Sitka Graywacke, undivided

Unit symbol: Ksg
Age range Cretaceous (100.5 to 66 Ma)
Lithology: Sedimentary
Group name: Chugach accretionary complex
Consists of sandstone and mudstone turbidites and subordinate conglomerate on Baranof, Chichagof, Kruzof, and Yakobi Islands (Loney and others, 1975; Decker, 1980; Johnson and Karl, 1985). Part of what is commonly called the Chugach accretionary complex, the Sitka Graywacke is lithologically similar to the Chugach flysch (see unit Kaf, above). Detrital zircon studies (Haeussler and others, 2006) suggest the unit includes two age-distinct subunits. The youngest and western part of the unit yields detrital zircons that have minimum age populations equivalent to the Late Cretaceous fossil ages of the Chugach flysch, which indicates a maximum depositional age of Campanian(?) or Maastrichtian. The eastern part of the unit yields an Albian maximum age on the baisis of detrital zircon populations, suggesting the presence of an earlier depositional system. As originally described, the Sitka Graywacke was thought to be Jurassic, in part; however, the detrital zircon data suggests this is unlikely. Strata represent deep-water marine trench, slope-basin, and fan deposits. Sitka Graywacke is moderately deformed and disrupted, regionally metamorphosed to as high a grade as greenschist facies in some areas, and thermally upgraded to hornblende-hornfels facies locally (Decker and others, 1979; Johnson and Karl, 1985). Common rock types in metamorphosed regions south of Cross Sound include metagraywacke and argillite. Early Cretaceous fossils were found in the Sitka Graywacke on Kruzof Island (Reed and Coats, 1941), and the detrital zircon data (Haussler and others, 2006) suggests a long depositional history for the Sitka Graywacke; minimum age of these strata is constrained by Eocene granodiorite (unit Toegr) on Baranof Island (Loney and others, 1975; Reifenstuhl, 1986; Bradley and others, 2003; S.M. Karl, unpub. data)

Source map information

Source map Karl, S.M., Haeussler, P.J., Himmelberg, G.R., Zumsteg, C.L., Layer, P.W., Friedman, R.M., Roeske, S.M.,and Snee, L.W., 2015, Geologic map of Baranof Island: U.S. Geological Survey Scientific Investigations Map SIM-3335, pamphlet 82 p., 1 sheet, scale 1:250,000.
Symbol Ks
Unit name Sitka Graywacke
Description Tan-weathering, light to dark gray-fresh, massive to thin-bedded, medium- to fine-grained graywacke, named the Sitka Group by Berg and Hinckley (1963) and the Sitka Graywacke by Loney and others (1963). Unit includes massive graywacke sandstone in beds up to 10 meters thick, amalgamated beds with rafts of sandstone and mudstone, medium- to thick-bedded sandstone, graded beds, laminated beds, slumped beds, thin rhythmic beds of sandstone and mudstone, full Bouma sequence turbidites, argillite with slump structures and soft sediment deformational features, and massive polymictic conglomerate. Sandstone locally contains web structures indicating deformation during a semilithified state (Cowan, 1982b). Sandstone and siltstone turbidites are dominant, with subordinate meter scale lenses of argillite, conglomerate, and basalt flows, sills, and dikes. Conglomerate is massive, mainly matrix-supported, but locally clast-supported. Typical proportions of conglomerate clast types are 50 per cent argillite, 25 per cent mafic to felsic volcanic rocks, 20 per cent graywacke; a few per cent granite, diorite, chert, and vein quartz, and rare limestone and greenstone. The Sitka Graywacke was correlated with flysch of the Valdez Group, the Kodiak Formation, and the Yakutat Group by Plafker and Berg (1994)
Lithology Sedimentary

Correlated geologic units

Label KJsgu
Description Sitka Graywacke, undivided
Geologic age Late-Cretaceous to Cretaceous
Geologic setting Sedimentary, slope-and-deep-water
Lithology Form Importance
Mica-schist < Schist < Metamorphic Greenschist Major