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.]