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.
| Fort St. John Group | |
|---|---|
| Stratigraphic range: | |
| Type | Geological formation |
| Sub-units | 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 2,000 metres (6,560 ft)[1] |
| Lithology | |
| Primary | Shale |
| Other | Sandstone, siltstone and conglomerates |
| Location | |
| Region | Northeast  British ColumbiaNorthwest  AlbertaSouthern  YukonSouthern  Northwest Territories |
| Country |  Canada |
| Type section | |
| Named for | Fort St. John |
| Named by | George Mercer Dawson, 1881 |
Lithology
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.
Distribution
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 700 metres (2,300 ft) to 2,000 metres (6,560 ft).
Relationship to other units
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.
Subdivisions
The Fort St. John Group is subdivided into the following formations:
Canadian Rockies foothills of British Columbia
| Sub-unit | Age | Lithology | Max. Thickness | Reference |
|---|---|---|---|---|
| Cruiser Formation | Albian - Cenomanian | marine shale, argillaceous siltstone and fine grained marine sandstone | 230 m (750 ft) | [3] |
| Goodrich Formation | late Albian | fine-grained, laminated sandstone, mudstone partings | 400 m (1,310 ft) | [4] |
| Hasler Formation | middle to late Albian | marine shale and siltstone, minor sandstone and pebble conglomerate | 265 m (870 ft) | [5] |
| Commotion Formation | early to middle Albian | sandstone, shale and conglomerate | 490 m (1,610 ft) | [6] |
| Gates Formation | early Albian | massive well-sorted sandstone, carbonaceous sandstone, mudstone, siltstone, coal | 263 m (860 ft) | [7] |
| Moosebar Formation | early Albian | marine shale and siltstone | 289 m (950 ft) | [8] |
| Sub-unit | Age | Lithology | Max. Thickness | Reference |
|---|---|---|---|---|
| Shaftesbury Formation | Albian | friable shale, fish scale siltstone, bentonite, ironstone | 400 m (1,310 ft) | [9] |
| Peace River Formation | middle Albian | Paddy Member - greywacke, coal Cadotte Member - coarse to fine marine sandstone Harmon Member - dark, fissile, non-calcareous shale | 60 m (200 ft) | [10] |
| Spirit River Formation | middle Albian | 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 m (1,140 ft) | [11] |
| Bluesky Formation | early Albian | brown, fine to medium grained, glauconitic, porous sandstone | 46 m (150 ft) | [12] |
Liard River and Fort Liard Area
| Sub-unit | Age | Lithology | Max Thickness | Reference |
|---|---|---|---|---|
| Sully Formation | early to Late Cretaceous | marine shale and siltstone | 300 m (980 ft) | [13] |
| Sikanni Formation | early Cretaceous | fine-grained, calcareous, glauconitic sandstone, argillaceous siltstone and shale | 240 m (790 ft) | [14] |
| Lepine Formation* | middle to late Albian | silty mudstone, sideritic concretions | 900 m (2,950 ft) | [15] |
| Scatter Formation* | early to middle Albian | Bulwell Member - glauconitic sandstone Wildhorn Member - silty mudstone Tussock Member - glauconitic sandstone, silty mudstone | 375 m (1,230 ft) | [16] |
| Garbutt Formation* | early Aptian | Lower Garbutt - mudstone, siltstone, siderite, bentonite Upper Garbutt - mudstone, sideritic weathering, argillaceous siltstone, laminated sandstone | 290 m (950 ft) | [17] |
| Chinkeh Formation | Barremian to early Albian | sandstone with marine shale, conglomeratic base | discontinuous | [18] |
*Buckinghorse Formation is equivalent to the sum of Lepine Formation, Scatter Formation and Garbutt Formation. It occurs north-east of the Canadian Rockies foothills in British Columbia, between the Halfway River and Muskwa River. It is composed of silty marine mudstone with fine grained marine sandstone interbeds.