Graywacke of Buchia Ridge and 

Correlative Lower Cretaceous Rocks 

in the Goodnews Bay and Bethel 

Quadrangles, Southwestern Alaska

CONTRIBUTIONS TO STRATIGRAPHY 

GEOLOGICAL SURVEY BULLETIN 1529-C

Graywacke of Buchia Ridge and 

Correlative Lower Cretaceous Rocks 

in the Goodnews Bay and Bethel 

Quadrangles, Southwestern Alaska

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

CONTRIBUTIONS TO STRATIGRAPHY

GEOLOGICAL SURVEY BULLETIN 1529-C

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.

Graywacke of Buchia Ridge and Correlative Lower Cretaceous Rocks in the Goodnews and Bethel Quadrangles, Southwestern , 

(Geological Survey Bulletin 1529-C Contributions to Stratigraphy) 

Bibliography: 17 p. 

Supl. of Docs. No.: I 19.3:1529-C

1. Graywacke Alaska. 2. Geology, Stratigraphic Cretaceous.

I. Coonrad, W. L. II. Title. III. Series: Geological Survey Bulletin 1529-C IV. Series: Contributions to Stratigraph

QE75.B9 No. 1529-C [QE471.15.S25] 557.3s 

81-607996 [552'.5] AACR2
CONTENTS

Page

Abstract ....................................... j

Introduction ..................................... 2

Buchia Ridge belt .................................. 3

Graywacke of Buchia Ridge .......................... g

Age ....................................... 9

Ungalikthluk belt .................................. 9

Mount Gratia belt .................................. 13

Eek Mountains belt ................................. 15

Other correlative(?) rocks .............................. jy

References cited ...................................

ILLUSTRATIONS

Page

FIGURE 1. Map showing location of four belts of correlative Lower Cretaceous

rocks in the Goodnews Bay and Bethel quadrangles ......... 3

2. Geologic map of parts of Goodnews Bay A-2, A-3, B-2, and B-3

quadrangles showing location of section A-B. ............ 4

3. Map of parts of Goodnews Bay A-2, A-3, B-2, and B-3 quadrangles 

showing location of composite section of lower part of graywacke 

of Buchia Ridge, representative outcrops of upper part of 

graywacke of Buchia Ridge, and measured section of correlative rocks 

in Ungalikthluk belt ......................... y

4. Cross section showing structural relations of graywacke of Buchia

Ridge and correlative rocks in Ungalikthluk belt ........ g

5. Composite measured section of lower part of graywacke of Buchia 

Ridge and representative section of correlative rocks of 

Ungalikthluk belt ...........................

TABLE

TABLE 1. Fossils from the graywacke of Buchia Ridge and correlative rocks 

in the Goodnews Bay and Bethel quadrangles .........

Page

12

in

CONTRIBUTIONS TO STRATIGRAPHY

GRAYWACKE OF BUCHIA RIDGE AND

CORRELATIVE LOWER CRETACEOUS

ROCKS IN THE GOODNEWS AND

BETHEL QUADRANGLES,

SOUTHWESTERN ALASKA

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

ABSTRACT

Lithologically distinct coeval strata of Lower Cretaceous age occur in four northeasttrending belts in the Goodnews Bay and Bethel quadrangles, southwestern Alaska. 

The belts are, from southeast to northwest, the Buchia Ridge belt, Ungalikthluk belt, 

Mount Gratia belt, and Eek Mountains belt. The belts are largely bounded by thrust 

faults. Rocks in the Mount Gratia and Eek Mountains belts are stongly deformed and 

infolded with older rocks, whereas the Ungalikthluk and Buchia Ridge belts are mildly 

deformed and (except for intrusive rocks of Tertiary age) include only rocks of Early 

Cretaceous age.

The Buchia Ridge belt is composed mainly of graywacke and related rocks defined 

as the graywacke of Buchia Ridge. This formation is exposed in a wedge-shaped area 

of about 350 km2 in the Goodnews Bay A-2, A-3, B-2, and B-3 quadrangles. The 

beds of this unit are deformed into a southeast-dipping homocline in a thrust sheet 

that is underlain and overlain by highly deformed older volcanic and sedimentary strata 

of Jurassic and Early Cretaceous(?) age. The graywacke of Buchia Ridge is estimated 

to be about 5,000 m thick. The lower part of the formation is thick-bedded graywacke, 

siltstone, and conglomerate; the upper part shale, shaly siltstone, and thin-bedded 

graywacke and calcarenite. The lower part contains abundant Buchia of Valanginian 

age; the upper part sparse Inoceramus and Belemnitesof Hauterivian age. If the section 

is not repeated by faults or folds, it is the thickest, least deformed section of Lower 

Cretaceous sedimentary rocks known in southwestern Alaska.

