DIGITAL DATA FOR\
GEOLOGIC FRAMEWORK OF THE ALASKA PENINSULA, SOUTHWEST\
ALASKA, AND THE ALASKA PENINSULA TERRANE

Frederic H. Wilson^1^, Robert L. Detterman^2^, and Gregory D. DuBois^3^

OPEN-FILE REPORT 99-317

This report is preliminary and has not been reviewed for conformity with
U.S. Geological Survey editorial standards or with the North American
Stratigraphic Code. Any use of trade, product, or firm names is for
descriptive purposes only and does not imply endorsement by the U.S.
Government.

1 U.S. Geological Survey, 4200 University Dr., Anchorage AK 99508-4667;
2 Deceased; ^3^ U.S. Geological Survey, 4200 University Dr., Anchorage
AK 99508-4667, now at Emcon Alaska Inc., 4701 Business Park Blvd.,
Anchorage AK 99503

1999

ABSTRACT

The Alaska Peninsula is composed of the late Paleozoic to Quaternary
sedimentary, igneous, and minor metamorphic rocks that record the
history of a number of magmatic arcs. These magmatic arcs include an
unnamed Late Triassic(?) and Early Jurassic island arc, the early
Cenozoic Meshik arc, and the late Cenozoic Aleutian arc. Also found on
the Alaska Peninsula is one of the most complete unmetamorphosed,
fossiliferous, marine Jurassic sedimentary sections known. Up to 8,500 m
of section of Mesozoic sedimentary rocks record the growth and erosion
of the Early Jurassic island arc.

A thinner, but still thick (up to 5,400 m), sequence of Tertiary
sedimentary rocks that are predominantly continental overlies the
Mesozoic section. A brief regression in early Tertiary time on the
Alaska Peninsula and granodiorite plutonism in the Shumagin, Semidi, and
Sanak Islands was followed by deposition of fluvial and minor marine
clastic strata. This was followed by deposition of transgressive marine
clastic strata and initiation of the Meshik arc, shown by an areally
extensive outpouring of volcanic and volcaniclastic rocks and debris
between late Eocene and earliest Miocene time. Late Miocene time was
marked by another brief transgression and northwest- to
southeast-directed compression, followed by renewed volcanism and
plutonism which initiated the modern Aleutian magmatic arc.

Extensive glacial and glacio-marine deposits of late Pleistocene age
create an extensive lowland physiographic province on the northwest side
of the Alaska Peninsula and join isolated mountain masses to the Alaska
Peninsula on the southwest. Multiple active volcanoes and volcanic peaks
dominate the skyline of the Alaska Peninsula and represent the
continuation of magmatic activity that has formed the Aleutian arc since
late Miocene time.

The Alaska Peninsula has had a long and involved history since Paleozoic
time. We propose that the Paleozoic and Mesozoic rocks that constitute
much of the Alaska Peninsula be called the Alaska Peninsula terrane.
Using the concept of subterranes, we divide the terrane into two
distinct but tectonically related subterranes: the Chignik and Iliamna
subterranes, which share a limited common geologic history. The Iliamna
subterrane has served at most times as a source area for the Chignik
subterrane; however, some rock units are in common across the
subterranes. The Iliamna and Chignik subterranes are in part separated
by the Bruin Bay fault system. The Iliamna subterrane is composed of
moderately deformed early Mesozoic marine sedimentary and volcanic rocks
and schist, gneiss, and marble of Paleozoic(?) and Mesozoic age and
plutonic rocks of the Alaska-Aleutian Range batholith. Characteristic of
the Chignik subterrane are little-deformed, shallow-marine to
continental clastic sedimentary rocks ranging in age from Permian to
latest Cretaceous. However, deep-marine, volcaniclastic, and calcareous
rocks form important components of the older rocks in the subterrane.

The two subterranes of the Alaska Peninsula terrane are characterized by
radically different structural and metamorphic styles. The non-plutonic
rocks of the Iliamna subterrane are characterized by metamorphism up to
amphibolite-facies grade and intense folding. In the Chignik subterrane,
the structural style is dominated by large, open, en echelon anticlinal
structures, normal faulting, and thrust and high-angle reverse faults
that have minor displacement in a northwest to southeast direction. In
the Outer Shumagin and Sanak Islands, rocks assigned to the Chugach
terrane are characterized structurally by tight, generally
northeast-trending folds. Dips in these rocks tend to be steep, rarely
less than 35°, and overturned beds are locally common.

The boundaries separating the Alaska Peninsula terrane from other
terranes are commonly indistinct or poorly defined. A few boundaries
have been defined at major faults, although the extensions of these
faults are speculative through some areas. The west side of the Alaska
Peninsula terrane is overlapped by Tertiary sedimentary and volcanic
rocks and Quaternary deposits.

INTRODUCTION

This map (pl. 1) of the Alaska Peninsula is based on the mapping
conducted as part of the Alaska Mineral Resource Assessment Program
(AMRAP). Mapping on the Alaska Peninsula under AMRAP began in 1977 in
the Chignik and Sutwik Island 1:250,000 quadrangles (Detterman and
others, 1981a). Continued mapping in the Ugashik, Bristol Bay, and
northwestern Karluk quadrangles (Detterman, Case, and others, 1987), the
Mount Katmai, eastern Naknek, and northwestern Afognak quadrangles
(Riehle and others, 1987; 1993) and the Port Moller, Stepovak Bay, and
Simeonof Island (Wilson and others, 1995) quadrangles on the Alaska
Peninsula was also conducted as part of AMRAP. The Cold Bay and False
Pass quadrangles were initially mapped by McLean and others (1978);
however, extensive modification of their map has resulted from field
work by F.H. Wilson, G.D. DuBois, R.L. Detterman, and W.H. White (DuBois
and others, 1989; unpub. data 1990), T.P. Miller and M.E. Yount (written
commun., 1989) and airphoto and Landsat interpretation by F.H. Wilson.
We have also incorporated the mapping of Kennedy and Waldron (1955) and
Waldron (1961) in the Cold Bay and False Pass quadrangles.

Geologic observations for mapping were made by helicopter-aided
foot-traverses and at spot localities. Coverage of areas between surface
observations was made using helicopter overflights, interpretation of
vertical aerial photography, and also enhanced and extensively processed
Landsat imagery in the Port Moller area (York and others, 1984; Wilson
and York, 1985). Field investigations were conducted from bases at
Battle Lake, approximately 8 km north of the map area, (1985, 1986),
Becharof Lake (1985, 1986), Chignik (1977, 1978), Cold Bay (1990), King
Cove (1988), King Salmon (1983, 1985), Naknek Lake (1984), Port Heiden
(1977, 1978), Port Moller (1984, 1985), and Sand Point (1982, 1984 to
1986, 1988). In addition, field work in 1977-1981, 1983, and 1984 was
also conducted using the U.S. Geological Survey Research Vessel *Don J.
Miller II*. The mapping of Burk (1965) provided an excellent base to
start the AMRAP mapping. However, important changes have been made to
Burk\'s pioneering work.

The reliability of the geologic mapping is variable, based, in part, on
the field time spent in each area of the map, the available support, and
the quality of the existing base maps. In addition, our developing
understanding of the geology of the Alaska Peninsula has required
revision of earlier maps, such as the Chignik and Sutwik Island
quadrangles map (Detterman and others, 1981a) to reflect this new
knowledge. We have revised the stratigraphic nomenclature (Detterman and
others, 1996) and our assignment of unit names to some rocks has been
changed. The major remaining uncertainties in the stratigraphy occur
primarily at the southwest end of the Alaska Peninsula and concern the
Belkofski Formation and the agglomerate of Cathedral Valley. The major
area of mapping uncertainty is also at the southwest end of the Alaska
Peninsula in the Cold Bay and False Pass quadrangles and is due to the
reconnaissance nature of the work that has been done there and
uncertainty due to the inaccuracy of the topographic base maps.
Comparison of the 1:250,000-scale topographic map with the few existing
1:63,360-scale sheets, NOAA nautical charts, and Landsat and
side-looking radar images shows that errors in the position of the
shoreline can exceed 1 km. As a result, the transfer of data from the
map of McLean and others (1978) to our revised digital base map produced
minor changes in the position of contacts. We attempted to transfer
mapped contacts and maintain topographic relationships between units.
For example, cliff lines were followed for units that appeared to be
\"cliff-formers.\"

All geologic maps on which this compilation is based were published
using the Universal Transverse Mercator projection (UTM; Zones 3, 4, and
5). Because of the distortions use of the UTM projection would produce
on a map of small scale and large area, the data for this map has been
converted to a more appropriate Albers Equal-area projection for
publication.

Definition and discussion of the Alaska Peninsula (Peninsular) terrane
in this paper is not without its difficulties. As knowledge of the
geology of the Alaska Peninsula and other parts of southern Alaska have
increased, identity of the Alaska Peninsula terrane as a fault-bounded
entity having a distinctive stratigraphy and unique geologic history
(see Berg and others, 1978; Jones and others, 1983) becomes less
convincing. Based on our knowledge of the rocks that characterize the
terrane we (1) question the location of the bounding faults that define
the Alaska Peninsula terrane, (2) question whether generally accepted
parts of the Peninsular terrane, such as the (informal) Border Ranges
ultramafic and mafic complex of Burns (1985), the Raspberry Schist of
Roeske (1986; Roeske and others, 1989), the Seldovia Schist of Roeske
(1986; equivalent to Seldovia schist terrane of Cowan and Boss, 1978),
and other units associated with the Border Ranges fault system should be
included in the Alaska Peninsula terrane, and (3) also find there to be
apparent genetic ties to several tectonostratigraphic terranes of
southern Alaska, including the Wrangellia, Chugach, and southern
Kahiltna terranes. By rigorous terrane analysis, we also find it is
difficult to show that the Alaska Peninsula terrane has a unique
stratigraphic and geologic history compared to its neighboring terranes.
However, we believe definition of the Alaska Peninsula terrane can be a
useful aid to the understanding of the geology and geologic history of
the region. Therefore, although we use the term terrane in this report,
we use it in a purely descriptive sense without the genetic implications
inherent in some usage\'s of the term \"terrane\" (see discussion by
Dover, 1990).

ACKNOWLEDGMENTS

Any compilation and interpretation of mapping as large as this relies
heavily on data and insights of prior workers. We also would not have
been as successful as we were without the aid of co-workers in the
mapping projects that preceded our work. The compilation of data for
this map was well along at the time of Robert L. (Buck) Detterman\'s
death; we would not have gotten very far along on this without Bob\'s
patience and mentorship over the years. Many a time during the
completion of the map and text we thought hard to try to determine the
way Bob would have done things and what he\'d have thought about
particular topics. We were fortunate to have had the opportunity to work
with Bob over the years and fortunate to have access to his tremendous
amount of experience and insight to the problems of Alaska Peninsula
geology.

Many geologists have contributed to the mapping of the Alaska Peninsula.
Workers whose pioneering efforts guided us are referenced in the text.
Participants in the projects that this map is a direct outgrowth of
included: Narin Albert, 1983; W.H. Allaway, Jr., 1980-81; L.M. Angeloni,
1983-85; Peter Burrell, 1983; J.E. Case, 1977-81, 1983-1985; C.L.
Connor, 1982; D.P. Cox, 1977-78, 1980; John Decker, 1985; R.L.
Detterman, 1977-86, 1988; G.D. DuBois, 1985, 1988, 1990; C.L.
Engelhardt, 1977; B.M. Gamble, 1983-84; S.A. Zurinski, 1986; C.D.
Holloway, 1977; D.G. Howell, 1977; T.E.C. Keith, 1984; L.B. Magoon,
1979-80; Louie Marincovich, Jr., 1978; 1982, 1984; Eric Martin, 1984;
Hugh McLean, 1977; J.W. Miller, 1979-86; T.P. Miller, 1973-75, 1977-78,
1979, 1981-82, 1988; M.A. Pernokas, 1983; B.L. Reed, 1983-84; J.R.
Riehle, 1981-86; Nora Shew, 1981-82; R.A. Spicer, 1977-78; F.R. Weber,
1983-85; F.H. Wilson, 1977-86, 1988, 1990; J.A. Wolfe, 1977-78; M.E.
Yount 1977-86, 1988. Their data, efforts, and insights are gratefully
acknowledged. In the office, E.E. Harris and M.W. Mullen were always
there when needed.

John W. Miller, Ralph W. Imlay, Louie Marincovich Jr., and John A. Wolfe
contributed fossil determinations and geologic mapping that help to
establish much of the stratigraphic control; potassium-argon
geochronology by Wilson, Nora Shew, and DuBois in combination with
fossil data help to establish age control. Florence R. Weber contributed
a great deal to the mapping of the Quaternary deposits, picking up and
carrying on where Bob left off. Thomas P. Miller made available
unpublished mapping in the vicinity of Mount Dutton and Emmons Caldera.
James E. Case, in addition to contributing a great deal of geologic
mapping, provided much in the interpretation of available geophysical
data. Many people on the Alaska Peninsula and helicopter pilots and
mechanics have made each field season possible, if not pleasurable.
Especially memorable were the contributions of Glenn Bell (pilot) and
Lewis Webb (mechanic and field assistant). The captain and crew of the
*RV* *Don J. Miller* during the years 1977 to 1984 were of immeasurable
aid in making it all happen, keeping us warm, dry, and fed. In
particular, Chief Engineer Edgar Maghalaes and Captain A.C. (Frosty)
Frothingham were there for us.

Reviews by J.R. Riehle, J.C. Moore, and C.M. Molenaar are gratefully
acknowledged for the errors they pointed out, the guidance freely
offered, and the support given. Discussions of geology and terrane
issues from areas beyond the map area were enjoyable and constructive
with B.L. Reed, D.C. Bradley, Bela Csejtey, and J.H. Dover.

THE ALASKA PENINSULA TERRANE

The Alaska Peninsula, long thought to be an area with a simple geologic
and tectonic history (see for example Burk, 1965, p. 75-78 and 119-122),
has had a long and involved history since Paleozoic time. Stone and
Packer (1977) reported results of paleomagnetic investigations which
suggested that since Jurassic time the Alaska Peninsula has migrated
thousands of kilometers northward from a position south of the equator.
Somewhat tongue-in-cheek, Stone and Packer (1977) called this
allochthonous terrane \"Baja Alaska\" and left its geologic definition
to others. Jones and Silberling (1979) designated a Peninsular terrane
and gave a brief definition based on its Mesozoic stratigraphy. Wilson,
Case, and Detterman (1985) redefined the Peninsular terrane and proposed
the terrane be called the Alaska Peninsula terrane; here we summarize
and update that definition of the Alaska Peninsula terrane (fig. 1, 2).
Moore (1974c, p. 813) suggested the rocks of the Alaska Peninsula
represent two \"regional tecto-stratigraphic terrains\"; however using
the concept of subterranes, as introduced by Berg and others (1978),
Wilson, Case, and Detterman (1985) suggested that Baja Alaska or the
Peninsular terrane comprises two distinct but tectonically related
subterranes. In this particular case, the two subterranes, the Chignik
and Iliamna subterranes, share a limited common geologic history; the
Iliamna subterrane has served at most times as a source area for the
Chignik subterrane; however, some rock units are in common across the
subterranes. The two subterranes have probably always been in close
proximity.

Paleomagnetic studies after Stone and Packer (1977) in southern Alaska
were summarized by Hillhouse (1987); these later studies in part
reinforced Stone and Packer\'s (1977) original conclusions and in part
showed that variability in the data was large, indicating a range of
Jurassic paleolatitudes for the Alaska Peninsula terrane from 4° to 40°.
Detterman (1988) presented a somewhat different view of the timing for
the migration history of the Alaska Peninsula based on
paleobiogeographic interpretation of Mesozoic fossil data. He suggested
that by Middle Jurassic time, the Alaska Peninsula was well into the
northern Pacific based on the presence of faunal elements of the Boreal
Realm and specifically the Boreal and Bering Provinces (Detterman, 1988,
p. 12).

The subterranes of the Alaska Peninsula terrane (fig. 1, 2) are (1) the
Iliamna subterrane, Paleozoic(?) and early Mesozoic rocks intruded by
and including the Mesozoic part the Alaska-Aleutian Range batholith
(Reed and Lanphere, 1973) and (2) the Chignik subterrane, Permian to
latest Cretaceous little-deformed fossiliferous sedimentary rocks of the
Alaska Peninsula and areas of southern Alaska including Cook Inlet and
the Matanuska Valley north of the map area.

The Iliamna and Chignik subterranes are separated by the Bruin Bay fault
system north of Becharof Lake. The boundary between the subterranes
southwest of Becharof Lake is not defined. Many authors have continued
the Bruin Bay fault system south of Becharof Lake (see Jones and
Silberling, 1979; von Huene and others, 1985; and Lewis and others,
1988); however, no actual data conclusively demonstrates its
continuation. At Becharof Lake, a structural high trends across the
Alaska Peninsula. South of the structural high the Iliamna terrane is
not exposed in outcrop; however, aeromagnetic (Case and others, 1988)
and drillhole data suggest the continuation of the Iliamna subterrane in
the subsurface. Small normal faults of short length occur along the east
shore of Ugashik Lakes, these faults are roughly aligned along the trend
of the Bruin Bay fault; however, the sense of motion of these faults is
opposite that of the Bruin Bay system. Similar faulting has not been
mapped farther southwest along the Alaska Peninsula, but if the Bruin
Bay fault system continues south of Becharof Lake, it must undergo a
significant change in character. In the Iliamna quadrangle, north of the
map area, it is a high-angle fault system (northwest up) that has a
possible sinistral strike-slip component and as much as 3 km of
stratigraphic throw (Detterman and Reed, 1980, p. B69). If the fault
system occurs south of Becharof Lake, drillhole data suggest the sense
of motion is apparently down on the north, as much as it is up farther
north. Nowhere north of Becharof Lake is the Naknek Formation known to
occur west of the fault system; southwest of Becharof Lake, rocks of the
Naknek Formation were reached by drilling in the General Petroleum Great
Basins No. 1 drillhole at a depth of 10,500 ft (3,360 m), west of the
extension of the Bruin Bay fault system (Detterman, 1990). Based on the
available constraints, a small rotational tilt (3° to 4°) along the axis
of the Alaska Peninsula can explain the depth to the Naknek as an
alternative to reliance on the Bruin Bay fault system. Case and others
(1988) interpretation of aeromagnetic data from the Ugashik
1:250,000-scale quadrangle postulated that \"If the geologic projection
of the fault is correct, it seems likely that the fault splays into
several strands southwest of Becharof Lake\*\*\*\" The aeromagnetic data
do not require or clearly define that the fault system is present;
rather it is permissive data that if, on the basis of other data, the
fault system must occur southwest of Becharof Lake the aeromagnetic data
can be interpreted to explain where it does occur.

Iliamna subterrane

The Iliamna subterrane, named after exposures in the Iliamna
1:250,000-scale quadrangle located north of the map area, is composed of
moderately deformed marine sedimentary and volcanic rocks of early
Mesozoic age; schist, gneiss, and marble of Paleozoic(?) and Mesozoic
age; and plutonic rocks of the Alaska-Aleutian Range batholith. In the
map area, the batholith, which has been well described by Reed and
Lanphere (1969, 1972, 1973, 1974), intrudes rocks of the Kakhonak(?)
Complex, Cottonwood Bay Greenstone, Kamishak Formation, and the
partially coeval Talkeetna Formation (Detterman and Reed, 1980). Each of
these units is correlative with parts of the Chignik subterrane or in
the case of the Kamishak and Talkeetna Formations, is actually part of
both subterranes. The southernmost exposure of rocks of the Iliamna
subterrane occurs in the vicinity of Becharof Lake. Here, quartz diorite
of Jurassic age (Reed and Lanphere, 1972) is exposed on a small island
on the south side of the lake and in the hills north of the lake in the
Naknek 1:250,000-scale quadrangle.

