Summit Island Formation, a New 

Upper Cretaceous Formation in 

Southwestern Alaska

GEOLOGICAL SURVEY BULLETIN 1529-B

Summit Island Formation, a New 

Upper Cretaceous Formation in 

Southwestern Alaska

By J. M. HOARE, W. L. COONRAD, and SCOTT McCOY 

CONTRIBUTIONS TO STRATIGRAPHY

GEOLOGICAL SURVEY BULLETIN 1529-B

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1983
UNITED STATES DEPARTMENT OF THE INTERIOR 

JAMES G. WATT, Secretary

GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Library of Congress Cataloging in Publication Data

Hoare, J. M.

Summit Island Formation, a new Upper Cretaceous formation in southwestern Alaska.

(Contributions to stratigraphy) (Geological Survey Bulletin 1529-B)

Bibliography: 18 p.

Supt. of Docs. No.: I 19.3:1529-8

1. Geology, Stratigraphic Cretaceous. 2. Geology Alaska Summit Island.

I. Coonrad, W. L. II. McCoy, Scott. III. Title. IV. Series. V. Series: Geological Survey Bulletin 1529-B.

QE75.B9 no. 1529-B 557.3s[551.7'7] 81-607195 AACR2

[QE688]

For sale by the Branch of Distribution, U.S. Geological Survey, 

604 South Pickett Street, Alexandria, VA 22304
CONTENTS

Abstract ....................................... 1

Introduction ..................................... 1

Distribution, structural relations, and thickness ................. 3

Lithologic character ................................. 10

Age and correlation ................................. 16

References cited ...........................'........ 18

ILLUSTRATIONS

FIGURE 1. Map of parts of the Hagemeister Island D-l and Nushagak Bay 

D-6 quadrangles, Alaska, showing location of type section and 

reference section of the Summit Island Formation .... 2

2. Generalized geologic map showing distribution of the Summit Island Formation ........................ 4

3-6. Photographs showing:

3. Summit Island Formation on Summit Island ......... 6

4. Sheared magnesite and dolomite vein in fault, southeast end Summit Island .......................... 7

5. Mostly fine-grained rocks in reference section of the Summit Island Formation on unnamed headland northeast of Summit 

Island ............................ 9

6. Summit Island Formation 6 km northwest of Right Hand Point. 10

7. Type and reference sections of the Summit Island Formation . 12

8. Photograph showing dikes and sills intruding the Summit Island

Formation on mainland 10 km northwest of Right Hand point 15

III

CONTRIBUTIONS TO STRATIGRAPHY

SUMMIT ISLAND FORMATION, A NEW

UPPER CRETACEOUS FORMATION IN

SOUTHWESTERN ALASKA

J. M. HOARE, W. L. COONRAD, and SCOTT McCov

ABSTRACT

The Summit Island Formation (new) in southwestern Alaska consists of nonmarine 

strata that were previously included in the Gemuk Group and in an unnamed unit 

thought to include strata of Cretaceous and early Tertiary age. The Summit Island 

Formation consists of thick-bedded conglomerate, sandstone, siltstone, and carbonaceous mudstone in which there is much carbonized plant material and a few thin coal 

seams. No complete section is exposed; the base and the thickest known section, 

about 850 m, are exposed in the sea cliffs on the northwest end of Summit Island. 

The top and about 160 m of section are exposed on the mainland northeast of Summit 

Island. The beds are gently folded and cut by several faults. The formation apparently 

rests with angular unconformity on highly deformed older Mesozoic rocks and is unconformably overlain by Tertiary volcanic rocks. Palynomorphs, fossil leaves, and the 

radiometric age of a dike that cuts the Summit Island Formation indicate a Late 

Cretaceous, probably Maestrichtian, age.

INTRODUCTION

Summit Island is in Togiak Bay, in the Hagemeister Island D-l 

quadrangle in southwestern Alaska (fig. 1). The island, 4.5 km long 

and .5-1.5 km wide, is elongated northwestward and shaped something 

like a dumbbell. Much of the island is underlain by moderately deformed nonmarine strata consisting of intertonguing beds of conglomerate, sandstone, siltstone, and carbonaceous mudstone of Late Cretaceous age. These rocks are here named the Summit Island Formation 

for exposures on Summit Island.

The type section for the Summit Island Formation (fig. 1), which 

includes the base and overlying 850 m, is here designated as the exposures in the sea cliffs on the northwest end of Summit Island, sec. 

