2012

U.S. Department of the Interior

U.S. Geological Survey

Geology of the Prince William Sound and Kenai Peninsula 

Region, Alaska

Including the Kenai, Seldovia, Seward, Blying Sound, Cordova, and Middleton 

Island 1:250,000-scale quadrangles

By Frederic H. Wilson and Chad P. Hults

Pamphlet to accompany

Scientific Investigations Map 3110

View looking east down Harriman Fiord at Serpentine Glacier and Mount Gilbert. (photograph by M.L. Miller)

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Contents

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

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

Geographic, Physiographic, and Geologic Framework ..........................................................................1

Description of Map Units .............................................................................................................................3

Unconsolidated deposits ....................................................................................................................3

Surficial deposits ........................................................................................................................3

Rock Units West of the Border Ranges Fault System ....................................................................5

Bedded rocks ...............................................................................................................................5

Metamorphic rocks ..................................................................................................................12

Igneous rocks ............................................................................................................................12

Rock Units within the Border Ranges Fault System ....................................................................14

Metamorphic rocks ..................................................................................................................14

Rock Units East or South of the Border Ranges Fault System ...................................................14

Bedded rocks .............................................................................................................................14

Igneous rocks ............................................................................................................................21

References Cited .........................................................................................................................................26

Tables

Table 1. Radiometric ages from the Prince William Sound and Cook Inlet region ..........................31

1

Abstract

The Prince William Sound and Kenai Peninsula region 

includes a significant part of one of the world’s largest accretionary complexes and a small part of the classic magmatic arc 

geology of the Alaska Peninsula. Physiographically, the map 

area ranges from the high glaciated mountains of the Alaska 

and Aleutian Ranges and the Chugach Mountains to the coastal 

lowlands of Cook Inlet and the Copper River delta. Structurally, 

the map area is cut by a number of major faults and postulated 

faults, the most important of which are the Border Ranges, Contact, and Bruin Bay Fault systems.

The rocks of the map area belong to the Southern Margin 

composite terrane, a Tertiary and Cretaceous or older subduction-related accretionary complex, and the Alaska Peninsula 

terrane. Mesozoic rocks between these two terranes have been 

variously assigned to the Peninsular or the Hidden terranes. The 

oldest rocks in the map area are blocks of Paleozoic age within 

the mélange of the McHugh Complex; however, the protolith 

age of the greenschist and blueschist within the Border Ranges 

Fault zone is not known. Extensive glacial deposits mantle the 

Kenai Peninsula and the lowlands on the west side of Cook Inlet 

and are locally found elsewhere in the map area.

This map was compiled from existing mapping, without 

generalization, and new or revised data was added where available.

Introduction

The Prince William Sound and Kenai Peninsula region 

displays a major portion of one of the world’s largest accretionary complexes (Plafker and others, 1994) and a small part of 

the Alaska Peninsula magmatic arc. The early Tertiary and Late 

Cretaceous age accretionary complex is represented by rocks 

of the Orca Group, Valdez Group, and McHugh Complex. The 

accretionary complex has been known by a variety of names 

as our understanding of the geology of southern Alaska has 

evolved: the Chugach terrane (Berg and others, 1972; Nielsen 

and Zuffa, 1982); Chugach and Prince William terranes (Plafker 

and others, 1977; Jones and others, 1981); and Southern Margin 

Composite terrane (Plafker, 1990; Plafker and others, 1994). In 

addition to the rocks of the Southern Margin Composite terrane, 

older sedimentary and igneous rocks of the Alaska Peninsula 

and Hidden terranes (Wilson and others, 1985; Wilson and 

others, 1999) occur on both sides of Cook Inlet. The Alaska 

Peninsula terrane is roughly equivalent to the Peninsular terrane of Jones and others (1981). Cook Inlet is a Tertiary forearc 

basin lying between the modern Aleutian magmatic arc and the 

outboard accretionary complex.

Many sources were used to produce this geologic map. In 

most cases, data from available maps was combined, without 

generalization, and new data added where available. Bedrock 

geology of the Seward and Blying Sound quadrangles was 

largely derived from Nelson and others (1985) within the 

Chugach National Forest, and from Tysdal and Case (1979) 

elsewhere. The northeastern area of the Seward quadrangle 

includes revised geology by Nelson and others (1999). Surficial 

geology for Seward and Blying Sound quadrangles is derived 

from Tysdal and Case (1979). The Cordova and Middleton 

Island quadrangles geology was compiled and edited by Richter 

and others (2005) and was primarily derived from Winkler 

and Plafker (1993) with minor additions by T.L. Pavlis and 

V.B. Sisson (written commun., 2002, to D.H. Richter). Seldovia quadrangle geology was largely derived from Bradley and 

others (1999) with addition of surficial geology by Karlstrom 

(1964). Kenai quadrangle geology was derived from Karlstrom 

(1964), Magoon and others (1976), Bradley and Wilson (2000), 

and Wilson and others (2006).

Geographic, Physiographic, and Geologic Framework

The map area borders and includes the northern part of the 

Gulf of Alaska, a formerly heavily glaciated region. From west 

to east, the map area extends from the high glaciated mountains 

of the Aleutian Range to the coastal lowlands of the Cook Inlet 

basin to the glaciated coastal mountains surrounding Prince 

William Sound. Relief is generally high except for the lowlands 

of the western Kenai Peninsula and the alluvial flats of the 

Copper River on the eastern margin of the map area. The area 

is best known for the fjords and ice fields surrounding Prince 

William Sound, particularly in Kenai Fjords National Park. 

Redoubt Volcano, on the west side of Cook Inlet, is a prominent 

physiographic feature and is the highest point in the map area at 

3,108 m.

Extensive glacial deposits on the Kenai Lowlands (Karlstrom, 1964) record Late Wisconsin and earlier advances; 

whereas glacial and volcanic deposits on the west side of 

Cook Inlet reflect only Late Wisconsin glaciation, Holocene 

readvances, and Holocene volcanism. More recently, glaciers 

covered much of the Prince William Sound region as a result 

of Little Ice Age cooling, reaching a modern maximum extent 

about 1900 (Grant and Higgins, 1910). Since that time, glaciers 

have rapidly receded: in some areas, such as in Northwestern 

Fiord as much as 15 km, or as much as 20 km in McCarty Fiord 

just since 1950. Detailed mapping of the limited surficial deposits of Prince William Sound has largely not been undertaken; 

however, extensive studies of recent history have been undertaken as a result of the 1964 Great Alaska Earthquake (see for 

example, Plafker, 1969; Plafker and Page, 1994) and the 1989 

Exxon Valdez oil spill.

The 1964 Great Alaska Earthquake had intense local 

impact and generated far-ranging tsunamis in the Pacific Basin. 

A brief summary of the effects of the earthquake in Alaska by 

Stover and Coffman (1993) described “* * * vertical displacement over an area of about 520,000 square kilometers. The 

major area of uplift trended northeast from southern Kodiak 

Island to Prince William Sound and trended east-west to the east 

of the sound. Vertical displacements ranged from about 11.5 

meters of uplift to 2.3 meters of subsidence relative to sea level. 

Off the southwest end of Montague Island, there was absolute 

vertical displacement of about 13–15 meters. * * * The zone of 
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subsidence covered about 285,000 square kilometers, including 

the north and west parts of Prince William Sound, the west part 

of the Chugach Mountains, most of Kenai Peninsula, and almost 

all the Kodiak Island group.” Much more extensive reports on 

the effects of the earthquake can be found in Wood (1967), Leopold (1969), and Committee on the Alaska Earthquake of the 

Division of Earth Sciences National Research Council (1971) 

and related volumes. Studies on Middleton Island and in Kenai 

Fjords National Park (Plafker and Rubin, 1978; Crowell and 

Mann, 1998) suggested a recurrence interval of 900 to 1,000 

years for similar great earthquakes in this region.

The dominant geologic feature of the map area is the accretionary flysch and associated mélange of the Southern Margin 

Composite (SMC) terrane. These deep marine rocks record 

intermittent subduction-related offscraping and underplating 

that probably has occurred since Late Triassic time (Plafker and 

others, 1994). Defined as a composite terrane, the SMC includes 

the earlier defined Chugach and Prince William terranes as 

well as the Ghost Rocks Formation of the Kodiak Island region 

which lies between these two terranes. According to Plafker and 

others (1994), the Chugach terrane portion of the SMC terrane 

is composed of three assemblages: (1) Late Triassic to Early 

Jurassic greenschist and blueschist, represented for example 

by the Seldovia metamorphic complex; (2) late Mesozoic and 

older? mélange of the McHugh Complex; and (3) Late Cretaceous flysch of the Valdez Group. The Prince William terrane 

portion of the SMC terrane is equivalent to the Orca Group. 

Plafker and others (1994) assigned the Ghost Rocks Formation 

of the Kodiak Island area to an intervening assemblage of the 

SMC terrane not represented in the map area.

Two terrane names have been assigned to the rocks west 

of the SMC in the map area, the Peninsular terrane of Jones and 

Silberling (1979) and Jones and others (1981), or the Alaska 

Peninsula terrane of Wilson and others (1985; 1999). Over time, 

the usage of the term Peninsular terrane has morphed, such 

that the term incorporates some aspects of the definition of the 

Alaska Peninsula terrane (Nokleberg and others, 1994) although 

significant differences remain in the definitions. For example, 

the incorporation of the plutonic rocks of the Alaska-Aleutian 

Range batholith of Reed and Lanphere (1972) was a core part 

of the definition of the Alaska Peninsula terrane, as was the 

subdivision of the terrane into two related subterranes, named 

the Chignik and Iliamna subterranes. The batholith was and is 

not part of the definition of the Peninsular terrane, nor was the 

definition of the subterranes. Within the map area, both of the 

subterranes defined for the Alaska Peninsula terrane are present 

on the west side of Cook Inlet, separated by the Bruin Bay Fault 

system. The essentially batholithic Iliamna subterrane is faulted 

against the Chignik subterrane. Clasts in the sedimentary rocks 

of the Chignik subterrane become finer grained as distance from 

the fault increases, suggesting the fault was active during deposition (R.L. Detterman, oral commun., 1980).

The older, early Mesozoic, rocks on the east side of Cook 

Inlet at the extreme southwestern end of the Kenai Peninsula 

have traditionally been assigned to the Peninsular terrane. 

Aspects of these early Mesozoic rocks are similar to the oldest 

rocks of the Chignik subterrane of the Alaska Peninsula terrane; however, Wilson and others (1999) suggested that these 

Mesozoic rocks may actually represent remnants of another, 

mostly lost, terrane which they called the Hidden terrane. The 

informally named Port Graham and Pogibshi formations of 

Kelley (1980) have some similarities to rocks of the Alaska 

Peninsula terrane; however, the Port Graham formation is lithologically distinct from the equivalent-age Kamishak Formation, and the Pogibshi formation is older than the lithologically 

equivalent Talkeetna Formation. Rock units assigned by Wilson 

and others (1999) to the Hidden terrane, such as the Seldovia metamorphic complex of Bradley and others (1999) and 

similar metamorphic complexes (Raspberry Schist of Roeske 

and others (1989), and Schist of Iceberg Lake and Schist of 

Liberty Creek of Winkler and others (1981) in south-central 

Alaska, and the Triassic Afognak pluton of Roeske and others 

(1989) in the Kodiak Island region have a different history 

than rocks more typical of the Alaska Peninsula terrane and 

were interpreted by Roeske and others (1989) to be part of a 

Late Triassic to Early Jurassic primitive island arc. Each of the 

metamorphic complexes, all of which are associated with the 

Border Ranges fault system, show evidence of blueschist facies 

metamorphism, whereas rocks typical of the Alaska Peninsula 

terrane are rarely metamorphosed. No other Triassic age plutons are known for the Alaska Peninsula terrane, another reason 

to separate these rocks from the Alaska Peninsula terrane. 

A recent U-Pb zircon age of 204.8±2.8 Ma by D.C. Bradley 

(written commun., June 7, 2007) on the diorite of Point Bede 

in the Seldovia quadrangle yielded a Triassic age very similar 

to the Afognak pluton. Kelley (1980) describes this pluton as 

intruding the Pogibshi formation; as such, the age assigned to 

the Pogibshi formation on the basis of fossils differs from and 

is inconsistent with the radiometrically determined age of the 

intruding pluton.

In the western part of the map area the Alaska-Aleutian 

Range batholith of Reed and Lanphere (1973) forms the 

backbone of the Alaska-Aleutian Range. The volcanoes of the 

Aleutian magmatic arc, such as Redoubt and Iliamna Volcanoes 

(most of Iliamna Volcano is just west of the western edge of 

the map area) are emplaced on top of this backbone and young 

volcanic rocks represent only a small fraction of the bedrock 

even in the immediate vicinity of these volcanoes. East of the 

batholith along the Cook Inlet coast, bedrock consists of the 

Talkeetna Formation and overlying Jurassic sedimentary rocks 

of the Tuxedni Group and other sedimentary rock units as 

young as Tertiary (Detterman and Hartsock, 1966; Detterman 

and Reed, 1980; Nelson and others, 1983).

On the west side of Cook Inlet, Late Cretaceous through 

Triassic age rocks are products of the development and erosion of a magmatic arc. Triassic siliceous carbonates of the 

Kamishak Formation represent part of the basement of the 

magmatic arc, rocks of the Talkeetna Formation are the volcanic portion of the arc, the Alaska-Aleutian Range batholith is 

the plutonic core, and the sedimentary rocks of Tuxedni Group 

and Chinitna and Naknek Formations are the products of the 

erosion of the arc. It is unclear whether these sedimentary rocks 

represent the fore-arc basin or backarc basin of the Jurassic arc. 

On the basis of Reed and others’ (1983) interpretation of the 

polarity of the arc, the sedimentary rocks would be part of the 

backarc.
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Structurally, the map area is cut by a number of major 

faults and postulated faults, in addition to many less significant 

faults mapped only locally. The Border Ranges Fault system is 

probably the most important of the major faults in the map area, 

separating the SMC terrane from older rocks. Plafker and others 

(1994) considered it the suture along which the Chugach terrane 

was juxtaposed against and beneath older rocks to the north; 

the fault system has subsequently been modified by strike-slip 

displacements. The Contact Fault is generally accepted as the 

boundary between the Chugach and Prince William terranes. 

Lithologically, the metasedimentary flysch units, the Valdez and 

Orca Groups, are very similar and the Contact Fault is a point 

on a continuum of decreasing metamorphic grade and deformation seaward in Prince William Sound. Another major fault 

system, the Bruin Bay Fault, is sub-parallel to the Cook Inlet 

coast east of the Alaska-Aleutian Range batholith. The Bruin 

Bay Fault separates the batholith from the sedimentary rocks 

derived from erosion of the batholith and what were the overlying volcanic rocks of the Talkeetna magmatic arc. The Bruin 

Bay Fault is a high-angle reverse fault and may also have leftlateral offset (Detterman and Hartsock, 1966). It was a growth 

fault during deposition of the Jurassic sedimentary sequence 

and has approximately 3 km of stratigraphic throw (Detterman and Hartsock, 1966). Finally, the Lake Clark Fault cutting 

through the northwest corner of the map area has demonstrated 

high-angle reverse motion (Detterman and others, 1976) and 

postulated right-lateral strike-slip motion ranging from 5±1 km 

(Plafker and others, 1975b) to 26 km in the past 34 to 39 m.y. 

(Haeussler and Saltus, 2005). Neither Detterman and others 

(1976) nor Plafker and others (1975b) saw evidence of Holocene movement.

Radiometric dating in the map area has been extensive 

and is still ongoing; however, the long history of geochronologic studies has resulted in some confusion. In particular, 

sample locations have not always been correctly reported and 

a number of reports of K-Ar age determinations do not reflect 

the change in accepted decay constants for potassium reported 

in Steiger and Jager (1977). As a result, we have assembled a 

table (table 1) of all available radiometric dates, using modern 

decay constants. We have also corrected sample locations, 

based on original data where available, and also ensured that 

the sample location falls within the map unit the sample is 

purported to date. The map unit and source reports are listed in 

the table.

DESCRIPTION OF MAP UNITS

[For sedimentary rock units, the stratigraphic position is reported. For igneous rock units, the age of the unit is reported. For metamorphic rock 

units: 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]

UNCONSOLIDATED DEPOSITS

SURFICIAL DEPOSITS

Qtf Modern tidal flat and estuarine deposits (Holocene)—Well-sorted, sub-tidal, stratified silt and 

some sand and local gravel deposited in shallow embayments

Qs Unconsolidated surficial deposits, undivided (Quaternary)—Mainly unsorted boulders, 

cobbles, gravel, sand, and silt produced, deposited, and reworked by action of wind, water, 

glaciers, and frost, including solifluction. Includes deposits of present streams, colluvial 

and alluvial fans; glaciofluvial, glaciolacustrine, and deltaic deposits; unsorted material of 

morainal deposits; glacial-lake silt, clay, and muskeg deposits; and locally interstratified 

beach gravel, sand, and clay (Tysdal and Case, 1979; Nelson and others, 1985; Winkler and 

Plafker, 1993). In the vicinity of the Drift River delta, this unit contains a significant proportion of volcanic debris derived from air fall, mudflows, and lahars from Redoubt Volcano 

and Iliamna Volcano (Till and others, 1993; Waythomas and Miller, 1999; F.H. Wilson, 

unpub. data). Includes artificial fill at the Drift River and West Foreland oil production facilities on the west side of Cook Inlet (Riehle and Emmel, 1980; Till and others, 1993). Locally 

subdivided into the following subunits:

Qat Alluvial and terrace deposits—Flood-plain alluvium and stream terrace deposits. Consists of 

poorly sorted to well-sorted silt, sand, pebbles, cobbles, and boulders. Also includes abandoned channel deposits in outwash above modern floodplains or underfit stream channels, 

inferred to be relict glacial melt water channels; may include some glacial deposits (Detterman and Hartsock, 1966; Riehle and Emmel, 1980; Till and others, 1993; Richter and others, 

2005; F.H. Wilson, unpub. data). Rare natural levee deposits on deltaic or fan delta deposits 

occur on west side of Cook Inlet (F.H. Wilson, unpub. data)

Qls Landslide and colluvial deposits—Unsorted, angular rock debris and mud in slumps, earthdebris flows, block glides, and debris avalanches, and poorly sorted to well-sorted silt to 

boulders in alluvial fans and cones, and unsorted rock talus (Riehle and Emmel, 1980; Waythomas and Miller, 1999; Richter and others, 2005; F.H. Wilson, unpub. data); in vicinity of 
4

Redoubt Volcano and Iliamna Volcano, may include some volcanic mudflow (lahar) deposits 

(Riehle and Emmel, 1980); in Cordova quadrangle includes large (more than 10 km2) and 

small landslide deposits caused by the 1964 Alaska earthquake (Winkler and Plafker, 1981; 

Post, 1967). On west side of Cook Inlet solifluction deposits thought to be poorly sorted 

sand, silt, and clay derived from local upslope bedrock sources are found associated with 

plutonic rocks of the Alaska-Aleutian Range batholith (F.H. Wilson, unpub. data)

Qsl Lacustrine, swamp, and fine silt deposits—Chiefly silt, clay, fine sand, peat, and other 

organic material generally of lacustrine origin (Riehle and Emmel, 1980; Richter and others, 

