Geologic Studies of the 

Lower Cook Inlet COST No.1 Wei 

Alaska Outer Contine tal Shelf 

GEOLOGIC STUDIES OF THE 

LOWER COOK INLET COST NO.1 WELL, 

ALASKA OUTER CONTINENTAL SHELF 
The ODECO Ocean Ranger, a semisubmersible drilling vessel, on location in lower Cook Inlet drilling the COST No.1 

well. The view is southwest with Augustine Volcano, an active andesiticvolcano, on the horizon.ln the summerof1977 

Atlantic Richfield , the operator, with 18 other participants from the petroleum industry drilled the well in Block No. 489 

to a total depth of 3,775 .6 m. The well penetrated rocks that ranged in age from Late Jurassic to early Cenozoic. This well, 

drilled just before the opening of OCS Lease Sale No. Cl, confirmed among other things that Lower and Upper 

Cretaceous rocks are present under lower Cook Inlet and, as an additional bonus , penetrated several Upper Cretaceous sandstone bodies with petroleum reservoir potential. 
Geologic Studies of the 

Lower Cook Inlet COST No.1 Well, 

Alaska Outer Continental Shelf 

Leslie B. Magoon, Editor 

U.S. GEOLOGICAL SURVEY BULLETIN 1596 
DEPARTMENT OF THE INTERIOR 

DONALD PAUL HODEL, Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas l. Peck, Director 

UNITED STATES GOVERNMENT PRINTING OFFICE 1986 

For sale by the 

Books and Open-File Reports Section 

U.S. Geological Survey 

Federal Center, Box 25425 

Denver, CO 80225 

Library of Congress Cataloging-in-Publication Data 

Geologic studies of the Lower Cook Inlet COST No. 1 

well, Alaska Outer Continental Shelf. 

(U.S. Geological Survey Bulletin 1596) 

Includes bibliographies. 

Supt. of Docs. No.: I 19.3: 1596 

1. Geology-Alaska-Cook Inlet. 2. BoringsAlaska-Cook Inlet. I. Magoon, Leslie B., 1941-

11. Series: Geological Survey Bulletin; 1596. 

QE75.B9 no. 1596 

[QE84.C69] 

557.3 s 

[557.98'3] 

86-600086 
CONTENTS 

Abstract 1 

Introduction, by Leslie B. Magoon 5 

Environmental geology, by Arnold H. Bouma and Monty A. Hampton 11 

Stratigraphic units of the COST No. 1 well, by Leslie B. Magoon 17 

Correlation of the COST No. 1 well data and the marine seismic data, by Michael A. Fisher 23 

Paleontology and biostratigraphy of the COST No. 1 well, by Ronald F. Turner 29 

Petroleum geochemistry, by George E. Claypool 33 

Present-day geothermal gradient, by Leslie B. Magoon 41 

Sandstone petrography, by Hugh McLean 47 

Sandstone diagenesis, by John G. Balm and Thane H. McCulloh 51 

Plutonism and provenance-implications for sandstone compositions, by Travis Hudson 55 

Petrography, provenance, and tectonic significance of Middle and Upper Jurassic 

sandstone from Tuxedni Bay, by Robert M. Egbert 61 

Framework geology and sandstone composition, by Leslie B. Magoon and Robert M. Egbert 65 

Conclusions, by Leslie B. Magoon 91 

References 93 

FIGURES 

Frontispiece. The ODECO Ocean Ranger, drilling the COST No. 1 well in lower Cook 

Inlet. 

1-3. Maps showing: 

1. Area of report and important features 6 

2. Bathymetry of lower Cook Inlet 12 

3. Geomorphology of lower Cook Inlet 14 

4. Stratigraphic chart showing wireline well-log curves, lithology, and rock units in 

COST No. 1 well 18 

5. Diagram showing age and depositional environment of sediments from COST 

No. 1 well 19 

6. Ternary diagrams and graphs showing grain size, porosity, and permeability of 

stratigraphic units penetrated in COST No. 1 well 20 

7. Graphs showing organic geochemistry of stratigraphic units penetrated in COST 

No. 1 well 21 

8. Map showing location of COST No. 1 well and seismic lines 24 

9. Graphs showing seismic velocity curves 24 

10. Graphs showing relation between well-log and seismic data 25 

11. Seismic line 7 52 26 

12. Cross section B-B' 27 

13-19. Graphs showing: 

13. Gas chromatographic analyses of saturated hydrocarbons 34 

14. Summary and comparison of analyses of organic carbon and 

hydrocarbon contents of rocks in COST No. 1 well 36 

15. Summary and comparison of H/C ratio, o 13C data, and pyrolytic 

hydrocarbon yield 37 

16. Summary and comparison of indicators of thermal history 38 

Contents V 
17. Temperature data 42 

18. Homer temperature plot 43 

19. Temperature increase at total depth 44 

20. Geothermal gradient map of Cook Inlet 45 

21. Graph showing "fossil" laumontite field 46 

22. Graph showing cross reference of the TAl scale to vitrinite reflectance 46 

23. Q-F-L ternary diagram 49 

24--26. Photographs showing: 

24. Authigenic clay 51 

25. Laumontite filling in pore space 52 

26. Oil in laumontite-lined fractures 52 

27. Map showing principal plutonic rocks in southern Alaska 56 

28. Ternary diagrams showing percentage of Q-K-P for groups of plutonic 

rocks 57 

29. Ternary diagrams showing influence of provenance on modal framework 

composition of sandstone 59 

30. Graph showing evolutionary trends of framework grain ratios 59 

31. Ternary Q-F-L diagrams and compositional data for sandstone from Tuxedni 

Bay 62 

32. Map showing geographic names and geomorphic features in Cook Inlet and 

adjacent areas 66 

33. Q-F-L diagrams for sandstones in the Cook Inlet-Shelikof Strait area 68 

34. Generalized geologic map of Cook Inlet, Shelikof Strait. and adjacent areas 72 

35. Geologic cross sections 74 

36. Major terrane boundaries in the Cook Inlet region 75 

37. Time-stratigraphic chart through COST No. 1 well 78 

38. Tertiary correlation chart for the Cook Inlet forearc basin 84 

TABLES 

1. Selected information from "Well Completion and Recompletion Report and 

Log" 7 

2. Type and depth interval of well logs run in COST No. 1 well 7 

3. Conventional cores acquired from COST No. 1 well 8 

4. Petroleum industry participants and contractors in COST No. 1 well 8 

5. Wells drilled in lower Cook Inlet outer continental shelf to 1980 9 

6. Petrophysical properties of sandstone, COST No. 1 well 21 

7. Organic carbon content of rock units penetrated in COST No. 1 well 22 

8. Hydrocarbon indications reported in the COST No. 1 well 22 

9. Late Cretaceous (Maestrichtian) foraminifers found between 1 ,402-1 ,539 m in 

the COST No. 1 well 30 

10. Early Cretaceous (Hauterivian into Barremian) foraminifers found between 

1,585 to 2,106 min the COST No. 1 well 31 

11. Conventional source rock analysis of extractable organic matter and kerogen in 

core samples from COST No. 1 well 33 

12. Thermal analysis of organic matter in the core samples from COST No. 1 

well 35 

13. Log runs for COST No. 1 well in 1977 42 

14. Temperature data for COST No. 1 well 43 

15. Sandstone petrography of COST No. 1 well 48 

16. Petrography of sandstone samples from northwest of Border Ranges fault 70 

17. Petrography of sandstone samples from southeast of Border Ranges fault 71 

18. Magmatic arcs, stratigraphic terranes, and accreted terranes of Cook InletShelikof Strait area 88 

VI Contents 
Geologic Studies of the Lower Cook Inlet COST 

No. 1 Well, Alaska Outer Continental Shelf 

Leslie B. Magoon, Editor 

Abstract 

1. The COST No. 1 well was 

drilled in 65 m (214 ft) of water and 

penetrated 3,776 m (12,387 ft) of sedi· 

mentary rocks in the Alaska Outer 

Continental Shelf (OCS) block 489 

about midway between the lniskin 

Peninsula on the northwest and the 

Kenai Peninsula on the southeast. The 

ages of the sedimentary units were 

determined by wireline logs and the 

analysis of foraminifers, radiolarians, 

calcareous nannoplankton, and 

marine and terrestrial palynomorphs. 

The section from 413 m (1 ,356ft) to 797 

m (2,615 ft) is Cenozoic. The underlying section, from 797 m (2,615 ft) to 

2,112 m (6,930 ft), is Cretaceous; at 

1,541 m (5,055 ft) the section is divided 

into Upper and Lower Cretaceous 

rocks. The bottom section, from 2,112 

m (6,930 ft), to 3,775.6 m (12,387 ft), is 

Upper Jurassic (Oxfordian through 

Tithonian). The base of this section 

probably was not penetrated so the 

1,663 m (5,455 ft) thickness is probably 

a minimum. The top of this unit is controversial. Drill cutting samples indicate that the top is at 2,109 m (6,918 ft), 

2,112 m (6,928 ft), or2,135 m (7003 ft). 

Paleontologic evidence indicates that 

the top is at2,109 m (6,918 ft) or2,417 m 

(7,930 ft). I place the top at 2,112 m 

(6,930 ft), just below the abundant 

Inoceramus fragments and just above 

the first occurrence of laumontite. 

The West Foreland Formation, of 

early Cenozoic age, extends down to 

797 m (2,615 ft). The sediments are 

sandstone, siltstone, coal, and some 

conglomerate. Modal analyses of the 

sandstone indicate that it is feldspatholithic with an average composition of Q 23F30L4 r The Kaguyak 

Formation (Upper Cretaceous), from 

797 m (2,615 ft) to 1,541 m (5,055 ft) 

contains marine and nonmarine sediments. The feldspathic sandstone has 

an average composition of Q23F61 L16• 

The Herendeen Limestone (Lower 

Cretaceous) contains sandstone, 

siltstone, shale and abundant 

Inoceramus fragments from 1,541 m 

(5,055 ft) to 2,112 m (6,930 ft). The 

lithofeldspathic sandstone has an 

average composition of Q36F 44L20 . The 

bottom section, the Naknek Formation (Upper jurassic), down to 3,776 m 

(12,387 ft), is dominantly siltstone, 

shale, and sandstone. The average 

composition of the lithofeldspathic 

sandstone is Q24F55L21 • 

The West Foreland Formation, 

the shallowest unit sampled, has a 

minimum thickness of 384m (1 ,259ft). 

Paleontologic data indicate Mesozoic 

rocks were reworked in a nonmarine 

environment. The organic carbon 

content of the unit is low and the section is thermally immature. No hydrocarbon shows were indicated. The 

Kaguyak Formation is 744 m (2,440 ft) 

thick and includes three sandstone 

bodies that are potential hydrocarbon 

reservoirs. Paleontologic and 

lithologic evidence indicate two prograding sedimentary sequences that 

prograde seaward from an upper 

bathyal environment to nonmarine. 

Although the organic carbon content 

is as much as 1.07 weight percent, the 

kerogen is low in oil-prone material 

and lacks an adequate thermal history 

to generate oil. Two different hydrocarbon shows were reported in this 

unit. The Herendeen Limestone is 572 

m (1 ,875 ft) thick. Foraminiferal data 

indicate that it formed in middle 

bathyal to inner neritic marine 

environments. The organic carbon 

content is low, and the thermal history 

is insufficient to generate hydrocarbons. Only a few hydrocarbon indications were reported. Reservoir properties of the sandstones are poor. The 

deepest rock unit penetrated, Naknek 

Formation, is at least 1,663 m (5,457 ft) 

thick. Even though the rock is thoroughly cemented with laumontite, it 

contained most of the hydrocarbon 

shows. Since the strata, mostly sandstone and siltstone, is low in organic 

content, and the organic matter is not 

the type to generate oil, the oil must 

have migrated into the area of the well 

bore from another part of the basin 

where mature oil-prone source rocks 

were present. 

2. Information about the 

environmental geology of the lower 

Cook Inlet basin is limited; and no 

quantitative information has been collected from the upper Cook Inlet 

basin. 

A number of geologic features 

and processes have been identified 

that may pose problems to engineering installations in the lower Cook 

Inlet and along the adjacent coastline. 

Strong earthquakes have caused 

major damage to the area by ground 

failure, surface warping, and 

tsunamis. Four active volcanoes on the 

west side of lower Cook Inlet are 

andesitic, so they can have violent 

eruptions. Very little is known about 

the swift bottom currents and sandwaves that have broken pipelines in 

the upper Cook Inlet. 

3. The seismic horizons were 

determined by comparing the measured seismic reflections with the calculated reflections from the COST No. 

1-well. Horizon A, at 900 m (2,950 ft) 

depth in the well, is near the top of the 

Upper Cretaceous rocks. Horizon B is 

at the depth of 2,005 m (6,575 ft) in the 

well. That depth represents the Cretaceous-jurassic contact where the 

rocks no longer have intergranular 

porosity but are cemented with calcite 

and laumontite. The third horizon, C, 

is 1,400 m (4,600 ft) below the bottom 

ofthe well and about 3,100 m (10,200 ft) 

Abstract 
below the top of the Upper jurassic 

rocks. It represents the contact 

between the top of the Middle Jurassic 

rocks and the base of the Upper 

Jurassic. 

4. The paleontology and biostratigraphy are based on detailed 

analysis of foraminifers, marine and 

terrestial palynomorphs, calcareous 

nannoplankton, and radiolarians. 

Eocene fossils from 413 to 552 m (1 ,356 

to 1,810 ft) represent a predominantly 

nonmarine depositional environment 

in a warm temperate climate. Paleocene, from 552 to 799 m (1 ,810 to 

2,620 ft) is characterized by a palynomorph assemblage containing Paraalnipollenites confusus. Well 

developed late Cretaceous 

(Maestrichtian) microfossil 

assemblages, from 799 to 1,539 m 

(2,620 to 5,050 ft), indicate nonmarine 

and marine intervals. Sparse but diagnostic assemblages of Early Cretaceous (Hauterivian to Valanginian?) 

microfossils, from 1,539 m (5,050), indicate a marine depositional environment whose paleobathymetry fluctuated twice from upper slope to 

middle-shelf water depths. late 

Jurassic (Tithonian to Oxfordian) 

microfossils, from 2,417 m (7,930), indicate an intertidal to inner neritic 

marine depositional environment. 

5. The COST No. 1 well does 

not contain prospective petroleum 

source rocks. The organic carbon content, which ranges from 0.08 to 0.65 

weight percent, is more abundant in 

the upper part of the well than in the 

lower. The thermal maturity, however, 

increases with increasing depth. The 

transition from immature to mature 

hydrocarbon generation seems to be 

from 2,100-2,700 m (6,900-8,900 ft) in 

the well. 

The Mesozoic sediments in the 

COST No. 1 well contain much less 

organic matter than the Mesozoic sediments in the upper Cook Inlet and the 

North Slope in Alaska. Although the 

thermochemical transformation of the 

organic matter in the Mesozoic rocks 

is marginally mature to mature, the 

organic matter is dominantly of the 

type that has a poor oil-generating 

capacity. However, because the COST 

No.1 well did not penetrate the source 

rocks for petroleum in the upper Cook 

Inlet-the shales of the Middle 

Jurassic-the prospect for petroleum 

in the lower Cook Inlet should not be 

judged only on the data obtained from 

the COST No. 1 well. The oil-generating system in the Cook Inlet basin is 

probably made up of migrating paths 

from the Middle Jurassic rocks. 

6. The present-day geothermal 

gradient for the COST No. 1 well is 

2.28°C/100 m. Even though this gradient is close to that of the Swanson 

River oil field gradient of2.37°C/100 m, 

the gradient is too low when compared to the geothermal trends in the 

upper Cook Inlet. The evidence from 

the stratigraphy of the rocks, the first 

occurrence of laumontite, and the TAl 

values indicate that the rocks in the 

well were subjected to high temperature gradients before uplift and 

erosion. The low vitrinite reflectance 

values do not substantiate this 

inference. 

7. The compositions of the 

sandstones in the COST No. 1 well 

indicated that they were derived from 

an igneous source terrane which had 

outcrops of both volcanic and plutonic 

rock types. The shallowest samples are 

volcanic lithic sandstone. The West 

Foreland Formation of early Cenozoic 

age contains feldspatholithic sandstone with an average composition of 

Q 23F30l 47 . The Kaguyak Formation 

(Upper Cretaceous) contains feldspathic sandstone with an average 

composition of Q 23F 61 l 16• Both the 

Herendeen limestone (lower Cretaceous) and Naknek Formation 

(Upper Jurassic) contain 

lithofeldspathic sandstone with average modal compositions of Q36F 44 l 20 

and Q24F55 l 211 respectively. 

8. The diagenetic changes most 

affected the volcanic rock fragments 

and plagioclase feldspar that are abundant in the sandstone samples. Sandstone diagenesis increases with 

increasing depth in the COST No. 1 

well. Pseudomatrix is more common 

in the deeper sandstones than in the 

higher sandstones. Authigenic clay, 

montmorillonite and chlorite, 

increases with depth, and some of it 

formed before laumontite crystallization. laumontite is present in most 

sandstone samples from depths 

greater than 2,134 m (7,000 ft) where 

2 Geologic Studies of the Lower Cook Inlet COST No. 1 Well 

most plagioclase is partly to completely albitized and intergranular 

porosity is destroyed. 

The major cause for the reduction of porosity and permeability in 

the COST No.1 well is due to the formation of authigenic clay matrix and 

pseudomatrix. The minor cause is that 

of compaction and diagenesis. 

The sandstones of the West Foreland Formation (lower Cenozoic) have 

a porosity of nearly 20 percent. The 

porosity and permeability of the sandstone of the Herendeen limestone 

(lower Cretaceous) has been reduced 

by compaction and pore fillings of clay 

and calcite. The original porosity of 

the sandstone of the Naknek Formation (Upper Jurassic) was probably 

high but is now substantially reduced 

by compaction and by precipitation of 

authigenic minerals, predominantly 

laumontite. 

9. The Middle Jurassic sandstones of the Tuxedni Group and the 

lower part of the Chinitna Formation 

were derived largely from a volcanic 

terrane. Important differences in composition occur between the lonnie 

Siltstone Member of the Chinitna Formation and its overlying Paveloff Siltstone Member; the lower member is 

volcanic-lithic rich, is virtually devoid 

of hornblende, and is cemented dominantly with chlorite; and the upper 

member is less lithic-rich (more feldspathic fragments from a dissected 

pluton), contains an average of 7 percent hornblende, and is cemented 

with laumontite as well as chlorite. 

The cementation and compaction of the sandstones in the Tuxedni 

Bay area in the western Cook Inlet 

should be considered when evaluating the hydrocarbon-reservoir potential of the Middle and Upper Jurassic 

sandstone in Cook Inlet. 

The new data about the composition of the sandstones from the Tuxedni Group and Chinitna and Naknek 

Formations provide evidence that the 

Alaska-Aleutian Range batholith was 

unroofed during the deposition of the 

Paveloff Siltstone Member of the 

Chinitna Formation. 

10. Eight separate episodes of 

plutonism in southern Alaska may 

have affected or have affected the 

provenance history of the forearc 
basin that includes Cook Inlet. The 

petrologic variations of the plutons 

representing these eight episodes 

reveal a general trend to higher felsic 

composition from the older to the 

younger plutonic groups. Generally, 

Jurassic plutonism produced mafic to 

intermediate compositions, Cretaceous plutonism produced mostly 

intermediate compositions, and Tertiary plutonism produced felsic compositions. 

Both short-term and long-term 

plate-convergent episodes have 

affected the provenance of the Cook 

Inlet forearc basin. Four relatively 

short-term episodes that affected the 

Cook Inlet basin occurred during the 

Late Triassic(?) and Early jurassic, Middle and Late Jurassic, Late Cretaceous 

and early Tertiary, and middle Tertiary. 

Widespread late Cenozoic volcanic 

rocks and the presence of many calcalkaline volcanoes in the region are 

evidence that a fifth episode is taking 

place now. 

Over the long term, the provenance evolved from simple magmatic 

arc sequences in the early Mesozoic to 

a composite of regional sedimentary, 

metamorphic, and magmatic arc 

sequences in the late Mesozoic and 

Cenozoic. In effect, each succeeding 

magmatic arc increased the proportion of crystalline rocks in the provenance. The simple magmatic arc 

provenance of the Late Triassic, 

jurassic, and Early Cretaceous produced sandstones changing from 

lithic to feldspathic compositions. The 

sandstones in the Late Cretaceous 

should reflect a composite provenance that includes magmatic arc and 

recycled orogen types, and, after a 

new magmatic arc cycle began in the 

Late Cretaceous, local development of 

volcanolithic sandstones. 

Late Cenozoic sandstones 

should reflect a complex composite 

provenance that includes magmatic 

arc, recycled orogen, and continental 

block provenance types. The diverse 

sandstones should range from lithic to 

quartzose types. 

The development of three major 

provenance types in southern Alaska 

should produce a complete range of 

sandstone framework modes. 

However, the younger sandstones of 

the Cook Inlet basin tend to have 

quartzose framework modes. This 

may be explained by the increasing 

proportion of metamorphic and plutonic rocks to volcanic rocks during 

provenance evolution and the more 

felsic compositions of the younger dissected plutons. 

11. Only the sandstones from 

the Lower jurassic through the Lower 

Cretaceous changed compositions in 

the way Dickinson and Suczek (1979) 

predicted for magmatic arc provenances. In the forearc basin, the 

Upper Cretaceous sandstones change 

composition from a feldspathic to a 

feldspatholithic sandstone with very 

little change in quartz content; this 

change is the reverse of the kind of 

change expected in a magmatic arc 

provenance and more like the dissection of the previous magmatic arc 

provenance. The composition of the 

Cenozoic sandstones in the forearc 

basin and in the adjacent accreted terranes are again derived from a magmatic arc; the sandstones were all 

feldspatholithic but they increased in 

quartz content very rapidly as they 

decreased in age, undoubtedly 

reflecting the more felsic provenance 

and a drainage system that reached 

into interior Alaska. 

Only in the Cenozoic sandstones 

was the quartz content high enough to 

provide adequate petroleum reservoirs consistently. The Upper jurassic 

sandstone would have been a good 

petroleum reservoir before the laumontite filled all the pores in the 

rocks. 

12. Considering all the data, 

only three sandstone beds of Upper 

Cretaceous age are potential hydrocarbon reservoirs in the COST No. 1 

well. These rocks, however, are not 

the source rocks for the petroleum in 

the upper Cook Inlet. COST No.1 well 

did not penetrate the shales of the 

middle jurassic, so the question of 

whether potential prospective 

petroleum rocks are present in the 

lower Cook Inlet basin still remains 

unanswered. 

Abstract 3 

Introduction 

By Leslie B. Magoon 

The lower Cook Inlet COST (Continental Offshore 

Stratigraphic Test) No. 1 well is 65 km southwest of Homer, 

Alaska in the Alaska Outer Continental Shelf block 489 (fig. 

1). The well was drilled in 65.2 m (214ft) of water to a total 

depth of 3,775.6 m (12,387 ft) from the rig Kelly bushing 

(KB). The well was spudded June 10, 1977, and it reached 

total depth on September 9, 1977. Other specific information 

is given in tables 1 through 4. 

COST No. 1 well was drilled in Cook Inlet Outer 

Continental Shelf waters to obtain stratigraphic information 

before the oil and gas Lease Sale No. CI was held October 27, 

1977. Atlantic Richfield Company, acting as operator, solicited support from industry to form a COST group to drill a 

stratigraphic test well; 18 petroleum companies responded. 

The proposed location of the well was about midway between 

the Iniskin Peninsula on the northwest and the Kenai Peninsula on the southeast. The granting of Federal leases within 

92.6 km (50 nautical miles) of the drill site on December 1, 

1979 has made possible the publication of detailed information about the geologic findings from the COST No. 1 well, in 

accordance with Title 30, Code of Federal Regulations, 

Chapter II, paragraph 251.15, which provides for public 

disclosure by the U.S. Geological Survey (USGS) of all 

geologic information on the well 60 days after such leasing. 

Open-File Report 78-145, entitled "Geological and operational summary, Atlantic Richfield lower Cook Inlet, Alaska, 

COST well No. 1" (Wills and others, 1978), was published at 

the time the detailed well information was released by the 

USGS in 1978. The present report discusses the detailed 

geologic information derived from the well and provides 

information on the framework geology and hydrocarbon 

potential of the Cook Inlet basin. 

This volume is divided into separate reports on geologic, geophysical, and geochemical topics. Some operational details from Wills and others (1978) are included. No 

information on the nine industry wells drilled offshore since 

Lease Sale No. CI is included (table 5). Special reports are 

included on the older rock units near Tuxedni Bay that were 

not penetrated by the COST No. 1 well; on the plutonic 

terranes of southern Alaska for sandstone provenance; and a 

compilation of the sandstone composition for most units 

present in the Cook Inlet forearc basin. Specific information 

about the COST No. 1 well is discussed first, and then special 

reports on framework geology with emphasis on sandstone 

composition follow. 

Acknowledgments-As editor, I wish to thank all 

the authors who contributed chapters to this report on the 

COST No. 1 well. Without their total support and cooperation, this report could not have been published. Special 

thanks go to David M. Hite of Atlantic Richfield Company 

who reviewed and checked the information in several chapters for accuracy and to the geologists of Chevron USA who 

reviewed the chapter on "Stratigraphic units of the COST No. 

1 well". Petroleum Information Corporation, using a Calma 

plotter, displayed the wireline log curves and lithology column used in figure 4. I gratefully acknowledge T. A. Coit, S. 

B. Griscom, M. A. McCall, P. A. Swenson, and L. M. Wood 

whose considerable efforts were crucial to getting this bulletin to the printer. 

Introduction 5 
154°00' 

60° 00' 

59° oo· 

. J N IS. 

0*:: 

0 

BOUNDARY OF 

OCS AREA 

IJ 

0 

151° 30' 

KENAI 

10 20 30 KILOMETERS ~~~--~~~~ 

Figure 1. Geographic features, leased OCS tracts, and wells drilled to date (1980) in the lower Cook Inlet OCS. Well 

numbers refer to table 5. 

6 Geologic Studies of the Lower Cook Inlet COST No. 1 Well 
Table 1. Selected information for "Well Completion and 

Recompletion Report and Log" 

[Information from U.S. Geological Survey Form 9-330] 

Operator: Atlantic Richfield CompF.y 

Well No.: Lower Cook Inlet, COST No. 1 

Category: Geot,~.>gic test 

Location: OCS block 1375 N., 105 E. 

Block No. 489 

Surface: 2,371.8 m FNL (from north line) and 

973.7 m FWL (from west line) 

Total depth: same location 

Permit: OCS-77-5 issued June 3, 1977 

Spud date: June 10, 1977 

Total depth date: Sept~mber 9, 1977 

Elevation Kelly bushing : 30.8 m (101 ft) MLLW (above mean lower low water) 

96.0 m (315 ft) RKB-ML (rig Kelly bushing above 

mud line or sea floor) 

Total depth: 3,775.6 m (12,387 ft) RKB 

Casing record: Size 

ll-:8"7m (30 in.) 

7.9 em (20 in.) 

Depth (RKB) 

Well logs: 

Cores: 

Status: 

5.3 em (13 3/8 in.) 

See table 2 

See table 3 

Plugged and abandoned 

1 COST - Continental Offshore Stratigraphic Test 

2 OCS - Outer Continental Shelf 

127.1 m (417ft) 

403.9 m (1,325 ft) 

1,475.2 m (4,840 ft) 

3 MLLW and RKB-ML corrected by D. M. Hite, Atlantic Richfield Company. 

Table 2. Type and depth of well logs run in COST No.1 well 

Type 

Dual induction laterologl 

Borehole compensated sonic 

log 

Sonic log-sequential long 

spaced 8-10 feet 

Sonic log-sequential long. 

spaced 10-12 feet 

Formation density log 

Compensated neutronformation density log 

Proximity log/microlog 

survey 

High resolution dipmeter2 

Temperature log 

Well-site geologist's 

lithologic log 

Mud log3 

Instantaneous drilling 

evaluation log4 

Delta chloride log4 

Shale density log4 

1 Mark of Schlumberger. 

Acronym 

DIL 

PL/ML 

PL/ML 

HDT1 

HDT1 

HDT1 

2 Dipmeter arrow plot available. 3 Borst and Giddens Logging Service, Inc. 

4 The Analysts, Inc. 

Depth interval 

m ft 

91.4-3,775.9 300-12,388 

403.9-3,776.8 1,325-12,391 

403.9-3,764.3 1,325-12,350 

403.9-3,764.3 1,325-12,350 

129.5-3,777.1 425-12,392 

403.9-3,777.1 1,325-12,392 

1,475.2-3,120.5 4,840-10,238 

2,987.0-3,775.6 9,800-12,387 

403.9-1,490.2 1,325- 4,889 

1,475.2-3,120.0 4,840-10,236 

3,048.0-3,775.6 10,000-12,387 

0 -3,775.9 0-12,388 

411.8-3, 7'7 :i.C.. 1,351-12,387 

411.8-3,775.6 1,351-12,387 

411.5-3,775.6 1,3 50-12,387 

426.7-3,775.6 1,400-12,387 

2,026. 9-3,764.3 6,650-12,350 

Introduction 7 
Table 3. Conventional cores acquired from COST No. 1 well 

Core 

No. 

1 

2 

3 

4 

8 

Age 

Early 

Cretaceous 

Late 

Jurassic 

Late 

Jurassic 

Late 

Jurassic 

Interval cored Core 

Top Bottom recovered Description 

m ft m ft m ft 

1,642.9 5,390 1,649.6 5,412 5.5 18 Interbedded 

2,165.3 7,104 2,173.5 7,131 7.6 25 

2,844.4 9,332 2,850.5 9,352 5.3 17.5 

3, 766.4 12,357 3,775.6 12,387 9.1 30 

Table 4. Petroleum industry participants and contractors in 

COST No.1 well 

Aminoil USA, Inc. 

Petroleum industry participants 

Getty Oil Company 

Gulf Energy & Minerals 

Hunt Oil Company 

sandstone and 

shale 

Sandstone 

Interbedded 

sandstone and 

siltstone 

Sandstone 

grading to 

siltsone 

Amoco Production Company 

Atlantic Richfield Company 

BP Alaska Exploration 

Champlin Petroleum Company 

Chevron USA 

Cities Service Oil Company 

Depco, Inc. 

Mobil Oil Corporation 

Murphy Oil Corporation 

Phillips Petroleum Company 

Shell Oil Company 

Exxon Company, USA 

Freeport Oil Company 

Texaco Incorporated 

Union Oil Company of California 

Petroleum industry contractors 

Principal contractor: ODECO International, Inc. 

Ocean Ranger-semisubmersible drilling vessel 

Support contractors and services: 

Adolf Curry-support boats 

Alaska Aeronautical, Inc.-air transportation 

Big T Oilfield Services-expediting services 

Christensen Diamond-bits 

Eastman Whipstock-directional survey 

Evergreen Helicopters of Alaska, Inc.-helicopters 

GBR Equipment Rental-tool rental 

Halliburton Services-cement 

Hughes Tool Company-bits 

International Technology Ltd.-position survey 

IMCO Services, Inc.-mud 

Land and Marine Rental Company-casing services and rentals 

Manley Terminals-warehouse, office space, and logging 

Oceaneering International, Inc.-diving 

ODECO International Corporation-rig 

Offshore Logistics, Inc.--boats 

Regan Offshore International, Inc.-wellheads 

Smith Tool Company-bits 

Tri-State Oil Tools-tool rentals 

Geoscience contractors and services: 

The Analysts, Inc.-mud logging 

BBN Geomarine Services Company-hazard survey 

Borst and Giddens Logging Service, Inc. 

Core Laboratories-core cutting and analysis 

Dames and Moore-meteorological services and bio-assay 

Geochem Laboratories, Inc.-geochemical analyses 

Schlumberger Ltd.-logging 

Seismic References Service-velocity survey 

Anderson, Warren and Associates-micropaleontology 

Geologic Studies of the lower Cook Inlet COST No. 1 Well 

Dip 

(degree) 

0-3, 

o, 

0-2, 

o, 
Table 5. Wells drilled in lower Cook Inlet outer continental shelf to 1980 

Well No. ocs Hydrocarbon 

on fig. 1 Operator Name API No. block shows 

1 Atlantic Richfield Company COST No.1 55-220-00001 489 Trace oil 

2 Marathon Oil Company OCS Y -0086 No. 1 55-220-00003 318 Some oil 

3 Phillips Petroleum Company OCS Y-012.4 No. 1 55-220-00004 668 None reported 

4 Marathon Oil Company OCS Y-0168 No. 1 55-219-Q0003 2 None reported 

5 Phillips Petroleum Company OCS Y -0136 No. 1 55-220-00005 798 None reported 

6 Marathon Oil Company OCS Y-0168 No.2 55-219-00005 2 None reported 

7 Atlantic Richfield Company OCS Y-0161 No. 1 55-250-00001 572 None reported 

8 Phillips Petroleum Company OCS Y-0152 No.1 55-220-00006 970 None reported 

9 Atlantic Richfield Company OCS Y -0097 No. 1 55-220-00002 401 None reported 

10 Atlantic Richfield Company OCS Y -0113 No. 1 55-220-00008 576 None reported 

Date De th 

Spud Abandoned Kelly bushing Water Total 

m ft m ft m ft 

1 10 June 1977 26 September 1977 30.8 101 65.2 214 3,775.6 12,387 

2 21 July 1978 22 December 1978 15.9 52 32.9 108 4,058.4 13,315 

3 9 October 1978 19July1979 15.2 50 3,448.5 11,314 

4 11 January 1979 27 April 1979 15.9 52 167.6 550 852.5 2,797 

5 23 April 1979 21 September 1979 23.8 78 3,146.8 10,324 

6 28 April 1979 21 August 1979 15.2 50 165.2 542 2,714.9 8,907 

7 15 July 1979 28 January 1980 31.4 103 39.9 131 4,564.4 14,975 

8 24 September 1979 22 April 1980 23.8 78 4,001.4 13,128 

9 6 April 1980 29 May 1980 65.5 215 2,283.3 7,491 

10 9 May 1980 28 June 1980 23.8 78 62.5 205 2,141.5 7,026 

Introduction 9 

Environmental Geology 

By Arnold H. Bouma1 and Monty A. Hampton 

INTRODUCTION 

Information about the environmental geology of the 

lower Cook Inlet is limited. Oceanographic studies and a 

monitoring program were conducted while the COST No. 1 

well was being drilled (Miller and others, 1978). The semisubmersible, Ocean Ranger, was moored at lat 59° 31' 

06.3445" N. and long 152 38' 36.6037" W. in lease block 

No. 489 in a water depth of 65.2 m (214ft). Also the U.S. 

Geological Survey made cruises to the lower Cook Inlet in 

1976-79. Data on sand waves and other bedforms were 

obtained from high-resolution seismic-reflection profiling, 

side-scan sonar surveying, bottom television and camera 

observations, bottom sampling, a limited number of vertical 

current profiles, and a few longer term current measurements. 

From these data and other published information, a number 

of geologic features and processes have been identified that 

may pose problems to engineering installations in lower 

Cook Inlet and along the adjacent coastline. Strong earthquakes (14 have occurred since 1920) have caused major 

damage to the area by ground failure, surface warping, and 

tsunamis. The four active volcanoes on the west side of lower 

Cook Inlet are andesitic; therefore they can have relatively 

violent eruptions. The most recent eruption was in 1979. 

Swift bottom currents and sand waves are known to break 

pipelines. Very little is known about their characteristics. 

Oil and gas have been produced for several years in the 

upper Cook Inlet, but little quantitative information has been 

accumulated that can be applied to the lower Cook Inlet. 

BATHYMETRY AND MORPHOLOGY 

The bathymetry of lower Cook Inlet is complex (Bouma 

and others, 1978b). A boomerang-shaped ramp at about lat 

59° 20' N. divides the area into a northern part that generally 

is shallower than 70 m (230 ft) and a southern part that is 

deeper than 120 m (394 ft) (fig. 2). In the northern part, a 

central axial depression (Cook Trough) is flanked by shallow 

shelflike areas. A relatively deep trough parallels the northeast shore of the Kenai Peninsula and ends in Kachemak Bay. 

The southern part is marked by local closed depressions. 

Water depth increases from south of the ramp into Shelikof 

Strait. Under Kennedy and Stevenson Entrances on either 

side of the Barren Islands, the sea floor has a complex 

morphology of ridges and troughs. 

1 Gulf Corporation. 

HAZARDS 

Water Currents 

The circulation of water in the lower Cook Inlet is 

affected by several factors. Water entering the inlet from the 

Gulf of Alaska is pushed by the counterclockwise Alaskan 

gyre through the Kennedy and Stevenson Entrances. Strong 

tidal effects are superimposed on the currents (Muench and 

others, 1978). Most of the water flows along the ramp and 

down into Shelikof Strait; the rest of the water flows up the 

inlet along the east side into Kachemak Bay. Cross currents 

move part of this water westward. The west side of the inlet is 

dominated by ebb tides (Burbank, 1977). 

Sand waves and other bedforms cover the sea floor off 

the coast of the Kenai Lowland and Kenai Peninsula. The area 

is roughly 50 km (80 mi) in diameter (fig. 3) and the bedforms 

range in height from 0.2 to 12m (0. 7-40ft). Swift bottom 

currents formed these bedforms in the past; how active the 

bottom currents are at present is unknown. 

Seismic Activity 

Since 1920, 14 earthquakes of magnitude 6 or greater 

have occurred within the lower Cook Inlet area. Earthquakes 

of this size typically can produce major damage to manmade 

structures, either directly by ground shaking, surface faulting, and surface warping, or indirectly by ground failure, 

sediment consolidation, and tsunamis (seismic sea waves). 

Tsunamis can be considered hazardous in the lower 

Cook Inlet area. They caused more loss of life than any other 

effect of the 1964 earthquake. They also caused damage along 

the east coast of the inlet, at Rocky Bay and Seldovia, but not 

along the west coast of the inlet. 

Tsunamis are generated when large volumes of water 

are suddenly displaced, either by tectonic displacement of the 

sea floor or by large rockfalls or landslides. The narrow, 

elongate geometry of Cook Inlet and its narrow entrances 

reduce the chance that a tsunami generated outside the inlet 

will propagate significant destructive energy into it. Nevertheless, local tsunamis can be exceptionally large; for 

example, a surge of water ran 530 m (1, 750ft) up the wooded 

slopes of Lituya Bay in the Gulf of Alaska during the southeastern Alaska earthquake in 1958. 

Sediment consolidation and its resulting subsidence 

can be another hazard in the lower Cook Inlet. This seismicrelated hazard causes flooding along coastlines and conEnvironmental Geology 11 
20 2,5 ~0 KILOMETERS 

10 15 NAUTICAL MILES 

UNIVERSAL TRANSVERSE MERCATOR PROJECTION 

+ 

+ 

SHELIKOF STRAIT 

Figure 2. Bathymetric map of lower Cook Inlet, Alaska. Water depth in meters. 

12 Geologic Studies of the Lower Cook Inlet COST No. 1 Well 
sequent damage to onshore installations. Consolidation of 

sediment was a significant cause of subsidence along Homer 

Spit during the 1964 earthquake (Waller, 1966). 

Surface warping caused by earthquakes can raise the 

level of the coastline, putting docks and facilities out of reach 

of water, as occurred at Cordova in the Gulf of Alaska area 

during the 1964 earthquake. Downwarping can cause flooding of structures and dry land. Any change in the level of a 

coastline can be destructive, and at present no system of 

forewarning is known. 

Faulting is not considered a serious problem in the 

Cook Inlet area. A few small faults have been observed on 

seismic-reflection profiles obtained north of the AugustineSeldovia arch, but these could not be found on adjacent 

tracklines 5 km (3 mi) away. South of the arch, a few surface 

faults are in the lease area. There are many normal faults with 

accompanying horst and graben structures near Augustine 

Island and near the Barren Islands (fig. 3; Bouma and 

Hampton, 1976). Faulting may create some minor problems 

with pipelines near landfalls. 

Usually ground failure caused by severe shaking, both 

on land and under water, is a major cause of destruction 

associated with large earthquakes, especially in areas containing thick sections of unconsolidated, fine-grained sediment. Deltas are especially susceptible to earthquakeinduced liquefaction and sliding. Underwater movement of 

slide sediment can cause physical damage to structures and 

exposed pipelines. Potential problems along coastlines are 

rock falls from steep cliffs, translatory block sliding, and 

ground fissuring with associated sand extrusions. 