25, T. 15 S., R. 66 W., Hagemeister Island D-l quadrangle. A section 

that includes the top and uppermost part of the formation is exposed 

in the sea cliffs on the east side of an unnamed headland on the mainland 6 km northeast of Summit Island, SE1A sec. 15, NE1A sec. 22, 

T. 15 S., R. 65 W., Hagemeister Island D-l quadrangle; this section

Bl
B2 CONTRIBUTIONS TO STRATIGRAPHY

160W

FIGURE 1. Parts of Hagemeister Island D-l and Nushagak Bay D-6 quadrangles, 

Alaska, showing location of type section and reference section of the Summit Island 

Formation.

is here designated a reference section. The total exposed thickness 

of Summit Island Formation is 1,000-1,200 m; a few hundred meters 

of additional section may be concealed by water northwest of Summit 

Island.

During earlier reconnaissance mapping (Hoare and Coonrad, 1961a, 

b), some of these strata were either included in an unnamed map 

unit thought to contain rocks of Cretaceous and Tertiary ages or were 

assigned to the upper part of the Gemuk Group (Carboniferous to 

Cretaceous). New fossil evidence and more detailed mapping (Hoare 

and Coonrad, 1978) showed that the rocks are of Late Cretaceous, 

probably Maestrichtian, age.
SUMMIT ISLAND FORMATION, ALASKA B3

DISTRIBUTION, STRUCTURAL RELATIONS, 

AND THICKNESS

The distribution of the Summit Island Formation in the Goodnews 

A-3, Nushagak Bay D-6, and Hagemeister Island D-l quadrangles 

is shown in figure 2. The Summit Island Formation presumably rests 

with angular unconformity upon older more highly deformed rocks 

of Mesozoic age, but no depositional contact is exposed. At three or 

four localities, massive beds of pebble-cobble conglomerate that constitute the base of the Summit Island Formation are in fault contact 

with the older rocks. Gently folded Tertiary volcanic rocks overlie 

the Summit Island Formation. The contact is apparently an angular 

unconformity because the Summit Island Formation was folded and 

eroded before the volcanic rocks were deposited and the volcanic rocks 

also overlie the older Mesozoic rocks. Little or no angular discordance, 

however, can be detected where the contact between the Summit Island and the overlying volcanic rocks is exposed because both units 

dip gently.

The bulbous ends of Summit Island are on the limbs of an asymmetrical anticline that trends northeastward across the island. Strongly 

deformed rocks of Jurassic age the graywacke of Kulukak Bay are 

exposed in the core of the anticline and are juxtaposed with the Summit Island Formation on the northwest and southeast limbs of the 

anticline along two northeast-trending faults that dip 70°-80° SE. The 

amount and direction of movement of the two faults are not known 

but may reflect slippage on the angular unconformity between overlying Summit Island Formation and the underlying more competent 

graywacke of Kulukak Bay. In the northwest limb of the anticline, 

the Summit Island Formation dips 45°-80° NW. (fig. 3A), and about 

850 m of section is exposed in the sea cliffs. In the southeast limb 

of the anticline, the Summit Island Formation dips 10°-15° SE. (fig. 

3£) and is overlain by pyroclastic volcanic rocks and intruded by many 

mafic sills. A vertical fault trending northwest, too small to show 

on the geologic map, truncates the beds near the southeast end of 

the island, juxtaposing a pile of mafic sills with the Summit Island 

Formation. The downthrown side of the fault is on the northeast. 

Defined by a vertical white vein of magnesite and dolomite several 

meters thick, the fault is highly visible (fig. 4).

The anticline and northeast-trending faults on Summit Island can 

be recognized on the mainland to the northeast. The Summit Island 

Formation forms the sea cliffs on the west and east sides of the unnamed headland 3-6 km northeast of Summit Island, and the 

graywacke of Kulukak Bay forms the cliffs on the southwest end of 

the headland. The rocks on the west side of the headland resemble
B4 CONTRIBUTIONS TO STRATIGRAPHY

160°00'

10 KILOMETERS

FIGURE 2. Generalized geologic map showing distribution of Summit Island Formation 

in Goodnews A-3, Hagemeister Island D-l, and Nushagak Bay D-6 quadrangles, 

Alaska.
SUMMIT ISLAND FORMATION, ALASKA B5

EXPLANATION

Graywacke of Buchia Ridge

Volcanic and sedimentary rocks

Graywacke of Kulukak Bay

Contact 

D

u

Fault

Dashed where approximately 

located; dotted where concealed or inferred. U, upthrown 

side; D, downthrown side

QUATERNARY

TERTIARY

Upper 

Cretaceous

Lower 

Cretaceous

CRETACEOUS

CRETACEOUS

AND 

JURASSIC

JURASSIC

Thrust fault

Dotted where concealed or inferred. 

