ABSTRACT: Permian to Jurassic Red Beds of the Williston Basin

CARLSON, CLARENCE G., Oil and Gas Division of the North Dakota Industrial Commission, Bismarck, ND

Red beds of Permian to Jurassic age are present in outcrops in the Black Hills of South Dakota. Red beds, which have been referred to as of Jurassic age, are present in outcrops in southern Manitoba. Equivalent strata have been referred to as the Spearfish Formation of Triassic or Permian-Triassic age in North Dakota and the Lower

Watrous or Amaranth formations of Jurassic-Triassic or Jurassic age in Saskatchewan and Manitoba. Paleontological data is lacking for resolution of this problem. Careful examination of physical evidence from the subsurface indicates that previously unrecognized regional unconformities are present within the clastic red beds. Areal distribution patterns of lithogenetic units favors a Jurassic age assignment for only a part of the Lower Watrous and Amaranth formations .

The Spearfish Formation, as originally defined, includes red beds that have been correlated with strata from Permian to Jurassic age in eastern Wyoming, although it was originally generally assigned a Triassic age. The Spearfish Formation will retain its greatest utility in the subsurface if it is restricted to strata of Permian to Triassic age. Redefinition of the Belfield and Pine units would avoid the introduction of new names and by applying them to the two lithogenetic units within the Spearfish would more accurately portray the depositional history of these strata. Similarly a redefinition of the Saude, closely resembling the original definition, would preserve the utility of that term. The Saude, as a member of the Piper Formation, would then be equivalent to the upper shaly un t of the lower Amaranth of Manitoba.

AAPG Search and Discovery Article #91017©1992 AAPG Rocky Mountain Section Meeting, Casper, Wyoming, September 13-16, 1992 (2009)