Enigmatic Uppermost Permian-Lowermost Triassic Stratigraphic Relations in the Northern Bighorn Basin of Wyoming and Montana

PAULL, RICHARD A., and RACHEL K. PAULL, University of Wisconsin-Milwaukee, Milwaukee, WI

Eighteen measured sections in the northern Bighorn basin of Wyoming and Montana provide the basis for an analysis of Permian-Triassic stratigraphic relations. This boundary is well defined to the south where gray calcareous siltstones of the Lower Triassic Dinwoody disconformably overlie the Upper Permian Ervay Member of the Park City Formation with little physical evidence of a significant hiatus. The Dinwoody is gradationally overlain by red beds of the Red Peak Formation.

The Dinwoody thins to zero near the state line. Northward, the erathem boundary is enigmatic because fossils are absent and there is no evidence of an unconformity. Poor and discontinuous exposures contribute to the problem.

Up to 20 m of Permian or Triassic rocks or both overlie the Pennsylvanian Tensleep Sandstone in the westernmost surface exposures on the eastern flank of the Bighorn basin with physical evidence of an unconformity. East of the exposed Tensleep, Ervay-like carbonates are overlain by about 15 m of Dinwoody-like siltstones interbedded with red beds and thin dolomitic limestones. In both areas, they are overlain by the Red Peak Formation.

Thin carbonates within the Dinwoody are silty, coarse algal laminates with associated peloidal micrite. Carbonates north of the Dinwoody termination and above probable Ervay are peloidal algal laminates with fenestral fabric and sparse coated shell fragments with pisoids. These rocks may be Dinwoody equivalents or they may be of younger Permian age than the Ervay. Regardless, revision of stratigraphic nomenclature in this area may be required.

AAPG Search and Discovery Article #91010©1991 AAPG Rocky Mountain Section Meeting, Billings, Montana, July 28-31, 1991 (2009)