The Big Snowy Group is a

stratigraphical 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: lithostrati ...

unit of Chesterian

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 Williston Basin. It takes the name from Big Snowy Mountains in

Montana Montana () is a state in the Mountain West division of the Western United States. It is bordered by Idaho to the west, North Dakota and South Dakota to the east, Wyoming to the south, and the Canadian provinces of Alberta, British Columbi ...

, and was first described on the north slopes of the mountain by H.W. Smith in 1935.

Lithology

Subdivisions

The Big Snowy Group is composed of three subdivisions, from top to base: *''Heath Formation'': black

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 ...

with

sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates) ...

lenses. *''Otter Formation'':

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...

and grey to green shale *''Kibbey Formation'': shaly sandstone

Distribution

The Big Snowy Group reaches a maximum thickness of in the Williston Basin. It is exposed in outcrop in the Big Snowy Mountains,

Little Belt Mountains The Little Belt Mountains are a section of the Rocky Mountains in the U.S. The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It cons ...

, Castle Mountains and Lombard Hills of central

. It occurs in the sub-surface throughout the central part of the Williston Basin and into a limited area of south-central

Saskatchewan Saskatchewan ( ; ) is a Provinces and territories of Canada, province in Western Canada, western Canada, bordered on the west by Alberta, on the north by the Northwest Territories, on the east by Manitoba, to the northeast by Nunavut, and on t ...

Relationship to other units

The Big Snowy Group is

unconformably An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval o ...

overlain by the

Tyler Formation Tyler may refer to: People and fictional characters * Tyler (name), an English name; with lists of people with the surname or given name * Tyler, the Creator (born 1991), American rap artist and producer * John Tyler, 10th president of the United ...

, and by the Watrous Formation in

; It disconformably overlays the Madison Group.

References

{{Western Canadian Sedimentary Basin, Saskatchewan=yes Geologic groups of North America Geologic formations of Montana Geologic formations of Saskatchewan Western Canadian Sedimentary Basin Carboniferous Montana Carboniferous Saskatchewan Carboniferous System of North America