2005; F.H. Wilson, unpub. data)

Qb Beach deposits—Sand, gravel, and cobbles of present and former beaches, beach ridges, spits, 

and tidal flats (Tysdal and Case, 1979; Winkler and Plafker, 1993; Detterman and Hartsock, 1966; Riehle and Emmel, 1980; F.H. Wilson, unpub. data). Wave-deposited, coarsegrained material and driftwood along seacoast; includes fine-grained sediment in mud flats 

of Copper River delta; and uplifted marine terraces. Successive spit-building episodes of 

progressive gradation are preserved in beach sequences along coastal lowland between Cape 

Yakataga and Bering Glacier (Richter and others, 2005). Includes beaches uplifted by the 

1964 Alaska Earthquake; most notably stranded beaches on southeastern side of Montague 

Island (Tysdal and Case, 1979)

Qes Estuarine deposits—Fine-grained silt rich in organic material, mud, and peat (Winkler and 

Plafker, 1993). Includes salt-marsh deposits of Detterman and Hartsock (1966) and grades 

into tidal flat deposits (Riehle and Emmel, 1980; F.H. Wilson, unpub. data). Also includes 

marine terrace deposits on west side of Cook Inlet (F.H. Wilson, unpub. data)

Qd Eolian deposits—Dunes consisting of well-sorted, fine-grained sand and silt aligned in longitudinal ridges on Copper River delta (Winkler and Plafker, 1993). Also includes dunes on 

top of river bluffs, windblown flats, and river bars (Richter and others, 2005). Includes informally named Bremner sand dunes, a large barchan field, located at confluence of Bremner 

and Copper Rivers (Richter and others, 2005). Includes cliff-head dunes in northern Kenai 

quadrangle on Turnagain Arm (Karlstrom, 1964)

Qdl Deltaic deposits—Deltaic deposits, including outwash and alluvial deposits (F.H. Wilson, 

unpub. data)

Qgl Glaciolacustrine deposits—Largely ephemeral glacial lake and lake-related deposits. Wellsorted, well-stratified clay, silt, and fine sand deposited in ephemeral glacial and postglacial 

lakes. Thought to be typically covered by several feet of muck and peat (F.H. Wilson, unpub. 

data). Includes stratified and locally foreset-bedded sand and gravel below 125 m elevation 

on West Foreland and from Katchin Creek to Redoubt Point; deposits are probable emerged 

delta and glaciolacustrine deposits (Riehle and Emmel, 1980). Also includes active outwash 

fans (Riehle and Emmel, 1980). On Kenai Peninsula, includes hanging delta deposits in 

former proglacial lakes associated with the last glacial maximum (Karlstrom, 1964)

Qm Glacial deposits, undivided—Glacial drift consisting of sand, gravel, and boulders. Includes 

ground, recessional, end, medial, and lateral moraine deposits, as well as colluvium, talus, 

landslide debris, alluvium, and silt, locally (Riehle and Emmel, 1980; F.H. Wilson, unpub. 

data). Includes active and recently active rock glaciers consisting chiefly of rubble and 

coarse rock debris (F.H. Wilson, unpub. data) and fresh, poorly sorted debris on surfaces 

of glaciers (Detterman and Hartsock, 1966; Winkler and Plafker, 1993). In the vicinity of 

Mount Redoubt and Mount Iliamna, may include a significant component of ash and other 

air-fall debris from eruptions (F.H. Wilson, unpub. data). Locally subdivided into the following subunits:

Qag Drift of Neoglacial age (Holocene)—Neoglacial terminal, recessional, lateral, and ground 

moraine consisting of unsorted boulders, cobbles, gravel, and sand deposited during retreat 

of glaciers. Includes deposits of the Tunnel and Tustumena Stades (Detterman and Hartsock, 

1966); Tunnel Stade moraines are barren deposits as much as 1.6 km in front of present 

glaciers. Tustumena Stade deposits consist of partially dissected spruce- and brush-covered 

moraine 1.6 to 10 km in front of present glaciers (Detterman and Hartsock, 1966). On the 

Kenai Peninsula, includes unmodified moraine deposits which may be mantled by loess, 

generally 2 to 6 ft thick (Karlstrom, 1964). Locally includes kames and outwash proximal to 

present glaciers (Richter and others, 2005)

Qao Outwash of the Neoglacial age (Holocene)—Outwash deposits associated with modern and 

Holocene glaciers (Karlstrom, 1964; F.H. Wilson, unpub. data). Consist of poorly sorted to 
5

well-sorted silt, sand, and gravel in terraces and outwash fan plains (Detterman and Hartsock, 1966; Riehle and Emmel, 1980)

Qgn Glacial deposits of the Naptowne and Brooks Lake Glaciations (Pleistocene)—Terminal, 

recessional, lateral, medial, and ground moraine and other deposits. Moderately weathered 

along Cook Inlet shoreline; heavily spruce and brush covered (Detterman and Hartsock, 

1966; F.H. Wilson, unpub. data). Internal contacts on west side of Cook Inlet reflect subdivisions of the Brooks Lake Glaciation; the Iliuk, Newhalen, and Iliamna advances. On east 

side of Cook Inlet, includes prominent and little modified morainal deposits, mantled by 2 to 

6 ft of loess (Karlstrom, 1964)

Qgno Outwash and valley train deposits (Pleistocene)—Silt, sand, and gravel sloping away 

from Naptowne age moraines (unit Qgn) on Kenai Peninsula (Karlstrom, 1964) and Brooks 

Lake age moraines on west side of Cook Inlet (F.H. Wilson, unpub. data)

Qog Older glacial deposits (Pleistocene)—Includes prominent, but modified morainal deposits 

of the Knik and Eklutna Glaciations. Deposits widely exposed north of Homer on Kenai 

Peninsula (Karlstrom, 1964). On Kenai Peninsula, Knik and Eklutna age moraines are 

terraced below approximately 228 m (750 ft), reflecting deposition in proglacial lakes. 

Also includes pebble- and boulder-bearing diamicton observed at one upland location at 

West Foreland and in sea cliffs along West Foreland, and from Katchin Creek south to near 

Redoubt Point (Riehle and Emmel, 1980)

Qogo Outwash associated with older glacial deposits (Pleistocene)—Silt, sand, and gravel 

graded to moraines of Knik and Eklutna age on Kenai Peninsula (Karlstrom, 1964)

Qch Moraine of the Caribou Hills Glaciation (Pleistocene)—Highly modified morainal 

deposits in Caribou Hills and exposed as remnant deposits elsewhere on Kenai Peninsula 

(Karlstrom, 1964). Deposits are terraced below elevations of about 305 m (1,000 ft) and 

discontinuously mantled by proglacial lake sediment deposits (Karlstrom, 1964)

ROCK UNITS WEST OF THE BORDER RANGES FAULT SYSTEM

BEDDED ROCKS

Tkn Kenai Group, undivided (Pliocene to Oligocene)—Coal-bearing clastic unit consisting of, in 

descending order, the Sterling, Beluga, and Tyonek Formations, and Hemlock Conglomerate in the vicinity of Cook Inlet. According to Calderwood and Fackler (1972), unit is at 

least 8,000 m thick in subsurface of Cook Inlet. Calderwood and Fackler (1972) included 

West Foreland Formation within the Kenai Group; however, West Foreland Formation was 

separated as a distinct unit by Magoon and others (1976). Subdivided into:

Ts Sterling Formation (Pliocene and Miocene)—Weakly lithified massive sandstone, conglomeratic sandstone and interbedded claystone; includes interbedded lignitic coals typically less 

than 1 m thick in upper part of unit, but may be as much as 3 m thick in lower part of unit 

(Calderwood and Fackler, 1972). According to Flores and others (1997, cited in Bradley 

and others, 1999), sandstone grades upward from coarse grained to very fine grained in 

trough crossbedded sequences; siltstone is typically ripple-laminated and contains roots or 

burrows. Bradley and others (1999) indicated that only the lowest 700 m of the more than 

3,000-m-thick Sterling Formation is exposed at the surface, consisting of siltstone, mudstone, carbonaceous shale, lignite coal, and minor volcanic ash. Triplehorn and others (1977) 

and Turner and others (1980) reported a number of K-Ar and fission-track age determinations from ash partings within coal beds within unit (table 1). Dates range from 4.2±1.4 

to 8.9±1.0 Ma; some older reported ages were interpreted to result from contamination by 

detrital material (Turner and others, 1980)

Tb Beluga Formation (Miocene)—Nonmarine, interbedded, weakly lithified sandstone, siltstone, 

mudstone, carbonaceous shale, coal, and minor volcanic ash (Bradley and others, 1999). 

Calderwood and Fackler (1972) reported a distinctive feature of Beluga Formation is its lack 

of massive sandstone beds and massive coal seams that characterize underlying Tyonek Formation; however, lignitic to subbituminous coal seams can be as much as 4 m thick in upper 

part of Beluga Formation. Contact between Beluga and overlying Sterling Formation may be 

an unconformity (Calderwood and Fackler, 1972), but in any case it can be difficult to pinpoint contact (Calderwood and Fackler, 1972; Turner and others, 1980). Triplehorn and others 

(1977) and Turner and others (1980) reported a number of K-Ar and fission-track age determinations from ash partings within coal beds ranging from 7.2±1.3 to 12.9±5.1 Ma (table 1)
6

Tty Tyonek Formation (Miocene and Oligocene)—Nonmarine conglomerate and subordinate 

sandstone, siltstone, and coal within the map area as described by Bradley and others (1999). 

However, in general, the Tyonek Formation is identified by the presence of massive sandstone beds and lignitic to subbituminous coal beds as much as 9 m thick (Calderwood and 

Fackler, 1972). Contact with overlying Beluga Formation is believed to be a disconformity 

where sandstone beds and coal beds become markedly thinner (Calderwood and Fackler, 

1972). Plant fossils representing Seldovian Stage of early and late Miocene age as well as 

plant fossils representing Angoonian Stage of Oligocene age provide age control (Wolfe and 

Tanai, 1980)

Th Hemlock Conglomerate (Oligocene)—Sandstone, conglomerate, and siltstone assigned to 

unit, as defined by Calderwood and Fackler (1972), occurs only in vicinity of Harriet Point 

in Kenai quadrangle on west side of Cook Inlet. On the basis of description in Detterman 

and others (1976), unit inferred to consist of fluvial conglomeratic sandstone and conglomerate containing minor interbeds of siltstone, shale, and coal. Magoon and others (1976) 

mapped these rocks in a unit consisting of the combined Tyonek Formation and Hemlock 

Conglomerate, whereas Detterman and others (1976) assigned these rocks to the Hemlock 

Conglomerate. Hemlock Conglomerate is lithologically transitional with Tyonek Formation, 

leading to some confusion. Hemlock Conglomerate is best known from the subsurface from 

well logs and not from surface mapping. Plant fossils suggest an Oligocene age; Wolfe and 

Tanai (1980) suggested early Oligocene and Wolfe (cited in Detterman and others, 1996) 

suggested late Oligocene for rocks assigned to this unit south of the map area. Wolfe and 

Tanai (1980) suggested rocks at Harriet Point may be isochronous with beds considered 

typical of the Tyonek Formation

Twf West Foreland Formation (Eocene and Paleocene)—Exposed only on west side of Cook 

Inlet, unit consists of tan to light-yellow-brown cobble conglomerate interbedded with 

lesser sandstone, laminated siltstone, and silty shale (Detterman and Hartsock, 1966). Thin 

coal beds are interbedded with the siltstone and shale. Detterman and Hartsock (1966) also 

described a lenticular bed of “carbonaceous ashstone breccia” that may have been referred to 

as obsidian by Martin and Katz (1912). However, Detterman and Hartsock (1966) indicate 

that along with quartz and feldspar grains and probably glass shards, the lenticular bed also 

contains plant fragments and is clearly of sedimentary origin. Clasts in conglomerate are 

mainly rounded to sub-rounded quartz diorite, volcanic rock, argillite, sandstone, siltstone, 

quartzite, tuff, and coal fragments. Intrusive and volcanic rock fragments each make up 

about 35 percent of the conglomerate clasts. Medium- to coarse-grained arkosic sandstone 

forms the matrix of the conglomerate as well as forming distinct lenticular beds. Siltstone 

and shale interbedded with conglomerate is a very fine grained sub-arkosic equivalent of 

the sandstone. Originally mapped as Kenai Formation by Detterman and Hartsock (1966), 

Calderwood and Fackler (1972) raised unit to group status and subdivided it into formations. 

Rocks here were assigned to the West Foreland Formation. West Foreland was assigned 

an Oligocene age by Kirschner and Lyon (1973) and later reassigned an early Eocene and 

late Paleocene age by Magoon and others (1976). Dating of zircon from an interbedded tuff 

about 1 m thick from north of map area yielded 43 Ma age (middle Eocene) (P.J. Haeussler 

and D.W. Bradley, USGS, oral commun., 2005). West Foreland Formation was removed 

from the Kenai Group because it is separated from overlying Hemlock Conglomerate by a 

major unconformity (Magoon and Egbert, 1986). Detterman and Hartsock (1966) described 

lower contact of these rocks as an angular unconformity with the Upper Jurassic Naknek 

Formation; subsequent work by Magoon and others (1980) showed there is a nonmarine 

Upper Cretaceous sedimentary unit between the West Foreland and Naknek Formations. 

Those Cretaceous sedimentary rocks are shown here as unit Ksm

Ksm Saddle Mountain section of Magoon and others (1980) (Upper Cretaceous, Maastrichtian)—Nonmarine sandstone, conglomerate, and minor siltstone and coal northeast of 

Chinitna Bay in an 83-m-thick section (Magoon and others, 1980). Consists dominantly of 

fine- to medium-grained sandstone that becomes finer grained upward. Unit is generally 

massive though some crossbedded sections are present. Sandstone is soft and friable except 

where calcite cemented. Conglomerate contains volcanic and plutonic rock boulders as 

much as 30 cm in diameter in a sandy matrix. Coal beds, which tend to occur in upper part 

of the section, are as much as 2.7 m thick and locally have underclay (“undersoils”, Magoon 

and others, 1980). Siltstone is most abundant in the middle part of the section. Sporomorphs 
7

Cranwellia striata (Couper) Srivastava, Balmeisporites spp., Wodehouseia spinata Stanley, 

Proteacidites spp., Aquilapollenites bertillonites Funkhouser, A. reticulatus Mtchedlishvili, A delicatus Stanley (Magoon and others, 1980) indicate a Maastrichtian age. Unit was 

assigned to the Kaguyak Formation by Bradley and others (1999), but is mapped separately here because lithology and depositional environment are distinctly different from the 

Kaguyak Formation. Unit overlies Jurassic Naknek Formation with angular unconformity 

and is in turn overlain by rocks of the West Foreland Formation with angular unconformity

Naknek Formation (Upper Jurassic; Tithonian to Oxfordian)—Originally named Naknek 

Series by Spurr (1900, p. 169–171, 179, 181) for exposures at Naknek Lake on the Alaska 

Peninsula. Largely consists of sandstone, conglomerate, and siltstone having a primarily 

plutonic provenance. Unit is widespread in southern Alaska, exposed in a long belt from 

south-central Alaska (Wilson and others, 1998) to southwestern end of the Alaska Peninsula 

(Wilson and others, 1999), a distance of about 1,150 km (Detterman and others, 1996). The 

aggregate thickness of the individual members exceeds 3,000 m, though average thickness is 

more typically 1,700 to 2,000 m (Detterman and others, 1996). Megafossils, particularly the 

pelecypod Buchia (Detterman and Reed, 1980, p. B38; J.W. Miller, written commun., 1982 

to 1988), are common, and fauna, which also includes ammonites, indicate an age range of 

Oxfordian to late Tithonian (Late Jurassic). Detterman and others (1996; see also, Detterman and Hartsock, 1966; Martin and Katz, 1912) have subdivided Naknek Formation into 

members of which the following appear in the map area:

Jnp Pomeroy Arkose Member (Kimmeridgian and Oxfordian?)—Massive, light-gray, mediumto coarse-grained arkose containing many interbedded thin beds of dark-gray to brownish 

siltstone and pebble conglomerate (Detterman and Hartsock, 1966). The sandstone is rich in 

quartz (40–45 percent), sodic feldspar (30–35 percent), and contains 15–20 percent hornblende and tourmaline. Volcanic lithic fragments make up 2–3 percent of unit. Grains are 

sub-angular to sub-rounded. The matrix is generally clay, but locally, is tuffaceous. Siltstone 

is mineralogically distinctive from the arkose and resembles the graywacke of older units 

according to Detterman and Hartsock (1966); we interpret this to indicate the siltstone contains a higher proportion of volcanic- and sedimentary-sourced components. Detterman and 

Hartsock (1966) indicated that most sections of the Pomeroy Arkose Member have a 70- to 

350-ft-thick (21 to 113 m) gray, medium-bedded to massive, arenaceous siltstone horizon, 

usually in the lower part of the member. The Pomeroy Arkose is sparsely fossiliferous containing Lytoceras, Phylloceras, and Buchia concentrica, suggesting an age no younger than 

early Kimmeridgian. Detterman and Hartsock (1966) suggested that the nearby Jurassic part 

of the Alaska-Aleutian Range batholith was the source for unit and the subangular character 

of the easy-to-destroy grains (hornblende, tourmaline, and feldspar) indicate short transport 

and rapid burial

Jnst Snug Harbor Siltstone Member (Kimmeridgian and Oxfordian)—Dominantly massive 

to thin-bedded, dark-gray to black siltstone; minor calcareous gray sandstone beds (Detterman and Hartsock, 1966). Hard gray limestone concretions and lenses are locally abundant; 

rare thin layers of volcanic ash and tuff are found locally (Detterman and Hartsock, 1966). 

