Joint 118th Annual Cordilleran/72nd Annual Rocky Mountain Section Meeting - 2022

Paper No. 42-10
Presentation Time: 4:50 PM


BONDE, Aubrey, Department of Geoscience, University of Nevada Las Vegas, Las Vegas, NV 89154, JONES, AnnMarie, Geosciences, University of Nevada, Las Vegas, Las Vegas, NV 89154, ROWLAND, Stephen, Department of Geoscience, University of Nevada, Las Vegas, 4505 S. Maryland Parkway, Las Vegas, NV 89154-4010, BREITHAUPT, Brent H., Wyoming State Office, Bureau of Land Management, Cheyenne, WY 82003 and MATTHEWS, Neffra A., Bureau of Land Managment, Wyoming State Office, 5353 Yellowstone Road, Cheyenne, WY 82009

The Muddy Creek Formation (MCF) preserves late Miocene fluvial, lacustrine, and paludal settings during infill of internally drained, structurally controlled basins. Deposits primarily consist of red-orange finely-bedded to massive sands, silts, and clays; beds are discontinuous due to stream channel incision and superimposed deposition. Exposures in southern Nevada preserve a small number of fossil localities, ranging from localized sites containing small and large-bodied body fossils mostly preserved in paludal sediments to more laterally extensive fluvial channel, floodplain, and mudflat deposits preserving ichnofossils. This project reports the most fossiliferous deposits yet discovered from the MCF; over 82 sites preserve hundreds of tracks on numerous horizons of laminated mudstone and sandstone. Camelid is by far the most frequent occurrence with over 100 tracks distributed throughout most all horizons. Avian tracks are the most abundant, over 200 tracks, although they are less frequent in their stratigraphic occurrence. Far fewer numbers are recorded of equid tracks, as well as predator tracks, ursid and felid. Sedimentary structures include small to large mudcracks, small to large ripple marks, and evaporitic mineral crystal casts.

The site is located on the western side of Lake Mead on Bureau of Reclamation (BOR) managed lands, and work was conducted under permit from the BOR. The MCF at this site is toward the top of the section corresponding with the latest Miocene to earliest Pliocene (~6-4.8 Ma). Previous researchers have proposed that the MCF may be representative of a savannah-like environment, where climates were slightly moderated and there were periods of higher levels of rainfall in comparison to today. The amount of channel incision at this locality supports increased water volume to this area compared to today. Fossil ichnotaxa and sediments suggest that this site was marginal to the fluctuating shoreline of the paleolake through the late Miocene, where these megafauna were likely moving through the area for hydration. Although, the lack of any plant fossils or impressions does not allow corroboration of a savannah-type environment directly at this site, that setting may have existed even more marginal within the basin than the floodplain/mudflat settings adjacent to the paleolake.