Sitkinak Formation

Unit symbol: Tsk
Age range Tertiary, Oligocene (33.9 to 23.03 Ma)
Lithology: Sedimentary
Group name: Sitkinak and Sitkalidak Formations
Conglomerate and cross-bedded sandstone and siltstone that contain coal fragments. Unit is 1,500 m thick in its type section along the south shore of Sitkinak Island. According to Nilsen and Moore (1979), the outcrops of the Sitkinak Formation consist of different facies on Sitkalidak and Sitkinak Islands. On Sitkalidak Island, the lower part of the unit is conglomerate that contains rounded clasts of volcanic rocks, graywacke, chert, and carbonate rocks. The upper section of the unit is turbidite sandstone and interbedded shale. The overall sequence represents and inner fan or lower slope environment with channel or canyon fill conglomerate (Nilsen and Moore (1979). On Sitkinak Island, “* * * the 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). The conglomerate contains well-rounded clasts of volcanic rocks, vein quartz, argillite, graywacke, red chert, and granite. The section probably represents conglomerate-sandstone channels enclosing interchannel, lagoonal, and interdistributary bay deposits (Nilsen and Moore, 1979). Very few marine fossils are known from the unit, whereas abundant plant fragments and fossil leaves have been collected from the siltstone-coal strata (Nilsen and Moore, 1979)

Source map information

Source map Moore, G.W., 1969, New formations on Kodiak and adjacent islands, Alaska, in Cohee, G.V., Bates, R.G., and Wright, W.B., Changes in stratigraphic nomenclature by the U.S. Geological Survey 1967: U.S. Geological Survey Bulletin 1274-A, p. A27-A35.
Symbol Tsk
Unit name Sitkinak Formation
Description Conglomerate and cross-bedded sandstone and siltstone containing coal fragments.
Lithology Sedimentary

Correlated geologic units

Label Tsk
Description Sitkinak Formation,
Geologic age Oligocene
Geologic setting Sedimentary, slope-and-deep-water
Lithology Form Importance
Conglomerate < Clastic < Sedimentary Bed Major
Sandstone < Clastic < Sedimentary Bed Indeterminate, major
Siltstone < Clastic < Sedimentary Bed Indeterminate, major