DETACHMENT STRUCTURES DOCUMENT WIDESPREAD FORELAND DEFORMATION IN THE BASAL MARCELLUS SHALE, CENTRAL NEW YORK STATE

Deformation in the basal Marcellus Subgroup (Union Springs, Cherry Valley and Oatka Creek Formations) is of interest to developers of gas resources in the Appalachian foreland of New York and Pennsylvania. Exposures in a 180 km along-strike zone in the Marcellus outcrop belt document a pattern of north-directed overthrusting. These detachment structures are present in areas lacking subsurface Silurian salt (Cherry Valley, (Bosworth, 1984), and Oriskany Falls, NY, and to the west where salt is present (northern Cayuga Lake Valley). Detachment is localized within the 25-60’ thick Union Springs, and in bentonite horizons in the underlying Onondaga Formation. Horizontal motion in the Union Springs was accommodated within carbonaceous shale, with striated and polished surfaces developed on stiffer, bounding carbonate units. Carbonate beds are cut by ramp faults with cm to m-scale displacement. Shale layers are thickened to form imbricate, cleaved and polished shale ‘wads’. Black shale of the overlying Oatka Creek Formation lacks evidence of thrust-related deformation at the detachment fault localities; however vertical jointing is intensified and may provide enhanced permeability.

Veins in the detachments document two episodes of mineralization; early calcite and quartz crystal growth occurred during evolution of fluid hydrocarbons; a second phase of calcite/dolomite mineralization was accompanied by emplacement of high-reflectance bitumen in vein pore space. Veins preserve significant vuggy porosity. Fluid inclusion and stable isotope data indicate temperatures of mineralization range from 145-160°C in the eastern site (Cherry Valley) to 120-135ºC in the western (Cayuga Lake) site. Hydrocarbon-rich fluid inclusions in quartz are common. Carbonate stable isotopes (δ¹³C range +0.5 to + 4.2 PDB; δ¹⁸O range -7.8 to -10.4 PDB) are consistent with mineral precipitation from an aqueous phase in equilibrium with the carbonate units of the Union Springs Formation with minor contribution of carbonate from hydrocarbon decarboxylation.

Décollement systems in the Onondaga and basal Marcellus may be recognized in seismic and well log data. Based on outcrop observations, these detachment faults should form zones of enhanced porosity and permeability within the Marcellus hydrocarbon system, and are accessible targets for horizontal development.