The Kamishak Formation is one of the few units that occur in both the
Iliamna and Chignik subterranes; lithologically and faunally equivalent
units, the Nizina Limestone and the McCarthy Formation (Detterman and
Reed, 1980), occur in the McCarthy 1:250,000-scale quadrangle 700 km
northeast of the Alaska Peninsula in the Wrangellia terrane (Jones and
others, 1977) and the Port Graham sequence occurs in the Seldovia area
(Kelley, 1985). The Cottonwood Bay Greenstone has been correlated(?)
with the Nikolai Greenstone of the McCarthy 1:250,000-scale quadrangle
(Detterman and Reed, 1980). The Nikolai Greenstone and Nizina Limestone
are fundamental units of the Wrangellia terrane (Jones and others,
1977); originally the Cottonwood Bay Greenstone and Kamishak Formation
were not considered part of a Wrangellian sequence (Jones and others,
1977, p. 2572); however, the correlation of these units with typical
Wrangellian units on lithologic, faunal, and structural grounds suggests
otherwise. The Cottonwood Bay Greenstone of the Iliamna subterrane may
also be coeval with the Triassic volcanic rocks at Puale Bay. However,
the Cottonwood Bay Greenstone has been interpreted as of oceanic
affinity, possibly ocean island basalt (Riehle and others, 1993; J.R.
Riehle, oral commun., 1991) in contrast to the possible island arc
derivation of the rocks at Puale Bay (Moore and Connelly, 1977; Hill,
1979; Wang and others, 1988). No chemical data presently exists to
conclusively categorize the Cottonwood Bay Greenstone.

Southwest of Becharof Lake the extent of the Iliamna subterrane is
masked by younger rocks, although its presence may be inferred from
geophysical and drill data. Brockway and others (1975) showed in a cross
section that the batholith had been reached by drilling south of
Becharof Lake on the Alaska Peninsula; however, a sample of the drill
core (supplied by Randy Billingsley, AMOCO, 1990) is a sheared
greenstone more likely part of either the Kakhonak(?) Complex or the
Cottonwood Bay Greenstone (B.L. Reed, oral commun., 1991).
Interpretation of aeromagnetic data (Case and others, 1981) suggested
that the subterrane may continue at least as far south as Port Heiden.
Pratt and others (1972) suggested continuation of the batholith into
southern Bristol Bay based on interpretation of shipborne magnetic data.

Chignik subterrane

Characteristic of the Chignik subterrane are little deformed,
shallow-marine to continental clastic sedimentary rocks. However,
deep-marine, volcaniclastic, and calcareous rocks are present as
important components of the older rocks in the subterrane. The rocks
range in age from Permian to latest Cretaceous and are found south or
east of the Iliamna subterrane.

The rocks of the Chignik subterrane in the map area record Permian to
Holocene sedimentary and igneous activity along an episodically active
convergent plate margin. The oldest rocks known in the Chignik
subterrane are found at Puale Bay and are fossiliferous late mid-Permian
limestone (Pls) and Permian(?) volcanic agglomerates and flows (Pv) (see
inset A, pl. 1). These Permian and Permian(?) units are structurally
overlain by Late Triassic carbonate and volcanic rocks of the Kamishak
Formation (Trk). Triassic limestone deposition and volcanism, possibly
in an island arc setting (Moore and Connelly, 1977; Hill, 1979; Wang and
others, 1988), occurred at low latitude (Detterman, 1988).
Lithologically similar Triassic rocks are also known in the subsurface
in the Cathedral River area (McLean, 1977) at the southwest end of the
Alaska Peninsula. Moore and Connelly (1977) suggested that rocks of the
Shuyak Formation (Connelly, 1978) on western Kodiak Island are possible
correlatives of the Triassic volcanic and volcaniclastic rocks at Puale
Bay. Finally, the Cottonwood Bay Greenstone of the Iliamna subterrane
may be coeval with the Triassic (Norian?) volcanic rocks at Puale Bay,
although the age control that does exist suggests that the Cottonwood
Bay Greenstone may be older than Norian.

Jurassic strata record the development and subsequent erosion of an
island arc; this may be the same island arc as suggested by
interpretation of the character of the Late Triassic strata at Puale
Bay. Early Jurassic volcaniclastic sedimentary rocks (Talkeetna
Formation) are derived from the volcanic part of the arc. The Talkeetna
Formation or laterally equivalent rocks crop out discontinuously from
the Matanuska Valley and the Talkeetna Mountains to Puale Bay;
stratigraphically and lithologically equivalent rocks were reported from
a drillhole in the Cathedral River area (McLean, 1977) and from the
Seldovia area (Kelley, 1985). The Talkeetna, within the Chignik
subterrane, undergoes a gradual lithologic transition from northeast to
southwest as sedimentary rocks come to dominate the unit; this suggests
the rocks of the unit become more distal from the arc volcanism to the
southwest. The Talkeetna Formation is succeeded by Early and Middle
Jurassic sandstone and conglomerate (Kialagvik Formation) indicating a
gradually subsiding depositional basin. The Kialagvik Formation is
exposed only in a limited area between Wide and Puale Bays; however,
correlative rocks of the Tuxedni Group are exposed on the west side of
Cook Inlet. The Kialagvik records the continued erosion of the volcanic
part of the island arc; however, a few granitic rock cobbles in some
conglomerate beds suggest the initial unroofing of the Alaska-Aleutian
Range batholith, the plutonic root of the island arc (Reed and others,
1983). Yet, not until the deposition of the Late Jurassic Naknek
Formation do granitic lithic fragments become an important component of
the sedimentary rocks. Northeast of the map area, the lower part of the
Tuxedni Group (Detterman and Hartsock, 1966; Imlay, 1984) is a
lithologic and stratigraphic correlative of the Kialagvik Formation. The
Tuxedni Group is a much thicker (as much as 3,000 m) and more complete
sequence than the Kialagvik (Detterman and others, 1996) and is a thick
sequence of graywacke, sandstone, conglomerate, siltstone, and shale.
The conglomerate of the Tuxedni is mainly composed of volcanic rocks in
a graywacke matrix. The Tuxedni Group crops out entirely north of the
map area from Iniskin Bay to the Talkeetna Mountains (Imlay, 1984).
Stratigraphically and lithologically equivalent rocks to both the
Talkeetna and Kialagvik Formations were reported from a drillhole in the
Cathedral River area (McLean, 1977).

Callovian rocks (Shelikof Formation) on the Alaska Peninsula mainly
reflect the continued erosion of the volcanic part of the arc and the
eventual shallowing of the depositional basin. Initially deposited in
deep water, the sedimentary rocks of the Shelikof record a shallow-water
environment at the close of deposition. The lithologic- and
age-equivalent Chinitna Formation north of the map area is divided into
the Paveloff Siltstone and Tonnie Siltstone Members and is fossiliferous
dark-gray siltstone with abundant calcareous concretions (Detterman and
Hartsock, 1966). The Chinitna ranges in areal extent from the Talkeetna
Mountains to the Iniskin Peninsula. The Shelikof and Chinitna Formations
represent the same or similar depositional environments. The source area
supplied volcanic debris and lay to the present northwest of the Chignik
subterrane.

Late Jurassic deposition of arkosic sandstone, conglomerate, and
siltstone (Naknek Formation) occurred in a shallow-water shelf and
nonmarine environment. The Naknek Formation contains the first
significant influx of granitic debris in the stratigraphic column and
records the initial unroofing of the Jurassic part of the
Alaska-Aleutian Range batholith. Abrupt lateral facies changes are
present in the Naknek. The facies changes within the Naknek Formation
occur both along, and transverse to, the Alaska Peninsula and result
from rapid uplift and erosion of the Alaska-Aleutian Range batholith and
older sedimentary rocks that flank it. The eroded sediments were carried
to the east by short, high-energy streams that debauched onto the
continental shelf and deposited their loads partly in subaerial and
partly in marine environments. The facies changes in the Naknek
Formation, as well as those in some of the other late Mesozoic
stratigraphic units, can best be seen in a fence diagram of the late
Mesozoic stratigraphic units, on the Alaska Peninsula (Detterman and
Miller, 1987). Several small starved basins are preserved on the east
side of the peninsula\--for example, near Hallo Bay and Amber Bay\--in
which the entire Naknek sequence which normally is 1,700 to 2,000 m
thick is compressed into a section of siltstone less than 500 m thick
(Detterman and Miller, 1986). The Naknek Formation is exposed from Black
Hill in the southwestern part of the map area to the Talkeetna Mountains
(Csejtey and others, 1978), 500 km northwest of the map area. Vallier
and others (1980) described Late Jurassic sandstone and siltstone
dredged from near the Pribilof Islands in the southern Bering Sea. They
correlated these rocks with the Naknek Formation and suggested that
these rocks \"underlie a large part of the continental margin on the St.
George Basin region of the southern Bering Sea.\" However, the Naknek is
not known to be exposed west or north of rocks that form the Iliamna
subterrane. The Naknek has also been correlated with the Root Glacier
Formation of the southern Wrangell Mountains (E.M. MacKevett, Jr., oral
commun., 1977; cited in Csejtey and others, 1978).

Early Cretaceous deposition of fine-grained siltstone, shale, and
calc-arenaceous sandstone (Staniukovich and Herendeen Formations)
followed the deposition of the Naknek Formation. Deposition into Early
Cretaceous time was continuous as the Staniukovich Formation conformably
overlies the Naknek. The depositional environment remained shallow
marine and nonmarine; however, the generally finer grain size of the
Staniukovich suggests a lower energy environment. Detterman and others
(1996) suggested that the three sandstone units in the Staniukovich
Formation in the Mount Katmai area probably represent stacked offshore
or barrier bars. The Staniukovich, like the Naknek was derived from a
gradually eroding source terrane composed in part of the Alaska-Aleutian
Range batholith (Reed and Lanphere, 1973; Reed and others, 1983).
Apparently, relief relative to the source area was decreasing with time
as witnessed by the absence of coarse clastic rocks. Also, the more
altered character of mineral fragments in the Staniukovich and typically
better rounding compared to the Naknek (Detterman, 1990) suggests that
reworking of older sedimentary units may have occurred. The record of
the Late Triassic to Early Cretaceous sedimentary regime culminates with
deposition of a thin, well-sorted, calc-arenaceous sandstone unit
(Herendeen Formation) composed of up to 40 percent *Inoceramus* prisms
(Detterman, 1990) and angular to sub-rounded quartz and feldspar grains
(Burk, 1965, p. 45), suggesting the contribution of new lithic clastic
debris from the source terrane was virtually eliminated. North of the
Mount Katmai area, rocks of Early Cretaceous age are generally not known
in the Chignik subterrane (J. Bolm, USGS, personal commun., 1981),
although a thin (100 m thick) Early Cretaceous clastic unit which
includes, in part, the Nelchina Limestone has been mapped by Csejtey and
others (1978) and Grantz (1960a, b) in the Talkeetna Mountains
1:250,000-scale quadrangle. The Nelchina Limestone is a lithologic and
stratigraphic equivalent of the Herendeen Formation.

Mid-Cretaceous (Aptian to Santonian) rocks are missing over much of the
Alaska Peninsula, presumably eroded. The widespread absence of rocks of
this age suggests uplift and erosion of the entire terrane during a part
of Aptian to Santonian time. Uplift through mid-Cretaceous time resulted
in removal of much of the Herendeen, the Staniukovich, and parts of the
Naknek Formation. The uplift appears to have been most pronounced in the
Puale Bay to Lake Iliamna area. However, in the Mount Katmai area, small
fault-bounded patches of Albian rocks (Pedmar Formation) have been found
which indicate some deposition, possibly local, occurred during Aptian
to Santonian time.

The stratigraphically uppermost part of the Chignik subterrane consists
of the Chignik, Hoodoo, Kaguyak, and Matanuska Formations. A short-lived
but vigorous marine transgression in Late Cretaceous time resulted in
the deposition of fluvial to deep-marine clastic rocks (Chignik, Hoodoo,
and Kaguyak Formations) on the Alaska Peninsula and deep-sea fan and
abyssal-plain turbidites (Shumagin Formation) in the Outer Shumagin and
Sanak Islands and presumably the Semidi Islands vicinity. The Shumagin
Formation and the lithologic- and age-equivalent Kodiak Formation and
Valdez Group (Moore, 1974c; Plafker and others, 1977) underlie much of
the continental margin along the Alaska Peninsula and in southern Alaska
and record an extensive Late Cretaceous trench system. The Chignik and
Hoodoo Formations are Campanian and Maestrichtian in age and exposed
southwest of Puale Bay and Becharof Lake. They are apparently in large
part derived from reworking of earlier strata although a volcanic-rock
component may have been added. In the Matanuska Valley of south-central
Alaska, the stratigraphically highest part of the Chignik subterrane is
the Matanuska Formation. The Matanuska Formation, named by Martin (1926)
is composed predominantly of dark-gray marine siltstone, shale, and
minor conglomerate (Grantz and Jones, 1960; Grantz, 1964); its type area
is defined as the Matanuska Valley. It ranges in age from Albian to
Maestrichtian, although several faunal gaps and unconformities divide
the formation. The much longer depositional history of the predominantly
Late Cretaceous Matanuska Formation, compared to the other Late
Cretaceous formations in the subterrane, indicates a return to a marine
depositional basin sooner in the northeastern part of the subterrane
than in more southwestern parts. However, the faunal gaps and
unconformities within the Matanuska Formation indicate the basin was not
stable. The Chignik Formation shows extensive evidence of a nearshore to
nonmarine environment, whereas conglomerate and coal seams are known
only locally in the Matanuska Formation. The Aptian to Santonian
geologic history of the Alaska Peninsula and Cook Inlet region parts of
the subterrane is unclear due to the virtual absence of Aptian to
Santonian rocks. Although erosion was apparently deep, as witnessed by
removal in some areas of rocks as old as Late Jurassic, structural
deformation was minimal. The cyclical marine to nonmarine depositional
history documented by Fairchild (1977) and Detterman (1978) in the
Chignik Formation may indicate lesser stability in the Chignik region in
Campanian and Maestrichtian time.

STRUCTURE OF THE ALASKA PENINSULA

The two subterranes of the Alaska Peninsula terrane are characterized by
radically different structural and metamorphic styles. The rocks of the
Iliamna subterrane, which lie west of the Bruin Bay fault, are
characterized by metamorphism up to amphibolite-facies grade and intense
folding. The severity of these effects is variable and is most intense
nearest the Alaska-Aleutian Range batholith. Both of these effects are
attributed to forcible intrusion of the batholith (B.L. Reed, personal
commun., 1981).

In much of the central part of the map area, which is dominantly
composed of the Chignik subterrane, the structural style of the Alaska
Peninsula is dominated by en echelon anticlines, normal faulting, and
thrust and high-angle reverse faults that have minor displacement in a
northwest to southeast direction. Folds in the northern and southern
parts of the map area are dominantly related to faulting, in contrast to
the well-developed, northeastwardly aligned, large en echelon anticlines
and synclines of the central Alaska Peninsula. In the northeast part of
the map area, these faults are dominantly normal faults, whereas in the
southwest, low-angle thrust and high-angle reverse faults are more
common. Structures on the Alaska Peninsula are typically aligned
sub-parallel to the general northeast-to-southwest trend of the Alaska
Peninsula. In general, as shown below, compressional features become a
relatively more important component of the structural style of the
Alaska Peninsula as one travels southwest.

North of the Wide Bay (Detterman, Case, and others, 1987) and Bear Creek
anticlines, in the northern part of the Ugashik and western part of the
Karluk 1:250,000 quadrangles, large en echelon anticlines and minor
thrusts and reverse faults tend to disappear to be replaced by closely
spaced normal faults of small displacement aligned perpendicular to the
general trend of the Alaska Peninsula. The dip of bedding tends to be
low in this area and, in general, rocks of Tertiary age are not present.
Farther north and northeast in the Katmai area, the structures can be
characterized as \"jostled\" homoclinally dipping blocks with no clear
regional structures developed southeast of the Bruin Bay fault (J.R.
Riehle, oral commun., 1989). Tertiary and younger rocks again become
important in the Katmai area, where the Meshik arc volcanic rocks lie
northwest of the Bruin Bay fault and Paleocene(?), Eocene, and Oligocene
sedimentary and Miocene and younger igneous rocks of the Aleutian arc
lie along the coast, southeast of the Bruin Bay fault. Compressional
structures are particularly absent except for the Bruin Bay fault which
is a high-angle reverse fault.

In the Port Moller area at the southwestern end of the Alaska Peninsula,
the regional structures are dominated by a zone of
northeast-to-southwest-trending high-angle reverse faults (Wilson, Case,
and Detterman, 1985). This zone extends from the vicinity of Tolstoi
Peak in the southwest to Mitrofania Bay in the northeast. Northwest of
this zone, the major structures are a series of ramped,
northwestward-directed low-angle thrust faults that carry the Mesozoic
rocks of the Chignik subterrane of the Alaska Peninsula terrane and
early Cenozoic rocks over the middle and late Miocene rocks of the Bear
Lake Formation. Southeast of the zone of high-angle reverse faults, the
only exposed Mesozoic rocks (Shumagin Formation) lie in the Outer
Shumagin and Sanak Islands and are part of the Chugach terrane (Plafker
and others, 1977). Early Cenozoic rocks southeast of the fault zone are
the same lithologically as early Cenozoic rocks northwest of it;
however, the Bear Lake Formation is not known to occur southeast of the
fault zone. Wilson, Case, and Detterman (1985) suggested that this fault
zone may represent a projection of the buried boundary between the
Chugach and Alaska Peninsula terranes through the overlying younger
rocks.

In the Outer Shumagin and Sanak Islands, the Shumagin Formation is
characterized structurally by tight, generally northeast-trending folds.
Dips in these rocks tend to be steep, rarely less than 35°, and
overturned beds are locally common, for example, on the north end of
Nagai Island. This is in marked contrast to the Mesozoic rocks of the
mainland part of the map area. Moore (1974a, b) indicated that the rocks
of the Shumagin Formation are pervasively faulted; however, he was only
able to trace individual faults for limited distances in most cases.

**History and character of Alaska Peninsula\
terrane overlap sequences**

Division of southern Alaska into tectonostratigraphic terranes (see
Jones and Siberling, 1979) emphasizes Mesozoic stratigraphy in the
definition of terranes. Certainly, post-Mesozoic terranes have been
defined, for example the Prince William terrane, yet in general,
Cenozoic rocks in southern Alaska are terrane overlap sequences. This
discussion of Cenozoic terrane overlap sequences is limited to those on
the Alaska Peninsula; the reader is referred elsewhere for discussion of
Cenozoic rocks of the Matanuska Valley (see Winkler, 1990).

On the Alaska Peninsula, a brief regression in early Tertiary time and
granodiorite plutonism in the Outer Shumagin, Semidi, and Sanak Islands
was followed by deposition of fluvial and marine clastic strata on the
mainland (Tolstoi and Copper Lake Formations) and in the Inner Shumagin
Islands. This was followed by deposition of transgressive marine clastic
strata (Stepovak Formation) and an areally extensive outpouring of
andesitic, dacitic, and basaltic volcanic and volcaniclastic materials
(Meshik Volcanics) between late Eocene and earliest Miocene time on the
southwestern part of the Alaska Peninsula. These volcanic and
volcaniclastic rocks have been included in the Meshik arc as defined by
Wilson (1985). In the Mount Katmai region of the map area, the only
sedimentary rocks between late Eocene and earliest Miocene time are
fluvial clastic strata (Hemlock Conglomerate) that were deposited during
the later stages (late Oligocene) of the Meshik arc. The Hemlock
contains clasts of varying lithologies and probably was not derived from
direct erosion of the volcanic rocks of the Meshik arc.