The Ungalikthluk belt consists of isolated synclinal outcrop areas of limestone, 

greenish limy grit, and conglomerate overlain by noncalcareous graywacke and grit. 

Although the total outcrop area is no more that 4 km2 , these erosional remnants form 

a distinct belt northwest of the Buchia Ridge belt.

Locally, Buchia crassicolis is found in the limestone, indicating that the Ungalikthluk 

sequence is coeval with the graywacke of Buchia Ridge.

The Mount Gratia belt, up to 25 km wide and extending more than 120 km northeast 

from the central part of the Goodnews Bay B-5 quadrangle, is bounded on the southeast 

and northwest sides by southeast-dipping reverse faults. Rocks within the belt are 

strongly folded and cut by many faults.

1
2 CONTRIBUTIONS TO STRATIGRAPHY

Stratigraphy in the Mount Gratia belt is not definitely known because of structural 

complexity, but in general consists of an upper section of thickbedded graywacke, shale, 

and other sedimentary rocks and a lower section of multicolored tuff and other volcanic 

rocks. Locally, there are small areas of Permian and Triassic rocks that are probably 

faulted in. The occurrence of Buchia crassicolis and radiolarians identified as forms 

of Early Cretaceous (Valanginian) age within the Mount Oratia belt again indicates 

contemporaneity with rocks in the other belts.

The Eek Mountains belt, 2-25 km in width, extends northeastward about 75 km 

from the Goodnews Bay C-6 quadrangle. The belt encompasses a large anticline consisting of older rocks flanked by Cretaceous rocks. The rocks in the belt are strongly 

folded and commonly overturned northwestward.

The Lower Cretaceous (Valanginian) section in the Eek Mountains belt consists of 

graywacke, shale, argillite, and conglomerate at least 1,000 m thick. The tuff and other 

volcanogenic rocks of the Mount-Oratia belt have not been identified within the Eek 

Mountains sequence.

Buchia crassicolis has been found in thin calcareous beds and in pebbly sandstone 

at several places within the Eek Mountains belt, thus substantiating an age coeval 

with rocks in the other three belts.

INTRODUCTION

New fossil collections and a better understanding of the geology 

obtained during a recent investigation has increased our knowledge 

of the lithologic character and distribution of Lower Cretaceous strata 

in the Goodnews Bay and Bethel quadrangles, Alaska (fig. 1). We 

now know that Lower Cretaceous strata include a thick section of 

fine- to coarse-grained clastic rocks, a thick section of tuff, tuffaceous 

sedimentary rocks, volcanic breccia, and other volcanic rocks, and a 

thin section of limy grit and limestone. Many of these rocks resemble 

older Paleozoic and Mesozoic rocks and were previously mapped in 

the Gemuk Group of Carboniferous to Cretaceous age; others were 

mapped in the Kuskokwim Group of Early and Late Cretaceous age 

(Hoare and Coonrad, 1959, 1961). With the discovery of many new 

fossil localities, it is now possible to recognize four northeast- trending 

belts that contain lithologically distinct coeval strata of Early Cretaceous age. For descriptive purposes, these belts, from southeast to 

northwest, are called the Buchia Ridge, Ungalikthluk, Mount Oratia, 

and Eek Mountains belts (fig. 1).

The four belts are apparently in thrust plates separated by reverse 

faults, most of which dip southeastward. The distance between the 

belts has been foreshortened an unknown amount by northwest-directed folding and thrusting.

The boundaries of the belts are drawn to enclose known Lower 

Cretaceous fossil localities, shown on the geologic map of the Goodnews and Hagemeister Island quadrangles region (Hoare and Coonrad, 1978, sheet 1) and rocks that resemble those in which the fossils 

occur. Most boundaries are approximate but are locally sharply de-
GRAYWACKE OF BUCHIA RIDGE, SOUTHWESTERN ALASKA 3

162° 161° 160° 159°

GOODNEWS BAY QUADRANGLE

FIGURE 1. Location of four belts (lined areas) of correlative Lower Cretaceous rocks in 

the Goodnews Bay and Bethel quadrangles, Alaska.

fined by faults. In the Eek Mountains and Mount Gratia belts, the 

rocks are strongly deformed, and the Lower Cretaceous rocks are 

locally faulted and folded in with older Paleozoic and Mesozoic rocks. 

In the Ungalikthluk and Buchia Ridge belts, the rocks are only mildly 

deformed and, except for a few Tertiary intrusive rocks, all of them 

are Early Cretaceous age. Other rocks of possible Early Cretaceous 

age lie east and west of the upper Togiak River in the northeast 

corner of the Goodnews Bay quadrangle.

BUCHIA RIDGE BELT

The Buchia Ridge belt consists of a thick section of fine-, medium-, 

and coarse-grained marine clastic sedimentary rocks. These rocks form 

Buchia Ridge, a prominent isolated ridge between the valleys of the 

Ungalikthluk and Kulukak Rivers in the Goodnews Bay A-2 and A-3 

quadrangles and underlie the Kulukak River valley north and east
4 CONTRIBUTIONS TO STRATIGRAPHY

of the Buchia Ridge (fig. 2). The rocks are deformed into a southeastdipping homocline, and, if not repeated by concealed faults, they con159'

FIGURE 2. Geologic map of parts of Goodnews Bay A-2, A-3, B-2, and B-3 quadrangles showing location of cross section A-B (see fig. 4).
GRAYWACKE OF BUCHIA RIDGE, SOUTHWESTERN ALASKA 5

stitute the thickest section of Lower Cretaceous sedimentary rocks 

known in southwestern Alaska. These rocks are here called the 

graywacke of Buchia Ridge.

EXPLANATION

Summit Island Formation

Graywacke of 

Buchia Ridge

Graywacke of 

Kulukak Bay

QUATERNARY

TERTIARY AND 

CRETACEOUS

Upper 

Cretaceous

Limestone and 

limy grit J

I Lower 

I Cretaceous

CRETACEOUS

CRETACEOUS 

AND JURASSIC

Upper and , umAccir Middle Jurassic f JURASSIC

Contact

Fault 

Dashed where covered or inferred

Thrust fault

Dashed where covered or inferred; 

sawteeth on upper plate
6 CONTRIBUTIONS TO STRATIGRAPHY 

GRAYWACKE OF BUCHIA RIDGE

The lower part of the graywacke of Buchia Ridge, at least 2,400 

m thick, is mostly thick-bedded sandstone, siltstone, and conglomerate. These relatively resistant rocks form Buchia Ridge and small 

rocky hills in the upper valley of the Kulukak River. A composite 

section for the lower part of the formation was measured on Buchia 

Ridge and in the hills north of the ridge. The lower part of the section 

was measured in a steep gully on the northwest side of Buchia Ridge 

in sec. 4, T. 12 S., R. 62 W., Goodnews Bay A-2 quadrangle. The 

upper part of the section, which is offset approximately along strike 

about 5 km to the northeast, is in the small rocky hills at the northeast 

end of Buchia Ridge near the east side of the Kulukak River valley 

(sees. 34, 27, and 26, T. 11 S., R. 62 W., Goodnews Bay A-2 quadrangle) (fig. 3).

The upper part of the formation, about 2,500 m thick, is mostly 

shale and thin-bedded sandstone. These relatively nonresistant rocks 

underlie the Kulukak River valley east of Buchia Ridge and are very 

poorly exposed. Representative outcrops occur in cutbanks along the 

Kulukak River (sees. 29, 31, and 32, T.12 S., R. 62 W., Goodnews 

A-2 and A-3 quadrangles), and in low hills near the river (sec. 2, 

T. 12 S., R. 62 W., Goodnews Bay A-2 quadrangle) (fig. 3). Probably 

no more than a total of 300-400 m of strata is exposed in these two 

areas, and no section has been measured in the upper part of the 

formation.

The graywacke of Buchia Ridge is apparently restricted to a wedgeshaped area of about 350 km2 truncated on the northwest by the 

Buchia Ridge fault, on the east by the Kulukak fault, and to the 

south by the East Kulukak fault (fig. 2). The faults dip southeastward 

and are probably thrust faults (fig. 4). In most areas, these faults 

juxtapose the graywacke of Buchia Ridge with older volcanic and finegrained volcanogenic sedimentary rocks of Jurassic and probable Early 

Cretaceous age, but on the west side of Buchia Ridge toward its southern end, the Buchia Ridge fault juxtaposes the formation with a 

younger section of soft black shale that is probably in the Summit 

Island Formation of Late Cretaceous age.

