Meshik Volcanics and similar rock units

Unit symbol: Tmv
Age range Tertiary, Oligocene to Eocene (47.8 to 28.1 Ma)
Lithology: Igneous - Volcanic
Group name: Older volcanic rocks, undivided
Typically consists of coarse andesitic and basaltic volcanic rubble, lahar deposits, andesite and basalt lava flows, tuff, hypabyssal basalt and andesite plugs, lesser dacite and rare rhyolite, and minor amounts of volcaniclastic sedimentary rocks. Includes the Meshik Volcanics of the Alaska Peninsula, the Mount Galen Volcanics of central Alaska (Decker and Gilbert, 1978), the Ship Creek, Windy Fork, and Terra Cotta volcanic fields of the McGrath quadrangle (Bundtzen and others, 1997a), and the Finger Bay Volcanics of the Aleutian Islands (Coats, 1956b). Also includes the Rat Formation (Lewis and others, 1960), Amchitka Formation (Powers and others, 1960; Carr and others, 1970), and other rocks included in the “Lower Series” (Vallier and others, 1994) of the Aleutian Islands, as well as other unnamed volcanic rocks. The Meshik Volcanics represent the early phase of the Aleutian magmatic arc, named the Meshik Arc, and correspond to the “Lower Series” rocks in the Aleutian Islands. Unnamed equivalent rocks extend as far north as the Lake Clark quadrangle. The sedimentary rocks are equivalent in age and lithology to the Stepovak and Andrew Lake Formations (unit Tarcs). As described by Detterman and Reed (1980), in many cases eruptive centers can be identified, either as volcanic necks of eroded volcanoes or as caldera complexes. Potassium-argon ages on multiple samples of the Meshik Volcanics and rocks in the Aleutian Islands range from about 42 Ma to about 25 Ma (Wilson and others, 1999, 2006b); the vast majority of age determinations are between 40 and 30 Ma. Megafauna fossil collections from the Meshik Volcanics are rare, but existing collections are Eocene and Oligocene in age. The Mount Galen Volcanics, originally considered part of the Cantwell Formation (Decker and Gilbert, 1978), can be considered the northernmost part of this magmatic arc. They consist of andesite and basalt flows, tuff, and breccia of calc-alkaline affinity. The Finger Bay Volcanics of Adak and Kagalaska Islands and most of the Andreanof Islands east of Adak Island (Coats, 1956a; 1956b; Fraser and Snyder, 1959), and similar rocks on southeastern Great Sitkin Island (Simons and Mathewson, 1955; Waythomas and others, 2003) are basalt flows, hornblende-basalt tuff, flow breccia, agglomerate, basalt dikes, and subordinate rhyolite tuff and quartz porphyry dikes, intruded by large masses of gabbro and small masses of rhyolite (Coats, 1956b). The Rat Formation of Hawadax (formerly “Rat”) Island (Lewis and others, 1960) consists of flows and flow breccia of porphyritic andesite and minor conglomerate composed of andesitic debris. Altered pyroclastic deposits, pillow lavas, and minor sedimentary rocks on Ulak Island (Coats, 1956a) are included here as well. Also included is the Amchitka Formation (Powers and others, 1960; Carr and others, 1970), which is mainly volcanic agglomerate, tuff-breccia, tuff, and pillow lava flows of andesitic to latitic composition. In the McGrath quadrangle, the Ship Creek, Windy Fork, and Terra Cotta volcanic fields of Bundtzen and others (1997a) include gray-green andesite flows, light- to medium-gray, porphyritic, hornblende-bearing massive dacite and potassium-feldspar porphyritic rhyodacite flows, and a similar chemical range of tuff and lahar deposits. Rocks in these volcanic fields have yielded K/Ar ages ranging between 45.5±1.4 and 37.2±2.9 Ma (Solie and others, 1991a)

Source map information

Source map Fraser, G.D., and Snyder, G.I., 1959, Geology of southern Adak and Kagalaska Island, Alaska: U.S. Geological Survey Bulletin 1028-M, p. 371-408, map, scale 1:125,000.
Symbol fb
Unit name Finger Bay Volcanics
Description Finger Bay Volcanics of Adak and Kagalaska Islands (Fraser and Snyder, 1959). Unit mapped as Paleozoic by Coats (1956), since revised. Consists of altered andesite and basaltic pyroclastic deposits and lavas: minor amounts of pillow lava, volcanic wacke, and argillite.
Lithology Igneous

Correlated geologic units

Label Tvb
Description Meshik Volcanics (Formation); Mt. Galen Volcanics; Ship Creek, Windy Fork, and Terra Cotta volcanic fields, McGrath
Geologic age Lutetian to Rupelian
Geologic setting Extrusive
Lithology Form Importance
Dacite < Felsic-volcanic < Volcanic < Igneous Pyroclastic Major
Andesite < Mafic-volcanic < Volcanic < Igneous Flow Major
Basalt < Mafic-volcanic < Volcanic < Igneous Flow Indeterminate, major
Hypabyssal-dacite < Felsic-hypabyssal < Hypabyssal < Igneous Dike or sill Incidental