Volcaniclastic sedimentary rocks

Unit symbol: Tarcs
Age range Tertiary, Oligocene to Eocene (38 to 23.03 Ma)
Lithology: Sedimentary
Group name: Volcanic and sedimentary rocks
Unit includes rocks of the Gunners Cove, Banjo Point, and Andrew Lake Formations of the Aleutian Islands and the Stepovak Formation of the Alaska Peninsula. Tuffaceous conglomerate and sandstone, crystal-vitric basaltic tuff, thin basalt flows, and basaltic dikes (Lewis and others, 1960) form two-thirds of Hawadax (formerly “Rat”) Island (Gunners Cove Formation) and are also exposed on Amchitka Island (Banjo Point Formation). Sandstone and conglomerate contain moderately to well-rounded mafic volcanic clasts, primarily basalt, in a matrix of basaltic glass and fragments of marine shells. One conglomerate outcrop along the north shore of Gunners Cove was especially rich in fragmentary barnacles, crinoids, echinoids, and pectinid bivalve fossils. Basalt occurs in crudely bedded masses of glassy scoria, in thin dike swarms, and in thin flows, some of which have pillows and local columnar jointing (Lewis and others, 1960). Identified fossils include Isocrinus aff. I. oregonensis (Moore and Vokes) and Chlamys aff. C. washburnei Arnold of probable Oligocene or early Miocene age. Andrew Lake Formation of Adak Island consists of tuffaceous sandstone and siltstone and siliceous and cherty shale interbedded with mafic flows or penecontemporaneous sills. “Graded sandstone beds as much as 0.3 m thick are common and alternating beds of sandstone and shale * * * form graded sequences 3 to 4 m in thickness” (Scholl and others, 1970, p. 3586). Stepovak Formation of the southwestern Alaska Peninsula was divided into two informal members by Detterman and others (1996). An incomplete reference section has 2,030 m exposed, approximately evenly divided into a lower siltstone member and an upper sandstone member (Detterman and others, 1996). Lower member is deep-water turbidite deposit that is composed of dark brown laminated siltstone and shale, as well as interbedded sandstone that commonly shows graded bedding and rip-up clasts. Upper member, rich in unaltered volcanic debris, was deposited in a shallow-water shelf environment; megafauna distributed throughout upper member are characteristic of water depths no greater than 30 to 50 m (Louie Marincovich, Jr., written commun., 1983 to 1986). Unit is age equivalent of volcanic rocks mapped as Meshik Volcanics (Tmv). Upper and lower contacts of Stepovak Formation are structurally conformable with Unga and Tolstoi Formations (units Tuu and Ttk, respectively) but are considered disconformities because considerable time gaps exist between younger and older units

Source map information

Source map Wilson, F.H., Weber, F.R., Dochat, T.M., Miller, T.P., and Detterman, R.L., 1997, Revised geologic map of the Cold Bay and False Pass quadrangles, Alaska Peninsula: U.S. Geological Survey Open-File Report 97-866, 34 p., 1 sheet, scale 1:250,000.
Symbol Ts
Unit name Stepovak Formation
Description In the map area, rocks exposed in the vicinity of Indian Head on Belkofski Bay, in part originally mapped as the arkose unit of Kennedy and Waldron (1955, unit Ta), are herein assigned to the Stepovak Formation, presumably the upper member. They consist of dusky yellow-green to green, very fine- to coarse-grained, angular to subrounded, volcaniclastic sandstone containing minor interbeds of dark-gray to olive-gray siltstone. Some sandstone layers are mottled green and white, indicating that they are laumontitic and tuffaceous. Unit is faintly bedded, having low-angle tabular crossbeds. Locally, it contains considerable pebble conglomerate and dark olive-gray siltstone containing plant debris; the siltstone is locally intensely bioturbated. Lower member is a deep-water turbidite deposit composed of dark-brown laminated siltstone and shale and interbedded sandstone that commonly shows graded bedding and rip-up clasts. Upper member, which is rich in unaltered volcanic debris, was deposited in a shallow-water shelf environment.
Lithology Sedimentary

Correlated geologic units

Label Tst
Description Stepovak and Andrew Lake Formations
Geologic age Late-Eocene to Early-Oligocene
Geologic setting Sedimentary, shallow-marine-siliciclastic
Lithology Form Importance
Sandstone < Clastic < Sedimentary Bed Major
Sandstone < Clastic < Sedimentary Bed Major
Siltstone < Clastic < Sedimentary Bed Major
Siltstone < Clastic < Sedimentary Bed Major