NORMAL FAULT-RELATED DOLOMITIZATION AND FRACTURING, MOHAWK VALLEY, EAST-CENTRAL NEW YORK STATE

The Cambro-Ordovician section of the Mohawk Valley provides an analog for fracture-related hydrocarbon plays associated with normal faults. This section is cut by normal faults with displacements of up to 450 m. These faults are associated with mineralization that occurred in two phases. Phase 1 mineralization caused dolomitization of the Tribes Hill Formation (Beekmantown Group, Early Ordovician) around the faults; Phase 2 formed veins and vugs in the Precambrian through Late Ordovician section, and cross-cuts dolomitization textures in the Tribes Hill Fm. Vug and vein fill is typically calcite, with subsidiary dolomite, quartz, pyrite and sphalerite. Lithology exerted a control on both phases of mineralization. Phase 1 dolomitization is patchy in the thin-bedded, shaly Sprakers Member, but pervasive in the thick-bedded limestones of the Wolf Hollow and Canyon Road members. Phase 2 vugs are most abundant in the primary dolostones of the Little Falls Formation (Beekmantown Group) and the upper- and lowermost Tribes Hill Formation. Veins and joints show distinct variations in orientation and frequency in contrasting lithologies. Stable isotope (carbon and oxygen) and trace element analyses indicate a hydrothermal origin for phase 2 mineralization. Carbon and oxygen isotope values are similar to early-formed Mississippi Valley-type hydrothermal carbonates. Previously published conodont alteration indices are higher in dolomitized Tribes Hill sections close to faults, consistent with localized elevated temperatures. Fracture orientation and spacing data were collected along 22 scanlines; additional orientation data was collected at 15 locations. Veins are typically cross-cut by late-stage joints, and multiple veining episodes during Phase 2 mineralization are evidenced by cross-cutting relationships, differing fill minerals and different vein orientations. A scanline across the only exposed fault shows an increase in the frequency of veins of the dominant set toward the fault.