The Fort St. John Group is a stratigraphic unit of Lower Cretaceous

age Age or AGE may refer to: Time and its effects * Age, the amount of time someone or something has been alive or has existed ** East Asian age reckoning, an Asian system of marking age starting at 1 * Ageing or aging, the process of becoming older ...

in the

Western Canada Sedimentary Basin The Western Canadian Sedimentary Basin (WCSB) underlies of Western Canada including southwestern Manitoba, southern Saskatchewan, Alberta, northeastern British Columbia and the southwest corner of the Northwest Territories. This vast sedimentary ...

. 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 Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especial ...

deposited in a

marine Marine is an adjective meaning of or pertaining to the sea or ocean. Marine or marines may refer to: Ocean * Maritime (disambiguation) * Marine art * Marine biology * Marine debris * Marine habitats * Marine life * Marine pollution Military * ...

environment. Bentonite is present in the shale, and it is interbedded with sandstone,

siltstone Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility.Blatt ''et al.'' 1980, p ...

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 The Northwest Territories (abbreviated ''NT'' or ''NWT''; french: Territoires du Nord-Ouest, formerly ''North-Western Territory'' and ''North-West Territories'' and namely shortened as ''Northwest Territory'') is a federal territory of Canada. ...

. It has a thickness of to .

Relationship to other units

The Fort St. John Group is conformably overlain by the

Dunvegan Formation The Dunvegan Formation is a stratigraphical unit of Cenomanian age in the Western Canadian Sedimentary Basin. It takes the name from the settlement of Dunvegan, Alberta, and was first described in an outcrop on Peace River near Dunvegan by George ...

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 The Canadian Rockies (french: Rocheuses canadiennes) or Canadian Rocky Mountains, comprising both the Alberta Rockies and the British Columbian Rockies, is the Canadian segment of the North American Rocky Mountains. It is the easternmost part ...
foothills of British Columbia

Peace River Country

Liard River The Liard River of the North American boreal forest flows through Yukon, British Columbia and the Northwest Territories, Canada. Rising in the Saint Cyr Range of the Pelly Mountains in southeastern Yukon, it flows southeast through British Co ...
and Fort Liard Area

*Buckinghorse Formation is equivalent to the sum of Lepine Formation, Scatter Formation and Garbutt Formation. It occurs north-east of the

Canadian Rockies The Canadian Rockies (french: Rocheuses canadiennes) or Canadian Rocky Mountains, comprising both the Alberta Rockies and the British Columbian Rockies, is the Canadian segment of the North American Rocky Mountains. It is the easternmost part ...

foothills in British Columbia, between the

Halfway River The Halfway River is a tributary of the Peace River in northeastern British Columbia, Canada. The river originates in the Muskwa Ranges at an elevation of . It flows from Robb Lake, between Mount Kenny and Mount Robb, then flows east to Pink Moun ...

and Muskwa River. It is composed of silty

mudstone Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from '' shale'' by its lack of fissility (parallel layering).Blatt, H., and R.J. Tracy, 1996, ''Petrology. ...

with fine grained marine sandstone interbeds.

References

{{Western Canadian Sedimentary Basin, Northwest_Plains=yes, Fort_Nelson=yes, Liard_River=yes Lower Cretaceous Series of North America Cretaceous Alberta Cretaceous British Columbia Cretaceous Northwest Territories Cretaceous Yukon Stratigraphy of Alberta Stratigraphy of British Columbia Stratigraphy of the Northwest Territories Stratigraphy of Yukon