Mechanism for joint saturation in mechanically layered rocks: An example from southern Israel

Over 700 joint spacings were measured along a 190-m scanline within a single limestone bed of the Turonian Gerofit Formation, southern Israel. By investigating joints belonging to a single systematic set confined to a bed of uniform thickness, one can factor out the major influences of bed thickness, lithology and mechanical properties on joint set development. Joint spacing distributions and fracture spacing ratios (FSR) vary dramatically along the length of the scanline, and correlate directly with throughgoing fracture zones belonging to the same tectonic stress regime. Regions cross-cut by fracture zones display narrower median joint spacings, FSR of similar to 1.3, and unimodal joint spacing distributions. Where fracture zones are absent, joint spacings have wider medians, FSR of similar to 0.8, and multimodal distributions. Because fracture zones most likely represent areas of high strain, and laboratory experiments demonstrate a systematic decrease in fracture spacing with increasing strain, regions of the bed cross-cut by fracture zones are likely at a more advanced stage of joint set development, and hence closer to saturation, than other regions. Joint saturation in the Gerofit Formation may be achieved by a shift in scale from single-layer to multi-layer jointing, as the development of throughgoing joints prevents the formation of additional bedding-confined joints.