2013

U.S. Department of the Interior

U.S. Geological Survey

Reconnaissance Geologic Map of the Kuskokwim Bay 

Region, Southwest Alaska

Including the Bethel, Goodnews Bay, Nushagak Bay, Hagemeister Island, Baird Inlet, 

Cape Mendenhall, Kuskokwim Bay, Nunivak Island, Saint Matthew, and 

Pribilof Islands 1:250,000-scale quadrangles

Compiled by Frederic H. Wilson, Chad P. Hults, Solmaz Mohadjer, and Warren L. Coonrad

Pamphlet to accompany

Scientific Investigations Map 3100

Migmatite north of Thumb Mountain, Goodnews Bay quadrangle. Border phase of plutons in the Kanetktok 

metamorphic complex. (Photograph by F.H. Wilson, 1975.)

i

Contents

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

Introduction and previous work .................................................................................................................1

Physiographic framework .......................................................................................................................... 1

Geologic framework .....................................................................................................................................2

Structural setting ..........................................................................................................................................4

Terranes .................................................................................................................................................4

Overlap assemblages ..........................................................................................................................5

Faults ....................................................................................................................................................5

Metamorphism and metamorphic rocks ..........................................................................................6

Quaternary geology ......................................................................................................................................7

Geochronology ..............................................................................................................................................8

Kanektok metamorphic complex—enigma in southwest Alaska .........................................................9

Description of map units ........................................................................................................................... 11

Unconsolidated deposits ................................................................................................................. 11

Sedimentary rocks .............................................................................................................................12

Igneous rocks .................................................................................................................................... 18

Togiak-Tikchik Complex ....................................................................................................................24

Metamorphic rocks of the Togiak-Tikchik Complex ............................................................27

References cited .........................................................................................................................................32

Figures

Figure 1. Index map showing 1:250,000-scale quadrangle coverage and prominent physiographic 

features of the map area. ...............................................................................................................2

Figure 2. Tectonostratigraphic terrane map of southwestern Alaska. .................................................6

Figure 3. Gneiss of the Kanektok metamorphic complex (<k) north of Thumb Mountain. .............10

Tables

Table 1. Radiometric Ages..........................................................................................................................36

1

Abstract

The rocks of the map area range from Proterozoic age 

metamorphic rocks of the Kanektok metamorphic complex 

(Kilbuck terrane) to Quaternary age mafic volcanic rocks of 

Nunivak Island. The map area encompasses much of the type 

area of the Togiak-Tikchik Complex. The geologic maps used 

to construct this compilation were, for the most part, reconnaissance studies done in the time period from the 1950s to 1990s. 

Pioneering work in the map area by J.M. Hoare and W.L. 

Coonrad forms the basis for much of this map, either directly 

or as the stepping off point for later studies compiled here.

Physiographically, the map area ranges from glaciated 

mountains, as much as 1,500 m high, in the Ahklun Mountains to the coastal lowlands of northern Bristol Bay and the 

Kuskokwim River delta. The mountains and the finger lakes 

(drowned fiords) on the east have been strongly affected by 

Pleistocene and Holocene glaciation.

Within the map area are a number of major faults. The 

Togiak-Tikchik Fault and its extension to the northeast, the 

Holitna Fault, are considered extensions of the Denali fault 

system of central Alaska. Other sub-parallel faults include the 

Golden Gate, Sawpit, Goodnews, and East Kulukak Faults. 

Northwest-trending strike-slip faults crosscut and offset 

northeast-trending fault systems.

Rocks of the area are assigned to a number of distinctive 

lithologic packages. Most distinctive among these packages are 

the high-grade metamorphic rocks of the Kanektok metamorphic complex or Kilbuck terrane, composed of a high-grade 

metamorphic orthogneiss core surrounded by greenschist and 

amphibolite facies schist, gneiss, and rare marble and quartzite. 

These rocks have yielded radiometric ages strongly suggestive 

of a 2.05 Ga emplacement age. Poorly known Paleozoic rocks, 

including Ordovician to Devonian and Permian limestone, are 

found east of the Kanektok metamorphic complex. A Triassic(?) ophiolite complex is on the southeast side of Kuskokwim 

Bay; otherwise only minor Triassic rock units are known. 

The most widespread rocks of the area are Jurassic and Early 

Cretaceous(?) volcanic and volcaniclastic rocks. The Kuskokwim Group flysch is restricted largely to the northeast part of 

the map area. It consists primarily of shelf and minor nearshore 

facies rocks. Primarily exposed in the lowlands west of the 

Ahklun Mountains, extensive latest Tertiary and Quaternary 

alkalic basalt flows and lesser pyroclastic rocks form much of 

the bedrock of the remaining area. On Saint Matthew Island, 

Cretaceous volcanic and pyroclastic rocks occur that are not 

found elsewhere within the map area. The Kuskokwim Group 

and older rocks, including on Saint Matthew Island, but not 

the Kanektok metamorphic complex, are intruded by widely 

dispersed Late Cretaceous and (or) Early Tertiary granitic 

rocks. Much of the lowland area is mantled by unconsolidated 

deposits that include glacial, alluvial and fluvial, marine, estuarine, and eolian deposits. These formed during several episodes 

of Quaternary glaciation.

Introduction and Previous Work

The map comprises the southwestern-most part of mainland Alaska (fig. 1), exclusive of the Alaska Peninsula, extending from Nushagak Bay on the east to the Saint Matthew and 

the Pribilof Islands on the west. In general, it is a relatively 

poorly known region but includes the oldest rocks known in 

Alaska as well as rocks that are geologically very young. The 

rocks of the map area range from the Proterozoic metamorphic 

rocks of the Kanektok metamorphic complex (Kilbuck terrane) 

to the Quaternary mafic volcanic rocks of Nunivak Island. The 

map area encompasses much of the type area of the TogiakTikchik Complex.

The geologic maps used to construct this compilation were, 

for the most part, reconnaissance studies done in the time period 

from the 1950s to 1990s. A few detailed studies, such as the 

work of Murphy (1987) or topical studies by Box and others 

(1990) and Moll-Stalcup and others (1996), provide better 

control on some aspects of the geology. Pioneering work in the 

map area by J.M. Hoare and W.L. Coonrad forms the basis for 

much of this map, either directly or as the stepping off point for 

later studies. A wide range of sources were used to construct 

this map, including the published mapping of Barth (1956) in 

the Pribilof Islands, Coonrad (1957) in the Baird Inlet, Cape 

Mendenhall, Kuskokwim Bay, and Nunivak Island 1:250,000-

scale quadrangles, Patton and others (1975) on Saint Matthew 

Island, Hoare and Coonrad (1959a) and Box and others (1993) 

in the Bethel 1:250,000-scale quadrangle, and Hoare and Coonrad (1961a,b; 1978) in the Goodnews Bay, Nushagak Bay, and 

Hagemeister Island 1:250,000-scale quadrangles. In addition, 

we relied on Box (1985a) for revisions to Hoare and Coonrad 

(1978) in the Hagemeister Island and southern Goodnews Bay 

1:250,000-scale quadrangles.

The authors gratefully acknowledge discussions with Steve 

Box and Sue Karl during map preparation. F. Wilson especially 

acknowledges the introduction to the area by Warren Coonrad 

and Joe Hoare during the 1975 field season and Coonrad’s 

continued encouragement and support. Technical review of this 

map compilation was by Arthur Schultz and Jeanine Schmidt. 

Map editing was by Theresa Iki.

Physiographic Framework

The map area encompasses the glaciated Ahklun and 

Kilbuck Mountains, the coastal lowlands of the southern Kuskokwim River delta and northern Bristol Bay, and a number of 

large islands in the southern Bering Sea. The Ahklun Mountains range up to 1,500 m in elevation and form the Ahklun 

Mountains province of Wahrhaftig (1965). The Ahklun Mountains supported a local ice cap during Pleistocene time that was 

the source for alpine and large valley glaciers that extended out 

to sea in Bristol Bay (Kaufman and Manley, 2004; Kaufman 

and others, 2001a,b; Manley and others, 2001) as well as to the 

east. The finger lakes of the east side of the map area in WoodTikchik State Park and southward were affected by Pleistocene 

and Holocene glaciation and are well-developed landlocked 
(!

(!

(!

(!

Kilbuck Mtns

Ahklun Mtns

Kanektok R

Kuskokwim

River

B

Kuskokwim Bay

E

R I N G S E A

WoodTikchik

State

Park

PRIBILOF

ISLANDS

KUSKOKWIM BAY

GOODNEWS BAY

ST MATTHEW

BAIRD INLET BETHEL

HAGEMEISTER ISLAND NUSHAGAK BAY

CAPE MENDENHALL

NUNIVAK ISLAND

DILLINGHAM

RUSSIAN MISSION

TAYLOR MTS

SLEETMUTE

Russian Mission

Hooper Bay

Dillingham Platinum

Bethel

168°W 162°W

60°N

58°N

ALASKA PENINSULA

Map Area

ALASKA CANADA

RUSSIA

2

fiords. West of the Ahklun Mountains, the southern part of the 

Kilbuck Mountains range up to 1,150 m high and lie in Wahrhaftig’s Kuskokwim Mountains section of the Western Alaska 

province (Wahrhaftig, 1965). Although remnant cirque glaciers 

occur locally in the Ahklun Mountains, only weathered cirques 

indicate past glaciation in the Kilbuck Mountains. Farther west 

are the marshy, lake-covered lowlands of northern Bristol Bay 

and the Kuskokwim River delta in Wahrhaftig’s Yukon-Kuskokwim Coastal Lowland section of the Bering Shelf province 

(Wahrhaftig, 1965). Glacial deposits are an important component of the unconsolidated deposits of this part of the map area; 

however, the dominant topographic features are the geologically young basaltic cinder cones and flows of the lowland and 

the volcanic islands rising from Wahrhaftig’s (1965) Bering 

Platform section of the Bering Shelf province. Nelson Island 

and Saint Matthew Island expose older rocks in the only other 

topographic high points of the western part of the map area. 

Permafrost is sporadic to discontinuous throughout the map 

area (Wahrhaftig, 1965) and both permafrost and the cirque 

glaciers are melting as a result of current climate conditions.

Geologic Framework

The map area contains a number of distinctive lithologic 

packages. We did not assign units to terranes because the area 

remains a region of reconnaissance-scale mapping and poor age 

control. However, in a section below we provide a summary of 

descriptions of the terrane assignments others have made.

Figure 1. Index map showing 1:250,000-scale quadrangle coverage and prominent physiographic 

features of the map area.
3

A significant geologic feature of the region is the Proterozoic, possibly Paleoproterozoic, Kanektok metamorphic complex of Hoare and Coonrad (1979), also known as the Kilbuck 

terrane of Jones and others (1981). Exposed at the western 

edge of the mountains north of Goodnews Bay, this high-grade 

metamorphic complex is composed of an orthogneiss core 

metamorphosed to granulite facies surrounded by greenschist 

and amphibolite facies schist, gneiss, and rare marble and 

quartzite (Turner and others, 2009) and may include some 

of the oldest rocks in Alaska. A more complete discussion of 

the Kanektok metamorphic complex is provided in a section 

below.

A variety of poorly known Paleozoic rocks, including 

Ordovician to Devonian and Permian limestone, crop out 

east of the Kanektok metamorphic complex. These rocks are 

included in a complex structural assemblage of Paleozoic and 

Mesozoic volcanic and sedimentary rocks and mélange traditionally assigned to the Gemuk Group (Hoare and Coonrad, 

1959a; 1961a,b). The Gemuk Group was originally loosely 

defined in the 1950s in the northwestern Taylor Mountains 

quadrangle and southward (Cady and others, 1955); subsequently its areal extent was expanded by Hoare and Coonrad 

(1959a,b; 1961a,b) and through unpublished mapping of J.N. 

Platt in the Taylor Mountains and J.N. Platt and E.H. Muller in 

the Dillingham 1:250,000-scale quadrangles in the late 1950s 

era. The Gemuk Group came to include many rocks ranging in age from Paleozoic to Cretaceous in southwest Alaska. 

Wilson and Coonrad (2005) discussed the history of usage of 

the term Gemuk Group and formally abandoned the Gemuk 

Group as a stratigraphic unit and term. Many of the rocks that 

were traditionally included in this unit are now assigned to the 

Togiak-Tikchik Complex. Within the map area, rocks that range 

in age from early Paleozoic to Cretaceous are included in the 

Togiak-Tikchik Complex structural assemblage. Box and others 

(1993) subdivided these rocks into a series of lithologic units in 

the Bethel quadrangle and Wilson and others (2006b) created 

similar subdivisions in the Taylor Mountains and Dillingham 

quadrangles to the east. However, other than the few subdivisions made of the }| unit by Hoare and Coonrad (1978), 

similar subdivisions have not been made here for the rocks in 

the Goodnews Bay quadrangle. However, reexamination of 

available data, especially the field notes of J.M. Hoare, W.L. 

Coonrad, W.H. Condon, and others, may allow some subdivision in the future. Among the rocks assigned to the Togiak-Tikchik Complex in the Bethel quadrangle are metamorphic rocks 

of unknown protolith age that locally yield Jurassic metamorphic ages. Jones and Silberling (1979), Jones and others (1981), 

Box (1985a), and Decker and others (1994) assigned the rocks 

of the Gemuk Group (now Togiak-Tikchik Complex) largely to 

the Goodnews terrane; some were also assigned to the Togiak 

terrane. Box and others (1993) interpreted these Mesozoic and 

Paleozoic rocks as an accretionary complex that progressively 

underthrust the volcanic arc rocks to the east.

At Cape Newenham, between Kuskokwim and Togiak 

Bays, a Triassic(?) ophiolite complex is associated with mafic 

and ultramafic intrusive rocks of possible Jurassic age in a complex structural relation, otherwise only minor Triassic rock units 

are known in the map area. The Jurassic ultramafic rocks were 

the source for the platinum group metals of the only commercial 

platinum deposit in Alaska (Hudson, 2001).

The eastern part of the map area consists largely of Triassic(?), Jurassic, and Early Cretaceous(?) volcaniclastic sedimentary rocks assigned to the Togiak terrane by Box (1985a), 

Box and others (1993), and Decker and others (1994). Wilson 

and others (2006b) suggested that these rocks may correlate in 

part with the lithologically similar Koksetna River sequence of 

Wallace and others (1989), exposed west of the Alaska-Aleutian 

Range. The two packages of rocks may represent the east and 

west sides of a Jurassic basin. The Kilbuck Mountains in the 

north-central Bethel quadrangle also consist largely of Jurassic and Early(?) Cretaceous volcaniclastic sedimentary rocks, 

but also include Middle Jurassic andesite and basalt flows and 

aquagene tuff (Box and others, 1993). These rocks in the Bethel 

quadrangle have been assigned to the Nyac terrane (Box and 

others, 1993; Decker and others, 1994) and are considered in 

part distinctive because they are intruded by mid-Cretaceous 

plutons, such as the Nyac pluton, which are not known to 

intrude rocks assigned to any other terrane in southwest Alaska. 

However, these Nyac terrane rocks are poorly known. Some 

Jurassic fossils have been collected. A number of igneous intrusive complexes that intrude the Nyac terrane are undated; Box 

and others (1993) assigned these complexes an early Tertiary to 

Jurassic age range.

Four distinctive sequences of Lower Cretaceous sedimentary rocks overlie the older rocks of the Ahklun Mountains. 

All of Valanginian age, each sequence has a slightly different 

mix of lithologies and character. The graywacke of Buchia 

Ridge (Hoare and Coonrad, 1983) is a fining upward clastic 

sequence rich in Buchia in its lower part and present in a largely 

undeformed state in a thrust sheet. West of Buchia Ridge, the 

limy grit and limestone of the Ungalikthluk belt of Hoare and 

Coonrad (1983) contains lithic clasts of low-grade metamorphic rocks. Farther west, volcanic and sedimentary rocks of the 

Mount Oratia belt of Hoare and Coonrad (1983) is a multi-hued 

package of widely varying lithology that includes chert, graywacke, andesitic crystal-lithic tuff and a few flows. Rocks of the 

Mount Oratia belt rocks overlie the Togiak-Tikchik Complex. 

The calcareous graywacke and siltstone of the Eek Mountains 

belt of Hoare and Coonrad (1983) is the westernmost of the four 

sequences. The sequence consists of strongly folded and commonly overturned calcareous turbidities of presumed Valanginian age and contains lithic clasts that include metamorphic rocks 

presumed to be of the Kanektok metamorphic complex (Hoare 

and Coonrad, 1983).

The Kuskokwim Group, an Early (Albian(?)) and Late 

Cretaceous flysch is restricted largely to the northeast part of the 

map area, although it is widely distributed throughout southwest 

Alaska. The Kuskokwim Group consists largely of interbedded 

graywacke and shale and, locally, has interbeds of argillite 

and conglomerate (Cady and others, 1955). In the map area, it 

consists largely of shelf facies rocks, although nearshore facies 

conglomeratic rocks are found in the vicinity of the Kanektok 

metamorphic complex rocks. Box and others (1993) considered 

the conglomeratic rocks, which include clasts probably derived 

from the Kanektok metamorphic complex, to be a basal facies 

of the Kuskokwim Group.
4

Widely dispersed latest Cretaceous and (or) early Tertiary 

granitic rocks intrude the Kuskokwim Group and most of the 

older rocks, but do not intrude the Kanektok metamorphic 

complex (Patton and others, 1975; Wilson, 1977; Hoare and 

Coonrad, 1978; Box and others, 1993). The plutons yield ages 

between approximately 60 and 70 Ma and show a crude compositional zoning. The more mafic plutons tend to occur in the 

western part of the map area, whereas more potassium-rich plutons tend to occur in the eastern part of the map area and farther 

eastward in the Dillingham quadrangle (Wilson, 1977; Wilson 

and others, 2006b). The plutons are conspicuously absent in the 

Kanektok metamorphic complex. Northwest of the Kanektok 

metamorphic complex, a number of plutons of unknown age 

intrude the Jurassic volcanic and volcaniclastic rocks of the 

Nyac terrane of Jones and others (1981) and Box and others 

(1993). Some of the plutons associated with the Nyac terrane 

may include latest Cretaceous to early Tertiary plutons, but they 

may also include plutonic rocks as old as Jurassic. Within the 

region, the rocks of the Nyac terrane are the only rocks intruded 

by plutons of mid-Cretaceous (~110 to ~120 Ma) age.

Extensive latest Tertiary and Quaternary alkalic basalt 

flows and lesser pyroclastic rocks form much of the bedrock 

of the western part of the map area. Primarily exposed in the 

lowlands west of the Ahklun Mountains, these volcanic rocks 

are also well known from Nunivak Island and the Pribilof 

Islands as well as north of Togiak Bay in the eastern part of the 

map area. The volcanic rocks are part of a belt of alkalic basalt 

that is widely distributed in western Alaska as far north as the 

Seward Peninsula. The rocks of Nunivak Island and the Pribilof 

Islands were extensively sampled for paleomagnetic analysis 

and radiometric age determination (Cox and Dalrymple, 1967; 

Cox and others, 1966) and these analyses were used to help 

establish the geomagnetic time scale of Cox and others (1968). 

In addition, xenoliths from the basalt of Nunivak Island have 

been the subject of a number of studies (see for example, Hoare 

and Condon, 1973; Francis, 1976; Roden and others, 1980).

Cretaceous volcanic and pyroclastic rocks that are exposed 

on Saint Matthew Island are not separately mapped elsewhere 

within the map area. However, rocks of this age may be present 

within other map units of the mainland. In the westernmost part 

of the map area, the largely volcanic and intrusive rocks of Saint 

Matthew Island (Patton and others, 1975) and Saint Lawrence 

Island to the north (Patton and Csejtey, 1979) provide the only 

exposures of the older rocks of the Bering Sea shelf.

Glacially derived materials are a major component of the 

unconsolidated deposits in the eastern part of the map area and 

are readily apparent glacial landforms. At maximum Pleistocene 

glaciation, glaciers flowing east from the Ahklun Mountains 

merged with glaciers flowing from the Alaska-Aleutian Range 

to the east. West of the Ahklun and Kilbuck Mountains, much 

of the map area consists of Quaternary surficial deposits of the 

Kuskokwim River delta. Largely not subdivided, these surficial 

deposits are thought to include glacial deposits of late Wisconsin 

and older age (W.L. Coonrad, oral commun., 2001) and uplifted 

marine deposits. W.L. Coonrad (oral commun., 2001) reported 

glacial deposits in stream and river cutbanks well away from the 

mountains in the Kuskokwim River delta lowlands. Small alpine 

glaciers remain in the higher mountains of the region.

Structural Setting

Terranes

The rocks in the Ahklun and Kilbuck Mountains area 

were mapped as a series of overlapping terranes and subterranes in the 1980s and 1990s by Jones and others (1981) and 

subsequently by S.E. Box and coworkers (Box, 1985a,b; Box 

and others, 1993; Decker and others, 1994). Decker and others 

(1994) summarized the application of terrane terminology to 

the region, dividing the map area into the Nyack1, Togiak, 

Goodnews, and Kilbuck terranes (fig. 2). However, it must be 

recognized that development of terrane terminology for this 

region preceded detailed geologic mapping; as a result, some of 

the map units shown on the geologic map may occur in more 

than one terrane.

Many of these terranes reflect various components of 

an accretionary arc system. However, the most distinct of the 

terranes is the Kilbuck terrane, which is synonymous with the 

Kanektok metamorphic complex. This is the sole terrane of 

the region not thought to reflect an accretionary arc. It can be 

described as multiply deformed, upper amphibolite to granulite 

facies metamorphic rocks and is thought to represent a fragment of continental crust of unknown affinity. It is also the only 

terrane not intruded by younger (late Mesozoic or Cenozoic) 

plutons. The Togiak terrane, the largest single terrane fragment in the map area, includes many of the rocks of the eastern 

part of the Goodnews Bay and Bethel quadrangles, much of 

the Hagemeister Island quadrangle, and all of the Jurassic-age 

stratified bedrock of the Nushagak Bay quadrangle. The East 

Kulukak Fault divides the Togiak terrane into two subterranes. 

According to Decker and others (1994, p. 293), the subterranes 

“differ in sedimentary facies and structural style, but are linked 

by common provenance and stratigraphic history.” The larger 

Hagemeister subterrane is thought to be an Upper Triassic to 

Lower Cretaceous volcanic island arc built upon an Upper 

Triassic ophiolitic basement. The Kulukak subterrane consists 

primarily of Jurassic volcaniclastic turbidites; although Box 

(1985a) suggested a provenance link with the Hagemeister 

subterrane, according to Decker and others (1994), no direct 

depositional relation has been observed.

“The Goodnews terrane is a collage of variably metamorphosed blocks of laminated tuff, chert, basalt, graywacke, 

limestone, gabbro, and ultramafic rocks, in roughly that order of 

abundance” (Decker and others, 1994, p. 295). The Goodnews 

terrane includes, generally in fault-bounded blocks, Devonian to 

Ordovician, Permian, and Triassic limestone and early Paleozoic, 

Triassic, and Jurassic chert. Jones and Silberling (1979) defined 

the Tikchik terrane, which became one of three subterranes 

included in the Goodnews terrane by Box (1985a) and one of 

four subterranes of the Goodnews terrane by Decker and others 

(1994). The Tikchik terrane consists of a mélange of Paleozoic 

and Mesozoic chert, Permian limestone and clastic rocks, Permian or Triassic(?) pillow basalt, and Late Triassic rocks (Decker 

and others, 1994). The Cape Pierce subterrane is described 

1

 The generally accepted spelling for the community that this terrane is named 

after is Nyac (Orth, 1967).
5

as “foliated metamorphic rocks * * * probably derived from 

protoliths of probable Permian and Triassic ages” (Box, 1985a). 

