Abstract

Nineteen measured sections in the northern Bighorn Basin of Wyoming and Montana provide the basis for an analysis of Permian-Triassic stratigraphic relations. The Permian-Triassic unconformity is well-defined in north-central Wyoming where gray, yellowish-weathering, calcareous siltstones of the Lower Triassic Dinwoody Formation disconformably overlie carbonates of 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 Lower Triassic Red Peak Formation of the Chugwater Group.

The Dinwoody depositionally thins to zero near the Wyoming-Montana state line. Northward, the erathem boundary is enigmatic because fossils are absent, Permian rocks change facies to resemble the Dinwoody, and there is no record of a Permian-Triassic unconformity. Discontinuous exposures and the restrictive access policies of the Crow Indian Reservation also contribute to the problem. Some previous workers assigned these rocks to the Permian, while others considered them Triassic in age.

Up to 12 m (37 feet) of Dinwoody-like Permian rocks unconformably overlie the Pennsylvanian Tensleep Sandstone and grade into the Lower Triassic Red Peak Formation in the westernmost surface exposures along the eastern flank of the Bighorn Basin in Montana. South and east of this area, but north of the Dinwoody termination, Ervay carbonate is overlain by up to 24 m (74 feet) of red siltstone capped by.25 (0.8 feet) to 0.5 m (1.6 feet) of laminated dolomitic limestone. This interval is assigned to the basal Red Peak.

Thin Lower Triassic dolomitic mudstones within the Dinwoody and basal Red Peak are commonly laminated, with associated peloidal micrite and coarse quartz silt. These carbonates are also poorly fenestral, and may contain small quantities (1% or less) of small peloidal rip-ups and very fine-grained quartz sand. Carbonates within the Dinwoody-like Permian are similar, but contain variable quantities of sand-size quartz and little to no coarse quartz silt.