Chapter A 

Stratigraphic Framework of the 

Alaska Peninsula 

By ROBERT L. DETTERMAN, JAMES E. CASE, 

JOHN W. MILLER, FREDERIC H. WILSON, 

and M. ELIZABETH YOUNT 

Sedimentary, volcanic, metamorphic, and plutonic rocks of 

Permian to Quaternary age are named, described, and correlated 

using standard stratigraphic sections 

U.S. GEOLOGICAL SURVEY BULLETIN 1969-A 

GEOLOGIC STUDIES ON THE ALASKA PENINSULA 
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary 

U.S. GEOLOGICAL SURVEY 

Gordon P. Eaton, Director 

Any use of trade, product, or firm names 

in this publication is for descriptive purposes only 

and does not imply endorsement by the U.S. Government 

Text and illustrations edited by Helen Gibbons 

Graphics prepared by Michael Mullen 

Layout and production by Helen Gibbons 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1996 

For sale by 

U.S. Geological Survey 

Information Services 

Box 25286, Federal Center 

Denver, CO 80225 

Library of Congress Cataloging-in-Publication Data 

Stratigraphic framework of the Alaska Peninsula / by Robert L. 

Detterman ... [et al.]. 

p. cm. - (U.S. Geological Survey bulletin ; 1969) (Geologic 

studies on the Alaska Peninsula ; A) 

Includes bibliographical references. 

Supt. of Docs. no.: 1 19.3: 1969A 

1. Formations (Geology)-Alaska-Alaska Peninsula. 2. GeologyAlaska-Alaska Peninsula. 3. Geology, Stratigraphic. 

I. Detterman, Robert L. It. Series. Ill. Series: Geologic studies 

on the Alaska Peninsula ; A. 

QE75.B9 no. 1969-A 

[QE841 

557.3 a-dc20 

[551.7'009798'4] 93-4701 0 

CIP 
CONTENTS 

Abstract 1 

Introduction 2 

Acknowledgments 2 

Geologic setting 4 

Paleozoic rocks 6 

Permian System 6 

Paleozoic(?) and Mesozoic rocks 6 

Permian(?), Triassic, and Jurassic Systems 6 

Metamorphic rocks 6 

Cottonwood Bay Greenstone 6 

Mesozoic rocks 7 

Triassic System 7 

Kamishak Formation 7 

Jurassic System 9 

Talkeetna Formation 9 

Kialagvik Formation 14 

Shelikof Formation 18 

Naknek Formation 19 

Chisik Conglomerate Member 19 

Northeast Creek Sandstone Member 22 

Snug Harbor Siltstone Member 22 

Indecision Creek Sandstone Member 23 

Katolinat Conglomerate Member 24 

Intrusive rocks 25 

Cretaceous System 25 

Staniukovich Formation 25 

Herendeen Formation 28 

Pedmar Formation 29 

Chignik Formation 29 

Hoodoo Formation 32 

Shumagin Formation 34 

Kaguyak Formation 34 

Cenozoic rocks 36 

Tertiary System 36 

Early Tertiary intrusive rocks 36 

Beaver Bay Group 37 

Tolstoi Formation 38 

Copper Lake Formation 42 

Stepovak Formation 43 

Early to middle Tertiary volcanic rocks 46 

Meshik Volcanics 46 

Popof volcanic rocks 48 

Belkofski Formation 48 

Hemlock Conglomerate 49 

Unga Formation 51 

Bear Lake Formation 55 

Tachilni Formation 57 

Milky River Formation 58 

Contents Ill 
Cenozoic rocks-Continued 

Tertiary System-Continued 

Late Tertiary igneous rocks 58 

Intrusive rocks 60 

Volcanic rocks 60 

Morzhovoi Volcanics 61 

Quaternary System 61 

Volcanic rocks 61 

Glacial deposits 62 

Summary 63 

References cited 64 

Appendix-List of fossil megafauna and megaflora collections 70 

FIGURES 

1. Map showing location of study area 3 

2. Generalized geologic map of Alaska Peninsula 4 

3. Generalized map of Cook Inlet area, Alaska 7 

4. Correlation chart of Mesozoic stratigraphic units, Alaska Peninsula 8 

5. Index map of Alaska Peninsula, showing location of sections 

of Mesozoic and Tertiary units 10 

6-1 1. Columnar sections and maps showing: 

6. Triassic and Lower Jurassic formations, Puale Bay, Alaska Peninsula 12 

7. Middle Jurassic Kialagvik Formation, Wide Bay, Alaska Peninsula 15 

8. Lower and Middle Jurassic formations, Puale Bay, Alaska Peninsula 17 

9. Members of Upper Jurassic Naknek Formation, Alaska Peninsula 20 

10. Lower Cretaceous formations, Alaska Peninsula 27 

11. Upper Cretaceous formations, Alaska Peninsula 30 

12. Correlation chart of Tertiary stratigraphic units, Alaska Peninsula 37 

13-16. Columnar sections and maps showing: 

13. Paleocene and Eocene formations, Alaska Peninsula 39 

14. Eocene, Oligocene, and Miocene(?) formations, Alaska Peninsula 44 

15. Oligocene and Miocene formations, Alaska Peninsula 52 

16. Pliocene Milky River Formation, Alaska Peninsula 59 

17. Map showing selected Quaternary volcanic centers and 

associated features on the Alaska Peninsula 62 

SECTIONS 

1. Reference section of the Kamishak Formation, Puale Bay 9 

2. Section of the Talkeetna Formation, Puale Bay 14 

3. Principal reference section of the Kialagvik Formation along Short Creek 14 

4. Reference section of the Kialagvik Formation at Puale Bay 16 

5. Type section of the Shelikof Formation 18 

6. Reference section of the Chisik Conglomerate Member 

of the Naknek Formation 21 

7. Type section of the Northeast Creek Sandstone Member 

of the Naknek Formation 22 

8. Reference section of the Snug Harbor Siltstone Member 

of the Naknek Formation 23 

9. Type section of the Indecision Creek Sandstone Member 

of the Naknek Formation 24 

10. Type section of the Katolinat Conglomerate Member of the Naknek Formation 25 

11. Type section of the Staniukovich Formation at Staniukovich Mountain 26 

12. Reference section of the Herendeen Formation 28 

IV Contents 
13. Type section of the Pedmar Formation 29 

14. Reference section of the Chignik Formation 32 

15. Reference section of the Hoodoo Formation 32 

16. Type section of the Kaguyak Formation 35 

17. Type section of the Tolstoi Formation 40 

18. Reference section of the Tolstoi Formation 41 

19. Section of the Copper Lake Formation 42 

20. Reference section of the Stepovak Formation 47 

21. Measured section of the Meshik Volcanics 48 

22. Generalized section of the Belkofski Formation 49 

23. Reference section of the Hemlock Conglomerate 50 

24. Type section of the Unga Formation 54 

25. Partial section of the Bear Lake Formation 55 

26. Partial section of the Bear Lake Formation 56 

27. Reference section of the Tachilni Formation 57 

28. Reference section of the Milky River Formation 58 

Contents V 

Stratigraphic Framework of the Alaska Peninsula 

By Robert L. Detterman, James E. Case, john W. 

and M. Elizabeth Yount 

Abstract 

This paper describes the sedimentary rocks exposed on the 

Alaska Peninsula, as well as some interlayered volcanic and 

intrusive rocks. This sequence of rocks has a cumulative thickness of about 14,000 m, but most localities contain no more than 

7,000-8,000 m. Locally the section is much thinner where units 

are missing through erosion or nondeposition. The sedimentary 

and volcanic rocks of Tertiary age and older are divided into 23 

formations, several of which are further subdivided into mappable members. Some of the stratigraphic units are newly 

named, and several others have been geographically extended 

from adjoining areas. Many formations are revised either in age 

or with regard to the positions of their contacts. 

Paleozoic rocks are known definitely from only one locality, a small islet at theentrance to Puale Bay. About 40 m of midPermian limestone are exposed on this islet. Isolated outcrops of 

metamorphic rocks are preserved as roof pendants in the AlaskaAleutian Range batholith northeast of Becharof Lake, and their 

protoliths may have been, in part, of Permian age, but the 

lithology indicates that their protoliths were more likely of 

Triassic or Early Jurassic age. 

The Mesozoic sequence ranges in age from Late Triassic to 

Late Cretaceous and has a maximum thickness of about 8,500 

m. In ascending stratigraphic order, the units consist of the 

Kamishak Formation (Upper Triassic), Talkeetna Formation 

(Lower Jurassic), Kialagvik Formation (Lower and Middle Jurassic), Shelikof Formation (Middle Jurassic), Naknek Formation 

(Upper Jurassic), herein-revised Staniukovich Formation (Lower 

Cretaceous), herein-redefined Herendeen Formation (Lower 

Cretaceous), Pedmar Formation (new) (Lower Cretaceous), 

herein-redescribed Chignik Formation (Upper Cretaceous), 

Hoodoo Formation (Upper Cretaceous), Shumagin Formation 

(Upper Cretaceous), and Kaguyak Formation (Upper Cretaceous). Additionally, the Naknek Formation is subdivided into 

five members. In ascendingorder, they are the Chisik Conglomerate Member, Northeast Creek Sandstone Member (new), Snug 

Harbor Siltstone Member, Indecision Creek Sandstone Member 

(new), and Katolinat Conglomerate Member (new). 

The Mesozoic sedimentary rocks are predominantly marine in origin and are feldspathic to arkosic in the upper part of 

the section and volcanogenic in the lower part. The provenance 

Manuscript approved for publication, January 4, 199 1. 

Miller, Frederic H. Wilson, 

fortheclasticsedimentswa~ initially anearly Mesozoicvolcanic 

arc; it shifted with time to the Alaska-Aleutian Range batholith 

(Middle Jurassic) and included some recycled older sediments. 

Clasts from this early Mesozoic volcanic arc are included within 

the Talkeetna Formation. 

The Tertiary sequence is somewhat thinner than the Mesozoic one; it has a maximum thickness of about 5,400 m, but it 

is considerably thinner at most localities due to nondeposition 

or postdepositional erosion. The rocks are mainly continental in 

origin, and the sequence contains a considerable amount of 

volcanic detritus and interbedded volcanic units. These rocks 

are divided into 1 1 formations, 1 of which is further subdivided 

into 2 informal members. In ascendingorder, they aretheTolstoi 

Formation (upper Paleocene to middle Eocene), Copper Lake 

Formation (Paleocene? and lower Eocene), Stepovak Formation 

and herein-redefined Meshi k Volcanics (upper Eocene and 

lower Oligocene), Belkofski Formation (upper Oligocene? and 

lower and middle Miocene?), Hemlock Conglomerate (upper 

Oligocene), herein-revised Unga Formation (upper Oligocene 

to middle Miocene), Bear Lake Formation (middle and upper 

Miocene), Tachilni Formation (upper Miocene), Milky River 

Formation (Pliocene), and Morzhovoi Volcanics (upper Miocene? to lower Quaternary?). Additionally, the Stepovak Formation is divided into informal (lower) siltstone and (upper) 

sandstone members. 

The provenance for sediments of the Tertiary formations 

was contemporaneous volcanic deposits and Mesozoic sedimentary and plutonic rocks of the Alaska Peninsula. The Mesozoic rocks were the main source for the Tolstoi and Bear Lake 

Formations, and the volcanic rocks werethe main source forthe 

other formations. 

Tertiary volcanic and intrusive igneous rocks form a considerable part of the stratigraphic sequence on the Alaska Peninsula. The volcanic rocks were formed mainly during two major 

pulses of activity. The first began during the Eocene (48 Ma) and 

continued into the early Miocene (22 Ma); these materials are 

termed the Meshik arc. Most of the rocks from this episode of 

volcanic activity are included within the Meshik Volcanics. 

Volcanic rocks of this age from the Shumagin Islands are called 

the (informal) Popof volcanic rocks. The second pulse of volcanic activity started during the late Miocene and is continuing at 

present. 

Intrusive bodies ranging in size from small stocks and plugs 

to large batholiths were emplaced throughout the Tertiary. The 

large batholithic intrusionsclustered nearthe beginning andend 
of the Tertiary: The Semidi and Shumagin Islands batholiths and As a result of the numerous investigations, the stratiSanak pluton were emplaced at 58 to 65.6 Ma; the Devils, graphic nomenclature of the Alaska Peninsula has undergone Agripina Bay, and other batholiths at 2 to 10 Ma. considerable change. Now additional changes are indicated, Quaternary rocks and surficial and undoubtedly other changes will be made as more detailed much of the Alaska Peninsula. Large stratovolcanoes are spaced investigations are accomplished~ The Unga Conglomerate alongtheentire peninsula and form the highest peaks; 37 major 

volcanic centers and numerous small vents have been identi- Member (of the Bear Lake Formation) is herein revised as the 

fied. Surficial deposits, primarily resulting from Pleistocene Unga Formation; the Herendeen Limestone is herein redeglaciation, cover most ofthe lowland along Bristol Bay. Deposits fined as the Herendeen Formation, and its age is clarified; and 

from four major glaciations are preserved on the peninsula. the Staniukovich Formation is herein revised, and its age is 

clarified. Additionally, we herein divide the Naknek Formation into five members, from oldest to youngest, the Chisik INTRODUCTION Conglomerate Member, Northeast Creek Sandstone Member 

The Alaska Peninsula is a 775-km-long southwestward 

extension of mainland Alaska (fig. 1). The peninsula is about 

225 krn wide at the northeast end and narrows to less than 10 

km near the southwest end. The southeast side is bordered by 

rugged mountains that rise sharply out of the Pacific Ocean. 

Many large bays and fiords cut the southeast shoreline. The 

northwest side, bordering Bristol Bay, is level and rarely more 

than 100 m above sea level; the coastline is nearly smooth and 

is broken by only a few large bays. At least 37 principal 

volcanic centers dot the length of the peninsula, and at least 30 

have erupted during the Holocene (Miller and Richter, 1994). 

The present Aleutian volcanic arc overlies an early Mesozoic 

magmatic arc and a middle Tertiary volcanic arc. 

A considerable amount of new stratigraphic and paleontologic data has become available as the result of recent work 

on the Alaska Peninsula during investigations for the Alaska 

Mineral Resource Assessment Program (AMRAP). This work 

includes detailed studies of lithostratigraphic units and the 

collection of nearly 1,000 megafauna and megaflora samples 

and more than 200 new K-Ar ages. As a result, we can more 

precisely date many of the units. The new stratigraphic data 

indicate that some named units should be subdivided and 

others combined. 

Geologic investigations on the AlaskaPeninsula started 

more than 200 years ago when G.W. Steller accompanied 

Vitus Bering in 1741 on the voyage that discovered Alaska. 

After Alaska was purchased from Russia in 1867, Dall(1870, 

1882,1896,1898) and Dall and Harris (1 892) explored many 

parts of the peninsula and established a stratigraphic nomenclature. A second wave of exploration started in 1903 and 

continueduntil about 1930, during which time Atwood (191 l), 

Capps (1923, 1934), Knappen (1929), Martin (1905, 1916, 

1921, 1925, and 1926), Mather (1925), Paige (1906), Smith 

(1925a, 1925b), Smith and Baker (1924), and Stone (1905) 

made reconnaissance surveys of parts of the peninsula and 

added to the stratigraphic nomenclature. The most recent and 

detailed investigation of the Alaska Peninsula southwest of 

Wide Bay (see fig. 2) was by Burk (1965). Most of the 

nomenclature in common use at the present time was established by Burk (1965). (For an extensive list of reports 

published through 1985 on the geology of the Alaska Peninsula, see the bibliography by Wilson and others, 1986.) 

(new), Snug Harbor Siltstone Member, Indecision Creek 

SandstoneMember (new), and Katolinat ConglomerateMember (new). Some of the other units contain age changes and 

(or) changes in position of their contacts, and several previously defined units in southern Alaska (Kamishak Formation 

and TalkeetnaFormation) are herein geographically extended 

to include strata on the Alaska Peninsula. 

ACKNOWLEDGMENTS 

This report was made possible through the cooperation 

and assistance, both technical and nontechnical, of many 

individuals in the field and in the office; it would be impossible to acknowledge all of them, and we apologize for any 

major omissions. 

Many individuals within the U.S. Geological Survey 

aided in the collection of field data or gave technical assistance in some of the more specialized aspects of the program. 

Some of the major contributors to the field investigations 

include W.H. Allaway, Jr., Naim Albert, L.M. Angeloni, C.D. 

Blome, D.P. Cox, R.M. Egbert, L.B. Magoon, T.P. Miller, 

D.H. Richter, J.R. Riehle, Nora Shew, and Florence Weber. 

Identification of the many megafauna, microfauna, and 

megaflora specimens was provided by C.D. Blome, R.W. 

Imlay, D.L. Jones, Louie Marincovich, Jr., N.J. Silberling, 

R.A. Spicer, and J.A. Wolfe. Identification of rocks in thin 

section and initial drafting of figures for this report were 

accomplished by Michael Mullen. 

We appreciate the efforts of many persons at Sand 

Point, Chignik Lagoon, Port Heiden, and Port Moller who 

provided logistical support without which the field work 

would have been impossible. A special thanks is due to the 

crew of the U.S. Geological Survey Research Vessel Don J. 

MillerII, especially "Frosty" Frothingham and Ed Magalhaes, 

and to helicopter pilots Bruce Reiche, Glenn Bell, Tom 

Robbins, Edward Svec, Lucky Wilson, and Ross Rossiter, 

who often worked under adverse weather conditions. ReFigure 1. Location of study area. A, Generalized map of Alaska 

showing location of Alaska Peninsula and outline of quadrangles shown in figure 1 B.B,1:250,000-scale quadrangles that 

encompass study area. 

2 Geologic Studies on the Alaska Peninsula 
0 400 KILOMETERS Kodiak I I I I I 

Island 

Peninsula 

Stratigraphic Framework of the Alaska Peninsula 3 
views by C.M. Molenaar and Louie Marincovich, Jr., of the 

U.S. Geological Survey greatly improved the manuscript. 

GEOLOGIC SETTING 

The Alaska Peninsula is part of an active continental 

margin that may have been accreted to North America during 

the Jurassic (Jones and others, 1978; Stone and others, 1982). 

The Alaska Peninsula constitutes the main part of what has 

been called the Peninsular terrane (Jones and others, 198 1) 

and, more recently, the Alaska Peninsula terrane (Wilson and 

others, 1985). The lithology and faunal content of its Triassic 

and Lower Jurassic rocks indicate a depositional source at low 

paleolatitudes, andpaleomagnetic studies suggest the site was 

at about 15" N. (Hillhouse, 1977). Faunal remains, mainly 

ammonites, indicate the Alaska Peninsula was near its present 

latitude by Middle Jurassic time (Taylor and others, 1984). 

Paleozoic rocks are known from only one locality on the 

peninsula. A few 10's of meters of midPermian limestone are 

exposed on one small islet at the entrance to Puale Bay (fig. 2; 

outcrop too small to show on maps in this report). 

Lower Mesozoic (Upper Triassic and Lower Jurassic) 

sedimentary rocks are mainly limestone and volcaniclastic 

sedimentary rocks that are interpreted to have been deposited 

along an island arc. Magmatic activity was associated with 

movement of the Alaska Peninsula to near its present position 

in Middle Jurassic time. This activity resulted in the emplacement of the Alaska-Aleutian Range batholith. Locally, the 

magma chamber vented to the surface and formed a thick 

volcanic sequence. These volcanic rocks were reworked into 

the sedimentary regime and constitute most of the Middle 

Jurassic sedimentary rocks on the Alaska Peninsula. The 

provenance for sediments of the Upper Jurassic and for the 

remainder of the Mesozoic section was mainly the AlaskaAleutian Range batholith; additional components were contributed by minor reworking of older volcaniclastic rocks. 

Lower Tertiary sedimentary rocks were derived mainly 

from erosion of Mesozoic sedimentary rocks and the batholith, 

but by late Eocene time, contemporaneous volcanic rocks 

were becoming a major source for the sedimentary sequence. 

The Meshik volcanic arc (Wilson, 1985) was active from late 

Eocene to early Miocene time, and was followed after a brief 

hiatus by the Aleutian volcanic arc, which began its history 

during the late Miocene and has been active to the present. 

Most post-Eocene sedimentary rocks, with the exception of 

the Bear Lake Formation, were formed from detritus derived 

from these volcanic deposits. 

Rocks at the northeast end of the Alaska Peninsula are 

bisected by a major high-angle fault, the Bruin Bay fault (fig. 

2). From the northeast edge of the field area, the Bruin Bay 

fault continues northeastward for about 325 km before intersecting the Castle Mountain fault, which strikes east-northeast north of Cook Inlet (fig. 3). The Alaska-Aleutian Range 

batholith, associated Lower Jurassic volcanic rocks, and meta- 

.- s 

r 

I-" 

4 Geologic Studies on the Alaska Peninsula 
Bristol 

Bay 

5 8 O 

57O 

56' 

IS 

z. 

9 

'0 

5 

0 

n PA CZFIC OCEAN 

55O n 

% 100 KILOMETERS 

2, 

CC 

3- 

e 

- 

B k 

2' 

2. 162O 1 60° 158' 156O 154O V) 

C- 

&I 

Ul 
morphic rocks lie west of the fault (fig. 2). Slightly deformed 

Mesozoic strata are mainly marine and were deposited in a 

shelf to lower-slope environment. Tertiary strata are largely 

nonmarine to inner neritic. 

PALEOZOIC ROCKS 

Paleozoic rocks were first reported on the Alaska Peninsula by Hanson (1957), when he described limestone of 

mid-Permian age from a small islet at the entrance to Puale 

Bay. Additionally, a Silurian or Devonian age was reported 

for two small areas of limestone north of Becharof Lake (fig. 

2; Detterman and others, 1979). After subsequent reevaluation of the poorly preserved fossils (W.A. Oliver, written 

commun., 1985) and reexamination of the outcrops north of 

Becharof Lake in 1985, we have concluded that the age of this 

limestone is Triassic rather than middle Paleozoic. Consequently, the only definite Paleozoic rocks on the Alaska 

Peninsula are on the small islet at Puale Bay. 

Permian System 

A section about 40 m thick of thin- to thick-bedded, 

medium-grained, crystalline, tan to gray limestone with thin 

interbeds of chert is exposed on a 100-by-200-m islet at the 

entrance to Puale Bay (Karluk C-4 and C-5 1 :63,360 quadrangles). The islet is located about 1 km from nearby islets that 

consist of only Triassic strata. The highly contorted beds dip 

about 40" NW., which would place them structurally under 

the nearby Triassic sequence, but the actual contact relations 

are unknown. 

The limestone contains a poorly preserved and silicified 

megafauna consisting of the horn coral Lophophyllidium sp. 

and the brachiopod Spiriferella aff. S. sarance (de Verneiul). 

Several genera of foraminifers were obtained also from these 

rocks including Bradyina? sp., Endothyra sp., Globivalvulina 

sp., Langella aff. L. conica, Langella sp., Monotaxinoides? 

sp., and Protonodosaria sp. Hanson (1957) reported the age of 

these fossils as late mid-Permian (early Guadalupian). 

Neither the upper nor the lower contact of this unit is 

exposed, so its relations to other stratigraphic units on the 

Alaska Peninsula are unknown. 

PALEOZOIC(?) AND MESOZOIC ROCKS 

Permian(?), Triassic, and Jurassic Systems 

Metamorphic Rocks 

Metamorphic rocks of uncertain age are exposed 

west of the Bruin Bay fault northeast of Becharof Lake 

(fig. 2, unit JPm). The rocks are mainly preserved as roof 

pendants in the Alaska-Aleutian Range batholith, but 

some outcrops are surrounded by surficial deposits, and 

contacts with other bedrock lithologic units are unknown. 

The rocks include metalimestone, quartzite, greenstone and other metavolcanic rocks, and schist. Most are 

weakly metamorphosed greenschist facies rocks in which 

relic bedding is still discernible. A few fragments of a 

silicified megafauna are present in the metalimestone and 

quartzite but are unidentifiable. The minimum age of the 

batholith enclosing the metamorphic rocks is 156 Ma 

(Reed and Lanphere, 1972). This batholith age places an 

upper age limit of early Late Jurassic on the metamorphic 

rocks. Lithologically, the rocks indicate that their precursors were limestone, sandstone, and volcanic rocks similar 

to those in the Upper Triassic and Lower Jurassic sections 

at Puale Bay, east of the fault. The metalimestone could 

also be derived from the Permian limestone sequence. 

These rocks probably are a part of the Kakhonak Complex 

as mapped in the Cook Inlet area northeast of the Alaska 

Peninsula (Detterman and Reed, 1980). 

Cottonwood Bay Greenstone 

The Cottonwood Bay Greenstone was named for a 

sequence of dark-green to gray metavolcanic rocks exposed along the south shore near the head of Cottonwood 

Bay on the west side of lower Cook Inlet (fig. 3; see 

Detterman and Reed, 1980). A similar sequence of 

metavolcanic rocks is exposed near the northeast end of 

the Alaska Peninsula. Whale Mountain on the north shore 

of Becharof Lake consists almost entirely of greenstone 

(fig. 2, unit Xm). A second small area of greenstone (not 

shown on map) is exposed about 18 km to the northeast, 

just north of the King Salmon River. 

The rocks at both these Alaska Peninsula localities are 

weakly metamorphosed to epidote-aIbite-actinolite assemblages suggestive of the greenschist facies. Original flow 

structure is locally discernible, and the rocks' precursors were 

probably porphyritic basalt. Similar-appearing rocks are locally included within the Kakhonak(?) Complex. 

The Cottonwood Bay Greenstone is a part of the 

sequence of rocks preserved as roof pendants in the 

Alaska-Aleutian Range batholith just west of the Bruin 

Bay fault. The thickness of the unit is unknown, but 

probably 400 to 500 m of it is present at Whale Mountain. Contact relations with other stratigraphic units 

are obscure, although at the locality north of the King 

Salmon River, the Cottonwood Bay lies between rocks 

of the Kakhonak(?) Complex and Kamishak Formation. The greenstone has not been dated, but its close 

association with the Kamishak Formation, the basal 

part of which contains clasts of similar-appearing rocks, 

suggests that the age of the Cottonwood Bay Greenstone is Late Triassic, probably Norian. 

6 Geologic Studies on the Alaska Peninsula 
MESOZOIC ROCKS 

Mesozoic sedimentary rocks form the backbone of the 

Alaska Peninsula as well as the adjacent Kodiak, Shumagin, 

and Sanak Islands (fig. 2). The stratigraphic units of the 

Alaska Peninsula sequence are shown in figure 4, which also 

includes an unnamed Permian limestone exposed on one 

small island at the mouth of Puale Bay (Hanson, 1957; 

Detterman and others, 1987a). Stratigraphic sections and 

locality maps are shown for most units (figs. 6-1 1, 13-16). 

The locality maps also contain many geographic names mentioned in the text. The locations of all sections described 

herein are shown on figure 5. 

Triassic System 

Kamishak Formation 

The oldest Mesozoic unit on the Alaska Peninsula is an 

Upper Triassic (Norian) limestone and volcanic rocks unit 

about 800 m thick called the Kamishak Formation (fig. 6) by 

Kellum (1945). The Kamishak Formation was originally 

called the Kamishak Chert by Martin and Katz (1912, p. 47) 

for highly contorted chert and limestone exposed at Bruin Bay 

and Ursus Cove in the Cook Inlet area (fig. 3). Recent 

investigations of the Upper Triassic rocks between Bruin Bay 

and Ursus Cove (Detterman and Reed, 1980) have determined 

that chert is only a minor part of the sequence, and consequently, the name "Kamishak Formation" was applied to 

those rocks by Detterman and Reed (1980, p. B 13). Initially in 

this study, we had concluded that the rocks at Puale and 

Alinchak Bays were not part of the Kamishak Formation, but 

additional thin-section investigations and a reevaluation of 

the fauna now indicate that they are part of the formation. The 

Kamishak Formation was divided into three members in the 

Cook Inlet area by Detterman and Reed (1980). In ascending 

order, they are theBruin LimestoneMember, middle member, 

and Ursus Member. The strata in the Puale Bay area are here 

assigned to the Bruin(?) Limestone Member of the Kamishak 

Formation. The strata in both areas are shallow-water shelf 

deposits containing reef and biohermal buildups. 

Figure 3. Generalized map of Cook Inlet area, Alaska, showing places and faults mentioned in this 

report. Faults, dotted where concealed by water, modified from Beikman (1 980). 

Stratigraphic Framework of the Alaska Peninsula 7 
Modified from Burk (1 965) 

Staniukovich Formation 

Staniukovich Formation 

Naknek Formation 

Kialagvik Formation 

Volcanic rocks 

Unnamed mid-Permian limestone 

Figure 4. Correlation chart of Mesozoic stratigraphic units, Alaska Peninsula. 

8 Geologic Studies on the Alaska Peninsula 
The only other exposures on the Alaska Peninsula that 

may be assignable to the Kamishak Formation are several 

small areas of recrystallized limestone and greenstone preserved as roof pendants in the Alaska-Aleutian Range batholith 

west of the Bruin Bay fault, north of Becharof Lake (fig. 2). 

The limestone and greenstone north of Becharof Lake were 

originally believed to be Silurian or Devonian in age (Detterman 

and others, 1979), but as described previously (see section 

"Paleozoic Rocks"), these rocks are now considered (Late) 

Triassic in age and thus part of the Kamishak Formation. 

Triassic rocks have been reported in the subsurface near the 

southwest end of the Alaska Peninsula, in the AMOCO 

Cathedral River #1 borehole (McLean, 1977). 

A reference section (section 1) for the Kamishak Formation was measured by R.M. Egbert (U.S. Geological 

Survey) in 1979 (fig. 6). The section starts at Cape Kekurnoi 

(Karluk C-4 and C-5 1:63,360 quadrangles), continues 4.8 

km southwest along the shore to the entrance to Puale Bay, 

then runs about 600 m northwest along the shore of Puale Bay 

to the contact with the overlying Talkeetna Formation. The 

strata are cut by numerous small faults and locally are highly 

contorted. Consequently, the thickness of the reference section described below can best be considered as only approximate. 

Section 1. Reference section of the Kamishak Formation, Puale 

Bay (fig. 6) 

Talkeetna Formation 

Gradational contact 

Kamishak Formation: 

Thickness 

(meters) 

14. Limestone, thin- to thick-bedded, medium-gray; 

contains few Monotis subcircularis (Dl 1288) ...... 13.7 

Fault, displacement unknown 

13. Limestone, thin- to medium-bedded, light-gray, 

highly contorted ................................................ 33.5 

Fault, displacement unknown 

12. Limestone, thin- to medium-bedded, yellowishgray; thin interbeds of siltstone, sandstone, and 

shale; contains few Monotis salinaria ................. 103.6 

11. Limestone, similar to unit 13 ................................... 22.2 

10. Basalt, brecciated and recemented with calcite ........ 38.7 

9. Limestone, similar to unit 13; contains Monotis ...... 60.9 

Fault, displacement unknown 

8. Basalt, columnar-jointed; dated at 197k12.0 Ma 

(Wilson and Shew, 1992) ...................................... 45.7 

7. Limestone, thin-bedded, gray ..................................... 6.0 

6. Siltstone and mudstone, thin-bedded, brownishgray, calcareous; contains few Monotis ................. 37.1 

5. Covered interval ....................................................... 98.1 

4. Limestone, thin-bedded, tuffaceous; few trace 

fossils ..................................................................... 38.1 

3. Volcanic breccia, green to red, vesicular; includes 

blocks of limestone ................................................ 9 1.4 

2. Limestone, thin- to medium-bedded, light-gray, 

bioclastic; reef and bioherms; contains Monotis 

subcircularis, hydrozoans, and ammonites 

(79AEg5-4G, 79AEg5-4D) ................................. 180.8 

Section 1. Reference section of the Kamishak Formation, Puale 

Bay (fig. 6)-Continued 

1. Limestone conglomerate, massive to thick-bedded, 

gray; clasts mainly limestone and limy mudstone, 

minor chert; contains bivalves, coral, gastropods, 

and ammonites (79AEg5-3,79AEg5-2A, 

79AEg5- 1) ............................................................. 29.7 

Total exposed section ..................................... 799.5 

Base of exposed section 

The nature of the contact between the KamishakFormation and the mid-Permian limestone is unknown. The midPermian rocks are exposed only on one small island, which is 

located about 3.5 km from the Triassic section. The northwest 

dip of these Permian rocks suggests that they underlie the 

Triassic section. The contact with the overlying Lower Jurassic Talkeetna Formation is conformable and gradational. The 

contact is arbitrarily placed at the point where clastic sediments-siltstone and sandstone-replace limestone as the 

major constituents of the rock sequence (fig. 6). 

The columnar basalt (unit 8) shown in section I and 

dated at 197*12 Ma (Wilson and Shew, 1992) may not 

actually be a part of the Kamishak Formation. The exact age 

of the Triassic-Jurassic boundary is not well constrained. 

There are many published ages that vary by as much as 13 m.y. 

A recently published age of 200 Ma (Olsen and others, 1987) 

based on vertebrate and invertebrate megafauna, pollen, and 

K-Ar ages on associated volcanic rocks is probably close to 

the true boundary age. If 200 Ma is the age of the boundary, 

then the basalt of unit 8, and probably unit 10, may be sills 

generated during the volcanic episode that produced the 

Talkeetna Formation; as such they would not be part of the 

Kamishak Formation. 

Jurassic System 

Talkeetna Formation 

Detterman and others (1983, 1987a) geographically 

extended the Talkeetna Formation from the Iliamna quadrangle, along the west side of Cook Inlet adjoining the Alaska 

Peninsula (see Detterman and Reed, 1980), to include 405 m 

of clastic sedimentary and volcanic rocks exposed on the 

northeast shore of Puale Bay. The section at Puale Bay (fig. 6), 

and in the mountains between the bay and Alinchak Bay, 

contains considerably more clastic debris than that in the 

Iliamna quadrangle, but it is similar to rocks of the Talkeetna 

Formation in the Talkeetna Mountains (fig. 1; Martin, 1926). 

The name "Talkeetna Formation" was introduced by Martin 

(1926) for a sequence of greenstone and tuff first described in 

the Talkeetna Mountains by Paige and Knopf (1907). The 

formation in the Talkeetna Mountains consists of lava, agglomerate, breccia, tuff, and interbedded sandstone and shale. 

This sequence of volcanic and sedimentary rocks has been 

recognizedas an important marker horizon in southern Alaska, 

Stratigraphic Framework of the Alaska Peninsula 9 
AREA OF MAP 

57' - 

56' - 

- 

PACIFIC OCEAN 

0 - 50 100 KILOMETERS 

- 

I I 1 1 I 
and the unit was geographically extended into the Cook Inlet 

area by ~etterman and Hartsock (1966) when they mapped the 

Talkeetna Formation in the Iniskin-Tuxedni Bay area (fig. 3). 

The stratigraphic sequence in the Iniskin-Tuxedni area was 

divided into three members whose type sections were described by Detterman and Hartsock (1966, p. 12-20). In 

ascending order the members are the Marsh Creek Breccia 

Member, Portage Creek Agglomerate Member, and Horn 

Mountain Tuff Member. The name "Talkeetna Formation7' 

was later applied to the volcanic sequence in the remainder of 

the Iliamna quadrangle (see Detterman and Reed, 1980), 

which borders the Alaska Peninsula. 

The tuffaceous sandstone and tuff exposed between 

Puale and Alinchak Bays (Karluk C-4 and C-5 1:63,360 

quadrangles; fig. 6) closely resembles the Horn Mountain 

Tuff Member in the Iniskin-Tuxedni area as well as the 

Talkeetna Formation in the Talkeetna Mountains. Because 

the same prolific marine megafauna occurs in the rocks at 

Puale Bay and in the Talkeetna Mountains, the Talkeetna 

Formation was geographically extended to include the rocks 

at Puale Bay by Detterman and others (1983, 1987a). 

