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Propagation |
Department of Geosciences, The Pennsylvania State University, University Park, PA 16801, USA
* ExxonMobil, 233 Benmar, Houston, TX 77060, USA christie.m.rogers{at}exxonmobil.com
Large NNW-trending slot canyons cut into, but generally not entirely through, the approximately 600 m-thick Jurassic Navajo Sandstone at Zion National Park (ZNP). These canyons sit immediately above, and parallel to, joint zones and exhibit a regular spacing (c. 450 m). The joint zones, in particular, consist of vertical and steeply dipping joints that tend to dip towards the axis of the canyon. These regularly spaced, joint-localized canyons are confined to the Navajo, suggesting a stress-shadow origin for their configuration; however, this explanation does not predict closely spaced joints in joint zones at each canyon. To explain the development of the joint zones, we treat the canyons themselves as cracks. Early, widely-spaced, NNW-trending joints propagated into the top of the Navajo, and later preferential erosion along these joints initiated the pattern of canyons with a cross-sectional profile consistent with blunt edge cracks spaced at about 450 m. Analogous to edge cracks, the canyons subsequently concentrated tensile stress at their tips while subjected to regional extension. Concentration of canyon-tip tensile stress was sufficient to drive steeply dipping secondary joints, reflecting principal stress rotation in a process zone ahead of the canyon tip. Joint density in each joint zone increases as a consequence of a gravity-induced shear traction that drives vertical wing cracks from the tips of steeply dipping secondary joints. Exfoliation jointing along canyon walls also contributes to the widening of canyons. The perferential erosion of slot canyons follows the joint zones, and thus, a feedback loop is set up between the growth of secondary jointing in the canyon-tip stress concentration and the downward erosion of the canyon.
