THE LINK BETWEEN FRACTURE SYSTEMS IN THE ADIRONDACK BASEMENT AND THE OVERLYING PALEOZOIC STRATA OF THE TUG HILL PLATEAU

The Adirondack basement massif is host to a suite of NE striking fault zones that include graben and evidence for minor strike-slip displacement. A transect across the southern Adirondacks to examine fracture systems associated with these faults was completed along and across strike of the fault zones, but continues westward across the Tug Hill plateau which is underlain by Middle Ordovician carbonates and the Taconic clastic wedge. Within the basement, there are three steeply dipping fracture systems: NE striking subparallel to the fault zones, ENE striking subparallel to the Prospect fault zone, and NW striking with evidence of minor dextral shear in many places. Immediately west of the Adirondacks, the basement rocks are overlain by the Black River Group carbonates containing the same three fracture sets with minor normal faults parallel to the ENE and NW striking fractures. The overlying Trenton Group limestone contains the same fractures and minor faults, except for 20 degree variation NW fracture strike from E to W across the Tug Hill plateau. The variation in strike is systematically gradual over a distance of about 70 km. Overall fracture intensity is pronounced in both carbonate units with typical spacing being sub-meter. The top of the carbonate sequence is marked by the Utica shale containing the same three steeply dipping fractures, with minor normal faults occurring parallel to the ENE striking set. The strata that overly the Utica shale include a sequence of interbedded shale, siltstone of the Whetstone Gulf Formation, the interbedded shale, siltstone and sandstone of the Pulaski Formation, and finally the massive sandstone of the Oswego Formation. The increase in amount of sandstone beds and the increase in sandstone bed thickness is gradual and occurs over 200 meters of strata. With the graduate increase in sandstone, the strike of the NE and NW fracture sets gradually refract eastward and northerly respectively. The ENE striking fractures that occur in the Utica shale do not occur higher in the sandstone rich portions of the Pulaski Formation, but appears to be overlapped or replaced by the NE striking fractures that refract toward the east. It is interesting to note that minor normal faults that occur in the basement, carbonates and shale, do not occur in the overlying sandstone bearing formations. Minor ENE striking sinistral faults and en-echelon fracture zones occur in the sandstone units.