Unfortunately, the description in Decker and others (1994) is not 

clear, but it seems that the Cape Pierce subterrane may include 

the Triassic ophiolitic rocks upon which the volcanic arc of the 

Togiak terrane was built. The Platinum subterrane is variably 

described as Lower and Middle Jurassic nonfoliated mafic flows, 

tuff, and volcaniclastic rocks, apparently containing interbedded calcareous tuff having Permian fossils or as the nonfoliated 

and less deformed equivalent of the lowest schistose nappe of 

the Permian to Triassic Cape Pierce terrane (Decker and others, 

1994). Clearly, there is an internal conflict in the definition of 

the subterrane; what is known for both of these subterranes is 

that they are intruded by zoned ultramafic bodies of presumed 

Jurassic age. The Platinum subterrane includes two of the Permian map units (Pv, Pvs) which we assign to the Togiak-Tikchik 

Complex. The Nukluk and Tikchik subterranes are similar in 

that both are structurally disrupted units or mélanges that contain 

blocks ranging in age from early Paleozoic to Jurassic or Early 

Cretaceous. The blocks include chert and limestone (units ^vs, 

Pls), clastic sedimentary rocks (unit Pcs), and Permian and 

Triassic volcanic rocks (units Pv, ^v). However, the Tikchik 

subterrane is spatially separated from the other parts of the 

Goodnews terrane; the Togiak terrane lies between the Tikchik 

subterrane and the rest of the Goodnews terrane.

The Nyac terrane is found in the Bethel quadrangle along 

the northwest part of the bedrock outcrop belt. Poorly defined, 

the Nyac (Nyack) terrane is thought to consist of an arc-related 

Jurassic volcanic and volcaniclastic rock assemblage (Decker 

and others, 1994). The Golden Gate Fault separates the Nyac 

and Kilbuck terranes where exposed. The Nyac terrane is 

distinctive in that it is the only terrane of the region intruded by 

mid-Cretaceous-age plutons.

The terrane nomenclature for this region is in need of 

revision as shown by the data and information used to create 

the present compilation. More detailed mapping and studies of 

the rock units will help to better define the appropriate packages and will better define the appropriate bounding structures. 

Paleontological and detrital zircon studies will help to better 

understand the affinities of the mapped rock units.

Overlap Assemblages

A number of Cretaceous sedimentary packages occur 

as overlap assemblages on the terranes. The oldest of these 

assemblages are of Valanginian age and occur in four belts 

of common age but distinctive lithology. The graywacke of 

Buchia Ridge (Kgbr) and the Ungalikthluk (Klg) belt of Hoare 

and Coonrad (1983) overlap the Togiak terrane, whereas the 

Mount Oratia (Kvm) and Eek Mountains (Kcgc) belts of Hoare 

and Coonrad (1983) overlap the Goodnews terrane. Rocks of 

the Eek Mountains belt include high-grade metamorphic clasts 

apparently derived from the Kanektok metamorphic complex.

A younger overlap assemblage, the Kuskokwim Group 

(Kk), a largely Late Cretaceous sedimentary unit, is widespread 

in southwest Alaska. In the map area, the Kuskokwim Group 

overlaps both the Goodnews and Kilbuck (Kanektok metamorphic complex) terranes. It locally underlies the Kilbuck terrane 

as well; Kuskokwim Group rocks crop out along the Kanektok 

River where the river cuts through the metamorphic complex 

that forms the ridges above.

The Summit Island Formation in the southern part of the 

map area is the youngest of the overlap assemblages. Apparently deposited in two adjacent(?) basins, the Summit Island 

Formation overlaps the Togiak terrane.

Latest Cretaceous or earliest Tertiary plutonic rocks intrude 

all terranes east of the Kilbuck terrane; plutonic rocks of this 

age are conspicuously absent west of the terrane boundary. As 

mentioned earlier, within the map area mid-Cretaceous plutonic 

rocks have been found only in the Nyac terrane.

Faults

Faults are mapped throughout the map area. One of the 

major faults, the Holitna Fault, enters the map area from the 

northeast and is considered part of the Denali-Farewell Fault 

System of southern Alaska. The Holitna Fault has a trace that 

cuts surficial deposits indicating it is presently active (Cady and 

others, 1955; F.H. Wilson, unpub. data). Field maps from the 

1969 and 1970 seasons of J.M. Hoare and W.H. Condon (unpub. 

data) indicate northwest side up and right-lateral offset. Cady 

and others (1955) indicated that the Holitna Fault had a highangle reverse sense of motion. The Holitna Fault is generally 

linked with the Togiak-Tikchik and Hagemeister Faults (Hoare 

and Coonrad, 1978), which strike offshore into Bristol Bay.

Other sub-parallel faults, such as the Golden Gate (Hoare 

and Coonrad, 1959a), Goodnews, and East Kulukak Faults 

(Hoare and Coonrad, 1978) provide strong evidence for a northeast-trending structural grain and are in some cases considered 

terrane-bounding faults (see Box and others, 1993; Decker and 

others, 1994). The Golden Gate Fault generally trends parallel to the Denali-Farewell Fault System and passes through the 

western Bethel quadrangle. The Golden Gate Fault is a highangle reverse fault dipping steeply to the southeast. A number of 

northwest-trending faults crosscut and offset rock units and the 

northeast-trending faults. Key among these northeast-trending 

faults are the Mount Oratia and Fork (or Lake) Creek high-angle 

reverse faults that are cut by the Trail Creek strike-slip fault of 

Hoare and Coonrad (1978); each of these northwest-trending 

faults follows topographic lineaments along which the rocks on 

either side generally indicate left-lateral offset. For example, the 

northwest-trending Trail Creek Fault appears to have 10 to 15 

km of left-lateral strike-slip movement.

Thrust and high-angle reverse faults are an important 

structural feature in the map area; however, as will be discussed 

more fully in the discussion of the Kanektok metamorphic 

complex rocks, agreement between the available map sources 

is limited at best. Box (1985a) mapped a complex system of 

thrust faults and folded thrust faults cutting Mesozoic rock units 

between Kuskokwim and Togiak Bays. While these faults are 

shown here as “thrusts”, the rock units separated by these faults 

have many common characteristics and the faults commonly 

emplace younger over older rocks.
6

Metamorphism and Metamorphic Rocks

Description of the metamorphic character of the rocks 

of the region, such as facies and grade, is incomplete at best. 

Although poorly known, the most thorough descriptions have 

been made for the Kanektok metamorphic complex, described 

later. Available descriptions indicate that many of the rocks of 

the Bethel quadrangle have undergone low-grade metamorphism, whereas the rocks of the Kisaralik Anticlinorium of 

Box and others (1993) in the north-central Bethel quadrangle 

are metamorphosed to greenschist and transitional greenschistblueschist facies. Similarly, metamorphic rocks in the vicinity 

of Goodnews Bay (see “Metamorphic Rocks of the TogiakTikchik Complex” section in the Description of Map Units) also 

are metamorphosed to greenschist and transitional greenschistblueschist facies. Deformation of rock units is even less well 

described; where data is available, it is included in the applicable unit description.

Figure 2. Tectonostratigraphic terrane map of southwestern Alaska, (from Decker and others, 1994). The 

included part of the Kuskokwim Bay region map area is shown outlined in black. See map explanation on 

facing page. 

FWb

?

BRISTOL

BAY

Yukon River

Kuskokwim

River

153°

156°

159°

63°

162°

61°

60°

62°

64°

?

?

?

59°

58°

LIME HILLS

LAKE CLARK

TAYLOR MOUNTAINS

DILLINGHAM

GOODNEWS

BAY

NUSHAGAK BAY HAGEMEISTER ISLAND

BETHEL

RUSSIAN

MISSION

SLEETMUTE

IDITAROD

MEDFRA

MCGRATH

MINCHUM NI A

S UTURE

SUSULATINA LINEAMENTSUSULATINA LINEAMENT

IDITAROD-NIXON FORK FAULT

BIG RIVER FAULT BIG RIVER FAULT

KULUKBUK THRUSTKULUKBUK THRUST

ANIAK - THOMPSON

CREEK FAULT

FAULT

GATE

GOLDEN

FAULT

GATE

GOLDEN

EAST KULUKBUK

GOODNEWS FAULT

KANEKTOK THRUST

FAULT

MULCHATNA

FAULT

MULCHATNA

FAUL

D

T

ENALI-FAREWELL FAULT

DENALI-FAREWELL FAULT

LAKE CLARK-CASTLE MTN FAULT

Kk

Kk

FWp

THA

ps

ps

Q

Q

TKU

{ {

{

PE

IN

Q

Q

Q

Q

Q is

RB

Q

YT

YT

Q

FMY

FMY

FMY

FMY

KH

FMY

THA

THA

GCP

THA

THA

TKU

TKU

Kk

Kk

Q

Q

Q

Q

Q

Q

FWp

FWb

NY

NY

K

K

K

K

FWp

FWp

{

SCALE IN KILOMETERS

0 50 100

GPT

GNU

KI GTC

FWt

FMY

ALASKA

Anchorage

Area of map
7

A distinctive sequence of lithologic units is exposed on a 

ridge in the Kilbuck Mountains just east of Greenstone Ridge 

in the Bethel quadrangle. These rocks were originally mapped 

by Hoare and Coonrad (1959a) as part of the Gemuk Group 

surrounded by rocks of the Kuskokwim Group. Box and others 

(1993) recognized the rocks as an antiformally folded structural 

sequence of variably metamorphosed rocks that they informally 

referred to as the Kisaralik anticlinorium. The anticlinorium 

is cored by a structurally imbricate package of Mesozoic to 

Paleozoic deep-marine sedimentary and mafic volcanic rocks 

(Box and others, 1993). The folding that created the anticlinorium also folded the rocks of the Kuskokwim Group nearshore facies (Kkn) that unconformably overlie the Mesozoic 

to Paleozoic core (Box and others, 1993). Deformation and 

metamorphic grade decrease structurally downward through the 

structural package. In structural order from top to bottom, map 

units }|p, MDm, and }|m are strongly foliated and are 

metamorphosed to greenschist and transitional greenschist-blueschist facies, whereas lower units in the anticlinorium, }|s, 

}|v, }|c, have a slaty cleavage and are metamorphosed 

to prehnite-pumpellyite facies (Box and others, 1993). Units 

are separated by antiformally folded low-angle faults (Box and 

others, 1993). Units making up the Kisaralik anticlinorium were 

included in the Goodnews terrane by Box and others (1993). 

Box and others (1993) used the basal conglomerate of the Kuskokwim Group, which they reported as Late Cretaceous (Cenomanian(?)) in age, to constrain the younger age limit of these 

map units. However, fossils of Albian (late Early Cretaceous) 

age are reported by Murphy (1987) for this map unit (Kkn).

Quaternary Geology

The Kuskokwim Bay region has a rich Quaternary geologic history of mafic volcanism, glacial advances, eolian 

deposition, varying sea level, and the development of periglacial features. Detailed mapping of surficial geology has 

not been undertaken, except for the mafic volcanic rocks on 

Nunivak Island and the Pribilof Islands (Cox and others, 1966; 

1968; Hoare and others, 1968); in much of the map area surficial deposits are not subdivided. A number of topical studies 

by Kaufman and co-workers (Kaufman and Manley, 2004; 

Kaufman and others, 2001a, 2003; Manley and others, 2001; 

Levy and others, 2004) and Lea and co-workers (Lea, 1989; 

Lea and Waythomas, 1990; Lea and others, 1991) have begun 

establishment of a framework for the Quaternary history and 

stratigraphy of the map area. In particular, Lea (1989) defined 

a number of formal and informal stratigraphic units and type 

sections in the southeast part of the map area. However, to date, 

maps showing the distribution of these units have not been 

produced.

Within the map area, glacial deposits range from Holocene 

to an indeterminate Pleistocene age. The oldest of the deposits 

form the informally defined Nichols Hills drift of Lea (1989) 

on the Nushagak Peninsula. These deposits are undated and 

designated oldest because they stratigraphically underlie better 

defined deposits of the Nushagak Formation and lie outboard 

of glacial deposits that still retain some of their original morphology. The Nichols Hills drift, according to Lea (1989, p. 88) 

may overlie a still older diamicton exposed at Cape Constantine 

reported by W.L. Coonrad (oral commun. to P.D. Lea, 1982). 

?

EXPLANATION

OVERLAP ASSEMBLAGES

TERRANES OF SOUTHWEST ALASKA

ADJACENT TERRANES

UNITS OF UNCERTAIN TERRANE AFFINITY

Quaternary Surficial Deposits

Kuskokwim Group

Cenozoic Deposits

Alaska Range and Kuskokwim Mountains

Farewell Terrane

Mystic Sequence

Basinal Facies

Transitional Facies

Platform Facies

Nukluk Subterrane

Tikchik Subterrane

Platinum Subterrane

White Mountain Sequence

Bristol Bay Region

Nyack Terrane

Togiak Terrane

Hagemeister Subterrane

Kulukak Subterrane

Goodnews Terrane

Kilbuck Terrane 

Cape Pierce Subterrane

Innoko Terrane

Yukon-Tanana Terrane 

Portage Sequence 

Ruby Terrane

Northern Kahiltna Terrane

Southern Kahiltna Terrane

Peninsular Terrane 

Idono Sequence 

Contact 

Q

K

{

FMY

FWb

FWt

FWp

NY

KH

Kk

PE

THA

TKU

GNU

GTC

GPT

GCP

KI

IN

RB

YT

ps

is

Fault—Dashed where approximate; dotted 

where concealed; queried where uncertain

Thrust fault—Dotted where concealed. 

Sawteeth on upper plate
8

Overlying the Nichols Hills drift are the deposits of the Nushagak Formation. This unit, originally called the Nushagak beds 

by Spurr (1900) and formalized as the Nushagak Formation by 

Mertie (1938), was originally designated as Pliocene or Miocene 

age and thought to represent nearshore marine deposits. However, re-examination of the type area by Lea (1989) indicates 

that the deposits are proglacial intertidal and fluvial sediments. 

According to Louie Marincovich, Jr., (USGS, oral commun., 

1988) to Lea (1989, p. 91), the originally reported molluscan 

fossil assemblage for the formation is now considered to be of 

Quaternary age. The Nushagak Formation includes morainal 

deposits designated as the Ekuk moraines, which Lea (1989) 

tentatively correlated with the Halfmoon Bay drift of Muller 

(1952). Locally, younger morainal deposits, informally named 

the Manokotak and Okstukuk moraines, which may record 

early? Wisconsin and late Wisconsin maxima, are mapped north 

of Nushagak Bay (Kaufman and others, 2001a,b).

Prior to this map compilation, virtually no maps of surficial 

deposits west of the Ahklun Mountains existed. W.L. Coonrad 

(USGS, oral commun., 2002?) reported the presence of glacial deposits in river cuts in the Yukon Kuskokwim Coastal 

Lowland section of the Bering Shelf province of Wahrhaftig 

(1965). Hoare and Coonrad (1959a, 1961b) showed a reconnaissance-level subdivision of surficial deposits in the Bethel 

and Goodnews Bay quadrangles and, where possible, that map 

data is shown here. We show a reconnaissance-level subdivision of surficial deposits in the Baird Inlet and Kuskokwim Bay 

quadrangles on the basis of topography and surface character. 

In the Nushagak Bay quadrangle, unpublished mapping of W.L. 

Coonrad in the northern part of the quadrangle was extended 

southward based on W.L. Coonrad’s field sheets and our topographic interpretation.

Geochronology

There is a long history of geochronologic studies in the 

map area, yet coverage of geologic units remains scant and data 

interpretation, particularly on the older rock units (table 1) is 

open to some controversy.

More than 60 K-Ar age determinations on young mafic 

volcanic rocks on Nunivak Island and the Pribilof Islands were 

used to provide precise age control for the paleomagnetic time 

scale developed by Cox, Dalrymple, and Hoare (Cox and Dalrymple, 1967; Hoare and others, 1968; Mankinen and Dalrymple, 1979). During mineral assessment studies of the mid-1970s, 

K-Ar dating plutonic rocks was an emphasis, particularly in the 

Goodnews Bay and Hagemeister Island quadrangles (Hoare and 

Coonrad, 1978; Kilburn and others, 1993). Wilson and Smith 

(1976) provided dates on many of the plutonic rocks of these 

quadrangles. Box and others (1993) reported K-Ar, 40Ar/39Ar, 

and U/Pb dates on igneous and metamorphic rocks in the Bethel 

quadrangle including a suite of Early Cretaceous plutons in the 

northernmost part of the map area. Box and others (1993) also 

reported fission-track cooling ages on a variety of rocks. Turner 

and others (1983) reported studies on the Kilbuck terrane rocks 

in the Goodnews Bay quadrangle; the extensive analytical data 

for these studies is presented in Turner and others (2009). Other 

studies (Globerman, 1985; Box, 1985a; Wittbrodt and others, 

1989) have added a number of individual dates in parts of the 

map area, all contributing to understanding of the history of the 

region.

Dates on most plutonic rocks in the map area yield ages 

that straddle the Tertiary and Cretaceous boundary (60 to 70 

Ma). Plutons of this age are widespread throughout much of 

southwest Alaska. A reverse discordance, in which hornblende 

consistently yields younger ages than co-existing biotite, is 

common among the plutonic rocks in the eastern part of the 

Goodnews Bay quadrangle (Wilson and Smith, 1976). The same 

reverse discordance is shown by one of the Early Cretaceous 

plutons in the Bethel quadrangle (Box and others, 1993). This 

discordance suggests an as yet unexplained problem with the 

age determinations. In normal conditions, where argon is locked 

in a mineral as it passes through the blocking temperature, 

hornblende should yield an older age because it has a higher 

blocking temperature. The age difference between minerals then 

would be dependent on the cooling rate.

The main focus of geochronologic studies in the mainland 

portion of the map area has been on the Kanektok metamorphic 

complex (fig. 3) or Kilbuck terrane. D.L. Turner and others 

(2009) measured 77 K-Ar ages on 58 samples of the Kanektok 

metamorphic complex, as well as dating zircon and sphene by 

the U/Pb method. They also attempted Rb/Sr geochronology 

with limited success. Extensive dating of the Kanektok metamorphic complex yielded widely varying results with respect 

to the age of rocks. If the K-Ar results, including 40Ar/39Ar, 

are considered simply as cooling ages, the results suggest that 

an Early Cretaceous thermal event affected the much older 

metamorphic complex and was of sufficient intensity to reset 

amphibole (about 500° C). Many of the K-Ar ages determined, 

both on metamorphic and plutonic rocks, were Mesozoic and 

commonly discordant when multiple minerals from the same 

sample could be dated. These highly discordant ages may suggest a long slow cooling. A few samples show the same reverse 

discordance as the plutonic rocks mentioned earlier in which 

hornblende yields younger ages than co-existing biotite or muscovite, or in which muscovite yields younger ages than coexisting biotite. However, for most samples, hornblende ages are 100 

m.y. or more older than coexisting biotite.

A number of the K-Ar and 40Ar/39Ar ages for the Kanektok 

metamorphic complex fall between 120 and 150 Ma; however, 

many more samples yield discordant ages significantly older 

than 150 Ma. One sample, DT72-82, yields concordant biotite 

and hornblende ages of about 212 Ma (table 1), suggesting a 

discrete event. Few other samples yielded concordant old ages; 

however, one was as old as about 480 Ma (Sample DT75-70, 

table 1). The minimum mica and amphibole ages are concordant 

at about 128 Ma.

Three pyroxene granulite samples yielded significantly 

older K-Ar ages than any other rocks in Alaska. Multiple analyses of biotite splits from these granulite samples yielded average 

ages between 2,255±71.2 and 2,474±75 Ma (table 1). A 40Ar/39Ar 

age of the oldest of these samples yielded a plateau age of 2,520 

Ma and showed no indication of excess argon. U/Pb ages of 

zircon from granite gneiss and tonalite gneisses from throughout 
9

the complex, including locations not distant from these granulite 

samples, cluster around 2,050 Ma (Turner and others, 2009; Box 

and others, 1990; Moll-Stalcup and others, 1996; table 1, herein). 

Turner and others (2009) made a case that the 2.5 Ga biotite 

samples contain excess argon by analogy with similar rocks from 

elsewhere and by the suggestion that the incremental heating plateau was an experimental artifact. Multiple studies, as reported 

in Faure and Mensing (2005), indicate that the 40Ar/39Ar method 

may not be able to distinguish excess argon; therefore, the U/Pb 

age of 2.05 Ga most likely represents the best estimate of the 

original emplacement age of the complex.

One sphene U/Pb age of 1,770 Ma was measured by 

Turner and others (2009) and Moll-Stalcup and others (1996) 

and was interpreted by Turner and others (2009) to represent the 

age of amphibolite-facies metamorphism of the terrane. This 

interpretation is supported by analysis of a suite of 13 samples 

collected for Rb/Sr analysis and by a single K-Ar hornblende 

age. Six of the Rb/Sr samples of Turner and others (2009) lie 

along a 1,780 Ma isochron. Moll-Stalcup and others (1996) similarly collected samples for Rb/Sr analysis, which they divided 

into two suites: granitic-gneiss and tonalite-gneiss. The graniticgneiss suite samples scatter about a reference line at 1,800 Ma, 

whereas the tonalite-gneiss suite samples scatter about a 2,070 

Ma reference isochron. Moll-Stalcup and others (1996) suggested either the results may indicate that the granitic gneisses 

are younger than the tonalite-gneiss suite or that the graniticgneiss suite Rb/Sr systematics have been disturbed. However, 

in common with Turner and others (2009), Moll-Stalcup and 

others (1996) did not consider the fit of the data adequate to 

yield reliable isochron ages. Both groups of workers collected 

amphibolite samples and in neither case did the data from these 

rocks define a statistically valid line on an Rb/Sr isotope plot. 

Moll-Stalcup and others (1996) indicated that the amphibolites 

are chemically distinct from the gneisses and form at least three 

groups based on their chemical affinity. They infer that the 

amphibolites may represent “dikes or sills that were intruded at 

different times.” Amphibolite samples for the complex yielded 

widely varying conventional K-Ar and 40Ar/39Ar ages, ranging 

from 130.6 to 1,778 Ma, suggesting significant thermal effects. 

Where multiple phases were dated, the discordance is great; for 

example, see sample DT76-70 in table 1.

Most samples from the tonalite-gneiss and granitic-gneiss 

suites yield Nd-Sm depleted-mantle model ages that range near 

2.1 Ga, concordant with the U/Pb zircon ages determined on 

these rocks, which suggests separation from a mantle reservoir 

at that time and that the Sm/Nd ratio has been unmodified since 

that time (Moll-Stalcup and others, 1996). One granitic-gneiss 

sample yielded a Sm/Nd model age of about 2.7 Ga (Box and 

others, 1990; Moll-Stalcup and others, 1996), which suggests 

incorporation of an Archean component. Miller and others 

(1991) reported a similar result of a 2.05 Ga U/Pb age and 2.5 

Ga Sm/Nd model age from the Idono complex in the Iditarod 

quadrangle, northeast of the map area. This result also suggests 

incorporation of Archean crust. The Idono complex and the 

Kanektok metamorphic complex are the only rocks in Alaska 

known to have a similar early history.

Kanektok Metamorphic Complex—

Enigma in Southwest Alaska

The Kanektok metamorphic complex was originally 

assigned a Precambrian age because the complex was more 

metamorphosed than nearby Paleozoic rocks (Hoare and Coonrad, 1959a, 1961b). Subsequently, a number of studies (Hoare 

and Coonrad, 1979; Box, 1982; Box and others, 1990; MollStalcup and others, 1996) of the Kanektok metamorphic complex and the Idono complex of Miller and others (1991) have 

shown that the complexes are thin slivers of Paleoproterozoic 

crust distinct from any in Alaska. To date, no cratonic source 

has been identified for these high-grade metamorphic rocks. The 

subsequent history of these rocks is also unclear, as geochronologic studies (Turner and others, 2009; Moll-Stalcup and others, 

1996) have yet been unsuccessful at deciphering their Paleozoic 

and Mesozoic history.

