Naknek Formation and Kotsina Conglomerate

Unit symbol: Jnk
Age range Upper Jurassic, Tithonian to Oxfordian (163.5 to 145 Ma)
Lithology: Sedimentary
Group name: Staniukovich and Naknek Formations, Kotsina Conglomerate, and similar rocks of southern Alaska
Sandstone, conglomerate, and siltstone whose clasts have a primarily plutonic provenance. Unit is widespread in southern Alaska, in a long belt that ranges from south-central Alaska (Wilson and others, 1998) to the southwest end of the Alaska Peninsula (Wilson and others, 1999)—about 1,150 km (Detterman and others, 1996). Aggregate thickness of the unit members exceeds 3,000 m, though the average thickness is more typically 1,700 to 2,000 m (Detterman and others, 1996). Megafossils, particularly the pelecypod Buchia and ammonites (Detterman and others, 1996), are age diagnostic and provide excellent control. Detterman and others (1996; see also, Detterman and Hartsock, 1966; Martin and Katz, 1912) subdivided unit into the following formal members, top to bottom: Pomeroy Arkose, Katolinat Conglomerate, Indecision Creek Sandstone, Snug Harbor Siltstone, Northeast Creek Sandstone, and Chisik Conglomerate. The Naknek is conformable with the overlying Staniukovich Formation (Kst) and unconformably overlies the Middle Jurassic Shelikof Formation (included here in unit Jsc). The Jurassic portion of the Alaska-Aleutian Range batholith (unit Jgr) was main source of sedimentary debris for Naknek Formation, which, on faunal evidence, ranges in age from about 145 to 163.5 Ma; hence, uplift and erosion of batholith occurred during and shortly after emplacement. The Kotsina Conglomerate is a stratigraphic equivalent of the Naknek and consists of mainly of well-indurated massive cobble and pebble conglomerate and minor boulder conglomerate, with some arenitic sandstone and siltstone interbeds. Conglomerate clasts in it are derived from Triassic limestone, Nikolai Greenstone, and the Skolai Group, reflecting local uplift and erosion (MacKevett and others, 1978)

Source map information

Source map Wilson, F.H., Detterman, R.L., and DuBois, G.D., 2015, Geologic framework of the Alaska Peninsula, southwest Alaska, and the Alaska Peninsula terrane: U.S. Geological Survey Bulletin 1969-B, scale 1:500,000, 2 plates, 34 p.
Symbol Jni
Unit name Naknek Formation -- Indecision Creek Sandstone Member
Description 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.
Lithology Sedimentary

Correlated geologic units

Label Jni
Description Naknek Formation, Indecision Creek Sandstone Member
Geologic age Tithonian
Geologic setting Sedimentary, shallow-marine-siliciclastic
Lithology Form Importance
Arkose < Sandstone < Clastic < Sedimentary Bed Major
Siltstone < Clastic < Sedimentary Bed Major