STRATIGRAPHY, DEPOSITIONAL ENVIRONMENTS, AND FOSSIL VERTEBRATE TAPHONOMY OF THE MAIN BODY OF THE WASATCH FORMATION, THE PINNACLES, WYOMING

The South Pass region of Wyoming exposes the fluvial Wasatch and Bridger formations interfingering with the lacustrine Green River Formation. The lowest unit, the Main Body of the Wasatch Formation, crops out at The Pinnacles, a series of isolated badland bluffs capped by limestones of the Tipton Member of the Green River Formation. This area is of special interest because of the diverse assemblage of late Early Eocene vertebrates found there.

The Pinnacles exposures are dominated by a sequence of 3 or 4 sandstones with a ribbon geometry and large festoon cross beds representing meandering stream channels. They fine upward, often have basal lag conglomerates, and are typically micaceous sublitharenties. The sandstones are interbedded with mudstones ranging from pure claystones to siltstones which are overprinted by weakly developed orange, pink, green and gray paleosols and are interpreted to be flood basin deposits. The overbank mudstones are in places associated with thin, tabular, and finer grained sandstones representing crevasse splay deposits. Well developed purple and green paleosol mudstones underlie the lowest channel complex.

The Pinnacles have yielded over 900 catalogued vertebrate specimens of Lostcabinian (latest Wasatchian, Wa7) age. Detailed sections were measured and examined to determine the fluvial architecture and occurrence of vertebrate fossils. Three fossilferous horizons were located with the abundance of bone decreasing upward. Vertebrate bone concentrations are primarily found in chute-modified point bars sandstones and basal lag conglomerates within channel deposits. These remains are typically fragmentary and abraded and are associated with abundant mudclasts suggesting that reworking from coeval proximal overbank mudstones may have been an important process in their formation. Fossil vertebrates are found in crevasse splay sandstones and some mudstones, but their lower abundance in these sediments suggests that proximal channel processes played the key role in fossil accumulation.

The Pinnacles vertebrates differ significantly from Lostcabinian assemblages derived primarily from overbank mudstones in the type area of the Wind River Basin. Analysis of the depositional, taphonomic, and paleogeographic causes for these differences is presented.