Deposited in moderately deep water, well below wave base and above carbonate compensation depth, in a basin having restricted circulation (Detterman and others, 1996). Unit is the 

lowest abundantly fossiliferous member of the Naknek; most common fossils present are of 

the genus Buchia, including Buchia concentrica, as well as the ammonites Amoeboceras, 

Phylloceras, and Perisphinctes

Jnn Northeast Creek Sandstone Member (Oxfordian)—Light-gray, thin-bedded to massive arkosic sandstone, graywacke, and siltstone. Originally called the “lower sandstone 

member” by Detterman and Hartsock (1966) who considered unit to have only local significance. Later work on the Alaska Peninsula (Detterman and others, 1996) showed that the 

lateral equivalent of this unit, which they named the Northeast Creek Sandstone Member, 

is present along the entire length of the Alaska Peninsula. According to Detterman and 

Hartsock (1966), some beds have a tuffaceous matrix, zones of small pebbles, and thin beds 

of arenaceous siltstone. Fossils are most common in lower part of unit and include ammonites, particularly Cardioceras, but also Phylloceras and Lytoceras. Detterman and Hartsock 

(1966) assigned a latest Callovian and early Oxfordian age to unit based Cardioceras martini

in lower part of the member and its association with Cardioceras distans, which then without Cardioceras martini continues to top of member. Detterman and others (1996) restricted 
8

unit to Oxfordian based on reinterpretation of available information. Pelecypods, including 

Pleuromya, Quenstedtia, Oxytoma, Thracia, and Astarte, and gastropods, echinoids, and 

belemnites are present but not common (Detterman and Hartsock, 1966). Lower contact 

intertongues with Chisik Conglomerate Member; upper contact is gradational with overlying 

Snug Harbor Siltstone Member

Jnc Chisik Conglomerate Member—Massive to thick-bedded conglomerate and interbedded, 

crossbedded, quartzose sandstone. Clasts, as large as 2 m, are mainly granitic rocks, but up 

to 20 percent are metamorphic and volcanic rocks (Detterman and others, 1996). In map 

area is mainly restricted to area adjacent to Tuxedni Bay. A K-Ar age determination (table 1, 

sample 62Ale 6e) on a quartz diorite cobble yielded a protolith age of 156.6 Ma on biotite 

and 159.7 Ma on hornblende (Detterman and others, 1965)

Chinitna Formation (Middle Jurassic, Callovian)—Massive gray arenaceous siltstone. Unit is 

best exposed along west coast of Cook Inlet where it is subdivided into two members; upper 

Paveloff Siltstone Member and lower Tonnie Siltstone Member (Detterman and Hartsock, 

1966). Unit is a partial age equivalent of the Shelikof Formation of the Alaska Peninsula 

(Detterman and others, 1996). Paveloff Siltstone and Tonnie Siltstone Members described 

below:

Jcp Paveloff Siltstone Member—Massive dark-gray arenaceous siltstone in upper part, thick 

sandstone unit at base (Detterman and Hartsock, 1966). Large ellipsoidal concretions and 

lenticular beds of limestone occur throughout and thin interbeds of sandstone occur in the 

siltstone. A few siltstone beds contain abundant finely disseminated pyrite, causing beds 

to weather rusty brown. Siltstone is well-indurated and uppermost part is thin-bedded and 

fractures into angular fragments. Graywacke sandstone of lower unit is “* * * thin bedded to 

massive, locally lenticularly bedded, fine to coarse grained, gray to greenish gray” (Detterman and Hartsock, 1966, p. 43). Limestone concretions and interbeds are common; very 

dark-gray on fresh surfaces, but weather buff to cream colored. Locally the limestone is 

bioclastic (Detterman and Hartsock, 1966). Many non-diagnostic pelecypods and gastropods have been collected from lower sandstone (Detterman and Hartsock, 1966), whereas 

a wide variety of ammonites have been collected from siltstone and limestone concretions 

higher in the section. Many genera of ammonites, including Cadoceras, Stenocadoceras, 

Pseudocadoceras, Kepplerites, Kheraiceras, and Lilloettia, have been collected (Detterman 

and Hartsock, 1966), indicating a Callovian age, although uppermost zone of the Callovian 

has not been identified. Paveloff Siltstone Member is the age equivalent of Shelikof Formation of the Alaska Peninsula; Shelikof contains a higher proportion of coarse volcanic debris 

(Detterman and others, 1996)

Jct Tonnie Siltstone Member—Massive dark-gray to brownish-gray arenaceous siltstone; 

weathers brownish-gray to red-brown (Detterman and Hartsock, 1966). Numerous small 

yellowish-brown weathering limestone concretions occur in parallel bands and randomly 

throughout section. Limestone concretions are generally ovoid, as much as 12–13 cm in 

diameter, extremely hard, and commonly fossiliferous (Detterman and Hartsock, 1966). 

Thin, fine-grained, greenish-gray sandstone interbeds occur in the siltstone; a more massive sandstone unit is found at the base of section. Sandstone interbeds are compositionally 

similar, but coarser grained than the siltstone. A thick (6–30 m) sandstone bed at base of the 

unit is medium bedded to massive, fine to medium grained, and grayish brown. On Chisik 

Island, a thick (65 m) channel conglomerate, consisting mainly of volcanic rock cobbles 

and boulders, is at base of the section. “The Chisik Island section also contains numerous thin beds of volcanic ash (Detterman and Hartsock, 1966, p. 41).” Unit is abundantly 

fossiliferous, yielding many mollusks, particularly ammonites, including Paracadocreas, 

Pseudocadoceras, Phylloceras, Lilloettia, Kheraiceras, Kepplerites, and Xenocephilites. 

Numerous pelecypods and rare belemnites, gastropods and brachiopods, with the ammonites 

indicate an early Callovian age (Detterman and Hartsock, 1966)

Tuxedni Group (Middle Jurassic, Bathonian to Aalenian)—Light- to dark-gray and green 

marine graywacke, conglomerate, siltstone, and shale (Detterman and Hartsock, 1966). 

Graywacke ranges from feldspathic to lithic to laumontitic; conglomerate composed mainly 

of volcanic clasts in graywacke matrix. Unit is locally subdivided into (from oldest to 

youngest): Red Glacier Formation, Gaikema Sandstone, Fitz Creek Siltstone, Cynthia Falls 

Sandstone, Twist Creek Siltstone, and Bowser Formation. Descriptions below are derived 

from Detterman and Hartsock (1966); age revised to Bathonian to Bajocian by Imlay and 
9

Detterman (1973) and to lower Bathonian to Aalenian by Detterman and Westermann 

(1992)

Jtb Bowser Formation (Bathonian)—Heterogeneous assemblage of sandstone, conglomerate, 

shale and siltstone characterized by rapid facies changes (Detterman and Hartsock, 1966). 

Massive light- to dark-gray sandstone and conglomerate are dominant lithologic types 

on Iniskin Peninsula. The sandstone and the conglomerate matrix are coarse grained and 

composed of angular fragments of feldspar and quartz, with biotite, augite, and magnetite 

as common accessory minerals. Light-gray sandstone is commonly calcareous and contains 

numerous coquina beds composed almost entirely of pelecypods Retroceramus (formerly 

most Middle Jurassic Alaska inoceramids were referred to the genus Inoceramus) and 

Trigonia and is also interbedded with dark-gray sandstone. Clasts in the conglomerate are 

dominantly felsic volcanic rocks and basalt, but include about 10 percent granitic rocks. 

Massive to thin-bedded, medium- to coarse-grained, dark-brownish gray siltstone beds 

weather to light brown. Siltstone beds as much as 100 m thick are interbedded with the conglomerate and sandstone; north of Chinitna Bay siltstone beds as much as 250 m thick form 

the dominant lithology of the Bowser Formation. Lenticular limestone concretions containing ammonites are common north of Chinitna Bay. Overall thickness ranges from 380 to 560 

m. Abundantly fossiliferous, containing ammonites and pelecypods. Most recently dated as 

late Bajocian to early Bathonian by Detterman and Westermann (1992)

Jtt Twist Creek Siltstone (Bajocian)—Soft, poorly consolidated, thin-bedded to massive siltstone 

and silty shale as much 125 m thick (Detterman and Hartsock, 1966). Dark-gray siltstone 

weathers to dark rusty brown, and contains many thin beds of volcanic ash that weather 

bright orange. Small, commonly fossiliferous, limestone concretions common throughout. 

Abundant ammonite fauna include Oppelia (Liroxyites), Megasphaerceras, Leptosphinctes, 

Lissoceras, and Normannites (Dettermanites), but is restricted to limestone concretions

Jtc Cynthia Falls Sandstone (Bajocian?)—Massive to thick-bedded graywacke sandstone and 

pebble conglomerate about 200 m thick (Detterman and Hartsock, 1966). Medium- to 

coarse-grained, greenish-gray to dark-green sandstone; graded bedding; weathers mottled 

light-gray due to presence of zeolites. Sandstone consists mainly of angular fragments of 

feldspar and volcanic rocks in a compositionally similar silt-size matrix. Pebble conglomerate, in thin lenticular beds within the sandstone, is well sorted within individual beds. 

Clasts consist of “red and green felsitic volcanic rocks, aphanitic igneous rocks, and a few 

metasedimentary rocks that are primarily dark-gray quartzite” (Detterman and Hartsock, 

1966, p. 32). Coarse-grained siltstone is interbedded with the sandstone and may contain a 

few limestone concretions. Siltstone makes up 10 to 20 percent of the formation. Like the 

underlying Fitz Creek Siltstone and Gaikema Sandstone, unit is coarsest grained in vicinity of Gaikema Creek and finer grained away from creek area. Fossils relatively uncommon 

in unit, thought in part due to rapid deposition in a nearshore environment. Fauna includes 

ammonites Chondroceras and Stephanoceras, as well as the pelecypods Retroceramus (formerly Inoceramus) sp. and Mytilus sp.

Jtf Fitz Creek Siltstone (middle Bajocian)—Massive, bluish dark-gray, arenaceous, coarse- to 

fine-grained siltstone up to 400 m thick; commonly weathers rusty orange and contains 

many small limestone concretions (Detterman and Hartsock, 1966). Fine-grained sandstone 

and, locally, conglomerate is interbedded. In upper part of unit, siltstone could possibly be 

called silty shale. Unit is coarsest in vicinity of Gaikema Creek and rapidly becomes finer 

grained in all directions away from Gaikema Creek. Unit is abundantly fossiliferous and is 

lowest unit of Tuxedni Group where ammonites are more numerous than pelecypods. A few 

non-diagnostic brachiopods are also present. Ammonites include Normannites, Teloceras, 

and Chondroceras and many other genera; pelecypods include Retroceramus (formerly 

Inoceramus) and Pleuromya, both in forms distinctly different than those found in lower 

parts of Tuxedni Group

Jtg Gaikema Sandstone (lower middle Bajocian)—Resistant, cliff-forming, massive to thinbedded graywacke sandstone and cobble conglomerate; 150 to 260 m thick (Detterman 

and Hartsock, 1966). Sandstone commonly occurs in graded beds up to 3 m thick, whereas 

conglomerate is well sorted within individual beds and rarely graded (Detterman and 

Hartsock, 1966). Conglomerate is confined to Iniskin Peninsula; clasts consist of “red and 

green felsitic volcanic rocks, aphanitic igneous rocks, and minor metasedimentary rocks” 

(Detterman and Hartsock, 1966, p. 26), all thought to be derived from the Talkeetna For-
10

mation. Rare granitic clasts are the first appearance of rocks presumably derived from the 

Alaska-Aleutian Range batholith. Siltstone, occurring mainly as thin interbeds in sandstone, 

is thin bedded to massive, generally coarse silt to sandy, gray to olive gray, and weathers 

brownish to rusty brown. In general, siltstone constitutes less than 10 percent of unit, though 

locally, it can constitute as much as 40 percent. Siltstone apparently does not occur in close 

proximity to conglomeratic parts of the formation (Detterman and Hartsock, 1966). Unit is 

fossiliferous throughout, containing pelecypods Meleagrinella, Trigonia, and Retroceramus

(formerly Inoceramus) and ammonites Witchellia(?), Stephanceras and locally, Sonninia

(Papilliceras), Lissoceras, and Emileia. Carbonized plant remains are locally present

Jtrg Red Glacier Formation (lower middle Bajocian to lower Bajocian)—Thin-bedded to massive, dark-gray to moderate olive-gray, highly arenaceous siltstone, weathers red brown and 

locally contains lenticular interbeds and concretions of reddish-gray, dense limestone and 

very minor coal seams (Detterman and Hartsock, 1966). Underlying siltstone is light-tan to 

buff arkosic sandstone and thick, black, silty to arenaceous, very fissile shale. Siltstone constitutes about 40 percent of unit, concentrated in upper part; sandstone about 25 percent, and 

shale the remainder. Overall thickness ranges from 600 to as much as 2,000 m. Fossils most 

abundant in upper part of unit; no fossils known from lowermost part (as much as 600 m) 

of unit. Pelecypods include Meleagrinella, Trigonia, Retroceramus (formerly Inoceramus), 

Camptonectes, and Pleuromya. Ammonites occur in two distinct faunal assemblages; lower 

assemblage faunal zone includes Erycites, Tmetoceras, and Pseudolioceras and ranges from 

450 to 1,400 m below the top of formation. Upper assemblage in upper 400 m of formation 

includes Soninia, Emileia, Parabigottes and in uppermost 150 m, Papilliceras, Strigoceras, 

Lissoceras, Stephanoceras, Stemmatoceras, and Skirroceras

Jtk Talkeetna Formation, undivided (Lower Jurassic)—Bedded volcanic rocks widely distributed 

in Seldovia and Kenai quadrangles west of Cook Inlet. Where undivided, unit consists of 

flows, breccia, tuff, and agglomerate and locally interbedded minor sandstone and shale, all 

typically somewhat altered or metamorphosed (Detterman and Hartsock, 1966; Detterman 

and Reed, 1980). Detterman and Hartsock (1966) formally divided unit into three members. Within map area, Talkeetna Formation is locally subdivided, from top to bottom, into 

the Horn Mountain Tuff, Portage Creek Agglomerate, and Marsh Creek Breccia Member, 

described below:

Jtkh Horn Mountain Tuff Member—Bedded tuff and tuffaceous feldspathic sandstone, locally 

containing porphyritic andesite flows. Tan, red, green, purple, or mottled-color, thin-bedded to massive and fine- to coarse-grained bedded tuff (Detterman and Hartsock, 1966). 

Locally, tree stumps preserved within tuff beds, indicating subaerial deposition. However, 

thin-bedded laminated units that have graded bedding and contain rare belemnite fragments 

indicating some parts of unit are marine. Measured thickness is as much as 870 m (Detterman and Hartsock, 1966). Above mentioned belemnites and plant fragments occur near the 

top of unit; fossils are not age diagnostic. In the Talkeetna Mountains north of the map area, 

fossils in upper part of Talkeetna Formation, which is considered correlative to rocks of the 

Horn Mountain Tuff Member, indicate a late Pliensbachian and Toarcian (Early Jurassic) age 

(Arthur Grantz, oral commun., 1963, cited in Detterman and Hartsock, 1966)

Jtkp Portage Creek Agglomerate Member—Reddish fragmental volcanic debris, primarily 

rounded volcanic bomb-like fragments and lapilli tuff; grading to fine-grained tuff, clastic 

sedimentary rocks and flows northward in map area (Detterman and Hartsock, 1966). Interbedded flows, tuff, and sedimentary rocks are thicker than in the underlying Marsh Creek 

Breccia Member, which suggested to Detterman and Hartsock (1966) a decrease in violent 

volcanism in the source area. These rocks are generally more felsic, although commonly 

described as andesitic (Detterman and Hartsock, 1966), than those of the Marsh Creek 

Breccia Member and their distribution suggests a separate source (Detterman and Hartsock, 

1966). Estimated thickness is between 685 and 870 m. No known fossil control

Jtkm Marsh Creek Breccia Member—Massive dark-green to green volcanic breccia having a tuff 

matrix (Detterman and Hartsock, 1966). Consists of angular fragments of aphanitic pink and 

green volcanic rocks ranging in size from 1 cm to nearly 1 m and in general, fine upward. 

Interbedded flows of andesite and basalt, thought to be partly submarine, are common and 

increase in abundance and thickness southward (Detterman and Hartsock, 1966; Detterman and Reed, 1980). Bedded tuff is locally important, thickest in southern exposures, and 

coarser in the more northern exposures. Unit has an estimated minimum overall thickness of 
11

1,000 m, although no complete section has been measured and sections that have been measured are cut by extensive faulting. Locally, unit has obscure contact relation with plutons 

of the Alaska-Aleutian Range batholith or has been assimilated into the Holocene magma 

chamber under Iliamna Volcano (Detterman and Hartsock, 1966). No fossils are known from 

unit in map area and age is inferred based on correlations with rocks in the Talkeetna Mountains type area north of map area

Jp Pogibshi formation of Kelley (1980), undivided (Lower Jurassic, Sinemurian to Hettangian 

and older?)—Informally named Pogibshi formation of Kelley (1980); exposed on east side 

of Cook Inlet. Consists of volcaniclastic rocks interbedded with small amounts of limestone, 

coal, and tuffaceous argillite. Kelley (1980) divided unit into three members on the basis of 

rock type, modal composition, and depositional texture. Stratigraphically lowest member, 

the Dangerous member, consists of volcaniclastic breccia, conglomerate, and sandstone 

and is in depositional contact with Kelley’s Port Graham formation (^pg herein). Locally 

tuffaceous dark-gray sedimentary rocks in Dangerous member make it hard to distinguish 

from Port Graham formation. July member consists of dacitic pyroclastic rocks, tuffaceous 

sandstone, granule conglomerate, and mudstone. Kelley (1980) indicated that the high quartz 

content and abundance of glassy debris help to distinguish this member from other members 

of his Pogibshi formation. The uppermost Naskowhak member, consists of greenish-gray 

tuffaceous mudstone and siltstone, and tuff. Locally, the basal part of Naskowhak member 

includes laterally extensive coal-bearing units. The coal-bearing units help distinguish the 

Pogibshi formation of Kelley (1980) from the otherwise lithologically similar Talkeetna Formation on the west side of Cook Inlet. The Pogibshi formation of Kelley (1980) is reportedly 

intruded by tonalite of Dogfish Bay of Bradley and others (1999) (^dt) and possibly by the 

diorite of Point Bedeof Bradley and others (1999) (^dt); if so the recently determined Triassic age on the diorite (D.C. Bradley, oral commun., 2007) may indicate that the Pogibshi 

is, in part, significantly older than the Talkeetna Formation. Martin (1915, 1926) reported 

a diverse Lower Jurassic fauna along the coast southwest of Seldovia. Bradley and others 

(1999) erroneously attributed these Jurassic fossil collections by Martin (1915) to the Port 

Graham formation of Kelley (1980); however, those collections were from localities within 

the outcrop area Bradley and others (1999) assigned to the Talkeetna Formation, which 

we reassign back to the Pogibshi formation of Kelley (1980). Fossils noted in two Martin 

reports (1915, 1926) include several species of scleractinian corals, numerous bivalves 

(mostly pectinaceans), gastropods, and ammonites. A brief visit by R.B. Blodgett (written 

commun., 2007) to an early Sinemurian age section exposed about 3 km west of Seldovia 

showed unit contains numerous pectinacean bivalves, gastropods, and several species of 

scleractinian corals. The bivalves were found both as articulated and disarticulated specimens, and most appear to belong to the genus Weyla, a Lower Jurassic index fossil found 

primarily along the western coast of North and South America. Lower Jurassic ammonites 

from these same rocks were discussed and, in part, illustrated in Imlay (1981), who recognized both Sinemurian and Hettangian age fossil assemblages. A fossil determination by 

A.K. Armstrong of a collection made by J.S. Kelley yielded poorly preserved Permian corals 

(sample 75JK-151B, www.alaskafossil.org). Due to the poor state of preservation of the 

coral material, we tentatively discount this collection until further material can be collected 

and identified from this locality. Connelly (1978) and Connelly and Moore (1979) suggested 

correlation of these rocks with the Upper Triassic Shuyak Formation of the Afognak Island, 

which is intruded by the Triassic age Afognak pluton (see also Wilson and others, 2005)

^pg Port Graham formation of Kelley (1980) (Upper Triassic, Norian)—Dominantly dark-gray, 

carbonaceous limestone and silty limestone containing varying amounts of silica cement 

