FRACTURES AND VEINS RELATED TO TACONIC-AGED FAULTS IN THE MOHAWK VALLEY

The Trenton/Black River (TBR) natural gas plays of New York State (NYS) are associated with dolomitized limestone. These regions of dolomitization are created by fluid flow along faults, associated fracture networks, and into nearby susceptible limestones. This fluid flow creates secondary porosity that is either destroyed by secondary mineralization or preserved as void space. These voids comprise much of the secondary porosity typical of the TBR gas reservoirs. The Cambro-Ordovician carbonates exposed in the Little Falls 7.5’ USGS Quadrangle in the Mohawk Valley are used as an outcrop analog for the subsurface TBR fields in NYS.

The study area is cut by Taconic-aged faults of three different orientations: NNE, WNW, and N. The NNE- and WNW-striking faults were active in Iapetan-opening time and re-activated during Taconic time. The N-trending Dolgeville Fault developed later, during Taconic time. Both the NNE and N-striking faults display down-dip kinematic indicators. Restriction of mineralization (vugs and veins) to areas near NNE- and WNW-striking faults indicates that these faults have acted as fluid conduits. For example, all outcrops (14 sites) of Little Falls dolomite near faults contain vugs, with the exception of the large outcrop of Little Falls dolomite on Dolge Avenue, adjacent to the Dolgeville Fault. Unlike the NNE- and WNW-striking faults, mineralization along the Dolgeville Fault appears to be restricted to a heavily mineralized fault breccia zone.

Outcrop analysis of fracture patterns within the Little Falls Quadrangle shows six distinct fracture populations (N, NNE, NE, ENE, WNW, and NW). Abutting relationships of the non-mutually abutting fractures indicate that the NNE-striking set is older than the WNW-striking set, which is older than the N-striking set. All fracture sets host veins near faults except N-striking fractures.

This field area demonstrates that secondary mineralization occurred along all fault trends to varying degrees. Secondly, the prevalent veins and fractures change orientation up-section, reflecting changes in the Taconic stress field.