People | 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.]
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