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 Johnson, B.R., and Karl, S.M., 1985, Geologic map of western Chichagof and Yakobi Islands, southeastern Alaska: U.S. Geological Survey Miscellaneous Investigations Series Map I-1506, pamphlet, 15 p., 1 sheet, scale 1:125,000.
Symbol Ksh
Unit name Hornfelsed Sitka Graywacke
Description Regionally metamorphosed to prehnite-pumpellyite facies with prehnite typically occurring in veinlets or clots. In the vicinity of Tertiary (?) plutons, has been thermally metamorphosed to at least hornblende hornfels facies. Metamorphic mineral include biotite, garnet, and andalusite. On western Yakobi Island, banded quartzo-feldspathic gneiss and migmatite occur at the contacts with Tertiary (?) tonalite and gabbronorite plutons.
Lithology Metamorphic

Correlated geologic units

Label KJsgs
Description Sitka Graywacke graywacke and argillite
Geologic age Late-Cretaceous to Cretaceous
Geologic setting Sedimentary, slope-and-deep-water
Lithology Form Importance
Graywacke < Sandstone < Clastic < Sedimentary Bed Major
Argillite < Metaclastic < Metasedimentary < Metamorphic Zeolitic (prehnite-pumpellyite) Indeterminate, major