Shelikof and Chinitna Formations

Unit symbol: Jsc
Age range Middle Jurassic, Callovian (166.1 to 163.5 Ma)
Lithology: Sedimentary
Group name: Shelikof and Chinitna Formations and Tuxedni Group
Primarily consists of siltstone that is best exposed along the west side of Cook Inlet and in the Anchorage quadrangle, within fault splays that are the eastern end of the Castle Mountain Fault System. The Chinitna Formation, found on the west side of Cook Inlet, is subdivided into two members: the upper Paveloff Siltstone Member, which is approximately equivalent to the Shelikof Formation; and lower Tonnie Siltstone Member (Detterman and Hartsock, 1966). Paveloff Siltstone Member is massive, dark-gray arenaceous siltstone in its upper part and thick graywacke sandstone at its base and ranges in thickness from about 275 m to more than 400 m (Detterman and Hartsock, 1966). Large ellipsoidal concretions and lenticular beds of limestone occur throughout, and thin interbeds of sandstone occur in the siltstone. Limestone concretions and interbeds common; fresh surfaces are very dark-gray, but weather buff to cream colored. Locally the limestone is bioclastic (Detterman and Hartsock, 1966). Siltstone is well indurated; uppermost part is thinly bedded and fractures into angular fragments. Graywacke of the lower part is “…thin bedded to massive, locally lenticularly bedded, fine to coarse grained, gray to greenish gray” (Detterman and Hartsock, 1966, p. 43). Ammonites within upper part of unit include species of Cadoceras, Stenocadoceras, Pseudocadoceras, Kepplerites, Kheraiceras, and Lilloettia (Detterman and Hartsock, 1966) and indicate Middle Jurassic (Callovian) age. Paveloff Siltstone Member is the age equivalent of Shelikof Formation of the Alaska Peninsula, but the Shelikof contains a higher proportion of coarse volcanic debris and is thought to have been deposited in a deep- to shallow-water environment (Detterman and others, 1996). Megafauna are locally abundant in the Shelikof, although, in general, formation is not fossiliferous. Many lithic intervals in upper part of section have a fining-upward sequence from conglomerate to sandstone or from sandstone to siltstone (Allaway and others, 1984). Tonnie Siltstone Member of the Chinitna Formation is massive, marine, gray arenaceous shale and siltstone that contains numerous large limestone concretions, typically about 250 m thick but thickens to as much as 400 m in southern exposures. Contains mixture of plutonic and volcanic detritus, which is thought to be derived from erosion of the Talkeetna Formation (unit Jtk) and related plutonic rocks of the Early to Middle Jurassic magmatic arc. Lower part of type section is mainly thick-bedded to massive, dusky-yellowish-green graywacke and conglomerate, and minor siltstone, whereas upper part is mainly volcanic sandstone interbedded with massive and laminated brownish-gray siltstone containing calcareous sandstone clasts. Contact with underlying Kialagvik Formation is considered conformable (see Detterman and others, 1996). Upper contact is an unconformity. In the northeastern Anchorage quadrangle, the Chinitna Formation is not subdivided and may be as thick as 600 m

Source map information

Source map Detterman, R.L., Case, J.E., Wilson, F.H., and Yount, M.E., 1987, Geologic map of the Ugashik, Bristol Bay, and western part of Karluk quadrangle, Alaska: U.S. Geological Survey Miscellaneous Investigations Series Map I-1685, 1 sheet, scale 1:250,000.
Symbol Jss
Unit name Shelikof Formation -- Sandstone member
Description Massive to thick-bedded, medium to coarse-grained, dark-green to gray volcanogenic sandstone containing abundant magnetite grains (Note, owing to rapid lateral facies changes, informal subdivisions of Capps (1923) have been found impractical for mapping and have been abandoned.) Shelikof is interpreted to have been deposited in a deep- to shallow-water environment (Detterman and others, 1996).
Lithology Sedimentary

Correlated geologic units

Label Js
Description Shelikof Formation and Chinitna Formation, Pavelof Siltstone Member (Jss, Sandstone member, Jsst, Siltstone member, and Jsc, Conglomerate member, all on I-1685 now abandoned)
Geologic age Callovian
Geologic setting Undivided
Lithology Form Importance
Graywacke < Sandstone < Clastic < Sedimentary Bed Major
Siltstone < Clastic < Sedimentary Bed Major
Conglomerate < Clastic < Sedimentary Bed Minor