2012

U.S. Department of the Interior

U.S. Geological Survey

Prepared in cooperation with the Alaska Department of Natural Resources Division of Oil and Gas

Geologic Map of the Cook Inlet Region, Alaska

Including parts of the Talkeetna, Talkeetna Mountains, Tyonek, Anchorage, Lake Clark, Kenai, Seward, 

Iliamna, Seldovia, Mount Katmai, and Afognak 1:250,000-scale quadrangles

Compiled by Frederic H. Wilson, Chad P. Hults, Henry R. Schmoll, Peter J. Haeussler, Jeanine M. Schmidt, 

Lynn A. Yehle, and Keith A. Labay

Pamphlet to accompany

Scientific Investigations Map 3153

Aleutian Range mountains south of Chakachamna Lake made up of Tertiary to Cretaceous quartz diorite of the 

Alaska-Aleutian Range Batholith.
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i

Contents

Conversion Factors .......................................................................................................................................ii

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

Acknowledgments ........................................................................................................................................1

Physiographic Setting ..................................................................................................................................1

Regional Framework .....................................................................................................................................2

Description of Map Units .............................................................................................................................4

Unconsolidated Deposits ...................................................................................................................4

Sedimentary Rocks ..............................................................................................................................9

Igneous Rocks ....................................................................................................................................22

Volcanic and Hypabyssal Rocks ............................................................................................22

Plutonic Rocks ...........................................................................................................................26

Mélange and Metamorphic Rocks .................................................................................................30

Bedrock ...............................................................................................................................................35 

References Cited .........................................................................................................................................35

Figure

1. Major faults in the map area ...................................................................................................................3

Table

1. Radiometric age data for the map of the Cook Inlet Region, south central Alaska .....................46
ii

Conversion Factors

Inch/Pound to SI

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

mile, nautical (nmi) 1.852 kilometer (km)

yard (yd) 0.9144 meter (m)

Area

acre 4,047 square meter (m2)

acre 0.4047 hectare (ha)

acre 0.4047 square hectometer (hm2)

acre 0.004047 square kilometer (km2)

square foot (ft2) 929.0 square centimeter (cm2)

square inch (in2) 0.09290 square meter (m2)

square foot (ft2) 6.452 square centimeter (cm2)

section (640 acres or 1 square mile) 259.0 square hectometer (hm2)

square mile (mi2) 259.0 hectare (ha)

square mile (mi2) 2.590 square kilometer (km2)
1

Introduction

In 1976, L.B. Magoon, W.L. Adkinson, and R.M. Egbert 

(Magoon and others, 1976) published a major geologic map 

of the Cook Inlet region, which has served well as a compilation of existing information and a guide for future research and 

mapping. The map in this report updates Magoon and others 

(1976) and incorporates new and additional mapping and interpretation. This map is also a revision of areas of overlap with 

the geologic map completed for central Alaska (Wilson and 

others, 1998). Text from that compilation remains appropriate 

and is summarized here; many compromises have been made in 

strongly held beliefs to allow construction of this compilation. 

Yet our willingness to make interpretations and compromises 

does not allow resolution of all mapping conflicts. More time 

and fieldwork may allow resolution of these conflicts. Nonetheless, we hope that geologists who have mapped in this region 

will recognize that in incorporating their work, our regional 

correlations may have required some generalization or lumping 

of map units (Wilson and others, 1998).

Many sources were used to produce this geologic map and 

in most cases, data from available maps were combined, without generalization, and new data added where available. Geology of the Mount Katmai and mainland part of the Afognak 

1:250,000-scale quadrangles is derived from Riehle and others 

(1987; 1993) and Riehle and Detterman (1993); the island part 

of the Afognak quadrangle is from Wilson (in press). Geology 

of the Iliamna, Lake Clark, and western Kenai quadrangles 

was derived from Wilson and others (in press) which, in turn 

was based on Detterman and Reed (1980), Nelson and others 

(1983), and Magoon and others (1976). Geology of the eastern Kenai and Seldovia quadrangles was derived from Wilson 

and others (2008), largely based on Bradley and others (1999), 

Karlstrom (1964), Bradley and Wilson (2000), and Magoon and 

others (1976). The Tyonek quadrangle is based on Magoon and 

others (1976) and is highly modified by more recent mapping 

by Solie and others (1991) and compilation by H.R. Schmoll, 

and sampling and analysis by P.J. Haeussler and D.C. Bradley 

(USGS). Geology of the Talkeetna quadrangle is from Reed 

and Nelson (1980) with additions from Weber (1961); geology of the Talkeetna Mountains quadrangle is based on Csejtey 

and others (1978) and Werdon and others (2002) and is highly 

modified by J.M. Schmidt on the basis of more recent mapping 

and interpretation. Finally, Anchorage quadrangle bedrock geology is largely from Winkler (1992). Mapping surficial deposits at scales of 1:63,360 and 1:25,000 during the last several 

decades was compiled by H.R. Schmoll from the following 

sources: Daniels (1981a,b); Kopczynski (2008); Kopczynski, S.E., Reger, R.D., and Evenson, E.B., unpub. data, 2009; 

Reger (1981a,b,c,d); Reger and others (1994a,b,c,d); Schmoll 

and Dobrovolny (1972, 1993); Schmoll and Gardner (1982); 

Schmoll and Yehle (1987, 1992); Schmoll and others (1981, 

1996, 1999); Trainer (1960); Trainer and Waller (1965); Updike 

and Ulery (1983); Updike and others (1988); Yehle and Schmoll 

(1987a,b, 1988, 1989, 1994); Yehle and others (1983a,b,c, 

1990, 1991, 1992), as well as unpublished mapping by Schmoll 

and Dobrovolny (Anchorage and Seward) and Yehle, Schmoll, 

Gardner, and J.K. Odum [USGS] (Tyonek). Mapping at smaller 

scales to the north and south (Weber, 1961; Reed and Nelson, 

1980; Karlstrom, 1964; Riehle and Emmel, 1980; F.H. Wilson, 

unpub. data, 2009; Detterman and Reed, 1973; Riehle and Detterman, 1993) was compiled by F.H. Wilson and C.P. Hults.

Rock unit descriptions are grouped as sedimentary, igneous, or metamorphic and listed in chronological order from 

youngest to oldest; upper and lower denote stratigraphic position, whereas early and late indicate age. Metamorphic rock 

units are listed in order of increasing age of their inferred or 

interpreted protolith age, which in many cases is subject to significant uncertainty. If we cannot interpret a protolith age, then 

we list the metamorphic units by increasing known or inferred 

age of metamorphism. All reported radiometric ages are listed 

in accompanying table 1. Unconsolidated units are subdivided 

by genetic origin; however, the boundaries are rarely sharp or 

regionally constant between units. The unconsolidated surficial 

deposits, undivided (unit Qs) contains a number of subunits 

that can be mapped only on a local basis, and are not shown 

separately here.

A preliminary version of this map was published as a U.S. 

Geological Survey Open-File Report (Wilson and others, 2009). 

The main differences between the versions concern revised 

mapping of surfical deposits in the northern and eastern parts of 

the map area. Minor error corrections have been made also.

Acknowledgments

A compilation on this scale could only be done with the 

assistance of many geologists, far more knowledgeable about 

the geology of various regions of Alaska than we are. We thank 

Chris Nye, Emily Finzel, and Dwight Bradley for discussions 

and information that assisted our compilation effort. The 

Alaska Department of Natural Resources, Division of Oil and 

Gas provided strong encouragement and financial support 

to assist with the preparation of this map; its assistance was 

invaluable. Technical review of the manuscript by Les Magoon, 

Michael Fisher, and Richard Stanley and editorial review by 

Theresa Iki was greatly appreciated and helped to improve the 

text and map.

Physiographic Setting

The map area borders the northern part of the Gulf of 

Alaska, a region heavily glaciated during Quaternary time. 

From west to east, the region extends from high glaciated 

mountains of the Aleutian Range through coastal lowlands of 

the Cook Inlet basin to glaciated coastal mountains on the west 

side of Prince William Sound. From north to south, the region 

extends from the Talkeetna Mountains and southern Alaska 

Range to the northern islands of the Kodiak Island archipelago. 

Throughout the mapped region, relief is generally high except 

for the lowlands of the Susitna River valley and the western 

Kenai Peninsula. Volcanoes on the west side of Cook Inlet, 

notably Mount Spurr (3,383 m), Redoubt Volcano (3,108 m), 

Iliamna Volcano (3,053 m), and Mount Douglas (2,140 m) are 
2

prominent physiographic features, as is Augustine Volcano (or 

Mount Saint Augustine) (1,227 m), an active volcano on Augustine Island in Cook Inlet. Relief on the east side of Cook Inlet 

is somewhat less, only locally exceeding 2,000 m. The mountains on the west side of the map area are part of the Alaska 

and Aleutian Ranges; the transition in name between these two 

mountain ranges is approximately at Chakachamna Lake, and 

in common usage, the mountains along the west side of Cook 

Inlet are called the Alaska-Aleutian Range. The Talkeetna 

Mountains, part of the Alaska Range, form the eastern boundary of the map area north of the Matanuska River. To the south 

of the Matanuska River, the map area is bounded on the east by 

the Chugach and Kenai Mountains, generally considered part of 

the “Coast” Range. The higher peaks in the region are presently 

glaciated; the Harding Icefield is a major center of glaciation in 

the Kenai Mountains, and a number of other areas of ice accumulation are found on the west side of Cook Inlet. Comparison 

between mid-1950’s aerial photographs and 2006 satellite imagery of ice-covered areas west of Cook Inlet indicates approximately 13 percent loss of surface area of glaciers in the 50-year 

time period (Wilson and Labay, unpub. data, 2007).

Regional Framework

Cook Inlet fills a largely Tertiary basin lying between 

two accreted belts of Mesozoic and younger rocks along the 

southern Alaska margin. Partially filling the basin and exposed 

on the east and west sides of the basin is a sequence of largely 

continental Tertiary sedimentary rocks as much as 7,000 m 

thick (Kirschner and Lyon, 1973; Fisher and Magoon, 1978). 

These Tertiary rocks are important oil and gas reservoirs for 

petroleum thought to be sourced from the underlying Jurassic 

and Triassic rocks (Magoon and Claypool, 1981). Bounding the 

western, and to a lesser extent, the northern side of the basin 

are the Mesozoic and Cenozoic plutonic rocks of the AlaskaAleutian Range batholith; the associated volcanic rocks of the 

Early Jurassic Talkeetna magmatic arc; and sedimentary rocks, 

largely of Jurassic age, derived from erosion of the arc and 

Jurassic portion of the batholith. Emplaced through and overlying these Mesozoic and Cenozoic rocks are the volcanic rocks 

of the modern Aleutian magmatic arc. The eastern and southern 

sides of the Cook Inlet basin are bounded by Late Cretaceous 

accretionary flysch and associated mélange. Also on the eastern 

side of the basin is a narrow belt of early Mesozoic igneous and 

metamorphic rocks.

A prominent feature of the western part of the map area 

is the Alaska-Aleutian Range batholith of Reed and Lanphere 

(1969, 1972, 1973). The batholith is a multiphase intrusive 

rock complex of Jurassic, Cretaceous, and Tertiary ages. The 

Lake Clark Fault system bifurcates the batholith and no Jurassic 

intrusive bodies are present north or west of the fault. However, 

Cretaceous and Tertiary rocks are found on both sides of the 

fault. Jurassic rocks that are lithologically and chronologically 

equivalent to the Alaska-Aleutian Range batholith occur in the 

Talkeetna Mountains of the northeast part of the map, north of 

the Castle Mountain Fault system.

Late Triassic and Early Jurassic plutonic rocks of similar composition to the Alaska-Aleutian Range batholith, but 

of slightly older age occur between the Border Ranges Fault 

system and Castle Mountain-Caribou Fault system in the Matanuska Valley; these plutons continue southward parallel to the 

Border Ranges Fault on the east side of Cook Inlet, extending 

to the Triassic Afognak pluton of Roeske and others (1989) on 

Afognak Island. Plutons of Triassic age are unknown on the 

Alaska Peninsula.

Two units closely associated with the Late Triassic and 

Early Jurassic plutons are the informally named Port Graham 

and Pogibshi formations of Kelley (1980). These two rock 

units have commonly been correlated with the Kamishak and 

Talkeetna Formations; however, the Port Graham formation of 

Kelley (1980) is lithologically distinct from the equivalent age 

Kamishak Formation, and the Pogibshi formation of Kelley 

(1980) is older than the lithologically equivalent Talkeetna 

Formation. Additionally, rock units tectonically associated with 

these units are distinctly different from rock units on the west 

side of Cook Inlet or in the Talkeetna Mountains. Amongst 

the distinctive units are the Seldovia metamorphic complex of 

Bradley and others (1999) and similar metamorphic complexes 

such as the Raspberry Schist of Roeske and others (1989), in 

the Kodiak Island archipelago, and the Schist of Iceberg Lake 

and Schist of Liberty Creek of Winkler and others (1981), in 

south central Alaska. These metamorphic rock assemblages 

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 is spatially associated with the Border 

Ranges Fault system and shows evidence of blueschist facies 

metamorphism, whereas rocks typical of the Alaska Peninsula 

on the west side of Cook Inlet rarely show evidence of metamorphism. The Triassic plutons are uniquely associated with the 

Port Graham and Pogibshi formations of Kelley (1980) and the 

blueschist facies metamorphic complexes.

A number of major fault systems bound or cross-cut the 

Cook Inlet region (fig. 1). The Border Ranges Fault system (in 

the past, called the Knik Fault; Magoon and others, 1976) lies 

on the eastern side of the Cook Inlet basin and separates the 

Late Cretaceous flysch and mélange from the older Mesozoic 

rocks on the east side of Cook Inlet and Shelikof Strait. The 

Border Ranges Fault system is exposed northeast of the Anchorage area (Winkler, 1992), on the southeast part of the map near 

Seldovia (Bradley and others, 1999), and on Afognak Island 

(Wilson, in press). Recent analysis of aeromagnetic data (R.W. 

Saltus, USGS, oral commun., 9/22/2010) suggests that along the 

Kenai Peninsula, the Border Ranges Fault system dips eastward, 

under the rocks of the McHugh Complex (KMm), whereas elsewhere it is generally considered a vertical feature. The Castle 

Mountain Fault system cuts across the Susitna River valley 

north of Cook Inlet and continues through the Matanuska River 

valley (Magoon and others, 1976; Winkler, 1992). The eastern 

part of the Castle Mountain-Caribou Fault system separates 

rocks, equivalent to those on the Alaska Peninsula, on the north 

from Mesozoic and younger rocks lying within and south of the 

fault. The western part of the Castle Mountain-Caribou Fault 

system transitions to the Lake Clark Fault system west of where 

the Bruin Bay Fault system is thought to splay off. Here, south 
3

Figure 1. Major faults in the map area.

of the Lake Clark Fault system lie rocks of the Alaska Peninsula; north of the Lake Clark Fault system are younger rocks, 

of different character. Haeussler and Saltus (2005) suggested 26 

km of right-lateral strike-slip offset along the Lake Clark Fault 

since Eocene time southwest of the map area; whereas Plafker 

and others (1975) reported 5 km of right-lateral offset in the 

same region. Detterman and others (1976) reported that near the 

Chulitna River, the fault has vertical offset in a reverse sense of 

500 to 1,000 m and they could demonstrate no horizontal offset 

along the fault. Hults and Wilson (2009) postulated that an older 

strand of the Castle Mountain-Caribou Fault system extends 

from the Matanuska River valley through the Chakachamna 

Lake area. This strand may be an early Tertiary fault that dextrally offset the Alaska-Aleutian Range batholith and associated 

Mesozoic sedimentary rocks 160 to 190 kilometers. The West 

Foreland and Arkose Ridge Formations may be deposits filling a 

trans-tensional basin associated with motion along the Chakachamna-Castle Mountain Fault system.
4

DESCRIPTION OF MAP UNITS

UNCONSOLIDATED DEPOSITS

Qs Surficial deposits, undivided (Quaternary)—Unconsolidated silt, sand, and gravel of fluvial, 

glacial, colluvial, and other origins. Mainly unsorted boulders, cobbles, gravel, sand, and 

silt produced, deposited, and reworked by action of wind, water, glaciers, frost (including 

solifluction), and gravity. 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. In the vicinity of the Drift River delta (Kenai quadrangle), 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, 2007). 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 

The Bruin Bay Fault system splays southward from the 

Castle Mountain Fault system on the west side of the Cook 

Inlet. The Bruin Bay Fault system cuts rocks of the Alaska 

Peninsula into a western pluton-dominated block and an eastern 

sedimentary rock-dominated block. The western block served 

as a source terrain for the eastern sedimentary rock-block 

during Jurassic and Cretaceous time. During an early part of 

the development of the Bruin Bay Fault system, the fault was a 

normal(?) growth fault bounding the Jurassic depositional basin; 

during Tertiary time the fault was reactivated having thrust and 

possibly strike-slip motion, which is most apparent along its 

northern sections. Fisher and Magoon (1978) and Magoon and 

Claypool (1981) show in cross sections an inferred fault through 

Cook Inlet that separates the older Mesozoic rocks of the Kenai 

and Alaska Peninsulas.

Quaternary deposits of the Cook Inlet Basin are reported as 

nearly 1,300 m thick in the lower Susitna River valley (Freethey 

and Scully, 1980), but elsewhere, maximum thicknesses of 

200 m to 300 m are more common (Trainer and Waller, 1965; 

Schmoll and Barnwell, 1984; W.W. Barnwell, USGS, unpub. 

data, 1984); the deposits record a history dominated by repeated 

glaciation as well as important estuarine, lacustrine, and alluvial 

deposits (Karlstrom, 1964; Schmoll and Yehle, 1986; Reger 

and others, 2007). Unconsolidated deposits at the surface were 

mainly derived from retreating glaciers that had advanced into 

the basin from surrounding mountains in the late Pleistocene, 

perhaps at times, filling the basin with ice. During glacial 

retreat, an ancestral Cook Inlet estuary invaded the basin as sea 

level rose, interacting with the glacial deposits and the glaciers 

themselves to form a central basin complex or terrain, the 

details of which are as yet incompletely understood. An excellent detailed discussion of the Quaternary history of the Municipality of Anchorage as it relates to Cook Inlet can be found in 

Schmoll and others (1999). The Quaternary unconsolidated 

deposits occur in four terrains that occur mainly at different 

levels, and generally decrease in age:

1. High-level terrain includes thin glacial drift at scattered localities on mountain ridges and glacially planed 

surfaces; 

2. Basin-margin terrain includes the principal welldeveloped moraines and associated features that extend 

basinward from the adjacent mountains and stand 

plateau-like above the lower basin;

3. Central-basin terrain includes the glacioestuarine 

Bootlegger Cove Formation and related deposits that 

form a relatively flat platform with low hills that lies at 

intermediate level;

4. Low-level terrain includes mainly Holocene deposits 

in valleys and estuarine embayments graded to present 

sea level.

Quaternary volcanic rocks and deposits, prominent on the 

west side of the basin, occur as tephra throughout the region 

(Riehle, 1985).

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, p. 55) described “vertical displacement over an area of about 520,000 square kilometers. 

The major area of uplift trended northeast from the offshore 

of southern Kodiak Island to Prince William Sound and then 

trended east across the Prince William 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 subsidence covered about 

285,000 square kilometers, including the northern and western 

parts of Prince William Sound, the western 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), 

Eckel (1970), and Committee on the Alaska Earthquake of the 

Division of Earth Sciences National Research Council (1971), 

and related volumes. Within the city of Anchorage, significant damage to structures occurred as a result of liquefaction 

of fine-grained sediments of the Bootlegger Cove Formation 

along exposed faces (Hansen, 1965). Studies in Turnagain 

Arm, on Middleton Island, east of the map area and in Kenai 

Fjords National Park, southeast of the map area (Plafker and 

Rubin, 1978, Bartsch-Winkler and Schmoll, 1992; Combellick, 1991a,b, 1994; Scott and others, 1998; Crowell and Mann, 

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

years for similar great earthquakes in this region.
5

others, 1993). Unit also includes older, semi-consolidated deposits exposed in sea bluffs 

and below bluffs where deposits are visible only at lowest tides. At Goose Bay, about 16 

km northwest of Anchorage (Anchorage quadrangle), older deposits are mainly gravel 

underlying peat; peat contains interbedded tephra dated at 378±0.67 ka by 40Ar/39Ar (Reger 

and others, 1995). Below sea level, at Goose Bay, deposits are mainly slightly indurated silt 

and clay with some comminuted shells and other organic material, as well as minor sand and 

gravel; Karlstrom (1964) also reported diamicton. Karlstrom’s (1964) correlation of shells in 

this unit with those of the Bootlegger Cove Formation is no longer valid. Deposits similar to 

those at Goose Bay also occur at Eagle Bay (Anchorage quadrangle) and at Point Woronzof, 

but shells were lacking. In the bluffs at Granite Point (southern Tyonek quadrangle), the 

informal Kaloa deposits of Schmoll and others (1984), included here as part of this map unit, 

consist mainly of diamicton. These Koloa deposits contain numerous horizons of cobblesized clasts that may be lag deposits (stone lines) and at least one prominent bed of probably 

transported coal debris that constitute gently dipping bedding surfaces; an apparent thickness 

of about 150 m is exposed. Reger (2009) considers the Kaloa deposits of subestuarine 

mass-movement origin, and late Pleistocene rather than older age. In the Eklutna valley 

(Anchorage quadrangle), somewhat oxidized poorly sorted gravel and sand are exposed in 

bluffs. These deposits served as the type deposits of the Eklutna Glaciation of Karlstrom 

(1964), but no reliable connection to the Eklutna moraines mapped elsewhere can be made. 

Similar deposits are thought to occur at Point Possession (Tyonek quadrangle) and perhaps 

elsewhere on the Kenai Peninsula (Schmoll and Yehle, 1986). Commonly subdivided into 

the following:

Qat Alluvium along major rivers and in terraces (Holocene)—Chiefly gravel and sand, well 

bedded and sorted within beds. Occurs (1) as actively reworked deposits in bars and very 

low terraces that are subject to continual reworking by streams and generally lack vegetation; thickness may be only a few meters; includes outwash from present-day glaciers where 

thickness may be 5–10 m; (2) in low to higher terraces where thickness may be 5–10 m; and 

(3) in small alluvial fans where thickness may be less than 5 m. Rarely preserved natural 

levee deposits on deltaic or fan delta deposits occur on the west side of Cook Inlet (F.H. 

Wilson, unpub. data, 2007)

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

and some sand and local gravel deposited in shallow embayments. Deposits reworked daily 

by tides in lower part, only during highest tides or extreme storms in upper part. Largely 

mapped from topographic maps

Qes Estuarine deposits (Holocene)—Chiefly silt and fine sand; somewhat coarser near tidal 

channels. Well bedded and sorted. Loose, commonly water saturated. Above zone of tidal 

activity but commonly marshy or occupied by numerous lakes. Locally includes thin beds 

of peat, driftwood, and other organic material, and some windblown material. Thickness 

of a few meters to a few tens of meters. Includes salt-marsh deposits of Detterman and 

Hartsock (1966) and grades into tidal flat deposits (Riehle and Emmel, 1980; F.H. Wilson, 

unpub. data, 2007). Also includes marine terrace deposits on the west side of Cook Inlet 

(F.H. Wilson, unpub. data, 2007). In upper Turnagain Arm area (Seward quadrangle) 

includes Placer River Silt (Ovenshine and others, 1976) deposited immediately following 

the 1964 earthquake. Homer Spit (Seldovia quadrangle) was shown as “elevated tidal silt 

and beach deposits” by Karlstrom (1964); however, his text described it as a subaqueous 

end moraine that he attributed to a late Skilak advance of the Naptowne glaciation. He 

described it as being deposited in deep water and subsequently being wave-reworked upon 

emergence

Qaf Alluvial fan deposits (Holocene and Upper Pleistocene)—Poorly sorted to well-sorted silt to 

boulders in alluvial fans, cones, and coalesced cones. May grade into colluvial deposits or 

talus. Some large fans graded to or may interfinger with glacioestuarine deposits

Qdf Debris-flow deposits (Holocene)—Debris-flow deposits from Redoubt Volcano in the Crescent River valley (Kenai quadrangle); includes several small (older?) deposits in the upper 

valley and a 3,500-year-old debris flow (Riehle and others, 1981) in the lower valley. Older 

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

Oldest(?) deposit is derived from west flank of Redoubt Volcano. Next oldest deposit 

appears derived from a presently glaciated valley draining to southwest off Redoubt. Deposits are of relatively limited extent and may have dammed North Fork of the Crescent River, 
6

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

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

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

debris flow is young enough to possibly explain lack of glaciers in the source valley, as all 

surrounding valleys have extensive glaciers and glacial deposits. Flow extended to coast of 

Cook Inlet and flowed back up the main fork of the Crescent River, creating Crescent Lake. 

Map unit also includes a deposit in upper Crescent River valley that may be a debris flow 

deposit. Includes Holocene mudflow deposits in Drift River valley draining north and east 

from Redoubt Volcano (Till and others, 1993); many of which are historic, including 1966 

and later flows

Qlc Landslide and colluvial deposits (Holocene and Upper Pleistocene)—Irregularly mixed 

fragments of various sizes and types derived by gravity processes, commonly in fast-moving 

events but also in slower downslope movement. Includes relatively large masses as well as 

deposits of smaller fragments of bedrock, diamicton, gravelly silt and sand, and relatively 

minor amounts of clay, boulders, and organic material. Only largest landslides mapped, 

developed mainly on Tertiary sedimentary rocks or lateral moraines on mountain slopes; 

some may have formed shortly after withdrawal of glacier ice. Most of those shown at 

Anchorage conceal Bootlegger Cove Formation and formed during the 1964 earthquake; 

others are older but proably of similar origin. Colluvial deposits shown mainly in small 

valleys where deposits occupy most of the area and where alluvium in valley bottoms is 

narrow. Includes angular rock debris and mud in slumps, earth-debris 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; F.H. 

Wilson, unpub. data, 2007); also includes remnants of lateral moraines and other glacial and 

unconsolidated deposits too small to map. In the vicinity of Redoubt Volcano and Iliamna 

Volcano, may include some volcanic mudflow (lahar) deposits (Riehle and Emmel, 1980). 

On the west side of Cook Inlet, solifluction deposits of poorly sorted sand, silt, and clay 

thought to be derived from local upslope bedrock sources are associated with plutonic rocks 

of the Alaska-Aleutian Range batholith (F.H. Wilson, unpub. data, 2007)

Qb Beach deposits (Holocene)—Sand, gravel, and cobbles of present and former beaches, beach 

ridges, spits, and tidal flats (Detterman and Hartsock, 1966; Riehle and Emmel, 1980; F.H. 

Wilson, unpub. data, 2007)

Qsl Lacustrine, swamp, and fine silt deposits (Holocene and Upper Pleistocene)—Chiefly 

silt, clay, fine sand, peat, and other organic material generally of lacustrine origin (Riehle 

and Emmel, 1980; F.H. Wilson, unpub. data, 2007). Includes deposits in peat bogs where 

mapped separately; may be underlain by permafrost

Qd Eolian deposits (Holocene)—Dunes consisting of well-sorted, fine-grained sand and silt. 

Includes cliff-head dunes adjacent to the mouth of Turnagain Arm southeast of Point Possession and Point Campbell, and on the southeast side of Fire Island (join area of Tyonek, 

Anchorage, Kenai, and Seward quadrangles). As mapped (Karlstrom, 1964), includes bluff 

exposures of underlying deposits, best seen on Fire Island

Qdl Deltaic deposits (Holocene and Upper Pleistocene)—Deltaic deposits, including outwash 

and alluvial deposits (F.H. Wilson, unpub. data, 2007); include deposits in hanging deltas 

marginal to glacial lakes and the ancestral Cook Inlet glacial-estuary

Glacial deposits—Range from clay to boulders, including till, consisting of gravelly sandy silt 

and variable amounts of clay. Chiefly unsorted, but locally moderately sorted as discontinuous lenses of sand and sandy pebbly gravel. Variably compact. May or may not show stratification or bedding. Forms end- and lateral-moraine ridges, hummocky ground moraine, 

and some relatively smooth ground-moraine plains. Also includes some glaciolacustrine, 

glacioalluvial, outwash, and other glacially related deposits. Thickness variable, as much 

as 25 m in some ridges, 10 m in hummocky terrain, and probably less than 7 m in plains 

(Karlstrom, 1964; Riehle and Emmel, 1980; Schmoll and Yehle, 1986, 1987, 1992; Yehle 

and Schmoll, 1987a,b, 1988, 1989, 1994; H.R Schmoll and L.A. Yehle, unpub. data, 2009; 

and F.H. Wilson, unpub. data, 2007). Unit ranges in age throughout the Quaternary. Locally 

subdivided into the following:

Qhg Young moraine deposits (Holocene)—Occurs mainly in prominent ridges within a few 

kilometers of large glaciers (late Holocene) and in well-developed to fragmentary ridges 

about 10 km downstream from large glaciers (early Holocene). Also occurs as a series of 
7

small ridges in some mountain valleys that head in small glaciers or valleys that no longer 

contain glaciers. Includes deposits of the Tunnel and Tustumena Stades (Karlstrom, 1964; 

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 Kenai Peninsula, includes unmodified moraine deposits, which may be 

mantled by loess, generally 2 to 6 ft thick (Karlstrom, 1964). As shown also includes active 

and recently active rock glaciers consisting chiefly of rubble and coarse rock debris (F.H. 

Wilson, unpub. data, 2007) as well as fresh, poorly sorted debris on surfaces of glaciers 

(Detterman and Hartsock, 1966). In vicinity of Redoubt Volcano and Iliamna Volcano, may 

include a significant component of ash and other air-fall debris from eruptions (F.H. Wilson, 

unpub. data, 2007)

Qho Younger outwash deposits (Holocene)—Chiefly gravel and sand, well-bedded and sorted 

within beds, in terraces, and plains higher than modern streams near and down valley 

from present-day glaciers (Detterman and Hartsock, 1966; Riehle and Emmel, 1980; H.R. 

Schmoll and L.A. Yehle, 1986, 1987, 1992). Thickness probably 5–10 m (Karlstrom, 1964; 

F.H. Wilson, unpub. data, 2007)

Qgl Glaciolacustrine deposits (Holocene and Upper Pleistocene)—Chiefly well-sorted, wellstratified, laminated to massive silt and clay, with some sand and pebbly sand, especially 

near margins of ephemeral glacial and postglacial lakes. Deposits of small lakes marginal to 

glaciers may include diamicton. Near Beluga and Tustumena Lakes, deposits reflect higher 

level ancestral lakes. These deposits are thought to be typically covered by several feet of 

muck and peat (F.H. Wilson, unpub. data, 2007). Thickness is as much as 10 m, with some 

glaciolacustrine deltaic deposits as much as 50 m thick. In Talachulitna River valley (about 

50 km north-northwest of Tyonek), includes by inference, deposits of ancestral glacial Lake 

Talachulitna (Karlstrom, 1964). Includes thin stratified and locally foreset-bedded sand and 

gravel over Hemlock Conglomerate of the Kenai Group sedimentary rocks (unit Tkh) that 

occur below 125 m elevation on West Foreland and from Katchin Creek to Redoubt Point; 

Riehle and Emmel (1980) inferred these deposits may be probable emerged delta and glaciolacustrine deposits. Includes active outwash fans (Riehle and Emmel, 1980)

Qm Major moraine and kame deposits (Upper Pleistocene)—Diamicton widespread in northern part of Cook Inlet basin and along margins of the basin farther south, produced by glaciers that originated in the adjacent mountains. Prominent end and lateral moraines indicated 

on map by blue line symbol. Most moraines represent glacier readvances during the general 

retreat from maximum positions in the central basin. Lakes, ponds, areas of peat deposits, 

and narrow channels are common locally. Kame fields, prominent locally, consist of irregularly shaped and relatively sharply defined hills as well as a few narrow and sinuous eskers 

that mainly consist of well-bedded to poorly bedded and sorted gravel and sand; some kame 

deposits and eskers might contain cores of diamicton. Especially between Big Lake and the 

Little Susitna River (Anchorage quadrangle), includes extensive esker fields (Kopczynski, 

2008) and an area of ribbed (“Rogen”) moraines (Reger and Updike, 1983) reinterpreted as 

subaqueous dunes resulting from a catastrophic outburst from glacial Lake Atna far up the 

Matanuska Valley (Wiedmer and others, 2010). Moderately weathered, heavily spruce- and 

brush-covered moraine along Cook Inlet shoreline; (Detterman and Hartsock, 1966; F.H. 

Wilson, unpub. data, 2007). Internal contacts on west side of Cook Inlet reflect subdivisions 

of the Brooks Lake Glaciation; Iliuk, Newhalen, and Iliamna advances. Includes stratified 

and locally foreset-bedded sand and gravel that occur below about 60 m elevation on West 

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

glaciolacustrine deposits (Riehle and Emmel, 1980). Includes active outwash fans (Riehle 

and Emmel, 1980). On the east side of Cook Inlet, unit includes deposits of Naptowne Glaciation, mantled by 2 to 6 ft of loess (Karlstrom, 1964). Includes areas of bedrock covered 

by thin or discontinuous glacial deposits

Qgm Modified glacial deposits (Upper Pleistocene)—Consist of glacial deposits modified 

by estuarine or lacustrine processes. Deposits originally most likely associated with Last 

Glacial Maximum (Naptowne?) and (or) Knik glacial advances; however, includes terraced 

deposits mapped by Karlstrom (1964) as Eklultna in age. Well-defined morainal morphology 

either not developed due to deposition in water or has been modified by later submergence. 

Deposits consist of a variety of interbedded materials including diamicton, gravel, stony silt, 
8

silt, fine-grained sand, and some clay and that generally have much fine-grained material. 

Winnowing of diamicton clasts and a cover of better sorted materials observed, especially 

in the Anchorage area (Karlstrom, 1964; Schmoll and others, 1999). Deposits are transitional between deposits of unit Qgm and map units Qge, Qbc, and minor other map units. 

Includes deposits of both units gm and mg of Schmoll and Dobrovolny (1972). Includes the 

Fire Island deposits of Schmoll and Gardner (1982) and Schmoll and others (1984), thought 

to have formed subaqueously in the glacioestuarine environment, perhaps in close proximity 

to glaciers

Qgc Glacioalluvium (Upper Pleistocene)—Gravel and sand that may be overlain by peat deposits; locally more than 1 m thick in channels within moraines. In some places, deposits may 

be lacking, and channel floored by diamicton of the enclosing moraine. Thickness generally 

less than 5 m. 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; F.H. Wilson, unpub. data, 2007)

Qgo Outwash in plains, valley trains, and fans (Upper Pleistocene)—Outwash extending 

directly from moraines in plains and in downstream valley trains in channels not related to 

present-day streams. Fans, more distantly related to glacier sources, commonly graded to 

or may interfinger with glacioestuarine deposits. Chiefly well-bedded and sorted gravel and 

sand within beds, but may be finer grained, mainly sand or gravelly sand near toes of fans. 

Thickness 5–10 m, thinner toward fan toes. Extends from moraines of Naptowne age on 

Kenai Peninsula (Karlstrom, 1964) and to Brooks Lake age moraines on west side of Cook 

Inlet (F.H. Wilson, unpub. data, 2007)

Qge Glacioestuarine deposits (Upper Pleistocene)—Chiefly well-bedded and sorted, medium 

to coarse sand, includes some interbeds of fine gravel. Probably deposited as a cover sand 

during waning phases of glacioestuarine environment in which underlying Bootlegger Cove 

Formation was deposited. Includes Hood Lake deposits of Schmoll and others (1999). 

Commonly overlain by peat that may be more than 1 m thick over large areas. Thickness 

of deposits is as much as 10 m, but locally much thinner or even absent where surface peat 

directly overlies Bootlegger Cove Formation. Includes some abandoned beach ridges that 

rise a few meters above surrounding plain

Qbc Bootlegger Cove Formation (Upper Pleistocene)—Silty clay and clayey silt containing 

minor interbedded silt, fine to medium sand, and thin beds of diamicton, and containing 

scattered pebbles and cobbles in widely varying concentrations (Updike and others, 1988). 

Miller and Dobrovolny (1959) defined the unit as the Bootlegger Cove Clay and considered 

the deposit of glaciolacustrine origin, as did Karlstrom (1964) except for one shell-bearing 

marine horizon. Established as glacioestuarine in origin throughout the exposed formation 

by Schmidt (1963). Radiocarbon ages of 14,000 to 16,500 years B.P. reported from shells at 

Anchorage by Schmoll and others (1972) and by Reger and others (1995) from shells near 

Kenai. Thickness as much as 35 m at type area in Anchorage (Updike and others, 1988), 

but poorly known in the Susitna River and Yentna River region (Tyonek quadrangle) where 

only small outcrops along those rivers have been observed and along Turnagain Arm where 

scattered evidence has been found. However, thought to be widely extensive beneath cover 

sand deposits (Qge). Includes deposits near Point Campbell and near Girdwood that might 

be younger than and discrete from other deposits of unit

Qig Intermediate age glacial deposits (Upper Pleistocene)—Prominent, but modified morainal 

deposits of Knik Glaciation of Karlstrom (1964) widely exposed north of Homer on Kenai 

Peninsula (Karlstrom, 1964). Occurs on relatively flat lying to sloping surfaces on some 

lower mountains where diamicton may be somewhat weathered and subjected to minor gravitational reworking. On Kenai Peninsula, moraines of this age are terraced below approximately 228 m (750 ft), reflecting deposition in proglacial lakes; these deposits are included 

in unit Qgm. In the vicinity of Tustumena Lake, Reger (1995) suggests that the innermost 

area of these deposits may more properly be assigned to the Moosehorn advance of the 

Naptowne Glaciation (Last Glacial Maximum). However, this would require extending the 

Moosehorn advance a significant distance beyond the defined type area 

Qog Older glacial deposits (Middle or Lower Pleistocene)—Prominent, but modified morainal 

deposits of Eklutna Glaciations of Karlstrom (1964). Occur on relatively flat lying to sloping surfaces on some lower mountains where diamicton may be somewhat weathered and 
9

subjected to minor gravitational reworking. Includes drift of the Caribou Hills glaciation 

on Kenai Peninsula (Karlstrom, 1964). Probably no more than a few meters to 10 m thick. 

Flat surfaces containing bedrock rubble and scattered glacial erratics, chiefly cobbles and 

small boulders, occur at highest elevations, notably on Mount Susitna. On Kenai Peninsula, 

moraines of Eklutna age are terraced below approximately 228 m (750 ft), reflecting modification in proglacial lakes. 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). Drift of the Caribou Hills 

of Karlstrom (1964) is highly modified in Caribou Hills and exposed as remnant deposits 

elsewhere on Kenai Peninsula; deposits are terraced below elevations of about 305 m (1,000 

ft) and discontinuously mantled by proglacial lake sediment deposits also reflecting modification in proglacial lakes (Karlstrom, 1964)

SEDIMENTARY ROCKS

Tsu Sedimentary rocks, undivided (Tertiary)—Clastic sedimentary rocks in Talkeetna Mountains 

quadrangle described as fluviatile conglomerate, sandstone, and claystone with a few interbeds of lignitic coal. Sequence is greater than 160 m thick; lithologically resembles Chickaloon Formation but lacks fossil evidence for definitive correlation (Csejtey and others, 

1978)

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

Kenai, and Tyonek quadrangles) consisting of, in descending order: Sterling, Beluga, and 

Tyonek Formations, and Hemlock Conglomerate in vicinity of Cook Inlet. According to 

Calderwood and Fackler (1972), unit is at least 8,000 m thick in the subsurface of Cook 

Inlet. Units are typically estuarine and nonmarine clastic sedimentary rocks. Calderwood 

and Fackler (1972) included West Foreland Formation within the Kenai Group; however, it 

was separated as a distinct unit by Magoon and others (1976). Swenson (1997) has proposed 

an alternative stratigraphic column for Cook Inlet basin that recognizes the time-transgressive nature of the units, wherein all units of Kenai Group and West Foreland Formation 

somewhat overlap in age. Dallegge and Layer (2004) suggested that the age range of the 

stratigraphic units be revised based on new 40Ar/39Ar dating of tephra from within Kenai 

Group. In particular, they document the time-transgressive nature of the formations and that 

Tyonek Formation may be as old as 49 Ma or early Eocene (Ypresian) in Matanuska River 

valley (see below), making its lower part age equivalent with Hemlock Conglomerate and 

West Foreland Formation. According to R.G. Stanley (USGS, written commun., 2009) the 

type sections for West Foreland Formation and the subdivisions of Kenai Group are not in 

outcrop but rather in the subsurface located in several different wells, both onshore and offshore. These wells, in turn, are located many miles from each other in an area of complicated 

structure and lateral facies changes; therefore, correlation of these subsurface type sections 

with the surface outcrops is poorly documented, difficult, and controversial. Subdivided into 

the following:

Tks Sterling Formation (Pliocene and Miocene)—Weakly lithified massive sandstone, 

conglomeratic sandstone and interbedded siltstone and claystone (in Kenai and Seldovia 

quadrangles); 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 sandstone, siltstone, mudstone, carbonaceous shale, lignite coal, and minor volcanic ash. Magoon and others (1976) tabulated plant 

fossil localities that provide the primary age control for Sterling Formation. Triplehorn and 

others (1977), Turner and others (1980), and Dallegge and Layer (2004) reported K-Ar, 

40Ar/39Ar, and fission-track age determinations from ash partings within coal beds within 

Sterling Formation (table 1). Dates range from 3.17±1.58 to 9.23±0.97 Ma; some older 

reported ages were interpreted to be contaminated by detrital material (Turner and others, 

1980), or contain excess argon or xenocrystic material (Dallegge and Layer, 2004)

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

mudstone, carbonaceous shale, coal, and minor volcanic ash (Bradley and others, 1999) 
10

in Kenai and Seldovia quadrangles. Calderwood and Fackler (1972) reported a distinctive 

feature of Beluga Formation is its lack of massive sandstone beds and massive coal seams 

that characterize the underlying Tyonek Formation; however, lignitic to subbituminous 

coal seams can be as much as 4 m thick in the upper part of Beluga Formation. The contact 

between Beluga and overlying Sterling Formation may be an unconformity (Calderwood 

and Fackler, 1972), but in any case can be difficult to pinpoint (Calderwood and Fackler, 

1972; Turner and others, 1980). Magoon and others (1976) tabulated plant fossil localities 

that provide the primary age control for Beluga Formation. Triplehorn and others (1977), 

Turner and others (1980), and Dallegge and Layer (2004) reported a number of K-Ar, 

40Ar/39Ar, and fission-track age determinations from ash partings within coal beds in this unit 

ranging from 4.57±0.72 to 12.9±5.1 Ma (table 1)

Tkt Tyonek Formation (Miocene and Oligocene)—Carbonaceous nonmarine conglomerate and 

subordinate sandstone, siltstone, and coal (Winkler, 1992; Bradley and others, 1999) in 

Tyonek, Talkeetna, and Anchorage quadrangles. Tyonek Formation is identified by 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). Reed and Nelson (1980) locally divided unit into two members: a sandstone 

member consisting of about 80 percent tan to light-gray sandstone, 20 percent light- to 

medium-gray siltstone and claystone, and less than 1 percent conglomerate, coal, and volcanic ash; and a conglomerate member consisting of 40 percent conglomerate, 20 percent 

sandstone, and less than 40 percent siltstone, claystone, and coal. Plant fossils comprising the Seldovian Stage of early and late Miocene age as well as plants representing the 

Angoonian Stage of Oligocene age provide age control (Wolfe and Tanai, 1980). Dallegge 

and Layer (2004) reported a 40Ar/39Ar age of 48.65±2.31 Ma on glass from a tephra sample 

collected from a well core; sample came from a depth of 593.8 m in the Matanuska River 

valley. According to Dallegge and Layer (2004), Smith (1995) considered this interval to 

represent the Tyonek Formation; however, there is no mention in Smith (1995) to suggest 

this correlation, which can only be inferred on the basis of an isopach map shown as figure 

8 in Smith (1995). However, correlation of the dated sample with Tyonek Formation is considered doubtful according to D.L. LePain (Alaska Division of Geological and Geophysical 

Surveys, oral commun., March 3, 2008)

Tkh Hemlock Conglomerate, undivided (Oligocene)—Sandstone, conglomerate, and siltstone in 

the vicinity of Harriet Point in Kenai quadrangle on west side of Cook Inlet and in Mount 

Katmai and Afognak quadrangles (Calderwood and Fackler, 1972); original description suggests unit may be about 170 m thick. On the basis of the description in Detterman and others 

(1976), rocks 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 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. Plant fossils indicate Oligocene 

age; Wolfe and Tanai (1980) suggested early Oligocene, and Wolfe (cited in Detterman and 

others, 1996) suggested late Oligocene in southern portion of map area, although he could 

not rule out an earliest Miocene(?) age. Wolfe and Tanai (1980) suggested the rocks at Harriett Point may be isochronous with beds considered typical of Tyonek Formation

Tts Tsadaka Formation (Miocene or Oligocene)—Poorly sorted cobble to boulder conglomerate, 

interbedded with lenses of feldspathic sandstone, siltstone, and shale (Winkler, 1992) at least 

200 m thick. Unit is terrestrial and of local provenance, showing rapid lateral lithologic and 

thickness changes; deposited on alluvial fans and in braided streams from a northerly source 

(Winkler, 1992). Clasts in the conglomerate are largely plutonic in contrast to the underlying 

Wishbone Formation (Winkler, 1992). Age considered Oligocene in Anchorage quadrangle 

(Winkler, 1992) on the basis of plant fossils described by Wolfe (1977)

Tvs Volcaniclastic sedimentary rocks (Eocene or younger)—Light-green to gray, bedded volcaniclastic sedimentary rocks; some beds contain volcanic breccia clasts. Volcanic rock clasts are 

light green. LA-ICPMS detrital zircon dates have youngest grain ages of 55–58 Ma (D.C. 

Bradley, USGS, written commun., 2008). Unit outcrops along western margin of Tyonek 

quadrangle at approximately latitude 61° 30’ N.
11

Tw Wishbone Formation (Eocene)—Fluviatile conglomerate having thick interbeds of sandstone, 

siltstone, and claystone and containing local partings of volcanic ash (Winkler, 1992); maximum thickness of about 560 m (Barnes and Payne, 1956). Deposited in a similar environment as overlying Tsadaka Formation; however, source terrain was largely volcanic, most 

likely the Talkeetna Formation (Winkler, 1992)

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 

describe 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 bed also contains plant fragments and is clearly of sedimentary origin. Conglomerate clasts are mainly 

rounded to subrounded 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 clasts in conglomerate. Medium- to coarse-grained arkosic sandstone forms 

the conglomerate matrix and forms distinct lenticular beds. Siltstone and shale interbedded 

with conglomerate is a very fine-grained subarkosic equivalent of the sandstone. Originally 

mapped as Kenai Formation by Detterman and Hartsock (1966); Calderwood and Fackler 

(1972) redefined Kenai Group and subdivided it into a number of formations. The rocks here 

were assigned to West Foreland Formation on the basis of correlation with the type section in a well (Pan American Petroleum Corp. West Foreland No. 1 well; Calderwood and 

Fackler, 1972) where the unit is about 275 m thick. West Foreland was assigned Oligocene 

age by Kirschner and Lyon (1973) and later reassigned early Eocene and late Paleocene age 

by Magoon and others (1976), presumably on the basis of plant fossils. Recent dating of 

zircon from an interbedded tuff about 1 m thick yielded a 43 Ma age (middle Eocene) (P.J. 

Haeussler, USGS, written commun., 2008). West Foreland Formation was removed from 

Kenai Group because it was separated by a major unconformity from the overlying Hemlock 

Conglomerate (Magoon and Egbert, 1986). Unit may be equivalent to Arkose Ridge Formation described below. Lower contact was described by Detterman and Hartsock (1966) as an 

angular unconformity with Upper Jurassic Naknek Formation; subsequent work by Magoon 

and others (1980) showed there to be a nonmarine Upper Cretaceous sedimentary unit 

between rocks of West Foreland and Naknek Formations. Those Cretaceous rocks are shown 

here as unit Ksm

Tar Arkose Ridge Formation (lower Eocene and upper Paleocene)—Fluviatile and alluvial 

feldspathic and biotitic sandstone, conglomerate, siltstone, and shale containing abundant 

plant fragments (Csejtey and others 1977; Winkler, 1992). Coarsening upward sequence 

was deposited on alluvial fans and by braided streams carrying sediment derived from rapid 

erosion of uplifted mountains to the north (Winkler, 1992). Thickness is as much as 700 m. 

Age control is largely based on late Paleocene fossil plants and radiometric ages on locally 

associated volcanic flows and dikes (Tvu). A pre-Tertiary age has been suggested for the part 

of the unit based on a questionable 67.5±2.4 K-Ar age determination on biotite from a metamorphosed part of the unit (Csejtey and others, 1977, 1978; sample 76ACy 19, table 1 here); 

otherwise the age is considered broadly coeval with Chickaloon Formation based on 46 to 

56 Ma whole rock ages from volcanic rocks in lower part of formation and 46.1±2.8 Ma 

whole rock age of basalt dike from the middle of formation as well as late Paleocene plant 

fossils (Silberman and Grantz, 1984). Unit may be equivalent to West Foreland Formation, 

representing a transtensional basin along the Castle Mountain Fault system

Tch Chickaloon Formation (lower Eocene and upper Paleocene)—Predominantly fluviatile and 

alluvial carbonaceous mudstone, siltstone, conglomeratic sandstone, and polymictic conglomerate (Winkler, 1992). Locally, upper and middle parts of unit contain numerous beds 

of bituminous coal (Winkler, 1992). Lower part of unit largely conglomerate and lithic 

sandstone derived from erosion of the Talkeetna Formation (Winkler, 1992). A strongly 

deformed, “southerly derived, green-weathering, noncarbonaceous basal sequence of 

poorly sorted, massive to crudely stratified cobble and boulder conglomerate ***” grades 

“upward into well-stratified, thick-bedded sandstone and conglomerate ***” having a 

chloritic matrix (Winkler, 1992). Little (1988, 1990, cited in Winkler, 1992), interpreted 

this sequence to represent a prograding alluvial fan derived from uplift and erosion of the 
12

Chugach accretionary complex (Chugach terrane) to the south (Winkler, 1992). Thickness 

is more than 1,500 m (Winkler, 1992). Age control derived from K-Ar and fission-track 

determinations on ash partings within coal beds ranging from 52 to 56 Ma (table 1) and the 

presence of Paleocene fossil leaves (Wolfe and others, 1966; Triplehorn and others, 1984)

Tcl Copper Lake Formation, undivided (lower Eocene and Paleocene?)—Thick clastic nonmarine sedimentary rock unit consisting of an upper and lower conglomerate member bounding 

a middle sandstone and siltstone member at the east end of Iliamna Lake (Detterman and 

Reed, 1980; Detterman and others, 1996). Detterman and others (1996) described a 1,025-

m-thick section in the Mount Katmai area. In the Iliamna Lake region (Detterman and Reed, 

1980), upper conglomerate unit consists of red-weathering pebble-cobble conglomerate 

consisting mainly of volcanic rock clasts and containing minor tuff. Member may be an 

agglomerate rather than conglomerate; clasts are 50 to 100 percent fresh-appearing volcanic 

rock. Clasts of quartzite, schist, greenstone, rose quartz, limestone, and granitic rocks 

are present in lower parts of this member. Sandstone and siltstone intervals of measured 

section vary from thin bedded to massive and are typically dark- to medium-gray; they are 

fine- to medium-grained lower in section and become medium- to coarse-grained toward 

top. Middle member is chiefly medium-gray to greenish-gray, lithic graywacke sandstone 

and siltstone. More fine-grained clastic parts of formation contain considerable carbonaceous debris and minor coaly material. Grains in the sandstone include abundant quartz, 

schist, volcanic, and granitic rock fragments. Interbedded siltstone is similar in color and 

composition to the sandy facies, whereas claystone interbeds are mainly micaceous clay 

containing a small amount of montmorillonite. Lower conglomerate member is red-weathering pebble-cobble conglomerate consisting mainly of volcanic rock clasts and containing 

minor tuff. Volcanic clasts constitute about 25 percent and appear to be derived from 

the Talkeetna Formation and not fresher appearing Tertiary volcanic units. Copper Lake 

Formation was derived from erosion of Mesozoic source area and is terrestrial. In Mount 

Katmai and Iliamna quadrangle areas, this source area was underlain by Alaska-Aleutian 

Range batholith (Reed and Lanphere, 1973) and associated Mesozoic sedimentary and 

metamorphic rocks. Towards the north, Copper Lake Formation undergoes transition from 

rocks of Mesozoic provenance to fresh volcanic clasts of probable Tertiary age (Detterman 

and Reed, 1980, p. B47). Age of the Copper Lake Formation is poorly constrained; sparse 

megaflora in type section in Iliamna area and abundant megaflora on the Alaska Peninsula 

are restricted to sandstone and siltstone intervals in middle part of unit (Detterman and 

others, 1996). Detterman and others (1996) correlated the Copper Lake Formation with the 

Tolstoi Formation of the southwest Alaska Peninsula where an early Eocene megaflora was 

collected from a sandstone and siltstone section overlying a basal conglomerate containing 

a late Paleocene flora and underlying beds containing a middle Eocene flora. By analogy, a 

Paleocene(?) to early Eocene age was assigned to the Copper Lake Formation by Detterman 

and others (1996). Upper and lower contacts of the Copper Lake Formation are disconformities with Hemlock Conglomerate and Kaguyak Formation, respectively

Kyh Graywacke of the Yenlo Hills (Cretaceous?)—According to Reed and Nelson (1980), graywacke in the Yenlo Hills (Willow Mountain and Mount Yenlo) of the Talkeetna quadrangle 

differs significantly from other sedimentary rocks in the vicinity “in that extremely angular, 

and oscillatory zoned plagioclase, volcanic rock fragments, hornblende, epidote, and calcite 

grains are more abundant, suggesting that the sandstone may have been derived from the 

Jurassic magmatic arc (Reed and Lanphere, 1973) now covered by younger rocks in Cook 

Inlet basin. Contemporaneous volcanism is suggested by rare interbedded light tuffaceous 

sediments in the Yenlo Hills (Reed and Nelson, 1980).” We have separated these rocks 

because of their distinct lithologic differences with other rocks assigned to the Kahiltna 

flysch sequence (units KJs, Ksf, Kes) 

Kkd Kodiak Formation (Upper Cretaceous)—Medium- to thick-bedded, graded-bed sequences that 

average 1 m thick, of arkosic wacke and shale and occasional beds of pebbly conglomerate, 

mostly known from Kodiak Island, south of the map area. Flute casts and complete Bouma 

sequences indicate deposition by turbidity currents. Unit crops out more extensively south of 

map area; crops out in a small area on northern end of Shuyak Island in Afognak quadrangle. 

Moore (1969) designated type section as rocks exposed along western shore of Uyak Bay in 

Kodiak quadrangle, about 70 km south of map area. Subsequent work by Nilsen and Moore 

(1979) indicated that unit is approximately 5,000 m thick, in contrast to estimated thickness 
13

of 30,000 m as originally suggested by Moore (1969). Nilsen and Moore (1979) found that 

unit is repeated structurally by folding and faulting, which makes internal stratigraphic correlation difficult; unit strikes northeast and dips steeply northwest and is generally deformed 

in tight, large-scale folds. Nilsen and Moore (1979) used a turbidite facies and facies-association scheme that generally follows the system of Mutti and Ricci Lucchi (1972, 1975, 

cited in Nilsen and Moore, 1979) and Nelson and Nilsen (1974, cited in Nilsen and Moore, 

1979); under this system, Kodiak Formation consists largely of basin-plain- and slope-facies 

associations. Basin-plain facies is structurally lowest part of Kodiak Formation and is characteristic of most of formation on southeast side of Kodiak Island (Nilsen and Moore, 1979). 

Slope-facies rocks are found primarily on northwest side of Kodiak Island and are primarily 

thick mudstone sequences. Conglomerate and sandstone channels are found locally within 

slope-facies associations; Nilsen and Moore (1979) mentioned channels in Uyak Bay in 

particular, where channels are about 50 m thick. Fossils from Kodiak Formation include 

Inoceramus kusiroensis of Late Cretaceous (Maastrichtian) age. Correlated with Shumagin 

Formation to southwest and Valdez Group to northeast 

Ksm Saddle Mountain section of Magoon and others (1980) (Upper Cretaceous, Maastrichtian)—Nonmarine sandstone, conglomerate, and minor siltstone and coal in a section 83 m 

thick (Magoon and others, 1980). Consists dominantly of fine- to medium-grained sandstone 

that becomes finer grained upward. Generally massive; some crossbedded sections. 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 the upper part of the section are as much as 2.7 m thick and locally have underclay 

(“undersoils”, Magoon and others, 1980). Siltstone most abundant in middle part of section. 

Sporomorphs Cranwellia striata (Couper) Srivastava, Balmeisporites spp., Wodehouseia 

spinata Stanley, Proteacidites spp., Aquilapollenites bertillonites Funkhouser, A. reticulatus

Mtchedlishvili, and A. delicatus Stanley (Magoon and others, 1980) indicate Maastrichtian 

age. Unit was assigned to Kaguyak Formation by Bradley and others (1999), but is separated here because its lithology and depositional environment are distinctly different from 

Kaguyak Formation. Unit overlies Upper Jurassic Naknek Formation with angular unconformity; unit overlain by West Foreland Formation with angular unconformity. Unit found 

northeast of Chinitna Bay in Seldovia quadrangle 

Kkg Kaguyak Formation (Upper Cretaceous, Maastrichtian and Campanian)—Dark-gray to 

pale-brown, typically thin-bedded, marine shale, siltstone, and fine-grained sandstone; 

measured thickness of more than 1,200 m (Detterman and others, 1996). Named by Keller 

and Reiser (1959) for rocks exposed near abandoned village of Kaguyak in Afognak C-6 

1:63,360-scale quadrangle, south of map area. Unit mapped on south side of Kamishak Bay. 

Proportion of sandstone increases up-section. Load and flute casts common; in upper part 

of unit, graywacke is graded with numerous rip-up clasts. Overall depositional environment 

was near mid-fan within multi-channeled system; however, uppermost part of unit may 

have been deposited in upper-fan regime (Detterman and others, 1996). In general, fossils 

are sparse; however, locally abundant in lower part of unit. Ammonites are most common 

and may range in size to as much as 1 m across. Fossils indicate latest Campanian and early 

Maastrichtian age. Unit unconformably overlies Naknek and Herendeen Formations; unit is 

unconformably overlain by Copper Lake Formation and younger units 

Kcc Cape Current terrane of Connelly (1978) (Upper Cretaceous)—Informal unit described by 

Connelly (1978) as consisting principally of medium- to thick-bedded arkosic and lithic 

sandstone that contains occasional sections of vesicular pillow lava and pillow breccia. 

Slightly metamorphosed and moderately deformed. Crops out more extensively south of 

map area; crops out in a small area on northern end of Shuyak Island in Afognak quadrangle. 

Two bodies of red pelagic limestone containing many siliceous layers are present within 

unit along southern shore of Shuyak Island. Limestone is thin-bedded and tightly folded; 

contains sparse coccoliths of indeterminate age and Late Cretaceous foraminifera (approximately Turonian to early Santonian age, according to W.V. Sliter, written commun., in Connelly, 1978) 

Km Matanuska Formation (Upper Cretaceous, Maastrichtian, to upper Lower Cretaceous, 

Albian)—Well-indurated, thinly bedded, dark-gray fossiliferous shallow marine shale containing conspicuous calcareous concretions, volcanic-lithic siltstone, sandstone, graywacke, 

and subordinate conglomerate (Winkler, 1992). According to Martin (1926), unit is at least 
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1,250 m thick. Diverse shallow to deep marine (in part, turbiditic) deposits derived from 

a northern source, either an unidentified mid-Cretaceous magmatic arc or the Jurassic arc 

represented in part by the Talkeetna Formation (Winkler, 1992). Upper part of unit is coeval 

with the flysch of the Valdez Group to the south. Rests with pronounced angular unconformity on Early Cretaceous and older strata (Csejtey and others, 1978) 

KJs Turbiditic sedimentary rocks of the Kahiltna flysch sequence (Cretaceous, Aptian and 

Valanginian or younger, to Upper Jurassic?)—Monotonous sequence of intensely 

deformed and locally highly metamorphosed turbidites described by Csejtey and others 

(1992) and Reed and Nelson (1980). Includes dark-gray to black argillite, fine- to coarsegrained, generally dark-gray graywacke, siltstone, and shale turbidites, thinly bedded and 

dense cherty argillite, dark-gray polymictic pebble conglomerate, subordinate black chert 

pebble conglomerate, a few thin layers of dark-gray to black radiolarian chert and thin, darkgray impure limestone interbeds. Sandstone includes graywacke in beds up to 2 m thick and 

feldspathic sandstone. Includes rocks mapped by Reed and Elliot (1970) as their units Km, 

Mzu, and Mzs. Locally contact metamorphosed on the margin of intrusions. Three fossils were found in this unit in the Tyonek quadrangle. Coquina beds south of McDoel Peak 

(southern edge of Talkeetna quadrangle) yielded Buchia sublaevis Keyserling of Valanginian age (Solie and others, 1991, their unit Ks). A middle Turonian Inoceramus hobetsensis 

Nagao and Matsumoto was also found within this unit north of the terminus of Hayes Glacier (William P. Elder, USGS, written commun., 1989 to Dwight Bradley). A float sample 

of chert also from this vicinity contained poorly preserved Spumellariina, which yielded a 

not-tightly constrained age between middle Toarcian (Lower Jurassic) to upper Valanginian 

(Lower Cretaceous) (E.A. Pessagno, Univ. of Texas, written commun., 2006, to Marti Miller, 

USGS); it is uncertain how this sample might constrain the age of this map unit. 40Ar/39Ar 

age determinations (table 1) on hornblende from igneous clasts in the conglomerate yielded 

Cenomanian ages. Fossils indicate the rocks are both Early and Late Cretaceous in age, and 

the radiometric ages on the conglomerate clasts indicate latest Early Cretaceous age igneous rocks formed a source for some of the unit. Thus, considering the Barremian to Aptian 

and Turonian to Santonian detrital zircon ages within unit Kes to the southwest, it appears 

there are at least two sequences of Cretaceous sedimentary rocks within unit KJs, which we 

are unable to distinguish based upon appearance. In the Talkeetna quadrangle to the north, 

similar rocks were assigned a Jurassic and Cretaceous age (map unit KJs, Reed and Nelson, 

1980); however, the Jurassic fossils were derived from spatially and lithologically distinct 

rocks which we do not consider part of this map unit. The rocks of unit KJs are traditionally assigned to the informally named Kahiltna assemblage (Nokleberg and others, 1994). 

