Admiralty Island and Cenotaph Volcanics

Unit symbol: Tca
Age range Tertiary, Miocene to Oligocene (33.9 to 5.333 Ma)
Lithology: Igneous - Volcanic
Group name: Older volcanic rocks, undivided
Admiralty Island Volcanics consist of a wide range of volcanic and volcaniclastic rocks primarily exposed on Admiralty, Kupreanof, and Kuiu Islands in southeast Alaska and range from rhyolite to basalt, although rhyolite is uncommon. Rhyolite is quartz and feldspar porphyritic, light-gray or light-brown when fresh and buff, white, green, lavender, maroon, or pink where altered; occurs in domes, as dikes up to several meters thick in swarms, or as breccia (Brew and others, 1984; Karl and others, 1999). Dacite found in flows that have large amygdules lined with silicate minerals and abundant inclusions of igneous rock types (Brew and others, 1984). Basalt or andesite flows are dark-gray and less commonly light-gray, grayish-green or red, commonly sparsely porphyritic and often contain phenocrysts of labradorite-bytownite feldspar, augite, and uncommon olivine and hypersthene. Tuff and breccia beds are interlayered with flows in lower part of the sequence. Locally the flows are altered and sheared along major linear features believed to be faults of considerable displacement. In several places sandstone and conglomerate are interbedded with flows and tuff beds at the base (Lathram and others, 1965). Cenotaph Volcanics are found in the Mount Fairweather quadrangle and are andesitic volcanic rocks, volcanic breccia and tuff, siltstone, sandstone, and conglomerate (D.A. Brew, written commun., 1997)

Source map information

Source map Karl, S.M., and Brew, D.A., 2002, Unpublished data.
Symbol Tavd
Unit name Felsic volcanic rocks
Description Hypabyssal dacite and felsic flows. On Kuiu Island, altered dellenite (quartz latite) extrusive rocks. Medium to light gray, flaggy-weathering, generally altered, felsic flow rock. Hypabyssal dacite and extrusive intermediate to felsic volcanic rocks. Northeast Kuiu Island and Conclusion Island. Overlies Tk sedimentary rocks. Flows on Conclusion Island have large amygdules lined with silicate minerals and abundant inclusions of igneous lithologies. On Kuiu Island, dacite overlies sedimentary rocks of the Kootznahoo Formation that contain Oligocene leaf fossils (ref.)
Lithology Igneous

Correlated geologic units

Label Tavd
Description Admiralty Island and Cenotaph Volcanics, dacite
Geologic age Oligocene to Early-Miocene
Geologic setting Extrusive
Lithology Form Importance
Hypabyssal-dacite < Felsic-hypabyssal < Hypabyssal < Igneous Indeterminate, major
Dacite < Felsic-volcanic < Volcanic < Igneous Flow Indeterminate, major