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 KJsp
Unit name Pelitic semischists
Description Dark gray semischistose mudstone, pelitic phyllite, and muscovite schist that locally contain biotite, almandine garnet, andalusite, sillimanite, and pyrite. The protolith is dominantly mudstone, subordinate thin sandstone layers are locally preserved and have relict turbidite bedding structures, slump structures, and soft sediment deformational features. At Mount Muravief, the protolith is mélange that consists of blocks of greenstone, volcanic wacke, and chert in a mudstone matrix. Along strike in Whale Bay the pelitic semischist includes metagraywacke layers, and along strike in Necker Bay, it contains lenses of limestone up to 2 meters thick. The pelitic semischist may represent a mudstone-rich facies of the Sitka Graywacke that contains olistostromal blocks of volcanic rocks and limestone. These rocks may correspond to argillite that contains lenses of chert with Jurassic? radiolarians (map locality F7, table 3) and limestone in the Sitka Graywacke in Sitka Sound. The pelitic semischist may represent toe of slope deposits within the Sitka Graywacke, or may represent out-of-sequence thrust panels of the mélange facies of the accretionary complex, and may correlate with the Khaz Complex (KJkk). At Mt. Muravief, the pelitic unit has a pervasive fabric, and has a sharp, steep, fault contact with the graywacke semischist unit. The pelitic semischist is tightly folded, locally rodded, highly strained, and sandstone and chert beds are extended into strings of boudins. Early quartz veins that cut the unit are folded and boudinaged; late quartz veins fill tension gashes and are locally offset on steep sinistral faults.
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