Studies by Ridgway and others (2002), Kalbas and others (2007), and Hampton and others 

(2007) have reported detrital zircon analyses that suggest the Kahiltna assemblage is, at 

least in part, Aptian or Albian or younger in age, similar to some of detrital zircon analyses 

reported for unit Kes below. However, these analyses are somewhat in conflict with the 

Valanginian fossils reported by Solie and others (1991). Reed and Nelson (1980) reported 

indeterminate broadleaf plant fossils occur in rocks of this unit that are lithologically similar 

to the largely Late Cretaceous Matanuska Formation (Km). The Kahiltna assemblage, widespread in southern Alaska, is often the repository for miscellaneous dark-colored sedimentary rock units and as such may not represent a coherent package of rocks. In the southwest 

part of the Tyonek quadrangle, the nature of the transition from this unit to the Kuskokwim 

Group is undefined. Both units have similar lithology and character and available mapping 

is insufficient to either distinguish the units or to indicate that they should be mapped as the 

same unit. Unit as mapped in the Tyonek quadrangle includes rocks mapped by Reed and 

Elliott (1970) as units Km, Mzu, and Mzs. Locally subdivided into the following: 

Ksf Feldspathic sedimentary rocks (Cretaceous)—Reddish-brown-weathering, turbiditic, 

feldspathic wacke containing interbeds of locally calcareous siltstone, shown as unit Kw by 

Reed and Elliot (1970) in the northwestern Tyonek quadrangle 

Kes Turbiditic sedimentary and volcanic rocks (Cretaceous, Aptian or younger)—Largely silty 

to sandy graywacke in beds up to 2 m thick. Sandy beds are medium to coarse grained, with 

quartz, feldspar, and chert clasts. Reed and Elliot (1970) reported that these rocks contain 

subordinate metavolcanic rocks; near the South Twin Glacier (western Tyonek quadrangle) 

outcrop is dominantly volcanic rocks. Unit contains detrital zircons with youngest grain 

ages of 138–122 Ma based on analyses from three samples (D.C. Bradley, USGS, written 
15

commun., 2008). A sample from the southwesternmost outcrop between Emerald Creek 

and the Skwentna River has youngest detrital zircon grain ages of 93–84 Ma (D.C. Bradley, 

USGS, written commun., 2008). Thus, considering the Barremian to Aptian detrital zircon 

ages and these younger detrital ages, it appears there are also two sequences of Cretaceous 

sedimentary rocks within unit Kes. Unit Kes is mapped only where detrital zircon analyses 

indicate the age of the sedimentary rocks. Detrital zircon ages are consistent with others 

from the informally named Kahiltna assemblage reported by Kalbas and others (2007) and 

Hampton and others (2007) and indicate the unit is Santonian or younger and suggest a long 

depositional history; the younger part of the sequence may be age correlative with the Kuskokwim Group exposed southwest of the map area

Khe Herendeen Formation (Lower Cretaceous, Barremian and Hauterivian)—Originally named 

Herendeen Limestone by Atwood (1911, p. 39) for exposures along east shore of Herendeen 

Bay on the southern Alaska Peninsula outside of the map area; renamed Herendeen Formation by Detterman and others (1996), who designate a 270-m-thick reference section south 

of the map area in hills southwest of Hot Spring in the Port Moller quadrangle (southern 

Alaska Peninsula). In the map area, unit crops out south of Kamishak Bay. Originally 

described as limestone, rocks are actually unusually uniform calc-arenaceous sandstone. 

Thin-bedded, medium-grained, dusky yellow to pale yellowish brown on freshly broken 

surfaces, and weather to conspicuous light-gray. Distinct platy fracture upon weathering and 

strong petroliferous odor when freshly broken. Inoceramus fragments form major component of formation, although complete specimens have only been found in the Mount Katmai 

area. A belemnite similar to Acroteuthis sp. A (Jones and Detterman, 1966) was found in 

rocks of formation just east of Staniukovich Mountain between Port Moller and Herendeen 

Bay. Ammonite fossils and other collections from Herendeen Formation in Mount Katmai 

area indicate Hauterivian and Barremian age (J.W. Miller, USGS, written commun., 1983–

85; Detterman and others, 1996)

Jn Naknek Formation, undivided (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, ranging in a long belt 

from south-central Alaska (Wilson and others, 1998) to the southwest end of the Alaska Peninsula (Wilson and others, 1999); about 1,150 km (Detterman and others, 1996). Aggregate 

thickness of the unit members exceeds 3,000 m, though the average thickness the formation 

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

the pelecypod Buchia (Detterman and others, 1996), are common and with ammonites indicate age range of Oxfordian to late Tithonian (Late Jurassic). Detterman and others (1996; 

see also, Detterman and Hartsock, 1966; Martin and Katz, 1912) subdivided unit into the 

following members:

Jnk Katolinat Conglomerate Member (Tithonian)—Type section, designated by Detterman and 

others (1996), is on unnamed mountain on northeast shore of Lake Grosvenor (secs. 33 and 

34, T. 17 S., R. 35 W., Mount Katmai C-4 1:63,360-scale quadrangle). About 450-m-thick 

pebble-boulder conglomerate and minor amounts of greenish-gray to yellowish-green 

sandstone containing abundant quartz, chert, and granitic and metamorphic rock clasts. 

Restricted to northern part of Alaska Peninsula, it is, in part, lateral facies equivalent of 

upper part of Indecision Creek Sandstone Member. Lower contact is gradational with Indecision Creek Sandstone Member. Upper contact is unconformity with overlying Lower Cretaceous Herendeen Formation. Unit mapped north of the Savonoski River in southwest part of 

map, in Mount Katmai quadrangle

Jni Indecision Creek Sandstone Member (Tithonian and Kimmeridgian)—First recognized by Keller and Reiser (1959); designated as informal member of Naknek Formation 

by Detterman and Reed (1980); formally named and described by Detterman and others 

(1996). Detterman and others (1996) designated an approximately 870-m-thick type section on east-facing slope of unnamed mountain near Mount Chiginagak in the Ugashik A-4 

1:63,360-scale quadrangle, south of the map area. Consists of thin-bedded to massive, locally 

crossbedded, medium-gray, fine- to medium-grained arkosic sandstone and siltstone. Fresh 

biotite and hornblende are minor, but important, components of sandstone because they are 

interpreted to indicate first-cycle erosion from Alaska-Aleutian Range batholith. Indecision 

Creek Sandstone Member is abundantly fossiliferous, but almost exclusively restricted to 
16

pelecypods of genus Buchia. Depositional environment is shallow-water shelf to inner neritic 

(Detterman and others, 1996). Lower contact is conformable and slightly gradational with 

Snug Harbor Siltstone Member. Upper contact is unconformity with overlying Upper Cretaceous or Tertiary strata except where conformably and gradationally overlain by Staniukovich 

Formation or, in Mount Katmai region, Katolinat Conglomerate Member. Indecision Creek 

Sandstone Member is most widely exposed member of Naknek Formation

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). Sandstone is rich in 

quartz (40–45 percent) and sodic feldspar (30–35 percent) and as well as 15–20 percent 

hornblende and tourmaline. Volcanic lithic fragments make up 2–3 percent of the rock. 

Grains are sub-angular to sub-rounded. 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 

siltstone contains a higher proportion of volcanic- and sedimentary-sourced components. 

Detterman and Hartsock (1966) indicated that most sections of Pomeroy Arkose Member 

have a 70- to 350-ft-thick (21- to 113-m-thick) gray, medium-bedded to massive, arenaceous 

siltstone horizon, usually in the lower part of the member. Unit is sparsely fossiliferous; 

Lytoceras, Phylloceras, and Buchia concentrica suggest 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 this 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; calcareous gray sandstone beds are minor part 

of the unit (Detterman and Hartsock, 1966); unit is about 250 m thick. 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). It is the lowest abundantly fossiliferous member of 

the Naknek; main fossils are Buchia, including Buchia concentrica, and 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” and indicated to be as much as 250 m thick by Detterman and Hartsock (1966) 

who considered unit of 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 the 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 based on Cardioceras martini in lower part of member and its 

association with Cardioceras distans, which then without Cardioceras martini continues to 

the top of the member. Detterman and others (1996) assigned only an Oxfordian age to the 

unit based on reinterpretation of the available information. Pelecypods including Pleuromya, 

Quenstedtia, Oxytoma, Thracia, and Astarte are present but uncommon as are gastropods, 

echinoids, and belemnites (Detterman and Hartsock, 1966). Lower contact intertongues with 

the Chisik Conglomerate Member. Upper contact gradational with overlying Snug Harbor 

Siltstone Member

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

crossbedded, quartzose sandstone, about 125 m thick on Chisik Island, but as much as 614 m 

thick south of map area. Clasts, as large as 2 m, are mainly granitic rocks, but up to 20 percent are metamorphic and volcanic rocks (Detterman and others, 1996). A K-Ar age determination (table 1, sample 62Ale 6e) on a quartz diorite cobble yielded an age of 156.6 Ma 

on biotite and 159.7 on hornblende, indicating provenance was likely the Alaska-Aleutian 

Range batholith (Detterman and others, 1965). Gradational and conformable contact with 

overlying Northeast Creek Sandstone Member
17

Jc Chinitna Formation, undivided (Middle Jurassic, Callovian)—Massive, marine, gray arenaceous shale and siltstone containing numerous large limestone concretions. Contains mixture 

of plutonic and volcanic detritus presumed to have been derived from erosion of the Talkeetna Formation and related plutonic rocks of the Early to Middle Jurassic magmatic arc. 

Unit is best exposed along the west coast of Cook Inlet and 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). Unit also mapped in the Anchorage quadrangle, north of 

the Castle Mountain Fault system. Unit age revised to Callovian by Imlay and Detterman 

(1973). Subdivided into the following:

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

a thick sandstone unit at its base; between 275 m to more than 400 m thick in Kenai and 

Iliamna quadrangles (Detterman and Hartsock, 1966). Large ellipsoidal concretions and 

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

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

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

and fractures into angular fragments. Graywacke sandstone of the 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 common; fresh 

surfaces are very dark-gray, 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 the lower sandstone (Detterman and Hartsock, 1966), whereas a 

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

higher in the section. Ammonites, include Cadoceras, Stenocadoceras, Pseudocadoceras, 

Kepplerites, Kheraiceras, and Lilloettia (Detterman and Hartsock, 1966) and indicate 

Callovian age, although the uppermost zone of the Callovian has not been identified. Paveloff Siltstone Member is the age equivalent of Shelikof Formation of the Alaska Peninsula; 

Shelikof Formation 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), typically about 

250 m thick but thickens to as much as 400 m in exposures to the south. Numerous small 

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

randomly throughout the section. Thin, fine-grained, greenish-gray sandstone interbeds 

occur in the siltstone and a more massive sandstone unit is found at the base of the section. 

Limestone concretions are generally ovoid and as much as 12–13 cm in diameter, extremely 

hard, and commonly fossiliferous (Detterman and Hartsock, 1966). Sandstone interbeds 

are compositionally similar but coarser grained than the siltstone. A thick (6–30 m) 

sandstone bed present at the base of the unit is medium-bedded to massive, fine- to mediumgrained and grayish-brown. On Chisik Island (Kenai quadrangle), a thick (65 m) channel 

conglomerate, consisting mainly of volcanic rock cobbles and boulders, is present at the 

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 Paracadocreas, Pseudocadoceras, Phylloceras, Lilloettia, 

Kheraiceras, Kepplerites, and Xenocephilites, numerous pelecypods, and rare belemnites, 

gastropods, and brachiopods indicate early Callovian age (Detterman and Hartsock, 1966)

Jt Tuxedni Group, undivided (Middle Jurassic, lower Bathonian to Aalenian)—Fossiliferous, 

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 a graywacke matrix. 

Unconformably overlies Talkeetna Formation and is disconformably overlain by Chinitna 

Formation. Locally subdivided into the Red Glacier Formation, Gaikema Sandstone, Fitz 

Creek Siltstone, Cynthia Falls Sandstone, Twist Creek Siltstone, and Bowser Formation. 

Descriptions shown below are derived from Detterman and Hartsock (1966); age revised to 

Bathonian to Bajocian by Imlay and Detterman (1973) and to lower Bathonian to Aalenian 

by Detterman and Westermann (1992). Undivided unit shown only in Anchorage quadrangle

Jtb Bowser Formation (Middle Jurassic, lower Bathonian and upper Bajocian)—Heterogeneous assemblage of sandstone, conglomerate, shale and siltstone characterized by rapid 
18

facies changes (Detterman and Hartsock, 1966). Massive, light- to dark-gray sandstone and 

conglomerate on the Iniskin Peninsula (Iliamna quadrangle). Coarse-grained sandstone and 

conglomerate matrix composed of angular fragments of feldspar and quartz; common accessory minerals biotite, augite, and magnetite. Light-gray sandstone is interbedded with darkgray sandstone and is commonly calcareous and contains numerous coquina beds composed 

almost entirely of pelecypods Inoceramus and Trigonia. Conglomerate clasts are dominantly 

felsic volcanic rocks and basalt but include about 10 percent granitic rocks. Siltstone as 

much as 100 m thick is interbedded with conglomerate and sandstone and north of Chinitna 

Bay siltstone occurs in units as much as 250 m thick, where it forms the dominant lithology 

in the Bowser Formation. Siltstone is massive to thin-bedded, medium- to coarse-grained, 

dark-brownish gray, weathering to light-brown. 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), unit can be 

divided into two faunal zones which occur on either side of the break between Callovian and 

Bathonian. Lower faunal zone immediately overlies unconformity with Twist Creek Siltstone and contains ammonites Cranocephalites, Arctocephalites, Siemiradzkia, Cobbanites, 

and Parareineckia and is middle Bathonian in age. Upper faunal zone contains ammonites 

Xenocephalites, Kheraiceras, and Kepplerites

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) in Kenai and 

Iliamna quadrangles. The siltstone is dark-gray, weathers to dark-rusty brown, and contains 

many thin beds of volcanic ash that weathers bright-orange. Small limestone concretions 

are common throughout the unit and commonly fossiliferous. Abundant ammonite fauna 

includes Oppelia (Liroxyites), Megasphaerceras, Leptosphinctes, Lissoceras, and Normannites (Dettermanites), restricted to the contained limestone concretions

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

pebble conglomerate about 200 m thick (Detterman and Hartsock, 1966) in Kenai, Seldovia, and Iliamna quadrangles. Medium- to coarse-grained, greenish-gray to dark-green 

sandstone, graded bedding, weathers mottled light-gray due to the presence of zeolites. 

Sandstone consists mainly of angular fragments of feldspar and volcanic rocks in a compositionally similar silt-size matrix. Pebble conglomerate occurs in thin lenticular beds within 

the sandstone and 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 makes up 10 to 20 percent of the formation, and is interbedded with the sandstone and 

may contain a few limestone concretions. Like underlying Fitz Creek Siltstone and Gaikema 

Sandstone, unit is coarsest grained in the vicinity of Gaikema Creek and finer grained away 

from this area. Fossils are relatively uncommon, thought in part due to rapid deposition in a 

nearshore environment. Sparse fauna includes ammonites Chondroceras and Stephanoceras

and pelecypods Inoceramus sp. and Mytilus sp.

Jtf Fitz Creek Siltstone (middle Bajocian)—Thick sequence (up to 400 m thick) of massive, 

bluish dark-gray, arenaceous, coarse- to fine-grained siltstone that commonly weathers rusty 

orange and contains many small limestone concretions (Detterman and Hartsock, 1966) in 

Kenai, Seldovia, and Iliamna quadrangles. Fine-grained sandstone and, locally, conglomerate is interbedded. Siltstone in upper part of unit, 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 section. Unit is abundantly fossiliferous and is the lowest unit 

of the Tuxedni Group where ammonites are more numerous than pelecypods. A few nondiagnostic brachiopods are also present. Ammonites include Normannites, Teloceras, and 

Chondroceras and many other genera; pelecypods include Inoceramus and Pleuromya, both 

in forms distinctly different than those found in the lower parts of the 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) in Kenai, Seldovia, and Iliamna quadrangles. Sandstone commonly occurs 

in graded beds up to 3 m thick, whereas conglomerate is well sorted within individual beds 

and rarely shows grading (Detterman and Hartsock, 1966). Conglomerate is confined to the 

Iniskin Peninsula; clasts consist of “red and green felsitic volcanic rocks, aphanitic igneous 
19

rocks, and minor metasedimentary rocks” (Detterman and Hartsock, 1966, p. 26), all thought 

to be derived from the Talkeetna Formation. Rare granitic clasts in the Gaikema Sandstone 

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 brownish to rusty brown weathering. 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 unit (Detterman and Hartsock, 1966). Unit is fossiliferous throughout, containing 

pelecypods Meleagrinella, Trigonia, and Inoceramus and ammonites Witchellia(?), Stephanoceras, and locally, Sonninia (Papilliceras), Lissoceras, and Emileia. Carbonized plant 

remains are locally present

Jtrg Red Glacier Formation (lower Bajocian to Aalenian)—Thin-bedded to massive, red-brownweathering, dark-gray to moderate olive-gray, highly arenaceous siltstone, locally containing lenticular interbeds and concretions of reddish-gray, dense limestone and very minor 

coal seams (Detterman and Hartsock, 1966) in Kenai, Seldovia, and Iliamna quadrangles. 

Underlying this 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 m 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). Pelecypods include Meleagrinella, Trigonia, Inoceramus, 

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

assemblage includes Erycites, Tmetoceras, and Pseudolioceras and faunal zone 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 the uppermost 150 meters, Papilliceras, 

Strigoceras, Lissoceras, Stephanoceras, Stemmatoceras, and Skirroceras. Detterman and 

Hartsock (1966) suggested that formation “probably was deposited by progressive onlap 

onto an irregular south-dipping {present geometry} erosional surface on the Talkeetna Formation; some of the increase in section {thickness} probably due to subsidence of the basin 

during deposition (italics added)”

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

in the Talkeetna Mountains and west of Cook Inlet. 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). First 

described by Paige and Knopf (1907); the name Talkeetna Formation was introduced by 

Martin (1926), recognized as a widespread and important marker horizon in southern Alaska. 

Detterman and Hartsock (1966) formally divided unit into three members. Within the map 

area, Talkeetna Formation is locally subdivided, from top to bottom, into the following:

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

containing porphyritic andesite flows. Bedded tuff is tan, red, green, purple, or mottledcolor, thin-bedded to massive and fine- to coarse-grained (Detterman and Hartsock, 1966). 

Locally, tree stumps are preserved within tuff beds, suggesting subaerial deposition. However, thin-bedded laminated units that have graded bedding and contain rare belemnite 

fragments indicate some parts of the 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 Talkeetna Mountains north of map 

area, fossils (Weyla) in upper part of Talkeetna Formation, which is considered correlative to 

the rocks of Horn Mountain Tuff Member, indicate a late Pliensbachian and Toarcian (Early 

Jurassic) age (Arthur Grantz, USGS, 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 the map area (Detterman and Hartsock, 1966). 

Interbedded flows, tuff, and sedimentary rocks are thicker than in the underlying Marsh 

Creek Breccia Member, suggesting 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 

Member and their distribution suggests a separate source (Detterman and Hartsock, 1966). 

Estimated thickness of unit is between 685 and 870 m. No known fossil control
20

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, fining 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 the southern exposures of the 

unit, and coarser in the more northern exposures. Member has an estimated minimum overall 

thickness of 1,000 m although a complete section has never been measured and those that 

have been measured are sections cut by extensive faulting, or have obscure contact relations 

with plutons of the Alaska-Aleutian Range batholith, or have been assimilated into the magma 

chamber under Iliamna Volcano (Detterman and Hartsock, 1966). Fossils are unknown in the 

map area and age is only inferred based on correlations with rocks in the Talkeetna Mountains 

type area

J^lm Limestone and marble (Lower Jurassic or Upper Triassic?)—Light- to dark-gray, fine to 

medium-grained, massive to poorly bedded, non-fossiliferous limestone in discontinuous 

lenticular bodies within shear zones in the Talkeetna Formation. Individual lenses as much 

as 30 m thick in broad shear zones in the Talkeetna Formation along the Castle Mountain 

Fault and in the Talkeetna Mountains quadrangle (Winkler, 1992; Csejtey and others, 1978). 

Near intrusions, recrystallized to marble. Winkler (1992) assigned a Triassic age to the unit; 

however, there is little evidence to support such an assignment and there is no evidence to 

suggest a Triassic age for the enclosing Talkeetna Formation. The Upper Triassic Kamishak 

Formation is associated with the Talkeetna Formation on the Alaska Peninsula and possibly 

some of these limestone and marble bodies may represent an equivalent unit

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 the 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 informal Port Graham formation of Kelley (1980) (^pg here). 

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

distinguish from the Port Graham formation of Kelley (1980). 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 

unit from other parts of his Pogibshi formation. The uppermost member, the Naskowhak 

member, consists of greenish-gray tuffaceous mudstone and siltstone, and tuff. Locally, the 

basal part of the Naskowhak member includes laterally extensive coal-bearing units that 

help to distinguish the Pogibshi formation of Kelley (1980) from the otherwise lithologically 

similar Talkeetna Formation on the west side of Cook Inlet. Unit is reportedly intruded by the 

tonalite of Dogfish (Koyuktolik) Bay (^qd) and possibly by the diorite of Point Bede (^qd) 

[both in the southern Seldovia quadrangle]; if so, the recently determined Triassic age on 

the diorite (D.C. Bradley, USGS, 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 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 the two Martin reports (1915, 

1926) included several species of scleractinian corals, numerous bivalves (mostly pectinaceans), gastropods, and ammonites. A brief visit by R.B. Blodgett (USGS contractor, written 

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

showed it to contain numerous pectinacean bivalves, gastropods, and several species of scleractinian corals. Bivalves, both articulated and disarticulated specimens, appear to belong to 

the genus Weyla, an Early Jurassic index fossil found primarily along 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 fossil 

assemblages. A fossil determination by A.K. Armstrong (USGS) of a collection made by J.S. 

Kelley yielded poorly preserved Permian corals (sample 75JK-151B, www.alaskafossil.org). 
21

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 Shuyak Island and farther south Afognak Island, which is intruded 

by the Afognak pluton of Triassic age (see also Wilson and others, 2005). Rioux and others 

(2007) reported an age on a metamorphosed volcaniclastic rock within the Border Ranges 

Fault system in the Anchorage quadrangle that may be equivalent to the informally defined 

Pogibshi Formation of Kelley (1980). The sample yielded two distinct populations of ages, 

reported as 202.1±1.2 and 205.8±0.4 Ma by Rioux and others (2007)

^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 the lower 

(middle Norian) part of the unit (R.B. Blodgett, USGS contractor, written commun., 2007 and 

unpub. data of Humble Oil Company [now Exxon-Mobil] reported by R.B. Blodgett); whereas 

the upper part (of late Norian age) is composed of considerably more volcaniclastic fragmentrich and shaly beds. Volcaniclastic fragment-rich beds contain a diverse, but uncommon molluscan fauna consisting of both bivalves and gastropods; shaly beds tend to have a monotaxic 

fauna of monotid bivalves. Fossils locally abundant, as reported by Kelley (1980); mostly thinshelled mollusks, but also corals, echinoids, ammonites, and trace fossils. Martin and others 

(1915) and Martin (1926) reported bivalves Halobia cf. H. superba Mojsisovics, Pseudomonotis (now placed in genus Monotis) subcircularis Gabb, Nucula?, and coral Astrocoenia? sp. 

Halobia cf. H. superba was suggested by Martin (1926) to possibly indicate Carnian age (this 

species was later re-identified as middle Norian age Halobia by Silberling and others, 1997), 

while Monotis subcircularis was noted to indicate late Norian age. Silberling and others 

(1997) provided a detailed analysis of known Late Triassic bivalve fauna known from the Port 

Graham area (Seldovia quadrangle) and reported that middle Norian age Halobia lineata and 

H. dilitata in collections reported by Martin (1915; USGS Mesozoic localities 6380 and 6382, 

respectively). Two different species of late Triassic Monotis were reported by Silberling and 

others (1997): Monotis (Pacimonotis) subcircularis and Monotis (Monotis) alaskana, and late 

middle Norian ammonite Steinmannites. Kelley (1984) and Bradley and others (1999) assigned 

an upper age limit of Early Jurassic to unit, although no fossils of this age are found. Early 

Jurassic fossils do occur in upper part of overlying Pogibshi formation of Kelley (1980)

Kamishak Formation (Upper Triassic, Norian)—Limestone, chert, porcellanite, and minor tuff. 

Detterman and Reed (1980) divided unit into three formal members, in descending order: 

Ursus Member, an unnamed middle Member, and Bruin Limestone Member. Originally 

named the Kamishak Chert by Martin and Katz (1912) and renamed Kamishak Formation 

by Kellum (1945, as cited in Detterman and Reed, 1980). Unit primarily found along west 

side of Cook Inlet, east of Alaska Range-Aleutian Range crest. Fossils in unnamed middle 

Member and Bruin Limestone Member yield a Norian age (Detterman and Reed, 1980; C.D. 

Blome, USGS, oral commun., 1981). Cut by abundant dikes and sills related either to Cottonwood Bay Greenstone or Talkeetna Formation. Subdivided into the following:

^ku Ursus Member—Thin-bedded, light-gray limestone, locally dolomitic, and minor interbedded gray chert and porcellanite and minor tuff, about 150 m thick. Limestone is fine-grained 

biomicrite. Depositional environment was moderate to high-energy, shallow water. Contact 

with overlying Talkeetna Formation is considered a disconformity; however, in most locations it is a fault, nonetheless units appear structurally conformable (Detterman and Reed, 

1980). Contact with underlying middle Member is gradational where thin-bedded, light-gray 

limestone changes to dark-gray, medium-bedded to massive limestone of middle Member 

(Detterman and Reed, 1980)

^km Middle Member—Thin- to medium-bedded, dark-gray to black limestone and calcilutite, 

locally dolomitic, and minor black chert and gray tuff, about 530 m thick. Limestone is 

fine-grained microsparite. Calcite is locally altered to chert, suggesting a deep-basin environment. Unit is nearly always complexly folded and commonly cut by dikes and sills of 

mafic igneous rocks (Detterman and Reed, 1980). Contact with underlying Bruin Limestone 

Member may be gradational; however, Bruin Limestone Member is more highly contorted 

(Detterman and Reed, 1980)
22

^kb Bruin Limestone Member—Massive to thin-bedded, light- to dark-gray limestone; coral and 

echinoid bioherms, banded green and white chert at least 610 m thick. Environment was 

apparently high energy, marine, shallow water. Sections of unit are incomplete and dikes 

and sills of mafic igneous rocks similar to underlying Cottonwood Bay Greenstone (^c) are 

ubiquitous; unit is considered at least 610 m thick. Contact with Cottonwood Bay Greenstone is not exposed, but where units are in close proximity, they appear concordant (Detterman and Reed, 1980)

Shuyak Formation (Upper Triassic)—Consists of two members: a sedimentary member in 

fault contact with a volcanic member. Sedimentary member is inferred to overlie volcanic 

member (Connelly and Moore, 1979). Unit structurally overlies schist of Kodiak Island 

(Jsch) and Uyak Complex (KMm) but usually is separated from them by the Afognak 

pluton (^qd). Estimated thickness is more than 7,000 m (Connelly, 1978). Divided into the 

following:

^ss Shuyak Formation, sedimentary member—Volcaniclastic sequence consisting of thin- to 

medium-bedded lithic sandstone that contains lesser amounts of conglomerate, argillite, and 

siliceous tuff and is intruded by mafic dikes and sills (Connelly and Moore, 1979) in the 

Barren Islands and on Shuyak Island. As described by Connelly and Moore (1979), unit is 

rich in primary andesitic material and displays flute casts and complete Bouma sequences 

that indicate deposition by turbidity currents. Rocks either are broadly folded or dip homoclinally to southeast and have undergone prehnite-pumpellyite-facies metamorphism 

(Connelly and Moore, 1979). Pelecypod Halobia halorica of Late Triassic (Norian) age as 

identified by N. Silberling was reported by Connelly and Moore (1979)

^sv Shuyak Formation, volcanic member—Vesicular pillow greenstone; locally contains beds 

of pillow-breccia agglomerate, tuff, and argillite (Connelly and Moore, 1979) on Shuyak 

Island. Greenstone is tholeiitic in composition

IGNEOUS ROCKS

VOLCANIC AND HYPABYSSAL 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 the vicinity of and 

west of Redoubt Volcano where these 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 may constitute 60 percent or more of rocks. Unit typically forms 

volcanic edifices and includes deposits related to the volcanic centers at Katmai, Kaguyak 

Crater, Four-Peaked Mountain, Mount Douglas, Augustine Island, Iliamna Volcano, Mount 

Spurr, and Redoubt Volcano. May include Tertiary volcanic rocks as mapped by Magoon 

and others (1976). South of Capps Glacier, rocks of this unit have been variously assigned 

by previous workers to glacial deposits or volcanic deposits. As described by Emily Finzel 

(written commun., 2009), these outcrops consist of clast- and matrix-supported conglomerate 

containing ashy stringers, and appearing volcaniclastic. Clasts are extrusive, mostly vesicular 

and porphyritic, dark-gray to black basalt(?) with white phenocrysts. Minor clasts have 

maroon and yellow-green groundmass. Unit is locally subdivided into the following:

Qvd Dacitic to rhyolitic domes (Holocene)—“Dacitic or rhyolitic vitrophyres having plagioclase, 

pyroxene, and (or) hornblende phenocrysts in a brown to grayish brown glassy groundmass 

that is locally devitrified. Local red to yellow orange fumarolic alteration” (Riehle and 

others, 1993). Dacitic domes at Kaguyak Crater

Qhv Volcanic rocks (Holocene)—Andesite and basalt lava flows, and sills. Primarily extrusive 

rocks, typically form present day edifices of historically active volcanoes. Unit caps ridges 

and includes massive lava flows, agglomerate, and lahar deposits, primarily associated with 

volcanic centers at Augustine, Redoubt, Spurr, Hayes, and Iliamna including North Twin and 

South Twin

Qad Andesite and dacite domes (Pleistocene)—Dome complexes of Double Glacier volcano 

(Reed and others, 1992) and Johnson Glacier dome complex of Iliamna Volcano (Waythomas and Miller, 1999). Composite dome complex of Double Glacier volcano consists 
23

of medium- to coarsely porphyritic hornblende andesite and dacite. 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) and one (sample 90AR 99), yielded 887±15 ka 

(Reed and others, 1992, table 1 here)

QTv Old volcanic rocks, undivided (Pleistocene or Pliocene)—Detterman and Reed (1980) mapped 

two eruptive centers within the Jurassic part of the Alaska-Aleutian Range batholith in the 

Iliamna quadrangle. Only one site was examined briefly by Detterman and Reed (1980). 

