Christie M. Rogers1, Terry Engelder2
(1) The Pennsylvania State University, University Park, PA
(2) The Pennsylvania State University
ABSTRACT: Mechanism for Closely Spaced Joint Propagation in Joint Zones of the Navajo Sandstone, Zion National Park, Utah
Lateral relief of stress as predicted by the stress shadow model preempts the development of joint zones by any mechanism that appeals to a joint normal stress. However, joint zones may constitute many high permeability pathways in reservoir settings thus making the search for a mechanism causing such joint zones important to industry. The flat lying Navajo Sandstone (Jurassic) in Zion National Park contains large straight canyons that exhibit regular spacing of approximately 500 m. These canyons form geomorphic slots by differential erosion of one or more joint zones that extend from the top to the base of the 500 m thick Navajo Sandstone. A two-stage process is required for the development of the joint zones at Zion. First, regularly spaced vertical joints propagated in intact Navajo Sandstone and relieved lateral confining stress possibly during the uplift and concomitant stretching of the Colorado Plateau. The pattern of early jointing led to a bed thickness to slot spacing ratio of approximately one. A 500 m spacing represents joint saturation in the Navajo Sandstone as explained by a stress-shadow model. Second, joint zones developed when small cracks in the zone of lateral stress relief were subjected to gravity induced shear tractions causing the propagation of wing cracks. The distribution of wing crack development leads to geomorphic slots that exhibit a "V" shape in cross section. The "V" shape is a consequence of the distribution of fractures where the density of fracturing decreases toward the base of the Navajo.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado