MORE STRUCTURES BEARING ON AN INITIAL ACADIAN EXTENSIONAL ORIGIN FOR THE DOMINANT STRUCTURES IN THE WESTERN APPALACHIAN FORELAND BASIN OF PENNSYLVANIA, AND NYS ANALOGS

Previously we proposed that 1) the dominant structures in the western PA and WV Appalachian Plateau originated initially from Late Devonian vertical loading and extension/sliding/slumping of the Upper Silurian/Devonian section, 2) the slumping resulted in prominent back-rotated blocks over locally flow-thinned and thickened Silurian units, 3) hinterland (easterly) verging thrusts initiated in Late Devonian, and 4) Carboniferous/Permian oil and gas migrated up the initially Acadian faults to Upper Devonian sands. Additional analyses of 2D and 3D seismic data in the western part of the PA Appalachian Plateau reveal structural features that support the slump hypothesis and the Acadian initial age of deformation.

Spectacular, distinct relay ramps that allow the main fault system to step over are observed in a 3D seismic survey. The relay ramps are on the order of 1000 ft to 5000 ft (0.3 to 1.5 km) wide and generally 10,0000 ft to 15,000 ft (3 to 4.6 km) long. Their structural style is strongly similar to relay ramps in other extensional regimes. We also recognized a rollover fold above a probable listric fault where lower units in the rollover fold are successively cut out along the more basal reaches of the fault. This geometry is also consistent with the hypothesis that the faults affecting the Onondaga were not initially compressional but extensional.

The data support a Late Devonian, neo-Acadian age for initial development of the decollement and thrusts, based on Upper Devonian sediments that onlapped and infilled the drape-synclines above the zone-of-removals associated with the slumping/sliding. Although many of the zones-of-removal appear to have initiated in Upper Devonian Bradford and Elk sand time, we have now also found older timings in other slumps such as Upper Devonian Hamilton (post Marcellus) and Rhinestreet times.

The decollement observed in seismic lines may have outcrop analogies in western NYS where stratigraphically-confined fracture cleavage/pencil cleavage zones occur that commonly include roll ups and other soft-sediment, boudined features. The soft sediment deformation is similar to seafloor-surface slide/slump folds, except that present-day observed seafloor-surface slides/slumps do not have cleavage and do have differential deposition over the rough upper surface of the slide material.