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Bearpaw Formation
The Bearpaw Formation, also called the Bearpaw Shale, is a geologic formation of Late Cretaceous (Campanian) age. It outcrops in the U.S. state of Montana, as well as the Canadian provinces of Alberta and Saskatchewan, and was named for the Bear Paw Mountains in Montana.〔 It includes a wide range of marine fossils, as well as the remains of a few dinosaurs. It is known for its fossil ammonites, some of which are mined to produce the organic gemstone ammolite.
==Lithology and depositional environment==
The formation was deposited in the Bearpaw Sea, which was part of the Western Interior Seaway that advanced and then retreated across the region during Campanian time.〔(Latest Cretaceous Western Interior Seaway )〕 It is composed primarily of dark grey shales, claystones, silty claystones and siltstones, with subordinate silty sandstones. It also includes bedded and nodular concretions (both calcareous and ironstone concretions) and thin beds of bentonite. As the seaway retreated toward the southwest, the marine sediments of the Bearpaw became covered by the deltaic and coastal plain sediments of the overlying formations.〔〔Glass, D.J. (editor) 1997. Lexicon of Canadian Stratigraphy, vol. 4, Western Canada including eastern British Columbia, Alberta, Saskatchewan and southern Manitoba. Canadian Society of Petroleum Geologists, Calgary, 1423 p. on CD-ROM. ISBN 0-920230-23-7.〕〔Wall, J.H., Sweet, A.R. and Hills, L.V. 1971. Paleoecology of the Bearpaw and contiguous Upper Cretaceous formations in the C.P.O.G. Strathmore well, southern Alberta. Bulletin of Canadian Petroleum Geology, vol. 19, no. 3, p. 691-702.〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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