Geologic Studies in the Basin and Range-Colorado 

Plateau Transition in Southeastern Nevada, 

Southwestern Utah, and Northwestern Arizona, 1992 

U.S. GEOLOGICAL SURVEY BULLETIN 2056 
Cover. False color composite TM image of the western half of the BARCO area. This 

mosaic was prepared by Kevin F. Mullins, U.S. Geological Survey, at Flagstaff, AZ. TM 

Bands 1, 4, and 7 are blue, green, and red, respectively. A polar orbit at 500 to 800 km elevation was used by the satellite that collected this image. These data were radiometrically 

corrected for instrument and atmospheric effects by Pat Chavez (1989, Radiometric calibration of landsat thematic mapper multispectral images: Photogrammetric Engineering 

and Remote Sensing, v. 55, no. 9, p. 1285-1294). The area shown extends from about lat 

38° N. , long 115° W. in the northwest comer to about 36°30'N., li3°30'W. in the southeast comer, about 165 km in a north-south direction and about 135 km in an east-west 

direction. The Escalante Desert in the northeastern area contains circular irrigation patterns; the city of St. George, Utah, is about 80 km to the south. The southern area is dominated by the domal Mormon Mountains flanked by the Virgin River to the southeast and 

Meadow Valley Wash to the west, all in Nevada. The northwestern area covers the North 

Pahroc Range, and east of that, Dry Lake Valley. Clouds cover much of the Bull Valley 

Mountains and the eastern end of the Caliente caldera complex about at the Nevada-Utah 

border. 
Geologic Studies in the Basin and RangeColorado Plateau Transition in 

Southeastern Nevada, Southwestern Utah, and 

Northwestern Arizona, 1992 

By Robert B. Scott and W C Swadley, Editors 

U.S. GEC>LOGICAL SURVEY BULLETIN 2056 

Chapter titles are listed on volume contents page 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1995 
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary 

U.S. GEOLOGICAL SURVEY 

Gordon P. Eaton, Director 

For sale by U.S. Geological Survey, Information Services 

Box 25286, 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 studies in the Basin and Range-Colorado Plateau transition in southeastern 

Nevada, southwestern Utah, and northwestern Arizona, 1992 I by Robert B. Scott and W C 

Swadley, editors. 

p. em. -(U.S. Geological Survey bulletin ; 2056) 

Includes bibliographical references. 

Supt. of Docs. no.: I 19.3:2056 

1. Geology-Nevada. 2. Geology-Utah. 3. Geology-Arizona. I. Scott, 

Robert B. II. Swadley, W C III. Series. 

QE75.B9 no. 2056 

[QE137] 

557.3 s-dc20 94-16278 

[557.93] CIP 
CONTENTS 

[Letters designate chapters] 

Introduction 

By Robert B. Scott and W C Swadley 

A. Stratigraphic relationships of Tertiary volcanic rocks in central Lincoln 

County, southeastern Nevada 

By Robert B. Scott, C. Sherman Gromme, Myron G. Best, Joseph G. 

Rosenbaum, and Mark R. Hudson 

B. Stratigraphy of Miocene ash-flow tuffs in and near the Caliente caldera complex, 

southeastern Nevada and southwestern Utah 

By Peter D. Rowley, L. David Nealey, Daniel M. Unruh, Lawrence W. Snee, 

Harald H. Mehnert, R. Ernest Anderson, and C. Sherman Gromme 

C. Preliminary geochemistry of Miocene ash-flow tuffs in and near the Caliente 

caldera complex, southeastern Nevada and southwestern Utah 

By L. David Nealey, Peter D. Rowley, Daniel M. Unruh, James R. Budahn, 

Lawrence W. Snee, Harald H. Mehnert, and R. Ernest Anderson 

D. Strontium and neodymium isotopic survey of ash-flow tuffs and related rocks from 

the Caliente caldera complex, southeastern Nevada and southwestern Utah 

By Daniel M. Unruh, L. David Nealey, Peter D. Rowley, Lawrence W. Snee, 

Harald H. Mehnert, and R. Ernest Anderson 

E. Evidence of the Kane Springs Wash caldera in the Meadow Valley Mountains, 

southeastern Nevada 

By Anne E. Harding, Robert B. Scott, Harald H. Mehnert, and Lawrence W. Snee 

F. Preliminary correlation of Quaternary and late Tertiary alluvial deposits in 

southeastern Nevada 

By W C Swadley, D.L. Schmidt, R.R. Shroba, V.S. Williams, andD.L. Hoover 

G. Low-angle normal faults in Devonian rocks of the southern Delamar Mountains, 

Lincoln County, Nevada 

By William R. Page 

H. Paleomagnetic data from the Miocene Hiko Tuff, southeastern Nevada, and their 

tectonic implications 

By Mark R. Hudson, Joseph G. Rosenbaum, Robert B. Scott, and Peter D. Rowley 

I. Decoupling of mid-Tertiary rocks, Red Hills-western Markagunt Plateau, 

southwestern Utah 

By Florian Maldonado 

1. Quaternary geology and tectonics of the Red Hills area of the Basin and RangeColorado Plateau transition zone, Iron County, Utah 

By VanS. Williams and Florian Maldonado 

III 

Introduction 

By Robert B. Scott and W C Swadley 

The scientific purpose of the H.asin f!nd Range to 

Colorado Plateau Transition (BARCO) Study Unit is to use 

the results of detailed geologic mapping and related 

research to investigate the tectonic and magmatic evolution 

of the eastern part of the Basin and Range province to the 

stable Colorado Plateaus province; the area includes the 

transition zone between the two provinces (figs. I and 2). 

BARCO is a U.S. Geological Survey multidisciplinary 

mapping Study Unit within the National Geologic Mapping 

Program, authorized by the National Geologic Mapping 

Act of 1992. This new National Geologic Mapping 

Program superseded the preexisting mapping program by 

0 

\ 

120° 

\ \ 

0 

the same name, under which the BARCO Study Unit 

operated from I988 to I992. The highest priority of the 

new program is the synthesis of I: I 00,000-scale, 

30x60-minute quadrangles compiled from more detailed 

general-purpose geologic maps, in areas where improved 

knowledge of the geologic framework is needed by the 

private sector, government agencies, and academic 

institutions for environmental purposes, geologic hazard 

reduction, land-use planning, and basic research. More 

detailed maps are being produced where they are needed to 

portray geologic relations too complex to be displayed at 

I: I 00,000 scale. 

500 KILOMETERS 

Figure 1. Regional map showing the BARCO Study Unit area, boundaries of the Basin and Range and Colorado 

Plateaus provinces, and transition zone between the provinces. 
2 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

Figure 2. Map of BARCO Study Unit area including locations of the eight 30x60-minute quadrangles: C, Caliente; CM, Clover Mts.; 0, 

Overton; CC, Cedar City; SG, St. George; L, Littlefield; P, Panguitch; K, Kanab. The Basin and Range province is shown by shaded ranges 

and unshaded basins, the transition zone is shown by a stipple pattern, and the Colorado Plateaus province is shown by a vertical line pattern. 

Heavy line fault; bar and ball on downthrown side. 

The Basin and Range province has been extensionally 

deformed during the Neogene in an east-west direction. It is 

topographically expressed in eastern Nevada and western 

Utah by a series of north- to north-northeast-trending, roughly 

10- to 20-km-wide bedrock ranges separated by 10- to 

25-km-wide alluvial basins, and it is magmatically expressed 

by the most voluminous terrestrial record of silicic ash-flow 

tuff volcanism. In contrast, the Colorado Plateaus province 

remained an essentially undeformed stable highland, subjected only to minor peripheral basalt volcanism during the 

Cenozoic. We refer readers to several summaries ofthe geology of these two intermontane provinces that describe the 

contrasting generalized tectonic and magmatic histories and 

the crustal geophysical character of the regions; these summaries include those in Geological Society of America Memoir 172 (Anderson, 1989; Pakiser, 1989; Smith and others, 

1989; Hamilton, 1989) and those by Gordon Eaton (Eaton, 

1979; 1982). Although most of the modem topographic 

expression in the Basin and Range is the result of high-angle 

late Cenozoic Basin-Range-style normal faulting formed 

mostly since about I 0 Ma, widespread evidence supports the 

existence of older, middle- Tertiary low-angle normal faults 

and detachment faults at depth (for example, Wernicke and 

others, 1988); this pattern of extensional faulting is further 

complicated by strike-slip faults related to the process of 

extension. Although in broad perspective, Cenozoic extension in the Basin and Range province is associated with silicic 

volcanism, in detail, volcanism and extension appear to be 

somewhat decoupled (Best and Christiansen, 1991 ). 

The transition zone between stable Colorado Plateau 

and extensionally deformed Basin and Range is characterized by faults that are similar to those in the Basin and Range 

but generally show a decrease in abundance and amount of 

offset toward the plateau. A distinction based on degree of 

deformation alone, however, cannot be applied universally 

because the transition zone includes areas that have been 

more extensively deformed than some relatively undeformed 

areas within the Basin and Range. In general, we consider the 

transition zone to be bounded on the west by the Grand 

Wash-Gunlock fault zone and bounded on the east by an 

east-stepping margin marked by the Hurricane fault in the 

southeastern part, by the Toroweap-Sevier fault systems in 

the eastern part, and by the Paunsaugunt fault in the northeastern part of the BARCO area (fig. 2). 

The status of mapping within the BARCO Study Unit 

area at the end of 1992 is shown in figure 3. Geologic maps 

published in conjunction with this Study Unit represent an 

area equivalent to about ninety 7.5-minute quadrangles. 
INTRODUCTION 

CEDAR CITY PANGUITCH 

CLOVER 

MTS 

30' OVERTON 

EXPLANATION 

~~ Published by BARCO I Mapped 

LITTLEFIELD 

Ounmapped 

KANAB 

(Published) 

I Published by others I Mapping in progress 

Figure 3. Map of the status of mapping in the BARCO Study Unit area at the end of 1992 by display of 7.5-minute quadrangles without 

regard to scale of mapping. The eight 30x60-minute quadrangles of the Study Unit are named on the margin. 

Most of these maps are at scales of 1:24,000, but they also 

include smaller scales, for example, the 1: 1 00,000-scale map 

of the Kanab 30x60-minute quadrangle. In addition, 

published maps covering an area equivalent to about 

twenty-five 7.5-minute quadrangles have been produced by 

geologists who have no association with this Study Unit. 

Unpublished mapping by geologists supported by the Study 

Unit covered an area equivalent to approximately fifty 

7.5-minute quadrangles at the end of 1992. Finally, areas 

covering about seventy 7 .5-minute quadrangles have been 

partially mapped, mostly by geologists associated with this 

Study Unit. Mapping scales range from 1:12,000 to 

I: 100,000 and are chosen to solve the geologic problems 

locally encountered. Many maps of the BARCO area are 

published as U.S. Geological Survey Open-File Reports to 

make them available for the public at the earliest possible 

date. Most maps are also being published as U.S. Geological 

Survey Geologic Quadrangle Maps (GQ maps), 

Miscellaneous Geologic Investigations Maps (I maps), or 

Miscellaneous Field Studies Maps (MF maps). Some 

mapped areas will remain unpublished at larger scales but 

will be directly incorporated into 1: 100,000-scale syntheses 

of 30x60-minute quadrangle I maps. Still other maps are 

published cooperatively by the Utah Geological Survey. 

3 
4 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

The first part of this volume addresses stratigraphic, 

magmatic, and tectonic topics in the Basin and Range part of 

the BARCO area (fig. 2). Stratigraphic nomenclature is 

established for Oligocene and Miocene ash-flow tuffs that 

cover much of the Basin and Range in the BARCO area by 

correlation of cooling units, using petrographic, geochemical, paleomagnetic, and isotopic age data, in addition to 

knowledge gained by establishing relative stratigraphic position during detailed mapping. The locations and boundaries 

of the sources for several of these ash-flow tuffs are established or confirmed to be in the Indian Peak, Caliente, and 

Kane Springs Wash caldera complexes. Initial petrological 

investigations of several ash-flow tuffs are provided by preliminary isotopic and trace..:element studies. Insight into the 

geometry, timing, and evolution of compressional and extensional tectonism that affected the BARCO area is also 

derived from relationships recognized from detailed mapping and paleomagnetic investigations. Stratigraphic studies 

also include correlations between some Quaternary and Tertiary alluvial units from separate basins within the BARCO 

area. 

The second part of the volume concerns the transition 

zone west of the stable Colorado Plateau. Early Miocene 

deformation in the transition zone includes chaotic 

deformation of Oligocene and younger volcanic strata that 

overlie undeformed older sedimentary strata; the degree and 

areal extent of this shallow crustal deformation are 

considerably greater than that previously recognized or 

reported, and they present a contrast with deeper crustal 

detachment found in the Basin and Range. Deformation 

adjacent to the Hurricane fault includes an impressive 60-m 

offset of a middle Pleistocene geomorphic surface. 

Acknowledgment.-We acknowledge the editing by 

L.M.H. Carter and the computer graphics efforts of Ramon 

E. Sabala, both of which improved the report and facilitated 

its completion. 

REFERENCES CITED 

Anderson, R.E., 1989, Tectonic evolution of the Intermontane System; Basin and Range, Colorado Plateau, and High Lava 

Plains, in Pakiser, L.C., and Mooney, W.D., eds., Geophysical 

framework of the Continental United States: Geological Society of America Memoir 172, p. 163-176. 

Best, M.G., and Christiansen, E.H., 1991, Limited extension during 

peak Tertiary volcanism, Great Basin of Nevada and Utah: 

Journal of Geophysical Research, v. 96, p. 13509-13528. 

Eaton, G.P., 1979, Regional geophysics, Cenozoic tectonics, and 

geologic resources of the Basin and Range province and 

adjoining regions, in Newman, G.W., and Goode, H.D., eds., 

Basin and Range Symposium and Great Basin Field Conference: Rocky Mountain Association of Geologists, Denver, 

Colo., and Utah Geological Association, Salt Lake City, p. 

11-39. 

---1982, Basin and Range province; Origin and tectonic significance: Annual Review of Earth and Planetary Sciences, v. 10, 

p. 409-440. 

Hamilton, W.B., 1989, Crustal geologic processes of the United 

States, in Pakiser, L.C., and Mooney, W.D., eds., Geophysical 

framework of the Continental United States: Geological Society of America Memoir 172, p. 743-781. 

Pakiser, L.C., 1989, Geophysics of the intermontane system, in 

Pakiser, L.C., and Mooney, W.D., eds., Geophysical framework of the Continental United States: Geological Society of 

America Memoir 172, p. 235-247. 

Smith, R.B., Nagy, W.C., (Smith) Ju1ander, K.A., Viveiros, J.J., 

Barker, C.A., and Gants, D.G., 1989, Geophysical and tectonic 

framework of the eastern Basin and Range-Colorado Plateau-Rocky Mountain transition, in Pakiser, L.C., and Mooney, W.D., eds., Geophysical framework of the Continental 

United States: Geological Society of America Memoir 172, p. 

205-233. 

Wernicke, Brian, Axen, G.J., and Snow, J.K., 1988, Basin and 

Range extensional tectonics at the latitude of Las Vegas, Nevada: Geological Society of America Bulletin, v. 100, p. 

1738-1757. 
Stratigraphic Relationships of 

Tertiary Volcanic Rocks in 

Central Lincoln County, 

Southeastern Nevada 

By Robert B. Scott, C. Sherman Gromme, Myron G. Best, 

Joseph G. Rosenbaum, and Mark R. Hudson 

GEOLOGIC STUDIES IN THE BASIN AND RANGE-COLORADO PLATEAU TRANSITION IN 

SOUTHEASTERN NEVADA, SOUTHWESTERN UTAH, AND NORTHWESTERN ARIZONA, 1992 

U.S. GEOLOGICAL SURVEY BULLETIN 2056-A 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1995 
CONTENTS 

Abstract........................................................................................................................... 7 

Introduction .. .............. .. . .. . . . . . . . . . . ... . . . . . . . . .. ............ .............. ....... .. . .. ... . . . . . . . . . .. .. . . . . . . . . ... . . . . . . . 8 

Purpose and Scope.................................................................................................. 8 

Acknowledgments.................................................................................................. 10 

Geologic Setting and Previous Work . . . . . ............ .. .. . . . . . . . . . . . ............ ... . . . . . . . . . . . ... . . . . . . . ......... 10 

Correlation Methods . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . ............ ............... ........... .............. ............ ........ 11 

Stratigraphic Relationships............................................................................................. 12 

Prevolcanic Sedimentary Rocks............................................................................. 12 

Calc-alkalic Volcanic Rocks ............................................................... ~.................. 13 

Needles Range Group..................................................................................... 13 

Petroglyph Cliff Ignimbrite of Cook (1965).... ............. .............. ........... ......... 14 

Monotony Tuff............................................................................................... 15 

Isom Formation and Relationships with other 

Isom Compositional-type Units.................................................................. 17 

Shingle Pass Tuff............................................................................................ 23 

Leach Canyon Formation . . . . . . . . . . . .... .. . . . . . . ... . . . . . . . . . . . . ........... .......................... ... . 24 

Condor Canyon Formation . . . . . . . . . . ... . . . . . . . ................ .. .. . . . . . . . .. .. .. . . . . . . . . .. . . . . . . . . . ..... 25 

Pahranagat Formation..................................................................................... 26 

Rhyolitic Lava Flows of Delamar Valley....................................................... 27 

Harmony Hills Tuff........................................................................................ 28 

Hiko Tuff........................................................................................................ 28 

Peralkaline Rhyolitic Ash-flow Tuffs and Mafic Lava Flows............................... 29 

Delamar Lake Tuff . . . . . ... . . . . . . . . . . . . . ............ ... . . . . . . . . . . . . ............. ... . . . . . . . . . .. . . . . . . . . . . . .. . 30 

Sunflower Mountain Tuff............................................................................... 31 

Peralkaline Precursor to the Kane Wash Tuff................................................ 31 

Kane Wash Tuff .............................................................................................. 32 

Tuff of Etna .................................................................................................... 33 

Mafic Lava Flows........................................................................................... 35 

Tuff and Bedded Volcanic Ash in Alluvium.................................................. 36 

Timing of Volcanism and Extension . . . . . . ... . . . . . . . . . . . . . . . . . . . . . . . . . . ... . . . . . . . . .............. .. . . . . . . . . . . . .. . . . 36 

References Cited............................................................................................................. 38 

FIGURES 

1. Map showing geographic and geologic features in southeastern and central Nevada................................................... 8 

2. Map showing location of 7.5-minute quadrangles in central Lincoln County, Nevada, discussed in this report.......... 9 

3. Diagram showing lUGS classification of ash-flow tuff units and distribution of samples from study area................. 14 

4. Diagram of modal composition of Oligocene and early Miocene ash-flow units......................................................... 16 

5. Diagram showing tilt-corrected site-mean paleomagnetic directions for ash-flow tuffs............................................... 22 

6. Correlation chart showing evolution of stratigraphic nomenclature used for the Kane Wash Tuff............................... 30 

7. Diagram showing tilt-corrected site-mean paleomagnetic directions for ash-flow tuffs collected from four sections. 34 

TABLE 

I. Published isotopic dates for ash-flow units, recalculated as necessary to new constants............................................. 18 
Stratigraphic Relationships of Tertiary Volcanic Rocks in 

Central Lincoln County, Southeastern Nevada 

By Robert B. Scott, C. Sherman Gromme, Myron G. Best, 1 

Joseph G. Rosenbaum, and Mark R. Hudson 

ABSTRACT 

Regional ash-flow sheets in central Lincoln County, 

Nevada, were deposited by major ash flows erupted initially 

from three major caldera complexes and finally from one 

small caldera complex in central and eastern Nevada active 

between about 30 and 13 Ma. The oldest of the outflow 

sheets (30-27 Ma) are large-volume, crystal-rich dacites; 

one sequence of ash flows, which formed the Needles Range 

Group, issued from the major Indian Peak caldera complex 

in northeastern Lincoln County and adjacent Utah, and a second sequence of ash flows ejected from the major Central 

Nevada caldera complex to form the Monotony Tuff. In contrast, younger pyroclastic rocks (27-22 Ma) are rhyolites 

that include the Leach Canyon Formation, probably derived 

from the major Caliente caldera complex in central Lincoln 

County, the Condor Canyon Formation, definitely derived 

from the Caliente caldera complex, and the Shingle Pass 

Tuff and Pahranagat Formation, certainly derived from the 

Central Nevada caldera complex. Intercalated between the 

Needles Range and Leach Canyon sheets are tuffs of the trachytic Isom Formation as well as other pre-Isom, 

Isom-compositional-type ash-flow tuffs, the sources for 

which are uncertain. The trachyandesitic to andesitic Harmony Hills Tuff (about 22 Ma), whose source close to the 

eastern end of the Caliente caldera complex remains uncertain, broke this chemical pattern. The last widespread calcalkalic outflow sheet in the southeastern Great Basin, the 

rhyolitic Hiko Tuff (18.6 Ma), erupted from the western end 

of the Caliente caldera complex, although minor calc-alkalic 

activity persisted within that complex for several more million years. Subsequently, erupted magmas became more 

alkalic, even mildly peralkaline, forming a high-silica rhyolite-basalt association ( 16-14.5 Ma); ash flows and lavas 

erupted from the relatively small Kane Springs Wash caldera 

complex to form the Kane Wash Tuff, several older, chemically related ash-flow sheets, and overlying basalts in central 

and southern Lincoln County. This southward shift of volcanism and associated shift to more alkalic compositions are 

generally mirrored in time by similar shifts in space and 

1Myron G. Best, Brigham Young University, Provo, UT 84602. 

composition by eruptive products of the Southwest Nevada 

volcanic field. 

Several ash-flow tuffs with similar petrography, age, 

chemical composition, and distribution are here combined 

with the previously named Pahranagat Lakes Tuff to form 

the newly defined Pahranagat Formation. Recent changes in 

stratigraphic terminology restrict the Kane Wash Tuff that 

was derived from the youngest feature of the Kane Springs 

Wash caldera complex, the Kane Springs Wash caldera, to 

three ash-flow tuff cooling units. The lower cooling unit is 

the Grapevine Spring Member, the middle cooling unit is the 

lower part of the Gregerson Basin Member, and the upper 

cooling unit is the upper part of the Gregerson Basin Member; their new sanidine 40Art39Ar dates are 14.67±0.22 Ma, · 

14.55±0.14 Ma, and 14.39±0.28 Ma, respectively. Two 

older units, whose rocks were formerly included in the Kane 

Wash Tuff, now have new formation-status names, in 

ascending order the Delamar Lake Tuff and Sunflower 

Mountain Tuff, proposed because the magmas that supplied 

these tuffs were erupted from undefined sources, probably 

northwest and southwest of the Kane Springs Wash caldera, 

respectively. Wall segments of a precursor to the Kane 

Springs Wash caldera, the Narrow Canyon caldera that also 

erupted peralkaline magmas, are found just north of the Kane 

Springs Wash caldera. The youngest ash-flow tuff between 

the Caliente caldera complex and the Kane Springs Wash 

caldera is the rhyolitic tuff of Etna (about 14 Ma); it is distinctly older than the youngest ash-flow tuff in the Bull Valley Mountains area of Utah, the petrographically similar 

rhyolitic Ox Valley Tuff (about 12.5 Ma). 

Detailed mapping in central Lincoln County indicates 

that drainage north of the Timpahute lineament was disrupted, probably by prevolcanic, top-to-the-east extension, 

forming a structural basin into which an intercalated 

sequence consisting of the Needles Range Group, lacustrine 

limestone, and fluvial clastic rocks accumulated, none of 

which are found south of the lineament. Progressive tilting 

and extension occurred between about 23 and 12 Ma in the 

South Pahroc Range and the Meadow Valley Mountains but 

not in the intervening Delamar Mountains. Although extension continued after 14.5 Ma, major tilting ceased after about 

12 Ma, followed by basin-range faulting to the present. 

7 
8 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

INTRODUCTION 

PURPOSE AND SCOPE 

The purpose of this report is to describe the progress of 

stratigraphic investigations of widespread Oligocene and 

Miocene volcanic units in the central part of Lincoln County, 

southeastern Nevada. These investigations are part of the 

Basin and Range to Colorado Plateau Transition (BARCO) 

Study Unit of the U.S. Geological Survey National Geologic 

Mapping Program (figs. 1, 2). (Also see figures 1, 2, and 3 in 

0 50 KILOMETERS 

8 

Introduction to volume.) Because most of the volcanic rocks 

are regional ash-flow tuffs, covering much of southeastern 

Nevada and southwestern Utah, correlation between tuffs is 

the key to recognition of stratigraphic relationships in this 

area. In addition to detailed mapping (fig. 2, and also fig. 3 

of Introduction to volume), we correlated these ash-flow 

sheets by using geochronologic, paleomagnetic, petrographic, and to some degree geochemical data. These stratigraphic studies allowed us to identify nested caldera 

complexes (fig. 1), but we do not discuss here post-collapse 

intracaldera volcanic units. Also these multidisciplinary 

NYECO 

LINCOLN CO 

Figure 1. Geographic and geologic features in southeastern and central Nevada. Light lines and pattern delineate ranges. Dotted lines 

delineate approximate margins of caldera complexes. Timpahute lineament from Ekren and others ( 1976). 
TERTIARY VOLCANIC STRATIGRAPHY, CENTRAL LINCOLN COUNTY, NEVADA 

WHEATGRASS DEADMAN 

SPRING SPRING 

Swadley 

and 

Simonds 

(1991) 

37°45' 

PAHROC PAHROC 

SPRING SPRING NE 

Scott, SWadley, Swadley 

and Byron and 

(1992) Simonds 

(1992) 

HIKOSE PAHROC PAHROC 

SUMMIT PASS SPRINGSE 

Scott and Swadley and Swadley (1992) Rowley (1991) 

DELAMAR NW DELAMAR SLIDY 

MOUNTAIN 

Swadley and 

Scott (1990) 

ALAMOSE DELAMAR GREGERSON ELGIN SW ELGIN 

LAKE BASIN 

Scott and others Scott, SWadle~ 

(1988, 1993) and others (19 ) 

LOWER DELAMAR3NW DELAMAR3 NE VIGONW VIGO NE 

PAHRANAGAT 

LAKE 

Scott, Page, and Scott, Novak, and Scott, Harding 

Swadley (1990) Swadley (1990) and others (1991) 

DELAMAR3SW DELAMAR3SE SUNFLOWER 

MOUNTAIN 

Page and 

others (1990) Swadley and 

others (1990) Pam;~an (1 ) 

37 0 

Figure 2. Location of 7 .5-minute quadrangles in central Lincoln County, Nevada, discussed in this report. 

Reference to published maps indicated. 

9 
10 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

stratigraphic studies have provided us with the means to recognize new ash-flow tuffs intercalated within the framework 

of previously recognized and named tuffs. Although we 

briefly discuss Tertiary sedimentary units because of their 

importance to tectonic interpretations, we do not attempt correlations of units between sedimentary basins. This report is 

supplemented by two accompanying stratigraphic studies in 

this volume, one by Harding and others on volcanic units 

associated with the eastern part of the Kane Springs Wash 

caldera, and the other by Rowley and others on volcanic units 

associated with the Caliente caldera complex. 

ACKNOWLEDGMENTS 

The authors of this manuscript received support through 

the U.S. Geological Survey National Geologic Mapping Program. We enjoyed the luxury of new K-Ar and 40 Arf39 Ar 

data from Harald H. Mehnert and Lawrence W. Snee that 

provided us with important time constraints. The background 

of knowledge in the area provided by R. Ernest Anderson and 

Peter D. Rowley was a critical resource and an inspiration to 

us. W C (Dub) Swadley, W. Ric Page, and Steve W. Novak 

contributed significantly to the mapping effort; Novak provided us with a copy of his dissertation and maps (Novak, 

1985). Two student volunteers to the U.S. Geological Survey, Anne E. Harding (University of Colorado) and Barbara 

Byron (Fort Lewis College) helped in the mapping. Myron 

G. Best's research in the Great Basin was also supported by 

Brigham Young University and the National Science Foundation through grants EAR-8604195, 8618323, and 

8904245. We thank RenA. Thompson and Peter D. Rowley 

for constructive review of the manuscript. 

GEOLOGIC SETTING AND 

PREVIOUS WORK 

Mackin ( 1960), followed by his students Cook ( 1965) 

and Williams ( 1967), pioneered work on the volcanic 

stratigraphy of southwestern Utah and southeastern Nevada, 

recognizing that individual cooling units of ash-flow tuffs 

could be correlated by comparison of phenocryst abundance 

ratios. Noble (1968), Novak (1984), and Novak and Mahood 

(1986) studied ash-flow tuffs in the area of Kane Springs 

Wash caldera. Ekren and others ( 1977) mapped the distribution of major chronologie groups of Tertiary volcanic rocks 

in Lincoln County, Nev., at a scale of 1:250,000. Pampeyan 

( 1989) mapped the Meadow Valley Mountains at a I :50,000 

scale. A review of previous isotopic dates for each stratigraphic unit is in table 1. 

By far, most Tertiary volcanic rocks in southeastern 

Nevada and southwestern Utah are ash-flow tuff. These 

regionally extensive ash-flow sheets are commonly tens, and 

in some places hundreds, of meters thick, extend more than 

100 km from their source, and total thousands of cubic kilometers of eruptive material (Best, Christiansen, and others, 

1989). Magma systems that fed these voluminous pyroclastic 

eruptions are marked by large caldera complexes tens of kilometers across, consisting of nested and partially overlapping 

calderas commonly identified on the basis of thick (commonly 2 km) prisms of intracaldera tuffs that are correlated 

to outflow deposits by their similar petrologic character, 

paleomagnetic direction, and precise 40 Art39 Ar dates. Intracaldera tuffs are commonly intercalated with coarse caldera 

collapse breccias and landslide blocks during and following 

explosive eruption. Numerous calderas occur in Lincoln 

County (fig. 1): the Oligocene Indian Peak caldera complex 

(Best, Christiansen, and Blank, 1989) lies along the NevadaUtah border in northeastern Lincoln County; the Miocene 

Caliente caldera complex (Rowley and others, this volume) 

extends from central Lincoln County, Nev., eastward to 

western Utah; and the smaller middle Miocene Kane Springs 

Wash caldera is exposed in southern Lincoln County in both 

the Delamar Mountains (Novak, 1984) and the Meadow Valley Mountains in southern Lincoln County (Harding and others, this volume). Most ash-flow sheets in Lincoln County 

were derived from these three sources or from the Central 

Nevada caldera complex 70 to 170 km to the west (fig. 1) 

(Best and others, 1992). Reconnaissance mapping by us indicates that additional sources of thin, densely welded, trachytic ash-flow tuffs (lsom Formation and chemically and 

lithologically similar units) probably exist in the vicinity of 

the Indian Peak and Caliente caldera complexes and west of 

the North Pahroc Range (fig. 1). The area between the three 

major caldera complexes was repeatedly flooded by largevolume ash flows erupted from the magma systems between 

late Oligocene and middle Miocene, forming a remarkable 

"outflow alley" of as many as 20 outflow sheets. Stacks of 

tuff sheets exposed in tilted and uplifted ranges include few 

intervening lava flows and sedimentary deposits (Best and 

Christiansen, 1991 ). 

Although rhyolite tuffs occur throughout the Tertiary of 

the Great Basin, petrographic and bulk chemical analyses 

reveal that ash-flow tuffs of particular compositions were 

deposited at particular times in the Great Basin (Scott and 

others, 1971; Best, Christiansen, and others, 1989). 

Northwest of central Lincoln County, early Oligocene tuffs 

are for the most part quartz-rich, biotite-bearing rhyolite. The 

most voluminous late Oligocene (about 31-27 Ma) tuffs 

between central Nevada and southwestern Utah are crystalrich dacite of a chemical compositional type characterized by 

the Monotony Tuff. These dacitic tuffs were overlain in the 

latest Oligocene and earliest Miocene (about 27-23 Ma) by 

widespread rhyolitic ash-flow tuffs, intercalated with 

widespread crystal-poor ash-flow tuffs of a distinct calcalkalic trachytic composition characterized by the Isom 

Formation (Christiansen and Best, 1989; Christiansen and 

others, 1989). These younger, mostly rhyolitic ash-flow tuffs 
TERTIARY VOLCANIC STRATIGRAPHY, CENTRAL LINCOLN COUNTY, NEVADA 11 

are again dominantly calc-alkalic. Still younger ash-flow 

tuffs (<about 17 Ma) are alkalic to peralkaline; peralkaline 

tuffs include the sanidine-quartz-hedenbergite-fayalitic 

olivine-bearing rhyolite outflow sheets (14.5 Ma) derived 

from the Kane Springs Wash caldera complex that includes 

those chemically related products from the precursor Narrow 

Canyon caldera (Scott, 8 lank, and Page, 1991 ). 

The timing and nature of Tertiary extensional tectonism 

were heterogeneous within the Basin and Range province, 

and tectonism appears not to be directly related in time or 

space to Tertiary volcanism (Best and Christiansen, 1991; 

Taylor and Bartley, 1992; Scott, 1990). In northern Lincoln 

County and surrounding areas to the east in southwestern 

Utah and the west in Nye County, Nev., three major tectonic 

periods are recognized (Bartley and others, 1988; Taylor and 

others, 1989). The earliest period is characterized by prevolcanic (pre-30.6 Ma) Paleogene faults, principally the top-tothe east, low-angle Stampede detachment fault and related 

high-angle normal faults (Axen and others, 1988; Rowley 

and others, 1992) that our mapping restricts to latitudes north 

of the east-striking Timpahute lineament ofEkren and others 

(1976). Taylor and Bartley (1992) suggested that the Stampede detachment may be part of a larger system of prevolcanic faults above their proposed detachment of the eastdirected Seaman breakaway fault. 

The main episode of extension occurred between at 

least 19 Ma and about 12 Ma; the faults of this episode form 

complex patterns of high-angle strike-slip, oblique-slip, and 

normal faults and low-angle normal faults (Rowley and others, 1992; Axen and others, 1988; Anderson and Hintze, 

1993). Unlike the prevolcanic episode and the youngest episode, the main episode of extension generally overlaps with 

the period of volcanic activity in the Caliente caldera complex (Rowley and others, 1992; Angelier and others, 1987; 

Bowman, 1985). Of the strike-slip faults, those with north to 

northeast strikes are sinistral, and those with east or northwest strikes are dextral (Rowley and others, 1992; Anderson 

and Hintze, 1993 ). The major low-angle fault of this episode 

is the top-to-the-west Highland detachment that was active 

about 15 Main the Highland Range (Axen and others, 1988) 

and has been mapped north of the Caliente caldera complex 

(Rowley and others, 1992; Rowley and others, 1991). 

Although many strike-slip faults have offsets of at least 1 km 

as far south as the northern margin of the Kane Springs Wash 

caldera, they are not present south of that caldera (Scott, 

Swadley, and others, 1990). Progressive tilting restricted to 

the northern and central Meadow Valley Mountains and to 

the South Pahroc Range records this episode of extension 

south and northwest of the Kane Springs Wash caldera in 

Lincoln County (Scott, Harding, and others, 1991 ). This 

main episode of extension overlaps partly with the slightly 

younger principal phase of extension that occurred between 

15 and 10 Ma near the latitude of Las Vegas, Nev., largely 

south of Lincoln County (Wernicke and others, 1988). 

Basin-range faults have controlled the major north- to 

north-northeast-trending topographic grain of this part of the 

Basin and Range province since at least 10 Ma into the Quaternary (Anderson and others, 1983; Tschanz and Pampeyan, 

1970; Ekren and others, 1977). These faults postdate basalts 

that have ages of 12.7 to 11.5 Ma (Snee and others, 1990). 

The direction of the minimum horizontal stress for these normal fault systems is west-northwest, a change of about 60° 

from the previous west-southwest minimum stress direction 

(Anderson, 1989; Zoback and others, 1981 ). 

Since 1988, a team of geologists in the U.S. Geological 

Survey, academic institutions, State geological surveys, and 

volunteers have been mapping in southwestern Utah, southeastern Nevada, and northwestern Arizona across the transition from the Colorado Plateau to the Basin and Range. As 

part of this team effort, a block of quadrangles in central Lincoln County, Nev., have been mapped at a scale of 1:24,000 

(fig. 2, and also see fig. 3 of the Introduction to volume); this 

work forms the basis for many conclusions reached in this 

report. The maps shown in figure 2 are exclusive of other 

mapping by the team in Lincoln County by Rowley and others (this volume), Keith and others (1992), Best and others 

(1991), Anderson and Hintze (1993), and Pampeyan (1989). 

CORRELATION METHODS 

In our study, careful attention to stratigraphic position, 

field characteristics, chemical composition, and phenocryst 

mineralogy enabled successful correlation of many ash-flow 

tuffs, but most remaining stratigraphic ambiguities were 

resolved by the added constraints provided by paleomagnetic data and isotopic dating. 

One of the most powerful correlation tools is modal 

analyses of phenocrysts, particularly the comparison of 

ratios of phenocryst abundances, as introduced by Mackin 

( 1960) and his students, Cook ( 1965) and Williams ( 1967); 

compaction in ash-flow tuffs changes abundances of phenocrysts by as much as a factor of two but not ratios of phenocryst abundance. Phenocryst volume percent was 

determined in this study by point counting 200-1,000 counts 

of phenocrysts per thin section stained for potassium feldspar. 

Where differences in age between stratigraphic units 

were greater than about 1 m.y., stratigraphic correlation 

ambiguities could be resolved by conventional K-Ar dating. 

H.H. Mehnert (U.S. Geological Survey) considerably aided 

our correlations by conventional K-Ardating, using the methods of Dalrymple and Lanphere ( 1969) and the constants of 

Steiger and Jager ( 1977). Where more precise isotopic dating 

was required to resolve differences less than 1 m.y., L.W. 

Snee (U.S. Geological Survey) used the 40 Arf39 Ar dating 

method to obtain dates with a 2-sigma error less than 0.3 m.y. 

using the methods of Shu bat and Snee ( 1992). 
12 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

Another powerful correlation technique used in our 

study has been the comparison of paleomagnetic directions 

of ash-flow cooling units. Ash-flow tuffs acquire stable 

thermoremanent magnetization parallel to the ambient geomagnetic field during initial cooling. In nearly all instances, 

plastic deformation (compaction or rheomorphism) ceases 

at a higher temperature than the Curie point ( 630° C or 

less), so that the magnetization direction remains undistorted; however, a notable exception to this rule is the Swett 

Tuff Member of the Condor Canyon Formation in eastern 

Condor Canyon (Gose, 1970). Typical intensities of natural 

thermoremanent magnetization range from 0.01 to 0.5 Aim 

(amperes/meter). 

The geomagnetic field exhibits two sorts of time variations in direction that enable correlation or lack of correlation of tuff sheets by means of their natural remanent 

magnetization directions. ( 1) Secular variation follows a 

randomly directed but continuous looping path with a characteristic time constant of one to a few centuries and an 

amplitude of 1 oo to 20° of arc. The time-averaged center of 

secular variation, the center of its probability distribution, is 

the direction corresponding to a geocentric axial dipole. (2) 

Reversal of polarity exhibits a characteristic frequency of a 

few thousand centuries but a random likelihood of occurrence. Thus, the geomagnetic field has two maximally stable 

states, normal and reversed. Clearly, if two ash-flow sheets 

have paleomagnetic directions that are opposite in polarity or 

differ greatly from one another, they cannot have been 

erupted at the same time. Conversely, if two otherwise similar ash-flow occurrences also have paleomagnetic directions 

that are not significantly different from one another, the 

probability that they could have resulted from the same eruption depends on how far their common direction is from the 

nearest stable axial dipole field direction (Cox, 1971; Bogue 

and Coe, 1981 ). The more unusual the magnetization direction is relative to the normal or reversed axial dipole direction, the greater is the strength of the correlation. For the 

purpose of this report, the stable normal axial dipole field 

direction is taken to be that at Caliente, Nev., having inclination 55.9° downward and declination 351.6° E., as calculated 

from the Oligocene reference paleomagnetic pole position 

for North America given by Diehl and others (1983). 

Where ash-flow sheets have been tilted, the measured 

paleomagnetic directions require corrections to directions 

one would observe in equivalent horizontal sheets. Where 

possible, structural attitudes are measured in the field on the 

planar tops of ash-flow tuff sheets. Because of exposure limitations, however, one must generally measure the attitude of 

the eutaxitic compaction foliation within the tuff sheet. 

Where tuffs have been draped over appreciable pre-eruption 

topographic relief, the distortion of foliation attitudes cannot 

be evaluated (Hagstrom and Gans, 1989); hence pre-eruption 

topography may result in errors in paleomagnetic directions 

if structural corrections have to be made. This effect would 

be expected to diminish upward in a single thick tuff sheet or 

in a sequence of several sheets (Cook, 1965, p. 44). For these 

reasons, we have always attempted to sample continuous 

stratigraphic sequences rather than isolated exposures of 

ash-flow tuffs. Nearly all samples were collected from outflow sheets, although some were obtained from intracaldera 

tuffs at the source. 

Eight or more oriented cores were collected from each 

site using a portable gasoline-powered diamond drill. Azimuthal orientation was both by magnetic and by sun compass. Remanent magnetizations were measured with a 

commercial superconducting magnetometer or a commercial 

spinner magnetometer, and all demagnetizations were done 

with commercial 400-Hz alternating-field (af) tumbling 

apparatus. For most sites, pilot demagnetizations of one 

specimen from each of two cores was done over 10 steps to 

100 mT. From these results, four appropriate steps were chosen to demagnetize one specimen from each remaining core. 

For the remaining sites, all samples were subjected to stepwise af demagnetization over eight steps to 80 mT. Demagnetization directions for each specimen were calculated 

using principal component analysis (Kirschvink, 1980). 

Mean directions and statistics and, in most cases, mean poles 

to bedding attitudes for each site were calculated by the 

method of Fisher (1953). 

STRATIGRAPHIC RELATIONSHIPS 

PREVOLCANIC SEDIMENTARY ROCKS 

In central Lincoln County, most of the oldest Tertiary 

volcanic strata rest directly on the angular unconformity 

above Paleozoic rocks deformed along the Sevier orogenic 

belt (Armstrong, 1968), but in some places prevolcanic Tertiary sedimentary deposits intervene. The character of these 

sedimentary rocks is summarized here because interpretation 

of their depositional environment constrains interpretation of 

prevolcanic tectonism in this part of the Basin and Range 

province. We have not attempted to correlate them with 

established older prevolcanic Tertiary stratigraphic units 

such as the Claron Formation of latest Cretaceous(?) or Paleocene(?) into Oligocene age (Anderson and Rowley, 1975; 

Taylor, 1993) because we have no reliable age control. 

The oldest prevolcanic unit noted here may be a wellcemented, poorly sorted, grayish-pink to light-gray, boulder 

to pebble conglomerate consisting of well-rounded Paleozoic 

carbonate and quartzite clasts. About 100 to 150 m of this 

conglomerate occurs roughly conformably between the 23-

Ma Bauers Tuff Member of the Condor Canyon Formation 

and the Cambrian Highland Peak Formation 7 km northnorthwest of the Kane Springs Wash caldera on the western 

side of the Delamar Mountains (Ekren and others, 1977; 

Scott, Swadley, and others, 1990) (fig. 1 ). Tschanz and 

Pampeyan ( 1970) suggested that the conglomerate may be 
TERTIARY VOLCANIC STRATIGRAPHY, CENTRAL LINCOLN COUNTY, NEVADA 13 

Cretaceous to Tertiary in age, but they had no age control. The 

highly cemented character of the conglomerate may have 

resulted not from greater age but from contact metamorphism 

by an adjacent Miocene stock that also recrystallized the 

limestone and dolomite of the Highland Peak Formation to 

marble. Boulders 0.5 m across occur at the base, but higher 

in the sequence the conglomerate consists of 70 percent 

pebbles in a sand matrix. 

At the southern end of the Delamar Mountains and on 

the western side of the Meadow Valley Mountains, prevolcanic Tertiary conglomerate and subordinate sandstone, 

shale, and limestone locally overlie an angular unconformity 

above Cambrian through Pennsylvanian strata. On the eastem side of the Delamar Mountains about 6 km south of the 

Kane Springs Wash caldera, 60 m of pale-red conglomerate 

with a ferruginous, calcareous cement are interbedded with 

sandstone, shale, and limestone (Swadley and others, 1990). 

The sandstone and shale are thin to medium bedded and are 

cross bedded in part. As much as 75 m of a lithologically similar unit is locally present along the western side of the southern end of the Delamar Mountains (Page and others, 1990). 

This unit thins to 7 m of pale-red conglomerate overlain by 

3-7 m of very light gray, medium-grained, recrystallized 

lacustrine limestone 10 km to the north on the western side 

of the Delamar Mountains (Scott, Page, and Swadley, 1990). 

A very light gray to pinkish-gray, lacustrine limestone containing recrystallized disrupted algal plates is locally present 

at the base of the volcanic sequence in the Meadow Valley 

Mountains and is as much as 55 m thick (Scott, Harding, and 

others, 1991). 

All these prevolcanic sedimentary units that occur 

south of the Timpahute lineament (fig. 1) are relatively thin 

and discontinuous. In the absence of evidence of erosion or 

angular unconformities above these deposits, we consider 

their depositional basins to have been isolated from one 

another. We also conclude that the small thicknesses of conglomerates and lacustrine limestones are evidence that only 

relatively minor tectonic disruption of drainage occurred in 

the Tertiary before initiation of voluminous volcanism south 

of the Timpahute lineament. However, the angular unconformity above prevolcanic conglomerate in the North Pahroc 

Range suggests that pre-30.6-Ma extension (Taylor, 1989) 

probably affected the area north of the Timpahute lineament 

to a greater degree than areas south of the lineament. 

CALC-ALKALIC VOLCANIC ROCKS 

NEEDLES RANGE GROUP 

The Needles Range Formation was originally defined 

by Mackin (1960); this definition was elaborated by Best and 

others (1973). Subsequently, S.K. Grant (Best and Grant, 

1987) recognized that, in addition to the crystal-rich dacite 

tuffs that make up the greatest volume erupted from the 

Indian Peak caldera complex, rhyolite tuffs, which are 

chiefly found within the caldera complex, were also erupted 

from the same long-lived magma system. Thus, Best and 

Grant (1987) raised the Needles Range unit to group rank to 

include all dacitic and rhyolitic intracaldera and outflow 

deposits associated with the Indian Peak caldera complex 

(Best, Christiansen~ and Blank, 1989) (fig. 1 ). 

The three large-volume, crystal-rich, dacite ash-flow 

sheets of the Needles Range Group are (from oldest to 

youngest) the Cottonwood Wash Tuff, the Wah Wah Springs 

Formation, and the Lund Formation and are restricted to the 

northern part of Lincoln County, north of the Timpahute lineament. All three units are compositionally similar to the 

dacite tuff of the Monotony Tuff (fig. 3) and therefore are 

included as part of the so-called Monotony compositional 

type. (See Best, Christiansen, and others, 1989, fig. 6.) The 

dacite tuffs of the Needles Range Group and the Monotony 

Tuff are crystal rich, and contain dominant plagioclase and 

lesser amounts of quartz, biotite, hornblende, and pyroxene, 

but are petrographically distinguishable (fig. 4). The Cottonwood Wash is distinguished by the presence of uncommonly 

large books of biotite (as much as 8 mm in diameter), large 

em bayed quartz, and lesser amounts of hornblende, the Wah 

Wah Springs by the absence of sanidine but relative abundance of hornblende, and the Lund by the relative abundance 

of quartz and accessory sphene (fig. 4 ). In addition to these 

dacitic units, a relatively small volume rhyolite tuff unit, the 

Mackleprang Tuff Member of the Ryan Spring Formation, 

characterized by phenocrysts of plagioclase and biotite, 

occurs locally near the southern and western margin of the 

Indian Peak caldera complex between the Wah Wah Springs 

and Lund units. 

The Cottonwood Wash Tuff, Wah Wah Springs Formation, and Lund Formation have been mapped in the North 

Pahroc Range (Taylor, 1989; Scott, Swadley, and Byron, 

1992). In the field, the Cottonwood Wash Tuff is distinguished by the largest biotite books and a distinctive palegreenish-yellow to pale-olive color, and the Lund Formation 

has the smallest biotite books, which are not aligned as well 

to the plane of compaction as those of the other two units. All 

three units are separated from one another by tens of meters 

of interbedded boulder conglomerate and lacustrine limestone. The clasts in the conglomerate consist of Paleozoic 

dolomite, limestone, and quartzite boulders as much as 2 m 

across. The southernmost exposures of all these volcanic and 

sedimentary units lie just north of the Timpahute lineament 

(fig. 1) (Ekren and others, 1976), suggesting that the lineament may have presented a topographic barrier to the southward spread of ash-flow tuffs about 30-28 Ma (Best, 

Christiansen, and Blank, 1989). Locally, the Lund Formation is overlain by interbedded lacustrine limestones and 

conglomerate (0-60 m thick), similar to those between formations of the Needles Range Group except that the clast 

size is smaller and the conglomerate is less abundant. 
14 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

/ v 10 inclusions in g 8 D,P 

~ 1- Trachyte BB 

z :s 

L1J T (..) 

a: p; g~ L1J TN S.J s a.. 8 p S N 

1-

I 

(!) 

[ij 

;.: L L 

LW Rhyolite 

6 Gvv we 

0 w C'1 

= z 

+ Andesite Dacite 

0 N 4 ~ 

60 65 70 75 

Si02 (WEIGHT PERCENT) 

Figure 3. lUGS classification (Le Maitre, 1989) of ash-flow tuff units. Data from Best, Christiansen, and 

Blank (1989), Williams (1967), Phillips (1989), Novak ( 1984 ), Harding and others (this volume), and unpublished data of M.G. Best and R.B. Scott. G, upper cooling unit of Gregerson Basin Member of Kane 

Wash Tuff; g, lower cooling unit of Gregerson Basin Member of Kane Wash Tuff; V, Grapevine Spring 

Member of Kane Wash Tuff; U, Sunflower Mountain Tuff; D, Delamar Lake Tuff; H, Harmony Hills Tuff; 

P, Pahranagat Formation: B, Bauers Tuff Member of Condor Canyon Formation; T, Swett Tuff Member of 

Condor Canyon Formation; N, Leach Canyon Formation; S, Shingle Pass Tuff; I, Isom Formation; M, Monotony Tuff; L, Lund Formation; W, Wah Wah Springs Formation; C, Cottonwood Wash Tuff. For chemical 

data on the tuff of Etna and the Hiko Tuff, see Rowley and others (this volume). 

These four formations of the Needles Range Group were 

sampled for paleomagnetic character east of Ely Springs, 

Nev. Our results for the dacite units do not differ significantly 

from those of Shuey and others ( 1976) and Nairn and others 

( 197 5 ). We have also obtained supplementary paleomagnetic 

data for the Wah Wah Springs and Lund Formations at White 

River Narrows and Condor Canyon. The Cottonwood Wash 

Tuff has normal magnetic polarity with large between-site 

dispersion of paleomagnetic directions that generally are 

within 30° of the Oligocene axial dipole field direction. The 

Wah Wah Springs is reversely magnetized with low betweensite dispersion of directions and a magnetic inclination that is 

systematically about 25° shallower than the Oligocene axial 

dipole field direction. The Mackleprang Tuff Member is also 

reversely magnetized, but with a steep magnetic inclination; 

its direction is about 40° different from that of the Wah Wah 

Springs. The Lund has normal magnetic polarity with a large 

between-site dispersion of directions, and its direction is similar to that of the Cottonwood Wash Tuff, though a pair of 

geomagnetic reversals intervened between them. At Ely 

Springs and at White River Narrows, the Lund Formation 

underlies the reversely magnetized trachytic Petroglyph Cliff 

Ignimbrite; at Ely Springs another reversely magnetized trachytic tuff of unknown provenance occurs between them. 

Ages of the dacitic formations of the Needles Range 

Group are not precisely known, in spite of numerous K-Ar 

dates of the units (table 1). For example, although the average of 16 dates for the Wah Wah Springs Formation is 29.5 

Ma (Best and Grant, 1987), subsequent 40 Arf39 Ar dating of 

volcanic units older and younger than the Wah Wah Springs 

indicates that this date is too young, possibly by as much as 

1 million years (Best and others, 1993, table 1 ). 

PETROGLYPH CLIFF IGNIMBRITE OF COOK (1965) 

The type section of the Petroglyph Cliff Ignimbrite of 

Cook ( 1965) is at White Rock Spring, a few kilometers north 
TERTIARY VOLCANIC STRATIGRAPHY, CENTRAL LINCOLN COUNTY, NEVADA 15 

of White River Narrows on the eastern flank of the Seaman 

Range (fig. 1 ). Martin ( 1957) referred to these same rocks as 

the White Rock Spring ignimbrite unit. The Petroglyph 

Cliff Ignimbrite is a trachytic(?) ash-flow tuff that contains 

10-15 percent phenocrysts consisting of about 80 percent 

plagioclase and 20 percent clinopyroxene and orthopyroxene. This unit has the phenocryst mineralogy and thin, highly 

welded ash-flow tuff morphology typical of the Isom Formation (described in a following section), but is unusual among 

Isom compositional-type ash-flow tuffs in containing a high 

concentration of apparently cognate lapilli and blocks (Best, 

Christiansen, and others, 1989). Commonly, lapilli have 

been welded to large fiamme flattened parallel to the plane 

of foliation; where chilled they are black vitrophyres, and 

where devitrified they range from moderate orange pink to 

moderate red in a lighter colored matrix. 

At White Rock Spring, the Petroglyph Cliff Ignimbrite 

underlies the Monotony Tuff. We should point out. 

moreover, that at the Petroglyph Cliff itself, the Petroglyph 

Cliff Ignimbrite is present only below ground level; the 

petroglyphs were carved in the Monotony Tuff. The 

Petroglyph Cliff Ignimbrite is reversely magnetized both at 

White Rock Spring and Ely Springs (fig. 1 ), the only 

localities where we have identified it paleomagnetically. 

In the North Pahroc Range, three and probably four 

Isom compositional-type cooling units occur at the general 

stratigraphic position occupied by the Petroglyph Cliff Ignimbrite of Cook ( 1965), that is, above the Lund Formation 

and below the Monotony Tuff. In ascending order these are 

( 1) a lowest cooling unit, (2) the cooling unit that is lithologically correlative with the Petroglyph Cliff Ignimbrite at 

White Rock Spring, (3) a vitrophyric cooling unit, and ( 4) a 

devitrified cooling unit. The lowest, the vitrophyric, and the 

devitrified cooling units have been recognized only locally 

in the North Pahroc Range. The lowest cooling unit ( 1) 

resembles the Petroglyph Cliff Ignimbrite (2) except for a 

greater abundance of large fiamme; the lowest cooling unit 

(1) is present only in the central part of the Wheatgrass 

Spring quadrangle, where it is separated from the overlying 

Petroglyph Cliff (2) by a bedded tuff. The largely vitrophyric 

cooling unit (3) differs from the Petroglyph Cliff (2) by containing distinctly smaller pumice fiamme and by having no 

cognate blocks. The vitrophyric cooling unit is locally 

present in both the Pahroc Spring and the Wheatgrass Spring 

quadrangles. The devitrified cooling unit (4) is lithologically 

indistinguishable from the Petroglyph Cliff Ignimbrite (2) 

but is separated from the Petroglyph Cliff (2) by a local 

andesite flow in the northern part of the Wheatgrass Spring 

quadrangle. The abundance of cooling units in the North 

Pahroc Range that closely resemble the Petroglyph Cliff Ignimbrite suggests that the source of these ash-flow tuffs is 

probably close by. Possible petrological associations with 

the Petroglyph Cliff Ignimbrite remain to be investigated. 

In both the Wheatgrass Spring and Pahroc Spring quadrangles a very light gray to medium-light-gray nonwelded 

tuff containing abundant unflattened pumice fragments commonly overlies the Petroglyph Cliff Ignimbrite. This nonwelded tuff definitely is not petrologically associated with 

the Petroglyph Cliff Ignimbrite because the nonwelded tuff 

has significantly lower rare earth element, zirconium, iron, 

and strontium abundances than the Isom compositional-type 

tuffs. The pore spaces in the nonwelded tuff commonly have 

been partly filled by calcite, probably derived from the depositional environment of thin lacustrine limestones, which 

occur above and below the nonwelded tuff (Scott, Swadley, 

and Byron, 1992). 

As previously mentioned, at Ely Springs (fig. 1 ), below 

the Petroglyph Cliff and an underlying lava, another 

unnamed trachytic tuff occurs above the Lund Formation; 

whether this oldest trachytic tuff is associated petrologically 

with the Petroglyph Cliff Ignimbrite or Isom compositionaltype tuffs also remains to be studied. If future investigations 

confirm that all these units are indeed related petrologically, 

the eruptive events related to the emplacement of the Petroglyph Cliff Ignimbrite may be considerably more complex 

than previously suspected. 

MONOTONY TUFF 

This crystal-rich dacite ash-flow tuff (figs. 3, 4) was 

referred to as the tuff of Monotony Valley by Cornwall 

( 1972) and was defined as the Monotony Tuff by Ekren and 

others ( 1971 ), who noted its widespread and thick occurrence in central Nye and northwestern Lincoln Counties. 

They suggested that the unit has a volume of more than 2,900 

km3 and a source in the southern Pancake Range. Subsequent geologic mapping in that area, summarized in Ekren 

and others (1974; see especially p. 607), disclosed segments 

of a northern caldera wall, labeled the Pancake Range caldron by Ekren and others ( 1972). Sargent and Roggensack 

( 1984) referred to the Pancake Range caldera, but did not 

associate it with the Monotony Tuff. Best and others ( 1992; 

see also Best, Christiansen, and others, 1989, p. 114) subsequently accepted the Pancake Range caldera in the Central 

Nevada caldera complex as the source area for the Monotony, somewhat modified its boundary and relation to other 

calderas, and assigned an age of 27.3 Ma to the Monotony 

Tuff (table 1). 

The Monotony compositional type is, of course, named 

after the Monotony Tuff and is represented by the 

compositional variation of the Monotony Tuff and the dacite 

tuffs of the Needles Range Group in figure 3. (Also see Best, 

Christiansen, and others, 1989, fig. 6.) The Monotony 

contains 20-60 percent phenocrysts that consist of 2-15 

percent quartz, 5-25 percent sanidine, 45-65 percent 

plagioclase, 5-20 percent biotite, 0-10 percent hornblende, 

0-10 percent clinopyroxene, and Fe-Ti oxides (fig. 4). Ekren 

and others (1971) noted the lithologic similarity of the 

Monotony Tuff to the Needles Range Group, but distinctions 
16 

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a: 

w 

a. 

w 

~ 

::> 

_J 

0 

> 

GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

Tuff of Etna 

(7) 

o~~--------~~--~--~ 

PI 

60 

40 

S a B H Px T 

I Kane Wash Tuff, 

Gregerson Basin Member, 

upper cooling unit 

(15) 

o~------------~--~--~ 

PI 

80 

60 

40 

20 

S a B He Fa T 

Kane Wash Tuff, 

Gregerson Basin Member, 

lower cooling unit 

(28) 

o~------------~--~--~ 

PI 

PI 

I 

Kane Wash Tuff, 

Grapevine Spring Member 

(24) 

S a B He Fa T 

Sunflower Mountain Tuff 

(xenocrystic PI, H, B, 

trace acmite) 

(22) 

S a B He Fa T 

Delamar Lake Tuff 

(xenocrystic H, B, 

trace acmite) 

(30) 

Hiko Tuff 

(+sphene) 

(28) 

L-----------~------~0 

PI 

I 

PI 

S a B H Px T 

Harmony Hills Tuff 

(40) 

S a B H Px T 

Pahranagat Formation 

(61) 

~--------~--~~----~0 

PI S a B He Fa T PI S a B He Fa T PI S a B H Px T 

Figure 4 (above and facing page). Modal composition (volume percent) of Oligocene and early Miocene ash-flow 

units. Number of samples encompassed in line-bar ranges is in parentheses after unit name. Data from Anderson and Rowley ( 1975), Siders and Shu bat ( 1986), Williams ( 1967), and unpublished data of M.G. Best, R.B. Scott, A. E. Harding, and 

R.E. Anderson. Phenocrysts include PI, plagioclase; S, sanidine; Q, quartz; B, biotite; H, hornblende; He, hedenbergite; 

Px, pyroxene; and Fa, fayalite. T, total phenocrysts relative to whole rock; tr, trace. 

between phenocrysts of the Monotony Tuff and the 

phenocrysts of the three dacite formations of the Needles 

Range Group have been made. In ascending order in the 

group, the lesser abundance of quartz and sanidine in the 

Cottonwood Wash Tuff, generally more hornblende than 

biotite and absence of sanidine in the Wah Wah Springs 

Formation, and the presence of trace amounts of sphene in 

the Lund Formation are distinctive from the phenocrysts of 

the Monotony Tuff (fig. 4 ). 

Although as many as three cooling units of the Monotony Tuff are known just northwest of the North Pahroc 

Range on the eastern and western side of the Grant Range, 

only one simple cooling unit occurs in central Lincoln 

County. In the southern Delamar Mountains (Swadley and 

others, 1990; Page and others, 1990; Scott, Page, and Swadley, 1990), the lowest ash-flow tuff in the section is the 

Monotony Tuff. This unit has a very distinctive magnetization direction, southeast and downward, about 80° away 
TERTIARY VOLCANIC STRATIGRAPHY, CENTRAL LINCOLN COUNTY, NEVADA 17 

80 

Condor Canyon Formation, Shingle Pass Tuff 

Bauers Tuff Member 

I 

{fayalite in 60 

I 

lower cooling unit) Lund Formation 

{+sphene) {49) {34) {16) 

40 

20 

en II 1- 0 tr? 0 en 

>- PI s a 8 H Px T PI s a 8 H Px T PI s a 8 H Px T 

a: 80 0 

0 Condor Canyon Formation, lsom Formation 

z (xenocrystic S, a, 8?) Q; Wah Wah Springs Formation 

w Swett Tuff Member {69) .0 (59) 60 I '-E 

(26) Q)Q) 

a.. ~:a: a>:::: 

1- ::2::::::1 _Iz ~1ij w 40 .!!1.~ 0 =a> 

.s:::..s:::. 

a: "0";" 

7ii.5 

w cocb 

a.. 0 20 J: 

II w 

~ II 

=> _J 0 0 

0 PI s a 8 H Px T PI s a 8 H Px T PI s a 8 H Px T 

> 

60 

Leach Canyon Formation 

(+sphene) 

{52) Monotony Tuff 

(13) 

Cottonwood Wash Tuff 

{7) 

40 

20 

0 L...___J. ______ _._ _ ___,o 

PI S a 8 H Px T PI S a 8 H Px T PI S a 8 H Px T 

from the reversed Oligocene dipole field direction. This 

same direction is present in the single Monotony cooling unit 

at White River Narrows (the tuff at the northernmost petroglyph site), and is also found in the Monotony Tuff at localities to the south and west of White River Narrows. At the 

Narrows, the Monotony Tuff underlies an Isom compositional-type tuff and overlies the type Petroglyph Cliff Ignimbrite as defined by Cook (1965). One Monotony cooling unit 

has been mapped above the Petroglyph Cliff Ignimbrite at 

the south end of the North Pahroc Range (Scott, Swadley, 

and Byron, 1992), and preliminary paleomagnetic data indicate that it is the same cooling unit. 

A new K-Ar date for biotite separated from the Monotony Tuff from the southern Delamar Mountains is 28.5±1.0 

Ma (H.H. Mehnert, written commun., 1990). This date falls 

within the error range of other dates for the Monotony Tuff 

given in table 1. 

ISOM FORMATION AND RELATIONSHIPS WITH OTHER 

ISOM COMPOSITIONAL-TYPE UNITS 

Generally densely welded, locally rheomorphic, trachytic ash-flow tuffs are the namesake of the so-called Isom 

compositional type, which is shown by the distribution of 

compositions of the Isom Formation in figure 3. (Also see 

Best, Christiansen, and others, 1989, fig. 6.) The tuffs are 

characterized by 4-20 percent phenocrysts that consist of 
18 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

Table 1. Published isotopic dates for ash-flow units, recalculated as necessary to new constants. 

[*, mineral pair from one sample. New or revised stratigraphic names are underlined. Errors are expressed as 2 sigma and dates have been recalculated as necessary to new constants. 

We conclude that an addition of 0.4 m.y. should be applied to dates reported by Novak (1984) because Novak's dates are consistently about 0.4 m.y. younger than dates for the 

same rocks reported by L.W. Snee and H.H. Mehnert. in this report] 

Unit Material Method Date (Ma) Reference 

dated 

Kane Wash Tuff, sanidine K-Ar 14.4 ± 0.6 Noble (1968). 

Gregerson Basin 

Mbr. upper 

cooling unit 

Gregerson Basin sanidine K-Ar 14.1 ± 0.4 Novak (1984). 

Mbr. 

Gra~evine S~ring sanidine K-Ar 14.0 ± 0.4 Novak (1984). 

Mbr. sanidine K-Ar 14.2 ± 0.4 Novak (1984). 

Sunflower sanidine K-Ar 14.7 ± 0.4 Novak (1984). 

Mountain Tuff 

Delamar sanidine K-Ar 15.5 ±0.4 Novak (1984). 

Lake Tuff sanidine K-Ar 15.8 ± 0.4 Novak (1984). 

Hiko Tuff *biotite/ K-Ar 18.1 ± 0.6 Armstrong (1970). 

hornblende 

mixture 

*plagioclase K-Ar 18.2 ± 1.0 Armstrong (1970). 

biotite K-Ar 18.4 ± 0.4 Armstrong (1970). 

sanidine K-Ar 20.1 ± 0.5 Noble and others 1968); 

Noble and McKee (1972). 

biotite 40Arf39Ar 18.5 ± 0.4 Taylor and others (1989). 

Harmony Hills biotite K-Ar 20.4 ± 0.5 Noble and McKee (1972). 

Tuff biotite K-Ar 20.3 ±0.4 Armstrong (1970). 

biotite K-Ar 21.9 ± 0.4 Armstrong (1970). 

biotite K-Ar 20.9 ± 0.4 Armstrong (1970). 

plagioclase K-Ar 21.5 ± 1.0 Armstrong (1970). 

Pahranagat biotite K-Ar 23.6 ± 1.0 Snyder and others (1972). 

Formation sanidine K-Ar 22.1 ± 1.0 Snyder and others (1972). 

sanidine K-Ar 23.4 ± 0.7 Ekren and others (1971). 

sanidine K-Ar 22.9 ± 0.7 Ekren and others (1971). 

sanidine K-Ar 21.8 ± 0.7 Ekren and others (1971). 

biotite K-Ar 23.8 ± 0.7 Ekren and others (1971). 

biotite K-Ar 21.5 ± 0.5 Ekren and others (1971). 

biotite K-Ar 20.0 ± 0.6 McKee and John (1987). 

biotite K-Ar 21.6 ± 0.6 Kleinhampl and Ziony (1985). 

sanidine 40Arf39Ar 22.64 ± 0.02 Deino and Best (1988). 

sanidine 40Arf39Ar 22.62 ± 0.03 Deino and Best (1988). 

sanidine 40Afj39Ar 22.66 ± 0.02 Deino and Best (1988). 

sanidine 40Afj39Ar 22.65 ± 0.02 Deino and Best (1988). 
TERTIARY VOLCANIC STRATIGRAPHY, CENTRAL LINCOLN COUNTY, NEVADA 19 

Table 1. Published isotopic dates for ash-flow units, recalculated as necessary to new constants-Continued. 

Unit Material Method Date (Ma) Reference 

dated 

Condor Canyon plagioclase K-Ar 25.0 ± 0.8 Fleck and others (1975). 

Formation, 

Bauers Tuff plagioclase K-Ar 22.7 ± 0.6 Fleck and others (1975). 

Member 

whole rock K-Ar 21.2 ± 0.5 Fleck and others (1975). 

whole rock K-Ar 23.5 ± 0.4 Fleck and others (1975). 

sanidine K-Ar 22.2 ± 0.4 Armstrong (1970). 

biotite K-Ar 21.9 ± 0.5 Armstrong (1970). 

biotite K-Ar 22.5 ± 0.9 Noble and McKee (1972). 

sanidine 40Ar/39Ar 22.78 ± 0.03 Best, Christiansen, and others (1989). 

Swett Tuff *biotite K-Ar 24.1 ± 0.5 Armstrong (1970). 

Member *plagioclase K-Ar 23.2 ± 1.7 Armstrong (1970). 

glass K-Ar 21.9 ± 0.4 Armstrong (1970). 

biotite K-Ar 23.9 ± 0.5 Armstrong (1970). 

*biotite K-Ar 23.9 ± 0.5 Armstrong (1970). 

*plagioclase K-Ar 25.4 ± 0.7 Armstrong (1970). 

Leach Canyon *biotite K-Ar 24.6 ± 0.5 Armstrong (1970). 

Formation *sanidine K-Ar 22.9 ± 0.5 Armstrong (1970). 

plagioclase 26.7 ± 1.0 Armstrong (1970). 

zircon fission 24.2 ± 2.0 Kowallis and Best (1990). 

track 

sanidine 40Arf39Ar 23.8 ± 0.05 Best and others (1993). 

Shingle Pass Tuff *biotite K-Ar 26.1 ± 0.6 Marvin and others (1973). 

*sanidine K-Ar 24.4 ± 0.6 Marvin and others (1973). 

biotite K-Ar 26.0 ± 0.7 Marvin and others (1973). 

*biotite K-Ar 28.1 ± 0.9 Marvin and others (1973). 

*split K-Ar 26.2 ± 1.0 Marvin and others (1973). 

of same 

*sanidine K-Ar 22.4 ± 0.7 Marvin and others (1973). 

*split K-Ar 23.1 ± 0.6 Marvin and others (1973). 

of same 

Upper member sanidine K-Ar 25.1 ± 1.0 Sargent and McKee (1969). 

sanidine 40Arf39Ar 26.00 ± 0.03 Best, Christiansen, and others ( 1989). 

intracaldera unit *sanidine 40Arf39Ar 26.2 ± 0.5 Taylor and others (1989). 

*biotite 40Arf39Ar 26.2 ± 0.5 Taylor and others (1989). 

sanidine K-Ar 25.8 ± 1.0 Sargent and McKee (1969). 

lower cooling unit sanidine 40Arf39Ar 26.68 ± 0.03 Best, Christiansen, and others (1989). 

Isom Formation, whole rock K-Ar 25.7 ± 0.5 Armstrong (1970). 

Baldhills Tuff plagioclase K-Ar 25.7 ± 0.4 Fleck and others (1975). 

Member 

Blue Meadows plagioclase K-Ar 25.9 ± 0.4 Fleck and others (19_75). 

Tuff Member 
20 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

Table 1. Published isotopic dates for ash-flow units, recalculated as necessary to new constants-Continued. 

Unit 

Monotony Tuff 

Needles Range 

Group, Lund 

Formation 

Wah Wah Springs 

Formation 

Cottonwood Wash 

Tuff 

Material 

dated 

biotite 

biotite 

biotite 

biotite 

biotite 

biotite 

biotite 

*biotite 

*hornblende 

sanidine 

unspecified, 

average of four 

analyses 

Method 

K-Ar 

K-Ar 

K-Ar 

K-Ar 

K-Ar 

K-Ar 

K-Ar 

40ArJ39Ar 

40ArJ39Ar 

40Ar/39Ar 

unspecified, 

average of sixteen 

analyses 

unspecified, 

average of four 

analyses 

70-80 percent plagioclase, 10 percent clinopyroxene and 

orthopyroxene, and minor magnetite (fig. 4 ); biotite, quartz, 

sanidine, and hornblende occur only in inconsistent trace 

amounts and may be xenocrystic. Considering its silica content (67-68 percent), the Isom (and the Isom compositional 

type) has relatively high Ti02 (0.8 percent), K20 (5.7 percent), Zr (500 ppm), Ba (1,500 ppm), and Rb (220 ppm) but 

is calc-alkaline (Best, Christiansen, and Blank, 1989), and 

therefore it is unlike much younger peralkaline trachyte 

magmas erupted from the Kane Springs Wash caldera and 

related younger centers. 

As originally defined by Mackin ( 1960) from sites west 

of Cedar City, Utah, and redefined by Anderson and Row ley 

(1975), the Isom Formation consists of three members, in 

ascending order, the Blue Meadows, Baldhills, and Hole-inthe-Wall Tuff Members. The oldest and youngest members 

are apparently simple tuff cooling units, whereas the 

Baldhills Tuff Member consists of several tuff cooling units 

and local lava flows. The Blue Meadows Tuff Member is 

confined solely to the Markagunt Plateau in south-central 

Utah. (See fig. 2 of Introduction to volume.) The younger 

two members, however, are widespread in the Great Basin 

and appear to have been erupted from an undetermined 

source just southeast of the Indian Peak caldera complex 

either from what is now the Escalante Desert (Anderson and 

Rowley, 1975) or from the mountains just to the west (Best, 

Date (Ma) 

28.1 ± 0.8 

28.5 ± 0.8 

27.7 ± 0.8 

26.8 ± 0.7 

29.2 ± 1.3 

27.1 ± "1.0 

29.1 ± 0.8 

27.1 ± 0.6 

26.7 ± 0.3 

27.31 ± 0.03 

27.9 

29.5 

30.6 

Reference 

Marvin and others (1973). 

Marvin and others (1973). 

Marvin and others (1973). 

Marvin and others (1973). 

Marvin and others (1973). 

Marvin and others (1973). 

Marvin and others (1973). 

Taylor and others (1989). 

Taylor and others (1989). 

Best, Christiansen, and others (1989). 

Best and Grant ( 1987). 

Best and Grant ( 1987). 

Best and Grant (1987). 

Christiansen, and others, 1989; Best, Christiansen, and 

Blank, 1989). But no caldera is exposed in these areas of 

widespread, thick, younger tuffs and lava flows. 

Published K-Ar dates for the Isom Formation range 

from 25.7 to 25.9 Ma (table 1). However, more precise 40 Ar/ 

39 Ar dates for the underlying Monotony Tuff and overlying 

Shingle Pass Tuff indicate that the age of the Baldhills Tuff 

Member of the Isom is between 27.3 and 26.7 Ma. A new KAr date for plagioclase from the Baldhills Tuff Member collected from the southern Delamar Mountains is 25.9±0.8 

Ma, compatible with these age constraints (H.H. Mehnert, 

written commun., 1990). The age of the Hole-in-the-Wall 

Tuff Member of the Isom is between 26.0 Ma, the age of the 

upper unit of the Shingle Pass Tuff, and 23.8 Ma, the age of 

the Leach Canyon Formation (table I and p. 24). 

Paleomagnetic data collected from stratigraphic 

sequences that demonstrate the relationships of the members 

of the Isom Formation with the members of the Shingle Pass 

Tuff and the Petroglyph Cliff Ignimbrite are shown in figure 

5. We sampled rocks from the type locality of the Isom Formation in the Bald Hills of southwestern Utah, at the canyon 

named Hole-in-the-Wall 3 km north of Mackin's ( 1960) designated type locality for the Isom. Here, the formation consists of four densely welded cooling units; the upper is 

Mackin's (1960) Hole-in-the-Wall Tuff Member, and the 

lower three are assigned to his Baldhills Tuff Member. The 
TERTIARY VOLCANIC STRATIGRAPHY, CENTRAL LINCOLN COUNTY, NEVADA 21 

Hole-in-the-Wall Tuff Member has reversed magnetic polarity and a southeast declination about 17° from the reversed 

axial dipole field direction. The lowest cooling unit of the 

Baldhills Tuff Member has normal polarity, but slightly 

shallow inclination and north-northeast declination, about 

35° from the normal axial dipole direction. The middle cooling unit of this member was too fractured to sample, and the 

uppermost cooling unit of this member has a magnetic direction similar to, but a shallower normal inclination than, the 

lowest cooling unit. These results are similar but not identical to those reported by Nairn and others (1975). 

In Condor Canyon, Cook ( 1965) originally showed in 

his columnar section a series of five tuffs underlying the 

Leach Canyon Formation and overlying a sequence of lava 

flows that in tum rest on the Needles Range Group. Later 

investigations (M.G. Best, unpub. data, 1981; G .J. Ax en, 

unpub. mapping, 1989) demonstrated that in the western part 

of the canyon (west of the Hamlight fault) at least seven Isom 

compositional-type welded tuffs are intercalated with the 

upper cooling unit of the Shingle Pass Tuff (fig. 5); all seven 

tuffs are contained within a thick sequence of mafic lava 

flows, rhyolitic tuffs, and minor sedimentary rocks (not 

shown in fig. 5). In the eastern part of the canyon (east ofthe 

Hamlight fault) in a repeated part of the section, at least four 

or five Isom compositional-type tuffs are present (fig. 5) 

beneath a thick mafic lava sequence. Axen showed "a probable upper member" of the Shingle Pass Tuff above the 

mafic lavas and below probable Blawn Formation in eastern 

Condor Canyon as shown in figure 5, but this unit was not 

sampled for paleomagnetic study. 

In both the eastern and western parts of Condor Canyon, 

the uppermost of the Isom compositional-type tuffs is 

reversely magnetized. This direction is nearly identical to 

that reported just previously in the type locality of the Holein-the-Wall Tuff Member. Moreover, to the west at White 

River Narrows, the upper unit of the Shingle Pass Tuff is 

directly overlain by a reversely magnetized Isom 

compositional-type tuff; this reversed direction is nearly 

identical to those in Condor Canyon and at Hole-in-the-Wall. 

From this evidence, it seems likely that the Hole-in-the-Wall 

Tuff Member extends from its type locality in southwestern 

Utah as far west as White River Narrows (fig. 5). Preliminary 

paleomagnetic data suggest that the Hole-in-the-Wall Tuff 

Member also occurs beneath the Leach Canyon Formation in 

the North Pahroc Range, as it does in Condor Canyon and at 

White River Narrows. We have not, however, found the 

Hole-in-the-Wall elsewhere, for example, in the south 

Delamar Mountains or in the area of Ely Springs. 

The paleomagnetic stratigraphy in normally magnetized 

Isom compositional-type tuffs in Condor Canyon is more 

complex than that described for other localities (fig. 5). Both 

east and west of the Hamlight fault, beneath the thick section 

of mafic lavas, the uppermost cooling unit has a shallow 

magnetic inclination; this direction, however, is both 

shallower and closer to due north than that at the top of the 

type locality of the Baldhills Tuff Member. Moving down 

section from this tuff, the paleomagnetic directions in the 

Isom compositional-type tuffs become progressively steeper, 

much more so than in units of the type Baldhills Tuff 

Member. West of the Hamlight fault, three Isom 

compositional-type tuffs occur beneath both the mafic lavas 

and the upper cooling unit of the Shingle Pass Tuff, whereas 

east of the Hamlight fault there are at least three and perhaps 

four. In each of these two nearby sections, the lowest Isom 

compositional-type cooling unit directly overlies the Lund 

Formation, but the two Isom compositional-type cooling 

units have quite different magnetization directions although 

both are normal and steeply inclined. 

Near Swiss Bob Well, about 8 km north-northeast of 

Ely Springs, two Isom compositional-type tuffs crop out; in 

this area they form the uppermost part of the Tertiary ashflow tuff section. The pattern of normal magnetization directions in these two tuffs is similar to that described for the 

lower tuffs at Hole-in-the-Wall in the Bald Hills, suggesting 

that these are also part of the Baldhills Tuff Member. 

In the southern Delamar Mountains, the Monotony Tuff 

at the base of the Tertiary volcanic section is overlain by an 

Isom compositional-type unit (Swadley and others, 1990; 

Page and others, 1990; Scott, Page, and Swadley, 1990). Its 

magnetization direction has normal polarity and shallow 

inclination, and although this direction is slightly shallower 

than that in the uppermost cooling unit of the type Baldhills 

Tuff Member, the two do not differ significantly. 

In summary, the time-progressions of magnetization 

directions in these lowest Isom compositional-type tuffs are 

similar at eastern and western Condor Canyon, Swiss Bob 

Well, and the Bald Hills, but Isom compositional-type tuffs 

in Condor Canyon are older than at the other two localities. 

On the paleomagnetic evidence illustrated in figure 5, 

however, it seems reasonable to designate them all as the 

Baldhills Tuff Member of the Isom Formation, including the 

single trachytic tuff in the southern Delamar Mountains. 

Further complicating the stratigraphy of the Isom 

compositional-type tuffs in Condor Canyon is the occurrence 

west of the Hamlight fault of three more Isom compositionaltype tuffs overlying mafic lavas and the upper member of the 

Shingle Pass Tuff and underlying the reversely magnetized 

Hole-in-the-Wall Tuff Member (fig. 5). These three tuffs are 

normally magnetized, with directions that are near the 

Oligocene axial dipole field direction. The same tuff 

sequence has been mapped east of the Hamlight fault, but 

there the exposure is insufficient for paleomagnetic 

sampling, and only one of these three Isom compositionaltype tuffs may be present there. Because these tuffs are 

stratigraphically within the Isom Formation as we have 

identified it, and have the same trachytic composition, we 

consider them to form a new member of the Isom and here 

informally name them the tuff member of Hamlight Canyon 

after the nearby northern tributary to Condor Canyon. 
Shingle 

Spring 

I Bauers] 

South 

Delamar 

Mts 

White 

River 

Narrows 

Ely 

Springs 

Condor 

Canyon 

West 

Condor 

Canyon 

East 

Bald 

Hills 

!Leach Canyon! !Leach Canyon! !Leach Canyon! ~~~Leach Canyon! !Leach Canyon! 

N N N N 

!*laval 

I trachytic tuff I 

0 ~r------------~ 

Wah Wah Springs 

N 

N 

0 

~ t""' 

0 

8 

n 

(/) 

§ 

m 

5'1 

to 

~ 0 

(/) 

--3 

c 

0 

>-<: 

~ 

~=3 

-\0 

\0 

N 
TERTIARY VOLCANIC STRATIGRAPHY, CENTRAL LINCOLN COUNTY, NEVADA 23 

These stratigraphic relationships are supported by 

detailed mapping and preliminary paleomagnetic data in the 

North Pahroc Range, where the Hole-in-the-Wall Tuff Member overlies two or three Isom compositional-type cooling 

units that may be correlative with the tuffs of Hamlight Canyon; these Isom compositional-type cooling units in turn 

overlie several mafic lavas, a tuff lithologically similar to the 

Blawn Formation, and two members of the Shingle Pass 

Tuff. In many localities, the cooling units proposed to be correlative with the tuffs of Hamlight Canyon are not present 

below the Hole-in-the-Wall Tuff Member. Below the Shingle Pass Tuff, the Baldhills Tuff Member overlies the 

Monotony Tuff (Scott, Swadley, and Byron, 1992). 

The stratigraphically highest Isom compositional-type 

ash-flow tuff in central Lincoln County occurs between the 

Swett and Bauers Tuff Members of the Condor Canyon Formation in the North Pahroc Range (Scott, Swadley, and 

Byron, 1992) and between the Leach Canyon Formation and 

overlying Bauers Tuff Member in the northeastern part of 

the South Pahroc Range (Scott and Swadley, 1992). At least 

two cooling units are present locally. Where the Swett Tuff 

Figure 5 (facing page). Tilt-corrected site-mean paleomagnetic 

directions for ash-flow tuffs of the Isom Formation, Shingle Pass 

Tuff, Petroglyph Cliff Ignimbrite, and Monotony Tuff. Locations of 

stratigraphic sections are shown in figure I ; the Bald Hills are in 

southwestern Utah, 20 km northwest of Cedar City. (See fig. 2 of 

Introduction to volume.) Solid circles are directions of normally polarized rocks on lower hemispheres of equal-area projections; open 

circles are directions of reversely polarized rocks on upper hemispheres. Confidence intervals are not shown. For the Shingle Spring 

section, lines connect directions at each of two sites in both cooling 

units. For the Baldhills Tuff Member and tuff member of Hamlight 

Canyon of the Isom Formation, numbers indicate the sequence of 

directions in multiple cooling units in ascending order, where 

present at a site. Stratigraphic positions of ash-flow sheets that enclose the Petroglyph Cliff Ignimbrite and the Isom Formation are 

indicated by names in boxes, as are intervening volcanic units; 

those units that have no paleomagnetic data are marked by asterisks. For the Ely Springs section, wavy line, angular unconformity. 

Bauers, Bauers Tuff Member of the Condor Canyon Formation; 

Leach Canyon, Leach Canyon Formation, undivided; upper, intermediate, and lower Shingle Pass, corresponding cooling units 

of the Shingle Pass Tuff; Hole-in-the-Wall, Hamlight Canyon, 

and Baldhills, Hole-in-the-Wall Tuff Member, tuff member of 

Hamlight Canyon, and Baldhills Tuff Member of the Isom Formation; Monotony, one cooling unit of the Monotony Tuff; Hancock 

Summit(?), informally named ash-flow tuff of Hancock Summit in 

White River Valley; Blawn(?), unnamed tuff unit tentatively correlated with the Blawn Formation; Petroglyph Cliff, densely welded 

cooling unit of the Petroglyph Cliff Ignimbrite; Lund, tuff member 

of the Lund Formation; Mackleprang(?), tentatively identified as 

the Mackleprang Tuff Member of the Ryan Spring Formation; Wah 

Wah Springs, outflow tuff member of the Wah Wah Springs Formation. Lines between columns indicate stratigraphic correlations; 

queried where probable. 

Member and Harmony Hills Tuff are absent along the northern part of the east-facing cliffs of the South Pahroc Range, 

this Isom compositional-type cooling unit is nearly 100 m 

thick; this abnormal thickness for Isom compositional-type 

tuffs may be related to a nearby source, but no caldera has 

been recognized. Where this Isom compositional-type cooling unit occurs in the well-exposed section northwest of Pahroc Spring, preliminary paleomagnetic data indicate that it 

has normal polarity; we are here giving it the informal name 

''tuff of Pahroc Valley." 

It is evident from the foregoing discussion that the Isom 

Formation and the upper member of the Shingle Pass Tuff 

are roughly coeval, although several geomagnetic reversals 

occurred between their eruptions. Preliminary paleomagnetic data from the North Pahroc Range together with the 

correlations suggested here imply that both the upper and 

lower members of the Shingle Pass Tuff were erupted after 

the Baldhills Tuff Member and before the tuff member of 

Hamlight Canyon, both members of the Isom Formation. 

The time during which all Isom compositional-type magmas 

were erupted covers a much greater period, between about 

27.9 Ma (age of the Lund Formation) and 22.8 Ma (age of 

the Bauers Tuff Member of the Condor Canyon Formation), 

or roughly 5 m.y. 

SHINGLE PASS TUFF 

At Shingle Spring in the Egan Range, Cook ( 1965) 

assigned the name "Shingle Pass Ignimbrite" to a rhyolitic 

tuff (fig. 3) overlying a dacite tuff that we now recognize as 

the outflow tuff member of the Wah Wah Springs Formation. Cook ( 1965) found the Shingle Pass over a wide area of 

southeastern Nevada, but subsequent work showed it to be 

distributed over an even larger area of 40,000 km2 (Best, 

Christiansen, and others, 1989, fig. Rl9) around a probable 

source area in the Quinn Canyon Range (Sargent and 

Roggensack, 1984; Best and others, 1992). Three additional 

rhyolitic ash-flow tuffs overlying the Shingle Pass at Shingle 

Spring were indicated but unnamed by Cook ( 1965, figs. 5, 

7). The rocks of the upper two rhyolite units we now recognize (Best, Christiansen, and others, 1989) as the Bauers 

Tuff Member of the Condor Canyon Formation and the outflow tuff member of the Pahranagat Formation (discussed 

later). Work by several U.S. Geological Survey geologists in 

and around the Central Nevada caldera complex has disclosed as many as three rhyolite cooling units that they have 

designated as the Shingle Pass Tuff (for example, Ekren and 

others, 1971 ); two of these cooling units are petrographically 

and paleomagnetically correlative with the two rhyolitic 

tuffs between the Wah Wah Springs Formation and Bauers 

Tuff Member at Shingle Spring (Best, Christiansen, and others, 1989) and are widespread over much the same part of 

southeastern Nevada around the apparent Quinn Canyon 

Range source area in the Central Nevada caldera complex. 
24 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

The lower of the two cooling units at the type section of 

the Shingle Pass Tuff contains sanidine and lesser plagioclase, but is petrographically unique among Basin and Range 

tuffs because of its mafic phenocryst assemblage of two 

pyroxenes, fayalite-rich olivine, amphibole, magnetite, 

ilmenite, allanite, and xenocrystic(?) biotite as well as its 

corroded and Christmas-tree-like quartz phenocrysts (fig. 4). 

The upper cooling unit, in contrast, has more plagioclase 

than sanidine, little or no quartz, no olivine or ilmenite, and 

biotite as the dominant mafic phenocryst; the upper cooling 

unit also has amphibole and xenocrystic pyroxenes. At Shingle Spring, the lower cooling unit is normally magnetized 

and the upper cooling unit is reversely magnetized; the directions are from 15° to 30° away from the respective axial 

dipole field directions, and hence they are not particularly 

diagnostic. 

Intercalated within the Isom Formation at White River 

Narrows and in western Condor Canyon is a single cooling 

unit of the Shingle Pass that is reversely magnetized, with 

approximately the same direction as the upper cooling unit at 

Shingle Spring. Preliminary paleomagnetic data from the 

North Pahroc Range indicate that both of the Shingle Pass 

cooling units found at Shingle Spring may be present there, 

having the same stratigraphic relationship to Isom compositional-type tuffs. These stratigraphic relationships have been 

confirmed during mapping in the North Pahroc Range 

(Scott, Swadley, and Byron, 1992). We have not, however, 

found the Shingle Pass Tuff in the area of Ely Springs. In the 

southern Delamar Mountains, a single Shingle Pass cooling 

unit overlies the Isom compositional-type tuff and underlies 

the Leach Canyon Formation (Swadley and others, 1990). 

This tuff is normally magnetized, having the same direction 

as the lower cooling unit at the type section, but petrographically it more closely resembles the upper cooling unit at the 

type section, so that we refer to it as a so-called intermediate 

Shingle Pass cooling unit. 

Sandwiched between the upper and lower cooling units 

of the Shingle Pass Tuff in the North Pahroc Range, a rhyolitic ash-flow tuff is tentatively correlated with the tuff of 

Hancock Summit, which is exposed at the White River Narrows section (fig. 5). This tuff is relatively phenocryst- and 

quartz-rich compared to the adjacent Shingle Pass cooling 

units; preliminary petrographic data indicate that the cooling 

unit contains about 25 percent phenocrysts that consist of 

about 40 percent quartz, 25 percent sanidine, 25 percent plagioclase, 10 percent biotite, minor Fe-Ti oxides, and common accessory zircon (Scott, Swadley, and Byron, 1992). 

Between the upper cooling unit of the Shingle Pass Tuff 

and the Hole-in-the-Wall Tuff Member of the Isom 

Formation in the North Pahroc Range, a nonwelded, nearly 

white, rhyolitic ash-flow tuff underlies a series of basalt 

flows (Scott, Swadley, and Byron, 1992). Preliminary 

petrographic data suggest that this nonwelded tuff is 

distinctive by having relatively high abundances of subequal 

biotite and hornblende and by abundances of quartz greater 

than those of plagioclase, which in turn are greater than those 

of sanidine. This nonwelded tuff is tentatively correlated 

with tuffs of the Blawn Formation noted in Condor Canyon 

(fig. 5). 

LEACH CANYON FORMATION 

The Leach Canyon Formation was originally named by 

Mackin (1960; see also Cook, 1965). Anderson and Rowley 

(1975) have adopted the Narrows Tuff Member and the 

overlying Table Butte Tuff Member as subdivisions of the 

Leach Canyon Formation and described the formation as the 

oldest in the Quichapa Group, following the terminology 

used by Williams (1967). The two members have similar 

phenocryst abundances (fig. 4). Total phenocrysts range 

from about 25 to 30 percent of the rock, and phenocrysts 

consist of 20-50 percent quartz, 10-30 percent sanidine, 

20-60 percent plagioclase, as much as 15 percent biotite, and 

trace amounts of hornblende, pyroxene, and, most characteristic of the formation, accessory sphene. Abundant concentrations of pumice lapilli and small dark volcanic lithic 

fragments characterize the Table Butte Tuff Member. 

The Leach Canyon Formation occurs in the same stratigraphic position as older units of the petrographically similar 

but sphene-free tuffs of the Blawn Formation, which we 

have not sampled for paleomagnetic study. Blawn eruptions 

were of small volume and did not produce known calderas; 

numerous vents are marked by rhyolite lava flows and lava 

domes in an east-northeast-trending zone extending from 

Pioche, Nev., to Milford, Utah (70 km north of Cedar City, 

Utah) (Best and others, 1987). Although some of the tuff 

shown by Williams as the Leach Canyon Formation ( 1967, 

fig. 9; compare Best, Christiansen, and others, 1989, fig. 

R40) near Milford is probably the Blawn Formation instead, 

Williams' isopach map nonetheless indicates a source for the 

Leach Canyon in the Caliente caldera complex. However, 

mapping has not yet disclosed any surface manifestation of 

an appropriate fault -bounded caldera and, like that of the 

Isom Formation, the source of the Leach Canyon Formation 

appears to be concealed beneath younger deposits. 

The average K-Ar age of the Leach Canyon Formation 

is 24.6 Ma, but the determinations range widely (table I); an 

average age of 23.8 Ma, determined on a coexisting sanidine 

and biotite pair (Armstrong, 1970), provides a better estimate of the age. This has recently been confirmed emphatically by the 40 Art39 Ar date of 23.8 Ma reported by Best and 

others (1993). 

Paleomagnetic sampling has been done in both members of the Leach Canyon Formation at seven localities: 

White River Narrows (Narrows Tuff Member), the North 

Pahroc Range (Narrows Tuff Member, preliminary results 

only), western and eastern Condor Canyon (both members), the southern Delamar Mountains (one undefined 

member), and at Table Butte in the Escalante Desert of 
TERTIARY VOLCANIC STRATIGRAPHY, CENTRAL LINCOLN COUNTY, NEVADA 25 

southwestern Utah (Table Butte Tuff Member). At all these 

localities, the Leach Canyon overlies either the Hole-inthe-Wall Tuff Member of the Isom Formation or, if this 

unit is absent, one or another of the cooling units of the 

Shingle Pass Tuff. All these sampling sites are reversely 

magnetized and have roughly the same paleomagnetic 

direction: east to east-southeast declination, and inclination 

between -35° and -60°. Neither the between-member nor 

the between-locality differences show any consistent pattern. Nairn and others (1975) had difficulty obtaining consistent paleomagnetic directions from the Leach Canyon 

Formation, and we conclude that the rock is a less than 

optimal magnetic recorder, perhaps because of the abundance of lithic fragments in it. 

The Leach Canyon Formation is widespread in central 

Lincoln County (Best, Christiansen, and others, 1989, fig. 

R40). It is present in all the quadrangles shown in figure 2 

except along the western side of the Delamar Mountains 

(Gregerson Basin quadrangle north to Pahroc Spring SE 

quadrangle). 

CONDOR CANYON FORMATION 

The Condor Canyon Formation was named by Cook 

(1965) to include two ash-flow tuff members, whose rocks 

were originally named the Swett Tuff and the overlying 

Bauers Tuff by Mackin (1960). Anderson and Rowley 

(1975) included the formation in the Quichapa Group, following the nomenclature used by Williams in his dissertation 

(1967). Anderson and Rowley also slightly revised Mackin's 

unit names when they adopted the Swett Tuff Member and 

the Bauers Tuff Member of the Condor Canyon Formation. 

Williams (1967) postulated that the source of the ash-flow 

tuffs of the Condor Canyon Formation lay in the Caliente 

area. Both members are largely densely welded biotite rhyolite, and are distinctive because neither contains phenocrysts 

of quartz (figs. 3, 4 ). The two members contrast petrographically: in addition to biotite and plagioclase common to both, 

the Bauers contains sanidine and a trace of clinopyroxene, 

whereas the Swett does not contain either. Average K-Ar 

ages of the two members differ by 1 million years (table 1); 

a more precise 40Ar/39Ar date of 22.78 Ma on the Bauers is 

in good agreement with its average K-Ar age of 22.7 Ma. 

Paleomagnetic sampling has been done in the Swett Tuff 

Member at four localities: (1) White River Narrows, where 

the unit is little more than a thin vitrophyre; (2) North Pahroc 

Range; (3) western Condor Canyon, where the unit is a 

relatively thick compound cooling unit; and (4) Eightmile 

Hills, 10 km northeast of Mackin's ( 1960) type locality at the 

western end of the Swett Hills in southwestern Utah. At White 

River Narrows and in western Condor Canyon, the Swett Tuff 

Member is reversely magnetized, but directions at the two 

sites differ by about 18° and are 20° or less from the reversed 

Oligocene axial dipole field direction. Examination of 

preliminary data from the North Pahroc Range confirms the 

reversed polarity. Gose ( 1970) obtained anomalous 

directions from the Swett Tuff Member exposed on the north 

wall of eastern Condor Canyon; our new data are a good 

match to those that he obtained from the southern wall in the 

same area. The direction reported by Nairn and others ( 1975) 

for the Swett Tuff Member at Table Butte in southwestern 

Utah corresponds well to our data. (See fig. 2 in the 

Introduction of volume for location.) However, in the 

Eightmile Hills 27 km to the southeast of Table Butte, we find 

that the Swett Tuff Member is almost completely overprinted 

magnetically by the overlying Bauers Tuff Member, but af 

demagnetization of one of eight specimens resulted in a 

trajectory of directions that intersects the reversed direction 

found in western Condor Canyon. The Swett is not found in 

the South Pahroc Range, southern Delamar Mountains, the 

Meadow Valley Mountains, or the Ely Springs area. The 

assumed source of the Swett is the Caliente caldera complex. 

The outflow sheet of the Bauers Tuff Member is almost 

everywhere a simple cooling unit. Pumice lumps increase in 

size closer to the source in the Caliente caldera complex. The 

source of the Bauers is the Clover Creek caldera in the Caliente caldera complex (Rowley and Siders, 1988; Rowley and 

others, this volume). The upper part of the unit in many 

places, as well as distal parts of the sheet, is more crystal 

poor relative to the lower part of the tuff and contains no 

large pumice fragments and smaller and less abundant phenocrysts than the lower part. 

The Bauers Tuff Member has exceptionally closely 

grouped paleomagnetic directions. We have sampled it at six 

localities: White River Narrows (top and base), North Pahroc Range (preliminary data only), southern Delamar Mountains (the main part and the thin vitric upper unit), eastern 

Condor Canyon, Shingle Spring in the Egan Range, and in 

Leach Canyon in southwestern Utah, 10 km southwest of 

Mackin's (1960) type locality at Bauers Knoll. It has been 

mapped throughout central Lincoln County (figs. 1, 2) 

except along the western side of the Delamar Mountains in 

the Gregerson Basin quadrangle. The Bauers Tuff Member 

is absent near Ely Springs; at our other sampling sites it overlies either the Swett Tuff Member at White River Narrows 

and Condor Canyon, an Isom compositional-type ash-flow 

tuff in the North Pahroc Range, the Leach Canyon Formation 

in the southern Delamar Mountains and Leach Canyon, or 

the Shingle Pass Tuff at Shingle Spring. The Bauers Tuff 

Member is normally magnetized; its paleomagnetic direction 

is within 1 oo of the Oligocene axial dipole field direction, 

and hence is quite ordinary. Our data from the Bauers Tuff 

Member match well the results of Nairn and others (1975) 

from other localities in southwestern Utah, including at 

Bauers Knoll and Table Butte, but our data do not match the 

direction for eastern Condor Canyon reported by Gose 

(1970) and quoted by Nairn and others (1975); the reason for 

the difference is not known. 
26 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

PAHRANAGAT FORMATION 

Although Deino and Best ( 1988) and Best, 

Christiansen, and others (1989) provided the first published 

account of the age, correlation, and description of the 

Pahranagat Lakes Tuff, more than 20 years earlier Williams 

(1967) first recognized these rocks as a simple cooling unit 

of rhyolite ash-flow tuff lying between the Bauers Tuff 

Member of the Condor Canyon Formation and the Harmony 

Hills Formation in exposures west of the Pahranagat Lakes 

in the Pahranagat Range in southeastern Nevada (figs. 1, 3). 

Williams ( 1967) placed this rhyolite ash-flow tuff within the 

Quichapa Group, but it is not included in the Quichapa 

Group as defined by Anderson and Rowley (1975) or by 

Rowley and others (1991). Williams described the 

occurrence of this rhyolite ash-flow tuff in White River 

Narrows and indicated that the unit extended as far northeast 

as Condor Canyon (however, at this locality we believe that 

the thin tuff lying between the Bauers and the Harmony Hills 

is a local tuff belonging to the Blawn Formation and not to 

the Pahranagat Lakes Tuff). Williams proposed that the 

source for the unit might lie just west of Pahranagat Lakes, 

but conceded that data were inconclusive. 

Based on precise single-crystal 40 Arf39 Ar dates (Deino 

and Best, 1988; A.L. Deino, unpub. data, 1992) and paleomagnetic and petrographic data, our study reveals a widespread distribution of the simple cooling unit of this rhyolite 

ash-flow tuff to the west and north of Pahranagat Lakes 

where it had been originally recognized. However, in the 

northern and western parts of its distribution, this petrographically, paleomagnetically, and chronologically similar 

cooling unit has been referred to locally as (1) the tuff of 

White Blotch Spring, (2) the granite-weathering tuff, (3) the 

tuff of Kawich Range and the tuff of Kawich caldera, and ( 4) 

the tuff of Saulsbury Wash. 

1. The tuff of White Blotch Spring (Ekren and others, 

1971) consists of two outflow cooling units at White Blotch 

Spring between the Groom and Belted Ranges; correlative 

tuffs were mapped in the Cactus, Kawich, northern Belted, 

and southern Reveille Ranges (fig. 1 ). Our paleomagnetic 

and chronologie data show that only the upper of the two 

cooling units at White Blotch Spring is equivalent to the 

Pahranagat Lakes Tuff; the lower unit, although petrographically similar, is about 3 m.y. older and has a distinctly different remanent magnetization. 

2. The granite-weathering tuff consists of an outflow 

cooling unit in the southern Hot Creek and Pancake Ranges 

(Quinlivan and Rogers, 1974; Ekren, Hinrichs, and Dixon, 

1972; and Ekren, Rogers, and Dixon, 1973; Snyder and others, 1972) (fig. 1 ). We conclude that this outflow cooling unit 

is also equivalent to the Pahranagat Lakes Tuff. 

3. Originally Rogers and others (1967) tentatively 

designated two petrographically similar cooling units in the 

Kawich and southern Reveille Ranges as the tuff of Kawich 

Range (fig. I). Ekren and others (1971) then superseded the 

name "tuff of Kawich Range" with the tuff of White Blotch 

Spring. In a reconnaissance map of the northern Kawich 

Range, within the Kawich caldera, Gardner and others 

( 1980) designated the intracaldera tuff facies as the tuff of 

Kawich caldera (which they included in the tuff of White 

Blotch Spring) and the overlying intracaldera tuff facies as 

the tuffs of Kawich Peak; Sargent and Roggensack (1984) 

adopted the terminology of Gardner and others (1980). We 

now believe this sequence of cooling units associated with 

the Kawich caldera in the Kawich and southern Reveille 

Ranges to be the intracaldera tuff facies equivalent of the 

outflow tuff facies of Pahranagat Lakes Tuff. 

4. Kleinhampl and Ziony (1985, p. 122; see also 

Whitebread, 1989) designated the outflow tuff facies sheet 

in the southernmost Monitor Range as the tuff of Saulsbury 

Wash and noted its petrographic similarity to the tuff of 

White Blotch Spring (fig. 1 ). We conclude that the tuff of 

Saulsbury Wash is also equivalent to the Pahranagat Lakes 

Tuff. 

The Pahranagat Formation is defined here to consist of 

two facies: ( 1) an intracaldera tuff facies restricted to within 

the Kawich caldera (consisting of rocks previously assigned 

to the tuff of Kawich Range, the tuff of Kawich caldera, the 

tuffs of Kawich Peak, and part of the tuff of White Blotch 

Spring) and (2) a widespread outflow tuff facies that is also 

called the outflow sheet (consisting of rocks previously 

assigned to the Pahranagat Lakes Tuff, the upper cooling 

unit of the tuff of White Blotch Spring at White Blotch 

Spring, the granite-weathering tuff, and the tuff of Saulsbury 

Wash). We follow Anderson and Rowley (1975) and Rowley and others (1991) by not including the Pahranagat Formation in the Quichapa Group as used by Williams ( 1967) 

because its rocks did not originate from the same long-lived 

magma system marked by the Caliente caldera complex, 

which was the source of at least the Bauers Tuff Member of 

the Condor Canyon Formation of the Quichapa (Rowley and 

others, this volume). 

The Pahranagat Formation takes its name from the 

Pahranagat Lakes and the East Pahranagat and Pahranagat 

Ranges south and west of Alamo, Nev., where good exposures of the outflow tuff facies (or outflow sheet) are abundant. The type section of the Pahranagat Formation is 

designated in its outflow sheet in the East Pahranagat Range 

just north of Medsger Pass at lat 37°15'18" N., long 

115°13'22" W., in the Alamo 7.5-minute quadrangle. Here, 

the outflow sheet is about 45 m thick, overlies the Bauers 

Tuff Member of the Condor Canyon Formation, and is overlain by the Harmony Hills Tuff. The Pahranagat Formation 

is a pumice-rich, generally pale purple to pinkish-gray tuff 

that contains approximately equal amounts of phenocrysts of 

embayed quartz (commonly as much as 5 mm in diameter), 

sanidine, and commonly intensely corroded plagioclase (fig. 

4 ); small phenocrysts of biotite increase in abundance 

upwards and are joined by sparse hornblende and a trace of 

clinopyroxene near the top of the unit. As much as one-third 
TERTIARY VOLCANIC STRATIGRAPHY, CENTRAL LINCOLN COUNTY, NEVADA 27 

of the rock consists of white, light-gray to grayish-orangepink pumice fragments. In the lowest 4 m of the partially 

welded basal part of the unit, pumice chunks are as much as 

25 em in diameter; in the overlying 40 m of moderately 

welded tuff, the pumice is lapilli- and small block-size; and 

in the top meter or so of partially welded tuff, pumice is as 

large as in the basal part. A reference section for the outflow 

sheet is located at the southern end of White River Narrows 

(see Williams, 1967, p. 81) on the west side of Nevada State 

Highway 318 in the White River Narrows 7 .5-minute quadrangle at lat 37°50'17" N., long 115°02'20" W. Here, the unit 

is about 12 m thick and lies between the Bauers and the Harmony Hills. Above about a meter of bedded tuff, the ashflow deposit in its lower half is nonwelded to partially 

welded and contains pumice lumps as much as 30 em in 

diameter. The overlying partially to moderately welded tuff 

has crude joint columns and white pumice lapilli in a grayish-pink matrix that has about the same modal composition 

as at the type section in the East Pahranagat Range. A second 

reference section, a partial section of the outflow sheet 

whose rocks were previously designated as the graniteweathering tuff, is located in the southern Pancake Range on 

the western side of Black Beauty Mesa on the Lunar Crater 

15-minute quadrangle (Snyder and others, 1972) at lat 

38° 16'40" N., long 116°06'25" W. Here 110m ofthe unit are 

exposed above a densely welded plagioclase-pyroxene-phyric Isom compositional-type tuff. The lower part of the unit 

is a white, pumice-bearing ash that grades upwards into a 

moderately welded lapilli tuff lithologically resembling the 

Pahranagat Formation in the White River Narrows. A third 

reference section in the outflow sheet in an area where its 

rocks have been previously referred to as the tuff of Saulsbury Wash is located in the southern Monitor Range at the 

southeast end of Woodchopper Canyon on the Mud Spring 

7.5-minute quadrangle at lat 38°09'53" N., long 116°50'52" 

W. Here, the outflow sheet overlies a local white, poorly 

exposed tuff and is overlain by a black lava flow containing 

small phenocrysts of olivine and pyroxene. At this reference 

section, the outflow sheet of the Pahranagat is about 144 m 

thick and grades upwards from a densely welded, devitrified 

pumice-poor tuff in its lowermost few tens of meters to more 

pumice rich, moderately welded tuff in its uppermost part 

that lithologically resembles the rocks in the White River 

Narrows. The section at White Blotch Spring on the Bombing and Gunnery Range has restricted access. 

We have paleomagnetic data for the outflow sheet of 

the Pahranagat Formation from the southern Delamar Mountains, the North Pahroc Range (preliminary only), White 

River Narrows, and Shingle Spring in the Egan Range 

(where it forms the topmost unit of the Tertiary tuff section), 

as well as many localities to the west and north. The outflow 

sheet of the Pahranagat Formation is not found in the Ely 

Springs area. The unit is reversely magnetized; a preliminary 

mean direction from 11 sites has a declination about 10° west 

of south and an inclination about 5° steeper than the reversed 

Oligocene axial dipole field direction, and thus is not particularly distinctive. Probably as a consequence of only weak to 

moderate welding, some sites have paleomagnetic directions 

that are as much as 18° of arc away from this preliminary 

mean. The formation has been mapped in the southern 

Delamar Mountains and the South and North Pahroc Ranges, 

but does not reach the Meadow Valley Mountains (fig. 1). 

Sanidine from the Pahranagat Formation has been precisely dated using the 40 Arf39 Ar method by Deino and Best 

( 1988 ), and its average age is about 22.6 Ma (early Miocene) 

based on four dates that only range from 22.62±0.03 to 

22.66±0.02 Ma (table l). 

RHYOLITIC LAVA FLOWS OF DELAMAR VALLEY 

A complex sequence of lava flows, lava domes, and 

subordinate encapsulating ash-fall and ash-flow tuffs that are 

distributed throughout the northern part of Delamar Valley 

(fig. 1) forms an important silicic volcanic center. Most of 

these rocks are phenocryst poor and contain about 10 percent 

sanidine and quartz; some are aphyric; and others contain 20 

percent phenocrysts of sanidine, quartz, plagioclase, and 

biotite. Commonly vitrophyric and frothy margins enclose 

devitrified flow interiors. At several localities, many glasses 

have been partly or totally hydrated to perlite; nonhydrated 

remnants (Apache tears) are common along the base of the 

east-facing cliffs of the northern South Pahroc Range; an 

active perlite mine is located about 7.5 km south of U.S. 

Highway 93 in that area. 

Along the cliffs on the east side of the northern South 

Pahroc Range, nonwelded ash-fall and ash-flow tuffs at the 

base of this sequence rest on the Pahranagat Formation, and 

the sequence of rhyolitic flows is covered by the Hiko Tuff. 

The absence of the Harmony Hills Tuff in the units exposed 

along the cliff was originally attributed to the presence of the 

rhyolitic domes and flows. However, samples of nonhydrated obsidian from the active perlite mine and adjacent 

devitrified aphyric rhyolite have been dated recently by 

whole rock K-Ar methods at 18.6±0.7 and 18.2±0.7 Ma, 

respectively (H.H. Mehnert, written commun., 1992). Thus, 

at least some of the rhyolitic sequence appears to be nearly 

as young as the overlying Hiko Tuff, too young to have created a barrier to ash flows from the source of the Harmony 

Hills Tuff. Although problems related to the absence of the 

Harmony Hills Tuff and the young dates of the rhyolite flows 

require further investigation, we assume that older rhyolites 

under those dated provided a barrier to emplacement of the 

Harmony Hills. 

Within Delamar Valley, the Hiko Tuff and the members 

of the younger Delamar Lake Tuff are plastered against hills 

formed by the lava domes; probably these domes were positive topographic features that were not completely covered 

by subsequent ash-flow tuffs (Scott and Swadley, 1992). The 

cooling units of the Gregerson Basin Member of the Kane 
28 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

Wash Tuff, the youngest cooling units in the area, occur in a 

few down-dropped blocks in the valley, and there is no evidence to suggest that they ever entirely covered the rhyolitic 

lava domes of Delamar Valley. If the domes remained 

uncovered by subsequent ash-flow tuffs, then the rhyolite 

domes may have had a local initial relief in excess of 500 m. 

Despite the thickness of the silicic rocks and their widespread distribution surrounding this volcanic center, evidence of associated hydrothermal activity is absent. 

HARMONY HILLS TUFF 

The Harmony Hills Tuff was originally defined as the 

youngest member of the Quichapa Formation by Mackin 

( 1960); these units were subsequently raised in rank by Cook 

(1965) to the Harmony Hills Tuff (formation rank) and the 

Quichapa Group; Williams also used this nomenclature in 

his dissertation ( 1967). Isopach data of Williams ( 1967) suggest that the source of the ash flows of the Harmony Hills lies 

in the Bull Valley Mountains of Utah, east of the Caliente 

caldera complex. Blank (1959) cited the occurrence of two 

cooling units in the Bull Valley area as evidence for a source 

in the Bull Valley area of westernmost Utah, but P.D. Rowley (written commun., 1992) has made similar observations 

in the Caliente area. Rowley and others (1979) and Ekren 

and others ( 1977) observed similar or greater thickness in the 

Clover Mountains of Nevada and have suggested that the 

Harmony Hills may have had its source in the southern part 

of the Caliente caldera complex. Because the tuff of the Harmony Hills is the least silicic volcanic rock of the ash-flow 

tuffs in this area and its composition plots in the trachyandesite and andesite lUGS fields (fig. 3), we consider that the 

Harmony Hills probably was not erupted from the Caliente 

caldera complex, which erupted rhyolitic ash flows. The 

Harmony Hills Tuff is also among the most crystal-rich ashflow tuffs in the Great Basin and contains about 50 percent 

phenocrysts, mostly plagioclase, and lesser (in diminishing 

amounts) biotite, clinopyroxene, hornblende, quartz, Fe-Ti 

oxides, and sanidine (fig. 4 ). 

