explanation is likely for some of the depressions observed on the east side of the Central Hill and on 

Edifice REX in the NESCA area. In both of these areas, 

the grabenlike features are the site of massive sulfide mineralization. However, the most likely origin for the majority of the channels is formation due to erosion of the 

surface sediment. Locally derived turbidites and debris 

flows apparently have sufficient energy to erode several 

meters into the surface sediment. Some channels were observed at the base of the steep mass-wasting scarps that 

form the sides of uplifted sediment hills. The channels appear to be sufficiently mature that some have developed 

meander patterns displayed by curved banks with asymmetric profiles (fig. 10.3B). The lack of evidence of downchannel sediment transport may be related to the high 

background sedimentation rate, relative to mass-flow 

events. If the channels are forming by active erosion, their 

widespread occurrence and extensive development show 

that redistribution of sediment in this setting is an important process. 

CURRENT EROSION 

Submersible observations suggest that sediment deposition and redistribution is also locally controlled by bottom currents. A current meter mounted on DSV Sea Cliff 

recorded bottom currents as strong as 2 knots. High current velocities appear to be associated with major topographic features, especially the edges of the uplifted sediment hills, where sediment ripples (fig. 10.3A) and lag deposits of coarser sediment were observed. High 

concentrations of near-bottom suspended flocculant matter often reduced visibility during parts of some dives. 

Local areas near the edges of hill tops displayed 

subhorizontal sedimentary layering enhanced by differential erosion and exposed by bottom currents. Areas of slow 

sediment accumulation and (or) subtle erosion are also recognizable by the effects on bottom fauna. Current-swept 

areas have lower abundance of organisms, many large 

bioturbation mounds and burrows, and were generally described as extensively bioturbated by divers. However, 

extensive bioturbation in areas of normal sedimentation 

homogenizes surface sediment, producing a smoother bottom microtopography where the larger burrows and 

mounds have been degraded by burrowing and grazing organisms. We therefore reinterpret these as areas with lower 

bioturbation due to reduced availability of organic matter 

in area of reduced sediment accumulation or erosion. 

HYDROTHERMAL PROCESSES 

LOCALIZATION AND TIMING OF 

HYDROTHERMAL DISCHARGE 

The spatial distribution of sulfide deposits within 

Escanaba Trough is indeterminate because only a small 

percentage of the sea floor has been observed. The known 

distribution of sulfide deposits shows a close spatial relationship to the volcanic edifices, even though mineralization is not found in areas of outcropping volcanic rock. 

Each of the four volcanic complexes investigated has associated sulfide mineralization, but the other volcanic edifices identified on figure 10.1 have not been investigated 

by methods capable of detecting sulfide mineralization. At 

three of the volcanic edifices, submersible dives found 

large sulfide deposits (SESCA, NESCA, and Edifice REX). 

A fourth volcanic edifice (latitude 40°53' N.) has been investigated only by sediment coring, so the extent of sulfide mineralization is unknown. The data base for the flat 

turbidite-covered plains between the volcanic edifices is 

limited, but sediment cores collected away from the volcanic centers show no signs of previous or active hydrothermal discharge (Karlin and Lyle, 1986; Normark and 

others, chap. 6, this volume). 

The largest massive sulfide deposits are located on the 

flanks of uplifted sediment hills developed above the 

volcanic edifices. The hills are interpreted as the consequence of igneous intrusion into the base of the sediment 

column, and the structural boundaries that accommodated 

uplift of the hills are interpreted as high-permeability, dilatant fault zones (Denlinger and Holmes, chap. 4, this volume) that focus hydrothermal discharge at the sea floor. If 
194 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

this interpretation is correct, fluid discharge and massive 

sulfide formation should immediately postdate uplift of the 

hills. Information on the timing of sulfide deposition is 

limited. Although their degree of oxidative weathering and 

surface degradation shows that the deposits at different uplifted hills formed at different times, the deposits around 

any given hill all appear to be of the same relative age, except that the SW Hill in the NESCA area appears to have 

had two episodes of mineralization. We infer that each significant episode of sill intrusion and sediment uplift is followed by a pulse of hydrothermal discharge and massive 

sulfide formation. The observations provide qualitative 

support for this model. The old, weathered sulfide deposits associated with the 3170 Hill in the SESCA area are 

cut by mass-wasting channels and are partly buried by sediment due to degradation of the uplifted hill by sediment 

slumping; the young and active deposits associated with 

the Central Hill in the NESCA area occur on a hill that has 

been modified less by mass wasting and still displays relatively unmodified normal fault scarps. A more detailed 

record of the relative timing of uplift and hydrothermal activity should be contained in the sediment aprons surrounding the hills. Holocene turbidites derived from the 3170 

Hill in the SES.CA area contain hydrothermal pyrrhotite 

(Karlin and Zierenberg, chap. 7, this volume), but higher 

resolution dating of these units is needed to document the 

onset and duration of hydrothermal activity. 

The scale of exposed massive sulfide. is similar to that 

of ore lenses mined at massive sulfide deposits on land. 

Extrapolation of the geologic observations at the four volcanic complexes investigated suggests that each of the volcanic centers in Escanaba Trough (fig. 10.1) may be 

associated with massive sulfide mineralization. Mineralization in Escanaba Trough appears to be developed on a 

scale similar to that observed in major mining districts of 

ancient massive sulfide deposits, with numerous large sulfide deposits forming within a geologically short period of 

time along the rift axis of Gorda Ridge. 

FLUID-SEDIMENT INTERACTION 

The presence of a relatively low-permeability sediment 

blanket on the spreading axis results in a hydrologic and 

thermal regime that is different from sediment-free spreading centers (Davis and Becker, chap. 3, this volume). A 

significant proportion of heat loss at sediment-free spreading centers occurs by hydrothermal convection (Wolery and 

Sleep, 1976; Morton and Sleep, 1985), but low cross-stratal 

permeability at sediment-covered spreading centers restricts hydrothermal circulation to higher permeability 

zones, such as turbiditic sand beds and zones of tectonic 

or igneous disturbance. Conductive transfer of heat by diffusion through the insulating sediment blanket results in a 

high geothermal gradient in the sediment pile, which affects sediment diagenesis, lithification, and alteration or 

metamorphism. Sediment-fluid interaction at elevated temperatures within the sediment of Escanaba Trough may be 

responsible for altered pore fluid composition in DSDP Site 

35 (Manheim and others, 1970; Normark and others, chap. 

6, this volume). 

Sediment-fluid interaction provides a primary control 

on the chemistry of hydrothermal fluid and precipitates in 

the Escanaba Trough sulfide deposits. The major-element, 

trace-element, and isotopic composition of hydrothermal 

fluid sampled from vents at the NESCA site are explained 

in terms of major input from sedimentary source material 

altered at high temperature (Campbell and others, chap. 11, 

this volume; Bohlke and Shanks, chap. 12, this volume). 

Escanaba Trough hydrothermal fluids have elevated alkali 

metal and boron contents, similar to fluids from the 

Guaymas Basin hydrothermal system (Campbell and others, chap. 11, this volume). Low o11B and radiogenic 

87186Sr ratios are further indications of sediment interaction (Campbell and others, chap. 11, this volume). Alteration of organic matter in the sediment results in elevated 

ammonium and iodide relative to vent fluid from sedimentfree spreading centers (Campbell and others, chap. 11, this 

volume). Terrigenous organic matter present in the sediment is the source material for hydrothermal hydrocarbon 

associated with the sulfide deposits (Kvenvolden and others, 1986; chap. 15, this volume). The only components 

of the hydrothermal fluid that require significant input from 

basaltic source material are 3He (T. Garno, oral commun., 

1989, referenced in Campbell and others, chap. 11, this 

volume) and 13C in C02 (B. Taylor, oral commun., 1989), 

both 9f which could be transported by a vapor phase separating from basalt, and therefore do not require direct basalt-fluid interaction. 

Fluid-sediment interaction provides controls on fundamental fluid properties such as oxygen and sulfur fugacities and pH (Bohlke and Shanks, chap. 12, this volume; 

Koski and others, chap. 16, this volume). Pyrrhotite is the 

dominant sulfide mineral in Escanaba Trough and is accompanied by minor to trace amounts of arsenopyrite, 

lollingite, alabandite, and native bismuth, minerals that indicate formation at relatively low sulfur fugacity. Elevated 

concentration of arsenic, antimony, bismuth, lead and tin 

in massive sulfide, relative to deposits on sediment-free 

spreading centers, are derived from sediment (Koski and 

others, chap. 16, this volume). The lead isotope composition of Escanaba Trough massive sulfide overlaps the composition of Escanaba Trough sediment (LeHuray and 

others, 1988). 

The geochemical data require extensive fluid-sediment 

interaction, but the data at hand are also consistent with 

fluid-basalt interaction as a contributing factor in evolution of fluid composition of the hydrothermal vents. Davis 
GEOLOGIC SETIING OF MASSIVE SULFIDE MINERALIZATION IN ESCANABA TROUGH 195 

and Becker (chap. 3, this volume) calculate the volume of 

magma necessary to supply the heat to form a large sulfide deposit and estimate that the lead and sulfur contained 

in the Escanaba Trough deposits could be leached from the 

volume of sediment that immediately underlies the deposits. Alternatively, if the temperature at depth in the sediment is sufficiently high, hydrothermal fluid may circulate 

in regionally extensive high-permeability turbidite layers 

and never penetrate significantly into basement. If this is 

true, the volcanic centers could serve principally as thermal and structural foci of hydrothermal discharge. The 

relative contribution of energy and material from the basaltic and sedimentary sources remains a critical unknown. 

SULFIDE DEPOSITION 

The larger sulfide deposits in Escanaba Trough that 

have not been degraded by sea-floor weathering are characterized by steep-sided constructional mounds composed 

predominantly of massive pyrrhotite. Chimneys occur on 

the tops of some mounds but are not abundant. Sulfate is 

less abundant than pyrrhotite in the bulk of the deposits, 

but barite chimneys appear to be more common than sulfide chimneys. Many fragments of massive sulfide have 

a layer of barite as an outer rim between the sulfide and 

seawater, and active high-temperature venting occurs from 

sinterlike carapaces of anhydrite and barite on massive sulfide mounds. 

Growth of sulfide mounds at sea-floor hydrothermal 

sites is preceded by growth and replacement of sulfate 

chimneys by sulfide, followed by collapse of gravitationally unstable chimneys and incorporation of the chimney 

talus into a massive structure by sulfide cementation due 

to diffusely circulating hydrothermal fluid (Goldfarb and 

others, 1983). However, the large sulfide mounds in 

Escanaba Trough do not appear to be growing primarily 

by incorporation of chimney fragments, because active or 

relict chimneys are uncommon on the mounds and recognizable chimney fragments do not occur in samples collected from the mounds. 

A surprising feature of the massive sulfide deposits is 

the degree of brecciation developed on and around the 

mounds. Many of the deposits are developed on slopes, 

and the constructional sulfide mounds are typically steepsided. Some brecciation is clearly related to formation of 

sulfide talus; however, large sulfide slabs are common and 

are often found on gentle slopes. The surfaces of several 

mounds are cut by open fissures exposing fresh sulfide (fig. 

10.8B). There is no evidence to suggest these fissures are 

tectonic as they do not occur in coherently oriented sets 

and are not observed in the surrounding sediment-covered 

areas. The fissures have the appearance of expansion 

joints. In other areas, sulfide crusts near the sedimentwater interface have buckled to form small ridges (fig. 

10.8C). These observations suggest that subsurface mineral precipitation within the mounds and the sediment 

causes fracturing and formation of sulfide breccia. 

Samples recovered from the mounds contain numerous anastomosing zones of coarser grained pyrrhotite interpreted to have formed in solution channelways (Koski 

and others, chap. 16, this volume). High thermal conductivity of the massive sulfide mounds could enhance cooling of hydrothermal fluids percolating through the mounds, 

causing mineral precipitation within the mounds. The 

abundance of diffuse, relatively low temperature, metaldepleted (Campbell and others, chap. 11, this volume) hydrothermal fluids leaking from the mound surfaces may 

result in part from efficient cooling and metal precipitation occurring within the large mound structures, in contrast to sulfide precipitation within the turbulent plumes of 

high-temperature black smoker vents. Reactions in this 

zone of high thermal gradients are important in concentrating metals such as gold, silver, antimony, mercury, and lead 

near and in the sulfate-rich crust that form on the outer surfaces of sulfide mounds where hydrothermal fluid can mix 

with seawater. There is limited textural evidence for replacement of sulfate by sulfide at the interface between 

massive sulfide and seawater (Koski and others, chap. 16, 

th'is volume), but this process does not appear to be a quantitatively important growth mechanism for formation of 

pyrrhotite-rich massive sulfide mounds. 

SUBSURFACE MINERALIZATION 

Bottom-towed camera and submersible observations of 

sea-floor sulfide deposits are limited to surface exposures 

with the rare exception of fault scarps. Three-dimensional 

exposures in ancient massive sulfide deposits indicate that 

sub-sea-floor processes can be a fundamental control on 

mineralization (Zierenberg and Schmidt, 1987; Zierenberg 

and others, 1988). Limited evidence exists for shallow subsea-floor mineralization in Escanaba Trough, such as the 

sulfide crusts formed near the sediment-water interface discussed above (fig. 10.8C). One example is a small chimney constructed on a sulfide crust that formed below the 

sediment-water interface. The sulfide substrate incorporated sediment during its growth (Holmes and Zierenberg, 

1990). Dredged samples include sediment clast breccias 

cemented with sulfide (Koski and others, 1988; Koski and 

others, chap. 16, this volume). Sediment cores have recovered vein and disseminated sulfide and barite mineralization in turbiditic sands (Normark and others, chap. 6, 

this volume; Zierenberg and Shanks, chap. 14, this volume). However, all these observations relate to processes 

occurring within the top few centimeters to meters of the 

sediment. 
196 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

The location of sulfide deposits along the structural 

boundaries of uplifted sediment hills suggests that the deposits are underlain by fracture-controlled mineralization, 

similar to some ancient sediment-hosted massive sulfide 

deposits (Turner, 1990). The highly permeable silty and 

sandy turbidite layers within the sediment fill of Escanaba 

Trough may act as conduits for both hydrothermal fluid and 

seawater entrained in secondary circulation cells above the 

upper portions of vigorous hydrothermal discharge sites. 

Fluid mixing and (or) cooling within the permeable turbidite units could result in subsurface precipitation of sulfide 

and other hydrothermal minerals. We infer that the extensive surface expression of the massive sulfide deposits represents only part of the total volume of precipitated sulfide. 

However, the vertical extent and zoning of subsurface mineralization will remain a matter of speculation until the deposits can be investigated by drilling. 

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R.J.W., Mineral deposits of the northern Canadian Cordillera, Yukon-Northeastern British Columbia, 8th IAGOD 

Symposium Field Excursion Guide Book, Trip 14: Geological Survey of Canada Open-File 2169, p. 137-175. 

Vallier, T.L., Harold, P.J., and Girdley, W.A., 1973, Provenances 

and dispersal patterns of turbidite sand in Escanaba Trough, 

northeastern Pacific Ocean: Marine Geology, v. 15, p. 67-

87. 

Van Dover, C.L., Grassle, J.F., and Boudrias, M., 1990, Hydrothermal vent fauna of Escanaba Trough (Gorda Ridge), in 

McMurray, G.R., ed., Gorda Ridge: A seafloor spreading 

center in the United States' Exclusive Economic Zone: New 

York, Springer-Verlag, p. 285-287. 

Wilson, D.S., 1989, Deformation of the so-called Gorda plate: 

Journal of Geophysical Research, v. 94, p. 3065-3075. 

Wolery, T.J., and Sleep, N.L., 1976, Hydrothermal circulation and 

geochemical flux at mid-ocean ridges: Journal of Geology, 

v. 84, p. 249-275. 

Zierenberg, R.A., and Schmidt, J.M., 1987, Subsurface mineralization and sulfur isotope systematics at the Red Dog 

Zn-Pb-Ag deposit, Noatak District, Alaska [abs.]: Geological Society of America Abstracts with Programs, v. 19, p. 

905. 

Zierenberg, R.A., Shanks, W.C., III, Seyfried, W.E., Jr., Koski, 

R.A., and Strickler, M.D., 1988, Mineralization, alteration 

and sub-seafloor metamorphism of the ophiolite-hosted 

Turner-Albright sulfide deposit, southwestern Oregon: Journal of Geophysical Research, v. 93, p. 4657-4674. 

Zierenberg, R.A., Morton, J.L., Reiss, C.A., and Holmes, M.L., 

1991, Preliminary results of the 1986 Sea Cliff dive program, Escanaba Trough: U.S. Geological Survey Open-File 

Report 91-137, 176 p. 

Zierenberg, R.A., and Shanks, W.C., III, Sediment alteration associated with massive sulfide formation in the Escanaba 

Trough, Gorda Ridge: The importance of seawater mixing 

and magnesium metasomatism, in Morton, J.L., Zierenberg, 

R.A., and Reiss, C.A., eds., Geologic, hydrothermal, and 

biologic studies at Escanaba Trough, Gorda Ridge, offshore 

northern California: U.S. Geological Survey Bulletin 2022, 

chapter 14 (this volume). 

Section II: 

Hydrothermal and Geochemical Studies 
View looking down on dark massive sulfide capped by a ledge of lighter colored barite with 

inactive barite chimneys projecting upward. This photo was provided by C. Van Dover from a 

camera mounted on the Alvin submersible; field of view is approximately 2m across. 
Chapter 11. Chemistry of Hydrothermal Fluids from 

Escanaba Trough, Gorda Ridge 

By Andrew C. Campbell, 1• 2 Chris R. German, 1• 3 Martin R. Palmer, 1• 4 Toshitaka Gamo,5 and 

John M. Edmond1 

CONTENTS 

Page 

Abstract ........ .. ............ .. .... .. ....... ............ ..... ....... ....... ...... .... .. 201 

Introduction.......................................................................... 201 

Acknowledgments . .... ....... .......... ...... .. ........ .... .. .. .. .. .. . .. 202 

Geologic setting .................... .............. ... . ........ ........ .... ....... .. 202 

Methods ............. ..... ........... .... ... .......... .... .......... .... .. .. .... .... .... 203 

Sampling and analytical techniques............................ 203 

Calibration of DSRV Alvin high-temperature probes 205 

Results . . ... . . . . . .. . . . . . . .. . . .. . . .. . . .. . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206 

Fluid temperatures .... .. ........... ............ .... .... .... .... .... ...... 206 

Fluid chemistry .... .............. ............ .... .... .... ...... .. .. .. .. .... 207 

Discussion .......... ....... ........... .......... ... . ... . ... . ... . ... . ... . ... . ... . ...... 210 

Fluid-sediment interactions .. ........ .... .. .. .. .. .. .... .. .. .. .. .... . 210 

Relative mobility of alkali elements................... 212 

Boron isotope systematics .. .. .. ............ .... .. ........... 213 

Alkaline earth elements .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . 214 

Quartz geobarometry .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . 216 

Thermal decomposition of organic matter.................. 216 

Formation of massive sulfide deposits ....................... 217 

Conclusions .............. ... . ....... ........ ... . ... . ... . ... . ... . ... . ... . ... ...... ... . 218 

References cited . ......... ...... ...... ... ........ ... . .... . ........ ... . ... . ... . .. .. . 218 

ABSTRACT 

Hydrothermal fluids from the sediment-covered 

Escanaba Trough are substantially different from sedimentstarved mid-ocean ridge (MOR) hot springs. They are enriched in K, Sr, Rb, Cs, B, and Tl, depleted in 

sulfide-forming metals, have lighter B isotope ratios, and 

more radiogenic 87Sr/86Sr ratios than MOR vent fluids. 

Similar trends are observed for the sediment-hosted 

Guaymas Basin vents; however, the Escanaba Trough fluids, which interact with a more felsic sediment, have higher 

Rb, Cs, and B concentrations, lower 811 B values, and more 

1 Massachusetts Institute of Technology, Cambridge, Massachusetts 2 Present address: U.S. Nuclear Regulatory Commission, Washington, D.C. 

3 Present address: Institute of Oceanographic Sciences, Wormley, 

Godalming, Surrey GUS SUB, England 4 Present address: University of Bristol, Bristol B58 IRJ, England 

5 Ocean Research Institute, University of Tokyo, Tokyo, Japan 

radiogenic 87Sr/86Sr ratios. In general, sediment-hosted 

systems have higher pH, alkalinity, NH4, I, and Br compared to sediment-starved MOR systems. The presence of 

less organic matter, sulfur, and carbonate in Escanaba 

Trough sediments relative to the Guaymas Basin produces 

lower pH, total alkalinity, NH4, and H2S in the fluids. 

However, I is significantly higher in the Escanaba Trough 

solutions due to the higher 1/C ratio of the sediments compared to the Guaymas Basin. The Escanaba Trough hotspring waters, perhaps because of their relatively alkaline 

composition and low end-member temperature of 217°C, 

have the lowest transition-metal content of any end-member vent fluid yet sampled. They likely reflect the waning 

phase of hydrothermal activity accompanying the cooling 

of a magmatic heat source. 

INTRODUCTION 

More than a decade has passed since the discovery of 

submarine hydrothermal vents at the Galapagos Rift 

spreading center (Corliss and others, 1979). Hot springs 

have now been sampled in a wide variety of tectonic settings, including sediment-starved systems along the slow 

rifting Mid-Atlantic Ridge (MAR) (Campbell and others, 

1988a), at intermediate and fast spreading parts of the East 

Pacific Rise (EPR) (Michard and others, 1984; Von Damm 

and others, 1985a; Campbell and others, 1988b; Bowers 

and others, 1988), at the intermediate spreading Juan de 

Fuca Ridge (JdF) (Von Damm and Bischoff, 1987; Massoth 

and others, 1989), from the Marianas Trough and Okinawa 

Trough back-arc basins (Campbell and others, 1987; Sakai 

and others, 1990), and sediment-hosted systems in 

Guaymas Basin, Gulf of California (Von Damm and others, 1985b, Campbell and others, 1988b), as shown on 

figure 11.1. This wide variety of environments allows us 

to compare and contrast the effects of depth, spreading rate, 

and reaction substrate on the resulting solution chemistry. 

Until the expedition to Escanaba Trough in June 1988, 

Guaymas Basin was the only sediment-hosted system 

sampled for hydrothermal fluids (Campbell and others, 

1988c ). Time series studies of the Guaymas Basin vents have 

shown that the inter-field variations in fluid chemistry, as in 

201 
202 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

sediment-starved systems, have remained stable for many 

years (Campbell and others, 1988b ). A number of characteristics of the Guaymas Basin fluids, such as enrichment 

in alkali metals and ammonium, high pH and total alkalinity, and depletion in sulfide-forming metals, were attributed to the interaction of the hydrothermal fluids with the 

sediment column (Von Damm and others, 1985b ). 

This paper presents the results of hydrothermal fluid 

chemistry studies of the Escanaba Trough vents. This work 

provides an important comparison to the Guaymas Basin 

and mid-ocean ridge (MOR) vents and allows us to draw 

some general conclusions about fluid chemistry in sediment-hosted hydrothermal systems in contrast to sedimentstarved vent systems. One important question is whether 

differences in the sediment composition at the two sites result in significant differences in fluid chemistry. Another 

question is to determine which elements can be used as indicators of variations in reaction substrate. In particular, 

one goal is to ascertain if the extent of fluid-sediment reaction can be determined by the isotope systematics and (or) 

chemical variations in the vent fluids. The world's largest 

massive sulfide deposits are sediment hosted; this work allows us to better understand the formation of these deposits. 

ASIA 

ACKNOWLEDGMENTS 

We wish to acknowledge the expertise and professionalism of the DSV Alvin group, chief pilot Ralph Hollis, 

Captain Gary Chiljean, and the officers and crew of the R/ 

V Atlantis II. The comments and suggestions of J. Gieskes, 

T. Bowers and reviewers R. Zierenberg, K. Von Damm, and 

J. Bischoff greatly improved the manuscript. Funding for 

this work was provided by the National Science Foundation through grant OCE-8721793. 

GEOLOGIC SETTING 

Escanaba Trough, located on the southern part of 

Gorda Ridge (fig. 11.1), is a slow spreading rate center (2.4 

cm/yr) possessing a typical deep axial valley (Atwater and 

Mudie, 1973). South of about latitude 41° 15' N. it is filled 

with several hundred meters of sediment derived from both 

the Klamath and Columbia river drainages (Vallier and others, 1973). The present sedimentation rates are about 10 

to 20 cm/kyr, although they have been much higher in the 

past (McManus and others, 1970). The sediment consists 

e DIVERGENT PLATE MARGINS 

0 CONVERGENT PLATE MARGINS 

.· JUAN DE FUCA RIDGE (5 AREAS)~ 

GORDA RIDGE { 

MARIANA TROUGH 

. . . 0" .. ·· .... ·· ..... · 0 

:.-· 

.. 

ESCANABA TROUGH 

GUAYMAS BASIN /1 

EAST 

PACIFIC 

RISE 

NORTH AMERICA 

Figure 11.1. Locations of submarine hydrothermal vent sites discussed in text. Mid-Atlantic Ridge Kane (MARK) site is located off the 

figure (arrow). 
CHEMISTRY OF HYDROTHERMAL FLUIDS FROM THE ESCANABA TROUGH, GORDA RIDGE 203 

mainly of hemipelagic and turbidite material, and smaller 

amounts of biological debris from the surface waters 

(Karlin and Lyle, 1986, McManus and others, 1970). Turbidite flows from Astoria Fan travel southward along the 

coast of Oregon through the Blanco Gap into Gorda Basin 

off northern California and make their way into Escanaba 

Trough from the south (Atwater and Mudie, 1973; Vallier 

and others, 1973). Thus felsic detritus from the upper Columbia and Snake rivers constitutes a significant fraction 

of the sedimentary fill in Escanaba Trough. The mineralogical composition of these sediments in comparison to 

those in Guaymas Basin is presented in table 11.1. Gravity cores taken in the area have about 1 to 2 percent carbonate, 0.1 to 0.3 percent sulfur, and 4 to 14 percent 

opaline silica (Karlin and Lyle, 1986). The carbonate content of the surface sediments is about 0.5 percent or less, 

and the organic carbon content is about 1 percent or less 

(Magenheim and Gieskes, chap. 13, this volume). 

A series of volcanic structures, displaying both intrusive and extrusive magmatism, occur at intervals of about 

15 km within this valley, and massive sulfide deposits are 

found at numerous locations associated with these features 

(Morton and others, 1990). In June 1988 a DSV Alvin diving program was conducted at the northern Escanaba 

(NESCA) site (Morton and others, 1990) between latitude 

40°58' and 41 °04' N. Massive sulfide deposits and hydrothermal venting activity were discovered at several places 

on an intra-rift hill at latitude 41 °00' N ., longitude 

127°29.3' W. (fig. 11.2). The greatest amount of hydrothermal activity was found at a large mound structure 

where fluids averaging 217±2°C were debouching from 

distinct orifices and also flowing diffusely from sulfide 

rubble. Several low-temperature ( 13°C to 18°C) vents 

were also found in the immediate vicinity of this mound. 

About 275 meters west, a smaller active mound was found 

where 108±2°C fluids emanated from beneath a thicket of 

apparently dying tube worms (fig. 11.2). Fluid samples 

were obtained from both vent sites. A third area on the 

southeast side of Central Hill (fig. 10.2) has extensive sulfide mineralization, abundant clams, and bacterial mats and 

was found to have diffuse, presumably low-temperature 

fluid issuing from a small mound (no fluid samples were 

obtained). 

METHODS 

SAMPLING AND ANALYTICAL TECHNIQUES 

The methods used for field collection and major-element analyses in this work have been reported previously 

(see Campbell and others, 1988a,b, and Von Damm and 

others, 1985a,b). High-temperature fluid samples were 

collected in titanium syringe bottles deployed from DSV 

Alvin (Von Damm and others, 1985a). Measurements of 

Table 11.1. Escanaba Trough and Guaymas Basin sediments: 

selected chemical and mineral components. 

[Escanaba data: Magenheim and Gieskes (chap. 13, this volume); Rex and Murray 

(1970); Karlin and Lyle( 1986). Guaymas data: Kastner (1982); Niemitz (1982)] 

Weight percent Escanaba Trough Guaymas Basin 

Organic carbon 0.3-0.5 2-4 

Sulfur .08- .2 1-2 

CaCO., 1-3 10- 15 

Opaliri'e silica 5- 14 30-50 

Detrital clay 42-65 30-45 

Feldspar 12-20 4-15 

Quartz 20-25 3-10 

pH and alkalinity were made as soon as possible on board 

the support ship RIV Atlantis II to avoid oxidation and precipitation artifacts, which drastically change the proton balance of the solutions. Hydrogen sulfide samples were 

drawn anaerobically into stoppered glass vials back-filled 

with argon. A subsample (100 J.LL) from each vial was 

fixed with reagents for the methylene blue dissolved sulfide technique (Cline, 1969) and measured on a spectrophotometer the next day. Dissolved silica was measured 

spectrophotometrically by the molybdate blue technique 

(Strickland and Parsons, 1968). The major elements were 

analyzed in the shore laboratory by an array of routine techniques (see Campbell and others, 1988a,b, and Von Damm 

and others, 1985a,b ). Some of the alkali and alkaline-earth 

elements (Rb, Cs, and Sr) were measured by isotope dilution-thermal ionization mass spectrometry (ID-TIMS), 

and 87Sr/86Sr was measured by isotope ratio TIMS (Palmer 

and Edmond, 1989a,b). Boron and o11B were measured 

by the method of Spivack and Edmond ( 1987). Thallium 

and other trace metals were analyzed by inductively 

coupled plasma-mass spectrometry (ICP-MS) multielement 

standard additions. 

Total Br and I were analyzed by ICP-MS; details of 

the Br analyses are given in Campbell and Edmond (1989). 

A significant memory effect in the sample introduction system proved to be a problem with the iodide determinations, 

and the previous I data are about 10 to 12 percent low. To 

solve this problem, a flow injection technique was adopted 

to minimize the amount of sample introduced into the tubing and glassware and to allow memory blank correction 

of each sample injection. Samples were prepared by making 1: 100 dilutions in 0.25 percent HCl (from triply distilled acid). Standards were made both in ambient seawater 

at 1:100 dilution and also in a pure 0.25 percent HCl 

matrix. No difference in response was observed for these 

two standards, hence suppression of the sample signal due 

to salinity variations is not a problem. An injection volume of 0.2 mL was used, and each sample and standard 

was run in duplicate. A mid-range standard was run every 

three samples, and all data were normalized to a constant 
204 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

monitor signal to correct for drift in instrumental response 

(approximately 9 percent in 4 hours). The signal was monitored with respect to time at 127 atomic mass units (amu). 

The peak area was determined by integrating from the onset 

127°29' 

41°00' 

0 

METERS 

Contour interval 5 meters 

58,500 59,000 

of the signal to where the remaining tail of the peak was less 

than 2 percent of the total peak area. Blanks were determined 

by integrating a similar time period just before onset of the 

sample signal. Standard additions on some samples gave the 

59,500 60,000 

-Sulfide ETI Sulfide mineralized area D Sediment D Pillow basalt 0 Lobate basalt 

Figure 11.2. Bathymetric map of active hydrothermal vents and associated massive sulfide deposits at Central Hill in NESCA area (modified 

from Zierenberg and others, chap. 10, this volume). Fluid samples were obtained from the 217°C and 108°C vents. Numbers along right and 

bottom give coordinates of local transponder net (in meters). Hachures indicate area of closed low. 
CHEMISTRY OF HYDROTHERMAL FLUIDS FROM THE ESCANABA TROUGH, GORDA RIDGE 205 

same values as those calculated from the calibration curve. 

Reproducibility was better than ±2 percent. 

CALIBRATION OF DSV ALVIN HIGHTEMPERATURE PROBES 

Submarine vent fluid temperatures have been of great 

interest since the discovery of high-temperature venting 

activity at the 21° N. East Pacific Rise site (Spiess and others, 1980). Measured fluid temperatures there were remarkably close to the "end-member" temperature predicted from 

studies of the highly diluted, low-temperature Galapagos 

Rift vent fluids (Edmond and others, 1979). Campbell and 

others (1988a) showed that the vast majority of measured 

hot-spring temperatures tend to a maximum around 355°C 

and only a few values have been reported at higher temperatures. However, experimental studies of hydrothermal 

fluid-basalt reactions in the laboratory are unable to reproduce the compositions of vent fluids below about 380°C 

(Berndt and others, 1989). Furthermore, invoking phase 

separation to explain different vent salinities requires substantial cooling of the fluids (as much as 200°C for the 

Escanaba Trough fluids) (Bischoff and Rosenbauer, 1984). 

However, the high flow rates of "black smoker" vents precludes large-scale conductive cooling of the fluids (Converse and others, 1984; Macdonald and others, 1980; Sleep 

and Wolery, 1978). 

Previous attempts to calibrate the DSV Alvin high-temperature (HT) probes using the melting points of metals 

have suggested they are systematically biased to low readings (Delaney and others, 1989; Bischoff and Rosenbauer, 

1984). In the absence of any published details, it is difficult to assess these results. Therefore, an experiment was 

conducted during the expedition on June 2, 1988, to establish a simple, at-sea HT probe calibration and determine 

the accuracy of the devices. The details are published here 

to facilitate future calibrations by other researchers. 

The calibration was done with a gas chromatograph 

oven (Hewlett Packard model5710a; fig. 11.3), which can 

be quickly set to any temperature between 10°C above ambient and 400°C and is stable to ±0.5°C. A "melting-point" 

thermometer ( -1 0°C to 360°C) and an air thermocouple 

probe (0°C to 816°C), both NBS traceable, were used as 

reference devices. The calibration of the mercury thermometer (Kessler model 2060-6M), which is accurate to 

±0.5°C, was checked in an ice bath and a beaker of boiling water. The air thermocouple probe (Fluke model 

80PK-4 Type K air probe) has a stated accuracy of ±1.1 oc 

up to 275°C and ±0.4 percent above 275°C. It was connected to a Fluke model 80TK thermocouple module that 

converts the output of the thermocouple to a temperature 

readout on a digital voltmeter (1 millivoltJOC). The module has an electronic reference junction, which was calibrated at room temperature. Disagreement between the 

mercury thermometer and the air probe was less than ±1 °C 

up to 360°C (upper limit of the thermometer; see table 

11.2). An oven thermocouple graduated in 10°C increments agreed with the reference probes. 

Two HT probes (SN 18HT and SN 25HT) were installed on DSV Alvin according to standard operating procedures and were subsequently used on this and other 

expeditions through the summer of 1988. Each probe consists of a "type K" Chromel-Alumel thermocouple that is 

mounted in a steel sheath attached to the pressure case containing the electronics package. The voltage signal is converted to a frequency, transmitted through the submarine 

pressure sphere via a 20-foot-long cable, and recorded on 

the data logger. The probes use an electronic reference 

junctiqn and are electronically calibrated at Woods Hole 

Oceanographic Institute against voltages corresponding to 

temperatures of ooc and 400°C (J. Aikens, DSV Alvin 

group, oral commun., 1988). The DSV Alvin probe, the 

air thermocouple probe, and the mercury thermometer were 

mounted so that all three were inserted about 3 inches into 

the oven cabinet (fig. 11.3). Glass wool insulation was 

packed around the holes. Probe SN 25HT was wrapped 

Mercury 

thermometer 

~ 

Gas chromatograph oven 

0 

ALVIN 

HTprobe 

Figure 11.3. Schematic drawing of experimental set-up for at sea 

calibration of DSV Alvin high temperature (HT) probes. 
206 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

Table 11.2. Calibration data for DSV Alvin high-temperature probes. 

[_, not measured] 

Mercury Air Alvin Corrected 

thermometer thermocouple probe Ambient Alvin 

(OC) (OC) (OC) (OC) (OC) 

DSV Alvin probe SN 2SHT in ambient air 

109 110 90 21 111 

196 197 176 21 197 

264 264 244 21 265 

323 322 300 21 321 

352 353 334 21 355 

393 373 21 394 

DSV Alvin probe SN 18HT in ice bath 

128 125 0 125 

196 196 0 196 

274 276 0 276 

333 335 0 335 

353 357 0 357 

396 398 0 398 

in a thick rubber sheet that acted as an insulation blanket 

protecting it from air currents. The reading displayed in 

the sub was corrected for ambient temperature (table 11.2). 

Probe SN 18HT was mounted in a fresh water and ice bath 

at 0°C, and no correction of the readout value was necessary (fig. 11.3). Six calibration points were measured for 

each probe; three below 300°C and three between 300°C 

and 400°C. 

RESULTS 

FLUID TEMPERATURES 

The results of the HT probe calibrations in table 11.2 

and figure 11.4 demonstrate that their accuracy is better 

than ±1 percent up to at least 400°C, which is within the 

stated National Bureau of Standards (NBS) specifications 

for this type of thermocouple. The precision of the probes, 

as determined by replicate measurements, is generally 

about ±2 to 3°C. Differences in temperature of less than 

about 4°C to 5°C are within the 2-standard-deviation error margin. This calibration demonstrates that the DSV 

Alvin HT probes are not biased to low temperature readings and no intrinsic flaw exists in their design. Recent 

at-sea calibration of three other DSV Alvin HT probes 

(Mid-Atlantic Ridge Vent Program, January 1990) have 

confirmed these results. Temperature measurements of 

vent fluids using this system are true to about ±1 percent 

when corrected for bottom water values, but only if the 

electronic calibration is done properly and no electronic or 

other technical problems occur. One such problem is 

accidental heating of the pressure case during measurement. Although gentle heating simply reduces the reading by the difference between 0°C and the true case 

temperature (see table 11.2), more intense heating causes 

the electronics to behave improperly, and seemingly stable 

high temperatures have been observed (Campbell and others, 1988b). 

The temperatures measured at the main active hydrothermal mound (fig. 11.2) varied from 217°C near the top 

to 215°C closer to the base. On a subsequent dive, fluids 

were measured at 216°C. This variability is typical of measurements of the same vent at other hot springs and, although these differences may be real, they are within the 

error margin of the HT probes. Two months after the expedition, the same mound was visited by DSV Sea Cliff, 

which measured a temperature of 220°C with a probe constructed and calibrated by the DSV Alvin group at Woods 

Hole (Reiss and others, 1992). The mean fluid temperature and standard deviation of the measurements at this 

mound is thus 217±2°C. Some fluid samples collected 

from this mound had magnesium concentrations less than 

2 mmol/kg. Therefore, this is an end-member temperature 

and is not due to subsurface dilution by seawater. In contrast, fluid samples obtained from an active mound 275 

meters away (see fig. 11.2), which had a measured temperature of 1 08±2°C, contain 50 percent or more seawater 

Mg. Mixing of seawater and hydrothermal fluids apparently occurs within the mound or subsurface region at this 

site, as discussed later. 

en 400 

:J 

(jj 

..J 

w 350 () 

C/) 

w 

w 

a: 300 (.!) 

w 

0 

z 

en 

250 

w 

m 

0 

a: 200 

a. 

z 

> ..J 150 < 

> a: 

C/) 

0 

REFERENCE TEMPERATURES, IN DEGREES CELSIUS 

Figure 11.4. DSV Alvin high temperature (HT) probe readings versus 

reference thermocouple and mercury thermometer readings. 

Measured HT probe readings plot along 1: 1 line. Hence, HT probes 

have no intrinsic bias with respect to true temperature and are 

completely linear over temperature ranges of all submarine 

hydrothermal vents sampled to date. 
CHEMISTRY OF HYDROTHERMAL FLUIDS FROM THE ESCANABA TROUGH, GORDA RIDGE 207 

FLUID CHEMISTRY 

The major-element concentrations of individual 

samples are plotted against magnesium in figures 11.5 

and 11.6. The zero-Mg intercept of the regression line 

(drawn on the figures) for each element is the end-member concentration. We assume that any significant 

amount of Mg in the sample is due to seawater entrainment during sampling or subsurface mixing prior to 

venting of the fluids (Von Damm and others, 1985a). It 

is important to note that small, non-zero Mg concentrations in the end-member fluid (on the order of 1-3 

mmol/kg) will not generate a significant error in the calculated end-member values of other elements. Within 

the scatter of the data, the samples from the 1 08°C vent 

plot along the same regression lines as the 217°C vent, 

except for silica (fig. 11.6A) and the alkali elements. 

This suggests a common fluid source at depth for these 

two mounds with some subsurface mixing and precipitation of alkali-containing silicates at the second mound 

before the fluids are vented. The end-member data for 

I 

c.. 

0 10 20 30 40 50 60 

(f) 

Iz 

w 

....J 

<( 

> 5 

0 

w 

:J 

....J 

~ 

~ 

~ 1-

z 

:J 

~ ....J 

<( 

~ 

<( 

c: 

(!J 

0 

....J 

S2 

a: 

w 

a.. 

(f) 

w 

....J 

0 

~ 

0 

a: 

u 

~ 

~ 

Escanaba Trough are compared in table 11.3 with the 

Guaymas Basin vent fields (Campbell and others, 1988b) 

and sediment-starved MOR vents at the Mid-Atlantic 

Ridge Kane (MARK) site (Campbell and others, 1988a) 

and the latitude 11 o to 13° N. East Pacific Rise (EPR) 

sites (Bowers and others, 1988). 

The pH and total alkalinity of hydrothermal vent fluids do not form linear mixing lines with respect to seawater because the process is essentially a weak acid-weak 

base titration. To estimate end-member values, we fit the 

pH and total alkalinity data with third-order and secondorder polynomials, respectively (fig. 11.5A,B). The endmember pH obtained by the fit is 5.3, and the total 

alkalinity is 3.2 meq/kg. These are lower values than the 

Guaymas Basin vents, which are buffered at a pH of 5.9 

and have a total alkalinity of about 9 meq/kg. Both systems have a higher total alkalinity than seawater (2.3 meq/ 

kg). Sediment-starved hydrothermal vents have pH values that range from 3 to 4 and possess negative total alkalinities, indicating an excess of protons over proton 

acceptors in solution. 

3.5 

B 

~ 

~ 3.0 

(!J 

0 

....J 

S2 

c: 

~ 2.5 

2.0 

2000 

1500 

1000 

500 

u)C\1 0 I 

0 10 20 30 40 50 60 

MAGNESIUM CONCENTRATION, IN MILLIMOLES PER KILOGRAM SOLUTION 

Figure 11.5. Magnesium content versus (A) pH, (B) total alkalinity, (C) ammonium, and (D) hydrogen sulfide in Escanaba Trough vent 

fluids. Open circles are data from 217±2°C vent, closed circles are data from 1 08±2°C vent. The pH data are fit with a third-order 

polynomial (r2=0.979) and the alkalinity data are fit with a second-order polynomial (r2=0.947). All other data are fit with linear regressions 

(NH4: r2=0.997; H2S: r2=0.99). 
208 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

10 700 

~ A 

<( 

a: 

(!) 

0 8 

~ 

<( 8 

0 217°C a: 

(!) 

0 

....J ....J 

S2 

a: 

w 

a.. 6 C/) 

w 

S2 650 a: 

w 

a.. 

C/) 

w 

....J ....J 

0 

~ 

:J 4 ....J 

5E 

~ 

<i 2 0 

:J 

(j) 

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MAGNESIUM CONCENTRATION, IN MILLIMOLES PER K•LOGRAM SOLUTION 

Figure 11.6. Magnesium concentration versus (A) dissolved silica, (B) chloride, (C) lithium, (D) sodium, (E) potassium, and (F) calcium in 

Escanaba Trough vent fluids. Open and closed circles as in figure 11.5. All data are fit with linear regressions (that for silica only includes 

samples from 217°C) with ,2 values as follows: Si(OH)4=0.999; Cl=0.998; Li=0.997; Na=0.974; K=0.994; and Ca=0.999. 
CHEMISTRY OF HYDROTHERMAL FLUIDS FROM THE ESCANABA TROUGH, GORDA RIDGE 209 

Table 11.3. End-member data for major and minor elements from ridge hydrothermal vents. 

[Data sources in text. Na calculated from charge balance. Concentration units in moles per kilogram solution. MAR, Mid-Atlantic Ridge; EPR, East Pacific Rise] 

Escanaba Trough Guaymas Basin, 

south field 

Fluid temperature (°C) - 108-217 250-308 

pH (NBS, 23°C) --------- 5.4 5.9 

Alkalinity (meqlkg) ----- 3.1 9.6 

NH4 (mmollkg) ---------- 5.6 15.3 

H2S (mmollkg) ---------- 1.1- 1.5 6 

Si(OH)4 (mmol/kg) ----- 5.6-6.9 12- 13.5 

Cl (mmollkg) ------------ 668 581-604 

Br (jlmollkg) ------------- 1,179 1,054-1,117 

Br/Clx1 000--------------- 1.76 1.8- 1.85 

I (jlmollkg) --------------- 99 80-83 

Li (jlmollkg) ------------- 1,286 993 

Na (mmollkg) ------------ 560 472 

K (mmollkg) ------------- 34-40.4 44.1 

Rb (jlmollkg) ------------ 80- 105 60 

Cs (jlmollkg) ------------- 6.0-7.7 5.5-6.0 

Tl (nmollkg) ------------- 110 114 

B (mmollkg) ------------- 1.71 -2.16 1.6- 1.68 

o 11 B ----------------------- 11.5-10.1 17.4-19.6 

Ca (mmollkg) ------------ 33.4 27.7 

Sr (jlmollkg) ------------- 209 158 

87 Sr/86Sr ------------------ .7099 .7059 

Mn (jlmollkg) ------------ 10-21 217 

Fe (jlmollkg) ------------- 0-10 8-24 

The Escanaba Trough fluids have 5.6 mmol/kg of NH4 

(fig. 11.5C) and 1.1 mmol/kg H2S (fig. 11.5D). These are 

significantly lower than the Guaymas Basin vents, where 

NH4 ranges from 10 to 14 mmol/kg and H2S varies from 

4.4 to 6 mmol/kg (table 11.3). The absolute magnitude of 

the scatter in the H2S data (up to 250 Jlillol/kg, fig. 11.5D) 

is due to the effects of sample degassing, oxidation and the 

intrinsic error in the method (see Campbell and others, 

1988b ). If degassing and oxidation systematically reduce 

the H2S in all the samples, the end-member concentration 

may be as much as 0.5 mmol/kg higher. At other vent systems, where H2S ranges from 4 to 12 mmol/kg, errors of a 

similar magnitude generate an error of only about 5 to 10 

percent in the calculated end-member, whereas at Escanaba 

Trough it is much greater because of the low sulfide content of the fluids. 

Chloride, bromide, and iodide were measured in the 

Escanaba Trough fluids to understand the halide systematics. Sediment-hosted vents have distinctly elevated Br/ 

Cl ratios and much higher I concentrations than sedimentstarved vents due to elevated levels of Br and I in sedimentary organic matter (Campbell and Edmond, 1989; 

table 11.3). At Escanaba Trough Br/Cl is 0.00176 compared to about 0.00152 to 0.00167 at sediment-starved 

MOR vents. This corresponds to an increase of about 160 

J...Lmol/kg Br relative to bromide originating from seawater. 

Similar bromide enrichments occur in the Guaymas Basin 

vent fluids. Total I is 99 J...Lmol/kg at Escanaba Trough and 

Guaymas Basin, MAR, MARK '86 EPR, l1°-13°N east hill 

290 335-350 347-380 

5.9 3.7-3.9 3.1-3.7 

8.9 -.06 to -.24 -.28 to -1.02 

13 <.01 <.01 

4.4 5.9 2.9-12.2 

13.8 18.2 14.3-21.9 

603 559 338-760 

1,063 847 533-1,242 

1.76 1.52 1.58- 1.67 

65 <1 <1 

849 843 484-884 

488 510 290-596 

41 23.6 18.7-32.9 

52 11 15-25 

5.2 .18 .117-.195 

104 34 23-40 

1.55 .52 .428-.468 

16.5-23.2 26.8 31.5-36.8 

33.8 10 10.6-54.8 

191 50 40-205 

.7052 .7028 . 7039 - . 7046 

128 491 742-2,932 

47 2,180 2,640- 10,760 

ranges from 65 to 83 J...Lmol/kg in Guaymas Basin. The Cl 

concentrations of the Escanaba Trough vents (668 mmol/ 

kg, fig. 11.6B) are higher than in the Guaymas Basin vents 

(580-603 mmol/kg), but well within the range of MOR hydrothermal vents (table 11.3). 

Dissolved silica is 6.91 mmol/kg in the end-member 

fluids that are 217°C and 5.6 mmollkg in the 108°C fluids (fig. 11.6A). The silica concentration at the 217°C 

vent is slightly elevated relative to quartz solubility at the 

vent temperature and sea-floor pressure (about 6 mmol/ 

kg), but is several times lower than amorphous silica 

solubility (Von Damm and others, 1991). This may indicate the fluids are in equilibrium with quartz at slightly 

higher temperatures and pressures within the system, but 

there are several problems with this interpretation that 

will be discussed later. 

The concentrations of Li (1 ,286 J...Lmol/kg) and K ( 40.4 

mmol/kg) in Escanaba Trough are similar to Guaymas Basin. Potassium enrichments in sediment-hosted fluids compared to sediment-starved fluids (table 11.3) suggest that 

the potassium content of the reaction substrate and its mobility with respect to hydrothermal alteration determine the 

K concentrations in high-temperature vent fluids. Only the 

high-chloride vents at the southern Juan de Fuca field (Von 

Damm and Bischoff, 1987) have K concentrations similar 

to the sediment-hosted systems. Lithium shows no trends 

between sediment-starved and sediment-hosted hydrothermal systems (table 11.3). 
210 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

The Escanaba Trough vents have somewhat higher 

concentrations of Rb, Cs, and B than the Guaymas Basin 

fluids, and both are greatly enriched compared to sedimentstarved systems (table 11.3). There is some inter-field variability in the Escanaba Trough fluids, with the lower 

temperature vent having the lowest concentrations of Rb 

(80 j.lmol/kg), Cs (6.0 IJ.mol/kg), and B (2.16 mmol/kg) (see 

table 11.3). Two different regions of the 217°C mound also 

show some variations. Fluids from a well-defined orifice 

near the top of the mound had end-member concentrations 

of 105 j.lmollkg Rb, 7.7 j.lmol/kg Cs, and 2.16 mmol/kg B, 

while fluids from near the base contained 95 j.lmol/kg Rb, 

7.1 j.lmol/kg Cs, and 1.9 mmol/kg B. These differences 

indicate possible removal into secondary mineral phases, 

not only at the lower temperature vent, but also within the 

plumbing system of the higher temperature mound. 

The o11 B values in Escanaba Trough waters range 

from 10.1 per mil at the top of the 217°C mound to 11.5 

per mil in the 108°C mound. These are much lower than 

the Guaymas Basin fluids that range from 17.4 to 19.6 per 

mil in the South Field vents and 16.5 to 23.2 per mil in 

the East Hill vents. Both sediment-hosted systems are 

much lighter than sediment-starved vent fluids along the 

EPR (31.1 to 36.8 per mil) and the MARK vents on the 

Mid-Atlantic Ridge (26.8 per mil). The o11 B values of 

MOR vents have been explained by a two-end-member 

model involving high-temperature reaction between heated 

seawater and fresh tholeiitic basalt (Spivack and Edmond, 

1987). Their boron content and isotopic composition is 

dominated by seawater because fresh tholeiitic basalt has 

so little boron. In contrast, the boron content and isotopic 

composition of sediment-hosted vent fluids are dominated 

by the sedimentary source (Spivack and others, 1987). 

The first analyses of thallium in submarine hydrothermal fluids are also reported here. The monovalent cation 

has an ionic radius of 0.154 nm similar to K (0.144 nm), 

Rb (0.158 nm), and Ag (0.127nm) and shows chemical 

similarities to them (Cotton and Wilkinson, 1972). In concert with the trends for Rb, Cs, B, and K, the sedimenthosted systems are enriched in Tl. The Escanaba Trough 

vents have about 110 nmol/kg and the Guaymas Basin 

vents range from 104 to 114 nmol/kg, whereas the MOR 

vents range from 23 to 40 nmollkg (table 11.3). The presence of higher thallium concentrations in sediment-hosted 

fluids reflects its greater concentrations in sedimentary 

material (about 1 to 10 ppm) compared to fresh mid-ocean 

ridge basalt (MORB) (up to 0.1 ppm) (Wedepohl, 1969). 

Unlike the alkalis, thallium is incorporated into sulfides, 

and its presence in ore-bodies is primarily due to solid solution with sulfide phases (Wedepohl, 1969). 

The alkaline-earth element calcium is enriched in the 

Escanaba Trough and Guaymas Basin fluids relative to 

MARK, but concentrations in some hydrothermal fluids at 

latitude 11 o to 13 ° N. EPR are higher (table 11.3). The 

strontium concentration in the Escanaba Trough vents is 

209 IJ.mol/kg, and 158 to 190 j.lmol/kg in the Guaymas Basin fluids. Only high-Cl sediment-starved vents have similar strontium concentrations. 

The Escanaba Trough vents are the first example of a 

hot-spring fluid having more radiogenic 87Sr/86sr ratios 

than seawater (0.70997 to 0.70967 versus 0.7092). The 

87Sr/86sr values for Escanaba Trough surface sediments 

range from 0. 7093 to 0. 7123 for unaltered material and 

0.7092 to 0.7095 for hydrothermally altered samples. In 

contrast to this, the 87Sr/86Sr values for the Guaymas Basin vents (0. 7052 to 0. 7059) are significantly lower than 

seawater, but more radiogenic than basalt (about 0.703). 

The 87Sr/86sr in unaltered surface sediments in Guaymas 

Basin are about 0.709 and about 0.705 in hydrothermally 

altered sediments (Gieskes and others, 1982a,b). Sediment-starved hydrothermal vent fluids have slightly 

higher 87 Sr/86sr than basaltic glass (Palmer and Edmond, 

1989b). 

The precipitation of sulfide minerals within the sediment column depletes fluids from sediment-hosted hydrothermal systems in iron, manganese, and other 

sulfide-forming metals compared to bare rock systems 

(table 11.3). The concentrations of trace elements in the 

Escanaba Trough samples were below the ICP-MS detection limits for Co, Cu, Zn, Ag, and Cd in the 100-fold diluted samples. The Fe and Mn data are presented as ranges 

in table 11.3 because of variability at different sampling 

sites on the 217°C mound and at the 108°C mound. These 

Mn and Fe concentrations are 10 times lower than in the 

Guaymas Basin vents and 100 to 1,000 times lower than 

in sediment-starved systems. Precipitation of small 

amounts of secondary minerals in the Escanaba Trough 

mounds are apparently affecting the fluid concentrations 

of Mn and Fe. In general, the ratio Fe/Mn in sedimenthosted fluids is much lower than in the sediment-starved 

vents (fig. 11.7). An exception is the low-salinity Axial 

Seamount Hydrothermal Emissions Study (ASHES) vent 

fluids that are thought to have undergone phase separation 

(Massoth and others, 1989). Since manganese-sulfides are 

many orders of magnitude more soluble than iron-sulfides, 

the effect of sulfide precipitation is to decrease Fe/Mn in 

the sediment-hosted and phase-separated fluids. 

DISCUSSION 

FLUID-SEDIMENT INTERACTIONS 

The enrichments of K, Rb, Cs, B, and Tl in both the 

Escanaba Trough and Guaymas Basin fluids compared to 

sediment-starved fluids (table 11.4) reflect compositional 

differences between tholeiitic basalt and the sediment fill 

at these two sites. The unaltered material in both the sediment-hosted systems are substantially enriched in these elements relative to fresh tholeiitic basalt. 
CHEMISTRY OF HYDROTHERMAL FLUIDS FROM THE ESCANABA TROUGH, GORDA RIDGE 211 

Table 11.4. Alkali elements, Tl, Sr, and B in Escanaba Trough and Guaymas Basin sediments and tholeiitic basalt. 

[Values in parentheses are average deviation for data. -, not determined; -, approximately. Escanaba data: B, Rb, Cs and Sr: this work; K: Zierenberg and Shanks 

(chap. 14, this volume). Guaymas data: Li, K, Rb, and Sr: Gieskes and others (l982b); Cs, Tl: H. Brumsack, Univ. Gottingen, oral commun., 1988; B: Spivack and 

others (1987). Basalt data: Wedepohl (1969); Krauskopf (1979); Li: Chan and Edmond (1988); B: Spivack and Edmond (1987)] 

Li K Rb 

(ppm) (pet) (ppm) 

Escanaba Trough sediment 

Unaltered -2 112(11) 

Altered to chlorite .01 3(2) 

Guaymas Basin sediment 

Unaltered 55(5) 2.1 90 

Altered to greenschist facies "0" "0" 2 

Tholeiitic basalt (unaltered) 7.5(1.5) -.1 -4 

Systematic change of K with respect to Cl provides an 

important constraint on distinguishing fluid-sediment from 

fluid-basalt interactions. Potassium increases in a linear 

fashion with Cl in the sediment-starved systems, while 

sediment-hosted systems show high K with no systematic 

trends relative to Cl (fig. 11.8A). The hjgh Kin the barerock vents at southern Juan de Fuca and latitude 13° N. 

EPR are clearly part of the overall K/Cl systematics at 

MOR vents. No regular trends with respect to Cl are observed for Rb and Cs in MOR systems because, unlike K, 

their concentrations in fresh basalt can range by as much 

as two orders of magnitude (Palmer and Edmond, 1989a). 

The high K/Cl at the sediment-covered vents reflects the 

much greater concentrations in sediments relative to 

MORB (table 11.4). 

ESCANABA 

S. FIELD GUAY. 

E. HILL GUAY. 

Mariana Trough 

VENT 3- S JdF 

VENT 1 -S JdF 

PLUME SJdF 

VENTS -11N 

VENT 5 -11N 

VENT 4-11N 

VENT 3 -13N 

VENT 2 -13N 

VENT 1 -13N 

HG-21N{85) 

SW-21N{85) 

NGS-21N 

OBS-21N 

MARK-1 --------

0 2 3 4 5 6 7 8 9 10 

Fe/Mn, MOLE RATIO 

Figure 11.7. Ratios of Fe to Mn in submarine hydrothermal vent 

fluids from sediment-hosted, back-arc basin, and sediment-starved 

mid-ocean ridge environments (data references listed in fig. 11.8 

caption). 

Cs Tl Sr 87srJB6sr B dllB 

(ppm) (ppm) (ppm) (ppm) 

7.6(.2) 290(50) 0.7108(15) 44(5) -4.8(.2) 

0.6(.3) 52(7) .7094(2) 1 

8 0.4 -320 .7073 60 -1.2(.4) 

-500 .7061 < 1 

-.06 < .1 -110 -.703 -.4 -3(1) 

Lithium shows no systematic trends with respect to Cl 

(fig. 11.8B). Despite significant enrichment in sediments 

compared to basalt (table 11.4), the fluids show little increase relative to sediment-starved systems (table 11.3). 

Chan and Edmond (1988) have shown that the lithium isotope ratios of both sediment-hosted and sediment-starved 

vent fluids are lighter than seawater and have similar values. They suggested incomplete extraction from the reaction substrate and uptake by a secondary mineral phase 

such as chlorite as possible causes. This is consistent with 

hydrothermal bomb experiments in which lithium is partially retained in the altered basalt (Seyfried and others, 

1984). 

Hydrothermally altered surface sediment in Escanaba 

Trough consists of assemblages rich in smectite, chlorite, 

and talc that show varying degrees of magnesium enrichment and alkali depletion (Zierenberg and Shanks, chap. 

14, this volume). The chlorite-rich sediments shown in 

table 11.4 represent the highest degree of alteration and 

show nearly complete removal of the alkalis (Na, K, and 

Rb) and the alkaline earths (Ca and Sr). Oxygen isotope 

measurements of these sediments suggest alteration temperatures between 200°C and 240°C (Zierenberg and 

Shanks, chap. 14, this volume). Analyses of 8180 in hydrothermal fluid and smectite from the high-temperature 

mound indicate equilibration temperatures of 220°C, similar to the measured fluid temperature (Bohlke and Shanks, 

chap. 12, this volume). This result does not necessarily 

imply, however, that the fluid chemistry is being determined solely by near-surface sediment-fluid interactions. 

The similarity of the major-element end-member fluid 

chemistry from two different mounds 275 meters apart (see 

fig. 11.2) suggests there is also a common fluid-sediment 

reaction zone at depth. 

Sediments from the Guaymas Basin hydrothermal system showed similar trends. Material from around sill intrusions drilled in Guaymas Basin by the Deep Sea Drilling 

Project (DSDP) Leg 64, particularly Sit~ 477, has been altered at around 200°C and shows Mg enrichments and 

leaching of K, Li, Rb, and B (Gieskes and others, 1982a,b; 
212 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

Spivack and others, 1987). In the deepest part of hole 477, 

where hydrothermal alteration to a greenschist-facies assemblage consisting of quartz, albite, chlorite, epidote, and 

EXPLANATION 

o Escanaba Trough • MARK - Mid-Atlantic Ridge 

• Guaymas Basin 85 • S. Juan de Fuca 

<> Guaymas Basin 82 • 13°N EPR 

o Mariana Trough I:J. 11 oN EPR 

o 21 °N. EPR + Axial Volcano, Juan de Fuca 

~ so~~--~~--~.--~--~~--~~--~.~~~ 

~ (!J 

g 50 

S2 

a: 

w 

a.. 40 (/} 

w 

....I 

0 

~ 30 

:J 

....I 

~ 

~ 20-

":i 

:::::> 

Ci5 10 (/} 

g 

a.. 

A 

+ 

• -

• 

• 

+ 

-

2000 r--~---r--~---r---~--r---~-..,..---.-----..~~__, 

~ 

<( 

a: 

(!J 

0 

....I 

521500 

a: 

w 

a.. 

(/} 

w 

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~ 1000 

0 

a: 

u 

~ 

~ 500-

B 

0 

+ 

+ 

• 

• 

-

0 

• -

o~~--._~--~·~~--L-•_.--~·--~~·--~-.....~ 

0 200 400 600 BOO 1 000 1200 

CHLORIDE, IN MILLIMOLES PER KILOGRAM 

Figure 11.8. Plots of end-member chloride at different submarine 

hydrothermal vents versus (A) potassium and (B) lithium. Data 

sources are as follows: Escanaba Trough, this work; Guaymas Basin 

85, Campbell and others (1988b ); Guaymas Basin 82, Von Damm 

and others (1985b); Mariana Trough, Campbell and others (1987); 

21 oN. EPR, Von Damm and others (1985a) and Campbell and others 

(1988a); MARK-Mid-Atlantic Ridge, Campbell and others (1988b ); 

S. Juan de Fuca, Von Damm and Bischoff (1987); 11° and 13° N. 

EPR, Bowers and others (1988); Axial Volcano, Juan de Fuca, 

Massoth and others (1989). 

pyrite occurs at about 300±50°C, the sediment is completely depleted in Li and K, and nearly depleted in Rb and 

B (Gieskes and others, 1982b; Kastner, 1982; Spivack and 

others, 1987). Presumably cesium and thallium are also 

leached from these sediments. The pore fluid chemistry 

in these sediments is remarkably similar to 300°C fluids 

venting near the drill site (Gieskes and others, 1982b; Von 

Damm and others, 1985b). The high-temperature vent fluids at Guaymas Basin derive their chemical composition 

primarily from deep reactions within the sediment column, 

but small inter-field differences suggest that near-surface 

reactions also affect the fluid composition (Campbell and 

others, 1988b). 

RELATIVE MOBILITY OF ALKALI ELEMENTS 

As a first approximation, the water-to-sediment (W/S) 

ratios of the sediment-hosted systems can be modeled on 

the basis of the alkali data. In this calculation, it is assumed 

that the difference between altered and unaltered sediments 

(table 11.4) corresponds to the amount of alkali extracted 

in a single-pass system and that basalt sources are minor. 

(This may not be true for Li.) W/S is calculated as the ratio of the alkali extracted from the sediment to the increased alkali concentration above seawater values in the 

fluid. In the Escanaba Trough, the alkali elements have a 

range of values from 7 to 18, with an average of about 

11±5, while boron has a significantly lower value of around 

2 (table 11.5). The lower fluid concentrations of Rb and 

Cs and B at Guaymas Basin compared to Escanaba Trough 

give higher W/S values with an average of 14±5 for alkalis and 4 for boron (table 11.5). A reaction-path model using oxygen isotope data predicts a W IS value of around 5 

for Escanaba Trough (Bohlke and Shanks, chap. 12, this 

volume). An estimate of W/S for the Guaymas Basin system based on mineralogical studies gives a value around 

3 to 4 (Kastner, 1982). In contrast to the sediment-hosted 

systems, the MOR basalt systems have alkali derived water-to-rock (W/R) ratios of about 1 to 2 (Von Damm and 

others, 1985a; Von Damm and Bischoff, 1987). An integrated reaction-path model for the latitude 21° N. EPR system produced W IR values between 0.5 and 0.8 (Bowers and 

Taylor, 1985). 

The high W/S ratios for the sediment-hosted vents 

compared to W/R in MOR vents may be explained by relatively incomplete extraction of alkalis per unit of fluid, perhaps due to lower reaction-zone temperatures and higher 

alkali concentrations in the reaction substrate. According 

to this hypothesis, a unit volume of sediment will undergo 

progressive leaching by successive volumes of fluid until 

it becomes depleted of alkali elements and is completely 

recrystallized to an alteration mineral assemblage. In general, relatively fresher sediment will generate higher alkali 

concentrations in the fluids (and lower W/S) than more 
CHEMISTRY OF HYDROTHERMAL FLUIDS FROM THE ESCANABA TROUGH, GORDA RIDGE 213 

Table 11.5. Water-to-sediment ratios for Escanaba Trough and Guaymas Basin 

[-,approximately. i-f sediment, unaltered sediment concentration minus altered sediment concentration (mol/kg sediment). 

i - f fluid, hydrothermal fluid concentration minus seawater concentration (mol/kg solution). W/S, amount of element extracted from 1 kg sediment/amount of element in 1 kg fluid (corrected for seawater)] 

Li K 

(mM!kg) (mM!kg) 

Escanaba Trough 

i - f sediment 8.7 550 

i-f fluid 1.26 30 

W/S (mass ratio) 7 18 

Guaymas Basin 

i - f sediment 8 641 

i-f fluid .9 33 

W/S (mass ratio) 9 19 

altered sediment of the same starting composition. Also, 

relatively mobile elements will have lower W/S values than 

less mobile elements. The trend in relative mobility indicated by the Escanaba Trough and Guaymas Basin data is 

as follows: B >> Cs -( Li )> Rb > K. (Note that the relatively low W/S value for Li may be an artifact of ignoring 

basaltic contributions.) 

If the progressive leaching model is correct, the alkali 

content of the fluid should decrease as the system ages. It 

is uncertain, however, what time scale would be necessary 

to observe changes due to progressive alteration. A time 

series study of the Guaymas Basin vents shows that the 

major-element composition and inter-field differences in 

fluid chemistry remain constant over 6 years (Campbell and 

others, 1988b). However, recent analyses of the time series samples by ID-TIMS show small decreases in B, Cs, 

and Rb, indicating that progressive leaching of the most 

reactive alkalis is occurring (Campbell and others, unpub. 

data). 

