2013

U.S. Department of the Interior

U.S. Geological Survey

Reconnaissance Geologic Map of Kodiak Island

and Adjacent Islands, Alaska

Compiled by Frederic H. Wilson

Pamphlet to accompany

Scientific Investigations Map 2999

i

Contents

Introduction and Previous Work ............................................................................................................... 1

Geologic and Physiographic Framework ................................................................................................. 1

Discussion ..................................................................................................................................................... 2

Description of Map Units ............................................................................................................................ 2

Surficial Deposits and Sedimentary Rocks .................................................................................... 2

Igneous Rocks ......................................................................................................................................4

Metamorphic and Tectonic Rocks .................................................................................................... 4

References Cited ...........................................................................................................................................6

Additional References ..................................................................................................................................7

1

Introduction and Previous Work

This map and its associated digital files (Wilson and others, 

2005) represent part of a systematic effort to release geologic 

map data for the United States in a uniform manner. The geologic data in this series have been compiled from a wide variety 

of sources, ranging from state and regional geologic maps to 

large-scale field mapping. The map data are presented herein for 

use at a nominal scale of 1:500,000, although individual datasets (see Wilson and others, 2005) may contain data suitable for 

use at larger scales; the metadata associated with each dataset 

provides more detailed information on sources and appropriate 

scales for use. Associated attribute databases accompany the 

spatial database of the geology and are uniformly structured for 

ease in developing regional- and national-scale maps.

Kodiak Island and its adjacent islands, located on the west 

side of the Gulf of Alaska, contain one of the largest areas of 

exposure of the flysch and melange of the Chugach terrane of 

southern Alaska. However, in the past 25 years, only detailed 

mapping covering small areas in the archipelago has been done. 

The map and associated digital files present the best available 

mapping compiled in an integrated fashion. The primary source 

for the map of Kodiak Island and islands to the south is the 

reconnaissance mapping of George W. Moore (Moore, 1967), 

augmented by his unit descriptions (Moore, 1969), as well as 

new mapping on the west coast by William Connelly and J. 

Casey Moore (Connelly, 1978; Connelly and Moore, 1979) and 

on the east coast by Moore and others (1983). George Moore’s 

map (Moore, 1967), in turn, is based heavily on mapping by 

Capps (1937), which also is the major source for the map of 

Afognak Island. However, a recent (2001-04) unpublished 

vegetation survey of Afognak Island by Page Spencer (National 

Park Service) and Michael Fleming (USGS) indicates that part 

of Afognak Island mapped by Capps (1937) as the Kodiak 

Formation consists of granitic rocks that are probably similar 

to rocks of the Kodiak batholith on Kodiak Island; thus, areas 

shown as granitic rocks on the map of Afognak Island have 

been inferred from photographs taken as part of this vegetation 

survey, in conjunction with available aeromagnetic data (Saltus 

and Simmons, 1997). A brief field visit to an area of magnetic 

rocks in the mountains east of Kazakof Bay showed the rocks at 

that location to be a highly indurated, possibly hornfelsed, clean 

sandstone, atypical of the graywacke of the Kodiak Formation. 

Mapping of the surficial deposits on Afognak Island has been 

derived from Landsat-image and topographic-map interpretation. The geology of Sitkinak Island has been derived from 

Clendenen and others (1992). Important interpretations of some 

of the geologic units as discussed in Plafker and others (1994) 

have been incorporated here where appropriate.

Geologic and Physiographic Framework

The accretionary flysch and melange of the late Mesozoic 

Chugach terrane (Plafker and others, 1977) is an important part 

of Kodiak Island and its adjacent islands. Also present in the 

map area are accreted components of the Prince William terrane 

(the Ghost Rocks and Sitkalidak Formations), which are present 

in the southeastern part of the map area, as well as Tertiaryoverlap rock units (Plafker and others, 1994). The flysch of 

the Chugach terrane, which here is represented by the Kodiak 

Formation, is intruded by Paleocene granitic rocks that typically 

are referred to as the Kodiak batholith. These Paleocene plutons 

were emplaced in a near-trench setting, possibly owing to the 

subduction of a spreading center (Marshak and Karig, 1977; 

Moore and others, 1983; Bradley and others, 2000). The Ghost 

Rocks Formation is the melange and trench-deposited flysch 

of the Prince William terrane; the Sitkalidak Formation is an 

outer-fan turbidite, thought to be related to the same depositional 

system (Plafker and others, 1994). Volcanic and mafic hypabyssal rocks within the Ghost Rocks Formation possibly are comagmatic with the granitic rocks of the Kodiak batholith (Hill and 

others, 1981); both tholeiitic basalt and calc-alkaline andesite are 

present. Because of this relation, Plafker and others (1994) suggested that the Ghost Rocks Formation, although younger than 

the Kodiak Formation, predates intrusion of the Kodiak batholith 

and, therefore, is of latest Cretaceous and early Paleocene age. 