Surprisingly, little ground failure of any kind took place 

along the coastline of the lower Cook Inlet during the 1964 

earthquake. Little effect from ground failure can be expected 

within the open-water area of lower Cook Inlet because the 

sediment on the nearly horizontal sea-floor slopes is typically 

coarse grained. 

Volcanic Activity 

Four volcanoes, Redoubt, Iliamna, Augustine, and 

Mount Douglas, are located along the west side of lower 

Cook Inlet (fig. 1). All the volcanoes are andesitic, so they 

can have relatively violent eruptions. Except for Mount 

Douglas, all the volcanoes have erupted in historic time. In 

fact, Augustine Volcano, the most active, erupted in 1964, 

1976, and 1979. Although its ejecta are not considered a 

serious problem more than 500 m (1 ,600 ft) away from the 

volcanic throat, its fine ash, which can be physically and 

chemically destructive, can travel for thousands of kilometers. Augustine Volcano also produces nuee ardentes, but 

little is known about the distance such hot ash clouds can 

travel over water. 

With an adequate warning system, most volcanic eruptions can be detected in time to shut machinery down and 

evacuate offshore structures. However, it is not safe to construct installations at the base of a volcano because nuee 

ardentes and lahars can cause significant damage and loss of 

life. 

Sand Waves and Other Bedforms 

A blanket of sand overlying a pebbly mud of glacial 

origin (Bouma and others, 1977, 1978a) covers the central 

part of lower Cook Inlet (fig. 3). This sand blanket, which 

ranges from a few centimeters to more than 12m (40ft) in 

thickness, makes up several different kinds of bedforms such 

as sand waves, sand ridges, dunes, sand ribbons, and comet 

marks (Bouma and others, 1977, 1978a,b, 1979; Whitney 

and others, 1979). 

An attempt was made to study the migration of the 

largest sand waves, which have heights from 6 to 8 m (20 to 

25 ft) and lengths from 400 to 800 m (1,300 to 2,600 ft). 

Within the accuracy of navigation, we could not detect migration of the larger sand waves when comparing 1973 industrial 

survey lines with our 1977 and 1978 resurveys (Whitney and 

others, 1979). Possibly the period of observation of sand, or 

perhaps its significant movement, may take place only during 

abnormal conditions such as severe storms, which did not 

happen during the 4-5 years of observation. 

We did observe changes in small sand waves superposed on larger sand waves. Time-lapse photography 

revealed significant ripple movement during a small storm as 

well as reversal of the ripple asymmetry during ebb and flood 

cycles. Moreover, by comparing 1977 and 1978 side-scan 

sonographs run along coincident tracklines, changes in the 

distribution and characteristics of small sand waves was discerned (Bouma and others, 1979; Whitney and others, 1979). 

Local bottom television, current measurements, and longterm observations during summer months revealed movement of sand grains only during spring tides for about one 

hour of each ebb and flood cycle. 

Comparable results were obtained from the semisubmersible Ocean Ranger, which was moored near the west 

side of a sand-wave field made up of bedforms ranging in 

height from 2 to 5 m (7 to 16ft). Several short cores collected 

around the semisubmersible contained drill cuttings from two 

horizons; the first from at 4.5 to 6.5 em (11 to 17 in.) depth 

and the second from 11.5 to 13.0 em (29 to 33 in.) depth. 

These buried drill cuttings are evidence that a certain amount 

of sand transport occurred, and also that the depth of the 

trough of the smallest bedforms was 6. 5 em (Bouma and 

others, 1978a). 

Environmental Geology 13 
59° 

58° 

0 50 100 Ml LES 

0 50 100 KILOMETERS 

I 

EXPLANATION 

'---J Fault-Hochures an downthrown side 

Q Area with bedforms ranging in height S;t from 0.2 to 2m 

....... ._.1 

Area with bed forms ranging in 

height from 2 to 12 m 

AUGUSTINE ({ 

ISLAND8 

('I' .) 

..;./J .) 

1._.1 

I KATMAI NATIONAL 

I MONUMENT 

·, 

14 Geologic Studies of the Lower Cook Inlet COST No.1 Well 

..,.>~ 4.tc... 

l D~f" 

~ BARREN ISLANDS 

ISLAND 

0 

0~ 
Movement of sand waves and other bedforms can cause 

severe problems to offshore installations by undermining or 

burying fixed structures, anchors, semisubmersibles (when 

pontoons are close to the bottom), and pipelines. This kind of 

damage commonly occurs to the pipelines in the upper Cook 

Inlet (Visser, 1969). There, pipelines rupture repeatedly over 

several years; however others may remain intact for a long 

time (F. Duthweiler, oral commun., 1978). Environmental 

data has not been collected continuously in the upper Cook 

Inlet so there is no quantitative information that can be 

applied to the problems in the lower Cook Inlet. 

Figure 3. Geomorphology of lower Cook Inlet. Surface 

faults that may be active are shown in two areas (1) 

Augustine Island and (2) around the Barren Islands. 

Environmental Geology 15 

Stratigraphic Units of the COST No. 1 Well 

By Leslie B. Magoon 

INTRODUCTION 

The COST NO. 1 well, drilled by the Atlantic Richfield 

Company, penetrated sandstone, siltstone, shale, conglomerate, and coal. The stratigraphic units represented are 

the Naknek Formation (Upper Jurassic), Herendeen Limestone and Kaguyak Formation (Lower and Upper Cretaceous, 

respectively), and West Foreland Formation (lower 

Cenozoic). These units are shown in relation to the wire line 

well-log curves in figure 4. 

Minor oil shows were found in the well but significant 

amounts of gas (CcC5 hydrocarbons) were not. The well also 

lacked organic matter capable of generating significant 

amounts of oil or gas. However, the sandstones of the West 

Foreland Formation and Kaguyak Formation, and possibly 

the Herendeen Limestone, were of potential reservoir quality. 

The interpretation of the COST No. 1 well in this report 

is based on the data of many workers. Data was submitted by 

industry, (Wills and others, 1978), as required by the U.S. 

government. Descriptions of conventional cores and sidewall 

cores were supplied by Arley and McCoy ( 1977) and Arley 

and Rathbun (1977). Petrographic analyses of sandstone were 

provided by Babcock (1977), Christensen (1977), Kuryvial 

(1977), Schluger (1977), and Swiderski (1977). Porosity and 

permeability data were supplied by Core Laboratories 

( 1977 a, b). Paleontologic reports were supplied by Boettcher 

(1977), Haga (1977), and Newell (1977a,b). Simpson (1977) 

and Van Delinder (1977) submitted the organic geochemical 

data. 

UPPER JURASSIC ROCKS 

This unit, dominantly siltstone, shale, and sandstone, 

extends from the bottom of COST No. 1 well at 3,775.6 m 

(12,387 ft) to 2,112 m (6,930 ft) for a minimum thickness of 

1,663 m (5,457 ft). Cass Arley (written commun., 1980) 

plact;!S the top of the unit at 2,135 m (7 ,005 ft) at the "abrupt 

change in character of (drill) cuttings, from a dirty-looking, 

but quartz-rich glauconitic sandstone above, to a light grey, or 

salt-and-pepper feldspathic sandstone below". According to 

the foraminiferal report, the top of the unit is at 2, 109 m 

(6,970 ft) where the "hard gray sandstone becomes the 

dominant lithology". Turner (this volume), basing his decision on paleontological evidence, places the top of this unit at 

2,417 m (7,930 ft). Wills and others (1978) place the top of 

the lowermost sandstone of the Lower Cretaceous at the large 

increase in interval velocity (decrease in interval transit time) 

that occurs at 2,088 m (6,850 ft). This calcite-cemented 

sandstone is diagenetically altered (Bolm and McCulloh, this 

volume) but does not contain zeolite (laumontite). The zeolite 

boundary in the well is 24 m (80 ft) below this altered 

sandstone. I place the top of this unit at 2,112 m (6,930 ft) just 

below the abundant Inoceramus fragments from the sidewall 

core at 2,109 m (6,920 ft) and just above the first extensive 

presence of zeolite (laumontite) at 2,146 m (7,040 ft). Here 

the resistivity, interval transit time, bulk density, and differential caliper curves do not change significantly because of the 

cements, laumontite below and calcite above. The high resolution dipmeter indicates a consistent dip of 2° to 8° to the 

northwest. 

Paleontological evidence in this unit is sparse. Haga 

(1977) assigns a Middle to Late Jurassic age (Callovian to 

Tithonian) to the palynological evidence; Turner (this volume; fig. 5) assigns a Late Jurassic age (Oxfordian to Tithonian) to the foraminifers. The fossils suggest a marginal to 

shallow marine environment, possibly ranging as deep as 

middle neritic. 

The dominant rock types found in the sidewall cores 

(fig. 6) are siltstone, shale, and sandstone. Modal analyses of 

28 thin sections indicate that the sandstone is lithofeldspathic 

(Crook, 1960) with an average composition ofQ24F55L21 , an 

average porosity of 17.9 percent, and an average permeability 

of 39 millidarcies (table 6). 

The lower part of this unit (1 ,200-1 ,600 m; 3, 900-5 ,200 

ft) correlates with the Naknek Formation exposed in the 

Iniskin-Tuxedni region (Detterman and Hartsock, 1966); the 

upper part (450-600 m; 1,500-2,000 ft) correlates with the 

rocks exposed along the southwest coast of the Kamishak 

Bay. 

Only 12 of the 172 recorded sidewall cores had favorable reservoir lithology. The organic carbon content from the 

core and drill cuttings ranges from 0.15 to 0.49 weight 

percent (fig. 7 and table 7). Traces of hydrocarbons were 

reported at many depths (table 8). 

LOWER CRETACEOUS ROCKS 

The COST No. 1 well penetrates 572 m (1,875 ft) of 

sandstone, siltstone, and shale from 1,540 m (5,055 ft) to 

2,112 m (6,930 ft). The top of this unit is placed at the large, 

abrupt excursion of the gamma ray and resistivity curves and 

the slight excursions of the interval transit and bulk density 

curves (fig. 4). The dipmeter indicates consistent dips from 5 

Stratigraphic Units of the COST No. 1 Well 17 
ERATHEM SYSTEM SERIES 

Q 

0 

w 

~ (.) 

w 

z 

w -u 

0 

w 

I 

Ow wz -.Jw 

~(.) 

STAGE 

FRANKLINIAN 

UNNAMED 

ROCK UNIT 

WEST 

FORELAND 

FORMATION 

1356 

1810 

~--r--r--~----~-----+2615 

Q 

0 

N 

0 

en 

w 

::E 

en 

::::> 

0 

w 

(.) 

~ w 

a: 

(.) 

a: 

w 

3: 

0 

...J 

MAESTRICHTIAN 

AND 

CAMPANIAN 

BARREMIAN 

AND 

HAUTERIVIAN 

PORTLANDIAN 

(TITHONIAN) 

KIMMERIDGIAN 

OXFORDIAN 

KAGUYAK 

FORMATION 

HERENDEEN 

LIMESTONE 

NAKNEK 

FORMATION 

12,387 

SPONTANEOUS INTERVAL BULK GAMMA RAY POTENTIAL RESISTIVITY TRANSIT TIME DENSITY DIFFERENTIAL 

(API units) (millivolts)LITHOLOG[ohm m2/m) (micro sift) (g/cm3J CA~i~ER ~--r---;-~ 

DEPTH 

<nil 

THICKNESS 

(m) 

o---+-----1 

413.3 

551.7 383.8 

--797.1 +-----1 

1000 

743.7 

1500 1540.8+----1 

571.5 

2000 

2112.3+------1 

2500 

1663.3 

3775.6-1-------1 

Figure 4. Wireline well-log curves, lithology, and rock units penetrated in the COST No.1 well. The lithology symbols are: 

(1) solid lines are coal; (2) thin dashes are shale; (3) thin dashes and dots are siltstone; (4) dots are sandstone; and (5) circles 

are conglomerate. The arrows on the left of the lithology are the depths for core Nos. 1-4. 

18 Geologic Studies of the Lower Cook Inlet COST No. 1 Well 
Summarized 

age 

(thia report) 

Lithology Summarized Foraminiferal Paleontologic Palynologic 

bathymetry 

(Arley and 

McCoy, 1977; 

Arley and 

Rathbun, 1977) 

Age Bathymetry Age Bathymetry 

.. Age Bathymetry 

FEET 

2000 

4000 

6000 

8000 

0,000 

2,000 

<Boettcher, 1977; 

Newell, I 977) 

No data 

Maestrichtian 

and 

Campanian 

Barremian 

and 

Hauterivian 

----- Valanginian(?) 

No data 

Middle(?) 

and 

Late(?) 

Jurassic 

I I I I 

~ 

'"I 

~ 

> 

~ 

(Turner, 

this report) 

Eocene 

Paleocene 

Maestrichtian 

Barremian 

and 

Hauterivian 

Valanginian(?) 

Tithonian 

to 

Oxfordian 

·~ 

e 

.. 

::::E 

Increasing 

abundanc 

~ 

e 

L 

(Haga, 1977) 

Eocene 

Paleocene 

Maestrichtian 

Barremian 

and 

Hauterivian 

------ Valanginian(?) 

Neocomlan and 

Late Jurassic 

Tithonian 

to 

Callovian 

METERS 

c .. I"" ..... 

~~ +> ~ 

c .. 0 +> 

o.!: 

I I 

II 

I 1000 

I 

2000 

~ 

,.. 

g 

~; 3000 

.. ... 

c 

.. 

a: 

Figure 5. Age and depositional environment of the sediments from the cores of COST No. 1 well, determined from 

bathymetry as interpreted from foraminiferal and palynological data and Inoceramus fragments. See figure 4 for explanation 

of lithologic symbols. 

to 10 to the east-northeast. Samples from the sidewall cores 

consist of sandstone, siltstone, shale, and fragments of 

Inoceramus. The Inoceramus fragments found below this 

unit were sloughed or caved. 

Modal analyses of 37 thin sections made from the 

sidewall cores revealed that the lithofeldspathic sandstone has 

an average composition of Q36F 44L22 , an average porosity of 

21.5 percent, and an average permeability of 80 millidarcies 

(table 6). The average porosity and permeability of the sandstone from the conventional cores are significantly lower than 

the averages from the sidewall cores; the samples from depths 

of 1,(j43 m (5,390 ft) to 1,650 m (5,410 ft) have an average 

porosity of 13.9 percent and an average permeability of 0.3 

millidlarcies. The large differences are attributed to the differem:e in sampling methods; sidewall cores are fractured by 

a core gun. 

A diverse assemblage of marine microplankton and 

calcareous nannofossils was found throughout this unit. 

These fossils indicate a Hauterivian to Barremian age except 

at the: base; there the fossils may be as old as Valanginian. 

Foraminifers found between 1,594 m (5,230 ft) and 2,109 m 

(6,920 ft) substantiate the Hauterivian to Barremian age; they 

indicate middle bathyal to inner neritic marine environments. 

This unit correlates with the Herendeen Limestone in 

the Kamishak Hills (Jones and Detterman, 1966). The rocks 

there are 215m (705ft) thick and are Lower Cretaceous in age 

(Magoon and others, 1978). 

The organic carbon content from core and drill cuttings 

ranges from 0.07 to 0.55 weight percent. Hydrocarbons were 

reported at 1,646 m (5,400 ft) on the mud log (table 8). 

UPPER CRETACEOUS ROCKS 

The marine and nonmarine sediments that make up this 

unit in the COST No. 1 well are 744 m (2,440 ft) thick. The 

unit extends from 1,541 m (5,055 ft) to 797 m (2,615 ft); the 

top is placed at the small, abrupt excursion of the resistivity, 

interval transit time, and bulk density curves (fig. 4). The 

dipmeter indicates fairly consistent 5° dips to the northnorthwest. 

Modal analyses of 21 thin sections taken from sidewall 

cores reveal that the feldspathic sandstone has an average 

composition of Q23F61L16, an average porosity of 22.8 percent, and an average permeability of 43 millidarcies (table 6). 

Stratigraphic Units of the COST No. 1 Well 19 
Q 

F l 

F l 

F l 

METERS 

Total depth 

3776 m 

5 

... 

... 

Porosity Permeability 

(%) (mD) 

15 25 <101 102 103 

:~ 

~~ '=-=-:: 

;~ 

~7 

Figure 6. Sandstone composition, grain size, porosity, and permeability of stratigraphic units penetrated in the COST No. 

1 well. Modal analyses done by oil company petrographers (Babcock, 1977; Christensen, 1977; Kuryvial, 1977; Schluger, 

1977; Swiderski, 1977) and reported on by Mclean (this volume), grain sizes from sidewall sample descriptions (Ariey and 

McCoy, 1977), and petrophysical properties from Core Laboratories (1977a,b). Abbreviations used are: Sh, shale; Sts, 

siltstone; Ss, sandstone; F, fine-grained sandstone; C, coarse-grained sandstone; and Cgl, conglomerate. See figure 4 for 

explanation of lithologic symbols. Triangles represent porosity or permeability values from cores. 

The palynomorphs in this unit are Maestrichtian; the 

foraminifers are Campanian and Maestrichtian in age. 

Throughout two intervals, at 797-1,036 m (2,615-3,400 ft) 

and 1,036-1,541 m (3,400-5,055 ft), the palynomorphs and 

foraminifers indicate that the sediments were deposited in 

shallowing depths. The sediments change from upper bathyal 

marine to nonmarine, therefore two prograding sedimentary 

sequences make up this unit (fig. 5). 

The Kaguyak Formation correlates with the Maestrichtian part of the Matanuska Formation north of Anchorage in 

the Matanuska Valley (Magoon and others, 1976a,b, 1980; 

Fisher and Magoon, 1978). 

The organic carbon content from core and drill cuttings 

range from 0. 22 to 1. 07 weight percent (table 7). Three 

sandstones penetrated from 1,036 to 1,088 m (3,400-3,570 

ft); from 1,286 to 1,294 m ( 4,220-4,245 ft); and from 1,353 

to 1,381 m ( 4,440-4,530 ft) show log characteristics of poten20 Geologic Studies of the Lower Cook Inlet COST No. 1 Well 

tial hydrocarbon reservoirs. Hydrocarbons were reported at 

two depths, at3,551-3,560m (11,650-11,680ft) and at2,847 

m (9,340 ft) (table 8). 

LOWER CENOZOIC ROCKS 

The first indication (minimum depth) of a lithologic 

break is 413 m (1 ,355ft) and the next lower break is at 797 m 

(2,615 ft); so this unit is at least 384m (1 ,260ft) thick (fig. 4). 

Though the dips are inconsistent, the dip 5° in either a 

northwest or southeast direction is approximately right. 

Samples from the sidewall cores are of sandstone, siltstone, coal, and some conglomerate (fig. 6). Modal analyses 

of eight thin sections of the sandstone indicate that it is 

feldspatholithic with an average composition of Q23F30L47 , 

an average porosity of 22.7 percent, and an average permeability of 108 millidarcies (table 6). 
Table t5. Petrophysical properties of sandstone, COST No.1 well 

Thick- Measure- Porosity (Eercent) PenDeability (millidarcy) 

Roc:k unit ness (m) ments Low High Average Low High Average 

Sidewall cores 

West Fore land Format ion 419 17 13.2 28.1 22.7 2.2 565 108 

Kaguyak Formation 709 35 15.6 28.9 22.8 1.8 153 43 

Herencieen Limestone 571 28 17.7 25.2 21.5 1.7 265 80 

Naknek Formation 1,664 12 12.6 25.5 17.9 .7 228 39 

COnventional cores 

Herendeen Limestone 6.7 12 8.5 19.5 13.9 0 0.8 0.3 

Naknek Format ion 7.6 27 1.8 6.7 3.5 0 7.8 0.6 

Naknek Formation 4.9 17 0.3 3.3 2.2 0 0 0 

Naknek Formation 9.2 31 1.1 4.1 2.9 0 0 0 

Vitrinite Headspa~ and 

Thermal reflectance cuttings gas Depth Age 

Lithol- Organic Hydrocarbon/ organic carbon 

alteration index (96) C 1 -C4 (ppm) (m) ogy carbon (96) (96) 

3 5 0.2 0.4 0.6 10 2 104 

0 0 1 2 0 2 4 6 8 10 

= 

·\ z:~ I :, 

-\< 1000 

:~ I CrC4 , ? 

\· 

-Y~ I :\ ~ I \· ~1; 2000 

. 6\ . f 

___ c,-c • 

. 

= 

. \ t 3000 -\ r I .\ I ·'· Total depth 

3776 m 

Figure 7. Organic geochemistry of stratigraphic units penetrated in the COST No.1 well, from data provided by Geochem 

Laboratories (Van Delinder, 1977) and ARCO (Simpson, 1977). See figure 4 for explanation of lithologic symbols. In vitri,-

nite reflectance, •, core; 6 , sidewall core; and •, drill cutting. In organic carbon, •, organic carbon in core. 

Stratigraphic Units of the COST No. 1 Well 21 
Table 7. Organic carbon content of rock units penetrated in 

COST No.1 well 

[Number of samples from each core is in parentheses, content in weight percent] 

The specimens of foraminifers, calcareous nannofossils, and radiolarians are nondiagnostic. The marine 

section, from 413 m (1 ,355ft) to 552 m (1 ,810ft), contains 

palynological evidence of Eocene age; and the nonmarine 

section, from 552 m (1 ,810ft) to 797 m (2,615 ft), contains 

palynological evidence of Paleocene age. The nonmarine 

section also contains Mesozoic fossils. 

Dri 11 cuttings 

Rock unit Caq>osite Picked 

~est Foreland Fonnation 0.93(5) 0.55(4) 

Kaguyak Fonnation 1.07(8) .2.2.(8) 

Herendeen Lbnestone .55(6) .07(8) 

Naknek Fonnation .15 ( 18) • 49(13) 

·sidewall 

0.2.0(2.) 

.56(12.) 

.2.6(10) 

.2.6(2.9) 

Core 

Conventional 

0.11 (1) 

.2.2.(3) 

This unit is assumed to be the West Foreland Formation 

which crops out on the west side of the Cook Inlet basin 

(Magoon and others, 1976b; Fisher and Magoon, 1978) . 

The content of organic carbon from the core and drill 

cuttings range from 0.20 to 0.93 weight percent (table 7). 

Table 8. Hydrocarbon indications reported in the COST No. 1 well 

[Oil in laumontite-lined fractures at 2,847 m, figure 18 this volume. NR, none reported] 

DeEth 

Rock unit m ft Mld log Geologist's log 

Kaguyak Fonnation 1,250 4,100 Olt fluorescence NR 

Kaguyak Fonnation 1,326 4,350 NR Cut fluorescence 

Herendeen Limestone 1,646 5,400 Cut fluorescence NR 

Naknek Fonnation 2,204-2,326 7,230-7,360 Cut fluorescence Cut fluorescence 

Naknek Fonnat ion 2,347-2,356 7,700-7,360 Cut fluorescence Cut fluorescence 

Naknek Fonnation 2,416-2,423 7,925-7,950 Cut fluorescence Cut fluorescence 

Naknek Fonnation 2,707-2,716 8,880-8,910 Cut fluorescence NR 

Naknek Fonnation 3,252-3,258 10,670-10,785 Cut fluorescence NR 

Naknek Fonnation 3,283-3,287 10,770-10,785 Cut fluorescence NR 

Naknek Fonnation 3,551-3,560 11,650-11,680 Cut fluorescence Trace oi 1 

Naknek Fonnation 3,598-3,621 11,805-11,880 Cut fluorescence NR 

Naknek Fonnation 3,766-3,776 12,357-12,387 NR Cut fluorescence 

22 Geologic Studies of the Lower Cook Inlet COST No. 1 Well 
Correlation of the COST No. 1 Well Data 

and the Marine Seismic Data 

By Michael A. Fisher 

INTRODUCTION 

The purpose of this report is to compare the velocity 

information from COST No. 1 well with the velocity information from multichannel seismic data and to determine the 

ages of prominent seismic horizons by correlating the measured seismic reflections with the reflections calculated from 

the well data. 

The COST No. 1 well is 7 km (4.3 mi) southwest of 

seismic line 752 and about midway between seismic lines 756 

and 757 (fig. 8). The well location, when projected perpendicularly onto seismic line 752, is closest to shot point 830. 

The direction of the projection is parallel to the regional 

struc1tural fabric of the basin which strikes northeast. 

Sonic velocity data are derived from the interval-transit 

time of the sonic logs (fig. 9). By integrating sonic velocity 

data, converting depth to two-way time, RMS (root-meansquare) velocities can be computed from the equation: 

where 

( 1 J 2 112 

vrms = - v dt) t son 

V rms RMS velocity, 

vson sonic velocity, and 

t two-way time. 

Stacking velocities are computed during the stacking of 

multichannel reflection data. From the stacking velocities, 

interval velocities are calculated by using the equation: 

where 

v;tk2 t2- v;tk1t1 112 

t2 -tl 

vint interval velocity, 

vstk stacking velocity, and 

t two-way time. 

Sonic velocities can be compared to interval velocities, 

and RMS velocities can be compared to stacking velocities. 

These velocities may compare poorly for several reasons. Dip of strata causes stacking velocities to vary whereas 

RMS velocities are unaffected. Different frequencies of 

sound waves are used to measure the velocities. Sonic 

velocities are measured with a 20 kHz wavelet over a distance 

of 2.4 m (8 ft) to 3. 7 m (12ft), whereas interval velocities are 

measured with frequencies between 5 and 50 Hz over a 

distance typically as large as 100m (300ft). Sonic logs may 

record higher velocities than those recorded by the seismic 

method because of frequency-dependent velocity dispersion. 

Fisher and Magoon (1978) computed interval-velocity 

curves at 22 locations in the lower Cook Inlet basin and 

grouped the curves having similar values. Four sets of interval-velocity curves resulted, and the geographic area that 

contained one set of curves was called a velocity domain (fig. 

8). An average of all interval-velocity curves in each set was 

used to convert reflection time to depth in each of the velocity 

domains. The COST No. 1 well is in domain 2. 

Although the interval-velocity curve of domain 1 is 

close to the average of its sonic velocity curve, the interval 

velocity curve of domain 2 has lower values, about 15 percent, than the sonic velocity curve of COST No. 1 well (fig. 

9A). This 15 percent difference between the interval velocity 

curve of domain 2 and the sonic velocity curve of the well 

means that the structure contour map of horizon C (Fisher and 

Magoon, 1978) gives depths in domain 2 that are too shallow 

by at most 15 percent. This difference can be expected 

because sonic logs give inherently higher velocities than 

interval velocities. 

The abrupt increase in sonic velocity from 3,500 m/s to 

4,500 m/s at just below the 2,000-m (6,560-ft) depth (fig. 9A) 

was evident in the sonic velocity curve but not in the interval 

velocity curve before averaging. The reason for the discrepancy is shown on the RMS velocity curve in figure 9Bthe abrupt increase in sonic velocities causes only a slight 

inflection in the RMS velocity curve. The resolution of the 

stacking velocity data apparently is too poor to discriminate 

between such subtle features; hence, the interval velocities do 

not show the same abrupt increase as the sonic velocities. 

RELATION BETWEEN WELL LOG 

AND SEISMIC DATA 

Reflection coefficients were calculated from the sonic 

and density logs (fig. 10C). Some of the largest reflection 

coefficients result from erroneous velocity and density data 

Correlation of Well and Marine Seismic Data 23 
(/) 

a: 

UJ 

1-

UJ 

:2: 

~ 

_j 

UJ 

> 

UJ 

..J 

<( 

UJ 

en 

3: 

0 

..J 

UJ 

0 

IN IS KIN Velocity Velocit.y ·.·'# •/ 

. . 

~ 

domain l~domam 2 

'·. ~ 1'\i 

Velocity 

domain 3 

10 KILOMETERS I 

• I 

··· .. §. 

. : '·I . • • • ~~"' "' 1\. 0) 0 '< .,, I\, . · <s0 ·, ~ o ... : J I .... ~ : <l"'so 

,' I '·. : ~ . I ··. ~ :"<o -QcosT : · . ," ! :' /Oo No.I well .:: ... <s · ..... ~o 

" ,' • I • • 0 ~ . . ~ 

~ .·' ~"s0 1..; 11 • • • ' 

.-.lsoo.._o 

·. 1? • 1..; , •• 118 7' • • 0o 

$3 ...... ~ 

8 /" I, $< 

• b 

. ·. $' 

/ 

Figure 8. Location of COST No.1 well and multifold seismic-reflection lines (dotted) in lower Cook Inlet. Numbers 

along seismic-reflection lines show the locations of shot points. Velocity domains (solid lines) are from Fisher and 

Magoon (1978). 

2000 3000 4000 0 

1000 

2000 

VELOCITY, IN METERS PER SECOND 

5000 

··· •.•. 

...... 

\ 

3000 

\\ 

\. 

\ ..... 

TWO-WAY TIME, IN SECONDS 

4000 5000 6000 0 0.4 0.8 1.2 1.6 2.0 

co 3000 \ RMS velocity I 

1- ··./ [J_ 

UJ 

0 

8 c 4000 

Figure 9. Velocity information from sonic logs and seismic data are compared. A, Comparison of interval-velocity curves of 

velocity domains 1 through 3 with the sonic velocity curve from the COST No.1 well. 8, Effect of large increase in sonic 

velocities or RMS velocities that are calculated from the sonic velocities. C, Time-depth curve from COST No.1 well data. 

Datum is sea level. 

that were obtained from zones where the borehole is washed 

out. In these washed out zones, both the sonic and density 

logs mimic the caliper log (fig. 4). This effect is most pronounced in the upper 900 m (2,950 ft) of the well but occurs 

also with lesser effect at depths of 1,200 m (3,935 ft), 2,000 

m (6,560 ft), and 3,600 m (11,810 ft). These erroneous 

24 Geologic Studies of the Lower Cook Inlet COST No. 1 Well 

reflection coefficients that result from the rough borehole 

would produce spurious reflections on a synthetic seismogram; consequently, a synthetic seismogram is not 

included here. 

The relation between well data and seismic data consists of a comparison of depths calculated from sonic logs to 
SONIC VELOCITY, 

IN METERS PER SECOND 

DENSITY, IN GRAMS PER 

CUBIC CENTIMETER REFLECTION COEFFICIENTS 

0 0 0 0 

0 0 0 0 

~ 0 0 0 C\J ('I) v 

0 

0 

0 

LO 

0 

0 

0 

CD 

0('1) C\J C\J 

0 

0 c09 9 9 0 0 r----r--~r----r----,----..---~----.----.~---r--~--.---.---~--r--,------~0 

0.2 

CJ) 0.4 0 

z 

8 0.6 l.U 

CJ) 

~ 0.8 

ui 

:::2: 

~ 1.0 

>-

<( 

$: 1.2 6 

$: 

1- 1.4 

1.6 

1.8 

2.0 

_ Eocene-Paleocene 

A 

CJ) 

500 ffi 

1-

l.U 

:::2: 

z 1000 --:. 

....J 

l.U 

> 

l.U 

....J 

1500 <( 

l.U 

CJ) 

$: 

2000 g 

l.U 

OJ 

2500 ~ a.. 

l.U 

0 

3000 

3500 

Figure 110. Relation between well-log and seismic data. A, Sonic velocity from the COST No.1 well. 8, Density from the COST 

No. 1 well. C, Reflection coefficients computed from the well data and formation tops. Datum is sea level. 

certain horizons on an interpreted seismic line with depths to 

geologic interfaces in the well. These depths compare favorably despite the 7-km (4.4-mi) distance between the well and 

the line. 

lhree seismic horizons are shown on a segment of line 

752 (fig. 11). Horizon A was thought to be at the base of 

Paleocene rocks by Fisher and Magoon (1978). However, the 

velocity data from the sonic log indicate that horizon A is at a 

depth of 900 m (2,950 ft), near the top of the Upper Cretaceous, at the projected well location. Although the washed 

out borehole invalidates much of the velocity and density data 

near this interface, the reflection coefficient at the base of the 

Paleocene rocks is near 0. 06. In comparison, some of the 

reflection coefficients that result from the washed-out 

borehole are as large as 0. 30. A reflection coefficient of 0. 06 

would produce a reflection of moderate strength. Line 752 

near the projected well location shows that horizon A is 

strong in the northeast, weakens toward the southeast, and is 

of moderate to low strength near the projected well location. 

The biostratigraphic evidence places the base of the 

Paleoc:ene rocks in the well at a depth of767 m (2,515 ft) (798 

m [2,615 ft] from KB). Thus, the qualitative reflection 

strength and the depth to the horizon on the line and in the 

well are similar. 

Horizon B is at a depth of 2,005 m (6,575 ft) at the 

projected well location on line 752. On seismic data, horizon 

B is a strong reflection that can be followed throughout the 

basin Estimates of velocity and density across the JurassicCretaceous contact suggest that the reflection coefficient is as 

large as 0.25. 

The reflection may be caused by the Jurassic-Cretaceous contact. Or regionally, the reflection may depict the 

boundary between the rocks cemented by calcite and laumontite and the porous rocks. If so, then the reflection level 

may not always coincide with the Jurassic-Cretaceous contact. The contact between the Jurassic and Cretaceous rocks is 

at a depth of2,082 m (6,830 ft) (2,112 m [6,930ft] fromKB) 

in the well. At that depth, the 20-m thick basal Cretaceous 

sandstone is cemented with calcite, and the Jurassic rocks are 

cemented with laumontite (Bolm and McCulloh, this volume). In the area of the well, then, horizon B is probably 

caused by the abrupt change in the physical properties of the 

rocks at the top of the thin Lower Cretaceous sandstone and 

the altered Jurassic rocks. Fisher and Magoon (1978) thought 

horizon B was at the base of the Lower Cretaceous, or more 

likely, at the base of the Upper Cretaceous. 

The reflection coefficient at the unconformity between 

the Lower and Upper Cretaceous rocks is estimated to be 

about -0.06. This value should produce a moderate reflection. 

The unconformity between Lower and Upper Cretaceous 

rocks spans nearly 40 m. y.; this amount of geologic time does 

not produce a strong reflection in seismic data. Although the 

sonic and density logs both decrease at 1,511 m (4,955 ft) 

(1,541 m [5,055 ft] from KB), the depth of the unconformity 

Correlation of Well and Marine Seismic Data 25 
(f) 

0 

z 

0 

~ 2 (f) 

~ 

uJ 

~ 

i= 

z 

0 

i= 3 

u 

w 

_J 

LL 

w 

0: 

0 1 KILOMETER 

NORTHWESr Slraligrapby 

In COST wall 

Eocene 

and 

Paleocene 

I 

Upper Cretaceous 

Lower Cretaceous + 

Upper + Jurassic 

Totall3776 m depth 

SOUTHEAST 

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

2 

3 

4 

5 

Figure 11. A part of seismic line 752 is split at shot point830 into which the COST No.1 well is projected. Horizon A is strong 

in northeast, weak toward southeast, and of moderate to low strength near projected well location. Horizon B is near the 

depth of the contact of the Jurassic and Cretaceous rocks. The stratigraphic position of Cis extrapolated from the timedepth curve (fig. 10C). Horizon C, 1,400 m below the bottom of the well, is believed to be in upper part of Middle Jurassic 

rocks. 

in the well, the caliper log shows that the hole is washed out in 

this area. If so, then this unconformity is not being delineated 

by a real seismic reflection. 

Horizon C is I ,400 m ( 4,600 ft) below the bottom of 

COST No. 1 well and about 3,100 m (10,200 ft) below the top 

ofthe Upper Jurassic rocks. This top is based on an extrapolation of the time-depth curve that was derived from well data 

(fig. 10C) to the seismic time of horizon C. Along the Alaska 

Peninsula a composite stratigraphic section of Upper Jurassic 

rocks may be as much as 3,050 m (10,000 ft) thick (Burk, 

1965), but at the Iniskin Peninsula the rocks are 1 ,435 m 

(4,700 ft) thick (Detterman and Hartsock, 1966) and at the 

Kamishak Hills they are 750 m (2,460 ft) thick (Magoon and 

others, 1976a). The COST No. 1 well penetrated only 1,663 

m (5,455 ft) of Upper Jurassic rocks, therefore, horizon C 

may be the top of the Middle Jurassic Chinitna Formation or 

the top of the Tuxedni Group. 

26 Geologic Studies of the Lower Cook Inlet COST No. 1 Well 

This estimate of the stratigraphic position of horizon C 

is approximate because the Chinitna Formation may thin 

southeastward from the Iniskin Peninsula across Cook Inlet 

(Fisher and Magoon, 1978) so the thickness of the formation 

measured onshore, even though the section may be 

incomplete, may be greater than the thickness of the formation at the location of the COST No. 1 well. 

A cross section B-B', constructed from seismic line 

752 (Fisher and Magoon, 1978), shows the relations of the 

seismic horizons to the formations and COST No. 1 well (fig. 

12). Horizons A, B, and C on the cross section can be 

compared to seismic line 752 in figure 11. The three seismic 

horizons that extend throughout the Cook Inlet basin appear 

to be caused by contacts at the base of the Paleocene rocks, 

near the base of the Lower Cretaceous rocks, and near the 

base of the Upper Jurassic rocks and the top of the Middle 

Jurassic rocks. 
ca Q) t\1 

- ~ 'E Q) c: Q) 

~ Q) c: c: 

:;:~ ci ·-- --C:Q) c: Q) z o.c B' B ci O.c (.)fl) UW 

NW z ..... ..... SE 

! Q) 

<C "S Kenai ~ ::l 

0 A 0 Peninsula 

1 well 

SEA 

SCALE LEVEL 

Lower Jurassic rocks Tertiary :L rocks 

A 

1000 Upper 0 5 Cretaceous 

KILOMETERS rocks 

Lower 

EXPLANATION Cretaceous 

rocks 2000 8 D Sedimentary rocks 

~ Volcanic rocks C/) 

Upper 

a: Jurassic w 3000 rocks t- D Metamorphic rocks w 

~ 

rn Plutonic rocks r 

4000 TO 3776 m 

A,B,C Seismic horizons 

Fault 

5000 

-?-----? c 

Middle 

Jurassic 

rocks 

Figure 12. Lower Cook Inlet COST No.1 well, placed on cross section B-B', is constructed from seismic line 752 (Fisher and 

Magoon, 1978). Major reflectors A, B, and C, are indicated on the cross section and in the enlarged inset at right. The 

location of this cross section is shown on figure 34 in this volume. 

Correlation of Well and Marine Seismic Data 27 

Paleontology and Biostratigraphy of the COST No. 1 Well 

By Ronald F. Turner2 

INTRODUCTION 

Biostratigraphic and paleoenvironmental interpretations of the COST No. 1 well are based on the detailed 

analysis of assemblages of foraminifers, marine and terrestrial palynomorphs, calcareous nannoplankton, and radiolarians. Well cuttings from 413 m (1,356 ft) to 3,776 m 

(12,837 ft) collected at 30 ft intervals were disaggregated by 

treatment with mineral spirits or by boiling in Quatemary-0 

industrial detergent and washed over a 200 mesh (75 micron) 

screen. Conventional cores and material taken from sidewall 

cores were prepared and examined in the same manner. 

Palynological slides, calcareous nannoplankton slides, 

and biostratigraphic reports prepared for the industry participants by Anderson, Warren and Associates (Boettcher, 1977; 

Haga, 1977; Newell, 1977a,b) were analyzed, interpreted, 

and integrated with the foraminiferal analysis. Radiolarian 

identifications were based on the work of Pessagno (1977) 

and Mickey (oral commun., 1978). Palynological identifications and interpretations generally follow Haga (1977), 

although the dinoflagellate taxonomy has been updated to 

conform with the systematics proposed by Stover and Evit 

(1978). Calcareous nannoplankton interpretations are based 

on Newell (1977a). Foraminiferal identifications and interpretations are based on the work of Bandy (1951 ), Bartenstein 

and Bettenstaedt (1962), Boettcher (1977), Church (1968), 

Dailey (1973), Martin (1964), Mayne (1973), McGugan 

(1964), Scott (1974), Sliter (1968, 1973), Sliter and Baker 

(1972), and Trujillo (1960). Ostracode identifications are 

from Holden (1964). and Swain (1973). 

Examination of paleontologic samples from wells is 

usually from top to bottom, so the biostratigraphic units are 

discussed from youngest to oldest. A summary of my interpretation is on figure 5. 

