Fractures Generated during Folding of the Palmerton Sandstone, Eastern Pennsylvania

Abstract

A syncline of Devonian Palmerton Sandstone exposed in a quarry in the fold-thrust belt of eastern Pennsylvania contains a set of fractures that are perpendicular to bedding and strike parallel to the fold axis. Fracture spacing is between 0.25 and 30 cm. These fractures are zones of cataclasis that have striated shear surfaces. Displacement on fractures, where measurable, is less than 1 cm. Sparse kinematic evidence suggests that slip on fractures could have contributed to growth of the fold. Petrographic evidence shows that slip is in part localized along pre-existing mineralized fractures which probably formed during burial, lithification, and diagenesis of the Palmerton Sandstone in an extensional structural setting. The unusual shear-fracture pattern illustrated by the Palmerton Sandstone may result from slip on these early fractures during thnist-sheet displacement along a thrust ramp. This exposure also illustrates the operation of contrasting deformation mechanisms in sandstone and in vertically adjacent limestone and shale units. Evidence of pressure-solution deformation, in the form of spaced cleavage, is present in limestone and shale, whereas in the Palmerton Sandstone, deformation was accomplished by pervasive macrofracturing and microfracturing.