The Togiak Basalt,

a New Formation

in Southwestern Alaska

GEOLOGICAL SURVEY BULLETIN 1482-C

The Togiak Basalt,

a New Formation

in Southwestern Alaska

By J. M. HOARE and W. L. COONRAD 

CONTRIBUTIONS TO STRATIGRAPHY

GEOLOGICAL SURVEY BULLETIN 1482-C

UNITED STATES GOVERNMENT PRINTING OFFICE: 1980
UNITED STATES DEPARTMENT OF THE INTERIOR 

CECIL D. ANDRUS, Secretary

GEOLOGICAL SURVEY 

H. William Menard, Director

Library of Congress Cataloging in Publication Data

Hoare, J. M.

The Togiak basalt. (Contributions to stratigraphy)

Geological Survey Bulletin 1482-C

Bibliography: p. C10-C11.

Supt. of Docs. No.: I 19.3:1482-C

I. Basalt-Alaska-Togiak Valley. 2. Geology, Stratigraphic-Quaternary. 

3. Geology-Alaska-Togiak Valley. I. Coonrad, W. L., joint author.

II. Title. III. Series. IV. Series: United States. Geological Survey.

Bulletin 1482-C.

QE462.B3H6 552'.26'097984 80-607886

For sale by the Superintendent of Documents, U. S. Government Printing Office 

Washington, D. C. 20402
CONTENTS

Page

Abstract __________________________________________________ Cl

Introduction _______________________________________________________ __ 1

Distribution, thickness, and stratigraphic relations __________________________ 2

Lithologic character ____________________________________ _________________ 3

Lower part _____________________________________________________________ 3

Upper part(the Togiak tuya) ______ -_________-____-____--__---_^-----_ 3

Geologic setting and origin ________________________________________________ 7

Age and correlation ________________________________________________________ 8

References cited_____ ________________________________________________________ 10

ILLUSTRATIONS

Page

FIGURE 1. Generalized geologic map of lower part of Togiak River valley,

southwestern Alaska, showing distribution of Togiak Basalt ____ C4

2. Type and reference sections of the Togiak Basalt _________________ 6

3. Map showing late Cenozoic basalt in western Alaska and some of the

major faults_______________________________^___________ 9

TABLE

TABLE 1. Chemical analyses of the Togiak Basalt

Page

C5

in

CONTRIBUTIONS TO STRATIGRAPHY

THE TOGIAK BASALT, A NEW 

FORMATION IN SOUTHWESTERN ALASKA

By J. M. HOARF. and W. L. COONRAD

XBSTRAC'I

The Togiak Basalt (new) in southwestern Alaska underlies an area of about 450 krrr 

in the lower part of the Togiak River valley. Most of this Quaternary formation is 

preserved in a graben between northeast-trending faults. It consists of tholeiite and 

alkali-olivine basalt flows and a prominent volcanic butte a tuya formed by a subglacial basaltic eruption. The tuya consists of palagonitized glassy tuff and pillow lava 

capped by glassy subaerial flows of alkali-olivine basalt. The magnetic polarity of the 

flows on the valley floor is normal. A potassium-argon age determination (0.758±0.2 

m.y.) shows that the flows erupted during the present (Brunhesl polarity epoch and are 

probably 0.55 to 0.7 million years old. The tuya is younger than the flows because it 

overlies them and erupted while the valley was filled with ice, probably during one of 

the major glacial events. Radiocarbon dating of glacial deposits indicates major glacial 

advances about 12,000 years ago and more than 39,000 years ago. The Togiak Basalt is 

petrologically similar to, and coeval with, late Cenozoic volcanic rocks in many other 

areas of western Alaska.

INTRODUCTION

The Togiak Basalt, here named for the Togiak River and Togiak 

Bay, consists of the volcanic rocks of late Cenozoic age that erupted in 

the Togiak River valley in the Goodnews Bay and Hagemeister Island 

quadrangles of southwestern Alaska. The first known reference to 

these rocks was made in 1898 by J. E. Spurr, who identified them as 

olivine basalt and assigned them a Tertiary age (Spurr, 1900, p. 140, 

231). During later reconnaissance mapping (Hoare and Coonrad, 

1961a, b), the basalt was assigned a Pliocene or early Pleistocene age 

because it was believed to have erupted after the main features of the 

valley had been cut by the Togiak River but before the valley was 

glaciated. The volcanic origin of a prominent butte that rises abruptly 

above the floor of Togiak River valley was also recognized, and the 

butte was assumed to be of the same age as, and genetically related 

to, the flows on the valley floor. During recent geologic mapping 

(Hoare and Coonrad, 1978 a, b), it became evident that the volcanic 

butte is younger than these flows and that it erupted while the valley 

vas full of glacial ice.

Cl
C2 CONTRIBUTIONS TO STRATIGRAPHY

The type section for the Togiak Basalt (figs. 1 and 2) is designated 

the exposures in the sea cliff east of Togiak Bay (SW1/4 sec. 1, T. 15 

S., R. 66 W., Hagemeister Island D-l quadrangle). The reference 

section (figs. 1 and 2) is designated the exposures in the steep gully on 

the northwest side of the prominent volcanic butte and about 1 km 

southwest of the northwest corner of the butte (SW1/4 sec. 7, T. 12 S., 

R. 64 W., Goodnews A-4 quadrangle).

Radiometric age-dating and chemical analyses of rocks of the Togiak Basalt obtained during the recent investigation confirm the late 

Cenozoic age and basaltic composition of the formation.

DISTRIBUTION, THICKNESS, AND STRATIGRAPHIC

RELATIONS

The Togiak Basalt is mostly concealed by overlying glacial deposits. It apparently underlies most of the lower part of the Togiak River 

valley east of the Togiak River as well as the coastal plain east of 

Togiak Bay and forms the prominent butte the Togiak tuya above 

the floor of the Togiak River valley. Three or more isolated flow remnants of the Togiak Basalt occur in the Nayorurun River valley 

(fig.l), and glacial erratics derived from the massive flows occur on 

the Walrus Islands at least 30 km south of the seacoast. The elevation 

of other ice-deposited basalt boulders on the mountains west of Togiak River valley suggests that they were deposited by a glacier at 

least 500 m thick.

Not included in the formation are two isolated hills near the east 

mouth of the Togiak River for which the village of Twin Hills is 

named. The hills consist of mafic and intermediate-composition intrusive and extrusive rocks that resemble an older volcanic unit of 

probable early Tertiary age, exposed on Hagemeister Island and in 

the sea cliffs about 15 km southeast of the map area.

The lower flows of the Togiak Basalt are well exposed in the sea 

cliffs east of Togiak Bay and in cutbanks along the Togiak, Pungokepuk, and Nayorurun Rivers (fig. 1). At most exposures only one 

or two flows are visible, and it seems likely that the formation is 

generally less than 20 m thick. North of the Pungokepuk River (fig. 

1), where horizontal flows form a low plateau that rises 75 to 100 m 

above the valley floor, the formation is much thicker. Evidence of the 

general thinness of the formation is a gravity profile made across the 

Togiak River valley near the seacoast. There is no appreciable difference between gravity measurements made over the basalt flows in
THE TOGIAK BASALT C3

the valley and measurements made over older rocks of Mesozoic age 

outside the valley.

In the Nayorurun Valley and at two or three localities along the 

east side of the Togiak Valley where the base of the Togiak Basalt is 

exposed, the flows rest uncomformably upon deformed sedimentary 

and volcanic rocks of Mesozoic age. The base of the formation is 

presumably exposed at one place in the sea cliffs east of Togiak Bay, 

where a massive flow overlies unconsolidated iron-stained sand and 

gravel.

LITHOLOGIC CHARACTER

LOWER PART

The Togiak Basalt consists largely of undeformed subaerial 

tholeiite and alkali-olivine basalt flows (table 1) characterized by 

well-developed columnar jointing and generally 3 m to 10 m thick 

with vesicular ropy tops. The columns are generally 1 to 2 m in 

diameter but are smaller in the finer grained flows. Most of the flows 

are gray, medium-grained, nonporphyritic rock with a loose diktytaxitic texture; some flows are dense fine-grained black rock with small 

olivine phenocrysts. The flows consist of labradorite, augite, olivine, 

and accessory magnetite. The fine-grained darker flows also have 

interstitial glass.

Most, or all, of the medium-grained flows with diktytaxitic texture 

contain many vertical or near vertical vesicle cylinders (Goff, 1976), 2 

to 6 cm in diameter and 10 to 60 cm long, of fine-grained, generally 

glassy, vesicular basalt. The walls of the vesicular cylinders are 

sharply defined in the host basalt containing sparse scattered vesicles. There is no gradation between the vesicular texture of the cylinders and the diktytaxitic texture of the host basalt. The fine-grained 

dark flows do not contain vesicle cylinders.

UPPER PART (THE TOGIAK TUYA)

In addition to the widespread flows, the Togiak Basalt forms an 

unusual and prominent butte. The shape, location, composition, and 

structure of this butte indicate that it was a subglacial volcano. Volcanoes of this type have been recognized in Iceland (Sigvaldason, 

1968, Jones, 1969, 1970) and in British Columbia, where Mathews 

(1947) defined them as "tuyas."

The Togiak tuya, 6 km by 2.5 km, is an isolated rectangular volcano that erupted from or near the Togiak fault near the east side of 

the Togiak River valley (fig. 1). It is elongate northeastward parallel 

to the regional structure and to the flow of glacial ice. The sides are 

steep to precipitous except at the southwest end, where the slope is
C4 CONTRIBUTIONS TO STRATIGRAPHY

160°00'

59°30

59'

/ GOODNEWS.BAY. . . (QUADRANGLE

' HAGEMEISTERYISLAND;. VQUADRANGLE

HAGEMEISTER ISLAND 

QUADRANGLE

10 15 20 25 KILOMETERS

10 15 MILES

FIGURE 1. Generalized geologic map of lower part of Togiak River valley, southwestern Alaska, showing location of type and reference sections and distribution of 

Togiak Basalt. Unconsolidated deposits and plutonic rocks not shown.
THE TOGIAK BASALT C5

EXPLANATION

Togiak Basalt

Qtt tuya consisting of glassy oliuine basalt flows, palagonitized basalt glass, and

pillow basalt 

Qtb, fine- and medium-grained columnar-jointed basalt flows

<

_ 0-

- o

Contact

Fault

Approximately located

Volcanic vent

e

Type section

X 

Reference Section

FIGURE 1. Continued.

TABLE 1. Chemical analyses of the Togiak Basalt

I Rapid-rock analysis by method of Shapiro 11975, p 4.3-541. Analyst' Hezekiah Smith. From Hoare and Coonrad,

1978a, p. 199]

Sample

SiO,

AloO,

Fe.,O.

FeO

MgO

CaO

Na.,0

K.,0

H.,0 H~,0+

TiO,

P..O-,

MnO

CO,

M124635W M124639W M124638W 

1 2 3

Total ___-__-_-_

51.7

15.4

1.3

8.7

8.5

8.6 

3.7 

.67 

.24 

.06 

2.0 

.36 

.12 

.03

10 1

49.5

14.7

1.0

9.6

10.0

8.0

3.8

1.0

.27

.13

2.2

.42

.13

.02

101

48.6

14.9

1.7

8.7

9.2

9.0

3.5

1.4

.93

.42

2.4

.48

.14

.03

101 ~"

1 Tholeilte from flow forming sea cliffs on east side of Togiak Bay.

2 Alkah-olivme basalt from valley floor 20 km north of tuya,

3. Alkah-ohvine basalt from capping flow on southeast rim of tuya.
C6 CONTRIBUTIONS TO STRATIGRAPHY

iiill

4- 00 *^ III*

"o I- > <° 

g "S to oc 

to

" 8

. O -r? IB <D

) . § > 00
THE TOGIAK BASALT C7

moderate. The top of the tuya, about 300 m above the valley floor, is a 

15-km2 plain that slopes a little more than 1° southwestward. The 

tuya consists of olivine basalt in the form of glassy tuff (sideromelane), fine-grained subaerial flows, and pillow basalt. The tuff 