BIOSTRATIGRAPHY 

Eocene 

The interval from 413 m (1,356 ft, first sample) to 552 

m (1,810 ft) represents a predominately nonmarine depositional environment. A warm temperate paleoclimate is indicated by a microfloral assemblage characterized by species of 

2 Minerals Management Service (Dept. of the Interior). 

Alnus, Carya, Ulmus, Tilia, Nyssa, Fagas, and !lex. Coal is 

present in most samples. No foraminifers are present. Calcareous nannoplankton are quite rare and poorly preserved in 

this interval, although specimens with affinities to Markalius 

sp. and Cyclococcolithina sp. support a Paleogene age. Rare 

reworked Inoceramus prisms were present in the first sample. 

Paleocene 

The nonmarine interval from 552 m (1,810 ft) to 799 m 

(2,620 ft) is characterized by a palynomorph assemblage 

containing Paraalnipollenites confusus associated with 

Alnus, Carya, and Tilia. Reworked specimens of Cretaceous 

and Jurassic palynomorphs occur sporadically throughout 

this interval. No foraminifers, calcareous nannoplankton, or 

siliceous microfossils were observed. 

Late Cretaceous 

The interval between 799 m (2,620 ft) and 1,539 (5,050 

ft) is Maestrichtian in age: A well developed microflora 

containing specimens of Aquilapollenites striatus, Aquilapollenites bertillonites, Cranwellia striata, Kurtzipites tripissatus, Sympolcacites sibiricus, and Wodehouseia spinata 

is present in a nonmarine coal-bearing sandstone section 

from 799 m (2,620 ft) to 963 m (3, 160 ft). 

A Late Cretaceous marine microfossil assemblage of 

foraminifers, ostracodes, dinoflagellates, coccoliths, and 

molluscan fragments is present from 963 m (3,160 ft) to 

1,036 m (3,400 ft). Rare specimens o(the dinoflagellate 

Palaeocystodinium cf. P. golzowense and the calcareous nannoplanktons Arkhangelskiella cymbiformis and 

Chiastozygus initialis at 963 m (3, 160 ft) presage the 

appearance of the relatively diverse Maestrichtian 

assemblage. The foraminiferal fauna is characterized by 

Gavelinella whitei, G. cf. G. stephensoni, Cribrostomoides 

cretaceus, C. trifolium, Haplaphragmoides kirki, H. 

excavatus, Trochammina cf. T. texana, Bathysiphon vitta, 

Gyroidinoides cf. G. nitidus, Praebulimina sp., and P. 

kickapooensis. Rare trachylebrid ostracodes are present. 

Deposition probably occurred in an outer shelf to upper slope 

environment (100 to 600 m water depth). A dinoflagellate 

assemblage containing relatively rare specimens of Isabelidinium cooksoniae, Palaeocystodinium cf. P. golzowense, 

Paleontology and Biostratigraphy 29 
Spiniferites spp., and Isabelidinium cf. /. cretaceum is also 

present. This marine component of the palynomorph 

assemblage persists to a depth of 1,197 m (3,926 ft) (sidewall 

core data) and reappears with new, less diagnostic elements at 

1,292 m (4,240 ft). 

The predominantly sandstone section between 1,036 m 

(3,400 ft) and 1,402 m (4,600 ft) represents intertidal to 

nonmarine deposition. The interval from 1 ,036 m (3 ,400 ft) 

to 1,197 m (3,926 ft) is considered to be intertidal on the basis 

of the sparse, low diversity dinocyst assemblage noted above. 

The interval from 1,197 m (3,926 ft) to 1,265 m (4,150 ft) is 

nonmarine based on the presence of an entirely terrestrial 

microflora. Rare specimens of the calcareous nannoplankton 

Micula cf. M. decussata, Watznaueria sp., and W. barnesae, 

associated with a marine palynomorph assemblage characterized by Ceratiopsis diebeli, Isabelidinium cf. /. cretaceum, I. belfastensis, Spiniferites spp., Palaeocystodinium 

cf. P. golzowense, and Baltisphaeridium sp. , indicate an 

inner neritic environment from 1 ,265 m ( 4,150 ft) to 1,402 m 

(4,600 ft). This dinocyst assemblage is present in samples 

down to the major unconformity between the Late and Early 

Cretaceous at 1,539 m (5,050 ft). 

An abundant and diverse Late Cretaceous (Maestrichtian) forminiferal fauna indicative of an upper slope paleoenvironment is present between 1,402 m (4,600 ft) and 1,539 m 

(5,050 ft). Forty-one genera containing ninety species were 

identified. The most significant are listed in table 9. 

The only planktonic foraminifer observed was a single 

crushed specimen tentatively assigned to Hedbergella. Rare 

fragments and crushed specimens of ostracodes with possible 

affinities to Cytheresis sp., Cletocytheresis sp., and Paracypris sp. are present. 

Early Cretaceous 

A major unconformity exists between the Late Cretaceous (Maestrichtian) and the Early Cretaceous 

(Neocomian). The Neocomian (Hauterivian to Valanginian) 

section from 1,539 m(5,050 ft) to2,417 m (7 ,930ft) contains 

a relatively diagnostic assemblage of foraminifers, calcareous nannoplankton, and marine and terrestrial palynomorphs, but the presence of reworked material, downhole 

contamination, poor preservation, and sparsely fossiliferous 

intervals complicate age and environmental interpretations. 

A marine dinoflagellate assemblage indicative of a 

Hauterivian to Barremian age is present from 1,539 m (5,050 

ft) to 2,085 m (6,840 ft). This assemblage is characterized by 

Cyclonephelium distinctum brevispinatum, Clathroctenocystis elegans, Dimidiadinium uncinatum, Omatia alaskaensis, 0. butticula, Oligosphaeridium cf. 0. complex, 

Prionodinium alaskense, P. alveolatum, Muderongia simplex, M. staurota, Gonyaulacysta serrata, and Gardodinium 

trabeculosum. The presence of Nelchinopsis kostromiensis at 

2,085 m (6,840 ft) suggests a possible Valanginian age. A 

calcareous nannoplankton assemblage of Hauterivian to Valanginian age containing Cruciellipsis cf. C. cuvillieri, Calcicalathina cf. C. oblongata, Cyclagelospheara sp., 

Diazonatolithus lehmanni, and Nannoconus colomi is presTable 9. late Cretaceous (Maestrichtian) foraminifers found between 1,402-1,539 min the COST No.1 well 

PsmDOOOsphaera spp. 

Hippocrepina sp. 

Saccamnina sp. 

Gyroidinoides trujilloi 

G. ni tidus 

g. globosa 

Q. quadratus 

Q • cf • .Q. bandyi 

Pul1enia spp. 

Bolivina incrassata 

B. cf. B. decurrens 

LenticuTina ovalis 

L. cf. L. willimnsoni 

1:· cf. ~· wa.rregonensis 

Praebulbnina kickapooensis 

P. venusae 

P. spinata 

P. cuslmani 

P. aspera 

~· lajollaensis 

Pyramid ina pro llxa 

Guadryina tailleuri 

G. cf. G. austinana 

:Q. pyramidata 

HOglundina supracretacea 

~angularina cordieriana 

Ga.velinella whitei 

G. stephensoni 

G. eriksdalensis 

G. nacatochensis 

G. cf. G. henbesti 

Cribrostarnoides cretaceus 

!Japlophragxmides kirki 

H. fraseri 

H. excavatus 

H. faxoosus 

~ophax globosus 

Trochamnina boebmi 

T. texana 

I· pi lea 

Silicosigmoilina californica 

Rzehakina epigona 

30 Geologic Studies of the Lower Cook Inlet COST No. 1 Well 

Dorothia bulletta 

Bathysiphon brosgei 

B. vitta ---- B. varans 

B. cali fornicus 

Glarnospira charoides corona 

Dentalina spp. 

Nodosaria spp. 

Lagena spp. 

Fissurina spp. 

Pseudonodosaria spp. 

~inqueloculina sandiegoensis 

Stilostamella pseudoscripta 

Marginulina curvatura 

M. bullata 

Sarcenaria triangularis 

Ma.rginullnopsis texasenis 

Guttulina adhaerens 

Pyrulina sp. 

Globulina lacrima 
ent between 1,841 m (6,040 ft) and 2,134 m (7,000 ft). 

The foraminiferal fauna present in the interval from 

1,585 m (5,200 ft) to 2,106 m {6,910 ft) indicates an Hauterivian to Barremian age. The most significant fauna are listed in 

table 10. 

Early Cretaceous paleobathymetry fluctuated twice 

from upper slope to inner to middle shelf depths, indicating 

two major periods of regression. The interval from 1,539 m 

(5,050 ft) to 1,585 (5,200 ft) represents an intertidal to 

middle shelf (0 to 100m) (328 ft) depositional environment. 

Upper slope faunas are present from 1,585 m (5 ,200 ft) to 

1,676 m (5,500 ft) and from 1,932 m (6,340 ft) to 2,106 m 

(6,910 ft). The strata between the two upper slope faunas 

generally reflect an inner or middle shelf paleoenvironment. 

Few dinoflagellates or calcareous nannoplankton are 

present in the predominantly intertidal marine section below 

2,143 m (7 ,030 ft). A nonmarine Neocomian to Late Jurassic 

microflora containing species of Deltoidospora, 

Lycopodiumsporites, Osmundacites, and Taurocusporites 

segmentatus is present from 2,160 m ((7 ,087 ft) to 2,417 m 

(7, 930 ft). The exact position and nature of the CretaceousJurassic contact could not be determined because of the 

absence of age diagnostic taxa. The hiatus may represent all 

of Berriasian time. 

Late Jurassic 

A Late Jurassic (Tithonian to Oxfordian) dinoflagellate 

assemblage characterized by Gonyaulacysta cladophora, G. 

jurassica, Pareodina certaphora, Pluriarvalium 

osmingtonense, Scriniodinium crystallinum, and Sirmiodinium grossi is present in the interval from 2,417 m 

(7 ,930ft) to 3, 776 m (12,387 ft; total depth). The calcareous 

nannoplankton assemblage contains species that are present 

in both the Cretaceous and Jurassic, although below 2,445 m 

(8,020 ft) forms with Jurassic affinities increase. The Jurassic 

paleoenvironment was predominately intertidal to inner 

neritic. The radiolarian fauna contains Parvicingula cf. P. 

hsui, Hsuum cf. H. stanleyensis, Cenosphaera spp., 

Spongiodiscus spp., and Archeodictyomitre cf. A. rigida. It 

is difficult to ascertain whether the radiolarians that sporadically occur in the Jurassic section are in situ, as reworked 

silicified and pyritized Late to Middle Jurassic radiolarians 

are present throughout the Mesozoic section. 

Table 10. Early Cretaceous (Hauterivian into Barremian) foraminifers found between 1,585 to 2,106 m in the COST No.1 

well 

Lent i cu 1 ina tDlens ter i 

L. bronni i 

L. saxonica 

L. gaul tina 

L. discrepans 

L. eichenbergi 

L. heiermanni 

L. sch 1 oenbach i 

L. cf. L. guttata 

L. turgida 

L. ouachensis 

Gavelinella andersoni 

G. c f. G. bar remi ana 

AStacolus onoanus 

A. calliopsis 

!!_. pachynota 

Sarcenaria aff. §_ • . triangular_is 

Marginu 1 inops is c f. M. co 11 ins i 

M· c f. M. graci llss ima 

M. c f. M· cephalotes 

Globulina prisca 

Ram! li na sp. 

Vaginulina sp. 

Ci thar ina sp. 

Rhizamnina sp. 

Glarnospira charoides corona 

Dentalina spp. 

D. di st incta 

D. aff. ~· porcatulata 

D. graci 1 is 

Bathysiphon sp. 

Dbrothia gradata 

Textularia cf. T. klaooathensis 

Haplophrasrnoides topagorukensis 

H. c f. H. incons tans 

Trochamnina spp. 

Lingulina buddencanyonensis 

Planama1ina buxtorfi 

Praebuliuma cf. P. churchi 

Rheinholdel1a aff. R. dreheri 

HDeglundina anterio; 

H. caracolla caraco1la 

~rssonella kummi 

M. cf. M. trochus 

GUbkinella californica 

Paleontology and Biostratigraphy 31 

Petroleum Geochemistry 

By George E. Claypool 

INTRODUCTION 

Four conventional cores (fig. 4), which were obtained 

from COST No. 1 well, were taken from the Herendeen 

Limestone (Lower Cretaceous) and the Naknek Formation 

(Upper Jurassic). The samples were taken from near the top, 

1,541 m (5,055 ft) of the Herendeen Limestone and near the 

top, 2,112 m (6,930 ft), middle, and bottom, 3,756m (12,387 

ft) of the Naknek Formation. All depths represent drilled 

depths measured from the rig Kelly bushing. 

The samples were analyzed to provide information on 

the content, type, and thermal maturity of the organic matter. 

The wet oxidation technique modified after Bush (1970) was 

used to determine the organic carbon content. These analyses 

were performed by Rinehart Laboratories, Arvada, Colo. 

Conventional source rock analyses (table 11) were also used 

to determine the organic carbon content, extractable organic 

matter, and kerogen. The saturated hydrocarbon fractions of 

the extractable organic matter were analyzed by gas chromatographic techniques (fig. 13). The MP-3 thermal analysis 

method was used to evaluate the content, type, and thermal 

maturity of the organic matter (table 12). 

The amounts of organic carbon, concentration of C15 + 

extractable hydrocarbon, and content of gas, methane 

through butanes (C1_C4 ), have been analyzed by Geochem 

Laboratories (Van Delinder, 1977), and these contents are 

shown plotted against sample depth in figures 7 and 14. 

In the sidewall core samples, the amount of organic 

carbon is generally less than 0.4 percent except for the 

samples of nonmarine rocks from depths of about 900 m 

(2,950 ft). The nonmarine samples contain as much as 1.44 

percent organic carbon. 

The concentration of C15 + extractable hydrocarbons 

was determined directly and was estimated from the weight of 

pentane-soluble extractable organic matter. The concentrations, determined by both methods, generally are less 

than 100 ppm for composite cuttings and for sidewall and 

conventional cores. The content of methane through butanes 

(C1_C4) of the drill cuttings is less than 0.01 standard volumes of gas per volume of rock. This low gas content 

decreases in both total concentration and C2 _C4 content with 

increasing depth. 

The organic matter was analyzed by three different 

methods to determine carbon isotopic composition and the 

capacity for generating hydrocarbons. The three methods, 

elemental hydrogen-to-carbon ratio of kerogen, the pyrolytic 

hydrocarbon yield per unit of organic carbon, and &13C 

values of kerogen and extractable organic matter, are comTable 11. Conventional source rock analysis of extractable organic matter and kerogen in core samples from COST No.1 

well 

[Organic matter analyzed by Carlos Lubeck, U.S. Geological Survey, Lakewood, Colo.; kerogen analyzed by Core Laboratories, Dallas, Tex.] 

Extractable organic matter Kerogen 

Total Hydrocarbon/ Vitrinite 

De)2th Litho logy I Bitumen hydrocarbons organic carbon Saturates/ Atcmic reflectance 

(m) (ft) core no. (ppm) (ppm) (%) aranatics H/C TAl (mean%) 

1,644.4 5,395 

1,646.5 5,402 Shale/core 1 84 48 1.9 1.3 NO 1.7 .41 

2,165.3 7' 104 Shale/core 2 61 36 1.8 3.0 NO 1.7 .42 

2,845.0 9,334 Shale/core 3 50 32 1.6 2.2 .74 z..o .so 

3,766.4 12,357 Shale/core 4 80 43 5.4 2.6 .79 2.5 .55 

3,775.6 1Z,387 Siltstone/core 4 144 77 8.5 2.9 NO 2.5 .58 

Petroleum Geochemistry 33 
/ 

1644.4m/ 1646.5m 

CPI 1.64 

2165.3m 

CPI 1.75 

27 

25 

v 

vl/ 

29 

31 

HERENDEEN LIMESTONE 

25 

27 

29 

31 

v lJ 

A 

8 

NAKNEK FORMATION 

c 

29 

CPI 1.19 

27 

25 31 

u 

NAKNEK FORMATION 

2845m 

D 

CPI 1.05 

25 

27 

29 

31 

NAKNEK FORMATION 

Figure 13. Gas chromatographic analyses of saturated hydrocarbons in samples from the COST No.1 well, 

A through D with depths as indicated in table 11. CPI (carbon preference index) ranges from 1.75 to 1.05. 

Peak numbers refer to the number of carbon atoms for each n-paraffin compound. 

34 Geologic Studies of the Lower Cook Inlet COST No. 1 Well 
Table 12. Thermal analysis of organic matter in the core samples from COST No.1 well 

[Thermal analysis (MP-3) by J.P. Baysinger, U.S. Geological Survey, Lakewood, Colo.; wet oxidation analysis of organic carbon by Rinehart Laboratories, 

Arvada, Colo.] 

Total 

pyrolytic 

Organic hydrocarbon 

Depth interval Age carbon (OC) yield (:I'IC) 

(m) ( ft) (weight%) (weight%) 

1,644.4 5,395 Early Cretaceous 0.65 0.062 

1,646.8 5,403 Early Cretaceous .41 .032. 

2,165.3 7.104 Late Jurassic .26 .020 

2,845.0 9,334 Late Jurassic .20 .014 

3,766.4 12,357 Late Jurassic .08 .009 

3,775.6 12,387 Late Jurassic .17 .023 

pared and summarized in figure 15. Only five analyses of 

elemental hydrogen-to-carbon ratio of kerogen are available, 

but all analyses show that the organic matter is relatively 

hydrogen-deficient (HIC<l.O) even at shallow depths of 

burial where the organic matter is immature. 

The pyrolytic hydrocarbon yield per unit of organic 

carbon, here defined as the total pyrolytic hydrocarbon yield 

minus the volatile hydrocarbon content normalized by 

organic carbon, is also low. The yield is from 62 to 90 mg 

HC/g OC, and its trend is not significantly related to depth. In 

the lower part of the well at 3,766.4 and 3,775.6 m (12,357 

and 12,387 ft), the Upper Jurassic shales have slightly higher 

hydrocarbon to carbon ratios and pyrolytic hydrocarbon per 

unit of organic carbon than shales higher up in the well, even 

though the shales higher up are at a less advanced stage of 

thermal maturity. This fact may indicate that the organic 

matter in the lower part of the well originally was more rich in 

hydrogen than the organic matter higher up in the well. The 

fact may also indicate a difference in environment: possibly 

the organic matter in the lower part of the well was produced 

in a marine environment, but the amount preserved was too 

low to generate significant amounts of hydrocarbon. 

The stable carbon isotope ratio, 813C, primarily reflects 

the nature of the source material or the environment in which 

the organic matter was generated by photosynthesis; in addition, the differences in 813C among the kerogen, total extract, 

and the saturated hydrocarbons may indicate the maturity of 

the organic matter-the extent to which hydrocarbons have 

been generated from the organic matter. 813C expresses the 

stable carbon isotope ratio, 13C/12C, in parts per thousand 

(per mil) deviations from the 13C/12C ratio of the PDB marine 

carbonate standard. 

In the COST No. 1 well, the 813C values of kerogen 

range between - 24 and - 27 per mil. These values are 

consistent with organic matter derived from a mixed marine 

and nonmarine sediment. 

The differences in values of 813C among the kerogen, 

total extract, and the saturated hydrocarbons changes with 

Ten\)erature :I'IC+V!C 

Volatile 0 f JlBX iiiUll --ochydrocarbon pyrolysis (hydrocarbon VIC/:I'IC 

content (VIC) yield :I'IC/OC index (transforma-

(weight ppm) (OC) (%) a:g lt::/g OC) tion ratio) 

35 475 9.6 90 0.06 

24 471 7.7 7Z .08 

21 481 7.8 69 .11 

17 473 7.2 62 .12 

27 480 12.2 79 .30 

78 483 13.7 89 .34 

depth in the well. Above about 2,100 m (6,900 ft) in the well 

the saturated hydrocarbons are lighter than the total extract by 

2-3 per mil, and the total extract is 1-2 per mil lighter than the 

kerogen. Below about 2,135 m (7 ,000 ft), the difference 

between the 813C values of saturated hydrocarbons and total 

extract decrease to about 1 per mil, but the difference between 

the total extract and the kerogen increases to about 2-2.5 per 

mil. This change probably reflects the increased maturity of 

organic matter in the lower part of the well. There the saturated hydrocarbons make up a greater proportion of the total 

extract, and the extractable organic matter is more 

petroleumlike in character. 

Both the thermal analysis and gas chromatographic 

analysis of the saturated hydrocarbons show fairly consistent 

trends that indicate increasing thermal maturity with increasing depth (fig. 16). The transition from immature to mature 

hydrocarbon generation seems to be in the well interval from 

about 2,100-2,700 m (6,900-8,900 ft). 

RESULTS AND DISCUSSION 

The extractable organic matter of the Upper Jurassic 

and Lower Cretaceous rocks from the COST No. 1 well was 

measured by a variety of techniques to determine the 

character of its content, the amount present, and its degree of 

thermal maturity. 

The organic carbon content in the core samples ranges 

from 0.08 to 0.65 weight percent (table 12). The organic 

carbon is more abundant in the upper part of the well than in 

the lower. 

The core samples were also analyzed by the MP-3 

thermal analysis method (Claypool and Reed, 1976). In this 

procedure, a small sample of rock is heated ( 40°C/min) in a 

flowing stream of helium. Using this procedure, measurements can be made of the total pyrolytic hydrocarbon yield, 

which is the total weight of hydrocarbons (or hydrocarbonlike compounds) given off during pyrolysis and distillation relative to the weight of the rock, the volatile 

Petroleum Geochemistry 35 
ORGANIC CARBON, 

IN WEIGHT PERCENT 

0 2 3 

0 

2 

3 0 

0 

1-

w 

w 4 LL 

LL 

0 

CJ) 

0 

z 5 

<( 

CJ) 

::) 

0 

I 6 1-

~ 

I 

1-

0.. 7 w 

0 

8 

9 

10 

c9 

0 

1 1 8 

12 

Total 

depth 

12,387 ft 

C 15+ HYDROCARBONS, 

IN WEIGHT 

PARTS PER MILLION 

0 

0 

1 -

2 -

0 

3 -

4 -

6 .0 

0 

7 -0 0 

0 

8 -

0 

9 -

0 

0 

0 

10 -

0 

1 1 -

0 

12 -

~ 

500 1000 

I 

C 1-C4 HYDROCARBONS, 

VOLUME GAS/VOLUME ROCK 

0.0001 0.001 0.01 0.1 

0 

2 

3 

4 

5 

6 

7 

8 

9 

10 

11 

1 2 

Figure 14. Summary and comparison of anatyses of organic carbon and hydrocarbon contents of rocks in the 

COST No.1 well. Data are from Van Delinder (1977). 

36 Geologic Studies of the Lower Cook Inlet COST No. 1 Well 
1-

L.U 

L.U 

u.. 

u.. 

0 

(/) 

0 

z 

<( 

(/) 

:::> 

0 

:I: 

1-

~ 

:r: 

1-

a.. 

L.U 

0 

ATOMIC H/C 

OF KEROGEN 

0 0.5 1.0 0....----~~---........ 

1 1- -

0 

2 - -

0 

3 - -

4 - o5 - -

6 - -

7 1- -

8- -

9 - -

0 

10 1- -

11 t-- -

12 1- - 0 I 

Total depth 12,387 ft 

-30 

• 

• 

8

13 C PER 1000 

-29 -28 -27 -26 

• 0 

e---------;:::r 

• 

• 0 

• o-o 

0 0 

• 0 0 

0 0 

• 0 

• 0 

• 0 0 

• 0 0 

«) 0 

• 0 0 

• 0 0 

• 0 0 

• 0 

• 

~ 

0 

• 0--D 

• 0 

• 0 0 

~ 

~ 

e-o 

• 0 0 

«) 0 

• ~ 

0 0 

•• 0 

0 

0 

EXPLANATION 

8

1 3 

C values 

• Saturated hydrocarbons 

0 Total extract (bitumen) 

0 Kerogen 

-25 -24 

0 

CD 

0 

0 

0 

0 

0 

0 

PYROLYSIS YIELD/ 

ORGANIC CARBON, 

IN mg HC/g OC 

0 100 200 

I 

1- -

- -

1- -

1- -

- -

- -

1- -

r- -

- -

r- -

- -

- -

PHC/OC VHC/OC 

Figure 15. Summary and comparison of kerogen atomic hydrogen/carbon (H/C) ratios (Core Lab and Geochem data) as 

determined by COST No.1 well group participant (Phillips Petroleum), 8 nc values of kerogen and extractable organic 

matter, and pyrolytic hydrocarbon yield normalized by organic carbon (USGS data, table 12) in rocks of the COST No. 1 

well. 813C expresses the stable carbon isotope ratio, 13Cf12C, in parts per thousand deviations from the 13Cf12C ratio of the 

PDB marine carbonate standard. PHC, pyrolytic hydrocarbon; VHC, volatile hydrocarbon; OC, organic carbon. 

Petroleum Geochemistry 37 
PYROLYSIS 

MAXIMUM 

TH~RMAL 

TEMPERATURE, N-ALKANE CARBON 

PREFERENCE INDEX 

CCP124-32) 

ALTERATION VITRINITE REFLECTANCE MEAN R0 , 

IN DE.GREES 

CENTIGRADE TRANSFORMATION 

INDEX IN PERCENT LO LO LO RATIO 

1 3 5 0.2 0.5 1.0 <D ,..._ <X) 

0.1 0.3 0.5 0.7 ..,. ..,. ..,. 0 1 2 

I I I I I I I 

EXPLANATION EXPLANATION EXPLANATION EXPLANATION 

o Arco o Core o Core 

0 0 <USGS <USGS data) 0 0 A Shell data) 2 0 0 0 0 D Core 

0 

~ Laboratories 

3 0 r- f- 0 

UJ 8 0 UJ 0 0 0 LL 4 0 0 - LL 

0 0 0 

0 8. ~ 5 0 0 r- 0 AA 0Q;J z 00 Q> <( 0 0 0 

~ 6 0 00 0 0 0 0 00 J: 

f- 0 0 0 7 - 0 ADA 0 0 

~ 0 0 

+ Cuttings West 

0 Core <USGS data) 

Foreland 

- A Core (sidewall) Formation 

0 CoreCconventional) 

Kaguyak 

Formation 

+ 

- 0 + Herendeen 

~ + Limestone 

+0 

:i 0 

f- 8 0 ~ 0 - a_ 0 

UJ 0 0 

D 0 0 

9 0 0 - - 0 0 0 A 0 0 0 0 0 

rr- + - + 0 Naknek 

10 0 0 

8 0 - - r- A - Formation + 

0 A 

11 0 - - 0 0 r0 

0 0 0 12 I Do 8 A AA 

-

Total I 00 I <;b 1 I 

tpSJ + 

depth 

12,387 ft 

Figure 16. Summary and comparison of thermal alteration index (Van Delinder, 1977), vitrinite reflectance as determined by 

COST No.1 well group participants and by Core Laboratories on USGS samples (table11), temperature of maximum pyrolysis 

yield and transformation ratio (USGS data, table 12), and n-alkane carbon perference index (CPI24_

32) of organic matter in 

rocks of the COST No.1 well. 

hydrocarbon content, which is the portion of the total hydrocarbon yield produced by evaporation or distillation at lower 

temperatures, and the temperature of maximum yield, which 

is the temperature at which the yield of pyrolysis products is 

at a maximum. The results of the MP-3 thermal analysis are 

given in table 12., 

The amount of total pyrolytic hydrocarbon yield, from 

0.009 to 0.062 mg/g (table 12), falls short of the 2 mg/g that 

Tissot and Welte (1978, p. 447) define as the amount of oilgenerating organic matter at an immature stage of thermochemical transformation necessarily present in a rock 

sample in order for it to be classified as a "potential oil source 

rock". 

The total pyrolytic hydrocarbon yield per unit of 

organic carbon, from 7.2 to 13.7 mg HC/g OC, is relatively 

low. Interpreting thermal maturity from samples with low 

proportions of hydrogen-rich ("sapropelic") organic matter 

is usually questionable, but the results for these samples seem 

to be meaningful because they are corroborated by similar 

estimates of thermal maturity determined by other methods. 

The volatile hydrocarbon-to-pyrolytic hydrocarbon 

ratio was determined for the core samples. This ratio is called 

38 Geologic Studies of the Lower Cook Inlet COST No. 1 Well 

the "transformation ratio" by Tissot and Welte (1978, p. 453) 

and the "production index" by Barker (197 4 ). If the volatile 

hydrocarbon content is indigenous, then values of 0.1 or 

more indicate organic matter that has advanced to the oilgenerating stage of thermal maturity. According to this interpretation, all the core samples below 2, 165. 3 m (7, 104 ft) in 

the well are mature. 

An empirical measure of thermal maturity, based on 

evidence collected in our laboratory, is provided by the temperature of maximum pyrolysis yield. We noticed that at a 

pyrolysis temperature of about 475°C the transition from an 

immature stage of thermal alteration to a mature stage of 

alteration occurs. By this criterion, the samples higher in the 

well are marginally mature and the lower samples are mature. 

Analysis of the core samples by conventional source 

rock evaluation techniques (Claypool and others, 1978) produces results that indicate the organic matter was transformed 

to hydrocarbons to a more advanced degree than that indicated by thermal analysis. 

The amounts of total extractable organic matter in the 

core samples (chloroform extractable bitumen) are low, from 

50 to 144 ppm (table 11). Hydrocarbon concentrations, as 
determined by elution chromatography, range from 32 to 77 

ppm. Vassoevich and others (1967) found that hydrocarbon 

concentrations in shales commonly average about 180 ppm. 

Therefore the hydrocarbon concentrations of the core samples of COST No. 1 well are below average for shales. The 

hydrocarbon content relative to organic carbon content in all 

ofthe core samples is greater than 1% or 10 mg HC/g OC, so 

that organic carbon has been transformed to hydrocarbons to 

an advanced degree. 

The kerogen analyses generally substantiate the results 

of the thermal analyses. The atomic H/C ratios of less than 1 

(table 11) confirm that the organic matter in the rocks at 

2,845.0 m (9,334 ft) and 3,766.4 m (12,357 ft) are lacking in 

hydrogen. 

The thermal alteration index and the vitrinite reflectance both increase consistently with increasing depth from 

1,644.4 m (5,395 ft) to 3,775.6 m (12,387 ft). This is consistent with the increasing degree of thermal maturity with 

depth. 

The alkane distributions of saturated hydrocarbon fractions of the extractable organic matter was determined by gas 

chromatography. Samples from shallower depths in the well 

have a distinct odd-numbered n-alkane predominance (figs. 

13A and 13B). This feature is greatly reduced or absent in 

samples deeper in the well. This pattern is consistent with a 

transition from thermally immature to mature organic matter. 

The carbon preference index decreases with increasing depth, 

from 1.75 to 1.05 (fig. 13). This index, which Bray and Evans 

(1961) based on peak heights ofn-alkanes with from 24 to 32 

carbon atoms, also indicates a change from thermal immaturity to maturity. 

CONCLUSIONS 

The fine-grained Lower Cretaceous and Upper Jurassic 

rocks in COST No. 1 well contain a very low amount of 

organic matter compared to fine-grained rocks of this age 

worldwide, which average about 1 percent organic carbon. 

Moreover, the Mesozoic shales in this well contain much less 

organic matter than the Mesozoic sediments in the upper 

Cook Inlet, the North Slope, and in the other petroleumproducing areas of Alaska. 

The section penetrated by COST No. 1 well does not 

contain prospective petroleum source rocks. Although the 

thermochemical transformation of the organic matter in the 

Mesozoic rocks is marginally mature to mature, the organic 

matter is dominantly of the type that has a poor oil-generating 

capacity. However, the prospect for petroleum of the lower 

Cook Inlet should not be judged on the data obtained from the 

COST No. 1 well alone. This well did not penetrate the shales 

of the Middle Jurassic-the source rocks for petroleum in the 

upper Cook Inlet (Magoon and Claypool, 1979, 1981). 

Migration paths from the Middle Jurassic rocks are crucial to 

the oil-generating system in the Cook Inlet basin. 

Petroleum Geochemistry 39 

Present-Day Geothermal Gradient 

By Leslie B. Magoon 

INTRODUCTION 

By extrapolating from present-day subsurface temperatures, the threshold temperatures for various stages of 

hydrocarbon generation and sediment diagenesis can be 

approximated. Knowing the constant heat flux, burial depth 

at its historical maximum, and the known ages of the rock 

units, the past threshold temperatures for various stages of 

hydrocarbon generation and sediment diagenesis can be 

extrapolated from the present geothermal gradient. Unfortunately, the unique set of geologic and thermal conditions 

rarely occur in nature, and reliable temperature data is difficult to obtain. Even if this ideal situation occurs, the presentday geothermal gradient is an approximation. 

Most borehole temperatures from exploratory wells are 

disequilibrium temperatures. Bottom-hole temperatures 

(BHT) are acquired when a well is logged; the BHT temperature is generally lower than the rock formation temperature because the drilling mud is uncirculated for only a 

few hours to a day or so while the well is logged. Other types 

of temperature data from exploratory wells, such as drill-stem 

test temperatures or temperature logs, may be more representative of rock formation temperatures than BHT temperatures 

because the drilling mud may be uncirculated for a longer 

period of time. More accurate temperature measurements are 

acquired after a hydrocarbon accumulation is discovered 

because of longer shut-in times. 

Even if reliable subsurface temperatures are acquired, 

this data cannot be directly projected into the geologic past. 

The problem of how to determine the variations of the temperature gradient below lower Cook Inlet through geologic 

time still remains unsolved. 

The problems in determining the present-day geothermal gradient of COST No. 1 well are complicated. In the 

COST No. 1 well, the Mesozoic section has many 

unconformities representing significant amounts of time. The 

COST No. 1 well is located off structure, north of the 

Augustine-Seldovia Arch, approximately 140 km (87 mi) 

southwest of Kenai and is drilled in a structural low and not in 

a structural high as most wells are. Moreover, the early 

Tertiary rocks penetrated near the surface in the well indicate 

that the geologic record for the Cenozoic is incomplete in the 

lower Cook Inlet basin. Therefore, the rocks in this area are no 

longer at maximum burial depth and have probably cooled 

since reaching their highest temperature. In the upper Cook 

Inlet just north of Kenai, a complete section of Late Cenozoic 

rocks is present, and it is probable that the entire Mesozoic 

and Cenozoic sections are very close to maximum burial 

depth. 

In spite of these difficulties, I will try to determine (1) 

the present -day geothermal gradient for COST No. 1 well 

using well log data, (2) whether the geothermal gradient of 

COST No. 1 well in lower Cook Inlet basin compares favorably with the gradients in the upper Cook Inlet basin, and (3) 

how present subsurface temperatures in COST No. 1 well 

compare to the first occurrence of laumontite, the vitrinite 

reflectance data, and the thermal alteration index. 

PRESENT-DAY TEMPERATURES 

The temperature log data for the COST No. 1 well and 

the BHT for each log run are shown in figure 17. The four log 

runs, as recorded on the log headers (table 13), were made on 

June 12, June 27, August 20-21, and September 17-19, 1977. 

The temperature data is plotted in figure 17 and listed in table 

14. 

If the BHT and time since last mud circulation are 

recorded correctly on the well headers, then there is no 

difficulty in reconstructing the sequence of log runs and the 

BHT's. Unfortunately, not all the data was recorded for COST 

No. 1 well, so the data on the log headers do not agree with the 

data in table 14. The correct data, given to me by the log 

analyst, Forrest Baker, who witnessed the last log run, is 

given in table 14. 

Bottom-hole Temperature 

The first log run covered the well interval from 91 m 

(300ft) to 405 m (1 ,327ft) (table 13). The BHT for both runs 

was 19.4°C (67°F) (table 14). The second log run covered the 

intervalfrom404m (1,325 ft) to 1,489 m(4,885 ft) (table 13). 

The three BHT's were for 43.9°C (ll1°F) (table 14). 

The BHT's for both log runs one and two do not 

increase even though the time since mud circulation stopped 

does increase. Two explanations are possible: (1) the temperature of the mud is in thermal equilibrium with the formation temperature, or more likely (2) because of the size of the 

well hole, 26 in. and 17.5 in. , there is too much mud in the 

hole to come to thermal equilibrium with the rocks in the 

formation in the short periods between mud circulation. 

The BHT's of the third log run, for the interval from 

1,475 m (4,840 ft) to 3,120 m (10,236 ft), ranged from 

66.1°C (151°F) to 71.1°C (160°F). The BHT's for the first 

Present-Day Geothermal Gradient 41 
(j) 

0: 

w 

0 10 20 

TEMPERATURE, IN DEGREES CELSIUS 

30 40 50 60 70 80 90 100 

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

5 ·.A 2 

• •• ......--Temperature log data 1.59°CI I 00 m 

• •• (0.93° F I I 00 ft) 

10 ·. 

Geothermal gradient 

2.3 7° Cl I 00 m ____...~ · ••• 4 

1- 15 w 

( I .30° F I I 00 ft) . ~ ... 

. A 2 . · .,..._ Casing depth 

~ ·· .. 

1-

w 

w 

u.. 

~ 0 

z 

Uncorrected bottom-hole temperature data :\,. •• 

1.52°CI I 00 m <0.83°FI 100ft)---~ "" :· •• 

0 

w 6 ~ 0: 

0 20 z 

:::> 

I 

z 

i: 25 

a.. 

w 

0 

30 

35 

A,B 

C,D 

4 

EXPLANATION 

COST No. 1 well 

Swanson River Oil Field 

Bottom-hole temperature 

Log-run number 

\ ~:-~ 

o\ 

\ ~ / Geothermal gradient 

:· 2.28°CI I OOm 

·.~ ( 1.25°FI I 00 ft) 

\ ·· .. ~ 3 . ~ 

\~-... .. ~~ 

\ · .. ,, 

·. ~ 

\ 4 ··· ... ''\. .... ........, .... __. . ..,___,,. .. ·. ~ . 

:::> 

0 

I 

1-

z 

8 i: 

10 

12 

a.. 

w 

0 

40 ~----_.--------~~--------~--------~--------~--------~--------~----~ 

50 75 100 125 150 175 200 

TEMPERATURE, IN DEGREES FAHRENHEIT 

Figure 17. Temperature data for the COST No. 1 well and the Swanson River Oil Field. See table 14 for bottom-hole 

temperatures. 

Table 13. log runs for COST No.1 well in 1977 

[See tables 2 and 14 for additional information on logs] 

Depth interval 

Log TOJ2 Bottan run Date m ft m ft 

1 June iz, 1977 91 300 405 1 ,3Z7 

z June Z7, 1977 404 1,3ZS 1,489 4,885 

3 August ZO-Z1, 1977 1,475 4,840 3,1ZO 10,236 

4 Septeoober 17-19, 1977 Z,987 9,800 3,776 12,387 

42 Geologic Studies of the Lower Cook Inlet COST No.1 Well 

three log types increased 5°C during a 12-hour period. Unfortunately, the temperature data were not recorded for the LSS 

(long-spaced sonic) log; and this log run was made at the time 

when the time since mud circulation stopped was more than 

doubled. 

The fourth log run covers the interval from 2,987 m 

(9,800 ft) to 3, 776 m (12,387 ft) (table 13). Core number four 

was taken over an 11-hour period. The mud circulation continued until total depth was reached and stopped when the 

core barrel began to be withdrawn from the hole. From this 

time, seven different temperatures were acquired over the 57-

hour period (table 14). The BHT's ranged from 71. 7°C 
(161°F) to 87.2°C (189°F). The BHT's increase 15.5°C 

(60°F) over a 46.5-hour period. A maximum temperature of 

92.4°C (198°F) was determined by a least squares linear 

regression. (The coefficient of determination, R2 , is 0.972). 

The Horner temperature plot (Horner, 1951; Timko and 

Ferti, 1972; Dowdle and Coss, 1975) was made to estimate 

the equilibrium formation temperature (fig. 18). Except for 

the first data point, 71. 7°C (161 °F) at 10.5 hours, all the data 

support this maximum equilibrium formation temperature. 

The BHT's from the first two log runs are probably not 

formation temperatures because of the short time since mud 

circulation stopped. The formation temperature was not calculated for the third log run because the mud circulation time 

after drilling to logging depth was not recorded. The formation temperature of 92.4°C (198°F), derived from making a 

plot of the data for the fourth log run, is considered the most 

reliable formation temperature for the COST No. 1 well. 