Granitic plutonic rocks intruding the Ghost Rocks Formation 

commonly are assumed (for example, Plafker and others, 1994) 

to be an extension of the Kodiak batholith; however, on this map 

they are shown as a distinct geologic unit because they are more 

altered and contain little biotite. If they truly are distinct from 

the Kodiak batholith, then the age of the Ghost Rocks Formation 

remains poorly constrained. The Sitkalidak Formation is also 

poorly age constrained; however, evidence suggests an Eocene 

and Oligocene age (Moore, 1969; Plafker and others, 1994). 

Tertiary overlap sequences indicate an initial terrestrial phase 

followed soon after by marine deposition at water depths of 100 

to 186 m (Clendenen and others, 1992). This was succeeded in 

turn by deposition of the shallow-water Narrow Cape Formation, which is of limited areal extent. Clendenen and others 

(1992) described a new unit as the informally named Albatross 

sedimentary sequence, which was originally mapped by Moore 

(1967) as part of the Sitkinak Formation on Sitkinak Island; 

they interpreted it to be equivalent to the Yakataga Formation in 

northern Southeast Alaska and suggested that it was deposited in 

conditions transitional between glaciomarine and fluvial conditions. The youngest sedimentary rock unit of the Kodiak Island 

area, the late Pliocene and early Pleistocene Tugidak Formation 

(Moore, 1969), is richly fossiliferous; its sedimentary characteristics suggest deposition in continuing glaciomarine conditions 

(Armentrout, 1979); part of this unit also has been correlated 

with the Yakataga Formation (Moore, 1969).

Physiographically, Kodiak Island and its adjacent islands 

show strong effects from Pleistocene and Holocene glaciation. 

Well-developed fjords and, locally, glacial deposits provide 

abundant evidence for the glaciers and ice cap that covered 

much of the islands (Karlstrom and Ball, 1969). Mann and 

Peteet (1994) provided dates and stratigraphic sections, which 

indicate that glaciers from Shelikof Strait flowed on land, as 

much as 10 km inland, at the same time that an ice cap covered 

much of the island. Nonetheless, very little mapping of Quaternary geology in the Kodiak Island area has been reported in the 

literature.
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Discussion

Ultimately, much better mapping is needed in the Kodiak 

Island area to resolve the age relations and distribution of geologic units. Unpublished anecdotal reports, such as those of Page 

Spencer and Michael Fleming (oral commun., 2004) mentioned 

above, suggest that additional outcrops of the Kodiak batholith may be found on densely forested Afognak Island. Plutons 

intruding the Ghost Rocks Formation need dating, as do the 

DESCRIPTION OF MAP UNITS

SURFICIAL DEPOSITS AND SEDIMENTARY ROCKS

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

moderately well stratified deposits. Consists predominantly of alluvial, colluvial, glacial, 

marine, lacustrine, eolian, and swamp deposits

QTtg Tugidak Formation (early Pleistocene and late Pliocene)—Consists of 1,500 m of interbedded 

sandstone and siltstone that contains randomly distributed pebbles and cobbles of glacialmarine origin (Allison, 1978). Single 1-m-thick cobble-conglomerate bed is reported 350 m 

above base of unit (Allison, 1978). Unit is richly fossiliferous, containing marine fossils of 

Pliocene age. Allison (1978, p. 177) reported occurrence of diverse fauna, which consists 

of more than 80 species that are “largely composed of living, cold-water, North Pacific and 