The graywacke of Buchia Ridge strikes N. 45 E. and dips 20-80 

SE. The trend of the Buchia Ridge fault parallels the trend of the 

formation, but the Kulukak fault trends N. 10-15 E., cutting obliquely 

across the unit. The two faults converge northward, apparently joining 

the head of the Kulukak River valley (fig. 2). Near the Kulukak fault, 

some of the fine-grained rocks in the graywacke of Buchia Ridge are 

highly contorted and sheared, but elsewhere the structure of the formation is homoclinal. If the formation is not repeated by concealed 

faults or folds, it is about 5,000 m thick. This is the thickest, least de-
GRAYWACKE OF BUCHIA RIDGE, SOUTHWESTERN ALASKA

159°45'

59°15'

R. 63 W. R. 62 W. R. 61 W.

10 KILOMETERS

FIGURE 3. Parts of Goodnews Bay A-2, A-3, B-2, and B-3 quadrangles showing location of composite measured section of lower part of graywacke of Buchia Ridge 

(A, A'), representative outcrops of upper part of graywacke of Buchia Ridge (1, 

2, 3), and measured section of Ungalikthluk belt of correlative rocks (B). See figure 

5 for measured sections.

formed section of Lower Cretaceous sedimentary rocks known in 

southwestern Alaska.

At the measured section on the northwest side of Buchia Ridge 

(fig. 3), the base and probably 100-200 m of the lowest beds of the 

formation are covered, but at least 1,000 m of the overlying section 

is continuously exposed in a steep gully (fig. 5). The section becomes
CONTRIBUTIONS TO STRATIGRAPHY

FIGURE 4. Cross section showing structural relations of graywacke of Buchia Ridge 

and correlative rocks in Ungalikthluk belt. See figure 2 for location of section and explanation of units. Arrows on faults indicate direction of relative movement. Vertical 

exaggeration x5.

coarser grained and more conglomeratic upward. The lower part of 

the section consists of well-bedded fine- and medium-grained 

sandstone with siltstone interbeds and a few thin layers of pebble 

conglomerate and pebbly sandstone. Beds range in thickness from 0.1 

m to 3-4 m. Siltstone beds are generally massive, characterized by 

hackly fracture, and weather spheroidally. Conglomerate beds become 

thicker and more numerous upward and contain large clasts. Several 

coquinas of Buchia crassicolis shells as thick as 1.5 m are interbedded 

with conglomerate and conglomeratic sandstone 100-200 m below the 

crest of Buchia Ridge. The upper part of the composite section (fig. 

5) consists of about 1,400 m of mostly pebble-cobble conglomerate, 

and pebbly sandstone. These rocks form many small rocky hills extending northeast from the northeast end of Buchia Ridge. As the 

hills are near the Kulukak fault (figs. 2,4), beds in this part of the 

section may be repeated by faulting.

Conglomerate clasts are well-rounded pebbles and cobbles as large 

as 10 cm of hard graywacke and siltstone, cherty tuff, and less abundant porphyritic volcanic rocks. The sedimentary clasts resemble, and 

apparently derive from, the graywacke of Kulukak Bay of Jurassic 

age, widely exposed to the south and southeast. The tuff and finegrained volcanic clasts probably derive from Jurassic and probable 

Lower Cretaceous strata that constitute most of the mountains to 

the east, west, and north. Sandstone in the graywacke of Buchia 

Ridge is hard, gray and brown weathering a typical graywacke. It 

is a poorly sorted mixture of angular to well-rounded grains, chiefly 

lithic fragments, with lesser amounts of quartz and feldspar. The cementing material is "argillite "and calcite. Reaction between the cement and the mineral and lithic grains has blurred the outline of many 

of the grains.
GRAYWACKE OF BUCHIA RIDGE, SOUTHWESTERN ALASKA 9

The upper, dominantly fine-grained part of the graywacke of Buchia 

Ridge consists of calcareous black shale, thin-bedded calcareous 

siltstone, sandstone, calcarenite, and minor gritstone. These rocks 

crop out in a few low cutbanks along the Kulukak River and in low 

hills in the Kulukak River valley east of Buchia Ridge (figs. 2, 3). 

The valley in this area is much wider than elsewhere, presumably 

because it is underlain by the less resistant thin-bedded and shaly 

rocks that constitute the upper part of the formation. The rocks dip 

40-70 SE. wherever exposed. This dip and the width of the valley 

suggest a minimum thickness of 2,500 m. The east side of the valley 

is defined by a southeast-dipping fault (figs. 2, 4), however, and it 

would not be surprising to find that the shaly rocks are repeated 

by faulting.

