ABSTRACT

The pre-Winnipeg sedimentary rocks of North Dakota and northwestern South Dakota are referred to as the Deadwood formation. The Deadwood formation, as defined in this paper, ranges in thickness from being absent in northeastern North Dakota to a maximum of 800+ feet in western North Dakota. It is composed of sandstone, carbonate rocks and shale of Upper Cambrian and Lower Ordovician age. A division into Upper Cambrian and Lower Ordovician formations is not feasible at this time since sedimentation appears to have been continuous from Upper Cambrian to Lower Ordovician time.

The Winnipeg formation of this area consists of sandstone, shale and siltstone ranging in thickness from 0 at its erosional edge in eastern North Dakota to a maximum of 334 feet in northwestern North Dakota. The Winnipeg formation lies unconformably on the Deadwood formation except in northeastern North Dakota where it lies on Precambrian rocks. The Winnipeg formation is overlain conformably and gradationally by the Red River formation.

The Winnipeg formation in North Dakota may be divided into three members. Comparisons of the subsurface members to the outcrops in the Black Hills indicate that the siltstone and shale units underlying the Whitewood dolomite of the outcrop area are equivalents of the upper two members of the Winnipeg formation. Therefore the terms Roughlock and Icebox, which have previously been applied to these units in the outcrop area, are here applied to the Winnipeg formation in the subsurface of the Williston Basin. The basal sandstone member of the Winnipeg formation does not have an equivalent in the Black Hills outcrop section.

The relationship of the Winnipeg formation of the Manitoba outcrop area to the subsurface cannot be as clearly demonstrated. The term Black Island has been previously applied to the basal sandstone member in the outcrop area of Manitoba and in the subsurface of the Williston Basin. Although the upper contact may not be the same in the outcrop as in the subsurface, this usage is herein accepted.