Arctic taxa” and indicate cold-water conditions, colder than present-day Gulf of Alaska. On 

the basis of available fauna, Allison (1978) suggested that deposition took place in upper 

part of outer-neritic zone, at water depths between 91 and 145 m

Ty Albatross sedimentary sequence of Clendenen and others (1992) (Pliocene or late Miocene)—Diamictite, sandstone, and siltstone. Also includes conglomerate horizons that 

contain clasts of granite, chert, melange, and slate. Conglomerate also contains distinctive 

calcareous-shale clasts that are unlike any nearby exposed units in that they contain late 

early Miocene fauna of foraminifers and flora of diatoms; however, unit in which clasts are 

enclosed is correlated with part of the Yakataga Formation of Yakutat area, 500 km to east

Tnc Narrow Cape Formation (middle Miocene)—Unit is 700 m thick: lower two thirds consists 

of sandstone and a few conglomerate beds; upper one third is siltstone. Basal sedimentary 

breccia and conglomerate contains clasts derived from the underlying Sitkalidak Formation; 

fossiliferous (Nilsen and Moore, 1979). Overlying breccia is highly bioturbated, massive, 

silty, fine-grained sandstone and siltstone that makes up more than 90 percent of formation 

(Nilsen and Moore, 1979). At irregular intervals, contains thin (as much as 70 cm thick) 

conglomeratic layers consisting of well-rounded granitic and volcanic pebbles, clasts of the 

underlying Sitkalidak Formation, and disarticulated, broken, and partly rounded megafossil fragments (Nilsen and Moore, 1979). Rich marine fauna is present in the Narrow Cape 

Formation; Allison (1978) indicated that formation contains at least 80 taxa that include 

Acila, Clinocardium, Cyclocardia, Mya, Colus, Cryptonatica, Anadara, Chione, Dosinia, 

and gastropod Ficus. Range and variety of invertebrate fossils provide strong indication of 

warm-water conditions within temperate-climate belt of Hall (1964)

Tsti Siltstone of Trinity Islands of Clendenen and others (1992) (earliest Miocene or late Oligocene)—Siltstone deposited in cool-temperature climactic conditions at outer-neritic water 

depths. Originally included in the Narrow Cape Formation, unit is now considered to be distinctive, older unit. Unit is highly bioturbated siltstone and very-fine-grained sandstone, containing sporadic conglomeratic layers (Nilsen and Moore, 1979). Nilsen and Moore (1979) 

reported that fossils are abundant but are of low species diversity. Allison and Marincovich 

(1981; see also, Allison, 1978) indicated that this unit “was deposited in the outer neritic 

zone of the continental shelf.” Their best interpretation of depth ranges of taxa found in unit 

suggested deposition in water depths of 100 to 200 m. Fauna also suggest cool-temperate 

marine climate, significantly cooler than that represented by the (younger) Narrow Cape 

Formation, to which this unit was originally assigned

volcanic rocks within the unit. Studies of the Quaternary history 

and glaciation are minimal (see, for example, Mann and Peteet, 

1994). The geologic units of the Kodiak Island area figure prominently in many tectonic reconstructions in the literature (see, 

for example, Marshak and Karig, 1977; Moore and others, 1983; 

Clendenen and others, 1992; Bradley and others, 2000), yet mapping of these units is, at best, only at reconnaissance scale.
3

Tsk Sitkinak Formation (Oligocene)—Conglomerate and cross-bedded sandstone and siltstone 

that contains coal fragments. Unit is 1,500 m thick in its type section along south shore of 

Sitkinak Island. According to Nilsen and Moore (1979), outcrops of the Sitkinak Formation 

on Sitkalidak and Sitkinak Islands consist of two different facies. On Sitkalidak Island, 

lower part of unit is conglomerate that contains rounded clasts of volcanic rocks, graywacke, 

chert, and carbonate rocks; upper section is turbidite sandstone and interbedded shale. 

Overall sequence represents an inner-fan or lower-slope environment that has channel or 

canyon fill (Nilsen and Moore, 1979). On Sitkinak Island, “formation consists of alternating 

conglomerate-sandstone units and fine-grained sandstone and siltstone units with some coal 

and carbonaceous shale strata” (Nilsen and Moore, 1979, p. 19–20); conglomerate contains 

well-rounded clasts of volcanic rocks, vein quartz, argillite, graywacke, red chert, and 

granite. Section probably represents conglomerate-sandstone channels enclosing interchannel, lagoonal, and interdistributary-bay deposits (Nilsen and Moore, 1979). Very few marine 

fossils are known from unit, whereas abundant plant fragments and fossil leaves have been 

collected from siltstone-coal strata (Nilsen and Moore, 1979)

Tsi Sitkalidak Formation (Oligocene and Eocene)—Uniform sandstone and siltstone in graded 

beds; few conglomerate beds. Total thickness about 3,000 m. Fossil crab, Callianassa aff. 

