VARIABLY DEVELOPED JOINTS IN THE ORDOVICIAN STRATA OF THE TUG HILL PLATEAU, NEW YORK

Ordovician strata of the Tug Hill Plateau (THP), NY, record the overlap between bedrock joints that are typical of the Appalachian basin and those associated with the uplift of the Adirondack dome. The sedimentary rocks of the THP (>500 m thick) consist of limestone of the Black River and Trenton Gps, overlain by the coarsening upward sequence of the Lorraine Gp. This sequence includes thinly-bedded black shales of the Utica and Whetstone Gulf Fms, that transition upward into interbedded shale and sandstone of the Pulaski Fm, capped off by thick-bedded sandstone of the Oswego Fm. The THP strata dip 2-5^o SW.

Steeply dipping, ENE and NW striking joint sets of the Appalachian basin are pervasive throughout the region. There are also two additional systematic joint sets found in specific locations of the THP. Both joints sets are steeply dipping, and generally strike NNW and NE. There is an apparent geographic, stratigraphic and lithologic relationship with the occurrence of these joints. They are pervasive in the Black River and Trenton Gps throughout the THP. In the Utica and Whetstone Gulf shale they are well developed in the east, but are less developed westward. They occur mostly in the Pulaski shale beds, but are rare in the sandstone beds, and in the Oswego sandstone.

Displacement on basement faults was proposed to explain the tilt of the plateau strata and uplift of the Adirondacks, specifically the steep NNW striking Black River fault, and a NE striking fault that forms the northern border of the THP. Perhaps the NNW and NE striking joint sets are genetically linked to these proposed faults explaining the restricted geographic distribution and highest joint densities in the northern and eastern sectors of the THP. Movement on these faults was dip-slip, and is supported by meter-scale offset on outcrop-scale faults found only in the lower limestone formations. Although highly jointed in places, a thick sequence of mechanically weak shale would have experienced a degree of plastic deformation during the regional tilting associated uplift, therefore limiting the transfer of stress into the overlying sandstone beds, explaining the rarity of these joint sets. Differential uplift resulted in the SW regional dip of the strata, and most likely reactivated the Appalachian basin joints (J2) to account for local sinistral displacement.