Paleoecological Successions from Shallow-marine Depositional Environments in Upper Silurian Carbonate Rocks of Blair County, Pennsylvania

Abstract

Studies of paleoenvironment and paleoecology are important for investigating relationships between the different environmental and biological variables that influenced biodiversification patterns during the Silurian Period. The mixed carbonate and siliciclastic rocks of the Bloomsburg and Mifflintown Formations in central Pennsylvania represent transitions between carbonate ramp and deltaic settings. The goal of this study was to assess changes in depositional environment and paleoecology of an undescribed Bloomsburg/Mifflintown locality in Bellwood, Pennsylvania. In central Pennsylvania, the interfingered relationship of mixed carbonate and siliciclastic mudstones of the Mifflintown Formation with red fine-grained sandstones and shales of the Bloomsburg Formation presented challenges for deciphering the formations. Five stratigraphic sections were measured and described, in addition to identification of fossils and petrographic analysis of rock samples. A relatively diverse range of marine invertebrate fossils was observed, including ostracods, brachiopods, mollusks, crinoids, tentaculitids, bryozoa. orthocerid nautiloids, sponges, tabulate corals, and a rare eurypterid tergite. Nine lithologies were identified, including: (1) bioclastic wackestone-packstone, (2) siliciclastic mudstone, (3) mottled nodular-bedded packstone (4) fossiliferous mudstone-wackestone, (5) laminated peloidal grainstone, (6) recrystallized skeletal packstone, (7) intraclastic conglomerate and detrital carbonates, (8) massive microcrystalline carbonate, and (9) laminated brown calcareous mudstone. The observed fossil taxa were identified to study patterns in paleoecological and faunal community composition across different depositional environments. Based on lithological associations, four lithofacies were identified: (1) distal slope, (2) open lagoon and subtidal channels, (3) intertidal channels, and (4) tidal flat and channels. Most paleoenvironmental shifts were gradual. Paleoecology patterns assessed at the community-level illustrated variation in fauna based on substrate consistency and energy levels. The majority of paleoecommunities consisted of open marine fauna, with one community dominated by semi-restricted, brackish fauna. Faunal communities showed more variation in open marine environments such as ramp, open lagoon, and reef shoals. The undifferentiated Bloomsburg and Mifflintown Formations at Bellwood, Pennsylvania illustrate a general coarsening-upward sequence that reflects a shallowing of depositional environments likely relating to fluxes in siliciclastic sediment input, relating to lateral migration of a coastal delta, relative sea level change, or potentially both. Deposition was dominated by storm-driven processes, with lesser influence of tidal energy during periods of fair weather. The stratigraphy exposed at the Bellwood outcrop represents the evolution of a semi-protected, muddy, low-gradient carbonate ramp system, as it became influenced by siliciclastic influx associated with coastal delta progradation.