The Fort St. John Group is a

stratigraphic Stratigraphy is a branch of geology concerned with the study of rock layers (strata) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks. Stratigraphy has three related subfields: lithost ...

unit of

Lower Cretaceous Lower may refer to: * ''Lower'' (album), 2025 album by Benjamin Booker * Lower (surname) * Lower Township, New Jersey *Lower Receiver (firearms) * Lower Wick Gloucestershire, England See also * Nizhny {{Disambiguation ...

age Age or AGE may refer to: Time and its effects * Age, the amount of time someone has been alive or something 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 Canada 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 ba ...

. It takes the name from the city of Fort St. John,

British Columbia British Columbia is the westernmost Provinces and territories of Canada, province of Canada. Situated in the Pacific Northwest between the Pacific Ocean and the Rocky Mountains, the province has a diverse geography, with rugged landscapes that ...

and was first defined by

George Mercer Dawson George Mercer Dawson (August 1, 1849 – March 2, 1901) was a Canadian geologist and surveyor. He performed many early explorations in western North America and compiled numerous records of the native peoples. Biography He was born in ...

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 mineral, clay minerals (hydrous aluminium phyllosilicates, e.g., Kaolinite, kaolin, aluminium, Al2Silicon, Si2Oxygen, O5(hydroxide, OH)4) and tiny f ...

deposited in a marine environment.

Bentonite Bentonite ( ) is an Absorption (chemistry), absorbent swelling clay consisting mostly of montmorillonite (a type of smectite) which can either be Na-montmorillonite or Ca-montmorillonite. Na-montmorillonite has a considerably greater swelli ...

is present in the shale, and it is interbedded with

sandstone Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains, Cementation (geology), cemented together by another mineral. Sand ...

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. Although its permeabil ...

and conglomerates.

Distribution

The Fort St. John Group occurs in the subsurface in the

Peace River Country The Peace River Country (or Peace Country; ) is an aspen parkland region centring on the Peace River in Canada. It extends from northwestern Alberta to the Rocky Mountains in northeastern British Columbia, where a certain portion of the region i ...

of northeastern

and north-western

Alberta Alberta is a Provinces and territories of Canada, province in Canada. It is a part of Western Canada and is one of the three Canadian Prairies, prairie provinces. Alberta is bordered by British Columbia to its west, Saskatchewan to its east, t ...

, in southern

Yukon Yukon () is a Provinces and territories of Canada, territory of Canada, bordering British Columbia to the south, the Northwest Territories to the east, the Beaufort Sea to the north, and the U.S. state of Alaska to the west. It is Canada’s we ...

and southern

Northwest Territories The Northwest Territories is a federal Provinces and territories of Canada, territory of Canada. At a land area of approximately and a 2021 census population of 41,070, it is the second-largest and the most populous of Provinces and territorie ...

. 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 The Canadian Rockies () 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 of the Canadian Cordillera, w ...
foothills of
British Columbia British Columbia is the westernmost Provinces and territories of Canada, province of Canada. Situated in the Pacific Northwest between the Pacific Ocean and the Rocky Mountains, the province has a diverse geography, with rugged landscapes that ...

Peace River Country The Peace River Country (or Peace Country; ) is an aspen parkland region centring on the Peace River in Canada. It extends from northwestern Alberta to the Rocky Mountains in northeastern British Columbia, where a certain portion of the region i ...

Liard River The Liard River of the Boreal forest of Canada, 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 sout ...
and
Fort Liard Fort Liard (Slavey language: ''Echaot'l Koe'' "people from the land of the giants" or ''Acho Dene Kue'') is a hamlet in the Dehcho Region of the Northwest Territories, Canada. It is located north of the British Columbia border. It became access ...
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 () 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 of the Canadian Cordillera, w ...

foothills in

, 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 marine

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.Blatt, H., and R.J. Tracy, 1996, ''Petrology.'' New York, New York, ...

with fine grained marine

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 Geologic groups of Alberta Geologic groups of British Columbia Geologic groups of the Northwest Territories Geologic groups of Yukon Shale groups of Canada Siltstone groups Sandstone groups of Canada Conglomerate groups