The Talkeetna Formation mapped west of the Bruin 

Bay fault, northeast of Becharof Lake (fig. 2, unit Je, in part) 

is more like the formation as it is mapped in the Iliamna area 

(see Detterman and Reed, 1980) in that it is mainly a volcanic 

unit that includes local deposits of volcanic conglomerate and 

sandstone. The area west of the Bruin Bay fault and the area 

near Puale Bay evidently were farther apart. when the strata 

were deposited than they are at present, and the Puale Bay 

locality received mainly deposits of volcanic ash and thin sills 

and flows. 

Lower Jurassic volcaniclastic sedimentary rocks, lithologically equivalent to the Talkeetna Formation, have been 

penetrated in several boreholes on the Alaska Peninsula 

southwest of the outcrop areaof the unit. The southwesternmost 

borehole is the AMOCO Cathedral River #1, near the end of 

the Alaska Peninsula, where a sequence of tuffaceous siltstone, sandstone, and limestone was penetrated. This section 

is similar to the rocks exposed at Puale Bay and indicates that 

the Talkeetna Formation may underlie the entire Alaska 

Peninsula. 

Amegafauna--consistingmainly of ammonites-similar to that found in the Talkeetna Formation in the Talkeetna 

Figure 6 (next page). Triassic and Lower Jurassic formations, 

Puale Bay, Alaska Peninsula. A, Reference section of 

Kamishak Formation and section of Talkeetna Formation. 

Ages, formation names, and lithic units (numbered to correspond to text) are shown for each section. B, Index map. 

Circled numbers label location of sections on index map and 

figure 5. Explanation applies to index maps in figures 6 

through 11 and 13 through 16. It lists all the Mesozoic and 

Cenozoic sedimentary units discussed in this report except 

for the Shumagin Formation (Upper Cretaceous), which 

does not appear on any of the index maps. 

Stratigraphic Framework of the Alaska Peninsula 11 
" A Section 1 

Section 2 

' 32302 

ault 

r 200 METERS 

Limestone 

EXPLANATION w1 0 o 001 conglomerate 

Limy siltsone m] - 

Tuff 

limestone 

Silty and tuffaceous A A A fi Dike or sill 

Volcanic breccia 

Conglomeratic limestone 

Covered interval 

Tuffaceous sandstone v. 

32303 Mesozoic fossil locality Fd Tuffaceous siltstone - 9 (listed in appendix) 

,,a,, ----- Megainvertebrate fossil 

Sandstone + 1 9 7_f 1 2 Potassium-argon age 

and analytical error, in 

Siltsone millions of years 
EXPLANATION OF INDEX MAPS FOR FIGURES 6 THROUGH 11 AND 13 THROUGH 16 

SURFlClAL DEPOSITS AND SEDIMENTARY ROCKS 

Surficial deposits (Holocene and Pleistocene) 

Milky River Formation (Pliocene) 

Tachilni Formation (Miocene) 

Bear Lake Formation (Miocene) 

Unga Formation (Miocene and Oligocene) 

Belkofski Formation (Miocene? and Oligocene?) 

Hemlock Conglomerate (Oligocene) 

( Stetepovak Formation (Oligocene and Eocene) 

Tolstoi Formation (Eocene and Paleocene) 

) Copper Lake Formation (Eocene and Paleocene?) 

Kaguyak Formation (Upper Cretaceous) 

Hoodoo Formation (Upper Cretaceous) 

Chignik Formation (Upper Cretaceous) 

Pedmar Formation (Lower Cretaceous) 

$ Herendeen Formation (Lower Cretaceous) 

Staniukovich Formation (Lower Cretaceous) u 

r - 

Naknek Formation (Upper Jurassic)-Divided into: 

z 

Pj 

Katolinat Conglomerate Member 

S Indecision Creek Sandstone Member 

% Snug Harbor Siltstone Member 

q Jnn Northeast Creek Sandstone Member rC 

r Chisik Conglomerate Member ID ' Shelikof Formation (Middle Jurassic) B 

k 

: Kialagvik Formation (Middle and Lower Jurassic) 4 - 

2 Talkeetna Formation (Lower Jurassic) C, 

VOLCANIC ROCKS 

Volcanic rocks (Holocene and Pleistocene) 

Volcanic rocks (Quaternary and Tertiary) 

Meshik Volcanics (Oligocene and Eocene) 

Popof volcanic rocks (Oligocene and Eocene) 

Volcanic rocks (Pliocene to Eocene) 

INTRUSIVE ROCKS 

Intrusive rocks (Pliocene to Eocene) 

Quartz diorite (Jurassic) 

Granodiorite (Jurassic) 

Granodiorite (Tertiary) 

Contact 

Fault-Dashed where inferred; dotted where concealed. U, upthrown side; D, downthrown 

side; arrows indicate direction of relative movement 

Thrust or high-angle fault-Dotted where concealed. Sawteeth on upper plate 

Anticline--Dashed where inferred; dotted where concealed 

Syncline-Arrow indicates direction of plunge. Dotted where concealed 

Strike and dip of bedding 

Measured section 

Exploratory well 

Mountain peak 

Spring 

Sefflement 

\ Kamishak Formation (Upper Triassic) 
Mountains indicates aHettangian and early Sinemurian (Early 

Jurassic) age for the Talkeetna Formation in the Puale Bay 

area. The unit's contact with the overlying Kialagvik Formation is structurally conformable, but it is considered to be a 

disconformity because the late Sinemurian, Pliensbachian, 

and most of the Toarcian Stages are apparently missing, 

owing to either nondeposition or erosion. A small fault occurs 

at the contact on the shore of Puale Bay but is not present 

elsewhere. The lower contact with the Kamishak Formation is 

gradational. 

A section measured along the northeast shore of Puale 

Bay (fig. 6) by L.B. Magoon and R.M. Egbert in 1979 begins 

at the contact with the underlying Kamishak Formation and 

continues northwest about 1.5 km (section 2). 

The Talkeetna Formation at Puale Bay is entirely marine and was deposited in aneritic environment. The megafauna 

is present in great abundance at a few horizons and may 

represent mass kills as a result of volcanic eruptions. 

Section 2. Section of the Talkeetna Formation, Puale Bay (fig. 6) 

Kialagvik Formation 

Disconformable contact Thickness 

Talkeetna Formation: (meters) 

9. Siltstone, thin-bedded, brownish-gray, calcareous; 

contains few bivalves (32302) ................................. 9.1 

Fault, displacement minor 

8. Sandstone, thick-bedded to massive, coarse-grained, 

light-brown; tuffaceous and crossbedded; thin 

interbeds of calcareous and tuffaceous siltstone; 

contains ammonite Arnioceras (32303) ................ 99.0 

7. Sandstone, thin-bedded at base becoming thicker 

bedded upward; tuffaceous and crossbedded ........ 86.8 

6. Sandstone, thick-bedded, coarse-grained, greenishgray; poorly sorted; crossbedded; carbonaceous 

debris on bedding plane; contains ammonites 

Psiloceras and Discamphiceras (3 1822) ............... 40.2 

-5. Tuff, massive, coarse-grained, green to red ............. 27.4 

4. Siltstone, thin-bedded, gray; tuffaceous; thin 

interbeds of tuff ..................................................... 35.0 

3. Tuff, similar to unit 5 ............................................... 15.2 

2. Limestone and tuff interbedded, thick-bedded; 

limestone grayish brown; tuff gray to green; tuff 

beds become thicker in lower part ......................... 30.5 

1. Limestone, sandstone, and siltstone interbedded, 

mostly thin-bedded; predominantly gray; 

laminations; contains ammonites 

Discamphiceras and Waehneroceras (3 1823) ....... - 61.0 

Total section ................................................... 404.2 

Gradational contact 

Kamishak Formation 

Kialagvik Formation 

The Kialagvik Formation was defined by Capps (1923, 

p. 94-97) for a few hundred feet of sandstone, shale, and 

conglomerate exposed along the northwest shore of Wide Bay 

(then known as Kialagvik Bay), between Pass Creek and the 

southwest end of the bay. Capps did not designate a "type" or 

publish specific stratigraphic details about the formation 

along the northwest side of Wide Bay. The outcrops there are 

discontinuous, and there is heavy brush cover. A fairly complete section along the south side of Short Creek, about 5 km 

southwest of Pass Creek, was investigated by L.N. Kellum in 

1944 and measured by R.W. Imlay and D.J. Miller in 1948. 

Imlay published a generalized version of this section (Imlay, 

1984). The present authors obtained more details on this 

section in 1980, during field work in the Ugashik and Karluk 

1:250,000 quadrangles. Combining our data with the data 

published by Imlay (1984), we here designate this section as 

the principal reference section for the Kialagvik Formation 

(section 3; fig. 7). The section lies along the south side of 

Short Creek, starting at VABM Creek (Sec. 34, T. 32 S., R. 

44 W., Ugashik B-2 1:63,360 quadrangle), continuing westward about 2 km to the east side of sec. 32, and then running 

northeastward up a ridge into sec. 28. A minor fault cuts the 

section a short distance below its contact with the Shelikof 

Formation, but strata can be correlated across the fault. The 

upper contact of the section with the Shelikof Formation is an 

unconformity. The lower contact is not exposed in the Wide 

Bay area. 

Section 3. Principal reference section ofthe Kialagvik Formation 

along Short Creek (fig. 7) 

Shelikof Formation 

Unconformable contact Thickness 

Kialagvik Formation: (meters) 

23. Mudstone, thin-bedded, brown; thin interbeds of 

fine-grained brown sandstone; contains 

Inoceramus lucifer (3 1974) ................................... 22.8 

22. Limestone, light-brown; contains Holcophylloceras, 

Inoceramus Lucifer, and Grammatodon (31975) ..... 6.0 

21. Siltstone and sandstone, thin-bedded, brown; few 

limestone concretions ............................................ 33.5 

20. Sandstone, medium-bedded, fine-grained, lightbrown; contains Parabigotites crassicostatus, 

Inoceramus lucifer, and Camptonectes (31976) ...... 6.0 

19. Siltstone, thin-bedded, fault through section ............ 28.9 

18. Sandstone, thin-bedded, fine-grained; few pebbles; 

contains Bradfordia cf. B. costidensa, and 

Parabigotites cf. P. crassicostatus (21 256) ............ 7.6 

17. Siltstone and shale interbedded; few limestone 

concretions; contains Arkelloceras, Pelekodites?, 

Lytoceras, and Macrophylloceras (2 1255) ............ 45.7 

16. Sandstone, similar to unit 18 ...................................... 6.0 

15. Shale, gray, blocky ................................................... 36.3 

14. Sandstone, medium- to thick-bedded, fine-grained, 

few pebbles; contains Pseudolioceras whiteavesi, 

Tmetoceras kirki, T. kirkiflexicostatum, Erycitoides 

howelli, and E. spinatus (21254 and 31978) ......... 25.9 

13. Sandy siltstone and shale ......................................... 10.6 

14 Geologic Studies on the Alaska Peninsula 
Section 3. Principal reference section of the Kialagvik Formation 

along Short Creek (fig. 7)-Continued 

12. Interbedded sandstone, conglomerate, and sandy 

siltstone; contains Erycitoides howelli, E. 

kialagvikensis, E. spinatus, E. levis, and 

Pseudolioceras whiteavesi (21 245 and 19748) ..... 32.0 

1 I. Sandy shale ............................................................. 12.2 

10. Covered .................................................................... 65.5 

9. Sandy siltstone and shale, brown ............................. 12.2 

8. Covered .................................................................... 67.0 

7. Sandstone, thick-bedded to massive, crossbedded; 

conglomerate lenses; muddy; abundant wood 

fragments; contains Trigonia ................................. 15.2 

6. Covered .................................................................... 50.2 

5. Sandy siltstone and shale ......................................... 12.2 

4. Covered .................................................................... 35.0 

3. Sandy siltstone and shale, brown; contains Tmetoceras 

scissum and Pseudolioceras (21248) ..................... 22.8 

Section 3. Principal reference section of the Kialagvik Formation 

along Shoit Creek (fig. 7)-Continued 

2. Covered .................................................................... 44.2 

1. Siltstone, brownish-gray; contains Tmetoceras 

scissum (21 247) .................................................. - 

28.9 

Total exposed section ..................................... 626.7 

Base of exposed section 

The strata in the Kialagvik Formation at Wide Bay 

indicate that the lower part of the section was deposited in a 

nearshore, shallow-water environment. The sandstone iscrossbedded and contains lenses of conglomerate and abundant wood and carbonaceous debris. Additionally, it contains 

the thick-shelled bivalve Trigonia sp. indicative of a nearshore 

high-energy environment. The upper part of the formation 

includes rhythmically bedded thin siltstone and sandstone 

A Section 3 EXPLANATION 

Carbonate concretions 

Covered interval 

-s- Conglomerate lens 

2 1245 Mesozoic fossil locality 

(listed in appendix) 

V Megainvertebrate fossil 

57O15' 

156' 30' Geology modified from Detterman and others (1987a) 

Figure 7. Middle Jurassic Kialagvik Formation, Wide Bay, Alaska Peninsula. A, Principal reference section. Age, formation 

name, and lithic intervals (numbered to correspond to text) are shown for section. 6, Index map. Circled number labels 

location of section on index map and figure 5. See figure 6 for explanation of other symbols. 

Stratigraphic Framework of the Alaska Peninsula 15 
units, limestone nodules and lenses, and other features characteristic of deeper water deposition. These features suggest 

that the area subsided rapidly. 

The formation at Wide Bay has been investigated numerous times since 1923, for example, by Smith and Baker 

(1924), Smith (1926), Martin (1926), Kellum and others 

(1945), Imlay (1961,1984), Imlay and Detterman (1977), and 

Westermann (1964, 1969). Smith (1926) was the first to 

recognize the formation at Puale Bay, and he geographically 

extended the unit to include the rocks there. The formation is 

abundantly fossiliferous, and most of these authors located the 

contact between the Kialagvik and Shelikof Formations on the 

basis of faunal evidence rather than lithologic characteristics. 

Allaway and others (1984) revised the contact between the 

Kialagvik and Shelikof Formations on lithologic evidence 

and, as a consequence, revised (that is, stratigraphically extended) the upper boundary of the Kialagvik Formation. To 

illustrate this boundary in the Puale Bay area, we here designate areference section (section4) of the Kialagvik Formation 

that runs along the northeast shore of Puale Bay and incorporates this contact. This section begins about 2 km northwest of 

the entrance point to the bay and continues northwest along 

the shore for approximately 1.75 km (secs. 29 and 19, T. 28 S., 

R. 37 W., Karluk C-4 and C-5 1:63,360 quadrangles). 

Section 4. Reference section ofthe Kialagvik Formation at Puale 

Bay (fig. 8) 

Shelikof Formation 

Conformable contact Thickness 

Kialagvik Formation: (meters) 

9. Siltstone, thin-bedded, gray; contains few ammonites, Cadoceras tenuicostatum (3 18 18) ............... 15.2 

8. Sandstone, thick-bedded, fine- to medium-grained; 

graded and laminated, gray .................................... 12.1 

7. Siltstone, thin- to thick-bedded, gray, micaceous; somewhat sandy; contains bivalve Inoceramus lucger and 

ammonites Pseudolioceras maclintocki and 

Eudmetoceras cf. E. amplectens (3 1819 and 32301) 27 1.2 

6. Sandstone, thick-bedded, fine- to medium-grained, 

grayish-green; laminated and graded; few rip-up 

clasts; minor siltstone ............................................ 47.7 

5. Conglomerate, massive, disorganized; clasts mainly 

volcanic rocks, minor intrusive rocks; clasts range 

from pebbles to boulders; channeled into 

underlying siltstone ............................................... 30.5 

4. Siltstone and sandstone, thin- to thick-bedded, 

rhythmically interbedded; siltstone dark gray; 

sandstone yellowish gray; lenticular limestone 

concretions throughout ........................................ 2 13.4 

3. Sandstone, thick-bedded, coarse-grained, tan .......... 19.2 

2. Siltstone and sandstone, rhythmically interbedded; 

similar to unit 4; few limestone concretions ........ 172.6 

1. Conglomerate, massive, pebble to cobble; minor 

sandstone; contains Haugia (19804) ....................... 7.6 

Total section ................................................... 789.5 

Unconformable contact 

Talkeetna Formation 

The reference section at Puale Bay is about 790 m thick 

(fig. 8). The lowermost 420 m consists of rhythmically 

interbedded gray siltstone and tan sandstone deposited in deep 

water and overlain by a massive, disorganized, and channeled 

conglomerate (unit 5). Limestone concretions occur throughout much of this part. The upper part of this reference section 

is gray siltstone and minor sandstone. The Kialagvik Formation at Puale Bay is similar to the upper part of the formation 

at Wide Bay. The nearshore beds in the basal part of the Wide 

Bay section are missing at Puale Bay. The two areas are about 

60 km apart, and the lithic differences between them may 

represent simply facies changes; additionally, they may mean 

that neither section is complete and that each represents a 

somewhat different time interval. The sequences at Wide Bay 

and Puale Bay are the only outcrops of the Kialagvik Formation on the Alaska Peninsula, but the unit has been encountered in the subsurface. The Humble Oil-Shell Oil Company 

Bear Creek well (see location in Detterman and others, 1983), 

about midway between WideBay andPualeBay, encountered 

about 1,000 m of siltstone and shale correlated with the 

Kialagvik Formation (R.L. Detterman, unpub. data, 1987). 

AMOCO Production Company drilled through about 530 m 

of the Kialagvik Formation in the Cathedral River#l borehole 

at the southwest end of the Alaska Peninsula (see McLean, 

1977). There the formation consists of siltstone, sandstone, 

and shale similar to the rocks of the Wide Bay section. 

Rocks in part lithologically equivalent to the Kialagvik 

Formation are well exposed along the west side of Cook Inlet 

(seeDetterman and Hartsock, 1966), where they are called the 

Tuxedni Group. The Tuxedni Group is divided into six formations (see Detterman and Hartsock, 1966) and is a much 

thicker (3,000 m) and more complete sequence than the 

Kialagvik Formation. The Kialagvik is similar to the lower 

part of the Tuxedni. The Tuxedni Group is also present in the 

Talkeetna Mountains (Imlay, 1984), 500 km northeast of the 

Alaska Peninsula. 

Thecontact of the Kialagvik with theoverlying Shelikof 

Formation is conformable at Puale Bay where the Kialagvik 

is as young as Callovian (Middle Jurassic) and unconformable 

at Wide Bay where the Kialagvik is entirely Bajocian (Middle 

Jurassic); the contact is placed at the base of a thick volcanic 

sandstone and conglomerate sequence. The contact of the 

Kialagvik with the underlying Talkeetna Formation at Puale 

Bay is an unconformity. 

Only sparse fossils are present in the deep-water deposits at Puale Bay in contrast to the abundant megafauna in the 

shallow-marine sedimentary rocks at Wide Bay. The 

megafauna at Wide Bay indicates adepositional event starting 

in the late Toarcian (Early Jurassic) and continuing until the 

Callovian (Middle Jurassic). Most of the megafauna is correlated with the lower and middle parts of the Bajocian Stage of 

the Middle Jurassic (Imlay, 1984). The age-diagnostic ammonites are Tmetoceras, Erycitoides, Eudmetoceras, and 

Pseudolioceras (Taylor and others, 1984; Westermann, 1964, 

1969). 

16 Geologic Studies on the Alaska Peninsula 
A Section 4 Section 5 EXPLANATION 

Sandstone ..... 

siltstone 

Conglomerate 

Limestone concretions 

W Covered interval 

Mesozoic fossil locality 

32026 (listed in appendix) 

W Megainvertebrate fossil 

Geology modified from Detterman and others (1 987a) 

Figure 8. Lower and Middle Jurassic formations, Puale Bay, Alaska Peninsula. A, Reference section of Kialagvik Formation and type section of Shelikof Formation. 

Ages, formation names, and lithic units (numbered to correspond to text) are shown for each section. B, Index map. Circled numbers label location of sections on 

index map and figure 5. See figure 6 for explanation of other symbols. 
Shelikof Formation 

The Shelikof Formation was named by Capps (1923, p. 

97-101) for exposures along the northwest shore of Shelikof 

Strait. He did not specify a "type," but Allaway and others 

(1984, p. 21-27) designated the section along the northeast 

shore of Puale Bay as the type section and established two 

reference sections at Alai and Big Creeks, near the southwest 

and northeast ends, respectively, of Wide Bay. The type 

section at Puale Bay begins at the contact with the Kialagvik 

Formation in sec. 19, T. 28 S., R. 37 W., Karluk C-4 and C5 1:63,360 quadrangle, and continues along the northeast 

shore to sec. 9, T. 28 S., R. 38 W., Karluk D-5 1:63,360 

quadrangle (fig. 8). 

The Shelikof Formation is the major stratigraphic unit 

forming the mountains along the west side of Shelikof Strait 

from Kashvik Bay to Wide Bay (fig. 2). The only other known 

exposures of the unit are along faults on the east shore of 

Upper and Lower Ugashik Lakes and in the Chignik area (fig. 

5; Detterman and others, 198 la, 1983,1987a). The formation 

has been identified in several boreholes including the AMOCO 

Cathedral River #1 near the southwest end of the Alaska 

Peninsula. Consequently, we presume that the Shelikof Formation underlies the entire Alaska Peninsula. Northeastward 

from the Alaska Peninsula, rocks of similar age are mapped as 

part of the Chinitna Formation, a major stratigraphic unit 

along the west side of Cook Inlet (Detterman and Hartsock, 

1966) and in the Talkeetna Mountains (Grantz, 1960). The 

Shelikof Formation contains much more coarse volcanic 

debris than does the Chinitna Formation, which consists 

mainly of shale and siltstone; and the Shelikof is correlative, 

lithologically and faunally, with the Paveloff Siltstone Member in the uppermost part of the Chinitna Formation. 

The Shelikof Formation is a deep- to shallow-water 

volcaniclastic unit 1,402 m thick in its type locality. The 

lower part of the unit is mainly thick-bedded to massive, 

dusky-yellowish-green graywacke and conglomerate. Clasts 

in the conglomerate are volcanic. The upper part of the 

formation is mainly brownish-gray siltstone containing limestone nodules. Capps (1923) divided the Shelikof into three 

informal members, called the siltstone, sandstone, and conglomerate members. These are useful for mapping locally, 

but owing to abrupt lateral facies changes, they are impractical for most mapping. 

The type section of the Shelikof Formation (section 5; 

fig. 8) was measured in part by L.B. Magoon and J.W. Miller 

in 1979 and in part by W.H. Allaway, Jr., and C.D. Blome in 

1981. 

Megafauna are locally very abundant in strata of the 

Shelikof Formation, but most of the unit is unfossiliferous. 

Where present, the fauna consists mainly of the ammonites 

Cadoceras, Pseudocadoceras, and Stenocadoceras, of early 

to middle Callovian (Middle Jurassic) age. A few specimens 

of Iniskinites at Wide Bay (Detterman and others, 1985) and 

a few species of the genus Cadoceras suggest a late 

Bathonian(?) age for some of the rocks. A few pelecypods are 

also present, but most are long-ranging forms. It may be that 

deposition was continuous from the Kialagvik through the 

Shelikof Formation; this appears likely as no erosional break 

between the Kialagvik and the unfossiliferous strata in the 

basal part of the Shelikof Formation is apparent at the type 

section at Puale Bay. Lacking positive stratigraphic evidence 

and from available faunal evidence we consider the contact 

between these units to be conformable at Puale Bay; however, there may be a diastem between the Kialagvik and 

Section 5. Type section of the Shelikof Formation (fig. 8) 

Naknek Formation 

Unconformable contact Thickness 

Shelikof Formation: (meters) 

16. Siltstone and sandstone rhythmically interbedded; 

siltstone, thin-bedded, dark-gray; sandstone, 

medium-bedded, medium-grained, dark-green; 

volcanogenic; lenticular limestone concretions in 

siltstone; contains ammonite Cadoceras 

(Stenocadoceras) striatum and bivalve 

Meleagrinella (32024, 32032) ............................. 260 

15. Sandstone, thick-bedded, fine- to mediumgrained, yellowish-gray; mainly volcanic sand ..... 31 

14. Siltstone and sandstone interbedded; similar to 

unit 16 .................................................................. 100 

13. Siltstone and sandstone; thick-bedded sandstone 

with thin siltstone interbeds; sandstone, 

olive-green ............................................................. 95 

12. Sandstone, massive, coarse-grained, olive-green; 

thin interbeds of siltstone; sandstone composed 

mainly of volcanic fragments .............................. 1 10 

11. Sandstone and siltstone interbedded, similar to 

unit 16 .................................................................. 205 

10. Siltstone, olive-gray; thin sandstone interbeds ....... 100 

9. Covered interval ....................................................... 30 

8. Siltstone and sandstone interbedded; contains 

ammonite Cadoceras (Stenocadoceras) striatum 

(32026) ................................................................ 100 

7. Sandstone, thick-bedded to massive, medium- to 

coarse-grained, moderate olive-brown; graded and 

amalgamated; graywacke-type; contains ammonite 

Cadoceras (Stenocadoceras) (32023) ................... 85 

6. Covered interval ....................................................... 40 

5. Volcanic graywacke, thick-bedded to massive, 

dusky-yellow-green, medium-grained; contains 

abundant plant debris ............................................. 75 

4. Conglomerate and sandstone, thick-bedded, 

channeled, greenish-black; poorly sorted; mainly 

volcanic clasts ........................................................ 80 

3. Siltstone, thin-bedded, dark-gray ............................. 15 

2. Siltstone and sandstone, light-olive-gray; contains 

ammonite Cadoceras (Stenocadoceras) cf. C. (S.) 

stenoloboide (32022) ............................................. 25 

1. Sandstone and conglomerate, medium- to thickbedded; mainly volcanic debris ............................. - 

5 1 

Total section ................................................ 1,402 

Conformable contact 

Kialagvik Formation 

18 Geologic Studies on the Alaska Peninsula 
Shelikof Formations. The upper contact of the Shelikof 

Formation with the Naknek Formation is an unconformity. 

Naknek Formation 

The Upper Jurassic Naknek Formation is the most 

persistent and widespread Mesozoic formation on the Alaska 

Peninsula, extending 650 km southwest from Kamishak Bay 

to Black Hill, and another 500 km to the northeast from 

Kamishak Bay. The formation was originally named the 

Naknek Series by Spurr (1900, p. 169-171). Martin (1905) 

first used the name "Naknek Formation" for the rocks in the 

Cook Inlet area. Early work (Kirschner and Minard, 1949), 

mainly in the Cook Inlet area, showed that the formation was 

divisible into four members; from oldest to youngest these are 

the Chisik Conglomerate Member (Martin and Katz, 1912), 

lower sandstone member, Snug Harbor Siltstone Member, 

and Pomeroy Arkose Member (Detterman and Hartsock, 

1966). The Chisik Conglomerate Member was originally 

considered a formational rank unit (= Chisik Conglomerate) 

by Martin and Katz (1912), but it was subsequently revised as 

a member of the Naknek Formation by Moffit (1927). 

A similar subdivision of its strata is appropriate for the 

NaknekFormation on the AlaskaPeninsula (fig. 9). Additionally, two members younger than those listed above have been 

mapped on the peninsula; the older of the two was called the 

"upper sandstone member" by Detterman and Reed (1980, p. 

B37). We herein geographically extend the Chisik Conglomerate Member and Snug Harbor Siltstone Member to include 

the equivalent strata on the Alaska Peninsula. Additionally, 

we herein introduce the name "Northeast Creek Sandstone 

Member" for rocks on the peninsula equivalent to the lower 

sandstone member of the Cook Inlet area, and the name 

"Indecision Creek Sandstone Member" for the upper sandstone member in the Kamishak Bay area (Detterman and 

Reed, 1980, p. B37). The name "Katolinat Conglomerate 

Member" is herein proposed for the lithic unit that is younger 

than the upper sandstone member in the Kamishak Bay area. 

ThePomeroy Arkose Member is not recognized on the Alaska 

Peninsula, probably owing to facies change or a change in 

source terrain. 

The division of the Naknek Formation into mappable 

members serves several purposes: (I) It indicates major shelf 

areas, depositional basins, source terranes, and the overall 

direction of sediment transport, and (2) it can be used to 

delineate the areas most likely to have clean sandstone intervals that may be potential oil-producing horizons. Three of the 

members, the Indecision Creek Sandstone, Snug Harbor Siltstone, and Northeast Creek Sandstone Members, are mappable throughout the length of the Alaska Peninsula. The 

Chisik Conglomerate Member is present as far south as the 

Chignik area, and the Katolinat Conglomerate Member is 

preserved for about 80 km in the Mount Katmai area, as well 

as in a small area near Wide Bay. 

The aggregate thickness of the members of the Naknek 

Formation on the Alaska Peninsula is approximately 3,205 m 

(fig. 9B). This is a maximum thickness as it incorporates the 

thicknesses from the various type or reference sections that 

show the maximum development of each member. The average thickness of the formation is approximately 1,700 to 2,000 

m. Abrupt lateral facies changes are present, so that only one 

or two members are of considerable thickness at any one 

locality, and the Katolinat Conglomerate Member is present 

only at the northeast end of the peninsula. Several small 

starved basins are preserved-for example, near Hallo Bay 

and Amber Bay-in which the entire Naknek sequence is 

compressed into a section of siltstone less than 500 m thick. 

The facies changes within the Naknek Formation occur both 

parallel and transverse to the Alaska Peninsula. The starved 

basin sequences at Hallo and Amber Bays lie on the southeast 

side of the peninsula. The abrupt changes are the results of 

rapid uplift anderosion of the Alaska-Aleutian Range batholith 

and older sedimentary rocks that flank it. The eroded sediments were carried to the southeast by short, high-energy 

streams that debauched onto the continental shelf and deposited their loads partly in subaerial and partly in marine environments. The facies changes in the Naknek Formation, as 

well as those in some of the other upper Mesozoic stratigraphic units, can best be seen in a fence diagram of the upper 

Mesozoic stratigraphic units on the Alaska Peninsula (see 

Detterman and Miller, 1987). 

The age of most of the Naknek Formation is well 

constrained by a biozonation of the bivalve Buchia (Imlay, 

1959; Jeletsky, 1965; Miller and Detterman, 1985). Buchia 

concentrica, the oldest form of Buchia in Alaska, occurs 

within the Snug Harbor Siltstone Member of the Cook Inlet 

area and is associated with the ammonite Cardioceras 

(Detterman and Hartsock, 1966). Cardioceras in association 

with other ammonites in the Cook Inlet area indicates an age 

of late Oxfordian (Imlay, 198 1) for the basal part of the Snug 

Harbor Siltstone Member. On the Alaska Peninsula, Buchia 

concentrica occurs sparsely in the upper part of the Northeast 

Creek Sandstone Member and in the lower part of the Snug 

Harbor Siltstone Member; its presence suggests that these 

beds are no younger than late Oxfordian. The thick sequence 

comprising the Chisik Conglomerate Member and the lower 

part of the Northeast Creek Sandstone Member could be as old 

as earliest Oxfordian. The succession of Buchia species on the 

Alaska Peninsula (Miller and Detterman, 1985) indicates that 

deposition of the Naknek was continuous from at least middle 

Oxfordian to latest Tithonian time. Ammonites in the Naknek 

Formation on the Alaska Peninsula are restricted to longranging forms, Phyllocerasand Lytocerns, that are of minimal 

value for age determinations. 

Chisik Conglomerate Member 

A reference section (section 6) of the Chisik Conglomerate Member, which is here geographically extended into the 

area of this report, is designated in the Ugashik C-1 1 :63,360 

quadrangle, located on an unnamed mountain in secs. 26 and 

Stratigraphic Framework of the Alaska Peninsula 19 
27, T. 30 S., R. 43 W., just west of two small lakes 3 km west 

of Ruth Lake (fig. 9B). The section, located 6.5 km southwest 

of the southernmost arm of Becharof Lake, begins at the top 

of the mountain and continues down its east-facing slope to 

near lake level. The 614 m of the member exposed at this 

locality (fig. 9A) consists mainly of massive to thick-bedded 

conglomerate with interbeds and lenses of clean, quartzose 

sandstone that is commonly crossbedded. Clast size in the 

conglomerate decreases upward in the section from a maximum of 120 cm at its base to about 15 cm at its top. Average 

composition of the polymictic clasts is about 30 percent 

granitic rocks, 30 percent quartzite, 20 percent metavolcanic 

rocks, 10 percent schist, and 10 percent chert and quartz. 

Section 7 

Section 6 - 

Clasts are generally well rounded and commonly decrease in 

size upward within each unit. Pebbles of similar composition 

are found in many of the sandstone beds. The contact between 

the Chisik Conglomerate Member and the underlying Shelikof 

Formation is not exposed at this locality, but it is exposed at 

several localities nearby where the Chisik unconformably 

overlies the Shelikof Formation. Consequently, the thickness 

of the member may be as much as 100 to 150 m more than 

measured in the section just described. The rocks at nearby 

localities are conglomerate similar to the lowest measured 

unit. At some localities near Wide Bay and Chignik Bay the 

basal conglomerate bed is channeled into the underlying 

ShelikofFormation, and that contact is obviously unconformSection 9 

lLgd Siltstone 

Shale 

Section 10 

EXPLANATION 

1-1 Conglomeratic sandstone - 0" 

Sandstone 

Silty sandstone and sandy 

-- 

Sandstone and 

conglomerate lenses 

Limestone beds C PI Carbonate concretions 

Fl sill 

Covered interval 

M7696 Mesozoic fossil locality 

(listed in appendix) 

Figure 9. Members of UpperJurassic Naknek Formation, Alaska Peninsula. A, Type sectionsof Northeast Creek Sandstone, Indecision 

Creek Sandstone, and Katolinat Conglomerate Members, and reference sections of Chisik Conglomerate and Snug Harbor Siltstone 

Members. Ages, formation names, and Iithic units (numbered to correspondtotext) are shown for each section. 6, Index maps. Circled 

numbers label location of sections on index maps and figure 5. See figure 6 for explanation of other symbols. 

20 Geologic Studies on the Alaska Peninsula 
Section 6. Reference section of the Chisik Conglomerate Section 6. Reference section of the Chisik Conglomerate 

Member of the Naknek Formation (fig. 9) Member of the Naknek Formation (fig. 9)-Continued 

Northeast Creek Sandstone Member 

Conformable contact Thickness 

Chisik Conglomerate Member: (meters) 

13. Conglomerate, massive, pebble-cobble; maximum clast size 26 cm; clasts 30 percent quartzite, 

15 percent metavolcanic rocks, 15 percent chert, 

10 percent quartz, and 10 percent schist and metalimestone; clasts well rounded; a coarse-grained, 

light-brownish-gray sandstone interval in middle 

part of conglomerate unit ....................................... 38.0 

12. Sandstone and conglomerate, thick-bedded to 

massive; sandstone is medium grained, light brown; 

conglomerate clasts are 60 percent quartzite, 20 

percent metavolcanic rocks, 15 percent chert (gray, 

Geology modified from Detterman and others (1987a) 

Chisik Conglomerate Member (reference section) 

green, red), 5 percent quartz; maximum clast size 

20 cm ..................................................................... 30.3 

11. Covered interval ....................................................... 23.6 

10. Conglomerate, massive, 60 percent metavolcanic 

rocks, 20 percent quartzite, 10 percent schist, 10 

percent chert and quartz; few coarse sandstone 

lenses ..................................................................... 53.3 

9. Sandstone, medium-bedded, fine-grained, olive-gray; 

high-angle crossbeds; frosted grains ...................... 30.4 

8. Conglomerate, massive, channeled; clasts are 60 percent schist and quartzite, 20 percent metavolcanic 

rocks, 10 percent granitic rocks, 10 percent chert 

and quartz; maximum clast size 120 cm ................ 42.6 

7. Covered interval ....................................................... 30.4 

Geology modified from Detterman and others (1987a) 

Indecision Creek Sandstone Member (type section) 

155" 00' 

Geology mod~fied from Riehle and others (1987a) 

Katolinat Conglomerate Member (type section) 

KILOMETERS 

Figure 9.-Continued. 