Beyond agreement that the Kanektok metamorphic 

complex is a thin sliver of early Proterozoic crust, Hoare and 

Coonrad (1978, 1979) and Box, Moll-Stalcup, and coworkers 

(Box, 1982; Box and others, 1990; Moll-Stalcup and others, 

1996) had diametrically opposed interpretations of the character of the contact with surrounding rocks and, therefore, the 

tectonic setting of the sliver of crust. The southeast bounding 

fault of the metamorphic complex was mapped and described 

by Hoare and Coonrad (1978, 1979) as a northwest-dipping 

low-angle thrust fault. Hoare and Coonrad (1979) reported that 

Cretaceous shale of the Kuskokwim Group (unit Kk) crops out 

in a number of places where the Kanektok River cuts through 

the complex. Because Kuskokwim Group rocks also crop out 

east of the complex, Hoare and Coonrad (1978, 1979) inferred 

that the contact was a thrust, placing the Kanektok metamorphic 

complex over the Cretaceous shale of the Kuskokwim Group. 

Conversely, Box and others (1990) and Moll-Stalcup and others 

(1996) interpreted the rocks of the metamorphic complex as 

being underthrust southeastward below Lower Cretaceous and 

older rocks. Box (1982; see also Box and others, 1990) inferred 

the metamorphic complex (Kilbuck terrane) collided with and 

partially underthrust the adjacent rocks that they assigned to 

the Goodnews and Togiak terranes, an oceanic-arc-subduction 

complex. However, we have difficulty reconciling Box’s interpretation with the mapping of Hoare and Coonrad (1978). Additionally, Box’s interpretation is difficult to reconcile with the 

available geochronologic data. For the most part, no contacts 

of the metamorphic complex with rocks on the west side of the 

contact are exposed; the Golden Gate Fault System is inferred 

to form the western boundary of the complex. The only portion of the complex in contact with a bedrock unit to the west 

is a very small exposure in the Bethel quadrangle. Hoare and 

Coonrad (1959a) showed this contact as a depositional contact 

with the Kuskokwim Group, whereas Box and others (1993) 

showed it as a fault contact, thrusting the metamorphic rocks 

over the Kuskokwim Group along a strand of the Golden Gate 

Fault. As this faulting must postdate the apparent thermal event 

that affected the west side of the terrane, the nature and cause 

of this thermal event remains to be determined. The nature of 

the contact and what rocks lie on the west side of the complex 
10

is unknown, especially in the vicinity of the Kanektok River 

and southward. R.W. Saltus (written commun., 2006) analyzed 

available aeromagnetic and gravity data, including a gravity 

traverse along the Kanektok River that crosses the metamorphic 

complex. In his interpretation, the metamorphic complex is a 

northwestward-dipping crustal block. This interpretation generally supports the mapping of Hoare and Coonrad (1978, 1979) 

and does not support the interpretation that the rocks of the 

metamorphic complex underthrust rocks to the east.

Spatial analysis of the K-Ar ages within the Kanektok 

metamorphic complex indicates a general pattern of decreasing age with distance from the southeast-bounding fault of 

the complex. Such a pattern has a number of possible interpretations. One possibility is that the easternmost parts of the 

complex were less affected, if at all, by the inferred Cretaceous 

(120–130 Ma) thermal event and may have been unroofed long 

before, as suggested by biotite ages of 190 Ma and older. Inclusion of metamorphic minerals and metamorphic rock clasts in 

the Valanginian age sedimentary rocks of the Eek Mountains 

belt (unit Kcgc), deposited at approximately the same time of 

the inferred thermal event in the western part of the complex, 

as well as in the younger basal conglomerate (unit Kkn) of the 

Kuskokwim Group of Albian age, indicates markedly different 

histories for the east and west sides of the exposed complex. A 

different interpretation by Turner and others (2009) suggested a 

cluster of K-Ar ages around 120–150 Ma, with biotite and hornblende ages extending back in time to more than 1.7 Ga, which 

suggests variable argon loss from an Early Cretaceous to Late 

Jurassic thermal event. They suggested this might have been 

related to widespread small intrusions of an alaskite granite that 

is undated and not shown on available maps. However, given 

the spatial distribution of the ages, we consider this unlikely. An 

analysis of K-Ar data for the metamorphic rocks of the Yukon 

Crystalline Terrane of central Alaska (Wilson and others, 1985) 

indicated little evidence for widespread resetting of metamorphic rock due to intrusion of plutons. 

Figure 3. Gneiss of the Kanektok metamorphic complex (<k) north of Thumb Mountain.
11

DESCRIPTION OF MAP UNITS

[Some unit exposures on the map are too small to distinguish the color for unit identification. These units 

are labeled where possible, and all units are identified by attributes in the digital database. Some unit labels 

listed in unit descriptions below refer to units not exposed within this map area]

UNCONSOLIDATED DEPOSITS

Qs Surficial deposits, undivided (Quaternary)—Unconsolidated, poorly sorted to well-sorted, 

poorly stratified to moderately well stratified deposits; consist predominantly of alluvial, 

colluvial, glacial, marine, lacustrine, eolian, and swamp deposits (Hoare and Coonrad, 

1959a, 1961a,b; Box and others, 1993; Coonrad, 1957). Locally includes reworked volcanic 

debris including block and ash flows. Locally subdivided into the following:

Qtf Tidal flat and active estuarine deposits (Holocene)—Fine-grained silt and mud in tidal flats 

and active estuaries as shown on topographic maps. Also includes older, topographically 

higher deposits on land. The abrupt ending of the tidal flat deposits at approximately long 

163°20 W. in the Kuskokwim Bay quadrangle is an artifact of the topographic maps; the 

tidal flats likely continue along the coast to the west

Qa Alluvial deposits—Flood-plain alluvium of sand, gravel, and boulders. Locally may include 

sand and small pebbles from beach deposits, clay-rich silt from estuarine deposits, finegrained eolian sand, and small areas of undivided surficial deposits (Hoare and Coonrad, 

1959a, 1961a; Box and others, 1993)

Qrs Reworked silt—Chiefly reworked silt, sandy silt, and bog deposits underlying a plain transitional with or slightly above areas of floodplain alluvium of Holocene age and separated 

from higher plains and residual islands of older silt deposits by an erosional scarp 3 to 15 m 

high (Hoare and Coonrad, 1959a)

Qb Beach deposits—Primarily fine sand to coarse gravel along modern and raised ancient 

beaches. Locally includes windblown dune sand. In the Kuskokwim Bay, Baird Inlet, and 

eastern Nunivak Island quadrangles, beach deposits extend 3 km inland as a series of raised 

beach ridges and may underlie the marine terraces farther inland. Along the Nushagak Peninsula, windblown sand derived from the adjacent tidal flats forms ridges interspersed with 

reworked glacially derived gravels

Qes Estuarine deposits—Generally fine silt to sand, but may include significant proportion of 

mud. Primarily found in the vicinity of Nushagak Bay; deposits extend well inland along the 

Nushagak and Igushik Rivers

Qmt Marine terraces—Raised terraces composed of many types of surficial deposits, which are 

primarily estuarine, tidal flat, and alluvial deposits, but may include glacial debris and outwash. Surfaces may be separated from younger active deposits by a bluff or may be gradational. Elevation ranges from less than 8 m in the western map area to as much as 12 m in 

the eastern map area

Qed Eolian deposits—In the Pribilof Islands, mostly composed of sand derived from alluvial 

sediments. Elsewhere, deposits primarily derived from glacial deposits or nearby tidal flats. 

In most places, the sand is loose and moves with the wind. Along the north shore of Saint 

Paul Island, impressive dunes occur (Barth, 1956). Deposits also widely distributed on the 

Nushagak Peninsula, where they overlie glacial, estuarine, and beach deposits. Lea (1989) 

described widespread eolian sand sheets in the Nushagak Lowland northeast of the map 

area and sand sheets have also been described locally (but not shown here) on the Nushagak 

Peninsula (W.L. Coonrad, unpub. data, 2007)

Qsd Silt deposits—Chiefly light- to dark-gray silt and sandy silt containing abundant permafrost. 

Deposits become sandier with depth and locally contain pebbles and wood fragments. 

Deposits are probably of nonmarine fluvial origin but may include eolian and marine members in some areas. Organic muck containing mammoth remains and nonmarine gastropods 

is found locally near top of deposits (Hoare and Coonrad, 1959a, 1961b)
12

Qg Glacial deposits, undivided—Glacial drift deposited during the last three glacial advances. Consists of sand, gravel, and boulders. Includes end and lateral moraine deposits, as well as colluvium, talus, landslide debris, alluvium, and local silt (Hoare and Coonrad, 1959a, 1961a,b; 

Box and others, 1993). Locally subdivided into the following:

Qgm Glacial moraine—Morainal ridges of generally late Wisconsin age, but may include deposits 

of other ages. Largely mapped from air-photo and topographic map interpretation. In the 

Nushagak Bay quadrangle, mapping is largely derived from the unpublished field data of 

W.L. Coonrad. Likely composed of poorly sorted silt to cobble and boulder-sized material

Qgo Outwash deposits—Poorly sorted to well-sorted sand and gravel with some silt and a few 

boulders, constituting terraces and outwash-fan plains (Hoare and Coonrad, 1959a, 1961a,b)

Qvs Volcaniclastic sediment (Quaternary)—Fossiliferous sediments consisting of sand and yellow 

tuff containing rounded basalt pebbles. On Saint Paul Island, sediments are polymictic, 

but composed primarily of basaltic boulders and clasts and locally crossbedded. On Saint 

George Island, sediments overlie a glaciated surface of peridotite upon which Pleistocene 

fossil shells have been cemented. The sediments are quite varied; however, a basal conglomerate has peridotite boulders but primarily consists of basalt boulders. A sand layer containing casts of fossil shells overlies the basal conglomerate; however, no CaCO3

 is left in the 

casts and the casts disintegrate on contact (Barth, 1956). Locally crossbedded

QTs Semiconsolidated marine beach deposits (Pleistocene or Pliocene)—Semiconsolidated marine 

beach deposits consisting of poorly bedded, soft, pebbly siltstone that caps sea cliffs of volcanic rock (unit Kv) on Hagemeister Island. Contains Pliocene or Pleistocene age shallowwater marine fossils

SEDIMENTARY ROCKS

Ks Summit Island Formation (Upper Cretaceous, Maastrichtian?)—Consists of lenses and intertonguing beds of nonmarine conglomerate, sandstone, carbonaceous siltstone, mudstone, 

and shale containing abundant plant detritus and a few coal seams (Hoare and others, 1983). 

In the type section, the unit “consists of about 200 m of massive and thick-bedded pebble 

[to] cobble conglomerate overlain by about 650 m of interbedded sandstone, siltstone, and 

mudstone and a few conglomerate beds” (Hoare and others, 1983). At the reference section of the unit, its character changes to consist of “about 75 percent carbonaceous mudstone, siltstone, and shale and 25 percent interbedded sandstone and pebble grit” (Hoare 

and others, 1983). Bedding is discontinuous and best developed in lower part of formation 

where thick conglomerate beds grade into pebble sandstone and sandstone beds (Hoare and 

others, 1983). Conglomerate clasts are well rounded, generally not larger than 10 cm, and 

primarily derived from flows, tuffs, and sedimentary rocks of Jurassic and Early Cretaceous 

age (Hoare and others, 1983). Hoare and others (1983) also reported white quartz clasts 

and sparse schist and plutonic clasts; they suggest the schist clasts are derived from tectonically metamorphosed rocks that occur locally along major faults in region. Sandstone and 

siltstone consist of 60 to 70 percent lithic fragments and 10 to 15 percent each of quartz and 

feldspar (Hoare and others, 1983). Lithic component of the sandstone and siltstone is very 

fine-grained cherty tuff and other volcanic rock fragments, as well as argillite, quartzite, 

and chert (Hoare and others, 1983). Hoare and others (1983) indicated the hardness of the 

rocks varies with their structure and, by implication, location. At the west end of Summit 

Island and the west side of the unnamed adjacent headland on the mainland, the rocks dip 

45° to 80° and are hard and well indurated. On the east end of the island and elsewhere on 

the mainland, dips are shallow and the rocks are less well indurated. Box (1985a) divided 

the unit into two subbasins along the trace of the Togiak-Tikchik Fault, based on contrasting 

sandstone compositions. Sandstone of his Summit subbasin (southeast of Togiak-Tikchik 

Fault) is composed mostly of slaty sedimentary lithic fragments, similar to underlying Jurassic sedimentary rocks. Rocks in the Summit subbasin are more than 1 km thick and consist 

of a lower conglomeratic member (200 m) of probable alluvial fan to braided stream origin 

and an upper carbonaceous shale member of probable meandering stream-flood plain origin, 

having channelized sandstone bodies that become finer grained and thinner upward. Paleocurrent indicators in the upper member generally indicate transport to the northeast. Sandstone of Box’s Hagemeister subbasin (northwest of Togiak-Tikchik Fault) is composed of 

nonfoliated volcanic detritus similar to underlying Jurassic rocks. The Hagemeister subbasin 
13

section is also more than 1 km thick, consisting of a lower channelized sandstone member 

(600 m) of probable meandering stream origin, and an upper sandy member (400 m) having 

coarsening and thickening upward cycles beginning with laminated siltstone-coal intervals, 

which are possibly lacustrine delta deposits. Paleocurrent indicators in the Hagemeister subbasin generally indicate transport to the northwest (Box, 1985a). Plant fossils from the base 

of the reference section indicate a latest Cretaceous or early Tertiary age (Jack Wolfe, USGS, 

oral commun. 1974, cited in Hoare and others, 1983). A dike that cuts the formation yielded 

an age of 64.6±2 Ma (74ACd 14d, table 1), which suggested to Hoare and others (1983) 

that a Late Cretaceous age for the Summit Island Formation was more probable. Subsequent 

dating of a dike (Box, 1985a) cutting the formation yielded an older age of 76.6±4.5 Ma 

(B78-1125G, table 1), further supporting the Late Cretaceous age assignment. Pollen, identified by R.H. Tschudy (USGS, written commun., 1975, cited in Hoare and others, 1983), 

formed a unique assemblage. Several of the taxa had been reported from Arctic Canada and 

western Siberia, but the complete assemblage was unique and therefore Tschudy was reluctant to assign the unit a definite Late Cretaceous age. However, on the basis of the fossil 

evidence and radiometric data, Hoare and others (1983) assigned a Late Cretaceous, possibly 

Maastrichtian age

Kk Kuskokwim Group, undivided (Cretaceous, Campanian? to Albian)—Interbedded graywacke and shale; local interbeds of argillite and conglomerate. Graywacke fine- to mediumgrained, gray, commonly micaceous and locally silty; in places is crossbedded, or contains 

siltstone partings. Contains rare argillite pebbles. Originally described by Cady and others 

(1955), the Kuskokwim Group is widespread in southwestern Alaska and, although largely 

a flysch deposit, unit represents a range of local depositional environments. In the Bethel 

quadrangle, Box and others (1993) subdivided the upper part of unit on the basis of provenance and depositional environment; however, data is insufficient to carry those distinctions 

throughout map area. They described three provenances (chert-clast, mixed, and volcanic) 

and three depositional environments (outer-fan turbidite, inner-fan turbidite, and deltaic). 

We have placed the deltaic environment, which is restricted to the chert-clast provenance, 

in the nearshore facies subdivision (Kkn) of this map. The other chert-clast provenance 

rocks are described as outer-fan facies and consist of thin-bedded, fine-grained, quartzose 

sandstone and shale with lesser thin- and thick-bedded medium-grained sandstone (Box 

and others, 1993). The mixed provenance rocks represent two depositional environments: 

an outer-fan turbidite facies consisting of a shale-rich sequence with lesser thin- to thickbedded, medium-grained sandstone sections and an inner-fan channel facies consisting of 

mixed shale-rich and sandstone-rich sections, with coarse sandstone and pebbly sandstone in 

thick-bedded, amalgamated sequences with minor interbedded shale (Box and others, 1993). 

Volcanic provenance rocks consist of interbedded shale, siltstone, sandstone, and conglomerate composed of rounded clasts of volcanic, volcaniclastic, and plutonic rocks deposited in 

slope and inner-fan environments (Box and others, 1993). Box and others (1993) divided the 

lower part of the Kuskokwim Group into two units: an upper shale and siltstone unit consisting of dark-gray to black, finely laminated shale, siltstone, and a thin-bedded, very finegrained sandstone and a basal conglomerate unit of pebble to boulder conglomerate, coarse 

sandstone, and minor interbedded medium-grained sandstone, siltstone, and shale. Box and 

others (1993) suggested the shale and siltstone represent sand-poor slope, interchannel, and 

distal basin-plain environments. Basal conglomerate is assigned to map unit Kkn herein, 

described below. The age of the Kuskokwim Group is not well constrained; multiple provenances and potentially multiple depositional systems are included within the map unit. 

Cady and others (1955) assigned an early Late Cretaceous age in part, but acknowledged 

collections outside their map area that might indicate an age older than Late Cretaceous and 

as young as Tertiary. Elder and Box (1992) and Box and others (1993) assigned an early 

Turonian to late Cenomanian age based on inoceramid fossils in the Bethel quadrangle, 

whereas Hoare and Coonrad (1959a) reported Albian and Cenomanian to Coniacian fossils 

from the same area, and Murphy (1989) reported Albian fossils. Other age assignments are 

as young as Santonian (Decker and others, 1994) or Campanian(?) (Miller and Bundtzen, 

1994). Locally subdivided into the following:

Kkn Nearshore facies (Cretaceous, Turonian to Albian)—Thick-bedded quartzose sandstone, 

pebbly sandstone, and subordinate siltstone and shale of the deltaic facies chert-clast provenance of Box and others (1993). Also includes pebble to boulder conglomerate, coarse 
14

sandstone, and minor interbedded medium-grained sandstone, siltstone, and shale of the 

basal conglomerate of Box and others (1993). This informal subdivision of the Kuskokwim 

Group was originally defined by Platt and Muller (USGS unpub. data, 1957, cited in Wilson 

and others, 2006b) in the northeastern Taylor Mountains quadrangle; subsequent mapping in 

the Iditarod quadrangle to the north (Miller and Bundtzen, 1994) also described a nearshore 

facies. As described by Hoare and Coonrad (1959a), unit chiefly consists of conglomerate 

facies of interbedded graywacke and siltstone, with lesser amounts of pebble grit conglomerate and a small amount of coal. Unit is well-indurated, commonly light to dark gray, and 

weathers brown. Varies from volcanic wacke to quartz-chert-feldspar-rich sandstone. Fossils 

collected from basal conglomerate of Box and others (1993) are considered Late Cretaceous 

(Cenomanian) age and fossils from the deltaic facies of Box and others (1993) are considered Late Cretaceous (Turonian). Murphy (1989) reported an Albian ammonite, Paragastroplites, from this unit in the Bethel quadrangle 

Kys Sandstone, shale, and conglomerate deltaic deposits (Upper Cretaceous?)—Fluvial and 

shallow marine deltaic deposits of sandstone, siltstone, shale, and conglomerate. Locally 

crossbedded and ripple marked. Unit contains abundant plant debris and fresh- and brackishwater mollusks. Unit is widely exposed in a broad belt that extends from northwest of the 

map area into the northern edge of the Marshall quadrangle (see Patton and others, in press, 

2009; Wilson and others, 2005, 2006a,c). Shallow water marine mollusks of middle Cretaceous age found in the Kwiguk quadrangle

Kygv Volcanic graywacke and argillite (Cretaceous?)—Thinly interbedded fine-grained graywacke 

and siliceous argillite (Patton and others, 1975) on Saint Matthew Island. Graywacke 

composed of angular to subrounded feldspar, volcanic lithic, and quartz grains in an altered 

argillaceous matrix (Patton and others, 1975). Argillite is siliceous and dense, breaks with 

conchoidal fracture, shows small-scale crossbedding and convolute laminations, and is interpreted to be a distal facies of a marine turbidite. No fossils are known in this unit; however, 

it is thermally altered by granodiorite (unit TKgd) which yielded an age of 62.2±2.0 Ma 

(71AMm 45, table 1). Unit appears to stratigraphically underlie pyroclastic rocks, which 

yield K-Ar ages of 76.0±2.0 and 76.3±2.0 Ma on biotite and hornblende, respectively 

(71AMm 50, table 1), which, in part, led Patton and others (1975) to assign a Cretaceous(?) 

age for the unit. Unit may correlate with unit Kygv of Patton and others (in press, 2009; 

Wilson and others, 2005, 2006a,c)

Kgbr Graywacke of Buchia Ridge (Lower Cretaceous, Valanginian and Hauterivian)—Lithic 

sandstone, conglomerate, and shale about 5 km thick (Hoare and Coonrad, 1983). Lower 

part of unit is about 2,400 m thick, composed mainly of well-bedded fine- to mediumgrained sandstone with siltstone interbeds, and pebble-cobble conglomerate. In lowest 

part of section, beds range from 0.1 to 3 to 4 m thick; the upper part of this lower section 

consists of 1,400 m of increasingly massive conglomerate (Hoare and Coonrad, 1983). 

Well-rounded clasts, as large as 10 cm, consist of “graywacke and siltstone, cherty tuff, and 

less abundant porphyritic volcanic rocks” (Hoare and Coonrad, 1983); the graywacke clasts 

resemble the graywacke of Kulukak Bay (unit Jkw: Jln, Jmm). The conglomerate, near the 

top of Buchia Ridge, contains coquinas of Buchia crassicollis shells (Hoare and Coonrad, 

1983). Box (1985a) interpreted this part of the unit to be of shallow marine origin. Upper 

part of unit, about 2,500 m thick, is “mostly shale and thin-bedded sandstone” (Hoare and 

Coonrad, 1983), which is poorly exposed. It is “dominantly fine-grained * * * calcareous 

black shale, thin-bedded calcareous siltstone, sandstone, calcarenite, and minor gritstone” 

(Hoare and Coonrad, 1983). Box (1985a) interpreted this upper part of the section to be of 

probable offshore marine origin. According to Hoare and Coonrad (1983), the graywacke 

of Buchia Ridge “is the thickest, least deformed section of Lower Cretaceous sedimentary rocks known in southwestern Alaska.” Outcrop area covers approximately 350 km2

 in 

a thrust plate separated from its neighbors by reverse faults (Hoare and Coonrad, 1983). 

Lower part of section contains abundant Valanginian fauna, primarily Buchia; upper part of 

section yields sparse Inoceramus, Belemnites, and a single Hauterivian ammonite according 

to Hoare and Coonrad (1983)

Klg Limy grit and limestone of the Ungalikthluk belt of Hoare and Coonrad (1983) (Lower 

Cretaceous, Valanginian)—Chiefly “limestone, greenish limy grit, and conglomerate 

overlain by noncalcareous graywacke and grit” (Hoare and Coonrad, 1983) exposed in a 

small area a few kilometers northwest of Buchia Ridge. Limy grit and conglomerate consists 
15

of “sub rounded green and maroon lithic fragments and a few well-rounded quartz pebbles” 

(Hoare and Coonrad, 1983) cemented by gray bioclastic limestone consisting of tiny shell 

fragments. Lithic clasts are fine-grained tuff and volcanogenic sedimentary rocks metamorphosed to phyllite and low-grade quartz-chlorite-sericite schist (Hoare and Coonrad, 1983). 