Scoriaceous olivine basalt at Seven Sisters (about 40 km west of Augustine Island) is 

thought to represent a volcanic neck and feeder-dike system. Second site, about 17 km 

northwest of Chinitna Bay, was thought to consist largely of andesitic tuff and breccia 

having a smooth to gently rounded topographic expression. No age control exists for either 

center; however, Detterman and Reed (1980) suggested olivine basalt of the Seven Sisters 

is most closely related to Intricate Basalt (unit Tb) of possible Pliocene age, whereas the 

andesite(?) at West Glacier Creek seemed more similar to Augustine Volcano and Iliamna 

Volcano products. Also included in this map unit, but too small to show, are basaltic andesite 

porphyry dikes and sills intruding Jurassic sedimentary rocks between Chinitna and Iniskin 

Bays and andesite (Detterman and Hartsock, 1966) and basalt lava flows, sills, and plugs 

in Mount Katmai area (Riehle and others, 1987). Extrusive rocks typically cap ridges and 

include massive lava flows, agglomerate, and lahar deposits (Riehle and others, 1987)

Tvu Tertiary volcanic rocks, undivided (Tertiary)—Andesite, basalt, and dacite lava flows, tuff, 

lahar deposits, volcanic breccia, and hypabyssal intrusions throughout the map area. In 

Anchorage and Talkeetna Mountains quadrangles, unit includes small lenses of fluviatile 

conglomerate. Csejtey and others (1978) and Winkler (1992) described a crude stratification, where felsic rocks and pyroclastic rocks occur stratigraphically lower and basaltic 

and andesitic flows occur in the upper part of the section. Age control is sparse and unit is 

generally considered to range throughout the Tertiary; however, most age determinations 

yield Eocene and older ages (54–60 Ma, Winkler, 1992). Outside the map area, Eocene plant 

fossils have been found in volcaniclastic rocks in unit. Flows possibly correlative with unit 

occur within the Arkose Ridge Formation and yield ages ranging 46 to 54 Ma. Detterman 

and Hartsock (1966) suggested middle to late Tertiary age for undivided volcanic rocks in 

Iliamna quadrangle. Locally subdivided into the following:

Tb Basaltic volcanic rocks (Pliocene)—Intricate Basalt of Detterman and Reed (1980), a “glassy 

to porphyritic black to dark-green olivine-augite basalt,” largely in the vicinity of Intricate 

Bay (west of map area, however a small outcrop of this unit appears at edge of map west of 

Kakhonak Lake), but also south of Gibraltar Lake outside of map area, and in mafic dikes 

too small to show on map. Detterman and Reed (1980) reported K-Ar dates of 4.4±0.5 and 

5.1±1.0 Ma on an olivine basalt dike that cuts Naknek Formation

Tpg Gibraltar Lake Tuff (Pliocene? to Oligocene?)—Defined by Detterman and Reed (1980), 

unit is divided into upper nonwelded member and lower welded member. Upper member, 

west of map area, consists of light-gray to white crystal ash-flow tuff having a maximum 

thickness of 152 to 182 m; locally capped by basalt flows of Intricate Basalt. Lower member 

is at least 300 m thick, consists of light- to medium-gray and tan rhyolitic crystal and lithic 

welded tuff; locally, interbedded porphyritic rhyolite flows. Unconformably overlies older 

Tertiary basalt and andesite. Detterman and Reed (1980) had little age control; however 

based on geomorphic character and comparison with other Tertiary age units, they suggested 

that the age was more likely Pliocene than Oligocene

Tmv Paleogene volcanic rocks, undivided (Oligocene and Eocene)—Ranges from rhyolitic 

breccia, ash-flow tuff, and flows to dark-gray to green, coarse andesitic and basaltic volcanic rubble, lahar deposits, glassy to porphyritic basaltic andesite and andesite lava 

flows. Includes minor volcaniclastic sedimentary rocks and hypabyssal felsic porphyry. As 

described by Detterman and Reed (1980), in many cases eruptive centers can be identified, 

either as volcanic necks of eroded volcanoes or as caldera complexes. Limited age determinations unit and subdivisions range from approximately 29.3 to 44.4 Ma (Thrupp, 1987; 

Nelson and others, 1983). Inferred to represent northern extension of Meshik Arc of the 

Alaska Peninsula (Wilson, 1985). Locally subdivided into the following:

Tmb Basalt and andesite—Dark-gray to green, glassy to porphyritic basaltic andesite and andesite lava flows. Also includes andesite to basalt plugs, volcanic rubble, and breccia, including 

some agglomerate; may include deposits of lahars (Detterman and Reed, 1980). Many of the 
24

rocks are associated with identifiable eruptive centers. Ages range from 29.5±1.5 to 44.4±1.7 

Ma

Tmf Tuffaceous felsic volcanic rocks (Eocene?)—Rhyolitic breccia, ash-flow tuff, and flows in 

Tyonek, Ilimna, and Lake Clark quadrangles. Cream, light-gray, green, and purple, bedded 

lithic, crystal, and vitric tuff; light-gray to tan, welded crystal and lithic tuff most common 

(Detterman and Reed, 1980)

Tfv Felsic volcanic and sub-volcanic rocks (Eocene)—Rhyolite, rhyodacite, and dacite. Predominantly subaerial lavas and tuffs, and lesser domes and other hypabyssal intrusions; includes 

minor quartz-feldspar-sericite tuffaceous sandstone of Anderson (1969a). Rhyolite flows 

are aphanitic to sparsely porphyritic with plagioclase and (or) hornblende phenocrysts and 

are locally flow-banded; dacitic tuff and subvolcanic intrusions contain oscillatory-zoned 

plagioclase, quartz, and amphibole (L.E. Burns, Division of Geological and Geophysical 

Surveys, written commun., 2010; Oswald, 2006). Individual flows and tuffs are meters to 

tens of meters thick; overall unit may be as much as several hundred meters thick. Includes 

plagioclase-quartz-hornblende dacite porphyry (Tdp) of Csejtey (1974), porphyritic rhyolite 

(rh) of Anderson (1969a), rhyolite, dacite, and ash flow tuff (Trt, Tdt, Tft), rhyolite, rhyodacite, and dacite (Tr, Trd, Td), and vitric and dacite flows (Tv, Tdf) of L.E. Burns (DGGS, 

written commun., 2010), dacite porphyry (Tdp), rhyolite tuff (Trt), felsic tuff and extrusive rhyolite of Oswald (2006). Not part of Meshik Arc (Wilson, 1985), typically found in 

Anchorage and Talkeetna Mountains quadrangles. Ages range from 35.6±0.2 to 45.2±0.5 Ma 

(table 1)

Tem Mafic volcanic rocks (Eocene)—Predominantly reddish-brown-weathering basaltic andesite 

and basalt lavas, often columnar-jointed, and including lesser tuff, and local cinder deposits. 

Basaltic lavas are aphanitic to porphyritic, having plagioclase and (or) pyroxene phenocrysts; 

andesite lavas locally are trachytic textured and plagioclase- and Fe- Ti-oxide rich (Oswald, 

2006). Also includes aphanitic and sparsely phenocrystic pyroxene andesite (Tpa) of Csejtey 

(1974); andesite and basalt of Anderson (1969b); andesitic and basaltic tuff (Tat, Tbt), andesite and basalt flows (Taf; Tmb, Tma) , basalt (Tb), and andesite (Ta) of L.E. Burns (DGGS, 

written commun., 2010); and andesite tuff (Tat), andesite, intermediate, and mafic flows (Taf, 

Tif, Tmf), basalt (Tb), and “mafic to intermediate composition lavas” of Oswald (2006). 

Individual flows and tuff beds are tens of meters thick; as a whole, unit may be several hundred meters thick. Not part of Meshik Arc (Wilson, 1985), typically found in Anchorage and 

Talkeetna Mountains quadrangles. Ages range from 44.4±0.2 to 60.1±4.6 Ma (table 1)

TKd Dikes and sills (Tertiary to Cretaceous)—Dikes, sills and small intrusive bodies of mafic to 

felsic composition. Dikes yield a wide range of ages (table 1), representing multiple intrusive events throughout region. Felsic and intermediate hypabyssal rocks from the Anchorage 

quadrangle include six whole-rock K-Ar ages of 40.0±1.6 to 54.8±2.7 Ma and three zircon 

fission track ages of 36.8±4.8 to 41.3±6.0 Ma (Winkler, 1992; table 1 here). Also in the 

Anchorage quadrangle, a basalt sill yielded a whole-rock K-Ar age of 40.9±1.6 Ma (Silberman and Grantz, 1984). A 40Ar/39Ar plateau age on amphibole from an andesite dike yielded 

an age of 58.64±0.52 Ma. Unit also includes unit TJds of Winkler (1992) that yielded widely 

different whole-rock K-Ar ages of 38±2 and 130±6 Ma. 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 (sample 

88ADw 230; W. Clendenin, South Carolina Department of Natural Resources, Geological 

Survey, written commun., cited in Bradley and others, 1999). Lytwyn and others (2000) 

described a suite of dikes ranging in composition from basalt to rhyolite that they associated 

with near-trench intrusions commonly related to the subduction of Kula-Farallon spreading 

center

Tpv Intermediate to mafic volcanic rocks (Paleocene)—Massive welded tuff beds greater than 

several meters thick; generally andesitic and dacitic composition. Unit includes informally 

named Porcupine Butte andesite of Solie and Layer (1993), a columnar jointed andesite plug 

forming the neck of a Paleocene volcanic center. Solie and Layer (1993) obtained a 40Ar/39Ar 

isochron age on biotite from the andesite at Porcupine Butte of 60.82±0.43 Ma (sample 

90DNS 31, table 1). A U/Pb zircon age of 61.9±0.3 Ma was determined on crystal-lithic 

andesite tuff east of the toe of the Trimble Glacier (Friedman, Univ. of British Columbia, 

written commun., 2005). Only basalt in this unit lies between Trimble and Hayes Glaciers 

(in Tyonek quadrangle) and includes rocks mapped by Solie and others (1991) as their unit 
25

TMzb. Several 40Ar/39Ar whole-rock ages were obtained on the basalt, yielding 66.0±0.9, 

57.9±1.9, and 62.1±0.4 Ma (Layer and Solie, 2008, table 1 here). The basalt was mapped by 

Solie and others (1991) in contact with the shale, siltstone, and limestone unit (KJsl) of Solie 

and others (1991); in the vicinity of the basalt they reported their KJsl unit may underlie the 

basalt and their KJcs unit. Some welded tuff of probable Cretaceous age appears similar in 

texture and composition; however, we distinguish these by their more greenish color

TKft Felsic tuff (early Tertiary or Cretaceous)—Crystalline tuff and lapilli tuff in north-central 

Tyonek quadrangle (Solie and others, 1991)

TKv Older volcanic rocks, undivided (Paleocene to Upper Cretaceous)—Plagioclase porphyritic 

andesite, dacite, and less common basalt. Rocks deposited as tuff have variable degrees 

of welding and variable thickness. In southwest part of Talkeetna quadrangle, Reed and 

Nelson (1980) mapped interbedded tuff, mafic volcanic flows, sandstone, shale, and minor 

calcareous mudstone as their unit Tvs. Volcanic rocks are chiefly medium- to coarse-grained 

greenish-gray crystal lithic lapilli tuff and volcanic rubble flows in units as much as 150 

m thick. They considered these rocks Tertiary age and related to early or middle Tertiary 

plutonism in the Alaska Range; however, mapping by Haeussler suggests that these rocks 

may be Cretaceous age on the basis of correlation with the volcanic rocks north of Dickason 

Mountain in north-cental Tyonek quadrangle, described below in unit Kv. Where possible, 

on the basis of field evidence, largely color, or geochronologic analysis, volcanic rocks were 

assigned to map units Tpv, Kv, or Kvs. Locally subdivided into the following:

Kv Intermediate and felsic volcanic rocks (Cretaceous)—Andesite, dacite, and rhyolite flows 

and tuff. Includes massive and crystal-rich tuff, containing either hornblende or plagioclase 

as phenocryst phases as well as flow-banded rhyolite. Hornblende andesite, north of Dickason Mountain in Tyonek quadrangle, yielded a whole-rock 40Ar/39Ar age of 58.3±1.1 Ma 

and a hornblende age of 89.7±3.0 Ma (table 1). The 58.3 Ma date is similar in age to that of 

a pluton exposed in the region surrounding this outcrop (Bear Cub pluton, Solie and Layer, 

1993); however, hornblende age suggests that eruption of the andesite may have been earlier. 

Rocks north of Dickason Mountain in north-central Tyonek quadrangle yielded a 98.2±0.4 

Ma age (table 1). Flow-banded rhyolite between north and south branches of Trimble Glacier yielded a 40Ar/39Ar age of 122.4±1.5 Ma (table 1), much older than age of the surrounding Paleocene granite of unit TKg (dated at the same locality at 60.4±0.8 Ma (table 1) and 

thus the volcanic rock age may quite likely be reset by this intrusion and as such is probably a minimum age. In Talkeetna Mountains quadrangle, Drake and Layer (2001) reported 

40Ar/39Ar ages of 100.5±0.7 and 132.8±0.7 Ma on a metarhyolite, however, phases dated 

yielded poor plateaus and were strongly discordant (table 1)

^vm Mafic dikes, sills, and plugs (Triassic?)—Diabasic bodies intruding Shuyak Formation (unit 

^ss and ^vs) on west side of Shuyak Island at Neketa Bay in Afognak quadrangle; unit 

does not have visible thermal aureoles (Connelly and Moore, 1979)

^n Nikolai Greenstone and related rocks (Upper and (or) Middle Triassic)—Massive, dark-graygreen, dark-gray-brown, and maroon-gray, subaerial and submarine basalt flows and minor 

interbedded volcaniclastic sedimentary rocks, aquagene and epiclastic tuff, breccia, argillite, 

and radiolarian chert (Nokleberg and others, 1992). Widely distributed and several thousands 

of meters thick. Includes unnamed Triassic greenstone units in Talkeetna Mountains 

quadrangle (Csejtey and others, 1978). Commonly associated with Late Triassic carbonate 

and cherty carbonate rocks. Together with Chitistone Limestone and Nizina Limestone 

units, unit is one of the diagnostic units of the Wrangellia terrane (Jones and others, 1977). 

Commonly metamorphosed to lower greenschist facies. A similar and correlative unit, 

Cottonwood Bay Greenstone of the Alaska Peninsula, is described below

^c Cottonwood Bay Greenstone (Upper Triassic, Carnian or older)—Largely dark-gray to 

dark-green, porphyritic to amygdaloidal basaltic flows altered to greenstone (Detterman and 

Reed, 1980). Locally includes andesite, chert, limestone, and tuffaceous sedimentary rock, 

all weakly metamorphosed. About 60 percent of unit is metabasalt. Unit occurs either as roof 

pendants of or east of Alaska-Aleutian Range batholith in Iliamna and Mount Katmai quadrangles. Basal beds of Bruin Limestone Member of Kamishak Formation contain volcanic 

rocks similar to Cottonwood Bay Greenstone and with this evidence Detterman and Reed 

(1980) assigned a Late Triassic age to unit. Because Kamishak Formation is Norian age, 

Detterman and Reed (1980) stated that Cottonwood Bay Greenstone is probably older than 

Norian
26

PLUTONIC ROCKS

Ti Intrusive rocks, undivided (Tertiary)—Generally consists of fine- to medium-grained 

granodiorite and quartz diorite, but also includes granite (Nelson and others, 1983) in Mount 

Katmai and Iliamna quadrangles. Typically surrounded by well-developed hornfels zones 

and sporadic hydrothermal alteration in country rocks, includes volcanic necks, sills, and 

dikes. Also includes dikes, sills, and stock-like masses of felsic to mafic composition. Parts 

were included in Alaska-Aleutian Range batholith of Reed and Lanphere (1969, 1972). 

Locally divided into the following:

Tpi Intermediate intrusive rocks (Pliocene and latest Miocene)—Fine- to medium-grained 

granodiorite and quartz diorite, in intrusive bodies located along Pacific coast at Cape 

Douglas in Afognak quadrangle. K-Ar ages range from 4.2±0.2 to 8.22±0.25 Ma (Shew and 

Lanphere, 1992)

Toegr Granitic rocks (Oligocene and late Eocene)—Compositionally variable suite of medium- to 

coarse-grained, grayish-white, granitic rocks in Lake Clark, Iliamna, and northern Mount 

Katmai quadrangles (Reed and Elliott, 1970; Detterman and Reed, 1980; Nelson and others, 

1983; Riehle and others, 1987). Rock types range from biotite granite and biotite-hornblende 

granite to granodiorite, tonalite, and quartz diorite. Plutons are considered part of the 

Tertiary phase of the Alaska-Aleutian Range batholith of Reed and Lanphere (1969, 1972) 

and tend to be exposed along its western margin. Potassium-argon ages range from 25.6±0.8 

to 43.9 Ma (Reed and Lanphere, 1972, 1973; Nelson and others, 1985; Winkler, 1992; table 

1, here). Many of the younger ages are discordant

Tgn Gabbronorite (Oligocene? or Eocene?)—Coarse-grained hornblende- and biotite-bearing 

olivine gabbronorite stock located along the western map area boundary in the Lake Clark 

quadrangle (Nelson and others, 1983). “Relations to surrounding igneous rocks are unknown, 

but the stock is most likely related to the Tertiary volcanic rocks in this part of the Lake Clark 

quadrangle” (Nelson and others, 1983)

Thg Hypabyssal granitic rocks (Paleocene?)—Granitic rocks commonly containing miarolitic 

cavities, beta quartz, or granophyric texture (Solie and others, 1991; P.J. Haeussler, unpub. 

data, 2008) in Tyonek quadrangle. Medium- to fine-grained and usually porphyritic granitic 

rocks containing few mafic minerals. Miarolitic cavities commonly contain native sulfur. 

Commonly orange to reddish weathering. Age likely Paleocene, although few of these 

granitic rocks have been dated. Granite porphyry of Dickason Mountain (Solie and others, 

1991) is included here as one of these hypabyssal intrusive bodies. Layer and Solie (2008) 

obtained a 40Ar/39Ar plateau age on hornblende from this pluton of 54.7±0.6 Ma (table 1). 

Thus, these granitic rocks may be slightly younger than the most common Paleocene granitic 

rocks (Tpgr) of this region

Tpgr Granitic rocks of Paleocene age (Paleocene)—Predominantly medium-grained composite 

plutons of granite, syenite, tonalite, quartz monzonite, quartz monzodiorite, quartz diorite, 

granodiorite, and minor diorite. Biotite is chief mafic mineral. Locally weakly foliated or 

containing flow structures. Unit is widespread in map area and includes rocks of map unit 

Ti7, Ti10, and Ti11 of Nelson and others (1983), granodiorite of Harding Icefield (Wilson 

and others, 2008), the Ruth pluton of the McKinley series of Reed and Nelson (1980) and 

unit Tbgd of Csejtey and others (1978), as well as other plutons throughout the map area. 

Unit includes “near-trench” plutons (Bradley and others, 1998) intruding flysch of the 

Kodiak Formation (Kkd) and the Valdez Group (see below). Numerous radiometric ages 

(table 1) predominantly fall within the Paleocene epoch (53.2 to 64 Ma) and decrease eastward in an abrupt step at the approximate longitude of Cook Inlet. Plutons to the north of 

Castle Mountain Fault system intrude the so-called Kahiltna flysch (KJs and subdivisions) 

but yield ages consistent with other plutons of this unit west of Cook Inlet

Tgd Biotite-hornblende-granodiorite—Medium-grained, hypidiomorphic-granular biotite-hornblende granodiorite including the Chilligan River pluton of Reed and Elliott (1970; see also 

Reed and Nelson, 1980; Nelson and others, 1983; and Wilson and others, 2006). Tourmaline 

is a characteristic accessory mineral. Also includes unit Thgd, hornblende granodiorite of 

Csejtey and others (1978). K-Ar ages on hornblende and biotite range from 54.7±2.1 to 62.8 

Ma (table 1)

Tpd Biotite diorite—Diorite, quartz diorite, quartz monzodiorite and quartz monzonite in a large 

compositionally variable pluton cropping out over an area of about 11 km2 near Dickason 

Mountain (Solie and others, 1991) in Tyonek quadrangle. Generally coarse- to medium-
27

grained; a fine-grained variant is also found. More mafic phases contain olivine, both clinoand orthopyroxene, and biotite, whereas more quartz- and potassium-feldspar-rich phases 

contain amphibole and (or) biotite (Solie and others, 1991). Referred to as the Bear Cub 

pluton, Solie and Layer (1993) obtained a 40Ar/39Ar isochron age on biotite of 60.78±0.42 

Ma (sample 90DNS 7, table 1). Isochron analysis indicated slight excess argon in the 

sample, which might explain the slightly older plateau age of 61.2±0.4 Ma reported by 

Layer and Solie (2008) for this sample. Layer and Solie (2008) also reported a whole-rock 

40Ar/39Ar average age of 55.9±0.5 Ma on a nearby sample (sample 90WG 119, table 1)

TKg Granitic rocks, undivided (Paleocene to Late Cretaceous)—Fine- to coarse-grained, epizonal, 

biotite and biotite-hornblende granite; granodiorite; quartz monzonite; and alkali granite. 

Unit crops out in Anchorage and Talkeetna Mountains quadrangles (Winkler, 1992; J.M. 

Schmidt, unpub. data, 2008) and in western part of Tyonek quadrangle (Layer and Solie, 

2008) and includes the Styx River batholith of Reed and Elliott (1970). Plutons of this age 

and composition are widespread throughout south and southwest Alaska. Within map area, 

K-Ar and 40Ar/39Ar ages typically range between 63.6±0.4 and 67.4±0.7 Ma in Anchorage and Talkeetna Mountains quadrangles with an outlier at 57.7±0.5 reported (Csejtey 

and others, 1978, Winkler, 1992, Drake and Layer, 2001) and 59.1 to 66.8 Ma on biotite 

in Tyonek quadrangle (Reed and Lanphere, 1972, 1973; P.J. Haeussler, USGS, written 

commun., 2008; Layer and Solie, 2008; see table 1 here). Plutons of unit and its subdivision, 

TKgd, west of Susitna River were generally included in Alaska-Aleutian Range batholith of 

Reed and Lanphere (1969, 1972). Locally subdivided into the following:

TKgd Granodioritic rocks—Widespread, predominately epizonal medium-grained plutons of granodiorite, quartz diorite, diorite, and tonalite as mapped by Reed and Lanphere (1969, 1972, 

1973), Magoon and others (1976), Csejtey and others (1978), Winkler (1992), Wilson and 

others (1998). Locally weakly foliated, subhedral and (or) seriate texture. K-Ar ages on biotite and hornblende range between 60.0 and 73.5 Ma in map area (Reed and Lanphere, 1972, 

1973; Magoon and others, 1976; Detterman and others, 1976; P.J. Haeussler, USGS, written 

commun., 2008). A hydrothermally altered sample in Anchorage quadrangle yielded slightly 

younger ages of 54.7±1.6 to 56.6±1.7 Ma (table 1)

TKgb Gabbroic rocks (Tertiary and (or) Cretaceous)—Poorly exposed small bodies of mafic rocks in 

Talkeetna Mountains and Tyonek quadrangles. In Talkeetna Mountains, Csejtey and others 

(1978) described a medium- to light-gray, coarse- to medium-grained, plagioclase and palegreen hornblende-bearing leucogabbro; unpublished data of J.M. Schmidt also described the 

presence of diorite associated with the gabbroic rocks. Reed and Elliot (1970) and Wilson 

and others (1998) described diorite and olivine and (or) hornblende gabbro which occurs as 

intrusive bodies and inclusions in other Tertiary intrusive rocks in Tyonek quadrangle. Age is 

poorly constrained; however, samples of hornblendite and gabbro from Tyonek quadrangle 

yielded 40Ar/39Ar plateau ages of about 84 and 70 Ma (see replicates in table 1), respectively

Kgr Granite (Cretaceous)—Coarse-grained, light-pink granite; contains subrounded quartz phenocrysts as much as 1 cm in size; medium-grained matrix of sodic plagioclase and granophyre 

and about 10 percent biotite and hornblende (Nelson and others, 1983; Detterman and Reed, 

1980). Exposed in small plutons on southern margin of Lake Clark quadrangle and northern Iliamna quadrangle. Unit is in fault contact with surrounding plutons. Radiometric age 

control is lacking

Kgd Granodiorite (Late Cretaceous)—Granodiorite, tonalite, and quartz monzodiorite bodies 

mapped by Nelson and others (1983) in eastern Lake Clark quadrangle and considered part 

of Alaska-Aleutian Range batholith of Reed and Lanphere (1969, 1972). Medium- to coarsegrained, light-to medium-gray, contain hornblende, biotite, and, rarely muscovite and locally 

have cataclastic textures. Although mapped as separate plutons by Nelson and others (1983), 

a number of these are thought to be fault offset extensions of each other. In Anchorage quadrangle includes Willow Creek pluton of Winkler (1992) described as “pervasively altered, 

zoned pluton occupying much of headwaters of Willow Creek***. Pluton has a 30- to 200-

m-wide outer margin of hornblende quartz diorite and lesser hornblende tonalite. Pluton core 

is hornblende-biotite granodiorite, and lesser hornblende-biotite quartz monzodiorite and 

biotite quartz monzonite. Foliation is common, particularly near margins. Mineralized veins 

of Willow Creek mining district are predominantly hosted in this pluton. Partial alteration 

of mafic minerals to chlorite+magnetite±pyrite-chalcopyrite is common throughout pluton; 

sericite-ankerite-rutile alteration is common within 30 m of major gold-bearing quartz veins 
28

(R.J. Newberry, Univ. of Alaska, Fairbanks, written commun., 1989).” Samples from Willow 

Creek Pluton yield discordant and subconcordant K-Ar ages ranging from 70.1 to 78.8±2.4 

Ma and K-Ar ages from propylitized plutonic samples and gold-bearing veins and dikes 

that cut pluton indicate episodes of alteration and quartz veining at 66 and 57 Ma (Winkler, 

1992). K-Ar and 40Ar/39Ar ages from the west side of Cook Inlet range from 63.1±1.8 to 80.7 

Ma (Reed and Lanphere, 1972; P.J. Haeussler, USGS, written commun., 2008); a number of 

ages are either discordant or have disturbed spectra. One of the plutons (Ki13 of Nelson and 

others, 1983) intrudes rocks of apparent Late Cretaceous age

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

quartz diorite, but includes hornblende-pyroxene gabbro and diorite, quartz diorite, tonalite, 

and minor granodiorite (Nelson and others, 1983). Biotite-hornblende ratio is quite variable 

although color index remains constant. Deuteric epidote is common. Crops out in a northeast 

to southwest trend from Tyonek quadrangle to Lake Clark quadrangle. Mapped as Tertiary or 

Cretaceous granodiorite, quartz diorite, and diorite by Magoon and others (1976). Shown as 

Cretaceous here based on five samples with K-Ar ages ranging from 65.6 to 75.5 Ma (table 

1 here; Reed and Lanphere, 1972, 1973; Magoon and others, 1976; Detterman and others, 

1976)

Kqms Quartz monzonite, monzonite, and syenite (Late Cretaceous)—Massive, coarse-grained, lightgray porphyritic quartz monzonite (Detterman and Reed, 1980; Nelson and others, 1983) in 

Iliamna quadrangle. In northern Tyonek quadrangle, fine- to medium-grained pink monzonite 

or syenite intrudes sedimentary rocks of unit KJs. Pluton contains primary pyroxene and has 

yielded a concordant U/Pb zircon age of 80.3±0.1 Ma (table 1 here, P.J. Haeussler, USGS, 

written commun., 2008). Pluton is distinctive in being more K-spar rich and quartz poor than 

other granitic rocks in the region. K-Ar ages range from 75.6 to 85.5 Ma (Reed and Lanphere, 

1972; table 1, here). Unit also includes undated plutons in Talkeetna Mountains quadrangle

Kum Ultramafic rocks (Late Cretaceous)—“Small, structurally bounded, pervasively sheared, 

discordant bodies of serpentinized ultramafic rocks wholly enclosed in pelitic schist (Jps) 

near Bald Mountain Ridge” (Winkler, 1992), in Anchorage quadrangle north of community of Wasilla. Age of ultramafic rocks unknown, but early Late Cretaceous K-Ar ages of 

88.9±4.4 and 91.0±4.6 Ma (samples 73ACy 17 and 74Asj 100a, table 1) are presumed to 

date emplacement of serpentinite (Winkler, 1992). However, dates on enclosing schist (unit 

Kps) are significantly younger than these ultramafic ages

Kogr Older granite (Cretaceous, Cenomanian)—Coarse-grained biotite granite mapped on boundary between Tyonek and Talkeetna quadrangles. Yielded a 40Ar/39Ar plateau age on biotite 

of 96.9±1.4 Ma (sample 02PH 375A, table 1). In Tyonek quadrangle, unit also includes a 

syenite body exposed in the ice field north of Chakachamna Lake that yielded a 109±3.2 

Ma K-Ar date on hornblende (Reed and Lanphere, 1972; Magoon and others, 1976; sample 

70AR 208, table 1 here). This syenite yields a significantly older age than other intrusive 

bodies assigned to this unit; it is likely that the determined age was partially reset by intrusion of the surrounding Tertiary plutons, suggesting the emplacement age may be older yet

Klt Leucotonalite and trondhjemite (Early Cretaceous, Albian to Barremian)—“Medium-grained 

plugs and elongate, irregular-shaped, sill-like bodies of leucocratic plutonic rocks in northern Chugach Mountains in a zone about 5 km wide near Border Ranges Fault. According to 

Pavlis and others (1988), bodies intrude crystalline rocks of the Peninsular terrane (see Jones 

and others, 1981), the McHugh Complex of the Chugach terrane, and the Border Ranges fault 

which separates the terranes. Rocks generally are foliated and contain less than 10 percent 

mafic minerals including muscovite, biotite, or hornblende (and minor garnet). Radiometric 

ages *** include: K-Ar ages on hornblende (126 and 124 Ma), biotite (116 Ma), and muscovite (110 Ma), and a concordant U/Pb zircon age of 103 Ma ***. These ages are comparable to 40Ar/39Ar plateau ages on hornblende (129, 125, and 114 Ma), biotite (123 Ma), and 

muscovite (118 Ma), and two Rb/Sr mineral isochrons (133 and 130 Ma) reported by Pavlis 

and others (1988). This spectrum of ages and observed fabrics in the rocks are interpreted to 

result from progressive syntectonic emplacement of the plutons during ductile and then brittle 

thrusting along Border Ranges fault in the Early Cretaceous (Pavlis and others, 1988). How 

these plutons formed and were injected into an active regional subduction boundary is enigmatic” (Winkler, 1992). Many of the age determinations reported in this quote from Winkler 

(1992) are listed in table 1 along with additional age determinations. However, we were not 

able to locate sample data for a number of the reported age determinations
29

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

hypidiomorphic granular hornblende-biotite quartz monzodiorite containing variable 

amounts of clinopyroxene located in extreme northwest part of Kenai quadrangle and 

northeast part of Lake Clark quadrangle (Nelson and others, 1983; Wilson and others, 2006). 

Flow structures locally present and hornblende and plagioclase are aligned north-northeast. 

Two samples yielded strongly discordant K-Ar biotite and hornblende ages between 58.8 

and 97.5 Ma (Reed and Lanphere, 1973; Nelson and others, 1983; samples 70AR 184 and 

68AR 261, also 69AR 399, table 1 here). Part of the Alaska-Aleutian Range batholith of 

Reed and Lanphere (1969, 1972)

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

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

containing 10 percent muscovite and about 5 percent interstitial, perthitic potassium feldspar 

(Nelson and others, 1983). 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, 

1992). Comprises a large body in central part of batholith in Iliamna quadrangle west of Chinitna Bay. In Talkeetna Mountains and Anchorage quadrangles, consists of altered, sheared, 

and locally faintly foliated trondhjemite in large, structurally discordant, northeast-trending 

plutons intruding quartz diorite and amphibolite (Winkler, 1992). Generally accepted K-Ar 

age determinations range from 129 to 148.5 Ma throughout exposure area and can be 

considered cooling ages; U/Pb zircon ages range between 152.7 to 156.9 Ma in Talkeetna 

Mountains quadrangle (see table 1)

Jqd Quartz diorite, tonalite, and diorite (Jurassic)—Locally foliated medium-grained quartz diorite and tonalite; the dominant map unit of Alaska-Aleutian Range and Talkeetna Mountains 

part of the batholith. Hornblende is the primary mafic mineral, biotite increases in proportion 

to the presence of quartz and potassium feldspar. Detterman and Reed (1980) reported that 

rocks of unit in Iliamna region grade to diorite but they did not observe it grading into quartz 

monzonite or granodiorite of their unit Jqm. Potassium-argon ages generally range from 146 

to 181 Ma, discounting discordant or suspect age determinations; most age determinations 

cluster around 160 Ma (table 1). U/Pb zircon ages range from 168.9 to 190.5 Ma for the 

batholith (table 1) in Talkeetna Mountains and Anchorage quadrangles. Two small exposures of gabbro and diorite are included in unit. Includes a group of lamprophyre and basalt 

dikes on west side of Cook Inlet that intrude the quartz diorite. Plutons are probably plutonic 

equivalent of volcanic rocks of Talkeetna Formation, usually found in close proximity to 

Talkeetna Formation. The Alaska-Aleutian Range batholith has been best described and dated 

by Reed and Lanphere (1969, 1972, 1973). Two samples (62Ale 2 and 65AR 906, table 1), 

collected northeast of Iliamna Lake yielded middle Cretaceous K-Ar age determinations and 

may indicate presence of an otherwise unrecognized middle Cretaceous magmatic event or 

may more likely indicate resetting by nearby middle Tertiary plutons

Jqm Granodiorite and quartz monzonite (Middle and late Early Jurassic)—Whitish-gray, 

medium-grained, biotite granodiorite, minor hornblende, and accessory primary muscovite. Medium-grained, light-gray with a pinkish cast quartz monzonite; locally may include 

quartz diorite and trondhjemite. In Mount Katmai quadrangle, unit exposed on eastern 

margin of Jurassic batholith on Alaska Peninsula part of the map. K-Ar age determinations 

range from 155 to 174 Ma and U/Pb zircon ages in Talkeetna Mountains and Anchorage 

quadrangles range from 168.3 to 177.5 Ma (table 1)

Jmu Mafic and ultramafic plutonic rocks (Middle and late Early Jurassic)—Small areas of 

gabbro, hornblende gabbro, hornblendite, and pyroxenite in Iliamna and Mount Katmai 

quadrangles within Jurassic Alaska-Aleutian Range batholith of Detterman and Reed 

(1980); appear unrelated to mafic and ultramafic rocks in Anchorage quadrangle (unit Jum, 

described below). K-Ar ages of 160 and 183 Ma were reported by Reed and Lanphere 

(1972; table 1 here). Locally show evidence of contact metamorphism by younger intrusions 

Plutons proximal to the Border Ranges Fault Zone

Jeqd Quartz diorite and tonalite (Middle and Early Jurassic)—“Series of discordant 

intermediate plutons between Nelchina Glacier and Lazy Mountain in the northern Chugach 

Mountains” (Winkler, 1992). They are relatively homogeneous, fine- to medium-grained 

quartz diorite, and tonalite, having extensive areas sheared and altered (Winkler, 1992). 