Another process affecting the alkali concentrations is 

uptake of these elements into secondary mineral assemblages after extraction from the sediments. In high-temperature alteration zones, the major-element composition 

of the fluids is apparently controlled by the formation of a 

greenschist-facies mineral assemblage (Bowers and others, 

1988; 1985). The time stability of the major-element composition of Guaymas Basin vent fluids is consistent with 

this (Campbell and others, 1988b). Since the high-temperature alteration phases show nearly complete depletion in 

the alkali elements (table 11.4 ), secondary uptake processes 

are most important at lower temperatures. In accord with 

this scenario, the 108°C vent fluids in the Escanaba Trough 

show identical major-element end-member values as the 

217°C fluids. However, the concentrations of K, Rb, Cs, 

and Si(OH)4 in the 108°C vent are lower (34 mmollkg, 80 

~ollkg, 6 J.lmollkg, and 5.6 mmollkg respectively; see 

figs. 11.5, 11.6, and table 11.3). This suggests subsurface 

mixing with seawater or porewater (to generate the high 

Rb Cs B I 

(~M!kg) (~Mikg) (mM!kg) (~M/kg) 

1,260 52 3.9 300-1,200 

102 8 1.8 99 

12 7 2 3-12 

900 60 5 -1,260 

53 6 1.2 -80 

17 10 4 -15 

Mg values and lower temperature for these fluids) and precipitation of an alkali-containing silicate phase such as 

smectite. 

A similar phenomenon appears to be occurring at the 

217°C mound. End-members calculated for samples from 

the base of the high-temperature mound show slightly 

lower Rb, Cs, and B concentrations (95 J.lmol/kg, 7.1 J.lmoll 

kg, and 1.9 mmol/kg, respectively) than those from the top 

(110 J.lmol/kg, 7.7 J.lmol/kg, and 2.2 mmol/kg, respectively), but no differences in Si(OH)4, Li, or K. These 

former fluids had only a marginally lower temperature 

compared to the latter (215°C versus 217-219°C). It seems 

unlikely this difference is sufficient to cause an increase 

in silicate precipitation, since the formation temperature of 

smectite from this mound (220°C) corresponds to the fluid 

temperature (Bohlke and Shanks, chap. 12, this volume). 

The samples from the top of the mound came from a welldefined orifice whereas those from the base of the mound 

came from a seep that had to be excavated to obtain good 

fluid flow. Uptake of the most reactive elements (Cs, Rb, 

and B) may occur during the precipitation of smectite in 

the rubble pile. Solutions from the top of the mound may 

experience less interaction with silicates because they are 

flowing in a conduit. Therefore, uptake of alkalis into secondary minerals occurs even at relatively high temperature, 

and estimates of W IS ratios from alkali data must be considered as maximum values only. The W/S ratios are, however, useful indicators of alkali mobility and progressive 

alteration in sediment-hosted hydrothermal systems. 

BORON ISOTOPE SYSTEMATICS 

The high boron concentrations in the sediment-hosted 

vent fluids and the light values (relative to MOR fluids) 

are a distinguishing feature of these systems (table 11.3). 

Boron is very low in basalts and greatly enriched in marine sediments (table 11.4 ). It is nearly quantitatively 
214 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

leached from hydrothermally altered sediment in Escanaba 

Trough. Previous studies have indicated that boron is 

highly mobile under hydrothermal conditions and is quantitatively removed from sediments below about 120 meters 

in DSDP Site 477 in Guaymas Basin. The other alkali elements, in contrast, become depleted below about 200 

meters, in the greenschist-facies alteration zone (Gieskes 

and others, 1982b; Spivack and others, 1987). The much 

lower calculated W /S for boron compared to the alkali elements (table 11.5) reflects its higher relative mobility and 

may be a truer representation of the W/S ratio. 

During uptake of boron into solid phases from solution, 1

<13 is preferentially partitioned into the solid phase 

thus enriching the fluid in the heavier isotope 11B. If the 

boron isotopic composition of the Escanaba Trough fluids 

were derived solely from sediments and residual seawater, 

then the expected o11 B value of the fluids would be about 

+ 3 to 4 per mil. The hydrothermal fluids, however, have 

heavier o11B values (+10.1 per mil to 11.5 per mil) than 

the sediments (-5.0 per mil), despite the fact that they contain 4 times more boron than seawater (o11B = 39 per mil). 

Various mixtures of sediment, seawater, and (or) basalt cannot generate the isotopic composition of the Escanaba 

Trough fluids, and uptake of boron onto secondary phases 

is required. The trends in the fluid data are consistent with 

this. The lowest temperature vent ( 1 08°C) has the lowest 

Band highest o11B (1.7 mmollkg and 11.5 per mil, respectively), fluids from the base of the 217° C mound have intermediate B and o11B values (1.9 mmol/kg and 10.9 per 

mil), and samples from the top of this mound have the 

highest Band lightest o11B values (2.1 mmol/kg and 10.1 

per mil; table 11.3). Hence even W/S ratios from boron 

data must be considered maximum values. 

ALKALINE EARTH ELEMENTS 

The systematic trends of the alkaline-earth elements 

and sodium relative to chloride reveal that the processes 

controlling sodium and calcium in vent fluids are linked 

by plagioclase alteration reactions and the formation of 

greenschist-facies mineral assemblages. Both Ca and Sr 

show increases with Cl that have a sharp upward trend in 

slope above about 550 mmol/kg Cl (fig. 11.9). In contrast 

to the alkali elements, it is not possible to distinguish sediment-hosted from sediment-starved vents on the basis of 

the trends for Ca with respect to Cl (fig. 11.9A). However, 

Sr shows distinct differences between the sediment-hosted 

and MORB systems (fig. 11.9B). Sodium is the major cation in hydrothermal vent fluids (about 80 to 90 percent of 

the charge) and follows the seawater Na/Cl ratio linearly 

up to Cl concentrations of about 600 mmol/kg (fig. 

11.1 OA ). At higher salinities, sodium is apparently taken 

up from solution by albitization (Von Damm and others, 

1985a; Shanks and Seyfried, 1987). 

In both sediment-hosted and sediment-starved hydrothermal systems, the increase in cationic charge from calcium exactly compensates the decrease in charge due to 

loss of sodium, which is especially evident at high chloride concentrations. This is demonstrated by plotting Na 

plus two times the Ca versus Cl, which has a slope essentially equal to the seawater ratio of (Na+2xCa)/Cl (fig. 

11.10B). This is not an artifact of obtaining a precise Na 

EXPLANATION 

o Escanaba Trough • MARK - Mid-Atlantic Ridge 

• Guaymas Basin 85 • S. Juan de Fuca 

o Guaymas Basin 82 • 13°N EPR 

o Mariana Trough t:. 11 oN EPR 

o 21 °N. EPR + Axial Volcano, Juan de Fuca 

~ 

100~~--~~~--~~--~~~~~--~~--T,--~.~ 

~ (!) 

g 80-

S2 

a: 

w 

a.. 

CJ) 

w 

....J 

0 

~ 

::::i 

....J 

~ 

~ 

60 ~ 

40 ~ 

~- 20 1- ::::> 

u 

....J 

<( 

u 

A 

+ 

+ 

0 

+ • r::f 

~ 

~ 

~ 

0 • 

• 

• 

o~~--._._. __ ~·--~~·--~--·~_. __ •• __ ._~ 

~400~~----.~--~.--~~.~~--~.~----.~~ 

a: 

§ 

S2 

ll:300 ~ w 

a.. 

CJ) 

w 

....J 

0 

~200 1-

a: 

u 

~ 

~ 

~100 1- ::::> 

i= 

z 

0 

a: 

B 

0 

oq,o. 

0 

• 

• -

• 0 

• -

-

~ o~~--~·_.--~·--~~·~~--~~--~~~ 0 200 400 600 800 1000 1200 

CHLORIDE, IN MILLIMOLES PER KILOGRAM 

Figure 11.9. Alkaline earths relative to chloride in submarine 

hydrothermal vents. A, Ca versus Cl. B, Sr versus Cl. (Data 

references as in fig. 11.8.) 
CHEMISTRY OF HYDROTHERMAL FLUIDS FROM THE ESCANABA TROUGH, GORDA RIDGE 215 

value by charge balance since the analytical errors in the 

Na, Ca, and Cl measurements are much smaller than the 

decrease in Na or increase in Ca. Thus for every two moles 

of sodium lost (relative to seawater) from the high-chloride solutions one mole of calcium is added to the solution. This is precisely what is expected from the 

EXPLANATION 

o Escanaba Trough • MARK - Mid-Atlantic Ridge 

• Guaymas Basin 85 • S. Juan de Fuca 

<> Guaymas Basin 82 "' 13°N EPR 

o Mariana Trough t:. 11 oN EPR 

o 21 o N. EPR + Axial Volcano, Juan de Fuca 

Seawater 

~ • 

• ...... 

... 

·~ 

~1200.-~----~--~--~-,--~----~------~~ 

<( 

a: 

(!) 

91000 

S2 

a: 

w 

a.. 

CJ) 

w 

.....J 

~ 600 

:::::i 

.....J 

~ 

~ 400 

ci 

() 

~ 200 

+ 

ctl 

z 

8 

y=15.1 +0.90x~ 

r2= 0.996 

200 400 600 800 1000 1200 

CHLORIDE, IN MILLIMOLES PER KILOGRAM 

Figure 11.10. Sodium and calcium systematics relative to chloride 

in submarine hydrothermal vents. A, Na versus Cl plotted with Na/ 

Cl ratio for seawater (data references as in fig. 11.8). B, Na plus 2 

times Ca versus Cl. Slope of linear regression line for all vent fluid 

data is virtually the same as seawater ratio for Na+2xCa/Cl. 

conversion of anorthite to albite in hydrothermal alteration 

zones (Von Damm and others, 1985a; Shanks and Seyfried, 

1987; Berndt and others, 1989). 

Von Damm and Bischoff (1987) and Bischoff and 

Rosenbauer (1989) have suggested that the fluid chemistry of most MOR vents can be explained in terms of fluidrock equilibrium with various mixtures of hydrothermal 

brines, chloride-depleted vapor phases, and solutions that 

have not experienced phase separation. The fact that the 

large shifts in Na and Ca concentrations occur in the highchloride fluids may indicate that conversion of anorthite 

to albite is enhanced in deep-seated brine-rock interactions. 

The increase in sodium metasomatism at high chloride concentrations may reflect the effects of chloro-complexation 

on the activity ratio of free sodium ions to free calcium ions 

in solution (about 90 percent of the Ca is tied up as a 

chloro-complex, but only 35 percent of Na is complexed 

by Cl; Bowers and others, 1988). There is not a one-toone relationship between the increases in chloride and calcium since the effect of chloro-complexation is to cause a 

shift in equilibrium. Further work is needed in order to 

establish the role chloro-complexation may have in this 

phenomenon . 

The formation of secondary mineral phases containing calcium and strontium plays a critical role in controlling the calcium and strontium content of vent fluids. 

Berndt and others, (1988, 1989) have argued that plagioclase recrystallization is primarily responsible for the calcium and strontium contents of MOR vent fluids and that 

equilibrium of the fluid with epidote plays a lesser role in 

determining composition. If this were the case however, 

the slope of the (Na+2xCa)/Clline would be greater than 

the seawater ratio for MORB systems and even steeper for 

sediment-hosted systems because of calcium sources not 

related to plagioclase alteration. The uptake of magnesium 

into basalt alteration phases during the initial heating of 

seawater (Mg metasomatism) releases additional calcium 

into the fluid (Mottl, 1983). The dissolution of carbonate 

in sediments also release calcium to vent fluids. Since 

these sources of calcium are not necessarily accompanied 

by the uptake of sodium, additional calcium sinks of similar magnitude to these sources must exist in order to maintain the seawater ratio of (Na+2xCa)/Cl in both 

sediment-hosted and sediment-starved vent fluids. One 

important calcium-containing mineral that is in equilibrium 

with these solutions is epidote. It is a common alteration 

mineral phase in sheeted dike complexes and oceanic 

greenstones (Alt and others, 1986; Richardson and others, 

1987; Humphris and Thompson, 1978). The high-temperature alteration of Guaymas Basin sediments results in complete recrystallization to an epidote-albite-quartz-chlorite 

assemblage (Kastner, 1982). In addition to epidote, calcite can be an important sink for calcium and strontium in 

sediment-hosted vent systems and it is major mineral phase 

in the Guaymas Basin mounds (Peter, 1986). 
216 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

The strontium isotope systematics and high Sr concentrations of the Escanaba Trough fluids appears to be controlled by complete re-equilibration with the local 

sediments. The 87Sr/86Sr values (0.70967 to 0.70997, table 

11.3) are indistinguishable from unaltered sediment (0.7093 

to 0.7123) and only slightly higher than altered sediment 

(0.7092 to 0.7095, table 11.4). Any basaltic component to 

the strontium isotope systematics, if it exits, is completely 

obscured by sediment-fluid interactions. In contrast, the 

Guaymas Basin fluids have 87Sr/86Sr values that range 

from 0.7052 to 0.7059 (table 11.3; Von Damm and others, 

1985b ), which are identical to pore fluids in the 

greenschist-facies alteration zone (0.7053), but less radiogenic than the altered sediments (0. 7061) and much lower 

than unaltered sediments (0.7089, table 11.4; Gieskes and 

others, 1982b). These trends were interpreted as indicating either preferential reaction of volcanic material in the 

sediment column or a basaltic 87 Sr/86Sr component in the 

fluids (Gieskes and others, 1982b). In both sedimenthosted systems, the enriched strontium content of the fluids compared to MOR systems requires a sedimentary 

source. This is particularly evident in the greenschist alteration zone in Guaymas Basin which is enriched in strontium relative to unaltered sediment (table 11.4). 

QUARTZ GEOBAROMETRY 

Quartz geobarometry has often been used to estimate 

the depth of the reaction zone at MOR vent systems (Von 

Damm and others, 1985a, Von Damm and Bischoff, 1987; 

Bowers and others, 1988; Campbell and others, 1988a) and 

at Guaymas Basin (Von Damm and others, 1985b). Three 

criteria must be met in order for this approach to be valid: 

the silica in solution is only in equilibrium with quartz, no 

silica-containing phase precipitates after equilibrium is established at depth, and the fluids only cool by adiabatic 

expansion during the transit to the sea floor (Von Damm 

and others, 1985a). In Escanaba Trough the silica content 

(6.9 mmol/kg, table 11.3) of the high-temperature vent fluids is oversaturated with respect to quartz at 217±2°C and 

the sea-floor pressure of 325 bars (the saturation value is 

6 mmol/kg, Von Damm and others, 1991). However, these 

solutions would be saturated with respect to quartz if the 

near-surface temperature were only a little higher (about 

230°C). 

A number of problems limit the usefulness of quartz 

geobarometry for the Escanaba Trough system. The isobars of quartz solubility are so closely spaced in this temperature region and at this silica concentration that it is not 

possible to obtain accurate information about the reaction 

zone depth. Temperature variations on the order of the error margin of the measurement result in huge differences 

in estimated pressure regime. Another problem is encountered in estimating adiabatic cooling of the fluids. At 

220°C and 300 bars, the decrease in temperature is less 

than 10°C for fluids originating from 1,000 bars pressure 

(6.8 km below sea floor) or less than 5°C from 600 bars 

(2. 75 km below sea floor) (Bischoff and Rosenbauer, 

1984). Since the fluids have clearly equilibrated with sediment, it is highly unlikely that the reaction zone is 3 km 

below the sea floor, and so some nonadiabatic cooling of 

the fluids must occur. A third problem is the assumption 

that the fluids have not precipitated a silica-containing 

phase after equilibrium with quartz is established. The alteration mineral assemblages in Escanaba Trough sediments have apparently formed close to the measured vent 

fluid temperature (Zierenberg and Shanks, chap. 14, this 

volume) and, even at 217±2°C, silicate phases are precipitating. It is likely that the silica content of the fluids is 

affected by this process. 

THERMAL DECOMPOSITION OF ORGANIC 

MATTER 

The proton activity in sediment-hosted fluids is much 

lower and the total alkalinities are higher than in sedimentstarved solutions due to the thermal decomposition of organic matter and dissolution of carbonate minerals in the 

sediment column (Von Damm and others, 1985b). Differences in organic carbon, sulfur, and carbonate content of 

Escanaba Trough and Guaymas Basin sediments may explain some of the variations in fluid chemistry (see table 

11.1 ). Escanaba Trough sediments have about 0.5 to 2 percent carbonate, 0.1 to 0.3 percent sulfur, and about 1 percent or less organic carbon (Karlin and Lyle, 1986; 

Magenheim and Gieskes, chap. 13, this volume). In contrast, the Guaymas Basin sediments contain 10 to 15 percent calcium carbonate, greater than 1 percent sulfur, and 

2 to 4 percent organic matter (Calvert, 1966; Kastner, 1982, 

Niemitz, 1982). The lower values for pH, total alkalinity, 

H2S, and NH4 of Escanaba Trough fluids compared to 

Guaymas Basin fluids are consistent with these differences 

in sediment composition if the water-to-sediment ratios are 

similar. 

Reaction of hydrothermal fluids with organic matter 

can generate significant quantities of organic acids such as 

acetate (Von Damm, 1991). At the relatively low temperatures in the Escanaba Trough fluids, these may be stable. 

The organic acids must exist at millimolar concentrations in 

the fluids to contribute significantly to the total alkalinity and 

affect the buffer capacity of the solutions. Although no measurements of organic acids were made, it is important to note 

that the end-member Na value determined analytically is 

within 0.5 percent of the Na calculated from the charge balance. This places an upper limit of 3 meq/kg of anionic 

charge due to organic acids in the Escanaba Trough fluids. 

The enrichments of bromide and iodide in the sediment-hosted vent fluids are derived from the thermal 
CHEMISTRY OF HYDROTHERMAL FLUIDS FROM THE ESCANABA TROUGH, GORDA RIDGE 217 

decomposition of organic matter in the sediments 

(Campbell and Edmond, 1989). Both sediment-hosted systems have higher Br/Cl ratios and I concentrations than 

sediment-starved systems (table 11.3). The absolute increase in Br (about 100 J.Lmol/kg) is small compared to the 

total Br signal in seawater (840 J.Lmol/kg), and differences 

between Guaymas Basin and Escanaba Trough are not discernible. However, iodide has a relatively low concentration in seawater (about 50 nmol/kg) and is a more useful 

diagnostic probe of sediment-fluid interactions than is bromide. 

Since the Escanaba Trough sediments have much 

lower organic carbon than Guaymas Basin (table 11.1 ), one 

might expect lower rather than the higher I values observed 

in the fluids. However, the geochemistry of iodide is complicated by low-temperature diagenesis, and a strict comparison is not possible. Both bromide and iodide are 

associated with organic matter in marine sediments, and 

both are remobilized during early diagenesis (Price and 

Calvert, 1977, Kennedy and Elderfield, 1987; Magenheim 

and Gieskes, chap. 13, this volume). Although both elements are taken up by live plankton in surface waters, iodide is also adsorbed onto seston at the sea floor in 

oxidizing environments, and these type of sediments have 

higher I/C than reducing sediments (Price and Calvert, 

1977). The threefold higher liN and IIC ratios in Escanaba 

Trough sediments compared to Guaymas Basin sediments 

are consistent with this (Magenheim, 1989). The I/N ratios of the fluids (table 11.3) reflect these differences in 

sediment composition and explain the higher I in the 

Escanaba Trough fluids. 

An estimate of the water-to-sediment ratio can be obtained if it is assumed that iodide is quantitatively leached 

during high-temperature alteration of the sediments. Since 

iodide is not taken up to any significant extent by secondary mineral formation, it is a more useful indicator of WI 

S then the alkalis. The liN of the fluids is similar to sediments with an I concentration of about 40-100 ppm in 

Escanaba Trough (Magenheim and Gieskes, chap. 13, this 

volume). The Escanaba Trough fluids have an end-member I concentration of about 100 ~ol/kg, requiring between 125 and 350 g of sediment per kilogram of fluid, or 

a water-to-sediment ratio of between 3 and 12 (table 11.5). 

This is similar to the W IR value of 5 obtained from a reaction path model using oxygen isotope data (Bohlke and 

Shanks, chap. 12, this volume). 

FORMATION OF MASSIVE SULFIDE DEPOSITS 

The relative proportion of basaltic and sedimentary 

derived components in sediment-hosted fluids is not readily 

discerned. Hydrothermal bomb experiments with Guaymas 

Basin sediment and seawater at appropriate temperature 

and pressure conditions reproduce most of the major ion 

trends observed in the Guaymas Basin hydrothermal vents 

(Von Damm, 1991; Thorton and Seyfried, 1987). As discussed above, the major and minor ions in the Escanaba 

Trough fluids apparently completely equilibrate with the 

sediment at around 220°C. The presence of 3He in the 

Escanaba Trough fluids (T. Garno, written commun., 1989), 

however, suggests that the fluids circulate through basaltic rock, picking up both heat and chemicals. The Guaymas 

Basin fluids also contain large amounts of 3He (Lupton, 

1979), and the lead isotope systematics (Chen and others, 

1986) and strontium isotope systematics in Guaymas Basin indicate contributions from intermediate to mafic volcanic rock, although the strontium may originate from ash 

layers or volcaniclastic components in the sediment 

(Gieskes and others, 1982a,b). 

If the hydrothermal fluids venting at sediment-covered 

spreading centers circulate within the basaltic basement, the 

metal content of any associated sulfide deposits may originate from high-temperature fluid interaction with the basaltic rocks. Models of the Guaymas Basin system 

assumed that fluids circulating in the basaltic basement 

might have a chemical composition similar to the latitude 

21° N. EPR vent fluids (Von Damm and others, 1985b; 

Bowers and others, 1985). Reaction of acidic, metal-rich 

waters with organic matter and carbonate in the sediment 

column titrates out the acidity, causing metal sulfides to 

precipitate and possibly results in the formation of a massive sulfide deposit at depth (Bowers and others, 1985). 

The Fe/Mn ratio of the fluids decreases significantly due 

to the much greater solubility of manganese sulfides compared to iron sulfides, and sediment-hosted systems display 

this effect (fig. 11. 7). If this model is correct, fluids debouching at the sea floor in Guaymas Basin represent spent 

sulfide-forming solutions. In the deepest parts of the 

Guaymas Basin system, large increases in iron and zinc 

occur in the sediment (Niemitz, 1982). The composition 

of mounds and chimneys in Guaymas Basin are also consistent with this hypothesis; they are primarily composed 

of the gangue minerals calcite and barite (Peter, 1986). 

Another source of metals is the sediment itself. If subjected to high-temperature leaching for any length of time, 

all the organic matter and carbonate will be removed, thus 

decreasing its ability to neutralize acidity. Reaction of 

high-temperature acidic fluids with these heavily altered 

sediments could extract significant quantities of some metals. This may explain the high arsenic and bismuth content of the Escanaba Trough sulfides, which Koski and 

others (1988) suggest are derived from felsic components 

in the Escanaba Trough sediment. 

The Escanaba Trough deposits are rich in metal sulfides that apparently formed at high temperatures (Morton 

and others, 1990). They could not have precipitated from 

the metal-depleted fluids currently emitted from the 

mounds. Subsurface cooling and deposition of metal sulfides may explain the paucity of dissolved metals in the 
218 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

fluids. Without drilling in this area, one cannot say if this 

is occurring or what spatial relationship exists between the 

deposits and the basaltic heat source. It is noteworthy that 

the volcanic hill on which the active deposits are situated 

has extrusive basaltic flows on its eastern side (Morton and 

others, 1990). This may indicate that the heat source(s) 

for driving hydrothermal circulation and forming sulfide 

deposits in Escanaba Trough exists at shallower depths in 

the sediment cover than in Guaymas Basin, where only intrusive magmatic activity is evident (Lonsdale and Becker, 

1985). 

CONCLUSIONS 

Submarine hydrothermal fluids from active vents in 

Escanaba Trough show a number of similarities in chemical composition to the fluids from Guaymas Basin, Gulf 

of California. Both can be readily distinguished from vent 

systems that only involve reactions between evolved, 

heated seawater and tholeiitic basalt by their enrichment 

in the alkali elements potassium, rubidium, and cesium and 

the alkali-like elements boron and thallium. These elements are extracted at 200°C to 300°C from sediment that 

is rich in alkalis and boron compared to MOR basalt. The 

systematic trends of potassium and strontium relative to 

chloride also differentiate sediment-hosted vents from sediment-starved vents. The W/S ratios calculated from the alkali data cannot be used to model the true water-to-rock 

ratio of the system due to incomplete leaching and uptake 

of alkalis into secondary mineral phases. The data suggest progressive leaching of alkali elements from sediment 

with the following relative mobility: B >> Cs- (Li) > Rb 

> K. Inter-field differences in the Si(OH)4 concentrations, 

and boron isotopic composition of the Escanaba Trough 

fluids also indicate uptake by a secondary silicate mineral, 

such as smectite, is occurring even at 220°C. In general, 

the boron isotopic composition of sediment-hosted vent fluids, which are much lighter than MOR fluids, is dominated 

by sedimentary-derived boron in contrast to MOR systems 

where the boron systematics are seawater dominated. The 

strontium isotope values at Escanaba Trough are more radiogenic than seawater and reflect complete equilibration 

of the fluids with the sediment. In contrast, the Guaymas 

Basin vent fluids must interact with some basaltic material to obtain the 8? Srt86sr values. 

Differences in the organic carbon and carbonate content of the Escanaba Trough and Guaymas Basin sediments 

have a significant effect on the buffer capacity of the fluids. Compared to bare rock vent systems, both vents have 

much higher pH, total alkalinity, ammonium, bromide, and 

iodide, which are derived from the thermal decomposition 

of organic matter and carbonate in the sediment (Von 

Damm and others, 1985b; Campbell and Edmond, 1989). 

The Escanaba Trough fluids have significantly lower concentrations of these chemicals and a lower pH than the 

Guaymas Basin vents because the organic carbon, carbonate, and sulfur content of the Escanaba Trough sediments 

is much lower. However, the Escanaba Trough vents have 

higher I values than Guaymas Basin because the ratio of 

iodide to organic carbon of the sediment is about 2 to 3 

times higher. This reflects the more oxidizing nature of 

the sediments at Escanaba Trough. The water-to-sediment 

ratio calculated from the I data is between 3 and 12, which 

agrees well with W/S calculated from oxygen isotope data 

(about 5). 

The precipitation of massive sulfide deposits in sediment-hosted environments is primarily induced by the pH 

and alkalinity changes accompanying the thermal decomposition of organic matter and carbonate in the sediment 

column. Decreasing temperature due to mixing of seawater with vent fluids is of secondary importance. The wide 

range in the metal content of MOR fluids suggest that fluidbasalt interactions play a major role in determining the 

metal content of sediment-hosted massive sulfide deposits. The sediments may, however, be an important source 

of metals if they are depleted of their organic matter and 

carbonate in a high-temperature portion of the system. In 

Guaymas Basin, the fluids are essentially spent ore forming solutions that are actively forming large mound structures on the sea floor mainly composed of gangue minerals. 

The cooler, metal-depleted fluids at the Escanaba Trough 

system are not responsible for the formation of the large 

sulfide-rich mounds there. It is possible that even at depth 

they do not contain significant metals and that they reflect 

the waning phase of a hydrothermal cycle. 

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Chapter 12. Stable Isotope Study of Hydrothermal Vents at 

Escanaba Trough: Observed and Calculated Effects of 

Sediment-Seawater Interaction 

By J.K. Bohlke 1 and W.C. Shanks 1112 

CONTENTS 

Page 

Abstract .. ... ... .... .. ... . ... ... .... . ... .... .. ........ .... .. .. .. .. .. .. .. .. .. .. .. ..... ... 223 

Introduction .... .... ..... .... ....... .................................... ..... ... . . ... . 223 

Acknowledgments . ........ ............... .... . . .. .... . . .. ......... .... .. 225 

Samples ................................................................................ 225 

Methods ..... .... ..... ... . ....... .... ... ...... .. .... .. . ... .. .. .. .. .. .. .. .. .. .... ..... . .. 225 

Results .. .. .. .. . .. .. .. . .. .. . . .. . . .. . . .. . .. . . .. . .. .. .. .. .. .. . .. . . .. . . .. . . .. . . .. . . .. . . . .. . . 226 

Oxygen isotopes . ......................................................... 226 

Hydrogen isotopes ............................ ........ ................... 226 

Sulfur isotopes . .. . .. .... .. .. ... . ... . ... . .. . . ... . ... . .. . . ... . ... . .. .. .. .. .. . 228 

Discussion .. ...... . . .. .. . .. . ... . ... ... . ... . ... .. .. .. .. .. .. . ... ... . . .. . . ... . ... . .... . . . 228 

Mineral-fluid isotopic equilibrium ............................. 228 

Comparison with other sites........................................ 229 

Isotopic effects of sediments....................................... 230 

Seawater-rock reaction models ............ ....................... 230 

Shallow subsurface mixing? ........................ ............... 235 

Conclusions.......................................................................... 235 

References cited................................................................... 237 

ABSTRACT 

Hydrothermal fluids venting at 217°C from sulfide 

mounds in the sediment-filled Escanaba Trough have 

o34SH2s = +7.8 ± 0.4, o180H20 = +0.43 ± 0.06, and 

BDH2o = -0.7 ± 0.3 (compared to local seawater with 

o180H2o = -0.17 and BDH2o = -0.6). The Escanaba 

Trough vent fluids have relatively low o18oHzO values 

compared to those at several other mid-ocean ridge sites, 

and are the first reported not to have a larger increase 

(relative to seawater) in BDHzO than in o18oHzO· Massive 

sulfide samples from various localities within Escanaba 

Trough have o34Ssolid ranging from -0.7 to + 11.6. The 

wide range of o34Ssolid values suggests variable contributions of relatively 34S-depleted sedimentary 

(bacteriogenic) ± igneous sulfide and 34S-enriched sulfide 

from reduced seawater sulfate in the hydrothermal systems. 

The vent fluid isotopes could be explained by reaction of 

heated seawater with sediment alone at relatively high 

1 U.S. Geological Survey, Reston, Virginia 2 U.S. Geological Survey, Denver, Colorado 

water to rock ratios (W/R), or possibly with some combination of basalt and sediment, but are less likely to result 

from reactions with basalt alone. 

Equilibrium reaction-path calculations indicate that 

heated seawater reacting with Escanaba Trough sediment 

should undergo relatively large positive shifts in o180H2

0 

but small or negative shifts in BDH2o, in contrast to seawater reacting with basalt, which exhibits roughly similar 

enrichments in both 180 and deuterium. Calculations also 

predict a negative correlation between B34SH2s and 

ol80H2o, and relatively large variability of o34SH2s. Isotopic ratios of oxygen, hydrogen, and sulfur in the 

Escanaba Trough vent fluid are reproduced by simulated 

equilibrium isothermal closed-system reactions with sediments at relatively high cumulative W/R (2 to 6 by weight 

at 220°C to 300°C), roughly an order of magnitude higher 

than W /R values inferred for seawater-basalt reactions at 

latitude 21 o N. on the East Pacific Rise. The measured 

Escanaba Trough isotope ratios are not reproduced by 

simulated closed-system reactions with basalt, or by opensystem reactions with either sediment or basalt. Relatively 

high effective water/rock ratios and low vent temperatures 

at Escanaba Trough could result from high permeabilities 

in the turbidite-hosted hydrothermal circulation system, a 

large amount of conductive cooling during fluid ascent, and 

(or) ongoing subsurface mixing of seawater with deeper 

hotter fluid reservoirs. The temperature and o180H2

o value 

of the 217°C vent fluid could be similar to those of fluids 

responsible for chloritization of near-surface sediments in 

Escanaba Trough, which probably required shallow circulation of heated magnesium-bearing seawater. 

INTRODUCTION 

Escanaba Trough occupies a sediment-covered slowspreading rift on Gorda Ridge in the northeast Pacific 

Ocean (fig. 12.1 ). The floor of the trough, 3,300 m below 

sea level, receives rapidly accumulating lithic and arkosic 

turbidite sand, silt, and mud, largely from the open southern end near the Mendocino Fracture Zone (McManus and 

others, 1970; Normark and others, chap. 6, this volume). 

The sedimentary section is at least 500 m thick in some 

223 
224 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

127° 30' 127° 

40° 30' 

Figure 12.1. Bathymetric map of Escanaba Trough showing location of volcanic edifices (dark gray) 

and SESCA and NESCA sites. Hachures indicate area of closed low. Contour interval is 200 m.~ 
STABLE ISOTOPE STUDY OF HYDROTHERMAL VENTS AT ESCANABA TROUGH 225 

parts of the trough, and it includes numerous basaltic 

intrusions and flows (Morton and Fox, chap. 2, this volume; Davis and Becker, chap. 3, this volume). Large massive sulfide bodies are known to occur at the sediment 

surface above the margins of buried igneous sills or stocks 

between approximate latitudes 40.5° and 41.5° N. (Morton 

and others, 1987; Koski and others, 1988; Zierenberg and 

others, chap. 10, this volume). Active hot springs currently 

are venting from areas of pyrrhotite-rich massive sulfide 

mounds near latitude 41° N. in the northern Escanaba 

Trough (NESCA) area (Campbell and others, 1989, chap. 

11, this volume). Both the vent fluids and the hydrothermal deposits at Escanaba Trough have chemical characteristics that are distinct from those of vent fluids and deposits 

from nonsedimented spreading centers and that indicate 

contributions from the underlying organic-bearing sediments (K venvolden and others, 1986, Koski and others, 

1988; 1987; LeHuray and others, 1988; Campbell and others, 1989, chap. 11, this volume). 

This paper presents new light stable isotope data (0, 

H, and S) for vent fluids and hydrothermal deposits at 

Escanaba Trough and discusses some of the implications 

of fluid-sediment interaction for the isotope compositions 

of venting fluids. Previous studies have indicated that massive sulfide deposits in sedimented mid-ocean ridge (MOR) 

settings such as Guaymas Basin (Gulf of California), 

Middle Valley (northern Juan de Fuca Ridge), and the 

Atlantis II Deep (Red Sea) have more variable o34Ssolid 

values (both higher and lower) than do those in 

nonsedimented MOR settings (for example, Shanks and 

Bischoff, 1980; Goodfellow and Blaise, 1988; Peter and 

Shanks, 1992). ODH2

o and o180H2o data for hydrothermal vent fluids from sedimented MOR settings have not 

been reported previously, except for the Red Sea brines. 

Simple equilibrium reaction-path calculations are presented 

here for isotopic alteration of seawater reacting with 

Escanaba Trough mud and basalt at 220°C and 300°C. The 

calculations indicate varied isotopic behavior, some of 

which matches observed vent fluid characteristics, and 

some of which has not yet been observed in MOR hot 

springs. 

ACKNOWLEDGMENTS 

We thank T. Coplen, C. Kendall, and C. Gwinn for assistance with the 0 and H isotopic analyses, T. Johnson for 

performing S isotopic analyses, M. Reed for supplying PCcompatible geochemical reaction-path computer codes, and 

R. Zierenberg for separating smectite from the 217°C vent 

site. We are also grateful to A.C. Campbell for providing 

unpublished partial chemical analyses of the vent fluid 

samples, and to A.C. Campbell, D.R. Janecky, R.O. Rye, 

and R. A. Zierenberg for comments on the manuscript. 

SAMPLES 

Clear hydrothermal fluids were sampled using DSV 

Alvin in 1988 from the tops of pyrrhotite-rich massive 

sulfide mounds at four locations in the NESCA area. The 

maximum measured fluid temperatures at the four 

sampled sites were 217°C, 108°C, and approximately 

18°C and l3°C, respectively (Campbell and others, chap. 

11, this volume; A. C. Campbell, written commun., 1989). 

The 217°C fluids were venting from a porous anhydrite-

(barite-silicate) crust on top of a sulfide mound. Fluid 

samples were collected with titanium syringe hot water 

samplers (Campbell and others, chap. 11, this volume), 

and splits for oxygen and hydrogen isotope analyses were 

transferred to evacuated glass tubes with soft rubber 

septa. Before being subsampled for these isotope analyses, each fluid sample was degassed into a vacuum 

through a needle inserted into the rubber stopper (B.E. 

Taylor, oral commun., 1990). Samples with low magnesium contents (high-temperature fluids) contained small 

quantities of whitish translucent solids (amorphous 

silica?) after storage. 

One sample of smectite (Mg-rich saponite) was obtained from the sulfate-rich crust at the active 217°C vent 

site. The bulk sample was crushed, and the smectite was 

separated from the sulfate minerals by suspension in distilled water followed by decanting and settling. One 

sample of pyrrhotite-rich massive sulfide from the vicinity of the 217°C vent site was also analyzed. 

Other sulfide mineral samples analyzed as part of this 

study were recovered using DSV Sea Cliff in 1986 and 

DSV Alvin in 1988. The samples include massive pyrrhotite-rich mound material and fragments of pyrrhotite-rich 

and polymetallic crusts (chimneys?) from various locations 

within the NESCA and southern Escanaba Trough 

(SESCA) areas, and from a volcanic edifice at the northern end of the NESCA area at latitude 41 °05' N. (Edifice 

REX). 

METHODS 

The oxygen isotopic compositions of vent fluids were 

measured using an automated C02 equilibration technique 

developed in the U.S. Geological Survey Water Resources 

Division stable isotope laboratory (T. Coplen and C. 

Kendall, oral commun., 1989). Two milliliters of each 

fluid sample was equilibrated with approximately 15 mL 

of tank C02 ( +9 per mil, SMOW) at approximately 0.33 

atmosphere for 5 to 10 hours in a rocking flask, then the 

C02 was released into the mass spectrometer. Mass balance corrections were made to the measured isotope ratios to account for isotopic shifts of the C02 and H20 

during equilibration. The Escanaba Trough samples were 
226 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

analyzed together with laboratory water standards calibrated as in Coplen and others (1983). The reproducibility (±cr) for individual analyses is estimated to be ±0.05 

to 0.10 per mil, based on replicate analyses of different 

subsamples. 

Hydrogen isotopic analyses were performed on hydrogen gas produced by zinc reduction (Kendall and Coplen, 

1985). Twenty-microliter aliquots of water were sealed 

into glass capillary tubes, each of which was loaded into 

either an evacuated glass reaction vessel or an evacuated 

sealed reaction tube with 700 mg of zinc shot and a glasssealed iron bar. The capillary was then broken with the 

iron bar, and the assembly was inserted into a heating block 

(for valved reaction vessels) or an oven (for sealed reaction tubes) and held at 455°C to 465°C for 70 minutes. No 

systematic differences in reproducibility were noted to correlate with either the type of reaction container used, or the 

mass spectrometer used. Approximately 5 to 10 percent 

of the hydrogen samples produced by zinc reduction 

yielded anomalous ODH2o values that were 

nonreproducible and clearly incorrect by more than 3cr; 

these outlying data were eliminated from consideration. 

The reproducibility (±cr) for individual analyses, after 

elimination of outlying data, is estimated to be ±0.5 to 1.0 

per mil, based on replicate analyses of different 

subsamples. The ODH2o values of Escanaba Trough 

samples were calibrated by interspersed analyses of 

VSMOW. Samples from the 108°C vent were not analyzed 

for hydrogen isotopes because all of them have greater than 

30 mmol/kg Mg (that is, they are all >60 percent contaminated by ambient seawater), and because the oxygen isotope results are approximately consistent with those of the 

217°C vent samples. 

Sulfur from mixed-sulfide mineral samples was obtained by sequential chemical extraction as described by 

Shanks and Seyfried (1987). Acid-volatile sulfide sulfur 

(mainly from pyrrhotite, sphalerite, and galena) was liberated from the samples with 6N HCl at 60°C. The remaining sulfide sulfur (mainly from pyrite, chalcopyrite, 

isocubanite, and marcasite), and native sulfur if present, 

were released by adding CrCh to the HCl at 85°C 

(Canfield and others, 1986). CrCb also was used to release total sulfide sulfur from samples that were not subjected to the first treatment. H2S released by each of these 

treatments was precipitated as Ag2S. H2S in the vent fluid 

samples was precipitated as CdS on board the ship within 

hours after collection, and later converted to Ag2S in the 

onshore laboratory. All Ag2S samples were subsequently 

com busted in the laboratory with Cu20 at 1 ,050°C to produce S02, which was purified by vacuum distillation and 

then analyzed in the mass spectrometer. The reproducibility and accuracy of the sulfur isotope data are estimated 

to be ±0.1 to 0.2 per mil, based on multiple analyses of 

NBS-123 sulfide (o34S = 17.1 per mil). 

RESULTS 

OXYGEN ISOTOPES 

The results of oxygen isotope analyses of vent fluids 

and vent smectite are listed in table 12.1 and the vent fluid 

data are summarized in figure 12.2. Analyses of seven 

Escanaba Trough water samples with more than 52 mmol/ 

kg Mg (ambient seawater) yield o18QH2o = -0.16 ± 0.05, 

indistinguishable from those measured by us in bottom 

waters from other northeast Pacific Ocean vent sites (average = -0.17), and from the value reported by Craig and 

Gordon (1965) for North Pacific deep water (-0.17), perhaps slightly lower than the value reported by Welhan and 

Craig (1983) for the East Pacific Rise (EPR) 21° N. vent 

site (-0.08). Average values for replicate analyses of 19 

water samples from the four vent sites define a single 

straight mixing line between seawater at -0.17 per mil and 

high-temperature vent fluid with a zero-Mg intercept at 

+0.43 ± 0.06 per mil. Considered separately, 16 samples 

from the 217°C vent yield a zero-Mg intercept at +0.45 per 

mil whereas three samples from the 1 08°C vent could be 

consistent with a slightly lower, but relatively poorly constrained, intercept at approximately +0.3 per mil (fig. 12.2). 

HYDROGEN ISOTOPES 

Ten analyses of Escanaba Trough vent fluid samples with 

Mg greater than 52 mmol/kg (ambient seawater) yield 

ODH2o = -0.6 ± 0.6 per mil, relative to nine analyses of 

VSMOW samples analyzed at the same times by the same 

techniques (0DH2

o = 0.0 ± 1.0 per mil; table 12.1). The analytical uncertainties of the hydrogen isotope analyses (± 0.5 

to 1.0 per mil) apparently are large compared to the real differences in the isotopic compositions of the analyzed samples. 

With one exception (2036-4C), all of the samples yielded average ODH2

o values that are statistically indistinguishable. 

Including all samples in a linear regression, we obtain a zero 

Mg-intercept for nine samples at -0.2 ± 0.7 per mil. The 

anomalous hydrogen isotopic composition of sample 2036-

4C was confirmed by analyses of three separate subsamples, 

but it is not known for certain if it was anomalous when 

sampled or if it may have been fractionated during storage 

or previous subsampling, possibly by evaporation. The same 

sample also has an anomalously low o34Sso4 value (table 

12.1), possibly because of sulfide oxidation in a leaky container. Ignoring 2036-4C, we obtain a (preferred) mixing line 

for eight Escanaba Trough samples between ambient seawater 

at -0.6 per mil and a zero-Mg intercept at 0.7. In this case, 

the difference between the high-temperature vent fluid and 

cold seawater values (dOD= -0.1 ± 0.3 per mil) apparently 

is known with greater precision than is either value with respect to the standard (fig. 12.3). 
STABLE ISOTOPE STUDY OF HYDROTHERMAL VENTS AT ESCANABA TROUGH 227 

Table 12.1. Oxygen, hydrogen, and sulfur isotope data for vent fluids and vent minerals from Escanaba Trough. 

[_, not determined. Samples were collected using DSV Alvin in 1988; o18oH2o and 8DH2o values represent either single analyses or averages of 

two to three analyses of separate splits (number of determinations given in parentheses); temperatures and Mg concentrations are from A.C. Campbell 

(written commun., 1989)] 

Dive Sample Latitude Longitude Temp. 

number N. w. (OC) 

2036 1C 41 °0.016' 127°29.623' 216 

2C 41 °0.016' 127°29.623' 216 

3C 41 °0.014' 127°29.621' 217 

4C 41 °0.014' 127°29.621' 217 

5C 41 °0.016' 127°29.623' 216 

6C 41 °0.014' 127°29.621' 217 

7C 41 °0.016' 127°29.623' 216 

2037 1C 40°59.998' 127°29.621' 13 

2C 41 °0.015' 127°29.670' 18 

3C 40°59.998' 127°29.621' 13 

4C 41 °0.015' 127°29.670' 2 

5C 41 °0.015' 127°29.670' 18 

7C 40°59.998' 127°29.621' 13 

2040 3C 41 °0.022' 127°29.843' 108 

4C 41 °0.022' 127°29.843' 108 

6C 41 °0.022' 127°29.843' 108 

2041 1C 41 °0.016' 127°29.623' 215 

7C 41 °0.016' 127°29.623' 215 

9C 41 °0.016' 127°29.623' 215 

2036 smectite 41 °0.014' 127°29.621' 217 

2036 massive sulfide 41 °0.014' 127°29.621' 

• ,.....,_ +0.4 -

~ t 

0 ·· ... 

~ 

~ ··· ... 

....:! 

~ ~ 0.0 

EXPLANATION 

• 217°C 

······~·111.~ .. g: ~, 0 

£ 

0 

... 108°C 

···~ 

• 00 

c-o -0.4 ~--_._ __ ---~.. ___ .L--__ ~ __ ___... __ __J 

0 20 40 60 

MAGNESIUM, IN MILLIMOLES PER KILOGRAM 

Figure 12.2. Variation of o180H2o with Mg concentration for 

mixtures of high-temperature vent fluids and ambient seawater. Each 

data point represents average of all isotope analyses of given sample 

(approximate uncertainty [la] of each point is indicated by error 

cross); Mg concentrations are from A. C. Campbell (written commun., 

1989); solid line indicates preferred mixing line for all samples; 

dotted line indicates possible mixing line for 1 08°C vent. 

Mg 

(mmol/kg) 

47.8 

39.1 

2.4 

5.0 

37.2 

1.6 

9.2 

52.5 

51.9 

52.6 

52.5 

52.1 

52.3 

35.2 

40.0 

33.0 

32.5 

29.9 

20.0 

0 

£ 

Q 

CtO -2 

ol8oH o 8DH~ csMoW) (SMO ) 

-0.24 (3) 

-0.05 (1) 

0.41 (3) -1.0 (3) 

0.45 (3) 1.4 (3) 

0.01 (1) 

0.41 (3) -0.8 (3) 

0.23 (1) -0.5 (2) 

-0.10 (1) -0.5 (2) 

-0.15 (1) 

-0.13 (1) -0.6 (3) 

-0.15 (1) -0.5 (2) 

-0.17 (2) -0.8 (3) 

-0.20 (1) 

-0.04 (1) 

-0.03 (1) 

0.02 (1) 

0.15 (1) 

0.12 (1) 

0.23 (1) -0.3 (2) 

6.84 

0 

------------·--------

o34sH s 

ccoB 

o34sso4 

(COT) 

8.2 17.5 

7.8 20.2 

20.3 

7.4 20.3 

4.8 

t 

' 

0 20 40 60 

MAGNESIUM, IN MILLIMOLES PER KILOGRAM 

Figure 12.3. Variation of ODH2o with Mg concentration for mixtures 

of high-temperature vent fluids and ambient seawater. Data are 

presented as in figure 12.2, except for sample 2036-4C (open square), 

which is considered to be anomalous; solid line indicates preferred 

mixing line (for all samples except 2036-4C); dashed line indicates 

best fit to all samples (including 2036-4C). 
228 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

SULFUR ISOTOPES 

The three analyses of 217°C vent fluid H2S are similar, and average +7.8 per mil (table 12.1). Pyrrhotite-rich 

massive sulfide from the 217°C vent site has ()34Ssolid = 

+4.8, while sulfide minerals from various localities within 

Escanaba Trough have variable ()34Ssolid values between 

-1 and+ 12 per mil (table 12.2 and fig. 12.4). In general, 

there are no consistent differences between ()34S values of 

sulfur released by the two stages of sequential chemical 

extraction. There are no obvious overall geographic trends 

in o34Ssolid values, nor is there a notable signature in 

samples associated with hydrocarbons (fig. 12.4), though 

this has not been thoroughly tested. 

DISCUSSION 

MINERAL-FLUID ISOTOPIC EQUILIBRIUM 

Smectite and hydrothermal fluid from the 217°C vent 

in the NESCA area are different in o18o by 6.4 ± 0.2 per 

mil, which corresponds to an isotope equilibration temperature of approximately 220°C (Yeh and Savin, 1977, as corrected by Savin and Lee, 1988), in excellent agreement 

with the measured vent temperature. Zierenberg and 

Shanks (chap. 14, this volume) report ()180solid values for 

several samples of hydrothermal chlorite (2.13, 2.42, and 

2.69 per mil) and smectite (6.04 per mil) separated from 

altered sediments collected in the SESCA area and between 

the NESCA and SESCA areas within Escanaba Trough. 

These minerals could have formed in equilibrium with fluids similar to those now venting in the NESCA area at temperatures between approximately 200°C and 240°C, 

calculated from fractionation equations of Wenner and Taylor (1971) and Savin and Lee (1988). If the inferred temperatures are meaningful, then they could indicate that 

surface hydrothermal discharge in Escanaba Trough is limited for some reason to lower temperatures than most other 

MOR hydrothermal vents. Yet, the abundance of massive 

coarse-grained pyrrhotite-isocubanite-bearing massive sulfide deposits and polymetallic chimneys may imply localized zones of higher temperature fluid flow either in the 

past or below the surface at the same sites (Campbell and 

others, chap. 11, this volume; Koski and others, chap. 16, 

this volume). 

Sulfur isotope data are not indicative of equilibrium 

either among sulfide minerals or between sulfide minerals 

and H2S in the vent fluids. H2S from the 217°C vent fluid 

has higher ()34S than a pyrrhotite-rich sulfide boulder from 

the same locality, opposite the small fractionation expected 

at equilibrium between aqueous H2S and pyrrhotite. Both 

positive and negative differences are observed between 

()34Ssolid values of acid-volatile and nonvolatile sulfides in 

the same samples (table 12.2). As the dominant sulfides 

in the samples (pyrrhotite, isocubanite, marcasite, and 

sphalerite) are present in varying proportions, it is not 

clearly established whether sulfur isotopic equilibrium was 

EXPLANATION 

II NESCA 217°C vent fluid 

ffiii]] NESCA minerals 

[] 40°54' N. minerals 

D SESCA minerals 

B 41 °05' N minerals 

Sulfide Sulfate 

+ + 

0~-----­

-5 0 +5 +10 +15 +20 +25 

834S, PER MIL (CDT) 

Figure 12.4. Histogram of 834S values for Escanaba Trough sulfide and sulfate minerals. Data are from 

Koski and others (1988), Zierenberg and Shanks (chap. 14, this volume), and table 12.2; dots indicate 

samples from which hydrocarbons have been reported (K venvolden and others, 1986, 1987). 
STABLE ISOTOPE STUDY OF HYDROTHERMAL VENTS AT ESCANABA TROUGH 229 

Table 12.2. Sulfur isotope data for sulfide minerals from Escanaba Trough. 

[Samples were collected using DSV Sea Cliff in 1986 and DSV Alvin in 1988. "HCl" and "CrCI2" refer to sulfur extraction methods (see text);-, not 

determined; po, pyrrhotite; cp, chalcopyrite; sl, sphalerite; wz, wurtzite; me, marcasite; icu, isocubanite; asp, arsenopyrite; HC, hydrocarbons] 

Dive Sample Latitude Longitude o34s 

number N. w. (HCl) 

Edifice REX 

2039 3 41 °5.507' 127°28.636' 

2039 5 41 °5.485' 127°28.642' 

NESCA 

658 R3-A1 40°59.998' 127°29.602' 5.9 

R3-A2 40°59.998' 127°29.602' 6.5 

R3-A2 40°59.998' 127°29.602' 6.1 

R4-1 41 °0.233' 127°29.644' 7.9 

R4-1 41 °0.233' 127°29.644' 4.8 

659 R3-6 41 °1.107' 127°29.627' -0.3 

R4-2 41 °1.107' 127°29.627' 0.4 

R4-2 41 °1.107' 127°29.627' -0.7 

2033 1 41 °0.219' 127°29.561' 1.3 

2035 3 40°58.836' 127°30.149' 

2036 1A 41 °0.014' 127°29.621' 

SESCA 

662 Rl-A 40°47.393' 127°30.299' 5.0 

RI-A 40°47.393' 127°30.299' 5.6 

R1-J 40°47.393' 127°30.299' 6.0 

Rl-J 40°47.393' 127°30.299' 5.9 

Rl-J 40°47.393' 127°30.299' 5.9 

Rl-J 40°47.393' 127°30.299' 

663 Rl-3 40°46.395' 127°31.150' 

Rl-3 40°46.395' 127°31.150' 

attained. Apparent nonequilibrium sulfur isotope systematics are commonly observed in MOR massive sulfides, 

and have been attributed in part to irregular temporal variations in the ()34SH2

s values of the pore fluids from which 

the sulfide minerals precipitated (Shanks and Seyfried, 

1987; Woodruff and Shanks, 1988). 

COMPARISON WITH OTHER SITES 

The range of ()34Ssolid values at Escanaba Trough ( -1 

to + 12) is large compared to those at most other MOR vent 

sites. It encompasses the ranges observed at sediment-free 

vent sites ( + 1 to +6 per mil; Arnold and Sheppard, 1981; 

Styrt and others, 1981; Zierenberg and others, 1984; Bluth 

and Ohmoto, 1988; Woodruff and Shanks, 1988), as well 

as some of the relatively low values observed at Guaymas 

Basin, EPR (-4 to +5 per mil; Peter and Shanks, 1992) and 

most of the relatively high values observed at Middle Valley, Juan de Fuca Ridge ( +2 to + 17 per mil; Goodfellow 

and Blaise, 1988). Relatively high ()34Ssolid values could 

be consistent with relatively large proportions of hydrothermal H2S formed by reduction of seawater sulfate (Shanks 

o34s Comments 

(CrCI2) 

1.8 cp-po-sl-wz (with barite) 

1.6 po-sl (blocky fine grained) 

5.1 po-mc-(sl-cp-icu) 

5.3 po-mc-sl-(icu-cp-asp) (fine-grained) 

5.9 po-mc-sl-(icu-cp-asp) (coarse-grained) 

4.0 po-(cp) 

5.1 po-(cp) 

-0.1 po (massive fine-grained) (HC in other R3 pieces) 

0.9 po (coarse-grained) 

1.0 po 

po-(+Cu-Fe sulfide) 

3.3 po (massive with silica?) 

4.8 po 

4.7 po-(mc-icu) (inner zone) 

5.5 po-(mc-icu-sl-cp) (outer crust) 

5.6 po-(mc?-icu?) (outer zone of chimney) 

5.4 po-(mc?-icu?) (middle zone of chimney) 

5.4 po (coarse inner chimney lining) 

4.0 po-(mc?-icu?) (outer zone of chimney) 

1.6 po (coarse bladed inner chimney lining) 

2.7 po (outer chimney wall) 

and others, 1981; Koski and others, 1988; Goodfellow and 

Blaise, 1988) whereas negative ()34Ssolid values may require contributions of biogenic sulfide from the sediments 

(Peter and Shanks, 1992). 

The inferred differences in oDH2o and o 180H2o between the high-temperature vent fluids and ambient seawater at Escanaba Trough (.1o18QH2o = +0.6 ± 0.1; 

.!loDH2o = -0.1 ± 0.3; t = 108°C to 217°C) are relatively 

small compared to most values reported for high-temperature vent fluids from latitude 13° N., EPR (,1()18QH2

o = 

+0.4 to +0.7; .!loDH2o = +0.7 to +1.6; t = 317°C to 335°C; 

Michard and others, 1984; Merlivat and others, 1987), from 

the southern Juan de Fuca Ridge (.1o18QH2o = +0.8 to 

+ 1.0; t = > 140°C to 284°C; Shanks and Seyfried, 1987), 

from latitude 21 o N., EPR (.1o18QH2o = + 1.6 to +2.0; 

.!loDH2o = +2.5; t = 273°C to 355°C; Craig and others, 

1980; Welhan and Craig, 1983; Von Damm and others, 

1985), and from the Kane site, Mid-Atlantic Ridge (MAR) 

(.1o18QH2o = +2.1 per mil; t = 335°C to 350°C; Campbell 

and others, 1988). These limited data do not clearly indicate that the hydrogen and oxygen isotopic compositions 

of MOR vent fluids are simply correlated either with the 

measured vent temperatures or with the presence or 
230 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

absence of sediments at the vent site; more data are needed. 

Vent fluid o180H2o values relatively close to ambient seawater values could be consistent with (1) relatively high 

water to rock ratios in high-temperature reaction zones 

(Bowers and Taylor, 1985; Bowers, 1989), (2) contamination of heavier high-temperature fluids by magnesiumdepleted seawater beneath the vents, or (3) seawater-rock 

reactions at relatively low temperatures ( <200-300°C) 

where product hydrothermal minerals form with oxygen 

isotope ratios close to those of fresh rocks. oDH2

o values 

relatively close to ambient seawater values could be consistent with (1) reactions at high water to rock ratios, (2) 

mixing with magnesium-depleted seawater beneath the 

vents, or (3) reactions with hydrous minerals that have 

negative oD values. 

ISOTOPIC EFFECTS OF SEDIMENTS 

Water-rock reactions cause the o18QH2o value of the 

reacting fluid to increase in most cases where the o180solid 

value of the reacting rock is higher than that of the solid 

reaction product (for example, Taylor, 1979). Because of 

the temperature dependence of oxygen isotope fractionation, seawater reacting with basalt (+5.7 per mil) is enriched in 18Q at temperatures above approximately 200°C, 

but can be depleted in 180 at lower temperatures (for example, Bohlke and others, 1984; Bowers and Taylor, 1985). 

Compared to basalts, the sediments filling Escanaba Trough 

have relatively high o180solid values. Unaltered bulk sand 

and mud have o180solid of 8. 7 to 10.3 per mil (Zierenberg 

and Shanks, chap. 14, this volume), whereas the clay fractions have o180solid greater than 12 per mil (Savin and 

Epstein, 1970; Zierenberg and Shanks, chap. 14, this volume). Seawater reacting with the sediment therefore could 

be enriched in 18Q over a wider range of temperatures, and 

could be more highly enriched in 18Q for a given amount 

of reaction, than seawater reacting with basalt, especially 

if low-o18Q detrital quartz is relatively unreactive. 

In contrast, it is possible that the initial oDsolid values 

of the sediment are less than or equal to the ODsolid values 

of seawater-sediment reaction products, so that the reacting fluid may tend to be relatively unchanged or it may be 

depleted in deuterium at all reasonable reaction temperatures. oDsolid values of Escanaba Trough sediments have 

not been measured; however, studies of continental weathering products and deep-sea detrital sediments indicate that 

they probably are between approximately -90 and -50 per 

mil (Savin and Epstein, 1970; Lawrence and Taylor, 1971). 

Hydrogen isotope fractionations between hydrous silicates 

and water are not well known, but most values for magnesium- and aluminum-rich phases probably are between approximately -50 and 0 per mil at temperatures between 

200°C and 500°C, while those for analogous iron-rich 

phases may be more negative by as much as 50 per mil 

(Taylor, 1979; Graham and others, 1980; Marumo and others, 1980; Bowers and Taylor, 1985). Natural chlorites and 

amphiboles formed in oceanic rocks altered by seawater 

above 200°C commonly have ODsolid values between -60 

and -30 per mil (Stakes and O'Neil, 1982). Thus, depending on the proportion of hydrogen dissolved from reacting 

sediment to hydrogen precipitated in hydrothermal phases, 

and depending on the temperature of reaction and the compositions of the hydrothermal phases, it is possible that seawater-sediment reactions could cause the ODH2

o value of 

reacting seawater to remain relatively unchanged or to decrease significantly while causing its o18QH2

o value to increase. This is in marked contrast to both observed and 

calculated effects of hydrothermal seawater-basalt reactions, in which the relative increases in fluid oDH2

o values are generally greater than or equal to those in fluid 

o18QH2o values (Craig and others, 1980; Bowers and Taylor, 1985; Merlivat and others, 1987; Bowers, 1989). 

SEAWATER-ROCK REACTION MODELS 

Simple mass balance models for equilibrium isotope 

exchange between hydrothermal fluids and rocks are summarized by Taylor (1979), whereas Bowers and Taylor 

(1985) and Bowers (1989) have presented more complex 

path-dependent exchange models. Both types of calculation predict a positive correlation between o18QH2

o and 

oDH2o values of fluids that have interacted with basalt at 

temperatures greater than about 200°C, such that the highest fluid values are attained at low "water to rock ratios." 

(The term "water to rock ratio" or W IR is retained here for 

comparative purposes, although its physical significance in 

real systems is open to debate; it is defined as the weight 

ratio of aqueous solution to reacted (equilibrated) solids, 

consistent with its use by Taylor, Bowers, and others.) The 

relatively low o18QH2

o value of the Escanaba Trough vent 

fluid, compared to those of other submarine vent sites, 

could be qualitatively consistent with basalt reactions at 

relatively high W/R, or at relatively low temperatures, but 

the apparent lack of deuterium enrichment may indicate 

significant sediment reactions. 

To illustrate the contrasting effects of basalt and sediment alteration on hydrothermal fluids, we have calculated 

simple isothermal equilibrium reaction-path models for isotope exchange between seawater and basalt, and between 

seawater and turbidite sediment typical of Escanaba 

Trough, at 220°C and 300°C. Temperatures of hydrothermal reaction and isotopic equilibration at Escanaba Trough 

are not well known, but must be higher than the measured 

vent temperature (217°C). Both the silica concentration 

(6.9 mmol/kg) and the Na/K molar ratio (14) of the 

sampled vent fluid (Campbell and others, chap. 11, this 

volume) could indicate equilibration with quartz and alkalialuminum silicates at less than 300°C (possibly less than 
STABLE ISOTOPE STUDY OF HYDROTHERMAL VENTS AT ESCANABA TROUGH 231 

250°C; Truesdell, 1984). Occurrences of hydrothermal 

smectite, talc, and chlorite denoting similar temperatures 

elsewhere within Escanaba Trough (Zierenberg and Shanks, 

chap. 14, this volume) may indicate that this is a consistent feature of the system; however, it is also possible that 

much higher temperatures prevail in the shallow subsurface, as inferred from the copper-rich compositions of some 

sulfide samples (Koski and others, chap. 16, this volume). 

Chemical and mineralogical changes as functions of 

incremental reaction progress during simple seawater-rock 

reactions were calculated using program CHILLER (Reed, 

1982; Spycher and Reed, 1989), with the thermodynamic 

data base SOLTHERM (Spycher and Reed, 1989) at the 

saturation pressure of water. The thermodynamic data were 

not altered to account for the effects of pressure at the bottom of Escanaba Trough (330 bars), as the dielectric constant of water is relatively insensitive to pressure at 

temperatures less than 300°C (Helgeson and Kirkham, 

1974). Aqueous NH3 and NH4+ were added to the data 

base (Shock and others, 1989) and included in the calculations because ammonium is a significant component of 

the Escanaba Trough vent fluids (Campbell and others, 

chap. 11, this volume). The effect of dissolved ammonium 

in the isothermal reaction path calculations is small, despite the fact that it has a major effect on the calculation 

of high-temperature fluid properties from the low-temperature vent fluid analyses. Isotope fractionations were calculated using program ISO (J.K. Bohlke, unpub. code, 

1989), which was written as a post-processor for CHILLER 

with general isotope mass balance equations similar to 

those summarized by Bowers and Taylor (1985). Isotope 

fractionation factors used by ISO are generally the same 

as those used by Bowers and Taylor (1985), Janecky and 

Shanks (1988), and Bowers (1989), except that hydrogen 

isotope fractionation factors for phyllosilicates and amphiboles were assigned values based on Taylor's (1979) estimated serpentine curve (for Mg and AI phases) or 50 per 

mil lighter than the serpentine curve (for Fe phases). The 

calculations proceeded as follows: (1) Seawater was heated 

by itself in an open system to the temperature of reaction; 

precipitated minerals (mainly anhydrite) were removed 

from the system after each step; the compositions of seawater heated to 220°C and 300°C are given in table 12.3. 

(2) The bulk composition of Escanaba Trough sediment or 

basalt was then titrated into the heated seawater in a 

"closed" system at constant temperature; precipitated minerals were retained in the system and allowed to re-equilibrate chemically and isotopically with the coexisting 

solution at all subsequent reaction steps. The initial compositions of reacting sediment and basalt used in the calculations are given in table 12.3. The calculated 

mineralogic variations of the solid reaction products are 

presented qualitatively as functions of W/R in figure 12.5; 

the corresponding oxygen, hydrogen, and sulfur isotope variations in reacting seawater are summarized in figure 12.6. 

The dominant phases at moderate to low W !R in the 

simulated basalt reactions are chlorite, albite, prehnite, 

quartz, actinolite, and epidote, approximately in agreement 

with those predicted by Bowers and Taylor (1985) and produced experimentally (Mottl, 1983). The dominant phases 

at moderate to low W !R in the simulated mud reactions are 

quartz, muscovite, albite, chlorite, and calcite. The calculated assemblages for each rock type are only slightly different at the different temperatures. In all of the 

calculations, anhydrite and hematite both are stable at high 

W !R in the presence of seawater sulfate, but they are replaced by more reduced assemblages at W !R less than 4 

to 6 (basalt) or W/R less than 10 to 12 (mud). From that 

point on, dissolved sulfur is essentially all reduced. The 

concentrations of dissolved sulfate just before reduction are 

low ( < 6 mmol/kg), as both mud and basalt supply significant amounts of calcium to form anhydrite isothermally. 

Throughout most of the range of W !R where sulfur is reduced, the pH and !:S concentration in the fluid phase are 

buffered at fairly constant values (pH is approximately 5.4 

for mud at 300°C, 5.6 for mud at 220°C, 6.1 for basalt at 

300°C, and 6.6 for basalt at 220°C; !:S is approximately 1 

to 2 mmol/kg for mud and basalt at 300°C and approximately 0.1 to 0.2 mmol/kg for mud and basalt at 220°C). 

The calculated final fluid pH and H2S concentrations in the 

seawater-mud reactions are approximately consistent with 

the observed values in the vent fluids (pH25 = 5.4, H2S = 

1.1 mmol/kg; Campbell and others, chap. 11, this volume). 

The isotopic effects of temperature are most notable 

in the evolution of ~)180H o, which is especially sensitive 

to temperature-dependent fractionation factors (as indicated 

by the bulk rock curves in fig. 12.6). The effects of rock 

type are significant for all of the isotope systems, but especially so for <5DH2

o, which evolves in opposite directions 

during reactions with mud and basalt. The calculations indicate that relatively large negative <5DH2

o values may be 

acquired by seawater reacting with argillaceous sediments 

at low values of W/R (for example, <5DH2

o = -10 per mil 

at W/R = 0.2 to 0.4). At comparable values of W/R, the 

highest fluid ()180H2

o values result from high-temperature 

reactions with mud, whereas the highest fluid <5DH2

o and 

()34SH2s values result from reactions with anhydrous basalt. The calculations predict negative correlations between 

()34SH2s and ()180H2o, and they indicate potentially large 

variations in ()34SH2s at intermediate to large values of 

W /R, as sulfide derived from quantitative reduction of seawater sulfate (()34Sso4 = +21, Rees and others, 1978) mixes 

with either basalt sulfide (()34S = +0.1, Sakai and others, 

1984) or diagenetic (bacteriogenic) sulfide in mud (()34S 

z -20, Goldhaber and Kaplan, 1974). 

In the simulated reaction with basalt at 300°C, the fluid 

oxygen, hydrogen, and sulfur isotope curves converge on 

their respective values at EPR latitude 21 o N. (d()180H2

o 

= + 1.6 to +2.0; ~()DH o = +2.5; ()34SH2s = + 1 to +6; Craig 

and others, 1980; Welhan and Craig, 1983; Woodruff and 
232 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

Table 12.3. Compositions of rocks and of heated seawater before and after calculated seawater-rock reactions. 