Stratigraphic Framework of the Alaska Peninsula 21 
Section 6. Reference section of the Chisik Conglomerate 

Member of the Naknek Formation (fig. 9)-Continued 

6. Conglomerate, massive, well-rounded clasts; clasts 

are 30 percent quartzite, 30 percent granitic rocks, 

20 percent metavolcanic rocks, 15 percent schist, 

10 percent chert and quartz; thick sandstone at top 

and bottom ............................................................ 60.8 

5. Covered interval ...................................................... 38.1 

4. Conglomerate, same as unit 8 .................................. 38.1 

3. Sandstone, thin-bedded, fine-grained, dusky-yellow .. 21.3 

2. Conglomerate, thick-bedded to massive, pebble to 

cobble; clasts are 30 percent granitic rocks, 20 

percent metavolcanic rocks, 30 percent quartzite, 

10 percent chert and quartz; sandstone lenses ....... 64.0 

1. Conglomerate, massive, cobble-pebble; contains 

sandstone lenses; forms vertical cliff; not measured 

in detail ................................................................ 143.0 

Total exposed section ..................................... 613.9 

Base of exposed section 

able. The upper contact of the Chisik with the Northeast Creek 

Sandstone Member is conformable and gradational. 

Northeast Creek Sandstone Member 

The Northeast Creek Sandstone Member is here named 

for exposures along Northeast Creek, Sutwik Island D-5 

1 :63,360 quadrangle. The Northeast Creek Sandstone Member is the lateral equivalent of the lower sandstone member of 

the Naknek Formation in the Cook Inlet area (see Detterman 

and Hartsock, 1966). The Northeast Creek Sandstone Member occupies the same stratigraphic position on the Alaska 

Peninsula with respect to the underlying Chisik Conglomerate 

Member and the overlying Snug Harbor Siltstone Member as 

does the lower sandstone member in the Cook Inlet area. The 

Northeast Creek Sandstone Member is present throughout the 

entire length of the Alaska Peninsula. 

The contact of the Northeast Creek with the underlying 

Chisik Conglomerate Member is conformable and is placed 

where thick sandstone replaces conglomerate in the section. 

Laterally, the contact may range either upward or downward 

due to facies changes. The upper contact of the Northeast 

Creek Sandstone Member with the Snug Harbor Siltstone 

Member is conformable and sharp. This contact may range 

upward or downward also, but to a lesser extent than does the 

lower contact. The upper contact represents a major shift in 

depositional environment from mainly nonmarine to marine 

conditions, and it varies both temporally and spatially depending on conditions at the local depositional site. 

The here-designated type section (section 7; fig. 9) of 

the Northeast Creek Sandstone Member is on an east-facing 

ridge in sec. 34, T. 37 S., R. 5 1 W., Seward Meridian, Sutwik 

Island D-5 1:63,360 quadrangle. It begins on a ridge top and 

extends down to a tributary of Northeast Creek. The 624-mthick section is, in part, alateral facies equivalent of the Chisik 

Conglomerate Member and, in part, a younger unit overlying 

that member. This relation is readily apparent in exposures of 

the two members along strike. Sandstone constitutes the main 

part of the Northeast Creek Sandstone Member, which is a 

nonmarine to shallow-marine deposit. The sandstone is mainly 

fine to medium grained, crossbedded, and laminated. Dark 

laminae in the rocks are commonly composed of magnetite. 

Some of thesand beds contain channels with lag gravel at their 

bases. The crossbeds are mostly high-angle aeolian crossbeds 

with variable flow directions; some are small-scale, tabular 

crossbeds with clay drapes characteristic of point-bar deposits. Siltstone intervals are locally bioturbated. The composition of the Northeast Creek Sandstone Member is remarkably 

Section 7. Type section of the Northeast Creek Sandstone 

Member of the Naknek Formation (fig. 9) 

Snug Harbor Siltstone Member 

Conformable contact Thickness 

Northeast Creek Sandstone Member: (meters) 

1 1. Sandstone, thick-bedded, fine- to medium-grained; 

yellowish- to brownish-gray; contains channels 

with lag gravel; abundant carbonaceous debris; 

laminae formed by magnetite grains ..................... 62.5 

10. Sandstone and siltstone, thin- to medium-bedded; 

brownish- to olive-gray; crossbedded; siltstone is 

carbonaceous ......................................................... 50.2 

9. Sandstone, thick-bedded, fine-grained, pale-olive; 

laminated; minor siltstone ..................................... 38.1 

8. Sandstone, thick-bedded to massive, medium- to 

coarse-grained, dusky-yellow; few pebbles; 

large-scale, high-angle crossbeds ......................... .60.9 

7. Siltstone and sandstone, thin- to thick-bedded, finegrained, brownish-gray to yellow-green; laminated 

and bioturbated ...................................................... 45.7 

6. Sandstone, massive, medium-grained, light-gray; 

minor carbonaceous siltstone at base ..................... 32.0 

5. Sandstone, thick-bedded, fine-grained, yellow-green; 

laminated and crossbedded .................................... 44.2 

4. Siltstone with thin sandstone lenses, thin-bedded, 

olive-gray ............................................................... 3 1.9 

3. Siltstone, thin-bedded, medium-dark gray; abundant 

plant debris; bioturbated; sandstone interbeds ....... 54.8 

2. Sandstone and siltstone rubble traces ..................... 189.0 

1. Sandstone, thick-bedded, fine-grained, yellowishgray; abundant plant debris .................................... 15.2 

Total exposed section ..................................... 624.5 

Base of exposed section 

consistent throughout the length of the Alaska Peninsula. The 

lower contact of the member is not exposed at the type section. 

Snug Harbor Siltstone Member 

A reference section (section 8; fig. 9) for the Snug 

Harbor Siltstone Member, which is here geographically extended into the area of this report, is designated on Northeast 

22 Geologic Studies on the Alaska Peninsula 
Creek about 1 km north of the type section of the Northeast 

Creek Sandstone Member in the Sutwik D-5 1:63,360 quadrangle; it is located along the top and down the southeast face 

of a flat-topped ridge in sec. 27, T. 37 S., R. 51 W. The 637- 

m-thick section consists mainly of dark-gray siltstone that 

contains numerous limestone concretions in its upper part and 

a few limestone beds at intervals throughout the section. 

Sandstone constitutes a minor part of the section but increases 

upward to a gradational contact with the overlying Indecision 

Creek Sandstone Meinber. Both the upper contact of the Snug 

Harbor with the Indecision Creek Sandstone Member and the 

lower contact with the Northeast Creek Sandstone Member 

are conformable but gradational. The lower contact is generally more sharply defined than the upper contact. Laterally, 

the contacts range upward or downward owing to facies 

changes. Sandstone is a minor part of the Snug Harbor 

SiltstoneMember; it ismost abundant in the northwesternmost 

exposures of the member and decreases southeasterly across 

the peninsula. The source area for this member, as well as for 

Section 8. Reference section of the Snug Harbor Siltstone 

Member of the Naknek Formation (fig. 9) 

Uppermost part of section missing 

Snug Harbor Siltstone Member: 

Thickness 

(meters) 

15. Silty sandstone, thin-bedded, fine-grained, darkyellowish-brown; rusty weathering ....................... 18.2 

14. Siltstone and silty shale, thin-bedded, darkyellowish-brown; limestone nodules in upper 

part; contains Buchia mosquensis and B. rugosa 

(M7822) ................................................................. 62.4 

13. Sandstone, thick-bedded, fine-grained, darkyellowish-brown; silty; few limestone nodules; 

contains bivalves Buchia mosquensis and B. 

rugosa and ammonite Lytoceras (M7823) ............ 12.1 

12. Siltstone, thin-bedded, olive-gray; abundant limestone concretions; contains Buchia rugosa and 

B. mosquensis (M7824 and M7825) ...................... 41.1 

11. Siltstone, similar to unit 12; contains a few limestone beds in addition to concretions; contains 

Buchia rugosa and B. concentrica (M7826 and 

M7827) .................................................................. 68.5 

10. Sandstone, medium-bedded, medium-grained, 

moderate-yellowish-brown; contains fossils as in 

unit 11 ...................................................................... 4.5 

9. Covered interval ....................................................... 24.7 

8. Sandstone, thin- to medium-bedded, fine-grained, 

olive-gray; silty; contains few bivalves and 

gastropods (M7828) ............................................... 25.8 

7. Siltstone, thin-bedded, olive-gray ............................ 30.5 

6. Sandstone, medium- to thick-bedded, fine-grained, 

light-olive-brown ................................................ 39.6 

5. Siltstone, thin-bedded, medium-gray; contains few 

bivalves (M7829) ................................................ 44.1 

4. Shale, dark-gray; contains Buchia concentrica 

(M7830) ................................................................ 19.8 

3. Siltstone, thin-bedded, sandy, olive-gray; few 

limestone layers ..................................................... 95.9 

Section 8. Reference section of the Snug Harbor Siltstone 

Member of the Naknek Formation (fig. 9)-Continued 

2. Siltstone, similar to unit 13; contains Buchia 

concentrica (M7831 and M7832) ........................ 128.0 

1. Claystone, olive-gray ............................................... - 

22.8 

Total exposed section ..................................... 638.0 

Conformable contact 

Northeast Creek Sandstone Member 

all the other members of the Naknek Formation, was located 

northwest of the present outcrop area. 

The Snug Harbor Siltstone Member is stratigraphically 

the lowest abundantly fossiliferous unit in the Naknek Formation. The main elements of the fauna are bivalves of the genus 

Buchia. A few brackish- and fresh-water bivalves occur in the 

older Naknek strata on the Alaska Peninsula, but Buchia is the 

oldest age-diagnostic marine fossil found in the formation on 

the peninsula. The species of Buchia present on the Alaska 

Peninsula can be divided into definite biozones (Miller and 

Detterman, 1985) that range in age from middle Oxfordian 

(Late Jurassic) to Valanginian (Early Cretaceous). Of note in 

the Snug Harbor reference section is the barren zone, units 7 

and 6, overlying rocks that contain Buchia concentrica in unit 

4. This 70-m interval is the same barren zone as that seen at 

Hal10 Bay and described by Detterman and Miller (1986, p. 

27-29). 

The Snug Harbor Siltstone Member was deposited in 

moderately deep water, not below the carbonate compensation depth but well below wave base, in a basin with restricted 

circulation. The depositional site could have been in a deep 

fiord-like setting similar to many of the bays along the Pacific 

coast of the Alaska Peninsula at present. The sandstone (unit 

8) overlying the barren zone contains only a small amount of 

volcanic detritus, but the sand grains are cemented by montmorillonite clay (identified by scanning electron microscope 

and microprobe analysis), indicating nearby volcanic activity 

(Mullen, 1987). 

Indecision Creek Sandstone Member 

The uppermost part of the Naknek Formation throughout most of the Alaska Peninsula is sandstone, which is here 

named the Indecision Creek Sandstone Member after a thick 

section measured on Indecision Creek in the Ugashik A-4 

1 :63,360 quadrangle. The Indecision Creek Sandstone Member is equivalent to the upper sandstone member mapped and 

described by Detterman and Reed (1980, p. B37) in the 

KamishakBay area. The sandstone was first recognized in the 

Mount Katmai area at the northeast end of the Alaska Peninsula by Keller and Reiser (1959), and it was made an informal 

member of the Naknek Formation by Detterman and Reed 

(1980). Recent work has shown that the Indecision Creek 

Sandstone Member is the most widespread unit of the Naknek 

Formation on the Alaska Peninsula; it is present along the 

Stratigraphic Framework of the Alaska Peninsula 23 
entire 650-km length of Mesozoic exposures on the peninsula. 

The type section (section 9; fig. 9) of the Indecision Creek is 

here designated in secs. 35 and 36, T. 35 S., R. 49 W., Seward 

Meridian, Ugashik A-4 1:63,360 quadrangle; it is located on 

the east-facing slope of an unnamed mountain opposite 

Mount Chiginagak, beginning at the top of the ridge and 

continuing down to stream level. The predominantly sandstone sequence is fine grained, thin bedded to massive, 

locally crossbedded, and abundantly fossiliferous. The depositional environment for this member is interpreted to be 

shallow-water shelf to inner neritic. The lithology and depositional environment of the member are the same throughout 

the Alaska Peninsula, and there are only very minor differences in its sand-shale ratio throughout its outcrop area. 

Changes in the thickness of the member along the peninsula 

are due almost entirely to post-depositional erosion. The 

lower contact of the Indecision Creek with the Snug Harbor 

Siltstone Member is conformable and slightly gradational. 

The upper contact is in most places an unconformity overlain 

by Upper Cretaceous or Tertiary strata. In a few places, 

mainly in the Mount Katmai area and near Wide Bay, the 

Katolinat Conglomerate Member conformably overlies the 

Indecision Creek Sandstone Member. The abundant 

megafauna of the Indecision Creek is restricted almost exclusively to bivalves of the genus Buchia. Ammonites are 

extremely rare, which suggests that the unit does not represent an open-marine environment. Ammonites were freeSection 9. Type section of the lndecision Creek Sandstone 

Member of the Naknek Formation (fig. 9) 

Chignik Formation 

Unconformable contact Thickness 

Indecision Creek Sandstone Member: (meters) 

17. Sandstone, thin-bedded, fine-grained, yellowishbrown; yellowish-orange weathered zone at 

upper contact with Chignik Formation .................. 27.4 

16. Sandstone, thin-bedded, very fine grained, silty 39.6 ...... 15. Sandstone, medium-bedded, fine-grained, yellowbrown, micaceous; mottled .................................... 83.8 

14. Sandstone, thick-bedded, fine-grained, greenishgray; in part rubble; contains Buchia 

blanfordiana (M7696) ........................................... 27.4 

13. Covered interval ....................................................... 19.8 

12. Sandstone, thin-bedded, fine-grained, grayishgreen; plant debris common; contains Buchia 

blanfordiana and other bivalves 

(M7697 and M7698) .............................................. 67.0 

1 1. Sandstone, thick-bedded to massive, fine-grained, 

light- to dark-gray; laminated; contains Buchia 

blanfordiana (M7699 and M7700) ........................ 46.2 

10. Covered interval ....................................................... 60.9 

9. Sandstone, thin- to thick-bedded, very fine grained, 

light-green; low-angle crossbedding; contains 

fossils as in unit 11 (M7701) ................................. 39.6 

8. Sandstone, medium-bedded, fine-grained, paleyellow; herringbone current crossbeds; contains 

fossils as in unit 11 (M7702 and M7703) .............. 45.7 

Section 9. Type section of the lndecision Creek Sandstone 

Member of the Naknek Formation (fig. 9)-Continued 

7. Sandstone, thin-bedded, very fine grained, paleyellowish-brown; current crossbeds; mostly 

rubble ..................................................................... 60.9 

6. Sandstone, thin-bedded, very fine grained, silty; 

contains Buchia blanfordiana, B. mosquensis, and 

other bivalves (M7704) ......................................... 39.6 

5. Sandstone, thin-bedded, fine-grained, pale-greenishyellow; contains Buchia mosquensis (M7705) ...... 67.0 

4. Porphyritic andesite sill .............................................. 9.1 

3. Sandstone, medium-bedded, fine-grained, greenishyellow; plant debris; contains Buchia mosquensis 

(M7706) ................................................................. 67.0 

2. Siltstone, thin-bedded, dark-gray ............................. 13.7 

1. Sandstone, medium-bedded, medium-grained, 

medium-green; contains Buchia mosquensis 

(M7707 and M7708) ............................................ 157.0 

Total section ................................................... 871.7 

Conformable contact 

Snug Harbor Siltstone Member 

swimmers and their presence normally indicates access to 

open-marine conditions. 

Katolinat Conglomerate Member 

A thick conglomerate sequence is present in part as a 

lateral facies equivalent of the upper part of the Indecision 

Creek Sandstone Member in the northeastern part of the 

Alaska Peninsula. This sequence of conglomerate in the 

uppermost part of the Naknek Formation was recognized on 

Mount Katolinat by Keller and Reiser (1959, p. 270), but it 

was not investigated in detail or mapped separately by them. 

Recent studies have shown it to be a thick mappable unit in 

the northeastern part of the Alaska Peninsula. It also crops out 

over an area of a few square kilometers between Wide Bay 

and Imuya Bay. This conglomerate is a distinctive mappable 

unit that is exposed for about 100 km in the Mount Katmai 

1:250,000 quadrangle, and we here name it the Katolinat 

Conglomerate Member after Mount Katolinat on the south 

shore of Iliuk Arm of Naknek Lake in the Mount Katmai B5 1:63,360 quadrangle. The member forms the upper part of 

Mount Katolinat, as well as other mountains along its outcrop 

belt. A section, here designated as the type section of the 

Katolinat, was measured on an unnamed mountain on the 

northeast shore of Lake Grosvenor, Mount Katmai C-4 

1:63,360 quadrangle (section 10; fig. 9). The section begins 

at peak 4470 in sec. 34, and continues southwest down the 

ridge into sec. 33, T. 17 S., R. 35 W. The upper part of the 

member at the type section forms vertical cliffs and was not 

measured, but because the rocks are nearly flat-lying the 

thickness estimate is believed to be reasonably accurate. At 

the type section the lower contact of the Katolinat is gradational with the Indecision Creek Sandstone Member. The 

24 Geologic Studies on the Alaska Peninsula 
Section 10. Type section of the Katolinat Conglomerate Member 

of the Naknek Formation (fig. 9) 

Top of exposed section Thickness 

Katolinat Conglomerate Member: (meters) 

12. Conglomerate, massive; contains sandstone lenses; 

not investigated but probably similar to underlying 

units ............................................. approximately 120 

1 1. Conglomerate, massive, pebble-boulder, reversegraded; maximum clast size 32 cm; clasts are 25 

percent quartz and quartzite, 25 percent red chert, 

20 percent granitic rocks, 10 percent schist and 

metalimestone, 5 percent gray chert; few coarse 

sandstone lenses ..................................................... 48.7 

10. Sandstone, thin-bedded, medium- to coarse-grained, 

greenish-gray; crossbedded ..................................... 3.6 

9. Conglomerate, similar to unit 11 .............................. 48.7 

8. Sandstone, thin-bedded, medium- to coarse-grained, 

greenish-gray; thin conglomerate lens ................... 12.1 

7. Conglomerate, massive, cobble-pebble; wellrounded clasts; clasts are 25 percent quartz and 

quartzite, 25 percent granitic rocks, 20 percent 

metamorphic rocks, 10 percent volcanic rocks, 

10 percent red chert, 10 percent gray chert; one 

schist boulder 4 m long ......................................... 76.2 

6. Sandstone, medium-bedded, coarse-grained, 

greenish-gray ........................................................... 9.1 

5. Conglomerate, massive pebble-cobble ..................... 15.2 

4. Sandstone, medium-bedded, coarse-grained with 

scattered pebbles, moderate yellowish-green; 

small-scale tabular crossbeds; contains Buchia 

mosquensis and Corbicula (M8350) ..................... 32.0 

3. Sandstone, thin-bedded, medium-grained, lightolive-gray; lenses of pebbles ................................. 32.0 

2. Conglomeratic sandstone, thin- to medium-bedded, 

medium-grained, light-olive-gray; crossbedded; 

contains Buchia mosquensis (M8351) ................... 35.0 

1. Sandstone and conglomerate, thin-bedded to massive, coarse-grained; fining upward; clasts are 60 

percent chert, quartz, and quartzite, 20 percent 

granitic rocks, 10 percent metamorphic rocks, 

10 percent volcanic rocks ...................................... 22.8 

Total exposed section ............. approximately 455.4 

Conformable contact 

Indecision Creek Sandstone Member 

upper contact is not exposed here, but about 12 km north, the 

Herendeen Formation disconformably overlies the Katolinat 

Conglomerate Member. 

The contacts between members of the Naknek Formation are gradational, and the formation probably represents 

nearly continuous deposition from the late Oxfordian through 

the Tithonian (Late Jurassic). This conclusion is based primarily on the Buchia zones of the Alaska Peninsula (Miller and 

Detterman, 1985). Lateral facies changes are common, so 

that the thickness of individual members may vary considerably within short distances. The contact between the Naknek 

Formation and the overlying Lower Cretaceous Staniukovich 

Formation is gradational. At StaniukovichMountain (see fig. 

lo), strata containing Buchia piochii (late Tithonian, Late 

Jurassic) directly underlie strata containing Buchia uncitoides 

(Berriasian, Early Cretaceous). At many localities, however, 

Upper Cretaceous or Tertiary strata rest unconformably on 

the Naknek Formation. 

Intrusive Rocks 

Jurassic intrusive rocks are exposed west of the Bruin 

Bay fault and northeast of Becharof Lake (fig. 2). They form 

the major part of the Alaska-Aleutian Range batholith (Reed 

and Lanphere, 1969, 1972, 1973a, 1973b) that continues 

northeastward from the Alaska Peninsula, along the west side 

of Cook Inlet. The batholith is not exposed southwest of 

Becharof Lake, but it is present in the subsurface where 

several boreholes on the lowlands bordering Bristol Bay have 

penetrated Jurassic intrusive rocks (Brockway and others, 

1975). Additionally, interpretation of aeromagnetic surveys 

(Case and others, 1981, 1988) indicates that the batholith is 

probably present in the subsurface throughout the lowlands 

southwest of Becharof Lake beneath a cover of surficial 

deposits and sedimentary rocks. 

The Alaska-Aleutian Range batholith was formed by 

multiple intrusive events. The main part of the batholith was 

emplaced during the Jurassic and has been radiometrically 

dated between 155 and 176 Ma (Reed and Lanphere, 1969, 

1972). Tertiary intrusive rocks, dated at between 25 and 36 

Ma (Reed and Lanphere, 1969, 1972), are present within the 

batholith near the northeast end of the Alaska Peninsula, and 

Late Cretaceous intrusive rocks are present farther north. 

Hornblende-biotite granodiorite constitutes most of 

the batholith on the Alaska Peninsula. Lesser amounts of 

quartz diorite and granite are present locally. Most of the rock 

is coarsely crystalline and weakly foliated to nonfoliated on 

the Alaska Peninsula. Metamorphosed roof pendants, whose 

precursors were presumably Upper Triassic and Lower Jurassic sedimentary and volcanic rocks, are fairly common 

near the northeast end of the peninsula. 

We infer that the batholith was uplifted and subjected 

to erosion shortly after it solidified because it was the main 

source of sediments for the Naknek Formation, which is 

dated on faunal evidence at about 155 to 145 Ma. The 

batholith continued to be amajor sediment source throughout 

the remainder of the Mesozoic and into the Tertiary. 

Cretaceous System 

Staniukovich Formation 

The Staniukovich Formation was originally named the 

Staniukovich Shale by Atwood (191 1, p. 38) for rocks exposed on Staniukovich Mountain, located on the peninsula 

Stratigraphic Framework of the Alaska Peninsula 25 
between Herendeen Bay and Port Moller (Port Moller D-2, D- Section 11. Type section of the Staniukovich Formation at 

3 1:63,36Oquadrangles). Atwood considered these rocks to be Staniukovich Mountain (fig. 10) - Early Cretaceous in age, although some of the fossils he listed 

from this area are now known to be Late Jurassic and may 

actually have been collected from the upperpart of theNaknek 

Formation. Burk (1965) changed the name to Staniukovich 

"Formation," and included within this unit many more of the 

rocks exposed on the peninsula than had Atwood. A considerable part of the section included by Burk in his Staniukovich 

is here considered to be part of the Naknek Formation. During 

recent mapping on the Alaska Peninsula that followed Burk's 

lead (Detterman and others, 1981a, 1983), we experienced 

considerable difficulty in trying to separate the Naknek and 

Staniukovich Formations. In 1985, we sampled and measured 

Atwood's section on Staniukovich Mountain, and the source 

of the mapping problem immediately became apparent. The 

formation as defined by Atwood is a mappable unit, but as 

changed by Burk, it is not a mappable unit. Consequently, we 

here stratigraphically restrict the Staniukovich Formation to 

that part of the section composed of siltstone and shale with a 

few sandstone beds at the base (fig. 10); as so restricted, its age 

is Early Cretaceous. Thus, the formation conforms to the 

lithology as originally defined by Atwood (191 1, p. 38). Some 

revisions of recently published maps are required as aresult of 

returning to the original definition of the Staniukovich Formation. For example, in the Ugashik and Karluk 1:250,000 

quadrangles (see Detterman and others, 1983,1987a) all rocks 

mapped as the Staniukovich Formation are here reassigned to 

the Indecision Creek Sandstone Member of the Naknek Formation. This same reassignment holds for most of the ChignikSutwik Island area (see Detterman and others, 1981a). The 

only rocks that remain assigned to the Staniukovich Formation in the Chignik-Sutwik Island area are those occurring 

along the northwest side of the mountains bordering Chignik 

Bay, where a thin sequence of dark siltstone conformably 

overlies the Indecision Creek Sandstone Member and underlies the Herendeen Formation. 

The section on Staniukovich Mountain (section 11; fig. 

lo), which Atwood (191 1) designated as the type section of 

the Staniukovich Formation, is located in sec. 30, T. 50 S., R. 

73 W., Port Moller D-2 1:63,360 quadrangle. It begins about 

400 m northwest of the peak and continues northwest along 

the ridge for about 1,000 m. The lower part of this 246-m-thick 

section consists of light-olive-gray siltstone with two lightolive-brown sandstone intervals. The upper part of the section 

is composed of thin-bedded to shaly olive-gray siltstone 

containing numerous limestone nodules and concretions. The 

concretions commonly formed around belemnite and bivalve 

fossil fragments, including, in rare cases, Inoceramus prisms. 

The rocks of the upper part of the section are very soft and 

typically form saddles along ridge lines rather than good 

exposures. An abundant megafauna, consisting mainly of the 

bivalve Buchia, is present in the sandy intervals in the lower 

part of the formation. Buchia uncitoides of Berriasian (Early 

Cretaceous) age occurs throughout the lower part as high as 

Herendeen Formation 

Conformable contact Thickness 

Staniukovich Formation: (meters) 

5. Siltstone and shale, shaly to thin-bedded, lightolive-gray; numerous irregularly shaped limestone concretions containing belemnite and 

bivalve fragments, including rare Inoceramus 

prisms (M8214) ................................................. 120.0 

4. Sandstone, medium- to thick-bedded, medium- to 

coarse-grained, light-olive-brown; quartz rich; 

layer of pebbles at top; contains abundant Buchia 

crassicollis solida (M82 13) ................................... 32.0 

3. Siltstone with thin sandstone interbeds, light-olivegray, contains Buchia uncitoides ........................... 30.4 

2. Sandstone, medium- to thick-bedded, mediumgrained, light-olive-brown; contains Buchia 

uncitoides (M8212) ................................................. 9.1 

1. Siltstone, thin-bedded, light-olive-gray; contains 

Buchia uncitoides (M8211) ................................... 54.8 

Total section ................................................... 246.3 

Conformable contact 

Naknek Formation 

the base of the second sandstone unit. The second sandstone 

unit is about 30 m thick, medium to coarse grained, and rich 

in quartz. Large, thick-shelled Buchia crassicollis solida of 

Valanginian (Early Cretaceous) age occurs throughout this 

sandstone. The only identifiable megafossil from the siltstone 

of the upper part of the section is the bivalve Pleuromya, 

which is of little value for age determinations as it is a longranging genus. 

The Staniukovich Formation is well exposed near the 

northeast end of the Alaska Peninsula in the Mount Katmai B2, B-3, and B-4 1:63,360 quadrangles. The formation in the 

Mount Katmai area is comparable in thickness to the type 

section at Staniukovich Mountain, but it contains three thick 

sandstone units rather than the two present in the type section. 

The three sandstone units contain low-angle crossbeds that are 

considered to represent stacked offshore or barrier bars. The 

units are abundantly fossiliferous, but contain only the bivalve 

Buchia uncitoides. Siltstone with thin interbeds of sandstone 

is present between the thick offshore sandstone units. 

The contacts of the Staniukovich Formation with both 

the underlying Naknek Formation and theoverlying Herendeen 

Formation are conformable at the type section. A minor 

diastem may be represented within the Staniukovich by a thin 

layer of pebble conglomerate that occurs at the top of the upper 

sandy unit underlying the thick siltstone part of the section. In 

the Mount Katmai area, the Staniukovich Formation 

unconformably overlies the Naknek Formation and is in turn 

unconformably overlain by the Herendeen Formation. Locally in the Mount Katmai area, the Herendeen Formation lies 

directly on the Naknek Formation. 

26 Geologic Studies on the Alaska Peninsula 
Section 12 Section 13 

EXPLANATION 

I - '. I Sandstone ... 

Calcareous sandstone 

IC-I-I-IA Shale ---- 

I %a I Carbonate concretions 

W Covered interval 

M82 1 1 Mesozoic fossil locality 

(listed in appendix) 

V Megainvertebrate fossil 

- 100 METERS 

Geology modified from Riehle and others (1987a) 

Pedmar Formation 

0 - 5 KILOMETERS 

Geology modified from Wilson and others (1995) 

Staniukovich and Herendeen Formations 

Figure 10. Lower Cretaceous formations, Alaska Peninsula. A, Type sections of Staniukovich and Pedmar Formations and reference 

section of Herendeen Formation. Ages, formation names, and lithic units (numbered to correspond to text) are shown for each section. 

B, Index maps. Circled numbers label location of sections on index maps and figure 5. See figure 6 for explanation of other symbols. 

Stratigraphic Framework of the Alaska Peninsula 27 
Herendeen Formation 

Atwood (191 1, p. 39) proposed the name "Herendeen 

Limestone" for about 800 ft of what he described as light-gray 

arenaceous limestone exposed along the east shore of 

Herendeen Bay, north of Coal Harbor (Mine Harbor) and near 

Marble Point in the Port Moller D-2 and D-3 1:63,360 

quadrangles. However, these rocks are not limestone, but 

rather calcareous sandstone. We, therefore, here redefine 

this unit as the Herendeen "Formation" and designate a 

reference section (section 12) for the Herendeen in the low 

Section 12. Reference section of the Herendeen Formation 

(fig. 10) 

Top of exposed section 

Herendeen Formation: 

Thickness 

(meters) 

14. Calcareous sandstone, thin-bedded, mediumgrained, yellowish-gray; contains abundant 

Inoceramus prisms; covered interval at base ......... 12.8 

13. Calcareous sandstone, thick-bedded, mediumgrained, yellowish-gray; few chert pebbles at 

base; contains Inoceramus prisms; strong 

petroliferous odor .............................................. 30.5 

12. Calcareous sandstone, thick-bedded, fine-grained, 

dusky-yellow; contains Inoceramus prisms; strong 

petroliferous odor; covered interval at top ............ 15.1 

11. Calcareous sandstone; thin-bedded, fine-grained, 

moderate-olive-brown; low-angle tabular crossbeds; contains Inoceramus prisms; partly covered .. 2 1.3 

10. Calcareous sandstone, thin- to thick-bedded, 

moderate-olive-brown; tabular crossbeds; 

contains Inocerarnus prisms; petroliferous odor ... 22.8 

9. Covered interval ......................................................... 5.2 

8. Calcareous sandstone, thin-bedded, fine-grained; 

contains few Inoceramus prisms ........................... 15.2 

7. Covered interval ....................................................... 12.2 

6. Calcareous sandstone, thin-bedded, fine-grained; 

poorly exposed and partly covered ........................ 1 1.4 

5. Calcareous sandstone, thin- to thick-bedded, finegrained, pale-yellowish-brown; high-angle crossbeds; contains Inoceramus prisms; strong 

petroliferous odor .................................................. 18.2 

4. Calcareous sandstone, shaly to thin-bedded, finegrained, light-olive-brown; crossbedded; contains 

Inoceramus prisms ................................................. 12.2 

3. Covered interval ....................................................... 15.2 

2. Calcareous sandstone, thin-bedded, fine-grained, 

pale-yellowish-brown; wavy crossbedding; 

contains Inoceramus prisms; petroliferous odor ..... 7.6 

1. Calcareous sandstone, shaly to thin-bedded, finegrained; layer of chert grit in middle; contains 

Inoceramus prisms ................................................. 60.9 

Total exposed section ..................................... 270.6 

Conformable contact 

Staniukovich Formation 

hills directly southwest of the hot springs in sec. 14, T. 50 

S., R. 73 W., Port Moller D-2 1:63,360 quadrangle. The 

reference section begins about 1,000 m southwest of the 

hot springs and continues across the low ridges for about 

1 km (fig. 10). The conspicuous light-gray color of the 

rocks makes the section readily apparent. 

The reference section at the hot springs is 270 m thick. 

The strata are uniform in composition and are mainly thinbedded, medium-grained calcareous sandstone, dusky yellow 

to pale yellowish brown on fresh surfaces, and light gray and 

yellowish gray on weathered surfaces. The rocks have a 

distinct platy fracture. Tabular crossbedding is common 

throughout the section, but is developed best in the lower part 

of the section. Nearly all of the rocks have a strong petroliferous 

odor when freshly broken. The yellowish cast of the rocks 

suggests oil staining, but this has not been checked. 

In thin section, rocks from throughout the Herendeen 

Formation are remarkably uniform in composition. Fifteen 

thin sections were examined. The average composition is as 

follows: quartz, 25 percent; feldspar, mainly plagioclase, 13 

percent; lithic fragments, mainly volcanic rocks, 4 percent; 

biotite, 7 percent; bioclastic grains, mainly Inoceramus prisms 

and shell fragments, 35 percent. The remaining 16 percent is 

composed of accessory minerals and cement. The grains are 

angular to subangular and in the fine to medium (0.125-0.264 

mm) range. The biotite is fresh; its presence gives the lightcolored rocks a speckled appearance. The sandstones are 

moderately clean, and the possible oil staining suggests good 

porosity; however, the porosity of the unit has not been 

measured. 

Prisms and shell fragments of Inoceramus form amajor 

part of the rocks, but complete specimens have not been found 

in the Herendeen Bay-Port Moller area. The Herendeen Formation in the Mount Katmai area, near the northeast end of the 

Alaska Peninsula, has yielded numerous complete specimens 

of Inoceramus ovatoides as well as prisms and shell debris; it 

also contains the ammonites Acrioceras and Hoplocrioceras 

and the belemnite Acroteuthis (Jones and Detterman, 1966). 

These fossils permit an age determination of Hauterivian and 

Barremian (Early Cretaceous) for the Herendeen Formation. 

The Herendeen Formation in the Mount Katmai area is 

composed of nearly 50 percent siltstone and shale that are 

interbedded with the calcareous sandstone. This composition 

suggests that these rocks were deposited in somewhat deeper 

water than the high-energy deposits in the Port Moller area. 

The occurrence of numerous complete Inoceramus shells, 

rather than only shell fragments, and the association with 

ammonites also suggest a deeper water depositional environment for the formation in the Mount Katmai area than in the 

Port Moller area. 

The contact of the Herendeen with the underlying 

Staniukovich Formation is conformable in the type section 

and is placed at the base of the lowest resistant calcarenaceous 

sandstone bed. However, in the Mount Katmai area the 

Herendeen unconformably overlies both the Naknek Forma28 Geologic Studies on the Alaska Peninsula 

EXPLANATION 

FI ..... Sandstone 

Siltstone 

Silty sandstone 

Shale 

F-3 claystone ml Conglomerate 

a a Concretions; limestone, 

sandstone, and clay-ironstone 

& Limestone 

Dikes or sills 

Covered interval 

M 7 8 1 4 Mesozoic fossil locality (listed in 

appendix) 

V Megainvertebrate fossil 

Section 16 

r 200 METERS 

Figure 11. Upper Cretaceous formations, Alaska Peninsula. A, Type section of Kaguyak Formation and reference sections of Hoodoo 

and Chignik Formations. Ages, formation names, and lithic units (numbered to correspond to text) are shown for each section. B, Index 

maps. Circled numbers label location of sections on index maps and figure 5. See figure 6 for explanation of other symbols. 