Locally foliated medium-grained quartz diorite and tonalite. As mapped by Rioux and others 

(2007), also includes trondhjemite, which yields significantly older ages (184.1 to 193.3 Ma) 
30

than trondhjemite of map units Jtr and Klt described above. Potassium-argon ages generally 

range from 167 to 194 Ma (table 1). U/Pb zircon ages range between 183 and 193.9 Ma 

(table 1) similar to, but marginally older than similar composition plutons of AlaskaAleutian Range batholith

Jum Mafic and ultramafic rocks, undivided (Middle Jurassic or older?)—Complexly intermixed series of mafic to intermediate plutonic rocks (Winkler, 1992). “Plutons consist of 

gabbronorite, hornblende gabbro, diorite, and tonalite. *** Xenoliths of gabbro show ductile 

deformation as though they still were warm when intruded by silicic magmas and migmatitic 

textures are common at contacts between lithologies” (Winkler, 1992). Plutons are cut by 

steeply dipping faults that form northern part of Border Ranges Fault system. Fault-bounded 

cumulate ultramafic and mafic rocks of Wolverine and Eklutna complexes of Winkler (1992) 

are also included in unit and have an inferred age of Middle and Early Jurassic based on correlation with Tonsina complex of adjacent Valdez quadrangle and intrusion by Middle and 

Early Jurassic dikes (Winkler, 1992)

^qd Quartz diorite to diorite (Late Triassic)—Quartz diorite, tonalite and diorite in Seldovia 

and Afognak quadrangles. In Seldovia quadrangle, unit includes diorite of Point Bede and 

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

of Cook Inlet. Unit also includes Afognak pluton in Afognak quadrangle, a quartz diorite 

pluton in the Barren Islands (Cowan and Boss, 1978), and diabasic hypabyssal intrusions in 

Shuyak Formation (Connelly and Moore, 1979). Diorite of Point Bede of Bradley and others 

(1999) is fine- to medium-grained, nonfoliated quartz diorite; the Afognak pluton is a large, 

multiphase hornblende diorite, quartz diorite, and tonalite pluton exposed along northwestern Afognak Island (Connelly and Moore, 1979; Roeske and others, 1989). Tonalite of 

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

chloritic alteration similar to that found in diorite of Point Bede and hence, was assigned a 

similar age. Bradley and others (1999) assumed plutons in Seldovia quadrangle were Jurassic based on correlation with the pluton in the Barren Islands that yielded a K-Ar hornblende 

age of 191±1.3 Ma (Cowan and Boss, 1978, table 1 here). However, a Triassic age was 

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

and others (1999) also mapped a light-gray, fine-grained to aphanitic felsite body south of 

the tonalite of Dogfish Bay. This felsite is undated and is tentatively assigned Triassic age 

due to its proximity and similar setting to this map unit. Bradley and others (1999) suggested 

the felsite could be Jurassic or Early Tertiary; however, no other rocks of Tertiary age are 

reported in this vicinity west of the Border Ranges Fault system in Seldovia quadrangle. 

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

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

of the available data. Possible scenarios include an incorrectly mapped contact, a wider 

range of age for Pogibshi formation of Kelley (1980), or inheritance in the zircon. Afognak 

pluton has a well-developed contact-metamorphic aureole in Shuyak Formation; its boundary with Schist of Kodiak Island (Jsch) is apparently a fault (Roeske and others, 1989). A 

fission-track age determination on zircon by Clendenen (1991) yielded 153 Ma. K-Ar ages 

on hornblende from the Afognak pluton and associated migmatite range from 187.5±5.5 to 

197±5.8 Ma (Roeske and others, 1989); however, a U/Pb age determination of 217±10 Ma is 

interpreted to indicate the emplacement age (Roeske and others, 1989)

MÉLANGE AND METAMORPHIC ROCKS

[Unless otherwise indicated, the stratigraphic position or age given is of the protolith of the metamorphic rock units]

TKc Cataclastite (Eocene? and Cretaceous, metamorphic age)—Chlorite-rich, fine-grained granular rocks formed by cataclasis and retrograde alteration of mafic and ultramafic plutonic 

rocks and mafic volcanic rocks (Winkler, 1992). May be equivalent lithologically to sheared 

gabbronorite included here in unit Jum. Deformation in the Border Ranges Fault is at least 

as old as Early Cretaceous; however, much of the fabric may be an Eocene overprint from 

reactivation of the old thrust as a strike-slip boundary. “Lithologies and fabrics of these 

rocks may resemble parts of the Haley Creek metamorphic assemblage (Plafker and others, 

1989), which to the east in the Valdez 1° x 3° quadrangle represents the southern margin of 

the Wrangellia terrane, and includes presumed basement for the terrane” (Winkler, 1992). 
31

However, because these rocks would be considered part of the Hidden terrane (Wilson and 

others, 2008), they are not assigned to Wrangellia terrane

TKgg Gneiss (Tertiary or Cretaceous, metamorphic age)—Gneiss, possibly of Mesozoic protolith 

age in Tyonek quadrangle west of Shamrock Glacier; extrapolated from adjacent Lime Hills 

quadrangle (B.M. Gamble and B.L. Reed, USGS, written commun., 1996)

Valdez Group (Upper Cretaceous)—A widespread unit of 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; Winkler, 

1992). Includes a variety of interbedded, tholeiitic metavolcanic and meta-intrusive rocks 

and locally a mélange facies. Unit ranges in metamorphic grade from laumontite- to midgreenschist facies and locally reaches amphibolite facies outside the map area 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 (Plafker and others, 1977; 1994). Valdez Group is subdivided here 

into the following:

Kvs Metasedimentary rocks, undivided—Dark-gray, thin- to thick-bedded, laumontite- to midgreenschist facies, moderately sorted to poorly sorted sandstone, siltstone, and mudstone 

flysch; sandstone is fine- to coarse-grained, mainly composed of plagioclase, quartz, and 

igneous rock fragments (Tysdal and Case, 1979; Dumoulin, 1987). Unit is a thick sequence 

of rhythmically alternating, multiply deformed, metamorphosed sandstone-siltstone turbidites having beds generally ranging from a few centimeters to a few meters thick and locally, 

massive beds as much as tens of meters thick (Winkler and Plafker, 1981; Nelson and others, 

1985; Winkler, 1992; 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 versus sedimentary rocks. Proportion of lithic fragments decreases 

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, 

1992; Winkler and Plafker, 1993). Metamorphic grade ranges from prehnite-pumpellyite to 

lower greenschist facies. Assigned Late Cretaceous (Campanian? to Maastrichtian) age on 

the basis of scattered occurrences of Inoceramus kusiroensis Nagao and Matsumoto and 

Inoceramus concentrica Ulrich (Jones and Clark, 1973; Tysdal and Plafker, 1978; Tysdal 

and Case, 1979; Nelson and others, 1985; Bradley and others, 1999). D.C. Bradley (USGS, 

oral commun., 2007) reported 70 Ma detrital zircons from unit near Anchorage along Turnagain Arm

Kvv Mafic metatuff—Altered chlorite-epidote-actinolite semischist interbedded with metasedimentary rocks in a small area near head of Metal Creek. Tuff may be analogous to widespread thicker metavolcanic rocks in Cordova and Valdez quadrangles according to Winkler 

(1992). Age inferred from association with sedimentary rocks of the Valdez Group, within 

which it is interbedded

Kvgs Schist—In the Seward quadrangle, Tysdal and Case (1979) describe “interbedded siltstone, 

graywacke, and less abundant tuff, tuffaceous sandstone, and basalt (pillow basalt?); igneous rocks are typically dark green, metasiltstone is shiny steel-gray, and metasandstone 

is dark-gray; 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; actinolite is present in some metavolcanic rocks; 

chlorite zone assemblages are similar but lack biotite”. Unit is an upper greenschist to lower 

amphibolite facies schistose equivalent of the Valdez Group; unit mapped along west side of 

Placer River Fault in Seward quadrangle. A K-Ar date on biotite semischist of 51.5±1.5 Ma 

(table 1) was reported by Nelson and others (1985) from a locality along Placer River Fault 

in Seward quadrangle
32

Kvm Mélange of Iceworm Peak of Kusky and others (1997)—Tectonic mélange consisting 

of blocks of graywacke in a phacoidally cleaved matrix of slate having a 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 Group contact (Kusky and others, 1997). Intruded by Paleocene dikes 

(Kusky and others, 1997) not shown here

Kivs Metamorphosed intermediate volcanic and sedimentary rocks (Cretaceous)—Dominantly 

andesitic composition on east side of the Tordrillo Mountains in Tyonek quadrangle (P.J. 

Haeussler, USGS, unpub. data, 2008). Most exposures consist of metamorphosed and 

altered volcanic tuff, breccia, or agglomerate. More mafic compositions are suggested by 

one outcrop of pillow lavas (small pillows, up to 30 cm in diameter). Metasedimentary 

rocks consist of volcaniclastic turbidites and rare non-volcaniclastic turbidites. Some rocks 

are light-green, but others are light-gray and not altered and not metamorphosed. Age best 

constrained at a locality near Hayes River Pass where the youngest detrital zircons in volcaniclastic sedimentary rocks were dated between 136–151 Ma, Hauterivian or younger (D.C. 

Bradley, USGS, written commun., 2008). As these rocks were typical of the sedimentary 

rocks of the unit, it seems likely they are the same age. Although age of remainder of unit is 

not constrained, all dated volcanic rocks in Tyonek quadrangle (see unit Kv) are Cretaceous 

or younger, which suggests that rocks of unit Kivs are no older than Cretaceous. Magoon 

and others (1976) included these rocks in their undivided metasedimentary rocks unit of 

Jurassic and (or) Cretaceous age. Other authors have included these rocks in the informally 

named Kahiltna assemblage (see for example, Jones and others, 1981)

Kps Pelitic schist (Cretaceous?)—Quartz-muscovite-albite-chlorite schist in southwestern Talkeetna 

Mountains, about 16 km north of Wasilla. Schist remarkably uniform in lithology; no 

correlative rocks known (Albanese and others, 1983; Winkler, 1992). Mineralogy indicates 

greenschist metamorphism, which Winkler (1992) interpreted probably was retrograded 

from amphibolite facies metamorphism. Jurassic age was inferred for the prograde 

metamorphism based on age of adjacent amphibolite of unit Jma (Winkler, 1992; shown as 

unit JPam here); analysis of detrital zircons collected from modern sediments by Van Wyck 

and Norman (2005) appears to support this interpretation. However, detrital zircon analysis 

from a bedrock sample reported by Bradley and others (2009) shows a peak at about 75 Ma, 

interpreted to indicate Late Cretaceous deposition of the sedimentary protolith of the schist. 

K-Ar muscovite ages of about 66 to 51 Ma (Csejtey and others, 1978; Silberman and others, 

1978a,b; Winkler, 1992; M.L. Silberman, USGS, written commun., 1978; table 1 here) from 

the schist are thought to reflect intrusion of adjacent tonalite and quartz monzonite (see table 

1). Corresponds to unit Jps of Winkler (1992) in Anchorage quadrangle, and unit MzPzs, 

schist at Willow Creek, of Magoon and others (1976)

}|m Metamorphosed mafic volcanic and sedimentary rocks (Mesozoic and Paleozoic)—Metamorphosed mafic volcanic rocks, phyllite, schist, quartzite, marble, calc-silicate rocks, 

serpentinite, gabbro, and chert. Mixed unit of varying affinity and protolith and metamorphic 

age. Dominant rock is metamorphosed mafic volcanic rocks which may be equivalent to the 

Chilikadrotna Greenstone and the Talkeetna Formation (Nelson and others, 1983)

KMm McHugh and Uyak Complexes, undivided (Cretaceous to Mississippian)—Tectonic mélange 

consisting largely of Mississippian through mid-Cretaceous protoliths of oceanic affinity (Clark, 1973; Connelly, 1978; Tysdal and Case, 1979; Winkler, 1992). Unit is found 

towards the Gulf of Alaska relative to the Border Ranges Fault system in the Anchorage, 

Kenai, Seldovia, and Afognak quadrangles. According to Clark (1973), unit consists of two 

lithologically distinct, but structurally juxtaposed packages. Dominant or most common is a 

metaclastic sequence “composed predominantly of gray, gray-green, and dark-green weakly 

metamorphosed *** siltstone, graywacke, arkose, and conglomeratic sandstone” (Clark, 

1973). A metavolcanic sequence consists of “greenstones of mostly basaltic composition and 

texture, that are commonly associated with radiolarian metachert, cherty argillite, and argillite. Small amounts of ultramafic rocks and marble occur locally as isolated, discontinuous 

outcrops or lenticular masses” (Clark, 1973). Uyak Complex of Connelly (1978) is similar in 

character though the metaclastic-equivalent part consists dominantly of deformed gray chert 

and argillite (Connelly, 1978). Within the McHugh and Uyak Complexes, “broad zones as 

wide as 1 km of intense shearing lack any stratal continuity and, in many places, are marked 

by angular, elongate phacoids, either enclosed in pervasively sheared matrix or juxtaposed 
33

against other phacoids. Larger phacoids are lithologically diverse, consisting of schist, 

amphibolite, marble, sandstone, conglomerate, diorite, gabbro, serpentinized ultramafic 

rocks, and mafic volcanic rocks” (Winkler, 1992). Metamorphic minerals include muscovite, 

epidote, calcite, chlorite, albite, and veinlets of prehnite (Tysdal and Case, 1979). Blocks 

of mafic and ultramafic rocks are serpentinized near their margins (Connelly, 1978). Slickensides are common both as subparallel, anastomosing fractures in competent rocks and as 

closely spaced fractures in less competent rocks (Connelly (1978). Connelly (1978) reported 

detailed structural analysis in the Uyak Complex indicated that mean slip direction during 

deformation was N 38o W (±11o) and that transport was southeast under northwest (present day) on the basis of work by Moore and Wheeler (1975). Ultramafic, gabbroic, basaltic 

(greenstone) rocks are interpreted as fragments of dismembered oceanic crust conveyed by 

plate motion into a subduction zone and deformed into a tectonic mélange (Connelly, 1978). 

In the McHugh Complex, sedimentary rocks of the matrix have yielded Early Cretaceous 

(Valanginian) fossils, whereas protolith ages on blocks in the mélange have yielded radiolarians of Cretaceous (Albian-Aptian), Jurassic, and Triassic age (Bradley and others, 1999; 

Winkler and others, 1981), Permian conodonts and fusilinids of Tethyan affinities (Connelly, 

1978; Stevens and others, 1997; Bradley and others, 1999), and Mississippian to Pennsylvanian conodonts (Nelson and others, 1986). D.C. Bradley (USGS, oral commun., 2007) 

reported 70 Ma detrital zircons from metasandstone within the McHugh Complex collected 

along Turnagain Arm in the Anchorage quadrangle. Bradley and others (1999) locally 

subdivided the McHugh Complex into distinct lithologic packages loosely similar to the two 

packages originally defined by Clark (1973)

}g Gabbro (Mesozoic)—Dark-green, medium- to coarse-grained gabbro and minor leucogabbro 

and plagiogranite (Bradley and others, 1999) in Seldovia quadrangle. Fault-bounded bodies 

in the McHugh Complex; following Bradley and others (1999), only the larger bodies are 

shown here. Associated with McHugh Complex basalt and chert unit (K^mc), which Bradley and others (1999) suggested may indicate a genetic relation. Bradley and others (1999) 

thought the relation may indicate Triassic to middle 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 (sample 91DW 87, table 1)

}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) in Seldovia quadrangle. 

Bradley and others (1999) reported at least 7 fault-bounded bodies, all associated with the 

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), it is 

not a klippe as interpreted by Magoon and others (1976), but rather is bounded by late stage, 

subvertical faults. However, Bradley and others (1999) did suggest a thrust may bound the 

Red Mountain body at depth. They also inferred that the gabbro (}g) and ultramafic rocks, 

while not spatially associated, are comagmatic and therefore by extension may be of Triassic 

to middle Cretaceous age; nonetheless, they assigned an undifferentiated Mesozoic age to 

these rocks

KJms McHugh Complex, graywacke and conglomerate (Early Cretaceous to Late Jurassic, 

Pliensbachian)—Fault-bounded blocks of massive conglomerate and graywacke that range 

up to several kilometers in structural thickness (Bradley and others, 1999) in Seldovia 

quadrangle. 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 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 radiolarians in conformably underlying ribbon chert at one 

locality and considered unit ranges in age regionally from Early Jurassic, Pliensbachian to as 

young as Early Cretaceous

K^mc McHugh Complex, basalt and chert (Early Cretaceous, Albian to Middle Triassic, 

Ladinian)—“Pillow and massive basalt, depositionally overlain by complexly folded and 

faulted radiolarian chert” (Bradley and others, 1999) in the Seldovia quadrangle. Bradley 

and others (1999, citing a C.D. Blome, USGS written commun. of 1994) reported radiolaria 

in bedded chert ranging in age from Middle Triassic, Ladinian to Early Cretaceous, Albian
34

Jsch Greenschist and blueschist (Jurassic, metamorphic age)—Diverse, well-foliated, multiplydeformed, variably foliated, metasedimentary and metavolcanic rocks. Metamorphism varies 

from greenschist to amphibolite facies (Winkler, 1992) and rocks are locally glaucophanebearing and crossite-bearing suggesting unit was metamorphosed under conditions transitional to and including blueschist facies (Forbes and Lanphere, 1973; Carden and Decker, 

1977; Carden and others, 1977; Connelly and Moore, 1979; Winkler and others, 1981; 

Bradley and others, 1999). Sedimentary protoliths were shale, chert, tuffaceous arenite, and 

limestone; volcanic protoliths were probably mostly basalt (Winkler, 1992). Outcrops along 

northern flank of Chugach Mountains in Anchorage, Seldovia, and Valdez quadrangles, 

closely associated with Border Ranges Fault system. In Anchorage quadrangle, includes 

the Knik River schist (JPzm of Winkler, 1992, or Knik River terrane of Pavlis, 1983; Pavlis 

and others, 1988) and the Seldovia Schist of Roeske (1986; see also Cowan and Boss, 1978; 

Bradley and others, 1999). Unit is correlative with the Raspberry Schist of Roeske and 

others (1989) in the Kodiak Island archipelago, exposed along Border Ranges Fault system 

to the southwest. Carden and others (1977) reported 10 K-Ar ages on mica (including 

chlorite) and amphibole from unit ranging from 157.8.3±4.8 to 196.6±5.8 Ma, and Bradley 

and Karl (2000) reported 40Ar/39Ar plateau ages on mica of 190.98±0.3 and 191.7±0.3 Ma 

and on hornblende of 191.92±0.6 Ma. Roeske and others (1989) reported Rb/Sr isochron 

ages ranging from 189 to 212 Ma (table 1, here). Dating of the blueschist facies metamorphism of this metamorphic complex supports a pattern of decreasing age from west to east 

as originally suggested by Sisson and Onstott (1986). Clark (1972, cited in Winkler, 1992) 

reported a single Permian fossil collection from limestone. The Raspberry Schist of Roeske 

and others (1989) is intruded by the Triassic Afognak pluton. Locally, schist probably of this 

unit is included in the McHugh Complex melange. Winkler (1992) suggested that the diverse 

protolith lithologies may indicate tectonic mixing prior to metamorphism

JPk Kakhonak Complex (Jurassic, Triassic, and Permian? or older?)—Lithologically diverse 

and complex assemblage of metamorphosed mafic plutonic, volcanic, and sedimentary 

rocks found on west side of Cook Inlet in Iliamna quadrangle, defined by Detterman and 

Reed (1980). 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 thought that 

the Kakhonak Complex represented, in part, the metamorphic equivalent of Upper Triassic 

and Lower Jurassic rocks, that is, 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. Because Permian rocks were known from Puale Bay, south of 

the map area, a possible Paleozoic age was not ruled out 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 in metamorphic 

grade from not metamorphosed to granulite facies. No known age control

Jpmu Plutonic and metamorphic rocks, undifferentiated (Middle to Early Jurassic, metamorphic age)—Poorly exposed, intricately intermixed schist, amphibole, and small dikes and 

veinlets of granodiorite (Csejtey and others, 1978) in Talkeetna Mountains. Two rock types, 

amphibolite and sheared quartz diorite, dominate unit (Csejtey and others, 1978). Locally 

subdivided into unit Jqd described above and JPam and JPmb, described below:

JPam Amphibolite (Lower Jurassic or older)—Dark-gray to dark-green, fine- to coarse-grained 

hornblende-plagioclase amphibolite, quartz-rich amphibolite, and mafic schist. Protolith 

is unknown. Amphibolite is intruded by foliated hornblende-biotite tonalite of unit Jgd; 

proportion of tonalite increases southeastward within map unit. Unit was mapped as unit 

Jmi by Csejtey and others (1978) and was correlated with their unit Jam, which was mapped 

north of map area. Csejtey and others (1978) map unit Jmi also includes minor greenschist 

and foliated diorite. Amphibolite from Csejtey’s map area yielded a K-Ar hornblende age of 

176.6±5.1 Ma (Csejtey and others, 1978) and a 40Ar/39Ar plateau age on hornblende from a 

biotite-hornblende gneiss of 161.9±0.3 Ma (J.M. Schmidt, unpub. data, 2008, table 1, here). 

A metaigneous rock sample yielded an apparent Permian or Pennsylvanian U/Pb zircon age 

(C.P. Hults and D.C. Bradley, USGS, unpub. data), which is consistent with the apparent 

protlith age of the marble, unit JPmb. Unit age is constrained as older than Jqd by crosscutting tonalite intrusions tentatively assigned to Jqd. Unit is overlain by Tertiary volcanic 
35

References Cited

Albanese, M., Kline, J.T., Bundtzen, T.K., and Kline, K., 1983, 

Reconnaissance geology and geochemistry of the Willow 

Creek-Hatcher Pass area, Alaska: Alaska Division of Geological and Geophysical Surveys Public-Data File 83-9, 1 

sheet, scale 1:63,360.

Amos, K.E., and Cole, R.B., 2003, Tertiary volcanic rocks of 

the central Talkeetna Mountains, Alaska, in Galloway, 

J.P., ed., Studies by the U.S. Geological Survey in Alaska, 

2001: U.S. Geological Survey Professional Paper 1678, p. 

71–82.

Anderson, R.E., 1969a, Geology and geochemistry, Diana 

Lakes area, western Talkeetna Mountains, Alaska: Alaska 

Division of Geological and Geophysical Surveys Geologic 

Report no. 34, 27 p., 1 sheet, scale 1:50,000.

Anderson, R.E., 1969b, Preliminary geochemistry and geology, 

Little Falls Creek area, Talkeetna Mountains quadrangle, 

Alaska: Alaska Division of Mines and Geology Geochemical Report 19, 16 p., 1 sheet, scale 1:42,240.

Atwood, W.W., 1911, Geology and mineral resources of parts 

of the Alaska Peninsula: U.S. Geological Survey Bulletin 

467, 137 p.

Barnes, F.F., and Payne, T.G., 1956, The Wishbone Hill district, 

Matanuska coal field: U.S. Geological Survey Bulletin, 

1016, 88 p., (including geologic maps, scale 1:6,000 and 

1:12,000).

Barnett, D.E., Bowman, J.R., Pavlis, T.L., Rubenstone, J.R., 

Snee, L.W., and Onstott, T.C., 1994, Metamorphism and 

near-trench plutonism during initial accretion of the Cretaceous Alaskan forearc: Journal of Geophysical Research, v. 

99, no. B12, p. 24,007–24,024.

Bartsch-Winkler, S.B., Schmoll, H.R., 1992, Utility of radiocarbon-dated stratigraphy in determining late Holocene 

earthquake recurrence intervals, upper Cook Inlet region, 

Alaska: Geological Society of America Bulletin, v.104, no. 

6, p. 684–694.

 Berry, A.L., Dalrymple, G.B., Lanphere, M.A., and Von Essen, 

J.C., 1976, Summary of miscellaneous potassium-argon 

age measurements, U.S. Geological Survey, Menlo Park, 

California, for the years 1972–74: U.S. Geological Survey 

Circular 727, 13 p.

Bradley, D.C., and Karl, S.M., 2000, Field guide to the Mesozoic accretionary complex in Kachemak Bay and Seldovia, south central Alaska: Anchorage, Alaska Geological 

Society, 19 p.

Bradley, D.C., and Miller, M.L., 2006, Field guide to south-central Alaska’s accretionary complex Anchorage to Seward: 

Anchorage, Alaska Geological Society, 32 p.

Bradley, D.C., and Wilson, F.H., 2000, Reconnaissance bedrock 

geology of the southeastern part of the Kenai quadrangle, 

rocks (Csejtey and others, 1978; Oswald, 2006) of unit Tvu. Unit description modified after 

Csejtey and others (1978) and Oswald (2006). North of the map area, but still within in the 

Talkeetna Mountains quadrangle, similar rocks were mapped by Kline and others (1990) and 

correlated with metamorphic complex of the Gulkana River of Nokelberg and others (1989) 

and Strelna Metamorphics of Plafker and others (1989)

JPmb Marble (Jurassic, metamorphic age; Permian?, protolith age)—White, coarse- to mediumgrained marble with porphyroblastic crystals of garnet and diopside (Csejtey and others, 

1978). In Talkeetna Mountains quadrangle, forms lenticular interbeds, as much as a few tens 

of meters thick, within amphibole of unit Jam of Csejtey and others (1978) (unit JPam, 

here) and basaltic to andesitic metamorphosed volcanogenic sequence (Pzv) of presumably 

late Paleozoic age of Csejtey and others (1978). As shown here, includes units Jmrb, Jmb, 

Pls of Csejtey and others (1978). Poorly preserved and generally unidentified crinoid columnals, brachiopods, bryozoans, and rarely, corals were reported from the rocks that Csejtey 

and others (1978) mapped as their unit Pls. Kline and others (1990) mapped similar unit in 

Talkeetna Mountains quadrangle north of the map area and reported late Wolfcampian to 

Leonardian (Cisuralian) and Leonardian (late Cisuralian) to Guadalupian conodonts

^*vs Basaltic to andesitic metavolcanic and sedimentary rocks (Triassic?, Permian, Cisuralian, 

and Pennsylvanian?)—Interlayered heterogeneous, dominantly marine sequence over 5000 

m thick (Csejtey and others, 1978). Consists primarily of metamorphosed flows and tuffs of 

basaltic and andesitic compositions; subordinate mudstone, bioclastic marble, and dark-gray 

to black phyllite. Includes olive green, medium-grained volcaniclastic crystal lithic tuff, 

pyroxene-hornblende meta-andesite flows, and meta-sandstone. Locally may also include 

Triassic age rocks that more commonly would be assigned to the Nikolai Greenstone (unit 

^n) if mapping allowed; hence inferred Triassic age. Low-grade regional metamorphism. 

Includes part of units Pzv and Pzt of Csejtey and others (1978)

BEDROCK

bu Bedrock, unknown—Areas of known or apparent bedrock exposure. Bedrock type unknown
36

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44

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Geological Survey Miscellaneous Field Studies Map MF–

1161–A, scale 1:31,680.
45

Table 1. Radiometric age data for the map of the Cook Inlet Region, south central Alaska.

[Samples listed by map units and ordered from south to north and east to west. Quadrangle is the 1:250,000-scale sheet. Methods: A, Lead-alpha, a largely 

tional potassium-argon; 40/39, 40Ar/39Ar; Rb/Sr, rubidium-strontium; Nd/Sm, neodymium-samarium, U/Pb, uranium-lead

Sample Map

unit

Quadrangle Latitude

°N

Longitude

°W

 Rock type Method Mineral Age

(Ma) 

Error

(m.y.)

Comment 

KL-027, KL081

Qod Anchorage 61.3922 149.8550 Tephra 40/39 Glass 0.378 0.067 Goose Bay tephra, composite of 

analyses

84AYb 71 Qv Mount Katmai 58.0217 155.565 Andesite K/Ar Plagioclase 0.582 0.085 Lava flow 

84ARj 134B Qv Mount Katmai 58.0783 155.4183 Andesite K/Ar Plagioclase 0.389 0.071 Lava flow 

84AYb 65 Qv Mount Katmai 58.1033 155.42 Dacite K/Ar Plagioclase 0.394 0.046 Lava flow 

85AYb 223 Qv Mount Katmai 58.14 155.5533 Andesite K/Ar Plagioclase 0.954 0.109 Andesite porphyry flow 

83ARj 143 Qv Mount Katmai 58.4867 154.2867 Andesite K/Ar Plagioclase 0.446 0.388 Lava flow 

84AYb 24 Qv Afognak 58.63 153.895 Andesite K/Ar Whole-rock 0.506 0.182 Basaltic andesite plug or dome 

84ARj 142B Qv Afognak 58.6567 153.63 Andesite K/Ar Plagioclase 0.884 0.246 Lava flow 

G-24-2 n.1 Qv Afognak 58.95 153.4633 Andesite K/Ar Whole-rock 0.70

1.00

2.13.0

85CNS 26 Qv Tyonek 61.075 152.1267 Andesite K/Ar Whole-rock 2.08 0.2 Mount Spurr, isolated remnant, 

mum age due to minor alteration± .20 my

AMS-A02 Qv Tyonek 61.2433 152.1167 Andesite K/Ar Whole-rock 0.112 0.008 Mount Spurr, also called 85CNS

AMS-B01 Qv Tyonek 61.245 152.3083 Andesite K/Ar Whole-rock 0.255 0.052 Mount Spurr, also called 85CNS

AMS-B02 Qv Tyonek 61.2467 152.3083 Andesite K/Ar Whole-rock 0.155 0.014 Mount Spurr 

AMS-B05 Qv Tyonek 61.2483 152.3083 Andesite K/Ar Whole-rock 0.065 0.015 Possible sill, also called 85CNS

AMS-A04 Qv Tyonek 61.255 152.145 Andesite K/Ar Whole-rock 0.134 0.021 Minimum age based on observa

tion of groundmass glass, also c85CNS22

AMS-B08 Qv Tyonek 61.2567 152.2967 Andesite K/Ar Whole-rock 0.139 0.009 Mount Spurr, also called 85CNS

AMS-A05 Qv Tyonek 61.2583 152.1467 Andesite K/Ar Whole-rock 0.110 0.006 Mount Spurr, also called 85CNS

AMS-A06 Qv Tyonek 61.26 152.1467 Andesite K/Ar Whole-rock 0.100 0.011 Mount Spurr, mean of 2 replicat

called 85CNS37

AMS-B10 Qv Tyonek 61.26 152.2967 Andesite K/Ar Whole-rock 0.119 0.016 Mean of 3 replicates, Mount Sp

called 85CNS40

AMS-A08 Qv Tyonek 61.2667 152.1567 Andesite K/Ar Whole-rock 0.059 0.014 Mount Spurr; mean of 3 replica

called 85CNS35

01PH 411A Qv Tyonek 61.3389 152.0917 Volcanic rock 40/39 plateau Whole-rock 1.8 0.5 Holocene? Mount Spurr volcan

rocks, 9 fractions, 76% 39Ar re

78AR 290DK Qad Kenai 60.7167 152.6667 Andesite K/Ar Whole-rock 0.763 0.017 Dark band in banded andesite 

78AR 290LT Qad Kenai 60.7167 152.6667 Andesite K/Ar Whole-rock 0.627 0.024 Light band in banded andesite 
46

Table 1. Radiometric age data for the map of the Cook Inlet Region, south central Alaska—continued.

[Samples listed by map units and ordered from south to north and east to west. Quadrangle is the 1:250,000-scale sheet. Methods: A, Lead-alpha, a largely 

tional potassium-argon; 40/39, 40Ar/39Ar; Rb/Sr, rubidium-strontium; Nd/Sm, neodymium-samarium, U/Pb, uranium-lead

Sample Map

unit

Quadrangle Latitude

°N

Longitude

°W

 Rock type Method Mineral Age

(Ma) 

Error

(m.y.)