AGE

The graywcke of Buchia Ridge is of Early Cretaceous age. The 

lower coarser-grained strata underlying Buchia Ridge contain abundant Buchia of Valanginian age; the upper fine-grained part has 

yielded sparse Inoceramus, thick-bodied Belemnites, and one ammonite of Hauterivian age (D. L. Jones, written commun., 1975, and 

table 1). These are the only strata of Hauterivian age yet recognized 

in this part of southwestern Alaska, but strata of Valanginian age, 

as indicated by the occurrence of Buchis crassicolis, are fairly widespread in the country northwest and north of Buchia Ridge. Most 

of these correlative rocks occur in three northeast-trending belts that 

are described below.

UNGALIKTHLUK BELT

The Ungalikthluk belt (fig. 1) consists of limestone, greenish limy 

grit, and conglomerate overlain by noncalcareous graywacke and grit 

that crop out at several isolated places on the ridge 3-5 km northwest 

of Buchia Ridge and the upper valley of the Kulukak River (figs. 

1, 2). The total area underlain by these rocks is no more than 2-4 

km2 . The structure of the rocks is synclinal. The isolated outcrops 

are apparently erosional remnants of a northeast-trending belt of 

rocks. At two localities (sec. 19, R. 62 W., T. 11 S., Goodnews Bay 

A-3 quadrangle; sec. 2, R. 62 W., T. US., Goodnews Bay B-2 quadrangle), the limy grit appears to be in depositional contact with highly 

deformed tuff and volcanogenic sedimentary rocks of Jurassic and 

probable Early Cretaceous age. At a third locality (sec. 23, R. 62 

W., T. 10 S., Goodnews Bay B-2 quadrangle), the contact is a steep 

southeast-dipping reverse fault.
10 CONTRIBUTIONS TO STRATIGRAPHY 

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DESCRIPTION

KULUKAK FAULTChiefly pebble-cobble conglomerate with 

interbedded pebbly sandstone and sandstone. 

Conglomerate clasts well rounded, as large as 

10 cm; mostly hard, fine-grained sandstone, 

argillite,and volcanic rocks. Buchia crassicolis 

occurs sporadically

-Break in section

Interbedded pebble-cobble conglomerate, 

sandstone, and pebbly sandstone

^ 

Mostly sandstone with some siltstone, minor grit, 

and pebble conglomerate

Mafic sill 5 m thick

Brown-weathering calcareous sandstone, grit, 

and pebble conglomerate; several coquina 

beds of Buchia cf. B. crassicolis

Chiefly fine- and medium-grained sandstone, 

some siltstone, and minor pebble grit. Rocks 

mostly calcareous and weather brown. 

Siltstone has hackly fracture and weathers 

spheroidally. Few calcareous concretions. 

Carbonaceous plant detritus in lower part. 

Buchia keyserlingi occurs sporadically

Covered interval 

BUCHIA RIDGE FAULT-
GRAYWACKE OF BUCHIA RIDGE, SOUTHWESTERN ALASKA 11

B

THICKNESS, 

METERS IN 

300- 

200- 

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LITHOLOGY

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- b-l-b- -I-B ! -oDESCRIPTION

Graywacke and grit, noncalcareous. Graywacke 

in beds of thin and medium thickness grades 

downward into thick, massive-bedded 

graywacke and grit. Grit contains abundant 

maroon and green schist clasts and minor white 

quartz

Graywacke, massive, dark-gray, hard, noncalcareous

Limy pebble-cobble conglomerate. Mostly 

fine-grained, subrounded green and maroon 

schist clasts as large as 8 to 10 cm in limestone 

matrix. Some weil-rounded quartz pebbles. 

Buchia crassicolis near base of unit

Gritty limestone and shell limestone, subrounded 

green and maroon schist, and nonschistose 

volcanic clasts in limestone matrix. Some 

massive dark beds of bioclastic shell-fragment 

limestone. Coquinoid limestone bed neartopof 

unit consists of Buchia crassicolis

 FAULT 

FIGURE 5. Composite measured section of lower part of graywacke of Buchia Ridge 

(A, A') and representative section of correlative rocks of Ungalikthluk belt (B). 

Locations of sections in figure 3.

The bioclastic limestone is mostly microscopic shell fragments. 

