ANALYSIS OF BEDROCK JOINTS IN THE EASTERN FLANK OF THE TUG HILL PLATEAU, OSWEGO AND JEFFERSON COUNTIES, NEW YORK

Strata of the Tug Hill plateau record the transition from marine to terrestrial deposition associated with the onset of the Taconic orogeny from the middle to late Ordovician. The Tug Hill sequence includes the Black River and Trenton Group limestone, overlain by shale and siltstone of the Whetstone Gulf fm. Increasing sandstone content defines the transition upward into the Pulaski fm, and is overlain by the medium to thick bedded sandstone of the Oswego fm. With support from the Institute for the Application of Geospatial Technology, detailed fracture maps were completed for the western Tug Hill region integrating traditional field methods, GPS and GIS. There are two prominent joint sets in all of the rock formations of the Tug Hill plateau, and both sets are subvertical. The joint sets can also be characterized as generally striking NW and NE, with attitude variation dependent on rock formation. The Trenton limestone is exposed in the Sandy Creek basin and along the Lake Ontario shore near Henderson Harbor. The average strike of the joints is 056 and 330, with a spacing of about 0.75 m. The Whetstone Gulf fm occurs in extensive outcrops within Mooney, Totman and Loraine Gulfs. The average joint strikes are 045 and 310. Joints in these gulfs have an average spacing of 0.5 m, and individual joints can be traced 10's of meters. Some joints form the high-walls of the gulfs and control the drainage. Finally, the Salmon River gorge exposes the Pulaski and Oswego formations. Within the Pulaski formation, the average joint spacing is ~0.5 m, and they have average strikes of 041 and 315. The Oswego fm forms the falls on the Salmon River. The two joint sets have average orientations of 057 and 320, with a spacing of ~1 m. The Tug Hill sequence represents a thick “sandwich” of relatively weak rock (shale-siltstone) between stronger limestone (below) and sandstone (above). Both joint sets show a counter-clock-wise variation in strike of about 15 to 20 degrees passing from the Trenton limestone into the overlying shale and siltstone. Conversely, the strike of the joint sets show a clock-wise variation of about 15-20 degrees passing upward from the Pulaski into the Oswego fm. Due to the systematic variation in strike of both joint sets in the Tug Hill plateau, it appears that rock competency may have controlled refraction of the joints on the scale of formations.