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The Exshaw Formation is a stratigraphic unit in the Western Canada Sedimentary Basin. It takes the name from the hamlet of Exshaw, Alberta in the Canadian Rockies, and was first described from outcrops on the banks of Jura Creek north of Exshaw by P.S. Warren in 1937.〔Warren, P.S., 1937. Age of the Exshaw shale in the Canadian Rockies. American Journal of Science, Series 5, volume 33, pages 454-457.〕 The formation is of Late Devonian (late Famennian) to Early Mississippian (middle Tournaisian) age as determined by conodont biostratigraphy, and it straddles the Devonian-Carboniferous boundary.〔〔Meijer Dries, N.C. and Johnston, D.I. 1996. Famennian and Tournaisian biostratigraphy of the Big Valley, Exshaw and Bakken Formations, southeastern Alberta and Southwestern Saskatchewan. Bulletin of Canadian Petroleum Geology, vol. 44, no. 4, p. 683-694.〕 The Exshaw strata were deposited in a marine setting during the Hangenberg event, an oceanic anoxic event associated with the Late Devonian extinction. The black shales of the Exshaw Formation are rich in organic matter and are one of the most important petroleum source rocks of the Western Canada Sedimentary Basin.〔Caplan, M.L. and Bustin, R.M. 1996. Factors governing organic matter accumulation and preservation in a marine petroleum source rock from the Upper Devonian to Lower Carboniferous Exshaw Formation, Alberta. Bulletin of Canadian Petroleum Geology, vol. 44, no. 3, p. 474-494.〕 ==Lithology and Environment of Deposition== The Exshaw Formation is informally subdivided into a lower shale member and an upper siltstone and limestone member. The lower shales are dark grey to black, thin-bedded to laminated, and rich in organic matter, with scattered sulphide and phosphate nodules.〔〔 There is no evidence of disturbance by biological activity. They are believed to have been deposited in an offshore environment on the outer continental shelf under anoxic conditions.〔〔 The shales are sparsely fossiliferous and contain some conodont elements, ostracods and brachiopod shells.〔 〕〔 The upper member consists of brown-weathering, medium- to thick-bedded, calcareous and dolomitic siltstones with subordinate silty limestones.〔〔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.〕 Bioturbation by burrowing marine organisms is common and indicates that they were deposited in an oxygenated environment, closer to the paleocoastline than the lower black shales were.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Exshaw Formation」の詳細全文を読む スポンサード リンク
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