Buchia crassicolis occurs in some limestone beds, and it seems likely 

that the bioclastic limestone is made up largely of Buchia shell fragments. The limy grit consists of subrounded green and maroon lithic 

fragments and a few well- rounded white quartz pebbles. The lithic
12 CONTRIBUTIONS TO STRATIGRAPHY

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GRAYWACKE OF BUCHIA RIDGE, SOUTHWESTERN ALASKA 13

fragments are fine- grained tuff and volcanogenic sedimentary rocks 

metamorphosed to phyllite and low-grade quartz-chlorite-sericite 

schist. The metamorphic clasts, which tend to be tabular in shape, 

are as large as 10 cm accross, but most are much smaller. They apparently derive from nearby strata that were tectomcally metamorphosed 

by movement on the Ungalikthluk fault 1-3 km to the northwest. 

Because the limestone matrix does not have a metamorphic fabric, 

the clasts came from rock that was metamorphosed in pre-Early Cretaceous (Late Jurassic?) time.

The measured section in sec. 23, R. 62 W., T. 10 S., Goodnews 

Bay B-2 quadrangle (B in figs. 3, 5) gave a thickness of about 175 

m for the limestone and grit sequence. Both here and at a second 

locality southwest of it (sec. 19, R. 62 W., T. 11 S., Goodnews Bay 

A-3 quadrangle), about 140 m of thin-bedded to massive noncalcareous 

graywacke and pebble conglomerate overlies the calcareous sequence. 

Clasts in the conglomerate include abundant green and maroon 

metamorphic rocks identical to those in the underlying calcareous grit 

and some white quartz pebbles. The graywacke and conglomerate sequence appears to be structurally conformable with the underlying 

calcareous sequence, but the abrupt change in lithologic character indicates that the two sequences are stratigraphically separated by an 

erosion interval. And in the southern locality, the graywacke not only 

overlies the calcareous grit but also rests directly upon schistose volcanogenic rocks that underlie the calcareous grit.

The occurrence of Buchis crassicolis in the limestone indicates that 

the limestone and limy grit sequence is coeval with the coarse conglomeratic section of the graywacke of Buchia Ridge. The present 

proximity of these dissimilar rocks suggests that the distance between 

them has been foreshortened by several kilometers of northwest transport on the Buchia Ridge fault (fig. 2).

MOUNT GRATIA BELT

The third belt (fig. 1), named for Mount Oratia, one of the highest 

peaks in this part of Alaska (1,420 m.), is 15-25 km wide and more 

than 120 km long. It extends from the headwaters of the Goodnews 

River, near the middle of the Goodnews Bay B-5 quadrangle, northeastward across the southern part of the Bethel A-3 quadrangle and 

probably into the adjoining Sleetmute quadrangle, where Cady (1955) 

originally defined the Gemuk Group. The description here of the rocks 

in the Mount Oratia belt applies only to rocks in the Goodnews Bay 

and Bethel quadrangles.

Rocks of the Mount Oratia belt are strongly folded, cut by many
14 CONTRIBUTIONS TO STRATIGRAPHY

faults, and commonly overturned northwestward. Within the belt are 

at least two small areas of Permian and Triassic rocks, probably 

faulted in. The northwest and southeast sides of the belt are defined 

by southeast-dipping low- and high-angle reverse faults. The northern 

part of the belt is thrust northwestward on younger rocks of Cretaceous (Albian?) age. To the southwest, the belt is juxtaposed with older 

rocks of Permian and Triassic ages. The northwest side of the belt 

is offset in three or four places by left-lateral displacement on northwest-trending faults. The southeast side is flanked by volcanic and 

sedimentary rocks, mostly Jurassic in age.

The rocks in the belt are a colorful assemblage of various kinds 

of tuffs, volcanogenic sedimentary rocks, massive graywacke, conglomerate, argillite, a few flows, and a few impure limestone beds. 

Structural complexity precludes certain knowledge of the stratigraphy, but in synclinal structures, the upper part of the section is 

generally thick-bedded graywacke and shale, the lower part tuff and 

other volcanic rocks. The Lower Cretaceous section probably grades 

downward into lithologically similar rocks of Jurassic age. No megafossils of Jurassic age have been found, but some cherty rocks contain 

Late Jurassic radiolarians (E. A. Pessagno, Jr., written commun., 

1977).

