Exshaw Formation
| Exshaw Formation Stratigraphic range: Late Famennian to Middle Tournaisian |
|
|---|---|
| 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) |
| Lithology | |
| Primary | Shale |
| Other | Siltstone, limestone |
| Location | |
| Coordinates | 51°05′29″N 115°09′29″W / 51.09139°N 115.15806°WCoordinates: 51°05′29″N 115°09′29″W / 51.09139°N 115.15806°W |
| Region |
 Alberta
|
| Country |
 Canada
|
| Type section | |
| Named for | Exshaw, Alberta |
| Named by | P.S. Warren, 1937 |
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. 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.
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.
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.
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