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Prince of Wales Island (Eberlein and others, 1983) and in the Ketchikan and Prince Rupert quadrangles (S.M. Karl, unpublished data)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 Gehrels, G.E., 1992, Geologic map of southern Prince of Wales Island, southeastern Alaska: U.S. Geological Survey Miscellaneous Investigations Series Map I-2169, 23 p., 1 sheet, scale 1:63,360.
Symbol Dkbv
Unit name Karheen Formation - basaltic to andesitic volcanic rocks
Description Basaltic to andesitic rocks present as flows, breccia, dikes, and small intrusive bodies in Tah Bay region and as a thin pillow flow just west of Bert Millar Cutoff. In both regions, these volcanic rocks are interbedded with, and overlain by, clastic strata of the lower part of the Karheen Formation (Dks and Dkcg). In thin section, rocks consist of highly altered, millimeter-scale plagioclase phenocrysts in a matrix of very fine grained opaque minerals, chlorite, calcite, epidote, and other secondary minerals. These rocks resemble andesitic breccia and flows present in uppermost part of the Karheen Formation north of study area (Herreid and others, 1978; Gehrels and Saleeby, 1987)
Lithology Igneous

Correlated geologic units

Label Dcv
Description Karheen Formation volcanic and hypabyssal rocks
Geologic age Devonian to Middle-Devonian
Geologic setting Extrusive
Lithology Form Importance
Basalt < Mafic-volcanic < Volcanic < Igneous Flow Major
Mafic-volcanic < Volcanic < Igneous Pyroclastic, tuff Minor
Andesite < Mafic-volcanic < Volcanic < Igneous Flow Minor
Hypabyssal-dacite < Felsic-hypabyssal < Hypabyssal < Igneous Incidental
Hypabyssal-rhyolite < Felsic-hypabyssal < Hypabyssal < Igneous Incidental
Clastic < Sedimentary Bed Incidental