Temperature Log 

The relation of temperature to depth for COST No. 1 

well and the Swanson River Oil Field (fig. 20) are plotted in 

figure 17. Using the temperature log, a temperature was 

plotted every 61 m (200ft). The temperature gradient (A in 

fig. 17) below 240 m (322 ft), determined by a least squares 

linear regression, is 1.59°CI100 m (0.93°F/100 ft). The coefficient of determination, R 2 , was plotted by using a temperature every 152m (500ft) from 300m (985ft) to 3,750 m 

150~~----~--~~--~~----~ 

1.0 1.2 1.4 1.6 1.8 2.0 2.1 

(tk+6t)/6t 

Figure 18. Horner temperature plot of the fourth log run for 

the COST No.1 well. Tws is the bottom-hole shut-in temperature measured at ~t. tk is 11 hours. 

(12, 300 ft ). It is 0. 997. Using this geothermal gradient, the 

temperature at the KB (Kelly bushing) was found to be 

22.2°C (71. 7°F). This temperature is quite a bit higher than 

the 8. 6°C that the measured water temperature is at the mud 

line (Miller and others, 1978). 

The maximum temperature at total depth on the temperature log is 94. 7°C. This temperature was measured, 

however, while the thermometer was at total depth for an 

unknown period of time (fig. 19). The temperature increased 

from 91°C (196°F) to 94.7°C (202°F) and asymptotically 

approached 95°C (203°F). 

This maximum temperature of 94. 7°C (202°F) agrees 

well with the formation temperature of 92.4°C (198°F) that 

was derived from a Horner plot (fig. 18). 

Geothermal Gradient 

The present -day geothermal gradient from the sea floor 

or mud line to total depth of the COST No. 1 well can be 

calculated by using either one of the formation temperatures; 

Present-Day Geothermal Gradient 43 
205 96 

1- C/) [ij ::::> I 

z Ci5 UJ 95 _J 

a: 202.4° F --- LU 

I -- 0 r4;.c <( C/) LL. UJ C/) UJ 

UJ 94 a: UJ CJ a: LU 

CJ 0 LU 

0 200 ~ 

~ 93 ui 

a: ui ::::> 

1- a: <( ::::> a: 1-

<( 92 UJ 

a.. a: 

UJ ::2: a.. LU 

::2: 1-

LU 

1- 91 

195 

INCREASING TIME ON BOTTOM 

Figure 19. Temperature increased at total depth on the 

temperature log over an unknown time duration for the 

COST No.1 well. The last recorded temperature is 94.7°C, 

and it asymptotically approached 95°C. 

the 92.4°C derived from the Horner plot or the 94.7°C 

calculated from the temperature log. Miller and others (1978, 

fig. 2.8-2) found that the mudline temperature at the COST 

No. 1 well was 8.6°C during flood tide on September 20, 

1977. By using the following figures: 

96.0 m (315 ft) from KB = sea floor or mud line, 

3,776 m (12,387 ft) = total depth of well, 

8. 6°C ( 4 7°F) = mudline water temperature, and 

92.4°C (198°F) = formation temperature derived by 

Horner plot, the geothermal gradient is 

92.4oC - 8·

6oc x 100 = 2.28°C/100 m 

3,776 m - 96.0 m (1.25oF/100ft). 

The geothermal gradient of profile B (fig. 17), 

2.28°C/100 m (1.25°F/100 ft), is higher than the geothermal 

gradient of 1.59°CI100 m (0.93°F/100 ft) that was calculated 

from the temperature log of profile A. The temperatures along 

profile A (fig. 17) are probably not at thermal equilibrium 

because of the lack of long shut-in periods. 

An example of an extremely long temperature survey is 

the one reported by Castano and Sparks (1974) in the Swanson River Oil Field. Temperatures recorded over a shut-in 

period of 100 days, after calculation, gave a geothermal 

gradient of 2.37°C/100 m (1.30°F/100 ft) and a surface temperature of 3. 9°C (39°F) (Castano, oral commun. , 1980). The 

geothermal gradient of the Swanson River Oil Field is profile 

C in figure 17. This geothermal gradient, 2. 3 7°C/l 00 m 

(1.30°F/100 ft), compares well with the geothermal gradient 

calculated from the COST No. 1 well data, 2.28°C/100 m 

(1.25°F/100 fi). 

44 Geologic Studies of the Lower Cook Inlet COST No. 1 Well 

In addition, a geothermal gradient was calculated from 

the uncorrected BHT data obtained from well log surveys run 

in the field, profile D in figure 17. The geothermal gradient of 

profileD, 1.52°C/100 m (0.83°F/100 ft), compares well with 

the geothermal gradient of profile A, 1.59°CI100 m 

(0. 93°F/100 fi) that was calculated from the temperature log 

data of COST No. 1 well. Since the BHT data are comparable 

with the temperature log data, the temperature log data must 

not be equilibrium temperatures. 

GEOTHERMAL GRADIENT OF THE AREA 

The geothermal data in a basin generally has lower 

gradients for the thickest parts of the basin and higher gradients for the perimeter where the older rocks are closer to the 

surface. This relation is shown for the Great Valley of California on the Geothermal Map of North America (Kinney, 1976). 

The west limb of the Cook Inlet basin is flanked by a 

batholith of Mesozoic and Cenozoic ages and five volcanoes: 

Mount Spurr, Redoubt Volcano, Iliamna Volcano, Augustine 

Volcano, and Mount Douglas (fig. 20). The geothermal gradient contours in upper Cook Inlet appear to reflect this heat 

source because the 2.7°CI100 m (1.6°F/100 ft) contour is 

located on the west near the heat source and the 2.1 °C/100 m 

(1.2°F/100 ft) contour is located on the east farthest from the 

source. The highest geothermal gradient contour is closer to 

the batholith near Tyonek in upper Cook Inlet than it is to the 

batholith near Homer in the lower Cook Inlet. 

The geothermal gradient in COST No. 1 well is lower 

than would be predicted from the regional trend (fig. 20; 

Kinney, 1976). The well is located a similar distance from the 

batholith as Tyonek, and it is also near an active volcanoAugustine Volcano, so one would expect the well to have a 

high geothermal gradient. Yet the geothermal gradient of the 

well is lower; it is very similar to the geothermal gradient of 

the Swanson River Oil Field. 

THERMAL HISTORY 

The thermal history or maximum temperature undergone by rocks can be inferred from the first occurrence of 

laumontite, from vitrinite reflectance, and from the TAl (thermal alteration index). 

Laumontite 

The first occurrence of laumontite recorded by Bolm 

and McCulloh (this volume) is at 1,891 m (6,205 ft) in the 

COST No. 1 well. It is also present in most samples from 

depths greater than 2,134 m (7 ,000 fi). 

The present pressure and temperature of the rocks that 

the laumontite first occurs in is 184 bars at 50°C (122°F); this 

temperature is 30°C (86°F) too cool to form laumontite. 
62°~--------~------------------~------------------~--------~------~------, 

Augustine-Seldovia 

arch 

I::J~~o~ 

Barren Islands 

0 

RIVER OIL 

FIELD 

(1.30°F/100 ft) 

EXPLANATION 

Geothermal gradient contours in 

° F/100 ft. dashed where inferred. 

Hachures indicate area of lower 

gradient 

0 50 MILES 

0 50 KILOMETERS 

Figure 20. Geothermal gradient map of Cook Inlet (Kinney, 1976), location of the Swanson River Oil Field, COST No.1 well, 

and the Alaska-Aleutian batholith. In upper Cook Inlet, the geothermal gradient on the west is much higher than that on the 

east. The COST No.1 well gradient of 2.28°C/100 m (1.25°F/100 ft) compares with the eastern gradient. 

Present-Day Geothermal Gradient 45 
Moreover, the entire T-P (temperature-pressure) profile for 

the COST No. I well is in the "fossil" laumontite field (fig. 

21; McCulloh and others, 1978). If the rock section is not 

overpressured, then the first occurrence of laumontite is a 

fossil occurrence. Greater burial depths probably would not 

provide a T-P field conducive to the formation of laumontite, 

but a much higher heat flow, possibly from one or more of the 

many periods of plutonism, could provide the proper T-P 

field. 

Vitrinite Reflectance Data 

The vitrinite reflectance value is about 0.65 percent at 

total depth in the COST No. 1 well. The rocks there are at a 

present-day temperature of about 92-95°C, a temperature that 

is very close to the threshold of oil generation for source 

shales that have been buried for at least 30-40 m.y. (Connan, 

1974). The vitrinite reflectance data obtained from conventional cores and sidewall cores substantiate these values. 

However, these reflectance values projected to the surface of 

the well are over 0. 3 percent. 

The vitrinite reflectance data determined by organic 

geochemical analysis by Simpson (1977) and Van Delinder 

(1977) (fig. 7) compare well with the vitrinite reflectance data 

determined by thermal analysis of organic matter supplied by 

Arco, Shell, and Core Laboratories (fig. 16). 

Even though the overall vitrinite reflectance gradient 

seems to be reasonable, the evidence that the rocks in the well 

were uplifted and eroded argues against accepting them as 

true values. Furthermore, the surface reflectance values 

TEMPERATURE, IN DEGREES CELSIUS 

0 40 80 120 160 200 

0 

' (f) ' T- P field beneath all 0: 

~ 200 1' shallowest occurrences 

of laumontite ~ 2' 

w " 0: ' e ~" . 

~ 400 ?;_~ (f) 

w o;~ 0: 

(L ~~-?. EXPLANATION 61?" 0 5 600 __J Laumontite occurrences LL 

o Los Angeles basin, California 

800 ~ Montebello Oil Field, California 

Figure 21. "Fossil" laumontite field. Figure modified from 

McCulloh and others (1978, fig.6). The dashed line with 

two solid triangles indicates the T-P (temperature-pressure) line for the COST No.1 well. The entire well profile is 

in the 11fossil" laumontite field. Triangle one indicates the 

laumontite at 1,891 m (6,205 ft) and triangle two is the 

laumontite at 2,134 m (7,000 ft). 

46 Geologic Studies of the Lower Cook Inlet COST No. 1 Well 

should be much higher than 0.3 percent, to fit in with the 

geothermal trend of the Cook Inlet basin. 

Thermal Alteration Index 

The TAl (thermal alteration index) of the rocks in the 

COST No. 1 well range from a low of 1.5 to a high of2.6. The 

relations of the TAl and vitrinite reflectance scales are shown 

on figure 22 (Bayliss and Smith, [1980]). The TAl values 

indicate a much higher thermal history than the vitrinite 

reflectance values of the well. The stratigraphic evidence of 

uplift and erosion and the evidence of the first occurrence of 

laumontite in the well substantiate the TAl values and confirm 

the validity of the higher thermal history. 

SUMMARY 

Even though the COST No. 1 well temperature gradient 

of about 2.28°C/100 m (1.25°F/100 ft) is close to the Swanson 

River Oil Field gradient of 2.37°C/100 m (1.30°FI100 ft), the 

present-day geothermal gradient for the COST No. 1 well 

seems to be too low when compared to the geothermal trends 

in upper Cook Inlet. All the evidence from the stratigraphy of 

the rocks, the first occurrence of laumontite, and the TAl 

values indicate that the rocks in the COST No. 1 well were 

subjected to high temperature gradients before uplift and 

erosion. The vitrinite reflectance values, which are low, do 

not substantiate this inference. 

1- 0.2 z 

LU 

0 

a: 

LU 

a. 

~ 

u.j 

0 

z COST No. I 

-< well 1-

0 0.5 Immature ':/ LU 

_J 

u.. 

LU 

a: 

LU Mature 

1-

z 

a: 

1-

> 1.0 

1 2 3 

THERMAL ALTERATION INDEX (T AI) 

Figure 22. Cross reference of the TAl (1-3) scale to 

vitrinite reflectance (0.2-1.0 percent) (Bayliss and Smith, 

[1980]). TAl values indicate a much higher thermal history 

for the COST No. 1 well than does the vitrinite reflectance. Points (1 to 6) represent corresponding TAl and 

vitrinite reflectance values from 610 m (2,000 ft) to 3,660 

m (12,000 ft) in intervals of 610 m. 
Sandstone Petrography 

By Hugh Mclean 

INTRODUCTION 

The petrographic characteristics of sandstone beds penetrated in the COST No. 1 well are important in 

~nde~stand~ng the diagenetic influences on reservoir quality 

m th1s part1cular well and in the surrounding area. 

Four conventional cores (table 3) and 101 sidewall core 

samples (table 15) were taken during drilling; one conventional core was cut from the Herendeen Limestone (Lower 

Cretaceous) and three cores were cut from the Naknek Formation (Upper Jurassic). Sidewall cores were taken from the 

interval between 545 m (1,788 ft) and 3,102 m (10,174 ft) 

(table 15), and include samples from the Naknek Formation, 

Herendeen Limestone, Kaguyak Formation (Upper Cretaceous), and West Foreland Formation (lower Tertiary). 

Twenty thin sections were prepared from three of the 

conventional cores; the remaining sections were made from 

sidewall cores. Grain-mount thin sections were made from 

sidewall cores that were badly fractured or disaggregated. The 

classification system used to indicate sandstone composition 

in figure 23 is modified from Crook (1960) and Dickinson 

(1970). 

The data shown in table 15 were compiled from 

petrographic reports prepared for the lower Cook Inlet COST 

~o. 1 well in 1977 by: Paul R. Schluger, Mobil Oil CorporatiOn, Denver, Colo.; E. W. Christensen, Chevron USA, Westem Region, San Francisco, Calif.; T. J. Swiderski, Phillips 

Petroleum Company, Denver, Colo.; Laurel Babcock, 

AMOCO Production Company, Tulsa, Okla.; R. J. Kuryvial, 

Cities Service Company, Tulsa, Okla.; and by Hugh McLean, 

U.S. Geological Survey, Menlo Park, Calif. 

PETROGRAPHIC ANALYSES 

Naknek Formation (Upper Jurassic) 

The Upper Jurassic Naknek Formation extends from 

2,112 m (6,930 ft) to 3, 776 m (12,387 ft) (fig. 4) from which 

32 sidewall core samples (table 15) and three conventional 

cores were taken (table 3). The uppermost conventional core, 

7.6 m (25ft) long, cut from between 2,165.3 m (7 ,104ft) and 

2,173.5 (7, 131 ft), consists entirely of sandstone. The middle 

core, 5.3 m (17.5 ft) long, cut from between 2,844.4 m 

(9,332 ft) and 2,850.5 m (9,352 ft) consists of interbedded 

sandstone and siltstone. The bottom core, 9.1 m (30ft) long, 

cut from between 3,766.4 m (12,357 ft) and 3,775.6 m 

(12,387 ft), grades from sandstone to siltstone (table 3). 

The average composition of 28 Naknek Formation 

sandstone thin sections is Q24F55L21 (fig. 23). Most are fineor medium-grained, with well sorted, subrounded grains. 

Samples however, between 2,145.8 m (7,040 ft) and 2,351.8 

m (7, 716 ft) consist of well rounded and very well sorted 

grains that may reflect winnowing, and possible deposition in 

a nearshore environment. 

Framework grains are packed tightly together, resulting 

in predominantly long and concave-convex grain contacts. 

Laumontite replaces plagioclase grains and glassy parts of 

volcanic rock fragments, and also fills intergranular pore 

space. Original porosity and permeability may have been 

high, but have been substantially reduced by mechanical 

compaction and by precipitation of authigenic minerals. 

Herendeen Limestone (Lower Cretaceous) 

The Herendeen Limestone extends from 1,541 m 

(5,055 ft) to 2,112 m (6,930 ft). Approximately 5.5 m (18ft) 

of core cut from 1,642.9 m (5,390 ft) to 1,649.6 m (5,412 ft) 

consists of interbedded sandstone and shale (table 3). Seven 

thin sections from conventional core and 32 thin sections 

from sidewall cores (tables 3 and 15) have an average composition of Q36F 44L20. Accessory minerals include green 

hornblende, biotite, epidote, and clinopyroxene. Rock fragments are mainly felsic and microlitic volcanics of intermediate composition. Metamorphic rock fragments include mica 

schist and quartzite. 

Sandstone samples are fine-and medium-grained, with 

subrounded to subangular, well-sorted framework aggregates 

that are typically framework supported. Adjacent grains display combinations of both point and long contacts. Although 

alteration of framework grains is minimal, porosity and permeability are nevertheless reduced by mechanical compaction and by pore-filling authigenic clay minerals and calcite. 

Samples from depths of 1,858.4 m (6,097 ft) and 

1,877.9 m (6,161 ft) contain Inoceramus prisms, a typical 

fossil in the Herendeen Limestone. 

Kaguyak Formation (Upper Cretaceous) 

The Kaguyak Formation extends from 797 m (2,615 ft) 

to 1,541 m (5,055 ft) (fig. 4). Thin sections from 22 sidewall 

cores have an average composition of Q23F61L16. Samples 

include a wide range of grain sizes including very fine, fine, 

medium, and coarse, which are typically poorly sorted and 

Sandstone Petrography 47 
Table 15. Sandstone petrography of COST No.1 well 

{Data compiled by R. M. Egbert, U.S. Geological Survey.] 

Feldspar Rock Fraga Other Ratios Cement 

Age Core Sample Core Q F L Quartz Chert K-spar Plag tot VRF URF MRF Bio Hbl C/Q P/F V/L Cly Cal Matrix 

0 

0 

N 

0 

c: 

Ql 

(.) 

"' :::> 

0 

C1> 

0 

"' 

C1> 

(.) 

C1> 

c. 

c. 

::> 

"' :::> 

0 

C1> 

0 

"' 

samples depth No. 

1 E 

2 E 

3 A 

4 E 

5 A 

6 A 

7 E 

8 A 

9 E 

10 E 

11 E 

12 A 

13 E 

14 E 

15 A 

16 A 

17 E 

18 A 

19 E 

20 A 

21 E 

22 A 

23 E 

24 A 

25 E 

(ft) 

1, 788 

1,858 

2,060 

2,230 

2,274 

2,475 

2,506 

2,594 

2,972 

3,100 

3,427 

3,442 

3,460 

3,478 

3,486 

3,508 

3,543 

3,553 

3,556 

3,587 

3,664 

3,670 

3,916 

3,974 

4,078 

26 A 4,080 

27 E 4,240 

28 E 4,317 

29 A 4,508 

30 E 4,520 

31 B 5,120 

32 D 5,120 

33 c 5,130 

34 B 5,210 

35 D 5,270 

24 29 47 12 

18 23 59 

15 26 59 

16 23 61 5 

32 13 55 18 

29 48 23 13 

27 44 29 17 

23 35 42 14 

9 21 70 

17 16 67 

39 55 6 30 

49 44 7 31 

16 78 6 12 

15 80 5 11 

27 63 10 19 

17 53 30 10 

16 78 6 12 

25 70 5 15 

29 66 20 

16 79 

30 65 5 22 

37 60 22 

9 86 

25 38 37 20 

216712 12 

12 58 30 

20 75 5 12 

23 70 7 15 

25 49 26 13 

28 65 7 18 

6 5143 

32 47 21 30 

17 48 35 14 

14 57 29 10 

32 47 21 30 

36 D 5,350 44 28 28 40 

37 E 5,391 1 42 SO 8 30 

38 A 5,393 1 25 53 22 19 

39 F 5,394 1 56 39 5 42 

40 B 5,396 29 60 11 26 

41 E 5,399 1 32 57 11 16 

42 A 

43 F 

44 E 

45 D 

46 c 47 D 

48 B 

49 D 

5,401 1 3148 21 19 

5,403 1 39 41 20 26 

5,404 1 30 51 19 17 

5,428 67 32 1 65 

5,472 15 70 15 15 

5,505 58 2121 55 

5,565 34 60 6 29 

5,648 40 30 30 32 

~ 50 8 5,692 18 62 20 14 

(.) 

C1> 

~ 

0 

...J 

51 D 

52 B 

53 c 54 D 

55 B 

5,692 

5,713 

5,713 

5,770 

5,839 

56 B 5,883 

57 D 5,932 

sa s 5,962 

59 D 6,035 

60 B 6,097 

61 D 6,097 

62 D 6,125 

63 B 6,161 

64 B 6,205 

65 B 6,302 

66 D 6,385 

67 B 6,404 

68 c 6,404 __ 69 D 6,435 

47 4211 45 

33 5512 27 

25 45 30 23 

39 44 17 35 

35 43 22 29 

26 44 30 21 

39 33 28 35 

29 5714 25 

52 32 16 so 38 42 20 28 

47 29 24 40 

66 22 12 55 

21 54 25 18 

37 49 14 29 

39 49 12 37 

50 22 28 45 

42 45 13 40 

5416 30 51 

55 20 25 55 

5 

10 

15 

2 

15 

2 

15 

7 

12 

20 

15 

15 

6 

20 

20 

6 

15 

20 

20 

20 

20 

20 

14 

8 

12 

15 

29 

10 

15 

11 

35 

15 

52 

48 

25 

20 

55 

25 

39 

25 

40 

15 

57 

15 

42 

20 

52 

47 

41 

39 

48 

40 

46 

40 

15 

20 

30 

57 

54 

20 

29 

20 

15 

35 30 

45 20 

40 29 

39 20 

45 20 

25 25 

34 

20 17 

29 

46 

36 

15 

28 

26 

48 

46 

43 

30 

31 

15 

10 

12 

70 15 

20 20 

25 25 

13 

40 10 

41 28 

40 15 

10 

35 18 

30 25 

47 

30 15 

31 13 

25 20 

18 10 

39 17 

34 6 

40 

20 25 

36 10 

15 28 

20 25 

angular to very angular. Most rock fragments (L) are volcanics that have altered to clay minerals that form 

pseudomatrix. Clay minerals also replace plagioclase and 

potassium feldspar. Porosity and permeability are mainly 

reduced by authigenic clays and traces of carbonate. 

West Foreland Formation (lower Cenozoic) 

The West Foreland Formation extends from 413 m 

(1,356 ft) to 797 m (2,615 ft). A total of 8 thin sections from 

sidewall cores from between 545 m (1, 788 ft) and 790 m 

(2,594 ft) (table 15) have an average composition of 

Q23F30L47 . Accessory minerals include hornblende, biotite, 

48 Geologic Studies of the Lower Cook Inlet COST No. 1 Well 

20 

15 

15 

15 

10 

10 

11 

5 

15 

11 

9 

.40 .83 

.40 .74 

.so .44 

.58 .71 

.22 .40 

10 

5 

10 

22 .28 .so .11 .94 

.18 .40 

.29 .88 

.36 .85 

0 .81 

.03 .so 0 .88 

.80 

.56 

.09 .57 95 

0 .92 

.63 .67 

5 

10 

.87 10 

18 0 .56 

0 .85 

16 0 .43 

3 .90 10 

.so 10 

.02 .86 

0 .so .95 10 

.25 .88 2 

.12 .89 

16 .13 .31 10 

16 

5 

.!In 

.10 89 

1.0 

.90 

15 21 10 

5 

10 

.06 .89 

0 .50 

0 1.0 .99 15 

- .87 .70 

.33 .84 

.14 .43 1 

0 .96 .81 

11 .11 81 

3 

0 High High 

14 .92 .40 

8 

10 19 .98 .87 

10 18 

8 

10 

16 

18 

10 

14 

5 

15 

17 

12 

12 

5 

10 

5 

- .96 .90 

.86 .56 

.97 .72 

- .96 .75 

87 

.87 .81 

.89 .ss .87 .82 

.84 .83 

23 

13 0 

10 

.!Is 

10 

11 

x-5 

x-3 

x-7 

20 

20 

10 

15 

20 

20 

25 

30 

20 

20 

10 

20 

15 

15 

15 

16 

10 

20 

14 

11 

15 

10 

10 

20 

2-20 

15 

21 

2-3 

10 

2-10 

10 

40 

8-10 

13 

epidote, and opaque minerals, and the assemblage of rock 

fragments is predominantly volcanic. Grains are fine and 

medium in size, angular to subangular in shape, and are 

poorly sorted. Framework grains tend to be surrounded by 

float in either a clay matrix or in calcite cement. In some 

samples, possible dissolution of calcite has increased porosity to nearly 20 percent (measured visually). 

DISCUSSION 

Sandstone porosity and permeability in the Kaguyak 

(Upper Cretaceous) and West Foreland Formations (lower 
Table 15. Continued. 

Feldsear 

Age Core Sample Core Q F L Quartz Chert K-spar Plag 

samples depth 

1ft) 

70 8 7,040 10 74 16 59 

71 D 7,104 30 20 50 30 

72 A 7,106 14 49 37 11 20 20 

73 F 7,107 20 46 34 17 37 

74 E 7,108 2 17 70 13 12 12 43 

75 A 7,110 2 20 58 22 11 15 25 

76 E 7,112 9 82 9 5 56 

77 A 7,114 2 17 51 32 14 15 25 

78 B 7,117 10 75 15 54 

79 E 7,117 2 19 71 10 12 50 

80 A 7,124 2 17 51 32 10 20 20 

<.l 81 E 7,126 2 21 63 16 14 46 

82 E 7,128 13 73 14 10 56 en 83 A 7,129 2 11 43 46 8 15 20 en 

., 84 F 7,1.l9 2 19 27 54 17 24 

85 B ::> 7,716 11 73 16 53 

...., 86 D 8,376 37 42 21 35 

;o 87 D 8,712 50 30 20 50 

c. 88 B 8,855 27 68 5 21 52 

c. 89 D 8,875 56 28 17 50 

::J 90 B 8972 28 68 4 17 41 

91 F 9,336 3 35 33 32 30 25 

92 c 9,341 3 29 32 39 29 

93 F 9,344 35 59 6 15 24 

94 c 9,348 3 22 45 33 22 

95 B 9,515 30 57 13 21 40 

96 D 9,515 64 24 12 55 

97 8 9,885 6 85 9 55 

98 D 9,928 67 22 11 60 

99 8 10,091 9 77 14 52 

100 B 10,145 4 68 28 49 

101 D 10,174 22 44 33 20 

Abbreviations and symbols: 

1\ Mobil = Quartz + Chert + Quartzite 

B Chevron USA = Total feldspar 

c Phillips = Total rock fragments 

D Amoco K-spar = Potassium feldspar 

E Cities Service Plag = Plagioclase feldspar 

F USGS J/t = Total feldspar lplag and K-spar) 

=~prisms 

Q 

F 3:1 1:1 1:3 L 

Figure 23. Averages of Q-F-L (quartz-feldspar-lithic fragments) for each formation. The classification presented is 

modified from Crook (1960) and Dickinson (1970). Quartz 

end member, Q, includes chert. 

Tertiary) have been reduced mainly by the formation of 

authigenic clay matrix and by pseudomatrix derived from 

unstable volcanic rock fragments. Precipitation of authigenic 

zeolite cements and mechanical compaction of framework 

tot 

Rock 

VRF URF 

Fraqs 

MRF Bio 

Other 

Hbl tot C/Q 

Ratios 

P/F V/L 

~ 

Cly Cal Matrix 

10 - .97 .77 x-2 12-13 

20 50 10 

30 0 .55 1 12 

24 0 .93 .83 13 

10 5 .14 78 

15 12 .!!1s .21 .63 10 

15 .17 .97 

25 .!16 0 .63 12 

- .98 .82 20-25 

.20 .91 

25 .23 .50 16 

12 .13 .96 

12 .09 .92 

38 .11 .57 10 

27 11 .58 10 

10 2 - .98 .83 20-25 

40 20 13 

30 20 

12 21 .50 2-5 

25 15 5 10 10 

16 30 7-10 3-5 

11 .93 .64 

32 39 .80 

0 .92 40 

45 33 - High 

15 18 .56 12-15 

20 10 10 12 15 

21 21 15-20 

20 10 10 10 20 

10 18 18 - .98 10-15 3-5 

21 11 14 - .94 7-10 3-5 

40 30 25 

VRF = Volcanic rock fragments C/Q = Chert/Quartz + Chert + Quartzite 

MRF = Metamorphic rock fragments P/F = Plagioclase/total feldspar 

URF = Undifferentiated rock fragments V/L = Volcanic rock fragments/total 

Bio = Biotite rock fragments 

Hbl = Hornblende Cly = Clay 

tot = Total other minerals Cal = Calcite 

z = Zeolite 

X = present in unknown amount 

x-5 = up to 5%, estimated 

grains combine to reduce porosity and permeability even 

farther. 

Although the lithofeldspathic sandstones of the Herendeen Limestone (Lower Cretaceous) and Naknek Formation 

(Upper Jurassic) contain low percentages of intergranular 

matrix, their framework grains are tightly compacted and 

locally cemented by laumontite and to a lesser extent by 

chlorite. Laumontite and heulandite together form as much as 

7 weight percent of a sandstone sampled between 2,166.5 m 

(7,108 ft) and 2,171.0 m (7,126 ft), as measured by X-ray 

diffraction techniques. Samples from progressively greater 

depths reveal that laumontite increasingly replaces stilbite 

and(or) heulandite and that chlorite increasingly replaces 

montmorillonite. 

Lower Cretaceous, Upper Cretaceous, and lower Tertiary sandstone samples all contain authigenic clay minerals 

and calcite cement. Intergranular pores of some Lower Cretaceous sandstone are filled with a mix of clay and calcite. 

Upper Cretaceous sandstone typically contains 

authigenic clays derived from volcanic rock fragments, plagioclase, and potassium feldspar, and most samples contain 

at least a trace of carbonate cement. Lower Tertiary samples 

contain extensive amounts of clay matrix and( or) calcite 

cement, with as much as 20 percent visual porosity in one 

sample. Porosity and permeability of most Upper Cretaceous 

and lower Tertiary rocks are reduced mainly by precipition of 

authigenic clay minerals and to a lesser extent, by the formation of zeolite cement and by mechanical compaction. 

Sandstone Petrography 49 

Sandstone Diagenesis 

By John G. Bolm3 and Thane H. McCulloh4 

INTRODUCTION 

The purpose of this study was to determine the composition of the sandstones, their origin, and their porosity and 

permeability. Cuttings and core samples were obtained from 

between the depth intervals of545.0 m (1,788 ft) and 3,775.6 

m (12,387 ft) in the COST No. 1 well (table 3; fig. 4). From 

these samples 161 standard petrographic thin sections were 

prepared by companies that participated in the drilling of the 

well (table 4 ). The thin sections were loaned to the U.S. 

Geological Survey. 

The compositions of the sandstones in the well indicate 

that they were derived from an igneous source terrane that had 

outcrops of both volcanic and plutonic rock types. Volcanic 

rock fragments and plagioclase feldspar, which are abundant 

in the sandstone samples, were most affected by the 

diagenetic changes. 

DIAGENETIC CHARACTERISTICS 

Four kinds of diagenetic changes occurred to the sandstones in the COST No. 1 well: ductile deformation of 

volcanic rock fragments, formation of authigenic clay, formation of carbonate cement, and crystallization of laumontite. 

Ductile Deformation of Volcanic Rock Fragments 

Volcanic rock fragments have commonly undergone 

ductile deformation, and in some sandstones this has produced pseudomatrix as defined by Dickinson (1970, p. 702). 

The pseudomatrix is more abundant in sandstones having a 

high content of volcanic rock fragments than in sandstones 

having a low content of volcanic rock fragments, and it is 

more abundant in sandstones from deeper depths than it is in 

sandstones from shallower depths. 

Authigenic Clay 

Yellow to light green authigenic clay is present in many 

samples from the COST No. 1 well (fig. 24). In the shallowest 

sample from the well (545 m or 1, 788 ft), the clay partly or 

completely coats framework grains; in deeper samples, clay 

coats are generally thicker and more widespread. Kuryvial 

3Minerals Management Service (Dept. of the Interior). 

4 Mobil Research and Development, Corp. 

0.1 MM 

Figure 24. Authigenic clay lining in pore space at 1 ,060-m 

(3,478-ft) measured depth. 

(1977) determined by X-ray analyses that the clay consists of 

various combinations of montmorillonite and chlorite as discrete phases or mixed layer combinations. Samples with 

abundant volcanic rock fragments contain the most clay. Clay 

coatings were probably formed mainly from material derived 

from the chemical breakdown of volcanic rock fragments. 

Carbonate Cement 

Minor amounts of carbonate cement are present in a few 

samples. The cement fills intergranular spaces in some samples, and it is scattered in intergranular areas in others. Both 

sparry and microsparry carbonate are present. The sparry 

carbonate occurs as single crystals filling intergranular 

spaces several millimeters in diameter. Individual crystals of 

micros parry carbonate are 0. 02-0.05 mm in diameter and are 

bounded by curved or wavy boundaries. The carbonate is 

probably of primarf depositional origin, and the grain size of 

any particular occurrence probably has been determined by a 

balance of aggradational recrystallization associated with 

fluid flow and diminutional recrystallization associated with 

structural strain. 

Laumontite 

Although Kuryvial (1977) reported finding laumontite 

in samples from a depth as shallow as 1,643.2 m (5,391 ft), 

Sandstone Diagenesis 51 
we first found laumontite at a depth of 1,891.3 m (6,205 ft). 

The method he used was X-ray analysis; ours was optical 

observation of thin sections. In our first sample, laumontite 

filled the intergranular spaces. Most of the sandstone samples 

from depths greater than 2,134 m (7 ,000 ft; fig. 25) contain 

laumontite; most of these samples also contain plagioclase 

that is partly to completely albitized. Release of calcium and 

aluminum from plagioclase during albitization together with 

chemical breakdown of volcanic rock fragments have 

undoubtedly contributed to the crystallization of laumontite. 

Kuryvial (1977) also found other sodium and calcium 

zeolites besides laumontite present in samples from depths 

greater than 545 m (1, 788 ft). He examined these zeolites 

with a scanning electron microscope and found evidence that 

they formed synchronously with the authigenic clay. Laumontite, however, crystallized after the clay formed, as is 

indicated by the presence of clay coats between framework 

clasts and laumontite pore-filling cements. 

Porosity 

In the upper half of the well, sandstone samples commonly have visible intergranular porosity. However, in most 

samples the pore spaces are lined with authigenic clay that 

obstructs pore throats and drastically reduces permeability. 

With depth, porosity decreases; framework grains are more 

tightly compacted and there is an increasing abundance of 

authigenic minerals. At 2,088 m (6,850 ft), there is a strong 

deflection in the resistivity, sonic velocity, and density curves 

on the geophysical logs (fig. 4); this deflection probably 

marks the base of significant intergranular porosity. Below 

this depth, fracture porosity is present in some samples. A 

sandstone sample from 2,847 m (9,340 ft) shows fractures 

that contain both oil and partial linings of laumontite (fig. 26). 

0.5 MM 

Figure 25. Laumontite filling in authigenic clay-lined pore 

space at 2,167.2-m (7,110-ft) measured depth. Note albitization of plagioclase. 

52 Geologic Studies of the Lower Cook Inlet COST No.1 Well 

0.25 MM 

Figure 26. Oil in laumontite-lined fractures, 2,846.8 m 

(9,340 ft); measured depth. 

DISCUSSION 

Porosity and Permeability Reduction 

The mineralogically immature sandstones in the COST 

No. 1 well are particularly susceptible to diagenetic processes 

because of their chemically unstable character. High temperatures from deep burial or from high geothermal gradients 

facilitate dissolution of unstable framework clasts and thus 

provide material for the precipitation of authigenic 

phases. 

Clay or zeolite minerals precipitate in the intergranular 

pore spaces of sandstone because of changes in temperature, 

pressure, or pore-fluid composition subsequent to deposition. 

The crystallization of some zeolite minerals, such as stilbite 

and clinoptilolite, may cause only moderate reduction in the 

reservoir properties of sandstones, but the crystallization of 

the calcium zeolite laumontite causes a great reduction. Some 

diagenetic processes are slowed or prevented if petroleum 

flows into the sandstone reservoir before the alteration episode occurs, but evidence gained from several producing oil 

fields in California shows that the crystallization of laumontite is not inhibited in this way. Laumontite crystallizes in 

mineralogically immature sandstones whenever the appropriate conditions of temperature, pressure, and pore-fluid chemistry prevail whether the principal fluid is petroleum or water. 

The laumontite in the COST No. 1 well is probably a 

product of an episode of thermal alteration that left a regional 

imprint throughout the Cook Inlet basin. Comparable alteration of Jurassic sandstones was observed during regional 

studies of the rocks of the Iniskin Peninsula on the west side 

of lower Cook Inlet (Detterman and Hartsock, 1966, p. 31). 

Laumontite also is present in rocks which crop out in the 

Kamishak Hills and Cape Douglas region. Several wells in 

the upper Cook Inlet basin contain laumontite, and Clark 

(1973) reported that laumontite was present in the rocks of the 
McHugh complex in the Chugach Mountains east of 

Anchorage. 

Most of the laumontite occurs in the mineralogically 

immature units of Jurassic and Cretaceous age, as in the 

COST No. 1 well; the youngest strata known to have been 

laumontitized belonged to the West Foreland Formation of 

Paleocene age that crops out near Cape Douglas. Evidently 

the upper surface of laumontitization (diagenetic front) is 

irregular and cuts discordantly across deformed strata and 

erosional unconformities. The thermal episode that partly 

controlled the formation of laumontite probably culminated 

in late Mesozoic or early Cenozoic time. 

Exploration Considerations 

Because of porosity and permeability reduction caused 

by laumontitization, sandstones of Jurassic, Cretaceous, and 

early Paleocene age are not highly prospective as reservoir 

rocks in Cook Inlet basin. However, information gathered 

from other oil fields suggests that there may be mitigating 

factors. 

Artificial stimulation procedures may permit hydrocarbon production in some laumontitized reservoirs, but such 

techniques have not generally worked. In other sedimentary 

basins, where oil wells are drilled in mineralogically immature sandstones below the shallowest occurrences of 

authigenic laumontite, the wells have had low or noncommercial production rates initially. However, the production of oil 

may be improved in wells whose oil-saturated sediments are 

incompletely laumontitized by using a recently developed 

method of hydraulic fracturing (Lavery, 1961). 

If parts of the lower Cook Inlet region were not subjected to the high thermal gradients required for the crystallization of laumontite, as the crosscutting character of the 

diagenetic front suggests, the sandstones in those areas may 

have retained sufficient intergranular porosity to be prospective. Several California oil fields, such as Santa Fe 

Springs, North Tejon, and Jacalitos, have producible reservoirs in strata that abruptly change laterally from destructively laumontitized rock to unaltered rock. Systematic 

mapping of the diagenetic front should identify potentially 

laumontite-free areas where reservoir quality should be 

better. 

Studies of sandstone diagenesis in other petroliferous 

basins have provided evidence that the chemistry of the fluids 

in the rock pores may inhibit or prevent the formation of 

laumontite. Even though the temperature and pressure are 

suitable, if the pore solutions are saline c~ 3 percent) or 

contain high levels of dissolved carbonate species, carbonate 

minerals and kaolinitic clays will form instead of laumontite 

(McCulloh and Stewart, 1979); as a result, reservoir properties will not be so seriously reduced. Possibly this situation 

may apply to the lower Cook Inlet basin. During the laumontite-forming hydrothermal episode, lateral or vertical variations in the chemistry of the fluids in the rock pores may have 

inhibited the crystallization of laumontite in parts of the lower 

Cook Inlet basin. 

Sandstone Diagenesis 53 

Plutonism and Provenance-Implications 

for Sandstone Compositions 

By Travis Hudsons 

INTRODUCTION 

In order to predict the kinds of sedimentary sequences 

in a well, the type of tectonic setting in the surrounding area 

must be identified first. 

The provenance history of the forearc basin that 

includes Cook Inlet is revealed in the temporal, petrologic, 

and regional relations of southern Alaska plutonic rocks. The 

plutonic history indicates that provenance evolved from simple magmatic arc sequences in the early Mesozoic to a 

complicated composite of magmatic arc, recycled orogen, 

and continental block provenances in the late Cenozoic. 

Because three major provenance types are present, a complete 

range of sandstone framework modes is to be expected. 

Generally, Jurassic plutonism produced rocks with mafic to 

intermediate compositions, Cretaceous plutonism produced 

mostly intermediate compositions, and Tertiary plutonism 

produced felsic compositions. During provenance evolution, 

the amounts of metamorphic and plutonic rocks increased 

proportionally and the compositions of the younger plutons 

became more felsic, so the younger sandstones of the Cook 

Inlet basin are most likely to have quartzose framework 

modes. 

For the purpose of this report, the provenance of the 

forearc basin is considered to be that part of southern Alaska 

south of, and including, the Aleutian-Alaska Range. This 

simplifying assumption focuses the discussion on the region 

proximal to the basin although it does not imply that other 

parts of Alaska could not have also been part of the provenance. 

