SHIFTS IN PALEOECOLOGICAL SUCCESSIONS FROM DEPOSITIONAL ENVIRONMENTS IN THE UPPER SILURIAN CARBONATES OF CENTRAL PENNSYLVANIA

Faunal successions of Appalachian Basin carbonates in Bellwood, PA provide an archive for studying how marine invertebrate biodiversity responded to environmental changes in the Silurian. The sequence stratigraphy is characterized by frequent fluctuations in sea level and oceanic oxygen levels, controlling trends in fossil community compositions and depositional lithofacies. This study has two main objectives; 1) assess long-term recovery of marine biodiversity from mass extinction events, and 2) evaluate response of marine fauna to frequently fluctuating sea level. The studied carbonate rocks are of Wenlockian-Ludlovian age and include calcareous mudstones of the Mifflintown Formation, overlain by red beds of the Bloomsburg Formation. Previous literature has interpreted these formations as representing epeiric ramp and delta plain depositional environments, respectively. The observed fossil taxa are classified to examine patterns of increasing and decreasing biodiversity across different populations through time. Marine transgressions and regressions are reconstructed by studying temporal and spatial shifts in lithofacies associations and faunal assemblages, correlating to shifts in depositional environment. Thin sections of rock samples will be examined to classify the various rock members based on carbonate lithofacies. Paleoecology and depositional environment will be reconstructed by examining lithofacies, sedimentary structures and paleontology in hand samples and outcrop. Paleoecological trends will be correlated with sea level changes and other environmental impacts to assess drivers of change in shallow marine community structures. A relatively diverse range of marine invertebrate fossils was observed on a preliminary visit to the study area, including ostracods, brachiopods, mollusks, crinoids, tentaculitids, and bryozoa. Additionally, orthocerid specimens, tabulate corals and a eurypterid tergite were retrieved, which are rare in other related localities. It is hypothesized that the rocks of this locality reflect a deeper marginal marine environment relative to other related localities, based on greater fossil diversity and abundance. Marine biodiversity is expected to be highest during marine transgressive periods due to increased shallow marine ecospace.