Evaluation and interpretation of faulting on the Rock Springs Uplift, SW Wyoming

The Rock Springs Uplift (RSU) in southwestern Wyoming is a N-striking, doubly-plunging anticline oblique to most NW-striking Laramide arches and faults. At the RSU, regional ENE Laramide compression, oriented on average at 066° (Erslev and Koenig, 2009), caused deformation along a blind, west-vergent reverse fault, and resulted in numerous minor faults cutting Cretaceous units that are now exposed at the surface (geologic maps and cross-sections included in supplementary files). The RSU is being examined as a candidate for CO2 sequestration, with injection proposed in the Madison Limestone and Weber Sandstone, at depths of 13,000 and 12,000 ft respectively, with a confining system of Triassic shales at depths of 9,700 ft to 10,900 ft. In order for CO2 injection to occur, faults exposed at the surface of the RSU must not compromise the Triassic confining system. Fracture analysis, well data interpretation, and twenty-five square miles of 3-D seismic data collected on the northeast flank were used to help determine the structural process(es) of fault initiation to indicate depth of fault penetration.;Fault orientations are mapped as changing across the uplift from ENE-striking on the western flank to E-striking on the eastern flank. Fractures were analyzed across the eastern and western flanks of the RSU by measuring major and minor faults and fractures, on and around mapped faults (data points identified in supplementary files). Although very few sense of motion indicators were observed, normal fault conjugate fracture planes were observed in some areas when data were displayed on stereonets. The majority of fracture planes have dips greater than 50°, which indicates the possible absence of low-angle reverse faulting. To further classify fault types present on the RSU, fracture planes were separated into normal faults and strike-slip faults/joints based on dip angle and dip frequency.;Normal fault and joint orientations suggest a dominant N-S direction of extension, perpendicular to strike. N-S extension corresponds with E-striking faults mapped on the eastern flank of the RSU, that are perpendicular to the anticline fold hinge. A possible secondary NNW extension direction is present on the southwest flank of the RSU, exhibited by ENE-striking normal faults, strike-slip faults, and joints. These features are parallel to Laramide compression, and oblique to the fold hinge.;Structure contour and isopach maps created from well data suggest that E-striking faults likely do not penetrate below the Cretaceous Rock Springs Formation, which is significantly shallower than the Triassic confining system. No mapped faults were observed offsetting reflectors in seismic data over a small area of the northeast flank, but subsurface faulting could be present outside of the coverage area or be below the resolution of the seismic data. Fracture analysis, well data, and seismic data, indicate that two structural processes may have initiated faults mapped across the RSU: oblique-slip on the blind reverse fault and outer-arc extension during folding aided by gravitational collapse. The interaction of these structural processes possibly caused the curvature of fault orientations across the anticline.;The angle between the N-striking blind reverse fault and the regional Laramide ENE-directed compression may have resulted in oblique-slip on the blind reverse fault on the SW flank. This resulted in normal faults on the western flank, near the thrust fault, that strike parallel to the average Laramide compression, oblique to the structure and thrust fault. As the RSU deformed into a N-striking anticline, extension was accommodated on the eastern flank, not in the vicinity of the thrust fault, by outer-arc extension aided by gravitational collapse, which resulted in E-striking faults perpendicular to the N-S fold hinge and oblique to Laramide compression.