The average K-Ar age of the Harmony Hills Tuff is 21.0 

Ma (table 1), but more recent dating of overlying rocks in 

Utah (Rowley and others, 1989) in addition to the age of the 

underlying Pahranagat Formation in eastern Nevada constrains the age of the Harmony Hills to between about 21.7 

and 22.6 Ma. 

The Harmony Hills Tuff was sampled for paleomagnetic study at three sites: the southern Delamar Mountains, 

eastern Condor Canyon, and the eastern end of Leach Canyon. At all three sites the Harmony Hills Tuff overlies the 

Bauers Tuff Member of the Condor Canyon Formation, and 

at the Nevada sites it underlies the Hiko Tuff. The Leach 

Canyon site is 4.5 km north of Mackin's ( 1960) type locality 

in the Harmony Mountains in southwestern Utah (Harmony 

Hills was dropped from official geographic names and 

replaced with Harmony Mountains). The Harmony Hills 

Tuff is normally magnetized, and our three sites show a relatively large dispersion in direction; the directions range 

from 5° to 27° of arc distant from the normal Oligocene axial 

dipole field direction. The paleomagnetic direction reported 

by Nairn and others (1975) for the Harmony Hills Tuff at 

Bauers Knoll in southwestern Utah is similar to our results. 

Gose (1970) reported results from the same site in eastern 

Condor Canyon as ours, and the two determinations are virtually the same. (See also Nairn and others, 1975.) 

The Harmony Hills Tuff has been mapped throughout 

the areas mapped in central Lincoln County (figs. 1, 2), 

except in the northern part of Delamar Valley where the 

thick complex of rhyolitic lava domes and lava flows occurs 

at the stratigraphic level of the Harmony Hills in parts of the 

Delamar NW and Pahroc Summit Pass quadrangles. Local 

sequences of olivine-, clinopyroxene-, and plagioclase-bearing mafic flows as thick as 25 m are present between the 

Harmony Hills Tuff and the Hiko Tuff at two localities in 

the North Pahroc Range. Also, in the Meadow Valley 

Mountains southeast of the Kane Springs Wash caldera, a 

local sequence of tuffaceous sedimentary rocks and nonwelded to partially welded ash-flow tuffs forms wedges as 

much as 65 m thick between the Harmony Hills and Hiko 

Tuffs on the hanging-wall side of south-dipping small-offset 

normal faults. Neither the sedimentary rocks nor the tuffs 

are as biotite rich as the rocks of the Harmony Hills or Hiko 

Tuffs. 

HIKOTUFF 

The most ubiquitous and distinctive ash-flow tuff unit 

throughout central Lincoln County is the rhyolitic Hiko 

Tuff. The tuff was named by Dolgoff ( 1963) after exposures 

in the Hiko Range where bold cliffs and knobby, joint-controlled landforms are characteristic of the outflow facies. In 

most areas the Hiko consists of one cooling unit; it is a 

devitrified, nonwelded to densely welded tuff; a eutaxitic 

texture formed by pumice fragments and shards deformed 

around phenocrysts occurs in the partly vitric, more welded 

zone close to local vitrophyres. Phenocrysts form about 

3~0 percent of the tuff and consist of about 10-35 percent 

very pale purple quartz, 15-35 percent sanidine, 30-65 percent plagioclase, 5-15 percent biotite, trace to 5 percent 

hornblende, traces of pyroxene and sphene, and several percent Fe-Ti oxides (fig. 4 ). Scott and others ( 1988) suggested 

that the magma from which the Hiko Tuff was erupted may 

have been compositionally zoned (Lipman and others, 1966) 

because they found a greater abundance of mafic minerals 

higher in the sequence of rocks of the Hiko Tuff. R.E. 

Anderson (written commun., 1992) also observed a distinct 

modal increase in plagioclase and ferromagnesian phases 

relative to quartz and sanidine in the upper part of the Hiko 

in southwestern Utah. 
TERTIARY VOLCANIC STRATIGRAPHY, CENTRAL LINCOLN COUNTY, NEVADA 29 

The Racer Canyon Tuff of Utah has a petrographic variability nearly identical to that of the Hiko Tuff of Nevada 

and, to add to stratigraphic complications, occurs at a stratigraphic level indistinguishable from that of the Hiko Tuff. 

After Cook ( 1965) correlated these two tuffs, attempts to distinguish between them have been numerous (Noble and 

McKee, 1972; Ekren and others, 1977; Rowley and others, 

1979; and Siders and Shubat, 1986). Rowley and Shroba 

(1991) considered that the two tuffs may be related genetically, building on the conclusions of Noble and others (1968) 

and Noble and McKee (1972), who proposed that both units 

were erupted from the Caliente caldera complex. Ekren and 

others ( 1977) first recognized that the Hiko Tuff formed 

thick intracaldera facies in the westernmost lobe of the Caliente caldera complex; Rowley and Siders (1988) named that 

part of the caldera complex the Delamar caldera and confirmed it as the source of the Hiko Tuff. 

Four K-Ar dates on mineral separates from the Hiko 

Tuff range from 18.1±0.6 to 20.1±0.5 Ma (Armstrong, 1970; 

Noble and others, 1968; Noble and McKee, 1972). However, 

40 Arf39 Ar dates provide a more precise measure of its age; 

Taylor and others (1989) obtained a 18.5±0.4 Ma-biotite plateau date, but concluded on the basis of additional unpublished dates that the best estimate of the age of the Hiko Tuff 

is 18.6 Ma. 

Paleomagnetic investigations of the Hiko Tuff, which 

were designed to test degrees of vertical-axis rotation and to 

provide constraints for tectonic models (Hudson and others, 

this volume), also provide some insight into the 

emplacement history of the Hiko Tuff. Although the Hiko 

Tuff generally consists of one cooling unit, at a thick section 

of the Hiko in the southern South Pahroc Range, it includes 

two cooling units. Five sites in the Hiko from the Delamar 

and Meadow Valley Mountains, thought to be unaffected by 

vertical-axis rotation, yield an apparently well-defined mean 

paleomagnetic direction of reversed polarity (D=166°, 

1=-45°, and alpha95=6.2°). However, inthe southern South 

Pahroc Range section, two vitrophyres are present. Samples 

obtained from below the upper vitrophyre yield a mean 

paleomagnetic direction that differs significantly from the 

direction derived from samples from the upper part of the 

ash-flow sheet. (See Hudson and others, this volume.) A 

period of at least a few hundred years is required between 

emplacement and cooling of these two cooling units if the 

difference in paleomagnetic directions is due to average rates 

of secular variation. We are uncertain whether the upper or 

lower cooling unit is the single cooling unit so widespread 

elsewhere. 

Paleomagnetic data for the Racer Canyon Tuff vary in 

direction and polarity from site to site (C.S. Gromme, unpub. 

data, 1991 ). These seeming ambiguities may be related to the 

presence of several cooling units within the tuffs of the Racer 

Canyon. 

The thickest part of the Hiko Tuff occurs along the eastfacing cliffs of the South Pahroc Range where more than 200 

m of a single cooling unit are exposed. Probably the Hiko was 

deposited in this locality in a valley trending westward away 

from the source (P.D. Rowley, written commun., 1992), for 

the Hiko thins south toward the southern Delamar Mountains 

and the Meadow Valley Mountains to 25-75 m and north 

toward the southern North Pahroc Range to at least 130m. 

Local olivine-bearing mafic flows overlie the Hiko Tuff 

southwest of Delamar Lake in the Pahranagat shear system. 

Although these flows are as much as 250 m thick here, they 

pinch out 6 km to the north and 7 km to the northeast and the 

southeast. Novak (1984) obtained a K-Ar whole-rock date of 

15.6±0.8 Ma for these flows, but a better estimate of the age 

of the basalt may be as much as 0.4 Ma older, about 16 Ma 

(see explanation in table 1 ). 

PERALKALINE RHYOLITIC ASH-FLOW 

TUFFS AND MAFIC LAVA FLOWS 

A significant change in the chemical composition of 

volcanic rock occurred between eruption of the Hiko Tuff 

(18.6 Ma) and the Delamar Lake Tuff (15.6 Ma). The Hiko 

Tuff is the youngest voluminous calc-alkalic ash-flow tuff in 

the central part of Lincoln County. Subsequent regional volcanism is characterized by a bimodal sequence of metaluminous to mildly peralkaline rhyolitic ash-flow tuffs 

commonly capped by alkalic olivine basalts. 

Rocks younger than the Hiko Tuff in the southern 

Delamar Mountains and Meadow Valley Mountains were 

originally defined as the Kane Wash Formation by Cook 

( 1965) based on his pioneering study of measured sections 

and his correlation of ash-flow tuffs by phenocryst modes. 

Cook proposed a type section for the formation at the western end of the Delamar Mountains that consists primarily of 

sanidine- and quartz-bearing ash-flow tuffs as well as sedimentary rocks and the basalt (reported in the previous section) that overlies the Hiko Tuff (fig. 6). Noble ( 1968) 

revised and renamed the unit the Kane Wash Tuff to exclude 

sedimentary rocks and basalts from Cook's definition (fig. 

6); Noble also recognized the peralkaline chemical composition of the ash-flow tuff. 

Initial K-Ar dating of the Kane Wash Tuff by Noble 

(1968) and Armstrong (1970) and of associated basaltic 

andesite by Best and others ( 1980) placed the age of the tuff 

at about 14 Ma. Subsequently Novak (1984) dated two ofthe 

three youngest cooling units of the Kane Wash Tuff that he 

established to have been erupted from the Kane Springs 

Wash caldera. He determined the K-Ar sanidine dates for the 

lower two cooling units, his members V 1 and V2, to range 

from 14.0±0.4 to 14.2±0.4 Ma (2 sigma errors); the youngest 

cooling unit, his member V3, was not dated. However, a better estimate of the age of these ash-flow tuffs may be as much 

as 0.4 Ma older, about 14.6 Ma (see explanation in table 1). 

Novak and Mahood (1986) established the petrologic evolution of outflow sheets and intracaldera units from the Kane 
30 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

Springs Wash caldera. Novak also recognized not only that 

two lower ash-flow tuffs included in Noble's Kane Wash 

Tuff predated the Kane Springs Wash caldera (15.5±0.4 and 

15.8±0.4 Ma for Novak's member 0 and 14.7±0.4 Ma for 

Novak's member W; better estimates of the ages of these 

units may be closer to 16 and 15.1 Ma, respectively, as 

explained in table 1 ), but also that these units had a distribution incompatible with a source from the Kane Springs Wash 

caldera and a chemical composition distinct from that of 

units derived from the Kane Springs Wash caldera. 

For the reasons outlined by Novak, Scott and others 

(1993) revised the stratigraphic nomenclature (fig. 6) previously applied to those units to exclude the two older units 

from the Kane Wash Tuff. Thus the redefined Kane Wash 

Tuff consists of only the three outflow cooling units derived 

from the Kane Springs Wash caldera; the lowest cooling unit 

(Novak's member V1) is called the Grapevine Spring Member and the upper two cooling units (Novak's members V2 

and V3) are called the Gregerson Basin Member. The two 

cooling units of the Gregerson Basin Member are commonly 

METERS 

300 

z 

0 

Cook (1965) 

Delamar Range 

(type section) 

vitric-crystal ignimbrites 

vitrophyre 

Three crystal-vitric ignimbrites 

Noble (1968) 

ttf-

::::> 

..... Restricted from 

indistinguishable both in the field and petrographically, particularly where complexly faulted or incompletely exposed. 

Rocks that make up Novak's (1984) members 0 and W have 

been raised to formation rank as the Delamar Lake Tuff and 

the overlying Sunflower Mountain Tuff, respectively (Scott 

and others, 1993). 

DELAMAR LAKE TUFF 

The rocks that now form the Delamar Lake Tuff were 

originally included as the lowest part of the Karie Wash Tuff 

(Noble, 1968; Novak, 1984, member 0). The Delamar Lake 

Tuff is a crystal-poor metaluminous rhyolitic sequence of 

quartz-, sanidine-, fayalitic olivine-, and hedenbergitebearing ash-flow tuffs (figs. 3, 4). The thickest accumulation 

is where four informal members (cooling units) are 

recognized on the western side of Delamar Valley about 10 

km north of Delamar Lake, the namesake of the tuff. Because 

these four members are not exposed in a single section, Scott 

Novak (1984) 

memberVa 

(unexposed at type 

section) LL. 

LL. 

::::> 

..... 

J: 

LL. 

LL. memberV2 ::::> ~ r- UJ 

- ~ 

I 

I 

I 

I 

I 

I 

This report 

and Scott and 

others (1993) 

Gregerson 

Basin 

Member 

Grapevine 

memberV Spring 1 

~ J: 

Tvk3 vitric-crystal ignimbrite Cook's (1965) Kane Member 

~ ~ r--Wash Formation by a: -

0 Tvk2x crystal-vitric ignimbrite J: removal of basalt or 

200 LL. 

~ other lavas and related sedimentary rocks 

from the definition. 

LL. 

LL. upper zone ::::> 

..... 

z 

< 

..... No description of indi- z 

::::> 

UJ 

z memberW ~ 

J: vidual units in the CJ) formation. 

~ Tvk 2 UJ 

0 

~ 

a: lower zone UJ 

vitric-lithic sillar ~ ;: 

0 

...J 

100 LL. 

z 

::::> 

memberO 

UJ f- -

~ c stal-vitric sillar 

Tvk 1 vitric-lithic ignimbrite 

CJ) 

~w-. Four informal members ,. .. at the composite :!:::5~ type section 

basalt (local) 

0 

Hiko Tuff 

Figure 6. Evolution of stratigraphic nomenclature used for the Kane Wash Tuff. Hiko Tuff continues below jagged line. From Scott and 

others (1993). 
TERTIARY VOLCANIC STRATIGRAPHY, CENTRAL LINCOLN COUNTY, NEVADA 31 

and others ( 1993) proposed a composite type section for the 

Delamar Lake Tuff consisting of four individual sections. 

From oldest to youngest. the cooling units are the (I) lower 

cooling unit consisting of less than 50 m of nonwelded to 

moderately welded rhyolitic ash-flow tuff, (2) crystal-rich 

cooling unit consisting of as much as 120 m of relatively 

phenocryst-rich (20-25 percent) moderately to densely 

welded tuff, (3) pumice-rich cooling unit consisting locally 

of less than 35 m of noncompacted, but sintered, pumice 

similar to pumice-flow tuffs recognized in Japan and New 

Zealand, and ( 4) orange cooling unit consisting of as much 

as 65 m of moderately welded to densely welded tuff 

characterized by a moderate-red to dusky-red vitrophyre 

below a light-red to moderate-orange-pink devitrified zone. 

The lowest two cooling units are extensively exposed in the 

southern Delamar Mountains and Meadow Valley 

Mountains, where they underlie the Sunflower Mountain 

Tuff. The pumice-rich cooling unit is present only in Delamar 

Valley near the composite type section. The orange cooling 

unit and the crystal-rich cooling units occur between the Hiko 

Tuff and the overlying Kane Wash Tuff in the northern end 

of Delamar Valley. None of the cooling units of the Delamar 

Lake Tuff or the Kane Wash Tuff are present north of the 

Timpahute lineament (fig. I). Although the source of the 

Delamar Lake Tuff is undetermined, it may be located 

northwest of the Kane Springs Wash caldera where the 

thickness of the cooling units is greater. 

Paleomagnetic characterization of the Delamar Lake 

Tuff is based upon a single site from the unit located on the 

western side of the central Meadow Valley Mountains, 

where the rock is normally magnetized. At this locality, only 

one cooling unit of the Delamar Lake Tuff is present; it is 

uncertain whether this cooling unit correlates with the lower 

cooling unit or the crystal-rich cooling unit of the Delamar 

Lake in Delamar Valley. 

SUNFLOWER MOUNTAIN TUFF 

The Sunflower Mountain Tuff is a newly defined formation (Scott and others, 1993) that derives its name from 

Sunflower Mountain in the central Meadow Valley Mountains (fig. 1 ). The rocks of this formation were originally designated as member W of the Kane Wash Tuff by Novak 

(1984 ). The Sunflower Mountain Tuff is a metaluminous 

rhyolitic tuff that consists of a thick compound ash-flow tuff 

cooling unit above thinner indistinctly bedded tuffs. The Sunflower Mountain Tuff contains two mappable zones, whose 

rocks Cook ( 1965) designated as his unit Tvk2 and its upper 

part, unit Tvk2x (fig. 6). The rocks of both these mappable 

zones have been defined as informal members in the new 

nomenclature. The lower member consists of all the nonwelded to partially welded lower part of the Sunflower 

Mountain Tuff, including indistinct bedded ash-flow tuffs 

near its base. The massive, upper part of the lower member 

consists of a pale-orange, devitrified, nonwelded to partially 

welded, lithic- and pumice-fragment-rich (20 percent of 

each), crystal-poor (about I 0 percent), ash-flow tuff that contains subequal amounts of quartz and sanidine phenocrysts 

and a trace of altered ferromagnesian minerals (fig. 4). 

Locally, near its middle part the lower member contains a 

thin interval of black fiamme. The lithologic distinction 

between the lower and upper members is based on an increase 

in degree of welding; no cooling break occurs between the 

members. The upper member is pinkish gray to pale red, 

devitrified, and partially to moderately welded to moderately 

to densely welded; it has few lithic fragments (1 percent) but 

has a phenocryst mineralogy indistinguishable from that of 

the lower member except for a greater total abundance (about 

20 percent). The upper member contains about 20 percent 

flattened pumice fragments and has distinctive moderateorange to pale-red mottled blotches. The type section of the 

Sunflower Mountain Tuff is 7.3 km north of Sunflower 

Mountain (Scott and others, 1993). Although the source of 

the Sunflower Mountain is unknown, its distribution suggests 

a source southwest of the Kane Springs Wash caldera. 

The Sunflower Mountain Tuff is present throughout the 

southern Delamar Mountains and the Meadow Valley 

Mountains; it is more than 150m thick 12 km west of the 

Kane Springs Wash caldera (fig. 1). Novak's (1984) reconnaissance mapping west of our mapping indicates that the 

thickest accumulation of the formation occurs about 25 km 

west of the Kane Springs Wash caldera; the tuff thins significantly to the north, east, and south. The tuff abruptly pinches 

out northeast of a line beginning about 4 km west of the Kane 

Springs Wash caldera and continuing to about 18 km northwest of that caldera. The Sunflower Mountain Tuff is notably absent northwest and north of the Kane Springs Wash 

caldera. 

The upper member, the more welded part of the Sunflower Mountain Tuff, was sampled for paleomagnetic characterization on the western side of the central Meadow 

Valley Mountains. At this site, the Sunflower Mountain Tuff 

is reversely magnetized. Because the lower part of the tuff 

contains bedded tuff interbedded with nonwelded ash-flow 

tuffs, significant secular variations, if not reversals, could 

have been recorded in the lower part of the unit. 

PERALKALINE PRECURSOR TO THE 

KANE WASH TUFF 

Several thick, undated ash-flow tuffs of peralkaline 

compositions similar to the comendites of the Kane Wash 

Tuff (Scott, Blank, and Page, 1991) are present stratigraphically beneath the Kane Wash Tuff and a precaldera trachyte 

of Novak (1984) along the northern wall ofthe Kane Springs 

Wash caldera (Scott, Swadley, and others, in press). Mapping in the area between 9 and 14 km north of the Kane 

Springs Wash caldera has identified structurally shredded 
32 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

fragments of a caldera wall in the vicinity of Narrow Canyon. Scott, Blank, and Page (1991) have considered these to 

be fragments of a precursor to the Kane Springs Wash 

caldera, which they called the Narrow Canyon caldera. East 

of Narrow Canyon, probable caldera-filling rhyolitic ashflow tuffs, rhyolitic lava flows, and a shallow trachytic intrusion(?) related to caldera resurgence are exposed near the 

intersection of the Delamar, Slidy Mountain, Elgin SW, and 

Gregerson Basin quadrangles (fig. 2). 

The extent of outflow sheets derived from the Narrow 

Canyon caldera remains relatively uncertain because mapping in the Elgin quadrangle and petrographic and geochemical investigations of potential outflow sheets are still in 

progress. Neither the Delamar Lake nor the Sunflower Mountain Tuffs are likely candidates for outflow sheets for this 

caldera because their distributions do not fit the Narrow Canyon source and because they are metaluminous, not peralkaline. The best candidates for outflow sheets are ( 1) a series of 

unnamed rhyolitic ash-flow tuffs found on the northeastern 

side of the Delamar Mountains between either the Kane Wash 

Tuff or the tuff of Etna and the Hiko Tuff, (2) a series of 

unnamed, post-Hiko Tuff/pre-Kane Wash Tuff ash-flow tuffs 

exposed near the Narrow Canyon caldera on the western side 

of the Delamar Mountains, (3) an undated peralkaline ashflow tuff found in the northern topographic wall of the Kane 

Springs Wash caldera close to the northwestern edge of the 

Meadow Valley Mountains, and ( 4) a similar unnamed ashflow tuff between the Kane Wash Tuff and the Hiko Tuff in 

the northern Meadow Valley Mountains. In all these cases, 

the ash-flow tuffs that are suspected to have been derived 

from the Narrow Canyon caldera have phenocryst assemblages similar to those of the Kane Wash Tuff. A thick 

sequence of peralkaline ash-flow tuffs exposed below the 

Kane Wash Tuff in the northern topographic wall of the Kane 

Springs Wash caldera in the central part of the Delamar 

Mountains is a good candidate for the intracaldera facies 

equivalent to the outflow sheets related to the Narrow Canyon 

caldera (Scott, Blank, and Page, 1991). But distinction 

between outflow units, intracaldera units, and post-collapse 

caldera-filling units is challenging in older, structurally disrupted, and largely buried calderas; more study of the Narrow 

Canyon caldera and related volcanic rocks is required. 

KANE WASH TUFF 

The Kane Wash Tuff caps the Tertiary volcanic succession throughout most of the Delamar Mountains and much of 

the Meadow Valley Mountains. Younger mafic lava flows 

and the tuff of Etna overlie the Kane Wash Tuff only in a few 

isolated exposures. 

The Grapevine Spring Member of the Kane Wash Tuff 

was named for the west-facing cliff exposures of its type section above Grapevine Spring in the central Meadow Valley 

Mountains in the Vigo NW 7 .5-minute quadrangle (Scott 

and others, 1993; Scott, Harding, and others, 1991). The 

Grapevine Spring Member is a slightly metaluminous rhyolite compound cooling unit (fig. 3); its peralkalinity index 

(PI=molar [Na+K]/Al) is slightly less than 1.0 (R.B. Scott, 

unpub. chemical data, 1990). The tuff includes a 3-m-thick, 

nonwelded, vitric basal zone, overlain by a 80-m-thick, 

densely welded, devitrified, moderate-brown, rough-weathering middle zone. The middle zone contains about 35 percent phenocrysts consisting of about 10-25 percent quartz, 

65-75 percent sanidine, as much as 10 percent hedenbergite, 

less than 5 percent fayalitic olivine, and traces of ilmenite 

(fig. 4). The middle zone is overlain by a 20-m-thick, moderately to densely welded, devitrified, light-brownish-gray, 

smooth-weathering upper zone. The upper zone is relatively 

crystal poor, containing about 10 percent phenocrysts consisting of about 25 percent quartz, 60 percent sanidine, and a 

total of about 15 percent hedenbergite, fayalitic olivine, and 

ilmenite (fig. 4 ). In one locality about 3 km southwest of the 

Kane Springs Wash caldera in the southern Delamar Mountains, minor trachytic cognate scoriaceous inclusions occur 

in the upper part of the Grapevine Spring Member. 

The Grapevine Spring Member is not uniformly distributed outside the Kane Springs Wash caldera. The member 

has a more or less uniform distribution northwest, west, and 

southwest of the caldera margin where the member thins to 

0-10 m about 10-15 km from the caldera. South of the margin, the Grapevine Spring Member does not thin nearly so 

much and is 50 m thick 22 km from the caldera, but north of 

the margin it pinches out in only 8 km. Both the Grapevine 

Spring and Gregerson Basin Members of the Kane Wash 

Tuff pinch out against the resurgent structure in the Narrow 

Canyon caldera about 10 km north of the Kane Springs Wash 

caldera (Scott, Swadley, and others, in press). 

The Gregerson Basin Member consists of two separate 

cooling units (members V2 and V3 of the Kane Wash Tuff of 

Novak, 1984 ), which are defined as a single member because 

the cooling units cannot be reliably mapped separately in 

complexly deformed terrane. The type section of the member 

(Scott and others, 1993) is about 5 km north- northwest of the 

Kane Springs Wash caldera; there, each of the two cooling 

units consists of a lower peralkaline rhyolite (PI> 1) interval 

overlain by an upper trachytic interval (fig. 3). The lower 

intervals are also comenditic according to the classification 

of peralkaline, quartz-normative extrusive rocks 

(Macdonald, 197 4 ). At the type section, comenditic intervals 

are devitrified, moderately to densely welded, and yellowish 

gray to light bluish gray. The rock has about 20 percent 

phenocrysts consisting of about 25 percent quartz, 60 percent 

sanidine, and a total of about 15 percent hedenbergite, 

fayalite and ilmenite (fig. 4). In the basal parts of the 

comenditic intervals, 1-2 m of welded pale-blue ash-flow 

tuff overlie 1-2 m of pale-yellowish-orange, nonwelded to 

partially welded ash-fall tuff. The bluish colors in the 

comenditic intervals appear to be related to vapor phase and 

deuteric(?) minerals, riebeckite and (or) arfvedsonite. The 
TERTIARY VOLCANIC STRATIGRAPHY, CENTRAL LINCOLN COUNTY, NEVADA 33 

upper cooling unit of the Gregerson Basin Member locally 

has striking bluish-gray colors and large altered riebeckite(?) 

crystals in lithophysal cavities and gas-expanded pumice 

fragments. The overlying trachytic intervals are devitrified, 

partially welded to moderately welded, and pale yellowish 

brown. The trachytic matrix is less mafic than common 

cognate inclusions of scoriaceous trachyte that contain about 

50 percent phenocrysts consisting of alkali feldspar, 

abundant opaque phases, and remnant pyroxene. At the type 

section of the Gregerson Basin Member, the lower cooling 

unit contains about 260 m of comendite below 30 m of 

trachyte and the upper cooling unit contains about 170 m of 

comendite below 30 m of trachyte. 

At the southern end of the Delamar Mountains, only the 

lower cooling unit is present and the Gregerson Basin Member has thinned to about 35 m thick, and it is only about 20 

m thick in the southern part of the Meadow Valley Mountains. About 40 m of these cooling units of the member are 

present in the northwestern part of Delamar Valley, but the 

cooling units are absent farther north and west. 

The new 40 Ar/39 Ar plateau sanidine dates from the 

Kane Wash Tuff are 14.67±0.22 Ma from the Grapevine 

Spring Member, 14.55±0.14 Ma from the lower cooling unit 

of the Gregerson Basin Member, and 14.39±0.28 Ma from 

the upper cooling unit of the Gregerson Basin Member 

(L.W. Snee, written commun., 1991). Although the 2-sigma 

errors overlap, the dates fall into the proper stratigraphic 

sequence supporting the probability that the time between 

eruptions of the cooling units is about 0.1-0.2 million years. 

As noted previously, these dates are about 0.4 Ma older than 

the K-Ar dates reported by Novak (1984). 

Paleomagnetic samples were collected from the Kane 

Wash Tuff in the Delamar and Meadow Valley Mountains. 

Three sites in the Grapevine Spring Member yield tightly 

grouped site-mean directions of normal polarity and northwesterly declination (fig. 7). This tight grouping of directions is good evidence for the absence of vertical-axis 

rotation between these sites. Paleomagnetic results from five 

sites in the lower cooling unit and from three sites in the 

upper cooling unit of the Gregerson Basin Member all indicate a reversed polarity and a southerly declination. The difference in magnetic polarity requires an interval of at least a 

few thousand years between emplacement of the two members, whereas 40 Art39 Ar data allow an interval as long as 

almost 600,000 years, but more probably nearer to 120,000 

years. The small but consistent difference between the mean 

directions of the lower and upper cooling units of the Gregerson Basin Member is great enough to require a minimum 

interval of a few hundred years between their emplacements. 

Correlation between members of the Kane Wash Tuff 

and a series of thin welded ash-flow sheets interbedded with 

nonwelded tuff in Rainbow Canyon requires both 

paleomagnetic and petrographic data. In thesis work by 

Bowman (1985), sheets of welded tuffs in Rainbow Canyon 

were informally referred to as numbered "C" units. The 

paleomagnetic directions for Bowman's units C2-C4 are 

shown in figure 7. Although the Grapevine Spring Member 

is not exposed in Rainbow Canyon, the lower and upper 

cooling units of the Gregerson Basin Member appear to be 

correlative with units C2 and C3, respectively, based on 

mineralogical evidence (Rowley and others, this volume). 

However, units C2 and C3 have a distinct counterclockwise 

paleomagnetic declination relative to the declinations of the 

lower and upper cooling units of the Gregerson Basin 

Member sampled close to the Kane Springs Wash caldera. 

We propose that a small but measurable vertical-axis rotation 

has affected the strata in Rainbow Canyon relative to the 

Gregerson Basin Member where it is exposed to the south 

near Kane Springs Wash caldera. 

We conclude that the Kane Springs Wash caldera is 

clearly nested into the southern part of the older Narrow 

Canyon, that the undetermined sources of the chemically and 

mineralogically similar Delamar Lake and Sunflower Mountains Tuffs are probably precursors to these two calderas, and 

therefore, that the four combined sources constitute a highly 

metaluminous to mildly peralkaline caldera complex, the 

Kane Springs Wash caldera complex. 

TUFFOFETNA 

As much as 60 m of partially to moderately welded rhyolitic ash-flow tuff has been preserved in a few localities in 

the northern and northeastern Delamar Mountains, particularly in down-dropped blocks between blocks of the Kane 

Wash Tuff and on east-facing dip slopes beneath capping 

mafic lavas. We correlate this tuff, using petrographic and 

paleomagnetic data, with the informally named tuff of Etna, 

proposed by Rowley, Shroba, and others (1991) for thick 

accumulations in Rainbow Canyon (unit C4 of Bowman, 

1985). Paleomagnetic studies of the tuff of Etna in Rainbow 

Canyon and in the northern Delamar Mountains identify a 

counterclockwise declination of the Rainbow Canyon rocks 

relative to those in the Delamar Mountains, similar to the differences in declinations found in the Gregerson Basin Member from those areas (fig. 7). 

The tuff of Etna contains about 30-50 percent phenocrysts consisting of about 20-30 percent quartz, 60-75 

percent sanidine, 1 percent plagioclase, and 2-3 percent 

altered biotite, and traces of hornblende, clinopyroxene, and 

opaque phases (fig. 4). Ekren and others (1977) reported a 

petrographically similar tuff, the Ox Valley Tuff, in the 

extreme eastern part of Lincoln County. Snee and others 

(1990) obtained a K-Ar date of 13.8±0.9 Ma for sanidine 

from a water-laid tuff above the tuff of Etna in Rainbow 

Canyon; that date and the 14.39±0.28 Ma date of the 

underlying upper cooling unit of the Gregerson Basin 

Member suggest that the age of the tuff of Etna is probably 

close to 14 Ma. The Ox Valley Tuff is apparently 

significantly younger (about 12.5 Ma) as noted by Rowley, 
34 

w 

GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

Meadow Valley Mtns 

N 

UGB 

s 

Mouth of Buckboard Canyon 

N 

TE 

C4 • .. ···· 

LGB ..... ·· 

UGB .. 9b C3 

s 

E w 

Boulder Canyon 

N 

GS• •TE 

LGB 

1\ 0 

s 

North end of Rainbow Canyon 

N 

C4 ........ TE 

• 

LGB 

UGB 

s 

E 

Figure 7. Tilt-corrected site-mean paleomagnetic directions for ash-flow tuffs collected from four sections. Alphags confidence circles are not shown because they are all less than 4 o. Meadow Valley Mountains site is from central part of western 

side of the mountains. Boulder Canyon section is located about 7 km north of intersection of the canyon mouth and northern 

margin of the Kane Springs Wash caldera. Mouth of Buckboard Canyon is about 4.5 km south of Caliente on western side of 

Rainbow Canyon. Northern end of Rainbow Canyon is about 0.5 km south of Caliente. GS, Grapevine Spring Member of the 

Kane Wash Tuff; LGB and UGB, lower and upper cooling units of the Gregerson Basin Member of the Kane Wash Tuff; TE, 

tuff of Etna; C2, C3, and C4 are informally named units of Bowman ( 1985) in Rainbow Canyon, referred to in text. Dotted 

lines connect stratigraphic correlatives and emphasize the basis for postulating slight counterclockwise vertical-axis rotation 

of Rainbow Canyon relative to surrounding areas to the south. 

Shroba, and others ( 1991 ); thus, the Ox Valley Tuff probably 

is not present in the Delamar Mountain-Rainbow Canyon 

area. For a more thorough discussion of possible correlation 

of the tuff of Etna and the Ox Valley Tuff, see Rowley and 

others (this volume). 

In one locality in the east-dipping dip slopes of the 

Delamar Mountains in the northeastern part of the Elgin SW 

7.5-minute quadrangle (fig. 2), an undated mafic lava flow 

underlies the tuff of Etna. Based on stratigraphic constraints, 

the age of this flow should be about 14 Ma. 
TERTIARY VOLCANIC STRATIGRAPHY, CENTRAL LINCOLN COUNTY, NEVADA 35 

MAFIC LAVA FLOWS 

Over much of central Lincoln County, the youngest 

volcanic rocks are mafic lava flows that appear to be 

conformable with underlying volcanic units in several areas. 

Where conformable relationships prevail, ages of these 

mafic lava flows generally provide maximum ages of 

Tertiary extension. Many of these mafic rocks cap the 

intracaldera-filling sequence in the Kane Springs Wash 

caldera and, therefore, are assumed to be related to the last 

phase of caldera-forming volcanism (Novak, 1984; Scott, 

Novak, and Swadley, 1990; Scott, Swadley, and others, 

1990; Scott, Harding, and others, 1991; Scott, Swadley, and 

others, in press). However, other mafic lava flows farther 

removed from the caldera may have independent origins, 

even though they have petrographic characteristics similar to 

those flows associated with the caldera. 

A series of extracaldera lava flows containing olivine 

phenocrysts and plagioclase megacrysts (classified as tholeiitic basalt and dated at 12.7±0.6 Ma by the whole-rock K-Ar 

method by Novak, I 984) occurs above the Kane Wash Tuff 

2 km southwest of the Kane Springs Wash caldera. (A better 

estimate of the age of this lava flow may be closer to 13.1 Ma, 

as explained in table I.) A large feeder dike cuts the older 

flows of this series, indicating a local extracaldera vent for at 

least some of these flows (Scott, Novak, and Swadley, 1990). 

A second series of mafic lava flows (classified as alkali 

basalt, trachybasalt, and trachyandesite according to Novak 

and Mahood ( 1986), but classified as basalt, trachybasalt, 

and basaltic trachyandesite, respectively, according to Le 

Maitre (1989)) conformably caps the rocks that fill the Kane 

Springs Wash caldera in the Delamar Mountains. In some 

cases, these intracaldera mafic flows have a phenocryst mineralogy similar to 1that of the extracaldera basalt flows. 

Although Best and others ( 1980) reported a younger wholerock K-Ar date of 12.1±0.5 Ma for a basaltic andesite from 

the intracaldera sequence, the dating error is large enough to 

leave the possibility that the intracaldera and extracaldera 

mafic flows are essentially coeval and were derived from the 

same magmatic system. The overlap in timing and location 

of eruption, petrographic character, and chemical composition of these two series permits several relationships: (1) 

These mafic lava flows were the result of the generation of 

mafic magmas regionally beneath this part of Lincoln 

County about 12.5 Ma, and they erupted both outside and 

coincidentally within the Kane Springs Wash caldera, independent of caldera magmatic evolution. (2) Both intracaldera 

and extracaldera mafic lava flows represent the final phase of 

caldera magmatism., some of which erupted outside the 

caldera. (3) Although these roughly coeval mafic lava flows 

may have been related to a regional mafic magma genesis 

within the mantle, those erupted from the caldera have been 

secondarily and independently involved with continued 

caldera magmatic evolution. ( 4) These mafic magmatic systems operated completely independently; their apparent 

coeval genesis is merely coincidental. The vanatwn in 

reported chemical compositions is large, from tholeiitic 

basalt outside the caldera to trachyandesite and basaltic 

andesite inside the caldera, suggesting significant differences in magma genesis in space in spite of similar ages 

(Novak and Mahood, 1986). Obviously, more work on petrogenesis of these mafic lava flows is needed. 

In the dip slopes on the eastern side of the northern 

Delamar Mountains, a mafic lava flow series conformable 

with the underlying tuff of Etna has been dated at 13.4±1.0 

Ma by whole-rock K-Ar methods (H.H. Mehnert, written 

commun., 1993). In the northeastern part of the Meadow 

Valley Mountains adjacent to Meadow Valley Wash, R.E. 

Anderson collected from two mafic lava flows for dating at 

a similar stratigraphic position; dates of 13.3±0.6 and 

13.24±0.6 Ma were obtained for these flows by H.H. Mehnert using whole-rock K-Ar methods (R.E. Anderson, written commun., 1992). Mafic flows at both these localities 

underlie a thick sequence of alluvial fill. A third mafic lava 

flow occurs within the thick sequence of alluvial fill between 

the northern end of the Meadow Valley Mountains and the 

Delamar Mountains; this mafic flow and the adjacent alluvial fill have been deformed by the left-lateral, oblique-slip 

Kane Springs Wash fault, which forms the northwestern 

boundary of the Meadow Valley Mountains. This flow has 

been dated by whole-rock K-Ar methods at 8.1±0.5 Ma by 

H. H. Mehnert (R.E. Anderson, written commun., 1992). The 

deformation of this flow and alluvium records movement . 

younger than about 8 Ma, but faulting in middle Pleistocene 

fans records the most recent movement on the Kane Springs 

Wash fault. 

One series of olivine-bearing mafic lava flows, some of 

which contain megacrysts of plagioclase, conformably caps 

the Kane Wash Tuff at the southwestern end of the Delamar 

Mountains, about 11 km from the Kane Springs Wash 

caldera. Similar flows are exposed a few kilometers 

southwest of the southwestern end of the Delamar 

Mountains in an area inhomogeneously deformed by the leftlateral Pahranagat shear system. At one locality, flows were 

deposited conformably on the underlying Kane Wash Tuff 

and were dated by whole-rock K-Ar methods at 14.5±1.0 

Ma; but at another nearby locality, flows angularly overlie 

small breached domes of the Kane Wash Tuff and were 

dated by whole-rock K-Ar methods at 15.1±1.3 Ma (H.H. 

Mehnert, written commun., 1993). These relationships 

suggest that some deformation spatially related to the 

Pahranagat shear zone occurred shortly after emplacement 

of the 14.5-Ma Kane Wash Tuff, between about 14.5 and 

13.8 Ma, given the large errors in dating of the mafic flows. 

In contrast to these conformable relationships, angular 

unconformities exist beneath mafic lava flows in two places 

in central Lincoln County; dating of these flows provides 

minimum ages of deformation of underlying volcanic strata. 

One example provides a minimum age for most of the 

progressive tilting that affected the eastern part of the Kane 
36 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

Springs Wash caldera in the Meadow Valley Mountains. A 

thick sequence of flows (classified as basalt by Novak, 1984) 

and associated scoriaceous material overlaps the 

southeastern wall of the Kane Springs Wash caldera and 

rests on both caldera filling units and outflow sheets of the 

Kane Wash Tuff (Harding and others, this volume; Scott, 

Harding, and others, 1991 ). Harding and others interpreted 

this sequence to have erupted from conduits that followed 

the caldera structural margin, ring-dike structures lying 

inboard of the caldera topographic wall. Novak (1984) 

determined whole-rock K-Ar dates for a basalt in this 

sequence to be 11.4±0.4 and 11.6±0.4 Ma. (A better estimate 

of the age of this lava flow may be closer to about 11.8-12.0 

Ma, as explained in table 1.) Although these lava flows dip 

at only 5°-8° to the east and southeast, underlying silicic 

strata both within and outside the caldera dip more steeply at 

20°-30°; and the oldest strata outside the caldera (Leach 

Canyon Formation and the Bauers Tuff Member of the 

Condor Canyon Formation) dip at 30°-40°, suggesting 

periods of progressive tilting and assumed extension in the 

Meadow Valley Mountains between about 24 and 12 Ma. 

The second example of angular relationships occurs in 

the northern part of the Pahroc Summit Pass and southern part 

of the Pahroc Spring 7 .5-minute quadrangles just south of the 

Timpahute lineament (figs. 1, 2). Here, on the downthrown 

side of the east-striking Pahroc Valley fault-a segment of 

the Timpahute lineament-nearly horizontal flows overlie 

tilted strata of the Hiko and Harmony Hills Tuffs (Scott, 

Swadley, and Byron, 1992). These flows also overlie an 

extremely coarse (clasts as great as 10m in diameter) Tertiary 

boulder alluvial fan(?) deposit that rests on the angular 

unconformity. The mafic flows, recently dated by wholerock K-Ar methods at 18.1±0.9 Ma (H.H. Mehnert, written 

commun., 1993), provide the minimum age of tilting and 

coarse fan growth associated with the Timpahute lineament. 

TUFF AND BEDDED VOLCANIC ASH IN ALLUVIUM 

About 1 km west of the Kane Springs Wash caldera, a 

tuff dipping 25° underlies an untilted ash in a complex 

sequence of alluvial deposits in a north-trending graben 

(Scott, Novak, and Swadley, 1990). The tilted tuff appears to 

be a deposit resulting from either a massive rhyolitic ash fall 

or a nonwelded ash flow and contains sanidine, quartz, 

minor biotite, and abundant pumice. A sanidine K-Ar date 

from the tilted tuff is 12.1±0.4 Ma, yet a hornblende K-Ar 

date from the untilted volcanic ash is 12.6±0.6 Ma (H.H. 

Mehnert, written commun., 1990). These dates are 

apparently out of stratigraphic order even though they are 

within the 2-sigma analytical error. Although the upper part 

of the volcanic ash was somewhat reworked by fluvial 

processes and contains evidence of bioturbation, it is 

unlikely that older hornblende was mixed with the ash 

because hornblende is sparse as a phenocryst phase in other 

volcanic units in the area. Probably the hornblende retained 

excess magmatic argon, providing an anomalously old date 

for the untilted ash; the hornblende is currently being redated 

by 40 Arf39 Ar methods to avoid the excess-argon problem 

(L.W. Snee, written commun., 1991). These dates are 

conformable with other evidence that suggests that the last 

major tilting event in the area ended about 12 Ma (Snee and 

others, 1990). 

Tertiary alluvial deposits in the southeastern side of 

Kane Wash Valley in the Delamar 3 SE quadrangle (Swadley 

and others, 1990) contain a bedded, biotite-bearing, silicic ash 

that is tilted at 24 o above an angular unconformity above the 

Hiko Tuff. Although the ash has not been dated using isotopic 

methods, the shard color, morphology, and major-element 

chemistry are similar to those of tephra erupted from the Yellowstone-Snake River Plain sequence of eruptive sources; 

the most likely age based upon preliminary correlation with 

those sources is between about 4.3 and 5.4 Ma (A.M. SarnaWojcicki, written commun., 1992). If this age range is corroborated by samples submitted for 40 ArP9 Ar dating, this is 

evidence for the youngest tilting in the area. 

TIMING OF VOLCANISM 

AND EXTENSION 

Although this report primarily concerns the stratigraphic framework of Tertiary volcanic rocks in central Lincoln County, the stratigraphic framework also records 

evidence of the nature and timing of tectonic events. We 

briefly summarize and comment on that evidence here. 

Some of this timing and nature of tectonic events has already 

been discussed in the previous sections, "Prevolcanic Sedimentary Rocks," "Mafic Lava Flows," and "Tuff and Bedded Volcanic Ash in Alluvium." 

Evidence for a prevolcanic basin exists north of the 

Timpahute lineament (fig. I); evidence in the North Pahroc 

Range consists primarily of thick pre volcanic limestones and 

clastic rocks including coarse boulder conglomerates (Scott, 

Swadley, and Byron, 1992; Taylor and Bartley, 1992). Axen 

and others (1993) and Taylor and Bartley (1992) attributed 

the sedimentary basin to prevolcanic (pre-32 Ma) extension 

related to the top-to-the-east Stampede detachment and postulated Seaman breakaway. Evidence for the Stampede 

detachment can be found no farther south than the northern 

boundary of the Caliente caldera complex, which coincides 

with the Timpahute lineament (Ekren and others, 1976; 

Rowley and others, 1992). In addition, the oldest volcanic 

units in that area, the formations of the Needles Range 

Group, are not present south of the Timpahute lineament, 

suggesting that the structural basin north of the lineament 

was still being filled by sediments and volcanic strata until at 

least about 28 Ma. 
TERTIARY VOLCANIC STRATIGRAPHY, CENTRAL LINCOLN COUNTY, NEVADA 37 

South of the Timpahute lineament, however, there is a 

conspicuous absence of evidence of significant sediment 

accumulation that would be expected if substantial prevolcanic extension had occurred. Ax en and others ( 1993) referred 

to an example of pre-24 Ma extension in the southern 

Delamar Mountains where the Devonian rocks are attenuated along west-directed low-angle normal faults (Page and 

Scott, 1991); Axen and others suggested that this may be an 

example of prevolcanic extension south of the Caliente 

caldera complex. However, Page (this volume) provides evidence and logic suggesting that the low-angle faults in the 

southern Delamar Mountains probably formed considerably 

earlier than the Stampede detachment and probably were 

associated with gravitational collapse of overthickened crust 

during or shortly after Sevier compression. 

Recent reevaluation of mapping by Swadley and others 

(1990) in the Delamar 3 SE quadrangle (fig. 2) in the 

Meadow Valley Mountains has identified at least one widespread attenuating low-angle normal fault, which is confined 

to Paleozoic rocks, between the Ordovician Pogonip Group 

and Ordovician Ely Springs Dolomite. Only highly 

deformed podlike remnants of the Eureka Quartzite exist 

along the fault zone. Overlying Tertiary volcanic rocks do 

not appear to be involved in typical upper plate extensional 

deformation, and therefore, the low-angle faults are probably 

prevolcanic. Evidence does not exist to determine whether 

this extension is related to Sevier compression or to a southem extension of a Stampede-age detachment. Only minor 

prevolcanic conglomerates exist on the angular unconformity above Paleozoic rocks. 

Essentially conformable stacks of late Oligocene and 

early Miocene tuff sheets exposed in tilted and uplifted 

ranges in central Lincoln County include few intervening 

lava flows and sedimentary deposits. These relations indicate relatively limited concurrent crustal extension during 

the major episode of explosive volcanism in southeastern 

Nevada about 32-22 Ma (Best and Christiansen, 1991). 

Beginning about 22 Ma and continuing to about 12 Ma, 

however, the stratigraphic record in central Lincoln County 

preserves local evidence of progressive tilting of Miocene 

ash-flow tuff sheets, requiring roughly coeval extensional 

tectonism and volcanism. 

In the east-facing cliffs of the South Pahroc Range, the 

Leach Canyon-Bauers-Pahranagat sequence dips between 

2S0 and 40° northwestward and is unconformably overlain 

by local rhyolitic tuffs and lavas. These rocks are overlain in 

tum by the Hiko Tuff that dips about 1S0 westward. All these 

rocks in tum are overlain by the Delamar Lake Tuff that dips 

about so westward. Between 22.6 and 18.6 Ma, 10°-2S0 of 

tilting clearly occurred, similar to the timing found by Best 

and others (1987) in the southern Wah Wah Mountains of 

southwestern Utah. Then, between 18.6 and about 16 Ma, 

another 1 oo of tilting occurred; and after the Delamar Lake 

Tuff was deposited, a final so of tilting occurred. Numerous 

normal faults cut the oldest strata, fewer cut the Hiko Tuff, 

and the least number cut the Delamar Lake Tuff; the amount 

of offset decreases on faults where they cut younger strata 

(Scott and Swadley, 1992). The downthrown sides of faults 

are generally filled with nonwelded tuffs. These "growth 

fault" relationships are similar to those observed by Bowman 

( 198S) and Angelier and others ( 1987) in Rainbow Canyon. 

Another example of progressive tilting is exposed on 

the eastern side of Delamar Valley, about 7.S km northwest 

of the Kane Springs Wash caldera, where the Hiko Tuff and 

older volcanic strata dip between about 8S0 and 4S0 eastward 

and are overlain by only slightly dipping younger strata 

(Scott, Swadley, and others, 1990). 

As described in the section, "Mafic Lava Flows," progressive tilting is also evident in the Meadow Valley Mountains, both within the intracaldera fill (Harding and others, 

this volume) and in outflow sheets south of the Kane Springs 

Wash caldera, where about 10°-1S0 southeastward tilting 

occurred between emplacement of the Leach Canyon Formation and the Kane Wash Tuff, followed by another 20° before 

emplacement of capping basalt flows ( 11.S Ma), and by a 

final tilting of so -10° (Scott, Harding, and others, 1991 ). 

Northeast of the Kane Springs Wash caldera in the 

northern Meadow Valley Mountains, the Harmony Hills 

Tuff dips about 60° northeastward, the Hiko Tuff dips about 

soo northeastward, ash-flow tuff from the Narrow Canyon 

caldera dips about 2S0 northeastward, Kane Wash Tuff dips 

about 1S0 , and the tuff ofEtna dips about so eastward. Thus, 

about sso of progressive tilting occurred over about 7 m.y., 

during which about five major ash flows erupted from local 

sources. 

We conclude that the distribution of progressive deformation in time was episodic. Also the degree and direction 

of extensional deformation were .heterogeneous in space. 

Present-day extension on basin-range faults in the Basin and 

Range province is also inhomogeneous in time and space, 

perhaps analogous to the past record. The scale of past episodic fault movement and tilting cannot be easily measured 

in geologic time, but the geographic heterogeneity of extension is obvious. In Lincoln County, extensional processes 

were clearly not uniformly distribute~; for example, the 

block of relatively undeformed strat~ in the southern 

Delamar Mountains is isolated within a "matrix" of more 

extended terrane (Scott, 1990). This stable pancake-like 

block is generally tilted less than 10° and is bounded by 

young basin-range faults. But even this block is intensely 

deformed locally by relatively small extensional structures, 

for example, the graben south and southwest of the Kane 

Springs Wash caldera (Scott, Novak, and Swadley, 1990) 

that splits the block in two. 1 

In summary, inhomogeneity in time and space is the 

primary relationship between extension and volcanism during the Miocene in central Lincoln County, a pattern generally true for the Great Basin as a whole (Best and 

Christiansen, 1991 ). 
38 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

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Stratigraphy of Miocene Ash-Flow Tuffs in and 

near the Caliente Caldera Complex, 

Southeastern Nevada and Southwestern Utah 

By Peter D. Rowley, L. David Nealey, Daniel M. Unruh, Lawrence W. Snee, 

Harald H. Mehnert, R. Ernest Anderson, and C. Sherman Gromme 

GEOLOGIC STUDIES IN THE BASIN AND RANGE-COLORADO PLATEAU TRANSITION IN 

SOUTHEASTERN NEVADA, SOUTHWESTERN UTAH, AND NORTHWESTERN ARIZONA, 1992 

U.S. GEOLOGICAL SURVEY BULLETIN 2056-B 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1995 
CONTENTS 

Abstract........................................................................................................................... 47 

Introduction . . . . . . ........... .. ...... ...... ............... ....... .. . . . ..... .... .. . . . ....... .. . . . .............. .. ....... .. ....... 4 7 

Acknowledgments.......................................................................................................... 49 

Geologic Setting of the Caliente Caldera Complex........................................................ 49 

Descriptions of Lithologic Units ........ .... ......... .............. ............ .............. .......... ............. 53 

Quichapa Group...................................................................................................... 54 

Leach Canyon Formation ............... ........... ............. ............ .. .......... ............. ... 55 

Condor Canyon Formation............................................................................. 55 

Harmony Hills Tuff........................................................................................ 56 

Racer Canyon Tuff................................................................................................. 58 

Hiko Tuff................................................................................................................ 59 

Tuff of Tepee Rocks............................................................................................... 61 

Delamar Lake Tuff................................................................................................. 62 

Tuff of Rainbow Canyon........................................................................................ 63 

Tuff of Kershaw Canyon........................................................................................ 64 

Tuff of Sawmill Canyon . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . ... . . . . . . . . . . .. . . . . . . . . . . . .. . . . . . . . . . .. . . . . . . . . . . . . . 65 

Kane Wash Tuff...................................................................................................... 66 

Tuff of Etna .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . .. . . . 68 

Ox Valley T.uff................... .. .. . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . .. . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . . .. . . . . . 69 

Conclusions ........ .. . . ............... .............. ..... ........ .... ........ ............. ............. ........... .. ........... 70 

References Cited............................................................................................................. 71 

Appendix 1. Modal analyses of rocks in southeastern Nevada and southwestern Utah 74 

Appendix 2. Chemical analyses (in parts per million except for Fe) 

of selected elements.................................................................................................... 86 

FIGURES 

I. Map showing aeromagnetic ridge and tectonic setting of Caliente caldera complex, Nevada-Utah............................ 48 

2. Map showing generalized geology and geography of Caliente caldera complex and vicinity .............. ............ ........... 50 

3. Generalized geologic map of western Caliente caldera complex and vicinity.............................................................. 51 

4. Stratigraphic chart of ash-flow tuffs, lava flows, and intrusive units in and near Caliente caldera complex................ 52 

5. Chemical classification of ash-flow tuffs in and near Caliente caldera complex.......................................................... 54 

6. Plot of phenocryst percentages in Leach Canyon Formation........................................................................................ 55 

7. Quartz-sanidine-plagioclase diagram of Leach Canyon Formation.............................................................................. 56 

8. Plot of selected elements determined by Kevex analysis of Leach Canyon Formation .......... ............... ............ ........... 56 

9. Plot of phenocryst percentages in Condor Canyon Formation...................................................................................... 57 

I 0. Quartz-sanidine-plagioclase diagram of Condor Canyon Formation and Harmony Hills Tuff.................................... 57 

II. Plot of selected elements determined by Kevex analysis of Swett Tuff Member of Condor Canyon Formation .... ..... 57 

I2. Plot of selected elements determined by Kevex analysis of Bauers Tuff Member of Condor Canyon Formation ....... 58 

13. Plot of phenocryst percentages in Harmony Hills Tuff................................................................................................. 58 

14. Plot of selected elements determined by Kevex analysis of Harmony Hills Tuff......................................................... 58 

15. Plot of phenocryst percentages in Racer Canyon Tuff and Hiko Tuff........................................................................... 59 

16. Quartz-sanidine-plagioclase diagram of Racer Canyon Tuff........................................................................................ 59 

17. Plot of selected elements determined by Kevex analysis of Racer Canyon Tuff .. ........................ ............ ................ .... 59 

18. Quartz-sanidine-plagioclase diagram of Hiko Tuff and tuff of Tepee Rocks................................................................ 60 

19. Plot of selected elements determined by Kevex analysis of Hiko Tuff......................................................................... 60 

20. Plot of phenocryst percentages in tuff of Tepee Rocks ...... .............. .... .. .... .. .. .. ...... .. .. .. ...... .. .. .. ...... .. .. .. .. .... .. .. .. ... ...... .. .. 61 
CONTENTS 

21. Plot of selected elements determined by Kevex analysis of tuff of Tepee Rocks . ... . . . . .. . . . . ... . .. . . . .............. .. . . . .. . . . ... .. . . . . .. 61 

22. Plot of phenocryst percentages in tuff of Sawmill Canyon and in Delamar Lake Tuff................................................. 62 

23. Quartz-sanidine-plagioclase diagram of tuff of Sawmill Canyon and of Delamar Lake Tuff....................................... 62 

24. Plot of selected elements determined by Kevex analysis of tuff of Sawmill Canyon and of Delamar Lake Tuff......... 63 

25. Plot of phenocryst percentages in tuff of Rainbow Canyon........................................................................................... 63 

26. Quartz-sanidine-plagioclase diagram of tuff of Rainbow Canyon................................................................................. 64 

27. Plot of selected elements determined by Kevex analysis of tuff of Rainbow Canyon................................................... 64 

28. Plot of phenocryst percentages in tuff of Kershaw Canyon and in McCullough Formation ... .............. ........... ............. 65 

29. Quartz-sanidine-plagioclase diagram of tuff of Kershaw Canyon and of McCullough Formation............................... 65 

30. Plot of selected elements determined by Kevex analysis of tuff of Kershaw Canyon and of McCullough Formation. 66 

31. Plot of phenocryst percentages in Gregerson Basin Member of Kane Wash Tuff......................................................... 67 

32. Quartz-sanidine-plagioclase diagram of Gregerson Basin Member of Kane Wash Tuff............................................... 67 

33. Plot of selected elements determined by Kevex analysis of Gregerson Basin Member of Kane Wash Tuff................. 68 

34. Plot of phenocryst percentages in tuff of Etna and in Ox Valley Tuff........................................................................... 68 

35. Quartz-sanidine-plagioclase diagram of tuff of Etna and of Ox Valley Tuff................................................................. 69 

36. Plot of selected elements determined by Kevex analysis of tuff of Etna... ........... ......................... ........... .............. ....... 69 

37. Plot of selected elements determined by Kevex analysis of Ox Valley Tuff.................................................................. 70 

Stratigraphy of Miocene Ash-Flow Tuffs in and 

near the Caliente Caldera Complex, 

Southeastern Nevada and Southwestern Utah 

By Peter D. Rowley, L. David Nealey, Daniel M. Unruh, Lawrence W. Snee, 

Harald H. Mehnert, R. Ernest Anderson, and C. Sherman Gromme 

ABSTRACT 

The Tertiary volcanic section of southeastern Nevada 

and southwestern Utah contains numerous regional and local 

ash-flow tuff sheets. Many of the tuffs in the upper 

(Miocene) part of this volcanic section were derived from 

the Caliente caldera complex of Nevada-Utah, centered 

southeast of Caliente, Nevada. The caldera complex is at 

least 80 kilometers east-west by 35 kilometers north-south 

and contains nested calderas that formed between at least 23 

and 13 Ma. The volcanism that formed this complex coincided with the main episode of extension in this part of the 

Basin and Range province, and thus, caldera margins consist 

of a complex array of faults and subsided areas. Many of the 

tuffs in the upper part of the section are lithologically similar 

(nearly all are rhyolites) yet were erupted at different times 

from different sources, including the Caliente caldera complex. This report summarizes the Miocene stratigraphy and 

attempts to utilize petrographic, chemical, and paleomagnetic data in concert with geologic mapping to correlate the 

tuffs. This report also sets the stage for preliminary conclusions in this volume about the magmatic evolution of the 

caldera complex. 

The oldest tuff for which we present data is the Leach 

Canyon Formation (23.8 Ma), which was thought to have 

been derived from the northern part of the Caliente caldera 

complex but whose source has not yet been found. The 

Bauers Tuff Member (22.8 Ma) and perhaps the underlying 

Swett Tuff Member (23.7? Ma), both of the Condor Canyon 

Formation, were derived from the Clover Creek caldera in 

the northwestern part of the Caliente complex. The Harmony 

Hills Tuff (22.5-22.0 Ma), although suggested to be from the 

Caliente complex, most likely was derived from well east of 

the complex. The source of the Racer Canyon Tuff 

(19.2?-18.7 Ma) is probably the east end of the Caliente 

caldera complex. The Hiko Tuff ( 18.6-18.2 Ma) was derived 

mostly from the Delamar caldera in the west end of the 

Caliente caldera complex. The Hiko cannot yet be 

distinguished from the Racer Canyon, so many questions 

remain about the relationship of each to the other; currently 

we suspect that they represent different eruptions from the 

same magma chamber. The tuff of Tepee Rocks ( 17.8 Ma) 

resembles the Racer Canyon and Hiko Tuffs but is a minor 

unit derived probably from directly east of the Delamar 

caldera. The change from calc-alkalic rocks to basalt and 

high-silica rhyolite then took place, and the Delamar Lake 

Tuff (15.8-15.5 Ma) was derived from a source suspected to 

be southwest of the Caliente caldera complex; outflow of 

this unit extended north from its source to just west of the 

complex. The tuff of Rainbow Canyon erupted at 15.6-15.2 

Ma from the Buckboard Canyon caldera, which is inset into 

the Delamar caldera. The tuff of Kershaw Canyon 

(15.3?-14.0 Ma) probably came from a caldera just east of 

the Buckboard Canyon caldera. The tuff of Sawmill Canyon 

is a minor cooling unit intertongued with the lower part of 

the tuff of Kershaw Canyon south of Caliente and 

presumably derived in or near this area. The two outflow 

sheets (14.6- 14.4 Ma) of the peralkaline Gregerson Basin 

Member of the Kane Wash Tuff are intertongued with the 

tuff of Kershaw Canyon, but the source is the Kane Springs 

Wash caldera 20 kilometers south of the Caliente caldera 

complex. The tuff of Etna (14.0 Ma) overlies the tuff of 

Kershaw Canyon; its source has not been found. The Ox 

Valley Tuff (12.6-11.4? Ma) of Utah may be derived from 

the same magma chamber as the tuff of Etna, but its source 

is farther east, probably in the southeast part of the complex. 

INTRODUCTION 

The Caliente caldera complex of southeastern Nevada 

and southwestern Utah is a large Miocene eruptive center that 

was the source of several of the most extensive regional ashflow sheets in the southern Great Basin. The complex, centered southeast of the town of Caliente, Nev., is within one 

of two connected major igneous belts, the Pioche-Marysvale 

47 
48 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

and Delamar-Iron Springs, that extend east-northeastward 

into Utah and are well delineated by aeromagnetic anomalies 

(fig. I). The belts also contain the major Indian Peak caldera 

complex (Best, Christiansen, and Blank, 1989), the source of 

the Oligocene Needles Range Group and Isom Formation, 

and at its east end the Marysvale volcanic field, a mineralized 

Oligocene and Miocene stratovolcano and caldera complex 

(Rowley and others, 1979; Steven and others, 1984, 1990; 

Rowley, Cunningham, and others, in press). The Caliente 

complex is dominated by silicic ash-flow tuffs and lava 

flows, but it also contains minor amounts of intermediate and 

mafic volcanic rocks. Volcanism in the complex began at 

least by 23 Ma and continued apparently to about 13 Ma 

(Rowley and others, 1990). Intracaldera deposits and outflow 

39° 115° 114° 

I 

z~c 

ml--1 

~)> 0 1I 

)>1 

I 

sheets from the complex are intercalated with outflow sheets 

from the nearby Kane Springs Wash caldera to the south. 

Outflow ash-flow tuffs from the Caliente complex also intertongue with outflow sheets from calderas to the west and 

northwest. 

This report is a product of the Basin and Range to Colorado Plateau transition (BARCO) Study Unit of the 

National Geologic Mapping program of the U.S. Geological 

Survey. This project includes detailed quadrangle mapping 

of the Caliente caldera complex and nearby areas in order to 

better understand its igneous and tectonic evolutionary history and the setting of epithermal gold deposits. In conjunction, we are also studying the area's igneous petrology and 

geochemistry, structural geology, isotopic ages, isotopic 

113° 

I 

, 

--n ,' u ? Cedar City 

I 

I 

I Area of figure 2 

I 

I 

I 

112° .; 

.; 

.; 

.; 

, 

I 

I 

I 

37°~~------------------~U-------------~----~--------------------~----~ 

~ 

~ 

0 

0 30 60 KILOMETERS 

EXPLANATION 

Boundary of aeromagnetic ridge (from Zietz and others, 1976, 1978) 

Caldera 

Colorado Plateau-Basin and Range boundary 

Axes of Pioche-Marysvale and Delamar-Iron Springs igneous belts 

Figure 1. Map showing aeromagnetic ridge and tectonic setting of Caliente caldera complex, Nevada-Utah. 
MIOCENE TUFF STRATIGRAPHY, CALIENTE CALDERA COMPLEX, NEVADA-UTAH 49 

geochemistry, and paleomagnetism. Before the project 

began, the area of the complex had been mapped only at 

1:250,000 scale (Ekren and others, 1977). Noble and others 

(1968) and Noble and McKee (1972) were the first to discover the caldera complex, which they called the Caliente 

depression. Ekren and others ( 1977) refined the shape of the 

complex, which they called the Caliente caldron complex. 

Numerous regional ash-flow tuffs had been predicted to be 

derived from the Caliente caldera complex: Leach Canyon 

and Condor Canyon Formations by Williams ( 1967) and 

Harmony Hills, Racer Canyon, Hiko, and Ox Valley Tuffs 

by Noble and others (1968) and Noble and McKee (1972). 

Yet before the current project began, only the Hiko Tuff had 

been confirmed to have erupted from the Caliente complex 

(Ekren and others, 1977), specifically from the west end of 

the complex. 

This report summarizes the stratigraphy of ash-flow 

tuffs in and around the Caliente caldera complex that span 

the interval from about 24 to about 13 Ma, the probable interval of Caliente volcanism. These tuffs make up the upper 

part of the volcanic section in this part of Nevada and Utah 

and include all tuffs known or suggested to have been 

derived from the caldera complex. Some tuffs of this interval, however, erupted from sources other than the Caliente 

complex, and data for them are included for comparison and 

for completeness of this part of the stratigraphic section. 

Presently the study at Caliente is in its early stages and 

entails considerable confusion about many field relations 

and about the identification of and sources for numerous ashflow tuffs that commonly look alike. As a first step in distinguishing the various tuffs, we here give data on lithology and 

on modal and chemical analyses and preliminary information on isotopic ages and paleomagnetism. Scott and others 

(this volume) present similar data on the stratigraphic section 

south and west of the Caliente complex; they also include 

information on the lower part of the section that we do not 

cover here. In work in progress we deal also with intercalated 

lava flows, including mafic sequences whose source plutons 

may represent the driving force for caldera magmatism (for 

example, Wiebe, 1993). Another purpose of the present 

report is to provide introductory information for two reports 

that follow, on preliminary geochemistry (Nealey and others, this volume) and isotopes (Unruh and others, this volume) of these tuffs. Together, the three studies provide our 

first impressions about the magmatic setting of the Caliente 

complex as well as of other calderas in the area that were 

active between about 24 and 13 Ma. 

Several new 40 ArJ39 Ar mineral dates help constrain 

temporal relationships of tuffs in the area of the Caliente 

caldera complex. The 40 ArJ39 Ar isotopic dating method is a 

variant of the conventional KJ Ar method; for 40 Arf39 Ar 

analysis, samples are irradiated in a nuclear reactor to 

produce 39 Ar from 39K. In this study, 50-100 mg of sanidine 

and biotite were irradiated in the USGS Denver TRIGA 

reactor for 30 hours at 1 MW (megawatt). Each sample was 

irradiated adjacent to an aliquant of standard MMhb-1 

hornblende for which K=1.555 percent, 40ArK=1.624x10-9 

mol/g, and KJ Ar date=520.4 Ma (Samson and Alexander, 

1987). Samples were experimentally degassed in a doublevacuum resistance furnace via step-heating in 12- 15 steps 

under ultra-high vacuum for about 20 minutes at each 

temperature step. The mass 40, 39, 38, 37, and 36 isotopes of 

argon were analyzed using a Mass Analyser Products series 

215 rare-gas mass spectrometer. Raw isotopic data were 

corrected for volume, mass discrimination, trap current, 

radioactive decay of 37 Ar and 39 Ar, and interfering isotopes 

of argon. Decay constants of Steiger and Jager (1977) were 

used to recalculate dates of any cited samples that were 

analyzed before 1977. 

ACKNOWLEDGMENTS 

This study could not have been possible without the 

help of numerous persons, to whom we are indeed grateful. 

It is an outgrowth of dissertation studies by the senior author 

many years ago, supervised by the late J.H. Mackin. Since 

then, many forms of data, advice, and conclusions from J.J. 

Anderson, M.G. Best, P.L. Williams, D.S. Barker, R.B. 

Scott, E.B. Ekren, T.A. Steven, C.G. Cunningham, R.F. Hardyman, A.E. Harding, A.L. Deino, and M.A. Siders have 

been invaluable to our work. We thank M.R. Hudson and 

J.G. Rosenbaum for use of their unpublished paleomagnetic 

data. E.A. du Bray convinced us that trace-element chemical 

data obtained by energy-dispersive X-ray fluorescence spectrometry (using a Kevex 700 spectrometer) are useful to correlate tuffs, taught us the technique, and shared his own data 

with us. H.R. Blank helped with information on tuffs from 

the Bull Valley Mountains and geophysics of the Caliente 

area. D.R. Lux shared isotopic dates on tuffs in the Caliente 

area that he has analyzed. We thank D.M. Cheney and P.L. 

FitzMaurice for mineral separations from most of the rocks 

we dated. M.A. Siders, F.W. Simonds, and F.M. Haggett did 

some modal analyses. Technical reviews by Florian Maldonado, R.B. Scott, and W C Swadley, and a non-technical 

review by L.M. Carter that was beyond the call of duty 

greatly improved the manuscript. We are grateful for the 

many forms of assistance from each and every Young of 

Young's R.V. Park, Caliente, Nev. 

GEOLOGIC SETTING OF THE 

CALIENTE CALDERA COMPLEX 

The Caliente caldera complex occupies an area of at 

least 80 km east-west by 35 km north-south, mostly in 

Nevada but partly in Utah. Its generalized shape is shown 

by reconnaissance mapping (Ekren and others, 1977) and 

gravity data (Blank and Kuchs, 1989) (fig. 2), but detailed 
50 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

0 20 40 KILOMETERS 

EXPLANATION 

D Quaternary and Tertiary sediments 

Tertiary igneous rocks 

' 

D Tertiary and pre-Tertiary sedimentary rocks 

0 Caldera boundary, dotted where concealed 

--Contact 

Figure 2. Map showing generalized geology and geographic features of Caliente caldera complex and vicinity. 

mapping of the western part of the complex shows the margin to be complicated and to be largely defined by synchronous tectonic faults (fig. 3). The caldera complex includes 

numerous nested calderas, only four of which so far have 

been named (Rowley and Siders, 1988). The oldest named 

is the Clover Creek caldera (22.8 Ma), which formed in the 

northern part of the complex (fig. 3) as the result of the 

eruption of the Bauers Tuff Member of the Condor Canyon 

Formation (fig. 4). About 4 million years after this, at 

18.6-18.2 Ma, the Hiko Tuff erupted from the western part 

of the complex, as Ekren and others (1977) determined; we 

call this the Delamar caldera (fig. 3). Later, at about 16 Ma, 

the Pine Park caldera (Rowley and Siders, 1988; Siders, 

1991) formed in the eastern part of the complex; we have 

not yet studied in detail the deposits of this caldera, nor have 

we mapped it. Then, at about 15.6-15.2 Ma, eruption of the 

tuff of Rainbow Canyon led to subsidence of the Buckboard 

Canyon caldera in the western part of the Caliente complex; 

it is inset into the Delamar caldera (fig. 3). 

In addition to the four named calderas, other vents or 

calderas formed in the Caliente complex, but their locations 

are uncertain or poorly defined. For example, the Racer Canyon Tuff (fig. 4) probably was derived from the east part of 

the complex (Siders and others, 1990; Siders, 1991 ), but we 

have not yet mapped that area. The tuff of Tepee Rocks (fig. 

4) also probably came from the complex east of the Delamar 

caldera; but we are not certain of its source. The tuff of Kershaw Canyon is exposed south of Caliente and north of the 

caldera complex, and it probably was derived from east of 

the Delamar caldera; but we have not determined whether 

this tuff represents outflow or intracaldera deposits, let alone 

where its source walls are located. Outflow of the tuff of 

Etna has been mapped in the caldera complex and in nearby 

areas outside the complex. Earlier mapping suggested that its 
MIOCENE TUFF STRATIGRAPHY, CALIENTE CALDERA COMPLEX, NEVADA-UTAH 51 

+ + + + + + 

Axis of Timpahute lineament 

+ 

/ 

® I 

/ 

PANACA 

BASIN 

+ + 

O.__ ___ s.__ __ ___.1 0 KILOMETERS 

EXPLANATION 

D Quaternary-Tertiary basin-fill deposits 

• Tertiary volcanic rocks, undivided 

• Tertiary intrusive rocks 

D Cambrian-Proterozoic sedimentary rocks 

Clover Creek caldera margin (concealed)-

Intracaldera Bauers Tuff Member of Condor 

Canyon Formation is patterned 

Fault margins of Delamar and related calderasdotted where concealed. Intracaldera Hiko and 

Hiko-like tuffs are patterned 

\ Fault - Dashed where approximately located, ~ Buckboard Canyon caldera margin - Intracaldera 

\ dotted where concealed tuff of Rainbow Canyon is patterned 

Figure 3. Generalized geologic map of western Caliente caldera complex and vicinity, based on 1 :24,000-scale 

mapping in progress. 

source was a fissure vent at the south edge of the Delamar 

caldera (Best and others, 1993), but recent mapping and petrologic studies now indicate otherwise; we do not know the 

exact location of the source of the tuff of Etna. The Ox Valley Tuff may have been derived from the southeastern part of 

the complex (Anderson and Hintze, 1993), but its specific 

source has not been located. 

Other regional outflow tuff sheets have a less certain 

connection to the Caliente caldera complex, or they are 

unlikely to have come from the complex. Thus, the Leach 

Canyon Formation may have been derived from the northern 

Caliente complex, as Williams ( 1967) suggested, but our 

mapping and reconnaissance investigations in the area have 

not disclosed evidence of its caldera. The Harmony Hills 
52 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

AGE UNIT 

Ox Valley Tuff {12.6?-11.4? Ma) 

tuff of Etna {14.0 Ma) 

Kane Gregerson upper unit {14.4 Ma)/ tuff of 

Wash Basin Kershaw 

Tuff Member lower unit {14.6 MaV Canyon 

y (15.3?-14.2 Ma) tuff of Sawmill Canyon 

tuff of Rainbow Canyon (15.6-15.2 Ma) 

Delamar Lake Tuff (15.8-15.5 Ma) 

Miocene 

tuff of Tepee Rocks {17.8 Ma) 

lava flows of Buckboard Spring (18.0 Ma) 

volcanic dome of Crows Nest Tank (18. 2 Ma) / *porphyry of 

Hiko Tuff (18.6-18.2 Ma) / Meadow Valley 

Racer Canyon Tuff {19.2?-18.7 Ma) /Wash 

lava flows of Indian Cove (21.9 Ma) '(19.4-<18.2 Ma) 

Harmony Hills Tuff (22.5-22.0 Ma) 

cu c: Bauers Tuff I intrusive rocks of Clover a. a. .... c: 0 

cu ::I Co:;:; Member {22.8 Ma) Creek caldera (22.8 Ma) .s:::. 0 -o >- cu 0 .... ·:; (!) g ~ E Swett Tuff Member {23. 7? Ma) 0 ooo LL. 

Leach Canyon Formation (23.8 Ma) 

Oligocene Cobalt Canyon stock (24. 8 Ma) 

*dikes 

Tuff, suggested by Noble and McKee (1972) to have been 

derived from the complex, more likely was derived from the 

Bull Valley Mountains of Utah (Blank, 1959; Williams, 

1967), although the caldera has not been identified. 

In addition to tuffs, plutons and lava flows were 

emplaced in and near the Caliente caldera complex at various times in the evolution of the complex. The Cobalt Canyon stock, which formed epithermal gold deposits in the 

Chief mining district 8 km north of Caliente (fig. 3) and 

whose exposed south edge is 3 km north of the caldera complex, may be related to caldera magmatism. However, the 

isotopic age of this quartz monzonite porphyry pluton, based 

on recently determined plateau 40 Arf39 Ar dates, is 24.8 Ma 

(Snee and others, 1990; Rowley and others, 1992), and thus 

older than presently known ages of caldera magmatism. Isotopic data on this stock are given by Unruh and others (this 

volume). A faulted dioritic stock exposed as close as 3 km 

from the southwest edge of the caldera complex (fig. 3) and 

younger dikes and small plutons of rhyolite that extend west 

from the edge of the Delamar caldera may also be related to 

magmatism in the caldera complex. These intrusive rocks, 

which are synchronous with caldera magmatism, formed 

epithermal gold deposits in the major Delamar (Ferguson) 

mining district (Emmons, 1901; Callaghan, 1937; Ferris, 

1991; P.D. Rowley, R.B. Scott, and W C Swadley, mapping 

in progress) just southwest of the caldera complex (fig. 3). 

Epithermal gold deposits also are known along east-striking 

faults at the Easter (Taylor) mine (fig. 3) and also occur in 

the Pennsylvania mining district (fig. 3) south of the Caliente 

caldera complex (Rowley and others, 1992; Best and others, 

SOURCE 

Caliente 

Caliente 

Kane 

Springs 

Wash 

Caliente 

Buckboard 

Canyon 

? 

Caliente 

Delamar 

Delamar 

Delamar 

Caliente 

Clover Creek 

? 

Clover Creek 

Caliente 

? 

Figure 4. Stratigraphic chart of igneous rocks 

in upper part of the Tertiary volcanic section in 

and near Caliente caldera complex. Ash-flow 

tuffs, which are discussed in this report, are 

shown in regular roman type; whereas lava flows 

and intrusive units discussed by Nealey and others (this volume) and Unruh and others (this volume) are shown in italic type. Numbers give 

isotopic or inferred ages, in Ma, of units; details 

are given in the text. "Source" refers to name of 

caldera source, where known. Use of "Caliente" 

for source indicates that the source is considered 

to be within the Caliente caldera complex but that 

the specific caldera has not been named or in 

some cases not found. Query, caldera has not 

been found; blank, not applicable. 

1993) and in the Goldstrike mining district just southeast of 

the caldera complex (Adair, 1986; Willden and Adair, 1986). 

Intracaldera plutons probably formed during development of most calderas and other vents, but few have been 

found in the Caliente caldera complex. So far, only a small 

part of the intracaldera pluton of the Clover Creek caldera has 

been mapped. It occurs as a narrow piece of a pluton roof 

exposed about 1 km north of Caliente. This rhyolite porphyry 

resembles the Bauers Tuff Member except that phenocrysts 

are larger than in Bauers and generally unbroken. The rock 

has a preliminary plateau 40 Arf39 Ar date on sanidine of 

22.8±0.1 Ma (L.W. Snee, unpub. data, 1991). Isotopic data 

on this intrusion are given by Unruh and others (this volume). 

Andesitic lava flows and volcanic mudflow breccia 

erupted from stratovolcanoes on the rims or within calderas 

of the Caliente complex. One such mafic volcanic dome, the 

lava flows of Indian Cove, rests on the apparent edge of the 

Clover Creek caldera several kilometers north of Caliente. 

Its eruption indicates consolidation of the magma chamber 

of the Clover Creek caldera. The dome consists of a thick 

sequence of lava flows and volcanic mudflow breccia of 

alkalic andesite, dacite, trachyandesite, and trachydacite. A 

sample received a plateau 40 Ar/39 Ar date on sanidine of 

21.9±0.1 Ma (Snee and others, 1991; Rowley and others, 

1992). Isotope data on this dome are given by Unruh and 

others (this volume). 

Rim and intracaldera rhyolite domes also formed 

throughout the caldera history as magma chambers leaked 

magma to the surface. One such rhyolite lava-flow sequence 

is the volcanic dome of Crows Nest Tank, which occurs on 
MIOCENE TUFF STRATIGRAPHY, CALIENTE CALDERA COMPLEX, NEVADA-UTAH 53 

the north edge of the Delamar caldera about 9 km west of 

Caliente. It much resembles in lithology and chemistry the 

Hiko Tuff, as is to be expected for a magma from the same 

source chamber as Hiko. It has two preliminary plateau 40 Ar/ 

39 Ar dates on sanidine and hornblende that average 18.2±0.1 

Ma (L.W. Snee, unpub. data, 1991). Isotope data on this 

dome are given by Unruh and others (this volume). This 

dome is identical in age and lithology to another small rhyolite dome, the volcanic dome of Barnes Canyon, also on the 

north edge of the Delamar caldera but 6 km east of Caliente, 

that has two preliminary plateau 40 ArJ39 Ar dates on sanidine 

and biotite that average 18.2±0.2 Ma (L.W. Snee, unpub. 

data, 1991). Other rhyolite domes are common in the Caliente caldera complex, notably along the east-trending south 

edge of the Delamar caldera; none of them have been isotopically dated but they appear to be generally of Hiko age 

(P.D. Rowley, unpub. mapping, 1992-1993). 

An unusual lava flow was analyzed by Unruh and others (this volume) and dated by L.W. Snee. It is a dark-gray 

and black, basaltic trachyandesite (fig. 5) containing a disequilibrium (commonly altered) assemblage of sanidine and 

plagioclase phenocrysts as long as 1.5 em and of scattered 

quartz and hornblende phenocrysts. These crystals occur in a 

fresh rock with sparse small phenocrysts of olivine, orthopyroxene, and plagioclase in a flow groundmass of the same 

minerals as well as of Fe-Ti oxides. The flows overlie the 

uppermost intracaldera ash-flow tuffs of the Hiko Tuff in the 

Delamar caldera and thus mark the cessation of Hiko volcanism and consolidation of the magma chamber of the 

Delamar caldera. L.W. Snee determined a plateau 40 Arf39 Ar 

date on sanidine of 18.0±0.2 Ma on a flow from this 

sequence, which we here call the lava flows of Buckboard 

Spring for exposures near this spring in upper Buckboard 

Canyon about 11 km west of Caliente. 

The Caliente caldera complex formed during the main 

episode of regional extension in the area, the second of three 

episodes that affected the Caliente area (Rowley and others, 

1992). Deformation during this episode occurred on complex mixtures of high-angle strike-slip, normal-slip, and 

oblique-slip faults, as well as low-angle detachment faults 

(Bowman, 1985; Michel-Noel and others, 1990; Rowley and 

others, 1992; Best and others, 1993). As such, the style of 

deformation in the Caliente area, and its synchroneity with 

volcanism, are similar to relationships described and origins 

interpreted in the Utah-Nevada-Arizona tricorner area by 

Anderson (1984, 1986, 1987, 1989, 1990), Anderson and 

Bohannon ( 1993 ), and Anderson and Barnhard ( 1993a, b). 

Distinctive porphyritic dacitic dikes and small plutons, characterized by similar modal ratios of minerals and by feldspar 

phenocrysts as long as 2 em, were emplaced along some of 

the high-angle faults in widely scattered areas in and near the 

caldera complex as well as in the Panaca Summit area about 

30 km northeast of Caliente (Best and others, 1991). One 

dike of this unit, the porphyry of Meadow Valley Wash, has 

40 Arf39 Ar dates averaging 19.4 Ma (Snee and others, 1990; 

Rowley and others, 1992); isotopic data on the unit are given 

by Unruh and others (this volume). The dates demonstrate 

that the main episode of major extension began before 19 

Ma; it may have begun as early as 25 Ma, and thus controlled 

emplacement of the Cobalt Canyon stock. The start of this 

episode of tectonism may correlate with a major reorganization of continental plates in western North America at about 

25 Ma (Mammerickx and Klitgord, 1982; Ward, 1991). 

Movement on related nearby high-angle faults of the same 

trend offset rocks as young as 12 Ma. 

Because of the synchroneity of major extension and 

caldera volcanism, most if not all tuff sources of the Caliente 

caldera complex are calderas that contain walls that are in 

part recurrent high-angle faults, that are trapdoor (half-graben) type calderas, or that are fissure vents now occupied by 

dikes and plugs. These tuff sources are largely controlled by 

synchronous faults, notably zones of east-striking faults that 

define the north and south sides of the caldera complex. Similar faults partly define numerous broad, east-trending structural zones ("lineaments") that are abundant in the Great 

Basin (Ekren and others, 1976; Rowley and others, 1978; 

Rowley, Cunningham, and others, in press) and appear to 

represent strike-slip accommodation zones. The lineaments 

that bound the Caliente caldera complex are the Timpahute 

lineament (Ekren and others, 1976) on the north and the 

Helene lineament (Best and others, 1993) on the south (fig. 

3). The resultant east-elongated caldera complex can be 

thought of as a volcanotectonic trough in the sense described 

by Burke and McKee (1979). The Caliente caldera complex 

differs from most described calderas in that Caliente caldera 

margins are commonly faults, the complex is unusually long 

lived (about 10 million years), many tuff eruptions are relatively small and do not form recognizable outflow sheets, 

intracaldera plutons are either sparse or deeper than the current surface of erosion, floors of older rocks may be exposed 

beneath the intracaldera fill, and venting of similar tuff 

batches can occur from widely separated sources presumably during chronologically different but similar episodes. 

Most post-caldera (after 13-12 Ma) volcanism was 

basaltic and basaltic andesitic, and their deposits predate and 

locally intertongue with basin-fill sedimentary rocks, including the upper Miocene and Pliocene Panaca Formation of the 

Panaca basin several kilometers north of Caliente. The basaltic volcanism (12 Ma; Mehnert and others, 1989) took place 

near the end of the main episode of extension, and after that 

the third episode of extensional deformation, basin-range 

faulting, became the dominant form of tectonism. 

DESCRIPTIONS OF LITHOLOGIC 

UNITS 

Many of the regional ash-flow sheets in and near the 

Caliente caldera complex have been described by Anderson 
54 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

and Rowley (1975), Rowley and others (1979), Siders and 

Shu bat ( 1986), Best, Christiansen, and others (1989), 

Rowley, Mehnert, and others (1994), and Scott and others 

(this volume). A brief discussion ofthose units specific to the 

Caliente area is given by Rowley and others (1992). More 

detailed descriptions of regional and local units are provided 

by geologic maps by Siders and others (1990), Rowley and 

Shroba (1991), Rowley, Shroba, and others (1994), and 

Swadley and Rowley (1994). 

The oldest rocks near the Caliente caldera complex consist of a thick sequence of orthoquartzite of Late Proterozoic 

and Early Cambrian age, overlain by thick limestone, dolomite, and shale sequences of Early through Late Cambrian 

age (Merriam, 1964; Tschanz and Pampeyan, 1970; Axen 

and others, 1988). These rocks are overlain by Oligocene 

and Miocene volcanic rocks, and in turn by upper Miocene, 

Pliocene, and Quaternary basin-fill sedimentary rocks (Rowley and others, 1992). Oligocene volcanic rocks consist 

mostly of ash-flow tuffs of the Needles Range Group from 

the Indian Peak caldera complex (Best, Christiansen, and 

Blank, 1989), but tuffs from calderas to the west and northwest are also represented (Best, Christiansen, and others, 

1989). These rocks are overlain by ash-flow tuffs of the Oligocene Isom Formation, probably also derived from the 

Indian Peak complex, and by local, mostly thin intertongued 

lava flows, mudflow breccia, and sedimentary units of 

Miocene age (Rowley and others, 1992; Rowley and Shroba, 

12 

1991; Rowley, Shroba, and others, 1994; Swadley and Rowley, 1994). The Miocene Quichapa Group (Anderson and 

Rowley, 1975), which overlies the Isom Formation, is the 

oldest unit that will be discussed in this report: most ash-flow 

tuffs in the group are known or suspected to be derived from 

the Caliente complex. 

We discuss the tuffs from oldest to youngest. Rock 

names are based on the classification of Le Bas and others 

(1986), shown in figure 5. Modal analyses are given in 

Appendix 1. Following a method of correlating ash-flow 

tuffs based on energy-dispersive X-ray fluorescence chemical analysis (EDXRF by Kevex instrumentation) suggested 

to us by E.A. du Bray (oral commun., 1990) and discussed 

by du Bray ( 1995), we give chemical data in Appendix 2 of 

elements that he considered most useful for correlation. As 

the reader will see herein, these methods are not always sufficient to distinguish rock units. 

QUICHAPA GROUP 

The name Quichapa Group was proposed by Cook 

(1957) for a sequence of informally named regional ash-flow 

tuffs in southwestern Utah and southeastern Nevada. Mackin 

(1960) reduced the unit in rank to the Quichapa Formation 

and divided it from base to top into the Leach Canyon, Swett, 

Bauers, and Harmony Hills Tuff Members. Anderson and 

EXPLANATION 

0 Ox Valley Tuff 

I- Trachyte Rhyolite + TuffofEtna z 

w ~'!!"! 6 Upper unit, Gregerson Basin Member 

(.) ~'!!"! 0::: T Lower unit, Gregerson Basin Member 

w 

a. Trachydacite ~'!!"! Grapevine Spring Member 

IJ: 8 e Sunflower Mountain Tuff 

(!) ~ Delamar Lake Tuff jjj 

~ 0 Tuff of Sawmill Canyon eo ~ 6 oe 0 Tuff of Kershaw Canyon • cS • Tuff of Rainbow Canyon N oX• ~ X Tuff of Tepee Rocks 

+ 

0 4 • HikoTuff 

~ 

N Dacite E9 Racer Canyon Tuff Andesite z Basaltic 

andesite 0 Harmony Hills Tuff 

2 • Bauers Tuff Member 

6 Sweet Tuff Member 

Leach Canyon Formation 

0 

53 57 61 65 69 77 81 

Si02, IN WEIGHT PERCENT 

Figure 5. Chemical classification of ash-flow tuffs in and near Caliente caldera complex, using total alkali-silica diagram of Le Bas and 

others (1986). Includes data of Blank (1959, table 1 ), Williams ( 1967, table 5), Hause I and Nash (1977, table 6), Novak (1984, table 2), 

Novak and Mahood (1986, table 10), Harding (1991, Appendix IIA), and Harding and others (this volume, Appendix 2A). 
MIOCENE TUFF STRATIGRAPHY, CALIENTE CALDERA COMPLEX, NEVADA-UTAH 55 

Rowley (1975) followed Williams (1967) in revising the 

Miocene Quichapa Group to consist of the Leach Canyon 

Formation, the Swett and Bauers Tuff Members of the Condor Canyon Formation, and the Harmony Hills Tuff. 

LEACH CANYON FORMATION 

The Leach Canyon Formation consists of numerous 

outflow cooling units of tan and gray, crystal-poor, poorly 

welded, rhyolite ash-flow tuff. Total thickness of the cooling 

units of the formation is generally less than 100m but locally 

is more than 300 m. These outflow tuffs are thickest on the 

north side of the Caliente complex, and overall tuff distribution is centered here as well, leading Williams ( 1967) to suggest that the source caldera was in the Panaca-Caliente area. 

Most of the area at the center of tuff distribution is the 

Panaca basin, which is covered by much younger basin-fill 

sedimentary rocks; thus we have at present confirmed no 

such caldera. 

The formation comprises two members. The lower is the 

Narrows Tuff ·Member, which covered at least 15,000 km2 

(Best, Christiansen, and others, 1989, fig. R40) after the 

effects of east-west extension (50 percent assumed) are 

removed from its present distribution of at least 22,000 km2 

(Williams, 1967). The overlying Table Butte Tuff Member 

covered 4,000 km2 if the same formula is applied to its current 

distribution of 6,000 km2 (Williams, 1967). Because the members are difficult to separate without detailed petrography, we 

generally do not separate them in our mapping. Distinguishing 

features of tuffs of the Leach Canyon Formation are their 

abundance of mostly red, angular volcanic rock fragments and 

their abundant, partly collapsed pumice fragments; volume of 

pumice fragments increases upward in the formation. 

We plot modal data to show phenocryst minerals relative to total phenocrysts (fig. 6) and to show quartz, sanidine, 

and plagioclase (QSP) relative to each other (fig. 7) from 

Appendix I and published sources. Modal amounts of minerals show considerable variation in the Leach Canyon Formation, but overall the unit consists of 10-30 percent 

phenocrysts of quartz (10-50 percent of total phenocrysts), 

sanidine (5-45 percent), and plagioclase (25-65 percent), 

biotite ( < 15 percent of total phenocrysts), hornblende ( <6 

percent), clinopyroxene ( < 1 percent in some thin sections), 

Fe-Ti oxides ( <7 percent), and accessory sphene. Generally 

plagioclase exceeds quartz, which in tum exceeds sanidine. 

Differences in amounts of minerals through a vertical stratigraphic sequence of the Leach Canyon are characterized 

generally by an upward increase in percent of plagioclase 

and a decrease in quartz and sanidine, as would be expected 

if the eruption were tapping increasingly lower parts of a 

compositionally zoned magma chamber (Lipman and others, 

1966). This trend in plagioclase versus quartz and sanidine is 

evident in samples collected in stratigraphic sections within 

members and in the overall petrographic difference between 

80 

(/) 

1-

(/) 

>- 0:: 

(.) 

0 

z 

w 

I 1-

a.. I 

_J I 

~ 0 

1-

u. I 0 jf1-

z 

w :-

(.) I I 

0:: 

~~ w 

a.. 

0 I: 

p s Q 8 H Px F 

PHENOCRYSTS 

Figure 6. Plot of percentages of major phenocrysts in theN arrows 

(solid lines) and Table Butte (dashed lines) Tuff Members of the 

Leach Canyon Formation. From data in Appendix 1 and from 

Williams ( 1967). Averages of most abundant minerals shown by 

arrows. P, plagioclase; S, sanidine; Q, quartz; B, biotite; H, 

hornblende; Px, pyroxene; F, Fe-Ti oxides. 

the members (fig. 6); in addition, in the QSP diagram, samples higher in the stratigraphic section plot closer to the plagioclase comer (fig. 7). Kevex EDXRF data (Appendix 2; 

fig. 8) are characterized by relatively low total iron and zirconium and variable strontium and neodymium compared 

with other tuffs we have examined; barium increases upward 

in the formation. Although these data are few, the values are 

similar for Leach Canyon samples analyzed by du Bray 

( 1995) during his mapping of the Seaman Range ( du Bray 

and Hurtubise, 1994; Hurtubise and du Bray, 1992), which is 

55 km northwest of Caliente. 

The average K-Ar age of the Leach Canyon Formation 

is 24.0 Ma based on three K-Ar dates by Armstrong (1970) 

and two fission-track dates by Kowallis and Best ( 1990), but 

considerable range (26.7-21.6 Ma) exists in published dates, 

causing doubt as to the actual age of the formation. Based on 

dating in progress by A.L. Deino and M.G. Best (Institute of 

Human Origins in Berkeley, Calif., and Brigham Young 

University, respectively, oral commun., 1991), the best estimate for the age of the Leach Canyon Formation is 23.8 Ma 

(Best and others, 1993). 

CONDOR CANYON FORMATION 

The Condor Canyon Formation, a name originally 

applied by Cook ( 1965), consists of mostly brown and purple, 
56 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

Sanidine 

Quartz 

EXPLANATION 

Narrows Tuff Member 

• From Appendix 1 

o From Williams (1967) 

Table Butte Tuff Member 

• From Appendix 1 

Plagioclase 

o From Williams (1967) 

Undifferentiated as to member 

x From Appendix 1 

+ From Shubat and Siders (1988) 

Figure 7. Ternary quartz-sanidine-plagioclase diagram of the 

Narrows and Table Butte Tuff Members and of samples undifferentiated as to member, Leach Canyon Formation. 

;: 1000 

ID.. 

w 

0 

>< 

~ 100 

w 

1-

~ 

c 

z 

~ 10 

0 

w 

-I 

a. 

::e 

<( 

U) 

Figure 8. Plot of selected elements determined by energy-dispersive X-ray fluorescence (Kevex) of samples of the Leach Canyon 

Formation. Member name and data given in Appendix 2. Iron (in 

weight percent) values result from dividing the raw data by 0.5; other elements (in parts per million) normalized by dividing by chondrite abundances according to values given by Thompson (1984). 

crystal-poor, low-silica rhyolite ash-flow tuff made up oftwo 

members, the Swett Tuff Member and Bauers Tuff Member. 

Outflow cooling units are densely welded and commonly 

contain a black basal vitrophyre zone overlain by a lenticulite 

zone; in some outcrops of the Bauers, the lenticulite zone is 

overlain by a gray vapor-phase zone. Where the Bauers is 

thick, its upper lenticulite or vapor-phase part is in places 

flow foliated probably due to movement during the latest 

stages of emplacement. Williams ( 1967) predicted that the 

source of the Bauers was the Caliente caldera complex, and 

this was confirmed by Rowley and Siders ( 1988), who named 

the source the Clover Creek caldera (fig. 3). Only faulted 

fragments of the caldera remain exposed at and north of Caliente (Rowley and Shroba, 1991; Rowley and others, 1992). 

Because of its petrologic similarity with the Bauers, the Swett 

may also be derived from the Clover Creek caldera or, more 

likely, a predecessor caldera in about the same place that was 

either largely destroyed by Clover Creek caldera subsidence 

or covered by younger basin-fill sedimentary rocks so that no 

trace remains exposed. 

The Swett Tuff Member consists of one and rarely 

several thin (generally less than 15m) ash-flow cooling units 

that formerly covered an area of at least 6,000 km2 

(Williams, 1967) after the effects of east-west extension are 

removed. The Swett Member (figs. 9, 10) contains 5-15 

percent phenocrysts of mainly plagioclase (70-90 percent) 

with small amounts of biotite (10-20 percent) and Fe-Ti 

oxides (<7 percent). Kevex data (fig. 11) show that the Swett 

contains variable strontium, high barium, and relatively high 

cerium. The average of six K-Ar dates of the Swett is 23.7 

Ma (Armstrong, 1970), but its true age is probably closer to 

the age of the Bauers. 

The Bauers Tuff Member consists of one or as many as 

three cooling units totalling generally about 50 m but locally 

as much as 120 m, that spread over an area of at least 23,000 

km2 (Best, Christiansen, and others, 1989, fig. R40). The 

Bauers is similar in appearance to the Swett except that it is 

thicker and better zoned; it is easily distinguished from the 

Swett by the presence of sanidine and pyroxene in the 

Bauers. The member (figs. 9, 10) contains 10-20 percent 

phenocrysts of sanidine (25-50 percent), plagioclase (40-70 

percent), biotite (2-1 0 percent), clinopyroxene ( <3 percent), 

and Fe-Ti oxides ( 1-8 percent). The relative amount of plagioclase to sanidine appears to generally increase upward in 

the member. The member (fig. 12) has similar Kevex values 

to the Swett. The preferred age of the Bauers is 22.8 Ma 

(Best, Christiansen, and others, 1989) based on the average 

of two 40 Arf39 Ar dates by A.L. Deino. 

HARMONY HILLS TUFF 

The Harmony Hills Tuff consists of a pink, tan, and 

gray, crystal-rich, moderately welded, andesitic ash-flow 

tuff that generally is exposed as a simple or multiple cooling 
MIOCENE TUFF STRATIGRAPHY, CALIENTE CALDERA COMPLEX, NEVADA-UTAH 57 

(J) 

1-

(J) 

~ (.) 

0 

z 

w 

:::c 

a.. 

...J 

~ 0 

ILL 

0 

1-

z 

w 

(.) 

0:::: 

w 

a.. 

100 

so~ 

so~ 

40~ 

20~ 

0 

l 

1-

I 

p 

-

-

·-I 

-

t_ I 

I I: I 

I I 

-

s Q 8 H Px F 

PHENOCRYSTS 

Figure 9. Plot of percentages of major phenocrysts in the Swett 

(solid lines) and Bauers (dashed lines) Tuff Members of the Condor 

Canyon Formation. From data in Appendix 1, Williams ( 1967), and 

Shubat and Siders (1988). Averages of most abundant minerals 

shown by arrows. P, plagioclase; S, sanidine; Q, quartz; B, biotite; 

H, hornblende; Px, pyroxene; F, Fe-Ti oxides. 

unit but locally consists of two cooling units. The unit generally is less than 100m thick but is as much as 150m thick. 

It was spread over about 13,000 km2 (Best, Christiansen, and 

others, 1989, fig. R40). Noble and others (1968) and Noble 

and McKee ( 1972) predicted that the source of the Harmony 

Hills is within the Caliente caldera complex, and Ekren and 

others (1977) cited field relations that suggest that the unit 

was derived from the southern part of the complex. Rowley 

(unpub. mapping, 1988) mapped deposits of Harmony Hills 

megabreccia in the southern Delamar caldera that suggest 

that the unit was unusually thick south of the Delamar 

caldera at the time this caldera subsided. Isopach and distribution data (Williams, 1967) and detailed geologic mapping 

(Blank, 1959; Blank and others, 1992; H.R. Blank, oral commun., 1990), however, suggest that the source is almost certain to be in the eastern Bull Valley Mountains of Utah. 

The Harmony Hills Tuff is distinguished from most 

other ash-flow tuffs in and near the Caliente caldera complex 

by its general andesitic chemical composition and abundant 

phenocrysts (fig. 5). It lithologically resembles the Rencher 

Formation (Cook, 1957, 1960; Blank, 1959; Mackin, 1960; 

Blank and others, 1992), which overlies the Harmony Hills 

Quartz 

Sanidine 

EXPLANATION 

Swett Tuff Member 

D. From Appendix 1, Williams (1967), 

and Shubat and Siders (1988) 

Bauers Tuff Member 

Plagioclase 

• From Appendix 1, Williams (1967}, and Shubat 

and Siders (1988) 

Harmony Hills Tuff 

o From Appendix 1 ~d Williams (1967) 

Figure 10. Ternary quartz-sanidine-plagioclase diagram of the 

Swett and Bauers Tuff Members of the Condor Canyon Formation 

and of the Harmony Hills Tuff. 

Fe Ba Rb Nb La Ce Sr Nd Zr Y 

Figure 11. Plot of selected elements determined by energydispersive X-ray fluorescence (Kevex) of samples from the Swett 

Tuff Member of the Condor Canyon Formation. From data in 

Appendix 2. Iron (in weight percent) values result from dividing the 

raw data by 0.5; other elements (in parts per million) normalized by 

dividing the chondrite abundances according to values given by 

Thompson (1984). 
58 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

Figure 12. Plot of selected elements determined by energydispersive X-ray fluorescence (Kevex) of samples from the Bauers 

Tuff Member of the Condor Canyon Formation. From data in 

Appendix 2. Iron (in weight percent) values result from dividing the 

raw data by 0.5; other elements (in parts per million) normalized by 

dividing the chondrite abundances according to values given by 

Thompson (1984). 

in the eastern Bull Valley Mountains and vicinity. Quartz 

and sanidine are absent in the Rencher, providing easy discrimination from the Harmony Hills Tuff. The Harmony 

Hills Tuff (figs. 10, 13) contains 25-60 percent phenocrysts 

of quartz (2-15 percent), sanidine (<3 percent), plagioclase 

(50-70 percent), biotite (10-25 percent), hornblende (5-15 

percent), clinopyroxene (1-8 percent), and Fe-Ti oxides 

(1-4 percent). The formation is chemically distinct from 

other tuffs in the Caliente area, bearing high iron and strontium (fig. 14). The actual isotopic age of the unit is difficult 

to establish. A broad range of K-Ar dates average 21.6 Ma 

(Armstrong, 1970; Noble and McKee, 1972), but the age is 

considered to be 22.5-22.0 Ma because dates of younger 

plutonic rocks in the Iron Springs mining district of Utah and 

of the younger Rencher Formation are in the same range of 

ages (Rowley and others, 1989). 

RACER CANYON TUFF 

The Racer Canyon Tuff was first proposed informally, 

as a member, by Blank (1959). Cook (1960) correlated the 

unit with the Kane Point Tuff Member of the Page Ranch 

Formation, but this usage was not adopted by subsequent 

workers (for example, Noble and others, 1968; Noble and 

McKee, 1972; Ekren and others, 1977), who used Racer 

Canyon Tuff at the formation level. Rowley and others 

(1979) abandoned Kane Point Tuff Member in favor of the 

formation name Racer Canyon Tuff. 

The Racer Canyon Tuff consists of a gray, tan, and 

pink, crystal-rich, poorly to moderately welded, low-silica 

rhyolite ash-flow tuff containing from 1 to 12 cooling units. 

It spread onto rough topography created by active tectonism; 

80 

(/) 

r 

I-

(/) 

>- 0:: 60 1- - (.) 

0 

z 

w 

I 

a.. 

_. 40r- - ~ 0 

Iu.. 

0 

I- 20 t-

~ z - w 

(.) 

~ ~ 0:: 

w 

a.. 

0 1- 1 

p s Q 8 H Px F 

PHENOCRYSTS 

Figure 13. Plot of percentages of major phenocrysts in the Harmony Hills Tuff. From data in Appendix 1 and from Williams 

(1967). Averages of most abundant minerals shown by arrows. P, 

plagioclase; S, sanidine; Q, quartz; B, biotite; H, hornblende; Px, 

pyroxene; F, Fe-Ti oxides. 

i 1000 

1-

a. 

w 

(..) 

X 

w 100 

w 

1-

a: 

c 

z 

0 10 

:I: 

(..) 

w 

...J 

a. 

:E 

<( 

en Fe Ba Rb Nb La Ce Sr Nd 'h. y 

Figure 14. Plot of selected elements determined by energydispersive X-ray fluorescence (Kevex) of samples from the 

Harmony Hills Tuff. From data in Appendix 2. Iron (in weight 

percent) values result from dividing the raw data by 0.5; other 

elements (in parts per million) normalized by dividing the chondrite 

abundances according to values given by Thompson (1984). 

outflow cooling units total generally less than 100 m thick 

but in some places are as much as 450 m thick. The unit is 

exposed over at least 3,000 km2. The source of the Racer 

Canyon is unknown, but it may have been derived from the 

eastern part of the Caliente caldera complex (Siders and others, 1990; Siders, 1991). The unit (figs. 15, 16) contains 

about 15-55 percent phenocrysts of quartz (20-50 percent), 
MIOCENE TUFF STRATIGRAPHY, CALIENTE CALDERA COMPLEX, NEVADA-UTAH 59 

sanidine (5-40 percent), plagioclase (15-60 percent), biotite 

(1-13 percent), hornblende ( <5 percent), clinopyroxene (0 to 

trace amounts), Fe-Ti oxides (1-4 percent), and sphene (<1 

percent). Generally quartz exceeds plagioclase, which in 

turn exceeds sanidine, but Racer Canyon shows considerable 

variation in modal amounts and ratios. The same high degree 

of variation upward in stratigraphic sections is also clear in 

samples of the Racer Canyon Tuff. In some sections, plagioclase increases upward, and more rarely, quartz and sanidine 

decrease upward. Kevex data (fig. 17) likewise show considerable variation from sample to sample and have no distinctive characteristics. 

Three K-Ar dates and one lead-alpha date (summarized 

by Siders and others, 1990) yield an average age of 19.4 Ma 

for the Racer Canyon. This average age is somewhat older 

than the average of two preliminary plateau 40 Arf39 Ar dates, 

18.7±0.1 Ma on sanidine and 19.2±0.8 Ma on biotite, 

recently determined by L.W. Snee from a sample collected 

south of Enterprise Reservoir in the eastern Bull Valley 

Mountains. Biotite ages are less reliable than sanidine ages 

by this technique, and the biotite age has a greater error bar, 

so the true age may be closer to 18.7 than 19.2 Ma. 

HIKOTUFF 

The Hiko Tuff of Dolgoff (1963; see also Noble and 

McKee, 1972) is a major ash-flow sheet that spread over at 

80.-------------------------------~ 

en 

1-

en 

>- 0:::: 

u 60t- - 0 

z 

w 

I 

a.. 1- _J I 

~ 40 t- I 

-

0 -t 

1- - LL 

0 

1- -t I 

z 20t- I If- - w 1-

u 

0:::: 

w I 

a.. -t 1- I 

I _J:- 0 I I· 

p s Q 8 H Px F 

PHENOCRYSTS 

Figure 15. Plot of percentages of major phenocrysts in the Racer 

Canyon Tuff (solid lines) and Hiko Tuff (dashed lines). From data 

in Appendix 1, Siders (1985), and Shubat and Siders (1988). P, plagioclase; S, sanidine; Q, quartz; B. biotite; H, hornblende; Px, pyroxene; F, Fe-Ti oxides. 

least 7,000 km2 of southeastern Nevada (Best, Christiansen, 

and others, 1989, fig. R40). The unit is a gray and tan, crystalrich, poorly to densely welded, low-silica rhyolite. In most 

places, outflow facies consist of one or two simple cooling 

Quartz 

X . • • • ,r • X 

• x'+ ._. + 

. 

Sanidine Plagioclase 

EXPLANATION 

Racer Canyon Tuff 

• From Appendix 1 

x From Siders (1985) 

+ From Shubat and Siders (1988) 

Figure 16. Ternary quartz-sanidine-plagioclase diagram of the 

Racer Canyon Tuff. 

Fe Ba Rb Nb La Ce Sr Nd Zr y 

Figure 17. Plot of selected elements determined by energydispersive X-ray fluorescence (Kevex) of samples of the Racer 

Canyon Tuff. From data in Appendix 2. Iron (in weight percent) 

values result from dividing the raw data by 0.5; other elements (in 

parts per million) normalized by dividing by chondrite abundances 

according to values given by Thompson (1984). 
60 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

units generally less than 50 m thick but locally as much as 

250 m thick. Locally, however, at least four cooling units 

have been identified. 

Noble and others (1968) and Noble and McKee (1972) 

suggested that the source of the Hiko Tuff was the Caliente 

caldera complex, and this was confirmed by Ekren and others (1977), who further noted that the Hiko came from the 

"western lobe" of the complex. The western lobe was named 

the Delamar caldera by Rowley and Siders ( 1988). In addition, the Hiko Tuff erupted from a vent south of the ghost 

town of Delamar that herein we name the Cedar Wash vent 

(fig. 3). This vent is about 2 km in maximum diameter and 

consists mostly of Hiko Tuff that contains subvertical flow 

foliation defined by pumice fragments and, at the vent's 

south edge, of caldera collapse megabreccia (Rowley, 

unpub. mapping, 1992). 

Cook ( 1965) correlated the Hiko Tuff with the Racer 

Canyon Tuff, and we also have yet to find firm field and petrographic bases for distinguishing the two units. Our current 

thinking, however, is that they are separate units erupted 

nearly synchronously from opposite ends of the Caliente 

caldera complex (Siders and others, 1990). The similarity in 

age and composition suggests that the two units erupted from 

the same magma chamber. This view is supported by preliminary paleomagnetic data by C.S. Gromme (unpub. data, 

1991) which suggest that the two units have different paleomagnetic signatures. In addition, other stratigraphic details 

of the Hiko Tuff are beginning to emerge as a result of recent 

unpublished mapping by Rowley and R.E. Anderson in the 

western Caliente caldera complex. For example, intracaldera 

Hiko Tuff southeast of Caliente differs somewhat in petrography from Hiko that makes up the main mass of the 

Delamar caldera southwest of Caliente (fig. 3) and from 

Hiko just east of the area of figure 3 (R.E. Anderson, unpub. 

data, 1992), and we are now entertaining the possibility of 

several batches of Hiko-like tuff, including Racer Canyon 

Tuff, that erupted at different times from separate calderas 

and vents of the same magma chamber. We have yet to determine the stratigraphic relationship of these batches to one 

another. 

Theovera11HikoTuff(figs.15, 18)contains l5-45percent phenocrysts of quartz (10-50 percent), sanidine (10-35 

percent), plagioclase (20-65 percent), biotite (5-15 percent), 

hornblende ( <9 percent), clinopyroxene (0 to trace amounts), 

Fe-Ti oxides (1-4 percent), and sphene (<1 percent). Generally plagioclase exceeds quartz, which in turn exceeds sanidine, but like the Racer Canyon, the Hiko shows 

considerable variation in modal amounts and ratios. In vertical sections, plagioclase generally increases in abundance 

and ratio from base to top. Plagioclase is generally more 

abundant and quartz less abundant in the Hiko Tuff than in 

the Racer Canyon Tuff, so the plots of the two units are 

slightly different (figs. 16, 18). Kevex data (fig. 19) are similar to those of Racer Canyon. The wide variability in modes 

and chemical data within samples of the Hiko and Racer 

Canyon, which currently prevents us from distinguishing 

them in the field, probably reflects the preliminary nature of 

our stratigraphic and petrologic studies. 

Sanidine 

Quartz 

X 

• X 

X 

X 

... X. 

X 

•• • ~ • •••• 

. . •••• ... 

EXPLANATION 

Hiko Tuff 

• From Appendix 1 

Tuff of Tepee Rocks 

x From Appendix 1 

Plagioclase 

Figure 18. Ternary quartz-sanidine-plagioclase diagram of the 

Hiko Tuff and the tuff of Tepee Rocks. 

i 1000 ._ 

c.. 

w 

(.) 

>< 

!:!:!. 

w 

._ 

0:: 

c 

z 

~ 10 

0 

w 

..J 

c.. 

::E 

<C 

(f) 

Fe Ba Rb Nb La Ce Sr Nd Zr Y 

Figure 19. Plot of selected elements determined by energydispersive X-ray fluorescence (Kevex) of samples of the Hiko Tuff. 

