Abstract

The most pervasive systematic joints hosted by Devonian black shale of the Appalachian Plateau include east-northeast joints and younger northwest-striking cross-fold joints. Both sets were driven exclusively by fluid pressure generated as a consequence of hydrocarbon-related maturation supplemented by subsequent tectonic compaction during the Alleghanian tectonic cycle. In the more deeply buried, proximal region of the Catskill Delta, joints of both sets cross-cut. However, in stratigraphically equivalent black shale of the distal, shallower region of the delta to the west, ~25% of the joint intersections are crosscutting whereas ~75% of the intersections are defined by east-northeast joints curving into and abutting cross-fold joints. East-northeast joints in the distal shale propagated early but were neither as long nor as pervasive as similarly oriented joints in the more deeply buried proximal delta deposits. However, these same joints are much better developed (more closely spaced) stratigraphically lower in the distal shale succession, where essentially all intersections are crosscutting. It is likely that relatively high contact stress, an effective stress on east-northeast joints, in the more deeply buried parts of the delta enabled the unimpeded transmission of elastic stress concentration at the tip of cross-fold joints across the older joints. In the distal delta, exhumation of the Devonian shale sequence following establishment of the contemporary stress field in middle Tertiary time to within ~1 km of the Earth's surface resulted in renewed propagation of east-northeast joints. The propagation path of the reactivated joints curved into and terminated against cross-fold joints at angles of 60°–90°. The curving-to-termination intersection style is a manifestation of crack-tip stress-field blunting as a consequence of slippage along the interface of cross-fold joints enabled by exhumation-induced relaxation of contact stress on these latter joints. It is noteworthy that the orientation of the densely formed east-northeast joints parallel to the maximum horizontal stress direction of the contemporary lithospheric stress field likely imparts a meaningful permeability anisotropy to these hydrocarbon source rocks.

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