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Prince of Wales Island (Eberlein and others, 1983) and in the Ketchikan and Prince Rupert quadrangles (S.M. Karl, unpublished data)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 Dph
Unit name Plagioclase-porphyritic hypabyssal rocks
Description Plagioclase porphyry. Euhedral plagioclase (interlocking andesine and labradorite) phenocrysts range up to 1.5 cm in length. Rock also contains 15 to 30 per cent diopsidic augite as subhedral grains intergrown with plagioclase. Locally clinopyroxene has rims of green or reddish-brown hornblende and opaque minerals. Interstitial microcline and microperthite. Variable abundance of K-feldspar. Skeletal opaque grains range up to 3 mm across. Miarolitic cavities are filled with chlorite, white mica, calcite, and opaque minerals. Unit intrudes rocks of the Karheen Formation.
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