From data in Appendix 2. Iron (in weight percent) values result 

from dividing the raw data by 0.5; other elements (in parts per 

million) normalized by dividing by chondrite abundances 

according to values given by Thompson (1984). 
MIOCENE TUFF STRATIGRAPHY, CALIENTE CALDERA COMPLEX, NEVADA-UTAH 61 

Tuffs of the Leach Canyon Formation resemble those of 

the Racer Canyon and Hiko, and little separates the three 

modally except that the Leach Canyon contains fewer phenocrysts. Kevex data are more distinctive, however, because 

the Leach Canyon has lower total iron, barium, strontium, 

and zirconium. Additional data on the Hiko Tuff and Leach 

Canyon Formation from du Bray (1995) support this comparison. 

Taylor and others (1989) preferred an age of 18.6 Ma for 

the Hiko, based on an average of several published 40 Arf39 Ar 

dates determined in the lab ofD.R. Lux (University of Maine, 

Orono). Recent: preliminary plateau 40 Arf39 Ar dates 

determined by L.W. Snee of sanidine from Hiko Tuff, from 

a standard used by Lux and sent to Snee, however, give a 

Hiko age of 18.2±0.1 Ma (Snee, unpub. data, 1993). Identical 

dates on the rhyolite volcanic domes of Crows Nest Tank and 

of Barnes Canyon, both of which are on the rim of the 

Delamar caldera and probably derived from the same magma 

chamber as Hiko Tuff, support an age of 18.2 Ma for Hiko 

Tuff. We are currently continuing geochronologic studies on 

this standard sample plus other samples of Hiko Tuff. 

The relative age of the Hiko Tuff and Racer Canyon 

Tuff has not been determined because their outflow sheets 

have not been identified in the same stratigraphic section. 

The dates on the Racer Canyon Tuff, however, are somewhat 

older than those on the Hiko. A higher average plagioclase 

content and lower average quartz content of the Hiko with 

respect to the Racer Canyon (fig. 15), plus the somewhat 

lower phenocryst content of the Hiko, supports the hypothesis that the Hiko is younger than the Racer Canyon if one 

assumes that their magma chambers were connected and 

compositionally layered during eruption (Lipman and others, 

1966). 

TUFF OF TEPEE ROCKS 

The name tuff of Tepee Rocks is used here for about 100 

m of ash-flow tuff exposed at Tepee Rocks, an area of hoodoos about 6 km east -southeast of Caliente. The unit is a gray 

and tan, crystal-poor, poorly welded, low-silica rhyolite ashflow tuff that lithologically resembles Hiko Tuff but may be 

distinguished from it by Kevex data and by a clearly younger 

isotopic age. Clearly the tuff of Tepee Rocks was derived 

from the Caliente caldera complex and probably from the 

same magma chamber as the Hiko, Racer Canyon, and other 

Hiko-like units. Current field mapping by R.E. Anderson and 

P.D. Rowley has not located a base to the unit at Tepee 

Rocks, so it may be an intracaldera sequence. Outflow deposits of the tuff of Tepee Rocks overlie intracaldera Hiko Tuff 

about 1 km west of Tepee Rocks. Outflow-facies rock of the 

unit outside the Caliente caldera complex underlies a butte 7 

km north of Caliente, where Rowley and Shroba (1991) misidentified the rock as Hiko Tuff. 

The tuff of Tepee Rocks (figs. 18, 20) contains 10-25 

percent phenocrysts of quartz (35-55 percent), sanidine 

(20-40 percent), plagioclase (10-35 percent), biotite (1-7 

percent), hornblende (generally trace amounts), Fe-Ti oxides 

(<3 percent), and sphene (trace amounts). Quartz generally 

exceeds sanidine, which in tum exceeds plagioclase. The low 

amount of hornblende and biotite helps distinguish the unit 

from Hiko and Racer Canyon, but the Kevex data (fig. 21) are 

much more diagnostic because of lower iron, neodymium, 

80 

en 

1-

en 

>- 0::: 

(.) 

0 

z 

~ w 

::r: 

a.. 

...J 

~ 

~ 0 

1-

LL. 

0 

1-

z 

w 

(.) 

0::: 

w 

a.. 

0 ~ P S Q B H Px F 

PHENOCRYSTS 

Figure 20. Plot of percentages of major phenocrysts in the tuff of 

Tepee Rocks. From data in Appendix 1. Averages of most abundant 

minerals shown by arrows. P, plagioclase; S, sanidine; Q, quartz; B, 

biotite; H, hornblende; Px, pyroxene; F, Fe-Ti oxides. 

Figure 21. Plot of selected elements determined by energydispersive X-ray fluorescence (Kevex) of samples of the tuff of 

Tepee Rocks. From data in Appendix 2. Iron (in weight percent) 

values result from dividing the raw data by 0.5; other elements (in 

parts per million) normalized by dividing by chondrite abundances 

according to values given by Thompson (1984). 
62 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

zirconium, and yttrium for the tuff of Tepee Rocks. A pre· 

liminary plateau 40ArJ39Ar date of 17.8±0.7 Ma was determined by L.W. Snee on a sample of biotite from the unit 

collected from Tepee Rocks. 

DELAMAR LAKE TUFF 

The Delamar Lake Tuff, defined by Scott and others 

(1993), was formerly called member 0 of the Kane Wash 

Tuff (Novak, 1984). It is the oldest known unit in the Caliente area that belongs to a basalt and high-silica rhyolite 

assemblage; this assemblage regionally postdates a calcalkalic andesite to low-silica rhyolite assemblage that 

includes the tuff of Tepee Rocks and older volcanic units 

described previously. In some nearby parts of the Basin and 

Range province, this petrologic change marked initiation of 

or an increase in extensional tectonism (for example, Christiansen and Lipman, 1972), but in the area of the Caliente 

caldera complex no such tectonic change is known to have 

taken place. 

The Delamar Lake Tuff consists of at least four cooling 

units of crystal-poor, nonwelded to densely welded, high-silica rhyolite ash-flow tuff (Harding, 1991; Harding and others, this volume; Scott and others, 1993; Scott and others, 

this volume). The thickest part of the outflow facies of the 

tuff is exposed in the central Delamar Valley near the west 

edge of the Caliente caldera complex (Swadley and Rowley, 

1994 ), and thus the Delamar Lake Tuff is summarized here 

to allow comparison with other units described in this report. 

More details about the unit are provided by Scott and others 

(this volume). R.B. Scott (written commun., 1992) suggested 

that the Delamar Lake Tuff came from a source somewhere 

west of the Delamar Mountains, close to its thickest outflow. 

Most outflow deposits of the Delamar Lake Tuff occur in an 

area of about 1,600 km2 surrounding the Kane Springs Wash 

caldera (Novak, 1984, fig. 2a; Scott and others, 1993). 

Samples of the Delamar Lake Tuff (figs. 22, 23) from 

Delamar Valley west of the Caliente caldera complex contain 7-20 percent phenocrysts of quartz (30-45 percent), 

sanidine ( 55-70 percent), plagioclase ( <2 percent, xenocrysts?), biotite (trace amounts, xenocrysts?), amphibole (0 

to trace amounts, xenocrysts?), hedenbergite pyroxene (1-5 

percent), Fe-Ti oxides (trace amounts), and fayalitic olivine 

(1-3 percent). Modal analyses of Delamar Lake Tuff from 

the Kane Springs Wash caldera area (R.B. Scott and A.E. 

Harding, written commun., 1991) are similar to the samples 

from near the Caliente caldera complex that we analyzed. 

Kevex analysis (fig. 24) of only one sample from near the 

Caliente caldera complex is similar to trace-element data for 

a single sample given for the unit by Harding (1991, Appendix IIA, unit Tdl; Harding and others, this volume, Appendix 

2A). The Delamar Lake Tuff has been dated by K-Ar methods at 15.8-15.5 Ma (Novak, 1984), but its true age is probably slightly older because Novak's dates are consistently 

80 

(f) 

I-

(f) 

>- 0:: 

0 -1-

0 I 

z 

UJ 

:c 

a. 

.....J 

~ I 

-I 

0 ._ I- I 

u.. 

0 

Iz 

UJ 

0 

0:: 

UJ 

a. 

0 

p s Q 8 H Px 0 

PHENOCRYSTS 

Figure 22. Plot of percentages of major phenocrysts in the tuff of 

Sawmill Canyon (solid lines) and in the Delamar Lake Tuff (dashed 

lines). From data in Appendix I. Averages of most abundant minerals shown by arrows. P, plagioclase; S, sanidine; Q, quartz; B, biotite; H, hornblende; Px, pyroxene; F, Fe-Ti oxides; 0, olivine. 

Sanidine 

Quartz 

EXPLANATION 

Tuff of Sawmill Canyon 

• From Appendix 1 

Delamar Lake Tuff 

x From Appendix 1 

Plagioclase 

Figure 23. Ternary quartz-sanidine-plagioclase diagram of the 

tuff of Sawmill Canyon and of the Delamar Lake Tuff. 
MIOCENE TUFF STRATIGRAPHY. CALIENTE CALDERA COMPLEX, NEVADA-UTAH 63 

younger by about 0.4 m.y. than new 40 ArJ39 Ar dates of other 

tuffs by L.W. Snee (see section on Kane Wash Tuff) (Scott 

and others, this volume). 

Southwest of the Kane Springs Wash caldera, the 

Delamar Lake Tuff is overlain by a similar ash-flow tuff, formerly called member W of the Kane Wash Tuff by Novak 

( 1984) and recently defined as the Sunflower Mountain Tuff 

by Scott and others (1993). It is not known to be present in 

the area of the Caliente caldera complex. Harding ( 1991 ), 

Harding and others (this volume), and Scott and others (this 

volume) describe the unit, which has been dated by K-Ar 

methods at 14.7 Ma (Novak, 1984). 

TUFF OF RAINBOW CANYON 

The name tuff of Rainbow Canyon is used here for nonwelded to poorly welded, crystal-poor, high-silica, rhyolite 

ash-flow tuff whose intracaldera facies is well exposed in 

Rainbow Canyon south of Caliente. The unit is lithologically 

i 1000 co----,----,--,--.----,-----.-----,------,--~ 

1-

a.. 

w 

~ 100 w 

w 

Iii: 

c 

z 

0 

::r: 

(.) 

w 

.J 

a. 

:E 

<C 

rn 

10 

i1000 I 

~ 100 

w 

Iii: 

c 

z 

~ 10 

(.) 

w 

.J 

a. 

:E 

<C 

rn 

B 

Figure 24. Plot of selected elements determined by energydispersive X-ray fluorescence (Kevex) of samples of A. the tuff of 

Sawmill Canyon and B, the Delamar Lake Tuff. From data in 

Appendix 2. Iron (in weight percent) values result from dividing the 

raw data by 0.5; other elements (in parts per million) normalized by 

dividing by chondrite abundances according to values given by 

Thompson ( 1984 ). 

distinct from the Hiko Tuff and tuff of Tepee Rocks. The 

name was first applied by Bowman ( 1985), but we restrict the 

name to the lowermost thick, mostly pink and light-yellow 

part of the intracaldera sequence. The rock is characterized 

by conspicuous and locally abundant volcanic rock fragments of dark comagmatic flow rock and abundant, partially 

collapsed pumice fragments. The exposures define a trapdoor-type caldera inset into the Delamar caldera and named 

the Buckboard Canyon caldera (fig. 3) by Rowley and Siders 

( 1988). Field relations indicate that the trapdoor nature of the 

caldera resulted from more subsidence on its north side than 

on its south side. Outflow ash-flow deposits are not recognized outside the Buckboard Canyon caldera. 

Intracaldera deposits of the tuff of Rainbow Canyon 

(figs. 25, 26) contain 5-25 percent phenocrysts of quartz 

(35-85 percent), sanidine (15-65 percent), plagioclase (<10 

percent), biotite ( <3 percent), and Fe-Ti oxides ( <2 percent). 

Kevex data (fig. 27) show that the rock is characterized by 

low iron and zirconium, and high rubidium, niobium, and 

barium relative to the other high-silica rhyolite tuffs. Mehnert 

and others (1989) reported a K-Ar date of 15.2±0.6 Ma from 

a sample at the top of the unit collected 6 km south-southeast 

of Caliente, and L. W. Snee recently determined a preliminary 

en 

rC/) 

~ (.) 

0 

z 

UJ 

I 

a. 

....J 

~ 0 

ru.. 

0 

rz 

UJ 

(.) 

0::: 

UJ 

a. 

100~------------------------------~ 

80f- -

60- -

40- -

20- -

1+-

o~------------~~------------·~ P S Q B H Px F 

PHENOCRYSTS 

Figure 25. Plot of percentages of major phenocrysts in the tuff of 

Rainbow Canyon. From data in Appendix 1. Averages of most 

abundant minerals shown by arrows. P, plagioclase; S, sanidine; Q, 

quartz; B, biotite; H, hornblende; Px, pyroxene; F, Fe-Ti oxides. 
64 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

Sanidine 

Quartz 

EXPLANATION 

Tuff of Rainbow Canyon 

• From Appendix 1 

Plagioclase 

Figure 26. Ternary quartz-sanidine-plagioclase diagram of the 

tuff of Rainbow Canyon. 

plateau 40 Arf39 Ar date of 15.6±0.2 Ma from sanidine collected from low in the unit 4 km south-southwest of Caliente. 

TUFF OF KERSHAW CANYON 

The upper part of Bowman's tuff of Rainbow Canyon is 

herein called the tuff of Kershaw Canyon. The unit is a relatively thin, mostly white and gray sequence of crystal-poor, 

nonwelded to poorly welded, high-silica rhyolite ash-flow 

tuff, pyroclastic-fall tuff, and water-laid and mudflow-laid 

tuff that contains locally abundant volcanic rock fragments 

and white pumice. Except in general color, hand samples of 

the unit resemble the tuff of Rainbow Canyon. 

The tuff of Kershaw Canyon is thickest and best 

exposed in Kershaw Canyon, a tributary that enters Rainbow 

Canyon 2 km south of Caliente. Here the unit is about 150 m 

thick and its base is not exposed. The tuff here may represent 

intracaldera deposits, but if so, a caldera wall has not been 

recognized. Alternatively, a caldera may lie east of Kershaw 

Canyon, in an area covered by younger fanglomerates 

informally named the conglomerate of Ash Canyon (Rowley 

and Shroba, 1991 ). The unit is thinner just west and south of 

Kershaw Canyon, where it rests on tuff of Rainbow Canyon 

and thus presumably represents outflow facies, and it thins 

and pinches out within about 6 km west and south of Kershaw 

Canyon. North of Kershaw Canyon, it intertongues with tan 

and reddish-brown sandstone and conglomerate referred to 

Fe Ba Rb Nb La Ce Sr Nd Zr Y 

;: 10000 

1-

c. 

~ 1000 

>< 

!:!:!. 

w 100 1-

a: 

c 

z 10 0 

::I: 

(.) 

w 

...J 

c. 

:E 

<( 0.1 tn 

Fe Ba Rb Nb La Ce Sr Nd Zr y 

Figure 27. Plot of selected elements determined by energydispersive X-ray fluorescence (Kevex) of samples of the tuff of 

Rainbow Canyon. A, samples 87-221, 87-223, 87-1110, 88-115, 

88-118, and 88-126d. B, samples 88-164, 88-220, 88-224, 88-

1158b, and 88-1264. From data in Appendix 2. Iron (in weight 

percent) values result from dividing the raw data by 0.5; other 

elements (in parts per million) normalized by dividing by chondrite 

abundances according to values given by Thompson (1984 ). 

as the sedimentary rocks of Newman Canyon (Rowley, 

Shroba, and others, 1994). These deposits are well exposed 

as coarse conglomerate on the east side of Caliente and as 

sandstone, mudflow, and conglomerate in Newman Canyon 

west of Caliente, in which are intertongued thin, gray and 

yellow airfall and ash-flow tuff beds of the tuff of Kershaw 

Canyon. Clear outflow ash-flow deposits, rarely more than 

50m thick, are exposed west of the Chief Range and about 

20 ,km northwest of the Caliente caldera complex (P.D. 

Rowley and F.W. Simonds, unpub. mapping, 1991). 

The tuff of Kershaw Canyon (figs. 28, 29) contains 

5-30 percent phenocrysts of quartz (20-55 percent), sanidine (25-65 percent), plagioclase (10-45 percent), biotite 

( <6 percent), hornblende ( <4 percent), clinopyroxene ( <4 

percent), Fe-Ti oxides ( <7 percent), and olivine ( < 1 percent, 

xenocrysts?). These modal data clearly distinguish the unit 

from the tuff of Rainbow Canyon; the tuff of Kershaw Canyon contains much more abundant plagioclase and more 
MIOCENE TUFF STRATIGRAPHY, CALIENTE CALDERA COMPLEX, NEVADA-UTAH 65 

abundant biotite than the tuff of Rainbow Canyon, and the 

tuff of Kershaw Canyon contains hornblende and pyroxene 

that the tuff of Rainbow Canyon does not. Kevex data (fig. 

30) are as diagnostic as modal data in distinguishing the unit 

from the tuff of Rainbow Canyon: the tuff of Kershaw Canyon contains higher iron, strontium, barium, lanthanum, and 

neodymium, and lower rubidium. 

The tuff of Kershaw Canyon is intertongued with outflow sheets of the tuff of Sawmill Canyon and Kane Wash 

Tuff (sections following) and is overlain by the tuff of Etna 

(section following). The sedimentary rocks of Newman Canyon, with which the tuff of Etna intertongues, contains a 

waterlaid pumice bed above the tuff of Etna that received a 

K-Ar date on sanidine by H.H. Mehnert of 13.8±0.6 Ma 

(Rowley and others, 1992). A white airfall tuff bed in the 

McCullough Formation (Axen and others, 1988, fig. 2d) 

about 45 km north-northwest of Caliente received a plateau 

40 Arf39 Ar date on biotite of 15.3±0.2 Ma (Axen and others, 

1988; Taylor and others, 1989). The McCullough is a 

sequence of landslides deposited syntectonically with movement on the Highland detachment fault. Axen and others 

(1988) suggested that the tuff correlates with Bowman's 

(1985) tuff of Rainbow Canyon, and this appears to be confirmed because modal analysis and Kevex data (Appendixes 

1, 2; figs. 28, 29, 30) suggest that the McCullough tuff correlates with the tuff of Kershaw Canyon. 

80.-------------------------------~ 

(f) 

I-

({) 

~ (.) 60 

0 

z 

w 

:::c 

a_ 

....J 

.:: 40 

0 

1-

u. 

0 

1-

a:i 20 

() 

0::: 

w 

a_ 

PHENOCRYSTS 

Figure 28. Plot of percentages of major phenocrysts in the tuff of 

Kershaw Canyon (solid lines) and from a sample of a tuff from the 

McCullough Fom1ation (open circles) of Ax en and others ( 1988). 

From data in Appendix 1. Averages of most abundant minerals 

shown by arrows. P, plagioclase; S, sanidine; Q, quartz; B, biotite; 

H, hornblende; Px, pyroxene; F, Fe-Ti oxides; 0, olivine. 

TUFF OF SAWMILL CANYON 

The name tuff of Sawmill Canyon is used here for a tan 

(commonly weathering dark brown), poorly to moderately 

welded, pumice-rich, crystal-poor, high-silica rhyolite ashflow tuff that is commonly about 50 m thick. The tuff is well 

exposed at stream level at the mouth of Sawmill Canyon 

where it enters Rainbow Canyon 6 km south-southwest of 

Caliente, and also just northwest of this location, about V2 km 

east up Buckboard Canyon from where it enters Rainbow 

Canyon. This single outflow cooling unit underlies only a 

small area in Rainbow Canyon south of Caliente, where it is 

interbedded with the lower part of the tuff of Kershaw Canyon; probably both units formerly occupied a valley or basin 

there. The source of the tuff of Sawmill Canyon is unknown 

but probably lies near its area of deposition. 

The tuff of Sawmill Canyon (figs. 22, 23) contains 3-7 

percent phenocrysts of quartz (25-55 percent), sanidine 

(45-80 percent), plagioclase (trace amounts), biotite (trace 

amounts), hedenbergite (<3 percent), and Fe-Ti oxides (<2 

percent). Quartz phenocrysts tend to be rounded by absorption, and sanidine phenocrysts are commonly adularescent 

(as this term is used by Mittwede, 1987) and occur as 

rounded to euhedral crystals; biotite is present but olivine is 

not, in contrast to the overlying Kane Wash Tuff. Kevex data 

Sanidine 

Quartz 

oo 

0 ~ 0 

0 0 0 

0 

X 

0 

0 

<o 0 0 

EXPLANATION 

Tuff of Kershaw Canyon 

0 From Appendix 1 

McCullough Formation 

x From Appendix 1 

Plagioclase 

Figure 29. Ternary quartz-sanidine-plagioclase diagram of the 

tuff of Kershaw Canyon and of a tuff from the McCullough 

Formation. 
66 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

~ 1000 c--,----,--------,-------r-----r-------,----,--------.------:J 

1-

a.. 

w 

0 

>< 

!:!:!.. 100 

w 

1-

~ 

c 

z 

0 

:I: 

0 

w 

..J 

a. 

:e 

c( 

en 

Qi' 1000 ~ 

1-

a.. 

w 

0 

>< 

!:!:!.. 100 

w 

1-

~ 

c 

z 

0 

:I: 

0 

w 

..J 

a. 

:e 

c( 

en 

10 

c 

Figure 30. Plot of selected elements determined by energydispersive X-ray fluorescence (Kevex) of samples of the tuff of 

Kershaw Canyon and of a tuff from the McCullough Formation. A, 

samples 88-120c, 88-123, 88-125c, 88-126c, 88-195a, 88-219f, and 

88-331. B, samples 89-695b, 89-696b, 89-698a, 89-698d, 89-698g, 

89-1097a, 89-1097e, and 89-1097f. C, McCullough Formation. 

From data in Appendix 2. Iron (in weight percent) values result 

from dividing the raw data by 0.5; other elements (in parts per 

million) normalized by dividing by chondrite abundances 

according to values given by Thompson (1984). 

(fig. 24) somewhat resemble those from the tuff of Rainbow 

Canyon but represent significantly higher values of cerium, 

neodymium, and zirconium. The tuff of Sawmill Canyon 

lithologically and petrographically resembles cooling units 

of the Delamar Lake, Sunflower Mountain, and Kane Wash 

Tuffs, but Kevex data and especially paleomagnetic data 

(C.S. Gromme, unpub. data, 1991) rule out a correlation with 

those units. The age of the tuff of Sawmill Canyon is constrained by its stratigraphic position above the tuff of Rainbow Canyon and below the Kane Wash Tuff, that is, between 

15.6 and 14.6 Ma. 

KANE WASH TUFF 

The Kane Wash Tuff was originally named by Cook 

( 1965) for quartz- and sanidine-bearing, high-silica rhyolite 

ash-flow tuffs in Nevada. The unit was redefined several 

times, most recently by Scott and others ( 1993 ), who placed 

the tuffs of Cook's Kane Wash Tuff into three formations, 

two of which (Delamar Lake Tuff and Sunflower Mountain 

Tuft) consist of metaluminous tuff and one of which (the 

overlying restricted Kane Wash Tuff of Scott and others) 

consists of a lower member of metaluminous tuff and an 

upper member of peralkaline tuff. Outflow sheets of these 

units are generally dark weathering, densely welded cooling 

units containing adularescent sanidine. The stratigraphic 

position of the Delamar Lake Tuff relative to the tuff of 

Rainbow Canyon shown in figure 4 is conjectural inasmuch 

as these units do not occur in the same area. 

As redefined by Scott and others (1993), the Kane 

Wash Tuff consists of the Grapevine Spring Member and 

overlying Gregerson Basin Member. Both members were 

derived from the Kane Springs Wash caldera (Noble, 1968; 

Scott and others, 1993; Scott and others, this volume). The 

Grapevine Spring is not found near the Caliente caldera 

complex but is confined to the area around the Kane Springs 

Wash caldera, extending especially to the southwest (Novak, 

1984, fig. 3a). The member is described by Harding (1991 ), 

Harding and others (this volume), and Scott and others (this 

volume). The Grapevine Spring Member is a crystal-poor, 

nonwelded to densely welded, metaluminous to nearly peralkaline, high-silica rhyolite ash-flow tuff that was earlier 

referred to informally as member V1 by Novak (1984). 

Trace-element data of two samples by Harding ( 1991, 

Appendix IIA, unit Tkg; Harding and others, this volume, 

Appendix 2A) show higher barium, lanthanum, neodymium, 

and zirconium, and lower cerium than values for the tuff of 

Sawmill Canyon; the Grapevine Spring Member has significantly higher cerium, lanthanum, neodymium, and zirconium than the Delamar Lake and Sunflower Mountain Tuffs. 

The Grapevine Spring Member has been dated by K-Ar 

methods at 14.2-14.0 Ma (Novak, 1984), but we favor a 

more precise preliminary 40 Arf39 Ar date on sanidine of 

14.7±0.2 Ma recently determined by L.W. Snee. (See Scott 

and others, this volume.) 

The Gregerson Basin Member consists of two ash-flow 

units, formerly called members V2 and V3 by Novak (1984). 
MIOCENE TUFF STRATIGRAPHY, CALIENTE CALDERA COMPLEX, NEVADA-UTAH 67 

Each is compositionally zoned from a comenditic base to a 

trachytic top. Outflow deposits of both units are widely 

exposed near Caliente, where Bowman (1985) informally 

called them units C2 (lower) and C3 (upper) of the tuff of 

Rainbow Canyon. Near Caliente, each of the Gregerson 

Basin units consists of one or two cooling units. Modal 

analyses of samples from the Caliente area of the lower unit 

of the Gregerson Basin Member (figs. 31, 32) show that it 

contains 3-15 percent phenocrysts of quartz (15-50 

percent), sanidine (45-80 percent), hedenbergite (1-10 

percent), Fe-Ti oxides ( <5 percent), fayalitic olivine ( <6 

percent), and local altered ferromagnesian minerals (as much 

as 10 percent). Tiny crystals of red acmite(?) occur in nearly 

all thin sections of the unit. Modal analyses of the Gregerson 

Basin Member from the Kane Springs Wash caldera (R.B. 

Scott and A.E. Harding, written commun., 1991) are similar 

to those from the Caliente area. The abundant crystals of 

hedenbergite and fayalitic olivine, and the absence of biotite, 

distinguish tuffs of the Kane Wash Tuff from other tuffs. 

Kevex data (fig. 33) also clearly identify Kane Wash Tuff, 

for (as befitting a peralkaline rock) they display especially 

high zirconium as well as high iron, niobium, lanthanum, 

cerium, and neodymium. These data are similar to those 

reported by Novak and Mahood (1986, table 10), Harding 

( 1991, Appendix IIA, unit Tkbi), and Harding and others 

en 

len 

~ 

80.------------------------------. 

g 60- I - z 

w 

:::c 

a.. 

...J 

~ 

~ LL 

0 

1-

z 

w 

{) 

0:::: 

w 

a.. 

I 

- 40- -

20- -

0~--~----------------~~~~~~~ : L I: 

P S Q 8 H Px F 0 

PHENOCRYSTS 

Figure 31. Plot of percentages of major phenocrysts in the lower 

unit (solid lines) and the upper unit (dashed lines) of the Gregerson 

Basin Member of the Kane Wash Tuff. From data in Appendix 1. 

Averages of most abundant minerals shown by arrows. P, plagioclase; S, sanidine; Q, quartz; B, biotite; H, hornblende; Px, pyroxene; F, Fe-Ti oxides; 0, olivine. 

(this volume, Appendix 2A). Modal analyses of Calientearea samples of the upper unit of the Gregerson Basin 

Member (figs. 31, 32) show that it contains 5-15 percent 

phenocrysts of the same minerals in similar proportions to 

the lower unit of the Gregerson Basin Member, except that 

the upper unit seems to have somewhat more quartz relative 

to sanidine than the lower unit. The same trend, with more 

quartz relative to sanidine upward, seems to characterize all 

individual cooling units in the Gregerson Basin Member. 

The upper unit is identified in the field by the fact that 

euhedral sanidine phenocrysts are significantly larger than 

those in the lower unit. Most phenocrysts are euhedral in 

both units of the member. Kevex data (fig. 33) of the upper 

unit likewise present a picture nearly identical to that of the 

lower unit, although the upper unit has trace-element 

abundances consistent with a more peralkaline composition 

than the lower unit. These data and comparisons are similar 

to those reported by Novak and Mahood (1986, table 10) and 

Harding (1991, Appendix IIA, unit Tkb2). The Gregerson 

Basin Member has been dated at 14.1 Ma (Novak, 1984) by 

the K-Ar method, but we favor more precise preliminary 

40 Ar/39 Ar dates recently determined by L.W. Snee (Scott 

and others, this volume) of 14.6±0.1 Ma for the lower unit 

(member V 2 of Novak, 1984) and 14.4±0.1 for the upper unit 

(member V3 of Novak, 1984). 

Quartz 

Sanidine Plagioclase 

EXPLANATION 

Gregerson Basin Member of Kane Wash Tuff 

• Lower unit, from appendix 1 

.a. Upper unit, from appendix 1 

Figure 32. Ternary quartz-sanidine-plagioclase diagram of the 

lower and upper units of the Gregerson Basin Member of the Kane 

Wash Tuff. 
68 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

TUFFOFETNA 

The informal name tuff of Etna was used by Rowley, 

Shroba, and others (1994) for a light- to medium-gray, outflow ash-flow tuff widely exposed in the Caliente area and 

named for exposures on the tops of buttes and mesas above 

the Etna railroad stop in Rainbow Canyon 7 km south-southwest of Caliente. The tuff was mapped by Bowman ( 1985) 

as unit C4 of his tuff of Rainbow Canyon, and we later incorrectly correlated it (Snee and others, 1990) with the Ox Valley Tuff, which it closely resembles. The unit consists of one 

to five crystal-poor to crystal-rich, poorly to highly welded, 

high-silica rhyolite ash-flow cooling units with a total thickness of about 60 m. The unit is also exposed about 20 km 

north of the west edge of the Caliente caldera complex in a 

few scattered areas west of the Chief Range; the southern 

distal edge is exposed on the northern flank of the Kane 

Springs Wash caldera (Scott and others, this volume). The 

source of the tuff of Etna was suggested by Best and others 

;: 1000 

1-

a. 

w 

(.) 

>< 

~ 100 

w 

1-

~ 

c 

z 

~ 10 

(.) 

jjj 

...1 

a. 

:e 

<( 

U) 

Figure 33. Plot of selected elements determined by energydispersive X-ray fluorescence (Kevex) of samples of A, lower, and 

B, upper units of the Gregerson Basin Member of the Kane Wash 

Tuff. From data in Appendix 2. Iron (in weight percent) values 

result from dividing the raw data by 0.5; other elements (in parts per 

million) normalized by dividing by chondrite abundances 

according to values given by Thompson ( 1984 ). 

(1993) to be about 9 km south-southwest of Caliente in the 

east-striking fault zone of the Helene lineament that defines 

the south side of the Caliente caldera complex, but recent 

field work and petrologic studies now rule out that hypothesis. We have not found the source but infer that it is somewhere in the central part of the Caliente caldera complex. 

The tuff of Etna (figs. 34, 35) contains 5-40 percent 

phenocrysts of quartz (10-40 percent), sanidine (50-85 percent), plagioclase (<3 percent), biotite (trace amounts), 

amphibole ( <5 percent), pyroxene ( <6 percent), Fe-Ti oxides 

( <4 percent), fayalitic olivine ( <9 percent), and altered ferromagnesian minerals (as much as 6 percent). As with the 

Gregerson Basin Member of the Kane Wash Tuff, there 

seems to be a trend in stratigraphic sections of the tuff of 

Etna toward greater quartz in samples collected higher in the 

section. Distinguishing features of the tuff of Etna include 

( 1) its brown amphibole, which probably replaced pyroxene 

(apparently hedenbergite, which is sparse and locally occupies cores in the amphibole grains) before tuff eruption; (2) 

plagioclase phenocrysts that generally occur in the cores of 

sanidine phenocrysts; (3) deeply embayed quartz; and (4) 

rounded, rarely euhedral sanidine crystals that are commonly 

en 

len 

~ (.) 

0 

z 

w 

:c 

Q. 

...J 

~ 0 

ILL 

0 

1-

z 

w 

(.) 

0::: 

w 

Q. 

100~--------------------------~ 

20 

-I 

I 

lf1 

:-

1 

OL-~------------~-L~-L~-L~-L--~ 

P S Q B H Px F 0 

PHENOCRYSTS 

Figure 34. Plot of percentages of major phenocrysts in the tuff of 

Etna (solid lines) and the Ox Valley Tuff (dashed lines). From data 

in Appendix 1. Averages of most abundant minerals shown by arrows. P, plagioclase; S, sanidine; Q, quartz; B, biotite; H, hornblende; Px, pyroxene; F, Fe-Ti oxides; 0, olivine. 
MIOCENE TUFF STRATIGRAPHY, CALIENTE CALDERA COMPLEX, NEVADA-UTAH 69 

Quartz 

Sanidine Plagioclase 

EXPLANATION 

• Tuff of Etna, from Appendix 1 

o Ox Valley Tuff, from Appendix 1 

Figure 35. Ternary quartz-sanidine-plagioclase diagram of the 

tuff of Etna and the Ox Valley Tuff. 

adularescent. Kevex data (fig. 36) show low zirconium and 

high barium relative to the Delamar Lake, Sunflower Mountain, and Kane Wash Tuffs. The tuff of Etna was emplaced 

between 14.4 and 13.8 Ma based on its stratigraphic position 

between the underlying Gregerson Basin Tuff Member and 

an overlying airfall tuff in the sedimentary rocks of Newman 

Canyon. D.R. Lux (University of Maine at Orono) has 

obtained a compatible 40 ArJ39 Ar date for the tuff of Etna 

(D.R. Lux, written commun., 1987). 

OX VALLEY TUFF 

The Ox Valley Tuff of Blank (1959) was formally proposed by Cook (1960) and adopted by Noble and McKee 

(1972) and Rowley and others (1979). The unit is a lightgray, crystal-poor, poorly to densely welded, high-silica rhyolite ash-flow tuff whose outflow consists in most places of 

one to four cooling units that generally total less than 40 m 

thick but may be as thick as 130 m. The Ox Valley Tuff is 

exposed over an area exceeding 500 km2 in southwest Utah 

and adjacent Nevada. The tuff varies greatly in thickness but 

it is not yet known whether or how those variations are 

related to volcanotectonic features. Anderson and Hintze 

(1993) suggested a possible source near the townsite of 

Motoqua in southwest Utah where the Ox Valley may 

exceed a kilometer in thickness, but more recent mapping by 

Fe Ba Rb Nb La Ce Sr Nd Zr Y 

Figure 36. Plot of selected elements determined by energydispersive X-ray fluorescence (Kevex) of samples of the tuff of 

Etna. From data in Appendix 2. Iron (in weight .percent) values 

result from dividing the raw data by 0.5; other elements (in parts per 

million) normalized by dividing by chondrite abundances 

according to values given by Thompson (1984). 

R.E. Anderson reveals that it is even thicker in the eastern 

Clover Mountai,ns, west of the Nevada/Utah State line, and 

this area is more likely as a source. 

The Ox Valley Tuff (figs. 34, 35) contains 30-40 percent phenocrysts of quartz (15-40 percent), sanidine (55-80 

percent), plagioclase (2-5 percent), biotite ( <2 percent), 

amphibole (1-3 percent), clinopyroxene (1 percent), and FeTi oxides ( <3 percent). Sanidine is commonly adularescent. 

Quartz is more abundant upward in the one stratigraphic section collected. The Ox Valley Tuff is distinguished from the 

Delamar Lake, Sunflower Mountain, and Kane Wash Tuffs 

by its conspicuously large rounded quartz phenocrysts that 

contain embayed edges, by its sanidine crystals that commonly have rounded and ragged edges, and by the presence 

of plagioclase, brown amphibole, and traces of sphene. It 

resembles the tuff ofEtna in modal amounts and ratios (figs. 

34, 35), in the presence of most plagioclase in the cores of 

sanidine phenocrysts (also noted by Blank, 1959), and in its 

brown amphibole. It contains somewhat more plagioclase, 

however, than the tuff of Etna, and it also appears to contain 

olivine and trace amounts of sphene. Kevex analyses (fig. 

37) show higher barium values than occur in the tuff ofEtna. 

The two units have somewhat different paleomagnetic signatures (C.S. Gromme, preliminary data, 1990), but the differences could be explained by vertical-axis structural rotations 

of the sample sites (M.R. Hudson and J.G. Rosenbaum, oral 

commun., 1991). 

Modal and Kevex data indicate that the Ox Valley Tuff 

is a similar yet different depositional unit than the tuff of 

Etna. As with the Hiko and Racer Canyon Tuffs, they appear 

to have erupted from widely separated sources in the Caliente 

caldera complex and probably were magmatically related, 

perhaps from the same magma chamber. Evaluation of such 

a possibility will require additional geologic mapping and 
70 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

Fe Ba Rb Nb La Ce Sr Nd Zr Y 

Figure 37. Plot of selected elements determined by energydispersive X-ray fluorescence (Kevex) of samples of the Ox Valley 

Tuff. From data in Appendix 2. Iron (in weight percent) values 

result from dividing the raw data by 0.5; other elements (in parts per 

million) normalized by dividing by chondrite abundances 

according to values given by Thompson (1984). 

isotopic analysis of the Ox Valley Tuff, whose actual age 

remains in doubt. Two discordant K-Ar dates by Noble and 

McKee (1972), one of 15.5±0.4 Ma from a sample from the 

Caliente caldera complex in Nevada and one of 12.6±0.3 Ma 

from a sample from near Ox Valley in southwestern Utah, 

were attributed by them to Ox Valley Tuff. More likely, however, the date of 15.5 Ma is from a sample of the tuff of Etna 

and only the 12.6 Ma date is of the Ox Valley Tuff (Rowley, 

Shroba, and others, 1994). This interpretation is supported by 

two K-Ar dates determined by H.H. Mehnert and discussed 

by Anderson and Hintze (1993); one is a published date of a 

basalt lava flow from Moody Wash, northwest of Gunlock, 

Utah, that underlies Ox Valley Tuff, of 12.3±0.6 Ma (Best 

and others, 1980) and the other is an unpublished date of a 

rhyolite lava flow that overlies it near Enterprise Reservoir, 

Utah, of 11.4±0.5 Ma (H.H. Mehnert, written commun., 

1980). Our best estimate of the age of the Ox Valley Tuff is 

12.6-12.3 Ma. 

CONCLUSIONS 

Eruptions from the Caliente caldera complex began at 

least by 22.8 Ma, the age of the Bauers Tuff Member of the 

Condor Canyon Formation, and continued until the apparent 

age of the Ox Valley Tuff, sometime between 12.6 and 11.4 

Ma. If the Swett Tuff Member of the Condor Canyon Formation were derived from the Caliente caldera complex, as 

seems certain, and if the Leach Canyon Formation also 

erupted from the complex, as seems possible, then the complex is as old as 24 Ma. The major episode of extension in 

this part of the Basin and Range province began before 19.4 

Ma, the age of the porphyry of Meadow Valley Wash, which 

was intruded as numerous widely scattered dikes along faults 

of significant offset. Extension along faults of similar trend 

and offset continued at least until after 14.0 Ma, the age of 

the tuff of Etna, and probably after 12.1 Ma, the youngest of 

two basalt flows (Mehnert and others, 1989). More than 

likely this deformational episode began earlier, probably as 

early as 25 Ma, but the evidence is equivocal. The synchroneity of caldera volcanism and extension means that many 

caldera margins in the Caliente caldera complex are regional 

faults. In particular, the north and south sides of the complex 

are bounded by faults of the east-trending Timpahute and 

Helene lineaments, respectively, and the caldera complex 

can be viewed as a volcanotectonic trough. 

The study of the Caliente caldera complex is just beginning and at the present time has involved mostly stratigraphy 

of tuff units and locating their source calderas. Mapping has 

concentrated on areas on the margin of the western part of 

the caldera complex, where outflow tuffs of the Caliente and 

other calderas are exposed, and on some parts of the interior 

of the complex. This report focuses on the stratigraphy and 

on our effort to correlate tuffs by petrography and chemical 

analyses. 

The oldest tuff discussed here is the poorly welded, 

widespread, rhyolite Leach Canyon Formation (23.8 Ma), 

which on the basis of distribution and isopach data has been 

proposed to be derived from the Caliente caldera complex 

but whose caldera has not yet been discovered. Densely 

welded, low-silica rhyolitic tuffs of the Swett (23.7? Ma) and 

Bauers (22.8 Ma) Tuff Members of the Condor Canyon Formation were widely spread over Nevada and Utah; the 

Bauers was derived from the poorly exposed Clover Creek 

caldera of the Caliente complex, and the Swett was derived 

probably from a nearby caldera-but no trace of it has been 

found. The source of the moderately welded, widespread, 

crystal-rich, andesitic Harmony Hills Tuff (22.5-22.0 Ma) 

has been proposed to be the Caliente caldera complex, but 

more likely the tuff erupted in or near the eastern Bull Valley 

Mountains of Utah. The poorly to moderately welded, lowsilica rhyolite Racer Canyon Tuff ( 19.2 ?-18. 7 Ma) is known 

only from Utah and probably came from the east end of the 

Caliente caldera complex. The poorly to moderately welded, 

widespread, low-silica rhyolite Hiko Tuff ( 18.6-18.2 Ma) 

was derived from the Delamar caldera, at the west end of the 

complex. The poorly welded, low-silica rhyolite tuff of 

Tepee Rocks (17.8 Ma) is a local Hiko-like unit; its source is 

unknown but probably lies just east of the Delamar caldera. 

The Racer Canyon, Hiko, and tuff of Tepee Rocks are similar in lithology and age and appear to have erupted from different vents from widely different parts of the Caliente 

caldera complex that tapped the same magma chamber. 

What we now map as the Hiko Tuff itself, in fact, probably 

consists of several more distinct tuff units erupted from isolated vents in and adjacent to the Delamar caldera. 

At some time after emplacement of the tuff of Tepee 

Rocks, about 17.5-16 Ma, the petrologic regime changed in 

this part of the Basin and Range from one of calc-alkalic 
MIOCENE TUFF STRATIGRAPHY. CALIENTE CALDERA COMPLEX, NEVADA-UTAH 71 

magmatism to one of basalt and high-silica rhyolite magmatism. Although such a change seems in some places in the 

Basin and Range to have coincided with initiation of or an 

increase in extensional tectonism, no tectonic change is 

known in the general area of the Caliente caldera complex. 

The first of the eruptive products of the new magmatic 

regime exposed in the Caliente area is the moderately to 

densely welded, high-silica rhyolite Delamar Lake Tuff 

(15.8-15.5 Ma), which was derived from an unidentified 

source well southwest of the Caliente caldera complex. The 

poorly welded, high-silica rhyolite tuff of Rainbow Canyon 

(15.6-15.2 Ma) is a local unit derived from the Buckboard 

Canyon caldera, which is inset into the Delamar caldera. The 

source of the poorly welded, high-silica rhyolite tuff of Kershaw Canyon (15.3?-14.0 Ma) has not been identified with 

certainty but probably is just east of the Buckboard Canyon 

caldera. The moderately welded, high-silica rhyolite tuff of 

Sawmill Canyon is a local outflow cooling unit interbedded 

with the lower part of the tuff of Kershaw Canyon; its source 

is inferred to be in the central Caliente caldera complex. Outflow cooling units of the moderately to densely welded, 

widespread, peralkaline high-silica rhyolite Gregerson Basin 

Member (14.6-14.4 Ma) of the Kane Wash Tuff, derived 

from the Kane Springs Wash caldera 20 km southwest of the 

Caliente caldera complex, are intertongued with the tuff of 

Kershaw Canyon in the caldera complex. The moderately to 

densely welded, widespread, high-silica rhyolite tuff of Etna 

(14.0 Ma) is derived probably from the central part of the 

Caliente caldera complex. The youngest known significant 

tuff in the area of the Caliente caldera complex is the moderately to densely welded, high-silica rhyolite Ox Valley Tuff 

(12.6?-11.4? Ma). This unit is distributed in the NevadaUtah border area between the Clover and Bull Valley Mountains and probably was derived from a vent in the southeastem part of the Caliente complex. It is lithologically similar 

to the tuff of Etna and may have been derived from the same 

magma chamber. Vents for this lithologic type could be scattered within the southern caldera complex. 

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Galyardt, G.L., 1990, Geologic map of the Newcastle quadrangle, Iron County, Utah: U.S. Geological Survey Geologic 

Quadrangle Map GQ-1690, scale 1:24,000. 

Siders, M.A., and Shubat, M.A., 1986, Stratigraphy and structure of 

the northern Bull Valley Mountains and Antelope Range, Iron 

County, Utah, in Griffen, D.T., and Phillips, W.R., eds., 

Thrusting and extensional structures and mineralization in the 

Beaver Dam Mountains, southwestern Utah: Utah Geological 

Association Publication 15, p. 87-102. 

Snee, L.W., Mehnert, H.H., Rowley, P.D., Anderson, R.E., and 

Scott, R.B., 1990, New isotopic ages demonstrate extensional 

faulting of 19-12 Ma in the western Caliente caldera complex 

and vicinity, Lincoln County, Nevada [abs.]: Eos, v. 71, no. 43, 

p. 1612. 

Steiger, R.H., and Jager, Emile, 1977, Subcommission on geochronology-Convention on the use of decay constants in geo- and 

cosmochronology: Earth and Planetary Science Letters, v. 36, 

p. 359-362. 

Steven, T.A., Morris, H.T., and Rowley, P.O., 1990, Geologic map 

of the Richfield 1 °X2° quadrangle, west-central Utah: U.S. 

Geological Survey Miscellaneous Investigations Series Map 

I-190 1, scale I :250,000. 

Steven, T.A., Rowley, P.D., and Cunningham, C.G., 1984, Calderas 

of the Marysvale volcanic field, west-central Utah: Journal of 

Geophysical Research, v. 89, no. B I 0, p. 8751-8764. 

Swadley, W C, and Rowley, P.D., 1994, Geologic map of the 

Pahroc Spring SE quadrangle, Lincoln County, Nevada: U.S. 

Geological Survey Geologic Quadrangle Map GQ-1752, scale 

1:24,000. 

Taylor, W.J., Bartley, J.M., Lux, D.R., and Axen, G.J., 1989, Timing of Tertiary extension in the Railroad Valley-Pioche 

transect, Nevada-Constraints from 40 Ar/39 Ar ages of volcanic 

rocks: Journal of Geophysical Research, v. 94, p. 7757-7774. 
74 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

Thompson, R.N., 1984, Magmatism of the British Tertiary volcanic 

province: Scottish Journal of Geology, v. 8, pt. 1, p. 49-108. 

Tschanz, C.M., and Pampeyan, E.H., 1970, Geology and mineral 

deposits of Lincoln County, Nevada: Nevada Bureau of Mines 

and Geology Bulletin 73, 188 p. 

Ward, P.L., 1991, On plate tectonics and the geologic evolution of 

southwestern North America: Journal of Geophysical 

Research, v. 96, p. 12479-12496. 

Wiebe, R.A., 1993, Basaltic injections into floored silicic magma 

chambers: Eos, v. 74, no. 1, p. 1-3. 

Willden, Ronald, and Adair, D.H., 1986, Gold deposits at Goldstrike, Utah, in Griffen, D.T., and Phillips, W.R., eds., Thrusting and extensional structures and mineralization in the Beaver 

Dam Mountains, southwestern Utah: Utah Geological Association Publication 15, p. 137-147. 

Williams, P.L., 1967, Stratigraphy and petrography of the 

Quichapa Group, southwestern Utah and southeastern Nevada: 

Seattle, Wash., University of Washington Ph. D. dissertation, 

182 p. 

Zietz, Isidore, Gilbert, F.P., and Kirby, J.R., Jr., 1978, Aeromagnetic map of Nevada-Color coded intensities: U.S. Geological Survey Geophysical Investigations Map GP-922, scale 

I: I ,000,000. 

Zietz, Isidore, Shuey, Ralph, and Kirby, J.R., Jr., 1976, Aeromagnetic map of Utah: U.S. Geological Survey Geophysical Investigations Map GP-907, scale I: I ,000,000. 

APPENDIX 1. 

Modal analyses of rocks in southeastern Nevada and southwestern Utah. Based on counting about 1,000 points on 

stained thin sections. 

Explanation of headings from left to right: Sample 

number-asterisk shows Kevex data are available (in 

Appendix 2); vertical arrows span samples from same stratigraphic section (arrow points toward top of section). 

Loc., location of sample (keys to letters given below). 

Phenocryst minerals are shown as volume percentages of 

total rock (percentage of phenocryst minerals to total phenocrysts given in parentheses): quartz, sanidine, plagioclase, 

biotite, hornblende, pyroxene, Fe-Ti oxides, olivine, altered 

ferromagnesian minerals, sphene. 

Total pheno., total phenocrysts; Q-S-P ratio, relative 

amounts of quartz, sanidine, and plagioclase; VRFs, 

volcanic rock fragments; Pum., collapsed pumice, where 

determined; G'mass, groundmass (for some samples 

includes pumice). T, trace; blank, not present. 

Key to location symbols, which are shown on the Caliente, Nevada-Utah; Cedar City, Utah; and Richfield, Utah 

1 °X2° quadrangles: 

A, Antelope Range, Utah; 8, Bald Hills, Utah; 8L, Black 

Mountains, Utah; C, Chief Range and hills to west, 

Nevada; CA, Caliente area, Nevada; CC, Condor Canyon, 

Nevada; CN, north of Caliente, Nevada; D, Delamar area, 

Nevada; E, Enterprise Reservoir area, Utah; EC, Eccles 

area, Nevada; EL, Elgin area, Nevada; ET, Etna area, 

Nevada; G, Gunlock-Veyo area, Utah; H, Harmony Mountains, Utah; I, Iron Springs mining district, Utah; 18, south 

of Islen-Barclay, Nevada; K, Kane Point, Utah; M, Markagunt Plateau, Utah; ME, north of Mount Escalante, Utah; N, 

Newcastle area, Utah; 0, Oak Spring Summit-Grey Dome 

area, Nevada; OV, Ox Valley, Utah; P, South Pahroc Range, 

Nevada; PE, east of South Pahroc Range, Nevada; R, Red 

Hills, Utah; S, Stine area, Nevada; ST, west of Stampede 

Gap, northern Highland Range, Nevada. 
Sample 

No. 

68-8 

68-321 

68-33 

68-61 

68-74 

68-182 

68-1841 

68-186 . 

71-3 

71-70 

71-1231 

71-124 

71-221 

72-179 

87-852b* 

68-56 

68-92 

68-181 

68-196 

68-200 

71-7 

71-46 

71-128 

71-2231 

71-224 . 

72-37 

72-182 

72-199 

87-851* 

Loc. 

B 

B 

B 

B 

BL 

M 

M 

B 

B 

I 

BL 

c 

BL 

BL 

I 

BL 

R 

I 

I 

B 

BL 

R 

c 

Quartz 

4.4 (34) 

6.5 (38) 

9.4 (36) 

8.3 (32) 

3.0 (19) 

8.0 (30) 

3.2 (17) 

6.8 (38) 

10.9 (46) 

1.8 (12) 

5.9 (34) 

4.6 (27) 

7.0 (42) 

2.5 (22) 

5.2 (28) 

6.7 (39) 

6.0 (30) 

1.0 (7) 

6.0 (28) 

4.2 (25) 

2.5 (27) 

1. 7 (9) 

2.2 (14) 

4.0 (22) 

2.0 (13) 

2.2 (15) 

3.1 (23) 

3.8 (25) 

5.4 (31) 

Sanidine 

2.5 (19) 

4.9 (28) 

7.1 (27) 

8.7 (33) 

4.6 (30) 

8.8 (33) 

6.4 (34) 

4.7 (26) 

6.2 (26) 

3.3 (22) 

5.9 (34) 

6.0 (35) 

4.4 (27) 

2.4 (21) 

8.2 (44) 

4.4 (25) 

5.5 (27) 

1.8 (12) 

4.9 (23) 

3.3 (19) 

2.6 (28) 

2.7 (15) 

4.4 (29) 

4.1 (23) 

2.8 (18) 

0.9 (6) 

0.9 (7) 

3.0 (19) 

3.8 (22) 

Plagioclase 

5.3 (41) 

4.6 (27) 

8.7 (33) 

8.2 (31) 

6.7 (43) 

8.0 (30) 

8.1 (43) 

4.5 (25) 

6.0 (25) 

8.7 (57) 

4.3 (25) 

5.4 (32) 

4.2 (25) 

4.4 (39) 

4.5 (24) 

5.1 (29) 

7.4 (37) 

9.0 (60) 

9.3 (43) 

8.2 (48) 

2.7 (29) 

12.0 (65) 

6.9 (45) 

7.6 (43) 

8.7 (55) 

9.7 (67) 

7.5 (56) 

7.0 (45) 

7.4 (43) 

Biotite 

0.4 (3) 

1.0 (6) 

0.6 (2) 

0.7 (3) 

0.4 (3) 

0.7 (3) 

0.4 (2) 

1.2 (7) 

0.3 (1) 

0.7 (5) 

1. 0 (6) 

0.6 (4) 

0.7 (4) 

1.4 (12) 

0.4 (2) 

0.6 (3) 

0.6 (3) 

1.1 (7) 

0.7 (3) 

0.6 (4) 

0.8 (9) 

1. 0 (5) 

1.5 (10) 

1.4 (8) 

1.1 (7) 

1.1 (8) 

1. 2 (9) 

1.1 (7) 

0.6 (3) 

Hornblende Pyroxene Fe-Ti 

Oxides 

Olivine 

Leach Canyon Formation, Narrows Tuff Member 

T 

T 

0.1 (T) 

0.3 (2) 

0.7 (3) 

0.4 (2) 

0.2 (1) 

0.1 ( 1) 

0.2 (1) 

0.1 (1) 

0.1 (1) 

0.1 ( 1) 

0.1 (1) 

T 

0.2 (1) 

T 

0.3 (2) 

0.3 (2) 

0.2 (1) 

0.2 (1) 

0.3 (2) 

0.5 (2) 

0.2 (1) 

0.4 (2) 

0.2 (1) 

0.6 (4) 

0.2 (1) 

0.4 (2) 

0.2 (1) 

0.5 (4) 

0.2 (2) 

Alt. 

Ferr. 

Leach Canyon Formation, Table Butte Tuff Membef 

0.2 (1) 

0.3 (1) 

0.9 (6) 

0.3 (1) 

0.2 (1) 

0.3 (3) 

0.4 (2) 

0.2 (1) 

0.5 (3) 

0.6 (4) 

0.3 (2) 

0.5 (4) 

0.2 (1) 

0.1 (T) 

0.1 (1) 

T 

T 

T 

0.3 (2) 

0.3 (1) 

1.0 (7) 

0.4 (2) 

0.5 (3) 

0.4 (4) 

0.6 (3) 

0.2 (1) 

0.2 (1) 

0.5 (3) 

0.2 (1) 

0.3 (2) 

0.3 (2) 

0.2 (1) 

Sphene 

T 

T 

T 

Total 

Pheno. 

12.9 

17.3 

26 

26.2 

15.5 

26.7 

18.7 

17.8 

23.6 

15.2 

17.5 

17.1 

16.6 

11.3 

18.6 

17.3 

20.1 

14.9 

21.6 

17 

9.3 

18.4 

15.4 

17.8 

15.7 

14.4 

13.5 

15.4 

17.4 

Q-S-P 

ratio 

36-21-43 

40-31-29 

37-28-35 

33-34-33 

21-32-47 

32-36-32 

18-36-46 

43-29-28 

47-27-26 

13-24-63 

37-37-26 

29-37-34 

45-28-27 

27-26-47 

29-46-25 

41-27-32 

32-29-39 

9-15-76 

30-24-46 

27-21-52 

32-33-35 

10-17-73 

16-33-51 

26-26-48 

15-21-64 

17-7-76 

27-8-65 

27-22-51 

33-23-45 

VRFs 

0.8 

3.0 

1.2 

0.2 

0.4 

1.6 

1.0 

0.4 

1.3 

6.1 

0.9 

1.4 

2.4 

4.2 

4.3 

10.0 

3.1 

3.6 

3.5 

4.5 

3.6 

4.4 

3.2 

3.4 

4.7 

4.6 

6.3 

15.3 

3.3 

Pum. 

1.8 

7.3 

G•mass 

86.3 

79.7 

72.8 

73.6 

84.1 

71.7 

80.3 

81.8 

75.1 

78.7 

81.6 

81.5 

81.0 

84.5 

75.3 

72.7 

76.7 

81.5 

74.9 

78.5 

87.1 

n.2 

81.4 

78.8 

79.6 

81.0 

80.2 

69.3 

72.0 

3:: 

a 

n 

~ 

CI'J 

~ >-3 

~ 

> >-3 

a 

~ 

> 'i:) 

::c: 

~-< 

n 

> r 

m 

~ tn 

n 

> r 

0 

tn 

~ 

> 

n 

0 

3:: 

'i:) 

~ 

~ 

z 

tn 

< 

> 0 

> c 

~ ::c: 

-....J 

VI 
Sample Loc. Quartz Sanidine Plagioclase Biotite Hornblende Pyroxene Fe-Ti Olivine Alt. 

No. Oxides Ferr. 

Leach Canyon Formation, undifferentiated as to member 

86-405 H 5.4 (25) 4.6 (21) 7.7 (36) 2.9 (14) 0.5 (2) 0.3 (1) 

88-654* H 6.0 (47) 3.3 (26) 2.9 (23) 0.4 (3) 0.1 ( 1) 0.1 ( 1) 

91-67b cc 6.9 (34) 6.7 (33) 4.2 (21) 0.6 (3) 0.5 (2) 0.2 (1) 1.3 (6) 

Condor Canyon Formation, Swett Tuff Member 

68-82 B 7.7 (88) 0.8 (9) T 0.3 (3) 

71-471 I 5.2 (81) 0.2 (3) 0.7 (11) 0.3 (5) 

71-48 I 7.6 (85) 1.1 (12) 0.2 (2) 

71-71 l I 10.7 (82) 1.7 (13) T 0.6 (5) 

71-72 I 7.5 (82) 1.5 (16) 0.1 ( 1) 

71-74 I 6.6 (88) 0.9 (12) Tr 

71-75 I 7.6 (84) 1.0 (11) T T 0.4 (4) 

71-132 J B T 5.4 (82) 0.9 (14) 0.3 (5) 

71-133 B 8.0 (88) 0.7 (8) 0.4 (4) 

71-135 B 6.0 (72) 1. 7 (20) 0.6 (7) 

71-136 B T 6.8 (88) 0.6 (8) 0.3 (4) 

71-1531 B 4.5 (79) 1.0 (18) T 0.2 (4) 

71-154 B T 6.8 (85) 0.9 (11) 0.3 (4) 

71-156 B T 7.4 (83) 1.1 (12) 0.4 (4) 

72-241 I 5.0 (82) 0.9 (15) T 0.2 (3) 

72-27 I T 5.4 (81) 1.0 (15) 0.3 (5) 

72-180 BL 5.4 (87) 0.7 (11) 0.1 (2) 

87-452a* l cc T 9.5 (88) 1.0 (9) 0.3 (3) 

87-452d* cc 6.0 (80) 1.1 (15) 0.4 (5) T 

87-844a c T? 9.6 (85) 1.3 (12) 0.4 (4) 

89-1103f* l D T? 8.1 (84) 1.1 (11) 0.4 (4) 

89-1103h D 6.6 (86) 0.9 (12) 0.2 (3) 

90-339 c T? 8.3 (86) 1.2 (12) T 0.2 (2) 

90-400c c 5.9 (87) 0.8 (12) 0.1 (1) 

Sphene Total Q-S-P VRFs 

Pheno. ratio 

21.4 31-26-43 1.5 

T 12.8 49-27-24 5.8 

20.4 39-38-23 2.6 

8.8 0-0-100 0.6 

6.4 0-0-100 0.7 

8.9 0-0-100 0.7 

13 0-0-100 0.3 

9.1 0-0-100 0.7 

7.5 0-0-100 0.1 

9 0-0-100 0.4 

6.6 0-0-100 T 

9.1 0-0-100 T 

8.3 0-0-100 0.7 

7.7 0-0-100 0.4 

5.7 0-0-100 0.5 

8 0-0-100 0.2 

8.9 0-0-100 0.2 

6.1 0-0-100 0.4 

6.7 0-0-100 0.3 

6.2 0-0-100 1.1 

10.8 0-0-100 0.6 

7.5 0-0-100 1.1 

11.3 0-0-100 0.4 

9.6 0-0-100 T 

7.7 0-0-100 2.4 

9.7 0-0-100 0.9 

6.8 0-0-100 0.8 

Pllll. G'mass 

77.1 

4.9 76.5 

77.0 

90.6 

92.9 

6.2 84.2 

86.7 

2.4 87.8 

2.2 90.2 

90.6 

93.4 

90.9 

91.0 

91.9 

93.8 

91.8 

90.9 

93.5 

8.0 85.0 

92.7 

1.5 87.1 

11.6 79.8 

1.0 87.3 

90.3 

89.9 

3.8 85.6 

0.4 92.0 

-...J 

0\ 

a 

tTl 

0 

r 

0 

a 

n 

C/'J 

..., 

c::: 

0 

m 

C/'J 

o::l 

;J> 

;::tl 

(") 

0 

C/'J 

..., 

c::: 

0 

><: 

c::: 

z 

~ 

::0 

\0 

N 
Sample 

No. 

Loc. 

68-671 BL 

68-68 BL 

68-69· BL 

68-80 BL 

69-88 BL 

69-38 BL 

71-9 

71-51 

71-521 

71-54 

71-76 

71-1571 

71-158 

71-204 

73-115 

73-118 

B 

B 

BL 

87-172a N 

87-452e*! CC 

87-452h* cc 

89-1103L* 0 

71-651 

71-68 

71-69 

72-391 72-40 

72-41 

72-43 

72-198 R 

73-105 A 

73-107 A 

73-109 A 

73-111 A 

73-112 A 

73-113 A 

87-142 N 

87-500 s 

87-501c s 

87-1182h* CA 

88-691 H 

88-1143 

89-558*1 

89-560 

c 

CA 

CA 

Quartz 

1.6 (3) 

1.9 (4) 

2.5 (5) 

0.8 (2) 

1.4 (3) 

4.3 (8) 

3.1 (7) 

1.2 (3) 

1.6 (3) 

2.5 (4) 

2.8 (5) 

4.5 (10) 

6.1 (11) 

3.8 (8) 

1.2 (5) 

2.1 (5) 

5.1 (13) 

2.5 (5) 

0.6 (3) 

2.6 (11) 

1.6 (7) 

2.1 (4) 

sanidine 

2.7 (27) 

4.5 (35) 

6.3 (46) 

5.1 (30) 

2.7 (32) 

4.1 (26) 

7.1 (35) 

9.2 (48) 

6.2 (47) 

7.3 (33) 

10.3 (50) 

7.7 (41) 

7.5 (35) 

3.8 (37) 

7. 7 (37) 

6.6 (48) 

6.0 (31) 

5.6 (36) 

5.2 (36) 

5.3 (29) 

0.2 (T) 

0.2 (T) 

0.6 (1) 

T 

0.2 (T) 

0.2 (T) 

0.2 (T) 

T 

0.1 (T) 

0.5 (1) 

0.1 (T) 

0.3 (1) 

0.6 (3) 

0.4 (2) 

0.2 (T) 

Plagioclase 

5.2 (53) 

6.8 (54) 

6.2 (46) 

9.5 (57) 

4.7 (56) 

9.8 (63) 

11.7 (58) 

8.7 (45) 

5.6 (42) 

12.6 (58) 

8. 7 (42) 

9.5 (51) 

12.5 (59) 

5.1 (50) 

12.3 (59) 

6.2 (45) 

11.9 (61) 

8.4 (54) 

8.0 (56) 

12.3 (68) 

34.4 (67) 

35.1 (71) 

35.5 (70) 

30.6 (67) 

28.8 (60) 

34.9 (65) 

30.5 (67) 

26.2 (63) 

35.6 (62) 

36.7 (65) 

34.6 (66) 

31.7 (69) 

35.7 (62) 

33.4 (67) 

12.8 (55) 

27.9 (66) 

23.9 (62) 

29.0 (64) 

16.2 (70) 

11.3 (48) 

12.8 (55) 

32.6 (66) 

Biotite 

0.9 (9) 

0.8 (6) 

0.6 (4) 

1.7 (10) 

0.6 (7) 

1.3 (8) 

1.0 (5) 

0. 7 (4) 

0.9 (7) 

1.2 (6) 

1.2 (6) 

1.1 (6) 

1.0 (5) 

0.9 (9) 

0.4 (2) 

0.6 (4) 

0.8 (4) 

1. 2 (8) 

0.8 (6) 

0.3 (2) 

6.2 (12) 

6.2 (13) 

6.0 (12) 

7.6 (17) 

9.2 (19) 

8.2 (15) 

7.0 (15) 

6.0 (14) 

10.6 (18) 

8.2 (14) 

7.4 (14) 

5.2 (11) 

8.2 (14) 

5.2 (10) 

2.2 (9) 

8.3 (20) 

6.2 (16) 

6.4 (14) 

2.5 (11) 

5.3 (23) 

4.1 (18) 

6.7 (14) 

Hornblende Pyroxene Fe-Ti 

Oxides 

Olivine 

Condor Canyon Formation, Bauers Tuff Member 

T 

6.2 (12) 

3.4 (7) 

3.2 (6) 

4.2 (9) 

4.8 (10) 

3.6 (7) 

1.8 (4) 

5.2 (13) 

4.6 (8) 

5.4 (10) 

3.8 (7) 

2.8 (6) 

4.6 (8) 

3.0 (6) 

5.6 (24) 

2.2 (5) 

1. 7 (4) 

3.0 (7) 

2.3 (10) 

1.6 (7) 

1.9 (8) 

3.5 (7) 

0.3 (3) 

0.3 (2) 

0.1 (1) 

0.2 (1) 

0.1 (1) 

0.2 (1) 

0.2 (2) 

0.3 (1) 

0.1 (T) 

0.2 (1) 

0.1 (1) 

0.1 (T) 

0.1 ( 1) 

0.3 (2) 

0.1 (1) 

0.1 ( 1) 

0.1 (1) 

0.8 (8) 

0.3 (2) 

0.4 (3) 

0.3 (2) 

0.3 (4) 

0.3 (2) 

0.3 (1) 

0.4 (2) 

0.4 (3) 

0.4 (2) 

0.3 (1) 

0.3 (2) 

0.3 (1) 

0.3 (3) 

0.3 (1) 

0.2 (1) 

0.5 (3) 

0.3 (2) 

0.2 (1) 

0.2 (1) 

Harmony Hills Tuff 

2.6 (5) 0.4 (1) 

1.4 (3) 1.4 (3) 

2.2 (4) 1.2 (2) 

1.4 (3) 1.2 (3) 

3.2 (7) 0.8 (2) 

1.8 (3) 0.6 (1) 

1.4 (3) 0.8 (2) 

2.0 (5) 1.0 (2) 

3.4 (6) 2.0 (3) 

2.0 (4) 1.8 (3) 

2.2 (4) 1.8 (3) 

1.0 (2) 0.6 (1) 

2.0 (3) 0.6 (1) 

2.8 (6) 1.6 (3) 

1.0 (4) 0.6 (3) 

0.9 (2) 0.8 (2) 

0.4 (1) 1.0 (3) 

3.1 (7) 1.5 (3) 

0.8 (3) 0.5 (2) 

1.0 (4) 1.1 (4) 

1.8 (8) 0.5 (2) 

3.6 (7) 0.4 (1) 

Alt. 

Ferr. 

Sphene Total 

Pheno. 

9.9 

12.7 

13.6 

16.8 

8.4 

15.5 

20.1 

19.2 

13.3 

21.8 

20.6 

18.8 

21.3 

10.2 

20.8 

13.7 

19.5 

15.6 

14.3 

18.2 

51.4 

49.4 

50.6 

46 

48.2 

53.6 

45.2 

41.6 

57.8 

56.6 

52.8 

46 

57.2 

50 

23.4 

42.3 

38.8 

45.6 

23.1 

49.1 

23.2 

23.5 

Q-S-P 

ratio 

0-34-66 

0-40-60 

0-50-50 

0-35-65 

0-36-64 

0-29-71 

0-38-62 

0-51-49 

0-53-47 

0-37-63 

0-54-46 

0-45-55 

0-38-62 

0-43-57 

0-39-61 

0-52-48 

0-34-66 

0-40-60 

0-39-61 

0-30-70 

4-0-96 

5-0-94 

7-0-93 

3-1-96 

5-0-95 

11-1-89 

9-2-89 

4-0-96 

4-0-96 

6-0-94 

7-1-92 

12-1-87 

15-0-85 

10-1-89 

9-0-91 

7-0-93 

17-2-81 

8-0-92 

11-3-86 

6-1-93 

3-2-95 

18-4-78 

VRFs 

0.4 

0.2 

0.5 

0.4 

0.2 

2.1 

0.2 

2.2 

0.2 

1.1 

1.2 

0.2 

0.8 

0.2 

0.3 

2.5 

0.5 

T 

0.8 

0.4 

0.8 

2.3 

0.6 

0.9 

9.8 

2.1 

Pum. 

5.6 

4.9 

6.7 

12.9 

5.1 

3.5 

0.9 

19.1 

0.5 

G'mass 

89.7 

87.1 

85.9 

82.8 

91.6 

84.3 

77.8 

80.6 

84.5 

78.0 

78.3 

80.0 

78.5 

89.0 

79.2 

86.1 

80.2 

76.3 

80.3 

81.8 

47.8 

50.6 

49.4 

54.0 

51.8 

46.4 

54.8 

58.4 

42.2 

43.4 

47.2 

54.0 

42.8 

50.0 

76.6 

50.6 

47.5 

47.0 

72.8 

49.1 

47.9 

73.9 

s= 

~ 

~ 

en 

~ o-3 

~ 

_;< 

~ n 

~ 

~ n 

> b 

tTl 

:::0 

> 

n 

~ 

~ ~ < E; 

> e: 

;; 

:I: 

-...] 

-...] 
Sample 

No. 

73-89 

73-90 

73-92 

73-94 

73-95 

~~:~idl 86-258f' 

86-258h 

86-258j 

86-25&n 

87-26-17 

87-26-18 

87-26-19 

87-1e*l 

87-1f 

87-1g* 

87-21b 

87-697a* 

87-697d 

87-697f 

87-697k 

87-697p* 

87-697s 

87-697u* 

87-697w* 

88-659a* 

88-819* 

90-457* 

ME3-211 

ME3-22 

ME3-23 

ME3-24 

Loc. Quartz 

A 12.6 (37) 

A 12.4 (39) 

A 13.9 (41) 

A 13.4 (35) 

A 16.0 (44) 

A 16.3 (48) 

K 9.0 (41) 

K 6.0 (40) 

K 11.7 (52) 

K 15.8 (46) 

K 11.6 (45) 

N 10.1 (33) 

N 12.5 (32) 

N 6.4 (25) 

N 9.8 (36) 

N 10.5 (35) 

N 13.3 (37) 

N 10.1 (43) 

E 10.6 (40) 

E 10.4 (41) 

E 14.4 (49) 

E 12.9 (41) 

E 12.1 (35) 

E 9.0 (36) 

E 10.4 (38) 

E 8.1 (30) 

H 5.2 (27) 

u 10.0 (28) 

E 7.3 (29) 

ME 11.3 (24) 

ME 18.8 (34) 

ME 11.2 (22) 

ME 9.4 (19) 

Sanidine 

7.3 (21) 

7.5 (24) 

9.7 (29) 

8.1 (21) 

7.9 (22) 

7.9 (23) 

4.3 (19) 

3.1 (21) 

4.9 (22) 

7.4 (22) 

5.3 (20) 

6.3 (21) 

4.8 (12) 

4.5 (18) 

6.0 (22) 

4.3 (14) 

6.8 (19) 

7.3 (31) 

11.8 (44) 

9.4 (37) 

8.2 (28) 

7.3 (23) 

9.3 (27) 

5.0 (20) 

6.4 (23) 

1.3 (5) 

6.0 (31) 

10.4 (29) 

1.1 (4) 

6.4 (13) 

2.8 (5) 

6.1 (12) 

4.5 (9) 

Plagioclase 

12.0 (35) 

8.5 (27) 

7.2 (21) 

13.1 (34) 

10.1 (28) 

8.1 (24) 

7.4 (33) 

4.1 (28) 

4.3 (19) 

8.3 (24) 

6.8 (26) 

10.3 (34) 

16.5 (43) 

11.7 (46) 

9.6 (36) 

12.3 (41) 

13.3 (37) 

4.7 (20) 

3.9 (15) 

4.5 (18) 

5.1 (17) 

8.7 (27) 

10.0 (29) 

8.4 (33) 

7.6 (28) 

14.0 (51) 

6.0 (31) 

12.3 (34) 

14.4 (58) 

21.8 (45) 

24.2 (44) 

27.2 (52) 

29.2 (59) 

Biotite 

1.9 (6) 

1.8 (6) 

2.3 (7) 

2.8 (7) 

2.1 (6) 

1.0 (3) 

1.0 (5) 

1.3 (9) 

0.9 (5) 

1.7 (5) 

1. 2 (5) 

3.1 (10) 

3.1 (8) 

2.0 (8) 

1.0 (4) 

1.3 (4) 

1.5 (4) 

0.6 (3) 

0.2 (1) 

0. 7 (3) 

0.9 (3) 

1.9 (5) 

2.6 (7) 

1.6 (6) 

1.9 (7) 

2.9 (11) 

1.6 (8) 

1.3 (4) 

2.0 (8) 

6.2 (13) 

4.9 (9) 

4.9 (9) 

4. 1 (8) 

Hornblende 

T? 

0.4 (1) 

T? 

T? 

T 

T? 

0.5 (2) 

0.2 (1) 

0.4 (2) 

0.6 (2) 

0. 7 (3) 

0.4 (1) 

T 

0.3 (1) 

0.2 (1) 

0.4 (1) 

0.6 (2) 

0.1 (T) 

0.2 (1) 

0.2 (1) 

0.7 (3) 

0.8 (3) 

0.3 (1) 

0.1 (1) 

1.4 (4) 

T 

1.6 (3) 

2.8 (5) 

1.9 (4) 

1.4 (3) 

Pyroxene Fe-Ti 

Oxides 

Racer Canyon Tuff 

T 

T 

0.2 (1) 

T 

T 

0.4 (1) 

0.8 (3) 

0.7 (2) 

0.6 (2) 

0.3 (1) 

0.4 (1) 

T 

0.2 (1) 

0.4 (2) 

0.5 (2) 

0.4 (2) 

0.2 (1) 

1.6 (4) 

0.5 (2) 

0.4 (1) 

0.5 (2) 

0.3 (1) 

0.5 (2) 

0.2 (1) 

0.4 (2) 

0.6 (2) 

0.7 (2) 

0.5 (1) 

0.3 (1) 

0.4 (1) 

0.6 (2) 

0.3 (2) 

0.4 (1) 

0.2 (1) 

0.7 (1) 

0.9 (2) 

0.6 (1) 

0.3 (1) 

Olivine Alt. 

Ferr. 

T 

Sphene Total 

Pheno. 

T 34.2 

T 31.4 

33.8 

38 

36.4 

T 33.7 

T 22.2 

T 14.9 

T 22.6 

T 34.3 

T 26 

T 30.4 

T 38.5 

T? 25.4 

T 27 

0.2 (1) 29.7 

T 35.8 

T? 23.3 

26.7 

T 25.4 

29.2 

T 31.7 

T 34.7 

0.1 (T) 25.1 

T 27.5 

T 27.2 

0.1 (T) 19.3 

0.1 (T) 35.9 

T 25 

T 48 

0.2 (T) 54.6 

0.1 (T) 52 

0.3 (1) 49.2 

Q-S-P 

ratio 

39-23-38 

44-26-30 

45-31-24 

39-23-38 

47-23-30 

51-24-25 

43-21-36 

45-24-31 

56-23-21 

50-24-26 

49-22-29 

38-24-38 

37-14-49 

28-20-52 

38-24-38 

39-16-45 

40-20-40 

46-33-21 

40-45-15 

43-39-19 

52-30-18 

45-25-30 

38-30-32 

40-22-38 

43-26-31 

35-6-59 

30-35-35 

30-32-38 

32-5-63 

29-16-55 

41-6-53 

25-14-61 

22-10-68 

VRFs 

0.1 

0.4 

1.5 

0.7 

0.8 

4.6 

4.3 

2.4 

2.1 

0.5 

3.0 

1.0 

2.0 

1.0 

1.5 

1.7 

1.3 

3.0 

3.0 

2.7 

4.4 

1.0 

1.9 

6.1 

1.7 

2.4 

4.7 

0.3 

1.3 

T 

0.7 

T 

2.5 

Pum. 

18.9 

22.1 

23.8 

17.7 

12.0 

2.0 

1.5 

3.0 

9.2 

14.0 

T 

5.0 

15.9 

7.3 

26.0 

7.7 

18.1 

18.7 

26.1 

14.8 

20.4 

17.7 

16.6 

G'mass 

65.7 

68.2 

64.7 

61.3 

62.8 

61.7 

54.6 

60.6 

51.5 

47.5 

59.0 

66.6 

58.0 

70.6 

62.3 

54.6 

62.9 

68.7 

54.4 

64.6 

40.4 

59.6 

45.3 

50.1 

44.7 

55.6 

55.6 

46.1 

57.1 

52.0 

44.7 

48.0 

48.3 

-....J 

00 

0 

~ 0 

0 

-

n 

CJ') 

§ 

s 

sn 

t:C 

:> 

:::tl 

n 

0 

CJ') 

...., 

c:::: 

0 

-<: 

c:::: 

z 

~~ 

\0 

\0 

N 
Sa~J1)le 

No. 

Loc. 

