VARIABILITY IN DEFORMATIONAL STYLE OF APPALACHIAN PLATEAU FOLDS, SULLIVAN COUNTY, PENNSYLVANIA

Within the Appalachian Plateau of northern Pennsylvania, strike-parallel zones of deformation occur within Devonian strata that are inconsistent with regional structural patterns. Traditionally, Appalachian Plateau structures consist of broad anticline and syncline with bedding dips in the 3 to 6 degree range. Field mapping in areas of Sullivan and Lycoming counties have recognized zones where structural dips often exceed 30 degrees. These areas are on the south limbs of the south-southeast verging, asymmetrical Towanda and Wilmot anticlines and are largely confined to strata of the Devonian Catskill Formation, but the scale of deformation has not yet been traced into the other stratigraphic units. Deformation extends along strike 8 – 20 km and is generally in zones 2 – 3 km wide. There is often a strong topographic expression of the folding that is evident on digital elevation models and the increased structural dip has caused localized slope stability problems.

Most of the field research has focused in northwestern Sullivan County along the east-west trending Little Loyalsock Creek which is bounded to the south by the Bernice syncline and to the north by the Wilmot anticline. Here, strata on the southern limb of the Wilmot anticline exhibit a trend of shallow dips (7-8 degrees) closer to the fold axis that increase in magnitude to the south. Structural dips in the most intensely deformed areas are 17 to 40 degrees to the south-southwest, with minor low-angle faulting present.

We hypothesize that these zones of more steeply dipping strata are likely associated with regions where the Silurian-age Salina Group salt in the subsurface (~2.5 km depth) is thicker and may exceed ~850 m (Mount, 2014). The thicker accumulations of salt, forming the cores of the anticlines, could influence regional structure in the shallower surface exposures resulting in steeper dip magnitudes. Areas where the salt is thinner in the subsurface would result more typical dip magnitudes of 3 to 6 degrees.