Mixed volcanic rocks of southeast Alaska

Unit symbol: Dmv
Age range Middle to Early Devonian (419.2 to 382.7 Ma)
Lithology: Igneous - Volcanic
Group name: Basalt, andesite, and sedimentary rocks
Consists of mafic to felsic amygdaloidal pillows, agglomerate, tuff, and breccia, which are locally metamorphosed to greenstone, and contains minor associated volcanic conglomerate, sandstone, siltstone, argillite, limestone, and small plugs and dikes (Muffler, 1967; Brew and others, 1984; Brew and Ford, 1985; D.A. Brew, written commun., 1997) which are primarily exposed in two areas of southeast Alaska. In the Glacier Bay area, unit is associated with the Black Cap Limestone (unit Dlse, here) in the Glacier Bay area (D.A. Brew, written commun., 1997). In the Craig quadrangle, the unit includes the St. Joseph Island and Coronados Volcanics, rhyolite of Kasaan Island, as well as volcanic rocks of the Karheen Formation. The St. Joseph Island Volcanics are as much as 3,000 m thick and although no fossils have been reported from included sedimentary rocks, a lamprophyre dike cutting the unit has been dated at 335±10 Ma (K/Ar, biotite) and provides a minimum age for the unit (Eberlein and others, 1983). Compositionally similar to the Glacier Bay region, these are also basaltic and subordinate andesitic pillow flows, breccia, aquagene tuff, and minor sedimentary interbeds (Gehrels, 1992). Early Devonian rhyolite and dacite is found in tuffs, flows, and dikes on east-central Prince of Wales Island (Eberlein and others, 1983) and in the Ketchikan and Prince Rupert quadrangles (S.M. Karl, unpublished data)

Source map information

Source map Brew, D.A., compiler, 1996, Geologic map of the Craig, Dixon Entrance, and parts of the Ketchikan and Prince Rupert quadrangles, southeastern Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-2319, 53 p., 1 sheet, scale 1:250,000.
Symbol Dsj
Unit name St. Joseph Island volcanics
Description Massive, locally amygdaloidal and pillowed, basalt flows; also basaltic breccia, tuff, agglomerate, and minor siltstone, mudstone, and sandstone; about 2000 to 3000 m thick; nonfossiliferous; cut by lamprophyre dike (K/Ar 335 +/- 10 Ma biotite)
Lithology Igneous

Correlated geologic units

Label Dv
Description Basalt and rhyolite on Prince of Wales Island and andesite associated with Black Cap Limestone
Geologic age Early-Devonian to Middle-Devonian
Geologic setting Extrusive
Lithology Form Importance
Andesite < Mafic-volcanic < Volcanic < Igneous Flow Major
Basalt < Mafic-volcanic < Volcanic < Igneous Flow Indeterminate, major
Dacite < Felsic-volcanic < Volcanic < Igneous Pyroclastic, tuff Minor
Rhyolite < Felsic-volcanic < Volcanic < Igneous Dome Minor