Volcanic rocks in southeast Alaska

Unit symbol: Sv
Age range Silurian (443.4 to 419.2 Ma)
Lithology: Igneous - Volcanic
Group name: Basalt, andesite, and sedimentary rocks
Includes volcanic rocks associated with the Bay of Pillars and Point Augusta Formations and the informal Berg Mountain formation of Brew (1997) in southern and northern southeast Alaska, respectively. The northern exposures of this unit consist of dark green, weakly metamorphosed basalt and subordinate white, green, and lavender chert and minor basaltic agglomerate, chert breccia, graywacke, argillite, and limestone (Gilbert, 1988). Basalt commonly displays pillow structures. The southern exposures of this unit consist of mafic to intermediate volcanic breccia, agglomerate, and flows, as well as greenschist and greenstone derived from these rocks (Brew and others, 1984). It also locally includes massive, dark-green to dark-red, dominantly matrix-supported conglomerate that has a matrix of massive volcanic sandstone and is associated with subordinate volcaniclastic turbidites. Conglomerate contains both rounded and angular clasts, as well as angular olistostromal blocks of contemporaneous, plastically-deformed sandstone turbidites and limestone. Gradationally overlies volcanic flows and mudstone turbidites of the Bay of Pillars Formations (S.M. Karl, USGS, and James Baichtal, U.S. Forest Service, unpub. data). Rocks of the northern part of unit have been variously assigned to or associated with the Berg Mountain, Point Augusta, Rendu, and Tidal Formations, whereas rocks of the southern exposures of this unit are typically associated with the Bay of Pillars Formation

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 DSbb
Unit name Basal graywacke and volcanic rock unit
Description ALEXANDER BELT: GLACIER BAY SEQUENCE: VOLCANIC AND ASSOCIATED ROCKS: Poorly documented basal conglomerate part of the thick volcanic pile; exposed only in central part of Chilkat Mountains.
Lithology Sedimentary

Correlated geologic units

Label DSv
Description Berg Mountain formation of Brew (1997), largely mafic volcanic rocks
Geologic age Silurian to Devonian
Geologic setting Extrusive
Lithology Form Importance
Basalt < Mafic-volcanic < Volcanic < Igneous Volcaniclastic, volcanic breccia Major