(Kelley, 1980). Fine-grained, dark-gray siliceous to limy mudstone, silty sandstone, and 

dark-gray to dark-olive-gray, thin- to medium-bedded chert having mudstone partings are 

also common lithologies according to Kelley (1980). Limy beds tend to be most common 

in lower (middle Norian) part of unit (R.B. Blodgett, written commun., 2007, and unpublished data of Humble Oil Company [now Exxon-Mobil] reported by R.B. Blodgett); 

whereas upper part (of late Norian age) is composed of considerably more volcaniclastic 

fragment-rich beds and shaly beds. The volcaniclastic fragment-rich beds contain a diverse, 

but uncommon molluscan fauna consisting of both bivalves and gastropods, while the 

shaly beds tend to have a monotaxic fauna of monotid bivalves. Fossils are locally abundant, as reported by Kelley (1980), and are mostly thin shelled mollusks, but also include 
12

corals, echinoids, ammonites, and trace fossils. Martin and others (1915) and Martin (1926) 

reported bivalves Halobia cf. H. superba Mojsisovics, Pseudomonotis subcircularis Gabb 

(this species is now referred in the genus Monotis), Nucula?, and the coral Astrocoenia? 

sp. The Halobia cf. H. superba was suggested by Martin to possibly indicate a Carnian age 

(this species was later re-identified as belonging to middle Norian age species of Halobia by 

Silberling and others, 1997), while Monotis subcircularis indicates a late Norian age. Silberling and others (1997) provided a detailed analysis of known Late Triassic bivalve fauna 

known from the Port Graham area and reported middle Norian age Halobia lineata and H. 

dilitata were found in collections reported by Martin (1915; USGS Mesozoic localities 6380 

and 6382, respectively). Silberling and others (1997) reported two different species of Late 

Triassic Monotis: Monotis (Pacimonotis) subcircularis and Monotis (Monotis) alaskana, 

as well as the late middle Norian ammonite Steinmannites. Kelley (1984) and Bradley and 

others (1999) assigned an upper age limit of Early Jurassic, although no Early Jurassic fossils have been found in unit: Early Jurassic fossils do occur in upper part of the overlying 

Pogibshi formation of Kelley (1980)

METAMORPHIC ROCKS

JPk Kakhonak Complex (Lower Jurassic to Permian? or older?)—The Kakhonak Complex, 

defined by Detterman and Reed (1980), is a lithologically diverse and complex assemblage 

of metamorphosed mafic plutonic, volcanic, and sedimentary rocks found on west side of 

Cook Inlet. Detterman and Hartsock (1966) mapped “metalimestone”, argillite, quartzite, 

metatuff, greenstone, and phyllite. Detterman and Reed (1980) described unit as largely 

consisting of roof pendants within the Alaska-Aleutian Range batholith and believed that the 

Kakhonak Complex represents, in part, the metamorphic equivalent of Upper Triassic and 

Lower Jurassic rocks, the Kamishak and Talkeetna Formations, of the vicinity. However, 

quartzite and quartz-mica schist within the Kakhonak Complex have no direct equivalent 

within the sedimentary rocks of the vicinity, indicating other protoliths may have contributed 

to the complex. As Permian rocks were known from Puale Bay, about 300 km south of the 

map area, a possible Paleozoic age was not excluded by Detterman and Reed (1980). Internal contacts are typically faults, resulting in a tectonic mix of lithologies. Although most of 

the rocks of this complex are at greenschist facies, the rocks range from nonmetamorphosed 

to granulite facies

IGNEOUS ROCKS

Qv Volcanic rocks, undivided (Quaternary)—Andesite, dacite, and basalt lava flows, volcanic 

breccia, lahar deposits, and debris-flow deposits. Includes air-fall tuff, volcanic dome 

deposits, block- and ash-flow deposits, ash-flow tuffs, volcanic-rubble flows, debris flows, 

and hot-blast avalanche deposits. Also includes tephra-rich colluvium in vicinity of and 

west of Redoubt Volcano where deposits both mantle and are incorporated into deposits 

of Holocene glaciation (Till and others, 1993; F.H. Wilson, unpub. data). Lava flows and 

clasts are porphyritic, typically glassy, gray to black, and commonly vesicular. Andesite is 

dominant composition and probably constitutes 60 percent or more of rocks. Unit typically 

forms volcanic edifices. Unit mapped at Redoubt and Iliamna Volcanoes includes Holocene 

rocks that cap ridges and include massive lava flows, agglomerate, and lahar deposits. May 

include Tertiary volcanic rocks, as mapped by Magoon and others (1976). Locally subdivided into:

Qdf Debris-flow deposits (Holocene)—Volcanic debris-flow and mudflow deposits from Redoubt 

Volcano in Crescent River valley. Includes several small (older?) deposits in the upper valley 

and a 3,500-yr-old debris flow in the lower valley (F.H. Wilson, unpub. data). The older 

deposits are from multiple debris flows off west and southwest flanks of Redoubt Volcano. 

Oldest(?) deposit is derived from west flank of Redoubt. Next oldest deposit appears to be 

derived from presently glaciated valley draining to the southwest off Redoubt. These older 

deposits are of relatively limited extent, but may have dammed the North Fork, Crescent 

River, creating a temporary lake in the valley. Deposits of the youngest, most extensive, and 

probably most fluid debris flow derived from southwest flank of Redoubt Volcano. Flow was 

apparently derived from the only presently glacier-free valley draining south from Redoubt. 

This debris flow was dated at 3,500 yr B.P. (Riehle and others, 1981), young enough to 
13

possibly explain the lack of glaciers in the source valley, as all surrounding valleys have 

extensive glaciers and glacial deposits. This debris-flow deposit extends to the coast of Cook 

Inlet and up the main fork of the Crescent River, creating a dam that contains Crescent Lake. 

Also includes Holocene age mudflow deposits in the Drift River valley draining north and 

east from Redoubt Volcano (Till and others, 1993); some Drift River deposits are historic 

including 1966 and later flows

Qad Andesite and dacite domes (Pleistocene)—Medium to coarsely porphyritic hornblende 

andesite and dacite of composite dome complex of Double Glacier Volcano (Reed and 

others, 1992). Three K-Ar andesite whole-rock ages were determined on these domes; two 

are considered minimum ages at 627±24 and 763±17 ka (sample 78AR 290, table 1) and one 

is 887±15 ka (sample 90AR 99, table 1) (Reed and others, 1992)

Tvu Lava flows (middle? to late? Tertiary)—A small exposure of dark-blue-gray to black, cryptocrystalline to vesicular basalt and andesite occurs on mainland northwest of Chisik Island 

(Detterman and Hartsock, 1966) and is one of a number of scattered exposures in their 

map area. As described, the flows are slightly altered and locally contain irregular veins of 

hematite and malachite in altered zones along shear planes. Flows unconformably overlie 

an exposure of the Talkeetna Formation. Detterman and Hartsock (1966) inferred the flows 

were of middle(?) or late(?) Tertiary age based on degree of dissection and correlation with 

flows in Iliamna Lake region where flows overlie Tertiary age sedimentary rocks

TKgd Granodiorite (Tertiary and (or) Cretaceous)—Medium-grained, hypidiomorphic granular, seriate granodiorite. Biotite tends to be more abundant than hornblende. Occurs in northwest 

corner of the Kenai quadrangle west of Lake Clark Fault and in adjacent Lake Clark quadrangle (Reed and Lanphere, 1972, 1973; Magoon and others, 1976; Detterman and others, 

1976). K-Ar ages on biotite and hornblende range from 65.1±1.9 to 70.5±2.0 Ma (Reed and 

Lanphere, 1972, 1973; Magoon and others, 1976; Detterman and others, 1976)

Kqd Quartz diorite (Cretaceous)—Locally foliated, largely medium-grained hornblende-biotite 

quartz diorite, but includes hornblende-pyroxene gabbro and diorite, quartz diorite, tonalite, and minor granodiorite (Magoon and others, 1976; Nelson and others, 1983; Reed and 

others, 1992). In northwestern part of map area, east of Lake Clark Fault, these rocks were 

mapped as Tertiary or Cretaceous by Magoon and others (1976). Biotite-hornblende ratio 

is quite variable although color index remains constant. Deuteric epidote is common. K-Ar 

ages on biotite and hornblende range from 67.2±1.9 to 74.4±2.2 Ma (Reed and Lanphere, 

1972, table 1 herein); shown as Cretaceous here

KJg Quartz monzodiorite (Cretaceous? and (or) Jurassic?)—Medium-gray, medium-grained, 

hypidiomorphic granular hornblende-biotite quartz monzodiorite; contains variable amounts 

of clinopyroxene; located in extreme northwestern part of Kenai quadrangle. Flow structures 

locally present and hornblende and plagioclase are aligned north-northeast. Two samples 

from outside map area yield strongly discordant K-Ar biotite and hornblende ages between 

58.8 and 97.5 Ma (Nelson and others, 1983). Part of the Alaska-Aleutian Range batholith of 

Reed and Lanphere (1969, 1972)

Alaska-Aleutian Range batholith (Jurassic phase)—Subdivided into:

Jtr Trondhjemite (Late Jurassic)—Medium- to coarse-grained, seriate, leucocratic trondhjemite; 

contains 10 percent muscovite and about 5 percent interstitial, perthitic potassium feldspar. 

Occurs in two bodies in Kenai quadrangle immediately west of Redoubt Volcano and in 

headwaters of Big River north of Double Glacier (Reed and others, 1983). Unit comprises a 

large body in central part of batholith in Iliamna quadrangle west of Chinitna Bay. K-Ar age 

on muscovite from Iliamna quadrangle was 148 Ma (Reed and Lanphere, 1972, analytical 

error not reported)

Jg Quartz diorite, tonalite, and diorite (Jurassic)—Locally foliated medium-grained quartz 

diorite and tonalite; this is by far the dominant map unit of the Alaska-Aleutian Range 

batholith in map area. Hornblende is primary mafic mineral; biotite increases in proportion 

to quartz and potassium feldspar. Detterman and Reed (1980) reported rocks of this unit 

grade to diorite but did not observe it grading into quartz monzonite or granodiorite of their 

unit Jqm. As mapped here, unit includes Detterman and Reed’s quartz monzonite unit that 

consists of medium-grained, pinkish-light-gray quartz monzonite and whitish-gray, mediumgrained, biotite granodiorite containing minor hornblende and accessory primary muscovite. 

Detterman and Reed’s quartz monzonite unit is exposed only on eastern margin of batholith 
14

within map area. Potassium-argon ages generally range from 146±4.3 to 183 Ma for the 

batholith; a number of samples yielded younger ages that are considered suspect or reset by 

younger plutonism. Maximum reported age within map area is 174 Ma (table 1). Two small 

exposures of gabbro and diorite are included in unit

^dt Quartz diorite and tonalite (Late Triassic)—In Seldovia quadrangle, unit includes diorite of 

Point Bede and tonalite of Dogfish Bay of Bradley and others (1999), both on east side of 

Cook Inlet. Diorite of Point Bede of Bradley and others (1999) is fine- to medium-grained 

nonfoliated quartz diorite that Bradley and others (1999) assumed to be of Jurassic age 

based on correlation with a quartz diorite pluton in the Barren Islands, just south of Seldovia 

quadrangle, that yielded a K-Ar hornblende age of 191±1.3 Ma (Cowan and Boss, 1978, 

age recalculated using constants of Steiger and Jager, 1977). However, a Triassic age was 

recently determined on zircon from the diorite (D.C. Bradley, oral commun., 2007). Tonalite 

of Dogfish Bay of Bradley and others (1999) is medium-grained nonfoliated tonalite which 

shows chloritic alteration similar to that found in the diorite of Point Bede and hence was 

assigned a Jurassic age by Bradley and others (1999). We show diorite and tonalite as a 

single map unit of Triassic age. Bradley and others (1999) also mapped a light-gray, finegrained to aphanitic felsite body south of the tonalite of Dogfish Bay. This felsite is undated 

and is tentatively assigned a Triassic age due to its proximity and similar setting to the 

tonalite and diorite of this map unit. Bradley and others (1999) suggested the felsite could 

be early Tertiary in age; however, no other rocks of Tertiary age are reported in this vicinity. 

Kelley (1980, 1984) mapped an intrusive contact between the diorite and his informal Pogibshi formation (Jp); however, both the newly determined Triassic zircon date from the diorite 

and Jurassic fossils from the Pogibshi indicate that there is some error in interpretation of 

available data. Possible scenarios include an incorrectly mapped contact, a wider age range 

for the Pogibshi formation of Kelley (1980), or inheritance in the zircon

ROCK UNITS WITHIN THE BORDER RANGES FAULT SYSTEM

METAMORPHIC ROCKS

Jsch Seldovia metamorphic complex (Early Jurassic)—Marble, quartzite, garnet-mica schist, and 

glaucophane-bearing amphibolite whose protoliths were interpreted by Bradley and others, 

(1999) to consist of limestone, chert, argillite, and greenstone. Unit occurs as narrow fault 

slices within the Border Ranges fault zone south of Seldovia and has been correlated with 

Raspberry Schist of Roeske and others (1989) to the southwest and to Schist of Liberty 

Creek and Schist of Iceberg Lake of Winkler and others (1981) to the northeast. Although 

primarily greenschist facies, these units preserve mineral assemblages that represent blueschist facies metamorphism. Carden and others (1977) reported a series of 10 K-Ar ages 

on micas (including chlorite) and amphiboles from this unit ranging from 158.3±8.3 to 

197.1±11.0 Ma, and Bradley and Karl (2000) reported 40Ar/39Ar plateau ages on mica of 

191.1±0.3 and 191.7±0.3 Ma and on hornblende of 191.9±0.6 Ma. Dating of the blueschist 

facies metamorphism of this metamorphic complex appears to support a pattern of decreasing age from west to east, as originally suggested by Sisson and Onstott (1986)

ROCK UNITS EAST OR SOUTH OF THE BORDER RANGES FAULT SYSTEM

BEDDED ROCKS

Qty Yakataga Formation (Pleistocene to Miocene)—“Consists of diverse marine and glaciomarine 

clastic rocks more than 1,670-m-thick on Kayak and Wingham Island and at least an additional 1,200-m-thick on Middleton Island (Plafker and Addicott, 1976). Much of the continental shelf between Middleton Island and Kayak Island also is underlain by the Yakataga 

[Formation] (Plafker and others, 1975a). Interbedded gray to dark-gray and greenish-gray 

siltstone, mudstone, and sandstone predominate in lower third of formation. Till-like 

diamictite is interbedded with siltstone and sandstone in all but the lowest part of the 

formation and is the dominant rock type in the upper part of the formation, particularly on 

Middleton Island. Conglomerate is a minor lithology throughout the formation and scattered 

clasts—presumably dropstones—are present in all lithologies. Sandstone and conglomerate 

combined constitute about 10 percent of the section on Kayak Island and about 12 percent 

on Middleton Island” (Winkler and Plafker, 1993). In most exposures the Yakataga Formation is conformable and gradational to the underlying Poul Creek Formation but in some 
15

areas there is an angular unconformity of up to 15 degrees (Plafker and Addicott, 1976). 

Age control derived from abundant mollusks and foraminifers, although most are identical 

to living species (Winkler and Plafker, 1981; Plafker and Addicott, 1976). Foraminifera are 

typical of the lower or middle Miocene Saucesian or Relizian Stages of Washington (Rau 

and others, 1977). Paleomagnetic and biostratigraphic dating indicate upper part of formation on Middleton Island was deposited during Matuyama reversed polarity chron (Plafker 

and Addicott, 1976)

Tr Redwood Formation (Pliocene to Oligocene?)—Marine mudstone, siltstone, sandstone, and 

conglomerate (Winkler and Plafker, 1981). “Lower unnamed sandstone member consists of 

about two-thirds thick-bedded sandstone and one-third silty sandstone and siltstone. Upper 

Puffy Member is more diverse and consists of about 50 percent siltstone, mudstone, and 

claystone, 30 percent conglomeratic mudstone and conglomerate, and 20 percent sandstone. 

Characteristic conglomeratic beds show complete gradation from coarse-grained, clastsupported conglomerate to matrix-supported conglomeratic mudstone and sandstone where 

coarse-grained clasts are suspended in the matrix. Siltstone, mudstone, and claystone are 

similar in appearance to parts of the underlying Poul Creek Formation but contain few or no 

concretions, no glauconitic or volcanic beds, and are sandier and more resistant to erosion” 

(Winkler and Plafker, 1993). “The fauna and lithology of the Redwood Formation indicate 

deposition below wave base in cold water at depth from neritic to probably bathyal. The well 

rounded character of clasts in the conglomerate indicates that the coarser clastic material 

was thoroughly abraded before being resedimented in the marine environment. The sparse 

and poorly preserved molluscan fauna from the formation suggests a correlation with the 

upper Galavinian (“Lincoln”) and Matlockian (“Blakeley”) Stages of the Pacific Northwest, or a range in age from late Eocene through Oligocene (Addicott and others, 1978). 

Foraminiferal control is extremely sparse, but suggests that the upper part of the formation 

may be as young as late Miocene (Rau and others, 1977); however, the USGS has accepted 

an age of late Oligocene(?) through early Pliocene for the unit (Plafker, 1987). Thus, the 

Redwood Formation in the Don Miller Hills apparently correlates with the upper part of the 

Poul Creek Formation and the lower part of the Yakataga Formation exposed on Kayak and 

Wingham Islands” (Winkler and Plafker, 1981)

Poul Creek Formation (lower Miocene to late Eocene)—Originally named Katalla Formation (Martin, 1905) whose definition was revised by Miller (1975); however, unit name was 

abandoned and Poul Creek Formation name was extended to map area from east by Nelson 

and others (1985). Unit consists of “Shale, and minor sandstone that is, in part, glauconitic, 

rich in organic material, and intercalated with intrabasinal, water-laid alkalic basaltic tuff, 

breccia, and pillow lava. Unit exposed intermittently along coastal belt in Gulf of Alaska 

region from Kayak Island through Ragged Mountain eastward to Icy Bay in the adjoining 

Bering Glacier quadrangle. Unit also penetrated in wells drilled along the coast and offshore 

in adjacent outer continental shelf. In Chugach National Forest area of Nelson and others 

(1985), unit conformably overlies the Tokun Formation; in Don Miller Hills, unit conformably underlies the Redwood Formation, but on Kayak Island unit is separated from the 

overlying Yakataga Formation by an apparent disconformity” (Winkler and Plafker, 1993). 

Although reported to conformably underlie Redwood Formation and overlie Tokun Formation, defined ages for the three units suggest the contacts may be locally time-transgressive. 

