| Fort St. John Group | |
| Type: | Geological formation |
| Period: | Early Cretaceous |
| Prilithology: | Shale |
| Otherlithology: | Sandstone, siltstone and conglomerates |
| Namedfor: | Fort St. John |
| Namedby: | George Mercer Dawson, 1881 |
| Region: | Northeast Northwest Southern Southern |
| Country: |  Canada |
| Subunits: | Cruiser Formation, Goodrich Formation, Hasler Formation, Gates Formation, Moosebar Formation, Shaftesbury Formation, Peace River Formation, Spirit River Formation, Bluesky Formation, Sully Formation, Sikanni Formation, Lepine Formation, Scatter Formation, Garbutt Formation, Buckinghorse Formation |
| Underlies: | Dunvegan Formation |
| Overlies: | Bullhead Group |
| Thickness: | up to 2000m (7,000feet)[1] |
The Fort St. John Group is a stratigraphic unit of Lower Cretaceous age in the Western Canada Sedimentary Basin.[2] It takes the name from the city of Fort St. John, British Columbia and was first defined by George Mercer Dawson in 1881.
The Fort St. John Group is mostly composed of dark shale deposited in a marine environment. Bentonite is present in the shale, and it is interbedded with sandstone, siltstone and conglomerates.
The Fort St. John Group occurs in the subsurface in the Peace River Country of northeastern British Columbia and north-western Alberta, in southern Yukon and southern Northwest Territories. It has a thickness of 700m (2,300feet) to 2000m (7,000feet).
The Fort St. John Group is conformably overlain by the Dunvegan Formation and conformably underlain by the Bullhead Group or may rest disconformably on older units.
The Fort St. John Group is subdivided into the following formations:
| Lithology | Max. Thickness | Reference | ||
|---|---|---|---|---|
| Cruiser Formation | 230-1NaN-1 | [3] | ||
| Goodrich Formation | fine-grained, laminated sandstone, mudstone partings | 400-1NaN-1 | [4] | |
| Hasler Formation | 265-1NaN-1 | [5] | ||
| Commotion Formation | 490-1NaN-1 | [6] | ||
| Gates Formation | 263-1NaN-1 | [7] | ||
| Moosebar Formation | 289-1NaN-1 | [8] | ||
| Lithology | Max. Thickness | Reference | ||
|---|---|---|---|---|
| Shaftesbury Formation | 400-1NaN-1 | [9] | ||
| 60-1NaN-1 | [10] | |||
| Notikewin Member - fine to medium grained argillaceous sandstone, dark shale, ironstone Falher Member - greywacke, shale, siltstone, coal Wilrich Member - dark shale thin sandstone and siltstone stringers | 348-1NaN-1 | [11] | ||
| 46-1NaN-1 | [12] | |||
| Lithology | Max Thickness | Reference | ||
|---|---|---|---|---|
| Sully Formation | 300-1NaN-1 | [13] | ||
| Sikanni Formation | 240-1NaN-1 | [14] | ||
| Lepine Formation* | silty mudstone, sideritic concretions | 900-1NaN-1 | [15] | |
| Scatter Formation* | Bulwell Member - glauconitic sandstone Wildhorn Member - silty mudstone Tussock Member - glauconitic sandstone, silty mudstone | 375-1NaN-1 | [16] | |
| Garbutt Formation* | 290-1NaN-1 | [17] | ||
| Chinkeh Formation | sandstone with marine shale, conglomeratic base | discontinuous | [18] | |