Comment 

90AR 99 Qad Kenai 60.77 152.7 Andesite K/Ar Whole-rock 0.887 0.015 Estimated latitude and longitude 

84AYb 69 QTv Mount Katmai 58.07 155.7317 Andesite K/Ar Whole-rock 2.45 0.21 Lava flow 

85AYb 214 QTv Mount Katmai 58.0783 155.8067 Andesite K/Ar Plagioclase 2.00 0.11 Basaltic andesite porphyry 

86ARj 62 QTv Mount Katmai 58.3867 155.2267 Andesite K/Ar Plagioclase 2.64 0.04 Basaltic andesite porphyry flow 

DT75-213 Tkn Tyonek 61.2483 151.1017 Ash K/Ar Plagioclase 15.8

15.9

1.81.5

Chuitna River section; Beluga Cfield. Ash bracketed by Seldovia

DT75-212 Tks Seldovia 59.77 151.1333 Ash K/Ar Plagioclase 32.2

30.1

1.91.8

33m stratigraphically above DTSterling Formation; age may be due to detrital contamination

DT75-211 Tks Seldovia 59.77 151.1333 Ash Fission-track Zircon 5.4 0.6 20-40m stratigraphically above 

210. Sterling Formation

DT75-211 Tks Seldovia 59.77 151.1333 Ash K/Ar Plagioclase 11.7

14.1

0.91.1

20-40m stratigraphically above 210. Sterling Formation; age matoo old due to detrital contamin

DT75-210 Tks Seldovia 59.77 151.1333 Ash K/Ar Plagioclase 7.2 0.6 Sterling Formation, Clamgulchi

Stage

DT75-209b Tks Seldovia 59.7967 151.1083 Tuff K/Ar Plagioclase 9.1

8.2

0.70.8

Sterling? Formation, age may bold due to detrital contamination

DT75-208 Tks Seldovia 59.7967 151.1117 Tuff K/Ar Hornblende

Plagioclase

4.74.2

0.61.4

35m stratigraphically higher thaDT75-206. Sterling Formation

DT75-207 Tks Seldovia 59.7967 151.1117 Tuff Fission-track Zircon 4.9 0.8 24m stratigraphically higher tha

DT75-206. Sterling Formation

DT75-207 Tks Seldovia 59.7967 151.11167 Tuff K/Ar Plagioclase 7.6

7.5

0.60.6

24m stratigraphically higher thaDT75-206. Believed to be too oto detrital contamination

DT75-206 Tks Seldovia 59.7967 151.1117 Ash Fission-track Zircon 5.6 0.9 Composite of four thin beds, Ste

Formation

DT75-206 Tks Seldovia 59.7967 151.1117 Ash K/Ar Plagioclase 4.6

8.4

0.70.7

Composite of four thin beds. Agbe too old due to detrital contam

6-25-77-1 Tks Seldovia 59.8233 151.0417 Ash K/Ar Plagioclase 6.9 0.5 Sterling Formation, Clamgulchi

Stage

7-14-73-3 Tks Kenai 60.0683 151.6467 Ash Fission-track Zircon 8.5 1.0 Kenai Peninsula, Sterling Form

Age revised in Turner and other(1980)

7-14-73-3 Tks Kenai 60.0683 151.6467 Ash K/Ar Plagioclase 8.9 1.0 Kenai Peninsula, Sterling Form

Age revised in Turner and other(1980)

7-13-73-9 Tks Kenai 60.1858 151.4617 Ash K/Ar Plagioclase 7.0 0.7 Kenai Peninsula, location revise

ling Formation. Age revised in Tand others (1980)
47

Table 1. Radiometric age data for the map of the Cook Inlet Region, south central Alaska—continued.

[Samples listed by map units and ordered from south to north and east to west. Quadrangle is the 1:250,000-scale sheet. Methods: A, Lead-alpha, a largely 

tional potassium-argon; 40/39, 40Ar/39Ar; Rb/Sr, rubidium-strontium; Nd/Sm, neodymium-samarium, U/Pb, uranium-lead

Sample Map

unit

Quadrangle Latitude

°N

Longitude

°W

 Rock type Method Mineral Age

(Ma) 

Error

(m.y.)

Comment 

7-13-73-6 Tks Kenai 60.205 151.4333 Ash K/Ar Hornblende

Plagioclase

7.48.8

0.70.5

Kenai Peninsula, location reviseling Formation. Age revised in Tand others (1980)

DT75-201 Tks Kenai 60.205 151.435 Tuff K/Ar Hornblende

Plagioclase

5.05.9

0.80.5

Crystal tuff, Sterling Formationis mean of 2 splits

DT75-200 Tks Kenai 60.2358 151.4008 Ash Fission-track Zircon 6.60 0.7 Sterling Formation, Clamgulchi

Stage

DT75-203 Tb Seldovia 59.7033 151.2783 Tuff Fission-track Apatite 12.9 5.1 Crystal vitric tuff mostly altered 

DT75-203 Tb Seldovia 59.7033 151.2783 Tuff K/Ar Plagioclase 8.8 0.9 Crystal vitric tuff mostly altered 

clay. Mean of 2 analyses

7-21-73-5 Tb? Seldovia 59.7058 151.28 Ash Fission-track Zircon 8.1 1.0 Age revised in Turner and other

(1980)

DT75-202 Tb Seldovia 59.71 151.2383 Tuff K/Ar Plagioclase 7.2 1.3 Crystal vitric tuff 

DT75-204 Tb Seldovia 59.7117 151.2683 Ash Fission-track Zircon 7.6 0.7 Crystal vitric tuff 

DT75-204 Tb Seldovia 59.7117 151.2683 Ash K/Ar Plagioclase 8.1 0.7 Mean of 2 analyses 

7-22-73-4 Tb? Seldovia 59.7617 151.1692 Ash Fission-track Zircon 8.1 1.0 Age revised in Turner and other

(1980)

7-22-73-4 Tb? Seldovia 59.7617 151.1692 Ash K/Ar Plagioclase 8.1 0.8 Age revised in Turner and other

(1980)

7-21-73-1 Tb? Seldovia 59.7683 151.1575 Ash Fission-track Zircon 8.8 1.0 Age revised in Turner and other

(1980)

7-21-73-1 Tb? Seldovia 59.7683 151.1575 Ash K/Ar Plagioclase 11.3 0.7 Age revised in Turner and other

(1980)

01PH 409A Twf Tyonek 61.2894 151.9364 Tuff U/Pb Zircon 43.4 0.2 Andesite tuff in West Foreland F

tion, 3 concordant, overlapping tions, 4 fractions run 

BIL99-14-C2 Tw Talkeetna 

Mountains

62.0403 147.6103 Dacite 40/39 Biotite 55.6 0.3 Pseudo-plateau age on dacite cl

Wishbone Formation

BC98-7-C5 Tw Anchorage 61.9792 147.7375 Granite 40/39 plateau Biotite 169.3 0.9 Granite clast in Wishbone Form

77AGz-Ar3 Tar Anchorage 61.695 149.7183 Basalt K/Ar Whole-rock 50.0 2.5 Approximate location, basalt flo

77AGz-Ar10 Tar Anchorage 61.75 149.24 Basalt K/Ar Whole-rock 46.1 2.8 Approximate location, basalt di

76ACy 19 Tar Anchorage 61.7722 149.1722 Meta-graywacke K/Ar Biotite 67.5 2.4 Metamorphosed graywacke 

78ASi-M19 Tar Anchorage 61.8083 149.9067 Basalt K/Ar Whole-rock 56.2 1.7 Approximate location 

78ASi-M21 Tar? Anchorage 61.8483 149.77 Basalt K/Ar Whole-rock 51.8 1.6 Approximate location, incorrect 

ID. in Winkler, 1992
48

Table 1. Radiometric age data for the map of the Cook Inlet Region, south central Alaska—continued.

[Samples listed by map units and ordered from south to north and east to west. Quadrangle is the 1:250,000-scale sheet. Methods: A, Lead-alpha, a largely 

tional potassium-argon; 40/39, 40Ar/39Ar; Rb/Sr, rubidium-strontium; Nd/Sm, neodymium-samarium, U/Pb, uranium-lead

Sample Map

unit

Quadrangle Latitude

°N

Longitude

°W

 Rock type Method Mineral Age

(Ma) 

Error

(m.y.)

Comment 

79AGz-38A Tar Anchorage 61.925 148.4783 Rhyolite K/Ar Whole-rock 45.5 1.8 Approximate location 

79AGz-38B Tar Anchorage 61.925 148.4783 Rhyolite K/Ar Potassium feldspar 51.4 1 Ash flow tuff, alkali feldspar. A

mate location 

78ASi-M23 Tar Anchorage 61.93 148.5 Rhyolite K/Ar Whole-rock 50.5 1.5 Arkose Ridge Formation, appro

location, tuff

6-27-77-6 Tch Anchorage 61.7717 148.935 Tuff Fission-track Zircon 53.1 1.3 Ash parting in coal bed no. 5, E

Jones mine, crystal vitric tuff

6-27-77-6 Tch Anchorage 61.7717 148.935 Tuff K/Ar Plagioclase 55.1 1.7 Ash parting in coal bed no. 5, E

Jones mine, crystal vitric tuff, cdant

8-7-78-8 Tch Anchorage 61.7717 148.935 Tuff K/Ar Plagioclase 52.2 1.6 Premier Group coal bed no. 5, E

Jones coal mine, crystal vitric tu

8-7-78-8 Tch Anchorage 61.7717 148.935 Tuff Fission-track Zircon 52.8 1.5 Premier Group coal bed no. 8, E

Jones coal mine, crystal vitric tu

8-7-78-4 Tch Anchorage 61.7717 148.935 Tuff K/Ar Plagioclase 55.8 1.7 Premier Group coal bed no. 8, E

Jones coal mine, crystal vitric tu

8-7-78-4 Tch Anchorage 61.7717 148.935 Tuff Fission-track Zircon 43.5 1.4 Premier Group coal bed no. 8, E

Jones coal mine, age inconsistenothers, crystal vitric tuff

M-25-88 TKd Afognak 58.2333 153.0417 Intrusive rock Fission-track Zircon 55 Mean age of individual crystals 

Ma; dike cutting Shuyak Formaelevation 0m

n.a. TKd Seldovia 59.3195 151.2917 Felsic dike 40/39 White mica 57.3 0.1 Port Dick prospect, hydrotherm

alteration preferred date from sistep (1000° C) and 67.8 % of ga

88ADw 230 TKd Seldovia 59.3933 150.665 Intermediate 

dike

40/39 isochron Hornblende 57.0 0.22 Leucocratic porphyry dike that 

the McHugh Complex near the Seldovia Bay

93ASB 66 TKd Seldovia 59.3938 151.2186 Basaltic 

andesite

40/39 plateau Hornblende 115.0 1.7 Hornblende-phyric basaltic and

dike cutting graywacke of McHComplex 4km SE of the head of Bay

K/Ar5 TKd Lake Clark 60.2772 154.8378 Andesite K/Ar Biotite 62.7 1.9 Augite andesite porphyry 

77E 216 TKd Lake Clark 60.37 155.2033 Dacite K/Ar Biotite 59.50 1.8 Biotite dacite 

82BB289 TKd Lake Clark 60.3833 153.8833 Mafic dike K/Ar Hornblende 79.6 2.4 Dike cutting Tlikakila Complex

Clark B-3 quadrangle, approximlocation, T.3N., R.27W., Section 

2203N TKd Seward 60.6667 149.7500 Felsic dike K/Ar Whole-rock 52.7 1.6 Hydrothermally altered felsic di

approximate location, Kenai Sta
49

Table 1. Radiometric age data for the map of the Cook Inlet Region, south central Alaska—continued.

[Samples listed by map units and ordered from south to north and east to west. Quadrangle is the 1:250,000-scale sheet. Methods: A, Lead-alpha, a largely 

tional potassium-argon; 40/39, 40Ar/39Ar; Rb/Sr, rubidium-strontium; Nd/Sm, neodymium-samarium, U/Pb, uranium-lead

Sample Map

unit

Quadrangle Latitude

°N

Longitude

°W

 Rock type Method Mineral Age

(Ma) 

Error

(m.y.)

Comment 

2237B TKd Seward 60.8000 149.6667 Granite K/Ar Muscovite 53.2 1.6 Hydrothermally altered albite-g

dike, approximate location, KenMine

91AKU 137 TKd Anchorage 61.0481 149.1078 Felsic intrusion 40/39 plateau Mica 54.1 .1 Crow Pass felsic intrusion, mino

loss, cooling age 

80AMS 34 TKd Anchorage 61.0533 149.1217 Dacite K/Ar Muscovite 54.5 1.6 Dacite dike 

80KMS 32I TKd Anchorage 61.0533 149.1217 Dacite K/Ar Whole-rock 54.8 2.7 Dacite dike 

CKA-1 TKd Anchorage 61.2717 149.3033 Felsic rock K/Ar Whole-rock 50.2 2.5 Felsic sill 

CKA-2 TKd Anchorage 61.32 149.3167 Felsic rock K/Ar Whole-rock 50.0 2.6 Felsic dike 

01PH 413B TKd Tyonek 61.5451 152.1696 Mafic dike 40/39 plateau Whole-rock 55.8 0.5 7 fractions, 95% 39Ar released 

81BS 116C TKd Seward 60.6155 149.5680 Felsic dike K/Ar Whole-rock 52.5 1.6 Dike, Oracle Mine, cutting Vald

Group (Kvs)

02PH 459B TKd Tyonek 61.6422 152.0575 Diabase 40/39 plateau Whole-rock 55.2

56.5

2.3

0.9

Diabase dike from dike swarm, cate analysis, 4 fractions, 69% 3releaseReplicate analysis, 5 fractions, 839Ar release

L & N 8 TKd Anchorage 61.6467 147.98 Dacite Fission-track Zircon 49.6 6.8 Dacite dike 

L & N 9 TKd Anchorage 61.66 147.91 Dacite Fission-track Zircon 47.3 5.3 Dacite dike 

02PH 332B TKd Tyonek 61.6699 152.0827 Basalt 40/39 plateau Whole-rock 51.6

52.0

2.10.8

Plagioclase basalt dike, replicatsis, 5 fractions, 86% 39Ar releaReplicate analysis, 4 fractions, 639Ar release; stair step up-no pl

L & N 6 TKd Anchorage 61.6833 147.8167 Dacite Fission-track Zircon 47.8 7.0 Oxidized dacite dike 

L & N 7 TKd Anchorage 61.6833 147.8333 Dacite Fission-track Zircon 42.9 5.5 Oxidized dacite dike 

02PH 461B TKd Tyonek 61.6868 152.0753 Mafic dike 40/39 plateau Whole-rock 57.0

57.9

0.80.9

Replicate analysis, 4 fractions, 739Ar releaseReplicate analysis, 5 fractions, 739Ar release

L & N 3 TKd Anchorage 61.6967 147.715 Dacite Fission-track Zircon 41.3 6.0 Dacite dike 

02PH 334C TKd Tyonek 61.7112 152.3290 Mafic dike 40/39 plateau Whole-rock 51.0 1.0 Mafic dike swarm dike, 4 fractio

91% 39Ar release

02PH 342C TKd Tyonek 61.7301 152.3342 Diabase 40/39 plateau Whole-rock 55.0 1.2 Diabase dike from dike swarm, 

tions, 86% 39Ar release 
50

Table 1. Radiometric age data for the map of the Cook Inlet Region, south central Alaska—continued.

[Samples listed by map units and ordered from south to north and east to west. Quadrangle is the 1:250,000-scale sheet. Methods: A, Lead-alpha, a largely 

tional potassium-argon; 40/39, 40Ar/39Ar; Rb/Sr, rubidium-strontium; Nd/Sm, neodymium-samarium, U/Pb, uranium-lead

Sample Map

unit

Quadrangle Latitude

°N

Longitude

°W

 Rock type Method Mineral Age

(Ma) 

Error

(m.y.)

Comment 

78ASi-M22A TKd Anchorage 61.75 148.5333 Rhyolite K/Ar Whole-rock 37.5 1.2 Rhyolite stock, approximate loc

81AWk 51B TKd Anchorage 61.755 148.275 Basalt K/Ar Whole-rock 38 2 Basalt dike, very low K2O, rese

81AWk 49A TKd Anchorage 61.755 148.33 Basalt K/Ar Whole-rock 130 6 Basalt dike 

78ASi-M6 TKd Anchorage 61.7667 148.52833 Rhyolite K/Ar Whole-rock 40.0 1.6 Rhyolite dike 

02PH 336B TKd Tyonek 61.7699 152.4027 Mafic dike 40/39 plateau Whole-rock 57.1

58.0

3.30.9

Replicate analysis, 4 fractions, 739Ar releaseReplicate analysis, 4 fractions, 739Ar release

78ASi-M8 TKd Anchorage 61.7733 148.505 Dacite K/Ar Whole-rock 43.5 1.7 Dacite dike 

MLS 5 TKd Anchorage 61.7767 149.42 Lamprophyre K/Ar Hornblende 66.2 2.0 Lamprophyre dike in tonalite 

L & N 1 TKd Anchorage 61.7783 147.4967 Dacite Fission-track Zircon 36.8 4.8 Dacite dike 

79AG-112 TKd Anchorage 61.7917 147.5717 Dacite K/Ar Whole-rock 45.5 2.3 Dacite stock, approximate locat

78ASi-M45 TKd Anchorage 61.8367 148.0933 Basalt K/Ar Whole-rock 40.9 1.6 Basalt sill, approximate location 

8-TMA3-1 TKd Talkeetna 

Mountains

62.0375 147.8842 Andesite 40/39 Whole-rock 45.5 0.3 Pseudo-plateau age; isochron ag

38 Ma

79AGz 102 TKd Talkeetna 

Mountains

62.0583 148.0 Intrusive rock K/Ar Whole-rock 46.7 2.3 Plug, approximate location 

01PJ35A TKd Talkeetna 

Mountains

62.3047 148.6214 Rhyolite 40/39 plateau Whole-rock 31.1 0.5 Vitric rhyolite intrusion. No iso

determined. Humped plateau, su

ing disturbance

99Pe 21 TKd Talkeetna 

Mountains

62.3901 149.0628 Felsite 40/39 plateau Biotite 52.1 0.4 Felsite or rhyodacite flow 

01PJ05A TKd Talkeetna 

Mountains

62.565 148.8899 Dacite 40/39 plateau Hornblende 44.6 1.1 Porphyritic dacite intrusion. Iso

age 45.0±1.0 Ma

6-27-1 Tb Iliamna 59.0733 154.0417 Basalt K/Ar Whole-rock 4.40

5.10

0.51.0

— 

79AGz 116 Tmf Talkeetna 

Mountains

62.1 148.16667 Tuff K/Ar Whole-rock 43.6 2.2 Pyroclast in tuff breccia, approx

location

77AMb 122 Tmv Afognak 58.88 153.2967 Andesite K/Ar Hornblende 29.3 8.4 Sill 

GT8 Tmv Iliamna 59.11 154.94 Andesite K/Ar Whole-rock 31.30 0.9 — 

GT4 Tmv Iliamna 59.64 154.45 Andesite K/Ar Plagioclase 33.80 6.8 — 
51

Table 1. Radiometric age data for the map of the Cook Inlet Region, south central Alaska—continued.

[Samples listed by map units and ordered from south to north and east to west. Quadrangle is the 1:250,000-scale sheet. Methods: A, Lead-alpha, a largely 

tional potassium-argon; 40/39, 40Ar/39Ar; Rb/Sr, rubidium-strontium; Nd/Sm, neodymium-samarium, U/Pb, uranium-lead

Sample Map

unit

Quadrangle Latitude

°N

Longitude

°W

 Rock type Method Mineral Age

(Ma) 

Error

(m.y.)

Comment 

GT9 Tmb Iliamna 59.31 154.62 Andesite K/Ar Whole-rock 29.50 1.4 — 

GT10 Tmb Lake Clark 60.57 154.34 Basalt K/Ar Whole-rock 44.4 1.7 — 

79AGz 112 Tem Talkeetna 

Mountains

62.0 148.2217 Basalt K/Ar Whole-rock 60.1 4.6 Basalt flow, approximate locatio

suspect age due to low K2O

79AGz 107 Tem Talkeetna 

Mountains

62.0733 147.7983 Andesite K/Ar Whole-rock 48.4 2.4 Andesite flow, approximate loca

79AGz 106 Tem Talkeetna 

Mountains

62.125 147.7633 Basalt K/Ar Whole-rock 55.5 3.5 Basalt flow, approximate locatio

75ASj 526 Tem Talkeetna 

Mountains

62.2808 148.7238 Andesite K/Ar Whole-rock 56.3 2.5 — 

01PJ40A Tem Talkeetna 

Mountains

62.2953 148.667 Basalt 40/39 plateau Whole-rock 46.1 0.4 Isochron age 46.0±0.5 Ma. Very 

calculated percent radiogenic ar

75ASj 520 Tem Talkeetna 

Mountains

62.4772 149.4917 Andesite K/Ar Hornblende 50.4 2.0 — 

00BP02 Tem Talkeetna 

Mountains

62.4780 148.7839 Basalt 40/39 Whole-rock 44.4 0.2 Basalt flow 

01PJ10D Tem Talkeetna 

Mountains

62.5889 148.8662 Basalt 40/39 plateau Whole-rock 44.9 1.0 Isochron age 44.8±1.0 Ma 

01PJ20B Tem Talkeetna 

Mountains

62.5889 148.8662 Basalt 40/39 plateau Whole-rock 21.6 2.1 Isochron age 25.5±5.8 Ma. Disc

dance between plateau and isocsuggests sample was highly distand therefore age is questionabl

75ASj 521B Tem Talkeetna 

Mountains

62.59 148.8945 Andesite K/Ar Whole-rock 51.3 2.5 — 

8-TMA3-1 Tem Talkeetna 

Mountains

62.0375 147.8842 Andesite 40/39 Whole-rock 45.5 0.3 Pseudo-plateau age; isochron ag

38 Ma

GLAC99-17 Tem Talkeetna 

Mountains

62.0219 147.9850 Basalt 40/39 plateau Whole-rock 45.6 5.1 — 

STM1-85 Tem Talkeetna 

Mountains

62.0456 147.7239 Basaltic 

andesite

40/39 plateau Whole-rock 48.3 1.6 — 

00BP01 Tfv Talkeetna 

Mountains

62.4780 148.7839 Rhyodacite 40/39 White mica 45.2 0.5 Rhyodacite dome. Total gas isoc

46.4±0.1 Ma

81AMH 65A Tfv Anchorage 61.3867 147.5367 Dacite K/Ar Whole-rock 43.6 1.6 Dacite dike 

70ACs 423 Tfv Anchorage 61.065 149.7967 Dacite K/Ar Hornblende 34.8 2.0 Dike or sill, age recalculated us

stants of Steiger and Jager (197

2-TMA3-3 Tfv Talkeetna 

Mountains

62.0389 147.9139 Rhyolite 40/39 Whole-rock 44.1 0.2 Pseudo-plateau age, disturbed a

spectrum
52

Table 1. Radiometric age data for the map of the Cook Inlet Region, south central Alaska—continued.

[Samples listed by map units and ordered from south to north and east to west. Quadrangle is the 1:250,000-scale sheet. Methods: A, Lead-alpha, a largely 

tional potassium-argon; 40/39, 40Ar/39Ar; Rb/Sr, rubidium-strontium; Nd/Sm, neodymium-samarium, U/Pb, uranium-lead

Sample Map

unit

Quadrangle Latitude

°N

Longitude

°W

 Rock type Method Mineral Age

(Ma) 

Error

(m.y.)

Comment 

7-TMA3-6 Tfv Talkeetna 

Mountains

62.0417 147.9017 Rhyolite 40/39 plateau;

40/39

Whole-rock;K-feldspar

35.633.7

0.20.4

Well defined plateau and isochr

K-feldspar yielded pseudo-plate

GLA99-18c Tfv Talkeetna 

Mountains

62.0219 147.9942 Rhyolite 40/39 Whole-rock 37.9 0.2 Pseudo-plateau age 

99ANS-9A Tfv Talkeetna 

Mountains

62.0472 148.0200 Dacite 40/39 plateau Whole-rock 39.9 0.7 Well defined plateau and isochr

90WG 143F Tpv Tyonek 61.8792 152.3361 Basalt 40/39 average Whole-rock 57.9 1.9 Maximum age thought to be abo

Ma

01PH 414A Tpv Tyonek 61.6275 152.2295 Tuff 40/39 plateau Whole-rock 56.9 0.7 Crystal tuff, 8 fractions, 94% 39

release

02PH 462A Tpv Tyonek 61.7174 151.9910 Tuff U/Pb Zircon 61.9 0.3 Crystal lithic tuff, 2 concordant

lapping fractions; 4 run in all 

90WG 154 Tpv Tyonek 61.8694 152.2386 Volcanic rock 40/39 plateau Whole-rock 62.1 0.4 — 

90MR 117C Tpv Tyonek 61.8829 152.3017 Basalt 40/39 average Whole-rock 66.0 0.9 — 

90DNS 31 Tpv Tyonek 61.9256 151.9880 Andesite 40/39 isochron Whole-rock 60.82 0.43 Location given as Porcupine Bu

90DNS 31 Tpv Tyonek 61.9256 151.9880 Andesite 40/39 plateau Whole-rock 60.5 0.6 Location given as Porcupine Bu

99ANS-18A Tpv Talkeetna 

Mountains

62.0242 147.8611 Andesite 40/39 Whole-rock 45.7 1.4 — 

99ANS-17a Tpv Talkeetna 

Mountains

62.0825 147.8256 Tuff 40/39 plateau Alkali feldspar 59.0 0.3 Andesitic tuff, plateau age; qual

not mentioned; a biotite analysithis sample had no plateau but sevidence of the 59 Ma age

02PH 348A TKft Tyonek 61.9026 151.8227 Tuff 40/39 plateau Whole-rock;

Hornblende

58.389.7

1.03.0

Massive crystal-lithic tuff, 7 fra82% 39Ar release3 fractions, 61% 39Ar release

01PH 415A Kv Tyonek 61.6519 152.2207 Rhyolite 40/39 plateau Hornblende 122.4 1.5 Flow-banded rhyolite, 7 fraction

39Ar released

02PH 348A Kv Tyonek 61.9026 151.8227 Tuff 40/39 plateau Whole-rock;

Hornblende

58.389.7

1.03.0

Massive crystal-lithic tuff, 7 fra82% 39Ar release3 fractions, 61% 39Ar release

02PH 317A Kv Tyonek 61.9860 151.5237 Tuff 40/39 plateau Whole-rock 98.2 0.4 Crystal-lithic tuff, 4 fractions, 6

39Ar release

99JS 48B Kv Talkeetna 

Mountains

62.2691 149.0267 Metarhyodacite 40/39 plateau Biotite; White mica 100.5 132.8 0.7 0.7 Plateau was not flat. Biotite “sev

altered”, strongly discordant wiexisting white mica. Porphyritic eye metarhyodacite 
53

Table 1. Radiometric age data for the map of the Cook Inlet Region, south central Alaska—continued.

[Samples listed by map units and ordered from south to north and east to west. Quadrangle is the 1:250,000-scale sheet. Methods: A, Lead-alpha, a largely 

tional potassium-argon; 40/39, 40Ar/39Ar; Rb/Sr, rubidium-strontium; Nd/Sm, neodymium-samarium, U/Pb, uranium-lead

Sample Map

unit

Quadrangle Latitude

°N

Longitude

°W

 Rock type Method Mineral Age

(Ma) 

Error

(m.y.)

Comment 

83APb 30 Tpi Mount Katmai 58.11 154.6833 Granodiorite K/Ar Biotite 5.00 0.18 Chloritized biotite, minimum ag

pluton

84ARj 79A Tpi Mount Katmai 58.2783 154.5 Granodiorite K/Ar Biotite 2.45 0.07 Serpent Tongue pluton 

79H9 Tpi Afognak 58.7 153.6217 Quartz diorite K/Ar Biotite 4.2 0.2 Dike or sill 

77AEg 74 Tpi Afognak 58.865 153.4133 Granodiorite K/Ar Biotite 4.8 0.5 Dike or sill 

84AEm 18A Tpi Afognak 58.6117 153.8433 Andesite K/Ar Whole-rock 8.22 0.25 Basaltic andesite porphyry sill 

84ADt 95 Tpi Afognak 58.635 153.5917 Andesite K/Ar Plagioclase 7.72 0.54 Andesite(?) sill 

83AR 28 Toegr Mount Katmai 58.78 154.965 Quartz diorite K/Ar Biotite;

Hornblende

35.835.3

1.11.1

Alaska-Aleutian Range batholit

A123 Toegr Mount Katmai 58.8083 154.63 Granodiorite K/Ar Biotite 29.1 0.9 Alaska-Aleutian Range batholit

recalculated using constants of and Jager (1977)

67AR 570 Toegr Mount Katmai 58.8283 154.73 Granodiorite K/Ar Hornblende 26.7 0.8 Alaska-Aleutian Range batholit

recalculated using constants of and Jager (1977)

83AR 29 Toegr Mount Katmai 58.8333 154.9117 Granodiorite K/Ar Biotite 37.6 1.1 Alaska-Aleutian Range batholit

A113 Toegr Mount Katmai 58.8717 154.5817 Granodiorite K/Ar Biotite 27.4 0.8 Alaska-Aleutian Range batholit

recalculated using constants of and Jager (1977)

83AR 22 Toegr Mount Katmai 58.8767 154.99 Granodiorite K/Ar Biotite 29.2 0.9 Alaska-Aleutian Range batholit

66ALe 5 Toegr Mount Katmai 58.895 154.8183 Granodiorite K/Ar Biotite;

Hornblende

28.226.0

0.80.8

Alaska-Aleutian Range batholitrecalculated using constants of and Jager (1977). Reverse disco

83AR 19 Toegr Mount Katmai 58.95 154.8133 Granodiorite K/Ar Biotite 26.6 0.8 Alaska-Aleutian Range batholit

83AR 18 Toegr Mount Katmai 58.96 154.7383 Quartz diorite K/Ar Hornblende 34.3 1.2 Alaska-Aleutian Range batholit

67AR 571 Toegr Mount Katmai 58.855 154.765 Quartz diorite K/Ar Biotite

Hornblende

27.826.1

0.60.8

Alaska-Aleutian Range batholitrecalculated using constants of and Jager (1977). Reverse disco

67AR 563 Toegr Mount Katmai 58.9183 154.5817 Quartz diorite K/Ar Biotite 26.8

25.6

0.80.8

Alaska-Aleutian Range batholitrecalculated using constants of and Jager (1977)

83AR 18 Toegr Mount Katmai 58.96 154.7383 Quartz diorite K/Ar Biotite 27.5 0.8 Alaska-Aleutian Range batholit

66AR 1289 Toegr Iliamna 59.0533 154.6517 Quartz diorite K/Ar Biotite

Hornblende

35.637.0

— —

Alaska-Aleutian Range batholitrecalculated using constants of and Jager (1977)
54

Table 1. Radiometric age data for the map of the Cook Inlet Region, south central Alaska—continued.

[Samples listed by map units and ordered from south to north and east to west. Quadrangle is the 1:250,000-scale sheet. Methods: A, Lead-alpha, a largely 

tional potassium-argon; 40/39, 40Ar/39Ar; Rb/Sr, rubidium-strontium; Nd/Sm, neodymium-samarium, U/Pb, uranium-lead

Sample Map

unit

Quadrangle Latitude

°N

Longitude

°W

 Rock type Method Mineral Age

(Ma) 

Error

(m.y.)

