INTERPRETATION OF TULE SPRINGS FOSSIL BEDS NATIONAL MONUMENT ENVIRONMENTAL CHANGES BASED ON MICROFOSSILS COLLECTED FROM THE LAS VEGAS FORMATION

Here we test the Eaton screenwashing method on Pleistocene deposits of the Las Vegas Formation. The purpose of this study is to collect sediment from Tule Springs Fossil Beds National Monument (TUSK) and adjacent land administered by the Nevada Division of State Parks in order to screen wash and identify microfossils to reconstruct the past environment. This technique has been mostly applied to Mesozoic units, and we are testing its use in more recent formations. Sediments preserved in and around TUSK are Pleistocene in age and located in the northern edge of the Las Vegas Valley in the Upper Las Vegas Wash. It was intended that sediment from each stratigraphic layer within the Las Vegas Formation be collected for screen washing purposes. However, the B2 unit was the first unit collected and it demonstrated to be highly fossiliferous. Due to the abundance of fossils within the B2 unit, it was decided that the study be narrowed down to the B unit and its subgroups in order to properly sort and identify the fossils within them. Above B2 is unit B3. Between units B2 and B3 there is a shift in Milankovitch cycles. Due to this shift, B3 was the second and last unit collected from TUSK in order to compare and contrast the different ecological environments. It is now known that the B2 unit was much wetter than present based upon the abundance of shallow aquatic mollusks and ostracods. Other key paleoclimate indicators are pollen records that favor a sagebrush dominated area. Overall, the B2 unit can be summarized as a marshy environment comprised of channels and spring fed ponds. B3 is the onset of a short drying period where the marshy ponds of the B2 unit begin to dry up. This is based on the lack of shallow aquatic mollusks, cicada burrows, and a decline in the pollen record that had previously suggested the dominated sagebrush valley. The Eaton method of screenwashing proved to be successful in the recovery of Pleistocene microfossils. We successfully recovered from unit B2: ostracods, gastropods (Gyraulus parvus and Physella virgate), bivalves (Pisidium sp.), fish, lizards, voles (Lemmiscus curtatus), and rabbits. B3 produced carbonized wood, but no fossils thus far.