FRACTURE PATTERN VARIABILITY IN THE ENTRADA FORMATION IN THE MOAB AREA, UTAH

Analysis of fracture traces in Google Earth led to field work concentrated in the Castle Mesa area and the NW end of the Arches anticline. Extensive bedding plane exposures of the Moab Tongue Member (MTM) presents an opportunity to see variation of within unit fracture patterns. Multiple episodes of mainly Cenozoic age salt tectonics and related faulting contributed to fracturing. Overall variation in the MTM of joint patterns a spans area dominated by one joint preferred orientation versus those with up to 3. In contrast, the underlying Slickrock can be devoid of joints, or has a much lower overall joint density, often in one dominant direction. Facies variations within the MTM influence fracture development, with 3 discernable sub-units. Stereoplots from field work show three distinct joint sets which include a dominant subvertical set running ≈E-W with an antithetic orthogonal set, and a more locally developed additional NE-SW joint set. Deformation bands formed first, followed by the E-W joints, and lastly the N-S set. A singular-appearing fracture in Google Earth was often a cluster of joints and deformation bands in the field. Deformation bands influenced later joint formation, with joints running parallel or perpendicular to the bands. In the Castle Mesa area, normal faults of varying sizes indicate overall N-S extension, with a dominant N-dipping set and a subordinate S-dipping set. Distributed deformation band bundles some of which evolved into small faults with less than a meter of offset occurred in the Castle Mesa area at distances greater than 700 m from the nearest substantial fault, but not in the immediate underlying Slickrock. From the cliff sides, joints appear to be locally conjugate. Maps of measures of the degree of strike organization in the MTM show an increase in complexity with an approach to the major fault. Weathering related jointing is common in the MTM, with sub-horizontal fractures, often along bedding. Exfoliation-like layers contain subvertical shallow joints that truncate against the sub-horizontal ones. These “slab break-up” joints often parallel deformation bands and older joints. Variation in fracture pattern is influenced by a combination of localized faulting and salt tectonics, the distribution of earlier structural elements, and facies changes (mechanical stratigraphy).