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, Gregory, 1999, Digital data for the geologic framework of the Alaska Peninsula, southwest Alaska, and the Alaska Peninsula terrane: U.S. Geological Survey Open-File Report 99-317. [http://geopubs.wr.usgs.gov/open-file/of99-317/]
Symbol Jns
Unit name Naknek Formation -- Snug Harbor Siltstone Member
Description 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. 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.
Lithology Sedimentary

Correlated geologic units

Label Jns
Description Naknek Formation, Snug Harbor Siltstone Member
Geologic age Oxfordian to Kimmeridgian
Geologic setting Sedimentary, slope-and-deep-water
Lithology Form Importance
Siltstone < Clastic < Sedimentary Bed Major
Sandstone < Clastic < Sedimentary Bed Minor