Sawteeth on upper plate

Fossil locality
B6 CONTRIBUTIONS TO STRATIGRAPHY

 ~ V* 1

FIGURE 3. Summit Island Formation on Summit Island. A, Steeply dipping siltstone 

and sandstone beds with pebble lenses, at type section of Summit Island Formation, 

northwest end Summit Island. Dip of bedding is shown by alinement of flowering 

plants that grow from bedding-plane fractures extending from upper left to lower 

right. View southwest. B, Gently dipping conglomerate, sandstone, and siltstone 

of Summit Island Formation, southeast end Summit Island. View northwest.
SUMMIT ISLAND FORMATION, ALASKA B7

FIGURE 3. Continued

FIGURE 4. Sheared vein of magnesite and dolomite in vertical fault between massive 

mafic Tertiary sills (left) and Summit Island Formation (right), southeast end of 

Summit Island. View south.
B8 CONTRIBUTIONS TO STRATIGRAPHY

those in the type section on Summit Island. They dip 25°-75° NW. 

and are chiefly massive pebble-cobble conglomerate and sandstone 

with some interbedded siltstone and mudstone. These rocks are 

faulted out on the east side of the headland, where the Summit Island 

Formation dips gently southeast and consists of at least 160 m of 

carbonaceous mudstone, siltstone, and shale with some interbedded 

sandstone and pebbly sandstone (figs. 5A, 5#).

The contact between the Summit Island Formation and overlying 

Tertiary volcanic rocks is exposed on the east side of the headland 

where coarse volcanic breccia overlies the gently dipping sedimentary 

rocks. The structure east of the unnamed headland is probably that 

of a broad, shallow syncline in which the Summit Island Formation 

and the graywacke of Kulukak Bay are juxtaposed by northeast-trending faults at three or four places. In the outcrop area about 10 km 

northwest of Right Hand Point (fig. 2), the Summit Island Formation 

dips 20° westward. Here it consists of about 40 m of soft massive 

carbonaceous mudstone with some sandstone beds overlain by a thick 

felsic sill.

The easternmost coastal exposure of Summit Island Formation, 

about 6 km northwest of Right Hand Point (fig. 2), consists of about 

50 m of massive sandy pebble-cobble conglomerate (fig. 6) and a few 

meters of soft carbonaceous siltstone, shale, and sandstone in which 

there are one or two thin coal seams. The rocks dip 25° NW. and 

form the sea cliffs for a distance of 100-150 m. They are overlain 

and flanked on either side by Tertiary volcanic rocks. The thickness 

and massive character of the conglomerate suggests that it may be 

near the base of the Summit Island Formation. If it is, this exposure 

of the Summit Island Formation is probably near an anticlinal axis 

or in an uplifted fault block that was truncated by erosion before 

the Tertiary volcanic rocks were deposited.

The distribution of the Summit Island Formation north of the coast 

(fig. 2) is rather poorly known because it is largely restricted to lowlying areas mantled by unconsolidated deposits. However, it clearly 

extends inland up the valley of the Ungalikthluk River, which is 

bounded on either side by faults and is structurally low. A section 

of gently dipping soft black shale that flanks Buchia Ridge on its 

northwest side probably is part of the Summit Island Formation. To 

the southeast, the shale is juxtaposed with much harder sedimentary 

rocks of the graywacke of Buchia Ridge of Early Cretaceous age along 

a southeast-dipping reverse fault. The south end of the ridge on the 

west side of the Ungalikthluk River valley is underlain by interbedded 

conglomerate, sandstone, and shale that dip gently southward and 

contain abundant carbonized plant debris. These rocks are assigned 

to the Summit Island Formation. Similar rocks that crop out along
SUMMIT ISLAND FORMATION, ALASKA B9