[Seawater compositions before reaction represent endpoints in open-system CHILLER calculations for heating seawater by itself 

(low-temperature seawater analysis from Von Damm and others, 1985). Mud composition includes major elements and o18o from 

sample L2-86-NC-4G, 138-145 (Zierenberg and Shanks, chap. 14, this volume); C and C02 are averages for analyses of sediment at 

DSDP Site 35 (Vallier, 1970); S is typical background (nonhydrothermal) value for Escanaba Trough sediment (Karlin and Lyle, 

1986); H20 was calculated by difference (allowing for other minor components reported by Zierenberg and Shanks, chap. 14, this 

volume); Fe2+JFet = 2/3 is typical value for subsurface terrigenous sediments; NH3 is from C/N(mol.) = 7 (marine plankton, Redfield, 

1958); oD is mid-range of values for detrital argillaceous sediments at similar latitudes (Savin and Epstein, 1970; Lawrence and 

Taylor, 1971); o34S is typical value for marine diagenetic (bacteriogenic) sulfide (Goldhaber and Kaplan, 1974). Basalt composition 

includes major elements and S from sample D17-10 (Clague and others, 1984, assuming all Sis reduced); Fe2+/Fet (0.85), o180, and 

o34S are typical fresh MORB values (Muehlenbachs and Clayton, 1972; Sakai and others, 1984). Reacted seawater compositions 

are from closed-system calculations at W/R = 2.8 (220°C) and W/R = 5.6 (300°C), where o180H 0 is equal to measured Escanaba 

Trough value ( +0.43 per mil). -, not determined] 2 

Seawater before reaction Rocks before reaction Seawater after reaction with mud 

220°C 300°C Mud Basalt 220°C 300°C 

(mmollkg) (mmollkg) (wt percent) (wt percent) (mmollkg) (mmollkg) 

Si02 ---------------- 0.006 0.004 52.8 50.7 4.0 7.9 

AI01.5 ------------- 0 0 17.3 15.7 .001 .003 

FeO 1_

5 ------------- 0 0 2.65 1.61 

FeO ------------- 0 0 4.77 8.18 .04 .05 

MgO ------------ 52.1 48.2 3.96 7.42 .11 .06 

CaO ------------- 2.51 .21 3.10 11.5 26.8 39.6 

N a005 ------------ 464 464 2.20 2.76 478 448 

KOo.s -------------- 9.80 9.80 3.13 .19 5.5 11.9 

H20 ------------- 7.2 0 

C02 ---------------- 1.47 1.47 0 0 69.8 139 

H2S ------------- 0 0 .11 .11 .12 1.1 

NH3 ---------------- 0 0 .07 0 15 7.5 

pH (T) ---------- 6.0 5.5 5.6 5.4 

oiso ------------ -0.17 -0.17 +9.2 +5.7 +0.43 +0.43 

oD -------------- -0.6 -0.6 -50 

o34s ------------- +21 +21 -20 

Shanks, 1988) at W/R = 0.3 to 0.5 (fig. 12.6C), similar 

to the degree of reaction required to reach those values in 

the more complex reaction paths with varying temperatures 

simulated by Bowers and Taylor (1985). The EPR latitude 

21° N. vent fluid data are not reproduced by the 220°C basalt calculations (fig. 12.6A) or by either of the mud calculations (fig. 12.6B, D). In contrast, the oxygen, 

hydrogen, and sulfur isotope data from Escanaba Trough 

(~o 18 0H o = + 0.60; ~0DH o = -0.1; o34SH2s = +8) are 

best reproduced by the simulated reactions with mud at 

much higher values ofW/R (approximately 2 to 3 at 220°C, 

5 to 6 at 300°C) (fig. 12.6B, D). These calculations do not 

rule out the possibility that Escanaba Trough vent fluids 

flowed through basalt within or below the sediment pile, 

but they do indicate that basalt reactions are not dominant 

and that mud reactions alone are sufficient to account for 

the fluid oxygen, hydrogen, and sulfur isotopic composition. 

The Escanaba Trough isotope data are slightly better 

reproduced by calculations at 300°C than at 220°C, while 

silica and alkali geothermometers indicate that the 

Escanaba Trough vent fluid may have equilibrated with altered sediment at temperatures significantly less than 

none -1.3 -0.9 

+0.1 +4.8 +9.7 

300°C. The temperature sensitivity of the isotope reaction 

path calculations is limited by large uncertainties in the input parameters and the simplicity of the isothermal closedsystem model, while confidence in temperature estimates 

from mineral equilibria is limited by the lack of information on hydrothermal assemblages in the subsurface. The 

calculated chemical and isotopic compositions of reacted 

220°C and 300°C seawater at the observed o18QH2o value 

( +0.43) (table 12.3) bracket the composition of the 217°C 

Escanaba Trough vent fluid for many of the measured components (Campbell and others, chap. 11, this volume). The 

aqueous Na/K ratios in the simulations are higher than the 

measured values, reflecting the presence of paragonite (absence of potassium-feldspar) in the calculated assemblages 

(fig. 12. 5B, D). The significant enrichment of chloride in 

the Escanaba Trough vent fluid is not reproduced by simulated reactions with mud, which instead dilute the reacting fluid by 1 percent (300°C, W/R = 5.6) to 2 percent 

(220°C, W/R = 2.8) because of mud dehydration. Hydration of basalt can increase fluid chloride concentrations, but 

the observed Escanaba Trough values would be reached 

only at W/R values too low to be consistent with the 

o18QH2o values. The relative timing of chloride enrichment 
STABLE ISOTOPE STUDY OF HYDROTHERMAL VENTS AT ESCANABA TROUGH 233 

and isotopic modification of the Escanaba Trough vent fluids is not known. W IR values inferred by Campbell and 

others (chap. 11, this volume) from alkali and iodine mass 

balance calculations (W/R = 10 ± 5) are generally higher 

than those indicated by the stable isotope reaction paths 

(except possibly at 300°C); nevertheless, all estimates are 

in agreement with the hypothesis that the Escanaba Trough 

hydrothermal system is relatively seawater-dominated in 

comparison to others such as at EPR latitude 21° N. 

To test some of the uncertainties in the closed system 

calculations, alternative mud reaction paths were simulated 

(fig. 12.6D). One alternative path was generated by assuming that unreacted mud has ODsolid = -50, instead of 

-70. This causes the fluid oDH2o value to be significantly 

less negative at low W/R, but has little effect at high 

W/R. Another alternative path was generated by assuming that quartz in unreacted mud was inert, but leaving the 

bulk o18Q value of the mud unchanged. As quartz composes less than 20 percent of the chosen mud by weight 

(Zierenberg and Shanks, chap. 14, this volume), this assumption has relatively little effect on the calculated fluid 

o18QH2o except at low W /R. Reasonable variations in the 

sulfur content, o34S value, and (or) Fe2+fFet of unreacted 

mud could cause significant shifts in the position of the 

BASALT, 220°C (closed) 

Clinochlore 

Mg-beidellite 

Anhydrite 

Hematite 

Pyrite 

Quartz 

Muscovite 

Paragonite 

Albite 

Epidote 

Actinolite 

Calcite 

Diopside 

Prehnite 

K-Feldspar A 

BASALT, 300°C (closed) 

Clinochlore 

Mg-beidellite 

Anhydrite 

Hematite 

Pyrite 

I I I 

calculated fluid o34SH2s curves, but would not change their 

general configurations. Some of the mineral-water hydrogen isotope fractionation factors used in our calculations 

at 220°C are more negative than those used by Bowers and 

Taylor (1985) by as much as 20 to 30 per mil. Substituting their values would have the effect of lowering the 

220°C calculated fluid ODH2o curves slightly, but would 

not alter our general conclusions. 

For comparison with the closed-system calculations, 

analogous open-system calculations also were performed 

(figs. 12.7 and 12.8). In the open-system calculations, solid 

reaction products were removed from the chemical and isotopic inventories at each reaction step between seawater 

and rock. ODH2o values in simulated open seawater-rock 

systems are almost identical to those in closed systems. 

o18QH2o values in open systems are significantly higher 

than those in closed systems at low W /R, but the differences are insignificant in the vicinity of the observed 

Escanaba Trough values (o18QH2

o = +0.43 at W/R = 3 at 

220°C, 6 at 300°C, for open-system reactions with mud). 

The most dramatic difference between the two sets of calculations is that fluid o34SH2s values approach the 

unreacted rock values much more rapidly in the open systems, such that the observed o34S value of the vent fluid 

MUD, 220 oc (closed) 

~-----------------------------------, 

Clinochlore 

Mg-beidellite 

Pyrite 

Quartz 

Hematite 

Anhydrite 

Muscovite 

Paragonite 

Albite 

Calcite 

Magnetite 

Minnesotaite 

Graphite 

B 

MUD, 300°C (closed) 

Clinochlore 

Mg-beidellite 

Pyrite 

Quartz 

Hematite 

Anhydrite 

Muscovite 

Paragonite 

Albite 

Epidote 

Magnetite 

Actinolite 

Min nesotaite 

Calcite 

Pyrrhotite 

I I I 

Quartz 

Paragonite 

Albite 

Epidote 

Magnetite 

Actinolite 

Tremolite 

Diopside 

Prehnite 

K-feldspar C Graphite D 

~------_.·--------~--------~~------~ ~--------~~--------~--------~------~ 3 2 1 0 -1 3 2 1 0 -1 

+ WATER-TO-ROCK RATIO BY WEIGHT, IN LOG UNITS 

Figure 12.5. Calculated mineral assemblages produced during reactions of seawater with Escanaba Trough basalt and mud, as functions of 

water to rock weight ratios in closed systems at 220°C and 300°C, corresponding to isotope fractionation trends shown in figure 12.6. 
234 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

occurs at much higher W IR ( 18 at 220°C, 20 at 300°C, for 

reactions with mud) than does the observed vent fluid 

o18QH2o value; that is, the observed sulfur and oxygen isotope ratios are not compatible in the open-system models 

(fig. 12.8). 

While it is generally agreed that relatively high 

o34SH2s values indicate relatively large proportions of reduced seawater sulfate in MOR vent fluids, it is debatable 

how and where H2S from reduced sulfate enters the hydrothermal systems (Styrt and others, 1981; Shanks and 

Seyfried, 1987; Janecky and Shanks, 1988; Woodruff and 

Shanks, 1988; Bluth and Ohmoto, 1988; Bowers, 1989). 

Experiments and calculations indicate that most of the 

original dissolved sulfate in recharging heated seawater in 

contact with basalt or sediment may be removed by 

precipitation ofCaS04 (Seyfried and Bischoff, 1979, 1981; 

Bowers, 1989; this study). If CaS04 precipitated during 

+5 ', ......... .... 

... «., 

······•······· ' 0 ................... .. 

-5 

Basalt, 220°C (closed) 

+10 ~------~------~----~,~~-------, 

c d'~ 

%, \ 

' +5 

-5 

Basalt, 300°C (closed) 

-10~----~------~------~------~ 

recharge (or an additional parcel of sulfate-rich seawater) 

is not made available at depth for subsequent reduction, 

then o34SH2s values of circulating heated seawater rapidly 

approach rock values (for example, 0 to 1 per mil for 

basalt) before significant changes occur in o18QH2Q, as in 

our open-system calculations. In this case fluids ascending from deep reaction zones with elevated o18QH2

o values would have low o34SH2s values, and higher observed 

vent fluid o34SH2s values would imply some form of sulfate reduction during discharge (Shanks and Seyfried, 

1987; Woodruff and Shanks, 1988; Bowers, 1989). If previously precipitated CaS04 (or an additional parcel of sulfate-rich seawater) is made available for reduction by hot 

deeply circulating seawater, as modeled crudely by our 

closed-system calculations, then vent fluids with highly 

variable o34SH2s values could be produced at depth without modification at the discharge site (Styrt and others, 

D 

1)18Q(f) \ ............................ ____ ~ \ 

OD(f) ::::::::····· \ 

····-.::~(:···· ..• 

\\,r:;'· •• ~ \ ··.%\~ \ ·.o-·.9' 

Mud, 300°C (closed) \ .._\'~<'J- ' -..s·-.s 

3 2 1 0 -1 3 2 1 0 -1 

+ WATER-TO-ROCK RATIO BY WEIGHT, IN LOG UNITS 

Figure 12.6. Calculated isotope shifts in oxygen, hydrogen, and 

sulfur for seawater reacting with Escanaba Trough basalt and mud, 

as functions of water to rock weight ratios in closed systems at 220°C 

and 300°C, corresponding to mineralogic variations shown in figure 

12.5. Curves were calculated using program CHILLER (Reed, 1982; 

Spycher and Reed, 1989) for geochemical reaction path simulation 

(see fig. 12.5) and program ISO (J.K. Bohlke, unpublished code) 

for corresponding isotopic fractionation; starting compositions of 

seawater, mud, and basalt are listed in table 12.3. Curves labeled (f) 

are for bulk fluid phase; curves labeled (r) are for bulk solid reaction 

products. Heavy bars on curves indicate approximate intercepts of 

calculated curves with observed Escanaba Trough fluid values, which 

are best reproduced for all three isotope systems by mud reactions 

between 220°C (W/R = 2 to 3) and 300°C (W/R = 5 to 6) (see table 

12.3). Observed isotope shifts for East Pacific Rise, latitude 21 oN., 

vent fluids (~o 18 0H o = + 1.6 to +2.0; ~0DH o = +2.5; o34SH2

s = 

+ 1 to +6) are nearly matched by 300°C basalt reaction curves at W/ 

R = 0.3 to 0.5. 
STABLE ISOTOPE STUDY OF HYDROTHERMAL VENTS AT ESCANABA TROUGH 235 

1981; Bluth and Ohmoto, 1988). This situation would be 

difficult to distinguish isotopically from the hybrid model 

of Janecky and Shanks (1988), which calls for reduction 

of previously precipitated CaS04 by ascending fluids with 

low o34SH2s in contact with unaltered rock reductants in 

the discharge region below the vents. The good agreement 

between our closed system equilibrium calculations and the 

observed hydrogen, sulfur, and oxygen isotope values at 

Escanaba Trough (and at EPR 21° N., allowing for differences in basalt composition and temperature), may be 

somewhat fortuitous; nevertheless, both our calculations 

and those of Janecky and Shanks (1988) imply that relatively high o34SH2

s values in MOR vent fluid may require 

that seawater sulfate in excess of the amounts available in 

completely open systems is reduced while the fluids are 

in contact with unoxidized rocks (that is, in the subsurface 

before venting). 

SHALLOW SUBSURFACE MIXING? 

Zierenberg and Shanks (chap. 14, this volume) and 

Zierenberg and others (chap. 10, this volume) suggest that 

much of the observed sediment alteration and sulfide mineralization at Escanaba Trough may be the result of shalBASALT, 220°C (open) 

Clinochlore 

Mg-beidellite 1------- . Anhydrite 

Hematite 

Pyrite 

Quartz 

Albite 

Andradite 

Epidote 

Actinolite 

Calcite 

Prehnite 

K-Feldspar A 

BASALT, 300 oC (open) 

Clinochlore 

Mg-beidellite 

Anhydrite 

Hematite 

Pyrite 

Quartz 

Albite 

Epidote 

Diopside 

Actinolite 

Prehnite 

K-feldspar c 

I I I 

low subsurface circulation of heated seawater, possibly 

mixing with ascending hotter hydrothermal fluids. Similar processes could account for some of the observed characteristics of the hydrothermal vent fluids, such as 

relatively low temperatures, oxygen and hydrogen isotope 

ratios relatively close to those of seawater, and o18QH2

0 

and temperature consistent with those required to produce 

magnesium-enriched chlorites (Zierenberg and Shanks, 

chap. 14, this volume). In the limiting case where no separate ascending hot fluid reservoir is involved, this hypothesis would correspond roughly to the assumptions made in 

the reaction-path calculations described above. If subsurface mixing of hydrothermal fluids from depth with shallower-circulating seawater in contact with sediment has 

obscured many of the characteristics of a hotter deeper 

fluid reservoir, then more work on altered sediment, hot 

pore waters, and vent fluids will be needed to document 

these processes. 

CONCLUSIONS 

Isotope ratios of oxygen, hydrogen, and sulfur in hydrothermal vent fluids at Escanaba Trough are closer to 

seawater values than those of most other MOR vents, esMUD, 220°C (open) 

Clinochlore 

Mg-beidellite 

Pyrite 

Quartz 

Hematite 

Anhydrite 

Muscovite 

Calcite 

Albite 

Annite 

Minnesotaite 

Graphite 

Paragonite B 

MUD, 300 oC (open) 

Clinochlore 

Mg-beidellite 

Pyrite 

Quartz 

Hematite 

Anhydrite 

Muscovite 

Albite 

Annite 

Minnesotaite 

Actinolite 

Calcite 

Pyrrhotite 

Graphite 

Paragonite D 

I I 

I I ~------~~--------~--------~------~ 3 2 1 0 -1 3 2 1 0 -1 

+ WATER-TO-ROCK RATIO BY WEIGHT, IN LOG UNITS 

Figure 12.7. Calculated mineral assemblages produced during reactions of seawater with Escanaba Trough basalt and mud, as functions of 

water-to-rock ratios in open systems at 220°C and 300°C, corresponding to isotope fractionation trends shown in figure 12.8. 
236 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

pecially those at latitude 21° N., EPR. In contrast to the 

hydrothermal system at EPR 21° N., the Escanaba Trough 

system occupies a sedimented rift valley, and hydrothermal fluids currently vent at lower temperatures. 

Isothermal reaction-path calculations indicate significant contrasts in isotope fractionation trends between seawater reacting with basalt and with typical Escanaba 

Trough mud. At comparable temperature and cumulative 

water to rock mass ratio (W /R), reactions with sediment 

can increase ~)18QH o more strongly, and decrease ODH2o, 

in contrast to basalt reactions that increase ()18QH2o and 

ODH2o at roughly comparable rates. The calculations predict potentially large variability in fluid ()34SH2

s, negative 

correlations between fluid ()34SH2s and ()18QH2o, and potentially significant negative fluid ODH2o values from reactions with sediments. Considered together, the ()18QH2

o, 

ODH2o, and ()34SH2s values of vent fluids at Escanaba 

Trough are consistent with seawater-sediment reactions at 

temperatures between 220°C and 300°C at W /R of about 

2 to 6 (closed system). This is in contrast to the apparent 

....:l 

::E -

~ 

~ 

0.. 

~ 

::J 

....:l 

<C 

> 

<C 

E--

....:l 

~ 

Q 

+5 

0 ................... . 

-5 

Basalt, 220°C (open) 

-10 

+10 \ 

c ~~ f.P, 

+5 S\ \ 

-5 

Basalt, 300°C (open) 

.. .. ........ 

················· 

-10~----~------~------~----~ 

constraints at EPR 21° N., where observed fluid ()18QH2o, 

ODH2o, and ()34SH2s values are reproduced by calculated 

seawater-basalt reactions at W/R of about 0.3 to 0.4 involving temperatures of 300°C and greater (Bowers and Taylor, 1985; Bowers, 1989). The calculations do not rule out 

the possibility of seawater-basalt reactions at higher 

temperatures at depth beneath Escanaba Trough, but they 

do indicate that seawater-sediment reactions alone at high 

W /R and temperatures less than 300°C are sufficient to account for the observed isotopic composition of the 217°C 

vent fluid. 

The modern vent fluid temperature and ()18QH2o may 

be similar to those of fluids responsible for chloritization 

of mud, a process thought to reflect near-surface circulation of magnesium-bearing seawater (Zierenberg and 

Shanks, chap. 14, this volume). It is possible that the high 

calculated W/R values for seawater-sediment isotopic 

equilibration at Escanaba Trough also indicate near-surface 

phenomena; however, a direct genetic link between 

chloritites and active vents has not yet been established. 

B 

ol8Q(r) 

8tso(f) 

I 

~I 

.j>.l 

(/)I 

"""'I ,:,I 

·····················-·m~····· .. ····· : ........ ":::.:::::::::··· .. . 

I •••• ••• ;:;.\t.:;. I \~\~ I "•;'<;;>-\9' 

Mud, 220°C (open) \.'':···.''. 

: \.~·-.. ~ 

D 

ol8Q(r) 

I 

I 

8tso<o , 

··················-·-·-- oD(~···•IIIf::····· 

\ ····.:.~:.::::::::::::··;;, •• r1 \%\% I \9- \9' 

Mud, 300°C (open) ··-...~'}.~'~ 

\S\S 

3 2 1 0 -1 3 2 1 0 -1 

+ WATER-TO-ROCK RATIO BY WEIGHT, IN LOG UNITS 

Figure 12.8. Calculated isotope shifts in oxygen, hydrogen, and 

sulfur for seawater reacting with Escanaba Trough basalt and mud, 

as functions of water-to-rock weight ratios in open systems at 220°C 

and 300°C, corresponding to mineralogic variations shown in figure 

12.7. Calculations and representations are similar to those of figure 

12.6, except that solid reaction products were removed from the 

open systems after each reaction step, and curves labeled (r) refer to 

the isotope compositions of only those solids produced within each 

reaction step. Escanaba Trough vent fluid sulfur and oxygen isotope 

values are not compatible in the open system simulations. 
STABLE ISOTOPE STUDY OF HYDROTHERMAL VENTS AT ESCANABA TROUGH 237 

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STABLE ISOTOPE STUDY OF HYDROTHERMAL VENTS AT ESCANABA TROUGH 239 

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922-929. 

Chapter 13. Evidence for Hydrothermal Fluid Flow Through 

Surficial Sediments, Escanaba Trough 

By Andrew J. Magenheim and J oris M. Gieskes 1 

CONTENTS 

Page 

Abstract ......... ..... ... ... . ....... ...... ... . ...... . ... . . ... ... . ... . ... . ... . ... ..... . .. 241 

Introduction . .... ..... .... ........... ... .... .. ... . ... . ... .... . ... . ... . ... .... . .... .. .. 241 

Acknowledgments....................................................... 242 

Methods................................................................................ 242 

Shipboard sampling ..................................................... 242 

Analytical methods...................................................... 242 

Interstitial waters .............. ................ ................... 242 

Bulk solids, major elements ................................ 243 

Bulk solids, minor elements .... .................... ...... .. 243 

Sites and sample description ............................................... 243 

Results and discussion .... ............. .... ..... .. .......... ....... .... .... .... 244 

Interstitial waters ......................................................... 244 

Bulk sediment chemical composition......................... 244 

Major elements ................................. ................... 244 

Hydrothermal calcite ........................................... 246 

Inorganic trace constituents ................................ 248 

Organic carbon and related elements.................. 249 

General discussion .. ........... ....... ..... . ... . ... . ... . ... . ... . ... . ... . .... .. .. . 249 

Hydrothermal calcite ................................................... 249 

Hydrothermal precipitates........................................... 252 

Alteration of the sediment........................................... 252 

Thermal alteration of organic matter .......................... 253 

Summary .............................................................................. 253 

References cited................................................................... 253 

ABSTRACT 

During DSV Alvin dives in 1988 three push cores 

were obtained in Escanaba Trough. Two of these cores 

were obtained in the vicinity of a hydrothermal vent. Interstitial water analyses suggest that the sediment sampled 

is not presently exposed to a hydrothermal fluid flux, but 

analysis of the solid phases indicates that at an earlier 

time sediment has undergone hydrothermal alteration. 

This alteration is evident from the presence of high-temperature calcite (-220°C) at a depth of 5 em from the surface, of sulfides associated with hydrothermally derived 

tar, and of large changes in major elements (Mg, Fe, S) 

1 Scripps Institution of Oceanography, La Jolla, California. 

and minor constituents (Ba, Sr, Cu, Zn) of the sedimentary phases. Hydrothermal sulfide minerals deposited in 

the sediment are presently undergoing oxidative weathering reactions. 

INTRODUCTION 

Hydrothermal activity in sediment-covered ridge systems (Escanaba Trough, Gorda Ridge, and Middle Valley, 

Juan de Fuca Ridge) and in marginal seas (Guaymas 

Basin, Gulf of California, and the Red Sea) is of special 

geochemical interest, because, as a result of hydrothermal 

fluid flow through the sediment, alteration of both inorganic and organic phases is enhanced. Hydrothermal fluids emanating from the sea floor are distinctly different in 

composition from those sampled from open ocean nonsediment-covered systems (Von Damm and others, 1985a, 

b; Von Damm, 1991). This is especially evident from 

higher pH values, lower trace-metal contents, and high 

concentrations of biogenic constituents such as ammonia 

and iodide (Von Damm and others, 1985b; Gieskes and 

others, 1988; Campbell and others, chap. 11, this volume). 

In sediment-covered systems, hydrothermal products are 

often deposited close to the sediment-water interface as a 

result of transport of inorganic and organic materials by 

diffuse fluid migration through the sediment. Especially 

in Guaymas Basin, these processes have been documented 

in an active mode over a variety of depth scales (for example, Gieskes and others, 1982, 1988; Simoneit and 

Lonsdale, 1982; Simoneit, 1983, 1984, 1985a; Stout and 

Campbell, 1983; Kawka and Simoneit, 1987; Bazylinski 

and others, 1988; Magenheim, 1989). In addition, retrograde reactions, for instance, redissolution of hydrothermal sulfate minerals (A. Sturz and others, unpub. data, 

1990) and the biodegradation and oxidation of petroleum 

products (Simoneit, 1984, 1985b) have been documented 

in Guaymas Basin. Similar studies have revealed such 

retrograde reactions in Escanaba Trough (Zierenberg and 

others, 1986; Kvenvolden and Simoneit, 1990). Sulfide 

samples recovered in the Escanaba Trough consist both of 

older deposits, partially buried in sediment and characterized by surface coatings of iron oxides and barite, as well 

241 
242 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

as fresher deposits, which are typically capped by chimneys of barite and anhydrite. Fluids discharging from 

these chimneys have temperatures of up to 217°C in 

Escanaba Trough (Zierenberg and others, 1988; Campbell 

and others, chap. 11, this volume). 

This paper presents evidence for previous hydrothermal fluid flow through the sediment of Escanaba Trough. 

This has led to the deposition of hydrothermal products just 

below the sediment-seawater interface, which presently are 

undergoing retrograde reactions. Materials for these observations were collected by DSV Alvin during dives in the 

summer of 1988 in the vicinity of an active hydrothermal 

vent(fig.13.1). 

ACKNOWLEDGMENTS 

We are grateful for the reviews of this manuscript by 

Andrew C. Campbell, Keith Kvenvolden, and Robert 

Zierenberg. Part of this work was supported by a grant 

from the National Science Foundation, NSF OCE 86-

13517. We also acknowledge the efforts of the crews of 

RN Atlantis II and of DSV Alvin. We thank Jim Leather 

for the isotope information. 

12JC32' 

41°00' 

56,000 

METHODS 

SHIPBOARD SAMPLING 

DSV Alvin push cores from 6 to 20 em long were 

subsampled immediately upon arriving on deck. An apparatus designed for sectioning the cores in a nitrogen atmosphere (Shaw, 1988, 1989) was used for processing all 

samples. Samples were sliced with clean plastic sheets and 

transferred into precleaned centrifuge tubes, then centrifuged and returned to the nitrogen-filled glove bag for processing. The fluids were decanted into precleaned syringes 

and filtered through 0.45 J.Lm millipore filters. The filtered 

solutions were separated into two aliquots. One untreated 

sample was taken for Cl, Mg, and Ca analysis. The remaining portion was acidified with 25 J.LL of 6 M ultrapure HCI. 

The solids were stored refrigerated in the centrifuge tubes. 

ANALYTICAL METHODS 

INTERSTITIAL WATERS 

The pore fluids were analyzed by techniques that have 

been outlined in a number of sources (Gieskes and 

( 56,000 

54,000 

52,000 

62,000 50,000 

Figure 13.1. Bathymetric map of study area in northern Escanaba Trough (depth in meters). Locations 

of cores from this study are shown. Numbers along right and bottom give coordinates of local transponder 

net (in meters). Hachures indicate area of closed low. 
EVIDENCE FOR HYDROTHERMAL FLUID FLOW THROUGH SURFICIAL SEDIMENTS, ESCANABA TROUGH 243 

Peretsman, 1986; Shaw, 1988). Chloride, calcium, and 

magnesium were analyzed by titration as described by 

Gieskes and Peretsman (1986). Silica, ammonium, and nitrate were analyzed by colorimetric techniques (Gieskes 

and Peretsman, 1986). Sulfate was analyzed by nephelometry (R. Jahnke, oral. commun., 1985). Five metals (Mn, 

Sr, V, Ni, and Cu) were analyzed by flameless atomic absorption spectrometry. 

BULK SOLIDS, MAJOR ELEMENTS 

Major-element composition of sediment was analyzed 

after dissolution using a lithium metaborate fusion technique (Brown, 1988, and references therein). Colorimetric techniques were used to analyze the solutions whenever 

possible (Fe, Ti, Si, P, and AI). Chloride was determined 

by leaching 20 mg of dried sediment in deionized water, 

filtering the sediment, and measuring Cl in the solution using ion chromatography. Other elements (Mg, Ca, K, Mn, 

and Na) were run using flame atomic absorption techniques. A detailed description of this procedure and subsequent analysis is given in Brown (1988). 

A Perkin and Elmer CHN elemental analyzer was used 

for total carbon and nitrogen determinations. Carbonate 

carbon analysis was performed by dissolving the sample 

in 1M HCl and analyzing the C02 evolved using a coulometric technique. Organic carbon was determined by difference between the total carbon and inorganic carbon. 

Some carbonate samples were dissolved in 50°C phosphoric acid (as were calcite standards), and carbon and oxygen isotopes of the C02 evolved were measured by 

standard mass spectrometric techniques. 

BULK SOLIDS, MINOR ELEMENTS 

Minor elements were analyzed using X-ray fluorescence spectroscopy following methods developed by 

Shimmield (1984). Great care was taken to make appropriate matrix corrections. All data have been corrected for 

salts and pore water contributions. Details of these corrections and standardization procedures are given in 

Magenheim (1989). 

SITE AND SAMPLE DESCRIPTION 

Three DSV Alvin push cores were obtained. Two of 

these cores (2040 AC-1 and 2040 AC-2) were located 

within 10m of an active hydrothermal vent (108°C) in the 

northern Escanaba Trough (NESCA) area (fig. 13.1). The 

other core (2035 AC-1) was sampled at the top of a flattopped sediment hill in the NESCA area (fig. 13.1). Visual observations made while extruding the two cores 

Table 13.1. Descriptions of Escanaba Trough surface cores. 

[Descriptions are based on visual observations on board and in the 

laboratory with an optical microscope. Calcite was identified below 4 

em by XRD; Fe, Cu, and Zn sulfide minerals were tentatively identified 

by XRD in 5-6 em section and in the tar mat] 

Interval (em) 

0-1.5 

1.5-3.0 

3.0-4.5 

6.0-7.5 

9.0-10.5 

12.0-13.5 

15.0-16.5 

17.0-18.5 

0-1 

1-2 

2-3 

3-4 

4-5 

5-6 

6-7 

7-8 

8-9 

9-10 

11-12 

0-1 

2-3 

3-4 

4-5 

5-6 

core catch 

General Description 

Core 2035 AC-1 

Brown, homogeneous 

Brown, homogeneous 

Lighter brown, homogeneous 

Brown, homogeneous, brassy flakes 

Brown, homogeneous, brassy flakes 

Brown, homogeneous, brassy flakes 

Brown, darker layer observed while sectioning, 

homogeneous, brassy flakes 

Brown, homogeneous, brassy flakes 

Core 2040 AC-1 

Brown, fine, homogeneous 

Brown, fine, homogeneous 

Darker brown-gray, brassy sheet-like flakes, 

black specks (bitumen?) 

Gray-brown, brassy sheets, some white crystals 

(calcite), black specks 

Gray-brown, white crystals more abundant and 

larger, brassy flakes, more black specks 

Darker gray, many white crystals, tar, black 

specks 

Gray, approximately 113 to 1/2 white to 

yellowish crystals with tar stuck to them 

(many are millimeter size), brassy flakes, 

black specks 

Gray, fewer crystals, brassy flakes, tar, and 

more black specks 

Darker gray, fewer crystals, brassy flakes, tar, 

small black specks 

Dark gray, white crystals, brassy flakes, tar, 

black specks 

Dark gray, white crystals, brassy flakes, tar, 

black specks 

Core 2040 AC-2 

Homogeneous, brown, brassy flakes 

Homogeneous, brown, brassy flakes, some 

small black grains 

Homogeneous, brown-gray, brassy flakes, small 

black grains 

Homogeneous, darker gray-brown, more black 

grains 

Gray, white crystals, tar, hard black glassy 

grains, small black grains 

Gray sediment, more white crystals, more tar, 

hard black glassy grains, small black crystals 

with white and yellow crusts. Tar was 

slightly malleable and faintly odorous 

collected at the hydrothermal site indicated evidence for 

hydrothermal alteration of the sediment (table 13.1). Core 

2040 AC-1 had a layer of large (about 1 mm) white crystals 

between 6 and 8 em depth. These crystals were examined 
244 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

under the microscope and appeared to be well-developed 

rhombs with a large amount of tar stuck to them. The morphology and veinlike occurrence of this deposit in the sediment indicate that it is authigenic. X-ray diffraction (XRD) 

analysis indicates that these crystals consist of calcite. The 

lower part of this core was significantly darker than the top. 

Core 2040 AC-2 had similar calcite crystals, although less 

abundant, below 4 em. The bottom sediment interval was 

considerably darker than the surface sediment. A piece of 

matlike tar was picked out of the core catcher, which appeared to contain many sand-size sulfide particles. XRD 

analysis tentatively confirmed the presence of marcasite, 

chalcopyrite, cubanite, barite, and calcite (and possibly 

pyrrhotite) in this tar mat. These minerals were also tentatively identified in the intervals below 4.5 em in this core. 

Core 2035 AC-1, taken in an area without sulfide mineralization, was basically uniform in appearance, except for 

a dark interval between 11 and 12 em. This core is used 

as a reference and is assumed to be representative of "average" sediment in this region. 

RESULTS AND DISCUSSION 

INTERSTITIAL WATERS 

Interstitial water profiles for the three cores from 

Escanaba Trough, presented in table 13.2 and figure 13.2, 

appear to be unaffected by present hydrothermal activity. 

All the Cl- profiles appear to be constant, with perhaps 

slightly higher concentrations in 2040 AC-2 (fig. 13.2A). 

The Mg2+ and Ca2+ profiles show no gradients and show 

seawater values (fig. 13.2B, C). The profiles of Sr2+ (fig. 

13.2/) show seawater conditions for both cores 2040 AC1 and 2040 AC-2, indicating no hydrothermal contribution. 

Silicon as orthosilicic acid (H4Si04) appears to increase with depth in all the push cores (fig. 13.2D). The 

profiles from the hydrothermal vent vicinity show a deficit when compared with 2035 AC-1. Past mobilization of 

silicon from the solids in 2040 AC-1 and 2040 AC-2 may 

have removed or altered the source of readily soluble silica 

(for example, radiolarian tests), which would ordinarily be 

present. 

Sulfate appears to be constant throughout cores 2040 

AC-1 and 2035 AC-1 at seawater values (fig. 13.2£). The 

data in core 2040 AC-2 indicate a slight decrease with 

depth, suggesting some bacterial sulfate reduction at greater 

depths. 

Nitrate profiles for all three cores show a decrease in 

N03- with depth to approximately 7 em (fig. 13.2F). This 

decrease suggests that denitrification is active, and 

conditions above about 7 em are suboxic. Some mobilization of NH4 +is observed in core 2035 AC-1 (fig. 13.2G) 

starting at the depth where N03- goes to zero. No increases in NH4 + are apparent in either of the other cores. 

The manganese profiles clearly show the mobilization 

of Mn for core 2035 AC-1 (fig. 13.2H), starting at 5 em. 

Small increases in Mn are observed for cores 2040 AC-1 

and 2040 AC-2. It appears that past hydrothermal fluid 

flow may have diminished the surface sediment manganese 

oxide concentrations. 

The profiles of vanadium (fig. 13.21) show surface 

maxima for 2035 AC-1 and 2040 AC-1. Core 2040 AC-2 

shows an apparent increase in vanadium with depth. This 

increase may be related to bacterial decay of the tar mat 

found at the bottom of the core. Nickel appears to have a 

subsurface maximum for all three cores (fig. 13.2K). Copper (fig. 13.2L) shows enrichment at the surface, with 2040 

AC-1 and 2040 AC-2 showing enrichments to greater 

depths than 2035 AC-1. 

Taken together, the interstitial water profiles suggest 

that the major process affecting the pore-water chemistry 

is the early diagenesis of organic matter, leading to suboxic 

conditions in those levels that show the presence of hydrothermal precipitates. 

BULK SEDIMENT CHEMICAL COMPOSITION 

The chemical composition of the hydrothermally altered sediment is influenced by a number of overlapping 

and competing processes (Magenheim, 1989): (1) precipitation of hydrothermal minerals, (2) alteration and dilution 

by hydrothermal precipitates of the pre-existing sediment 

matrix, and (3) bacterial diagenesis of organic matter. 

These processes can affect both major constituents and minor constituents of the sediment, as will be shown below. 

Data for major elements are presented in table 13.3 and for 

minor elements in table 13.4. 

MAJOR ELEMENTS 

To represent the data for the purpose of comparison, 

we have normalized the data to the titanium content. The 

latter component is of low mobility and is at the same time 

a good marker of inorganic detrital input. Titanium-ratio 

profiles for all cores are shown in figure 13.3. 

To evaluate the contribution of hydrothermal processes to the composition of cores 2040 AC-1 and 2040 

AC-2, it is first necessary to examine the trends with depth 

in the reference core 2035 AC-1. In the latter core titanium shows a slight increase with depth with an average 

concentration of 93 Jlmollg. A slight decrease in the molar 

Al/Ti ratio is observed for 2035 AC-1 from 35 at the surface to 32 at 19 em. The average value, Al/Ti = 32.7, for 

this core is close to the average value of terrigenous matter, Al/Ti = 34.1 (Bostrom and others, 1976). The Si/Ti 

profile is somewhat noisy, but the average value of Si/Ti, 

102, is comparable to that of average terrigenous matter 
EVIDENCE FOR HYDROTHERMAL FLUID FLOW THROUGH SURFICIAL SEDIMENTS, ESCANABA TROUGH 245 

Table 13.2. Escanaba Trough interstitial fluid compositions. 

[-, not determined] 

Depth Cl Mg Ca H4Si04 S04 NH4 N03 Mn Sr v Ni Cu 

(em) (mM) (mM) (mM) (J.LM) (mM) (J.LM) (J.LM) (J.LM) (J.LM) (nM) (nM) (nM) 

Core 2040 AC-1 

0.5 563 54 9.8 180 31 2.0 51 0 90 234 313 205 

1.5 567 55 9.8 229 30 1.2 42 0 84 196 72 224 

2.5 565 55 9.7 256 31 .9 43 0 84 159 32 206 

3.5 563 54 9.5 265 30 0 27 0 84 51 32 205 

4.5 569 53 9.9 301 31 0 23 0 88 42 43 202 

5.5 566 54 9.9 292 30 0 12 2 84 52 163 19 

6.5 551 53 9.7 265 30 0 11 14 84 38 214 84 

7.5 553 53 9.8 270 26 0 7 24 92 33 240 83 

8.5 573 54 9.8 292 29 0 9 29 84 47 445 52 

9.5 568 58 10.1 292 31 0 13 32 92 70 492 56 

10.5 58 10.4 292 33 1.6 35 97 80 602 58 

12.5 297 0 27 92 75 124 28 

Core 2040 AC-2 

0.5 566 54 9.9 270 26 5 40 0 89 121 0 169 

1.5 588 56 10.4 310 29 3 31 0 93 116 0 346 

2.5 514 52 10.1 246 26 0 18 0 89 74 0 197 

3.5 582 52 10.2 259 27 0 9 .3 89 53 60 269 

4.5 588 55 10.4 324 26 0 9 2 89 146 268 185 

5.5 592 57 10.2 315 22 0 6 18 89 218 316 169 

7.0 597 56 10.5 265 22 3 16 89 239 219 140 

Core 2035 AC-1 

0.75 561 54 9.4 412 30 30 

2.25 564 51 9.3 430 28 2 

3.75 553 52 9.3 434 30 2 

5.25 561 52 9.3 399 29 0 

6.75 562 52 9.0 412 29 0 

8.25 562 54 9.1 425 29 5 

9.75 571 53 9.0 403 29 2 

11.25 550 52 9.0 465 28 6 

12.75 561 54 9.2 452 29 11 

14.25 550 52 9.0 456 27 14 

15.75 553 50 8.8 495 28 20 

17.25 568 51 8.9 509 29 24 

18.75 561 51 8.8 517 30 27 

20.25 571 52 8.9 482 30 37 

(Si/Ti = 110, Bostrom and others, 1976). The K/Ti ratio 

for 2035 AC-1 appears relatively constant with an average 

value of 5.8. Both Fe/Ti and Mg/Ti appear to decrease with 

depth in 2035 AC-1. The decrease in Fe/Ti may be due to 

the association of iron with manganates in the surface sediments. A maximum in manganese is observed at 3.75 em 

(table 13.3). 

In core 2040 AC-1, authigenic calcite dilutes the preexisting sediment matrix in the intervals between 5 and 10 

em. The titanium profile (fig. 13.3A) shows this quite 

clearly. The titanium concentration drops to one-third of 

its previous value in the calcite-rich interval between 6 and 

7 em. In spite of this large dilution in titanium, only a 

0 730 344 

0 128 145 85 

13 2 67 21 77 

11 10 82 8 90 

3 38 97 649 117 

1 68 267 560 92 

4 84 143 26 

3 105 214 38 

2 123 128 214 65 

2 127 10 155 36 

3 156 10 122 27 

5 178 10 11 

5 179 17 

3 186 

slight increase in Al/Ti (fig. 13.3B) is observed in this interval. Virtually no changes are noticed in the K/Ti (fig. 

13.3£) and Si/Ti (fig. 13.3C) ratios in this interval. This 

lack of change suggests that the precipitation of calcite diluted the pre-existing sediment by almost two-thirds. 

The observed trends in Fe/Ti and Mg/Ti (fig. 13.3F, 

G) are similar for both hydrothermally affected cores. Between 2.5 and 7.5 em, an enrichment in both Mg/Ti and 

Fe/Ti over values from 2035 AC-1 is observed in core 2040 

AC-1. Below this layer deficits (relative to 2035 AC-1) 

in both of these elements are observed. While 2040 AC-2 

does not extend as deep, the enrichment in both Fe/Ti and 

Mg/Ti is apparent to a depth of 7.5 em. 
246 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

0 ,. ' ~ • ... A . 8 c D • ... .... • ... ... • .. . ... • • .. ... •• p . • • ... .... ~ • .... 5 ... ... . . ... .. .... • 

•• ... f#J ... .. .... ' • • • •• • .. • • .. 10 • • • • • • • • • • • • • 

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• • • • 15 r- - - • • • • 

• • • • 

• • • • 

20 

450 500 550 600 650 30 40 50 60 0 5 10 15 20 0 200 400 600 

CJ mM MgmM CamM Si(OH)4 J,JM 

0 ..... - .... • . ... ' I 

C/) E F • G H 

a: .... ... • .... I 

w .... . ... • ... I- • 

w .... .... • • •• ~ ... • ... • ~ i-... i= 5 ... • - .... z ... fiJ 

w • • ... t • 

() • • • I ~ • • • z • 

•• • • ~ • • u.i 10 • • • 

a: • • • • 

0 • • ~ () • • • 

z • • • • 

I 15 

I- • • • • 

a. 

w • • • • 

0 • • • • 

20 . 0 10 20 30 40 0 20 40 60 0 10 20 30 40 0 100 200 

S04mM N03J,JM NH4JJM MnJJM 

0 • J ... • • I • K L ... • • .... ... . I • • ... .... ..... I~ • • ... • .... ' .... • • ... • • ... •• ... .... 5 .... .. ... • ... • ... • 

\ •• ... '· • • • 

• . ... 

• • • • • • • 

• • • . • • r-•. . 10 • • • • • • 

• •• .. • • 

• • • 15 

EXPLANATION • • • Core 2035 AC-1 • • 

• Core 

Core 

2040 

2040 

AC-1 

AC-2 -

• 

20 I . 0 100 200 0 400 800 0 400 800 0 200 400 

Sr J,JM VnM NinM CunM 

Figure 13.2. Interstitial water composition of cores 2035 AC-1 (dots), 2040 AC-1 (squares), and 2040 AC-2 (triangles). 
EVIDENCE FOR HYDROTHERMAL FLUID FLOW THROUGH SURFICIAL SEDIMENTS, ESCANABA TROUGH 247 

Table 13.3. Major-element composition of Escanaba Trough solids. 

[Weight percent oxides, salt free] 

Depth SiOz TiOz AI2o 3 Fe MnO MgO CaO NazO KzO P2os c c N Total 

(em) inorg org 

Core 2040 AC-1 

0.5 54.88 0.75 15.05 6.15 0.99 4.14 1.80 2.37 2.33 0.24 0.07 1.26 0.12 90.16 

1.5 55.79 .75 15.31 5.89 .98 3.96 3.47 2.19 2.37 .24 .06 1.30 .16 92.47 

2.5 53.07 .75 15.53 6.35 .92 4.27 3.44 2.83 2.46 .24 .06 1.46 .16 91.54 

3.5 56.22 .76 15.73 6.86 .73 4.09 3.32 2.68 2.46 .25 .09 1.20 .15 94.54 

4.5 54.76 .74 14.38 6.66 .48 4.02 2.31 2.92 2.31 .27 .09 1.22 .12 90.28 

5.5 41.55 .59 11.41 5.67 1.46 3.21 14.23 2.17 5.88 .23 3.37 1.97 .09 91.85 

6.5 18.04 .24 5.52 2.80 4.05 1.36 39.23 .90 .83 .10 7.02 1.66 .05 81.79 

7.5 41.41 .51 10.50 3.91 1.98 2.25 17.71 2.15 1.83 .15 2.92 1.68 .08 87.09 

8.5 54.93 .66 13.06 4.14 .76 2.61 8.34 2.99 2.40 .16 1.36 2.25 .09 93.76 

9.5 63.11 .76 15.30 3.11 .26 2.42 3.55 2.56 2.73 .18 .41 2.30 .10 96.78 

10.5 63.51 .76 14.51 2.94 .20 2.56 3.43 4.40 2.74 .16 .23 1.34 .06 96.86 

12.5 63.92 .74 14.67 3.51 .27 2.78 5.47 2.82 2.66 .26 .46 1.35 .07 98.98 

Core 2040 AC-2 

0.5 61.96 0.71 15.14 5.91 1.09 3.91 1.93 2.31 2.32 0.16 0.09 1.32 0.17 97.01 

1.5 45.29 .68 16.45 6.24 1.14 3.99 1.70 2.52 2.78 .17 .08 1.42 .17 82.63 

2.5 47.53 .70 14.81 6.57 1.01 4.00 2.11 1.86 2.41 .18 .08 1.36 .17 82.80 

3.5 49.53 .68 16.50 7.47 .51 3.97 1.72 2.67 2.40 .18 .08 1.39 .17 87.27 

4.5 49.77 .69 15.26 6.63 .26 3.96 2.10 2.37 2.43 .14 .10 1.71 .16 85.58 

5.5 41.64 .65 12.90 4.40 .63 0 5.51 2.18 2.15 .12 .44 5.62 .15 76.41 

7 49.76 .78 15.85 5.70 .94 3.82 9.23 2.65 2.72 .11 .85 10.20 .14 102.7 

Core 2035 AC-1 

0.75 65.89 0.71 15.94 6.06 1.73 4.09 1.79 

2.25 49.85 .70 15.56 5.89 2.48 4.02 1.55 

3.75 51.97 .76 14.74 5.77 2.97 3.96 2.24 

6.75 45.90 .72 14.84 5.28 .31 3.49 2.01 

9.75 64.14 .75 15.22 4.39 .08 3.46 2.35 

12.75 52.84 .76 16.41 4.66 .13 3.80 2.01 

15.75 58.24 .76 14.72 4.05 .10 3.28 2.58 

18.75 65.83 .78 15.97 4.72 .11 3.72 2.34 

HYDROTHERMAL CALCITE 

The presence of a calcite layer in the 6 to 7 em interval of core 2040 AC-1 has been mentioned previously. 

Calcite is present as well-crystallized rhombs on the order 

of 0.5 to 2 mm in size. Calcite was also found below 4 

em in core 2040 AC-2. The background core 2035 AC-1 

shows relatively little inorganic carbon, indicating that biogenic carbonate is not an important component of the sediment. Slightly greater amounts of calcium than inorganic 

carbon in the calcite vein indicate that not all calcium is 

associated with calcite, some probably being contributed 

by clays. Manganese and chromium both increase considerably in the calcite layer of 2040 AC-1. 

In core 2040 AC-2 calcite crystals are observed below 

4 em and are more concentrated in the deepest interval and 

the tar mat. Increases in the concentrations of manganese 

and chromium are observed in the bottom of 2040 AC-2 

in association with the increases in calcium and inorganic 

2.12 2.48 0.19 0.06 1.22 0.14 102.4 

2.31 2.53 .19 .05 1.22 .15 86.52 

2.38 2.33 .19 .05 .88 .12 88.37 

2.45 2.51 .23 .04 .70 .08 78.57 

2.75 2.58 .15 .10 .40 .03 96.40 

2.52 2.69 .15 .05 .97 .09 87.08 

2.79 2.47 .14 .11 .75 .04 90.03 

2.72 2.66 .14 .07 .42 .03 99.52 

carbon. The strong similarity between the distributions of 

manganese and chromium in the calcite layers suggests that 

these elements are trapped in the calcite or an associated 

phase. Though manganese is enriched in hydrothermal fluids, the source of chromium is less certain. We submit that 

chromium becomes mobilized from the surrounding sediment through which the highly reducing hydrothermal fluids have percolated and is subsequently adsorbed in the 

calcite layer at the time of calcite precipitation. 

The results of carbon and oxygen isotope measurements on calcite are given in table 13.5. One sample was 

taken from the calcite vein of 2040 AC-1 and one from the 

7.5 em interval of 2040 AC-2. The sample from 2040 AC1 consisted of almost pure crushed calcite crystals, and the 

sample from 2040 AC-2 was taken from the bulk sediment. 

This was done because individual crystals of calcite were 

not common enough to separate in this core. The oxygen 

isotopic results show very similar compositions for the two 

samples, as do the carbon isotopic ratios. Using a range 
248 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

Table 13.4. Minor-element compositions of Escanaba Trough solids. 

[Compositions are salt free. All data in parts per million. n.d., not determined] 

Depth Sr Zn Cu Ni Co Cr Ba v Mo Br s 

(em) 

2040AC-1 

0.5 329 230 181 129 21 139 2594 169 30 13 1849 344 

1.5 331 244 214 135 24 199 2795 176 32 5 1615 361 

2.5 293 261 220 132 24 145 2312 203 31 40 1848 466 

3.5 332 271 264 123 25 208 2531 210 29 32 1766 462 

4.5 310 273 247 99 22 156 2356 181 22 16 1664 350 

5.5 350 295 491 81 25 364 2978 169 26 11 2460 278 

6.5 329 321 800 50 20 553 2613 154 18 4 2199 166 

7.5 332 319 644 61 19 297 2940 185 26 n.d. 2675 130 

8.5 350 459 712 65 16 198 2975 185 33 7 4369 96 

9.5 355 466 504 62 12 138 2916 162 26 n.d. 5205 84 

10.5 352 422 372 63 11 70 2208 147 27 n.d. 7347 n.d 

12.5 349 392 361 63 14 209 2343 166 38 n.d. 12,472 n.d 

2040AC-2 

0.5 312 240 241 143 24 189 2821 194 27 8 1983 333 

1.5 265 231 223 134 22 34 2251 176 11 19 2023 402 

2.5 272 239 235 123 22 85 2093 178 18 n.d. 1595 423 

3.5 262 301 362 110 25 116 2031 203 23 7 1887 348 

4.5 312 325 464 119 19 88 2498 184 23 14 2441 302 

5.5 452 1120 1302 85 15 52 4332 183 27 5 10,048 323 

7 772 6371 4802 102 22 393 8668 187 57 n.d. 25,846 607 

2035AC-1 

0.75 267 221 151 165 24 233 

2.25 233 215 127 158 20 87 

3.75 273 183 113 115 19 95 

6.75 256 123 60 58 13 0 

9.75 298 120 32 65 13 79 

12.8 253 154 242 72 13 14 

15.8 292 114 36 60 13 75 

18.8 279 136 49 68 17 142 

of o18o of water between 0 and 2 per mil (relative to 

SMOW) and using the calcite fractionation factors given 

by O'Neil and others (1969), isotopic temperatures of 

208°C to 279°C were calculated. Bohlke and Shanks 

(chap. 12, this volume) report a value of o18o of 0.5 per 

mil for Escanaba Trough hydrothermal fluids, which would 

yield a formation temperature of approximately 220°C for 

the calcite. The carbon isotopes indicate a biogenic origin of the C02, probably from the decomposition of organic matter at depth. 

INORGANIC TRACE CONSTITUENTS 

Five trace elements (Zn, Cu, Sr, S, and Ba) are enriched in cores 2040 AC-1 and 2040 AC-2 relative to 2035 

AC-1 (table 13.4; Magenheim, 1989). This enrichment is 

evident for copper, zinc, and sulfur in and below the calcite layer in 2040 AC-1 and particularly below 5 em in core 

1978 260 38 121 1812 534 

1637 194 44 97 1387 395 

1570 159 40 88 1340 467 

835 114 23 58 1152 143 

720 158 31 45 1886 41 

1056 161 26 96 166 170 

788 178 26 51 3919 n.d. 

779 164 35 34 3839 39 

2040 AC-2, where significant increases in these elements 

occur. Strontium and barium, on the other hand, appear 

to be enriched throughout the two cores. The dark appearance of the lower portions of these two cores is probably 

the result of the presence of sulfide minerals. 

The molar Ti ratios for Ba, Sr, Co, Cu, Zn, and S (fig. 

13.3K-P) all show peaks in the calcite-rich layer of 2040 

AC-1, even though their absolute concentrations are not all 

that different from the values in the surrounding sediments 

(table 13.4; Magenheim, 1989), with the exception of Cu, 

which shows a significant increase in the calcite layer. 

Thus it appears that the calcite has not diluted these elements or, more likely, that these elements have been relatively concentrated in the calcite layer, perhaps associated 

with sulfides and barite. 

Overall increases in Zn/Ti, Cuffi, and Sffi in the lower 

portions of both hydrothermally affected cores suggest that 

these elements are associated with sulfides. Indeed, marcasite (FeS2), chalcopyrite (FeCuS2), cubanite (Fe2CuS3)' 
EVIDENCE FOR HYDROTHERMAL FLUID FLOW THROUGH SURFICIAL SEDIMENTS, ESCANABA TROUGH 249 

and barite (BaS04) have been identified tentatively in 

samples from the tar mat and the 7 em interval of core 2040 

AC-2. The enrichments in Sr and Ba throughout these 

cores suggests that barite may have been precipitated along 

with calcite and base-metal sulfide minerals. 

ORGANIC CARBON AND RELATED ELEMENTS 

Organic matter can preserve a record of present and 

past bacterial and thermal decomposition. The organic carbon concentrations appear to be greatest at the surface in 

core 2035 AC-1, with a slight decrease in abundance with 

depth. At 12.75 em there appears to be an anomalously 

high value (0.97 percent organic carbon) with a decrease 

below this interval. This increased organic carbon may be 

the result of variable deposition of organic matter, and is 

located close to a black layer observed while sectioning the 

core. The C0 r/fi ratio also shows this slightly increased 

organic carbon concentration (fig. 13.3R). However, the 

relatively small decrease in organic carbon with depth and 

the absence of any apparent reduction in sulfate in the pore 

fluid profiles suggest that sulfate reduction is not presently 

active in the upper 20 em of this sediment core. Decomposition of the organic matter in 2035 AC-1 is apparent 

from the N/Ti and 1/Ti profiles, which show significant decreases with depth. 

Similarly a general decrease is observed in the bromine 

concentration (table 13.4), but accuracies of the determinations do not warrant a representation in terms of the titanium ratio. The VITi (fig. 13.3V) profile suggests that 

vanadium is enriched in the surface interval of core 2035 

AC-1. The Ni/Ti (fig. 13.3U) also suggests enrichment in 

the surface interval. Whereas the vanadium enrichment 

may be related to organic carbon, the nickel enrichment is 

more likely associated with manganese oxides. Shaw 

(1988) has shown that nickel, molybdenum, and cobalt are 

associated with manganese-oxides. 

Above 5 em in core 2040 AC-1, organic carbon appears to have approximately the same concentration as 

2035 AC-1 (table 13.5). Below and throughout the calciterich layer, organic carbon is enriched to values between 1.3 

and 2.3 percent. The presence of tar on the calcite crystals suggests that some of this organic carbon is of 

thermogenic origin, supplied to the surface with the migrating hydrothermal fluids. Core 2040 AC-2 shows a relatively constant organic carbon concentration to a depth of 

4.5 em, where it begins to increase toward the bottom. In 

the 7.5 em interval the organic carbon concentration is 10.2 

percent. Profiles of N/Ti and I/Ti (fig. 13.3S, T) for the 

2040 cores show a decrease with depth, notwithstanding 

increases in organic carbon. Bromine concentrations also 

become very small, below our detection limit. 

Because the decay and (or) supply of organic matter 

is not uniform with depth, as indicated in core 2035 AC1, the 1/Corg and N/Corg profiles were plotted (fig. 13.4A, 

B). Core 2035 AC-1 shows a gradual decrease in N/Corg 

and I/Corg' as would be expected in the suboxic conditions 

observed. The more rapid decrease 1/Corg suggests that iodine is relatively more labile than nitrogen during early diagenesis. The profiles of both these ratios in cores 2040 

show more rapid decreases in the ratios with depth, again 

indicating increased diagenesis. Values of N/Corg this low 

are indicative of thermally altered organic matter (Simoneit 

and Bode, 1982), and, in addition, thermogenically derived 

organic carbon may cause a dilution effect. 

The Ni/Ti and V/Ti profiles are shown in figure 13.3U 

and V. The resemblance to the N/Ti and I/Ti profiles suggests that these elements may also be related to the decay 

and alteration of organic matter. For this reason Ni/Corg 

and V/Corg profiles were also examined (fig. 13.4C, D). 

The surface enrichment in nickel could be related to the 

presence of manganese oxyhydroxides (Shaw, 1988). 

Rapid depletion in both vanadium and nickel is apparent 

from the profiles for core 2035 AC-1 and also for core 2040 

AC-2. Core 2040 AC-1 shows a steady decrease in V/Corg 

and Ni/Corg profiles, except in the calcite-rich intervals, 

where an apparent maximum is evident. In part, this may 

be due to the presence of metalloporphyrins, containing these 

elements, associated with the tar on the calcite crystals. 

GENERAL DISCUSSION 

The results from the Escanaba Trough samples clearly 

show that at one time hydrothermal fluids did flow through 

the near-surface sediment. No presently active flow is indicated by the interstitial water compositions. However, 

retrograde reactions of precipitated hydrothermal minerals 

may be indicated. Three lines of evidence support the conclusion that the sediment has been exposed to hydrothermal 

activity in the past: (1) the presence of a high-temperature 

authigenic carbonate, (2) the presence of other hydrothermal precipitates and petroleum condensates, and (3) subtle 

alteration of the sediment. Without age dates on the sediment it is difficult to ascertain when hydrothermal activity ceased in the past, but the first few centimeters of each 

core do not indicate great alteration and may have been 

deposited under normal conditions. 

HYDROTHERMAL CALCITE 

The most striking observation is the authigenic precipitation of calcite within these sediments. Calcite has been 

reported in other hydrothermal situations. Peter and Scott 

(1988) reported high abundances of calcite in actively venting hydrothermal spires in Guaymas Basin. These calcites 

were found to be enriched in iron and manganese, similar 

to the manganese enrichment in the calcite layer of core 
250 

en 

a: 

w 

Iw 

:2 

i= 

z 

w 

0 

z 

w 

a: 

0 

0 

z 

I 

Ia.. 

w 

0 

GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

0 

5 

10 

15 

20 

20 

0 

5 

10 

15 

20 

4 

0 

5 

10 

15 

40 60 80 100 20 

Ti J.Lmole/g 

E 

6 

K/Ti RATIO 

I 

EXPLANATION 

---e-- CORE 2035 AC-1 

--- CORE 2040 AC-1 

__,.___ CORE 2040 AC-2 

8 4 

20 .____......___...~--_ __.__ _ __. 

c 

30 40 50 60 50 100 150 200 0 

AI/Ti Si/Ti RATIO 

8 12 16 6 8 10 12 0 

Fe/Ti RATIO Mg/Ti RATIO 

K 

0 1 00 200 0 1 00 200 300 400 0 200 400 600 800 20 

INORGANIC C/Ti RATIO Cr/Ti RATIO x 1000 Ba/Ti RATIO x 1000 

5 10 15 

Mn/Ti RATIO 

H 

100 200 300 

Ca/Ti RATIO 

60 100 140 

Sr/Ti RATIO x 1 000 

Figure 13.3. Titanium concentrations and molar ratios relative to titanium of major and minor constituents of solid phases cores 2035 AC1 (dots), 2040AC-1 (squares), and 2040AC-2 (triangles). 
EVIDENCE FOR HYDROTHERMAL FLUID FLOW THROUGH SURFICIAL SEDIMENTS, ESCANABA TROUGH 251 

o M N 0 p 

5 

10 

15 

20 

0 4 8 12 0 200 400 600 800 0 250 500 750 1000 0 2 4 6 8 10 

Co!Ti RATIO x 1000 Cu!Ti RATIO x 1 000 Zn!Ti RATIO x 1 000 S!Ti RATIO 

(/) 0 R a: 5 

w 

1--

w 

~ 

i= 5 

z 

w 

(.) 

z 

w- 10 

a: 

0 

(.) 

z 

I 15 

1--

0.. 

w 

0 

2 3 4 5 6 7 0 40 80 120 0.0 0.4 0.8 1.2 1.6 0 10 20 30 40 50 

Mo!Ti RATIO x 1000 Corg!Ti RATIO N!Ti RATIO I!Ti RATIO x 1000 

0 u 

5 

10 

15 

20 ._____.__----~. _ _._____L._...______, 

10 20 30 40 20 40 60 80 100 

Ni!Ti RATIO x 1000 V!Ti RATIO x 1000 

Figure 13.3. Continued. 
252 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

Table 13.5. Isotope data for calcite from cores 2040 AC-1 and 2040 

AC-2. 

[Temperatures of formation were calculated for a range of isotopic compositions of equilibrating water of 0 to +2 per mil. PDB, Pee Dee belemnite; SMOW, standard mean ocean water] 

Core -------------------------------------------------------

<5 13C {per mil)-------------------------------- 18 PDB () 0 (per mil)-------------------------------- 18 PDB () OSMOW (per mil) -----------------------------

Temperature (°C) at <5 180sMowCH20): 

0 per mil -------------------------------------------

1 per mil -------------------------------------------

2 per mil -------------------------------------------

2040 

AC-1 

-11.07 

-21.63 

8.61 

208 

230 

254 

2040 

AC-2 

-11.03 

-22.49 

7.72 

227 

251 

279 

2040 AC-1. Koski and others (1985) described a dredged 

sample of lithified sediment with large calcite veins from 

Guaymas Basin. Similar calcite veins have been observed 

in sediment samples from the hydrothermally active Middle 

Valley, Juan de Fuca Ridge (Goodfellow and Blaise, 1988). 

The temperatures of formation calculated for the calcites 

from this study clearly indicate that they were formed from 

a high-temperature fluid (about 220°C). Peter and Scott 

(1988) and Koski and others (1985) reported temperatures 

on the order of 230°C, based on microthermometry of fluid 

inclusions from the calcite crystals. Precipitation of calcite could result from degassing of the C02 from the hydrothermal fluids near the sediment surface. The 

precipitation of pure calcite suggests an absence of magnesium in the hydrothermally affected solutions, in contrast 

to Guaymas Basin where protodolomite is often formed 

(Magenheim, 1989). 

en 

cr: 

w 

1-

w 

:2 

~ 5 

z 

w 

() 

z 

-_ 10 

w 

cr: 

0 

() 

z 15 

I 

1-

a... 

w 

Cl 

HYDROTHERMAL PRECIPITATES 

The appearance of sulfide minerals and barite below 

the calcite layer in 2040 AC-1 and associated with calcite 

and petroleum condensates in 2040 AC-2 supports the conclusion that the sediment was exposed to active hydrothermal flow in the past. The low abundance of Fe-S minerals, 

Fe-Cu-S minerals, and possibly zinc sulfides suggests that 

the fluid was enriched in trace metals and sulfide. Many 

observations and samples of massive sulfide deposits have 

been reported from Escanaba Trough (Benninger and 

Koski, 1987; Koski and others, 1988; Morton and others, 

1990, chap. 1, this volume). It has been suggested that 

these sulfide deposits are formed by uniform, relatively low 

temperature ( <250°C) diffuse flow onto the sea floor 

(Koski and others, 1988), forming mounds like the ones 

observed in Guaymas Basin. Sulfur isotope data from pyrrhotite and marcasite recovered from Escanaba Trough indicate a mixture of seawater- and basalt-derived sulfur 

(Koski and others, 1988). 

Visual and SEM observations of sulfide grains from 

core 2040 AC-2 (Magenheim, 1989) showed evidence for 

weathering, including the presence of a Cu-Cl phase. The 

latter is probably atacamite (Cu2Cl(OH)3), a commonly 

observed mineral in chalcopyrite-rich samples that have 

undergone weathering in contact with seawater. 

ALTERATION OF THE SEDIMENT 

It appears from the profiles of iron and magnesium 

(fig. 13.3F, G) that these elements are affected by hydrothermal alteration. The deficit in both elements below the 

calcite vein in 2040 AC-1 suggests that these elements may 

be mobilized by the hydrothermal fluids. Similar decreases 

E 

0.1 

N/Corg RATIO 

0.2 0 2 4 6 0 1 2 3 4 5 0 2 4 6 8 10 12 0.0 0.5 1.0 1.5 

1/Corg RATIO x 1000 Ni/Corg RATIO x 1000 V/Corg RATIO x 1000 Mo/Corg RATIO x 1000 

Figure 13.4. Organic carbon ratios for nitrogen, iodine, nickel, vanadium, and molybdenum in cores 2035 AC-1 (dots), 2040 AC-1 (squares), 

and 2040 AC-2 (triangles). 
EVIDENCE FOR HYDROTHERMAL FLUID FLOW THROUGH SURFICIAL SEDIMENTS, ESCANABA TROUGH 253 

in magnesium were observed in cores from Guaymas Basin, in which hydrothermal fluids were actively flowing 

(Gieskes and others, 1988; Magenheim, 1989). Mobilization of magnesium from clays by the high-temperature, 

low-Mg fluids could explain these observations. The similarity between magnesium and iron profiles would suggest 

that a similar process controls the distribution of these two 

elements. Especially in deeper zones of cores 2040 AC-1 

and 2040 AC-2 a large increase occurs in sulfur, with the 

content of sulfur at 7.5 em in 2040 AC-2 being almost 

equivalent to that of iron. Presumably iron is mobilized 

from iron-oxides as well as from clays. 

