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Source map Karl, S.M., and Brew, D.A., 2002, Unpublished data.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 Tavb
Unit name Basalt
Description On Kupreanof Island, basalt and other mafic extrusive rocks. Dense, dark gray aphanitic basalt. Flows as much as several meters thick. Commonly vesicular and amygdaloidal, containing fillings of calcite, epidote, chalcedony, chlorite, and zeolites. Flows may contain magnetite, pyroxene, olivine. Flows may be separated by lenses or cm- to meter-scale layers of tuff or flow breccia. Dark gray, rusty-weathering, platy, blocky, or columnar jointed flows 50 cm to several meters thick. Commonly vesicular and amygdaloidal; amygdule fillings include calcite, epidote, chalcedony, chlorite, and zeolites. In Bradfield Canal area, basalt and andesite(?) flows and breccia are cut by sharp-edged dikes of medium-grained, massive-appearing gabbro that is presumed to be congenic with the basalt. Admiralty, Medium to dark gray basalt flows and breccia, with sparse plagioclase, augite, and olivine phenocrysts. Locally amygdaloidal. Massive, bench-forming meter-scale cooling units. Basal flows of the Admiralty volcanic pile are basalt, also late dikes that cut all volcanic phases are basalt. Medium to dark gray basalt flows and breccia, with sparse plagioclase, augite, and olivine phenocrysts. Gray, green, and brown massive flow breccia of intermediate to mafic composition at Whitewater Bay. Locally amygdaloidal. Massive, bench-forming meter-scale cooling units. Basal flows of the Admiralty volcanic pile are basalt, also late dikes that cut all volcanic phases are basalt. Icy Strait
Lithology Igneous

Correlated geologic units

Label Tavb
Description Admiralty Island and Cenotaph Volcanics, basalt
Geologic age Oligocene to Early-Miocene
Geologic setting Extrusive
Lithology Form Importance
Basalt < Mafic-volcanic < Volcanic < Igneous Flow Major