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Prince of Wales Island (Eberlein and others, 1983) and in the Ketchikan and Prince Rupert quadrangles (S.M. Karl, unpublished data)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Source map Karl, S.M., 2012, Unpublished data.
Symbol Dmf
Unit name Dacite and rhyolite
Description Massive, buff to light gray, albite and quartz porphyritic rhyolite, and welded rhyolite tuff, interlayered with fossiliferous Emsian limestone on Kasaan Island. The limestone contains rhyolite clasts. Plugs of flow-banded rhyolite also intrude the Emsian limestone on Kasaan Island. Thin sections of rhyolite contain corroded quartz phenocrysts and euhedral albite phenocrysts in a groundmass of albite and orthoclase microlites with trachytic texture (Sainsbury, 1961). Sainsbury also reports the presence of phenocrysts of oligoclase and corroded garnet, and minor hornblende in the groundmass of some thin sections. In the Port Saint Nicholas area, felsic volcanic rocks are interbedded with dark gray argillite. In the Brownson Bay area, tan, red, and gray plagioclase porphyritic dacite(?) tuff, tuff breccia, and flows are interbedded with red-weathering sandstone, mudstone, and conglomerate (Gehrels, 1992). Dacite(?) is also present as dikes, sills, and hypabyssal intrusive bodies near the head of Tah Bay. Thin sections show subhedral andesine in a matrix of altered plagioclase microlites, calcite, iron oxides, and ferro-magnesian grains with pigeonite cores (Gehrels, 1992).
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