THE ORIGIN AND DEVELOPMENT OF FRACTURES WITHIN THE BEEKMANTOWN GROUP CARBONATES OF EAST CENTRAL NEW YORK STATE

Fracture information was collected from a series of cores taken from the shallow-water Beekmantown Group (Sauk Sequence) carbonates from both the Mohawk River Valley and Champlain Valley. Fracture information of interest included type of fracture, fracture spacing and the spatial relationship that the fractures have with the normal faults that bisect both valleys.

The majority of the fractures found were extension (mode I) fractures. This is consistent with the tectonic forces that affected the area during the Taconic orogeny. During this time, upwarping due to the cratonward migration of a peripheral bulge resulted in extension in the upper portion of the lithosphere. A more substantial cause of extension in the upper lithosphere was the bending of the plate as it entered the subduction zone beneath the Taconic allochthon. Cores that did not intersect fault zones usually contained only extension fractures whereas cores that were very close to, or intersected fault zones, contained both extension and shear fractures. Both extension and shear fractures increased in number as a core approached a fault zone. As a result, the average fracture spacing was found to decrease as the fault zones were approached.

The spatial relationship between fractures and faults in individual cores was determined using chi-square goodness of fit testing. The regional spatial relationship between fractures and faults was determined graphically, by plotting the average number of fractures that occurred in equal length intervals along selected cores against the distance between the core locations and the closest surface exposures of adjacent faults. In both cases, there is a direct relationship between the occurrence of fractures and faults.

It was determined that the fractures found within the Beekmantown Group carbonates are clustered in a series of fracture swarms. The fracture swarms preceded faulting by preparing the rock for eventual fault slip. Fracture swarms that did not experience faulting are also found within these rocks. Despite the fact that these fracture swarms did not experience faulting, they still identify areas of stress build up and were produced by the tectonic forces that produced the faulting.