PRELIMINARY INVESTIGATION OF FRACTURES AND FAULTS IN THE MORGANTOWN SHEET, NEWARK BASIN, SOUTHEASTERN PENNSYLVANIA

The Morgantown Sheet, located in southeastern Pennsylvania in the westernmost Newark basin, is a diabase intrusion that was part of the Central Atlantic Magmatic Province (CAMP). The Morgantown Sheet comprises an interconnected network of dikes and sills that fed CAMP flood basalts. Magmatic layering in the diabase dips approximately 20° to the NW indicating post-crystallization rotation of rocks in the basin. As a result, the southern portion of the basin exposes rocks that were 6 to 7 km deeper than those at the northern margin at the time of intrusion.

This study will examine fractures in diabase that crystallized at different depths. Efforts thus far are focused on an exposure of the Morgantown Sheet along highway I-176 in Morgantown PA. This is the longest exposure of the Sheet and is located in the lower part of the intrusion. Measurements of 47 moderate to steeply dipping fractures taken at uniform intervals along the road cut are somewhat dispersed, but exhibit maxima at strikes of approximately 210 and 295 (rhr). Steeply dipping (> 60°) mineralized (both chlorite and calcite) fractures (n = 10), show strikes which range from 099 to 224, with the majority between 99 to 109 degrees. Two biotite coated fracture surfaces show similar orientation. Four fault surfaces, which exhibit chlorite(?) slickenfibers, strike approximately N-S. Chatter mark asymmetry and shallowly plunging slickenlines indicate right lateral strike-slip motion. The variation in orientation of these is such that they could not have formed under the same stress orientations. Future work will focus on establishing cross-cutting relationships and examine additional fractures at other depths. Preliminary observations at an exposure representing a shallow paleo-depth indicates that columnar jointing is more prevalent.