Lithic fragments thought to be derived from “nearby strata that were tectonically metamorphosed by movement on the Ungalikthluk fault” (Hoare and Coonrad, 1983); the matrix 

does not have a metamorphic fabric suggesting that metamorphism and therefore fault 

movement were pre-Early Cretaceous (Hoare and Coonrad, 1983). Bioclastic limestone is 

composed mostly of microscopic shell fragments. Buchia crassicollis is found in some limestone beds and Hoare and Coonrad (1983) suggested that the shell fragments in the bioclastic limestone were derived from Buchia. The presence of Buchia crassicollis indicates these 

rocks are coeval with the graywacke of Buchia Ridge (Kgbr)

Kvm Volcanic and sedimentary rocks of the Mount Oratia belt of Hoare and Coonrad (1983) 

(Lower Cretaceous, Valanginian)—Consists of a variety of lithologies including thin-bedded tuffaceous chert, massive graywacke, conglomerate, argillite, a few volcanic flows and 

impure limestone beds, and massive, coarse-grained crystal-lithic tuff (Hoare and Coonrad, 

1983). Rocks range widely in color, mostly green and gray but also red, yellow, brown, or 

black (Hoare and Coonrad, 1983). Most distinctive rock type in the belt according to Hoare 

and Coonrad (1983) is the massive andesitic crystal-lithic tuff, which is at least 1,000 m 

thick. Fine-grained tuff and some graywacke commonly contain laumontite. Buchia crassicollis of Early Cretaceous, Valanginian age is found in calcareous graywacke, conglomerate, 

and impure limestone. In cherty tuff, Early Cretaceous radiolaria has been found at three 

localities (Hoare and Coonrad, 1983). As in unit Klg, the presence of Buchia crassicollis

indicates unit Kvm is coeval with the graywacke of Buchia Ridge (Kgbr)

Kcgc Calcareous graywacke and siltstone of the Eek Mountains belt of Hoare and Coonrad 

(1983) (Lower Cretaceous, Valanginian)—Turbidites composed of highly calcareous sandstone interbedded with non-calcareous micaceous siltstone and shale. Hoare and Coonrad 

(1983) inferred the unit to be coarsening upward to conglomerate. Generally thick-bedded 

to massive with alternating sandstone and shale intervals 5 to 20 m thick, locally has thinbedded sections (Hoare and Coonrad, 1983). The base of the unit is not recognized and as 

such Hoare and Coonrad (1983) suggested that rocks of Jurassic age may be present in the 

unit. Rocks are strongly folded and commonly overturned (Hoare and Coonrad, 1983). Box 

and others (1993) reported the presence of lithic clasts likely derived from the Kanektok 

metamorphic complex, as well as other local units. Buchia crassicollis is found in several 

areas on the northwest side of the belt. Unit is exposed in the Bethel and Goodnews Bay 

quadrangles and appears to correspond with similar rocks that extend in a broad belt from 

the southeast corner of Saint Michael quadrangle and the northwest corner of the Holy Cross 

quadrangle to the south-central part of the Kwiguk quadrangle (map unit Kygc, Patton and 

others, in press, 2009; Wilson and others, 2005, 2006a,c). No fossils have been found in this 

similar unit in the Saint Michael and Holy Cross quadrangles, but in the Yukon-Koyukuk 

Basin to the north, similar rock assemblages have yielded marine mollusks of probable late 

Early or early Late Cretaceous age (Patton and others, in press; Wilson and others, 2006c), 

slightly younger than within this map area

K^vs Volcanic and sedimentary rocks, undivided (Lower Cretaceous to Upper Triassic?)—Thick 

unit consisting of low-grade metamorphic or contact metamorphosed marine volcanic and 

sedimentary rocks. According to Hoare and Coonrad (1978), “the volcanic rocks range in 

composition and type from mafic pillow basalts to more abundant andesitic and trachytic 

flows, tuffs, and breccias. Interbedded with the volcanic rocks are thick sections of tuffaceous siltstone, tuffaceous cherts, and massive or thin-bedded argillite. Tuffs and tuffaceous 

sedimentary rocks associated with the intermediate-composition volcanic rocks are commonly laumontized.” Radiolarians of Late Jurassic to Early Cretaceous age and fragmentary 

ammonites of Jurassic age have been collected from this unit in the Goodnews Bay quadrangle (Hoare and Coonrad, 1978). The unit may include some areas of rocks of Triassic and 

Permian age. Box and others (1993) suggested that this unit may be subdivided into the units 

KJc, KJb, Ja, and Jvs in the Bethel quadrangle, whereas Box (1985a) divided the unit in 

the Hagemeister Island and southern Goodnews Bay quadrangles into the subdivisions Jlt, 

Jvt, and J^cv. On the map, no contact is shown between these subdivisions and this undivided unit due to lack of mapping. Locally subdivided into the following:
16

KJc Argillite and tuffaceous chert (Lower Cretaceous and (or) Upper Jurassic)—Thin-bedded, 

green to brown argillite having local 1- to 4-cm-thick tuffaceous chert, siltstone, and finegrained tuff interbeds (Box and others, 1993). A Late Jurassic and (or) Early Cretaceous age 

is suggested from poorly preserved radiolarians (Box and others, 1993). Mapped only in the 

Bethel quadrangle (Box and others, 1993)

KJb Pillow basalt (Lower Cretaceous and (or) Upper Jurassic)—Highly altered pillow basalt 

sequence exposed along a narrow trend about 1 km wide and 10 km long south of Crooked 

Mountain in the Bethel quadrangle (Box and others, 1993). Interbedded with argillite and 

thin-bedded chert of unit KJc. Age inferred from enclosing map unit. Mapped only in the 

Bethel quadrangle (Box and others, 1993)

Jvs Marine volcaniclastic sandstone, conglomerate, and argillite (Jurassic)—“Turbidite-facies 

volcaniclastic strata, locally as coarse as fine-grained pebble conglomerate” (Box and others, 

1993). Clasts are predominantly intermediate volcanic rock fragments and detrital feldspar, 

quartz, clinopyroxene, and hornblende. Minor plutonic rock fragments, felsic volcanic and 

hypabyssal sedimentary rock fragments, and low-grade metamorphic rock fragments are 

a persistent component (Box and others, 1993). According to Box and others (1993), age 

control is based on stratigraphic position above Early Jurassic and Late Triassic phyllite and 

chert of map unit J^p and below Early Cretaceous and (or) Late Jurassic argillite and tuffaceous chert of map unit KJc. Mapped only in the Bethel quadrangle (Box and others, 1993)

Ja Marine arkosic sandstone and argillite (Jurassic)—“Turbidite-facies arkosic sandstone and 

shale” (Box and others, 1993). Sandstone, apparently derived from weathering of plutonic 

rocks, consists primarily of detrital plagioclase, quartz, and potassium feldspar, and minor 

hornblende, biotite, and clinopyroxene, in contrast to sandstone of map unit Jvs, which 

had a largely volcanic source (Box and others, 1993). Age inferred based on stratigraphic 

position above Early Jurassic and Late Triassic phyllite and chert (unit J^p) and on strike 

position relative to unit Jvs. Mapped only in the Bethel quadrangle (Box and others, 1993)

Jvt Volcaniclastic turbidites of Togiak Bay of Box (1985a) (Middle Jurassic)—Volcaniclastic 

turbidite sandstone and conglomerate composed entirely of volcanic and hypabyssal igneous clasts; unit is at least 1.7 km thick (Box, 1985a). Box (1985a) divided a tightly folded 

section along the seacoast into a buff-colored lower member, 1 km thick, and a bright green 

upper member, 0.7 km thick. Lower unit consists of thin-bedded fine-grained sandstone and 

siltstone turbidites and channelized medium- to coarse-grained sandstone and minor chaotic 

interbeds indicating downslope mass movement, which suggests an inner fan to midfan 

environment (fig. 10, Walker and Mutti, 1973). Upper, more tuffaceous member is mostly 

thin-bedded fine-grained sandstone and siltstone turbidites, having coarse conglomerate beds 

in upper part showing poorly defined thinning and fining upward cycles. “Entire sequence 

interpreted as deposited on proximal channelized portion (i.e., upper to midfan) of turbidite basin fill complex” (Box, 1985a). Similar conglomerate intruded by gabbro at Aeolus 

Mountain; gabbro yielded 174±8 Ma K-Ar age on amphibole (B78-1171G, table 1). “A few 

tool marks and interbedded slump folds suggest ENE-directed paleocurrents. * * * Unit is 

folded around sub-horizontal, northeast-trending axes and steep axial planes, and lacks a 

penetrative cleavage” (Box, 1985a). Box (1985a) constrained the age to Middle Jurassic on 

the basis of Bajocian pelecypods northeast of his map area in Togiak River valley reported 

by Hoare and Coonrad (1978). These rocks were included in map unit KJvs of Hoare and 

Coonrad (1978) and are likely to be present in undivided parts of that unit as shown herein 

in unit K^vs. Mapped only in the Hagemeister Island and southern Goodnews Bay quadrangles (Box, 1985a)

Jlt Laminated tuff and associated rocks of Box (1985a) (Jurassic)—Banded to finely laminated crystal-lithic tuff having interbedded green, white, and black tuffaceous chert (Box, 

1985a). Unit contains minor interbedded sections of massive, coarse-grained lithic tuff and 

(or) pillow basalt (mapped as unit Jpb by Box, 1985a). Highly deformed, commonly having 

folded slaty or spaced cleavage. Partially altered to prehnite-pumpellyite facies mineral 

assemblage. Several chert samples have yielded Radiolaria of Jurassic age, one of which 

is identified as upper Kimmeridgian to upper Tithonian (latest Jurassic) (see Box, 1985a). 

Mapped only in the Hagemeister Island and southern Goodnews Bay quadrangles (Box, 

1985a)

J^cv Coarse volcaniclastic rocks (Lower Jurassic to Upper Triassic)—Dense green tuff, tuff 

breccia, pillow breccia, minor pillow basalt, and associated sedimentary rocks (Box, 1985a). 
17

Breccia clasts are mostly angular aphanitic or plagioclase-clinopyroxene porphyritic rocks, 

which are commonly vesicular. Interbedded aquagene tuff and crystal tuff commonly having 

devitrified glass shards. Rocks are altered to prehnite-pumpellyite facies mineral assemblages and locally altered to hornblende and biotite hornfels around gabbroic plutons. Unit is 

exposed on both sides of Togiak-Tikchik and Kulukak Faults; age northwest of Togiak-Tikchik Fault constrained by underlying Upper Triassic basalt and crosscutting plutons of early 

Middle Jurassic age; southeast of Togiak-Tikchik Fault, constrained only to pre-Valanginian 

age (Box, 1985a). Mapped only in the Hagemeister Island and southern Goodnews Bay 

quadrangles (Box, 1985a)

Jvc Volcaniclastic conglomerate, sandstone, and shale (Upper and Middle Jurassic)—Poorly 

to moderately exposed sequence of tuffaceous marine sandstone, shale, and conglomerate 

having minor interbedded basalt and andesite lava flows in the northwest part of the Bethel 

quadrangle (Box and others, 1993). Unit is as much as 1 to 2 km thick; coarser parts are 

composed primarily of rounded clasts of basalt and andesite and minor mafic to intermediate-composition plutonic rock fragments (Box and others, 1993). Felsic pyroclastic rocks 

are found locally in the middle part of the unit within the map area and in the lower part of 

the unit north of the map area (Box and others, 1993). Marine pelecypods of Bajocian age 

(Middle Jurassic) are found near the base of the sequence and marine pelecypods of late 

Kimmeridgian to early Tithonian age (Late Jurassic) were reported from the Russian Mission quadrangle north of map area (Box and others, 1993)

Jkw Graywacke of Kulukak Bay (lower Upper to Middle Jurassic)—A thick marine sedimentary 

unit consisting of very hard, dark-green or gray, massive graywacke and siltstone containing local conglomerate horizons. Typically consists of sandstone, although coarse pebble 

conglomerate is locally present. Compositionally, “varies from quartz- and plagioclase-rich 

wackes to quartz-poor volcanic wackes. Generally contains black argillite or tuff chips” 

(Hoare and Coonrad, 1978). These rocks are the thick marine sedimentary unit referred to as 

the “Weary graywacke” by Hoare and others (1975). Unit is widely exposed in the southern 

Goodnews Bay and the Nushagak Bay quadrangles as well as the adjacent southeastern Dillingham quadrangle (Hoare and Coonrad, 1978). Buchia, Inoceramus, belemnites, and rare 

ammonite fragments ranging from Middle to early Late Jurassic age have been found in this 

unit. Locally subdivided by Box (1985a) into the following:

Jmm Turbidites of Metervik Bay of Box (1985a) (Upper and Middle Jurassic)—Volcaniclastic 

turbidite-facies sedimentary rocks southeast of Right Hand Fault of Box (1985a), which is 

roughly equivalent to unnamed fault of Hoare and Coonrad (1978) that parallels the valley 

of Ualik Lake. Box (1985a) reconstructed a 6-km-thick section, which he divided into four 

turbidite environment units (using criteria of Walker and Mutti, 1973, cited in Box, 1985a) 

from bottom to top: (1) conglomeratic inner fan; (2) well-bedded, outer fan lobe; (3) sandy, 

typically massive, midfan channel; and (4) conglomeratic inner fan unit. According to Box 

(1985a), “sandstone compositions almost entirely derived from plagioclase-rich volcanic 

sources. Minor slaty clasts near base of lowest unit, and minor component of slate, dioritic 

plutonic and chert detritus in upper unit. Interbedded slump folds suggest southeast-facing 

paleoslope. * * * Tightly folded around gently northeast-plunging axes and sub vertical axial 

planes.” Box (1985a) suggested deformation was probably during latest Jurassic to earliest 

Cretaceous time, based on occurrence of clasts of similar lithology in the lower unit of the 

graywacke of Buchia Ridge (unit Kgbr). Unit located southeast of map unit Jln and separated from it by the Right Hand Fault of Box (1985a); unit Kgbr is located northwest of the 

Kulukak Fault and northwest of map unit Jln; hence units Jmm and Kgbr are not in contact 

Jln Dismembered volcaniclastic turbidites (Lower Jurassic?)—Predominately argillaceous, 

tuffaceous sandstone-shale sequence that is pervasively deformed and structurally dismembered (Box, 1985a). Sandstone is composed of volcanic detritus and a very minor 

chert component. “Sedimentary structures variably preserved and indicate deposition from 

turbidity currents and other mass flow processes” (Box, 1985a). Partially altered to prehnitepumpellyite mineral assemblages, the unit is divisible into two structural facies types, which 

occur as alternating, fault-bounded blocks (Box, 1985a). One structural facies is characterized by generally well-bedded sections showing varying degrees of disruption prior to lithification; the second structural facies is distinguished by pervasive cleavage and occurrence 

of sandstone boudins in an argillaceous matrix (Box, 1985a). Poorly preserved radiolaria 

from structural facies 1 have been identified as post-Triassic Mesozoic forms (Box, 1985a). 
18

Clasts of slate similar to structural facies 2 occur stratigraphically below Bajocian fossil 

occurrences in adjacent Middle Jurassic turbidite facies sedimentary rocks (unit Jmm). 

Box (1985a) interprets this to indicate that deposition and deformation of these rocks were 

therefore Early Jurassic in age. This unit and map unit Jmm above are found southeast of 

the Kulukak Fault; their outcrop area corresponds to the area shown by Hoare and Coonrad 

(1978) for the graywacke of Kulukak Bay (unit JKw)

IGNEOUS ROCKS

QTv Volcanic rocks, undivided (Pleistocene or Pliocene)—Widely distributed basalt flows in the 

western part of the map area (Coonrad, 1957). Possibly equivalent in part to flows of map 

unit Qvb. In the Pribilof Islands, widespread olivine hylobasanite (containing normative 

nepheline) flows and sills range in thickness from 20 cm to 7 m or more (Barth, 1956); 

on Otter Island, hylobasanite flows are at least 100 m thick. Dated samples in the Pribilof 

Islands range from less than 0.10 to 2.25±.10 Ma (table 1). These rocks can be vesicular or 

dense and many eruptive features, such as hornitos (chimneys surrounded by foam and splatters), blowholes, diatremes, and miniature craters are present (Barth, 1956) suggesting very 

recent eruption. Other flows lack these features and are overgrown with vegetation. No dates 

are available on rocks of this unit except in the Pribilof Islands. Locally subdivided into the 

following:

Qpd Pyroclastic rocks (Quaternary)—Pyroclastic rocks in the Pribilof Islands, largely basaltic tuff 

(Barth, 1956). A K-Ar whole-rock age of 1.63±.06 Ma (sample P21, table 1) was determined 

on basalt from this unit

Qcs Alkalic basalt of Karon Lake (Quaternary)—Cinder cones, flows, and tephra; normally 

polarized rocks of Brunhes polarity chron (Hoare and others, 1968). K-Ar ages range from 

0.03±.02 to 0.74±.09 Ma (table 1)

QTab Older alkalic basalt of Karon Lake—Cinder cones, flows, and tephra; reversely and normally polarized rocks of Matuyama(?) chron (Hoare and others, 1968) on Nunivak Island. A 

sample of basalt from this unit yielded an age of 0.88±.07 Ma (sample A50, table 1) 

Qvb Basalt flows, undivided (Pleistocene)—Widely distributed fine- and medium-grained, columnar jointed tholeiite and alkali-olivine basalt flows. Columnar-jointed subaerial alkali olivine 

basalt flows have ropy tops and include minor interflow tuff and breccia. Generally nonporphyritic, dark-gray to black and contain fine microcrysts of labradorite, augite, olivine, 

and accessory magnetite in a diktytaxitic texture (Hoare and Coonrad, 1980). Includes the 

Togiak Basalt of Hoare and Coonrad (1980) on the floor of the Togiak River valley as well 

as flows in the Bethel, Nunivak Island, Cape Mendenhall, Pribilof Islands, and Baird Inlet 

quadrangles. The Togiak Basalt is divisible into a lower unit of low-lying horizontal flows, 

less than 100 m thick, and an upper unit, about 300 m thick, comprising an isolated volcanic edifice, regarded by Hoare and Coonrad (1978) as a tuya (subglacial volcano). Wholerock K-Ar age on lower unit was 0.758±0.2 Ma (Hoare and Coonrad, 1978, 1980; Box, 

1985a, sample 74AHr 77, table 1, herein). In the Bethel quadrangle, includes fresh olivine 

tholeiite basalt flows having diktytaxitic texture and containing olivine, subophitic clinopyroxene, plagioclase, and iron oxides as well as sparse interstitial volcanic glass yielding 

a whole-rock K-Ar age of 0.418±0.016 Ma (Box and others, 1993, sample 86ASb 9a, table 

1, herein). Unit also includes Tholeiitic Basalt of Mekoryuk on Nunivak Island (Hoare and 

others, 1968); massive, columnar-jointed flows including both normally polarized rocks of 

the Brunhes polarity chron and reversely magnetized rocks of the Matuyama polarity chron 

(shown as unit Qvbm). Samples dated by K-Ar for this unit on Nunivak Island yielded ages 

as old as 1.69±.09 Ma (table 1). Locally subdivided on Nunivak Island into:

Qvbm Basalt flows of Matuyama Chron—Reversely and normally magnetized flows of the 

Matuyama polarity chron on Nunivak Island (Hoare and others, 1968); similar rocks may 

occur on the mainland but cannot be distinguished because no magnetic studies have been 

undertaken 

Ttb Tholeiitic basalt of Binakslit Bluff (Pliocene)—Tholeiitic basalt of Binakslit Bluff on Nunivak 

Island; massive, columnar-jointed flows; normally polarized flows of Gauss polarity chron as 

well as normally and reversely polarized flows older than Gauss polarity chron (Hoare and 

others, 1968). Multiple samples yielded ages between 3.24±0.10 and 5.01±0.15 Ma (table 1)

Talb Alkalic basalt of Ahzwiryuk Bluff (Pliocene)—Alkalic basalt of Ahzwiryuk Bluff on Nunivak 

Island; nubbly mottled flows and pyroclastic ejecta; includes both normally and reversely 
19

polarized rocks older than Gauss polarity chron (Hoare and others, 1968). Two samples 

yielded K-Ar ages of 5.19±0.15 and 6.28±0.18 Ma (samples A15 and A1, respectively, table 

1)

Tfi Felsic intrusive rocks (Miocene?)—Light-colored, fine-grained, commonly porphyritic felsic 

intrusive rocks. Chiefly rhyolitic to dacitic dikes and sills. K-Ar age on biotite of 13.00±0.50 

Ma (Hoare and Coonrad, 1978, sample 74AHr 26, table 1, herein)

Tad Aplite on Saint George Island (Eocene)—Aplite dike, about 400 m wide, has chilled margins 

where it intrudes peridotite on Saint George Island (Barth, 1956). The dike and peridotite 

surface is overlain by till and fine-grained sedimentary deposits, which are in turn overlain 

by basalt and basanite flows of map unit Qvb. Radiometric dating (Hopkins and Silberman, 

1978; table 1 herein) indicates an age range of about 49.5±2.0 to 57.2±2.3 Ma

Tnr Nukluk Volcanic Field, rhyolite (Eocene)—Moderately altered rhyolite flows, domes, and ashflow tuff. Rhyolite of the main volcanic field contains phenocrysts of sanidine, riebeckitic 

amphibole, and rare quartz. A 40Ar/39Ar total-fusion age on riebeckite was 54.7±1.6 Ma (Box 

and others, 1993, sample 87ACz 75m, table 1 herein)

Tnba Nukluk Volcanic Field, basalt and andesite (Eocene)—Massive, columnar-jointed basalt and 

subordinate andesite flows. Minor latite and dacite. Overlies Tnr in Fog River area. Elsewhere, interbedded with rhyolite (Box and others, 1993)

Tvep Rhyolite and dacite tuff (Tertiary)—Chiefly rhyolite and dacite welded tuff and tuff breccia 

and dark rhyolite vitrophyre on the northern part of Saint Matthew Island (Patton and others, 

1975) containing minor intercalated andesite and basalt flows and dikes. On southwestern 

part of island, chiefly light-colored rhyolite and dacite hypabyssal rocks (Patton and others, 

1975). These felsic rocks appear to overlie mafic flows and volcaniclastic rocks mapped 

herein as unit Kv and may be extrusive and hypabyssal cogenetic equivalent of granodiorite 

mapped herein as unit TKm (Patton and others, 1975). Age thought to be Eocene or Paleocene (see Wittbrodt and others, 1989)

T}p Peridotite on Saint George Island (Tertiary or older)—Peridotite forming the basement of 

Saint George Island in the Pribilof Islands. The peridotite is massive, originally was dunite, 

and now is largely altered to serpentine and magnetite (Barth, 1956). The erosional surface 

of the peridotite appears polished, has glacial striae and a brownish-black varnish (Barth, 

1956, p. 117). Intruded by an aplite dike of unit Tad, suggesting age is greater than 57 Ma

TKr Rhyolite and dacite flows, tuff, dikes, and sills (Early Tertiary and (or) Late Cretaceous)—Rhyolite and dacite flows and tuff including block-and-ash flows as well as rhyolite domes. Unit includes felsic rocks of the Swift Creek, Tulip, and Eek volcanic fields of 

Box and others (1993). Consists of fresh to moderately altered, densely welded rhyolitic 

ash-flow tuff, black glassy vitrophyre, and pale pink to orange, fine-grained, porphyritic 

albite-bearing rhyolite. The rhyolite commonly has well-developed flow structures and 

forms rhyolite domes found as small knobs or large domal hills as much as 4 km in diameter which have 300 to 500 m of relief (Box and others, 1993), as well as small stock-like 

intrusions, dikes, and sills (Hoare and Coonrad, 1959a). The block-and-ash flow in the 

Tulip volcanic field consists of broken blocks of dacite in an ashy dacitic matrix and was 

interpreted to be a hot avalanche deposit formed during dome collapse. Dacite and subordinate andesite flows in the Tulip volcanic field, which crop out as columnar-jointed cliffs, 

are also included here. K-Ar ages range from 59.3±1.8 Ma on a whole-rock to 62.5±1.9 

Ma on biotite (table 1)

TKa Eek and Swift Creek volcanic fields of Box and others (1993); andesite flows (Early Tertiary 

and (or) Late Cretaceous)—Small outcrops of columnar jointed andesite flows. Subordinate altered andesite porphyry is exposed in the northern part of the Eek volcanic field. Map 

unit also includes minor olivine basalt, andesitic lithic tuff, dacite, and a small amount of 

obsidian and siliceous tuff (Hoare and Coonrad, 1959a). Primarily exposed on the west side 

of the Bethel quadrangle along ridges and in river cuts in the Eek volcanic field; however, 

the Swift Creek volcanic field also contains exposures of columnar jointed andesite capping 

small hills in the southeastern part of the Bethel quadrangle (Box and others, 1993). According to Hoare and Coonrad (1959a) this map unit is moderately folded and unconformable on 

steeply dipping rocks of Cretaceous age and older. A 40Ar/39Ar total fusion age of 59.5±12.3 

Ma was determined on plagioclase from andesite in the central Bethel quadrangle (Box and 

others, 1993, sample 88Al 12, herein)
20

TKg Granitic rocks, undivided (Tertiary and (or) Cretaceous)—Fine- to coarse-grained or porphyritic, light- to dark-gray, rarely pink, granitic rocks. Range in composition from granite to 

quartz diorite, including granodiorite, quartz monzonite, and quartz monzodiorite. Biotite 

and hornblende are locally common; muscovite is uncommon. K-Ar and 40Ar/39Ar ages 

range from 60.7 to 75 Ma throughout the map area for rocks of this unit and its subdivisions 

(table 1) and 68.7±3.0 and 75.0±2.9 Ma for dated rocks that are in this undivided unit. Unit 

includes the Fisher Dome and Marvel Creek plutons of Box and others (1993). Also includes 

map unit TKg of Hoare and Coonrad (1978) except where subdivision of these rocks is possible on the basis of composition. Rocks assigned to this map unit by Hoare and Coonrad 

(1978) on Hagemeister Island were subsequently assigned to map unit Jmf of Middle Jurassic age by Box (1985a). Locally subdivided into the following:

TKgr Granite—Coarse- to medium-grained granite and granitoid porphyry. Biotite is dominant 

mafic phase, hornblende is sparsely present; feldspars are microcline and albite-oligoclase 

(Wilson, 1977). Unit includes the Akuluktok and Gechiak plutons of Wilson (1977) as well 

as Aniak Lake pluton of Box and others (1993) and the Nayorurun River intrusive complex 

of Hoare and Coonrad (1978) consisting of granite porphyry dikes, sills, and related tuff and 

breccia. The Aniak Lake pluton is a coarse-grained, porphyritic to seriate leucocratic biotite 

granite pluton 10 km2 in area in the Bethel quadrangle (Box and others, 1993). It has a distinctive speckled appearance due to randomly oriented microperthite feldspar phenocrysts. 

