FRACTURE ORIENTATIONS AND GAMMA COUNTS IN DEVONIAN MARCELLUS SHALE OUTCROPS IN THE VALLEY AND RIDGE PROVINCE ADJACENT TO THE ALLEGHENY FRONT

Three outcrops exposing Marcellus Shale along the westernmost margin of the Appalachian Fold and Thrust Belt along the Allegheny Mountain front are being investigated. Lithologic units bounding the Marcellus are identified where possible to determine the stratigraphic position of the outcrop. Gamma counts are also taken to compare to wireline logs from nearby Allegheny Plateau boreholes for stratigraphic control. Fracture sets in these outcrops are measured and described. Here we present our initial results from one well-exposed outcrop. Our goal is to better understand fracture orientation and spacing of the Marcellus shale beneath the nearby portions of the Allegheny Plateau, and at the same time, improve our understanding of the history of tectonic stresses the rocks have experienced. Detailed transects of fracture measurements have been made at an outcrop in Tyrone, PA to document the relative age of different fracture sets. We identified three distinct fracture geometries, two of which are generally not mineralized and have a conjugate geometry, and one that is commonly filled with fibrous calcite. The conjugate fractures display intersection lineations approximately normal to bedding and we tentatively interpret these to have formed due to tectonic loading when bedding was nominally horizontal. The acute bisector of these fractures in a bedding-horizontal reference frame suggests shortening oriented toward 333°. These fractures display mutual cross cutting relations and clear conjugate geometries suggesting that they do not record two distinct episodes of jointing. The calcite-filled fractures are subvertical and were sampled for petrographic analyses to gain insight into the incremental strain and possibly the geometry of tectonic forces that drove the opening of the fracture. These fractures may post-date the conjugate fractures however to date we have not found cross-cutting relations. Ongoing analyses are aimed at establishing a possible temporal relation between these fractures and a late-stage fault zone that cuts the outcrop. Overall our initial findings suggest that a subset of fractures in Valley and Ridge exposures of the Marcellus shale might be useful for predicting the orientation and spacing of fractures beneath the Allegheny Plateau.