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{{Short description|Stratigraphic unit in the Western Canada Sedimentary Basin}}
{{Infobox Rockunit
| name = Exshaw Formation
| image =
| caption =
| type = [[Geological formation]]
| age = {{Fossil range|360|350|Late [[Famennian]] to Middle [[Tournaisian]]}}
| period = Paleozoic
| prilithology = [[Shale]]
| otherlithology = [[Siltstone]], [[limestone]]
| namedfor = [[Exshaw, Alberta]]
| namedby = P.S. Warren, 1937<ref name=Warren />
| region = [[Alberta]]
| country = [[Canada]]
| coordinates = {{coord|51|05|29|N|115|09|29|W|name=Exshaw Formation|display=inline,title}}
| unitof =
| subunits = Lower and Upper Exshaw
| underlies = [[Banff Formation]]
| overlies = [[Wabamun Formation]], [[Kotcho Formation]], [[Palliser Formation]]
| thickness = up to {{convert|50|m|ft|-1}}<ref name=lexicon/>
| extent =
| area =
| map =
| map_caption =
}}
The '''Exshaw Formation''' is a [[stratigraphic unit]] in the [[Western Canadian Sedimentary Basin|Western Canada Sedimentary Basin]].<ref name=Mossop13>{{Cite web|url=http://www.ags.gov.ab.ca/publications/wcsb_atlas/a_ch13/ch_13.html|title=The Geological Atlas of the Western Canada Sedimentary Basin, Chapter 13: Devonian Wabamun Group of the Western Canada Sedimentary Basin|author=Mossop, G.D. and Shetsen, I., (compilers), Canadian Society of Petroleum Geologists and Alberta Geological Survey|year=1994|access-date=2014-10-18}}</ref> It takes the name from the hamlet of [[Exshaw, Alberta]], in the [[Canadian Rockies]], and was first described from [[outcrop]]s on the banks of Jura Creek north of Exshaw by P.S. Warren in 1937.<ref name=Warren>Warren, P.S., 1937. Age of the Exshaw shale in the Canadian Rockies. [[American Journal of Science]], Series 5, volume 33, pages 454-457.</ref> The formation is of [[Late Devonian]] (late [[Famennian]]) to [[Mississippian (geology)|Early Mississippian]] (middle [[Tournaisian]]) [[Geochronology|age]] as determined by [[conodont biostratigraphy]], and it straddles the [[Devonian]]–[[Carboniferous]] boundary.<ref name=Warren /><ref name=MD>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.</ref>

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 [[shale]]s of the Exshaw Formation are rich in [[organic matter]] and are one of the most important [[petroleum]] [[source rock]]s of the Western Canada Sedimentary Basin.<ref name=Caplan>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.</ref>

==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-[[bedding (geology)|bedded]] to laminated, and rich in organic matter, with scattered sulphide and [[phosphate nodule]]s.<ref name=lexicon /><ref name=Glass /> 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.<ref name=MD /><ref name=Caplan /> The shales are sparsely fossiliferous and contain some [[conodont element]]s, [[ostracod]]s and [[brachiopod]] shells.<ref name=lexicon>
{{Cite web|url=http://cgkn1.cgkn.net/weblex/weblex_litho_detail_e.pl?00053:004767
|title=Exshaw Formation
|author=[[Lexicon of Canadian Geologic Units]]
|access-date=2009-03-01}}
</ref><ref name=Caplan />

The upper member consists of brown-weathering, medium- to thick-bedded, [[calcareous]] and [[Dolomite (mineral)|dolomitic]] siltstones with subordinate silty limestones.<ref name=lexicon /><ref name=Glass>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}}.</ref> [[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.<ref name=Caplan />

==Thickness and Distribution==
The Exshaw Formation has a thickness of {{convert|46.7|m|ft|-1}} at its [[type locality (geology)|type locality]] near Exshaw and typically ranges between {{convert|7|m|ft|-1}} and {{convert|50|m|ft|-1}} 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, Alberta|Peace River]] Arch, and the upper siltstone member is absent in [[central Alberta]] and [[northern Alberta]].<ref name=lexicon/>

==Relationship to other units==
Because the Exshaw Formation is a widespread unit, it overlies different formations in different areas. It rests [[unconformity|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.<ref name=lexicon />

The Exshaw Formation is equivalent to the lower and middle members of the [[Bakken Formation]] in [[southern Alberta]], southern [[Saskatchewan]], [[Montana]] and [[North Dakota]].<ref name=lexicon /><ref name=MD />

== References ==
{{reflist}}

{{Western Canadian Sedimentary Basin|Northwest_Plains=yes|Northeast_Plains=yes|South AB=yes}}

[[Category:Carboniferous Alberta]]
[[Category:Devonian Alberta]]
[[Category:Western Canadian Sedimentary Basin]]
[[Category:Mississippian Series]]
[[Category:Famennian Stage]]
[[Category:Carboniferous System of North America]]
[[Category:Devonian System of North America]]
[[Category:Devonian southern paleotropical deposits]]
[[Category:Geologic formations of Alberta]]
[[Category:Shale formations of Canada]]
[[Category:Siltstone formations of Canada]]
[[Category:Limestone formations of Canada]]

Exshaw Formation - Revision history

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