Subdivided into sedimentary rocks and volcanic rocks members, described below:

Tps Sedimentary rocks—“Concretionary, pyritic, glauconitic, reddish-weathering, dark-gray to 

greenish-gray siltstone, claystone, and sandstone; subordinate dark-brown, laminated shale 

that is rich in organic material, silty shale, and gray calcareous sandstone; locally includes 

thin interbeds of basaltic tuff” (Winkler and Plafker, 1993). Thickness is approximately 

1,600 m (Miller, 1975). “Deposited in cool neritic to bathyal marine environment, mostly 

below wave base. Occurrence of shales as much as 244-m-thick with high organic carbon 

contents (to 7.57 percent), extractable petroleum (0.8 gallons per ton), abundant glauconite, 

and common pyrite are suggestive of deposition in part under conditions of restricted bottom 

circulation. Intercalated basaltic fragmental rocks and less common pillow basalt (map unit 

Tpv) indicate episodic submarine mafic volcanism in the basin (Plafker, 1974). Mollusks 

indicate a range in age from late Eocene (Galvinan or “Keasey”) through much of the Oligocene (Matlockian or “Blakeley”) (Addicott and others, 1978). In general, foraminifera from 

the same localities indicate slightly younger ages, ranging from late Eocene through early 
16

Miocene (Refugian, Zemorrian, and Saucesian Stages) (Rau and others, 1977)” (Winkler 

and Plafker, 1981)

Tpv Volcanic rocks—“Basaltic pyroclastic and flow rocks, including minor pillowed lava flows; 

locally interbedded with marine sedimentary rocks, including tuffaceous or glauconitic 

strata, probably related genetically to mafic dikes, sills, and plugs unit (Tm)” (Winkler and 

Plafker, 1993). Winkler and Plafker (1981) reported a K-Ar whole rock date of 31.2±1.3 Ma 

on a sample from the mafic dike unit (Tm) intruding unit Tps on southeast side of Kayak 

Island, providing a minimum age for unit

Tt Tokun Formation (Eocene)—“Consists predominantly of concretionary siltstone and lesser, 

variable amount of interbedded sandstone, chiefly in lower part of formation. Thick sandstone beds exposed near Point Hey and on Kayak and Wingham Islands presumably 

correlate with lower part of formation to the north but were closer to the sediment source. 

Siltstone generally is medium to dark gray and nearly massive; locally, thin beds and lenses 

of lighter gray, brown-weathering calcareous siltstone and silty limestone are found within 

darker siltstone. Spheroidal calcareous concretions as much as 1 m in maximum dimension are distributed randomly or along bedding surfaces in siltstone. Near Bering Lake, 

thin beds of glauconitic sandstone are found near top of formation. Interbedded sandstone 

in Tokun, which generally is lighter gray than the siltstone, is micaceous, feldspathic, and 

brown weathering. Isolated, intertidal outcrops of coarse- to fine-grained, brown-weathering 

feldspathic sandstone on Wessels Reef, which are exposed only at low tide, may correlate 

with the Tokun Formation” (Winkler and Plafker, 1993). “Lithology and megafauna indicate 

general deposition under quiet bottom conditions seaward of the surf zone in tropical to 

warm temperature water (Miller, 1975)” (Winkler and Plafker, 1981). Gradational contact 

with underlying mostly nonmarine Kulthieth Formation and has gradational to abrupt contact with overlying Poul Creek Formation (Winkler and Plafker, 1981). Represents a transgressive marine sequence approximately 1,070 m thick (Winkler and Plafker, 1981). Fossil 

crabs are abundant, especially in upper part of formation and occur intact in concretions 

(Winkler and Plafker, 1981). Sparse mollusks indicate middle and upper Eocene “Tejon” and 

“Keasey” Stages of the Pacific Coast standard section (Addicott and others, 1978)

Tkt Kulthieth Formation (Eocene)—“Includes at least 1,500 m of interbedded, massive to thinbedded, coal bearing arkosic sandstone, dark-gray to black carbonaceous siltstone and shale, 

and minor coal. Sandstone to shale ratios in measured sections of the Kulthieth Formation 

(Martin, 1908) average about 1:1. Sandstone varies from massive intervals as much as 

150-m-thick to thin-bedded and shaly intervals. Bituminous to semi-anthracite coal in beds 

as much as 3-m-thick is conspicuous, but minor part of sequence. Commonly intensely 

deformed into imbricated stacks of fault-bounded chevron folds displaying shearing and 

structural thinning and thickening of coal beds” (Winkler and Plafker, 1993). “Mostly 

nonmarine, with minor tongues of transitional marine strata lithologically similar to the 

underlying Stillwater Formation (Tsu [in their report, Tsw herein]) and overlying Tokun 

Formation (Tt). Represents major progradational cycle into an otherwise marine lower 

Tertiary sequence” (Winkler and Plafker, 1981). Age control derived from widespread fossil 

plant collections and single mollusk collection near top of section (Winkler and Plafker, 

1981). Mollusk collection indicates late Eocene (Tejon) age, whereas fossil plant collections 

indicate wider range from lower Ravenian (late middle Eocene) to lower Kummerian (early 

Oligocene) (Winkler and Plafker, 1981; Wolfe, 1977); however, the accepted age is Eocene

Tsw Stillwater Formation (Eocene)—“Primarily consists of dense hard dark-gray siltstone. Where 

siltstone is carbonaceous, it has a coal-like appearance; where it is calcareous, it may be 

variegated from reddish brown to pale green and usually contains foraminifers” (Winkler 

and Plafker, 1993). Lithology and microfauna of lower section indicate marine deposition 

in neritic to upper bathyal depths (Tysdal and others, 1976; Winkler and Plafker, 1981); 

in “* * * exposures north of the Bering River, which may constitute a higher part of the 

formation, * * *” lithology and megafauna indicate regressive shallow marine deposition 

(Winkler and Plafker, 1981). Grades upward into nonmarine rocks of overlying Kulthieth 

Formation (Miller, 1951; MacNeil and others, 1961). “Dominantly marine strata of the 

Stillwater Formation in the map area may be coeval with the nonmarine lower part of the 

Kulthieth Formation 65 km to the east in the Bering Glacier quadrangle (Miller, 1961; 

MacNeil and others, 1961)” (Winkler and Plafker, 1981). “The Stillwater Formation is 

complexly deformed and is characterized by tight folds and shearing in incompetent strata; 
17

hence its thickness can be estimated only crudely to be at least 1,500 m (Plafker, 1971)” as 

reported in Winkler and Plafker (1981). Age control derived from poorly constrained ages 

of forams and mollusks. “According to W.W. Rau, the foraminifera indicate a range from 

possibly the Paleocene and early Eocene Bulitian Stage to the middle Eocene Ulatisian 

Stage. The mollusks indicate a younger age, late middle or early late Eocene (F.S. MacNeil 

in Wolfe, 1977)” (Winkler and Plafker, 1981)

Orca Group (lower middle Eocene to late Paleocene)—Originally named by Schrader (1900), 

unit consists of widespread complexly deformed flyschoid sedimentary and intercalated 

mafic volcanic and intrusive rocks (Winkler, 1976; Tysdal and Case, 1979; Nelson and 

others, 1985). Thick monotonous sequence of turbiditic sedimentary rocks including thin- to 

thick-bedded sandstone, siltstone, mudstone, and local conglomerate represent middle (and 

possibly inner) submarine fan deposits (Winkler and Tysdal, 1977; Dumoulin, 1987). Mafic 

rocks include ultramafic intrusive rocks, gabbro, sheeted basalt dikes, and pillow basalt 

(Winkler, 1976; Winkler and Tysdal, 1977; Nelson and others, 1985). The igneous rocks, 

while remaining part of the defined Orca Group, are described here under the heading of 

“Ophiolitic rocks of Prince William Sound” in the igneous rocks section of the Description 

of Map Units. Orca Group as a whole variably metamorphosed from laumontite (prehnitepumpellyite) to lower-greenschist facies; hornfels is developed near plutons (Nelson and 

others, 1985). “The lowest grade rocks form a northeast-southwest-trending belt that 

includes Montague and Hinchinbrook Islands; metamorphic grade increases to the north and 

the northwest” (Dumoulin, 1987). Northeastern contact between the Orca and Valdez Groups 

generally accepted to lie along Jack Bay, Gravina, and Bagley Faults in Cordova quadrangle 

(Bol and Roeske, 1993; Winkler and Plafker, 1993; Dumoulin, 1987). Western contact with 

the Valdez Group is less well established due to poor age control and similarities between 

the two units (Dumoulin, 1987; Bol and Gibbons, 1992). Western contact is shown here to 

lie along the Contact Fault, but alternatively the contact may instead lie along Johnstone Bay 

or Bainbridge Bay Fault of Helwig and Emmet (1981) as suggested by Dumoulin (1987), or 

elsewhere. Dumoulin (1987), on the basis of sandstone petrography, suggested “* * * that 

the Valdez and Orca Groups form a single flysch sequence derived from one progressively 

unroofing source * * *.” In the Cordova quadrangle, the Orca Group is bounded on southeast 

along the Wingham, Ragged Mountain, and Chugach Faults by rocks of Stillwater (Tsw) 

and Tokun (Tt) Formations (Winkler and Plafker, 1981, 1993). Unit subdivided into:

Tos Sedimentary rocks, undivided—Thin- to thick-bedded graywacke sandstone, siltstone, 

mudstone, slate, and locally, minor conglomerate. Abundant sedimentary structures, such as 

graded bedding, crossbedding, and ripple marks, along with flute, groove, and load casts, 

indicate deposition from turbidity currents (Tysdal and Case, 1979; Nelson and others, 

1985). Graywacke sandstone is more abundant than finer grained rocks (Tysdal and Case, 

1979). Bioturbated limestone lenses and concretions are found locally, and along with 

conglomerate, are characteristic of sedimentary rocks belonging to the Orca Group (Moffat, 

1954; Tysdal and Case, 1979; Nelson and others, 1985). Lenticular matrix-supported conglomerate and pebbly mudstone and sandstone lenses are widespread, and contain clasts of 

intrabasinal and extrabasinal sedimentary rocks, felsic igneous rocks, and quartz (Winkler 

and Plafker, 1993). Thin-section petrography indicates that most of the sandstone is feldspathic to feldspatholithic (Nelson and others, 1985) and contains abundant monocrystalline quartz, indicating a plutonic provenance (Dumoulin, 1987, 1988). Winkler and Plafker 

(1993) reported unit is metamorphosed to zeolite or prehnite-pumpellyite facies in Cordova 

and Middleton Island quadrangles; Nelson and others (1985) reported alteration ranges 

from diagenetic recrystallization of the matrix to low greenschist facies metamorphism. 

In Seward and Blying Sound quadrangles, unit also includes rocks mapped as siltstone by 

Tysdal and Case (1979); their siltstone unit is locally tightly folded and metamorphosed to 

slate; medium-gray and green lenses of micritic limestone as much as 2 m thick are present locally. At several places in the islands south of Knight Island Passage, Tysdal and Case 

(1979) describe locally folded and contorted sequences of siltstone and isolated greenstone 

blocks mixed in with sandstone and siltstone within otherwise uniformly layered sections. 

Tysdal and Case (1979) thought these sequences were olistostromes. Tysdal and Case (1979) 

reported the same sedimentary rock types are included in their sedimentary rock-greenstone 

map units containing basalt sills (Togs) and pillow basalt (Tops) which are herein assigned 

to units Tosv and Tov, respectively. Fossils reported by Nelson and others (1985) include 
18

Alnus (Alder) pollen; foraminifers Globogerina sp., G. senni, Globogerina sp. (hispid), and 

Globorotalia sp.; and echinoids Holaster sp., ?Hypsopygaster sp., and Nucleopygus

Toc Conglomerate—“Ranges from matrix-supported pebbly mudstone and sandstone to massive 

clast-supported pebble, cobble, and boulder conglomerate” (Nelson and others, 1985). 

Generally well rounded clasts consist primarily of extrabasinal felsic volcanic and igneous 

rocks (felsic porphyry and tuff, granitic rocks, and white quartz) and intrabasinal sedimentary 

and mafic rocks (greenstone, sandstone, siltstone argillite, and limestone) (Nelson and others, 

1985; Winkler and Plafker, 1993; Winkler and Tysdal, 1977; Moffit, 1954). Usually occurs as 

lenses 90 to 210 m thick within flyschoid rocks; though the thickest lens, located in Miners 

Bay (approximately 7 km north of the map area in Unakwik Inlet), measured 900 m thick 

(Nelson and others, 1985). Matrix-supported conglomerate and pebbly mudstone may have 

been formed by submarine landslides on unstable slopes, whereas inversely and normally 

graded clast-supported conglomerate beds are channel fill deposits in feeder and distributary 

channels (Winkler and Tysdal, 1977). Fossils reported by Addicott and Plafker (1971, cited 

in Nelson and others, 1985) include a crab, Branchioplax washingtoniana, and a pelecypod, 

Acila decisa, that indicate Paleocene(?) to late Eocene age

Valdez Group (Upper Cretaceous)—A widespread unit of the coastal region of south central 

Alaska. Consists primarily of complexly deformed metasedimentary graywacke, siltstone, 

and shale generally considered to be deposits of turbidity currents in an oceanic trench 

(Tysdal and Case, 1979; Nelson and others, 1985; Winkler and Plafker, 1981, 1993). 

Includes a variety of interbedded, tholeiitic meta-volcanic and meta-intrusive rocks and 

locally a mélange facies. Valdez Group ranges in metamorphic grade from laumontite- to 

mid-greenschist-facies and locally reaches amphibolite facies east of Copper River (Nelson 

and others, 1985; Winkler and Plafker, 1981). Valdez Group is correlative with Kodiak Formation (Capps, 1937; Wilson and others, 2005), Shumagin Formation (Wilson and others, 

1999), and partially with Sitka Graywacke (Gehrels and Berg, 1992), which together 

make up the Chugach terrane that extends for more than 1,700 km along southern coast of 

Alaska. Subdivided into:

Kvs Metasedimentary rocks, undivided—Dark-gray, thin- to thick-bedded, laumontite to midgreenschist facies metamorphosed, moderately to poorly sorted sandstone, siltstone, and 

mudstone flysch; fine- to coarse-grained sandstone mainly composed of plagioclase, 

quartz, and igneous rock fragments (Tysdal and Case, 1979; Dumoulin, 1987). Unit is 

thick sequence of rhythmically alternating, multiply deformed, metamorphosed sandstonesiltstone turbidites; beds generally range from a few centimeters to a few meters thick to 

locally, massive beds as much as tens of meters thick (Winkler and Plafker, 1981; Nelson 

and others, 1985; Winkler and Plafker, 1993; Bradley and others, 1999). Point count analysis 

by Dumoulin (1987) showed Valdez Group sandstone contains between 6 to 30 percent 

quartz, 23 to 45 percent feldspar, and 28 to 68 percent lithic fragments; lithic fragments are 

dominantly volcanic rocks. Proportion of lithic fragments decreases from west to east, as 

feldspar and quartz increases (Nelson and others, 1985). Conglomeratic sandstone containing clasts of quartzite, intermediate and felsic volcanic rocks, and rare sandstone, limestone, 

and granitic rocks is uncommon but widely distributed, occurring at base of some sandstone beds (Bradley and others, 1999; Bradley and Miller, 2006). In some places, primary 

sedimentary structures such as graded bedding, current-ripple cross-lamination, convolute 

bedding, and sole markings are preserved (Nelson and others, 1985; Winkler and Plafker, 

1993). Inoceramus kusiroensis, Inoceramus ulrichi, and Inoceramus concentrica of Maastrichtian or Late Cretaceous age have been reported (Tysdal and Plafker, 1978; Tysdal and 

Case, 1979; Nelson and others, 1985; Bradley and others, 1999). A K-Ar date on biotite 

semischist of 51.5±1.5 Ma was reported by Nelson and others (1985). D.C. Bradley (USGS, 

oral commun., 2007) reported 70 Ma detrital zircons from unit near Anchorage

Kvvs Interbedded metavolcanic and metasedimentary rocks—Approximately equal portions of 

interbedded metavolcanic and metasedimentary rocks (Winkler and Plafker, 1981; Nelson 

and others, 1985; Winkler and Plafker, 1993). Metavolcanic rocks include semischistose 

volcanic breccia, tuff, tuffaceous sediment, and minor pillow basalt; metasedimentary rocks 

include slate and phyllite, although metamorphosed sandstone, siltstone, and chert are interbedded in many places (Winkler and Plafker, 1981; Nelson and others, 1985; Winkler and 

Plafker, 1993). Unit crops out in two locations: north central Cordova quadrangle, and east 

side of Resurrection Peninsula in Seward quadrangle. Winkler and Plafker (1993) report unit 
19

tends to be “variably metamorphosed regionally from zeolite to lower greenschist facies, 

although near Cordova, Woodworth, and Schwan Glaciers rocks are transitional to amphibolite facies”

Kvv Metavolcanic rocks, undivided—Tholeiitic metabasalt, massive greenstone, and basaltic 

metatuff, including local pillow lava, pillow breccia, and gabbroic dikes and sills (Winkler 

and Plafker, 1993) in Cordova quadrangle. Metabasalt forms rugged, nearly massive outcrops, whereas semischistose metatuff forms more subdued outcrops (Winkler and Plafker, 

1993)

Kvt Aquagene tuff—Green to dark-gray, fine-grained, and finely laminated tuff contains 

plagioclase phenocrysts; metamorphosed to chlorite and biotite zones of greenschist facies; 

chlorite, epidote, and, locally, actinolite are abundant; remnants of glossy shards are present 

(Tysdal and others, 1977; Tysdal and Case, 1979). Whole rock age for metatuff on Fox 

Island of 54.4 ±2.7 Ma was interpreted by Miller (1984) as possibly reset during a thermal 

event; alternatively, metatuff on Fox Island (formerly Renard) is closely associated with 

ophiolitic rocks of Resurrection Peninsula, now considered Eocene age. Whole-rock age 

determination on metamorphosed tuff is considered suspect

Kvgs Schist—Schistose metamorphic rocks found in two general modes of occurrence. In Cordova 

quadrangle, Winkler and Plafker (1993) describe a “chiefly homogenous pelitic schist and 

minor amphibolite; [unit which] also includes spotted biotite-plagioclase-quartz (±muscovite) schist containing porphyroblasts of cordierite, andalusite, garnet, and staurolite 

in several places. Common banding probably represents original layering inherited from 

protoliths; transitional into gneissose rocks to the east and into greenschist-facies rocks 

of the metasedimentary rocks unit (Kvs) to the west.” In contrast, in Seward and Blying 

Sound quadrangles, Tysdal and Case (1979) describe a schist unit as “interbedded siltstone, 

graywacke, and less abundant tuff, tuffaceous sandstone, and basalt (pillow basalt?) * * *.” 

