Exshaw Formation

Exshaw Formation Stratigraphic range: Late Famennian to Middle Tournaisian PreЄ Є O S D C P T J K Pg N
Type	Geological formation
Sub-units	Lower and Upper Exshaw
Underlies	Banff Formation
Overlies	Wabamun Formation, Kotcho Formation, Palliser Formation
Thickness	up to 50 metres (160 ft)^[1]
Lithology
Primary	Shale
Other	Siltstone, limestone
Location
Coordinates	51°05′29″N 115°09′29″W / 51.09139°N 115.15806°W / 51.09139; -115.15806 (Exshaw Formation)Coordinates: 51°05′29″N 115°09′29″W / 51.09139°N 115.15806°W / 51.09139; -115.15806 (Exshaw Formation)
Region	Alberta
Country	Canada
Type section
Named for	Exshaw, Alberta
Named by	P.S. Warren, 1937^[2]

The Exshaw Formation is a stratigraphic unit in the Western Canada Sedimentary Basin.^[3] 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.^[2] 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.^[2]^[4]

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.^[5]

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.^[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]

Relationship to other units

Because the Exshaw Formation is a widespread unit, it overlies different formations in different areas. It rests disconformably on the Wabamun Formation in the Alberta plains, the Kotcho Formation in northeastern British Columbia, and the Palliser Formation in the Canadian Rockies and foothills. It is disconformably overlain by the Banff Formation in most areas.^[1]

The Exshaw Formation is equivalent to the lower and middle members of the Bakken Formation in southern Alberta, southern Saskatchewan, Montana and North Dakota.^[1]^[4]

References

1 2 3 4 5 6 7 Lexicon of Canadian Geologic Units. "Exshaw Formation". Retrieved 2009-03-01.
1 2 3 Warren, P.S., 1937. Age of the Exshaw shale in the Canadian Rockies. American Journal of Science, Series 5, volume 33, pages 454-457.
↑ Mossop, G.D. and Shetsen, I., (compilers), Canadian Society of Petroleum Geologists and Alberta Geological Survey (1994). "The Geological Atlas of the Western Canada Sedimentary Basin, Chapter 13: Devonian Wabamun Group of the Western Canada Sedimentary Basin". Retrieved 2014-10-18.
1 2 3 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.
1 2 3 4 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.
1 2 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.

Western Canadian Sedimentary Basin

Hydrocarbon history	Oil sands and heavy oil Frontier exploration and development Natural gas liquids Natural gas

Depositional Regions	Southern Alberta Central Alberta Northwestern Alberta Plains South-central Canadian Rockies foothills North-east Plains North-central foothills Liard River Fort Nelson Northern Rocky Mountains Fort St. John Saskatchewan Western Manitoba

Northwest Plains	Paskapoo FM Scollard FM Wapiti GRP Smoky GRP Puskwaskau FM Badheart FM Muskiki FM Cardium FM Kaskapau FM Pouce Coupe MBR Doe Creek MBR Dunvegan FM Fort St. John GRP Shaftesbury FM Peace River FM Spirit River FM Bullhead GRP Bluesky FM Gething FM Cadomin FM Nikanassin FM Fernie GRP Rock Creek MBR Poker Chip Shale Nordegg MBR Schooler Creek GRP Baldonnel FM Charlie Lake FM Halfway FM Doig FM Montney FM Belloy FM Stoddart GRP Taylor Flat FM Kiskatinaw FM Golata FM Rundle GRP Debolt FM Shunda FM Pekisko FM Banff FM Exshaw FM Wabamun FM Trout River FM Kakisa FM Redknife FM Jean Marie MBR Woodbend GRP Irteton FM Fort Simpson FM Leduc FM Duvernay FM Cooking Lake FM Beaverhill Lake GRP Waterways FM Swan Hills FM Fort Vermilion FM Elk Point GRP Watt Mountain FM Gillwood MBR Presqu’ile FM Sulphur Point FM Pine Point FM Muskeg FM Zama MBR Black Shale Keg River FM Rainbow MBR Chinchaga FM Cold Lake FM Ernestina FM Basal Red Beds Granite Wash Canadian Shield

Northeastern Plains	Bearpaw FM Belly River GRP Lea Park FM La Biche GRP Viking FM Joli Fou FM Colony SS Mannville GRP McLaren MBR Grand Rapids FM Waseca SS Sparky SS Gen. Petr. SS Clearwater FM Rex SS Lloydminster SS Wabiskaw MBR Cummings MBR McMurray FM Dina MBR Banff FM Exshaw FM Wabamun FM Winterburn FM Woodbend GRP Grosmont FM Irteton FM Cooking Lake FM Beaverhill Lake GRP Elk Point GRP Canadian Shield

Southern Alberta Plains	Cypress Hills FM Porcupine Hills FM Ravenscrag FM Edmonton GRP Belly River GRP Oldman FM Foremost FM Pakowki FM Milk River FM Colorado GRP Mannville GRP Ellis GRP Swift FM Rierdon FM Sawtooth FM Rundle GRP Banff FM Exshaw FM Bakken FM Wabamun FM Crowfoot FM Nisku FM Ireton FM Leduc FM Cooking Lake FM Beaverhill Lake GRP Elk Point GRP Canadian Shield

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