The Appalachian Plateau, extending from the Folded Appalachians to the Central Stable Interior, contains Pennsylvanian and Permian cyclic sedimentary rocks which permit study of joints in coal and shale throughout a region of deformation that diminishes to the northwest.
The basic unit of jointing is the fundamental joint system, comprised of an approximately orthogonal systematic and nonsystematic set of joints. Complex systems of jointing arise from overprinting of two or more fundamental systems, each of which may occur alone in adjacent areas. In north-central Pennsylvania, the local and regional joint patterns of coals and shales differ: systematic joints in coals are tangent to an arc, convex to the south, with a center of curvature at approximately 78° W. long., in northern Pennsylvania; systematic joints in shales strike north or northwest grossly transverse to northeast-trending fold axes of the Folded Appalachians and Appalachian Plateau. Locally, systematic joints in shale extend into underlying coals and have been overprinted upon the older arcuate pattern of joints in coal. Although systematic joints in shale are grossly transverse to fold axes, no one set of joints remains perpendicular to folds, which bend from N. 30° E. to N. 70° E., throughout the region. Rather, the regional joint pattern consists of overlapping joint trends of remarkably uniform orientation, each trend being dominant (showing greatest size, frequency, and persistence) where it is most nearly perpendicular to fold axes.
The joint pattern of the region is cumulative, presenting a record of all events which have produced stress differences sufficient to induce fracture. Systematic joints formed early, independent of folds and faults. This is evinced by the complete independence of joints in coal and fold axis trends, by greater consistency of regional strike of joints in shale than of fold axis orientation, by the rotation of joints adjacent to faults, and by the presence of systematic joints west of folds of Appalachian trend. Although systematic joints may be polygenetic, their mutual relations, relations to nonsystematic joints, surface character, open, lensing nature, and lack of lateral movement suggest that they are of extensional origin. Nonsystematic joints of the special type, here called “truncated joints,” are also of extensional origin although they are probably late release-type fractures resulting from a combination of residual tectonic stress differences and surficial stress differences arising in blocks between successive systematic joints during erosion and unloading.