Comment 

66ALe 23 Toegr Lake Clark 60.04 154.17 Quartz monzonite K/Ar Biotite 43.9 — Age recalculated using constant

Steiger and Jager (1977) 

66ALe 25 Toegr Lake Clark 60.415 153.61 Quartz monzonite K/Ar Biotite 39.6 — Age recalculated using constant

Steiger and Jager (1977) 

70AR 159 Toegr Lake Clark 60.705 153.28 Granodiorite K/Ar Biotite;

Hornblende

38.639.3

1.11.9

Age recalculated using constantSteiger and Jager (1977)

68AR 251 Toegr Lake Clark 60.7233 153.5383 Granodiorite K/Ar Biotite

Hornblende

39.441.9

1.11.3

Age recalculated using constantSteiger and Jager (1977)

CHK00-1 Toegr Talkeetna 

Mountains

62.0544 148.1911 Granite 40/39 plateau K-feldspar 39.0 0.4 Fine-grained granite 

L & N 2 Toegr Anchorage 61.7517 148.5083 Granodiorite K/Ar Hornblende 37.7 2.2 — 

L & N 2 Toegr Anchorage 61.7517 148.5083 Granodiorite Fission-track Zircon 37.3 4.7 — 

70AR 181 Toegr Lake Clark 60.7683 153.3367 Granodiorite K/Ar Biotite 34.8 1 Age recalculated using constant

Steiger and Jager (1977)

W4A Toegr Anchorage 61.7917 149.325 Granite K/Ar Muscovite 56.6 n.a. Altered granite, Bullion Mounta

68AR 248 Toegr Lake Clark 60.8383 153.5817 Quartz monzonite K/Ar Biotite 38.6 1.1 Age recalculated using constant

Steiger and Jager (1977) 

70AR 165 Toegr Lake Clark 60.8683 153.415 Granodiorite K/Ar Biotite 37.5 1.0 Age recalculated using constant

Steiger and Jager (1977)

68AR 245 Toegr Lake Clark 60.93 153.345 Quartz monzonite K/Ar Biotite 34.6 1.3 Age recalculated using constant

Steiger and Jager (1977) 

68AR 244 Toegr Lake Clark 60.9467 153.545 Granodiorite K/Ar Biotite;

Hornblende

31.936.3

0.91.8

Age recalculated using constantSteiger and Jager (1977). Disco

70AR 104 Toegr Talkeetna 62.7972 152.1528 Granodiorite K/Ar Biotite;

Hornblende

37.139.8

1.11.6

Age recalculated using constantSteiger and Jager (1977)

70AR 109 Toegr Talkeetna 62.8187 151.6555 Granodiorite K/Ar Biotite;

Hornblende

35.236.6

0.91.4

Age recalculated using constantSteiger and Jager (1977)

70AR 1-12 Toegr Talkeetna 62.8738 151.5738 Granodiorite K/Ar Biotite;

Hornblende

33.236.3

0.91.6

Age recalculated using constantSteiger and Jager (1977)

70AR 127 Toegr Talkeetna 62.8867 151.787 Quartz diorite K/Ar Biotite;

Hornblende

35.339.5

1.01.4

Age recalculated using constantSteiger and Jager (1977). Disco

M2-1Z Tpgr Afognak 58.2017 153.2067 Intrusive rock Fission-track Apatite 64 — Afognak pluton, elevation 0m 

M-25-88 Tpgr Afognak 58.2333 153.0417 Intrusive rock Fission-track Apatite 55 — Dike cutting Shuyak Formation

tion 0m 
55

Table 1. Radiometric age data for the map of the Cook Inlet Region, south central Alaska—continued.

[Samples listed by map units and ordered from south to north and east to west. Quadrangle is the 1:250,000-scale sheet. Methods: A, Lead-alpha, a largely 

tional potassium-argon; 40/39, 40Ar/39Ar; Rb/Sr, rubidium-strontium; Nd/Sm, neodymium-samarium, U/Pb, uranium-lead

Sample Map

unit

Quadrangle Latitude

°N

Longitude

°W

 Rock type Method Mineral Age

(Ma) 

Error

(m.y.)

Comment 

90ADW 809 Tpgr Seldovia 59.343 151.8212 Andesite 40/39 plateau Amphibole 58.64 0.52 Andesite dike, might have intru

Port Graham Formation near PoGraham

88ACy 9 Tpgr Seldovia 59.4733 150.3367 Granodiorite 40/39 isochron Biotite 54.2 0.08 Nuka pluton 

88ACy 9 Tpgr Seldovia 59.4733 150.3367 Granodiorite U/Pb Monazite 56 0.5 Nuka pluton 

92PH 454B Tpgr Seldovia 59.5428 150.1806 Granite 40/39 plateau Biotite 53.7 0.1 Thunder Bay granitic sill, slight

disturbed spectrum

Paguna Tpgr Seldovia 59.7083 150.135 Granodiorite 40/39 plateau n.a. 53.4 1.5 Paguna stock, approximate loca

91ADw 55g Tpgr Seldovia 59.8828 150.4638 Granodiorite 40/39 plateau Biotite 54.2 1.1 Chernof Stock 

92AKu 71b Tpgr Seldovia 59.9774 150.1611 Granodiorite 40/39 plateau Biotite 53.2 1.1 Tustemena pluton 

92PH 215C Tpgr Anchorage 61.0803 148.275 Granite 40/39 plateau Mica 52.8 0.1 Granite at Homestake mine, min

argon loss, cooling age

92PH 215D Tpgr Anchorage 61.0803 148.275 Granite 40/39 plateau White mica 53.7 0.1 Homestake Mine, hydrothermal

altered granite, minor argon los

02PH 410A Tpgr Tyonek 61.2915 152.0274 Quartz monzonite U/Pb Zircon 58.8 0.5 Three concordant, overlapping f

tions, 4 fractions run 

70AR 195 Tpgr Tyonek 61.2917 152.0183 Quartz monzonite K/Ar Biotite 58.4 1.7 Age recalculated using constant

Steiger and Jager (1977) 

01PH 419A Tpgr Tyonek 61.3298 152.0520 Quartz monzonite 40/39 plateau Hornblende 55.5 1.9 10 fractions, 91% 39Ar release 

70AR 191 Tpgr Tyonek 61.3317 152.5767 Quartz monzonite K/Ar Biotite 57.0 1.6 Age recalculated using constant

Steiger and Jager (1977) 

70AR 200 Tpgr Tyonek 61.425 151.955 Quartz monzonite K/Ar Biotite 58.0 1.6 Age recalculated using constant

Steiger and Jager (1977) 

70AR 194 Tpgr Tyonek 61.4583 152.6333 Granite K/Ar Biotite 57.4 1.6 Age recalculated using constant

Steiger and Jager (1977)

70AR 199 Tpgr Tyonek 61.4945 152.0125 Quartz monzonite K/Ar Biotite 59.8 1.7 Age recalculated using constant

Steiger and Jager (1977) 

 02PH 413A Tpgr Tyonek 61.5451 152.1696 Granite U/Pb Zircon 57.3 0.3 Three concordant, overlapping f

tions; 5 run in all 

02PH 385A Tpgr Tyonek 61.5585 150.9107 Granodiorite 40/39 plateau Hornblende 57.7

58.3

1.51.1

Hornblende-biotite granodioritelicate analysis, 4 fractions, 76% releaseReplicate analysis, 2 fractions, 639Ar release

90DNS 62 Tpgr Tyonek 61.6927 152.0561 Granite 40/39 average Biotite 61.9 0.7 90WG 121 Tpgr Tyonek 61.7906 151.9093 Quartz monzonite 40/39 plateau Hornblende 53.7 0.4 — 90DNS 17 Tpgr Tyonek 61.8591 151.8734 Granite 40/39 plateau Hornblende 54.7 0.6 Porphyritic granite porphyry 67AR 404 Tpgr Talkeetna 62.4425 152.8133 Quartz monzonite K/Ar Biotite 57.6 1.6 Cathedral pluton. Age recalcula

using constants of Steiger and J(1977)
56

Table 1. Radiometric age data for the map of the Cook Inlet Region, south central Alaska—continued.

[Samples listed by map units and ordered from south to north and east to west. Quadrangle is the 1:250,000-scale sheet. Methods: A, Lead-alpha, a largely 

tional potassium-argon; 40/39, 40Ar/39Ar; Rb/Sr, rubidium-strontium; Nd/Sm, neodymium-samarium, U/Pb, uranium-lead

Sample Map

unit

Quadrangle Latitude

°N

Longitude

°W

 Rock type Method Mineral Age

(Ma) 

Error

(m.y.)

Comment 

70AR 114 Tpgr Talkeetna 62.6022 151.5453 Quartz monzonite K/Ar Biotite 56.6 1.6 Kahiltna pluton. Age recalculate

constants of Steiger and Jager (

75ACy 7 Tpgr Talkeetna 

Mountains

62.625 149.41 Quartz monzonite K/Ar Biotite 58.6 1.8 — 

70AR 121 Tpgr Talkeetna 62.7005 152.6072 Quartz monzonite K/Ar Biotite 57.9 1.6 Ruth pluton. Age recalculated u

constants of Steiger and Jager (

70AR 112 Tpgr Talkeetna 62.7103 151.1745 Quartz monzonite K/Ar Muscovite Biotite 55.9 57.1 1.6 1.6 Kahiltna pluton. New muscovite measurement published in Lanp

and Reed, (1985); old analysis w53.6 Ma. Biotite age recalculateconstants of Steiger and Jager (

75ACy 2 Tpgr Talkeetna 

Mountains

62.7628 149.9388 Quartz monzonite K/Ar Biotite 56.3 1.7 — 

67AR 427 Tpgr Talkeetna 62.837 152.202 Quartz monzonite K/Ar Biotite 57.4 1.6 Tonzona pluton. Age recalculate

constants of Steiger and Jager (

70AR 116 Tpgr Talkeetna 62.898 150.5722 Quartz monzonite K/Ar Biotite 57.6 1.5 Ruth pluton. Age recalculated u

constants of Steiger and Jager (

70AR 115 Tpgr Talkeetna 62.9375 150.9033 Quartz monzonite K/Ar Biotite 56.7 1.6 McKinley pluton. Age recalcula

using constants of Steiger and J(1977)

70AR 117 Tpgr Talkeetna 62.998 150.3972 Quartz monzonite K/Ar Biotite 57.1 1.5 Ruth pluton. Age recalculated u

constants of Steiger and Jager (

75ACy 92 Tpgr Talkeetna 

Mountains

62.9972 149.3967 Quartz monzonite K/Ar Biotite 57.8 1.7 — 

66AR 1393 Tgd Lake Clark 60.1383 154.07 Granodiorite K/Ar Biotite

Hornblende

62.861.0

— —

Age recalculated using constantSteiger and Jager (1977)

K/Ar2 Tgd Lake Clark 60.3075 154.6433 Granodiorite K/Ar Biotite 56.2 1.7 Hornblende biotite granodiorite 69AR 330 Tgd Tyonek 61.26 152.7633 Granodiorite K/Ar Biotite

Hornblende

54.760.7

2.12.3

Age recalculated using constantSteiger and Jager (1977). Disco

73ACy 101 Tgd Talkeetna 

Mountains

62.3117 149.7755 Tonalite K/Ar Biotite 54.8 1.6 — 
57

Table 1. Radiometric age data for the map of the Cook Inlet Region, south central Alaska—continued.

[Samples listed by map units and ordered from south to north and east to west. Quadrangle is the 1:250,000-scale sheet. Methods: A, Lead-alpha, a largely 

tional potassium-argon; 40/39, 40Ar/39Ar; Rb/Sr, rubidium-strontium; Nd/Sm, neodymium-samarium, U/Pb, uranium-lead

Sample Map

unit

Quadrangle Latitude

°N

Longitude

°W

 Rock type Method Mineral Age

(Ma) 

Error

(m.y.)

Comment 

75ACy 146 Tgd Talkeetna 

Mountains

62.957 148.805 Granodiorite K/Ar Biotite 58.6 1.8 — 

TT-1-72 Tgd Talkeetna 

Mountains

62.9745 148.4305 Granodiorite K/Ar Biotite 58.7 1.7 Age recalculated using constant

Steiger and Jager (1977)

90WG 119 Tpd Tyonek 61.7606 151.8975 Diorite 40/39 average Whole-rock 55.9 0.5 — 

90DNS 7 Tpd Tyonek 61.8376 152.0187 Diorite 40/39 isochron;

40/39 plateau

Biotite 60.78

61.2

0.420.4

Location given as west of DickaMountain. Isochron shows slighexcess argon

66ALe 29 TKg Tyonek 61.21 152.3817 Quartz monzonite K/Ar Biotite 59.7 — Age recalculated using constant

Steiger and Jager (1977) 

69AR 328 TKg Tyonek 61.2117 152.9183 Quartz monzonite K/Ar Biotite 53.9 1.5 Age recalculated using constant

Steiger and Jager (1977) 

69AR 393 TKg Tyonek 61.2433 152.5133 Quartz monzonite K/Ar Biotite; Hornblende 54.4 65.2 1.6 1.9 Age recalculated using constant

Steiger and Jager (1977) 

70AEr 255 TKg Tyonek 61.3067 152.7033 Granodiorite K/Ar Biotite;

Hornblende

59.659.1

1.71.7

Age recalculated using constantSteiger and Jager (1977)

01PH 420A TKg Tyonek 61.3614 152.1672 Granite 40/39 plateau Biotite 62.5 0.8 Hornblende granite, 11 fraction

39Ar release

69AR 383 TKg Tyonek 61.365 152.945 Quartz monzonite K/Ar Biotite 62.0 1.6 Age recalculated using constant

Steiger and Jager (1977) 

70AR 200 TKg Tyonek 61.425 151.955 Quartz monzonite K/Ar Hornblende 60.2 1.8 Age recalculated using constant

Steiger and Jager (1977) 

69AR 239 TKg Tyonek 61.44 152.94 Quartz monzonite K/Ar Biotite; Hornblende 61.7 56.9 1.8 1.7 Age recalculated using constant

Steiger and Jager (1977) 

02PH 329A TKg Tyonek 61.4615 151.3570 Granite 40/39 plateau Biotite 59.3 0.2 Fine-grained biotite granite, 8 fr

tions, 82% 39Ar release 

01PH 415A TKg Tyonek 61.6519 152.2207 Granite 40/39 plateau Biotite 60.4 0.8 8 fractions, 80% 39Ar released 

70AR-8-11 TKg Tyonek 61.6667 152.0833 Quartz monzonite K/Ar Biotite 60.5 2.4 Age recalculated using constant

Steiger and Jager (1977) 

02ADW 313A TKg Tyonek 61.6674 152.3029 Granite U/Pb Zircon 60.5 0.5 Four concordant fractions 

69AR 207 TKg Tyonek 61.705 152.9233 Granodiorite K/Ar Biotite;

Hornblende

62.164.2

1.82.5

Age recalculated using constantSteiger and Jager (1977)

02PH 426A TKg Tyonek 61.7336 152.0739 Granite U/Pb Zircon 60.2 0.3 Fine-grained granite, 4 concord

overlapping fractions, 4 run in a

MLS 6 TKg Anchorage 61.81 149.3483 Pegmatite K/Ar Muscovite;

Feldspar

66.864.7

2.01.9

Concordant, mineralized pegmadike, alkali feldspar

W41A TKg Anchorage 61.8183 149.2917 Pegmatite K/Ar Muscovite 66.0 — Holland Prospect, copper sulphi

bearing pegmatite

90SAL 44A TKg Tyonek 61.8809 152.2577 Granite 40/39 average Biotite;

Hornblende

66.363.6

1.50.4

Reverse discordant 
58

Table 1. Radiometric age data for the map of the Cook Inlet Region, south central Alaska—continued.

[Samples listed by map units and ordered from south to north and east to west. Quadrangle is the 1:250,000-scale sheet. Methods: A, Lead-alpha, a largely 

tional potassium-argon; 40/39, 40Ar/39Ar; Rb/Sr, rubidium-strontium; Nd/Sm, neodymium-samarium, U/Pb, uranium-lead

Sample Map

unit

Quadrangle Latitude

°N

Longitude

°W

 Rock type Method Mineral Age

(Ma) 

Error

(m.y.)

Comment 

02ADW 305B TKg Tyonek 61.8872 152.3143 Granite 40/39 plateau Hornblende 65.1

67.0

1.61.1

Hornblende granite, replicate an6 fractions, 69% 39Ar released

69AR 310 TKg Tyonek 61.9017 152.8967 Quartz monzonite K/Ar Biotite 66.8 1.9 Age recalculated using constant

Steiger and Jager (1977) 

90DNS 36 TKg Tyonek 61.896 152.1395 Granodiorite 40/39 plateau Hornblende 68.0 0.7 Dacite sill, good plateau 

75ACy 135 TKg Anchorage 61.9883 149.4167 Granodiorite K/Ar Biotite;

Muscovite

65.067.2

2.02.0

Concordant 

69AR 305 TKg Talkeetna 62.0633 152.9167 Granodiorite K/Ar Biotite 67.5 1.9 Age recalculated using constant

Steiger and Jager (1977)

01WE01 TKg Talkeetna 

Mountains

62.1363 149.2603 Granodiorite 40/39 plateau Biotite;

Hornblende 

66.067.4 

0.50.7 

Approximate location, near Gunprospect. Slightly discordant agHornblende isochron 67.5±0.7, isochron 66.6±0.6

01WE02 TKg Talkeetna 

Mountains

62.1363 149.2603 Granodiorite 40/39 plateau White mica 64.7 0.5 Approximate location, near Gun

prospect. Altered medium-graintite granodiorite. Mafic minerals to chlorite, abundant sericite altof plagioclase. Hydrothermal ag

70AR 132 TKg Talkeetna 62.2367 152.7858 Quartz monzonite K/Ar Biotite 66.2 1.9 Kohlsaat pluton. Age recalculat

using constants of Steiger and J(1977)

99MBW 403 TKg Talkeetna 

Mountains

62.3948 149.2916 Granodiorite 40/39 plateau Hornblende 57.7 0.5 Biotite-hornblende granodiorite 

99MBW 458 TKg Talkeetna 

Mountains

62.4244 149.0722 Granite 40/39 plateau Potassium feldspar 68.5 0.6 Biotite granite. Flat plateau, no 

evidence of argon loss. Howevea K-feldspar, it must be considesuspect

70AR 2-14 TKg Talkeetna 62.5478 152.1883 Granodiorite K/Ar Biotite 66.2 1.8 Cascade pluton. Age recalculate

constants of Steiger and Jager (

77BT 224 TKgd Lake Clark 60.6317 154.5783 Diorite K/Ar Hornblende 69.4 2.1 Hornblende diorite 

70AR 146 TKgd Kenai 60.7433 152.9583 Granodiorite K/Ar Biotite 68.2 1.9 Age recalculated using constant

Steiger and Jager (1977)

70AR 147 TKgd Kenai 60.865 152.68 Granodiorite K/Ar Biotite; Hornblende 65.4

67.6

1.82.0

Age recalculated using constantSteiger and Jager (1977)

70AR 140 TKgd Kenai 60.9433 152.8617 Granodiorite K/Ar Biotite; Hornblende 65.1

70.5

1.92.0

Age recalculated using constantSteiger and Jager (1977)
59

Table 1. Radiometric age data for the map of the Cook Inlet Region, south central Alaska—continued.

[Samples listed by map units and ordered from south to north and east to west. Quadrangle is the 1:250,000-scale sheet. Methods: A, Lead-alpha, a largely 

tional potassium-argon; 40/39, 40Ar/39Ar; Rb/Sr, rubidium-strontium; Nd/Sm, neodymium-samarium, U/Pb, uranium-lead

Sample Map

unit

Quadrangle Latitude

°N

Longitude

°W

 Rock type Method Mineral Age

(Ma) 

Error

(m.y.)

Comment 

70AR 188 TKgd Tyonek 61.0083 152.3967 Quartz diorite K/Ar Biotite; Hornblende 64.2 

66.4

1.8 1.9

Age recalculated using constantSteiger and Jager (1977)

68AR 258 TKgd Tyonek 61.1017 152.4817 Quartz diorite K/Ar Biotite; Hornblende 62.3 

63.5

1.8 1.9

Age recalculated using constantSteiger and Jager (1977)

70AR 134 TKgd Tyonek 61.1617 152.095 Granodiorite K/Ar Biotite; Hornblende 64.4 

70.4

1.8 2.1

Age recalculated using constantSteiger and Jager (1977)

66ALe 28 TKgd Tyonek 61.2167 152.6183 Quartz diorite K/Ar Biotite;

Hornblende

50.5;51.163.0

— — —

Age recalculated using constantSteiger and Jager (1977). Probadiscordant.

MLS 9 TKgd Anchorage 61.7783 149.3033 Tonalite K/Ar Plagioclase; 

Chlorite

54.7 56.6

1.6 1.7

Propylitized tonalite, concordan

73ACy 95 TKgd Talkeetna 

Mountains

62.1075 148.9955 Granodiorite K/Ar Biotite; Hornblende 61.7 

71.3

1.9 2.1

Discordant 

73ACy 94 TKgd Talkeetna 

Mountains

62.0772 149.1872 Tonalite K/Ar Biotite; Hornblende 61.7 

61.0

1.9 1.8

— 

72ACy 127 TKgd Talkeetna 

Mountains

62.1462 149.3083 Tonalite K/Ar Biotite; Hornblende 66.4 

64.3

22

Age recalculated using constantSteiger and Jager (1977), biotiteblende tonalite, reverse concord

72ACy 117 TKgd Talkeetna 

Mountains

62.15 149.225 Quartz diorite K/Ar Biotite; Hornblende 67.3 

64.6

22

Age recalculated using constantSteiger and Jager (1977), reversconcordant

02PH 388A TKgb Tyonek 61.5700 150.7670 Hornblendite 40/39 plateau Hornblende 83.5

84.2

5.21.3

Replicate analysis, 3 fractions, 839Ar releaseReplicate analysis, 3 fractions, 739Ar release

02ADW 317A TKgb Tyonek 61.7732 152.2407 Gabbro 40/39 plateau Hornblende 69.0

72.3

1.12.6

Replicate analysis, 5 fractions, 539Ar released; poor plateau

K-2-88 Kkd Afognak 58.1717 152.895 Metagraywacke Fission-track Zircon 72 Kodiak Formation graywacke, e

tion 0m 

78ASi-M7 Km Anchorage 61.75 148.55 Hornfels K/Ar Whole-rock 40.0 1.2 Hornfelsed shale, approximate l

M-19-88 KMm Afognak 58.21 153.21 Intrusive rock Fission-track Apatite 53 Dike cuts Uyak Complex, eleva

M-19-88 KMm Afognak 58.21 153.0017 Intrusive rock 40/39 isochron Hornblende 59.3 2.2 Dike cuts Uyak Complex, 58.3 

total fusion age 
60

Table 1. Radiometric age data for the map of the Cook Inlet Region, south central Alaska—continued.

[Samples listed by map units and ordered from south to north and east to west. Quadrangle is the 1:250,000-scale sheet. Methods: A, Lead-alpha, a largely 

tional potassium-argon; 40/39, 40Ar/39Ar; Rb/Sr, rubidium-strontium; Nd/Sm, neodymium-samarium, U/Pb, uranium-lead

Sample Map

unit

Quadrangle Latitude

°N

Longitude

°W

 Rock type Method Mineral Age

(Ma) 

Error

(m.y.)

Comment 

Clark73 KMm Anchorage 61.0027 149.66 Granitic clast K/Ar Hornblende 146. 7.0 Granitic clast in metaconglomer

atic sandstone within the McHuComplex

70AR 169 Kgd Lake Clark 60.5983 153.405 Granodiorite K/Ar Biotite; Hornblende 63.1

69.0

1.82.0

Age recalculated using constantSteiger and Jager (1977)

60AGz 45 Kgd Tyonek 61.4633 150.7433 Granodiorite K/Ar Hornblende 73.5 — Age recalculated using constant

Steiger and Jager (1977)

01PH 417A Kgd Tyonek 61.5013 150.7534 Quartz diorite 40/39 plateau Hornblende 73.8 1.1 Hornblende quartz diorite, 8 fra

96% 39Ar released

02PH 322A Kgd Tyonek 61.5790 150.5509 Granodiorite 40/39 plateau Biotite 74.9 0.4 Biotite granodiorite, 4 fractions

39 Ar released; Ar loss

02PH 345A Kgd Tyonek 61.7472 152.2916 Granodiorite 40/39 plateau Hornblende 78.4 1.1 Hornblende granodiorite, 5 frac

94% 39Ar release

W077A Kgd Anchorage 61.77 149.3183 Granite K/Ar Biotite;

Hornblende

58.170.1

— —

Chloritized biotite, Hatcher Pas

MLS 10 Kgd Anchorage 61.7833 149.313 Tonalite K/Ar Muscovite 56.6 1.7 Altered tonalite, quartz-sericite 

on gold-bearing vein at Bullion 

WKW Kgd Anchorage 61.785 149.3083 Granite K/Ar Biotite 78.4 — Kelley Willow Prospect 

66AGz W4 Kgd Anchorage 61.7867 149.2183 Tonalite K/Ar Biotite;

Hornblende

72.074.4

2.22.2

Concordant 

MLS 4 Kgd Anchorage 61.7917 149.325 Tonalite K/Ar Biotite;

Hornblende

78.872.2

2.42.2

Reverse discordant 

66AGz W2 Kgd Anchorage 61.8133 149.2133 Tonalite K/Ar Biotite;

Hornblende

69.073.3

2.12.2

Subconcordant 

60AGz 40 Kgd Anchorage 61.8267 149.2417 Tonalite K/ar Hornblende 73.1 2.2 — 

84AWk29 Kgd Anchorage 61.88 149.0267 Tonalite K/Ar Biotite;

Hornblende

68.073.0

1.41.7

Discordant 

73ACy 97 Kgd Anchorage 61.942 148.9938 Quartz diorite K/Ar Biotite;

Hornblende

67.471.8

2.02.2

Discordant 

66ALe 30 Kqd Tyonek 61.0333 152.1433 Quartz diorite K/Ar Biotite;

Hornblende

72.673.9

— —

Age recalculated using constantSteiger and Jager (1977)

70AR 168 Kqd Lake Clark 60.6417 153.1583 Quartz diorite K/Ar Biotite; Hornblende 65.6 

68.7

1.82

Age recalculated using constantSteiger and Jager (1977)

70AR 173 Kqd Kenai 60.7483 152.8117 Diorite K/Ar Biotite;

Hornblende

67.271.5

1.92.1

Age recalculated using constantSteiger and Jager (1977)
61

Table 1. Radiometric age data for the map of the Cook Inlet Region, south central Alaska—continued.

[Samples listed by map units and ordered from south to north and east to west. Quadrangle is the 1:250,000-scale sheet. Methods: A, Lead-alpha, a largely 

tional potassium-argon; 40/39, 40Ar/39Ar; Rb/Sr, rubidium-strontium; Nd/Sm, neodymium-samarium, U/Pb, uranium-lead

Sample Map

unit

Quadrangle Latitude

°N

Longitude

°W

 Rock type Method Mineral Age

(Ma) 

Error

(m.y.)

Comment 

70AR 158 Kqd Kenai 60.8317 152.5817 Quartz diorite K/Ar Biotite;

Hornblende

70.074.4

2.02.2

Age recalculated using constantSteiger and Jager (1977)

70AR 179 Kqd Kenai 60.985 152.28 Quartz diorite K/Ar Biotite;

Hornblende

71.370.6

2.02.1

Age recalculated using constantSteiger and Jager (1977)

70AR-3-1 Kqd Talkeetna 62.0767 152.45 Diorite K/Ar Biotite 69.2 2.0 Kichatna pluton, recalculated us

constants of Steiger and Jager (

99ARj 28 Kqd Talkeetna 

Mountains

62.2988 149.1572 Diorite 40/39 plateau Hornblende 75.5 0.6 Biotite-hornblende diorite. Isoch

age 73.2 ± 1.3

65AR 1034 Kqms Iliamna 59.805 154.2467 Quartz monzonite K/Ar Hornblende 75.6 Alaska-Aleutian Range batholit

recalculated using constants of and Jager (1977)

65AR 910 Kqms Iliamna 59.8267 154.2067 Quartz diorite K/Ar Biotite

Hornblende

82.785.5

Alaska-Aleutian Range batholitrecalculated using constants of and Jager (1977)

65AR 818 Kqms Iliamna 59.8467 153.8583 Quartz diorite K/Ar Biotite 79.8 Alaska-Aleutian Range batholit

recalculated using constants of and Jager (1977)

02PH 321A Kqms Tyonek 61.9595 152.3774 Syenite-monzonite U/Pb Zircon 80.3 0.1 2 concordant, overlapping fracti

run in all 

69AR 388 Kogr Tyonek 61.3417 152.7417 Quartz diorite K/Ar Hornblende 97.9 3.7 Age recalculated using constant

Steiger and Jager (1977)

02PH 375A Kogr Talkeetna 62.0007 151.4733 Granite 40/39 plateau Biotite 96.9 1.4 Coarse biotite granite, 3 fraction

39Ar release

70AR 208 Kogr Tyonek 61.4067 152.4117 Syenite K/Ar Hornblende 109.4 3.2 Age recalculated using constant

Steiger and Jager (1977). Age lireset by surrounding plutons

74ASj 100a Kum Anchorage 61.7342 149.3937 Serpentinite K/Ar Actinolite 91.0 4.6 K2O by isotope dilution 

73ACy 17 Kum Anchorage 61.7508 149.4188 Serpentinite K/Ar Actinolite 88.9 4.4 K2O by isotope dilution 
62

Table 1. Radiometric age data for the map of the Cook Inlet Region, south central Alaska—continued.

[Samples listed by map units and ordered from south to north and east to west. Quadrangle is the 1:250,000-scale sheet. Methods: A, Lead-alpha, a largely 

tional potassium-argon; 40/39, 40Ar/39Ar; Rb/Sr, rubidium-strontium; Nd/Sm, neodymium-samarium, U/Pb, uranium-lead

Sample Map

unit

Quadrangle Latitude

°N

Longitude

°W

 Rock type Method Mineral Age

(Ma) 

Error

(m.y.)

Comment 

Pavlis H Klt Anchorage 61.575 148.7283 Trondhjemite K/Ar Biotite;

Hornblende

116126

56

Concordant 

85paw 7a Klt Anchorage 61.5833 148.7000 Trondhjemite? 40/39 total 

fusion

Hornblende 134.8 1.6 Approximate location. Thought 

contain excess argon, low tempeture heating steps contaminatedthought too old

85paw 7a Klt Anchorage 61.5833 148.7000 Trondhjemite? 40/39 total 

fusion

Biotite 119.0 0.5 Approximate location. No plate

hence total fusion age

85paw 7a Klt Anchorage 61.5833 148.7000 Trondhjemite? Rb/Sr Isochron 130.0 1.0 Approximate location. Whole-ro

plagioclase-biotite isochron 

85paw 27 Klt Anchorage 61.5833 148.7417 40/39 total 

fusion

Hornblende 115.9 1.5 Approximate location. Strongly 

dant heating spectra 

85paw 71 Klt Anchorage 61.5833 148.7583 Trondhjemite? Rb/Sr Isochron 132.6 2.6 Approximate location. Whole-ro

plagioclase-biotite isochron 

85paw 38 Klt Anchorage 61.5833 148.8000 Trondhjemite? Nd/Sm Isochron 121.5 9.5 Approximate location 

85paw 75 Klt Anchorage 61.5900 148.8000 Trondhjemite? 40/39 plateau Hornblende 126 2.3 Approximate location 

85paw 102 Klt Anchorage 61.6000 148.4333 Tonalite? 40/39 plateau Hornblende 125.4 0.7 Approximate location 

85paw 93 Klt Anchorage 61.6000 148.4333 Trondhjemite? 40/39 total 

fusion

Hornblende 105.4 0.9 Approximate location. Strongly 

turbed argon spectra 

AK341 Klt Anchorage 61.6167 149.6667 Trondhjemite U/Pb Zircon 103 — — 

C80-14 (Pavlis A)

Klt Anchorage 61.6283 148.5183 Trondhjemite K/Ar Hornblende 124 8 — 

85puc 100 Klt Anchorage 61.6583 148.4500 Tonalite 40/39 total 

fusion

Biotite 122.1 0.4 Approximate location. Total fus

no plateau

86pac 12 Klt Anchorage 61.6667 148.4500 Tonalite 40/39 total 

fusion

Biotite 120.3 0.4 Approximate location. Total fus

no plateau

85pac 31a Klt Anchorage 61.6833 148.4333 Tonalite? 40/39 plateau Hornblende 125.4 1.1 Approximate location 

81AWk 44B Klt Anchorage 61.7017 148.3633 Trondhjemite K/Ar Muscovite 110 3 — 

85paw 40 Klt Anchorage 61.6167 148.6667 Trondhjemite? Rb/Sr Isochron 133.0 0.2 Approximate location. Whole-ro

plagioclase-biotite isochron 

81KMS 25 Kvgs Seward 60.3974 149.2185 Biotite semischist K/Ar Not reported 51.5 1.5 Metamorphic age or cooling age 

Valdez Group schist unit (Kvgs) Placer River Fault

73ACy 85 Kps Anchorage 61.7167 149.5417 Schist K/Ar Muscovite 59.0 1.8 Muscovite schist, reset 

73ACy 27 Kps Anchorage 61.7305 149.4347 Schist K/Ar Muscovite 65.9 3.0 Muscovite schist, reset 
63

Table 1. Radiometric age data for the map of the Cook Inlet Region, south central Alaska—continued.

