FAULT-RELATED FRACTURES, VEINS, AND FLUID MIGRATION; MOHAWK VALLEY, NYS

The Trenton/Black River (T/Br) play in the Finger Lakes region, New York State, is the most prolific recent natural gas play in the Appalachian Basin. This study utilizes the surface outcrops of the Trenton/Black River Groups in the Mohawk Valley as an analog to the subsurface T/Br play in the Finger Lakes. The T/Br Finger Lakes fields are structurally controlled and are situated in narrow grabens where fluid circulation resulted in secondary porosity and extensive dolomitization. Similar styles of reservoir formation are found within other T/Br plays such as the Albion-Scipio and Stoney Point Fields of the Michigan Basin and within Devonian carbonates in western Alberta. However, the structures in southern Michigan and western Alberta are associated with wrench-faulting, whereas slickenlines, drag folds, and outcrop patterns indicate that northerly-striking faulting in the Mohawk Valley is dominantly dip-slip.

Outcrop analysis of faulting, fracturing, veining, and other mineralization features in the Mohawk Valley show that the normal faults of the study area have acted as fluid conduits in much the same manner as the wrench faults of Michigan and Alberta. Along NNE-trending splays of the Little Falls Fault, mineralization features (vugs, veins, incipient breccia, dolomitized limestone) are observed outward to a distance of approximately 500m from the fault. The mineralization events also bear a Mississippi Valley Type (MVT) signature (e.g., an abandoned zinc mine with sphalerite near a splay of the Little Falls fault). Along the N-trending Dolgeville Fault, mineralization is largely restricted to the fault plane. Veins occurring in stratigraphically low units near the Little Falls Fault strike parallel with the fault plane, whereas veins occurring higher in the section near the Dolgeville Fault strike nearly perpendicular to the fault plane.

In earlier Taconic times, the faults in this region were extensional related to stretching of the craton over the peripheral bulge and into the trench. Mineralization at this time would occur in the open tension fractures parallel to the faults. In later Taconic times, the stress field would have rotated in response to the jammed subduction zone, and mineralization would now occur in open tension fractures perpendicular to the faults.