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Approximately 2,500 stations were occupied for joint analysis in the western Valley and Ridge and eastern Allegheny Plateau of West Virginia and Maryland. Structural positions range from the Georges Creek-Stony River syncline atop the Allegheny Plateau to the Nittany anticlinorium and western Broad Top synclinorium in the Valley and Ridge. Rocks exposed range in age from Middle Ordovician carbonate within the core of the Wills Mountain anticline to Pennsylvanian coal measures on the Allegheny Plateau. The highest percentage of joint readings was obtained from Middle to Upper Devonian sandstone, siltstones, and shales because of the widespread areal distribution of these rocks within the Bedford and Clearville synclines. Here, fracture trends are similar to those observed n Devonian shale cores taken farther west.
As many as eight different joint sets are present within the study area although only four to five major systematic sets are pervasive throughout the entire region. Most commonly only two, or at the most three, joint sets are present at the scale of the individual outcrop. Consideration of joint crosscutting and offset relationships, tendential and transient features, fibrous mineralization, stylolites, and slickenlines has permitted the establishment of a consistent chronology of joint development throughout the region. Joint set I (N30°to 50°W) formed first as extension fractures early in the lithification history of all formations, followed by a less commonly developed orthogonal set trending N40° to 60°E. Both sets predated Alleghenian folding. Fracture plume d ta indicate upward propagation for joint development, perhaps associated with regional northeastward extension into the deepest part of the Paleozoic depositional basin. Joint set II (N55° to 70°W) formed second in response to early Alleghenian compressive stress, coupled with continuous subsidence, before folding. Locally, minor and nonregionally pervasive fractures showing shear joint geometry also developed at this time. Joint set III (N20° to 30°E) formed as extension fractures parallel to fold axes, with fracture inception early in fold development. Joint set IV (N75°E to N75°W) shows slickenlines more commonly than other sets and formed late in the folding history, possibly as shear joints where structures were effectively "locked." More likely this se formed as extension joints in response to post-folding stresses, perhaps consistent with the present-day regional stress field. A moderately to poorly developed joint set V (N10°E to N10°W) formed last in the region as an orthogonal set to IV.
The dominant joint set or sets at any location within the study area depend on the bedding thicknesses, lithology, structural position, and early fracture history. Prediction for joint trends, and possible hydrocarbon migration timing at depth in potential fractured reservoirs, must consider this aspect as well as chronological development, especially in view of the different stress fields within lower and upper thin-skinned plates.
The study did not reveal large-scale zones of high joint frequency except for the confirmation of increased fracturing in linear belts such as the Petersburg lineament and Parsons lineament previously reported by Sites, Dixon, Wheeler and Dixon, and Wilson.
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