Porphyritic rhyolite dikes common in the uppermost part of this pluton and in surrounding biotite hornfels. K-Ar and 40Ar/39Ar total-fusion ages for this unit range from 60.7±1.8 

Ma to 69.6±2.1 Ma (table 1). The Akuluktok pluton yielded reversely discordant ages with 

biotite at 69.6±2.1 Ma and hornblende at 63.4±3.0 Ma (Wilson, 1977; sample 74B57, table1 

herein). To the east of the map area, rocks of this map unit range from 54.5±1.6 Ma on 

hornblende, considered a minimum age and not an emplacement age by Wallace and others 

(1989) to 66.64±0.08 Ma on mica (Wilson and others, 2003)

TKgs Monzonite—Medium-grained pyroxene-bearing monzonite. Biotite is the dominant mafic 

mineral; however, these plutons are characterized by the presence of orthopyroxene. 

Includes the Ualik Lake and Kulukak plutons of Wilson (1977). The chemically and texturally similar Okstukuk Hills and Kulik Lake plutons occur in the adjacent Dillingham 

quadrangle (Wilson, 1977; Wilson and others, 2006b). Potassium content is high relative to 

SiO2, as high as 4.50 percent K2O at a SiO2 value of 57.4 percent (Wilson, 1977). A K-Ar 

age of 71.9±2.0 Ma (sample 74AHr 51, table 1) was determined on a pluton in the Nushagak 

Bay quadrangle, whereas the Okstukuk Hills pluton in the adjacent Dillingham quadrangle 

yielded an age of 84.49±0.05 Ma (Iriondo and others, 2003; Wilson and others, 2006b) 

which is unusually old for this region

TKqm Quartz monzonite and quartz monzodiorite—Medium- to coarse-grained, light-gray monzogranite or quartz monzodiorite plutons which may contain biotite, hornblende, sodic 

amphibole, and (or) clinopyroxene and orthopyroxene (Wilson, 1977). Locally contain 

phenocrysts of perthitic feldspar; biotite to hornblende ratio is variable but generally subequal (Wilson, 1977). These plutons are the most common of the Tertiary and (or) Cretaceous plutons of the map area. Includes Crooked Mountain and Canyon Creek plutons of 

Box and others (1993) in the Bethel quadrangle. As described by Box and others (1993), 

Crooked Mountain is a large composite pluton having a thin gabbro and diorite margin 

that partially rims a massive core of augite-biotite-hornblende and (augite-) hornblendebiotite granodiorite, quartz monzonite, and granite. Canyon Creek pluton is a small (~1 

km2

), medium- to coarse-grained, intergranular to hypidiomorphic-granular augite-biotite 

quartz diorite to hornblende-biotite granodiorite. K-Ar and 40Ar/39Ar ages range from 

63.7 to 71.1 Ma (table 1); the Togiak Lake pluton of Wilson (1977) yielded a discordant 

K-Ar age of 63.7±2.0 Ma on biotite and 69.5±3.0 Ma on hornblende (74AHr 118, table 

1). 40Ar/39Ar total-fusion ages of 69.8±2.1 Ma were determined on biotite from Crooked 

Mountain pluton and 70.3±2.1 Ma on biotite from Canyon Creek pluton (87ATf 51 and 

87AJm 36a, respectively, table 1)

TKgd Granodiorite—Consists of plutons in the Saint Matthew, Goodnews Bay, and Bethel quadrangles. On Saint Matthew Island, a fine-grained hornblende granodiorite yielded a 62.3±2.0 

Ma K-Ar age on hornblende (Patton and others, 1975, 71AMm 45, table 1 herein). In the 

Goodnews Bay quadrangle, the Sunday Creek and Mt. Waskey plutons of Wilson (1977) 

are medium- to coarse-grained tonalite and granodiorite. The Sunday Creek pluton yielded 
21

a K-Ar age on biotite of 72.5±2.2 Ma (GC1-1381, table 1), whereas the Mt. Waskey pluton 

yielded slightly discordant K-Ar ages of 62.5±1.9 and 65.2±2.0 Ma on biotite and hornblende (GD1-3154, table 1). In the Bethel quadrangle, includes the two-phase North Fork 

pluton of Box and others (1993) and the Cripple Mountain pluton. Aplite dikes are common 

near these plutons. Main felsic phase of the North Fork pluton is seriate to hypidiomorphic 

granular (augite-) hornblende-biotite quartz monzodiorite, granodiorite, and minor biotite 

granite. The North Fork mafic phase is a coarse-grained, cumulate-textured augite quartz 

gabbro and quartz diorite. Cripple Mountain pluton is an elliptical, coarse-grained, hypidiomorphic-granular hornblende-biotite granodiorite and granite, and lesser quartz monzodiorite and quartz monzonite covering about 40 km2

 in the Bethel quadrangle (Box and others, 

1993). The pluton is erosionally recessive wherein the contact aureole and surrounding 

country rock are more resistant to erosion than the pluton. North Fork and Cripple Mountain 

plutons yield 40Ar/39Ar total-fusion ages of 64.3±1.9 and 62.2±1.9 Ma, respectively (87ATf 

56 and 87ATf 72, respectively, table 1)

TKqd Monzodiorite and quartz diorite—Consists of the Eek River, Gemuk Mountain, Mount 

Plummer plutons of Box and others (1993) and the hornblende diorite stocks and dikes 

assigned a Late Cretaceous age by Box and others (1993) as well as the Wattamuse pluton 

of Wilson (1977). Plutons are fine- to coarse-grained monzodiorite and diorite, but locally 

include gabbro. Eek River pluton is a sill 50 to 500 m thick cropping out in discontinuous exposures over 10 km length which intruded and contact-metamorphosed rocks of the 

Kuskokwim Group (Kk). It is dominantly medium- to coarse-grained, porphyritic to diabasic 

(hornblende) augite diorite, quartz diorite, and mafic quartz monzodiorite. Gemuk Mountain pluton is elongate, north-trending, erosionally resistant, mafic to intermediate quartz 

gabbro to mafic granodiorite pluton. The pluton also intruded and contact-metamorphosed 

the Kuskokwim Group as well as Lower Cretaceous marine volcaniclastic sandstone, conglomerate, shale, and interbedded tuff of map unit Kvm. Mt. Plummer pluton is medium- to 

coarse-grained, erosionally resistant, hypidiomorphic granular augite-biotite mafic granodiorite, quartz monzodiorite, and quartz diorite; it intruded and contact metamorphosed the 

Kuskokwim Group. The Mt. Plummer pluton has yielded K-Ar biotite ages of 65.1±2.0 Ma 

(87ACz 61, table 1) and 66.6±2.0 Ma in the adjacent Russian Mission quadrangle north 

of the map area (Box and others, 1993). Hornblende diorite stocks and dikes are small, 

extensively altered, and contain relict euhedral plagioclase; samples yielded a 40Ar/39Ar totalfusion age of 68.0±2.0 Ma on hornblende (87AJm 271b, table 1) and a conventional K-Ar 

age of 71.3±2.1 Ma on biotite (GB7-1479a, table 1)

TKm Mafic intrusive rocks (Paleocene and Cretaceous?)—Dark-colored dikes and sills of diabase, basalt, and dioritic, gabbroic, and biotite lamprophyre (Hoare and Coonrad, 1978). 

K-Ar age of 64.6±2 Ma on biotite (74ACd 14d, table 1) from the western Nushagak Bay 

quadrangle. Also includes medium- to coarse-grained pyroxene gabbro dike(?) on Saint Matthew Island (Patton and others, 1975)

TJih Hypabyssal felsic intrusive rocks (earliest Tertiary? to Late Jurassic?)—Highly altered 

porphyritic rocks consisting of orange-weathering rhyolite and dacite. Rhyolite contains 

as much as 10 percent phenocrysts, locally in graphic intergrowths, in an altered groundmass of quartz, potassium feldspar, and plagioclase. Dacite contains as much as 5 percent 

phenocrysts of plagioclase and biotite in a groundmass of plagioclase, quartz, and opaque 

oxides (Box and others, 1993). Some of these rocks have similar trace element signatures to 

the Slate Creek pluton (unit TJis) of Box and others (1993). Age is uncertain though rocks 

intrude the rocks of units Jab and Jvc

TJis Slate Creek pluton of Box and others (1993) (earliest Tertiary? to Late Jurassic?)—Small, 

medium-grained hypidiomorphic-granular, hornblende tonalite and granodiorite pluton, less 

than 2 km2. Intrudes rocks of map unit Jab without development of a prominent contactmetamorphic zone. Pluton is relatively quartz-rich, containing 20 to 25 percent quartz, 

along with green hornblende, plagioclase, and opaque oxides (Box and others, 1993). Pluton 

erodes similarly to country rocks, hence is not prominent and was considered “erosionally 

neutral” by Box and others (1993). Age is uncertain

TJik Little Kasigluk River pluton of Box and others (1993) (earliest Tertiary? to Late Jurassic?)—Medium-grained, hypidiomorphic granular hornblende diorite and gabbro in a 

number of small plutons (Box and others, 1993) in the central to western Bethel quadrangle. 

Age is uncertain
22

Kv Volcanic rocks (Late Cretaceous, Maastrichtian and Campanian)—Volcanic rocks ranging 

from rhyolite to olivine basalt flows, dacitic to andesitic tuff and tuffaceous sandstone (line 

pattern), and rhyolitic domes (Hoare and Coonrad, 1978; Patton and others, 1975; Box 

1985a; Box and others, 1993). Includes rhyolite domes and flows of the Tulip volcanic 

field, lithic air-fall tuff of the Swift Creek volcanic field, and andesite and basalt flows, tuff, 

tuffaceous sandstone, and rhyolite domes and flows of the Kipchuk volcanic field (Box and 

others, 1993). Also includes moderately to steeply dipping, fresh to weakly altered porphyritic basaltic andesite flows of calc-alkaline affinity and interbedded sandy to boulder-rich 

volcaniclastic rocks mapped by Globerman (1985) and Box (1985a) in the Hagemeister 

Island quadrangle which were originally mapped as unit Tv by Hoare and Coonrad (1978). 

Associated dikes cut underlying units. In the Hagemeister Island and southern Goodnews 

Bay quadrangles, unit overlies Upper Cretaceous Summit Island Formation (Ks) on Summit 

Island and adjacent mainland. On Hagemeister Island, unit consists of individual 2- to 

5-m-thick subaerial flows aggregating to at least 2 km thick with very minor fluvial sandstone interbeds. On Crooked Island, unit has similar flows but has more voluminous (25 

percent) interbedded epiclastic volcanic sandstone and coarse boulder conglomerate. On 

mainland coast, northwest of Right Hand Point, consists almost entirely of boulder-rich volcaniclastic sedimentary rocks of probable debris-flow origin and very minor flows. On Saint 

Matthew Island, consists of flat-lying to gently dipping mafic flows of andesite and basalt 

and volcaniclastic rocks consisting of andesitic and basaltic tuff and conglomerate. K-Ar and 40Ar/39Ar ages range from 64.5±2.0 Ma to 79.2±2.4 Ma (table 1); however, a basalt sill from 

Saint Matthew Island yielded a whole-rock K-Ar age of 52.2±1.6 Ma (Wittbrodt and others, 

1989, sample 12-106, table 1 herein). As this sample yields a significantly younger age than 

other dated rocks from Saint Matthew Island, Wittbrodt and others (1989) suggested that 

the basalt sill was derived from a significantly younger event, which intruded unit Kv. In the 

Bethel quadrangle, internal contacts reflect distinction between flow and tuff units of Box 

and others (1993); pyroclastic and felsic subunits are shown with a dot pattern

Kpd Intermediate and silicic pyroclastic rocks (Late Cretaceous)—Intermediate and silicic pyroclastic rocks on Saint Matthew Island including dacite, andesite, and rhyolite nonwelded 

tuff breccia and crystal tuff, dacite and rhyolite welded tuff, massive dacite breccia (lahar?), 

fine ash-fall tuff, volcanic conglomerate, and andesite and dacite porphyritic and vitrophyric 

plugs, dikes, sills, and flows (Patton and others, 1975). K-Ar ages were 76.0±2.0 and 

76.3±2.0 Ma on biotite and hornblende (sample 71AMm 50, table 1). Late Cretaceous pollen 

assemblages from carbonaceous tuffs (R.A. Scott, oral commun., 1973; reported in Patton 

and others, 1975)

Ksp Serpentinite (Late Cretaceous?)—Small, pervasively slickensided bodies of serpentinite, 

serpentinite-matrix melange, and silica-carbonate-altered serpentinite north of mouth of 

Crooked Creek (Box and others, 1993). Protolith and intrusion age uncertain; Box and 

others (1993) infer intrusion during Late Cretaceous tectonism

Kg Granodiorite and granite (Early Cretaceous)—Consists of the Nyac pluton in Kilbuck Mountains, only a small part of which is exposed in the northernmost part of the Bethel quadrangle; the bulk of the exposure of the pluton is in the adjacent Russian Mission quadrangle 

(Box and others, 1993). About 200 km2

 in size, pluton is crudely concentrically zoned from 

hornblende-biotite granodiorite and granite at margin to hornblende-biotite granite at core 

(Box and others, 1993). K-Ar age determinations yield a range of ages, from 101.1±3.0 

Ma (Decker and others, 1984; Box and others, 1993) on hornblende in the Russian Mission 

quadrangle north of the map area to 120.0±3.3 Ma on biotite (Wilson, 1977; Box and others, 

1993) also in the Russian Mission quadrangle, as well as a K-Ar biotite age of 115±3.5 Ma 

from within the Bethel quadrangle (sample 87ATf 30, table 1) and a U/Pb crystallization age 

reported as a range of 104 to 129 Ma (upper Concordia intercept) on zircon (see table 1, also 

for additional data see Box and others, 1993; Patton and others, 2009; Wilson and others, 

2006a) 

KJba Basaltic and andesitic lava flows (Early Cretaceous and (or) Late Jurassic)—Poorly to moderately exposed basalt and andesite lava flows in the west-central part of the Bethel quadrangle (Box and others, 1993). Overlies volcaniclastic sedimentary rocks of map unit Jvc

and generally spatially associated with rocks of map unit Jab

Jum Ultramafic rocks (Jurassic?)—Serpentinite, serpentinized dunite, and other ultramafic rocks 

form a number of intrusive bodies and tectonic blocks(?) within fault zones separating 
23

pillow basalt (^ob) and pyroxene gabbro (^og), respectively (Hoare and Coonrad, 1978; 

Box, 1985a). At Red and Susie Mountains, south of Goodnews Bay, the rocks consist of 

medium-grained, partially serpentinized dunite, wehrlite, and clinopyroxenite (Box, 1985a); 

Hoare and Coonrad (1978) reported websterite rather than wehrlite. The margins of these 

bodies are cut by numerous coarsely pegmatitic hornblendite dikes which have contact metamorphic zones as much as 800 m wide (Box, 1985a). Unit, as mapped herein, also includes 

serpentinite at Cape Newenham, which apparently had a protolith that was predominately 

harzburgite, subordinate dunite, and rare clinopyroxenite (Box, 1985a). This serpentinite 

“varies from blocky, unsheared protoliths [harzburgite?] to slickensided serpentinite enclosing blocks with original fabrics” (Box, 1985a). According to Box (1985a), the northeast 

flank of the ultramafic body at Susie Mountain appears to grade into hornblende gabbro of 

map unit Jgb, which yielded an age of 162.4±4.9 Ma (GA7-1450, table 1), significantly 

younger than the dated dikes that cut the ultramafic body at Red Mountain. K-Ar dates were 

176.4±5.3 and 186.9±5.6 Ma (HD6-2272a and HD6-1453e, respectively, table 1) on amphibole from the crosscutting hornblendite dikes

Jab Andesite and basalt (Middle Jurassic)—Massive andesitic and basaltic flows; minor trachyte, 

trachyandesite, and dacite; some interbedded tuffaceous sedimentary rocks; and, locally, 

several hundred feet of volcanic breccia at or near base (Box and others, 1993). An estimated 25 percent of the unit is interbedded graywacke, siltstone, impure limestone, and 

pebble conglomerate (Hoare and Coonrad, 1959a). This rock unit forms a belt of low rolling 

hills on the west flank of the Kuskokwim Mountains from the Kwethluk River and northward (Hoare and Coonrad, 1959a). Marine pelecypods of Bajocian (Middle Jurassic) age 

occur in sedimentary strata of this unit (Hoare and Coonrad, 1959a; Box and others, 1993). 

Assigned to Nyac terrane by Box and others (1993)

Jmv Interbedded volcanic and sedimentary rocks (Middle Jurassic)—Basaltic and andesitic volcanic and volcaniclastic rocks on Hagemeister Island (Hoare and Coonrad, 1978; Box, 1985a); 

extensively altered to prehnite-pumpellyite facies mineral assemblage. Volcanic rocks, 

characterized by plagioclase + clinopyroxene ± hornblende porphyritic rocks, more common 

in lower third of exposed section. Interbedded clastic rocks are composed almost entirely of 

volcanic lithic detritus, except for a lower conglomerate, which contains a minor proportion 

of granitic plutonic rock clasts similar to adjacent pluton. Sedimentary facies range from fluvial deltaic to nearshore marine to subwave base turbidites. Box (1985a) mapped the contact 

between this unit and similar older volcanic and sedimentary rocks (unit Jlvs) structurally 

and stratigraphically below it as a low-angle thrust(?) that he suggested is probably a faulted 

unconformity. Fossils of Middle Jurassic age are locally common (Box, 1985a). Conglomerate includes clasts of granitic rocks similar to nearby Middle Jurassic Hagemeister pluton 

(Box, 1985a), mapped herein as unit Jlgd

Jgb Gabbroic rocks (Middle and Early Jurassic)—Medium- to coarse-grained, locally pegmatitic 

hornblende gabbro to diorite (Hoare and Coonrad, 1978). Plutons locally contain olivine and 

commonly are compositionally layered (Hoare and Coonrad, 1978). Box (1985a) described 

these rocks as slightly altered, containing sericitized plagioclase, hornblende partially 

replaced by actinolite and chlorite, and prehnite-quartz veinlets. Hoare and Coonrad (1978) 

reported that these gabbroic intrusions are commonly associated with ultramafic rocks. Five 

K-Ar amphibole and hornblende ages have a bimodal distribution, having peaks at about 187 

and 161 Ma (table 1)

Jlvs Volcanic and sedimentary rocks (Lower Jurassic)—Marine unit of mafic flows, some displaying pillow structure, volcanic breccia, and massive fine- to medium-grained volcanogenic 

sedimentary rocks (Hoare and Coonrad, 1978). Fractures commonly coated with laumontite. 

Mapped as unit Jlv by Box (1985a) who described the unit as grading from a lower massive, 

sub-wave-base, angular pebbly mudstone to upper abundantly fossiliferous, shallow marine, 

well-bedded, locally crossbedded sandstone and shale. Unit only exposed on Hagemeister 

Island. An impure sandy limestone that outcrops in the Taylor Mountains quadrangle to the 

east of map area was originally included in this unit by Hoare and Coonrad (1978) because 

of the common occurrence of Weyla pelecypods of Early Jurassic age; however, the limestone was mapped as a distinctive unit by Wilson and others (2006b)

Jlgd Hagemeister pluton of Box (1985a) (Early Jurassic?)—Medium-grained biotite-hornblende 

granodiorite of Hagemeister Island; slightly altered (that is, sericitized plagioclase, chlorite 

partially replacing biotite and hornblende) as described by Box (1985a). Intrudes Lower 
24

Jurassic volcanic and sedimentary rocks (map unit Jlvs) with a narrow contact metamorphic zone of hornblende-biotite-plagioclase hornfels. Box (1985a) reported that clasts of 

similar granodiorite occur in conglomerate of adjacent interbedded volcanic and sedimentary rocks of map unit Jmv. An age reported variously by Box (1985a) as on biotite (p. 26) 

or hornblende (p. 18) yielded K-Ar age of 183±7 Ma (B78-1152G, table 1); the reported 

K2O content for this sample is well below that typical for biotite and, conversely, very high 

for hornblende. As presented, the date must be considered suspect. No similar age granitic 

rocks are known elsewhere in southwestern Alaska, but rocks of this age are common in the 

Alaska-Aleutian Range batholith to the east (Wilson and others, 2006b). Pluton originally 

mapped as part of map unit TKg by Hoare and Coonrad (1978)

TOGIAK-TIKCHIK COMPLEX

}|t Togiak-Tikchik Complex, undivided (Lower Cretaceous to Lower Paleozoic)—Thick, 

marine unit consisting of volcanic and sedimentary rocks including pillow basalts, intermediate to mafic flows, breccia, crystal-lithic tuff, thin-bedded to massive tuffaceous chert 

and siltstone, argillite, graywacke, pebble-cobble conglomerate, and limestone. Formerly 

called the Gemuk Group (Hoare and Coonrad, 1959a, 1961a,b), this unit consists of a wide 

variety of rock types in a structural collage of blocks. The Gemuk Group as originally 

defined consists chiefly of dense, dark, massive siltstone having interbeds of chert, volcanic 

rocks, limestone, graywacke, and breccia (Cady and others, 1955). The assemblage was 

subsequently redefined as the Togiak-Tikchik Complex on the basis of its structural character (Wilson and Coonrad, 2005). Unit crops out in a wide belt through the Goodnews Bay 

quadrangle from the Eek Mountains in the north southwest to Goodnews Bay making up 

most of the northwestern Ahklun Mountains. It is unconformably overlain by rocks of the 

Late Cretaceous Kuskokwim Group (Kk and Kkn) and Early Cretaceous (Valanginian) calcareous graywacke and siltstone (Kcgc). Permian limestone (Pl) containing Atomodesma

sp. is interbedded throughout the unit. Hoare and Coonrad (1978, see also www.alaskafossil.org) collected samples containing corals of Permian age and crinoids, bryozoans, and 

Halobia of Triassic age. They also collected brachiopods (including several productoid 

forms, cleiothyridinoid genus, and a spiriferoid), a pleurotomarian gastropod, and pelecypods of late Paleozoic (probably Permian) age. Hoare and Coonrad (1978) mentioned the 

unit contains volcanic rocks of Triassic, Permian, and Devonian ages. Unit was included 

in the Goodnews terrane by Box (1985a) and Box and others (1993); however, rocks also 

mapped as unit }| by Hoare and Coonrad (1978) and included in the Togiak-Tikchik 

Complex were assigned to the Togiak terrane by Box and others (1993; see also Decker and 

others, 1994). Unit was subdivided by Box (1985a) in the Hagemeister Island and southernmost of the Goodnews Bay quadrangles into units J^cs (JPcs, herein), J^mvs (JPmvs, 

herein), J^mb (JPmb, herein), Pvs, and Pmv. Where possible, subunit assignments were 

extrapolated from the Box (1985a) map area into the Hoare and Coonrad (1978) map area. 