30 Geologic Studies on the Alaska Peninsula 
The Chignik Formation has its greatest thickness (500 

to 600 m) in the area between Port Moller and Chignik Bay. 

This is the area that contains tongues of nonmarine coalbearing strata mapped by Burk (1965) as his Coal Valley 

Member. The formation thins rapidly both northeast and 

southwest from this area and becomes entirely marine. The 

massive units of pebble-cobble conglomerate and coarsegrained sandstone interbedded with carbonaceous shale, coal, 

and siltstone on the peninsula between Port Moller and 

Herendeen Bay indicate that the area was part of a fluvial 

deltaic system adjacent to a source area northwest of the 

peninsula. It should be mentioned that we here consider the 

volcaniclastic sequence exposed at Coal Bluff, on the east 

shore of Herendeen Bay, to be unassigned Tertiary strata 

rather than the Coal Valley Member of the Chignik Formation 

as mapped by Burk (1965). This is an important exposure that, 

owing to its ready accessibility on the shoreof the bay, is often 

investigated by persons studying the geology of the Alaska 

Peninsula. Much of the shoreline of the Port Moller-Herendeen 

Bay peninsula, as well as the west shore of Herendeen Bay, 

consists of deformed Tertiary sedimentary rocks. The mountains west of Herendeen Bay, and those on the peninsula along 

the east side of the bay, are underlain mainly by Mesozoic 

sedimentary rocks. We consider the Mesozoic rocks to be part 

of the upper plate of a low-angle thrust fault that has placed 

Geology modified from Detterman and others (1981) 

Chignik Formation 

them over the deformed Tertiary sequence on the shore of the 

bay (Wilson and others, 1995). An inspection of the Coal 

Bluff section (Fairchild, 1977) shows that it is richer in 

volcaniclastic rocks than exposures of the Chignik Formation 

a few kilometers distant, whereas it is similar to nearby 

volcanic-rich sedimentary rocks of Tertiary age. 

The type section of the Chignik Formation was established on Whalers Creek, one-half mile from the shore of 

Chignik Lagoon, by Atwood (191 1, p. 41). A slightly thicker 

and more accessible section along the shore of Chignik 

Lagoon is here designated as a reference section (section 14) 

for the unit. The reference section, which is located along the 

northwest shore of Chignik Lagoon in secs. 24,26, and 34, T. 

44 S., R. 59 W., Chignik B-2 1:63,360 quadrangle, begins at 

Boomer Cove and continues northeast along the shore for 3.7 

km (fig. 11). This section is 490 m thick; it consists of 

nearshore sandstone and siltstone and two intervals of floodplain and fluvial deposits. Coal and carbonaceous shale are 

present in the nonmarine parts of the section, and some of the 

sandstone is oil stained (Keller and Cass, 1956). Marine 

fossils, mainly the bivalves Inoceramus balticus var. 

kunimiensis and I. schmidti and the ammonite Canadoceras 

newberryanum, indicate a late Campanian to early 

Maestrichtian (Late Cretaceous) age for the Chignik Formation. 

Geology modified from Detterman and others (1981) 

Geology modified from Riehle and others (1 987a) 

Kaguyak Formation 

0 - 5 KILOMETERS 

. . 

Hoodoo Formation 

Stratigraphic Framework of the Alaska Peninsula 31 
Section 14. Reference section of the Chignik Formation (fig. 1 1 ) 

Top of exposed section Thickness 

Chignik Formation: (meters) 

21. Sandstone, massive, fine-grained, light-olive-gray; 

few scattered pebbles; micaceous .......................... 19.8 

20. Covered interval ....................................................... 15.2 

19. Sandstone, massive, fine-grained, light-olive-gray; 

conglomeratic ....................................................... 13.7 

18. Siltstone, thin-bedded, olive-black; bioturbated ...... 12.2 

17. Sandstone, thick-bedded, light-olive-gray ............... 10.8 

16. Siltstone, thin-bedded, olive-black; contains 

Inoceramus (M6977) ............................................. 24.4 

15. Sandstone, medium-bedded, fine-grained, darkgreenish-gray; ripple marks; conglomerate bed 

at top ...................................................................... 21.3 

14. Siltstone, thin-bedded, olive-gray .............................. 9.1 

13. Sandstone and conglomerate, medium-bedded, 

greenish-gray; channeled and crossbedded ........... 24.4 

12. Siltstone, thin-bedded, medium-dark-gray; micaceous; contains Canadoceras newberryanum and 

Inoceramus schmidti (M6976 and M7084) ........... 24.4 

11. Sandstone and conglomerate, massive ....................... 4.5 

10. Siltstone, thin-bedded, medium-dark-gray ............... 18.3 

9. Sandstone, silty, and conglomerate, mediumbedded, greenish-gray; micaceous; oil stained ...... 33.4 

8. Sandstone, thin-bedded, medium-grained, 

medium-gray ......................................................... 27.4 

7. Sandstone, thin-bedded; channeled into black 

carbonaceous shale ............................................. 14.7 

6. Sandstone, conglomeratic, medium-bedded, 

medium- to coarse-grained olive-gray; tabular 

crossbeds; oil-stained ............................................. 21.3 

5. Covered interval .................................................... 12.1 

4. Siltstone, thin-bedded, dark-yellowish-brown 62.5 ......... 3. Sandstone, thin-bedded, fine-grained, moderateyellowish-brown; silty; carbonaceous, thin coal 

seams ..................................................................... 42.6 

2. Sandstone, medium-bedded, fine-grained; contains 

Inoceramus balticus and I. schmidti (M6975) ....... 21.3 

1. Sandstone, medium- to thick-bedded, fine-grained, 

light-gray; few pebbles; contains Anomia and 

Calva (M6974) ................................................... - 57.3 

Total exposed section ..................................... 490.7 

Fault 

Naknek Formation 

The 490-m-thick reference section of the Chignik Formation is incomplete; it is in fault contact with the underlying 

Naknek Formation, and its top is an erosional surface. The 

lower contact of the ChignikFormation is amajor unconformity 

that represents a considerable part of Cretaceous time. The 

Chignikcommonly overlies the Naknek Formation, but it may 

locally overlie either the Herendeen or Staniukovich Formation. There is only a minor structural discordance between the 

Chignik and underlying rocks; bedding attitudes generally 

differ by no more than a few degrees. The Alaska Peninsula 

must have been fairly stable tectonically during the 45 to 75 

m.y. represented by the missing strata. The upper contact of 

the Chignik is also an unconformity in most localities, where 

the Tolstoi Formation of late Paleocene to early middle 

Eocene age overlies the Chignik Formation with only minor 

discordance. Locally thechignikis in contact with the Hoodoo 

Formation, which is, in part, the age equivalent of the Chignik 

and is generally considered to be a deep-water lateral facies 

equivalent of the Chignik rather than an overlying unit. 

Hoodoo Formation 

The Hoodoo Formation was named by Burk (1965, p. 

59) for the black-siltstone and shale flysch unit at the top of the 

Cretaceous sequence on the Alaska Peninsula. The type 

section of the unit is located north of Beaver Bay (fig. 5), along 

the southeast side of Hoodoo Mountain and the west side of 

the upper Beaver River valley (Port Moller C-3 1:63,360 

quadrangle), where Burk reported 2,000 to 3,000 ft of section 

(1965, p. 59). A structurally uncomplicated section of the 

Hoodoo has not been located; its strata are commonly folded 

and probably repeated by faulting. A partial section, 630 m 

thick, was measured on a ridge north of Foot Bay (Chignik A3 1:63,360 quadrangle) and is here designated as a reference 

section (section 15). The section begins at the southwest 

corner of sec. 30 and continues south along the west border of 

sec. 31, T. 47 S., R. 60 W. (fig. 11). 

Section 15. Reference section of the Hoodoo Formation (fig. 1 1) 

Tolstoi Formation 

Unconformable contact Thickness 

Hoodoo Formation: (meters) 

19. Siltstone and sandstone, thin-bedded, olive-black; 

sandstone laminated; convolute bedding common .. 15.2 

18. Covered interval ....................................................... 15.2 

17. Siltstone and sandstone, thin-bedded, olive-gray; 

pencil fracture in siltstone ................................... 30.4 

16. Covered interval ..................................................... 25.9 

15. Siltstone, thin-bedded ................................................. 7.6 

14. Sandstone, medium- to thick-bedded, fine-grained, 

medium-gray; laminated, graded ........................... 12.2 

13. Siltstone and sandstone, thin- to thick-bedded, finegrained, olive-gray; laminated, graded; convolute 

bedding common ................................................... 64.0 

12. Sandstone, thick-bedded, fine-grained, brownishgray; laminated .................................................... 24.3 

11. Siltstone and shale, shaly to thin-bedded, olive-gray; 

pencil fracture ....................................................... 97.5 

10. Sandstone, massive, fine-grained, light-brownishgray ........................................................................ 16.7 

9. Shale, olive-gray ....................................................... 27.4 

8. Siltstone, thin-bedded, olive-gray ............................ 25.9 

7. Shale, olive-black; pencil fracture ............................ 22.8 

6. Siltstone, thin-bedded; convolute lamination ........... 53.3 

5. Sandstone and siltstone, thin- to thick-bedded, 

olive-gray to olive-black; laminated and graded ... 36.5 

4. Shale, olive-black ..................................................... 15.2 

32 Geologic Studies on the Alaska Peninsula 
Section 15. Reference section of the Hoodoo Formation (fig. quence becomes more sandy upward, and the upper part of the 

1 1 )-Continued formation contains medium-bedded, fine-grained sandstone 

3. Siltstone, thin-bedded, olive-gray; plant debris 30.4 units as much as 60 m thick. Chert-pebble lag gravel is present ....... 2. Conglomerate, massive, pale-brown; chaotic; clasts near the tops of some beds. Siltstone and shale rip-up clasts are 

of chert. .. auartz. siltstone. and shale ....................... 24.4 common in beds. Clasts in the , 

1. Siltstone, thin-bedded, olive-gray; laminated; well- units become larger and more numerous upward in the secdeveloped cleavage ................................................ 85.3 tion. Most are well rounded and less than 15 cm in diameter, 

Total exposed section ..................................... 630.2 but a few reach 0.5 m in diameter. Clasts include volcanic and 

Base of exposed section plutonic rocks as well as chert and quartz. 

Deposition of the Hoodoo Formation is attributed to 

The Hoodoo Formation may be considerably thicker 

than the 630 m measured at the reference section, but complex 

folding and faulting and the absence of good marker beds 

make it impossible to ascertain the formation's true thickness. 

A bore-hole (Pure Oil Company #1 Canoe Bay) at Canoe Bay 

(Port Moller C-4 1:63,360 quadrangle, sec. 8, T. 54 S., R. 78 

W.) drilled 2,024 m (6,640 ft) entirely within the Hoodoo 

Formation. The section in this borehole was probably repeated several times by faulting, but the exact positions of the 

faults are difficult to ascertain. 

The Hoodoo Formation is thicker and more continuously exposed between Pavlof Bay and Chignik Bay than 

elsewhere on the Alaska Peninsula. The northernmost good 

exposures are at Amber Bay (fig. 5). Nearly all exposures 

border or are within a few kilometers of the Pacific Ocean 

coast of the AlaskaPeninsula. The thickest part of the Hoodoo 

Formation, and the part containing the most sandstone and 

conglomerate, is located between Pavlof Bay and Herendeen 

Bay. This part of the Hoodoo is southwest of the thickest part 

of the partially coeval Chignik Formation, and its location is 

evidence that the source for the two formations was to the 

northwest and that a deep basin lay adjacent to the fluvial plain 

on which the Chignik Formation was deposited. 

The lower contact of the Hoodoo Formation is not 

exposed at the reference section, nor is it exposed at the type 

section. In the Chignik Bay area the Hoodoo Formation is seen 

to overlie the Chignik Formation with apparent conformity. 

Some investigators (Mancini and others, 1978; Molenaar, 

1980) consider the Hoodoo Formation to be a deep-water 

lateral facies equivalent of the Chignik Formation. On the 

basis of our investigation of the two formations, we conclude 

that they were formed during a major Late Cretaceous marine 

transgression and that they represent two distinct facies of the 

same time-stratigraphic interval. The contact between the 

Hoodoo and Chignik Formations should be considered gradational. Continued deposition during subsidence of the basin 

suggests that the uppermost part of the Hoodoo Formation 

may be slightly younger than the upper part of the Chignik 

Formation. 

The lower part of the HoodooFormation consists mainly 

of dark-gray to black, thin-bedded siltstone that fractures in 

pencil-shaped splinters. Subordinate, thin interbeds of finegrained sandstone are present at regular intervals. The sesubmarine slumps and turbidity currents. The predominantly 

thin-bedded siltstone and thin, regularly spaced sandstone 

intervals in the lower part of the unit were probably deposited 

on the lower slope of a submarine fan. Both siltstone and 

sandstone are finely laminated locally, and rip-up clasts are 

common in the sandstone beds, but other sedimentary structures characteristic of turbidites are missing. The splintery 

fracture of the siltstone possibly has obliterated other features. 

The thick sandstone and conglomerate units in the upper part 

of the formation are interpreted as submarine fan build-ups, 

locally including upper-fan deposits. The sand beds are broadly 

lenticular and graded, and they contain flute and load casts; the 

lag gravels are interpreted as inner-fan channel deposits. 

Marine megafossils are present, but sparse, throughout 

the formation; they are somewhat more abundant in the upper 

part. Inoceramus schmidti, I. balticus, and I. vancouverensis 

are found throughout the formation. Ammonites including 

Diplomoceras notabile, Ncophylloceras hetonaise, and 

Canadoceras newberryanurn are found mainly in the upper 

part of the formation. These megafossils indicate a late 

Campanian to early Maestrichtian (Late Cretaceous) age for 

the Hoodoo Formation, which is approximately the age of the 

Chignik Formation. However, the two formations have different faunal assemblages and therefore may not be exact age 

equivalents. Canadoceras, Inoceramus schmidti,and I. balticus 

are found mainly in the Chignik, whereas Diplomoceras and 

the other Inoceramus species are restricted to the Hoodoo and 

correlative rocks. Additional evidence that the Hoodoo is in 

part younger than the Chignik Formation is seen at several 

localities in the Chignik area where the Hoodoo Formation 

conformably overlies the Chignik Formation. Of course, 

another possible interpretation is that the different faunal 

assemblages were coeval and their distribution patterns were 

governed by depositional environments. 

In structurally complex areas, the dark siltstone of the 

Hoodoo Formation is extremely difficult to differentiate from 

similar-appearing dark siltstone in the overlying Tolstoi Formation (upper Paleocene to middle Eocene) or in the lower 

part of the herein-adopted StepovakFormation of Burk (1965) 

(upper Eocene and lower Oligocene). There are a few distinguishing features, however, in each of these units. The Hoodoo 

tends to fracture into pencil-shaped splinters or small chips 

more commonly than either of the other units; siltstone in the 

Tolstoi commonly contains abundant plant debris and complete leaves; small concretions in the siltstone of the Stepovak 

Stratigraphic Framework of the Alaska Peninsula 33 
generally contain small glendonite crystals and are typically 

brownish black. 

The upper contact of the Hoodoo with the Tolstoi 

Formation in the Port Moller area is structurally conformable. 

The oldest flora (Paleocene) in the lowermost part of the 

Tolstoi is found only where the Tolstoi overlies the Hoodoo in 

the Pavlof Bay-Port Moller area. Likewise, the Hoodoo Formation contains its youngest (early Maestrichtian) megafauna 

in this same area. The contact between the formations in this 

area is probably a disconformity. From Chignik northeastward, the contact between these same units is unconformable. 

The Hoodoo Formation is, at least in part, the age 

equivalent of other Late Cretaceous flysch deposits on or near 

the AlaskaPeninsula; these deposits are the Kaguyak, Kodiak, 

and Shumagin Formations. Faunal remains are sparse in all 

these formations owing to the deep-water depositional environment that they represent; the few fossils collected from 

them indicatea Late Cretaceous (Campanian to Maestrichtian) 

age for each of the units. 

Shumagin Formation 

The Shumagin Formation was named by Burk (1965) 

for the Cretaceous flysch deposits on the outer Shumagin 

Islands, particularly Nagai and Big Koniuji Islands, and the 

small islands between them (fig. 5). The least disturbed 

sequence occurs at Falmouth Harbor on the west side of Nagai 

Island, and these rocks were termed "typical exposures" of the 

unit by Burk (1965, p. 64); Moore (1974a) designated this area 

as the type area of the formation. The highly contorted deepwater turbidites are intruded by a granodiorite batholith dated 

at 57.4 to 65.6 Ma (Burk, 1965; Moore, 1974a; Kienle and 

Turner, 1976, recalculated using constants of Steiger and 

Jager, 1977). A considerable amount of detailed stratigraphic 

and structural information about these rocks was obtained by 

Moore (1973,1974a, 1974b). During the course of our investigations on the Alaska Peninsula and offshore islands, we 

have not added much new data on the Shumagin Formation. 

However, we have obtained additional megafauna collections, samples for K-Ar determinations, and samples for thinsection analysis. 

The Shumagin Formation consists of interbedded 

graywacke, siltstone, mudstone, and shale at least 3,000 m 

thick (Burk, 1965). The dark-gray sandstone, mostly lithic 

graywacke, ranges from beds 2 cm thick to massive units 20 

m thick. Most beds are graded and contain abundant shale and 

siltstone chips. Their contacts with underlying siltstone and 

shale units are sharp, but their upper contacts with overlying 

fine clastic units are gradational. Convolute bedding is common, and the bases of many beds contain abundant flute and 

load casts as well as drag and prod marks. These features are 

all commonly associated with turbidite deposition. The thick 

to massive sandstone units pinch and swell abruptly along 

strike, and were probably deposited as deep-sea fans. The 

thin-bedded, very fine to fine-grained sandstone units are 

rhythmically interbedded with the siltstone, and both their 

upper and lower contacts are sharp. Bed thickness is uniform. 

These sandstone and siltstone interbeds are characteristic of 

abyssal plain deposits. 

In many cases it is extremely difficult to identify tops 

and bottoms of beds. However, on a large scale the rhythmically bedded abyssal plain sequence is the lowest part of the 

Shumagin Formation and is overlain by the submarine fan 

sequence composed of more massive sandstone units. There 

are no exposed contacts with other sedimentary units. The 

only exposed contact is with the Shumagin batholith, which 

has altered the sedimentary rocks to hornfels near its contacts. 

Megafaunal remains areextremely rare in the Shumagin 

Formation, and most were probably deposited by the turbidity 

currents. Inoceramus kusiroensis is the most common bivalve; many were found along bedding planes with both 

valves attached and in an open position. Jones and Clark 

(1973, p. 134) suggested that this species of Inoceramus may 

have had a pseudoplanktonic habit in nearshore water. Upon 

the death of the animal, its shell would open and sink to the 

bottom. One collection also contains Inoceramus cf. I. 

shikotanensisand Terebellinapalachii, amarine worm. These 

fossils indicate an early Maestrichtian age; thus the Shumagin 

Formation is age equivalent to the upper parts of the Hoodoo 

and Kaguyak Formations. The Shumagin Formation was 

deposited farther offshore than either of those formations; it 

was deposited at or near the paleo-Aleutian trench. Thinsection analysis of sandstone samples from the Shumagin 

suggests that it has a major lithic component similar to the 

Kodiak Formation on Kodiak Island and a minor lithic component similar to the Hoodoo Formation. This composition 

suggests that most of the sediments that make up the Shumagin 

were transported from the northeast along the axis of the 

paleo-Aleutian trench (John Decker, oral commun., 1985). 

The source areas for the Hoodoo and Kaguyak Formations 

were located on the Alaska Peninsula. 

Kaguyak Formation 

The Kaguyak Formation was named by Keller and 

Reiser (1959) for the sequence of rocks of Late Cretaceous age 

exposed in seacliffs along the shore of KaguyakBay between 

the Swikshak and Big Rivers (Afognak C-6 1:63,360 quadrangle) (fig. 11). These rocks had been described earlier by 

Martin (1926) and Hazzard and others (1950) but had not been 

named. This sequence of rocks wasremeasured and described 

by Detterman and Miller (1985) as part of the Upper Cretaceous flysch sequence in southern Alaska. The formation is 

mapped only in the northeastern part of the Alaska Peninsula, 

from Katmai Bay northeastward to Kamishak Bay. 

The type section in the seacliffs (section 16) begins at 

the mouth of the Swikshak River and continues westward 

along the cliffs for about 5 km in secs. 19 and 20, T. 18 S., R. 

27 W., and secs. 13 and 14, T. 18 S., R. 28 W., Afognak C-6 

1:63,360 quadrangle. Additionally, beds at the base of the 

34 Geologic Studies on the Alaska Peninsula 
formation on a narrow cape about 2 krn southwest (sec. 34, T. 

18 S., R. 28 W.) areprojectedinto the section; these basal beds 

were measured by Keller and Reiser (1959) and listed as their 

unit 15 (p. 276). We also investigated these beds and sampled 

the abundant megafauna in them but did not measure the 

section. We have added these beds measured by Keller and 

Reiser to the section presented below (unit 2). Neither the 

upper nor the lower contact of the Kaguyak Formation is 

exposed at the type section, but both are exposed nearby. 

Section 16. Type section of the Kaguyak Formation (fig. 11) 

Top of exposed section Thickness 

Kaguyak Formation: (meters) 

22. Sandstone and siltstone, thin-bedded to massive, 

medium-dark-gray to pale-brown; rip-up clasts .... 44.2 

21. Sandstone, thick-bedded, fine-grained, pale-brown; 

rip-up clasts; load clasts ......................................... 10.6 

20. Siltstone, thin-bedded, medium-gray; crossbedded . 12.2 

19. Sandstone and siltstone, thin-bedded to massive, 

olive-gray; rip-up clasts and graded bedding ........ 56.3 

18. Sandstone and siltstone, thin-bedded to massive, 

fine-grained, medium-dark-gray; bedding graded, 

laminated to convolute; load and flute casts along 

amalgamation surfaces; limy sandstone concretions 42.6 

17. Covered interval ....................................................... 15.2 

16. Siltstone and sandstone, thin-bedded, dark-gray; 

convolute; contains Inoceramus subundatus 

(M7813) ............................................................... 30.5 

15. Sandstone, massive, fine-grained, medium-darkgray; rip-up and load clasts along amalgamation 

surfaces; limy sandstone concretions 45.7 .................... 14. Siltstone and sandstone, thin-bedded to massive, 

fine-grained, dark-gray; rhythmically bedded; 

rip-up and load clasts in sandstone; flame 

structures in siltstone ............................................. 67.0 

13. Siltstone and sandstone, thin-bedded, fine-grained, 

medium-gray; flame structures, sole and drag 

marks ..................................................................... 74.6 

12. Sandstone, massive, fine-grained, medium-gray; 

amalgamated; ball and pillow structures ............... 11.5 

11. Siltstone and sandstone, thin-bedded to massive, 

fine-grained, medium- to dark-gray ....................... 15.2 

10. Andesite porphyry sill .............................................. 16.7 

9. Covered interval ....................................................... 442 

8. Shale, dark-gray ........................................................ 9.1 

7. Covered interval ....................................................... 18.2 

6. Andesite porphyry sill .............................................. 12.1 

5. Covered interval ..................................................... 1 12.7 

4. Siltstone, thin-bedded, medium-dark-gray; limestone and clay-ironstone concretions; contains 

Diplomoceras notabile, Didymoceras, and 

Inoceramus balticus var. kunimiensis (M7903) . . 137.0 

3. Covered interval .................................................... 9 1.4 

2. Claystone and limestone, shaly to thin-bedded, 

dark-gray ........................................................... 30.4 

1. Siltstone, massive, medium-gray; siliceous; very 

dense; breaks down into hackly fracturing shale; 

Section 16. Type section of the Kaguyak Formation (fig. 

1 1 )-Continued 

abundant large spheroidal limy concretions; abundant ammonites and bivalves aligned on bedding 

planes, including Didymoceras aff. D. hornbeyense, 

Diplomoceras notabile, Gaudryceras tenuiliratum, 

Neophylloceras ramosum, Pachydiscus 

(Pachydiscus) hazzardi, Inoceramus balticus 

var. kunimiensis, and Glycymeris (M78 14) ......... - 

2 19.4 

Total exposed section .................................. 1 ,1 16.8 

Base of exposed section 

The thickness of the Kaguyak Formation as given here 

is somewhat less than that published by Keller and Reiser 

(1959, p. 275). The discrepancy is apparently within the 

uppermost part of the formation. We did not observe the upper 

contact at the type section, but have seen it in other exposures 

very nearby where Tertiary sedimentary and volcanic rocks 

unconformably overlie the Kaguyak Formation. We suggest 

that unit 1 of the Keller and Reiser section (1959, p. 275) may 

actually be part of the Tertiary sequence. This suggestion is 

supported by their report of carbonaceous partings in this unit. 

Carbonaceous material is very common in the Tertiary sequence, but is uncommon in the Kaguyak Formation. 

Like theupper contact, the lower contact of the Kaguyak 

Formation is not exposed at the type section but can be seen 

at outcrops nearby. We estimate that this contact lies 91 m 

below the base of the exposed section at the type section. At 

this contact, the Kaguyak Formation unconformably overlies 

the Herendeen Formation, and not the Naknek Formation as 

reported by Keller and Reiser (1 959, p. 277). The 30-ft-thick 

sandstone bed that they considered to be the Naknek Formation actually contains Inoceramus ovatoides, which is characteristic of the Herendeen Formation. Prisms from this 

Inoceramus compose most of the sandstone bed. The 30-ftthicksandstone bedin turn unconformably overlies theNaknek 

Formation. 

The Kaguyak Formation is a turbidite deposit representing a submarine fan prograding into a deep-water environment. The lower part of the unit consists of thin-bedded to 

massive dark-gray siltstone with numerous silty limestone 

concretions and a few thin limestone beds. A few of the 

siltstone beds are laminated, load and flute casts are common, 

and drag marks and flame structures are present. Thin, graded 

sandstone beds become more numerous upward. 

The upper part of the Kaguyak is an interbedded sequence of siltstone and graywacke in units 10 to 20 m thick. 

The graywacke is graded and contains numerous rip-up clasts; 

load and flute casts up to 1 m long indicate a current direction 

from about N. 30" E. Some of the sandstone beds are channeled into the underlying siltstone. Siltstone overbank and 

levee deposits are commonly interbedded with the sandy 

units. These rocks were deposited in a submarine fan complex 

Stratigraphic Framework of the Alaska Peninsula 35 
and have been assigned to the middle-fan facies association of 

Mutti and Ricci-Lucchi (1972). 

Megafossils are locally abundant in the lower part of the 

Kaguyak at the type section. Elsewhere they occur randomly 

throughout the formation. Most represent forms that were probably carried in by turbidity currents. Ammonites are common 

and includeDiplomoceras notabile, Neophylloceras ramosum, 

Pachydiscus kamishakensis, P. hauardi, and Didymoceras 

aff. D. hornbeyense. Some of thepachydiscids are 1 m ormore 

across. lnoceramus balticus var. kunimiensis, I. subundatus, 

and I. kusiroensis also are common. These fossils have been 

assigned to the Pachydiscus kamishakensis zone of latest 

Campanian to early Maestrichtian age (Jones, 1963). 

The lower contact of the Kaguyak Formation is an 

unconformity, and the formation overlies strata ranging in age 

from Kimmeridgian (Late Jurassic) to Albian (Early Cretaceous). The upper contact also is unconformable; beds of early 

Tertiary age (Copper LakeFormation) locally overlie the Kaguyak 

Formation. In the Katmai Bay-Geographic Harbor area, nonmarine upper Oligocene strata overlie the formation; elsewhere it is 

overlain by Tertiary and Quaternary volcanic rocks. 

CENOZOIC ROCKS 

Tertiary System 

Rocks of all the Tertiary epochs are present on the 

Alaska Peninsula. They are mainly volcaniclastic sedimentary rocks, both marine and continental, interlayered with 

volcanic rocks and unconformably overlying Mesozoic rocks. 

As much as 5,000 to 6,000 m of Tertiary sedimentary rocks 

mantle the southwest end of the Alaska Peninsula. The Tertiary strata become thinner northeastward, and only about 

2,000 m are exposed at the northeast end of the peninsula. 

Most of the Tertiary sedimentary rocks are rich in contemporaneous volcanic debris. Exceptions are the Tolstoi and Bear 

Lake Formations, which are made up largely of nonvolcanic 

sediments. Mesozoic strata, in contrast to the Tertiary rocks, 

contain much less volcanic debris and considerably more 

intrusive and metamorphic rock fragments. 

The Tertiary sequence on the Alaska Peninsula is divided into 11 formations, 1 of which is subdivided into 2 

informal members (fig. 12). In ascending order these units are 

the Tolstoi Formation (upper Paleocene to middle Eocene), 

Copper Lake Formation (Paleocene? and lower Eocene), 

Stepovak Formation and Meshik Volcanics (upper Eocene 

and lower Oligocene), Belkofski Formation (upper Oligocene? 

and lower and middle Miocene?), Hemlock Conglomerate 

(upper Oligocene), Unga Formation (upper Oligocene to 

middle Miocene), Bear Lake Formation (middle and upper 

Miocene), Tachilni Formation (upper Miocene), Milky River 

Formation (Pliocene), and Morzhovoi Volcanics (upper Miocene? to lower Quaternary?). The Stepovak Formation is 

divided into informal (lower) siltstone and (upper) sandstone 

members. In some cases the distinction between these formations is rather subtle, but we consider them all to be mappable 

units and therefore separate formations. 

Intrusive and volcanic rocks form a considerable part of 

the Tertiary sequence on the Alaska Peninsula. The large 

intrusive bodies aremapped as the Shumagin Islands batholith 

(Paleocene), Devils Bay batholith (Miocene and Pliocene), 

and Agripina Bay batholith (Miocene and Pliocene). Smaller 

plutons of similar ages are located in the Sanak and Semidi 

Islands, and at American Bay, Moss Cape, andMount Becharof. 

Additionally, numerous small plugs, domes, and stocks of 

hypabyssal intrusive rocks associated with volcanism are 

present and range in age from Oligocene to Pliocene. The 

outpouring of basalt and andesite of the Meshik arc (see 

Wilson, 1985) began in the late Eocene and continued into the 

early Miocene. The present Aleutian volcanic arc began its 

development in late Miocene time and is still active today. 

Early Tertiary Intrusive Rocks 

Granitic rocks were first reported on the outer Shumagin 

Islands by Dall and Harris (1 892) and Dall(1896), but no map 

or description of these rocks was provided. Grantz (1963) was 

the first to report biotite granodiorite, on the basis of samples 

obtained duringa brief visit to the islands. Burk(1965) was the 

first to conduct an extensive mapping program on the outer 

Shumagin Islands, and he determined that the granitic rocks 

had intruded the Upper Cretaceous flysch deposits (Shumagin 

Formation) on Nagai and Big Koniuji Islands. Burk also 

obtained samples of granitic rocks for age determinations. 

Moore (1974a) did detailed mapping of the islands and obtained megafossils and intrusive rock samples for age determinations. Samples of intrusive rocks were also obtained 

from the Shumagin Islands by Kienle and Turner (1976). The 

present investigators visited the islands only briefly, but we 

did obtain more megafossils and samples for K-Ar age determinations. 

The Shumagin Islands batholith (Shumagin-Kodiak 

batholith of Kienle and Turner, 1976) consists mainly of 

medium-grained biotite granodiorite and quartz monzonite; it 

has hypidiomorphic granular texture. Potassium feldspar phenocrysts are locally as much as 1 cm across, and minor 

muscovite is present. A hornfels zone a few meters to as much 

as 500 m wide is present where the batholith intrudes the 

Upper Cretaceous flysch. 

The Shumagin Islands batholith is one in a line of 

northeast-trending hybrid granodiorite batholiths (Hill and 

others, 1981) that begin on Sanak Island, 50 km southeast of 

the tip of the Alaska Peninsula. The line of batholiths continues northeast of the Shumagin Islands to the Semidi and 

Kodiak Island groups. The batholiths parallel the southeast 

side of the AlaskaPeninsula and also roughly parallel the edge 

of the continental shelf. 

The age of the Shumagin Islands batholith is well 

constrained by numerous K-Ar age determinations and ranges 

36 Geologic Studies on the Alaska Peninsula 
between 65.6f3.3 and 57.4f2.9 Ma (Burk, 1965; Moore, 

1974a; Kienle and Turner, 1976). Corresponding ages have 

been obtained from other batholiths in this northeast-trending 

line, and all intrude an Upper Cretaceous flysch sequence that 

is dated by its megafauna as early Maestrichtian. 

Beaver Bay Croup 

The name "Beaver Bay Group" was proposed by Burk 

(1965, p. 88-89) for a thick sequence of volcanic sandstone 

and conglomerate, breccia, and black siltstone exposed along 

the west shore of Beaver Bay (fig. 5) and in the mountains 

bordering the lower part of the Beaver River, which flows 

south into Beaver Bay. Burk's Beaver Bay Group consists of 

the Tolstoi and Stepovak Formations. Burk felt that it was 

impossible to tell the difference between these two formations 

when viewing them in isolated outcrops. However, there are 

major differences between the Tolstoi and Stepovak Formations that we will discuss below. 

In Burk's report (1965, p. 88-89), he designated a type 

locality for the Beaver Bay Group in which he did not separate 

the Tolstoi and Stepovak Formations. However, on Burk's 

geologic map (1965, part 2, sheets 1 and 2), he did not map the 

Beaver Bay Group in its type locality; instead he mapped the 

Figure 12. Correlation chart of Tertiary stratigraphic units, Alaska Peninsula. 

Stratigraphic Framework of the Alaska Peninsula 37 

AGE 

a, Piacenzian a, 

8 .- Volcanic rocks 

E Zanclean Tachilni Formation 

-7-? 

Bear Lake Formation (including 

Unga Conglomerate Member) 

Unga Formation 

Tolstoi Formation 

Tolstoi Formation 

STAGE 

STRATIGRAPHIC UNITS 

This report Modified from Burk (1 965) 
Tolstoi and Stepovak Formations as separate units there. Burk 

mapped the Beaver Bay Group only between Stepovak Bay 

and Chignik Bay, and even in that area, he mapped the Beaver 

Bay Group as two subunits corresponding, respectively, to the 

Tolstoi and Stepovak Formations. Thus Burk assigned rocks 

to either the Stepovak or the Tolstoi Formation, even when 

using the Beaver Bay Group designation, apparently using the 

subunits of the Beaver Bay Group to indicate uncertainty 

about his assignments in the area between Stepovak Bay and 

Chignik Bay. 

We believe that during Burk's broad reconnaissancescale mapping he did not recognize that the volcaniclastic 

sedimentary rocks of the Stepovak Formation and the mainly 

volcanic sequence of the Meshik Volcanics constitute 

intertonguing facies of a time-stratigraphic unit. The presence 

of this additional unit, the Meshik Volcanics, may have 

contributed to Burk's confusion about the distinction between 

the Stepovak and Tolstoi Formations. The area between 

Stepovak Bay and Chignik Bay, where he mapped the Beaver 

Bay Group, is the same area where the Stepovak Formation 

and the Meshik Volcanics intertongue. The abundant volcanic 

rocks that he noted as belonging to the Tolstoi Formation in 

that area are herein considered to be the Meshik Volcanics. 

Stratigraphically below these rocks are sedimentary rocks 

which we have assigned to the Tolstoi Formation (Wilson and 

others, 1995). 

