PALEOENVIRONMENTS AND PALEOLIMNOLOGY OF THE EOCENE WAGON BED FORMATION, SOUTHERN WIND RIVER BASIN, CENTRAL WYOMING

Shallowing of the Farallon plate beneath North America has long been proposed to explain contractile deformation that led to the Rocky Mountains and associated basins, but questions remain about the extent of emplacement, timing of slab removal, and its influence on the land surface above. Fluvial and lacustrine sediments of the Eocene Wagon Bed Formation of Central Wyoming may provide insights into the land surface response to slab removal. The Wagon Bed Formation crops out along the margins of the Wind River Basin, adjacent to two anticlinal basement uplifts: the Granite Mountains and the Owl Creek Mountains, both of which formed during Laramide deformation and have subsequently collapsed. Wagon Bed Formation strata adjacent to the Granite Mountains are divided by an uplift into east and west depositional zones by a prominent anticline. A detailed field study was undertaken to better understand the paleogeography and paleolimnology during deposition.

Initial results of 8 measured stratigraphic sections of the Wagon Bed Formation reveal distinct vertical and lateral lithofacies variations outlining several depositional environments. In the west, lithofacies include cross bedded sandstones, finely laminated mudstone, volcanic ash beds containing authigenic zeolite minerals, carbonate cemented tuffaceous sandstone, sandy oolitic and stromatolitic limestone, and matrix-supported volcanic conglomerate. These lithofacies are interpreted to have been deposited in alkaline-saline lacustrine fluvial environments, and by an Absaroka derived volcanic lahar. In the east, volcaniclastic lithofacies are predominant including clay-rich volcanic tuff, tuffaceous sandstone, and intraclastic volcanic breccia. These lithofacies contain evidence for fluvial reworking and paleosol formation. The corresponding depositional environment is interpreted as a floodplain filled with volcaniclastic sediments from the Rattlesnake Hills. New ⁴⁰Ar/³⁹Ar dates for ash beds within the Wagon Bed Formation add to legacy geochronology to constrain the age and duration of deposition. Results of these analyses show the basin experienced three distinct depositional stages: 1) fluvial reworking of clastic input, 2) lake formation, expansion, and salinity increase, and 3) delivery and filling by volcaniclastic material.