The relative enrichments in barium and strontium 

throughout cores 2040 AC-1 and 2040 AC-2, when compared to core 2035 AC-1, suggest precipitation of barite 

from diffusely flowing hydrothermal fluids, mixed to some 

extent with sulfate-containing seawater. The precipitation 

of barite was also observed in cores from Guaymas Basin, 

especially at the sediment-seawater interface, where mixing occurs between hydrothermal fluid and overlying bottom water. It appears that strontium is associated with 

barite in both cases. Figure 13.5 shows strontium plotted 

against barium for the Escanaba Trough cores, as well as 

Guaymas Basin cores (Magenheim, 1989). A reasonable 

correlation is apparent between these two elements for both 

sites. The scatter in strontium concentrations at low barium 

is probably due to strontium associated with biogenic carbonate at Guaymas Basin. The slope of the correlation line 

indicates that the ratio of strontium to barium is around 

1:10. This result is in agreement with experimental results 

and the frequency distribution of natural barites (Hanor, 

1968). 

1sr---~--~----~---r--~~--~--~--~ 

Y= 1.6043 + 0.115x 

,2 = 0.964 

10 

~ Q) 

0 

E 

:::J... 

(jj 

5 

Explanation 

• Escanaba Trough 

x Gauymas Basin 

0~--~--~----._---L----~--~--~--~ 

0 25 50 75 100 

Ba J.Jmole/g 

Figure 13.5. Strontium versus barium for solid phases from 

Guaymas Basin and Escanaba Trough. 

THERMAL ALTERATION OF ORGANIC MATTER 

The presence of a tar mat in core 2040 AC-2 and tar 

stuck to the calcite crystals of 2040 AC-1, suggests that 

petroleum was supplied from below. Molar N/Corg ratios 

on the order of 0.01 in the 7 em interval of core 2040 AC1 are indicative of thermal alteration (Simoneit and Bode, 

1982). In these same intervals iodine, bromine, nickel, and 

vanadium are also depleted. All of the results indicate that 

thermally altered organic matter has reached the surface 

sediment in both cores. While core 2040 AC-1 did not 

have the obvious odor of petroleum when sampled, a slight 

sulfurous smell was observed. Elsewhere we demonstrate 

that the petroliferous material of cores 2040 AC-1 and 2040 

AC-2 at present is undergoing oxidative alteration (Gieskes 

and others, 1990). 

SUMMARY 

Surface sediment cores from active hydrothermal vents 

in Escanaba Trough show evidence of hydrothermal alteration, precipitation of hydrothermal minerals, and thermally 

altered organic matter. The presence of hydrothermal calcite indicates that high-temperature fluid did flow through 

the sediment. 

Elevated concentrations of copper, zinc, barium, and 

sulfur were observed in cores 2040 AC-1 and 2040 AC-2, 

associated with precipitation of sulfide minerals and of barite. Sulfide grains appear to be weathered and altered. 

Barite (and associated strontium) appears to be enriched 

in the hydrothermally altered cores. Evidence for diffuse 

flow through the sediments is given by subtle changes in 

sediment composition. Depletion of magnesium and iron 

seems to occur where precipitation of sulfide minerals is 

evident. 

Interstitial water chemical data indicate an absence of 

fluid flow at this time. Instead, suboxic conditions, established after cessation of the hydrothermal fluid flow, lead 

to the oxidation of both sulfides and hydrocarbons. 

REFERENCES CITED 

Bazylinski, D.A., Farrington, J.W., and Jannash, H.W., 1988, Hydrocarbons in surface sediments from the Guaymas Basin 

hydrothermal vent site: Organic Geochemistry, v. 12, p. 

547-558. 

Benninger, L.M., and Koski, R.A., 1987, Descriptions and chemical analyses of sulfide samples dredged in 1986 from 

Escanaba Trough, southern Gorda Ridge: U.S. Geological 

Survey Open-File Report 87-375-B, 26 p. 

Bohlke, J.K., and Shanks, W.C., III, Stable isotope study of hydrothermal vents at Escanaba Trough: Observed and 

calculated effects of sediment-seawater interaction, in 

Morton, J.L., Zierenberg, R.A., and Reiss, C.A., eds., Geo-
254 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

logic, hydrothermal, and biologic studies at Escanaba 

Trough, Gorda Ridge, offshore northern California: U.S. 

Geological Survey Bulletin 2022, chapter 12 (this volume). 

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EVIDENCE FOR HYDROTHERMAL FLUID FLOW THROUGH SURFICIAL SEDIMENTS, ESCANABA TROUGH 255 

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Transactions American Geophysical Union, v. 69, p. 1488. 

Chapter 14. Sediment Alteration Associated with Massive 

Sulfide Formation in Escanaba Trough, Gorda Ridge: 

The Importance of Seawater Mixing and Magnesium 

Metasomatism 

By Robert A. Zierenberg1 and Wayne C. Shanks III2 

CONTENTS 

Page 

Abstract ................................................................................ 257 

Introduction.......................................................................... 258 

Acknowledgments....................................................... 260 

Methods . ..... .. .. ... ... . ... .. .... .. .. ... . ... .... ... . ... . ... . . .. . ..... .. . . ... . ... . .. .. .. 260 

Sedimentology .. .... ... ... . ... . ... .. .. .. .. .. ... . . .. . .. .. .. .. .. .. .. .. .. .. .. .. ... ... . 260 

Sediment history.......................................................... 260 

Depositional environment ............ ......................... ...... 260 

Mineralogy ...... ... ... .. .. . ..... ... . ... .... ... . ... . . .. ..... ... . ... . ... . ... . . ... . .. .. . 261 

Unaltered sediment...................................................... 261 

Altered sediment . . ... . ... ... . ..... .. .. .. .. .. . ... . .. . . . .. . . . . . . .. . . .. .. ... . 262 

Smectite alteration............................................... 262 

Chlorite alteration................................................ 262 

Talc alteration .. ... ..... .. . ... . ... . .. .. ... . ... . ... . ... . ... . . .. . . .. .. 265 

Sediment chemistry .. ........ .... ... ........ ... . ... . . ........ .. . . ... . ... . .... .. . 265 

Unaltered sediment...................................................... 265 

Altered sediment ...... .. .. .... ... . ... . ... . ... . ... . ... . ... . ... . ... ... .. .. . . 265 

Reaction mechanisms and mass balance constraints . 266 

Smectite alteration............................................... 266 

Chlorite alteration................................................ 267 

Talc alteration...................................................... 269 

Isotope chemistry................................................................. 270 

Oxygen isotopes .......................................................... 270 

Detrital sediment ...................... .... ........ .... ........... 270 

Hydrothermal clays ...................... ....................... 272 

Sulfur isotopes .............................. ...................... ......... 273 

Comparison to ore deposit alteration.................................. 274 

Summary . . ... ... .. .. .. ... ... . ... . ... ...... .. .. .... ... . .. .. .. .. .. .. .. .. .. .. .. .. .. ...... 27 4 

References cited................................................................... 275 

ABSTRACT 

Escanaba Trough is a slow spreading ridge buried to 

an average depth of 500 m by turbiditic and hemipelagic 

sediment. The sediment is predominantly muddy but locally contains turbiditic silt and sand. Silt and sand grains 

1 U.S. Geological Survey, Menlo Park, California. 2 U.S. Geological Survey, Denver, Colorado. 

are predominantly quartz, plagioclase, biotite, muscovite, 

chlorite, and amphibole. The muddy sediment is composed 

predominantly of detrital quartz, chlorite, illite, and 

smectite. The finest grain fraction of clay separates is predominantly smectite. 

Massive sulfide deposits occur within the sediment 

near areas of recent sea-floor spreading volcanism. Sediment alteration associated with formation of massive sulfide is classified as smectite-, chlorite-, or talc-dominated 

based on the mineralogy. Smectite alteration is characterized by the overgt"owth of sand and silt grains by iron-magnesium smectite. Aluminum, titanium, and zirconium 

behave conservatively during smectite alteration; magnesium is enriched relative to unaltered sediment. Hydrothermal smectite gives more coherent X-ray diffraction patterns 

and has lower 8180 values than detrital and diagenetic 

smectite due to formation of hydrothermal smectite at temperatures near 200°C. Chlorite alteration is characterized 

by replacement of detrital minerals by magnesium-rich 

chlorite. Aluminum, titanium, and zirconium behave conservatively; silica, sodium, potassium and calcium are 

strongly depleted and magnesium is highly enriched relative to unaltered sediment. Chlorite 8180 values near 2.5 

per mil reflect formation at temperatures near 220°C. Talc 

alteration proceeds by direct precipitation of talc in sediment and on massive sulfide resulting in significant addition of magnesium to the rock. 

Each type of alteration involves addition of magnesium 

from seawater that mixes with hydrothermal fluid in the 

upper part of hydrothermal circulation cells. This magnesium metasomatism releases hydrogen ions, which leads to 

hydrolytic alteration of detrital sediment. Magnesium removal during mixing with seawater also lowers the pH and 

increases the oxygen fugacity of the hydrothermal fluid 

leading to conversion of primary pyrrhotite to secondary 

marcasite and Fe-oxyhydroxides. Pyrite precipitated in altered sediment has 834s approximately 2 per mil higher 

than hydrothermal H2S due to incorporation of minor 

amounts of sulfur derived from reduced seawater sulfate 

with 834s of 21 per mil. 

257 
258 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

The hydrothermal alteration observed at Escanaba 

Trough is similar to alteration observed at sediment-covered spreading centers in Guaymas Basin on the East Pacific Rise and Middle Valley on Juan de Fuca Ridge, and 

the processes involved are similar to those that result in 

wallrock alteration at both volcanic- and sediment-hosted 

massive sulfide deposits. Magnesium metasomatism 

caused by mixing of seawater into the upper parts of the 

discharge zones of sea-floor hydrothermal convection cells 

provides an important control on rock alteration and the 

transport and deposition of metal. 

INTRODUCTION 

Active hydrothermal systems at sea-floor spreading 

centers offer an unprecedented opportunity to investigate 

the processes leading to formation of massive sulfide deposits. Massive sulfide mineralization has been discovered 

at several sites within Escanaba Trough (Morton and others, 1987a, 1987b, 1990; Holmes and Zierenberg, 1990). 

The large size of individual deposits and the extensive areas where mineralization is known (Zierenberg and others, 

chap. 10, this volume) make Escanaba Trough one of the 

premier sites to study massive sulfide formation. Escanaba 

Trough is the slow spreading (2.4 cm/yr total spreading 

rate, Riddihough, 1980) southern portion of Gorda Ridge, 

and is bounded by the Mendocino Fracture Zone on the 

south and by a right-lateral offset of the ridge axis near latitude 41 °25' N., longitude 127°25' W. on the north (Clague 

and Holmes, 1987). Escanaba Trough has a morphology 

typical of slow spreading ridges: a flat-floored axial valley 5 to 20 km wide at a depth of 3,300 m flanked by steep 

ridge crests rising 800 to 1,200 m above the axial valley 

floor (fig. 14.1). The southern two-thirds of the trough is 

covered by hemipelagic and turbiditic sediment that averages 500 m in thickness over the spreading axis. Turbiditic 

sediment enters the trough through a topographic low area 

of the east flank of the ridge north of the Mendocino transform fault, and sediment thickness decreases northward 

along the ridge axis (Vallier and others, 1973; Normark and 

others, chap. 6, this volume). Several igneous complexes 

3 to 5 km in diameter occur along the spreading axis beneath the sediment cover, and basaltic rocks are locally exposed on the sea floor at these volcanic centers (Morton 

and others, 1987a; Clague and Holmes, 1987). These volcanic edifices are built by both intrusion and extrusion of 

basaltic rock (Morton and Fox, chap. 2, this volume). Detailed geologic mapping and sampling at two of the volcanic edifices utilizing surface ship and submersible, and 

reconnaissance work at two other sites, have revealed at 

least six major massive sulfide deposits and numerous sulfide occurrences (Morton and others, 1990; Holmes and 

Zierenberg, 1990). Although the sulfide deposits are spatially and genetically associated with the volcanic edifices, 

they all occur on or within sediment (Zierenberg and others, chap. 10, this volume). The presence of a sediment 

blanket over the hydrothermal system influences the form 

of the sulfide deposits by controlling the hydrologic regime. The composition of the hydrothermal fluids and deposits reflects chemical interaction of the hydrothermal 

fluid with the sediment (Campbell and others, chap. 11, this 

volume; Koski and others, chap. 16, this volume). The 

relatively impermeable sediment cover acts as a thermal 

blanket limiting large-scale convective cooling of the ridge 

crest (Davis and Becker, chap. 3, this volume; Denlinger 

and Holmes, chap. 4, this volume). Hydrothermal recharge 

and discharge are restricted to zones where tectonic and 

volcanic processes provide cross-stratal permeability. Sand 

and silt turbidites are presumed to be hydrothermal aquifers, because of their relatively high permeability. The volcanic edifices serve as both a topographic and thermal 

focus for hydrothermal fluids, resulting in long-lived discharge systems capable of forming large massive sulfide 

deposits. 

Venting hydrothermal fluids have been sampled near 

latitude 41° N., longitude 127°30' W.; other known sulfide 

occurrences do not appear to be presently active. Clear 

hydrothermal fluid at temperatures up to 217°C issues from 

porous anhydrite-barite chimneys on top of large mounds 

of pyrrhotite-rich massive sulfide. Active vents are colonized by dense clusters of tube worms and associated palm 

worms, anemones, limpets, gastropods, and bacterial mats 

(Van Dover and others, 1990). Bivalves are common in 

the sediment near some sulfide deposits. 

The circulation of fluid through near-surface sediment in hydrothermal discharge zones results in alteration 

of the sediment. This chapter focuses on the nature of 

this alteration and suggests that mixing of seawater with 

hydrothermal fluid in the shallow subsurface is an important genetic process leading to magnesium metasomatism 

of sediments adjacent to the sulfide deposits. Subsurface 

mixing will also cause deposition of sulfide below the sea 

floor. Sulfide veining in sediment cores and sedimentary 

breccias with a sulfide matrix indicate that some sulfide 

minerals are deposited beneath the sediment surface. The 

extent of subsurface mineralization cannot be determined 

from surface observations, but it is possible that the large 

amount of sulfide exposed on the sea floor represents only 

a small part of the total deposits. The processes involved 

in sulfide deposition and sediment alteration at the 

Escanaba Trough deposits are similar to those occurring 

in Guaymas Basin on the East Pacific Rise (Kastner, 

1982) and at Middle Valley on Juan de Fuca Ridge 

(Goodfellow and Blaise, 1988). Geologic and geochemical similarity to mineralization and alteration processes 

that formed some ancient massive sulfide deposits (Roberts and Reardon, 1978; Franklin, 1986) make Escanaba 

Trough an excellent natural laboratory in which to investigate ore deposit genesis. 
SEDIMENT ALTERATION ASSOCIATED WITH MASSIVE SULFIDE FORMATION IN ESCANABA TROUGH 259 

127° 30' 127° 

, SESCA .. 

I . DSDP Site 35 

40° 30' 

Figure 14.1. Bathymetric map of Escanaba Trough showing location of volcanic edifices (dark 

gray), SESCA and NESCA sites, DSDP Site 35, and core Ll-86-270 in axial valley of Escanaba 

Trough outside of SESCA area. Hachures indicate area of closed low. Contour interval is 200 m. 
260 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

ACKNOWLEDGMENTS 

We thank the captain and crew of the RN S.P. Lee for 

their efforts, often under adverse weather conditions, in 

collecting the samples used in this study. We thank Mary 

Horan, U.S. Geological Survey, Reston, Virginia, for 

oxygen isotope analyses. X-ray powder photographs used 

for the identification of chlorite polytype were kindly 

provided by R.C. Erd, Jr., U.S. Geological Survey, Menlo 

Park, California. 

METHODS 

Sediment was collected by both gravity coring and 

dredging at locations shown on figures 14.1 and 14.2. The 

bulk sediment and clay fraction mineralogy were determined by X-ray diffraction (XRD). Samples for clay mineralogy were repeatedly dispersed in deionized water by 

ultrasonic probe and settled by centrifugation. Three smear 

mounts on glass slides were prepared from the finest grainsize fraction of centrifuged samples to allow XRD examination of clays that had been air dried, treated with ethylene 

glycol, and heated to 400°C. 

Bulk chemical analyses of unaltered and altered sediment samples were conducted in the Analytical Geochemistry Laboratory, Department of Geology and Geophysics, 

University of Minnesota (R. Knoche, analyst). Samples 

were leached of interstitial water and salts by repeated 

dispersal in deionized water followed by centrifugation. 

Major and minor elements were determined by DC-plasma 

atomic emission spectrometry (Bankston and others, 1979). 

Carbonate carbon and organic carbon were determined by 

automatic coulometric titration. 

Samples for oxygen isotope analyses were reacted with 

ClF3 (Borthwick and Harmon, 1982) and standardized 

using NBS-28, which has a value of 9.6 per mil relative 

toSMOW. 

Samples for sulfur isotope analyses were treated 

chemically to separate mineral phases. Pyrrhotite was liberated in 6N HCl at 60°C and includes minor amounts of 

coexisting sphalerite. Isocubanite in residues from pyrrhotite reaction was subsequently dissolved in chromous chloride solution (Canfield and others, 1986). Chalcopyrite is 

a minor constituent in some of the isocubanite and is included in the chromous chloride fraction. Pyrite samples 

were also treated by chromous chloride solution. The sulfur in all sulfide samples was converted first to Ag2S and 

then oxidized by combustion with Cu20 at 1 ,050°C to produce S02. Barite was purified by reaction with NaC03 

(Breit and others, 1985), reprecipitated as pure BaS04, 

then reacted with Cu20 and Si02 at 1,140°C to produce 

S02• B34S values were standardized to Canyon Diablo 

troilite using the McMaster University reference series 

(Rees, 1978). 

SEDIMENTOLOGY 

SEDIMENT HISTORY 

The proximity of Gorda Ridge to the North American 

continental margin results in significant deposition of continental clastic sediment on the southern part of the spreading center (Normark and others, chap. 6, this volume). 

Normark and others (chap. 6, this volume) propose that the 

bulk of this sediment was deposited within the last 100 kyr 

during the last two glaciallowstands of sea level. The sediment is predominantly turbiditic sand, silt, and mud transported from the continental shelf (Vallier and others, 1973; 

Normark and others, chap. 6, this volume). Vallier and others (1973) established that the source areas for the upper 

360 m of sediment are igneous and metamorphic terranes 

in the Columbia River drainage basin. A channel system, 

which can be seen in the GLORIA long-range side-scan 

mosaics of this area (EEZ-SCAN 84 Scientific Staff, 1986 ), 

directs sediment transport southward from the Columbia 

Fan, past the Blanco Fracture Zone to the southern end of 

Escanaba Trough (Vallier and others, 1973). Turbidity currents flow north from the Mendocino Fracture Zone and 

bury the spreading center. Sediment cover thins to the 

north and ridge-axis basalt is exposed north of a topographic high at approximately latitude 41 °17' N. (Morton 

and Fox, chap. 2, this volume). 

DEPOSITIONAL ENVIRONMENT 

A drill hole at DSDP Site 35 penetrated the upper 390 

m of sediment without reaching basement (McManus and 

others, 1970). Only 95 m of core was recovered from the 

hole, and many sections of core were not opened or described by the shipboard scientific party. Preliminary studies and descriptions of the sediment cored were reported 

by McManus and others (1970). The previously unopened 

core sections were subsequently logged in 1988 and are 

described in Normark and others (chap. 6, this volume). 

The sedimentology, mineralogy, and sediment chemistry of 

the uppermost 4 m of sediment recovered by piston and gravity coring in Escanaba Trough are described by Karlin and 

Lyle (1986) and Normark and others (chap. 6, this volume). 

Presently accumulating sediment in Escanaba Trough 

is predominantly hemipelagic mud and locally derived silty 

to sandy turbidites (Karlin and Lyle, 1986; Normark and 

others, chap. 6, this volume). The transition from Holocene 

surface hemipelagic mud to Pleistocene turbiditic sediment 

is generally marked by a subtle change to grayer, more 

compacted mud with higher magnetic susceptibility and is 

generally found at 1 to 2 m depth in undisturbed cores 

(Karlin and Lyle, 1986). Cores that recover the sedimentwater interface are capped by a 1- to 5-cm layer of oxidized red-brown mud formed in the zone ventilated by 
SEDIMENT ALTERATION ASSOCIATED WITH MASSIVE SULFIDE FORMATION IN ESCANABA TROUGH 261 

bioturbation (Karlin and Lyle, 1986; Normark and others, 

chap. 6, this volume). Mud below this level is more reduced and is typically olive-brown to olive-gray. 

Sandy intervals 10 to 40 em thick were deposited by 

turbidity currents (Normark and others, chap. 6, this volume). Grain size ranges from fine to medium sand. The 

sandy intervals are generally massive to poorly graded and 

occasionally have scoured bases. The sands are moderately 

to poorly sorted and often contain some mud matrix. 

Sandy intervals typically compose 0 to 5 percent of the vol12r36' 12JD34' 

6000 8000 10,000 

ume of sediment in the upper 3 m typically recovered by 

piston and gravity cores. 

MINERALOGY 

UNALTERED SEDIMENT 

The mineralogy of unaltered sediment shows little 

variation between cores. The mud intervals are dominantly 

12JD32' 12JD28' 

16,000 

14,000 

'10,000 

8000 

0 2 KILOMETERS 

Contour interval 10 meters 6000 

12,000 14,000 16,000 

Figure 14.2. Bathymetric maps showing locations of core samples (open and closed circles) and dredge samples (solid lines) from the 

SESCA study area used in this study. Numbers along right and bottom give coordinates of local transponder net (in meters). Hachures 

indicate area of closed low. 
262 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

quartz, feldspar, mica, and clay minerals. Carbonate and 

zeolite minerals were not detected by XRD. The finest 

grain-size fraction separated by centrifugation was dominantly green-brown to red-brown smectite. This smectite 

gives low-amplitude, broad XRD peaks due to very fine 

grain size and poor crystallinity. Coarser clay fractions are 

brown to gray and contain variable amounts of chlorite, illite, quartz, and smectite. Kaolinite contents appear to be 

low, but the presence of kaolinite peaks in the XRD traces 

could be partially masked by the more abundant chlorite. 

Sandy intervals are predominantly quartz with abundant feldspar, predominantly plagioclase. Mica is common, and chlorite and minor amounts of amphibole were 

detected by XRD in many samples. Microscopy shows 

that biotite is more abundant than muscovite. The sands 

also contain minor amounts of pyroxene, igneous and 

metamorphic amphibole, epidote, ilmenite, magnetite, 

chromite, and occasionally pyrrhotite (Normark and others, chap. 6, this volume; Karlin and Zierenberg, chap. 7, 

this volume). 

ALTERED SEDIMENT 

We identified three mineralogically distinct types of 

hydrothermal alteration, which are referred to as smectite, 

chlorite, or talc alteration based on the dominant hydrothermal mineral in the samples. The differences in alteration mineralogy reflect differing environments and 

mechanisms of formation of the hydrothermal phases and 

provide some insight into the processes leading to mineral 

deposit formation in this system. The degree of mixing of 

hydrothermal fluid with seawater in part controls the mineralogy of the altered sediment. 

SMECTITE ALTERATION 

Hydrothermal smectite has been observed in samples 

from two settings from Escanaba Trough: as an alteration 

product in mineralized sediment, and as a hydrothermal 

precipitate with anhydrite in active vents. Hydrothermal 

smectite is easily distinguished from smectite occurring in 

unaltered sediment by XRD peaks that are narrower and 

increased in intensity by 1 to 3 orders of magnitude. The 

clay fraction of one sample of altered sediment (L1-86-3D) 

gives XRD patterns indicative of smectite-chlorite 

interlayering. 

Hydrothermal smectite separated from altered sediment is deep green and gives very strong XRD peaks. 

Smectite is the dominant alteration mineral occurring in the 

mineralized section of core L1-86-27G. The turbiditic 

sands recovered at the base of this core are mineralized by 

pyrrhotite, accompanied by barite, isocubanite, sphalerite, 

and galena. The hydrothermal fluid that precipitated the 

sulfide also altered the host sediment. The smectite in the 

mineralized interval occurs as replacements and 

overgrowths of the sand grains. Examination of the altered 

sand by scanning electron microscope (SEM) shows that 

virtually all grains have coatings of hydrothermal smectite 

(fig. 14.3A). Smectite is also intergrown with hexagonal 

pyrrhotite (fig. 14.3B). Energy dispersive X-ray spectroscopy indicates that the smectite is an intermediate Mg-Fe 

smectite. 

Smectite also occurs with anhydrite as a hydrothermal 

precipitate in deposits at active vents, where it occurs as 

light yellow-green reniform masses (Koski and others, 

chap. 16, this volume). It is the only silicate mineral in 

the chimney structures. This smectite is Mg-rich and Alpoor, and is probably near stevensite in composition. Formation temperatures for this smectite, calculated by Bohlke 

and Shanks (chap. 12, this volume) from oxygen isotope 

values measured on the smectite and hydrothermal vent 

fluid, are 220°C, in excellent agreement with measured 

vent temperatures of 217°C. 

CHLORITE ALTERATION 

Benninger and Koski ( 1987) identified chlorite in 

mineralized and altered sediment samples recovered by 

dredging from Escanaba Trough. A spectacular occurrence of chlorite is in hydrothermally altered sediment in 

core L2-86-9G. This core recovered a sulfide debris flow 

in the interval 50 to 77 em that was deposited on a zone 

of intense chlorite alteration from 77 em to the base of 

the core at 200 em. The hemipelagic and turbiditic sediment in this interval has been replaced by a nearly 

monomineralic hydrothermal clinochlore with minor 

amounts of talc and trace amounts of pyrite, barite, 

isocubanite, sphalerite, gypsum, and rutile. The only remaining detrital minerals observed are coarse-grained 

quartz and mica in an interval formerly occupied by a 

sandy turbidite; finer grained detrital minerals have been 

completely replaced by chlorite. Talc occurs in some intervals as 2- to 10-mm white clots in the bluish-gray 

chlorite matrix. The overlying sulfide debris flow contains clasts of pyrrhotite-rich massive sulfide with barite, 

isocubanite, sphalerite, and galena (fig. 14.3C), typical of 

most Escanaba Trough massive sulfide. In contrast, the 

predominant sulfide in the altered chlorite is disseminated 

euhedral pyrite (fig. 14.3D). Isocubanite (fig. 14.4A) and 

gypsum (fig. 14.3B) are also observed. Despite its fine 

grain size, chlorite displays good crystallinity as demonstrated by strong, narrow XRD peaks and euhedral crystals seen by SEM (fig. 14.4C). 

The less-than-2-)..lm fraction separated from the altered 

sediment in core L2-86-9G is also nearly monomineralic 
c 0 20 MICROMETERS 

L_____j D 0 20 MICROMETERS I I 

Figure 14.3. Scanning electron micrographs of Escanaba Trough sediments. A, Sand grains coated by Fe-Magnesium smectite (Sm). Uncoated, bladed grains are barite (Ba). Ll86-270 82 em. B, Euhedral hexagonal pyrrhotite (Po) crystals intergrown with Fe-magnesium smectite (Sm). Ll-86-270 82 em. C, Euhedral barite (Ba) grains overgrown by small 

euhedral galena (On) crystals and colloform aggregates of sphalerite (Sl). L2-86-90 62 em. D, Euhedral pyrite (Py) crystal in magnesium-chlorite (Ch). L2-86-90 107 em. 

en 

tTJ 

tj 

~ tTJ 

z 

>-3 

> 

C3 

tTJ 

;:o 

~ 

0 

z 

> en 

en 

0 

(') 

~ tTJ 

tj 

~ 

::] 

::c: 

~ 

> en 

en 

< tTJ 

en 

c 

r 

'"rl 

8 

tTJ 

'"rl 

0 

;:o 

3:: 

~ 

0 

z 

z 

tTJ 

en 

(') 

> z 

> t:X:I 

> 

>-3 

;:o 

0 

c 

Q 

::c: 

N 

0\ 

w 
A 0 40 MICROMETERS 

c 0 10 MICROMETERS 

B 0 4 MICROMETERS L__j 

Figure 14.4. Scanning electron micrographs of Escanaba Trough sediments. A, Euhedral 

isocubanite (Icb) grain on talc (Ta) from a magnesium-chlorite rich sample. L2-86-9G 

111 em. B, Gypsum (Gyp) crystals intergrown with magnesium-rich chlorite (Ch). L2-

86-9G 107 em. C, Well-crystallized platelets of magnesium-rich chlorite (Ch). L2-86-9G 

107 em. 

N 

0\ 

.+::-. 

Cl 

m 

0 

L"' 

0 

Cl 

0 

::c: 

~ 

t;j 

:::z:; 

0 

...-3 

::c: 

m 

:::0 

s: 

)> 

r 

)> 

z 

t;j 

t::cl 

5 

L"' 

0 

Cl 

n 

C/.l 

...-3 

c 

t;j 

m 

C/.l 

~ trl 

C/.l 

n 

)> 

z 

)> 

t::cl 

)> 

~ 0 

c 

Cl 

~::c: 

Cl 

0 

:::0 

t;j 

)> 

:::0 

8 

Cl 

m 
SEDIMENT ALTERATION ASSOCIATED WITH MASSIVE SULFIDE FORMATION IN ESCANABA TROUGH 265 

chlorite with minor amounts of talc. The separated chlorite is silver-gray, reflecting the low iron content. Even 

the finest grain-size fractions give very strong, sharp XRD 

peaks. Powder camera photographs show that the chlorite 

is the lib structural polytype, which is the stable polytype 

at elevated temperatures (Bailey and Brown, 1962). 

TALC ALTERATION 

Talc is the most commonly observed hydrothermal silicate in massive sulfide samples from Escanaba Trough. 

The typical occurrence of talc is rinds 5 to 20 mm thick 

on the outer surfaces (seawater side) of massive sulfide. 

Talc also occurs interstitial to sulfide near the outer edges 

of samples and in some mineralized sediment samples, 

where it is often associated with chlorite (Benninger and 

Koski, 1987). Talc grains are fibrous to platy and occur 

either as intergrown reticulate networks or as botryoidal to 

radial fibrous aggregates. In some samples talc is overgrown by botryoidal amorphous silica and rarely by radial 

fibrous chalcedony. Some talc is associated with a more 

highly birefringent phyllosilicate interpreted to be smectite. 

The pyrrhotite in massive sulfide associated with talc is 

commonly replaced by marcasite; in some samples pyrrhotite and secondary marcasite are replaced by lepidocrocite 

and (or) goethite. 

SEDIMENT CHEMISTRY 

UNALTERED SEDIMENT 

Bulk chemical analyses of near-surface samples of altered and unaltered mud and sand were obtained to assess 

mass transfer during hydrothermal alteration (table 14.1). 

Sands recovered at DSDP Site 35 from deeper in the sedimentary section were also analyzed to characterize potential metal sources and aquifers for the hydrothermal fluids. 

The major chemical variations among unaltered 

samples are primarily a function of the relative proportions 

of clay minerals to quartz and feldspar. Mud samples have 

lower Si02 and higher Al20 3, MgO, Fe20 3 T (total iron as 

Fe20 3), Cu, and Zn relative to sand. For example, the nearsurface sand from core L 1-86-80 contains more mud matrix than sands sampled from DSDP Site 35, which is 

reflected in the chemical analyses of this sample. The covariance of Na and Ca in unaltered samples (fig. 14.5A ) 

is probably controlled by the abundance of detrital igneous feldspar. 

A sample of sediment occurring above the sulfide debris flow in core L2-86-9G (36 to 40 em) has elevated concentrations of Fe20 3 T, Ba, Pb, Cu, and Zn relative to other 

samples of unaltered mud. Although there are no indications of hydrothermal alteration of the silicate phases in this 

sample, it does contain abundant hydrothermal Fe-oxide 

codeposited with the hemipelagic sediment (Normark and 

others, chap. 6, this volume). Two possible sources for this 

hydrothermal component are ( 1) direct sedimentation from 

a hydrothermal plume, either lower temperature venting 

forming primary iron oxide or "black smoker" plume sulfide fallout that was oxidized to Fe-oxide; or (2) the Feoxide formed by sea-floor weathering of a nearby sulfide 

deposit. The presence of a sulfide debris flow above highly 

altered sediment in this core requires the presence of a 

nearby sulfide deposit. The lack of either sulfide mineralization or sediment alteration above the sulfide debris flow 

suggests that hydrothermal activity ceased at this site prior 

to the codeposition of sediment and Fe-oxide, and favors 

sea-floor oxidation of sulfide as the source of the Fe-oxide. 

Base-metal and barium enrichment in the Fe-oxide-rich 

sediment is consistent with derivation by oxidation of massive sulfide at the sea floor, but could also result from deposition from a hydrothermal plume. The enrichment of 

P20 5, Mn, and Th may suggest deposition of hydrothermal oxides, accompanied by scavenging of some elements 

from seawater, from a hydrothermal plume (Trefry and 

others, 1990). 

ALTERED SEDIMENT 

The three mineralogically distinguishable types of alteration (smectite, chlorite, talc) show chemical variations 

(table 14.1) that can be correlated with differences in the 

processes that form the different alteration types. The 

chemical differences between the alteration types are 

clearly shown on a plot of Al20 3 and MgO (fig. 14.5B). 

Magnesium addition is common to all of the alteration 

types, and the extent of magnesium enrichment serves as 

a rough index of alteration intensity. The large variation 

in Al20 3 content is not caused by mobility of Al but rather 

by addition or removal of Si02 and MgO, as discussed below. In fact, several elements appear to have behaved conservatively. This is illustrated by a plot showing the strong 

covariation of Al20 3 and Ti02 (fig. 14.5C). Along with 

these two species, Cr, Ni, Sc, V, Y, and Zr all show strong 

interelement correlations consistent with conservative behavior of these elements. 

A second feature common to each of the alteration 

types is depletion of alkali and alkaline-earth elements during alteration. The magnesium-rich samples that are the 

most altered show near-total removal of Na, K, Rb, Ca, and 

Sr (for example, fig. 14.5D, E). Barium is depleted in 

most of these samples but is locally enriched due to precipitation of hydrothermal barite in some samples. 

The behavior of base metals in altered samples is controlled in large part by the precipitation of sulfide miner-
266 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

Table 14.1. Major- and minor-element composition of unaltered, altered, and mineralized sediment from Escanaba Trough. 

Sample --------- Ll-86- Ll-86- L1-86- L1-86- Ll-86- Ll-86- Ll-86- L1-86- Ll-86-

30-1 8G 150 2-5 240 8-1 240 21-2 27G 27G 27G 27G 

Depth (em) ---- Surface 174-179 Surface Surface Surface 72.5-74 74-77 77-80 84-87 

Alteration------ Smectite- Unaltered Talc Talc Talc Sulfide Smectite Smectite Smectite 

chlorite veins 

Grain size ----- Mud Sand Mud Mud Mud Sand Sand Sand Sand 

Major elements, in weight percent 

Si02 -------------- 54.0 60.1 59.3 44.9 35.4 15.6 44.5 60.4 62.7 

Ti02 -------------- .912 .774 .113 .088 .050 .175 .497 .518 .646 

Al20 3 ----------- 19.1 15.8 1.5 1.2 .2 3.5 9.9 12.2 13.2 

Fe203 T ---------- 7.17 6.62 2.76 27.20 23.60 54.40 25.40 6.62 9.97 

MgO ---------- 6.63 4.07 28.5 16.4 15.6 .68 6.25 9.01 3.79 

CaO ----------- .1 2.1 .1 .1 .0 .5 10.2 1.3 2.0 

Na20 --------- .4 2.6 .3 .2 1.2 .6 1.5 2.2 2.6 

K20 ----------- 2.20 2.58 .13 .14 .13 .56 1.04 1.26 1.92 

P205 ------------- .211 .188 .054 .205 .005 .040 .115 .084 .137 

Minor elements, in parts per million 

Ba ------------- 746 727 167 458 

Co------------- 12 21 11 136 

Cr ------------- 136 109 14 13 

Cu ------------- 77 37 204 925 

Mn ------------ 352 688 644 592 

Ni ------------- 47 62 20 13 

Pb ------------- 40 20 20 20 

Rb ------------- 74 96 12 5 

Sc ------------- 20 17 2 4 

Sr ------------- 83 281 19 38 

Th ------------- 17 10 0 0 

v -------------- 203 143 36 120 

y -------------- 27 25 9 9 

Zn ------------- 161 Ill 34 123 

Zr ------------- 120 132 10 10 

als. Contents of Cu, Zn, and Pb increase as the Fe20 3 T 

content of the samples increases due to the precipitation 

of pyrrhotite accompanied by accessory base-metal sulfide 

minerals. The samples with the smallest amounts of sulfide 

minerals generally have base-metal values below those of 

the starting sediment, suggesting that in the absence of sulfide mineralization metals were leached from the sediment 

during alteration. 

REACTION MECHANISMS AND 

MASS BALANCE CONSTRAINTS 

SMECTITE ALTERATION 

Hydrothermal smectite is forming in two distinct environments by different reaction mechanisms. The precipitation of magnesium-rich smectite within the active vent 

edifices is analogous to the formation of talc and is discussed 

further below. Alteration of sediment to magnesium-iron 

smectite is the result of reaction with hydrothermal fluid that 

is also precipitating sulfide minerals and barite within the 

235 8,200 1,649 378 803 

437 10 17 6 17 

5 21 70 51 75 

0 6,300 2,500 98 343 

91 185 566 425 382 

3 22 39 21 41 

26 1,500 160 10 20 

11 23 43 48 64 

3 8 10 9 12 

16 326 165 220 281 

0 7 4 5 0 

13 31 74 79 101 

6 7 16 14 20 

25 12,000 2,320 67 381 

3 60 120 109 137 

sediment. Lateral flow of hydrothermal fluid through the 

more porous sandy turbidite layers in core Ll-86-27G resulted in alteration of the sand grains and precipitation of 

both vein and disseminated sulfide and barite mineralization 

(Normark and others, chap 6, this volume). Sulfur, iron, copper, zinc, lead, and barium have been added to the sediment 

by precipitation from the hydrothermal fluid. However, hydrothermal fluid is not a likely source of magnesium or sulfate; end-member hydrothermal fluid is assumed to have 

none of either (Campbell and others, chap. 11, this volume). 

Measured magnesium concentration in 217°C hydrothermal 

fluid sampled in Escanaba Trough is as low as 1.6 mmol/kg 

(A.C. Campbell, written commun., 1989, referenced by 

Bohlke and Shanks, chap. 12, this volume). These components must be derived by mixing with either ambient seawater or seawater trapped as interstitial pore fluid in the 

sediment. Sulfur isotope data for barite (discussed below) 

are consistent with a seawater source for the sulfate. The 

low pH (5.3; Campbell and others, chap. 11, this volume) 

of the hydrothermal fluid enhances hydrolytic attack of minerals such as feldspars and micas resqlting in depletion of 

sodium, calcium and potassium. However, the overgrowth 
SEDIMENT ALTERATION ASSOCIATED WITH MASSIVE SULFIDE FORMATION IN ESCANABA TROUGH 267 

Table 14.1. Major- and minor-element composition of unaltered, altered, and mineralized sediment from Escanaba Trough-Continued. 

Sample --------- L2-86- L2-86- L2-86- L2-86- L2-86- L2-86- DSDP5 DSDP5 DSDP5 

4G 9G 9G 9G 9G 28G 35 3-5 35 5-4 35 17-1 

Depth (em) ---- 138-145 36-40 60-64 106-109 115-122 119-126 85 m 103m 383m 

Alteration ------ Unaltered Unaltered Massive Chlorite Chlorite Unaltered Unaltered Unaltered Unaltered 

sulfide 

Grain size ------ Mud Mud Sand Mud Mud Sand Sand Sand 

Major elements, in weight percent 

Si02 -------------- 52.8 52.9 4.8 30.6 31.3 51.9 70.3 71.1 71.6 

Ti02 -------------- .790 .675 .048 .995 .925 .825 .572 .745 .502 

Al20 3 ------------ 17.3 14.4 .9 20.9 19.7 17.5 12.4 11.9 12.8 

Fe203 T ---------- 7.93 11.00 43.40 8.25 12.40 8.45 3.78 5.00 3.05 

MgO ---------- 3.96 3.80 .90 25.60 24.10 4.24 2.01 1.87 1.55 

CaO ----------- 3.1 1.4 .5 .1 .1 3.1 3.6 3.5 4.0 

Na20 --------- 2.2 1.6 .2 .0 .1 2.2 2.7 2.8 3.6 

K20 ----------- 3.13 2.23 .15 .03 .01 2.98 2.19 1.98 .81 

p 2°5 ------------- .202 .363 .050 .044 .038 .215 .130 .136 .104 

Minor elements, in parts per million 

Ba ------------- 853 6,290 4,870 186 

Co ------------- 24 36 1,050 30 

Cr ------------- 112 148 13 147 

Cu ------------- 65 428 na 78 

Mn ------------ 922 1,220 90 322 

Ni ------------- 69 97 29 74 

Pb ------------- 20 210 4,800 30 

Rb ------------- 124 93 13 17 

Sc ------------- 20 18 6 19 

Sr -------------- 296 256 955 15 

Th ------------- 4 24 5 16 

v -------------- 168 190 46 168 

y -------------- 24 20 4 35 

Zn ------------- 141 269 21,000 145 

Zr ------------- 102 100 15 217 

of all sand grains, including quartz, by smectite (fig. 14.3A) 

suggests that much of the material that forms the smectite is 

carried by the hydrothermal fluid and is not derived by acid 

leaching of the sediment. 

One sample (L1-86-3D-1) composed of interlayered 

smectite-chlorite shows behavior intermediate between 

smectite and chlorite type alteration. This sample shows 

only moderate magnesium enrichment, similar to the 

smectite alteration (fig. 14.5B). However, the sample with 

interlayered smectite-chlorite shows strong depletion of 

sodium and calcium, typical of the more intense chlorite 

alteration (fig. 14.5D, E). Potassium and rubidium are not 

depleted in this sample, and it is possible that this smectitechlorite also contains significant interlayered illite. 

CHLORITE ALTERATION 

Total replacement of sediment from the base of L2-

86-9G by a nearly monomineralic chlorite assemblage is 

the result of extreme magnesium metasomatism caused by 

reaction of seawater with hot sediment and (or) hydrother108 791 675 670 240 

47 26 12 13 8 

183 106 66 65 53 

228 77 15 11 58 

714 1,220 449 576 295 

184 70 33 25 22 

20 20 5 20 10 

0 124 65 60 29 

24 22 9 10 8 

10 282 342 369 403 

12 7 0 7 6 

223 165 77 106 69 

35 24 19 19 17 

288 142 106 122 55 

147 110 152 137 133 

mal fluid percolating through sediment. Magnesia content 

has increased from approximately 4 weight percent in unaltered mud to 25 weight percent; silica has decreased from 

approximately 53 weight percent to 31 weight percent 

(table 14.1). The apparent increases in alumina and other 

"immobile" elements (fig 14.5C) are the result of residual 

enrichment caused by silica removal. The alkali and 

alkaline-earth elements have been stripped from the sediment (fig. 14.5D, E). 

Because the altered sediment in sample L2-86-9G 115 

to 122 em is nearly monomineralic chlorite, the major-element 

chemical data for this sample (table 14.1) can be used to 

calculate an approximate composition for the chlorite. This 

calculated composition (table 14.2) falls within the clinochlore 

field (Hey, 1954) with an Fe/Fe+Mg ratio of0.21. 

Sample L2-86-4G 138 to 145 em is typical unaltered 

mud representative of the protolith for the chlorite alteration. Using the major-element chemical data for this 

sample and assuming a simple mineralogy consistent with 

the XRD data, the proportions of the minerals composing 

the sample can be calculated, using the mineral compositions given in table 14.2. Because immobile elements, 
268 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

5 30 

I- EXPLANATION A I- 8 z c Unaltered z w w () • Smectite () 

a: • Smectite-chlorite a: 25 w 4 A Chlorite w a.. x Talc a.. 

I- II I 

C!J ~ 20 

w w • 

3;: 3 3;: 

~ ~ 

I-- ~ z z w 2 w I- Iz z 

0 0 

() () 

0 d" m C\J C\J 5 

z <i: 

• • 0 

1 2 3 4 5 0 5 10 15 20 25 30 

CaO CONTENT, IN WEIGHT PERCENT MgO CONTENT, IN WEIGHT PERCENT 

25 30 

Ic • D 

z Iw z 

() , w 2 

a: 20 () 

w a: 

a.. w 

Ia.. 

I I- 20 

C!J I 

15 C!J w w 3;: 3;: • ~ 15 • ~ I--

10 I--

z 

w z 

I- w 10 

z I0 z 

() 0 

5 () • 

d" 0 5 C\J C) 

<i: ~ 

.. 

0 0 

0 0.2 0.4 0.6 0.8 1 1.2 0 1 2 3 4 5 

Ti02 CONTENT, IN WEIGHT PERCENT Na2

0 CONTENT, IN WEIGHT PERCENT 

30 30 

• E • F 

I- Iz z ' w w 25 () () 

a: a: w w a.. a.. 

I- I- 20 

I I 

~ C!J w w 

3;: 3;: 15 • ~ ~ 

~ ~ z z w 10 w 10 I- Iz z 

0 • 0 

() () 

0 5 0 5 C) C) 

~ ~ 

0 0 

0 2 3 4 5 0 2 4 6 8 10 12 

CaO CONTENT, IN WEIGHT PERCENT 8 180 (SMOW), IN PER MIL 
SEDIMENT ALTERATION ASSOCIATED WITH MASSIVE SULFIDE FORMATION IN ESCANABA TROUGH 269 

including aluminum, behaved conservatively during alteration (fig 14.5C), the reaction converting sediment to chlorite can be calculated by conserving aluminum during 

alteration: 

0.266 Qtz + 0.071 Ab + 0.055 An+ 0.033 Bi + 0.023 Kaol 

+ 0.426 Mg2+ + 0.038 Fe2+ + 1.704 H20 

~ 

0.0745 Ch + 0.422 Si(OH)4 + 0.071 Na+ + 0.066 K+ 

+ 0.055 Ca2+ + 0.764 H+ (1) 

This equation illustrates the importance of magnesium 

addition and silica removal during alteration. This intense 

Mg-metasomatism generates large quantities of hydrogen 

ions, which further attack the silicate structures. The strong 

acid generation resulting from Mg-metasomatism is analogous to the reactions occurring during experimental interaction of seawater with basalt (Bischoff and Dickson, 1975; 

Seyfried and Bischoff, 1977) or sediment (Bischoff and 

others, 1981; Thornton and Seyfried, 1985) at elevated temperatures. The low pH created by precipitation of magnesium silicates is an important control on both the alteration 

assemblage formed and the fluid geochemistry and reactions occurring "downstream" from the zone of magnesium-metasomatism. Hydrolytic alteration by the low-pH 

fluids generated by Mg-metasomatism will enhance dissolution of silicate grains and removal of cations from the 

sediment, as indicated by equation 1, consistent with the 

low concentrations of alkali and alkaline-earth elements in 

the altered sediment (table 14.1 ). 

The high-temperature vent fluids at Escanaba Trough 

contain essentially no magnesium (Campbell and others, 

chap. 11, this volume), similar to other sea-floor hydrothermal systems (Von Damm and others, 1985a, b). Furthermore, the amount of magnesium contained in interstitial 

pore fluid trapped in the sediment prior to alteration is in-

<= 

Figure 14.5. A, Plot of Na20 versus CaO for unaltered and altered 

sediment samples. Weak correlation ofNa20 and CaO in unaltered 

samples reflects abundances of plagioclase. Highly altered samples 

have been stripped of Na, K, Rb, and Ca. B, Plot of Al20 3 versus 

MgO delineates different alteration types defined by predominant 

alteration mineralogy. C, Plot of Al20 3 versus Ti02 shows strong 

correlation indicating that these elements have behaved 

conservatively during alteration. Cr, Ni, Sc, V, Y, and Zr are also 

strongly correlated with Al20 3 and Ti02. D, Plot of MgO versus 

Na20 shows decrease of Na20 with increasing MgO, which increases 

with increasing degree of alteration; K20, and Rb20 show similar 

behavior. E, Plot of MgO versus CaO shows decrease of CaO with 

increasing MgO, which increases with increasing degree of alteration. 

F, Plot of MgO versus o18o shows decreasing 5180 values with 

increasing alteration due to high-temperature exchange with 

hydrothermal fluid. 

Table 14.2. Mineral compositions used for mass balance 

calculations. 

Chlorite (Ch) 

Quartz (Qtz) 

Albite (Ab) 

Anorthite (An) 

Biotite (Bi) 

Kaolinite (Kaol) 

Mg7 .osFe I.84Al2.69(All.86Si6.14)0zo(OH) 16 

Si02 

NaA1Si30 8 

CaA12Si20 8 

K2Mg3Fe3Si6Al20 20(0H)4 

Al4Si40 10(0H)8 

sufficient to drive the reaction to completion. The only reasonable source of magnesium is seawater that has mixed 

with high-temperature hydrothermal fluid or with sediment 

heated by the flux of high-temperature fluids. The high 

thermal gradient of these systems near the sea floor drives 

secondary convective circulation cells, which pull seawater into a high-temperature reaction zone beneath and adjacent to active hydrothermal discharge zones. A minimum 

water-to-rock ratio can be calculated for the alteration zone 

by assuming that magnesium is quantitatively removed 

from seawater as it flushes through the reaction zone. The 

stoichiometry of equation 1 requires a minimum seawaterto-sediment weight ratio greater than 90. Alternatively, if 

we assume that the minimum measured Mg concentration 

in hydrothermal fluid (1.6 mmoVkg; Campbell and others, 

chap. 11, this volume) is the only source of magnesium, 

then quantitative removal into the alteration products requires a minimum seawater-to-sediment weight ratio 

greater than 1,000. Seawater mixing is the most likely 

source for the magnesium added to the sediment, but the 

large amount of seawater needed to supply the magnesium 

necessary for total conversion of sediment to chlorite will 

also affect the thermal regime in the alteration zone. Isotopic data presented below indicate that chlorite formed at 

high temperature (above 200°C). Large amounts of hydrothermal fluid and (or) high conductive heat flux is therefore required to maintain the alteration zone at temperatures 

near 200°C while mixing with large volumes of 1. 7°C seawater. 

TALC ALTERATION 

Talc-rich samples have levels of magnesium enrichment similar to the chlorite alteration, but are clearly distinguished by lower aluminum content (fig 14.5B). The 

other "immobile" elements are correlated with aluminum 

and have low values in these samples as illustrated by the 

plot of Al20 3 versus Ti02 (fig. 14.5C). Both petrographic 

and chemical data suggest that talc is a direct chemical precipitate and not a hydrothermal replacement of detrital minerals in the sediment. The low contents of Al20 3, Ti02, 

Cr, V, and Zr reflect minor amounts of sediment incorporated in the talc-rich samples. The formation of talc on 
270 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

the outer edges of massive sulfide samples and the depositional textures indicating growth into open space support 

direct precipitation of talc. Mixing of high-temperature 

silica-rich hydrothermal fluid with cold magnesium-rich 

seawater at the seawater-sulfide interface results in supersaturation and precipitation of talc (Janecky and Seyfried, 

1984 ). Removal of magnesium from seawater by talc precipitation also causes hydrogen ion generation: 

3 Mg2+ + 4 Si(OH)4 <=> 

Mg3Si40 10(0H)2 + 4H20 + 6 H+ (2) 

The decrease in pH that accompanies talc precipitation 

may be responsible for the alteration of pyrrhotite and the 

precipitation of marcasite commonly observed in talc-rich 

samples (Murowchick and Barnes, 1986). 

The Mg-rich, Al-poor smectite that coprecipitates with 

anhydrite in the active vents is also formed by mixing of 

hydrothermal fluid and seawater. The hydrothermal fluid 

contains abundant calcium and silica, but essentially no 

sulfate or magnesium, which are supplied by mixing with 

seawater. Anhydrite and magnesium-silicates are common 

constituents of active vent edifices (Styrt and others, 1981; 

Brett and others, 1987), and have been shown by isotopic 

(see below) and thermodynamic (Janecky and Seyfried, 

1984) studies to form by mixing hydrothermal fluid with 

seawater. Higher silica activity favors the formation of talc 

relative to magnesium-smectite. Talc formation on massive sulfide may result from slower, more diffuse flow 

through sulfide mounds compared to the more rapid, focused discharge at active vents. The slower flow through 

the thermally conductive sulfide mounds would facilitate 

conductive cooling of hydrothermal fluid, leading to increased silica oversaturation (Janecky and Seyfried, 1984). 

The formation of amorphous silica, which overgrows talc 

in some samples, requires cooling of the hydrothermal fluid 

by thermal conduction as opposed to mixing with cold seawater (Janecky and Seyfried, 1984). Precipitation of talc 

at the seawater-sulfide interface may seal the system 

enough to allow hydrothermal fluid in the sulfide mounds 

to conductively cool to the point where amorphous silica 

can precipitate. 

ISOTOPE CHEMISTRY 

OXYGEN ISOTOPES 

Thirteen samples of altered and unaltered sediment 

were analyzed for oxygen isotopes to constrain the extent 

and conditions of alteration. Unaltered sediment samples 

range in value from 8.7 to 10.3 per mil (table 14.3; fig. 

14.6), primarily reflecting the value of detrital igneous and 

metamorphic quartz that is the dominant component of the 

sediment. Altered sediment samples range from 2.2 to 9.0 

per mil (table 14.3; fig. 14.6). A plot of magnesium conTable 14.3. Oxygen isotope values for bulk sediment samples 

Sample Depth Grain size oiso 

(per mil) 

Unaltered sediment 

L 1-86-8G ---------------------- 174-179 em sand 9.2 

L2-86-4G ---------------------- 138-145 em mud 9.2 

L2-86-28G -------------------- 119-126 em mud 9.2 

DSDP 5-35 3-5-29-33 ------- 85 m sand 10.3 

DSDP 5-35 5-4-127-129---- 103m sand 8.7 

DSDP 5-35 17-1-130-135 -- 383m sand 8.7 

Altered sediment 

L1-86-3D-1 ------------------- surface smec-chl 7.7 

L 1-86-15D-2 ------------------ surface talc 5.8 

L 1-86-24 D-21 ---------------- surface talc 3.5 

L 1-86-27G -------------------- 77-80 em smectite 8.2 

L 1-86-27 G -------------------- 84-87 em smectite 9.0 

L2-86-9G ---------------------- 106-109 em chlorite 2.2 

L2-86-9G ---------------------- 115-122 em chlorite 2.6 

tent, which serves as an approximate index of the extent 

of alteration, versus 0180 shows that 0180 decreases as 

samples become more altered (fig. 14.5F). The decrease 

in 0180 is related to formation of alteration minerals from 

hydrothermal fluid at high temperature. 

To quantitatively evaluate the conditions of formation 

of the alteration minerals, o18o values were determined for 

the fine-grained fraction of the sediment separated for XRD 

analyses of clay minerals. Samples separated by grain size 

for clay mineralogy were used in an attempt to obtain pure 

or nearly pure separates of hydrothermal clay minerals. 

Generally, only the finest grain size fraction was analyzed 

because this fraction was often nearly monomineralic and 

because the finest grain size fraction should more readily 

approach isotopic equilibrium upon heating by hydrothermal fluid (Yeh and Savin, 1976). The isotope values for 

the clay separates are presented in table 14.4, which also 

lists the dominant and minor minerals present in each 

sample. 

DETRITAL SEDIMENT 

The fine fraction of unaltered sediment is generally 

dominated by smectite and has 0180 values that overlap 

the range measured in bulk unaltered sediment (fig. 14.6), 

consistent with previous studies that document the preservation of original 0180 values in detrital clay in low-temperature authigenic minerals (Yeh and Savin, 1976). 

Several samples have 8180 values higher than the values 

for the bulk sediment. All of these isotopically heavy 

samples are dominantly smectite, and the isotope values are 

a reflection of lower temperature of formation of the 

smectite. o18o values above 15 per mil are only observed 

in smectite-rich samples from the surface sediment recovered in the cores. Most cores recovered an oxidized sur-
SEDIMENT ALTERATION ASSOCIATED WITH MASSIVE SULFIDE FORMATION IN ESCANABA TROUGH 271 

face sediment layer 1 to 5 em thick formed in the zone ventilated by bioturbation (Normark and others, chap. 6, this 

volume). Surface samples differ from deeper intervals in 

their red-brown color, due to the presence of iron oxides, 

and generally higher abundance of smectite compared to 

samples deeper in the cores. Burial of sediment below the 

zone of bioturbation is followed by reduction of iron ox-

>- (.) 

6 

5 

4 

3 

2 

0 

-

~ 

-

~ 

-

A. Hydrothermal samples 

I I I I I I I I 

z 

w 

::> 

a 

w 

a: 

0 2 4 6 8 

LL 6 

5 

4 

3 

2 

0 

0 

B. Detrital sediments 

EXPLANATION 

EL] Unaltered sediment 

• Clay fraction 

2 4 6 8 

ides that destroys the red-brown pigmentation, resulting in 

the gray-green mud typical of the hemipelagic sediment. 

The high o18o values for the samples from the surface 

oxidized zone are a reflection of the low temperatures of 

formation of the diagenetic oxides and (or) smectite. 

Smectite from surface sediment from core L2-86-4G is not 

enriched in o18o to the extent of samples from the oxidized 

EXPLANATION 

• Chlorite 

~ Talc 

m Smectite 

I I I I I I I I I 

10 12 14 16 18 

10 12 14 16 18 

3 

18

0 (SMOW), IN PER MIL 

Figure 14.6. A, Histogram of 8180 values for bulk sediment and 

clay fraction separates of different alteration types defined in text. 

Mineral separates of chlorite and smectite have 8180 values similar 

to bulk sediment from the same sampling intervals. Bulk sediment 

samples labeled "talc" show peaks for talc only on XRD traces. B, 

Comparison of bulk-sediment 8180 values with values from clay 

fraction separates. Smectite-rich clay fractions with 8180 values 

greater than 15 per mil are from zone of red-brown surface oxidized 

sediment that occurs at top of most cores. 
272 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

Table 14.4. Oxygen isotope values for clay fraction separates. 

[smec, smectite; chl, chlorite; qtz, quartz; ill, illite] 

Sample Depth 

(em) 

Unaltered sediment 

L 1-86-8G -------------- 174-175 

L 1-86-19G ------------ surface 

L1-86-27G-Fine ------ surface 

L1-86-27G-Coarse --- surface 

L2-86-27G ------------ 72.5-74 

L 1-86-33G ------------ surface 

L 1-86-33G ------------ 104-105 

L2-86-4G -------------- surface 

L2-86-7G -------------- surface 

L2-86-9G -------------- 36-40 

L2-86-28G ------------ surface 

L2-86-28G ------------ 119-126 

Hydrothermal clay 

L2-86-27G ------------ 74-77 

L2-86-27G ------------ 77-80 

L2-86-27G ------------ 84-87 

L2-86-9G-Fine ------- 106-109 

L2-86-9G-Coarse ---- 106-109 

L2 86-9G -------------- 115-122 

surface layer, consistent with the absence of this layer in 

the core (Normark and others, chap. 6, this volume). It was 

possible to separate the finer grained smectite-rich fraction 

from a coarser clay fraction dominated by detrital quartz, 

chlorite, and illite in the surface sediment sample from core 

L1-86-27G. The smectite-rich fraction has a o180 value 

of 16.2 per mil compared to 8.8 per mil for the detrital fraction. The latter is similar to the isotopic value of unaltered 

sediment. 

HYDROTHERMAL CLAYS 

Hydrothermal smectite separates from the mineralized 

part of core L1-86-27G have o180 values ranging from 6.0 

to 8.7 per mil (table 14.4). The lowest value is from the 

sample with the smallest amount of detrital clays. The 

8180 values of hydrothermal smectite are only slightly 

lower than bulk-sediment o18o values from the same intervals, even though the bulk of the sediment in these 

samples is detrital quartz sand. Isotopic fractionation temperatures, calculated using the smectite-water fractionation 

equation given in Savin and Lee (1988) and assuming equilibrium with hydrothermal fluid with an isotope value equal 

to Escanaba Trough vent fluid of 0.4 per mil (Bohlke and 

Shanks, chap. 12, this volume), range from 181 °C to 

240°C. These values are similar to the measured vent temperature of 217°C at the NESCA site and similar to the 

220°C temperature calculated from oxygen isotopes for 

smectite precipitated in the 217°C vent structure (Bohlke 

Mineralog:y 8Iso 

Major Minor (per mil) 

smec chi, qtz 7.9 

smec chi, qtz 15.9 

smec 16.2 

qtz,chl, ill smec, pi 8.8 

qtz chi, ill 6.3 

smec chi, ill 15.3 

chi, smec, ill 12.7 

smec 12.0 

smec 16.6 

smec 13.7 

smec ill, chi 15.9 

smec ill, chi 11.1 

smec chi, ill 8.1 

smec 6.0 

smec chi, ill 8.7 

chi 

chi 

chi 

2.4 

2.1 

2.7 

and Shanks, chap. 12, this volume). These o180 values of 

hydrothermal smectite are lower than those of smectite 

from the unaltered samples, with the exception of sample 

L1-86-8G 174 to 175 em, which has a value of7.9 per mil. 

Examination of magnetic separates from this core by SEM 

revealed the presence of hydrothermal pyrrhotite in contrast with other cores, which contain only detrital magnetite and ilmenite. Morton and others (1987b) identified the 

lower part of core L 1-86-8G as hydrothermally altered 

sediment based on the bleached gray color and texture of 

the sediment. While the chemistry and mineralogy of this 

core indicate that it is predominantly composed of detrital 

minerals, the presence of pyrrhotite and the low o18o value 

from this core suggest that the turbiditic lower part of this 

core contains some hydrothermal component. 

Hydrothermal chlorite separates from the lower altered 

part of core L2-86-9G have very low o180 values ranging 

from 2.1 to 2.7 per mil (table 14.4). The isotopic values 

of the bulk-sediment samples overlap those of the chlorite 

separates because the sediment is nearly monomineralic 

chlorite. Calculated temperatures of chlorite formation using the fractionation equation of Wenner and Taylor ( 1971) 

and a hydrothermal fluid value of 0.4 per mil (Bohlke and 

Shanks, chap. 12, this volume) range from 199°C to 221 °C. 

These temperatures are very close to the measured temperature of active hydrothermal vents at the NESCA site 

(217°C). The calculated temperatures of formation are consistent with the occurrence of the type lib structural 

polytype, stable at elevated temperatures (Bailey and 

Brown, 1962). The high temperature indicated for forma-
SEDIMENT ALTERATION ASSOCIATED WITH MASSIVE SULFIDE FORMATION IN ESCANABA TROUGH 273 

tion of chlorite limits the rate at which cold magnesiumbearing seawater, necessary for Mg-metasomatism, can 

flow into the alteration zone. 

Mineral separates were not prepared for talc-rich 

samples; however, the samples analyzed for bulk-sediment 

o18o values are essentially monomineralic as determined 

by XRD, which shows talc as the only mineral present. If 

we assume that the samples are composed of talc and apply 

the talc-water fractionation equation of Savin and Lee 

(1988), then using a hydrothermal fluid value of 0.4 per 

mil (Bohlke and Shanks, chap. 12, this volume) we calculate formation temperatures of 222°C and 277°C. These 

temperatures are similar to the temperature of the hydrothermal vents and values calculated for chlorite and 

smectite alteration, but extend to higher range. A higher 

temperature for talc formation might be expected because 

talc is closely associated with pyrrhotite-rich massive sulfide. The presently venting hydrothermal fluids carry little 

dissolved base metal (Campbell and others, chap. 11, this 

volume) and are not precipitating massive sulfide where 

they vent to the sea floor. Mineral assemblages in massive 

sulfide samples indicate formation temperatures of approximately 300°C (Koski and others, chap. 16, this volume). 

SULFUR ISOTOPES 

The sulfur isotope compositions of sulfide and sulfate 

samples from the sediment cores (table 14.5) were determined to allow comparison to massive sulfide samples previously measured (Koski and others, 1988; Bohlke and 

Shanks, chap. 12, this volume). Disseminated and vein sulfide from the mineralized sandy turbidite layers at the base 

of core L1-86-27G range in o34S value from 3.9 to 7.1 per 

mil, similar to the range of values reported for Escanaba 

Trough massive sulfide samples by Koski and others 

(1988). Sulfur isotope values of coexisting pyrrhotite and 

chalcopyrite are similar, but isotopic equilibrium among 

sulfide minerals is not commonly observed in sea-floor 

massive sulfide deposits (Zierenberg and others, 1984 ). 

Sulfur in barite from this core is slightly heavier than sulfur in seawater sulfate. Barite is interpreted to form by 

mixing of sulfate-bearing pore fluid of seawater composition with barium-rich hydrothermal fluid, perhaps accompanied by a minor amount of sulfate reduction. 

Barite intergrown with massive sulfide fragments from 

the sulfide debris flow in core L2-86-9G has a o34s value 

indistinguishable from that of seawater sulfate and is interpreted to have formed from mixing of hydrothermal fluid 

with seawater on the sea floor. Sulfide minerals in the massive sulfide fragments have o34s values ranging from 2.9 

to 4.8 per mil, which overlap with the lower range of values previously measured (Koski and others, 1988). Only 

minor amounts of sulfide occur in the chlorite-altered sediment that underlies the sulfide debris flow. The dominant 

Table 14.5. Sulfur isotope values for sediment-hosted mineralization. 

Core Depth Mineral 034s 

(em) (per mil) 

L1-86-27G ------ 64 Barite 22.3 

Pyrrhotite 5.0 

Isocubanite 3.9 

L1-86-27G ------ 72.5-74 Pyrrhotite 5.9 

Isocubanite 6.2 

L1-86-27G ------ 74-77 Pyrrhotite 6.4 

Isocubanite 6.3 

L1-86-27G ------ 77-80 Pyrrhotite 6.1 

Isocubanite 5.3 

L1-86-27G ------ 80-84 Pyrrhotite 6.8 

Isocubanite 6.1 

L1-86-27G ------ 84-87 Pyrrhotite 6.7 

Isocubanite 7.1 

L2-86-9G -------- 62 Barite 20.5 

Isocubanite 2.9 

L2-86-9G -------- 60-64 Pyrrhotite 4.8 

Isocubanite 4.6 

L2-86-9G -------- 106-109 Pyrite 7.8 

L2-86-9G -------- 111 Pyrite 6.1 

L2-86-9G -------- 115-122 Pyrite 7.4 

sulfide in the altered sediment is euhedral pyrite. Pyrite 

is rarely observed in massive sulfide samples from 

Escanaba Trough and is usually present as rare framboidal 

pyrite or as late-stage botryoidal pyrite associated with 

marcasite. The dominance of pyrite in the altered sediment 

requires higher sulfur fugacity conditions than generally 

prevailed during precipitation of the pyrrhotite dominated 

massive sulfide. Pyrite o34S values are distinctly higher 

than values measured in sulfide minerals from the overlying massive sulfide debris flow, although still within the 

range of values common to Escanaba Trough massive 

sulfide (Koski and others, 1988). The chemistry of the silicate alteration requires mixing of large amounts of seawater 

with hydrothermal fluid in the alteration zone. Seawater 

supplies sulfate that potentially could react with the hydrothermal fluid to raise both the f s2 and o34s values of the 

fluid. We assume that the o34S value of pyrrhotite in the 

overlying massive sulfide ( 4.8 per mil) is representative of 

H2S in the local hydrothermal fluid and that seawater sulfate has a o34s value of 21 per mil (Rees and others, 1978), 

similar to the barite in this core. If pyrite in the mixing 

zone formed by the following reaction: 

4 Fe2+ + 7 H2S +Sol-<:::> 4 FeS2 + 4 H20 + 6 H+ (3) 

then the expected o34s value of pyrite can be calculated 

by isotope mass balance to be 6.8 per mil, in reasonable 

agreement with the observed value. It appears that abiotic 

reduction of seawater sulfate mixed into the alteration zone 

increases the f s2 value of the fluid and leads to the precipitation of isotopically enriched pyrite. 
274 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

COMPARISON TO ORE DEPOSIT 

ALTERATION 

Sediment alteration in Escanaba Trough is a result of 

interaction of hydrothermal fluid with sediment accompanied by mixing with pore water and (or) seawater. Flow 

of seawater into the alteration zones beneath and adjacent 

to the Escanaba Trough deposits is the critical factor that 

leads to magnesium-metasomatism of the sediment. Removal of magnesium from the fluid during alteration has 

important effects on the fluid chemistry, primarily by production of hydrogen ion. Seawater mixing also influences 

fluid parameters such as temperature, f 0 2, and f S2, and 

therefore exerts a major control on the transport, deposition, and phase chemistry of base-metal sulfide minerals 

as well as silicate alteration minerals. 

