A widely accepted structural model for folds in the outer central Appalachian foreland is partially based on the geologic structure of the North Summit field. The model includes a simple surface anticline that is detached in Silurian Salina Group salts and cored by imbricated Devonian Tully-Helderberg rocks thrust inward toward a depressed axial low.
New data from wells at North Summit show that the core of the Chestnut Ridge anticline is not filled with imbricated reservoir rocks but that the reservoir is deformed into a series of faulted folds. Gas was trapped by a combination of closure and sealing faults. Space problems within collapsing synclines above competent reservoir rocks, the Huntersville-Helderberg lithostructural unit, are resolved by distortion and evacuation of overlying, incompetent Hamilton rocks. Huntersville-Helderberg rocks deformed into a variety of structural shapes, not solely the imbricated model that traditionally has been applied to Plateau folds of the central Appalachian foreland.
Disparity in relief between pre- and post-Salina rocks indicates that primary detachment occurs in evaporite beds of the Salina Group. Erratic dips in presalt units and unresolved differences in structural relief between synclines flanking the Chestnut Ridge anticline, however, suggest that the basal detachment lies within the Martinsburg-Reedsville shales under the Allegheny Mountains. Regional decollement in Martinsburg shales ends in a triangle zone at the Intra-Plateau Front, at the Chestnut Ridge anticline, whereas the Salina decollement continues westward under the Pittsburgh Plateau.