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Source map Karl, S.M., and Brew, D.A., 2002, Unpublished data.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 Tava
Unit name Andesite
Description Light to medium gray-weathering, commonly plagioclase porphyritic, locally hornblende or augite porphyritic, andesite flows and breccia form massive, bench-forming meter-scale cooling units, alternate with centimeter- to meter-scale lenses of water-laid gravel and conglomerate, forming distinctive topography on southeastern Admiralty Island. Andesite is dark gray fresh, green to maroon altered, blocky weathering, pyroxene and feldspar porphyritic, massive to vesicular and amygdaloidal flows 10-50 cm thick. On the Sisters Islands in Icy Strait, basalt and andesite flow breccia are dark greenish gray and locally altered to epidote. At the intersection of Icy Strait and Chatham Strait, Hanus Reef is composed of basalt, andesite, and dacite flows, agglomerates, and tuff. The Knob on Pleasant Island consists of undated augite basalt that overlies andesitic breccia and volcaniclastic rocks. Pyroclastic deposits and massive andesite of unknown age that range up to hundreds of feet in thickness are interlayered on Kupreanof Island. Unit also includes and andesite stock in Chilkat Mountains
Lithology Igneous

Correlated geologic units

Label Tava
Description Admiralty Island and Cenotaph Volcanics, andesite
Geologic age Oligocene to Early-Miocene
Geologic setting Extrusive
Lithology Form Importance
Andesite < Mafic-volcanic < Volcanic < Igneous Flow Indeterminate, major
Conglomerate < Clastic < Sedimentary Bed Incidental