86-666a P 

86-666c* P 

86-666f p 

86-666h* p 

86-666i 

87-452n 

p 

p 

cc 

cc :~~~:~[ 

87-452o 

87-452p 

87-452q cc 

cc 

cc 

87-846* 1 c 

87-847* c 

87-858bl c 

87-858c. C 

90-186 c 

91-208 c 

91-463 c 

91-470c c 

Hk-1 PE 

Hk-2 PE 

Hk-3 PE 

Hk-4a PE 

Hk-4b PE 

Hk-5 PE 

Hk-6 PE 

15-7-7 G 

1-1-48-10 G 

~~~~~~~[14 : 

1-45-12 G 

1-45-13 G 

1-45-14 G 

1-45-15 G 

Quartz 

7.1 (48) 

6.9 (21) 

14.0 (35) 

9.3 (25) 

5.6 (13) 

4.5 (11) 

3.5 (29) 

4.5 (21) 

5.5 (25) 

5.4 (19) 

5.4 (19) 

6.2 (22) 

8.6 (39) 

8.1 (26) 

7.7 (35) 

4.8 (21) 

4.4 (22) 

7.3 (27) 

6.9 (26) 

4.9 (12) 

7.3 (18) 

7.4 (18) 

4.3 (10) 

4.0 (9) 

5.5 (13) 

6.9 (16) 

4.3 (10) 

6.1 (16) 

5.1 (14) 

4.0 (21) 

6.8. (24) 

7.2 (23) 

4.4 (13) 

5.6 (12) 

Sanidine 

3.1 (21) 

11.2 (34) 

11.8 (29) 

8.3 (22) 

9.4 (22) 

9.6 (24) 

1.9 (16) 

5.2 (25) 

3.9 (18) 

3.3 (12) 

4.4 (15) 

5.3 (19) 

7.1 (32) 

7.2 (23) 

3.7 (17) 

3.0 (13) 

4.7 {24) 

6.3 (23) 

6.0 (22) 

6.6 (16) 

7.2 {17) 

3.5 (8) 

3. 7 {9) 

3.8 {8) 

3.6 (8) 

3.8 {9) 

5.6 {13) 

5.8 {15) 

4.6 {13) 

6.4 (34) 

6.3 {22) 

6.8 (22) 

6.2 (18) 

8.2 {18) 

Plagioclase Biotite Hornblende 

3.5 {24) 

11.0 {34) 

9.6 {24) 

15.3 {41) 

22.1 {51) 

9.3 {48) 

5.0 {42) 

8.2 {39) 

9.1 {41) 

16.5 {59) 

15.7 (55) 

12.7 {45) 

5.2 {23) 

12.1 {39) 

8.6 {39) 

7.3 (32) 

9.1 (46) 

12.0 (44) 

10.8 {40) 

19.9 (49) 

17.1 (42) 

22.8 {55) 

26.5 (62) 

29.8 (66) 

25.3 {58) 

26.5 (60) 

26.6 (64) 

20.2 (53) 

20.3 (57) 

7.4 {39) 

12.2 (43) 

14.2 (45) 

17.9 {51) 

24.7 (54) 

0.6 (4) 

3.0 (9) 

3.0 (7) 

2.6 (7) 

3.8 (9) 

4.5 (11) 

1.2 (10) 

2.5 (12) 

2.1 (10) 

2.0 (7) 

1.9 (7) 

2.6 (9) 

0.9 (4) 

2.1 (7) 

1.3 (6) 

2.0 (9) 

0.9 {5) 

1.0 (4) 

2.0 (7) 

7.3 (18) 

5.9 (14) 

6.0 (14) 

4.5 (10) 

4.2 {9) 

6.0 (14) 

5.3 (12) 

2.3 (6) 

3.1 (8) 

3.8 (11) 

0.9 (5) 

1.8 (6) 

2.0 (6) 

3.7 (11) 

2.2 (5) 

T 

0.2 (1) 

1.1 (3) 

1.1 (3) 

1.5 (3) 

1.0 (3) 

0.2 (2) 

0.3 (1) 

1.2 (5) 

0.7 (2) 

0.7 {2) 

0.6 (2) 

0.3 (1) 

0.7 (2) 

0.2 (1) 

0.1 (T) 

0.4 (2) 

0.5 (2) 

0.5 (2) 

0.8 (2) 

2.3 (6) 

1. 0 (2) 

2.5 {6) 

2.2 (5) 

2.1 {5) 

0.9 (2) 

2.0 (5) 

1.1 (3) 

T 

0.7 (2) 

0.5 {2) 

1.6 (5) 

4.3 (9) 

Pyroxene Fe-Ti 

Oxides 

Hiko Tuff 

0.4 (4) 

0.4 (1) 

0.8 (2) 

T 0.9 (2) 

0.7 (2) 

1.0 (3) 

0.2 (2) 

0.5 (2) 

0.3 (1) 

0.3 (1) 

0.7 (2) 

T 0.7 (2) 

T 0.2 (1) 

T 0.4 (1) 

0.5 (2) 

T? 0.5 (2) 

0.2 {1) 

0.4 {1) 

0.6 {2) 

0.8 (2) 

1.3 {3) 

0.4 (1) 

1.3 {3) 

0.7 {2) 

0.7 {2) 

0.5 {1) 

0.9 (2) 

1.1 {3) 

1.0 (3) 

0.2 {1) 

0.7 (2) 

0.5 (2) 

0.8 {2) 

0.1 {T) 0.7 {2) 

Olivine Alt. 

Ferr. 

0.2 (1) 

1.5 {4) 

0.1 {T) 

Sphene 

T 

T 

0.1 

0.1 

0.1 

0.1 

T 

T 

T 

0.1 

0.2 

T 

T 

0.1 {T) 

T 

0.2 (1) 

T 

0.2 {1) 

0.6 (1) 

0.1 {T) 

0.3 {1) 

0.1 {T) 

0.3 (1) 

0.5 (1) 

0.3 (1) 

0.1 (T) 

0.2 (1) 

0.2 (1) 

0.2 (1) 

0.1 (T) 

0.2 (1) 

0.2 (T) 

Total 

Pheno. 

14.7 

32.7 

40.3 

37.5 

43.1 

39.9 

12.0 

21.2 

22.1 

28.2 

28.8 

28.1 

22.3 

30.7 

22 

22.7 

19.9 

27.5 

27 

40.9 

41.2 

41.4 

42.9 

45 

43.7 

44.2 

41.8 

38 

35.9 

19.1 

28.7 

31.4 

34.8 

46 

Q-S-P 

ratio 

52-23-25 

24-38-38 

40-33-37 

28-25-47 

15-25-60 

13-29-58 

34-18-48 

25-29-46 

30-21-49 

21-13-66 

21-17-62 

26-22-52 

41-34-25 

30-26-44 

38-19-43 

32-20-48 

24-26-50 

28-25-47 

29-25-46 

16-21-63 

23-23-54 

22-10-68 

12';.11-77 

11-10-79 

16-10-74 

19-10-71 

12-15-73 

19-18-63 

17-15-68 

22-36-42 

27-25-48 

26-24-50 

15-22-63 

15-21-64 

VRFs 

3.6 

2.6 

1.2 

T 

0.4 

T 

0.3 

0.5 

0.6 

0.2 

T 

T 

2.4 

0.8 

1. 7 

0.6 

5.2 

1.4 

2.3 

T 

0.1 

2.0 

0.4 

T 

T 

0.3 

1.4 

1.5 

0.3 

0.2 

3.6 

0.2 

Pum. 

23.0 

0.1 

5.5 

10.6 

10.2 

8.3 

7.1 

3.4 

8.9 

17.8 

0.9 

8.7 

0.6 

10.1 

13.0 

9.9 

G1mass 

58.7 

64.6 

58.4 

62.4 

56.4 

44.5 

77.2 

68.2 

69.1 

64.4 

67.6 

63.0 

57.5 

67.6 

67.6 

80.9 

64.8 

58.1 

60.8 

59.1 

58.7 

56.6 

57.1 

54.6 

56.3 

55.8 

57.9 

60.6 

64.1 

79.4 

71.0 

68.4 

61.6 

53.8 

~ 

0 

n 

~ tTl 

>-3 

e 

~ (/1 

>-3 

:Al 

~ Cl 

~ ~>-< 

n 

> t""' 

m 

~ tTl 

n 

F: 

0 

tTl 

:Al 

> 

n 

0 

~ 

~ z 

tTl 

< 

> 0 

> e 

~ ::I: 

-l 

\0 
Sample 

No. 

88-30b* 

88-311* 

Loc. Quartz 

EC 11.2 (48) 

EC 8.2 (42) 

88-910* EC 10.0 (45) 

88-1268* 

89-495b*! CN 

89-495c CN 

91-599a 

EC 

c 

5.3 (55) 

5.3 (34) 

10.2 (53) 

3.7 (35) 

87-221* 

87-223* 

87-241 

87-1110 

87-1111 

87-1113* 

88-115*1 

88-118* 

88-126d* 

88-164* 

88-224* 

ET 6.8 (67) 

ET 5.3 (69) 

ET 4.9 (60) 

ET 4.6 (70) 

ET 2.0 (40) 

ET 

EC 

EC 

EC 

EC 

CA 

CA 

88-220*1 

88-225c 

88-226a 

88-1158b* EC 

88-1264* 

CA 

CA 

CA 

5.0 (34) 

3.5 (38) 

4.0 (64) 

3.8 (60) 

4.5 (85) 

3.1 (53) 

3.9 (63) 

6.5 (67) 

3.9 (51) 

4.4 (56) 

6.0 (48) 

89-696c 

91-206 

CA 10.2 (44) 

c 4.0 (49) 

Sanidine 

4.2 (18) 

6.7 (34) 

7.5 (34) 

3.2 (33) 

6.0 (38) 

4.4 (23) 

1.9 (18) 

2.6 (25) 

2.1 (27) 

2.6 (32) 

1.8 (27) 

2.9 (58) 

9.3 (63) 

5.6 (60) 

1.9 (30) 

1.9 (30) 

0.8 (15) 

1.9 (33) 

1.5 (24) 

2.6 (27) 

3.1 (41) 

3.0 (38) 

5.9 (48) 

13.0 (56) 

4.0 (49) 

Plagioclase 

7.2 (31) 

3.9 (20) 

3.9 (17) 

0.9 (9) 

3.7 (24) 

4.5 (23) 

3. 7 (35) 

0.6 (6) 

0.3 (4) 

0.5 (6) 

0.2 (3) 

0.1 (2) 

0.2 (1) 

0.2 (2) 

0.4 (6) 

0.3 (5) 

T 

0.5 (9) 

0.6 (10) 

0.3 (3) 

0.3 (4) 

0.2 (3) 

0.3 (2) 

0.1 (T) 

0.2 (2) 

Biotite 

0.5 (2) 

0.8 (4) 

0.6 (3) 

0.1 (1) 

0.4 (3) 

0.2 (1) 

0.8 (7) 

0.1 (1) 

T 

0.1 (1) 

T 

T 

0.1 (1) 

T 

T 

0.2 (3) 

T 

0.1 (2) 

0.1 (2) 

0.1 (1) 

0.2 (3) 

0.2 (3) 

0.1 ( 1) 

Hornblende 

T 

T 

T 

T? 

0.2 (2) 

0.1 (2) 

Pyroxene Fe-Ti 

Oxides 

Tuff of Tepee Rocks 

T 

0.1 (1) 

T 

0.2 (1) 

0.1 (1) 

0.2 (1) 

T 

0.3 (3) 

Tuff of Rainbow canyon 

0.1 (1) 

T 

T 0.1 (1) 

T 

0.2 (1) 

T 

T 

0.1 (2) 

T 

0.1 (2) 

0.1 (2) 

0.2 (2) 

0.1 (1) 

T 

0.1 ( 1) 

0.1 (T) T 

T 

Olivine Alt. 

Ferr. 

T 

T 

Sphene 

T 

T 

0.1 

T 

T 

T 

T 

T 

Total 

Pheno. 

Q-S-P 

ratio 

23.1 50-19-31 

19.7 44-36-20 

22.3 47-35-18 

9.8 56-34-10 

15.6 35-40-25 

19.3 53-23-24 

10.7 40-20-40 

10.2 

7.7 

8.2 

6.6 

5 

14.8 

9.3 

6.3 

6.3 

5.3 

5.8 

6.2 

9.7 

7.6 

7.8 

12.4 

23.4 

8.2 

68-26-6 

69-27-4 

61-33-6 

70-27-3 

40-58-2 

34-64-2 

38-60-2 

64-30-6 

63-32-5 

85-15-0 

56-35-9 

65-25-10 

69-28-3 

53-43-4 

58-39-3 

49-48-3 

44-56-0 

49-49-2 

VRFs Pun. 

1.7 17.3 

1.0 21.5 

3.6 40.0 

0.4 7.4 

3.8 7.9 

1.6 16.6 

1.4 

1.2 

10.9 

5.4 

0.5 

3.0 

3.1 

1.9 

1.9 

4.4 

1.8 

3.6 

6.3 

8.5 

1.3 

1.1 

0.2 

1.8 

15.0 

25.5 

29.4 

29.6 

19.3 

7.7 

24.0 

24.1 

10.5 

19.6 

33.3 

25.1 

24.7 

18.0 

25.0 

16.3 

10.6 

22.8 

G1mass 

57.9 

56.7 

34.1 

90.4 

76.6 

69.0 

71.1 

73.4 

65.6 

51.5 

58.4 

75.2 

74.5 

63.6 

67.7 

81.4 

70.7 

59.1 

65.1 

59.3 

65.9 

65.9 

70.2 

65.8 

67.2 

00 

0 

a 

t'T1 

0 

r 

0 

a 

n 

Cl:l 

2 

0 

m 

s.n 

I:::C 

> :::0 

n 

0 

Cl:l 

-3 

c:::: 

0 

-<: 

c:::: 

z 

~~ 

1.0 

1.0 

N 
~ 

Sa~le Fe-n Alt. Total Q-S-P 

-0 Loc. Quartz Sanidine Plagioclase Biotite Hornblende Pyroxene Olivine Sphene VRFs Pum. G1mass n 

No. Oxides Ferr. Pheno. ratio trl 

z 

trl 

Tuff of Kershaw Canyon >-:3 

c:::: 

~ 87-177f CA 1. 7 (53) 0.9 (28) 0.3 (9) T T 0.1 (4) 0.2 (7) T? 3.2 58-29-13 0.3 3.0 93.5 en 

>-:3 87-720a CA 3.7 (19) 12.2 (64) 1. 7 (9) 0.4 (2) 0.8 (4) 0.1 (1) 0.2 (1) 19.1 21-69-10 0.3 1.8 78.8 :;:tl 

88-70b EC 5.3 (43) 3.7 (31) 2.7 (22) 0.1 (1) 0.1 (1) 0.2 (2) 12.1 45-32-23 1.7 17.9 68.3 > >-:3 

88-120c* EC 8.5 (30) 10.8 (38) 8.3 (29) T 0.3 (1) 0.3 (1) 0.4 (1) 0.2 (1) 28.8 31-39-30 1.3 13.0 56.9 a 

88-123* r EC 13.3 (53) 7.8 (31) 3.7 (15) 0.1 (T) T T T T 24.9 34-31-15 12.0 17.5 45.6 :;:tl 

> 

88-125c* EC 5.8 (45) 3.1 (24) 3.6 (28) 0.3 (2) T T 0.2 (2) T 13.0 46-25-29 4.5 21.4 61.1 d! 

88-126c* EC 7.3 (48) 5.2 (34) 2.2 (14) 0.3 (2) T 0.2 (1) 15.2 50-35-15 10.2 11.6 63.0 ~-< 

88-195a* CA 6.0 (49) 3.6 (30) 1.9 (16) 0.1 (1) 0.2 (2) 0.1 (1) 0.2 (2) 0.1 (1) 12.2 52-31-17 0.4 16.3 71.1 n 

88-219al CA 3.2 (50) 1.9 (30) 0.7 (11) 0.2 (3) T 0.1 (2) 0.3 (5) 6.4 55-38-12 2.9 1.9 88.8 > r-' 

88-219c CA 13.2 (56) 5.4 (23) 3.8 (16) 0.4 (2) 0.2 (1) T 0.4 (2) 23.4 59-24-17 6.9 3.0 66.7 s 

88-219f* CA 4.9 <47) 2.8 (27) 1.8(17) 0.4 (4) 0.1 (1) 0.1 (1) 0.4 (4) 10.5 52-29-19 3.1 9.9 76.5 z 

>-:3 

88-331* EC 6.3 (44) 5.5 (38) 2.2 (15) 0.3 (2) T 0.1 ( 1) 14.4 45-39-16 17.0 16.3 52.4 trl 

n 89-695a~ CA 5.3 (34) 7.5 (48) 2.5 (16) 0.2 (1) T T 0.1 (1) T 15.6 35-49-16 2.7 11.7 70.0 > 

89-696b* CA 2.5 (27) 3.2 (35) 2.7 (30) 0.3 (3) T T 0.4 (4) T 9.1 30-38-32 1.5 26.0 63.4 r-' 

C1 

89-698a*l CA 3. 9 (37) 2.4 (23) 3.4 (32) 0.1 (1) 0.1 (1) 0.1 ( 1) 0.5 (5) 10.5 40-25-35 7.5 5.0 77.0 tTl 

:;:tl 

89-698d* CA 1.6 (20) 2.5 (31) 3.4 (43) 0.2 (3) T 0.1 (1) 0.2 (3) 8 21-33-46 6.3 13.3 72.4 > 

89-698g* CA 1.3 (24) 2.3 (42) 1.8 (33) 0.1 (2) T T T 5.5 24-43-33 1.2 20.5 72.8 n 

0 

89-1097a* Jl CA 1.1 (28) 1.4 (35) 1.1 (28) 0.1 (3) 0.1 (3) T 0.2 (5) 4 31-39-30 0.4 32.3 63.3 ~ 89-1097d CA 0.9 (19) 1.6 (34) 1. 7 (36) 0.3 (6) T 0.1 (2) 0.1 (2) 4.7 21-38-41 1.1 22.2 72.0 ts 

89-1097e* CA 1.3 (20) 2.6 (39) 2.1 (32) 0.3 (5) 0.1 (2) 0.1 (2) 0.1 (2) 6.6 22-43-35 1.3 27.5 64.6 trl 

:>< 

89-1097f* CA 3.2 (25) 3.3 (26) 5.1 (40) 0.2 (2) 0.1 (1) 0.2 (2) 0.5 (4) 12.6 28-28-44 8.2 17.0 62.2 ~ 

z 

trl 

McCullough Formation < 

> C1 

88-1* ST 1.6 (41) 0.8 (21) 1.3 (33) 0.2 (5) T T > 3.9 43-22-25 0.1 1.6 94.4 C::: 

>-:3 

> 

:::c: 

00 
Sample 

No. 

Loc. 

87-476a EC 

87-479 EC 

87-547a ET 

87-553a CA 

87-573b ET 

87-577b ET 

87-578a*l EC 

87-578d EC 

87-1104a 1 ET

87-1104b ET 

88-283a*j CA 

88-283b CA 

88-283c* CA 

88-1265* CA 

90-801a*f PE 

90-801b PE 

90-881a PE 

90-885 PE 

87-177a CA 

87-331 CA 

87-332 

87-437Kl 

87-437L EC 

87-456a I CA 

87-456c .J.,. CA 

CA 

EC 

87-467a CA 

87-547b ET 

87-556a CA 

87-556b CA 

87-557a CA 

88-1157tf EC 

87-578f ~ EC 

87-669a CA 

87-1104c I ET 

87-1104d~ ET 

88-85b EC 

88-125a* EC 

88-218a CA 

Quartz 

1.6 (35) 

2.2 (46) 

2.2 (33) 

1.9 (56) 

1.1 (38) 

1.4 (35) 

2.5 (39) 

0.9 (28) 

1.8 (27) 

1.0 (32) 

2.1 (44) 

2.6 (46) 

0.9 (33) 

2.2 (32) 

2.9 (28) 

2.3 (30) 

4.8 (43) 

7.7 (39) 

2.1 (40) 

1.5 (38) 

1.0 (20) 

0.5 (13) 

2.3 (43) 

2.2 (24) 

1.3 (20) 

1.2 (26) 

1.3 (19) 

1.6 (52) 

0.5 (15) 

1.1 (20) 

3.2 (34) 

3.2 (23) 

3.2 (35) 

1.5 (38) 

1.2 (41) 

1.6 (21) 

1.9 (37) 

2.7 (25) 

'\'\-~41r, ~c 1.1\ <~fl) 

Sanidine 

3.0 (65) 

2.6 (54) 

4.1 (62) 

1.5 (44) 

1.8 (62) 

2.6 (65) 

3.8 (59) 

2.3 (72) 

4.5 (68) 

2.0 (65) 

2.7 (56) 

3.1 (54) 

1. 7 (63) 

4.4 (65) 

6.6 (68) 

4.9 (64) 

6.0 (54) 

11.4 (58) 

2.9 (56) 

1.9 (48) 

3.8 (75) 

2.8 (72) 

2.4 (45) 

6.6 (73) 

4.4 (68) 

3.0 (65) 

5.1 (76) 

1.4 (45) 

2.7 (79) 

3.8 (70) 

5.7 (60) 

10.1 (71) 

5.7 (62) 

1.8 (46) 

1.5 (52) 

5.1 (65) 

2.9 (56) 

7.7 (72) 

6.0 {7'3) 

Plagioclase 

T 

T 

T 

T 

0.2 (2) 

0.1 ( 1) 

0.2 (2) 

0.2 (1) 

T 

T 

T 

T 

Biotite 

T 

T 

T 

T 

T 

T 

T 

T 

T 

0.1 (4) 

T 

T 

T 

T 

T 

Hornblende 

T 

T 

T 

T 

Pyroxene Fe-Ti 

Oxides 

Tuff of Sawmill Canyon 

T 

T? 

T 

T 

0.1 (2) 

T 

0.1 (2) 

0.1 (3) 

T 

T 

T 

0.1 ( 1) 

T 

T 

0.1 (2) 

T 

T 

T 

T 

T 

0.1 (2) 

T 

T 

T 

T 

0.1 ( 1) 

Delamar Lake Tuff 

0.5 (5) 

0.1 (1) 

0.1 (1) 

0.1 (1) 

0.1 (1) 

T 

T 

T 

Olivine Alt. 

Ferr. 

T 

0.2 (3) 

T 

0.1 (2) 

0.2 (2) T 

0.2 (3) 

0.1 ( 1) 

0.1 (1) T 

Kane Wash Tuff, Gregerson Basin Member, lower unit 

T 

T 

0.2 (4) T 

0.3 (8) 0.1 (3) 

0.2 (4) 0.1 (2) 

0.4 (10) T 

0.5 (9) 0.1 (2) 

0.2 (2) T 

0.6 (9) 0.1 (2) 

0.2 (4) 

0.2 (3) 

0.1 (3) 

0.2 (6) 

0.5 (9) 

0.4 (4) 

0.5 (4) 

0.1 (1) 

0.5 (6) 

0.1 (2) 

0.1 (1) 

T 

0.1 (2) 

T 

T 

T 

T 

T 

T 

0.1 ( 1) 

0.2 (5) 

. T 

0.1 (1) 

0.1 (2) 

0.1 (1) 

0.1 {1) 

T 

0.2 (5) 

T 

0.2 (5) 

T 

0.1 ( 1) 

0.1 (2) 

T 0.1 (2) 

0.1 ( 1) 

T 

T 

T 

0.2 (2) 

0.4 (3) 

0.1 (1) 

T 

0.5 (6) 

0.2 (4) 

0.1 (1) 

0.1 {1) 

0.4 (10) 

0.2 (7) 

Sphene Total 

Pheno. 

4.6 

4.8 

6.6 

3.4 

2.9 

4 

6.4 

3.2 

6.6 

3.1 

4.8 

5.7 

2.7 

6.8 

10.5 

7.6 

11.2 

19.5 

5.2 

4 

5.1 

3.9 

5.3 

9.1 

6.5 

4.6 

6.7 

3.1 

3.4 

5.4 

9.5 

14.2 

9.2 

3.9 

2.9 

7.8 

5.2 

10.7 

~.9 

Q-S-P 

ratio 

35-65-0 

46-54-0 

35-65-0 

56-44-0 

38-62-0 

35-65-0 

40-60-0 

28-72-0 

29-71-0 

33-66-0 

44-56-0 

46-54-0 

35-65-0 

33-66-0 

30-68-2 

32-67-1 

44-54-2 

40-59-1 

42-58-0 

44-56-0 

21-79-0 

15-85-0 

49-51-0 

25-75-0 

23-77-0 

29-71-0 

20-80-0 

53-47-0 

16-84-0 

22-78-0 

36-64-0 

24-76-0 

36-64-0 

45-55-0 

44-56-0 

24-76-0 

40-60-0 

26-74-0 

21-?9~0 

VRFs 

0.7 

0.1 

0.2 

T 

0.3 

0.3 

0.5 

4.1 

1.7 

0.1 

0.8 

0.5 

0.3 

0.8 

0.8 

2.1 

4.8 

0.4 

1.0 

0.3 

0.7 

0.2 

0.5 

0.4 

1.1 

2.2 

0.2 

0.9 

0.3 

0.3 

0.1 

0.1 

0.1 

0.3 

0.1 

0.2 

1.4 

0.2 

Pllll. 

10.7 

6.3 

21.6 

15.8 

16.8 

8.1 

19.8 

9.3 

13.3 

5.8 

6.3 

14.6 

3.8 

10.8 

3.8 

4.1 

5.5 

0.5 

1.0 

22.4 

1.5 

1.0 

5.1 

3.2 

2.6 

3.4 

3.5 

14.1 

4.0 

2.5 

2.9 

2.2 

4.5 

9.7 

7.6 

1.6 

2.4 

2.1 

G'mass 

84.0 

88.8 

71.6 

80.8 

80.0 

87.6 

73.3 

83.4 

78.4 

91.0 

88.1 

79.2 

93.2 

81.6 

85.7 

87.5 

81.2 

75.2 

93.4 

72.6 

93.1 

94.4 

89.4 

87.3 

90.5 

90.9 

87.6 

82.6 

91.7 

91.8 

87.3 

83.5 

86.2 

89.3 

89.2 

90.5 

92.2 

85.8 

!39.? 

00 

N 

a 

tTl 

0 

r 

0 

a 

n 

C/J 

§ 

tTl 

~C/J 

tti 

> :::0 

n 

0 

C/J 

-3 

c:::: 

ti 

-< 

c:::: 

z 

~=3 

\0 

\0 

N 
Sample Loc. Quartz Sanidine Plagioclase Biotite Hornblende Pyroxene Fe-Ti Olivine Alt. 

No. Oxides Ferr. 

Kane Wash Tuff, Gregerson Basin Member, upper unit_ 

87-177c 1 EC 2.2 (44) 2.7 (54) T 0.1 (2) T T 

87-177d EC 1.9 (35) 3.3 (61) T T T 0.1 (2) T 0.1 (2) 

87-437m ! EC 2.0 (26) 5.5 (71) T T 0.2 (3) T T 

87-437o EC 1.3 (38) 1.3 (38) T 0.2 (6) T T 0.6 (18) 

87-458a CA 3.6 (36) 6.4 (63) 0.1 (1) T T 

87-553c CA 0.8 (16) 3.8 (78) T 0.1 (2) T 0.2 (4) 

88-1157t EC 5.9 (47) 6.3 (50) T T 0.3 (2) T 0.1 (1) 

87-578h EC 3.9 (65) 1.6 (28) T 0.2 (3) T 0.3 (5) 

87-669c CA 3.3 (39) 4.8 (56) T 0.3 (4) T 0.1 ( 1) 

87-1105al ET 1.4 (21) 4. 7 (70) 0.3 (4) 0.1 (1) 0.2 (3) 

87-1105b ET 1.4 (16) 5.9 (69) T 0.5 (6) 0.1 (1) 0.6 (7) 

87-1105c* ET 2.7 (38) 3.9 (54) 0.4 (6) T 0.2 (3) 

87-1105d ET 2.6 (43) 3.0 (50) 0.1 (2) T T 0.3 (5) 

88-124* EC 2.0 (32) 4.1 (65) 0.2 (3) T T 

88-197 CA 1.8 (41) 2.0 (45) T 0.2 (5) 0.1 (2) 0.1 (2) 0.2 (5) 

Sphene Total Q-S-P VRFs 

Pheno. ratio 

5 45-55-0 0.2 

T? 5.4 37-63-0 0.4 

7.7 27-73-0 T 

3.4 50-50-0 1.9 

10.1 36-64-0 0.3 

4.9 17-83-0 1.7 

12.6 48-52-0 0.4 

6.0 71-29-0 0.7 

8.5 41-59-0 0.5 

6.7 23-77-0 0.3 

8.5 19-81-0 0.7 

7.2 41-59-0 3.4 

6 46-54-0 0.6 

6.3 33-67-0 0.6 

4.4 47-53-0 1.4 

Pun. 

3.3 

2.0 

0.5 

13.9 

0.7 

3.6 

5.5 

9.5 

3.0 

1.2 

2.2 

1.5 

1.6 

4.9 

5.9 

G1mass 

91.5 

92.2 

91.8 

80.8 

88.9 

89.8 

81.5 

83.8 

88.0 

91.8 

88.6 

87.9 

91.8 

88.2 

88.3 

........ 

~ 

0 

(") 

t'I1 

z 

t'I1 

...., 

c 

~ C/.l 

...., 

~ 

>- ...., 

a 

~ 

., >-

::c: 

~ 

(") 

>- t'""' 

........ 

t'I1 

z ...., 

t'I1 

(") 

>- t'""' 

t:l 

t'I1 

~ 

>-

(") 

0 

., 

~ 

t'""' 

t'I1 

~;>< 

z 

t'I1 

< 

>- t:l 

>-

C::: 

...., 

>- ::c: 

00 

w 
Sa~le .. 

No. 

88-667 

87-177b* 

87-177d 

87-177f* 

87-177h* 

87-177i 

87-177j* 

87-177k 

87-178al 

87-178e · 

87-184 

Loc. Quartz 

IB 11.8 (28) 

CA 

CA 

CA 

CA 

CA 

CA 

CA 

CA 

CA 

CA 

El 

CA 

CA 

87-267 

87-304a! 

87-304b 

87·329a r CA 

87-329b CA 

0.7 (13) 

3.0 (14) 

6.3 (30) 

3.2 (21) 

3.2 (18) 

1.1 (9) 

1.3 (15) 

0.9 (18) 

2.3 (19) 

2.5 (16) 

5.6 (24) 

1.8 (20) 

3.2 (18) 

4.7 (39) 

1.2 (41) 

1. 7 (37) 

3.8 (18) 

5.3 (34) 

1.7 (29) 

2.3 (12) 

2.2 (14) 

7.2 (24) 

6.3 (27) 

7.4 (27) 

4.2 (29) 

87-425 

87-467dl 

87-467e 

87-527 

87-545al 

87-545b 

87-557b 

87-570al 

87-570b 

87-570d 

87-580bl 

87-580c 

87-581a 

87-581c 

87-720b 

87-856c* 

87-1268 

88-121b! 

88-121d 

88-1209* 

ET 

CA 

CA 

s 

ET 

ET 

CA 

ET 

ET 

ET 

EC 

EC 

EC 

EC 

CA 

c 

EC 

EC 

2. 7 (30) 

1.5 (26) 

1.9 (27) 

1.9 (24) 

1.5 (15) 

2.5 (21) 

8.1 (27) 

1.9 (18) 

EC 13.0 (36) 

0 4.2 (43) 

89-1099* CA 3.9 (14) 

90-31a* s 6.8 (23) 

90-41a* S 5.2 (24) 

Sanidine 

29.2 (69) 

4.2 (76) 

17.0 (81) 

13.6 (65) 

11.5 (75) 

13.4 (77) 

10.5 (86) 

6.8 (81) 

3.5 (70) 

9.2 (76) 

12.5 (81) 

16.9 (71) 

6.4 (70) 

12.7 (73) 

6.9 (58) 

1.5 (52) 

2.9 (63) 

16.4 (80) 

9.8 (62) 

3.9 (67) 

16.7 (84) 

13.2 (85) 

22.0 (72) 

15.3 (66) 

18.6 (68) 

9.8 (67) 

5. 7 (63) 

3.3 (57) 

5.0 (68) 

5.8 (73) 

7.6 (76) 

8.0 (68) 

20.8 (69) 

7.6 (72) 

21.6 (60) 

5.3 (55) 

23.4 (81) 

21.2 (73) 

15.1 (71) 

Plagioclase 

0.2 (T) 

0.1 (2) 

0.1 (T) 

T 

0.1 (1) 

0.2 (1) 

0.1 (1) 

T 

T 

T 

T 

0.6 (3) 

T 

0.1 (1) 

T 

T 

T 

0.2 (1) 

0.2 (1) 

T 

0.2 (1) 

T 

0.3 (1) 

0.3 (1) 

0.1 (T) 

T 

T 

T 

T 

0.2 (2) 

0.2 (2) 

0.2 (1) 

0.1 ( 1) 

T 

T 

0.1 (T) 

0.2 (1) 

0.2 (1) 

T 

T 

T 

T 

T 

Biotite 

T 

0.1 (1) 

T 

0.1 (1) 

T 

T 

T 

Hornblende 

T 

0.1 (2) 

0.1 (T) 

0.1 (2) 

0.4 (4) 

0. 7 (4) 

0.1 (1) 

T 

0.2 (1) 

0.1 (1) 

T 

0.3 (1) 

0.4 (2) 

0.3 (1) 

T 

0.3 (5) 

0.1 (1) 

0.1 (1) 

0.1 ( 1) 

0.3 (3) 

0.5 (2) 

0.3 (3) 

T 

0.4 (1) 

0.1 (T) 

0.6 (3) 

Pyroxene Fe-Ti 

Oxides 

Tuff of Etna 

T 

0.2 (4) 

0.1 (T) 

0.1 (T) 

0.1 (1) 

T 

0.1 (1) 

T 

T 

0.2 (1) 

T 

0.1 (3) 

T 

T 

0.2 (3) 

T 

T? 

0.1 (T) 

0.1 (T) 

0.2 (1) 

0.3 (3) 

0.2 (3) 

T 

0.1 ( 1) 

0.1 (1) 

T 

0.1 (1) 

T 

0.3 (1) 

0.1 (2) 

0.3 (1) 

0.2 (1) 

0.2 (1) 

0.1 (1) 

0.1 (1) 

0.1 (1) 

0.2 (4) 

0.1 (1) 

0.1 ( 1) 

0.1 (T) 

0.2 (2) 

0.5 (3) 

0.1 (1) 

T 

T 

T 

0.3 (2) 

T 

0.3 (2) 

T 

0.2 (1) 

0.2 (1) 

0.2 (1) 

0.2 (1) 

T 

T 

T 

T 

0.1 (1) 

0.2 (2) 

0.4 (1) 

0.2 (2) 

0.3 (1) 

T 

0.2 (1) 

0.1 (T) 

0.1 (T) 

Olivine Alt. 

Ferr. 

1.1 (3) 

T 0.1 (2) 

0.5 (2) 

T 0.8 (4) 

T 0.3 (2) 

0.3 (2) 

T 0.4 (3) 

T 0.2 (2) 

0.3 (6) 

0.4 (3) 

T 0.3 (2) 

0.6 (3) 

T 0.3 (3) 

0.1 (1) 

0.1 (3) 

T 

T 

0.1 ( 1) 

T T 

0.3 (2) 

T? 0.1 (1) 

0.4 (1) 

0.1 (T) 0.4 (2) 

0.2 (1) 0.3 (1) 

0.4 (3) 

0.3 (3) 

0.5 (9) 

0.1 (1) 0.2 (3) 

T 0.2 (3) 

0.3 (3) 

T 0.5 (4) 

0.3 (1) 

0.1 (1) 0.4 (4) 

0.1 (T) 1.2 (3) 

0.1 ( 1) 

T 0.8 (3) 

0.1 (T) 0.7 (2) 

T 0.1 (T) 

Sphene Total 

Pheno. 

42.6 

5.5 

21.1 

21 

15.3 

17.3 

12.2 

8.4 

5 

12.1 

15.4 

23.8 

9.1 

17.5 

11.9 

2.9 

4.6 

20.6 

15.7 

5.8 

19.8 

15.5 

30.5 

23.1 

27.3 

14.6 

9 

5.8 

7.4 

8 

10 

11.8 

30.3 

10.6 

36.2 

9.7 

28.8 

29.2 

21.3 

Q-S-P 

ratio 

29-71-0 

14-84-2 

15-85-0 

32-68-0 

22-78-1 

19-80-1 

9-90-1 

16-84-0 

20-80-0 

20-80-0 

17-83-0 

24-73-3 

22-78-0 

20-79-1 

41-59-0 

44-56-0 

37-63-0 

19-80-1 

35-64-1 

30-70-0 

12-87-1 

14-86-0 

24-75-1 

29-70-1 

28-71-1 

30-70-0 

32-68-0 

31-69-0 

28-72-0 

25-75-0 

16-82-2 

23-75-2 

28-71-1 

20-79-1 

38-62-0 

44-56-0 

14-85-1 

24-75-1 

25-74-1 

VRFs 

T 

4.1 

1.1 

0.3 

0.2 

0.2 

0.2 

T 

0.8 

T 

T 

T 

0.6 

0.9 

1.3 

1.0 

0.2 

2.7 

0.9 

1.7 

0.1 

1.5 

3.0 

0.1 

0.8 

0.3 

T 

0.6 

0.3 

1.4 

T 

0.3 

1.1 

0.1 

1.8 

0.1 

1.9 

0.3 

Pl.lll. 

4.0 

3.1 

7.9 

3.3 

2.0 

1.8 

3.1 

2.8 

2.9 

1.6 

2.0 

23.4 

6.0 

6.8 

6.2 

1.3 

5.5 

17.3 

16.8 

4.1 

33.9 

3.8 

8.5 

11.5 

5.8 

6.2 

3.3 

1.6 

3.8 

4.2 

1.5 

1.2 

1.8 

1.3 

5.9 

1.2 

2.8 

1.4 

G•mass 

53.4 

87.3 

69.9 

75.4 

82.5 

80.7 

84.5 

88.8 

91.3 

86.3 

82.6 

52.8 

84.3 

74.8 

80.6 

94.8 

89.7 

59.4 

66.6 

88.4 

46.2 

79.2 

58.0 

65.3 

66.1 

78.9 

87.7 

92.0 

88.5 

92.0 

84.4 

86.7 

68.2 

86.5 

62.4 

82.6 

69.9 

66.1 

77.0 

00 

+::-

0 

~ r 

0 

0 

n 

en 

...., 

c 

0 

m 

sn 

o::l 

;J> 

;:tl 

("') 

0 

en 

...., 

c 

0 

-<: 

c 

z 

~~ 

\.0 

\.0 

N 
• 

Sample Loc. Quartz Sanidine Plagioclase Biotite Hornblende Pyroxene Fe-Ti Olivine 

No. Oxides 

Ox Valley Tuff 

87-697Y' 1 E 9.4 (27) 21.6 (62) 1.3 (4) 0.3 (1) 0.8 (2) 0.4 (1) 1.0 (3) T? 

87-697z* E 8.2 (22) 23.8 (64) 2.0 (5) 0.9 (2) 1.0 (3) 0.5 (1) 0.4 (1) T? 

87-697aa E 10.0 (25) 26.3 (67) 1.4 (4) 0.3 (1) 0.5 (1) 0.5 (1) 0.3 (1) T? 

87-697bb* E 14.0 (38) 19.7 (53) 1.1 (3) 0.3 (1) 0.6 (2) 0.3 (1) 1.0 (3) T? 

89-672* ov 4.3 (15) 22.9 (81) 0.6 (2) T 0.4 (1) T 

Alt. Sphene Total Q-S-P 

Ferr. Pheno. ratio 

T? 34.8 29-67-4 

0.3 (1) T 37.1 24-70-6 

0.1 (T) T 39.4 27-70-4 

0.3 (1) T 37.3 40-57-3 

0.1 (T) T 28.3 15-82-2 

VRFs Pum. 

0.6 6.1 

3.0 3.4 

1.0 2.2 

1.1 3.6 

0.5 2.6 

G•mass 

58.5 

56.5 

57.4 

58.0 

68.6 

a: 

~ 

...., 

~ 

c 

~ en 

...., 

...., 

~ 

a 

~ 

~ ~>-< 

n 

> r 

tri 

~ n 

~ ~ n 

0 

-~ ~ < E; 

> 

~ ::z:: 

00 

Vl 
86 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

APPENDIX2. 

Chemical analyses in parts per million (except for Fe, which is in weight percent) of selected elements, from powdered samples whose modal analyses are given in Appendix 1. Determined by energy-dispersive X-ray fluorescence 

techniques using a Kevex 7000 instrument. 

Sample No. Fe Rb Sr y Zr Nb Ba La Ce Nd 

Leach Canyon Formation, Narrows Tuff Member 

87-852b 1.23 122 436 13 84 13 440 35 72 7 

Leach Canyon Formation, Table Butte Tuff Member 

87-851 1.62 158 227 15 135 14 901 38 91 43 

Leach Canyon Formation, undifferentiated 

88-654 1.14 163 167 17 103 12 507 44 77 13 

Condor Canyon Formation, Swett Tuff Member 

87-452a 1.52 141 242 30 287 16 1,302 74 121 49 

87-452d 1.60 149 243 24 287 14 1,306 69 114 46 

89-1103f 1.50 69 1,030 23 269 21 1,158 62 100 41 

Condor Canyon Formation, Bauers Tuff Member 

87-452e 1.39 176 258 21 260 14 1,240 74 128 so 

87-452h 1.40 181 276 22 254 12 1,320 70 114 so 

89-1103L 1.40 60 1,054 24 241 17 1,056 59 101 45 

Harmony Hills Tuff 

87-1182h 5.53 127 849 22 152 12 727 51 79 38 

89-558 4.36 132 588 17 158 11 1,028 58 92 31 

Racer Canyon Tuff 

87-1e 2.04 157 343 14 136 16 726 30 79 23 

87-1g 2.07 151 345 14 142 15 722 47 82 30 

87-697a 1.68 179 68 66 253 35 500 72 118 58 

87-697p 1.98 135 437 17 121 15 739 so 96 43 

87-697u 1.73 55 756 14 95 8 11196 39 66 35 

87-697w 2.50 100 600 14 135 7 1,039 51 81 26 

88-659a 2.27 126 326 22 149 19 683 64 108 35 

88-819 3.02 144 570 16 164 13 1,140 64 98 so 

90-457 2.74 107 671 10 135 9 1,168 73 81 so 
Sample No. 

86-666c 

86-666h 

87-846 

87-847a 

88-30b 

88-311 

88-910 

88-1268 

89-495b 

87-221 

87-223 

87-110 

88-115 

88-118 

88-126d 

88-164 

88-220 

88-224 

88-1158b 

88-1264 

88-120c 

88-123 

88-125c 

88-126c 

88-195a 

88-219f 

88-331 

89-695b 

89-696b 

89-698a 

89-698d 

89-698g 

89-1097a 

89-1097e 

89-1097f 

MIOCENE TUFF STRATIGRAPHY, CALIENTE CALDERA COMPLEX, NEVADA-UTAH 

Fe 

2.20 

2.58 

2.55 

1.55 

0.93 

0.89 

0.98 

0.66 

0.66 

0.73 

0.73 

0.89 

0.78 

0.76 

1.02 

0.81 

0;81 

0.72 

0.63 

0.80 

2.86 

1. 74 

1.72 

1.98 

1.63 

1. 70 

1.77 

1.98 

1.93 

2.83 

2.23 

1.41 

1.41 

1.83 

2.41 

Rb 

144 

127 

137 

150 

120 

138 

131 

205 

270 

371 

229 

276 

287 

311 

159 

371 

238 

297 

214 

215 

97 

122 

153 

145 

139 

199 

136 

134 

126 

147 

173 

174 

111 

140 

163 

Sr 

347 

467 

532 

317 

572 

472 

156 

57 

79 

27 

22 

136 

410 

424 

542 

65 

46 

6 

80 

26 

681 

618 

653 

333 

229 

135 

522 

189 

190 

289 

408 

193 

142 

382 

260 

y 

11 

17 

20 

14 

Zr 

Hiko Tuff 

141 

166 

180 

118 

tuff of Tepee Rocks 

8 

6 

11 

6 

10 

79 

76 

90 

81 

73 

Nb 

14 

16 

15 

14 

11 

14 

13 

20 

23 

tuff of Rainbow Canyon 

25 

19.3 

30 

24 

28 

18 

19 

so 

29 

22 

25 

106 

108 

129 

107 

115 

117 

123 

128 

116 

96 

118 

34 

48 

46 

41 

45 

29 

43 

so 

44 

37 

43 

tuff of Kershaw Canyon 

35 

33 

17 

19 

25 

29 

20 

28 

24 

21 

32 

27 

21 

26 

33 

223 

186 

140 

177 

216 

206 

146 

219 

221 

190 

230 

214 

114 

203 

273 

25 

31 

20 

18 

24 

27 

20 

24 

30 

17 

24 

28 

19 

34 

32 

Ba 

854 

11197 

1,216 

675 

1,268 

389 

330 

120 

194 

so 

60 

83 

93 

20 

206 

49 

73 

16 

88 

77 

1,412 

619 

573 

612 

653 

570 

568 

459 

575 

701 

568 

489 

324 

455 

496 

59 

59 

48 

38 

35 

43 

47 

37 

30 

31 

35 

53 

27 

35 

29 

40 

63 

42 

24 

33 

61 

53 

56 

52 

91 

60 

46 

38 

51 

36 

57 

56 

38 

30 

52 

La 

96 

97 

84 

62 

47 

64 

69 

58 

31 

62 

67 

71 

54 

69 

79 

70 

64 

63 

58 

70 

95 

87 

85 

89 

140 

103 

81 

78 

104 

88 

103 

103 

64 

85 

104 

Ce Nd 

36 

49 

24 

16 

7 

25 

13 

16 

2 

17 

22 

25 

0 

14 

10 

14 

24 

11 

4 

22 

20 

21 

24 

24 

52 

34 

14 

33 

27 

18 

39 

42 

23 

19 

30 

87 
88 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

Sample No. Fe Rb Sr y Zr Nb Ba La Ce Nd 

McCullough Format;on 

88-1 1.29 113 941 27 140 24 427 38 109 7 

tuff of Sawm;ll Canyon 

87-578a 1.50 170 28 51 378 59 72 68 127 51 

88-283a 1.44 220 10 42 373 57 39 65 127 34 

88-283c 1.45 232 11 71 359 64 39 59 123 42 

88-1265 1.24 132 94 50 304 49 43 53 110 37 

Delamar lake Tuff 

90-801a 1.94 149 30 52 437 52 86 87 182 72 

Kane Wash Tuff, Gregerson Bas;n Member, lower unH 

88-1157b 2.68 263 20 94 955 102 56 131 259 104 

88-125a 2.77 271 22 104 958 116 28 90 221 70 

Kane Wash Tuff, Gregerson Bas;n Member, upper un;t 

88-1157a 2.96 272 41 81 1,109 107 90 112 236 101 

87-1105c 2.07 183 37 75 771 77 77 90 161 61 

88-124 3.14 300 31 148 1,299 121 21 173 281 123 

tuff of Etna 

87-177b 1.06 179 64 55 158 48 205 54 92 51 

87-177f 1.47 216 40 64 239 44 310 57 114 53 

87-177h 1.47 245 32 68 227 52 249 65 129 33 

87-177j 1.41 246 31 67 231 51 257 67 136 48 

87-856c 1.48 274 32 89 239 54 225 64 141 57 

88-1209 1.79 159 28 48 430 56 74 79 149 58 

89-1099 1.40 222 37 60 222 48 318 45 126 52 

90-31a 1.52 196 62 71 224 44 396 57 121 51 

90-41a 1.46 207 35 58 230 49 325 49 129 32 

Ox Valley Tuff 

87-697y 1.48 132 74 42 209 33 497 55 97 51 

87-697z 1.52 147 80 43 223 32 542 47 108 32 

87-697bb 1.95 172 94 50 274 35 623 59 134 36 

89-672 1.47 215 47 66 233 46 418 56 133 42 
Preliminary Geochemistry of Miocene Ash-Flow 

Tuffs in and near the Caliente Caldera Complex, 

Southeastern Nevada and Southwestern Utah 

By L. David Nealey, Peter D. Rowley, Daniel M. Unruh, James R. Budahn, 

Lawrence W. Snee, Harald H. Mehnert, and R. Ernest Anderson 

GEOLOGIC STUDIES IN THE BASIN AND RANGE-COLORADO PLATEAU TRANSITION IN 

SOUTHEASTERN NEVADA, SOUTHWESTERN UTAH, AND NORTHWESTERN ARIZONA, 1992 

U.S. GEOLOGICAL SURVEY BULLETIN 2056-C 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1995 
CONTENTS 

Abstract........................................................................................................................... 91 

Introduction . . .. .. . . . . . . . . . . ..... . . . . . . . . . . .. . . . . . . . . . . . .. .... . . . .. . . . . . .... . . . . . . . . . . . .. . . . . . . . . . . . ...... . . . . . . . ..... . . . . . . . . . . 91 

Acknowledgments.......................................................................................................... 94 

Geologic Setting . . . . . . . . . . .... .. . . . . . . . . .. .. . . . . . . . . . .. . ..... . . . . . . . . .... . . . . . . . . . . . . . . . . . . . . . . . . . . .... . . . . . . . . . .. .. . . . . . . . . . 94 

Geochemistry.................................................................................................................. 95 

Analytical Methods................................................................................................. 95 

Major-element Geochemistry................................................................................. 95 

Classification . .... . . . . . . . . ... .. . . . . . . . . . . . . .... . . . . . . . . . .... . . . . . . . . . . . ... . . . . . . . . . .. .... . . . . .. . . ... .. . . . . . . . . . 95 

Silica Variation Diagrams .. . . . .. .. .. .... .. . .. .. ...... .. .. . . .. .. ... .. .. . . . .. .. .. .... .. .. .... ...... .. .. . . 95 

Trace-element Geochemistry.................................................................................. 1 03 

Spider Diagrams .. .. .. . . .. .... .. .. . . . .. .. .. ...... . . .. ................................ .... .. .. .. .. ...... .. .. . . 1 05 

Comparative Geochemistry . .. . . . . . . . ................... .. . . . . . . . . .............. .. ... . . . . . . . . . . . ... .. . . . . . . . ........... I 07 

Summary and Conclusions .... .. .. .. .. .. .. .. . .. . . . .. .. .. .. . . . .. ................... .... . .. .. .. .. .... .. .. .. .. .... .... .. . . I 07 

References Cited ............................................................................................................. I 09 

FIGURES 

1. Generalized geologic map for western part of Caliente caldera complex..................................................................... 92 

2. Stratigraphic column of ash-flow tuffs and related rocks in and near Caliente caldera complex and vicinity, 

Nevada-Utah.................................................................................................................................................................. 94 

3. Potassium variation diagram for ash-flow tuffs in and near Caliente caldera complex ................................................ 10I 

4. Silica versus FeO*/MgO variation diagram for ash-flow tuffs in and near Caliente caldera complex and vicinity ...... 10I 

5. Silica variation diagrams for ash-flow tuffs in and near Caliente caldera complex ...................................................... 102 

6. Molar Al20/Ca0+Na20+K20 variation diagram for ash-flow tuffs in and near Caliente caldera complex ................ 101 

7. Trace-element variation diagrams for ash-flow tuffs in and near Caliente caldera complex ........................................ 104 

8. Zr/Nb variation diagram for ash-flow tuffs in and near Caliente caldera complex ....................................................... 105 

9. BalLa versus Ba/Nb diagram for ash-flow tuffs in and near Caliente caldera complex ............................................... 105 

10. Spider diagrams for ash-flow tuffs in and near Caliente caldera complex .................................................................... 106 

11. Rb versus Y + Nb variation diagram for ash-flow tuffs in and near Caliente caldera complex...................................... 108 

TABLES 

1. Chemical analyses for ash-flow tuffs in and near Caliente caldera complex................................................................ 96 

2. Sample localities for ash-flow tuffs in and near Caliente caldera complex ................................................................... 100 
Preliminary Geochemistry of Miocene Ash-Flow Tuffs 

in and near the Caliente Caldera Complex, 

Southeastern Nevada and Southwestern Utah 

By L. David Nealey, Peter D. Rowley, Daniel M. Unruh, James R. Budahn, Lawrence W. Snee, 

Harald H. Mehnert, and R. Ernest Anderson 

ABSTRACT 

Chemical compositions of ash-flow tuffs in and near the 

Caliente caldera complex, Nevada-Utah, are distinctive. 

Selected samples of ash-flow tuffs were analyzed by X-ray 

fluorescence, instrumental neutron activation, and inductively coupled plasma. The units under discussion are the 

Swett Tuff (23.7 Ma) and Bauers Tuff (22.8 Ma) Members 

of the Condor Canyon Formation, Harmony Hills Tuff 

(22.5-22.0 Ma), Hiko Tuff ( 18.6-18.2 Ma), tuff of Tepee 

Rocks (17.8 Ma), tuff of Rainbow Canyon (15.6-15.2 Ma), 

tuff of Kershaw Canyon (15.3?-14.2 Ma), Gregerson Basin 

Member (14.6-14.4 Ma) of the Kane Wash Tuff, tuff ofEtna 

(14.0 Ma), and Ox Valley Tuff (12.6?-11.4? Ma). Most of 

these units erupted from calderas or vents within the Caliente 

caldera complex, but probably the Harmony Hills and certainly the Kane Wash Tuff were derived from sources outside the complex. 

Ash-flow tuffs erupted from the Caliente caldera complex and other calderas in the area range in composition from 

high-potassium andesite to very high silica rhyolite. Most of 

the tuffs are calc-alkaline, with metaluminous and peraluminous affinities. In contrast, the Gregerson Basin Member of 

the Kane Wash Tuff is tholeiitic, with mildly peralkaline 

affinities. Very high silica rhyolite tuffs show evidence of 

alkali loss and silica and calcium gain during postemplacement alteration processes. Calc-alkaline rocks have low concentrations of Zr ( <500 ppm), Nb ( <60 ppm), and Y (<50 

ppm) compared with the tholeiitic Gregerson Basin Member 

(Zr >800 ppm; Nb >80 ppm; Y > 70 ppm). The Gregerson 

Basin Member, tuff of Etna, Ox Valley Tuff, and tuff of 

Tepee Rocks have low Sr contents ( < 100 ppm) compared 

with the other units, which contain more than 250 ppm Sr. 

The Gregerson Basin Member, tuff of Etna, tuff of Rainbow 

Canyon, and tuff of Tepee Rocks show low Ba contents 

( <200 ppm) that distinguish them from other units that contain at a minimum about 400 ppm Ba. The Gregerson Basin, 

tuff of Tepee Rocks, Ox Valley Tuff, and tuff of Kershaw 

Canyon also have low BalLa values (~10). 

Chemical differences between the various units show 

up clearly in spider diagrams. In these diagrams the Harmony Hills Tuff is characterized by small negative Ba and Ti 

anomalies compared with large Ba and Ti anomalies in the 

Hiko Tuff, tuff of Etna, and the Gregerson Basin Member. 

Small negative Sr anomalies distinguish the Harmony Hills 

Tuff and tuff of Rainbow Canyon from the tuff of Etna and 

Gregerson Basin Member, units with strong negative Sr 

anomalies. Small negative Nb and Ta anomalies characterize 

the tuff of Etna and Gregerson Basin Member, whereas large 

negative Nb and Ta anomalies typify the Swett Tuff and 

Bauers Tuff Members, Harmony Hills Tuff, Hiko Tuff, and 

tuff of Tepee Rocks. 

INTRODUCTION 

The Caliente caldera complex is one of the largest Cenozoic caldera complexes in the western United States, with 

estimated dimensions of 35x80 km (fig. 1). The complex is 

centered southeast of the town of Caliente, Nev. (fig. 1), in 

the eastern part of the Basin and Range province. Like many 

other middle and upper Cenozoic volcanic fields, the complex is dominated by silicic lava flows and ash-flow tuffs, 

but it also contains minor amounts of mafic and intermediate 

volcanic rocks. (See Rowley and others, this volume.) Intracaldera deposits and outflow sheets from the Caliente 

caldera complex are intercalated with outflow sheets from 

other late Cenozoic calderas. The best known of these 

calderas is the Kane Springs Wash caldera, which is located 

about 30 km south of the Caliente caldera complex (Novak, 

1984; Scott and others, this volume; Harding and others, this 

volume). The Caliente caldera complex is south of the much 

larger Oligocene Indian Peak caldera complex (Best and others, 1989a), which is 80x 120 km. The Caliente caldera complex lies southwest and west of Oligocene and early Miocene 

91 
92 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

114 °45' 37° 45',..--------r------r-

+ + + + + + 

Axis of Timpahute lineament 

37°30' 

+ 

0 5 10 KILOMETERS 

/ 

® I 

/ 

PANACA 

BASIN 

+ + 

~------~----~ 

EXPLANATION 

D Quaternary-Tertiary basin-fill deposits 

Ill Tertiary volcanic rocks, undivided 

• Tertiary intrusive rocks 

Cambrian-Proterozoic sedimentary rocks 

~ Clover Creek caldera margin (concealed)-

Intracaldera Bauers Tuff Member of Condor 

Canyon Formation is patterned 

Fault margins of Delamar and related calderasdotted where concealed. Intracaldera Hiko and 

Hiko-like tuffs are patterned 

\ Fault - Dashed where approximately located, ~ Buckboard Canyon caldera margin - lntracaldera 

\ dotted where concealed tuff of Rainbow Canyon is patterned 

Figure 1 (above and facing page). Generalized geology and location of western part of Caliente caldera 

complex, Nevada. 

volcanic centers that extend from eastern Nevada to the 

Marysvale volcanic field in southern Utah. Like the source 

of many other ash-flow tuffs in the western United States, the 

Caliente caldera complex is a nested caldera complex that 

resulted from multiple eruptions from closely spaced 

calderas, lava domes, and satellitic stratovolcanoes. 

This report is a product of the USGS Basin and 

Range-Colorado Plateau (BARCO) Study Unit (see 

Introduction, this volume) and an earlier project that began 

in 1986. Both efforts consisted of 7 .5-minute quadrangle 

mapping of a large area that includes the Caliente caldera 

complex, and related topical studies. We give a preliminary 

geochemical analysis of ash-flow tuffs in and around the 

caldera complex. We focus on that part of the ash-flow tuff 

section that spans the interval from 24 to 12 Ma, when the 

caldera complex is known or suspected to have been active. 
GEOCHEMISTRY OF MIOCENE TUFFS, CALIENTE CALDERA COMPLEX 93 

39° 115° 114° 113° 112° , ; ; 

; 

BASIN AND RANGE , I 

zlc 

mi-t 

~)> o~:r: 

·)>I 

I 

38 

0 30 

PROVINCE 

I 

I 

I 

I 

I 

I 

I 

I 

COLORADO PLATEAUS 

PROVINCE 

60 KILOMETERS 

EXPLANATION 

Boundary of aeromagnetic ridge (from Zietz and others, 1976, 1978) 

Caldera 

Colorado Plateau-Basin and Range boundary 

Axes of Pioche-Marysvale and Delamar-Iron Springs igneous belts 

Additional details on these tuffs are provided by the 

companion report by Rowley and others (this volume). We 

include here data for the Gregerson Basin Member of the 

Kane Wash Tuff (Scott and others, this volume) and the 

Harmony Hills Tuff. The Gregerson Basin Member erupted 

from the Kane Springs Wash caldera, which is not a part of 

the Caliente caldera complex. The Harmony Hills Tuff was 

probably derived from a source east of the Caliente caldera 

complex. Data for these rocks are included for comparative 

purposes and because they are intercalated with ash-flow tuffs 

known to have erupted from the Caliente caldera complex. 

The ultimate goal of this study is to use geochemical data to 

shed light on the tectonic and magmatic evolution of the 

Caliente caldera complex. And toward this same end, isotopic 

data for many of the same rock units are given in the 

companion paper by Unruh and others (this volume). 

We present here whole-rock major- and trace-element 

analyses of the ash-flow tuffs. The data suffer from a variety 

of unquantifiable uncertainties. First, that an unknown 

amount of vitric ash was lost during the eruption of the 

caldera results in phenocryst enrichment compared to the 

parent magma (Cameron and Hanson, 1982). Second, alteration of tuffaceous material probably began as soon as it was 

deposited. Although we selected samples that showed no 

obvious evidence of hydrothermal alteration, our analyses 

show small to moderate amounts of postemplacement vaporphase crystallization, devitrification, weathering, and 

ground-water interaction. We use the terms "alteration" and 

"altered" in a generic way for all these processes. Lastly, 

analyzed material contains small but variable amounts of 

xenolithic material derived from the wallrock material and 

older volcanic units. Future analyses of vitric lavas, tuffs, 
94 GEOLOGIC STUDIES, BARCO ST{}DY UNIT, 1992 

and pumices should more closely represent magmatic compositions than the analyses discussed here. Ideally, pumice 

and nonhydrated obsidian should be used to characterize the 

composition of silicic volcanic rocks, but that would mean 

that some of the most voluminous volcanic units in the Caliente area would never be described chemically. We emphasize that our purpose here is to characterize the units in the 

vicinity of the Caliente caldera complex, without regard to 

eruptive differentiation, postemplacement alteration, or 

xenolith concentration. 

ACKNOWLEDGMENTS 

We thank R.B Scott and A.E. Harding for sharing data 

from their mapping south of the Caliente caldera complex. 

Scott and Harding also helped in the correlation of tuffs and 

provided conclusions about their sources. Mel Kuntz, Dave 

Sawyer, Lorna Carter, and Bob Scott provided excellent 

reviews of early drafts of this report. We also thank all of the 

analysts who have done their best to provide us with superior 

results. 

GEOLOGIC SETTING 

Geologic mapping and geochronologic studies indicate 

that the Caliente caldera complex was episodically active for 

at least 10 m.y., beginning about 23 Ma. Volcanism during 

the interval consisted of caldera- and stratovolcano-forming 

AGE UNIT 

Ox Valley Tuff (12.6?-11.4? Ma) 

tuff of Etna (14.0 Ma) 

Kane Gregerson upper unit (14.4 Ma)/ tuff of 

Wash Basin Kershaw 

Tuff Member lower unit (14.6 MaV canyon Y (15.3?-14.2 Ma) tuff of Sawmill Canyon 

tuff of Rainbow Canyon (15.6-15.2 Ma) 

Delamar Lake Tuff (15.8-15.5 Ma) 

Miocene 

tuff of Tepee Rocks (17.8 Ma) 

lava flows of Buckboard Spring (18. 0 Ma) 

volcanic dome of Crows Nest Tank (18. 2 Ma) / *porphyry of 

Hiko Tuff (18.6-18.2 Ma) /Meadow Valley 

Racer Canyon Tuff (19.2?-18. 7 Ma) /Wash 

lava flows of Indian Cove (21 .9 Ma) '(19.4-<18.2 Ma) 

Harmony Hills Tuff (22.5-22.0 Ma) 

co c: O.o. .._ c: 0 Bauers Tuff I intrusive rocks of Clover co :::J co:;:; Member (22.8 Ma) Creek caldera (22. 8 Ma) .r:.o -o >- co (.) .._ g lij E ·:; (!) Swett Tuff Member (23.7? Ma) 0 UUo u.. 

Leach Canyon Formation (23.8 Ma) 

Oligocene Cobalt Canyon stock (24. 8 Ma) 

*dikes 

events, and ended with basalt- and basaltic andesite-dominated volcanism. This progression is typical of late Cenozoic 

volcanism of western North and Central America and it characterizes volcanic activity that extends from the Sierra 

Madre Occidental to the Mogollon-Datil field in New Mexico and Arizona and from there to the northern Great Basin 

(Elston and Bornhorst, 1979; Best, Christiansen, and others, 

1989; Wark and others, 1990). 

Previous research in the Caliente area predicted that 

numerous ash-flow tuffs in and near the complex were 

derived from the Caliente caldera complex: Leach Canyon 

and Condor Canyon Formations according to Williams 

( 1967), and Harmony Hills, Racer Canyon, Hiko, and Ox 

Valley Tuffs according to Noble and others ( 1968) and 

Noble and McKee (1972). Later mapping by Ekren and others ( 1977) confirmed that the Hiko Tuff erupted from the 

western lobe of the Caliente caldera complex, which is now 

called the Delamar caldera. Four nested calderas are known 

to compose the Caliente caldera complex: Clover Creek, 

Delamar, Pine Park, and Buckboard Canyon (Rowley and 

Siders, 1988). The oldest of these is the Clover Creek 

caldera, which formed in the northern part of the complex as 

the result of the eruption of the Bauers Tuff Member (22.8 

Ma) of the Condor Canyon Formation (fig. 2). The underlying Swett Tuff Member of the Condor Canyon Formation 

may have also erupted from the Clover Creek caldera. 

About 4 m. y. after the Clover Creek caldera formed, the 

Hiko Tuff ( 18.6-18.2 Ma) erupted from the Delamar caldera 

in the western part of the complex. At about 16 Ma, the eastem part of the complex erupted to form the Pine Park caldera 

SOURCE 

Caliente 

Caliente 

Kane 

Springs 

Wash 

Caliente 

Buckboard 

Canyon 

? 

Caliente 

Delamar 

Delamar 

Delamar 

Caliente 

Clover Creek 

? 

Clover Creek 

Caliente 

? 

Figure 2. Stratigraphy of ash-flow tuffs and 

related rocks in and near Caliente caldera complex and vicinity, Nevada-Utah. Note that Kane 

Springs Wash caldera is not part of Caliente 

caldera complex. K-Ar, 40 Ar/39 Ar ages and interpreted ages in parentheses; see text and Rowley and others (this volume) for details. Use of 

"Caliente" for the source indicates that source is 

considered to be within Caliente caldera complex, but exact caldera has not been named or in 

some cases has not been mapped. Query, source 

unknown; blank, not applicable. 
GEOCHEMISTRY OF MIOCENE TUFFS, CALIENTE CALDERA COMPLEX 95 

(Rowley and Siders, 1988; Siders, 1991 ), but deposits from 

this caldera have not been included here because they have 

not been studied in detail. At about 15.6-15.2 Ma, another 

major eruption produced the tuff of Rainbow Canyon from 

an area in the western part of the complex, referred to as the 

Buckboard Canyon caldera. The Kane Springs Wash 

caldera, 30 km south of the Caliente caldera complex, produced the Kane Wash Tuff at about 14.7-14.4 Ma (Scott and 

others, this volume; Harding and others, this volume). 

In addition to the four named calderas noted, other 

calderas and vent areas formed in the complex; but their 

locations and geometry are poorly known (Rowley and others, this volume). These were the sources of the tuff of Tepee 

Rocks, tuff of Kershaw Canyon, tuff of Etna, and Ox Valley 

Tuff. Intermediate to silicic lavas and volcanic mudflow 

breccia erupted from lava domes and stratovolcanoes at various times during the evolution of the Caliente caldera complex. Intracaldera megabreccias in the caldera complex were 

derived from collapsing walls of the complex. 

Volcanism in the area occurred during the main episode 

of regional late Cenozoic extension. Faulting comprised 

high-angle strike-slip, normal-slip, oblique-slip, and lowangle normal types. Some of the calderas that form the Caliente caldera complex are trapdoor calderas or have complex 

margins that are locally faults, suggestive of concurrent 

faulting during caldera subsidence (Rowley and others, 

1990; Rowley and others, 1992). The inception of this volcanic and tectonic event correlates with a major reorganization 

of plates in western North America at about 25 Ma (Mammerickx and Klitgord, 1982; Ward, 1991 ). 

Mafic and intermediate magmas erupted from local volcanic centers, probably throughout the history of the complex. Rocks produced by this activity predate and locally 

intertongue with the upper Miocene and Pliocene Panaca 

Formation, a sequence of basin-fill sedimentary rocks. The 

volcanism occurred near the end of the main episode of 

extension as basin-range faulting took over as the dominant 

form of crustal deformation. After about 2-4 m.y., volcanism 

ceased in the area. The cessation of volcanic activity correlates with another reorganization of tectonic plates in western 

North America between 12 and 1 0 Ma (Ward, 1991 ) . 

GEOCHEMISTRY 

ANALYTICAL METHODS 

Analyzed whole-rock samples of ash-flow tuffs from 

the Caliente caldera complex range in composition from 

andesite to high-silica rhyolite, but most are rhyolite. Due to 

the inhomogeneity within individual ash-flow tuffs, as many 

as eight samples from a single unit were analyzed. Before 

analysis, all samples were crushed in an aluminum shatterbox 

and ground with an agate mortar and pestle. Major elements 

are given in table 1 and are plotted in figures 3-8 on a volatile-free basis. Table 2 describes sample localities. 

Chemical analyses of the samples were determined in 

Lakewood, Colo., and Menlo Park, Calif., laboratories of the 

USGS using standard procedures. Major-element compositions were determined by X-ray fluorescence spectroscopy 

using methods described in Taggart and others ( 1987). 

Minor elements (Nb, Rb, Sr, Zr, Y, Ba, La, Cu) were determined using X-ray fluorescence methods described by 

Johnson and King (1987). Trace elements (Ba, Co, Cr, Th, 

Zn, Sc, La, Ce, Nd, Sm, Eu, Gd, Tb, Tm, Yb, Lu) were determined by inductively coupled plasma-atomic absorption 

spectroscopy (ICPAAS) and instrumental neutron activation 

analysis (INAA). Methods for ICPAAS and INAA followed 

those described by Lichte and others (1987) and Baedecker 

and McKown (1987), respectively. Samples analyzed by 

INAA provided more complete rare-earth element analyses, 

whereas samples analyzed by ICPAAS lack data for the middle and heavy rare-earth elements (table 1). 

MAJOR-ELEMENT GEOCHEMISTRY 

CLASSIFICATION 

In this report, we classify ash-flow tuffs in the Caliente 

area by the potassium versus silica scheme of Peccerillo and 

Taylor (1976) and the FeO* /MgO versus silica scheme of 

Miyashiro ( 1974). These two schemes allow for the application of common, yet meaningful rock names and are also 

used commonly to classify subduction-related magmas. In 

all diagrams, major elements are plotted recalculated volatile-free. Using the potassium versus silica scheme, the 

majority of Caliente rocks are high-potassium rhyolite (fig. 

3). A few samples plot slightly into the high-potassium dacite field. Samples with greater than 76 weight percent silica 

are definitely altered. A single sample of the Harmony Hills 

Tuff is classified as high-potassium andesite. On the basis of 

FeO* /MgO and Si02, the majority of Caliente samples are 

calc-alkaline (fig. 4 ). The Gregerson Basin Member of the 

Kane Wash Tuff and a sample of the tuff ofEtna show strong 

tholeiitic affinities, using Miyashiro's classification scheme. 

SILICA VARIATION DIAGRAMS 

A variety of techniques exist for showing chemical 

variations in igneous rocks. The most common method is the 

Harker diagram, a type of binary diagram in which the abundance of a major or trace element is plotted against another 

element. Harker diagrams are used herein to show chemical 

variations within and between the ash-flow tuffs in the vicinity of Caliente (fig. 5). 

On a volatile-free basis, our suite of samples contains 

between 62 and 84 weight percent Si02, forming a rock 
96 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

Table 1. Chemical analyses for ash-flow tuffs in and near Caliente caldera complex. 

I Major elements in weight percent. trace elements in parts per million. ICPAAS and INAA indicated by*; nd, not determined; LOI, loss on ignition. Analysts: J.E. Taggart. A. Bartel. D.F. Siems. T.L. Fries, J.R. Evans, J.R. Budahn. R.J. Knight. D.M. McKown, Hezekiah Smith, J.W. Marinenko, W.B. Crandell, D.L. Fey, C.S.E. Papp, 

G.O. Riddle, K.C. Curry. and F.W. Tippitt. Petrographic data for all samples given in Rowley and others (this volume)! 

Unit Bauers Member 

Sample No 87-1181C 87-452E 87-452H 88-1065A 

Major-element composition 

Si02 

Al203 

FeT03 

MgO 

CaO 

Na20 

K20 

Ti02 

P20s 

MnO 

LOI 

68.4 

14.7 

1.91 

0.62 

1.84 

3.97 

4.28 

0.35 

0.08 

0.05 

3.46 

70.0 

14.0 

1.62 

0.42 

1.21 

3.52 

5.63 

0.31 

<0.05 

0.05 

2.66 

Trace-element composition 

Nb 16 18 

Rb 

Sr 

Zr 

y 

Ba 

La 

Cu 

*Ba 

*Co 

*Cr 

*Hf 

*Ta 

*Th 

*U 

*Zn 

*Sc 

*La 

*Ce 

*Nd 

*Sm 

*Eu 

*Gd 

*Tb 

*Tm 

*Yb 

*Lu 

224 

422 

325 

25 

1,110 

66 

9 

1,100 

4 

3 

nd 

nd 

31 

nd 

46 

5 

66 

110 

42 

nd 

nd 

nd 

nd 

nd 

2 

nd 

198 

272 

292 

26 

1,180 

69 

8 

1,100 

2 

2 

nd 

nd 

33 

nd 

39 

4 

67 

110 

41 

nd 

nd 

nd 

nd 

nd 

2 

nd 

72.8 

13.8 

1.66 

0.32 

1.17 

3.51 

5.40 

0.30 

0.06 

0.04 

0.47 

17 

199 

306 

289 

24 

1,220 

61 

5 

1,200 

3 

nd 

nd 

30 

nd 

33 

4 

63 

110. 

39 

nd 

nd 

nd 

nd 

nd 

2 

nd 

70.7 

14.8 

1.88 

0.32 

1.26 

3.79 

5.39 

0.33 

0.06 

0.04 

0.71 

15 

190 

356 

280 

21 

1,390 

58 

8 

1,200 

nd 

nd 

nd 

nd 

29 

nd 

39 

5 

63 

110 

40 

nd 

nd 

nd 

nd 

nd 

2 

nd 

88-1035A 

67.3 

14.8 

2.06 

0.62 

2.11 

3.31 

5.02 

0.35 

0.08 

0.05 

3.55 

16 

241 

584 

301 

19 

1,430 

47 

14 

1,430 

5 

5 

nd 

nd 

29 

nd 

44 

5 

63 

110 

41 

6.0 

1.3 

3.8 

0.62 

0.28 

1.9 

nd 

Swett Member 

87-452A 

69.6 

14.0 

1.73 

0.56 

1.29 

3.52 

5.22 

0.35 

<0.05 

0.04 

3.08 

14 

157 

267 

309 

25 

1,280 

66 

7 

1,200 

2 

nd 

nd 

21 

nd 

42 

5 

66 

110 

44 

nd 

nd 

nd 

nd 

nd 

3 

nd 

87-4520 

71.8 

14.1 

1.78 

0.53 

1.39 

3.78 

5.06 

0.36 

0.06 

0.03 

0.62 

21 

168 

268 

315 

24 

1,270 

58 

4 

1,200 

4 

3 

nd 

nd 

21 

nd 

38 

4 

62 

110 

43 

nd 

nd 

nd 

nd 

nd 

2 

nd 

Hannony Hills Tuff 

87-452K 

60.9 

15.6 

5.67 

2.93 

5.12 

3.16 

3.46 

0.73 

0.28 

0.08 

1.5 

8 

102 

771 

185 

17 

941 

nd 

nd 

974 

nd 

nd 

5.24 

0.70 

16 

3.52 

nd 

nd 

45.9 

90.8 

36.9 

6.74 

1.57 

nd 

0.65 

nd 

1.82 

0.27 
GEOCHEMISTRY OF MIOCENE TUFFS, CALIENTE CALDERA COMPLEX 

Table 1. Chemical analyses for ash-flow tuffs in and near Caliente caldera complex-Continued. 

Unit 

Sample No. 86-666C 86-666H 

Ma.jor-element composition 

Si02 69.4 69.4 

Al203 13.9 14.6 

FeT03 

MgO 

CaO 

Na20 

K20 

Ti02 

P205 

MnO 

LOI 

2.39 

0.98 

2.12 

3.16 

4.46 

0.37 

0.14 

0.05 

2.29 

Trace-element composition 

2.83 

0.77 

2.43 

3.56 

4.36 

0.46 

0.16 

0.05 

0.81 

Nb 16 14 

Rb 

Sr 

Zr 

y 

Ba 

La 

Cu 

*Ba 

*Cr 

*Cs 

*Hf 

*Ta 

*Th 

*U 

*Zr 

*La 

*Ce 

*Nd 

*Sm 

*Eu 

*Tb 

*Yb 

*Lu 

148 

374 

151 

17 

782 

nd 

nd 

826 

5.13 

6.53 

4.6 

1.3 

21.1 

5.29 

156 

48.6 

84.6 

29.3 

4.7 

0.94 

0.44 

1.5 

0.24 

124 

494 

194 

18 

1,120 

nd 

nd 

1,240 

5.52 

4.18 

5.84 

1.37 

17.9 

4.25 

210 

51.8 

101 

36.4 

6.36 

1.3 

0.60 

1.65 

0.26 

87-2060 

71.1 

13.2 

1.79 

0.98 

1.79 

2.89 

4.07 

0.28 

0.08 

0.04 

3.33 

19 

164 

318 

129 

15 

704 

nd 

nd 

762 

3.97 

14.6 

4.27 

1.63 

22.5 

6.3 

137 

48.9 

87.2 

28 

4.65 

0.89 

0.42 

1.59 

0.25 

HikoTuff 

87-452N 

67.5 

14.9 

2.01 

1.16 

2.20 

2.99 

4.76 

0.33 

0.11 

0.04 

3.27 

16 

164 

420 

158 

18 

852 

nd 

nd 

861 

nd 

nd 

4.45 

1.35 

21.9 

5.02 

150 

51.7 

89.2 

29.2 

4.76 

0.92 

0.40 

1.44 

0.22 

87-4520 

67 

15.2 

2.08 

1.30 

2.49 

3.27 

3.97 

0.33 

0.11 

0.06 

3.55 

17 

125 

447 

151 

17 

935 

nd 

nd 

981 

nd 

nd 

4.98 

1.44 

23.4 

4.82 

171 

52.4 

96 

31 

5.12 

1.03 

0.45 

1.54 

0.24 

87-506 

67.6 

15.1 

2.88 

0.94 

2.37 

3.09 

4.41 

0.52 

0.18 

0.04 

2.13 

15 

123 

531 

222 

17 

1,230 

nd 

nd 

1,260 

nd 

nd 

5.38 

1.27 

17.1 

4.23 

197 

57.8 

109 

38.6 

6.61 

1.33 

0.56 

1.62 

0.24 

87-1231 

70.2 

14.0 

2.65 

0.80 

2.04 

3.26 

4.32 

0.43 

0.15 

0.03 

1.31 

17 

150 

404 

180 

16 

952 

nd 

nd 

944 

nd 

nd 

4.91 

1.47 

19.9 

5.39 

nd 

51.6 

93.7 

33.2 

5.77 

1.11 

0.52 

1.68 

0.26 

88-1267 

67.7 

14.6 

2.93 

1.05 

3.25 

3.19 

4.31 

0.46 

0.16 

0.05 

1.76 

13 

136 

628 

196 

16 

1,120 

49 

9 

1,000 

nd 

nd 

nd 

nd 

17 

nd 

nd 

65 

100 

37 

nd 

nd 

nd 

2 

nd 

97 
98 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

Table 1. Chemical analyses for ash-flow tuffs in and near Caliente caldera complex-Continued. 

Unit Tuff of Tepee Rocks 

Sample No. 88-1268 

Major-element composition 

Si02 

Al203 

FeT03 

MgO 

CaO 

Na20 

K20 

Ti02 

P20s 

MnO 

LOI 

73.1 

12.6 

0.77 

0.67 

0.94 

2.43 

5.18 

0.11 

<0.05 

0.03 

3.89 

Trace-element composition 

Nb 26 

Rb 

Sr 

Zr 

y 

Ba 

La 

Cu 

*Ba 

*Co 

*Cr 

*Cs 

*Hf 

*Ta 

*Th 

*U 

*Zr 

*La 

*Ce 

*Nd 

*Sm 

*Eu 

*Gd 

*Tb 

*Tm 

*Yb 

*Lu 

235 

73 

107 

16 

97 

39 

4 

96 

nd 

nd 

nd 

nd 

nd 

31 

nd 

nd 

37 

53 

12 

nd 

nd 

nd 

nd 

nd 

nd 

Tuff of Rainbow Canyon 

87-221 87-1111 

67.5 

10.7 

0.8 

0.90 

2.76 

1.38 

3.30 

0.1 

0.05 

0.07 

76.2 

8.1 

0.57 

0.49 

1.74 

0.65 

2.66 

0.07 

0.05 

0.05 

12.0 9.01 

43 38 

398 224 

32 321 

114 102 

29 18 

62 43 

nd nd 

nd nd 

75 66 

0.5 0.2 

1.3 0.7 

17.4 9.9 

5.69 4.18 

3.62 2.79 

39.4 29.9 

6.15 8.1 

114 <70 

33.4 23 

59.0 44.4 

14.8 11.0 

2.78 2.13 

0.12 0.05 

nd nd 

0.37 0.30 

nd nd 

3.42 

0.58 

2.14 

0.34 

87-1113 

76.2 

8.1 

0.59 

0.43 

1.52 

1.28 

2.26 

0.07 

0.05 

0.06 

8.85 

36 

230 

540 

102 

26 

65 

nd 

nd 

35 

0.2nd 

<.4 

9.3 

4.08 

2.73 

27.7 

15.6 

<71 

37.7 

50.5 

16.9 

3.05 

0.07 

nd 

0.42 

nd 

2.81 

0.43 

88-226A 

73.5 

10.2 

0.86 

0.60 

1.21 

2.33 

2.58 

0.1 

0.05 

0.06 

8.01 

45 

154 

590 

124 

25 

96 

nd 

nd 

123 

0.4 

0.6d 

9.4 

5.26 

3.37 

37.6 

8.85 

<98 

33.7 

58.8 

15.2 

2.63 

0.11 

nd 

0.36 

nd 

3.01 

0.48 

Tuff of Kershaw Canyon 

88-195A 89-6980 

65.1 

13.3 

2.08 

0.69 

2.32 

2.69 

3.65 

0.27 

0.05 

0.08 

9.44 

31 

145 

262 

261 

29 

598 

nd 

nd 

610 

1.3 

3.1 

5.1 

7.76 

2.03 

18.6 

4.75 

250 

78.7 

152 

47.5 

8.03 

0.70 

nd 

0.83 

nd 

3.09 

0.47 

67.6 

12.6 

2.46 

1.05 

2.45 

1.79 

4.15 

0.31 

0.08 

0.06 

6.94 

26 

211 

468 

250 

29 

546 

54 

9 

520 

nd 

nd 

nd 

nd 

nd 

17 

nd 

nd 

56 

100 

40 

6.3 

0.65 

5.1 

0.81 

0.55 

2.9 

nd 
GEOCHEMISTRY OF MIOCENE TUFFS, CALIENTE CALDERA COMPLEX 

Table 1. Chemical analyses for ash-flow tuffs in and near Caliente caldera complex-Continued. 

Unit Gregerson Basin Gregerson Basin Gregerson Basin 

Mbr., upper unit Mbr., upper unit Mbr., lower unit 

Sample No. 87-l105C 88-1157A 

Major-element composition 

Si02 80.8 75.1 

Al203 7.96 11.3 

FeT03 

MgO 

CaO 

Na20 

K20 

Ti02 

P205 

MnO 

LOI 

2.15 

0.21 

0.42 

2.81 

3.35 

0.13 

0.05 

0.05 

1.41 

Trace-element composition 

Nb 84 

Rb 

Sr 

Zr 

y 

Ba 

La 

Cu 

*Ba 

*Co 

*Cr 

*Hf 

*Ta 

*Th 

*U 

*Zr 

*Zn 

*La 

*Ce 

*Nd 

*Sm 

*Eu 

*Gd 

*Tb 

*Tm 

*Yb 

*Lu 

194 

25 

825 

80 

65 

102 

12 

67 

nd 

2 

nd 

nd 

23 

nd 

nd 

140 

85 

170 

68 

nd 

nd 

nd 

nd 

nd 

9 

nd 

2.98 

0.17 

0.25 

4.34 

4.62 

0.19 

<0.05 

0.07 

0.27 

104 

261 

13 

1110 

77 

75 

127 

9 

77 

nd 

nd 

nd 

nd 

30 

nd 

nd 

200 

120 

240 

93 

nd 

nd 

nd 

nd 

nd 

11 

nd 

88-1157B 

75.3 

11.4 

2.78 

0.15 

0.19 

4.27 

4.77 

0.19 

<0.05 

0.06 

0.31 

108 

249 

7 

928 

80 

26 

128 

13 

39 

nd 

nd 

nd 

nd 

27 

nd 

nd 

180 

130 

250 

96 

nd 

nd 

nd 

nd 

nd 

9 

nd 

Tuff of 

Etna 

88-177B 

80.1 

9.02 

1.19 

0.20 

0.58 

2.36 

4.07 

0.09 

0.05 

0.03 

1.76 

52 

171 

67 

189 

49 

191 

59 

3 

170 

3 

nd 

nd 

21 

nd 

nd 

71 

56 

92 

44 

nd 

nd 

nd 

nd 

nd 

5 

nd 

Tuff of 

Etna 

88-1209 

76.3 

11.7 

1.96 

0.21 

0.22 

3.99 

4.62 

0.13 

0.05 

0.04 

0.67 

61 

164 

18 

462 

44 

57 

78 

4 

57 

nd 

nd 

nd 

nd 

17 

nd 

nd 

98 

77 

140 

58 

nd 

nd 

nd 

nd 

nd 

5 

nd 

Ox Valley 

Tuff 

87-697Y 

75.6 

11.7 

1.70 

0.26 

0.72 

3.19 

4.58 

0.18 

0.05 

0.03 

1.35 

35 

137 

86 

212 

43 

457 

nd 

nd 

451 

nd 

nd 

7.24 

2.51 

16.9 

3.28 

237 

nd 

43.8 

94.1 

39.3 

8.52 

0.60 

nd 

1.14 

nd 

4.26 

0.62 

99 
100 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

Table 2. Sample localities for ash-flow tuffs in and near Caliente caldera complex. 

Sam_ele No. Unit Coun~, State Latitude N. Long!tude W. Oescri_etion 

87-1181C Bauers Member Lincoln, Nev. 37° 38' 05" 114° 29' 50" Glass, intracaldera. 

87-452E Bauers Member Lincoln, Nev. 37°51'29" 114° 19' 28" Glass, outflow. 

87-452H Bauers Member Lincoln, Nev. 37°51' 29" 114° 19' 30" Outflow. 

88-1065A Bauers Member Lincoln, Nev. 37° 37' 05" 114° 29' 08" Intracaldera. 

88-1035A Bauers Member Lincoln, Nev. 37° 37' 37" 114° 29' 28" Glass, intracaldera. 

87-452A Swett Member Lincoln, Nev. 37°51' 29" 114° 19' 22" Glass, outflow. 

87-4520 Swett Member Lincoln, Nev. 37°51'29" 114° 19' 24" Outflow. 

86-666C Hiko Tuff Lincoln, Nev. 37° 32' 15" 115° 01' 15" Glass, outflow, type section. 

86-666H Hiko Tuff Lincoln, Nev. 37° 32' 15" 115° 01' 40" Outflow, type section. 

87-206G Hiko Tuff Lincoln, Nev. 37° 35' 32" 114° 36' 04" Glass, intracaldera. 

87-452N Hiko Tuff Lincoln, Nev. 37° 51' 34" 114° 19' 25" Lower outflow unit. 

87-4520 Hiko Tuff Lincoln, Nev. 37° 51 36" 114° 19' 22" Upper outflow unit. 

87-506 Hiko Tuff Lincoln, Nev. 37° 30' 33" 114° 37' 16" Glass, intracaldera. 

87-1231 Hiko Tuff Lincoln, Nev. 37°3127" 114° 43' 03" Intracaldera. 

88-1267 Hiko Tuff Lincoln, Nev. 37° 35' 03" 114° 38' 10" Top, intracaldera. 

88-1268 tuff of Tepee Lincoln, Nev. 37° 35' 47" 114° 26' 17" Outflow?, type area. 

Rocks 

87-452K Harmony Hills Iron, Utah 37° 51' 31" 114° 19' 31" Glass, Condor Canyon. 

Tuff 

87-697Y Ox Valley Tuff Iron, Utah 37° 30' 04" 113° 51' 38" Outflow, near type section. 

87-221 tuff of Rainbow Lincoln, Nev. 37° 30' 47" 114° 30' 07" Intracaldera. 

Canyon 

87-1111 tuff of Rainbow Lincoln, Nev. 37° 34' 04" 114° 34' 13" Glass, intracaldera. 

Canyon 

87-1113 tuff of Rainbow Lincoln, Nev. 37° 34' 27" 114° 34' 40" Glass, intracaldera. 

Canyon 

88-226A tuff of Rainbow Lincoln, Nev. 37° 34' 53" 114° 32' 52" Intracaldera. 

Can on 

88-195A tuff of Kershaw Lincoln, Nev. 37° 35' 37" 114° 32' 50" Upper (post-C3) tuff, Little 

Canyon Kershaw Canyon. 

89-6980 tuff of Kershaw Lincoln, Nev. 37° 34' 21" 114° 33' 02" Post -C2 tuff. 

Can on 

87-1105C Gregerson Basin Lincoln, Nev. 37° 34' 02" 114° 33' 46" Upper unit (C3). 

Member 

88-1157A Gregerson Basin Lincoln, Nev. 37° 33' 34" 114° 29' 56" Upper unit (C3). 

Member 

88-1157b Gregerson Basin Lincoln, Nev. 37° 33' 34" 114° 29' 55" Lower unit (C2). 

Member 

88-177B tuff ofEtna Lincoln, Nev. 37° 36' 08" 114° 32' 03" Outflow. 

88-1209 tuffofEtna Lincoln, Nev. 37° 34' 20" 114° 40' 14" Outflow. 
GEOCHEMISTRY OF MIOCENE TUFFS, CALIENTE CALDERA COMPLEX 101 

7 EXPLANATION 

D Swett Tuff Member of Condor Canyon Formation 

6 • Bauers Tuff Member of Condor Canyon Formation 

0 Harmony Hills Tuff 

1- 5 • Hiko Tuff z 

w X Tuff of Tepee Rocks 0 

0:: 

w Tuff of Rainbow Canyon 4 • D.. 

1- 0 Tuff of Kershaw Canyon 

:I: Gregerson Basin Member of Kane Wash Tuff (!) .... w 3 Tuff of Etna 3: + 

~ 0 Ox Valley Tuff 

~ ~ 

2 

SiOz, IN WEIGHT PERCENT 

Figure 3. Potassium variation diagram for ash-flow tuffs in and near Caliente caldera complex and vicinity, Nevada-Utah. Rock nomenclature from Peccerillo and Taylor (1976). 

18 

16 

14 

0 12 ~ 

~ 10 -

0 * 

~ 8 ~ 

6 

4 

2 

0 

+ ... 

g__ l Tholeiitic -~- + 

o(alc~alfal~in•~o. t ·~ x. • I 

60 65 70 75 80 85 

Si02 CONTENT, IN WEIGHT PERCENT 

EXPLANATION 

/j, Swett Tuff Member of Condor Canyon Formation 

+ Bauers Tuff Member of Condor Canyon Formation 

o Harmony Hills Tuff 

• Hiko Tuff 

X Tuff of Tepee Rocks 

• Tuff of Rainbow Canyon 

0 Tuff of Kershaw Canyon 

.l. Gregerson Basin Member of Kane Wash Tuff 

+ Tuff of Etna 

D Ox Valley Tuff 

Figure 4. Silica versus Feo* /MgO variation diagram for ashflow tuffs in and near Caliente caldera complex and vicinity, 

Nevada-Utah. Fields modified from Miyashiro (1974). FeO* is 

total iron, reported as FeT03, multiplied by 0.8993. 

series from andesite to high-silica rhyolite (greater than 75 

weight percent silica). Many samples cluster between about 

69 and 74 weight percent Si02. Some samples of the tuff of 

Rainbow Canyon, Ox Valley Tuff, Gregerson Basin Member 

of the Kane Wash Tuff, and tuff of Etna have unrealistically 

high silica contents (silica greater than 76 wt. percent Si02), 

probably due to silica gain during alteration. In general, the 

suite shows an initial increase in potassium with increasing 

silica content to about 72 wt. percent silica, and a decrease in 

potassium in yet more silicic samples. 

Calcium variability suggests low-temperature alteration 

of low-potassium, high-silica rocks. Such rocks have high 

calcium contents compared to the rest of our suite (fig. SA). 

Unaltered rocks show an almost monotonic decrease in calcium with increasing silica content. Sodium contents give 

support to the hypothesis that high-silica rhyolite is the product of postemplacement alteration (fig. SB) because all highsilica rhyolites have low and highly variable sodium contents 

when compared to the other tuffs in the Caliente area. The 

diagrams suggest that unaltered ash-flow tuffs in the Caliente 

area originally contained between about 3 and 4.5 weight 

percent Na20. Total alkali contents confirm the disparity 

between unaltered and altered rocks (fig. 5C). Altered rocks 

apparently lost both sodium and potassium, and gained calcium and silica during alteration. The chemical patterns of 

altered rocks are consistent with the alteration of glass during 

advanced stages of hydration. Alumina and titania contents 

decrease with increasing silica content (fig. SD and 5£). 

Caliente samples straddle the boundary between peraluminous and metaluminous rocks on plots of the molar 

Al203/(CaO+Na20+K20) (A/CNK) versus silica (fig. 6). 

The Gregerson Basin Member of the Kane Wash Tuff has 
102 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

6 1- 16 

1- A - z 

z w D 

w 5 r 

0 0 

0 - ~ ~ w 14 

w ~ - ~ 

~ 1- X 1- 4 - J: J: - (!) (!) iii 12 

iii !- • ~~ ~ 3 ~ • ~ 

z • ~ 

"o<> ...: r- ...: 10 

z 2 r- ~ . 

z 

w .- w + 

1- • 1-

z f-- • *• z 

0 • 0 8 ! 

0 1 f-- xo 

0 

0 1"'1 

~ 0 ~ r- - N u 

0 I I I I I 1&+1 I I < 6 

60 65 70 75 80 85 60 65 70 75 80 85 

0.8 

1-

z l 1- 0 • B- E w 4 - + z 

0 

~..:~. w - -

~ 0 

w - - ~ 0.6 ~ w - -

0 0 ~ 

1- 3 • 0 • 1- • J: - (!) ! J: -

~ (!) 

X • •• + - w 

~ 0.4 - • ~ 2 - () - ~ 

1-

...: 

z 

w f-- • .-

...: 

z 

w "~. z 1- 0.2 f-- 1 f-- z £o -

0 - 0 0 • 0 ~ .. 0 r- - N f-- • + - N 

~ 0 • z 0 I I I I I I I I I E=: I I I I I I I I I 0.0 60 65 70 75 80 85 60 65 70 75 80 85 

SiO 2CONTENT, IN WEIGHT PERCENT 

1-

z 10 

w 

.. ~. EXPLANATION 0 9 r- ~ j+ c- !'::, Swett Tuff Member of Condor Canyon Formation w 

~ 8 f-- .... Xo -

• Bauers Tuff Member of Condor Canyon Formation 1-

J: 7 f-- • (!) () - 0 Harmony Hills Tuff 

~ 0 () +._ 6 f-- -

• Hiko Tuff 

~ 5 • • X Tuff o.f Tepee Rocks 

...: r- -

z 4 f-- • Tuff of Rainbow Canyon w 

·- 1-

z 3 0 Tuff of Kershaw Canyon 0 -

0 .. Gregerson Basin Member of Kane Wash Tuff 0 2 - N 

~ 1 + Tuff of Etna 

0 -

N 0 

0 Ox Valley Tuff ~ I I I I I I I I I z 60 65 70 75 80 85 

SiO 2CONTENT, IN WEIGHT PERCENT 

Figure 5. Silica variation diagrams for ash-flow tuffs in and near Caliente caldera complex. A, CaO; B, Na20; C, Na20+K20; D, Al203; 

E,Ti02. 
GEOCHEMISTRY OF MIOCENE TUFFS, CALIENTE CALDERA COMPLEX 103 

1.2 

ON 

~ 1.1 + 

ON 

C': 

z 1.0 

+ 

0 

C': 

u 0.9 

~ N 0.8 < 

... 

.! 

0 0.7 

:!!! 

0.6 

• 

X • • ... 0 

~ + 

• l .&. 

0 

60 65 70 75 80 85 

Si02 CONTENT, IN WEIGHT PERCENT 

EXPLANATION 

D. Swett Tuff Member of Condor Canyon Formation 

+ Bauers Tuff Member of Condor Canyon Formation 

o Harmony Hills Tuff 

• Hiko Tuff 

X Tuff of Tepee Rocks 

• Tuff of Rainbow Canyon 

O Tuff of Kershaw Canyon 

• Gregerson Basin Member of Kane Wash Tuff 

+ Tuff of Etna 

o Ox Valley Tuff 

Figure 6. Molar Al203/CaO+Na20+K20 versus silica variation 

diagram for ash-flow tuffs in and near Caliente caldera complex. 

A/CNK<1, and is technically peralkaline, having molar 

Na20+K20 in excess of molar Al203. Altered (>76 weight 

percent Si02) samples show considerable scatter both above 

and below the A/CNK= I line. 

TRACE-ELEMENT GEOCHEMISTRY 

Despite the demonstrable fact that several of our 

samples are altered, we are compelled to discuss their traceelement abundances because these data are essential to 

characterize ash-flow tuffs in the Caliente area. Moreover we 

are not sure that we can unequivocally distinguish altered 

from unaltered samples. Obviously samples with more than 

76 weight percent Si02 are altered, but some less silicic rocks 

may also be altered. So, in this section, we describe the traceelement patterns of all the samples discussed previously. The 

composition of the Gregerson Basin Member is commonly 

discussed for the sake of comparison and because this unit 

occurs within the Caliente caldera complex. Because the 

Gregerson Basin Member is peralkaline, concentrations of 

high-field-strength elements (such as Zr, Y. Nb) are higher 

in it than in the peraluminous and metaluminous rocks. 

Peraluminous and metaluminous samples show moderate Zr levels that tend to decrease along convergent trends 

with increasing silica content (fig. 7 A). Among the low-silica 

rhyolites, the Hiko Tuff and tuff of Tepee Rocks have lower 

Zr contents than the Bauers Tuff Member, Swett Tuff Member, and tuff of Kershaw Canyon. The Gregerson Basin 

Member has high Zr contents (800-1, 100 ppm Zr), consistent with its peralkaline chemistry. 

The abundance of Nb in rocks in the Caliente area 

shows an overall increase with increasing silica content (fig. 

7B). Post-Hiko units have higher Nb contents than the older 

units (Swett and Bauers Tuff Members, Harmony Hills Tuff, 

Hiko Tuff). The Gregerson Basin Member has the highest 

Nb levels (>80 ppm Nb ). The Gregerson Basin also has the 

highest concentration of Y (fig. 7C). Hiko Tuff is distinguished from other peraluminous and metaluminous rhyolites by lower Y concentrations. Thus, as noted by previous 

workers (Novak, 1984; Novak and Mahood, 1986; Harding 

and others, this volume; Scott and others, this volume) the 

Gregerson Basin Member has high concentrations of highfield-strength elements, a chemical characteristic of peralkaline magmas. 

The abundance of Sr in most of our samples decreases 

with increasing silica content, consistent with plagioclasecontrolled fractionation (fig. 7D). But interestingly, none of 

the samples have Sr contents between 100 and 250 ppm, 

allowing the samples to be separated into high-Sr and low-Sr 

groups. High Sr abundances characterize the Swett Tuff 

Member, Hiko Tuff, Bauers Tuff Member, and the tuff of 

Kershaw Canyon. Low Sr abundances characterize the tuff 

of Tepee Rocks, Gregerson Basin Member, tuff of Etna, and 

one sample of the tuff of Rainbow Canyon. Altered samples 

of the tuff of Rainbow Canyon are displaced away from the 

remainder of the samples toward the high-silica side of figure 7 D, but they have Sr contents that overlap values of unaltered(?) rocks. 

Rubidium shows a four-fold range in ash-flow tuffs of 

the Caliente area (fig. 7 E). but Rb is expected to be readily 

mobilized by postemplacement alteration. The amount of 

scatter in figure 7 E is probably evidence of this alteration. 

The tuff of Rainbow Canyon, for example, shows more variability in Rb content than exists for all other units. The pattern in figure 7 E may indicate that Rb content for 

peraluminous and metaluminous rhyolites was the same as it 

was in peralkaline rhyolite. 

Barium shows an overall decrease with increasing silica 

content (fig. 7 F). This element ranges in concentration from 

about 1 ,400 ppm Ba in the Bauers Tuff Member to less than 

I 00 ppm Ba in the Gregerson Basin Member and some highsilica rhyolite samples. A small gap at about 300 ppm Ba 

separates high-Ba and low-Ba rocks. 

Trace-element ratios are commonly used as petrologic 

indicators. At similar silica contents. Zr/Nb values of the 

Bauers and Swett Tuff Members of the Condor Canyon Formation are higher than those of the Hiko Tuff (fig. 8 ). Lowsilica rhyolite samples of the Hiko Tuff have Zr/Nb greater 

than 8, whereas samples of high-silica rhyolite Hiko Tuff 
104 GEOLOGIC STUDIES. BARCO STUDY UNIT, 1992 

1200,----------------, 

~ 1000 

::J 

...J 

:iii. 800 Ill:: 

w 

c.. 

~ 600 

~ 

!!: 400 

'""-

N 

200 

A 

0 ~~~ _, . 

+ 

0 

x• 

+ • • I 

75 0 60 I 

120.------------------------------, 

I I 

65 I I 70 80 85 

~ 100 

::J 

...J 

:iii 80 Ill:: 

w 

c.. 

Cl) 

~ 

~ 

!!: 

.cz 

60 

40 

20 

B 

0 

_L I I I I I 

65 70 75 

+ 

X 

• 

0 

I I 

80 

+ -

• 

.--

-

-

85 

90.---------------------------~ 

80r- c 

70 

60 

50 

40 

30 

20 0 

~ -

-

-

-

-

+ --

-

-

• -

. ·- X .--

-

-

I I I I 

65 70 75 80 85 

Si02 CONTENT, IN WEIGHT PERCENT 

800~--------------------------D~ 

700 

600 

500 

400 

300 

ri 200 

100 

060 

• 

• • 

• 0 ..... • (). 

• • 

• 

85 

450~--------------------------~ 

400 

350 

300 

250 

200 

150 

100 

50 

0

60 

0 

I 

65 

• 

.. \ ~~ • __:_t; • + ... ~ 0 

I I I I 

711 75 

~ 

• + 

I I 

80 

E -

-

• 

-

-

-

-

-

-

-

85 

1600~------------------, 

z 

0 

1400 

3 1200 

:iii 

Ill:: 1000 w 

c.. 

Cl) 800 ~ 

if 600 

!!: 

ci 400 

= 

0 

65 

•• 

. Lh. 

.... 

• • • • 

70 

0 

+ 

75 80 

Si02 CONTENT, IN WBGHT PERCENT 

F 

I 

85 

EXPLANATION 

6 Swett Tuff Member of Condor Canyon Formation 

+ Bauers Tuff Member of Condor Canyon Formation 

o Harmony Hills Tuff 

• Hiko Tuff 

X Tuff of Tepee Rocks 

• Tuff of Rainbow Canyon 

0 Tuff of Kershaw Canyon 

_. Gregerson Basin Member of Kane Wash Tuff 

+ Tuff of Etna 

o Ox Valley Tuff 

Figure 7. Trace element variation diagrams for ash-flow tuffs in and near Caliente caldera complex. A, zirconium; B, niobium; C, yttrium: 

D, strontium: E, rubidium; F, barium. 
.c 

..._ 

z 

N ""

GEOCHEMISTRY OF MIOCENE TUFFS, CALIENTE CALDERA COMPLEX 105 

25 I I I I I I I I I 

0 r-

~ -

20 f- • -

f- •• •• -

15 r- ... .A. -

f- • -

10 f- • • • •oO ~ -

r- • .+ -

• D 5 f- -

X + f- • • 

0 I I I I I I I I ·- 60 65 70 75 80 85 

Si02 CONTENT, IN WEIGHT PERCENT 

EXPLANATION 

D. Swett Tuff Member of Condor Canyon Formation 

+ Bauers Tuff Member of Condor Canyon Formation 

o Harmony Hills Tuff 

• Hiko Tuff 

X Tuff of Tepee Rocks 

• Tuff of Rainbow Canyon 

0 Tuff of Kershaw Canyon 

.A Gregerson Basin Member of Kane Wash Tuff 

+ Tuff of Etna 

D Ox Valley Tuff 

Figure 8. Zr/Nb versus Si02 variation diagram for ash-flow tuffs 

in and near Caliente caldera complex. 

have Zr/Nb less than 8. More silicic and locally altered samples of the tuffs of Rainbow Canyon and Kershaw Canyon 

have low Zr/Nb values ( ::::3 ), suggesting that Zr/Nb values did 

not change during alteration. The decrease in Zr enrichment 

relative to that of Nb in tuff of the Bauers, Swett, and Hiko 

also characterizes calc-alkaline rocks from the Chinati 

Mountains area, West Texas (Cameron and Cameron, 1986). 

Metaluminous granites, to which the majority of the ash-flow 

tuffs of the Caliente area are analogous, have similar Zr/Nb 

values (5-20). Davies and Macdonald ( 1987) attributed high 

Zr/Nb and high Rb/Zr (>3) in metaluminous granitic melts to 

the involvement of significant amounts of crustal material. 

Cameron and Cameron (1986) suggested that the decrease in 

Zr/Nb in Chinati Mountains rocks resulted from the fractionation of zircon, an accessory phase in those rocks. 

The variability of BalLa and Ba/Nb serves to distinguish Gregerson Basin Member, tuff of Rainbow Canyon, 

tuff of Kershaw Canyon, and Ox Valley Tuff from the 

remainder ofthe ash-flow tuffs (fig. 9). The tuff of Rainbow 

Canyon, Gregerson Basin Member, and tuff of Etna have 

low Ba/Nb and BalLa values. The tuff of Kershaw Canyon 

and the Ox Valley Tuff have slightly higher values for both 

ratios. The remainder of our suite has high BalLa (> 15) and 

Ba/Nb (>39). 

~ 

~ ~ 

~ 

25 

• •• 0 

20 ~ ••• 

15 • ... .. 

• 

10 oo 

0 

5 

0 

0 25 50 75 100 125 150 

Ba/Nb 

EXPLANATION 

D. Swett Tuff Member of Condor Canyon Formation 

+ Bauers Tuff Member of Condor Canyon Formation 

o Harmony Hills Tuff 

• Hiko Tuff 

X Tuff of Tepee Rocks 

• Tuff of Rainbow Canyon 

0 Tuff of Kershaw Canyon 

.A Gregerson Basin Member of Kane Wash Tuff 

+ Tuff of Etna 

D Ox Valley Tuff 

Figure 9. BalLa versus Ba!Nb diagram for ash-flow tuffs in and 

near Caliente caldera complex. 

SPIDER DIAGRAMS 

In this section we show spider diagrams (fig. 1 0) for ashflow tuffs from the Caliente area. These diagrams provide a 

convenient way of comparing elemental abundances in volcanic rocks. Raw data are normalized to values of Thompson 

and others ( 1984 ), most of which are average chondri tic 

abundances; normalization values forK, Rb, and Pare not 

chondritic. In the spider diagrams, elements are arranged to 

produce a downward slope from left to right in the patterns 

of most igneous rocks. Gaps in the patterns appear where no 

elemental data exist, but allow for easy comparison between 

all the units. 

In general, ash-flow tuffs from the Caliente area show 

elemental patterns similar to those of many silicic volcanic 

rocks in western North America. The patterns allow the samples to be separated into four distinct geochemical types: (I) 

Condor Canyon Formation, Harmony Hills Tuff, Hiko Tuff, 

tuff of Tepee Rocks, and tuff of Kershaw Canyon; (2) Ox 

Valley Tuff; (3) tuff of Rainbow Canyon; and (4) Gregerson 

Basin Member of the Kane Wash Tuff. Type I units are characterized by moderate negative Ba, Nb, Ta, Sr, P, and Ti 

anomalies. Our single sample of the Harmony Hills Tuff is 

distinguished from the other Type I units by the absence of 
106 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

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GEOCHEMISTRY OF MIOCENE TUFFS, CALIENTE CALDERA COMPLEX 107 

a Sr anomaly. The single sample of the tuff of Tepee Rocks 

shows a much larger Ba anomaly than the other Type 1 units. 

The Ox Valley Tuff (Type 2) is distinctive in that Ba, Sr, and 

Ti anomalies tend to be larger, and Nb and Ta anomalies are 

slightly smaller than they are for Type 1 units. The tuff of 

Rainbow Canyon (Type 3) lacks many of the negative anomalies exhibited by Types 1 and 2. This unit shows a strong Ba 

and a moderate negative Ti anomaly but small or no anomalies at Nb, Ta, Sr, and only a small one at P. One sample of 

the tuff of Rainbow Canyon does show a moderate negative 

Sr anomaly. The Gregerson Basin Member and tuff of Etna 

have similar elemental patterns. They show large negative 

Ba and Sr anomalies, and slightly smaller negative P and Ti 

anomalies. These units lack the negative Nb anomalies of 

Type 1 units and have Ba and Sr anomalies that are significantly larger than the Ox Valley Tuff (Type 2). Type 4 units 

are distinguished from the tuff of Rainbow Canyon (Type 3) 

by large Sr and P anomalies. 

The majority of the older units (Condor Canyon Formation to Hiko Tuff) show Type 1 chemical signatures. PostHiko Tuff units derived from the Caliente caldera complex 

tend to have larger negative Ba anomalies than the Hiko Tuff 

and pre-Hiko Tuff units. Although not derived from the Caliente caldera complex, the Gregerson Basin Member also 

shows a large negative Ba anomaly. The tuff of Kershaw 

Canyon and the Ox Valley Tuff show moderate negative Ba 

anomalies. 

COMPARATIVE GEOCHEMISTRY 

In this section we compare trace-element abundances of 

rocks from the Caliente area to those of silicic volcanic rocks 

from other volcanic fields, mostly those in the western Cordillera. We assume here that the compositions of tuffs from 

the Caliente area are similar to those of the magma systems 

from which they erupted. However, we realize that many of 

the analytical results are not strictly magmatic. Deviations 

from magmatic compositions, as we have already indicated, 

probably resulted from physical mixing of juvenile and 

xenolithic material and from postemplacement alteration. 

A variety of chemical diagrams are available for discriminating volcanic rocks erupted in different tectonic environments. Although each type of diagram has been tested 

over a relatively large range of compositions and tectonic 

Figure 10 (facing page). Spider diagrams for ash-flow tuffs in 

and near Caliente caldera complex. Normalization values from 

Thompson and others ( 1984 ). A, Swett Tuff Member of Condor 

Canyon Formation; B, Bauers Tuff Member of Condor Canyon 

Formation; C, Harmony Hills Tuff; D, Hiko Tuff; E, tuff of Tepee 

Rocks; F, tuff of Rainbow Canyon; G, tuff of Kershaw Canyon; H, 

Gregerson Basin Member of Kane Wash Tuff; I, tuff of Etna; J, Ox 

Valley Tuff. 

environments, none of the diagrams has been shown to work 

in all cases. Here we use diagrams proposed by Pearce and 

others ( 1984) for discriminating between collisional granite, 

volcanic-arc granite, within-plate granite, and ocean-ridge 

granite. 

Ash-flow tuffs in the Caliente area show mostly volcanic-arc and within-plate granite signatures (fig. 11 ). The 

calc-alkaline (metaluminous to peraluminous) rocks have 

low total Y +Nb contents compared with the tholeiitic (peralkaline) Gregerson Basin Member. Data for the calc-alkaline 

units overlap with those for pumices and whole-rock samples of ash-flow tuffs from several volcanic terranes in the 

Cordillera of North and Central America, including samples 

of the 19-Ma metaluminous Peach Springs Tuff in northwest 

Arizona (L.D. Nealey, unpub. data, 1988), the 26.5-Ma peralkaline Amalia Tuff of the Latir volcanic field, New Mexico 

(Johnson and Lipman, 1988), tuffs in the Sierra Madre Occidental (Cameron and Hanson, 1982), Miocene Thirsty Canyon Tuff of the Black Mountain caldera, Nevada (Vogel and 

others, 1989), and the 11.6-Ma Rainier Mesa and 11.4-Ma 

Ammonia Tanks Members of the Timber Mountain Tuff, 

Nevada (Mills, 1991 ). Although the Gregerson Basin Member is demonstrably peralkaline, it is transitional between 

strongly peralkaline rhyolite like that of the Naivasha volcanic center in the Kenya rift (Macdonald and others, 1987) 

and the peralkaline Amalia Tuff in New Mexico (Johnson 

and Lipman, 1988; this report, fig. 11 ). 

Although all the ash-flow tuffs erupted in a within-plate 

environment, their Y +Nb contents in the Pearce plot (fig. 11) 

suggest that the older units (Swett and Bauers Tuff Members, Harmony Hills Tuff, Hiko Tuff, and tuff of Tepee 

Rocks) erupted in a volcanic-arc setting. Younger units (tuff 

of Rainbow Canyon, tuff of Kershaw Canyon, Gregerson 

Basin Member, tuff of Etna, and Ox Valley Tuff), in contrast, tend to show signatures of within-plate granites. Similar trends have been identified in late Cenozoic ash-flow 

tuffs of the Mogollon-Datil volcanic field in New Mexico 

(W.E. Elston, oral commun., 1987). 

SUMMARY AND CONCLUSIONS 

Major compositional differences exist among ash-flow 

tuffs in the Caliente area. Rowley and others (this volume) 

have shown that the ash-flow tuffs can be distinguished petrographically into groups using relative proportions of phenocrysts and accessory minerals and chemically using minor 

element concentrations. We have further shown here that 

these groups can be distinguished into petrochemical assemblages using other geochemical data. The majority of the units 

derived from the Caliente caldera complex are calc-alkaline 

and metaluminous to peraluminous. The Gregerson Basin 

Member, which erupted from the nearby Kane Springs Wash 

caldera, shows tholeiitic affinities and is mildly peralkaline. 
108 

z 

0 

...J 

...J 

:2 

a:= 

w 

c.. 

C/) 

1-

o:= 

<C 

c.. 

z 

GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

1000~--------------------------------------~------------~ 

100 

10 

10 

syn-COLG 

VAG 

ORG 

100 

Y+Nb, IN PARTS PER MILLION 

EXPLANATION 

6 Swett Tuff Member of Condor Canyon Formation j;l_. Amalia Tuff 

+ Bauers Tuff Member of Condor Canyon Formation 8 Naivasha Rhyolite 

0 Harmony Hills Tuff ~ Superstition Mountains Tuff 

• Hiko Tuff > Timber Mountain Tuff 

X Tuff of Tepee Rocks < Sierra Madre Occidental tuff 

• Tuff of Rainbow Canyon * Peach Springs Tuff 

0 Tuff of Kershaw Canyon l:l Thirsty Canyon Tuff 

A Gregerson Basin Member of Kane Wash Tuff 

+ Tuff of Etna 

0 Ox Valley Tuff 

1000 

Figure 11. Rb versus Y +Nb variation for ash-flow tuffs in and near Caliente caldera complex, and selected rhyolites. Data 

for the Amalia Tuff from Johnson and Lipman ( 1988); Naivasha Rhyolite from Macdonald and others ( 1987); silicic rocks 

from the Superstition Mountains from L.D. Nealey (unpub. data, 1987); tuffs from Timber Mountain caldera from Mills 

( 1991 ); tuff from the Sierra Madre Occidental from Cameron and Hanson ( 1982); Peach Springs Tuff from L.D. Nealey 

(unpub. data, 1988); and Thirsty Canyon Tuff of the Black Mountain caldera from Vogel and others (1989) shown for comparison. Tectonic fields from Pearce and others ( 1984 ): VAG, volcanic arc granite; syn-COLG, syncollisional granite; 

WPG, within-plate granite; ORG, oceanic island granite. 

These two groups are easily distinguished using high-fieldstrength elements (Zr, Y, Nb) which show high concentrations in the Gregerson Basin Member. 

Zr. The tuff of Rainbow Canyon is consistently the most 

altered unit in the Caliente area. It characteristically has high 

silica and low alkali contents compared with other ash-flow 

tuffs. The tuff of Kershaw Canyon is distinguished from the 

underlying tuff of Rainbow Canyon by lower silica and AI, 

and higher Ba contents. The tuff of Etna and Ox Valley Tuff 

are characterized by relatively low Ba and Sr compared with 

other calc-alkaline units. 

Harmony Hill Tuff is the only andesite ash-flow tuff 

and probably was erupted from a source other than the Caliente caldera complex. It has high Ti and Sr that distinguish it 

from other units. The Hiko Tuff is distinguished from the 

Condor Canyon Formation by its lower total alkalies, Y, and 
GEOCHEMISTRY OF MIOCENE TUFFS, CALIENTE CALDERA COMPLEX 109 

Original composition of source magmas of ash-flow 

tuffs in the Caliente area was probably related to a combination of magmatic processes. Melting of intermediate-composition crustal material is likely to have been the ultimate origin 

of the older (23-17.8 Ma) calc-alkaline magmas. Different 

degrees of melting, different proportions of crustal materials, 

and crystal fractionation probably contributed to variations in 

the composition of these melts. Younger (16-12 Ma) highsilica magmas appear to have originated by variable amounts 

of crystal fractionation, probably combined with different 

amounts of crustal assimilation. The ultimate source of the 

younger magmas may have also been the crust. 

Anomalous elemental concentrations in many of the 

younger high silica rhyolites are due to postemplacement 

weathering, vapor-phase crystallization, devitrification, and 

removal and deposition of ions by ground water. These 

processes are manifested in low K20 and Na20, high Si02 

and CaO contents, and anomalous trace-element abundances. Probably most of the secondary depletions and 

enrichments occurred in the glass component of tuffaceous 

materials. 

Although our data are presently insufficient to confirm 

it, the wide variations in the composition of some units may 

be due in part to the tapping of compositionally zoned 

magma chambers, and to differences in the relative proportions of juvenile magma and xenolithic material. Our wholerock compositions are likely to be intermediate in composition between the extremes of a dominant volume of preemptive dacitic magma and a less voluminous rhyolitic 

component (Smith, 1979). Magmatic compositions are better 

represented by individual pumices, which we have not yet 

analyzed. 

REFERENCES CITED 

Baedecker, P.A., and McKown, D.M., 1987, Instrumental neutron 

activation analysis of geochemical samples, Chapter H, in Baedecker, P.A., ed., Methods for geochemical analysis: U.S. Geological Survey Bulletin 1770, p. H1-H14. 

Best, M.G., Christiansen, E.H., and Blank, R.H., Jr., 1989, Oligocene caldera complex and calc-alkaline tuffs and lavas of the 

Indian Peak volcanic field, Nevada and Utah: Geological Society of America Bulletin, v. 101, p. 1076-1090. 

Best, M.G., Christiansen, E.H., Deino, A.L., Gromme, C.S., 

McKee, E.H., and Noble, D.C., 1989, Excursion 3A-Eocene 

through Miocene volcanism in the Great Basin of the western 

United States: New Mexico Bureau of Mines and Mineral 

Resources Memoir 47, p. 91-133. 

Cameron, K.L., and Cameron, Maryellen, 1986, Geochemistry of 

quartz-normative igneous rocks from the Chinati Mountains 

and Teringua areas, West Texas, a comparison with Cenozoic 

volcanic rocks from Chihuahua and Baja California Sur, Mexico: University of Texas Guidebook, Bureau of Economic 

Geology, v. 23, p. 143-163. 

Cameron, K.L., and Hanson, G.N., 1982, Rare earth element evidence concerning the origin of voluminous mid-Tertiary rhyolitic ignimbrites and related rocks, Sierre Madre Occidental, 

Chihuahua, Mexico: Geochimica et Cosmochimica Acta, v. 46, 

p. 1489-1503. 

Davies, G.R., and Macdonald, Ray, 1987, Crustal influences in the 

petrogenesis of the Naivasha basalt-comendite complex-Combined trace element and Sr-Nd-Pb isotope constraints: Journal of Petrology, v. 28, p. 1009-1031. 

Ekren, E.B., Orkild, P.P., Sargent, K.A., and Dixon, G.L., 1977, 

Geologic map of Tertiary rocks, Lincoln County, Nevada: U.S. 

Geological Survey Miscellaneous Investigations Series Map 

I-1041, scale 1:250,000. 

Elston, W.E., and Bornhorst, T.J., 1979, The Rio Grande rift in context of regional post-40 m.y. volcanism and tectonic events, in 

Riecker, R.E., ed., Rio Grande Rift-Tectonics and magmatism: American Geophysical Union, p. 416-438. 

Johnson, C.M., and Lipman, P.W., 1988, Origin of metaluminous 

and alkaline volcanic rocks of the Latir volcanic field, northern 

Rio Grande rift, New Mexico: Contributions to Mineralogy 

and Petrology, v. 100, p. 107-128. 

Johnson, R.G., and King, B.-S., 1987, Energy-dispersive X-ray 

fluorescence spectrometry, Chapter Fin Baedecker, P.A., ed., 

Methods for geochemical analysis: U.S. Geological Survey 

Bulletin 1770, p. F1-F5. 

Lichte, F.E., Golightly, D.W., and Lamothe, P.J., 1987, Inductively 

coupled plasma atomic emission spectroscopy, Chapter B in 

Baedecker, P.A., ed., Methods for geochemical analysis: U.S. 

Geological Survey Bulletin 1770, p. B 1-B 10. 

Macdonald, Ray, Davies, G.R., Bliss, C.M., Leat, P.T., Bailey, 

D.K., and Smith, R.L., 1987, Geochemistry of high-silica peralkaline rhyolites, Naivasha, Kenya Rift Valley: Journal of 

Petrology, v. 28, p. 979-1008. 

Mammerickx, Jacqueline, and Klitgord, K.D., 1982, Northern East 

Pacific Rise-Evolution from 25 m.y. to the present: Journal of 

Geophysical Research, v. 87, p. 6751-6759. 

Mills, J.G., Jr.. 1991, The Timber Mountain Tuff, southwestern 

Nevada volcanic field; Geochemistry, mineralogy and petrogenesis: East Lansing, Mich., Michigan State University Ph. D. 

dissertation, 332 p. 

Miyashiro, Akiho, 197 4, Volcanic rock series in island arcs and 

active continental margins: American Journal of Science, v. 

274, p. 321-355. 

Noble, D.C., and McKee, E.H., 1972, Description and K-Ar ages of 

volcanic units of the Caliente volcanic field, Lincoln County, 

Nevada, and Washington County, Utah: Isochron/West, no. 5, 

p. 17-24. 

Noble, D.C., McKee, E.H., Hedge, C.E., and Blank, H.R., 1968, 

Reconnaissance of the Caliente depression, Lincoln County, 

Nevada [abs.]: Geological Society of America Special Paper 

115, p. 435-436. 

Novak, S.W., 1984, Eruptive history of the rhyolitic Kane Wash 

volcanic center, Nevada: Journal of Geophysical Research, v. 

89,p. 8603-8615. 

Novak, S.W., and Mahood, G.A., 1986, Rise and fall of a basalttrachyte-rhyolite magma system at the Kane Springs Wash 

caldera, Nevada: Contributions to Mineralogy and Petrology, 

V. 94, p. 352-373. 
110 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

Pearce, J.A., Harris, N.B.W., and Tindle, A.G., 1984, Trace element discrimination diagrams for the tectonic interpretation of 

granitic rocks: Journal of Petrology, v. 25, p. 956-983. 

Peccerillo, A., and Taylor, S.R., 1976, Geochemistry of Eocene 

calc-alkaline volcanic rocks from the Kastamonu area, Northem Turkey: Contributions to Mineralogy and Petrology, v. 58, 

p. 63-81. 

Rowley, P.D., Anderson, R.E., Snee, L.W., and Mehnert, H.H., 

1990, Geology and structural setting of the western Caliente 

caldera complex, Lincoln County, Nevada: Geological Society 

of America Abstracts with Programs, v. 22, no. 3, p. 79. 

Rowley, P.D., and Siders, M.A., 1988, Miocene calderas of the Caliente caldera complex, Nevada-Utah: Eos, v. 69, no. 44, p. 

1508. 

Rowley, P.D., Snee, L.W., Mehnert, H.H., Anderson, R.E., Axen, 

G.J., Burke, K.J., Simonds, F.W., Shroba, R.R., and Olmore, 

S.D., 1992, Structural setting of the Chief Mining District, 

eastern Chief Range, Lincoln County, Nevada, Chapter H in 

Thorman, C.W., ed., Application of structural geology to mineral and energy resources of the central region: U.S. Geological 

Survey Bulletin 2012, p. H1-H17. 

Siders, M.A., 1991, Geologic map of the Mount Escalante quadrangle, Iron County, Utah: Utah Geological and Mineral Survey 

Map 131, 9 p., scale 1:24,000. 

Smith, R.L., 1979, Ash flow magmatism, in Chapin, C.E., and 

Elston, W.E., eds., Ash flow tuffs: Geological Society of 

America Special Paper 180, p. 5-27. 

Taggart, J.E., Lindsay, J.R., Scott, B.A., Vivit, D.V., Bartel, 

A.J., and Stewart, K.C., 1987, Analysis of geologic materials 

by wavelength-dispersive X-ray fluorescence spectrometry, 

Chapter E in Baedecker, P.A., ed., Methods for geochemical 

analysis: U.S. Geological Survey Bulletin 1770, p. El-El9. 

Thompson, R.N., Morrison, M.A., Henry, G.L., and Parry, S.J., 

1984, An assessment of the relative roles of crust and mantle in 

magma genesis-An elemental approach: Philosophical 

Transactions of the Royal Society of London, v. A31 0, p. 

549-590. 

Vogel, T.A .. Noble, D.C., and Younker, L.W., 1989, Evolution of 

chemically zoned magma body; Black Mountain volcanic center, southwestern Nevada: Journal of Geophysical Research, v. 

94, p. 6041-6058. 

Ward, P.L., 1991, On plate tectonics and the geologic evolution of 

southwestern North America: Journal of Geophysical 

Research, v. 96, p. 12479-12496. 

Wark, D.A., Kempter, K.A., and McDowell, F.W., 1990, Evolution 

of waning, subduction-related magmatism, northern Sierra 

Madre Occidental: Geological Society of America Bulletin, v. 

102, p. 1555-1564. 

Williams, P.L., 1967, Stratigraphy and petrography of the 

Quichapa Group, southwestern Utah and southeastern Nevada: Seattle, Wash., University of Washington Ph. D. dissertation, 182 p. 
Strontium and Neodymium Isotopic Survey of 

Ash-Flow Tuffs and Related Rocks from the 

Caliente Caldera Complex, Southeastern 

Nevada and Southwestern Utah 

By Daniel M. Unruh, L. David Nealey, Peter D. Rowley, Lawrence W. Snee, 

Harald H. Mehnert, and R. Ernest Anderson 

GEOLOGIC STUDIES IN THE BASIN AND RANGE-COLORADO PLATEAU TRANSITION IN 

SOUTHEASTERN NEVADA, SOUTHWESTERN UTAH, AND NORTHWESTERN ARIZONA, 1992 

U.S. GEOLOGICAL SURVEY BULLETIN 2056-D 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1995 
CONTENTS 

Abstract ........................................................................................................................... 113 

Introduction .......... ....... .... . . . . .. ........... ................ .......... .............. ............ .......................... 113 

Description of Lithologic Units ...................................................................................... 115 

Cobalt Canyon Stock .............................................................................................. 116 

Intrusive Rocks of Clover Creek Caldera ............................................................... 116 

Lava Flows of Indian Cove ............ ...................................... .............. .......... .......... 116 

Porphyry of Meadow Valley Wash .......... ............ .............. .............. .......... ............ 117 

Lava Flows of Buckboard Spring ........................................................................... 117 

Volcanic Dome of Crows Nest Tank...................................................................... 117 

Analytical Procedures..................................................................................................... 117 

Major-element Chemistry............................................................................................... 117 

Analytical Results ........................................................................................................... 118 

Strontium and Neodymium Concentrations .......... ............................ .......... ........... 118 

Strontium Isotopic Variations ................................................................................. 120 

Neodymium Isotopic Variations ............................................................................. 121 

Discussion ....................................................................................................................... 123 

Evolution of the Silicic Magmas ............................................................................ 124 

Evolution of the Parental Magmas ......................................................................... 126 

Summary and Conclusions ............................................................................................. 128 

References Cited............................................................................................................. 128 

FIGURES 

1. Location map and general geologic and geographic features of Caliente caldera complex and vicinity .. .. .. .. ...... .. .. . 114 

2. Partial stratigraphic column of ash-flow tuffs and related rocks in and near Caliente caldera complex .................... 115 

3. Diagram of potassium vs. silica and total alkalies vs. silica for Caliente caldera complex samples analyze 

in this study.................................................................................................................................................................. 118 

4. Sr and Nd vs. silica variation diagrams for samples from Caliente caldera complex ................................................. 120 

5. Diagram of initial 87Sr/86Sr for Caliente caldera complex samples shown as functions of silica content, age, 

and Sr content.............................................................................................................................................................. 121 

6. Diagram of initial ENd values for Caliente caldera complex samples as functions of silica content, Nd content, 

and age ......................................................................................................................................................................... 122 

7. 143Ndi144Nd vs. 87Srf86sr diagram for data from Caliente caldera complex and other selected areas in the 

western United States .................................................................................................................................................. 123 

8. Diagram of assimilation/fractional crystallization models for Delamar caldera-related samples based on Nd 

concentration and ENd ................................................................................................................................................. 125 

9. 143Nd/144Nd vs. 87Sr/86Sr diagram for Caliente caldera complex samples ................................................................ 127 

TABLES 

1. Major-element chemistry for selected samples from Caliente caldera complex ........................................................ 116 

2. Rb/Sr and Sm/Nd data for selected samples from Caliente caldera complex and Kane Springs Wash caldera........ 119 
Strontium and Neodymium Isotopic Survey of Ash-Flow Tuffs 

and Related Rocks from the Caliente Caldera Complex, 

Southeastern Nevada and Southwestern Utah 

By Daniel M. Unruh, L. David Nealey, Peter D. Rowley, Lawrence W. Snee, 

Harald H. Mehnert, and R. Ernest Anderson 

ABSTRACT 

Intermediate to silicic intrusive rocks, lava flows, and 

tuffs of the Caliente caldera complex exhibit isotopic features common among Cenozoic caldera complexes in the 

western United States. Correlated chemical and isotopic 

trends are observed among the samples as a group, but are 

more easily recognized among samples related to a single 

caldera-forming event. As silica contents increase, strontium 

and neodymium contents decrease, 87 Sr/86Sr increases, and 

143Nd/144Nd decreases. These trends suggest that the silicic 

magmas were generated by fractionation of mafic (andesitic 

or perhaps basaltic) parental magmas open to modification 

by crustal contamination or assimilation. However, crustal 

interaction appears to be subordinate to fractional crystallization in the generation of the silicic magmas. 

Similar correlated variations exist among samples of 

Hiko Tuff (Delamar caldera) and a related volcanic dome, 

which suggests that the subvolcanic magma chamber was 

chemically and isotopically zoned as a result of open system 

modification of the magma by crustal interaction. Calculations based on neodymium contents and isotopic ratios suggest that high-silica rhyolite (75 percent Si02) could be 

generated from low-silica rhyolite (70 percent Si02) by fractional crystallization and assimilation of 5-15 percent wall 

rock. The low-silica rhyolite can be related to an andesitic 

parent magma (represented by the lava flows of Buckboard 

Spring) by fractional crystallization and assimilation of 

10- 30 percent crustal material, depending upon the particular model chosen. The calculations also suggest that crustal 

interaction became less important as magmas evolved to 

more silicic compositions. 

Initial 143Nd/144Nd values for the parental magmas of 

the Caliente complex increased from ENdz -10 at 21-23 Ma, 

to - 8 at 17-19 Ma, to -8 to -5 at 14-15 Ma. Similar 

temporally related increases in ENd have been observed in 

the Timber Mountain-Oasis Valley caldera complex in 

southern Nevada and the central caldera complex of the San 

Juan volcanic field in Colorado. The apparent temporal 

evolution of the parental magmas may reflect progressive 

intrusion of mantle-derived magmas into the lower crust, 

resulting in a progressive increase in ENd at site of meltgeneration near the crust/mantle boundary. Alternatively, the 

increase in ENd may reflect variations within mantle-derived 

magmas caused by progressive erosion of lithospheric 

mantle by upwelling asthenospheric magma. 

INTRODUCTION 

The Cenozoic Caliente caldera complex, one of the 

largest caldera complexes in the western United States, is 

centered in southeastern Nevada southeast of the town of 

Caliente, Nev. (fig. 1). The complex is intermediate in size 

and age between the larger Indian Peak caldera complex 

(27- 32 Ma; Best, Christiansen, and Blank, 1989) to the north 

and the Kane Springs Wash caldera (13-16 Ma; Novak, 

1984) to the southwest. The southward progression of Cenozoic nonbasaltic volcanism observed in this area is common 

throughout the Great Basin. (See, for example, Farmer and 

others, 1 989.) 

The Caliente complex includes at least four recently 

named clustered calderas (Rowley and Siders, 1988) that 

range in age from about 23 to 15 Ma. The oldest recognized 

caldera, Clover Creek, formed at about 22.8 Ma (Best, Christiansen, and others, 1989) with the eruption of the Bauers 

Tuff Member of the Condor Canyon Formation in the northem part of the complex (fig. 2). The Delamar caldera in the 

western part of the complex formed during eruption of Hiko 

Tuff at 18.6-18.2 Ma (Taylor and others, 1989; L.W. Snee, 

unpub. data, 1993). At 16 Mathe Pine Creek caldera formed 

in the eastern part of the complex (Siders, I 99 I), and at 15.6 

Ma (L.W. Snee, unpub. data, 1991) the Buckboard Canyon 

caldera formed in the western part of the complex, producing 

the tuff of Rainbow Canyon. 

113 
114 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

0 20 40 KILOMETERS 

EXPLANATION 

D Quaternary and Tertiary sediments 

Tertiary igneous rocks 

, ' 

D Tertiary and pre-Tertiary sedimentary rocks 

0 Caldera boundary, dotted where concealed 

--Contact 

Figure 1. Location map and general geologic and geographic features for western portion of Caliente caldera complex 

and vicinity, Nevada and Utah (from Rowley and others, this volume). 

The Kane Springs Wash caldera formed at 14.7-14.4 

Ma (L.W. Snee, unpub. data, 1991), producing the Grapevine Spring and Gregerson Basin Members of the Kane 

Wash Tuff(Novak, 1984; Scott and others, 1993). In the central part of the Caliente complex, these two members of the 

Kane Wash Tuff form thin, densely welded, peralkaline beds 

that are interlayered within upper parts of the tuff of Ker-. 

shaw Canyon. The tuff of Kershaw Canyon is overlain by the 

tuff ofEtna, which was emplaced between 14.4 Ma and 13.8 

Ma, based on its stratigraphic position between the underlying upper unit of the Gregerson Basin Member and an ashflow tuff bed in overlying sedimentary rocks (H.H. Mehnert, 

unpub. data, 1990). The tuffs of Kershaw Canyon and Etna 

are thought to have originated from the Caliente complex 

(Rowley and others, 1991; Rowley and others, this volume). 

The locations of their sources have not as yet been identified 

with certainty, although recent mapping by P.D. Rowley 

suggests that the tuff of Etna originated from just south of the 

Delamar caldera. The last major ash-flow eruption in the 

Caliente area produced the Ox Valley Tuff at between 12.6 

and 11.4 Ma (H.H. Mehnert, unpub. data, 1990). 

Volcanic rocks of the Caliente complex are predominantly rhyolite ash-flow tuffs. However, temporally associated, intermediate to silicic lava flows, dikes, and plutonic 

rocks are also present. Postcaldera volcanic rocks are predominantly basalt and basaltic andesite, as is common 

among Cenozoic caldera complexes in the western United 

States. (See, for example, Best, Christiansen, and others, 

1989.) For a more detailed discussion of the geologic setting 

of the Caliente complex, the reader is referred to companion 

reports in this volume by Nealey and others and Rowley and 

others, as well as to Rowley and others (1991), Rowley and 
STRONTIUM AND NEODYMIUM, CALIENTE CALDERA COMPLEX 115 

AGE UNIT SOURCE 

Ox Valley Tuff (12.6?-11.4? Ma) Caliente 

tuff of Etna (14.0 Ma) Caliente 

Kane Gregerson Basin upper unit (14.4 Ma) 7 tuff of Kane 

Member (l) 

Wash lower unit (14.6 Ma) Kershaw Springs ....... 

c 

Tuff Canyon Wash .~ 

Grapevine Spring Member (14.7 Ma) / (15.3?-14.2 Ma) "@ 

0 

tuff of Sawmill Canyon / Caliente 

tuff of Rainbow Canyon (15.6-15.2 Ma) Buckboard 

Canyon 

Delamar Lake Tuff (15.8-15.5 Ma) ? Miocene 

tuff of Tepee Rocks (17.8 Ma) Caliente 

lava flows of Buckboard Spring (18.0 Ma) Delamar r-7 

volcanic dome of Crows Nest Tank (18.2 Ma) II Delamar 

Hiko Tuff (18.6-18.2 Ma) ~porphyry of Delamar 

Racer Canyon Tuff 

* Meadow Valley 

--;; Wash (19.2?-18. 7 Ma) (19.4-< 18.2 Ma) Caliente 

lava flows of Indian Cove (21.9 Ma) Clover Creek 

0... Harmony Hills Tuff (22.5-22.0 Ma) ? :::J 

0 

\,... c C) 0 Bauers Tuff Intrusive rocks of Clover \,... c ·~ Clover Creek 

C\S 0 0 C\S Member (22.8 Ma) Creek caldera (22.8 Ma) 0... u >- E C\S c c \,... .c 0 C\S 0 

g OOLL Swett Tuff Member (23.7? Ma) Caliente :::J 

a Leach Canyon Formation (24.0 Ma) ? 

Oligocene Cobalt Canyon stock (24.9 Ma) --

*dikes 

Figure 2. Partial stratigraphic column of ash-flow tuffs and related rocks in and near Caliente caldera complex. Rock units analyzed in 

this study are shown in bold type. Those units whose source is simply listed as "Caliente" were derived from within Caliente caldera complex, but their relationship to specific calderas within the complex is presently unknown. Sources with unknown locations are indicated by 

queries. A dash indicates a local source. 

others ( 1992), Rowley and Shroba ( 1991 ), and Swadley and 

Rowley (1992). 

In this report we present results from a preliminary Nd 

and Sr isotopic survey of the Caliente caldera complex. The 

objectives of this study are to determine the isotopic signatures of the various units and the relationships of these signatures to compositional, spatial, and temporal variations 

within the complex. This study represents one aspect of an 

integrated mapping, geochronologic, and geochemical 

investigation of the Caliente caldera complex done under the 

auspices of the USGS Basin and Range-Colorado Plateau 

Transition (BARCO) Study Unit (see the introduction to this 

volume). 

DESCRIPTION OF LITHOLOGIC 

UNITS 

Whole-rock tuff samples from three of the four named 

calderas in the Caliente complex (the Pine Creek caldera was 

not included) as well as samples from other major tuffs of the 
116 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

complex were selected for analyses. These units are 

described elsewhere in this volume by Rowley and others 

and by Nealey and others. In addition, our study included 

samples from three intrusive units- the Cobalt Canyon 

stock, intracaldera intrusive rocks of the Clover Creek 

caldera, and the porphyry of Meadow Valley Wash; two 

andesitic flows- lava flows of Indian Cove and the lava 

flows of Buckboard Spring; and a rhyolite dome- the 

volcanic dome of Crows Nest Tank. A brief description of 

these units follows, because these units were not treated in 

detail by Nealey and others and by Rowley and others. 

Major-element chemistry (recalculated to I 00 percent) of the 

samples analyzed is given in table 1. 

COBALT CANYON STOCK 

The Cobalt Canyon stock consists of discordant dikes 

and plugs exposed on the southeast side of the Chief Range, 

Nev. (fig. 1) that are thought to be related to a large stock at 

depth (Rowley and others, 1991 ). The rocks are primarily 

porphyritic quartz monzonite, although compositions as 

mafic as monzogabbro are found locally. Phenocrysts compose 35- 80 percent of the rock and consist of plagioclase ( 65 

percent of the total phenocrysts), biotite (0- 10 percent), clinopyroxene ( 1- 25 percent), orthopyroxene ( 1- 10 percent), 

Fe-Ti oxides (5 percent), and occasional trace amounts of 

hornblende. The groundmass is holocrystalline and consists 

of sanidine, quartz, plagioclase, and minor clinopyroxene, 

biotite, and Fe-Ti oxides. The unit has been dated at 24.9 Ma 

(Rowley and others, 1992). 

INTRUSIVE ROCKS OF CLOVER CREEK 

CALDERA 

A dikelike mass, interpreted to be the upper part of a 

shallow intracaldera intrusive body of the Clover Creek 

caldera, is exposed on the east side of the canyon of Meadow 

Valley Wash about 1 km north of Caliente. The rock is a rhyolite porphyry that resembles the Bauers Tuff Member of the 

Condor Canyon Formation, except that phenocrysts are 

larger and generally unbroken. Phenocrysts make up 15- 35 

percent of the rock and consist of sanidine (3-4:0 percent), 

plagioclase (50- 90 percent), biotite (5 percent), and Fe-Ti 

oxides ( 1 percent). A preliminary 40 Arf39 Ar age of 22.8 Ma 

(L.W. Snee, unpub. data, 1991) has been obtained from this 

unit. 

LAVA FLOWS OF INDIAN COVE 

Andesitic and dacitic lava flows are found m the 

vicinity of Indian Cove, near the northern margin of the 

Caliente caldera complex, and about 4 km north of the town 

of Caliente (Rowley and Shroba, 1991). These flows are 

probably the remnants of volcanic domes and may represent 

the last stages of volcanic activity related to the Clover Creek 

caldera. A sample of one of the flows has been dated by the 

40Arf39Ar method at 21.9 Ma (Snee and others, 1990; 

Rowley and others, 1992). 

Lava flows of Indian Cove show a wide compositional 

range. They contain 5-45 percent phenocrysts consisting of 

plagioclase ( 40-80 percent), clinopyroxene (5-60 percent), 

Table 1. Major-element chemistry for selected samples from Caliente caldera complex. 

[In weight percent, recalculated to 100 percent on a volatile-free basis. Analyses performed by XRF at the U.S. Geological Survey, Denver, CO 80225] 

SAMPLE1 89-854 89-530 87-1194 

Si02 68.24 73.37 59.51 

AI203 13.79 14.33 15.58 

Fe203 1.97 1.43 2.51 

FeO 3.10 0.13 5.82 

MgO 1.70 0.34 3.08 

CaO 2.97 0.96 5.36 

Na20 3.12 3.19 3.09 

K20 4.43 6.15 3.58 

Ti02 0.49 0.03 1.01 

P205 0.15 0.05 0.39 

MnO 0.05 0.02 0.08 

189-854, Cobalt Canyon Stock 

89-530, intrusive rocks of Clover Creek caldera. 

87-1194, lava flows of Indian Cove. 

88-382 

66.87 

13.33 

1.99 

1.33 

1.31 

6.82 

2.76 

4.97 

0.43 

0.15 

0.04 

88-382, 88-4868, 88-1271, porphyry of Meadow Valley Wash. 

87-786, volcanic dome of Crows Nest Tank. 

87-997, lava flows of Buckboard Spring. 

88-4868 88-1271 87-786 

70.38 67.05 74.65 

14.22 15.68 13.42 

1.94 2.23 1.38 

1.75 2.54 0.41 

1.28 1.92 0.48 

2.35 2.49 1.34 

3.32 3.37 3.23 

4.12 4.23 4.76 

0.42 0.7 0.22 

0.15 0.32 0.07 

0.06 0.07 0.04 

87-997 

55.86 

15.48 

2.48 

6.62 

5.29 

6.68 

3.33 

2.67 

0.99 

0.49 

0.11 
STRONTIUM AND NEODYMIUM, CALIENTE CALDERA COMPLEX 117 

orthopyroxene (0-40 percent), biotite (0-1 0 percent), sanidine (0-7 percent), hornblende (0-5 percent), and Fe-Ti 

oxides (2-1 0 percent) in a glassy groundmass. Some flows 

contain black basal vitrophyres. 

PORPHYRY OF MEADOW VALLEY WASH 

Several exposures of coarse-grained, hypabyssal plugs 

and dikes occur along the traces of major fault zones in and 

near the Caliente caldera complex, from north of Delamar 

southwest of the Caliente complex to Panaca Summit about 

40 km northeast of the town of Caliente. The rocks are characterized by large (> 1.5 em) abundant phenocrysts of feldspar. These scattered exposures have been correlated by 

Rowley and Shroba (1991) and Rowley and others (1991) 

and informally named the porphyry of Meadow Valley Wash 

for exposures in the canyon of the same name. 

The rocks range in composition from dacite to rhyolite. 

Phenocrysts constitute 35-50 percent of the rock and consist 

of plagioclase (60-90 percent), quartz (0-10 percent), sanidine (0-1 0 percent), biotite (7 percent), hornblende ( 1-10 

percent), clinopyroxene (1-10 percent), Fe-Ti oxides (3 percent), and local trace amounts of orthopyroxene in a glassy 

groundmass. 

Analyses of sanidine and hornblende from a sample 

collected from within the Clover Creek caldera gave an average 40 Arf39 Ar age of 19.4 Ma (Snee and others, 1990; Rowley and others, 1992). However, dikes also intrude Hiko Tuff 

in some areas, and therefore some of these dikes must be 

younger than 18.6-18.2 Ma. A tentative age of 18.5 Ma has 

been assigned to those samples that postdate Hiko Tuff. The 

porphyry of Meadow Valley Wash is a key unit because its 

emplacement coincides with the major episode of extension 

in the Caliente area (Rowley and others, 1992). 

LAVA FLOWS OF BUCKBOARD SPRING 

A distinctive fine-grained basaltic andesite lava flow is 

found near Buckboard Spring in Buckboard Canyon, about 

11 km west of the town of Caliente. The rock contains a disequilibrium assemblage of 10-15 percent megacrysts of 

quartz (5-25 percent), sanidine (5-10 percent), plagioclase 

(40 percent), hornblende (2 percent), orthopyroxene (10-15 

percent), Fe-Ti oxides (1-3 percent), and olivine (20 percent). Large megacrysts of quartz, sanidine, plagioclase, and 

hornblende are altered or have reaction rims that indicate 

that they were not in equilibrium with the melt which crystallized to form the rest of the rock. 

The flow rests on intracaldera Hiko Tuff, so it probably 

marks the cessation of volcanism and consolidation of the 

magma chamber of the Delamar caldera. One sample from 

the flow has been dated by the 40 Arf39 Ar method at 18.0 Ma 

(L.W. Snee, unpub. data, 1991). 

VOLCANIC DOME OF CROWS NEST TANK 

A rhyolite dome, herein called the volcanic dome of 

Crows Nest Tank for exposures at the north edge of the 

Delamar caldera, occurs 9 km west of Caliente. Phenocrysts 

make up about 35 percent of the rock and consist of quartz 

(25 percent), sanidine (30 percent), plagioclase ( 40 percent), 

biotite (5 percent), hornblende (1 percent), and Fe-Ti oxides 

(1 percent). 

The rock is lithologically similar to Hiko Tuff, although 

slightly more silicic, and may have tapped the same magma 

chamber from which Hiko Tuff erupted. The dome has 

received two preliminary concordant 40 Arf39 Ar ages that 

average 18.2 Ma (L.W. Snee, unpub. data, 1991). 

ANALYTICAL PROCEDURES 

Concentrations of Rb, Sr, Sm, and Nd and isotopic 

compositions of Sr and Nd were determined by isotope dilution-mass spectrometry using the techniques described by 

Stille and others (1986). Samples analyzed were powdered 

splits of the same samples used for major- and trace-element 

analyses (table 1, and Nealey and others, this volume; Rowley and others, this volume). For a detailed discussion of 

sample selection and preparation procedures, the reader is 

referred to the companion report by Nealey and others in this 

volume. Isotopic measurements of Sr and Nd were obtained 

on a VG 54R single-collector mass spectrometer. 87Srf86sr 

values were normalized to 88Sr/86Sr=0.1194, and 143Nd/ 

144Nd values were normalized to 146Ndi 144Nd=0.7219. 

Replicate analyses of the NBS Sr standard SRM-987 gave a 

mean 87srt86Sr=0.71 0273±0.000012 (95 percent C.l.), and 

replicate analyses of the La Jolla Nd standard produced a 

mean 143Nd/144Nd=0.511858±0.000008. 

MAJOR-ELEMENT CHEMISTRY 

Samples chosen for this study represent a suite of intrusive, extrusive, and pyroclastic units that range in composition from andesite to high-silica rhyolite. Caliente complex 

samples analyzed in this study are all metaluminous to 

weakly peraluminous. The sample from the upper unit of the 

Gregerson Basin Member of the Kane Wash Tuff is mildly 

peralkaline as are samples from other units of the Grapevine 

Spring and Gregerson Basin Members (units V2 and V3 of 

Novak, 1985; and see Harding and others, this volume; Rowley and others, this volume). This sample also shows the distinctive trace-element characteristics exhibited by other 

samples of Kane Wash Tuff (low Sr, Ba; high Zr; Novak and 

Mahood, 1986; Nealey and others, this volume). 

Major-element data for Caliente complex samples analyzed in this study are given in table I (recalculated to I 00 
118 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

percent) and are shown on potassium vs. silica and total alkalies vs. silica variation diagrams in figure 3. (For a more 

complete data set, see Nealey and others, and Rowley and 

others; this volume.) In the notation of Peccerillo and Taylor 

.... 

z 

w 

0 

a: 

w 

a. 

.... 

:X: 

CJ 

w 

3: 

3E 

.... 

z 

w 

0 

a: 

w 

a. 

.... 

:X: 

CJ 

w 

3: 

3E 

0 

N 

cu 

z 

+ 

0 

N 

~ 

7.-~-,.--~-r-.~-,.--~-r-,~~.~~---.~--

6 ~ 

5 

4 

3 -

2 -

BB 

• 

IC 

• 

. 

~~0--~5~·5~-6~0~~6~5~-7~0---7~5~-8~0~~85 

A Si02, IN WEIGHT PERCENT 

10 B 

H t~A KW,G 

8 MV~ CNe =TR 

IC 

BB • Q CC KC 

6 • 

4 RC 

2 

0 

so 70 75 80 85 

B Si02, IN WEIGHT PERCENT 

Figure 3. Variation diagrams for A, potassium and B, total alkalies vs. silica for Caliente samples analyzed in this study. For a more 

complete data set see Nealey and others and Rowley and others, this 

volume. CC, Cobalt Canyon stock; B, Bauers Tuff Member, Condor Canyon Formation (includes the intrusive rocks of the Clover 

Creek caldera); IC, lava flows of Indian Cove; MV, porphyry of 

Meadow Valley Wash; H, Hiko Tuff; CN, volcanic dome of Crows 

Nest Tank; RA, tuff of Rainbow Canyon; RC, Racer Canyon Tuff; 

BB, lava flows of Buckboard Spring; TR, tuff of Tepee Rocks; KC, 

tuff of Kershaw Canyon; G, Gregerson Basin Member of Kane 

Wash Tuff; KW, Kane Wash Tuff (from Novak, 1985). Open triangle, sample related to the Kane Springs Wash caldera. Solid circle, 

solid triangle, samples related to the Delamar caldera and Clover 

Creek caldera (Caliente caldera complex), respectively. Remaining 

samples from the Caliente caldera complex are shown by open 

squares. Samples from the same unit are connected by tie lines. 

( 1976), samples range from high-K andesite to high-K rhyolite with K20 contents that range from about 3 percent to 

more than 5 percent as Si02 ranges from 56 to 76 percent. 

The two mafic flows analyzed, Indian Cove and Buckboard 

Spring, are both andesites with 56 percent and 59 percent silica, respectively. The sample of the Cobalt Canyon stock has 

a quartz monzonite composition, and those of the porphyry 

of Meadow Valley Wash have dacitic compositions. The 

intrusiv·e rocks of Clover Creek caldera, the volcanic dome 

of Crows Nest Tank, and all tuffs analyzed in this study are 

rhyolites . 

Some samples of the tuff of Rainbow Canyon and the 

Racer Canyon Tuff show anomalously high silica and low 

alkali contents. Nealey and others (this volume) attributed 

the high recalculated silica and low alkali contents to 

postemplacement alteration of matrix glass. Samples from 

the tuff of Rainbow Canyon are commonly altered. The 

effects of alteration of samples from these two tuffs are also 

observed in the Sr contents and Sr isotopic data discussed in 

the next section. 

ANALYTICAL RESULTS 

Concentrations ofRb, Sr, Sm, and Nd and 87Sr/86Sr and 

143Nd/144Nd for selected Caliente complex samples are 

shown in table 2. Both measured and age-corrected (initial) 

Sr and Nd isotopic values are given in table 2, but only the 

age-corrected values are shown in the subsequent illustrations. However, the age corrections to the Nd data seldom 

exceed the analytical uncertainties. Initial Nd isotopic data 

are presented as both the actual ratios and using the conventional ENd notation (DePaolo and Wasserburg, 1976). 

STRONTIUM AND NEODYMIUM 

CONCENTRATIONS 

Concentrations of Sr and Nd are summarized as a function of silica content in figure 4 (data from table 2 and from 

Nealey and others, this volume). Strontium concentrations, 

in general, exhibit a strong negative correlation with silica 

content, and vary by more than an order of magnitude (820 

ppm in the lava flows of Buckboard Spring to 64 ppm in the 

tuff of Tepee Rocks, fig. 4A) as silica varies from about 56 

to 76 percent. This type of correlation is common among differentiation series and probably reflects the fractionation of 

feldspar (plagioclase or sanidine) into which Sr is strongly 

partitioned (Dsr=4.5-7.3 and 7-33 for sanidine and plagioclase, respectively, in silicic rocks; for example, Nash and 

Crecraft, 1985). 

Exceptions to the overall trend of the Sr data are found 

among samples from the tuff of Rainbow Canyon and the 

Racer Canyon Tuff. These samples show anomalously high 

Sr contents and are all apparently altered, as shown by the 
Table 2. Rb/Sr and Sm/Nd data for selected samples from Caliente caldera complex and Kane Springs Wash caldera. 

Sample No. Rock unit 

KANE SPRINGS WASH CALDERA: 

87-580A Gregerson Basin 

Mbr. 

Do do 

CALIENTE CALDERA COMPLEX: 

Agel 

in Ma 

14.4 

89-1097F TuffofKershaw (14.2) 

Do 

88-1264 

Canyon 

do 

Tuff of Rainbow 

Canyon, sanidine 

15.6 

88-226A Tuff of Rainbow 15.6 

88-1268 

88-1271 

Do 

88-486B 

88-382 

87-697 A 

87-697 p 

Do 

87-697U 

89-854 

Canyon 

Tuff of Tepee Rocks 17.8 

porphyry of (18.5) 

Meadow Valley W. 

do 

porphyry of 

Meadow Valley W. 

porphyry of 

Meadow Valley W. 

(18.5) 

19.4 

Racer Canyon Tuff 18.8 

Racer Canyon Tuff 18.8 

do 

Racer Canyon Tuff 1 8.8 

Cobalt Canyon 24.9 

Stock 

Delamar caldera: 

88-997 lava flows of 

87-786 

Do 

88-1185 

Buckboard Spring 

Volcanic dome of 

Crows Nest Tank 

do 

Hiko Tuff 

(intracaldera) 

Do 

88-666H 

do 

Hiko Tuff 

(outflow) 

Clover Creek caldera: 

18.0 

18.2 

{18.6) 

18.6-

18.2 

87-1194 lndianCove 21.9 

89-530 Intrusive rocks of 22.8 

87-452H 

Clover Creek 

caldera 

Bauers Tuff Mbr. 

(outflow) 

22.8 

Si02 

77.5 

72.0 

77.5 

80.5 

76.2 

67.1 

70.4 

66.9 

76.4 

73.1 

78.3 

68.2 

55.9 

74.7 

71.1 

70.0 

59.5 

73.4 

73.3 

Rb 

ppm 

228.6 

232.5 

169.3 

166.7 

52.5 

138.5 

223.9 

137.3 

137.3 

164.6 

206.5 

129.8 

129.9 

131.4 

56.6 

205.0 

54.0 

203.9 

201.9 

132.6 

139.4 

124.1 

123.1 

218.5 

181.1 

Sr 

ppm 

15.44 

Sm Nd 

ppm ppm 

17.15 95.96 

1 5.50 17.29 97.30 

273.00 12.23 59.34 

286.60 

64.50 

514.20 2.40 14.19 

63.75 1.58 12.04 

582.40 9.40 48.56 

587.70 

352.30 

346.80 

117.10 

443.50 

440.40 

827.60 

294.70 

819.80 

219.00 

215.20 

413.70 

415.00 

6.93 36.47 

6.64 34.00 

3.86 20.52 

5.06 29.67 

5.18 30.35 

3.59 22.58 

6.73 38.08 

8.09 46.09 

3.66 23.00 

5.24 35.0 

494.30 6.56 40.03 

739.40 8.21 45.74 

209.80 6.42 42.81 

290.20 6.18 41.10 

87Rbt86sr 87srt86sr 87srt86sq 2 147smt144Nd 143Ndf144Nd 143Nd/144Nd12 £Nd1 

42.86 ± 0.50 0.717654 ± 21 0.709133 ± 102 0.1079 ± 1 0.512350 ± 14 0.512340 ± 14 -5.4 

43.42 ± 0.50 0.717687 ± 44 0.709054 ± 109 0.1073 ± 1 0.512364 ± 14 0.512354 ± 14 -5.2 

1. 794 ± 0.021 0. 708626 ± 20 0. 708264 ± 21 

1.682 ± 0.028 0. 708588 ± 20 0. 708249 ± 21 

2.354 ± 0.040 0.708027 ± 21 0.707505 ± 23 

0.1244 ± 1 0.512219 ± 15 0.512207 ± 15 -8.0 

0.512207 ± 12 

0.780 ± 0.009 0.711752 ± 30 0.711579 ± 30 0.1020 ± 1 0.512163 ± 11 0.512153 ± 11 -9.0 

10.160 ± 0.120 0.710036 ± 19 0.707648 ± 36 

0.682 ± 0.016 0.707218 ± 20 0.707039 ± 20 

0.686 ± 0.008 0.707301 ± 27 0.707121 ± 27 

1.351 ± 0.049 0.707209 ± 20 0.706854 ± 24 

1.722 ± 0.059 0.707207 ± 21 0.706735 ± 27 

3.208 ± 0.035 0.708750 ± 20 0.707893 ± 22 

0.847 ± 0.009 0.707741 ± 34 0.707515 ± 34 

0.862 ± 0.009 0.707735 ± 20 0.707505 ± 20 

0.198 ± 0.002 0. 708947 ± 29 0. 708894 ± 29 

2.013 ± 0.037 0.710128 ± 21 0.709419 ± 25 

0.0796 ± 1 

0.1168 ± 5 

0.1147 ± 3 

0.1179 ± 1 

0.1137 ± 1 

0.1030 ± 1 

0.1031 ± 1 

0.0960 ± 1 

0.1067 ± 1 

0.512090 ± 15 0.512081 ± 15 -10.4 

0.512183 ± 14 0.512169 ± 14 -8.6 

0.512106 ± 15 0.512092 ± 15 -10.2 

0.512055 ± 14 0.512040 ± 14 -11.1 

0.512156 ± 14 0.512142 ± 14 -9.2 

0.512095 ± 14 0.512082 ± 14 -10.3 

0.512080 ± 12 0.512067 ± 12 -10.6 

0.512104 ± 14 0.512092 ± 14 -10.1 

0.512001 ± 18 0.511984 ± 18 -12.1 

0.190 ± 0.003 0.706494 ± 20 0.706445 ± 20 0.1060 ± 1 0.512220 ± 18 0.512208 ± 18 -7.9 

2.693 ± 0.031 0.708276 ± 14 0.707580 ± 16 0.0962 ± 22 0.512093 ± 13 0.512082 ± 13 -10.4 

2.714 ± 0.103 0.708274 ± 21 0.707573 ± 34 

0.927 ± 0.034 0.707435 ± 21 0.707190 ± 23 0.0904 ± 5 0.512138 ± 12 0.512127 ± 12 -9.5 

0.972 ± 0.011 0.707479 ± 20 0.707222 ± 20 

0.727 ± 0.019 0.707420 ± 17 0.707228 ± 18 0.0990 ± 1 0.512150 ± 11 0.512138 ± 11 -9.3 

0.481 ± O.D15 0.706597 ± 24 0.706447 ± 25 0.1084 ± 1 0.512122 ± 15 0.512106 ± 15 -9.8 

3.012 ± 0.107 0.707700 ± 21 0.706725 ± 41 0.0906 ± 1 0.512089 ± 15 0.512075 ± 15 -10.4 

1.805 ± 0.040 0.707226 ± 21 0.706642 ± 25 0.0908 ± 1 0.512107 ± 15 0.512093 ± 15 -10.0 

1 Ages in parentheses are assumed. Age data from Snee and Mehnert {unpub. data, 1 990, 1991 ), Snee and others { 1990), Best, Christiansen, and Blank ( 1989), Taylor and others {1989), and Rowley and others 

{1992). 

2calculated initial ratios. Uncertainties reflect those of the parent-daughter ratios, but not the age uncertainties. 

(/) 

...., 

~ 

0 

z ...., 

2 

a= 

> z 

tJ 

z 

ti1 

0 

u 

--< 

a= 

2 

~ 

n 

> ['""" 

m 

z 

...., 

ti1 

n 

> ['""" 

tJ 

m 

~ 

> 

n 

0 

a= 

rs 

m 

><: 

-\0 
120 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

51000 

:J 

:::! 800 :a: 

a: 

w 600 c.. 

en 

1- 400 a: 

<t 

c.. 200 

z 

~ ... 0 

r.Fl 50 

A 

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Q 100 -I 

-I 

~ 80 

a: 

w 

c.. 

en 

1-

a::: 

<t 

c.. 

z 

60 

60 70 80 

Si02, IN WEIGHT PERCENT 

o~~~~-L~~~~~~-L-L~~~ 

50 60 70 80 

B Si02, IN WEIGHT PERCENT 

Figure 4. Variation diagrams for A, Sr and B, Nd vs. silica content for samples from Caliente caldera complex. Data from this 

study and from Nealey and others, this volume. Silica contents recalculated to I 00 percent on a volatile-free basis. CC, Cobalt Canyon stock; B, Bauers Tuff Member, Condor Canyon Formation 

(includes the intrusive rocks of the Clover Creek caldera); IC, lava 

flows of Indian Cove; MY, porphyry of Meadow Valley Wash; H, 

Hiko Tuff (includes volcanic dome of Crows Nest Tank); RA, tuff 

of Rainbow Canyon; RC, Racer Canyon Tuff; BB, lava flows of 

Buckboard Spring; TR, tuff of Tepee Rocks; KC, tuff of Kershaw 

Canyon; KW, Kane Wash Tuff (from this work and Novak, 1985). 

low alkali and high recalculated silica contents (fig. 3, and 

Nealey and others, this volume). The altered Rainbow Canyon sample (88- 226A) shows an apparent initial 87Sr/86sr of 

0.7116 (table 2), a value much higher than found in any other 

Caliente complex sample thus far analyzed. A sanidine separate from another sample (88- 1264 ), collected from near 

88- 226A, shows a much lower initial 87Srf86sr of 0.7075. 

Extremely high and anomalous Sr contents and anomalous 

87Sr/86Sr were also observed in altered (zeolitized) samples 

of the Paintbrush Tuff in southern Nevada by Peterman and 

others (1991 ). Consequently, we suggest that the lower ratio 

from sample 88-1264 is more representative of unaltered 

tuff of Rainbow Canyon, and this value will be used in the 

ensuing discussion. The altered sample of the Racer Canyon 

Tuff (87- 679U) also exhibits a somewhat elevated initial 

87Srf86Sr and will be deleted from the following discussion. 

Neodymium concentrations for most of the samples 

also exhibit a negative correlation with silica contents (fig. 

4B). However, the overall relative variation in Nd concentrations (50- 12 ppm) is much less than for Sr, and Nd abundances show only a weak negative covariance with silica 

contents of about 56-70 percent. 

The trend exhibited by the Nd data is also consistent 

with trends predicted by Nd mineral/liquid distribution coefficients and reflects the strong dependency of DNd for mafic 

mineral phases on bulk composition. DNd values for clinopyroxene and orthopyroxene range from less than 0.2 and 0.5, 

respectively, in basaltic systems (Grutzeck and others, 1974; 

Weil and McKay, 1975) to values of 0.6- 1.3 and 2.3-5.8, 

respectively, in silicic systems (for example, Henderson, 

1984; Nash and Crecraft, 1985). Nd is also strongly partitioned into biotite (DNd=0.9- 5.8), hornblende (DNd=3-4; 

Henderson, 1984; Nash and Crecraft, 1985), and sphene 

(DNd=88; Simmons and Hedge, 1978). However, Nd is not 

strongly partitioned into feldspar at any bulk composition 

(DNd<0.2; Nash and Crecraft, 1985). Based upon the phenocryst assemblages and the aforementioned distribution 

coefficients, we estimate bulk DNd values of about 1.5 to 2.2 

for most Caliente tuffs. 

The altered samples from the tuff of Rainbow Canyon 

and the Racer Canyon Tuff do not show anomalous Nd concentrations. In their study of the Paintbrush Tuff, Peterman 

and others ( 1991) found that REE were not significantly 

affected by alteration, and that the Nd isotopic data were 

similarly unaffected (Z.E. Peterman, oral commun., 1992). 

We therefore will use the Nd data for the altered Rainbow 

Canyon sample in the ensuing discussion, but we recognize 

that these data need further evaluation and confirmation. 

Kane Wash Tuff (table 2; Novak, 1985; Nealey and others, this volume) exhibits considerably lower Sr contents 

(5-15 ppm) and higher Nd contents (95-110 ppm) than tuffs 

from the Caliente complex. These differences may reflect primary differences in parental magmas and (or) differences in 

the crystallizing mineral phases among the parental magmas. 

STRONTIUM ISOTOPIC VARIATIONS 

Initial 87Sr/86Sr values for Caliente samples from table 

2 are shown as functions of silica content, age, and Sr content 

in figure 5. Shown for comparison are data from two analyses of Kane Wash Tuff by Novak (1985). The 87Sr/86Sr initial ratios show a rather restricted range of 0.7064 to 0.7094. 

The Cobalt Canyon stock has the highest initial ratio, 

whereas the two mafic flows (Buckboard Spring and Indian 

Cove) have the lowest initial ratios (fig. SA). These latter two 

samples also have the highest Sr contents. 

With the exception of the Cobalt Canyon stock, the 

data define fairly well correlated trends on all three 
STRONTIUM AND NEODYMIUM, CALIENTE CALDERA COMPLEX 121 

[/1 ""' 

\0 

00 

-

~ ""' r--

00 

[/1 ""' \0 

00 

-s... 

[/1 

r---

00 

0. 71 3 

0. 711 ·wj 

cc 

D G 

0.709 A 

KC 

D RC 

H o-<"Bo 

0.707 ~ CN T R R A 

• .... ~ MV B BB IC 

0. 705 

so 55 60 65 70 75 

A Si02, IN WEIGHT PERCENT 

0. 71 3 

0. 71 1 I KW 

GA 0.709 CN 

KC D T R I RC 

D ~l -H 0.707 RA 

BB • MV 

0.705 

1 2 1 4 1 6 1 B 20 

B AGE, IN Ma 

0. 71 3 

KW 

0. 711 

0.709o,G 

cc 

0 

RC 0 KC 

o- H 

oD • -o / 

TR C N r. e _:!-.a 

R A A---A e----- - B MV 

0.707 

.... . 

B 

IC 

22 24 

BB ... . 

IC 

cc 

D 

80 

26 

0.705~~--~--~--------~------~·----~ 

0 200 400 600 BOO 1000 

C Sr, IN PARTS PER MILLION 

diagrams. Initial 87Sr/86Sr increases with increasing silica 

and decreasing age and Sr contents. The trends are 

somewhat fan-shaped in fig. SA and SC and show increasing 

variability in initial 87 Sr/86Sr with increasing silica and 

decreasing Sr contents. 

The apparent correlation between initial 87Sr/86sr and 

age could, in part, reflect the fact that no rocks with 

intermediate compositions are included among samples 

Figure 5. Initial 87Srf86Sr for samples from Caliente caldera complex shown as functions of A, silica content; B, age; and C, Sr content. CC, Cobalt Canyon stock; B, Bauers Tuff Member, Condor 

Canyon Formation (includes the intrusive rocks of the Clover Creek 

caldera); IC, lava flows of Indian Cove; MY, porphyry of Meadow 

Valley Wash; H, Hiko Tuff; CN, volcanic dome of Crows Nest 

Tank; RA, tuff of Rainbow Canyon; RC, Racer Canyon Tuff; BB, 

lava flows of Buckboard Spring; TR, tuff of Tepee Rocks; KC, tuff 

of Kershaw Canyon; G, Gregerson Basin Member of Kane Wash 

Tuff; KW, Kane Wash Tuff (from Novak, 1985). Open triangle, 

sample related to the Kane Springs Wash caldera. Solid circle, solid 

triangle, samples related to the Delamar caldera and Clover Creek 

caldera (Caliente caldera complex), respectively. Remaining samples from the Caliente caldera complex are shown by open squares. 

Samples from the same unit are connected by tie lines. 

dated at less than about 18 Ma. However, when compared at 

approximately constant silica (fig. SA), and constant Sr 

contents (fig. SC), initial 87Sr/86Sr increases from 0.7067 in 

the Bauers Tuff Member (22.8 Ma), to 0.707S in Hiko Tuff 

(18.6-18.2 Ma), to 0.7083 in the tuff of Kershaw Canyon 

(14.2 Ma). 

Radioactive decay of 87Rb in the source region of these 

magmas could account for apparent relationship between 

age and initial 87Sr/86Sr. However, an 87Rb/86Sr of about 

12-13 is required to account for the observed variations. 

This ratio is much higher than is observed among any of the 

units (87Rb/86Sr=0.7-3.0; table 2) except the Gregerson 

Basin Member of the Kane Wash Tuff and tuff of Tepee 

Rocks. Consequently, such an explanation seems quite 

unlikely. 

The "fan-shaped" trends between initial 87 Sr/86Sr and 

both silica and Sr contents (fig. SA, C) suggest that the individual units could be related to a common, or at least similar, 

source represented by the data for the two mafic flows. 

Given such a model, the open end of the "fans" (high Si02 

and low Sr contents) would reflect variations in fractional 

crystallization and crustal-interaction processes that have 

modified the parental magmas and produced the individual 

units. 

The Cobalt Canyon stock predates known calderarelated magmatism in the area, and the stock is not exposed 

within the caldera complex. Furthermore, the Sr data for the 

Cobalt Canyon stock do not plot on the trends defined by the 

other data in figure S. Consequently, we suggest that this 

stock is not directly related to any of the other units under 

present consideration (Rowley and others, 1992). 

NEODYMIUM ISOTOPIC VARIATIONS 

Initial 143Nd/144Nd for most samples shows a restricted 

range of O.S120 to O.S122 (ENd I= -7.9 to -11.1; table 2; fig. 

6). The single exception, the sample from the Cobalt Canyon 
122 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

-= 

~ 

-= z 

w 

- 4 

- 6 -

- 8 -

-, 0 

-, 2 

-, 4 

50 

A 

- 6 

- 8 -

-, 0 

- 1 2 -

-, 4 

0 

B 

- 4 

- 6 

- 8 -

-, 0 

-, 2 ~ 

-, 4 , 2 

c 

. 

88 

• 

I 

55 

• IC 

. 

60 

. 

65 70 

. 

G 

6 

KW~ 

75 

Si02, IN WEIGHT PERCENT 

88 

e C KC 

RA 

0 RC ~NV ~ 4 .1C 

C e ~ B 

TR CN 

c 

cc 

I I 

20 40 60 80 

6 

G 

100 

Nd, IN PARTS PER MILLION 

. . I 

KW A 

G 

88 

0 

KC • 

0 IC 

RA 

~c • ~ B 

TR 

N 

NV 

0 

cc 

I 

14 1 6 , 8 20 22 24 

AGE,IN Ma 

80 

-

-

120 

-

-

-

26 

stock, shows the lowest initial 143Nd/144Nd=0.51198 (ENdJ= 

-12.1). The sample of the Gregerson Basin Member exhibits 

a higher initial 143Nd/ 144Nd of 0.51235 (ENdJ= -5.3) which 

is similar to those previously reported for Kane Wash Tuff 

by Novak (1985). Neodymium concentrations range from 

10-15 ppm in the tuffs of Rainbow Canyon and Tepee 

Rocks, to 59 ppm in the tuff of Kershaw Canyon. 

Figure 6. Initial £Nd values for Caliente samples plotted as functions of A, silica content; B, Nd content; and C, age. CC, Cobalt 

Canyon stock; B, Bauers Tuff Member, Condor Canyon Formation 

(includes the intrusive rocks of the Clover Creek caldera); IC, lava 

flows of Indian Cove; MV, porphyry of Meadow Valley Wash; H, 

Hiko Tuff; CN, volcanic dome of Crows Nest Tank; RA, tuff of 

Rainbow Canyon; RC, Racer Canyon Tuff; BB, lava flows of 

Buckboard Spring; TR, tuff of Tepee Rocks; KC, tuff of Kershaw 

Canyon; G, Gregerson Basin Member of Kane Wash Tuff; KW, 

Kane Wash Tuff (from Novak, 1985). Open triangle, sample related 

to the Kane Springs Wash caldera. Solid circle, solid triangle, samples related to the Delamar caldera and Clover Creek caldera (Caliente caldera complex), respectively. Remaining samples from the 

Caliente caldera complex are shown by open squares. Samples 

from the same unit are connected by tie lines. 

Initial Nd ratios (expressed as ENd in fig. 6) do not 

show a correlation with silica content (fig. 6A). Virtually the 

entire range of Nd ratios is found among the rhyolite tuffs, 

and the two andesites have intermediate initial Nd values 

that differ from one another by about 2 ENd units (I ENd 

unit=O.Ol percent). However, among those units related to 

the Delamar caldera (Hiko Tuff, volcanic dome of Crows 

Nest Tank, lava flows of Buckboard Spring), the data exhibit 

a trend of decreasing ENd with increasing silica. A similar 

trend may exist for the units related to the Clover Creek 

caldera (lava flows of Indian Cove, Bauers Tuff Member, 

and intrusive rocks of the Clover Creek caldera,) but additional analyses will be required because this apparent trend 

consists of essentially only two points. The Cobalt Canyon 

stock shows an anomalously low ENd, which reinforces the 

suggestion made in the previous section, that this stock is not 

related to the other units. 

A similar lack of correlation exists among ENd and Nd 

concentrations (fig. 6B). Although a weak positively correlated trend exists among the Delamar caldera-related samples, the majority of the samples are clustered between ENd= 

-8 to -11 and 15-50 ppm Nd, with no correlation within this 

group as a whole. Samples related to the Clover Creek 

caldera tend to have slightly higher Nd concentrations at 

similar ENd than samples related to the Delamar caldera; and 

the two youngest tuffs (Rainbow Canyon and Kershaw Canyon) have the highest ENd values among Caliente caldera 

tuffs, although their Nd concentrations differ greatly. 

In contrast to figure 6A and B, the initial Nd ratios yield 

a good correlation with age (fig. 6C), although there is considerable overlap among three general recognizable age 

groups. Although the data base is limited, average ENd values for the tuffs appear to increase from -10 in Bauers Tuff 

Member (22.8 Ma) to -9.4 in Hiko Tuff (18.6 Ma) to -8.0 in 

the tuff of Kershaw Canyon (14.2 Ma). A more pronounced 

apparent trend is observed between the two mafic flows. The 

ENd value of the lava flows of Buckboard Spring (18.0 Ma) 

is 2 units higher than that of the lava flows of Indian Cove, 

although both units have similar silica and Nd contents. 
STRONTIUM AND NEODYMIUM, CALIENTE CALDERA COMPLEX 123 

Unfortunately, no younger (14-16 Ma) mafic flows were 

included in the present study and the confirmation of the 

apparent correlation between eNd and age must await further 

sampling. Similar positive correlations between eNd and age 

have been observed in the San Juan volcanic field in Colorado (Riciputi and Johnson, 1990) and the Timber Mountain- Oasis Valley caldera complex in southwestern Nevada 

(Farmer and others, 1991). 

In summary, Nd isotopic variations appear to be related 

primarily to age differences, with a secondary, more subtle 

compositional dependence within at least the Delamar 

caldera samples. This general relationship is in marked contrast to that indicated by the Sr isotopic data presented previously. The Sr data show pronounced correlations between 

87 Sr/86sr and silica and Sr contents, as well as age. 

DISCUSSION 

Initial 87Srf86sr and 143Nd/144Nd values for Caliente 

caldera complex samples are summarized in figure 7. Shown 

for comparison are data from the Kane Springs Wash caldera 

0.5130 

0.5128 

~ 0.5126 z 

~ 

~ 

'!"""'( 

--~ 0.5124 

z 

~ 

~ 

'!"""'( 0.5122 

0.5120 

0.5118 

0.702 0.704 0.706 

(Novak, 1985) and the Timber Mountain-Oasis Valley 

caldera complex (TMOV; Tegtmeyer and Farmer, 1990; 

Farmer and others, 1991). Also shown are basalt data from 

Kane Springs Wash (Novak, 1985), and from various locations in the northern Colorado Plateaus and Basin and Range 

provinces (Menzies and others, 1983; Farmer and others, 

1989; Kempton and others, 1991). 

Data for the Caliente caldera complex as a whole exhibit 

a somewhat smaller range in 87Sr/86Sr than do those for the 

TMOV, and only about one-half the range shown by Kane 

Springs Wash data. In contrast, N d isotopic data show a range 

(0.03 percent=3 eNd units) similar to those observed among 

intermediate and silicic rocks of the Kane Springs Wash 

caldera and the TMOV. In the latter two caldera complexes, 

the highest 143Nd/144Nd values are found among trachytes 

and the lowest 143Nd/144Nd are found among rhyolites. However, in the Caliente complex the entire range of 143Nd/144Nd 

values is found among rhyolite tuffs (table 2). 

The most striking features of the isotopic data in figure 

7 are the nearly horizontal trends exhibited by the data for 

intermediate to silicic rocks from caldera complexes, in contrast to the steep negatively correlated trends exhibited by 

Rwr ~ 

TMOV 

CALIENTE 

0.708 0.710 0.712 

87Srf86Sr 

Figure 7. I43Ndfl44Nd vs. 87Srf86Sr diagram for data from Caliente caldera complex and other selected areas in the western United States. 

BR, Basin and Range basalts; NC, Nye Canyon, Nev., basalts; SPB, Sierran Province basalts, southern Nevada, and southeastern California; 

SWUB, southwestern Utah basalts; SNB, southwestern Nevada basalts; KSB, basalts from Kane Springs Wash caldera; KSTR, trachytes from 

Kane Springs Wash caldera; KWT, Kane Wash Tuff; TMOVB, basalts from Timber Mountain-Oasis Valley caldera complex; TMOV, trachytes and rhyolites from Timber Mountain- Oasis Valley caldera complex. Fields compiled from data taken from Menzies and others (1983; 

SPB); Novak (1985; KSB, KSTR, KWT); Farmer and others (1989; NC, SNB); Farmer and others (1991 ; TMOVB, TMOV); and Kempton 

and others (1991; BR, SWUB). Estimated "bulk-earth" values are shown by the horizontal and vertical lines at 143Nd/ 144Nd=0.512636, and 

87Sr/86Sr=0.7045, respectively. 
124 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

much of the basalt data. The different trends are related to the 

different sources and processes involved in the generation of 

the two general types of volcanism. Basalts are thought to 

derive their isotopic characteristics from two principal 

sources: "depleted" asthenospheric mantle (as exemplified 

by the "Basin and Range" field in fig. 7), and remnant Proterozoic(?) lithospheric mantle (for example, Menzies and 

others, 1983; Perry and others, 1987; Farmer and others, 

1989; Kempton and others, 1991) that may range from 

slightly depleted to moderately enriched (isotopically). The 

isotopic characteristics of the basaltic magmas may be further modified by interaction with the crust, either near the 

crust-mantle boundary (Johnson and others, 1990; Riciputi 

and Johnson, 1990, among others) or during ascent of the 

magmas to the surface. 

Correlated isotopic and geochemical trends observed in 

a number of studies of silicic volcanic centers (see fig. 7 for 

example) have been interpreted to reflect open-system modification of crystallizing parental magmas by assimilation of 

crustal material (Noble and Hedge, 1969; Novak and 

Mahood, 1986; Leeman and Hawkesworth, 1986; Musselwhite and others, 1989; Johnson and others, 1990; Tegtmeyer and Farmer, 1990; Farmer and others, 1991 ). In most 

of these studies, assimilation of crustal material was interpreted to be subordinate to fractional crystallization (see also 

Halliday and others, 1984; Cameron and Cameron, 1985), 

particularly as the magma systems evolve to more silicic 

compositions (Novak and Mahood, 1986; Musselwhite and 

others, 1989, among others). Because Sr is more effectively 

partitioned into the fractionating phases than Nd, Sr contents 

decrease more rapidly than Nd (fig. 4). This decrease coupled with exceptionally high 87Sr/86Sr in crustal material 

causes 87 Sr/86Sr in the magma to be much more affected 

than 143Nd/144Nd by crustal assimilation. Consequently, a 

large range in 87Sr/86Sr may be observed where only limited 

variation in 143Nd/ 144Nd is present. 

Kane Wash Tuff has the lowest Sr contents ( < 10 ppm; 

Novak, 1985) among the tuffs represented in figure 7, and 

the Kane Springs Wash data (KWT, KSTR) exhibit the largest variation in Sr isotopic ratios. Novak and Mahood ( 1986) 

suggested that Kane Wash Tuff could have been derived by 

fractionation of parental trachytic magmas (KSTR field in 

fig. 7) and that the elevated 87Srf86sr in Kane Wash Tuff 

samples may represent as little as 1 percent crustal contamination/assimilation. They also suggested that the trachytic 

magmas may have been derived by fractionation of alkali 

basalt magma with 10-20 percent assimilation of crustal 

material. 

The samples with the highest 87 Sr/86Sr among the 

TMOV volcanics also exhibit the lowest Sr contents (25-28 

ppm Sr in sanidine; Farmer and others, 1991). Tegtmeyer 

and Farmer (1990) and Farmer and others (1991) suggested 

that the high-silica rhyolite of the Topopah Spring Member 

of the Paintbrush Tuff could have been derived by fractionation of parental trachytic magma accompanied by 10--40 

percent assimilation of crustal material, depending upon the 

type of model invoked. The smaller variation in 87 Sr/86Sr 

among Caliente complex silicic samples relative to those of 

Kane Springs Wash and the TMOV may reflect the more 

restricted range of Sr contents among Caliente samples. 

EVOLUTION OF THE SILICIC MAGMAS 

The 87Sr/86Sr and 143Nd/ 144Nd of the silicic tuffs from 

the Caliente complex analyzed in this study are in the range 

of 0.7067-0.7082, and 0.5121-0.5122 (ENd= -:-8 to -10), 

respectively (figs. 5, 6). These 143Nd/144Nd values are considerably higher, and the 87Sr/86Sr are considerably lower 

than one would expect if these tuffs represented crustal anatexis of Proterozoic felsic basement (ENd= -15 to -20, 87Sr/ 

86Sr z 0.713 to >0.72; Bennett and DePaolo, 1987; Kistler 

and Lee, 1989; Farmer and others, 1991 ). However, the isotopic variations exhibited by the tuffs and their elevated 87Sr/ 

86sr relative to basalts with similar ENd values (fig. 7) suggest that some crustal interaction is involved. 

The trends exhibited by the Sr data in figure 5, and to a 

lesser extent by the Nd data in figure 6 (at least among samples related to the Delamar caldera), suggest that the magmas 

related to a single caldera-forming "event" may also be 

related to a single parental magma, or at least similar parental 

magmas. In following discussion we assume this to be the 

case, and we use the Nd data from samples related to the 

Delamar caldera to outline conditions under which the more 

silicic samples could have been derived from these parental 

magmas. Due to the limited data base and the lack of tight 

constraints on DNd values and the composition of the crustal 

component involved, the calculations presented herein are 

somewhat speculative and subject to revision. 

The sample of the lava flows of Buckboard Spring is the 

least silicic among this group and has the highest ENd (-7.9) 

and lowest 87 Sr/86Sr (0. 7064) among these samples. The Sr 

and Nd isotopic ratios of this sample are similar to those of 

isotopically "enriched" basalts from southern Nevada and 

southwest Utah (fig. 7). Although we recognize that this unit 

contains a disequilibrium assemblage of phenocrysts which 

may be isotopically heterogeneous, we suggest that this sample may still approximate the parental magma to the Delamar 

"system," and may represent either a fractionated mantlederived melt or a hybrid melt produced within the lower 

crust or at its base (Johnson and others, 1990). The crustal 

contaminant is assumed to contain 30--40 ppm Nd, with 

ENd= -15 to -18 (for example, Bennett and DePaolo, 1987; 

Farmer and others, 1991 ). 

Results of an assimilation/fractional crystallization 

model (DePaolo, 1981) are summarized in figure 8. In this 

model, fractional crystallization is presumed to occur 

concomitantly with assimilation of wall-rock material. This 

process may occur as magmas "stope" their way through the 

crust or as magmas differentiate within a chamber. In the 
STRONTIUM AND NEODYMIUM, CALIENTE CALDERA COMPLEX 125 

-7 

-8 

-9 

c 

-10 r = 0.1-0.3 

D = 1.4-2.0 

"C -11 z 

c = 0.1-0.2 

F = 0.1-0.6 

w 

-12 

-13 

-14 

-15 

0.00 0.01 0.02 0.03 

1/Nd 

Nest 

Tank 

r 

D 

c 

F 

Crust 

= 

= 

= 

= 

8 

0.1-0.5 

1.1-1.5 

0.1-0.15 

0.2-0.85 

£Nd =-15to-18 

0.04 

A 

r = 0.1-0.3 

D = 1.6-2.0 

c = 0.06-0.15 

F = 0.45-0.65 

0.05 0.06 

Figure 8. Assimilation/fractional crystallization models for Delamar caldera-related samples based on Nd concentration and ENd 

(DePaolo, 1981). The parental magma to the system is assumed to have ENd equal to that of BB, the Java flows of Buckboard Spring 

(ENd= -7 .9), and the crustal contaminant an ENd of - 15 to -18 and 30-40 ppm Nd (Bennett and DePaolo, 1987). Line segment A reflects 

the evolution from low-silica (Hiko Tuff) to high-silica (volcanic dome of Crows Nest Tank) rhyolite, and segments B and C reflect 

possible evolution paths from the lava flows of Buckboard Spring (with variable initial Nd concentrations) to Hiko low-silica rhyolite. 

Numerical values for each of the variables used in the calculations of the evolution paths are given in the respective boxes: r represents 

the assumed mass ratio of assimilation to fractional crystallization per unit time, 0 represents bulk Nd mineral/melt partition coefficients, 

F represents the mass fraction of original magma remaining at the end point of each trend, and C represents the mass ratio of total material 

assimilated to original magma (=Ma!Mmo of Farmer and DePaolo, 1983) at the end point. Silica contents for individual samples are 

shown in parentheses. 

latter case, a chemically and isotopically zoned magma 

chamber may be produced, and the chemical and isotopic 

zoning may be preserved within the erupted products 

(Tegtmeyer and Farmer, 1990). In the former case, only the 

end-product may be erupted to the surface. 

Limited chemical and isotopic zoning is observed 

within the three rhyolite samples (fig. 8), although we note 

that the two low-silica rhyolites are ofHiko Tuff whereas the 

high-silica rhyolite is from the volcanic dome of Crows Nest 

Tank and may be 0.4 m.y. younger than Hiko Tuff. The calculations suggest that the high-silica rhyolite could have been 

derived from the low-silica rhyolite by extensive fractional 

crystallization and about 5-15 percent assimilation of wal1-

rock material (line segment A in fig. 8), if reasonably low values for rare assumed (r equals the mass ratio of assimilation 

to fractional crystallization per unit time; DePaolo, 1981 ). 

Possible conditions under which the low-silica rhyolites could be related to the lava flows of Buckboard Spring 

are shown by line segments B and C in figure 8. Segment B 

suggests a scenario in which a magma initially equivalent to 

the lava flow of Buckboard Spring stapes its way to a level 

at which the subvolcanic magma chamber forms. At this 

level the magma has trace element and isotopic characteristics of the low-silica rhyolites. Segment C represents extrapolation of the Hiko trend to the eNd= - 7.9 line (parental 

magma), and represents an "initial" magma that is a fractionated, but uncontaminated derivative of the lava flows of 

Buckboard Spring with 110 ppm Nd and eNd= - 7.9. The 

actual evolution path (if it exists) probably lies between 

these two extremes. Although the amount of fractionation 

required is strongly dependent upon the assumed value for r, 

and is therefore not well constrained, the amount of assimilant appears to be restricted to about 10- 20 percent 

(C=O.l- 0.2) of the original magma mass, regardless of the 

path chosen. Thus, the high-silica rhyolite could be related to 

the lava flows of Buckboard Spring by rather extensive fractional crystallization accompanied by about 15- 30 percent 

assimilation of crustal material. 

Similar calculations have been performed for samples 

from the porphyry of Meadow Valley Wash (not shown in 

fig. 8). These results suggest that Meadow Valley Wash 

samples could have been derived from a magma with trace 
126 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

element and isotopic characteristics of the lava flows of 

Buckboard Spring with assimilation of20-30 percent crustal 

material. However, these calculations are not entirely justified, because the three samples of porphyry of Meadow Valley Wash are from different locations within the complex 

and are not related to a single parent magma, and because the 

variations in Nd data are not correlated with silica content 

(fig. 6). Interestingly, the sample with lowest ENd and low 

silica is also at least 1 m.y. older than the other two (19.4 Ma 

as opposed to post-18.6-18.2 Ma). Furthermore, this sample 

was collected from a location near the Clover Creek caldera; 

thus it may have been derived from a low-ENd parental 

magma related to Clover Creek caldera activity rather than to 

Delamar caldera activity. 

The data for Hiko Tuff and the volcanic dome of Crows 

Nest Tank have also been evaluated in terms of a bulk-assimilation model wherein compositional gradients develop in an 

isotopically homogeneous magma chamber prior to assimilation of crustal material (Johnson, 1989; see fig. 6 of Farmer 

and others, 1991). If this magma originally had the ENd of 

the lava flows of Buckboard Spring, then the Hiko and 

Crows Nest Tank data may reflect approximately 30 percent 

addition of crustal material to this magma at all levels. 

Although the calculations just presented are rather 

speculative and model dependent, one important conclusion 

can be obtained from them: fractional crystallization was 

much more important than crustal anatexis or contamination 

in the generation of even the high-silica rhyolites (Musselwhite and others, 1989; Tegtmeyer and Farmer, 1990; 

Farmer and others, 1991; Johnson, 1991). In fact, comparison of the calculations for line segments B and C in figure 8 

with those for segment A suggests that crustal interaction 

becomes less significant as magmas become more silicic 

(Novak and Mahood, 1986; Johnson, 1989; Musselwhite and 

others, 1989). 

EVOLUTION OF THE PARENTAL MAGMAS 

The Nd data presented in figure 6 suggest a good correlation between the initial 143Nd/144Nd values and the ages of 

the samples. A similar trend is observed among the Sr data 

in figure 5. However, apparent age-correlated variations in 

87Sr/86Sr, particularly among tuffs, may simply result from 

variations in Sr contents and crustal assimilation processes 

outlined previously. On the other hand, variations in initial 

143Nd/144Nd, particularly when compared at similar silica 

contents and initial 87Sr/86Sr, are more likely related to 

143Nd/144Nd variations inherited from the mantle (± lower 

crust) source regions. Consequently, the 2 ENd-unit variation 

observed among the lava flows of Indian Cove (ENd= -9.8) 

and Buckboard Spring (ENd= -7.9) should represent primary 

features of all magmas derived from these presumed parental 

magmas. 

This hypothesis is at least partially supported by the 

87sr;86sr and 143Nd/144Nd values for the two groups of Caliente complex samples representing the Clover Creek and 

Delamar calderas as shown in figure 9. The trend of the data 

for the Delamar samples appears to be displaced from that for 

the Clover Creek caldera toward higher ENd, although additional data will be required in order to confirm the existence 

of these two apparent trends. We note that data for the other 

two tuffs of approximate Delamar caldera age (Racer Canyon 

Tuff and the tuff of Tepee Rocks) also plot on or near the 

trend defined by the data for Delamar caldera samples. 

Kane Wash Tuff, on the other hand, was clearly derived 

from a source with ENd~ -5.3, the measured value within 

this tuff (table 2 and Novak and Mahood, 1986). The parental magma to the Kane Wash Tuff probably had 87sr;86sr 

and 143Nd/144Nd values similar to those of Kane Springs 

Wash trachytes or intracaldera basalts (ENd~ -5; Novak, 

1985; Novak and Mahood, 1986). 

The tuff of Kershaw Canyon may have been derived 

from a parental magma with isotopic characteristics similar 

to that parental to Kane Wash Tuff. The tuff of Kershaw 

Canyon has a much higher Sr/Nd value (4.6) than does Kane 

Wash Tuff (0.16). Consequently, its evolution path in figure 

9 would have been steeper than that for Kane Wash Tuff, and 

more similar to those of the other Caliente complex samples. 

Alternatively, the tuff of Kershaw Canyon could have been 

derived from a parental magma with isotopic characteristics 

intermediate between those of Kane Wash Tuff and the 

Delamar samples (ENd= -7 to -5). 

The suggestion presented in the previous section, that 

the sample of porphyry of Meadow Valley Wash with the 

lowest ENd may be related to Clover Creek rather than 

Delamar activity, is supported by the combined Sr and Nd 

data in figure 9. The Sr-Nd isotopic trend for the three 

Meadow Valley Wash samples cuts across both Delamar and 

Clover Creek trends, and we speculate that isotopic analyses 

of samples from this unit could be used to monitor the isotopic 

evolution in the parental magmas throughout the complex. 

The combined data suggest that ENd in the magmas 

parental to Caliente caldera tuffs increased from -9.8 at 

22-23 Ma to -7.9 at 18 Ma to -8 to -5 at about 14.2 Ma. In 

their study of the Timber Mountain-Oasis Valley caldera 

complex, Farmer and others ( 1991) observed an increase of 

about 1 ENd unit between the Rainier Mesa (ENd= -10.5) 

and Ammonia Tanks Members (ENd= -9.5) of the Timber 

Mountain Tuff, which have an age difference of only about 

0.2 m.y. (11.6-11.4 Ma). Riciputi and Johnson (1990) have 

suggested that ENd in the parental magmas in the San Juan 

volcanic field, Colorado, increased by about 2 ENd units during a 2 m.y. interval, from -8 at 28 Ma to -6 at 26 Ma. Therefore, the larger range in ENd values exhibited by the Caliente 

complex data probably reflects the prolonged period of nonbasaltic volcanism relative to these other areas, rather than 

unusual Nd isotopic evolution of the source region of the 

Caliente complex magmas. 
STRONTIUM AND NEODYMIUM, CALIENTE CALDERA COMPLEX 127 

""0 

z 

~ 

~ 

~ 

--~ 

z 

~ 

~ 

,......: 

0.5125 

0.5123 

0.5121 

G 

--~···:iii!:::~:ii;i;!i!iiD 

0.707 0.709 0.711 0.713 

87Srf86Sr 

Figure 9. 143Nd/144Nd vs. 87Sr/86Sr diagram for Caliente samples. Possible assimilation-fractional crystallization evolution paths (heavy 

dashed lines) for silicic tuffs from mafic parental magmas are shown for the Clover Creek caldera and Delamar caldera groups of samples. 

Data from the Kane Springs Wash caldera are also shown (Novak, 1985; KSB, KSIB, KSTR, KW=basalts, intracaldera basalts, trachytes, 

and Kane Wash Tuff, respectively). CC, Cobalt Canyon stock; B, Bauers Tuff Member, Condor Canyon Formation (includes the intrusive 

rocks of the Clover Creek caldera); IC, lava flows of Indian Cove; MV, porphyry of Meadow Valley Wash; H, Hiko Tuff (includes volcanic dome of Crows Nest Tank); RA, tuff of Rainbow Canyon; RC, Racer Canyon Tuff; BB, lava flows of Buckboard Spring; TR, tuff 

of Tepee Rocks; KC, tuff of Kershaw Canyon; G, Gregerson Basin Member of Kane Wash Tuff. Open triangle, sample related to the Kane 

Springs Wash caldera. Solid circle, solid triangle, samples related to the Delamar caldera and Clover Creek caldera (Caliente caldera complex), respectively. Remaining samples from the Caliente caldera complex are shown by open squares. 

The variations in ENd observed among Caliente complex parental magmas, as well as in the other areas just mentioned, require that at least two isotopically distinct sources 

exist at depth within each area. Although, in the present study, 

we cannot rule out the possibility that these two sources are 

being randomly sampled, the apparent time-dependence of 

these variations suggests that the higher-ENd source is, in 

effect, progressively replacing the lower-ENd source. The 

data are consistent with two general models as outlined in the 

following discussion (see Farmer and others, 1991 ). 

The variations in ENd may reflect progressive erosion 

of lithospheric mantle by upwelling asthenospheric mantle 

(Perry and others, 1987). If so, then the isotopic 

characteristics of the asthenospheric mantle in this area may 

be approximated by the basalt from the Kane Springs Wash 

caldera which exhibits the highest 143Ndf144Nd (ENd2 0) 

and lowest 87Srf86sr (~ 0.7043; Novak, 1985). The isotopic 

ratios of the lava flows of Indian Cove are similar to those of 

enriched (isotopically) basalts in southern Nevada and 

southwestern Utah (fig. 7). These isotopic characteristics 

(ENd~ -9.8, 87Sr/86Sr20.7064) may approximate those of 

Proterozoic lithospheric mantle in this area (Menzies and 

others, 1983; Farmer and others, 1989; Menzies, 1989; 

Kempton and others; 1991). 

Alternatively, the variations may reflect progressive 

intrusion of mantle-derived magmas into the crust (Johnson 

and others, 1990). In this model, magmas parental to the 

caldera-related magmas are derived from near the crustmantle boundary, and repeated injections of magma into the 

lower crust cause the lower crust to progressively adopt a 

more mantle-like isotopic signature. In the Caliente area, the 

lithospheric mantle might have isotopic characteristics similar to those of Kane Springs Wash intracaldera basalts (ENd2 

-5) or the more depleted (ENd20) extracaldera basalts. The 
128 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

isotopic characteristics of the lower crust are not well constrained. The ENd of the lower crust in this area might resemble that of Proterozoic felsic rocks ( -15 to -19; Bennett and 

DePaolo, 1987) or it may resemble that of the lava flows of 

Indian Cove (ENd::; - 10). We are presently unable to choose 

between these two alternatives, but Pb isotopic analyses currently in progress may help to clarify the role of the lower 

crust in the generation of these parental magmas (Johnson, 

1989; 1991; Johnson and others, 1990). 

SUMMARY AND CONCLUSIONS 

The Sr and Nd isotopic data for samples from the Caliente caldera complex provide significant constraints on the 

sources and processes involved in the generation of these 

magmas. Although contamination or assimilation of crustal 

material is required to account for the isotopic characteristics of the silicic tuffs, fractional crystallization of more 

mafic magmas appears to be the primary process responsible for the generation of the caldera-forming magmas (Musselwhite and others, 1989; Farmer and others, 1991; 

Johnson, 1991 ). 

Analyses of two samples of Hiko Tuff and one of a late 

caldera-related rhyolite dome suggest that the subvolcanic 

magma chamber that produced the Delamar caldera was 

chemically and isotopically zoned as a result of fractionation 

and assimilation of crustal material. Calculations, based on 

Nd data using several different models, suggest that the highsilica rhyolite of the volcanic dome of Crows Nest Tank 

could have been derived from low-silica (Hiko) rhyolite by 

fractional crystallization and addition of about 5-15 percent 

crustal material. The low-silica rhyolite could have been 

derived from andesitic or perhaps even basaltic parental magmas with the addition of about 10- 20 percent crustal material. Thus the entire evolution from mafic parental magma to 

high-silica rhyolite may have involved no more than a 20- 30 

percent contribution from the crustal wall rocks. 

Volcanic and intrusive rocks from the Clover Creek 

caldera (lava flows of Indian Cove, Bauers Tuff Member, 

intrusive rocks of the Clover Creek caldera) were derived 

from parental magmas with 87Sr/86Sr:s;0.7064 and ENd2:: -10. 

The parental magmas of volcanic rocks related to the 

Delamar caldera (lava flows of Buckboard Spring, Hiko 

Tuff, volcanic dome of Crows Nest Tank) appear to have had 

a similar 87Sr/86Sr to those related to the Clover Creek 

caldera, but a somewhat higher 143Nd/ 144Nd (ENd2:: -8). The 

isotopic character of the parental magmas to the post-16 Ma 

tuffs (tuffs of Rainbow Canyon and Kershaw Canyon) is not 

well constrained and may have been similar to that for the 

Delamar caldera (ENd2:: - 8) or as high as that for Kane Wash 

Tuff (ENd2:: -5). The apparent temporal evolution of the 

parental magmas may reflect repeated injection of mantlederived melts into the lower crust (Johnson and others, 

1990), or lithospheric thinning as a result of erosion by 

upwelling asthenosphere (Perry and others, 1987). The 

period of major extensional faulting in this area roughly 

corresponds to the formation of the Caliente caldera complex 

(for example, Rowley and others, 1992). Therefore, isotopic 

data for each of the individual calderas may also reflect local 

changes in the tectonic setting. 

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---1985, Geology and geochemical evolution of the Kane 

Springs Wash volcanic center, Lincoln County, Nevada: Stanford, Calif., Stanford University Ph. D. dissertation. 

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Caldera, Nevada: Contributions to Mineralogy and Petrology, 

V. 94, p. 352-373. 

Perry, F.V., Baldridge, W.S., and DePaolo, D.J., 1987, Role of 

asthenosphere and lithosphere in the genesis of late Cenozoic 

basaltic rocks from the Rio Grande Rift and adjacent regions of 

the southwestern United States: Journal of Geophysical 

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Peterman, Z.E., Spengler, R.W., Futa, K., Marshall, B.D., and 

Mahan, S.A., 1991, Assessing the natural performance of felsic 

tuffs using the Rb-Sr and Sm-Nd systems-A study of the 

altered zone in the Topopah Springs Member, Paintbrush Tuff, 

Yucca Mountain, Nevada: Materials Research Society 

Symposium Proceedings, v. 212, p. 687-694. 

Riciputi, L.R., and Johnson, C.M., 1990, Nd- and Ph-isotope variations in the multicyclic central caldera cluster of the San Juan 

volcanic field, Colorado, and implications for crustal hybridization: Geology, v. 18, p. 975-978. 

Rowley, P.O., and Shroba, R.R., 1991, Geologic map of the Indian 

Cove quadrangle, Lincoln County, Nevada: U.S. Geological 

Survey Geologic Quadrangle Map GQ-170 1, scale 1 :24,000. 

Rowley, P.D., Shroba, R.R., Simonds, F.W., Burke, K.J., Axen, 

G.J., and Olmore, S.D., 1991, Geologic map of the Chief 

Mountain quadrangle, Lincoln County, Nevada: U.S. Geological Survey Open-File Report 91-135, scale 1:24,000. 

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p. 1508. 

Rowley, P.D., Snee, L.W., Mehnert, H.H., Anderson, R.E., Axen, 

G.J., Burke, K.J., Simonds, F.W., Shroba, R.R., and Olmore, 

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Geological Survey Bulletin 2012, p. H1-H17. 

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of the Delamar Lake quadrangle, Lincoln County, Nevada: 

U.S. Geological Survey Geologic Quadrangle Map GQ-1730, 

scale I :24,000. 

Siders, M.A., 1991, Geologic map of the Mount Escalante quadrangle, Iron County, Utah: Utah Geological and Mineral Survey 

Map 131, 9 p., scale I :24,000. 

Simmons, E.C., and Hedge, C.E., 1978, Minor-element and Sr-isotope geochemistry of Tertiary stocks, Colorado mineral belt: 

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Snee, L.W., Mehnert, H.H., Rowley, P.D., Anderson, R.E., and 

Scott, R.B., 1990, New isotopic ages demonstrate extensional 

faulting of 19-12 Main the western Caliente caldera complex 

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p. 1612. 

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7757-7774. 

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upper crustal magma chambers: Evidence for in situ 

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Ce, Sm, Eu, and Yb in lunar igneous systems and a possible 

origin of KREEP by equilibrium partial melting: Proceedings 

of the 6th Lunar Science Conference, p. 1143-1158. 

Evidence of the Kane Springs Wash Caldera 

in the Meadow Valley Mountains, 

Southeastern Nevada 

By Anne E. Harding, Robert B. Scott, Harald H. Mehnert, and Lawrence W. Snee 

GEOLOGIC STUDIES IN THE BASIN AND RANGE--COLORADO PLATEAU TRANSITION IN 

SOUTHEASTERN NEVADA, SOUTHWESTERN UTAH, AND NORTHWESTERN ARIZONA, 1992 

U.S. GEOLOGICAL SURVEY BULLETIN 2056--E 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1995 
CONTENTS 

Abstract ........................................................................................................................... 135 

Introduction . . . . . ... ... . . . . . . .... . . .... . . . . .. . . . . ... . . . . . . .. .. . . ... . . . . .. .. . . . . . . . . . . . . .. ..... . . . . .... . . .... . . .. .. . .... . . . . . . .. . 135 

Regional Geology................................................................................................... 136 

Previous Investigations in the Study Area .............................................................. 137 

Acknowledgments .................................................................................................. 138 

Methods of Study............................................................................................................ 139 

Stratigraphy .................................................................................................................... 139 

Southern Stratigraphic Sequence ............................................................................ 139 

Hiko Tuff........................................................................................................ 142 

Delamar Lake Tuff ......................................................................................... 142 

Sunflower Mountain Tuff ............................................................................... 143 

Older Tuff ....................................................................................................... 143 

Precaldera Trachyte Flows . . . .... . . .. .. . . . . .. .. . .. . . . . . . . ... . . . . . . . . . .... . . .. .. . . . . .... .. . . . . . .... . . .. . 143 

Kane Wash Tuff .............................................................................................. 143 

Northern Stratigraphic Sequence ............................................................................ 144 

Lithic-rich Tuff ............................................................................................... 144 

Columnar-jointed Rhyolite ............................................................................. 144 

Breccia ............................................................................................................ 146 

Flow-lineated Rhyolite ................................................................................... 146 

Lower Basalt Flow.......................................................................................... 149 

Gray Ash-flow Tuff ........................................................................................ 149 

Yell ow Tuff.................................................................................................... 150 

Red Lithic Rhyolite ........................................................................................ 151 

Biotite-rhyolite Flow ...................................................................................... 152 

Pumice-rich Ash-flow Tuff ............................................................................ 152 

Flow-banded Rhyolite .................................................................................... 152 

Middle Basalt Flow ........................................................................................ 153 

Lithophysal Rhyolite Flow ............................................................................. 153 

Youngest Rhyolite Flows ............................................................................... 153 

Upper Basalt Flow .......................................................................................... 153 

Intrusive Rocks in the Northern Area ..................................................................... 153 

Rhyolite Dikes ................................................................................................ 153 

Volcanic Center.............................................................................................. 154 

Chemistry........................................................................................................................ 154 

Structure.......................................................................................................................... 156 

Discussion ....................................................................................................................... 158 

Caldera Wall ........................................................................................................... 158 

Correlation of the Northern Stratigraphic Sequence .............................................. 158 

Structural Implications .... . . .. . . . . . . ... . . ...... . . . . .. .. .. . . . . . .. .. . . . . .. . . . . ... ..... . . . . . .... . . ... . . .. .. . . .... . . . 160 

Conclusions . . . . . . . . ..................... .... . . . . ... . . .... . . . . . . . . .... .. . . . . . .. . . ...... . . .. ... .... . . . . . ... . . .... . . ... . . .. .. . . . . . 165 

References Cited............................................................................................................. 165 

Appendix 1. Thin Section Modal Analysis .. ..... .. .... .. .. ...... ..... ...... .... .. .. ...... .. .. .. ..... .. .... 168 

Appendix 2. Chemical Data: EDXRF Trace-element Analysis; 

WDXRF Major-element Analysis; INAA Analysis; ICP Analysis ............................ 171 

Appendix 3. Isotopic Dates ......................................................................................... 179 
CONTENTS 

FIGURES 

1. Index map showing study area, southeastern Nevada ................................................................................................. 136 

2. Location map of study area in Meadow Valley Mountains and vicinity ..................................................................... 137 

3. Generalized geologic map of the study area ................................................................................................................ 140 

4. Chart showing stratigraphic nomenclature changes for the Kane Wash Tuff ............................................................. 142 

5. Photograph showing southern stratigraphic sequence in the Meadow Valley Mountains .......................................... 142 

6. Photomicrograph of hedenbergite and fayalitic olivine microphenocrysts in columnar-jointed rhyolite ................... 145 

7. Photomicrograph of relict glass shard morphology in columnar-jointed rhyolite ....................................................... 146 

8. Photograph showing flow lineation of the flow-lineated rhyolite ............................................................................... 147 

9. Photograph showing layering a few meters above base offlow-lineated rhyolite ...................................................... 148 

10. Photograph showing layering in basal zone of flow-lineated rhyolite ........................................................................ 148 

11. Photomicrograph of eutaxitic glass shard texture in basal, layered zone of flow-lineated rhyolite ............................ 149 

12. Photograph of trachytic inclusions in upper zone of flow-lineated rhyolite ............................................................... 150 

13. Photograph of upper zone of gray ash-flow tuff.......................................................................................................... 151 

14. Sketch of stratigraphic relationships between the yellow tuff and rhyolite flows .. ..................................................... 151 

15. Photomicrograph of phenocryst minerals of biotite-rhyolite flow .............................................................................. 152 

16. Diagram classifying peralkaline, quartz-normative extrusive rocks ........................................................................... 154 

17. Graph showing REE patterns for units from lower part of northern stratigraphic sequence ...................................... 155 

18. Spider diagram for units from lower part of northern stratigraphic sequence ............................................................. 155 

19. Graph showing REE patterns for units from upper part of northern stratigraphic sequence .. .. . . . . . .. ... . . . . . . . .. .. . . .. ... . ..... 155 

20. Spider diagram for units from upper part of northern stratigraphic sequence ............................................................. 156 

21. Equal-area lower hemisphere projection showing relative density of poles to fault-plane attitudes.......................... 156 

22. Equal-area lower hemisphere projection of 20 fault planes showing direction and sense of slip ............................... 157 

23. Rose diagram showing strikes of faults with left-lateral and right-lateral oblique slip............................................... 158 

24. Rose diagram showing trends of 25 dikes ................................................................................................................... 158 

25. Graph showing REE patterns for units from lower part of northern stratigraphic sequence and Kane Wash Tuff. ... 159 

26. Spider diagram for units from lower part of northern stratigraphic sequence and Kane Wash Tuff .......................... 159 

27. Spider diagram for caldera-fill units in Meadow Valley Mountains and early caldera-fill units in 

Delamar Mountains ..................................................................................................................................................... 161 

28. Spider diagram for caldera-fill units in Meadow Valley Mountains and late caldera-fill units in 

Delamar Mountains .. . . . . . . . . .. . . ... . . . . . . . . .. . . . . .. . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . .. . . . . . . . . . . .. . . . . .. . . ... . . .. .. . . . . . . . . ... .. .. . . 161 

29. Generalized geologic map of northern Meadow Valley Mountains and Delamar Mountains and vicinity................ 162 

30. Sketch of major features of northern Meadow Valley Mountains ...... ............. ........................................................... 165 

TABLES 

1. Zones in the columnar-jointed rhyolite ............................. .................. ........................................................................ 146 

2. Zones in the flow-lineated rhyolite .............................................................................................................................. 148 

3. Classification of rhyolitic rocks in northern stratigraphic sequence ........................................................................... 154 

4. Comparison of dates for lower units of northern stratigraphic sequence with dates for the Kane Wash Tuff ........... 160 

Evidence of the Kane Springs Wash Caldera in the 

Meadow Valley Mountains, Southeastern Nevada 

By Anne E. Harding, 1 Robert B. Scott, Harald H. Mehnert, and Lawrence W. Snee 

ABSTRACT 

New stratigraphic, structural, chemical, petrographic, 

and age data suggest that the 14.5-Ma Kane Springs Wash 

caldera extends into the Meadow Valley Mountains, more 

than 10 kilometers east of its established boundary in the 

Delamar Mountains. The newly recognized eastern part of 

the caldera in the Meadow Valley Mountains has been offset 

left-laterally between 4 and 7 kilometers from the established western part of the caldera in the Delamar Mountains 

by the northeast-striking Kane Springs Wash fault located 

along the western side of the Meadow Valley Mountains. 

In the Meadow Valley Mountains, the Grapevine Spring 

and Gregerson Basin Members of the Kane Wash Tuff, eruptive products of the Kane Springs Wash caldera, cap a southem stratigraphic sequence that terminates abruptly against 

the southern wall of this newly recognized eastern part of the 

caldera. Caldera-wall breccia contains clasts of the oldest 

outflow unit, the Grapevine Spring Member; the youngest 

intracaldera unit, equivalent to the upper cooling unit of the 

Gregerson Basin Member, encases a 200-meter-long megaclast of the Grapevine Spring Member. A northern stratigraphic sequence that accumulated within the caldera abuts 

the caldera wall breccia. After major eruption and collapse, 

caldera-filling tuff and lava flows covered intracaldera units 

in the eastern part of the caldera and overflowed southward 

onto outflow units in the Meadow Valley Mountains. The 

two youngest intracaldera units are mildly peralkaline and 

are chemically indistinguishable from the equivalent lower 

and upper cooling units of the Gregerson Basin Member; no 

intracaldera equivalent to the metaluminous Grapevine 

Spring Member is exposed. Comenditic lower parts and trachytic upper parts of each of the two youngest intracaldera 

units are exposed along the footwall block of the Kane 

Springs Wash fault and correlate with equivalent parts of 

respective outflow units. Phenocryst assemblages of quartz, 

sanidine, hedenbergite, and fayalitic olivine of the Gregerson 

Basin Member are indistinguishable from the phenocryst 

1 Anne E. Harding, University of Colorado, Boulder, CO 80302. 

mineralogy of equivalent intracaldera units; also, post-collapse lava flows and tuff filling the eastern part of the caldera 

are similar petrographically to those of the western part. New 

K-Ar sanidine dates for the two youngest intracaldera units 

(lower unit, 14.6±0.5; upper unit, 14.6±0.4 and 14.9±0.5 Ma) 

and a 40 Ar/39 Ar sanidine date for the upper intracaldera unit 

(14.43±0.14 Ma) are within analytical uncertainty for dates 

of equivalent outflow units. A new K-Ar biotite date of 

13.1±0.5 Ma from a dike intruding eastern caldera fill is comparable to dates from western caldera fill. 

In contrast to the nearly undeformed western part of the 

caldera, the eastern part is moderately faulted, tilted, and 

injected by dikes; as much as 30° of eastward tilt occurred 

between major caldera-forming eruptions and emplacement 

of capping basalts 3 million years later. Apparently, the eastern 

part of the caldera actively extended during caldera-filling 

events while the western part passively rode out extension. 

INTRODUCTION 

This study defines and describes the volcanic stratigraphy of a complex suite of Tertiary volcanic rocks that are 

well exposed on the rugged northwest flank of the northern 

Meadow Valley Mountains, Lincoln County, southeastern 

Nevada, in the Basin and Range province (figs. 1, 2). A 

major feature recognized during mapping in this part of the 

Meadow Valley Mountains is an abrupt boundary that separates a southern area largely containing regionally correlated 

ash-flow tuffs from a northern area containing an unknown 

stratigraphy. The principal objective of this study became 

the investigation of this boundary and the rocks north of it. 

As discussed following, several relatively small scale 

field studies of the northern Meadow Valley Mountains east 

and northeast of the recognized Kane Springs Wash caldera 

in the Delamar Mountains (fig. 2) did not find that caldera in 

the Meadow Valley Mountains. In fact, we also had 

difficulty recognizing, and then becoming confident of, the 

presence of the Kane Springs Wash caldera in the Meadow 

Valley Mountains until we had amassed nearly all the 

stratigraphic, structural, petrographic, chemical, and age 

135 
136 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

NEVADA 

Study area 

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··•··•··•..•..• BASIN AND RANGE r .... ·· .. ··~ PROVINCE l ··........ ! 

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~---,_ 

Figure 1. Location of study area, Basin and Range province, southeastern Nevada. Areas of transition 

zone between stable Colorado Plateau and Basin and Range province are also indicated. 

data we present here. Because of this long history of 

investigation, we have presented our data first, free from 

conclusions, so that the reader is able to parallel our process 

of recognizing that the Kane Springs Wash caldera is also in 

the Meadow Valley Mountains. 

REGIONAL GEOLOGY 

The major volcano-stratigraphic framework in the 

northern Meadow Valley Mountains and surrounding areas 

was first established from Cook's (1965) correlation of 

widespread ash-flow tuffs in the eastern Basin and Range 

province in conjunction with Armstrong's (1970) isotopic 

dating of these tuffs. 

Two major volcanic centers were the sources of most of 

the ash-flow tuffs present in the northern Meadow Valley 

Mountains (fig. 2). The Caliente caldera complex, located 

about 30 km north of the study area, produced major regional 

outflow sheets of ash-flow tuff from about 24 to at least 18 

Ma (Rowley and others, 1990; Rowley and others, this 

volume); its eruptive products include the 18.6-Ma Hiko 

Tuff, the oldest unit exposed in the study area. In the 

Meadow Valley Mountains, the Hiko Tuff represents the last 

of the voluminous eruptions of calc-alkalic quartz-latitic and 

rhyolitic ash-flow tuffs that typified eruptions in Nevada 

between about 34 and 17 Ma (Stewart, 1980). In the Delamar 

Mountains west of the study area, the 14- Ma Kane Springs 

Wash caldera erupted the Kane Wash Tuff (Noble, 1968; 

Novak, 1984; Scott and others, 1993; Scott and others, this 

volume; Best and others, 1993). Both Noble and Novak 

reported the Kane Springs Wash caldera to be elongate in the 

east-west direction (about 19 by 13 km), with its eastern 

boundary buried beneath alluvial deposits in Kane Springs 

Valley just west of the study area. In the Meadow Valley 

Mountains, the Kane Wash Tuff and a capping basalt are 

representative of the bimodal assemblage of rhyolite and 

basalt, including high-silica subalkaline and peralkaline 

rhyolite, that followed calc-alkalic activity in Nevada. 

Prevolcanic Tertiary extension (Axen and others, 1988; 

Taylor and Bartley, 1992) and middle-Miocene low-angle 

normal faulting (Axen and others, 1988; Rowley and others, 

1991) north of the Caliente caldera complex apparently did 

not affect the Meadow Valley Mountains and adjacent areas 

south of the Caliente caldera complex. In central Lincoln 
KANE SPRINGS WASH CALDERA, MEADOW VALLEY MOUNTAINS 137 

37°07'30" 

0 10 KILOMETERS 

Figure 2. Location of study area in Meadow Valley Mountains and vicinity. Geology generalized from Ekren and others (1977). Bedrock 

is shaded; surficial deposits unshaded. Heavy lines, faults; dotted where concealed; bar and ball on downthrown side; barbs show relative 

motion on strike-slip and oblique-slip faults. Single-hachured line marks boundary of Kane Springs Wash caldera; double-hachured line 

marks boundary of Caliente caldera complex. 

County south and west of the Caliente caldera complex, 

Scott and others (this volume) have recognized progressive 

tilting of strata associated with normal fault activity between 

about 23 and 14 Ma, although large areas within the region 

remained essentially unaffected by extension (Scott, 1990). 

Between about 15 and 10 Ma, major west-southwestdirected extension affected areas southeast of the Meadow 

Valley Mountains in southeastern Nevada and southwestern 

Utah (Wernicke and others, 1988). After about 10 Ma, the 

direction of extension for this part of the Basin and Range 

province changed to west-northwest (Zoback and others, 

1981 ), the rate of extension decreased (Wernicke and others, 

1988), and the dominant style of extension in the upper crust 

appeared to have been high-angle, basin-range block faulting 

(Anderson, 1989). 

PREVIOUS INVESTIGATIONS IN THE 

STUDY AREA 

Previous work in the study area falls into two categories: mapping in the Meadow Valley Mountains and investigations of the Kane Wash Tuff. Previous mapping in the 

Meadow Valley Mountains has been mainly reconnaissance 

in scope. Tschanz and Pampeyan ( 1970) mapped (I :250,000 

scale) and described the geology and mineral deposits of 
138 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

Lincoln County, concentrating on pre-Tertiary sedimentary 

rocks. They showed the northern Meadow Valley Mountains 

as consisting of undivided Cretaceous or Tertiary volcanic 

rocks, and younger Tertiary volcanic rocks. They named the 

western range-front fault the Kane Springs Wash fault and 

described it as a normal fault that cuts valley fill as young as 

Pliocene and early Quaternary. 

Ekren and others ( I977) mapped the distribution of the 

Tertiary rocks in Lincoln County, also at a scale of 

1:250,000, and correlated some of the widespread ash-flow 

tuffs. In the study area, they reported three rhyolitic units and 

a younger basalt, most of which could not be correlated with 

recognized stratigraphic units. Ekren and others ( I977) also 

recognized several ash-flow tuffs that are conspicuously 

flow laminated and layered and that apparently flowed during the welding process. They cautioned that these rocks are 

extremely difficult to distinguish from true lavas. 

They interpreted the Kane Springs Wash fault as a 

strike-slip or oblique-slip fault having a minimum of 8 km of 

left -lateral displacement, based on (I) horizontal slickenlines 

in and adjacent to the fault zone near the head of Kane Springs 

Valley, (2) correlation of thick rhyolitic lavas in the northern 

Meadow Valley Mountains with lavas in the Delamar Mountains to the southwest, and (3) apparent offset of Paleozoic 

strata in the southern part of Kane Springs Wash. 

During reconnaissance mapping for a wilderness study, 

Pampeyan (1989) mapped the geology of the Meadow Valley Mountains at a scale of 1:50,000. In all but the northern 

part of his map area, Pampeyan correlated units in the northem Meadow Valley Mountains with the Kane Wash Tuff as 

subdivided by Novak (1984), but in the northern part, he 

grouped units as "rhyolite lavas" or "rhyolite lavas and Kane 

Wash Tuff, undivided." 

In written communication (1988), Pampeyan stated that 

"the complex lithologic, structural, and stratigraphic relations in the volcanic terrane at the northern end of the range 

may signify that this area is an eastward continuation of the 

Kane Springs Wash caldera." Although he proposed no 

caldera boundaries, Pampeyan suggested left-)-ateral offset 

of about 8 km across the Kane Springs Wash fault based on 

his interpretation that two trachyte dikes in the northernmost 

part of the range may mark the northern edge of the Kane 

Springs Wash caldera. Alternatively, he interpreted other 

trachyte lavas as evidence for both right-lateral and no lateral 

offset. In addition, Pampeyan gave several reasons why the 

apparent offset of Paleozoic strata proposed by Ekren and 

others (1977) cannot be used with certainty to indicate leftlateral offset. 

Members of the Kane Wash Tuff, eruptive products of 

the Kane Springs Wash caldera, are among the youngest 

rocks previously mapped in the study area. Cook (1965) 

originally applied the name "Kane Wash Formation" to 

exposures in the Delamar and Meadow Valley Mountains. 

His type section for the unit in the southern Delamar Mountains contains a basalt flow in addition to eight rhyolitic ashflow tuffs. 

After reconnaissance mapping, Noble ( 1968) revised 

the definition ofthe Kane Wash Formation to exclude basalt, 

other lavas, and unrelated sedimentary rocks, and renamed it 

the Kane Wash Tuff. He proposed the presence of as many 

as 12 cooling units in the Kane Wash Tuff, but did not 

describe or map them individually. Noble delineated the 

boundaries of the source caldera (fig. 2) and produced a generalized geologic map of the distribution of the Kane Wash 

Tuff, showing rocks in the northern Meadow Valley Mountains as outflow units derived from that caldera. Noble recognized the peralkaline chemistry of the rocks and reported 

a K-Ar date of I4.0±0.5 Ma for the tuff. 

Novak (1984) and Novak and Mahood (1986) 

described the eruptive history and the chemical evolution of 

the Kane Springs Wash volcanic center based on geochemistry, K-Ar dates, and field mapping (published at a scale of 

about I: 130,000). Novak (1984) divided the Kane Wash 

Tuff into five mappable units, members V 3, V 2· V 1, W, and 

0 (in order of decreasing age), and described them along 

with younger caldera-filling lava flows and tuffs. Based on 

the distribution of the tuffs and on dating, Novak recognized 

that members V 1, V 2· and V 3 (14.1 ±0.4 Ma) were erupted 

from the Kane Springs Wash volcanic center, but that the 

older members Wand 0 (14.7±0.4 and 15.6±0.4 Ma, respectively) came from sources south and west of the Kane 

Springs Wash volcanic center. (Novak reported his dates 

with one standard deviation; here his results are reported 

with two standard deviations to be consistent with other literature.) East of the caldera in the Meadow Valley Mountains, in the southern part of the study area, Novak mapped 

Kane Wash Tuff and basalt, and in the northern part he 

mapped pre-Kane Wash volcanic rocks. 

ACKNOWLEDGMENTS 

This study was done in partial fulfillment of Anne Harding's M.S. degree at the University of Colorado, Boulder. 

During the study she was a volunteer with a U.S. Geological 

Survey mapping project that is studying the transition 

between the Basin and Range province and the Colorado 

Plateau (Basin and Range- Colorado Plateau Transition in 

southeastern Nevada, southwestern Utah, and northwestern 

Arizona Study Unit of the National Geologic Mapping Program). See Introduction, this volume, for more information 

on the project. The U.S. Geological Survey provided her 

with maps, aerial photographs, field trailer, and laboratory 

equipment. USGS analytical laboratories performed most of 

the chemical analyses. 

Steve Novak suggested the study area and provided a 

copy of the geologic map from his Ph. D. dissertation (Novak, 

1985). Support from a large number of USGS scientists and 

technicians is deeply appreciated. Dave Nealey helped us 

present our chemical data by making REE and spider diagrams. We especially thank reviewers Mark Hudson and 

Scott Minor for their constructive advice and encouragement. 
KANE SPRINGS WASH CALDERA, MEADOW VALLEY MOUNTAINS 139 

METHODS OF STUDY 

A variety of techniques was used to define and describe 

the volcanic stratigraphy in the study area and to correlate 

these strata with regional volcanic units. 

About 60 days of detailed field mapping between 1988 

and 1990 delineated the stratigraphic and structural relationships in the study area. Most contacts and faults were walked 

out, as little contrast in color or features is apparent between 

many of the units both on the photos and in the field. Field 

data were recorded on color aerial photographs (scale of 

about 1 :30,000) and were compiled on a 1: 12,000-scale, stable mylar topographic base map by means of a Kern PG-2 

photogrammetric plotter. 

About 75 samples were selected and cut for thin-section 

analysis. The thin sections were stained for identification of 

potassium-bearing phases. Modes were obtained from about 

50 thin sections (Appendix 1). For most of the modal analyses, a minimum of 200 phenocryst points and a minimum of 

2,000 total points were counted. 

Twenty-three samples were analyzed by energy-dispersive X-ray fluorescence (EDXRF) spectrometry (Appendix 

2A) by means of the U.S. Geological Survey Kevex 7000 

spectrometer, using methods outlined by Johnson and King 

(1987). A 109Cd source was used to analyze for Rb, Sr, Y, Zr, 

and Nb. A 241 Am source was used to analyze for Ba, La, Ce, 

and Nd. The U.S. Geological Survey standard rock GSP-1 

was used as a calibration standard. In the illustrations and 

tables, only the Zr data from the EDXRF analyses were used; 

more precise data were obtained for other elements by other 

methods. 

Major-element oxides were determined in 21 samples 

by wavelength-dispersive X-ray fluorescence (WDXRF) 

spectrometry (Appendix 2B) using the methods of Taggart 

and others (1987). All major-element data in the diagrams 

and tables come from WDXRF analyses. 

Instrumental neutron activation analysis (INAA) of 15 

samples provided data for many major, minor, and trace 

elements (Appendix 2C), particularly the rare earth elements 

(Baedecker and McKown, 1987). All minor- and traceelement data except for the elements Zn, Ga, Sr, Y, Nb, and 

Pb that appear in the diagrams and tables were determined by 

INAA. 

The same 15 samples were also analyzed (Appendix 

2D) by inductively coupled plasma-atomic emission 

spectrometry (ICP-AES) by means of the methods described 

by Lichte and others ( 1987). This method provided 

semiquantitative analyses for some elements not analyzed by 

another method. The ICP-AES values for the elements Zn, 

Ga, Sr, Y, Nb, and Pb were used in the diagrams and tables. 

To refine the stratigraphic framework, five samples 

were collected for isotopic dating using potassium-argon 

dating methods (Appendix 3). Conventional K-Ar dates 

were obtained from five samples (for methods see Dalrymple and Lanphere, 1969; for constants see Steiger and Jager, 

1977). One sample was redated using the more precise 

40 Ar/39 Ar method (Shubat and Snee, 1992). 

STRATIGRAPHY 

The two stratigraphic sequences separated by the abrupt 

boundary mentioned in the Introduction are described separately, beginning with the southern sequence. Unit~ overlying both sequences and the boundary between them are 

described with the northern sequence. Informal field names 

are used for new stratigraphic units. Fresh rock colors are 

given in all descriptions, and color names are from the RockColor Chart (Goddard, 1948). A generalized geologic map of 

the study area is shown in figure 3; detailed field relationships are not shown on this map but are documented elsewhere at scales of 1: 12,000 (Harding, 1991) and 1:24,000 

(Scott, Harding, and others, 1991). 

The nomenclature for the Kane Wash Tuff used in this 

study follows the usage of Scott and others (1993). After 

mapping this tuff in nine 7 .5-minute quadrangles, Scott and 

others removed the units of its lower part, members W and 0 

of the Kane Wash Tuff of Novak (1984), from the Kane 

Wash Tuff because they are now regarded to have separate 

sources. They reassigned Novak's members W and 0 to two 

newly named formations. The name Kane Wash Tuff is now 

restricted to the three petrologically related ash-flow tuff 

cooling units that were derived from the Kane Springs Wash 

caldera. Novak ( 1984) defined these cooling units as members V3, V2, and V1. Locally, members V3 and V2 are indistinguishable in the field and, therefore, these have been 

combined into one consistently mappable member, the 

Gregerson Basin Member (fig. 4). Rocks of Novak's member V 1 have been reassigned to the Grapevine Spring Member (Scott and others, 1993). 

Novak (1984) also described and named two older cooling units, members W and 0 of his Kane Wash Tuff, and recognized that these members are distinctly different in age, 

geographic distribution, and genesis from the younger peralkaline tuffs, members V 1, V 2· and V 3· The rocks of these two 

units have been designated as separate formations (Scott and 

others, 1993): in descending order, the Sunflower Mountain 

Tuff (=member W) and the Delamar Lake Tuff (=member 

0) (fig. 4). 

SOUTHERN STRATIGRAPHIC SEQUENCE 

The southern stratigraphic sequence consists of units 

which are correlated with regional ash-flow tuffs. These 

units can be traced from the southern part of the study area 

to about the midpoint of the map (fig. 3), where the units end 

abruptly against the northern stratigraphic sequence. 
~ 

ffi 

~ a 

~ 

~ 

~ 

~ 

c:: 

~ 

u 

.Q 

0::: 

~ 

@ 

u 

0 

~ 

EXPLANATION 

Alluvium and colluvium 

Upper basalt flow 

~ Youngest ITdl Rhyolite 

Flow-banded rhyolite ~ rhyolite flows ~ dike 

Lithophysal rhyolite flow~ Pumice-rich I Tvc I Volcanic 

Middle basalt flow ash-flow tuff center 

Yellow ~ Biotite-rhyolite flow 

tuff ~ Red lithic rhyolite 

Gray ash-flow tuff 

Lower basalt flow 

Flow-Jineated 

myolite 

Columnar-jointed 

rhyolite 

Lithic-rich tuff 

EJ Wall 

breccia 

Older tuff 

Gregerson Basin Member 

of the Kane Wash Tuff 

Grapevine Spring Member 

of the Kane Wash Tuff 

Precaldera trachyte flows 

Sunflower Mountain Tuff 

Delamar Lake Tuff 

HikoTuff 

3r1~ 11no· ~ .. :lff-- u Try 

114°37'30" 

,.<? 

/:>·· 

l 

.. ~; 

./ j QTa 

Tcj_ ••• 

•·· ,:' 

.. ··· ,./ 

.. ··· ;': 

.·· ./ .. 

.l... •• •• // 

iq .. ·· ,/ . ,' . / "' ... ,/ "' .. / 

,"l: .... / ~ o I 

.... :~' / /:/// /00 _:: QTa 

•· ~ .:! .. ·· .:::::- ... / •• h.. .. ! •• ••• "'<. / ; 

•• ~ n. ; 

........... 0 /iii ... ··· ...... / 

...... iq .. </ .. ~ :.' 

... ' :"/ 

• L"f" f{ 

""' 1 f./ •I 

~ /' ~ // ·$ ;. A/: "'""''' . . 11/,' / ~ l : 

\ ... / ... : 

; : : QTa • /./ ...... ~= 

/// .-! .. ..... : .. : / ......... : 

_.../) 

.//Th 

........ ! 

... /b .... 

+::>. 

0 

a 

tTl 

0 

r 

0 

a 

n 

en 

-3 

c::: 

0 

tT1 

s-n 

t:C 

> :;r::l 

n 

0 

en 

-3 

c::: 

0 

-< 

c::: 

z 

~~ 

\0 

\0 

N 
8 

' 

~ 

~ 

II II IIIII II 

___2§ 

... 

X 

Contact--Showing dip where known 

High-angle normal fault--Dotted where concealed; queried where 

uncertain; bar and ball on downthrown side 

Strike-slip or oblique-slip fault-Arrows show relative direction 

of offset; dotted where concealed; queried where uncertain 

Fault scarp along which younger unit has been depositedHachures on side of postfault deposit 

Strike and dip of compaction foliation of ash-flow 

tuffs--Showing inclined strata 0:.> 

.· .. ·· Caldera margin-Half-circles on down-dropped 

caldera interior; dotted where concealed. This 

symbol also marks the general boundary between 

the northern and southern stratigraphic sequences 

in text 

........ =··· 

Highest elevation 

37°10' 

" .... v-v ... ~'it .... 

~i ~,'it .. : 