Sea-floor hydrothermal deposits on sediment-covered 

spreading centers have been described from Guaymas 

Basin in the Gulf of California and at Middle Valley on 

the northern Juan de Fuca Ridge. Inactive hydrothermal 

deposits sampled in the Guaymas North Basin are predominantly talc and pyrrhotite (Lonsdale and others, 1980). The 

talc in these deposits was interpreted to have formed at 

about 280°C by the mixing of hydrothermal fluid with seawater (Lonsdale and others, 1980). Talc and magnesiumrich smectite identified as stevensite are also forming in 

hydrothermally active mounds and chimneys in the 

Guaymas South Basin (Koski and others, 1985; Peter and 

Scott, 1988), even though measured Mg concentrations in 

hydrothermal fluid are as low as 0.60 mmol/kg (Von Damm 

and others, 1985b ). Mixing of hydrothermal fluid with seawater is interpreted as the mechanism leading to precipitation of magnesium-rich silicates in the Guaymas 

hydrothermal deposits (Koski and others, 1985; Peter and 

Scott, 1988). Magnesium-rich phyllosilicates, including 

talc, are also described from hydrothermally altered sediment from the Middle Valley (Goodfellow and Blaise, 

1988). Sulfide-rich horizons in the sediment cores have 

elevated magnesium contents (Goodfellow and Blaise, 

1988). 

Magnesium enrichment in alteration zones underlying 

massive sulfide deposits has also been well documented. 

Examples include the Archean deposits at Mattagami Lake 

(Roberts and Reardon, 1978) and Millenbach (Riverin and 

Hodgson, 1980), Canada; the Proterozoic Sullivan deposit, 

Canada (Hamilton and others, 1983); the Devonian Arctic 

Prospect, Alaska (Schmidt, 1988); Cretaceous deposits on 

Cyprus (Lydon and Galley, 1986; Richards and others, 

1989); and Tertiary Kuroko deposits, Japan (Urabe and others, 1983). The cited examples span both geologic time 

and a wide variety of tectonic settings. Roberts and 

Reardon (1978) recognized the importance of mixing between hydrothermal fluid and seawater in the formation of 

alteration zones in the upper parts of massive sulfide hydrothermal systems. The chemical and isotopic effects of 

fluid mixing have been thermodynamically modeled by 

Janecky and Seyfried (1984) and Janecky and Shanks 

(1989). Both field and theoretical studies clearly establish 

the importance of fluid mixing in the shallow subsurface 

as a factor controlling the formation of massive sulfide deposits. Studies that attempt to understand ore fluid composition, fluid inclusion and isotopic systematics, mineral 

precipitation, and mineralogical and chemical zonation in 

massive sulfide deposits and associated shallow alteration 

zones need to evaluate the effects of seawater mixing. 

Examination of the alteration zones of some massive 

sulfide deposits demonstrates that the cores of alteration 

pipes and the deeper zones of alteration differ from the alteration found in the shallow mixing zone. Some massive 

sulfide deposits, including Kidd Creek, Canada (Slack and 

Coad, 1989), and ophiolite-hosted deposits in Cyprus 

(Lydon and Galley, 1986; Richards and others, 1989) and 

Oregon (Zierenberg and others, 1988), have alteration 

zones that are characterized by the presence of iron-rich 

chlorite. These alteration zones are interpreted to reflect 

hydrothermal fluid-rock interaction below the zone of seawater mixing. This type of alteration is less well studied 

than the shallower alteration because exploration drilling 

and mine workings are often too shallow to intersect these 

zones, and because the deeper alteration zones are often 

separated from the sulfide deposits by tectonic disruption. 

This type of alteration has not been encountered in 

Escanaba Trough because our samples are restricted to 

near-surface zones. Further understanding of this important alteration type will require drilling below an actively 

forming massive sulfide deposit, such as those found in 

Escanaba Trough. 

SUMMARY 

Hydrothermally altered sediment is associated with 

sediment-hosted sea-floor massive sulfide deposits in 

Escanaba Trough. Three types of alteration have been distinguished based on the dominant alteration mineralogy: 

smectite, chlorite, and talc. The different alteration types 

have distinct geochemical signatures. Petrographic and 

geochemical data established that the different alteration 

types formed in distinct environments. Smectite alteration 

occurs mainly in the more permeable sandy aquifers. Sand 

grains are coated by Fe-Mg smectite formed by mixing of 

hydrothermal solutions with minor amounts of seawater or 

seawater-like pore fluid. Hydrothermal smectite is distinguished from detrital and diagenetic smectite in the degree 

of crystallinity and in lighter oxygen isotope composition, 

reflecting formation at temperatures near 200°C. Intensely 

altered sediment is completely replaced by hydrothermal 

clinochlore. Extensive mixing of seawater with hydrothermal fluid at temperatures above 200°C are required to explain the chemical and isotopic composition of the chlorite 
SEDIMENT ALTERATION ASSOCIATED WITH MASSIVE SULFIDE FORMATION IN ESCANABA TROUGH 275 

alteration. The formation of talc and Mg-smectite on massive sulfide and in active vents, respectively, proceeds primarily by direct precipitation in the zone of mixing of 

hydrothermal fluid with seawater without extensive replacement of detrital silicates. Magnesium metasomatism 

caused by mixing has important consequences for alteration 

and mineralization processes, primarily due to production 

of hydrogen ion during alteration. Geochemical characteristics common to each alteration type are an increase of 

magnesium coupled with a decrease in N a, K, Rb, and Ca. 

High-field-strength elements including AI, Ti, Cr, Ni, Sc, 

V, Y, and Zr remained relatively immobile during hydrothermal alteration. 

The alteration processes and products observed in 

Escanaba Trough are comparable to the shallow alteration 

zones found below and adjacent to ancient massive sulfide 

deposits formed over the span of geologic time in diverse 

tectonic environments. Escanaba Trough is therefore an 

important natural laboratory where the processes of mineral deposit genesis can be investigated. 

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SEDIMENT ALTERATION ASSOCIATED WITH MASSIVE SULFIDE FORMATION IN ESCANABA TROUGH 277 

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10 (this volume). 

Chapter 15. Hydrocarbons in Sediment from 

Escanaba Trough 

By Keith A. Kvenvolden, John B. Rapp, and Frances D. Hostettler1 

CONTENTS 

Page 

Abstract ..... ......... .... ...... .... .......... ... . ............ .......................... 279 

Introduction .. .... ... ..... .... .... ....... ... ...... .. .. .. ............ .. .. .. .. .. .. ...... 279 

Acknowledgments .... .. .. .. ........ ... ..... ... . .... ... ............ ...... 279 

Geologic setting . ... .......... ... ... .... .. .............. ........................... 279 

Methods ..... ..... .... .............. ...... ................... ... ..... .... ... ............ 280 

Samples .. ...... ......... .... ... ...... ....... ........ ................................... 280 

Results . . .. . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . .. . . .. . . .. . . .. . . .. . . . . . . .. . . . . . . .. . . .. . . .. . . . . . . 281 

Normal alkanes and isoprenoid hydrocarbons ........... 281 

Triterpanes and steranes .............................................. 284 

Discussion ............................................................................ 286 

Pleistocene and Holocene sediments ..... ... . .... ......... .... 286 

Altered Quaternary sediments..................................... 288 

Hydrothermal oil.......................................................... 289 

Summary .... ...... .. . ... ...... ... . ....... ..... .. .. .......... .. .... .. .. .. .. .... .. ...... 290 

References cited ..... ... . ... ............. .... ... . ... . ................ ... ........... 291 

ABSTRACT 

Normal alkanes, isoprenoid hydrocarbons, triterpanes, 

and steranes are present in low concentrations in sediment 

that fills Escanaba Trough at the southern end of the Gorda 

Ridge spreading axis. Distributions of n-alkanes are generally dominated by odd-carbon-number molecules suggesting terrigenous sources. Epimer ratios of triterpanes 

and steranes have a wide range of maturity values, with 

Pleistocene sediment having lower maturity values than 

Holocene sediment. Hydrothermal activity in Escanaba 

Trough has altered the distribution of hydrocarbons in the 

sediment and has led to the generation of petroleum that 

is found in sediment-hosted metallic sulfide deposits. 

INTRODUCTION 

Gorda Ridge is an active oceanic spreading axis located about 200 to 300 km offshore from southern Oregon 

and northern California (fig. 15.1). The southernmost segment of this ridge, Escanaba Trough, is partly filled with 

1 U.S. Geological Survey, Menlo Park, California. 

Quaternary turbidite sediment. The trough is the site of 

hydrothermal activity as first indicated by the discovery of 

various hydrothermal mineral deposits (Clague and others, 

1984 ), including massive metallic sulfide samples (Morton 

and others, 1987a; Koski and others, 1988). Active hydrothermal vents were discovered in the trough in 1988 

(Campbell and others, 1988; Zierenberg and others, 1988). 

One result of the hydrothermal activity within the sediment 

of the trough is the formation of petroleum, first discovered in 1985 (Kvenvolden and others, 1986). The hydrocarbon geochemistry of four examples of hydrothermally 

derived petroleum has been described by Kvenvolden and 

others (1987). The most likely source for this petroleum 

is the organic material in the sediment of the trough. This 

report examines a portion of this organic material, namely, 

the alkanes that were extracted from sediment recovered 

by gravity coring during RIV S.P. Lee cruises Ll-86-NC 

and L2-86-NC. This is the second report to describe the 

hydrocarbons in sediment of Escanaba Trough; the first report (Simoneit, 1977) presented limited information on hydrocarbons from DSDP Site 35 (fig. 15.1). Other studies 

have been made previously of hydrocarbons in sediments 

of Guaymas Basin, an active spreading axis in the Gulf of 

California (Simoneit and others, 1979, 1984; Kawka and 

Simoneit, 1987; Bazylinski and others, 1988). 

ACKNOWLEDGMENTS 

We thank R.A. Koski and B.R.T. Simoneit for very 

helpful reviews, B.V. Fulop and P.A. Swenson for drafting the figures, and J.S. Gibbs for typing the manuscript. 

We particularly appreciate the comments of R.A. 

Zierenberg, who brought new insight into the interpretation of our results. 

GEOLOGIC SETTING 

For the purpose of geologic mapping, two areas of 

Escanaba Trough have been described by Morton and 

others (1987b) and Koski and others (1988). These areas are called SESCA and NESCA (fig. 15.1). In the 

279 
280 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

southern area (SESCA), sulfide deposits are spatially related to three steep-sided, sediment-capped blocks that 

rise above the axial valley floor. Massive sulfide deposits are concentrated on the flanks of the hills. The northern area (NESCA) is dominated by two hills, one capped 

by sediment and the other by sediment and lava flows. 

Massive sulfide deposits also occur on the flanks of these 

hills. The SESCA and NESCA areas overlie volcanic 

edifices that penetrate and locally breach the sediment. A 

third volcanic edifice underlies sediment between the 

SESCA and NESCA areas; one sampling site was located 

there. 

~ l--~~----i 

y .27G 

'~------------, , -~ SESCA : 

: 

: \ 9G•320-2• ~~; .. ~:i,~ 7G I 

: • 7G• : 

l ____________ : 

.osoPJS 

Figure 15.1. Locations of samples recovered by gravity coring (G), 

dredge (D), and submersible (R), in SESCA and NESCA survey 

areas of Escanaba Trough. Deep Sea Drilling Project (DSDP) Site 

35 is located south of SESCA. Hachures indicate area of closed low. 

METHODS 

Sediment samples for geochemical studies were collected from sediment cores that were obtained by gravity 

coring using a barrel 3 m long and 9 em in diameter with 

a 500-kg weight stand (Morton and others, 1987b ). Sediment core subsamples, 8 em long, were placed into 

precleaned glass jars and immediately frozen. Organic carbon (OC) was determined on a small portion of each 

sample by wet combustion following the procedure of Bush 

(1970). 

The procedures for extraction and analysis of hydrocarbons were adapted from Kvenvolden and others (1985). 

A 1 00-g portion of dried, pulverized ( < 32 mesh) sediment 

was triple extracted with dichloromethane by shaking on 

a wrist-action shaker. The combined filtered extracts were 

concentrated by rotary evaporation. Sulfur, if present, was 

removed by filtration through activated copper. A portion 

of the filtrate was removed for the determination of the 

weight of extract (EOM = extractable organic material). 

The dichloromethane solvent was then exchanged with nhexane in preparation for chromatography. 

The extracts in n-hexane were fractionated by liquidsolid chromatography using a column packed with activated silica gel and deactivated alumina. The column was 

sequentially eluted with n-hexane, benzene, and methanol 

to yield three fractions containing aliphatic hydrocarbons, 

medium-polarity organic compounds (including aromatic 

hydrocarbons), and polar organic compounds, respectively. 

Portions of each fraction were removed for weight determinations. For four samples, the elution sequence was 

modified, and six fractions were obtained: n-hexane; 20, 

40, 60 percent benzene in n-hexane; benzene; and methanol. Only then-hexane fractions (aliphatic hydrocarbons) 

are considered in detail in this paper. The aliphatic hydrocarbons were analyzed by capillary gas chromatography for 

distributions of n-alkanes and isoprenoid hydrocarbons, and 

by gas chromatography/mass spectrometry. Single-ion 

monitoring of mass-to-charge ratio ( rnlz) 191 and 217 produced mass chromatograms from which triterpanes and 

steranes, respectively, were identified and compared. Ratios 

of various organic compounds were determined from peak 

heights on gas chromatograms and mass chromatograms. 

SAMPLES 

Sediment samples came from nine core sites in 

SESCA, one core site to the west of SESCA, one core site 

from NESCA, and one core site between SESCA and 

NESCA. The cores are described in table 15.1. The nine 

cores from SESCA and one core west of SESCA consist 

predominantly of gray-green mud with occasional sandy 

turbidite layers (Normark and others, chap. 6, this volume). 
HYDROCARBONS IN SEDIMENT FROM ESCANABA TROUGH 281 

Table 15.1. Descriptions of Escanaba Trough sediment cores from which samples were taken for organic geochemical analyses. 

Sample Location Length of 

Latitude N. Longitude W. core (em) Description (Morton and others, 1987b) 

L 1-86-1 G ---------- 40°45.55' 127°28.93' 273 Gray-green, slightly porous mud with two distinct gray layers. 

L 1-86-7G ---------- 40°45.84' 127°30.48' 233 Gray-green mud with thin brown and gray layers and gray silty turbidite 

beds at base. 

Ll-86-8G ---------- 40°46.11' 127°29.93' 197 Gray-green mud overlying thick gray silty-sand turbidite beds. Minor 

amounts of pyrrhotite and resedimented hydrothermally altered 

sediment (Zierenberg and Shanks, this volume). 

L 1-86-19G --------- 40°46.44' 127°31.24' 255 Gray-green mud. 

Ll-86-26G --------- 40°45.78' 127°30.73' 239 Gray-green mud with gray sandy turbidite beds and disrupted gray mud 

intervals in upper 100 m of core. 

L 1-86-27G --------- 40°53.35' 127°31.65' 105 Blotchy gray-green mud (0-42 em); gray mud and silt with sandy blebs 

(42-72 em); gray sand with massive sulfide layers (72-105 em). 

L 1-86-3 3G --------- 40°58.73' 127°30.46' 124 Partly indurated smooth gray mud with black streaks, basalt glass chips, 

and a few silty blebs. 

L2-86-2G ---------- 40°46.16' 127°31.14' 96 Gray-green mud with discontinuous brown mud layers in upper part and 

silty sand layer with black fragments at base. 

L2-86-4G ---------- 40°45.79' 127°69.70' 263 Gray-green mud. 

L2-86-7G ---------- 40°43.79' 127°35.20' 241 Gray-green mud with thin discontinuous silty inner beds at base. 

L2-86-9G ---------- 40°45.59' 127°33.05' 200 Red-brown metalliferous mud; zone of massive sulfide nodules and sandsize sediment; blue-gray chloritized mud with white (talc?) nodules. 

L2-86-28G --------- 40°46.49' 127°30.17' 244 Gray-green mud with gray fine-sand turbidite bed near top. 

An exception is the sample from core L2-86-9G. This 

sample is composed of blue-gray chloritized mud. In addition to the cores, a dredge sample L1-86-32D-2 from 

SESCA contains fine-grained sandstone intermixed with 

metallic sulfide minerals and asphaltic petroleum. The 

geochemistry of this petroleum is described elsewhere 

(Kvenvolden and others, 1986, 1987). 

Gravity core Ll-86-27G, recovered between SESCA 

and NESCA, was more heterogeneous in composition compared to the majority of cores from SESCA. The analyzed 

sample from the 87 to 95 em depth interval is a gray, finegrained sand containing layers of fine-grained massive sulfide minerals. The sample obtained from NESCA (core 

L1-86-33G) is composed mainly of partly indurated gray 

mud. In addition to this core sample, a sample recovered 

by dredging (L2-86-14D-6) and two samples recovered by 

submersible (659-R1 and 659-R3) from NESCA were 

shown to contain hydrothermal petroleum, and the hydrocarbon chemistry of these samples has been discussed previously by Kvenvolden and others (1987). 

RESULTS 

Geochemical results for 18 sediment samples (summarized in table 15.2) include measurements of organic carbon (OC), extractable organic material (EOM), and the 

weights of the hexane fractions from liquid chromatography. The distributions of n-alkanes, isoprenoid hydrocarbons, triterpanes, and steranes are summarized by means 

of various ratios that will be defined as they are discussed 

in the text. The OC content of these samples is low, ranging from 0.13 to 0. 72 percent. EOM concentrations in our 

samples range from 2 to 90 mg/g, and aliphatic hydrocarbons in the hexane fraction are less than 15 mg/g. 

NORMALALKANESAND 

ISOPRENOID HYDROCARBONS 

The chromatographic distributions of n-alkanes and 

isoprenoid hydrocarbons for six samples (shown in fig. 

15.2) were selected to demonstrate the variety of compound 

distributions that are present in sediment of Escanaba 

Trough. Typically the n-alkanes range from about n-C12 

to as high as n-C40. In seven samples [L1-86-19G (90-

100), L1-86-19G (237-245), L1-86-26G (82-90), L1-86-

26G (221-229), L1-86-33G (106-114), L2-86-7G 

(133-141), L2-86-28G (136-144)], then-alkane distributions are bimodal, having maxima in two regions, one centered at n-C16 or n-C17 and the other at n-C27 or n-C29. 

The bimodal distributions of n-alkanes observed in samples 

L1-86-26G (82-90) and L1-86-330 (106-114) are shown 

in figure 15.2C,E. The n-alkanes in the region of n-C29 

have a distinct odd-carbon-number dominance, whereas in 

the region of n-C16 then-alkane distributions have neither 

odd- nor even-carbon-number dominance. The n-alkane 

distributions in seven samples [L 1-86-1 G (82-90), L 1-86-

lG (182-190), Ll-86-1G (255-263), L1-86-70 (82-90), 

L1-86-7G (215-237), L2-86-2G (78-86), L2-86-4G (145-

153)] have a prominent mode usually centered at n-C29, 

as illustrated for sample L1-86-7G (82-90) in figure 15.2A. 
N 

00 

N 

Cl 

ti1 

0 

r 

0 

Cl 

fl -

::I: 

~ 

Table 15.2. Organic geochemical data for sediment core samples and one oil from Escanaba Trough. tj 

:;:tl 

0 

[Age: H, Holocene; P, Pleistocene; QA, Quaternary Altered. n.d., not determined] >-3 

::I: 

ti1 

:;:tl 

Hexane I so- c s a~ Sample Interval Age oc EOM fraction n-Alkanes prenoids c3ol3j3 (em) (pet.) (Jlg/g) (Jlg/g) CPI Pr/Ph 31S+R 

Triterpanes Steranes ~ 

> ~a Tm a~ C3o s c 29a(R) r 

C3o- C21- C29 S+R > a~ Ts C29 C27a(R) z 

tj 

Ll-86-lG 82-90 H 0.53 44 6 1.8 3.0 0.52 6.5 0.19 2.0 2.0 0.26 1.0 tc 

182-190 p .46 25 3 4.7 3.0 .30 6.4 - 255-263 p .43 34 3 4.9 2.7 .29 7.0 

.28 3.7 2.1 .12 1.9 0 

.23 2.7 2.1 .13 1.4 r 

0 

Ll-86-7G 82-90 p .52 15 2 4.6 3.2 .23 3.2 .31 3.5 1.8 .13 1.7 Cl 

215-223 p .43 15 2 5.1 3.2 .27 4.0 - Ll-86-8G 82-90 QA .31 58 10 1.0 .8 .25 7.3 

179-187 QA .36 82 13 .9 .7 .24 9.0 

Ll-86-19G 90-100 H .51 40 6 2.0 3.0 .50 4.7 

.29 3.0 1.9 .13 1.6 n 

.75 8.7 .9 .05 1.1 t:f) 

>-3 

.71 7.8 .9 .06 1.1 e 

tj 

.22 2.1 2.0 .24 1.0 -ti1 

237-245 p .45 15 3 4.7 2.6 .30 6.1 

Ll-86-26G 82-90 H .72 29 3 2.0 2.9 .51 5.3 

.27 3.4 1.8 .14 1.4 t:f) 

.21 2.3 1.8 .24 .8 ~ 221-229 p .60 26 2 4.0 2.9 .27 4.0 

Ll-86-27G 87-95 QA .13 21 1 1.1 1.9 .40 n.d. 

Ll-86-33G 106-114 p .57 90 5 3.7 3.5 .33 5.0 

.27 4.5 1.8 .14 1.3 ti1 

t:f) 

.74 6.5 1.0 .12 1.0 n 

.40 3.0 1.6 .18 1.6 > z 

L2-86-2G 78-86 p .44 34 3 3.2 3.4 .30 6.1 

L2-86-4G 145-153 p .53 27 2 4.6 2.6 .27 12.3 

.31 3.4 1.5 .24 1.8 > 

.38 2.8 1.7 .19 1.6 tc 

> 

L2-86-7G 133-141 H .43 34 3 3.2 2.7 .40 6.5 

L2-86-9G 132-140 QA .36 2 n.d. n.d. 1.6 n.d. n.d. 

.27 2.7 1.4 .22 1.0 >-3 

n.d. n.d. n.d. n.d. n.d. :;:tl 

0 

L2-86-28G 136-144 p .54 26 3 5.0 2.8 .24 5.2 

L6-85-32D-2 Surface dredge 5.6 55,000 1,000 1.2 1.7 .47 n.d. 

.44 4.2 1.5 .12 2.6 e 

.66 21 .73 .24 .82 Cl 

~::I: 

Cl 

0 

:;:tl 

tj 

> 

:;:tl 

-tj 

Cl 

ti1 
HYDROCARBONS IN SEDIMENT FROM ESCANABA TROUGH 283 

In contrast, the n-alkane distributions in samples L1-86-

27G (87-95) and L2-86-9G (132-140) have a maximum 

at n-C16 and n-C15, respectively, and no odd/even-carbonnumber dominance (fig.15.2D,F). Samples L1-86-8G (82-

90) and L 1-86-8G ( 179-187) are unusual in that the n-alkanes 

have a distinct even-carbon-number dominance in the region 

from n-C18 to n-C28 with a maximum at n-C24, as shown for 

sample L1-86-8G (82-90) in figure 15.2B. 

r 

w 

Cf) 

z 

0 

a.. 

Cf) 

w 

0: 

A 

25 

23 

27 29 

7G(82-90) 

Pleistocene 

31 

33 

18 I . I, 

~JJltLUllJ J '~- c 

14 

E 

16 

7 

15 

18 

Pr 20 23 

Ph 

u..~ ..... 1 •• ' I. 

25 

25 

27 

J 

29 

lA 

26GC82-90) 

Holocene 

33G(106-114) 

Pleistocene 

29 

27 

31 

23 

151fluul~---·, ... "'-l,.J33~ 

Distributions of n-alkanes can be partially described 

by the carbon preference index (CPI), which is a measure 

of the distribution of odd-carbon-number n-alkanes relative to even-carbon-number n-alkanes, calculated over the 

range n-C24 to n-C34 following the method of Cooper and 

Bray (1963). With the exception of two samples from station L1-86-8G, all samples from the SESCA area (fig. 15.1) 

have high CPI values (1.8 to 5.1), indicating a dominance 

8 

13 

D 

F 

24 

22 26 

20 

18 

17 

uJ Ph 

. 

29 

LJ 

8G(82-90) 

Quaternary 

Altered 

31 

33 

40 I 

27G(87-95) 

Quaternary 

16 Altered 

15 17 

18 

Pr 

22 

25 29 

31 

~--'--'--l.-ll-LLLlJj_U_~_o 

9G(132-140) 

Quaternary 

Altered 

TIME --• 

Figure 15.2. Representative gas chromatograms showing distributions of n-alkanes, pristane (Pr) and phytane (Ph) in extracts from 

six sediment core samples from Escanaba Trough. Numbers specify carbon numbers of n-alkanes. 
284 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

of odd-carbon-number n-alkanes of high molecular weight 

(table 15.2). CPI values for samples L1-86-80 (82-90) and 

L1-86-80 (179-187) are low (0.9 and 1.0, respectively), 

reflecting the averaging influence of the dominant evencarbon-numbered n-alkanes in the lower part of the CPI 

range and the dominant odd-carbon-numbered n-alkanes in 

the upper part of the CPI range. A CPI value could not be 

determined for sample L2-86-90 (132-140) because of the 

very low concentrations of high molecular weight n-alkanes. Sample L1-86-270 (87-95) from the area between 

SESCA and NESCA, has a low CPI value of 1.1, reflecting a slight odd-carbon-number dominance; in contrast, the 

one sample L1-86-330 (106-114) from NESCA has a CPI 

value of 3. 7, reflecting a significant odd-carbon-number 

dominance. 

The two major isoprenoid hydrocarbons in sediment 

from Escanaba Trough are pristane and phytane, which are 

identified in figure 15.2 for six samples. In all samples, 

except L1-86-80 (82-90) and L1-86-80 (179-187), pristane is more abundant than phytane as reflected in the Pr/ 

Ph ratios greater than one (table 15.2). 

TRITERPANES AND STERANES 

The aliphatic hydrocarbons in sediment from Escanaba 

Trough contain a wide range of pentacyclic triterpanes. Only 

in sample L2-86-90 (132-140) were these kinds of compounds not found. Distributions of triterpanes are shown by 

mass chromatograms (m/z 191) in figure 15.3 for five 

samples. Compounds are identified in table 15.3. Typically, 

the samples contain the 17a(H),21f3(H)-hopane series, and 

17f3(H),21 a(H)-hopane (moretane) series. With the exception of samples L1-86-270 (87-95) and L2-86-90 (132-

140), all the samples contain the 17f3(H),21 f3(H)-hopane 

series. Both 22R and 22S epimers of the 17a(H),21f3(H)-

homohopanes are present, and the 22R epimer is usually more 

abundant than its corresponding 22S epimer. 

Steranes as well as diasteranes were found in all samples, 

except L2-86-90 (132-140), and typical mass chromatograms (rnlz 217) are shown in figure 15.4. Compounds are 

identified in table 15.3. The sediment samples contain both 

20R and 20S epimers of the C2r, C28-, and C29-

5a(H),14a(H),17a(H)-cholestane series. In addition, the 20R 

and 20S epimers of 13f3(H),17a(H)-diacholestane are present. 

Distributions of triterpanes and steranes are summarized through various ratios in table 15.2. Only in sample 

L2-86-90 ( 132-140) were these compounds not detected, 

and, therefore, ratios were not determined. C31 S/(S+R) 

is the ratio of the 22S and 22R epimers of 17a(H),21f3(H)-

homohopane. Only the 22R epimer is expected in immature mixtures. With increasing maturity, the preference for 

22R is lost in favor of about a 60:40 mixture of 22S and 

22R epimers, or an S/(S+R) ratio of 0.6 (Ensminger and 

others, 1974). In our sediment samples, this ratio varies 

Table 15.3. Identification of triterpanes and steranes. 

Carbon 

Peak Compound number 

Triterpanes (m/z 191) 

1 18a(H)-22,29 ,30-trisnorneohopane (Ts) C21 

2 17a(H)-22,29,30-trisnorhopane (Tm) C21 

3 17B(H)-22,29,30-trisnorhopane C21 

4 17a(H),21 BCH)-30-norhopane C29 

5 17 BCH) ,21 a( H)-30-normoretane C29 

6 17a(H),21 BCH)-hopane C3o 

7 17B(H),21 BCH)-norhopane + ? C29 

8 17B(H),21 a(H)-moretane C3o 

9 17a(H),21B(H)-homohopane(22S) and (22R) C31 

10 17B(H),21 BCH)-hopane C3o 

11 17B(H),21a(H)-homomoretane C31 

12 17a(H),21B(H)-bishomohopane(22S) and (22R) C32 

13 17B(H),21 a(H)-bishomomoretane(22S) and (22R) C32 

14 17B(H),21 BCH)-homohopane C31 

15 17a(H),21B(H)-trishomohopane(22S) and (22R) C33 

16 17B(H),21a(H)-trishomomoretane(22S) and (22R) C33 

17 17B(H),21 BCH)-bishomohopane C32 

18 17a(H),21 BCH)-tetrakishomohopane(22S) 

and (22R) C34 

19 17 BCH) ,21 a(H)-tetrakishomomoretane(22S) 

and (22R) C34 

20 17B(H),21 BCH)-trishomohopane C33 

21 17 a(H),21 BCH)-pentakishomohopane(22S) 

and (22R) C3s 

22 17B(H),21 a(H)-pentakishomomoretane(22S) 

and (22R) C3s 

23 17B(H),21 BCH)-tetrakishomohopane C34 

24 17B(H),21 BCH)-pentakishomohopane C3s 

Steranes and diasteranes (m/z 217) 

A 13 BCH), 17 a(H)-diacholestane(20S) c27 

B 13 BCH), 17 a(H)-diacholestane(20R) C21 

c 5a(H), 14a(H), 17a(H)-cholestane(20S) C21 

D 5a(H),14a(H),17a(H)-cholestane(20R) C21 

E 24-methyl-5a(H), 14a(H), 17a(H)-

cholestane(20S) + ? C2s 

F 24-me thy 1-5a(H), 14a(H), 1 7 a(H)-cholestane(20R) C2s 

G 24-ethyl-5a(H), 14a(H), 17a(H)-cholestane(20S) C29 

H 24-ethyl-5a(H), 14B(H), 17B(H)-cholestane(20R) C29 

I 24-ethyl-5a(H), 14a(H), 17a(H)-cholestane(20R) C29 

widely from 0.23 to 0.52, indicating a range of maturities. 

C30 af3/f3f3 is the ratio of 17a(H), 21 f3(H)-hopane, the most 

stable C30 hopane, to 17f3(H),21f3(H)-hopane, the least 

stable c30 hopane that is found in natural (biological) products (Ourisson and others, 1984). In sediments from 

Escanaba Trough, the af3-hopane is consistently more 

abundant than the f3f3-hopane (ratios range from 3.2 to 

12.3). In sample L1-86-270 (87-95), f3f3-hopanes are not 

present. C30 f3a/af3 is the ratio of the 17f3(H),21 a(H)-

hopane (moretane) to the 17a(H),21f3(H)-hopane. 

Moretanes are intermediates in the maturity scheme between unstable, natural (biological) product precursor f3f3-

hopanes and the stable af3-hopanes (Philp, 1985). The 

highest values of this ratio are found in samples from core 
HYDROCARBONS IN SEDIMENT FROM ESCANABA TROUGH 285 

L1-86-8G where the ratios are 0.75 and 0.71, and in sample 

L1-86-27G (87-95) where the ratio is 0.74. In the other 

samples this ratio ranges from 0.19 to 0.66. C27 Tmffs is 

the ratio of 17a(H)-22,29,30-trisnorhopane to 18a(H)-

22,29,30-trisnorneohopane, described originally by Seifert 

and Moldowan (1978) as a source and maturity index. In 

our sediment samples this ratio ranges from 2.0 to 8.7. Finally, a~ C3ofC29 is the ratio of 17a(H),21~(H)-hopane to 

A 6 

4 

B 4 6 

8 

2 5 

c 

1 

6 

w 4 

(f) 

z 

0 

a_ 

(f) 

w 

a: 

D 

2 4 6 

8 

5 

E 

6 

7 

9 

R 

9 

78(82-90) 

Pleistocene 

R 12 

9 

11 R 13 

R 

278(87-95) 

Quaternary 

Altered 

338(106-114) 

Pleistocene 

TIME __... 

88(82-90) 

21 

R 22 

R 24 

Quaternary 

Altered 

268(82-90) 

Holocene 

21 

s R 

Figure 15.3. Representative mass chromatograms ( rnlz 191) showing distribution of triterpanes in extracts 

from five sediment core samples from Escanaba Trough. Identifications are listed in table 15.3. 
286 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

f 

w 

(f) 

z 

0 

a_ 

(f) 

w 

0: 

A 7G(82-90) 

D 

~ Pleistocene 

8 D 8G(82-90) 

Quaternary 

Altered 

JJ»J~l1H c D 26G(82-90) 

F 

c I 

tNfrJ 

D 

A B 

E 

c 

Holocene 

D 27G(87-95) 

Quaternary 

Altered 

F 

33G(106-114) 

Pleistocene 

D F H 

TIME 

\ 

17a.(H),21J3(H)-30-norhopane. This ratio has been used for 

source-rock correlations (Palacas and others, 1984). In our 

sediment samples from Escanaba Trough, this ratio does 

not vary greatly and ranges between 0.9 and 2.1. Only 

samples from core L1-86-8G have ratios less than 1.0. 

Two ratios are used to show sterane distributions in 

sediments from Escanaba Trough (table 15.2). C29 S/(S+R) 

is the ratio of the 20S to 20R epimers of 24-ethyl5a.(H), 14a.(H), 17 a.(H)-cholestane. During maturation the 

biologically derived 20R epimer is converted to nearly 

equal mixtures of 20R and 20S (Mackenzie and others, 

1980). In our sediment samples, these ratios are significantly below the equilibrium value of 0.5 and range from 

0.12 to 0.26, except samples from core L1-86-8G, which 

have very low values of 0.05 and 0.06. A second sterane 

ratio is C29a.(R)/C27a.(R), which is the ratio of the 20R 

epimer of 24-ethyl-5a.(H),14a.(H),17a.(H)-cholestane to the 

20R epimer of 5a.(H), 14a.(H), 17 a.(H)-cholestane. The ratio of the c29 to c27 steranes has been used as a source 

indicator (Huang and Meinschein, 1979) with C29 steranes 

attributed to terrigenous sources and C27 steranes to marine sources; however, Volkman (1986) has shown limitations to the application of this ratio as a source indicator. 

In our sediment samples, this ratio ranges from 0.8 to 2.6 

with most values greater than 1.0. 

DISCUSSION 

The distribution of various classes of aliphatic hydrocarbons in sediment of Escanaba Trough can be interpreted 

relative to the histories of sedimentation and processes of 

hydrothermal alteration. 

PLEISTOCENE AND HOLOCENE SEDIMENTS 

The sedimentary geology and detailed lithologic descriptions are discussed by Normark and others (chap. 6, 

this volume). A lithologic transition is recognizable in 

many Escanaba Trough sediment cores. Coarse-grained 

siliceous surficial sediment with high water content and 

low magnetic susceptibility is underlain by calcareous 

sediment that is fine grained with lower water content and 

higher magnetic susceptibility. Karlin and Lyle (1986) correlate this lithologic change with a transition from a glacial to interglacial climate at approximately 11 ka, the 

Pleistocene-Holocene boundary. Chemical variations 

<= 

Figure 15.4. Representative mass chromatograms (m/z 217) 

showing distribution of steranes in extracts from five sediment core 

samples from Escanaba Trough. Identifications listed in table 15.3. 
HYDROCARBONS IN SEDIMENT FROM ESCANABA TROUGH 287 

across this boundary likely reflect decreased terrigenous 

input and increased siliceous biogenic input in the Holocene due to decreased continental weathering and enhanced coastal upwelling (Karlin and Lyle, 1986). 

Holocene sediment in Escanaba Trough includes turbiditic 

silt and sand, some of which has local sources, but also includes sediment derived from the Astoria submarine fan 

(Karlin and Lyle, 1986; Karlin and Zierenberg, chap. 7, this 

volume; Normark and others, chap. 6, this volume). Magnetic susceptibility profiles of the core samples for this 

study (Karlin and Zierenberg, chap. 7, this volume) provide a basis for assigning our samples a probable age of 

Pleistocene (10 samples) and Holocene (4 samples). Core 

descriptions are given in table 15.1. 

The Pleistocene and Holocene samples can be differentiated on the basis of distributional relationships among 

aliphatic hydrocarbons reflected in the average values of 

the following molecular ratios (table 15.4): CPI, C31 Sf 

(S+R), C30 ~a/a~, C27 Tm/Ts, C29 S/(S+R), and C29a(R)/ 

C27a(R). Other average ratios in table 15.4, that is, Pr/ 

Ph, C30 a~/~~. and a~ C3cJC29, are not significantly different in the Pleistocene and Holocene samples. 

The high molecular weight n-alkanes (n-C24 to nC34) in the Pleistocene sediment samples have a greater 

odd-carbon-number dominance than do the Holocene 

samples, as shown by a comparison of average CPI values (table 15.4). This dominance of odd-carbon-number 

n-alkanes represents contributions to the sediment from 

terrigenous plant waxes (Eglinton and others, 1962). Our 

results suggest that more terrigenous plant material entered the sediment during Pleistocene time (CPI = 4.5) 

than during Holocene time (CPI = 2.3). This interpretation correlates with that of Karlin and Lyle (1986) based 

on lithological and chemical considerations. The apparent decrease in terrigenous input to sediment from Pleistocene to Holocene times is also reflected in the values 

of the C29a(R)/C27a(R) ratios, which average 1.7 in our 

Pleistocene samples and 1.0 in our Holocene samples. If 

the arguments of Huang and Meinschein ( 1979) are valid, 

then our interpretations of the sterane and n-alkane ratios 

are mutually supportive. 

Microscopic examination of a Pleistocene and a Holocene sediment sample (table 15.5) shows that the bulk 

organic matter in both samples has a large terrigenous component, including woody and coaly fragments, pollen, and 

spores, but the Holocene sample has a significant content 

of marine diatoms and radiolarians. Our observations generally agree with those of McManus and others (1970), 

Moore (1970), and Vallier and others (1973), who concluded that sediment of Escanaba Trough contains terrigenous organic debris derived in part from the drainage of 

the Klamath and Columbia rivers. Besides n-alkanes of 

terrigenous origin (>n-C24), both Pleistocene and Holocene 

samples contain n-alkanes in the molecular region of n-C16 

(see fig. 15.2A, C, E); these compounds likely originate in 

marine sources (Simoneit, 1978). 

Although the CPI values for n-alkanes in Pleistocene 

and Holocene sediments are consistent with the idea of 

more terrigenous input during the Pleistocene than in the 

Holocene, these values can also be interpreted relative to 

maturity. In the maturation process, CPI values tend to decrease as the odd-carbon-number dominance of n-alkanes 

gives way to mixtures in which odd- and even-carbon-number n-alkanes become more equal in abundance (Bray and 

Evans, 1961). In the case of our samples, organic matter 

in Holocene sediment (average CPI value of 2.3) would be 

considered to be more mature than the organic matter in 

Pleistocene sediment (average CPI value of 4.5). This observation is consistent with the results observed for terpane 

and sterane maturity parameters. 

The average values of the terpane and sterane maturity parameters, C31 S/(S+R), C30 ~ala~, C27 Tm/Ts, and 

C29 S/(S+R), clearly differentiate Pleistocene from Holocene sediment; however, the values of the c30 a~/~~ ratio are similar. On the other hand, these sediments cannot 

be differentiated on the basis of the more source-related 

parameters, Pr/Ph and a~ C30/C29, suggesting that the 

Pleistocene and Holocene sediments come from a similar 

provenance. The values of most of the maturity ratios indicate, however, that the organic matter in the Holocene 

samples is more mature than that in the Pleistocene 

samples, a relationship that is contrary to what is expected 

in a normal sedimentary sequence, where older sediments 

should contain more mature components. 

The anomalous maturity measurements for sediments 

of Escanaba Trough suggest that the process of sedimentation involved a complex history, the details of which are 

obscure. One scenario is presented here as a working hypothesis (W.R. Normark, oral commun., 1989). One component of sedimentation in Escanaba Trough comes from 

the mass wasting of older sediments on the flanks of the 

trough. The rate of input of this material is more or less 

constant with time, unaffected by sea-level changes or rates 

of erosion on the continent. During the Pleistocene, high 

rates of continental erosion contributed large volumes of 

younger sediment to the trough, which diluted the older 

sediment entering the trough by mass wasting. During the 

Holocene, continental erosion diminished with rising sea 

level so that the trough received less of the younger, eroded 

sediment but continued to receive the older sediment from 

the mass wasting process. Because of these dilution effects, the results are higher organic maturity values in Holocene sediment and lower organic maturity values in 

Pleistocene sediment. Much of the sediment involved in 

mass-wasting was originally derived from erosion of the 

continent. Thus the terrigenous components of both old 

and young sediment is continental, and this ,common source 

accounts for the similarity in values of organic source-
288 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

Table 15.4. Average values of molecular ratios in sediments and an oil from Escanaba Trough. 

[n.d., not determined] 

CPI ------------------------------

Pr/Ph ----------------------------

C31 S/(S+R) -------------------

C3o a~/~~ ----- C3o ~ala~ ---- c27 Tmffs ---------------------

a~ C3o/C29 -------------------------

C29 S/(S+R) -------------------

C2ga(R)/C27a(R) ------------

Pleistocene 

4.5 ±0.6 

3.0±0.3 

.28 ±0.03 

5.9 ± 2.5 

.32 ± 0.07 

3.4 ± 0.6 

1.8 ± 0.2 

.15 ± 0.04 

1.7±0.4 

related parameters in both the Pleistocene and Holocene 

sediments of Escanaba Trough. 

ALTERED QUATERNARY SEDIMENTS 

Four samples from three cores (table 15.2) have been 

altered lithologically by hydrothermal processes associated 

with deposition of base-metal sulfide minerals. These altered cores could not be assigned a Pleistocene or Holocene 

age because of sediment disruptions; the cores are indicated 

as "Quaternary altered" in table 15.2. Sediment from cores 

L1-86-8G contains minor amounts of pyrrhotite and is interpreted to be a locally derived mass-flow deposit that includes resedimented hydrothermally altered sediment 

(Zierenberg and Shanks, chap. 14, this volume; Karlin and 

Zierenberg, chap. 7, this volume; Normark and others, 

chap. 6, this volume). The sample from core L1-86-27G 

comes from a sandy turbidite unit containing veins and 

disseminations of hydrothermal pyrrhotite and barite accompanied by minor base-metal sulfide minerals and hydrothermally precipitated smectite formed at 180°C to 

235°C (Zierenberg and Shanks, chap. 14, this volume). 

The sample from core L2-86-9G consists of blue-gray chlorite formed by replacement of hemipelagic mud through 

high temperature (200°C to 225°C) magnesium metasomatism adjacent to a massive sulfide deposit (Zierenberg and 

Shanks, chap. 14, this volume). 

The molecular parameters measured in the four 

samples designated "Quaternary altered" (tables 15.2 and 

15.4) have been strongly affected by the hydrothermal processes that apparently have altered the organic matter in 

various ways. Almost all of the values of the molecular 

ratios obtained for these samples are distinct from the values obtained either from the Pleistocene or Holocene 

samples. This distinction is well illustrated on table 15.4. 

Only average values for C31 S/(S+R) in Pleistocene 

samples and for C29a(R)/C27a(R) in Holocene samples are 

similar to the values found in the "Quaternary altered" 

samples. 

Holocene 

2.3 ±0.6 

2.9 ± 0.1 

.48 ± 0.06 

5.8 ±0.9 

.22 ± 0.03 

2.3 ±0.3 

1.8 ± 0.3 

.24 ± 0.02 

1.0 ± 0.1 

Quaternary altered 

1.0 ± 0.1 

1.3 ± 0.6 

.30 ±0.09 

8.2 ± 1.2 

.73 ± 0.02 

7.7 ± 1.1 

.93 ± 0.06 

.08 ±0.04 

1.1 ± 0.1 

Oil 

1.2 

1.7 

.47 

n.d. 

.66 

21 

.73 

.24 

.82 

The n-alkanes in samples from core L1-86-27G and 

L1-86-8G have CPI values that average 1.0, a distribution 

in the n-C24 to n-C34 range that suggests the results of thermal maturation leading to approximately equal abundance 

of odd- and even-carbon-numbered n-alkanes (Bray and 

Evans, 1961). In sample L1-86-27 (82-90), the oddcarbon-numbered n-alkanes in this range are slightly more 

abundant than are their even-carbon-numbered neighbors 

(fig. 15.2D). The distribution of this same range of nalkanes in sample L1-86-8G (82-90) is unusual in that 

from n-C24 to n-C27 the even-carbon-numbered n-alkanes 

dominate, whereas in the range from n-C29 to n-C34 the 

odd-carbon-numbered n-alkanes are slightly more abundant 

than the even-carbon-numbered n-alkanes (fig. 15.2B). 

Both samples from core L 1-86-8G contain similar 

distributions of hydrocarbons (table 15.2) including the 

unusual even-carbon-numbered dominance, which actually 

begins at about n-C16 and extends to n-C28 (fig. 15.2B). 

These two samples are the only ones from our collection 

that show this unusual distribution of n-alkanes; however, 

DSDP drilling at Sites 33, 34, 36, and 37, near to Escanaba 

Trough, recovered samples with n-alkanes having even-carbon-numbered predominances (Simoneit, 1977). This 

even-carbon-numbered dominance in samples from core 

L1-86-8G probably reflects an unusual source, perhaps 

anoxic microenvironments as suggested by Welte and 

Ebhardt (1968). The slight odd-carbon-number dominance 

at n-C29 to about n-C37 (fig. 15.2B) may result from hy"' 

drothermal alteration of a previous n-alkane mixture in 

which the odd-carbon-numbered molecules were much 

more abundant, as in the Pleistocene and Holocene samples 

(fig. 15.2A, C, E). 

The distribution of n-alkanes in cores L1-86-27G and 

L2-86-9G are similar in that most of the compounds are 

in the range from n-C 13 to n-C20 with no significant oddor even-carbon-number dominance (fig. 15.2D, F). The 

sample from core L2-86-9G has only trace amounts of nalkanes and other aliphatic hydrocarbons larger than about 

n-C20. The n-alkanes that are present in these two samples 

likely reflect the results of thermal processes, because 
HYDROCARBONS IN SEDIMENT FROM ESCANABA TROUGH 289 

Table 15.5. Descriptions of sediment organic matter in four samples from Escanaba Trough. 

Core Interval oc no. (em) Area (pet.) Age 

7G 89-90 SESCA 0.52 Pleistocene 

8G 82-90 SESCA .31 Quaternary 

altered 

26G 82-90 SESCA .72 Holocene 

27G 87-95 Between .13 Quaternary 

SESCA and altered 

NESCA 

lower carbon number molecules are enhanced during 

catagenesis (Tissot and Welte, 1984) and, therefore, should 

be expected where there is hydrothermal alteration of sediment. Samples from core L 1-86-8G do not show this 

dominance of n-alkanes in the n-C13 to n-C20 region (fig. 

15.2B), perhaps indicating that the intensity of hydrothermal processes affecting this sediment was lower than in the 

other altered sediments. 

Although the molecular ratios of the "Quaternary altered" sediments are generally distinct from the unaltered 

Pleistocene and Holocene samples, the values of these ratios do not always follow consistent patterns. For example, 

the values of some of the maturity parameters (tables 15.2 

and 15.4), such as C31 S/(S+R), C27 Tm!Ts, and C29 S/ 

(S+R), suggest that the organic matter in the altered sediment is generally less mature than that in the unaltered 

sediment, whereas the values of c30 a~/~~ and c30 ~ala~ 

are consistent with increased maturation of the altered sediment. Source-related ratios, such as Pr/Ph, a~ C3ofC29, and 

C29 a(R)/C27a(R), are generally lower in the altered sediment (tables 15.2 and 15.4). Clearly our results show that 

the lithologically altered sediment contains altered organic 

matter, but the course of organic matter alteration, as indicated by the molecular ratios, cannot be consistently predicted. Nevertheless, the cause of these alterations is 

attributed to hydrothermal processes. 

HYDROTHERMAL OIL 

The molecular distributions of the aliphatic hydrocarbons in Escanaba Trough sediment are, in general, different from the distributions in a hydrothermal oil 

(Kvenvolden and others, 1986, 1987) obtained from a 

sediment-hosted sulfide sample (L6-85-32D-2) that was 

dredged from the SESCA area (fig. 15.1). Molecular ratios 

Description 

(Clark Geological Services, written commun., 1987) 

Abundant woody and coaly fragments. Common fine 

amorphous kerogen, aggregated amorphous kerogen and 

cuticles. Traces of pollen, spores, and fungal parts. 

Fragments of diatoms and radiolarians present. 

Predominantly coaly fragments. Very common 

aggregated amorphous kerogen. Common fine amorphous 

kerogen and woody fragments. Traces of cuticles, pollen, 

spores, dinoflagellates, and fungal parts. 

Predominantly aggregated amorphous kerogen. Very common 

very fine-grained coaly fragments. Traces of pollen, 

spores, and woody fragments. Abundant diatom and 

radiolarian fragments present. 

Predominantly coaly fragments. Very common woody 

fragments. Common fine amorphous kerogen. Traces of 

aggregated amorphous kerogen, pollen, and spores. 

for sediment samples and the oil are shown in table 15.2, 

and average values of these ratios for Pleistocene, Holocene, and Quaternary altered sediment samples and for 

the oil are listed in table 15.4. Comparison of figures 15.2 

and 15.5A and information in table 15.4 illustrate some of 

the distributional differences in n-alkanes and isoprenoid 

hydrocarbons. The CPI value of the oil is smaller than the 

average CPI values in unaltered sediment and slightly 

larger than values in the altered sediment; Pr/Ph ratios follow the same trend. The n-alkanes in the oil have a slight 

odd-carbon-numbered dominance at high molecular 

weights as do altered sediments from cores L1-86-8G and 

L1-86-27G (fig. 15.2B,D) and an even-carbon-numbered 

dominance in the intermediate molecular weight range as 

in altered sediments from core L 1-86-8G. The oil lacks, 

however, the high concentration of n-alkanes in the n-C13 

to n-C20 range as seen in altered sediments from cores L 1-

86-27G and L2-86-9G (fig. 15.2D,F). 

Comparison of triterpane and sterane ratios also shows 

some difference between Escanaba Trough sediment and 

a hydrothermal oil (table 15.4). The ~~-hopanes are 

present in all of the unaltered sediments but not in two 

samples of altered sediment (cores L1-86-27G and L2-86-

9G) and in the oil sample. The compounds are known to 

be thermally unstable, and thus their absence in altered 

sediment and in the oil formed by hydrothermal processes 

is not unexpected. The sediment hydrocarbons show a 

wide range of maturities based on the C31 homohopane and 

the C29 sterane epimer ratios. These ratios, determined for 

the oil, have values similar to the average values measured 

in the Holocene sediment and exceed the average values 

measured in the Pleistocene sediment and in the altered 

sediment. None of these epimer ratios from sediments or 

the oil is at equilibrium. The values of the other molecular ratios, such as C30 ~ala~, C27 Tm!Ts, a~ C3ofC29, and 

C29a(R)/C27a(R), in the oil are distinct from the values of 
290 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

the same ratios in both altered and unaltered sediment 

(table 15.4). Based on this comparison of molecular ratios, we conclude that although the hydrocarbons in the 

Escanaba Trough sediment likely contribute to the final oil 

product that is generated from the sedimentary organic 

matter, the amount of this contribution must be small, having only minimal influence on the distribution of hydrocarbons in the oil. However, certain features, such as the 

dominance of odd-carbon-numbered n-alkanes in the sediments and the oil, suggest that terrigenous organic matter 

is an important source material in the hydrothermal regime 

of Escanaba Trough. 

A 

20 

18 

17f Ph 

14 15 / 

.u J.J J.J i ... . 

1 

8 

2 

4 

6 

w 5 8 

C/) 

z 

0 

c.. ~ 

C/) 

w 

a: c 

D 

c 

TIME 

SUMMARY 

In Escanaba Trough, Pleistocene and Holocene sediments can be differentiated on the basis of distributional 

relationships among aliphatic hydrocarbons as reflected in 

various ratios of n-alkanes, isoprenoids, terpanes, and 

steranes. More terrigenous plant material entered the sediment during Pleistocene time than during Holocene time, 

based on considerations of n-alkanes and steranes. Terpane 

and sterane maturity parameters indicate that the organic 

matter in the Holocene sediment is more mature than that 

in the Pleistocene sediment. This unusual relation is 

27 

24 29 Alkanes 

31 

_a.J ..J !11 I 1 1 40 

Triterpanes 

mlz 191 

Steranes 

mlz 217 

Figure 15.5. Gas chromatogram and mass chromatograms showing distributions of 

hydrocarbons in oil extracted from sediment-hosted, massive sulfide sample L6-85-

32D-2 from Escanaba Trough. A, Gas chromatogram showing n-alkanes, pristane (Pr), 

and phytane (Ph). Numbers specify carbon numbers of n-alkanes. B, Mass 

chromatogram (rnlz 191) oftriterpanes. C, Mass chromatogram (rnlz 217) of steranes. 

Identifications listed in table 15.3. 
HYDROCARBONS IN SEDIMENT FROM ESCANABA TROUGH 291 

explained by a dilution effect caused by increased sedimentation of continentally derived material during Pleistocene 

time. 

Some Escanaba Trough sediment has been affected by 

hydrothermal fluids, and this alteration has caused not only 

lithological changes but also changes in the distributions 

of organic compounds. Molecular ratios of various hydrocarbons in the altered sediment are distinct from the values obtained for unaltered Pleistocene and Holocene 

sediments. The course of organic matter alterations, as indicated by molecular ratios, cannot, however, be consistently predicted. 

Hydrothermally derived oil found with sedimenthosted sulfide deposits contains aliphatic hydrocarbons 

with distributions that are distinct from the distributions 

found in both altered and unaltered sediment samples. Although hydrocarbons from these sediments likely contribute to the hydrothermal oil generated in Escanaba Trough, 

these hydrocarbons have only minimal influence on the distributions of hydrocarbons in the oil. 

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292 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

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Kaplan, I.R., 1979, Organic geochemistry of recent sediments from Guaymas Basin, Gulf of California: Deep-Sea 

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dispersal patterns of turbidite sand in Escanaba Trough, northeastern Pacific Ocean: Marine Geology, v. 15, p. 67-87. 

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Geochimica et Cosmochimica Acta, v. 32, p. 465-466. 

Zierenberg, R.A., Koski, R.A., Shanks, W.C., III, and Slack, J.F., 

1988, Preliminary results of ALVIN dives on active sediment-hosted massive sulfide deposits in the Escanaba 

Trough, southern Gorda Ridge [abs.]: Eos, Transactions 

American Geophysical Union, v. 69, p. 1488. 

Zierenberg, R.A., and Shanks, W.C., III, Sediment alteration associated with massive sulfide formation in the Escanaba 

Trough, Gorda Ridge: The importance of seawater mixing 

and magnesium metasomatism, in Morton, J.L., Zierenberg, 

R.A., and Reiss, C.A., eds., Geologic, hydrothermal, and 

biologic studies at Escanaba Trough, Gorda Ridge, offshore 

northern California: U.S. Geological Survey Bulletin 2022, 

chapter 14 (this volume). 
Chapter 16. Composition and Growth History of Hydrothermal 

Deposits in Escanaba Trough, Southern Gorda Ridge 

By Randolph A. Koski, Laura M. Benninger, Robert A. Zierenberg, 1 and Ian R. Jonasson2 

CONTENTS 

Page 

Abstract .. .... ...... ............ ..... ....... ...... ... . ........... .... ......... ... ..... .. 293 

Introduction ..... ... .... ..... ..... ...... ........ .. ...... .. .. ...... .................... 294 

Acknowledgments ....................................................... 294 

Hydrothermal deposit types in Escanaba Trough............... 294 

Sulfide-dominant deposits ............ ....................................... 302 

Pyrrhotite-rich massive sulfide................................... 302 

Polymetallic sulfide..................................................... 307 

Sulfide-cemented sediment and sediment breccia ..... 307 

Clastic sulfide deposits ................ ................................ 308 

Sulfate-dominant deposits .............................. .... .... ........ ..... 310 

Barite-rich crusts ......................................................... 310 

Barite chimneys ........................................................... 311 

Sulfate sinter .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . .. .. .. .. .... .. . 311 

Bulk chemistry of deposits .................................................. 311 

Analytical techniques .................................................. 311 

Sulfide-dominant samples ...... ....... ..... ......................... 311 

Sulfate-dominant samples .............. ............................. 314 

Genesis of sulfide- and sulfate-dominant deposits ............ 316 

Distribution and size considerations ........................... 316 

Sediment as a source of metals and barium ...... ......... 318 

Formation of massive sulfide mounds........................ 318 

Formation of the polymetallic sulfide structure......... 321 

Conclusions.......................................................................... 321 

References cited................................................................... 322 

ABSTRACT 

Large-scale discharge of hydrothermal fluids through 

the sequence of turbidites and hemipelagic sediment 

infilling Escanaba Trough, the axial valley of southern 

Gorda Ridge, has resulted in widespread formation of sulfide- and sulfate-dominant deposits. On a regional scale, 

mineralization is spatially related to several volcanic edifices that penetrate and locally breach the sediment pile. 

In the NESCA and SESCA study areas, hydrothermal deposits are most abundant on the flanks of sediment hills 

upraised during emplacement of basaltic sills. 

1 U.S. Geological Survey, Menlo Park, California 2 Geological Survey of Canada, Ottawa, Canada 

Sulfide-dominant deposits are grouped into two cornpositional types: (1) pyrrhotite-rich sulfide that forms 

mounds, chimneys, cement in sediment breccia, and subsurface clastic aggregates; and (2) less abundant 

polymetallic sulfide that forms the wall of a chirnneylike 

sulfide structure and cement in sand turbidite. Pyrrhotiterich massive sulfide contains variable amounts of 

isocubanite and chalcopyrite, and minor sphalerite and 

lOllingite; texturally, it is characterized by a latticework texture of pyrrhotite and numerous fluid channel ways. Chemically, pyrrhotite-rich sulfide samples have high Cu (to 20.6 

percent), As (to 1.55 percent), Bi (to 820 ppm), Co (to 

3,000 ppm), Se (to 525 ppm), and Au (to 10.1 ppm). The 

Cu/Zn ratio generally exceeds unity, and Pb is very low. 

The highest Au occurs in Cu-rich samples. 

A 40-rnrn-thick section of the polymetallic sulfide 

structure contains a telescoped growth zonation that includes sphalerite, isocubanite, pyrrhotite, galena, arsenopyrite, lOllingite, and boulangerite. A coarse sand turbidite 

sample is cemented by a polymetallic sulfide assemblage 

of galena, sphalerite, pyrrhotite, and lOllingite. Two tinbearing phases, stannite and franckeite, are present in trace 

amounts. Polymetallic samples are especially enriched in 

Zn (to 42.6 percent), Pb (to 13.7 percent), As (to 2.75 percent), and Ag (to 681 ppm). 

Sulfate-dominant deposits include barite-rich crusts 

and chimneys on sulfide mounds, isolated barite chimneys 

on sediment, and sulfate (anhydrite and barite)-rich sinters 

deposited where hydrothermal fluids are venting through 

massive sulfide mounds (NESCA site). Barite-rich crusts 

and chimneys on sulfide mounds also contain abundant 

sphalerite and galena and have high zinc, lead, and silver 

contents. Anhydrite-rich sinters contain high magnesium 

and silicon related to the presence of stevensite, whereas 

barite-rich sinters contain high silicon related to abundant 

amorphous silica. 

The widespread distribution and large size of hydrothermal deposits are the results of long-lived, sedimenthosted hydrothermal convection cells driven by heat 

sources located at depth within the volcanic edifices. The 

abundance of barite and enrichment of metals such as lead, 

arsenic, antimony, and bismuth in sulfide deposits are indicators of extensive contributions from a sedimentary 

293 
294 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

source rock. Massive sulfide mounds are formed by (1) an 

early stage of sulfate-dominant deposition on the stepfaulted margins of the uplifted sediment hills, (2) a mature 

stage including less fluid mixing, low oxygen fugacity and 

sulfur fugacity, higher fluid temperatures, and deposition 

of the main-stage sulfide assemblage beneath the sulfaterich shell, and (3) a near-dormant declining stage marked 

by erosion, oxidation, and partial burial by unconsolidated 

Holocene sediment. Hydrothermal activity in this final 

stage is restricted to localized venting of moderate-temperature (to 220°C) fluids through sulfide mounds, deposition of sulfate sinter, and diffuse seepage of 

low-temperature fluid through the sediment. Unlike the diffuse upwelling associated with pyrrhotite-rich sulfide 

mounds, the polymetallic sulfide structure was formed 

around a focused "black smoker" type vent. The telescoping of high- and low-temperature sulfide assemblages in 

this structure resulted from complex bidirectional growth 

of the chimney wall under the influence of steep temperature and chemical gradients. 

INTRODUCTION 

A variety of sulfide- and sulfate-dominant hydrothermal deposits are the product of large-scale fluid discharge 

through the sediment-covered floor of Escanaba Trough, 

southern Gorda Ridge. The concentration of deposits in the 

northern Escanaba (NESCA) and southern Escanaba 

(SESCA) areas (fig. 16.1) is spatially related to two volcanic edifices that penetrate and locally breach the 500-mthick sequence of turbidite and hemipelagic sediments 

within the axial valley (Morton and others, 1987, 1990). 

The largest hydrothermal deposits are massive sulfide 

mounds with lateral and vertical dimensions measured in 

hundreds and tens of meters, respectively, that formed on 

the flanks of sediment hills rising above the surrounding 

sea floor (Holmes and Zierenberg, 1990; Zierenberg and 

others, chap. 10, this volume). Small chimneys (less than 

1 min height) composed of massive sulfide and barite± 

sulfide occur on the tops of massive sulfide mounds and 

on the sediment surface between the hills. Sinter deposits 

largely composed of anhydrite, barite, and stevensite are 

forming around active hydrothermal vents discharging 

through the tops of massive sulfide mounds at the NESCA 

site. A sediment core taken from the SESCA site contains 

a subsurface layer of clastic sulfide. 

The composition and distribution of hydrothermal deposits, the presence of interstitial and vein sulfide minerals in sediment samples, and the alteration of sediment to 

aggregates of chlorite and talc indicate that the circulation 

of hydrothermal fluids through sediment is widespread in 

Escanaba Trough. The mineral and chemical compositions 

of sulfide- and sulfate-rich hydrothermal samples reflect 

fluid-sediment interaction (Benninger and Koski, 1987; 

Koski and others, 1988), and these compositions contrast 

with those of deposits formed at sediment-free ridge axes 

of the northeast Pacific Ocean (Haymon and Kastner, 1981; 

Styrt and others, 1981; Koski and others, 1984). 

Benninger and Koski (1987), Koski and others (1988), 

and Morton and others (1990) presented mineralogical and 

chemical data for sulfide samples dredged from Escanaba 

Trough in 1985 and 1986, and developed preliminary models for the deposition of distinct massive sulfide types. In 

this chapter, we present additional chemical data for the 

dredge samples along with descriptions and compositional 

data for samples recovered by gravity coring in 1986 (R/ 

V S.P. Lee, U.S. Geological Survey) and 1989 (CSS J.P. 

Tully, Geological Survey of Canada) and by submersibles 

in 1986 (DSV Sea Cliff) and 1988 (DSV Alvin and Sea 

Cliff). These data are used to further define the growth history of sulfide- and sulfate-dominant deposits in Escanaba 

Trough. 

ACKNOWLEDGMENTS 

We thank J.M. Edmond and J.F. Grassle (co-chief scientists, Massachusetts Institute of Technology and Woods 

Hole Oceanographic Institution) and E.E. Davis and J.M. 

Franklin (co-chief scientists, Geological Survey of Canada) 

for inviting us to participate in and collect samples during 

their 1988 RIV Atlantis II and 1989 CSS J.P. Tully cruises, 

respectively. We appreciate the dedication and professionalism of ship's crews from the RIV S.P. Lee (U.S. Geological Survey ), CSS J.P. Tully (Geological Survey of Canada), 

RIV Atlantis II (Woods Hole Oceanographic Institution), 

and DSVSS Laney Chouest (Edison Chouest Offshore, 

Inc.) and the crews of research submersibles DSV Sea Cliff 

(Submarine Development Group One, U.S. Navy) and DSV 

Alvin (Woods Hole Oceanographic Institution). J.F. Slack 

provided many of the original sample descriptions from the 

1988 Alvin dive program. Franckeite in a polymetallic sulfide sample was identified by E. Oudin. Data sources for 

mid-ocean-ridge basalts were kindly provided by J .A. 

Philpotts. J.A. Reid and V.K. Smith assisted in compilation of data tables and drafting of figures. 

HYDROTHERMAL DEPOSIT TYPES IN 

ESCANABA TROUGH 

During the period 1985 to 1988, approximately 57 kg 

of sulfide- and sulfate-rich hydrothermal material was recovered by dredge and another 245 kg was recovered by submersible from the NESCA and SESCA areas of Escanaba 

Trough. Subsurface sulfide deposits were sampled by two 

gravity cores. The locations of individual sample stations are 

shown in figure 16.2. Brief descriptions of these samples 

along with their locations and depths of recovery are 
COMPOSITION AND GROWTH HISTORY OF HYDROTHERMAL DEPOSITS IN ESCANABA TROUGH 295 

127° 30' 

40° 30' 

Figure 16.1. Location and regional bathymetry of Gorda Ridge and Escanaba Trough. Also shown 

are locations of Deep Sea Drilling Project (DSDP) Site 35 and gravity core station L 1-86-27G. Northern 

Escanaba (NESCA) and southern Escanaba (SESCA) sites are shown in greater detail in figure 16.2. 

