STRUCTURAL TRANSITIONS FROM VALLEY-AND-RIDGE TO PLATEAU: ALONG-STRIKE CHANGES AROUND THE BEND IN CENTRAL PENNSYLVANIA

The nature of the structural transition from the Valley-and-Ridge to the Appalachian plateau varies along strike around the bend in central Pennsylvania. These changes can be attributed to changes in the stratigraphic level of the active detachment under the plateau. At the northeast end of the bend, the transition corresponds to a change in the basal detachment from lowest Cambrian to the Salina salt with a progressive decay of imbrication northward. Along the central portion of the bend, the transition is a monocline that is underlain by backthrusts rising from the upper Ordovician shale detachment and cutting across the anticlinal stack of Cambro-Ordovician strata that forms the core of the Nittany Valley. At the southwest end of the bend, the transition is marked by a dramatic decrease from valley-and-ridge to plateau in the displacements on the ramps rising from the lower Cambrian detachment, without an accompanying change in the overall structural style; this transition continues southwestward and creates the more diffuse frontal edge of the Valley-and-Ridge in West Virginia.

The boundaries between these segments is not entirely abrupt, with structural features being traceable into adjoining segments. The boundary between the northeastern and central portions occurs around the village of Woolrich as the thrusts of the shallow imbricating structures die out westward, apparently forming a composite lateral ramp that is virtually invisible in the field. As that lateral ramp is approached, the ramp-bend folding disappears and all strata dip northwestward in an apparent congruent monocline, the stratigraphic offsets being obscured in the very repetitive middle Devonian shale-siltstone sequences. The boundary between the central and southwestern portions occurs around the village of Bald Eagle along a very prominent tear fault. Despite the apparent abruptness of this transition, the backthrusting can be traced to the very southwestern end of the Nittany antiformal structure and the deep-seated imbrications under the plateau appear to extend north of that boundary.