FRACTURE ANALYSIS ALONG THE SOUTHERN SHORE OF LAKE ONATRIO IN THE OSWEGO FORMATION, OSWEGO COUNTY, NEW YORK

Detailed fracture maps, of the southern shore of Lake Ontario in Oswego County, NY, were produced for the bedrock exposures of the Ordovician Oswego Formation. The maps reveal systematic variation in fracture orientations, and densities relative to stratigraphic position and bed-thickness. Three subvertical fracture sets were identified, and the relative timing was determined based on intersecting and cross-cutting relationships. A set of extension fractures strikes about 310, has a density (fractures per meter, measured perpendicular to fracture strike) that ranges from <1 in thick (0.5-1 m) bedded sandstone to 10 in thin siltstone beds. A second set of extension fractures strikes 015, has a density variation similar to the first set. These fracture sets are interpreted to be extensional based on the occurrence of plumose structure, and ramp- and cuspate-shaped rib marks on the fracture surfaces. They are also interpreted to have developed simultaneously because they mutually terminate against one another in the same outcrops; however, they also cross-cut each other in some places as well. A third set of shear fractures cross-cuts the first two with an strike variation of 065 to 080. The older fractures serve as markers, and reveal minor sinistral shear on the third fracture set. In some places, the third fracture set developed as m-scale fracture zones containing minor cataclasite. Like the other fracture sets, the density of the shear fractures varies with bed thickness in the Oswego Formation with a range from <1 to 10 fractures/meter. The shear fractures range in length from a few meters to 10’s of meters where there is continuous exposure. Some shear fractures terminate by curving into each other, while some terminate into small (4-20 cm wide) en-echelon fracture zones. The asymmetry of the en-echelon fracture zones is consistent with sinistral shear. In some places, the earlier extension fractures re-mobilized as shear fractures. All three fracture sets are similar in orientation to those mapped in the Appalachian basin by earlier researchers.