Hachures indicate area of closed low. Contour interval is 500 m. 
296 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

A 

12J030' 12JC28' 

41°06' r------r------------------.------------------.--~r-------------, 

66,000 

64,000 

62,000 

60,000 

I 

58,000 

56,000 

54,000 

52,000 

0 

56,000 58,000 60,000 62,000 50,000 

Figure 16.2. Bathymetry and sampling locations for NESCA (A) and SESCA (B) sites, Escanaba 

Trough. Solid circles indicate location of samples collected by submersible. Heavy solid lines are 

tracks of dredge stations that recovered hydrothermal samples. Open circle at SESCA is location of 
COMPOSITION AND GROWTH HISTORY OF HYDROTHERMAL DEPOSITS IN ESCANABA TROUGH 297 

presented in table 16.1 at the end of this chapter; the mineralogy of various deposit types is given in table 16.2. 

Sulfide-dominant deposits in Escanaba Trough include 

massive sulfide mounds, massive sulfide chimneys and 

chimneylike(?) vent structures, sulfide-cemented sediment 

and sediment breccia, and clastic sulfide aggregates. Massive sulfide mounds have a wide range in morphology from 

rugged, steep-sided stacks and ridges to subdued, flat-lying ledges (fig. 16.3A, B, C). Nearly all of the observed 

mounds are actively eroding and partly covered or surrounded by blocky sulfide debris (fig. 16.3D). The flanks 

of many mounds are buried by unconsolidated sediment. 

It is uncertain whether the exposed massive sulfide bodies 

are connected at depth, but several mounds at the NESCA 

site appear to be joined by narrow sulfide ridges. 

Chimneys composed of massive sulfide form small 

jagged pinnacles and eroded stumps on the tops of sulfide 

mounds (fig. 16.3A, B) and isolated spires on the sediment 

surface. Hydrothermal venting through massive sulfide 

chimneys has not been observed, and the few sampled 

chimneys, with one exception, are plugged or contain many 

small fluid channelways. In the one exception, massive sulfide fragments from dredge L2-86-14D in the NESCA area 

(fig. 16.2A) appear to be part of a chimneylike sulfide 

structure with a large central conduit. 

Sulfide also forms the cement in sedimentary breccias 

that probably represent debris-flow deposits derived by 

\ 

B 

mass wasting of the nearby sediment hills (Zierenberg and 

others, chap. 10, this volume). In addition, massive sulfide 

fragments occur in poorly sorted clastic aggregates as thick 

as 27 em that have been recovered by gravity cores. These 

deposits may represent debris flows shed from massive sulfide mounds that were subsequently buried by turbidite 

flows and hemipelagic mud. Sulfide deposits with similar 

clastic textures have been cored on the flank of a sedimentcovered dome in the Middle Valley segment of Juan de 

Fuca Ridge (Davis and others, 1987; Goodfellow and 

Blaise, 1988). 

Although less abundant than sulfide deposits, sulfate-dominant deposits including crusts, chimneys, and 

sinter are equally widespread at the NESCA and SESCA 

sites. Barite-rich encrustations and cavity infillings occur on the surface of massive sulfide mounds. Numerous thin (less than 2 em) slabs of massive barite 

recovered in several dredges probably represent crusts 

spalled from massive sulfide blocks during sampling. 

Barite is also the most abundant phase in small (less 

than 1 m) knobby chimneys that occur in clusters on top 

of sulfide mounds (fig. 16.3£) and in stubby cylindrical 

chimneys rooted in the sediment. Sinter deposits composed of anhydrite, barite, stevensite, and minor sulfide 

are precipitating from moderate-temperature (about 

220°C) fluids venting through the tops of massive sulfide mounds at NESCA (fig. 16.3F). 

12r3o· 

gravity core station L2-86-9G. Numbers along right and bottom give coordinates 

of local transponder net (in meters). Hachures indicate area of closed low. 
298 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

Table 16.1. Location and description of hydrothermal samples from Escanaba Trough 

[X andY coordinates refer to local navigation grid for NESCA and SESCA shown in figure 16.2] 

Station or Location Depth (m) Sample number and description 

dive number 

Dredge Samples 

L6-85-32D East and south flank of 3180-3280 1 through 26: Rounded, knobby fragments of massive sulfide, predomi3170 hill at SESCA area nantly pyrrhotite. One fragment (sample 32D-2) of interlayered 

pyrrhotite-rich massive sulfide and sediment is saturated with petroleum. Total weight approximately 5 kg. 

L1-86-3D South flank of 3170 hill 3200-3280 2: Two chips of gray mudstone cut by 1-mm-thick pyrite veins (0.002 kg). 

at SESCA }: A dish-shaped fragment (10 em x 9 em x 4 em) of gray indurated silt 

and sand. The concave surface is coated with a thin crust of finegrained pyrrhotite (0.145 kg). 

.6: Two chips (approximately 3 em x 2 em x 2 em) of pyrrhotite-rich 

massive sulfide. Total weight 0.01 kg. 

L1-86-15D Top and northern flank of 3170-3260 1: Slabs and cobble-size fragments of sulfide-cemented sediment 

3170 hill at SESCA breccia. Sulfide is mostly marcasite after pyrrhotite; sediment 

fragments contain abundant chlorite and talc. Total weight 0.75 kg. 

2...1: Pebble-size and smaller pieces of gossanous sediment with veins 

and crusts of black Fe oxide, mostly goethite as pseudomorphous 

replacement of pyrrhotite. Total weight 0.15 kg. 

Other: Blocky, pebble-size fragments of concentrically zoned massive 

sulfide composed of isocubanite, sphalerite, and marcasite. Total 

weight 0.02 kg. 

L1-86-16D Northern flank of east hill 3220-3240 1 through 4: Cobble-size fragments and numerous pebble-size fragments 

at SESCA of pyrrhotite-rich massive sulfide with 1-cm-thick crusts of barite. 

Pyrrhotite in some fragments pseudomorphed by marcasite. Some 

fragments also have cavities lined with barite and surfaces with 

patches of sponge spicules. 

5 through 18: Slabby fragments, 1 to 2 em thick, of interlayered sulfide 

and barite. 

Total weight of all samples 5.05 kg. 

L1-86-22D Southern and eastern flanks 3210-3260 1: A flat slab (4.5 em x 4.5 em x 2 em, 0.35 kg) of massive sulfide 

of north hill at SESCA largely composed of intergrown pyrrhotite and barite. 

2: Pebbles and smaller fragments of poorly indurated sediment breccia 

with a few clasts composed of massive sulfide. Total weight 0.315 kg. 

L1-86-24D Southeast flank of 3170 hill 3230-3210 2. 5 6 9 12 through 17. 20. 21: Numerous blocky boulder- to 

at SESCA pebble-sized fragments and slabs of pyrrhotite-rich massive sulfide 

some of which contain crusts and interstitial concentrations of talc or 

barite. Total weight approximately 25 kg. 

~: Two tabular crusts (average 7 em x 5 em x 2 em) of dense granular barite. Total weight 0.195 kg. 

3. 4. 7: Assorted fragments of sediment breccia cemented by a mixture of 

Fe sulfide (pyrrhotite and marcasite) and barite. Total weight 1.20 kg. 

8. 18. 19. 22: Assorted cobble- and pebble-size fragments of indurated 

gray sediment containing disseminated and fracture-controlled 

sulfide, mostly pyrrhotite. Oxidation of sulfide results in orangeyellow coloration and narrow ribbons of black vitreous Fe oxide, 

mostly goethite. Total weight approximately 0.4 kg. 

L1-86-29D West flank, top, and southeast 3160-3260 .L Pebble-size fragments of pyrrhotite-rich massive sulfide (<0.15 kg). 

flank of SW hill at NESCA 

L1-86-30D Sediment plain northwest 3260-3290 1: Four pebble-size fragments of coarse-grained pyrrhotite-rich massive 

of central hill at NESCA sulfide. Total weight 0.02 kg. 

Ll-86-34D Southeast flank of SW hill 3240-3250 1 through 4: Angular block (7 em x 5 em x 3 em), curved fragment 

at NESCA (10 em x 8 em x 5 em), and rounded pebbles of massive sulfide 

largely composed of pyrrhotite and isocubanite. Some surfaces 

altered to Fe oxides. Total weight 0. 73 kg. 

L1-86-35D Southeast flank of SW 3240-3290 Unconsolidated mud containing small fragments of pyrrhotite-rich 

hill at NESCA sulfide, mudstone, and volcanic glass. Total weight 2.6 kg. 
COMPOSITION AND GROWTH HISTORY OF HYDROTHERMAL DEPOSITS IN ESCANABA TROUGH 299 

Table 16.1. Location and description of hydrothermal samples from Escanaba Trough-Continued 

Station or 

dive number 

Location 

L2-86-14D West side of trough north 

of central hill at NESCA 

L2-86-16D 

L2-86-18D 

L2-86-25D 

SC658 

SC658 

SC659 

SC659 

West of L2-86-14D 

West side of trough 2 km 

south of central hill at NESCA 

South and east flank of SW 

hill at NESCA 

Northern flank of central 

hill at NESCA 

(Rl) X:58800, Y:55812 

(R2) X:58800, Y:55812 

(R4) X:58800, Y:55812 

Approximately 400 m 

south of 658-R1 

(R3) X:58862, Y:55375 

In trough near west wall 

of axial valley, 1.5 km north 

of central hill at NESCA 

(R1) X:58810, Y:57430 

(R3) X:58810, Y:57430 

(R4) X:58810, Y:57430 

Approximately 500 m 

northeast of 659-Rl 

(R2) X:59000, Y:57934 

Depth (m) Sample number and description 

Dredge Samples-Continued 

3310-3330 1 through 4: One large (28 em x 21 em x 5 em) bowl-shaped fragment 

3300-3340 

3280-3360 

3160-3270 

and several smaller curved fragments composed of dark-gray 

massive sulfide. The polymetallic sulfide assemblage includes 

sphalerite, pyrrhotite, galena, and isocubanite. Oxidation is very 

minor. The large sample, part of a sulfide edifice, is mineralogically 

and texturally zoned. Total weight 11.84 kg. 

Q.: One blocky fragment (0.1 0 kg) of coarse sand cemented by sulfide 

that is predominantly pyrrhotite, galena, and sphalerite. Hydrocarbon is also present in interstices between sediment clasts. 

I: Cobble- and pebble-size blocks of sandstone and silty mudstone. 

One sandstone fragment contains discontinuous fractures lined with 

partly oxidized isocubanite. Another fragment of mudstone is 

crosscut by veins of pyrrhotite and also contains hydrocarbon. Total 

weight 5.53 kg . 

.1....1: Pebble-size angular fragments of altered sediment containing talc, 

granular barite, and sulfide veinlets. Total weight 0.10 kg . 

.1....1: Fragments of pyrrhotite-rich massive sulfide with dimensions 

averaging 6 em x 4 em x 3 em; some replacement of pyrrhotite by 

marcasite. Total weight 0.15 kg . 

.l....1.....4: Blocky fragments (maximum 10 em x 9 em x 5 em) of sulfidecemented sediment breccia. Sediment clasts are partly altered to 

chlorite. Pyrrhotite is the predominant mineral in the cement and 

also occurs in veins cutting the clasts. Total weight 1.245 kg. 

3 and 5 through 7: Subangular fragments (average 7 em x 5 em x 4 em) 

of porous pyrrhotite-rich massive sulfide. Pyrrhotite in some 

samples is replaced by marcasite, and several samples have surfaces 

covered with thin encrustations of talc. Total weight 0.98 kg. 

Samples Collected by DSV Sea Cliff 

3265 

3265 

3265 

3238 

3322 

3322 

3322 

3322 

R1: Three cobble-size and approximately 20 smaller fragments of 

pyrrhotite-rich massive sulfide with abundant barite and isocubanite. 

Some pyrrhotite altered to marcasite. Some surfaces have 1-mmthick rind of Fe oxide. Total weight 0.65 kg. 

R2: Barite chimney approximately 25 em in height and 10 em in diameter (5.4 kg) with no continuous central conduit recovered from sediment substrate. Outer surface coated with 1 to 2 mm of Mn oxide. 

R4: Four flat slabs of pyrrhotite-rich massive sulfide recovered from 

beneath sediment surface. Total weight 2. 7 kg. 

R3: Rounded talus block of pyrrhotite-rich massive sulfide with 2-mmthick rind of Fe oxides including goethite (9.5 kg). 

Rl: Chimney composed of pyrrhotite-rich massive sulfide (3.5 kg). The 

upper part, 25 em high and 10 em in diameter, has many fluid channelways partly to completely filled with coarse-grained pyrrhotite. The 

basal platform was embedded in sediment, has a layered structure, 

and contains abundant pyrrhotite, isocubanite, native sulfur, and 

silicate sediment grains. 

R3: Fragments of partly oxidized pyrrhotite-rich massive sulfide containing sediment grains and hydrocarbon. Total weight 1.0 kg. 

R4: Fragments of pyrrhotite-rich massive sulfide including a few small 

chimney tops with open fluid conduits. Total weight 0.4 kg. 

R2: Fragments of partly oxidized pyrrhotite-rich massive sulfide 

incorporating sediment grains and hydrocarbon. Total weight 0.7 kg. 
300 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

Table 16.1. Location and description of hydrothermal samples from Escanaba Trough-Continued 

Station or 

dive number 

SC662 

SC663 

SC663 

SC766 

ALV2033 

ALV2035 

ALV2036 

ALV2037 

Location 

Approximately 500 m 

southwest of north hill 

at SESCA 

(Rl) X:13610, Y:15550 

North flank of the 3170 

hill at SESCA 

(R1) X:12419, Y:13700 

Approximately 70 m west 

of 663-Rl 

(R2) X: 12353, Y: 13695 

Active vent area on upper 

eastern flank of central hill 

at NESCA 

X:59220, Y:54904 

Northern flank of central hill 

at NESCA 

(1) X:58916, Y:55771 

(2) X:58886, Y:55716 

(3) X:58858, Y:55848 

(5) X:58760, Y:55460 

Base of eastern slope of 

SW hill at NESCA 

(1) X:58103, Y:53215 

(2) X:58122, Y:53223 

(3) X:58114, Y:53223 

Active 217°C vent site on 

northwest flank of central 

hill at NESCA 

(1) X:58848, Y:55399 

(2) X:58832, Y:55400 

Northwest flank of central 

hill at NESCA near ALV2036 

X:58836, Y:55376 

Depth (m) Sample number and description 

Samples Collected by DSV Sea Cliff -Continued 

3237 

3246 

3246 

3227 

R 1: Many fragments of massive sulfide containing abundant pyrrhotite, 

chalcopyrite, and barite from a short chimney structure projecting 

from a massive sulfide mound. Total weight 6.3 kg. 

Rl: Fragments of deeply-weathered pyrrhotite-rich sulfide in which 

pyrrhotite is completely replaced by marcasite, goethite, and 

hematite. Total weight 1.2 kg. 

R2: Rubbly fragments containing a mixture of talc and Fe-oxide minerals. 

Some fragments contain hydrocarbon. Total weight 0.5 kg. 

Hourglass-shaped, barite-rich chimney from top of massive sulfide 

mound (approximately 10 kg). 

Samples Collected by DSV Alvin 

3274 

3257 

3264 

3242 

3277 

3282 

3277 

3234 

3228 

3217 

1: Angular block (25 em x 25 em x 12 em, 6.72 kg) of pyrrhotite-rich 

massive sulfide with weathered (to depth of 3 em) surfaces composed of Fe oxide. 

2.: Knobby weathered tip of barite chimney (12 em x 10 em x 6 em, 

0.26 kg) coated with Mn oxide. 

3A B: Two finger-shaped barite-rich chimneys (25 em x 15 em x 42 

em and 22 em x 10 em x 50 em) from top of massive sulfide mound. 

Knobby surfaces are deeply weathered and covered in part by a film 

of Mn oxide. Total weight 35.5 kg. 

5A. B. C: Blocky fragments (maximum 42 em x 40 em x 25 em) of 

pyrrhotite-rich massive sulfide with barite lining cavities. Some 

surfaces have thin alteration rinds of Fe oxide. Total weight 61.6 kg. 

1: Six angular blocks (the largest 20 em x 18 em x 8 em) of pyrrhotiterich massive sulfide containing 1- to 3-cm-wide fluid conduits lined 

with Cu-Fe sulfides. Total weight 6.07 kg. 

2.: Two angular fragments (average 25 em x 15 em x 10 em) of 

pyrrhotite-rich massive sulfide recovered from top of sulfide mound. 

Surfaces weathered to Fe oxide. Total weight 15.34 kg . 

.3.: Two angular fragments (the larger 15 em x 10 em x 7 em) of 

pyrrhotite-rich massive sulfide from weathered sulfide mound. Total 

weight 1.33 kg. 

1A: Large angular block (28 em x 25 em x 18 em, 18.9 kg) of pyrrhootite-rich massive sulfide coated by Fe oxide. 

ill: Five angular fragments of vuggy massive sulfate (barite and 

anhydrite) with minor sulfide (mostly marcasite and sphalerite). 

Total weight 1.77 kg. 

1 C: Sixteen small angular fragments (the largest has dimensions 7 em x 

5 em x 3 em) of porous massive sulfate, mostly barite with minor 

anhydrite and oxidized Fe sulfide. Total weight 0.28 kg. 

1D: Miscellaneous sulfate-rich and sulfide rubble (0.46 kg). 

2.: Eight fragments of massive sulfate with numerous drusy cavities. 

Barite and anhydrite are equally abundant; minor marcasite. Several 

of the sulfate fragments contain equant clasts of pyrrhotite-rich 

massive sulfide. Total weight 5.89 kg. 

1A: Blocky fragment (18 em x 15 em x 10 em, 6.21 kg) of pyrrhotiterich massive sulfide with local concentrations of Cu-Fe sulfide and 

barite. Surface is partly covered with dark red Fe oxide. 

1B: Angular dense fragment (15 em x 13 em x 7 em) of pyrrhotite-rich 

massive sulfide. Some surfaces are encrusted by a thin rind of 

orange-red Fe oxide. Estimated weight is 5 kg. 
COMPOSITION AND GROWTH HISTORY OF HYDROTHERMAL DEPOSITS IN ESCANABA TROUGH 301 

Table 16.1. Location and description of hydrothermal samples from Escanaba Trough-Continued 

Station or 

dive number 

ALV2038 

ALV2039 

ALV2040 

ALV2041 

ALV2042 

Ll-86-27G 

L2-86-9G 

TUL89D-8G 

Location Depth (m) Sample number and description 

Samples Collected by DSV Alvin-Continued 

South flank of sediment- 1: Two arcuate slabs (12 em x 7 em x 8 em and 10 em x 6 em x 5 em) 

covered hill approximately of pyrrhotite-rich massive sulfide from an inactive sulfide mound. 

9 km north of central hill at Much of the pyrrhotite is altered to marcasite; weathered surfaces 

NESCA have 1 to 3 mm thick crusts of earthy, orange-red Fe oxide. Barite is 

(1) X:58785, Y:65250 3264 abundant in cavities. Total weight 0.65 kg. 

(2) X:59020, Y:65280 3268 2: One large fragment (38 em x 20 em x 18 em) and 11 smaller pieces 

(3) X:59020, Y:65280 3268 of pyrrhotite-rich massive sulfide; pyrrhotite is coarse grained in 

cavities. Total weight 15.80 kg. 

.3.: Columnar fragment of pyrrhotite-rich sulfide, 30 em long and 18 to 

23 em in diameter (10.62 kg), possibly part of a chimney. Deeply 

weathered regions on the sample surface may mark sites of side 

spouts. Oxidized surface contains abundant sponge spicules. 

Small hill 1.2 km northeast .3.: Elongate block (13 em x 8 em x 5 em, 0.43 kg) of pyrrhotite-rich 

of ALV2038 massive sulfide with abundant Cu-Fe sulfides. 

(3) X:60082, Y:65643 3212 4A: Arcuate slab (15 em x 10 em x 4 em, 0.43 kg) of massive sulfide 

(4) X:60128, Y:65582 3199 largely consisting of chalcopyrite and sphalerite. 

(5) X:60201, Y:65500 3215 4B: Rounded elongate fragment ( 12 em x 9 em x 7 em, 0.67 kg) of 

pyrrhotite-rich massive sulfide with earthy yellow to red Fe-oxide 

crust. 

.5_: Angular block (14 em x 10 em x 9 em, 1.38 kg) of pyrrhotite-rich 

massive sulfide with numerous cavities and layered structure. 

Sample surfaces partly covered with red Fe oxides and black sooty 

Mn (?)oxide. 

Active vent area on upper 1: Irregular angular block (23 em x 17 em x 15 em, 3.86 kg) of 

eastern flank of central hill pyrrhotite-rich massive sulfide with a 2-cm-thick drusy crust of 

at NESCA barite and amorphous silica on one edge. 

(1) X:59179, Y:54879 3231 2: Conical sulfate-rich (predominantly barite with minor sphalerite and 

(2) X:59179, Y:54879 3231 Fe sulfide) fragment (26 em x 18 em x 10 em, 2.99 kg) with 

(4) X:59179, Y:54879 3231 numerous worm tube structures and cavities. 

~: Vuggy fragment (14 em x 9 em x 7 em, 0.21 kg) of massive sulfate 

(barite with minor sphalerite and Fe sulfide) with closely spaced 

worm tubes. Many of the tubes are partly filled with barite. 

Active vent area on upper 1: Porous, Fe-oxide-covered slab ( 13 em x 10 em x 3 em, 0.24 kg) of 

northwestern flank of central massive sulfate composed of barite and minor sulfide with numerous 

hill at NESCA vugs and worm tube structures. 

(1) X:58834, Y:55402 3219 

Active vent area on upper 1: Nearly equidimensional block (21 em x 17 em x 17 em, 9.81 kg) of 

eastern flank of central hill pyrrhotite-rich massive sulfide with minor sphalerite, Cu-Fe sulfide, 

at NESCA and barite. 

(1) X:59249, Y:54937 3226 

Gravity Cores 

Sediment plain midway Sediment core with pyrrhotite along fractures cutting sand layers in the 

between SESCA and NESCA interval 72 em to 105 em (base of core). Talc nodules are dispersed 

X:11698, Y:26586 3309 through sand and silt layers. 

Sediment plain 2.5 km south- Sediment core with layer of pyrrhotite-rich massive sulfide fragments 

west of 3170 hill at SESCA (50 to 77 em) between surface layer of reddish-brown metalliferous 

X:9756, Y:12209 3240 mud and underlying chlorite-rich sediment (77 to 200 em). 

Approximately 30 m south- The core recovered a 10-cm-long, cylindrical fragment of pyrrhotitewest of the active 220°C vent rich massive sulfide, 1 to 2 em of sulfide rubble, and small pieces of 

site on the central hill at NESCA lithified and altered sediment from the top of a sulfide mound. The 

X:58806, Y:55393 3240 sulfide rubble includes fragments of pyrrhotite-rich massive sulfide 

and polymetallic aggregates of barite, sphalerite, galena, and pyrite. 
302 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

SULFIDE-DOMINANT DEPOSITS 

Massive sulfide samples are grouped according to their 

mineralogy and texture into pyrrhotite-rich and 

polymetallic types. The mineralogy of these sulfide types 

is summarized in table 16.2. 

A 

c 

E 

PYRRHOTITE-RICH MASSIVE SULFIDE 

Massive sulfide composed of 50 to 85 percent pyrrhotite appears to form the bulk of sulfide mounds. Open fissures and slump blocks reveal that pyrrhotite-rich sulfide 

extends at least 1 m below the oxidized mound surfaces. 

B 

D 

F 
COMPOSITION AND GROWTH HISTORY OF HYDROTHERMAL DEPOSITS IN ESCANABA TROUGH 303 

Most samples of this sulfide type are derived from talus 

blocks or rubble (fig. 16.4A), but a massive sulfide chimney (sample SC659-Rl) is also included in this group (fig. 

16.4B). The surfaces of pyrrhotite-rich samples typically 

are weathered and mantled by iron oxides; some have surfaces covered with massive talc. 

The characteristic texture of this sulfide type is a latticework of interpenetrating, randomly oriented pyrrhotite 

grains in which interstices are partly filled by other sulfide minerals, sulfate minerals, sulfur, or iron oxides (fig. 

16.5A). The porosity of unaltered samples varies between 

10 and 20 percent. Pyrrhotite aggregates have a radial orientation and coarser grain size (to 1.5 mm) around fluid 

channel ways. Individual pyrrhotite grains have a platy morphology, and hexagonal tablets are abundant in cavities. Xray diffraction analysis identified only the hexagonal 

polymorph of pyrrhotite (Arnold, 1966); the metal content 

(Fe + Cu + Co + Ni), determined from the d-spacing of 

the (102) reflection (Arnold and Reichen, 1962), ranges between 47.5 and 48.7 atom percent. 

Pyrrhotite generally exhibits some degree of alteration 

to marcasite or iron-oxide minerals (Koski and others, 

1988). This alteration can vary considerably on the scale 

of a single thin section; in many samples, pyrrhotite is completely pseudomorphed by secondary minerals. In the least 

altered samples, marcasite forms discrete selvages along 

cracks and grain margins (fig. 16.5B). With increasing alteration, remnant cores of pyrrhotite are enclosed by mar-

~ 

Figure 16.3. Examples of sea-floor hydrothermal deposits in 

Escanaba Trough. A, Inactive sulfide mound and 1-m-high sulfide 

chimney at NESCA site. Organisms include anemones and sponges. 

Photographed from submersible DSV Alvin by R.A. Zierenberg. B, 

Craggy, steep-sided sulfide mound topped by eroded sulfide 

chimneys at NESCA site. Height of mound and chimneys in field of 

view is approximately 3 m. Photographed from submersible DSV 

Alvin by R.A. Zierenberg. C, Flat-lying ledges of partly oxidized 

massive sulfide overlapped by sediment at NESCA site. Prominent 

sulfide ledge at right center is approximately 0.5 m thick. Note several 

white galatheid crabs on sediment and sulfide outcrop. Photographed 

from submersible DSV Sea Cliffby R.A. Zierenberg. D, DSV Alvin 

external photograph showing talus blocks of massive sulfide resting 

on steep flank of sulfide mound at NESCA site. Sediment infills 

spaces between blocks and forms thin coating on some fragments. 

Largest blocks are approximately 0.5 m across. E, DSV Alvin external 

photograph showing cluster of knobby, barite-rich chimneys 

projecting upward from massive sulfide mound at NESCA. Two 

elongate broken surfaces with white rims mark location of two 

chimneys (for example, ALV2033-3; see fig. 16.4F) sampled with 

submersible manipulator. Maximum dimension of base of chimneys 

is about 0.2 m. F, DSV Alvin external photograph showing sulfate 

sinter deposit (white areas) on flank of massive sulfide mound on 

Central Hill at NESCA. Fluids discharging through sinter (areas of 

blurred focus) have temperatures as high as 220°C. Vertical extent 

of sinter in field of view is approximately 3 m. 

casite or an anisotropic iron-sulfide phase that has lower 

reflectivity and is pale brownish-gray. This "intermediate 

product" may be marcasite with anomalous optical properties owing to poor crystallinity or nonstoichiometry 

(Fleet, 1978). Where alteration is complete, the entire grain 

is pseudomorphed by marcasite, or the outline of the grain 

is filled by dull earthy mixtures of iron oxides including 

goethite and lepidocrocite. In a few grains, the cores of 

pyrrhotite grains are replaced by covellite (fig. 16.5C). 

Dark zones in the altered pyrrhotite grains may also be related to void space created by the negative volume change 

in the pyrrhotite ~ marcasite reaction (Fleet, 1978; 

Murowchick and Barnes, 1986). Koski and others (1988) 

attributed the formation of marcasite to an oxidative latestage hydrothermal alteration of pyrrhotite. 

The second most abundant mineral (5 to 50 percent of 

the mode) is the cubic polymorph of cubanite, now officially named isocubanite (Caye and others, 1988), which 

occurs as equant blebs occupying the corners of interstices 

in the latticework of pyrrhotite grains (fig. 16.5A). 

Isocubanite grains typically enclose an orthogonal arrangement of spindle-shaped chalcopyrite lamellae and are usually rimmed by the same mineral (fig. 16.5D). This texture 

has been documented in other sea-floor sulfide deposits 

(Haymon and Kastner, 1981; Styrt and others, 1981; Oudin 

and others, 1981; Koski and others, 1984) and may form 

by the combination of unmixing of chalcopyrite from a primary grain of intermediate solid solution (iss) composition 

and marginal replacement of isocubanite by chalcopyrite. 

Isocubanite has been obtained synthetically by transformation of the orthorhombic polymorph of CuFe2S3 above 

200°C; annealing recrystallization below the 200°C inversion point resulted in exsolution of chalcopyrite (Cabri and 

others, 1973 ). 

Sphalerite is ubiquitous in pyrrhotite-rich sulfide, but 

is generally less abundant (1 to 5 percent of the sample) 

than isocubanite. It is concentrated along cavities and 

fluid channel ways and on the outer surfaces of pyrrhotiterich sulfide fragments, where it is associated with barite 

and galena. Sphalerite typically forms subhedral crystals 

occupying interstices between pyrrhotite grains and also 

occurs as overgrowths on isocubanite (fig. 16.5A, D); the 

reverse relationship in which tiny grains of isocubanitechalcopyrite have grown on the surface of sphalerite is 

less common. Intergrowths of isocubanite-chalcopyrite 

and sphalerite form embayments in pyrrhotite. Sphalerite 

is generally opaque because of partial replacement by 

very fine grained Cu-Fe sulfide (Barton and Bethke, 

1987). These textural relations indicate that sphalerite and 

Cu-Fe sulfides are, in large part, coprecipitated, and both 

are paragenetically later than pyrrhotite. This textural relation between pyrrhotite, Cu-Fe sulfides, and sphalerite 

is also recognized in massive sulfide samples from 

Guaymas Basin (Koski and others, 1985; Peter and Scott, 

1988). 
304 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

Table 16.2. Mineralogy of hydrothermal samples from Escanaba Trough 

[XXX, abundant; XX, common; X, rare; -, not observed] 

Mineral General formula Pyrrhotite-rich Polymetallic 

sulfide sulfide 

Pyrrhotite ------------------

Isocu bani te ----------------

Chalcopyrite --------------

Sphalerite------------------

Marcasite ------------------

Galena ---------------------

Pyrite -----------------------

Arsenopyrite --------------

Lollingite ------------------

Wurtzite --------------------

Boulangerite --------------

Tetrahedri te ---------------

Acanthite(?) ---------------

S tanni te --------------------

Jordanite -------------------

F ranckei te -----------------

Alabandite-----------------

Covellite -------------------

Barite -----------------------

Anhydrite ------------------

Gypsum --------------------

Talc -------------------------

Stevensite ------------------

Amorphous silica --------

Goethite--------------------

Lepi docroci te -------------

Mn oxide (amorphous)--

Sulfur -------------:---------

Native bismuth -----------

Sulfides 

Fe0 _x1s ------------------- CuFe2s3 -------------------

CuFeS2 ---------------------

(Zn,Fe )S -------------------

FeS2 ------------------------

PbS -------------------------

FeS2 ------------------------

FeAsS ----------------------

FeAs2 ----------------------

(Zn,Fe )S -------------------

(Pb,Zn)5Sb4S11 -----------

Cu 12 S bS 13 -----------------

Ag2S ------------------------

Cu2FeSnS4 ----------------

Pb14 As7S24 ----------------

Pb5Sb2Sn3S 14 -------------

MnS ------------------------

Cu S -------------------------

Sulfates 

BaSO 4 ----------------------

CaSO 4 ----------------------

CaS04·2H20 --------------

XXX 

XXX 

XX 

XXX 

XXX 

X 

X 

X 

X 

X 

X 

X 

X 

XX 

X 

Silicates, oxides, and native elements 

Mg3Si40 10(0H)2 ---------

trioctahedral smectite ---

Si02-nH20 ----------------

FeO(OH) ------------------

FeO(OH) ------------------

? -----------------------------

s -----------------------------

Bi ---------------------------

Other 

XX 

X 

XX 

X 

XX 

X 

Bi telluride ---------------- ? ----------------------------- X 

Xenotime ------------------ YPO 4 ----------------------- X 

XXX 

XXX 

X 

XXX 

XXX 

XX 

XX 

X 

X 

X 

X 

X 

X 

X 

X 

X 

Sulfate 

dominant 

X 

X 

X 

XX 

XX 

X 

X 

X 

XXX 

XXXtoXX 

X 

XX 

XX 

X 

X 

X 

In several samples, clusters of small (20 Jliil) wedgeshaped grains of lollingite appear to overprint pyrrhotite 

and other sulfide minerals around fluid channelways (fig. 

16.5£). The 16llingite grains contain inclusions of native 

bismuth approximately 1 to 2 Jlii1 across. Microprobe 

analyses of lollingite yield as much as 7 percent sulfur, indicating the presence of intergrown -arsenopyrite (Koski and 

others, 1988). 

Pyrrhotite-rich sulfide samples also contain small 

amounts of pyrite, galena, lead-arsenic and lead-antimony 

sulfosalts, barite, amorphous silica, sulfur, hydrocarbons, 

and iron oxides. In sulfide samples where pyrite is present, 

it is typically framboidal. In one sample (a fragment of 

massive sulfide from gravity core TUL89D-8G), however, 

pyrite and marcasite form anhedral masses enclosing and 

partly replacing plates of pyrrhotite. The pyrite-marcasite 

mixture also pseudomorphs crystals with a dendritic or plumose habit, probably barite. Fine-grained galena and 

sulfosalt minerals such as jordanite and boulangerite occur as isolated grains and aggregates nucleated on the surfaces of coarser grained sulfides (fig. 16.5F). Barite 

typically forms individual tablets or rosettes that project 

into voids or channelways. Away from channelways, barite is partly replaced by sulfide minerals. Amorphous silica 

and goethite are present in many samples as a coating on 

pyrrhotite and isocubanite; in strongly oxidized samples, 

iron oxide fills interstices between sulfide grains. Yellowbrown asphaltic hydrocarbon in the pore space of pyrrhotite-rich sulfide interlayered with sand turbidite (fig. 16.4C) 

was derived from the hydrothermal alteration of terrigenous 
COMPOSITION AND GROWTH HISTORY OF HYDROTHERMAL DEPOSITS IN ESCANABA TROUGH 305 

A 

L6-NC-32-2 

-

,. .' . \ . . 

"" . • ~ 0 ... ~ 

·~ ~ • ., ..;."::?:- • • .:• 

" •• ,.. " !- •• ~ 

em 

0 1 2 3 4 

• 

c 

E 

Figure 16.4. Hydrothermal deposit types from Escanaba Trough. A, 

Talus fragment of pyrrhotite-rich massive sulfide with iron-oxide 

coating on weathered surfaces. Sample from dredge Ll-86-24D, 

NESCA site. B, Small chimney (sample SC659-Rl) composed of 

pyrrhotite-rich massive sulfide, shown on its side. Basal crust of 

mineralized sediment is on left. Sample recovered by submersible 

DSV Sea Cliff from NESCA area. C, Small fragment of interlayered 

pyrrhotite-rich massive sulfide and altered sand turbidite (sample 

L6-85-32D-2) dredged from SESCA site. Sulfide and sediment are 

B 

D 

F 

0 

--- - ~ 

---- CM 

saturated with hydrothermal petroleum. D, Bowl-shaped fragment 

of polymetallic massive sulfide, part of chimney like sulfide structure. 

Sample L2-86-14D-1, dredged from sediment plain north of Central 

Hill at NESCA. E, Sediment breccia cemented by pyrrhotite-rich 

sulfide. Sediment fragments are altered to chlorite and talc. Sample 

from dredge Ll-86-15D at SESCA site. F, Knobby barite-rich 

chimney with thin coating of manganese oxide. Sample ALV2033-

3, recovered by DSV Alvin from NESCA site. 
306 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

A B 

C D 

E F 

Figure 16.5. Petrography of pyrrhotite-rich massive sulfide samples. 

A, Reflected-light photomicrograph (RLP) of sample L6-85-32-25 

showing open latticework texture of pyrrhotite (po) plates with 

interstitial grains of corroded, intergrown Cu-Fe sulfides (isocubanite 

and chalcopyrite, cfs) and sphalerite (sp). Width of field is 0.25 mm. 

B, Backscattered electron (BSE) image of pyrrhotite grains (po) 

altered to anisotropic iron sulfide phase ("intermediate product") 

along grain margins and cracks. Width of field is 0.46 mm. C, RLP 

of altered pyrrhotite grain (po) with core partly replaced by covellite 

(cv), sample L1-86-15-1A. Width of field is 0.11 mm. D, RLP of 

mixed Cu-Fe sulfide grains in sample L 1-86-22-1 in which 

chalcopyrite (cp) occurs as oriented lamellae within isocubanite (icb) 

and also rims it. These grains are, in turn, overgrown by sphalerite 

(sp). Width of field is 0.11 mm. E, BSE image of sample L6-85-32-

1 0 showing fine-grained aggregate of wedge-shaped lOllingite grains 

(lo) partly replacing pyrrhotite (po ). Small white specks in lollingite 

are inclusions of native bismuth. Width of field is 0.15 mm. F, BSE 

image of sample ALV2035-1 A showing euhedral grains of pyrrhotite 

(po) and barite (ba) with fine-grained globular overgrowths of 

jordanite Ud). Width of field is 0.75 mm. 
COMPOSITION AND GROWTH HISTORY OF HYDROTHERMAL DEPOSITS IN ESCANABA TROUGH 307 

organic matter in the underlying sediment (Kvenvolden and 

others, 1986, 1990, chap. 15, this volume). 

Trace amounts of two bismuth minerals were identified by scanning electron microscope (SEM) examination 

of sample ALV2039-3 (table 16.1). Tiny (less than 5 ~m) 

anhedral blebs of a bismuth telluride phase form inclusions 

in grains of isocubanite-chalcopyrite. Equally small 

anhedra of native bismuth are loosely attached to the surfaces of these Cu-Fe sulfide grains. The SEM analysis also 

revealed a cluster of xenotime crystals in the interstice between adjacent Cu-Fe sulfide grains. In addition to yttrium 

and phosphorous, the energy-dispersive X-ray spectrum of 

the xenotime grains has lines for uranium, silicon, and rareearth elements. 

Subhedral grains of alabandite are present in the basal 

part of the pyrrhotite-rich chimney sample (SC659-R1; fig. 

16.4B). The alabandite grains occur within a zone of clastic 

sediment that also contains interstitial yellow hydrocarbon. 

POLYMETALLIC SULFIDE 

Approximately 12 kg of dark-gray massive sulfide 

with a diverse polymetallic mineral assemblage was recovered in a single dredge (L2-86-14D) from the sedimentcovered plain north of the Central Hill at NESCA (fig. 

16.2A). The largest fragment is bowl shaped (fig. 16.4D) 

and exhibits considerable variation in grain size, texture, mineralogy, and chemical composition over a distance of about 

40 mm between the concave inner and the convex outer surfaces. Koski and others (1988) interpreted these samples to 

be fragments of a chimneylike sulfide structure enclosing a 

large fluid channelway. The textural and structural characteristics of this sample resemble "black smoker" sulfide chimneys formed at sediment-free spreading axes (Haymon and 

Kastner, 1981; Styrt and others, 1981; Haymon, 1983; Paradis 

and others, 1988); however, differences in the mineralogy and 

mineral zonation are readily apparent. 

The sulfide structure of sample L2-86-14D-1 is subdivided into inner, middle, and outer wall sections that encompass six separate growth zones (fig. 16.6). The inner 

wall (zone 1) is a layer of coarse-grained sphalerite and 

isocubanite 4 to 6 mm thick that forms the concave surface of the sample. The subhedral sphalerite grains contain plumose cores and numerous inclusions of isocubanite 

(fig. 16.7A); these two minerals precipitated directly from 

hydrothermal fluid within the sulfide structure. Coarsegrained arsenopyrite occurs with pyrrhotite, sphalerite, and 

isocubanite near the contact with zone 2 (fig. 16.7B). These 

arsenopyrite grains have near-stoichiometric compositions 

and contain tiny (about 1 J..Un) inclusions of native bismuth 

(Koski and others, 1988). 

The middle wall of the sulfide structure consists of 

growth zones 2, 3, and 4. Zone 2 is a 10- to 12-mm-wide 

layer composed of pyrrhotite + sphalerite + isocubanite + 

arsenopyrite+ lollingite. In this zone, large (to 5 mm) platy 

pyrrhotite grains with a subparallel orientation extend 

toward zone 1 from a layer of intergrown sphalerite and 

isocubanite that mimics the texture of zone 1. Arsenopyrite 

in the sphalerite-cubanite layer of zone 2 is rimmed by 

lollingite (fig. 16.7 C). 

Zone 3 is approximately 10 mm wide and 

mineralogically similar to zone 2. Compared to zone 2, 

however, the pyrrhotite, arsenopyrite, and 16llingite are 

finer grained. Zone 3 meets zone 4 at an undulating 

unconformity that truncates pyrrhotite plates in the latter 

zone. Zone 4 is 6 to 10 mm thick and consists of sphalerite, isocubanite, and pyrrhotite in a textural association 

similar to zone 2. Mineralogically, this zone is distinct in 

that it contains abundant galena (in interstices and as a replacement of other sulfides) and much less arsenopyrite and 

lollingite. 

Growth zones 5 and 6 comprise the outer wall. Zone 

5 is a dense fine-grained layer of variable thickness (maximum 6 mm) largely composed of dendritic sphalerite 

intergrown with randomly oriented, stubby plates of pyrrhotite (less than 0.25 mm in length). Isocubanite is present 

as anhedral blebs in sphalerite dendrites, whereas galena 

forms overgrowths on sphalerite grains. Compared to zone 

4, galena is more abundant, isocubanite is less abundant, 

and arsenic-bearing phases are much less abundant. This 

low-porosity zone may represent the original wall of the 

sulfide structure. 

The outermost zone 6 is 1 to 2 mm thick and consists 

of porous fine-grained masses of dendritic sphalerite 

intergrown with galena and trace amounts of boulangerite, 

tetrahedrite, and a silver sulfide mineral, probably acanthite 

(fig. 16.7D; Koski and others, 1988). This zone is also characterized by clusters of extremely thin (less than 5 ~m) 

reedlike plates of pyrrhotite. Locally, these pyrrhotite crystals are nucleation sites for deposition of dendritic sphalerite. The "primitive" texture of zone 6 is the result of rapid 

crystallization during the mixing of hydrothermal fluid and 

seawater. 

SULFIDE-CEMENTED SEDIMENT AND 

SEDIMENT BRECCIA 

Several dredge hauls contained samples of sediment 

and sediment breccia with either pyrrhotite-rich or 

polymetallic sulfide assemblages as cement. Mineralized 

breccia samples consist of angular pebble-size mudstone 

and siltstone clasts in a cement composed of pyrrhotite and 

minor isocubanite (fig. 16.4£). The pyrrhotite grains are 

partly to completely pseudomorphed by marcasite. Corroded blebs of isocubanite within or between altered pyrrhotite grains are rimmed by covellite. In a few samples 

of this type, the interstitial space in the sulfide cement is 

partly filled with talc. 
308 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

A sample (L2-86-14D-6; table 16.1) of coarse sand 

cemented by polymetallic sulfide was recovered with 

polymetallic massive sulfide in a dredge from the NESCA 

site. In the sediment sample, the sulfide matrix encloses 

well-rounded sand-size clasts composed of altered mudstone (fig. 16.7£). The principal minerals in the cement are 

galena, sphalerite, pyrrhotite, and lollingite; void spaces are 

coated with hydrothermal petroleum (Koski and others, 

1988). Textural relations suggest that the paragenetic sequence is pyrrhotite -t sphalerite -t galena -t 16llingite. 

Isocubanite is relatively minor, occurring as small blebs in 

sphalerite. The margins of sediment clasts exhibit minor 

replacement by these minerals, and some clasts contain disseminated cubes of galena. Lollingite occurs as elongate 

clusters of fine-grained, wedge-shaped crystals that are superimposed on both the sulfide cement and adjacent sand 

grains. 

Two tin-bearing minerals are also present in trace 

amounts in this sample. Stannite is present as thin lamellae oriented parallel to the margins of coarse subhedral 

0 

somm Inner Wall 

sphalerite grains (fig. 16.7 F). A tin-bearing sulfosalt mineral, probably franckeite, occurs as clusters of curved platy 

crystals that typically radiate outward from the edges of 

pyrrhotite grains. 

The clasts in sulfide-cemented sediment breccia and 

sediment are predominantly indurated mudstone and siltstone that have been partly to completely altered to talc and 

chlorite (Benninger and Koski, 1987; Zierenberg and 

Shanks, chap. 14, this volume). A smaller proportion of the 

sediment is composed of sand- and silt-size grains of 

quartz, feldspar, muscovite, and pyroxene. Of these detrital mineral grains, only muscovite is altered and replaced 

by sulfide along cleavage planes. 

CLASTIC SULFIDE DEPOSITS 

Approximately 27 em of pyrrhotite-rich massive sulfide with a clastic texture was recovered in core L2-86-9G 

in the SESCA area (table 16.1; fig. 16.2B). The sulfide inSeawater 

Hydrothermal Fluid 

L2-86-14 D-1 Zone 1 Seawater 

Zone6 

(1-2mm) 

ZoneS 

(4~mm) 

Zone4 

(6-10 mm) 

Zone3 

(10-12mm) 

Zone 2 

(10-12 mm) 

Zone 1 

(4~mm) 

Porous, fine-grained, dendritic sphalerite with minor disseminated pyrrhotite, galena, 

boulangerite, and acanthite 

Low-porosity layer of fine-grained dendritic sphalerite with interstitial granular sphalerite, 

pyrrhotite, and galena. 

Subhedral sphalerite intergrown with isocubanite overlies interior layer of partly altered, radiating, 

fine-grained pyrrhotite laths. Sphalerite occupies interstices between laths. Galena is 

disseminated throughout zone. 

Similar to zone 4. Pyrrhotite laths are coarser and are partly replaced by medium-grained 

sphalerite. Disseminated galena is absent. Arsenopyrite and lollingite are concentrated in two 

irregular "wavefronts" along outer region of zone. 

Similar to zone 3. Pyrrhotite laths are much coarser grained and are largely replaced by 

sphalerite. Arsenopyrite-lallingite front coincides with region of intergrown isocubanite and 

sphalerite. 

lsocubanite intergrown with sphalerite projects toward open interior. Arsenopyrite is 

concentrated along contact with zone 2. 

Outer 

Wall 

Middle 

Wall 

Inner 

Wall 

Hydrothermal Fluid 

Figure 16.6. Cross section of polymetallic sulfide wall (sample L2-86-14D-l) subdivided into inner, middle, and outer wall sections and 

six distinct mineral zones that formed between hydrothermal fluid and seawater. Location of area studied in polished thin section is also 

shown. 
COMPOSITION AND GROWTH HISTORY OF HYDROTHERMAL DEPOSITS IN ESCANABA TROUGH 309 

A 

c 

E 

Figure 16.7. Petrography of polymetallic sulfide samples. A, 

Reflected-light photomicrograph (RLP) showing subhedral sphalerite 

grain (sp) with plumose core and numerous inclusions of isocubanite 

(icb) from inner wall (zone 1) of sulfide sample L2-86-14D-1. Width 

of field is 0.4 mm. B, RLP of inner wall (zone 1) of sample L2-86-

14D-1 showing arsenopyrite grain (asp) along contact between 

altered pyrrhotite (po) and sphalerite (sp). Sphalerite grains contain 

inclusions of isocubanite (icb). Width of field is 0.5 mm. C, 

Backscattered electron image from middle wall (zone 2) of sample 

L2-86-14D-1 showing rhombic arsenopyrite grains (asp) with thin 

rims of lOllingite (lo). Sphalerite (sp) and isocubanite (icb) are also 

B 

D 

F 

present. Black area is void space. Width of field is 0.11 mm. D, RLP 

of outer wall (zone 6) of sulfide structure (sample L2-86-14D-1) 

showing porous texture and occurrence of boulangerite (bl), 

sphalerite (sp), and galena (gl). Width of field is 0.2 mm. E, RLP of 

sulfide-cemented sediment (sample L2-86-14D-6) composed of 

rounded sand grains and interstitial aggregate of pyrrhotite, galena, 

sphalerite, and lollingite. Width of field is 0.25 mm. F, RLP of sulfidecemented sediment (sample L2-86-14D-6) showing sphalerite grain 

(sp) that contains narrow oriented intergrowths of stannite (st). Width 

of field is 0.11 mm. 
310 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

terval is overlain by reddish-brown metalliferous mud and 

underlain by chlorite-rich mud and turbidite (Zierenberg 

and Shanks, chap. 14, this volume). The sulfide clasts are 

subrounded and consist of a pyrrhotite latticework with interstitial isocubanite-chalcopyrite and barite. 

SULFATE-DOMINANT DEPOSITS 

BARITE-RICH CRUSTS 

Barite-rich crusts as thick as 2 em occur on the outer surfaces of massive sulfide mounds where they precipitated as 

discharging hydrothermal fluids mixed with seawater. These 

drusy encrustations have narrow gradational contacts with the 

pyrrhotite-rich sulfide substrate. Within the crusts, barite 

forms bladed aggregates, dendrites, and rosettes with preferred growth orientations outward toward seawater (fig. 

A 

c 

Figure 16.8. Sulfate-dominant samples. A, Reflected-light 

photomicrograph (RLP) of barite-rich crust (sample L1-86-24D-3) 

showing radiating barite blades (ba) overgrown by sphalerite (sp). 

Width of field is 0.12 mm. B, Backscattered electron (BSE) image 

of composite overgrowth of jordanite (jd) and galena (gl) in barite 

crust, sample L 1-86-16D-2. Dark zones in galena (sb-gl) are enriched 

in antimony. Width of field is 0.13 mm. C, RLP of complex 

16.8A). Locally, barite encrustations extend into massive sulfide along fluid passageways where crystals project inward 

toward hydrothermal fluid. Barite exhibits considerable grainsize variation with maximum crystal dimensions up to 3 mm. 

Barite in crusts exhibits a variety of intergrowth and 

replacement textures with sulfide minerals. In one association, radiating blades of barite are overgrown by colloform 

sphalerite and granular galena. A less common association 

is barite overgrown by mixtures of galena, antimony-bearing galena, and jordanite (fig. 16.8B). In barite crusts, 

sphalerite, galena, chalcopyrite, and pyrrhotite exhibit a 

number of disequilibrium textures and parageneses that reflect rapid variations in fluid composition within the zone 

of fluid mixing (fig. 16.8C). Most barite crusts are covered 

with thin films of iron oxide and (or) amorphous silica that 

contain fine-grained sulfosalt phases such as boulangerite 

and jordanite, granular aggregates of marcasite and pyrite, 

and rare pyrite framboids. 

B 

D 

overgrowth paragenesis in barite crust sample L 1-86-16D-1 that 

includes, from core to rim, pyrrhotite (po), sphalerite (sp), 

chalcopyrite (cp), sphalerite, pyrrhotite, chalcopyrite, and sphalerite. 

Width of field is 0.25 mm. D, BSE showing alternating layers of 

anhydrite (anh), barite (ba), and anhydrite+ gypsum (gp) from sulfate 

sinter deposit, sample ALV2036-1B. Width of field is 3.6 mm. 
COMPOSITION AND GROWTH HISTORY OF HYDROTHERMAL DEPOSITS IN ESCANABA TROUGH 311 

BARITE CHIMNEYS 

Massive barite chimneys recovered from the tops of 

hydrothermal mounds and from the sediment surface between mounds are characterized by a porous spongy texture and the absence of mineral zonation around fluid 

channelways. The chimneys have either a narrow (2 to 4 

mm) central fluid conduit or lack any continuous 

channelways. In sample SC658-l (table 16.1), dendritic and 

plumose barite aggregates make up approximately 98 percent of the total mineralogy; in other chimney samples, 

however, barite is intergrown with variable amounts of 

sphalerite, galena, marcasite, pyrite, amorphous silica, and 

native sulfur. The central channelway near the apex of 

chimney sample ALV2033-2 is composed of barite, amorphous silica, and native sulfur, all of which are stained by 

petroleum. All of the barite chimneys recovered were hydrothermally inactive and are coated with thin films of 

amorphous manganese oxide (fig. 16.4F). 

SULFATE SINTER 

Clear fluids with temperatures near 220°C are depositing aprons of sulfate-rich sinter where they discharge 

through massive sulfide mounds at the NESCA site (fig. 

16.3F; Campbell and others, chap. 11, this volume). Sinter samples from within the area of active discharge are the 

youngest materials collected, and they consist of coarsegrained vuggy aggregates of anhydrite, gypsum, barite, and 

stevensite. Our identification of stevensite, a trioctahedral 

smectite (Brindley, 1980; Newman and Brown, 1987), was 

based on its X-ray diffraction pattern and chemical data 

(high Mg, Si; low AI, Fe) obtained during SEM analysis. 

Other sinter samples taken a few meters away from a 

108°C active vent are predominantly barite but contain local concentrations of marcasite, pyrite, sphalerite, wurtzite, 

and galena. Textural relations suggest that euhedral sphalerite in the barite-rich samples has formed from transformation of metastable wurtzite. The abundance of tubular 

structures in barite-rich sinter shows that these deposits 

serve as a substrate for tube worms and other organisms. 

The sinter sample shown in figure 16.8D (ALV2036-

1B) is constructed from alternating layers of fibrous 

anhydrite crystals and bladed barite. The anhydrite fibers 

are as long as 2 em; gypsum occurs as an infilling within 

aggregates of anhydrite crystals. The contacts between 

sulfate layers often truncate earlier-formed sulfate grains 

and are marked by disseminated fine-grained pyrite and 

marcasite. 

Stevensite occurs in compact semiopaque clusters of 

birefringent fibers scattered through the sulfate layers. The 

margins of the stevensite aggregates truncate and embay 

adjacent sulfate crystals, indicating that it is paragenetically 

late. Stevensite has also been identified in mound and 

chimney samples from the hydrothermal vent fields in 

Guaymas Basin, Gulf of California (Peter, 1986; Peter and 

Scott, 1988). 

Sulfate sinter also contains partly oxidized fragments 

of pyrrhotite-rich sulfide probably derived from the collapse of nearby mound or chimney material. Stevensite is 

concentrated between pyrrhotite grains and in narrow rims 

around the corroded margins of the sulfide fragments. 

BULK CHEMISTRY OF DEPOSITS 

ANALYTICAL TECHNIQUES 

Analytical data for 32 samples of sulfide- and sulfatedominant deposits, analyzed at two separate laboratories, 

are presented in tables 16.3, 16.4, and 16.5. At the U.S. 

Geological Survey, Si02, CaO, MgO, Al203, Fe, Cu, Pb, 

Zn, Au, Bi, Cd, Hg, Sb, Se, Sr, and Tl were analyzed by 

quantitative direct current arc optical emission spectroscopy; As, Ag, Ba, Mn, Sn, Co, Cr, Ge, Ni, V, and Y by 

semiquantitative emission spectroscopy; and totalS and C 

using combustion techniques combined with infrared detector (comb-IR). Organic carbon was determined by subtracting carbonate carbon, analyzed by coulometric 

titration, from total carbon. Arsenic greater than 0.2 weight 

percent was analyzed by inductively coupled plasmaatomic emission spectrometry (ICP-AES). At the Geological Survey of Canada, Si02, CaO, MgO, Al203, Fe, Cu, 

Pb, Zn, Ba, Ag, Co, Cr, Mn, Mo, Ni, Sr, and V were determined by ICP-AES; As, Bi, Mo, Sb, and Se by atomic 

absorption spectrometry; totalS by comb-IR; Au by a combined fire assay/DC plasma method; and Sn by ICP-mass 

spectrometry. Sulfate sulfur (reported as S03) was calculated as a proportion of the total sulfur according to the 

ratio of sulfate cations (Ba + Ca) to sulfide cations (Fe + 

Cu + Zn + Pb ). Because the complete dissolution of barite 

is difficult during routine laboratory analysis, the Ba and 

total S contents reported here should be considered minimum values. The low totals in many samples are also related to the presence of additional volatiles (especially 

H20) and the partial oxidation of sulfides. 

SULFIDE-DOMINANT SAMPLES 

The analytical results for 26 sulfide-dominant samples 

are presented in table 16.3. In addition to abundant Fe and 

S, pyrrhotite-rich massive sulfide samples have high Cu 

contents (average 3.12 percent), the highest being 20.6 percent for a NESCA mound sample (ALV2039-3). The CuZn-Pb(x10) ratios of sulfide-dominant samples from 

Escanaba Trough are plotted in the ternary diagram of figure 16.9A. The Cu-to-Zn ratio exceeds unity in 18 of 23 

pyrrhotite-rich samples, although Zn ranges between 0.02 
312 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

Table 16.3. Bulk composition of sulfide-dominant samples. 

[USGS, U.S. Geological Survey; GSC, Geological Survey of Canada; NA, not analyzed; <, less than; >, greater than; - ' interference from other elements] 

Sample ----------------- L1-86 L1-86 L1-86 

16D-1 24D-1 24D-1 

Laboratory------------- USGS USGS GSC 

Weight percent: 

Fe ----------------------- 33.3 42 41.5 

Cu ----------------------- 4.62 1.21 2.90 

Zn ----------------------- 2.3 .34 .08 

Pb ----------------------- .65 .2 .02 

As ----------------------- .2 .03 .001 

BaO --------------------- 3.01 .04 .12 

s ------------------------- 35.33 45.16 39.00 

so3 --------------------- 5.43 .10 .24 

Si02 -------------------- .95 <.02 .40 

CaO --------------------- .025 .02 .11 

MgO -------------------- .07 .11 .14 

Al20 3 ------------------ <.02 <.02 .03 

Total -------------- 86.91 89.25 84.54 

Parts per million: 

Ag ---------------------- 67.5 14 8 

Au ---------------------- 1.4 <.02 .74 

Bi ----------------------- 44 25 43 

Cd ----------------------- 270 4.4 NA 

Co ----------------------- 500 10 460 

Cr ----------------------- <2 <2 <1 

Ge ----------------------- 15 <2 NA 

Hg ---------------------- <2 <2 NA 

Mn ---------------------- 50 100 <100 

Mo ---------------------- NA NA 45 

Ni ----------------------- 7 2 <1 

Sb ----------------------- 530 24 4 

Se ----------------------- 95 180 72 

Sn ----------------------- 70 300 115 

Sr ----------------------- 130 <50 6 

v ------------------------ <2 <2 <2 

y ------------------------ <10 <10 .1 

Total C (wt. pet.) ----- .14 .32 NA 

Organic C (wt. pet.) - .14 .32 NA 

and 5.9 percent. The amount of Pb is generally subordinate to Zn in pyrrhotite-rich sulfide, but exceeds 1 percent 

in two samples (ALV2039-4A and ALV2039-5B). Figure 

16.9B shows that pyrrhotite-rich sulfide samples have a 

higher Cu-to-Zn ratio and a larger proportion of Pb relative to Cu and Zn than massive sulfide samples from sediment-free spreading axes of the northeast Pacific Ocean 

and the Mid-Atlantic Ridge, as well as from Middle Valley, the sediment-covered northern segment of Juan de Fuca 

Ridge. 

The average Au content for 23 samples of pyrrhotiterich sulfide is approximately 2 ppm; the highest Au value 

of 10.1 ppm occurs in the Cu-rich sample ALV2039-3. The 

association of gold and copper in some pyrrhotite-rich 

Pyrrhotite-rich sulfide 

L1-86 L1-86 L2-86 L2-86 ALV 

24D-16-3 24D-16-2 18D 25D 2033-1 

GSC GSC USGS USGS GSC 

35.2 40.1 35.2 57 27.5 

.78 .72 1.01 .42 1.2 

.13 .16 .07 .11 4.6 

.21 .20 <.02 <.02 .12 

.027 .25 .55 <.02 .42 

.02 .20 <.0006 .004 .010 

40.48 14.04 42.1 36.6 39.19 

.01 .15 .00 .00 .03 

.19 4.7 <.02 .34 .20 

.08 .25 .036 .01 .07 

.11 .71 .12 .05 .10 

.03 .17 <.02 .16 .06 

77.30 61.65 79.15 94.73 73.50 

4 2 <3 <3 33 

.04 .16 <.2 <.02 <.003 

29 20 27 12 13 

NA NA 4.8 4.5 NA 

220 120 2 <2 5 

<1 <1 <2 <2 <1 

NA NA <2 <2 NA 

NA NA <2 <2 NA 

230 160 7 7 230 

20 275 NA NA <2 

13 <1 1.5 1.5 15 

21 262 132 172 130 

194 25 525 380 179 

28 9 500 200 620 

6 91 <50 <50 4 

<2 75 <2 <2 4 

.1 1.8 <10 <10 .3 

NA NA .13 .11 NA 

NA NA .13 .11 NA 

samples, evident in table 16.3, is contrary to the commonly 

observed association of gold in sea-floor sulfide samples 

enriched in zinc and lead (Hannington and Scott, 1990). It 

is also apparent from the ternary Cu-Zn-Au(x104) diagram 

of figure 16.9C that pyrrhotite-rich samples from Escanaba 

Trough are enriched in gold relative to sulfide samples from 

sediment-free spreading axes as well as Middle Valley. 

The pyrrhotite-rich massive sulfide samples from 

Escanaba Trough also contain high As (to 1.55 percent, average 0.31 percent), Bi (to 820 ppm, average 83 ppm), Co 

(to 3,000 ppm, average 571 ppm), Sb (to 749 ppm, average 167 ppm), and Se (to 525 ppm, average 142 ppm). Of 

these, As, Bi, Sb, and Se are considerably enriched in comparison to deposits from sediment-free ridges (see, for 
COMPOSITION AND GROWTH HISTORY OF HYDROTHERMAL DEPOSITS IN ESCANABA TROUGH 313 

Table 16.3. Bulk composition of sulfide-dominant samples-Continued. 

Pyrrhotite-rich sulfide 

Sample ----------------- ALV ALV ALV ALV ALV ALV ALV 

2033-5A 2033-5B 2035-1A 2035-2 2037-1A 2037-1B 2038-2A 

Laboratory------------- GSC GSC GSC GSC GSC GSC GSC 

Weight percent: 

Fe ----------------------- 40.5 26.4 31.0 54.9 50.6 51.2 31.9 

Cu ----------------------- 1.6 3.3 3.0 1.8 2.3 2.6 2.3 

Zn ----------------------- 1.2 .61 2.7 .02 .025 .025 .038 

Pb ----------------------- .027 .0007 .34 .002 .0005 .001 .004 

As ----------------------- .171 .609 .932 .008 .049 .052 .091 

BaO --------------------- 6.4 6.5 11.2 .02 .06 .04 .004 

s ------------------------- 27.6 29.3 20.2 38.4 36.0 37.0 36.6 

so3 --------------------- 9.1 14.0 13.7 .03 .09 .05 0 

Si02 -------------------- 1.68 .25 .35 .2 .24 .19 .23 

CaO --------------------- .10 .14 .2 .1 .08 .07 .1 

MgO -------------------- .04 .10 .09 .05 .03 .03 .07 

Al20 3 ------------------ .06 .04 .07 .03 .03 .03 .03 

Total -------------- 88.38 81.22 83.74 95.55 89.46 91.37 71.37 

Parts per million: 

Ag ---------------------- 25 60 52 2 9 10 13 

Au ---------------------- 1.29 3.20 8.89 1.58 1.08 1.44 1.16 

B i ----------------------- 25 43 820 

Cd ----------------------- NA NA NA 

Co ----------------------- 320 320 3,000 

Cr ----------------------- <1 <1 110 

Ge ----------------------- NA NA NA 

Hg ---------------------- NA NA NA 

Mn ---------------------- <100 <100 100 

Mo ---------------------- 2 3 17 

N i ----------------------- <3 <3 <3 

Sb ----------------------- 73 79 296 

Se ----------------------- 22 3.6 3 

Sn ----------------------- 15 23 405 

Sr ----------------------- 110 120 500 

v ------------------------ <2 3 24 

y ------------------------ .1 .1 

Total C ( wt. pet.) ----- NA NA NA 

Organic C (wt. pet.) - NA NA NA 

example, Bischoff and others, 1983). High arsenic is consistent with the presence of arsenopyrite and lOllingite, and 

high bismuth with the presence of native bismuth and bismuth telluride phases in samples of this type. The barium 

and strontium contents are proportional to the amount of 

barite present in the pyrrhotite-rich samples. 

The polymetallic sulfide samples from dredge L2-86-

14D have a high Zn-to-Cu ratio and are enriched in lead 

relative to pyrrhotite-rich sulfide deposits at Escanaba 

Trough and sulfide deposits from other ridge axes (fig. 

16.9A,B). High arsenic, antimony, silver, and tin contents 

are consistent with their diverse mineral assemblages. It 

is noteworthy that gold is not concentrated in the 

polymetallic sulfide samples (fig. 16.9C). The 

polymetallic massive sulfide sample L2-86-14D-1 has the 

highest Ag (681 ppm) and sample L2-86-14D-2 has the 

.1 

73 54 66 25 

NA NA NA NA 

1,200 1,000 510 660 

<1 <1 <1 130 

NA NA NA NA 

NA NA NA NA 

100 100 <100 230 

47 24 7 14 

12 4 6 61 

4 5 4 12 

139 108 81 164 

22 6 8 40 

4 5 3 3 

<2 <2 <2 2 

1.5 NA .7 1.4 

NA NA NA NA 

NA NA NA NA 

highest Zn ( 42.6 percent) among all of the bulk samples 

analyzed (table 16.3). 

Chemical data for representative samples of the inner, 

middle, and outer walls of the polymetallic sulfide structure 

(L2-86-14D-1) are presented in table 16.4 and illustrated in 

figure 16.1 0. The sample is strongly zoned: Pb, Sb, Ag, Ba, 

Sn, and Hg increase and As, Cu, Cd, Se, Bi, and Mn decrease 

between the inner, high-temperature surface (growth zones 

1 and 2) and the outer, low-temperature surface (growth zones 

5 and 6). The largest increases from inner to outer walls are 

for lead (15 times), silver (20 times), antimony (33 times), 

and tin (20 times). Arsenic is greatly enriched in this sample 

and is more abundant than copper in the inner and middle 

walls. Although silver is concentrated to a high level in the 

sample, especially in the outer wall (3,000 ppm), gold was 

not detected at the 0.1 ppm level. 
314 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

Table 16.3. Bulk composition of sulfide-dominant samples-Continued. 

Sample ----------------- ALV ALV ALV 

2039-3 2039-4A 2039-5B 

Laboratory------------- GSC GSC GSC 

Weight percent: 

Fe ----------------------- 25.5 30.3 43.6 

Cu ----------------------- 20.6 1 1.7 

Zn ----------------------- 1.2 5.9 4.8 

Pb ----------------------- .34 1.1 1.1 

As ----------------------- .162 1.55 .476 

BaO --------------------- .030 2.3 .75 

s ------------------------- 37.1 37.3 32.7 

so3 --------------------- .05 5.1 1.1 

Si02 -------------------- .2 2.02 .22 

CaO --------------------- .06 .07 .08 

MgO -------------------- .04 .05 .06 

Al20 3 ------------------ .02 .03 .04 

Total -------------- 85.28 86.73 86.56 

Parts per million: 

Ag ---------------------- 160 170 150 

Au ---------------------- 10.14 3.27 2.94 

B i ----------------------- 125 8 54 

Cd ----------------------- NA NA NA 

Co ----------------------- 890 210 370 

Cr ----------------------- 1 3 <1 

Ge ----------------------- NA NA NA 

Hg ---------------------- NA NA NA 

Mn ---------------------- <100 100 100 

Mo ---------------------- 51 <2 16 

Ni ----------------------- 34 <3 <3 

Sb ----------------------- 69 749 521 

Se ----------------------- 130 88 148 

Sn ----------------------- 60 NA 63 

Sr ----------------------- 3 37 40 

v ------------------------ 6 3 <2 

y ------------------------ .2 NA .9 

Total C ( wt. pet.) ----- NA NA NA 

Organic C ( wt. pet.) - NA NA NA 

The sample of clastic sediment (L2-86-14D-6) with 

polymetallic sulfide cement has the highest Pb (13.7 

percent), As (2.75 percent), Mn (7,000 ppm), Sb (8,700 

ppm), and Sn (> 1,500 ppm) among the samples analyzed, 

and also contains high Zn (table 16.3). The high organic 

carbon content (2.24 percent) is related to the presence of 

hydrothermally derived petroleum that coats interstices between sediment and sulfide grains. 

