EVALUATING THE STRUCTURE ACROSS A TRANSITIONAL DECOLLEMENT: THE ALLEGHENY FRONT, LYCOMING COUNTY, PENNSYLVANIA

Pennsylvania is divided by two structurally diverse provinces; the Appalachian Plateau in northwest PA, and the Valley and Ridge in southeastern PA. The Plateau is characterized by gentle folds and low relief topography, overlying a decollement surface in Silurian evaporites. The Valley and Ridge province is a blind fold and thrust belt with a deep decollement surface in Cambrian shale, and an overlying roof thrust detachment in Ordovician shale. How does the mechanical differences of these decollements and the transition from the deep to the more shallow accommodate slip, shortening, and control surface and subsurface geometry?

Because segments of the Allegheny front are now targeted for hydrocarbon exploration, new data is available in this region. High resolution seismic and well data allow unprecedented subsurface detail. We integrate subsurface data with field data (detailed geologic mapping, and outcrop observations) to ensure that geophysical interpretations are supported by observations in outcrops.

Valley and Ridge seismic data reveal thickening of the Ordovician Bald Eagle formation through the Silurian Bloomsburg formation. This strata is predominantly sandstone and is immediately above the Reedsville shale detachment. In thickened areas, the reflector which marks the top of this package is repeated, indicating that it has been faulted. Returning to normal thickness, the seismic reflectors marking the top of the package converge at oblique angles, similar to hanging wall cutoff angles seen in outcrops. We interpret these observations as large scale wedge faulting that shorten the Late Ordovician through Early Silurian strata.

The transition zone between the Valley and Ridge and Plateau show similar structures. The repeated stratigraphic section is the Devonian Marcellus to Keyser formations, consisting of shale and limestone lithology, all above the Silurian Salina evaporite detachment. In seismic, this section doubles in thickness in several locations. As it thickens, horizontal reflectors are truncated by reflectors of ~20°. We interpret the truncation as a ramp, and the resulting structure as small scale wedge faulting. Overlying strata exhibit tight kink folding in seismic, which is propagated up and observed in strike and dip data. Limbs of these folds range from a 30° dip to near vertical.