Although the Beaver Bay Group has been used and 

might be useful for generalized mapping on the Alaska Peninsula, there are major differences between the Tolstoi and 

Stepovak Formations that can be used for detailed mapping. 

Sedimentary rocks of the Stepovak Formation are almost 

entirely derived from a volcanic source terrain. Clasts in its 

conglomerate beds are mostly unaltered volcanic rocks. Its 

fine-grained siltstone and shale contain montmorillonite clay 

as a major component, and its sandstones are commonly 

tuffaceous. The Tolstoi Formation, in contrast, generally 

contains more than 50 percent nonvolcanic materials. Clasts 

in its conglomerate beds are composed mainly of chert (black, 

gray, and red), quartz, quartzite, and metalimestone; these 

conglomerates generally contain no more than 25 to 35 

percent volcanic rocks. Many of the volcanic clasts are highly 

altered and silicified. Sandstones from each formation are of 

the same general composition, but some of those of the 

Stepovak contain a higher percentage of volcanic fragments. 

Carbonaceous debris and plant remains are abundant in the 

Tolstoi Formation, as are coal and carbonaceous shale. On the 

basis of the above, we don't consider the name "Beaver Bay 

Group" to be a useful stratigraphic term and suggest that its 

usage be discontinued on the Alaska Peninsula. 

Tolstoi Formation 

The oldest Tertiary deposits overlying Mesozoic strata 

were named the Tolstoi Formation by Burk (1965). He designated a type locality along the east shore of Pavlof Bay, 

between Tolstoi Peak and Cone Peak (Port Moller B-5 and C5 1:63,360 quadrangles). These rocks, which are here regarded as the type section of the Tolstoi (section 17), are not 

typical of the rest of the formation as mapped by either Burk 

or us to the northeast of the type locality. The Tolstoi Formation at Pavlof Bay is in part a shallow-water marine deposit, 

whereas northeastward along the peninsula the formation is a 

nonmarine fluvial deposit containing abundant conglomerate, 

crossbedded sandstone, coal, and carbonaceous shale. For this 

reason, we here designate a reference section (section 18) 

along the east shore of Ivanof Bay, in the Stepovak Bay D-5 

1 :63,360 quadrangle, beginning at the northeast corner of sec. 

3 and continuing south along the shore to the southeast corner 

of sec. 10, T. 50 S., R. 66 W. (fig. 13). Both the type and 

reference sections were measured during the course of the 

present investigation. Dikes and sills are noted in the descriptions of the measured sections but are not included in the 

sections' total thicknesses. The 659 m measured here as the 

thickness of the type section of the Tolstoi Formation at Pavlof 

Bay is about 20 percent less than that given by Burk (1965, p. 

189). The discrepancy may result from difficulties in correlating beds across the several small anticlinal and synclinal folds 

in the section, and at present the actual thickness is unknown. 

The reference section measured at Ivanof Bay (fig. 13) is 

characteristic of the fluvial flood-plain and delta sequence that 

is common in the Tolstoi Formation northeast of its type 

locality. 

Volcaniclastic debris forms a conspicuous but not major part of the sedimentary rocks of the Tolstoi Formation. A 

few thin tuff beds are present in all sections, and some of the 

sandstones are tuffaceous. Clasts in the conglomerate beds 

consist of 25 to 35 percent volcanic rocks. Most of the 

volcanic clasts are not fresh-appearing, suggesting that they 

have been reworked from a Mesozoic source rather than 

derived from contemporaneous Tertiary volcanic deposits. 

The sandstones contain much arkosic and granitic detritus, 

likewise indicating a Mesozoic source terrane. This is in sharp 

contrast to the overlying Stepovak Formation and the Meshik 

Volcanics, whose sandstones are composed almost entirely of 

fresh volcanic debris. 

The Tolstoi was deposited by onlap onto the underlying 

Mesozoic rocks. Deposition of shallow-marine sediments at 

Pavlof Bay and the inner Shumagin Islands was, in part, 

contemporaneous with deposition of delta-plain and fluvial 

(mainly braided-stream) deposits elsewhere on the peninsula. 

Mollusks at Pavlof Bay and Korovin Island indicate a late 

middle Eocene age for the Tolstoi Formation according to 

Marincovich (1988b). Marine mollusks are restricted almost 

entirely to the Pavlof Bay and Korovin Island localities where 

the marine beds are interlayered with nonmarine strata. The 

main elements of the fauna are Turritella uvasana stewarti 

Merriarn and Microcallista (Costacallista) conradiana (Gabb). 

The fauna also includes Nuculana sp., Tellina sp., Ostrea sp., 

Solena sp., Corbicula sp., Volsella sp., and Glycymeris sp. 

This fauna represents an extremely shallow-water, warm38 Geologic Studies on the Alaska Peninsula 
temperate to subtropical marine environment (Marincovich, 

1988b). 

Dinoflagellates obtained from the same bed on Korovin 

Island that yielded the Turritella uvasana stewarti indicate an 

early to middle Eocene age (Nairn Albert, written commun., 

1985). The dinoflagellates include Impagidinium californiense, 

Adnatosphaeridium rnultispinosom, Turbiosphaera filosa, 

Glaphyrocysta retiintexta, and Rhombodinium sp. 

An abundant fossil megaflora was obtained from the 

reference section at Ivanof Bay. The main elements of this 

A Section 17 

EXPLANATION ml Conglomerate 

Conglomeratic sandstone ..-..a. r] . . . . . . . . . 

..... Sandstone 

-. - I-X-~ Siltstone 

I0_0 I Concretions 

F-J run ml Dike or sill 

Covered interval 

1 141 6 Cenozoic fossil locality (listed in 

appendix) 

flora include Acerarcticum Heer,Alnus sp., Cocculus~obella 

(Newberry) Wolfe, Grewiopsis curiculaecrodatus (Hollick) 

Wolfe, Liquidambar sp., Metasequoia occidentalis 

(Newberry) Channey, Protophyllum semotum Hickey, and 

Thuites interruptus (Newberry) Bell. The flora was identified 

by J.A. Wolfe (198 1) as late Paleocene to middle Eocene in 

age. Wolfe (198 1) stated that the flora indicates a paratropical 

rain-forest environment with a mean annual temperature of 

about 22°C. Thus, both the megaflora and megafauna indicate 

a warm temperature for the area at the time of deposition of the 

Section 18 Section 19 

4 

400 METERS 

200 

V Megainvertebrate fossil l- 0 

Plant fossil 

Figure 13. Paleocene and Eocene formations, Alaska Peninsula. A, Type section (1 7) of Tolstoi Formation at Pavlof Bay, 

reference section (1 8) of Tolstoi Formation at lvanof Bay, and section (19) of Copper Lake Formation. Ages, formation 

names, and lithic units (numbered to correspond totext) are shown for each section. 6, Index maps. Circled numbers label 

location of sections on index maps and figure 5. See figure 6 for explanation of other symbols. 

Stratigraphic Framework of the Alaska Peninsula 39 
Geology modified from Wilson and others (1995) 

Tolstoi Formation 

Geology modified from Wilson and others (1995) 

Tolstoi Formation 

0 - 5 KILOMETERS 

Geology modified from Riehle and others (1987a) 

Copper Lake Formation 

Section 17. Type section of the Tolstoi Formation (fig. 13) 

Meshik Volcanics 

Unconformable contact Thickness 

Tolstoi Formation: (meters) 

24. Andesite porphyry dike .............................................. 5.4 

23. Sandstone and siltstone; rubble; mostly covered ...... 67.6 

22. Siltstone, thin-bedded, olive-gray; andesite 

porphyry sill at top ................................................. 18.2 

21. Siltstone; rubble, mostly covered ............................. 30.4 

20. Andesite porphyry dike .............................................. 6.7 

19. Sandstone, thin-bedded, olive-gray to massive, paleyellowish-brown; small-scale crossbedding and 

ripple marks; plant debris ...................................... 53.3 

18. Andesite porphyry sill ................................................ 5.4 

17. Covered interval ....................................................... 60.9 

Section 17. Type section of the Tolstoi Formation (fig. 13)- 

Continued 

16. Andesite porphyry sill ................................................ 3.6 

15. Covered interval ....................................................... 28.9 

14. Sandstone; rubble ..................................................... 30.4 

13. Sandstone, thin-bedded, fine-grained, light-olivegray; thin tuff bed at base ...................................... 13.7 

12. Siltstone, thin-bedded, light-olive-gray; plant debris . 16.7 

1 1. Covered interval ....................................................... 16.7 

10. Sandstone, massive, fine-grained, olive-gray; clay 

ironstone nodules; plant debris; sill at base ........... 15.2 

9. Sandstone, thin-bedded to massive, fine-grained, 

pale-olive; sandstone concretions; current-ripple 

marks; plant debris; andesite dike in middle part; 

bivalves and gastropods in upper part, include 

40 Geologic Studies on the Alaska Peninsula 
Section 17. Type section of the Tolstoi Formation (fig. 13)- 

Continued 

Ostrea, Microcallista (Costacallista) conradiana, 

and Turritella uvasana stewarti (M8906) ............. 9 1.4 

8. Sandstone and siltstone, thin- to medium-bedded, 

pale-olive to light-gray; plant debris; bivalves and 

gastropods, Turritella uvasana stewarti (M8382) ... 48.7 

7. Siltstone with thin sandstone interbeds and two thin 

andesite dikes; ripple marks; bivalves Solena cf. 

S. clarki and Corbicula (M8380 and M8386) ....... 39.6 

6. Sandstone, thin- to medium-bedded, fine-grained, 

light-olive-gray; crossbedded; plant leaves; 

bivalves Nuculana and Microcallista 

(Costacallista) conradiana (M8379 and M8385) . 24.4 

5. Sandstone, thick-bedded, medium-grained, lightgray; large-scale tabular crossbeds ........................ 25.9 

4. Siltstone, thin-bedded, irregular; plant leaves .......... 15.2 

3. Sandstone, medium-bedded, medium-grained, 

olive-gray; carbonaceous; plant leaves; thin dike .. 19.8 

2. Siltstone, thin-bedded; sandstone concretions ........... 8.5 

1. Sandstone, massive, olive-gray; crossbedded .......... - 1 2.2 

Total exposed section ..................................... 658.8 

Base of exposed section 

Section 18. Reference section of the Tolstoi Formation (fig. 13) 

Stepovak Formation 

Unconformable contact Thickness 

Tolstoi Formation: (meters) 

40. Andesite porphyry sill .............................................. 13.7 

39. Sandstone, massive, medium-grained, greenishgray; plant debris ..................................................... 7.6 

38. Siltstone, sandstone, and tuff, thin-bedded; sandstone 

channeled into siltstone; leaves common, Acer, 

Alnus, Cocculus, Liquidambar, Metasequoia 

(11416) .................................................................. 17.4 

37. Andesite sill .............................................................. 18.2 

36. Covered interval ....................................................... 18.2 

35. Sandstone and siltstone, massive to thin-bedded, 

fine- to coarse-grained, medium-light-gray; wood 

and plant debris; sandstone channeled into siltstone 33.5 

34. Andesite sill ................................................................ 6.0 

33. Covered interval ....................................................... 15.2 

32. Siltstone and sandstone, thin- to medium-bedded, 

medium- to greenish-gray; coal and carbonaceous 

shale; abundant Acer and Alnus (1 1415) ............... 45.7 

3 1. Conglomeratic sandstone and conglomerate, massive, 

pale-yellowish-brown; numerous channels containing pebble lag gravel; carbonaceous ................ 92.0 

30. Covered interval ....................................................... 5 1.8 

29. Conglomeratic sandstone, massive, pebble, greenishgray; carbonaceous ................................................ 15.2 

28. Andesite sill .............................................................. 18.2 

27. Siltstone, thin-bedded, dark-gray; plant debris ........ 74.6 

26. Conglomeratic sandstone, massive, pebble, paleyellowish-brown; crossbedded and channeled ...... 48.7 

25. Siltstone, thin-bedded, dark-gray; leaves, Cocculus 

(1 1413) .................................................................. 42.6 

24. Sandstone, massive, fine-grained, pale-yellowishSection 18. Reference section oftheTolstoi Formation (fig. 13)- 

Continued 

brown; large tabular crossbeds; carbonaceous ...... 21.3 

23. Siltstone, thin-bedded, dark-gray ; carbonaceous ....... 9.0 

22. Sandstone, thin-bedded, fine-grained; crossbedded . 17.6 

2 1. Covered interval ....................................................... 65.5 

20. Sandstone, thin- to thick-bedded, fine-grained, lightolive-gray; crossbedded; carbonaceous 53.3 ................. 

19. Siltstone, thin-bedded, medium-gray; carbonaceous .. 19.8 

18. Sandstone, massive, medium-grained, olive-gray; 

lag gravel in channels; coal; carbonaceous; plant 

debris ..................................................................... 83.8 

17. Covered interval ....................................................... 5 1.8 

16. Sandstone, massive, medium-grained, paleyellowish-brown; crossbedded, channeled; plant 

fossils Metasequoia and Thuites (1 1412) .............. 38.1 

15. Covered interval ....................................................... 36.6 

14. Sandstone, massive, medium-grained, lightbrownish-gray; large tabular crossbeds; plant 

debris ..................................................................... 19.8 

13. Shale, fissile to thin-bedded, dark-gray ; 

carbonaceous ......................................................... 33.5 

12. Sandstone, thick-bedded, medium-grained, 

medium-gray; crossbedded; carbonaceous ............ 12.2 

11. Shale, fissile to thin-bedded, dark-gray; 

carbonaceous ......................................................... 7 1.6 

10. Sandstone, massive, fine-grained, medium-gray; 

carbonaceous ........................................................... 7.6 

9. Siltstone, massive, fine-grained, medium-gray; 

carbonaceous ......................................................... 39.6 

8. Sandstone, massive, medium-grained, paleyellowish-brown; crossbedded .............................. 76.1 

7. Covered interval ....................................................... 22.8 

6. Sandstone, massive. medium-grained, brownish-gray; 

crossbedded; plant debris; contains Chaetoptelea, 

Dicotylophyllum, Grewiopsis, Protophyllum, and 

Metasequoia (1 141 1) ........................................... 5 1.8 

5. Covered interval ....................................................... 13.7 

4. Siltstone, thin-bedded, dark-gray; sandstone 

concretions; laminated ........................................... 19.8 

3. Sandstone, thick-bedded, fine-grained, brownishgray; crossbedded .............................................. 19.8 

2. Siltstone, dark-gray; laminated; carbonaceous ......... 41.1 

1. Sandstone and siltstone, brownish-gray; laminated 

and crossbedded ..................................................... 10.6 

Total exposed section .................................. 1,355.4 

Base of exposed section 

Tolstoi Formation. The flora suggests an older and longer age 

range (late Paleocene to middle Eocene) than does the fauna 

(middle Eocene) for Tolstoi deposition. We believe that there 

is a logical explanation for this apparent discrepancy in age. 

The sequence of the Tolstoi Formation at Ivanof Bay is twice 

as thick as at the type section at Pavlof Bay (fig. 13A), and we 

believe the Ivanof Bay section is more complete. The floristic 

elements of late Paleocene age came from the lower part of the 

Ivanof Bay sequence and are, therefore, older than any of the 

Stratigraphic Framework of the Alaska Peninsula 41 
strata exposed at Pavlof Bay. Thus, the overall age of the 

Tolstoi Formation is regarded in this report as late Paleocene 

to middle Eocene. 

The marine strata of the Tolstoi Formation constitute 

only a minor part of the formation as defined and mapped by 

Burk (1965) and by us. The dominant lithologies of the 

formation are sandstone and conglomerate with subordinate 

interbeds of siltstone, shale, carbonaceous shale, and coal. 

The sand-shale ratio in the unit at Pavlof Bay is 1:5; it 

increases to 2:6 near Chignik Bay and increases even more 

northeastward from Chignik. Volcanic debris is a conspicuous, but not a major, component of the unit's sandstones and 

conglomerates. Detritus derived from the Mesozoic sedimentary sequence and the Alaska-Aleutian Range batholith constitutes the major part of the Tolstoi Formation. 

Carbonaceous shale and coal beds in the Tolstoi Formation are more numerous and thicker from Chignik Bay northeastward, as are conglomerate and sandstone units. The sandstone is generally crossbedded and channeled. A thin bed of 

blue pebble conglomerate occurs near the top of the Tolstoi in 

the northeastern part of its exposure area. The pebbles are 

black chert. We have been unable to confirm the composition 

of the bluecoating, but we believe it is probably vivianite. This 

conglomerate underlies a lignite bed. 

Throughout most of the Alaska Peninsula, the lower 

contact of the Tolstoi Formation is a major unconformity and 

the base of the formation rests locally on the Hoodoo, Chignik, 

Staniukovich, or Naknek Formation. A yellowish-orange 

weathered zone commonly occurs at the contact between the 

Tolstoi and the underlying formations and marks a definite 

erosional break. However, between Pavlof Bay and Port 

Moller, the Tolstoi Formation concordantly overlies the 

Hoodoo Formation and the contact is a disconformity. Orogenic activity, possibly associated with subduction along a 

paleo-Aleutian trench, was greatest in the Becharof Lake area 

of the-peninsula. A structural high was centered in the Becharof 

Lake area (Fisher and others, 1981), and the Tolstoi Formation pinches out against the south flank of this structural high. 

The northeasternmost exposures of the Tolstoi form a thin 

sequence south of Lower UgashikLake (fig. 2), where the unit 

lies on theNaknekFormation. The upper contact of the Tolstoi 

is disconformable where it is overlain by the sedimentary 

rocks of the Stepovak Formation. Where the Tolstoi is overlain by the Meshik Volcanics, the contact is an unconformity. 

Copper Lake Formation 

p. 62) briefly mentioned the coal-bearing rocks in the Cape 

Douglas area and concluded also that they are part of the Kenai 

Formation. Keller and Reiser (1959, p. 278-281) described 

Tertiary sedimentary rocks in the Mount Katmai area as 

probably of Eocene age. Recent investigations at the northeast 

end of the Alaska Peninsula, first by Magoon and others 

(1976a, 1976b) and more recently by us, have shown that the 

Tertiary sedimentary sequence there consists of two separate 

rock units of different ages. The rocks are dissimilar in 

composition and are herein mapped as two formations, the 

Copper Lake Formation and Hemlock Conglomerate. 

Continental lower Tertiary strata in the Cape Douglas 

area were mapped as part of the West Foreland Formation by 

Magoon and others (1976a, 1976b). However, the lower 

Tertiary strata in the Cape Douglas area are very different 

from the type West Foreland Formation (Calderwood and 

Fackler, 1972; Adkison, 1975; Adkison and others, 1975) in 

the upper Cook Inlet area. Consequently, we here reassign 

these strata to the Copper Lake Formation (see Detterman and 

Reed, 1980). The stratigraphic sequence exposed in the Cape 

Douglas area, particularly the section along Spotted Glacier 

(fig. 13), consists of thick pebble-cobble conglomerate in the 

uppermost and lowermost parts and sandstone and siltstone in 

the middle part. Considerable carbonaceous debris and minor 

coal are found in the finer clastic sediments. This sequence of 

strata is very similar to the type section of the Copper Lake 

Formation, located in the Iliamna 1 :250,000-scale quadrangle 

south of Upper and Lower Copper Lakes (Detterman and 

Reed, 1980). The strata in the Cape Douglas areaare unlike the 

white claystone and sandstone of the West Foreland Formation (Calderwood and Fackler, 1972). 

The section along Spotted Glacier (section 19; fig. 13) 

was measured by Magoon and others (1976b) and examined 

several times by us during the present investigation. The 

section is along the east side of the glacier in secs. 16 and 21, 

T. 14 S., R. 25 W., AfognakD-5 1:63,36Oquadrangle. Seacliff 

exposures of much of this section are present in secs. 14 and 23. 

Section 19. Section of the Copper Lake Formation (fig. 13) 

Hemlock Conglomerate 

Disconformable contact Thickness 

Copper Lake Formation: (meters) 

24. Conglomerate, massive, units to 15 m thick, pebblecobble; clasts about 30-40 percent volcanic rocks, 

20 percent granitic rocks, 0-20 percent chert and 

auartz. remainder quartzite, schist, and limestone . 134.0 

..... Lower Tertiary rocks are missing for nearly 200 km 23. Siltstone, thin-bedded, dark-gray; carbonaceous 16.7 

.............................. across the Becharof Lake structural high, from southwest of 22. Conglomerate; similar to unit 24 21.3 

21. Sandstone, thick-bedded, medium-gray; conglomerate Lower Ugashik Lake to the Swikshak River area near Cape lag gravel in channels; minor siltstone containing Douglas on the northeast flank of the structural high. Tertiary plant fossils ............................................................ 27.4 

sedimentary rocks along Shelikof Strait at the northeast end of 20. Siltstone and sandstone, thin-bedded, dark-gray ...... 12.2 

the Alaska Peninsula were first described by Atwood (191 1, 19. Sandstone and siltstone: similar to unit 20: thin sill 

p. 49). Atwood considered these rocks to be equivalent to the at top ...................................................................... 21.3 

KenaiFormation and presumably of Eocene age. Smith (1939, 18. Conglomerate, massive, pebble; sandstone matrix .. 30.5 

42 Geologic Studies on the Alaska Peninsula 
Section 19. Section of the Copper Lake Formation (fig. 13)- 

Continued 

17. Conglomeratic sandstone, thick-bedded, mediumto coarse-grained; tabular crossbeds ...................... 2 1.3 

16. Sandstone with few pebbles, thick-bedded; few 

channels; crossbedded ........................................... 57.9 

15. Sandstone and siltstone; mostly rubble .................. 1 12.7 

14. Sandstone, medium- to thick-bedded, mediumgrained, medium-gray; crossbedded ...................... 54.8 

13. Covered inter~al ...................................................... 10.6 

12. Sandstone and siltstone, thin- to medium-bedded, 

fine-grained, dark-gray; carbonaceous .................. 77.7 

11. Sandstone, massive; contains conglomerate 

channel fills ......................................................... 50.3 

10. Sandstone and siltstone; similar to unit 12 ............... 5 1.8 

9. Conglomerate, massive, pebble .................................. 6.0 

8. Sandstone and siltstone; similar to unit 12 ............... 18.2 

7. Covered interval ....................................................... 57.9 

6. Sandstone, medium-bedded, fine-grained .................. 6.0 

5. Conglomerate, thick-bedded to massive, pebblecobble .................................................................... 33.5 

4. Conglomerate, massive; poorly exposed ................ 125.0 

3. Conglomerate, massive, pebble-cobble .................... 25.9 

2. Sandstone, medium-bedded, medium-grained; silty; 

carbonaceous .................................................... 15.2 

1. Conglomerate, massive, pebble; minor sandstone 

lenses .................................................................... 36.5 

Total exposed section .................................. 1,024.7 

Base of exposed section 

The fossil megaflora from seacliff exposures of the 

Copper Lake Formation, especially from Sukoi Bay at Cape 

Douglas, indicates correlation with the Franklinian Stage 

(Ypresian) of the early Eocene (Magoon and others, 1976a; 

J.A. Wolfe, written commun., 1984). Thus, the age of the 

Copper Lake Formation on the Alaska Peninsula is approximately the same as that of the Tolstoi Formation in the 

northeastern part of its outcrop area (Detterman and others, 

1985, 1987a) and at its referelice section (fig. 13A). (At the 

reference section of the Tolstoi, an early Eocene megaflora 

was collected from sandstone and siltstone strata that overlie 

beds containing a late Paleocene flora and underlie beds 

containing a middle Eocene flora.) The age of the Copper 

Lake Formation is poorly constrained at its type section 

(Detterman and Reed, 1980). The Paleocene(?) and early 

Eocene age favored here for the type section of the Copper 

Lake Formation is based on the early Eocene megaflora in the 

sandstone and siltstone member overlying the lower conglomerate member. In outcrops at the northeast end of the 

Alaska Peninsula, the abundant megaflora is also confined to 

siltstone and sandstone beds overlying a thick basal conglomerate. Therefore, we consider the age of the Copper Lake 

Formation in the Cape Douglas area to be the same as at the 

type section: Paleocene(?) and early Eocene. The provenance 

for the sediments, as determined from clast content, was the 

Alaska-Aleutian Range batholith and Mesozoic sedimentary 

and metamorphic rocks exposed in the Iliamna quadrangle 

north of the Cape Douglas area. 

The contacts of the Copper Lake Formation with the 

underlying Kaguyak Formation and the overlying Hemlock 

Conglomerate are disconformable. A considerable amount of 

time elapsed between the deposition of each of the units, but 

apparently there was little orogenic activity as there is no 

structural discontinuity present between the formations. 

Stepovak Formation 

The name "Stepovak Formation" was used by Burk 

(1 965) for rocks exposed along the east side of Chichagof Bay 

(see fig. 14B); we here agree with this usage. These rocks had 

originally been called the Stepovak Series by Palache (1904). 

Burk (1965) included the Stepovak Formation as the upper 

unit of his Beaver Bay Group. We here divide the Stepovak 

Formation into two informal members, a (lower) siltstone 

member and an (upper) sandstone member. We do not map the 

formation herein as part of Burk's Beaver Bay Group. 

The volcaniclastic rocks of the Stepovak Formation are 

geographically restricted to the southwestern endofthe Alaska 

Peninsula from Pavlof Bay to near Chignik (fig. 2). They are 

age equivalents of the sequence of volcanic rocks to the 

northeast mapped as the Meshik Volcanics (fig. 12) and of 

rocks to the south in the Shumagin Islands mapped as the 

informally named Popof volcanic rocks. The Meshik Volcanics 

and the Stepovak Formation intertongue between Stepovak 

Bay and Chignik, and the Popof volcanic rocks and the 

Stepovak Formation intertongue on Popof and Unga Islands. 

Northeast of Chignik, the rocks are almost entirely volcanic 

and are mapped as the Meshik Volcanics. 

Burk (1965) designated a type locality for the Stepovak 

Formation along the east shore of Chichagof Bay. We here 

designate a reference section (section 20) for the Stepovak 

about 2.5 km east of the type locality, along ridge tops where 

the stratigraphic section is more completely exposed (fig. 14). 

The upper and lower contacts of the Stepovak were not 

observed at either locality. The reference section begins on a 

ridge top in the middle of sec. 12, T. 52 S., R. 71 W., Port 

Moller C-1 1:63,360 quadrangle, continues southwesterly 

along the ridge into sec. 13, and thence continues southerly 

across sec. 24 to the coast (fig. 14B). 

The Stepovak Formation is a thick volcaniclastic sedimentary unit. About 2,030 m of the unit are exposed at the 

reference section (fig. 14A) and are about evenly divided 

between its (lower) siltstone and (upper) sandstone members. 

Figure 14. Eocene, Oligocene, and Miocene(?) formations, 

Alaska Peninsula. A, Reference section of Stepovak Formation, 

measured section of Meshik Formation, and partial sections of 

Belkofski Formation and Hemlock Conglomerate. Ages, formation names, and lithic units (numbered to correspond to text) are 

shown for each section. B, Index maps. Circled numbers label 

location of sections on index maps and figure 5. See figure 6 for 

explanation of other symbols. > 

Stratigraphic Framework of the Alaska Peninsula 43 
Section 20 

I I I-. ..\ 

'J.~.V.s".V"."" 

......... - .. - ... 8 ----" 

......... 6 -. a, ? ?+'-, .t-.t- 

.. - ... - . C 4 .a ...... - . C 0 . - (I) - 

.- 

C 3 - 

. .- -. - - - - - - - - - 

1 400 METERS 

Section 21 Section 

Conglomerate 

Conglomeratic sandstone 

Sandstone 

Tuffaceous sandstone 

Silty sandstone or sandy 

siltstone 

Siltstone 

Tuffaceous siltstone 

Shale 

Limestone 

Section 

Concretions; limestone, 

sandstone, and glendonitic 

Tuff 

Volcanic flows 

Volcanic breccia 

Covered interval 

Cenozoic fossil locality 

(listed in appendix) 

Megainvertebrate fossil 

Plant fossil 

Potassium-argon age and 

analytical error, in millions 

of years 

Geologic Studies on the Alaska Peninsula 
Hemlock 

Geology from Detterman and others (1981) 

Meshik Volcanics 

Conglomerate 

0 

w 

5 KILOMETERS 

Belkofski Formation 

Stratigraphic Framework of the Alaska Peninsula 45 
The two members of the Stepovak Formation represent very 

different depositional environments. The siltstone member, 

particularly units I through 10, is a deep-water turbidite 

deposit with dark laminated siltstone and shale beds and 

minor sandstone interbeds that contain numerous rip-up clasts 

and commonly show graded bedding. The sandstone member 

is particularly rich in volcanic debris and was deposited in a 

shallow-water shelfenvironment. Megafossils, mainly bivalves 

and gastropods, are found throughout the sandstone member 

and the upper part of the siltstone member. The lower part of 

the deep-water siltstone member is unfossiliferous. The 

megafossils were deposited in water depths no greater than 30 

to 50 m and are correlated with Eocene or early Oligocene 

forms found elsewhere around the north Pacific Ocean (Louie 

Marincovich, Jr., written commun., 1985). The abrupt change 

from deep to shallow-water deposition may indicate the 

renewal of orogenic activity along the Alaska Peninsula, or it 

may represent an interval during the middle Oligocene worldwide lowering of sea level (Vail and others, 1977). 

The volcaniclastic sedimentary rocks of the Stepovak 

Formation and the volcanic sequence of the coeval Meshik 

Volcanics record the beginning of a major magmatic episode 

on the Alaska Peninsula that produced the Meshik arc of 

Wilson (1985). The sandstone of the Stepovak Formation is 

composed almost completely of detritus from the volcanic 

part of this arc. The grains and clasts are fresh-appearing and 

unaltered. These clasts are in sharp contrast to the chloritized 

volcanic rock fragments that form a minor part of the underlying sedimentary sequence, and they indicate a change in 

provenance from the Alaska-Aleutian Range batholith and 

Mesozoic sedimentary rocks to the contemporaneous volcanic arc. 

The contact of the Stepovak Formation with the underlying Tolstoi Formation is structurally conformable and is 

considered a disconformity. A major break in time between 

these two formations cannot be demonstrated by the enclosed 

megafauna, but the change in provenance and the change from 

the shallow-marine and nonmarine beds of the Tolstoi Formation to the deep-water siltstone of the overlying Stepovak 

Formation indicate a considerable interval of time. The upper 

contact of the Stepovak with the Unga Formation is a disconformity also. The lateral facies change from the sedimentary 

rocks of the Stepovak Formation to the coeval volcanic rocks 

of the Meshik Volcanics is gradual. Few outcrops exposing 

either formation are completely devoid of thin interbeds of the 

other rock unit. The depositional basin for the Stepovak 

Formation was centered in the area between the head of Port 

Moller and the northeast shore of Dorenoi Bay (fig 14B). This 

area contains only minor interbedded volcanic rocks of the 

MeshikVolcanics. It appears that volcanic centers surrounded 

this area because the marine volcaniclastic rocks abruptly 

pinch out in all directions and are replaced by volcanic rocks 

of the Meshik Volcanics or the coeval (informal) Popof 

volcanic rocks. The interbedded volcanic rocks have been 

dated by the K-Ar method numerous times, and their ages 

range from 32 to 36 Ma (Wilson, 1985; DuBois and others, 

1987). The radiogenic ages obtained from volcanic rocks that 

were the source for the contemporaneous volcaniclastic sediments of the upper part of the Stepovak Formation provide a 

minimum age of early Oligocene for the Stepovak Formation. 

The K-Ar ages and the data provided by the megafossils 

indicate that the Stepovak Formation is late Eocene and early 

Oligocene in age. 

Early to Middle Tertiary Volcanic Rocks 

The Alaska Peninsula was the site of widespread volcanic and hypabyssal igneous activity during the Tertiary. 

Recent field and geochronologic investigations have shown 

that the igneous activity can be separated into two main 

episodes. One began during the Eocene and continued until 

the earliest Miocene. The second episode began in late Miocene time and is continuing now; it is represented by the 

present-day Aleutian volcanic arc. The older magmatic arc 

was termedtheMeshik arc by Wilson (1985). The two arcs are 

subparallel to one another and roughly parallel to the northeastward trend of the Alaska Peninsula. The Aleutian arc lies 

along the west side of the Meshik arc in the southern part of the 

peninsula, but at the general latitude of Port Heiden the 

Aleutian arc shifts to the east side of the Meshik arc. The 

change in location of the arcs probably reflects a change in rate 

or direction of subduction along the Aleutian Trench. 

The dacitic to basaltic flows, agglomerate, lahars, and 

coarse volcanic rubble produced by the Meshik arc in the 

central part of the peninsula were named the Meshik Formation by Knappen (1929, p. 198-200). Age-equivalent and 

lithologically equivalent rocks on the inner Shumagin islands 

were informally named the Popof volcanic rocks (Wilson and 

others, 1995) for exposures on Popof Island. The two units are 

described separately. 

Meshik Volcanics 

The name "Meshik Formation" was first used by 

Knappen (1929, p. 198) for volcanic and volcanogenic sedimentary rocks exposed along the Meshik River and near 

Meshik Lake. We here redefine this unit as the Meshik 

Volcanics to reflect its lithologic character. We have not 

measured any sections of the formation, but we do include 

herein a description of the section (section 21) encountered in 

the Gulf Port Heiden Unit No. 1 borehole (Brockway and 

others, 1975). The borehole is located in the Chignik D-3 

1:63,360 quadrangle, sec. 20, T. 37 S., R. 59 W., approximately 3.2 km northwest of the Port Heiden airstrip. Approximately 1,7 16 m of section was drilled in this well beginning 

at -9,280 ft (-2,816 m) and continuing to the contact with 

Jurassic rocks; the section consists almost entirely of volcanic 

flows, tuffs, and breccia. A few thin intervals of sedimentary 

rocks (Stepovak Formation) were noted interfingering with 

the Meshik Volcanics. 

46 Geologic Studies on the Alaska Peninsula 
Section 20. Referencesection ofthe Stepovak Formation (fig. 14) 

Top of exposed section Thickness 

Sandstone member: (meters) 

48. Siltstone, thin-bedded, medium-light-gray; 

tuffaceous ............................................................... 12.2 

47. Sandstone, thin-bedded, fine-grained, light-olivegray; tuffaceous ..................................................... 10.6 

46. Siltstone, thin-bedded, dark-yellowish-brown 19.8 ......... 

45. Sandstone, thin-bedded, fine-grained, light-olivegray; contains bivalves (M8352) of indeterminate 

- genus ........................................................................ 9.1 

44. Siltstone and shale, dark-yellowish-brown .............. 24.3 

43. Sandstone, conglomeratic, thin-bedded; thin limestone 

layer at base; contains bivalve and scaphopod 

fragments ................................................................. 4.6 

42. Shale, dark-yellowish-brown; limestone concretions; 

siltstone at base ...................................................... 44.1 

41. Sandstone, thin-bedded, fine-grained, olive-gray; 

volcanic; well-rounded grains; contains Spisula 

callistaeformis, Unio, and Acila (M835 1) ............... 9.1 

40. Shale and siltstone, dark-yellowish-brown .............. 42.7 

39. Sandstone, thin-bedded, fine-grained, olive-gray; 

minor siltstone ..................................................... 32.0 

38. Shale and siltstone, dark-yellowish-brown 41.1 .............. 

37. Sandstone, thin- to medium-bedded, fine- to 

medium-grained, light-olive-gray to dark-yellowish-brown; volcanogenic; subangular grains; 

several thin tuff layers; few bivalve casts ............ 149.3 

36. Tuff, thin, olive-gray; water-laid ................................ 6.0 

35. Conglomeratic sandstone, thick-bedded, grit to 

pebble; volcanic clasts ........................................... 12.2 

34. Sandstone, medium-bedded, medium-grained, 

olive-gray; contains Spisula (M8353) ................... 12.2 

33. Silty sandstone, very fine grained, grayish-orange; 

lower part tuffaceous; few bivalves ....................... 89.9 

32. Conglomerate, sandy, medium-bedded, grit to 

pebble; volcanogenic; subangular clasts ................ 13.7 

3 1. Siltstone, sandy, thin-bedded; tuffaceous ................. 24.4 

30. Silty sandstone, thin-bedded, olive-gray; 

volcanogenic .......................................................... 45.7 

29. Siltstone, thin-bedded, olive-gray ............................ 45.7 

28. Sandstone, thin- to medium-bedded, olive-gray; 

volcanogenic; contains few bivalves ..................... 18.3 

27. Siltstone and sandstone, rhythmically interbedded, 

moderate-yellowish-brown; contains few bivalves . 48.7 

26. Sandstone, thin-bedded to massive, fine- to mediumgrained, light-olive-gray; grit beds formed of volcanic rocks in middle part of unit; abundant fossils 

throughout, Acila, ?Yoldia palachei, ?Yoldia 

breweri, Spisula callistaeformis, gastropod 

(M8349) .............................................................. 48.7 

25. Siltstone, thin-bedded, dark-yellowish-brown ........... 6.6 

24. Sandstone, thin- to medium-bedded, fine- to 

medium-grained, dark-greenish-gray; volcanogenic; contains fossils similar to unit 26 (M8350) ... 27.4 

23. Siltstone, thin-bedded, olive-gray; few limestone 

concretions ........................................................... 42.7 

22. Siltstone and shale, dark-greenish-gray; pencil 

fracture ................................................................... 5 1.8 

Section 20. Reference section of the Stepovak Formation (fig. 