SOUTHERN ALASKA PLUTONISM 

Plutonic rocks are very extensive in southern Alaska; all 

larger outcrops are shown in figure 27. The distribution, age, 

and setting of these rocks define several regional plutonic 

belts (Reed and Lanphere, 1973b; Hudson, 1979a), and these 

belts have important implications for southern Alaskan tectonic history (Hudson, 1979b,c; Hudson and others, 1979). 

Southern Alaska plutons (fig. 27) can be subdivided into the 

following groups or episodes of plutonic activity based on 

their age and geologic setting: 

1. Late Triassic(?) and Early Jurassic (ca. 195 to 185 

Ma) quartz diorite and tonalite in elongate deformed plutons 

5 ARCO Exploration Technology. 

are discontinuously exposed on northwestern Kodiak Island, 

the Barren Islands, and southern Kenai Peninsula. Some 

plutons intrude Upper Triassic sedimentary rocks that 

include volcanogenic sandstone and tuff. Because the K-Ar 

ages (about 193 Ma) on these plutonic rocks are all single 

mineral (hornblende) minimum ages, some could be slightly 

older. 

2. Early Jurassic(?) gabbro, quartz diorite, and tonalite 

make up elongate and deformed plutons of the northern 

Chugach Mountains; the easternmost plutonic complex in 

this group (fig. 27) is mostly gabbro. The age of these poorly 

known plutons is uncertain. Because they share certain 

petrologic characteristics and a similar geologic setting with 

plutonic rocks on western Kodiak Island they may be Early 

Jurassic in age. 

3. Middle and Late Jurassic (ca. 175 to 145 Ma) 

gabbro to granite but mostly quartz diorite, tonalite (including trondhjemite) and granodiorite make up large elongate 

batholithic complexes in the Aleutian Range and the Talkeetna Mountains. The batholiths locally intrude, and are 

regionally in spatial association with, Early Jurassic volcanogenic rocks of the Talkeetna Formation. 

4. Late Jurassic (ca. 160 to 140 Ma) quartz diorite, 

monzodiorite, tonalite, and granodiorite are present in a large 

batholithic complex and some smaller plutons of the Chitina 

Valley area (Chitina Valley batholith of MacKevett, 1978). 

The plutons are known to have correlatives to the south east, 

and they apparently overlap in age and share some petrologic 

characteristics with Middle and Late Jurassic plutons to the 

west. Unlike Jurassic plutons to the west, however, spatially 

associated volcanogenic rocks have not been identified with 

plutons of the Chitina Valley area. 

5. Middle Cretaceous (ca. 120 to 105 Ma) diorite to 

granite is present in shallow-seated small to moderate-sized 

plutons in the Nutzotin Mountains-correlative plutons have 

been identified to the southeast but not to the west. These 

plutons have local areas of extensive hydrothermal alteration 

and the volcanogenic rocks of the Lower Cretaceous Chisana 

Formation (Richter, 1976) are probably related to them. 

6. Late Cretaceous and early Tertiary (ca. 7 5 to 50 Ma) 

gabbro to granite form many large plutons and coalescing 

batholithic complexes that make up a major part of the 

plutonic rocks in the Alaska Range and Talkeetna Mountains. 

Most of the more mafic rocks are ~62 Ma old and tend to be 

located in the southern and eastern plutons of the group. 

Plutonism and Provenance-Implications for Sandstone Compositions 55 
RANGE t) 

.& Active calc-alkaline volcano 

8 ~ Middle Tertiary (ca 40-25 Ma) 

7 - Early Tertiary (ca 60-50 Ma) 

6 ~ Late Cretaceous and early Tertiary (ca 75-50 Ma 

5 ~ Middle Cretaceous (ca I 20-1 05 Ma) 

4 liiiiiiiiJIII] Late Jurassic (ca 160-140 Ma) 

3 ;R Middle and Late Jurassic (ca I 75-145 Ma) 

2 !:~:·:{:"..~::,:] Early Jurassic(?) 

1 [t'i!'!ii!i!!i!!i!i!::::::::::::j Late Triassic(?) and Early Jurassic 

(ca I 95- I 85 Ma) 

0 50 100 MILES 

0 50 100 KILOMETERS 

I I I 

Figure 27. Principal groups of plutonic rocks in southern Alaska subdivided on the basis of their age and geologic setting. 

Subdivisions are keyed to the text and to figure 28 by age and number. A summary of the field petrology and age relations of 

these groups, based on the available literature, is presented in Hudson (1979a). 

Related volcanic rocks probably developed although they 

have not been identified in many areas. 

7. Early Tertiary (ca. 60 to 50 Ma) biotite tonalite, 

granodiorite, and granite in small to large plutons, are distributed along the Gulf of Alaska margin from Sanak Island 

on the west to Baranof Island on the east. These predominantly epizonal plutons are mostly granodiorite and granite; 

related volcanic rocks are not present. They are believed to 

have formed by melting of accreted terranes-an origin different from that for most other plutons in southern Alaska 

(Hudson and others, 1979). 

56 Geologic Studies of the Lower Cook Inlet COST No. 1 Well 

8. Middle Tertiary (ca. 40 to 25 Ma) quartz diorite, 

tonalite, granodiorite, and granite, in small to large plutons, 

are scattered through the southern and central Alaska Range. 

Plutons of similar age are present on the Alaska Peninsula and 

possibly in eastern parts of southern Alaska. The plutons are 

commonly shallow-seated and in spatial association with 

subarea! volcanic rocks. 

The ternary diagrams in figure 28 summarize the 

petrologic variation in the groups of plutons outlined above. 

The diagrams show a general shift toward more felsic compositions from the older to the younger plutonic groups. The 
Q 

Tonalite 

Quartz diorite 

'L=!=-==:t=====~t=::t~p Diorite/gabbro 

Quartz monzodiorite 

Q Q 

8. Middle Tertiary 

(ca 40-25 Ma) 

7. Early Tertiary 

(ca 60-50 Ma) 

6. Late Cretaceous and early Tertiary 

(ca 75-50 Ma) 

Q 

5. Middle Cretaceous 

(ca 120-1 05 Ma) 

K 10 

Q 

35 65 

Q 

4. Late Jurassic 

(ca I 60- I 40 Ma) 

Q 

Q 

3. Middle and Late Jurassic 

(ca I 75- 145 Ma)' 

2. Early Jurassic(?) I. Late Triassic(?) and Early Jurassic 

(ca I 95- I 85 Ma. possibly older) 

Figure 28. Ternary diagrams showing quartz (Q), K-feldspar 

(K), and plagioclase (P) ratios for groups of plutonic rocks 

shown in figure 27. Most of the granite in group 6 is early 

Tertiary in age. Group 2 includes a large proportion of 

gabbro. The classification scheme follows that suggested 

by the JUGS Subcommission on the Systematics of Igneous 

oldest rocks (Early Jurassic as presently known) make up a 

comparatively primitive suite with little or no K-feldspar. The 

voluminous Middle or Late Jurassic plutons display a shift to 

more intermediate compositions with development of quartz 

monzodiorite, granodiorite, and some large trondhjemite plutons. Petrologic variation becomes well-developed in the 

Cretaceous plutons and, although most of these are still of 

Rocks (Geotimes, 1973). Data for groups 1, 2, and 4 from 

Travis Hudson and j.G. Arth (unpub. data, 1980), group 3 

from Reed and Lanphere (1973b), group 5 from Richter and 

others (1975), group 6 from Reed and Lanphere (1973b) and 

Reed and Nelson (1977), group 7 from Hudson and others 

(1979), and group 8 from Reed and Lanphere (1973b). 

intermediate calc-alkaline character, granodiorite and granite 

are important parts of them. The Tertiary plutons, including 

the early Tertiary ones of group 6 (figs. 27 and 28), are 

dominantly granodiorite and granite. Thus, in a general 

sense, Jurassic plutonism produced mafic to intermediate 

compositions, Cretaceous plutonism produced mostly intermediate compositions, and Tertiary plutonism produced felPlutonism and Provenance-Implications for Sandstone Compositions 57 
sic compositions. These shifts in petrologic character, 

combined with specific considerations of the timing, extent, 

and impact of volcanic and metamorphic processes related to 

plutonism, indicate some general evolutionary changes in 

regional provenance during development of the Cook Inlet 

basin. 

PROVENANCE EVOLUTION 

There are two types of provenance evolution indicated 

by the plutonic history of southern Alaska. The first type 

includes those comparatively short-term changes that accompanied emplacement of a particular group of plutons. The 

second type includes those more gradual and longer term 

changes that took place as repeated episodes of plutonism 

affected the region. 

Short- and Long-term Changes 

Most of the individual groups of plutons identified in 

figure 27 and outlined above are believed to have formed as 

part of magmatic arcs that developed during episodes of plate 

convergence along the continental margin. Such plate convergent episodes generally modify the continental margin 

through sequential or overlapping cycles of tectonism, magmatism, and sedimentation that are marked by initial and 

ongoing volcanism, development of comagmatic and indirectly-related plutonism, and they culminate with uplift and 

erosion of the magmatic arc. Metamorphic and hydrothermal 

processes, accompanying regional tectonism and magma 

emplacement, recrystallize and mobilize large volumes of 

provenance material throughout this cycle. The plutonic history of southern Alaska indicates that such cycles were relatively short-term and affected the provenance marginal to the 

Cook Inlet forearc basin during at least four separate episodes: Late Triassic(?) and Early Jurassic (1, 2); Middle and 

Late Jurassic (3, 4); Late Cretaceous and early Tertiary (6, 7); 

and middle Tertiary (8). A fifth episode, ongoing today, is 

evidenced by widespread late Cenozoic volcanic rocks and 

the many active calc-alkaline volcanoes of the region 

(fig. 27). 

The repetition of magmatic arc cycles and the shifting 

petrologic nature of plutonism combine to produce some 

longer-term changes in provenance character during development of the Cook Inlet forearc basin. The Jurassic provenance 

may have included the primitive magmatic (probably island) 

arc sequence (group 1, figs. 27 and 28) represented by the 

Late Triassic volcanogenic rocks and the Early Jurassic plutonic rocks of Kodiak Island, the Barren Islands, and Kenai 

Peninsula, but it was dominated by the magmatic arc rocks of 

the Aleutian Range and Talkeetna Mountains (group 3 ). PreJurassic metasedimentary and metavolcanic rocks are locally 

a part of this arc but until the Late Cretaceous, the provenance 

was mostly volcanic and plutonic rocks of mafic to intermedi58 Geologic Studies of the Lower Cook Inlet COST No. 1 Well 

ate composition (fig. 28). The volume of plutonic rocks in 

southern Alaska virtually doubled in the Late Cretaceous and 

early Tertiary (groups 6 and 7). This increase, combined with 

widespread thermal and dynamothermal metamorphism, led 

to a composite provenance that, for the first time in the history 

of the forearc basin, included a high proportion of crystalline 

rocks. The Late Cretaceous plutonic rocks in part contained 

more quartz and K-feldspar than their predecessors, but it was 

during the Tertiary (groups 6, 7, and 8) that a large amount of 

felsic plutonic rocks were formed; the proportion of plutonic 

rocks increased by about a third in the Tertiary and these 

rocks were dominantly felsic in character. Through the Tertiary, the composite aggregate of diverse Mesozoic igneous, 

metamorphic, and sedimentary rocks that make up the bulk 

of the provenance became thoroughly welded by plutonism 

and metamorphism into what is now a relatively youthful but 

truly continentalized crust. 

Therefore, provenance evolved over the long term from 

simple magmatic arc sequences in the early Mesozoic to a 

composite of regional sedimentary, metamorphic, and magmatic arc sequences in the late Mesozoic and Cenozoic. In 

effect, each succeeding magmatic arc increased the proportion of crystalline rocks in the provenance. It is against this 

background of long-term evolution that shorter-term changes, 

accompanying development of individual magmatic arcs, 

need to be considered. Consideration of the long- and shortterm provenance changes together defines a history that 

should be reflected in the composition of Mesozoic and 

Cenozoic sandstones of the Cook Inlet forearc basin. 

Implications for Sandstone Compositions 

The influence of provenance on model framework compositions of sandstone has been summarized recently by 

Dickinson and Suczek (1979), who identified framework 

modes that have formed from three provenance types-magmatic arc, recycled orogen, and continental block provenances (fig. 29). 

The Mesozoic and Cenozoic evolution of southern 

Alaska provenance, as indicated by the plutonic history of the 

region, involves the complicated interplay of all three provenance types. Even so, the following general implications of 

provenance evolution on sandstone composition in the Cook 

Inlet forearc basin can be made: (1) Late Triassic, Jurassic, 

and Early Cretaceous sandstones should reflect simple magmatic arc provenance and show Q-F-L modes changing from 

lithic to feldspathic compositions (fig. 29). Such changes 

have been identified qualitatively by stratigraphic studies 

(Burk, 1965; Detterman and others, 1965; Grantz and others, 

1963) and in some cases quantitatively by sandstone modal 

studies (Egbert, this volume; McLean, 1979). (2) Late Cretaceous sandstones should reflect a composite provenance 

that includes magmatic arc and recycled orogen types. The 

continued dissection of Jurassic arcs would have helped 
F 

F 

CONTINENTAL 

BLOCK 

PROVENANCE 

~ 

Q 

A 

Q 

B 

MAGMATIC ARC 

PROVENANCE 

L 

SOUTHERN ALASKA 

J/ARC BASIN 

Older 

sandstone 

L 

Figure 29. Relation of provenance on modal framework 

composition of sandstone. A, Ternary Q-F-L diagram 

showing framework mode fields from sandstones derived 

from different types of provenance (from Dickinson and 

Suczek, 1979, fig.1). 8, Q-F-L diagram showing expected 

framework mode field for sandstones in the forearc basin 

of southern Alaska. Modal trend, from older to younger 

sandstones, is the expected general trend for the forearc 

basin that is suggested by provenance evolution. 

produce quartzofeldspathic sandstone compositions but a 

new magmatic arc cycle should be evidenced in the Late 

Cretaceous with at least local development of volcanolithic 

sandstones (Magoon and others, 1980). (3) Tertiary sandstones should reflect an even more complicated composite 

provenance that includes all three types. The sandstones 

should be diverse and range from lithic to quartzose types. 

Volcanic, metamorphic, and sedimentary rock fragments 

could each dominate the lithic component in particular cases. 

Feldspar, commonly K-feldspar as well as plagioclase and 

quartz, could dominate framework modes of sandstones 

derived directly from plutonic terranes. In a very general 

sense, the Tertiary sandstones (assuming a derivation from a 

southern Alaska provenance) are the most likely to contain a 

significant K-feldspar and quartz component. 

Figure 30 schematically illustrates the generalized 

framework grain ratios that are predicted by provenance 

evolution. The trend for the lithic volcanic (Lv) to total 

feldspar (F) ratio shows the initiation of each magmatic arc as 

distinct short-term shifts to higher ratios. These shifts to 

higher ratios are superimposed on a general trend of decreasing Lv to F ratios that reflect the predicted longer-term 

changes in provenance. The longer term changes should be 

more clearly shown by the ratio of total quartzose grains (Q) 

to total feldspar (F). The Q/F ratio should increase gradually 

while the dissected Jurassic arc is the principal provenance, 

then should increase more by the Late Cretaceous, and 

increase sharply through the Tertiary. The ratio of K-feldspar 

(K) to plagioclase (P) in the framework modes is the ratio that 

is most likely to reflect the development of felsic rocks in the 

provenance. This ratio is expected to be low in the Early to 

Middle Jurassic, increase slightly to a uniform volume in the 

Early and middle Cretaceous, and increase more rapidly by 

the Late Cretaceous. Like Q/F, KIP probably increased 

sharply through the Tertiary. 

t 

(/) 

0 

f= 

.q: 

0:: 

(!) 

z 

(ij 

<t 

L&.J 

0::: 

() 

~ ~K/~P--------------------

200 150 100 50 

MILLIONS OF YEARS BEFORE PRESENT 

0 

Figure 30. Schematic diagram showing possible evolutionary trends of average framework grain ratios in sandstones of the Cook Inlet forearc basin: lithic volcanic to 

total feldspar(Lv/F),total quartzose grains to total feldspar 

( Q/F), and K-feldspar to plagioclase (KIP) (symbology after 

Dickinson and Suczek, 1979). Models are based on the 

assumption of continuous erosion of source terranes and 

sedimentation in the basin. 

Plutonism and Provenance-Implications for Sandstone Compositions 59 
CONCLUSION 

The Mesozoic and Cenozoic plutonic history of southem Alaska provides insight into the provenance evolution for 

the forearc basin that includes Cook Inlet. This plutonic 

history indicates that provenance evolved from simple magmatic arc sequences in the Early Mesozoic to a complicated 

composite that includes magmatic arc, recycled orogen, and 

continental block provenance types in the late Cenozoic. 

Provenance with continental block characteristics is not 

widespread adjacent to Cook Inlet; it is mostly represented by 

those areas dominated by felsic plutonic rocks-these types 

of plutonic rocks were primarily emplaced in southern Alaska 

during the Tertiary. 

60 Geologic Studies of the Lower Cook Inlet COST No. 1 Well 

Sandstone compositions in the forearc basin can be 

expected to reflect the changes in provenance and to show an 

increasing diversity from the early Mesozoic to late 

Cenozoic. Because all three major provenance types (fig. 

29A) have developed in southern Alaska, a complete range of 

sandstone framework modes is to be expected (fig. 29B). 

However, the increasing proportion of metamorphic and plutonic rocks during provenance evolution combined with a 

shift to more felsic compositions in younger plutons suggests 

that younger sandstones of the Cook Inlet basin are the most 

likely to have quartzose framework modes. Although younger 

sandstones are expected to show an increasing diversity of 

composition, the general evolutionary trends shown in figure 

29B are probably developed in the forearc basin. 
Petrography, Provenance, and Tectonic 

Significance of Middle and Upper Jurassic 

Sandstone from Tuxedni Bay 

By Robert M. Egbert6 

INTRODUCTION 

Modal analyses of 88 thin sections from Middle and 

Upper Jurassic sandstone in the Tuxedni Bay area, western 

~ook Inlet, show systematic changes in framework composition, hornblende content, and type of cement with stratigraphic position. The changes appear to reflect uplift and 

unroofing of the Alaska-Aleutian Range batholith in Jurassic 

time. Compaction and cementation, which have occluded all 

visible porosity in the sandstone, should be considered in 

evaluating the hydrocarbon-reservoir potential of Middle and 

Upper Jurassic sandstone in Cook Inlet. 

The Middle and Upper Jurassic sequence in Tuxedni 

Bay is over 3,000 m (10,000 ft) thick, and includes the 

Middle Jurassic Tuxedni Group and Chinitna Formation and 

Upper Jurassic Naknek Formation (Detterman and Hartsock 

1966; Imlay and Detterman, 1973). I examined 21 thin sec~ tions from the Tuxedni Group, 23 thin sections from the 

Chinitna Formation (11 from the Tonnie Siltstone Member 

and 12 from the Paveloff Siltstone Member), and 44 thin 

sections from the Naknek Formation. 

The sandstones were analyzed according to a modified 

classification system based on Crook (1960) and Dickinson 

(1970). An average of 4 70 points were counted for each thin 

section in order to obtain a minimum count of 300 framework 

grains <q = quartz, F = feldspar, L = lithic fragments); 

these grams were used to determine framework composition 

(fig. 31). The Q-F-L populations were normalized to 100 

percent. The remaining points counted were cements and 

accessory minerals that are shown on figure 31 as percentages 

of total constituents, including quartz, feldspar, and lithic 

fragments. In addition, secondary parameters in the form of 

r~tios, C/Q (polycrystalline quartz to total quartz), P/F (plagiOclase to total feldspar), and V/L (volcanic lithic to total 

lithic frag~ents) were calculated to aid in differentiating 

sandstone m the three stratigraphic units. 

RESULTS 

The average framework compositions (Q-F-L) change 

systematically up section from the Tuxedni Group through 

6SOHIO Petroleum Company. 

the Chinitna Formation and into the Naknek Formation 

becoming progressively more feldspathic and less lithic. Th; 

quartz content remains nearly constant. 

Quartz 

All the sandstones studied contain less than 10 percent 

quartz, most of which is monocrystalline. However, in samples from the Tuxedni Group, quartz has features indicative of 

volcanic derivation (clear, unstrained, resorbed margins, or 

bleb inclusions), but the quartz in samples from the Chinitna 

and Naknek Formations has features indicative of plutonic or 

metamorphic derivation. Poly crystalline quartz, although not 

abundant, systematically decreases upward through the section from the Tuxedni Group to the Naknek Formation (see 

C/Q ratios, fig. 31). Most of the polycrystalline quartzose 

fragments probably represent silicified volcanic detritus. 

Feldspar 

Feldspars are the most abundant mineral grains in the 

sandstone and increase in abundance from an average of 43 

percent of the framework grains in the Tuxedni Group to 73 

percent in the Naknek Formation. Plagioclase, the dominant 

feldspar, averages greater than 90 percent of the feldspar 

population throughout the stratigraphic section (see P/F 

ratios, fig. 31 ). The plagioclase is highly altered and is mostly 

albitized and(or) replaced by laumontite. 

Lithic 

The lithic fragment populations decrease up section as 

the feldspar populations increase. Lithic fragments average 

51 percent in the Tuxedni Group, 35 percent in the Chinitna 

Formation ( 43 percent in the Tonnie Siltstone Member and 29 

percent in the Paveloff Siltstone Member), and 21 percent in 

the Naknek Formation. Most of the lithic fragments are 

volcanic grains of intermediate composition. V /L ratios 

decrease slightly up section; this decrease can be related to 

the appearance of plutonic rock fragments in the upper part of 

the Chinitna Formation and in the Naknek Formation. 

Petrography, Provenance, and Tectonic Significance of Jurassic Sandstone 61 
Q 

t 

F L F L 

Q Compositon Naknek Formation 

(N 44) 

Q 6 

F 73 

L 20 

C/Q .06 

P/F .96 

V/L .84 

Hornblende (%) 11 

Chlorite (%) 4 

Laumontite F L 

(%) 10 

F 

TUXEDNI 

GROUP 

Q 

t 

Chinitna Formation 

Paveloff Tonnie 

Sts. Mbr. Sts. Mbr. 

(N 12) (N 11) 

5 4 

66 53 

29 43 

.11 .12 

.96 .96 

.93 .90 

7 0 

11 12 

5 

L 

Tuxedni Grou,E 

(N 21) 

6 

43 

51 

.27 

.92 

.96 

0 

17 

2 

Figure 31. Q-F-l ternary diagrams and compositional data of sandstone from Tuxedni Bay. Sandstone classification after 

Crook (1960) and Dickinson (1970). N, number of thin sections; Q, quartz; F, feldspar; and l, lithic fragments; C, 

polycrystalline quartz; P, plagioclase; V, volcanic lithic fragments. 

Accessory Minerals 

Accessory minerals include: hornblende, epidote, biotite, and opaque minerals. Hornblende, which is absent or 

present in only trace amounts in samples from the Tuxedni 

Group and Tonnie Siltstone Member of the Chinitna Formation, is the dominant accessory mineral in the Paveloff Siltstone Member of the Chinitna Formation and in the Naknek 

Formation, averaging 7 and 11 percent respectively. 

lntergranular Spaces 

Three types of cement, chlorite, laumontite, and calcite, 

fill intergranular spaces in the sandstones. Chlorite is ubiquitous in the Tuxedni Group and in the lower part of the 

Chinitna Formation, apparently forming from the chemical 

breakdown of the abundant volcanic rock fragments. Laumontite is a common cement in the upper part of the Chinitna 

Formation and is the dominant cement in the Naknek Formation; laumontite probably formed in response to chemical 

alteration of plagioclase that is abundant in these units. Sampling was biased towards noncalcareous sandstone in order to 

obtain accurate modal analyses because calcite can replace 

62 Geologic Studies of the Lower Cook Inlet COST No. 1 Well 

framework grains. However, though calcite-cemented sandstone was found in all units, it was not common. No porosity 

was observed in any of the sandstone. Initial pore space has 

been totally occluded by compaction and deformation of 

volcanic lithic grains, and by chlorite, laumontite, and calcite 

cements. 

DISCUSSION 

Sandstone of the Tuxedni Group was derived largely 

from a volcanic terrane as indicated by the high percentages 

of volcanic lithic grains and volcanic quartz. Several important changes in sandstone composition occur between the two 

members of the Chinitna Formation. The Tonnie Siltstone 

Member is volcanic-lithic rich, is virtually devoid of 

hornblende, and has chlorite as the dominant cement, 

whereas the overlying Paveloff Siltstone Member is much 

more feldspathic (less lithic rich), contains an average of 7 

percent hornblende, and has laumontite as a common cement 

in addition to chlorite (fig. 31 ). These differences indicate a 

change from a dominantly volcanic source for the Tonnie 

Siltstone Member to a mixed volcanic/plutonic provenance 
for the Pavel off Siltstone Member. Sandstone of the Naknek 

Formation was derived mainly from a plutonic source, as 

indicated by the high percentages of feldspar and hornblende. 

The composition of sandstone from the Tuxedni Group 

and Chinitna and Naknek Formations is directly related to 

Jurassic tectonic events in south-central Alaska. During the 

Early Jurassic, 1,800 to 2,700 m (6,000-9,000 ft) of volcanic 

and volcaniclastic rocks (Talkeetna Formation) accumulated 

in the area of the present-day Alaska-Aleutian Range forming 

a volcanic arc at least 800 km (500 mi) long (Detterman and 

Hartsock, 1966; Reed and Lanphere, 1973a). During the 

Middle to Late Jurassic, gabbro to granite but mostly quartz 

diorite, and tonalite to granodiorite of the Alaska-Aleutian 

Range batholith intruded the Early Jurassic volcanic arch 

sequence and older rocks (Detterman and others, 1965; Hudson, this volume). Uplift and erosion of the ancestral Aleutian 

Range in the Middle Jurassic shed volcanic detritus into the 

adjacent Cook Inlet forearc basin-the deposits represented 

by the Tuxedni Group. Continued uplift and erosion of the 

Aleutian Range in the late Middle Jurassic partially unroofed 

the Alaska-Aleutian Range batholith, and both volcanic and 

plutonic detritus was being deposited in the adjacent basin; 

these deposits are represented by the Chinitna Formation. By 

Late Jurassic time, when rocks ofthe Naknek Formation were 

being deposited, the Alaska-Aleutian Range batholith was 

mostly unroofed, thereby supplying dominantly plutonic 

detritus to the Cook Inlet basin. These conclusions are consistent with previous detailed work by Detterman and Reed 

(1980). They base their conclusions mainly on comparison of 

age and composition of conglomerate clasts in the Naknek 

Formation to underlying units. I suggest, however, that the 

Alaska-Aleutian Range batholith was unroofed earlier-during the deposition of the Paveloff Siltstone Member of the 

Chinitna Formation. 

Petrography, Provenance, and Tectonic Significance of Jurassic Sandstone 63 

Framework Geology and Sandstone 

Composition 

By Leslie B. Magoon and Robert M. Egbert7 

INTRODUCTION 

. The purpose of this paper is to review the stratigraphic 

history and tectonic evolution of the Cook Inlet forearc basin 

in order to understand the evolution of the sandstone composition so that potential hydrocarbon reservoirs can be predicted. 

The tectonic evolution of the Cook Inlet -Shelikof Strait 

area and adjacent areas is complex because the areas are a part 

of the northern Pacific margin that has been the site of 

continuous convergence throughout the Mesozoic and 

Cenozoic eras. The Cook Inlet forearc basin is bounded on 

the northwest by the Alaska-Aleutian Range and the Talkeetna Mountains and on the southeast by Kodiak Island and 

a~jacent islands, the Barren Islands, and the Chugach Mountams. The main discussion will focus on the rock units and 

tectonic elements of the Cook Inlet-Shelikof Strait area 

which includes the Alaska Peninsula from Puale Bay north: 

east to the Matanuska Valley (fig. 32). If pertinent, rock units 

and tectonic elements adjacent to this area also will be 

discussed. 

The stratigraphic and structural history of each rock 

unit will be summarized; all the available information on 

s_andstone composition will be summarized; and the implications of sandstone composition and provenance in relation to 

the tectonic evolution and development of a hydrocarbon 

~servoir will be discussed. The parallel change in composit10n of the plutonic bodies of southern Alaska (Hudson, this 

volume) to that of the sandstones of the Cook Inlet forearc 

basin, and in some instances, to that of the sandstones of the 

accreted terranes, will be demonstrated. 

The stratigraphic history will include discussions on: 

the early Mesozoic geologic framework of the Cook Inlet 

forearc basin northwest of the Border Ranges fault, the Mesozoic and Cenozoic accreted terranes southeast of the Border 

Ranges fault, and the Mesozoic Peninsular terrane within the 

Cook Inlet forearc basin that overlaps the Border Ranges 

fault. Information from the lower Cook Inlet COST No. 1 

well, as well as other wells, measured sections, and plutonic 

bodies, is incorporated in this discussion. 

The compositions of the sandstones in the Cook Inlet 

forearc basin have an important bearing on the development 

7 SOHIO Petroleum Company. 

of hydrocarbon reservoirs in the Cook Inlet-Shelikof Strait 

area. New data and data from many other sources, modal 

analyses, and petrographic descriptions of approximately 

700 sandstone samples from the Cook Inlet-Shelikof Strait 

and adjacent areas, are included here. The petrographic data 

are summarized in figure 33 and in tables 16 and 17. 

Dickinson's (1970) classification is used for the sandstone, and his classification with modifications from Crook 

(1960) is used for subquartzose sandstone. If the published 

data did not conform to the conventions of Dickinson (1970), 

the data was recalculated and this is noted in tables 16 and 17. 

The sandstones are classified using three primary grain 

parameters: Q (quartz), F (feldspar), and L (lithic fragments). 

To further differentiate the sandstones, secondary parameters 

in the form of ratios, C/Q (polycrystalline quartz to total 

quartz), P/F (plagioclase to total feldspar), and V/L (volcanic 

lithic to total unstable lithic fragments) are listed in tables 16 

and 17 and discussed in the text. New point-count data and 

the published point -count data of Egbert and Magoon (1981) 

were made by counting at least 300 framework grains (quartz, 

feldspar, and lithic fragments) following the method of 

Dickinson (1970). All the sandstone from the Cook InletShelikof Strait and adjacent areas is subquartzose (Q<75) 

and is classified using the following system: feldspathic (F/ 

L>3), lithofeldspathic (3>F/L> 1), feldspatholithic (3>L/ 

F> 1), and lithic (L/F>3). 

By applying the techniques of Dickinson and Suczek 

(1979), the compositions of the sandstones in the forearc 

basin are related to those of the plutonic bodies and of the 

accreted terranes, thus two provenance terranes, a magmatic 

arc and a recycled orogen, are identified in the Cook InletShelikof Strait area from the Q-F-L (quartz-feldspar-lithic) 

ternary diagrams. 

GEOLOGIC FRAMEWORK 

Rocks of the Cook Inlet -Shelikof Strait are part of a belt 

of Mesozoic and Cenozoic sedimentary rocks that extend 

northeastward to the Matanuska Valley and southwestward 

along the Alaska Peninsula and Shelikof Strait (figs. 32, 34, 

35). Four major northeast -trending geologic features flank the 

Cook Inlet-Shelikof Strait area; the Alaska-Aleutian Range 

batholith and the Bruin Bay and Castle Mountain faults are on 

Framework Geology and Sandstone Composition 65 
0 

0 50 

I KATMAI 

50 100 MILES 

100 KILOMETERS 

NATIONAL 

Redoubt 

Volcano A 

Iliamna 

Volcano 

I MONUMENT 

~ 

Capps Glacier 

)( 

J!J,: ,, BARREN ISLANDS 

Talkeetna 

Mountains 

Figure 32. Geographic names and geomorphic features in Cook Inlet, Shelikof Strait, and adjacent areas. The COST No.1 

well is located in lower Cook Inlet. 

66 Geologic Studies of the Lower Cook Inlet COST No. 1 Well 
the northwest side of the Cook Inlet-Shelikof Strait area, and 

the Border Ranges fault and the accreted terranes are on the 

southeast side. The Mesozoic rocks of the Cook InletShelikof Strait area may be more than 10,750 m (35,250 ft) 

thick; the continental Cenozoic rocks are as much as 7,000 m 

(25 ,000 ft) thick (figs. 37, 38). 

The Mesozoic sequence of the Cook Inlet-Shelikof 

Strait area, together with the Alaska-Aleutian Range 

batholith, constitute the Peninsular terrane of Jones and Silberling (1979) (fig. 36). The Cenozoic rocks that overlie the 

Peninsular terrane are not designated a terrane name. On the 

southeast side of the Border Ranges fault, the tectonic terranes comprise the Kachemak (Triassic rocks), Chugach, and 

Prince William terranes (Jones and Silberling, 1979). 

Alaska-Aleutian Range Batholith 

The Alaska-Aleutian Range batholith, exposed on the 

northwest side of the region, is composed of a series of 

elongate plutonic belts that extend from the Becharof Lake 

area on the Alaska Peninsula northeast to the Talkeetna 

Mountains (fig. 34 ). These calc-alkaline plutonic belts Hudson (1979a,c) has designated the Aleutian Range-Talkeetna 

Mountains belt (ca. 175-145 Ma), the Alaska Range-Talkeetna Mountains belt (ca. 75-50 Ma), and the Alaska Range 

belt (ca. 40-25 Ma). The ages of the plutonic belts in figure 30 

are those of Hudson (this volume). Three intrusive episodes 

are indicated by concordant mineral ages on hornblende and 

biotite pairs: (1) Middle and Late Jurassic plutons emplaced 

between 176 and 154 Ma, (2) Late Cretaceous and early 

Tertiary plutons emplaced between 83 and 58 Ma, and (3) 

middle Tertiary plutons emplaced between 38 and 26 Ma 

(Reed and Lanphere, 1969, 1972, 1973a,b; Lanphere and 

Reed, 1973). 

Bruin Bay and Castle Mountain Faults 

The Bruin Bay fault (fig. 34), a high-angle reverse fault, 

can be traced along the northwest side of the Alaska Peninsula and Cook Inlet basin for 500 km (300 mi). The fault plane 

dips 60° northwest in Kamishak Bay. Just north of Chinitna 

Bay, stratigraphic throw across the fault is as much as 3 km 

(1.9 mi); and here beds on the west, granitic rocks and rocks 

as old as Early Jurassic are juxtaposed against sedimentary 

strata as young as Middle Jurassic (Detterman and Hartsock, 

1966). 

The Castle Mountain fault, an extension of the Lake 

Clark fault, can be traced for over 200 km (125 mi), from the 

north end of the Bruin Bay fault, northeast through the Susitna River Valley to the southeast flank of the Talkeetna 

Mountains. The fault apparently has large right-lateral strikeslip displacement. The Aleutian Range-Talkeetna Mountains 

belt has been displaced by about 150 km (95 mi). 

Vertical offset has occurred along both the Bruin Bay 

and Castle Mountain faults; the Cook Inlet-Shelikof Strait 

block dropped relative to the northwest block. 

Border Ranges Fault 

The Border Ranges fault, a major tectonic feature in 

southern Alaska, extends from southeastern Alaska (MacKevett and Plafker, 1974), throughout southern Alaska 

(Plafker and others, 1977; Fisher, oral commun., 1980), at the 

surface near Anchorage, Seldovia, and on the northwest side 

of Kodiak Island and adjacent islands. The part of the Border 

Ranges fault near Anchorage is designated the Knik fault 

zone by Clark (1972); the fault near Seldovia is designated the 

Seldovia fault by Fisher and Magoon (1978). The Seldovia 

fault is interpreted to extend northeast through Homer, 

beneath the Kenai lowland to connect with the Knik fault, and 

southwest through the Barren Islands to Kodiak Island and 

adjacent islands. The fault is overlapped at the surface by 

Tertiary (Neogene) sedimentary rocks in the Kenai lowland 

northeast toward Anchorage. However, in the areas of 

Matanuska Valley, Seldovia, Kodiak Island and adjacent 

islands, the Border Ranges fault separates the moderately 

deformed Mesozoic sequence, Peninsular terrane of Cook 

Inlet-Shelikof Strait on the northwest, from the severely 

deformed rocks, Chugach and Prince William terranes, on 

the southeast. 

Kodiak-Seldovia Schist Terrane 

The schist terrane, or glaucophane-bearing rock 

assemblage, was first recognized by Martin and others (1915) 

on the Kenai Peninsula and was described and dated in more 

detail by Forbes and Lanphere (1973). Later it was designated 

the Kodiak-Seldovia schist terrane by Moore and Connelly 

(1979). Similar schist terranes have been identified on Kodiak 

Island and adjacent islands (Carden and others, 1977), east of 

Anchorage (Clark, 1972), and in the northern Chugach 

Mountains (Winkler and others, 1980). 

The terrane contains a variety of metamorphic rocks, 

marble, metachert, blueschist, and greenschist, in addition to 

quartz-mica schist. The original sediments of this terrane, 

marble and metachert, should predate the K-Ar ages that 

range from 192 to 181 Ma (Forbes and Lanphere, 1973) or 

Early Jurassic time; in fact, Permian fusilinids were identified 

from limestone east of Anchorage (Clark, 1972). Although 

the blueschist mineralogy suggests a low-temperature, highpressure metamorphic event, the greenschist mineralogies 

suggest that a higher temperature event formed the schist 

(Forbes and Lanphere, 1973; Carden and others, 1977). 

On the Kenai Peninsula, the schist terrane is separated 

from the Mesozoic shelf deposits by the Port Graham fault on 

the northwest (Carden and others, 1977) and is separated 

Framework Geology and Sandstone Composition 67 
West .,.4.,._ ___ Border Ranges -----t.,• East 

0 Upper Cenozoic 

0 Lower Cenozoic 

0 Upper Cretaceous 

0 TuronianSantonian (?) 

0 Albian 

0 Jurassic into 

0 Triassic 

Q fault 

Q L 

Q 

68 Geologic Studies of the Lower Cook Inlet COST No. 1 Well 

@ 

@ 

Q 

Q 

Q 

® 

® 

EXPLANATION 

Sandstone classification 

I F eldspa thic 

II Lithofeldspa thic 

Ill F eldspa tho lithic 

IV Lithic 

~ Trend--Showing change in sandstone 

composition. Arrow in direction 

of decreasing age 

Average sandstone composition 

Table 16 

Table 17 
from the Kachemak (Triassic rocks) and Chugach terranes by 

the Border Ranges fault on the southeast (Plafker and others, 

1977). In the northern Chugach Mountains, the schist terrane 

has been, in part, tectonically incorporated into the McHugh 

Complex (Winkler and others, 1980). On Kodiak Island and 

adjacent islands, the schist terrane is bounded on the east by 

the Border Ranges fault and on the west by the Kodiak-Kenai 

plutonic belt. 

Kodiak-Kenai Plutonic Belt 

The Kodiak-Kenai plutonic belt, ca. 195-185 Ma, (Hudson, 1979b; fig. 27, this volume) extends from the Seldovia 

area southwest to Kodiak Island; it includes the Afognak 

pluton (Hill and Morris, 1977). The K-Ar age of the plutonic 

body on the Barren Islands is 187 Ma (Cowan and Boss, 

1978); and on Kodiak Island and adjacent islands the age 

ranges from 193 to 184 Ma (Carden and others, 1977). 

The structural relation of the Kodiak-Kenai plutonic 

belt to the Kodiak-Seldovia schist terrane is unclear. In the 

Seldovia area the rock units are geographically separated 

(Carden and others, 1977). The similarity in K-Ar age and 

close geographic proximity of the pluton to the schist terrane 

leads Carden and others ( 1977) to infer that emplacement of 

the pluton did not create the schist, but the two rock units 

"experience(d) a similar crystallization and cooling history". The pluton is in intrusive contact with the Mesozoic 

shelf rocks on Kodiak and the Barren Islands (Carden and 

others, 1977; Cowan and Boss, 1978; Connelly and Moore, 

1979). 

ACCRETED TERRANES 

The severely deformed Mesozoic and Cenozoic rocks 

southeast of the Border Ranges fault can be subdivided into 

two fault-bounded terranes (Jones and Silberling, 1979; Jones 

and others, 1984 )-the Chugach terrane, and the Prince 

William terrane (fig. 36). 

Chugach Terrane 

The Chugach terrane (Berg and others, 1972) lies 

southeast of the Border Ranges fault (fig. 36). It consists of 

Figure 33. Q-F-L ternary diagrams for sandstones from 

Upper Triassic through upper Cenozoic rocks of the Cook 

Inlet forearc basin northwest of the Border Ranges fault and 

of the accreted terranes southeast of the fault. Average Q-FL values are plotted; numbers and letters refer to samples 

listed in tables 16 and 17. 

the Triassic rocks and the melange and flysch sequences. The 

melange consists of the McHugh and Uyak Complexes and 

the deformed flysch sequences consist of the Cape Current 

terrane, the Valdez Group, and the Kodiak and Shumagin 

Formations. 