Unit }| of Hoare and Coonrad (1978) was further subdivided into two mélange units 

(}|a; }m) by Box and others (1993) in the Bethel quadrangle. Included rock packages 

are the following:

}m Mélange (Mesozoic)—Mélange containing chert, cherty tuff, siliceous siltstone, limestone and 

dolostone, pillowed and massive basalt, gabbro, and graywacke in an argillite matrix (Box 

and others, 1993; Hoare and Jones, 1981; Wilson and others, 2006b). Chert was described by 

Hoare and Jones (1981) as gray and thin bedded with shale partings or red, siliceous silt and 

massive or thick bedded, gray, black, white and brownish. They also described calcareous 

sandstone and shale, white crystalline limestone, and pyroclastic rocks altered to greenstone. 

Consists of unit ^|m of Box and others (1993) in the Bethel quadrangle and subdivided 

from unit }| of Hoare and Coonrad (1978) in the Goodnews Bay and adjacent Dillingham 

and Taylor Mountains quadrangles based on work reported in Wilson and others (2006b). 

Unit crops out in the Tikchik Lakes area of the Bethel and Goodnews Bay quadrangles and 

extends into the adjacent Taylor Mountains and Dillingham quadrangles. The mélange may 

contain rocks of multiple map units, including the argillite and graywacke (K^ag), volcanic rock (Pv and ^v), clastic rocks (Pcs), limestone (Pls), greenstone and schist (MDv), 

and chert (|c and ^|rc) of Wilson and others (2006b). Unit is structurally complex and 

is dismembered along anastomosing cleavage (Box and others, 1993). According to Hoare 
25

and Jones (1981) “the rocks were deformed twice by isoclinal folding and offset by northwest trending high-angle faults and northwest-dipping low- and high-angle reverse faults. 

The older folds trend west or northwest; the younger folds are post-Cretaceous in age, 

trend northeast, and are commonly recumbent to the southeast.” Unit structurally underlies 

greenstone and schist (MDv) to the northwest beneath a low-angle(?) fault and is juxtaposed 

against volcanic and sedimentary rocks (K^vs) along the high-angle Togiak-Tikchik Fault 

(Box and others, 1993). Units that can be distinguished, such as the limestone of Permian 

age are mapped separately. Reported are Late Triassic and Permian megafossils and radiolarians of Paleozoic(?), Devonian, pre-Late Devonian, and Triassic age including the radiolarians Eucyrtis sp., Mirifusus(?), Parvicingula blowi Pessagno, Parvicingula sp., Ristola(?) 

altissima Rust, Ristola sp., and Spongocapsula sp.(?) of Early Cretaceous (Berriasian/early 

Valanginian) to Late Jurassic (late Tithonian) age, and conodonts Epigondolella, Epigondolella abneptis, Neogondolella, Neogondolella navicula, Xaniognathus sp. indet, of Late 

Triassic, Norian age (Hoare and Jones, 1981; Box and others, 1993; see www.alaskafossil.

org). Unit was included in the Tikchik terrane by Box and others (1993) 

J^p Phyllite and chert (Lower Jurassic and Upper Triassic?)—Gray, green, and black phyllite, 

fine-grained tuff, and tuffaceous chert (Box and others, 1993). Unit crops out in the southeast corner of the Bethel quadrangle near the heads of Upnuk and Chikuminuk Lakes and 

trends north-northeast where it continues into the adjacent Taylor Mountains quadrangle. 

Unit depositionally overlies map unit ^b in the southeastern Bethel quadrangle and is overlain by map units Jvs and Ja (Box, 1985a). Box and others (1993) constrained age based on 

unit overlying ^b and an Early Jurassic age derived from the single radiolarian collection. 

On the basis of field notes of J.M. Hoare and W.H. Condon, in the adjacent Taylor Mountains quadrangle, the unit could also be described as gray to black cherty phyllitic shale, 

highly contorted thin-bedded siliceous argillite and local cherty dark calcareous phyllite in 

association with greenstone of map unit MDv. The phyllite is locally sulfide-bearing along 

fault zones according to the field notes of J.M. Hoare and W.H. Condon (USGS, unpub. 

data, 1969, 1970) 

^vs Volcanic and sedimentary rocks (Upper Triassic)—“Marine unit consisting of chert, 

tuffaceous cherty rocks, argillite, siltstone, volcanic wackes, conglomerate, limestone, and 

mafic flows and breccias” (Hoare and Coonrad, 1978). Limestone is generally white to 

cream colored and recrystallized; however, it locally may be dark gray and finely crystalline. Possibly occurs in fault-bounded settings. Unit crops out in two areas: near the head of 

Lake Chauekuktuli in the northeast corner of the Goodnews Bay and adjacent Dillingham 

quadrangles and also near where the Kanektok River has cut through the Ahklun Mountains. 

Fossils include the ammonite Arcestes sp. of Late Triassic age and pelecypods Monotis 

subcircularis Gabb and Monotis(?) sp. of Norian age (see www.alaskafossil.org). Similar 

rocks are included in the }| unit of Hoare and Coonrad (1978, unit }|t, herein) as well 

as in the ^|m melange unit of Box and others (1993, unit }m in part, herein). Hoare and 

Coonrad (1978) mapped these rocks as a separate unit only in the vicinity of fossil localities 

“because the rocks resemble other rocks of Paleozoic and Mesozoic ages with which they 

are tectonically associated.” Unit included both in the Goodnews and Tikchik terranes by 

Box and others (1993) 

^v Basalt and chert (Upper Triassic)—“Massive and pillow basalt and basaltic breccia interbedded with thin-bedded tuffaceous chert and shale” (Box and others, 1993). Unit crops 

out near the heads of Upnuk and Nishlik Lakes along the southwestern edge of the Bethel 

quadrangle and continues eastward into the adjacent Taylor Mountains quadrangle. West 

of Milk Creek Fault, hydrothermally altered basalt contains secondary chlorite, quartz, and 

calcite but retains original subophitic and fine-grained plagioclase porphyritic textures (Box 

and others, 1993). East of Milk Creek Fault, a strong foliation and recrystallization has 

produced fine-grained schist or phyllite composed of chlorite, epidote, calcite, and pumpellyite (Box and others, 1993). “Whole-rock trace-element chemistry is characterized by flat 

to light REE-depleted chondrite-normalized REE pattern, and by high La/Nb elemental 

ratios, characteristic of island-arc tholeiitic basalts” (Box and others, 1993). Box and others 

(1993) linked the units on either side of the Milk Creek Fault on the basis of similar stratigraphic position and trace-element chemistry. Box and others (1993) reported interbedded 

shale yielded pelecypods of Late Triassic age just east of Aniak Lake as well as in the Taylor 

Mountain quadrangle east of map area (W.L. Coonrad, written commun., 1991, cited in Box 
26

and others, 1993). In the Taylor Mountains quadrangle, unit is associated with a limestone 

that yields Monotis and Halobia fossils of Late Triassic age (Wilson and others, 2006b). 

Unit was included in the Togiak terrane by Box and others (1993) 

}|a Argillaceous mélange (Mesozoic to Paleozoic)—Rock unit described by Box and others 

(1993) as a weakly to intensely fractured foliated black to green argillaceous mélange containing discontinuous phacoids of radiolarian chert, limey sandstone, blocks of limestone, 

and subphyllitic, amygdaloidal basalt. Unit crops out in the Eek Mountains near the southern 

boundary of the Bethel quadrangle. Unit is unconformably overlain by Cretaceous rocks 

of the Kuskokwim Group (Kk) and calcareous graywacke and siltstone (Kcgc). Conodonts 

Neogondolella bitteri and Neogondolella phosphoriensis of Permian age were found in 

limy siltstone (Box and others, 1993; see also www.alaskafossil.org). Box and others (1993) 

reported Permian pelecypods (Atomodesma sp. fragments) from a basalt-limestone block 

originally reported by Murphy (1989) and Permian conodonts recovered from limy siltstone 

in the matrix of the unit. Unit was included in the Goodnews terrane by Box and others 

(1993) 

Pv Volcanic rocks (Permian)––Pillow and columnar-jointed amygdaloidal, mildly altered basalt 

flows, breccia, diabasic intrusive, and a few sandy tuffs; unit Pb of Box (1985a) and Pv of 

Hoare and Coonrad (1978). Unit crops out near Goodnews Bay and north of Nuyakuk Lake. 

In the Goodnews Bay area, unit is interbedded with volcaniclastic sedimentary rocks (unit 

Pvs herein; Box, 1985a). Hoare and Coonrad (1978) did not separate these rocks from their 

undivided }| unit in the vicinity of Goodnews Bay. In the Nuyakuk Lake area, unit lies 

stratigraphically above and grades downward into the Permian limestone (unit Pls herein; 

Mertie, 1938, p. 45–46). Near lowest part of section north of Nuyakuk Lake, fossils identified in an interbed or fault sliver of limestone include brachiopods Calliprotonia sp., Neochonetes(?), Neophricadothyris sp., Neospirifer(?) sp., Thamnosia sp., Waagenoconcha(?) 

sp., and Yakovlevia sp. of Permian age; foraminifera Schwagerina jenkinsi Thorsteinnson of 

Permian age; stenoporoid bryozoans of Permian age; and echinoderm debris of Permian age 

(see sample 79AHr 2, www.alaskafossil.org). Outcrops in vicinity of Goodnews Bay were 

included in the Goodnews terrane by Box (1985a); outcrops near Nuyakuk Lake lie within 

his Togiak terrane 

Pls Limestone (Permian)—Thin-bedded and massive, light- to dark-gray, locally cream-colored, 

fine-grained, recrystallized limestone; unit Pl of Hoare and Coonrad (1978) and Box 

(1985a). Tuffaceous and locally cherty; unit has a fetid odor upon breaking (Hoare and 

Coonrad 1978; Box, 1985a). Unit is widely distributed throughout the map area and is commonly intercalated in the Togiak-Tikchik Complex and mélange units (}|t, }|a, }m). 

Hoare and Coonrad (1978) reported the unit “commonly closely associated with mafic 

volcanic rocks into which it grades through medium of calcareous breccias and tuffs.” Commonly contains the pelecypod Atomodesma of Permian age as well as crinoid stems, brachiopods, other pelecypods, and possible bryozoan reported (J.M. Hoare and W.H. Condon, 

unpub. data; Box, 1985a; see also www.alaskafossil.org). Unit included in the Goodnews 

terrane by Box (1985a); however, rocks of this unit are also found within his Togiak terrane 

(Box and others, 1993; Decker and others, 1994) 

Pvs Volcaniclastic sedimentary rocks (Permian)—“Volcaniclastic sedimentary rocks, ranging from coarsely bedded volcanic breccias to finely bedded calcareous tuffaceous rocks. 

Limestone cobble conglomerate locally north of Goodnews Bay. Red to black laminated 

argillite with radiolarian ghosts locally north and east of Goodnews Bay. Age constrained 

by several occurrences of fragmentary Atomodesma sp. Prehnite-pumpellyite facies” (Box, 

1985a). Interbedded with Permian mafic volcanic rocks (Pv). Unit included in the Platinum 

subterrane of the Goodnews terrane by Box (1985a); however, rocks that are similar and 

stratigraphically correlated are found in the western Taylor Mountains quadrangle associated 

with map units Pls and Pv and apparently extend into the eastern Bethel quadrangle (map 

unit Pcs of Wilson and others, 2006b). These rocks were assigned to the Togiak terrane by 

Box (1985a) 

Pcs Clastic rocks (Permian?)—“Well-bedded, cleaved sandstone, shale, thin limestone interbeds, 

and cobble conglomerate * * * sandstone and conglomerate are composed predominantly of 

chert clasts (containing radiolarian ghosts and internal quartz veins) and minor phyllite and 

porphyritic volcanic clasts; shale-rich section have thin limestone turbidite beds and thin 

sand to gravel beds of volcaniclastic composition” (Box and others, 1993). Unit crops out 
27

in the Bethel quadrangle north and south of the head of Upnuk Lake and extends outside the 

map area into the adjacent Taylor Mountains quadrangle where it is mapped as the clastic 

rocks unit (Pcs) of Wilson and others (2006b). Unit depositionally overlies the greenstone 

and schist unit (MDv) (Box and others, 1993). Permian limestone (Pls) is interbedded 

with this unit in many places in the Taylor Mountains quadrangle to the east (Wilson and 

others, 2006b). Conodonts of Middle(?) Triassic and Early Mississippian to Late Devonian age were reported by Box and others (1993). With respect to the Devonian conodonts, 

Anita Harris (USGS, written commun., 1989) suggested they may have been derived from 

redeposited clasts or “interweaved” tectonic slices (see comments for sample 87AMM 69 at 

www.alaskafossil.org). Unit was included in the Tikchik terrane by Box and others (1993) 

MDv Greenstone and schist (Lower Mississippian or Upper Devonian)—Box and others (1993) 

described this unit as consisting of “Weakly to moderately foliated and flattened pillowed and 

massive basalt, andesite, dacite, rhyolite flows, and breccia with greenschist metamorphicmineral assemblages.” They also stated that the rocks that have a volcanic protolith have 

whole-rock trace-element signatures indicating an arc-alkaline arc affinity. In the adjacent 

Taylor Mountains quadrangle, Wilson and others (2006b) described “Fine- to coarse-grained, 

massive green to purple altered greenstone, occasionally calcareous or schistose, rarely 

interbedded with green chert, argillite, and tuff. Greenschist with occasional chert boudins is a 

minor part of unit as is shale-chip agglomerate. Greenstone is locally cut by quartz and calcite 

veins containing epidote and copper sulfides” based on the 1969 and 1970 field notes of J.M. 

Hoare and W.H. Condon. Unit crops out near the head of Chikuminuk Lake. Unit is interbedded with the basal part of unit Pcs; clastic rocks and phyllite of map unit J^p may overlie 

this map unit (Wilson and others, 2006b). Age inferred by Box and others (1993) on the basis 

of conodonts of Late Devonian to Early Mississippian age near the base of unit shown herein 

as Pcs (note, the Pcs unit was mapped as Triassic to Devonian age by Box and others, 1993). 

Age assignment questionable because conodont-bearing samples may have been collected 

from redeposited clasts or “interweaved” tectonic slices (see comments by Anita Harris for 

sample 87AMM 69, www.alaskafossil.org). Unit may be Permian in age because of association with clastic rocks of presumed Permian age owing to association with Permian limestone 

(Pls) interbeds in the Taylor Mountains quadrangle (Wilson and others, 2006b). Unit included 

in the Tikchik terrane by Box and others (1993) 

DOl Thin-bedded limestone (Devonian to Ordovician)—“Thin-bedded to massive, fine-grained 

gray limestone, highly fractured and veined with white calcite, contains algal reefs and 

reef breccias. Locally contains interbedded tuffs and mafic volcanic rocks. Recrystallized 

to marble with interbedded quartzite and quartz-chlorite schist * * *” (Hoare and Coonrad, 

1978). Algal mounds indicate shallow water deposition (Decker and others, 1994). Unit is 

in scattered outcrops along a north-northeasterly trend from Goodnews Bay to the northern edge of the Goodnews Bay quadrangle. Unit thrust over Permian limestone (Pls) and 

calcareous schist unit (^Pcs) and intruded by gabbro of Jurassic age (Jgb). Hoare and 

Coonrad (1978) reported sparse fossils of Middle Devonian to Early(?) Ordovician age. One 

sample collected by Stephen Box in northern part of Goodnews Bay quadrangle contains 

the conodonts Acanthodus lineatus (Furnish), Drepanoistodus sp., Variabiliconus bassleri

(Furnish), “Scolopodus” cf. “S.” sulcatus Furnish, Eucharodus parallelus (Branson and 

Mehl), and Rossodus(?) sp. of Early Ordovician age (see sample 89AJM 60a, www.alaskafossil.org). A sample collected by J.M. Hoare contained ostracodes, echinoderms, fragments 

of brachiopods, algae, and small colonies of tabulate coral, possibly the genus Dania of 

Silurian age (A.K. Armstrong, written commun., 1975). Unit was included in the Goodnews 

terrane by Box (1985a) 

METAMORPHIC ROCKS OF THE TOGIAK-TIKCHIK COMPLEX

[In general, where stratigraphic position is reported, this is the inferred stratigraphic position for the protolith; where age is reported, this is the metamorphic age of the unit]

Rock units of the Kisaralik anticlinorium of Box and others (1993) (Mesozoic and (or) 

Paleozoic)—A structurally imbricate package of Mesozoic to Paleozoic deep-marine sedimentary and mafic volcanic rocks (Box and others, 1993). Units listed here are in structural 
28

order in the anticlinorium, top to bottom. Unless otherwise noted, the following unit descriptions are quoted from Box and others (1993)

}|m Metachert and phyllitic metachert—“Finely crystalline, thin-bedded quartzites (metachert) 

and finely interlayered quartzite and black phyllite. Found east of southern part of Greenstone Ridge in central part of map area [Bethel quadrangle]. Composed of fine aggregates of 

quartz with seams to centimeter-thick bands rich in fine-grained white mica. Recrystallized 

radiolarian tests present in some horizons. Thickness is uncertain. Structurally overlies units 

}|s and }|v on north and is structurally overlain by unit MDm on south. Unconformably overlain by conglomeratic rocks [Kkn] of Kuskokwim Group along Kisaralik River. 

Age constrained by presence of radiolarians (Phanerozoic) and by late Early Cretaceous age 

of overlying unit [Kkn].” Unit is strongly foliated and metamorphosed to greenschist and 

transitional greenschist-blueschist facies 

}|s Arkosic sandstone and slate—“Strongly cleaved, medium-grained, generally thin-bedded, 

arkosic sandstone and slate. Exposed on prominent ridge east of Greenstone Ridge in central 

part of map area [Bethel quadrangle]. Detrital grains in sandstones include monocrystalline quartz (30 to 50 percent), plagioclase (10 percent), and potassium feldspar (5 percent), 

and minor mica, slate, and volcanic rock fragments. Alteration products include secondary 

prehnite, calcite, quartz, and white mica. Turbidite depositional features locally preserved. 

Thickness is uncertain. Intercalated (structurally?) over 100-m-thick zones with thin-bedded 

chert-argillite sections (}|m); unconformably overlain by conglomeratic rocks [Kkn] of 

Kuskokwim Group. Age is uncertain, but older than Late [Early] Cretaceous age of unit 

[Kkn].” Unit has a slaty cleavage and is metamorphosed to the prehnite-pumpellyite facies 

(Box and others, 1993) 

}|v Volcaniclastic sandstone and argillite—“Turbiditic, thin-bedded, medium- to fine-grained, 

volcaniclastic sandstones, and dark-green to black argillites with weak to nonexistent 

slaty cleavage. Exposed along prominent ridge east of Greenstone Ridge in central part of 

map area [Bethel quadrangle]. Detrital grains include volcanic rock fragments containing 

plagioclase phenocrysts, plagioclase, and minor potassium feldspar, clinopyroxene, and 

quartz. Thickness is uncertain. Structurally overlain and underlain by unit }|s; structurally 

overlain by unit }|m along Kisaralik River; unconformably overlain by conglomeratic 

strata [Kkn] of Kuskokwim Group north of Kisaralik River. Age is uncertain, but older than 

Late [Early] Cretaceous age of unit [Kkn].” Unit has a slaty cleavage and is metamorphosed 

to the prehnite-pumpellyite facies (Box and others, 1993) 

}|c Chert and argillite—“White, gray-green, and blue-green crystallized chert in 2- to 5-cmthick beds interbedded with black to dark-green phyllite or slate beds of similar thickness. 

Exposed on prominent ridge east of Greenstone Ridge in central part of map area [Bethel 

quadrangle]. Structurally(?) intercalated with unit }|s; unconformably overlain by 

conglomeratic rocks [Kkn] of Kuskokwim Group. Age constrained by presence of recrystallized radiolarian (Phanerozoic) and Late [Early] Cretaceous age of overlying unit [Kkn].” 

Unit has a slaty cleavage and is metamorphosed to the prehnite-pumpellyite facies (Box and 

others, 1993) 

}|p Chloritic phyllite—“Relatively homogenous unit of finely foliated and crenulated phyllite 

* * *. Found east of southern part of Greenstone Ridge in south-central part of the map area 

[Bethel quadrangle]. Protolith was probably fine-grained tuffaceous sediment of uncertain 

age. Whole-rock major- and trace-element chemical compositions are similar to those of 

modern mid-ocean ridge basalts and strikingly similar to those of lavas from unit }|b. 

Structurally overlies unit MDm and is intruded by serpentinite (Ksp). Depositionally overlain by conglomeratic rocks [Kkn] of Kuskokwim Group. Age is uncertain, but older than 

Late Cretaceous age of unit [Kkn].” Unit is strongly foliated and metamorphosed to greenschist and transitional greenschist-blueschist facies. (Note, on this map, the Kuskokwim 

Group nearshore facies rocks of map unit Kkn are assigned an age that includes Albian or 

late Early Cretaceous) 

MDm Marble and metabasaltic dikes (Lower Mississippian and (or) Upper Devonian)—“Lightgray to white calcitic and dolomitic marble cut by pre-metamorphic basaltic dikes. Only 

found in small area east of Greenstone Ridge in central part of map area [Bethel quadrangle]. Locally contains as much as 20 percent clastic grains of feldspar, quartz, and plutonic 

rock fragments. Rare metamorphic blue amphiboles (magnesioriebeckite) in metabasaltic 

dikes indicate relatively high-pressure, low-temperature metamorphism (Sarah Roeske, 
29

written commun., 1988). Ranges from 100 to 300 m thick. Unit found structurally above 

unit }|m and structurally below unit }|p; original depositional relations are uncertain. 

Age constrained by conodonts of latest Devonian to earliest Mississippian age” (Stephen 

Box and others, unpub. data, reported in Box and others, 1993). Unit is strongly foliated and 

metamorphosed to greenschist and transitional greenschist-blueschist facies 

Jgs Green amphibole-bearing schist (Late Jurassic)—Schist recrystallized in part to greenschist 

metamorphic facies mineral assemblage of albite-epidote-chlorite-actinolite. Schist contains 

interbeds of thin-bedded, white or green, meta-chert having white mica-rich laminae (Box 

and others, 1993); overlain by crystalline limestone, phyllite, and minor amounts of chlorite, 

graphite, and quartz-sericite schist (Hoare and Coonrad, 1959a). Mapped as Mesozoic to 

Paleozoic metabasalt and minor meta-chert (}|b) by Box and others (1993); protolith was 

diabase and basalt having MORB trace-element (REE) signature (Box and others, 1993). 