14)-Continued 

21. Sandstone, thin-bedded, fine-grained, paleyellowish-brown; clean ............................................ 6.0 

20. Siltstone, thin-bedded; tuffaceous ............................ 21.3 

19. Sandstone, thin-bedded, medium-grained, paleyellowish-brown; volcanogenic; subangular to 

angular grains ....................................................... 44.2 

1 8. Conglomeratic sandstone; volcanogenic .................. 10.6 

Total exposed section, sandstone member .. 1,004.1 

Contact 

Siltstone member: 

17. Siltstone and shale, thin-bedded, dark-gray ............. 85.3 

16. Sandstone and siltstone, thin-bedded, dark-gray; 

small-scale crossbeds; few bivalves ...................... 24.3 

15. Shale, dark-gray ....................................................... 30.5 

14. Sandstone, thin-bedded, fine-grained; few pumice 

fragments ................................................................. 6.1 

13. Siltstone, thin-bedded, dark-gray; concretion with 

glendonites; fossils in upper part, ?Spisula 

callistaeformis, ?Ostrea tigiliana, Crepidula 

(M8354 and M8355) ............................................ 198.1 

12. Shale, dark-gray; concretions resistant to 

weathering; concretions containing glendonites; 

contains few fossils (M8356) .............................. 134.1 

11. Sandstone, silty, thin-bedded, fine-grained, darkgray; rhythmically bedded; rip-up clasts ................. 6.1 

10. Sandstone, silty, thin-bedded, fine-grained, darkgray; rhythmically bedded; rip-up clasts ............... 70.1 

9. Sandstone, massive, medium- to fine-grained, 

graded, olive-gray; rip-up clasts; crossbedded ...... 5 1.8 

8. Siltstone, sandy, thin-bedded, dark-gray .................. 48.7 

7. Conglomeratic sandstone, medium-bedded; grit to 

small pebble; clasts mainly chert ............................. 3.0 

6. Siltstone, thin-bedded, dark-gray ............................. 36.5 

5. Sandstone, conglomerate, and minor siltstone ......... 15.2 

4. Sandstone, silty, thin-bedded to massive, oliveblack; "floating" quartz grains; few limestone 

concretions ........................................................... 45.7 

3. Siltstone, thin-bedded, grayish-black ....................... 88.4 

2. Shale, grayish-black ................................................. 65.5 

1. Siltstone, thin-bedded, grayish-black ..................... 1 14.3 

Total exposed section, siltstone member .... 1,023.7 

Total exposed section, Stepovak Formation . 2,027.8 

Base of exposed section 

The measured section from the Gulf Port Heiden Unit No. 

1 well contains considerably more tuff than do exposures of the 

MeshikVolcanics in the MeshikRiver valley and northward into 

the Ugashik 1 :250,000 quadrangle. These exposures consist 

mainly of basalt and andesite flows, coarse volcanic rubble, 

breccia, and lahars. Apparently rocks in the exposures were 

deposited closer to vent areas than were rocks in the well section. 

Numerous samples from the Meshik Volcanics have 

been dated by the K-Ar method (Wilson, 1978, 1980, 1982, 

Stratigraphic Framework of the Alaska Peninsula 47 
Section 21. Measured section of the Meshik Volcanics from the 

Gulf Port Heiden No. 1 drillhole well log by Borst and Giddens 

Oilwell Logging Service (fig. 14) 

Bear Lake Formation 

Unconformable contact Thickness 

Meshik Volcanics: (meters) 

34. Tuff, aphanitic, grayish-green; pyritic ........................ 9.1 

33. Siltstone and shale, thin-bedded, dark-brown; 

tuffaceous ............................................................... 30.5 

32. Andesite flow ........................................................... 15.2 

31. Tuff, white to light-gray; minor siltstone ................. 30.5 

30. Siltstone and shale; tuffaceous ................................. 15.2 

29. Tuff, greenish-gray; minor volcanic sandstone ........ 24.4 

28. Tuff, medium-gray to chalky-white; welded ............ 61.0 

27. Andesite flow ........................................................... 10.6 

26. Tuff, porphyritic, rhyolitic; interbedded brown 

volcanic flow .................................................... 1 14.3 

25. Tuff, porphyritic, rhyolitic; locally lithic ................. 57.9 

24. Dacite and rhyolite (and probably andesite), graygreen to dark-gray and black; thick flow units; 

porphyritic; locally glassy ................................... 381.0 

23. Volcanic breccia, medium-dark-green; tuffaceous .. 24.3 

22. Andesite, dark-green; chloritic; K-Ar age 42k4 Ma 

(Brockway and others, 1975) ................................. 12.2 

21. Tuff; pyroclastic flow and volcanic breccia ............. 71.6 

20. Silicic flow, dark-gray-green to brown; glassy; 

porphyritic ........................................................... 181.3 

19. Tuff, white to light-gray; welded ............................. 13.7 

18. Flow, medium-dark-green, porphyritic .................... 53.3 

17. Tuff and breccia, white to light-gray ........................ 22.8 

16. Flow, medium-dark-green; glassy ............................ 13.7 

15. Tuff, white; welded; pyritic ...................................... 45.7 

14. Flow, aphanitic, gray-green ...................................... 12.2 

13. Tuff, white; welded; pyritic ...................................... 41.1 

12. Flow, light-green ...................................................... 13.7 

11. Tuff, white to light-gray; welded ............................. 35.0 

10. Flow, dark-gray to black; glassy ................................ 6.0 

9. Tuff, white to light-gray; welded ............................. 15.2 

8. Flow, felsic, light-gray to green; fine-grained .......... 41.1 

7. Tuff; same as unit 9 .................................................. 15.2 

6. Siltstone and sandstone, light-gray ; tuffaceous ........ 2 1.3 

5. Flow; same as unit 8 ................................................. 19.8 

4. Volcanic breccia, dark-green .................................... 23.1 

3. Flow, dark-gray to brown; porphyritic; K-Ar age 

36f 8 Ma (Brockway and others, 1975) ............... 143.2 

2. Tuff; same as unit 9 .................................................. 30.4 

1. Flow; same as unit 3 ............................................... 11 1.2 

Total section ................................................ 1,716.8 

Unconformable contact 

Jurassic intrusive rocks 

reported by Wolfe (J.A. Wolfe, written commun., 1980) as 

late Eocene to early Oligocene; this age range correlates 

closely with the K-Ar ages. Therefore, the overall age of the 

Meshik Volcanics is here considered to be late Eocene and 

early Oligocene. 

The contact between the Meshik Volcanics and the 

underlying Tolstoi Formation is considered an unconformity. 

The upper contact of the Meshik is unconformable with either 

the overlying Bear Lake or Milky River Formation. 

Popof volcanic rocks 

The informal term "Popof volcanic rocks" is applied by 

Wilson and others (1995) to the volcanic rocks that form most 

of Popof, Unga, Korovin, and Andronica Islands of the inner 

Shumagin Islands (see figs. 5 and 15B). These rocks are best 

exposed on Popof Island. They are lithologically and temporally equivalent to the Meshik Volcanics and gradationally 

interfinger with the contemporaneous Stepovak Formation. 

The rocks are spatially separated from the Meshik Volcanics 

and had a local source on the islands. 

The total thickness of the Popof volcanic rocks is 

unknown, but it may be at least 1,000 m on the basis of seacliff 

exposures. Volcanic flows, lahar deposits, debris flows, 

ashflow tuff, and tuff constitute the unit. The flows range from 

dacite to basalt in composition and include minor rhyolite on 

Unga Island. Andesite makes up most of the flow rocks and is 

interbedded with numerous volcanic rubble flows and lahar 

deposits. Many of these rocks are pervasively altered and 

mineralized. Propylitized andesite and dacite at the Apollo 

Mine on Unga Island contain gold, silver, galena, sphalerite, 

chalcopyrite, pyrite, and native copper (Berg and Cobb, 1967, 

p. 5). The mine produced about 2 million dollars in gold and 

silver between 189 1 and 1904. The mine has been closed since 

World War I, but some exploration and assessment work is 

currently being done. 

The Popof volcanic rocks locally interfinger with the 

Stepovak Formation along the north side of Unga Island and 

also on Korovin Island. Marine megafossils are very abundant 

locally in some of the sedimentary interbeds and include the 

bivalve Ostrea tigiliana and the gastropods Whitneyella cf. W. 

sinuata aragoensis and Neverita cf. N. washingtonensis. This 

megafauna, along with other species, indicates an Eocene and 

early Oligocene age according to Marincovich (Louie 

Marincovich, Jr., written commun., 1985). Potassium-argon 

ages on samples from several flows and an ash flow tuff range 

from 37.13k1.16 to 3 1.22f 1.38 Ma(DuBois and others, 1987; 

F.H. Wilson andNoraShew, unpub. data, 1991). Thus, the age 

of the Popof volcanic rocks corresponds to that of the ~eshik 

1985; Wilson and others, 1981; Wilson and Shew, 1992; Volcanics. 

DuBois and others, 1987; F.H. Wilson andNora Shew, unpub. 

data, 1991). The most reliable ages of the volcanic rock ~~lk~f~ki Formation 

samples range from 24.9k0.49 to 43.1k 1.3 Ma, and most of 

the ages range from about 28 to 38 Ma. An excellent collection Kennedy and Waldron (1955) first used the name 

of megaflora fossils from a tuff bed exposed on the south flank "Belkofski Tuff' for volcaniclastic rocks found principally 

of AniakchakCrater (fig. 2; Detterman andothers, 1981c) was around Belkofski Bay and on Belkofski Point (fig. 14B). 

48 Geologic Studies on the Alaska Peninsula 
McLean (1979b) renamed the unit the Belkofski "Formation," following Burk's (1965) usage, to better describe the 

varied lithologies occurring within the unit. The formation is 

present only near the southwest end of the Alaska Peninsula; 

it crops out between False Pass and Pavlof Bay and on the 

Pavlof Islands at the mouth of Pavlof Bay. The unit's type 

locality was considered by Burk (1965) to be between Cold 

and Pavlof Bays, and McLean (1979a) gave a generalized 

description of a section located on the northwest shore of 

Belkofski Bay, about 5 km north of Indian Head; this section 

(section 22) is here considered the unit's type section. 

Section 22. Generalized section of the Belkofski Formation 

(fig. 14) 

[Thicknesses are approximate] 

Volcanic rocks 

Unconformable contact Thickness 

Belkofski Formation: (meters) 

16. Volcanic breccia, massive, gray; clasts mainly 

pale-green volcanic rocks ...................................... 25 

15. Sandstone and siltstone, medium-bedded, green to 

red; tuffaceous ....................................................... 45 

14. Tuff, thin-bedded, pink ............................................ 10 

13. Volcanic breccia, massive, red; clasts of volcanic 

rocks mostly about 1 cm, angular .......................... 20 

12. Siltstone, thin-bedded, pink to red; tuffaceous ......... 30 

11. Volcanic breccia, similar to unit 13 ......................... 30 

10. Sandstone, medium-bedded, red; tuffaceous ............ 40 

9. Conglomerate, pebble-cobble, thick-bedded, volcanic 25 

8. Tuff, similar to unit 14 ............................................. 10 

7. Sandstone, thin-bedded, pink to purple; tuffaceous . 15 

6. Siltstone, similar to unit 12 ...................................... 20 

5. Tuff, lithic, pink ....................................................... 10 

4. Volcanic breccia, massive, pink to red; clasts to 

2 cm ....................................................................... 30 

Valley fill of unknown thickness consisting of 

younger volcanic rocks between units 3 and 4 

3. Volcanic breccia, massive, red ................................. 40 

2. Sandstone and siltstone, thin-bedded, dark-gray ...... 30 

1. Conglomerate, thick-bedded, pebble ........................ - 30 

Total exposed section ..................................... 410 

Base of exposed section 

The Belkofski Formation is a tuffaceous volcaniclastic 

sandstone, siltstone, and conglomerate with interbeds of tuff 

and volcanic breccia; it totals about 1,830 m in thickness 

(McLean, 1979b). The lower part of the unit is marine, and it 

grades upward into a nonmarine sequence. The rocks along 

Belkofski Bay are mainly gray and greenish-gray to graybrown. On the Pavlof Islands the rocks are mainly red, pink, 

and purple. This difference in color may be due to thermal 

alteration of the rocks on the Pavlof Islands, which are near 

exposures of intrusive bodies. A green arkose mapped separately by Kennedy and Waldron (1955) was included in the 

Belkofski by Burk (1965). This green arkosic sandstone, 

considered to be the basal part of the Belkofski Formation by 

Burk (1965), McLean andothers (1 978), and McLean (1 979b), 

probably should be considered a separate stratigraphic unit, as 

it was by Kennedy and Waldron (1955). However, we do not 

have enough data to justify such a reassignment at this time. 

The green arkosic sandstone is tuffaceous and has 

blotchy green and white spots characteristic of a laumontiterich rock. The sandstone is not massive as described by 

McLean and others (1978), but is instead composed of numerous low-angle tabular crossbed sets with pebbles between the 

bed sets. Some of the bed sets are bioturbated, and mudstone 

interbeds contain numerous plant remains and leaves. The 

dark-brown to black coaly mudstone associated with the 

sandstone indicates that the depositional site was a beach 

fronting a swamp. These lithologic features are in sharp 

contrast to those of the varicolored lithic tuff and volcanic 

breccia characteristic of the main part of the Belkofski Formation. 

We investigated theBelkofski Formation only cursorily 

at its type locality and on the Pavlof Islands. The relations 

between the Belkofski Formation and the other stratigraphic 

units exposed near the southwest end of the Alaska Peninsula 

are not clear. Most likely, the Belkofski Formation can be 

correlated with the Unga Formation; this idea is based on 

several lines of evidence: (1) The one megafossil collection 

obtained from the type locality of the Belkofski includes 

Anadara sp., Macoma sp., and Mya sp. of probable Miocene 

age (Marincovich and McCoy, 1984); (2) plant fossils consisting of broad deciduous leaves, logs, and stumps indicate an 

Oligocene age (J.A. Wolfe, oral commun., 1978); and (3) 

volcaniclastic sandstone, siltstone, and conglomerate are 

interbedded with tuff and volcanic breccia, all of which are 

dark greenish-brown to red. All of these features are matched 

in the Unga Formation. Additionally, a thick sequence of the 

UngaFormation has been mapped on Ukolnoi Island about 10 

km northeast of exposures of the Belkofski Formation on 

Dolgoi Island. We believe that these two formations may be, 

in part, correlative; however, correlation ofpart of the Belkofski 

(specifically the green arkose) with the StepovakFormation is 

also possible. 

The lower contact of the Belkofski Formation has not 

yet been found. The formation is overlain unconformably by 

volcanic flows and breccias as well as fluvial sedimentary 

rocks. Samples from some of the flows have been dated by KAr analysis as 1 1.82k0.50 and 1 1.79k0.40 Ma (late Miocene) 

(F.H. Wilson and Nora Shew, unpub. data, 1991). Because the 

Belkofski Formation is overlain by late Miocene lava flows 

and contains plant fossils of Oligocene age and a probable 

Miocene megafauna, we regard its age as late Oligocene(?) 

and early and middle Miocene(?). 

Hemlock Conglomerate 

Sedimentary rocks approximately equivalent in age to 

the Belkofski Formation are present in the Mount Katmai area 

Stratigraphic Framework of the Alaska Peninsula 49 
at the northeast end of the Alaska Peninsula. Magoon and 

others (1 976a, b) applied the name "Hemlock Conglomerate" 

to these rocks and correlated them with the type Hemlock 

Conglomerate noted at Capps Glacier in the Cook Inlet area by 

Calderwood and Fackler (1972) and Adkison (1975). The 

rocks at both localities are very similar in composition, but the 

rocks in the Mount Katmai area are considerably thicker than 

those measured at the type section in the Cook Inlet area. 

The Hemlock Conglomerate in the Mount Katmai area 

consists mainly of fluvial sandstone and conglomerate; there 

are also minor interbeds of siltstone, shale, and coal. Minor 

tuff units are present in some exposures. The provenance for 

these sediments was different from that of the Belkofski 

Formation at the southwest end of the peninsula. In the Mount 

Katmai area, chert, quartz, and granitic and metasedimentary 

rocks form a major part of the conglomeratic clasts. Volcanic 

rocks compose a small percentage of these clasts, whereas 

volcanic rocks are the major constituents of the Belkofski 

Formation at the southwest end of the peninsula. 

Tertiary fluvial sedimentary rocks that are here considered to be part of the Hemlock Conglomerate are exposed in 

the mountains bordering Shelikof Strait from Katmai Bay 

northeastward to Cape Douglas. In most exposures the Hemlock unconformably overlies the Kaguyak Formation (Upper 

Cretaceous). In a few localities, such as near Hallo Bay, the 

Hemlock lies unconformably on the Naknek Formation (Upper Jurassic). In the Cape Douglas area, the Hemlock Conglomerate disconformably overlies the Copper Lake Formation of early Tertiary age. 

A reference section (section 23) of the Hemlock Conglomerate was measured on the northwest shore of Kukak 

Bay, in sec. 16, T. 21 S., R. 29 W., Mount KatmaiB-1 1:63,360 

quadrangle, beginning at VABM Yugnat and continuing 

northeastward to Cape Nukshak (fig. 14). 

Section 23. Reference section of the Hemlock Conglomerate 

(fig. 14) 

Tertiary volcanic rocks 

Unconformable contact Thickness 

Hemlock Conglomerate: (meters) 

28. Sandstone and conglomerate, thick-bedded, fineto medium-grained, dark-yellowish-orange; 

tuffaceous; conglomerate as lag gravel in channels; clasts composed of chert, quartz, volcanic 

rocks; maximum clast size 20 cm; wood fragments 18.3 

27. Covered interval ......................................................... 6.0 

26. Lithic tuff, thick-bedded, pale-yellowish-brown ........ 9.1 

25. Conglomeratic sandstone, massive, coarse-grained, 

pale-yellowish-brown; channeled .......................... 15.2 

24. Sandy siltstone, thin-bedded, pale-yellowish-brown; 

coaly seams; abundant deciduous leaves ............... 12.2 

23. Covered interval ..................................................... 100.0 

22. Sandstone, thin- to thick-bedded, medium-grained, 

light-olive-gray; tuffaceous; deciduous leaves 12.2 ...... 21. Covered interval ....................................................... 22.8 

Section 23. Reference section of the Hemlock Conglomerate 

(fig. 1 4)-Continued 

20. Conglomeratic sandstone, thick-bedded to massive, 

fine- to coarse-grained, medium-dark-gray; conglomerate in channels; maximum clast size 60 cm; 

clasts composed of volcanic rocks, chert, quartz, 

granitic rocks ....................................................... 56.3 

19. Conglomerate, massive, light-olive-gray; mainly 

volcanic rocks .......................................................... 9.1 

18. Siltstone, thin-bedded, olive-gray; tree stump in 

growth position ...................................................... 12.2 

17. Sandstone, thin-bedded to massive, brownish-gray; 

several coal seams .............................................. 18.2 

16. Siltstone, thin-bedded, medium-gray; fossil leaves . 12.2 

15. Conglomerate and sandstone, massive, olive-gray; 

channeled; clasts to 8 cm, mainly volcanic rocks .. 24.4 

14. Conglomeratic sandstone, massive, medium-grained, 

light-olive-gray ...................................................... 24.4 

13. Tuff, massive, dark-greenish-gray; laminated; fossil 

leaves ....................................................................... 6.0 

12. Conglomeratic sandstone, massive, coarse-grained, 

light-olive-gray; clasts mainly pebbles .................. 24.4 

1 1. Conglomerate, massive; clasts composed of chert, 

quartz, volcanic rocks; maximum clast size 15 cm; 

surface weathers orange ........................................... 9.1 

10. Sandstone, massive, fine-grained, silty, mediumdark-gray; crossbedded; carbonaceous .................. 18.2 

9. Conglomeratic sandstone, massive, medium-gray; 

carbonaceous; volcanic rock clasts ........................ 15.2 

8. Sandstone, massive, fine-grained, medium-gray; 

carbonaceous ........................................................... 6.1 

7. Conglomerate, massive, greenish-gray; volcanic 

rock clasts .............................................................. 10.6 

6. Sandstone, thin- to thick-bedded, fine-grained ........ 10.6 

5. Siltstone, thin-bedded, olive-gray; laminated; 

sandstone concretions; coal seams; fossil leaves ... 42.6 

4. Sandstone, massive, medium-grained, olive-gray; 

carbonaceous; numerous stumps; tuffaceous .......... 9.1 

3. Sandstone, thin-bedded; carbonaceous .................... 10.6 

2. Lithic tuff; volcanic rocks and pumice fragments .... 15.8 

1. Siltstone and sandstone, thin-bedded, olive-gray; 

laminated; tuffaceous ............................................. 27.4 

Total section ................................................... 558.3 

Intrusive contact 

Strata mapped as the Hemlock Conglomerate in the 

Mount Katmai area contain considerably more tuffaceous 

material than in the area of the type section (Calderwood and 

Fackler, 1972). The greater abundance of tuffaceous material 

is probably due to the greater proximity of the Mount Katmai 

area to centers of active volcanism on the Alaska Peninsula. 

Some volcanoes must have been fairly close, as pumice 

fragments are present in a number of Hemlock beds in the 

Mount Katmai area. 

Megaflora fossils found in most outcrops of the Hemlock are mainly broadleaf deciduous plants; evergreen needles 

are also present. These fossils suggest a late Oligocene age 

50 Geologic Studies on the Alaska Peninsula 
(J.A. Wolfe, written commun., 1988) though an earliest Miocene(?) age can not be ruled out (J.A. Wolfe, written commun., 

1988). On the basis of Wolfe's interpretations, we consider 

the age of the Hemlock Conglomerate as late Oligocene. 

Unga Formation 

Volcaniclastic and volcanic rocks that are well exposed 

west of Zachary Bay on the north end of UngaIsland were first 

investigated by Dall (1882, 1896) who applied the name 

"Unga Conglomerate" to the coarse clastic beds at White 

Bluff (see fig. 15B). Atwood (191 1) renamed the unit the 

Unga "Formation." Burk (1965) reassigned these strata to the 

Unga Conglomerate Member, the basal member of the Bear 

LakeFormation; Allison and Addicott (1973) followed Burk's 

(1965) usage. We here return this unit to formational rank as 

the Unga "Formation" because it is a mappable unit lithologically distinct from the Bear Lake Formation. 

The Unga Formation is geographically limited mainly 

to UngaIsland and neighboring islands; it is also present along 

the southeast coast of the Alaska Peninsula facing Unga 

Island. The volcaniclastic rocks and interbedded volcanic 

rocks indicate a local volcanic provenance for the Unga. The 

BearLake Formation is found mainly on the northwest side of 

the Alaska Peninsula. Sedimentary rocks in the Bear Lake 

contain chert, quartz, and other nonvolcanic debris. Volcanic 

detritus constitutes only a minor part of the Bear Lake Formation, whereas volcanic clasts are the major part of the Unga 

Formation. The provenance for the Bear Lake Formation was 

different from that of the UngaFormation; the Bear Lake was 

derived mainly from the older Mesozoic formations and the 

Alaska-Aleutian Range batholith. Additionally, the two units 

are of different ages. 

Atwood (191 1, p. 66) established the type section of the 

Unga along the west shore of Zachary Bay, Unga Island. As 

thus defined, the type section is 200 ft (61 m) thick. Burk 

(1965) later included all of the rocks exposed along the west 

shore of Zachary Bay as part of his Unga Conglomerate 

Member. As part of the present investigation, we herein 

provide a more detailed description of the type section (section 24) than was publishedpreviously. Additionally, we have 

herein added about 30 m to the section measured by Burk 

(1965, p. 212). The breakdown of stratigraphic units described below and shown on figure 15 is not the same as that 

used by either Atwood (191 1, p. 66) or Burk (1965, p. 212). 

The type section as defined by Atwood (191 I, p. 66) consists 

of our stratigraphic units 14 through 18, which are equivalent 

to unit 5 of Burk (1965, p. 212). The section of the Unga 

Formation described here is the same as that described by 

Burk (1965), except that we define more units and have added 

about 30 m to the section's total thickness. Our measured 

section runs along the west shore of Zachary Bay from White 

Bluff northwestward along the coast to Unga Spit, Port Moller 

B-2 and B-3 1:63,360 quadrangles. 

The volcaniclastic rocks composing the Unga Formation were derived locally from the (informal) Popof volcanic 

rocks that form the inner Shumagin Islands and adjacent 

islands, as well as from volcanic rocks dated as Miocene (F.H. 

Wilson and Nora Shew, unpub. data, 1991) that crop out on 

Unga and Wosnesenski Islands. The relatively small size of 

this source terrane may in large part explain the restricted areal 

extent of the Unga (Wilson and others, 1995), as well as the 

lack of major facies changes between its various exposures. 

The type section, being close to the source, contains more 

coarse volcanic debris than other exposures and is mainly a 

subaerial deposit. The more distal parts of the formation, such 

as the exposures forming Ukolnoi Island and Cape Aliaksin, 

are generally finer grained clastic rocks, some of which 

represent a shallow-marine environment. These thin marine 

sequences commonly contain a fairly abundant molluscan 

fauna that lived in the inner part of the inner shelf at approximately 25-35 m depths (Marincovich, 1988a). 

Da11(1896), Dall and Harris (1892), and Atwood (I 91 1) 

reported fossils, primarily Crepidula sp., from the type section. Both Burk (1965) and the present investigators failed to 

locate the so-called "Crepidula bed." We found only agatized 

burrows, probably Crustacean burrows, near the top of the 

extended type section (unit 23). The burrows are in a highly 

bioturbated sandstone (unit 23) that is exposed for about 1 km 

along the north shore of Unga Island, east of Unga Spit. The 

bioturbated sequence contains both agatized and sand-filled 

burrows 2 to 3 cm across and up to 45 cm long. The burrows 

are terminated at the top by a layer of grit and pebbles. Some 

of the agatized burrows are hollow, but most are solid. 

Unfortunately, no fossils were found associated with the 

burrows. Crepidula ungana is common at West Head on Unga 

Island, but at that locality the enclosing beds are part of the 

Stepovak Formation, not the Unga Formation. Fossils are 

locally very abundant elsewhere in the Unga Formation where 

thin marine sequences intertongue with thick nonmarine sequences. These thin marine layers contain numerous specimens of a few genera. Good examples of such layers are 

present on Ukolnoi Island and at Cape Aliaksin. The fauna at 

these localities include the bivalves Mya (Mya) truncata, 

Acila sp., Yoldia sp., Macoma sp., Clinocardium sp., and 

Lucinoma acutilineata, and the gastropods Beringius 

crebricostatus, Natica (Cryptonatica) clausa, and Turritella 

(Hataiella) sagai. Turritellasagai indicates an earliest middle 

Miocene age (15-16 Ma) for the Unga Formation on Ukolnoi 

Island according to Marincovich and Kase (1986) and 

Marincovich (1988a). The gastropod Turritella sagai was 

recently described by Marincovich and Kase (1986) as being 

from the oldest part of the Bear Lake Formation. However, we 

now consider the volcaniclastic beds from which Turritella 

sagai was obtained to be part of the Unga Formation rather 

than the Bear Lake Formation. The tuffaceous sandstone 

containing the Turritella sagai grades laterally into a thick tuff 

bed associated with lahars. 

The remains of numerous petrified logs and smaller 

pieces of wood found in a debris flow (fig. 15, unit 14) 

Stratigraphic Framework of the Alaska Peninsula 51 
Upper Oligocene to 1 01 middle Miocene 

Upper Miocene 

l ;Enaa Formation cn 
Unga Formation 

(type and measured sections) 

Geology modified from Detterman and others (1981) 

and Wilson and others (1995) 

0 - 5 KILOMETERS 

Geology modified from Detterman and others (1981) 

and Wilson and others (1995) 

Bear Lake Formation 

163' 00' 

Geology from Wilson and others (1992) 

Tachilni Formation 

Bear Lake Formation 
Section 24. Type section of the Unga Formation (fig. 15) 

Top of exposed section Thickness 

Unga Formation: (meters) 

23. Sandstone and grit, medium-bedded, fine- to 

coarse-grained, light-olive-brown; grit and small 

pebbles of volcanic rocks; limy concretionary 

lenses in sandstone; burrows filled with agate ...... 12 

22. Lahar or volcanic rubble flow, massive; angular 

clasts of wide size range in volcanic mud matrix .... 9 

21. Sandstone and grit to small-pebble conglomerate, 

medium- to thick-bedded, light-olive-brown ........... 9 

20. Lahar, similar to unit 22 ............................................. 6 

19. Sandstone, thick-bedded, medium- to coarse-grained, 

light-brownish-gray; interbedded yellowish-gray 

mudstone; wood fragments common ....................... 9 

18. Lahar, similar to unit 22; clasts to 1.5 m .................... 8 

17. Sandstone, massive; volcanic and tuffaceous; few 

clasts; possibly a debris flow ................................. 12 

16. Lahar, massive, brownish-gray; clast size to 2 m; 

angular ................................................................... 12 

15. Conglomerate, massive, grit to cobble; all clasts are 

volcanic rocks ......................................................... 6 

14. Debris flow, massive; contains logs and carbonized 

wood fragments ................................................. 18 

13. Tuff, massive, gray; partly welded ............................. 6 

12. Conglomerate, thick-bedded; volcanic pebbles 12 ....... 

11. Lahar, brown; angular clasts 1 cm to 1 m in size ....... 6 

10. Siltstone, mudstone, carbonaceous shale, and tuff, 

medium-brown ........................................................ 9 

9. Conglomerate, massive, cobble-boulder; all clasts 

are volcanic rocks .................................................... 5 

8. Sandstone and siltstone, thin- to medium-bedded, 

light-olive-gray; low-angle crossbedding; few 

pebbles; carbonaceous ........................................... 18 

7. Carbonaceous shale and coal ...................................... 6 

6. Sandstone, siltstone, and carbonaceous shale; coaly 

debris in sandstone; low-angle crossbedding; 

poorly consolidated ............................................... 30 

5. Carbonaceous shale and coal; several coal seams 

to 0.5 m thick; shale weathers yellowish orange ... 25 

4. Sandstone, siltstone, and coal, thin-bedded, mediumgray; poorly consolidated ...................................... 30 

3. Siltstone and shale, thin-bedded ............................... 18 

2. Carbonaceous shale and coal ...................................... 6 

1. Tuff, massive, gray ................................................. 3 - 

Total exposed section ..................................... 275 

Base of exposed section 

probably formed during the same event that produced the 

petrified forest on the northwest side of Unga Island (Eakins, 

1970). Hundreds of petrified logs now lie along the beach and 

in the surf zone along the northwest shore of Unga Island, 

between Unga Spit and Bay Point (which lies about 13 km 

southwest of Unga Spit). Stumps larger than 2 m in diameter 

and still in their growth positions occur in the cliff face of the 

debris flow. The trees apparently are all Metasequoia (Eakins, 

1970). The Unga Island locality contains the largest concentration of petrified wood, but logs and stumps occur in other 

outcrops of the formation as well; the wood was petrified 

either in a widespread event or in several related events. The 

presence of Metasequoia does not help in determining the age 

of the rocks, as the tree lived in southern Alaska from the Late 

Cretaceous through most of the Tertiary. However, a sample 

obtained for pollen analysis from the petrified forest horizon 

on the northwest side of Unga Island contained a pollen flora 

of about 30 taxa. This rich pollen assemblage was described 

as middle Miocene in age by T.A. Ager (written commun., 

1986). Thus, the horizons containing abundant petrified wood 

in the Unga Formation probably can all be considered middle 

Miocene in age. This is approximately the same age as 

indicated by the gastropod Turritella sagai (Marincovich and 

Kase, 1986) from Ukolnoi Island. Petrified logs and stumps 

are common in the formation on Ukolnoi Island, as well as on 

nearby Wosnesenski Island. 

The age of the lowermost part of the Unga Formation is 

not well constrained. The Crepidula sp. bed reported by early 

workers (Dall, 1896; Dall and Harris, 1892; and Atwood, 

191 1) as being near the base of the formation has not been 

located by recent workers (Burk, 1965; and this report). 

Crepidula sp. is present in nearby beds of the Stepovak 

Formation but not in the Unga Formation. A sparse pollen 

flora from the lower part of the type section (see Atwood, 

191 1, p. 66) indicates a late Oligocene age for that part of the 

formation (Virgil Wiggins, written commun., 1986). Thus, 

the overall age of the Unga is here considered late Oligocene 

to earliest middle Miocene, on the basis of megafauna and 

pollen. This age range is supported by K-Ar data obtained 

from associated volcanic rocks. One sample (83AWs 125) 

from an intrusive plug on Ukolnoi Island, which is believed to 

be the source of the tuffaceous sediments containing the 

molluscan fauna described by Marincovich (1988a) and 

Marincovich and Kase (1986), was dated by the K-Ar method 

at 15.69f 1.6 Ma (DuBois and others, 1987); this information 

directly confirms the upper age limit for the Unga Formation. 

The lower age limit for the unit (late Oligocene) is less 

precisely substantiated. Several K-Ar ages (DuBois and others, 1987) from the postulated source beds on Unga Island 

range in age from 3 1.78kO.61 to 37.13f 1.16 Ma. Thus, the 

lower part of the Unga can be no older than early Oligocene. 

The late Oligocene pollen obtained from near the base of the 

type section along the west side of Zachary Bay is considered 

indicative of a more definitive age assignment. 

The contact relations of the Unga with underlying and 

overlying rock units are poorly defined. The lower contact is 

considered an unconformity. A seacliff exposure 11 km east 

of Cape Aliaksin contains a mixed molluscan fauna characteristic of both the Unga and Stepovak Formations. This mixed 

fauna indicates that Stepovak materials were eroded and 

reworked into the Unga sediments (Marincovich, 1988a). 

Also, a reef exposed at low tide offshore from White Bluff 

(fig. 15B) consists of sandstone that is highly discordant to the 

strike and dip of beds in White Bluff. The age of the reef beds 

is unknown, but they are lithologically similar to the Stepovak 

54 Geologic Studies on the Alaska Peninsula 
Formation, which is exposed on the east side of Zachary Bay. 

The Unga is nearly everywhere overlain by volcanic rocks that 

range in age from 10.4M.49 Ma at Cape Aliaksin to 8.43k0.45 

Ma on Wosnesenski Island (DuBois and others, 1987). 

