Joint initiation in bedded clastic rocks

Abstract

Two general types of sedimentary structures serve as initiation points (IP) for cross-fold joints in siltstone beds of the Ithaca Formation, Appalachian Plateau, New York: planar bed boundaries and geometrically complex features such as bedforms, trace fossils, and softsediment deformation structures. The relative abundance of each IP type varies with the stratigraphic position of the jointed body. When they are mechanically isolated by thicker shales, siltstone bodies have a higher percentage of IP associated with complex features. This association supports the hypothesis that sedimentary structures serve to concentrate stress, thus becoming preferred sites for primary joint initiation. Secondary joint initiation occurs at planar bed boundaries through interaction with pre-existing joints in adjacent siltstone beds that are mechanically coupled to form composite joints. The propagation path of joints passing through planar bed boundaries of composite joints can be traced backward to primary IP at complex sedimentary structures. Modeling suggests that when beds are mechanically coupled, the modest joint-tip stress concentration across a shale layer (e.g., , 3 £ for a 1-cm thick bed) is equivalent to the stress concentration in the vicinity of a bedform. Consequently, in mechanically coupled siltstone beds, sedimentary structures such as bedforms compete with preexisting joint tips in adjacent beds to initiate new joint segments. q

Mechanisms controlling rupture shape during subcritical growth of joints in layered rocks

“…The surface roughness in glass results from postcritical secondary crack branching in the process zone ahead of the crack tip, whereas the uniform roughness (i.e., the plume pattern) on rock joints is due to out-of-plane propagation of the crack tip in response to a much weaker subcritical K I interacting with much larger pores and grain boundaries within a process zone about the crack tip (e.g., Kulander et al, 1979;Scott et al, 1992;McConaughy and Engelder, 2001). In contrast to relatively large grain boundaries and pores spaces in rock (10 −2 -10 −3 mm), fl aws in glass are more than 3 orders of magnitude smaller and, therefore, require a much larger K I to induce process zone cracking.…”

“…The best examples are found within the Ithaca Formation (Genesee Group) in the Finger Lakes District of New York (McConaughy and Engelder, 2001) and in its stratigraphic equivalent, the Brallier Formation, located in central Pennsylvania (Ruf et al, 1998). For this study, we collected surface morphology data from over 250 joints located in and around Huntingdon, Pennsylvania, Watkins Glen, New York, and further east in Broome and Cortland counties, New York.…”

“…They are thought to form during small-scale, out-of-plane crack propagation as the rupture deviates into pore spaces and along grain boundaries (Kulander et al, 1979;Scott et al, 1992). Plumes (either lines or curves) radiate away from joint initiation sites where local inhomogeneities serve to concentrate the crack driving stress (McConaughy and Engelder, 2001). Plume patterns are maps of the paths taken by ruptures as they cut through intact rock to form a joint (e.g., Bahat and Engelder, 1984;DeGraff and Aydin, 1987;Helgeson and Aydin, 1991;Lacazette and Engelder, 1992).…”

Mechanical aspects of thrust faulting driven by far-field compression and their implications for fold geometry

“…For example, strata that are domed by an expanding magma chamber experience considerable stretching, which is not included in bending models [46]. The interaction between deforming layers with different constitutive properties is known to affect the local stress field [3,4,18,54,87], as does the interfacial conditions of the layer boundaries [23,24,48]. The stress distribution changes over time as the fault slips, the fold evolves, and the bedding rotates [30,38].…”

Geometrical classification and estimation of stresses of the joints in the ShewaSur-Chamchamal Anticline, NE Iraq

“…Among these works are : Dyer, 1988;Lash & Engelder, 2007;McConaughy & Engelder, 2000;ZHAO and JACOBI, 1996. The relation of joints and stress was discussed by ENOELDER & Geiser (1980), they concluded from field studies on the Appalachian Plateau that regional systematic joints reflect the principal directions of the regional stress field which gave rise to the joint.…”