Although protolith age is unknown, it was suggested to be Devonian to Ordovician by Hoare 

and Coonrad (1959a). Greenstone Ridge near the center of the Bethel quadrangle is composed of this map unit. Unit is bounded on west by Golden Gate Fault; nearshore facies Kuskokwim Group rocks (Kkn) depositionally overlie unit at the north end of its exposure. Age 

of metamorphism may be Late Jurassic based on 146.0±15.0 Ma age of actinolite (sample 

B78-1213, table 1). Unit was included in the Goodnews terrane by Box and others (1993) 

^Pmb Metabasaltic schist (Triassic? to Permian?)—Partially to completely recrystallized schistose rocks derived from mafic igneous and associated sedimentary rocks, including pillow 

basalt, angular volcanic breccia, pebbly mudstone, fine-grained tuffaceous sedimentary 

rocks, diabase, gabbro, and rare chert (Box, 1985a). Mineral assemblages indicate greenschist to transitional greenschist-blueschist facies metamorphism (Box, 1985a). Unit crops 

out on the south and west side of Goodnews Bay and near Cape Pierce. Low-angle faults 

separate unit from overlying nonfoliated mafic rocks (ophiolite units ^ob, ^od, ^og) and 

from underlying schistose clastic rocks (^Pvs). Schistose fabrics indicative of multiple 

deformation events include moderately to shallowly dipping foliation and a locally developed lineation; sporadically developed crenulation cleavage and crenulation microfolds; 

and northeast-trending open folds folding both deformation fabrics (Box, 1985a). Metamorphic fabric is similar to that of structurally underlying meta-volcaniclastic glaucophanehornblende epidote schist (^Pvs) and calcareous schist (^Pcs) (Box, 1985a). Similarity 

of metamorphic fabrics suggests all three units were deformed during the same event (Box, 

1985a). West of Goodnews Bay, the unit structurally overlies Permian volcaniclastic rocks 

(Pvs) and underlies Permian limestone (Pls). Metamorphic age is between Middle Jurassic 

and Middle Triassic based on age constraints as described for unit ^Pvs below. Intruded 

by nonfoliated Middle Jurassic hornblende gabbro (Jgb). Schist yielded K-Ar mica ages of 

150±8 and 155±8 Ma (table 1), possibly related to a Late Jurassic thermal event suggested 

by Box (1985a); however, the ages are suspect because the K2

O content of the mica (2.94 

and 1.71 percent) is much lower than typical for mica (8 to 10 percent), suggesting alteration. Box (1985a) suggests the protolith age is similar to the structurally overlying ophiolite 

(units ^ob, ^od, and ^og) because of the lithologic similarity of the protolith. However, 

on the basis of Box’s (1985a) terrane model, the similarity of protolith may not be relevant, 

as he assigned this unit to his Goodnews terrane and the ophiolite to his Togiak terrane. An 

alternative protolith age assignment could be Permian or older, based on metamorphic ties to 

unit ^Pvs and association with Permian rocks of the Goodnews terrane 

^Pvs Volcaniclastic glaucophane-hornblende-epidote schist (Triassic? to Permian?)—Metamorphosed conglomerate, sandstone, and shale. Unit includes map units J^mvss and JPvss of 

Box (1985a), which he reports grade together in the vicinity of Goodnews Bay. Protolith for 

both was derived from a feldspar-rich volcanic source (Box, 1985a). Rounded cobble metaconglomerate contains mostly plagioclase-phyric amygdaloidal volcanic clasts (Box, 1985a). 

Mineral assemblages indicate greenschist to transitional greenschist-blueschist facies 

metamorphism (Box, 1985a). Unit shows evidence of multiple metamorphic events and has 

a more pronounced structural fabric to the southwest (Box, 1985a). The latest metamorphic 

event created a strong fabric by syn-metamorphic flattening or mylonitization and a lineation 

formed by clast elongation and mineral alignment (Box, 1985a). This latest metamorphic 

fabric deformed non-coplanar isoclinal folds and slaty cleavage of a previous metamorphic 

event (Box, 1985a). Metamorphic fabric is similar to the structurally overlying metabasaltic schist (^Pmb) and underlying calcareous schist (^Pcs) suggesting all three units were 
30

deformed during the same Late Jurassic metamorphic event (Box, 1985a). However, this is 

contradicted by K-Ar cooling age of amphibole of 231.2±6.9 Ma (sample 82SB-147, table 1) 

suggesting Middle Triassic metamorphism; age is constrained to be older than the intrusion 

of Middle Jurassic gabbroic rocks (unit Jgb). On and adjacent to Susie Mountain, unit is 

intruded by pyroxenite and hornblende gabbro bodies which yielded Middle Jurassic K-Ar 

amphibole ages. Box (1985a) indicates unit structurally overlies Permian mafic volcanic 

rocks (Pv) and volcaniclastic rocks (Pcs), which he concludes suggests the protolith age 

could be as old as Permian. Unit crops out near Cape Pierce and on the south and west sides 

of Goodnews Bay. Unit was included in the Cape Pierce subterrane of the Goodnews terrane 

by Box (1985a) 

^Pcs Calcareous schist (Triassic? to Permian?)—Recrystallized schistose and phyllitic calcareous sandstone, shale, limestone, limestone conglomerate, greenish tuffaceous rocks, mafic 

volcanic rocks, and volcanic conglomerate. Includes units J^mc of Box (1985a) and |cs

of Hoare and Coonrad (1978). Turbidite characteristics are locally preserved in clastic 

metasedimentary rocks (Box, 1985a). Mineral assemblages indicate greenschist to transitional greenschist-blueschist facies metamorphism (Box, 1985a); however, structurally the 

unit shows evidence of multiple metamorphic events (Box, 1985a). The latest metamorphic 

event created a strong fabric by syn-metamorphic flattening or mylonitization and a lineation 

formed by clast elongation and mineral alignment (Box, 1985a). This latest metamorphic 

fabric deformed tight folds and axial planar slaty cleavage of a previous deformation event 

(Box, 1985a). Metamorphic fabric is similar to the structurally overlying metavolcaniclastic 

glaucophane-hornblende epidote schist (^Pvs) and metabasaltic schist (^Pmb) suggesting 

all three units were deformed during the same metamorphic events (Box, 1985a). Metamorphic age is constrained as described above for unit ^Pvs. Unit crops out in two locations, 

near Cape Pierce and near Jacksmith Bay. Near Jacksmith Bay, unit is structurally overlain 

by Devonian to Ordovician limestone (DOl) and the Kanektok metamorphic complex (<k

and <km). Intruded by nonfoliated Middle Jurassic gabbroic rocks (Jgb). Age of protolith 

may be Permian or older (Hoare and Coonrad, 1978; Box, 1985a) because of metamorphic 

ties to unit ^Pvs and association with Permian rocks of the Goodnews terrane. Unit was 

included in the Cape Pierce subterrane of the Goodnews terrane by Box (1985a) 

Dismembered ophiolite of Box (1985a) (Lower Jurassic to Middle Triassic)—The following units were described by Box (1985a) and Decker and others (1994) as the Newenham 

ophiolite complex, a dismembered ophiolite. The defined ophiolite assemblage also included 

the serpentinized ultramafic rocks of Cape Newenham, which are here included in map 

unit Jum. Hoare and Coonrad (1978) mapped the trondhjemite as a distinct map unit, but 

included the remaining lithologies in their }| or Jum units. Box (1985a) placed these 

rocks in his Hagemeister subterrane of the Goodnews terrane. As described by Hoare and 

Coonrad (1978), little information is available about specific lithologies and their distribution in the }| unit, hence the ophiolite may also occur outside of the area covered by 

Box’s dissertation area and therefore be more extensive than shown. Box (1985a) inferred 

the igneous rocks of the complex to be cogenetic and inferred the ophiolite complex to be 

Late Triassic in age on the basis of the radiolaria in the included chert. Subdivided into the 

following: 

J^ot Trondhjemite—Light-gray, medium-grained trondhjemite consisting of abundant quartz, 

plagioclase, and minor chlorite, sericite, and clinozoisite (Hoare and Coonrad, 1978; Box, 

1985a). Found in small bodies mapped in a number of areas of the Hagemeister Island 

and Goodnews Bay quadrangles by Hoare and Coonrad (1978). Box (1985a) indicated 

that in addition to the mapped bodies, other small bodies occur within altered diabase near 

Cape Newenham. At Tokomarik Mountain, Box (1985a) described the trondhjemite as “a 

northeast-dipping slab faulted above and below against schistose rocks * * *.” Box (1985a) 

tentatively assigned a Late Triassic age based on what he thought was a probable co-genetic 

relation with the Upper Triassic pillow basalt (^ob) and other components of the ophiolite, 

whereas Hoare and Coonrad (1978) assigned a Jurassic age based on the association of the 

trondhjemite with Jurassic gabbro and ultramafic rocks 

^ob Pillow basalt (Triassic)—Aphanitic to porphyritic pillow basalt, containing interbedded 

pillow breccia, aquagene tuff, and inter-pillow and interbedded red and white radiolarian 

chert as described by Box (1985a). Porphyritic basalt has plagioclase and clinopyroxene 

phenocrysts in an intergranular to subophitic groundmass of plagioclase, clinopyroxene, and 
31

secondary replacement minerals. Basalt is commonly amygdaloidal; abundant vesicles are 

filled with a combination of chlorite, clinozoisite, prehnite, pumpellyite, and calcite. Box 

(1985a) reported a number of radiolarian collections; however, only two were age diagnostic, yielding Middle Triassic to Early Jurassic and Middle to Late Triassic ages. Box (1985a) 

considered the pillow basalt to be Late Triassic 

^od Diabasic intrusive rocks (Triassic)—Subophitic, holocrystalline plagioclase-clinopyroxene 

diabase (Box, 1985a). Shown here west of Jagged Mountain at Cape Newenham, at Chagvan Mountain, on the coast south of Downdraft Mountain, and along the coast east of Cape 

Pierce; Box (1985a) also mapped 0.3- to 1.0-m-wide diabase sills and dikes containing 

basalt and chert screens west of Matogak River mouth, thick diabase sills intruding chert at 

Pyrite Point, and diabase dikes cutting gabbro south of Chagvan Bay, which are not shown. 

Box (1985a) described the unit as “mostly massive and highly altered to low greenschist 

facies assemblage.” Unit is altered to hornblende hornfels west of Matogak River mouth, 

which Box (1985a) reported is due to an underlying, but not shown, gabbroic pluton; a K-Ar 

amphibole age on the hornfels was 187±10 Ma (sample 81SB 116b, table 1). Unit believed 

to be cogenetic with associated, overlying Upper Triassic pillow basalt (^ob) (Box, 1985a) 

^og Pyroxene gabbro (Triassic)—Altered clinopyroxene gabbro, locally containing up to 5 

percent orthopyroxene, which is commonly kinked and fractured and locally rimmed or 

replaced by pale-green amphibole (Box, 1985a). Plagioclase partially to completely replaced 

by fine aggregate of albite, epidote, chlorite, calcite, and iron oxides. Compositional layering is rare in contrast to gabbro of map unit Jg. Locally grades into diabase (^od) south 

at Chagvan Mountain due to increasing proportion of crosscutting diabasic dikes. Box 

(1985a) mapped gabbro as a thrust sheet above the Osviak Fault emplaced over schistose 

rocks of map unit ^Pcs around Security Cove and Cape Pierce. Locally, mylonite is found 

along contact and similar mylonite is found sporadically along contact between gabbro and 

structurally underlying diabase near Cape Newenham. Box (1985a) believed gabbro was 

co-genetic with associated diabase (^od), pillow basalt (^ob), trondhjemite (J^ot) and 

serpentinized ultramafic rocks, all part of a dismembered ophiolite 

<k Kanektok metamorphic complex, undivided (Paleoproterozoic)—Gneiss and schist derived 

from sedimentary, volcanic, and plutonic rocks metamorphosed to upper greenschist through 

granulite facies (Hoare and Coonrad, 1978; Turner and others, 2009). “Includes mediumto coarse-grained, massive and well-foliated, biotite-hornblende gneisses, garnetiferous 

amphibolites, quartz-mica schists, and marble” (Hoare and Coonrad, 1978). It is an antiformal crystalline complex cored by high-grade orthogneiss intercalated with metavolcanic and 

metasedimentary rocks and flanked by greenschist-facies rocks (Turner and others, 2009). 

Rocks in core of complex include varieties of orthogneiss whose protoliths may have ranged 

in composition from granite to diorite (Turner and others, 2009; Moll-Stalcup and others, 

1996). As described by Turner and others (2009), orthogneiss of the core is intercalated 

with pyroxene granulite, garnet amphibolite, garnet-mica schist (locally kyanite-bearing), 

and rare quartzite and marble (marble locally mapped separately as unit <km). The core of 

amphibolite facies rocks apparently grade into greenschist facies rocks that dip away from 

the core on the northwest and southeast. These lower grade rocks contain a variety of schist 

and quartzite, as well as calc-phyllite, marble, and meta-conglomerate (Turner and others, 

2009). Mineral foliation tends to parallel compositional layering and both consistently strike 

northeast; “dip of the foliation changes from northwest to southwest two to four times across 

the width of the complex” (Hoare and Coonrad, 1979) south of Kanektok River. Compositional layering and foliation are disrupted by northwest-trending faults including a significant left lateral offset at the Kanektok River. North of the river, the rocks are more highly 

deformed and the dip and trend of foliation are more variable (Hoare and Coonrad, 1979). 

The Kanektok metamorphic complex is fault bounded on the southeast against calcareous 

schist (unit ^Pcs), the Kuskokwim Group (unit Kk), and the undivided Togiak-Tikchik 

Complex (unit }|t) in the Goodnews Bay quadrangle and green amphibole-bearing schist 

(Jgs) in the Bethel quadrangle. The metamorphic complex is depositionally overlain by 

the Kuskokwim Group (unit Kkn) in the Bethel quadrangle. The Kanektok metamorphic 

complex crops out as a narrow belt trending northeast, extending from near Jacksmith Bay 

in the Goodnews Bay quadrangle northward 160 km to the southern part of the Bethel quadrangle. The Idono complex of Gemuts and others (1983; Miller and others, 1991; Miller and 

Bundtzen, 1994; Wilson and others, 1998) is similar in some respects to the Kanektok meta-
32

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35

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2,116–2,129.
36

Table 1. Radiometric ages, Kuskokwim Bay region, southwest Alaska.

[Method: Ar/Ar, 40Argon/39Argon; FT, Fission-track; K/Ar, conventional potassium-argon; U/Pb, Uranium-lead. Material dated: AC, actinolite; AM, amphibole

biotite; CL, chlorite; HO, hornblende; IC, isochron; MI, mica; MU, muscovite; PL, plagioclase; RI, riebeckite; SE, sericite; SP, sphene; WR, whole-rock; ZI, zirc

Sample Map unit Quadrangle Latitude

(N.)

Longitude

(W.)

Rock type Method Material

Dated

Age

(Ma)

Error

(Ma)

A125 Qcs Cape Mendenhall 59.983 166.367 Basalt K/Ar AN 0.03 0.02 Cox and Da

1968; Mank

A102 Qcs Nunivak Island 60.000 166.333 Basalt K/Ar WR .06 .02 Cox and Da

1968; Mank

A126 Qcs Nunivak Island 60.017 166.183 Basalt K/Ar AN .09 .02 Cox and Da

1968

A96 Qcs Nunivak Island 60.017 166.2 Basalt K/Ar WR .11 .03 Cox and Da

1968; Mank

A114 Qcs Cape Mendenhall 59.95 166.167 Basalt K/Ar AN .14 .02 Cox and Da

1968; Mank

A71 Qcs Cape Mendenhall 59.978 165.657 Basalt K/Ar WR .28 .06 Cox and Da

1968; Mank

A91 Qvb Nunivak Island 60.033 166.083 Basalt K/Ar WR .23 .09 Cox and Da

1968; Mank

A86 Qvb Nunivak Island 60.017 166.067 Basalt K/Ar WR .29 .05 Cox and Da

1968; Mank

A69 Qvb Nunivak Island 60.053 165.682 Basalt K/Ar WR .30 .07 Cox and Da

1968; Mank

A74 Qvb Cape Mendenhall 59.933 165.562 Basalt K/Ar WR .34 .12 Cox and Da

1968

A88 Qvb Nunivak Island 60.033 166.083 Basalt K/Ar WR .35 .05 Cox and Da

1968; Mank

A78 Qvb Cape Mendenhall 59.8667 165.85 Basalt K/Ar WR .36 .09 Cox and Da

1968; Mank

86ASb 9a Qvb Bethel 60.355 159.863 Basalt K/Ar WR .418 .016 Box and othA70 Qvb Nunivak Island 60.017 165.633 Basalt K/Ar WR .50 .06 Cox and Da

1968; Mank

A59A Qvb Nunivak Island 60.223 165.693 Basalt K/Ar WR .63 .09 Cox and Da

1968; Mank

A108 Qvb Nunivak Island 60 166.233 Basalt K/Ar WR .67 .07 Cox and Da

1968; Mank

A97 Qvb Nunivak Island 60.017 166.267 Basalt K/Ar WR .69 .09 Cox and Da

1968; Mank

A101 Qvb Nunivak Island 60.017 166.317 Basalt K/Ar WR .74 .09 Cox and Da

1968; Mank
37

Table 1. Radiometric ages, Kuskokwim Bay region, southwest Alaska—continued.

[Method: Ar/Ar, 40Argon/39Argon; FT, Fission-track; K/Ar, Conventional potassium-argon; U/Pb, Uranium-lead. Material dated: AC, actinolite; AM, amphibole

biotite; CL, chlorite; HO, hornblende; IC, isochron; MI, mica; MU, muscovite; PL, plagioclase; RI, riebeckite; SE, sericite; SP, sphene; WR, whole-rock; ZI, zirc

Sample Map unit Quadrangle Latitude

(N.)

Longitude

(W.)

Rock type Method Material

Dated

Age

(Ma)

Error

(Ma)

A61 Qvb Nunivak Island 60.21 165.682 Basalt K/Ar WR .75 .75 Cox and Da

1968; Mank

74AHr77 Qvb Goodnews Bay 59.297 159.97 Basalt K/Ar WR .758 .2 Hoare and C

A46 Qvbm Nunivak Island 60.367 166.1 Basalt K/Ar WR .81 .10 Cox and Da

1968; Mank

A48 Qvbm Nunivak Island 60.348 166.085 Basalt K/Ar WR .86 .08 Cox and Da

1968; Mank

A45 Qvbm Nunivak Island 60.422 166.132 Basalt K/Ar WR .87 .07 Hoare and o

Dalrymple, 

A35 Qvbm Nunivak Island 60.365 166.3317 Basalt K/Ar WR .93 .07 Cox and Da

1968; Mank2 analyses.

A41 Qvbm Nunivak Island 60.387 166.242 Basalt K/Ar WR .95 .08 Cox and Da

1968; Mank2 analyses.

A23 Qvbm Nunivak Island 60.327 166.703 Basalt K/Ar WR 1.55 .15 Hoare and o

analyses.

A57 Qvbm Nunivak Island 60.267 165.685 Basalt K/Ar WR 1.58 .09 Cox and Da

1968; Mank

A51 Qvbm Nunivak Island 60.323 166 Basalt K/Ar WR 1.69 .09 Cox and Da

1968; Mank2 analyses.

P21 Qpd Pribilof Islands 56.596 169.514 Basalt K/Ar WR 1.63 .06 Cox and oth

Dalrymple, 

P52 QTv Pribilof Islands 57.131 170.291 Basalt K/Ar WR <0.10 -- Cox and oth

P35 QTv Pribilof Islands 57.195 170.405 Basalt K/Ar WR .124 .058 Average ag

and Dalrym1979

P35 QTv Pribilof Islands 57.195 170.405 Basalt K/Ar WR .096

.065.200

.10.10.10

Cox and oth

P38 QTv Pribilof Islands 57.184 170.416 Basalt K/Ar WR .33 .04 Cox and oth

1967; Mank

P38 QTv Pribilof Islands 57.184 170.416 Basalt K/Ar WR .33

.36

.10.10

Cox and oth
38

Table 1. Radiometric ages, Kuskokwim Bay region, southwest Alaska—continued.

[Method: Ar/Ar, 40Argon/39Argon; FT, Fission-track; K/Ar, Conventional potassium-argon; U/Pb, Uranium-lead. Material dated: AC, actinolite; AM, amphibole

biotite; CL, chlorite; HO, hornblende; IC, isochron; MI, mica; MU, muscovite; PL, plagioclase; RI, riebeckite; SE, sericite; SP, sphene; WR, whole-rock; ZI, zirc

Sample Map unit Quadrangle Latitude

(N.)

Longitude

(W.)

Rock type Method Material

Dated

Age

(Ma)

Error

(Ma)

P11 QTv Pribilof Islands 56.594 169.474 Basalt K/Ar WR 1.82 .07 Cox and othP11 QTv Pribilof Islands 56.594 169.474 Basalt K/Ar WR 1.85 .06 Cox and Da

1968; Mankothers, 1966

P20 QTv Pribilof Islands 56.609 169.527 Basalt K/Ar WR 1.85 .07 Cox and othP5 QTv Pribilof Islands 56.580 169.573 Basalt K/Ar WR 1.88

1.94

.09.08

Cox and othDalrymple, 

P19 QTv Pribilof Islands 56.603 169.495 Basalt K/Ar WR 1.97 .06 Cox and othP3 QTv Pribilof Islands 56.604 169.566 Basalt K/Ar WR 1.97

2.04

.08.08

Cox and oth1967; Mank

P2 QTv Pribilof Islands 56.605 169.556 Basalt K/Ar WR 2.00 .10 Cox and oth

1967; Mank

P12 QTv Pribilof Islands 56.598 169.472 Basalt K/Ar WR 2.12 .08 Cox and oth

1967; Mank

P13 QTv Pribilof Islands 56.597 169.471 Basalt K/Ar WR 2.15 .08 Cox and oth

1967; Mank

P67 QTv Pribilof Islands 56.583 169.584 Basalt K/Ar WR 2.16 .06 Cox and othP10 QTv Pribilof Islands 56.594 169.474 Basalt K/Ar WR 2.19 .06 Cox and othP15 QTv Pribilof Islands 56.600 169.486 Basalt K/Ar WR 2.19 .08 Cox and othP66 QTv Pribilof Islands 56.578 169.504 Basalt K/Ar WR 2.25 .10 Cox and othA50 QTab Nunivak Island 60.317 166.017 Basalt K/Ar WR .88 .07 Cox and Da

1968; Mank

A19 Ttb Nunivak Island 60.2 166.917 Basalt K/Ar WR 3.24 .10 Cox and Da

1968; Mank

A6 Ttb Nunivak Island 60.233 167.295 Basalt K/Ar WR 3.36 .10 Hoare and o

Dalrymple, 

A7 Ttb Nunivak Island 60.233 167.215 Basalt K/Ar WR 3.40 .12 Cox and Da

1968; Mank

A3 Ttb Nunivak Island 60.208 167.435 Basalt K/Ar WR 4.15 .12 Cox and Da

1968; Mank

A2 Ttb Nunivak Island 60.213 167.458 Basalt K/Ar WR 4.25 .12 Cox and Da

1968; Mank
39

Table 1. Radiometric ages, Kuskokwim Bay region, southwest Alaska—continued.

[Method: Ar/Ar, 40Argon/39Argon; FT, Fission-track; K/Ar, Conventional potassium-argon; U/Pb, Uranium-lead. Material dated: AC, actinolite; AM, amphibole

biotite; CL, chlorite; HO, hornblende; IC, isochron; MI, mica; MU, muscovite; PL, plagioclase; RI, riebeckite; SE, sericite; SP, sphene; WR, whole-rock; ZI, zirc

Sample Map unit Quadrangle Latitude

(N.)

Longitude

(W.)

Rock type Method Material

Dated

Age

(Ma)

Error

(Ma)

A12 Ttb Nunivak Island 60.225 166.95 Basalt K/Ar WR 4.96 .14 Cox and Da

1968; Mank

A8 Ttb Nunivak Island 60.22 167.063 Basalt K/Ar WR 4.97 .15 Cox and Da

1968; Mank

A11 Ttb Nunivak Island 60.225 166.95 Basalt K/Ar WR 5.01 .15 Cox and Da

1968; Mank

A17 Ttb Nunivak Island 60.212 166.945 Basalt K/Ar WR 5.01 .15 Cox and Da

1968; Mank

A15 Talb Nunivak Island 60.22 166.94 Basalt K/Ar WR 5.19 .15 Dalrymple 

Dalrymple, 

A1 Talb Nunivak Island 60.213 167.458 Basalt K/Ar WR 6.28 .18 Dalrymple 

Dalrymple, 

74AHr26 Tfi Hagemeister Is. 58.95 160.01 Rhyodacite K/Ar BI 13.00 .50 Wilson, 197

1985a

G199 Tad Pribilof Islands 56.575 169.5 Quartz diorite K/Ar SE

PL

49.555.0

2.02.2

Silberman aSilberman, 

ISOCHRON Tad Pribilof Islands 56.575 169.5 Quartz diorite K/Ar IC 51.7 1.9 Silberman a

Silberman, 

G200 Tad Pribilof Islands 56.575 169.5 Quartz diorite K/Ar AN

CL

52.757.2

2.12.3

Silberman aSilberman, 

87ACz 75M Tnr Bethel 60.766 160.23 Rhyolite Ar/Ar RI 54.7 1.6 Box and oth

82JD 450B TKr Bethel 60.583 160.272 Rhyolite K/Ar WR

BI

59.362.5

1.81.9

Robinson an1993

88AMl 12 TKa Bethel 60.576 160.427 Andesite Ar/Ar PL 59.5 12.3 Box and oth

GD5-1771 TKg Goodnews Bay 59.853 160.813 Dacite K/Ar HO 68.7 3.0 Hoare and CB78-1133G TKg Hagemeister Is. 58.847 160.232 Diorite K/Ar BI 75.0 2.9 Box, 1985aGC4-1300 TKgr Goodnews Bay 59.618 160.142 Granite porphyry K/Ar HO 60.7 1.8 Wilson and 

and Coonra

87ATf 62 TKgr Bethel 60.521 159.164 Granite Ar/Ar BI 60.8 1.8 Box and oth

74B57 TKgr Goodnews Bay 59.627 159 Granite K/Ar HO

BI

63.469.6

3.02.1

Wilson and and Coonra

74AHr51 TKgs Nushagak Bay 58.897 159.48 Monzonite K/Ar BI 71.9 2.0 Wilson, 197

1985a
40

Table 1. Radiometric ages, Kuskokwim Bay region, southwest Alaska—continued.

