Print this pageAbstract: Carbonate Environments of Middle Ordovician Interval, Central Pennsylvania
The Middle Ordovician interval of eastern North America displays a sedimentary sequence which was imprinted by the effects of the developing Taconic orogeny. In central Pennsylvania this interval is a carbonate bank-to-basin transition. The lower part (Black River) can be divided into four paleoenvironmentally significant facies. These facies are recognized by characteristic cycles of lithologies and are interpreted as having formed primarily by processes within and inherent to each environment.
(1) The 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. (2) The 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. (3) The mottled facies varies from a mottled calcisiltite to a mottled micrite. The mottling is due to bioturbation and the deposits are interpreted as having formed in a quiet shelf environment. (4) The nodular facies contains a nodular, lumpy-bedded limestone. The character of the beds most probably is due to an increased terrigenous influx from th eastern part 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 bank-to-basin transition.
AAPG Search and Discovery Article #90961©1978 AAPG Annual Convention and Exhibition, Oklahoma City, Oklahoma