REGIONAL JOINTS AND THE OCCURRENCE OF ORIENTED ARCHES IN ARCHES NATIONAL PARK, UTAH

A natural arch is a perforation in a rock wall that is larger than ~3 ft across. Arches National Park, Utah, hosts thousands of natural arches with several hundred of them being classified as freestanding arches, a density unique to this area. The perforations are widely regarded to have formed from increased erosion at portions of the rock walls that are affected by dense, secondary fracturing. The location of the arches is generally attributed to the regional structural pattern, caused by gentle folding of surface units due to salt migration at depth. The majority (82%) of the freestanding arches are located in the Slick Rock Member of the Entrada Sandstone, which, together with the Navajo Sandstone, is systematically pervaded by several sets of large, regularly spaced joint sets that follow the trends of the fold hinge lines. Detailed mapping of ~5000 of these joints reveals that there are three major joint sets. Joint spacing in all sets ranges from < 1 m to tens of meters and varies with distance from the fold hinges and the geological unit they fracture. This joint pattern was geospatially correlated to the location, arch orientation, and perforation size of the 527 most prominent arches. The orientation of the arches −as measured by the azimuths of the arch span− generally correlates with the orientation of the predominant joint set in the particular sub-area of the park. The highest density of arches coincides with areas where two joint sets intersect at acute angles of less than 30°. The three major joint sets intersect each other in different parts of the park, defining triangular zones. In particular, two of the major sets curve to assume parallel orientations to one another in each of the parks Devils Garden, Wolf Ranch, and Fiery Furnace/The Windows sub-areas. Interestingly, a large number of arches, including the ones with the largest perforations, cluster in these areas. The location of arches along intersecting curved joint sets and the joint curvature itself both indicate that arch localization is governed by joint intersection and joint growth interaction. While the previous hypothesis of arch formation remains applicable to individual arches, the relationships presented in this abstract show additionally that the locations, orientations, and formation of the arches in Arches National Park are all causally linked.