Type Locality:

Rierdon Gulch, about 43 km (27 mi) west of Choteau, Montana, W/2 Sec. 23, Twp. 24N, Rge. 9W.

Distribution:

The Rierdon Formation is recognized in the southern Alberta Plains, extreme southwestern Saskatchewan, northern and central Montana and the North Dakota Williston Basin. The Rierdon is 42 m (138 ft) thick at the type locality and ranges up to 60 m (197 ft) thick in southeastern Alberta and north-central Montana. It thins regionally over the Sweetgrass Arch, pinching out over the southwestern part of the Arch near Great Falls. The northern subcrop edge lies between Twps. 8 and 20 in Alberta and Saskatchewan, and is deeply dissected by Late Jurassic and Early Cretaceous erosion.

Lithology:

Three informal members can be recognized: a basal grey-green, calcareous shale with micritic limestone beds, a medial dark grey-green, fissile, slightly calcareous to noncalcareous shale with minor limestone beds, and an upper grey-green, calcareous shale with nodular limestones or limestone beds. Bedding throughout the formation is on the scale of tens of centimetres and marine molluscan fossils are common in all three members. The Rierdon is generally more calcareous to the east, especially in the Williston Basin.

Relationship:

The Rierdon conformably overlies the Sawtooth Formation and equivalents, or unconformably overlies Paleozoic strata where the Sawtooth is absent. The Swift Formation unconformably overlies the Rierdon; north of the Swift subcrop edge lower Mannville Group strata overlie the Rierdon unconformably. Stratigraphically equivalent rocks, generally in more arenaceous and less calcareous facies are assigned to the lower Sundance Formation in southeastern Montana and adjacent Wyoming and South Dakota. Christopher (1974) subdivided Rierdon-equivalent strata in southern Saskatchewan into the Rush Lake Shale and sandier Roseray Formation. Westward, in Alberta the Rierdon grades to less calcareous shales of the Grey Beds of the Fernie Group.

Other Citations:

Christopher, 1974; Cobban, 1945; Hayes, 1982; Peterson, 1972.