Seismic and Stratigraphic Evidence for Recurrent Movement along Precambrian Age Fault Systems throughout Middle to Late Paleozoic in Central Appalachians

Craig A. Eckert
EQT Production Co., Pittsburgh, PA, [email protected]

Multiple stages of deformation and thrusting appear to have created a faulted basement complex which continued to reactivate during orogenic and inter-orogenic times throughout much of the Paleozoic. Along-strike and cross-strike (CSD) basement fault patterns established as result of the multiple tectonic events comprising the 100 million year duration Grenville orogeny may be seen reflected in the development of the middle Cambrian Rome Trough, as well as numerous structural and depositional patterns in mid to late Paleozoic. Seismic and stratigraphic evidence abound for these relationships. Examples range from the Silurian through the upper Devonian in the central Appalachians. Methodologies included time stratigraphic well correlations, seismic well ties and correlation of horizons exhibiting impedance contrasts throughout a ~8,000 mile 2D seismic grid from New York to central Kentucky. Faulted and depth converted time horizons were mapped throughout the data set along with existing well control for a basin wide structural framework from basement to base of Devonian shale.

During Silurian time, Salina depositional patterns were apparently influenced and somewhat defined by CSDs and other deep seated tectonic elements. The resulting stratigraphic variations in these evaporate deposits not only influenced the structural development of the Allegheny Plateau fold belt, but also resulted in an earlier gravity-sliding event that contributed to depositional patterns observable in the overlying Devonian section.

Within the Upper Devonian, discrete stratigraphic sequences were mapped by identifying and correlating fourth order flooding surfaces in Famennian section of northern West Virginia and southwestern Pennsylvania. These mapped sequences contain mostly distal turbidites, and reveal three dimensional geometries attributable to the subtle, but active movement of basement blocks. The gross sequence geometries also reflect the geometries of the internal clastic reservoir units contained within. Mapping of basement based on 2D and 3D seismic data strongly suggest that a series of en echelon basement faults reactivated along depositional strike, which abruptly changed bathymetric levels and thus resulted in strike oriented turbidite belts. Recurrent movement along many of the CSDs may be seen reflected in termination of linear turbidite bars whose extent was determined by existing deep seated transform faults.

Other evidence for the influence of basement fabric on Devonian deposition may be seen in recent 3D surveys in which amplitude attributes in the Marcellus exhibit a striking correlation to the underlying basement. In addition, changes in position of Alleghenian age decollement zones occur along pre-existing major CSDs where lateral ramps form and allow translation of bedding plane movement from one stratigraphic level to another. These changes in decollements may be in part responsible for over- and under-pressure seen in the organic shales.

AAPG Search and Discovery Article #90154©2012 AAPG Eastern Section Meeting, Cleveland, Ohio, 22-26 September 2012