FIGURE 5. Carbonaceous mudstone, siltstone, shale, and sandstone in reference section 

of Summit Island Formation on east side of unnamed headland on mainland, 6 

km northeast of Summit Island (figs. 1, 2). A, North view of nearly flat lying 

Summit Island Formation exposed in central part of reference section. B, Sea-cliff 

exposures at north end of reference section. Plant fossils occur at base of cliff 

near center of photograph. Boulders in foreground from a large rhyolite sill. Beds 

dip eastward; view north.
BIO CONTRIBUTIONS TO STRATIGRAPHY

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FIGURE 6. Summit Island Formation on mainland 6 km northwest of Right Hand 

Point (fig. 2). Rock is chiefly massive sandy pebble-cobble conglomerate. Man at 

base of cliff provides scale. Beds dip northwest; view north.

the east side of the Ungalikthluk River valley southeast of Buchia 

Ridge juxtaposed with volcanic and sedimentary rocks of Jurassic and 

Early Cretaceous age by a southeast-dipping reverse fault are also 

assigned to the Summit Island Formation.

LITHOLOGIC CHARACTER

The Summit Island Formation consists of lenses and intertonguing 

beds of conglomerate, sandstone, carbonaceous siltstone, mudstone, 

and shale. The discontinuous character of the bedding is best seen 

in the lower part of the formation, where thick conglomerate beds 

commonly grade laterally and (or) vertically into pebbly sandstone 

and sandstone. Bedding in the Summit Island Formation tends to be 

thick to massive. Sandstone and conglomerate beds 1-3 m thick are 

common (fig. 3B) and some conglomerate beds are much thicker. Carbonaceous siltstone and mudstone in beds 1-3 m thick are also common, particularly in the upper part of the formation (figs. 5A, 5£). 

Beds and lenses of pebbly sandstone occur throughout the Summit 

Island Formation, but in general the formation becomes finer grained 

upward.

At its type section on the north end of Summit Island, the Summit 

Island Formation (fig. 1) consists of about 200 m of massive and thickbedded pebble- cobble conglomerate overlain by about 650 m of inter-
SUMMIT ISLAND FORMATION, ALASKA Bll

bedded sandstone, siltstone, and mudstone and a few conglomerate 

beds. At its reference section, about 6 km northeast of Summit Island, 

the formation (figs. 5A, 55) is about 75 percent carbonaceous 

mudstone, siltstone, and shale and 25 percent interbedded sandstone 

and pebble grit (fig. 7). About 6 km farther east along the seacoast, 

the uppermost part of the Summit Island Formation consists of about 

40 m of soft carbonaceous mudstone capped by a massive felsic sill 

(fig. 8).

Conglomerate clasts are well rounded, generally no larger than 10 

cm in diameter, and are mostly derived from flows, tuffs, and sedimentary rocks of Jurassic and Early Cretaceous age. The conglomerate 

also contains white quartz and sparse schist and plutonic clasts. The 

schist clasts probably derive from tectonically metamorphosed rocks 

that occur locally along some of the larger faults. The Summit Island 

Formation consists of rocks that are mostly various shades of gray 

except for the highly carbonaceous rocks, which are black. Sandstone 

and conglomerate beds are commonly brown on weathered surfaces. 

By its color, texture, and composition, the sandstone is graywacke. 

Sandstone and siltstone beds are made up largely of lithic fragments 

(60-70 percent) and lesser amounts of quartz and feldspar (commonly 

10-15 percent). The lithic fragments are chiefly very fine grained 

cherty tuff and other volcanic rocks, "argillite," quartzite, and chert. 

Quartz appears to be more abundant in sandstone and siltstone than 

in conglomerate. Most feldspar is twinned plagioclase. There is a little 

secondary calcite on some of the feldspar, but the rocks are essentially 

noncalcareous. The cementing material is a claylike paste. The rocks 

are fairly hard but not tough; under the hammer, they generally fracture around the larger rounded grains. The hardness of the rocks 

appears to vary with their structure. On the northwest end of Summit 

Island and on the west side of the unnamed headland northeast of 

Summit Island, the rocks dip 45°-80° and are relatively hard and well 

indurated; on the southeast end of Summit Island and elsewhere on 

the mainland where the dip angle is low, the rocks are not so hard.