~~~ .... ~ 

~ ... : 

rS / QTa ~ y;J .. . 

-l!~ ..... .;, . . . . T :! : • Q a ./ \ / 

y:.:;·· .... ~ y· • • .· : r. / ·._ Tkb Tpr ~ 

....... Tpr ~\ \ . . .. "-.. ~ -:>1: : 

: Tsm 

1W40' 

0 1 2 KILOMETERS 

Figure 3. Generalized geologic map of the study area. 

Trlr 

QTa 

114°37'30" 

~ 

> z 

tTl 

C/) 

;g 

z 

a 

C/) 

~ 

> C/) 

:I:: 

n 

> l' 

0 

tTl 

~ 

? 

~ > 0 

0 

~ 

< 

> 

~ -< 

:: 

0 

c 

z ...., 

> z 

C/) -+::- -
142 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

HIKOTUFF 

The oldest unit exposed in the southern stratigraphic 

sequence is the Hiko Tuff, a devitrified, moderately welded, 

rhyolitic ash-flow tuff that forms gentle slopes (fig. 5). The 

Novak (1984) Scott and others (1993) 

MemberV3 

=s Gregerson Basin t-

~ 

en Member CIS 

MemberV2 3: 

Q) 

c: 

CIS 

~ 

::::: 

~ 

tMemberv1 

Grapevine Spring 

.r: Member en 

CIS 

!: 

Upper member 

Q) 

c: =s CIS t- :::.::: c: ·.s 

c: 

~ 

MemberW 0 

:::!: 

Q) Lower member ~ 

..Q 

'E 

~ 

C/) 

Delamar Four informal MemberO Lake 

Tuff members 

Figure 4. Stratigraphic nomenclature changes for the Kane Wash 

Tuff. 

tuff is slightly but pervasively hydrothermally altered 

throughout the study area and is grayish orange to dark yellowish orange; elsewhere, unaltered rock is light brownish 

gray. The Hiko Tuff contains 30-40 percent phenocrysts 

based on two thin-section modal analyses, one from the 

study area (Appendix I) and one from a locality north of the 

study area. The phenocrysts consist of 17- 3I percent very 

pale purple quartz, I4-32 percent sanidine, 28- 55 percent 

plagioclase, 5-I2 percent biotite, and 2 percent opaque minerals. The tuff contains I-I6 percent volcanic and distinctive 

Paleozoic argillitic lithic fragments. Pumice fragments as 

long as 3 em form I 0-25 percent of the rock. Taylor and others (1989) reported a 40 Ar/39 Ar biotite date of I8.5±0.4 Ma 

for the Hiko Tuff, but suggested that the best age estimate 

may be I8.6 Ma based on additional data. The thickness of 

the Hiko Tuff cannot be determined in the study area because 

its base is not exposed; however, a thickness of about 75 m 

for the Hiko Tuff was measured south of the study area in the 

Vigo NW 7.5-minute quadrangle (Scott, Harding, and others, I99I). 

DELAMAR LAKE TUFF 

The Delamar Lake Tuff (Scott and others, 1993), 

which overlies the Hiko Tuff, is a devitrified, moderately 

welded, rhyolitic ash-flow tuff that forms both gentle slopes 

and sharp ledges and ranges from about 60 to I20 m thick. 

The Delamar Lake Tuff is the oldest high-silica rhyolite ashflow tuff representative of the bimodal basalt-rhyolite volcanism in this part of the Basin and Range province. Like the 

Hiko Tuff, it is slightly but pervasively hydrothermally 

altered over an area extending nearly to the southern edge of 

the study area. Altered rocks are pale pink, pale red purple, 

pale yellowish orange, and moderate reddish orange; in 

Figure 5. Southern stratigraphic 

sequence in the Meadow Valley 

Mountains. In descending order, 

the Gregerson Basin (Tkb) and 

Grapevine Spring (Tkg) Members 

of the Kane Wash Tuff form the 

bold cliffs on the skyline. The 

Sunflower Mountain Tuff (Tsm) 

forms the major bench and the 

overlying small, bold cliff. The 

Delamar Lake Tuff (Tdl) forms 

the lowest sharp ledge above the 

poorly exposed, slope-forming 

Hiko Tuff (Th). View to southeast. 
KANE SPRINGS WASH CALDERA, MEADOW VALLEY MOUNTAINS 143 

some localities, they display Liesegang rings of yellowishgray to light-red iron oxides. The tuff contains about 20 percent phenocrysts that consist of 25 percent quartz, 70 percent sanidine, and 5 percent altered ferromagnesian and 

opaque minerals. Less than 2 percent of the rock consists of 

volcanic lithic fragments. Flattened pumice fragments as 

long as 4 em form about 10 percent of the tuff. Sanidine KAr dates for the tuff are 15.5±0.4 and 15.8±0.4 Ma (Novak, 

1984 ). The overlying Sunflower Mountain Tuff is not 

hydrothermally altered, thereby restricting the time of alteration of the Delamar Lake and Hiko Tuffs. 

SUNFLOWER MOUNTAIN TUFF 

Stratigraphically above the Delamar Lake Tuff, the 

Sunflower Mountain Tuff (Scott and others, 1993) is a pinkish-gray to grayish-orange, devitrified, rhyolitic ash-flow 

tuff that typically forms gentle slopes below bold cliffs (fig. 

5). The Sunflower Mountain Tuff contains a lower, nonwelded to partially welded zone and an upper, partially to 

moderately welded zone. A K-Ar date of 14.7±0.4 Ma for 

the tuff was obtained from sanidine (Novak, 1984 ). 

The lower, nonwelded to partially welded zone of the 

Sunflower Mountain Tuff is pale orange to grayish orange 

and forms gentle to moderate slopes (fig. 5). The lower zone 

ranges in thickness from about 120 to 220m. The rock contains about 10 percent phenocrysts that consist of subequal 

amounts of quartz and sanidine and sparse altered ferromagnesian minerals. Pumice fragments, which form about 20 

percent of the rock, are 0.5-3 em long. Rhyolitic volcanic 

lithic fragments form about 20 percent of the rock. 

The upper, partially to moderately welded zone, which 

is mottled pinkish gray to pale red, forms moderate slopes to 

bold cliffs. Mottling consists of distinctive moderate-orangepink to very pale orange altered blotches in a pale-red matrix. 

In the southern part of the study area, the upper zone of the 

Sunflower Mountain Tuff is as much as 150 m thick. The 

rock contains 9-14 percent phenocrysts that consist of 34-37 

percent quartz, 60-63 percent sanidine, less than I percent 

plagioclase, and 2-3 percent altered ferromagnesian and 

opaque minerals (Appendix I). Pumice fragments form about 

20 percent of the rock and are as long as I5 em. Rhyolitic volcanic lithic fragments form less than I percent of the rock. 

OLDER TUFF 

Three small exposures of a pale-red-purple, devitrified, 

densely welded, rhyolitic ash-flow tuff form a small bench (2 

m high) along the range front, on the footwall of the Kane 

Springs Wash fault. The tuff contains 4 percent phenocrysts 

that consist of 20 percent quartz, 70 percent sanidine, and I 0 

percent altered ferromagnesian and opaque minerals 

(Appendix I). The older tuff is considered to be the outflow 

facies from a peralkaline precursor of the Kane Springs 

Wash caldera, the Narrow Canyon caldera (Scott, Blank, and 

Page, 199I ); the older tuff lies below the precaldera trachyte 

flows in the study area. Based on regional correlations made 

during unpublished mapping, the older tuff is younger than 

the Hiko Tuff and older than the Kane Wash Tuff. Because 

the older tuff is mildly peralkaline, like the Kane Wash Tuff, 

it is probably younger than the less geochemically evolved 

metaluminous Delamar Lake and Sunflower Mountain Tuffs 

(based on unpublished geochemical studies by R.B. Scott). 

PRECALDERATRACHYTEFLOWS 

Stratigraphically above the older tuff at one locality and 

above the Sunflower Mountain Tuff elsewhere, pale-red to 

grayish-red, devitrified, trachytic lava flows form rugged, 

steep slopes and small cliffs. These trachyte flows were 

referred to as the precaldera trachyte lavas by Novak (1984) 

because he found the trachyte flows to be restricted to localities near the Kane Springs Wash caldera margin below the 

Kane Wash Tuff and to have an age indistinguishable from 

that of the Kane Wash Tuff. The precaldera trachyte flows 

are massive and lack megascopic flow banding; however, 

flow banding can be seen in thin section. A very dusky red to 

black basal vitrophyre is present locally. Just south of the 

boundary between the stratigraphic sequences, two cooling 

breaks indicate the presence of at least three flows. The precaldera trachyte contains 36 percent phenocrysts, 95 percent 

of which are conspicuous alkali-feldspar phenocrysts that 

range from 0.25 to 1 em in diameter. The remainder of the 

phenocrysts consists of 4 percent pyroxene and 2 percent 

opaque and altered ferromagnesian minerals (Appendix 1). 

The precaldera trachyte is about I80 m thick at the northern 

limit of exposure in the study area, and it pinches out to the 

south. K-Ar sanidine dates for the precaldera trachyte flows 

are 13.6±0.4 and 14.2±0.4 Ma (Novak, 1984). 

KANE WASH TUFF 

The Kane Wash Tuff consists of two newly defined 

members, the Grapevine Spring Member and the Gregerson 

Basin Member (Scott and others, 1993), whose rocks were 

erupted from the Kane Springs Wash caldera. Although 

Noble (1968) described the rocks of his Kane Wash Tuff as 

peralkaline, only the upper Gregerson Basin Member (equivalent to members V 2 and V 3 of Novak, I984) of the Kane 

Wash Tuff is mildly peralkaline (Novak, 1984). The lower 

Grapevine Spring Member (equivalent to member V 1 of 

Novak, 1984) of the Kane Wash Tuff is metaluminous 

(Novak, 1984 ), making it chemically distinct from the 

Gregerson Basin. 

The Grapevine Spring Member overlies either the precaldera trachyte or the Sunflower Mountain Tuff and is a 
144 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

light-brownish-gray to brownish-gray, devitrified, moderately to densely welded, rhyolitic to trachytic ash-flow tuff 

that forms rugged slopes and cliffs (fig. 5). The tuff is characterized by a moderate abundance of sanidine phenocrysts 

that are adularescent. (See Mittwede, 1987.) Throughout its 

regional distribution, phenocryst abundances range from 16 

to 35 percent and consist of 8-27 percent quartz, 64-78 percent sanidine, and 4-16 percent hedenbergite, fayalite, and 

opaque minerals. The basal part of the unit contains 38 percent phenocrysts that consist of less than 1 percent quartz, 94 

percent sanidine, less than 1 percent plagioclase, 3 percent 

hedenbergite, and 2 percent opaque minerals (Appendix 1 ). 

The tuff contains few lithic fragments or recognizable pumice fragments. Novak (1984) reported K-Ar dates of 

14.0±0.4 and 14.2±0.4 Ma for sanidine from the Grapevine 

Spring Member (his member V 1 ). A new 40 Art39 Ar date of 

sanidine from this unit of 14.67±0.22 Ma has been determined (analysis by L.W. Snee) (Appendix 3). 

The Gregerson Basin Member of the Kane Wash Tuff 

consists of a lower and an upper cooling unit equivalent to 

members V2 and V3, respectively, of the Kane Wash Tuff as 

used by Novak (1984) (fig. 4). 

The lower cooling unit is a devitrified, moderately to 

densely welded ash-flow tuff that forms moderate slopes and 

cliffs (fig. 5) and is characterized by conspicuous adularescent sanidine phenocrysts. Most of the lower cooling unit is 

pinkish gray to yellowish gray, but it is pale blue and light 

bluish gray in some zones. The lower cooling unit is about 

100 m thick. Regionally, the tuff contains 24-36 percent 

phenocrysts that consist of 6-51 percent quartz, 47-78 percent sanidine, and 3-15 percent altered ferromagnesian 

(some relict hedenbergite and fayalite) and opaque minerals. 

Distinctive vapor-phase amethyst and blue-green amphibole, probably riebeckite, are present. Samples at and near the 

base of the lower cooling unit contain 8-11 percent phenocrysts that consist of 30-44 percent quartz, 55-65 percent 

sanidine, and 1-5 percent altered ferromagnesian (some 

relict hedenbergite) and opaque minerals (Appendix 1 ). 

Highly flattened pumice fragments about 5 em long that 

enhance parting parallel to the plane of compaction form 

about 20 percent of the rock. The lower cooling unit is 

capped by pale-yellowish-brown, partially to moderately 

welded ash-flow tuff containing as much as 20 percent cognate trachytic inclusions. These inclusions are from 1 to 30 

em long, scoriaceous, and pale brown to grayish brown; they 

contain about 50 percent phenocrysts of alkali feldspar. 

Novak (1984) reported a K-Ar date of 14.1±0.4 Ma for the 

sanidine from the lower cooling unit (equivalent to his member V 2) of the Gregerson Basin Member. A new 40 Ar/39 Ar 

date of 14.55±0.14 Ma has been determined for sanidine 

from the lower cooling unit of the Gregerson Basin Member 

(analysis by L.W. Snee) (Appendix 3). 

The upper cooling unit of the Gregerson Basin Member 

is a devitrified, moderately to densely welded ash-flow tuff 

that is nearly indistinguishable lithologically from the lower 

cooling unit. Modal analysis of a thin section of a sample 

near the base of the upper cooling unit indicates that the rock 

contains 10 percent phenocrysts that consist of 33 percent 

quartz, 64 percent sanidine, and 3 percent altered ferromagnesian (some relict hedenbergite) and opaque minerals 

(Appendix I). Some acicular dark-blue-green, vapor-phase 

crystals, probably riebeckite, are also present. The upper 

cooling unit is about 20m thick in the southern part of the 

study area and pinches out to the north. A new 40 Art39 Ar 

date of 14.39±0.28 Ma has been determined for sanidine 

from the upper cooling unit (analysis by L.W. Snee) (Appendix 3). 

NORTHERN STRATIGRAPHIC SEQUENCE 

The northern stratigraphic sequence cannot be correlated directly with regional ash-flow tuff units. Therefore, 

this sequence must have a local source. The sequence is continuous from the northern part of the map area southward to 

the midpoint of the map (fig. 3). 

LITHIC-RICH TUFF 

A lithic-rich tuff forms the oldest rock unit in the northem volcanic sequence. The rock is a quartz-sanidine-bearing, rhyolitic tuff that contains abundant lithic fragments and 

some pumice fragments. This tuff is poorly exposed, and 

typically it is brecciated and altered. Only the upper 5 m of 

this rock unit are exposed over a lateral distance of about 300 

m in the footwall block along Kane Springs Wash fault in the 

northern part of the map area. The lithic-rich tuff has not 

been dated, and its relative age is uncertain. Welding of pumice fragments and groundmass around lithic fragments and 

the lack of sorting of the lithic fragments indicate that the 

lithic-rich tuff was probably emplaced as an ash flow. 

The lithic-rich tuff is light brownish gray to grayish red 

purple and contains as much as 75 percent inclusions. Pumice fragments of the tuff, probably most representative of its 

magmatic source, contain 10 percent phenocrysts that consist of 42 percent quartz, 54 percent sanidine, and 4 percent 

altered ferro magnesian minerals (Appendix 1 ). 

Two types of pale-red to grayish-red trachyte, which 

are probably foreign lithic fragments, are the most conspicuous inclusions in hand samples. Other inclusions, noticeable 

in thin section, are probably cognate because they were still 

ductile when incorporated into the lithic-rich tuff (Harding, 

1991). 

COLUMNAR-JOINTED RHYOLITE 

Along the range front in the northern part of the study 

area, a columnar-jointed rhyolite ash-flow tuff forms steep 
KANE SPRINGS WASH CALDERA, MEADOW VALLEY MOUNTAINS 145 

slopes and cliffs and weathers into small boulders that form 

talus cones on the slopes. The columnar-jointed rhyolite contains quartz, sanidine, and ferromagnesian phenocrysts. 

Most of the quartz phenocrysts are 2 mm or less in diameter. 

Sanidine phenocrysts are as much as 5 mm long, and some 

of them are adularescent. Although most of the ferromagnesian phenocrysts were altered to opaque minerals, remnant 

green-tinged clinopyroxene and olivine can be identified in 

a few thin sections. The olivine is probably fayalitic, as it 

coexists with quartz. The columnar-jointed rhyolite also contains accessory phenocrystic zircon, some of which is prismatic. A new K-Ar sanidine date for this unit is 14.6±0.5 Ma 

(analysis by H.H. Mehnert) (Appendix 3). The columnarjointed rhyolite, which is about 130m thick, can be divided 

into three zones: a basal, layered zone; a middle, densely 

welded zone; and an upper, partially welded zone. Crude 

columnar jointing characterizes the densely welded zone in 

places. The rock has a weak flow foliation along which a 

parting has developed. Table 1 summarizes the phenocryst 

mineralogy and thickness for each zone. Complete petrographic modal analyses are in Appendix 1. 

The basal, layered zone is exposed only near the Kane 

Springs Wash fault adjacent to the lithic-rich tuff. Layering 

is evident from color, contrasting styles of devitrification, 

degrees of welding, and in one case, a difference in phenocryst abundance. Both a black vitrophyre and the lightbluish-gray layer are densely welded. The pale-red layer is 

devitrified, moderately welded, and contains abundant 

microscopic cavities, now partly filled with vapor-phase 

amphibole and unknown dark acicular crystals. Secondary 

quartz completely fills the cavities between the vapor-phase 

minerals. 

The layered zone is generally less than 5 m thick and at 

some exposures is less than 30 em thick. Where thin, this 

zone consists of several relatively phenocryst-rich layers 

(14-17 percent phenocrysts, Appendix 1 ), separated by several densely welded, phenocryst-poor layers ( <2 percent 

phenocrysts) that are less than 3 em thick. The basal layered 

zone is chilled and has preserved its diagnostic ferromagnesian phenocryst mineralogy, particularly in the vitrophyre 

(fig. 6). Dark-grayish-blue-green amphibole (riebeckite?) 

can be observed in the bluish-gray layer of the basal zone. In 

contrast, the middle and upper zones contain only altered ferromagnesian phases. 

The middle, densely welded zone consists of devitrified, light-gray to medium-light-gray rhyolite and is about 

90 m thick. Both the densely welded and the partially welded 

zones have less abundant phenocrysts than the layered zone 

(table 1). The felsic groundmass of the densely welded zone 

contains aggregates of secondary opaque minerals with 

irregular crystal boundaries that in some places give the rock 

a "peppered" appearance in hand sample. 

The upper, partially welded zone of the columnarjointed rhyolite is grayish pink to pale red purple and about 

35 m thick. This zone contains abundant unflattened pumice 

Figure 6. Photomicrograph of green-tinged hedenbergite (bottom) and partly altered fayalitic olivine (top) microphenocrysts in 

vitrophyre in basal, layered zone of columnar-jointed rhyolite. 

View is 0.64 mm high; plane light. 

fragments that are as large as 12 em in diameter, and the zone 

also contains grayish-red trachytic inclusions tens of centimeters in diameter that consist of as much as 40 percent 

alkali-feldspar phenocrysts and no quartz. These trachytic 

inclusions were deformed during compaction ofthe ash-flow 

tuff and are considered to be cognate. Flattened pumice fragments and deformed glass shards can be seen in thin sections 

of the partially welded zone (fig. 7). Abundant small, secondary opaque minerals and aggregates of opaque minerals 

are present in the groundmass. The partially welded zone is 

porous and contains abundant microscopic vapor-phase cavities, many of which are lined with dark, acicular, vaporphase crystals (amphibole?). 

Although the columnar-jointed rhyolite has flow foliation and lineation, features commonly associated with lava 

flows, other features indicate that this unit was emplaced as 

an ash flow. Supportive evidence includes (1) a layered basal 

zone, (2) the presence of glass shards and pumice fragments, 

(3) a decrease in the degree of welding toward the top of the 
146 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

Table 1. Zones in the columnar-jointed rhyolite. 

[Percent of phenocrysts refers to percent of rock. Mineral percentages are percent of phenocrysts; nd, not determined! 

Zone Phenocrysts Quartz Sanidine Ferromagnesian and opaque 

(thickness) (percent) (percent) (percent) (percent) 

Partially welded zone (35 m) 3-7 24-30 62-71 5-10 

Densely welded zone (90 m) 6 

Layered zone ( <5 m) 

Light-bluish-gray 16 

Pale-red 9 

Black vitrophyre 13 

Greenish-gray vitrophyre nd 

Figure 7. Photomicrograph of relict glass shard morphology preserved in upper, partially welded zone of columnar-jointed rhyolite. 

View is 1.26 mm high; plane light. 

unit (Smith, 1960), ( 4) an increase in ferromagnesian phenocryst content toward the top of the unit, and (5) the presence 

of trachytic inclusions in the uppermost part of the unit. Ashfall deposits commonly underlie ash-flow tuffs, and the layered basal zone of this unit with its variation in phenocryst 

abundance may have resulted from several bedded ash 

deposits that were subsequently welded by the heat of the 

overlying tuff (Smith, 1960). The layered basal zone lacks 

30 

20 

28 

30 

nd 

66 4 

75 5 

71 I 

67 3 

nd nd 

textures indicative of pyroclastic surge deposits (Wohletz 

and Sheridan, 1979). The increase in mafic phases and cognate trachytic inclusions toward the top of the columnarjointed rhyolite is similar to zonation reported for other ashflow tuffs that were erupted from chemically zoned magma 

chambers (Lipman and others, 1966). The weak flow foliation and lineation probably formed during post-emplacement 

flow. 

BRECCIA 

A coarse, angular boulder breccia is discontinuously 

exposed along the boundary between the northern and southem stratigraphic sequences (fig. 3). The breccia is included 

with the northern stratigraphic sequence because in some 

places the breccia interfingers with the northern sequence. 

This unit may be as thick as 40 m locally. The breccia consists mostly of clasts of precaldera trachyte and Grapevine 

Spring Member of the Kane Wash Tuff, rock units exposed 

in the abutting southern stratigraphic sequence. These clasts 

are in a pale-reddish-brown, partially welded, devitrified, 

tuffaceous matrix that contains phenocrysts of quartz and 

sanidine. Locally the matrix of the breccia is similar to partially welded flow-lineated rhyolite, a unit described below. 

At the southern end of the westernmost segment of wall 

breccia (fig. 3), a 200-m-long megabreccia clast composed 

of the Grapevine Spring Member is partly enveloped in a vitrophyre of the flow-lineated rhyolite. 

The breccia displays a massive to crude bedding that 

dips away from the southern stratigraphic sequence into the 

northern sequence at attitudes close to the angle of repose. 

There is a lack of planar shear in the breccia. The lateral margin of the flow-lineated rhyolite has chilled against the breccia; in some localities the breccia is interbedded with chilled 

layers of the flow-lineated rhyolite above the westernmost 

segment of the breccia (fig. 3). 

FLOW -LINEA TED RHYOLITE 

A flow-lineated, rhyolite ash-flow tuffthat forms prominent cliffs below gentle, rounded slopes is widely exposed 
KANE SPRINGS WASH CALDERA, MEADOW VALLEY MOUNTAINS 147 

along the mountain front in the northern part of the area. This 

unit contains quartz, sanidine, and ferromagnesian phenocrysts. Most of the quartz phenocrysts are 2 mm or less in 

diameter. Sanidine phenocrysts are as much as 5 mm long 

and some are adularescent. Although most ferromagnesian 

phenocrysts were altered to opaque minerals, a few remnant 

green-tinged clinopyroxene phenocrysts can be identified in 

thin section. The flow-lineated rhyolite also contains accessory phenocrystic zircon, only some of which is prismatic. 

Two new K-Ar sanidine dates obtained for this unit are 

14.6±0.4 and 14.9±0.5 Ma (analyses by H.H. Mehnert), and 

a more precise 40 Ar/39 Ar sanidine date is 14.43±0.14 Ma 

(analysis by L.W. Snee) (Appendix 3). Although within a 

few meters of its base the rhyolite has layering that is parallel 

to the base, higher in the unit it is strongly flow foliated and 

flow lineated (figs. 8, 9). This unit is about 110 m thick and 

can be divided into three zones: a basal, layered zone; a middle, densely welded zone; and an upper, partially welded 

zone. Table 2 summarizes the phenocryst mineralogy and 

thickness of each zone; complete petrographic modal analyses are given in Appendix 1. 

The basal, layered zone, which is less than 3 m thick, is 

commonly covered with colluvium; however, this zone can 

usually be identified in the field by the presence of distinctively colored float. A black vitrophyre occurs locally at the 

base of the zone. Layering in the basal, layered zone results 

from differences in the abundance of phenocrysts as well as 

from color differences (fig. 1 0). 

Several layers within the basal, layered zone are distinctive. The pale-red, phenocryst-rich layer, ranging from 3 

to 5 em thick, contains vapor-phase amphibole and dark, 

acicular crystals in cavities. Deformed glass shards and a 

eutaxitic texture are evident in thin section in the grayishred, phenocryst-poor layer, which ranges from 1 to 5 em 

thick (fig. ll ). The pale-red and light-greenish-gray layer is 

conspicuous in float. Some sanidine phenocrysts in this layer 

are altered, appearing white in hand sample. In some places 

this layer is dark yellowish brown. 

The 70-m-thick middle, densely welded zone is composed of devitrified, light-gray rhyolite with very light gray 

stringers aligned parallel to foliation. A strong flow lineation 

in the plane of the foliation is characteristic of this zone. The 

groundmass contains granular clumps and sheaf like aggregates of an unknown, pale-green, secondary mineral. 

The upper, partially welded zone of the flow-lineated 

rhyolite is pale red and about 40 m thick. It weathers to form 

Figure 8. Flow lineation of the flow-lineated rhyolite. Hammer for scale. 
148 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

Table 2. Zones in the flow-lineated rhyolite. 