These rocks are primarily localized on west side of Placer River Fault and “* * * metamorphosed chiefly to biotite zone of greenschist facies, but locally to chlorite zone; typical 

metamorphic-mineral assemblages of biotite zone are biotite-muscovite-chlorite-quartz-epidote-calcite-albite * * *” (Tysdal and Case, 1979). Nelson and others (1985) did not map the 

schistose rocks mapped by Tysdal and Case (1979) as a distinct unit because they reported 

the rocks to be only semi-schistose and interlayered with rocks that have no penetrative 

deformation; Nelson and others (1985) reported that rocks such as these were common 

throughout the Valdez Group, especially near faults and in areas of finer grained sedimentary 

protolith. However, we have retained the belt of schistose rocks along the Placer River Fault 

by Tysdal and Case (1979), which they reported as being conspicuously schistose and the 

only extensive area of schistose rocks in their map area. Nelson and others (1985) reported 

other areas of schistose rocks are common in the Valdez Group, yet none are distinguished 

on maps to date except by Tysdal and Case (1979) for areas on Fox [Renard] Island and Resurrection Peninsula; on Resurrection Peninsula, the unit occurrence is included in unit Kvs

here, following the mapping of Bradley and Miller (2006)

Kvgg Gneiss—“Muscovite-biotite-quartz-plagioclase (± sillimanite and garnet) schist, banded 

gneiss, and migmatitic gneiss; potassium-feldspar-bearing segregations in migmatite; 

many local intrusive stringers, dikes, and sills of granodiorite and granite; transitional into 

schistose rocks to the north, south, and west” (Winkler and Plafker, 1993). Nelson and 

others (1985) describe unit as a paragneiss of “predominantly metasedimentary rocks with 

well-developed gneissic foliation consisting of alternating quartz and feldspar-rich layers 

with biotite-rich layers. * * * Locally some layers in the paragneiss contain 1-in.-long [2.5 

cm] porphyroblasts of andalusite.” Unit contains as much as 10 percent cream-colored or 

light-gray, coarse-grained orthogneiss that locally grades into normal textured granitic rocks 

containing abundant inclusions of country rock (Nelson and others, 1985). Metamorphic 

grade ranges from middle to upper amphibolite facies (T.L. Pavlis and V.B. Sisson, written 

commun., 2002). Hudson and others (1979) reported hornblende and biotite ages of 52.8±2 

Ma and 47.9±2 Ma respectively on amphibolite and schist in Bering Glacier quadrangle, east 

of map area. Unit is gneissic equivalent of Valdez Group found along Wernicke Glacier

Kvm Mélange of Iceworm Peak of Kusky and others (1997)—Tectonic mélange of blocks of 

graywacke in a phacoidally cleaved matrix of slate; Valdez Group protolith for both matrix 

and blocks (Bradley and others, 1999; Bradley and Wilson, 2000). Mapped only in Seldovia and Kenai quadrangles but may be more extensive along McHugh Complex and Valdez 
20

Group contact (Kusky and others, 1997). Intruded by Paleocene dikes (Kusky and others, 

1997) not shown here

KCm McHugh Complex, undivided (Cretaceous to Carboniferous, Mississippian)—“Mélange of 

siltstone, sandstone, mud-chip sandstone, conglomeratic sandstone, tuff, and less abundant gabbro, serpentinite, bedded chert, and pillow basalt; fine-grained sedimentary strata 

commonly streaked out; sedimentary and igneous rocks are broken and discontinuous; 

metamorphic minerals include muscovite, epidote, calcite, chlorite, albite, and veinlets 

of prehnite” (Tysdal and Case, 1979). Paraphrasing Clark (1981), McHugh Complex is a 

heterogeneous, chaotic assemblage that includes metamorphosed clastic and volcanic rocks 

of diverse ages. Clastic rocks, which are the most abundant, comprise thick, fault-bounded 

sequences of weakly metamorphosed graywacke, arkose, siltstone, and conglomeratic sandstone. Bedding is rarely seen and commonly discontinuous. Volcanic rocks in the McHugh 

Complex, also in fault-bounded sequences, are greenstone, mostly of basaltic composition, 

and are associated with radiolarian metachert, metasiltstone, and argillite. Clark (1981) 

thought the volcanic rocks represent oceanic crust and that the clastic sedimentary rocks 

were deposited in an oceanic trench. Sedimentary rocks of the mélange matrix yield Early 

Cretaceous (Valanginian) fossils, whereas protolith ages on blocks in the mélange yield 

radiolarians of Cretaceous (Albian-Aptian), Jurassic, and Triassic age (Bradley and others, 

1999; Winkler and others, 1981), Permian conodonts and fusilinids (Bradley and others, 

1999), and Mississippian to Pennsylvanian conodonts (Nelson and others, 1986). D.C. 

Bradley (USGS, oral commun., 2007) reports ~90 Ma detrital zircons from metasandstone 

within the McHugh Complex collected along Turnagain Arm in Anchorage quadrangle to 

the north of map area. Bradley and others (1999) locally subdivided the McHugh Complex 

into distinct lithologic packages:

KJms McHugh Complex, graywacke and conglomerate (Lower Cretaceous to Lower Jurassic, 

Pliensbachian)—Consists of fault-bounded blocks of massive conglomerate and graywacke 

that range up to several kilometers in structural thickness (Bradley and others, 1999). Bradley and others (1999) reported that deformation has generally obliterated primary sedimentary features; they interpreted the conglomerate and massive graywacke to be of turbiditic 

origin and locally noted the presence of thin- and medium-bedded turbiditic graywacke. 

Graywacke is matrix-supported, poorly sorted, and has clasts consisting primarily of chert 

and volcanic rock fragments (Bradley and others, 1999). Bradley and others (1999) reported 

Pliensbachian radiolaria in conformably underlying ribbon chert at one locality and considered unit ranges in age regionally from Lower Jurassic, Pliensbachian, to as young as Lower 

Cretaceous

K^mc McHugh Complex, basalt and chert (Lower Cretaceous, Albian, to Middle Triassic, 

Ladinian)—“Pillow and massive basalt, depositionally overlain by complexly folded and 

faulted radiolarian chert” (Bradley and others, 1999). Bradley and others (1999, citing a 

C.D. Blome, USGS written commun. of 1994) reported radiolaria in bedded chert range in 

age from Middle Triassic, Ladinian, to Lower Cretaceous, Albian

}g Gabbro (Mesozoic)—Dark-green, medium- to coarse-grained gabbro and minor leucogabbro 

and plagiogranite (Bradley and others, 1999). Fault-bounded bodies in McHugh Complex; 

following Bradley and others (1999), only the larger bodies are shown on the map. Associated with McHugh Complex basalt and chert unit (K^mc), which Bradley and others (1999) 

suggested may indicate a genetic relation and therefore they thought may indicate Triassic to 

mid-Cretaceous age. However, they assigned an undifferentiated Mesozoic age to the unit. 

Bradley and Karl (2000) reported a concordant U-Pb zircon age of 227.7±0.6 Ma

}u Ultramafic plutonic rocks (Mesozoic)—Predominantly layered, variably serpentinized dunite 

containing rare to locally abundant layers of chromite and pyroxenite and fault slices of 

garnet pyroxenite and serpentinite (Bradley and others, 1999). Bradley and others (1999) 

reported at least 7 fault-bounded bodies, all associated with McHugh Complex. Most bodies 

are bounded by low-angle thrust faults. This was originally suggested for the Red Mountain 

body, but as discussed in Bradley and others (1999), the Red Mountain body is not a klippe 

as interpreted by Magoon and others (1976) but rather is bounded by late-stage, sub-vertical 

faults. However, Bradley and others (1999) did suggest a thrust may bound the Red Mountain body at depth and also inferred that the gabbro (}g) and ultramafic rocks, while not 

spatially associated are comagmatic and therefore, by extension, may be Triassic to midCretaceous age; nonetheless, they are assigned a Mesozoic age
21

IGNEOUS ROCKS

Tdc Dacite of Cape Saint Elias (Pliocene or Miocene)—Described by Winkler and Plafker (1993) 

as follows: “Prominent very pale gray dacite plug complex that forms Cape Saint Elias 

and Pinnacle Rock at the southwestern end of Kayak Island (Plafker, 1974). Dacite is very 

dense and hard and is conspicuously jointed. Unit has a microgranitic and porphyritic 

texture and consists of about 35 percent plagioclase, 35 percent quartz, 25 percent 

orthoclase, and 5 percent relict brown hornblende and biotite. Unit has sharp, nearly 

vertical contacts with adjacent dark-gray argillaceous rocks of the Yakataga and Poul 

Creek Formations, which are metamorphosed to dark hornfels for at least 100 m around the 

intrusion. Intrudes lower part of Yakataga Formation (unit QTy) and, therefore, must be at 

least as young as late Miocene and more likely Pliocene because unit was emplaced after 

enclosing strata were deformed.” A whole-rock K-Ar age of 6.2±0.3 Ma (table 1, herein) 

was reported by Nelson and others (1985)

Tm Mafic plugs (Oligocene?)—Strongly altered dikes, sills, and plugs of diabase, alkalic basalt, 

olivine basalt, and lamprophyre, mostly on or near Kayak Island. Winkler and Plafker (1993) 

suggested these hypabyssal rocks were probably genetically related to extrusive volcanic 

rocks in the Poul Creek Formation (Tpv). Winkler and Plafker (1981) described these rocks 

as coarse grained and diabasic in the plugs west of the Nichawak Hills and fine grained and 

glassy elsewhere. Immediately west of the Nichawak Hills, two plugs are diabasic, containing 45 percent euhedral plagioclase intergrown with 30 percent anhedral augite and enstatite, 

5 percent opaque minerals, and 20 percent secondary chlorite. East of Ragged Mountain, an 

isolated plug is nepheline syenite. Lithologic associations and crosscutting relations suggest Oligocene age; nowhere does unit intrude rocks younger than the upper part of the Poul 

Creek Formation (Winkler and Plafker, 1981). K-Ar whole rock date of a mafic dike on 

Kayak Island was 31.2±1.3 Ma (Winkler and Plafker, 1981)

Tg Granitic rocks (Oligocene? and latest Eocene)—Plutons in Seward quadrangle are dominantly 

light-gray, medium- to coarse-grained biotite±hornblende granodiorite and granite stocks; 

larger stocks commonly grade inward from marginal zones containing more biotite (and 

locally hornblende) to more leucocratic zones; large areas of intrusions are porphyritic and 

contain orthoclase phenocrysts in a medium-grained groundmass; plagioclase composition ranges from oligoclase to andesine, and biotite constitutes between 5 and 15 percent 

(Tysdal and Case, 1979). Also includes Cedar Bay granite of Tysdal and Case (1979), a 

light-gray and pink, medium- and coarse-grained muscovite (±biotite) granite; plagioclase 

composition ranges from oligoclase to andesine, and biotite is commonly less than 1 percent. 

Hypidiomorphic-granular texture is most common, but some varieties are equigranular or 

porphyritic. Biotite is the most abundant mafic mineral and is sometimes accompanied by 

hornblende; towards the pluton margins, hornblende commonly is predominant (Nelson 

and others, 1985). K-Ar dates on biotite and hornblende range from 34.2±1.7 to 37.1±1 

Ma (Tysdal and Case, 1979; Nelson and others, 1985; ages recalculated using constants of 

Steiger and Jager, 1977). 40Ar/39Ar ages were 29.2±0.1 and 31.7±0.1 on potassium feldspar; 

however these dates are inherently suspect because potassium feldspar is not generally considered suitable for dating emplacement of plutonic rocks

Tgd Gabbro and diorite (Oligocene and older)—Dominantly dark-gray, medium- to coarsegrained gabbro and subordinate diorite, both having finer grained borders of quartz gabbro 

and quartz diorite (Tysdal and Case, 1979; Nelson and others, 1985). Gabbro generally 

composed of subequal amounts of labradorite and clinopyroxene; locally contains olivine 

on Esther Island; pyroxene largely altered to hornblende north of Paddy Bay (Tysdal and 

Case, 1979). Mostly subophitic texture; however, some hypidiomorphic to allotriomorphic granular and a few are porphyritic (Nelson and others, 1985). Unit considered early 

mafic phase associated with granitic plutons of unit Tg. Tysdal and Case (1979) originally 

assigned biotite K-Ar date of 37.6±1.0 Ma (age recalculated using constants of Steiger and 

Jager, 1977) to Granite of Passage Canal; Nelson and others (1985) assigned the sample 

locality to this map unit although the reported rock type for the date was granodiorite. 

Grant and Higgins (1910) and Tysdal and Case (1979) suggested, and Nelson and others 

(1985) supported, a genetic and chronological relation of these mafic rocks to the granitic 

rocks unit (Tg)

Tgg Granite and granodiorite (Eocene)—Generally medium-grained and medium- to coarsegrained hypidiomorphic-granular biotite granite having border phases of biotite-hornblende 
22

granite, granodiorite, and tonalite (Nelson and others, 1985) in Cordova quadrangle. 

Also includes uncommon dacite (George Plafker, written commun., 1985). Mafic mineral 

content and color index varies with composition; color index of the granite ranges from 

5 to 10, granodiorite ranges from 10 to 20, and tonalite ranges from 15 to 35. In Cordova 

quadrangle, plutons also include biotite-muscovite±garnet tonalite and trondhjemite that 

include strongly foliated and lineated plutons, weakly foliated, syn-deformation plutons, 

and nonfoliated, post-deformation plutons (T.L. Pavlis and V.B. Sisson, written commun., 

2002). K-Ar and 40Ar/39Ar ages range from 47.23 Ma on dacite to 53.5±1.6 Ma on hornblende from a sample reported only as granitic rock; a U-Pb monazite age on a sill located 

near Van Cleve glacier was about 54 Ma (Poole, 1996). U-Pb age determinations on zircon 

from sill were unsuccessful due to inheritance and lead loss (J.N. Aleinikoff, USGS, written 

commun., 2007). Also included in unit is a small pluton in northern part of Seward quadrangle on west side of Port Wells. Surrounded by plutons assigned to unit Tg, this small 

pluton is assigned to map unit Tgg on the basis of reported age (56±1 Ma, listed in table 1 

for the Granite Mine in the section on hydrothermal alteration dates). Other small plutons 

of similar but slightly younger age are mapped in adjacent Anchorage quadrangle north of 

map area

Tgh Granitic rocks of Harding Icefield region (Eocene and Paleocene)—Primarily “medium- to 

dark-gray foliated medium- to coarse-grained biotite-muscovite-(hornblende) granite and 

granodiorite; marginal phases are locally biotite-muscovite-(hornblende) tonalite” (Tysdal 

and Case, 1979). Consists of a large batholith extending from Nuka and Aialik Bays and 

offshore islands northward more than 60 km into Harding Icefield where unit is exposed in 

many nunataks and westward into Seldovia quadrangle near Harris Bay; “* * * mafic minerals, almost wholly biotite, constitute 20 to 30 percent of minerals in tonalitic rocks, 15 to 20 

percent of granodiorite, and 5 to 10 percent of granite; muscovite, a late but primary mineral, 

commonly makes up less than 2 percent” (Tysdal and Case, 1979). Unit includes Nuka, 

Aialik, and Tustumena plutons, and Chernof stock of Bradley and others (1999). A K-Ar 

date from a sample collected at Aialik Cape yielded 61.0±1.8 Ma on biotite and 56.0±1.6 Ma 

on muscovite (Tysdal and Case, 1979, ages recalculated using constants of Steiger and Jager, 

1977), whereas a sample of the Aialik pluton from Harris Bay yielded a weighted mean 

40Ar/39Ar age of 52.2±0.9 Ma (Bradley and others, 2000) and the Paguna stock of Bradley 

and others (1999) yielded a 40Ar/39Ar plateau age of 53.4±1.5 Ma. The apparent large size 

of the pluton and wide-ranging age determinations (from 52.2 to 61.0 Ma), which overlap 

the age of unit Tgg, suggest these rocks are part of a multiphase batholith rather than a 

single pluton. Lytwyn and others (2000) described a suite of dikes (part of map unit TKd, 

herein) that they associate with the same ridge subduction related system thought responsible for these plutons

Ophiolitic rocks of Prince William Sound (Eocene)—The ophiolitic rocks of Prince William 

Sound are composed of ultramafic rocks, gabbro, plagiogranite, sheeted basalt dikes, and 

pillow basalts that represent mid-oceanic-ridge rocks (Crowe and others, 1992; Tysdal and 

others, 1977) and are generally considered part of the Orca Group. These ophiolitic rocks 

crop out in a fault(?) bounded window in the Valdez Group along the Resurrection Peninsula 

and are interbedded and intruded within the Orca Group sedimentary rocks on Knight and 

Glacier Islands and to the northwest to Valdez Arm. The chemistry of the mafic rocks indicates a mid-ocean-ridge origin (Crowe and others, 1992; Tysdal and others, 1977; Lytwyn 

and others, 1997). An aeromagnetic high, associated with the ophiolitic rocks of Knight 

Island, extends to similar rocks on Glacier Island, which suggests the ophiolite is a continuous belt through Prince William Sound (Case and others, 1979; Barnes, 1991).

Historically, the Resurrection Peninsula rocks had been included within the Upper 

Cretaceous Valdez Group, although age control was lacking. With the report of a 57 Ma age 

on a zircon from a plagiogranite (Nelson and others, 1989), it was recognized that Resurrection Peninsula rocks are similar in age to the Knight Island ophiolite, which is assigned 

to the Orca Group. Workers have consistently commented on the similarities between the 

two ophiolitic complexes (Tysdal and others, 1977; Tysdal and Case, 1979; Miller, 1984; 

Nelson and others, 1985; Lytwyn and others, 1997). Grant and Higgins (1910) originally 

showed these two ophiolitic complexes as part of the same unit. Because the units are the 

same age and are similar in character, herein we include Resurrection Peninsula ophiolitic 

rocks and Knight Island ophiolitic rocks as part of the Orca Group. Resurrection Peninsula 
23

ophiolitic rocks represent a single-sided west-dipping sequence (Tysdal and others, 1977). 

Knight Island ophiolitic rocks represent a mirrored or antiformal sequence having a core 

of gabbro cut by sheeted dikes with tops of pillow basalt on the west facing west and tops 

of pillow basalt on the east facing east (Tysdal and others, 1977). Although these ophiolitic rocks might be mapped as a separate unit from the Orca Group trench fill sedimentary 

rocks, the interfingering nature of the contact makes assigning a definitive demarcation 

difficult. Following Lytwyn and others (1997), we include the metavolcanic rocks described 

by Winkler and Plafker (1981; 1993, unit Tov) included in the Orca Group in the Cordova 

quadrangle in the ophiolitic rocks of Prince William Sound. On the Resurrection Peninsula, 

the contact between the Valdez Group and the ophiolitic rocks is in dispute (Bradley and 

Miller, 2006; Kusky and Young, 1999). Bradley and Miller (2006) interpreted the contact 

between the Valdez Group and the ophiolitic rocks as faulted on all sides. Kusky and Young 

(1999) interpreted the contact as stratigraphic on the west side of the peninsula near Humpy 

and Thumb Coves. Complicating matters are tuffaceous rock outcrops on Fox Island, which 

are petrographically and metamorphically different from typical rocks of the Valdez Group 

(Bradley and Miller, 2006; Kusky and Young, 1999; Tysdal and Case, 1979) and are mapped 

as a distinct unit of the Valdez Group by all workers. Also, Tysdal and Case (1979) map an 

intrusive contact between gabbroic and sedimentary rocks of the Valdez Group. Tysdal and 

Case (1979) mentioned a chilled margin and a 200-m-wide thermal aureole along the contact 

on the east side of the peninsula, but this was not observed by Nelson and others (1985). 

Tysdal and Case (1979) also mention an intrusive contact near the pass at the head of Likes 

Creek. Future study of the tuffaceous rocks and the contact between the ophiolitic rocks and 

surrounding sedimentary rocks is necessary to resolve the controversy regarding placement 

of the ophiolitic rocks in the section.

