STRATIGRAPHIC AND DEPOSITIONAL CONTROLS ON FOSSIL ASSEMBLAGES OF THE NONMARINE MEMBER OF THE UPPER CRETACEOUS MESAVERDE FORMATION ALONG THE TEAPOT DOME, WYOMING, USA

Numerous studies have shown that fossils are non-randomly distributed in the stratigraphic record. However, few case studies have examined the dispersal of fossils in nonmarine deposits. To gain a better understanding of the stratigraphic distribution and depositional controls on fossils in nonmarine strata, this study examined the stratigraphy and fossil assemblages of the nonmarine member of the Mesaverde Formation along the Teapot Dome in the Powder River Basin of Wyoming. The Mesaverde Formation in the Powder River Basin is relatively poorly known compared to adjacent regions, providing an excellent opportunity to also better understand the tectonic, eustatic, and sedimentological influences in northern Wyoming during the middle to late Campanian. To conduct this study, multiple high-resolution stratigraphic sections were measured through the marine Parkman Sandstone and overlying nonmarine member. Data were collected on lithology, sedimentary structures, paleocurrent directions, ichnofossils, body fossils, and types of fossil occurrences (e.g., macro-, micro-bonebeds) to correlate strata and interpret depositional environments and/or controls. Results show that this stratigraphic succession records a switch from a regressive to transgressive shoreline, which was controlled by tectonic and eustatic processes. The marine Parkman Sandstone consists of shoreface to foreshore lithofacies deposited within a prograding delta. In contrast, the overlying nonmarine member is composed of carbonaceous shales, channel sandstones, and mudstones deposited within backswamp, fluvial channel, and floodplain settings along a coastal plain. Above the nonmarine member is a laterally extensive, massive to rippled sandstone, which represents the basal flooding surface of the unnamed marine shale member. The nonmarine member is highly fossiliferous with numerous isolated bones and bonebeds concentrated in various depositional settings, including fluvial channel, floodplain, backswamp, and levee deposits. Micro-bonebeds are situated on or above the nonmarine member indicating a preferential concentration along a flooding surface. These findings show that depositional processes influence the preservation and distribution of fossils in non-marine strata, leading to their concentration in specific horizons and/or depositional environments.