Asking the pikas politely…
ABSTRACT: Along the northeast flank of the Uncompahgre Plateau, forced folds developed in a 2-km-thick (6,500 ft thick) sequence of sandstones and shales above high-angle basement faults during the Laramide(?) orogeny. The structures developed in the aeolian Wingate Sandstone, which lies near the base of this section of sedimentary rocks, are well exposed in many of the canyons within the Colorado National Monument. Within these structures, Wingate beds have been substantially flexed and attenuated with little attendant fracturing or major faulting. The primary deformational features producing the observed strain within the Wingate are microfaults. Microfaults are roughly planar zones across which small, but discernible, amounts of shear displacement have occurred. These features rarely continue through major bedding-unit contacts. Microfaults are conspicuous in outcrop by virtue of a light-colored and relatively resistant gouge zone. The gouge consists of a 0.3-mm-wide (0.01 in. wide) zone of mechanically comminuted and compacted sand grains. The porosity of this initially high-porosity sandstone is substantially reduced both within and along the boundaries of the gouge zone. Shear displacement along a microfault of greater than ~5 mm (0.2 in.) produces additional gouge zone segments, which results in a braided or anastomosing texture. Microfaults form in conjugate shear systems that intersect at 20° to 40°, roughly in accord with the predictions of the Mohr-Coulomb criterion. This organized arrangement of lowporosity gouge zones produces a reduction in the permeability of this sandstone, perpendicular to the microfaults, by as much as three orders of magnitude. Microfaults forming parallel to cross-beds and cross-bed-set boundaries occur locally within the Wingate structures, as do microfaults arranged in a Riedel shear-zone fabric. Assessment of mechanisms involved in gouge-zone de-velopment, generation of the anastomosing gouge-zone fabric, and offsetting relationships suggests that deformation by microfaulting is a strain-hardening process. This strain hardening terminates when a major fault zone has developed through the Wingate Formation.
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