Tov Volcanic rocks, undivided (Eocene)—“Thick and thin tabular bodies of altered tholeiitic 

basalt consist of pillowed, massive, or crudely columnar flows, but also include pillow 

breccia, aquagene tuff, and diabase or gabbro sills; pillows have chilled margins that are 

palagonitic and amygdaloidal. Minor mudstone and siltstone that have interbedded basalt are 

included locally. Commonly contains green, gray, or red chert in interstices between pillows, 

rarely includes interpillow clots of pink limestone or black mudstone. * * * Metamorphosed 

to zeolite or prehnite-pumpellyite facies” (Winkler and Plafker, 1993). Unit also includes 

areas mapped as greenstone that were probably pillow basalt, basaltic tuff, and sills (Tysdal 

and Case, 1979, unit Tog; Nelson and others, 1985, unit Tov). May be equivalent in part to 

units Top, Tovs, and Tod. Locally subdivided into:

Top Pillow basalt—“Pale- to dark-green aphyric to porphyritic, frequently amygdaloidal, pillow 

basalt composed of plagioclase and clinopyroxene; plagioclase commonly replaced by albite 

and epidote, and clinopyroxene by hornblende, actinolite, and(or) chlorite; vesicles mostly 

filled with chlorite; pillow basalt in some areas is interlayered with subordinate pillow breccia and tuff” (Tysdal and Case, 1979). Discrete pillows average 2 ft [50 cm] in diameter on 

Resurrection Peninsula and average about 1 m where they were mapped in the Orca Group 

by Nelson and others (1985) and are metamorphosed to as high as low greenschist facies 

(Nelson and others, 1985). Fractured surfaces, chilled margins, palagonitic coatings, and 

cracks that radiate outward from pillow centers indicate rapid cooling (Tysdal and others, 

1977; Tysdal and Case, 1979). Altered brown-colored volcanic glass makes up much of the 

matrix, especially in the outer parts of pillows (Nelson and others, 1985). Inter-pillow material consists of extrusive breccia, agglomerate, and hyaloclastite and, locally, thin sequences 

of siliceous mudstone and argillite, sandstone, and carbonate rocks, and minor red, green, 

and gray chert (Capps, 1915; Tysdal and others, 1977; Tysdal and Case, 1979; Miller, 1984; 

Nelson and others, 1985). Interbedded sedimentary layers locally display crossbedding and 

graded beds (Tysdal and others, 1977; Tysdal and Case, 1979). “Pillow basalt and lesser 

massive basalt and broken pillow breccia make up most of the western flank of the mafic 

complex on the Resurrection Peninsula” (Nelson and others, 1985; Tysdal and Case, 1979) 

where flows dip westward roughly 30 ̊ and are as much as 1,000 m thick (Tysdal and others, 

1977). Flows dip westward on the west side of Knight Island and eastward on the east side 

of the island and are more than 5,000 m thick (Tysdal and others, 1977). Unit is gradational 

into and cut by sheeted dikes of unit Tod over a wide zone on Glacier and Knight Islands 

and interbedded with volcanic and sedimentary rocks of unit Tovs in eastern Prince William 

Sound (Nelson and others, 1985; Tysdal and Case, 1979)
24

Tovs Volcanic and sedimentary rocks—Consists of locally variable amounts of tholeiitic basalt 

and tuffaceous and generally minor turbiditic sedimentary rocks (Winkler and Plafker, 1993; 

Nelson and others, 1985; Tysdal and others, 1976). Lithology is quite variable throughout 

map area. “Basalt consists of pillowed and massive flows, pillow breccia, and tuff. Turbidites, in most places, are mudstone, siltstone, and fossiliferous volcanogenic sandstone; in a 

few places nonvolcanogenic sandstone is interbedded” (Winkler and Plafker, 1993). “In the 

Ragged Mountains of the Cordova quadrangle, volcaniclastic and tuffaceous sedimentary 

rocks are abundant, and pillow basalt is rare” (Nelson and others, 1985; Tysdal and others, 

1976). In Ragged Mountains area, lower part of unit is interlayered with abundant aquagene 

tuff, tuff breccia, and calcareous sandstone containing shells (Tysdal and others, 1976). 

Another variation of the unit is found north of Martin River where Nelson and others (1985) 

mapped a unit, Tots, consisting of tuffaceous sedimentary rocks, volcaniclastic sandstone, 

and minor chert. Majority of Tots unit is found south of Martin Fault where minor pillow 

basalt and abundant volcanic breccia is included with the sedimentary rocks of the unit and 

the “rocks are altered to a bright orange-weathering gossan * * * intruded by numerous porphyritic dikes for a distance of 10 mi along the south side of the Martin fault” (Nelson and 

others, 1985). As shown here, in vicinity of Knight Island, map unit is composed as much 

as 92 percent volcanic rocks; in this area these were mapped by Tysdal and Case (1979) as 

a pillow basalt and sedimentary rocks unit (Tops) and a greenstone and sedimentary rocks 

unit (Togs); and by Nelson and others (1985) as a pillow basalt and sedimentary rocks unit 

(Tops). Nelson and others (1985) distinguished the pillow basalt and sedimentary rocks unit 

from a separately mapped interbedded sedimentary and mafic volcanic rocks unit (Tosv) on 

the basis of the predominance of pillow basalt in the section; however, we have combined 

the units herein. Locally, this combined unit is marginal to and probably gradational to 

thicker sequences composed of pillowed and massive basalt flows (Top). Nelson and others 

(1985) reported a number of varieties of foraminifers of Paleocene to late middle Eocene age 

in Seward and Cordova quadrangles, and north of Martin River the tuffaceous sedimentary 

rocks contain locally abundant radiolarians and diatoms of late Paleocene to Eocene age 

(Tysdal and others, 1976; Nelson and others, 1985). A 47.6±1.4 Ma K-Ar hornblende age 

from a greenschist sample (table 1, sample number 71Apr 23A, Winkler and Plafker, 1981; 

George Plafker field notebook, 1971; Nelson and others, 1985) collected from the head of 

Port Fidalgo in Cordova quadrangle was originally assigned to the Valdez Group by Winkler 

and Plafker (1981) and Nelson and others (1985); however, Winkler and Plafker (1993) reassigned rocks in the vicinity of this sample to the Orca Group, which would constrain the age 

of this unit to no younger than early Eocene

Tod Sheeted basalt dikes—Mafic sheeted-dike complexes consisting of dark-green, gray, and 

brown, aphanitic to porphyritic, chiefly basaltic, but locally gabbroic to dioritic, dikes 

(Tysdal and others, 1977; Tysdal and Case, 1979). In Knight Island area, felsic plagioclasequartz dikes occur locally and plagioclase-clinopyroxene-olivine dikes are present but not 

common (Tysdal and Case, 1979). On Resurrection Peninsula, olivine is present in a few 

dikes (Tysdal and Case, 1979). Dikes are commonly 1 to 2 m thick, vertical or nearly so, 

and generally strike north (Tysdal and others, 1977; Tysdal and Case, 1979). Dikes contain 

greenschist facies mineral assemblages ascribed to ocean-floor metamorphism (Bradley and 

Miller, 2006); according to Tysdal and Case (1979), plagioclase replaced by albite, clinopyroxene by hornblende and actinolite, and olivine partly altered to serpentine. Dikes crosscut one another, intrude the adjacent pillow basalt (unit Top), and on Knight Island, dikes 

locally intrude sedimentary rocks of Orca Group (Tysdal and others, 1977). Double- and 

single-sided chilled margins observed on dikes indicate a spreading center setting (Bradley 

and Miller, 2006; Tysdal and Case, 1979). Pillow basalt screens are common in the up-to2,000-m-wide transition zone between pillow basalt and sheeted dike units (Top and Tod) 

(Tysdal and others, 1977; Tysdal and Case, 1979; Miller, 1984; and Bradley and Miller, 

2006). Xenoliths of gabbro and peridotite are present locally on Knight Island (Richter, 

1965; Nelson and others, 1985). Small irregular pods, veins, and dikes of plagiogranite are 

also present in dike complex north of Bay of Isles on Knight Island (Nelson and others, 

1985). Dikes are intruded by and also intrude gabbro in transition zone between gabbro and 

sheeted dike units (Togb and Tod) (Tysdal and Case, 1979; Tysdal and others, 1977). Dikes 

make up the topographically high and rugged cores of the Resurrection Peninsula, Knight 

Island, and Glacier Island (Tysdal and others, 1977). Whole rock K-Ar ages on greenstone 
25

reported by Miller (1984) are 35.0±1.3 Ma and 38.8±1.9 Ma on Knight Island and 49.6±2.5 

Ma, 45.7±2.3 Ma, and 47.3±2.4 Ma on Resurrection Peninsula. Miller (1984) interpreted 

ages to represent minimum ages of accretion, reflecting argon loss due to heating

Togb Gabbro—“Medium- and dark-gray to green coarse-grained chiefly clinopyroxene-plagioclase rocks with ophitic to subophitic texture and gabbroic to dioritic composition; locally 

pegmatitic with clinopyroxene crystals as much as 5 cm long; clinopyroxene commonly is 

partly replaced by uralitic amphibole and chlorite; plagioclase is partly replaced by albite; 

epidote and minor opaque minerals are common” (Tysdal and Case, 1979). Several small 

intrusive bodies of gabbro occur on Knight Island, the largest crops out east of Drier Bay, 

where gabbro intrudes sheeted dikes (Nelson and others, 1985). Other gabbro bodies, too 

small to show at map scale, are elongate parallel to trend of the sheeted dikes. At north end 

of Latouche Island, a 30-m-wide exposure of gabbro intrudes slate and sandstone that is cut 

by several small gabbro-pegmatite dikes (Grant and Higgins, 1910, p. 49–50; Tysdal and 

Case, 1979; Nelson and others, 1985). On Resurrection Peninsula, Miller (1984) described 

local occurrences of west-dipping magmatic mineral layering and cumulate textures within 

the gabbro. Dikes and plugs of fine- to medium-grained plagiogranite (Miller, 1984; Bradley 

and Miller, 2006), the largest of which crop out northwest of head of Drier Bay on Knight 

Island and south side of Killer Bay on Resurrection Peninsula (Tysdal and Case, 1979; 

Miller, 1984). Gabbro grades into sheeted dike unit (Tod herein) and is generally elongate 

and parallel to the sheeted dikes, but also crosscuts dikes locally (Tysdal and Case, 1979; 

Miller, 1984). Tysdal and Case (1979) state that on Resurrection Peninsula, “The gabbro 

intrudes slate and sandstone of the Valdez Group, crosscuts the bedding, and forms aphanitic 

sills in other places. A blue-gray and whitish thermal aureole, at least 200 m wide, marks the 

contact zone with the sedimentary rocks.” However, Nelson and others (1985) and Bradley 

and Miller (2006) interpreted contacts between the gabbro and Valdez metasedimentary 

rocks as faults. On Knight Island, gabbro intrudes rocks of the Orca Group. Nelson and 

others (1989) reported a 57 Ma U-Pb zircon age on a plagiogranite intruding gabbro on 

Resurrection Peninsula

Tou Ultramafic rocks—“Dunite, locally with layers of chromite, moderately altered to mostly 

altered to serpentine, serpentine-talc, and talc schist; lustrous pale- to dark-green and black, 

locally reddish weathering rock that forms dike-like and irregularly shaped intrusive bodies” 

(Tysdal and Case, 1979) on Resurrection Peninsula. “In most places enough relict texture 

and mineralogy remains to recognize original clinopyroxenite, dunite, and harzburgite” 

(Miller, 1984). Occurs as small pods in gabbro and fault-bounded slices within Valdez Group 

metasedimentary rocks (Miller, 1984; Nelson and others, 1985). On Knight Island, xenoliths 

of peridotite in sheeted dikes were observed by Richter (1965) but not by subsequent workers (Tysdal and Case, 1979; Nelson and others (1985). However, Nelson and others (1985) 

mapped three peridotite bodies; two lie within or near a shear zone in the sheeted dikes unit 

(Tod), the other occurs as a xenolith in sheeted dikes unit. Weathers orangish brown and 

forms subdued rubble outcrops

Tmu Mafic and ultramafic plutonic rocks (Eocene and Paleocene?)—“Compositionally and texturally variable unit consists mainly of clinopyroxene gabbro, two-pyroxene gabbro, and 

local diabase, hornblende gabbro, peridotite, and orthopyroxenite. Gabbro commonly is 

medium-grained and equigranular and is slightly to moderately foliated except near contacts 

with country rocks where it locally is strongly foliated or mylonitic” (Winkler and Plafker, 

1993). In Cordova quadrangle, near head of Port Fidalgo, Nelson and others (1985) mapped 

a small area of blocks of both banded and massive dark- to light-green serpentinized dunite 

and peridotite that crop out within metasedimentary rocks unit of Valdez Group (Kvs). 

Strongly sheared margins in dunite and peridotite suggest blocks have been tectonically 

emplaced (Nelson and others, 1985). Within country rocks of Orca Group, the intrusions 

commonly have well-developed thermal aureoles; in sedimentary rocks, intrusions have 

prominent zones of iron-staining (Winkler and Plafker, 1993). Winkler and Plafker (1993) 

considered age of unit as uncertain; two hornblende K-Ar ages of 64.5±2.1 and 48.5±1.5 Ma 

were reported; the older age was considered suspect because it is older than the Orca Group, 

which unit Tmu intrudes

TKd Dikes (early Tertiary to Early Cretaceous)—Dikes of dacite, rhyolite, andesite, and rare 

basalt intrude rocks of the Valdez Group (Kvs) and McHugh Complex (KMm) (Bradley 

and others, 1999). Analyzed samples of dikes yield a wide range of ages, as shown in table 
26

1. A basaltic-andesite dike intruding McHugh Complex yielded an 40Ar/39Ar hornblende 

plateau age of 115±2 Ma, whereas an intermediate composition dike yielded an 40Ar/39Ar 

isochron(?) age of 57.0±0.22 Ma (W. Clendenin, written commun., cited in Bradley and 

others, 1999). Felsic dikes mapped by Nelson and others (1999) yielded a 40Ar/39Ar isochron age as young as 31.1±0.2 Ma on potassium feldspar, as well as a 40Ar/39Ar plateau of 

38.6±0.6 Ma on biotite and a 40Ar/39Ar disturbed age of 40±1 Ma, also on biotite. Lytwyn 

and others (2000) described a suite of dikes ranging in composition from basalt to rhyolite 

that they associated with the near-trench intrusions commonly related to the subduction of 

the Kula-Farallon spreading center; a 40Ar/39Ar plateau amphibole age on andesite from this 

suite yielded an age of 58.64±0.52 Ma. Lytwyn and others (2000) considered the dikes they 

studied to represent the same system as the plutons we have included in map unit Tgh here. 

Even discounting the potassium feldspar age as a minimum age, data indicate that the dikes 

include rocks of multiple ages, possibly representing multiple events throughout the region

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29

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the Cordova and Middleton Island quadrangles, southern 

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and MacKevett, E.M., Jr., 1981, Geologic map and summary geochronology of the Valdez quadrangle, southern Alaska: U.S. Geological Survey Open-File Report 

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31

of the Orca Group, Prince William Sound, southern Alaska, 

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Engineering seismology: U.S. Department of Commerce, 

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Geodetic Survey, Publication 10–3, v. 2, part A, 392 p.
32

Table 1. Radiometric ages from the Prince William Sound and Cook Inlet region.

[Table sorted by map unit, from youngest to oldest. 1:250,000-scale quadrangles: BS, Blying Sound; CV, Cordova; KN, Kenai; MI, Middleton Island; SR, Seward; SV, Seldov

Unit

sampled

Sample

number

Quad. Latitude

° N

Longitude

° W

Rock type Method Mineral Age

(Ma)

Error

(m.y.)

Notes 

Qad 78AR 290DK KN 60.7167 152.6667 Andesite K-Ar Whole-rock 0.763 0.017 Dark band in banded andesite; minimum age 

Qad 78AR 290LT KN 60.7167 152.6667 Andesite K-Ar Whole-rock 0.627 0.024 Light band in banded andesite; 

minimum age

Qad 90AR 99 KN 60.77 152.7 Andesite K-Ar Whole-rock 0.887 0.015 Estimated latitude and longitude 

Ts 7-14-73-3 KN 60.0683 151.6467 Ash Fission-track Zircon 8.5 1.0 Kenai Peninsula. Age revised in 

Turner and others, 1980.

Ts 7-14-73-3 KN 60.0683 151.6467 Ash K-Ar Plagioclase 8.9 1.0 Kenai Peninsula. Age revised in 

Turner and others, 1980.

Ts 7-13-73-9 KN 60.1858 151.4617 Ash K-Ar Plagioclase 7.0 0.7 Kenai Peninsula, location revised. 

Age revised in Turner and others, 1980.

Ts 7-13-73-6 KN 60.205 151.4333 Ash K-Ar Hornblende

Plagioclase

7.48.8

0.70.5

Do. 

Ts DT75-200 KN 60.2358 151.4008 Ash Fission-track Zircon 6.60 0.7 Sterling Formation, Clamgulchian 

Stage

Ts DT75-201 KN 60.205 151.435 Tuff K-Ar Hornblende

Plagioclase

5.05.9

0.80.5

Crystal tuff. Mean of 2 splits for both minerals.

Ts DT75-208 SV 59.7967 151.1117 Tuff K-Ar Hornblende

Plagioclase

4.74.2

0.61.4

35m stratigraphically higher than DT75-206.

Ts DT75-207 SV 59.7967 151.1117 Tuff Fission-track Zircon 4.9 0.8 24m stratigraphically higher than 

DT75-206.

Ts DT75-207 SV 59.7967 151.1117 Tuff K-Ar Plagioclase 7.5

7.6

0.60.6

24m stratigraphically higher than DT75-206. Believed to be too old duto detrital contamination.

Ts DT75-206 SV 59.7967 151.1117 Ash Fission-track Zircon 5.6 0.9 Composite of four thin beds. 

Ts DT75-206 SV 59.7967 151.1117 Ash K-Ar Plagioclase 4.6

8.4

0.70.7

Composite of four thin ash beds. 8.4 Ma age may be too old due to detritacontamination.

Ts? DT75-209b SV 59.7967 151.1083 Tuff K-Ar Plagioclase 9.1

8.2

0.70.8

Sterling? Formation. Age may be too old due to detrital contamination.

Ts DT75-212 SV 59.77 151.1333 Ash K-Ar Plagioclase 33m stratigraphically above DT75-

211. Ages may be too old due to detrital contamination.

Ts DT75-211 SV 59.77 151.1333 Ash Fission-track Zircon 5.4 0.6 20-40m stratigraphically above 

DT75-210. 

Ts DT75-211 SV 59.77 151.1333 Ash K-Ar Plagioclase 11.7

14.1

0.70.7

20-40m stratigraphically above DT75-210. Ages may be too old due to detrital contamination.

Ts DT75-210 SV 59.77 151.1333 Ash K-Ar Plagioclase 7.2 0.6 Sterling Formation, Clamgulchian 

Stage
33

Table 1. Radiometric ages from the Prince William Sound and Cook Inlet region — continued

Unit

sampled

Sample

number

Quad. Latitude

° N

Longitude

° W

Rock type Method Mineral Age

(Ma)

Error

(m.y.)

Notes 

Ts 6-25-77-1 SV 59.8233 151.0417 Ash K-Ar Plagioclase 6.9 0.5 Sterling Formation, Clamgulchian 

Stage

Tb 7-21-73-1 SV 59.7683 151.1575 Ash Fission-track Zircon 8.8 1.0 Age revised in Turner and others, 

1980.

Tb? 7-21-73-1 SV 59.7683 151.1575 Ash K-Ar Plagioclase 11.3 0.7 Beluga? Formation. Age revised in 

Turner and others, 1980.

