Type Locality:

Named after the Big Snowy Mountains of central Montana. The type section is located on the north flank of these mountains, in Sec. 6, Twp. 12N, Rge. 20E.

Distribution:

According to Smith's (1935) description the Big Snowy Group at the type section is 240 m (787 ft) thick and attains a maximum outcrop thickness of 366 m (1,200 ft). However, these thicknesses probably include an interval that Mundt (1956) described as the Tyler Formation and separated from the Big Snowy Group. Bluemle et al. (1980) indicated a maximum thickness of 135 m (443 ft) for the Big Snowy in the Williston Basin. The group underlies 51,800 km2 (20,234 mi2) of central Montana, outcropping in the Little Belt Mountains, Castle Mountains and Lombard Hills as well as in the type locality. It extends eastward in the subsurface into the Williston Basin, where it underlies much of the central portion of the basin, extending as far north as extreme south-central Saskatchewan. In Saskatchewan the Big Snowy underlies an area between Rges. 7 and 26W2M, to a maximum distance of 19 km (12 mi) north of the U.S. border in the area east of Ratcliffe (Fuzesy, 1960).

Lithology:

Smith (1935) divided the Big Snowy Group into three formations: Heath, consisting of black shales and cross-bedded sandstone lenses, Otter, composed of limestones and grey to green shales; and the basal Kibbey, a red, shaly sandstone. In the Williston Basin Bluemle et al. (1980) identified the Big Snowy Group as consisting of an upper greenish to reddish grey sometimes carbonaceous shale with minor occurrences of marly fossiliferous and oolitic limestone, and a lower interval of reddish and light grey, fine- to medium-grained quartz sandstone accompanied by a dolomitic limestones some reddish to variegated silty shale and gypsum.

Relationship:

In the type locality the Big Snowy Group disconformably rests on the Madison Group. Smith (1935) reported relief of as much as 1.2 to 1.6 m (4 to 5 ft) at the contact between the two units. The upper contact has been somewhat problematical in the past but Mundt (1956) proposed that a disconformity existed within the Heath Formation and consequently placed the overlying strata into the Tyler Formation, which he considered to mainly Pennsylvanian in age. Thus the overlying strata including the Tyler can be a Minnelusa equivalent. This subdivision and relationship was followed by Bluemle et al. (1980) in the Williston Basin as well. However, in the basin there is little evidence of a basal unconformity as the Kibbey Formation appears to be conformable and transitional with the Poplar Beds. In Saskatchewan the Big Snowy Group is unconformably overlain by the Watrous Formation. It is thought to be primarily Chesteran in age.

Other Citations:

Bluemle, Anderson and Carlson, 1980; Fuzesy, 1960; Gardner, 1959; Mundt, 1956; Smith, 1935.

References:

Bluemle, J.R., Anderson, S.B. and Carlson, C.G., 1980. North Dakota stratigraphic column.

Fuzesy, L.M., 1960. Correlation and subcrops of the Mississippian strata in southeastern and south-central Saskatchewan. Saskatchewan Dept. of Min. Res., Rept. 51.

Mundt, Philip Amos, 1956. "Heath-Amsden strata in central Montana"; American Association of Petroleum Geologists (AAPG), AAPG Bulletin, vol. 40, no. 8 (August), pp. 1915-1934.