SEDIMENTARY AND ICHNOLOGICAL ANALYSIS OF THE JUNIATA FORMATION AT POTTERS MILLS AND REEDSVILLE, PENNSYLVANIA: A POST-TACONIC CLASTIC WEDGE

The Upper Ordovician Juniata Formation spans a large portion of the Appalachian Basin and is sourced from the eroding highlands formed during the Taconic Orogeny. Road cut exposures at Potters Mills and Reedsville, Pennsylvania were analyzed for lithology, sedimentary structures, and trace fossils. Interpretations of the depositional setting for the Juniata Formation have been a controversial topic, with proposed environments ranging from meandering fluvial to shallow marine conditions. The study section exhibits an overall fining upward trend, ranging from medium- to coarse-grained sandstone to cyclically graded, fine-grained sandstone to paleosol packages at the top of the formation. The lower section of the Potters Mills exposure exhibits cross-bedded and planar-bedded sedimentary structures. Convolute bedding, flaser bedding, and flute marks are more abundant near the top of the formation. Flaser bedding indicates deposition of mud during slack currents and is interpreted as a marginal marine, tidally influenced depocenter. Conversely, flute marks within the same succession may be representative of a fluvial-influenced setting. The overlying paleosols indicate subaerial hiatuses. Ichnofossils are abundant in the upper half of the Juniata Formation. Skolithos >10 cm in length are common in muddy and paleosol units. Planolites-like burrows were observed in medium-to-fine sandstone units. These burrows have been previously described as some of the oldest terrestrial trace fossils of this type. Facies relationships were utilized to further decipher the environment consistent with these traces. Our interpretation of the exposed section of the Juniata Formation is a coastal plain setting, similar to the modern Atlantic coastal plain. The clastic wedges are characterized by deposition from fluvial systems, which are reworked by marine processes due to eustatic sea-level fluctuations.