[Samples listed by map units and ordered from south to north and east to west. Quadrangle is the 1:250,000-scale sheet. Methods: A, Lead-alpha, a largely 

tional potassium-argon; 40/39, 40Ar/39Ar; Rb/Sr, rubidium-strontium; Nd/Sm, neodymium-samarium, U/Pb, uranium-lead

Sample Map

unit

Quadrangle Latitude

°N

Longitude

°W

 Rock type Method Mineral Age

(Ma) 

Error

(m.y.)

Comment 

73ACy 11a Kps Anchorage 61.7422 149.4245 Schist K/Ar Muscovite 59.6 1.8 Muscovite schist, reset 

W077 Kps Anchorage 61.77 149.3183 Schist K/Ar Muscovite;

Chlorite

57.156.0

Muscovite-chlorite, Hatcher Pas

MLS 11 Kps Anchorage 61.775 149.305 Schist K/Ar Biotite;

Chlorite

54.550.6

1.62.5

Concordant, reset 

90JK 191A KJs Tyonek 61.9083 152.2871 Conglomerate 40/39 average Hornblende 97.7 2.0 Igneous clast from conglomerat

90JK 191B KJs Tyonek 61.9083 152.2871 Conglomerate 40/39 average Hornblende 101.0 2.0 Igneous clast from conglomerat

plateau showing some argon los

90JK 191C KJs Tyonek 61.9083 152.2871 Conglomerate 40/39 plateau Hornblende 94.8 1.4 Igneous clast from conglomerat

70AR 184 KJq Lake Clark 60.7367 153.1033 Quartz diorite K/Ar Biotite;

Hornblende

72.495.8

2.02.8

Strongly discordant. Age recalcusing constants of Steiger and J(1977)

68AR 261 KJq Lake Clark 60.8867 153.0317 Granodiorite K/Ar Biotite;

Hornblende

58.897.5

1.73.8

Strongly discordant. Age recalcusing constants of Steiger and J(1977)

68AR 260 KJq Tyonek 61.0833 152.8317 Quartz diorite K/Ar Biotite;

Hornblende

60.0 80.7

1.72.3

Age recalculated using constantSteiger and Jager (1977)

62ALe 6e Jnc Kenai 60.1133 152.585 Quartz diorite K/Ar Biotite;

Hornblende

156.6159.7

— Rounded granitic boulders with

Chisik Conglomerate member oNaknek Formation. Age recalulusing constants of Steiger and J(1977)

2710M01 Jp? Anchorage 61.6243 148.6905 Metavolcanicclastic rock U/Pb Zircon 202.1 1.2 Single grain analysis, 3 grains, fi

population 

2710M01 Jp? Anchorage 61.6243 148.6905 Metavolcanicclastic rock U/Pb Zircon 205.8 0.4 Single grain analysis, 3 grains, s

population 

65AR 905 Jtr Iliamna 59.92 153.6083 Trondhjemite K/Ar Muscovite 148.0 — Alaska-Aleutian Range batholit

recalculated using constants of and Jager (1977)

82ARM 21 Jtr Anchorage 61.8083 148.9083 Trondhjemite K/Ar Biotite;

Muscovite

136.2142.5

2.24.1

Concordant 

74ACy 151 Jtr Anchorage 61.8272 148.8972 Trondhjemite K/Ar Muscovite 134.0 4.0 — 

74ACy 146 Jtr Anchorage 61.9463 148.6845 Trondhjemite K/Ar Muscovite 129 3.9 — 
64

Table 1. Radiometric age data for the map of the Cook Inlet Region, south central Alaska—continued.

[Samples listed by map units and ordered from south to north and east to west. Quadrangle is the 1:250,000-scale sheet. Methods: A, Lead-alpha, a largely 

tional potassium-argon; 40/39, 40Ar/39Ar; Rb/Sr, rubidium-strontium; Nd/Sm, neodymium-samarium, U/Pb, uranium-lead

Sample Map

unit

Quadrangle Latitude

°N

Longitude

°W

 Rock type Method Mineral Age

(Ma) 

Error

(m.y.)

Comment 

1721M04 Jtr Talkeetna 

Mountains

62.0177 148.5618 Trondhjemite U/Pb Zircon ~157 — Age given as range 157.1 to 159

Ma, no error reported. Interpretea mixing age of two similarly agzircon populations

1721M04 Jtr Talkeetna 

Mountains

62.0177 148.5618 Trondhjemite U/Pb Zircon 155.6 2.6 Weighted mean age 

73ACy 115 Jtr Talkeetna 

Mountains

62.0803 148.508 Trondhjemite K/Ar Biotite;

Muscovite

99.4135.0

3.04.0

Reset, discordant 

73ACy 114 Jtr Talkeetna 

Mountains

62.0808 148.7662 Trondhjemite K/Ar Biotite 67.8 2.0 Reset 

62AE 4 Jtr Talkeetna 

Mountains

62.3175 148.1692 Trondhjemite K/Ar Biotite;

Muscovite

143.0146.0

4.34.4

Concordant 

1723M12 Jtr Talkeetna 

Mountains

62.3807 148.1555 Trondhjemite U/Pb Zircon 156.9 2.5 Weighted mean age 

TropS54 Jtr Talkeetna 

Mountains

62.4103 147.9462 Trondhjemite U/Pb Zircon 152.7 1.3 Weighted mean age 

73ASt 275 Jtr Talkeetna 

Mountains

62.512 147.8763 Trondhjemite K/Ar Biotite 148.5 4.3 Age recalculated using constant

Steiger and Jager (1977)

73ASt 256 Jtr Talkeetna 

Mountains

62.5812 147.6978 Trondhjemite K/Ar Biotite 146.5 4.3 Age recalculated using constant

Steiger and Jager (1977)

6-14-6 Jqm Iliamna 59.9217 153.285 Quartz monzonite K/Ar Whole-rock 170 174 44 — 

1723M07 Jqm Talkeetna 

Mountains

61.8277 148.794 Granodiorite U/Pb Zircon 169.4 1.5 Weighted mean age 

1723M07 Jqm Anchorage 61.8277 148.794 Granodiorite U/Pb Zircon ~169 — Age given as 172.2 to 174.4, no 

error reported, ~best~ age assigInterpreted due to minor inheritolder zircon

1723M08 Jqm Anchorage 61.8702 148.7488 Quartz diorite U/Pb Zircon ~169 — Age range given as 169.8 to 171

Interpreted as due to minor inheof older zircon

1723M08 Jqm Anchorage 61.8702 148.7488 Quartz diorite U/Pb Zircon 168.3 1.8 Weighted mean age 

1723M04 Jqm Anchorage 61.872 148.5598 Granodiorite U/Pb Zircon 176.9 0.4 — 

82AWk 65A Jqm Anchorage 61.9067 148.5083 Granodiorite K/Ar Biotite;

Hornblende

169.0173.2

3.52.3

Concordant 

1721M03 Jqm Anchorage 61.9655 148.3242 Granodiorite U/Pb Zircon 177.5 0.8 — 

74ACy 149 Jqm Anchorage 61.9925 148.4375 Granodiorite K/Ar Biotite 168.0 5.0 — 

1721M01 Jqm Talkeetna 

Mountains

62.0193 148.3187 Granodiorite U/Pb Zircon 175.5 0.3 — 

01JS04A Jqm Talkeetna 

Mountains

62.132 148.831 Granodiorite 40/39 Biotite — — Biotite granodiorite. Disturbed a

reported as 115 to 130 Ma, probgeologically meaningless
65

Table 1. Radiometric age data for the map of the Cook Inlet Region, south central Alaska—continued.

[Samples listed by map units and ordered from south to north and east to west. Quadrangle is the 1:250,000-scale sheet. Methods: A, Lead-alpha, a largely 

tional potassium-argon; 40/39, 40Ar/39Ar; Rb/Sr, rubidium-strontium; Nd/Sm, neodymium-samarium, U/Pb, uranium-lead

Sample Map

unit

Quadrangle Latitude

°N

Longitude

°W

 Rock type Method Mineral Age

(Ma) 

Error

(m.y.)

Comment 

59AGz M25-1

Jqm Talkeetna 

Mountains

62.165 147.745 Granodiorite A Zircon 180 — Quartz diorite-granodiorite 

59AGz M25-1

Jqm Talkeetna 

Mountains

62.165 147.745 Granodiorite K/Ar Biotite 160 — Quartz diorite-granodiorite 

59AGz M25-2

Jqm Talkeetna 

Mountains

62.165 147.745 Quartz monzonite K/Ar Biotite 155.0 — Granodiorite-quartz monzonite 

1723M09 Jqm Talkeetna 

Mountains

62.2498 148.152 Granodiorite U/Pb Zircon 173.7 0.3 — 

59AGz M57 Jqm Talkeetna 

Mountains

62.3547 147.82 Granodiorite A Zircon 125 15 — 

59AGz M58 Jqm Talkeetna 

Mountains

62.3562 147.8217 Granodiorite A Zircon 165 20 — 

59AGz M58 Jqm Talkeetna 

Mountains

62.3562 147.8217 Granodiorite K/Ar Biotite;

Hornblende

174.0167.0

—6.0

Age recalculated using constantSteiger and Jager (1977); no errreported for biotite

83AR 46 Jqd Mount Katmai 58.455 155.8933 Quartz diorite K/Ar Biotite;

Hornblende

166.0167.0

5.05.0

Alaska-Aleutian Range batholit

83AR 34 Jqd Mount Katmai 58.7083 155.1217 Quartz diorite K/Ar Biotite;

Hornblende

164.0153.0

4.94.6

Discordant. Alaska-Aleutian Rabatholith

A126 Jqd Mount Katmai 58.86 154.8917 Quartz diorite K/Ar Hornblende 173.0 5.2 Alaska-Aleutian Range batholit

recalculated using constants of and Jager (1977)

69AR 1 Jqd Mount Katmai 58.975 154.5483 Quartz diorite K/Ar Hornblende 174.7 8.8 Alaska-Aleutian Range batholit

recalculated using constants of and Jager (1977)

64AR 612 Jqd Iliamna 59.3 154.305 Quartz diorite K/Ar Biotite;

Hornblende

167158

— —

Alaska-Aleutian Range batholitrecalculated using constants of and Jager (1977)

64ADt 863 Jqd Iliamna 59.3417 154.45 Quartz diorite K/Ar Biotite;

Hornblende

161159

— —

Alaska-Aleutian Range batholitrecalculated using constants of and Jager (1977)

64ADt 420A Jqd Iliamna 59.375 154.4667 Quartz diorite K/Ar Biotite 158 — Alaska-Aleutian Range batholit

recalculated using constants of and Jager (1977)

66ALe 45 Jqd Iliamna 59.3767 154.3217 Quartz diorite K/Ar Biotite;

Muscovite

158.0160.0

4.64.7

Alaska-Aleutian Range batholitrecalculated using constants of and Jager (1977)
66

Table 1. Radiometric age data for the map of the Cook Inlet Region, south central Alaska—continued.

[Samples listed by map units and ordered from south to north and east to west. Quadrangle is the 1:250,000-scale sheet. Methods: A, Lead-alpha, a largely 

tional potassium-argon; 40/39, 40Ar/39Ar; Rb/Sr, rubidium-strontium; Nd/Sm, neodymium-samarium, U/Pb, uranium-lead

Sample Map

unit

Quadrangle Latitude

°N

Longitude

°W

 Rock type Method Mineral Age

(Ma) 

Error

(m.y.)

Comment 

64ADt 715 Jqd Iliamna 59.3967 154.3083 Quartz diorite K/Ar Biotite;

Muscovite

156164

— —

Alaska-Aleutian Range batholitrecalculated using constants of and Jager (1977)

66ALe 22 Jqd Iliamna 59.4017 154.6333 Quartz diorite K/Ar Biotite;

Hornblende

135.0151.0

5.04.5

Discordant. Alaska-Aleutian Rabatholith, recalculated using conof Steiger and Jager (1977)

66ALe 13 Jqd Iliamna 59.4217 154.4433 Quartz diorite K/Ar Biotite;

Hornblende

156.0157.0

4.64.6

Alaska-Aleutian Range batholitrecalculated using constants of and Jager (1977)

6-26-1 Jqd Iliamna 59.6083 153.5617 Quartz diorite K/Ar Feldspar 155 — — 

62ALe 1 Jqd Iliamna 59.7017 153.7 Quartz diorite K/Ar Biotite;

Hornblende

163.0172.0

— —

Alaska-Aleutian Range batholitrecalculated using constants of and Jager (1977)

62ALe 2 Jqd Iliamna 59.7683 153.915 Granodiorite K/Ar Biotite;

Hornblende

87.192.3

— —

Alaska-Aleutian Range batholitrecalculated using constants of and Jager (1977)

65AR 906 Jqd Iliamna 59.8833 153.745 Quartz diorite K/Ar Biotite;

Hornblende

78.0

90.1

— —

Alaska-Aleutian Range batholitrecalculated using constants of and Jager (1977)

65AR 827 Jqd Iliamna 59.7717 153.645 Diorite K/Ar Biotite;

Hornblende

161160

— —

Alaska-Aleutian Range batholitconstants, Detterman and Reed, 

62ALe 5 Jqd Kenai 60.2533 152.886 Granodiorite K/Ar Biotite;

Hornblende

174.0171.9

— —

Alaska-Aleutian Range batholitrecalculated using constants of and Jager (1977)

70AR 175 Jqd Kenai 60.59 152.78 Granodiorite K/Ar Biotite 97.8 2.8 Age recalculated using constant

Steiger and Jager (1977). Age sumay be reset by younger pluton

70AR 177 Jqd Kenai 60.615 152.6283 Quartz diorite K/Ar Biotite;

Hornblende

163.0161.0

4.74.7

Age recalculated using constantSteiger and Jager (1977)

70AR 178 Jqd Kenai 60.6767 152.4517 Granodiorite K/Ar Biotite;

Hornblende

165.0162.0

4.84.8

Age recalculated using constantSteiger and Jager (1977)

66AR 1464 Jqd Kenai 60.8033 152.355 Quartz monzonite K/Ar Biotite 159 — Age recalculated using constant

Steiger and Jager (1977) 
67

Table 1. Radiometric age data for the map of the Cook Inlet Region, south central Alaska—continued.

[Samples listed by map units and ordered from south to north and east to west. Quadrangle is the 1:250,000-scale sheet. Methods: A, Lead-alpha, a largely 

tional potassium-argon; 40/39, 40Ar/39Ar; Rb/Sr, rubidium-strontium; Nd/Sm, neodymium-samarium, U/Pb, uranium-lead

Sample Map

unit

Quadrangle Latitude

°N

Longitude

°W

 Rock type Method Mineral Age

(Ma) 

Error

(m.y.)

Comment 

70AR 156 Jqd Kenai 60.805 152.4917 Diorite K/Ar Hornblende 146.0 4.3 Age recalculated using constant

Steiger and Jager (1977)

66AR 1 Jqd Tyonek 61.0967 151.48 Quartz diorite K/Ar Biotite;

Hornblende

167166

— —

Stedatna Creek well, depth 74527459ft, recalculated using constSteiger and Jager (1977)

L & N 5 Jqd Anchorage 61.7167 147.8383 Tonalite Fission-track Zircon 186 29 — 81AWk 14 Jqd Anchorage 61.7183 147.8417 Tonalite K/Ar Biotite;

Hornblende

172181

58

— 

L & N 4 Jqd Anchorage 61.7367 147.9067 Tonalite Fission-track Zircon 170 23 — 

82AWk62 Jqd Anchorage 61.9233 148.4933 Tonalite K/Ar Biotite;

Hornblende

169.1154.2

2.94.5

Reverse discordant 

1723M01 Jqd Anchorage 61.941 148.627 Quartz diorite U/Pb Zircon 168.9 0.3 — 

1721M05 Jqd Anchorage 61.9672 148.5283 Quartz diorite U/Pb Zircon ~169 — Age given as 169.1 to 172.0, no 

reported error, interpreted to havminor inherited zircon

1721M05 Jqd Anchorage 61.9672 148.5283 Quartz diorite U/Pb Zircon 168.9 3.3 Weighted mean age 

82SK 305 Jqd Anchorage 61.97 148.5033 Quartz diorite K/Ar Hornblende 168.7 1.0 — 

59AGz M25-3

Jqd Talkeetna 

Mountains

62.165 147.745 Quartz diorite A Zircon 150 — — 

59AGz M25-3

Jqd Talkeetna 

Mountains

62.165 147.745 Quartz diorite K/Ar Biotite 160 — — 

59AGz M26 Jqd Talkeetna 

Mountains

62.2138 148.1097 Quartz diorite K/Ar Biotite 161 — Age recalculated using constant

Steiger and Jager (1977), no errreported

59AGz M26 Jqd Talkeetna 

Mountains

62.2138 148.1097 Quartz diorite K/Ar Biotite 173 — Age recalculated using constant

Steiger and Jager (1977), no errreported

59AGz M26 Jqd Talkeetna 

Mountains

62.2138 148.1097 Quartz diorite K/Ar Biotite

Hornblende

170.0163.0

66

Age recalculated using constantSteiger and Jager (1977)

2724M01 Jqd Talkeetna 

Mountains

62.2317 148.8497 Tonalite U/Pb Zircon ~192 — Age given as range, 191.5 to 19

error reported. Probably include

ited Carboniferous to Triassic zi

2724M01 Jqd Talkeetna 

Mountains

62.2317 148.8497 Tonalite U/Pb Zircon 187.4 2.2 Weighted mean age 

2712M06 Jqd Talkeetna 

Mountains

62.2523 148.7028 Tonalite U/Pb Zircon ~191 — Age given as range, 190.0 to 19

error reported. Analysis indicatesion of Paleozoic xenocrystic zi

2712M06 Jqd Talkeetna 

Mountains

62.2523 148.7028 Tonalite U/Pb Zircon 188.3 2.2 Weighted mean age. 

2712B2A Jqd Talkeetna 

Mountains

62.3727 148.2543 Tonalite U/Pb Zircon 190.5 6.8 Weighted mean age 
68

Table 1. Radiometric age data for the map of the Cook Inlet Region, south central Alaska—continued.

[Samples listed by map units and ordered from south to north and east to west. Quadrangle is the 1:250,000-scale sheet. Methods: A, Lead-alpha, a largely 

tional potassium-argon; 40/39, 40Ar/39Ar; Rb/Sr, rubidium-strontium; Nd/Sm, neodymium-samarium, U/Pb, uranium-lead

Sample Map

unit

Quadrangle Latitude

°N

Longitude

°W

 Rock type Method Mineral Age

(Ma) 

Error

(m.y.)

Comment 

73ACy 109 Jqd Talkeetna 

Mountains

62.4578 148.3095 Quartz diorite K/Ar Biotite

Hornblende

144.0154.0

4.34.6

Discordant 

KA431 Jqd Talkeetna 

Mountains

62.6417 147.3833 Diorite K/Ar Biotite 169 — Oshetna River 

64AE 98 Jmu Iliamna 59.2683 154.49 Gabbro K/Ar Hornblende 183 — Alaska-Aleutian Range batholit

blende gabbro. Age recalculated constants of Steiger and Jager (

65ADt 1084 Jmu Iliamna 59.73 153.8483 Hornblendite K/Ar Hornblende 160 Alaska-Aleutian Range batholit

recalculated using constants of and Jager (1977). Minimum age

ANCH-1 Jeqd Anchorage 61.4633 149.36 Quartz diorite K/Ar Biotite 161 5 — 

SClark 1 Jeqd Anchorage 61.465 149.3617 Quartz diorite K/Ar Biotite 165 5 — 

AK316 Jeqd Anchorage 61.5967 148.92 Quartz diorite K/Ar Hornblende 167 9 — 

AK319 Jeqd Anchorage 61.645 148.745 Quartz diorite U/Pb Zircon 183 — Concordant, no error reported 

W80-48 (Pavlis F)

Jeqd Anchorage 61.6467 148.7317 Quartz diorite K/Ar Hornblende 194 7 — 

W80-25B (Pavlis E)

Jeqd Anchorage 61.6533 148.685 Quartz diorite K/Ar Hornblende 191 7 — 

2714M02 Jeqd Anchorage 61.6562 148.7982 Tonalite U/Pb Zircon 186.1 0.3 — 

0719P05B Jeqd Anchorage 61.6588 148.6835 Trondhjemite U/Pb Zircon 193.3 0.4 — 

81AWk 34 Jeqd Anchorage 61.66 148.6883 Granodiorite K/Ar Biotite;

Hornblende

174175

58

Concordant 

C80-43B (Pavlis C)

Jeqd Anchorage 61.67 148.5233 Diorite K/Ar Hornblende 135 6 — 

2717M04 Jeqd Anchorage 61.6968 148.5352 Diorite U/Pb Zircon 193.9 0.5 — 

AK320 Jeqd Anchorage 61.72 148.4867 Quartz diorite K/Ar Hornblende 165 10 — 

0729G02 Jeqd Anchorage 61.7243 147.576 Quartz diorite U/Pb Zircon 186.1 0.8 — 

0717B03 Jeqd Anchorage 61.726 147.0935 Trondhjemite U/Pb Zircon 184.1 1.9 — 

0716P01 Jeqd Anchorage 61.7268 147.0928 Trondhjemite U/Pb Zircon 185.1 0.5 — 

AK338 Jeqd Anchorage 61.7733 147.2467 Quartz diorite K/Ar Hornblende 167 10 — 

W80-17 (Pavlis G)

Jum Anchorage 61.6367 148.6583 Diorite K/Ar Hornblende 189 8 

AK317 Jum Anchorage 61.645 148.675 Quartz diorite U/Pb Zircon 171 — Concordant, no error reported 

AK332 Jum Anchorage 61.6933 148.5383 Diorite K/Ar Hornblende 172 11 — 

82AWk59 Jum Anchorage 61.715 148.4933 Tonalite K/Ar Hornblende 176.4 3.7 — 
69

Table 1. Radiometric age data for the map of the Cook Inlet Region, south central Alaska—continued.

[Samples listed by map units and ordered from south to north and east to west. Quadrangle is the 1:250,000-scale sheet. Methods: A, Lead-alpha, a largely 

tional potassium-argon; 40/39, 40Ar/39Ar; Rb/Sr, rubidium-strontium; Nd/Sm, neodymium-samarium, U/Pb, uranium-lead

Sample Map

unit

Quadrangle Latitude

°N

Longitude

°W

 Rock type Method Mineral Age

(Ma) 

Error

(m.y.)

Comment 

AK336 Jum Anchorage 61.74 147.22 Gabbro K/Ar Hornblende 177 11 Hornblende gabbro 

V5C Jsch Afognak 58.145 153.1867 Schist K/Ar Muscovite 196.4 5.8 Quartz-mica schist. Age recalcu

using Steiger and Jager, 1977 co

R83M Jsch Afognak 58.2167 153.0333 Schist Rb/Sr Isochron 212 — Five whole rock samples, 4 Ras

berry sch., 1 Seldovia sch., R83MlN11, Ml05, SlU3, 7-24-S6

ML-08 Jsch Afognak 58.23 153.0317 Blueschist Rb/Sr Isochron 207 — Whole rock-phengite date from 

ceous quartzite, Raspberry schis

ML-05 Jsch Afognak 58.23 153.0333 Blueschist Rb/Sr Isochron 189 — Whole rock-phengite date from 

ite, Raspberry schist 

M-26-88 Jsch Afognak 58.23 153.0367 Schist Fission-track Apatite;

Zircon

77148

— Raspberry schist, elevation 0 m 

ML-N11 Jsch Afognak 58.2317 153.0367 Blueschist Rb/Sr Isochron 193 — Whole rock-phengite date from 

metavolcanic, Raspberry schist

M1D Jsch Afognak 58.2417 153.0467 Schist K/Ar Amphibole;

Mica

172.8191.9

5.15.6

White mica-crossite schist, crosdate, recalculated using Steiger Jager, 1977 constants

7AF23-9 Jsch Seldovia 59.0033 151.7067 Schist K/Ar Muscovite 194.7 5.7 Quartz-mica schist, approximat

tion, recalculated using Steiger Jager, 1977 constants

74PG79 Jsch Seldovia 59.37 151.81 Schist K/Ar Muscovite 196.1 5.8 Quartz-mica schist, approximat

tion, recalculated using Steiger Jager, 1977 constants

92ATi 316D Jsch Seldovia 59.4479 151.7145 Schist 40/39 plateau White mica 190.98 0.3 Seldovia metamorphic complex

Plateau age

92ATi 309B Jsch Seldovia 59.4547 151.7153 Schist 40/39 plateau Barroisite;

Muscovite

191.92191.7

0.60.3

Seldovia metamorphic complexPlateau age, quartz-white mica-schist

SD3-3 Jsch Seldovia 59.4617 151.7067 Greenschist K/Ar Actinolite;

Chlorite;Mica

195.0185.1192.3

11.08.310.0

Approximate location. Age recalated using constants of Steiger Jager (1977)

74AF4B.1 Jsch Seldovia 59.4617 151.7067 Schist K/Ar Amphibole 166.8 4.9 Blueschist, crossite date, approx

location, recalculated using SteiJager, 1977 constants

74AF23-10 Jsch Seldovia 59.4617 151.7067 Schist K/Ar Amphibole;

Amphibole;Muscovite

188.6188.3196.6

5.55.55.8

Amphibole-mica schist, approxlocation, replicate amphibole anAge recalculated using constantSteiger and Jager (1977)

SD9-3 Jsch Seldovia 59.4667 151.7333 Schist K/Ar Crossite;

Phengite

157.8192.9

4.85.7

Blueschist, possible argon loss, imate location. Age recalculated constants of Steiger and Jager (

JD-1 (76181) Jsch Anchorage 61.4883 149.2317 Greenschist K/Ar Actinolite 177 7 Actinolite epidote schist greens

Age recalculated using constantSteiger and Jager (1977)
70

Table 1. Radiometric age data for the map of the Cook Inlet Region, south central Alaska—continued.

[Samples listed by map units and ordered from south to north and east to west. Quadrangle is the 1:250,000-scale sheet. Methods: A, Lead-alpha, a largely 

tional potassium-argon; 40/39, 40Ar/39Ar; Rb/Sr, rubidium-strontium; Nd/Sm, neodymium-samarium, U/Pb, uranium-lead

Sample Map

unit

Quadrangle Latitude

°N

Longitude

°W

 Rock type Method Mineral Age

(Ma) 

Error

(m.y.)

Comment 

F80-6A (Pavlis B)

Jsch Anchorage 61.6083 148.595 Amphibolite K/Ar Hornblende 107 5 Impure hornblende separate 

C80-28 (Pavlis D)

Jsch Anchorage 61.6633 148.5183 Amphibolite K/Ar Hornblende 121 8 Reset 

R2Y ^qd Afognak 58.06 153.4133 Dioritic migmatite K/Ar Hornblende 187.5 5.5 Dioritic migmatite, mean of 2 d

minations. Age recalculated usinconstants of Steiger and Jager (

R4A ^qd Afognak 58.1467 153.2517 Diorite K/Ar Hornblende 192.7 5.7 Hornblende diorite, approximat

tion. Age recalculated using conof Steiger and Jager (1977)

M2-1Z ^qd Afognak 58.2017 153.2067 Intrusive rock Fission-track Zircon 153 — Afognak pluton, elevation 0m 

M2-1Z ^qd Afognak 58.2017 153.2067 Intrusive rock 40/39 Feldspar 134.5 — 40/39 incremental heating, integ

age, range 150-120 Ma

ML-I1 ^qd Afognak 58.2150 153.1650 Quartz diorite U/Pb Zircon 217

203.5

— —

Coarse fractionFine fraction

B10-9Z ^qd Afognak 58.375 152.7817 Diorite K/Ar Hornblende 197.0 5.8 Hornblende diorite, recalculated 

Steiger and Jager, (1977) consta

BARREN ^qd Afognak 58.9433 152.2067 Quartz diorite K/Ar Hornblende 191 1.3 Ushagat Island, approx. location 

Steiger and Jager (1977) consta

91DW 87 }g Seldovia 59.6075 151.1378 Gabbro U/Pb Zircon 227.7 0.6 Gabbro of Halibut Cove 

72ASt 290 JPam Talkeetna 

Mountains

62.76 147.3083 Diorite K/Ar Hornblende 176.6 5.1 Diorite or amphibolite 

Unknown rock units76JF 649 — Iliamna 59.3167 154.0667 Unknown K/Ar Whole-rock 29.0 1.5 Estimated latitude and longitude

tion given as SE1/4 Sec. 7 T10S however location must be in R2Location near Contact Point. Agrecalculated using constants of and Jager (1977)

n.a. — Seldovia 59.5392 150.478 Not reported 40/39 plateau Sericite 55.6 0.1 Valdez Group, Beauty Bay Min

altered rock, clear sericite

n.a. — Seldovia 59.5392 150.478 Not reported 40/39 Sericite 55.9 0.1 Valdez Group, Beauty Bay Min

altered rocks, gold-colored sericPreferred date based on 37.2 pegas, disturbed spectra
71

Table 1. Radiometric age data for the map of the Cook Inlet Region, south central Alaska—continued.

[Samples listed by map units and ordered from south to north and east to west. Quadrangle is the 1:250,000-scale sheet. Methods: A, Lead-alpha, a largely 

tional potassium-argon; 40/39, 40Ar/39Ar; Rb/Sr, rubidium-strontium; Nd/Sm, neodymium-samarium, U/Pb, uranium-lead

Sample Map

unit

Quadrangle Latitude

°N

Longitude

°W

 Rock type Method Mineral Age

(Ma) 

Error

(m.y.)

Comment 

n.a. — Seldovia 59.5428 150.18 Vein 40/39 plateau White mica 52.9 0.1 Thunder Bay gold occurrence, p

metalllic gold-sulfide vein 

GT13 — Iliamna 59.73 154.72 Andesite K/Ar Whole-rock 36.4 Rabbit Island, Iliamna Lake 

RG075 — Anchorage 61.043 149.1038 Not reported 40/39 plateau White mica 54.3 0.1 Hydrothermal alteration at Jewe

Valdez Group

02PH 394A — Tyonek 61.4372 150.6489 Tuff U/Pb Zircon ~32 Andesite tuff, 1 concordant frac

2710M01 — Anchorage 61.6243 148.6905 Metavolcanic / 

clastic

U/Pb Zircon 202.1 1.2 

88pac 24 — Anchorage 61.6417 148.6333 Not reported 40/39 total 

fusion

Hornblende 180.4 1.0 Approximate location 

W32W — Anchorage 61.7817 149.4017 Vein K/Ar Muscovite 66.3 Lucky Shot Mine, gold bearing 

MLS 8 — Anchorage 61.7833 149.405 Vein K/Ar Muscovite 66.3 2.0 Gold-bearing quartz vein in ton

99Pe 66 — Talkeetna 

Mountains

62.2960 149.0562 Diorite 40/39 plateau Hornblende 251.1 2.1 Sample thought to contain signi

excess argon. Isochron age was ± 10.9

99MBW 529 — Talkeetna 

Mountains

62.3583 149.0070 Schist 40/39 plateau Sericite 145.7 1.0 Quartz-sericite-pyrite schist 

99MBW 541B

— Talkeetna 

Mountains

62.3699 149.2170 Hornblendite 40/39 isochron Hornblende 63.0 8.4 Sample has had complex therma

tory and significant excess argonthought to be “reset.” “Saddle” age was 84.0 ± 1.2 may be too o

CC36-659 — Talkeetna 

Mountains

62.993 149.861 Greisen 40/39 plateau Muscovite 53.4 0.2 Coal Creek tin deposits. Plateau 

isochron age. Integrated age 53.

N.A. — Talkeetna 

Mountains

n.a. n.a. Rhyolite U/Pb Zircon 45.2 1.1 Weighted mean age, based on se

analysis of a suite of zircon; incof all samples in weighted mean lation yielded 45.5 ± 1.1 Ma