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.[1][6] 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.[4][5] The shales are sparsely fossiliferous and contain some conodont elements, ostracods and brachiopod shells.[1][5]
The upper member consists of brown-weathering, medium- to thick-bedded, calcareous and dolomitic siltstones with subordinate silty limestones.[1][6]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.[5]
Thickness and Distribution
The Exshaw Formation has a thickness of 46.7 metres (150 ft) at its type locality near Exshaw and typically ranges between 7 metres (20 ft) and 50 metres (160 ft) thick. It is a widespread unit that can be seen in outcrop at many locations in the Canadian Rockies, and it is present in the subsurface from the prairies of northern Montana to southern Northwest Territories. It is absent at the Peace River Arch, and the upper siltstone member is absent in central Alberta and northern Alberta.[1]
^ abcMeijer 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.
^ abcdCaplan, 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.
^ abGlass, 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. ISBN0-920230-23-7.