A major gap in the Meshik arc occurs between the southern end of Ugashik
Lakes and the north side of Naknek Lake. Exposures of early Cenozoic
volcanic rocks in the Naknek 1:250,000-scale quadrangle, shown as the
Meshik Volcanics on this map, are on trend with exposures of the Meshik
arc southwest of Ugashik Lakes. However, both surface exposures (see
map) and drillhole data (Detterman, 1990) provide no evidence of the
magmatic arc in the gap between. Erosion is not a good explanation for
this gap, as no plutonic root or alteration of the presumably underlying
Mesozoic rocks is apparent. It appears that during late Eocene to
earliest Miocene time, magmatic activity and sedimentary deposition did
not occur across this gap. It is in this gap that the oldest rocks known
on the Alaska Peninsula, mid-Permian limestone, are exposed. Finally,
the last known exposure of the Bruin Bay fault system occurs in the
northern part of this gap. Could the southern limit of the known
Alaska-Aleutian Range batholith have served as a \"plug\" around which
magmatic activity was displaced or contained below? Does the batholith
and the \"plug-contained\" Cenozoic igneous rocks then provide the extra
buoyancy to uplift the Paleozoic and older part of the Mesozoic section?
Alternatively, does the marine record contain evidence that the
subducted sea floor had some irregularity that resulted in uplift of the
continental margin and elimination of subduction related magmatism?
Interestingly, the modern Aleutian volcanic arc is offset in a
left-lateral sense roughly 50 km at the southern end of this gap. Does
the postulated batholithic plug or sea floor irregularity still affect
the convergent margin at this location?

Late Miocene time was marked by another brief transgression and
northwest- to southeast-directed compression, followed by renewed
volcanism and plutonism which initiated the modern Aleutian magmatic
arc. Engelbretson and others (1985, p. 27) suggest that the extinct
Kula-Pacific ridge arrived at the Aleutian trench at roughly 10 Ma and
postulated that given its relatively young age it might have been
sufficiently buoyant to have produced compressive tectonics in the
Aleutian arc. Burk (1965) ascribed most of the tectonic features, that
is folding and faulting, seen on the Alaska Peninsula to a Pliocene
deformational event. Low-angle thrust faults, not recognized by Burk
(1965), in the Port Moller-Herendeen Bay area emplaced Mesozoic and
early Tertiary rocks over the deformed middle and late Miocene Bear Lake
Formation. In some areas, Quaternary volcanic centers are apparently
emplaced into the upper plate rocks, constraining thrusting to no older
than the late Miocene and no younger than the Quaternary. However, at
most exposures, the Bear Lake Formation is little deformed. The local
angular unconformities seen between the Bear Lake and Stepovak
Formations suggest that most of the deformation in Cenozoic time
occurred in the Miocene, prior to deposition of the Bear Lake; however,
as the Bear Lake is only known northwest of the Alaska Peninsula ridge
crest, distinction between post-middle Miocene and Pliocene or later
deformation is difficult. At the southwest end of the Alaska Peninsula
the late Miocene Tachilni Formation is mildly deformed, having dips
usually less than 20° (Marincovich, 1983). Subsequent rapid uplift and
erosion has exposed the roots of late Miocene and Pliocene and older
volcanic centers.

Outcrop patterns of the late Miocene to Holocene volcanic and intrusive
rocks indicate that the Aleutian magmatic arc has migrated in a
northwestward direction from the Pacific coast. This is shown by a
number of exposures of late Miocene or Pliocene plutons and coeval
volcanic rocks at the Pacific coast and by emplacement of progressively
younger hypabyssal or volcanic rocks inland toward the modern volcanic
front. Among the numerous examples of this pattern are the trends from
the Agripina Bay batholith and associated volcanic rocks to Chiginagak
and Kialagvik Volcanoes, southwest of Wide Bay; and from Weasel
Mountain, through the Bee Creek prospect (Wilson and Cox, 1983), to
Black Peak, northwest of Chignik Bay.

Finally the youngest overlap sequences include the many Holocene
volcanic edifices that occur in the map area. These include the
well-known volcanoes of Katmai National Park, Aniakchak Caldera, Mount
Veniaminof, and the volcanoes of the Pavlof group, which is the most
active volcanic center in the continental United States. Complex
interaction between glacial advances and retreats, volcanic activity,
and sea level variation has resulted in a rich Quaternary geologic
history (see Detterman and others, 1981b; Detterman, Wilson, and others,
1987; Riehle and Detterman, 1996).

**Terrane Boundaries**

The boundaries separating the Alaska Peninsula terrane from other
terranes are commonly indistinct or poorly defined. A few of these
boundaries have been defined at major faults, yet even in the best
cases, the extensions of these faults are speculative through some
areas. The most clearly defined boundary of the Alaska Peninsula terrane
is the Border Ranges fault (MacKevett and Plafker, 1974; BRFS herein),
which separates it from the Chugach terrane (Plafker and others, 1977).
However, implicit in our interpretation is that the BRFS is a fault
system and not a simple thrust as described by MacKevett and Plafker
(1974). The BRFS is considered the southern and southeastern boundary of
the Alaska Peninsula (Peninsular) terrane in the Matanuska Valley;
however, the rocks bounded on the north of the Chugach terrane within
the BRFS cannot be unequivocally related to the Alaska Peninsula terrane
(see Pavlis, 1986; Roeske and others, 1989). The BRFS extends from the
Matanuska Valley southwesterly, to a point southwest of Kodiak Island
near longitude 156° west (Fisher, 1981). In the Kodiak Islands and on
the Kenai Peninsula, rocks of the Shuyak Formation, the Raspberry Schist
of Roeske (1986), and the Seldovia Schist of Roeske (1986) (D.C.
Bradley, oral commun., 1991) are sufficiently different from the known
Alaska Peninsula terrane stratigraphy as to possibly constitute another,
probably exotic, block, here called the \"hidden block\" (fig. 2) and
probably equivalent to the Knik River terrane of Pavlis (1986). A major
fault is shown just offshore of the Alaska Peninsula in Shelikof Strait
by von Huene and others (1985); this may well constitute the true
terrane boundary of the Alaska Peninsula terrane in this area.

The BRFS has been projected southwest through the Shumagin Islands from
its last known position (Fisher, 1981), where it forms the southern
boundary of the Alaska Peninsula terrane out to its western edge. Recent
work on the mainland adjacent to Sand Point in the Shumagin Islands
(Wilson, Detterman, and Case, 1985) suggests that, if the BRFS exists at
all in the area, it lies inland of the Shumagin Islands on the Alaska
Peninsula mainland. Wilson, Detterman, and Case (1985) defined a zone of
structurally disrupted Oligocene to Miocene rocks that may overlie the
extension of the fault system.

The position of the BRFS in the vicinity of the Sanak Islands, 160 km to
the southwest of Sand Point cannot be determined by available data but
may lie as far north as Cold Bay as the last known exposure of Alaska
Peninsula terrane rocks occurs in the Black Hill area northeast of Cold
Bay. Although this boundary may be clearly defined in a structural
sense, the probable facies relations between the Chignik, Hoodoo, and
Shumagin Formations lead to the conclusion that the Alaska Peninsula and
Chugach terranes on the southwestern Alaska Peninsula may be part of a
larger, as yet undefined, Late Cretaceous terrane. The stratigraphic
relationship between the geologic units of the Alaska Peninsula and
Chugach terranes indicates that the hidden block or Knik River terrane
was probably in place before deposition of the Chugach terrane flysch.

The northwestern boundary of the Alaska Peninsula terrane is largely
speculative. Northwest of the terrane, a sequence of Late Jurassic and
Cretaceous flyschoid rocks overlie the Iliamna subterrane (for example,
in the Lake Clark quadrangle, Nelson and others, 1983). This flysch
sequence is not well mapped and often is not separated from
lithologically similar rocks of the Kuskokwim Group (Cady and others,
1955; Nelson and others, 1983). The Lake Clark-Castle Mountain fault
system, traversing this area, does not represent a terrane-bounding
fault because zenolithic blocks of rocks which are lithologically
similar to rocks of the Talkeetna and Kamishak Formations are present in
Cretaceous and Tertiary plutonic rocks of the Alaska-Aleutian Range
batholith north of the fault system (B.L. Reed, oral commun., 1991).
Likewise the apparent continuity of the informally named Tlikakila
complex of Wallace and others (1989) (here considered equivalent to the
Kakhonak Complex) on both sides of the fault system suggests that
displacement is small. The southern Kahiltna terrane as defined by
Wallace and others (1989) lies between the Late Jurassic and Cretaceous
flysch sequence and the Alaska Peninsula (Peninsular) terrane. The
lithologic similarity and general age equivalency of the Chilikadrotna
Greenstone of Wallace and others (1989) of the southern Kahiltna terrane
and the combined Cottonwood Bay Greenstone, Kamishak Formation, and
Talkeetna Formation, and the absence of any mapped terrane-bounding
fault indicates the southern Kahiltna terrane is actually part of the
Iliamna subterrane of the Alaska Peninsula terrane. Wallace and others
(1989) define the southern Kahiltna terrane boundary as the contact with
the Alaska-Aleutian Range batholith; however, the batholith and its
contact rocks are an integral part of the Iliamna subterrane. The
Koksetna River sequence of Wallace and others (1989, p. 1392) could, in
part, represent forearc sediments derived from the Jurassic magmatic
arc, as suggested by Wallace and others (1989, p. 1397-1398) and
therefore would not be part of a different terrane. Wallace and others
(1989, p. 1392) show a discontinuously exposed Chilchitna fault; this
may be the northwest boundary of the Alaska Peninsula terrane. However,
in light of the reinterpretation by Reed and others (1983) of the
polarity of the Jurassic magmatic arc and the mapping and interpretation
of the Late Jurassic and Cretaceous flyschoid rocks by John Decker (oral
commun., 1984); the Late Jurassic and Cretaceous flysch northwest of the
terrane may also represent, in part, forearc sediments derived from the
Jurassic magmatic arc and not a distinct terrane.

In the Talkeetna Mountains, the boundary of the Alaska Peninsula terrane
is a zone of intense shearing and possible thrusting (Csejtey and St.
Aubin, 1981; Csejtey and others, 1978; called the West Fork fault by
W.L. Nokleberg, written commun., 1991) where the Alaska Peninsula
terrane is juxtaposed against the Wrangellia terrane. Stratigraphic ties
dating from Middle Jurassic time between the Wrangellia and the Alaska
Peninsula (Peninsular) terranes lead to definition of the Talkeetna
superterrane by Csejtey and St. Aubin (1981). We have shown in this
report that those stratigraphic ties can be reasonably extended to
Triassic time and that the Alaska Peninsula and Wrangellia terranes may
be more closely related than previously thought (Jones and others, 1979;
Csejtey and St. Aubin, 1981). Csejtey and St. Aubin (1981) and Csejtey
and others (1978; 1982) briefly described the Talkeetna thrust as the
terrane boundary that separates the Talkeetna superterrane from the
flysch basins to the northwest. Southwest of the Talkeetna Mountains
1:250,000-scale quadrangle in the Talkeetna (Reed and Nelson, 1977),
Lime Hills (B.L. Reed, oral commun., 1991) and Lake Clark (Nelson and
others, 1983) quadrangles, the Talkeetna thrust is not mapped or is not
recognized; however, it may be represented by the Chilchitna fault.

On the Alaska Peninsula, the west side of the Alaska Peninsula terrane
is overlapped by Tertiary sedimentary and volcanic rocks and Quaternary
deposits of the Nushagak-Bristol Bay Lowland physiographic province of
Wahrhaftig (1965). Although it is not now possible to define the
northwest boundary of this part of the Alaska Peninsula terrane,
extrapolation of structural trends and limited aeromagnetic data (U.S.
Geological Survey, 1978) suggests that it is sub-parallel to the Alaska
Peninsula.

The Border Ranges ultramafic and mafic complex problem

The informally named Border Ranges ultramafic and mafic complex of Burns
(1985) (approx. age range 163-194 Ma) has, on the basis of chemistry and
proximity, been considered the root of the Talkeetna volcanic arc
(Burns, 1985), a role also ascribed to the apparently younger (approx.
age range 155-175 Ma) Alaska-Aleutian Range batholith of Reed and
Lanphere (1969, 1972, 1973, 1974; Reed and others, 1983). The Border
Ranges ultramafic and mafic complex, considered by some to be an
integral part of the Alaska Peninsula terrane (W.J. Nokleberg, oral
commun. 1991), is consistently found within or seaward of the BRFS. For
example, in the Seldovia region, the Border Ranges ultramafic and mafic
complex is found as klippen east of the Seldovia Schist, separated from
the Alaska Peninsula terrane rocks by the Port Graham and Seldovia
faults. Direct age control on rocks of the Border Ranges ultramafic and
mafic complex is suggestive of correlation with the rocks of the
Talkeetna Formation; however, given the expected great depth of
emplacement and slow cooling of the mafic rocks (Burns, 1985, p.
1034-1035), it is unlikely that the reported ages represent emplacement
ages. In addition the increasing spatial separation of the Border Ranges
ultramafic and mafic complex and the Talkeetna Formation southwest of
the Kenai Peninsula argues against a close genetic association. Finally,
inclusion of the Border Ranges ultramafic and mafic complex in the
Alaska Peninsula terrane requires that its rocks be preferentially
emplaced seaward of the main Alaska Peninsula terrane. This suggests
that faulting and uplift along the BRFS operated to dismember the
terrane rather than to override the Chugach terrane.

Rocks of the Talkeetna Formation have been found as outcrops on both
sides and as roof pendants of the Alaska-Aleutian Range batholith.
Potassium-argon age determinations on the batholith are generally
younger than fossil or apparent ages from the Talkeetna Formation, which
may be ascribed to cooling. The reasonably close age and close spatial
relationships between the batholith and the Talkeetna Formation provide
good evidence for a close genetic association. If so, the Border Ranges
ultramafic and mafic complex, Shuyak Formation, Raspberry Schist,
Seldovia Schist, and other rocks may constitute a newly recognized
\"hidden\" block or terrane, extending northeastward parallel to the
BRFS from Shelikof Strait to the Matanuska Valley. This hidden terrane
may also extend in the offshore southwestward of Shelikof Strait;
however, it is not known to and probably does not extend much farther
than the Sutwik Island area. Rocks of this hidden terrane appear to
consist of an early Mesozoic island arc and a juxtaposed, coeval
subduction complex. The Knik River terrane of Pavlis (1986) may well
represent this terrane in the Matanuska Valley; its spatial relationship
to the Chugach and Alaska Peninsula terranes and the character of the
rocks that constitute it are sufficiently similar to the hidden block to
suggest they may be the same terrane.

REFERENCES CITED

Adkison, W.L., 1975, West Foreland and Tyonek Formations at Capps
Glacier, *in* Yount, M.E., ed., United States Geological Survey Alaska
Program, 1975: U.S. Geological Survey Circular 722, p. 45.

Allaway, W.H., Jr., Detterman, R.L., Miller, J.W., and Magoon, L.B.,
1984, Stratigraphic clarification of the Shelikof Formation, Alaska
Peninsula, *in* Stratigraphic notes, 1983: U.S. Geological Survey
Bulletin 1537-A, p. A21-A29.

Allison, R.C., and Addicott, W.O., 1973, The *Mytilus* *middendorffi*
group (*Bivalva*) of the North American Miocene \[abs.\]: Geological
Society of America Abstracts with Programs, v. 5, no. 1, p. 2-3.

Atwood, W.W., 1911, Geology and mineral resources of parts of the Alaska
Peninsula: U.S. Geological Survey Bulletin 467, 137 p.

Berg, H.C., Jones, D.L., and Coney, P.J., 1978, Map showing pre-Cenozoic
tectonostratigraphic terranes of southeastern Alaska and adjacent areas:
U.S. Geological Survey Open-File Report 78-1085, 2 plates, scale
1:1,000,000.

Brockway, Ronald, Alexander, B., Day, P. Lyle, W.M., Hiles, R., Decker,
W., Polski, W., and Reed, B.L., 1975, Bristol Bay region, stratigraphic
correlation section, southwest Alaska: Anchorage, The Alaska Geological
Society.

Burk, C.A., 1965, Geology of the Alaska Peninsula \-- Island arc and
continental margin: Geological Society of America Memoir 99, 250 p.,
scales 1:250,000 and 1:500,000, 3 sheets.

Burns, L.E., 1985, The Border Ranges ultramafic and mafic complex,
south-central Alaska: cumulate fractionates of island-arc volcanics:
Canadian Journal of Earth Sciences, v. 22, p. 1020-1038.

Cady, W.M., Wallace, R.E., Hoare, J.H., and Webber, E.J., 1955, The
Central Kuskokwim Region, Alaska: U.S. Geological Survey Professional
Paper 268, 132 p.

Calderwood, K.W., and Fackler, W.C., 1972, Proposed stratigraphic
nomenclature of Kenai Group, Cook Inlet Basin, Alaska: American
Association of Petroleum Geologists Bulletin, v. 56, no. 4, p. 739-754.

Capps, S.R., 1923, Recent investigations of petroleum in Alaska; the
Cold Bay district: U.S. Geological Survey Bulletin 739-C, p. C77-C116.

Case, J.E., Cox, D.P., Detra, David, and Detterman, R.L., and Wilson,
F.H., 1981, Geologic interpretation of aeromagnetic map of the Chignik
and Sutwik Island quadrangles, Alaska: U.S. Geological Survey
Miscellaneous Field Studies Map MF-1053-J, scale 1:250,000.

Case, J.E., Detterman, R.L., Wilson, F.H., Chuchel, B.A., and Yount,
M.E., 1988, Maps showing aeromagnetic survey and geologic interpretation
of parts of the Ugashik and Karluk quadrangles, Alaska: U.S. Geological
Survey Miscellaneous Field Studies Map MF-1539-D, 12 p., 2 sheets, scale
1:250,000.

Connelly, William, 1978, Uyak Complex, Kodiak Islands, Alaska: A
Cretaceous subduction complex: Geological Society of America Bulletin,
v. 89, no. 5, p. 755-769.

Connelly, William, and Moore, J.C., 1978, Geologic map of the northwest
side of the Kodiak and adjacent islands, Alaska: U.S. Geological Survey
Miscellaneous Fields Studies Map MF-1057, scale 1:63,360.

Cowan, D.S., and Boss, R.F., 1978, Tectonic framework of the
southwestern Kenai Peninsula, Alaska: Geological Society of America
Bulletin, v. 89, p. 155-158.

Csejtey, Bela, Jr., Cox, D.P., Evarts, R.C., Stricker, G.D., and Foster,
H.L., 1982, The Cenozoic Denali fault system and the Cretaceous
accretionary development of southern Alaska: Journal of Geophysical
Research, v. 87, no. B5, p. 3741-3754.

Csejtey, Bela, Jr., Nelson, W.H., Jones, D.L., Silberling, N.J., Dean,
R.M., Morris, M.S., Lanphere, M.A., Smith, J.G., and Silberman, M.L.,
1978, Reconnaissance geologic map and geochronology, Talkeetna Mountains
quadrangle, northern part of Anchorage quadrangle, and southwest corner
of Healy quadrangle, Alaska: U.S. Geological Survey Open-File Report
78-558-A, 60 p., scale 1:250,000.