The general character of the Summit Island Formation suggests 

that it was probably deposited by shifting streams on a coastal plain 

that was underlain by previously deformed rocks of earlier Mesozoic 

age. The nonmarine character of the rocks is indicated by the occurrence of large-scale crossbedding, abundant ripple marks, lensing conglomerate beds, cut-and-fill structures, leaf impressions, abundant carbonized plant remains, carbonaceous mudstone and shale with coal 

seams, and the apparent absence of marine fossils.

A large number of dikes and sills intrude the Summit Island Formation. Dikes and sills also intrude the older Mesozoic rocks nearby but 

they appear to be much more numerous in the Summit Island Forma-
B12 CONTRIBUTIONS TO STRATIGRAPHY

REFERENCE SECTION

THICKNESS METERS IN 

200-

150- 

100- 

50-

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Summit Formation Island 

LITHOLOGY

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DESCRIPTION

 Unconformity (angular?) 

Mudstone; some beds massive and resistant, 

other beds fractured and nonresistant

Medium-grained sandstone and sandy grit

Mudstone and siltstone

^ Thick-bedded gray sandstone

\Dike or sill

Mudstone with some siltstone and claystone

/ Dike or sill

Interbedded mudstone, shale, and sandstone; 

pebbly sandstone at base

/Dike or sill

Interbedded mudstone, sandstone, and shale

/ Dike or sill

Black carbonaceous mudstone; a few sandstone 

and siltstone beds; more sandy near base; 

abundant carbonaceous plant fragments; a few 

lenses and beds of black shale

Medium-grained poorly bedded sandstone grit 

lenses

Grit and sandy grit 

, Dark-gray shaly mudstone with sandstone 

\ stringers

Covered interval

FIGURE 1. Sections of the Upper Cretaceous Summit Island Formation in southwestern Alaska. Location of sections shown on figure 1.
SUMMIT ISLAND FORMATION, ALASKA B13

TYPE SECTION

THICKNESS IN METERS

900-

800-

700-

600-

500-

UNIT

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LITHOLOGY

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DESCRIPTION

Covered interval

Massive carbonaceous siltstone and very fine 

grained sandstone; ripple laminated, good 

ripple marks; many dikes and sills

Siltstone with thin sandstone beds; thin coal 

seam; many dikes and sills

^Channeled pebbly sandstone and conglomerate

Blocky massive silty mudstone; plant stems; 

many dikes and sills

Covered interval

Siltstone and minor sandstone; plant stems

/Fine-grained sandstone with pebble lenses

//Massive siltstone, rare sandstone, conglomerate

/ with plant stems

Hard calcareous mudstone with thin sandstone 

beds containing shale pebbles and plant stems; 

load structures and ripple marks

Continued

FIGURE 1. Continued
B14 CONTRIBUTIONS TO STRATIGRAPHY

(/) CC 

LU LU

HI UNIT LITHOLOGY DESCRIPTION

400-

300-

200-

100

-0-

Hard poorly sorted crossbedded sandstone and

conglomerate; cut-and-fill structure; flat 

\ shale-chip zones

Hard ripple-laminated calcareous mudstone; 

sandstone lenses; wood imprints

o'.o'.o'.o: bvo'.o'.ovo;

Pebble-cobble conglomerate with sandstone 

lenses; large crossbeds

Interbedded siltstone and sandstone, ripple 

laminated; many dikes and sills

Poorly sorted sandstone and conglomerate; large 

crossbeds; many channels___________

Thin- and thick-bedded calcareous siltstone and

mudstone; plant material; pyrite concretion; 

\ many dikes and sills

Interbedded carbonaceous mudstone and 

\ sandstone; dikes and sills

Massive sandstone, granule conglomerate;

cut-and-fill structure; large crossbeds; wood 

v imprints

Hard interbedded sandstone, siltstone, and 

carbonaceous mudstone; many dikes and sills

Massive poorly sorted pebble-cobble conglomerate; channel red; clasts include volcanic 

rock, argillite, metamorphic rock, sandstone, chert, and rare quartz

Dike cutting Summit Island Formation

^Conglomerate

\Dike cutting Summit Island Formation

Massive poorly sorted pebble-cobble conglomerate; channeled; clasts include volcanic 

rock, chert, metamorphic rock, argillite, 

and rare quartz

Fault intruded by rhyolite dike

FIGURE 7. Continued

tion. Sea cliffs on the mainland that expose the gently dipping darkcolored rocks of the Summit Island Formation are crisscrossed by
SUMMIT ISLAND FORMATION, ALASKA B15

a maze of near-horizontal sills and near-vertical dikes (fig. 8). These 

intrusive rocks are highly visible because they are harder and lighter 

colored than the enclosing sedimentary rocks. On Summit Island, however, the intrusive rocks are generally more difficult to recognize because they are mostly dark-colored sills that are concordant with, and 

about the same color as, the enclosing sedimentary rocks.

