Structural Style of Appalachian Plateau Folds, North-Central Pennsylvania

Mount, Van S.; Harris, Ronald E.; Casillas, Hector A.

The structural style of the Allegheny front and folds underlying the Appalachian Plateau have been studied for well over a century based on surface geology. Over the past 50 years, Appalachian Plateau deep oil and gas exploration wells and 2-D seismic reflection data have shown that the broad, subtle, long wavelength folds observed in Devonian, Mississippian and Pennsylvanian age strata at the surface are detached from deeper, older strata along evaporites within the Upper Silurian Salina Group. Over the past few years, industry activity in north-central Pennsylvania (Potter, Tioga, Bradford, Sullivan, and northern Lycoming, Clinton and Centre counties) associated with exploration and development of the Middle Devonian Marcellus shale, has included acquisition of high quality seismic data (both long offset, regional 2-D lines and 3-D surveys covering large areas) and the drilling of horizontal wells with long lateral reaches (typically over a mile in opposite directions from a vertical pilot well). These new data provide further insight to the structural style of the Appalachian Plateau detachment folds developed above the Upper Silurian Salina Group evaporites, and allow the Plateau fold belt to be partitioned into three structural domains based on detachment fold characteristics (as observed on the recent seismic data and delineated by well results). The depositional thickness of original salt is interpreted to have been one of the factors controlling the structural style of the overlying folds. Systems of short wavelength, low amplitude detachment folds developed over areas of relatively thin original salt (Domain 1). Large amplitude, long wavelength, salt-cored, detachment fold anticlines (Domain 2) developed adjacent to areas of relatively thick original salt; and large synclines (Domain 3) developed over areas that were originally underlain by relatively thick salt. Salt originally beneath Domain 3 synclines is interpreted to have been mobilized, or evacuated, into the cores of adjacent Domain 2 anticlines during folding. Further, the interpretation suggests that lateral heterogeneity in halite net-to-gross within the Upper Silurian Salina Group evaporite section resulted in a non-planar base of salt detachment, which also influenced the evolution of overlying detachment fold domains.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013