Basalt and keratophyre

Unit symbol: Tbk
Age range Tertiary, Paleogene, or older (66 to 28.1 Ma)
Lithology: Igneous - Volcanic
Group name: Basalt and keratophyre
Largely tuff and tuff agglomerate of generally basaltic composition and interbedded pillow lava flows on Attu and Agattu Islands, called the “Basement Rocks” by Gates and others (1971). Pyroclastic rocks range from coarse, bomb-filled lapilli tuff to fine-grained, silt-sized tuff, agglomerate, and breccia. Beds, up to 30 m thick, range from massive and poorly sorted to well bedded and graded (Gates and others, 1971). Agglomerate consists of angular to subrounded blocks up to 1 ft (30 cm) in diameter of basalt, keratophyre, and, rarely, albite granite in a mottled pale-green and white tuffaceous matrix (Gates and others, 1971, p. 718–719). Pillow lava flows form an important part of the basement series. Individual flows range in thickness from 1 ft (30 cm) to several tens of feet and contain pillows typically 3 to 6 ft (1 to 2 m) in horizontal dimension. Columnar jointing is locally present. Pillows have glassy-appearing selvages on their outer surfaces; in cross section, some display concentric layering of amygdules. Basaltic rocks are commonly altered to spilite; more siliceous volcanic rocks are altered to keratophyre and quartz keratophyre (Gates and others, 1971). Interstices between pillows are filled with varicolored chert, chalcedony, red jasper, or calcareous mud, and, locally, red limestone (Gates and others, 1971). DeLong and McDowell (1975) report two K/Ar ages from this unit, interpreted as minimum ages: 29.0 Ma on a pillow basalt flow and 30.7 Ma on amphibolite

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Correlated geologic units

Label TMzbv
Description Basement rocks, including basaltic, spilitic, and keratophyric lava and tuff. Pillow lava very common, western Aleutian Islands
Geologic age Paleocene to Rupelian
Geologic setting Extrusive
Lithology Form Importance
Volcanic < Igneous Pyroclastic Major
Basalt < Mafic-volcanic < Volcanic < Igneous Flow Major