LITHOLOGIC CONTROL ON THE ORIENTATION OF THE BEDROCK JOINTS IN THE TUG HILL PLATEAU, NEW YORK

Sedimentary rocks of the Tug Hill plateau record the middle Ordovician carbonate to clastic marine transition in New York. From bottom to top, the plateau is underlain by the Black River and Trenton Group limestones, shale of the Utica and Whetstone Gulf formations, interbedded shale-siltstone-sandstone of Pulaski formation, and capped off by the Oswego quartz sandstone. This stratigraphic sequence overlies Mesoproterozoic crystalline basement, exposed in the west Adirondack region. Respectively, there are 550 m and 360 m of strata exposed on the west and east flanks of the Tug Hill plateau. In general the strata consist of a mechanically weak shale-siltstone body that is underlain and overlain by more competent limestone and sandstone. During the current investigation, fractures were mapped in each stratigraphic unit at three general regions around the flank of the plateau (SW, NW and NE), to search for any systematic variations that may be associated with the geographic location and/or lithology. All of the well developed joint sets in the Tug Hill plateau are steeply dipping, but there are three major sets delineated by strike orientation: E-W, NE-SW, and NW-SE. On the western flank of the plateau, there is a ~20 degree counter-clock-wise variation in strikes of the joint sets (NE-SW & NW-SE) between the Oswego sandstone and the Pulaski formation. The transition from shale downward into the limestone on the NW flank of the plateau is marked by the same joint sets varying in strike by ~20 degrees, but in the clock-wise direction. Although all three joint sets can be found in rocks of the eastern flank, the NW-SE striking joints are most prominent in the Whetstone Gulf shale while the E-W striking joints are most common in the Trenton limestone. As well, there appears to be little strike variation between the formations. However, the attitudes of all the joint sets show considerable variation in the underlying crystalline basement, as well as the addition of several joint sets that do not occur in the Paleozoic strata. From these observations, it is apparent that the variation in the orientation of joint is linked to the stratigraphic units of the Tug Hill plateau on the western flank, but must be related to other factors in the Black River region.