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 Brew, D.A., compiler, 1996, Geologic map of the Craig, Dixon Entrance, and parts of the Ketchikan and Prince Rupert quadrangles, southeastern Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-2319, 53 p., 1 sheet, scale 1:250,000.
Symbol QTvb
Unit name Basaltic to rhyolitic breccia and tuff
Description Poorly consolidated, iron-stained; weathers brown and reddish brown. Breccia is associated with similar dikes that contain xenoliths of agglomerate derived from Descon Fm. Unit exposed on NE Prince of Wales Is.
Lithology Igneous

Correlated geologic units

Label Tav
Description Admiralty Island volcanics, undivided
Geologic age Oligocene
Geologic setting Extrusive
Lithology Form Importance
Andesite < Mafic-volcanic < Volcanic < Igneous Flow Indeterminate, major
Basalt < Mafic-volcanic < Volcanic < Igneous Flow Indeterminate, major
Rhyolite < Felsic-volcanic < Volcanic < Igneous Flow Minor
Conglomerate < Clastic < Sedimentary Bed Incidental
Sandstone < Clastic < Sedimentary Bed Incidental