[Method: Ar/Ar, 40Argon/39Argon; FT, Fission-track; K/Ar, Conventional potassium-argon; U/Pb, Uranium-lead. Material dated: AC, actinolite; AM, amphibole

biotite; CL, chlorite; HO, hornblende; IC, isochron; MI, mica; MU, muscovite; PL, plagioclase; RI, riebeckite; SE, sericite; SP, sphene; WR, whole-rock; ZI, zirc

Sample Map unit Quadrangle Latitude

(N.)

Longitude

(W.)

Rock type Method Material

Dated

Age

(Ma)

Error

(Ma)

74AHr118 TKqm Goodnews Bay 59.66 159.672 Quartz monzonite K/Ar BI

HO

63.769.5

2.03.0

Wilson and and Coonra

87ATf 51 TKqm Bethel 60.275 159.633 Granite Ar/Ar BI 69.8 2.1 Box and oth87AJm 36a TKqm Bethel 60.152 159.936 Granodiorite Ar/Ar BI 70.3 2.1 Box and oth73AHr1 TKqm Goodnews Bay 59.918 159.915 Granodiorite K/Ar BI 71.1 2.1 Wilson and 

and Coonra

71AMm45 TKgd St. Matthew 60.4 172.698 Granodiorite K/Ar HO 62.3 2.0 Patton and 

Hopkins, 19

87ATf 72 TKgd Bethel 60.644 159.661 Granite Ar/Ar BI 62.2 1.9 Box and othGD1-3154 TKgd Goodnews Bay 59.843 159.182 Granodiorite K/Ar BI

AM

62.565.2

1.81.9

Wilson and and Coonra

GB1-3156 TKgd Goodnews Bay 59.465 159.177 Quartz diorite K/Ar HO

BI

63.469.5

1.82.0

Wilson and and Coonra

87ATf 56 TKgd Bethel 60.468 159.409 Granite Ar/Ar BI 64.3 1.9 Box and othGA5-1310 TKgd Goodnews Bay 59.172 160.648 Quartz diorite K/Ar BI 67.4 2.0 Wilson and 

and Coonra

GC1-1381 TKgd Goodnews Bay 59.665 159.36 Tonalite - Quartz 

diorite

K/Ar BI 72.5 2.2 Wilson and 

and Coonra

87ACz 61 TKqd Bethel 60.941 159.659 Quartz diorite K/Ar BI 65.1 2.0 Box and oth

87AJm 271b TKqd Bethel 60.394 159.118 Diorite Ar/Ar HO 68.0 2.0 Box and othGB7-1479a TKqd Goodnews Bay 59.347 161.325 Quartz diorite K/Ar BI 71.3 2.1 Wilson and 

and Coonra

74ACd 14d TKm; dike 

in Ks

Nushagak Bay 58.852 159.978 Diabase K/Ar BI 64.6 2.0 Hoare and C

12-106 Kv St. Matthew 60.335 172.433 Basalt K/Ar WR 52.2 1.6 Wittbrodt a

81SB-35b Kv Hagemeister Is. 58.777 160.787 Andesite K/Ar WR 64.5 3.2 Box, 1985a8106301 Kv Hagemeister Is. 58.8 160.75 Basalt K/Ar WR 64.5 3.2 Globerman, 71APa10 Kv St. Matthew 60.562 172.942 Basalt K/Ar WR 66.5 2.0 Patton and o

Hopkins, 19

5-29 Kv St. Matthew 60.583 172.967 Andesite K/Ar WR 68.0 2.3 Wittbrodt an8107126 Kv Hagemeister Is. 58.78 160.757 Andesite K/Ar WR 68.3 .4 Globerman, 8107131 Kv Hagemeister Is. 58.785 160.748 Andesite K/Ar WR 68.7 3.7 Globerman, 
41

Table 1. Radiometric ages, Kuskokwim Bay region, southwest Alaska—continued.

[Method: Ar/Ar, 40Argon/39Argon; FT, Fission-track; K/Ar, Conventional potassium-argon; U/Pb, Uranium-lead. Material dated: AC, actinolite; AM, amphibole

biotite; CL, chlorite; HO, hornblende; IC, isochron; MI, mica; MU, muscovite; PL, plagioclase; RI, riebeckite; SE, sericite; SP, sphene; WR, whole-rock; ZI, zirc

Sample Map unit Quadrangle Latitude

(N.)

Longitude

(W.)

Rock type Method Material

Dated

Age

(Ma)

Error

(Ma)

82JD 435A Kv Bethel 60.770 159.107 Andesite K/Ar PL

WR

69.071.4

2.12.1

Robinson an1993

1-2 Kv St. Matthew 60.57 172.997 Andesite K/Ar WR 69.2 2.1 Wittbrodt an

88ASb 99a Kv Bethel 60.782 159.170 Dacite Ar/Ar BI 69.5 2.1 Box and oth4-16 Kv St. Matthew 60.482 172.953 Basalt K/Ar WR 73.0 2.2 Wittbrodt an5-33 Kv St. Matthew 60.583 172.963 Andesite K/Ar WR 73.8 2.2 Wittbrodt an86ASb 1 Kv Bethel 60.323 159.627 Andesite Ar/Ar WR 74.3 2.2 Box and oth

5-123 Kv St. Matthew 60.583 172.963 Andesite K/Ar WR 76.0 2.3 Wittbrodt anB78-1125G Kv?; Dike 

in Ks

Hagemeister Is. 58.853 160.213 Andesite K/Ar HO 76.6 4.5 Box, 1985a

5-27 Kv St. Matthew 60.583 172.963 Andesite K/Ar WR 76.5 2.3 Wittbrodt an12-105 Kv St. Matthew 60.335 172.433 Basalt K/Ar WR 78.1 2.3 Wittbrodt an5-125 Kv St. Matthew 60.583 172.963 Andesite K/Ar WR 78.7 2.4 Wittbrodt an71APa15 Kv St. Matthew 60.583 172.975 Basalt K/Ar WR 78.7 2.0 Patton and o

Hopkins, 19

5-31 Kv St. Matthew 60.583 172.963 Andesite K/Ar WR 79.2 2.4 Wittbrodt a

71AMm50 Kpd St. Matthew 60.6533 173.06 Tuff K/Ar BI

HO

76.076.3

2.02.0

Patton and oHopkins, 19

87ATf 30 Kg Bethel 60.577 160.788 Granodiorite K/Ar BI 115.0 3.5 Box and oth88ATF24d Kg Bethel 60.158 159.822 Granodiorite U/Pb ZI 104-

129

-- Box and oth

HD6-2272a Jum Hagemeister Is. 58.9112 161.772 Amphibolite K/Ar AM 176.4 5.3 Hoare and C

HD6-1453e Jum Hagemeister Is. 58.918 161.775 Amphibolite K/Ar AM 186.9 5.6 Hoare and C

74AHr112 Jgb Hagemeister Is. 58.987 160.97 Gabbro K/Ar HO 159.3 3.5 Wilson, 197

1985a. Mat

GA7-1450 Jgb Goodnews Bay 59.022 161.523 Diorite K/Ar AM 162.4 4.9 Wilson and 

and Coonra

B78-1171G Jgb Hagemeister Is. 58.93 160.698 Gabbro K/Ar HO 174. 8. Box, 1985a74AHr111 Jgb Hagemeister Is. 58.963 161.05 Gabbro K/Ar HO 186.9 6.0 Wilson, 197

1985a. Dow
42

Table 1. Radiometric ages, Kuskokwim Bay region, southwest Alaska—continued.

[Method: Ar/Ar, 40Argon/39Argon; FT, Fission-track; K/Ar, Conventional potassium-argon; U/Pb, Uranium-lead. Material dated: AC, actinolite; AM, amphibole

biotite; CL, chlorite; HO, hornblende; IC, isochron; MI, mica; MU, muscovite; PL, plagioclase; RI, riebeckite; SE, sericite; SP, sphene; WR, whole-rock; ZI, zirc

Sample Map unit Quadrangle Latitude

(N.)

Longitude

(W.)

Rock type Method Material

Dated

Age

(Ma)

Error

(Ma)

81SB116b Jgb/^od Hagemeister Is. 58.853 160.99 Hornfels K/Ar AM 187. 10. Box, 1985a 

Jgb

B78-1152G Jlgd Hagemeister Is. 58.615 161.073 Granodiorite K/Ar BI or 

HO?

183. 7. Box, 1985a

content of mJlgd

89ASb 101 MDv Bethel 60.178 160.128 Dacite FT AP 47.5 6.9 Box and oth

81SB107A ^Pmb Hagemeister Is. 58.843 161.025 Phyllite K/Ar MI 150. 8. Box, 1985a81SB133a ^Pmb Hagemeister Is. 58.752 161.307 Schist K/Ar MI 155. 8. Box, 1985a82SB147 ^Pvs Hagemeister Is. 58.637 161.883 Schist K/Ar HO 231.2 6.9 Box, 1985aB78-1213 Jgs Bethel 60.342 160.313 Schist K/Ar AC 146.0 15.0 Box and oth

88ML 22b <k Bethel 60.093 160.6 Gneiss FT AP 66.6 4.6 Box and oth

DT76-21 <k Goodnews Bay 59.92 161.932 Hornblende diorite K/Ar HO 122.2 3.6 Turner and DT76-57 <k Goodnews Bay 59.955 160.898 Biotite-muscovite 

gneiss

K/Ar MU

BI

127.7131.9

3.93.9

Turner and 

DT75-67 <k Goodnews Bay 59.55 161.44 Biotite-muscovite 

granodiorite gneiss

K/Ar BI

MU

128.0135.8

3.74.0

Turner and 

DT76-58 <k Goodnews Bay 59.953 160.893 Hornblende diorite K/Ar HO 128.7 3.8 Turner and DT75-66 <k Goodnews Bay 59.562 161.432 Biotite-hornblende 

granodiorite gneiss

K/Ar BI

HO

128.9140.5

3.84.1

Turner and 

DT76-41 <k Goodnews Bay 59.947 160.912 Biotite-hornblende 

epidote schist

K/Ar HO

BI

129.6158.8

3.84.7

Turner and 

DT75-79 <k Goodnews Bay 59.543 161.593 Biotite-epidote 

schist

K/Ar BI 130.4 3.8 Turner and 

DT76-22 <k Goodnews Bay 59.903 161.92 Garnet amphibolite K/Ar HO 130.6 3.8 Turner and DT76-31 <k Goodnews Bay 59.922 161.902 Biotite-muscovite 

gneiss

K/Ar MU

BI

130.6196.8

3.85.8

Turner and 

DT76-52 <k Goodnews Bay 59.922 160.887 Quartz muscovite 

gneiss

K/Ar MU 130.6 3.9 Turner and 

DT75-64 <k Goodnews Bay 59.572 161.442 Biotite-hornblende 

granodiorite gneiss

K/Ar BI

HO

130.8141.7

3.84.1

Turner and 

DT75-63 <k Goodnews Bay 59.577 161.45 Biotite granodiorite gneiss K/Ar BI 133.2 3.9 Turner and 
43

Table 1. Radiometric ages, Kuskokwim Bay region, southwest Alaska—continued.

[Method: Ar/Ar, 40Argon/39Argon; FT, Fission-track; K/Ar, Conventional potassium-argon; U/Pb, Uranium-lead. Material dated: AC, actinolite; AM, amphibole

biotite; CL, chlorite; HO, hornblende; IC, isochron; MI, mica; MU, muscovite; PL, plagioclase; RI, riebeckite; SE, sericite; SP, sphene; WR, whole-rock; ZI, zirc

Sample Map unit Quadrangle Latitude

(N.)

Longitude

(W.)

Rock type Method Material

Dated

Age

(Ma)

Error

(Ma)

DT76-42 <k Goodnews Bay 59.947 160.913 Biotite-muscovite 

granodiorite gneiss

K/Ar BI 133.3 4.0 Turner and 

75AFS-18ck <k Goodnews Bay 59.62 161.2 Biotite-bearing 

amphibolite

K/Ar HO 133.7 3.9 Turner and 

75AFS-18ak <k Goodnews Bay 59.617 161.205 Biotite-muscovite 

granodiorite gneiss

K/Ar MU

BI

135.6143.7

3.94.2

Turner and 

DT75-78 <k Goodnews Bay 59.54 161.588 Greenschist K/Ar AC 137.1 4.0 Turner and 75AFS-12k <k Goodnews Bay 59.577 161.235 Garnet amphibolite K/Ar BI

HO

136.5163.2

3.94.8

Turner and 

75AFS-35ak <k Goodnews Bay 59.582 161.397 Biotite-hornblende 

granodiorite

K/Ar BI

HO

138.8235.5

4.16.9

Turner and 

DT76-34 <k Goodnews Bay 59.925 161.898 Biotite gneiss K/Ar BI 140.8 4.1 Turner and 

DT76-72 <k Bethel 60.127 160.567 Biotite-hornblende-epidoteplagioclase rock K/Ar BI HO 143.6 153.4 4.2 4.5 Turner and 

DT75-68 <k Goodnews Bay 59.547 161.437 Biotite garnet 

amphibole

K/Ar HO 144.2 4.2 Turner and 

75AFS-3k <k Goodnews Bay 59.607 161.225 Epidote amphibolite K/Ar BI HO 146.3 170.5 4.3 4.9 Turner and 

DT75-81 <k Goodnews Bay 59.543 161.593 Biotite-bearing 

greenschist

K/Ar AC 146.6 4.3 Turner and 

DT75-73 <k Goodnews Bay 59.563 161.382 Biotite-muscovite 

gneiss

K/Ar MU 153.2 4.5 Turner and 

DT75-73 <k Goodnews Bay 59.563 161.382 Biotite-muscovite 

gneiss

K/Ar BI 196.1 5.7 Turner and 

DT76-36 <k Goodnews Bay 59.93 161.895 Biotite-muscovitequartz schist K/Ar MUBI 156.5 395.6 4.6 12 Turner and 

DT76-74 <k Bethel 60.127 160.567 Biotite-hornblende-epidote 

rock

K/Ar BI

HO

162.1328.7

4.79.7

Turner and 

75AFS-1k <k Goodnews Bay 59.6117 161.228 Biotite-muscovite 

gneiss

K/Ar BI 169.7 4.9 Turner and 

DT75-60 <k Goodnews Bay 59.543 161.462 Garnet amphibolite K/Ar HO 169.7 5.0 Turner and 
44

Table 1. Radiometric ages, Kuskokwim Bay region, southwest Alaska—continued.

[Method: Ar/Ar, 40Argon/39Argon; FT, Fission-track; K/Ar, Conventional potassium-argon; U/Pb, Uranium-lead. Material dated: AC, actinolite; AM, amphibole

biotite; CL, chlorite; HO, hornblende; IC, isochron; MI, mica; MU, muscovite; PL, plagioclase; RI, riebeckite; SE, sericite; SP, sphene; WR, whole-rock; ZI, zirc

Sample Map unit Quadrangle Latitude

(N.)

Longitude

(W.)

Rock type Method Material

Dated

Age

(Ma)

Error

(Ma)

75AFS-22ak <k Goodnews Bay 65.45 161.205 Biotite-hornblende 

diorite gneiss

K/Ar BI

HO

170.3197.3245.3

4.95.87.2

Turner and 183.9±19.3 

very low

75AFS-28ak <k Goodnews Bay 59.57 161.442 Biotite-hornblende 

diorite gneiss

K/Ar HO 186.6 5.5 Turner and 

DT76-56 <k Goodnews Bay 59.97 160.847 Biotite-muscovitehornblende schist K/Ar MUBI 186.9 351.3 5.5 10.3 Turner and 

DT76-46 <k Goodnews Bay 59.94 160.923 Biotite-hornblende 

muscovite schist

K/Ar HO

BI

192.2223.7

5.66.6

Turner and 

75AFS-23ak <k Goodnews Bay 59.65 161.207 Biotite-hornblende 

diorite gneiss

K/Ar BI 199.7 5.8 Turner and 

DT76-61 <k Goodnews Bay 59.945 160.882 Porphyoblastic 

amphibolite

K/Ar HO 201.0 5.9 Turner and 

DT75-83 <k Goodnews Bay 59.65 161.21 Biotite-hornblende 

diorite gneiss

K/Ar BI 212.6 6.2 Turner and 

as reported and possibly 

DT75-82 <k Goodnews Bay 59.65 161.212 Biotite-hornblende 

diorite gneiss

K/Ar BI

HO

212.6215.8217.8

6.26.36.4

Turner and 214.2±6.3 a

DT76-66 <k Goodnews Bay 59.902 160.902 Unknown K/Ar BI 246.9 7.2 Turner and 

DT75-74 <k Goodnews Bay 59.567 161.385 Garnet amphibole K/Ar HO 247.1 7.2 Turner and 

DT75-62 <k Goodnews Bay 59.578 161.455 Garnet amphibolite K/Ar HO 247.8 7.2 Turner and DT76-71 <k Bethel 60.095 160.628 Biotite-hornblende 

gneiss

K/Ar HO 266 7.8 Turner and 

DT76-39 <k Goodnews Bay 59.913 160.897 Granodiorite 

gneiss

K/Ar BI 292.4 8.6 Turner and 

DT76-62 <k Goodnews Bay 59.905 160.903 Amphibolite K/Ar HO 406.4 18 Turner and 50AHr 747a <k Goodnews Bay 59.725 161.108 Quartz diorite K/Ar BI

HO

439.2450.2540.1544.2

131315.916.1

Turner and 444.7±13 MMa average 

DT75-69 <k Goodnews Bay 59.563 161.38 Garnet amphibole K/Ar HO 466.3 13.7 Turner and DT75-69 <k Goodnews Bay 59.563 161.38 Garnet amphibole Ar/Ar, total 

fusion

HO 478 4.2 Turner and 
45

Table 1. Radiometric ages, Kuskokwim Bay region, southwest Alaska—continued.

[Method: Ar/Ar, 40Argon/39Argon; FT, Fission-track; K/Ar, Conventional potassium-argon; U/Pb, Uranium-lead. Material dated: AC, actinolite; AM, amphibole

biotite; CL, chlorite; HO, hornblende; IC, isochron; MI, mica; MU, muscovite; PL, plagioclase; RI, riebeckite; SE, sericite; SP, sphene; WR, whole-rock; ZI, zirc

Sample Map unit Quadrangle Latitude

(N.)

Longitude

(W.)

Rock type Method Material

Dated

Age

(Ma)

Error

(Ma)

DT75-70 <k Goodnews Bay 59.558 161.38 Biotite quartz 

monzonite gneiss

Ar/Ar, total 

fusion

BI 487 4.3 Turner and 

argon loss

DT75-70 <k Goodnews Bay 59.558 161.38 Biotite quartz 

monzonite gneiss

K/Ar BI 498.0 14.7 Turner and 

DT76-76 <k Goodnews Bay 59.857 160.903 Garnet amphibolite K/Ar HO 637 19 Turner and DT76-70 <k Bethel 60.093 160.6 Biotite amphibolite K/Ar BI

HO

685915

2027

Turner and 

DT76-70 <k Bethel 60.093 160.6 Biotite amphibolite Ar/Ar, 

Plateau

HO 936 36 Paul Layer, 

conventionaindicating c

DT76-70r <k Bethel 60.093 160.6 Biotite amphibolite Ar/Ar, 

Plateau

HO 974 10.7 Paul Layer, 

rerun of conspectra, indi

DT75-76 <k Goodnews Bay 59.547 161.403 Garnet amphibole K/Ar HO 1037

1063

3132

Turner and 1049±32 av

DT75-76 <k Goodnews Bay 59.547 161.403 Garnet amphibole Ar/Ar, Total 

Fusion

HO 1124 8 Turner and 

K/Ar analysplateau age 

50AHr49 <k Goodnews Bay 59.55 161.333 Garnet amphibole K/Ar HO 1088. 32. R.B. Forbes

ten commun

DT76-69a <k Goodnews Bay 59.9 160.897 Garnet amphibolite K/Ar HO 1770 53 Turner and DT76-69a <k Goodnews Bay 59.9 160.897 Garnet amphibolite Ar/Ar, 

Plateau

HO 1778 89 Paul Layer, 

conventiona

DT75-86 <k Goodnews Bay 59.648 161.205 Quartz monzonite 

orthogneiss

U/Pb SP 1770 Turner and 

metamorphi

DT75-86 <k Goodnews Bay 59.648 161.205 Quartz monzonite 

orthogneiss

U/Pb ZI 2050 30 Turner and 

emplacemen

88ML22b <k Bethel 60.093 160.600 Granitic gneiss U/Pb ZI 2040 74 Box and oth

87ASB125b <k Bethel 60.300 160.334 Trondhjemite 

gneiss

U/Pb ZI 2070 16 Box and oth

DT75-61 <k Goodnews Bay 59.543 161.458 Retrograded 

pyroxene granulite

K/Ar BI 2231

2312

6770

Turner and 2271±70 av
46

Table 1. Radiometric ages, Kuskokwim Bay region, southwest Alaska—continued.

[Method: Ar/Ar, 40Argon/39Argon; FT, Fission-track; K/Ar, Conventional potassium-argon; U/Pb, Uranium-lead. Material dated: AC, actinolite; AM, amphibole

biotite; CL, chlorite; HO, hornblende; IC, isochron; MI, mica; MU, muscovite; PL, plagioclase; RI, riebeckite; SE, sericite; SP, sphene; WR, whole-rock; ZI, zirc

Sample Map unit Quadrangle Latitude

(N.)

Longitude

(W.)

Rock type Method Material

Dated

Age

(Ma)

Error

(Ma)

DT75-71 <k Goodnews Bay 59.555 161.372 Pyroxene granulite K/Ar BI 1921

215925312409

57.6657673

Turner and 2255±271.2 

DT75-72 <k Goodnews Bay 59.56 161.355 Pyroxene granulite K/Ar BI 2434

2500243224412564

7377737477

Turner and 2474±75 av

DT75-72 <k Goodnews Bay 59.56 161.355 Pyroxene granulite Ar/Ar, 

Plateau

BI 2520 Turner and 

tional K/Ar 

89AJm 34 K^vs Bethel 60.056 159.257 Sandstone FT AP 29.7 4.2 Box and oth

89ASb 38 Jvs Bethel 60.088 159.269 Sandstone FT AP 90.5 17.0 Box and oth

44-7 — offshore 58.648 177.215 Basalt K/Ar WR .38

.33

.05.05

Davis and o

43-2 — offshore 58.648 177.215 Basalt K/Ar WR .33

.50

.15.15

Davis and o

44-3 — offshore 58.648 177.215 Basalt K/Ar WR .60 .20 Davis and oDR5-34 — offshore 56.383 169.625 Basalt K/Ar WR .77 .08 Simpson an

others, 1979approximate 

DR5-8 — offshore 56.383 169.625 Basalt K/Ar WR .84 .08 Simpson an

others, 1979approximate 

44-12 — offshore 58.648 177.215 Basalt K/Ar WR 1.10 .20 Davis and o