CHARACTERIZING BEDROCK JOINT AND STREAM LINEAMENT PATTERNS FOR A REGIONAL WATERSHED ON THE APPALACHIAN PLATEAU, NORTHWESTERN PENNSYLVANIA

More than 4,000 joints located at ~400 sites were measured in bedrock exposures across the French Creek watershed (2850 km²) by a summer research team of 11 undergraduate students. Bedrock consists of Devonian and Mississippian siliciclastic rocks covered by Quaternary glacial sediments and Holocene stream deposits. The majority of measurements were made where bedrock was exposed in stream bottoms. Bedrock joints occur in three predominant orientations: a NW/SE and NE/SW set that occurs in nearly equal amounts coupled with a N/S pattern that occurs less frequently. Joint patterns do not vary appreciably with changes in lateral and vertical space, bedrock age, or lithology. Stream segments (n= 3,804), mapped as lineaments on aerial photographs, have the same orientation patterns observed in the bedrock joints. More than 96% of the stream lineaments are less than 800 m in length; among these, all three orientations are present with similar frequency. Longer stream segments (800 - 2,000m) exhibit much less variability in orientation and have a greater frequency of NE/SW trends with subordinate N/S and few NW/SE trends. This regional dataset clearly demonstrates the influence of bedrock joints on stream orientation. These joint patterns can be attributed to the regional tectonic setting (both ancient and modern). The N/S joints are consistent with the direction of shortening during the early (Lackawana) phase of the Alleghanian orogeny, while the NW/SE joints are consistent with the orientation of shortening during the main orogenic phase. The NE/SW joints are less straightforward; they are subparallel to the orientation of contemporary maximum horizontal stress but are also orientated parallel to basement structures that may have been reactivated during orogenesis. Detailed mapping work at some of these sites may yield additional insight into the timing of and mode of origin for these joints. Other future work will be directed to examine the relationship between stream order and the size of the stream valley to joint and stream lineament orientation.