is altered to brown palagonite containing small unaltered black 

vesicular glass fragments. The structure of the tuff indicates that it 

probably was erupted from vents forming two cones, both near the 

northeast end of the tuya. The structural apex of one cone is on or 

near the northeast rim of the tuya. The summit is capped by 40 to 50 

m of the black fine-grained, highly fractured subaerial flows that 

overlie the palagonitized tuff to the northeast and pillow basalt to the 

southwest. The only known exposure of pillow basalt is a few meters 

below the northwest rim and a few hundred meters southwest of a 

valley that opens northwestward near the center of the tuya. A more 

complete description of this tuya was published in an earlier paper 

(Hoare and Coonrad, 1978 a, p. 193-201).

GEOLOGIC SETTING AND ORIGIN

Most of the Togiak Basalt is in a graben between northeasttrending faults (fig. 1). A high west-facing scarp developed along the 

Togiak fault defines the east side of the graben; a concealed fault that 

coincides with the course of the Togiak River defines the west side. 

The concealed fault is inferred from the observation that the course of 

Togiak River is alined with the southeast side of Hagemeister Island, 

a southeast-facing fault scarp (Hoare and Coonrad, 1961b; Hoare, 

1961). In addition, contours on the aeromagnetic map of the Goodnews quadrangle (Alaska Division of Geological and Geophysical 

Surveys, 1973; Griscom, 1978) are offset along a line that coincides 

with the course of the Togiak River. The upthrown side of the fault is 

apparently on the west because deformed rocks of Mesozoic (Jurassic?) age underlie the valley west of the river but are overlain by 

horizontal basalt flows a short distance east of the river. The north 

end of the graben is probably defined by a fault trending about 

N. 70° E.

The distribution of the flows and their geologic setting suggest that 

they probably erupted from vents localized along the faults and were 

later displaced by the faults. But most of the vents have been destroyed by glacial ice or buried by glacial deposits. Only one cone 

remnant of preglacial age has been recognized, a highly oxidized, 

scoriaceous breccia that caps a small shelf projecting out from the 

east valley wall about 170 m above the valley floor 25 km north of the 

coast (fig. 1). The location of the cone remnant on the west-facing 

fault scarp suggests about 170 m of vertical uplift on the Togiak fault
C8 CONTRIBUTIONS TO STRATIGRAPHY

since the cone formed. About 9 km south of the cone remnant at the 

same elevation is a larger shelf underlain by horizontal basalt flows. 

This shelf curves around the south end of a ridge and extends a short 

distance up a tributary valley where the flows overlie deformed rocks 

of Mesozoic (Jurassic?) age.

A second probable center of volcanic activity is the plateau north of 

the Pungokepuk River. The greater thickness of the flows in this area 

suggests that they accumulated near a volcanic vent. The top of the 

plateau is covered with glacial deposits and vegetation and has not 

been examined on the ground. From the air, several lakes are visible; 

most are irregular in shape and very shallow. But one small round 

lake is deep blue and appears to be much deeper. It may be an old 

crater. The flow remnants in the Nayorurun River valley are at a 

lower elevation than the basalt plateau. They probably came from a 

volcano on top of the plateau and flowed up the Nayorurun River 

valley, where they overlie deformed sedimentary rocks of Mesozoic 

(Jurassic?) age.

The third and most prominent eruptive center is the site of the 

Togiak tuya described here.

AGE AND CORRELATION

The Togiak Basalt is Quaternary in age. The widespread flows on 

the valley floor erupted after the main surface features of the valley 

had been eroded. The radiometric age of a flow on the east side of the 

basalt plateau was determined by the potassium-argon method to be 

0.758±0.2 m.y. (J. G. Smith, written commun., 1976). The magnetic 

polarity of the flows was determined at several places and found to be 

normal. The polarity of the flows, together with their radiometric age, 

indicates that they erupted near the beginning of the present 

(Brunhes) polarity epoch, which began approximately 0.7 m.y. ago 

(Cox and others, 1966).