Tb? 7-21-73-5 SV 59.7058 151.28 Ash Fission-track Zircon 8.1 1.0 Beluga? Formation. Age revised in 

Turner and others, 1980.

Tb DT75-203 SV 59.7033 151.2783 Tuff Fission-track Apatite 12.9 5.1 Crystal vitric tuff mostly altered to 

clay.

Tb DT75-203 SV 59.7033 151.2783 Tuff K-Ar Plagioclase 8.8 0.9 Crystal vitric tuff mostly altered to 

clay. Mean of 2 analyses

Tb DT75-202 SV 59.71 151.2383 Tuff K-Ar Plagioclase 7.2 1.3 Crystal vitric tuff. 

Tb DT75-204 SV 59.7117 151.2683 Ash Fission-track Zircon 7.6 0.7 

Tb DT75-204 SV 59.7117 151.2683 Ash K-Ar Plagioclase 8.1 0.7 Mean of 2 analyses 

Tb? 7-22-73-4 SV 59.7617 151.1692 Ash Fission-track Zircon 8.1 1.0 Beluga? Formation. Age revised in 

Turner and others, 1980.

Tb? 7-22-73-4 SV 59.7617 151.1692 Ash K-Ar Plagioclase 8.1 0.8 Beluga? Formation. Age revised in 

Turner and others, 1980.

Tovs 71APr 23A CV 60.8675 146.1692 Amphibolite K-Ar Hornblende 47.60 1.4 Greenschist facies amphibolite, location revised. 

Tdc 80AMH 159A MI 59.7969 144.5864 Dacite K-Ar Whole-rock 6.2 .3 Dacite of Cape St. Elias. 

Tpv 72APr 76A1 MI 59.9133 144.3817 Basalt K-Ar Whole-rock 31.20 1.3 Basaltic dike from Kayak Island. 

Analytical data does not match reported age.

Tm 72APr 22A1 CV 60.0000 144.1958 Diorite K-Ar Hornblende 200.5 29 Recalculated using constants of 

Steiger and Jager, 1977. Age highly suspect, too old.

Tov 72APr 2A3 CV 60.5567 145.7517 Basalt K-Ar Whole-rock 43.00 6.1 Recalculated using constants of 

Steiger and Jager, 1977.

Tov 83APr 312 CV 60.9750 146.7183 Diabase K-Ar Whole-rock 130.00 14.0 Johnstone Cove. Recalculated using 

constants of Steiger and Jager, 1977. Age highly suspect due to excess argon.

Tod 92SN 5a SR 60.8722 147.179 Mafic dike 40/39 Plateau Plagioclase 37.6 0.6 Sheeted dike from ophiolite, approxi

mate location. 

Tod 81BS 011A BS 59.953 149.268 Basaltic rock K-Ar Whole-rock 49.6 2.5 

Tod 81BS 011B BS 59.953 149.268 Basaltic rock K-Ar Whole-rock 45.7 2.3 

Tod 81BS 013K BS 59.883 149.278 Basaltic rock K-Ar Whole-rock 47.3 2.4 

Tod 81BS 017B SR 60.350 147.6484 Mafic rock K-Ar Whole-rock 35 1.3 
34

Table 1. Radiometric ages from the Prince William Sound and Cook Inlet region — continued

Unit

sampled

Sample

number

Quad. Latitude

° N

Longitude

° W

Rock type Method Mineral Age

(Ma)

Error

(m.y.)

Notes 

Tod 81BS 034E SR 60.373 147.708 Mafic rock K-Ar Whole-rock 38.8 1.9 

Togb n.a. BS 59.9491 149.2228 Plagio-granite U-Pb Zircon 57 n.a. Plagiogranite intruding gabbro. 

Tg PW8 SR 60.4503 148.1063 Quartz diorite K-Ar Biotite

Hornblende

37.135.3

1.01.2

Eshamy Granite, age recalculated using constants of Steiger and Jager, 1977.

Tg PW9 SR 60.4917 148.3833 Quartz

monzonite

K-Ar Biotite 37.1 0.9 Nellie Juan Granite, age recalculated 

using constants of Steiger and Jager, 1977.

Tg PW2 SR 60.8227 148.0752 Granodiorite K-Ar Biotite 36.4 .9 Esther Granite, age recalculated usin

constants of Steiger and Jager, 1977.

Tg 92SN 14 SR 60.9772 147.1485 Granite 40/39 Plateau K-feldspar 29.2 0.1 Granite Cove. K-feldspar is suspect 

material for dating, probably too young.

Tg 92SN 12 SR 60.9817 147.42 Granite 40/39 Isochron K-feldspar 31.7 0.1 Cedar Bay. K-feldspar is suspect 

material for dating, probably too young.

Tg 80ANS 60A SR 60.74 147.9578 Granitic rock K-Ar Whole-rock 34.2 1.7 Granite/granodiorite on Perry Island

Inherently suspect because it is a whole-rock age on a granitic rock.

Tgd PW1 SR 60.8325 148.4675 Granodiorite K-Ar Biotite 37.6 1.0 Granite of Passage Canal, age recalculated using constants of Steiger an

Jager, 1977.

Tgg 85APr 140 CV 60.5383 145.1867 Dacite K-Ar Whole-rock 47.23 n.a. Dacite porphyry. 

Tgg 84APr 9 CV 60.5733 145.2517 Granodiorite K-Ar Biotite 51.4 1.5 North of McKinley Peak. 

Tgg 84APr 8 CV 60.7017 145.4867 Granodiorite K-Ar Biotite

Hornblende

52.349.9

1.61.5

Near head of Orca Inlet. Hornblende age is considered a minimum age.

Tgg 80ANS 148A CV 60.589 144.4096 Granitic rock K-Ar Biotite

Hornblende

52.751.3

1.62.9

South side of Miles Glacier. 

Tgg 67APr 1 CV 60.465 145.299 Granitic rock K-Ar Phlogopite 51.6 2 Northwest of McKinley Peak; location revised. 

Tgg 71APr 25C CV 60.4883 144.385 Granitic rock K-Ar Hornblende 53.5 1.6 

Tgg n.a. CV 60.4883 144.395 Granodiorite K-Ar Biotite 52.2 n.a. 

Tgg 71APr 20B CV 60.61 144.19 Granitoid K-Ar Hornblende 50.6 1.5 
35

Table 1. Radiometric ages from the Prince William Sound and Cook Inlet region — continued

Unit

sampled

Sample

number

Quad. Latitude

° N

Longitude

° W

Rock type Method Mineral Age

(Ma)

Error

(m.y.)

Notes 

Tgg 71APr 20C CV 60.6617 144.1783 Tonalite K-Ar Biotite 50.9 1.5 

Tgg 71APr 22c CV 60.7067 146.1283 Granodiorite K-Ar Biotite

Hornblende

53.250.5

1.61.5

Location revised. 

Tgg 94APo 361 CV 60.7636 144.1897 Tonalite U-Pb Zircon See 

footnote

Sill, Van Cleve Glacier. 

Tgg 94APo 361 CV 60.7636 144.1897 Tonalite U-Pb Monazite 54.3

54.454.0

n.a. Sill, Van Cleve Glacier. Age dependent on size fraction, 207Pb/235U 

reported.

Tgg 94APo 361 CV 60.7636 144.1897 Tonalite U-Pb Xenotime See 

footnote

Sill, Van Cleve Glacier. 

Tgg 94APo 471 CV 60.7625 144.2120 Tonalite U-Pb Zircon See 

footnote

Van Cleve Glacier area. 

Tgh 88ACy 9 SV 59.4733 150.3367 Granodiorite U-Pb Monazite 56.0 0.5 

Tgh 92AKu 71b SV 59.9774 150.1611 Granodiorite 40/39 Plateau Biotite 53.2 1.1 Tustemena pluton. 

Tgh 88ACy 9 SV 59.4733 150.3367 Granodiorite 40/39 Isochron Biotite 54.2 0.08 Nuka pluton. 

Tgh 88ACy 9 SV 59.4733 150.3367 Granodiorite U-Pb Monazite 56 0.5 Nuka pluton. 

Tgh 91ADw 55g SV 59.8828 150.4638 Granodiorite 40/39 Biotite 54.2 1.1 Chernof Stock. 

Tgh 91AKu 3 BS 59.7917 149.9917 Granodiorite 40/39 Plateau Biotite 51.1

53.549.652.2

0.50.51.70.9

Aialik pluton, three biotite analyses and weighted mean age. Quadrangle incorrect in source.

Tgh n.a. SV 59.7083 150.135 Granodiorite 40/39 Plateau n.a. 53.4 1.5 Paguna stock, approximate location. 

Tgh n.a. BS 59.7067 149.5233 Granite K-Ar Biotite

Muscovite

61.056.0

1.81.6

Granite of Harding Icefield region, southern tip of Aialik Peninsula. Age recalculated using constants of Steiger and Jager, 1977.

Tgh 91AKu 3 BS 59.7917 149.9917 Granodiorite 40/39 Biotite 52.2 0.9 Aialik pluton, Harris Bay. 

Tgh 95TD 2b BS 59.89 149.3733 Granodiorite 40/39 Biotite 53.3

53.553.453.4

0.61.00.50.4

Hive Island pluton. Three biotite analyses and weighted mean age.

Tgh 92PH 454B SV 59.5428 150.1807 Granite 40/39 Plateau Biotite 53.7 0.1 Thunder Bay granitic sill, slightly 

disturbed spectrum.

Tgh?/TKd

90ADW 809 SV 59.3430 151.8212 Andesite 40/39 Plateau Amphibole 58.64 0.52 Andesite dike, might have intruded 

Port Graham formation near Port Graham. Age may be suspect due to excess argon.

Tmu 85ANk 96B CV 60.6967 145.5183 Gabbro K-Ar Hornblende 64.5 2.1 Suspect, age is older than country 

rock which gabbro intrudes.
36

Table 1. Radiometric ages from the Prince William Sound and Cook Inlet region — continued

Unit

sampled

Sample

number

Quad. Latitude

° N

Longitude

° W

Rock type Method Mineral Age

(Ma)

Error

(m.y.)

Notes 

Tmu 85APr 177 CV 60.6762 145.4978 Altered 

gabbro

K-Ar Hornblende 48.5 1.5 Altered clinopyroxene gabbro in 

written report, reported as hornblendgabbro on map.

TKgd 70AR 146 KN 60.7433 152.9583 Granodiorite K-Ar Biotite 68.2 1.9 Age recalculated using constants of 

Steiger and Jager, 1977.

TKgd 70AR 147 KN 60.865 152.68 Granodiorite K-Ar Biotite

Hornblende

65.467.6

1.82.0

Age recalculated using constants of Steiger and Jager, 1977.

TKgd 70AR 140 KN 60.9433 152.8617 Granodiorite K-Ar Biotite

Hornblende

65.170.5

1.92.0

Age recalculated using constants of Steiger and Jager. 1977.

TKd 92SN 4 SR 60.9867 147.5138 Porphyry 40/39 Isochron K-feldspar 31.1 0.2 Unakwik Inlet, approximate location

K-feldspar is suspect material for dating, probably too young.

TKd 81BS 116C SR 60.6155 149.5680 Felsic igneous 

rock

K-Ar Whole-rock 52.5 1.6 Dike, near Gilpatrick Mountain, cutting Valdez Group (Kvs). 

TKd 88ADw 230 SV 59.3933 150.665 Intermediate 

igneous rock

40/39 Isochron Hornblende 57 0.22 Leucocratic porphyry dike containin

feldspar and hornblende phenocrystscuts the McHugh Complex near head of Seldovia Bay.

TKd 93ASB 66 SV 59.3938 151.2186 Basaltic 

andesite

40/39 Plateau Hornblende 115 1.7 Hornblende-phyric basaltic andesite 

dike cutting graywacke of McHugh Complex 4 km SE of the head of Tutka Bay.

Kqd 70AR 158 KN 60.8317 152.5817 Quartz diorite K-Ar Biotite

Hornblende

70.074.4

2.02.2

Age recalculated using constants of Steiger and Jager, 1977.

Kqd 70AR 179 KN 60.985 152.28 Quartz diorite K-Ar Biotite

Hornblende

71.370.6

2.02.1

Age recalculated using constants of Steiger and Jager, 1977.

Kqd 70AR 173 KN 60.7483 152.8117 Diorite K-Ar Biotite

Hornblende

67.271.5

1.92.1

Age recalculated using constants of Steiger and Jager, 1977.

Kvv 81BS 001A BS 59.929 149.324 Metatuff K-Ar Whole-rock 54.4 2.7 Fox Island. 

Kvgs 81KMS 25 SR 60.3974 149.2185 Biotite semischist K-Ar Unknown 51.5 1.5 Metamorphic age of Valdez Group schist unit (Kvgs) along Placer Rive

Fault.

Jnc 62ALe 6e KN 60.1133 152.585 Quartz diorite K-Ar Biotite

Hornblende

156.6159.7

n.a.n.a.

Rounded granitic boulders within the Chisik Conglomerate Member of the Naknek Formation. Recalculated age using constants of Steiger and Jager, 1977.

Jg 62ALe 5 KN 60.2533 152.886 Granodiorite K-Ar Biotite

Hornblende

174.0171.9

n.a.n.a.

Alaska-Aleutian Range batholith, recalculated using constants of Steiger and Jager, 1977
37

Table 1. Radiometric ages from the Prince William Sound and Cook Inlet region — continued

Unit

sampled

Sample

number

Quad. Latitude

° N

Longitude

° W

Rock type Method Mineral Age

(Ma)

Error

(m.y.)

Notes 

Jg 70AR 175 KN 60.59 152.78 Granodiorite K-Ar Biotite 97.8 2.8 Age recalculated using constants of 

Steiger and Jager, 1977. Age suspectmay be reset by younger plutonism.

Jg 70AR 177 KN 60.615 152.6283 Quartz diorite K-Ar Biotite

Hornblende

163.0161.0

4.74.7

Age recalculated using constants of Steiger and Jager, 1977.

Jg 70AR 178 KN 60.6767 152.4517 Granodiorite K-Ar Biotite

Hornblende

165.0162.0

4.84.8

Age recalculated using constants of Steiger and Jager, 1977.

Jg 66AR 1464 KN 60.8033 152.355 Quartz monzonite K-Ar Biotite 159.0 n.a. Age recalculated using constants of Steiger and Jager, 1977. 

Jg 70AR 156 KN 60.805 152.4917 Diorite K-Ar Hornblende 146.0 4.3 Age recalculated using constants of 

Steiger and Jager, 1977.

Jsch 7AF23-9 SV 59.0033 151.7067 Schist K-Ar Muscovite 194.7 5.7 Quartz-mica schist, approximate 

location, age recalculated using constants of Steiger and Jager, 1977.

Jsch 74PG79 SV 59.37 151.81 Schist K-Ar Muscovite 196.1 5.8 Quartz-mica schist, approximate 

location, age recalculated using constants of Steiger and Jager, 1977.

Jsch SD3-3 SV 59.4617 151.7067 Greenschist K-Ar Actinolite

ChloriteWhite mica

195.0185.1192.3

11.08.310.0

Approximate corrected location, con

stants of Steiger and Jager, 1977. 

Jsch 74AF4B.1 SV 59.4617 151.7067 Schist K-Ar Crossite 166.8 4.9 Blueschist, crossite date, approximat

location, recalculated using constantof Steiger and Jager, 1977.

Jsch 74AF23-10 SV 59.4617 151.7067 Schist K-Ar Amphibole

Muscovite

188.4196.6

5.55.8

Amphibole-mica schist, approximate location, amphibole is average of 2 splits, age recalculated using constants of Steiger and Jager, 1977.

Jsch SD9-3 SV 59.4667 151.7333 Schist K-Ar Crossite

Phengite

157.8192.9

4.85.7

Blueschist, crossite date may reflect argon loss, approximate location, recalculated using constants of Steiger and Jager, 1977.

Jsch 92ATi 316D SV 59.4479 151.7145 Schist 40/39 Plateau White mica 190.98 0.3 Seldovia metamorphic complex. 

Jsch 92ATi 309B SV 59.4547 151.7153 Schist 40/39 Plateau Barroisite 191.92 0.6 Seldovia metamorphic complex, 

quartz-white mica-chlorite schist

Jsch 92ATi 309B SV 59.4547 151.7153 Schist 40/39 Plateau Muscovite 191.7 0.3 Seldovia metamorphic complex, 

quartz-white mica-chlorite schist

Mzg 91DW 87 SV 59.6075 151.1378 Gabbro U-Pb Zircon 227.7 0.6 Gabbro of Halibut Cove. Sample 

number uncertain.
38

Table 1. Radiometric ages from the Prince William Sound and Cook Inlet region — continued

Unit

sampled

Sample

number

Quad. Latitude

° N

Longitude

° W

Rock type Method Mineral Age

(Ma)

Error

(m.y.)

Notes 

Dates recording hydrothermal alteration

-- 92PH 216f SR 60.972 148.2112 Granite 40/39 White mica >53 -- Granite Mine, hydrothermally altered 

granite. 30% of mica was light green (chlorite?); analysis shows argon losspectra, no plateau.

-- n.a. SV 59.3195 151.2917 Felsic dike 40/39 White mica 57.3 0.1 Port Dick prospect, preferred hydrothermal alteration date from single 

gas release step (1000° C) and 67.8 percent of gas.

-- n.a. SV 59.5392 150.4780 n.a. 40/39 Plateau Sericite 55.6 0.1 Beauty Bay Mine, altered rock, clear 

sericite.

-- n.a. SV 59.5392 150.4780 n.a. 40/39 Sericite 55.9 0.1 Beauty Bay Mine, altered rocks, 

gold-colored sericite. Preferred date based on 37.2 percent of gas, disturbed spectra.

-- n.a. SV 59.5428 150.1800 Vein 40/39 Plateau White mica 52.9 0.1 Thunder Bay gold occurrence, polymetalllic gold-sulfide vein. 

TKd 2203N SR 60.6667 149.75 Felsic igneous 

rock

K-Ar Whole-rock 52.7 1.6 Kenai Star Mine(?), hydrothermally 

altered felsic dike, approximate location.

TKd 2237B SR 60.8 149.667 Granite K-Ar Muscovite 53.2 1.6 Hydrothermally altered albite-granite 

dike, approximate location.

Tg? 92PH 216e SR 60.972 148.2112 Granite 40/39 Biotite 56. 1. Granite Mine, hydrothermally altered 

granite. Disturbed spectrum, chlorite present, date is weighted average and of low confidence.

1These U-Pb data are of minimal and generally unusable quality. All fractions of zircons contain significant inheritance, severely complicating interpretations. Regressions thlower intercepts have huge uncertainties due to a combination of inheritance and subsequent Pb-loss. The monazite data probably are good. However, the age should be calcu

there is frequently excess 206Pb in young monazite, making the 206Pb/238U age too old and the 207Pb/206Pb age too young). Thus, the age of 94APo36 probably is about 54 M

without uncertainties). From the 206Pb/238U ages of zircons from the other samples, a crude estimate of an age constraint for 94APo47 is <62 Ma. In summary, the age for 94

Ma. All other ages are relatively unknown. Thus, any tectonic scenario from these age data are untenable (J.N. Aleinikoff, USGS, written commun., 2007).