Big Snowy Group

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

Big Snowy Group
Stratigraphic range:
TypeGeological group
Sub-unitsHeath, Otter & Kibbey Formations
UnderliesTyler Formation
Watrous Formation
OverliesMadison Group
Area51,800 square kilometres (20,000 sq mi)
Thicknessup to 135 metres (440 ft)[1]
Lithology
PrimaryShale, limestone, sandstone
Location
RegionWilliston Basin
WCSB
Country United States
 Canada
Type section
Named forBig Snowy Mountains
Named byH.W. Smith
Year defined1935

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]

References
  1. Lexicon of Canadian Geologic Units. "Big Snowy Group". Archived from the original on 2013-02-21. Retrieved 2010-02-03.
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