NEW SCREEN-WASHING APPROACHES TO BIOSTRATIGRAPHY AND PALEOECOLOGY OF NONMARINE ROCKS, CRETACEOUS OF UTAH

Surface prospecting methods used for identifying microvertebrate localities suitable for wet screen-washing yield highly biased samples limited to horizons containing obvious accumulations of fossil remains. The technique is restricted to areas with well-exposed strata and results in the recovery of hydrologically concentrated faunas that are not in situ. In many areas of Utah, Cretaceous strata are either exposed in cliff faces (if the sequences are sandstone dominated), or in road cuts (if the sequences are dominated by mudstones). In order to recover fossils from these kinds of exposures in either critical stratigraphic intervals or geographic areas, I have simply “blind washed” fine-grained horizons that lack little or any evidence of fossils at the surface. This method has yielded well-preserved dentitions and skeletal elements of mammals, lizards, amphibians, and other vertebrates as well as ostracodes and gastropods. This approach permits sampling throughout the entire stratigraphic thickness of formations (providing fine-grained intervals are present) including bases and tops of units. This method also permits the recovery of biostratigraphic materials in areas where outcrops are very steep or limited to road cuts. Faunas recovered from fine-grained horizons show little evidence of transport and provide a more reliable basis for paleoecologic interpretation than hydrologically concentrated fossil accumulations. Multidisciplinary approaches to nonmarine biostratigraphy that incorporate all vertebrates, invertebrates, and palynomorphs as well as “blind washing” techniques should be able to establish biostratigraphies with resolutions that rival marine biostratigraphic schemes.