The tuffaceous rocks are mostly green and gray; some are red, yellow, brown, or black. They range from thin-bedded cherty rocks to 

massive coarse-grained crystal-lithic tuff. The most distinctive rock 

in the belt is the massive andesitic crystal- lithic tuff (at least 1,000 

m thick) that forms many high peaks and jagged ridge crests. This 

tuff consists of green, rarely red or black, cherty tuff fragments, 

plagioclase (oligoclase- andesine), and minor amounts of fine-grained 

porphyritic volcanic rock fragments. Quartz is rare or absent. The 

angular to subrounded lithic clasts range in size from microscopic to 

20 cm but are mostly less than 1 cm. Plagioclase grains are mostly 

broken twinned laths less than 1 mm in size. Massive beds several 

meters thick are common, but most of the tuff shows light-gray and 

dark-green bedding bands 1 mm to several centimeters thick. The 

light bands are plagioclase-rich; the green bands are mostly lithic 

clasts. Lithic clasts in plagioclase-rich beds commonly grade from 

coarse to fine upward. Layers of fine-grained gray and green cherty 

tuff are interbedded with the massive crystal-lithic tuff. These cherty 

layers resemble the clasts in the crystal-lithic tuff. Much of the finegrained tuff, including that interbedded with the crystal-lithic tuff, 

is laumontitized and has developed a chalky mottled appearance; the 

clasts and plagioclase in the massive crystal-lithic tuff are not laumontitized. In the Bethel A-3 quadrangle, some of the graywacke is tuffaceous and is laumontitized with a mottled appearance; massive black
GRAYWACKE OF BUCHIA RIDGE, SOUTHWESTERN ALASKA 15

"argillite" beds are crisscrossed with tiny laumontite veinlets on irregular fracture surfaces.

The Early Cretaceous age of this belt of rocks is based on the occurrence of Buchia crassicolis and radiolarians (table 1). Radiolarians 

found in cherty tuff at three localities in the Goodnews Bay CM quadrangle near the southwest end of the belt were identified and assigned 

an Early Cretaceous (Valanginian) age by E. A. Pessagno, Jr. (written 

commun., 1977). Near the northeast end of the belt in the Goodnews 

Bay D-3 and Bethel A-3 quadrangles, Buchia crassicolis occurs in 

calcareous graywacke and conglomerate and in impure limestone. 

These megafossils were identified by D. L. Jones (written commun., 

1977) and R. W. Imlay (written commun., 1948, 1950).

EEK MOUNTAINS BELT

The fourth and north westernmost belt (fig. 1), named for the Eek 

Mountains, extends northeastward about 75 km from the Goodnews 

Bay C-5 and C-6 quadrangles into the Bethel A-4 quadrangle. The 

overall structure of the belt is that of a large northeast-plunging anticline tapering in width northeastward from a maximum of about 25 

km to 2 or 3 km in the Bethel A-4 quadrangle. The anticline is cored 

by older rocks of Paleozoic and Mesozoic age and flanked by Cretaceous rocks. The rocks in the belt are strongly folded and commonly 

overturned northwestward. They are cut by southeast-dipping reverse 

faults and locally offset to the northwest by northwest-trending tear 

faults.

The lower Cretaceous rocks in the Eek Mountains belt that are 

correlative with the graywacke of Buchia Ridge contain Buchia crassicolis of Valanginian age. No rocks of Hauterivian age, such as form 

the upper part of the graywacke of Buchia Ridge, have been recognized in or near the Eek Mountains belt, but they may be present, 

as the age of much of the thick Cretaceous section is only approximately known. For most of its length, the Eek Mountains belt is 

flanked on either side by the Kuskokwim Group (Cady and others, 

1955; Hoare and Coonrad, 1959, 1961, 1978) which is of late Early 

Cretaceous (Albian) and Late Cretaceous (Cenomanian and Turonian) 

age. A regional unconformity separates the Kuskokwim Group from 

the Lower Cretaceous section. There is no obvious angular discordance between the two units but the Kuskokwim Group is generally 

not difficult to recognize.

The Kuskokwim Group consists of a thick basal conglomerate overlain by a thick section of black micaceous shaly rocks that grade upward into several thousand meters of interbedded graywacke and 

shale. The conglomerate is noncalcareous and no fossils have been
16 CONTRIBUTIONS TO STRATIGRAPHY

found in it. It is a poorly sorted mixture of well-rounded to subangular 

pebbles, cobbles, and boulders of gneiss, schist, white quartz, volcanic 

rocks, and rare sandstone derived from the underlying Lower Cretaceous section. The source of the quartz and metamorphic clasts in the 

conglomerate and the mica in the shaly rocks was a belt of Precambrian metamorphic rocks that is parallel to, and a short distance northwest of,, the Eek Mountains belt. Apparently the Precambrian rocks 

were not exposed in Early Cretaceous time, because no metamorphic 

clasts of Precambrian rocks are known in the calcareous fossiliferous 

Lower Cretaceous conglomerate.

