BOUGUER GRAVITY STUDY OF THE ST. JAMES FAULT COMPLEX WITHIN THE NITTANY ANTICLINORIUM, CENTRAL PENNSYLVANIA

The St. James Fault Complex in Nippenose Valley, Central Pennsylvania, is mapped as a folded, low-angle thrust fault separating Ordovician carbonate and clastic rocks (Faill and Wells, 1977; Lloyd and Carswell, 1981). Although the fault complex is not exposed at the surface within this doubly-plunging anticline, offset formation contacts require a 7.5 km long composite trace length with a maximum 150 m stratigraphic separation. The carbonate-bearing formations of the Nittany anticlinorium are known to be karstified, typically along fracture networks. Thus, we examined the extent to which karst is localized by the St James Fault Complex. We attempt to overcome the lack of surface exposure with multiple geophysical methods, such as ground-based gravity readings. Nearby limestone surface mines generate obfuscating seismic noise that required us to develop a new approach to collect both accurate and precise gravity data sets on both limbs of the anticlinorium using an auto-leveling, tide corrected gravimeter. We collected 50,000+ gravity measurements at 300+ locations, and report the average values, along with the respective 95% confidence intervals, at each location. Gravity readings with confidence intervals greater than ±0.020 mgal, were recollected to limit uncertainty. We used an RTK-GPS combined with OPUS-correction to measure the latitude, longitude, and elevation of each location with an accuracy and precision of ±0.05 m. The combination of high quality spatial information and the modified collection procedure lead to a majority of the gravity readings with precisions ±0.003 to 0.007 mgal. Modeling efforts are ongoing; however, these data clearly show a regional increase in gravity towards the fold hinge from the middle Ordovician clastic and carbonate formations to the lower Ordovician dolomites within the Bellefonte Formation. Local gravity deficits, typically ranging from 0.1-1.4 mgals constrained within 10-80 m, will be used to model karst features within the anticlinorium. Our gravity survey clearly demonstrates that high resolution gravity data, in corroboration with complementary geophysical data sets, can be used to constrain the locations of the bedrock contacts and karst features within Nippenose Valley and allow us to confirm or adjust the location of the fault trace previously mapped.