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 Kss
Unit name Graywacke semischist
Description Semischistose graywacke and mudstone containing biotite, almandine garnet, andalusite, and sillimanite, mapped south of the Crawfish Inlet pluton. The dominant protolith is massive to thick-bedded sandstone; rhythmic turbidite bedding structures are locally preserved. Mudstone is metamorphosed to sericitic phyllite. The semischist and phyllite show evidence for two generations of folding and have a strong lineation that predates thermal metamorphism. The semischist is highly strained; pelitic layers are locally rodded and have quartz pressure shadows around porphyroclasts. The deformation fabric is northwest-striking and is truncated by the Eocene Redfish Bay and Crawfish Inlet pluton margins. The protolith of the semischistose graywacke is inferred to be the Sitka Graywacke (Ks) that lies along strike with the semischist unit on the north side of the Crawfish Inlet pluton. South of the Crawfish Inlet pluton, the graywacke has a pervasive regional metamorphic fabric and greenschist to amphibolite facies metamorphic mineral assemblages. The Sitka Graywacke has a minimum age of 50 Ma based on the age of the Crawfish Inlet and other crosscutting plutons; the maximum pre metamorphic age is constrained by detrital zircon age peaks as young as 72 Ma in outboard structural panels (table 2, Haeussler and others, 2006)
Lithology Metamorphic

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