USING GEOPHYSICAL METHODS TO DELINEATE LITHOLOGIC BOUNDARIES IN THE PENNSYLVANIA PIEDMONT

The Pennsylvania Piedmont was examined through fieldwork and geophysical methods to define the boundaries of varying lithologies. The regional geology of the Piedmont was most heavily influenced by the Taconic orogeny (470 Ma) when the Taconic island arc collided with the North American continent, obducting, accreting, and creating a wide range of intermingled lithologies. Due to millions of years of erosion, soil build up, and anthropogenic development, contacts between lithologies in the study area were often concealed. Geophysical methods were employed to reveal subsurface lithological changes.

Eleven surveys were conducted at three separate sites using geophysical methods to perform both magnetic (Magnetometer) and induced conductivity (EM-31) surveys. The EM-31 was used in both horizontal and vertical dipole modes to obtain shallow (~3m) and deep (~6m) conductivity data respectively. Total magnetic data was obtained with a Geometrics^Ⓡ proton precession magnetometer and drift corrections were provided. The lithologies present within the study sites were ultramafic, granodiorite, amphibolite, schist, and reaction rocks (ultramafic lithologies siliceously altered through contact metamorphism). The data obtained from surveys were consistent with values reviewed in literature and based on outcrops examined at the study sites was also consistent with expected lithologies. Conductance was observed to be highest in the amphibolite lithologies (3.70 mS/m) and lowest in the Wissahickon Schist (0.9 mS/m). Magnetic susceptibility was highest in the ultramafic bodies (~51767.1 nT) and again lowest in the Wissahickon Schist (~51262.9 nT). Most lithologies exhibited consistent signals with low variance (≥ 0.1%) from the mean; however, ultramafic rocks displayed variance up to 1% from the mean. Transition zones of high slope bridge regions of flat, consistent data signal. Transition zones were sharpest (~4m) where little contact metamorphism was observed. When evidence of contact metamorphism was present, wide transition zones (~30m) were observed. The information collected can be used to help locate concealed contacts, better define boundaries between lithologic zones, and form conclusions about the way these rock types interact with one another.