THE ROLE OF BASEMENT DEFORMATION AND STRATA COMPETENCY ON THE DEVELOPMENT OF FRACTURES, FAULTS, AND FAULT KINEMATICS IN THE FORELAND STRATA OF THE NORTHERN APPALACHIAN BASIN, NEW YORK

The development of unconventional shale gas plays in the Paleozoic rocks of the Appalachian Basin (AB) has focused attention on the structural evolution of joints and fracture zones, in addition to the role that basement faults play in the basin structural evolution. One of few places in the Appalachians where faults in autochthonous basement can be traced directly into the overlying strata is the western margin of the Adirondacks, where the Ordovician strata of the Tug Hill plateau are in direct contact with the basement. The Adirondack region has a distinct, NNE- and ENE-trending topography that strongly correlates with fault. It is assumed that at least some of these faults may have served to accommodate Mesozoic uplift of the region that is responsible for the dome outline of the highlands. Graben associated with a few Adirondack faults contain Ordovician strata, demonstrating that the basement was once covered by AB strata . Offset of Mesoproterozoic structures, reveals a complex kinematic fault history including normal and transcurrent displacement. In places, fault breccias occur in zones upward of 20m wide. The breccia includes fragments of basement lithologies in a fine chloritized matrix, but some also include fragments of limestone where no Paleozoic strata occur otherwise. There appears to be a geometric, and most likely genetic relationship, between the basement faults and deformation in the Ordovician strata to the west. Deformation in the basement produced different responses in the strata depending on structural competency and physical proximity to the basement. Near the basement contact, the overlying carbonates (Black River-Trenton Groups) contain southeast striking normal faults, closely spaced joints, and fracture zones. Within the Utica and Whetstone Gulf shales, there are well developed joint sets in addition to local fracture cleavage consistent with a less competent rock body. The overlying sandstone-dominated units of the Pulaski and Oswego Formations were not as severely affected by joint development, but instead there was reactivation of pre-existing joints as left lateral faults. It appears that the thick underlying shale units probably absorbed much of the joint forming stress through development of fracture cleavage and low-temperature plastic flow.