Acadian Sliding: Anatomy of Styles for Gravitational Fault Development and Hydrocarbon Migration in the Western Appalachian Foreland Basin of Pennsylvania and West Virginia
Jacobi, Robert; Starr, Joel; Jackson, David; Warner, Travis; Eckert, Craig
Outstanding imaging in 3-D and 2-D seismic data reveals details of the complicated geometries and timing of structural elements in the western part of the PA/WV Appalachian Plateau. These structures are consistent with Late Devonian extension/sliding/slumping of the Upper Silurian/Devonian section easterly toward the basin axis in response to vertical loading. Prominent back-rotated blocks over locally downslope flow-thickened Silurian units, and easterly verging thrusts all initiated in Late Devonian, based primarily on growth fault geometries. Normal faults are associated with the early slumping. Further motion occurred on a number of these faults during Alleghanian. The Acadian faults developed before the Devonian sediments entered the oil window. Subsidence models suggest the faulted area entered the oil window in Late (neo) Acadian, and the gas window in early Alleghanian. Near the faults J1 trajectories rotate into orthogonality with the fault strike, indicating these particular faults were "open" when J1 developed; the J1 fractures would have delivered gas to these faults, which in turn may have provided hydrocarbon migration pathways to the Elk and Bradford sands, as well as surface seeps.Part of the Acadian structural system in a 3-D seismic survey includes 1) an upslope zone-of-removal where the basal decollement cut a linear, 1050 m wide trough downsection deep (~65m) into the Silurian Vernon shale, 2) an adjacent down-slope, slide-thickened, Vernon/Lower Salina (D-Salt) section that exhibits spectacular box folds, kink folds, and disharmonic folds indicating multiple decollement and thrust ramps, and 3) steeply dipping faults with down-on-the-east throw, some of which are normal faults that accommodated downslope transport with slump block back-rotation on the Vernon and D-salt section. A Late Devonian age is suggested for the decollement because Upper Devonian Elk and Bradford sands onlap and infill the drape-synclines above the zones-of-removal. Additionally, basement fault-influenced paleoslope changes occurred then, implying a possible cause of the Salina collapse zone. The thick F-salt section does not display significant thinning or thickening in the zone of removal or in the slide-thickened area of the back-rotated blocks, but is internally deformed. The primarily easterly-verging thrusts climb upsection from nickpoints primarily at the Onondaga/Helderberg or the Tully. Thrust fault-associated anticlines have F-salt and Vernon thickening.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013