Bear Lake Formation 

Burk (1965, p. 89) gave the name "Bear Lake Formation" to a thick sequence of sandstone and conglomerate 

exposed along the east shore of Port MolIer and in the 

mountains around Bear Lake (fig. 15B). He designated the 

unit's type locality as the mountains above and eastward from 

Bear Lake. Detterman and others (198 la) designated the type 

locality more specifically as the southeast slope of the mountains in secs. 27,28, and 33, T. 48 S., R. 69 W., Chignik A-7 

and Port Moller D-1 1:63,360 quadrangles. Two recently 

measured partial sections of the Bear Lake are herein included 

(sections 25, 26; fig. 15) to aid in the description of the 

formation. Additional measured sections are shown in areport 

by Lyle and others (1979). Section 25 was measured on the 

southflankofMount Veniarninof in sec. 19, T. 48 S., R. 65 W., 

Chignik A-5 1 :63,360 quadrangle; it is about 3 15 m thick (fig. 

15). Section 26 is about 240 m thick and was measured on a 

south-facing slope of Sandy Ridge in sec. 32, T. 47 S., R. 68 

W., and sec. 5, T. 48 S., R. 68 W., Chignik A-6 1:63,360 

quadrangle. The thickness of the Bear Lake at both sections is 

much less than the thicknesses estimated by previous investigators. Burk (1965, p. 89) estimated the thickness of the Bear 

Lake as 5,000 ft (1,525 m), and Brockway and others (1975) 

identified 2,360 m of rock in the Gulf Oil Corporation Sandy 

River #1 borehole as the Bear Lake Formation. The present 

investigators consider the thickness of the Bear Lake Formation to be no more than about 1,000 m. The section may be 

repeated by faulting in the Sandy River borehole, as a major 

thrust fault has been identified a few miles to the east (Wilson 

and others, 1995). 

Section 25. Partial section of the Bear Lake Formation (fig. 15) 

Volcanic rocks 

Unconformable contact Thickness 

Bear Lake Formation: (meters) 

17. Sandstone, thin-bedded to massive, mediumgrained, olive-gray; slightly tuffaceous; abundant 

molluscan fossils, including Acila empirensis, 

Clinocardium, Cyclocardia, Mizuhopecten 

mollerensis, Glycymeris, Mya cf. M. cuneifomis, 

Mya truncata, Spisula, Natica, and Neptunea 

(M7181 and M7371) ............................................. 16.7 

16. Siltstone, thin-bedded, moderate-yellowish-brown ... 9.1 

15. Conglomerate, thick-bedded, pebble; coarse-sand 

matrix; fossiliferous (M7372) .................................. 3.0 

14. Sandstone, thick-bedded, coarse-grained, olive-gray ... 3.0 

13. Sandstone, massive, fine-grained, pale-yellowishbrown; silty .......................................................... 10.7 

12. Sandy siltstone, thin-bedded, grayish-green ............ 15.2 

Section 25. Partial section ofthe Bear Lake Formation (fig. 15)- 

Continued 

1 1. Conglomeratic sandstone, massive, dark-yellowishbrown; abundant fossils, including Chione, 

Clinocardium, Mizuhopecten mollerensis, Mya 

elegans, Mytilus, and Remondella waldroni 

(M7182 and M7373) ........................................... 18.3 

10. Siltstone, thin-bedded, olive-gray ............................ 10.7 

9. Conglomeratic sandstone, massive, mediumgrained, light-olive-gray; poorly sorted ................... 3.6 

8. Siltstone, thin-bedded, pale-yellowish-brown 9.7 ............ 

7. Sandstone, thin-bedded, medium-grained, 

light-olive-gray ...................................................... 10.7 

6. Conglomeratic sandstone and siltstone, thin- to 

thick-bedded; grayish-olive-green; megafauna in 

upper part, ?Neptunea (M7374) ............................. 19.8 

5. Siltstone, thin-bedded, greenish-gray; few shale 

chips ....................................................................... 18.3 

4. Sandstone, thin-bedded, medium-grained, lightbrownish-gray; crossbedded; channels containing 

lag gravel ............................................................... 42.6 

3. Siltstone and shale, thin-bedded, olive-gray ............ 35.9 

2. Conglomeratic sandstone, medium- to thick-bedded, 

medium-grained, olive-gray; crossbedded and 

channeled; some siltstone interbeds ...................... 39.6 

1. Siltstone, thin-bedded, olive-gray; crossbedded; 

plant fragments; few pebbles ................................. - 

47.2 

Total exposed section ..................................... 314.1 

Base of exposed section 

The Bear Lake Formation is exposed mainly along the 

northwest side of the Alaska Peninsula from Port Moller 

northeastward to the Upper and Lower Ugashik Lakes area, 

and it continues in the subsurface at least as far as Becharof 

Lake (Brockway and others, 1975). Rocks that are partly ageequivalent to the Bear Lake are mapped as the Tachilni 

Formation southwest of Port Moller. 

The Bear Lake Formation differs from most of the other 

Tertiary sedimentary units exposed on the peninsula, with the 

exception of the Tolstoi Formation, in its greater abundance of 

nonvolcanic detritus. Chert and quartz form a considerable 

part of the clastic detritus in its sandstones, which exhibit 

better sorting than is common in the other Tertiary units. 

These rocks evidently had a source area in the older Mesozoic 

sedimentary strataand, possibly, in the Alaska-AleutianRange 

batholith. Apparently, the Bear Lake depositional basin was 

isolated from the volcanic-rich sequence of rocks on the 

southeast side of the Alaska Peninsula. Conglomerate clast 

composition varies somewhat between conglomerate beds, 

but an average composition is about 30-40 percent chert, 20- 

30 percent volcanic rocks, 10-15 percent quartz, and 10-15 

percent granitic rocks. All clasts are well rounded and range 

in size from pebbles to small cobbles. 

The Bear Lake Formation was deposited mainly in an 

inner neritic to tidal flat environment: tidal and fluvial chanStratigraphic Framework of the Alaska Peninsula 55 
nels commonly cut into its deposits (Nilsen, 1984; Wisehart, 

1971). Strong tidal currents were apparently the cause of the 

better sorting of the clastic sediments. The bulk of the marine 

invertebrate fauna whose fossils are preserved in the formation lived in shallow, near-shore environments in water depths 

of less than 100 m (Louie Marincovich, Jr., 1983, and written 

commun., 1978 and 1985). 

The two sections of the Bear Lake Formation illustrated 

in figure 15 are incomplete, and their exact positions within 

the total formation are questionable. However, section 25 

from the south flank of Mount Veniaminof contains a high 

percentage of siltstone that may be correlative with the lower 

part of the section encountered in the Sandy River # 1 borehole 

(Brockway and others, 1975). The Sandy Ridge section (section 26), in contrast, contains more sandstone and conglomerate and has a fairly high content of tuffaceous material; this 

composition is more characteristic of the upper part of the 

borehole section. 

The Sandy Ridge section (fig. 15, section 26) was 

measured in part by John Bolm (U.S. Minerals Management 

Service) in 1982 and in part by the present authors in 1982. 

Section 26. Partial section of the Bear Lake Formation (fig. 15) 

Top of exposed section Thickness 

Bear Lake Formation: (meters) 

26. Sandstone, thick-bedded, fine- to coarse-grained, 

olive-gray; tuffaceous; scattered pebbles; abundant 

megafossils including Clinocardium and 

Mya truncata ........................................................... 4.3 

25. Siltstone, thin-bedded, medium-brown; poorly 

exposed .................................................................. 4.8 

24. Sandstone, medium-bedded, fine- to mediumgrained, medium-olive-gray; abundant 

disarticulated bivalves ............................................. 5.5 

23. Sandstone, thick-bedded, medium-grained, lightgray; scattered pebbles and small cobbles; 

crossbedded; tuffaceous; scattered bivalves .......... 10.0 

22. Conglomerate, massive; well-rounded clasts to 

30 cm, consist mainly of volcanic fragments .......... 2.1 

21. Sandstone, thick-bedded, fine- to medium-grained, 

medium-olive-gray; scattered pebbles to 10 cm; 

trough crossbeds; contains few fossils, including 

Mya truncata, Chlamys, Clinocardium, and 

?Neptunea (M8511) ............................................... 12.8 

20. Sandstone, thin-bedded to massive, medium- to 

coarse-grained, light-olive-gray; few pebbles to 

4 cm; trough crossbeds; bioturbated ........................ 7.9 

19. Sandstone, massive, fine-grained, light-olive-brown; 

pebble-to-cobble layer near top; few Clinocardium 

and Pecten ............................................................... 3.6 

18. Sandstone, thin- to medium-bedded, fine-grained, 

light-olive-gray; tuffaceous; 60-cm-thick pebbleconglomerate channel at base .................................. 5.5 

17. Sandstone, medium-bedded to massive, fine-grained, 

olive-gray to moderate-yellow-brown; tabular 

crossbeds; Clinocardium fragments ........................ 6.0 

Section 26. Partial section ofthe Bear Lake Formation (fig. 15)- 

Continued 

16. Conglomerate, massive, well-rounded pebbles; 

sandy tuffaceous layer in middle part ...................... 1.5 

15. Sandstone, massive, medium- to coarse-grained, 

light-olive-gray; slightly tuffaceous; few pebbles ... 9.1 

14. Conglomeratic sandstone, thick-bedded to massive, 

fine- to medium-grained, light-olive-brown; 

volcanic rock clasts to 45 cm; trough crossbeds ...... 6.1 

13. Sandstone, thin-bedded to massive, fine- to mediumgrained, light-olive-gray; few pebbles to 10 cm; 

bivalves in upper part; bioturbated in lower part ... 10.6 

12. Sandstone, massive, medium- to coarse-grained, 

medium-gray; conglomerate bed in middle part; 

trough crossbeds; bioturbated .................................. 9.1 

11. Sandstone; rubble ................................................. 7.6 

10. Sandstone, thick-bedded, medium- to coarse-grained, 

gray; bioturbated; crossbedded .............................. 13.7 

9. Conglomeratic sandstone, thick-bedded, mediumdark-gray; trough crossbeds; bivalves ................... 10.0 

8. Sandstone, medium-bedded, fine- to coarse-grained, 

medium-gray; conglomerate layer in middle part; 

bivalves Cardium and Acila .................................... 7.3 

7. Sandstone, massive, very fine to fine-grained, lightto medium-gray; pebbles and cobbles at base ....... 18.3 

6. Sandstone, thin- to thick-bedded, fine- to mediumgrained, medium-gray to brown ............................ 18.5 

5. Sandstone, massive, fine- to coarse-grained, grayishbrown; scattered pebbles and bivalves .................. 17.9 

4. Sandstone, thick-bedded, fine-grained, dusky-yellow 

to medium-gray; few pebbles; trough crossbeds; 

few bivalves ........................................................... 22.5 

3. Sandstone, medium-bedded, fine- to mediumgrained, medium- to dark-gray; scattered pebbles. 18.6 

2. Sandstone, thin-bedded, fine-grained, olive-gray to 

dusky-yellow; laminated and crossbedded .............. 7.9 

1. Conglomerate, massive; well-rounded clasts to 4 cm ... - 

2.4 

Total exposed section ..................................... 243.7 

Fault or unconformable(?) contact 

All exposures of the Bear Lake Formation are abundantly fossiliferous, containing mainly bivalves and gastropods. These fossils have been studied in considerable detail by 

Louie Marincovich, Jr. (1988a, and written commun., 1978, 

1980,1984,1985) and Allison (1978). They have been correlated with the Newportian to Graysian Stages of middle and 

late Miocene age; this age assignment is based on about 30 

genera of mollusks, two-thirds of which are bivalves. On the 

basis of its contained megafauna, the Bear Lake Formation is 

here considered to be middle and late Miocene in age. Some 

of the genera also are present in the Tachilni Formation, 

exposed at the southwest tip of the Alaska Peninsula, and the 

two formations are partly correlative according to Marincovich 

(1983). The two formations have about 20 genera in common 

(Marincovich, 1983). 

56 Geologic Studies on the Alaska Peninsula 
The Bear LakeFormation overlies the Meshik Volcanics, 

Stepovak Formation, and Tolstoi Formation at various localities. Most of the contacts are disconformities; a few are 

angular unconfonnities. The Bear Lake Formation is not in 

contact with the Unga Formation, which has heretofore been 

considered to be the basal member of the Bear Lake Formation 

by numerous authors, including Burk (1965), Allison and 

Addicott (1973), and Marincovich (1988a). The Bear Lake 

Formation is overlain at some localities by volcaniclastic and 

volcanic rocks of the Milky River Formation-most of the 

contacts are disconformities and a few are angular 

unconformities-but more commonly the Bear Lake is the 

uppermost sedimentary rock unit exposed. 

Tachilni Formation 

The Tachilni Formation was named by Waldron (1961, 

p. 686-687) for several hundred feet of marine sedimentary 

rocks exposed along Morzhovoi Bay at the tip of the Alaska 

Peninsula. The formation is present only near the southwest 

end of the Alaska Peninsula. We studied the Tachilni Formation only briefly during the present investigation. However, 

other U.S. Geological Survey workers have studied the formation in considerabledetail (see McLean, 1979a, 1979b; McLean 

and others, 1978; and Marincovich, 1983). We show herein 

(fig. 15) a reference section (section 27) located on South 

Walrus Peak near Cape Tachilni that was measured during a 

U.S. Geological Survey project in the area (see McLean, 

1979b; Marincovich, 1983). The section is located in the SElI 

4 NE114 sec. 19, T. 60 S., R. 89 W., Seward Meridian, False 

Pass D-3 1:63,360 quadrangle. 

Section 27. Reference section of theTachiIni Formation (fig. 15) 

Volcanic rocks 

Unconformable contact Thickness 

Tachilni Formation: (meters) 

7. Conglomerate, thick-bedded to massive, pebble-size 

clasts; coarse sandstone matrix; contains fossils 

Bulbus fragilis, Neptunea lyrata altispira, 

Polinices pallidus, Mya truncata (M7 143) ............. 8 

6. Sandstone, thin-bedded, fine-grained, silty, gray; 

grades downward to interbedded dark mudstone; 

laminated; festoon crossbeds ................................. 30 

5. Sandstone; contains mollusks, including Clinocardium and Mizuhopecten mollerensis (M7144) ... 4 

4. Sandstone, medium-bedded, dark-gray; laminated 

and crossbedded; few pebble layers ...................... 11 

3. Conglomerate, pebble ................................................. 1 

2. Sandstone, dark-gray; bimodal crossbeds ................ 17 

1. Sandstone, medium-bedded, fine- to mediumgrained, gray with black laminae; bimodal trough 

crossbedding; pebble layers with fossils, 

including 1 1 genera of bivalves and 6 of 

gastropods (M7145 and M7146) ........................... - 61 

Total measured section .................................. 132 

Section continues, not measured 

The 132 m of the Tachilni Formation exposed at the 

reference section is not a complete section; the basal part was 

not investigated. Stratigraphic and structural data suggest that 

the Tachilni may be as much as 200 m in total thickness, which 

is considerably more than the 60 to 65 m exposed at the 

formation's type locality at Cape Tachilni. 

The Tachilni Formation is composed mainly of poorly 

consolidated subgraywacke sandstone containing 30-35 percent angular quartz, 10-15 percent feldspar, 5 percent pyroxene and amphibole, 30 percent volcanic rock fragments, and 

20 percent clay matrix. The gray to brown sandstone is 

commonly crossbedded and interbedded with volcanic-pebble 

conglomerate and siltstone. The Tachilni (upper Miocene) 

and Bear Lake (middle and upper Miocene) Formations are, 

in part, age equivalents (Allison, 1978; Marincovich, 1983), 

and they have molluscan fossils in common; but they are 

distinctly different lithologically even though their exposure 

areas are separated by only about 125 km. This lithologic 

difference reflects different source areas for the two formations. The Tachilni Formation is geographically restricted to 

the southeast side of the Alaska Peninsula, and its source 

terrane is the abundant volcanic rocks along the Pacific Ocean 

side of the Alaska Peninsula. In contrast, the Bear Lake 

Formation was deposited on the Bristol Bay side of the 

peninsula and was derived from the erosion of the older 

Mesozoic sedimentary rocks and the Alaska-Aleutian Range 

batholith. The two formations have about 20 molluscan genera in common, so there apparently was an open seaway 

between the two depositional areas. The faunal differences 

between the two formations may reflect the organisms' preference for a particular type of substratum, a difference in 

preservation of the fossils, or merely poor collecting techniques. Horizons containing abundant bivalve and gastropod 

fossils are common throughout both units. The fossils indicate 

shallow-water deposition, generally in less than 100-m water 

depths (Marincovich, 1983). 

The reference section of the Tachilni Formation is only 

part of the total Tachilni sequence exposed in the Cape 

Tachilni area. McLean (1979b) reported the formation to be 

about460m thick in this area, but structural complications and 

poor exposures preclude measuring a complete section. 

Marincovich (1983, p. 66) stated that the fauna from the strata 

at South Walrus Peak is significantly younger than the fauna 

obtained from the beds at Cape Tachilni, suggesting that the 

formation is thicker than shown here. 

The Tachilni Formation is richly fossiliferous, containing 36 genera of bivalves and 11 genera of gastropods. The 

Tachilni fossils show a close correlation with the Wishkahan 

Stage of the late Miocene; it is on this basis that the age of the 

unit is regarded as late Miocene in this report. This age assignment is based on such forms as Acila empirensis, Clinocardium 

hannibali, Felaniella parilis, Glycymeris grewingki, Macoma 

astori, Protothaca staleyi, Buccinum sp., Natica janthostoma, 

and Neptunea sp. The complete list of Tachllni fossils is contained in a report by Marincovich (1983). 

Stratigraphic Framework of the Alaska Peninsula 57 
The Tachilni Formation contains both marine and nonmarine strata that consist mainly of sandstone, mudstone, and 

conglomerate. The sandstone and conglomerate are composed chiefly of volcanic rock fragments; this composition 

contrasts sharply with that of the partly coeval Bear Lake 

Formation, which is mainly nonvolcanic. Lateral and vertical 

facies changes are common in the Tachilni, and many of its 

beds are channeled. Bimodal crossbedding in the unit suggests strong tidal influences. The provenance for its sediments 

is the older Tertiary volcanic rocks located along the southeast 

side of the peninsula, whereas the provenance for the Bear 

Lake Formation is the Mesozoic deposits located on the 

northwest side of the peninsula. 

The Tachilni Formation is unconformably overlain by 

volcanic rocks, presumably part of the Milky River Formation. 

The lower contact of the formation is unknown, but the Tachilni 

may overlie the Belkofski Formation (McLean, 1979b). 

Milky River Formation 

The name "Milky River Formation" was used by Galloway (1974) for volcanic and sedimentary rocks overlying 

the Bear Lake Formation along the Milky River. Prior to 1974, 

these rocks were informally referred to as Pliocene volcanic 

and sedimentary rocks (see Waldron, 1961; Burk, 1965). The 

formation was formally defined and a type locality designated 

by Detterman and others (1981a). The type locality is located 

on the northeast spur of a mountain in sec. 14, 15, and 22, T. 

48 S., R. 69 W., Seward Meridian, Chignik A-7 1:63,360 

quadrangle. The Milky River in its type locality is about 465 

m thick. A considerably thicker section was identified in the 

Gulf Oil Corporation Sandy River #1 borehole by Brockway 

and others (1975), approximately 25 km northwest of the type 

locality (fig. 16). The section from the borehole is in sec. 10, 

T. 46 S., R. 70 W. Chignik A-7 1:63,360 quadrangle. The 

section identified as the Milky River Formation, from a depth 

of -160 ft (-48 m) to -3,480 ft (-1,060 m) is here designated 

as a reference section of the unit (section 28). Details from the 

borehole logged by Baroid Well Logging and from ditch 

samples are shown in section 28. 

The Milky River Formation in the Sandy River #1 

borehole reference section is more than twice as thick as the 

section at the type locality. This is probably due to the strata 

in the type locality being subjected to greater postdepositional 

erosion than the strata in the area of the borehole. The strata in 

the type locality are also much more conglomeratic than those 

at the reference section, indicating their relative proximity to 

the source area. The formation is entirely nonmarine both at 

the type locality and in the reference section; however, northeast along the Alaska Peninsula, near Becharof Lake, General 

Petroleum Corporation's Great Basin #1 borehole encountered bivalves at -1,185 m depth in rocks correlative with the 

Milky River Formation (Brockway and others, 1975). NO data 

are available on the kind(s) of fossils or their age(s). The 

fossils may actually be from beds of the Bear Lake Formation, 

rather than beds correlative with the Milky River Formation. 

The fossils cannot be recovered, so additional information 

about them cannot be obtained. A K-Ar age of 3.53kO.27 Ma 

(Wilson and others, 1981) obtained from a lava flow just 

below the top of the section at the type locality, as well as 

another ageof 3.87&0.06Ma(F.H. Wilson, unpub. data, 1991) 

from near the base of the formation near Port Moller, indicates 

that the Milky River Formation is of Pliocene age. 

Section 28. Reference section of the Milky River Formation 

(fig. 16) 

Quaternary glacial deposits 

Unconformable contact Thickness 

Milky River Formation: (meters) 

18. Basalt flow, fine-grained, porphyritic, black ............ 41.1 

17. Sandstone, coarse-grained, brown to orange; 

conglomeratic; all volcanic rock clasts; tuffaceous 57.9 

16. Siltstone, yellow; tuffaceous .................................... 16.7 

15. Andesite flow; porphyritic ......................................... 9.1 

14. Tuff, olive-brown to yellow ..................................... 18.3 

13. Sandstone and conglomerate, brown, basaltic; 

minor tuffaceous siltstone ...................................... 5 1.8 

12. Tuff, lapilli, pale-yellowish-orange .......................... 19.8 

1 1. Tuff and scoria ......................................................... 54.8 

10. Sandstone and conglomerate, coarse-grained, lightgrayish-yellow; tuffaceous; all volcanic debris ..... 10.6 

9. Sandstone, tuffaceous, fine- to coarse-grained, 

light- to dark-gray; some scoria; few pebbles; all 

volcanic debris ....................................................... 27.4 

8. Siltstone, dark, carbonaceous; pumice fragments 18.2 .... 

7. Sandstone, tuffaceous, fine- to coarse-grained, 

light- to dark-gray; some scoria; few pebbles; all . 

volcanic debris ....................................................... 6 1.6 

6. Basalt flow, black to reddish-brown ........................ 30.5 

5. Siltstone, tuffaceous, black; thin lignite beds ........... 54.8 

4. Sandstone, coarse-grained, basaltic; interbedded 

white tuff ............................................................... 64.0 

3. Sandstone, coarse-grained, tuffaceous; interbeds of 

tuff and scoria ...................................................... 198.0 

2. Siltstone, light-gray, tuffaceous ............................... 54.8 

1. Sandstone and sandy siltstone, tuffaceous, 

light-gray ............................................................. 156.7 

Total section ................................................ 1,011.7 

Unconformable contact 

Bear Lake Formation 

Late Tertiary Igneous Rocks 

Igneous activity on the Alaska Peninsula during Tertiary time can be separated into two main episodes. The first 

episode, which formed the Meshik arc (Wilson, 1985), ceased 

in earliest Miocene time. During late Miocene time, after a 10- 

to 12-m.y. hiatus recognized over much of the Alaska Peninsula, igneous activity resumed with the formation of the 

58 Geologic Studies on the Alaska Peninsula 
A Section 28 

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16 

15 

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XIX*IX 

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Conglomeratic sandstone Es5<j Shale 

Fl Sandstone ..... r] coal (lignite) 

Tuffaceous sandstone v. .v m] Tuff 

Fd . Silty sandstone Scoria and pumice 

pj Tuffaceous siltstone v - Volcanic flow 

EJ Siltstone 

4 

3 

2 

1 200 METERS 

vvvvvvv 

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. v .V..l . . . . . . . . . -. ."-.I y -. -." v - -. "LT-ITi- 

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-. " 2727 v-."- . -. v-. , 

B INDEX MAP 

Sandy River #l well 

- 5 KILOMETERS 

160" 00' 

Geology modified from Detterman and others (1981) 

Figure 16. Pliocene Milky River Formation, Alaska Peninsula. A, Reference section from Sandy River #1 

borehole. Age, formation name, and lithic units (numbered to correspond to text) are shown for section. B, 

Index map. Circled number labels location of section on index map and figure 5. See figure 6 for explanation 

of other symbols. 

Stratigraphic Framework of the Alaska Peninsula 59 
Aleutian magmatic arc. (Note: In common usage, the term 

"Aleutian arc" is used for rocks that are herein called the 

"Meshik arc" and the "Aleutian arc." Here, we restrict usage 

of the term "Aleutian arc" to the late Tertiary to present 

magmatic arc in southern Alaska.) Igneous activity along a 

convergent plate margin is generally believed to be associated 

with subduction of one plate beneath another, and cessation of 

activity indicates that little or no subduction is taking place. 

On the Alaska Peninsula, magmatism has been produced by 

subduction of the Kula and Pacific plates beneath the North 

American plate along the ancient and modem Aleutian trenches. 

DeLong and others (1978,1980) and Farrar and Dixon (1980) 

suggested that subduction of the Kula-Pacific ridge occurred 

during the early Oligocene, about 35 Ma, and that subduction 

of the ridge resulted in a hiatus in subduction-related igneous 

activity. Wilson (1985) suggested that the waning of igneous 

activity on the Alaska Peninsula between the end of the 

Meshik arc at about 25 Ma and the beginning of the Aleutian 

arc at about 10 to 15 Ma may reflect the hiatus predicted by 

DeLong and others (1978, 1980). However, more recent 

analysis of the history of the Kula plate based on new data 

(Lonsdale, 1988) changes our understanding of the timing of 

events affecting the Kula-Pacific ridge; Lonsdale presents 

evidence suggesting that the Kula-Pacific spreading center 

ceased activity at 43 Ma. Plate reconstructions by Engebretson 

and others (1985) suggest that this old spreading center would 

have been subducted in latest Miocene or early Pliocene time, 

and Lonsdale (1988, p. 753) doubted "...that the presence of 

this old plate boundary had any major influence on arc 

volcanism ...." Lonsdale' s conclusions are specific to the Aleutian Islands segment of the magmatic arc; however, the timing 

of ridge subduction in the Alaska Peninsula segment was not 

greatly different (Engebretson and others, 1985, p. 19-22), 

and therefore Lonsdale's conclusions should also apply to the 

Alaska Peninsula. The Miocene hiatus in magmatism may 

reflect cessation of. subduction while the Alaska Peninsula 

was coupled with the Pacific plate; however, limited paleomagnetic data (Thrupp and Coe, 1987) do not suggest any 

post-Eocene latitudinal displacement of the Alaska Peninsula 

and therefore do not support coupling of the Alaska Peninsula 

to the Pacific plate. The resumed magmatic activity appears to 

have started near Chignik, in the middle of the peninsula, 

about 10-15 Ma and then spread northward and southward 

during the course of a few million years. Quaternary volcanoes along the Alaska Peninsula indicate continued activity 

along the Aleutian arc. 

Intrusive Rocks 

Bay 1 :250,000 quadrangle), Devils Bay (Chignik 1 :250,000 

quadrangle), Agripina Bay and Cape Igvak (Ugashik and 

Karluk 1 :250,000 quadrangles), and finally to Kukak Bay and 

Cape Douglas (Mount Katmai and Afognak 1 :250,000 quadrangles) at the northeast end of the peninsula. Smaller stocks and 

hypabyssal plugs and domes are interspersed between the larger 

batholithic bodies, generally somewhat farther inland than the 

batholiths. Some of these smaller intrusions are hypabyssal 

bodies directly associated with extinct volcanic centers. 

The plutonic rocks range in composition from diorite 

and quartz diorite to granodiorite and also include minor 

quartz monzonite and monzodiorite. They range from fine to 

coarse grained, though most are medium grained; they are 

equigranular and contain pyroxene, hornblende, and (or) 

biotite in varying amounts. Well-developed homfels zones 

surround the batholiths, and many of the batholiths have areas 

of hydrothermal alteration and mineralization (see, for example, Wilson and others, 1988). Many of the plutonic bodies 

have been radiometrically dated; ages range from 10.08H. 14 

Ma for the Devils batholith (Wilson and others, 1981) to 

2.1Ok0.28 Ma for the Agripina Bay batholith (Wilson and 

Shew, 1992). 

The hypabyssal rocks are mainly andesitic to dacitic, 

fine to medium grained, and hornblende-bearing. A few small 

plugs and sills are as silicic as rhyolite or as basic as olivine 

basalt. Alteration and low-grade mineralization of the rocks 

are fairly common. Many dikes and sills intrude the sedimentary rocks of the Alaska Peninsula. It is difficult to distinguish 

hypabyssal rocks of late Tertiary age from similar rocks of 

Eocene and Oligocene age unless there is good stratigraphic 

control or else radiometric age determinations are available. 

The radiometric age determinations available for the late 

Tertiary hypabyssal rocks have a range similar to that for the 

plutonic rocks discussed above. Though many of the measured sections of sedimentary rocks in this report show dikes 

and sills, few of these dikes are radiometrically dated. Although some of these dikes and sills resulted from Aleutian arc 

magmatism, someundoubtedly were emplacedduring Meshik 

arc magmatism. 

Volcanic Rocks 

Large volcanic fields associated with hypabyssal intrusive centers are not common on the Alaska Peninsula. Several 

are present, however, and the largest covers approximately 

850 km2 between the Katmai River and Kukak Bay, near the 

northeast end of the Alaska Peninsula. Several other moderate-sized areas (less than 300 km2) of late Tertiary volcanic 

rocks are near Chiginagak Bay and Cape Douglas, between - - - 

Moderate-sized batholiths of late Tertiary age were Cape Aliaksin and Dorenoi Bay, between Morzhovoi Bay and 

emplaced along the present Pacific coast of the Alaska Penin- False Pass, and between King Cove and Cold Bay. Small areas 

sula at fairly regular spacing. These plutons start at King Cove are also present on the southwestern part of the peninsula 

(Cold Bay 1 :250,00Oquadrangle) and continue northeastward where flow remnants cap ridges. Perhaps the major reason for 

to Moss Cape, Pyramid Mountain, and American Bay (Port the relatively small areal extent of Tertiary extrusive rocks is 

Moller 1 :250,000 quadrangle), Mitrofania Island (Stepovak erosion, particularly glacial erosion during the Pleistocene. 

60 Geologic Studies on the Alaska Peninsula 
Also, structural data indicate that the late Miocene and Pliocene 

was a time of intense structural deformation and uplift, which 

would accentuate erosion. 

The late Tertiary volcanic rocks are mainly basaltic to 

andesitic in composition and dark greenish gray to black in 

color. The rocks commonly are porphyritic and contain phenocrysts of plagioclase and of clinopyroxene, orthopyroxene, 

or hornblende. Flows, locally columnar jointed, are as much 

as 15 m thick and are commonly interbedded with lithic tuff 

and volcanic breccia. Hypabyssal plugs of similar composition, probable erosional remnants of volcanic edifices that 

produced the flows, are present within or around the volcanic 

fields. Local alteration and silicification of the volcanic rocks 

is particularly prevalent near the plugs. Some minor mineralization is found in the altered rocks. 

Morzhovoi Volcanics 

The name "Morzhovoi Volcanics" was introduced by 

Waldron (1961) for a sequence of interbedded lava flows and 

pyroclastic rocks, and some associated volcaniclastic rocks, 

that are exposed east of Morzhovoi Bay and south of Frosty 

Peak(fig. 15B). Reynolds Head, on theeast shoreof Morzhovoi 

Bay, in sec. 5, T. 59 S., R. 90 W., Cold Bay A-3 1:63,360 

quadrangle, was designated as the type locality by Waldron 

(1961, p. 688), who suggested that North and South Walrus 

Peaks, as well as other high peaks in the area, were remnants 

of an ancient volcano. The arrangement of the peaks suggest 

a caldera rim, but we cannot prove or disprove a caldera at this 

location. 

Lava flows of the Morzhovoi Volcanics are light- to 

pinkish-gray porphyritic basalt composed mainly of plagioclase and augite. The flows contain minor amounts of either 

hypersthene or olivine, but not both in the same flow. Volcanic breccia and agglomerate enclosed in a tuffaceous matrix 

are interbedded with the lava flows. These rocks are locally 

hydrothermally altered and mineralized in the Walrus Peaks 

area, particularly around South Walrus Peak. 

The age of the Morzhovoi Volcanics is not known 

precisely, but it can be determined fairly accurately by indirect 

methods. The Morzhovoi overlies the Tachilni Formation, 

whose age has been established as late Miocene according to 

Marincovich (1983) and the present authors. The upper part of 

the Tachilni Formation contains several ash beds that were 

possibly derived from the Morzhovoi volcano. The Morzhovoi 

volcanic flows commonly contain olivine, which is rare in 

Quaternary volcanic rocks on the AlaskaPeninsula. Additionally, glaciers have eroded deep, broad, U-shaped valleys that 

are separated by knife-like ridges in Morzhovoi strata, indicating that the Morzhovoi is in part older than the Pleistocene 

glacial interval. The best estimate at present for the age of the 

Morzhovoi Volcanics is late Miocene(?), Pliocene, and early 

Quaternary(?). 

The contact between the Morzhovoi Volcanics and the 

underlying Tachilni Formation is generally conformable, but 

it is locally unconformable where volcanic flows have filled 

former stream valleys cut into the Tachilni Formation. 

Quaternary System 

Deposits of Quaternary age cover about 75 percent of 

the land area of the Alaska Peninsula. Two processes, volcanism and glaciation, are responsible for the formation of most 

of these deposits. Subsequent minor modification of these 

deposits has occurred through the action of water, wind, and 

gravity; however, only the major deposits attributable to 

volcanism and glaciation will be described here. 

Volcanic rocks 

An increase in magmatic activity that began in the late 

Miocene continues to form the present Aleutian arc. Thirtyseven Quaternary volcanic centers have been identified along 

the Alaska Peninsula, 30 of which have erupted in Holocene 

time (fig. 17; Miller and Richter, 1994). Eight caldera-forming eruptions have occurred along the Alaska Peninsula during the Holocene (Miller and Smith, 1987), one of which, the 

Novaruptaeruption of 19 12, is the largest volcanic eruption of 

this century. Two eruptive centers have formed since 1900. 

Some of the highest peaks along the peninsula are formed by 

its stratovolcanoes, the highest of which is Pavlof Volcano at 

just over 2,500 m. 

The Quaternary volcanic centers define a narrow belt of 

volcanic activity that trends northeast-southwest along the 

axis of the peninsula about 15 to 30 km from the Pacific coast. 

The most marked offset of this volcanic line is in the central 

peninsula where Mount Kialagvik, Mount Chiginagak, and 

Yantarni Volcano (Riehle and others, 1987c) are approximately 32 km closer to the trench than their neighbors on 

either side. This offset may be due to structural control exerted 

by faults in the upper crust, or it may be related to deep 

transcurrent fractures, perhaps in the Pacific plate being 

subducted beneath the peninsula. Nakamura and others (1980) 

used a 60-krn-long belt of cinder cones transcurrent to the arc 

across Mount Veniaminof as an indicator of crustal stress 

patterns. 

The spacing of Quaternary volcanic centers along the 

volcanic front is variable, normally ranging from 20 to 54 km 

between the major centers. Volcanic vents are more closely 

spaced in the Mount Katmai area, in the Pavlof Volcano area, 

and southwest of Mount Kupreanof. Mount Katmai, Trident 

Volcano, Novarupta, Mount Griggs, Falling Mountain, Mount 

Cerberus, Mount Mageik, and Mount Martin are no more than 

10 km apart. In the vicinity of Pavlof Volcano, the most active 

volcano in the Aleutian arc, several late Pleistocene to Holocene stratovolcanoes trend northeastward from Mount 

Emmons to Pavlof Sister at intervals of 5 to 8 km. About 15 

km southwest of Mount Kupreanof is a series of at least three 

Holocene volcanic vents 3.6 to 6 km apart (Yount and others, 

1985; Wilson, 1989). 

