George Davis | Conjugate Riedel Deformation Band Shear Zones

Our investigations have disclosed that individual Riedel shear zones may organize themselves into broadly distributed thought rigorously oriented intraformational conjugate systems which may form without relationship to, or dependence upon, an underlying basement fault zone.  The Riedel shear zones we mapped are zones of deformation bands, which developed as the preferred deformation mechanism in porous Navajo Sandstone (Jurassic).  In the Cottonwood area, located at the northern end of the Kaibab Uplift, a conjugate normal Riedel deformation band shear system developed during the Laramide in the uppermost Navajo Sandstone on the outer arc of the upper hinge zone of the East Kaibab monocline.  In the Sheets Gulch area, located at the northern end of the Waterpocket Fold, a conjugate strike-slip Riedel deformation band shear system developed during the Laramide in upper Navajo Sandstone within an imperfect transfer zone between the northeast-vergent Circle Cliffs Uplift and the southwest-vergent Miners Mountain Uplift.  Within both the Cottonwood and Sheets Gulch areas there are tens to hundreds of Riedel shear zones, 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 ~15? from the zone as a whole and are arranged in an en echelon, overstepping geometry.   The antithetic R’-shears depart by ~75? 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 shear zones occur in two sets that intersect at ~60? The Riedel shear geometry is self-similar from the scale of hand samples (and thin sections) 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 study areas, and the limits imposed by the strain-hardening nature of deformation banding, we may be seeing a rare snapshot that records an image of early, arrested fault-system development in relatively homogenous, porous sandstone.

The literature on classic Riedel shear zones postulates that displacement and shear along Riedel shears bring about a localized reorientation of stress.   This interpretation can be tested and confirmed, using the geometry and kinematics of conjugate Riedel systems.  Detailed understanding of the total nested geometric characteristics of the conjugate Riedel deformation band shear zone systems also provides insight regarding controls on reservoir-scale fluid flow.  The low permeability of the deformation band shear zones tends to compartmentalize the Navajo Sandstone into chambers along which fluid flow is channeled.  The geometry and spacing of the deformation band patterns control 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.

[Davis, G.H., Bump, A, Garcia, P., and Ahlgren, S., 2000, Conjugate Riedel shear zones:
Journal of Structural Geology, v. 22, p. 169-190.]