Big Snowy Group
Stratigraphic range: Chesterian PreꞒ Ꞓ O S D C P T J K Pg N
Type	Geological group
Sub-units	Heath, Otter & Kibbey Formations
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
Region	Williston Basin WCSB
Country	United States  Canada
Type section
Named for	Big Snowy Mountains
Named by	H.W. Smith
Year defined	1935

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 and 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]