Volcanic rocks of the Aleutian Islands and Alaska Peninsula

Unit symbol: Tvm
Age range Tertiary, Miocene (23.03 to 5.333 Ma)
Lithology: Igneous - Volcanic
Group name: Younger volcanic rocks, undivided
Consists of subaerial hornblende- and pyroxene-andesite and basalt flows, sills, and plugs largely restricted to the Aleutian Islands and the Pacific coast of the Alaska Peninsula. Extrusive rocks of unit typically cap ridges and consist of massive lava flows, agglomerate, and lahar deposits; unit also includes minor small intrusive bodies. Minor propylitic alteration is characteristic of these rocks. Locally, these rocks are the extrusive rocks associated with the plutons of map unit Tmi. In the Aleutian Islands, also includes breccia, tuff, and marine conglomerate on Amchitka Island that exceeds 1,000 ft (305 m) in thickness (Powers and others, 1960). Includes the Chitka Point Formation as redefined by Carr and others (1970) who extended the outcrop area of the Chitka Point by including andesitic rocks originally defined as part of the Amchitka and Banjo Point Formations (Powers and others, 1960) and excluding basaltic rocks that they considered to be part of the Banjo Point Formation. The conglomerate of the Chitka Point Formation on Amchitka Island consists primarily of well-rounded to subrounded cobbles of porphyritic andesite; also less abundant, but common, are clasts derived from the Amchitka Formation (Powers and others, 1960). At the type locality of the Chitka Point Formation, the conglomerate contains abundant carbonized fragments of woody material, which suggests proximity to land. A coal sample from the conglomerate yielded numerous pollen and spores, probably of middle to late Miocene age (E. Leopold, cited in Carr and others, 1970). In addition, Carr and others (1970) reported late Miocene K/Ar ages (minimum age of 14.6±1.1 to 12.8±1.1 Ma) from flows within the Chitka Point Formation and therefore assigned a Miocene age, revising the Quaternary or Tertiary age assignment by Powers and others (1960). Unit also includes dikes and small intrusive bodies largely of basalt and andesite on southern Adak and Kagalaska Islands (Coats, 1956b, c). In the Russian Mission, Dillingham, and Iditarod quadrangles of southwest Alaska, rocks assigned to this unit generally consist of very fine-grained to aphanitic, dark- to medium-gray, locally vesicular basalt and basaltic andesite; K/Ar ages are between 19.35±0.58 and 6.19±0.19 Ma (Patton and others, 2006; Wilson and others, 2006a, in press [SIM 2942]; Miller and Bundtzen, 1994)

Source map information

Source map Wilson, F.H., Detterman, R.L., Miller, J.W., and Case, J.E., 1995, Geologic map of the Port Moller, Stepovak Bay, and Simeonof Island quadrangles, Alaska: U.S. Geological Survey Miscellaneous Investigation Map I-2272, 2 sheets, scale 1:250,000.
Symbol Tv
Unit name Volcanic rocks
Description Andesite and basalt flows, sills, and plugs. Extrusive rocks of unit typically cap ridges and consist of massive lava flows, agglomerate, and lahar deposits; unit also includes minor small intrusive bodies, for example, in San Diego Bay-Albatross Anchorage area of Port Moller C-2 1:63,360-scale quadrangle. Minor propylitic alteration is characteristic of these rocks, except in area around San Diego Bay, where sericitic and argillic alteration is pervasive.
Lithology Igneous

Correlated geologic units

Label Tv
Description Miocene volcanic rocks, undivided
Geologic age Miocene to Late-Miocene
Geologic setting Extrusive
Lithology Form Importance
Volcanic < Igneous Major
Hypabyssal-andesite < Mafic-hypabyssal < Hypabyssal < Igneous Dike or sill Indeterminate, major
Hypabyssal-basalt < Mafic-hypabyssal < Hypabyssal < Igneous Dike or sill Indeterminate, major
Andesite < Mafic-volcanic < Volcanic < Igneous Flow Indeterminate, major
Andesite < Mafic-volcanic < Volcanic < Igneous Flow Indeterminate, major
Basalt < Mafic-volcanic < Volcanic < Igneous Flow Indeterminate, major
Basalt < Mafic-volcanic < Volcanic < Igneous Flow Indeterminate, major