STRUCTURES IN UTICA BLACK SHALE CORES NEAR FAULT SYSTEMS IN THE MOHAWK VALLEY REGION, NEW YORK STATE: VARIATIONS WITH FAULT PROXIMITY AND FAULT THROW

Structure data from four unoriented cores in the Ordovician Utica black shale are used to determine the effect distance from “Taconic” NNE-striking faults and their throw may have on structure characteristics, including vein-filled fracture length, aperture, frequency, slickenside/slickenfiber orientation, and other rarer structures. Secondly, the rake of slickenfibers can be used to determine whether a component of strike-slip motion was sustained in addition to the generally assumed dip-slip motion (both normal and reverse).

Cores 75-NY-2, 74-NY-12, and 75-NY-11 have all been previously studied in detail by members of the UB Rock Fracture Group. Core 75-NY-2 penetrated 137.4 m of Utica, 35.22 m of Frankfort, and 139.48 m of Schenectady. Cores 74-NY-12 and 75-NY-11 retrieved 53.6 m and 6.1 m of Utica, respectively. See table for core information.

core #	Fault Name	throw (m)	distance from thrust front (km)	core distance from fault (km)	hi side/ lo side	cluster frac freq* max per meter
75-NY-2	Saratoga-McGregor	137	8	~1.5	hi	3
74-NY-12	Hoffmans	381	23	0.55	hi	13
75-NY-11	Hoffmans	381	23	1.73	lo	7
74-NY-10	E. Stone Arabia	24	58	1.46	lo	9

*downhole fracture frequency

We completed a detailed structural analysis of core 74-NY-10, which is located 1.46 km E of a splay of the E. Stone Arabia (or “Northville”) Fault. Only 48 vein-filled fractures in the 83.2 m of Utica core were observed. 39 were steeply dipping (ranging from 78°-90°) and two were horizontal. The two horizontal veins exhibited variable thickness from <1 mm to 5 mm. All of the steeply dipping veins had apertures of 1 mm or less. The steep fractures generally sparsely populate the core, averaging < 0.3/downhole m.

Three clusters of higher fracture frequency are observed in each core except for 75-NY-11, which only shows one cluster. In all cores a peak in downhole fracture frequency occurs in the upper part of the Utica. The fracture clusters typically correspond to higher proportions of silt/sand. The general downhole fracture frequency for the Utica is relatively low. Structure metrics (vs. distance and throw) are quasi-consistent, e.g., cores 75-NY-2, 75-NY-11, and 74-NY-10 (with similar distances to faults), have lower fracture frequency maxima than 74-NY-12. Only core associated with Hoffmans Fault shows indications of oblique-slip, and this slip was relatively “late”.