Turbidite deposits of southeast Alaska

Unit symbol: St
Age range Silurian (443.4 to 419.2 Ma)
Lithology: Sedimentary
Group name: Shale, chert, and argillite
Thin- to thick-bedded lithic graywacke, siltstone, and argillite turbidites, and interbedded conglomerate, limestone, and volcanic rocks (Rossman, 1963; Muffler, 1967; Loney and others, 1975; Eberlein and others, 1983; Brew, 1996). Turbidite beds commonly have laminar, cross-bedded, graded, and load cast structures typical of full and partial Bouma sequences (Muffler, 1967; Karl and Giffen, 1992). The olistostromal conglomerate is commonly polymictic and contains clasts of granitic and gabbroic rocks, greenstone, graywacke, chert, and limestone blocks thought to have been derived from local sources, but is locally monomictic (Muffler, 1967; Brew and others, 1984; Karl and Giffen, 1992; Brew, 1996; Karl and others, 1999). Sandstone grains consist of volcanic rock fragments, mudstone, slate, chert fragments, and grains of plagioclase, calcite, and quartz (Muffler, 1967; Karl and Giffen, 1992). Mafic to intermediate flows, breccia, and tuff are reported by several sources (Brew and others, 1984; Brew, 1996; Karl and others, 1999); where separately mapped, they are included in unit Sv here. Includes clastic rocks of the Rendu and Tidal Formations in Glacier Bay (Rossman, 1963), the Point Augusta Formation on Chichagof Island (Loney and others, 1975), Bay of Pillars Formation on Kupreanof Island (Muffler, 1967; Brew, 1996), and polymictic olistostromal conglomerate interbedded within the Heceta Limestone (unit DSl) in the Prince of Wales Island area (Brew, 1996; Karl and others, 1999). Also includes shallow water carbonaceous sedimentary rocks of the Staney Creek area on Prince of Wales Island, which consists of limestone, siltstone, calcareous mudstone, and polymictic conglomerate (Eberlein and others, 1983). Age is primarily derived from Silurian graptolites in the Bay of Pillars and Point Augusta Formations (Muffler, 1967; Loney and others, 1975) and the sedimentary rocks of the Staney Creek area (Eberlein and others, 1983), but the unit is also locally interbedded with fossiliferous Silurian limestone (Shpa). Poorly preserved Early Jurassic radiolarians (C.D. Blome, written commun. (1996), reported by D.A. Brew, written commun., 2004), in an argillite and chert section of rocks unconformably overlying this unit in the northwestern Juneau quadrangle, are unique to southeast Alaska. As the outcrop area is extremely small, and documentation of this occurrence is lacking or conflicting, it is not shown separately here

Source map information

Source map Brew, D.A., and Ford, A.B., 1985, Preliminary reconnaissance geologic map of the Juneau, Taku River, Atlin, and part of the Skagway 1:250,000 quadrangles, southeastern Alaska: U.S. Geological Survey Open-File Report 85-395, pamphlet 23 p., 2 sheets, scale 1:250,000.
Symbol Stag
Unit name Graywacke, mudstone, turbidites, and limestone
Description ALEXANDER BELT: GLACIER BAY SEQUENCE: TURBIDITES AND ASSOCIATED ROCKS-- Interpreted to be the dominant feature of a long lived island arc environment. The limestones that are mapped separately probably vary in age and do not represent persistent stratigraphic horizons. POINT AUGUSTA FORMATION: Grayish brown to gray fresh, tan to gray weathering graywacke, mudstone and calcareous mudstone; graywackes typically very thin to medium bedded, with some thicker and amalgamated beds. Soft sediment deformation is common. The dominant rock type is calcareous graywacke with carbonate clasts, fossil fragments, subordinate feldspar, quartz, and volcanic rock fragments, and patchy recrystallized carbonate matrix.
Lithology Sedimentary

Correlated geologic units

Label Stg
Description Silurian turbidite deposits including the Bay of Pillars Formation, Point Augusta and Tidal Formation and related units
Geologic age Silurian
Geologic setting Sedimentary, slope-and-deep-water
Lithology Form Importance
Graywacke < Sandstone < Clastic < Sedimentary Bed Major
Mudstone < Clastic < Sedimentary Bed Indeterminate, major
Siltstone < Clastic < Sedimentary Bed Indeterminate, major
Conglomerate < Clastic < Sedimentary Olistrostrome Minor
Limestone < Carbonate < Sedimentary Bed Incidental