The tuya is clearly younger than the flows on the valley floor because it is built upon them. Because the structure and composition of

FIGURE 3. Map showing late Cenozoic basalt in western Alaska and some of the 

major faults. Localities are: 1, in the valley of the Kwethluk, Kisaralik. and 

Aniak Rivers in the Bethel quadrangle (Hoare and Coonrad, 1959a; Hoare and 

Condon, unpub. data, 1970); 2, on Nelson Island and elsewhere on the YukonKuskokwim delta (Coonrad, 1957; Hoare and Condon, 1966, 1968, 1971a); 3, on 

Nunivak Island (Hoare and others, 1968); 4, in the St. Michael area (Hoare and 

Condon, 1971b); 5, near Imuruk Lake and elsewhere on Seward Peninsula 

(Hopkins, 1963; Sainsbury, 1972, 1976); 6, on St. Lawrence Island (Patton and 

Csejtey, 1971; Hoare and Condon, unpub. data, 1966); 7. Pribilof Islands (Barth, 

1956; Cox and others, 1963); 8, Yukon River (Harrington, 1918; Hoare and 

Coonrad, 1959b).
C9

the tuya shows that it erupted beneath a glacier, it must be about the 

same age as one of the major ice advances. Several radiocarbon age

170° 165'

60° h

55'

0 100 200 300 400 500 KILOMETERS

100 200 300 MILES
CIO CONTRIBUTIONS TO STRATIGRAPHY

determinations (M. Rubin, written commun., 1977) made on glacial 

deposits 80 km southeast of the Togiak River valley indicate that the 

last two major ice advances occurred about 12,000 years ago and more 

than 39,000 years ago. It is uncertain when the tuya erupted, but it 

probably was during the older ice advance, for the northeast end and 

sides of the tuya have been strongly truncated by glacial ice.

The Togiak Basalt is about the same age and chemical composition 

as basalt erupted in several other volcanic centers in western Alaska 

and on the Bering Sea shelf. Most of the other volcanic centers 

include slightly older basalt and some of them include younger 

basalt, but all of the centers are late Cenozoic in age. In some of the 

other volcanic areas, alkali-rich basalt (basanite) occurs in minor 

amounts, generally <2 percent of the volcanic field.

Late Cenozoic basalt occurs in western Alaska at the localities 

shown on figure 3.

Numerous radiometric age determinations have been made on 

these rocks. The oldest age so far recorded, 6 m.y., comes from a 

sea-level flow at the west end of Nunivak Island (Hoare and others, 

1968, p. 385, 386). The youngest rocks in two or three eruptive centers 

are several hundred or a few thousand years old.

Most, if not all, of the late Cenozoic eruptive centers are localized 

along large faults. It seems likely that they formed during a period of 

tension and tectonic relaxation following a long period of compressional tectonics that began in early Mesozoic time and continued into 

early Cenozoic time in western Alaska.

REFERENCES CITED

Alaska Division of Geological and Geophysical Surveys, 1973, Aeromagnetic map, 

Goodnews quadrangle, Alaska: Alaska Division of Geological and Geophysical 

Surveys Open-File Report 15, 1 sheet, scale 1:250,000.

Earth, T. F. W., 1956, Geology and petrology of the Pribilof Islands, Alaska: U.S. 

Geological Survey Bulletin 1028-F, 60 p.

Coonrad, W. L., 1957, Geologic reconnaissance in the Yukon-Kuskokwim delta region, 

Alaska: U.S. Geological Survey Miscellaneous Geologic Investigations Map 1-223, 

scale 1:500,000.

Cox, A. V., Hopkins, D. M., and Dairymple, G. B., 1963, Geomagnetic polarity epochs: 

Pribilof Islands, Alaska: Geological Society of America Bulletin, v. 77, p. 883-910.

Cox, A. V., Doell, R. R., and Dalrymple, G. B., 1966, Geomagnetic polarity epochs and 

Pleistocene geochronometry: Nature, v. 198, no. 4885, p. 1049-1051.

Goff, E. F., 1976, Vesicle cylinders formed in vapor-differentiated basalt flows [abs.]: 

Geological Society of America Bulletin, v. 8, no. 6, p. 887-888.

