TECTONIC CONTROLS ON CLIMATE AND THE DEVELOPMENT OF A PLAYA IN THE UPPER PART OF THE CLOVERLY FORMATION (LOWER CRETACEOUS), BIGHORN AND WIND RIVER BASINS, WYOMING

The upper part of the Cloverly Formation in the Bighorn and Wind River Basins of Wyoming consists of smectite-rich, laminated mudstone interlayered with smectite-rich diamictites and pebbly mudstones, wave-rippled, fine- to very fine-grained sandstones, tuffaceous siltstones, gypsum beds, nodular limestone and chalcedony, and discrete layers of barite nodules. Stable sulfur isotope values of barite, d³⁴S_CDT=12.83 and 8.76 per mil, and gypsum, d³⁴S_CDT=10.67 per mil, from the upper part of the Cloverly Formation are consistent with continental evaporite formation. The d³⁴S values of barite and gypsum from the Cloverly Formation indicate mixing of sulfate derived from volcanism and weathering of marine evaporites. Mudstones within the upper part of the Cloverly Formation contain predominantly dioctahedral smectite, and individual beds within the upper part of the Cloverly Formation can be traced laterally up to 100's of meters. The smectite-rich, mud-dominated, upper part of the Cloverly Formation covers ~40,000 km² in central Wyoming and is interpreted to represent deposition on a playa.

The occurrence of extensive playa deposits in the upper part of the Cloverly Formation in the Bighorn and Wind River Basins of Wyoming may suggest the presence of a rain shadow induced by the uplift of the Sevier Mountains in the Early Cretaceous. The eastward progression of lithofacies, from smectite-rich mudstones, evaporites, and nodular limestones in the Bighorn and Wind River Basins to organic-rich mudstones and trough cross-stratified sandstones in the Black Hills region, is also consistent with the diminishing influence of the rain shadow farther away from the mountain front. The extent of the rain shadow preserved in the Lower Cretaceous Cloverly Formation, up to ~500 km into the basin, is comparable to the Patagonia Desert of Argentina, a modern rain shadow produced from the deflection of westerlies over the Andes Mountains.