CHARACTERISTICS OF JOINT PROPAGATION ACROSS LAYER INTERFACES IN SEDIMENTARY-ROCKS

Interfaces between dissimilar layers play a fundamental role during joint propagation in layered sedimentary rocks, limiting the vertical extent and physical continuity of joint traces. Joints do, however, communicate across interfaces between dissimilar layers, forming an overall composite joint, which is the collection of vertically aligned but discrete joint segments. Detailed fractographic analysis of the surface features of these segments reveals several characteristics of the incremental propagation of a composite joint in the alternating siltstone and shale turbidite sequence of the Genesee Group of the Appalachian Plateau, central New York. (1) Joint segments confined by interfaces are arranged in-plane with each other in a sequential manner for layers of similar properties. (2) Out-of-plane arrangement is common for propagation across thin or discontinuous inhibiting layers. (3) Thick inhibiting layers do not allow communication among joints occurring in adjacent layers above and below. (4) If the thick inhibiting layers fractured, usually these joints initiate at the tip of a pre-existing joint, in an adjacent layer, and propagate away with a slightly different orientation. An analysis of the maximum principal stress in an unjointed layer, due solely to a joint in an adjacent layer, separated by a thin resistant layer, provides a conceptual basis for understanding the incremental nature of composite joints and their step-like geometry.