!Percent of phenocrysts refers to percent of rock. Mineral percentages are percent of phenocrysts; nd, not determined;*, estimated] 

Zone Phenocrysts 

(thickness) (percent) 

Partially welded zone (40 m) 18-39 

Densely welded zone (70 m) 13- 21 

Layered zone ( <3 m) 

Pale-red, hackly fractured *20 

Pale-red and light-greenish-gray *10 

Grayish- red, phenocryst-poor 4 

Pale-red. phenocryst-rich 45 

Black vitrophyre nd 

Figure 9. Layering observed a few meters above base of flowlineated rhyolite in its middle, densely welded zone. Here layering 

is subparallel with base of flow-lineated rhyolite, but higher in its 

densely welded zone, flow foliation can be at attitudes significantly 

different from those at stratigraphic base and top. Boulder in 

foreground shows flow lineation. Sledge hammer for scale. 

rounded slopes and cavities as large as 6 by 3 m. The upper, 

partially welded zone contains 1-15 percent grayish-red 

trachytic inclusions as much as 2m long; many of these are 

Quartz Sanidine Ferromagnesian and opaque 

(percent) (percent) (percent) 

23- 29 57-73 12-14 

29-37 59- 71 3-4 

nd nd nd 

*30 *65 *5 

54 40 6 

39 60 

nd nd nd 

Figure 10. Distinctive layering in basal, layered zone of flowlineated rhyolite. The grayish-red (2) and light-greenish-gray (3) 

layers are crystal poor relative to the pale-red layer (I) near the top 

of this sample. Nickel for scale. 

deformed with their long axes parallel to the foliation of the 

rock, suggestive of a cognate origin (fig. 12). This zone 

contains abundant microscopic cavities partially filled with 
KANE SPRINGS WASH CALDERA, MEADOW VALLEY MOUNTAINS 149 

Figure 11. Photomicrograph of eutaxitic glass shard texture in basal, layered zone of tlow-lineated rhyolite. Quartz 

phenocryst at left of photo. Plane light; view is 1.26 mm wide. 

vapor-phase amphibole and dark acicular amphibole-like 

crystals. Secondary quartz completely fills the cavities 

between vapor-phase growths. One thin section contains 

lustrous, euhedral, green vapor-phase amphiboles that line 

cavities. Also, sheaflike aggregates of an unknown, palegreen, secondary mineral occur in the groundmass. 

The flow-lineated and columnar-jointed rhyolites are 

nearly indistinguishable in hand sample and in the field. In 

faulted areas, discriminating between the two units can be 

especially difficult. Both have similar types, sizes, and relative proportions of phenocrysts; however, the flow-lineated 

rhyolite has a higher percentage of phenocrysts (compare 

tables 1 and 2). Although near their tops both units have trachytic inclusions, they appear to be more abundant in the 

flow-lineated rhyolite. The flow-lineated rhyolite is interpreted to be an ash-flow tuff for the same reasons given for 

the columnar-jointed rhyolite. 

Several discontinuous, thin tuffaceous layers are 

present above the flow-lineated rhyolite, each less than 5 m 

thick; these layers were mapped as part of the flow-lineated 

rhyolite. They include a yellowish-gray tuffaceous sandstone, a light-greenish-gray nonwelded tuff, and a lightbrownish-gray nonwelded tuff. 

LOWER BASALT FLOW 

A thin, olivine-bearing basalt flow lies above the flowlineated rhyolite nearly everywhere in the northern area. The 

best exposures of the lower basalt flow are in gullies, where 

the unit is grayish black and generally massive. Elsewhere, 

the lower basalt flow is discontinuously exposed under 

colluvium. The flow is typically less than a few meters thick, 

but toward the north it reaches a thickness of 25 m. The 

lower basalt flow contains fewer than 2 percent plagioclase 

and altered olivine phenocrysts in a groundmass of a 

subophitic intergrowth of microcrystalline plagioclase and 

clinopyroxene. 

GRAY ASH-FLOW TUFF 

Next in the stratigraphic sequence is a gray devitrified 

rhyolitic ash-flow tuff that forms a ledge and an overlying 

gentle slope. This welded tuff contains phenocrysts of quartz, 

sanidine, plagioclase, biotite, altered ferromagnesian and 

opaque minerals, and accessory zircon. Basaltic lithic fragments from tenths of millimeters to 5 em long give the rock 

a characteristic spotted appearance, although they form less 
150 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

Figure 12. Trachytic inclusions in upper, partially welded zone of flow-lineated rhyolite. This boulder has rolled out of place. Note 

that inclusions are deformed with their long axes parallel to the foliation of rock. Sledge hammer for scale. 

than 1 percent of the tuff. The gray ash-flow tuff forms a distinctive topographic break between the underlying basalt and 

the overlying tuff. The gray ash-flow tuff is thin, ranging 

from 5 to 25 m thick, and it consists of two zones: a lower, 

moderately welded zone and an upper, partially welded zone. 

The lower, moderately welded zone of the gray ash-flow 

tuff is grayish orange pink with moderate-brown basaltic 

lithic fragments; it forms a ledge as much as 15 m thick. The 

tuff contains 14 percent phenocrysts that consist of 10 percent quartz, 71 percent sanidine, less than 1 percent plagioclase, a trace of biotite, and 19 percent altered 

ferromagnesian and opaque minerals (Appendix 1). Some of 

the sanidine phenocrysts are adularesoent. Deformed glass 

shards and a eutaxitic texture can be observed in thin section. 

The upper, partially welded zone is very light gray a~d 

forms gentle slopes. Typically, the upper zone is characterized by elongate, rounded exposures just above the lower, 

moderately welded zone (fig. 13). The maximum thickness 

of the upper, partially welded zone is about 10m. The tuff in 

this zone contains 19 percent phenocrysts that consist of 22 

percent quartz, 72 percent sanidine, 3 percent plagioclase, a 

trace of biotite, and 3 percent altered ferromagnesian and 

opaque minerals (Appendix 1 ). Some of the altered ferromagnesian and opaque minerals can be identified as biotite 

on the basis of phenocryst shape and relict centers. 

YELLOW TUFF 

Above the gray ash-flow tuff, a thick, crudely bedded, 

nonwelded to partially welded yellow tuff forms distinctive 

steep, grayish-yellow, rounded slopes. Deposition of the 

yellow tuff spanned the eruption of two rhyolite lava 

flows-a red lithic rhyolite and a biotite rhyolite-and 

minor sedimentary tuffs and welded ash-flow tuffs (fig. 14). 

At least three discontinuous, minor tuffaceous layers are 

present in the basal part of the yellow tuff; these are a tuffaceous sandstone and two densely welded ash-flow tuffs 

(Harding, 1991). 

The yellow tuff is devitrified and is very pale orange, 

grayish orange pink, and grayish yellow. It contains phenocrysts of quartz, sanidine, plagioclase, biotite, and altered ferromagnesian and opaque minerals. The tuff contains abundant 

pumice and lithic fragments, many of which are devitrified 

rhyolite. Numerous crudely bedded ash-fall layers, 0.5-2 m 

thick, are present in several places. At the southern limit of 

its exposure, the yellow tuff is undivided and ranges from 0 
KANE SPRINGS WASH CALDERA, MEADOW VALLEY MOUNTAINS 151 

Figure 13. Typical exposure of upper, partially welded zone of gray ash-flow tuff. 

NE sw 

Tyt 

Figure 14. Diagrammatic sketch of probable stratigraphic relationships between the yellow tuff and rhyolite lava flows that erupted during accumulation of the yellow tuff. Dashed line 

approximates present erosion surface. Not to scale. Tgt, gray ashflow tuff; Tytl, lower part of the yellow tuff; Trlr, red lithic rhyolite; 

Tbr, biotite-rhyolite flow; Tytu, upper part of the yellow tuff; Tyt, 

yellow tuff, undivided; Tfb, flow-banded rhyolite. 

to 180m thick. To the north, it is physically separated into an 

upper and a lower part by rhyolite lava flows. The lower part 

of the yellow tuff ranges from 0 to 15 m thick, whereas the 

upper part of the yellow tuff reaches a maximum thickness 

of I 00 m. The upper and lower parts are indistinguishable 

except where intervening rhyolite flows are present. 

The phenocryst content of the yellow tuff ranges from 

2 to 6 percent and consists of 31- 60 percent quartz, 21- 57 

percent sanidine, 2-11 percent plagioclase, a trace of biotite, 

and 1-10 percent altered ferromagnesian and opaque minerals (Appendix 1 ). The tuff contains 5-10 percent lithic fragments and abundant pumice fragments. 

RED LITHIC RHYOLITE 

The red lithic rhyolite occurs within the yellow tuff and 

separates it into lower and upper parts throughout much of 

the northern study area. The red lithic rhyolite ranges in 

thickness from 0 to about 85 m. Where this unit is thin, it 

forms a ledge; where it is thick, it forms gentle slopes to 

steep cliffs. The red lithic rhyolite is pale red to dusky red 

and contains grayish-red basaltic lithic fragments as much as 

15 em long. The size and abundance of lithic fragments 

change greatly among localities. Flow-banding in the rhyolite is thin, ranging from I to 10 mm. The red lithic rhyolite 

is massive but autobrecciated on a fine scale, so that fragments of flow-banded material are contained in a rhyolite 

matrix. 

The red lithic rhyolite contains only 2-3 percent phenocrysts, but abundant devitrification centers give the 
152 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

appearance of a greater phenocryst content. The phenocrysts 

consist of 22-43 percent quartz, 38-69 percent sanidine, 

1-19 percent plagioclase, and less than I percent opaque 

minerals (Appendix l ). 

Several features indicate that the red lithic rhyolite is a 

lava flow rather than a welded tuff that flowed after emplacement. (See Bonnichsen and Kauffman, 1987.) These features 

are (I) an abrupt lateral termination of the rhyolite, (2) the 

presence of flow layering that was brecciated after cooling, 

(3) chaotic, tightly folded flow layering, (4) an absence of 

pumice fragments and glass shards, (5) a lack of internal subhorizontal layering indicative of multiple ash emplacements, 

and (6) an absence of vertical zonation in phenocryst type or 

abundance. 

BIOTITE-RHYOLITE FLOW 

A biotite-rhyolite lava flow was emplaced above the 

lower part of the yellow tuff in some places and above the 

red lithic rhyolite in others. This unit is a pinkish-gray to 

pale-red-purple, devitrified, massive lava flow that displays 

columnar jointing in vertical cliffs. Locally, it has a vitrophyric base about 20 em thick. The biotite-rhyolite flow is 

exposed along the eastern edge of the study area, where it is 

as much as 55 m thick; it pinches out to the north. 

The biotite-rhyolite flow contains 17 percent phenocrysts that consist of 31 percent quartz, 46 percent sanidine, 

18 percent plagioclase, 5 percent biotite, a trace of amphibole, and less than l percent opaque minerals (Appendix l; 

fig. 15). The biotite-rhyolite flow contains accessory phenocrystic zircon. 

PUMICE-RICH ASH-FLOW TUFF 

A nonwelded to moderately welded, vitric, rhyolitic 

ash-flow tuff that contains conspicuous blackish-red to darkgray pumice fiamme lies above the Kane Wash Tuff in the 

southern part of the study area. The pumice-rich ash-flow 

tuff, which is pale red, grayish orange pink, and pale reddish 

brown, forms gentle to moderate slopes and is as much as 35 

m thick. The non welded part of this tuff contains lithic fragments of devitrified rhyolite and white pumice fragments 

that range from 0.1 to 15 em long. The tuff forms wedgeshaped layers between the underlying, more steeply dipping 

Kane Wash Tuff and the overlying, less steeply dipping 

upper basalt. Locally, a thin, biotite-bearing tuffaceous sandstone is present between the pumice-rich ash-flow tuff and 

the overlying basalt and is included within this map unit. 

The pumice-rich ash-flow tuff contains 7 percent phenocrysts that consist of 12 percent quartz, 25 percent sanidine, 42 percent plagioclase, 5 percent biotite, 3 percent 

hornblende, 4 percent clinopyroxene, 9 percent opaque minerals, and 0.5 percent zircon. The phenocrysts are unusually 

Figure 15. Photomicrograph of some of the phenocryst minerals 

characteristic of biotite-rhyolite flow. Q, quartz; S, sanidine; P, plagioclase; B, biotite. Plane light; view is 1.26 mm high. 

small, from less than 0.1 to 0.5 mm. Eighty-five percent of 

the groundmass consists of pumice fragments that range 

from 0.25 to 0.8 mm across. 

Although this tuff is observed only in the southern part 

of the study area, it is included with the northern stratigraphic sequence because the tuff cannot be correlated with 

regional ash-flow tuffs; the large size of pumice fragments in 

the tuff suggests it had a local source; and the tuff underlies 

a series of rhyolitic lava flows and basalt flows, some of 

which overlie the boundary between the northern and southern stratigraphic sequences. 

FLOW-BANDED RHYOLITE 

Two aphyric rhyolite lava flows form steep slopes 

above the yellow tuff in the northern part of the study area 

and continue unbroken southward over the boundary 

between the two stratigraphic sequences to overlie the Kane 

Wash Tuff (fig. 3). The lower flow is thinly flow banded in 
KANE SPRINGS WASH CALDERA, MEADOW VALLEY MOUNTAINS 153 

layers 1-2 mm thick and is more widespread than the upper 

massive flow. Both flows of the unit locally have 5-m-thick 

glassy bases; distinctive Apache tears erode from the basal 

part of the lower flow. The de vitrified part of this flow is 

very light gray, light brownish gray, and pale red. The flowbanded rhyolite is as thick as 110 m and pinches out to the 

south. 

New whole-rock K-Ar dates are 13.7±0.5 and 12.9±0.7 

Ma for devitrified and vitric parts of the flow-banded rhyolite, respectively; the vitric material has probably lost argon, 

resulting in a date that is too young, and therefore the older 

age is considered to be more representative (analyses by 

H.H. Mehnert) (Appendix 3). 

MIDDLE BASALT FLOW 

A 15-m-thick, dark-gray basalt flow that contains about 

5 percent olivine phenocrysts occurs at one locality above 

the flow-banded rhyolite in the central eastern part of the 

study area. The flow has scoriaceous upper and lower zones 

and a massive middle zone, all of which form moderate to 

steep slopes. Although the basalt lies south of the projected 

boundary between the northern and southern stratigraphic 

sequences, it is between units of the northern stratigraphic 

sequence that overlie the boundary; therefore, it is included 

as part of the northern sequence. 

LITHOPHYSAL RHYOLITE FLOW 

A light-brownish-gray, devitrified, massive rhyolite 

flow forms moderate to steep slopes above the middle basalt 

flow at one locality in the central eastern part of the study 

area. The lithophysal rhyolite flow contains about 10 percent 

lithophysal cavities 1-2 em in diameter. The lithophysae are 

coated with concentric layers of fine-grained vapor-phase 

crystals. This rhyolite is nearly aphyric, containing less than 

1 percent phenocrysts that consist of hornblende and opaque 

minerals. The lithophysal flow is considered to be part of the 

northern stratigraphic sequence for the same reasons given 

for the middle basalt flow. 

YOUNGEST RHYOLITE FLOWS 

One exposure of rhyolite lava flow occurs on each side 

of Kane Springs Valley in the northern part of the study area; 

based on stratigraphic relationships found elsewhere, this is 

the youngest unit filling the Kane Springs Wash caldera in the 

Delamar Mountains (Scott, Harding, and others, 1991 ). Most 

of this unit is vitric; some zones contain devitrification spherulites that are from 0.5 to 2 em in diameter. The youngest rhyolite flows are medium light gray to light gray, are massive 

to flow banded, and contain 2-5 percent phenocrysts of 

quartz, sanidine, biotite, and amphibole. The rhyolite flows 

have a K-Ar sanidine date of 13.3±0.4 Ma (Novak, 1984). 

UPPER BASALT FLOW 

Capping both the northern and southern stratigraphic 

sequences are dark-gray to medium-dark-gray basalt and 

light-brownish-gray basaltic andesite(?) flows that form 

gentle to steep slopes and cliffs. The upper basalt flow 

contains sparse plagioclase phenocrysts in a groundmass of 

flow-oriented plagioclase laths and equant olivine. K-Ar 

whole-rock dates for the basalt are 11.4±0.4 and 11.6±0.4 

Ma (Novak, 1984 ). 

In the eastern part of the study area, this unit forms a 

basaltic volcano approximately centered over the highest 

topographic point in the map area (shown by x in fig. 3). The 

flanks of the volcano consist exclusively of massive and 

vesicular dark-gray basalt flows. Northwest of the topographic high, deep erosion has exposed light-brownish-gray 

basaltic andesite(?) and blackish-red, dusky-red, and moderate-red, highly scoriaceous mounds of basaltic ejecta intercalated with dark-gray basalt flows. This heterogeneous 

sequence of eruptive products is probably close to the volcanic vent for this unit. 

INTRUSIVE ROCKS IN THE 

NORTHERN AREA 

RHYOLITE DIKES 

Several rhyolite dikes have intruded rocks in the 

northern part of the study area; one rhyolite dike extends 

about 550 m southwest of the boundary between the northern 

and southern stratigraphic sequences (fig. 3 ). Three types of 

rhyolite dikes are present: ( 1) aphyric rhyolite dikes, (2) 

biotite-absent rhyolite dikes containing quartz and (or) 

feldspar phenocrysts, and (3) biotite-bearing rhyolite dikes 

(complete descriptions in Harding, 1991 ). 

Several biotite-bearing rhyolite dikes, which are 

present within and just to the east of the study area, form 

sharp ridges and are typically 2-5 m thick. The dikes are 

massive, devitrified, and very light gray to pale red purple. 

The new K-Ar biotite date of one of these dikes is 13.1 ±0.5 

Ma, and the sanidine date is 12.8±0.5 Ma (analyses by H.H. 

Mehnert) (Appendix 3). 

The biotite-bearing rhyolite dikes and the biotiterhyolite t1ow were initially assumed to have originated from 

the same source because they contain similar types and 

abundances of phenocrysts. However, in the eastern part of 

the study area, a biotite-bearing rhyolite dike intrudes the 

flow-banded rhyolite, which is younger than the biotiterhyolite flow (fig. 14). Two (or more) similar sources or, 

more likely, a single long-lived magmatic source may have 
154 GEOLOGIC STUDIES. BARCO STUDY UNIT. 1992 

produced both the biotite-rhyolite flow and the biotitebearing rhyolite dikes. 

VOLCANIC CENTER 

In the northern part of the study area, a rhyolitic volcanic center consisting of a capping rhyolite lava flow, a lithicrich tuff, a vitrophyric lava flow, and cross-cutting dikes, 

truncates older shallowly dipping volcanic rocks along 

steeply dipping contacts. The volcanic center, which is 

exposed in steep slopes, is about 100 m thick and 250 m 

wide. Rocks in the volcanic center cannot be correlated with 

other map units; they probably represent a local eruptive 

center. 

The capping rhyolite lava flow is devitrified, and contains quartz, sanidine, plagioclase, and ferromagnesian and 

opaque phenocrysts. This flow is very light gray and has 

pale-red-purple streaks. A vitrophyre of a petrographically 

similar rhyolite is present along the northwestern edge of the 

unit. The rhyolite contains 6-8 percent phenocrysts that consist of 39-48 percent quartz, 43-55 percent sanidine, 4-7 

percent plagioclase, and 2-5 percent mafic and opaque minerals (Appendix 1). Amphibole and clinopyroxene phenocrysts are observed in thin section, as well as shattered quartz 

and sanidine phenocrysts. 

Between the devitrified lava flow and the vitrophyre, a 

nonbedded and nonwelded, very light gray to pinkish-gray 

tuff contains rhyolitic lithic fragments as large as 1.5 m in 

diameter, some of which resemble volcanic bombs. In some 

places the tuff contains as much as 80 percent lithic fragments. The tuff is cut by a 1.5-m-wide, grayish-pink, devitrified rhyolite dike containing about 3 percent phenocrysts of 

7 5 percent quartz and 25 percent sanidine. Under the tuff, a 

complex network of cross-cutting dikes is poorly exposed in 

talus slopes. These dikes are partly devitrified and partly vitric; they contain quartz, sanidine, and in one case, biotite 

phenocrysts. 

CHEMISTRY 

Chemical analyses of representative samples of rocks 

from the northern stratigraphic sequence provide data to classify these previously uninvestigated rocks that had not been 

correlated with regional ash-flow tuffs. The chemical classifications of Shand (1951) and Macdonald (197 4) are used. 

Major-element chemistries (Appendix 2B) separate 

silicic rocks of the northern stratigraphic sequence into 

weakly peralkaline and metaluminous groups (table 3). The 

lower ash-flow tuffs, the columnar-jointed and flow-lineated 

rhyolites, can be further classified as comenditic (fig. 16), 

which indicates that the tuffs are transitional between nonperalkaline rhyolites and more strongly peralkaline 

pantellerites. 

Table 3. Classification of rhyolitic rocks in northern stratigraphic 

sequence. 

[PI=peralkalinity index=(molar [Na+Kl/AI). PI=I.O to l.l=weakly peralkalinel 

Unit 

Flow-banded rhyolite 

Biotite-rhyolite flow 

Yellow tuff 

Red lithic rhyolite 

Gray ash-flow tuff 

Flow-lineated rhyolite 

Columnar-jointed rhyolite 

p 

z 

w 

0 

0:: 

~ 13 

II 

C) 

~ 11 

7 

0 

Sample No. 

6AHI8.4.88 

I AH23.1 0.88 

IAHI8.11.88 

2AH16.5.88 

3AH20.1 1.88 

9AHI9.5.88 

7AHI9.5.88 

4AHI9.5.88 

SAH 12.11.88 

2AH 12.11.88 

IAHI9.5.88 

IRON AS FeO 

PI Classification 

0.9 Metaluminous. 

0.8 Metaluminous. 

0.5 Metaluminous. 

0.8 Metaluminous. 

0.9 Metaluminous. 

1.0 Peralkaline. 

1.1 Peralkaline. 

1.0 Peralkaline. 

1.0 Peralkaline. 

1.0 Peralkaline. 

1.0 Peralkaline. 

4 6 

(WEIGHT PERCENT) 

Figure 16. Classification of peralkaline, quartz-normative extrusive rocks (Macdonald, 1974 ). Oxide data from+, columnar-jointed, 

and o. flow-lineated ash-flow tuffs are plotted. 

The lower ash-flow tuffs have very tightly grouped 

enriched REE patterns with large negative Eu anomalies (fig. 

17) as is typical for peralkaline rocks (Ferrara and Treuil, 

1974). In REE diagrams, some of the analyses do not include 

Gd data (Appendix 2C); a value for Gd was estimated visually in order to express the Eu anomalies uniformly. A trachytic inclusion from the upper part of the flow-lineated 

·rhyolite is less enriched in heavy REE and has a distinctly 

smaller Eu anomaly. 

Incompatible-element patterns (spidergrams) were also 

determined according to the method of Thompson ( 1982), 

who chose elements that are incompatible relative to basalts 

and arranged them in decreasing degree of incompatibility 

from left to right. Spider diagrams are used here for comparison only. As in REE patterns, spidergrams for individual 
KANE SPRINGS WASH CALDERA, MEADOW VALLEY MOUNTAINS 155 

samples from the lower part of the northern stratigraphic 

sequence cluster tightly with the exception of the trachytic 

inclusion from the flow-lineated rhyolite (fig. 18 ). The columnar-jointed and flow-lineatedrhyolites are geochemically distinctive for their low relative abundances of Ba, Sr. P, and Ti. 

Units in the upper part of the northern stratigraphic 

sequence show a greater range in REE enrichments than do 

w 

ta:: 

c 

z 

0 

:::t: 

0 

w 

...J 

D.. 

::E 

<( 

tn 

100 

10 Trachytic inclusion 

1 

La Ce Nd Sm Eu Gd Tb Tm Yb Lu 

EXPLANATION 

Columnar-jointed rhyolite 

--D- 4AH19.5.88 

----tr- 5AH12.11.88 

-o- 1AH19.5.88 

Flow-lineated rhyolite 

-+- 1AH21.5.88 

--- 3AH21.5.88 

__._ 9AH19.5.88 

--- 7 AH19.5.88 

Figure 17. REE patterns for units from lower part of northern 

stratigraphic sequence. Results shown for columnar-jointed and 

flow-lineated rhyolites. Elements normalized to chondrite values of 

Hanson ( 1980). 

the lower units, particularly in light rare earths (fig. 19). 

Degrees of light-REE enrichments do not differ systematically with stratigraphic order. A strong negative Eu anomaly 

characterizes these units. The spider diagram for the upper 

units, although similar to that of the lower units, shows more 

variation in element values (fig. 20). In general, the upper 

units are less enriched in trace elements than the lower units, 

w 

~ 

i= 

c 100 

z 

0 

:I: 

0 

w 

.J 10 D. 

:1 

c( 

U) 

1 

La Ce Nd Sm Eu Gd Tb 

EXPLANATION 

- 1AH23.10.88 Td 

__._ 6AH18.4.88 Tfb 

-o- 1AH18.11.88 Tyt 

-<>- 2AH16.5.88 Trlr 

--o--- 3AH20.11.88 Tgt 

Tm Yb Lu 

Figure 19. REE patterns for units from upper part of northern 

stratigraphic sequence. Results are shown for Td, biotite-bearing 

rhyolite dike; Tfb, flow-banded rhyolite; Tyt, yellow tuff; Trlr, red 

lithic rhyolite; Tgt, gray ash-flow tuff. 

Figure 18. Spider diagram for units from lower 

part of northern stratigraphic sequence, the columnar-jointed and flow-lineated rhyolites. Elements 

are normalized to chondritic abundances (except 

for Rb, K, and P which have been normalized to 

terrestrial abundances) according to the model of 

Thompson ( 1982). 

~1000 ~=-~~--~~~~--~~_,-,,-,--r-.--r-,--,~ 

'C 

c 

C'IS 

~ 100 

~ tD.. 

w 

0 

X 

~ 

w 

to:: 

c 

z 

0 

:::t: 

10 

0 0.1 L_~_L __ L_~-L~--~_L_J __ L_~~--~~~~~~~ 

~ &~ThK~Th~~&~P~~lliTITbYTmYb D.. 

::E 

~ EXPLANATION 

Columnar-jointed rhyolite 

--D- 4AH19.5.88 

----tr- 5AH 12.11.88 

-o- 1AH19.5.88 

Flow-lineated rhyolite 

-+- 1AH21.5.88 

------ 3AH21.5.88 

__._ 9AH19.5.88 

------ 7 AH19.5.88 
156 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

~ 1000 r-~-r--,-.--.--,-~~~--~~~--~~~--~~~ Figure 20. Spider diagram for units from upper 

part of northern stratigraphic sequence. Results 

are shown for Td, biotite-bearing rhyolite dike; 

Tfb, flow-banded rhyolite; Tyt, yellow tuff; Trlr, 

red lithic rhyolite; Tgt, gray ash-flow tuff. 

"0 

c 

"' 

~~ 100 

~ ID. 

~ 10 

>< !!:!. 

w 

1-

a: 

c 

z 

0 

J: 

0 

w 

0.1 L___l.___l. __ ..J_____.J____l __ _j____L __ [___L___j_ __ .L___j___L __ _L____l____j_ __ _L__j 

...J 

D. 

:IE 

<C 

UJ 

h~ThKmTh~~&~P~hlliTIThY~Th 

EXPLANATION 

--- 1AH23.10.88 Td 

___.___ 6AH18.4.88 Tfb 

-o- 1AH18.11.88 Tyt 

---<>--- 2AH16.5.88 Trlr 

-o- 3AH20.11.88 Tgt 

with the exception of Zr and Hf, which are much less 

enriched, and of Ba and Sr, which are less depleted. The red 

lithic rhyolite is distinctive from the other upper units by 

being significantly higher in Sr and Ti. This difference may 

reflect contributions from small ( < 2 mm) basaltic inclusions 

that could not be completely removed from the sample. 

STRUCTURE 

Deformation in the study area was accommodated by 

normal and strike-slip faulting and tilting of strata. In 

addition, numerous dikes cut the rocks of the northern part of 

the study area. 

Most faults strike north to northeast and dip steeply 

west and northwest; the greatest population of these faults 

strikes about N. 25° E., essentially parallel to the range-front 

fault (fig. 21 ). A secondary group strikes slightly west of 

north and dips west; only a few north-striking faults dip 

steeply east. A few faults strike nearly east-west and dip 

steeply south. 

Twenty of the observed fault surfaces have slickenlines 

that constrain the directions of fault slip (fig. 22). Most of the 

faults have stratigraphic offsets that require a component of 

normal dip slip; those faults without stratigraphic constraints 

are assumed to also have normal fault components. Five 

faults have slickenlines that indicate pure dip-slip movement, but movement on the remaining 15 faults also includes 

a component of strike slip. Of those oblique-slip faults, eight 

strike more than 22° east of north and exhibit left-lateral slip; 

two strike more than 3 o west of north and exhibit right -lateral 

N 

w E 

s 

Figure 21. Equal-area lower hemisphere projection. Contours express the relative density of poles to 55 measurements of fault-plane 

attitudes within the area. 

slip; and the intervening 25° segment contains an overlap of 

both left-lateral and right-lateral slip (figs. 22, 23). Strands of 

the northeast-striking Kane Springs Wash fault also display 

left-lateral oblique slip (fig. 3). (See plate I, Harding, 1991.) 

Most dikes strike northeast, similar to the strike of most 

of the measured faults, whereas a smaller group strikes about 
w 

KANE SPRINGS WASH CALDERA, MEADOW VALLEY MOUNTAINS 

N 

s 

EXPLANAnON 

1 Trend and plunge of left-lateral oblique slickenlines 

~ Trend and plunge of right-lateral oblique slickenlines 

j Trend and plunge of dip-slip slickenlines 

E 

Figure 22. Equal-angle lower hemisphere projection of 20 fault planes showing direction and sense of slip, as determined 

from slickenlines. 

157 
158 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

N 

Figure 23. Rose diagram showing strikes of faults with leftlateral oblique-slip movement (northeast diagonals) and rightlateral oblique-slip movement (northwest diagonals). Overlap is 

cross-hatched. Full radius=five faults. Dip-slip faults are omitted 

here. 

N. 10° W. (fig. 24). Vertical offset of strata across some 

dikes and a similarity between dike and fault trends suggest 

that other dikes also intruded along faults (fig. 3). Brecciation of one dike on fault surfaces parallel to the dike suggests 

that fault movement renewed after dike emplacement. 

Throughout the study area, strata dip southeastward, 

presumably as a result of tilting during extensional deformation. In the southern part of the study area, older strata (Hiko 

and Delamar Lake Tuffs) dip about 25°-30° whereas the 

youngest capping basalts dip about 5° (fig. 3 ). In the northern 

area a similar relationship is found, although it is less obvious in the field because contrasts in dip are difficult to 

observe there. 

DISCUSSION 

CALDERA WALL 

The breccia that locally marks the boundary between 

the northern and southern stratigraphic sections (fig. 3) has 

characteristics that are commonly associated with caldera 

collapse (Lipman, 1976). These features, described previously in the stratigraphy section, include a massive to crude 

bedding that dips approximately at the angle of repose away 

from the regional ash-flow tuffs of the southern sequence, an 

absence of shear in the breccia parallel to the boundary 

between the northern and southern sequences, and the buttress unconformity geometry of the lateral margin of the 

flow-lineated rhyolite chilled against the breccia. Based on 

these features, the breccia unit (Twb, fig. 3) is interpreted to 

be caldera-wall breccia. 

This part of the inferred caldera cannot be older than the 

14.7-Ma Grapevine Spring Member of the Kane Wash Tuff 

because clasts derived from the Grapevine Spring are 

included in the breccia. The upper part of the breccia was 

being deposited coevally with the 14.4-Ma flow-lineated 

N 

Figure 24. Rose diagram showing trends of 25 dikes. Largest 

cluster represents 11 dikes. Full radius= 15 dikes. 

rhyolite. Also, the caldera cannot be younger than the 

13.7-Ma flow-banded rhyolite that stratigraphically overlies 

the caldera wall (fig. 3 ). Thus, field relations and dating 

indicate that the breccia is between 14.7 and 14.4 Ma and 

that it marks the topographic wall of either the Kane Springs 

Wash caldera or another caldera that formed between 14.7 

and 13.7 Ma. Because no outflow sheets younger than the 

Kane Wash Tuff have been mapped in the study area, the 

Kane Springs Wash caldera is the most likely candidate. 

CORRELATION OF THE NORTHERN 

STRATIGRAPHIC SEQUENCE 

Results of petrographic, chemical, and geochronologic 

studies of the northern stratigraphic sequence were compared with results of studies of Kane Wash Tuff outflow 

sheets and rocks from the Kane Springs Wash caldera in the 

Delamar Mountains to test whether this caldera extends into 

the Meadow Valley Mountains. 

The lower ash-flow tuffs of the northern stratigraphic 

sequence, the columnar-jointed and flow-lineated rhyolites 

(Tcj and Tfl, respectively, in fig. 3) are mineralogically similar to the Kane Wash Tuff. Quartz and sanidine are the dominant phenocrysts in both cases, and these minerals are 

present in similar ranges of abundances; plagioclase and 

biotite are absent. Novak (1984) reported hedenbergite, fayalitic olivine, and opaque minerals as ferromagnesian phenocrysts, and amethyst and riebeckite as vapor-phase 

minerals in the Gregerson Basin Member (equivalent to 

members V2 and V3 of Novak's Kane Wash Tuff). Although 

most ferromagnesian minerals in the columnar-jointed and 

flow-lineated rhyolites have been altered beyond recognition 

in the devitrified tuff, olivine (probably fayalitic) and clinopyroxene were observed in a few thin sections from the 

chilled margins (fig. 6). The clinopyroxene is green and 

resembles what Novak (1984) identified as hedenbergite in 

his Kane Wash Tuff. Dark-blue, vapor-phase riebeckite(?) 

occurs in a few thin sections of the columnar-jointed rhyolite. 
KANE SPRINGS WASH CALDERA, MEADOW VALLEY MOUNTAINS 

The mineralogical similarity between the Kane Wash 

Tuff and the lower tuffs of the northern stratigraphic 

sequence can be attributed to their chemical similarity. Both 

the Gregerson Basin Member of the Kane Wash Tuff and the 

lower tuffs of the northern stratigraphic sequence are mildly 

peralkaline comendites. Trace-element abundances for the 

two groups of rocks are indistinguishable in the REE (fig. 

25) and spider diagrams (fig. 26). The chemical equivalence 

of these two groups of rocks indicates that the lower ashflow tuffs of the northern stratigraphic sequence could be 

equivalent to the intracaldera facies of the Gregerson Basin 

Member of the Kane Wash Tuff. 

w 

1-

il2 

c 

z 

0 

J: 

0 

w 

..J 

a.. 

:::!: 

<( 

UJ 

1000 

100 

10 

La Ce Nd Sm Eu Gd Tb 

159 

Tm Yb Lu 

Conventional K-Ar dates for the columnar-jointed and 

flow-lineated rhyolites overlap those for the Kane Wash Tuff 

within analytical uncertainty (table 4). This overlap is 

increased if an interlaboratory difference of about 0.4 m.y. is 

added to Novak's dates (Scott and others, this volume). 

Moreover, the 40 Art39 Ar date for the flow-lineated rhyolite 

(14.43±0.14 Ma) is comparable to the 40Arf39Ar dates for 

the lower and upper cooling units of the Gregerson Basin 

Member (14.55±0.14 and 14.39±0.28 Ma, respectively). 

These dates further confirm the proposed correlation. 

EXPLANATION 

Northern stratigraphic sequence 

lower units 

----- 3AH21.5.88 

--*- 9AH19.5.88 

___._ 7AH19.5.88 

---+- 4AH19.5.88 

--- 5AH12.11.88 

-t-- 1AH19.5.88 

Kane Wash Tuff 

-o- 58519.5.89 

-tr- 98 

---o- 28527.3.90 

--o- 89 

-<>-- 2AH24.3.90 

- 38527.3.90 

Thus, it is likely that the uppermost intracaldera-facies --7<C- 245 

unit, the flow-lineated rhyolite (Tfl in fig. 3), is equivalent to 

the uppermost outflow-facies unit, the upper cooling unit of 

the Gregerson Basin Member (upper cooling unit of Tkb in 

fig. 3). The next lower intracaldera-facies unit, the columnar-jointed rhyolite (Tcj in fig. 3), is then equivalent to the 

Figure 25. REE patterns for lower units of northern stratigraphic 

sequence (flow-lineated and columnar-jointed rhyolites) and members of the Kane Wash Tuff. Data for samples 245, 98, and 89 from 

Novak and Mahood (1986). 

~ 1000 ~~~~--~~~--~~~~~~~~--~~~--,-~ 

"'0 

c 

ca 

~~ 

~ .... 

c.. 

w 

0 

X 

~ 

w 

.... 

~ 

0 

z 

0 

J: 

0 

w 

...J 

c.. 

:E 

c( 

U) 

100 

10 

0.1 

&~ThK~Th~~&~P~~lliTIThY~Th 

EXPLANATION 

Northern stratigraphic sequence 

lower units 

---- 4AH19.5.88 -+- 3AH21.5.88 

~ 5AH12.11.88 --9AH19.5.88 

---- 1AH19.5.88 ---- 7AH19.5.88 

Kane Wash Tuff 

-fr--- 58519.5.89 

--o-- 28527.3.90 

--o- 2AH24.3.90 

--D-- 38527.3.90 

Figure 26. Spider diagram for lower units of northern stratigraphic sequence (flowlineated and columnar-jointed rhyolites) and members of the Kane Wash Tuff. 
160 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

Table 4. Comparison of dates (in Ma) for lower units of northern stratigraphic sequence with dates 

for the Kane Wash Tuff. 

[Data analyst heads each column. Tfl, tlow-lineated rhyolite; Tcj, columnar-jointed rhyolite; Tkbu, upper cooling unit of the 

Gregerson Basin Member of the Kane Wash Tuff; Tkbl, lower cooling unit of the Gregerson Basin Member of the Kane Wash 

Tuff; Tkg, Grapevine Spring Member of the Kane Wash Tuff] 

Northern stratigraphic sequence 

K-Ar 40 Ar;39 Ar 

Mehnert Snee 

Tfl 14.6±0.4+ 14.43±0.14+ 

14.9±0.5+ 

Tcj 14.6±0.5+ 

+see Appendix 3 for analytical data. 

#From Novak (1984). 

* See Scott and others (this volume) for analytical data. 

lower cooling unit of the Gregerson Basin Member. This 

correlation is further strengthened by the chemical zonation 

from comenditic lower parts to trachytic upper parts of both 

the cooling units of the Kane Wash Tuff and the intracaldera 

equivalents. Sample 1AH21.5.88 (marked trachytic inclusion) in figures 17 and 18 is representative of this upper part 

in the flow-lineated rhyolite (Appendix 2). 

If the lower ash-flow tuffs of the northern sequence are 

intracaldera facies, then the upper units of the northern 

sequence are probably caldera fill. Based on conventional KAr dates and their analytical uncertainties, the upper units of 

the northern stratigraphic sequence (13.7±0.5 to 12.8±0.5 

Ma, Appendix 3) are coeval with Kane Springs Wash caldera 

fill (14.4±0.6 to 12.5±0.4 Ma) (Novak, 1984 ). This correlation is further strengthened by the metaluminous character of 

both the upper part of the northern sequence (table 3) and the 

caldera fill in the Delamar Mountains (Novak, 1984). Novak 

divided the caldera-fill sequence in the Delamar Mountains 

into edenite-bearing early fill and biotite-bearing late fill. 

Caldera fill in the Meadow Valley Mountains contains 

biotite and plagioclase in addition to sanidine and quartz 

phenocrysts, as does late caldera fill of the Delamar Mountains. However, caldera fill in the Meadow Valley Mountains is slightly chemically distinct from late fill and more 

similar to early fill in the Delamar Mountains, based on spider diagrams (figs. 27, 28). Mineralogical evidence of correlation between late Delamar Mountains fill and Meadow 

Valley Mountains fill is probably strong enough to outweigh 

the small differences in incompatible-element compositions. 

The correlation of the lithic-rich tuff remains unresolved. This lowest ash-flow tuff of the intracaldera facies in 

the Meadow Valley Mountains has a high Zr content (946 

ppm; Appendix 2A). This composition is most similar to the 

mildly peralkaline composition of the Gregerson Basin 

Member, rather than to that of the Grapevine Spring Member 

of the Kane Wash Tuff, which has Zr values that range from 

497 to 733 ppm (Appendix 2A and Novak and Mahood, 

Kane Wash Tuff 

K-Ar 40Ar/39 Ar 

Novak Snee 

Tkbu 14.39±0.28 * 

Tkbl 14.1±0.4# 14.55±0.14 * 

Tkg 14.0±0.4# 14.67±0.22 * 

14.2±0.4# 

1986). No correlative lithic-rich uppermost part of an outflow cooling unit has been observed (Scott and others, this 

volume), but the upper parts of intracaldera equivalents may 

have been restricted to the caldera. 

In the Meadow Valley Mountains part of the caldera, 

post-collapse volcanic units filled and overflowed the 

caldera wall, with many of the latest units pouring onto the 

outflow facies, including the pumice-rich tuff (Tpr), the 

flow-banded rhyolite (Tfb), possibly the middle basalt (Tmb) 

and lithophysal rhyolite (Tir), and the upper basalt flow (Tb) 

(fig. 3). The flow-banded rhyolite directly overlies the yellow tuff in the north and the Kane Wash Tuff in the south. 

Basalt flows (Tb) capping the stratigraphic sequence also 

overlie caldera fill and caldera outflow facies (fig. 3). The 

thickest accumulation of basalt and the probable volcanic· 

vent for the flows are located just inboard from the projected 

topographic caldera wall (fig. 3); the basaltic magmas may 

have used concealed caldera ring fractures as conduits to the 

surface. 

STRUCTURAL IMPLICATIONS 

Several important structural implications follow from 

the interpretation that the intracaldera facies and caldera fill 

in the Meadow Valley Mountains are interpreted to represent 

the eastern extension of the Kane Springs Wash caldera. The 

eastern projection of the southern wall of the caldera from 

the Delamar Mountains into Kane Wash Valley is sinistrally 

offset at least 4 km and probably closer to 7 km from its continuation in the Meadow Valley Mountains (fig. 29). This 

confirms the earlier prediction of 8 km of sinistral offset on 

the Kane Springs Wash fault by Ekren and others ( 1977) and 

the generalized hypothesis of E.H. Pampeyan (written commun., 1989). 

Location of the fault strands on which this left -lateral 

movement occurred is constrained by two exposures of the 
KANE SPRINGS WASH CALDERA, MEADOW VALLEY MOUNTAINS 

~ 1 000 r-~-=----,-.--,-----,--,-----,---r-,-----,--------,-r---.---..------,----.-------:J 

'C 

r:::: 

ra 

~ 100 

~ 1-

a. 

w 

(J 

>< w 

w 

.... 

~ 

c 

z 

0 

:::t 

(J 

w 

...J 

a. 

::! 

< en 

10 

0.1 l____l__~__j _ __L___.L_____j_ _ _j___j_____l__l____J___l__L____L______L___j _ _j___j 

&~ThK~Th~~&~P~blliTIThY~Th 

EXPLANATION 

Caldera fill 

Meadow Valley Mountains 

----- 1AH23.10.88 

-A- 6AH18.4.88 

----- 1AH18.11.88 

--+- 2AH16.5.88 

---- 3AH20.11.88 

Early caldera fill 

Delamar Mountains 

-o-- N342 

----!'s- N335 

-o-- N295 

-o-- N354 

Figure 27. Spider diagram for caldera-fill units in Meadow Valley Mountains and 

early caldera-fill units in Delamar Mountains. Data for samples N342, N335, N295, 

and N354 from Novak and Mahood ( 1986). 

ii:' 1 0000 o:---.--.----,-,---,-----,-,-----,-------,-r---.--.----r-.---.-----,-,------:1 

'C 

c: 

ra 

~ 1000 

~ .... 

a. 

w 

(J 

>< 

~ 

w 

.... 

~ 

c 

z 

0 

::1: 

(J 

w 

...J 

a. 

::! 

< UJ 

100 

10 

0.1 

&~ThK~Th~~&~P~blliTIThY~Th 

EXPLANATION 

Caldera fill 

Meadow Valley Mountains 

----- 1 AH23.1 0.88 

-A- 6AH18.4.88 

----- 1AH18.11.88 

--+- 2AH16.5.88 

---- 3AH20.11.88 

~te caldera fill 

Delamar Mountains 

-o-- N284 

----!'s- N352 

Figure 28. Spider diagram for caldera-fill units in Meadow Valley Mountains and late 

caldera-fill units in Delamar Mountains. Data for samples N284 and N352 from Novak 

and Mahood (1986). 

161 
162 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

>-

~ 

QTa ~ ~ ~ w Tot 0 ! 

.f 

10 20 KILOMETERS 

Figure 29 (above and facing page). Generalized geology of northern Meadow Valley and Delamar Mountains and vicinity. Geology 

outside of study area from R.B. Scott (unpub. mapping, 1991) and Ekren and others (1977). 

youngest rhyolite flows along the northern edge of the 

study area (unit Try, fig. 3). This unit, which is the youngest 

Kane Springs Wash caldera fill in the Delamar Mountains, 

does not appear to have been offset along the Kane Springs 

Wash fault. Therefore, most of the left-lateral movement 

occurred on fault strands east of these exposures. The 

method of regional accommodation of the 4-7 km of leftlateral slip on the northern projection of the Kane Springs 

Wash fault is limited by the apparent lack of offset on the 

margin of the older Caliente caldera complex (fig. 29). 

Probably the segment of the Kane Springs Wash fault that 

has a more northerly strike north of Meadow Valley Wash 

has a more significant component of dip-slip, making that 

segment a minor breakaway zone (mapping in progress by 

R.B. Scott). 

Although the role of strike-slip faulting in Cenozoic 

extension of the Basin and Range province is debated 

(Shawe, 1965; Anderson, 1973, 1984; Bohannon, 1979; Ron 

and others, 1986), the observation by Shawe (1965) of 

numerous northeast-striking left-lateral and northwest-striking right-lateral faults in western Nevada also applies elsewhere in the Basin and Range. Examples of this fault pattern 

in southern Nevada are the northwest-striking, right-lateral 

Walker Lane belt, the northeast-striking, left-lateral Lake 

Mead fault zone, the local northwest-striking, right-lateral 

faults in the area of the Caliente caldera complex (Best and 

others, 1993), and the local left-lateral Pahranagat shear system (fig. 2; Stewart and Carlson, 1978). Oblique slip on 

major and minor faults in the northern Meadow Valley 

Mountains also fits this pattern (figs. 22, 23). 
KANE SPRINGS WASH CALDERA, MEADOW VALLEY MOUNTAINS 

~ Alluvium 

~ Younger tuffs and flows 

~ Basalt flows (12-13 Ma) 

I }!~ : I Caldera-filling rhyolitic tuffs and basaltic flows 

Caldera-filling syenite and trachyte dome 

El 

Trachytic intrusion 

EXPlANATION 

Kane Wash Thff 

intracaldera facies 

~ 

~ 

~ 

~ 

~ 

CCJ 

Caldera-filling rhyolitic tuffs and flows ~ Tuff of Narrow Canyon L:__l intracaldera facies 

Thff of Narrow canyon 

outflow facies 

Rhyolitic intrusion 

Older tuffs- Sunflower Mountain Tuff (14.7 Ma) stratigraphically through 

Monotony Tuff (27.3 Ma) 

Sedimentary rocks 

Contact 

Kane 

Springs 

Wash 

caldera 

}

Narrow 

Canyon 

caldera 

' High-angle normal fault-- Dotted where concealed; bar and ball on downthrown side 

25 _L_ 

I I 

Ill Ill Ill 

Strike-slip fault-May be combined with normal fault symbol where oblique slip; dotted 

where concealed; barbs show direction of relative movement 

Strike and dip of bedding 

Caldera outlines-Dashed where inferred 

Kane Springs Wash caldera 

Narrow Canyon caldera 

Caliente caldera complex 

Miocene 

} 

Miocene and 

Oligocene 

163 

QUATERNARY 

AND 

TERTIARY 

TERTIARY 

PERMIAN 

} TIIROUGH 

CAMBRIAN 
164 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

As discussed earlier, the eastern part of the caldera in 

the Meadow Valley Mountains shows evidence of faulting, 

dike injection, and progressive tilting after major collapse 

and during filling of the caldera between 14.4 and 11.5 Ma. 

Thus, late-stage volcanism and extension were contemporaneous in the eastern part of the caldera. This relationship 

stands in contrast to the relatively undeformed and slightly 

tilted western part of the caldera in the Delamar Mountains, 

indicating that extensional and strike-slip processes operated 

primarily east of the Kane Springs Wash fault while the 

western area in the Delamar Mountains passively rode out 

extension (Harding and others, 1991 ). Because the eastern 

part of the Kane Springs Wash caldera is on the footwall of 

the Kane Springs Wash fault, stratal tilts and synthetic 

oblique-slip faults should be expected to be concentrated 

adjacent to such a half-graben range-bounding fault zone. 

During our reconnaissance mapping outside the study 

area, we searched for the remainder of the caldera wall in the 

Meadow Valley Mountains. The eastern margin of the 

caldera is covered both by overflow of caldera fill and by 

basin deposits in Meadow Valley Wash. Another calderawall segment was recognized about 11 km north of the 

caldera-wall breccia described previously (Best and others, 

1993); this distance is consistent with the width of the Kane 

Springs Wash caldera in the Delamar Mountains. The 

caldera-wall segment is continuously exposed for 5 km 

across the northern part of the Meadow Valley Mountains. 

Along those exposures, the wall breccia dips 10°-40° southwestward, perpendicular to the trend of the wall, as would be 

expected for the northeastern wall of the caldera. The northem segment of the caldera wall does not contain clasts 

derived from the Kane Wash Tuff, but rather contains clasts 

of outflow units of the Narrow Canyon caldera, peralkaline 

precursor to the Kane Springs Wash caldera, and clasts of 

older precaldera volcanic units (Scott, Blank, and Page, 

1991 ). 

Although the northern and southwestern caldera walls 

of the Kane Springs Wash caldera trend generally westnorthwest along the easternmost exposures in the Delamar 

Mountains, the trends of the eastern continuation of the 

caldera in the Meadow Valley Mountains are more diverse, 

from almost east-west to north. The caldera wall in the 

Delamar Mountains generally follows a smooth "cookie cutter" curvature, whereas the caldera wall in the Meadow Valley Mountains is highly irregular (fig. 29). We attribute the 

variability of the trend of the caldera wall in the Meadow 

Valley Mountains to the influence of preexisting north- to 

northwest-striking faults on caldera collapse; this contrasts 

with the absence of such faults in the Delamar Mountains. 

Such heterogeneity of fault patterns is common in central 

Lincoln County, as is emphasized by Scott and others (this 

volume); Rowley and others (this volume) and Best and others (1993) noted a profound influence of preexisting fault 

attitudes on caldera margin attitudes in the Caliente caldera 

complex. 

The general offset geometry of the Kane Springs Wash 

caldera is mirrored in gravity and aeromagnetic data (Blank 

and Kucks, 1991 ). The contrast in magnetic signature 

between the areas underlain by the caldera and areas underlain by outflow sheets to the south and east ranges from 

about 250 to 300 nanoteslas. The magnetic anomaly patterns 

to the north are obscured by the presence of the Narrow Canyon caldera (Scott, Blank, and Page, 1991). Although the 

caldera underlies a steep regional gravity gradient that coincides with a regional thinning of the ash-flow tuffs southward, a distinct left -lateral offset of the gradient occurs along 

the Kane Springs Wash fault. The projected eastern boundary of the caldera under caldera-filling units that overflow 

the caldera in the Meadow Valley Mountains and under alluvium in Meadow Valley Wash is based upon these geophysical signatures. 

We doubt that a convincing case can be made for two 

calderas, one in the Delamar Mountains with its southeastern 

wall hidden beneath alluvium in Kane Springs Valley, and a 

second caldera in the Meadow Valley Mountains with its 

northwestern wall also hidden beneath alluvium in Kane 

Springs Valley. Although such a hypothesis would remove 

the need for about 4-7 km of left -lateral offset of the caldera 

along the Kane Springs Wash fault, objections to a "two 

fixed calderas" hypothesis include: ( 1) The age constraints of 

the intracaldera units require that the caldera segment in the 

Meadow Valley Mountains be the source of the Gregerson 

Basin Member of the Kane Wash Tuff. (2) The distributions 

of the outflow facies of the Kane Wash Tuff are incompatible 

with the two segments being separate sources for specific 

members of the Kane Wash Tuff. (3) By using abrupt, and 

fortuitously hidden, curvatures to the caldera wall, it is possible, but in our opinion contrived, to terminate the Delamar 

Mountains caldera segment beneath the alluvium in Kane 

Springs Valley as proposed by Noble (1968) and Novak 

( 1984 ). The Kane Springs Valley narrows at its northern end, 

allowing less than 1 km for the northern wall to curve from 

aN. 40° W. trend to aS. 60° W. trend, a change of about 80°; 

this makes a termination of the Meadow Valley Mountain 

caldera segment buried beneath alluvium of Kane Springs 

Valley highly unlikely (fig. 29). Also, recent unpublished 

mapping by R.B. Scott in the northeastern Delamar Mountains indicates that the northwest-trending segment of the 

northeastern caldera wall in the Meadow Valley Mountains 

does not continue into the eastern part of the Delamar Mountains. (4) A hypothesis that assumes "two fixed calderas" 

must accept the coincidences that not only are both abrupt 

changes of caldera wall attitudes hidden by alluvium in Kane 

Springs Valley, but also the trace of the Kane Springs Wash 

fault coincidentally separates the two calderas. We conclude 

that the simplest interpretation is that the once-continuous 

caldera walls were offset by left-lateral oblique motion along 

strands of the Kane Springs Wash fault. 
KANE SPRINGS WASH CALDERA, MEADOW VALLEY MOUNTAINS 165 

CONCLUSIONS 

1. The Kane Springs Wash caldera extends into the 

Meadow Valley Mountains (Harding and others, 1991) (figs. 

29, 30). 

2. The lower ash-flow tuffs of the northern stratigraphic sequence are intracaldera facies of the Kane Wash 

Tuff; the columnar-jointed rhyolite is equivalent to the lower 

cooling unit of the Gregerson Basin Member of the Kane 

Wash Tuff, and the flow-lineated rhyolite is equivalent to the 

upper cooling unit of the Gregerson Basin Member of the 

Kane Wash Tuff. 

3. The upper units of the northern sequence represent 

post-collapse caldera-fill rocks that are mineralogically similar to the late caldera-fill rocks of the Delamar Mountains. 

4. The southern caldera wall in the Delamar Mountains is offset left-laterally across the Kane Springs Wash 

fault about 4- 7 km from its continuation in the Meadow Valley Mountains (fig. 29). 

5. Extensional deformation significantly affected the 

eastern part of the caldera in the Meadow Valley Mountains, 

whereas the western part in the Delamar Mountains was generally unaffected. 

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166 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

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KANE SPRINGS WASH CALDERA, MEADOW VALLEY MOUNTAINS 167 

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168 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

APPENDIX 1. THIN SECTION MODAL ANALYSIS 

Northern stratigraphic sequence 

Sample 3AH15 2AH18 7AH30 5AH19 1AH14 4AH19 

No. .11.88 .10.88 .10.90 .11.88 .10.88 .11.88 

Map unit Td Td Td Tvc Tvc Tvc 

Qtz 43.4 46.8 31.3 47.8 39.1 39.9 

San 53.3 51.1 60.9 43.3 54.9 51.1 

Plag 2.9 0 6.9 4.5 4.2 6.7 

other 0.2 Op tr Bt 0.9 Op 4.5 Op 0.1 Op 0.4 Bt 

pheno Al 0.2 Op 0.9 Am 

Zir tr Cpx 

0.9 Op 

Ap+Zir 

% pheno 23.4 5.0 16.6 6.8 7.8 6.4 

% lithic .3 .5 8.8 2.1 0.2 0 0.4 

granoph granoph 

pheno pts 486 188 335 201 215 223 

total pts 2,073 3,724 2,014 2,967 2,972 3,498 

other gr in gr in vit 

Sample 3AH15 1AH23 5AH21 8AH27 5AH28 1AH18 

No. .10.88 .10.88 .5.88 .10.90 .10.90 .11.88 

Map unit Tbr Tbr Trlr Trlr Trlr Tyt 

Qtz 30.6 29.0 40.4 22.1 43.4 30.7 

San 46.3 43.5 53.5 68.6 38.0 57.4 

Plag 17.6 32.5 0.6 9.3 17.8 10.9 

other 4.7 Bt 3.2 Bt 0.8 Op tr Bt 

pheno trAm trAm 0.9 Am 

0.8 Op 0.9 Op 1.0 Op 

Zir Zir 

%- pheno 17.0 16.2 2.7 2.3 3.4 2.4 

%- lithic 0 0 7.5 7.4 10.2 10.2 

bsltic bsltic bsltic pumice 

pheno pts 363 345 99 86 129 101 

total pts 2,139 2,135 3,613 3,764 3,760 4,121 

Sample 9AH18 1AH24 4AH21 2AH21 3AH25 2AH18 

No. .4.88 .5.88 .5.88 .11.88 .10.88 .5.88 

Map unit Tyt Tyt Tyt Tyt Tyt Tyt 

Qtz 43.6 59.7 58.8 52.6 49.2 14.4 

San 45.3 20.9 34.4 36.7 39.7 80.7 

Plag 1.7 9.7 5.7 0 0.8 0 

other 1.7 Bt tr Bt 1.0 Op 10.6 Op 10.3 Op tr Bt 

pheno 7.7 Op 9.7 Op 4.9 Op 

%- pheno 3.0 5.7 7.3 6.1 3.5 13.7 

%- lithic 5.0 8.9 5.9 44.2 24.3 0.2 

pumice pumice some 

bsltic 

pheno pts 117 206 209 245 126 223 

total pts 3,930 3,616 2,878 4,013 3,598 1,629 

other def gl 
Sample 

No. 

Map unit 

Qtz 

San 

Plag 

other 

pheno 

% pheno 

% lithic 

pheno pts 

total pts 

other 

Sample 

No. 

Map unit 

Qtz 

San 

Plag 

other 

pheno 

% pheno 

% lithic 

pheno pts 

total pts 

other 

Sample 

No. 

Map unit 

Qtz 

San 

Plag 

other 

pheno 

% pheno 

% lithic 

pheno pts 

total pts 

KANE SPRINGS WASH CALDERA, MEADOW VALLEY MOUNTAINS 

Northern stratigraphic sequence--Continued 

4AH16 

.5.88 

Tyt 

25 

7 

1 

tr Bt 

lOp 

Zir 

40 

10 

est 

2AH21 

.5.88 

Tfl 

30.6 

68.7 

0 

0.7 Op 

sec Min 

34.3 

0 

710 

2,069 

5AH12 

.11.88 

Tcj 

30.0 

61.7 

0 

8.3 Op 

3.1 

0.4 

120 

3,923 

gl sh 

6AH14 

.5.88 

Tyt 

25 

69 

2 

2 Bt 

20p 

Zir 

45 

5 

est 

1AH28 

.10.90 

Tfl 

37.0 

59.4 

0 

3.6 Op 

Zir 

20.9 

0 

446 

2,133 

gr in 

4AH12 

.11.88 

Tcj 

23.5 

71.1 

0 

5.4 Op 

Zir 

6.1 

0 

204 

3,356 

3AH20 

.11.88 

Tgt 

22.2 

72.5 

2.7 

tr Bt 

2.7p 

Zir 

18.7 

0.1 

374 

2,000 

4AH26 

.5.88 

Tfl 

29.0 

71.0 

0 

sec Min 

12.9 

0.6 

trachy 

269 

2,092 

4AH19 

.5.88 

Tcj 

27.2 

62.4 

0 

8.0 

7.2 

0 

250 

3,483 

gr in 

Cp? 

5AH14 

.5.88 

1AH21 

.3.90 

Tfl 

23.1 

73.4 

0 

Tgt 

11.3 

83.1 

0.4 

tr Bt 

5.2 

3.5 Op 

Op vap Am 

12.0 

0.6 

248 

2,065 

eutax 

5AH19 

.5.88a 

Tfl 

53.7 

40.0 

0 

6.3 Op 

Zir 

sec Min 

4.5 

1.6 

95 

2,122 

xl-p b 

eutax 

5AH26 

.5.88 

Tcj 

29.7 

66.2 

0 

4.1 Op 

01 

Zir 

5.9 

0 

219 

3,690 

18.2 

0.3 

398 

2,184 

5AH19 

.5.88b 

Tfl 

38.8 

60.1 

0 

0.5 Op 

44.8 

0.4 

1,015 

2,266 

xl-r b 

2AH3 

.11.90 

Tcj 

20.4 

74.9 

0 

4.6 Op 

Zir 

15.6 

0 

323 

2,073 

bg base 

3AH21 

.5.88 

Tfl 

32.2 

62.8 

0 

5.0 Op 

sec Min 

19.9 

0 

538 

2,707 

7BS26 

.4.90 

Tyt 

30.0 

45.0 

18.4 

1.2 Bt 

2.5 Am 

2.5 Op 

2.6 

12.1 

pumice 

80 

3,041 

3AH3 

.11.90 

Tcj 

27.8 

70.9 

0 

1.3 Op 

8.7 

0 

230 

2,643 

r base 

169 
170 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

Northern stratigraphic sequence--Continued 

Sample 

No. 

Map unit 

Qtz 

San 

1AH5 

.11.90b 

Tcj 

31.4 

6b.6 

0 

1AH5 

.11.90a 

Tcj 

33.4 

63.6 

0 

5AH10 

.4.89 

Tcj 

30.1 

67.2 

0 

4AH3 

.11.90 

Tlt 

41.6 

54.3 

Plag 0 

other 

pheno 

2.1 Op 0.6 01 2.7 Cp 4.1 Op 

% pheno 

% lithic 

pheno pts 

total pts 

other 

Sample 

No. 

Map unit 

Qtz 

San 

Plag 

other 

pheno 

% pheno 

% lithic 

pheno pts 

total pts 

Other 

Zir 

14.2 

0 

287 

2,015 

ul base 

5AH16 

.4.89 

Tkbu 

32.6 

63.8 

0 

0.4 01 

2.3 Cp 

0.9 Op 

Zir 

vap Ri? 

10.4 

0 

221 

2,118 

Abbreviations used: 

2.4 Op 01 01 

Zir Zir 

16.6 12.6 10.0 

0 0 0 

341 259 221 

2,059 2,061 2,208 

11 base 

Southern stratigraphic sequence 

1AH18 6AH20 2AH8 1AH22 

.4.89 .4.89 .4 .88 .3.90 

Tkbl Tkg Tpt Tot 

44.0 0.1 0 19.7 

54.6 94.2 94.7 70.1 

0 0.1 0 0 

tr Cp 3.0 Cp 3.6 Cp 10.2 Op 

1.3 Op 2.5 Op 1.7 Op 

Zir Zir 

7.9 37.8 36.4 3.6 

0 0 0 0 

227 800 777 137 

2,866 2,119 2,137 3,825 

base base 

3AH20 

.4.89 

Tsm 

36.7 

60.3 

0.3 

2.6 Op 

Zir 

14.5 

0.05 

305 

2,105 

2AH21 

.4.89 

Tsm 

33.8 

63.3 

0.5 

0.5 Bt 

1.9 Op 

9.0 

0.1 

210 

2,333 

1AH14 

.4.89 

Th 

16.8 

13.7 

55.2 

11.7 Bt 

2.5 Op 

29.5 

0.6 

786 

2,664 

Op, opaque; Bt, biotite; Cp, clinopyroxene; Am, amphibole; Ri, riebeckite; 01, olivine; Zir, zircon; Ap, apatite; 

AI, allanite; Qtz, quartz; San, sanidine; Plag, plagioclase 

est, estimate; pts, points; pheno, phenocrysts; tr, trace 

trachy, trachytic; sec Min, secondary green, sheaflike minerals; gr in, granophyric intergrowth of quartz and 

feldspar (lithic fragments); vap, vapor phase; bsltic, basaltic; gl sh, glass shards; def gl, deformed glass shards; eutax, 

eutaxitic texture 

xl-p b, crystal-poor base; xl-r b, crystal-rich base; b g base, blue-gray base; r base, red base; ul, upper layered; 11, 

lower layered; vit, vitrophyre 

Td, rhyolite dike; Tvc, volcanic center; Tbr, biotite rhyolite flow; Trlr, red lithic rhyolite; Tyt, yellow tuff; Tgt, 

gray ash-flow tuff; Tfl, flow-lineated rhyolite; Tcj, columnar-jointed rhyolite; Tlt, lithic-rich tuff; Tkbu, upper cooling 

unit of the Gregerson Basin Member of the Kane Wash Tuff; Tkbl, lower cooling unit of the Gregerson Basin Member of 

the Kane Wash Tuff; Tkg, Grapevine Spring Member of the Kane Wash Tuff; Tpt, precaldera trachyte; Tot, older tuff; 

Tsm, Sunflower Mountain Tuff; Th, Hiko Tuff 
KANE SPRINGS WASH CALDERA, MEADOW VALLEY MOUNTAINS 171 

APPENDIX2. CHEMICAL DATA 

2A. EDXRF TRACE-ELEMENT ANALYSIS 

Values reported as ppm. 

Northern stratigraphic sequence 

Map Sample Rb Sr y Zr Nb Ba La Ce Nd 

Unit No. 

Tbr 1AH23.10.88 206 36 31 143 26 280 49 85 40 

Td 1AH23.4.89 181 56 27 139 16 508 56 87 36 

Td 3AH15.11.88 183 37 35 133 38 110 54 116 51 

Tb 5AH21.4.89 36 653 24 162 17 593 28 60 38 

Tfb 6AH18.4.88 185 30 45 164 36 123 80 138 59 

Tyt 1AH18.11.88 313 47 46 189 55 66 20 43 25 

Tyt 7BS26.4.90 215 259 39 128 31 24 40 69 37 

Tyt 2BS26.4.90 153 170 22 121 30 21 31 50 34 

Trlr 2AH16.5.88 178 81 49 169 37 141 40 86 43 

Tgt 3AH20.11.88 229 61 62 249 55 233 66 134 56 

Tbl 4AH14.5.88 23 552 32 190 18 644 27 54 29 

Tfl 1AH21.5.88 100 39 56 589 49 145 113 251 89 

Tfl 3AH21.5.88 249 29 88 952 90 74 129 228 88 

Tfl 9AH19.5.88 262 27 104 1,097 110 46 120 204 95 

Tfl 3AH14.5.88 271 26 99 1,081 115 63 127 227 97 

Tfl 7AH19.5.88 299 27 129 1,376 131 49 139 265 106 

Tcj 5AH12.11.88 272 50 91 938 103 82 120 207 86 

Tcj 4AH19.5.88 279 32 87 999 107 75 120 226 96 

Tcj 2AH12.11.88 271 22 104 979 107 56 106 199 90 

Tcj 1AH19.5.88 263 28 90 909 103 64 99 207 78 

Tcj 2AH3.11.90 246 12 108 1,018 96 27 114 217 90 

Tlt 4AH3.11.90 226 37 86 946 100 80 103 195 80 

Southern stratigraphic sequence 

Tkbu 5BS19.5.89 265 22 66 1,134 119 32 114 225 96 

Tkbl 2BS27.3.90 255 29 75 860 112 76 104 201 93 

Tkg 2AH24.3.90 182 45 67 725 80 154 142 265 106 

Tkg 3BS27.3.90 214 39 71 733 83 109 149 253 121 

Tsm 13BS16.11.87 266 156 58 231 62 208 27 66 27 

Tdl 2AH11.5.88 194 24 55 467 68 47 43 116 38 
172 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

2B. WDXRF MAJOR-ELEMENT ANALYSIS 

Values reported as weight percent. 

Northern stratigraphic sequence 

Oxide Map unit symbol and sample number 

Tbr Td Td Tbu Tfb Tyt 

AH23. 1AH23. 3AH15. 5AH21. 6AH18. 1AH18. 

10.88 4.89 11.88 4.89 4.88 11.88 

Si02 74.3 74.3 75.0 51.8 75.4 66.3 

Al2o 3 12.4 12.8 11.9 16.3 12.5 11.4 

Fe2o 3T 1.23 1.12 1.42 10.8 1.28 1.17 

MgO 0.18 0.24 0.36 5.61 0.16 0.96 

CaO 0.46 0.70 0.97 8.09 0.39 3.58 

Na2o 2.67 3.47 3.55 3.54 3.61 0.94 

K~O 5.51 5.64 4.65 1.60 5.40 3.54 

T102 0.11 0.13 0.14 2.10 0.11 0.07 

P205 <0.05 <0.05 <0.05 0.61 <0.05 <0.05 

MnO <0.02 <0.02 <0.02 0.14 <0.02 <0.02 

LOI 0.89 0.78 0.76 1.85 <0.01 0.52 

Tyt Tyt Trlr Tgt Tbm Tf1 

7BS26. 2BS26. 2AH16. 3AH20. 4AH14. 1AH21. 

4.90 4.90 5.88 11.88 5.88 5.88 

Si02 73.5 72.2 71.1 72.7 46.8 62.1 

Al 203 10.4 9.98 13.2 12.7 16.0 16.9 

Fe20 3T 0.99 0.95 3.36 1. 73 13.8 5.12 

MgO 0.65 0.74 0.57 0.31 4.27 0.42 

CaO 2.22 2.66 1.26 1.46 8.17 0.66 

Na2o 0.89 0.58 3.69 3.82 3.56 5.59 

K~O 3.06 2.76 4.68 4.97 1.32 6.23 

T102 0.07 0.06 0.52 0.15 2.73 0.62 

P205 <0.05 <0.05 0.09 0.05 0.66 0.11 

MnO <0.02 <0.02 0.04 0.05 0.10 0.16 

LOI 8.65 9.29 0.56 1.24 2.31 1.14 

Tfl Tfl Tfl Tcj Tcj Tcj Tcj 

3AH21 9AH19. 7AH19. 5AH12. 4AH19 2AH12. 1AH19. 

5.88 5.88 5.88 11.88 5.88 11.88 5.88 

Si02 73.5 74.8 73.4 72.9 73.7 74.5 74.5 

Al203 11.8 11.1 11.4 10.4 10.9 11.6 11.4 

Fe2o 3T 3.13 2.77 3.17 2.57 2.68 2.89 2.79 

MgO 0.30 <0.10 0.14 0.48 0.12 0.14 0.15 

CaO 0.25 0.65 0.28 1. 74 1.25 0.12 0.42 

Na2o 2.94 3.90 3.97 3.23 3.22 3.65 3.81 

K~O 5.41 4.56 5.74 4.91 5.10 5.07 4.71 

T102 0.20 0.16 0.19 0.16 0.17 0.19 0.17 

P2o5 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 

MnO 0.03 0.05 0.06 0.02 0.03 0.05 0.04 

LOI 1.20 0.66 0.49 2.46 1.40 0.58 0.86 

Southern stratigraphic sequence 

Tkbu Tkbl Tkg Tkg Tsm Tdl 

5BS19. 2BS27 2AH24. 3BS27. 13BS16. 2AH11. 

5.89 3.90 3.90 3.90 11.87 5.88 

Si02 74.7 75.5 68.0 70.0 70.6 75.2 

Al 203 11.1 11.1 13.5 14.0 11.7 11.8 

Fe2o 3T 3.18 2.46 3.38 3.34 1.66 2.04 

MgO <0.10 0.17 0.26 0.17 0.64 0.15 

CaO 0.26 0.33 1.96 0.34 2.13 0.13 

Na2o 4.29 3.73 4.22 4.35 1. 70 3.70 

K~O 4.61 4.74 5.41 5.67 3.98 5.29 

T102 0.18 0.15 0.26 0.29 0.13 0.12 

P205 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 

MnO 0.07 0.04 0.07 0.06 <0.02 <0.02 

LOI 0.57 0.74 1.85 0.74 6.72 0.55 
KANE SPRINGS WASH CALDERA, MEADOW VALLEY MOUNTAINS 173 

2C. INAA ANALYSIS 

Values reported as ppm. 

Northern stratigraphic sequence 

Element Map unit symbol and sample number 

Tbr Tbf Tyt Trlr Tgt Tfl 

1AH23. 6AH18. 1AH18. 2AH16. 3AH20. 1AH2l. 

10.88 4.88 11.88 5.88 11.88 5.88 

Fe 9,070 9,210 7,830 23,300 12,400 35,200 

Na 20,000 27,200 6,430 27,200 28,000 40,100 

Rb 209 183 297 174 233 91.4 

Sr 17.6 17.6 54.2 54.9 34.0 19.9 

Cs 2.34 2.49 7.09 2.01 2.34 4.23 

Ba 259 120 71.6 137 226 142 

Th 26.5 23.7 32.4 19.7 28.4 16.2 

u 6.22 5.16 8.62 4.24 6.03 2.93 

La 47.9 75.2 18.5 35.1 59.4 101 

Ce 97.1 158 34.9 82.2 141 289 

Nd 36.2 59.1 22.4 37.4 54.8 86.3 

Sm 6.91 11.3 6.77 8.98 11.8 14.8 

Eu 0.345 0.214 0.146 0.509 0.370 1.15 

Gd 6.41 11.0 0.00 8.49 0.00 14.3 

Tb 0.862 1.36 1.16 1.30 1.76 1.81 

Ho 1.24 1.87 1.88 1.87 2.54 2.30 

Tm 0.00 0.829 0.883 0.797 1.16 1.01 

Yb 3.87 5.11 5.84 5.14 7.24 6.83 

Lu 0.567 0.718 0.822 0.715 1.02 0.976 

Zr 160 207 218 218 303 593 

Hf 5.25 6.34 8.13 5.95 9.43 11.6 

Ta 2.87 3.00 6.38 2.77 4.42 2.33 

w 0.820 1.75 0.522 2.19 1.24 3.18 

Sc 2.01 1.87 2.04 5.45 1.76 7.47 

Cr 0.00 1.65 0.00 3.02 2.45 1.79 

Co 0.661 0.547 0.518 6.24 1.25 0.553 

Ni 0.00 1.39 0.00 0.00 2.39 0.00 

Zn 24.3 38.8 64.1 66.2 77.5 148 

As 0.911 20.2 2.69 3.66 2.65 4.03 

Sb 0.224 0.201 0.494 0.00 0.195 0.841 

Au 0.570 0.043 0.492 0.662 0.00 0.894 
174 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

Northern stratigraphic sequence--Continued 

Element Map unit symbol and sample number 

Tfl Tfl Tfl Tcj Tcj Tcj 

3AH21. 9AH19. 7AH19. 5AH12. 4AH19. 1AH19. 

5.88 5.88 5.88 11.88 5.88 5.88 

Fe 21,700 19,400 22,200 17,700 18,600 18,900 

Na 20,700 28,300 29,000 23,000 23,200 26,700 

Rb 237 252 278 252 254 244 

Sr 18.3 0.00 0.00 35.5 18.4 0.00 

Cs 2.04 2.76 1.67 2.26 2.58 5.48 

Ba 64.4 20.9 67.1 79.9 95.3 37.5 

Th 26.8 26.7 39.5 27.0 28.3 27.6 

u 7.85 4.24 7.46 7.20 9.78 9.24 

La 123 112 117 102 107 84.5 

Ce 265 222 304 230 245 221 

Nd 104 101 107 90.7 93.7 75.9 

Sm 19.2 20.0 21.9 18.9 18.9 16.2 

Eu 0.338 0.246 0.255 0.227 0.247 0.226 

Gd 18.3 19.9 21.6 17.5 19.30 16.8 

Tb 2.69 2.87 3.33 2.61 2.69 2.53 

Ho 3.71 4.09 5.18 3.85 4.00 3.74 

Tm 1.62 1.86 2.63 1.66 1.75 1.68 

Yb 10.7 11.5 16.7 10.5 10.7 10.6 

Lu 1.48 1.58 2.27 1.43 1.47 1.50 

Zr 956 1,030 1,270 894 927 915 

Hf 24.6 27.2 32.6 23.7 24.8 24.5 

Ta 6.96 7.96 9.55 7.02 7.37 7.43 

w 1.28 0.651 0.00 1.33 0.00 2.20 

Sc 1.18 0.452 0.537 0.478 0.514 0.476 

Cr 2.61 0.00 0.00 0.00 3.02 0.00 

Co 0.233 0.170 0.174 0.174 0.215 0.148 

Ni 0.00 2.04 0.00 2.75 4.42 0.00 

Zn 140 164 200 144 171 141 

As 4.97 1.09 6.84 6.84 7.01 2.32 

Sb 0.773 0.882 1.28 0.660 1.21 0.772 

Au 1.57 0.00 0.630 1.32 0.669 0.00 
KANE SPRINGS WASH CALDERA, MEADOW VALLEY MOUNTAINS 175 

Southern stratigraphic sequence 

Element Map unit symbol and sample number 

Tkbu Tkbl Tkg Tkg Tsm Tdl 

5BS19. 2BS27. 2AH24. 3BS27. 13BS16. 2AH11. 

5.89 3.90 3.90 3.90 11.87 5.88 

Fe 18,800 17,100 23,900 22,600 10,700 14,300 

Na 28,500 26,800 31,400 32,200 11,900 27,100 

Rb 243 261 186 200 252 201 

Sr 0.00 0.00 42.8 12.6 139 0.00 

Cs 2.29 3.72 1.75 5.96 8.04 3.10 

Ba 35.2 82.2 139 91.6 198 52.6 

Th 29.3 29.8 23.8 24.4 24.2 21.7 

u 3.43 8.27 5.69 5.23 3.77 7.22 

La 98.2 103 135 142 28.4 45.9 

Ce 229 209 290 280 68.4 128 

Nd 83.6 89.7 110 112 32.5 43.8 

Sm 16.5 18.1 18.7 18.2 9.63 10.2 

Eu 0.195 0.250 0.456 0.446 0.309 0.089 

Gd 15.6 16.6 14.5 16.5 10.10 10.7 

Tb 2.37 2.56 2.16 2.22 1.50 1.54 

Ho 3.50 3.76 3.02 0.00 2.16 2.16 

Tm 1.54 1.69 1.26 1.34 0.957 0.936 

Yb 9.87 10.4 7.89 8.25 6.10 5.97 

Lu 1.41 1.41 1.17 1.17 0.818 0.838 

Zr 985 846 761 823 265 448 

Hf 26.9 24.1 20.3 21.0 9.56 12.7 

Ta 7.81 8.08 5.64 5.78 5.25 5.27 

w 1.87 2.28 2.97 3.57 0.00 1.41 

Sc 0.416 0.496 3.13 2.83 1.19 0.953 

Cr 2.13 0.00 2.38 0.00 0.642 0.00 

Co 0.186 0.206 0.846 0.642 1.14 0.331 

Ni 0.00 3.51 0.00 0.00 0.00 0.00 

Zn 155 154 99.5 113 80.0 89.7 

As 3.36 4.16 3.85 0.00 3.99 3.62 

Sb 0.246 0.629 0.388 0.490 0.419 0.430 

Au 1.50 0.712 0.616 0.00 0.462 0.00 
176 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

2D. ICP ANALYSIS 

Values reported as ppm except where noted. 

Northern stratigraphic sequence 

Element Map unit symbol and sample number 

Tbr Tfb Tyt Trlr Tgt Tfl 

1AH23. 6AH18. 1AH18. 2AH16. 3AH20. 1AH21. 

10.88 4.88 11.88 5.88 11.88 5.88 

Al%' 6.5 6.6 6.1 7.0 6.7 8.5 

Ca%' 0.34 0.31 2.6 0.96 1.1 0.48 

Fe%' 0.93 0.86 0.84 2.5 1.2 3.6 

K %' 4.5 4.5 3.0 3.9 4.2 5.0 

Mg%' 0.09 0.08 0.59 0.35 0.17 0.23 

Na%' 1.9 2.6 0.72 2.7 2.8 3.8 

Ti%' 0.08 0.06 0.05 0.34 0.09 0.34 

Mn 220 180 170 420 380 1,200 

Ag <2 <2 <2 <2 <2 <2 

As <10 20 <10 <10 <10 <10 

Au <8 <8 <8 <8 <8 <8 

B 

Ba 240 110 65 130 210 120 

Be 4 5 12 5 5 4 

Bi <10 <10 <10 <10 <10 <10 

Cd <2 <2 <2 <2 <2 <2 

Ce 83 140 41 82 130 260 

Co 1 <1 <1 8 2 1 

Cr <1 <1 <1 2 <1 <1 

Cu 2 3 4 8 4 3 

Eu <2 <2 <2 <2 >2 >2 

Ga 19 23 26 24 23 32 

Ge 

Ho <4 <4 <4 <4 <4 <4 

La 48 80 21 39 66 110 

Li 58 26 38 24 34 29 

Mo <2 <2 <2 <2 <2 5 

Nb 19 19 40 29 48 38 

Nd 31 53 23 36 51 85 

Ni <2 <2 2 4 <2 <2 

Pb 18 20 18 22 17 15 

Sc 2 <2 2 6 2 8 

Sn <5 <5 5 <5 <5 <5 

Sr 32 22 41 70 45 23 

Ta <40 <40 <40 <40 <40 <40 

Th 25 22 33 20 27 17 

u <100 <100 <100 <100 <100 <100 

v 3 24 3 58 7 <2 

w 

y 28 47 40 48 64 57 

Yb 3 5 6 5 6 7 

Zn 30 35 75 70 78 160 

Zr 
KANE SPRINGS WASH CALDERA, MEADOW VALLEY MOUNTAINS 177 

Northern stratigraphic sequence--Continued 

Element Map unit symbol and sample number 

Tfl Tfl Tfl Tcj Tcj Tcj 

3AH21. 9AH19. 7AH19. 5AH12. 4AH19. 1AH19. 

5.88 5.88 5.88 11.88 5.88 5.88 

Al%- 6.2 5.9 6.0 5.6 5.8 6.0 

Ca%- 0.18 0.49 0.22 1.3 0.93 0.32 

Fe%- 2.2 2.1 2.4 1.9 2.0 2.1 

K %- 4.4 3.8 4.8 4.1 4.3 3.9 

Mg%- 0.16 0.03 0.06 0.29 0.05 0.07 

Na%- 2.1 2.9 2.9 2.4 2.4 2.7 

p %- 0.009 0.006 0.007 0.01 0.02 0.01 

Ti%- 0.10 0.10 0.13 0.10 0.11 0.11 

Mn 280 470 560 260 310 360 

Ag <2 <2 <2 <2 <2 <2 

As <10 <10 <10 <10 <10 <10 

Au <8 <8 <8 <8 <8 <8 

B 

Ba 59 39 37 68 62 52 

Be 7 9 14 9 9 7 

Bi <10 <10 <10 <10 <10 <10 

Cd <2 <2 <2 <2 <2 <2 

Ce 230 210 280 220 230 200 

Co <1 <1 <1 <1 <1 <1 

Cr <1 <1 <1 <1 <1 <1 

Cu 4 4 3 3 4 4 

Eu <2 <2 <2 <2 <2 <2 

Ga 31 32 36 28 30 31 

Ge 

He <4 <4 <4 <4 <4 <4 

La 130 120 130 120 120 91 

Li 35 32 82 80 43 33 

Me <2 <2 <2 <2 <2 <2 

Nb 70 89 110 66 68 81 

Nd 98 96 100 88 90 73 

Ni <2 <2 <2 <2 <2 <2 

Pb 36 38 22 27 36 32 

Sc <2 <2 <2 <2 <2 <2 

Sn <5 <5 <5 <5 <5 <5 

Sr 17 9 10 38 17 12 

Ta <40 <40 <40 <40 <40 <40 

Th 19 27 37 28 26 23 

u <100 <100 <100 <100 <100 <100 

v 2 <2 2 3 <2 <2 

w 

y 83 110 110 99 99 90 

Yb 9 12 15 11 10 8 

Zn 160 180 220 160 190 180 

Zr 
178 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

Southern stratigraphic sequence 

Element Map unit symbol and sample number 

Tkbu Tkbl Tkg Tkg Tsm Tdl 

SBS19. 2BS27. 2AH24. 3BS27. 13BS16. 2AH11. 

5.89 3.90 3.90 3.90 11.87 5.88 

Al% 5.8 5.8 7.1 7.2 6.2 6.3 

Ca% 0.20 0.23 1.4 0.26 1.6 1.5 

Fe% 2.2 1.7 2.4 2.4 1.2 1.5 

K % 3.8 3.9 4.5 4.6 3.3 4.6 

Mg% 0.03 0.08 0.14 0.08 0.38 0.07 

Na% 3.1 2.7 3.1 3.1 1.3 2.7 

p % 0.009 0.01 0.02 0.01 0.01 0.007 

Ti% 0.12 0.08 0.13 0.19 0.08 0.09 

Mn 560 380 540 560 210 170 

Ag <2 <2 <2 <2 <2 <2 

As <10 <10 <10 <10 <10 <10 

Au <8 <8 <8 <8 <8 <8 

B 

Ba 29 69 130 93 190 42 

Be 8 8 6 7 8 6 

Bi <10 <10 <10 <10 <10 <10 

Cd <2 >2 >2 >2 >2 <2 

Ce 230 180 260 260 68 120 

Co <1 <1 1 <1 2 <1 

Cr <1 <1 <1 1 2 2 

Cu 3 4 5 4 4 3 

Eu <2 <2 <2 <2 <2 <2 

Ga 33 31 29 29 29 29 

Ge 

Ho <4 <4 <4 <4 <4 <4 

La 120 110 150 160 32 51 

Li 63 57 29 28 so 42 

Mo <2 <2 <2 4 <2 <2 

Nb 90 78 63 76 48 56 

Nd 89 81 100 110 33 43 

Ni <2 <2 <2 <2 <2 <2 

Pb 38 34 20 25 20 20 

Sc <2 <2 4 4 <2 <2 

Sn <5 <5 5 <5 <5 <5 

Sr 7 13 33 20 160 14 

Ta <40 <40 <40 <40 <40 <40 

Th 32 20 23 25 26 23 

u <100 <100 <100 <100 <100 <100 

v 4 2 3 3 7 4 

w 

y 72 63 60 69 58 54 

Yb 11 6 6 8 6 5 

Zn 190 170 120 130 97 100 

Zr 
KANE SPRINGS WASH CALDERA, MEADOW VALLEY MOUNTAINS 

APPENDIX3. ISOTOPIC DATES 

K-Ar determinations 

Sample 3AH18 3AH18 1AH26 

No. .5.89 .5.89 .5.88 

Map unit Td Td Tfb 

vit 

Material sanidine biotite whole 

rock 

K20% 10.7 8.36 .4 .97 

10.8 8.37 4.95 

*Ar40 1.9833 1.5812 0.9262 

1o- 1Dmol/g 

%*Ar40 62.3 57.1 38.9 

Age, Ma 12.8 13.1 12.9 

± 2 sigma ±0.5 ±0.5 ±0.7 

Constants: K40A8 = 0.581x1o-10jyr 

Ap = 4.962x1o-10jyr 

40K/K = 1.167x1o- 4 

2AH26 4AH26 

.5.88 .5.88 

Tfb Tfl 

devit 

whole sanidine 

rock 

4.84 6.71 

4.86 6.71 

0.9619 1.4188 

77.7 85.7 

13.7 14.6 

±0.5 ±0.4 

40Ar/39Ar determinations 

Sample 4AH26 SBS19 2BS27 

No. .5.88. .5.88 .3.90 

Map unit Tfl Tkbu Tkbl 

Material sanidine sanidine sanidine 

Sample wt.mg 60 59.3 68 

Ar40/36 meas atm 298.8 298.9 298.9 

J-val.ue 0.007432 0.007412 0.007415 

error 0.25% 0.25% 0.1% 

Plateau 14.43 14.39 14.55 

age, Ma ± 0.14 ± 0.28 ± 0.14 

4AH.26 

.5.88 

Tfl 

sanidine 

6.71 

6.71 

1.4426 

69.5 

14.9 

±0.5 

3BS27 

.3.90 

Tkg 

sanidine 

50 

298.9 

0.007368 

0.1% 

14.67 

± 0.22 

179 

SAH26 

.5.88 

Tcj 

sanidine 

6.41 

6.42 

1.3513 

72.5 

14.6 

±0.5 

Preliminary Correlation of Quaternary and 

Late Tertiary Alluvial Deposits in 

Southeastern Nevada 

By W C Swadley, D.L. Schmidt, R.R. Shroba, V.S. Williams, and D.L. Hoover 

GEOLOGIC STUDIES IN THE BASIN AND RANGE-COLORADO PLATEAU TRANSITION IN 

SOUTHEASTERN NEVADA, SOUTHWESTERN UTAH, AND NORTHWESTERN ARIZONA, 1992 

U.S. GEOLOGICAL SURVEY BULLETIN 2056-F 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1995 
CONTENTS 

Abstract........................................................................................................................... 183 

Introduction . . . . . . . . . . . . . . . . . . . . . . ... .. . . . . . . . . . . . ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . 183 

Chief Mountain Area...................................................................................................... 183 

Delamar Mountains Area................................................................................................ 189 

Moapa Valley Area......................................................................................................... 190 

Mesquite Area................................................................................................................. 194 

Nevada Test Site Area .................................................................................................... 198 

Discussion....................................................................................................................... 199 

References Cited............................................................................................................. 199 

FIGURES 

1. Map showing location of the four study areas discussed in this report, and the Nevada Test Site area..................... 184 

2. Sketch map of Chief Mountain area............................................................................................................................ 188 

3. Sketch map of Delamar Mountains area showing the major areas of mapped alluvium............................................ 189 

4. Sketch map of Moapa Valley and Mesquite areas .. . . . . .. ........ .. . . . .............. .. . . . .............. .. ............... .. . . . . ............. .. . . . . . . .... 192 

5. Idealized composite cross section showing Quaternary and late Tertiary terraces of the Moapa Valley area . . . . . .. .... 193 

6. Sketch map of part of Nevada Test Site area showing major areas where surficial deposits were mapped............... 198 

TABLES 

1. Stages of calcium carbonate morphology observed in calcic soils developed in noncalcareous surficial deposits 

under arid and semiarid climates of the southwestern United States .......................................................................... 185 

2. Characteristics used in distinguishing alluvial-fan deposits in Chief Mountain area ...... .................. .. ................... .. . . 186 

3. Characteristics for distinguishing alluvial-fan deposits in Delamar Mountains area .................................................. 191 

4. Characteristics for distinguishing alluvial deposits in Moapa Valley area . . . ................ ................. .. . . ................. .. . . .. .. 195 

5. Characteristics for distinguishing alluvial deposits in Mesquite area ......................................................................... 197 

6. Characteristics for distinguishing alluvial-fan deposits in Nevada Test Site area...................................................... 200 

7. Correlation of alluvial deposits in five areas in southeastern Nevada and age control for deposits in NTS area ....... 201 
Preliminary Correlation of Quaternary and Late Tertiary 

Alluvial Deposits in Southeastern Nevada 

By W C Swadley, D.L. Schmidt, R.R. Shroba, V.S. Williams, and D.L. Hoover 

ABSTRACT 

Surficial mapping in parts of the Basin and Range-Colorado Plateau Transition (BARCO) Study Unit area in southeastern Nevada has defined the sequence of Quaternary and 

late Tertiary alluvial deposits in four areas based chiefly on 

age-dependent geomorphic criteria. Alluvial-fan, pediment, 

terrace, and flood-plain deposits of these areas are correlated 

on the basis of similarities in these geomorphic aspects. 

Using these same criteria, correlations are extended to similar deposits in the Nevada Test Site area for which isotopic 

ages have been determined. Tentative ages for the deposits 

of the four study areas are inferred from the correlations to 

the NTS area. With minor exceptions, the sequence of alluvial deposits in the four areas of this study consists of, in 

order of increasing age, a late Holocene unit, an early 

Holocene to late Pleistocene unit, three middle Pleistocene 

units, and an early Pleistocene to Pliocene(?) unit. 

INTRODUCTION 

Alluvial deposits are common geologic features in 

southern Nevada and in much of the southern Great Basin, 

forming extensive alluvial piedmonts along the flanks of 

mountain ranges and prominent flood plains and terraces 

along major streams. Alluvial deposits in four areas in southeastern Nevada (fig. 1) were mapped as part of the Basin and 

Range-Colorado Plateau Transition (BARCO) Study Unit 

(see the Introduction to this volume for a discussion of the 

BARCO Study Unit, its history, activities, and goals). The 

mapped areas discussed in this report are (I) Chief Mountain, by Shroba; (2) Delamar Mountains, by Swadley~ (3) 

Moapa Valley, by Schmidt~ and (4) Mesquite area, by Williams and Hoover. 

The sequences of alluvial deposits recognized in each 

of the four areas are similar; however, variations in drainagebasin characteristics, tectonic settings, bedrock lithologies, 

and climatic conditions have resulted in minor differences. 

Deposits in the Chief Mountain and Delamar Mountains 

areas are mostly alluvial-fan deposits, whereas deposits in 

the Moapa Valley and Mesquite areas include flood-plain, 

terrace, pediment, and alluvial-fan deposits. 

The numerical ages of alluvial deposits in the four areas 

are difficult to determine, because the deposits commonly 

lack fossils or other materials that are suitable for age determination by radiogenic methods. Therefore, in order to make 

tentative correlations, we have used traditional stratigraphic 

correlation, based chiefly on age-dependent geomorphic criteria, to show correspondence in character and in stratigraphic sequence among deposits. Criteria used include 

geomorphic position, preservation and modification of original depositional topography, soil development, rock and 

mineral weathering, and formation of desert pavements. 

(See, for example, Colman and others, 1987; Hoover and 

others, 1981.) These same criteria were used to make tentative correlation with similar surficial deposits at and near the 

Nevada Test Site (NTS; fig. 1), because many NTS deposits 

have numerical ages determined by isotopic, radiogenic, and 

tephrochronologic methods. 

A standardized nomenclature is not in place for the four 

areas. In this report, the alluvial deposits of each area are designated by informal map unit names and symbols that have 

been used by the authors in their respective areas. Some map 

units have been given informal place names; others have 

only been given relative age designations. 

CHIEF MOUNTAIN AREA 

The Chief Mountain area is near the eastern margin of 

the Great Basin in southeastern Nevada near Caliente (fig. 

1 ). The area covers primarily the north-trending Chief Range 

and the Panaca basin, a deep, fault-bounded basin on the east 

side of the Chief Range. Chief Mountain (2,278 m), in the 

western part of the area, is the highest peak in the Chief 

Range. Two gently sloping alluvial piedmonts along the 

southern and eastern flanks of the Chief Range characterize 

the area. The piedmonts range in altitude from about 1,425 

to 1,885 m, are typically 3-12 km wide, and slope southward 

toward Antelope Canyon and eastward toward Meadow Valley. Both are drained by southward- and eastward-flowing 

183 
184 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

116° 115° 38°.-------------~------~----------------------------~r---------------------------1 

NEVADA 

·-·-·'·,·'·,·-·-·-·-.......... ! 

\ ! '\ ·-·-·-·-·-·-·-·-! ., NEVADA 

; 

i 

i 

i 

i 

i 

i 

i 

i 

i 

i 

i 

i 

i 

i 

! 

TEST 

SITE 

0 

I 

.... 

E "' 

~ "' 

50 KILOMETERS 

I 

Chief 

a Mountain 

area 

Caliente 

I I 

Figure 1. Location of the four study areas of this report, and the Nevada Test Site area. 

intermittent streams that are tributary to the unnamed wash 

in Antelope Canyon and Meadow Valley Wash (fig. 2). 

Meadow Valley Wash flows south and ultimately joins the 

Colorado River about 165 km south of Caliente. 

Bedrock in the Chief Range is the source of the alluvialfan deposits on the piedmonts. Composition of the bedrock 

is mostly Late Proterozoic and Cambrian quartzite, a minor 

amount of Cambrian limestone and shale, Oligocene and 

Miocene volcanic rocks, and Miocene intrusive porphyry. 

The volcanic rocks are mostly rhyolitic ash-flow tuffs and 

andesitic to basaltic lava flows, flow breccias, and volcanic 

mudflow breccias (Rowley and Shroba, 1991; Rowley and 

others, 1992). 

The climate of the area is semiarid and is characterized 

by warm, dry summers and cool, moist winters. Mean annual 

temperature and mean annual precipitation vary with elevation and are about 11-12 °C and 22-34 em, respectively 

(Borup and Bagley, 1976). 

The surficial deposits on the piedmonts are mostly alluvial-fan deposits that were deposited by minor intermittent 

streams about 5-15 km long. The fans are commonly about 

0.5-1 km wide, 1-4 km long, and have surfaces that slope 

4-5 percent. Some of the older fans coalesced into fan aprons 

that were dissected by subsequent stream erosion. Some of 

the older alluvial-fan deposits may grade into pediment 

deposits near the mountain front. 
CORRELATION OF ALLUVIAL DEPOSITS, SOUTHEASTERN NEVADA 185 

The alluvial-fan deposits on the piedmonts are typically 

clast supported pebble and cobble gravel in a sand matrix 

that is interbedded with matrix-supported sandy pebble 

gravel and pebbly sand. These deposits locally contain minor 

beds of clast-supported, cobbly, small-boulder gravel and 

are typically poorly sorted to moderately sorted and thin to 

medium bedded. The clasts in most of these deposits are 

chiefly angular to subangular quartzite and a minor amount 

of subrounded volcanic rocks. Near the south end of 

Meadow Valley, the clasts are chiefly subrounded tuffs and 

lavas. Near the north end of the Chief Range, the clasts are 

mostly angular to subrounded limestone. 

The soils developed in the alluvial-fan_ deposits are 

characterized by horizons of secondary calcium carbonate 

accumulation. The thickness and the morphology of these 

calcic horizons are useful age indicators for these deposits 

(table 1). The youngest alluvial-fan deposits have stage I carbonate morphology (Gile and others, 1966), which forms 

thin coatings on the bottoms of clasts. These coatings are 

found to depths of greater than 3.5 m. The older alluvial-fan 

deposits are distinguished from the youngest ones by the 

presence of soil K horizons that range in thickness from 0.3 

m to as much as 2.5 m in the oldest deposits. K horizons less 

than 1 m thick have stage III carbonate morphology, and 

those greater than 1 m thick commonly have stage IV carbonate morphology in the upper part and stage III carbonate 

morphology in the lower part. Thick K horizons with stage 

IV carbonate morphology in the upper part are resistant to 

·erosion and locally form resistant ledges at the top ofthe oldest alluvial-fan deposits. 

The alluvial-fan deposits on the piedmonts were 

subdivided into six map units chiefly on the basis of their 

geomorphic position, topographic expression, and extent of 

calcic horizon development (table 2). The six units, from 

oldest to youngest, are: fan alluvium of Antelope Canyon 

(Tfa), fan alluvium of Chief Mountain (QTfc), fan alluvium 

of Ox borrow Ranch (Qfo ), fan alluvium of Miller Spring 

Wash (Qfms), fan alluvium of 1001 Ranch (Qfr), and fan 

alluvium of Meadow Valley (Qfm) (Rowley and Shroba, 

1991; Rowley and others, 1991). These alluvial-fan deposits 

are of Pliocene to Holocene age and underlie five wellexpressed geomorphic surfaces that range in elevation from 

5 m or less above stream level to as much as 120m above 

stream level near the lower part of Antelope Canyon. These 

deposits are interpreted to postdate the development of a 

through-flowing drainage in the Panaca basin via Meadow 

Valley Wash (Rowley and others, 1992). The oldest alluvialfan deposits (unit Tfa) are of Pliocene age. They are eroded, 

Table 1. Stages of calcium carbonate morphology observed in calcic soils developed in noncalcareous surficial deposits under 

arid and semiarid climates of the southwestern United States 

[Modified from Gile and others (1966, table I) and Machette (1985, table I)] 

Stage 

I 

n 

m 

IV 

v 

VI 

Diagnostic morphologic characteristics 

Thin, discontinuous coatings on clasts, usually on 

undersides. 

Continuous, thin to thick coatings on tops and 

undersides of clasts. 

Massive accumulations between clasts, becomes 

cemented in advanced form. 

Thin ( <0.2 em) to moderately thick (1 em) 

laminae in upper part of K horizon. Thin 

laminae may drape over fractured surfaces. 

Thick laminae ( > 1 em) and thin to thick 

pisolites. Vertical faces and fractures are 

coated with laminated carbonate (casehardened surface). 

Multiple generations of laminae, breccia, and 

pisolites; recemented. Many case-hardened 

surfaces. 

Calcium carbonate distnbution 

Coatings sparse to common. 

Coatings common, some calcium carbonate in matrix, 

but matrix still loose. 

Essentially continuously dispersed in matrix (K 

fabric). 

Cemented platy to weak tabular structure and 

indurated laminae. 

Indurated dense, strong platy to tabular structure. 

Indurated and dense, thick strong tabular structure. 
Table 2. Characteristics used in distinguishing alluvial-fan deposits in Chief Mountain area. 

Map unit and 

symbol 

Fan alluvium of 

Meadow Valley 

(Qfm). 

Fan alluvium of 

1001 Ranch 

(Qfr). 

Fan alluvium of 

Miller Spring 

Wash (Qfms). 

Fan alluvium of 

Ox borrow 

Ranch (Qfo). 

Topographic expression 

Preservation of depositional features Erosional modification of 

depositional features 

Depositional surfaces are 

preserved; little or no 

modification of bar-and-swale 

topography. 

Depositional surfaces are 

preserved; little or no 

preservation of bar-and-swale 

topography. 

Depositional surfaces are mostly 

preserved in areas between 

major washes. 

Depositional surfaces are mostly to 

locally preserved in areas 

between major washes. 

Commonly incised by 

narrow, shallow, steepsided main washes; little 

or no surface drainage 

development. 

Commonly incised by 

narrow, shallow, steepsided main washes; minor 

surface drainage 

development 

characterized by a few 

shallow washes. 

Commonly incised by steepsided major washes that 

are as much as 20 m deep; 

moderate surface drainage 

development 

characterized by 

numerous minor washes 

on broad interfluves. 

Commonly incised by wide 

major washes as much as 

30 m deep; moderate 

surface drainage 

development 

characterized by 

numerous minor washes 

on broad interfluves and a 

few minor washes on 

narrow interfluves. 

Calcic horizon development 

Thickness of Typical 

conspicuous 

carbonate1 

(m) 

0 

03 

0.8 

1.7 

maximum 

stage of 

carbonate 

morphology2 

I 

III 

III 

IV 

Pavement development 

None. 

No data. 

Smooth, relatively continuous. 

Smooth, relatively continuous; numerous calcrete 

fragments on ground surface. 

-

00 

0\ 

a 

tTl 

0 

l' 

0 

a 

(5 

en 

...., 

c:: 

0 

m 

~en 

co 

;J> 

~ 

(') 

0 

en 

...., 

c:: 

0 

--< 

c:: 

z 

!=3 

~ \0 

N 
Map unit and 

symbol 

Fan alluvium of 

Chief Mountain 

(QTfc). 

Fan alluvium of 

Antelope 

Canyon (Tfa). 

Topographic expression 

Preservation of depositional features Erosional modification of 

depositional features 

Depositional surfaces are locally 

preserved on narrow interfluves. 

Depositional surfaces are not 

preserved. 

Commonly incised by wide 

major washes as much as 

60 m deep; well-developed 

surface drainage 

characterized by 

numerous deep washes 

that have rounded side 

slopes. 

Commonly incised by 

numerous major washes as 

much as 70 m deep; well 

developed surface 

drainage characterized by 

numerous deep washes 

and by sharp to narrow 

rounded ridges. 

Calcic horizon development 

Thickness of Typical 

conspicuous 

carbonate! 

(m) 

2.5 

e) 

maximum 

stage of 

carbonate 

morphologyl 

IV 

IV 

Pavement development 

Do. 

No data. 

!Thickness values are for soil K horizons that have stage III and IV carbonate morphology. Values for fan alluvium of Oxborrow Ranch and fan alluvium of Chief Mountain are 

minimum values because the upper part of the K horizon in these units is eroded. 

2stages of carbonate morphology are those of Gile and -others (1966). See table 1. 

3-rhe soil that is formed in fan alluvium of Antelope Canyon is poorly exposed and has an eroded K horizon of unknown thickness. 

n 

0 

:::0 

:::0 

tTl 

t""" 

;l> 

::j 

0 

z 

0 

'"l'1 

;l> 

t""" 

t""" 

c 

< 

> t""" 

u 

tTl 

a 

en 

~ ~en 

en 

0 

c 

>-3 

s 

;l> 

en 

>-3 

tTl 

~ z 

tTl 

< ;l> 

u 

;l> 

-00 

..._] 
188 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

5 KILOMETERS 

I 

Figure 2. Sketch map of Chief Mountain area. Light dot pattern, distribution of alluvial piedmonts. Shade, distribution 

of bedrock in the Chief Range and adjacent area. Vertical pattern, distribution of alluvial surficial deposits in Meadow 

Valley and Antelope Canyon. Unpattemed area, distribution of undifferentiated surficial deposits and bedrock. 

lack original depositional topography, and overlie an eroded 

surface cut on the Panaca Formation of late Miocene and 

Pliocene age (Row ley and Shroba, 1991 ~ Rowley and others, 

1991). They are locally overlain by younger alluvial-fan 

deposits and are most extensive along the mountain front and 

in and near Antelope Canyon where they make up the upper 

part (possibly 300m or more) of a thick sequence of basinfill deposits (Rowley and Shroba, 1991~ Rowley and others, 

1991). The younger alluvial-fan deposits (units QTfc 

through Qfm) are of Pliocene(?) and Quaternary age 

(Rowley and Shroba, 1991 ~Rowley and others, 1991 ). These 

deposits retain some or all of their original depositional 

topography, are commonly 2-20 m thick, and underlie 

geomorphic surfaces that are inset into geomorphic surfaces 

that are underlain by older alluvial-fan deposits. The 

younger alluvial-fan deposits overlie surfaces that are cut on 
CORRELATION OF ALLUVIAL DEPOSITS, SOUTHEASTERN NEVADA 189 

the oldest alluvial-fan deposits (unit Tfa) or on the Panaca 

Formation. The youngest alluvial-fan deposits (units Qfr and 

Qfm) form small fans that are 0.2 km2 or less in area and 

commonly are graded to surfaces that are 5 m or less above 

the modem washes. 

Meadow Valley and Antelope Canyon are major valleys that border the piedmonts of the Chief Mountain area. 

These valleys are underlain by stream alluvium and locally 

by minor deposits of hillslope colluvium and fan alluvium. 

Most of the alluvium in Meadow Valley is flood-plain alluvium; a minor amount of fan and low-terrace alluvium 

occurs near the mouths of intermittent streams tributary to 

Meadow Valley Wash. On the west side of the valley, a volcanic-rich, high terrace gravel (unit QTam of Rowley and 

Shroba, 1991) is present at one or more levels above 

Meadow Valley Wash. The flood-plain alluvium is a silty 

aggradational sequence composed mostly of fine-grained 

sediment eroded from the Panaca Formation, which is 

exposed along washes that flow into Meadow Valley. The 

alluvium is at least 7 m thick (Rowley and Shroba, 1991), 

and driller's logs of water wells in Meadow Valley suggest 

that the silty alluvium may be about 10-40 m thick (Rush, 

1964 ). The alluvium that underlies the flood plain and low 

terraces in Antelope Canyon is derived mostly from quartzite 

and volcanic rocks and is much coarser than the flood-plain 

alluvium in Meadow Valley. The alluvium in Antelope Canyon may be as much as 15m thick and is composed mostly 

of thin- to medium-bedded, clast-supported, cobbly pebble 

gravel, pebble gravel, and matrix-supported pebbly sand 

(Rowley and others, 1991). Minor terrace deposits in Antelope Canyon are not discussed in this report. 

DELAMAR MOUNTAINS AREA 

The Delamar Mountains are near the east edge of the 

Great Basin in southeastern Nevada (fig. 1 ), an area drained 

chiefly by tributaries of the Colorado River. This region is 

characterized by short, northerly trending mountain ranges 

separated by intermontane valleys, some of which have 

internal drainage. The mountains are commonly bounded by 

linear range-front faults. 

The bedrock that is the major source of alluvial deposits 

in the Delamar Mountains area consists chiefly of Paleozoic 

marine limestone, dolomite, and quartzite overlain unconformably by Tertiary rhyolitic ash-flow tuff, minor rhyolitic 

lava, basaltic lava, and sparse shallow silicic to intermediate 

intrusive rocks. The Paleozoic sedimentary rocks weather to 

coarse gravel; the Tertiary volcanic rocks locally weather to 

gravel, but many of them commonly weather to sand-sized 

detritus. 

The climate of the area is arid. The average rainfall is 

less than 20 em per year. In the valley of Pahranagat Wash 

the mean annual temperature is 15 °C. At Caliente, Nev., 

25 km east of Dry Lake Valley, annual temperature averages 12 oc (Houghton and others, 197 5). The altitude of the 

alluvial deposits ranges from about 850 m at the south end 

of the Delamar Range to about 1, 7 50 m in the northern part 

of Dry Lake Valley (fig. 3). 

0 

I 

10 

I 

20 

I 

30 KILOMETERS 

I 

Figure 3. Sketch map of Delamar Mountains area and vicinity. 

Unpatterned areas within Delamar Mountains area show the major 

areas of mapped alluvium. 
190 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

Quaternary alluvial-fan deposits of the Delamar Mountains area have been mapped and described over an area of 

about 2,100 km2. The principal mapped areas are along the 

Pahranagat, Kane Springs, Delamar, and Dry Lake Valleys 

(fig. 3). Pahranagat Wash is a south-flowing intermittent 

stream on the southwest flank of the Delamar Mountains. At 

the south end of the Delamar Mountains, Pahranagat Wash 

is joined by the southwest-flowing intermittent Kane Springs 

Wash. Delamar Valley and Dry Lake Valley are contiguous 

elongate, north-trending closed basins along the northwest 

side of the Delamar Mountains and along their northward 

extension, the Burnt Springs Range. 

The alluvial fans of the area were deposited by small 

intermittent streams 5-10 km long that flow into the main 

valleys approximately perpendicular to the range fronts. In 

the upper and middle parts of the piedmonts, successively 

younger fans typically are inset into one or more older fans. 

Locally, younger fans overlie older fans in the middle parts 

of the piedmonts. On the lower piedmont slopes in closed 

basins, older fan deposits are buried by successively younger 

deposits toward the center of the basins. 

The fans are commonly 1-3 km wide and have 

coalesced into continuous fan aprons. The aprons vary in 

width from 1 to 3 km along the valleys of Pahranagat and 

Kane Springs Washes and the aprons on both sides of the 

valleys have similar topographic expression. The fan aprons 

in the Delamar and Dry Lake Valleys are typically 2-6 km 

wide. The aprons on the east side of these valleys commonly 

are more deeply dissected, and younger fans are more deeply 

inset into older fans than those on the west side. Minor 

terrace deposits of limited extent along Kane Springs and 

Pahranagat Washes are not described in this report. 

The alluvial fans consist of interbedded gravel, gravelly 

sand, and sand. The relative amounts of each of these components and the proportion of the various lithologies of 

gravel clasts vary from fan to fan and reflect the bedrock 

composition within the drainage basin of the depositing 

wash. Deposits are typically poorly sorted and poorly to 

moderately well bedded. Clasts in the deposits are com-

. monly angular to subrounded. 

Soils developed in the alluvial fans are typical of aridclimate soils. The horizon of secondary calcium carbonate 

enrichment is the most conspicuous feature and is a useful 

age indicator for all but the youngest soils. The amount of 

carbonate enrichment varies from no visible carbonate accumulation in the youngest soils to a 2-m-thick carbonatecemented horizon that locally forms a resistant ledge in 

exposures of the oldest Quaternary deposits. 

Pliocene(?) and Quaternary alluvial-fan deposits of the 

area have been divided into six map units based largely on 

topographic sequence, soil development, preservation of 

depositional features, and amount of surface dissection 

within each map unit. This sequence of fan deposits is estimated to range in age from late Pliocene(?) to late Holocene. 

Although age designations are included in the informal 

names of some of the map units (QTa, Qae, and Qal), these 

age designations are preliminary and are based chiefly on the 

amount of carbonate enrichment in the soil developed in 

each unit. The six map units, in order of decreasing age, are: 

Quaternary and Tertiary alluvium (QTa); alluvium of Coyote Spring Valley (Qac), of Willow Spring (Qaw), and of 

Jumbo Wash (Qaj); late Pleistocene and early Holocene alluvium (Qae); and late Holocene alluvium (Qal). The mapping criteria for each of the map units are shown in table 3. 

No ages have been determined for these deposits using 

isotopic dating. Samples of an ash bed exposed at one locality at the base of unit Qaw and of an ash bed in fluvial deposits that are locally overlain by fan units Qaw and Qac have 

been submitted for possible age determinations by chemical 

correlation methods. Preliminary results are inconclusive. 

The age of unit Qae is somewhat constrained in Dry Lake 

Valley by its field relations with lacustrine deposits of a latest Pleistocene pluvial lake (Mifflin and Wheat, 1979). Here, 

lacustrine beach deposits locally overlie unit Qae, and wavecut scarps related to the beach deposits locally cut unit Qae, 

indicating that Qae is in part as old as latest Pleistocene. 

Other deposits of unit Qae that extend into the area formerly 

occupied by the lake seem not to have been eroded, suggesting that the deposition of Qae continued after the lake disappeared and, therefore, that part of the unit may be as young 

as early Holocene. 

MOAPA VALLEY AREA 

The verdant Muddy River flood plain of the Moapa 

Valley (fig. 4) is set in a stark, barren landscape exhibiting 

spectacular terraces that were produced and preserved in a 

generally arid climate (Gardner, 1972a). The arid climate has 

favored the development of thick, resistant carbonate-rich 

soils that protect the old terrace surfaces from erosion. These 

surfaces have steep scarps from tens of meters to more than 

200m high (fig. 5; Gardner, 1972b). The high, old terraces, 

including the Mormon Mesa terrace about 200 m above the 

Muddy River, are of late Tertiary age and are the products of 

erosional isolation caused by deep dissection since the Colorado River was integrated through the region about 5 m.y. 

ago (Longwell, 1928; Damon and others, 1978). Four conspicuous terraces at lower levels are the products of pluvialarid climatic cycles of Quaternary age and are discussed 

herein. 

