A Paleoenvironmental Study Of The Middle Ordovician (Black Review) Interval In Central Pennsylvania

Abstract

The Middle Ordovician interval of Eastern North America displays a sedimentary sequence which was imprinted by the effects of the developing Taconic Orogeny. This interval in central Pennsylvania is comprised of a carbonate bank to basin transition. The lower portion (Black River), the focus f this study, can be divided into four paleoenvironmentally significant facies. These facies are are recognized by characteristic cycles of lithologies which they exhibit. These cycles are interpreted as having been primarily formed by processes within and inherent to each environment. The first facies (Laminite Facies) is comprised of cycles of micritic intraclastic conglomerates, varying algal structures and shales. These lithologies are interpreted as having formed on an intertidal-supratidal flat. The second facies (Calcarenite Facies) is comprised of varying calcarenitic lithologies and a micrite containing the schizocoral Tetradium cellulosum. These deposits are interpreted as representing a high energy intertidal ("wave baffle") zone. The third facies (Mottled Facies) varies from a mottled calcisilitite to a mottled micrite. The mottling is due to bioturbation and the deposits are interpreted as having formed in a quiet shelf environment. The fourth facies (Nodular Facies) contains a nodular, lumpy bedded limestone. The character of the beds is most likely due to an increased terrigenous influx from the eastern portion of the basin. Segregation of the nodules was accomplished by either physical or chemical means/ This environment of deposition is interpreted as a deeper shelf. These four facies thus display the shallow water environments of the entire Middle Ordovician transition.