Marcellus Shale Deformation and Related Devonian and Silurian Age Structural Styles and Fabrics, Appalachian Foreland, Susquehanna County, Pennsylvania, USA
Wylie, Albert S.; Parker, Ronald L.
Distinctly different structural styles characterize the Ordovician through upper Devonian stratigraphic sections as observed in 3-D seismic data recorded across Susquehanna County, Pennsylvania. Ordovician through Lower Silurian stratigraphic intervals appear only slightly deformed due to compression. Silurian Salina Group salt to middle Devonian Hamilton Group stratigraphic intervals display an intense combination of compressional, extensional and possibly strike-slip structural features, compounded by salt movement and related deformation. Upper Devonian stratigraphic intervals and upper Devonian surface outcrops display compressional features.
Examination of seismic data from the Salina salt through the Marcellus shale reveals narrow graben-like features formed by late salt withdrawal. The graben-like features terminate along strike, shift hundreds of feet and then develop again with similar patterns and geometries. Adjacent anticlines are long, tight and salt-cored. Near vertical fold axes and fault planes rotate from north to south and south to north and are parallel to the graben-like features. Deformation fabrics interpreted from the seismic data at the Union Springs and Marcellus levels include detachment folds, faults, and numerous stratal discontinuities. Evidence of these features is observed indirectly in horizontal well bores drilled into the Marcellus shale using stratigraphic correlations, formation dip interpretations and cuttings analyses. Analogous deformation features are observed in Marcellus Formation exposures in central and eastern New York and in cored Marcellus intervals indicating the widespread presence of these features in this portion of the Appalachian Foreland basin.
Well known structural detachments are observed at the base of the Salina salt section. However, detachments also are observed (1) at one or more stratigraphic positions in the Marcellus Formation, and (2) at multiple positions above the upper Devonian Tully Limestone. Patterns in borehole breakout and drilling induced fractures measured by image logs collected through the Tully section suggest a clockwise rotation in the maximum stress direction near this stratigraphic level, further supporting the possible presence of shallow detachments. The interpretation of vertical changes in structural style and stress patterns indicate that upper Devonian strata are likely detached from the underlying middle Devonian and older strata beneath Susquehanna County.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013