Csejtey, Bela, Jr., and St. Aubin, D.R., 1981, Evidence for
northwestward thrusting of the Talkeetna superterrane, and its regional
significance, *in* Albert, N. R. D., and Hudson, Travis, eds., 1981, The
United States Geological Survey in Alaska: Accomplishments during 1979:
U.S. Geological Survey Circular 823-B, p. B49-B51.

Dall, W.H., 1882, Notes on Alaska Tertiary deposits: American Journal of
Science, 3d. series, v. 24, p. 67-68.

\_\_\_\_\_1896, Report of coal and lignite of Alaska: U.S. Geological
Survey 17th Annual Report, Part 1, p. 771-914.

Detterman, R.L., 1978, Interpretation of depositional environments in
the Chignik Formation, Alaska Peninsula, *in* Johnson, K.M., ed., The
United States Geological Survey in Alaska: Accomplishments during 1977:
U.S. Geological Survey Circular 772-B, p. B62-B63.

\_\_\_\_\_1988, Mesozoic biogeography of southern Alaska with
implications for the paleogeography: U.S. Geological Survey Open-File
Report 88-662, 27 p.

\_\_\_\_\_1990, Stratigraphic correlation and interpretation of
exploratory wells, Alaska Peninsula: U.S. Geological Survey Open-File
Report 90-279, 51 p., 2 sheets.

Detterman, R.L., Case, J.E., Wilson, F.H., and Yount, M.E., 1987,
Geologic map of the Ugashik, Bristol Bay, and western part of Karluk
quadrangles, Alaska: U.S. Geological Survey Miscellaneous Investigations
Series Map I-1685, scale 1:250,000.

Detterman, R.L., Case, J.E., Wilson, F.H., Yount, M.E., and Allaway,
W.H., Jr., 1983, Generalized geologic map of the Ugashik, Bristol Bay,
and part of Karluk quadrangles, Alaska: U.S. Geological Survey
Miscellaneous Field Studies Map MF-1539-A, scale 1:250,000.

Detterman, R.L., and Hartsock, J.K., 1966, Geology of the
Iniskin-Tuxedni region, Alaska: U.S. Geological Survey Professional
Paper 512, 78 p.

Detterman, R.L., and Miller, J.W., 1985, Kaguyak Formation \-- an Upper
Cretaceous flysch deposit, *in* Bartsch-Winkler, Susan, and Reed, K.M.,
eds., The United States Geological Survey in Alaska: Accomplishments
during 1983: U.S. Geological Survey Circular 945, p. 49-51.

\_\_\_\_\_1986, A widespread catastrophic event in the Naknek Formation
on the Alaska Peninsula, *in* Bartsch-Winkler, Susan, and Reed, K.M.,
eds., The United States Geological Survey in Alaska: Accomplishments
during 1985: U.S. Geological Survey Circular 978, p. 27-29.

\_\_\_\_\_1987, Lithofacies fence diagrams of the upper Mesozoic, Alaska
Peninsula: U.S. Geological Survey Open-File Report 87-688, 3 sheets,
scale 1:250,000.

Detterman, R.L., Miller, J.W., and Case, J.E., 1985, Megafossil locality
map, checklists, and pre-Quaternary stratigraphic sections of Ugashik,
Bristol Bay, and part of Karluk quadrangles, Alaska: U.S. Geological
Survey Miscellaneous Field Studies Map MF-1539-B, 3 sheets, scale
1:250,000.

Detterman, R.L., Miller, J.W., Case, J.E., Wilson, F.H., and Yount,
M.E., 1996, Stratigraphic framework of the Alaska Peninsula: U.S.
Geological Survey Bulletin, 1969-A, 74 p.

Detterman, R.L., Miller, T.P., Yount, M.E., and Wilson, F.H., 1981a,
Geologic map of the Chignik and Sutwik Island quadrangles, Alaska: U.S.
Geological Survey Miscellaneous Investigations Series Map I-1229, scale
1:250,000.

Detterman, R.L., Miller, T.P., Yount, M.E., and Wilson, F.H., 1981b,
Quaternary geologic map of the Chignik and Sutwik Island quadrangles,
Alaska: U.S. Geological Survey Miscellaneous Investigations Series Map
I-1292, scale 1:250,000.

Detterman, R.L., and Reed, B.L., 1980, Stratigraphy, structure, and
economic geology of the Iliamna quadrangle, Alaska: U.S. Geological
Survey Bulletin 1368-B, 86 p., 1 map, scale 1:250,000.

Detterman, R.L., Wilson, F.H., Yount, M.E., and Miller, T.P., 1987,
Quaternary geologic map of the Ugashik, Bristol Bay, and western part of
Karluk quadrangles, Alaska: U.S. Geological Survey Miscellaneous
Investigations Series Map I-1801, scale 1:250,000.

Dover, J.H., 1990, Opinion: Problems of terrane terminology\--causes and
effects: Geology, v. 18, p. 487-488.

DuBois, G.D., Wilson, F.H., Detterman, R.L., and Hopkins, R.T., Jr.,
1989, Reconnaissance geology and exploration geochemistry of King Cove,
Alaska Peninsula: U.S. Geological Survey Open-File Report 89-350, 23 p.

DuBois, G.D., Wilson, F.H., and Shew, Nora, 1987, Potassium-argon age
determinations from the Port Moller and Stepovak Bay quadrangle, Alaska
Peninsula: U.S. Geological Survey Open-File Report 87-191, scale
1:250,000.

Eakins, G.R., 1970, A petrified forest on Unga Island, Alaska: Alaska
Division of Mines and Geology Special Report 3, 19 p.

Engelbretson, D.C, Cox, Allan, and Gordon, R.G., 1985, Relative motions
between oceanic and continental plates in the Pacific basin: Geological
Society of America Special Paper 206, 59 p.

Fairchild, D.T., 1977, Paleoenvironments of the Chignik Formation,
Alaska Peninsula: Fairbanks, Alaska, University of Alaska, M.S. thesis,
168 p.

Fisher, M.A., 1979, Structure and tectonic setting if continental shelf
southwest of Kodiak Island, Alaska: American Association of Petroleum
Geologists Bulletin, v. 63, no. 3, p. 301-310.

\_\_\_\_\_1981, Location of the Border Ranges fault southwest of Kodiak
Island, Alaska: Geological Society of America Bulletin, Part 1, v. 92,
no. 1, p. 19-30.

Galloway, W.E., 1974, Deposition and diagenetic alteration of sandstone
in northeast Pacific arc-related basins: Implications for graywacke
genesis: Geological Society of America Bulletin, v. 85, no. 3, p.
379-390.

Grantz, Arthur, 1960a, Geologic map of the Talkeetna Mountains A-2
quadrangle, Alaska and the contiguous area to the north and northwest:
U.S. Geological Survey Miscellaneous Geologic Investigations Map I-313,
scale 1:48,000.

\_\_\_\_\_1960b, Geologic map of the Talkeetna Mountains A-1 quadrangle
and the south third of the Talkeetna Mountains B-1 quadrangle, Alaska:
U.S. Geological Survey Miscellaneous Geologic Investigations Map I-314,
scale 1:48,000.

\_\_\_\_\_1964, Stratigraphic reconnaissance of the Matanuska Formation
in the Matanuska Valley, Alaska: U.S. Geological Survey Bulletin 1181-I,
p. I1-I33.

Grantz, Arthur, and Jones, D.L., 1960, Stratigraphy and age of the
Matanuska Formation, south-central Alaska: U.S. Geological Survey
Professional Paper 400-B, p. B347-B350.

Hanson, B.M., 1957, Middle Permian limestone on Pacific side of Alaska
Peninsula: American Association of Petroleum Geologists Bulletin, v. 41,
no. 10, p. 2376-2378.

Hildreth, Wes, 1983, The compositionally zoned eruption of 1912 in the
Valley of Ten Thousand Smokes, Katmai National Park, Alaska: Journal of
Volcanology and Geothermal Research, v. 18, p. 1-56.

\_\_\_\_\_1987, New perspectives on the eruption of 1912 in the Valley
of Ten Thousand Smokes, Katmai National Park, Alaska: Bulletin of
Volcanology, v. 49, p. 680-693.

Hill, M.D., 1979, Volcanic and plutonic rocks of the Kodiak-Shumagin
shelf, Alaska: Subduction deposits and near-trench magmatism: Santa
Cruz, California, California University, Ph.D. dissertation, 274 p.

Hillhouse, J.W., 1987, Accretion of southern Alaska: Tectonophysics, v.
139, p. 107-122.

Imlay, R.W., 1984, Early and middle Bajocian (Middle Jurassic) ammonites
from southern Alaska: U.S. Geological Survey Professional Paper 1322, 38
p.

Imlay, R.W., and Detterman, R.L., 1973, Jurassic paleobiography of
Alaska: U.S. Geological Survey Professional Paper 801, 34 p.

Jones, D.L., and Detterman, R.L., 1966, Cretaceous stratigraphy of the
Kamishak Hills, Alaska Peninsula, *in* Geological Survey Research 1966:
U.S. Geological Survey Professional Paper 550-D, p. D53-D58.

Jones, D.L., Howell, D.G., Coney, P.J., and Monger, J.W.H., 1983,
Recognition, character, and analysis of tectono-stratigraphic terranes
in western North America, *in* Hashimoto, M., and Uyeda, S. eds.,
Accretion tectonics in the Circum-Pacific region: Tokyo, Terra, p.
21-35.

Jones, D.L., and MacKevett, E.M., Jr., 1969, Summary of Cretaceous
stratigraphy in part of the McCarthy quadrangle, Alaska: U.S. Geological
Survey Bulletin 1274-K, 19 p.

Jones, D.L., and Silberling, N.J., 1979, Mesozoic stratigraphy\--the key
to tectonic analysis of southern and central Alaska: U.S. Geological
Survey Open-File Report 79-1200, 37 p.

Jones, D.L., Silberling, N.J., Berg, H.C., and Plafker, George, 1987,
Lithotectonic terrane map of Alaska (west of the 141st meridian): U.S.
Geological Survey Miscellaneous Field Studies Map MF-1874A, scale
1:2,500,000.

Jones, D.L., Silberling, N.J., and Hillhouse, John, 1977, Wrangellia\--A
displaced terrane in northwestern North America: Canadian Journal of
Earth Science, v. 14, p. 2565-2577.

Keller, A.S., and Reiser, H.N., 1959, Geology of the Mount Katmai area,
Alaska: U.S. Geological Survey Bulletin 1058-G, p. G261-G298, 1 map,
scale 1:250,000.

Kelley, J.S., 1985, Geology of the southwestern tip of the Kenai
Peninsula, *in* Sisson, Alexander, ed., Guide to the geology of the
Kenai Peninsula: Anchorage, Alaska, Alaska Geological Society Guidebook,
p. 50-68.

Kennedy, G.C., and Waldron, H.H., 1955, Geology of Pavlof Volcano and
vicinity, Alaska: U.S. Geological Survey Bulletin 1028-A, 19 p., scale
1:100,000.

Kellum, L.B., 1945, Jurassic stratigraphy of Alaska and petroleum
exploration in northwest America: New York Academy of Sciences
Transactions, Series 2, v. 7, no. 12, p. 201-209.

Kienle, Juergen, and Turner, D.L., 1976, The Shumagin-Kodiak batholith,
a Paleocene magmatic arc?: Alaska Division of Geological and Geophysical
Surveys Geologic Report 51, p. 9-11.

Kirschner, C.E., and Minard, D.L., 1949, Geology of the Iniskin
Peninsula, Alaska: U.S. Geological Survey Oil and Gas Investigations Map
OM-95, 1 sheet, scale approximately 1:80,000.

Knappen, R.S., 1929, Geology and mineral resources of the Aniakchak
district: U.S. Geological Survey Bulletin 797, p. 161-223.

Lewis, S.D., Ladd, J.W., and Bruns, T.R., 1988, Structural development
of an accretionary prism by thrust and strike-slip faulting: Shumagin
region, Aleutian Trench: Geological Society of America Bulletin, v. 100,
p. 767-782.

MacKevett, E.M., Jr., 1969, Three newly named Jurassic Formations in the
McCarthy C-5 quadrangle, Alaska, *in* Changes in stratigraphic
nomenclature by the U.S. Geological Survey 1967: U.S. Geological Survey
Bulletin 1274-A, p. A35-A49.

\_\_\_\_\_1978, Geologic map of the McCarthy quadrangle, Alaska: U.S.
Geological Survey Miscellaneous Investigations Series Map I-1032, scale
1:250,000.

MacKevett, E.M., Jr., and Plafker, George, 1974, The Border Ranges Fault
in southcentral Alaska: U.S. Geological Survey Journal of Research, v.
2, n. 3, p. 323-329.

Magoon, L.B., Adkison, W.L., Chmelik, F.B., Dolton, G.L., Fisher, M.A.,
Hampton, M.A., Sable, E.G., and Smith, R.A., 1976a, Hydrocarbon
potential, geologic hazards, and infrastructure for exploration and
development of the lower Cook Inlet, Alaska: U.S. Geological Survey
Open-File Report 76-449, 123 p.

Magoon, L.B., Adkison, W.L., and Egbert, R.M., 1976b, Map showing
geology, wildcat wells, Tertiary plant-fossil localities, K-Ar age
dates, and petroleum operations, Cook Inlet area, Alaska: U.S.
Geological Survey Miscellaneous Investigations Series Map I-1019, 3
sheets, scale 1:250,000.

Mancini, E.A., Deeter, T.M., and Wingate, F.H., 1978, Upper Cretaceous
arc-trench gap sedimentation on the Alaska Peninsula: Geology, v. 6, no.
7, p. 437-439.

Marincovich, Louie, Jr., 1983, Molluscan paleontology, paleoecology, and
North Pacific correlations of the Miocene Tachilni Formation, Alaska
Peninsula, Alaska: Bulletin of American Paleontology, v. 84, no. 317,
155 p.

Martin, G.C., 1926, The Mesozoic stratigraphy of Alaska: U.S. Geological
Survey Bulletin 776, 493 p.

Martin, G.C., and Katz, F.J., 1912, A geologic reconnaissance of the
Iliamna region, Alaska: U.S. Geological Survey Bulletin 485, 138 p.

McLean, Hugh, 1977, Organic geochemistry, lithology, paleontology of
Tertiary and Mesozoic from wells on the Alaska Peninsula: U.S.
Geological Survey Open-File Report 77-813, 63 p.

\_\_\_\_\_1979a, Tertiary stratigraphy and petroleum potential of the
Cold Bay-False Pass area, Alaska Peninsula: American Association of
Petroleum Geologists Bulletin, v. 63, no. 9, p. 1522-1526.

\_\_\_\_\_1979b, Observations on the geology and petroleum potential of
the Cold Bay - False Pass area, Alaska Peninsula: U.S. Geological Survey
Open-File Report 79-1605, 34 p.

McLean, Hugh, Englehardt, C.L., and Howell, D.G., 1978, Reconnaissance
geologic map of the Cold Bay and False Pass quadrangles, Alaska: U.S.
Geological Survey Open-File Report 78-323, scale 1:250,000.

Miller, T.P., and Smith, R.L., 1977, Spectacular mobility of ash flows
around Aniakchak and Fisher calderas, Alaska: Geology, v. 5, p. 173-176.

\_\_\_\_\_1987, Late Quaternary caldera-forming eruptions in the eastern
Aleutian arc, Alaska: Geology, v. 15, p. 434-438.

Molenaar, C.M., 1980, Cretaceous stratigraphy, Chignik area, Alaska
Peninsula, Alaska \[abs.\]: American Association of Petroleum Geologists
Bulletin, v. 64, no. 5, p. 752.

Moore, G.W., 1969, New formations on Kodiak and adjacent islands,
Alaska, *in* Changes in stratigraphic nomenclature by the U.S.
Geological Survey 1967: U.S. Geological Survey Bulletin 1274-A, p.
A27-A35.

Moore, J.C., 1974a, Geologic and structural map of part of the Outer
Shumagin Islands, southwestern Alaska: U.S. Geological Survey
Miscellaneous Investigations Series Map I-815, scale 1:63,360.

\_\_\_\_\_1974b, Geologic and structural map of the Sanak Islands,
southwestern Alaska: U.S. Geological Survey Miscellaneous Investigations
Series Map I-817, scale 1:63,360.

\_\_\_\_\_1974c, The Ancient continental margin of Alaska, in Burk,
C.A., and Drake, C.L., eds., The Geology of Continental Margins:
Springer-Verlag, New York, p. 811-816.

Moore, J.C., and Connelly, William, 1977, Tectonic history of the
continental margin of southwestern Alaska: Late Triassic to earliest
Tertiary, *in* Sisson, Alexander, ed., The relationship of plate
tectonics to Alaskan geology and resources: Alaska Geological Society
Symposium, 6th, Anchorage, Alaska, 1977, Proceedings, p. H1-H29.

Muller, E.H., 1952, The glacial geology of the Naknek district, the
Bristol Bay region, Alaska: Urbana, Illinois, Illinois University, Ph.D.
dissertation, 98 p.

Nelson, W.H., Carlson, Christine, and Case, J.E., 1983, Geologic map of
the Lake Clark quadrangle, Alaska: U.S. Geological Survey Miscellaneous
Field Studies Map MF-1114A, scale 1:250,000.

Nilsen, T.H., 1984, Miocene back-arc tidal deposits of the Bear Lake
Formation, Alaska Peninsula, *in* Reed, K.M., and Bartsch-Winkler,
Susan, eds., The United States Geological Survey in Alaska:
Accomplishments during 1982: U.S. Geological Survey Circular 939, p.
85-88.

Paige, Sidney, and Knopf, Adolph, 1907, Stratigraphic succession in the
region northeast of Cook Inlet, Alaska: Geological Society of America
Bulletin, v. 18, p. 327-328.

Palache, Charles, 1904, Geology about Chichagof Cove, Stepovak Bay; with
notes on Popof and Unga Islands (Alaska): Harriman Alaska Expedition, v.
4, p. 69-88.

Pavlis, T.L., 1983, Pre-Cretaceous crystalline rocks of the western
Chugach Mountains, Alaska: Nature of the basement of the Jurassic
Peninsula terrane: Geological Society of America Bulletin, v. 94, p.
1329-1344.

\_\_\_\_\_1986, Geologic map of the Anchorage C-5 quadrangle, Alaska:
Alaska Division of Geological and Geophysical Surveys Public-data File
86-7, 44 p., 1 sheet, scale 1:63,360.

Plafker, George, Jones, D.L., and Pessagno, E.A., Jr., 1977, A
Cretaceous accretionary flysch and melange terrane along the Gulf of
Alaska margin, *in* Blean, K.M., ed., The United States Geological
Survey in Alaska: Accomplishments during 1976: U.S. Geological Survey
Circular 751-B, p. B41-B43.

Plafker, George, Nokleberg, W.J., and Lull, J.S., 1989, Bedrock geology
and tectonic evolution of the Wrangellia, Peninsular, and Chugach
terranes along the Trans-Alaska crustal transect in the Chugach
mountains and southern Copper River basin, Alaska: Journal of
Geophysical Research, v. 94, no. B4, p. 4255-4295.

Pratt, R.M., Rutstein, M.S., Walton, F.W., and Buschur, J.A., 1972,
Extension of Alaskan structural trends beneath Bristol Bay, Bering
Shelf, Alaska: Journal Geophysical Research, v. 77, p. 4994-4999.

Reed, B.L., and Lanphere, M.A., 1969, Age and chemistry of Mesozoic and
Tertiary plutonic rocks in south-central Alaska: Geological Society of
America Bulletin, v. 80, p. 23-44.

\_\_\_\_\_1972, Generalized geologic map of the Alaska-Aleutian Range
batholith showing potassium-argon age of the plutonic rocks: U.S.
Geological Survey Miscellaneous Field Studies Map MF-372, 2 sheets,
scale 1:1,000,000.

