CHARACTERISTICS AND IMPLICATIONS OF FRACTURES IN CORE AND OUTCROP IN THE ORDOVICIAN UTICA BASIN IN THE MOHAWK VALLEY OF NEW YORK STATE

We collected fracture data from seven un-oriented cores of Utica black shale and 49 field sites in the Mohawk Valley (MHV), New York with the intent of defining the structural history based on fracture sets and kinematic indicators. Sense-of-slip was deduced from releasing bends (rhombochasms) in calcite-filled fractures, bedding offsets, and slickenside/fiber orientation. Oblique and/or strike-slip is observed on veins and fractures in two cores and at four field sites.

Jacobi (2013) proposed different structural domains for fault blocks in the MHV based on fracture sets and abutting relationships, and suggested that the fault blocks had undergone different structural histories. Kinematic indicators from core and outcrop enhance the definitions of two fault block domains and indicate that these fault blocks do have different motion histories.

Domain	Bounding Faults	Defining Characteristics
A	Hoffmans & Saratoga-McGregor	Oldest NNE/NE fractures; dip slip
B	Fonda & Hoffmans	Oldest ENE fracture set; oblique/strike slip
C	Noses & Fonda	No ENE fracture set
D	Little Falls & Noses	Oldest E-striking fracture set

Cores 75-NY-11 and 75-NY-2, in Domain A, contain dip-slip kinematic indicators, but none with a strike-slip component. The presence of both normal and reverse motion is consistent with reactivation of the Mohawk Valley faults during multiple orogenies.

Domain B can be defined by oblique and strike-slip motion. Two field sites within the domain have conjugate ENE/WNW vein sets with strike-slip kinematic indicators. Sub-horizontal slickenfibers are present on veins in core 74-NY-9 and oblique striations are present on bitumen-coated fractures in core 74-NY-12.

In cores with TOC data, possible beef structures correspond to higher TOC intervals, which could indicate overpressure from gas generation was approximately contemporaneous with calcite veining. This relationship provides a check of possible gas/vein ages when coupled with subsidence curves. Based on the subsidence curve from a NYS well, veined fractures with bitumen are likely to be Acadian or later (assuming no hydrothermal circulation). Veins on separate fault blocks may have formed at different times from different fluid circulation events and/or sources.