SULFATE-DOMINANT SAMPLES 

Analytical data for representative sulfate-dominant 

samples are presented in table 16.5. The barite-rich crust and 

chimneys are characterized by high Ba-to-Ca ratios, which 

is consistent with the absence of anhydrite in these samples. 

Pyrrhotite-rich sulfide 

sc sc sc sc TUL 89-

658-R3 658-R4 659-Rl 662-Rl G8 

USGS USGS USGS USGS GSC 

48.1 52.5 17.9 48.2 38.8 

.55 2.06 7.7 2.22 2.3 

.61 .12 1.38 2.05 3.0 

.07 .08 .26 .35 .16 

<.02 <.02 .2 .05 .29 

.80 .01 .26 .3 4.7 

39.5 38.8 53.8 35.2 27.7 

1.4 .03 1.1 .50 6.6 

.35 <.02 1.86 <.02 8.2 

.02 .01 .14 .02 .20 

.07 .04 .17 .06 1.23 

<.02 <.02 .76 <.02 1.51 

91.57 93.7 85.51 89.02 94.62 

15 15 150 150 73 

<.1 <.1 2.20 <.1 1.11 

57 53 68 22 102 

16 6 47 67 NA 

500 300 700 700 410 

<2 <2 2 <2 32 

<2 <2 <2 30 NA 

<2 <2 <2 51 NA 

100 15 300 15 200 

17 

5 5 30 10 <3 

8 17 200 140 162 

71 <10 324 <10 67 

<2 <2 30 20 96 

100 3 30 90 130 

<2 <2 <2 <2 43 

10 10 <10 10 2.4 

<.18 <.18 1.38 <.24 NA 

.05 .09 NA 

The presence of amorphous silica in all crust and chimney 

samples results in silica contents greater than 1 percent. The 

barite-rich crust and two of the chimney samples also contain high zinc, lead, and silver. The high zinc contents and 

Zn-to-Fe ratios in samples SC766-1 and ALV2033-3B indicate that the sphalerite present has a low iron content. The 

barite-rich crust has the highest Y (30 ppm) and Ge (150 

ppm) contents of all the hydrothermal samples analyzed. The 

nearly monomineralic barite chimney (SC658-R2) is depleted 

in metals (for example, Fe+ Cu + Zn + Pb <1.5 percent and 

Ag = 2 ppm), but mercury, manganese, and vanadium are 

enriched relative to other sulfate-rich and most sulfide 

samples. The high concentration of strontium in this sample 

results from its substitution for barium in barite. 

Samples of sulfate sinter have distinct calcium-rich, 

barium-poor or barium-rich, calcium-poor compositions, 
COMPOSITION AND GROWTH HISTORY OF HYDROTHERMAL DEPOSITS IN ESCANABA TROUGH 315 

Table 16.3. Bulk composition of sulfide-dominant samplesContinued. 

Sample -----------------

Laboratory-------------

Weight percent: 

Fe -----------------------

Cu -----------------------

Zn -----------------------

Pb -----------------------

As -----------------------

BaO ---------------------

S -------------------------

S03 ---------------------

Si02 --------------------

CaO ---------------------

MgO --------------------

Al203 ------------------

Total --------------

Parts per million: 

Ag ----------------------

Au ----------------------

B i -----------------------

Cd -----------------------

Co -----------------------

Cr -----------------------

Ge -----------------------

Hg ----------------------

Mn ----------------------

Mo ----------------------

Ni -----------------------

Sb -----------------------

Se -----------------------

Sn -----------------------

Sr -----------------------

V ------------------------

y ------------------------

Total C (wt. pet.) -----

Organic C (wt. pet.) -

=> 

Polymetallic 

massive sulfide 

L2-86- L2-86-

14D-1 14D-2 

USGS USGS 

33.1 

1.82 

20.3 

7.6 

1.7 

2.8 

31.4 

3.1 

<.02 

.016 

.05 

<.02 

101.89 

681 

<.2 

40 

630 

<2 

<2 

7 

<2 

15 

<1 

960 

1,000 

700 

<50 

<2 

<10 

.07 

.07 

17.9 

.99 

42.6 

1.05 

2.5 

.01 

32.7 

.01 

.34 

.028 

.09 

.11 

99.13 

184 

<.2 

64 

1,975 

<2 

<2 

25 

22 

35 

1.5 

850 

510 

500 

<50 

<2 

<10 

<.06 

<.06 

Polymetallic 

sulfide cement 

L2-86-

14D-6 

USGS 

22.9 

.26 

7.1 

13.7 

2.75 

.39 

23.8 

.5 

13.8 

.038 

.25 

3.40 

88.87 

339 

<.2 

9.7 

150 

3 

20 

<2 

<2 

7,000 

15 

8,700 

<10 

>1,500 

<50 

<2 

15 

2.24 

2.24 

Figure 16.9. A, Ternary plot showing Cu-Zn-Pb(xlO) ratios for 

pyrrhotite-rich sulfide samples (open circles) and polymetallic sulfide 

samples (closed circles) from Escanaba Trough. B, Comparison of 

Cu-Zn-Pb(x10) ratios for Escanaba Trough sulfides (averages for 

pyrrhotite-rich and polymetallic samples) with sulfide samples from 

other ocean ridges. C, Comparison of Cu-Zn-Au (x1o4) ratios for 

Escanaba Trough sulfides and sulfide samples from other ocean 

ridges. Chemical data for samples from other ridges are compiled 

from Hannington and others (1991), Kappel and Franklin (1989), 

and Bischoff and others (1983). Abbreviations used: ET, Escanaba 

Trough; GAL, Galapagos Spreading Center; EPR, East Pacific Rise; 

EXP, Explorer Ridge; TAG, Trans-Atlantic Geothermal site, MidAtlantic Ridge; END, Endeavour segment of Juan de Fuca Ridge; 

MV, Middle Valley segment of Juan de Fuca Ridge; AX, Axial 

Seamount, Juan de Fuca Ridge; SJDF, southern Juan de Fuca Ridge. 

which reflect the relative abundances of anhydrite and 

barite. The combination of high magnesium and silicon in 

calcium-rich sinters is largely related to the presence of 

stevensite, but some magnesium may be present as 

caminite. The barium-rich sinter has higher iron and zinc 

contents owing to the presence of marcasite, pyrite, 

sphalerite, and wurtzite; the high silicon content is related 

Cu 

A 

Pb X 10 

Cu 

B 

Pb X 10 

Cu 

c 

MV 

• EPA, 21°N -

SJDF,.. 

Zn 

Zn 

ET 

(polymetallic) 

eMv 1 

~---------AX~~--------~~-·SJDF EPA, 21°N Zn 
316 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

Table 16.4. Composition of polymetallic sulfide sample L2-86-

14D-1. 

[Analyses by U.S. Geological Survey. <,less than; -,interference 

from other elements] 

Inner Wall Middle Wall Outer Wall 

Weight percent: 

Fe ----------------------- 21.1 29.3 21.2 

Cu ----------------------- 4.64 1.62 .24 

Zn ----------------------- 35.1 26.1 22.5 

Pb ----------------------- 1.4 2.3 20.7 

As ----------------------- 5.4 1.76 .1 

BaO --------------------- .036 .108 . 28 

s ------------------------- 31.68 33.64 28.40 

S03 --------------------- .04 .16 .28 

Si02 --------------------- .44 <.02 <.02 

CaO --------------------- .033 .005 .078 

MgO -------------------- .075 .01 .22 

Al20 3 ------------------- .10 <.02 <.02 

Total ----------------- 100.04 95.04 94.12 

Parts per million: 

Ag ---------------------- 150 200 3,000 

Au ---------------------- <.1 <.1 <.1 

Bi ----------------------- 95 88 73 

Cd ----------------------- 1,800 940 76 

Ge ----------------------- <2 <2 100 

Hg ---------------------- <2 <2 127 

Mn ---------------------- 50 15 15 

Mo ---------------------- 20 

Ni ----------------------- 3 <1 3 

Sb ----------------------- 480 550 16,000 

Se ----------------------- 1,400 500 400 

Sn ----------------------- 70 70 1,410 

Sr ----------------------- 3 3 15 

Tl ----------------------- <10 <10 24 

Total C (wt. pet.) ----- <.15 <.15 .70 

Organic C ( wt. pet.) - .50 

to the abundance of amorphous silica. Samples of bariterich crust (L1-86-16D-2), barite chimney (ALV2033-3B), 

and barite-rich sinter (ALV2040-2) also have notably high 

gold contents. 

GENESIS OF SULFIDE- AND 

SULFATE-DOMINANT DEPOSITS 

DISTRIBUTION AND SIZE CONSIDERATIONS 

Seismic-reflection data show that the distribution of 

large sulfide deposits and currently active hydrothermal 

vent fields in Escanaba Trough is related to the emplacement of volcanic edifices along the sediment-covered 

Gorda Ridge axis (Morton and others, 1987, 1990). High 

heat-flow measurements (greater than 1,200 mW/m2, 

Abbott and others, 1986; Davis and Becker, chap. 3, this 

volume) above two of the sediment-covered edifices indicate that these volcanic centers are the heat sources driving fluid circulation. 

The overall form of the volcanic edifices is domelike, 

but details of their subsurface structure are poorly constrained. The importance of satellitic intrusions including 

plugs, dikes, or sills in the establishment of local hydrothermal convection cells is not known. In a similar setting 

in Guaymas Basin, Gulf of California, mineral and oxygen isotope studies indicate that the emplacement of shallow sills into the sediment pile generated low-temperature, 

short-lived hydrothermal convection (Kastner, 1982) . 

Lonsdale and Becker ( 1985) suggested that pore waters 

expelled by these shallow intrusions produced 50°C to 

130°C hot springs and barite-rich deposits. By analogy, 

some hydrothermal deposits (such as small, metal-poor 

barite chimneys) between the sediment hills of Escanaba 

Trough may be related to shallow intrusions and smallscale fluid circulation within the sediment section. 

100 r-----------------, 

1-

z 

w 

() 

10 

a: 1 

w 

a.. 

1-

J: 

CJ 

w 

3: 0.1 

0.01 

0.001 

Sample L2-86-14D-1 

Zn 

Fe 

As 

Cu 

Pb 

Cd 

Se 

Sb 

Bi 

Sn 

Mn 

INNER 

WALL 

MIDDLE 

WALL 

Sb 

As 

Ag 

Cu 

Se 

Cd 

Bi 

Mn 

OUTER 

WALL 

Figure 16.10. Chemical variations across wall of polymetallic 

sulfide structure (sample L2-86-14D-1). Inner wall mineralogy is 

inferred to have evolved in direct contact with hydrothermal fluid, 

whereas outer wall developed in contact with seawater. 
COMPOSITION AND GROWTH HISTORY OF HYDROTHERMAL DEPOSITS IN ESCANABA TROUGH 317 

Table 16.5. Bulk composition of sulfate-dominant samples. 

[USGS, U.S. Geological Survey; GSC, Geological Survey of Canada. NA, not analysed; <,less than; -,interference from other 

elements] 

Barite-rich crust Barite chimney Sulfate sinter 

Sample number----------- Ll-86- sc sc ALV ALV ALV 

16D-2 658-R2 766-1 2033-3B 2036a 2040-2b 

Laboratory ---------------- USGS USGS GSC GSC GSC GSC 

Weight percent: 

Fe ----------------------- 6.4 1.3 3.18 3.37 .62 5.3 

Cu ----------------------- .49 .02 .06 .27 .05 .25 

Zn ----------------------- 17.3 .02 19.0 24.9 .05 1.6 

Pb ----------------------- 4.38 <.01 2.80 1.1 .04 .19 

As ----------------------- .72 <.02 .26 .56 .003 .084 

BaO --------------------- 25.7 34.6 14.5 10.7 .31 16.2 

s ------------------------- 11.69 .59 13.8 16.6 .60 5.4 

so3 --------------------------- 23.5 33.8 18.0 13.5 41.3 26.6 

Si02 -------------------------- 2.21 1.6 1.34 2.98 10.44 15.6 

CaO --------------------- .11 .30 .16 .22 28.77 .21 

MgO -------------------- .13 .27 .04 .06 6.1 .17 

Al20 3 ----------------------- .16 .38 .02 .06 .41 .27 

Total -------------- 92.79 72.89 73.29 74.31 88.68 71.79 

Parts per million: 

Ag ---------------------- 387 2 240 570 1 156 

Au ---------------------- 2.00 <.1 .005 1.64 .83 6.45 

B i ----------------------- 2 6 .1 <.5 5 1 

Cd ----------------------- 830 .8 NA NA NA NA 

Co ----------------------- 30 15 21 32 20 13 

Cr ----------------------- 5 3 20 11 11 16 

Ge ----------------------- 150 <2 NA NA NA NA 

Hg ---------------------- 43 106 NA NA NA NA 

Mn ---------------------- 500 1,500 200 200 <100 200 

Mo ---------------------- 3 3 4 17 

Ni ----------------------- 2 15 <1 <1 1 <1 

Sb ----------------------- 2,700 9 1,228 10 12 70 

Se ----------------------- <10 <10 .16 <.5 37 244 

S n ----------------------- 150 <2 145 139 9 26 

Sr ----------------------- <400 15,000 140 770 1,500 2,800 

v ------------------------ <2 70 21 10 9 18 

y ------------------------ 30 10 .1 NA NA NA 

Total C ( wt. pet.) ---- .16 .14 NA NA NA NA 

Organic C (wt. pet.) - .15 .08 NA NA NA NA 

aAverage of three samples (ALV2036-1B, ALV2036-2A, ALV2036-2C). 

b Average of two samples (ALV2040-2A and ALV2040-2B ). 

It is more likely that the heat source controlling largerscale hydrothermal circulation and the formation of large 

sulfide mounds was generated from the top of the axial 

magma chamber emplaced at some point below the base 

of the volcanic edifices. In addition to supplying heat, 

magma cupolas or pipelike subvolcanic intrusions above 

the volcanic edifices may have caused uplift and concentric and radial fracturing of the overlying sediment (Koide 

and Bhattacharji, 1975). Denlinger and Holmes (chap. 4, 

this volume) hypothesize that the uplift of sediment blocks 

resulted from the emplacement of laccolithic intrusions at 

the interface between basaltic basement and overlying sediment. The fracturing and faulting that accompanied uplift 

of the sediment blocks was an important factor in creating 

cross-stratal permeability for fluid discharge and recharge. 

The observations made from submersibles and towed 

camera systems indicate that the Escanaba Trough sulfide 

mounds are large features with lateral and vertical dimensions measured in hundreds and tens of meters, respectively 

(Holmes and Zierenberg, 1990; Zierenberg and others, 

chap. 10, this volume). The construction of large mounds 

is dependent on the longevity of the hydrothermal system, 

and this is increased by the insulating effect of the sediment blanket (Levi and Riddihough, 1986). The "black 
318 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIESATESCANABATROUGH, GORDA RIDGE 

smoker" type of hydrothermal discharge from large chimneys at sediment-free spreading axes results in the rapid 

dissipation of heat from the axial magma chamber 

(Macdonald and others, 1980). In contrast, the massive 

mound structures and the low-velocity, diffuse seepage of 

fluid through the sediment observed in Escanaba Trough 

indicate that hydrothermal fluid discharge and the convective heat flux are muted by the sediment cover. 

SEDIMENT AS A SOURCE OF 

METALS AND BARIUM 

All of the known sulfide and sulfate deposits in 

Escanaba Trough occur on a variably thick layer of sediment overlying basaltic volcanic basement. Both the sulfide- and sulfate-dominant deposits are characterized by 

high lead, bismuth, arsenic, and antimony contents compared to sulfide deposits from sediment-free ridges 

(Bischoff and others, 1983; Hannington and others, 1991 ). 

From a volumetric standpoint, sulfate-dominant deposits 

are also more abundant at Escanaba Trough (and at 

Guaymas Basin), and barium is a major element in crusts, 

sinters, and chimneys. In order to evaluate these differences 

in deposit composition as a function of source rock, the 

amount of these metals in Escanaba Trough turbidite 

samples is compared to mid-ocean ridge basalt values in 

table 16.6. 

Table 16.6 shows that the enrichment of lead, bismuth, 

arsenic, antimony, and barium in Escanaba Trough deposits is consistent with their greater abundance in turbidite 

samples compared to mid-ocean ridge basalt. Although 

comparative data for Escanaba Trough sediment samples 

are not available, it is probable that enrichments of silver, 

tin, and mercury in some Escanaba Trough sulfide samples 

are also related to their greater relative abundance in the 

sediment. 

The presence of sediment-derived lead in Escanaba 

Trough sulfide deposits is confirmed by lead isotope data 

presented by LeHuray and others (1988). In their study, 

lead isotope ratios for pyrrhotite-rich and polymetallic massive sulfides are closely matched to values for terrigenous 

sediment; there is no indication of any lead contribution 

from basalt. 

The ubiquitous nature of arsenic-bearing phases in 

Escanaba Trough sulfide samples, the rare occurrence of 

these minerals in samples from sediment-free ridges, and 

the low amount of arsenic in mid-ocean ridge basalt are 

indirect evidence that a significant proportion of the arsenic 

is derived from the continentally-derived sediment. 

Bischoff and others (1981) showed that arsenic is readily 

leached from sediment during interaction with NaCl-rich 

solutions. High arsenic contents and the presence of arsenic 

minerals (orpiment and realgar) have also been reported in 

Table 16.6. Comparison of metal contents (ppm) in mid-ocean ridge 

basalt samples from northeast Pacific Ocean and sediment samples 

from Escanaba Trough. 

[-, not determined] 

Element Mid-ocean ALV2039-lb DSDP Site 35c 

ridge basalta (sand turbidite) (sand-silt turbidite) 

Pb --------------- 0.89 5 15 

Bi --------------- .007 .38 

As --------------- .15 3.6 

Sb --------------- .007 .19 

Ba --------------- 40 750 528 

a Data for Pb, Bi, and Ba are for samples from Gorda Ridge: Pb is 

average value for eight samples analyzed by Church and Tatsumoto 

(1975); Bi is average value for two samples determined by neutron activation analysis (Laul and others, 1972); Ba is average value for six 

samples analyzed by isotope-dilution mass spectrometry (Kay and others, 1970). Values for As and Sb were obtained by radiochemical 

epithermal neutron activation analysis of a sample from Galapagos Ridge 

(Sims and others, 1990). 

b Pb and Ba determined by inductively coupled plasma-atomic emission spectrometry; Bi, As, and Sb by atomic absorption spectrometry. 

Analyses performed at the Geological Survey of Canada. 

c Pb (average of two samples) and Ba (average of three samples) 

determined by direct current argon plasma-optical emission spectrophotometry. Analyses performed at the University of Minnesota, R.L. 

Knoche, analyst. 

massive sulfide from the Okinawa Trough, where the 

associated volcanic rocks have a calc-alkaline composition 

(Halbach and others, 1989). 

Although a contribution from igneous sources may 

also be important, the relatively high barium in Escanaba 

Trough hydrothermal deposits can be attributed to its 

greater abundance in sediment (table 16.6) where it is concentrated in detrital feldspar, mica, and clay minerals as 

well as siliceous and calcareous microfossil tests (Martin 

and Knauer, 1973; Church, 1979). In addition, the burial 

and alteration of barium-enriched organic matter leads to 

authigenic precipitation of barite in marine sediment 

(Church and Wolgemuth, 1972). The dissolution of fossil 

tests and marine barite, alteration of barium-bearing detrital grains, and entrainment of pore waters by upwelling 

hydrothermal fluids should favor the formation of bariterich deposits on the sea floor. 

FORMATION OF MASSIVE SULFIDE MOUNDS 

There is no direct evidence for the early stage of 

mound-building, but construction probably began with the 

precipitation of small sulfate-dominant mounds or chimneys in clusters or curvilinear arrays on the step-faulted 

margins of the uplifted sediment hills (fig. 16.11A). Dur-
COMPOSITION AND GROWTH HISTORY OF HYDROTHERMAL DEPOSITS IN ESCANABA TROUGH 319 

ing the early stages of tectonism, uplift of the sediment 

blocks and hydrothermal activity were contemporaneous 

events. Although the hydrothermal solutions migrated upward along the bounding fault structures, it is likely that 

there was a considerable lateral flow along antithetic fractures and through permeable turbidite beds, especially 

where sandy units were overlain by less permeable clayrich layers that formed effective caprocks. The movement 

of fluids through sediment talus at the base of fault scarps 

resulted in the formation of sulfide-cemented sediment deposits (fig. 16.4£). 

The process of outward growth and expansion of the 

nascent deposits included ongoing deposition of sulfaterich crusts and chimneys at the zone of mixing between 

exiting hydrothermal fluids and seawater. Although the 

early stage of mineralization probably involved the precipitation of some anhydrite, the predominance of barite deposition over a wide range of temperatures was controlled by 

fluids reacting with a barium-enriched sediment substrate. 

The deposition of a thin but relatively impermeable 

sulfate-sulfide shell above the exit points for hydrothermal 

discharge led to the mature mound-building stage (fig. 

16.11B) characterized by less fluid mixing, more reduced 

conditions, higher fluid temperatures, and the deposition 

of the main-stage sulfide paragenesis: pyrrhotite ~ sphalerite + Cu-Fe sulfides ~ lOllingite + native bismuth in 

mounds and chimneys. The buildup of temperatures in the 

interior of maturing sulfide mounds resulted in the interstitial deposition of Cu-Fe sulfides and sphalerite in the interstices between pyrrhotite grains and along fluid 

channelways. The high gold content in copper-rich samples 

suggests that gold was deposited in the mounds at this 

stage. 

The deposition of pyrrhotite and lollingite indicate 

very lowj02 (less than 10-45) in the fluid (fig< 16.12; 

Heinrich and Eadington, 1986) during this stage. The low 

oxygen fugacity may have resulted from the oxidation and 

decomposition of organic carbon during hydrothermal alteration of the sediment and from the reduction of dissolved 

sulfate by reactions such as 

CH3COOH + S042- + 2H+ = 2H2C03 + H2S 

(Thornton and Seyfried, 1987), which would also provide 

reduced sulfur for the formation of sulfide minerals. 

Escanaba Trough sediment recovered at Deep Sea Drilling 

Project (DSDP) Site 35 contains 0.35 percent organic carbon (average for 19 analyses; Vallier, 1970), and the presence of petroleum in sulfide deposits indicates that organic 

matter has been thermally altered and transported by hydrothermal fluids (Kvenvolden and others, 1986, 1990, 

chap. 15, this volume). The interaction between hydrothermal fluids and organic matter has other geochemical implications for metal transport and sulfide deposition in 

sedimentary environments. Recent experimental work by 

A 

B 

c 

EARLY STAGE 

SULFATE-DOMINANT 

MOUNDS AND ?HIMNEYS"' I· 

---------~ I, I I / 11 SULFIDE-

-- ..... . -- CEMENTED 

~·~~,}~.4t?;,:':·\;;'~~c;;~;;;~;:s .. , FLUIDFLOW ·. :- :·>: ·· 

"' FAULT MOVEMENT 

/~ DISSEMINATED AND 

\~i FRACTURE CONTROLLED 

SULFIDE AND ALTERED SEDIMENT 

MATURE STAGE 

DECLINING STAGE 

SEDIMENT 

PYRRHOTITE-RICH 

SULFIDE MOUND 

AND CHIMNEYS 

Figure 16.11. Generalized model for development of massive sulfide 

mounds from (A) early stage through (B) mature stage to (C) present 

declining stage. See text for detailed description. 
320 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

Hennet and others (1988) and Giordano (1989) has shown 

that the solubility and transport of metals, especially lead 

and zinc, in mildly acidic NaCl-rich solutions at temperatures up to 200°C are enhanced by complexing with carboxyl ligands such as acetate. 

The occurrence of IOllingite, arsenopyrite, native bismuth, and bismuth telluride in pyrrhotite-rich sulfide deposits indicates a hydrothermal fluid depleted in total 

sulfur. The sulfur deficiency could have resulted from 

deposition of pyrite and other sulfides at depth in the sediment where fluid pH is buffered by reaction with calcium 

carbonate and organic matter (Von Damm and others, 

1985). The proportion of pyrrhotite, arsenopyrite, and 

IOllingite in the deposit are also controlled by the as laAs 

ratio in the fluid (Barton, 1969). 2 

During the main stage of development, fluid flow 

within the mounds was locally focused and vented through 

-20 

en 

C: 

z 

::::> __ hem 

CJ -30 

0 py-------- d ----... (/) 

a: 

<( 0 co H3As0 z 3 

>=' 

~ 

0 -40 <( 

CJ 

::::> 

u. 

z 

w 

CJ 

>- X 

0 

-50 

Fe 

0 2 4 6 

sulfide-rich chimneys; sulfate-dominant chimneys may 

have become more abundant near the end of this stage as 

the hydrothermal activity waned and increasing amounts 

of seawater mixed with hydrothermal fluid. A considerable 

volume of fluid also percolated through the tops of mounds 

(fig. 16.11B). This diffuse discharge resulted in the outward 

migration of sulfate shells with quench deposition of new 

sulfate phases along the interface with seawater and the replacement of sulfate by sulfide phases along the contact 

with massive sulfide. The zone refining of metals soluble 

at low temperatures resulted in deposition of sphalerite, 

galena, and sulfosalt phases in the sulfate shells. The flow 

through a myriad of channelways in mounds is expressed 

by the grain-size heterogeneity and radial growth orientation in pyrrhotite-rich samples. The pattern of diffuse fluid 

discharge through mounds was inherited from the broadly 

distributed flow out of the sediment substrate, but was also 

Fe As 

8 

pH 

2-

HAs04 

I 

I 

- _h~f!!---

...... I .................. I ...... ...., I 2-~ 

HAs03 '~ --------1 mag ' -------------- -- Fe AsO~-

10 12 14 

Figure 16.12. Phase diagram of pH versus oxygen fugacity showing calculated predominance fields for arsenic species 

in the system Fe-As-S-0-H at equilibrium with a sulfur-bearing aqueous fluid at 250°C. Activity for all sulfur species 

is w-2. Field boundaries for Fe, Fe-0, and Fe-S species are also shown. Abbreviations used: asp, arsenopyrite; lo, 

lOllingite; hem, hematite; mag, magnetite; py, pyrite; po, pyrrhotite. From Heinrich and Eadington (1986). 
COMPOSITION AND GROWTH HISTORY OF HYDROTHERMAL DEPOSITS IN ESCANABA TROUGH 321 

enhanced by passage of fluid through layers of sulfide 

rubble created by the continual collapse of unstable sulfide and sulfate chimneys. 

The large massive sulfide mounds in Escanaba Trough 

appear to be the products of a long-lived (on the order of 

hundreds or thousands of years) hydrothermal system that 

has now waned to a near-dormant condition (fig. 16.11C). 

In this declining stage of hydrothermal activity, the surfaces 

of deposits are undergoing oxidation and erosion. Sulfide 

talus blocks have resulted from mass wasting of the sediment hills and sulfide deposits. In many places, the flanks 

of sulfide mounds are buried by unconsolidated Holocene 

sediment. The known extent of active hydrothermal discharge is entirely within the Central Hill area of the 

NESCA site and is restricted to ( 1) localized vents where 

fluids at temperatures up to 220°C are depositing sulfate 

+ silicate sinter on the flanks of sulfide mounds and (2) 

diffuse low-temperature seepage through the surrounding 

sediment (fig. 16.11 C). The collapse of the hydrothermal 

system resulted in the lower-temperature deposition of barite, sphalerite, and galena in channelways within the sulfide mounds, and the alteration of pyrrhotite to marcasite 

and covellite as the hydrothermal fluids became more oxidized with time. 

FORMATION OF THE POLYMETALLIC 

SULFIDE STRUCTURE 

The polymetallic sulfide samples were dredged from 

the sediment plain north of the Central Hill at the NESCA 

site (fig. 16.2A). The shape of the large sample L2-86-14D1 and the absence of pyrrhotite-rich massive sulfide or sulfate-dominant samples in dredge L2-86-14D indicate that 

the polymetallic sulfide fragments were part of an upright 

chimneylike sulfide structure with a large central cavity 

that formed on sediment substrate. The telescoped zonation 

of higher-temperature (sphalerite-isocubanite-pyrrhotitearsenopyrite) and lower-temperature (sphalerite-galenaboulangerite) sulfide assemblages formed in response to 

steep temperature and chemical gradients in the chimney 

wall. Unlike the diffuse upwelling associated with the 

deposition of pyrrhotite-rich sulfide mounds, the 

polymetallic sulfide structure apparently was constructed 

around a focused, high-velocity "black smoker" type vent 

(Koski and others, 1988). The galena-rich sulfide-cemented 

sediment sample recovered in dredge L2-86-14D may represent associated sulfide mineralization in the sediment 

substrate. 

The wall of the sulfide structure (fig. 16.6) is the result of a complex bidirectional growth process that evolved 

under the influence of an intensifying hydrothermal system. The original wall of the sulfide structure, consisting 

of fine-grained dendritic sphalerite, galena, pyrrhotite, barite, and probably anhydrite, is only partly preserved in zone 

5. This early-formed assemblage resulted from quench 

crystallization as the initial discharge of hydrothermal fluid 

was rapidly cooled by mixing with ambient seawater. Although anhydrite is not preserved because of its retrograde 

solubility, the supersaturation of heated seawater with respect to CaS04 around the margin of the discharging fluid 

jet may have triggered its precipitation in the primitive 

chimney wall (Haymon, 1983). 

The zone 5 assemblage provided a physical barrier that 

greatly reduced fluid mixing, and the first of several 

sphalerite-isocubanite-pyrrhotite layers, zone 4, was precipitated from hotter and less diluted hydrothermal fluid. 

The coarse-grained zones 3, 2, and 1, each with the paragenetic sequence sphalerite (subhedral) __., isocubanite (in 

sphalerite cores) __., pyrrhotite (subparallel plates), were 

subsequently accreted to the inner chimney wall according to fluctuations in fluid temperature and chemistry (for 

example, the variation in the aculaFe ratio). The deposition of the basal part of each succeeding sphaleriteisocubanite-pyrrhotite cycle was in part a replacement of 

the earlier-formed layer of subparallel pyrrhotite. Sphalerite and isocubanite of the inner wall (zone 1) represent the 

latest stage of high-temperature sulfide deposition. During 

the accretion of each high-temperature zone, sphalerite and 

isocubanite were also deposited interstitially and partly replaced pyrrhotite in the earlier-formed zones. 

Each cycle of deposition in zones 4, 3, 2, and 1 included a stage of deposition of arsenopyrite and (or) 

lollingite along the contact with the preceding layer. These 

arsenopyrite-lollingite "fronts" may have been deposited 

when the high-temperature fluids were depleted in H2S 

(Ballantyne and Moore, 1988). 

Concurrent with the inward accretion of sphaleriteisocubanite-pyrrhotite cycles, the lower-temperature assemblage of sphalerite, galena, pyrrhotite, barite, boulangerite, 

tetrahedrite, and acanthite (zone 6) was deposited on the 

outer surface of the structure by fluids seeping through 

zone 5. Fluid temperatures in this part of the sulfide wall 

were probably too low to precipitate anhydrite. The thickening of the sulfide structure also resulted in the inward 

migration of galena deposition into zone 3 and the partial 

replacement of arsenopyrite by lollingite in zones 2, 3, and 

4 of the middle wall. Except for incipient oxidative alteration of pyrrhotite in zone 2, the sulfide structure is remarkably free of sea-floor weathering effects. Thus it is probable 

that at the time of sampling the polymetallic sulfide structure was actively venting fluid. 

CONCLUSIONS 

The size, distribution, and composition of hydrothermal deposits in Escanaba Trough reflect extensive interaction between hydrothermal fluid and the insulating 

layer of turbidite and hemipelagic sediment. Mineral and 
322 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

chemical characteristics of sulfide- and sulfate-dominant 

deposits including the abundance of pyrrhotite, galena, arsenopyrite, and barite, presence of unusual minerals such 

as lollingite, alabandite tin-bearing phases, native bismuth, 

and bismuth telluride, and high Pb, Ba, As, Sb, Bi, Ag, Hg, 

and Sn contents are related to fundamental control by the 

source rock composition. High values for Au (maximum 

10.1 ppm) in Cu-rich (maximum 20.6 percent) samples of 

pyrrhotite-rich sulfide suggest coprecipitation of these metals. Large mounds may contain significant reserves of both 

copper and gold. 

The construction of massive sulfide mounds on the 

margins of sediment hills consists of an early stage of sulfate-dominant deposition, a mature stage of sulfide-dominant deposition under conditions of high temperature and 

low fo and fs , and the present declining stage marked by 

erosio~, oxid~tion, and partial burial by unconsolidated 

sediment. Hydrothermal activity at this final stage includes 

localized deposition of sulfate + silicate sinter from fluids 

at temperatures up to 220°C and seepage of low-temperature fluid through the sediment. At the NESCA site, steep 

temperature and chemical gradients associated with focused 

"black smoker" type discharge resulted in the formation of 

a chimneylike sulfide structure characterized by a highly 

telescoped polymetallic (Fe-Zn-Pb-Cu-As-Sb-Ag) sulfide 

assemblage. 

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chapter 14 (this volume). 
Section III: 

Biological Studies 
Brecciated pyrrhotite-rich massive sulfide colonized by thin tubeworms. Vent-specific anemones 

colonize the tubeworms and the massive sulfide and can be seen with their red tentacles extended 

and retracted. The massive sulfide is also colonized by abundant limpets, gastropods, and galatheid 

crabs and locally covered by filamentous bacteria. This photograph was provided by C. Van Dover 

from a camera mounted on the Alvin submersible; field of view is approximately 2 m across. 
Chapter 17. Soft-Sediment Hydrothermal Vent Communities 

of Escanaba Trough 

By J. Frederick Grassle and Rosemarie Petrecca 1 

CONTENTS 

Abstract ............................................................................... . 

Introduction ......................................................................... . 

Acknowledgments ...................................................... . 

Materials and methods ........................................................ . 

Results ................................................................................. . 

Community similarities among sites ......................... . 

Community diversity .................................................. . 

Hard-surface fauna ..................................................... . 

Discussion ........................................................................... . 

References cited .................................................................. . 

ABSTRACT 

Page 

327 

327 

327 

329 

329 

330 

332 

332 

333 

335 

In June 1988, during DSV Alvin exploration of the 

northern Escanaba Trough (NESCA), three active hydrothermal vents and an active soft-sediment vent were located 

and sampled. Samples from nonactive sulfide mounds and 

the surrounding ambient soft sediment indicated a heterogeneous community. These samples were, however, much 

more similar to one another than to a sample adjacent to a 

submerged tree limb or samples in a soft-sediment hydrothermal area. The hydrothermal hard-surface fauna was 

generally similar to that from the Juan de Fuca Ridge to the 

north but with significant differences. Fauna from tree limb 

and hydrothermal hard surfaces were quite different, indicating that pieces of wood are unlikely to serve as stepping 

stones for hydrothermal vent fauna to move from one vent 

to another. The diversity of species in soft sediment both adjacent to the wood and from the soft-sediment vent was reduced in comparison with the ambient deep-sea community. 

INTRODUCTION 

In June 1988, RN Atlantis II with DSV Alvin conducted a program of geochemical, geological, and biological sampling in the Escanaba Trough region of Gorda 

1 Institute of Marine and Coastal Sciences, Rutgers University, New 

Brunswick, New Jersey 

Ridge. The axial valley of Gorda Ridge closely resembles 

that of the Mid-Atlantic Ridge and is quite unlike the broad 

flat axis of Juan de Fuca Ridge to the north. The valley is 

composed of three segments: ( 1) the northern segment, 

studied for the past nine years; (2) the middle segment, 

which is the most tectonically complex; and (3) Escanaba 

Trough, the southernmost segment. Escanaba Trough extends 130 km northward from the intersection of the ridge 

with the Mendocino Fracture Zone (latitude 40°25' N., longitude 127°30' W.) to a right-lateral offset of the ridge axis 

near latitude 41 °35' N., longitude 127°25' W. (Morton and 

others, 1990). Unlike the northern two segments, the axis 

of Escanaba Trough is covered by turbiditic and 

hemipelagic sediment (Moore, 1970; Vallier and others, 

1973; Karlin and Lyle, 1986). The spreading rate along 

Escanaba Trough is approximately 2.4 cm/yr, which is 

characteristic of a slow-rate spreading center (Morton and 

others, 1990). Located along the axial valley are sedimenthosted polymetallic massive sulfide deposits and active 

hydrothermal vents (Morton and others, 1987; Clague and 

Holmes, 1987; Zierenberg and others, 1988). 

The eight DSV Alvin dives pertaining to our softsediment work were located in the northern part of the 

Escanaba Trough or NESCA site (fig. 17.1). In this region three active hydrothermal vents and one active softsediment vent were located and sampled. The soft-sediment 

area was dominated by the vent clam Calyptogena 

phaseoliformis (Petrecca and Grassle, 1990; Van Dover and 

others, 1990). This species was previously known only 

from soft -sediment cold seeps near Japan (Metvier and others, 1986; Juniper and Sibuet, 1987). This soft-sediment 

hydrothermal site appears similar to the sites in Guaymas 

Basin dominated by Vesicomya gigas (formerly thought to 

be Calyptogena pacifica). Areas of bacterial mats or petroleum seepage such as those observed in Guaymas Basin were not observed. 

ACKNOWLEDGMENTS 

We thank the following systematists: James Blake, 

Brigitte Hilbig, Marian Pettibone, Meredith Jones, James 

McLean, Richard Gustafson, and Vivianne Solis-Weiss. We 

327 
328 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

12J032' 12J030' 

66,000 

64,000 

62,000 

56,000 

54,000 

52,000 

56,000 58,000 60,000 62,000 50,000 

Figure 17 .1. Location of samples identified in table 17 .1. Numbers along right and bottom give coordin(;ltes 

for local transponder net (in meters). Hachures indicate area of closed low. Depth contours are in meters. 
SOFf-SEDIMENT HYDROTHERMAL VENT COMMUNITIES OF ESCANABA TROUGH 329 

Table 17.1. DSV Alvin dives on Escanaba Trough 1988. 

Dive Sample Reference Latitude Longitude Depth Comments 

(fig. 17.1) north west (m) 

2034 Box CoreA 1 41 °00.86' 127°28.93' 3,366 Near tree limb 

Box Core B 2 41 °00.82' 127°28.92' 3,366 3 meters from wood 

2035 Box CoreA 3 40° 58.73' 127°30.74' 3,180 Top of hill 

2038 Box CoreA 4 41 °05.35' 127°29.34' 3,268 Ambient 

Box Core B 4 41 °05.35' 127°29.34' 3,268 Ambient 

2039 Box Core B 5 41°05.51' 127°28.70' 3,254 Non-active sulfide mound 

2040 Box CoreA 6 41 °00.01' 127°29.86' 3,274 Active sulfide mound 

Box Core B 6 41 °00.01' 127°29.86' 3,274 Active sulfide mound 

2041 Box CoreA 7 41 °00.01' 127°29.62' 3,244 Active sulfide mound 

2042 Box CoreA 8 40° 59.50' 127°29.41' 3,260 Ambient 

Box Core B 8 40°59.50' 127°29.41' 3,260 Ambient 

2042 Scoop sample 9 40°59.71' 127°29.31' 3,254 Hydrothermal soft-sediment site 

also extend our gratitude to Janet Morton, Susan BrownLeger, Cindy Van Dover, John Edmond, the pilots of DSV 

Alvin , and the crew of RN Atlantis II. This work was supported by National Science Foundation grant OCE-8812733. 

MATERIALS AND METHODS 

Stainless-steel box cores with a 225-cm2 footprint, 

specifically designed for use by DSV Alvin, were used to 

collect our samples. Each box core was equipped with an 

improved lid design that activates the closing of the doors 

by rotation of a manipulator-operated T-handle (instead of 

the rollerspring-operated doors used previously). 

Eleven quantitative box cores were obtained during 

this dive series (table 17.1). Three box cores were taken 

in sediment containing sulfide deposits at the base of active hydrothermal mounds. Another six were taken in the 

surrounding ambient soft sediment, including nonactive 

sulfide mounds. One core was obtained immediately adjacent to a large submerged tree limb lying on the bottom, 

which contained a rich faunal assemblage, and another core 

was taken 3 meters away in an attempt to compare the animal communities at the two locations. On the final dive, 

a soft-sediment hydrothermal area was discovered, a 50-

m-wide patch of soft sediment venting low-temperature 

hydrothermal fluid. A qualitative sample was obtained 

using a DSV Alvin tube core as a scoop to shovel sediment 

into a collection box on DSV Alvin's basket. The amount 

of sediment was the equivalent of at least two box cores. 

Samples were sieved through 300-J.Lm and 63-J.Lm 

screens, fixed in 10 percent formalin, and stored in 80 

percent ethanol. After staining with protein-specific Rose 

Bengal, these samples were carefully sorted to the species level under a dissecting microscope. Only the 

macrofauna retained on a 300-J.Lm screen are summarized 

below. The 63-J.Lm material will be used for future 

meiofaunal studies. 

Similarity among samples were analyzed using the 

NESS similarity measure (Grassle and Smith, 1976) and 

the Bray-Curtis similarity measure (Clifford and 

Stephenson, 1975). Diversity was estimated using a 

rarefaction measure (Smith and Grassle, 1977). 

RESULTS 

Samples taken in sediment at the base of active and 

nonactive mounds did not differ in densities or species 

composition from those samples taken tens of meters 

away in ambient sediment. For purposes of comparison 

with hydrothermal areas and the sediment adjacent to 

wood on the bottom, these samples are combined. These 

10 box core samples are inhabited by a fauna similar to 

that of deep-sea environments at similar depths elsewhere 

(table 17.2). All of the genera identified are often common at sites in the western Atlantic (Grassle and 

Morse-Porteous, 1987), and equatorial Pacific (Grassle 

and others, 1985). The second most abundant species, 

Aurospio dibranchiata, is the numerical dominant in these 

and other deep-sea environments. The genera found contain species known to be surface deposit feeders; however, the maldanid feeds head down in the sediment, and 

the polynoid is a predator. 

The species in sediment adjacent to a piece of wood 

were similar to some of the species associated with vegetation at Deep Ocean Site (DOS) 2 off New England (table 

17.3; Turner, 1977; Grassle and Morse-Porteous, 1987), 

such as the two most common species of Ophryotrocha and 

hesionids. Several individuals of the archaeogastropod species Provanna were also present. This genus as well as 

Ophryotrocha and hesionids, are common at the Guaymas 

Basin hydrothermal vents (Grassle and others, 1985). A 

new genus of phyllodocid polychaete was found associated 

with the wood (James Blake, Science Application International Corporation, oral commun., 1990). 
330 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

Table 17.2. Most common species in box core samples from ambient Escanaba Trough soft-sedimentcommunities. 

[Fifteen most common species in ten samples, each 225 cm2 in surface extent] 

Species Number/225 cm2 Number/m2 Percent contribution 

Chaetozone cf. abyssa (Polychaeta) -------------- 34 1,511 8.1 

Aurospio dibranchiata (Polychaeta) -------------- 25 1,111 5.9 

Aricidea nr. no/ani (Polychaeta) ------------------- 20 889 4.8 

Macrostylis sp. (lsopoda) --------------------------- 16 711 3.8 

Cirratulidae sp.juv. (Polychaeta) ------------------ 15 667 3.6 

Cossura sp. (Polychaeta) --------------------------- 12 533 2.9 

Prionospio nr. steenstrupi (Polychaeta) ---------- 12 533 2.9 

Chelator sp. juv. (lsopoda) ------------------------- 11 489 2.6 

Prionospio sp. 1 (Polychaeta) --------------------- 10 444 2.4 

Leptognathia sp. BBBBB (Tanaidacea) ---------- 10 444 2.4 

Paramphinome profunda (Polychaeta) ----------- 9 400 2.1 

Maldanid sp.II juv. (Polychaeta) ------------------ 9 400 2.1 

Aricidea sp.II (Polychaeta) ------------------------- 9 400 2.1 

Bivalve sp.l [Thyasirid] (Bivalvia) --------------- 9 400 2.1 

Polynoidae sp.juv. (Polychaeta) ------------------- 8 356 1.9 

Total Individuals ------------------------------- 421 18,709 

Total Species------------------------------------ 103 

Table 17.3. The fifteen most common species in a single 225 cm2 box core sample taken adjacent to a piece of wood. 

Species Number/225 cm2 

Hesionidae sp.juv (Polychaeta) -------------------------------- 49 

Ophryotrocha n.sp. (Polychaeta) ------------------------------ 48 

Protanais n.sp. (Tanaidacea) ----------------------------------- 44 

Chaetozone curvata (Polychaeta) ----------------------------- 41 

Asabellides n.sp. (Polychaeta) --------------------------------- 41 

Fauvelopsis n.sp. (Polychaeta) -------------------------------- 30 

Pseudotanais sp.Z (Tanaidacea) ------------------------------- 23 

Ophelina pallida (Polychaeta) --------------------------------- 21 

Ophelina abranchiata (Polychaeta) -------------------------- 10 

Cossura sp. (Polychaeta) --------------------------------------- 8 

Prionospio ( Minuspio) n.sp. (Polychaeta) ------------------- 8 

Prionospio nr. steenstrupi (Polychaeta) ---------------------- 7 

Aricidea catherinae (Polychaeta) ----------------------------- 5 

Gastropod sp.I [Provanna ? sp.] (Archaeogastropoda)---- 5 

Paramphinome profunda (Polychaeta) ----------------------- 4 

Total lndi vi duals -------------------------------------------

Total Species------------------------------------------------

381 

36 

Number/m2 

2,089 

2,133 

1,955 

1,822 

1,822 

1,333 

1,022 

933 

444 

356 

356 

311 

222 

222 

178 

16,932 

Percent contribution 

12.9 

12.6 

11.6 

10.8 

10.8 

7.9 

6.0 

5.5 

2.6 

2.1 

2.1 

1.8 

1.3 

1.3 

1.0 

The soft-sediment hydrothermal site was dominated by 

the cosmopolitan hydrothermal vent polychaete 

Amphisamytha galapagensis, found at most hydrothermal 

and cold seep environments both in washings of hardsurface fauna and soft-sediment samples but never in sediment away from hydrothermal or cold seep fluids (table 

17 .4; Petrecca and Grassle, 1990). At least three new species are present. The two most abundant, the orbiniid polychaete genus Leitoscoloplos and the spionid polychaete 

genus Prionospio (Minuspio ), are common in a wide variety of shallow-water environments. 

COMMUNITY SIMILARITIES AMONG SITES 

Relative to other deep-sea areas, the fauna from the 

ambient community is quite heterogeneous (fig. 17 .2). 

NESS similarities may be interpreted as the average percentage of species shared, and these are generally greater 

than 0. 7 in deep-sea communities. Heterogeneity in faunal distribution has also been observed in the rich 

megafauna! populations from the area (Carey and others, 

1990). Hydrothermal mounds in NESCA are closer together than in Guaymas Basin, so there is less expanse of 
SOFf-SEDIMENT HYDROTHERMAL VENT COMMUNITIES OF ESCANABA TROUGH 

Table 17.4. Abundance of the thirteen species from the Escanaba Trough soft-sediment hydrothermal 

qualitative scoop sample. 

Species 

Amphisamytha galapagensis (Polychaeta) ------------------

Leitoscoloplos n.sp. (Polychaeta) -----------------------------

Prionospio ( Minuspio) n.sp. (Polychaeta) -------------------

Neolepetopsis gordensis (Archaeogastropoda) -------------

Nuculana sp. (Bivalvia)-----------------------------------------

Gastropoda sp.III [Provanna?] (Archaeogastropoda)------

Yoldiella? sp. (Bivalvia) ----------------------------------------

Gastropoda sp.IV [Turrid] (Archaeogastropoda)-----------

Harpinia sp. (Amphipoda) -------------------------------------

M unnid? s p. (Isopod a) ------------------------------------------

Levensteiniella n.sp. (Polychaeta) ----------------------------

Sphaerosyllis sp.l (Polychaeta) --------------------------------

Gastopoda sp.l [Provanna?] (Archaeogastropoda)---------

Total Indi victuals -------------------------------------------

Number 

45 

36 

15 

9 

4 

4 

2 

2 

2 

2 

1 

1 

1 

120 

Total Species------------------------------------------------ 13 

Percent contribution 

37.5 

30 

12.5 

7.5 

3.3 

3.3 

1.7 

1.7 

1.7 

1.7 

0.83 

0.83 

0.83 

331 

soft sediment. For this reason and because most of the rock 

and hydrothermal fluid sampling effort was concentrated 

on the mounds, samples were taken relatively short distances from hydrothermal mounds. Two of the samples 

were not used because rocks in the sediment prevented us 

from obtaining a full sample. The sample near wood and 

the sediment scoops from the soft-sediment hydrothermal 

vent area are very different from all the other samples. The 

Bray-Curtis similarity method gives a similar result (fig. 

17.3). 

0.0 

0.1 

0.2 

0.3 

X 

w 

0 

z 0.4 

>- Ia: 0.5 <X: 

_J 

~ 

Ci3 0.6 en 

en 

w 

z 

0.7 

0.8 

0.9 

ff-

-

-

-

rfffBase of 

mound 

Active 

Ambient 

I 

I 

Ambient Top of 

hill 

I 

I 

I 

Base of Ambient Ambient 3 meters Adjacent Hydrothermal 

mound from wood to wood scoop 

Active 

Figure 17.2. NESS similarity among Gorda Ridge soft-sediment samples. Comparison at 20 individuals. Group average sorting. 
332 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

0.0 

0.1 

0.2 

X 

w 

Cl 0.3 

z 

>- f- 0.4 a: 

<( 

_J 

~ 0.5 U5 

CfJ 

~ 

a: 0.6 ::> 

0 

>- <( 0.7 a: 

Ol 

0.8 

0.9 

f..-

-

-

1--

f--

f..-

f--

f--

f--

Base of 

mound 

Active 

I 

Ambient 

I 

Base of 

mound 

Active 

I 

Ambient Top of Ambient 

hill 

I 

I 

[ 

Ambient 3 meters Adjacent Hydrothermal 

from wood to wood scoop 

Figure 17.3. Bray-Curtis similarity among Gorda Ridge samples. Group average sorting. 

COMMUNITY DIVERSITY 

The ambient community diversity of individual 

samples tends to be low in comparison to samples from the 

ambient community in Guaymas Basin and other deep-sea 

areas (Petrecca and Grassle, 1990). This may be because 

of the proximity of hydrothermal mounds to all areas 

sampled, as suggested above. A few species tend to be 

very abundant, and rare species are absent. When the 

samples are added together, the curve is more similar to 

that of other deep-sea communities (fig. 17.4). Counts 

from the individual cores are all within the envelope designated by the cross-hatching. The wood community in the 

soft sediment has a lower diversity, and the soft-sediment 

hydrothermal vent area has the lowest diversity. The relative abundance of species in the three communities is 

shown in figure 17.5. 

HARD-SURFACE FAUNA 

The fauna from mound and sulfide rock surfaces are 

similar to the fauna from vestimentiferan washings; how110 

Ambient 

w 100 _J 

a.. 90 :2 

<( 

CfJ 80 

a: 

w 

a.. 70 

CfJ 

w 60 

0 

w 50 a.. 

CfJ 

u. 40 

0 

a: 

w 30 

Ol 

:2 20 ::> 

z 10 

1 00 200 300 400 500 

NUMBER OF INDIVIDUALS PER SAMPLE 

Figure 17 .4. Rarefaction diversity curves (Hessler and J umars, 197 4; 

Smith and Grassle, 1977) showing relations between species and 

individuals of three communities: ambient soft-sediment, sediment 

adjacent to wood, and hydrothermal soft-sediment. Individual 

samples all fall within cross-hatched envelope. 
SOFf-SEDIMENT HYDROTHERMAL VENT COMMUNITIES OF ESCANABA TROUGH 333 

ever, these hard surfaces had a greater variety of gastropods and more phyla because more habitats are represented 

(table 17 .5). Several species of filter feeders were found 

on the sulfide surfaces but not in the vestimentiferan 

washings. The fauna on the tree limb was quite different 

than either the washings or the mound and sulfide surfaces. 

On sulfides and in vestimentiferan washings, four new genera and several new species endemic to this hydrothermal 

area were found (table 17 .5). These are an amphinomid 

20 -

I-

- z 

w 10 

() 

a: 

w 

a.. 

z 

w~ 0 

() 

z 

<( 

0 

z 

::::> 

m 

<( 40 - w 

> 

~ 

:5 

w 

a: 

30 -

20 -

10 -

I I I 0 I 

polychaete worm related to the genus Amphisamytha, two 

new limpet genera, one of which has two new species, and 

a new genus of aplacophoran mollusk. 

DISCUSSION 

The ambient community composition has many of the 

polychaete genera such as Aurospio, Aricidea, Chaetozone, 

Soft-sediment fauna 

adjacent to wood 

I I I 

Soft-sediment fauna 

adjacent to vent 

I I 

SPECIES 

Figure 17.5. Relative abundance of 15 most common species in three sediment communities. Hydrothermal soft sediment had only 13 

species in scoop sample taken. 
334 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

Table 17.5. Fauna from hard surfaces, Escanaba Trough. 

[V, vestimentiferan washings; H, hydrothermal mounds; T, submerged tree limb; X, species present; *, abundant species] 

v 

Vestimentifera 

* Ridgeiidae sp. ----------------------------- X 

Polychaeta 

*Amphisamytha galapagensis ------------ X 

Arctonoinae n. gen., n. sp. ----------------- X 

Branchinotogluma grasslei --------------- X 

Capitellid sp. A ------------------------------

Capitellid sp. B ------------------------------

Capitellid nr. sp. B --------------------------

Chane sp. -------------------------------------

nr. Dodecaceria n. sp. ----------------------

Eulalia sp. ------------------------------------

Grassleia hydrothermalis ------------------ X 

Harmothoe n. sp. ----------------------------

Harmothoe sp. -------------------------------

Hesionidae sp. juv. -------------------------- X 

Levensteinella n. sp. ------------------------ X 

Nereis n. sp. ----------------------------------

Nereis sp. II ----------------------------------

Nicomache? sp. ------------------------------

Ophryotrocha n. sp. ------------------------ X 

Opisthotrochopodus tunnicliffeae -------- X 

* Paralvinella palmiformis ---------------- X 

* Protis sp. -----------------------------------

Oligochaeta 

Oligochaete sp. 1 ----------------------------

Oligochaete sp. 3 ----------------------------

Bivalvia 

*Calyptogena phaseoliformis ------------- X 

*Xylophaga washingtonia -----------------

*Xylophaga n. sp. ---------------------------

Gastropoda 

*Gastropoda sp. I [Provanna?] -----------

*Gastropoda sp. II [Glob snail]----------- X 

H 

X 

X 

X 

X 

X 

X 

T 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

and Prionospio that might be expected in a normal deepsea soft-sediment community. Cossura and Paramphinome 

tend to be found in shallower water in the western Atlantic (Maciolek and others, 1987a, b), and isopods are somewhat more abundant than in the western North Atlantic and 

equatorial Pacific. Cossura and Chaetozone were common 

at 1,310 m depth in the Santa Catalina Basin (Jumars and 

Eckman, 1983) and at depths up to 1,500 m in the Beaufort Sea (Bilyard and Carey, 1979). It is possible that disturbance from hydrothermal plumes has resulted in a fauna 

more characteristic of shallower depths elsewhere in the 

deep sea, where external perturbations may be more frequent in comparison to deeper areas. Twenty-nine new species were found, and most of these were at sites away from 

hydrothermal activity. 

Four new genera were found from the vestimentiferan 

washings and the hydrothermal sulfides. A single new 

v 

Gastropoda-Continued 

*Gastropoda so. III [Provanna?] --------- X 

*Gastropoda sp. V. [Provanna?] ----------

*Gastropoda sp. VI [Melanodrymia?] ---

* Lepetodrilis fucenis ----------------------- X 

* Neolepetopsis gordensis ------------------ X 

Pseudococculinid ---------------------------

Pseudococculinid n. gen. A, n. sp. A X 

Pseudococculinid n. gen. A, n. sp. B - X 

Aplacophora 

* Aplacophora n. gen. H, n. sp. J ---------

Amphipoda 

Dyopedos? sp. ------------------------------- X 

Lysianassidae sp. ----------------------------

Metopa sp. ----------------------------------- X 

Copepoda 

Copepoda spp. ------------------------------- X 

Decapoda 

Munidopsis sp. ------------------------------

Isopoda 

Munnidae sp. --------------------------------

Tanaidacea 

Leptognathia sp. CCCC ------------------- X 

Leptognathia sp. FFFFF -------------------

* Protanais sp. -------------------------------

Pseudotanais sp. AA ----------------------- X 

*Anemones-----------------------------------

Asteroidea ------------------------------------

*Echiurans -----------------------------------

N emertean ------------------------------------

Pycnogonida ---------------------------------

Si puncula ------------------------------------

Tunicata --------------------------------------

H 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

T 

X 

X 

X 

X 

X 

X 

X 

genus of phyllodocid polychaete was found associated with 

the wood. The ambient community had no new genera. 

Most deep-sea hydrothermal sites have endemic higher taxa 

specific to that area. The discovery of the four new genera from Gorda Ridge indicates that these vents are not an 

exception to the rule that each new vent area will have 

endemic higher taxa. 

Very little similarity was found between the communities in sediment adjacent to wood and those in hydrothermal sediment. The hydrothermal sediment site had large 

numbers of Amphisamytha galapagensis (table 17.4) characteristic of most vent and cold seep areas and four species of Archaeogastropoda among the total of 13 species 

obtained in the qualitative scoop sediment sample. These 

results and the comparison of hard-surface communities 

indicate that pieces of wood are unlikely to be used as stepping stones by true vent species such as A. galapagensis 
SOFT-SEDIMENT HYDROTHERMAL VENT COMMUNITIES OF ESCANABA TROUGH 335 

or species in vent genera such as Provanna and Ridgeia. 

Most hydrothermal vent species are unlikely to be found in 

association with wood or even such concentrated sources of 

organic material as whale carcasses (Smith and others, 1989). 

The comparison of diversity supports the observation 

that concentrated sources of organic matter such as wood 

or hydrothermally based microbial productivity will result 

in local reductions in species diversity as well as different 

species compositions that contribute to regional diversity 

(Grassle and Morse-Porteous, 1987). The patterns follow 

the generalization that relatively disturbed deep-sea environments such as trenches, upwelling areas, regions of intense 

bottom currents, and hydrothermal vents have lower diversities, with the patterns of relative abundance skewed in the 

direction of the few most common species (Grassle, 1989). 

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Zierenberg, R.A., Koski, R.A., Shanks, W.C., III, Slack, J.F., 

1988, Preliminary results of Alvin dives on active 

sediment-hosted massive sulfide deposits in the Escanaba 

Trough, southern Gorda Ridge [abs.]: Eos, Transactions 

American Geophysical Union, v. 69, p. 1488. 

Chapter 18. Quaternary Foraminifers from Escanaba Trough, 

Northeast Pacific Ocean 

By Paula J. Quinterno1 

CONTENTS 

Page 

Abstract................................................................................ 337 

Introduction.......................................................................... 337 

Sampling methods and procedures............................. 338 

Oceanography .. ........ ....... ... . ....... ..... ... . . .. . ... .. .. ..... ... . . ... . 340 

Previous studies ........................................................... 342 

Acknowledgments ....................................................... 342 

Foraminifers from Escanaba Trough ...... .. .. .. .. .. .. .. .. .. .. .. ...... 342 

Gravity and piston core samples .. .... .. .. .. .. .. .. .. .. .. ..... ... . 343 

L2-86-2G .............................................................. 343 

Ll-86-70 .............................................................. 343 

Ll-86-250 ........................................................... 343 

Ll-86-260 ........................................................... 346 

L2-86-7G .............................................................. 347 

TUL-89D core samples ........ .. .. .. .... .. ...... .. . . ... ... . .. 348 

Dredge samples .. .... ... .... ......... .. .. ...... .... .... .. .. .. .. .. .. . ..... .. 348 

Discussion ............................................................................ 349 

Relationship to vents ................................................... 349 

Relationship to "flysch-type" faunas .......................... 350 

Taxonomic notes ...... ... .... ... ........ ..... ... . ... . ... . ... . . .. . ..... ... . 351 

Conclusions ........ .. .. .......... ... .... ... ............. ... . ... . ... . .. . . . .. . ..... ... . 351 

References cited................................................................... 352 

ABSTRACT 

Benthic faunas recovered from Escanaba Trough are 

dominated by agglutinated foraminifers typical of abyssal 

depths and include large specimens of Bathysiphon, 

Cribrostomoides, Hormosina, Hyperammina, Reophax, 

Rhabdammina ? , and Rhizammina. An unusual feature of 

the Escanaba Trough foraminifers is the use of large, angular fragments of basaltic volcanic glass in test construction. In samples containing basaltic glass, some 

foraminifers attach to the glass as a substrate. Calcareous 

foraminifers are rare or absent in the surface samples studied. The shallowest water depth sampled was 3,146 m; 

therefore, a present-day carbonate compensation depth 

(CCD) near 3,146 m is indicated for this area. Large agglutinated foraminifers generally are less abundant at depth 

1 U.S. Geological Survey, Menlo Park, California 

in the cores, and in some cores small calcareous foraminifers predominate. This could be due to the destruction of 

agglutinated tests after burial by physical and chemical processes, but it is more likely indicative of a different depositional environment. The presence of calcareous 

foraminifers at these water depths probably indicates deposition in shallower water and subsequent downslope transport by turbidity currents. Dominance by sinistral 

Neogloboquadrina pachyderma at depth in several gravity cores indicates cooler temperatures than at present and 

indicates a Pleistocene age. An agglutinated foraminifer 

resembling Rhabdammina, but with interior ridges, occurs 

in most dredge samples and may be a new genus. 

INTRODUCTION 

Dredge samples and gravity cores were collected from 

northern (NESCA), southern (SESCA), and central 

Escanaba Trough in 1986 during three U.S. Geological Survey cruises of RIV S.P. Lee (L1-86-NC, L2-86-NC, and L3-

86-NC). Additional cores from NESCA were obtained 

during a Geological Survey of Canada (GSC) cruise of the 

RIV Tully in 1989 (TUL-89D). The main purpose of the 

cruises was to study the processes and products of hydrothermal circulation at a sediment-covered spreading center (Morton and others, 1987). Although the collection of 

foraminifers was not a primary purpose of sampling, foraminifers from dredge samples and core tops were analyzed 

to determine the present-day distribution and especially to 

note whether or not a relationship could be seen between 

the occurrence of foraminifers and active hydrothermal 

vents. Foraminifers from subsurface samples of some 

cores were also analyzed and provided evidence for turbidite deposition during Pleistocene and Holocene time. 

Gorda Ridge is a 300-km-long active spreading center in the northeast Pacific Ocean, situated between the 

Blanco Fracture Zone to the north and the Mendocino Fracture Zone to the south (fig. 18.1). Escanaba Trough, which 

occupies the southern third of Gorda Ridge, is an axial 

valley with water depths of approximately 3,200 to 3,300 

m. It contains about 500 m of Quaternary turbiditic sediment thought to be derived largely from the Columbia and 

337 
338 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

Klamath rivers during the Pleistocene when sea level was 

lower (Vallier and others, 1973; Normark and others, chap. 

6, this volume). 

Active hydrothermal vents and associated vent communities were discovered in 1988 in northern Escanaba 

Trough (latitude 41 °00' N., longitude 127°29' W.) at 3,200 

to 3,250 m water depth using the submersible DSV Alvin 

(Zierenberg and others, 1988; Grassle and Van Dover, 

1989, Van Dover and others, 1990). Grassle and Petrecca 

(chap. 17, this volume) discuss the soft-sediment 

macrofauna! (larger than 0.3 mm) communities from the 

Escanaba Trough vent area. Lnfortunately, sediment collected by submersible in close proximity to active vents 

40° 

40' 

() 

127°30' 

*SG 

NESCA 

'---------_I 

SESCA 

I 

I 

I 

I I 

[_~ __________ _! 

WASH 

127°20' 

;() 

I 

15 KILOMETER 

Figure 18.1. Bathymetric map of Escanaba Trough showing sample 

locations from central area. Symbols as follows: NESCA, northern 

Escanaba Trough; SESCA, southern Escanaba Trough; solid dot, 

Ll-86-NC; asterisk, L3-86-NC; open triangle, Tully cruise of 1989. 

Hachures indicate area of closed low. 

was not subsampled for foraminifers; therefore, it is not 

known if vent-specific foraminifers are present in Escanaba 

Trough. However, samples collected in the general vicinity of both active and inactive vents yielded similar faunas, which were dominated by large agglutinated 

foraminifers. 

SAMPLING METHODS AND PROCEDURES 

Dredge samples were collected with a round-frame 

chain bag dredge with a small cloth-lined pipe dredge 

mounted inside the mouth of the main dredge to collect 

unconsolidated sediment. A 3-m-long, 9-cm-diameter 

barrel gravity corer with a 500-kg weight stand was used 

to collect the USGS sediment cores. Cores provided by 

the GSC included gravity, piston, and trigger weight 

cores. 

Subsample nomenclature is as follows: Cruises are 

designated as L1, L2, and L3 for USGS cruises L1-86-NC, 

L2-86-NC, and L3-86-NC respectively; and TUL-89D for 

the 1989 GSC RIV Tully cruise. Following the cruise designator is the sample number and sample type (D, dredge; 

G, gravity core; P, piston core; TR, trigger weight core), 

and when appropriate, the depth in the core. For example, 

subsample L1-25G-15-16 em is from 15-16 em depth in 

gravity core 25 from cruise L1-86-NC. Sediment 

subsamples for foraminiferal analysis were obtained from 

the small pipe dredges and cores (figs. 18.1, 18.2, and 

18.3). The sediment was washed using a 0.063-mm sieve 

and air dried. It was dry-sieved with a 0.150-mm sieve, 

and the sediment portion greater than 0.150 mm was split 

(if many more than 350 specimens were present) using a 

microsplitter. All specimens in the split were picked and 

mounted on cardboard slides for identification. Thirteen 

samples provided by Florence Wong had been treated with 

hydrogen peroxide during processing for her study of mineral compositions of turbiditic sands (Normark and others, 

chap. 6, this volume). This treatment may have affected 

the preservation state of the foraminifers; therefore, these 

samples are marked with an asterisk in the preservation 

column of table 18.1. 

A quantitative study of the foraminifers was not practical because of limitations of the sampling equipment 

used. Dredges provide large samples, but dredge stations 

often extend along the sea floor for a kilometer or more 

and, therefore, may sample more than one environment. 

