BIOLOGICAL AND SEDIMENTOLOGICAL CONDITIONS OF THE NOPAH FORMATION AND POGONIP GROUP (LATE CAMBRIAN TO ORDOVICIAN) OF THE ANTELOPE VALLEY REGION OF NEVADA

The late Cambrian, here defined as the Miaolingian and Furongian Series, is not as well studied as the preceding early Cambrian and succeeding Early Ordovician as it has typically been associated with lower diversity levels. However, proximity of this interval to two diversification events (the Cambrian Radiation and Great Ordovician Biodiversification Event) has raised questions as to whether environmental conditions of the late Cambrian prevented early metazoans from diversifying at a continuous rate through the late Cambrian. Our goal is to identify any potential environmental disturbances in the late Cambrian and early Ordovician as well as assess biological diversity in response to any disturbances.

Our study briefly investigated the Nopah Formation of the Late Cambrian. Detailed sedimentological analysis was not performed, but outcrop observations revealed assemblages dominated by gastropods and microbial fabrics. There was a general lack of faunal diversity and gastropods that were present were significantly smaller than those found in overlying formations.

The primary focus of our study was the Pogonip Group (specifically the Antelope Valley Limestone) of the early/middle Ordovician using sedimentological analysis accompanied by point counts of both thin sections and outcrop. Sedimentological analysis showed progressively shallowing marine carbonate settings with high amounts of terrestrial input. Thin sections and outcrop show a high abundance of receptaculitalean and gastropod assemblages. We interpreted the environment here as a restricted, shallow marine setting similar to a lagoon. The fauna in the early/middle Ordovician was more diverse and had a larger mean body size in comparison to late Cambrian deposits in the same region. Future geochemical and more detailed analysis of the Nopah Formation is required, but the observations provided here tentatively suggest a reduction in inhibiting environmental conditions in the early Ordovician as compared to the late Cambrian. Additional investigation into this time period may answer longstanding questions about the connectedness of the Cambrian and Ordovician Radiations.