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 Karl, S.M., and Brew, D.A., 2002, Unpublished data.
Symbol DSal
Unit name Calcareous mudstone, wacke, and limestone
Description Light tan to gray, thin to medium bedded, calcareous mudstone and siltstone turbidites, with subordinate green and tan calcareous graywacke turbidites, siliceous turbidites, and limestone. In Glacier Bay area, includes thin-bedded argillaceous and calcareous sedimentary rocks of the Rendu Formation, named by Rossman, (1963), which overlies the Pyramid Peak Formation. Rendu contains more limestone than the Tidal Formation, and is locally silicified to cherty brown, red, yellow, green, blue, black, and white 'hornstone' (Rossman, 1963). South of Tsirku River, unit includes white, gray, and black siliceous argillite interbedded with light gray to black marble, locally with alternating bands of argillite and marble, 1 mm to 10 cm thick, locally crossbedded, and light gray, very argillaceous schistose marble, and buff weathering, tan calc-schist and calc-phyllite. Commonly contains quartz segregations up to 15 cm thick. Contains minor intervals of muscovite schist (metatuff), and metabasite, argillaceous marble, and black phyllite, in intervals up to 3 m thick. Thin-bedded, fine-grained, rhythmically bedded, purple and tan marble. Gray and black marble also common, with argillite and greenstone locally present. Also includes 11-km long band of fine-grained, black to dark gray marble west of Takhin Glacier, consisting of limestone beds a few cm to 10 m in thickness, with thin bands of light colored argillite up to 2 cm in thickness, alternating with carbonate bands 1 mm to 10 cm thick. Marble locally retains primary crossbedding. On southern Admiralty Island, unit includes calcareous turbidites, transitional between Silurian turbidites and Upper Devonian limestones at Carroll Island. On Prince of Wales Island, unit includes the Staney Creek Formation of Eberlein and others (1983), consisting of light gray to pale yellow and pink laminated limestone, buff, light gray or tan mudstone, and gray, green, and tan calcareous sandstone. Locally includes fossiliferous limestone, forming meter-scale bioherms that are laterally and vertically gradational to thin-bedded brown calcareous, locally fossiliferous mudstone and green, rhythmically bedded fossiliferous volcaniclastic sandstone that has turbidite structures. Limestone contains brachiopods, corals, gastropods, dasycladacean algae. Subordinate clast-supported and matrix-supported conglomerate contain rounded clasts of dominantly intermediate volcanic rocks, with a few per cent each of limestone, brown chert, green chert, and intermediate to mafic intrusive rocks. Only the limestone clasts are angular; some limestone clasts show soft-sediment deformation. In some places there are cm to m scale channels of conglomerate scoured into the bedded limestone, and both lithologies are overlain by greenish brown lime mudstone grading to siltstone. In some places massive conglomerate overlies dark gray, petroliferous, fossiliferous limestone. In some places, dark limestone with brachiopods in growth position overlies the calcareous mudstone. Sandstone contains calcite, feldspar, quartz, volcanic rock fragments, and locally contains detrital biotite. The Staney Creek unit depositionally overlies Heceta limestone and is gradational to the overlying redbeds of the Karheen Formation. Unit also depositionally overlies Descon Formation and Luck Creek Breccia and is transitional to the Karheen Formation. On Luck Creek Breccia, 200 m of conglomerate with dominantly volcanic clasts overlies andesitic breccia; the conglomerate is overlain by 10-50 meters of grayish brown or grayish green mm laminated limestone and calcareous siltstone, which is overlain by up to 100 m of medium-grained, medium-bedded red and green sandstone, which is overlain by up to 100 m of medium-bedded dark gray limestone that contains corals and brachiopods in growth position. Rapid lateral and vertical facies changes are interpreted to represent a shallow water depositional environment.
Lithology Sedimentary

Correlated geologic units

Label DSr
Description Rendu Formation and related units, argillite and silty limestone turbidite
Geologic age Ludlow to Silurian
Geologic setting Undivided
Lithology Form Importance
Marble < Metacarbonate < Metasedimentary < Metamorphic Major
Argillite < Metaclastic < Metasedimentary < Metamorphic Major
Siltstone-mudstone < Mixed-clastic < Clastic < Sedimentary Bed Indeterminate, major