\_\_\_\_\_1973, Alaska-Aleutian Range batholith: Geochronology,
chemistry, and relations to circum-Pacific plutonism: Geological Society
of America Bulletin, v. 84, no. 8, p. 2583-2610.

\_\_\_\_\_1974, Chemical variations across the Alaska-Aleutian Range
batholith: U.S. Geological Survey Journal of Research, v. 2, p. 343-352.

Reed, B.L., Miesch, A.T., and Lanphere, M.A., 1983, Plutonic rocks of
Jurassic age in the Alaska-Aleutian Range batholith: Chemical variations
and polarity: Geological Society of America Bulletin, v. 94, no. 10, p.
1232-1240.

Reed, B.L., and Nelson, S.W., 1977, Geologic map of the Talkeetna
quadrangle, Alaska: U.S. Geological Survey Miscellaneous Field Studies
Map MF-870A, scale 1:250,000.

Roeske, S.M., 1986, Field relations and metamorphism of the Raspberry
Schist, Kodiak Island, Alaska, *in* Evans, B.W., and Brown, E.H., eds.,
Blueschist and ecologites: Geological Society of America Memoir 164, p.
169-184.

Roeske, S.M., Mattinson, J.M., and Armstrong, R.L., 1989, Isotopic ages
of glaucophane schists on the Kodiak Islands, southern Alaska, and their
implications for Mesozoic tectonic history of the Border Ranges Fault
system: Geological Society of America Bulletin, v. 101, p. 1021-1037.

Riehle, J.R., and Detterman, R.L., 1993, Quaternary geologic map of the
Mount Katmai quadrangle and adjacent parts of the Naknek and Afognak
quadrangles, Alaska: U.S. Geological Survey Miscellaneous Investigations
Series Map I-2032, scale 1:250,000.

Riehle, J.R., Detterman, R.L., Miller, J.W., and Yount, M.E., 1993,
Geologic map of the Mt. Katmai quadrangle and portions of the Afognak
and Naknek quadrangles, Alaska: U.S. Geological Survey Miscellaneous
Investigations Series Map I-2204, scale 1:250,000.

Riehle, J.R., Detterman, R.L., Yount, M.E., and Miller, J.W., 1987,
Preliminary geologic map of the Mt. Katmai quadrangle and portions of
the Afognak and Naknek quadrangles, Alaska: U.S. Geological Survey
Open-File Report 87-593, 1 sheet, scale 1:250,000.

Spurr, J.E., 1900, A reconnaissance in southwestern Alaska in 1898: U.S.
Geological Survey 20th Annual Report, Part 7, p. 31-264.

Steiger, R.H., and Jager, E., 1977, Subcommission on Geochronology:
Convention on the use of decay constants in geo- and cosmochronology:
Earth and Planetary Science Letters, v. 36, p. 359-362.

Stone, D.B., and Packer, D.R., 1977, Tectonic implications of Alaska
Peninsula paleomagnetic data: Tectonophysics, v. 37, p. 183-201.

U.S. Geological Survey, 1978, Aeromagnetic map of Chignik and Sutwik
Island quadrangles, Alaska: U.S. Geological Survey Open-File Report
78-262, scale 1:250,000.

Vallier, T.L., Underwood, M.B., Jones, D.L., and Gardner, J.V., 1980,
Petrography and geologic significance of Upper Jurassic rocks dredged
near Pribilof Islands, southern Bering Sea continental shelf: American
Association of Petroleum Geologists Bulletin, v. 64, p. 945-950.

von Huene, Roland, Keller, G., Bruns, T.R., and McDougall, Kristin,
1985, Cenozoic migration of Alaskan terranes indicated by paleontologic
study, in Howell, D.G., ed., Tectonostratigraphic terranes of the
Circum-Pacific region: Houston, TX, Circum-Pacific Council for Energy
and Mineral Resource, p. 121-136.

Wahrhaftig, Clyde, 1965, Physiographic divisions of Alaska: U.S.
Geological Survey Professional Paper 482, 52 p.

Waldron, H.H., 1961, Geologic reconnaissance of Frosty Peak volcano and
vicinity, Alaska: U.S. Geological Survey Bulletin 1028-T, p. 677-708.

Wallace, W.K., Hanks, C.L., and Rogers, J.F., 1989, The southern
Kahiltna terrane: Implications for the tectonic evolution of
southwestern Alaska: Geological Society of America Bulletin, v. 101, p.
1389-1407.

Wang, Jason, Newton, C.R., and Dunne, Lorie, 1988, Late Triassic
transition from biogenic to arc sedimentation on the Peninsular terrane:
Puale Bay, Alaska Peninsula: Geological Society of America Bulletin, v.
100, p. 1466-1478.

Wilson, F.H., 1985, The Meshik arc \-- an Eocene to earliest Miocene
magmatic arc on the Alaska Peninsula: Alaska Division of Geological and
Geophysical Surveys Professional Report No. 88, 14 p.

\_\_\_\_\_1989, Geologic setting, petrology, and age of Pliocene to
Holocene volcanoes of the Stepovak Bay area, western Alaska Peninsula,
*in* Dover, J.H., and Galloway, J.P., eds., Geologic studies in Alaska
by the U.S. Geological Survey, 1988: U.S. Geological Survey Bulletin
1903, p. 96-103.

Wilson, F.H., Case, J.E., and Detterman, R.L., 1985, Preliminary
description of a Miocene zone of structural complexity in the Port
Moller and Stepovak Bay quadrangles, Alaska, *in* Bartsch-Winkler,
Susan, and Reed, K.M., eds., The United States Geological Survey in
Alaska: Accomplishments in 1983: U.S. Geological Survey Circular 945, p.
54-56.

Wilson, F.H., and Cox, D.P., 1983, Geochronology, geochemistry, and
tectonic environment of porphyry mineralization in the central Alaska
Peninsula: U.S. Geological Survey Open-File Report 83-783, 24 p.

Wilson, F.H., Detterman, R.L., and Case, J.E., 1985, The Alaska
Peninsula terrane: A definition: U.S. Geological Survey Open-File Report
85-450, 19 p.

Wilson, F.H., Detterman, R.L., Miller, J.W., and Case, J.E., 1995,
Geologic map of the Port Moller, Stepovak Bay, and Simeonof Island
quadrangles, Alaska: U.S. Geological Survey Miscellaneous Investigations
Series Map I-2272, scale 1:250,000.

Wilson, F.H., Gaum, W.C., and Herzon, P.L., 1981, Map and tables showing
geochronology and whole-rock geochemistry of the Chignik and Sutwik
Island quadrangles, Alaska: U.S. Geological Survey Miscellaneous Field
Studies Map MF-1053-M, scale 1:250,000, 3 sheets.

Wilson, F.H., and Shew, Nora, 1992, Map and tables showing geochronology
and whole-rock geochemistry of selected samples, Ugashik and part of
Karluk quadrangles, Alaska: U.S. Geological Survey Miscellaneous Field
Studies Map MF-1539-E, 12 p., 1 map, scale 1:250,000.

Wilson, F.H., and York, J.E., 1985, Processing of Landsat imagery to map
surface mineral alteration on the Alaska Peninsula, Alaska \[abs.\],
*in* Krafft, Kathleen, ed., U.S. Geological Survey research on Mineral
Resources - 1985, Program and Abstracts: U.S. Geological Survey Circular
949, p. 56-57.

Winkler, G.R., 1990, Preliminary geologic map, cross sections, and
summary geochronology of the Anchorage quadrangle, southern Alaska: U.S.
Geological Survey Open-File Report 90-83, 2 sheets, scale 1:250,000.

Wisehart, R.M., 1971, Paleoenvironmental analysis of the Bear Lake
Formation (upper and middle Miocene), Alaska Peninsula, Alaska: Los
Angeles, Calif., University of California, M.S. thesis, 112 p.

York, J.E., Wilson, F.H., and Gamble, B.M., 1984, Detection of surface
alteration on the Alaska Peninsula using Landsat multispectral scanner
data \[extended abs.\]: Remote Sensing for Exploration Geology
Symposium, Colorado Springs, Colo., April 16-19, 4 p.

Yount, M.E., Wilson, F.H., and Miller, J.W., 1985, Newly discovered
Holocene volcanic vents, Port Moller and Stepovak Bay quadrangles,
Alaska Peninsula, *in* Bartsch-Winkler, Susan, and Reed, K.M., eds., The
United States Geological Survey in Alaska, Accomplishments during 1983:
U.S. Geological Survey Circular 945, p. 60-62.

DESCRIPTION OF MAP UNITS

SURFICIAL DEPOSITS AND SEDIMENTARY ROCKS

**Surficial deposits**\--Typically unconsolidated, poorly sorted to well
sorted, poorly to moderately well-stratified sand, gravel, and silt.
Includes alluvial, colluvial, glacial, marine, lacustrine, and eolian
deposits, as well as locally reworked volcanic-ash and debris-flow
deposits. Consist of:

**Qa Alluvial deposits (Holocene and Pleistocene)**\--Ranges from
coarse, subangular rock fragments to fine sand and silt in grainsize;
locally includes considerable amounts of pumice near volcanic centers.
As mapped, unit locally incorporates deposits of fluvial, colluvial,
glacial outwash, lacustrine, estuarine, swamp, and eolian origin, in
addition to minor areas of other Quaternary surficial units. Large areas
are covered by organic-rich silt deposits. Unit includes much volcanic
ash and volcanic debris in vicinity of Katmai, Aniakchak, Kametolook,
and Joshua Green Rivers, Knife Creek, and unnamed river west of
Kametolook River, as well as in vicinity of most volcanoes, particularly
those of Pavlof group and on Unimak Island. Eolian deposits form dunes
10 to 20 m high that are composed largely of sand and pumice

**Qaf Alluvial fan and landslide deposits (Holocene and
Pleistocene)**\--Locally mapped; grainsize in this unit ranges from
coarse, subangular rock fragments to fine sand and silt. Alluvial fans
either typically have a well-developed cone shape, or multiple cones
coalescing into aprons. Landslide deposits have poorly developed to
well-developed lobate morphology

**Qb Marine beach and estuarine deposits (Holocene and
Pleistocene)**\--Moderately well stratified and sorted sand and gravel
on beaches; mud and silt in estuaries

**Qm Moraines and other glacial deposits (Holocene and
Pleistocene)**\--Poorly sorted, nonstratified, glacial drift that forms
end, lateral, and ground moraines. Locally includes moderately well
sorted and stratified ice-contact and outwash deposits. Morphologically
distinct drift units indicated by dash-dot internal contacts shown
within unit

**Qmt Marine terrace deposits (Holocene and Pleistocene)**\--Stratified
and moderately well sorted sand and gravel deposits that form nearly
level plains that end locally at prominent wave-cut scarps. Terraces
occur at 15 to 18 m elevation along Bristol Bay and between 40 and 45 m
elevation along Pacific Ocean

**Tmr Milky River Formation (Pliocene)**\--Named by Galloway (1974, p.
381, 384) and defined by Detterman and others (1981a), who designated
type locality as northeast spur of unnamed mountain 12 km east of Bear
Lake (see sec. 14, 15, and 22, T. 48 S., R. 69 W. of Chignik A-7
1:63,360-scale quadrangle). Formation is of variable thickness (as much
as 600 m) and consists of volcanogenic, nonmarine sedimentary rocks and
interlayered flows and sills; volcanic rocks are thicker and more
abundant stratigraphically upward in unit. Lower part of unit consists
nearly entirely of coarse, highly crossbedded and channeled, fluvial
volcaniclastic sandstone and cobble-boulder conglomerate. Rocks are
poorly indurated, are dark brown to gray, and have clasts composed
almost entirely of volcanic lithologies. Upper part of unit contains
numerous porphyritic andesite flows, lahar deposits, and tuff beds
interlayered with sedimentary rocks. Volcanic rock clasts in lahar
deposits are as large as 2 m. A flow unit near top of stratigraphic
section in type locality was dated at 3.530.27 Ma (Wilson and others,
1981). Another flow unit in lower part of the section exposed in the
Port Moller D-1 1:63,360-scale quadrangle has been dated at 3.870.06 Ma
(F.H. Wilson and Nora Shew, unpub. data, 1987). Unit unconformably
overlies Bear Lake Formation (Tbl) and conformably underlies Pliocene(?)
and Quaternary volcanic flows (QTv) and surficial deposits. Included in
unit in area northwest of Pavlof group of volcanoes are rocks mapped as
agglomerate of Cathedral Valley by Kennedy and Waldron (1955), described
as a thick sequence of agglomerate beds and subordinate tuff beds and
lava flows that are well exposed in Cathedral Valley. These volcanic
rocks crop out north of line of volcanoes from Mount Dutton to Pavlof
volcanoes. According to Kennedy and Waldron (1955), rocks are
predominantly basalt and basaltic andesite and dip north toward Bering
Sea. Kennedy and Waldron (1955) suggested that they are probably
comparable in age to Belkofski Tuff, later renamed the Belkofski
Formation by Burk (1965); we suggest here that a better lithologic and
stratigraphic correlation is with Milky River Formation and map rocks as
such

**Tbl Bear Lake Formation (late and middle Miocene)**\--Named by Burk
(1965, p. 89-92; see also, Allison and Addicott, 1973) for exposures on
ridges along and running eastward from Bear Lake. Type locality was
clarified by Detterman and others (1981a) to be located on southeast
slope of unnamed mountain 10 km east of Bear Lake (see secs. 27 and 28,
T. 48 S., R. 69 W. of Chignik A-7 and Port Moller D-1 1:63,360-scale
quadrangles). Unit locally reaches thickness of 300 to 500 m and
consists of inner-neritic marine and nonmarine (Wisehart, 1971; Nilsen,
1984) sandstone, conglomerate, siltstone, and shale. Rocks are dark
brown to pale yellowish brown. Bear Lake unit is distinguished from
other local Tertiary units by its greater abundance of nonvolcanic
debris and its better degree of sorting. Sandstone is moderately well
sorted and grains are moderately well rounded. Conglomerate beds are
made up of well-rounded clasts, 40 to 55 percent of which are quartz and
chert, 20 to 30 percent are volcanic fragments, 10 to 15 percent are
felsic plutonic clasts, and remainder are lithic sedimentary clasts.
Formation is abundantly fossiliferous; fossils are mainly pelecypods,
gastropods, and echinoids of late Miocene age (Louie Marincovich, Jr.,
and C.W. Allison, written commun., 1978, *in* Detterman and others,
1981a). Most of these fossils lived in shallow, nearshore environments
in water less than 100 m deep (Marincovich, 1983, and written commun.,
1985, *in* Detterman and others, 1996). Contact between Bear Lake
Formation and locally underlying Meshik Volcanics (Tm) and Stepovak and
Tolstoi Formations (units Ts and Tt, respectively) varies from
disconformity to angular unconformity. Contact between Bear Lake and
overlying Milky River Formation (Tmr) generally is disconformable;
however, contact locally is an angular unconformity. Marincovich (1983)
noted partial age correlation of Bear Lake with late Miocene Tachilni
Formation (Tta)

**Tta Tachilni Formation (late Miocene)**\--Named by Waldron (1961) for
marine sedimentary rocks roughly 100 meters thick at Morzhovoi Bay at
southwest end of Alaska Peninsula. Unit composed of gray to brown,
poorly consolidated, crossbedded subgraywacke sandstone commonly
interbedded with volcanic-pebble conglomerate and siltstone (Detterman
and others, 1996). Sandstone is composed of 30 to 35 percent angular
quartz, 30 percent volcanic rock fragments, 10 to 15 percent feldspar, 5
percent pyroxene and amphibole, and 20 percent clay matrix. Measured
reference section 130 m thick was designated by McLean (1979b) at South
Walrus Peak in False Pass 1:250,000-scale quadrangle; however, section
is incomplete, as basal part was not examined. McLean (1979b) reported
thickness of about 460 m for Tachilni; however, Detterman and others
(1996) suggested that structural complications and poor exposures
preclude measurement of complete section. Unit is richly fossiliferous,
containing 36 genera of bivalves and 11 genera of gastropods (Detterman
and others, 1996). Marincovich (1983) showed that these fossils have
close correlation with Wishkahan Stage of late Miocene, resulting in
late Miocene age assignment for unit, in contrast to late Miocene and
early Pliocene age reported by McLean (1979a, b). Tachilni is
unconformably overlain by late Tertiary and Quaternary volcanic and
volcaniclastic rocks that are presumably partially correlative with
Milky River Formation. Contact of Tachilni and Morzhovoi Volcanics
generally is structurally conformable although locally unconformable
where volcanic flows fill former stream valleys cut into Tachilni. No
lower contact of formation is known although it may overlie Belkofski
Formation

**Tu Unga Formation (middle Miocene to late Oligocene)**\--Originally
named Unga Conglomerate by Dall (1882, 1896) for exposures west of
Zachary Bay on Unga Island (see Port Moller B-2 and B-3 1:63,360-scale
quadrangles); later redefined by Burk (1965; see also, Allison and
Addicott, 1973) as lower member (Unga Conglomerate Member) of Bear Lake
Formation; and subsequently excluded from Bear Lake Formation and
returned to formational rank by Detterman and others (1996). Atwood
(1911, p. 66) designated type section of unit as sequence of rocks
located along west shore of Zachary Bay; Burk (1965) later included in
unit coal-bearing rocks exposed below Atwood\'s section on Unga Island.
Detterman and others (1996) stratigraphically and geographically
extended Burk\'s Unga to include rocks westward from Zachary Bay to Unga
Spit; extended section is 275 m thick, 25 percent or more of which is
volcanic rocks (lahar deposits, debris-flow deposits, and tuff).
Volcanic rocks are dominant in upper part of extended section, whereas
carbonaceous shale and coal are restricted to lower part; sandstone and
conglomerate throughout unit are composed of poorly sorted and typically
loosely consolidated volcanic debris. Unit is geographically restricted
to Unga Island, Deer Island, Pavlof Islands, and along Pacific coast of
Alaska Peninsula adjacent to Unga Island. Fossils are locally abundant
but are restricted to thin zones that contain numerous specimens of a
few genera. Petrified wood, including logs and stumps in growth
position, is common and is typically associated with debris-flow
deposits that engulfed then-existent forests of *Metasequoia* sp.
(Eakins, 1970). A well-known petrified forest is found within unit on
Unga Island (Eakins, 1970), and another is found on Wosnesenski Island.
Neither top nor base of formation is exposed, and, therefore, its
relation to other units is not well defined; however, because formation
has similar structural attitudes as underlying and overlying units, we
infer that it disconformably overlies Stepovak Formation (Ts) and is
disconformably overlain by late Miocene volcanic rocks (Tv)

**Tbe Belkofski Formation (middle Miocene? to late
Oligocene?)**\--Originally named Belkofski Tuff by Kennedy and Waldron
(1955); later renamed Belkofski Formation by Burk (1965; see also,
McLean, 1979a). McLean (1979a, b) gave generalized description of
section about 1,830 m thick along northwest shore of Belkofski Bay in
the Cold Bay 1:250,000-scale quadrangle; it consists of tuffaceous,
volcaniclastic sandstone, siltstone, and conglomerate and contains
interbeds of tuff and volcanic breccia. Along Belkofski Bay, unit is
mainly gray or greenish gray to gray brown; however, on Pavlof Islands,
rocks are dominantly red, pink, and purple and are very well indurated.
Stratigraphic relations of this unit with other units on Alaska
Peninsula are not known with certainty; however, most likely correlative
unit, on the basis of lithologic similarity and apparent stratigraphic
position, is Unga Formation (unit Tu; Detterman and others, 1996). Lower
contact of Belkofski Formation is nowhere exposed; upper part of unit is
overlain unconformably by volcanic flows of late Miocene age.
Potassium-argon age determination on clast from volcanic agglomerate,
tentatively mapped as Belkofski Formation on Dolgoi Island, is
11.790.41 Ma (Nora Shew, oral commun., 1988)

