Big Snowy Group

Big Snowy Group Stratigraphic range: Chesterian PreЄ Є O S D C P T J K Pg N
Type	Geological formation
Underlies	Tyler Formation Watrous Formation
Overlies	Madison Group
Area	51,800 square kilometres (20,000 sq mi)
Thickness	up to 135 metres (440 ft)[1]
Lithology
Primary	Shale, limestone, sandstone
Location
Named for	Big Snowy Mountains
Named by	H.W. Smith, 1935
Region	Williston Basin WCSB
Country	United States  Canada

The Big Snowy Group is a stratigraphical unit of Chesterian age in the Williston Basin.

It takes the name from Big Snowy Mountains in Montana, 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: ^[1]

• Heath Formation: black shale with sandstone lenses.
• Otter Formation: limestone and grey to green shale
• Kibbey Formation: shaly sandstone

Distribution

The Big Snowy Group reaches a maximum thickness of 135 metres (440 ft) in the Williston Basin.^[1] It is exposed in outcrop in the Big Snowy Mountains, Little Belt Mountains, Castle Mountains and Lombard Hills of central Montana. It occurs in the sub-surface throughout the central part of the Williston Basin ind into a limited area of south-central Saskatchewan.

Relationship to other units

The Big Snowy Group is unconformably overlain by the Tyler Formation in Montana, and by the Watrous Formation in Saskatchewan; It disconformably overlays the Madison Group.^[1]

References

1. ^ ^{a b c d} Lexicon of Canadian Geologic Units. "Big Snowy Group". http://cgkn1.cgkn.net/weblex/weblex_litho_detail_e.pl?00053:001329. Retrieved 2010-02-03.