The Fort St. John Group is a stratigraphic unit of Lower Cretaceous age in the Western Canada Sedimentary Basin. It takes the name from the city of Fort St. John, British Columbia and was first defined by George Mercer Dawson in 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 to .
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 Goodrich Formation late Albian fine-grained, laminated sandstone, mudstone partings Hasler Formation middle to late Albian marine shale and siltstone, minor sandstone and pebble conglomerate Commotion Formation early to middle Albian sandstone, shale and conglomerate Gates Formation early Albian massive well-sorted sandstone, carbonaceous sandstone, mudstone, siltstone, coal Moosebar Formation early Albian marine shale and siltstone
Peace River Country Sub-unit Age Lithology Max. thickness Reference Shaftesbury Formation Albian friable shale, fish scale siltstone, bentonite, ironstone Peace River Formation middle Albian Paddy Member - greywacke, coalCadotte Member - coarse to fine marine sandstoneHarmon Member - dark, fissile, non-calcareous shale Spirit River Formation middle Albian Notikewin Member - fine to medium grained argillaceous sandstone, dark shale, ironstoneFalher Member - greywacke, shale, siltstone, coalWilrich Member - dark shale thin sandstone and siltstone stringers Bluesky Formation early Albian brown, fine to medium grained, glauconitic, porous sandstone
Liard River and Fort Liard Area Sub-unit Age Lithology Maxthickness Reference Sully Formation early to Late Cretaceous marine shale and siltstone Sikanni Formation early Cretaceous fine-grained, calcareous, glauconitic sandstone, argillaceous siltstone and shale Lepine Formation* middle to late Albian silty mudstone, sideritic concretions Scatter Formation* early to middle Albian Bulwell Member - glauconitic sandstoneWildhorn Member - silty mudstoneTussock Member - glauconitic sandstone, silty mudstone Garbutt Formation* early Aptian Lower Garbutt - mudstone, siltstone, siderite, bentoniteUpper Garbutt - mudstone, sideritic weathering, argillaceous siltstone, laminated sandstone Chinkeh Formation Barremian to early Albian sandstone with marine shale, conglomeratic base discontinuous * 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.
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