Conjugate Riedel deformation band shear zones

Our investigations have disclosed that individual Riedel shear zones may or ganize themselves into broadly distributed though rigorously oriented intra formational conjugate systems which may form without relationship to, or de pendence upon, an underlying basement fault zone. The Riedel shear zones we mapped are zones of deformation bands, which developed as the preferred de formation mechanism in porous Navajo Sandstone (Jurassic). In the Cottonwoo d area, located at the northern end of the Kaibab Uplift, a conjugate norma l Riedel deformation band shear system developed during the Laramide in the uppermost Navajo Sandstone on the outer are of the upper hinge zone of the East Kaibab monocline. In the Sheets Gulch area, located at the northern e nd of the Waterpocket Fold. a conjugate strike-slip Riedel deformation band shear system developed during the Laramide in upper Navajo Sandstone withi n an imperfect transfer zone between the northeast-vergent Circle Cliffs Up lift and the southwest-vergent Miners Mountain Uplift. Within both the Cott onwood and Sheets Gulch areas there are tens to hundreds of Riedel sheer zo nes, the largest of which are up to hundreds of meters in trace length. In classic Riedel fashion. the synthetic R-shears within each Riedel shear zone depart by similar to 15 degrees From the zone as a whole and are arran ged in an en echelon, overstepping geometry. The antithetic R'-shears depar t by similar to 75 degrees from the Riedel shear zones of which they are a part, and are especially abundant in transfer zones where they create hard linkages between overstepping: R-shears. At both localities the Riedel shea r zones occur in two sets that intersect at similar to 60 degrees. The Ried el shear geometry is selfsimilar from the scale of hand samples (and thin s ections) where offsets are measured in centimeters (or millimeters), to the map scale where displacements are measured in meters. Because of the small amount of deformation which had to be accommodated in each of the two stud y areas, and the limits imposed by the strain-hardening nature of deformati on banding, we may be seeing a rare snapshot that records an image of early , arrested fault-system development in relatively homogeneous, porous sands tone. The literature on classic Riedel shear zones postulates that displacement a nd shear along Riedel shears brings about a localized reorientation of stre ss. This interpretation can be tested and confirmed, using the geometry and kinematics of conjugate Riedel systems. Detailed understanding of the tota l nested geometric characteristics of the conjugate Riedel deformation band shear zone systems also provides insight regarding controls on reservoir-s cale fluid flow. The low permeability of the deformation band shear zones t ends to compartmentalize the Navajo Sandstone into chambers alone which flu id flow is channeled. The geometry and spacing of the deformation band patt erns controls shapes and sizes of the compartments, which in these examples tend to be long, polyhedral, porous chambers marked by either diamond- or rhombic-shaped cross-sections. (C) 2000 Elsevier Science Ltd. All rights re served.