Triassic Rocks 

The chert and ellipsoidal lavas that are mapped east of 

the Border Ranges fault and along the south shore of 

Kachemak Bay (Martin and others, 1915, pl. III; Magoon and 

others, 1976b) constitute the Triassic rocks (fig. 36). A Triassic age has been assigned to the rocks on the basis of the 

stratigraphic relations of the nearby units (Martin and others, 

1915, p. 61) and on the paleontologic evidence. A radiolarian 

assemblage, recovered from chert on Yukon Island in 

Kachemak Bay by E. A. Pessagno in 1975, was assigned a 

Late Triassic age (Jones and others, 1977, p. 2572). This chert 

unit was found to be in thrust-fault contact with the McHugh 

Complex by J. S. Kelley (Magoon and others, 1976b). Other 

workers have included the Triassic rocks in the Seldovia Bay 

Complex (Cowan and Boss, 1978) or in the McHugh Complex (Plafker and others, 1977). 

McHugh and Uyak Complexes 

The McHugh and Uyak Complexes consist of metasedimentary and metavolcanic rocks in the Chugach 

Mountains near Anchorage (Clark, 1972, 1973), on the Kenai 

Peninsula (Kelley, 1977b), and on the northwest part of 

Kodiak Island and adjacent islands (Connelly, 1978; Connelly and Moore, 1979) (fig. 34). 

Quantitative modal analyses of sandstone are limited 

for the McHugh Complex. Clardy (1974) described one 

sample from the McHugh east of Anchorage as having a 

modal composition of Q33F25L42. Nineteen sandstone samples from the Uyak Complex on the Kodiak Island and 

adjacent islands have an average modal composition of 

Q29F61L10 (Connelly, 1978). Plagioclase to total feldspar 

ratios (P/F), which range from 1.0 to 0.91, indicate that 

nearly all the feldspar is plagioclase. The very high ratios of 

volcanic lithic to total lithic fragments (V /L) indicate a volcanic source terrane. 

Quartz from both the McHugh and Uyak Complexes is 

present in moderate amounts. Polycrystalline to total quartz 

ratios (C/Q) are low, therefore most of the quartz is monocrystalline. Connelly (1978) concluded that the monocrystalline quartz was mainly of volcanic derivation. 

However, Connelly (1978) described several samples of 

"chert-clast wackes" from the Uyak Complex as having high 

polycrystalline quartz to total quartz ratios. 

Framework Geology and Sandstone Composition 69 
Table 16. Petrography of sandstone samples from northwest of Border Ranges fault 

[Sandstone classification of Dickinson (1970) with modifications from Crook (1960). Sample numbers are referred to in QFL plots, fig. 33 J 

Sample 

Number Formation 

45 Tyonek Formation 

4 4 Hemlock/Tyonek 

Undifferentiated 

43 Hemlock Conglomerate 

Area 

Middle Ground Shoal 

Field 

Cape Douglas 

Middle Ground Shoal 

Field 

42 Tsadaka Formation Matanus'ka Valley 

41 West Foreland Formation Cape Douglas 

40 West Foreland Formation COST No. 1 Well 

39 West Foreland Formation Pan American West 

38 Wishbone Formation 

37 Chickaloon Formation 

36 Chickaloon Form.at.ion 

35 Arkose Ridge Formation 

34 Arkose Ridge Formation 

33 Kaguyak Formation 

Upper Part 

32 Kaguyak Formation 

Lower Part 

31 Kaguyak Formation 

Upper Part 

30 Kaguyak Formation 

Lower Part 

29 Kaguyak Formation 

28 Matanuska Formation 

Upper Part 

27 Matanuska Formation 

Upper Part 

26 Chignik Formation 

25 Chignik Formation 

24 Matanuska Formation 

Lower Part 

23 Unnamed 

22 Herendeen Limestone 

21 Herendeen Limestone 

20 

19 

18 

17 

16 

15 

14 

13 

12 

11 

10 

Herendeen Limestone 

Staniukovich Formation 

Staniukollich Formation 

Naknek Formation 

Naknek Formation 

Naknek Formation 

Naknek Formation 

Naknek Formation 

Naknek Formation 

Naknek Formation 

Shel ikof Formation 

Chinitna Formation 

Paveloff Siltstone 

Chinitna Formation 

Tonnie Siltstone 

Kialagvik Formation 

Tuxedni Group 

Talkeetna Formation 

Talkeetna Fonnation 

(Pogibshi Formation, 

Kell8y 1980) 

Unnamed 

Shuyak Formation 

Unnamed 

Foreland No. 1 

Matanuska Valley 

Matanus'ka Valley 

Matanuska Valley 

Matanuska Valley 

Matanuska Valley 

Cape Douglas 

Cape Douglas 

COST No. 1 Well 

COST No. 1 llell 

Saddle Mounta1n 

Matanuska Valley 

Matanuska Valley 

Alaska Peninsula 

Alaska Peninsula 

Matanuska Valley 

Alaska Peninsula 

Alaska Peninsula 

Cape Douglas 

COST No. 1 Nell 

Alaska Peninsula 

Alaska Peninsula 

Puale Bay 

Alaska Peninsula 

Alaska Peninsula 

Cape Douglas 

Cost No. 1 llell 

Iniskin-Tuxedni Region 

Tuxedni Bay 

Puale Bay 

Tuxedni Bay 

Tuxedni Bay 

Puale Bay 

Tuxedni Bay 

Puale Bay 

Kenai Pen1nsula 

Pual~ Bay 

Kodiak Island 

Kenai Peninsula 

Age 

Oligocene-middle Miocene 

Oligocene-middle Miocene 

Late 01 igocene 

Late 01 igocen0 

Paleocene-Early Eocene 

Paleocene-Early Eocene 

Paleocene-Early Eocene 

Paleocene-Early Eocene 

Paleocene 

Paleocene 

Paleocene 

Paleocene 

Late Cretaceous 

(Maestrichtian) 

Late Cretaceous 

(Maestrichtian) 

Late Cretaceous 

(Maestrichtian) 

Late Cretaceous 

(Maestrichtian) 

Late Cretaceous 

(Maestrichtian) 

Late Cretaceous 

( Campanian-Maestrichtian) 

Late Cretaceous 

( Campanian-Maestrichtian) 

Late Cretaceous 

(Campanian) 

Late Cretaceous 

(Campanian) 

Early Cretaceous 

(Albian) 

Early Cretaceous 

(Albian) 

Early Cretaceous 

(Hauterivian-Barremian) 

Early Cretaceous 

(Hauterivian-Barremian) 

Early Cretaceous 

(Hauterivian-Barremian) 

Late Jurassic-Early Cretaceous 

(late J<immeridgian-Valang in ian) 

Late Jurassic 

Late Jurassic 

Late Jurassic 

Late Jurassic 

Late Jurassic 

(Kimmeridgian) 

Late Jurassic 

(Kimmeridgian) 

Late Jurassic 

Late Jurassic 

(Oxfordian-early Kimmeridgian) 

Middle Jurassic (Callovian) 

Middle Jurassic (Callovian) 

Middle Jurassic (Callovian) 

Middle Jurassic 

(early-middle Bajocian) 

Middle Jurassic 

(Bajocian-Bathonian) 

Early Jurassic 

( Hettangian-early Sinernurian) 

Early Jurassic 

(Hettangian-early Sinemurian) 

Late Triassic 

Late Triassic 

Late Triassic 

Reference 

Stewart ( 19 76 ) 

Lankford and Magoon ( 1978) 

Stewart ( 1976) 

Clardy ( 1974) 

Lankford and Magoon (1978) 

J<uryvial ( 1977) 1 Schluger 

! 1977 I 

Stewart (1976) 

Clardy ( 1974) 

Clardy ( 1974) 

Winkler ( 1978) 

Clardy ( 1974) 

Winkler ( 1978) 

Lankford and Magoon (1978) 

Lankford and Magoon ( 1978) 

Schluger ( 1977) 

Kuryvial ( 1977) 1 Schluger 

( 1977) 

Magoon, Griesbach and 

Egbert ( 1980) 

Grantz ( 1964) 

Clardy ( 1974) 

Fairchild ( 1977) 

Burk ( 1965) 

Grantz (1964) 

Egbert (this volume) 

Burk (1965) 

Lankford and l4agoon ( 1978) 

Babcock ( 1977}, Christensen 

(1977), Kuryvial (1977), 

Schluger ( 1977), Swiderski 

( 1977) 

Burk (1965) 

Franks and Hite (1980) 

Egbert (this volume) 

McLean ( 1979) 

Burk ( 1965) 

Lankford and Magoon (1978) 

Babcock ( 1977), Christensen 

(1977), Kuryvial (1977), 

Schluger ( 1977), Swiderski 

( 1977) 

Franks and Hite (1980) 

Egbert and Magoon ( 1981 ) 

Egbert (this volume) 

Egbert and Magoon (1981) 

Egbert and Magoon ( 1981) 

Egbert (this volume) 

Egbert and Magoon ( 1981) 

Egbert (this volume) 

Kelley ( 1980) 

Moore and Connelly ( 1 979) 

Moore and Connelly (1979) 

Moor~ and Connelly ( 1979) 

¥; Recalculated from original data. 

- Estimated percentages from QFL ternary diagrams in reference (no tabular data present in reference). 

]./ Triassic sandstone from Puale Bay, Kodiak Island, and the Kenai Peninsula averaged together by Moore and Connelly ( 1979). 

Sandstone 

classification 

Feldspatholithic 

Feldspatholi thic 

Feldspatholithic 

Lithofeldspathic 

Feldspatholithic 

Feldspatholithic 

Feldspatholithic 

Feldspatholithic 

Feldspatholithic 

Feldspatholithic 

Feldspathic 

Feldspathic 

Feldspatholithic 

Feldspathic 

Lithic 

Feldspathic 

Li thofeldspathic 

Feldspathic 

Li thofeldspathic 

Li thofe ldspa thic 

Feldspatholithic 

Lithic 

Feldspatholithic 

Feldspathic 

Feldspathic 

Lithofeldspathic 

Feldspathic 

Feldspathic 

Feldspathic 

Feldspathic 

Feldspathic 

Feldspathic 

Li thofe ldspathic 

Lithofeldspathic 

Feldspathic 

Lithofeldspathic 

Lithofeldspathic 

Lithofeldspathic 

Feldspatholithic 

Feldspatholithic 

Feldspatholithic 

Lithic 

Feldspatholithic 

Feldspatholithic 

Feldspatholithic 

Q = Quartz + Chert + Quartzite 

F c •rotal feldspar 

C/Q = Chert/Quartz + Chert + Quartzite 

P/F = Plagioclase/Total feldspar 

N = nwnber of samples 

L = Total rock fragments V/L = Volcanic rock fragments/Total rock fragments 

~.Y-~!9~ 

Q F L ~e~ C/Q P/F V/L 

61 19 20 0.17 0.46 0.20 

39 24 37 

66 15 19 

37 40 23 

29 22 49 

23 30 47 

22 27 51 

23 22 55 

30 22 48 

28 20 52 

22 68 10 

28 20 52 

27 31 42 

34 55 11 

13 18 69 

22 64 14 

32 40 28 

26 58 16 

21 48 31 

43 31 26 

34 19 47 

4 92 

11 40 49 

62 33 

38 59 

36 44 20 

55 41 

48 42 10 

28 63 9 

38 52 10 

47 52 

36 60 

24 55 21 

58 38 

73 21 

46 45 

66 29 

53 43 

35 57 

43 51 

42 52 

20 77 

2 39 59 

3 27 70 

0 35 65 

2 34 64 

.0 

.14 • 50 .21 

.14 • 78 26 

.0 .95 .98 

.33 .62 1.0 

.15 • 87 • 79 

.23 

.18 

.21 

• 06 

.05 

.0 

. 72 .89 

• 74 • 73 

.96 .86 

• 72 .28 

.92 . 71 

• 90 • 82 

.0 89 

• 33 .87 1.0 

12 

12 

12 

8 

43 

16 

8 

4 

20 

12 

.06 . 71 .99 20 

.04 • 59 . 67 

.11 .97 

.23 • 81 .68 

.16 .31 73 ..!.169 

Y1 

.o .75 (high) ..!./2 

.03 • 99 • 97 

.o 

.06 

.0 

.o 

.08 

.06 

.04 

.11 

.12 

• 77 .64 

.88 

• 90 

.97 

• 87 

• 96 

.97 

.96 

83 

.66 

• 75 

.88 

.84 

.95 

. 93 

.96 .90 

.23 • 95 • 94 

.27 .92 .96 

. 0 • 97 .98 

(low) 1.0 1.0 

0.10 1.0 0.9 

37 

42 

11 

28 

44 

13 

12 

11 

21 

114 

Clark ( 1972, 1973) was uncertain about the age of the 

McHugh Complex; she considered it to be Late Jurassic 

and( or) Cretaceous. Recently radiolarians were recovered 

from the chert at the type locality of the McHugh Complex, 

and Karl and others (1979) assigned the radiolarians an Early 

Cretaceous (Valanginian) age. Connelly and Moore (1979) 

found that the Uyak Complex on Kodiak Island and adjacent 

islands contained fossils ranging in age from Paleozoic to 

70 Geologic Studies of the Lower Cook Inlet COST No. 1 Well 
Table 17. Petrography of sandstone samples from southeast of Border Ranges fault 

[Sandstone classification of Dickinson (1970) with modifications from Crook (1960), Letter refers to QFL plots, fig. 33] 

Sandstone Aver!!S,e Aver!!ie 

Letter Formation Area Age Reference classification Q F L C/Q P/F V/L N 

R Narrow Cape Formation Kodiak Island Late Oligocene(~)- Winkler (1979) Litho feldspathic 50 26 24 0.25 0.75 0.47 

middle Miocene 

Q Narrow Cape Formation Kodiak Island Late Oligocene(?)- Lyle and others (1977) Lithofeldspathic 70 18 12 .75 .47 

middle Miocene 

p Sitkinak Formation Kodiak Island Oligocene Winkler (1979) Feldspatholithic 40 16 44 .34 .98 .75 4 

0 Sitkinak Formation Kodiak Island Oligocene Lyle and others (1977) Feldspatholithic 45 18 37 .90 .75 

N Sitkinak Formation Kodiak Island Oligocene Stew art ( 197 6) Feldspatholithic 30 33 37 .12 .84 .28 

M Sitkalidak Formation Kodiak Island Eocene-Oligocene Stewart (1976) Feldspatholithic 40 22 38 .25 .81 .41 

L Sitkalidak Formation Kodiak Island Eocene-Oligocene Lyle and others (1977) Feldspatholithic 46 17 37 .90 .75 

K Sitkalidak Formation Kodiak Island Eocene-Oligocene Winkler (1979) Feldspatholithic 26 26 48 .16 .91 .74 

J Ghost Rocks Formation Kodiak Island Paleocene-Eocene Winkler (1979) Feldspatholithic 21 37 42 .08 .92 .95 

Ghost Rocks Formation Kodiak Island Paleocene-Eocene Lyle and others (1977) Feldspatholithic 23 25 52 .92 .95 2 

H Shumagin Formation Shumagin Island Late Cretaceous Moore (1973) Feldspatholithic 17 32 51 .15 .97 .96 61 

(Maestrichtian) 

G Kodiak Formation Kodiak Island Late Cretaceous Connelly (1978) Lithofeldspathic 50 33 17 .3 1.0 

(Maestrichtian) 

F Kodiak Formation Kodiak Island Late Cretaceous Winkler (1979) Feldspatholithic 24 3Z 44 .10 .95 .96 

(Maestrichtian) 

E sliver wacke Kodiak Island Late Cretaceous Connelly (1978) Feldspathic 48 47 5 .20 1.0 

(Maestrichtian) 

D Valdez Group Chugach Mountains Late Cretaceous Clardy (1974) Lithic 17 80 .80 1.0 .90 

(Maestrichtian) 

c Cape Current terrane Shuyak-Afognak Islands Late Cretaceous Connelly (1978) Lithofeldspathic 12 52 36 .2 1.0 1.0 

(Turonian-Santonian?) 

B Uyak Complex Kodiak Island Middle Permian(?)- Connelly (1978) Feldspathic 29 61 10 .3 1.0 1.0 19 

Early Cretaceous 

(Valanginian) 

A McHugh Complex Chugach Mountains Middle Permian(?)- Clardy (1974) Feldspatholithic 33 25 42 .06 .91 .74 

Early Cretaceous 

(Valanginian) 

Q =Quartz + Cllert +Quartzite C/Q = Cllert/Quartz N = mmi>er of sarq>les 

F = Total Feldspar P/F = Plagioclase/total feldspar 

L = Total rock fragments V/L = Volcanic rock fragments/total rock fragments 

Early Cretaceous; however, they believed the petrologic and 

stratigraphic data indicated an Early Cretaceous (late Valanginian to Aptian) age. 

The sandstone compositions of these two rock complexes are considered to be correlative (table 17; fig. 34). The 

McHugh and Uyak Complexes are inferred to represent 

oceanic crust, abyssal floor, and trench deposits of a subduction complex (Plafker and others, 1977; Connelly and Moore, 

1979). 

Cape Current Terrane 

The Cape Current terrane is between, and in fault contact with, the Uyak Complex and the Kodiak Formation 

(Connelly and Moore, 1979). This terrane (Connelly, 1978) 

consists dominantly of lithofeldspathic sandstone with minor 

amounts of pillow lava and pelagic limestone that dip steeply 

to the northwest (Connelly, 1978). 

Sandstones from the Cape Current terrane are 

lithofeldspathic with an average modal composition of 

Q12F52L36 (table 17; fig. 34; Connelly, 1978). Quartz content 

is low, and the low polycrystalline quartz to total quartz ratios 

( C/Q) indicate that most of the quartz is monocrystalline. 

Plagioclase to total feldspar ratios (P/F) of 1 indicate that all 

the feldspar in the Cape Current sandstones is plagioclase. A 

volcanic source area can be inferred because the ratio of 

volcanic lithic to total lithic fragments (V/L) is 1. The rock 

composition of the Cape Current terrane is inferred to be 

mostly medium- to thick-bedded turbidites, and like the 

McHugh and Uyak Complexes, the Cape Current terrane is 

thought to be part of a subduction complex. 

The age of the Cape Current terrane, which Connelly 

and Moore ( 1979) based on paleontologic data, is Late Cretaceous (Turonian-Santonian?). 

Valdez Group and Kodiak and Shumagin 

Formations 

The correlative rock units of the Valdez Group, the 

Kodiak and Shumagin Formations, and the "extraformational slivers wacke" of Connelly (1978) are located on the 

Kenai Peninsula, Kodiak Island, and Shumagin and Sanak 

Islands southwest of Kodiak Island (fig. 34). The rock units 

consist of deformed and metamorphosed flysch sequences of 

sandstone, siltstone, shale, and some conglomerate. The 

McHugh and Uyak Complexes and the Cape Current terrane 

structurally overlie the Valdez Group and the Kodiak Formation, and they, in tum, structurally overlie younger rocks. 

However, on Kodiak and adjacent islands, fault-bounded 

outliers or klippe of the Kodiak Formation overlie the U yak 

Complex (Connelly and Moore, 1979). 

Framework Geology and Sandstone Composition 71 
o6V~ 

~ 

~ 4 

4 ~" 

0(): 

.::: • • (). "Y: 

•""' .. ~: " . "Y: 

~fi) 

~ 

~ ~ 

~"'."'_:· 

•""' ~. . ~-" ">· ": -4.· • eJ 

"Y : : 

oO'~; 4:lJ'if 1:!!AOPJ9S 

• "',';:,~ -•unsn6n~t $ 

oOS~ 0 ~ s ~ 0 (; s ~ o8S~ 

naM ~ "ON lSOJ tafUI 'fOO) JO)MOl alJt JO sa,pnts :>!SOfoa~ Zl 

oLS 

o8S 

o6S 

o09 

St:E3L3l/IJOll~ OS 0 

S::Jllli\J OS 0 

0 ~ 9 

oG9 

oVS~ oSS ~ o9S ~ oLS~ 
EXPLANATION 

Quaternary ~0 

Tertiary 

Tertiary D { 

Sterling Formation 

Kenai Group Beluga Formation 

Tyonek Formation 

Hemlock Conglomerate ., 

0:: 

~ 

{

Narrow Cape Formation 

Sitkinak Formation 

Sitkalidak Formation 

Ghost Rocks Formation 

"' 0:: 

"' 

"' 

WELL 

NO. 

II 

Ill 

IV 

v 

VI 

VII 

"' 0:: 

"' 

., 

Cretaceous ~ 

Upper and I' :::: <: ''1 Middle Jurassic { } 

Lower Jurassic ~ 

and Triassic ~ 

COMPANY 

Standard Oil of Calif. 

Standard Oil of Calif. 

Pan American Petroleum Corp. 

Richfield Oil Corp. 

Pan American Petroleum Corp. 

Standard Oil of Calif. 

Union Oil of Calif. 

T sad aka Formation 

West Foreland Formation 

Wishbone Formation 

Chickaloon Formation 

Arkose Ridge Formation 

Ma tanuska Formation 

Kaguyak Formation 

Chignik Formation 

Unnamed Albian rocks 

Herendeen Limestone 

Naknek Formation 

Chinitna Formation 

Shelikof Formation 

Tuxedni Group 

Kialagvik Formation 

(

Talkeetna Formation 

Shuyak Formation 

Kamishak Formation 

Cottonwood Bay Greenstone 

Unnamed Triassic rocks 

WELL NAME AND NUMBER 

North Fork Unit No. 4 I -35 

Anchor Point Unit No. I 

West Foreland Unit No. I 

Swanson River Unit No. I (34- 1 0) 

Tyonek State I 7587 No. 2 

Beluga River Unit No. 2 I 2-35 

Sterling Unit No.23- I 5 

Cenozoic { 

Mesozoic { 

OIL FIELDS 

GP Granite Point 

TB Trading Bay 

SR Swanson River 

Cretaceous 

and Triassic l 

Kodiak Formation 

Shumagin Formation 

Valdez Group 

Cape Current terrane 

McHugh Complex 

Uyak Complex 

Triassic rocks (unnamed) 

Volcanic rock 

Alaska- Aleutian batholith 

Location of cross section 

Fault-Dotted where concealed 

MeR MacArthur River 

MGS Middle Ground Shoal 

BC Beaver Creek 

/zoO.,__..... Isobath 

GAS FIELDS 

K Kenai 

BR Beluga River 

NCI North Cook Inlet 

_, Oil field 

0 Gas field 

e Oil show in abandoned well 

A Surface indication of oil 

Figure 34. Generalized geologic map of Cook Inlet, Shelikof Strait, and adjacent areas (modified from Beikman, 1978). The 

COST No.1 well is located in lower Cook Inlet. Cross sections A-A', B-B', and C-C' are shown in figure 35. Indication of oil in 

wells and at the surface are shown around lower Cook Inlet and on the Alaska Peninsula. 

These rock units are in thrust-fault contact with both 

older and younger rocks. In the Anchorage area, the thrust 

fault is designated the Eagle River fault (Clark, 1972; Tysdal 

and Case, 1979); in the Seldovia area, the Chugach Bay fault 

(Cowan and Boss, 1978); and in the Kodiak Bay area, the 

Uganik thrust (Connelly and Moore, 1979). For all these 

areas, Beikman (1978) designated the name Eagle River fault 

(fig. 34). 

Fossil evidence indicates that these three rock units are 

correlatives, however, the sandstones in these rock units have 

significantly different compositions. Moreover, some of the 

determinations of the average modal sandstone compositions 

from the same rock unit made by different workers (table 17) 

do not agree closely. Although Moore (1973) and Winkler 

(1979) determined similar average modal compositions for 

the Shumagin and Kodiak Formations of Q17F32L51 and 

Q24F32L44, respectively, Connelly (1978) determined the 

average modal sandstone compositions for the Kodiak Formation and the "extraformational slivers wacke" of the 

Kodiak Formation incorporated within the Uyak Complex to 

be Q50F33L17 and Q48F47L5 , respectively. Connelly's sandstone samples are much more rich in quartz and poor in lithic 

fragments than those of Moore (1973) and Winkler (1979). 

One sandstone sample from the Valdez Group, 

described by Clardy (1974), differs from the sandstone in all 

the other units. Its modal composition of Q3F 17L80, which is 

poor in quartz and feldspar and rich in volcanic lithic fragments, is opposite to that of the Kodiak and Shumagin 

Formations. 

The quartz content seems to be variable in the Upper 

Cretaceous rocks east of the Border Ranges fault. The low 

ratios of polycrystalline quartz to total quartz indicate that 

most of the quartz is monocrystalline except for one sample 

from the Valdez Group which contains 80 percent polyFramework Geology and Sandstone Composition 73 
A BBF UPPER COOK INLET 

CL TB MeR MGS CL A' 

SR BRF 

B CL LOWER COOK INLET CL B' 

c BBF ALASKA PENINSULA CL SHELIKOF STRAIT CL C' 

BRF 

Figure 35. Geologic cross sections A-A' (modified from Boss and others, 1976) through upper Cook Inlet, B-B' (modified 

from Fisher and Magoon, 1978) through lower Cook Inlet, and C-C' through the Alaska Peninsula and the Shelikof Strait. 

Cross section C-C' is conceptual except for onshore surface geology. BBF, Bruin Bay fault; BRF, Border Ranges fault; Cl, 

coastline. Refer to figure 34 for location of cross sections and explanation of other symbols. 

crystalline quartz. Winkler (1979) suggested that much of the 

polycrystalline quartz represents silicified volcanic rock fragments. 

The very high ratios of volcanic lithic to total lithic 

fragments (V/L) indicate that the volcanic rocks were derived 

from a predominantly volcanic terrane. 

The plagioclase to total feldspar ratios (P/F) are very 

high in the Upper Cretaceous sandstone east of the Border 

Ranges fault, so in those sandstones almost all the feldspar is 

plagioclase. 

Work by Jones and Clark (1973) showed that the age 

diagnostic fossils of the Valdez are latest Cretaceous. 

Prince William Terrane 

The Prince William terrane consists of the Cenozoic 

rocks south of the Chugach terrane and is separated from the 

Chugach terrane by the Contact fault (Jones and Silberling, 

1979) (fig. 36). Four formations, the Ghost Rocks, Sitkalidak, Sitkinak, and Narrow Cape Formations, make up the 

terrane. 

74 Geologic Studies of the Lower Cook Inlet COST No.1 Well 

Ghost Rocks Formation 

The Ghost Rocks Formation crops out in a long belt on 

the east side of Kodiak Island and extends from Cape Chiniak 

on the northeast to the Geese Islands on the southwest. The 

formation is bounded by faults between the older Kodiak 

Formation on the northwest and the younger Sitkalidak Formation on the southeast. The strata consists mostly of highly 

deformed shale, argillite, and mudstone that contain isolated, 

thin-bedded turbidites (Nilsen and Moore, 1979). 

Sandstones from the Ghost Rocks Formation are generally feldspatholithic with average modal compositions of 

Q21F37L42 (Winkler, 1979) and Q23F25L52 (Lyle and others, 

1977). The content of quartz is relatively low and that of 

volcanic lithic fragments high. The low ratios of polycrystalline quartz to total quartz (C/Q) indicate that most of 

the quartz is monocrystalline. The high ratios (>0.90) of 

plagioclase to total feldspar (P/F) indicate that most of the 

feldspar is plagioclase. Both the high ratios of volcanic lithic 

to total lithic fragments and the high percentages of total 

lithic fragments indicate a predominantly volcanic source 

area. 
156° 154° 

62° 

0 50 MILES 

0 50 KILOMETERS 

I I 

61° 

152° 150° 

Cz 

~ I 

r>f.. u ~ I 

I (!)~ ~ I ~ ~ 

u~ ~q;; I 

" I 

0 / 

/ 

~ / 

Kodiak 

/ 

I 

I 

I 

I 

I 

Figure 36. Major terrane boundaries in the Cook Inlet region. Terrane boundaries dashed 

where queried, dotted where buried under younger sediments. Cz stands for Cenozoic postaccretionary cover deposits. (Modified from Jones and others, 1984). 

Moore (1969) considered the Ghost Rocks Formation, 

on the basis of lithologic and paleontologic evidence, to be 

Paleocene and Eocene in age. 

Although the sedimentary and tectonic relations of the 

Ghost Rocks Formation are not well understood, Nilsen and 

Moore (1979) think that the sediments may have been deposited in a basin plain or slope environment. 

Sitkalidak Formation 

The Sitkalidak Formation consists of interbedded sandstone and shale that forms a thick series of turbidites. The 

rocks crop out in separate areas along the southeastern parts 

of Kodiak, Sitkalidak, and Sitkinak Islands (off south tip of 

Kodiak Island). On Sitkinak Island it is separated from the 

Framework Geology and Sandstone Composition 75 
Sitkinak Formation by faults; on Kodiak Island it is faulted 

from the older Ghost Rocks Formation; on Sitkalidak Island, 

it is conformably overlain by the Sitkinak Formation; and at 

Cape Narrow (at the northeast end of Kodiak Island), it is 

overlain unconformably by the Narrow Cape Formation. At 

its type section in eastern Sitkalidak Island, Moore (1969) 

estimated the formation to be about 3,000 m (10,000 ft) 

thick. 

Sandstones from the Sitkalidak Formation (table 17) 

are generally feldspatholithic with average modal compositions of Q26F26L48 (Winkler, 1979), Q46F17L37 (Lyle and 

others, 1977), and Q40F22L38 (Stewart, 1976). The moderate 

amounts of quartz present are mostly monocrystalline, as 

indicated by the polycrystalline quartz to total quartz ratios 

(C/Q). Polycrystalline quartz is about twice as abundant in 

the Sitkalidak Formation as in the Ghost Rocks Formation. 

The ratios of plagioclase to total feldspar (P/F), which 

range from 0.81 to 0.91, indicate that most of the feldspar is 

plagioclase in the Sitkalidak Formation. K-feldspar, also 

present, is slightly more abundant in the Sitkalidak than in 

older units. Volcanic lithic to total lithic fragment ratios (VI 

L), which range from 0.41 to 0. 75, indicate that volcanic 

lithic fragments are not the important framework grains. Most 

of the lithic fragments are sedimentary rock fragments, and 

there are lesser amounts of metamorphic rock fragments. 

The sandstone and shale of the Sitkalidak Formation 

were deposited in middle and outer submarine fan environments in the Eocene and Oligocene (Nilsen and Moore, 

1979). 

Sitkinak Formation 

Small isolated outcrops of the Sitkinak Formation are 

exposed on Kodiak Island, Sitkalidak Island, Sitkinak Island, 

and Chirikof Island (southwest from Kodiak Island). The 

Sitkinak Formation consists of two distinct sedimentary 

deposits-marine and nonmarine facies. The marine facies 

on Sitkalidak Island consists of interfan channel conglomerate and slope facies siltstone and shale (Nilsen and 

Moore, 1979); these beds conformably overlie the Sitkalidak 

Formation and are faulted against the Ghost Rocks Formation 

(Nilsen and Moore, 1979). The nonmarine facies on Sitkinak 

Island consist of conglomerate, sandstone, and siltstone with 

some coal and carbonaceous shale; this unit is faulted against 

the Sitkalidak Formation. The siltstone and fine-grained 

sandstone strata contain abundant fossil plants of Oligocene 

age (Moore, 1969). 

Sandstone, generally feldspatholithic (table 17) from 

the Sitkinak Formation, have average modal compositions of 

Q30F33L37 (Stewart, 1976), Q45F18L37 (Lyle and others, 

1977), and Q40F16L44 (Winkler, 1979). Quartz is present in 

moderate amounts. The low ratios of polycrystalline quartz to 

total quartz (C/Q) indicate that most of the quartz is monocrystalline. However, polycrystalline quartz is slightly more 

76 Geologic Studies of the Lower Cook Inlet COST No. 1 Well 

abundant in the Sitkinak Formation than in the Sitkalidak 

Formation. 

The high ratios of plagioclase to total feldspar, ranging 

from 0.84 to 0.98, indicate that plagioclase is the most 

abundant feldspar. The content of potassium feldspar is as low 

in the Sitkinak Formation as it is in the Sitkalidak Formation. 

A great diversity in type of lithic fragments is present, 

as indicated by the range from 0.28 to 0. 75 of volcanic lithic 

to total lithic fragments. Sedimentary and metamorphic lithic 

fragments make up a significant part of the total lithic fragment assemblage, but generally volcanic lithic fragments are 

more abundant. 

Narrow Cape Formation 

The Narrow Cape Formation, at its type locality at 

Narrow Cape on eastern Kodiak Island, consists of silty 

sandstone and siltstone and sedimentary breccia and conglomerate. The formation rests with angular unconformity on 

the Sitkalidak and Ghost Rocks Formations. A smaller outcrop on Sitkinak Island has a general lithology similar to that 

at Narrow Cape. The Narrow Cape Formation here is disconformable with the Sitkinak Formation. The exposures at both 

places contain abundant fossils; however, at Narrow Cape the 

fossils indicate an early and middle Miocene age, and at 

Sitkinak Island, they indicate a late Oligocene or early 

Miocene age (Allison and Addicott, 1976). The strata represent marine shelf deposits. 

Sandstone from the Narrow Cape Formation is 

lithofeldspathic with average modal compositions of 

Q70F18L12 (Lyle and others, 1977) and Q5of26L24 (Winkler, 

1979). Quartz content is higher in this formation than in the 

older formations. The low ratios of polycrystalline quartz to 

total quartz (C/Q) indicate that most of the quartz is monocrystalline. The Narrow Cape Formation contains similar 

percentages of polycrystalline quartz (as much as 25 percent 

of the total quartz) as the Sitkinak Formation. 

Plagioclase to total feldspar ratios (P/F), which average 

0. 7 5, indicate that most of the feldspar is plagioclase and that 

25 percent of the total feldspar assemblage is K-feldspar. Of 

the lithic fragment assemblage, 48 percent is sedimentary, 4 7 

percent are volcanic, and 5 percent are metamorphic rock 

fragments. The Narrow Cape Formation has the greatest 

diversity in types of lithic fragments than any other formation 

east of the Border Ranges Fault. 

Peninsular Terrane 

Moderately deformed upper Paleozoic and Mesozoic 

sedimentary rocks at least as old as Permian are present 

within the confines of the basin framework (fig. 36). The 

Permian rocks (Hanson, 1957) are known from only one 

small area near Puale Bay, so they will not be discussed here; 

only the Mesozoic rocks will be discussed in this section. 

Formerly these Mesozoic sedimentary rocks were designated 
the Matanuska-Wrangell terrane (Berg and others, 1972), but 

they are now renamed the Peninsular terrane (Jones and 

Silberling, 1979). The age, general lithology, and stratigraphic relations of these rocks are shown on figure 37. The 

important distinction between this terrane and the previous 

terranes is that the stratigraphic units are deposited one on top 

of another and many stratigraphic units are separated by 

unconformities, whereas most of the previously discussed 

terranes are in fault contact and become progressively younger southward from the Border Ranges fault to the Aleutian 

trench. 

Upper Triassic Rocks 

On the northwest side of lower Cook Inlet, Upper 

Triassic rocks are exposed in scattered outcrops on both sides 

of the Bruin Bay fault from Tuxedni Bay to Kamishak Bay. At 

the head of Tuxedni Bay, unnamed Upper Triassic rocks, as 

much as 395 m thick (1,300 ft; fig. 37), consist of metamorphosed limestone, tuff, chert, sandstone, shale, and 

basaltic lava flows (Detterman and Hartsock, 1966). West of 

the Bruin Bay fault in the Kamishak Bay area, the Upper 

Triassic mafic-volcanic rocks of the Cottonwood Bay Greenstone presumably underlie the fossiliferous limestone, chert, 

and tuff of the Kamishak Formation (Detterman and Reed, 

1980). The Cottonwood Bay Greenstone is probably more 

than 600 m thick (2,000 ft; fig. 37), and a composite thickness 

for the three members of the Kamishak Formation is as much 

as 1,290 m thick (4,230 ft; fig. 37; Detterman and Reed, 

1980). 

In the Puale Bay area unnamed Upper Triassic rocks, as 

much as 800 m thick (2,625 ft; fig. 37), consist of fossiliferous limestone and volcanic agglomerate intruded by 

basaltic dikes and sills (Imlay and Detterman, 1977). On the 

southeast side of lower Cook Inlet, unnamed Upper Triassic 

rocks exposed northwest of the Border Ranges fault on the 

Kenai Peninsula co~sist of fossiliferous limestone, finegrained tuffs, and a few thin sandstone beds (Martin and 

others, 1915; Moore and Connelly, 1979). 

Unnamed Upper Triassic rocks at least 600 m thick 

(2,000 ft; fig. 37) depositionally overlie undeformed pillow 

basalts on the westernmost island of the Barren Islands 

(Cowan and Boss, 1978). These rocks, which contain Late 

Triassic fossils, consist of gently folded greenish tuff, 

tuffaceous sandstone, minor black siliceous mudstone, and 

chert (Cowan and Boss, 1978). On the northwestern part of 

the Kodiak Islands, pillowed greenstones are overlain by 

fossiliferous volcaniclastic turbidites of the Upper Triassic 

Shuyak Formation (Connelly, 1978). 

Sandstones within Upper Triassic rocks from widely 

spaced areas around Cook Inlet and Shelikof Straits have 

similar compositions (table 16, fig. 33). The sandstones are 

lithic and feldspatholithic and are distinctly poor in quartz. 

Average modal compositions determined by Moore and Connelly (1979) are Q0F35L65 from the Kenai Peninsula, 

Q3F27L70 from the Shuyak Formation, Kodiak Island, and 

Q2F39L59 from Puale Bay. 

Polycrystalline quartz to total quartz ratios (C/Q) are all 

low, therefore most of the quartz is monocrystalline. Plagioclase to total feldspar ratios (P/F) of one indicate that 

plagioclase is the only feldspar present. The very high ratios 

of volcanic lithic to total lithic fragments (V/L) reflect the 

lithic nature of the sandstones and indicate a prominent 

volcanic source terrane. 

Two assumptions need to be made to reconstruct the 

generalized paleogeology of the Late Triassic rocks: (1) the 

structural grain of the Triassic is parallel to the Kodiak-Kenai 

plutonic belt of Late Triassic(?)-Early Jurassic age; and (2) 

the Triassic rocks, on both sides of the Cook Inlet-Shelikof 

Strait west of the Border Ranges fault, are genetically related 

but are not fragments of other land masses that were later 

welded together. If these assumptions are true, then the 

northwest and southeast flanks of the basin can be compared. 

Shallow-water shelf carbonates and agglomerates occur on 

the northwest flank compared to turbidite sandstones and 

siltstones on the southeast flank; mafic igneous flows, sills, 

and dikes with a few pillow basalts occur mainly on the 

northwest compared to many pillow basalts on the southeast; 

chert occurs on both flanks. In other words, the Late Triassic 

rocks formed a broad volcanic Island arc that included shallow-water carbonates, turbidites, mafic volcanic rocks, and 

chert. 

Lower Jurassic Rocks 

Lower Jurassic rocks are exposed on both sides of Cook 

Inlet. On the northwest side, the outcrop belt is as much as 15 

km (10 mi) wide and is cut in several places by the Bruin Bay 

fault. South of Tuxedni Bay, unfossiliferous Lower Jurassic 

rocks dip generally southeast and consist of poorly bedded 

volcanic agglomerates, breccias, and lava flows of the Talkeetna Formation (Detterman and Hartsock, 1966). In the 

Iniskin-Tuxedni area the formation is as much as 2,575 m 

(8,500 ft) thick (fig. 37). 

Near Seldovia, fossiliferous Lower Jurassic strata consist of a pyroclastic sequence that is about 4, 000 m ( 13,000 ft) 

thick (fig. 37; Kelley, 1978). Three lithofacies were 

mapped from bottom to top: a graded pyroclastic lithofacies 

with each graded sequence as much as 50 m (165ft) thick, a 

pyroclastic debris-flow and fossil-bearing turbidite 

lithofacies, and a reworked volcaniclastic lithofacies with 

coal beds as much as 4 m (15ft) thick (Kelley, 1978). On the 

basis of lithology and age, the rocks near Seldovia have been 

assigned to the Talkeetna Formation (Magoon and others, 

1976b ). Lower Jurassic rocks are not exposed northwest of the 

Border Ranges fault on Kodiak Island (Connelly and Moore, 

1979) or on the Barren Islands. 

