People | George
Davis
Jointing in Circle Cliffs Uplift, Utah
Analysis of Spacing and Orientation of Jointing
in Wingate Sandstone, Circle Cliffs, Colorado Plateau
The Laramide Circle Cliffs Uplift
in Utah is an ideal location to systematically investigate jointing. The Uplift is
a north-south doubly-plunging anticline with a steeply dipping (up
to 80?) monoclinal eastern limb (the Waterpocket fold). The other
limbs are gently dipping (≤5?). The Uplift is roughly 80km
by 33km, and averages 340? in trend. At the crest of the Uplift,
a giant amphitheater exposure provides 360? access to vertically jointed
cliffs of Jurassic Wingate Sandstone, which averages 105m thick. Structural
relief on the Uplift is estimated to be roughly 450-600m.
Joints in the Wingate include long,
continuous, planar to curvi-planar surfaces expressing huge classic
Mode I surface morphologies, including ribs and hackles. Joint data were gathered along tape
transects at more than 70 locations around the Uplift, and include
orientations, vertical continuity, cross-cutting relationships, surface
morphologies and presence/absence of mineral fill. These
data are being analyzed in order to relate orientation, spacing and
timing of joints to folding.
Preliminary analysis reveals three
main joint orientations: NW
(327?-1?), NE (19?-64?) and EW (78?-101?). Most locations contain
two dominant joint sets, with one usually occurring roughly parallel
with the local trend of the monocline and the other set sub-orthogonal. Joint
spacing ranges from 1m – 7m across most of the Uplift, but just
to the west of the Uplift the spacing soars to ≥50m in essentially
flat-lying Wingate. Joint spacing also exceeds 50m on the monoclinal
limb where joints crosscut tectonic deformation bands that are found
exclusively on the eastern limb.
Preliminary interpretation suggests 1) on the eastern
limb, deformation bands helped to accommodate folding; 2) jointing
on the eastern limb postdates deformation banding, the presence of
which possibly inhibited joint development; 3) jointing on the rest
of the Uplift is oriented rationally with respect to folding, suggesting
A) that folding and jointing were at least partly synchronous, or B)
jointing postdated folding, but formed in response to release of residual
stresses.
[Swanberg, K.A., and Davis, G.H., 1999, Analysis of
spacing and orientation of jointing
In Wingate Sandstone, Circle Cliffs, Colorado Plateau: Geological Society
of America
Abstracts with Programs, v. 31, no. 4, p. A58.]
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