Joint interaction with embedded concretions: joint loading configurations inferred from propagation paths

Abstract

The interaction between propagating joints and embedded concretions in a Devonian black shale near Seneca Lake, NY, permits identi®cation of the loading con®gurations responsible for two joint sets of dierent ages striking at nearly the same orientation. The earlier set consists of systematic joints cut by later Alleghanian joints of the Appalachian Plateau. The later set consists of non-systematic curving cross joints that abut these same Alleghanian joints. Field evidence shows that concretions functioned as sti inclusions in a compliant black shale. As a consequence of this elastic contrast, local perturbations in the remote stress ®eld persisted around the concretions during burial, tectonic deformation, and exhumation. These stress perturbations in¯uenced joint propagation paths of both joint sets. Our conclusions about loading con®gurations are based on ®nite-element modeling of the eect of the local stress perturbation on concretion-modi®ed joint propagation. Modeling shows that the local stress perturbation from a thermoelastic loading was responsible for de¯ecting cross joints away from concretions in a curved propagation path near the concretion. This load con®guration also led to arrest of cross joints before they penetrated the shale±concretion interface. At greater distances from the concretion, the propagation path of cross joints was controlled by the contemporary tectonic stress ®eld. The interface between concretions and the surrounding shale was strongly bonded, as indicated by the crossing of the interface by some of the systematic ENE joints. Higher compressive stress levels within the concretions relative to the shale suppressed joint development in the concretion, causing the arrest of those joints once they had driven across the interface and a short distance into the concretion. Numerical modeling shows that interface penetration by the systematic ENE joints is consistent with a¯uid load, the same loading con®guration postulated for the subsequent Alleghanian joints. The traces of the systematic ENE joints align on opposite sides of concretions, rather than curving toward the concretion as predicted by two-dimensional models of the¯uid load. Co-planar traces are indicative of large, planar joints propagating in-plane around the concretion, making it energetically inecient for the crack front to curve as it enters the local stress perturbation near the concretion. A¯uid load for the systematic ENE joints came from high pore pressure during the pre-Alleghanian stages of burial of the Devonian Catskill delta complex. #

Joint initiation in bedded clastic rocks

“…The siltstone balls act as large, rigid inclusions in a less rigid matrix. As such, they are analogues for concretions in the black shales of the Geneseo Formation (McConaughy and Engelder, 1999). In both cases, an ENE joint set propagates in shale but does not cut through the rigid inclusion.…”

Revisiting the Hubbert–Rubey pore pressure model for overthrust faulting: Inferences from bedding-parallel detachment surfaces within Middle Devonian gas shale, the Appalachian Basin, USA

“…A number of structures are a manifestation of elevated pore pressure in the Appalachian Basin including natural hydraulic fracturing in several black shale units ranging from the Middle Devonian Marcellus to the Upper Devonian DunkirkeHuron (Engelder et al, 2009;Lacazette and Engelder, 1992;McConaughy and Engelder, 1999;Sheldon, 1912), slickensides arising from bedparallel slip through these same shales (CliffsMinerals, 1982;Evans, 1994), cleavage duplexes, particularly in the Marcellus (Bosworth, 1984;Engelder et al, 2011;Kepferle et al, 1981;Nickelsen, 1986;Wheeler, 1978), fibrous veins including beef and cone-in-cone structures (Gilman and Metzger, 1967;Taber, 1918) and other detachment surfaces found cutting several shales (Hatcher et al, 1989). To examine whether the HubberteRubey model operated in shale of the foreland portion of the Appalachian foldethrust belt because of maturation-related detachment, we document the distribution of detachment surfaces at two scales based on thickness and slip magnitude.…”

“…Other indirect indicators of the presence of seal rocks with concomitant abnormal pore pressure include the distribution of volume-loss strain by pressure solution (Engelder, 1984) and the present stress profile in the Upper Devonian section indicative of poroelastic relaxation (Evans et al, 1989). Finally, natural hydraulic fractures are one of the most direct manifestations of high fluid pressures at some point in the burial history of the Marcellus (Engelder et al, 2009;Lacazette and Engelder, 1992;Lash and Engelder, 2007;McConaughy and Engelder, 1999). Natural hydraulic fracturing requires a mechanism capable of recharging which is not provided by compaction disequilibrium.…”

Multiple failures in or around a stiff inclusion embedded in a soft matrix under a compressive loading

“…Cracks inside or in the vicinity of concretions: (a) 5 closely spaced cracks in a concretion (Bessinger et al 2003); (b) overview of cracks inside and around concretions in a sandstone matrix, (Bessinger et al 2003) ; (c) multiple cracks in a pebble (Eidelman and Reches 1992) ; (d) various kinds of failure in pebbles (Eidelman and Reches 1992) ; (e) matrix cracking in the vicinity of two concretions (Mc Conaughy and Engelder 1999).…”