The main streams of the Moapa Valley area are Pahranagat Wash and Meadow Valley Wash (fig. 4), which 

slope from about 750 m altitude in the north and west parts 

of the area to 335 m in the south at the confluence of the 

Muddy River with the Virgin River; Pahranagat Wash 

becomes the Muddy River at Muddy River Springs. Wide 

piedmont slopes rise above these main streams to an altitude 

of about 760 mat the surrounding range fronts. The Muddy 
Table 3. Characteristics for distinguishing alluvial-fan deposits in Delamar Mountains area. 

Map unit and 

symbol 

Late Holocene 

alluvium (Qal). 

Early Holocene 

and late 

Pleistocene 

alluvium (Qae). 

Alluvium of 

Jumbo Wash 

(Qaj). 

Alluvium of 

Willow Spring 

(Qaw). 

Alluvium of 

Coyote Spring 

Valley (Qac). 

Quaternary and 

Tertiary 

alluvium (QTa). 

Topographic expression 

Preservation of depositional features Erosional modification of 

depositional features 

Depositional surface intact; bar- None 

and-swale topography commonly 

preserved. 

Depositional surface intact; local Minor local dissection. 

preservation of subdued bar-andswale topography. 

Depositional surface mostly intact; Minor dissection. 

soil commonly preserved on 

interfluves. 

Depositional surface mostly intact; Moderate dissection by 

soil locally to commonly stripped v-shaped washes; flat 

to upper part of K horizon on interfluves. 

interfluves. 

Depositional surface moderately Moderate dissection by 

preserved; soil on interfluves washes with rounded 

commonly stripped to upper part edges. 

of K horizon. 

Depositional surface completely Extensive dissection; 

eroded. rounded interfluves. 

lstages follow standards defmed by Gile and others (1966). See table 1. 

Calcic horizon development 

Thickness of Typical 

conspicuous 

carbonate (m) 

None 

0.4-0.7 

1 

1-1.5 

1-2 

1-2 

maximum 

stage of 

carbonate! 

None 

I 

II 

III 

III 

III 

Pavement development 

None. n 

0 

:;tl 

:;tl 

tTl 

t""' 

> ....., 

None. ....... 

0 

z 

0 

"Tl 

> t""' 

t""' 

Loosely packed pavement locally developed. c 

< ;; 

t""' 

0 

tTl 

""0 

Moderately to tightly packed pavement common; 0 

en pedogenic carbonate chips common in surface layer. :::] 

~en 

en 

0 

c 

....., 

Tightly packed pavement common; pedogenic ::c: 

tTl 

carbonate chips common in surface layer. > en 

....., 

tTl 

:;tl 

z 

z 

Tightly packed pavement on some rounded tTl 

< 

interfluves. > 0 

> 

-\0 
192 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

37"00' ,.--..,-------

0 10 

Figure 4. Sketch map of Moapa Valley and Mesquite areas. 

River drains to the Colorado River via the Virgin River in the 

lower Virgin Valley (now the Overton Arm of Lake Mead). 

The climate of the area is arid. At Logandale on the 

lower Muddy River, the average annual rainfall is 10 em and 

the average annual temperature is 18 °C; the average summer 

temperature is 29 °C, the average winter temperature is 8 °C, 

and the daily winter temperature seldom falls below freezing 

(Bagley, 1980; records for 1968-7 5). 

The Moapa Valley area is situated in the extensionally 

deformed Basin and Range province and includes parts of 

two basins and five ranges. The upper Moapa Valley is centered in the Glendale basin (lower Meadow Valley 

Wash-California Wash), which is bounded by the Arrow 

Canyon Range (1,555 m maximum altitude) and Meadow 

Valley Mountains (1,738 m maximum altitude) on the west 

and the Mormon Mountains (2,260 m maximum altitude) on 

the east. The lower Moapa Valley occupies the southwestern 

part of the large Mesquite basin, which is bounded by the 

Muddy Mountains ( 1,006 m maximum altitude) on the west 

20 30 KILOMETERS 

and the Virgin Mountains (2,356 m maximum altitude) on 

the east. The five ranges consist mostly of Paleozoic carbonate rocks, with the exception that the Virgin Mountains contain about 50 percent Precambrian crystalline rocks and the 

Muddy Mountains contain abundant Mesozoic clastic and 

carbonate rocks. The two basins are separated by a narrow 

ridge composed of Paleozoic and Mesozoic rocks. 

The Glendale and Mesquite basins contain thick, synextensionally deformed, basin-fill deposits (equivalent to 

Horse Spring Formation, less than 20 m.y. to about 11.5 m.y. 

old; Bohannon, 1984) overlain by a thick fill of postextensional, only slightly deformed sediments of the Muddy Creek 

Formation (about 11.5 m.y. to 5.5 m.y. old; Bohannon, 

1984 ). Since deposition of the Muddy Creek Formation, the 

two basins have been open basins with exterior drainage to 

the Colorado River, in contrast to closed basins of the Great 

Basin that have interior drainage. 

The water table along the Muddy River and Meadow 

Valley Wash is at channel level and, being so shallow, has 
CORRELATION OF ALLUVIAL DEPOSITS, SOUTHEASTERN NEVADA 193 

260r-------------------------------------------------------------------~ 

/ / 

______________________ -::::_ ......... ___ - ~.....;;......;_....;._...;....;;,_~ 

en 

a: 

w 

...._ 

w / 

~ ~ / ~ / 

Trgm ·C /"' 

-----~---------~~ 

Tmc 

0 2 3 4 KILOMETERS 

Figure 5. Idealized composite cross section showing Quaternary and late Tertiary terraces of the Moapa Valley area. Vertical scale 

greatly exaggerated. Dashed lines across valley connect mainstream terraces on left to side-stream terraces on right. Terrace and alluvium 

symbols as on table 4: Qo, alluvium of old terrace; Qm, alluvium of middle terrace; Qy, alluvium of young terrace; Qaz, silty alluvium of 

lowest terrace; Qa, modem channel and flood-plain alluvium; added m (as, Qmm), mainstream deposit; added s (as, Oms), side-stream 

deposit. Tmc, lacustrine deposits of the Muddy Creek Formation; Tmm, Mormon Mesa soil developed on Muddy Creek Formation; Tag, 

aggradational gravel deposit graded to top of Muddy Creek Formation; and Trg, regraded gravel deposit, representing initial grading to 

entrenched Colorado River. 

strongly influenced mainstream deposition during the Quaternary. The shallow ground water along the Muddy River is 

largely supplied by discharge of the voluminous, deep, 

regional ground-water system at Muddy River Springs at the 

head of the Muddy River (Eakin, 1964 ). This regional system is recharged in the high, carbonate-rock ranges some 

200 km farther north near Ely, Nev. 

The regional ground-water system has existed since the 

beginning of extensional deformation less than 20 m.y. ago, 

and subsequently a large component of its flow (about 

37,000 acre-feet per year) has been through the Paleozoic 

carbonate rocks of the northern Arrow Canyon Range and 

southern Meadow Valley Mountains. The ground water is 

saturated in bicarbonate ions, some of which are precipitated 

as limestone (travertine) on discharge, a process which over 

time has greatly added to the carbonate content of the upper 

Tertiary sediments. Carbonate materials, transported by 

stream, rain, and wind, have been abundantly available for 

carbonate soil development. This carbonate availability is a 

consequence of ( 1) long-term carbonate discharge from 

springs, (2) abundant carbonate detrital rocks from the surrounding bedrock ranges, and (3) abundant carbonate 

reworked from the basin fill. 

The Quaternary mainstream alluvial deposits of the 

Muddy River and lower Meadow Valley Wash are unusual 

in the arid part of the Basin and Range province because they 

were deposited under conditions of a shallow ground-water 

table and exterior drainage. These two factors mean that the 
194 GEOLOGIC STUDIES, BARCO STUDY UNIT, I 992 

main-stream depositional regime was especially sensitive to 

climatic fluctuations during the Pleistocene, that is, fluctuations between pluvial (moist) and arid (dry) episodes. In contrast, the adjacent side streams on the long piedmont slopes 

had deep ground-water tables, making the effects of Pleistocene climatic fluctuations much less dramatic. As a suggested general rule for these integrated basins: pluvial 

mainstream aggradation equates to side-stream pedimentation whereas arid main-stream entrenchment equates to sidestream entrenchment. 

During the pluvial episodes of the Pleistocene, the 

Muddy River and lower Meadow Valley Wash flowed 

through perennially wet flood plains and aggraded as fine 

detritus accumulated on lushly vegetated flood plains 

(Quade, 1986). Aggradation was probably enhanced by a 

more uniform annual distribution of rainfall during a pluvial 

part of the climatic cycle as compared to an uneven seasonal 

distribution of rainfall and localized flash flooding during an 

interpluvial (arid) part of the climatic cycle. 

On broad piedmonts, the side streams did not aggrade 

in the absence of a shallow water table during the pluvial episodes. In much of the Moapa Valley area where the shallow 

suballuvial bedrock is the easily erodible Muddy Creek Formation, the side streams mostly cut laterally (incipient pedimentation) during the pluvial episodes. However, on either 

side of the narrow axial mainstream valley, the piedmont 

side-stream alluvium thickens to form a narrow alluvial 

wedge against the mainstream aggradational deposit with 

which it intertongues. During the ensuing arid part of the climatic cycle, localized flash flooding strongly affected the 

piedmont, and the side streams entrenched into their pediments (Lustig, 1965). 

Entrenchment of the mainstreams into their aggradational deposits and entrenchment of the side streams into 

their piedmont deposits during the interpluvial episode 

resulted in terrace remnants in both deposits that were isolated from vigorous fluvial action. Once isolated, carbonaterich soils characteristic of the arid environment developed in 

these deposits. This isolation was particularly enhanced by 

deep entrenchment, a characteristic of tributary streams near 

the Colorado River. Deep entrenchment of the main streams, 

however, was also a function of the thickness of the pluvial 

aggradational deposit, which in tum was dependent in part 

on the length of the pluvial episode. Where the deposits were 

isolated long enough for thick carbonate-rich soils to 

develop on both the mainstream terraces and the piedmont 

pediments, the resistant soils greatly slowed the erosion of 

the underlying deposits. 

In the Moapa Valley area, the recent valley floor of the 

Muddy River is underlain by a wide, aggradational silt 

deposit (Qaz, fig. 5) that has been sharply entrenched (from 

3 to as much as 9 m) by the Muddy River. This silt deposit 

mostly accumulated during the latest Wisconsin pluvial 

aggradation and may have been mostly entrenched since 

about 1880 (Longwell, 1928). If the last pluvial episode 

ended about I 0,000 yr ago, then the deposit has survived 

entrenchment, at least in many places, for about I 0,000 yr 

during the current arid episode and probably would have survived much longer had man not upset the balance of nature. 

A narrow, youthful mainstream terrace deposit (unit Qym, 

fig. 5) is preserved about 7.5 m above the recent silt aggradational terrace (Qaz); it is probably a higher and older Wisconsin aggradational deposit. 

Four alluvial terrace deposits of Quaternary age are recognized in the Moapa Valley area. These are designated by 

relative age as old, middle, young, and recent (Qo, Qm, Qy, 

and Qaz, respectively). The deposits of the active stream 

channels and low flood plains (Qa) were deposited during 

the arid part of the current climatic cycle. The terrace deposits are diagrammatically shown in figure 5, and some of the 

distinctive characteristics of their surfaces and soils are 

described in table 4. 

MESQUITE AREA 

The large, perennial Virgin River flows in a narrow, 

deeply entrenched valley across the Mesquite basin along the 

east edge of the Basin and Range province in southeastern 

Nevada (fig. I). The Mesquite basin, about 28 km wide, lies 

between the Mormon Mountains (maximum altitude 2,260 

m) to the northwest and the Virgin Mountains (maximum 

altitude 2,356 m) to the southeast and east (fig. 4). The Mormon Mountains are composed largely of Paleozoic carbonate 

rocks and minor Precambrian and Mesozoic rocks, whereas 

the Virgin Mountains are composed chiefly of Precambrian 

igneous and metamorphic rocks with lesser areas of Paleozoic carbonate rocks and Mesozoic clastic rocks in approximately equal amounts. Along the west side of the basin, 

Mormon Mesa, altitude about 560 m, is an erosional remnant 

of lacustrine and fluvial basin-fill deposits, the flattish top of 

which is underlain by an ancient, thick, resistant carbonaterich soil (Gardner, 1972a). The Virgin River flows westward 

and southward from an altitude of about 550 m where it 

debouches from its gorge through the Virgin Mountains to 

about 335 mat its confluence with the Muddy River at the 

south end of Mormon Mesa. The piedmont plains rise steeply 

from the river to about 1,000 m altitude against the rugged 

Virgin and Mormon Mountains. Along the Virgin River at 

Littlefield, Ariz., near the east edge of the Mesquite basin 

(fig. 4), the mean annual temperature is 18 oc and the mean 

annual rainfall is 16 em (U.S. Department of Commerce, 

1975). 

The Mesquite basin is a large tectonic basin that was 

extensionally deformed during late Tertiary time and filled 

with a thick sequence of basin-fill deposits. The exposed part 

of the basin fill is the postextensional Muddy Creek Formation, which consists of several hundreds to many hundreds of 

meters of mostly well bedded, moderately consolidated silt 

and sand that are probably largely fluvial (Kowallis and 
Table 4. Characteristics for distinguishing alluvial deposits in Moapa Valley area. 

Map unit and 

symbol 

Modem channel 

and flood-plain 

alluvium (Qa). 

Silty alluvium of 

lowest terrace 

(Qaz). 

Alluvium of young 

terrace (Qy). 

Alluvium of 

middle terrace 

(Qm). 

Alluvium of old 

terrace (Qo). 

Topographic expression 

Preservation of depositional features Erosional modification of 

depositional features 

Active alluvial channel and flood Noneplain, abandoned channelways 

conspicuous. 

Depositional surface intact; bar- Abrupt, narrow 

and-swale topography entrenchment by parent 

conspicuous. stream; surface not 

eroded. 

Depositional surface partly intact, A few short, narrow gullies 

some bar-and-swale relief serrate terrace scarp; 

preserved. surface not eroded. 

Surface well preserved but Surface slightly eroded near 

depositional surface features not shallowly incised washes 

preserved. of local, intraterrace 

origin; scarp edge in many 

places exposes a thin soilcalcrete rim. 

Slightly rolling surface, no Surface decidedly eroded by 

depositional surface features broad, shallow washes of 

preserved. local origin; scarp edge 

mostly exposed as a thick 

soil-calcrete rim. 

1stages follow standards defined by Gile and others (1966). See table 1. 

Calcic horizon development 

Thickness of Typical 

conspicuous 

carbonate (m) 

None 

None conspicuous. 

Thinly 

disseminated, powdery 

carbonate. 

2m 

conspicuous 

carbonate 

includes 0.3 

m calcrete 

upper part. 

3.1 m 

conspicuous 

carbonate, 

includes 0.8 

m calcrete 

upper part. 

maximum 

stage of 

carbonate1 

None 

None 

Stage I 

Stage III 

Stage IV to 

IV+ 

Pavement development 

No pavement; depositional gravel, sand, and silt (1 

0 exposed. ~ 

~ 

tTl 

t'""" 

~ 

No pavement; surface is silty; exposed gravel ...., 

0 uncommon. z 

0 

'TI 

~ 

t'""" 

t'""" 

Firm, light-brown-varnished cobble pavement c 

< 

overlying several centimeters loose cobbly silt; less ......., 

~ 

than 1 mm carbonate undercoat on cobbles. t'""" 

u 

tTl 

""0 

0 

Very firm, brown-varnished cobble pavement C/.l 

::::3 

overlying about 5 em of silt, as much as 1 em sn 

carbonate undercoat on cobbles; terrace surface C/.l 

0 

shows about 10 percent undercoat carbonate. c 

...., 

::r: 

tTl 

~ C/.l 

...., 

tTl 

Very ftrm, darkly varnished, resistant cobble ~ 

pavement overlying silt as thick as 15 em; terrace 

z 

z 

surface is distinctly light colored showing about 40 tTl 

< 

percent carbonate soil fragments and cobble ~ 

t:i undercoating. ~ 

-\0 

VI 
196 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

Everett, 1986). The Muddy Creek Formation is about 11.5 

m.y. to 5.5 m.y. old. To the west and southwest the formation 

is composed mostly of lacustrine deposits. Muddy Creek 

lake was abruptly drained when the Colorado River was integrated through the present-day Lake Mead area. 

The top of the Muddy Creek Formation and the adjacent 

piedmont slopes were deeply entrenched by newly established tributary drainages of the Virgin River system after the 

Muddy Creek lake was drained. The result of the entrenchment was the erosional isolation of large parts of the lake 

floor and adjacent piedmont slopes. A thick carbonate-rich 

soil developed during the Pliocene and Quaternary on these 

flat and gently sloping deposits; the cap rock on Mormon 

Mesa is a classic example of this ancient carbonate-rich soil. 

By the beginning of Quaternary time the Colorado- Virgin River system had entrenched itself to its present depth. 

All the Quaternary terrace deposits along the perennial Virgin River are the result of cyclical aggradation and degradation caused mostly by climatic fluctuations. The 

aggradational silt and sand deposits were probably the products of pluvial episodes during the Quaternary (Quade, 

1986). The piedmont side streams had base levels controlled 

by the Virgin River. These side streams deposited inset fan 

and inset channel gravels in response to climatic fluctuations. 

The Quaternary and Tertiary alluvial deposits of the 

Mesquite area (fig. 4) are divided into four categories 

according to source of sediment and environment of deposition. For each category two to seven generations of deposits 

have been distinguished by the geomorphic and pedogenic 

criteria in table 5. 

Mainstream deposits of the Virgin River are predominantly sand and gravel and include little silt or clay. The 

gravel consists of a mixture of subangular Precambrian, 

Paleozoic, and Mesozoic detritus from the adjacent ranges as 

well as distinctive well-rounded cobbles of chert and quartzite from the uplands of southwestern Utah. Flights of terraces 

indicate at least four prior intervals of deposition at levels 

about 6, 20, 55, and 85 m above the modern channel. The 

surface at 55 m tops a fill more than 55 m thick (units Qav, 

Qgv) and records a major period of aggradation during the 

early middle Pleistocene or early Pleistocene, probably during a major pluvial episode. 

Alluvium from major tributaries on the north side of the 

river, such as Toquop Wash, is dominated by rocks eroded 

from the Mormon Mountains, and includes abundant limestone. Toquop Wash terraces record substantial lateral shifting of the channel and at least five prior intervals of 

deposition at about 6, 20, 35, 55, and 80 m above the modem 

channel. Toquop Wash terraces are all graded to corresponding Virgin River terraces except that the 35-m terrace is not 

preserved along the Virgin River. 

Minor tributaries on the north side of the river deposited 

alluvium derived by erosion of the escarpment of Muddy 

Creek Formation exposed on the side of Mormon Mesa and 

by reworking of high terrace deposits of the Virgin River. 

Such tributaries deposited thin layers of sandy pediment 

alluvium that includes well-cemented angular boulders 

eroded from the carbonate-enriched soil on the surface of 

Mormon Mesa. The deposits cover a bench between the 

Mormon Mesa escarpment and the inset canyon of the Virgin River, and are graded to resistant ledges of cemented Virgin River terrace gravels 50 to 60 m above the modern Virgin 

River channel. Only two generations of such deposits have 

been distinguished. 

Tributaries on the south and east side of ·the Virgin 

River have deposited coarse alluvium dominated by angular 

clasts of metamorphic and igneous Proterozoic rocks. These 

deposits form alluvial fans, thin alluvial mantles on pediment surfaces cut on older alluvium, and stream terraces 

inset into alluvial fans. At least six prior depositional intervals are recorded. The last five correspond approximately to 

terrace levels along the Virgin River and Toquop Wash, 

whereas the oldest depositional interval, represented by alluvium of Bunkerville Ridge (Tab), was probably contemporary with the upper part of the Muddy Creek Formation. 

The oldest and most widespread piedmont deposit in 

the Mesquite area is the alluvium of Bunkerville Ridge (Tab) 

that underlies about two-thirds of the piedmont slope below 

the Virgin Mountains in the central part of the area (fig. 4). 

The alluvium consists of coarse poorly sorted bouldery 

gravel, sandy gravel, and gravelly sand eroded from the Virgin Mountains during late Tertiary time. The deposit is dissected 20-150 m deep by tributary streams of the Virgin 

River, and its surface, entirely erosional, has rounded bouldery interfluves. Remnants of a stage IV-V carbonate-rich 

soil 3-6 m thick underlie these eroded interfluves. 

On the lower part of the piedmont slope, remnants of 

five Pleistocene fan deposits are deeply inset below the 

ridges ofthe alluvium of Bunkerville Ridge (Tab) and above 

the modem depositional surface (Qany). They overlie the 

easily eroded silt and sand of the Muddy Creek Formation. 

The fan deposits consist mostly of eroded detritus from the 

alluvium of Bunkerville Ridge. The oldest of the inset fan 

deposits (Qof) is preserved only as a few scattered ballenas. 

None of the original depositional surface is preserved. The 

next youngest (Qmf) has been reduced to ballenas in the 

western part of the area but retains a greater proportion of its 

original depositional surface toward the east (upstream). It is 

entrenched as much as 55 m and locally contains remnants of 

a stage IV carbonate-rich soil. The third youngest fan deposit, 

the alluvium of Nevada Highway 170 (Qah), consists mostly 

of sandy gravel and has a preserved depositional surface that 

overlies a stage II-III carbonate-rich soil capped by a tightly 

packed cobble and pebble pavement. Modern washes are 

entrenched 20--40 m below the depositional surface of this 

deposit. The second youngest deposit, the alluvial fan deposits of Meadowlands Farm (Qam), consists of sandy gravel 

and has a well-preserved depositional surface that overlies a 

stage II-III carbonate-rich soil capped by a firm cobble and 
CORRELATION OF ALLUVIAL DEPOSITS, SOUTHEASTERN NEVADA 197 

Table 5. Characteristics for distinguishing alluvial deposits in Mesquite area. 

Topographic expression Calcic horizon development 

Map unit and symbol Preservation of depositional features Erosional modification of Thickness of Typical Pavement 

depositional features conspicuous maximum development 

carbonate (m) stage of 

carbonate1 

Channel sand of Depositional surface intact; bar-and- None. None. None. None. 

Virgin River (Qsv). swale topography common. 

Younger alluvium of do None. None. None. None. 

Nickel Creek (Qany). 

Aoodplain deposits of Depositional surface intact; traces of Eroded by lateral None. None. None. 

Virgin River (Qfp). abandoned channels remain. migration of the river; 

entrenched 1-2m. 

Older alluvium of Depositional surface locally Eroded by migration of 0.5 I None. 

Nickel Creek (Qano). preserved in terrace remnants. tributary streams; 

entrenched 0.3-1 m. 

Sandy terrace deposits Depositional surface preserved in Most of deposit removed 0.3 I None. 

of the Virgin River small remnants commonly covered by river; entrenched 6 m. 

(Qtv). with thin eolian sand. 

Young terrace Depositional surface extensively Entrenched about 20 m. I II Moderately packed 

alluvium of Virgin preserved in terrace remnants pavement where not 

River (Qyv) and commonly covered with thin eolian covered by sand. 

Toquop Wash (Qyt). sand. 

Alluvium of About 40 % of depositional surface Entrenched 30 m by 0.8-1.3 II Moderately packed 

Meadowlands Farm preserved. steep-sided washes with pavement. 

(Qam). sharp edges. 

Young sandy Depositional surface mostly intact. Moderately dissected. 0.5-1 II None. 

pediment and fan Generally entrenched 10 

alluvium (Qsy). m, but 30 m at lower end. 

Younger middle Depositional surface preserved in Entrenched about 35 m by 1-2 III Moderately packed 

terrace alluvium of small remnants along a former steep-sided washes with pavement. 

Toquop Wash (Qyml). course of Toquop Wash. sharp edges. 

Alluvium of Nevada Depositional surface extensively Moderately dissected by 0.5-1 II-III Moderately to tightly 

Highway 170 (Qah). preserved. steep-sided washes with packed pavement. 

rounded edges. 

Older sandy pediment Depositional surface mostly eroded. do 0.5-1.5 III None. 

and fan alluvium 

(Qsh). 

Middle terrace do Moderately dissected by 1-2 III-IV Tightly packed 

alluvium of the Virgin steep-sided washes with pavement locally 

River (Qav, Qgv) and rounded edges; entrenched developed on gravelToquop Wa<>h (Qmt). 55 m. rich part. 

Middle mountain fan Small areas of the depositional do 1-1.5 III-IV Moderate to tightly 

alluvium (Qmf). surface are locally preserved. packed pavement. 

Old terrace alluvium Depositional surface entirely eroded. Extensively dissected; 1-2 IV Tightly packed 

of Virgin River (Qov). entrenched 85 m. pavement. 

Old terrace alluvium Depositional surface extensively Moderately dissected; 1-2 IV Tightly packed 

of Toquop Wash preserved as a terrace capping a entrenched 85 m. pavement. 

(Qot). mesa along a former course of 

Toquop Wash. 

Old mountain fan Depositional surface entirely eroded. Most of deposit removed; 1-2 III-IV None. 

alluvium (Qof). remnants are isolated 

ballenas. 

Alluvium of Depositional surface entirely eroded. Rounded interfluves; 3-6 v None. 

Bunkerville Ridge entrenched 60 m. 

(Tab). 

1

Stages follow standards defined by Gilc and others (1966). See table 1. 
198 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

pebble pavement. Modern washes are entrenched 20-30 m 

below the depositional surface of this deposit. 

Holocene alluvium on the piedmont is restricted to narrow washes entrenched into the alluvium of Bunkerville 

Ridge (Tab) and Pleistocene alluvial deposits. The older 

alluvium of Nickel Creek (Qano) of late(?) Pleistocene to 

early Holocene age forms deposits about I m above the 

younger alluvium of Nickel Creek (Qany). The younger 

alluvial unit of late Holocene age constitutes the flood plains 

of the modern piedmont washes. 

The narrowness (0.3 to I km wide) of the entrenched 

valley of the Virgin River demonstrates the dominance of 

alluviation by the high-gradient, piedmont, side-stream 

washes over valley-widening lateral erosion by the Virgin 

River. The Virgin River meanders across the full width of its 

valley floor, giving it the appearance of an overfit stream. Its 

channel deposit is the channel sand of the Virgin River (Qsv) 

that consists of fine to medium sand, silt, and gravel. About 

2--4m above the active river channel are flood-plain deposits 

of the Virgin River (Qfp) that consist of overbank deposits of 

sand and subordinate amounts of sandy gravel and gravel. 

NEVADA TEST SITE AREA 

The Nevada Test Site (NTS) is in the southern part of 

the Great Basin (fig. 1). Surficial deposits in part of the NTS 

area were mapped in 1978-87 as part of a study of a 

proposed repository for radioactive waste at Yucca 

I 

! 

! 

! Jackass ! 

i 

~\ !} 

s; . 

I ~~ • 

! / NEVADA ··-·-··:!'-···-·-·-·-·-·-·-·-

Amargosa ,, .. ~··/ .// Desert 

// 

' 0 5 

I II II I 

10 KILOMETERS 

I 

Mountain in the western part of NTS. Recognition of 

surficial map units and their correlation within and among 

valleys in the area was based on correlation characteristics 

similar to those described in this report. These 

characteristics, which include topography, drainage, 

topographic relations, soils, desert pavement, and 

depositional environment, are described in detail by Hoover 

and others (1981 ). The relative ages, which were based on 

geomorphic data, were quantified by isotopic ages for some 

of the surficial units as discussed following. 

The physiography and the stratigraphy of the surficial 

deposits in the NTS area are described by Hoover ( 1989). 

This information and the bedrock geology of the NTS area 

are briefly summarized as follows. Bedrock of the NTS area 

consists of Proterozoic and Paleozoic sedimentary rocks, 

chiefly marine limestone, dolomite, quartzite, and argillite, 

and Miocene and Pliocene volcanic rocks, chiefly rhyolitic 

ash-flow tuff and lava. These rocks are exposed in northward-trending, fault-block mountain ranges separated by 

alluvial valleys (Frizzell and Shulters, 1990). 

Surficial deposits, chiefly alluvial fan deposits, were 

mapped in an area of approximately 2,400 km2, principally 

in Crater Flat and Jackass Flats, west and east of Yucca 

Mountain, respectively, and in the northern part of the Amargosa Desert, south of Yucca Mountain (fig. 6). These areas 

are drained by southward-flowing intermittent washes that 

are tributary to the Amargosa River. The climate of the 

region is arid. Average annual precipitation is less than I 8 

em and mean annual temperature is about 17 oc (Houghton 

Figure 6. Sketch map of part of 

Nevada Test Site area showing major 

areas where surficial deposits were 

mapped. Pattern indicates bedrock 

areas. 
CORRELATION OF ALLUVIAL DEPOSITS, SOUTHEASTERN NEVADA 199 

and others, 1975). The altitude ofthe alluvial deposits ranges 

from about 1 ,200 m at the north end of Crater Flat to about 

730 m along the Amargosa River near the south border of the 

area (fig. 6). 

Alluvial deposits of the NTS area form almost continuous piedmont aprons along the mountain fronts. These 

aprons consist of coalescing fan and pediment remnants 

along with numerous inset fans and minor terrace and 

stream-channel deposits. Washes that head in bedrock are 

typically incised into the piedmont aprons, and successively 

younger deposits form inset fans or terraces at successively 

lower levels in these washes. Interfluves are generally 

smooth but commonly exhibit greater dissection of successively older fans by small washes that head within the piedmont. The fans consist of interbedded gravel and gravelly 

sand. The relative amounts of gravel and sand vary greatly 

among fans as does the composition of the gravel; both 

depend largely on the bedrock exposed in the drainage basin 

of the depositing wash. 

These alluvial deposits have been divided into seven 

map units of Pliocene(?) and Quaternary age. Table 6 lists 

these units by map symbol in order of increasing age and 

shows the characteristics used to distinguish these units in 

the field. 

Numerical ages have been determined for some of the 

surficial deposits of the NTS area. These ages were obtained 

using radiocarbon dating, fission-track dating, tephrochronologic identification, and the experimental uranium-trend 

method (Rosholt, 1980, 1985). The ages of the deposits 

range from about 8 to <2, 100 ka as shown in table 7. The 

methods of age determination and the sample localities for 

each unit are discussed by Hoover (1989). 

DISCUSSION 

Similarities in some of the physical characteristics used 

to identify and map the alluvial deposits in the four study 

areas permit tentative correlation of these deposits (table 7). 

These correlations are also extended to similar deposits of 

known and inferred ages in the NTS area. 

Several differences among the sequences of alluvial 

deposits of the four study areas are apparent. Unit Qfm in the 

Chief Mountain area probably includes deposits equivalent 

to the two youngest units that are identified and mapped in 

the other three areas. More detailed studies in the Chief 

Mountain area may identify criteria that will permit the subdivision of unit Qfm into two separate units. Late Pleistocene 

deposits that correlate with those mapped in the Moapa and 

Mesquite areas were not observed in the Chief Mountain and 

Delamar Mountains areas. In the areas where late Pleistocene deposits were mapped, they occur along permanent 

streams that have large drainage basins. Streams of this type 

do not occur in the Chief Mountain and Delamar Mountains 

areas. 

The largest difference between the sequences of deposits in the four areas is the absence of deposits of early Quaternary age in the Moapa Valley area (table 7). In that area 

all the terrace deposits higher and older than the old Quaternary deposit (unit Qo) are of late Tertiary age, according to 

D.L. Schmidt. These late Tertiary alluvial deposits (estimated 3.5 to 5 m.y. old) have terrace surfaces that are well 

preserved by resistant stage V and VI carbonate soils. These 

thick soils developed because of long development time, low 

rainfall, and a large aerial carbonate flux in the Moapa Valley area. 

Comparison of the alluvial deposits of the study area 

with those of the NTS area indicates two significant differences. The age of unit Qlc of the NTS area is reported as 

early Holocene. However, data from the Delamar Mountains 

area indicate the deposits here correlated with unit Qlc are 

late Pleistocene and early Holocene age. Unit Q2c (middle 

Pleistocene) of the NTS area includes two soils that yielded 

uranium-trend ages of about 270 and 440 ka (table 7). These 

ages suggest that unit Q2c includes deposits of at least two 

depositional episodes that could not be mapped separately 

because of similar topographic expression. The deposits of 

these two depositional episodes at NTS may correlate with 

the two oldest units of middle Pleistocene age in the Chief 

Mountain, Delamar Mountains, Moapa Valley, and Mesquite areas. In part of the Delamar area, the two units that 

probably correlate with unit Q2c (units Qaw and Qac) may 

locally be mapped as a single unit (Qaw) where the topographic separation between the two units is slight. 

REFERENCES CITED 

Bagley, D.O., 1980, Soil survey of Virgin River area, Nevada-Arizona-Parts of Clark and Lincoln Counties, Nevada and part of 

Mohave County, Arizona: U.S. Department of Agriculture, 

Soil Conservation Service, 149 p. 

Bohannon, R.G., 1984, Nonmarine sedimentary rocks of Tertiary 

age in the Lake Mead region, southwestern Nevada and northwestern Arizona: U.S. Geological Survey Professional Paper 

1259, 72 p. 

Borup, H.J., and Bagley, D.O., 1976, Soil survey of Meadow Valley area, Nevada-Utah-Parts of Lincoln County, Nevada and 

Iron County, Utah: U.S. Department of Agriculture, Soil Conservation Service, 167 p. 

Colman, S.M., Pierce, K.L., and Birkeland, P.W., 1987, Suggested 

terminology for Quaternary dating methods: Quaternary 

Research, v. 28, p. 314-319. 

Damon, P.E., Shafiqullah, Muhammed, and Scarborough, R.B., 

1978, Revised chronology for critical stages in the evolution of 

the lower Colorado River: Geological Society America 

Abstracts with Programs, v. 10, no. 3, p. 101. 

Eakin, T.E., 1964, Ground-water appraisal of Coyote Spring and 

Kane Springs Valleys and Muddy River Springs area, Lincoln 
Table 6. Characteristics for distinguishing alluvial-fan deposits in Nevada Test Site area. 

[Modified from Hoover and others, 1981] 

Map unit 

Qla 

Qlb 

Qlc 

Q2a 

Q2b 

Q2c 

QTa 

Topographic expression 

Preservation of depositional features Erosional modification of 

depositional features 

Depositional surface intact; local None-------------------------

bar-and-swale topography. 

Depositional surface intact; bar- None------------------------------

and-swale topography common. 

Depositional surface intact. Bar-and-swale topography 

largely obliterated by 

postdepositional 

smoothing. 

Depositional surface intact. Minor local dissection. 

Depositional surface largely intact. None to local minor 

dissection by steep-sided 

washes with sharp edges. 

Depositional surface largely intact. Moderate dissection by 

steep-sided washes with 

sharp edges. 

Depositional surface completely Extensive dissection; 

eroded. rounded interfluves. 

lstages follow standards defined by Gile and others (1966). See table 1. 

Calcic horizon development 

Thickness of Typical 

conspicuous 

carbonate (m) 

None 

0-<0.2 

<0.2 

0.3 

0.3 

1-1.5 

2-3 

maximum 

stage of 

carbonate! 

None 

I 

I 

I 

I-II 

III-IV 

IV 

N 

8 

Pavement development 

None. 

a 

Do. tTl 

0 

r 

0 

a 

Weak pavement locally developed. n 

c.n 

..., 

e 

d 

m 

sn 

o:; 

Loosely packed pavement. ;1> 

:::0 

n 

Loosely to moderately packed p_avement. 0 

c.n 

..., 

e 

d 

--< 

e 

Moderately to tightly packed pavement common. z 

.:::3 

::0 

\0 

N 

Tightly packed pavement locally preserved on 

rounded interfluves. 
Table 7. Correlation of alluvial deposits in five areas in southeastern Nevada and age control for deposits in NTS area. 

[Refer to tables 2-5 for names of map units. Time subdivisions of the Pleistocene correspond to those of Richmond and Fullerton ( 1986 ). Time subdivisions of the Quaternary are informal. FR, field relations with radiocarbon dated 

deposits; FT, fission-track age of volcanic ash underlying QTa; no, not observed; RC, radiocarbon age of charcoal; T, tephrochronologic identification of Bishop ash in deposits that intertongue with Q2c; UT, uranium-trend age. Query 

indicates age boundary uncertain] 

Age control for deposits in NTS area 

Age Chief Delamar Moapa Mesquite area NTS Age Dating Source of data 

Moun- Moun- Valley area (ka) method 

tain tains area 

areal area 

Oal Oa Oany Osv Ola <1 FR Hoover, 1989. Cl) 

~ .! Ofp Olb >1 FR Do. Cl) 

u 

..9 Ofm 0 - 0:: / / / >5 FR Do. " ; 

~ /" 

'"' Oae / ~ v/ Oano Ole 8 RC Do. Cl) 

~ Oaz 

~ - Cl) 

.! no no Otv 02a 40±15 UT Rosholt and others, 1985. 

~ Qy ::> 

0 

Ofr Oaj Qyv Qyt Oam Osy 02b 170±40 UT Do. 

Cl) 

~ 

Cl) Ofms Oaw Om Qymt Oah Osh 270±50 UT Do. u 

0 Cl) 

~ ;a 

"0 02c 

~ ·e 

440±60 UT Do. 

Ofo Oac Oo Oav Ogv Omt Omf 738 T Hoover, 1989. 

~ OTfc OTa OTa <2100 Ff Do. Q no 

~ Oov Oot Oof 

~ 

~ Cl) 

~ -?- -?- Trg Tab -?- Cl) 

u 

0 

~ ;.:::: Tfa Ta Tag ~ 

lAge assignments for units Ofr, Ofo, and OTfc reflect current interpretation and differ from those in Rowley and Shroba (1991). 

n 

0 

i'O 

~ > ::j 

0 

z 

0 

'Tl 

~ r 

~ 

> r 

tj 

tr:1 

a 

Vl 

~ ~Vl 

Vl 

0 

c 

>-3 

s 

> Vl 

>-3 

~ 

~ < 

> tj 

> 

N 

0 

-
202 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

and Clark counties, Nevada: Nevada Department of Conservation and Natural Resources, Ground-Water ResourcesReconnaissance Report 25, 40 p. 

Frizzell, V.A., Jr., and Shulters, Jacqueline, 1990, Geologic map of 

the Nevada Test Site, southern Nevada: U.S. Geological Survey Miscellaneous Investigations Series Map 1-2046, scale 

1:100,000. 

Gardner, L.R., 1972a, Pediments and terraces along the Moapa Valley, Clark County, Nevada: Geological Society of America 

Bulletin, v. 83, p. 3479-3486. 

---1972b, Origin of the Mormon Mesa caliche, Clark County, 

Nevada: Geological Society of America Bulletin, v. 83, p. 

142-156. 

Gile, L.H., Peterson, F.F., and Grossman, R.B., 1966, Morphological and genetic sequences of carbonate accumulation in desert 

soils: Soil Science, v. 101. p. 347-360. 

Hoover, D.L., 1989, Preliminary description of Quaternary and late 

Pleistocene surficial deposits at Yucca Mountain and vicinity, 

Nye County, Nevada: U.S. Geological Survey Open-File 

Report 89-359, 45 p. 

Hoover, D.L., Swadley, W C, and Gordon, A.J., 1981, Correlation 

characteristics of surficial deposits with a description of surficial stratigraphy in the Nevada Test Site region: U.S. Geological Survey Open-File Report 81-512,27 p. 

Houghton, J.C., Sakamoto, C.M., and Gifford, R.O., 1975, Nevada's weather and climate: Nevada Bureau of Mines and Geology Special Publication 2, 78 p. 

Kowallis, B.J., and Everett, B.H., 1986, Sedimentary environments 

of the Muddy Creek Formation near Mesquite, Nevada, in 

Griffen, D.T., and Phillips, W.R.. eds., Thrusting and extensional structures and mineralization in the Beaver Dam Mountains, southwestern Utah: Utah Geological Association 

Publication 15, p. 69-75. 

Longwell, C.R., 1928, Geology of the Muddy Mountains, Nevada: 

U.S. Geological Survey Bulletin 798, 152 p. 

Lustig, L.K., 1965, Clastic sedimentation in Deep Springs Valley, 

California: U.S. Geological Survey Professional Paper 352-F, 

p. Fl31-F192. 

Machette, M.N ., 1985, Calcic soils of the southwestern United States, 

in Weide, D.L., ed., Soils and Quaternary geomorphology of the 

southwestern United States: Geological Society of American 

Special Paper 203, p. 1-21. 

Mifflin, M.D., and Wheat, M.M., 1979, Pluvial lakes and estimated 

pluvial climates of Nevada: Nevada Bureau of Mines and 

Geology Bulletin 94, 57 p. 

Quade, Jay, 1986, Late Quaternary environmental changes in the 

upper Las Vegas Valley, Nevada: Quaternary Research, v. 26, 

p. 340-357. 

Richmond, G.M., and Fullerton, D.S., 1986, Introduction to Quaternary glaciations in the United States of America, in Sibrava, 

V., Bowen, D.Q., and Richmond, G.M .. eds., Quaternary glaciations in the northern hemisphere: Quaternary Science 

Reviews, v. 5, p. 3-10. 

Rosholt, J.N., 1980, Uranium-trend dating of Quaternary sediments: 

U.S. Geological Survey Open-File Report 80-1087. 65 p. 

---1985, Uranium-trend systematics for dating Quaternary 

sediments: U.S. Geological Survey Open-File Report 85-298, 

34p. 

Rowley, P.O., and Shroba, R.R., 1991, Geologic map of the Indian 

Cove quadrangle, Lincoln County, Nevada: U.S. Geological 

Survey Geologic Quadrangle Map GQ-170 1, scale I :24,000. 

Rowley, P.D., Shroba, R.R., Simonds, F.W., Burke, K.J., Axen, 

G.J., and Olmore, S.D., 1991, Geologic map of the Chief 

Mountain quadrangle, Lincoln County, Nevada: U.S. Geological Survey Open-File Report 91-135, scale 1 :24,000. 

Rowley, P.D., Snee, L.W., Mehnert, H.H., Anderson, R.E., Axen, 

G.J., Burke, K.J., Simonds, F.W., Shroba, R.R., and Olmore, 

S.D., 1992, Structural setting of the Chief mining district, eastem Chief Range, Lincoln County, Nevada, chapter H in Thorman, C.H., ed., Application of structural geology to mineral 

and energy resources of the central and western United States: 

U.S. Geological Survey Bulletin 2012, p. Hl-H17. 

Rush, F.E., 1964, Ground-water appraisal of the Meadow Valley 

area, Lincoln and Clark Counties, Nevada: State of Nevada 

Department of Conservation and Natural Resources, GroundWater Resources-Reconnaissance Series Report 27, 43 p. 

U.S. Department of Commerce, 1975, Climatological data annual 

summary, Nevada. 
Low-Angle Normal Faults in Devonian Rocks 

of the Southern Delamar Mountains, 

Lincoln County, Nevada 

By William R. Page 

GEOLOGIC STUDIES IN THE BASIN AND RANGE-COLORADO PLATEAU TRANSITION IN 

SOUTHEASTERN NEVADA, SOUTHWESTERN UTAH, AND NORTHWESTERN ARIZONA, 1992 

U.S. GEOLOGICAL SURVEY BULLETIN 2056-G 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1995 
CONTENTS 

Abstract. .......................................................................................................................... 205 

Introduction .................................................................................................................... 205 

Geologic Setting ............................................................................................................. 207 

Description of Low-angle Fault System ......................................................................... 209 

PWL Fault. .............................................................................................................. 210 

Ramping Low-angle Faults .................................................................................... 210 

Discussion ....................................................................................................................... 211 

Conclusions .................................................................................................................... 216 

References Cited ............................................................................................................. 216 

FIGURES 

1. Index map showing location of study area, southeastern Nevada ............................................................................... 206 

2. Generalized geologic map of study area ..................................................................................................................... 208 

3. Generalized stratigraphic column of Tertiary volcanic and Paleozoic rocks exposed in the 

southern Delamar Mountains ...................................................................................................................................... 209 

4. Photographs of PWL fault plane .............. ............ ....... ...... ............... ........... .............. ........... .............. ....... .. . . ........ ...... 211 

5. Present-day and restored cross sections along A-A' from figure 2, showing interrelations of PWL fault .................. 212 

6. Photograph of west-vergent fold exposed in upper plate of PWL fault.. .................................................................... 214 

7. Photograph and sketch of fault network at locality RL-1 . . . .. . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . .. .. . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . 214 

8. Restored cross section of the Delamar ramp anticline to the time of thrust emplacement and prior to 

low-angle normal faulting ........................................................................................................................................... 215 
Low-Angle Normal Faults in Devonian Rocks 

of the Southern Delamar Mountains, 

Lincoln County, Nevada 

By William R. Page 

ABSTRACT 

A system oflow-angle normal faults occurs in the Paleozoic rocks of a major west-dipping ramp above the Sevierage Delamar thrust fault on the west flank of the southern 

Delamar Mountains, southeastern Nevada. The most prominent fault in the system is a west-dipping low-angle fault that 

places Middle and Upper Devonian Guilmette Formation and 

the uppermost member of the Middle Devonian Simonson 

Dolomite in the hanging wall above Lower Devonian Sevy 

Dolomite in the footwall, omitting parts of the Simonson and 

the Sevy Dolomites. The hanging wall of this fault contains 

west-vergent folds, high-angle normal faults, and beddingplane faults. 

The normal fault system also includes networks of 

ramping low-angle faults within the Guilmette Formation. 

These faults dip westward, occur at multiple stratigraphic 

levels, and are distinguished by local bedding discordances 

as great as 35°. The faults generally follow bedding planes, 

but locally ramp down section to the west. 

The age of the normal faults is poorly constrained; however, map relations suggest that these normal faults predate 

Oligocene volcanic rocks (27 Ma). The faults are interpreted 

to have formed in Late Cretaceous or early Tertiary time by 

gravitational collapse off the west flank of the Delamar 

allochthon following ramping of the Sevier-age Delamar 

thrust fault. Restoration of the prominent low-angle fault in 

the system to postfault, pre-erosion geometry indicates that a 

minimum of about 1.7 kilometers of Paleozoic and Mesozoic 

rocks was eroded off the upthrown block of the fault prior to 

the emplacement of the volcanic rocks, which strongly suggests that faulting occurred either during or shortly following 

Sevier compression, rather than during regional, middle Tertiary prevolcanic extension. 

INTRODUCTION 

This report examines the geometry, timing, and tectonic 

implications of low-angle normal faults in Paleozoic rocks in 

the southern part of the Delamar Mountains, southeastern 

Nevada. The southern Delamar Mountains are located about 

110 km north of Las Vegas, Nev. (fig. 1), in the eastern part 

of the Basin and Range province. The mountains are 

bounded by Coyote Spring Valley on the west, Kane Springs 

Valley on the southeast, and the Pahranagat shear system on 

the northwest. Topographic relief from the base of Coyote 

Spring Valley to the top of adjacent mountains in the range 

is approximately 1 km. Steeply dipping, folded Paleozoic 

rocks underlie a gentle irregular erosional surface below a 

veneer of gently dipping Tertiary volcanic rocks. The study 

area, shown in figure 2, has been mapped in detail as part of 

the Basin and Range to Colorado Plateau Transition 

(BARCO) Study Unit (see Introduction to this volume) 

(Page and others, 1990; Scott and others, 1990; and Swadley 

and others, 1990). The low-angle normal fault system is 

located on the west flank of the mountain range, near the 

southwest terminus of the Delamar Mountains. 

The southern Delamar Mountains are within the Sevier 

orogenic belt that trends northeast from southeastern California, through southern Nevada, and into southwestern and 

central Utah (Armstrong, 1968; Fleck, 1970) (fig. 1). The 

belt consists of east- to southeast-directed decollement-style 

thrust faults that formed during the Cretaceous in southern 

Nevada. The Delamar thrust fault, the major structure in the 

southern Delamar Mountains, is one of these thrusts. 

Numerous low-angle normal faults attenuate strata in 

the Sevier orogenic belt. Some of these low-angle normal 

faults are interpreted as middle Tertiary structures formed as 

a result of regional crustal extension. Examples near the 

study area include the Mormon Peak and Tule Springs 

detachments in the Mormon Mountains of southern Nevada 

(Axen and others, 1990; Wernicke and others, 1985), and the 

Stampede and Highland Peak detachments in east-central 

Nevada (Axen, 1986; Axen and others, 1988; Axen and others, 1993). 

In contrast, some low-angle normal faults in the hinterland of the Sevier orogenic belt attenuate strata and were 

described as Cretaceous to early Tertiary extensional structures. Examples include faults in the Drum Mountains in 

205 
206 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

120° 420 .---~NE=V~A~D~A~-.--r----+~ 

.· .. ··· 

Basin 

and 

Range 

province 

~til- •• •• -~1i •• -~IY •• •• 

~·()' .·· 

.· .. ···· 

0 10 KILOMETERS 

Figure 1. Index map showing location of study area, southeastern Nevada. 

west-central Utah and the southwestern Deep Creek Range, 

western Utah and eastern Nevada (Nutt and Thorman, 1992; 

Nutt and others, 1992), and in the Raft River and Black Pine 

Mountains in northeastern Utah and southern Idaho (Wells 

and others, 1990). 

Many questions arise concerning the age, origin, and 

tectonic implications of the low-angle normal faults in the 

southern Delamar Mountains. A prevolcanic age of these 

normal faults is inconsistent with Miocene and younger 

regional extensional structures observed in the Delamar 

Mountains. In this report, I address the possibility that these 

normal faults represent localized gravitational collapse of 

probable Late Cretaceous or early Tertiary age, either 

synchronous with or following regional compression and 

crustal thickening related to ramping of the Delamar 

allochthon. 
FAULTS IN DEVONIAN ROCKS, DELAMAR MOUNTAINS 207 

GEOLOGIC SETTING 

About 4.3 km of Cambrian through Lower Permian 

rocks are exposed in the southern Delamar Mountains (figs. 

2, 3). These rocks comprise predominantly dolomite and 

limestone and subordinate sandstone and shale, deposited 

near the eastern margin of the Cordilleran miogeocline. 

The Paleozoic rocks are overlain unconformably by as 

much as 450 m of gently dipping Tertiary volcanic rocks 

(figs. 2, 3) that are subdivided into two groups. The older 

group consists of late Oligocene to early Miocene ash-flow 

tuffs deposited from regional ash flows erupted from the 

Central Nevada caldera complex (Best and others, 1993 ), 

and the Caliente caldera complex (Rowley and others, this 

volume). These rocks range in age from about 27 Ma to 18.6 

Ma. The younger group of rocks include the Kane Wash 

Tuff, erupted from the Kane Springs Wash caldera in the 

central Delamar Mountains (fig. 2) about 14.5 Ma (Scott and 

others, this volume), and two other units, the Delamar Lake 

and Sunflower Mountain Tuffs that erupted from undetermined sources and were dated at 15.6 Ma and 14.7 Ma, 

respectively, by Novak (1984). 

The structural history of the southern Delamar 

Mountains records compression of the northeast-trending 

Sevier orogenic belt followed by middle Cenozoic to modern 

extension. During the Sevier orogeny in Cretaceous time, the 

Paleozoic rocks in the study area formed an allochthon that 

ramped eastward on the Delamar thrust fault. The hangingwall block consists of lower Paleozoic rocks that form a 

ramp anticline, and the footwall block consists of upper 

Paleozoic rocks that form a tightly folded overturned 

syncline (fig. 2). 

Following the Sevier orogeny and an erosional period 

of about 35-50 million years, extensional tectonics dominated southern Nevada. The onset of extension in this area 

remains poorly understood, but early phases of extension in 

the southern Basin and Range province are believed to have 

begun in latest Eocene or Oligocene time (Wernicke and others, 1987). Major regional extensional episodes occurred 

mostly during the Miocene in the Delamar Mountains (Scott, 

1990) and to the south, in northern Clark County (Bohannon, 

1984; Guth, 1981; Guth and others, 1988; Wernicke and others, 1985, 1988). However, examples of regional Oligocene, 

prevolcanic extensional faulting have been reported from 

areas north of the Delamar Mountains in Lincoln and northeastern Nye Counties (Axen, 1986; Axen and others, 1988; 

Axen and others, 1993; Jayko, 1990; Lewis, 1987; Taylor 

and others, 1989). 

The major extensional structures in the southern 

Delamar Mountains are Miocene and younger and include 

the Kane Springs Wash fault, and Coyote Spring fault, the 

southernmost strand of the Pahranagat shear system (fig. 2). 

The Pahranagat shear system is a zone of northeast-trending, 

steeply northwest dipping faults that show both dip-slip and 

left-lateral strike-slip displacement (Tschanz and Pampeyan, 

1970; Ekren and others, 1977). Liggett and Ehrenspeck 

(1974) interpreted the system as a fault zone that accommodates differentially extending domains, a model also used to 

explain the Las Vegas Valley shear zone, which separates 

highly extended blocks in the Sheep Range north of the shear 

from less extended blocks in the Spring Mountains south of 

the shear (Guth, 1981). Tschanz and Pampeyan (1970) estimated approximately 9-16 km of left-lateral displacement 

on the shear zone. The maximum age of major deformation 

of the Pahranagat shear system is unknown; however, 

Tschanz and Pampeyan ( 1970) postulated that the shear system might have originally been active as a right-lateral tear 

fault during Sevier orogenic thrusting. Fault scarps and fissures in alluvial deposits within the shear system suggest a 

modern age for the minimum age of deformation in the shear 

system (Swadley and Scott, 1988; Jayko, in press). Current 

seismicity on faults within the shear system suggests that it 

is presently active (Rogers and others, 1987). 

The Coyote Spring fault consists of two segments: a 

western segment strikes north and dips west, and a northern 

segment strikes northeast and dips northwest, and is parallel 

to other strands of the Pahranagat shear system (fig. 2). The 

northern segment is interpreted as a sinistral oblique fault; 

however, where the fault strike bends about 60°, the fault 

becomes the western segment interpreted as a minor breakaway with predominantly dip-slip displacement on the west 

side of the southern Delamar Mountains block (Scott and 

others, 1990). Stratigraphic offset on the western segment is 

estimated to be about 300 m by Tschanz and Pampeyan 

(1970), and I estimate at least 650 m based on exposure of 

Tertiary volcanic rocks (Kane Wash Tuff) on the 

downthrown block of the Coyote Spring fault in Coyote 

Spring Valley. The age of the Coyote Spring fault is not well 

established. The youngest units cut by the fault are Tertiary 

alluvial sediments of probable Pliocene to Miocene age, but 

the earliest time of movement cannot be determined. 

The Kane Springs Wash fault is a northeast-striking, 

northwest-dipping sinistral oblique-slip fault that bounds the 

west flank of the Meadow Valley Mountains. Progressive 

tilting of volcanic strata in the Meadow Valley Mountains 

both south of and within the Kane Springs Wash caldera 

indicates that this area was being affected by extension at 

least as early as about 16 Ma (Scott and others, this volume; 

Harding and others, this volume). Yet no progressive tilting 

has been observed west of the Kane Springs Wash fault in 

the Delamar Mountains. This suggests that the Kane Springs 

Wash fault formed the western limit of that tilting and therefore was active at least as early as 16 Ma. 

The Kane Springs Wash caldera is a 12x25 km ovalshaped caldera located in the northwestern part of the study 

area (fig. 1 ). Outflow sheets of the Kane Wash Tuff erupted 

from the caldera 14.5 Ma (Scott and others, this volume). 

Rocks in the western part of the caldera in the Delamar 

Mountains have been undisturbed structurally for the most 
208 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

Tv 

Tv 

::0)::: : ... . 

]§:: :::" 

:: : 

.... ....... . ... .... .. .... .. 

::: ::::: :::::: : . ..... ····· ··· ........ 

. ·····-·· ··· ·· · ·· ··· ····· .... ... . .. .. 

.. . ... ·· ·· ···· 

ora:::: 

0 

I 

KANE SPRINGS WASH CALDERA 

5 KILOMETERS 

I 

Tv 

. . . . . . . . . . . . . . . 

::::ora::::: ::: 

Tv 

Figure 2 (above and facing page). Generalized geology of study area. Geology modified from Page and others 

( 1990), Scott and others ( 1990), and Swadley and others ( 1990). Cross section A-A' in figure 5. 

part, except for a slight 5°- 10° dip to the east. In contrast, 

rocks in the eastern part of the caldera in the Meadow Valley 

Mountains were tilted eastward about 15°- 20° on normal 

faults between 14.5 and 11.5 Ma (Harding and others, this 

volume) and were subsequently offset 4-7 km by the sinistral Kane Springs Wash fault (fig. 1 ). 

Rocks in the southern Delamar Mountains appear to be 

only mildly extended by high-angle normal faults, but are 

surrounded by highly extended regions to the north across the 

Pahranagat shear system (Hudson and others, 1991; Scott and 

others, 1993), to the southeast in the Mormon Mountains 

(Wernicke and others, 1985), and to the southwest in the 

Sheep Range (Guth, 1981; Guth and others, 1988). The 

southern Delamar Mountains are considered as "stable 

terrane" in the context of regional extensional tectonics 

(Wernicke and others, 1985). Although exposed regional 
FAULTS IN DEVONIAN ROCKS, DELAMAR MOUNTAINS 209 

T 

EXPLANATION 

Quaternary and Tertiary alluvium 

Tertiary volcanic rocks 

Permian and Pennsylvanian rocks 

Pennsylvanian to Devonian rocks 

Devonian rocks 

Devonian to Cambrian rocks, undivided 

Silurian rocks 

Ordovician rocks 

Cambrian rocks 

Contact 

High-angle normal fault -Dashed where approximately located; dotted where concealed; bar and 

ball on downthrown side 

4 T + Normal fault superposed on preexisting thrust fault --

Dotted where concealed; sawteeth on upper plate of 

thrust; bar and ball on downthrown side of normal 

II II 

-

fault 

Low-angle normal fault-Dotted where concealed; 

hachures on upper plate of fault 

+ 

Oblique-slip normal fault --Arrows show relative 

direction of lateral offset; bar and ball on downthrownside 

Anticline 

ft Overturned syncline 

Caldera edge-Dotted where concealed; hachures on 

caldera interior 

Strike and dip of sedimentary beds and compaction 

foliation of ash-flow tuffs 

_.I._ Inclined 

-P- Overturned 

PWL fault Prominent west-dipping low-angle normal fault 

RL-1 Ramping low-angle normal fault 

detachment faults are absent, these structures are proposed to 

underlie the southern Delamar Mountains at depth (Scott, 

1990). In Scott's interpretation, large stable island-like 

blocks, such as the southern Delamar Mountains, are 

envisioned to be adrift, riding passively above detachment 

systems at depth that connect adjacent extended areas. The 

presence within the block of Miocene and younger structures 

such as gravity-slides, high-angle normal faults bounding 

strike-slip faults, and grabens floored by steeply dipping 

middle Miocene strata demonstrates local denudation and 

internal breakup of the stable block, reflecting differential 

movement of the block on the underlying detachment system. 

AGE THICKN7 

(m) STRATIGRAPHIC UNIT 

Kane Wash Tuff (14.5 Ma) 

~ 200 Sunflower Mountain Tllf (14.7 Ma) 

111 Delamar Lake Tuff (15.6 Ma) 

t: 

Gl 

1-

250 Older tuffs (27to 18.6 Ma) 

c 

.!!! 

E 

Ill 

0.. 

~ 1060 Bird Spring Formation 

c 

.!!! 

c 

111 

> 

>. 

~ c 

If. 

-

c 58 Indian Springs Formation 111 

a. 

a. 

~ 204 Chainman Shale 

0 

0 

~ 64 Joana Limestone 

417 Guilmette Formation 

Iii 

~ 247 Simonson Dolom~e 

~ 0 

235 Sevy Dolomite 

c 

111 

~ 291 Laketown Dolomite 

a; 

135 Ely Springs Dolomite 

Iii 

~ 

~ 38 Eureka Quartzite 

0 

575 Pogonip Group 

c 564 Desert Valley Formation 

-~ .0 

E 76 Dunderberg Shale 111 

u 

459 Highland Peak Formation 

Figure 3. Generalized stratigraphic column of Tertiary volcanic 

and Paleozoic rocks exposed in the southern Delamar Mountains. 

DESCRIPTION OF LOW-ANGLE 

FAULT SYSTEM 

The low-angle, ramping properties of the fault system 

in the southern Delamar Mountains are geometrically similar 

to those of minor thrust faults, except that one of the fundamental rules of thrust geometry is violated (Dahlstrom, 

1970). Detailed stratigraphic and structural studies have 

shown that the low-angle faults attenuate rather than thicken 

the stratigraphic section; in most cases these faults place 
210 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

younger rocks over older rocks in contrast to older over 

younger as generally shown by thrust faults. Therefore, these 

faults are proposed to be low-angle normal faults rather than 

compressional thrust faults. 

The prominent west-dipping low-angle (PWL) normal 

fault and ramping low-angle faults are viewed as one integrated fault system. Although the PWL and ramping faults 

occur at different scales, observed similarities among them 

suggest that these low-angle normal faults are contemporaneous, and have a common genesis. These similarities 

include (I) restriction of these structures to one specific geographic area within the mountain range, (2) restriction to 

Devonian rocks, (3) consistent low-angle, westward dip, and 

( 4) offset by the Coyote Spring fault. 

PWLFAULT 

The PWL fault is exposed along the west flank of the 

southern Delamar Mountains. Here the Guilmette Formation 

and uppermost member of the Simonson Dolomite in the 

hanging wall overlie the lower member of the Sevy Dolomite in the footwall, eliminating about 270 m of the stratigraphic section. 

Canyons that dissect the western range front expose the 

scoop-shaped PWL fault plane along 1.5 km from its north 

to its south boundary (fig. 2). The fault plane steepens at its 

boundaries, and flattens between boundaries, making a 

scoop or shovel-shaped fault. Along the southern fault 

boundary, the fault plane dips approximately 55° NNW. 

Although exposures are limited along the northern boundary, 

the fault plane appears to dip south at about 50°- 60°. In the 

deepest canyon on the southern interior of the fault block, the 

fault plane dips southwest at approximately 15° (fig. 4A, 4B), 

and slickenlines on the fault plane trend N. 67° W. and 

plunge 12° SW. (Page and others, 1990). The fault is also 

exposed at several other places within the interior of the fault 

block and dips gently to the west. About 1 m of fault gouge 

is exposed along the fault plane, and abundant small-scale, 

high-angle west-dipping normal faults sole into the master 

fault plane (fig. 4A). 

I interpret the eastern projection of the PWL fault to be 

covered by tuff of the Leach Canyon Formation (dated at 

23.8 Ma; Best and others, 1993). The projection of the PWL 

fault is probably covered by the Shingle Pass Tuff (26. 7 Ma; 

Best and others, 1989) and the Monotony Tuff (27 .3 Ma; 

Best and others, 1989) as well, because these units are 

exposed north and south of the PWL fault and conformably 

underlie the Leach Canyon Formation (fig. 5A) (Page and 

others, 1990). This interpretation requires that the fault 

formed prior to deposition of these late Oligocene and early 

Miocene volcanic rocks and can be assigned a minimum date 

of about 27 Ma (late Oligocene); however, field evidence is 

somewhat equivocal because landslide deposits largely conceal a poorly exposed Paleozoic-Tertiary contact at this 

locality. Discontinuous, high-angle, small-offset normal 

faults are present in the volcanic rocks near the east edge of 

the PWL fault (Page and others, 1990), but these high-angle 

normal faults do not appear to be associated with the PWL 

fault. Restoration of the PWL fault to postfault, pre-erosion 

geometry (fig. 5B) indicates that minimum offset on the 

PWL fault must have been at least 1.7 km; no faults of this 

magnitude cut the volcanic strata, which are nearly undeformed. For further discussion on the age of faulting, see the 

section, "Discussion." 

The PWL fault cuts up stratigraphic section in the direction of transport because the strata dip about 30° W. whereas 

the fault only dips at 15° W. Although this geometry seems 

atypical for normal faults, it can be explained by normal 

faulting of the west-dipping panel of rocks previously 

deformed in the Mesozoic during the Sevier orogeny. The 

volcanic rocks which overlie both the Paleozoic rocks and 

the eastern extension of the PWL fault dip about 10° E., indicating that the west side of the mountain range has been 

slightly tilted at least since late Oligocene time. Therefore, 

the original tilt of the Paleozoic rocks probably was about 

I 0° greater during faulting, or about 40° W., making the 

original dip of the PWL fault about 25° W. 

Several west-vergent folds are present in the upper plate 

of the PWL fault, some with amplitudes as great as 20m and 

wavelengths of about 30 m (fig. 6). These folds are interpreted as local ductile shear structures that probably formed 

in response to drag as upper layers in hanging-wall rocks 

were transported westward over lower plate rocks. Local 

contractional structures associated with attenuated regimes 

have been found in other areas of the Basin and Range (Bartley and Wernicke, 1984; Campagna and Aydin, 1991; 

Hodges and others, 1984; Lund and others, 1991; Wernicke 

and others, 1985). 

RAMPING LOW-ANGLE FAULTS 

Exposed directly south of the PWL fault are complex 

fault networks described as ramping low-angle faults (Page 

and Scott, 1991 ). These fault networks were examined on outcrop scale (fig. 2) to better define their structural geometry. 

An example of these structures (RL- 1) is shown in figure 7. 

Exposures of RL- 1 (fig. 7) show two major discordances- both of which are faults. The two faults occur at different stratigraphic levels and separate the rocks into lower, 

middle, and upper panels. Rocks in the lo.wer panel dip about 

30° W. whereas beds in the middle panel dip about 15° W. 

and SW. The fault that separates lower from upper panels 

dips about 30° W. and is bedding-plane parallel to the lower 

panel. A discordance of about 15° is recorded between lower 

and middle panels. I do not know whether or not the lower 

panel rocks are bounded by a buried fault; however, as 

regional dips from this part of the mountain range average 

about 30° W., a buried fault is not required. Rocks in the fault 
FAULTS IN DEVONIAN ROCKS, DELAMAR MOUNTAINS 

PWL 

FAULT 

PLANE 

yPWL 

FAULT 

PLANE 

Figure 4. Views of PWL fault plane. A, View toward north, of fault plane dipping about 15° W. (left); Guilmette 

Formation in upper plate, Sevy Dolomite in lower plate; note small-scale high-angle normal faults that sole into PWL 

fault. Barbs show direction of relative movement. B, Same location as A, except view is to west, looking down fault 

plane. Coyote Spring Valley is in background. Hammer on lower right for scale. 

211 

zone of the lower fault are highly brecciated, altered, and 

preferentially dolomitized. The upper fault at RL-1 occurs at 

a higher stratigraphic level and separates the middle and 

upper panels. The upper fault shows a typical footwall ramp 

that dips approximately 50° W. The ramp truncates bedding 

sharply in the footwall and abruptly flattens to the west, 

becoming a west-dipping bedding-plane fault. Along the 

ramp, strata in the upper panel dip approximately 50° W., 

whereas in the middle panel they dip about 15° W., a discordance of about 35°. 

DISCUSSION 

The minimum age of the low-angle normal fault system in the southern Delamar Mountains is believed to be 
212 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

WEST 

A 

(J) 1.5 a: 

PWL 

FAULT 

::!! 

~ 1.0 L--~~3;:J.--7 0 

g 0.5 

Tv 

EAST 

AI 

1.5 

---~-.....----..--4 1.0 

0.5 

SEA LEVEL L.___L..__,L__L.__ ___ ____L:..___L ______ .l..___L_:l::::...__:___j~ __ ::::::....J SEA LEVEL 

A 0 5 KILOMETERS 

EXPlANATION 

OTa Quaternary and Tertiary alluvium 

Tv Tertiary volcanic rocks 

Dg Guilmette Formation (Devonian) 

Dsi Simonson Dolomite (Devonian) 

Dse Sevy Dolomite (Devonian) 

Sl Laketown Dolomite (Silurian) 

0 Ordovician rocks 

£ Crunbrianrocks 

pC Precambrian rocks 

Contact 

_..;;,£._=..- Prominent west-dipping low-angle normal fault (PWL)-

Barb shows direction of reiative movement 

High-angle normal fault 

Figure 5 (above and facing page). Views of cross section A-A' from figure 2. A, Present-day 

cross section A-A'. PWL fault is restricted to tilted Paleozoic rocks below overlying volcanic rocks. 

B, Restored cross section of PWL fault along A-A' to postfault, pre-erosion geometry. 

pre-27 Ma (late Oligocene) based on the age of the oldest 

volcanic strata that overlie the PWL fault. Although the 

maximum age is poorly constrained, faulting is proposed to 

be either Late Cretaceous or early Tertiary based on restoration of the PWL fault to postfault, pre-erosion geometry 

(fig. 5B); that geometry shows vertical offset of about 1. 7 

km. This offset represents the minimum amount of erosion 

of Paleozoic and Mesozoic rocks off the upthrown block of 

the fault necessary to form the topography onto which the 

volcanic rocks were deposited. The volcanic rocks above the 

PWL fault show no thickness variation across the fault indicating that no relief was left across the fault at the time of 

ash flow emplacement. The length of time required for this 

erosion is probably on the order of tens of millions of years, 

suggesting that faulting is more likely to have occurred 

either during or shortly following Sevier thrusting rather 

than during middle Tertiary prevolcanic extension reported 

north of the southern Delamar Mountains, in Lincoln and 

Nye Counties. 

These . extensional structures in the southern Delamar 

Mountains are interpreted to have formed as a result of 

gravitational collapse in response to relief produced by 

overthickening of the crust during Sevier thrusting. The 

concept of gravitational collapse along orogenic belts has 

been reviewed by many workers. Coney ( 1987) and 

Wernicke and others ( 1987) proposed that thickening of the 

crust in the Sevier orogenic belt was the ultimate driving 

force of regional Tertiary crustal extension in the Basin and 

Range province. Dewey ( 1988) reported that orogenic belts 

with topographic relief of 3 km or greater have the 

gravitational potential to collapse and spread laterally due to 

a decrease in boundary forces controlled by inhomogeneities 
(/) 

a: 

w 

1-

w 

:::E 

g 

S2 

z 

z 

0 

i= 

WEST 

A 

8 

6-

4-

<( 2-

> w 

_J 

w 

B 

FAULTS IN DEVONIAN ROCKS, DELAMAR MOUNTAINS 

Mz 

pp 

M 

Dg 

Dsi 

Dse 

Sl 

0 

-£ 

p£ 

0 

EAST 

A' 

PWLFAULT~ 

Mz 

Mz 

EXPLANATION 

Mesozoic rocks 

Permian and Pennsylvanian rocks 

Mississippian rocks 

Guilmette Formation (Devonian) 

Simonson Dolomite (Devonian) 

Sevy Dolomite (Devonian) 

Laketown Dolomite (Silurian) 

Ordovician rocks 

Cambrian rocks 

Precambrian rocks 

Contact 

p-£ 

5 KILOMETERS 

Prominent west-dipping low-angle normal fault (PWL)-

Barb shows direction of relative movement 

8 

6 

4 

2 

0 

213 

within thickened continental crust. The decrease in boundary 

forces is also dependent on thermal constraints within the 

continental lithosphere. Sonder and others ( 1987) also 

discussed the concept of thickened crust as being 

gravitationally unstable, leading to spreading under its own 

weight. They concluded that the time interval between 

compression and the onset of extension depends on rates of 

thermal relaxation in the crust; the higher the thermal 

regime, the shorter time interval between compression and 

the response of gravitational spreading or collapse. Burchfiel 
214 

West 

GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

Figure 6. West-vergent fold, amplitude about 20m, exposed in upper plate of PWL fault. View 

toward northwest. 

Figure 7. Photograph and sketch of ramping low-angle fault network at RL- 1. View toward north. 

East 

30 METERS 
FAULTS IN DEVONIAN ROCKS, DELAMAR MOUNTAINS 215 

and others (1992), Burchfiel and Royden (1985), Chen and 

others ( 1990), and Hodges and others ( 1992) have 

documented gravitational collapse in the Himalayan 

orogenic belt that is believed to be contemporaneous with 

compression. In these examples, basic requirements for 

gravitational collapse were met: crustal thickening, 

topographic relief (represented by the elevated Tibetan 

plateau), and a high thermal regime. Other examples of 

gravitational collapse in collisional plate-tectonic settings 

occur in the Caledonides in northern Norway (Cashman, 

1990), and the eastern Alps (Ratschbacher and others, 1989). 

The Raft River and Black Pine Mountains in northwestern 

Utah and southern Idaho provide an example of gravitational 

collapse in the hinterland of the Sevier orogenic belt (Wells 

and others, 1990). 

These examples support the conclusion that gravitational collapse related to regional crustal extension is contemporaneous with or follows crustal thickening. However, 

no evidence exists to suggest that the low-angle normal 

faults in the southern Delamar Mountains are related to 

regional crustal extension. Although the mechanics of both 

regional and localized gravitational collapse are probably 

similar, an alternative explanation, independent of a regional 

extensional process, must be postulated for these upper 

crustal faults. Height of the Delamar ramp anticline above 

CJ) 

a: 

w 

Iw 

~ 

9 

S2 

~ 

z 

0 

w 

~ ....J 

w 

w 

10 

5 

0 

-5 

0 

I 

5 

I 

areas to the west prior to erosion and at the time of thrust 

emplacement is estimated to have been about 5 km, based on 

restoration of the ramp anticline to the time of thrust 

emplacement (fig. 8). This estimate includes about 2.5 km of 

Mesozoic rocks assumed to have been deposited in the 

southern Delamar Mountains, but subsequently eroded. The 

thickness of the Mesozoic rocks is based on exposures of 

these rocks just 15 km east of the study area on the west flank 

of the Meadow Valley Mountains (Pampeyan, 1989). I propose that gravitational instabilities occurring along incompetent bedding planes on a 40° -dipping thrust ramp elevated by 

Sevier thrusting induced local gravity-sliding westward off 

that ramp. 

In the southeast part of the study area in the Meadow 

Valley Mountains, Swadley and others (1990) mapped two 

stratigraphic levels of low-angle faults (fig. 2). The upper 

fault placed rocks of the brown cliff member of the Simonson Dolomite in the hanging wall above the middle to upper 

part of the Sevy Dolomite (Swadley and others, 1990). The 

fault followed the the stratigraphic level of the 13-m-thick 

quartzite at the base of the Simonson Dolomite (Swadley 

and others, 1990), converted the quartzite to fault gouge, 

and removed about 130 m of strata including middle and 

lower parts of the Simonson, and upper parts of the Sevy. 

The lower fault generally followed the stratigraphic level of 

DELAMAR RAMP ANTICLINE 

10 KILOMETERS 

I 

E 

- 10 

EXPI.ANAnON 

Mz Mesozoic rocks 

~ Paleozoic rocks 

pS Precambrian rocks 

Figure 8. Restored cross section of the Delamar ramp anticline to the time of thrust emplacement (Cretaceous), and prior to low-angle normal faulting. Restoration shows sufficient relief of ramp anticline to induce gravity-sliding off the west flank. 
216 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

the Eureka Quartzite, converted the quartzite to fault gouge, 

and truncated strata of the underlying Pogonip Group, and 

overlying Ely Springs Dolomite. At many locations in the 

Meadow Valley Mountains, the Eureka Quartzite is absent, 

or is only several meters thick where it has been tectonically 

omitted or thinned by low-angle faulting. Although upper 

plate Ordovician strata are locally altered to jasperoid and 

are highly faulted, overlying Tertiary volcanic rocks are neither altered nor highly faulted. This suggests that these lowangle faults predate Tertiary volcanism and major regional 

extension in the Meadow Valley Mountains. Perhaps these 

faults formed as gravity-slides similar to the low-angle normal faults in the southern Delamar Mountains during Late 

Cretaceous or early Tertiary time, or they may be related to 

some other regional prevolcanic episode of extension such 

as the prevolcanic Oligocene regional extension noted in 

northern Lincoln and northeastern Nye Counties (Axen and 

others, 1988; Axen and others, 1993; Taylor and others, 

1989). 

Kellogg ( 1992) reported evidence for Late Cretaceous 

or early Tertiary gravity-sliding in the Deep Creek Mountains of southeastern Idaho. He interpreted the Deep Creek 

fault as a west-dipping, west-directed, low-angle fault 

formed by localized gravity-sliding off topographic relief 

developed by ramping of a Sevier-age thrust, much like the 

case in the Delamar Mountains. Vandervoort and Schmitt 

( 1990) suggested that the presence of Upper Cretaceous 

scarp-front alluvial facies in the Sevier hinterland was evidence of normal-faulting-related localized gravitational collapse of Late Cretaceous to early Tertiary age; this high-level 

crustal deformation may also be similar to that of the southern Delamar Mountains. 

CONCLUSIONS 

A system of low-angle normal faults occurs in Devonian rocks along the west flank of the southern Delamar 

Mountains. Faults of that system dip westward, attenuate the 

stratigraphic section, and place younger rocks over older 

rocks. The age of the fault system is poorly constrained; 

however, evidence from geologic mapping suggests that the 

minimum age is greater than 27 Ma. Although the maximum 

age of faulting cannot be determined, the fault system is 

interpreted as Late Cretaceous or early Tertiary, based on 

restoration of the most prominent low-angle fault (PWL) of 

the fault system to postfault, pre-erosion geometry. The lowangle fault system is interpreted to have formed by processes 

of brittle, gravitational collapse off the steeply west dipping, 

west flank of the Delamar allochthon, following ramping of 

the Sevier-age Delamar thrust fault. This deformation is 

viewed as localized gravity-sliding in contrast to regional 

crustal extension. 

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FAULTS IN DEVONIAN ROCKS, DELAMAR MOUNTAINS 217 

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90p. 

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scale 1 :24,000. 

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Preliminary geologic map of the Delamar 3 SE quadrangle, 

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Report 90-221, scale 1 :24,000. 
218 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

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69, p. 1459. 

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203-221. 
Paleomagnetic Data from the Miocene 

Hiko Tuff, Southeastern Nevada, and 

Their Tectonic Implications 

By Mark R. Hudson, Joseph G. Rosenbaum, Robert B. Scott, and Peter D. Rowley 

GEOLOGIC STUDIES IN THE BASIN AND RANGE-COLORADO PLATEAU TRANSITION IN 

SOUTHEASTERN NEVADA, SOUTHWESTERN UTAH, AND NORTHWESTERN ARIZONA, 1992 

U.S. GEOLOGICAL SURVEY BULLETIN 2056-H 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1995 
CONTENTS 

Abstract. .......................................................................................................................... 221 

Introduction .................................................................................................................... 221 

Acknowledgments .......................................................................................................... 221 

Geologic Setting ............................................................................................................. 222 

Methods . . . . . . . . . . . . .. . . . . . . . . . . . .. . . . . . . . . . . . . .. .. . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . 223 

Paleomagnetic Results .................................................................................................... 223 

Discussion ....................................................................................................................... 227 

Conclusions .................................................................................................................... 230 

References Cited ............................................................................................................. 230 

FIGURES 

1. Generalized geologic map of southeastern Nevada..................................................................................................... 222 

2. Representative orthogonal demagnetization diagrams from the Miocene Hiko Tuff ................................................. 225 

3. Equal-area projections of uncorrected and tilt-corrected site mean directions for the Hiko Tuff ............................... 226 

4. Diagrams of paleomagnetic data from the stratigraphic section of Hiko Tuff sampled at site B91-6 ... ........ ............ 227 

5. Generalized geologic map for part of southeastern Nevada showing estimates of vertical-axis rotation for 

individual sites from the Hiko Tuff ............................................................................................................................. 229 

TABLE 

1. Paleomagnetic data from the Miocene Hiko Tuff, southeastern Nevada .................................................................... 224 
Paleomagnetic Data from the Miocene Hiko Tuff, 

Southeastern Nevada, and Their 

Tectonic Implications 

By Mark R. Hudson, Joseph G. Rosenbaum, Robert B. Scott, and Peter D. Rowley 

ABSTRACT 

Paleomagnetic data from the 18.6-Ma Hiko Tuff constrain the nature of fault -block rotations during Miocene and 

younger extensional deformation in an area of southeastern 

Nevada. A mean remanent magnetization direction of five 

sites from the relatively undeformed southern Delamar 

Mountains and Meadow Valley Mountains (D=165.6°, 

I=-44.5°, a9s=6.2°) established a reference for comparison 

to mean directions from sites from fault blocks in nearby 

deformed areas. After tilt correction, mean directions from 

three sites from tilted fault blocks above the Highland 

detachment fault, west of the Chief Range, had little or no 

significant declination difference from the reference direction, suggesting that rotation of these blocks occurred principally by tilt about their strike axes. 

In contrast, mean declinations for 18 sites that lie within 

or north of the Pahranagat shear system, distributed over a 

30-kilometer-long belt from the southern Delamar Mountains to the South Pahroc Range, are moderately but consistently counterclockwise from the reference direction. These 

data suggest that this area was affected by 15°-20° of counterclockwise vertical-axis rotation. This interpretation is 

complicated, however, by the identification of a lower cooling unit of the Hiko Tuff locally in the South Pahroc Range 

whose declination is 13° counterclockwise of that recorded 

by upper parts of the tuff. Comparison of declinations from 

these different cooling units of the Hiko Tuff could suggest 

an apparent counterclockwise rotation of magnitude similar 

to that inferred for the South Pahroc Range. However, the 

declination discordance for most sites in the South Pahroc 

Range is most likely due to tectonic rotation, because five 

sites that we are confident were from the upper part of the 

Hiko have declinations that are 13°-26° discordant to the 

reference direction, whereas declinations from three sites 

from the lower cooling unit are 27°-40° discordant. These 

data suggest that sinistral shear in the South Pahroc Range 

area was accomplished not only by offset along sinistral 

faults but also by counterclockwise rotation of fault blocks 

within and north of the presently recognized Pahranagat 

shear system. Offset on the sinistral faults and the counterclockwise block rotations both may be components of deformation within a broad transfer zone that accommodated 

differential extension of the area. 

INTRODUCTION 

Rigid body rotation of fault blocks has been an 

important mechanism in accommodating strain during 

Cenozoic deformation within the Basin and Range province 

of western North America (Emmons and Garrey, 1910; 

Thompson, 1960; Proffett, 1977). Although components of 

rotation of fault blocks about horizontal axes can commonly 

be estimated from dips of stratified rocks within the blocks, 

components of rotation about vertical axes are in most places 

difficult to constrain. In appropriate rocks, paleomagnetic 

information can be used to assess components of verticalaxis rotation (Magill and others, 1982; Hudson and 

Geissman, 1987, 1991; Wells and Hillhouse, 1989; 

Rosenbaum and others, 1991 ). This report describes 

paleomagnetic data from the Miocene Hiko Tuff obtained to 

assess the importance of vertical-axis rotation of fault blocks 

in an area of southeastern Nevada (fig. 1 ). These data suggest 

that rocks within an area including and extending at least 15 

km north of the sinistral Pahranagat shear system in the South 

Pahroc Range were affected by 15°-20° of counterclockwise 

vertical-axis rotation. 

ACKNOWLEDGMENTS 

Laboratory analyses for this study were conducted by 

W.C. Rivers and S. Morikawa, whose services are greatly 

appreciated. Discussions with C.S. Gromme during the 

course of this study have been particularly helpful. Useful 

reviews were provided by G.J. Calderone and H.R. Blank, Jr. 

221 
222 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

i ~ 

. 

. 

EXPLANATION 

Quaternary and Tertiary alluvium 

Tertiary volcanic rocks 

Pre-Tertiary sedimentary rocks 

High-angle fault-Dotted where uncertain 

~ 

.._--

Strike-slip fault-Dotted where uncertain, 

barbs show direction of relative movement 

--1.J...._ 

Detachment fault- Dotted where uncertain, 

hachures on hanging wall 

Caldera complex 

0 10 zo 30 KILOMETERS 

Figure 1. Generalized geology of southeastern Nevada. Only selected faults are shown. 

GEOLOGIC SETTING 

Within the Basin and Range province in southeastern 

Nevada (fig. 1 ), regions that were only mildly tilted and 

faulted during Cenozoic deformation contrast with more 

highly extended regions. This study compares paleomagnetic data from relatively undeformed Tertiary strata in the 

Meadow Valley and southern Delamar Mountains with data 

from equivalent rocks from more extended regions in the 

adjacent South Pahroc and Chief Ranges (fig. 1). Tertiary 

volcanic strata in this region overlie Paleozoic rocks, commonly with angular unconformity (Tschanz and Pampeyan, 

1970). Tertiary rocks predominantly are Oligocene to 

Miocene silicic ash-flow tuffs (Cook, 1960, 1965; Ekren and 

others, 1977; Best and others, 1989; Scott and others, this 

volume). Although Oligocene ash-flow tuffs erupted from 

calderas outside the study area (Best and others, 1989), two 

Miocene caldera complexes lie within the area. The Caliente 

caldera complex was the source of several calc-alkaline ashflow tuffs (Williams, 1967; Noble and McKee, 1972; Ekren 

and others, 1977; Rowley and Siders, 1988) between about 

23 and 17 Ma. The younger Kane Springs Wash volcanic 

center includes the 14-Ma Kane Springs Wash caldera 

(Noble and others, 1968; Novak, 1984) and a probable precursor caldera (Scott and others, 1991) that erupted mildly 

peralkaline ash-flow tuffs between about 15 and 14 Ma. 

The Hiko Tuff is a rhyolitic ash-flow tuff erupted from 

the western Caliente caldera complex (Ekren and others, 

1977; Rowley and Siders, 1988). The Miocene Hiko Tuff 

(Dolgoff, 1963) was selected for paleomagnetic study 

because it is lithologically distinctive, widely distributed, and 

sufficiently old to have been involved in much Cenozoic 

deformation of the area. Taylor and others (1989) concluded 

that the best estimated 40 Arf39 Ar age for Hiko Tuff is 18.6 

Ma based on unpublished data ofD.R. Lux and W.J. Taylor. 

In many regions the outflow Hiko Tuff appears to be a simple 

cooling unit lacking evidence of significant internal cooling 

breaks (Dolgoff, 1963). In the central and northern South 

Pahroc Range, however, outflow Hiko Tuff is notably thicker 

than elsewhere, and its basal portions are anomalously lithic 
PALEOMAGNETIC DATA, HIKO TUFF. SOUTHEASTERN NEVADA 223 

rich. In this area, the Hiko Tuff appears to be a compound 

cooling unit with cooling breaks indicated by small reversals 

of degree of welding and by the presence of multiple vitrophyres. 

Cenozoic deformation of the study area was heterogeneous in both style and degree. Tertiary volcanic rocks are 

least deformed in the southern Delamar and Meadow Valley 

Mountains, where generally west dipping normal faults are 

widely spaced and volcanic strata are generally east dipping 

at less than 15°. Only narrow zones within this belt are more 

highly deformed. In areas due north of the Kane Springs 

Wash caldera, including the Caliente caldera complex, rocks 

are broken by numerous northwest-striking faults having 

both dextral and normal components of slip (Bowman, 1985; 

Michel-Noel and others, 1990; Scott and others, 1990). 

Caldera formation was in part coeval with activity of these 

faults (Rowley and others, 1990; Rowley and others, 1992; 

Snee and others, 1990). Still farther north, Tertiary volcanic 

rocks are present in several east-tilted normal-fault blocks 

that lie in the upper plate of the Highland detachment fault of 

Ax en and others (1988 ). Several northeast -striking sinistral 

faults that constitute the Pahranagat shear system (Tschanz 

and Pampeyan, 1970; Ekren and others, 1977) separate the 

southern Delamar Mountains from the South Pahroc Range. 

Tschanz and Pampeyan estimated total sinistral displacement across the shear system to be about 16 km. North of the 

Pahranagat shear system, along the east side of the South 

Pahroc Range, Tertiary rocks are broken by numerous 

closely spaced normal faults and are tilted as much as 30° to 

the east (Scott and others, 1993). Farther west in the South 

Pahroc Range, the volcanic rocks dip more gently. 

METHODS 

We collected all samples for the paleomagnetic study 

with a portable rock drill, orienting them with a clinometer 

and both magnetic and solar compasses. Most sites consisted 

of seven to nine samples collected over several tens of square 

meters. Forty-three samples at one site spanned 235 m 

through a thick stratigraphic section of Hiko Tuff to assess 

possible intraunit dispersion of remanent magnetization 

directions. Averaging 3 to I 0 independent measurements of 

compaction foliations gave stratal attitudes for the sites. 

These attitudes were generally concordant with independently derived attitudes obtained from field measurements of 

contact surfaces or from attitudes calculated from the geometric trace of contacts across topography. 

The laboratory procedure subjected samples to progressive alternating field demagnetization. Six or more samples 

from each site were demagnetized at eight steps to peak 

inductions of 80 mT (millitesla). We plotted demagnetization results on vector diagrams and determined directions of 

components by principal component analysis (Kirschvink, 

1980) for visually linear segments. For most samples, 

demagnetization path trajectories visually trended toward 

the origin of vector diagrams, and we included the zero magnetization value (representing the diagram origin) as an additional data point in principal component analysis. We 

calculated site mean directions and their dispersion parameters (table 1) using statistics of Fisher (1953) and corrected 

site mean directions for tectonic tilt by rotation about their 

strike axes. 

PALEOMAGNETIC RESULTS 

The Hiko Tuff carries a component of remanent magnetization interpreted to be a thermoremanent magnetization 

acquired during cooling of the tuff after deposition. For most 

samples, a single reverse-polarity component of magnetization was isolated at demagnetization levels greater than 15 

mT (fig. 2A, B). These characteristic components have wellgrouped directions within a site, as shown by the high values 

(typically exceeding 1 00) of the site mean concentration 

parameter, k (Fisher, 1953, table 1). At some sites the ch;lfacteristic component was overprinted to a variable extent by 

a low-coercivity, large-moment component that is interpreted to be a lightning-induced isothermal remanent magnetization (fig. 2C). Data for 10 samples for which 

demagnetization did not convincingly remove lightninginduced magnetization components were omitted from site 

mean calculations. For samples from a few sites (for example, B90-14 ), demagnetization path trajectories slightly 

missed the origin of vector diagrams (fig. 2D), indicating 

that a low-moment, high-coercivity component of magnetization having a direction different than the characteristic 

component was present in these samples. The maximum offsets of demagnetization trajectories from diagram origins, 

however. are small relative to the magnitude of the characteristic components, and thus potential contamination 'of 

characteristic directions by the high-coercivity component is 

unlikely to be significant. Principal component analysis for 

samples with this type of demagnetization behavior did not 

include the origin. At one site (B90-27) the Hiko was highly 

altered, and the component of magnetization isolated during 

demagnetization was more dispersed and of greatly different 

direction than at other sites. Results from this site were 

excluded from further analysis because they probably reflect 

at least partial remagnetization during alteration. 

Dispersion of Hiko site mean directions is reduced by 

tilt correction (fig. 3 ). For averages of the 30 site mean 

directions, k increases from 17 to 46 after tilt correction. 

Declinations of the tilt-corrected site means, however, still 

vary from 125.6° to 181.8°. The k values based only on 

inclinations (McFadden and Reid, 1982), which are more 

appropriate to examine if the Hiko Tuff was affected by 

vertical-axis rotation, increased from 20 to 85 after tilt 

correction. The increase of k values after tilt correction is in 
224 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

Table 1. Paleomagnetic data from the Miocene Hiko Tuff, southeastern Nevada. 

I NINo, number of samples accepted/number of samples demagnetized; R, resultant vector length; k, concentration parameter (Fisher, 1953 ); <X95· radius of 95 percent confidence 

cone about mean; Du, lu, uncorrected declination and inclination of site mean; St/Dp. strike and dip of site (dip direction 90° clockwise from strike); De, lc, tilt-corrected declination 

and inclination of site mean; V AR. vertical-axis rotation with respect to reference direction with 95 percent confidence limits (Demarest, 1983) (positive and negative values indicate 

clockwise and counterclockwise rotation, respectively! 

------------------------------------------------------------------------------------------------------------------------------------------------------

Site Lat 0 N Long ow NINo R a95 k Du Iu St/Dp De Ic VAR 

-------Th-------------------------------------------------------------------------------------------------------------------------------------------

B90-1RD 37.0833 114.8680 6/8 5.9900 3.0 501 155.8 -55.5 285/4.5 159.5 -51.9 -6.1 ± 7.7 

B90-3RD 37.2100 114.8585 8/8 7.9852 2.6 475 178.9 -52.3 235/16 170.8 -38.4 +5.2 ± 7.2 

B90-6RD 37.2083 114.9563 8/8 7.9837 2.7 430 165.7 -47.2 0/0 165.7 -47.2 +0.1 ± 7.4 

B90-7RD 37.2098 114.9662 8/8 7.9855 -2.5 482 151.7 -65.6 270/30 165.7 -38.4 +0.1 ± 7.1 

B90-8 37.0665 114.7510 717 6.9946 1.8 1117 180.8 -47.1 175/15 164.7 -46.5 -0.9 ± 7.0 

B90-9 37.2260 114.9823 8/8 7.9116 6.3 79 184.7 -45.0 193/3 181.8 -44.4 +16.2 ± 9.6 

B90-10 37.2318 114.9980 8/8 7.9622 4.1 185 138.6 -28.3 13/35 161.5 -53.0 -4.1 ± 8.5 

B90-11 37.2587 114.9667 8/8 7.9661 3.9 207 144.6 -23.4 359/17 152.4 -32.1 -13.2 ± 7.6 

B90-12 37.2678 114.9685 8/8 7.9828 2.8 407 134.8 -44.8 335/16 151.5 -48.1 -14.1 ± 7.4 

B90-13 37.2630 114.9273 717 6.9934 2.0 916 160.5 -41.5 32/11 168.1 -49.7 +2.5 ± 7.1 

B90-14 37.2958 114.8867 8/8 7.9580 4.3 167 107.4 -31.5 325/26 125.6 -44.2 -40.0 ± 8.2 

B90-15 37.2725 114.9933 6/8 5.9837 3.8 307 157.4 -53.3 343/15 177.3 -52.1 + 11.7 ± 8.3 

B90-16 37.2705 115.0000 6/8 5.9900 3.0 501 100.6 -58.1 276/15 133.0 -48.7 -32.6 ± 7.6 

B90-17 37.2840 114.9960 6/8 5.9564 6.1 123 131.2 -34.8 352/26 151.5 -48.4 -14.1 ± 9.8 

B90-18 37.3090 114.9882 8/8 7.9636 4.0 192 133.3 -33.9 15/19 142.7 -49.9 -22.9 ± 8.3 

B90-19 37.3363 114.9423 8/8 7.9862 2.5 506 140.0 -28.3 15/24 152.3 -46.6 -13.3 ± 7.3 

B90-20 37.3422 114.9973 717 6.9899 2.5 595 123.2 -38.7 343/29 150.4 -52.1 -15.2 ± 7.4 

B90-21 37.3658 114.9977 8/8 7.9655 3.9 203 130.1 -34.1 341/24 148.3 -43.2 -17.3 ± 7.9 

B90-221 37.3698 114.9747 8/8 7.9916 1.9 835 130.2 -51.2 287/9 139.4 -47.0 -26.2 ± 7.0 

B90-231 37.3632 114.9658 8/8 7.9900 2.1 698 121.8 -16.9 0/17 126.1 -31.0 -39.5 ± 7.0 

B90-24 37.4110 114.9978 8/8 7.9760 3.3 291 138.4 -33.0 0/0 138.4 -33.0 -27.2 ± 7.4 

B90-25 37.4333 114.9993 8/8 7.9865 2.4 519 151.2 -46.3 010 151.2 -46.3 -14.4 ± 7.2 

B90-26* 37.3533 114.7623 8/8 7.9770 3.2 305 102.1 -61.5 294/36 157.1 -50.2 -8.5 ± 7.7 

B90-27 37.3717 114.7725 7/8 6.8535 9.5 41 171.5 68.0 20/82 129.2 -2.7 -36.4 ± 10.0 

B90-28 37.4173 114.9385 8/8 7.8959 6.8 67 124.7 -16.7 11127 131.6 -40.9 -34.0 ± 9.7 

B90-29 37.4395 114.9417 8/8 7.9864 2.5 513 130.2 -23.7 14/33 144.7 -51.7 -20.9 ± 7.4 

B90-30 37.5033 114.9290 7/8 6.9923 2.2 783 140.9 -34.0 1125 159.7 -47.1 -5.9 ± 7.2 

B90-32 37.6974 114.6655 8/8 7.9619 4.1 184 125.5 -30.0 338/37 152.5 -42.8 -13.1 ± 8.0 

B90-33 37.7696 114.5967 617 5.9830 3.9 294 127.2 -47.7 340/30 165.5 -55.4 -0.1 ± 8.6 

B90-34 37.7939 114.5993 9/9 8.9853 2.2 545 154.9 -39.9 11114 166.4 -47.0 +0.8 ± 7.2 

B91-6 (all) 37.3908 114.9992 43/43 42.5711 2.2 98 138.8 -44.2 196/3 137.3 -41.7 -28.3 ± 7.3 

(lower) 20/20 19. 8600 2.8 136 131.6 -41.1 130.6 -38.4 -35.0 ± 7.5 

(upper) 23/23 22.9195 1.8 273 145.5 -46.5 143.7 -44.1 -21.9 ± 7.2 

Reference direction 4.9738 6.2 153 165.6 -44.5 

------------------------------------------------------------------------------------------------------------------------------------------------------ RD Site used to calculate reference direction. 

1 Site that probably sampled the lower cooling unit of the Hiko Tuff in the South Pahroc Range. 

*Site from altered Hiko whose mean was not used for further analysis. 

both cases statistically significant and yields a positive fold change of remanent magnetization direction. In this stratitest (McElhinny, 1964) at the 99 percent confidence level graphic section, a relatively thick zone of poorly welded, 

based on an F-test of k ratios (Watson, 1956). Although lithic-rich basal tuff grades upward into densely welded vitMcElhinny's ( 1964) fold test makes assumptions that may be rophyre, about 15 m thick, which in tum grades upward into 

invalid in detail (McFadden and Jones, 1981), it generally a platy-weathering devitrified tuff. Still farther upward, a 

errs by being too conservative. These results indicate that second thinner (about 2m thick) densely welded vitrophyre 

characteristic magnetizations in the Hiko predated tectonic is present which also grades upward into devitrified tuff. 

tilting. Lithic content of the tuff appeared to decrease in the vicinAn atypically thick section of outflow Hiko Tuff that ity of this second vitrophyre, although this was not quantiwas present in parts of the South Pahroc Range was sam- fied. The upper devitrified tuff contains two topographic 

pled at site B91-6 to assess any stratigraphically controlled ledges separated by a mostly covered interval. The upper 
PALEOMAGNETIC DATA, HIKO TUFF, SOUTHEASTERN NEVADA 225 

N,Up Site 890-1, Sample 4 

¢·¢ 10 mT I 

0.4 Aim 1¢ 15 

¢20 

I 

,¢ 25 

,<> 30 

,¢ 40 

~8~0 

E, Hor. 

S,Down 

A 

N,Up 

Site 890-11, Sample 8 

0.1 Aim E, Hor. 

S,Down 

B 

w 

c 

Site 890-1, Sample 6 

N,Up 

2 A/m 

S,Down 

10 mT I 

Site 890-14, Sample 8 

N,Up 

20 ~..¢10 mT 

..¢'¢ \ 30.,¢25 ¢ 

<>'<> 

80 ~/ 40 ...... ¢ 0.1 Aim 

E, Hor. ~ ........ . 

........... I 

• 

S,Down 

D 

E, Hor. 

Figure 2. Representative orthogonal demagnetization diagrams, uncorrected for tectonic tilt, from the Miocene Hiko 

Tuff. Solid diamond, projection on horizontal plane; open diamond, projection on vertical plane. A, B, Typical 

demagnetization behavior. C, Lightning-induced magnetization overprinting the characteristic magnetization. The 

trajectory of the horizontal demagnetization path (dashed line) for D does not intersect the origin, indicating presence 

of small-magnitude component of different direction than the characteristic magnetization. Demagnetization levels are 

given in millitesla (mT). Magnetizations are given in Amperes/meter (Aim). Scales are the same for both axes of each 

diagram. 

ledge forms the ridge crest. Forty-three samples were collected, starting in the basal poorly welded part, spanning 

about a 235-m thickness of the Hiko Tuff, and stopping 

about 30 m below the top of the upper topographic ledge to 

avoid lightning-prone areas. 

Declinations and inclinations of remanent magnetizations for the 43 samples show systematic changes when displayed versus height (fig. 4 ). A shift in declination is present 

near the level of the second vitrophyre (fig. 4A). The average 

declination of 20 samples below the vitrophyre is 130.6°, 

whereas the average declination of 23 samples above this 

horizon is 143.7°. The general concordance of this declination shift with the second vitrophyre and an apparent upward 

decrease in lithic content suggest that a complete cooling 

break occurred between deposition of these lower and upper 

parts of the Hiko. Sufficient time must have elapsed between 

emplacement of the lower and higher flows to allow a detectable change of the geomagnetic field direction. The exact 
226 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

N 

k=17 

k;= 20 

w 

@e@ @®t~ • 

A s 

N 

B 

k= 46 

k;= 85 

E 

@ 

Reference declination 

= 165.6° 

Figure 3. Equal-area projections of A, uncorrected, and B, tilt-corrected site mean directions for the Hiko Tuff with their 

associated cones of 95 percent statistical confidence. All projections are on the upper hemisphere. Values fork are concentration parameters (Fisher, 1953) for an average of the site mean directions. Values for ki are concentration parameters 

based solely on inclinations (McFadden and Reid, 1982) of the site mean directions. In B, arrow labeled 165.6° represents 

the declination of the reference direction calculated for the Hiko Tuff (table 1 ). 

location of the cooling break is debatable because declinations of three samples beneath the second vitrophyre, down 

to the 90-m level, are intermediate between mean declinations for the lower and upper units. The cooling break could 

occur as low as the 90-m level. Alternatively, the cooling 

break could lie just beneath the second vitrophyre and the 

intermediate declinations of the underlying samples might 

be caused by a partial remagnetization of the top of the 

underlying cooling unit. 

Inclinations of remanent magnetizations vary in a more 

complicated way through the section (fig. 4B). The postulated cooling break near the second vitrophyre is less marked 

as a change in inclination, although the lower part has, on 

average, shallower inclinations than the upper part (fig. 4C). 

Systematic variations of inclination, however, exist within 

both the lower and upper parts of the Hiko. We suspect that 

these variations may reflect inclination shallowing caused by 

rotation of magnetic grains at temperatures below their magnetic blocking temperatures during welding, as was documented by Rosenbaum (1986) for a silicic welded ash-flow 

tuff from southwestern Nevada. Post-blocking welding 

explains particularly well the inclination pattern of the lower 

lithic-rich unit, in which inclinations in the lowest poorly 

welded tuff are steepest but shallow upward to a minimum 

within the vitrophyre and then gradually steepen upwards 

toward the devitrified top of the flow. Two steepeningupward intervals are seen in inclination data from the upper 

part of the Hiko, with the maximum of the lower interval 

present in samples from the top of the first topographic 

ledge. This inclination pattern, if interpreted in terms of a 

welding-related shallowing, implies that the upper part of the 

Hiko at this section contains two flows separated by a short 

cooling break that presently lies within a mostly covered 

interval between the two topographic ledges (that is, the 

upper cooling unit of the Hiko is compound). However, if a 

hiatus occurred during deposition of the upper part of the 

Hiko, it was not recorded as a detectable difference of remanence declination. 

The recognition that, at least locally, separate cooling 

units of the Hiko recorded two geomagnetic field directions, 

and that inclinations of magnetizations within these cooling 

units may have been shallowed during welding may explain 

some dispersion that remains among the tilt -corrected site 

means of figure 3. In particular, although tilt-corrected 

inclinations of Hiko site mean directions cluster at about 

-48° (fig. 3), inclinations of several site mean directions are 

significantly shallower (as low as -28°). These low 

inclinations may largely be due to welding-related 

shallowing and (or) to a shallower geomagnetic field 

inclination recorded by the lower cooling unit. Likewise, an 

approximately 13° difference between the declinations of 

magnetizations from the lower and upper parts of the Hiko 

Tuff in the thick section can explain some, although not all, 
PALEOMAGNETIC DATA, HIKO TUFF, SOUTHEASTERN NEVADA 227 

(/J 

a: 

w 

1--

w 

~ 

z 

....: 

:::c 

C) 

w 

:::c 

250 

200 

150 

100 

891-6 

•• 

• • • • • • • ______ b.~$~_Q{_~_I)_d_!~_c;f~~---------------- • • 

mostly covered 

_______ top __ of.lslJedg_e ___________ _ • 

• • 

• 

:'!':':':':':':':':':':vttrophyre:':':':; :':'!'!'!':':'!':'!':':':'! • 

~ 

• • • • 

• • 

• ·. --------------------

• 

• 

• • • ... • so nllllllllli!:lllllllllllllllllllllllllllllllllmin\;, .. ~\ :llllli~lll911Ytllllllllllillllliil • 

•• • • poorly welded 

0 ~~--~--~_.--~--~~--~--~~ 

110 120 130 140 150 

DECLINATION, IN DEGREES 

A 

160 

• 

• 

891-6 

250 .---..-----.-----.---r----.-----,----..----. 

200 

150 

100 

50 

II • 

• I 

•• B 

·------------------------------ • ---~-----J~~$-~_gf_~[l_c;f_j~~g_~_ • 

mostly covered 

------------------- • • -- -~-- -~------- -- ___ \QP-_Q_f__1_~~.~.~~9.~--- • • 

: . 

• . • 

• • 

• 

• • 

• 

• 

•• . • • .. • 

poorly welded 

0 L---~--~--~--~--~--~--~~ 

-60 -50 -40 -30 

INCLINATION, IN DEGREES 

B 

E 

-20 

+:t·~· ··" +. -t ••• 

+ • 

c s 

Figure 4. Paleomagnetic data from the stratigraphic section of Hiko Tuff sampled at site B91-6. A, Declinations of remanent magnetizations versus sample height B, Inclinations of remanent magnetizations versus sample height C, Directions of remanent magnetizations from lower (solid dot) and upper (plus) parts of the tuff on an equal-area net Projections are on the upper hemisphere. 

of the approximately 56° range of declinations (125.6° to 

181.8°) among the tilt-corrected Hiko site means. 

DISCUSSION 

If vertical-axis rotation has affected some sites where 

we sampled the Hiko Tuff. this may be apparent from the 

declinations of their tilt-corrected site means. To determine 

relative vertical-axis rotation (V AR), we have compared 

individual site mean directions to a reference direction 

(D=l65.6°, 1=-46.0°, ag5=6.2°) calculated by averaging 

five site mean directions from the relatively little deformed 

southern Delamar and Meadow Valley Mountains (table 1). 

If samples from all sites initially recorded the same geomagnetic field direction, then the difference between the declination of individual site means and the declination of the 

reference direction is, within error limits, an estimate of relative vertical-axis rotation of the individual sites. The potential complication raised by comparison of results from 
228 GEOLOGIC STUDIES. BARCO STUDY UNIT. 1992 

noncontemporaneous cooling units of the Hiko Tuff. such as 

found at site B91-6, is discussed later in this section. The 

V AR values for individual sites calculated with respect to 

the reference direction (fig. 5) range from+ 11.7° to -40.0°, 

where positive and negative VAR values suggest clockwise 

and counterclockwise rotations, respectively. Uncertainties 

for such rotation estimates (table 1) can be partly quantified 

as functions of the dispersions and inclinations of the individual site and reference mean directions (Demarest, 1983 ). 

This type of analysis. however. does not account for possible errors introduced by tilt corrections for individual sites, 

which may be significant for some ash-flow tuffs (Hagstrum 

and Gans, 1989; Rosenbaum and others, 1991 ). Tilt-correction errors for individual sites in the Hiko Tuff are difficult 

to quantify. but most are probably less than 10° considering 

(I) the general concordance of attitudes estimated from 

compaction foliations and map patterns and (2) the conformable stratigraphic relations of the Hiko Tuff with underlying and overlying ash-flow tuffs throughout the region of 

study. For the five individual site mean directions averaged 

to calculate the reference direction, tilt-correction errors, if 

random and less than about 20° (Calderone and Butler, 

1991 ), should partly cancel and increase dispersion about 

the reference mean. Therefore, the V AR uncertainties of 

table I may at least partly account for tilt-correction error of 

the reference direction. 

The V AR values obtained from the Hiko Tuff paleomagnetic data suggest several points of tectonic interest. 

Tilt-corrected mean directions for the five sites from the 

southern Delamar and Meadow Valley Mountains that were 

used to derive the reference direction show little dispersion 

of declinations (table 1; fig. 5). This observation suggests 

that these sites probably were in the same cooling unit of the 

Hiko Tuff and that there was little relative vertical-axis rotation between the sites. These points justify the use and combination of data from these five sites to estimate a reference 

direction. The declination of the reference mean (165.6°) is 

similar, although slightly clockwise of declinations of site 

means for the Hiko at Condor Canyon (D= 162.5°) and White 

River Narrows (D=157.9°) sections to the northeast and 

northwest of our study area, respectively (C.S. Gromme, 

written commun., 1990). 

Small V AR values for three sites (B90--32, B90-33, 

and B90-34) from tilted fault blocks that lie above the 

Highland detachment fault west of the Chief Range suggest 

that these sites experienced little or no significant verticalaxis rotation, assuming the Hiko Tuff here is the same 

cooling unit as was sampled in the reference sites. This result 

suggests that rotation of fault blocks above the Highland 

detachment fault occurred principally as tilt about their strike 

axes. 

In contrast, a band of sites distributed over approximately 30 km from just northwest of the stable southern 

Delamar Mountains into the South Pahroc Range have moderate but consistently negative V AR values (table 1, fig. 5). 

For most sites these V AR values range between -15° and 

-20°. These data imply that much of the South Pahroc 

Range was affected by moderate counterclockwise verticalaxis rotation. There is no obvious difference between the 

magnitude or range of estimated rotation for sites within and 

north of the Pahranagat shear system. A mean direction of 

all 18 sites (B90-I0-12, 15-25, 28-30, B91-6) that lie 

within or north of the shear system (D= 146.4 o, 1=-45 .8°, 

a9s=4.7°) suggests counterclockwise rotation of 19.2°±8.7° 

of this area with respect to the southern Delamar Mountains. 

This conclusion must be further qualified, however, because 

it is likely that not all sites in the South Pahroc Range sampled the same cooling unit. Results from site B91-6 (fig. 4) 

show that the declination of remanence in the lower cooling 

unit is 13° counterclockwise of that recorded by the upper 

cooling unit. A moderately negative VAR value that does 

not reflect tectonic rotation could be calculated if a site 

mean direction from the lower cooling unit was compared to 

a reference direction derived from sites in the upper cooling 

unit. 

Several lines of evidence suggest that the declination 

discordance of most sites from the South Pahroc Range 

with respect to the reference direction derived from sites in 

the southern Delamar and Meadow Valley Mountains is due 

to tectonic rotation rather than to an inappropriate comparison of data from different cooling units of the Hiko Tuff. 

For site B91-6, V AR estimates for both the lower and 

upper cooling units of the Hiko are significant (-35.0°±7.5° 

and -21.9°±7.2°, respectively), and the value from the 

upper unit is more typical of those for most sites in the 

South Pahroc Range. We believe that several other sites 

with significant negative VAR values (for example, 

B90-1 L B90-17, B90-20-22) were in the upper part of the 

Hiko because of their position within a few tens of meters 

of the upper contact of the tuff. Conversely, we are fairly 

certain that two sites (B90-23 and B90-24) sampled the 

lower cooling unit of the Hiko Tuff on the basis of their 

location within basal, lithic-rich vitrophyres of thick sections of Hiko Tuff near the B91-6 section. Both sites 

B90-23 and B90-24 have VAR values (-39.5° and -27.2°, 

respectively) that are greater than typical for this area (for 

example, -17.3° to -26.2° from sites B90-2 L B90-22, 

upper B91-6) and both sites have anomalously shallow 

Figure 5 (facing page). Generalized geology for part of southeastern Nevada and estimates of vertical-axis rotation (angle between line and geographic north) for individual sites from the Hiko 

Tuff. Location of reference sites shown by squares whereas remaining sites are shown by dots. Amount of rotation is the angle between 

the declination of an individual site mean and the declination for the 

Hiko reference mean. Numbers by site locations are keyed to individual B90 sites on table I. Two rotation estimates shown for site 

B91-6 are calculated from means from the lower and upper parts of 

the Hiko Tuff. 
PALEOMAGNETIC DATA, HIKO TUFF, SOUTHEASTERN NEVADA 

EXPlANATION 

Quaternary and Tertiary alluvium 

Tertiary volcanic and 

minor intrusive rocks 

Pre-Tertiary sedimentary rocks 

._~;;;;;;;;;._ Fault -- Dashed where approximately located; 

....--- barbs show relative movement ; includes 

caldera boundaries 

• • 21 Paleomagnetic site 

\ Vertical-axis rotation 

229 
230 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

inclinations (-31 o and -33°, respectively). In the B91-6 

section, inclinations this shallow were found only in the 

basal vitrophyre of the lower cooling unit (fig. 4B). If mean 

directions from sites B90-23 and B90-24 are omitted and 

the mean direction from only the upper part of B91-6 is 

used, the average of mean directions from the remaining 16 

sites that lie within or north of the shear system is 

D=149.1 °, I=-47.4°, a9s=4.1 °. This mean direction suggests a revised average counterclockwise rotation of 

-16.5°±8.4° of this area with respect to the southern 

Delamar Mountains. These V AR calculations assume that 

the reference sites were in the upper cooling unit of the 

Hiko Tuff. If, instead, the reference sites were in the lower 

cooling unit of the Hiko, V AR values calculated for sites 

from the upper part of the Hiko would underestimate counterclockwise vertical-axis rotation by about 13°. 

