The Spearfish Formation in the Williston Basin of western North Dakota
The Spearfish Formation of the Black Hills has been traced into the Williston Basin of western North Dakota. In the basin, the formation can be divided into three members. In ascending order these are: (1) a lower gray shale and red siltstone unit herein named the Belfield Member, (2) a middle salt unit, the Pine Salt Member, and (3) an upper red siltstone and fine grained sandstone unit, the Saude Member. Type sections for all three members are herein established. Isopachous maps and stratigraphic cross sections have been prepared to delireate the thickness and distribution of each of the units in western North Dakota. The Belfield Member and the Pine Salt Member are considered to Be Permian in age and are correlated with the Permian part of the type Spearfish Formation the outcrop sections north of the Black Hills. The Saude Member is predominantly Triassic in age although its upper part in extreme northern North Dakota and Canada may contain the Triassic-Jurassic boundary. The Saude in North Dakota is probably non-marine in origin and was deposited in a different depositional environment than its equivalent south of the Cedar Creek Anticline. South of the anticline the upper part of the Saude has been removed by erosion and a sharp angular unconformity separates the Saude from over lying Jurassic sediments. A persistent anhydrite bed in the lower part of the Saude in southwestern North Dakota is correlated with the top of the Goose Egg Formation in eastern Wyoming and Montana.
Geology--North Dakota; Geology--Williston Basin; Formations (Geology)--North Dakota, Geology
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