The composition of the intrusive rocks ranges from fine-grained 

dark-colored basaltic and andesitic rocks to light-colored quartz and 

orthoclase porphyries of rhyolitic and dacitic composition. Most of the 

dikes and sills are a few centimeters to several meters thick, but 

at least two light-colored rhyolitic bodies are much larger. One of 

these sills, probably 50-150 m thick and at least 8 km2 in area, caps 

a high sea cliff on the mainland 12 km northeast of Summit Island, 

apparently fed by a felsic dike that cuts up through the underlying 

Summit Island Formation (fig. 8).

Radiometric dating shows that the dikes and sills that intrude the 

Summit Island Formation are both early and late Tertiary age. Biotite 

from one of the large rhyolitic sills on the mainland gave a potassiumargon age of about 13 m.y., biotite from a mafic dike an age of 64 ±2 

m.y. (Hoare and Coonrad, 1978, table 1, samples B and D).

FIGURE 8. Dikes and sills intruding the Summit Island Formation on mainland 10 

km northwest of Right Hand Point (fig. 2). Light-colored rocks in sea cliff are 

rhyolitic and basaltic dikes and sills. Height of cliff, about 100 m. Fault in gully 

to right juxtaposes Jurassic graywacke of Kulukak Bay (foreground) with Summit 

Island Formation (background). View northwest.
B16 CONTRIBUTIONS TO STRATIGRAPHY

AGE AND CORRELATION

Identifiable plant fossils were found near the base of the reference 

section for the Summit Island Formation on the east side of the unnamed headland on the mainland 6 km northeast of Summit Island, 

SEV4 sec. 15, T. 15 S., R. 65 W., Hagemeister Island D-l quadrangle. 

R. W. Brown (written commun., 1952) identified the following forms:

Metasequoia occidentalis (Newberry) Chancy

Populus zaddachi Heer?

Cercidiphylum arcticum (Heer) Brown

Fraximus inordinate, Hollick

Viburnum contortum Lesqrereux

Brown stated: "The foregoing identifications are chiefly in accordance 

with those of Hollick in USGS Professional Paper 182 and in some 

instances are not the names I would now use in publication. The material is somewhat fragmentary and my age assignment of early Tertiary 

should, therefore, be considered tentative."

Jack Wolfe, after examining a second collection from the same locality, stated (oral commun., 1974) that the fossil plants could be either 

Late Cretaceous or early Tertiary in age. A Late Cretaceous age 

is more probable on the basis of the radiometric age of 64 ±2 m.y. 

(Hoare and Coonrad, 1978, table 1, sample D) obtained on a mafic 

dike that intrudes the formation.

The best fossil evidence of the age of the Summit Island Formation 

is provided by palynomorphs (loc. No. D4733) collected in a small 

cutbank exposure on the Ungalikthluk River near the south edge of 

the Goodnews A-3 quadrangle, sec. 31, T. 13 S., R. 64 W. (fig. 2). 

The fossils occur in about 15 m of carbonaceous mudstone and siltstone 

containing coaly seams. R. H. Tschudy identified the following 

palynomorphs (written commun., 1975):

Expressipollis accuratus

Inoperturopollenites

Stereisporites cf. S. princeps

Stereisporites sp.

Cicatricosisporites

Hamulatisporis

aff. Liliacidites (Schizoporis) complexus

Osmundacidites

Alisporites grandisl

Lycopodiumsporites

Ovoidites

Tsugaepollenites

Gleicheniidites

Tricolpites

Aquilapollenites saginatusl
SUMMIT ISLAND FORMATION, ALASKA BIT

Aquilapollenites sp. 