Griscom, Andrew, 1978, Aeromagnetic map and interpretation of the Goodnews and 

Hagemeister Island quadrangles region, southwestern Alaska: U.S. Geological 

Survey Open-File Report 78-9-C, 2 sheets, scale 1:250,000, 19 p.

Harrington, G. L., 1918, The Anvik-Andreafsky region, Alaska: U.S. Geological Survey Bulletin 683, 70 p.
THE TOGIAK BASALT Cll

Hoare, J. M., 1961, Geology and tectonic setting of lower Kuskokwim-Bristol Bay 

region, Alaska: American Association of Petroleum Geologists Bulletin, v. 45, no. 

5, p. 594-611.

Hoare, J. M., and Condon, W. H., 1966, Geologic map of the Kwiguk and Black quadrangles, western Alaska: U.S. Geological Survey Miscellaneous Geologic Investigations Map 1-469, scale 1:250,000.

 1968, Geologic map of the Hooper Bay quadrangle, Alaska: U.S. Geological 

Survey Miscellaneous Geologic Investigations Map 1-523, scale 1:250,000.

 1971a, Geologic map of the Marshall quadrangle, western Alaska: U.S. Geological Survey Miscellaneous Geologic Investigations Map 1-668, scale 1:250,000. 

-197Ib, Geologic map of the St. Michael quadrangle, Alaska: U.S. Geological

Survey Miscellaneous Geologic Investigations Map 1-682, scale 1:250,000. 

Hoare, J. M., Condon, W. H., Cox, A. V., and Dalrymple, G. B., 1968, Geology,

paleomagnetism, and potassium-argon ages of basalts from Nunivak Island,

Alaska: Geological Society of America Memoir 116, p. 377-413. 

Hoare, J. M., and Coonrad, W. L., 1959a, Geologic map of the Bethel quadrangle,

Alaska: U.S. Geological Survey Miscellaneous Geologic Investigations Map 1-285,

scale 1:250,000.

 1959b, Geologic map of the Russian Mission quadrangle, Alaska: U.S. Geological Survey Miscellaneous Geologic Investigations Map 1-292, scale 1:250,000.

 196 la, 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.

 1978a, A tuya in Togiak Valley, southwest Alaska: U.S. Geological Survey 

Journal of Research, v. 6, no. 2, p. 193-201. 

-1978b, 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. 

Hopkins, D. M., 1963, Geology of the Imuruk Lake area, Seward Peninsula, Alaska:

U.S. Geological Survey Bulletin 1141-C, 101 p. 

Jones, J. G., 1969, Intraglacial volcanoes of the Laugarvatn region, southwest

Iceland 1: Geological Society of London Quarterly Journal, v. 124, p. 197-211.

 1970, Intraglacial volcanoes of the Laugarvatn region, southwest Iceland: Journal of Geology, v. 78, no. 2, p. 127-140.

Mathews, W. H., 1947, "Tuyas," flat-topped volcanoes in northern British Columbia: 

American Journal of Science, v. 245, no. 9, p. 560-570.

Patton, W. W., Jr., and Csejtey, Bela, Jr., 1971, Preliminary geologic investigations of 

western St. Lawrence Island, Alaska: U.S. Geological Survey Professional Paper 

684-C, 15 p.

Sainsbury, C. L., 1972, Geologic map of the Teller quadrangle, western Seward Peninsula, Alaska: U.S. Geological Survey Miscellaneous Geologic Investigations Map 

1-685, scale 1:250,000.

 1976, Geology, ore deposits, and mineral potential of the Seward Peninsula,

Alaska: U.S. Bureau of Mines Open-File Report 73-75, 108 p. 

Shapiro, Leonard, 1975, Rapid analysis of silicate, carbonate, and phosphate rocks (rev.

ed.): U.S. Geological Survey Bulletin 1401, 76 p. 

Sigvaldason, G. E., 1968, Structure and products of subaquatic volcanoes in Iceland:

Contributions to Mineralogy and Petrology, v. 18, p. 1-16. 

Spurr, J. E., 1900, A reconnaissance in southwestern Alaska in 1898: U.S. Geological

Survey 20th Annual Report, pt. 7, p. 31-264.
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