STRUCTURE AND VEIN ISOTOPES OF A CAMBRO-ORDOVICIAN 360m CORE FROM THE SARATOGA SPRINGS REGION, NEW YORK STATE: IMPLICATIONS FOR TECTONICS AND UTICA GAS EXPLORATION

The Ordovician section in the Mohawk Valley has served as a model for Taconic plate tectonics, for Trenton-Black River gas production, and now for Utica black shale gas production. We conducted a detailed fracture analysis on a 635m (2084ft) unoriented core (75-NY-2, Charlton, NY) that penetrates the Cambro-Ordovician section from the Schenectady Formation down to Grenville basement. Detailed analysis of kinematic indicators (e.g., calcite-filled rhombochasms, slickensides, vein offsets) provides structural guidelines for future gas production from the Utica Shale, and constraints on timing of fluid migration and faulting.

We measured 455 vein-filled fracture/faults in the core, including horizontal (0-10⁰), shallow dipping (10⁰ to 45⁰), and steeply dipping (>45⁰) vein-filled fractures, as well as vein-filled normal and thrust faults. In contrast to the Utica, the Schenectady and Frankfort formations are highly fractured and exhibit discrete deformed zones at 67m and 132m bgs (Schenectady Fm.,) and at 156m, 162m, and 171m bgs. (Frankfort Fm.) These zones have 94 slickensided/fiber calcite surfaces coated with anthraxolite; the upper 3 zones include soft-sediment deformation and scaly cleavage. The deformed intervals are mid Katian (late Caradocian) in age (~452-450Ma). The soft sediment deformation, scaly cleavage, and localization of fractures in soft sediment deformation zones suggest these deformation zones were dewatered during late Taconic deformation. Stable isotope d¹³C and d¹⁸O values from 24 samples indicate two geochemical groups of veins. One group has -14.2 to -11.7 d¹⁸O, and is consistent with a metamorphic clay dewatering source in the Taconic. The second group has higher d¹⁸O values of -11.2 to -8.1, and is similar in composition to fluids Selleck (2010) suggested leached Grenvillian basement during fluid circulation along active Taconic-aged faults, and is consistent with other geochemical data of veins in the Mohawk Valley. New isotope data in carbonate units below the Utica have signatures similar to the veins in Utica-Frankfort formations. In total, the data suggest that much of the fluid circulation evident in the Glens Falls-T/Br-Utica-Frankfort occurred during the Taconic and that circulation cells extended into basement, even with the shale cover.