Azonia pulchella 

Tschudy stated:

"This assemblage is unlike any Cretaceous assemblage from the 

United States or from southern Canada. Several of the taxa have 

been reported previously from Arctic Canada (Ellef Ringnes Island) 

and from the Ust-Yenisey botanical-geographical subprovince of 

Samoilovich in the western Siberian lowlands. The assemblage does 

not correlate exactly with plant microfossil assemblages from either 

region, but no other reference material from Arctic Canada or Alaska 

yielding assemblages even remotely resembling the assemblage from 

this sample is available. The presence of specimens Expressipollis 

accuratus, Aquilapollenites, aff. Liliacidites complexus, and Azonia 

pulchella almost certainly indicates a Late Cretaceous age. I strongly 

suspect that this sample represents the Maestrichtian, but in the absence of adequate well-dated reference material, I am reluctant to 

assign this sample to a definite Late Cretaceous stage."

The fossil evidence indicates that the Summit Island Formation is 

Late Cretaceous, probably Maestrichtian, in age.

The Summit Island Formation may be correlative with the rocks 

that underlie the interior lowland of Hagemeister Island about 40 km 

southwest of Summit Island. These rocks consist of interbedded sandstone, siltstone, and shale containing abundant fine carbonized plant 

debris and a few thin coal seams. Massive carbonaceous mudstone 

and siltstone and thick beds of pebble-cobble conglomerate such as 

occur on Summit Island are either absent or covered by unconsolidated 

deposits. The sandstone and siltstone generally contain much primary 

and secondary calcium carbonate and chlorite, neither of which is 

characteristic of the Summit Island Formation.

These rocks underlie a belt 2-3 km wide that extends from the 

south coast of Hagemeister Island northward to, or nearly to, the 

northwest coast. Except for one or two minor reversals, the beds 

dip 60°N-85° eastward. If not duplicated by faults, the section is at 

least 1,000-1,500 m thick. The belt is apparently bounded on the west 

and east by two steep southeast-dipping reverse(?) faults that juxtapose older volcanic and sedimentary rocks of earlier Mesozoic age.

Lithologically, the Hagemeister Island section resembles the Lower 

and Upper Cretaceous Kuskokwim Group (Cady and others, 1955) 

more than the Summit Island Formation. But the nearest exposed 

rocks of the Kuskokwim Group are almost 100 km north, whereas 

Summit Island is only 40 km northeast. At this time, the assignment 

of the Hagemeister Island section is uncertain.

The only diagnostic fossil in the Hagemeister Island section is 

Metasequoia, which ranges in age from Early Cretaceous (Albian) into 

the Tertiary (Jack Wolfe, written commun., 1976); Metasequoia also
B18 CONTRIBUTIONS TO STRATIGRAPHY

occurs in the Summit Island Formation on the mainland. The relative 

nearness of Summit Island and the possible Late Cretaceous age of 

the Hagemeister Island section suggest a possible correlation with 

the Summit Island Formation, but if the same rocks were exposed 

150 km north, they would undoubtedly be mapped as part of the Kuskokwim Group.

Other possibly correlative rocks occur on the Alaska Peninsula 300 

km south and southeast, where strata of Late Cretaceous age are 

widespread. According to Burk (1965), the Chignik Formation of Campanian age and the overlying Hoodoo Formation of late Campanian 

and Maestrichtian age constitute a transgressive sequence grading upward from nonmarine to marine strata. The probable Maestrichtian 

age and the character of the Summit Island Formation suggest that 

it may be the nonmarine equivalent of Burk's Hoodoo Formation.

REFERENCES CITED

Burk, C. A., 1965, Geology of the Alaska Peninsula Island arc and continental margin

(Pt. 1): Geological Society of American Memoir 99, 250 p. 

Cady, W. M., Wallace, R. E., Hoare, J. M., and Webber, E. J., 1955, The central

Kuskokwim region, Alaska: U.S. Geological Survey Professional Paper 268, 132

PHoare, J. M., and Coonrad, W. L., 1961a, Geologic map of the Goodnews quadrangle, 

Alaska: U.S. Geological Survey Miscellaneous Geologic Investigations Map 1-339, 

scale 1:250,000.

 1961b, Geologic map of the Hagemeister Island quadrangle, Alaska: U.S. 

Geological Survey Miscellaneous Geologic Investigations Map 1-321, scale 

1:250,000.

 1978, Geologic map of the Goodnews and Hagemeister Island quadrangles region, southwestern Alaska: U.S. Geological Survey Open-File Report 78-9-B, scale 

1:250,000, 2 sheets.

U.S. GOVERNMENT PRINTING OFFICE: 676-041/24-1983