**Th** **Hemlock Conglomerate (late Oligocene)**\--Originally described
by Calderwood and Fackler (1972) and Adkison (1975) for exposures in
Cook Inlet area. Magoon and others (1976a, b) applied this name to rocks
in northern part of Mount Katmai 1:250,000-scale quadrangle and
correlated thicker section there with type Hemlock Conglomerate.
Detterman and others (1996) described reference section about 560 m
thick of Hemlock Conglomerate in Mount Katmai 1:250,000-scale quadrangle
that is composed primarily of fluvial sandstone and conglomerate that
contain interbeds of siltstone, shale, and coal. Chert, quartz, and
granitic and metasedimentary rocks form most of clasts in conglomerate;
volcanic rocks are small but significant proportion of clasts. Minor
tuff is present; however, erosion of underlying Mesozoic rocks is
probably main source for Hemlock Conglomerate. In most areas, unit
unconformably overlies Kaguyak Formation; in a few localities, it
unconformably overlies Naknek Formation. At Cape Douglas, it
disconformably overlies Copper Lake Formation. Megaflora fossils of
broadleaf deciduous plants and evergreen needles suggest late Oligocene
age (J.A. Wolfe, written commun., 1988, *in* Detterman and others, 1996)

**Ts Stepovak Formation (early Oligocene and late Eocene)**\--Originally
called Stepovak Series by Palache (1904) for rocks exposed on east side
of Chichagof Bay; later renamed as Stepovak Formation by Burk (1965; see
also, Detterman and others, 1996); and divided into two informal members
by Detterman and others (1996). Unit is age equivalent of volcanic rocks
mapped to north as Meshik Volcanics (Tm) and of rocks informally called
Popof volcanic rocks (Tp) on inner Shumagin Islands. Reference section
described by Detterman and others (1996) begins on ridge top in sec. 12,
T. 52 S., R. 71 W., continues southwesterly into sec. 13, and then
southerly into sec. 24 and on to Pacific coast (Port Moller C-1
1:63,360-scale quadrangle). Reference section has 2,030 m exposed,
approximately evenly divided into an (informal) lower siltstone member
and an (informal) upper sandstone member (Detterman and others, 1996).
This reference section is not complete section of formation, as neither
upper nor lower contacts are exposed. Lower member is deep-water
turbidite deposit that is composed of dark-brown laminated siltstone and
shale, as well as interbedded sandstone that commonly shows graded
bedding and rip-up clasts. Upper member, rich in unaltered volcanic
debris, was deposited in shallow-water shelf environment; megafauna
distributed throughout upper member are characteristic of water depths
no greater than 30 to 50 m (Louie Marincovich, Jr., written commun.,
1983 to 1986). Upper and lower contacts of formation are structurally
conformable with Unga and Tolstoi Formations (units Tu and Tt,
respectively) but are considered disconformities because considerable
time gaps exist between younger and older units

**Tt Tolstoi Formation (middle Eocene to late Paleocene)**\--Named and
defined by Burk (1965; see also, Detterman and others, 1981a), who
designated type locality along east shore of Pavlof Bay north of Tolstoi
Peak. Detterman and others (1996) have designated stratigraphic section
exposed at type locality between Tolstoi Peak and Cone Peak (Port Moller
B-5 and C-5 1:63,360-scale quadrangles) as type section. They have also
designated reference section that is more representative of unit\'s
lithology, located along east shore of Ivanof Bay that begins at
northeast corner of sec. 3 and continues south along east shore of
Ivanof Bay to southeast corner of sec. 10, T. 50 S., R. 66 W., in
Stepovak Bay D-5 1:63,360-scale quadrangle. In approximately 640-m-thick
type section, lithology is characteristic of shallow marine environment
that is succeeded northward (stratigraphically upward) by rocks
characteristic of nonmarine delta-plain and fluvial deposits, mainly of
braided-stream character. Much thicker (1,319 m) reference section has
lithology that is typical of nonmarine fluvial flood-plain, delta
sequence, which is common in main part of Tolstoi Formation. Marine
rocks are not known in Tolstoi north of Ivanof Bay (C.M. Molenaar,
written commun., 1991). In type section, sandstone is dominant lithology
of formation; sandstone intervals grade upward from light gray to olive
gray and tend to become more thin bedded. Siltstone intervals in section
are consistently thin bedded and are usually light olive gray. Plant
debris is present throughout type section, whereas megafauna is only
reported from lower 280 to 290 m. In reference section, proportion of
sandstone and siltstone thickness is sub-equal; additionally,
conglomerate and conglomeratic sandstone intervals make up roughly 15
percent of section. Sandstone intervals tend to be massive or thick
bedded, medium grained, and various shades of gray and brown. Siltstone
intervals in reference section are usually thin bedded and consistently
dark gray. Conglomerate and conglomeratic sandstone intervals are
typically pale yellowish brown and massive. Lithic clasts in
conglomerate and conglomeratic sandstone are dominantly granitic and
arkosic detritus but also include as much as 30 percent volcanic clasts.
Most volcanic clasts are altered or weathered, which is in sharp
contrast to most overlying units (Detterman and others, 1996);
additionally, presence of granitic and arkosic detritus suggests a
Mesozoic source rather than derivation from contemporaneous magmatic
activity. Plant debris, including well-preserved leaves, are common
throughout reference section; however, no megafauna has been reported.
In southern part of map area, Tolstoi Formation disconformably overlies
Late Cretaceous Hoodoo Formation (Kh); in central part of map area,
Tolstoi overlies Late Cretaceous Chignik Formation (Kc) with slight
unconformity. Tolstoi is disconformably overlain by both Stepovak and
Bear Lake Formations (units Ts and Tbl, respectively). In its
northeastern exposures Tolstoi overlies Hoodoo Formation with angular
unconformity and is overlain by Meshik Volcanics with angular
unconformity (C.M. Molenaar, written commun., 1991)

**Tc Copper Lake Formation (early Eocene and Paleocene?)**\--Named by
Detterman and Reed (1980) for exposures in Iliamna 1:250,000-scale
quadrangle directly north of map area, name \"Copper Lake Formation\"
also was applied to rocks in Mount Katmai area by Riehle and others
(1987; see also Riehle and others, 1993). Detterman and others (1996)
described 1,025-m-thick section in Mount Katmai area (see secs. 16 and
21, T. 14 S., R. 25 W., Afognak D-5 1:63,360-scale quadrangle);
formation there consists of thick pebble-cobble conglomerate in its
uppermost and lowermost parts and contains sandstone and siltstone in
its middle part. In measured section, conglomerate intervals are massive
and have clasts that are mixture of volcanic, granitic, and metamorphic
rocks in subequal proportions. 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. More fine grained clastic
parts of formation contain considerable carbonaceous debris and minor
coaly material. In map area, Copper Lake Formation, like Tolstoi
Formation, was derived from erosion of Mesozoic source area and is
nonmarine. In Mount Katmai and Iliamna areas (north of map area), this
source area was underlain by Alaska-Aleutian Range batholith (Reed and
Lanphere, 1973) and associated Mesozoic sedimentary and metamorphic
rocks. North of map area, Copper Lake undergoes transition from rocks of
Mesozoic provenance to fresh volcanic clasts of probably Tertiary age
(Detterman and Reed, 1980, p. B47). Age of Copper Lake is not well
constrained; sparse megaflora in type section in Iliamna area and
abundant megaflora in map area are restricted to sandstone and siltstone
intervals that are present in middle part of unit (Detterman and others,
1996). Detterman and others (1996) consider age of Copper Lake to be
Paleocene(?) and early Eocene. Upper and lower contacts of Copper Lake
Formation are disconformities with Hemlock Conglomerate and Kaguyak
Formation, respectively. Unit also includes, only for map display
purposes, rocks mapped by Riehle and others (1987) as their unit Ts,
Tertiary sedimentary rocks (Riehle and others, 1987, unit Tu, and Riehle
and others, 1993, unit Ts)

**Kh Hoodoo Formation (Late Cretaceous; Maestrichtian and
Campanian)**\--Named by Burk (1965, p. 59-63, 182; see also, Detterman
and others, 1981a) for exposures southeast of Hoodoo Mountain between
Herendeen and Pavlof Bays; type section of unit is located on southeast
side of Hoodoo Mountain and along west side of upper Beaver River valley
in Port Moller C-3 1:63,360-scale quadrangle. Reference section,
630-m-thick, was designated by Detterman and others (1996) in Chignik
A-3 1:63,360-scale quadrangle. Detterman and others (1996) indicate
Hoodoo may be considerably thicker than measured at reference section,
but complex folding and faulting, including well-developed olistotromes,
and absence of good marker beds make it difficult, if not impossible, to
determine true thickness of formation. Unit is typically dark-gray to
black, thin-layered and rhythmically bedded, splintery to
pencil-fracturing shale, siltstone, and fine sandstone, becoming more
sandy stratigraphically upwards. At Hoodoo Mountain, section contains
ammonite-bearing channel conglomerate composed of clasts of plutonic and
volcanic rocks, chert, and quartz. Sandstone beds are 0.3 to 1 m thick
and siltstone and shale beds are 1 to 2 m thick, although individual
layers are as thin as 1 cm (Detterman and others, 1981a). Depositional
environment for most of unit is characteristic of lower slope of a
submarine fan; structures imply submarine slumping and turbidity current
flow. Locally, thick sandstone and conglomerate in upper part of unit
imply upper fan-regime environment. Hoodoo can be easily mistaken for
similar-appearing dark-green or brown siltstone of overlying Tolstoi
Formation (Tt) or lower part of the Stepovak Formation (Ts), although
color, style of fracturing, and fossils can help to distinguish units.
Hoodoo is, in part, age equivalent to Chignik Formation (Kc), which can
be considered shallow-water-facies equivalent of Hoodoo (Mancini and
others, 1978; Molenaar, 1980; Detterman and others, 1996). However,
where two units are in contact, Hoodoo conformably overlies Chignik
Formation, which indicates a generally transgressive sequence. Hoodoo is
in turn unconformably overlain by Tolstoi Formation, although locally
the contact can be structurally conformable (Detterman and others,
1996). Sparse megafauna indicates an age of late Campanian and early
Maestrichtian for Hoodoo (J.W. Miller, written commun., 1983-85)

**Kc Chignik Formation (Late Cretaceous; Maestrichtian and
Campanian)**\--Named by Atwood (1911, p. 41-48) for exposures at Chignik
Lagoon and Chignik Bay; unit has type section located on Whalers Creek
(all locations in Chignik B-2 1:63,360-scale quadrangle). Burk (1965, p.
50) subdivided lower, nonmarine part of formation as his Coal Valley
Member; however, Detterman and others (1996) consider Coal Valley Member
to be useful mapping unit only between Port Moller and Herendeen Bay and
state that exposures along east shore of Herendeen Bay at Coal Bluff are
not Coal Valley Member but refer them, rather, to unnamed Tertiary
strata. Chignik is cyclic, nearshore-marine, tidal-flat, and nonmarine
flood-plain and fluvial deposit (Fairchild, 1977; Detterman, 1978); its
thickness is as much as 600 m in area between Port Moller and Chignik
Bay. Unit consists dominantly of light-olive-gray to olive-gray
sandstone containing interbedded olive-gray to olive-black siltstone and
conglomerate. Conglomerate intervals are composed of multicolored chert,
white quartz, felsic plutonic, and minor volcanic clasts. Nonmarine part
locally contains coal beds as much as 2-m-thick, that were mined at Mine
Harbor on Herendeen Bay around turn of twentieth century. As mapped,
includes rocks mapped as undivided Chignik and Hoodoo Formations in
Ugashik 1:250,000-scale quadrangle by Detterman, Case, and others
(1987). Marine fossils, mainly pelecypods, indicate late Campanian and
early Maestrichtian age (J.W. Miller, written commun., 1983-85) for
Chignik. Formation unconformably overlies Herendeen, Staniukovich, and
Naknek Formations (units Khe, Kst, and Jn, respectively) and conformably
interfingers and underlies Hoodoo Formation (Kh) at Chignik Bay; Hoodoo
Formation is both a deep-water-facies equivalent and an overlying unit
of Chignik (Mancini and others, 1978; Molenaar, 1980; Detterman and
others, 1996). Tolstoi Formation (Tt) unconformably overlies Chignik

**Kk Kaguyak Formation (Late Cretaceous; Maestrichtian and
Campanian)**\--Named by Keller and Reiser (1959) for rocks exposed near
abandoned village of Kaguyak in Afognak C-6 1:63,360-scale quadrangle.
Detterman and Miller (1985) remeasured and described type section that
begins at mouth of Swikshak River and continues westward in cliffs for
approximately 5 km. Measured thickness is more than 1,200 m of dark-gray
to pale-brown, typically thin bedded shale, siltstone, and fine grained
sandstone. Proportion of sandstone in unit increases up-section. Load
and flute casts are common; in upper part of unit, graywacke is graded
with numerous rip-up clasts. Overall depositional environment of
formation is 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,
in lower part of unit they are locally abundant. Ammonites are most
common and may range in size to as much as 1-m-across. Fossils allow age
assignment of latest Campanian and early Maestrichtian for Kaguyak.
Kaguyak unconformably overlies rocks as young as Early Cretaceous; upper
contact is unconformity with Copper Lake Formation

**Ks Shumagin Formation (Late Cretaceous; Maestrichtian)**\--Named and
defined by Burk (1965; see also, Moore, 1974a) for typical exposures at
Falmouth Harbor on Nagai Island, which was later designated as type area
of unit by Moore (1974a). Shumagin Formation consists of interbedded
sandstone, siltstone, mudstone, and shale at least 3,000-m-thick (Burk,
1965). Sandstone is dominantly very fine- to medium-grained,
medium-light-gray to medium-dark-gray, highly indurated, lithic
graywacke (Moore, 1974a). Sandstone beds in unit vary in thickness from
2 cm to 20 m, are graded, and contain abundant shale and siltstone chips
(Moore, 1974a; Detterman and others, 1996). Thin (less than 10 cm)
grayish-black mudstone layers are interbedded with sandstone; however,
in some areas, mudstone forms dominant lithology. Areas of outcrops
containing 80 percent or more mudstone were mapped separately by Moore
(1974a); that distinction is also made on this map (unit Ksm).
Thin-bedded siltstone, mudstone, and sandstone sequences are
rhythmically bedded and have sharp upper and lower contacts that are
indicative of turbidity-current deposition in deep-sea-fan and
abyssal-plain environments. Only observed contact of Shumagin with a
younger unit is where early Tertiary Sanak Island and Shumagin Islands
batholiths intrude (Kienle and Turner, 1976; Detterman and others,
1996); at these contacts, Shumagin has been contact-metamorphosed. In
Sanak Islands, Shumagin depositionally overlies structurally conformable
sequence of chert and volcanic rock (unit KJcv). Fossils are uncommon in
Shumagin, but existing collections indicate early Maestrichtian age for
formation (J.W. Miller, written commun., 1983-88; oral commun., 1991),
which is equivalent to upper parts of Hoodoo, Kaguyak, and Chignik
Formations (units Kh, Kk, and Kc, respectively). Shumagin is trench- and
abyssal-plain-facies equivalent of Hoodoo Formation (Mancini and others,
1978); Plafker and others (1977) also correlated Shumagin Formation with
Kodiak Formation of Kodiak Islands and Valdez Group of south-central
Alaska. Thin-section analysis of unit samples by John Decker (oral
commun., 1985) also suggests close lithologic relation with Kodiak
Formation on Kodiak Island. According to Decker, protolith for Shumagin
was derived from north and transported along paleo-Aleutian trench,
although a protolithic component of apparently local derivation has same
provenance as Hoodoo Formation. Locally, divided into:

**Ksm Mudstone\--**Thin-bedded (less than 10 cm) grayish-black mudstone
layers; mapped in areas where outcrops contain 80 percent or more
mudstone (Moore, 1974a)

**Kp Pedmar Formation (Early Cretaceous; Albian)**\--Named by Detterman
and others (1996) for thin (82 m) sequence of thick-bedded, fine- to
medium-grained, gray to olive-gray sandstone exposed along coast of
Katmai Bay near Mount Pedmar in Mount Katmai A-3 1:63,360-scale
quadrangle. Carbonaceous debris is present throughout, and pebbles as
large as 4 cm in diameter are found in tabular crossbedded sandstone in
middle part of formation. Clasts are typically quartz; volcanic clasts
are conspicuously absent. Two poorly exposed siltstone and shale
intervals are present in upper part of formation; siltstone is much more
common in a measured section east of Ikagluik Creek, 42 km north of type
section in Mount Katmai B-3 1:63,360-scale quadrangle. Abundant ammonite
and *Aucellina* sp. fossils help to establish well-controlled Albian age
for formation. Upper contact of formation is disconformity with
overlying by Kaguyak Formation. In type section, lower contact is fault
that juxtaposes Pedmar and Naknek Formations. At Ikagluik Creek section,
formation disconformably overlies Herendeen Formation

**KJcv Chert and volcanic sequence (Early Cretaceous or
Jurassic)**\--Unit was originally described by Moore (1974b) for rocks
found on northeast coast of Long and Clifford Islands in the Sanak
Islands (False Pass B-3 1:63,360-scale quadrangle). Moore\'s (1974b)
description of these rocks is as follows: \"Reddish-brown, light-gray,
and grayish-green bedded chert; dark-greenish-gray pillow lava and
tuffs, thin (5-10 cm) medium-gray, medium- to fine-grained sandstone
interbedded with dark-gray mudstone.\" Maximum exposed thickness
reported by Moore (1974b) is 250 m; however, as no base of unit is
exposed, total thickness of unit is unknown. Clasts of chert, presumably
derived from this unit, are found in depositionally overlying massive
sandstone of Shumagin Formation (Ks); hence, a Jurassic or Early
Cretaceous age is assigned to these rocks (Moore, 1974b), because no
fossil or radiometric-age control exists. No other locality on Alaska
Peninsula or adjacent islands has similar lithologies, except for parts
of Early Cretaceous Uyak Formation (Moore, 1969; Fisher, 1979) on Kodiak
Island, with which Moore (1974b) made only lithologic correlation. Since
publication of Moore (1974b), Uyak has been revised in age from Triassic
to Early Cretaceous (Fisher, 1979), which further supports correlation
of this chert and volcanic sequence with Uyak Formation (J.C. Moore,
written commun., 1991). However, recent chemical analysis (F.H. Wilson,
unpub. data, 1991) suggests that these volcanic rocks are significantly
more siliceous than greenstone of the Uyak Formation (Hill, 1979)

**Khe Herendeen Formation (Early Cretaceous; Barremian and
Hauterivian)**\--Originally named Herendeen Limestone by Atwood (1911,
p. 39) for exposures along east shore of Herendeen Bay, north of Mine
Harbor; renamed Herendeen Formation by Detterman and others (1996), who
designate a 270-m-thick reference section in hills southwest of Hot
Spring on Port Moller (sec. 14, T. 50 S., R. 73 W., Port Moller D-2
1:63,360-scale quadrangle). Originally described as limestone, rocks of
formation are actually an unusually uniform calc-arenaceous sandstone.
Rocks are thin bedded, medium grained, and dusky yellow to pale
yellowish brown on freshly broken surfaces and weather to conspicuous
light gray. They have 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 in Mount Katmai
area allow an age assignment of Hauterivian and Barremian for formation
(J.W. Miller, written commun., 1983-85; Detterman and others, 1996).
Herendeen conformably overlies Staniukovich Formation (Kst) and is
unconformably overlain by Chignik (Kc) and Pedmar (Kp) Formations