C. porterensis, indicates Oligocene age (Moore, 1969); Moore (1969) assigned Eocene and 

Oligocene age on the basis of this fossil and evidence from superposition. Clendenen and 

others (1992) reported Eocene foraminifers but only reported Eocene age for unit. Because 

of reported presence of Oligocene-age fossil crab in unit, unit age is herein revised to conform to Moore’s (1969) original age assignment (Eocene and Oligocene)

Kcct Cape Current terrane of Connelly (1978) (Late Cretaceous)—Informal unit described by 

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

sandstone that contains sparse sections of vesicular pillow lava and pillow breccia. Slightly 

metamorphosed and moderately deformed. Two bodies of red pelagic limestone containing 

many silicieous layers are present within unit along southern shore of Shuyak Island. Limestones, which are thin bedded and tightly folded, contain sparse coccoliths of indeterminate 

age and Late Cretaceous foraminifera (approximately Turonian to early Santonian age, 

according to W.V. Sliter, written commun., in Connelly, 1978)

Kkd Kodiak Formation (Late Cretaceous)—Consists of medium- to thick-bedded, graded-bed 

sequences, which average 1 m thick, of arkosic wacke and shale and occasional beds of 

pebbly conglomerate. Flute casts and complete Bouma sequences indicate deposition by 

turbidity currents below wave base. Strikes to northeast and dips steeply to northwest; is 

generally deformed in tight, large-scale folds. Moore (1969) designated type section as rocks 

exposed along western shore of Uyak Bay. Base of section is 4 km south of head of bay, 

where lower part of formation is intruded by pluton; top of section is 3 km south of village of 

Uyak, where rocks of the Uyak Complex (Kmk) are thrust over the Kodiak Formation. As a 

result, neither a true lower or upper contact is known for the Kodiak Formation. Subsequent 

work by Nilsen and Moore (1979) indicated that unit is approximately 5,000 m thick, in 

contrast to estimated thickness of 30,000 m as originally suggested by Moore (1969). Nilsen 

and Moore (1979) found that unit is repeated structurally by folding and faulting, which 

makes internal stratigraphic correlation difficult. Nilsen and Moore (1979) used a “turbidite 

facies and facies-association” scheme that generally follows system of Mutti and Ricci 

Lucchi (1972, 1975, as cited in Nilsen and Moore, 1979) and Nelson and Nilsen (1974, as 

cited in Nilsen and Moore, 1979); under this system, the Kodiak Formation consists largely 

of basin-plain- and slope-facies associations. Basin-plain facies is structurally lowest part of 

the Kodiak Formation and is characteristic of most of formation on southeast side of Kodiak 

Island (Nilsen and Moore, 1979). It “consists of repetitively interstratified graded sandstone 

beds and hemipelagic shales, typically 30 cm thick. * * * Sandstone:shale ratios range in 

general from 1:1 to 1:5. The interstratified hemipelagic shale is generally bioturbated; trace 

fossils typically are those of the surface-grazing deep-marine Nereites facies and include 

Helminthoida and Helminthopsis as well as larger trace fossils possibly formed by moving 

gastropods” (Nilsen and Moore, 1979, p. 6). Slope-facies rocks are found primarily on northwest side of Kodiak Island and are primarily thick mudstone sequences. These “may form 

imbricate slices juxtaposed during multiple phases of synsedimentary slumping [and they] 

contain chaotically oriented blocks, slabs, and disordered fragments of hemipelagic mudstone 
4

that probably slid from upper slope depositional sites to the lower slope or base of slope 

under the influence of gravity. * * * Sandstone:shale ratios are typically very low, ranging 

from 1:30 to 1:10” (Nilsen and Moore, 1979, p. 6). Conglomerate and sandstone channels 

are found locally within slope-facies associations; Nilsen and Moore (1979) mentioned their 

presence in Uyak Bay in particular, where they are about 50 m thick. Fossils from the Kodiak 

