PETROGRAPHIC AND GEOCHEMICAL EVALUATION OF THE UPPER ORDOVICIAN JUNIATA FORMATION IN CENTRAL PENNSYLVANIA: IMPLICATIONS FOR CHANGES IN SEDIMENTATION RATE

A well-preserved section of the Upper Ordovician Juniata Formation, forming from Taconic highland sourced sediments is exposed in an outcrop at Potters Mills, PA. Previous work by Seminack et al. (2016) has divided this formation into seven distinct units based on visual observations which represent a transition from a shallow, marine-dominated depositional environment at the base of the formation to a fluvial-dominated depositional environment in the upper units. This study identifies potential changes in provenance, sedimentation, and geochemistry across these seven units as depositional environments within the Juniata Formation shift from marine to fluvial. Twenty-one thin sections were prepared from samples collected from the base, middle, and upper portion of each unit. Provenance and the detrital mode of these samples was determined through >3000 points counted per thin section. Geochemical (major, minor and trace element) analysis utilizes the ratio of immobile to mobile elements to offer insight into sedimentation and weathering of these units. Tectonic setting was interpreted by plotting point counts on a QFL plot following the methods of Dickinson et al. (1983). Basal units (1-4) plot within the dissected arc region of the QFL diagram, closer to the F-pole. There is a shift towards to the Q-pole in the upper units (5-7), plotting within the dissected arc region, but further towards the basement uplift/transitional continent line. This shift is likely due to a general unroofing trend as the Taconic uplands to the east eroded, but may also partially reflect sediment rejuvenation due to reactivated uplift (as seen in Unit 7). Geochemical analysis was conducted using bulk samples from each unit. Three samples per unit (base/mid/upper) were powdered and analyzed using x-ray fluorescence. A general shift is apparent during the transition from basal to upper units (Unit 5), similar to that observed on the QFL diagram. Measures of SiO₂/Al₂O₃, and Ti/Zr show sediments of Unit 5 to be less mature. Furthermore, previous studies documented convolute bedding, bidirectional cross bedding, and flute casts within Unit 5. We hypothesize that Unit 5 represents a rapid increase in sedimentation rate due to observed sedimentary structures and geochemical signatures likely tied to previously documented climatic changes.