JOINT ANALYSIS ACROSS THE TRENTON-UTICA BOUNDARY IN THE MOHAWK RIVER HEADWATERS REGION, NEW YORK

The upper members (~200 m) of the Trenton Group are well exposed in the headwaters to the Mohawk River, including Lansing Kill and Wells Creek. The top of the Trenton limestone is marked by irregular beds that include nodules and blocks of limestone in a mudstone matrix. Immediately overlying this surface, the Utica Formation is predominately black shale with minor thin beds of siltstone and there is about 100-150m of exposure. The Trenton-Utica contact was previously interpreted as a disconfomity in the western Mohawk valley. In this region, the Trenton limestone and Utica shale display similar consistent joint sets, in addition to joint sets that do not fit the regional orientations and distribution. The Trenton limestone contains four subvertical joint sets that strike NNE, NE, ENE and SE. The ENE and NNE striking joints are consistent with the Appalachian basin, J1 joints and J2 cross-fold joints. The NE and SE striking joints are parallel to two dominant joints that occur in the crystalline bedrock of the western Adirondacks. Within the Utica shale, the NE striking Adirondack joints and the J1 joints are well developed, however, there is a third joint set with variable strike from NNE to NE, making it difficult to distinguish between the J2 cross-fold joints and those associated with deformation in the crystalline basement. Joint spacing varies for each joint set by formation with the tightest spacing occurring in the Utica shale (~20 cm) and the widest spacing in the underlying limestone (50-100 cm). Although the Trenton limestone recorded Alleghanian cross-fold joints (J2), it is not apparent that this phase of brittle deformation equally impacted the Utica shale. We attribute the differences in joint development within the Trenton limestone and Utica Formation to be related to the difference in competency between relatively rigid limestone and weaker shale.