Formation include Inoceramus kusiroensis of Late Cretaceous (Maestrichtian) age. Correlated 

with the Shumagin Formation to southwest and the Valdez Group to northeast

Shuyak Formation (Late Triassic)—Consists of two members: a sedimentary member in 

fault contact with a volcanic member. Sedimentary member is inferred to overlie volcanic 

member (Connelly and Moore, 1979). Unit structurally overlies the schist of Kodiak Island 

(Jsch) and the Uyak Complex (Kmk) but usually is separated from them by the Afognak 

pluton (^qd). Divided into the following:

^ss Sedimentary member—Volcaniclastic sequence consisting of thin- to medium-bedded lithic 

sandstone that contains lesser amounts of conglomerate, argillite, and siliceous tuff and is 

intruded by mafic dikes and sills (Connelly and Moore, 1979). As described by Connelly 

and Moore (1979), unit is rich in primary andesitic material and displays flute casts and 

complete Bouma sequences that indicate deposition by turbidity currents. Rocks either are 

broadly folded or dip homoclinally to southeast and have undergone prehnite-pumpellyitefacies metamorphism (Connelly and Moore, 1979)

^sv Volcanic member—Vesicular pillow greenstone; locally contains beds of pillow-breccia 

agglomerate, tuff, and argillite (Connelly and Moore, 1979). Greenstone is tholeiitic in 

composition

IGNEOUS ROCKS

Tegr Younger granitic rocks (Eocene?)—Small, altered granitic plutons that intrude the Ghost 

Rocks Formation (TKgh) between Portage and Kaiugnak Bays, as well as north of Ugak 

Bay on southeast coast of Kodiak Island. Examination of a few thin sections of these rocks 

show them to be distinctive from rocks of Kodiak batholith (mapped as Tg) in that they are 

more altered and contain little biotite (W.L. Coonrad and F.H. Wilson, unpub. data, 1976). 

Although shown on Moore’s (1967) map as part of his granodiorite unit, they are here separated because of their distinctive character

Tg Granitic rocks (Paleocene)—Unit generally consists of fine-grained biotite quartz monzonite 

of Kodiak batholith; ranges in composition from two-mica granite to granodiorite. Intrudes 

flysch deposits of the Kodiak Formation (Kkd) and, in vicinity of Japanese Bay, rocks of 

the Ghost Rocks Formation (TKgh). K-Ar ages range from 57.1 to 62.6 Ma, majority of 

which cluster around 60 Ma (Marvin and Dobson, 1979; Moore and others, 1983; Shew 

and Wilson, 1981); falling within this range are 40Ar/39Ar ages of 57.8±2.5 and 58.3±0.2 Ma 

reported by Bradley and others (2000). On Afognak Island, presence of rocks of Kodiak 

batholith is inferred from weak aeromagnetic anomalies (Saltus and Simmons, 1997), as 

well as from anecdotal reports that note the presence of granitic rocks (see discussion in 

text). All contacts on Afognak Island are hypothetical

^qd Afognak pluton (Triassic)—Large, multiphase pluton of hornblende diorite, quartz diorite, and 

tonalite exposed along northwestern side of Kodiak Island and neighboring islands to north 

(Connelly and Moore, 1979; Roeske and others, 1989). Pluton has well-developed contactmetamorphic aureole in the Shuyak Formation; boundary with the Schist of Kodiak Island 

(Jsch) is apparently a fault (Roeske and others, 1989). K-Ar ages on hornblende range 

between 188±11 and 197±11 Ma (Roeske and others, 1989); a U-Pb age determination of 

217±10 Ma is interpreted to indicate emplacement age (Roeske and others, 1989)

^vm Mafic dikes, sills, and plugs (Triassic?)—Diabasic bodies intruding the Shuyak Formation but 

having no visible thermal aureoles (Connelly and Moore, 1979)