Paleomagnetic data from the Hiko Tuff suggest that 

parts of the South Pahroc Range were affected not only by 

sinistral offset along northeast-striking faults of the Pahranagat shear system but also by counterclockwise rotation 

of fault blocks within and north of the identified shear system. We interpret this sinistral shear to have accommodated 

a difference in extension between the mildly extended southem Delamar Mountains and the more extended terrane of the 

South Pahroc Range to the northwest. If the sinistral shear 

zone trends roughly east-west, its width must be at least 30 

km to encompass the northward extent of sites where counterclockwise rotation has been inferred. Gibbs ( 1984) used 

the term "transfer fault" to describe a discrete fault having a 

substantial strike-slip component of offset that has acted to 

transmit extensional displacement between spatially offset 

loci of extension. We suggest that both counterclockwise 

block rotation and displacement along sinistral faults in the 

South Pahroc Range are deformation components of a broad 

"transfer zone" that transmitted extensional strain from areas 

north and northeast of the relatively stable southern Delamar 

Mountains to areas to the southwest (perhaps to highly 

extended areas west of the Sheep Range (Guth, 1981; Guth 

and others, 1988) ). 

CONCLUSIONS 

Paleomagnetic data from the 18.6 Ma Hiko Tuff allow 

constraints to be placed on components of vertical-axis 

rotation of fault blocks in an area of southeastern Nevada. A 

mean direction of five sites from the relatively little 

deformed southern Delamar and Meadow Valley Mountains 

was used as a reference direction for comparison with paleomagnetic data from more deformed adjacent areas. Tiltcorrected directions for three sites from fault blocks above 

the Highland detachment fault, west of the Chief Range, are 

little different from the reference direction, indicating that 

rotation of these blocks principally occurred by tilt about 

their strike axes. Mean directions of sites distributed over a 

30-km-long belt extending into the South Pahroc Range 

from the southern Delamar Mountains are rotated 

moderately but consistently counterclockwise from the 

reference direction. Interpretation of these data is 

complicated by recognition of a lower cooling unit of the 

Hiko Tuff in parts of the South Pahroc Range that recorded 

a declination of remanent magnetization that is about 13 o 

counterclockwise from magnetizations in the overlying 

Hiko. Comparisons to data from sites that were probably 

restricted to either upper or lower parts of the Hiko suggests 

that about 15°-20° of the declination discordance found at 

sites from the South Pahroc Range was probably caused by 

counterclockwise vertical-axis rotation of fault blocks. 

Areas affected by counterclockwise rotation include but also 

extend north of a system of northeast-striking sinistral faults 

of the Pahranagat shear system. Sinistral shear 

accommodated by counterclockwise rotation and 

displacement on northeast-striking faults is interpreted to 

have occurred within a broad transfer zone that transmitted 

extensional strain from areas northeast of the relatively 

stable southern Delamar Mountains to areas to the 

southwest. 

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PALEOMAGNETIC DATA, HIKO TUFF, SOUTHEASTERN NEVADA 231 

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Decoupling of Mid-Tertiary Rocks, 

Red Hills-Western Markagunt Plateau, 

Southwestern Utah 

By Florian Maldonado 

GEOLOGIC STUDIES IN THE BASIN AND RANGE-COLORADO PLATEAU TRANSITION IN 

SOUTHEASTERN NEVADA, SOUTHWESTERN UTAH, AND NORTHWESTERN ARIZONA, 1992 

U.S. GEOLOGICAL SURVEY BULLETIN 2056-I 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1995 
1. 

2. 

3. 

4-11. 

12-14. 

15. 

16-18. 

CONTENTS 

Abstract ........................................................................................................................... 235 

Introduction .................................................................................................................... 235 

Acknowledgments .......................................................................................................... 237 

Geology of the Upper Plate ............................................................................................ 237 

Geology of the Lower Plate ............................................................................................ 237 

Evidence for Low-angle Shear Zone .............................................................................. 240 

Possible Mechanisms for Decoupling of Upper Plate .................................................... 243 

Origin of Megabreccia.................................................................................................... 249 

Relationship of Paragonah Fault to Red Hills Shear Zone ............................................. 249 

Features Related to Low-angle Shear Zone .................................................................... 251 

Conclusions .................................................................................................................... 252 

References Cited ............................................................................................................. 252 

FIGURES 

Location map of the Red Hills and surrounding area, southwestern Utah ............................................................ 236 

Generalized geologic map of the Red Hills and Markagunt Plateau ..................................................................... 238 

Schematic diagram showing relationships of gravity-slide blocks, upper and lower plates, upper plate faults, 

reactivated upper plate faults or new faults, and Red Hills shear zone .................................................................. 240 

Photographs showing: 

4. Intense faulting and fragmentation of upper plate ........................................................................................ 240 

5. Flat-lying and essentially unfragmented lower plate .................................................................................... 241 

6. Northern Red Hills, upper plate and lower plate and Brian Head unit. ........................................................ 241 

7. Southern Red Hills, upper plate and lower plate; Brian Head unit missing ................................................. 242 

8. Well-rounded clasts in boulder bed of the Brian Head unit .......................................................................... 242 

9. Intensely sheared tuffaceous sandstone of the Brian Head unit ................................................................... 243 

10. Folded and sheared tuffaceous sandstone of the Brian Head unit ................................................................ 243 

11. Chaotic mixture of rocks of upper plate interpreted as megabreccia ........................................................... 244 

Maps showing: 

12. Distribution of intrusive bodies, southwestern Utah .................................................................................... 245 

13. Aeromagnetic data for the Red Hills-western Markagunt Plateau area ....................................................... 246 

14. Gravity data for the Red Hills-western Markagunt Plateau area ................................................................. 24 7 

Diagram showing normal fault model as possible mechanism for decoupling of upper plate .............................. 248 

Photographs showing: 

16. Mafic dike truncated by inferred Paragonah fault along western margin of the Markagunt Plateau ........... 250 

17. Mafic dike cutting the Brian Head unit ........................................................................................................ 250 

18. Iron Peak laccolith ........................................................................................................................................ 252 

TABLE 

1. Geochemical analyses of chalcedony zone in the Brian Head unit, Brian Head (Peak) area ................................ 251 
Decoupling of Mid-Tertiary Rocks, 

Red Hills-Western Markagunt Plateau, 

Southwestern Utah 

By Florian Maldonado 

ABSTRACT 

A succession of Oligocene and Miocene volcanic and 

volcaniclastic rocks about 610 meters thick is inferred to 

have been decoupled at a shallow crustal level from 

underlying Tertiary rocks within the Red Hills. Evidence of 

detachment of the upper plate along a basal low-angle shear 

zone is ( 1) a low-angle, dipping pulverized zone, interpreted 

as fault gouge, that separates an upper (allochthonous) plate 

from an underlying lower (autochthonous) plate; (2) 

attitudes of upper plate rocks that are steeper than those of 

underlying lower plate rocks and that locally dip into lower 

plate rocks; (3) intense faulting and fragmentation of upper 

plate rocks; (4) faults that appear to be restricted to the upper 

plate; and (5) omission of strata along the shear zone. The 

upper plate is overlain by blocks of megabreccia composed 

of chaotically mixed, upper plate rock types. These blocks 

are inferred to be gravity-slide blocks emplaced during 

decoupling and breakage of the upper plate. 

Because the stratigraphy of the Red Hills is almost 

identical to that in the Markagunt Plateau, about 15 kilometers east of the Red Hills, and the same low-angle shear 

zone is also present in the Markagunt Plateau, the shear zone 

documented in the Red Hills may be of regional extent. The 

age of the shear zone is bracketed between 22.5 Ma (age of 

the youngest rocks in the upper plate) and 20 Ma (approximate age of dikes cutting the shear zone). The shear zone 

apparently formed due to decoupling of strata through tilting. Tilting may have resulted from crustal extension and 

associated block faulting, or from emplacement of magma at 

shallow depths. 

INTRODUCTION 

The Red Hills, approximately 20 km north-northeast of 

Cedar City, in southwestern Utah (fig. 1 ). lie along the eastern margin of the Basin and Range province, just west of the 

High Plateaus subprovince of the Colorado Plateaus province. In its structure, the High Plateaus subprovince is a 

structural transition zone between the two provinces. The 

Red Hills have been mapped by Thomas and Taylor ( 1946), 

Threet ( 1952), Rowley and Threet ( 1976), and most recently 

by Maldonado and Williams (1990, 1993a, b). The results of 

mapping discussed in this report are part of the Basin and 

Range to Colorado Plateau Transition (BARCO) Study Unit 

described in the Introduction to this volume. 

Low-angle extensional faults (detachment faults) and 

associated upper plate faults in the Basin and Range pro~­

ince of the western United States have been described extensively (Anderson, 1971: Wernicke, 1981; Gross and 

Hillemeyer, 1982; Wernicke and Burchfiel, 1982; Gans and 

Miller, 1983; Reynolds and Spencer, 1985; Hamilton, 1988; 

Maldonado, 1990). Some detachment faults may extend to 

the middle crust and possibly deeper, as suggested by the 

presence of high-grade metamorphic rocks, mineral assemblages indicative of high temperature-pressure regimes, and 

mylonitic metamorphic fabrics in their lower (autochthonous) plates presumably acquired at depths greater than 15 

km (Davis and Coney, 1979; Davis and others, 1980; Wernicke, 1981; Wernicke and Burchfiel, 1982; Davis, 1983; 

Hamilton, 1988 ). This type of faulting has not been identified in the Colorado Plateau. In this report, I present evidence of a low-angle but otherwise dissimilar shear zone and 

associated upper plate fault complex in the Red Hills. I stress 

''dissimilar,'' because I believe that this low-angle shear zone 

may have formed entirely as a shallow structure, rather than 

as a deep-seated detachment fault analogous to those suggested for the Basin and Range. The Red Hills shear zone 

probably developed at less than 2 km depth as indicated by 

the absence of features that would suggest formation in the 

middle crust (greater than 15 km). A similar shallow type of 

structure has been described by Pierce in Wyoming and 

Montana ( 1973, 1987 ). 

The author first mapped the low-angle shear zone in the 

Red Hills (Maldonado and Williams, 1990) (fig. 2). and by 

inference extended it into the adjacent Markagunt Plateau 

235 
236 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

113°30' 113°00' 112°30' 112°00' 

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Figure 1. Location map of the Red Hills (shaded area) and surrounding area, southwestern Utah. BH, Brian Head 

(Peak); H, Hurricane fault (dotted where speculative); PC. Parowan Canyon; YM, Yankee Meadow Reservoir; P, 

Parowan; PG, Paragonah; LC. Little Creek; IP, Iron Peak; CM, Cottonwood Mountain; BR, Basin and Range province; 

HPS, High Plateaus subprovince; CP, Colorado Plateaus province. Solid outline, area of figure 2. 

east of the Red Hills (Maldonado and others, 1989 and 

1992). The fault zone separates an upper plate composed of 

Oligocene and Miocene volcanic and volcaniclastic rocks 

from an underlying lower plate composed of Tertiary, Cretaceous, and Jurassic rocks. Rocks in the upper plate are 

intensely broken and tilted, as expected of a detached plate; 

the underlying lower plate, in contrast, is structurally less 

complex. In some areas, the top of the shear zone is at the top 

of an Eocene and Oligocene sequence of rocks originally 

called the sedimentary and volcanic rocks of the Red Hills 

(Trs unit of Maldonado and others, 1992, and Maldonado 

and Williams, 1993a, b) but now referred to as the Brian 

Head unit (Tbh unit of fig. 2) for rocks exposed near the 

Brian Head (Peak) area (fig. 1) that will be formalized (E.G. 

Sable and Florian Maldonado, work in progress). This unit is 

described in the next section. In other areas, the shear zone 

may occur stratigraphically lower in the sequence. Thus, the 

upper plate in some places includes rocks correlative with 

the uppermost part of the Brian Head unit (fig. 3). The shear 

zone is shown as a single fault (fig. 2), arbitrarily placed 

between the Brian Head unit and the overlying Oligocene 

and Miocene volcanic rocks. 
DECOUPLING IN RED HILLS-MARKAGUNT PLATEAU 237 

ACKNOWLEDGMENTS 

This study is part of the Basin and Range to Colorado 

Plateau Transition Study Unit described in the Introduction 

to this volume. I appreciate discussions with J.J. Anderson, 

P.D. Rowley, D.L. Schmidt, R.E. Anderson, and H.R. Blank, 

Jr., that aided in presentation of ideas in this report. I thank 

K.F. Fox, Jr. and E.G. Sable for their review comments. 

GEOLOGY OF THE UPPER PLATE 

Upper plate rocks in the Red Hills contain a sequence of 

ash-flow tuffs and minor volcaniclastic rocks approximately 

610 m thick that consists of the following units, in ascending 

order (ages corrected for new decay constants of Steiger and 

Jager, 1977): uppermost part of the Brian Head unit (map 

unit Tbh) (locally present); Wah Wah Springs Formation 

(29.5 Ma, Best and Grant, 1987) and Lund Formation (27.9 

Ma, Best and Grant, 1987), both of the Needles Range 

Group; Baldhills Tuff Member of the Isom Formation (27 

Ma, Best, Christiansen, and Blank, 1989); Bear Valley Formation (25 Ma, Fleck and others, 1975); Leach Canyon Formation (24 Ma, Row ley and others, 1994) of the Quichapa 

Group; Bauers Tuff Member (22.8 Ma, Best, Christiansen, 

and others, 1989) of the Condor Canyon Formation of the 

Quichapa Group; Harmony Hills Tuff (22-22.5 Ma, Rowley 

and others, 1989; Siders and others, 1990) of the Quichapa 

Group; and Mount Dutton Formation (21-26 Ma, Anderson 

and Rowley, 1975)-map unit Tv (figs. 2, 3) includes these 

units. The upper plate is discordantly overlain by masses of 

Tertiary megabreccia (shown as Tm in figs. 2, 3) inferred to 

be gravity-slide blocks. These blocks contain rock types of 

the upper plate and are interpreted to have formed as a consequence of action of the Red Hills shear zone (see section, 

"Origin of megabreccia"). 

Upper plate rocks dip gently (5°) to very steeply (as 

much as 75°) and are locally concordant and elsewhere discordant to the underlying rocks. The rocks are intensely 

faulted and fragmented (fig. 4), in contrast to structures in 

the lower plate rocks (fig. 5). The faults are mostly high 

angle, but some low-angle or bedding-plane faults are also 

present. In general, the high-angle faults strike northwest and 

northeast. Most of the high-angle faults cutting the upper 

plate terminate at its base, but some extend below the base. 

This relationship suggests that most faults are upper plate 

faults that propagated upward from the Red Hills shear zone 

(fig. 3). Those faults that extend below the base of the volcanic sequence may represent reactivation and upward growth 

of lower plate faults or new faults (fig. 3). 

The upper plate may also contain faults unrelated to and 

earlier than the Red Hills shear zone. Anderson ( 1985, 1988) 

has presented evidence from the northern Markagunt Plateau 

that indicates west- and northwest-trending block faulting 

that began about 26 Ma. These faults are older than the upper 

plate faults that propagated from the Red Hills shear zone, 

and may be difficult to separate from those caused by movement of the upper plate. The older faults may have been 

rotated as well as reactivated by decoupling of the upper 

plate. 

GEOLOGY OF THE LOWER PLATE 

Rocks in the lower plate generally dip less than 15°, 

and are less faulted than rocks in the overlying allochthonous plate. They are composed of sedimentary rocks and 

minor volcaniclastic rocks and thin ash-flow tuffs (part of 

Tbh map unit). These rocks as exposed in the Red Hills consist of the Navajo Sandstone (Lower Jurassic), Temple 

Cap(?) Sandstone (Middle Jurassic), Carmel Formation 

(Middle Jurassic), Iron Springs Formation (Upper? Cretaceous), Grand Castle unit of Goldstrand (1991) (Paleocene) 

(equivalent to conglomerate of Parowan Gap, Maldonado 

and others, 1992 and Maldonado and Williams, 1993b ), 

Claron Formation (Paleocene and Eocene) (Goldstrand, 

1994) (all included in TKJ, fig. 3), and part of the Brian 

Head unit (Eocene and Oligocene). 

The Brian Head unit (Tbh map unit) provides one key 

to understanding the Red Hills shear zone. The unit is 

approximately 213m thick (fig. 6) in the northern part of the 

Red Hills, thin to absent in the southern Red Hills (figs. 2, 7) 

and as thick as 350 m in the Markagunt Plateau (Anderson 

and Kurlich, 1989). The upper half of the unit is equivalent 

to "local volcanic and sedimentary strata," a unit mapped on 

the Markagunt Plateau by Anderson and others ( 1987, 

1990). The lower half is equivalent to the upper "white" 

subunit (Doelling and others, 1989) of the Claron Formation. The unit is composed mostly of sandstone, minor limestone and limy shale beds, and tuffaceous sandstone. 

Pebble- to boulder-size conglomerate (boulder beds), mudflow breccia, thin ash-fall(?) tuff, and ash-flow tuff are 

present in the uppermost part of the unit. Plagioclase and 

biotite from the ash-flow tuff have been dated respectively 

at 34.2±2.1 Ma and 26.3±1.3 Ma using the K-Ar method 

(H.H. Mehnert, written commun., 1992), and 33.00±0.13 

Ma and 33.70±0.14 Ma using the Ar40-

39 method (L.W. 

Snee, written commun., 1994 ). The younger age appears to 

be discordant in contrast to the older ages. The boulder beds 

contain well-rounded clasts as large as 0.6 m in diameter in 

the Red Hills and about 1.5 min diameter along the western 

margin of the Markagunt Plateau (fig. 8). Rocks composing 

some clasts closely resemble rock types of the upper plate 

units, principally those of the Needles Range Group and 

Is om Formation. If the boulder beds do indeed contain clasts 

of these upper plate rocks, then the boulder beds are much 

younger than the underlying uppermost part of the Brian 

Head unit. One possible interpretation is that the upper plate 
... 00' I 

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t

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plate 

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graphic profile 

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RHSZ 

Figure 2. Generalized geology of the Red Hills and Markagunt Plateau, southwestern Utah, and geologic section on line A-A'. RHSZ, Red Hills shear zone; queried in 

cross section where uncertain. Dashed lines within insert indicate dip planes. Geology of Markagunt Plateau modified from Anderson ( 1965), Judy ( 1974 ), Moore ( 1982), 

Maldonado and Moore (1993), F. Maldonado (unpub. data, 1991 ), and F. Maldonado and E.G. Sable (unpub. data, 1992). 

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240 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

Tv 

TKJ 

- ................. ___ 

TKJ 

or new fault 

TKJ 

Upper (allochthonous) 

plate 

Lower 

(autochthonous) plate 

Figure 3. Schematic relationships of gravity-slide blocks (Tm), 

upper and lower plates, upper plate faults, reactivated upper plate 

faults or new faults, and the Red Hills shear zone (RHSZ). Tv, 

Tertiary volcanic rocks undivided; Tbh , Brian Head unit; TKJ. 

Tertiary, Cretaceous, and Jurassic rocks undivided. Light line, 

contact; heavy line, fault; barbs show direction of relative movement. Dashed line indicates bed within TKJ. 

slid over its own erosional debris. In areas where the shear 

zone forms the contact between the base of the overlying 

volcanic rocks and the underlying boulder beds, the clasts 

may, in fact, have acted as ball bearings, allowing the upper 

plate to translate on those beds. An alternate interpretation is 

that the clasts are debris of older units that only resemble 

units of the upper plate, but additional data are needed to test 

this hypothesis. In areas where the shear zone is within the 

uppermost part of the Brian Head unit, the shear zone lies 

within a tuffaceous sandstone interval. The tuffaceous sandstone is comminuted and pulverized, forming a soft fault 

gouge of intensely sheared (fig. 9) and folded rock (fig. 10). 

The structurally incompetent nature of the tuffaceous sandstone perhaps caused slip to be localized within this stratigraphic interval. Other sheared and comminuted lithologies 

include the thin ash-flow tuff and mudflow breccia units. 

EVIDENCE FOR LOW-ANGLE 

SHEAR ZONE 

Evidence for the proposed low-angle shear zone is ( l) 

the comminuted zone within the Brian Head unit that separates upper plate from lower plate; (2) attitudes in upper 

plate rocks that are steeper than those in lower plate rocks, 

and upper plate rocks that locally dip into and are cut off by 

Figure 4. Intense faulting and fragmentation of upper plate (TI, Leach Canyon Formation), northern part of Red Hills. 

Pumice foliations of unit dip from 10° to 50°. Dashed line, fault; bar and ball on downthrown side. View looking southwest. 
DECOUPLING IN RED HILLS-MARKAGUNT PLATEAU 

Figure 5. Flat-lying and essentially unfragmented lower plate, Parowan Canyon area (fig. I). Tgc, Grand Castle unit; Kiu, 

upper part of Iron Springs Formation. View looking northwest. 

Figure 6. Northern part of Red Hills showing upper plate (UP) and lower plate (LP) (Brian Head unit, Tbh). View 

looking northwest. 

241 

the shear zone (northern Red Hills area, fig. 2); (3) intense 

faulting and fragmentation of upper plate rocks in contrast 

to relatively simple structures in lower plate rocks (fig. 2); 

( 4) faults in the upper plate that do not appear to penetrate 

into the lower plate (fig. 3); and (5) omission of strata along 

the contact between the plates (fig. II). 
242 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

Figure 7. Southern part of Red Hills showing upper plate (UP) and lower plate (LP); upper plate is attenuated, and Brian 

Head unit is missing. View looking southwest. 

Figure 8. Well-rounded clasts in boulder bed of the Brian Head unit, south of Cottonwood Mountain (fig. 1). 
DECOUPLING IN RED HILLS-MARKAGUNT PLATEAU 243 

Field relationships in the Red Hills and Markagunt Plateau record at least two major episodes of late Tertiary and 

Quaternary deformation. An early episode formed the lowangle Red Hills shear zone and a later episode fragmented 

the Markagunt Plateau and offset the Red Hills shear zone, 

'\ 

\• 

Figure 9. Intensely sheared tuffaceous sandstone of the 

Brian Head unit near Yankee Meadow Reservoir (fig. 1 ). 

Hammer for scale. 

breaking away the Red Hills from the Markagunt Plateau. 

The date of the earliest episode is probably less than 22-22.5 

Ma (age of the Harmony Hills Tuff), the youngest unit in the 

upper plate. The later episode of faulting in the area was 

high-angle faulting during the late Tertiary to middle or late 

Pleistocene (Anderson and Christenson, 1989) that resulted 

in the present physiography. 

POSSIBLE MECHANISMS FOR 

DECOUPLING OF UPPER PLATE 

Decoupling was a consequence of regional tilting of 

the area in which units of the upper plate were deposited. At 

least two possible mechanisms exist for tilting of this area: 

( 1) local uplift caused by magmatic intrusion (such as in the 

Iron Springs mining district, fig. 1 ), and (2) block rotations 

associated with normal faulting and regional extension. 

Numerous exposures of intrusive bodies are scattered 

within the region. Figure 12 shows the distribution of these 

intrusive bodies. Most intrusive bodies are laccoliths and 

stocks; however, only one body of batholithic dimensions is 

exposed, in the central and northern Mineral Mountains (fig. 

12) approximately 40 km north of the Red Hills. Exposed 

intrusive bodies within the immediate vicinity of the Red 

Hills include laccoliths at Three Peaks, Granite Mountain, 

and Iron Mountain at the Iron Springs mining district (fig. 

12) and Iron Peak (fig. 12) approximately 20 km east of the 

Red Hills in the western part of the Markagunt Plateau. 

Figure 10. Folded and sheared tuffaceous sandstone of the Brian Head unit, eastern Little 

Creek (fig. I). Scale is approximate. 
244 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

Figure 11. Chaotic mixture of rocks of upper plate interpreted as megabreccia. Heavy lines, faults. Dashed line is extent 

of outcrop of megabreccia exposure. Tl, Leach Canyon Formation; Tbv, Bear Valley Formation; Tib, Baldhills Member 

of the Isom Formation; Tnw, Wah Wah Springs Formation. The Wah Wah Springs Formation and the Brian Head unit 

(Tbh) toward the horizon, not part of megabreccia deposit, separated by a fault in wash not shown. 

These intrusive bodies form parts of a belt that has been 

referred to as the "Iron Axis" (Tobey, 1976) that is part of 

the larger east-northeast-trending Delamar-Iron Spring 

igneous belt (Shawe and Stewart, 1976; Rowley and others, 

in press). Slide blocks forming a zone of structural chaos 30 

km2 in area in the Iron Springs mining district were attributed to tilting associated with emplacement of the Iron 

Mountain pluton (Blank and Mackin, 1967; Rowley and 

others, 1989; Blank and others, 1992). 

Aeromagnetic and gravity highs appear to be associated with some intrusive bodies in this region, for example, 

the Three Peaks, Granite Mountain, and Iron Mountain 

intrusive bodies (Blank and Mackin, 1967; Blank and others, 1992; Blank and Kucks, 1989). Figures 13 and 14 show 

aeromagnetic and gravity data respectively for the Red 

Hills- western Markagunt Plateau area. The data indicate a 

magnetic and gravity high for the southernmost part of the 

Red Hills. Magnetic and gravity signatures like those at the 

Iron Springs district suggest the presence of an intrusive 

body in the subsurface under the southwest part of the Red 

Hills, which was confirmed by a drill hole (fig. 2) that penetrated a quartz monzonite body at 1,064 m and bottomed in 

it at 1,340 m (Tompkins and others, 1963). In Parowan Valley, the data indicate a magnetic high but gravity low. The 

absence of a gravity high does not preclude the presence of 

an intrusive body under Parowan Valley, however, because 

low-density basin fill may mask a higher density intrusive 

body at depth (H.R. Blank, Jr., oral commun., 1990). 

The potential role of intrusion in tilting and decoupling of an upper plate is apparent. However, in order to 

decouple an intact plate the size of the upper plate here postulated in the Red Hills- Markagunt Plateau area, an intrusive body would have to be of batholithic dimensions; no 

such body is exposed at the surface, but geophysical data 

suggest a batholith at depth (Blank and Mackin, 1967; 

Blank and others, 1992) for the Iron Springs mining district 

that may extend into the Markagunt Plateau area (Rowley 

and others, in press). The smaller intrusive bodies (laccoliths and stocks) that are exposed may conceivably be apophyses connected to an underlying batholith at great depth. 

Alternatively, the "upper plate" may in fact have been fragmented as a result of decoupling and independent 

movement over several bulges, each developed above a 

small pluton. The distribution of these smaller plutons may 

be great enough and the separation between them short 

enough to have the same effect as a batholith. Decoupling 

would result in movement of upper plate fragments in differing directions. 

Block rotations associated with normal faulting may 

also explain decoupling of the upper plate. Figure 15 shows 

normal faults at various stages of development. Early, 

down-to-the-west normal faults or breakaway faults may 

have resulted in formation of a regional, east-dipping slope 

on the Markagunt Plateau. This proposed east-dipping slope 

may have formed during a period of uplift, forming an eastdipping homocline (fig. 15B). The combination of the slope 
DECOUPLING IN RED HILLS-MARKAGUNT PLATEAU 

0 50 

EXPLANATION 

Quaternary-Tertiary basin-fill sedimentary rocks 

Quaternary-Tertiary basalt and andesite 

Tertiary calc-alkaline volcanic rocks 

Lower Tertiary sedimentary rocks 

Pre-Cenozoic rocks 

-- Contact 

100 KILOMETERS 

Quaternary-Tertiary rhyolite 

Tertiary intrusive rocks 

Figure 12. Distribution of intrusive bodies, southwestern Utah (from Rowley and others, 1979). B, Beaver; CC, Cedar City; 

E, Enterprise; IP, Iron Peak; lSD, Iron Springs mining district; M, Minersville; Mi, Milford; MM, Mineral Mountains; P, 

Panguitch; Pa, Parowan; SG, Saint George. 

245 

and thin succession of upper plate rocks (at least 610 m 

thick) within the uplifted block allowed the upper plate succession of rocks to decouple along the incompetent Brian 

Head unit, thus forming the Red Hills low-angle shear zone 

(fig. 15C). The transport direction of the upper plate is not 

well known and may be difficult to determine. The proposed 

slope direction (and thus the transport direction of the upper 

plate) has been tilted in various directions by subsequent 

faulting. In the Red Hills, the low-angle shear zone surface 

dips to the northeast, but weak evidence (fault geometry and 

stratal rotation) suggests a westerly direction of movement. 

Southwest of Cottonwood Mountain (fig. l ), the shear zone 
113° ()()' 

38°00' 

37°52'301 

37°45' L--------- ~----

0 2 4 6 8 KILOMETERS 

I I I I I I I I I 

112° 45' 

UfAH 

.. 

Figure 13. Aeromagnetic data for the Red Hills-western Markagunt Plateau area (Blank and Kucks, 1989). 

Quaternary basalt 

Quaternary and Tertiary alluvium 

Tertiary mega breccia slide block 

Tertiary mafic dike 

Tertiary intrusive and associated rocks 

of Iron Peak laccolith 

Tertiary volcanic rocks, undivided 

Brian Head unit 

Tertiary, Cretaceous, and Jurassic 

Claron Formation, Orand CasUe unit, 

Iron Springs and Carmel Formations, 

Temple Cap? and Navajo Sandstones 

!

Upper 

plate 

Lower 

plate 

----Contact 

20 

"'<... Strike and dip 

...L.......L... Red Hills low-angle shear moe--Dotted where concealed; hacbures on upper plate 

........... 

~ 

Me10zoic thrust fault--Sawteeth on upper plate 

Tertiary thrust fault--Sawteeth on upper plate 

! Normal fault --Dotted wbcre c:oocealed; bai and ball an 

dowothrown side 

+ Drillhole 

Rcaidual-toeal. intensity aeromagnetic contour -

contour interval20 nT; hacbures indicate area af 

closed low 

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0 2 4 6 8 KILOMETERS 

I I I I I I I I I 

Figure 14. Gravity data for the Red Hills-western Markagunt Plateau area (Blank and Kucks, 1989). 

112° 45' 

UTAH 

.. 

Quaternary basalt 

Quaternary and Tertiary alluvium 

Tertiary megabreccia slide block 

Tertiary mafic dike 

Tertiary intrusive and associated rocks 

of Iron Peak laccolith 

Tertiary volcanic rocks, undivided 

Brian Head unit 

Tertiary, Cretaceous, and Jurassic 

Claron Formation, Orand Castle unit, 

Iron Springs and Carmel Formations, 

Temple Cap ? and Navajo Sandstones 

t

Upper 

plate 

Lower 

plate 

----Contact 

20 

""<.. Strike and dip 

1 1 1 R.ed Hills low-angle shear zone--Dotted where conL..& 

l:t.......1:t. 

cealed; hachures on upper plate 

Mesozoic thrust fault--Sawteetb on upper plate 

Tertiary thrust fault--Sawteeth on upper plate 

! Normal fault --Dotted where concealed; bar and ball on 

downthrown side 

+ Drillhole 

Ccmplefe Bouguer gravity--Col*lur ildlernl2 JDCJal 

0 

tTl 

n 

0 

c::: 

""0 

r 

z 

0 

z 

:;:cl 

tTl 

0 

:::c: 

r 

r 

(/) 

~ ;J;. 

:;:cl 

;;o::: 

;J;. 

0 

c::: 

z 

...., 

""0 

r 

;J;. 

~ ;J;. 

c::: 

N 

~ 

-..l 
248 

WEST 

A 

B 

GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

EAST 

(,z/ (MARKAGUNT PLATEAU) 

---~,_------------Datum line 

Tv 

r Tbh 

/--- Early breakaway TKJ 

I 

(MARKAGUNT PLATEAU) 

-------------- ----Datum line 

r------------------=~~====jT~b~h Tv l~~=r Lower 

TKJ plate 

Early breakaway 

(MARKAGUNT PLATEAU) 

bh 

Early breakaway TKJ A RHSZ 

c 

CEDAR 

VALLEY 

BLOCK 

D 

RED HILLS 

BLOCK 

I \ Later breakaway 

(Paragonah fault) 

MARKAGUNT 

PLATEAU 

BLOCK Tv 

Reactivated (?) 

upper plate faults 

or new faults 

Figure 15. Normal fault model as possible mechanism for decoupling of upper plate at 

various stages of development (A-D). QTa, Quaternary and Tertiary alluvium; Tv, Tertiary 

volcanic rocks undivided; Tbh, Brian Head unit; TKJ, Tertiary Claron Formation and older 

rocks (Cretaceous and Jurassic); RHSZ, Red Hills shear zone. Light line, contact; heavy 

line, fault; dashed where inferred or where projected above topographic profile; barbs show 

direction of relative movement; triangle, gravity-slide block. 

surface dips to the north. Weak evidence here suggests a 

southerly direction of movement. Thus, transport direction 

for the upper plate is not conclusive. 

Decoupling of the upper plate resulted in fragmentation 

of upper plate rocks, forming upper plate faults that in part 

emplaced gravity-slide blocks (fig. 15C). Most of the upper 
DECOUPLING IN RED HILLS-MARKAGUNT PLATEAU 249 

plate faults are high angle, but some are low-angle or bedding 

plane faults that are difficult to map due to their geometry. 

Later deformation as represented by the Paragonah fault 

resulted in separation of the Red Hills from the Markagunt 

Plateau (fig. 15D) offsetting the Red Hills shear zone. This 

period of deformation also formed grabens that preserved the 

shear zone (fig. 2) and further fragmented the upper plate; 

new faults formed that extend from the lower plate into the 

upper plate and generally strike northeast. These faults may 

have reactivated previously formed upper plate faults and 

emplaced additional gravity-slide blocks (fig. 15D). 

ORIGIN OF MEGABRECCIA 

In the Red Hills, numerous megabreccia blocks (map 

unit Tm, figs. 2, 3), composed of relatively simple mono lithologic breccia to chaotically mixed poly lithologic breccia, lie 

atop the upper plate. The megabreccia is composed of brecciated units present within the upper plate. As indicated earlier, these blocks are inferred to be gravity-slide blocks 

emplaced during formation of upper plate faults. The gravityslide blocks may have been emplaced as sheets along lowangle faults, forming megabreccia deposits that are presently 

found as scattered blocks and may represent erosional remnants of those sheets. Other gravity-slide blocks may have 

been shed from scarps that formed along high-angle faults. 

On the Markagunt Plateau numerous blocks of megabreccia, as much as 2.5 km2 in areal extent and 100 m thick 

(Sable and Anderson, 1985; J.J. Anderson, 1993) are scattered over an area of as much as 1,000 km2 (E.G. Sable, oral 

commun., 1994 ). Most of these blocks are interpreted here to 

have been emplaced during breakage of the upper plate by 

faults propagating from or as a consequence of movement 

along the Red Hills shear zone. Some blocks, however, may 

be related to later deformation; others may be due to secondary movements as slide blocks; others may have been 

emplaced during intrusion of local magmatic bodies of 

Miocene (21.5-22 Ma) age, as suggested by Mackin ( 1960, 

p. 121), Rowley and others (1989), and Blank and others 

( 1992) for the Iron Springs mining district (approximate! y 15 

km southwest of the Red Hills); and still others may be 

related to late stages of Mount Dutton volcanism. In some 

areas, extensive erosion of the upper plate has left only erosional remnants that resemble slide blocks. Differentiating 

remnant blocks of the upper plate from slide blocks may be 

impossible in some areas. 

RELATIONSHIP OF PARAGONAH 

FAULT TO RED HILLS SHEAR ZONE 

The Paragonah fault (fig. 2) is interpreted as a breakaway fault with possible listric geometry at great depth, 

along which the Red Hills block broke away from the westem Markagunt Plateau, carrying with it the Red Hills shear 

zone. The fault may be manifested by small scallop-shaped 

blocks of Claron Formation (included with unit TKJ in fig. 

2) that have been mapped along the west edge of the Markagunt Plateau in the Parowan Valley area east of Paragonah. 

These blocks may be relicts of the footwall block on the 

downdropped side of the Paragonah fault. Measured fault 

planes that bound these blocks are generally of intermediate 

angle rather than high angle, suggesting a listric geometry in 

the subsurface. In some areas also along the west edge of the 

Markagunt Plateau, similar blocks overlie what appears to be 

Quaternary colluvium. This suggests that some of the blocks 

may simply represent landslides or block slides. 

The Paragonah fault bounds Parowan Valley on the east 

side and the Red Hills fault bounds the valley on the west 

(fig. 2). The Red Hills fault is herein assumed to be antithetic 

to the Paragonah fault, propagating from it at depth. Development of both the Paragonah and Red Hills faults thus may 

have formed the Parowan Valley. 

The history of the Paragonah fault may be similar to 

that of the Hurricane fault discussed by previous workers. 

Anderson and Christenson (1989) stated that the Hurricane 

fault probably formed after the late Miocene and remained 

active into the Pleistocene. The Paragonah fault may be the 

same age as the Hurricane fault. The following data support 

this interpretation: (1) A regional ash-flow tuff, the Harmony 

Hills Tuff, whose source area was thought to be the Caliente 

caldera complex (Noble and others, 1968; Ekren and others, 

1977) in eastern Nevada, is now thought to be from the Bull 

Valley Mountains of southwestern Utah by Rowley and others (this volume), as originally suggested by Blank (1959) 

and Williams (1967). This unit has been mapped on the 

Markagunt Plateau (Maldonado and Moore, 1993) in the 

Parowan Canyon area (fig. 1 ). The presence of this unit on 

the Markagunt Plateau suggests that it spread from its source 

area in southwestern Utah across the present boundary that 

separates the Markagunt Plateau from the Basin and Range 

province. This implies that no major barriers blocked the 

spread of the Harmony Hills Tuff and that the Markagunt 

Plateau did not exist until after the emplacement of the Harmony Hills Tuff (22-22.5 Ma). (2) In Parowan Valley, along 

the west edge of the Markagunt Plateau, south of the town of 

Paragonah, the Paragonah fault offsets a basalt lava flow 

referred to here as the basalt of Water Canyon. The basalt 

flowed down a paleovalley in the Markagunt Plateau and 

over the Paragonah fault scarp, flowing out onto the Parowan 

Valley where it was subsequently cut by the Paragonah fault. 

This basalt has been dated at 0.45±0.04 Ma (Fleck and others, 1975, age corrected for new decay constants of Steiger 

and Jager, 1977) using the K-Ar method. This relationship 

indicates recurrent movement along the Paragonah fault in 

relatively recent geologic time. (3) Along the same fault 

strand, north of the town of Paragonah, the Paragonah fault 

truncates a set of N. 10° W.-striking faults filled with mafic 
250 GEOLO~JIC STUDIES, BARCO STUDY UNIT, 1992 

dike material (figs. 2, 16) (Anderson, 1965; J.J. Anderson 

and F. Maldonado, unpub. data, 1990). These dikes have 

been dated at 20 Ma using the K-Ar method (H.H. Mehnert 

and R.E. Anderson, written commun., 1988). The dikes are 

likely related to the nearby Iron Peak gabbroic laccolith (fig. 

2) described by Anderson (1965), Judy (1974), and Spurney 

( 1984) and dated at about 20.2±0.5 Ma (Fleck and others, 

1975, age corrected for new decay constants of Steiger and 

Jager, 1977). The laccolith is also offset by high-angle faults 

that are inferred to be of the same age as the Paragonah fault. 

Figure 16. Mafic dike truncated by inferred Paragonah fault along western margin of the Markagunt Plateau north of 

Paragonah (fig. 1). Tmd, mafic dike; Tc, Claron Formation. Bar and ball on downthrown side of fault. View looking east 

from Parowan Valley. 

Figure 17. Mafic dike (Tmd) cutting the Brian Head unit (Tbh) west of Cottonwood Mountain. Tnw, Wah Wah Springs 

Formation; RHSZ, Red Hills shear zone; ticks on upper plate, dashed where projected. 
DECOUPLING IN RED HILLS-MARKAGUNT PLATEAU 251 

The relations just discussed suggest that the Paragonah 

fault and related faults have been active in the Quaternary 

and are not older than approximately 20 Ma. Southwest of 

Cottonwood Mountain (figs. 1, 2), the aforementioned 

20-Ma dikes intruded the Claron Formation, the Brian Head 

unit (Tbh, fig. 17), and upper plate rocks of the Red Hills 

low-angle shear zone. The age of the shear zone is thus 

bracketed between 22.5 Ma (age of Harmony Hills Tuff) and 

20 Ma (age of mafic dikes) or very likely 20.2±0.5 Ma (age 

of the Iron Peak gabbroic laccolith). 

FEATURES RELATED TO LOW -ANGLE 

SHEAR ZONE 

Several zones of replacement chalcedony Gasper) are 

found within the Brian Head unit in the Red Hills and 

Markagunt Plateau. The relationship between the chalcedony 

and the Red Hills shear zone is not well understood. The shear 

zone may have formed low-angle barriers to flow, channeling 

fluids within the strata of the Brian Head unit. The chalcedony 

deposits may have formed between the barriers within 

permeable strata. In the southern Red Hills, the chalcedony 

locally replaces limestone. In the Markagunt Plateau, the 

chalcedony zone is well developed in the Brian Head (Peak) 

area, approximately 8 km south of the area of figure 2 and 

shown in figure 1, where it was first described by Gregory 

( 1949). Samples of chalcedony from the zone at Brian Head 

were collected for geochemical analyses to determine 

possible metal association with the zone. The results of 

analyses are shown in table 1. Analysis indicates generally 

low metal values and suggests a hydrothermal source. Metals 

with some anomalous values are Mn, As, Ba, Co, Mo, and V 

(Richard Van Loenen, oral commun., 1993). Gold is at limit 

of detection. 

The Iron Peak gabbroic laccolith (fig. 18) intrudes the 

Brian Head unit. Evidence discussed previously suggests 

that the age of the shear zone is pre-20 Ma, while the age of 

the laccolith has been determined to be 20±0.5 Ma (Fleck 

and others, 1975, age corrected for new decay constants of 

Steiger and Jager, 1977). I propose here that magma was 

Table 1. Geochemical analyses of chalcedony zone in the Brian Head unit, Brian Head (Peak) area. 

[Analyses in parts per million; in percent for AI, Ca, Fe, K, Mg, Na, P, Ti (analysis by D.L. Fey and C.A. Motooka, U.S. Geological Survey). Analysis by 40 element ICP (induetively coupled plasma) method and by flame method for Au. Limit of detection for the following elements in parentheses in ppm for Au (0.05), Cr ( l ), Cu (I), Sr (2), and V (2). 

Leaders(--), no detection; *, anomalous value! 

Sample Sample 

No. RH738A RH738B RH738C RH738D No. RH738A RH738B RH738C RH738D 

AI 0. 1 2.1 2 0.06 0.05 La <2. 12. 3. <2. 

Ca . 20 .95 .05 .21 Li 3. 9 . <2. <2. 

Fe . 12 .83 .14 . 07 Mo <2 . <2 . 6.* <2. 

K . 06 .48 . 13 <.05 Nb 4. <4 . <4. <4. 

Mg . 01 .27 .01 .01 Nd <4. 8. <4 . <4. 

Na . 01 .42 .0 1 .008 Ni <2. 3. 6 . <2. 

p <.005 .03 <.005 <.005 Pb 19. 12. <4. <4. 

Ti <.005 .09 <.005 <.005 Sc <2. 3. <2. <2. 

Mn 74. 297.00 17,000.0* 181. Sn <10. <10. <10. <10. 

Ag <2. <2. <2.0 <2. Sr 3. 125. 46. 2. 

As 10. <10. 20.0* <10. Ta <40. <40. <40. <40. 

Au . 05 .05 .05 .05 Th <4. <4. 5 . <4. 

B u <100. <100. <100. <100. 

Ba 94. 339. 2,070.0* 54. v 3. 20. 132.* 5. 

Be <I. <I. <1.0 <I. w 

Bi <10. <10. <10. <10. y <2. 4. <2. <2. 

Cd <2. <2. <2. <2. Yb <I. <I. <I. <I. 

Ce <4. 19. 7. <4. Zn <2. 16. 15. 9. 

Co <I. 5. 13.* <I. Zr 

Cr 2. 10. 3. 2. 

Cu 2. 4. 5. 4. 

Eu <2. <2. <2. <2. 

Ga <4. <4. 8. <4. 

Ge 

Ho <4. <4. <4. <4. 
252 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

Figure 18. Iron Peak laccolith, Markagunt Plateau. Tip, Iron Peak laccolith; Tc, Claron Formation; dashed line, contact; 

view looking northeast from Parowan Valley. 

intruded along the Red Hills shear zone, forming the laccolithic body. 

CONCLUSIONS 

Field evidence from the Red Hills and adjacent Markagunt Plateau of Utah records two major episodes of deformation following mid-Tertiary deposition of the Brian Head 

unit. An early episode between 22.5 and 20 Ma resulted in 

decoupling of the upper plate, forming the Red Hills lowangle shear zone and the upper plate faults. The upper plate 

faults are mostly high angle that may become listric to the 

shear zone, but some low-angle or bedding plane faults are 

also present. The gravity-slide blocks may have been 

emplaced as sheets along some of the low-angle faults forming megabreccia deposits, presently found as scattered 

blocks that may represent erosional remnants of those sheets. 

Other blocks may have been shed from scarps that formed 

along high-angle faults. A later episode (post-20 Ma) fragmented the Markagunt Plateau and offset the Red Hills shear 

zone. This later episode of faulting cut the Red Hills shear 

zone and created slopes along which blocks on top of the 

upper plate were mobilized as young gravity slides. The 

upper plate contains gravity-slide blocks emplaced during 

formation of the Red Hills shear zone and gravity-slide 

blocks emplaced during later deformation. 

At least two possible mechanisms could have triggered 

decoupling of the upper plate: ( 1) uplift caused by intrusion, 

or (2) normal faulting (extension). The mechanism for 

decoupling of the upper plate is enigmatic; intrusion, normal 

faulting, or a combination of the two models appears to be 

valid. 

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Sable, E.G., and Anderson, J.J., 1985, Tertiary tectonic slide megabreccias, Markagunt Plateau, southwestern Utah: Geological 

Society of America Abstracts with Programs. v. 17, no. 3, p. 

263. 

Shawe, D.R., and Stewart, J.H., 1976, Ore deposits as related to tectonics and magmatism, Nevada and Utah: Transactions of 

American Institute of Mining, Metallic, and Petroleum Engineers, v. 260, p. 225-232. 

Siders, M.A., Rowley, P.O., Shubat, M.A., Christenson, G.E., and 

Galyardt, G.L., 1990, Geologic map of the Newcastle quadrangle, Iron County, Utah: U.S. Geological Survey Geologic 

Quadrangle Map GQ--1690, scale 1 :24,000. 

Spurney, J.C., 1984, Geology of the Iron Peak intrusion. Iron 

County, Utah: Kent, Ohio, Kent State University M.S. thesis, 

84 p. 

Steiger, R.H., and Jager, Emile, 1977, Subcommission on 

geochronology-Convention on the use of decay constants in 

geo- and cosmochronology: Earth and Planetary Science 

Letters, v. 36, p. 359-362. 

Thomas, H.E., and Taylor, G.H., 1946, Geology and ground-water 

resources of Cedar City and Parowan Valleys, Iron County, 

Utah: U.S. Geological Survey Water-Supply Paper, 210 p. 

Threet, R.L., 1952, Geology of the Red Hills area, Iron County, 

Utah: Seattle, Wash., University of Washington Ph. D. dissertation, 107 p. 

Tobey, E.F., 1976, Geology of the Bull Valley intrusive-extrusive 

complex and genesis of the associated iron deposits: Eugene, 

Oregon, University of Oregon Ph. D. dissertation, 244 p. 

Tompkins, F.V., Goodwin, J.C., and Malin, W.J., 1963, Cedar City, 

Utah, to Parowan, Utah, via Iron Springs mining district and 

Parowan Gap, Road log 3, in Heylmun, E.B., ed., Geology of 

southwestern Utah: Intermountain Association of Geologists 

Guidebook 12th Annual Field Conference, 1963, p. 222-226. 

Wernicke, Brian, 1981, Low-angle faults in the Basin and Range 

province-Nappe tectonics in an extending orogen: Nature, v. 

291, p. 645-648. 

Wernicke, Brian, and Burchfiel, B.C., 1982, Modes of extensional 

tectonics: Journal of Structural Geology, v. 4, p. 105-115. 

Williams, P.L., 1967, Stratigraphy and petrography of the Quichapa 

Group, southwestern Utah and southeastern Nevada: Seattle, 

Wash., University of Washington Ph. D. dissertation, 182 p. 
Quaternary Geology and Tectonics of the Red Hills 

Area of the Basin and Range-Colorado Plateau 

Transition Zone, Iron County, Utah 

By VanS. Williams and Florian Maldonado 

GEOLOGIC STUDIES IN THE BASIN AND RANGE-COLORADO PLATEAU TRANSITION IN 

SOUTHEASTERN NEVADA, SOUTHWESTERN UTAH, AND NORTHWESTERN ARIZONA, 1992 

U.S. GEOLOGICAL SURVEY BULLETIN 2056-J 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1995 
CONTENTS 

Abstract ........................................................................................................................... 257 

Introduction .................................................................................................................... 257 

Acknowledgments .......................................................................................................... 260 

Quaternary and Upper Tertiary Deposits ........................................................................ 260 

Old Alluvial Deposits ............................................................................................. 260 

Parowan Valley ............................................................................................... 260 

Northern Cedar City Valley ............................................................................ 262 

Young Alluvial Deposits ........................................................................................ 264 

Playa Deposits ........................................................................................................ 265 

Quaternary Basalts .................................................................................................. 265 

Quaternary Faults ........................................................................................................... 266 

General Pattern ....................................................................................................... 266 

Little Salt Lake Fault .............................................................................................. 266 

Structure .......................................................................................................... 266 

Morphology .................................................................................................... 268 

History ............................................................................................................ 272 

Enoch Graben ......................................................................................................... 273 

Conclusions .................................................................................................................... 273 

References Cited ............................................................................................................. 274 

FIGURES 

I. Map showing area surrounding the Red Hills, and location of other maps and cross sections in this report............. 258 

2. Generalized cross section A-A' across the Red Hills . . ...... .. . . . . . .... ............ .. ......... ...... .. .. . . ..... ........ ...... ..... ...... .......... .... 259 

3. Map showing Quaternary deposits and faults, and generalized bedrock geology of the Red Hills and 

surrounding basins....................................................................................................................................................... 261 

4. Cross section B-B' across the Hieroglyph horst and Enoch graben............................................................................ 262 

5. Map showing geology along a section of the Little Salt Lake fault zone ................................................................... 263 

6. Aerial view of Little Salt Lake fault scarp showing location of profiles and of figure 5 ... . . . . .............. ........... ............ 267 

7. Photograph of Little Salt Lake fault and small antithetic faults.................................................................................. 268 

8. Map and cross section of fault and bedding attitudes in alluvium near Little Salt Lake fault, in walls of 

north gully ................................................................................................................................................................... 269 

9. Map and cross section of fault and bedding attitudes in alluvium near Little Salt Lake fault, in walls of 

south gully . . . . . . .... ....... .. . . . . . . ............ .............. .............. ...... ....... .... .. .......... ........... .............. ... .......... ........... .......... .. . . ...... 270 

I 0. Profiles across Little Salt Lake fault measured at four locations ......... ............ ..... ........ .. ......... ........ ..... ..... ......... ........ 27I 

II. Plot of slope angle against log of scarp height for profiles across Little Salt Lake fault, referenced to 

measurements on Lake Bonneville shoreline scarps and Drum Mountain fault scarps ......... .............. ... ........ ........ .... 272 
Quaternary Geology and Tectonics of the Red Hills Area of the 

Basin and Range-Colorado Plateau Transition Zone, 

Iron County, Utah 

By VanS. Williams and Florian Maldonado 

ABSTRACT 

Near the Red Hills in southwestern Utah, upper Tertiary 

and Quaternary deposits and faults give some indication of 

the late Cenozoic history of geologic processes active in the 

transition zone between the relatively undisturbed structure 

of the Colorado Plateau and the highly faulted and tilted 

extensional structure of the Basin and Range province. The 

Red Hills, a north-northeast-trending, fault-bounded range 

formed of Tertiary volcanic rocks overlying Mesozoic and 

Tertiary sedimentary rocks, are part of a fault-block system 

that separated from the Colorado Plateau along west-dipping 

normal faults, slid westward, and tilted as much as 20° 

eastward. 

Ages of the extensive alluvial deposits around the Red 

Hills are indicated by their stratigraphic relation to isotopically dated Quaternary basalts, coupled with their relative 

geomorphic position, modification of original depositional 

form, and degree of soil development, especially the morphology and thickness of carbonate-enriched horizons. 

These alluvial deposits consist for the most part of old alluvium (late Tertiary and early Quaternary) containing sparse 

intercalated tephra beds. The deposits are more than 1 km 

thick where preserved in down-faulted basins east and west 

of the Red Hills, but they are discontinuous and thinner on 

the uplifted Red Hills block. Post-depositional tectonism has 

tilted, faulted, and uplifted the old deposits in the basins; erosion, dominant over deposition since middle Pleistocene, has 

also modified original forms. Thus post-middle Pleistocene 

alluvial deposits are mostly very thin, overlying erosion surfaces cut on old alluvium and soft bedrock. 

Besides alluvium, Quaternary deposits include playa 

silt, as well as basalt flows. The silt beds were deposited in 

Little Salt Lake during late Pleistocene and Holocene time. 

Basalt erupted during middle Pleistocene time in the eastern 

Parowan Valley and in early Pleistocene time in the Enoch 

graben. 

The Little Salt Lake fault, one of a series of normal 

faults along the east side of the Red Hills, strikes N. 45° E. 

and dips 75° SE. It exhibits the highest fault scarp in alluvium near the Red Hills: the scarp's maximum height is 18 

meters at a point where the maximum slope of the scarp is 

19°. We postulate at least three episodes of movement on 

Little Salt Lake fault beginning before middle Pleistocene 

and continuing through late Pleistocene and perhaps into the 

Holocene; offset of middle Pleistocene geomorphic surfaces 

across the fault exceeds 60 meters, and offset of the late 

Pleistocene surface exceeds 10 meters. The average rate of 

vertical motion since middle Pleistocene has been about 0.12 

mm/yr±50 percent, whereas the average rate since late Pleistocene has been at least 0.5 mm/yr±50 percent. Vertical offset on the fault bounding the east side of the Hieroglyph 

horst, in the southwestern Red Hills, has been as much as 70 

meters since eruption of basalt flows 1.28±0.4 million years 

ago; vertical offset on the west-side-bounding fault is as 

much as 230 meters. Offset on the fault bounding the east 

side of the Enoch graben during the same period is as much 

as 50 meters. These relationships indicate vertical slip rates 

in the range of 0.054 mm/yr±30 percent on the east side and 

0.178 mm/yr±30 percent on the west side of the horst, and 

0.039 mm/yr±30 percent on the east side of the graben. 

INTRODUCTION 

Near the Red Hills, in southwestern Utah, geologic terrain evolves across a transition zone from the relatively less 

disturbed structure of the Colorado Plateau to the highly 

faulted and tilted extensional structure of the Basin and 

Range province. Examination of the upper Tertiary and Quaternary deposits and faults indicates the late history of geologic processes active in this transition zone. 

The Red Hills are a north-northeast-trending, faultbounded range of Tertiary volcanic rocks overlying Mesozoic and Tertiary sedimentary rocks. The range is located 

about 40 km north of Cedar City in the northeast comer of 

Iron County, southern Utah (fig. 1 ). Maldonado and others 

(1990) and Maldonado (this volume) have interpreted the 

257 
258 GEOLOGIC STUDIES, BARCO STUDY UNIT, I 992 

38° 15' 

WOO' 

37"45' 

BASIN AND RANGE PROVINCE 

A 

MUD 

SPRING 

HILLS 

CedarCity 0 

112" 45' 112" 30' 

BEAVER BASIN 

UI'AH 

Figure 1. Map showing area surrounding the Red Hills, and location of other maps and cross sections in this report. Heavy line, fault; bar 

and ball on downthrown side. USS, U.S. Steel drill hole. MFS, Mountain Fuel Supplies drill hole. 

Red Hills as part of a structural block that has been transported away from the Colorado Plateau along the Paragonah 

fault, detached from the basement along listric and lowangle faults, and migrated westward in response to gravitational forces and crustal extension (fig. 2). Normal faulting 

antithetic to the Paragonah fault has broken the detached 

block so that the western third of the block now forms a horst 

relative to the eastern two-thirds, which have dropped down 

to form a graben. The horst forms the Red Hills, and the graben underlies the alluvium-filled Parowan Valley, between 

the Red Hills on the west and the Markagunt Plateau on the 

east (fig. 1). 

West of the Red Hills an alluvium-filled graben forms 

the north end of the Cedar City basin. Subsequent to 

deposition of a thick alluvial fill in the graben, the block has 

been uplifted and tilted eastward, and the Tertiary and 

Quaternary alluvium deeply dissected. Along the west side 

of that structural block, erosion has completely removed the 

alluvium to expose Tertiary volcanic rocks that now form 

part of the Black Mountains. The pattern of deformation that 

formed the Red Hills continues, as reflected by deformation 

of the bounding Tertiary and Quaternary alluvial deposits, 

and offset of Pleistocene and Holocene depositional surfaces 

by faults. 
A 

ESCALANTE 

DESERT 

BLACK NORTHERN CEDAR 

MOUNTAINS CITY VAu.EY 

REOHIUS PAROWAN VAllE.V 

KILOMETERS USS drill hole 

2 1 nT .. /Tv~ ~~rote7d) Jif¥F TK.I ~ Tre 

0 

-2 

-4 

-6 

-8 

-10 

-12 

-14 

~ 

8 

a 

EJ 

a 

8 

Holocene-upper Pleistocene alluvium 

Quaternary-Tertiary alluvium (undifferentiated) 

of Long Hollow 

of Long Hill 

of Little Salt Lake fault 

of Slough Bench 

NO VERTICAl- EXAGGERATION 

EXPLANA110N 

l:::):j[)Tij[j[jjj[jjj Tertiary volcanic rocks 

G Tertiary sedimentary and volcanic rocks of the Red Hills 

G Tertiary, Cretaceous, and Jurassic rocks-Claron Formation, 

conglomerate of Parowan Gap, Iron Springs Formation, 

Carmel Formation, and Navajo Sandstone 

Contact 

Fault-Barbs show direction of relative movement 

A' 

MARKAGUNT PLATEAU 

Figure 2. Generalized cross section A-A' across the Red Hills, showing westward movement and eastward tilting of structural blocks extending from the Markagunt Plateau west to 

the Escalante Desert basin. Queried at depth where relations uncertain. East-dipping faults are antithetic. 

0 

c 

> @ 

~ > 

:;;a::l 

~ 

a 

tr1 

0 

r 

0 

a 

~ 

> z 

0 

@ 

n 

...., 

0 

z 

n 

.en 

f§ 

::r: 

F 

r 

C/) 

N 

Vl 

\0 
260 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

ACKNOWLEDGMENTS 

Several residents of Cedar City assisted this study by 

sharing their knowledge of the area acquired over many 

years. These include Dr. Fred Lohrengel, Professor of Geology at Southern Utah University; Gorden Crandell, soil scientist with the U.S. Soil Conservation Service; Woodard 

Sandberg, hydrologist with the U.S. Geological Survey, and 

Gerald Staker, Utah State Engineer's Office. Profile surveys 

of the fault scarps would not have been completed without 

the much-appreciated assistance of four geology students at 

Southern Utah University: Dean Child, David Bateman, Dan 

Barton, and Paula Prichard. 

QUATERNARY AND UPPER TERTIARY 

DEPOSITS 

Thick, unconsolidated, alluvial fills are preserved in the 

deep, down-faulted basins east and west of the Red Hills. 

These are mostly of late Tertiary and early Quaternary age, 

except for thick upper Quaternary fan alluvium deposited on 

the east side of the Parowan Valley (fig. 3) by sedimentladen streams from the Markagunt Plateau. Tectonism 

within the basins has deformed and uplifted the older deposits so that they crop out in many places. Several generations 

of extensive but thin deposits of middle Pleistocene, upper 

Pleistocene, and Holocene alluvium overlie erosion surfaces 

cut on upper Tertiary and lower Pleistocene alluvium or 

weakly consolidated bedrock. Little Salt Lake Playa presently covers 21 km2 on the floor of Parowan Valley, but 

older playa deposits have not been found at the surface or in 

drill holes (Thomas and Taylor, 1946). Ages of alluvial 

deposits are estimated based on their relation to isotopically 

dated Quaternary basalts, relative geomorphic position, 

modification of original depositional form, and degree of 

soil development, especially the morphology and thickness 

of carbonate-enriched horizons. No isotopic dates are available that directly apply to alluvial materials. 

OLD ALLUVIAL DEPOSITS 

The upper Tertiary and lower Quaternary basin fills on 

both sides of the Red Hills are quite thick. A U.S. Steel drill 

hole (USS) on the west side of the range in sec. 31, T. 33 S., 

R. 10 W. (fig. 4), only 2 km west of the range-front fault, 

reached a depth of 918 m without penetrating the base of the 

alluvium (Rowley, 1975). A gravity low (Cook and Hardman, 1967) centered south of the drill hole suggests a maximum depth to bedrock of about 1 ,200 m. On the east side of 

the range the alluvium may also be thick, but no deep holes 

have been drilled. A gravity profile across the southern 

Parowan Valley (Pe and Cook, 1980) placed the base of the 

alluvium at about 1,200 m. The deepest water well penetrated more than 180 m of alluvium without reaching the 

base (Bjorklund and others, 1977). 

Uplift of the Red Hills has deformed old basin fill alluvium and raised it to a level where it is subject to erosion. On 

the west side, much of the bedrock on the uplifted block is 

still partially covered by eroded basin fill deposits (alluvium 

of Long Hill, unit QTh, fig. 3 ). To the east of the Red Hills, 

old alluvium on the west side of the Parowan Valley graben 

has been exposed by eastward tilting ofthe graben along normal faults, so that low hills of old alluvium protrude above 

depositional level west of the axis of the Parowan Valley 

(alluvium of Slough Bench, unit QTb, fig. 3). 

PAROWAN VALLEY 

Two distinct types of old basin fill alluvium are 

exposed on the west side of the Parowan Valley. The deposits forming low hills west of the axis of the valley consist of 

eroded remnants of uplifted distal alluvial fan and basin-center sheet flow deposits (unit QTb, fig. 3). The coarsest beds 

are sandy, pebbly silt and contain about 35 percent quartzite 

and volcanic pebbles less than 3 em in diameter. The finest 

beds contain as much as 50 percent clay. Original calcium 

carbonate content of the sediment is variable depending on 

source rock, but probably is less than the 14 percent average 

determined for young sediment eroded from the Paleocene 

and Eocene Claron Formation, the main local source of calcium carbonate. Surface soils formed under surfaces cut on 

unit QTb are thicker than 1.5 m, and include as much as 46 

percent total calcium carbonate, small amounts of gypsum, 

and some salt (Gorden Crandell, U.S. Soil Conservation Service, Cedar City, written commun., 1989). The deposits are 

poorly exposed and commonly veneered with windblown 

sand and silt from the Little Salt Lake playa. They are overlain by younger playa deposits and alluvium (units Ql, Qfa, 

and Of, fig. 3). Direct evidence of the numerical age of unit 

QTb is scant, but its stratigraphic position, thick carbonateenriched soil, and lack of original depositional form all indicate that it is relatively old. 

Calculations based on the measured calcium carbonate 

of horizons in the top 1.5 m of the soil, an assumed bulk density of 1.6, and assumed original calcium carbonate content 

of about 14 percent indicate that at least 36 g CaC03 /cm2 

has been added to the soil profile. This compares approximately to the accumulation in soils developed in alluvium of 

North Creek in the Beaver Basin (Machette, 1985, p. 32), 

which is 50 km northeast of the study area. The age of alluvium of North Creek indicated by U-trend dating is 290±80 

ky (Steer, 1980), but unit QTb is probably much older, 

because it was uplifted and eroded prior to the start of soil 

carbonate accumulation. 

Old alluvium (QTf, fig. 5) that is distinct from that in 

the center of the valley (QTb, fig. 3) is exposed at a few 
QUATERNARY GEOLOGY AND TECTONICS, RED HILLS 261 

113° 00' 

38° . 

00' ; 

3r 

52' 

30" 

.______) KILOMETER 

EXPlANATION 

0 Holocene-upper Pleistocene playa deposits 

G Holocene-upper Pleistocene alluvium 

• Upper Plcistoccnc alluvium 

Middle Pleistocene alluvium 

• Quaternary basalt 

Quaternary-Tertiary alluvium 

of l.ong Hollow 

of l.ong Hill 

of Slough Bench 

Tertiary volcanic rocks 

Tertiary sedimentary and volcanic rocks of the Red Hills 

Tertiary, Cretaceous, and Jurassic rocks-Qaron Formation; 

conglomerate of Parowan Gap, Iron Springs Formation, 

Carmel Formation, and Navajo Sandstone 

LSL FAULT Little Salt Lake fault zone 

Contact 

Quaternary normal fault-Dashed where concealed; 

bar and ball on downthrown side 

Figure 3. Quaternary deposits and faults, and generalized bedrock geology of the Red Hills and surrounding basins. Parowan Gap (left) 

and Paragonah (right) 7.5- minute quadrangles. 

places along ravines cut into the upthrown block on the west 

side of the Parowan Valley, west of the Little Salt Lake 

fault. Unit QTf is coarser, better bedded, more intensely 

deformed than unit QTb, and includes more mafic volcanic 

clasts and beds of silicic volcanic ash. It is primarily yellowish brown, medium-grained, subangular, weakly indurated 

tuffaceous sandstone interbedded with light-brownish-gray, 

silty, tuffaceous, weakly indurated mudstone, and reddishblack, subangular, coarse-grained, weakly indurated, mafic 

volcanic sandstone. The mafic volcanic sandstone beds 

locally contain matrix-supported, subrounded mafic lava 

clasts as large as 50 em in diameter, probably derived from 

mafic lava blocks in the Mount Dutton Formation (20- 26 

Ma). The Mount Dutton Formation is missing from the Red 
262 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

B 

METERS 

2000 

1500 

1000 

Northern 

Cedar City 

Valley 

'IN/ Quaternary eluvium 

.-· --\ 

Quaternary-Tertiary eluvium 

B' 

ENOCH GRABEN 

HIEROGLYPH 

HORST 1.28 Ma basalt 

Lower Jurassic Navajo Sandstone 

Quartz monzonite 

500 NO VERTICAL EXAGGERATION 

Figure 4. Cross section B-B' across the Hieroglyph horst and Enoch graben. Geologic contacts from Maldonado and 

Williams (1993b), Rowley (1975), and Rowley and Threet (1976). Heavy line, fault; barbs show direction of relative 

movement. USS, U.S. Steel drill hole; WW, Endrody Ranch water well; MFS, Mountain Fuel Supplies drill hole. 

Hills except for three small erosional remnants on the east 

side of the range (fig. 3, outliers of unit Tv) (Maldonado and 

Williams, 1993a); however, it is several hundred meters 

thick 10 km north of the Little Salt Lake fault and probably 

existed on the Red Hills prior to its removal by erosion. The 

exposures of unit QTf near the Little Salt Lake fault (fig. 5) 

are extensively sheared, faulted, folded, and tilted as much 

as 41°. Some fault planes are coated by calcium carbonate. 

The mudstone in unit QTf locally contains at least one 

bed of reworked gray silicic volcanic ash more than 1 m 

thick. At the base of the reworked ash bed, a thin layer of 

white air-fall ash averages about 1 em thick. This ash is 

weathered, and it has not yet been possible to identify its 

source or determine its age. In a structural basin similar to 

the Parowan Valley but 40 km to the north, lake deposits in 

the Beaver Basin contain at least five different Tertiary and 

early Pleistocene ash beds ranging in age from 2.4 to 1.6 Ma, 

and post-lake Quaternary alluvium contains the 0.55 Ma 

tephra of Ranch Canyon (Machette, 1985). 

No direct evidence of the numerical age of unit QTf has 

been found, but it seems to be the oldest alluvial unit exposed 

in the Parowan Valley. It is folded and dips as steeply as 41 o, 

but it is overlain by unfolded Pleistocene alluvium (units Qf 

and Qfo). Unit Qfo has lost much of its original depositional 

form but locally retains a thick, well-developed carbonateenriched soil suggesting that unit Qfo is at least as old as 

middle Pleistocene. Unit QTf is rich in mafic lava clasts from 

the Mount Dutton Formation, a thick volcanic deposit that 

has been completely stripped from the drainage area that formerly contributed sediment to unit QTf. Parts of the unit are 

well consolidated and highly oxidized. The tephra beds are 

more altered and appear older than the 0.55 Ma tephra of 

Ranch Canyon, and they probably correlate to one of the Tertiary tephras found in the Beaver Basin, although the correlation has not been proven. 

NORTHERN CEDAR CITY VALLEY 

Most of the basin fill in the northern Cedar City Valley 

(alluvium of Long Hollow, unit QTI, figs. 2, 3) has been 

raised and tilted southwest as much as 20° as part of the 

uplift of the Black Mountain structural block (Rowley, 

1975). The eastern part of the basin fill (alluvium of Long 

Hill, unit QTh, figs. 2, 3), however, rests on the Red Hills 

structural block, and has been further uplifted by the growth 

of the Red Hills to a level above that of the adjacent east edge 

of the Black Hills block in the northern Cedar City Valley. 

This difference in structural settings is the primary basis for 

distinguishing two old alluvial units on the west side of the 

Red Hills (Maldonado and Williams, 1993b ). 

Both unit QTI and unit QTh are poorly sorted, poorly 

bedded, gravelly fan alluvium, but unit QTh is distinguished 

from unit QTI by its relative thinness, greater deformation, 

inclusion of large allochthonous blocks of volcanic rocks, 

greater proportion of quartzite clasts, and greater dissection. 

Unit QTh probably correlates to the older, basal part of unit 

QTI that is buried at great depth west of the range-front fault. 

An early Pleistocene or older age is indicated for unit QTh 

because it underlies 1.28±0.4 Ma basalt flows in the Enoch 

graben (figs. 3, 4). 

Unit QTI consists of subangular to angular gravel of 

mostly volcanic rocks and sparse rounded quartzite cobbles 

supported by a matrix of light-reddish-brown sand. The 

deposits are generally uncemented but locally are moderately 

well cemented by calcium carbonate. Clasts average 6 em in 

diameter and are as large as 50 em in diameter. The original 

depositional form is not preserved because the deposit has 

been uplifted and tilted, and the beds truncated by lateral erosion that has cut pediments across the deposits at several levels. Streams are now entrenched as deep as 60 m below the 

pediment surfaces. 
QUATERNARY GEOLOGY AND TECTONICS, RED HILLS 263 

112'52'30" 11 2'51'50" 

37'56'10" 

Qfo 

Qfa 

100 200 

37'55'40" 

EXPLANATION 

ffi Holocene-playa margin deposits Contact 

Holocene-upper Pleistocene playa I __.i1 Strike and dip of beds 

deposits 

Qfa Holocene-upper Pleistocene alluvium 172 

Quaternary normal faults-Dotted Upper Pleistocene alluvium where concealed; arrow indicates 

Qfo Middle Pleistocene alluvium direction and degree of dip; bar 

and ball on downthrown side 

. ott< Quaternary-Tertiary alluvium of Little 

Salt Lake fault 

~ ' Tertiary volcanic rocks 

18 m/18' Fault scarp measurements, scarp 

height/maximum scarp slope 

Figure 5. Geologic map along a section of the Little Salt Lake fault zone showing details of faulting. 

Despite the small amount of calcium carbonate in the 

alluvium of unit QTI, thick petrocalcic soils are developed 

under the relict pediment surfaces, which are much younger 

than the underlying unit QTI. Upper soil horizons have been 

stripped leaving stage IV laminar calcium carbonate (ofGile 

and others, 1966) exposed from the surface down to 25 em 
264 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

depth and a strongly cemented stage III carbonate-enriched 

horizon from 25 to 150 em depth. These are the most 

strongly developed soils in the Red Hills area; they are 

approximately comparable in development to the soils below 

the Table Grounds Surface in the Beaver Basin, 50 km to the 

northeast, which are estimated to be 750 ky old (Machette, 

1985, p. 32). If the comparison is valid, it indicates a minimum age of latest early Pleistocene for the pediments and a 

much older age for deposition of unit QTI. 

Unit QTh, resting on the Red Hills block, is poorly bedded, poorly sorted, and locally weakly cemented by calcium 

carbonate. Clasts are angular to subangular and consist primarily of volcanic rocks averaging 6 em in diameter and as 

large as l m in diameter in the northern part of the range; 

southward the predominant clast type changes gradually to 

rounded quartzite cobbles less than l 0 em in diameter. Clasts 

are supported by a matrix of light-reddish-brown sand that is 

redder and more calcareous southward. The original depositional form of unit QTh is not preserved. 

Within unit QTh are allochthonous blocks of volcanic 

rock as long as 400 m formed of the same rock units that are 

exposed in the northern Red Hills. These blocks probably 

represent the syntectonic deposit that resulted from opening 

of a depression west of the present Red Hills through downto-west offset at the western margin of the Red Hills structural block. Although such detached blocks are exposed only 

along the western margin of the northern Red Hills, they may 

also be present within the basal alluvial deposits of the northem Cedar City Valley (unit QTI) to the west. 

YOUNG ALLUVIAL DEPOSITS 

During the middle and late Pleistocene, erosional processes have been more effective than depositional ones in the 

area surrounding the Red Hills. Consequently, young alluvial deposits are generally quite thin except on the east side 

of the Parowan Valley, where sediment-laden streams from 

the Markagunt Plateau have built up alluvial fans thick 

enough to completely bury the underlying older alluvial sediments (unit Qfa, figs. 2, 3). The extensive low-level surfaces 

east and west of the Red Hills consist typically of pediments 

cut on old alluvium (units QTI, QTh, QTf, and QTb) and are 

veneered with a thin layer of young alluvium (units Qfa, Qf, 

and Qfo ). Similar surfaces have been cut on soft bedrock 

exposed within the Red Hills; these are now preserved on 

high benches or in embayments of the mountain front. The 

young alluvium resembles the old alluvium in its composition, but it is less consolidated and less oxidized, retains depositional form, and is not tilted. 

At least four distinct levels of pediment surfaces, estimated on the basis of soil development and geomorphology 

to range in age from middle Pleistocene to Holocene, record 

periods when downcutting was minimal and local base level 

relatively stable. The relation of most late Pleistocene and 

some Holocene surfaces to fault scarps implies that some of 

the episodic instability was caused by uplift of these surfaces 

relative to local base level in the Parowan Valley, uplift that 

occurred along normal faults on the west side of the Parowan 

Valley. Late Pleistocene surfaces thinly covered with unit Qf 

are extensively preserved on the upthrown side of faults 

(figs. 3, 5), although the surfaces are interrupted by 

entrenched streams. Late Pleistocene surfaces are preserved 

as high as 10m above modem surfaces of planation, and late 

Pleistocene and Holocene surfaces (unit Qfa, figs. 3, 5) as 

high as 1 m above those surfaces. Middle Pleistocene surfaces are generally preserved as small remnants near the apical areas of the younger surfaces and cannot be directly 

related to fault scarps. Two middle Pleistocene surfaces covered with unit Qfo (figs. 3, 5) are preserved at about 60 m and 

18m above modem, active surfaces of planation. 

Soils in unit Qfo on the middle Pleistocene surfaces 

have been truncated by erosion in most places, but locally 

profiles are preserved that have stage III (Gile and others, 

1960) development of carbonate-enriched horizons 55 em 

thick that extend from 35 to 90 em depth; in addition, they 

have calcium carbonate rinds and pendants as thick as 5 mm 

beneath clasts (Gorden Crandell, written commun., 1989). 

Where the soils were observed, units Qfo and Qf contain little or no Claron Formation detritus and have little original 

calcium carbonate. Soils on unit Qfo depositional surfaces 

are approximately comparable in development to the middle 

Pleistocene soils in alluvium of Greenville in the Beaver 

Basin that are estimated to be 140 ky old (Machette, 1985, 

p. 32). 

Soils on unit Qf have stage II development of carbonate-enriched horizons 40 em thick that extend from 35 to 75 

em depth, and the bottoms of clasts have thin carbonate coatings (Gorden Crandell, written commun., 1989). These soils 

are slightly more developed than the late Pleistocene soils in 

alluvium of Beaver in the Beaver Basin that are estimated to 

be 12-15 ky old (Machette, 1985, p. 32). 

The youngest (late Pleistocene to Holocene) alluvial 

deposits (unit Qfa) generally thinly cover active pediments 

cut on units QTb and QTf on the west side of the Parowan 

Valley and accumulate as thick fan deposits on the east side. 

Small fans of unit Qfa have also grown along the sides of 

Parowan Gap Canyon and in the narrow graben east of the 

Little Salt Lake fault. The base of the deposits is not exposed. 

Surfaces underlain by unit Qfa are generally not eroded, 

although they are slightly disrupted by faults at a few places. 

The material in unit Qfa is similar to that in the older alluvial 

deposits, although a surface layer of sand and silt exists that 

has been largely stripped from the older deposits. Virtually 

no soil has developed beneath the Holocene surfaces. The 

large fans on the east side of the Parowan Valley must 

include lower, middle, and upper Pleistocene alluviums, but 

they are mapped as unit Qfa because this unit is exposed at 

the surface. 
QUATERNARY GEOLOGY AND TECTONICS, RED HILLS 265 

PLAY A DEPOSITS 

On the west side of the Parowan Valley are young 

deposits of Little Salt Lake playa (unit Ql, figs. 3, 5) that are 

dammed by the combination of late Cenozoic uplift of the 

Red Hills relative to the Parowan Valley graben (Threet, 

1952; Anderson and Christenson, 1989) and deposition of 

alluvial fans within Parowan Gap Canyon near its east end 

(fig. 3) (Neilson, 1983). The great depth of Parowan Gap 

Canyon cutting across the Red Hills indicates that significant 

stream flow westward from the Parowan Valley has occurred 

throughout the history of Red Hills uplift, although little 

flow occurs at present, and even less occurred before local 

farmers cut a trench 3m deep through the alluvial fan deposits within the canyon. Whether the present lack of significant 

flow through the canyon is representative of conditions prior 

to settlement 150 years ago is uncertain. Since agricultural 

development, the loss of water from Parowan Valley is much 

greater due to evapotranspiration by crops, and peak runoff 

is reduced by impoundment in reservoirs and infiltration to 

replace ground-water withdrawals. 

Under natural conditions, the balance among several 

competing processes determines the situation in the canyon 

and the lowest portion of the Parowan Valley floor. 

Increased precipitation on the Markagunt Plateau, such as 

occurred during the pluvial periods of the Pliocene and Pleistocene, would tend to increase stream flow and sediment 

production, and would work to destroy Little Salt Lake, both 

by filling the basin with sediment and by eroding the threshold that retains the lake. When increased stream flow filled 

the playa basin to overflowing, flow through Parowan Gap 

Canyon carried away sediment from the canyon walls and 

prevented it from building up on the canyon floor, and it may 

have also eroded the bedrock threshold. Increased deposition 

of sediment in the lake tended to fill the lake basin above the 

level of the canyon threshold, so that all runoff tributary to 

Parowan Valley flowed through the Parowan Gap Canyon, 

increasing the efficiency of erosion and counteracting the 

damming effects of both uplift of the bedrock threshold and 

deposition of sediment from the canyon sides. High evapotranspiration in the Parowan Valley, low sediment influx, 

and rapid uplift of the Red Hills relative to the Parowan Valley worked in combination to prevent flow through Parowan 

Gap Canyon and consequently to promote blockage of the 

canyon by sediment from the canyon walls and expansion of 

the lake floor. 

Unlike many ephemeral lakes in the Basin and Range 

province, Little Salt Lake was probably not significantly 

larger during pluvial periods. No shorelines are observed 

more than a few meters above the present playa floor, and the 

many water wells drilled in the Parowan Valley have not 

encountered playa deposits at depth; thus the playa probably 

was no more extensive during the Pleistocene than it is at 

present. 

Deposits on the playa floor (unit Ql) are calcareous, 

saline, and gypsiferous gray beds of clay, silt, and sand. 

Deposits on the playa margin (unit Qlm) up to former high 

water level are similar but sandier and include pebbles transported by sheet wash. Sand and sand-sized aggregates of silt 

and clay are eroded from the playa floor by wind and transported to the northeast and east, where they accumulate as 

low, saline dunes and sheets that support a distinctive 

sodium-tolerant brushy vegetation, dominated by greasewood. 

QUATERNARY BASALTS 

Quaternary basalts are found at two different locations 

around the Red Hills. Volcanism from centers in the Enoch 

graben in the southwestern Red Hills has produced flows 

(figs. 1, 3, 4) that extend into the southwest corner of the 

Parowan Gap quadrangle (fig. 3) and have an isotopically 

determined age of 1.28±0.4 Ma (Anderson and Mehnert, 

1979). Those flows are localized along and offset by a system of normal faults. 

Basalt flows that extend into the southeast corner of the 

Paragonah quadrangle from Water Canyon (figs. 1, 3) have 

an isotopically determined age of 0.44±0.04 Ma (Fleck and 

others, 1975). The vent for these flows is about 4.5 km south 

of the Paragonah quadrangle and 2.5 km southeast of the 

Paragonah fault, on the upthrown side. The flow surface 

drops down about 130 m where it crosses the Paragonah 

fault, and at least 70 m more where it crosses a parallel 

range-front fault 1.5 km east of the Paragonah fault (Hamblin and others, 1981 ). The westward extension of the flow is 

buried at unknown depth beneath alluvium in the Parowan 

Valley graben. 

If the steps in the basalt flow surface are entirely caused 

by post-flow displacement along the faults rather than cascades over preexisting scarps, combined average rate of vertical displacement across the two mountain-front faults since 

the middle Pleistocene exceeds 0.46 mm/yr±9 percent. The 

Paragonah fault alone has averaged 0.30 mm/yr±9 percent. 

The uncertainty of 9 percent reflects only the analytical 

uncertainty of the date. 

In addition to the Quaternary basalts, another source of 

basaltic sediment that is probably of Tertiary age is a string 

of three small, basaltic exposures along the range-front fault 

on the west side of Parowan Valley in the northwest corner 

of the Paragonah quadrangle (fig. 3, outliers of unit Tv). This 

group of exposures is probably a remnant of Mount Dutton 

Formation, which has otherwise been completely stripped 

from the Red Hills but is extensively preserved 5 km to the 

north of Black Knoll (fig. I) (Maldonado and Williams, 

1 993a). The range-front fault in this part of the range is the 

northward extension of the Little Salt Lake fault. The northernmost and largest of these exposures, Black Knoll, includes 

dikes, large, angular blocks of mafic lava, and beds of scoria 
266 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

dipping radially outward at about I7°. Flow rocks and dikes 

within Black Knoll have reversed magnetic polarity. 

QUATERNARY FAULTS 

GENERAL PATTERN 

Figure 3 shows faults around the Red Hills that displace 

alluvium and volcanic rocks of Quaternary age. Where there 

is an absence of Quaternary deposits within bedrock areas of 

the Red Hills, many more faults may also have been active 

during the Quaternary. All the faults cutting upper Tertiary 

and Quaternary alluvial deposits have a northeasterly trend, 

but within the Oligocene volcanic bedrock of the northern 

Red Hills many older faults have other trends. The randomly 

oriented faults do not extend downward into the Eocene 

Claron Formation sediments, and probably reflect only local 

stresses that developed as the volcanic rocks slid along their 

contact with the underlying sedimentary rocks. 

With a few important exceptions, faults within the alluvium on both sides of the Red Hills are down to the west. The 

cross section in figure 2 shows most faults in the Parowan 

Valley to be antithetic to the Little Salt Lake fault, which 

forms the western boundary of the Parowan Valley graben, 

but synthetic to the Paragonah fault, the major fault that 

defines the western margin of the Markagunt Plateau as well 

as the base of the Red Hills horst and Parowan Valley graben 

(fig. 2). Displacement of aquifers within the alluvium of the 

Parowan Valley by faulting has produced abundant springs 

along the fault traces (Thomas and Taylor, 1946). In some 

cases, no offset of the ground surface is apparent, yet the 

trace of the fault is prominent on the aerial photographs 

because of the aligned springs. 

On the east side of the Parowan Valley and on the west 

side of the Red Hills, faults dip in the same direction as the 

ground-surface slope, and fault scarps as high as 7. 7 m are 

preserved. On the west side of the Parowan Valley, all faults 

but the westernmost dip opposite to the direction of ground 

surface slope, and fault scarps tend to be buried by sediment 

from upslope. Some buried fault scarps maintain surface 

expression in the form of a subtle break in slope where the 

pediment surface slopes opposite to the fault dip slightly less 

steeply than in other parts of the pediment. Other upslopefacing faults have scarps protruding as high as 3 m above the 

postfaulting young alluvium burying the downthrown side. 

Several west-facing scarps on the east side of the 

Parowan Valley are estimated to have formed during the 

Holocene or late Pleistocene (Anderson and Christenson, 

1989, p. 15), and one clearly offsets Holocene alluvium 

about 1 m (Maldonado and Williams, 1993a). Several westfacing scarps west of the Red Hills are estimated to have 

middle to late Pleistocene ages, and the age of one· on the 

west side of the Red Hills horst was estimated as early to 

middle Pleistocene (Anderson and Christenson, 1989, p. 14 ). 

At only two localities around the Red Hills are downto-the-east Quaternary faults prominent. One is the east side 

of the Red Hills (fig. 3), where mountain-front faults are 

antithetic to the Paragonah fault. Two right-stepping enechelon range-front faults on the east side of the Red Hills 

generally form the boundary between bedrock and alluvium 

without significantly offsetting the alluvium, except where 

the northernmost fault extends southward through alluvium 

as the Little Salt Lake fault. The other major east-dipping 

faults bound the west side of the Enoch graben (Threet, 

1 963) (figs. I, 4) on the west side of the southern part of the 

range. The faults along the Enoch graben offset Quaternary 

basalt flows at the north end and Quaternary alluvium at the 

south end, near Enoch. 

LITTLE SALT LAKE FAULT 

The east side of the Red Hills is bounded by a series of 

right -stepping, en-echelon, northeast -trending, down-to-theeast faults, but only the fault near the center of the range front 

has produced a prominent scarp in Quaternary alluvium 

(figs. 3, 5). In the southern part of the range, the Red Hills 

fault (figs. I, 3) is along the bedrock alluvium contact, and 

forms few scarps in alluvium (Anderson and Christenson, 

I989). In the northern part, the range-front fault is covered, 

and there are no east-facing scarps in the alluvium. 

In the central part of the Red Hills, where the mountain 

front jogs to the right (fig. 3), a 3.5-km-long fault scarp (fig. 

6) that offsets alluvial surfaces down to the east by I 0 m 

projects southward from the northern mountain front. This 

scarp disappears southward beneath deposits of Little Salt 

Lake playa, and was named Little Salt Lake fault by Thomas 

and Taylor ( 1946). It is the highest Quaternary fault scarp in 

alluvium in the area surrounding the Red Hills. 

STRUCTURE 

Strands of the Little Salt Lake fault, small antithetic 

faults, and a cross fault are exposed in two gullies where 

streams have eroded 15 m into the upthrown block (figs. 5, 

7, 8, 9). The fault planes can be recognized only where they 

cut the semiconsolidated alluvium of unit QTf. The strands 

of the main fault are indistinct because they mostly cut uniformly brown, poorly bedded, coarse sand within the small 

areas of exposure. Measurements were made by carefully 

brushing the exposure to emphasize weak preferential 

cementation along the anastomosing fault strands (fig. 7). 

The strike of the exposed east-dipping fault surfaces averages 

about N. 45° E., dipping 75° SE. (figs. 5, 8, 9). Beds east of 

the fault plane could not be matched with those to the west 

to estimate vertical offset. In most places the fault is covered 

by the boulder-rich colluvium that mantles the fault scarp. 
QUATERNARY GEOLOGY AND TECTONICS, RED HILLS 267 

Figure 6. Aerial view of Little Salt Lake fault scarp showing location of profiles C-C', D-D', E-E', and F-F'; north and south 

gullies; and location of figure 5. 

Around the area of exposures, the Little Salt Lake fault 

is not a single plane, but splits into several strands, only some 

of which are exposed (figs. 5, 8, 9). Between the strands, 

blocks of downdropped unit QTf are preserved that expose 

small west-dipping antithetic faults. These antithetic faults 

are more obvious in exposures than the east-dipping faults, 
268 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

Figure 7. Exposure of the Little Salt Lake fault and small antithetic faults cutting tilted Tertiary and Quaternary alluvium unit 

associated with the fault (unit Qtf). North wall of north gully at intersection of Little Salt Lake fault with cross fault. Hammer between fault strands indicates scale. 

because they offset well-bedded strata of contrasting lithology. The antithetic faults dip northwestward at 37° to 46° 

and strike about S. 30° W., converging toward the Little Salt 

Lake fault northward. 

The Tertiary and Quaternary alluvium (unit QTf) is 

tilted on both sides of the fault. Strikes of beds measured in 

a limited number of exposures in the downthrown block are 

rotated counterclockwise about 50° relative to those on the 

upthrown block (fig. 5). On the downthrown side of the Little Salt Lake fault exposure, the beds strike about N. 20° E. 

and dip from 26° to 37° E. Immediately west of the Little 

Salt Lake fault exposure, beds in the upthrown block strike 

about N. 70° E. and dip from 23° to 41 o S. About 500 m west 

of the Little Salt Lake fault, the beds strike west but dip 20° 

N. (figs. 5, 8, 9), indicating that proximity to the fault is not 

the sole cause of folding. 

In both gullies where the Little Salt Lake fault is 

exposed, the structure is further complicated by cross faulting 

and splaying (figs. 5, 8, 9). At the north gully (fig. 8) the fault 

scarp is displaced about 75 m to the left along an apparent 

cross fault exposed along the wall of the gully. The crossfault surface consists of several parallel calcite-coated surfaces with slickensides and mullions parallel to the direction 

of dip, indicating normal rather than strike-slip or obliqueslip fault movement. These surfaces strike generally S. 70° 

E. and dip 60° SW. (figs. 5, 8). 

South of the south gully (figs. 5, 9), the Little Salt Lake 

fault scarp is offset to the right through splaying rather than 

crossfaulting. The west splay is expressed as a continuation 

of the scarp south of the south gully that dies out at the gully. 

The east splay crosses the gully and merges with the scarp 

north of the south gully. The west splay is well exposed in 

the south wall of the southern gully but could not be traced 

north of the gully. The east splay is well exposed in the 

northern wall of the gully. 

MORPHOLOGY 

At its south end, the Little Salt Lake fault scarp disappears beneath the Holocene deposits of Little Salt Lake 

playa. No offset of the playa surface, cracking, or tonal contrast was detected along the surface projection of the fault 

(fig. 6). Northward, the down-to-the-east scarp increases in 

height, and splays, cross faults, and en-echelon steps offset 

the fault scarp slightly to the right (figs. 5, 6). Near the north 

end, the fault splits into two strands with about equal vertical 
QUATERNARY GEOLOGY AND TECTONICS, RED HILLS 269 

NORTH SIDE OF NORTH GULLY 

ATTITUDES OF FAULTS AND TERTIARY-QUATERNARY ALLUVIUM 

UTTLE SALT LAKE FAULT -........... 

~ 

\ 7~ 

UTILE SALT LAKE FAULT ..-

!-~,. 

---------------------------------------------------·-·; 

-------·-··--·-----------·-··--·-··-----------NO·----RTH··----·--------~~---GULLY·-- BED ---·-------------------_j----·-

-----·-··-··-··--·--·-·--··-.... -.. -.. -.. -... -. MAPVIFEW --·--·-··-··-··-··---------------- -··-··-----·-··--·-··-··--·-··-

PROFILE 

Upper Pleistocene alluvium 

METERS 

20 

10 

UT1J.E SALT LAKE FAULT 

0 

100METERS 

Figure 8. Map and cross section of fault and bedding attitudes in tilted Tertiary and Quaternary alluvium (unit Qtf) near Little Salt Lake 

fault exposed in walls of north gully. Map at same scale as profile. 

scarp height, and a total scarp height about the same as farther south (figs. 3, 6). Both strands appear to end where they 

butt into basalt exposures of the Miocene Mount Dutton Formation that extend about 700 m east of the main mountain 

front (fig. 6). The bedrock was evidently strong enough to 

resist the forces that broke and displaced the alluvium. North 

of the basalt exposures, another fault on the same trend as the 

east splay of the Little Salt Lake fault extends from the basalt 

exposures north to the mountain front, but the scarp is down 

to the west (figs. 3, 6), opposite to the Little Salt Lake fault. 

On the upthrown side of the Little Salt Lake fault, remnants of at least two pediment surfaces are preserved. The 

lower is relatively planar except where incised by gullies. 

Four to eight meters higher, rounded ridges of alluvium represent the remnants of at least one older surface. These pediment surfaces are covered by a relatively thin veneer of 

unfolded alluvium (units Qfo and Qf) and are cut on much 

older alluvial sediments (unit QTf) deformed by folding and 

faulting. 

East of the Little Salt Lake fault is a graben, filled with 

upper Pleistocene and Holocene alluvium (unit Qfa), that is 

bounded on its east side by a down-to-the-west fault antithetic to the Little Salt Lake fault (figs. 3, 5, 6, 10). The graben narrows southward from 1 ,000 to 100 m in width over a 

distance of 4 km as the antithetic fault converges with the 

Little Salt Lake fault. The pediment surface east of the graben appears, on the basis of slope, geomorphic form, and soil 

development, to be equivalent to the uplifted pediment surface west of the graben, which is underlain by unit Qf. 

A measure of the age of the fault scarps can be estimated from their slope morphology. Four profiles across the 

Little Salt Lake fault scarp (figs. 6, 1 0) were measured using 

a surveying level and rod. Profile C-C' (fig. 1 0) extends 

across the graben and has a maximum slope of 18°, and a 

scarp height of 17.7 m. The upslope-facing scarp of the antithetic fault is expressed as a flat bench along the slope, 

because the scarp is mostly buried. A scarp rise of 17.7 m 

over a horizontal distance of 75 m and a depositional slope 

of about 5.6° indicates that the late Pleistocene fault offset is 

about 10.3 m plus the unknown thickness of graben fill. 

Equivalent surfaces east and west of the graben are offset 

about 8.5 m down to the east as projected parallel to the depositional slope angle. This geometry implies that displacement on the antithetic fault is about 1.8 m plus the thickness 

of graben fill. Direct evidence on the antithetic fault is lacking because the antithetic fault scarp has been buried by the 

graben fill, and there are no exposures of the antithetic fault 

surface or the graben floor beneath the Holocene fill. 

In profile D-D' the scarp height is 9.5 m and the 

maximum slope is 19.2°. The horizontal distance from top to 
270 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

ATIITUDES OF FAULTS AND TERTIARY-QUATERNARY ALLUVIUM 

.,. um.ESALTIAKEFAULT L es· ~ { ~ 

·--·---~----·-·--··-. ~- ~ :r !I FAULtS\ 

- -~ .. -..... -.. ____________ .. __ .. _ .... _ ... _ .... -·--.... _ -_-_-_-__ ----SOUTH--------------·--------------G--U--U.:·-·~----------------·-··-B-·E--D·-··--·-··--·-------·--·--··--·-··-··-·\.,- ~· 

··-··--·-..... -.. -.... - ----------··-··--·-··-· ~-

UTilE SALT LAKE FAULT SPLAY as• ··-··-··-··-··-··-·· ·--·-··-··-··-··-··-··--·-··-··-··-··--·-··--,. 

NORTH SIDE OF SOUTH GULLY 

Upper Pleistocene alluvium 

20 

Quaternary-Tertiary alluvkln (QTI) 

10 

~ 

! 

i 

MAP VIEW 

PROFILE 

Holocene 

alluvium 

UTilE SALT LAKE FAULT 

0 

100METERS 

SOUTH SIDE OF SOUTH GULLY PROFILE 

METERS PROJECTIONS FROM NORTH WALL 

100 METERS 

Figure 9. Map and cross section of fault and bedding attitudes in tilted Tertiary and Quaternary a11uvium (unit Qtf) near Little Salt Lake 

fault exposed in wa11s of south gu11y. Map at same scale as profile. Dip of faults and strike and dip of bedding shown in map view; broken 

line, inferred or concealed. 

bottom of the scarp is about 30 m and the depositional slope 

5.6°. This indicates late Pleistocene fault offset of about 6.9 

m plus the thickness of graben fill. The low scarp height at 

D-D' compared to C-C' probably reflects an unusually thick 

graben fill of Holocene fan alluvium from a stream that has 

entrenched the scarp only 50 m north of D-D'. 

Profile E-E', just 53 m north of D-D', crosses the Little 

Salt Lake fault in a ravine cut into the upthrown block and 

extends down an active Holocene fan within the graben. Profile E-E' shows a slight inflection in slope where it crosses 

the fault scarp that could be interpreted as a very low fault 

scarp modified by active alluvial fan deposition. On aerial 

photographs, a tonal difference across this Holocene alluvial 

fan and across the next one to the north can also be interpreted as representing the trace of the fault. The offset of the 

Holocene depositional surface across the scarp estimated 

from the profile is about 1 m, and the slope about 5°. Scarp 

morphology has little meaning in such an active depositional 

environment, and the slope inflection may be caused by the 

transition from confined stream flow in the gully to unconfined flow on the fan. The slope inflection is linear exactly 

along the fault plane rather than radial about the fan apex. 

The evidence for Holocene fault movement across profile 

E-E' is thus not conclusive, but is permissive. It is the only 
ME'l'ERs 

1840 

1820 

1800 

1780 

0 

METERS 

1840 

1820 

1800 

1780 

1760 

E E' 

Lml.E SALT LAKE FAULT 

c 

0 

100 

UlTLESALT 

LAKE FAULT 

18"=Maxlmum acarp angle 

100 200 

D D' 

METERS 

1840 

Lml.E SALT LAKE FAULT 

1820 

1800 

1780 

0 100 

C' 

AntithetiC fa~ 

300 400 

Figure 10. Profiles across Little Salt Lake fault at the four locations shown in figure 6. Vertical exaggeration x2; 

altitudes above mean sea level. In D-D', dotted line is profile E-E' projected. In E-E', dotted line is profile D-D' 

projected. 
272 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

indication of Holocene movement on the Little Salt Lake 

fault. The closest confirmed Holocene movement is a I m 

scarp on a west-dipping fault near the center of the Parowan 

Valley, about 6.5 km to the east. 

Profile F-F' was measured north of the point where the 

Little Salt Lake fault splits into two strands. Each strand has 

a scarp height of about I 0 m, about the same as that measured in profile D-D', but the combined height of the two 

separate scarps is 19.9 m, about the same as that measured in 

profile C-C'. The slope of the scarp for each of the strands is 

less than at places where a single high scarp occurs, as at 

C-C' and D-D' to the south. Calculated offset of the western 

strand is about 2.6 m, plus the unknown thickness of postfaulting sediment at the foot of the scarp. No graben occurs 

at the foot of the western strand, and no sharp break in slope, 

so the sediment is probably thin. Calculated offset of the 

eastern strand is about 5.6 m plus the unknown thickness of 

post-faulting graben fill, which is thicker than usual because 

of a nearby ravine and alluvial fan. The combined offset of 

8.2 m plus unknown thickness of graben fill for both strands 

may be comparable to 10.3 mat C-C' and 6.9 mat D-D'. 

HISTORY 

The various episodes of movement on the Little Salt 

Lake fault have not been radiometrically dated; however, the 

morphology of the scarp and the degree of soil development 

on uplifted surfaces can be used to constrain an estimate. A 

reasonable inference is that movement on the Little Salt 

Lake fault began prior to the late Pleistocene (more than 130 

ka) and has continued into the Holocene (less than 10 ka). 

Two middle Pleistocene, one late Pleistocene, and possibly 

one Holocene geomorphic surfaces have been offset across 

the fault, implying a minimum of three or four distinct pulses 

of movement. Each of the pre-Holocene pulses totaled more 

than 10 m, and probably consisted of several distinct events. 

Total uplift of the highest surface is about 60 m. 

The youngest evidence of displacement is lineations in 

Holocene graben-filling alluvial fans that correspond to an 

inflection where the slope steepens slightly down slope. 

These lineations are limited in extent and are ambiguous. If 

they do represent fault offset, the amount was small, probably 

less than 1 m. Late Pleistocene offset exceeds 10 m. Two 

uplifted pediment surfaces with middle Pleistocene soils are 

preserved about 8 and 50 m above the late Pleistocene surface, 

implying one pulse of multiple movement events totaling 

about 42 m followed by another pulse totaling about 8 m. 

The present scarp, which indicates a minimum fault offset of 10m, was formed mostly during the late Pleistocene 

and was probably preceded by other scarps that were obliterated by erosion. The thickness of the carbonate-enriched soil 

horizon on unit Qf implies that start of formation of the 

present scarp probably was more than 15 ka. 

Figure 11 plots log of scarp height against maximum 

slope angle for four profiles measured along the Little Salt 

Lake fault. Those points can be compared with lines fit to 

similar measurements on Lake Bonneville shoreline scarps 

cut in gravelly alluvium and known to date from about 15 ka 

and on Drum Mountain fault scarps considered to date from 

about I 0 ka (Bucknam and Anderson, 1979). The Little Salt 

Lake fault scarps are also developed in gravelly alluvium 

and are located only about 40 km southeast from the southern Bonneville shoreline, so the geographic proximity and 

similarities in climate and deposits between the shoreline 

profiles and the Little Salt Lake fault profiles enhance the 

validity of the comparison. However, the shoreline scarps 

have degraded from a particular moment in time and never 

been rejuvenated, whereas the higher fault scarps are likely 

to have been produced by multiple events separated by substantial time intervals. The fault scarp height reflects the 

summation of all events, whereas the maximum scarp slope 

angle is more likely to be controlled by the most recent 

event. For the high, composite, fault scarps of the Little Salt 

Lake fault, the point on figure 11 represents neither the 

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./ * 

X 

X 

5 

SCARP HEIGHT, IN METERS 

EXPLANATION 

X Profile F-F' 

+ Profile£-£' * Profile D-D' 

0 Profile C-C' 

Drwn Mtn ( 10 ka?) 

Bonneville ( 15 ka) 

0 

Figure 11. Plot of maximum slope angle in degrees against log of 

scarp height for the four profiles measured along Little Salt Lake 

fault and reference lines for similar measurements on the 15 ka Lake 

Bonneville shoreline scarps and the 10 ka(?) Drum Mountain fault 

scarps cut in alluvium (modified from Bucknam and Anderson, 

1979). 
QUATERNARY GEOLOGY AND TECTONICS, RED HILLS 273 

inception of faulting nor the most recent event, but should 

fall between. Profile E-E' lies near the 15 ka line, and all the 

other profiles plot older than the 15 ka line. 

Profile E-E' (fig. 1 0) crosses a very low, indistinct break 

in slope that can be interpreted as a degraded scarp from faulting that offsets young fan alluvium where it extends across 

the fault trace into an embayment in the high fault scarp. Soils 

on both sides of the scarp show virtually no pedogenic calcium carbonate~ hence the alluvium is thought to be of 

Holocene age. That location is subject to direct deposition 

from a ephemeral stream draining about 1 km2, so the maximum fault scarp slope and height has probably been reduced 

faster than in the case of the Bonneville shorelines and the 

other Little Salt Lake scarp profiles. No valid comparison can 

be made between profile E-E' and any other data in figure 11. 

Study of points plotted for the three profiles (fig. 10) 

that include the full scarp height reveals some differences. 

The scarp at D-D' plots "younger" than scarps on profiles 

C-C' and F-F'; it may represent a slightly younger fault offset than those scarps. Profile D-D' is only 53 m south of profile E-E', so Holocene faulting at profile E-E' is likely to 

affect the older scarp at profile D-D' and tend to rejuvenate 

it. 

The degree of development of soils in unit Qf on the 

alluvial surface offset by the Little Salt Lake fault scarp suggests a late Pleistocene age for the fault movement that 

ended alluvial deposition on the surface. On the upthrown 

side of the fault and about 8 m higher than the late Pleistocene (unit Qf) alluvial surface are remnants of the older 

depositional surface of unit Qfo that have a more developed 

carbonate-enriched soil, probably of middle Pleistocene age. 

The elevation of this surface suggests at least 8 m of midPleistocene uplift along the Little Salt Lake fault prior to the 

formation of the late Pleistocene scarp. 

The absence of numerical dating of the offset pediment 

surfaces introduces great uncertainty in estimating the average rate of fault slip. The following calculations use age estimates of 20 ka for the late Pleistocene surface ( + > 10 m ), 150 

ka for the lower middle Pleistocene surface ( + 18 m), and 700 

ka for the higher middle Pleistocene surface ( +60 m), based 

on their soil development, degree of erosion, and relative 

elevation. The uncertainty of these age estimates is at least 

50 percent. On that basis, the average rate of vertical motion 

since the middle Pleistocene has been about 0.12 mrnlyr±50 

percent, whereas the average rate since the late Pleistocene 

has been at least 0.5 mrn/yr±50 percent. 

ENOCH GRABEN 

Quaternary basalt with a K-Ar age of 1.28±0.4 Iv1a 

(Anderson and Mehnert, 1979) is faulted near the southwest 

comer of the Parowan Gap quadrangle (figs. 3, 4 ). The vents 

and central part of the lava flows have been dropped down 

along two parallel fault zones that define the Enoch graben. 

The east side of the graben has dropped from 15 to 50 m 

along a narrow zone or single fault. The Hieroglyph horst 

(Threet, 1963 ), which forms the west wall of the graben, 

apparently has been uplifted about 70 m relative to the graben floor across a fault zone about 200 m wide. Faults along 

the west side of the horst have dropped flows down about 

230 m across a zone about 400 m wide into the low valley 

north of Rush Lake (fig. 1), where they are buried beneath 

alluvium. The basalt was intersected in a drill hole for a 

water well beneath 30 m of alluvium, 2 km west of its last 

exposure at the surface (Gerald Staker, Utah State Engineers 

Office, Cedar City, Utah, written commun., 1989). 

The average rate of fault movement over about the last 

1.28±0.4 m.y. along cross section B-B' is about 0.012 mrn/ 

yr±35 percent for the fault bounding the east side of the 

Enoch graben, about 0.054 mrn/yr±35 percent for the fault 

between the Enoch graben and the Hieroglyph horst, and 

about 0.178 mrnlyr±35 percent for the fault on the west side 

of the Hieroglyph horst. The analytical uncertainty in the 

date and these rates is at least ±31 percent. The amount of 

offset of the basalt and thus the rate of offset also vary along 

the strike of the faults. Throw increases southward to a maximum of about 50 m on the east side of the graben, indicating 

a slip rate averaging as much as 0.039 mrn/yr±35 percent. At 

their south end, the faults bounding the Enoch graben cut late 

Pleistocene, and possibly Holocene alluvium (Anderson and 

Christenson, 1989, p. 14). 

The Hieroglyph horst is an anomalous structural block. 

At the south end the basalt rests on Tertiary and Quaternary 

alluvium, and at the north it rests on steeply dipping, highly 

sheared Lower Jurassic Navajo Sandstone cut by narrow siliceous dikes. This horst extends northward to Parowan Gap 

Canyon, where the rapid uplift across established drainage 

has produced "the Narrows," a short section of canyon 

through the Navajo Sandstone that is much narrower than the 

rest of Parowan Gap Canyon. 

The basalt in the Enoch graben overlies primarily 

Paleocene and Eocene Claron Formation, but also overlies 

Upper(?) Cretaceous Iron Springs Formation, Tertiary and 

Quaternary alluvium (unit QTh), and exotic blocks of 

Tertiary volcanic rocks. Immediately east of the Enoch 

graben, a hole drilled by Mountain Fuel Supply (fig. 4) in sec. 

9, T. 34 S., R. 10 W. penetrated quartz monzonite porphyry 

at a depth of 1,341 m (Tompkins and others, 1963) (fig. 4), 

establishing that the southern part of the Red Hills is underlain 

by an intrusive body. The quartz monzonite porphyry is 

overlain by 171 m of Lower Jurassic Navajo Sandstone, and 

893 m of Upper(?) Cretaceous Iron Springs Formation. 

CONCLUSIONS 

The Red Hills, and the alluvium-filled basins on either 

side, formed as a result of down-to-west normal block 
274 GEOLOGIC STUDIES, BARCO STUDY UNIT, 1992 

faulting, eastward tilting of the faulted blocks, and secondary 

down-to-east antithetic faulting. Thick alluvial fills are 

preserved in the deep, down-faulted basins east (> 1,000 m) 

and west (>900 m) of the range. Tectonism within the basins 

has deformed and locally uplifted the older alluvial deposits 

so that they crop out in many places. Several generations of 

widespread but thin deposits of younger alluvium overlie 

erosion surfaces cut on the older alluvium and areas of soft 

bedrock. The youngest deposits are lacustrine silts and clays 

of Little Salt Lake playa that cover 21 km2 on the west side 

of Parowan Valley. 

Most faults within alluvium on both sides of the Red 

Hills are down to the west and synthetic to the Paragonah 

fault (figs. 1, 2), a major west-dipping fault that forms the 

eastern and lower bound of the detached terrain. Two major 

east-dipping faults antithetic to the Paragonah fault form the 

boundary between the Red Hills horst and the Parowan Valley graben. The southern part is bounded by the Red Hills 

fault (fig. 3), and the northern by the Little Salt Lake fault. 

The Little Salt Lake fault is best expressed where it extends 

3.5 km through alluvium from Little Salt Lake north to the 

mountain front (figs. 5, 6). A segment without young scarps 

continues northward at least 8 km parallel to the range front 

of the northern Red Hills (fig. 3). 

The Little Salt Lake fault strikes generally N. 45° E. and 

dips about 75° SE. It is expressed as an 18-m-high scarp with 

maximum slope of 19° offsetting upper Pleistocene pediment alluvium. On the upthrown block, remnants of middle 

Pleistocene pediment surfaces record uplift prior to formation of the late Pleistocene scarp. Parallel strands of the fault 

can be seen beneath the pediment alluvium in isolated exposures of folded upper Tertiary(?) to lower Pleistocene(?) 

alluvial sediments that contain silicic volcanic ash. Minor 

antithetic faults dipping 45° NW. are exposed in the folded 

sediments a few meters east of the main fault. Larger faults 

antithetic to the Little Salt Lake fault bound the. southeast 

side of a scarp-front graben, filled with Holocene alluvium, 

that narrows southward from 1,000 to 100m width in a distance of 4 km. 

Movement on the Little Salt Lake fault probably began 

prior to middle Pleistocene time and has continued through 

the late Pleistocene and perhaps into the Holocene. The 

fault moved during at least three major pulses of activity. A 

small event may have offset a short segment of the fault 

about 1 m during the Holocene, but evidence is not conclusive. Total offset of mid-Pleistocene(?) geomorphic surfaces across the fault exceeds 60 m. The present scarp, 

which was formed by vertical fault movement of at least 

10.3 m, was initiated during the late Pleistocene and was 

preceded by other scarps that have been obliterated by erosion. The average rate of vertical motion since the middle 

Pleistocene has been about 0.12 mm/yr±50 percent, 

whereas the average rate since the late Pleistocene has been 

at least 0.5 mm/yr±50 percent. 

The Hieroglyph horst in the southwestern Red Hills has 

moved up 70 m relative to the Enoch graben on its east side 

and about 230m relative to the northern Cedar City Valley 

on its west side since eruption of basalt flows 1.28±0.4 Ma, 

indicating fault movement averaging 0.054 mm/yr±35 percent on the east side and 0.178 mm/yr±35 percent on the 

west. This recent rapid uplift is responsible for the narrowing 

of the Parowan Gap Canyon where it crosses the horst exposing Navajo Sandstone. The east side of the Enoch graben has 

moved down 15-50 m relative to the part of the Red Hills to 

the east, indicating average fault movement between 0.012 

and 0.039 mm/yr±35 percent. 

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QUATERNARY GEOLOGY AND TECTONICS, RED HILLS 275 

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Manuscript approved for publication February 14, 1994 

Published in the Central Region, Denver, Colorado 

Photocomposition by Denny Welp 

Graphics by Ramon E. Sabala; 

final graphics preparation by Denny Welp 

Edited by Lorna Carter 

*U.S. OOVERNMENT PRINTING OFFICE: 1995-673-046/86118 

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