**Kst Staniukovich Formation (Early Cretaceous; Valanginian and
Berriasian)**\--Originally named Staniukovich Shale by Atwood (1911, p.
25, 38) for exposures on Staniukovich Mountain. Burk (1965; see also,
Imlay and Detterman, 1973, and Detterman and others, 1981a) changed name
to Staniukovich Formation and included within unit a variety of rocks of
latest Jurassic and Early Cretaceous age; however, Detterman and others
(1996) stratigraphically restrict formation to original usage of Atwood
(1911), and, as so restricted, its age is Early Cretaceous. Type section
(Atwood, 1911), in sec. 30, T. 50 S., R. 73 W., Port Moller D-2
1:63,360-scale quadrangle, is composed of 246 m of light-olive-gray
siltstone containing two light-olive-brown sandstone intervals, overlain
by shaly olive-gray siltstone containing numerous calcareous nodules and
concretions (Detterman and others, 1996). Upper part of formation erodes
readily and, therefore, is typically not well exposed; additionally, it
contains few age-diagnostic fossils, whereas lower part has abundant
megafauna fossils, particularly pelecypod *Buchia*, which indicates
Berriasian and Valanginian age (J.W. Miller, written commun., 1982-88;
Detterman and others, 1996). Upper and lower contacts of Staniukovich
are conformable with Herendeen (Khe) and Naknek (Jn) Formations,
respectively

**Jn Naknek Formation (Late Jurassic; Tithonian to
Oxfordian)**\--Originally named Naknek Series by Spurr (1900, p.
169-171, 179, 181) for exposures at Naknek Lake. Detterman and others
(1996; see also, Detterman and Hartsock, 1966; Martin and Katz, 1912)
have subdivided unit into five members on Alaska Peninsula. Megafossils,
particularly pelecypod *Buchia* (Detterman and Reed, 1980, p. B38; J.W.
Miller, written communs., 1982-88), are common, and fauna, which also
includes ammonites, indicate age range of Oxfordian to late Tithonian
(Late Jurassic). Naknek is conformable with overlying Staniukovich
Formation (Kst) and unconformably overlies Middle Jurassic Shelikof
Formation (Js). Alaska-Aleutian Range batholith was main source of
sedimentary debris for Naknek Formation, which on faunal evidence ranges
in age from about 138 to 155 Ma; hence, uplift and erosion of batholith
occurred shortly after emplacement. Locally, divided into:

**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); and 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
(see secs. 35 and 36, T. 35 S., R. 49 W., Seward Meridian, Ugashik A-4
1:63,360-scale quadrangle). Consists of medium-gray, fine- to
medium-grained arkosic sandstone and siltstone. It is thin bedded to
massive; where bedded, it is locally crossbedded. Fresh biotite and
hornblende are minor, but important, components of sandstone, as they
are interpreted to indicate first-cycle erosion from Alaska-Aleutian
Range batholith. Indecision Creek Sandstone Member is abundantly
fossiliferous; however, fossils are restricted almost exclusively to
pelecypods of genus *Buchia*. Depositional environment is shallow-water
shelf to inner neritic (Detterman and others, 1996). Its lower contact
is conformable and slightly gradational with Snug Harbor Siltstone
Member. Upper contact is unconformity with overlying Late 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 and occurs throughout Alaska
Peninsula

**Jnk Katolinat Conglomerate Member (Tithonian)**\--Type section
designated by Detterman and others (1996) is on unnamed mountain on
northeast shore of Grosvenor Lake (secs. 33 and 34, T. 17 S., R. 35 W.,
Mount Katmai C-4 1:63,360-scale quadrangle). Thick (about 450 m)
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 Early Cretaceous Herendeen Formation

**Jns Snug Harbor Siltstone Member (Kimmeridgian and
Oxfordian)**\--Originally described by Kirschner and Minard (1949) and
named by Detterman and Hartsock (1966) for exposures in Iniskin
Peninsula area of Cook Inlet. Detterman and others (1996) established
reference section on Northeast Creek in sec. 27, T. 37 S., R. 51 W., in
Sutwik Island D-5 1:63,360-scale quadrangle. Reference section consists
of more than 638 m of dark-yellowish-brown and dark-gray, thin-bedded
siltstone and minor amounts of thin- to medium-bedded olive-gray
sandstone. Limestone nodules are locally abundant, and limestone beds
are present in some siltstone intervals. It is lowest abundantly
fossiliferous member of Naknek; main fossils present are of genus
*Buchia*. Depositional environment was interpreted by Detterman and
others (1996) to have been moderately deep water, well below wave base
and above carbonate compensation depth, in a basin that had restricted
circulation. Both upper and lower contacts of unit are gradational with
Indecision Creek Sandstone and Northeast Creek Sandstone Members,
respectively

**Jnn Northeast Creek Sandstone Member (Oxfordian)**\--Named and
described by Detterman and others (1996) for exposures along Northeast
Creek in Sutwik Island D-5 1:63,360-scale quadrangle (sec. 34, T. 37 S.,
R. 51 W., Seward Meridian). Type section consists of 624 m of fine- to
coarse-grained, light-brownish-gray arkosic sandstone and minor amounts
of olive-gray to dark-gray, thin-bedded siltstone in lower part of
section. Sandstone is typically thick bedded and crossbedded and
contains magnetite laminae and thin beds of conglomerate. Fossils are
uncommon in unit although carbonaceous debris is common locally.
Depositional environment is mainly nonmarine. Some sand beds are
channeled with lag gravel at bases of channels. Crossbedding is mostly
high-angle and variable directional eolian type, some is small-scale,
tabular crossbedding with clay drapes characteristic of point bar
deposits (Detterman and others, 1996). Lower contact is conformable on
underlying Chisik Conglomerate Member and is placed where thick
sandstone replaces conglomerate in section. Upper contact is sharp and
conformable with overlying Snug Harbor Siltstone Member. At this
contact, depositional environment shifts from mainly nonmarine to
marine; position of contact varies temporally depending on local
conditions

**Jnc Chisik Conglomerate Member (Oxfordian)**\--Limited to northern
half of map area, reference section is located 6.5 km south of Becharof
Lake (secs. 26 and 27, T. 30 S., R. 43 W., Ugashik C-1 1:63,360-scale
quadrangle) is composed of 614 m of massive to thick-bedded conglomerate
and interbedded, crossbedded, clean quartzose sandstone. Clasts range in
size from maximum of 120 cm at bottom to 15 cm at top. Clast composition
is 30 percent granitic rocks, 30 percent quartzite, 20 percent
metavolcanic rocks, 10 percent schist, and 10 percent chert and quartz.
Clasts are well-rounded and commonly decrease in size stratigraphically
upward within each lithic interval of member. Lower contact is
unconformity with Shelikof Formation; upper contact is conformable and
gradational with Northeast Creek Sandstone Member

**Js Shelikof Formation (Middle Jurassic; Callovian)**\--Named by Capps
(1923) for exposures at Cold (now named Puale) Bay along the northwest
shore of Shelikof Strait. Type section designated by Allaway and others
(1984) is 1,402 m of volcaniclastic sedimentary rocks along northeast
shore of Puale Bay (see sec. 19 , T. 28 S., R. 37 W. to sec. 9, T. 28
S., R. 38 W., Karluk C-4, C-5, and D-5 1:63,360-scale quadrangles).
Allaway and others (1984) also designated 1,524-m-thick principal
reference section at Wide Bay along ridge southwest of Big Creek (see
sec. 5, T. 31 S., R. 43 W., Ugashik C-1 to sec. 3, T. 32 S., R. 43 W.,
Ugashik B-1 1:63,360-scale quadrangles). Another reference section 823 m
thick was designated by Allaway and others (1984) at Wide Bay along
ridge southwest of Alai Creek, between sec. 13, T. 33 S., R. 46 W. and
sec. 19, T. 33 S., R. 45 W., Ugashik B-2 1:63,360-scale quadrangle.
Lower part of type section is mainly thick-bedded to massive,
dusky-yellowish-green graywacke and conglomerate, and minor amounts of
siltstone, whereas upper part is mainly volcanic sandstone interbedded
with massive and laminated brownish-gray siltstone containing calcareous
sandstone clasts. Many lithic intervals in upper part of section have a
fining-upward sequence from conglomerate to sandstone or sandstone to
siltstone (Allaway and others, 1984). Owing to rapid lateral facies
changes, informal subdivisions of Capps (1923) have been found
impractical for mapping and have been abandoned. Shelikof is interpreted
to have been deposited in a deep- to shallow-water environment
(Detterman and others, 1996). Megafauna are locally abundant in
Shelikof, although, in general, formation is unfossiliferous. Vast
majority of fossils collected from formation are ammonites of Callovian
age; however, a few specimens suggest a Bathonian(?) age for some rocks
(Allaway and others, 1984, p. A23; Detterman and others, 1985, and
1996). Contact with underlying Kialagvik Formation is considered
conformable (see Detterman and others, 1996). Upper contact of Shelikof
with Naknek Formation is an unconformity. Shelikof is lithologically and
faunally correlative with Paveloff Siltstone Member of Chinitna
Formation (Detterman and Hartsock, 1966) of Cook Inlet region (Detterman
and others, 1996)

**Jk Kialagvik Formation (Middle and Early Jurassic; Callovian to late
Toarcian)**\--Originally defined by Capps (1923) for sandstone, shale,
and conglomerate exposed on northwest shore of Kialagvik (now named
Wide) Bay, between Pass Creek and southwest end of bay in Ugashik
1:250,000-scale quadrangle. Capps did not designate type section;
however, Detterman and others (1996) designated principal reference
section more than 620 m thick on south side of Short Creek (see secs. 34
to 32 to 28, T. 32 S., R. 44 W., Ugashik B-2 1:63,360-scale quadrangle),
and a second reference section 395 m thick at Puale Bay (secs. 29 and
19, T. 28 S., R. 37 W., Karluk C-4 and C-5 1:63,360-scale quadrangles).
Only well-known subaerial exposures of Kialagvik are found in vicinity
of these two localities; however, it has been encountered in subsurface
in a number of drillholes along Alaska Peninsula. Additionally, because
of redefinition of Shelikof Formation on lithologic grounds, C.M.
Molenaar (written commun., 1991) suggested that it may also crop out on
crest of Bear Creek anticline east of Becharof Lake. Unit consists of
brown, fossiliferous sandstone, siltstone, and minor conglomerate,
grading upward into fossiliferous siltstone, mudstone, and rare
limestone at Wide Bay. Lower part of unit was deposited in near-shore,
shallow-water environment; sandstone is crossbedded and contains lenses
of conglomerate and fossils indicative of high-energy environment. Upper
part of unit has characteristics of deeper water deposition including
thin, rhythmically bedded siltstone and sandstone packages and limestone
nodules and lenses. At Puale Bay, Kialagvik consists dominantly of gray,
thin-bedded siltstone but contains two thin conglomerate beds, thicker
of which is submarine channel deposit. Nearshore beds of Wide Bay
section do not crop out at Puale Bay, either because reference sections
are not complete or because of facies change. Shallow-water deposits of
Kialagvik Formation are abundantly fossiliferous; most megafauna
indicate age of early and middle Bajocian, although overall age range of
fossils collected from unit is late Toarcian to Callovian (Imlay, 1984;
see also, Detterman and others, 1996). Contact of Kialagvik with
overlying Shelikof Formation is conformable at Puale Bay, where
Kialagvik is as young as Callovian (Middle Jurassic), but unconformable
at Wide Bay where Kialagvik is entirely Bajocian (Middle Jurassic);
contact is placed at base of thick volcanic sandstone and conglomerate
sequence. Contact of Kialagvik with underlying Talkeetna Formation at
Puale Bay is disconformity

**Jt Talkeetna Formation (Early Jurassic)**\--Name introduced by Martin
(1926) for series of greenstone and tuff deposits first described in
Talkeetna Mountains of south-central Alaska by Paige and Knopf (1907).
Detterman and Hartsock (1966) geographically extended unit into Cook
Inlet area. Detterman and others (1983) further geographically extended
unit to include 405 m of clastic sedimentary rocks and tuff exposed on
northeast shore of Puale Bay. Stratigraphically and lithologically
equivalent rocks have been encountered in drillholes as far southwest as
vicinity of Cathedral River (AMOCO Cathedral River #1 drillhole) in
southwest part of map area. In measured section at Puale Bay, described
by Detterman and others (1996), Talkeetna Formation is composed of
primarily of gray-green, coarse-grained tuffaceous sandstone, lesser
amounts of green to red, massive coarse-grained tuff, and minor amounts
of brownish-gray siltstone and gray to gray-brown limestone. At Puale
Bay, formation records inner-neritic to sublittorial environment.
Northeast of map area, formation has much higher proportion of volcanic
rocks (Detterman and Hartsock, 1966). Age of unit, Early Jurassic
(Hettangian and early Sinemurian), is based on abundant megafauna;
however, this megafauna is present in great abundance in only a few
horizons and may represent mass kills as a result of volcanic eruptions
(Detterman and others, 1996). Contact of Talkeetna with underlying
Kamishak Formation is conformable and gradational; it is arbitrarily
placed where clastic sedimentary rocks replace limestone as major
constituents of rock sequence. Contact of Talkeetna with overlying
Kialagvik Formation is structurally conformable; however, it is
considered a disconformity, as rocks of late Sinemurian, Pliensbachian,
and most of Toarcian Stages are missing

**Trk Kamishak Formation (Late Triassic; Norian)**\--Originally named
Kamishak Chert by Martin and Katz (1912) for rocks in vicinity of Bruin
Bay, in Cook Inlet area. Kellum (1945; see also, Kirschner and Minard,
1949) changed name to Kamishak Formation for similar rocks at Puale Bay.
Detterman and Reed (1980) divided unit into three members, in descending
order, Ursus Member, middle member, and Bruin Limestone Member. In map
area (see inset A, pl. 1), Triassic limestone strata was assigned to
Bruin(?) Limestone Member of Kamishak Formation by Detterman and others
(1996). Reference section of Kamishak was measured near Puale Bay by
R.M. Egbert (U.S. Geological Survey, 1979, cited in Detterman and
others, 1996, section in Karluk C-4 and C-5 1:63,360-scale quadrangles).
Section consists of approximately 800 m of light- to brownish-gray,
thin- to medium-bedded limestone, minor amounts of brownish-gray,
calcareous siltstone and mudstone, and limestone conglomerate. An
interval of brecciated and calcite-recemented basalt occurs near top of
section, as does a volcanic breccia interval in lower part of the
section. A 46-m-thick columnar-jointed basalt flow or sill from upper
part of section has yielded a potassium-argon age of 19712 Ma (Wilson
and Shew, 1992); however, it remains to be determined whether basalt is
sill from overlying Early Jurassic Talkeetna Formation or,
alternatively, a flow from Kamishak that yields an analytically
\"minimum\" age. Depositional environment of unit was shallow water and
high energy; intervals of unit include both reefs and biohermal
buildups. Fossils that have been found yield Norian age (Detterman and
Reed, 1980; C.D. Blome, U.S. Geological Survey, oral commun., 1981). In
map area, no lower contact is exposed. Upper contact is conformable and
gradational with overlying Talkeetna Formation; Detterman and others
(1996) arbitrarily placed contact at point where clastic sedimentary
rocks replace limestone as major constituent of rock sequence. As Bruin
Limestone Member is lowest member of Kamishak Formation, this contact
indicates Bruin Limestone Member is time-transgressive unit

**Pls Limestone (Permian; early Guadalupian)**\--Only known exposure is
40 m of thin- to thick-bedded, medium-grained, crystalline tan to gray
limestone containing thin interbeds of chert, located on a small islet
(100 by 200 m) at entrance to Puale Bay (see inset A, pl. 1). Hanson
(1957) reported age of late mid-Permian (early Guadalupian) for these
rocks on the basis of poorly preserved and silicified coral, brachiopod,
and foraminifer fossils. No contacts are exposed, although highly
contorted beds dip about 40° northwest, which places them structurally
beneath Triassic rocks that are located on other islands about 1 km away

VOLCANIC ROCKS

**Qv Volcanic rocks (Holocene and Pleistocene)**\--Andesite, dacite, and
leucobasalt lava flows, volcanic breccia, lahar deposits, and
debris-flow deposits. Lava flows and clasts in other volcanic deposits
of unit are porphyritic, typically glassy, gray to black, and commonly
vesicular. Andesite is overwhelmingly dominant composition and probably
constitutes 60 percent or more of rocks. Includes basalt of Black Point,
Arch Point Basalt, and Dushkin Basalt (Kennedy and Waldron, 1955), and
Frosty Peak Volcanics (Waldron, 1961). Also includes lava flows,
breccias, and older pyroclastic deposits of Yantarni Volcano (Riehle and
others, 1987). Unit typically forms volcanic edifices; it also forms
isolated outcrops that cap ridges, providing a good example of
topography reversal caused by erosion. Individual flows are locally as
thick as 30 m and are laterally continuous over large areas.
Potassium-argon ages are as old as 1.780.034 Ma (Wilson and Shew,
1992). Morphologically distinct flows separated by internal long and
short dash contacts. Unit also includes basaltic, basaltic andesite, and
dacite parasitic cinder and spatter cones. Cones are commonly 30 to 300
m high, are steep sided, and have small crater at top. Rocks at Mount
Veniaminof are mainly basaltic andesite, whereas those at Aniakchak
Crater are mainly dacite (T.P. Miller, oral commun., 1990). Basaltic
scoria cones occur at three separate locations in the Mount Katmai
1:250,000-scale quadrangle (Riehle and others, 1993). Rocks are highly
scoriaceous to vitrophyric, ranging in size from cinder-size fragments
to 1-m-long bombs (Detterman and others, 1981b; T.P. Miller, oral
commun., 1991). Primarily located in vicinity of Mount Veniaminof,
Aniakchak Crater, in Mount Katmai area, and on Unimak Island

**Qpd Pyroclastic and debris-flow deposits (Holocene and late
Pleistocene?)**\--Dacite and rhyolite ash-flow tuff and debris-flow,
block-and-ash-flow, explosion debris, and air-fall deposits. Mapped near
Pavlof Volcano, Mount Dana, Kupreanof Volcano and other nearby unnamed
volcanoes (Wilson, 1989), Mount Veniaminof (Detterman and others,
1981b), Aniakchak Crater (Miller and Smith, 1977), Ugashik Caldera
(Detterman, Wilson, and others, 1987), and Valley of Ten Thousand Smokes
(Riehle and others, 1987 and 1993). Pyroclastic deposits \"typically are
composed of pumice and scoria bombs \*\*\* and subordinate lithic
fragments in a matrix of fine to coarse ash, pumice, and lithic
material\" (Miller and Smith, 1977, p. 174). Miller and Smith (1977)
reported that composition ranges from basaltic andesite to rhyolite,
although most are dacite. Valley of Thousand Smokes is well known for
its compositionally zoned rhyolite to andesite ash flows erupted in 1912
(see, for example, Hildreth, 1983). Hildreth (1987) estimated
composition of lavas from 1912 Katmai eruption to have been roughly 54
to 59 percent rhyolite, 35 to 43 percent dacite, and 3 to 5 percent
andesite. Map unit is interbedded with, and overlies, Quaternary glacial
debris (Qm). Unit records multiple, late Pleistocene and (or) Holocene
eruptions at each of following volcanic centers: Roundtop, Pavlof and
Pavlof Sister volcanoes, Mount Dana, Mount Veniaminof, and Aniakchak
Crater, as well as possibly other volcanic centers. Radiocarbon ages
(Wilson and others, 1995; Miller and Smith, 1987) have been determined
on peat, charcoal, and other organic material at numerous localities
beneath air-fall and block-and-ash-flow deposits. Youngest deposits are
clearly historic (Valley of Ten Thousand Smokes) or postglacial; older
deposits may be coeval with latest glaciation and show reworking into
shoreline deposits at low elevations, particularly west and southwest of
Mount Dana and in vicinity of Aniakchak River. Where known, individual
deposits are shown by long and short dash internal contacts

