HOW DO THE PROPERTIES OF INDIVIDUAL MECHANICAL LAYERS AFFECT JOINT ORIENTATION?

The spacing of joints in rock layers with varying mechanical properties has been studied in great detail, but few studies have focused on subtle differences in joint orientation within different layers in a mechanical sequence. Previous mathematical modeling suggests that joint orientation may differ among layers with contrasting strength properties and thicknesses. The current field study seeks evidence of these relationships in multiple outcrops of sedimentary rocks ranging from Ordovician through Triassic in Pennsylvania.

Joint orientations in sedimentary layers were measured and plotted using stereographic projection software. Joint spacing was measured for each systematic joint set, and a rebound value (a proxy for unconfined compressive strength) was obtained for competent jointed layers using a Type N Schmidt Hammer in the ASTM evaluation mode. In most outcrops, joints generally represent two sets that are perpendicular to bedding and to each other. However, in several instances joints in adjacent layers occupy slightly different orientations. When examined bed by bed, subtle variations in joint orientation and spacing emerge. Joint orientations are more dispersed in layers with lower rebound values, in thicker layers, and in layers with varying thickness. Joints are more closely spaced in thinner and/or stronger layers, and the joint spacing distribution is wider in thicker and/or weaker layers. These relationships were observed within individual outcrops, but comparison of data across multiple outcrops is problematic because of Pennsylvania’s complex deformation history.

The relationships shown by the preliminary data are elusive. Multiple variables influenced jointing in each outcrop, including thicknesses of both competent and incompetent layers, strength contrasts, and total strain. Future work will include searching for an outcrop with a simple burial, deformation, and exhumation history and only minor variations in thickness or strength of the interbedded mechanical layers in order to clarify any correlation between joint orientation and mechanical layer properties.