STRAIN ANALYSIS ON FOSSILS FROM THE SUSQUEHANNA RIVER VALLEY, SOUTHERN VALLEY AND RIDGE PHYSIOGRAPHIC PROVINCE

We present new analyses of finite strain from the hinge of the Pennsylvania salient in the southern Valley and Ridge physiographic province. This study focuses on a north-south trending swath running along the Susquehanna River from Harrisburg to approximately four miles north of Halifax, PA. Folded and faulted Silurian through Mississippian age clastic sediment is exposed throughout the study area. Outcrops of the Devonian Trimmers Rock Formation, an olive grey shale and siltstone interbedded with fine sandstone, were found to be consistently fossiliferous. Crinoid columnals and brachiopod fossils were collected from a single first order anticline and used to quantify the orientations of the principle axes of finite strain within the Trimmers Rock Formation. The fold is a steeply inclined asymmetric anticline with a fold hinge oriented 06° 071°. Best-fit solutions for strain ellipses are determined using the Fry and Wellman methods for crinoid columnals and brachiopod samples, respectively. In general, the long axes of the strain ellipses are oriented parallel to strike of the fold limbs. The best-fit strain ellipses are more elliptical along the southern limb of the fold than on the northern fold limb. Our strain ellipses are combined with the orientations of strain ellipses from previous studies across the Pennsylvania salient to elucidate regional-scale trends in the distribution of strain across the orogen.