The Lower Cretaceous section consists of graywacke, shale, argillite, and conglomerate. No tuff, tuffaceous sedimentary rocks, or 

laumontitized rocks that constitute much of the Lower Cretaceous 

section in the Mount Oratia belt have been recognized in the Eek 

Mountains belt. The stratigraphic succession is not certainly known, 

but it seems likely that most of the massive finer grained rocks are 

in the lower part of the section; conglomerate and well-bedded 

sandstone and shale are probably in the upper part of the section. 

Locally the rocks are thin bedded, but in general they are thick bedded to massive with alternating sandstone and shale intervals 5-20 

m thick. The sandstone is gray, brown-weathering, hard, and fine 

to medium grained. It has a blocky fracture and bedding is commonly 

difficult to recognize. Shaly rocks commonly show good fracture cleavage. Massive dark-colored beds of argillite are characterized by a fine 

hackly fracture. The sandstone is mostly somewhat calcareous, locally 

highly calcareous, and contains layers of calcareous pebble conglomerate. Most of the fossils occur in these highly calcareous beds. Calcareous pebble-cobble conglomerate with interbedded pebbly limestone 

containing Buchis crassicolis crops out at several places along the 

northwest side of the Eek Mountains belt. Near the southwest end 

of the belt, conglomerate containing fossiliferous pebbly limestone 

beds is at least 100 m thick; elsewhere it is much thinner. Clasts 

in the conglomerate are well rounded and chiefly, or entirely, finegrained cherty tuff and porphyritic volcanic rock.

The base of the Lower Cretaceous section has not been recognized 

because the structure is complex and strata of Permian and Triassic 

age also include sedimentary rocks that are very similar to those of 

Early Cretaceous age. No Jurassic fossils have been found in the Eek 

Mountains belt, but since sparsely fossiliferous rocks of Jurassic age 

are widespread both to the northwest and southeast, it seems likely 

that they are present in the Eek Mountains belt.
GRAYWACKE OF BUCHIA RIDGE, SOUTHWESTERN ALASKA 17

The thickness of the Lower Cretaceous section is indeterminate because the rocks are highly deformed, the base of the section has not 

been recognized, and an unknown amount has been eroded off the top. 

It probably is at least 1,000 m thick and may be as thick as 3,000-4,000 

m.

The age of the rocks is based on the occurrence of Buchia crassicolis 

(table 1) found at several localities on the northwest and southeast 

flanks of the Eek Mountains belt (fig. 1). These fossils occur in thin 

beds of impure limestone, highly calcareous pebble conglomerate, and 

pebbly sandstone. The thin fossil horizons generally occur in thick 

sections of nonfossiliferous rocks, but the location of the fossils in 

the section is generally not known.

OTHER CORRELATIVE(?) ROCKS

Other rocks of probable Early Cretaceous age lie north of Buchia 

Ridge on both sides of the upper Togiak River (fig. 1). No diagnostic 

fossils of Early Cretaceous age have been found in this large area, 

but many of the rocks, including a thick sequence of laumontitized 

tuffs and tuffaceous sedimentary rocks, resemble the Lower Cretaceous section in the Mount Gratia belt. Radiolarians found in cherty 

rocks a short distance east of the upper Togiak River include a species 

of Parvicingula that ranges in age from Late Jurassic (Kimmeridgian) 

into Early Cretaceous (Valanginian) (E. A. Pessagno, written commun., 1977). The stratigraphic position of the radiolarians is not certainly known, but their occurrence suggests the general age range 

of the rocks, some of which are probably correlative with the 

graywacke of Buchia Ridge.

REFERENCES CITED

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, 134 p. 

Hoare, J. M., and Coonrad, W. L., 1959, Geology of the Bethel quadrangle, Alaska:

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

1:250,000. 

 1961, Geologic map of the Goodnews quadrangle, Alaska: U.S. Geological Survey

Miscellaneous Geologic Investigations Map 1-339, scale 1:250,000. 

-1978, Geologic map of the Goodnews and Hagemeister Island quadrangles region: U.S. Geological Survey Open-File Report 78-9-B, 2 sheets. 

Hoare, J. M., Coonrad, W. L., Detterman, R. L., and Jones, D. L., 1975, Preliminary 

geologic map of the Goodnews A-3 quadrangle and parts of the A-2 and B-2 quadrangles, Alaska: U.S. Geological Survey Open-File Report 75-308.

U.S. GOVERNMENT PRINTING OFFICE: 676-041/19 1983