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 Riehle, J.R., Detterman, R.L., Yount, M.E., and Miller, J.W., 1993, Geologic map of the Mount Katmai quadrangle and adjacent parts of the Naknek and Afognak quadrangles, Alaska: U.S. Geological Survey Miscellaneous Investigations Series Map I-2204, 1 sheet, scale 1:250,000.
Symbol Tab
Unit name Volcanic rocks of Barrier Range
Description Breccia, lava flows, sills, and local pyroclastic and epiclastic tuff located southeast of active part of Aleutian volcanic arc, from Katmai River to Kukak Bay. Chiefly dark-reddish-brown or greenish-brown, porphyritic lava flows of andesitic and dacitic composition that have phenocrysts of plagioclase and pyroxene (± hornblende). Tuffs and breccias are light to medium greenish brown. Propylitic alteration is extensive and argillic or potassic alteration, together with quartz and (or) calcite veining and silicification, occur locally such as near contacts with hypabyssal intrusive rocks of unit Ti. Large areas of oxidized iron marked by red and orange are common. The maximum thickness of about 800 m occurs along axis of open syncline that trends through Kukak Bay. Unit overlies unit Th, in part along irregular or gradational contact marked by sills in unit Th and by tightly folded sedimentary rocks. Deformed sedimentary rocks are inferred to have been nonlithified at time of intrusion or extrusion, indicating near-synchroneity of oldest volcanic deposits with youngest strata of unit Th
Lithology Igneous

Correlated geologic units

Label Tab
Description Miocene and Pliocene andesite and basalt
Geologic age Late-Oligocene to Early-Pliocene
Geologic setting Extrusive
Lithology Form Importance
Hypabyssal-andesite < Mafic-hypabyssal < Hypabyssal < Igneous Dome Major
Hypabyssal-basalt < Mafic-hypabyssal < Hypabyssal < Igneous Dome Major
Hypabyssal-dacite < Felsic-hypabyssal < Hypabyssal < Igneous Dome Incidental
Mafic-volcanic < Volcanic < Igneous Pyroclastic Incidental