SEDIMENTOLOGICAL CHARACTERISTICS OF AN EXTRAORDINARILY RICH MAMMALIAN FOSSIL LOCALITY WITHIN THE EARLY EOCENE WASATCH FORMATION, GREAT DIVIDE BASIN, WYOMING: EVIDENCE EARLY PRIMATES CONGREGATED AT THE BAR

A mammalian fossil deposit (WMU-VP 2009-01) discovered during the 2009 field season within the Early Eocene Wasatch Formation of the Great Divide Basin (GDB) in Sweetwater County, Wyoming contains the most dense concentration of skeletal remains of any other locality collected in the vicinity over the last 20 years. Twelve prior field seasons in the GDB yielded ca. 7,500 cataloged mammals and ca. 254 jaws with teeth from 100 localities, an average of 75 mammals and 2.5 jaws per locality. Six days of surface collecting and dry screening (with 1/8 inch mesh screen) at WMU-VP 2009-01 yielded 400 jaws with teeth, 2,250 teeth, and 1,350 postcranial fossils of Eocene mammals. Sedimentological characteristics of the Wasatch Formation are typical of immature fluvial sandstones and associated floodplain and deltaic deposits. The fossil-bearing deposits at WMU-VP 2009-01 are typified as fine-medium grained, poorly sorted, very angular - subrounded, arkosic sandstone. Depositional structures include well developed trough cross-beds, less common planar cross-beds, and convolute bedding, with skolithos burrows near the top of the section. The outcrop includes a point bar geometry that has 4.6 meters offset between the channel bottom and bar top. Fossil remains are primarily concentrated within accretion deposits associated with the bar complex. We suppose that sedimentoligic controls on deposition and availability of fossil remains make this an extraordinary site. Furthermore, we predict that identification of similar facies in deposits of similar stratigraphic position within the region will also yield impressive fossil assemblages.