METAMORPHIC AND TECTONIC ROCKS

TKgh Ghost Rocks Formation, undivided (Paleocene and Late Cretaceous?)—Consists of zeolitebearing tuffaceous sandstone and basalt, as well as hard claystone, sandstone, and tuff, 

in graded beds (wildflysch of Moore, 1969). Formation is sheared, faulted, and folded; 
5

estimated thickness is 5,000 meters (Nilsen and Moore, 1979). Other than intrusive contacts, 

all contacts with other units are faults. “The formation is lithologically distinct, as it is a 

coherent belt of rocks that contains pillow basalt and tuff, which are not found in either the 

underlying or the overlying formations” (Moore, 1969, p. A31). Although no fossils were 

found, Moore (1969) suggested that the Ghost Rocks Formation straddles Paleocene-Eocene 

boundary, on the basis of “analysis of the thickness of similar lithologies in the successive 

formations and a hypothetical correlation of the volcanic rocks in the Ghost Rocks Formation with nearby intrusive rocks of known age.” Poorly preserved fossils reported by Plafker 

and others (1994) suggest latest Cretaceous and Paleocene age for limestone blocks associated with pillow basalt within this unit. Locally subdivided into the following:

TKghc Coherent sedimentary rocks—Unit B of Byrne (1981). Rocks are exposed in vicinity of Japanese Bay. Consists of “medium-bedded (10-40 cm thick beds) sandstone interbedded with 

similar thicknesses of shale. The sandstones and shales are rhythmically bedded and sole 

markings and graded beds indicate turbidite deposition” (Byrne, 1981, p. 42). Byrne (1981) 

interpreted these rocks as “slope basin sequences” and indicated that they were distinctive in 

the Ghost Rocks Formation because of their lack of contact metamorphism and because they 

contain only rare thick-bedded (more than 1 m thick) sandstone. In addition, Byrne (1981) 

noted absence of conglomerate and pebbly mudstone, as well as lack of igneous rocks and 

relative lack of deformation, as compared with other rocks in the Ghost Rocks Formation. 

Rocks may be inferred to be lithologically similar to basin-plain-facies association of Nilsen 

and Moore (1979), as described for the Kodiak Formation (unit Kkd; see description above)

TKghs Sandstone-rich unit—Massive sandstone beds, lesser amounts of argillite, and medium- to 

thin-bedded sandstone. Sandstone beds show Bouma sequences indicative of deposition 

by turbidity currents. Locally, channelized conglomerate units as much as 100 m thick are 

found on Aliulik Peninsula; clasts in conglomerate are as large as 30 cm and consist of chert, 

sandstone, limestone, and greenstone. Andesite also is interbedded on Aliulik Peninsula. Age 

of unit is Paleocene(?)

TKgha Argillite-rich unit—Thick sections of argillite, local massive sandstone, and chert-rich pebble 

conglomerate. Locally contains tuff beds. Limestone containing planktonic foraminifers is 

present locally at depositional contacts with pillow basalt. Unit typically is found to southeast of sandstone-rich unit (TKghs). Three collections of planktonic foraminifers yielded 

Late Cretaceous and early Tertiary ages. Age of unit is Paleocene(?)

TKgvb Basaltic volcanic and hypabyssal rocks—Tholeiitic basalt within both the sandstone- and 

argillite-rich subunits (TKghs and TKgha, respectively) of the Ghost Rocks Formation. 

Rocks typically are altered by shearing and low-grade metamorphism, yet “these lavas 

cannot have been derived from a single source and in many respects exhibit chemical affinities to magmas found in a variety of tectonic environments” (Moore and others, 1983, p. 

270). Age of unit is Cretaceous(?) or Tertiary

Kmk Uyak Complex (Early Cretaceous)—Includes, in “lithologically chaotic assemblage of deep-sea 

rock types” (Connelly and Moore, 1979), argillite and gray chert containing minor tuff; massive arkosic sandstone; pillowed and massive greenstone; bedded radiolarian chert; minor 

limestone; and tuffaceous argillite. According to Connelly and Moore (1979), “most of the 

Uyak Complex is tectonic melange with foliation dipping steeply northwest” and “contains 

fossils ranging in age from Paleozoic to Early Cretaceous.” The Uyak Complex is a classic 

melange, containing large tectonic blocks of varying lithology in matrix of gray chert and 

argillite. “Gabbroic and ultramafic rocks occur as kilometre-sized slabs in the northernmost 

exposures of the Uyak [and] are always fault bounded, and serpentinization is pronounced 

near their margins” (Connelly, 1978, p. 758). Sparse tectonic blocks of rhythmically bedded 

radiolarian chert are widely scattered throughout the Uyak Complex. Intensely deformed 

gray chert and argillite, which forms about 45 percent of the Uyak, is matrix to other rock 

types of the Uyak. Limestone blocks are uncommon in the Uyak; Connelly (1978) reported 

