| Big Snowy Group | |
| Type: | Geological group |
| Period: | Mississippian |
| Prilithology: | Shale, limestone, sandstone |
| Namedfor: | Big Snowy Mountains |
| Namedby: | H.W. Smith |
| Year Ts: | 1935 |
| Region: | Williston Basin WCSB |
| Country: |  United States Canada |
| Subunits: | Heath, Otter & Kibbey Formations |
| Underlies: | Tyler Formation Watrous Formation |
| Overlies: | Madison Group |
| Thickness: | up to 135m (443feet) |
| Area: | 51800km2 |
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.
The Big Snowy Group is composed of three subdivisions, from top to base:[1]
The Big Snowy Group reaches a maximum thickness of 135m (443feet) 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.
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]