On the northwest coast of Shelikof Strait in Puale Bay, 

as much as 555 m (1 ,820ft) (fig. 36) of sparsely fossiliferous 

Framework Geology and Sandstone Composition 77 
(.)~ Dates of Bruin Bay fault 

'51~ intrusion 0'-' I Bear No. I 0 Q) Era System Series Stage Alaska- COST No. I 

~:E Aleutian Zappa Nol-9¢- ~ld Mans Bay State No. I 

JJ batholith -¢- -- -

3 Q) u c ....._ Q) 

4 \ (.) Clamgulchian Sterling Formation 

.2 

0:: L 

__ 5_ 277m ~ 

.......... ~~ ~~ 

~ 

10 u Homerian 911m Beluga Formation Q) 

c 

Q) ~ __...._ ~ ~~~ 

15 (.) M '1 0 

~ r- Seldovian 1865m 0 

20 L Tyonek Formation c. 

~ 22.5 .~ 

25 \ 0 >. 

N ctl Q) ---- 0 c u Angoonian /\'_.,.'/\_.,.""'; 216m Hemlock Conglomerate \ T? 30 c 

Q) Q) (.) 

Q) ,,, () 1- 0 /l ...../1:._ Unnamed 

s~L:zzzzzLV .21 r- 35 /~/ /~// 6 L Kummerian r-/ /~..,/ /:.... 

40 I/\,..,./\"" 

u Raveman Q) 

45 c - Q) 

0 

(.) M Fultonian 

50 w 

L Franklinian ~ ....... /~//, 265m CD~31m West Foreland Formation 384m 

55 / !'::..,/ 1~./ Q) 

c u ~/ /~ ....... /\ 

60 Q) - (.) Unnamed /l,/1'/ 0 L ~,..,.)~_.,.), ~ 65 

~ ;'/,,/ 

Maestncht1an ~,..,.)~ ....... ""'/, 

70 /f,/1'/ c Campanian ~,..,./~ ....... /\ 

v;j// v 

75 ctl 

·c: 

0 

c 80 Q) Santonian (/) 

85 en Coniacian ::J 

88 0 

90 Q) 

(.) Turonian /y/ / L/ ctl 

Upper 95 ~ 

() Cenomanian 

10..0 

Albian // / ~UnnameC!~ 

110 Lower Aptian / / / / / 

/ /~ L. 11~}... Barremian ,(rlr 

120 c //m @~15m Herendeen Umestone 572m (.) ctl Hauterivian ·a '§ 

Valanginian / -v/~7/ // / 130 N 0 0 (.) 

~/ / << // 

en 0 Berriasian 140 Q) Q) 

~ z 141.r Portlandian 

~ 270m .~ ®\:50 585 1828 m Naknek Format;on 1663m 150 Upper Kimmeridgian 

160 Oxfordian )'t''/1'/ 1 1612m <.<_120m (.) 

'Cii allovian z,..,./~,.-/~ 558-728m Ch;n;tna Formatron~sm170 en Middle H~thonan /I,/ I'/ 2964m Ll2782m -~~87m Tuxedni Group 730m ctl Bajocian 176 :; _i - .. \ ........ 

180 ""') Toarcian ~ 459m l ~188m ).. 

~555m Pliensbachian Lower Sinemurian 575m Talkeetna Formation 

190 Hettangian. 195 

200 / \ / / L / / / / / / 

(.) Upper \395m Unnamed @\.1290m Kamishak Format1on BOOm\ 

210 en 600m_'\.'Pcottonwood Bay Greenstone en /7//////// ctl 

220 ~ Lower 

~-

JJ Van Eysinga (I 975) 

Figure 37. Time-stratigraphic chart through COST No. 1 well coincident with B-B'. Wells (vertical lines) and measured 

sections (diagonal lines) are projected parallel to the Alaska-Aleutian Range. The thickness of each rock unit is placed to the 

right of the well or outcrop. The circled numbers refer to the references for the measured sections: (1) Detterman and 

Hartsock (1966), (2) Detterman and Reed (1980), (3) Kellum and others (1945), Imlay and Detterman (1977), and the authors of 

78 Geologic Studies of the Lower Cook Inlet COST No. 1 Well 
Port Graham fault ll Border Ranges fault 

Deep Creek Anchor Point Caribou Hills Eagle River fault 

No. I N 1 No. I 

1 

NinilchikA _AK0 " ANorth Fork ____A_ ~A_Anchor River No. 

No. IY YY Y41-35 y Y 

TECTONIC EVENT 

744m 1192m 690m 

277m I ~ Oil field structures 

471m formed. 

921m 1073m I 

57 3m 1003m 942m ) Kenai Mountains uplifted. 

863m 

2012m 1890m 1862m 1818m 

180m 165m 76m 18m 29m 

Renewed movement along 

Bruin Bay and Border 

Ranges faults to form 

Cook Inlet Tertiary bastn. 

I / 

~ Mesozoic deformed, uplifted, 

f '"' "''''' mlloot tlmo Valdez Group-Kodiak 

~~I Forma.tion tectonically 

© emplaced. 

Uplift and erosion of 

Cretaceous plutonic bodies. 

Orocline in vicinity of 

Matanuska Valley. 

Minor deformation in Cook 

Inlet-Alaska Peninsula. 

Earliest time McHugh 

Complex tectonically 

emplaced. 

r-'" 

- -

r "·"" .. , ,; ... ,.,, .. 256m Talkeetna arc sequence; 

292m first movement along Bruin 

(Kialagvik Formation) 

Bay or related fault. 

© f Tolk""'"' "'"'""· 

unpublished information, (4) Cowan and Boss (1978), (5) Kelley (1978), (6) Magoon and others (1976a), (7) unpublished 

section, and (8) Magoon and others (1980). letters refer to (A) Seldovia schist terrane, (B) Kachemak terrane (Triassic rocks), 

(C) Chugach terrane, and (D) Peninsular terrane. Hachure lines indicate time of nondeposition or erosion. 

Framework Geology and Sandstone Composition 79 
Lower Jurassic volcaniclastic sandstone, conglomerate, and 

limestone are exposed (Imlay and Detterman, 1977). According to Detterman (oral commun., 1980), Lower Jurassic rocks 

of Puale Bay are very similar to the Talkeetna Formation in 

the Talkeetna Mountains. 

The presence of the Lower Jurassic Talkeetna Formation on both sides of Cook Inlet, in some offshore areas (Boss 

and others, 1976; Fisher and Magoon, 1978), and in the Puale 

Bay area suggests that this unit is continuous beneath Cook 

Inlet and Shelikof Strait. 

Lower Jurassic sandstones from widely spaced areas in 

Cook Inlet and Shelikof Strait and adjacent areas are similar 

and they are also similar to sandstones in the underlying 

Upper Triassic sequences. The Lower Jurassic sandstones are 

generally volcanic lithic rich, reflecting a primary volcanic 

nature of associated rocks interbedded with the sandstones. 

Average modal compositions from the Puale Bay area and 

from the Kenai Peninsula are Q6F 42L52 (Egbert, this volume) 

and Q3F20L77 (Kelley, 1980), respectively. 

The sandstones are distinctly quartz poor with low 

polycrystalline quartz to total quartz ratios (C/Q) indicating 

that most of the quartz that is present is monocrystalline. The 

quartz has features indicative of volcanic derivation: clear, 

unstrained, resorbed margins or bleb inclusions. Plagioclase 

to total feldspar ratios (P/F) are high (0.97 to 1.0); the high 

ratios indicate that almost all the feldspar is plagioclase. 

Volcanic lithic to total lithic fragment ratios (V/L), which are 

also high (0. 98 to 1. 0), reflect the volcanic lithic rich nature of 

the sandstones and indicate an extensive volcanic source 

terrane. 

Middle Jurassic Rocks 

Unconformably overlying the Lower Jurassic volcaniclastic rocks is a thick sequence of Middle Jurassic 

marine sedimentary rocks. These rocks are exposed only on 

the west side of Cook Inlet and on the Alaska Peninsula east 

of the Bruin Bay fault. In the subsurface, 292 m (960 ft) of 

Middle Jurassic rock has been penetrated in the North Fork 

41-35 well on the Kenai lowland (fig. 37). 

Middle Jurassic rocks are divided, in upward order, into 

the Tuxedni Group and the Chinitna Formation in the Cook 

Inlet area and into the Kialagvik and the Shelikof Formations 

on the Alaska Peninsula. 

Tuxedni Group and Kialagvik Formation 

The Tuxedni Group, Bajocian to Callovian in age, 

includes as many as six formations and ranges in thickness 

from 536 to 2,748 m (1,760 to 9,015 ft) in the IniskinTuxedni area (Detterman and Hartsock, 1966, pl. 5). According to Detterman and Hartsock (1966, pl. 5), the thickness of 

the Tuxedni Group on the Chisik Island-Tuxedni Bay section 

measures 1,687 m (5,535 ft) (fig. 37), is abundantly fossiliferous and contains 35 percent sandstone and 65 percent 

80 Geologic Studies of the Lower Cook Inlet COST No. 1 Well 

siltstone with minor amounts of tuff. All the sediments were 

deposited in a shallow marine shelf environment. 

The Kialagvik Formation, Toarcian and Bajocian in age 

and in part correlative to the Tuxedni Group, crops out along 

the north shore of Puale Bay on the Alaska Peninsula (Imlay 

and Detterman, 1977). The unit is at least 730 m (2,400 ft) 

thick (fig. 37), is sparsely fossiliferous and contains 10 percent sandstone-conglomerate and 90 percent siltstone representing submarine slope-channel fill and slope deposits, 

respectively. 

Sandstones from the Tuxedni Group and Kialagvik 

Formation from widely spaced areas have similar compositions. The sandstones are feldspatholithic with average modal 

compositions of Q6F 43L51 (Egbert and Magoon, 1981) and 

Q8F35L57 (Egbert, this volume) from the Tuxedni Group and 

Kialagvik Formation respectively. The sandstones are quartz 

poor with low polycrystalline quartz to total quartz ratios 

indicating that most of the quartz is monocrystalline. The 

quartz, mostly of volcanic origin, was probably derived from 

the underlying volcanic and volcaniclastic Talkeetna Formation. 

The high (0.92-0.95) plagioclase to total feldspar ratios 

(P/F) indicate that most of the feldspar is plagioclase. The 

high ratios of volcanic lithic to total lithic fragments (V/L) 

reflect the lithic-rich nature of the sandstones and also their 

derivation from an extensive volcanic source terrane (Talkeetna Formation). 

Chinitna and Shelikof Formations 

The Chinitna Formation of Callovian age, has two 

members, and ranges in thickness from 558 m (1,830 ft) at 

the Chisik Island-Tuxedni Bay section to 728 m (2,375 ft) in 

the Iniskin-Tuxedni area (fig. 37; Detterman and Hartsock, 

1966, pl. 5). The Chisik Island-Tuxedni Bay section contains 

10 percent sandstone-conglomerate and 90 percent siltstone. 

These rocks represent submarine channel fill conglomerate 

and slope facies siltstone and thin-bedded turbidite sandstone 

of the forearc basin. 

The Shelikof Formation, of Callovian age and a correlative unit to the Chinitna Formation, overlies the Kialagvik 

Formation in the Puale Bay area (Kellum and others, 1945; 

Imlay and Detterman, 1977). Here the Shelikof Formation is 

at least 1,525 m thick (5 ,000 ft) (fig. 37), is sparsely fossiliferous, and contains 80 percent sandstone and 20 percent 

siltstone. These rocks mainly represent submarine fan 

deposits. 

Sandstones from the Chinitna and Shelikof Formations 

are generally lithofeldspathic with average modal compositions of Q5F 59L36 and Q9F 46L45 , respectively (Egbert and 

Magoon, 1981; Egbert, this volume). Quartz content is low in 

both units; the low polycrystalline quartz to total quartz ratios 

indicate that most of the quartz is monocrystalline. Present in 

only minor amounts, the polycrystalline quartz probably 

represents silicified volcanic detritus. 
Feldspars are the most abundant framework grains in 

the Chinitna and Shelikof Formations. The high plagioclase 

to total feldspar ratios (P/F) indicate that most of the feldspar 

is plagioclase. Lithic fragments are the second most abundant 

framework grains; the high volcanic lithic to total lithic 

fragment ratios (V/L) indicate a predominantly volcanic 

source terrane. The high feldspar content may indicate plutonic source areas in addition to volcanic sources. 

Hornblende becomes a dominant accessory mineral in the 

Shelikof Formation and in the upper part of the Chinitna 

Formation, however, its virtual absence in underlying units 

indicates the unroofing of the Jurassic plutonic rocks. 

Upper Jurassic Rocks 

Naknek Formation 

The Naknek Formation of Oxfordian to Tithonian age 

unconformably overlies the Chinitna and Shelikof Formations. The Naknek Formation crops out from the Alaska 

Peninsula to Tuxedni Bay and is also present in the 

Matanuska Valley. It has greater areal exposure than the 

underlying or overlying rocks. The depositional environment 

of the Naknek Formation varies from fluvial, alluvial and 

shallow marine deposits to deep-marine channel, slope, and 

submarine fan deposits. 

In the Iniskin-Tuxedni area, as much as 1,612 m (5,285 

ft) of the Naknek is exposed and the outcrops represent four 

members (fig. 37; Detterman and Hartsock, 1966) and in the 

Kamishak Bay area, the thickness ranges from 1,585 to 1,828 

m (5,200 to 5,995 ft) (fig. 37; Detterman and Reed, 1980). 

The Naknek Formation does not crop out on the southeast 

side of the basin, but 256m (840ft) of Upper Jurassic rocks 

were penetrated in the North Fork 41-35 well and 1,663 m 

(5,455 ft) were penetrated in the COST No. 1 well (fig. 36). 

Along the Alaska Peninsula, the Naknek varies in thickness 

and may be as much as 3,050 m (10,000 ft) thick (Burk, 

1965). 

Sandstones from the Naknek Formation on the Alaska 

Peninsula are all generally feldspathic with moderate 

amounts of quartz and average modal compositions of 

Q28F 63L9 (Egbert, this volume), Q38F 52L10 (McLean, 1979), 

Q36F60L4 (Lankford and Magoon, 1978), and Q47F52L1 

(Burk, 1965). These rocks are similar to sandstones penetrated in the COST No. 1 well that have an average modal 

composition of Q24F 55L21 (McLean, this volume). This composition contrasts to Naknek sandstones exposed in the Iniskin-Tuxedni region that are distinctly quartz poor but still 

feldspathic; they have average modal compositions of 

Q6F73L21 (Egbert and Magoon, 1981) and Q4F58L38 (Franks 

and Hite, 1980). 

Although there is a distinct contrast in quartz content 

between the Naknek exposed on the Alaska Peninsula to that 

exposed in the Iniskin-Tuxedni area, the polycrystalline 

quartz to total quartz ratios are all similar and indicate that 

almost all of the quartz is monocrystalline. Plagioclase to 

total feldspar ratios (P/F), which are all high, indicate that 

most of the feldspar is plagioclase, however K-feldspar is 

more abundant in the Naknek Formation than in the underlying units. Potassium feldspar in the Naknek Formation, which 

averages 13 percent of the total feldspar population, is most 

abundant in samples from the COST No. 1 well. 

Volcanic lithic to total lithic fragment ratios (V/L) are 

high in the Naknek Formation but are lower than the ratios in 

underlying units; this variation indicates greater diversity in 

the source area. Plutonic, sedimentary, and metamorphic 

lithic fragments account for the remaining lithic fragment 

population. 

Upper Jurassic and Lower Cretaceous Rocks 

Staniukovich Formation 

The Staniukovich Formation of Late Jurassic (Kimmeridgian) through Early Cretaceous (Valanginian) age 

crops out on the southwest end of the Alaska Peninsula where 

it conformably overlies the Naknek Formation. The formation is unknown in the Cape Douglas area and areas farther 

northeast. The Staniukovich Formation is as much as 549 m 

(1,800 ft) thick and consists mainly of marine sandstone and 

siltstone (Burk, 1965). 

Two sandstone samples described by Burk (1965) are 

feldspathic; their average modal composition, Q55F 41L4 , is 

similar to the average modal composition, Q48F 42L10, of 

Franks and Hite (1980). The sandstone samples in the 

Staniukovich are more quartz rich than those in the underlying Naknek Formation and are nearly compositionally 

mature. 

Lower Cretaceous Rocks 

Herendeen Limestone 

Lower Cretaceous (Valanginian to Barremian) rocks 

crop out in many areas from the Matanuska Valley and 

Wrangell Mountains in the north (Grantz and others, 1966; 

MacKevett, 1976, 1978) to Port Moller-Herendeen Bay area 

on the Alaska Peninsula in the south (Atwood, 1911; Burk, 

1965; Molenaar, 1980). 

In this area, thick beds of the Naknek Formation (91 + 

m; 300 + ft), Staniukovich Formation (550 m; 1,800 ft), and 

Herendeen Limestone (152 m; 500 ft) (Burk, 1965; Jones, 

1973) seem to be substantial evidence that sedimentation was 

continuous from Late Jurassic through Early Cretaceous; 

however, Molenaar (1980) demonstrates an unconformity 

between Lower Cretaceous and Upper Jurassic strata in the 

Chignik area 160 km (100 mi) to the northeast. 

In the Kamishak Hills, 215 m (700ft) of Lower Cretaceous rock (Herendeen Limestone) unconformably overlies 

Upper Jurassic strata (Jones and Detterman, 1966; Jones, 

Framework Geology and Sandstone Composition 81 
1973; Magoon .and others, 1976a, 1978). The beds consist of 

7 percent sandstone and 93 percent siltstone and are rich in 

Inoceramus fragments. These bioclastic fragments occur in 

very high concentrations and are described as limestone in 

this predominantly siltstone rock unit. 

In the subsurface, Lower Cretaceous rocks were 

depicted by Boss and others (1976) in a cross section through 

Cook Inlet basin north of Kalgin Island. On the Kenai 

Lowland 215 m (700 ft) of Lower Cretaceous rocks (Herendeen Limestone) were penetrated in the Anchor Point No. 1 

well and 572 m (1 ,875 ft) were penetrated in the COST No. 1 

well (fig. 37, table 16). 

The Lower Cretaceous Nelchina Limestone crops out 

east of the Matanuska Valley (Grantz and others, 1966) and in 

the Wrangell Mountains (MacKevett, 1976, 1978). The similarity of both the Herendeen and Nelchina Limestones, in 

age and lithology, has been acknowledged by many workers. 

In the Wrangell Mountains the undeformed Lower Cretaceous rocks unconformably overlie much older severely 

deformed rocks (MacKevett, 1976, 1978). 

Sandstone from the Lower Cretaceous sequences are 

generally feldspathic and lithofeldspathic with average modal 

compositions of Q62F33L5 from the Alaska Peninsula (Burk, 

1965), Q38F 59L3 from the Cape Douglas area, (Lankford and 

Magoon, 1978) and Q36F44L20 from the COST No. 1 well 

(McLean, this volume). Quartz content is similar to the upper 

part of the underlying Naknek and Staniukovich Formations 

reflecting increased compositional maturity from underlying 

units. 

Polycrystalline quartz to total quartz ratios are very low; 

the low ratio indicates that most of the quartz is monocrystalline. Plagioclase to total feldspar ratios (P/F) are high 

but K-feldspar ranges from 12 to 23 percent of the total 

feldspar populations. Volcanic lithic to total lithic fragment 

ratios (V/L) are moderately high, but total lithic fragment 

populations are low. 

Unnamed Albian Rocks 

Rocks of Albian age have been reported from the 

Matanuska Valley (Grantz, 1964) and Wrangell Mountains 

(Jones, 1973). Fossils collected recently by the authors on the 

Alaska Peninsula, which were identified by D. L. Jones, 

indicate that Albian rocks are also present on the Alaska 

Peninsula. The fossils were collected north of the Katmai 

River from rocks in fault contact with the Naknek Formation. 

Albian sandstone samples from the Matanuska Valley 

and on the Alaska Peninsula are lithic and feldspatholithic 

with average modal compositions ofQ4F4L92 and Q11F40L49 

respectively (Grantz, 1964; Egbert, this volume). These compositions are in sharp contrast to the more feldspathic nature 

of the Lower Cretaceous (Valanginian to Barremian) and 

Upper Jurassic sandstone. 

Polycrystalline quartz to total quartz ratios (C/Q) are 

very low. Plagioclase to total feldspar ratios (P/F) are high, 

82 Geologic Studies of the Lower Cook Inlet COST No. 1 Well 

however K-feldspar accounts for 25 percent of the total feldspar population from samples from the Matanuska Valley and 

account for only 1 percent of the total feldspar from the 

Alaska Peninsula. The very high volcanic lithic to total lithic 

fragment (V/L) and total lithic fragment populations from 

both areas possibly reflect local but prominent volcanic 

sources during the Albian. 

Upper Cretaceous Rocks 

Upper Cretaceous (Campanian and Maestrichtian) 

rocks in the Cook Inlet area north of Kalgin Island are 

assigned to the Matanuska Formation; south of Kalgin Island, 

they are assigned to the Kaguyak Formation. Magoon and 

others (l976b) and Fisher and Magoon (1978) suggested the 

latitude at Seldovia as an arbitrary boundary separating these 

two formations, but that suggestion was prior to drilling the 

COST No. 1 well (Magoon and Claypool, 1979, 1981) and 

recognizing the Saddle Mountain outcrop northeast of 

Chinitna Bay (Magoon and others, 1980) and the Augustine 

Island outcrop (Buffler, 1976; Detterman and Reed, 1980). In 

addition, the continuity or mapability of Upper Cretaceous 

rocks south of Kalgin Island to the Katmai River can be 

demonstrated using reflection seismic data (Fisher and 

Magoon, 1978), well control, and outcrop patterns. 

The Kaguyak Formation (Keller and Reiser, 1959) 

crops out in the Kamishak Hills-Cape Douglas area where 

1,385 m (4,545 ft) of rock unconformably overlies Lower 

Cretaceous and Upper Jurassic strata (fig. 37). In upward 

succession this formation consists of a thin shallow-marine 

basal conglomerate (5 m; 16ft) that grades upward into thick 

sandstones (32m; 105ft), bioturbated gray siltstone (600 m; 

1,970 ft), and turbidite sandstone interbedded with siltstone 

(460 m·; 1,500 ft). The COST No. 1 well penetrated 744 m 

(2,325 ft) of the Kaguyak Formation. In addition, 83 m (275 

ft) of Upper Cretaceous nonmarine sandstone, conglomerate, 

and coal are exposed northwest of the COST No. 1 well; the 

Upper Cretaceous rocks unconformably overlie Upper 

Jurassic rocks near Saddle Mountain in the Iniskin-Tuxedni 

area (Magoon and others, 1980). 

On the Kenai Lowland, several wells penetrated the 

Kaguyak Formation, including the Anchor Point No. 1 well, 

which penetrated 1,530 m (5,000 ft) of Upper Cretaceous 

strata (figs. 34 and 37, table 16). 

In the Matanuska Valley at least 1,250 m (4,100 ft) of 

Campanian and Maestrichtian rocks of the Matanuska Formation are exposed (Grantz, 1964). In this area the 

Matanuska Formation is composed of claystone, siltstone, 

sandstone, and conglomerate, and the rocks have features 

indicative of deposition from turbidity currents (Grantz, 

1964). 

South of Wide Bay on the Alaska Peninsula (fig. 32), 

the Upper Cretaceous rocks are known as the Chignik Formation (fig. 37) and consist dominantly of sandstone with lesser 

amounts of conglomerate, siltstone, shale and mudstone. 
Depositional environments represented within the formatioil. 

range from fluvial to marine (Burk, 1965; Imlay and Detterman, 1977; Fairchild, 1977; Molenaar, 1980). 

Sandstones from the Matanuska, Kaguyak, and Chignik Formations are of variable composition, ranging from 

lithic to feldspathic with intermediate quartz content. Sandstones from the Kaguyak Formation are represented by two 

compositional modes. In the Cape Douglas area and in the 

COST No. 1 well, the lower part of the Kaguyak Formation 

has average modal compositions of Q34F 55L11 (Lankford and 

Magoon, 1978) and Q22F64L14 (McLean, this volume) 

respectively. Similarly, sandstone from the Kaguyak Formation near Saddle Mountain, which has been correlated with 

the lower part of the Kaguyak Formation penetrated in the 

COST No. 1 well (Magoon and others, 1980), has an average 

modal composition of Q32F 40L28 . 

In contrast, sandstones from the upper part of the 

Kaguyak Formation from the Cape Douglas area and COST 

No. 1 well have average modal compositions of Q27L31F42 

and Q13F18L69 , respectively; these compositions reflect 

increased volcanic lithic detritus from source areas in the 

latest Cretaceous. 

Stratigraphically located sandstone samples were not 

collected from the Matanuska Formation, however, Grantz 

(1964) and Clardy (1974) indicate average modal compositions of Q26F58L16 and Q21F 48L31 , respectively; these compositions are similar to the modal compositions of the lower 

part of the Kaguyak Formation. 

Sandstone samples from the Chignik Formation are 

mostly lithofeldspathic and feldspatholithic with average 

total quartz percentages higher than in the Kaguyak and 

Matanuska Formations. Average modal compositions from 

the Chignik are Q43F31L26 (Fairchild, 1977) and Q34F19L47 

(Burk, 1965). 

Quartz content is variable from the Kaguyak, 

Matanuska, and Chignik Formations, but the low ratios of 

polycrystalline quartz to total quartz (C/Q) indicate that most 

of the quartz is monocrystalline. Plagioclase to total feldspar 

ratios (P/F), although variable in the three formations, generally are lower in the three formations than in the underlying 

units; this variability indicates increased K-feldspar content 

(as much as 41 percent of the total feldspar from the Kaguyak 

Formation and 69 percent from the Chignik Formation, table 

16). The moderate to high ratios of volcanic lithic to total 

lithic fragment (V/L) reflect a prominent volcanic source, but 

plutonic and sedimentary sources are also indicated. 

CENOZOIC SEDIMENTARY ROCKS 

The early Cenozoic sequence (ca. 50-65 Ma) includes 

the Arkose Ridge, Chickaloon, West Foreland, and Wishbone 

Formations (fig. 37). The late Cenozoic sequence (ca. 0-30 

Ma) includes the Kenai Group (Hemlock Conglomerate and 

Tyonek, Beluga, and Sterling Formations) and Quaternary 

rocks. 

Lower Cenozoic Rocks 

Arkose Ridge and Chickaloon Formations 

The Arkose Ridge Formation crops out north of the 

Castle Mountain fault north of the Matanuska Valley. The age 

of this unit has not been established, but it is considered Late 

Cretaceous by Csejtey and others (1977) and Early and 

Late(?) Cretaceous by Grantz and Wolfe (1961) and Paleocene by Magoon and others (1976b) (fig. 38). The outcrops of 

the Arkose Ridge have not been correlated with the subsurface units, so at present it cannot be considered a regional 

unit. 

The Chickaloon Formation, Paleocene in age (Magoon 

and others, 1976b), consists of volcanic lithic sandstone, 

siltstone, and coal. The Chickaloon crops out south of the 

Castle Mountain fault in the Matanuska Valley and can be 

correlated into the subsurface from here to west of the Susitna 

River (Alaska Geological Society, 1969c, 1970a, b). The 

Chickaloon is of regional importance inasmuch as it extends 

into the subsurface and is compositionally similar to and 

correlates, in part, with the West Foreland and Wishbone 

Formations. 

Sandstone from the Arkose Ridge and Chickaloon Formations are feldspathic and feldspatholithic, respectively. 

Average modal sandstone compositions from the Arkose 

Ridge Formation are Q37F57L6 (Winkler, 1978) and 

Q22F68L10 (Clardy, 1974), whereas average modal compositions from the Chickaloon Formation are Q28F20L52 

(Winkler, 1978) and Q30F22L48 (Clardy, 1974). Arkose Ridge 

sandstone compositions reflect the composition of granitic 

and local metamorphic rocks that are directly below stratigraphically whereas the sandstone compositions of the 

Chickaloon Formation indicate a volcanic source in part, 

however the sandstones also contain a heavy mineral population high in epidote and garnet that partly reflects a metamorphic source terrane (Kirschner and Lyon, 1973; Clardy, 

1974). 

Both the Arkose Ridge and Chickaloon Formations 

have moderate amounts of quartz and low pofycrystalline 

quartz to total quartz ratios, however polycrystalline quartz in 

Chickaloon samples, which is three to four times more abundant than in samples from the Arkose Ridge Formation, may 

be silicified volcanic rock fragments. 

Plagioclase to total feldspar ratios, which are moderate 

to high, indicate that most of the feldspar is plagioclase, but 

as much as 28 percent of the feldspar is K-feldspar in both 

units (Clardy, 1974). Volcanic lithic to total lithic fragment 

ratios are low to moderate in the Arkose Ridge Formation and 

high in the Chickaloon Formation, and total lithic fragments 

are much higher in the Chickaloon Formation than in the 

Arkose Ridge Formation. 

West Foreland and Wishbone Formations 

The type section of the West Foreland Formation, as 

designated by Calderwood and Fackler (1972), is the section 

Framework Geology and Sandstone Composition 83 
Dall and Dall Spurr 

Van Eysinga (1975) Harns c1898) c1900) 

(1892) 

Martin Barnes Barnes 

and Moffit and and 

Katz (1954) Payne Cobb 

(1912) (1956) (1959) 

Parkinson 

(1962) Kelly (1963) 

Wolfe 

and 

others 

(1966) 

Kelly (1968) 

Q) 

Ol 

<( Kenai Kenai Wishbone Kenai Wishbone Kenai Matanuska Kenai Cook Inlet IMatanuska Kenai 

Pen. Pen. Tyonek Hill Pen. Hill Pen. Valley Peninsula basin I Valley J Peninsula 

04--r-Q-ua-te-r-na-ry~H~~~~~s~c~~~~e~~rl-.r~~~~~~rl-~~+----+~~rl----------+-AI-Iuv-iu_m_a-nd~gla-c-ia-ld-e-po-s-its+_•~~,~~~~Y~~~~-'A~IIu-vi-um~aLn-d-gl-ac-ia-l-de-p-os~its 

cene ,.-,..... Clam- t- Clamgulchian 

10 

20 

30 

40 

50 

60 

Neogene 

Paleogene 

Pliocene 

Miocene 

Oligocene 

Eocene 

Paleocene 

Kenai 

Group 

hl 

Kenai 

Group 

Tyonek 

Fm. 

Eska 

Cgl. 

~ 

Kenai 

Fm. 

Ch"k'- I loon 

Fm. 

Tsadaka 

Fm. 

/ 

Wishbone 

Fm. 

/ 

/ 

/ 

/ 

Chickaloon 

Fm. 

Kenai 

Fm. 

' --

upper 

Kenai 

Formation 

middle 

lower 

Eska 

Formation 

Chickaloon 

Formation 

Arkose 

Ridge 

Cgl. 

Kenai 

Formation 

Hemlock 

zone 

Basal 

Tertiary 

nonmarine 

g~~~h Staoe 

8

.!.9:... Homerian Stage 

Kenai 

Group 

\ 

1\ 

1\ 

~ >-

w;>hb.J l Hill r t----

Chicka- Chickaloon loon 

Fm. Fm. 

Seldovian 

Stage 

1\~ 

Hemlock 

zone 

Figure 38. Tertiary correlation chart for the Cook Inlet forearc basin. The chart shows the evolution of stratigraphic 

nomenclature for the Cook Inlet area from the Matanuska Valley to Cape Douglas. The names formally proposed by 

Calderwood and Fackler (1966, 1972) for each rock unit are unchanged, but the ages vary from author to author. 

penetrated between 3,237.0 to 3,508.3 m (10,617 to 11,507 

ft) in the Pan American West Foreland No. 1 well (fig. 34; 

table 16). The lithology is described as "A tuffaceous siltstone-claystone unit containing a few conglomeratic sandstone and thin coal beds" (Calderwo9d and Fackler, 1972). 

Other workers (Adkison and Newman, 1973; Adkison and 

others, 1975a,b; Boss and others, 1976; Rite, 1976; Magoon 

and others, 1976a) describe the lithology of this unit as 

variable but tuffaceous, and much of the composition reflects 

the Mesozoic basement rock. 

Hartman and others (1972) made isopach maps of the 

West Foreland Formation using industrial well information. 

The isopach map indicates that the West Foreland Formation 

thickness ranges from 0 m on the basin periphery to more 

than 600 m (2,000 ft) in the basin center. Kirschner and Lyon 

(1973) and others (Clardy, 1974; Boss and others, 1976; 

Magoon and others, 1976b) correlated this unit with outcrops 

on the west flank of the basin and to the Wishbone Formation 

in the Matanuska Valley. 

84 Geologic Studies of the Lower Cook Inlet COST No.1 Well 

The age of the West Foreland Formation is late Paleocene and early Eocene (Magoon and others, 1976b). Previous age assignments ranged from Oligocene (Crick, 1971; 

Kirschner and Lyon, 1973; Clardy, 1974) to Paleocene 

(Clardy, 1974; Boss and others, 1976). The age is based on 

leaf fossils and palynomorphs from localities in the Cape 

Douglas, East Glacier Creek, and Capps Glacier areas 

(Magoon and others, 1976b). A similar age is indicated for 

the Wishbone Formation in the Matanuska Valley. 

The heavy mineral composition of the West Foreland 

Formation is variable, but it usually contains substantial 

amounts of epidote and hornblende (Kelley, 1973, 1977a; 

Rite, 1976). The Wishbone Formation has a heavy mineral 

composition comparable to the underlying Chickaloon Formation that is high in epidote and garnet; the garnet apparently comes from a local source in the Talkeetna Mountains. 

The contact between the Chickaloon and Wishbone Formations on Wishbone Hill is gradational. However, the lower 

Cenozoic rocks are bounded by unconformities. 
Calderwood 

and Fackler Crick (1971) 

( 1966, 1972) 

Kirschner and Lyon 

(1973) Clardy (1974) Magoon and others 

(1976) 

Boss and others 

(1976) 

Wolfe and Tanai 

(1980) 

Wolfe (1981) 

Cook Inlet basin Cook Inlet basin Cook Inlet basin Cook Inlet 

basin 

Matanuska 

Valley 

Cook Inlet I Matanuska 

basin Valley Cook Inlet basin Cook Inlet basin 

Alluvium and glacial deposits Not discussed Not discussed Not discussed Not discussed Not discussed Not discussed Not discussed 

Sterling 

Formation ---~~~ 

c: 

Beluga ·~ ~ 

Formation ~ ~ J: 

Sterling ClamgulchFormation ian Stg 

Beluga ~ ·;: 

Fo~~:;ion ~ J: 

Sterling Formation 

Beluga Formation 

Tyonek ~ ~ ~ ~ g. 1------~--1 

Form~aion ~ 

Tyonek 

Formation 

0 

c'5 

·~ 

:.:: 

Tyonek ~ a. sadaky CJ 

FormatiOn ·~ ~ Fm.l ·c:;;; 

:2 C) 

~ ·Q)~ I ~ Hemlock r---H-em-1-oc~k---7'/ 

Conglomerate :.:: Conglomerate A 

Sterling 

Formation 

Beluga 

Formation 

Tyonek 

Formation 

---- ~ 

rf~ Sterling Clamgulchian 

Tillite Formation Stage 

,;'1 nn" ~ 

Beluga Homerian Formation Stage 

Sterling Formation 

Beluga Formation 

g. 

0 g.v c'5 0 

-~ c'5 Seldovian 

:.:: ·~ Tyonek Stage -~ Tyonek Formation 

:.:: :.:: Formation 

Tyonek Formation 

West Foreland 

Formation 

~ West Foreland \ 

Hemlock 

Cgl. 

Tsadaka 

Formation Hemlock~; Hemlock Angoonian 

~· Stage 

Hemlock 

Conglomerate 

West 

Foreland 

Formation 

Figure 38. Continued. 

Formation "-

Wishbone 

r.-,...<.,.....,..."1""'1...,......""""~c-,if" Hill L. Formation 

I 

"'~,Y If 

I I Arkose 

Ridge l::gl Chickaloon 

Fm. J!!l Formation 

~/ lgt 

'8t 

II 

L! 

West 

Foreland 

Formation 

I I 

I'§/ 

Arkose /J!!f 

Ridge IS/ 

Fm. 1!1 

It;/ 

ICJI 

J_j_ 

Wishbone Hill 

Formation 

Chickaloon 

Formation 

The depositional environments of the West Foreland, 

Chickaloon, and Wishbone Formations are similar; all are 

nonmarine-fluvial units derived from local source terranes 

(Kirschner and Lyon~ 1973; Clardy, 1974; Rite, 1976). Conglomerate on the west flank of the Cook Inlet indicates 

proximity to the source terrane. Coalescing alluvial fans from 

the northwest flank spread southeastward. Ash from volcanic 

eruptions covered the entire area, in some places burying 

trees in growth position as at East Glacier Creek; locally, coal 

swamps developed. 

Sandstones from the Wishbone and West Foreland Formations are feldspatholithic, similar to the Chickaloon Formation. The average modal composition for the Wishbone 

Formation is Q23F22L55 (Clardy, 1974) and for the West 

Foreland Formation average modal compositions are 

Q22F27L51 (Stewart, 1976), Q23F30L47 (Schluger, 1977; 

Kuryvial, 1977), and Q29F22L49 (Lankford and Magoon, 

1978). 

The ratios of polycrystalline quartz to total quartz (C/Q) 

ranged from zero to moderate (0. 33) for the two units indicatConglomerate Tsadaka 

Formation 

West ~ 

Foreland 5: Wishbone 

Formati~ Formation 

13!~ Arkose 1~1 

Ridge IE I Chickaloon 

Fm. 1::;;1 Formation 

I~ I 

l:gl 

u, 

West Foreland 

Forma lion 

I! I 

West Foreland Formation 

---------- Not discussed 

ing that most of the quartz is monocrystalline. The polycrystalline quartz that is present probably represents silicified 

volcanic fragments (Lankford and Magoon, 1978). 

Plagioclase to total feldspar ratios (P/F), which are 

moderate to high (0.62 to 0.95), indicate that most of the 

feldspar is plagioclase, but as much as 30 percent of the total 

feldspar is K-feldspar in some areas (table 16). The percentages of lithic fragments are high for the Wishbone and West 

Foreland Formations and reflect prominent volcanic source 

terrane. 

Upper Cenozoic Rocks 

Kenai Group 

The Kenai Group, first proposed by Calderwood and 

Fackler (1972), consisted, in ascending order, of the West 

Foreland Formation, Hemlock Conglomerate, Tyonek Formation, Beluga Formation, and Sterling Formation (fig. 38). 

The West Foreland Formation was later excluded from the 

Framework Geology and Sandstone Composition 85 
Kenai Group because of a major unconformity between the 

two units (Clardy, 1974; Boss and others, 1976; Magoon and 

others, 1976b). The Kenai Group now represents almost 

continuous continental deposition from the Hemlock Conglomerate through the Sterling Formation, but each formation reflects a different source terrane or depositional 

environment. 

The stratigraphic committee of the Alaska Geological 

Society (l969a-d, 1970a,b) correlated the Kenai Group, 

which at that time included the West Foretand Formation, by 

making a series of stratigraphic cross sections throughout the 

Cook Inlet basin. Hartman and others (1972) made an isopach 

map of the Kenai Group and Quaternary rocks using industrial well information, and Kirschner and Lyon (1973) correlated the subsurface units of the Kenai Group with the outcrop 

units. The outcrops of equivalent units of the Cook Inlet basin 

from the Matanuska Valley to Cape Douglas, which have 

been correlated and dated, represent two stratigraphic 

sequences-an early Cenozoic sequence and a late Cenozoic 

sequence (Kirschner and Lyon, 1973; Clardy, 1974; Magoon 

and others, 1976b); the two sequences are separated in time 

by a 20 m. y. hiatus. 

Hemlock Conglomerate and Tsadaka Formation 

The type section of the Hemlock Conglomerate, as 

designated by Calderwood and Fackler (1972), was penetrated from 3,392.4 to 3,566.2 m (11,697 to 12,890 ft) in the 

Richfield Swanson River No. 1 (34-10) and is described as 

"conglomeratic sandstones and conglomerates." The 

detailed description includes coal and siltstone. Boss and 

others (1976) described the "Hemlock sandstone member" 

as a blanket sandstone that consists of 50 to 70 percent 

sandstone with the remainder siltstone and a few thin coal 

seams. Similar descriptions of the Hemlock Conglomerate 

were made by Crick (1971), Hartman and others (1972), and 

Adkison and Newman (1973). 