Stratigraphic Framework of the Alaska Peninsula 61 
164' 162' 160" 158' 156' 154" 

155'30' I 

58', 

30' 

20 KILOMETERS 

Se" - 

50 100 KILOMETERS 

Peaks 

I I 1 I I 

162' 160" 158" 156" 154" 

Figure 17. Selected Quaternary volcanic centers and associated features on the Alaska Peninsula. Solid triangles, 

named volcanic centers; open triangles, unnamed volcanic centers. "Mount Kejulik" is an informal name. 

Eight calderas of varying sizes and ages occur along the 

Alaska Peninsula. Emmons Lake caldera, which has a diameter of 19 km and a total postulated eruptive volume of more 

than 50 km3, is one of largest and oldest (Miller and Smith, 

1987). Its caldera-forming eruption is pre-Holocene in age but 

probably no older than early Wisconsin according to Miller 

and Smith (1987). Veniaminof, Black Peak, and Aniakchak 

calderas in the central peninsula formed 3,700 yr B.P., between 3,660 and 4,170-4,700 yr B.P., and 3,430 yr B.P., 

respectively, according to Miller and Smith (1987). Ash 

deposits on the Seward Peninsula approximately 970 km 

northwest of Aniakchak caldera (= Aniakchak Crater) have 

been identified as originating from Aniakchak's calderaforming eruption (Riehle and others, 1987b). A spectacular 

60-hr eruption in 1912 formedNovarupta and Katmai calderas 

and filled the Valley of Ten Thousand Smokes with more than 

11 km3 of ash-flow tuff (Fenner, 1920, 1926; Griggs, 1922; 

Curtis, 1968; Hildreth, 1983, 1987; Fierstein and Hildreth, 

1986). 

Clusters of dacitic domes at The Gas Rocks and Blue 

Mountain (on the west shore of Upper Ugashik Lake) give KAr ages ranging from 0.134M.058 Ma to 1.66k0.072 Ma 

(Wilson and Shew, 1992). The domes may have been associated with early Quaternary calderas; if so, subsequent glacial 

erosion has removed associated cone-building lava flows and 

ash-flow tuff sheets. Other Quaternary dome clusters occur at 

Kaguyak caldera (= Kaguyak Crater), in Ugashik and Black 

Peak calderas, and in the vicinity of Katmai Pass. 

Several of the Quaternary volcanic sequences near the 

southwest end of the Alaska Peninsula were formally named 

as the result of investigations initiated in 1945 by the War 

Department (now the Department of Defense) and the U.S. 

Geological Survey. The volcanic sequences so named consist 

of the Frosty Peak Volcanics (Waldron, 1961) and the Arch 

Point Basalt, BlackPointBasalt, and DushkinBasalt (Kennedy 

and Waldron, 1955). The last three units are part of the Pavlof 

Volcano sequence. These named volcanic rock sequences are 

mainly basalt; they will not be discussed individually in this 

report. 

Glacial deposits 

The Alaska Peninsula contains abundant evidence that 

it was extensively glaciated during the Pleistocene. The low, 

nearly flat land along the northwest side of the peninsula is 

constructed mainly of glacial drift, and the surface is covered 

by arcuate morainal hills and dotted by numerous kettle lakes. 

The mountainous southeastern side of the peninsula contains 

62 Geologic Studies on the Alaska Peninsula 
many U-shaped valleys with hanging tributary valleys. Numerous small valley glaciers are present on the peninsula. 

Most originate on the higher stratovolcanoes. 

Evidence obtained as the result of recent studies on the 

Alaska Peninsula indicates that glaciers crossed the peninsula 

at least four times (Detteman, 1986). Thedata further indicate 

that significant intervals of time elapsed between these major 

glacial advances. During these intervals the climate ameliorated, the glacial deposits were reworked, soil was formed, 

and vegetation was reestablished. The record for the last two 

major glaciations is well preserved and indicates that each was 

made up of two or more minor advances. 

The first detailed investigations of the glacial record on 

the AlaskaPeninsula were by Muller (1952a, 1952b, 1953) in 

the Naknek Lake area. This was followed by a study by Funk 

(1973) at Cold Bay. More recently the glacial deposits in the 

central part of the peninsula were mapped (Detterman and 

others, 1981b, 1987b). 

The glacial chronology for the Alaska Peninsula is still 

poorly understood, owing in part to a scarcity of radiocarbon 

ages. In the absence of radiocarbon ages, and because the 

older deposits are beyond the range of present dating methods, 

other criteria have been developed to identify the different 

deposits. These criteria are based on morphology, extent of 

weathering, and position of the various deposits. To facilitate 

mapping and discussion, the drift sheets were named, except 

for the poorly defined oldest deposit. In ascending order the 

drift sheets are (1) oldest drift, (2) drift of Johnston Hill 

glaciation (Abrahamson, 1949; Muller, 1953), (3) drift of 

Mak Hill glaciation (Abrahamson, 1949; Muller, 1953), and 

(4) drift of Brooks Lake glaciation (Muller, 1953). Welldefined morainal deposits of the Brooks Lake drift sheet can 

be further subdivided into deposits of informally named 

advances, namely the Kvichak, Iliamna, Newhalen, and Iliuk 

stades (Detterman and Reed, 1973). In addition, several minor 

glacial advances have left deposits on the Alaska Peninsula 

during the Holocene. These deposits are unnamed and are 

referred to as deposits of the Neoglaciation of Porter and 

Denton (1967). Modern glaciers are remnants of the last pulse 

of the Neoglaciation, which probably had its maximum extent 

about 200 years ago. 

The oldest drift sheet is exposed only in coastal bluffs 

along Bristol Bay near Naknek, where it unconformably 

underlies drift of Johnston Hill age. The deposit consists of a 

deeply weathered, compact boulder clay containing clasts of 

granitic, metavolcanic, and sedimentary rocks. All of the 

clasts have a thick weathering rind, which, along with the 

deeply weathered character of the boulder clay, suggests great 

age. 

Johnston Hill drift also is found only along the Bristol 

Bay coastline, where morainal ridges of this drift sheet are 

preserved. Mass wasting has greatly altered the moraines and 

filled in swales and kettle lakes. We do not have any age data 

for this drift, but it is undoubtedly pre-Wisconsin. 

Drift of the next youngest glaciation, the Mak Hill drift 

sheet, retains a well-defined arcuate morainal character although the sharp crests of the moraine are considerably 

subdued by mass wasting. Outwash plains are still present but 

somewhat modified. Radiocarbon samples obtained from this 

drift all indicate an age greater than 40,000 yr B.P. 

Drift of the Brooks Lake glaciation covers most of the 

lowland northwest of the Alaska-Aleutian Range. Its individual advances are well defined, and little modification of 

arcuate morainal ridges and kettle lakes has occurred. The 

modern topography is largely the result of this glaciation. The 

positions of the moraines indicate that the glaciers moved 

northwestward across the peninsula, probably from an ice-cap 

located on the continental shelf. 

Deposits of the Neoglaciation are generally restricted to 

valleys that still contain glaciers. These deposits are little 

altered and lie no more than 1 to 2 km in front of the presentday glaciers. 

The large lakes on the AlaskaPeninsula are all remnants 

of formerly much larger lakes that were dammed by moraines 

of the glacial advance that tookplace during the Kvichak stade 

of the Brooks Lake glaciation. These lakes are now dammed 

by moraines of the advance that took place during the later, 

Iliamna stade. 

A general warming trend began on the AlaskaPeninsula 

about 10,000 yr B.P., with maximum deglaciation occurring 

before 8,000-8,500 yr B.P. High lake levels resulting from 

this deglaciation formed terraces still present around the large 

lakes. Radiocarbon ages from these terracesrange from 8,000- 

8,500 yr B.P. (Detterman, 1986). Minor glacial advances 

during the Neoglaciation have taken place since then, but their 

deposits are undated on the peninsula. 

SUMMARY 

The Alaska Peninsula contains a well-exposed and 

extremely thick sequence of Mesozoic and Cenozoic rocks, as 

well as a very thin Paleozoic section at Puale Bay. The 

cumulative thickness is about 14,000 m, but the average 

thickness at most localities probably does not exceed 7,000- 

8,000 m; this is due mainly to lateral depositional thinning of 

strata and postdepositional erosion. Volcanic rocks are included in the overall thickness where they are interbedded 

with the sedimentary sequence. 

Paleozoic rocks are known from only one locality, a 

small islet at the mouth of Puale Bay, where about 40 m of 

mid-Permian limestone is exposed (Hansen, 1957; Detterman 

and others, 1985, 1987a). Metamorphic rocks exposed in the 

Mount Katmai area (Riehle and others, 1987a) may include 

some strata with Paleozoic protoliths. 

The Mesozoic section on the Alaska Peninsula is about 

8,500 m thick and is divided into 12 formations (fig. 4). In 

ascending stratigraphic order, these are the Kamishak Formation (Upper Triassic); Talkeetna Formation (Lower Jurassic); 

Kialagvik Formation (Lower and Middle Jurassic); Shelikof 

Stratigraphic Framework of the Alaska Peninsula 63 
Formation (Middle Jurassic); Naknek Formation (Upper Jurassic); Staniukovich Formation, Herendeen Formation, and 

Pedmar Formation (Lower Cretaceous); and Chignik Formation, Hoodoo Formation, Shumagin Formation, and Kaguyak 

Formation (Upper Cretaceous). In addition, the Naknek Formation is subdivided into five members; in ascending order, 

these are the Chisik Conglomerate, Northeast Creek Sandstone, Snug Harbor Siltstone, Indecision Creek Sandstone, 

and Katolinat Conglomerate Members. 

The Mesozoic strata are predominantly marine in origin, but they include a significant thickness of continental 

beds in the upper part of the section, particularly in the Upper 

Jurassic and Upper Cretaceous. The provenance for these 

sediments was initially a volcanic arc that was subsequently 

sutured to southern Alaska in Middle Jurassic time. During the 

late Mesozoic, the provenance shifted to the Alaska-Aleutian 

Range batholith and associated metamorphic rocks. The reworking of previously deposited sediments is increasingly 

common upward in the Mesozoic section. 

Marine Mesozoic strata contain the fossils of an abundance of life forms that are very valuable in assigning ages to 

the enclosing beds. Nearly 1,000 collections of megafossils 

were obtained during recent field work on the Alaska Peninsula; this large quantity of samples greatly reduces possible 

errors in age assignments that can result from inadequate 

sampling. Megafaunal data for the central part of the Alaska 

Peninsula were reported by Detterman and others (1981c, 

1985). In addition to age data, the marine megafauna may 

provide a key to the latitudinal position of the depositional 

area at the time of its formation (Taylor and others, 1984). 

Tertiary strata are nearly as thick as the Mesozoic 

sequence but are mostly restricted to the southwestern part of 

the Alaska Peninsula. Tertiary rocks, in contrast to the Mesozoic ones, are mainly of continental origin. Marine beds 

locally intertongue with thecontinental deposits, and in nearly 

all cases they are nearshore to tidal-flat facies. Volcanic 

detritus composes the main part of the sedimentary constituents. Their provenance was the volcanic terrane of the Alaska 

Peninsula. The older Mesozoic strata and the Alaska-Aleutian 

Range batholith provided some detrital input to the sedimentary regime, but only locally were they major contributors, as 

for example, in the Bear Lake and Tolstoi Formations. 

TheCenozoic section is divided into 1 1 formations (fig. 

12). In ascending stratigraphic order, these are the Tolstoi 

(upper Paleocene to middle Eocene) and Copper Lake Formations (Paleocene? and lower Eocene), Stepovak Formation 

and Meshik Volcanics (upper Eocene and lower Oligocene), 

Hemlock Conglomerate (upper Oligocene), Belkofski Formation (upper Oligocene? and lower and middle Miocene?), 

Unga Formation (upper Oligocene to lowermost middle Miocene), Tachilni Formation (upper Miocene), Bear Lake Formation (middle and upper Miocene), Milky River Formation 

(Pliocene), and Morzhovoi Volcanics (upper Miocene? to 

lower Quaternary?). The Stepovak Formation is further subdivided into informal (lower) siltstone and (upper) sandstone 

members. 

Several of the formations that are mainly volcanic units, 

namely the Meshik Volcanics and the Belkofski, Unga, and 

Milky River Formations, are interbedded with the sedimentary sequence and are included and described with the sedimentary rocks. 

Marine megafauna are greatly restricted in the Tertiary 

sedimentary rocks on the Alaska Peninsula because most of 

these rocks are composed of continental debris. Well-preserved megafloral specimens are abundant and diversified 

and have been used in conjunction with available megafauna 

to determine the ages of the various stratigraphic units. K-Ar 

age determinations from interlayered volcanic rocks in the 

sedimentary sections have helped to further refine the ages of 

these units. The age of the Belkofski Formation is poorly 

constrained and may be subject to change with additional 

work. 

Volcanic and intrusive rocks constitute a large part of 

the exposed stratigraphic sequence on the Alaska Peninsula. 

Most of these rocks were formed during three well-defined 

time intervals-Early to Middle Jurassic (Talkeetna Formation and Alaska-Aleutian Range batholith), late Eocene to 

early Miocene (Meshik arc = Meshik Volcanics, Popof volcanic rocks, and associated hypabyssal, intrusive, and 

volcaniclastic rocks), and late Miocene(?) to Holocene (Milky 

River Formation; Morzhovoi Volcanics; Pleistocene to Holocene volcanoes; and Devils, Agripina Bay, and similar-age 

batholiths). One group of intrusive rocks does not fit any of 

these igneous episodes. This group consists of the batholiths 

on the outer Shumagin Islands, Sanak Island, and the Semidi 

Islands. These intrusive rocks probably belong to a different 

province that was juxtaposed against the Alaska Peninsula 

after their formation. 

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Stratigraphic Framework of the Alaska Peninsula 69 
APPENDIX-LIST OF FOSSIL MEGAFAUNA 

AND MEGAFLORA COLLECTIONS 

Megafauna and megaflora are abundantly preserved in the 

Mesozoic and Tertiary rocks of the Alaska Peninsula. The nearly 

1,000 collections obtained during the present investigation have 

aided greatly in assigning ages to the rock sequences. The following 

list includes only those collections obtained from sections shown in 

figures 6 through 16. The complete list of fossils obtained from the 

central part of the Alaska Peninsula is included in reports by 

Detterman and others (1981b, 1985). Data on fossil collections 

from the Port Moller and Mount Katmai areas are being prepared for 

publication. 

Megafauna from the Kamishak Formation, reference section, 

Puale Bay (section 1; fig. 6). Fossils identified by N.J. Silberling, 

U.S. Geological Survey. Numbers are field locality numbers. 

Locality Dl 1288 

Bivalve 

Monotis subcircularis (Gabb) (early late Norian) 

Locality 79AEg5-4G 

Ammonite 

Holoritinid (Indeterminate) 

Locality 79AEg5-4D 

Bivalve 

Monotis subcircularis (Gabb) (early late Norian) 

Hydrozoan 

Heterastridium sp. 

Locality 79AEg5-3 

Bivalve 

Plicatula sp. 

Coral 

Colonial scleractinian (Indeterminate) 

Locality 79AEg5-2A 

Ammonite 

Alloclionites sp. (late middle Norian) 

Gastropod 

-(Indeterminate) 

Locality 79AEg5- 1 

Coral 

Colonial scleractinian (Indeterminate) 

Megafauna from the Talkeetna Formation, Puale Bay (section 2; 

fig. 6). Fossils identified by R.W. Imlay, U.S. Geological Survey. 

Numbers are U.S. Geological Survey Mesozoic locality numbers, 

Washington, D.C. 

Locality 32302 

Bivalve 

?Corbicellopsis sp. 

Gastropod 

(Indeterminate) 

Locality 32303 

Ammonite 

Arnioceras cf. A. densicostata (Queenstedt) 

(late early Sinemurian) 

Bivalve 

Oxytoma sp. 

Locality 3 1822 (middle Hetangian) 

Ammonites 

Discamphiceras cf. D. toxophorum (Gumbel) 

Psiloceras sp. 

Waehneroceras cf. W. portlocki (Wright) 

Locality 3 1823 (middle Hettangian) 

Ammonites 

?Discamphiceras sp. 

Waehneroceras cf. W. portlocki (Wright) 

Early Bajocian (Middle Jurassic) megafauna from the Kialagvik 

Formation, principal reference section, Wide Bay (section 3; fig. 7). 

Fossils identified by R.W. Imlay, U.S. Geological Survey. Numbers areU.S. Geological Survey Mesozoic locality numbers, Washington, D.C. 

Locality 3 1974 

Bivalve 

Inoceramus lucifer (Echwald) 

Locality 31975 

Ammonite 

Holcophylloceras sp. 

Bivalves 

Inoceramus lucifer (Echwald) 

Grammatodon sp. 

Locality 3 1976 

Ammonite 

Parabigotites crassicostatus Imlay 

Bivalves 

Inoceramus lucijer (Echwald) 

Camptonectes Sp. 

Locality 21256 

Ammonites 

Parabigotites cf. P. crassicostatus Imlay 

Bradfordia cf. B. costidensa Imlay 

Bivalve 

Inoceramus lucifer (Echwald) 

Locality 21255 

Ammonites 

Arkelloceras sp. 

Pelekodites? sp. 

Lytoceras sp. 

Macrophylloceras sp. 

Locality 21254 and 3 1978 

Ammonites 

Pseudolioceras whiteavesi (White) 

Tmetoceras kirki Westermann 

T. kirkiflenicostatum Westermann 

T. scissum (Benecke) 

Erycitoides howelli (White) 

E. spinatus Westermann 

Locality 19748 and 21 245 

Ammonites 

Erycitoides howelli (White) 

E. (Kialagvikes) kialagvikensis (White) 

E. (K.) spinatus Westermann 

E. (K.) levis Westermann 

Pseudolioceras whiteavesi (White) 

Locality 21 248 

Ammonites 

Pseudolioceras sp. 

70 Geologic Studies on the Alaska Peninsula 
Tmetoceras scissum (Benecke) 

Locality 2 1247 

Ammonites 

Tmetoceras scissum (Benecke) 

Megafauna from the Kialagvik Formation, reference section, 

hale Bay (section 4; fig. 8). Fossils identified by R.W. Imlay, U.S. 

Geological Survey. Numbers are U.S. Geological Survey Mesozoic 

locality numbers, Washington, D.C. 

Locality 3 181 8 (late early Callovian) 

Ammonite 

Cadoceras tenuicostatum (Imlay) 

Locality 32301 (early Bajocian) 

Ammonites 

Eudmetoceras cf. E. amplectens (Buckman) 

Pseudolioceras maclintocki Westermann 

Tmetoceras scissum (Benecke) 

Bivalves 

Oxytoma sp. 

Pleuromya sp. 

Locality 3 18 19 (early Bajocian) 

Bivalves 

Inoceramus lucifer (Echwald) 

Inoceramus cf. I. lucifer (Echwald) 

Pleuromya dalli (White) 

Trigonia sp. 

Locality 19804 (late Toarcian) 

Ammonite 

Haugia sp. 

Late early Callovian to early late Callovian (Middle Jurassic) 

megafauna from the Shelikof Formation, type section, hale Bay 

(section 5; fig. 8). Fossils identifiedby R.W. Imlay and J.W. Miller, 

U.S. Geological Survey. Numbers are U.S. Geological Survey 

Mesozoic locality numbers, Washington, D.C. 

Locality 32024 

Bivalve 

Meleagrinella sp. 

Locality 32032 

Ammonite 

Cadoceras (Stenocadoceras) cf. C. (S.) striatum Imlay 

Locality 32026 

Ammonite 

Cadoceras (Stenocadoceras) cf. C. (S.) striatum Imlay 

Locality 32023 

Ammonite 

Cadoceras (Stenocadoceras) sp. 

Locality 32022 

Ammonite 

Cadoceras (Stenocadoceras) cf. C. (S.) stenoloboide 

Pompeckj 

Megafauna from the Snug Harbor Siltstone Member of the 

Naknek Formation, reference section, Northeast Creek (section 8; 

fig. 9). Fossils identified by J.W. Miller, U.S. Geological Survey. 

Numbers are U.S. Geological Survey Mesozoic locality numbers, 

Menlo Park, Calif. 

Locality M7822 (late Kimmeridgian) 

Bivalves 

Buchia mosquensis (von Buch) 

Buchia rugosa (Fischer) 

Locality M7823 (late Kimmeridgian) 

Bivalves 

Buchia mosquensis (von Buch) 

Buchia rugosa (Fischer) 

Ammonite 

Lytoceras sp. 

Locality M7824 (late Kimmeridgian) 

Bivalves 

Buchia mosquensis (von Buch) 

Buchia rugosa (Fischer) 

Locality M7825 (Kimmeridgian) 

Bivalve 

Buchia rugosa (Fischer) 

Gastropod 

Trochus sp. 

Locality M7826 (early Kimmeridgian) 

Bivalves 

Buchia concentrica (Sowerby) 

Buchia rugosa (Fischer) 

Pleuromya sp. 

Locality M7827 (early Kimmeridgian) 

Bivalves 

Buchia concentrica (Sowerby) 

Buchia rugosa (Fischer) 

Entolium sp. 

Locality M7828 

Bivalves 

Nuculana sp. 

Pleuromya sp. 

Pseudolimnea sp. 

Gastropod 

Trochus sp. 

Scaphopod 

Dentalium sp. 

Locality M7829 

Bivalves 

Corbicula sp. 

Oxytoma sp. 

Locality M7830 (late Oxfordian) 

Bivalve 

Buchia concentrica (Sowerby) 

Locality M7831 (late Oxfordian) 

Bivalves 

Buchia concentrica (Sowerby) 

Oxytoma sp. 

Locality M7832 (late Oxfordian) 

Bivalves 

Buchia concentrica (Sowerby) 

Oxytoma sp. 

Placunopsis sp. 

Gastropod 

Trochus sp. 

Megafauna from the Indecision Creek Sandstone Member of the 

Naknek Formation, type section, Indecision Creek (section 9; fig. 

9). Fossils identified by J.W. Miller, U.S. Geological Survey. 

Numbers are U.S. Geological Survey Mesozoic locality numbers, 

Stratigraphic Framework of the Alaska Peninsula 71 
Menlo Park, Calif. 

Locality M7696 (middle Tithonian) 

Bivalve 

Buchia blanfordiana (Stoliczka) 

Locality M7697 (middle Tithonian) 

Bivalves 

Buchia blanfordiana (Stoliczka) 

Entolium sp. 

Nuculana sp. 

Locality M7698 (middle Tithonian) 

Bivalves 

Buchia blanfordiana (Stoliczka) 

Camptonectes sp. 

Locality M7699 (middle Tithonian) 

Bivalves 

Buchia blanfordiana (Stoliczka) 

Pholadomya sp. 

Locality M7700 (middle Tithonian) 

Bivalve 

Buchia blanfordiana (Stoliczka) 

Locality M7701 (middle Tithonian) 

Bivalves 

Buchia blanfordiana (Stoliczka) 

Pleuromya sp. 

Ammonite 

Phylloceras sp. 

Locality M7702 (middle Tithonian) 

Bivalves 

Buchia blanfordiana (Stoliczka) 

Entolium sp. 

Locality M7703 (middle Tithonian) 

Bivalves 

Buchia blanfordiana (Stoliczka) 

Entolium sp. 

Locality M7704 (early Tithonian) 

Bivalves 

Buchia blanfordiana (Stoliczka) 

Buchia mosquensis (von Buch) 

Pholadomya sp. 

Pinna sp. 

Pleuromya sp. 

Locality M7705 (early Tithonian) 

Bivalves 

Buchia mosquensis (von Buch) 

Pholadomya sp. 

Pleuromya sp. 

Locality M7706 (early Tithonian) 

Bivalves 

Buchia mosquensis (von Buch) 

Pleuromya sp. 

Locality M7707 (early Tithonian) 

Bivalve 

Buchia mosquensis (von Buch) 

Locality M7708 (early Tithonian) 

Bivalve 

Buchia mosquensis (von Buch) 

Late Jurassic megafauna from the Katolinat Conglomerate Member of the Naknek Formation, type section, east of Lake Grosvenor 

(section 10; fig. 9). Fossils identified by J.W. Miller, U.S. Geological Survey. Numbers are U.S. Geological Survey Mesozoic locality 

numbers, Menlo Park, Calif. 

Locality M8350 

Bivalve 

Buchia mosquensis (von Buch) 

Locality M835 1 

Bivalve 

Buchia mosquensis (von Buch) 

Early Cretaceous megafauna from the Sta~ukovich Formation, 

reference section, Staniukovich Mountain (section 11; fig. 10). Fossils 

identified by J.W. Miller, U.S. Geological Survey. Numbers are U.S. 

Geological Survey Mesozoic locality numbers, Menlo Park, Calif. 

Locality M8214 

Bivalves 

Inoceramus sp. (fragments) 

Pleuromya sp. 

Belemnite 

Cylindroteuthis sp. (reworked) 

Locality M8213 (Valanginian) 

Bivalve 

Buchia crassicollis solida (Lahusen) 

Locality M8212 (Berriasian) 

Bivalve 

Buchia uncitoides (Pavlow) 

Locality M8211 (Berriasian) 

Bivalve 

Buchia uncitoides (Pavlow) 

Early Cretaceous (late Albian) megafauna from the Pedmar Formation, type section, Mount Pedmar (section 13; fig. 10). Fossils identified 

by J.W. Miller, U.S. Geological Survey. Numbers are U.S. Geological 

Survey Mesozoic locality numbers, Menlo Park, Calif. 

Locality M7373 

Ammonites 

Calliphylloceras cf. C. aldersoni (Anderson) 

Marshallites cumshewaensis (Whiteaves) 

Locality M7374 

Ammonites 

Desmoceras (Pseudoughligella) dawsoni (Whiteaves) 

Turrilites(?) sp. 

Locality M7260 

Ammonites 

Desmoceras (Pseudoughligella) dawsoni (Whiteaves) 

Lytoceras sp. 

Bivalve 

Mesopuzosia sp. 

Late Cretaceous (Campanian) megafauna from the Chignik 

Formation, reference section, Chignik Lagoon (section 14; fig. 1 1). 

Fossils identified by J.W. Miller and D.L. Jones, U.S. Geological 

Survey. Numbers are U.S. Geological Survey Mesozoic locality 

numbers, Menlo Park, Calif. 

Locality M6977 

Ammonite 

Canadoceras sp. 

Bivalve 

Inoceramus sp. 

72 Geologic Studies on the Alaska Peninsula 
Locality M6976 

Ammonite 

Canadoceras sp. 

Locality M7084 

Ammonite 

Canadoceras newberryanum (Meek) 

Bivalve 

Inoceramus schmidti (Michael) 

Locality M6975 

Bivalves 

Inoceramus balticus var. kunimiensis (Nagao and Matsumoto) 

Inoceramus schmidti (Michael) 

Locality M6974 

Bivalves 

Anomia sp. 

Calva sp. 

Late Cretaceous (early Maestrichtian) megafauna from the 

Kaguyak Formation, type section, along the shore between the 

Swikshak and Big Rivers (section 16; fig. 1 1). Fossils identified by 

J.W. Miller, U.S. Geological Survey. Numbers are U.S. Geological 

Survey Mesozoic locality numbers, Menlo Park, Calif. 

Locality M7813 

Bivalve 

Inoceramus ex. gr. I. subundatus (Meek) 

Locality M7903 

Ammonites 

Didymoceras sp. 

Diplomoceras notabile (Whiteaves) 

Bivalve 

Inoceramus balticus var. kunimiensis (Nagao and Matsumoto) 

Locality M7814 

Ammonites 

Didymoceras aff. D. hornbeyense (Whiteaves) 

Diplomoceras notabile (Whiteaves) 

Gaudryceras tenuiliratum (Yabe) 

Neophylloceras ramosum (Meek) 

Pachydiscus (Pachydiscus) hazzardi Jones 

Bivalves 

~noceramus bc)lticus var. kunimiensis (Nagao and Matsumoto) 

Glycymeris sp. 

Late middle Eocene megafauna from the Tolstoi Formation, 

type section, Pavlof Bay (section 17; fig. 13). Fossils identified by 

Louie Marincovich, Jr., U.S. Geological Survey. Numbers are U.S. 

Geological Survey Cenozoic locality numbers, Menlo Park, Calif. 

Locality M8906 

Bivalves 

Microcallista (Costacallista) conradiana (Gabb) Turner 

Corbicula? sp. 

Ostrea sp. indet. 

Gastropod 

Turritella uvasana stewarti Merriam 

Locality M8382 

Gastropod 

Turritella uvasana stewarti Merriam 

Plants 

(Indeterminate) 

Locality M8380 and M8386 

Bivalves 

Solena cf. S. clarki (Weaver and Palmer) 

Corbicula sp. 

Locality M8379 and M8385 

Bivalves 

Microcallista (Costacallista) conradiana (Gabb) Turner 

Nuculana sp. 

Corbicula? sp. 

Late Paleocene megaflora from the Tolstoi Formation, reference section, Ivanof Bay (section 18; fig. 13). Fossils identified by 

J.A. Wolfe, U.S. Geological Survey. Numbers are U.S. Geological 

Survey Cenozoic locality numbers, Menlo Park, Calif. 

Locality 11416 

Acer arcticum Heer 

Alnus new species 

Cocculus new species 

Hamameidacaea new species 

Liquidambar new species 

Platycarya new species 

Protophyllum semotum Hickey 

Metasequoia occidentalis (Newberry) Chaney 

Thuites interruptus (Newberry) Bell 

Locality 1 1415 

Acer arcticum Heer 

Alnus new species 

Locality 1 141 3 

Cocculusflabela (Newberry) Wolfe 

Locality 11412 

Metasequoia occidentalis (Newberry) Chaney 

Thuites interruptus (Newberry) Bell 

Locality 1 141 1 

Chaetoptelea microphylla (Newberry) Hickey 

Dicotylophyllum richardsoni (Heer) Wolfe 

Grewiopsis auriculaecordatus (Hollick) Wolfe 

(late Paleocene) 

Protophyllum semotum Hickey 

Metasequoia occidentalis (Newberry) Chaney 

Thuites interruptus (Newberry) Bell 

Megafauna from the Stepovak Formation, reference section, 

ridge east of Chichagof Bay (section 20; fig. 14). Fossils identified 

by Louie Marincovich, Jr., U.S. Geological Survey. Numbers are 

U.S. Geological Survey Cenozoic locality numbers, Menlo Park, 

Calif. 

Locality M8352 

Bivalve 

(Indeterminate) 

Locality M835 1 

Bivalves 

Spisula callistaeformis Dall 

Unio sp.? indet. 

Acila sp. indet. 

Locality M8353 

Bivalve 

Spisula sp. 

Locality M8349 

Bivalves 

Acila sp. indet. 

?Ostrea tigiliana Slodkewitsch 

Stratigraphic Framework of the Alaska Peninsula 73 
Spisula callistaeformis Dall 

?Yoldia breweri Dall 

?Yoldia palachei Dall 

Gastropod 

Trochidae genus indet. 

Locality M8350 

Bivalves 

Acila sp. indet. 

Ostrea tigiliana Slodkewitsch 

Spisula callistaeformis Dall 

Spicula cf. S. Callistaeformis Dall 

Gastropods 

Naticidae genus indet. 

Trochidae genus indet. 

Locality M8354 

Bivalve 

Spisula sp. 

Locality M8355 

Bivalves 

?Ostrea tigiliana Slodkewitsch 

?Spisula callistaeformis Dall 

Gastropod 

Crepidula sp. 

Locality M8356 

Bivalve 

Spisula sp. indet. 

Late Miocene megafauna from the Bear Lake Formation, section 25, Mount Veniaminof (fig. 15). Fossils identified by Louie 

Marincovich, Jr., U.S. Geological Survey. Numbers are U.S. Geological Survey Cenozoic locality numbers, Menlo Park, Calif. 

Locality M7181 and M7371 

Bivalves 

Acila (Truncacila) empirensis Howe 

Acila (Truncacila) sp. 

Clinocardiurn hannibali Keen 

Cyclocardia sp. 

?Cyclocardia sp. 

Mizuhopecten mollerensis (MacNeil) 

Glycymeris sp. 

Macoma sp. 

Mya (Mya) cf. M. (M.) cuneiformis (Boehm) 

Mya (Mya) truncata Linnaeus 

Nuculana sp. 

?Pandora sp. 

Spisula sp. 

?Spisula sp. 

?Thracia 

Gastropods 

Natica sp. 

Neptunea sp. 

Turritella sp. 

Locality M7372 

Bivalves 

Clinocardium sp. 

Mizuhopecten mollerensis (MacNeil) 

Mya (Mya) cf. M. (M.) truncata Linnaeus 

Mytilus (Plicatomytilus) gratacapi Allison and Addicott 

Gastropod 

?Neptunea sp. 

Locality M7373 

Bivalves 

74 Geologic Studies on the Alaska Peninsula 

Clinocardium sp. 

Mizuhopecten mollerensis (MacNeil) 

Glycymeris sp. 

Mya (?Arenomya) aff. M. (A.) elegans (Echwald) 

?Mytilus sp. 

Locality M7182 

Bivalves 

Chione sp. 

Clinocardiurn hannibali Keen 

Mya sp. 

?Thracia sp. 

Mytilus sp. 

Gastropods 

?Colus sp. 

~e~tunea (Neptunea) sp. 

Echinoid 

Remondella waldroni (Wagner) 

Locality M7374 

Gastropod 

?Neptunea sp. 

Early late Miocene megafauna from the Tachilni Formation, 

South Walrus Peak (section 27; fig. 15). Fossils collected and 

identified by Louie Marincovich, Jr., U.S. Geological Survey. 

Numbers are U.S. Geological Survey Cenozoic locality numbers, 

Menlo Park, Calif. 

Locality M7 143 

Bivalves 

Mya (Mya) truncata Linnaeus 

Panomya izumo Nomura and Hatai 

Gastropods 

Bulbus fragilis (Leach) 

Polinices (Euspira) pallidus (Broderip) 

Neptunea (Neptunea) lyrata altispira Gabb 

Oenopota cf. 0. candida Yokoyama 

Locality M7 144 

Bivalves 

Clinocardium sp. 

Mizuhopecten mollerensis MacNeil 

Locality M7 145 and M7 146 

Bivalves 

Chlamys (Swiftopecten) cosibensis (Yokoyama) 

Clinocardiurn cf. C. ciliatum (Fabricius) 

Clinocardium hannibali (Keen) 

Clinocardium meekianium (Gabb) 

Clinocardiurn cf. C. pristinum Keen 

Cyclocardia cf. C. roundiformis Ilyina 

Glycymeris grewingki Dall 

Macoma uncongrua (von Martens) 

Musculus (Musculus) niger (Gray) 

Mya (Arenomya) elegans (Echwald) 

Periploma (Periploma) cf. P. (P.) aleutica (Krause) 

Peronidia aff. P. lutea alternidentata (Broderip and Sowerby) 

Siliqua cf. S. alta Broderip and Sowerby 

Spisula (mactromeris) brevirostrata Packard 

Spisula (mactromeris) polynyma Simpson 

Gastropods 

Beringius herleini MacNeil 

Buccinum cf. B. planeticum Dall 

Fusitriton oregonensis (Redfield) Smith 

Margarites (Papillaria) new species 

Natica (Cryptonatica) clausa Broderip and Sowerby 

Neptunea (Neptunea) aff. N. ( N.) modesta (Kuroda) 

*U.S. GOVERNMENT PRINTING OFFICE: 1996 - 774-045 / 20051 REGION NO. 8