The dredge digs into the sea floor to some unknown depth, 

so the sample may consist of sediment mixed from the upper several centimeters. Gravity and piston core samples 

may be more accurately located, but samples are small, and 

the surface sediment may be disturbed or missing due to a 

pressure wave created by the corer just before impact or 

by overpenetration of the corer into the sediment (sediment 

forced out the top of the corer). The core logs indicate that 
QUATERNARY FORAMINIFERS FROM ESCANABA TROUGH, NORTHEAST PACIFIC OCEAN 

12JC32' 12JC30' 

66,000 

64,000 

62,000 

60,000 

58,000 

56,000 

54,000 

52,000 

0 2 

KILOMETERS --"', 

Contour interval 10 meters 

56,000 58,000 60,000 62,000 50,000 

Figure 18.2. Northern Escanaba Trough sample locations. Symbols same as in figure 18.1; open circle, L2-

86-NC, solid lines, Ll-86-NC dredge tracks, dashed lines, L2-86-NC dredge tracks. Numbers along right 

and bottom give coordinates of local transponder net (in meters). Hachures indicate area of closed low. 

339 
340 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

sediment from the uppermost intervals of cores is 

frequently disturbed or missing. Box cores are better suited 

to quantitative analysis because the sediment surface is usually less disturbed, a large sample is available for study, 

and a known volume of the surface can be obtained. Unfortunately, box core samples were not available for this 

study. The type of foraminiferal taxa present also limit the 

value of quantitative studies. Large, tubular foraminifers 

dominate many of the Gorda Ridge samples; however, they 

often break during collection and processing, so an accurate count cannot be obtained. 

12r36' 

6000 8000 10,000 

OCEANOGRAPHY 

Although Escanaba Trough is in a deep oceanic setting, it deviates from typical open oceanic areas in several 

respects. For example, the complex topography (figs. 18.2, 

18.3) and the presence of hydrothermal activity presumably affect current patterns, water mass characteristics, 

sediment, and faunas. At present, however, only a limited 

amount of detailed oceanographic data is available. 

Escanaba Trough is seaward of influence from the California Current and the area of intense upwelling that occurs 

12JC30' 12JC28' 

8000 

0 2 KILOMETERS 

Contour interval 10 meters 6000 

12,000 14,000 16,000 

Figure 18.3. Southern Escanaba Trough sample locations. Explanations and symbols same as in figure 18.2. 
QUATERNARY FORAMINIFERS FROM ESCANABA TROUGH, NORTHEAST PACIFIC OCEAN 341 

Table 18.1. List of samples, water depths, and summary of foraminiferal faunal characteristics 

[A (abundant), more than 500 individuals; C (common), 100-500; R (rare), fewer than 100; o, absent or barren;+, present; aggl, agglutinated foraminifers; calc, 

calcareous foraminifers;>, greater than;>>>, much greater than; H (high), more than 20 species; M (moderate), 10-20 species; L (low), fewer than 10 species; g, 

good; m, moderate; p, poor;*, samples treated with hydrogen peroxide for mineral analysis] 

Large Tests 

Water agglutinated Assemblage with 

Sample number depth Benthic Planktonic tests composition volcanic Species Preservation 

(m) abundance abundance (>1.0 mm) by wall type glass diversity 

NESCA 

L1-29D ------------------------------------- 3,260-3,230 A R + aggl>>>calc + H m 

L 1-31 D ------------------------------------- 3,300-3,290 A R + aggl>>>calc + H m 

Ll-33G 0-1 em---------------------------- 3,299 R 0 + aggl>>ca1c + H m 

Ll-33G 104-105 em---------------------- 3,299 0 0 0 0 0 0 0 

L1-35D ------------------------------------- 3,280-3,240 A 0 + aggl>>>calc + H m 

L1-37D ------------------------------------- 3,310-3,290 A R + aggl>>>calc + H m 

L 1-3 8D ------------------------------------- 3,310-3,260 A R + aggl>>>calc + H m 

L2-14D ------------------------------------- 3,325-3,315 A R + aggl>>>calc + H m 

L2-16D ------------------------------------- 3,345-3,295 A 0 + aggl>>>calc + H m 

L2-17D ------------------------------------- 3,200-3,120 c R + aggl>>calc + M p 

L2-18D ------------------------------------- 3,350-3,250 A 0 + aggl>>>calc + H m 

L2-23D ------------------------------------- 3,325-3,295 A R + aggl>>>calc + H m 

L2-25D ------------------------------------- 3,270-3,205 A 0 + aggl>>>calc + H m 

L3-4G 15-16 em -------------------------- 3,262 R R 0 calc>>agg1 0 M m 

L3-4G 39-40 em -------------------------- 3,262 R R 0 calc>>aggl 0 M m 

L3-5G 3-4 em ----------------------------- 3,266 0 0 0 0 0 0 0 

L3-5G 30-31 em -------------------------- 3,266 R 0 0 calc 0 L m 

L3-5G 60-61 em -------------------------- 3,266 R R 0 calc 0 L m 

TUL-89D-lP Top (6-7 em) ------------- 3,305 R R 0 calc>>aggl 0 L p 

TUL-89D-l TR Top (3-4 em) ----------- 3,305 R R + aggl>calc 0 L p 

TUL-89D-l TR 50-51 em---------------- 3,305 R R 0 calc>aggl 0 L p 

TUL-89D-l TR 52-53 em---------------- 3,305 R 0 + aggl>>calc + H m 

TUL-89D-1TR 98.5-99.5 em----------- 3,305 R R 0 calc>>aggl + L p 

TUL-89D-2G Top (2.5-3.5 em) -------- 3,250 R R 0 calc>>aggl + L p 

TUL-89D-2G 94-95 em----------------- 3,250 R R 0 calc 0 L m* 

TUL-89D-4G 0-1 em -------------------- 3,235 R R + aggl + M m 

TUL-89D-6G Top (2.5-3.5 em) -------- 3,220 R 0 + aggl 0 M m 

TUL-89D-9TR Top (2 em) -------------- 3,240 R 0 + aggl + H m 

TUL-89D-9TR 7-8 em------------------- 3,240 R 0 0 aggl>calc 0 L p 

TUL-89D-9TR 12-14 em---------------- 3,240 c c 0 calc>>>aggl 0 M m* 

SESCA 

Ll-IG 0-l em ----------------------------- 3,247 R 0 + aggbcalc 0 H m 

Ll-7G 0-l em ----------------------------- 3,240 R 0 + aggl 0 M m 

Ll-7G 211-212 em ----------------------- 3,240 R R 0 calc 0 M m* 

Ll-8G 0-l em ----------------------------- 3,236 R 0 0 aggbcalc 0 M m 

Ll-l5D ------------------------------------- 3,261-3,251 A 0 + aggb>>calc 0 H m 

Ll-l6D ------------------------------------- 3,236-3,258 R 0 + aggl>calc 0 L p 

Ll-l9G 0-1 em---------------------------- 3,258 R 0 + aggl>calc 0 M m 

Ll-22D ------------------------------------- 3,247-3,264 A R + aggb>calc 0 H m 

L l-25G 0-l em---------------------------- 3,232 R 0 + aggl 0 H m 

Ll-25G 48-49 em------------------------- 3,232 R R 0 calc 0 L p* 

Ll-25G ll9-l20 em---------------------- 3,232 R c 0 calc 0 M m* 

L 1-26G 0-l em ---------------------------- 3,225 R 0 0 aggl>calc 0 M m 

Ll-26G 33.5-35 em ---------------------- 3,225 R R 0 calc 0 L p* 

Ll-26G 40-41 em------------------------- 3,225 R R 0 calc 0 L p* 

L2-2G 0-l em ----------------------------- 3,160 R 0 + aggl>calc 0 H p 

L2-2G 72-74 em-------------------------- 3,160 R c 0 calc 0 H g* 

L2-4G 7-8 em ----------------------------- 3,199 R 0 + aggl>calc 0 L p 

L2-7G 7-8 em ----------------------------- 3,146 R 0 + aggl>calc 0 L p 

L2-7G 235-237 em ----------------------- 3,146 c c 0 calc 0 L g* 

L2-9G 20-21 em-------------------------- 3,232 R 0 + aggl>calc 0 L p 

L2-9G 48-50 em -------------------------- 3,232 R 0 0 aggbcalc 0 L p 

L2-9G 75-76 em -------------------------- 3,232 0 0 0 0 0 0 0 

L2-28G 2-3 em---------------------------- 3,251 R 0 + aggl>calc 0 M p 

Central Escanaba Trough 

Ll-27G 0-l em---------------------------- 3,318 R 0 + aggbcalc 0 H m 

Ll-27G 64-65 em------------------------- 3,318 0 0 0 0 0 0 o* 

Ll-27G 71-72 em------------------------- 3,318 0 0 0 0 0 0 o* 

Ll-27G 87-95 em------------------------- 3,318 R 0 0 l aggl 0 L m* 

L3-8G 10-11 em -------------------------- 3,279 R R 0 calc>>aggl 0 M m 

L3-8G 84-85 em -------------------------- 3,279 0 0 0 0 0 0 0 

L3-9G 8/9-10111 em --------------------- 3,322 0 R 0 I calc 0 L m 

TUL-89D-7TR Top (6 em) -------------- 3,350 R R 0 aggl>calc 0 M m 

TUL-89D-7P Top (2.5-3.5 em)--------- 3,350 R R + aggl>calc 0 H m 

TUL-89D-7P 240-242 em--------------- 3,350 R R 0 calc 0 L m* 
342 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

along the continental slope. The water mass properties in 

Escanaba Trough are probably influenced to some extent 

by the North Pacific Gyre, but it is likely that local topography significantly affects the bottom currents. 

Carey and others ( 1990) suggested that the food supply is more abundant in the Gorda Ridge axial valley than 

at equivalent depths on oceanic abyssal plains. The three 

possible sources they listed are (1) continental runoff via 

the Columbia River plume in the summer (Barnes and others, 1972); (2) entrainment by the valley walls of currents 

and concentration of particulate organic matter; and (3) 

bacterial chemosynthetic primary production associated 

with active hydrothermal vents (Cavanaugh, 1985; Rona 

and others, 1988; Zierenberg and others, 1988). 

PREVIOUS STUDIES 

Deep-water foraminifers from the North Pacific have 

been studied by a number of workers including Cushman 

(1910-1917), Smith (1973), Hessler and Jumars (1974), 

Saidova (1975), Bernstein and others (1978), Snider and 

others (1984), and Schroder and others (1988). Hessler and 

Jumars (1974) and Bernstein and others (1978) studied the 

biota in box cores from the CLIMAX II area, located in 

the central North Pacific Ocean about 770 km north of Hawaii. This is a nutrient-poor area that lies beneath the central North Pacific gyre at approximately 5,800 m water 

depth. The sedimentation rate is very low, and the sediment type is red clay with manganese nodules. Agglutinated benthic foraminifers were abundant and diverse, and 

the authors concluded that agglutinated foraminifers might 

affect the structure and dynamics of deep-sea benthic communities more than the metazoan macrofaunas that are usually the objects of deep-sea ecological studies. In a later 

study at a nearby site, Snider and others ( 1984) estimated 

the biomass of biota greater than 0.01 mm taken from box 

cores collected at water depths of approximately 5,800 m. 

They found that foraminifers dominated the fauna, making up 55.2 percent of the total biomass of all size classes 

of identifiable organisms. Bernstein and others (1978) 

identified foraminifers only to the generic level; species 

were distinguished by numbers, and none of the specimens 

were illustrated. Schroder and others ( 1988) extended this 

study by formally identifying and illustrating the North 

Pacific specimens in order to compare them with Atlantic 

Ocean assemblages from the Nares Abyssal Plain. 

To date, only Fowler and Kulm (1970), Vallier and others (1973), and Clague and others (1984) have discussed 

the foraminifers from Gorda Ridge. Fowler and Kulm 

( 1970) reported displaced benthic foraminifers in a dredge 

haul collected at a water depth of approximately 2,000 m 

from the western wall of Escanaba Trough. Sublittoral 

forms from this dredge haul made up more than 76 percent of the benthic assemblage in a semiconsolidated 

clayey sandstone derived from the Columbia and Klamath 

rivers and transported by turbidity currents into Escanaba 

Trough. A Pleistocene age for the sediment was derived 

from a typical subarctic planktonic fauna dominated by 

sinistrally coiled Neogloboquadrina pachyderma (Bandy, 

1960; Ingle, 1973). Agglutinated species, if present, were 

not discussed. Vallier and others (1973) reported that Deep 

Sea Drilling Project (DSDP) Site 35 in Escanaba Trough 

yielded cold-water Pleistocene foraminifers in the upper six 

cores (to a depth of 166m). 

Clague and others (1984) analyzed 19 dredge samples 

from the northern part of Gorda Ridge (between latitudes 

41 °27.65' N. and 42°56.99' N.) and found that agglutinated 

foraminifers were dominant in most samples. Calcareous 

benthic and planktonic foraminifers were present at most 

sites and dominant in three of the samples (D3, Dll, and 

D 17). I recently examined the planktonic foraminifers in 

these three samples and found that planktonic faunas in 

samples D3 and Dll (3,165 and 3,105 m water depths, respectively) are dominated by sinistrally coiled 

Neogloboquadrina pachyderma, indicative of cool surface 

water temperatures (Bandy, 1960; Ingle, 1973), and a possible Pleistocene age. Although sinistrally coiled N. 

pachyderma dominates over the dextrally coiled form in 

sample D 17 ( 1, 700 m water depth), warmer (possibly Holocene) conditions are suggested by an abundance of 

Globigerina sp. aff. G. umbilicata (Kennett and Srinivasan, 

1983). However, because D17 is a dredge sample, more 

than one environment or stratigraphic horizon may have 

been sampled. 

ACKNOWLEDGMENTS 

I thank Jere Lipps (University of California, Berkeley) 

and William Sliter (U.S. Geological Survey) for reviewing the manuscript and Brad Carkin and Carol Reiss (both 

U.S. Geological Survey) for technical support. Robert 

Oscarson (U.S. Geological Survey) provided the scanning 

electron micrographs. The Dynaphot photographs were 

taken at Dalhousie University (Audio-Visual Services) under the direction of David Scott. 

FORAMINIFERS FROM 

ESCANABA TROUGH 

One hundred sixteen benthic taxa were identified from 

14 dredge and 45 core samples; 43 taxa are calcareous and 

71 are agglutinated. Planktonic foraminifers are present 

in 31 samples. Agglutinated foraminifers far outnumber 

calcareous forms in all the dredge samples and in most 

cores. Many of these organisms are shown in figures 18.4 

through 18.9 at the end of this chapter. Figures 18.4, 18.5, 

18.6, and 18.8 are scanning electron photomicrographs; 
QUATERNARY FORAMINIFERS FROM ESCANABA TROUGH, NORTHEAST PACIFIC OCEAN 343 

colored figures 18.7 and 18.9 are light microscope photographs and are placed on pages facing figures 18.6 and 18.8 

to help distinguish basaltic glass from other mineral grains. 

Table 18.1 summarizes some characteristics of the faunas. 

Tables 18.2 and 18.3 list the agglutinated and calcareous 

foraminiferal taxa, respectively, identified from Escanaba 

Trough and show their presence or absence in the samples. 

Benthic foraminifers, especially agglutinated specimens 

larger than 1 mm, are more abundant in the dredge samples 

than in the tops of the gravity cores (table 18.1). Although 

dredge subsamples were larger and, therefore, would yield 

more specimens than the tops of gravity cores, this may 

not be the only reason for the difference in abundance. 

Dredge sites were chosen where rock exposures were common and sediment cover thin; gravity core sites were chosen where sediment cover was thicker and more 

widespread. It is likely that substrate and other conditions 

at the dredge sites provided more favorable environments 

for large agglutinated foraminifers. 

GRAVITY AND PISTON CORE SAMPLES 

Agglutinated foraminifers dominate the assemblages 

in the tops of the majority of gravity cores from Escanaba 

Trough; calcareous foraminifers predominate at depth 

(table 18.1). A similar trend has been noted by others who 

have studied deep-sea foraminifers from the Atlantic and 

Pacific Oceans (Kaminski, 1985; Schroder, 1986). A possible explanation is that reducing conditions at depth in the 

sediment cause the iron in the cement of the agglutinated 

test to become unstable when it is reduced from the ferric 

to the ferrous state; it becomes mobilized and the test disintegrates (Sidner and McKee, 1976). This process may 

be partly responsible for the lack of agglutinated foraminifers at depth in the Escanaba Trough cores, but it probably 

is not the main reason in the core samples studied. Sedimentological and foraminiferal evidence indicates sediment 

transport and deposition by turbidity flows which were especially prevalent during Pleistocene time. The turbidites 

are recognized by mineral composition and texture 

(Normark and others, chap. 6, this volume), magnetic susceptibility measurements (Karlin, 1989; Karlin and 

Zierenberg, chap. 7, this volume), and the presence of calcareous foraminifers indicative of shallower water and (or) 

cooler water temperatures than at present. Agglutinated 

faunas occurring above turbidites in several gravity cores 

from Escanaba Trough probably indicate colonization after 

the flows. 

Although planktonic foraminifers are sparse in most 

samples, I have used the coiling ratios of sinistrally and 

dextrally coiled Neogloboquadrina pachyderma to determine relative paleotemperatures (table 18.4). Descriptions of cores containing foraminifers indicative of 

turbidites follow. 

L2-86-2G 

This core was taken from a hilltop at SESCA from 3,160 

m water depth (fig. 18.3). Agglutinated foraminifers dominate the assemblage in the upper centimeter and are typical 

of faunas from this water depth. In contrast, calcareous 

benthic and planktonic foraminifers are abundant, and agglutinated forms are absent at 72 to 7 4 em. Specimens of the 

planktonic foraminifer, Neogloboquadrina pachyderma, at 72 

to 74 em are 86 percent sinistrally coiled and 14 percent 

dextrally coiled (table 18.4); this indicates cooler temperatures than at present and supports Karlin's (1989) interpretation, based on high magnetic susceptibility values, that 

the core is made up of Pleistocene sediment. 

The presence of two shelf foraminifers, Buccella sp. 

aff. B. frigida and Elphidium clavatum (table 18.3), at 70 

to 72 em indicates downslope transport. Other calcareous 

foraminifers typical of lower middle bathyal (less than 

2,000 m) or shallower depths include Buliminella tenuata, 

Epistominella pacifica, Globobulimina sp., and Uvigerina 

peregrina. These displaced foraminifers indicate transport 

by turbidity flow across the shelf and slope into the deeper 

waters of Escanaba Trough during the Pleistocene when sea 

levels were lower. Evidence of reworking from Miocene 

sediment is implied by one poorly preserved orange-stained 

specimen of Nonion costiferum. 

L1-86-7G 

Core L1-86-7G is located approximately 1 km southeast of L2-86-2G (fig. 18.3). The top of the core contains 

an agglutinated foraminiferal fauna typical of the 3,240 m 

water depth from which it was collected; calcareous foraminifers are absent (tables 18.1 and 18.3). In contrast, the 

assemblage at 211 to 212 em is exclusively calcareous and 

most species are typical of water depths less than 2,000 m. 

Although sinistrally coiled Neogloboquadrina pachyderma 

is more abundant (60 percent) than the dextrally coiled 

morphotype (40 percent), the dominance is not as pronounced as in L2-86-2G (86 and 14 percent, respectively) 

and may indicate cooler temperatures than at present but 

slightly warmer than indicated in L2-86-2G at 72 to 74 em. 

Karlin and Zierenberg (chap. 7, this volume) assign a Pleistocene age to sediment below 30 em in L 1-86-7G. 

L1-86-25G 

This core is located in a valley approximately 1 km 

southeast of L2-86-2G at a water depth of 3,232 m (fig. 

18.3). Sediment in the uppermost centimeter is brown mud 

with abundant fecal pellets and a foraminiferal fauna that 

is exclusively agglutinated. A sparse, very poorly preserved calcareous fauna is present in a sandy interval at 
344 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

Table 18.2. Agglutinated foraminifers from Escanaba Trough. 

[x, present;?, questionable identification] 

Species 

Adercotryma glomerata ----------

Ammobaculites americanus -----

A. microformis ---------------------

A. sp. aff. A. agglutinans ---------

A. spp. -------------------------------

Ammodiscus incertus -------------

Ammoglobigerina sp. -------------

Ammomarginulina foliacea ------

A. foliacea f. recurva -------------

A. sp. aff. A. ensis -----------------

Ammoscalaria tenuimargo -------

Astrammina rara ------------------

Astrorhiza sp. ----------------------

Bathysiphon rufus -----------------

Bathysiphon sp. --------------------

Crib~ostom_oides subglobosus ---

C. wtesnen -------------------------

C. sp. --------------------------------

Cyclammina sp. -------------------

Cystammina pauciloculata ------

Eggerella spp. ---------------------

Glomospira gordialis -------------

Haplaphragmoides spp. ----------

H. columbiense --------------------

H. rotalutum -----------------------

H. sphaeriloculus -----------------

Hormosina carpenteri ------------

H. globulifera ----------------------

Hype rammina friabilis -----------

H. laevigata ------------------------

H. sp. --------------------------------

Jaculella sp. ------------------------

Karreriella novangliae ----------

K. sp. --------------------------------

Labrospira crassimargo ---------

Martinotiella spp. -----------------

Nodellum membranaceum -------

Nodulina dentalinaformis --------

Pelosina cylindrica ---------------

Psammosphaera parva -----------

Pseudonodosinella nodulosa ----

Recurvoides contortus ------------

R. scitulum -------------------------

R. spp. -------------------------------

Reophax bacillaris ----------------

R. bilocularis ----------------------

R. difflugiformis -------------------

R. distans ---------------------------

R. guttifer ---------------------------

R. horridus -------------------------

R. scorpiurus -----------------------

R. subfusiformis -------------------

R. spp. -------------------------------

Rhabdammina spp. ----------------

R. sp. (with ridges)----------------

Rhizammina algaeformis --------

Saccammina sphaerica -----------

S. sphaerica var. catenulata -----

S. socialis ? ------------------------

S. tubulata --------------------------

Saccorhiza ramosa ----------------

Spiroplectammina sp. -------------

Subreophax aduncus --------------

Textularia spp. ---------------------

Tholosina vesicularis -------------

Thurammina sp. -------------------

Tolypammina ? --------------------

Trochammina globulosa ---------

T. inconspicua ---------------------

T. spp. -------------------------------

Vanhoeffenella sp. -----------------

Foraminifers on rocks ------------

s s 

u u 

~ 

s 

s u u 0\ 

~ ~ 

X X X X X X X 

X 

? X X X X 

X 

X X X 

X X X X X X X 

? X 

? 

X X X X X 

X ? X 

X X X X 

X X X X X X 

? X X X X ? X X X X X 

X X 

X X 

X X ? ? 

X X 

X ? X 

? 

X X X X X X X X 

X 

X X X 

X 

X X X X X X X 

X 

? X X 

X ? X ? X ? X X X 

X ? X ? X ? X X X 

X 

X X X X X 

? X ? 

X ? X X 

? 

X X X X X X X 

X X X X X 

X X 

X X X 

X ? 

X X 

? ? ? X ? X X 

X 

? 

X 

X X X X 

X X X X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

? 

X 

? 

X 

X 

X 

X 

? 

X 

X 

X 

? 

X 

X 

X 

X 

X 

X 

X 

X 

? 

? 

X 

X 

X 

? 

X 

? 

X 

X 

? 

X 

X 

X 

X 

? 

X 

X 

X 

X 

X 

X 

X 

? 

X 

X 

X 

X 

X 

X 

? 

X 

X X 

X X 

X 

X 

X 

? 

X X 

? 

X 

X 

X 

X 

X X 

X 

X 

X X X 

X 

X X X 

? X 

X 

X 

X X 

X 

X 

X 

X 

? 

X 

X X 

X 

X X 

X 

X 

X ? 

X X 

X X X 

X X 

X 

X 

X 

X 

X X 

X 

X 

X 

X 

X 

X 

X 

X X 

X 

X 

X 

X X 

X 

X 

X X 

? X 

X X 

? X 

X 

X X 
QUATERNARY FORAMINIFERS FROM ESCANABA TROUGH, NORTHEAST PACIFIC OCEAN 

Table 18.2. Agglutinated foraminifers from Escanaba Trough-Continued. 

[X, present; ?, questionable identification] 

Species 

Adercotryma glomerata ------------- - x Ammobaculites americanus -------- -

A. microformis ------------------------ - x A. sp. aff. A. agglutinans ------------ - X 

A. spp. ---------------------------------- -

Ammodiscus incertus ---------------- - X 

Ammoglobigerina sp. ---------------- - X 

Ammomarginulina foliacea --------- - X 

A. foliacea f. recurva ---------------- - x A. sp. aff. A. ensis -------------------- -

Ammoscalaria tenuimargo ---------- - x Astrammina rara --------------------- -

Astrorhiza sp. ------------------------- -

Bathysiphon rufus -------------------- - X 

Bathysiphon sp. ----------------------- - x 

Crib~ostom_oides subglobosus ------ - X 

C. w1esnen ---------------------------- - x C. sp. ----------------------------------- - x 

Cyclammina sp. ---------------------- -

Cystammina pauciloculata --------- - x 

Eggerella spp. ------------------------ - x Glomospira gordialis ---------------- - x 

Haplophragmoides spp. ------------- - ? 

H. columbiense ----------------------- - x 

H. rotalutum -------------------------- - x 

H. sphaeriloculus -------------------- - X 

Hormosina carpenteri --------------- -

H. globulifera ------------------------- - X 

Hyperammina friabilis -------------- - X 

H. laevigata --------------------------- - x 

H. sp. ----------------------------------- - x 

Jaculella sp. --------------------------- - X 

Karreriella novangliae ------------- - X 

K. sp. ----------------------------------- - x Labrospira crassimargo ------------ - X 

Martinotiella spp. -------------------- - x 

Nodellum membranaceum ---------- -

Nodulina dentalinaformis ----------- - X 

Pelosina cylindrica ------------------ -

Psammosphaera parva -------------- - ? 

Pseudonodosinella nodulosa ------- - X 

Recurvoides contortus --------------- - X 

R. scitulum ---------------------------- - x R. spp. ---------------------------------- -

Reophax bacillaris ------------------- - x R. bilocularis ------------------------- -

R. dijjlugiformis ---------------------- - X 

R. distans ------------------------------ - x R. guttifer ------------------------------ - x R. horridus ---------------------------- -

R. scorpiurus -------------------------- - ? 

R. subfusiformis ---------------------- - ? 

R. spp. ---------------------------------- - x Rhabdammina spp. ------------------- - x R. sp. (with ridges)------------------- - x Rhizammina algaeformis ----------- - X 

Saccammina sphaerica -------------- - ? 

S. sphaerica var. catenulata -------- - X 

S. socialis ? --------------------------- -

S. tubulata ----------------------------- -

X X 

X X 

X 

X X 

X 

X 

X 

X 

X 

X X 

X X 

X X 

X ? X 

X 

X 

X 

X 

X X 

X ? X 

X 

? 

? 

? 

X ? X 

X X 

X 

X 

? 

X X 

X X 

X X 

? 

? 

? ? 

X 

X ? 

X 

X 

X 

? 

? 

X X 

Saccorhiza ramosa ------------------- - x x x Spiroplectammina sp. ---------------- -

Subreophax aduncus ----------------- - x x 

Textularia spp. ------------------------ - ? 

Tholosina vesicularis ---------------- - ? ? 

Thurammina sp. ---------------------- - ? ? 

Tolypammina ? ----------------------- - ? 

Tr~chamm~na globulosa ------------ - X X 

T. mconsp1cua ------------------------ -

T. spp. ---------------------------------- - X X 

Vanhoeffenella sp. -------------------- -

Foraminifers on rocks --------------- - x x x 

X 

X 

X 

X 

X 

X 

X 

X 

X 

? 

X 

X 

X 

X 

X 

? 

X 

? 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

? 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

? 

? 

X 

X 

X 

X 

? 

X 

X 

X 

X 

X 

X 

X 

? 

X 

X 

X 

X 

X 

X 

X 

X 

? 

X 

X 

X 

X 

X 

X 

? 

X 

X 

X 

? 

X 

X 

X 

X 

X 

X 

? 

X 

X 

? 

X 

? 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

? 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

? 

X 

X 

X 

? 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

? 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

? 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

? 

X 

X 

X 

X 

X 

X 

X 

X 

X 

? 

X 

X 

X 

X 

X 

X 

? 

X 

X 

? 

X 

? 

X 

X 

? 

X 

X 

X 

X 

? 

X 

X 

X 

X 

X 

? 

X 

X X X X X X X X X 

X X X X 

X X X X X X 

? 

X X X X 

? 

X X X X X X X X 

X 

X X X X X X X X 

§ § \C) 0 s I 1 u 

~ ~ l 

X 

? 

? 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X X 

X X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

? 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

? X 

X 

X 

X 

X X X 

X X X 

345 

X 

X 
346 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

Table 18.3. Calcareous foraminifers from Escanaba Trough. 

[X, present; ?, questionable identification] 

Species 

Astrononion gallowayi -----------

Bolivina sp. ------------------------

Buccella frigida -------------------

Bulimina spp. ----------------------

Buliminella tenuata ---------------

B. sp. --------------------------------

Cassidulina spp. -------------------

Chilostomella sp. ------------------

Chilostomellina sp. ---------------

Cibicides spp. ----------------------

Ehrenbergina sp. ------------------

Elphidium spp. ---------------------

Epist~m~~ella pacifica -----------

£. smtthu ---------------------------

£. sp. --------------------------------

f!:s~~~~~ ~~~--~~~~~~~~~~~~~~~~~~~~~~ Fursenkoina spp. ------------------

Globobulimina spp. ---------------

Gyroidina gemma -----------------

G. spp. ------------------------------

Hoeglundina sp. -------------------

lslandiella sp. aff. I. teretis ------

Lagena spp. ------------------------

Melonis barleeanum --------------

M. pompilioides -------------------

M. sp. --------------------------------

Miliolidae --------------------------

Nonion costiferum ----------------

Nonionellina labradorica --------

Nonionellina sp. -------------------

Oolina seminuda ------------------

Pullenia bulloides -----------------

Pullenia sp. ------------------------

Pyrgo sp. ---------------------------

Quinqueloculina spp. -------------

Trifarina sp. ------------------------

Uvigerina hispida -----------------

U. peregrina -----------------------

U. senticosa ------------------------

U. spp. ------------------------------

Uvigerinella sp. -------------------

Valvulineria sp. --------------------

Planktonic foraminifers ----------

X X - X - - X - - - - - -

- - X X - X - - - - X - -

X - X X - X 

- - X - X - - - - - X - - ? - - - - - - - X 

- - X - - X ? - - - - - X X - - - - - - X 

X - - X - - - - - - - - X - - - - X X -

- - - - - - - - - - X - X X X 

- - - - - - ? X - - X - - - - X X 

X -

- X -

X 

X 

X - - - - - - - - - - - - - - - X - - - - - - - - -

? - - - - - - - X X - X - -

- X - - - - -

- - - - X - -

- - X - X - X - - - - X 

? - - - - -

- - - - - - X - - - - - - ? 

- - - - X - - - - -

X - - X - - X - - - - -

X - - - - - - X - - - - - - X - -

- - X 

? 

X 

X 

- - - X - - - - -

- X - -

- - - - - - - - - - - - - X 

- - - - X - - - - - - - - X - - X 

X -

X -

X -

- - X - - - -

- - X 

- X - - - -

- - - - - - - - - - X - - - - - - - X -

- - - - - X - - - X 

? - ? X -

- - - ? X 

- - X - X - - X - ? - - - - - - X 

- - X - - - - - - - - - - - - - X 

- - - - - - - - - - X 

X - X - X X - X X - - - X X - - - - X - X X X 

48 to 49 em, and a small amount of plant material is 

present. At 119 to 120 em calcareous foraminifers are 

abundant, and agglutinated forms are absent in sandy sediment with abundant fecal pellets. Ninety percent ( 141 

specimens) of theN. pachyderma at 119 to 120 em are 

sinistrally coiled (table 18.4), indicating cooler water temperatures than at present. Karlin (1989) and Karlin and 

Zierenberg (chap. 7, this volume) place the PleistoceneHolocene boundary at approximately 130 em in this core 

and show a turbidite layer at about 112 to 120 em. If their 

placement of the boundary is correct, then the sinistrally 

coiled N. pachyderma at 119 to 120 em represents 

redeposition of Pleistocene sediment during Holocene time. 

L1-86-260 

This core is located adjacent to L1-86-250 at a water 

depth of 3,225 m (fig. 18.3). The top 1 em interval is 

brown clay with abundant fecal pellets; the foraminiferal 

fauna is sparse, poorly preserved, and entirely agglutinated. 

In contrast, the faunas at 33.5 to 35 em and 40 to 41 em 

are exclusively calcareous, but are also sparse and partially 

dissolved. Specimens of Elphidium clavatum and Buccella 

sp. aff. B. frigida, which indicate transport from shelf 

depths, occur with bathyal species such as Bulimina sp., 

Buliminella tenuata, Epistominella pacifica, and 

Globobulimina sp. 
QUATERNARY FORAMINIFERS FROM ESCANABA TROUGH, NORTHEAST PACIFIC OCEAN 347 

Table 18.3. Calcareous foraminifers from Escanaba Trough-Continued. 

[X, present;?, questionable identification] 

Species 

Astrononion gallowayi -------------- - - - - - - - - - - - - - - - - - - - - - X 

Bolivina sp. --------------------------- - x - - - - - - - x - - - - - - - - - - - - - X 

Buccella frigida ---------------------- - x - - - - - - - - - - - - - - - X 

Bulimina spp. ------------------------- - x - x - - - - - - - - - - - - - - X 

Buliminella tenuata ------------------ x - - - - - - - - - - - - - - - - - X 

B. sp. ----------------------------------- - - - - - x - - - - - - - - - - - Cassidulina spp. ---------------------- - X - - - - - - - - - - - - - X X X - -

Chilostomella sp. --------------------- - x - - - - x x - - - - x Chilostomellina sp. ------------------ - - - - - - - - -

Cibicides spp. ------------------------- x - - - - - - - x - - x - - - - X -

Ehrenbergina sp. --------------------- - - - - - - - x - - - - - - -

Elphidium spp. ------------------------ - x - ? - - - - - - - - - - - - - - - X - - X 

Epist~m~~ella pacifica -------------- - - - - - - - - - - - - - - - - - - - x - - - E. smlthu ------------------------------ - x - - - - - - - - - - - - - - - - - - -

- - X 

E. sp. ----------------------------------- - - - - - - - - - - - - - - x - - - - - - - -

lf!::S~~~~ ~~~·-~~~~~~~~~~~~~~~~~~~~~~~~~ ~ : = = ~ = ~ = - X - ~ : = X X -

Fursenkoina spp. --------------------- - X - - - - - - - - - - - X - X X - X - - -

Globobulimina spp. ------------------ - x - x - x x - - x x - - - X 

Gyroidina gemma -------------------- - - - - - - - - - - - X 

G. spp. --------------------------------- - X X - - X - X X X X ? ? - - - - - - - - - - - X -

Hoeglundina sp. ---------------------- - - - - - - - - - - - - - -

Islandiella sp. aff. /. teretis --------- - - - - - - - - - - - - - - - - - X - - -

Lagena spp. --------------------------- - - - - - - x - - - - - - X 

Melonis barleeanum ----------------- - x ? - - - - - - - - - - - - - - - M. pompilioides ---------------------- - - - - - -

M. sp. ----------------------------------- - x x x - x - X - -

Miliolidae ----------------------------- X - - X - - - - - - - X X - X - - X X - -

Nonion costiferum ------------------- - - - - - - - - - - - - - - - - - - -

Nonionellina labradorica ----------- - X - - - - - - - - - - - - - - - - - - -

Nonionellina sp. ---------------------- - - - - - - - - ? - - - - - x x - - - - - - - - - - - X 

Oolina seminuda --------------------- - - - - - - - - - - - - - - - - - - - -

Pullenia bulloides -------------------- - X X - - - x X - - - - - - - - - - -

Pullenia sp. --------------------------- - - - - - - x x - - - - - - x X - - - - - - - ? 

Pyrgo sp. ------------------------------ - x - - - - - - - - x - - - - - - - - - - Quinqueloculina spp. ---------------- - x - - - - x x - - - - - - - - - X 

Trifarina sp. --------------------------- - x - - - - - - - - - - - - - - - - - - - - - - Uvigerina hispida -------------------- X - - - - - - - - - - - - - - - - -

U. peregrina -------------------------- X X - - - - - - - - - - - - - - - - -

U. senticosa --------------------------- - x x - - - - - - - x - x - - - - - x X -

U. spp. --------------------------------- X X - - - - X X X X - - X - - - - - - - X - - -

Uvigerinella sp. ---------------------- - - - - - - - - - - - - - x X - x - - - - - Valvulineria sp. ----------------------- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - X 

Planktonic foraminifers ------------- X X X - - - X X X - X - X - X X - - X X X X - X X X - X - - X 

The planktonic species N. pachydemza is present in equal 

proportions (two sinistral, two dextral) at 33.5 to 35 em depth 

in the core; at 40 to 41 em, the sinistral form is represented 

by six specimens and the dextral form by one questionable 

specimen (table 18.4). Normark and others (chap. 6, this volume) describe sediment from these intervals as turbidite mud 

and sand. If the Pleistocene-Holocene boundary is at 200 

em as suggested by Karlin (1989) and Karlin and Zierenberg 

(chap. 7, this volume), then the dominance of sinistrally 

coiled N. pachyderma at 40 to 41 em indicates redeposition 

of Pleistocene sediment during Holocene time. This seems 

reasonable considering that L1-86-26G is located in a valley downslope from L2-86-2G, which is thought to be of 

Pleistocene age based on magnetic susceptibility (Karlin, 

1989) and planktonic foraminiferal evidence. 

L2-86-7G 

L2-86-7G is from the southwest extreme of the 

SESCA area on a terrace in the west wall of the ridge at 

3,146 m water depth (fig. 18.3). The topmost sample (7 

to 8 em) contains a sparse and poorly preserved agglutinated fauna and one calcareous specimen. A silty sample 

from 235 to 237 em in the core has abundant but very 

small, unidentifiable planktonic foraminifers and fragile, 
348 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

Table 18.4. Ratio of sinistrally to dextrally coiled Neogloboquadrina pachyderma. 

Sample number Number of specimens Percentage 

Sinistral Dextral Sinistral Dextral 

Ll-7G 211-212 em------------------- 12 

L 1-25G 119-120 em ----------------- 141 

Ll-26G 33.5-35 em------------------ 2 

L 1-26G 40-41 em -------------------- 6 

Ll-29D --------------------------------- 19 

L2-2G 72-74 em---------------------- 70 

L2-7G 235-237 em------------------- 2 

L3-4G 15-16 em ---------------------- 36 

L3-4G 39-40 em ---------------------- 48 

L3-8G 10-11 em ---------------------- 5 

TUL-89D-1P Top (6-7 em)--------- 16 

TUL-89D-1 TR Top (3-4 em)------- 0 

TUL-89D-1 TR 50-51 em ----------- 12 

TUL-89D-1 TR 52-53 em ----------- 0 

TUL-89D-1 TR 98.5-99.5 em------- 7 

TUL-89D-2G Top (2.5-3.5 em)---- 13 

TUL-89D-2G 94-95 em------------- 4 

TUL-89D-7TR Top (6 em)---------- 6 

TUL-89D-7P Top (2.5-3.5 em) ---- 13 

TUL-89D-7P 240-242 em ---------- 19 

TUL-89D-9TR Top (2 em) ---------- 0 

TUL-89D-9TR 7-8 em -------------- 0 

TUL-89D-9TR 12-14 em ----------- 12 

transparent calcareous benthic foraminifers including 

Bolivina sp., Buliminella sp., Epistominella sp., Fursenkoina 

sp., and Nonionella sp. derived from bathyal depths. Karlin 

and Zierenberg (chap. 7, this volume) assign a Pleistocene 

(11 ka) age to this core at approximately 145 em. 

TUL-89D CORE SAMPLES 

Fifteen samples from eight cores were analyzed for 

foraminifers. Seven were from NESCA, and one was approximately halfway between NESCA and SESCA (fig. 

18.1). Cores TUL-89-40, 6G, and 9TR were taken from 

a hill in the vicinity of the only known active vent in 

Escanaba Trough (fig. 18.2). The actual distances from the 

vent to the sites are uncertain, but no exotic vent species 

occur in the foraminiferal faunas at these sites. 

As in most cores from Escanaba Trough, agglutinated 

foraminifers dominate the upper part of the TUL-89D 

cores, and calcareous forms are dominant below (table 

18.1); however, TUL-89-lP and 2G are exceptions. The 

top of the piston core (TUL-89D-1P) is dominated by calcareous foraminifers with 80 percent of the N. pachyderma 

sinistrally coiled (table 18.4), indicating cooler water than 

at present. This may be an artifact of sampling in which 

the topmost sediment (with dominant agglutinated foraminifers) was lost during sampling, or it may be a result 

of redeposition of Pleistocene calcareous sediment by 

Holocene turbidites. 

A trigger weight core (TUL-89D-1 TR) sent down with 

the piston core (TUL-89D-1P) had the usual sequence of 

8 

15 

2 

1 

0 

11 

2 

18 

36 

9 

4 

0 

0 

0 

5 

3 

6 

3 

2 

6 

0 

0 

6 

60 

90 

50 

86 

100 

86 

50 

67 

57 

36 

80 

0 

100 

0 

58 

81 

40 

67 

87 

76 

0 

0 

67 

40 

10 

50 

14 

0 

14 

50 

33 

43 

64 

20 . 

0 

0 

0 

42 

19 

60 

33 

13 

24 

0 

0 

33 

more agglutinated forms at the top; a subarctic planktonic 

fauna ( 100 percent sinistrally coiled N. pachyderma) at 50-

51 em; a relatively diverse agglutinated fauna at 52-53 em; 

and a temperate planktonic foraminiferal fauna (Holocene) 

at 98.5 to 99.5 em (table 18.4). The presence of the agglutinated fauna at depth in this core and the occurrence 

of 100 percent sinistrally coiled N. pachyderma above it 

(table 18.4) are evidence for redeposition of Pleistocene 

sediment during Holocene time. 

The top of gravity core TUL-89D-2G is dominated by 

calcareous foraminifers, and 81 percent of the N. 

pachyderma are sinistrally coiled (table 18.4). At 94 to 95 

em, temperate conditions are indicated by 40 percent 

sinistrally and 60 percent dextrally coiled N. pachyderma. 

Large agglutinated foraminifers dominate the upper 

part of core TUL-89D-9TR at 2 em and 7 to 8 em. A sand 

layer at 12 to 14 em contains mica and plant material, along 

with abundant, diverse, calcareous benthic and planktonic 

foraminifers. The relatively low percentage of sinistrally 

coiled N. pachyderma (67 percent) and the presence of several other planktonic species suggest temperate conditions 

and a Holocene age for this probable turbidite deposit. 

Normark and others (chap. 6, this volume) provide further sedimentological and mineralogical evidence for Holocene turbidites in Escanaba Trough. 

DREDGE SAMPLES 

Large, agglutinated foraminifers, up to 3 em long, are 

abundant in the dredge samples examined from Escanaba 
QUATERNARY FORAMINIFERS FROM ESCANABA TROUGH, NORTHEAST PACIFIC OCEAN 349 

Trough; calcareous benthic foraminifers are also present 

but are rare. Planktonic foraminifers are rare or absent in 

all dredge samples (table 18.1). The agglutinated fauna is 

similar to bathyal and abyssal faunas from the North Pacific Ocean (Cushman 1910-1917), the central Pacific 

Ocean (Bernstein and others, 1978, Schroder and others, 

1988), the Atlantic Ocean (Schroder, 1986; Schroder and 

others, 1988), and shallower depths off Antarctica (Echols, 

1971; Herb, 1971). However, the NESCA faunas are 

unique to other agglutinated deep-sea faunas in the common use of basaltic glass to construct the tests (figs. 18.5-

18.9). The widespread occurrence of basaltic glass exposed 

on the sea floor at NESCA and the restricted occurrence 

of it at SESCA has been documented by underwater photography, sample collection, and submersible observations 

(Morton and others, 1990; Ross and Zierenberg, chap. 8, 

this volume). When large, angular pieces of glass are used 

in test construction, the resulting tests are irregular in 

shape, making identifications difficult. Glass is also used 

as a substrate for attachment (figs. 18.4F; 18.6A, B, C), and 

sometimes it is difficult to distinguish if a foraminifer is 

using the glass as a substrate or if it has incorporated a 

large piece as one of the building blocks of the test wall 

(fig. 18.5B). 

Of the 14 dredge samples examined for foraminifers, 

only L1-86-29D contained enough specimens of the planktonic foraminifer Neogloboquadrina pachyderma to calculate the coiling ratio of this species. All 19 specimens 

present in the dredge sample were the sinistrally coiled subarctic form, which indicates cooler conditions and implies 

a Pleistocene age; however, this might be a result of reworking during Holocene time. 

Dredge sample Ll-86-16D from the northern flank of 

the eastern hill at SESCA (fig. 18.3) emitted a strong odor 

of oxidizing sulfide and contained a sparse, entirely agglutinated foraminiferal fauna of only four species and many 

test fragments. Unlike most Escanaba Trough foraminifers, which are brownish-orange (possibly due to the presence of iron), those in Ll-86-16D are white. Brownish 

orange agglutinated specimens from other samples, when 

treated with dilute hydrochloric acid, first turn white and 

then become fragile, probably as a result of the dissolution of the iron cement. The oxidation of sulfide in sample 

L1-86-16D (possibly pyrrhotite) may have occurred after 

the sample was stored in a closed jar (R.A. Koski, oral 

commun., 1990); therefore, the low abundance and white 

color of the specimens in Ll-86-16D may not reflect seafloor conditions. 

DISCUSSION 

Although agglutinated foraminifers are abundant and 

diverse in the deep sea, there are several reasons why little 

is known about the living animal. Biologists focus on the 

soft parts of organisms, and the tests of foraminifers enclose the soft parts making them difficult to study from a 

biological standpoint. Micropaleontologists concentrate 

their studies on the test, because it is preserved in the sediment and can provide biostratigraphic and 

paleoenvironmental information. Collection of undisturbed 

surface samples from the deep sea is difficult, time-consuming, and expensive. Observing live foraminifers at 

great water depths is difficult, so little is known about their 

feeding habits, substrate requirements, locomotion, growth, 

reproduction, and interactions with other organisms. 

RELATIONSHIP TO VENTS 

Since the discovery of hydrothermal vents on the East 

Pacific Rise (Corliss and Ballard, 1977; Lonsdale, 1977; 

Corliss and others, 1979), much has been published about 

a megafauna unique to the vent sites that consists of the 

vestimentiferal tube worm Riftia pachyptila, the mussel 

Bathymodiolus thermophilus, and the clam, Calyptogena 

magnifica. However, little has been published about the 

relationship of foraminifers to hydrothermal vents, and the 

information that is available implies that, although vents 

may affect foraminiferal faunas to some extent, the effect 

is not as obvious as it is on the megafauna. 

Clague and others ( 1984 ), in their preliminary report 

on Gorda Ridge, suggested that the dense biota from Gorda 

Ridge, particularly benthic foraminifers, may be related to 

abundant food in the proximity of vents. However, the nutritional requirements of foraminifers are poorly understood, and at the time of the study, no active vents had been 

observed; their presence was inferred from mineralogic and 

geochemical evidence. Foraminifers are known to eat detrital material such as bacteria, algae, other animals, and 

fecal matter (Lipps, 1983). Some species of foraminifers 

use dissolved organic material for nutrition (DeLaca and 

others, 1981). Algal symbionts within the protoplasm may 

provide part or all of the nutritional requirements for some 

foraminifers. Nonphotosynthesizing symbiotic bacteria 

that rely on reduced sulfur compounds for their energy 

source are the food source for many hydrothermal vent organisms that harbor symbionts (Childress, 1988); however, 

the importance of these symbionts as a food source for 

foraminifers is unknown. Sulfur compounds occur in proximity of vents in Escanaba Trough, but their importance 

to foraminiferal nutrition is uncertain. 

Lee and others ( 1991) investigated the possibility that 

the agglutinated foraminifer, Abyssotherma pacifica 

(Bronnimann and others, 1989), from near a hydrothermal 

vent on the East Pacific Rise might prey upon or harbor 

endosymbiotic chemolithotrophic bacteria. Although some 

bacteria were observed in vacuoles, there was no evidence 

that chemosynthesizing endosymbiotic bacteria were 

present. The inconclusive results of this study were due 
350 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

to poor preservation of the cell fine structure caused by the 

fixation methods used during field collection (Lee and others, 1991). 

The effect of hydrothermal processes on foraminifers 

has been studied by Nienstedt and Arnold (1988) from seamounts east of the East Pacific Rise crest; by Molina-Cruz 

and Ayala-Lopez (1988) from Guaymas Basin, Mexico; and 

by Van Dover and others (1988) from the Galapagos 

spreading center and the East Pacific Rise at latitude 21 o 

N. Unfortunately, the first two studies do not discuss agglutinated foraminifers in detail, and the third deals mainly 

with one new agglutinated species, which is not present in 

the Escanaba Trough samples considered in this report. 

Although some Escanaba Trough samples were collected near active hydrothermal vents (fig. 18.2), the importance of the vents on benthic foraminifers could not be 

determined because the distance of samples from the vents 

is not accurately known; furthermore, the samples are not 

of the quality for quantitative study, and they were not preserved or stained to permit recognition of live foraminifers. One obvious characteristic of some Escanaba Trough 

foraminifers that is related to recent volcanism and possible vent proximity is the use by the foraminifers of basaltic glass to construct the tests (figs. 18.5-18.9). Pieces 

of glass are often large and angular, giving the tests an unusual appearance and making identifications difficult. 

Foraminifers from L1-86-37D, a dredge sample collected near an active vent at NESCA, are similar to those 

from areas at both NESCA and SESCA where active venting has not been reported. Approximately half of the taxa 

present in L1-86-37D are also present in L1-86-15D 

(SESCA), L1-86-22D (SESCA), and L2-86-23D (NESCA). 

Large agglutinated forms, such as Bathysiphon rufus, 

Cribrostomoides subglobosus, Hormosina globulifera, 

Saccorhiza ramosa, and the Rhabdammina-like form with 

internal ridges, are abundant in these samples. Smaller agglutinated species are represented by Adercotryma 

glome rata, Ammobaculites folicea recurva, Ammobaculites 

microformis, and Reophax spp. 

The large size of agglutinated foraminifers from 

Escanaba Trough is not unique to hydrothermal vent areas, 

and large size does not necessarily imply an abundant food 

source. Large foraminifers, similar to those from Escanaba 

Trough, are known from the deep sea and from shallow 

seas surrounding Antarctica-environments where food 

sources may be limited much of the time. At Escanaba 

Trough, these very large foraminifers may have adapted to 

the low abundance of food in the deep-sea environment and 

variable food supplies near vents by developing a slow 

metabolism and the ability to exist for long periods of time 

without food. Individuals might grow slowly and become 

very large before reaching reproductive maturity. Lipps and 

Hickman (1982) describe such "giants" as caloric dwarfs. 

RELATIONSIDP TO "FLYSCH-TYPE" FAUNAS 

"Flysch-type" foraminiferal assemblages were first 

described in rocks of Cretaceous and Paleogene age from 

the Alpine-Carpathian flysch basin and are characterized 

by a predominance of large agglutinated cosmopolitan foraminifers (Gradstein and Berggren, 1981 ). Various factors 

such as water depth, high organic carbon, low 02, high 

C02, low pH, low Eh, and poor circulation have been proposed to explain the occurrence of "flysch-type" faunas, 

but the importance of these factors is not well understood 

(Gradstein and others, 1983). 

Gradstein and others (1983) noted that modern agglutinated foraminiferal taxa from the HEBBLE site on the 

Newfoundland slope ( 4,800 m water depth) were similar 

to "flysch-type" assemblages of Late Cretaceous to Paleogene age from the Newfoundland shelf and slope. They 

attributed the similarities to short-term catastrophic events 

that affected the benthic populations at both locations. In 

the HEBBLE area the events were periodic high-velocity 

boundary currents capable of resuspending and transporting bottom sediment; in the flysch basins the events were 

turbidites. Gradstein and others (1983) concluded that 

foraminiferal populations were destroyed by turbidity associated with high-energy events and repopulated the areas during times of relative quiescence. 

Verdenius and van Hinte (1983) described "flyschtype" foraminiferal faunas in clastic sediment from DSDP 

holes in the North Atlantic. They suggested that these Atlantic Ocean faunas represented the uppermost layers of a 

turbidite (an environment in which the foraminifers actually lived) not the transported individuals carried by the 

flow. They further suggested that the first faunas to colonize an area after a flow would be a frontier-area fauna, 

represented by simple agglutinated forms such as 

Rhabdammina; gradually, more sophisticated forms would 

become established. 

Agglutinated foraminiferal assemblages from Escanaba 

Trough have taxonomic similarity to "flysch-type" assemblages, at least at the generic level. Both simple (tubular 

and single chamber) and complex (coiled and 

multichamber) forms are present. Escanaba Trough is similar to the HEBBLE area in having high-energy environments locally. Strong bottom currents, noted during DSV 

Alvin dives in Escanaba Trough (David Clague, oral 

commun., 1990), may be capable of destroying the benthic 

foraminiferal populations. Other evidence of high-energy 

environments in Escanaba Trough includes turbidites, 

which can be recognized in the cores (see previous chapters). The agglutinated foraminifers in Escanaba Trough 

may be opportunistic species well adapted to high-energy 

environments where the food supply is variable and often 

scarce, and vent fluids may be toxic to other foraminifers. 
QUATERNARY FORAMINIFERS FROM ESCANABA TROUGH, NORTHEAST PACIFIC OCEAN 351 

TAXONOMIC NOTES 

Agglutinated foraminifers may use a variety of materials to construct their tests, depending largely on what is 

available. However, some species show a preference for 

certain grain sizes and possibly compositions (Towe, 1967), 

but the reasons for these preferences are unknown. 

Nodulina dentaliniformis (fig. 18.5A) constructed its test 

mostly of flat, light-colored grains of plagioclase and 

quartz with minor amounts of dark (heavy) minerals such 

as hornblende (F. L. Wong, oral commun., 1990). The reason for selecting these grains-composition, shape, transparency, weight-is unknown. Taxonomy of agglutinated 

foraminifers is difficult because of the variety of materials used, and the Gorda Ridge foraminifers further complicate the task when they use large angular fragments of 

basaltic glass. I do not think the type of material used by 

agglutinating foraminifers for test construction should be 

the basis for defining genera or species, so I have attempted 

to assign the Gorda Ridge specimens to already existing 

taxa. Some species from Escanaba Trough, such as the 

Rhabdammina-like form with internal ridges (figs. 18.8A, 

B, C; 18.9A, B, C, D), always incorporate coarse, angular 

material into their external test walls. When basaltic glass 

is available in the sea-floor sediment as at NESCA, it is 

used; when glass is not available as at SESCA, detrital 

grains, smaller foraminifer tests, or fragments of large tests 

are used. Species such as Reophax (figs. 18.5B; 18.6/, J) 

and Haplaphragmoides rotalutum use coarse material to 

varying degrees. Many other species, such as Bathysiphon, 

Hormosina, Adercotryma glomerata, Cribrostomoides 

subglobosus, and C. weisneri, are typically very 

finegrained species, and rarely or never use coarse material (figs. 18.4B, G, H, I; 18.5C-J). 

A foraminifer that has not previously been described 

occurs in one gravity core and all but three dredge samples 

from Escanaba Trough, and it may be a new genus. It is 

listed on table 18.2 as "Rhabdammina sp. with ridges" and 

is figured in figs. 18.8A-C and 18.9A-D. It resembles 

Rhabdammina in having a straight tubular shape and being open at both ends; however, internal ridges extend the 

length of the tube and give a petal-like appearance when 

the specimen is viewed looking down either end of the 

tube. A slight constricture at one end of the tube resembles 

a poorly developed neck. The species resembles 

Nodosinella in possessing internal ridges but differs in being unilocular, not multilocular. Also, the ridges are not 

continuous in Nodosinella; they are only near the apertures of the last chamber and each of the previous chambers. The Rhabdammina-like test is brownish orange, very 

fine grained and firmly cemented, similar in texture to 

Bathysiphon rufus from Escanaba Trough (fig. 18.4B, C), 

but the "Rhabdammina-like form" has very coarse material, usually large pieces of dark volcanic glass embedded 

in the exterior of the test wall, giving the test a spiny appearance. I do not know if this form is unique to the Gorda 

Ridge area or if the occurrence is related to hydrothermal 

activity. It is present in a dredge sample near a hydrothermal soft sediment site (37D) as well as throughout the 

SESCA and NESCA areas. It is not abundant in any of 

the samples and is only present in one gravity core, possibly because the subsample volume from cores is much less 

than subsample volumes from dredges. 

A few samples contain specimens that resemble 

Ammobaculites in some respects, but the apertures differ 

from the typical ones that Loeblich and Tappan ( 1988) described for that genus as terminal and rounded. For example, one specimen in Ll-86-31D has a terminal aperture 

that is triangular in shape with rounded apices (figs. 18.6G, 

H; 18.7 F, G). This specimen bears some resemblance to 

Ammobacularia triloba Kristan-Tollmann, figured by 

Loeblich and Tappan (1988, Plate 59, figs. 4 and 5), which 

resembles Ammobaculites but differs in having a triangular aperture with toothlike projections from each of the 

three edges. Ammobacularia triloba is known from a 

single specimen of Late Triassic age from Austria. Another 

specimen from Ll-86-31D has a slightly arcuate slit surrounded by lips and located near the center of the apertural 

face. A third specimen has what appears to be a shallow 

U-shaped aperture surrounded by lips; however, if this 

structure is an aperture it is somewhat obscured by finegrained material. Specimens that resemble Ammobaculites 

in L2-86-25D have variable slitlike apertures including a 

nearly straight slit with lips, an arcuate slit with a rim, and 

two arc-shaped structures side-by-side (roughly shaped like 

the number 8) and surrounded by lips (figs. 18.6£; 18.7£). 

For now, these questionable specimens are assigned to 

Ammobaculites sp. aff. A. agglutinans; further study may 

show that one or more are new genera, but not necessarily 

vent specific. 

CONCLUSIONS 

Although the samples available from Escanaba Trough 

do not permit detailed quantitative analysis of the relationship of foraminifers to hydrothermal vents, they allow a 

description of the foraminiferal faunas present in the vicinity of vents and provide the groundwork for future studies. Box core samples or samples obtained from 

submersibles at known vent and nonvent sites are needed 

to determine which, if any, species are restricted to vents. 

A new genus, resembling Rhabdammina, but with internal ridges, may be unique to this area. Specimens resembling Ammobaculites, but with variable apertures. also 

appear to be undescribed in the literature. 
352 GEOLOGIC, HYDROTHERMAL, AND BIOLOGIC STUDIES AT ESCANABA TROUGH, GORDA RIDGE 

Shallow-penetration cores from Escanaba Trough 

containing calcareous benthic and planktonic foraminifers 

substantiate conclusions by Vallier and others (1973 ), 

Karlin and Zierenberg (chap. 7, this volume), and Normark 

and others (chap. 6, this volume) of turbidite deposition in 

Escanaba Trough during the Pleistocene and Holocene. 

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northeastern Pacific Ocean: Marine Geology, v. 15, p. 67-

87. 

Van Dover, C.L., Berg, C.J., Jr., and Turner, R.D., 1988, Recruitment of marine invertebrates to hard substrates at deep-sea 

hydrothermal vents on the East Pacific Rise and Galapagos 

'spreading center, in Childress, J.J., ed., Hydrothermal vents, 

a case study of the biology and chemistry of a deep-sea hydrothermal vent of the Galapagos Rift; The Rose Garden in 

1985: Deep-Sea Research, v. 35, nos.10/11A, p. 1833-1849. 

Van Dover, C.L., Grasssle, J.F., and Boudrias, Michel, 1990, 

Hydrothermal vent fauna of Escanaba Trough (Gorda 

Ridge), in McMurray, G.R., ed., Gorda Ridge: A seafloor 

spreading center in the United States' Exclusive Economic 

Zone: New York, Springer-Verlag, p. 285-290. 

Verdenius, J.G., and Van Hinte, J., 1983, Central NorwegianGreenland Sea: Tertiary arenaceous foraminifera, 

biostratigraphy and environment: Proceedings of the First 

Workshop on Arenaceous Foraminifera, 7-9 September 

1981: Institutt for Kontinentalsokkelundersokeler [Continental Shelf Institute], Norway, p. 173-223. 

Zierenberg, R.A., Koski, R.A., Shanks, W.C., III, and Slack, J.F., 

1988, Preliminary results of ALVIN dives on active sediment-hosted massive sulfide deposits in the Escanaba 

Trough, southern Gorda Ridge [abs.]: Eos, Transactions 

American Geophysical Union, v. 69, p. 1488. 

Zierenberg, R.A., Morton, J.L., Koski, R.A., and Ross, S.L., Geologic setting of massive sulfide mineralization in the 

Escanaba Trough, in Morton, J.L., Zierenberg, R.A., and 

Reiss, C.A., eds., Geologic, hydrothermal, and biologic studies at Escanaba Trough, Gorda Ridge, offshore northern 

California: U.S. Geological Survey Bulletin 2022, chapter 

10 (this volume). 
A 0 ~ MILLIMETER B 0 1 MILLIMETER c 0 100 MICROMETERS 

D 0 400 MICROMETERS L______j E 0 100 MICROMETERS F 0 100 MICROMETERS L__j 

G 0 100 MICROMETERS L_____j H 0 100 MICROMETERS I 0 100 MICROMETERS 

Figure 18.4. Scanning electron micrographs of foraminifers from Escanaba Trough. A, Hyperammina laevigata (Wright) [center of photograph]; site L1-86-29D. B,C, Bathysiphon 

rufus de Polin; site L1-86-29D; (B) side view [center of photograph]; (C) end view showing aperture. D,E, Rhizammina algaeformis Brady; site L1-86-29D; (E) detail of D. F, 

Placopsilinella sp. attached to basaltic glass; site Ll-86-29D. G-/, Adercotryma glomerata (Brady); site L1-86-1 G, 0-1 em; apertural views showing variation in height of tests; within 

same sample, test may be high (G), medium (H), or elongate(/). 

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Figure 18.5. Scanning electron micrographs of foraminifers from Escanaba Trough. 

A, Nodulina dentaliniformis (Brady); test is constructed mostly of flat, light-colored grains 

of plagioclase and quartz with minor dark minerals such as hornblende (F.L. Wong, oral 

commun., 1990); site Ll-86-290. B, Reopha.x sp. with large, angular chip of volcanic glass 

used as building material for test wall; site Ll-86-290. C, Cribrostomoides wiesneri (Parr); 

site L1-86-19G, 0-1 em. D, Cribrostomoides subglobosus (Sars), apertural view; site L1-86-

290. E,F, Cribrostomoides subglobosus (Sars); (E) involute side; (F) Close-up of wall texture; 

three pores of unknown origin; site L1-86-310. G, Cribrostomoides subglobosus; slightly 

evolute side of another specimen from site L1-86-310. H,I, Hormosina globulifera Brady; 

site L1-86-290; (H) single-chambered form;(/) three-chambered form. 

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Figure 18.6. Scanning electron micrographs of foraminifers from Escanaba Trough. 

A, Tolypammina? sp. (Brady); site L1-86-29D; area in box shown in D; see also figure 18. ?A. 

B, Tholosina vesicularis? (Brady) attached to basaltic glass; note well-defined neck; site 

L1-86-29D; see also figure 18.7B. C, Tholosina vesicularis? (Brady); siteL1-86-29D; three 

specimens attached to basaltic glass; a three-chambered foraminifer resembling Placopsilinella 

is present near center of basalt fragment; see also figure 18.7 C. D, Placopsilinella sp.; detail 

of boxed area in A. E, Ammobaculites sp. aff. A. agglutinans; site L1-86-31D; apertural 

I 0 200 MICRO- L______j METERS J 

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view; aperture difficult to see, but shaped somewhat like a figure 8; see also figures 18.7 D, E. 

F, Tholosina vesicularis ? and Placopsilinella; detail of specimens near center of C. G,H, 

Ammobaculites sp. aff. A. agglutinans; site L1-86-31 D; (G) oblique apertural view showing 

short rectilinear part of test offset to one side from plane of coiling; much use of volcanic 

glass; see also figure 18.7 F; (H) detail of aperture showing triangular shape; see also figure 

18.7G. /, Reophax sp.; site Ll-86-29D; see also figure 18.7H. J, Reophax sp.; site Ll-86-

29D; large horizontal blade is basaltic glass; see also figure 18.7/. 

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sp. (Brady); site L1-86-29D; see also figure 18.6A; orange tube (foraminifer?) attached to 

basalt and also possibly using basalt grains as building material for its test (attaching it to 

various surfaces of the tube). B, Tholosina vesicularis? (Brady); site L1-86-290; see also 

figure 18.6B. C, Tholosina vesicularis? (Brady) and Placopsilinella sp.; site L1-86-290; 

see also figures 18.6C, F. D, E, Ammobaculites sp. aff. A. agglutinans; site L1-86-310; 

(D) side view showing coil and short rectilinear part of test; large angular fragments of 

dark-colored volcanic glass are incorporated into test; (E) apertural view; aperture difficult 

to see but shaped somewhat like a figure 8; green is food color applied to aid in viewing 

c 

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aperture; see also figure 18.6£. F,G, Ammobaculites sp. aff. A. agglutinans, site L1-86-

310; (F) side view showing coil; a very short rectilinear part which is offset from the 

plane of coiling is not visible; much volcanic glass used in test; variety of sizes and shapes 

of material used make it difficult to discern coil and individual chambers; (G) apertural 

view showing triangular-shaped aperture; green is food color; see also figures 18.6G, H. 

H, Reophax sp.; site L1-86-290; light grains are mostly feldspar and quartz; dark grains 

are basaltic volcanic glass; see also figure 18.6/. /, Reophax sp.; site L1-86-290; large 

horizontal bladelike grain is basaltic glass; light grains are mostly feldspar and quartz; see 

also figure 18.61. 

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Figure 18.8. Scanning electron photomicrographs of foraminifers from Escanaba Trough. 

A-C, Rhabdammina? sp. (with ridges); site L1-86-310; (A) side view; (B) longitudinal 

section of another specimen showing interior ridges; (C) end view showing petal-like 

shape formed by interior ridges; see also figures 18.9A-D. D-F, Karreriella? sp.; site 

L1-86-19G, 0--1 em; (D) oblique apertural view; (E) detail of aperture; (F) detail of wall 

showing very fine texture. G, Reophax bacillaris? Brady; site L1-86-290. H, 

Ammomarginulina ensis Wiesner; site Ll-86-1G, 0--1 em. I,Ammobaculites microformis 

Saidova; site L1-86-1G, 0--1 em. 

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Figure 18.9. Dynaphot photomicrographs of foraminifers from Escanaba Trough. AD, Rhabdammina? sp. (with ridges); site Ll-86-31D; black grains are mostly basaltic 

glass; lighter grains are mostly feldspar and quartz; (A) side view; (B) longitudinal section 

of specimen in A showing interior ridges; (C) end view showing petal-like shape formed 

by intet,i.or ridges; (D) oblique end view showing ridges; see also figures 18.8A, B, C. 

E,F, Ammobaculites sp. aff. A. agglutinans; site Ll-86-31D. (E) Side view showing 

indistinct coil and rectilinear part; (F) oblique apertural view showing arcuate aperture; 

green is food color. G,H, Ammobaculites sp. aff. A. agglutinans; site Ll-86-25D. (G) 

Side view; (H) apertural view showing sinuous aperture with lips. /, J, Ammobaculites 

sp.; site Ll-86-25D. (/)Side view; much use of volcanic glass; (J) apertural view showing 

arcuate aperture. 

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