The Hemlock Conglomerate crops out in the Cape 

Douglas and Harriet Point areas. In the Matanuska Valley, 

correlative rocks are known as the Tsadaka Formation 

(Magoon and others, 1976b). The Hemlock Conglomerate 

thickens from 0 on the periphery of the basin to 244m (800ft) 

in the center of the basin (Hartman and others, 1972). In the 

subsurface, two other thick sections are shown: (1) a section 

near the west flank at Trading Bay (374 m; 1,225 ft) of the 

Hemlock Conglomerate, and (2) a section near the Castle 

Mountain fault (448 m; 1,470 ft)-a lateral equivalent of the 

Bell Island Sandstone. Coal in the Hemlock Conglomerate is 

concentrated along both flanks of the basin in the vicinity of 

Trading Bay and the Kenai Lowlands (Hite, 1976). 

The age of the Hemlock Conglomerate and the Tsadaka 

Formation is late Oligocene (Clardy, 1974; Magoon and 

86 Geologic Studies of the Lower Cook Inlet COST No. 1 Well 

others, 1976b); the age is based on leaf fossils and palynomorphs in the Cape Douglas and Harriet Point areas and 

in the Matanuska Valley (Magoon and others, 1976b). Recent 

studies by Wolfe and Tanai (1980) show that the Hemlock 

Conglomerate is of early Oligocene age. Previous age designations ranged from Eocene to Miocene (Crick, 1971; 

Adkison and Newman, 1973; Kirschner and Lyon, 1973; 

Boss and others, 1976; fig. 38). 

The heavy mineral composition is high in garnet and 

epidote (Kelley, 1973; Kirschner and Lyon, 1973; Hite, 1976). 

Clardy (1974) indicates that the Tsadaka Formation, a lateral 

equivalent of the Hemlock Conglomerate, is high in epidote. 

The depositional environments of the Hemlock Conglomerate are fluvial-deltaic to estuarine (Hartman and others, 1972; Kirschner and Lyon, 1973; Boss and others, 1976; 

Hite, 1976). The lower contacts of both the Hemlock Conglomerate and Tsadaka Formation are sharp unconformities, 

whereas the upper contact of the Hemlock Conglomerate 

with the Tyonek Formation has been depicted as an unconformity (Hite, 1976) or a gradational boundary (Crick, 1971; 

Kirschner and Lyon, 1973; Boss and others, 1976; Magoon 

and others, 1976b). 

Sandstones of the Hemlock Conglomerate and the 

Tsadaka Formation are generally feldspatholithic and 

lithofeldspathic. Average modal compositions are variable 

for the two units, but the quartz content is greater for both 

units than it is in the underlying units. The Tsadaka Formation has an average modal composition of Q37F 40L23 (Clardy, 

197 4 ), whereas the Hemlock Conglomerate has an average 

modal composition of Q66F15L19 (Stewart, 1976), and the 

undifferentiated Hemlock Conglomerate and Tyonek Formation has an average modal composition of Q39F24L37 (Lankford and Magoon, 1978). 

The very low ratios of polycrystalline quartz to total 

quartz (C/Q) for both units indicate that most of the quartz is 

monocrystalline. Plagioclase to total feldspar ratios range 

from 0. 78 for the Tsadaka Formation to 0. 50 for the Hemlock 

Conglomerate; this variability indicates that K-feldspar averages 22 and 50 percent of the total feldspar from the Tsadaka 

Formation and Hemlock Conglomerate, respectively (Stewart, 1976; Clardy, 1974). 

Volcanic lithic to total lithic fragment ratios (V/L) are 

low for both the Tsadaka Formation (0.26) and Hemlock 

Conglomerate (0.21); these low ratios completely contrast 

with all the other underlying units. Metamorphic and plutonic rock fragments, in subequal proportions, account for 

most of the lithic fragment populations. 

Tyonek Formation 

The type section of the Tyonek Formation, as designated by Calderwood and Fackler (1972), was penetrated 

from 1,310.6 to 3,642.4 m (4,300 to 11,947 ft) in the Pan 

American Tyonek State 17587 No. 2 well. The lithology is 

described as "massively bedded sandstones and thicker coal 
beds which contrast with the more thinly bedded sandstones, 

claystones, and thin lignitic coal beds of the overlying Beluga 

Formation." Crick (1971), Hartman and others (1972), 

Adkison and Newman (1973), Kirschner and Lyon (1973), 

and Rite (1976) give similar lithologic descriptions. 

The Tyonek Formation crops out in the Cape Douglas, 

Harriet Point, and Susitna River areas (Magoon and others, 

1976b). In the subsurface this unit thickens from 0 on the 

periphery of the basin to more than 2,134 m (7,000 ft) in the 

center of the basin (Hartman and others, 1972). A sandstone/ 

shale ratio map indicates that in the Trading Bay area, the unit 

is more than 50 percent sandstone (Hartman and others, 

1972). 

The age of the Tyonek Formation, late Oligocene to 

middle Miocene, is based on leaf fossils and on palynomorphs in the Cape Douglas, Harriet Point, and Capps 

Glacier areas (Magoon and others, 1976b). Wolfe and Tanai 

( 1980), however, believe that the age of the Tyonek Formation 

ranges from early Oligocene to middle Miocene. 

The depositional environment of the Tyonek is 

described as fluvial, deltaic, and estuarine (Hartman and 

others, 1972; Kirschner and Lyon, 1973), or as alluvial (Boss 

and others, 1976; Rite, 1976). In outcrops at Capps Glacier, 

near Seldovia, and in the subsurface of the Kenai Low land 

where the Hemlock is missing, the Tyonek Formation unconformably overlies the West Foreland Formation. The upper 

contact of the Tyonek Formation with the Beluga Formation 

is described as gradational and intertonguing (Kirschner and 

Lyon, 1973; Boss and others, 1976; Hayes and others, 1976) 

or as an unconformity (Crick, 1971; Calderwood and Fackler, 

1972). 

The heavy mineral concentrations for the Tyonek Formation and underlying Hemlock Conglomerate are high in 

garnet and epidote (Kelley, 1973, Rite, 1976; C. E. Kirschner 

and others, written commun., 1979). These units are also 

thick in the subsurface near the Matanuska Valley. These 

rocks compare in both age and lithology to the Nenana coalbearing rocks farther north, and the wide distribution of the 

rocks suggest that both rock sequences had source areas in 

interior Alaska or western Canada, northeast of the present 

Alaska Range (Wahrhaftig and others, 1969; Kirschner and 

Lyon, 1973; Buffler and Triplehorn, 1976). From late 

Oligocene to middle Miocene time, one or more river systems brought garnet-rich sediments from interior Alaska or 

Canada through the Nenana and Susitna River areas to the 

Trading Bay and Matanuska Valley areas. 

Sandstones from the Tyonek Formation are generally 

feldspatholithic with an average modal composition of 

Q6IF19L20 (Stewart, 1976). Quartz content is relatively high, 

and a low polycrystalline quartz to total quartz ratio (C/Q) 

indicates that most of the quartz is monocrystalline. Plagioclase to total feldspar ratios are low ( <0.50); K-feldspar 

averages 54 percent of the total feldspar population. Volcanic 

lithic to total lithic fragment ratios are low for the Tyonek 

Formation; the ratios are similar to those of the underlying 

Hemlock Conglomerate and Tsadaka Formation that have 

high percentages of metamorphic and plutonic rock fragments. 

Beluga Formation 

The type section of the Beluga Formation, as designated 

by Calderwood and Fackler (1972), was penetrated between 

1,100.3 to 2,365.3 m (3,610 to 7,760 ft) in the. Standard Oil 

CompanyofCaliforniaBelugaRiverNo. 1 (212-35) well, and 

"consists of thin intercalated beds of claystone, sandstone, 

siltstone, and lignitic to sub bituminous coal." Similar 

descriptions were given by Crick (1971), Hartman and others 

(1972), Adkison and Newman (1973), Adkison and others 

(1975b), Boss and others (1976), and Hayes and others 

(1976). 

The Beluga Formation crops out in cliffs near Homer 

and northeast of Trading Bay (Magoon and others, 1976b). In 

the subsurface, this unit thickens from 0 on the basin edge 

where it is truncated by the Sterling Formation to as much as 

1,525 m (5,000 ft) in the basin axis under the Kenai Lowland 

(Hartman and others, 1972). 

The age of the Beluga Formation, based on leaf fossils 

and palynomorphs from outcrops north of Trading Bay and 

north of Kachemak Bay, is late Miocene (Magoon and others, 

1976b) or middle and late Miocene according to Wolfe and 

Tanai (1980). The upper contact with the overlying Sterling 

Formation is considered to be unconformable by some workers (Crick, 1971; Calderwood and Fackler, 1972; Hayes and 

others, 1976), transitional in part by Kirschner and Lyon 

(1973), and intertonguing by Boss and others (1976). A high 

epidote content is typical of the Beluga Formation (Kirschner 

and Lyon, 1973; Rite, 1976; Biddle, 1977). 

The depositional environment is believed to have been 

alluvial fans (Kirschner and Lyon, 1973; Hayes and others, 

1976) or glacial deposits (Boss and others, 1976). Both 

depositional models designate the Chugach terrane on the 

southeast as the primary source of sediments with a secondary source of sediments from the north (Hartman and others, 

1972). In late Miocene time, the Alaska Range became a 

prominent topographic feature on the north that cut off drainage from interior Alaska (Wahrhaftig and others, 1969; 

Kirschner and Lyon, 1973; Buffter and Triplehorn, 1976). 

After the Chugach terrane on the southeast was elevated and 

eroded, it provided epidote-rich sediments to the Cook Inlet 

basin. 

Sterling Formation and Quaternary Rocks 

The type section of the Sterling Formation, as designated by Calderwood and Fackler (1972), was penetrated 

from 320.0 to 1,688.6 m (1,050 to 5,540 ft) in the Union Oil 

Company Sterling Unit No. 23-15 well, and it is described as 

"a thick sequence of massive sandstones, conglomeratic 

sandstones, and interbedded claystones." Other authors give 

Framework Geology and Sandstone Composition 87 
a similar description; 50 to 75 percent or more of the section 

is medium grained, well to fairly well sorted, and slightly 

conglomeratic sandstone (Crick, 1971; Boss and others, 

1976; Hayes and others, 1976). Sandstones are composed of 

quartz, plagioclase, biotite, and glassy volcanic rock fragments (Hayes and others, 1976). Hornblende, hypersthene, 

and some epidote are the major heavy minerals (Kirschner 

and Lyon, 1973; Hayes and others, 1976; Hite, 1976; Biddle, 

1977). Primary and secondary porosity are as much as 30 

percent (Boss and others, 1976; Hayes and others, 1976). 

The Sterling Formation crops out in the cliffs of the 

Kenai Lowland. An isopach map of the formation and the 

Quaternary age rocks in the subsurface reveals that these 

rocks are more than 3,050 m (10,000 ft) thick (Hartman and 

others, 1972). The age of the Sterling Formation, late 

Miocene and Pliocene (Magoon and others, 1976b; Wolfe 

and Tanai, 1980), is based on leaf fossils and palynomorphs. 

The depositional environment was alluvial. The primary source terrane was the Alaska-Aleutian Range batholith 

to the northwest (Kirschner and Lyon, 1973; Boss and others, 

1976; Hayes and others, 1976), but the Chugach terrane was 

and still is a contributor of some sediment (Hartman and 

others, 1972). Throughout the Pliocene and Quaternary and 

even up to the present, volcanic material was erupted over the 

entire basin. 

TECTONIC EVOLUTION 

The northern Pacific margin has been the site of continuous convergence throughout the Mesozoic and Cenozoic 

eras; the Cook Inlet-Shelikof Strait and adjacent areas are a 

part of this margin. Many authors have discussed the tectonic 

evolution of various parts of southern Alaska in relation to a 

convergent margin model (Burk, 1965; Plafker, 1969; MacKevett and Plafker, 1974; Moore and Connelly, 1977; Plafker 

and others, 1977; Fisher and Magoon, 1978; Hudson, 1979a, 

b,c). Three major geologic elements are included in many of 

these models: the magmatic arc, the forearc basin, and the 

accreted terranes (table 18). In general, the magmatic arc, 

represented by the Alaska-Aleutian batholith, and the subduction zone, whose present location is the Aleutian trench, 

are directly or indirectly responsible for the temporal, compositional, and spatial relations of the intervening rocks of the 

forearc basin and accreted terranes. 

Beck and others (1980) distinguished two types of 

tectonic (tectonostratigraphic) terranes. First, the "stratigraphic terranes wherein major contacts between principal 

lithic subdivisions are depositional". The Peninsular terrane 

and the Cenozoic sedimentary rocks fall into this category 

(fig. 36). Second, "disrupted terranes wherein major contacts 

between principal lithic subdivisions are faults". The 

Chugach and Prince William terranes (fig. 36) fall into this 

category. 

These stratigraphic terranes of the forearc basin, which 

include the Peninsular terrane and the Cenozoic sedimentary 

rocks, seem to be genetically linked to the intrusive bodies 

that make up the Alaska-Aleutian batholith. The more difficult relationship to demonstrate is the connection between the 

stratigraphic terranes of the forearc basin to the disrupted or 

accreted terranes seaward of the Border Ranges fault. There, 

the rocks are highly deformed, lack adequate fossil control, 

and are bounded by faults. 

Table 18. Magmatic arcs, stratigraphic terranes, and accreted terranes of Cook lnlet-Shelikof Strait area 

[Numbers and letters in parentheses refer to figure 33 and tables 16 and 17] 

Episode 

Post-Eocene 

Pre-01 igocene 

Late Cretaceous 

Late Early and early 

Late Cretaceous 

Early Jurassic into 

Early Cretaceous 

Permian(?) to 

Early Jurassic 

Magmatic arc (plutonic belt) 

Alaska-Aleutian Range (granite to 

granodior i tel 

Alaska Range-Talkeetna Mmntains 

Belt (granite to quartz diorite) 

Alaska Range-Talkeetna Mmntains 

Belt (granite to quartz diorite) 

Not represented 

Aleut ian Range-Talkeetna M>untains 

Belt (granodiorite to quartz 

diorite) 

Kodiak-Kenai belt (tonalitequartz dior i tel 

1 Prince William terrane 

2. Olugach terrane 3 Peninsular terraneo_ 

Stratigraphic terrane (unconformity bounded) 

Hemlock Conglomerate and Tyonek Format ion (43-49) 

Tsadaka Format ion (42.) 

West Foreland Formation (39-41) 

Wishbone Formation (38) 

Olickaloon Formation (36, 37) 

Arkose Ridge Formation (34, 35) 

Kaguyak Formation (2.9, 33)3 

Matanuska Formation (part) (2.7-2.8)3 

Olignik Formation (2.5, 2.6)3 

Matanuska Format ion (Albian part, 2.4) 3 

Unnamed Albian rocks (2.3)3 

Herendeen Limestone (2.0-2.2.)3 

Staniukovich Formation q8, 19)3 

Naknek Formation (11-17) 

Shelikof and Olinitna Formations (8-10)3 

Thxedni Group and Kialagvik Format ion (6, 7) 3 

Talkeetna Format ion (4, 5) 3 

Shuyak Formation (2.)3 

Unnamed ( 1, 3) 3 

Unnamed Permian rocks3 

88 Geologic Studies of the Lower Cook Inlet COST No. 1 Well 

Accreted terrane (fault bounded) 

Narrow Cape Format ion (Q, R) 

Si tkinak Format ion (N-P) 

Sitkalidak Formation (K-M) 

Glast Rocks Formation (1, J)l 

Kodiak and Shumagin Format ions (F-H) Z 

Slivers wacke (E~ 

Valdez Group (D) 

Cape Current terrane (C) Z 

M:Hlgh and Uyak Ca!Iplexes (A, B) Z 

Kachemak terranez 

Kodiak-Seldovia schist terrane(?) 
Paleomagnetic data from both the stratigraphic and 

disrupted terranes strongly suggest that southern Alaska is a 

conglomeration of terranes that originated in a more southerly latitude and subsequently moved north to finally collide 

or accrete in southern Alaska (Packer and Stone, 1974; Jones 

and others, 1977; Hillhouse, 1977; Stone, 1979). The 

petrographic data and depositional history of the sandstones 

in the Cook Inlet basin and adjacent terranes do not preclude 

this interpretation, but the Cook Inlet evidence does constrain 

the interpretation. For example, sandstones in the Cook Inlet 

basin and from adjacent accreted terranes that are of the same 

age and composition as the sandstones of southern Alaska 

may well have had the same source areas. However, we feel 

that the terranes of southern Alaska need not have evolved in 

their present location, and therefore they are not necessarily 

in conflict with the paleomagnetic data. 

The development of the magmatic arc, the forearc basin 

(stratigraphic terrane), and accreted terrane (disrupted terrane 

of the Cook Inlet-Shelikof Strait and adjacent areas) can be 

discussed in terms of the tectonic cycle. Each cycle represents 

one episode in the development of the crust adjacent to this 

convergent margin (Hudson, this volume); each succeeding 

episode affects the crust landward of the trench. The crust, 

which ranged from near oceanic to continental in composition, evolved in six episodes that took place over the following 

time span: (1) Permian(?) through Early Jurassic; (2) Early 

Jurassic into Early Cretaceous; (3) late Early and early 

Late(?) Cretaceous; (4) Late Cretaceous; (5) early Cenozoic, 

and (6) late Cenozoic. The rocks of episodes 1 and 2, and 4 

and 5 may overlap temporally, but they are different geologically, whereas the rocks of episode 3 are different temporally but are poorly represented geologically. 

According to this model, a complete tectonic cycle 

goes through three stages of development; first, plutonism 

and volcanism construct a magmatic arc along a convergent 

margin; second, the tectonic forces associated with the convergent margin and magmatic arc interact to form the outer 

arc ridge, shelf, slope, trench, and abyssal plain. Finally uplift 

and erosion dissect the magmatic arc and the outer arc ridge, 

and the eroded sediments fill the arc basins, shelf, slope, and 

trench, and then spill onto the abyssal plain. Throughout the 

cycle, tectonism, magmatism, and sedimentation proceed at 

varying intensities. Present-day evidence for the existence of 

the cycle include: (1) a plutonic belt with radiometric age 

dates that correspond to the cycle; (2) evidence for coeval 

extrusive volcanic activity of calc-alkaline composition; (3) 

undeformed sedimentary rocks bounded by unconformities 

(stratigraphic terrane) whose provenance is the magmatic arc 

and whose age, from fossils or radiometric age dates, is 

similar to the plutonic belt, and ( 4) accreted (disrupted) 

terrane(s), bounded by faults, whose age based on fossils is 

no younger than the undeformed sedimentary rocks of the 

forearc basin and whose rocks were probably deformed 

towards the end of the cycle. The disrupted terrane need not 

evolve adjacent to nor be intimately associated with the 

stratigraphic terrane. The time needed to complete a stage in 

the tectonic cycle is called an episode. 

Permian(?) through Early Jurassic Episode 

The first episode took place during the Triassic, but 

fossil evidence suggests it may have extended back into the 

Permian(?) and forward into the Early Jurassic. The magmatic arc is represented by the Kodiak-Kenai plutonic belt, 

the extrusive activity by the pillow basalts and greenstones, 

and the erosion of the volcanic terrane is shown by the lithicrich turbidite sandstones (numbers 1 through 3 on table 16 

and fig. 33). Behind or adjacent to the arc, biothermal 

carbonate reefs were periodically inundated by volcanic 

agglomerates. The accretionary wedge may be represented by 

the Triassic rocks in Kachemak Bay or by the Kodiak-Seldovia schist terrane, but the latter rocks may not be related to 

the rocks of the accretionary wedge because the schist is 

presently defined as being inboard of the Border Ranges fault. 

Early Jurassic into Early Cretaceous Episode 

The second episode, which took place from Early 

Jurassic through Early Cretaceous, is the most complete, 

longest, and best understood. The magmatic arc is represented by the Aleutian Range-Talkeetna Mountain plutonic 

belt, the extrusive activity is represented by the volcanogenic 

Talkeetna Formation, and the unroofing of the magmatic arc 

is exquisitely shown by the change in composition of the 

sandstone (numbers 4 through 22 in table 16 and fig. 33) and 

conglomerate in the middle Jurassic through Lower Cretaceous sedimentary rocks of the Peninsular terrane. The 

change in sandstone composition compares closely with the 

magmatic arc provenance of Dickinson and Suczek (1979). 

The fault-bounded accretionary terrane may be represented 

by the McHugh and Uyak Complexes because its youngest 

age is Aptian. The compositions of the sandstone within this 

terrane are variable (letters A and B in table 17 and fig. 33), 

but the terrane is inadequately sampled. The rock sample 

from the Chugach Mountains is more quartz rich compared 

with the lithic content than any other rocks involved in this 

episode from the Peninsular terrane, but the rock samples 

from Kodiak Island do compare fairly well with rocks of the 

same age in the Peninsular terrane. 

Late Early and Early Late(?) Cretaceous Episode 

The third episode, which occurred during the late Early 

and early Late(?) Cretaceous, is poorly represented in the 

rock record. The middle Cretaceous plutons, which are 

located in the Nutzotin Mountain on the USA-Canadian 

border, probably are not represented in the immediate vicinity 

of the Cook Inlet; the only rocks in the forearc basin are lithicrich sandstones of Albian age (numbers 23 and 24 in table 16 

and fig. 33). The accreted terrane is represented by the younFramework Geology and Sandstone Composition 89 
ger Cape Current terrane that occurs on the Shuyak and 

Afognak Islands which are questionably of Turonian-Santonian age (letter C in table 17 and fig. 33). 

Late Cretaceous Episode 

The fourth episode, which occurred during the Late 

Cretaceous, is well represented in the rock record and isolated 

by unconformities. Although the plutonic belt contains intrusive bodies of both ages, the Late Cretaceous episode is 

considered distinct from the early Cenozoic episode because 

a major unconformity occurs at the base of the early Cenozoic 

rocks. The unconformity at the Late Cretaceous-early 

Cenozoic boundary, the most important structural 

unconformity because the Mesozoic rocks were folded and 

truncated before the Cenozoic rocks were deposited, separates the two episodes. The magmatic arc is represented, in 

part, by the Alaska Range-Talkeetna Mountain plutonic belt 

of granite to quartz diorite composition. The composition of 

the Upper Cretaceous sandstone (numbers 30 and 32 in table 

16 and fig. 33) reflects the reworking of the underlying 

feldspathic-rich rocks and deeply dissected magmatic arc of 

the second episode, but eventually the sandstone composition 

becomes more lithic rich high in the section. The primary 

extrusive volcanic rock of the Late Cretaceous may have been 

reworked into the lithic-rich sandstones in the upper part of 

the Kaguyak Formation in the COST No. 1 well (number 31 

in table 16 and fig. 33) and in the Cape Douglas area (number 

33 in table 16 and fig. 33). In general, Upper Cretaceous 

sandstones are more quartz rich than the underlying Mesozoic units (numbers 25 through 33 in table 16 and fig. 33). 

The accreted terrane contains sandstones ranging from those 

having few volcanic-lithic fragments (letter E in table 17 and 

fig. 33) to those having abundant volcanic-lithic fragments 

(letter Din table 17 and fig. 33). From a petrographic study of 

rock fragments from Upper Cretaceous sandstone of the 

Chugach terrane, Zuffa and others (1980) concluded that the 

sandstone was derived primarily from a volcanic arc with 

secondary inputs from a magmatic arc, a subduction complex 

and recycled sedimentary debris. To date (1981), the range in 

compositions of the sandstones in the forearc basin and 

accreted terrane does not preclude that the provenance is the 

90 Geologic Studies of the Lower Cook Inlet COST No. 1 Well 

same for both and that the accreted terrane may be farther 

from the source than the forearc basin. 

Early Cenozoic Episode 

The fifth episode, which occurred during the early 

Cenozoic, includes the same plutonic belt as the previous 

episode. These early Cenozoic stratigraphic units of the 

forearc basin unconformably overlie the Mesozoic rocks and 

the accreted terrane seaward of the Border Ranges fault. The 

composition of the sandstone is approximately 50 percent 

lithic (numbers 36 through 41 in table 16 and fig. 33) except 

for the Arkose Ridge Formation, which contains very few 

lithic fragments (numbers 34 and 35 in table 16 and fig. 33). 

The Ghost Rocks Formation of the accreted (Prince William) 

terrane, (letters I and J in table 17 and fig. 33), is very similar 

in composition to the lithic-rich sandstone. In addition, the 

Sitkalidak and Sitkinak Formations (letters K through P in 

table 17 and fig. 33 ), which are more quartz rich than the 

lithic-rich sandstone, also contain a lot of lithic material. 

Late Cenozoic Episode 

The last episode, which is still going on today, is demonstrated by the granitic to granodioritic plutonic rocks 

exposed in the Alaska Range and the stratigraphic terrane by 

the late Cenozoic Kenai Group; however, there is no evidence 

of an accretionary terrane. Provenance of the lower part of the 

Kenai Group sedimentary rocks in the forearc basin (numbers 

42 through 45 in table 16 and fig. 33) is more quartz rich in the 

lower part than in the upper part; the plutonic body follows 

the same trend as the sedimentary rocks (Hudson, this volume). Heavy-mineral content (garnet and epidote) suggest 

that the provenance for the Hemlock Conglomerate and 

Tyonek Formation is from the interior of Alaska. High concentrations of epidote and volcanic-lithic fragments in the 

Beluga Formation suggest that the provenance is the Kenai 

Peninsula after the Alaska Range was uplifted. The provenance for the Sterling Formation and Quaternary rocks is 

probably the Alaska-Aleutian Range, and to a lesser degree, 

the Kenai Peninsula. Except for today's active volcanism, 

there is no evidence of a plutonic body with K-Ar age dates 

that correlate with the Kenai Group and Quaternary rocks. 
Conclusions 

By Leslie B. Magoon 

1. The COST No. 1 well was drilled in 65 m (214ft) of 

water and penetrated 3,776 m (12,387 ft) of sedimentary 

rocks in the Alaska Outer Continental Shelf block 489 about 

midway between the Iniskin Peninsula on the northwest and 

the Kenai Peninsula on the southeast. The ages of the sedimentary units were determined by wireline logs and the 

classification of Foraminifera, Radiolaria, calcareous nannoplankton, and marine and terrestrial palynomorphs. The 

section from 413 m (1 ,356ft) to 797 m (2,615 ft) is Cenozoic. 

The underlying section, from 797 m (2,615 ft) to 2,112 m 

(6,930 ft), is Cretaceous; at 1,541 m (5,055 ft) the section is 

divided into Upper and Lower Cretaceous rocks. The bottom 

section, from 2,112 m (6,930 ft) to 3,775.6 m (12,387 ft), is 

Upper Jurassic (Oxfordian through Tithonian). The base of 

this section probably was not penetrated so the 1,663 m 

(5,455 ft) thickness is probably a minimum. The top of this 

unit is controversial. Drill cutting samples indicate that the 

top is at 2,109 m (6,918 ft), 2,112 m (6,928 ft), or 2,135 m 

(7 ,003 ft). Paleontologic evidence indicates that the top is at 

2,109 m (6,918 ft) or 2,417 m (7,928 ft). I place the top at 

2,112 m (6,928 ft), just below the abundant Inoceramus 

fragments and just above the first occurrence of laumontite. 

The West Foreland Formation, of lower Cenozoic age, 

extends down to 797 m (2,615 ft). The sediments are sandstone, siltstone, coal, and some conglomerate. Modal analyses of the sandstone indicate that it is feldspatholithic with 

an average composition of Q23F30L47 . The Kaguyak Formation (Upper Cretaceous), from 797 m (2,615 ft) to 1,541 m 

(5,055 ft) contains marine and nonmarine sediments. The 

feldspathic sandstone has an average composition of 

Q23F61L16 . The Herendeen Limestone (Lower Cretaceous) 

contains sandstone, siltstone, shale, and abundant 

Inoceramus fragments from 1,541 m (5,055 ft) to 2,122 m 

(6,930 ft). The lithofeldspathic sandstone has an average 

composition of Q36F 44L20 . The bottom section, the Naknek 

Formation (Upper Jurassic), down to 3,776 m (12,387 ft), is 

dominantly siltstone, shale, and sandstone. The average composition of the lithofeldspathic sandstone is Q24F 55L21 . 

The West Foreland Formation, the shallowest unit 

sampled, has a minimum thickness of 384 m (1 ,259 ft). 

Paleontologic data indicate Mesozoic rocks were reworked 

into a nonmarine environment. The organic carbon content of 

the unit is low and the section is thermally immature. No 

hydrocarbon shows were indicated. The Kaguyak Formation 

is 744 m (2,440 ft) thick and includes three sandstone bodies 

that are potential hydrocarbon reservoirs. Paleontologic and 

lithologic evidence indicate two prograding sedimentary 

sequences that prograde seaward from upper bathyal to nonmarine. Although the organic carbon content is as much as 

1.07 weight percent, the kerogen is low in oil-prone material 

and lacks an adequate thermal history to generate oil. Two 

different hydrocarbon shows were reported in this unit. The 

Herendeen Limestone is 572 m (1 ,875 ft) thick. Foraminiferal data indicate middle bathyal to inner neritic 

marine environments. The organic carbon content is low, and 

the thermal history is insufficient to generate hydrocarbons. 

Only a few hydrocarbon indications were reported. Reservoir 

properties of the sandstones are poor. The deepest rock unit 

penetrated, the Naknek Formation, is at least 1,663 m (5,457 

ft) thick. Even though the rock is thoroughly cemented with 

laumontite, it contained most of the hydrocarbon shows. 

Since the strata, mostly sandstone and siltstone, is low in 

organic content, and the organic matter is not the type to 

generate oil, the oil must have migrated into the area of the 

well bore from another part of the basin where mature oilprone source rocks are present. 

2. Information about the environmental geology of 

the lower Cook Inlet basin is limited; and no quantitative 

information has been collected from the upper Cook Inlet 

basin. 

A number of geologic features and processes have been 

identified that may pose problems to engineering installations 

in the lower Cook Inlet and along the adjacent coastline. 

Strong earthquakes have caused major damage to the area by 

ground failure, surface warping, and tsunamis. Four active 

volcanoes on the west side of lower Cook Inlet are andesitic, 

so they can have violent eruptions. Very little is known about 

the swift bottom currents and sandwaves that have broken 

pipelines in the upper Cook Inlet. 

3. The seismic horizons were determined by comparing the measured seismic reflections with the calculated 

reflections from the COST No. 1 well. Horizon A, at 900 m 

(2,950 ft) depth in the well, is near the top of the Upper 

Cretaceous rocks. Horizon B is at the depth of 2, 005 m 

(6,575 ft) in the well. That depth represents the CretaceousJurassic contact where the rocks no longer have intergranular 

porosity but are cemented with calcite and laumontite. The 

third horizon, C, is 1,400 m (4,600 ft) below the bottom of 

the well and about 3,100 m (10,200 ft) below the top of the 

Upper Jurassic rocks. It represents the contact between the 

top of the Middle Jurassic rocks and the base of the Upper 

Jurassic. 

Conclusions 91 
4. The paleontology and biostratigraphy are based on 

detailed analysis of foraminifers, marine and terrestrial palynomorphs, calcareous nannoplankton, and Radiolaria. 

Eocene fossils from413 to 552 m (1 ,356 to 1,810 ft) represent 

a predominantly nonmarine depositional environment in a 

warm temperate climate. The Paleocene rocks from 552 to 

799 m (1 ,810 to 2,620 ft) are characterized by a palynomorph 

assemblage containing Paraalnipollenites confusus. Well 

developed Late Cretaceous (Maestrichtian) microfossil 

assemblages, from 799 to 1,539 m (2,620 to 5,050 ft), 

indicate nonmarine and marine intervals. Sparse but diagnostic assemblages of Early Cretaceous Hauterivian to Valanginian(?) microfossils, from 1,539 to 2,417 m (5,050 to 

7, 930 ft), indicate a marine depositional environment whose 

paleobathymetry fluctuated twice from upper slope to middle-shelf water depths. Late Jurassic (Tithonian to Oxfordian) 

microfossils, from 2,417 to 3, 776 m (7 ,930 to 12,387 ft), 

indicate an intertidal to inner neritic marine depositional 

environment. 

5. The COST No. 1 well does not contain prospective 

petroleum source rocks. The organic carbon content, which 

ranges from 0. 08 to 0. 65 weight percent, is more abundant in 

the upper part of the well than in the lower. The thermal 

maturity, however, increases with increasing depth. The transition from immature to mature hydrocarbon generation 

seems to be from 2,100-2,700 m (6,900-8,900 ft) in the well. 

The Mesozoic sediments in the COST No. 1 well contain much less organic matter than the Mesozoic sediments in 

the upper Cook Inlet and the North Slope in Alaska. 

Although the thermochemical transformation of the organic 

matter in the Mesozoic rocks is marginally mature to mature, 

the organic matter is dominantly of the type that has a poor 

oil-generating capacity. However, because the COST No. 1 

well did not penetrate the source rocks for petroleum in the 

upper Cook Inlet-the shales of the Middle Jurassic-the 

prospect for petroleum in the lower Cook Inlet should not be 

judged only on the data obtained from the COST No. 1 well. 

The oil-generating system in the Cook Inlet basin is probably 

made up of migrating paths from the Middle Jurassic rocks. 

6. The present-day geothermal gradient for the COST 

No. 1 well is 2.28°C/100 m. Even though this gradient is 

close to that of the Swanson River oil field gradient of 

2.37°CI100 m, the gradient is too low when compared to the 

geothermal trends in the upper Cook Inlet. The evidence from 

the stratigraphy of the rocks, the first occurrence of laumontite, and the TAl values indicate that the rocks in the well were 

subjected to high temperature gradients before uplift and 

erosion. The low vitrinite reflectance values do not substantiate this inference. 

7. The compositions of the sandstones in the COST 

No. 1 well indicate that they were derived from an igneous 

source terrane which had outcrops of both volcanic and 

plutonic rock types. The shaliowest samples are volcaniclastic sandstone. The West Foreland Formation of lower 

Cenozoic age contains feldspatholithic sandstone with an 

92 Geologic Studies of the Lower Cook Inlet COST No. 1 Well 

average composition of Q23F30L47 . The Kaguyak Formation 

(Upper Cretaceous) contains feldspathic sandstone with an 

average composition of Q23F61L16 • Both the Herendeen 

Limestone (Lower Cretaceous) and Naknek Formation 

(Upper Jurassic) contain lithofeldspathic sandstone with 

average modal compositions of Q36F 44L20 and Q24F 55L21 , 

respectively. 

8. The diagenetic changes most affected the volcanic 

rock fragments and plagioclase feldspar that are abundant in 

the sandstone samples. Sandstone diagenesis increases with 

increasing depth in the COST No. 1 well. Pseudomatrix is 

more common in the deeper sandstones than in the higher 

sandstones. Authigenic clay, montmorillonite and chlorite, 

increases with depth, and some of it formed before laumontite 

crystallization. Laumontite is present in most sandstone samples from depths greater than 2,134 m (7 ,000 ft) where most 

plagioclase is partly to completely albitized and intergranular 

porosity is destroyed. 

The major cause for the reduction of porosity and 

permeability in the COST No. 1 well is due to the formation 

of authigenic clay matrix and pseudomatrix. The minor cause 

is that of compaction and diagenesis. 

The sandstones of the West Foreland Formation (lower 

Cenozoic) have a porosity of nearly 20 percent. The porosity 

and permeability of the sandstone of the Herendeen Limestone (Lower Cretaceous) has been reduced by compaction 

and pore fillings of clay and calcite. The original porosity of 

the sandstone of the Naknek Formation (Upper Jurassic) was 

probably high but is now substantially reduced by compaction and by precipitation of authigenic minerals (laumontite). 

9. The Middle Jurassic sandstones of the Tuxedni 

Group were derived largely from a volcanic terrane. Important differences in composition occur between the Tonnie 

Siltstone Member of the Chinitna Formation and the overlying Paveloff Siltstone Member; the lower member is volcanic-lithic rich, is virtually devoid of hornblende, and is 

cemented mostly with chlorite; and the upper member is less 

lithic-rich (more feldspathic ), contains an average of 7 percent hornblende, and is cemented with laumontite as well as 

chlorite. 

The cementation and compaction of the sandstones in 

the Tuxedni Bay area in the western Cook Inlet should be 

considered when evaluating the hydrocarbon-reservoir potential of the Middle and Upper Jurassic sandstone in Cook Inlet. 

The new data about the composition of the sandstones 

from the Tuxedni Group and Chinitna and Naknek Formations provide evidence that the Alaska-Aleutian Range 

batholith was unroofed during the deposition of the Pavel off 

Siltstone Member of the Chinitna Formation. This event 

would have happened earlier than previously thought. 

10. Eight separate episodes of plutonism in southern 

Alaska may have affected or have affected the provenance 

history of the forearc basin that includes Cook Inlet. The 

petrologic variations of the plutons representing these eight 

episodes reveal a general trend to higher felsic composition 
from the older to the younger plutonic groups. Generally, 

Jurassic plutonism produced mafic to intermediate compositions, Cretaceous plutonism produced mostly intermediate 

compositions, and Tertiary plutonism produced felsic compositions. 

Both short-term and long-term plate-convergent episodes have affected the provenance of the Cook Inlet forearc 

basin. Four relatively short-term episodes that affected the 

Cook Inlet basin occurred during the Late Triassic(?) and 

Early Jurassic, Middle and Late Jurassic, Late Cretaceous 

and early Tertiary, and middle Tertiary. Widespread upper 

Cenozoic volcanic rocks and the presence of many calcalkaline volcanoes in the region are evidence that a fifth 

episode is taking place now. 

Over the long term, the provenance evolved from simple magmatic arc sequences in the early Mesozoic to a 

composite of regional sedimentary, metamorphic, and magmatic arc sequences in the late Mesozoic and Cenozoic. In 

effect, each succeeding magmatic arc increased the proportion of crystalline rocks in the provenance. The simple magmatic arc provenance of the Upper Triassic, Jurassic, and 

Lower Cretaceous produced sandstones changing from lithic 

to feldspathic compositions. The sandstones in the Upper 

Cretaceous should demonstrate a composite provenance that 

includes magmatic arc and recycled orogen types, and, after a 

new magmatic arc cycle began in the Late Cretaceous, local 

development of volcanolithic sandstones. 

Late Cenozoic sandstones should demonstrate a complex composite provenance that includes magmatic arc, 

recycled orogen, and continental block provenance types. 

The diverse sandstones should range from lithic to quartzose 

types. 

The development of three major provenance types in 

southern Alaska will produce a complete range of sandstone 

framework modes. However, the younger sandstones of the 

Cook Inlet basin are the most likely to have quartzose frame'-

work modes because the increasing proportion of metamorphic and plutonic rocks during provenance evolution 

occurred as the compositions in younger plutons became 

more felsic. 

11. Only the sandstones from the Lower Jurassic 

through the Lower Cretaceous changed compositions in the 

way Dickinson and Suczek ( 1979) predicted for magmatic arc 

provenances. In the forearc basin, the Upper Cretaceous 

sandstones change composition from a feldspathic to a feldspatholithic sandstone with very little change in quartz content; this change is the reverse of the kind of change expected 

in a magmatic arc provenance and more like the dissection of 

the previous magmatic arc provenance. The changes in the 

Cenozoic sandstones in the forearc basin and the accreted 

terranes also are those of a magmatic provenance; the sandstones were all feldspatholithic but they increased in quartz 

content very rapidly as they decreased in age, undoubtedly 

demonstrating the more felsic provenance and indicating that 

the drainage system reached into interior Alaska. 

Only in the Cenozoic sandstones was the quartz content 

high enough to provide adequate petroleum reservoirs consistently. The Upper Jurassic sandstone would have been a good 

petroleum reservoir before the laumontite filled all the pores 

in the rocks. 

12. Considering all the data, only three sandstone beds 

of Upper Cretaceous age are potential hydrocarbon reservoirs 

in the COST No. 1 well. These rocks, however, are not the 

source rocks for the petroleum in the upper Cook Inlet. COST 

No. 1 well did not penetrate the shales of the middle Jurassic, 

so the question of whether potential prospective petroleum 

accumulations are present in the lower Cook Inlet basin still 

remains unanswered. 

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