that only three localities have bodies of mappable size. Two of three limestone bodies are 

contorted sequences of nonfossiliferous, thin-bedded white limestone containing thin layers 

of argillite; third body consists of “thick-bedded, variably recrystallized grainstone with 

occasional layers of tuff and tuffaceous lime” (Connelly, 1978, p. 760). Variously sized 

tectonic blocks, none of which have remaining primary sedimentary structures, of massive 

wacke and contorted thinly interbedded wacke and argillite constitute about 20 percent of 
6

the Uyak. Petrographically similar, massive and thinly interbedded wackes are “typically 

medium grained, poorly sorted, and have diffuse grain boundaries surrounded by finegrained matrices of phyllosilicates (mostly chlorite), prehnite, and/or pumpyellite” (Connelly, 1978, p. 760). In the Uyak Complex, mesoscopic shear fractures and innumerable 

faults of unknown offset have disrupted stratigraphic continuity and juxtaposed contrasting 

sedimentary and igneous rocks. Slickensides are common both as subparallel, anastomosing 

fractures in competent rocks and as closely spaced fractures in less competent rocks. Connelly (1978) reported detailed structural analysis indicating that mean slip direction during 

deformation was to N. 38o W. (±11o) and that transport was southeast under northwest (present day), on the basis of work by Moore and Wheeler (1975). Locally subdivided into the 

following:

Kmu Gabbroic and ultramafic rocks—Includes layered gabbro, clinopyroxenite, dunite, and 

plagioclase peridotite. Found in northwesternmost exposures of the Uyak Complex as 

kilometer-sized slabs that are always fault bounded (Connelly, 1978); margins of slabs show 

pronounced serpentinization

Jsch Schist of Kodiak Island (Jurassic)—Thinly layered and complexly folded quartz-mica schist, 

greenschist, crossite-epidote schist (blueschist), and epidote amphibolite (Connelly and 

Moore, 1979). Roeske and others (1989), who called unit the “Raspberry Schist,” subdivided it into two subunits: first consists of metabasite and metasedimentary rocks that have 

some relict textures and phases preserved, including pillow shapes and bedding; second unit. 

is completely recrystallized, and no original textures or phases are preserved. Carden and 

others (1977) reported Jurassic K-Ar metamorphic ages of 164.7±19.4 and 173.0±5.1 Ma on 

crossite and 192.2±5.6 and 196.7±5.8 Ma on white mica; Roeske and others (1989) reported 

U-Pb isochron age of 204±8 Ma and Rb-Sr isochron age of 195±10 Ma age for unit

References Cited

Allison, R.C., 1978, Late Oligocene through Pleistocene molluscan faunas in the Gulf of Alaska region: The Veliger, v. 

21, no. 2, p. 171–188.

Allison, R.C., and Marincovich, Louie, Jr., 1981, A Late Oligocene or earliest Miocene molluscan fauna from Sitkinak 

Island, Alaska: U.S. Geological Survey Professional Paper 

1233, 11 p., 3 plates.

Armentrout, J.M., 1979, Cenozoic stratigraphy, Kodiak Island 

Archipelago, Alaska [abs.]: Geological Society of America 

Abstracts with Programs, v. 11, p. 66.

Bradley, D.C., Parrish, Randall, Clendenen, William, Lux, 

Daniel, Layer, P.W., Heizler, Matthew, and Donley, D.T., 

2000, New geochronological evidence for the timing of 

early Tertiary ridge subduction in southern Alaska, in

Kelley, K.D., and Gough, L.P., Geologic studies in Alaska 

by the U.S. Geological Survey, 1998: U.S. Geological 

Survey Professional Paper 1615, p. 5–21.

Byrne, Timothy, 1981, Tectonic and structural evolution of the 

Ghost Rocks Formation, Kodiak Islands, Alaska: Santa 

Cruz, University of California, Ph.D. dissertation, 174 p.

Capps, S.R., 1937, Kodiak and adjacent islands, Alaska: U.S. 

Geological Survey Bulletin 880–C, p. 111–184.

Carden, J.R., Connelly, William, Forbes, R.B., and Turner, D.L., 

1977, Blueschists of the Kodiak Islands, Alaska: An extension of the Seldovia schist terrane: Geology, v. 5, no. 9, p. 

529–533.

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