THREE GENERATIONS OF FRACTURES AND THE LANDSCAPE DEVELOPMENT OF BRYCE CANYON NATIONAL PARK: GEOCORPSTM 2015

Each year, millions of people travel to Bryce Canyon for a glimpse of the dramatic southeast-facing escarpment located in the western Colorado Plateau. Stream erosion of the Paleocene-to-Eocene Claron Formation generates vast amphitheaters and alcoves, with elaborate fins, windowed walls, and hoodoos. Geomorphic models of hoodoo development describe the influence of differential weathering and ice wedging along systematic vertical fractures. Conjugate shear fractures are also found in the footwall of the Rubys Inn thrust fault, located just north of the park. During a summer 2015 GeoCorpsTM of America internship, we investigated the contribution of joint sets to focused erosion of exposed fins and hoodoo development in Bryce Canyon National Park. Our field mapping documents the nature and spatial distribution of known fractures as well as a previously undocumented third generation characterized by steeply-dipping conjugates and zones of distributed deformation. Evidence for normal reactivation of contractional structures suggests that distributed deformation evolved during Basin and Range extension, possibly associated with the nearby Paunsaugunt fault. Cross-cutting relations among fracture sets suggest vertical jointing prior to collapse of the Marysvale volcanic complex and subsequent Rubys Inn thrust fault(~22-20 Ma) which also precedes more recent Basin and Range extension. Spatial trends in fracture density illustrate a systematic increase of vertical fractures, conjugate shear fractures, and reactivated zones to the north, proximal to thrust faulting. The increase in fracture density leads to accelerated weathering and erosion, this contributes to an increase in the development of windows, hoodoos, and gentle slopes. While erosional windows commonly develop along vertical fractures intersecting relatively weak lithologies, approximately 60% of the 133 documented windows also coincide with conjugate shear fractures or reactivated zones. In the southern portion of the park, high vertical cliffs and relatively few hoodoos record the response of a competent limestone-rich facies to increased fracture spacing distal from the Rubys Inn thrust fault.