GEOPHYSICAL CHARACTERIZATION OF THE LIGHT STREET FAULT, COLUMBIA AND MONTOUR COUNTIES, PENNSYLVANIA

The Light Street fault (LSF) is recognized as a zone of brecciated and broken rock that extends approximately 47 km from north of Danville, PA northeast to Berwick, PA within the Valley and Ridge province. Its inferred trace is oriented 77º-257º and lies on the north limb of the Berwick anticlinorium. The fault offsets and juxtaposes carbonates of the Silurian-age Wills Creek-Keyser-Tonoloway succession with intervals of Devonian strata of the Old Port Sandstone-Marcellus-Mahantango formations. Previous studies used field evidence to characterize the fault zone, sense of motion and displacement and interpreted the LSF to either be a low-angle, south-dipping reverse fault or a north-dipping detachment fault (Inners, 1978, 1981). The location of much of the fault and associated structures is only postulated due to the vegetative cover, soil development and human modification of the landscape. The goal of this study was to image the Light Street fault using magnetic field data and resistivity profiling along transects across the fault trace. The magnetic data were acquired from 168 stations with a Geometrics G-856 portable proton magnetometer. Resistivity data were acquired along a 84 m long transect with a SYSCAL Kid resistivity meter in a Wenner spacing of 3.5 m between electrodes.

Results indicate that the magnetic survey data is inconsistent in imaging the fault across its postulated trace. In some areas the zone of faulting is characterized by steep gradient, magnetic highs, while in other areas low gradient, magnetic lows correspond to the approximate fault location. The variability in the magnetic field signature is probably due to differences in magnetic susceptibility between lithologies on either side of the fault, thus total field magnetic surveys are not adequate to identify these types of structures in the Valley and Ridge province. The resistivity profile, acquired at Briar Creek Lake, shows an abrupt increase in the resistivity that corresponds to the inferred fault trace. The faulting in the area of the resistivity survey is further supported by a subsurface core dataset that was drilled prior to construction of the Briar Creek dam and reservoir. These cores show increasing fracturing and the occurrence of vein-filling calcite in the Marcellus Shale where it is in close proximity to the fault.