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 Jnc
Unit name Naknek Formation -- Chisik Conglomerate Member
Description Limited to northern half of Alaska Peninsula, 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.
Lithology Sedimentary

Correlated geologic units

Label Jnc
Description Naknek Formation, Chisik Conglomerate Member
Geologic age Oxfordian
Geologic setting Sedimentary, shallow-marine-siliciclastic
Lithology Form Importance
Conglomerate < Clastic < Sedimentary Bed Major
Sandstone < Clastic < Sedimentary Bed Major