**QTv Volcanic rocks (Quaternary and Pliocene?)**\--Andesite and basalt
lava flows, sills, and plugs. These primarily extrusive rocks typically
cap ridges and include massive lava flows, agglomerate, and lahar
deposits. Includes basalt flows of Mount Simeon of Waldron (1961).
Primarily located on southwest part of Alaska Peninsula and on Unimak
Island

**QTp Pyroclastic deposits (Pleistocene? and Pliocene)**\--Welded
ash-flow tuff, block-and-ash-flow deposits, and air-fall deposits of
dacitic(?) composition. Largest exposure is found southeast of Left Head
in Port Moller D-1 1:63,360-scale quadrangle. Similar rocks, which are
composed of variably indurated deposits of ash, vitrophyre blocks, and
(or) pumiceous lapilli, are found in Mount Katmai 1:250,000-scale
quadrangle near Devils Desk, Mount Griggs, and east of Blue (Whale)
Mountain (see Riehle and others, 1993, unit QTap). In Port Moller area,
pyroclastic rocks are pale green, propylitically altered, and form base
of southernmost of unnamed Quaternary volcanic centers (Yount and
others, 1985; Wilson, 1989) northwest of Clark Bay. Unit has very
limited distribution but reaches thickness of at least 500 m (1,600 ft)
in Port Moller area and 200 m (650 ft) in Mount Katmai area. Age of
these deposits is poorly controlled; however, because in Port Moller
area deposits are capped by Pliocene(?) and Quaternary volcanic rocks
(QTv, Qv), and on the basis of outcrop patterns we think they overlie
Tolstoi Formation (Tt), we infer partial age equivalence of these rocks
with Milky River Formation (Tmr)

**QTm Morzhovoi Volcanics (early Quaternary?, Pliocene, and late
Miocene?)**\--Named by Waldron (1961) for sequence of lava flows,
interbedded pyroclastic rocks, and minor amounts of volcanic sedimentary
rocks that overlie Tachilni and Belkofski Formations. Unit comprises
\"the eroded remnants of an ancient large composite cone \*\*\* called
Morzhovoi volcano, in the area south of Frosty Peak\" (Waldron, 1961, p.
688). Waldron (1961, p. 688) designated type locality along Pacific
coast south and east of Reynolds Head (see sec. 5, T. 59 S., R. 90 W.,
Cold Bay A-3 1:63,360-scale quadrangle). At type locality, unit consists
of poorly consolidated, dark sandy shale, sandstone, and fine- to
coarse-grained conglomerate composed of subrounded to rounded fragments
of volcanic debris, which are conformably overlain by interbedded lava
flows, coarse agglomerates, and volcanic breccias of light- to
pinkish-gray porphyritic basalt. Olivine is present in small amounts in
nearly all rocks of Morzhovoi Volcanics that were examined (Waldron,
1961); however, olivine is not common in Alaska Peninsula volcanic rocks
in general. Age of Morzhovoi Volcanics is not as yet well controlled.
Waldron (1961) inferred an age of no older than latest Tertiary nor
younger than middle Pleistocene. Detterman and others (1996) reported
that upper part of the Tachilni Formation contains several ash beds that
were possibly derived from Morzhovoi volcano. From this they inferred an
earlier age limit of late Miocene for Morzhovoi Volcanics. Unit is
probably correlative with parts of unit QTv elsewhere on Alaska
Peninsula

**Tvu Volcanic rocks, undivided (Tertiary)**\--Andesite, dacite, and
basalt lava flows, tuffs, lahar deposits, volcanic breccia, and
hypabyssal intrusions, all locally hydrothermally altered or hornfelsed.
No potassium-argon ages are available and little stratigraphic control
exists for these rocks, but outcrop and erosional patterns are similar
to other Tertiary volcanic rocks. Includes volcanic rocks of Thinpoint
Lagoon of Waldron (1961)

**Tv Volcanic rocks (late Miocene)**\--Andesite and basalt flows, sills,
and plugs. Extrusive rocks of unit typically cap ridges and consist of
massive lava flows, agglomerate, and lahar deposits; unit also includes
minor small intrusive bodies, for example, in San Diego Bay-Albatross
Anchorage area of Port Moller C-2 1:63,360-scale quadrangle. Minor
propylitic alteration is characteristic of these rocks, except in area
around San Diego Bay, where sericitic and argillic alteration is
pervasive. Potassium-argon ages range from 10.40.49 to 6.10.23 Ma
(F.H. Wilson, unpub. data, 1987)

**Tm Meshik Volcanics (early Oligocene and late Eocene)**\--Originally
named Meshik Formation by Knappen (1929, p. 198-201) for exposures along
Meshik River and near Meshik Lake in Chignik 1:250,000-scale quadrangle.
Detterman and others (1996) have renamed unit Meshik Volcanics and have
included measured section from Gulf Oil Co. Unit No. 1 drillhole,
located near Port Heiden (see Chignik D-3 1:63,360-scale quadrangle).
This drillhole penetrated almost 1,765 m of Meshik before reaching total
depth, still within Meshik (Brockway and others, 1975). Drillhole
section contains considerably more tuff than do outcrop exposures of
Meshik Volcanics. In map area, formation is well exposed in mountains
north of Ivanof and Chignik Bays; it consists of coarse andesitic and
basaltic volcanic rubble, lahar deposits, andesite and basalt lava
flows, tuff, hypabyssal basalt and andesite plugs, and minor amounts of
volcaniclastic sedimentary rocks. Volcaniclastic sedimentary rocks are
equivalent in age and in lithology to Stepovak Formation (Ts). In
Stepovak Bay region, Meshik gradationally interfingers with Stepovak
Formation; unit shown on map depends on which lithology dominates. On
Unga Island, volcanic rocks of Meshik also include dacite and rare
rhyolite flows and domes. Potassium-argon ages on multiple samples of
Meshik Volcanics range from about 25 Ma to about 42 Ma (Wilson and
others, 1981; DuBois and others, 1987; F.H. Wilson and Nora Shew, unpub.
data, 1990; Wilson and Shew, 1992); vast majority of age determinations
are between 30 and 40 Ma. Megafauna fossil collections from Meshik are
rare; however, collections from Kupreanof Peninsula are Eocene and
Oligocene in age. Contact of Meshik with underlying Tolstoi Formation
(Tt) is considered a disconformity, although Meshik Volcanics
unconformably overlie Tolstoi Formation in area northwest of Chignik
Bay. Upper contact of Meshik Volcanics is unconformable with both Bear
Lake Formation (Tbl) and late Miocene volcanic rocks (Tv). As mapped,
unit includes informally defined Popof volcanic rocks of Wilson and
others (1995), and also is herein geographically extended to include
volcanic rocks of equivalent age, composition, and stratigraphic
position mapped by Riehle and others (1987) as units Tvb and Tvm in
Mount Katmai and Naknek 1:250,000-scale quadrangles

**Trv Volcanic rocks (Late Triassic)**\--Columnar-jointed basalt flows,
volcanic breccia, and agglomerate (see inset A, pl. 1). Chemical data
(Hill, 1979; Wilson and Shew, 1992) indicate these rocks are of
tholeiitic affinity. Potassium-argon age determination on flow unit was
19712 Ma (Wilson and Shew, 1992); age is thought to be minimum age, as
it is younger than stratigraphic position suggests. Interbedded with
Late Triassic Kamishak Formation (Trk)

**Pv Volcanic rocks (Permian?)**\--Massive, dark-green to black volcanic
breccia, agglomerate, and andesitic flows (Hanson, 1957; Hill, 1979).
These rocks are only exposed on offshore islets east of Puale Bay and
were not examined during this study (see inset A, pl. 1). Well-developed
joint system has obscured original structure, and because of this, in
part, thickness is unknown. No definite age control exists; however,
these rocks appear to structurally underlie mid-Permian limestone (Pls).
Hill (1979) briefly examined exposures and sampled rocks. Limited
chemical data from Hill (1979) suggest these volcanic rocks are of
calc-alkaline affinity, in contrast to Late Triassic tholeiitic volcanic
rocks of nearby mainland (Trv)

INTRUSIVE ROCKS

**Qi Intrusive rocks (Holocene and Pleistocene)**\--Hypabyssal dacite
plugs and domes at Quaternary volcanic centers, particularly at Frosty
Peak, Trader Mountain, Mount Dana, Ugashik Caldera, Blue Mountain (west
of Ugashik Lakes), and The Gas Rocks. Porphyritic dacite at Trader
Mountain has yielded potassium-argon age of 0.980.05 Ma (Wilson and
Shew, 1992). Potassium-argon ages on other intrusive bodies range from
about 1 Ma to about 165 ka (Wilson and others, 1981; Wilson and Shew,
1992). Pyroclastic deposits (Qpd) form an apron of ash around Mount
Dana; radiocarbon ages (see table 1, Wilson and others, 1995) of samples
from stratigraphic sections containing these deposits suggest Holocene
age for dome at Mount Dana

**Ti Intrusive rocks (Pliocene and late Miocene)**\--Medium- to
coarse-grained, equigranular, granodiorite to quartz diorite plutons and
stocks containing hornblende, biotite, and pyroxene as mafic minerals
and typically are surrounded by well-developed hornfels zones and
sporadic hydrothermal alteration in country rocks. Intrusive bodies are
typically located along Pacific coast and include, but are not limited
to, large plutons at Moss Cape, American Bay, Pyramid Mountain,
Mitrofania Island, Devils Bay (Devils batholith, Detterman and others,
1981), Agripina Bay, Mount Becharof, Cape Igvak, and Cape Douglas.
Potassium-argon ages range from 9.430.26 to 3.210.14 Ma (Wilson and
others, 1981; F.H. Wilson and Nora Shew, unpub. data, 1990; Wilson and
Shew, 1992)

**Tiu Intrusive rocks, undivided (Tertiary)**\--Small, typically
hypabyssal dikes, sills, and stocks of andesite, quartz diorite, or
diorite containing phenocrysts of pyroxene or hornblende in fine-grained
groundmass. No potassium-argon ages are available for these rocks; unit
may include rocks of other Tertiary intrusive rock units

**Tgd Granodiorite (Oligocene)**\--Riehle and others (1993) describe
these rocks as \"Light-gray to gray-green, medium-grained equigranular
to marginally porphyritic rocks having subhedral plagioclase and (or)
hornblende phenocrysts \*\*\* Biotite is subequal to clinopyroxene or
hornblende and secondary epidote and chlorite are common \*\*\* Average
color index \*\*\* is 18. Modal quartz rarely exceeds 25 percent, and
unit includes samples that marginally classify as quartz monzodiorite,
quartz diorite, or uncommonly, tonalite.\" Found in northern part of
Mount Katmai 1:250,000-scale quadrangle

**Tqd Quartz diorite (Oligocene)**\--Riehle and others (1993) describe
these rocks as \"Gray, medium-grained equigranular rocks consisting
chiefly of plagioclase, pyroxene, and quartz and having accessory
hornblende in excess of biotite. Average color index is 23 \*\*\*\"
Modal quartz content varies widely, and rocks composing unit include
tonalite and monzonite (Riehle and others, 1987). Found in northern part
of Mount Katmai 1:250,000-scale quadrangle

**Tg Granodiorite (Paleocene)**\--Medium-grained biotite granodiorite,
quartz monzonite, and granite(?) plutons of Sanak Island, outer Shumagin
Islands, and Semidi Islands; exhibits hypidiomorphic-granular texture
and locally contains potassium feldspar phenocrysts as long as 1 cm.
Rare muscovite is found in limited areas near contacts with Shumagin
Formation (Ks). Minor hornfels zone as wide as 500 m is mapped at
contact with Shumagin Formation (Moore, 1974a). Potassium-argon ages
reported by Burk (1965), Moore (1974a), and Kienle and Turner (1976)
range from 65.63.3 to 57.41.8 Ma when recalculated using currently
accepted constants (Steiger and Jager, 1977)

**Granitic rocks**\--Granitic rocks, typically hornblende-biotite
granodiorite of Alaska-Aleutian Range batholith (Reed and Lanphere,
1969; 1972; 1973), although quartz diorite and granite are present
locally. Most of batholith is composed of coarsely crystalline, weakly
foliated to nonfoliated rocks. Metamorphosed roof pendants are most
common at northern end of Alaska Peninsula. Potassium-argon age
determinations yield ages that range from 1534.6 to 174.78.8 Ma (Reed
and Lanphere, 1969; 1972; Nora Shew and M.A. Lanphere, unpub. data,
1990; Wilson and Shew, 1992) in map area. Mapped separately as:

**Jgd Granodiorite (Late? and Middle Jurassic)**\--Fine- to
coarse-grained, gray, hornblende-biotite granodiorite. In Naknek Lake
area, \"Color index averages 16; either hornblende or biotite can be
dominant mafic phase. Modal quartz comprises 22-44 percent of the rock,
in excess of that found in unit Tgd\" (Riehle and others, 1993).
Generally is found in vicinity of Naknek Lake in Mount Katmai
1:250,000-scale quadrangle; however, southernmost outcrop of this unit
of Alaska-Aleutian Range batholith is mapped on an unnamed island on
south side of Becharof Lake in Ugashik 1:250,000-scale quadrangle

**Jgr Granite (Late? and Middle Jurassic)**\--\"Light-gray,
medium-grained equigranular or fine-grained porphyritic rocks. Average
color index is 8 and biotite equals or exceeds hornblende. Rounded
quartz grains weather prominently from the rock and, where elliptical,
define a foliation \*\*\*\" (Riehle and others, 1993). Generally found
between Becharof and Naknek Lakes in Mount Katmai and Naknek
1:250,000-scale quadrangles

**Jqd Tonalite and quartz diorite (Late? and Middle
Jurassic)**\--\"Medium-grained, gray equigranular rocks. Average color
index of tonalite is 28, of quartz diorite is 37. Either clinopyroxene
or hornblende is the dominant mafic (mineral) and biotite is an
accessory. Inclusions of fine-grained mafic rocks or of
porphyroaphanitic volcanics (rocks) are common. Both a foliation and a
layering commonly occur together \*\*\*\" (Riehle and others, 1993).
Generally found north of Naknek Lake in Mount Katmai 1:250,000-scale
quadrangle

**Jgb Diorite and gabbro (Late? and Middle Jurassic)**\--\"Small bodies
of dark gray, diabasic- and gabbroic-textured rocks having either or
both of two pyroxenes and hornblende. In some, hornblende is secondary;
others are hornfelses in which biotite and muscovite have partly
replaced hornblende and clinopyroxene \*\*\*\" (Riehle and others,
1993). Generally found south of Naknek Lake in Mount Katmai
1:250,000-scale quadrangle

METAMORPHIC ROCKS

**QTc Contact-metamorphosed rocks (early Quaternary or late
Tertiary)**\--Contact-metamorphosed and hydrothermally altered rocks
found in mountains east of Mount Dana in Port Moller C-4 1:63,360-scale
quadrangle. Protoliths probably consist of, in order of importance, Late
Cretaceous Hoodoo Formation (Kh), Tertiary volcanic rocks (Tvu), and
Late Cretaceous Chignik Formation (Kc); may also include Tertiary
intrusive rocks (Tiu) at lower elevations. Pervasive alteration prevents
separation of altered rocks into their respective units. Rocks are hard,
very fine grained, and intensely fractured; sulfide mineralization is
common, and resultant iron staining is ubiquitous on weathered surfaces

**JPk Kakhonak(?) Complex (Late Jurassic? to Permian?)**\--Name
introduced by Detterman and Reed (1980) for heterogeneous assemblage of
metamorphic rocks exposed in Iliamna 1:250,000-scale quadrangle north of
map area. In map area, metamorphic rocks of unknown age exposed west of
Bruin Bay fault north of Becharof Lake are tentatively correlated with
Kakhonak Complex (Detterman and others, 1996). Rocks are mainly
preserved as roof pendants in Alaska-Aleutian Range batholith, but some
outcrops are found in areas surrounded by surficial deposits. Includes
metalimestone, quartzite, greenstone and other metavolcanic rocks, and
schist. Most are weakly metamorphosed, greenschist-facies rocks in which
relict bedding is often discernable. Minimum age of enclosing batholith
is about 155 Ma, which places upper age limit of early Late Jurassic on
metamorphic rocks. Lithology of rocks indicates that protoliths were
similar to Permian limestone and rocks of Kamishak and Talkeetna
Formations exposed at Puale Bay and on nearby islands. Unit age is
Permian(?) to early Late Jurassic(?)

**Trc Cottonwood Bay Greenstone (Late Triassic; Norian?)**\--Named by
Detterman and Reed (1980) for sequence of dark-green to gray
metavolcanic rocks exposed on west side of Cook Inlet. In map area, unit
mapped only as roof pendants in Alaska-Aleutian Range batholith north of
Becharof Lake, where it consists of weakly metamorphosed
epidote-albite-actinolite assemblages that are suggestive of
greenschist-facies metamorphism. Thickness may exceed 400 m; however,
contact relations with other stratigraphic units are not well
understood. Some rocks locally included herein in Kakhonak(?) Complex
(JPk) are Cottonwood Bay Greenstone. No isotopic age determinations
exist for greenstone, but its close association with Kamishak Formation
near Iliamna Bay northeast of map area suggests Late Triassic (Norian?)
age.

Note for map base on field personnel: Geology mapped by Narin Albert,
1983; W.H. Allaway, Jr., 1980-81; L.M. Angeloni, 1983-85; Peter Burrell,
1983; J.E. Case, 1977-81, 1983-1985; C.L. Connor, 1982; D.P. Cox,
1977-78, 1980; John Decker, 1985; R.L. Detterman, 1977-86, 1988; G.D.
DuBois, 1985, 1988, 1990; C.L. Engelhardt, 1977; B.M. Gamble, 1983-84;
S.Z. Gajewski, 1986; C.D. Holloway, 1977; D.G. Howell, 1977; T.E.C.
Keith, 1984; L.B. Magoon, 1979-80; Louie Marincovich, 1978; 1982, 1984;
Eric Martin, 1984; Hugh McLean, 1977; J.W. Miller, 1979-86; T.P. Miller,
1973-75, 1977-78, 1979, 1981-82, 1988; M.A. Pernokas, 1983; B.L. Reed,
1983-84; J.R. Riehle, 1981-86; Nora Shew, 1981-82; R.A. Spicer, 1977-78;
F.R. Weber, 1983-85; F.H. Wilson, 1977-86, 1988, 1990; J.A. Wolfe,
1977-78; M.E. Yount 1977-86, 1988. Some geology modified from Burk
(1965), Kennedy and Waldron (1955), Waldron (1961), and Moore (1974a,
b).

List of map symbols and overprint patterns

Contact\--Dashed where inferred; dotted where concealed; long and short
dashes where internal

Fault\--Dashed where inferred; dotted where concealed.

Thrust or high-angle reverse fault\--Dotted where concealed. Sawteeth on
upper plate

Folds\--Showing approximate trace of axial plane. Dotted where concealed

Anticline

Syncline

Crater rim

Volcanic crater

Glacier

Alteration

Hornfelsed zone