ABSTRACT

The Lower Triassic Dinwoody Formation in the Bighorn Basin of Wyoming and Montana records the northeasternmost extent of the widespread and rapid Griesbachian transgression onto the Wyoming shelf. Depositional patterns document a progressive change from inner shelf marine conditions in the southwest and west to restricted, marginal marine environments to the north and east. Characteristic lithologies are greenish-gray dolomitic siltstone, very thin- to thick-bedded gypsum, thin-bedded, often laminated dolomite, and sandstone. A formational thickness of approximately 20 meters persists throughout most of the basin, but diminishes rapidly near the northern and eastern limits of deposition.

The Dinwoody is disconformable on the Ervay Member of the Permian Park City Formation except in the northeastern part of the basin, where it locally overlies the Pennsylvanian Tensleep Sandstone. Considering the significant time interval involved (1 to 6 m.y.), physical evidence at the Permian-Triassic boundary is generally limited to an abrupt lithologic change from light-colored shallow marine or intertidal Permian dolomite to greenish-gray Dinwoody siltstone. The Dinwoody grades vertically, as well as laterally to the east and north, into red beds of the Lower Triassic Red Peak Formation of the Chugwater Group.

The Early Triassic depositional environment in the present-day Bighorn Basin was hostile. A sparse molluscan fauna was observed at only one of twenty sections studied, and no acid insoluble microfossils were recovered from Dinwoody carbonates. Significant amounts of gypsum within the Dinwoody suggest periodic high evaporation from hypersaline waters on a low energy, shallow shelf during intervals of reduced terrigenous sediment supply from land areas to the north and east. However, sufficient organic material was present to create reducing conditions, as evidenced by greenish rock color and abundant pyrite.