STRATIGRAPHY AND SEDIMENTOLOGY OF THE LIMEROCK CANYON FORMATION: IMPLICATIONS FOR THE RUBY'S INN THRUST, SEVIER FAULT, AND MARKAGUNT GRAVITY SLIDE, SOUTHWEST UTAH

The Cenozoic geology of southwestern Utah’s Markagunt and Paunsaugunt plateaus is influenced by the complicated interactions of widespread bi-modal volcanism, south-vergent compressional deformation, large-scale catastrophic gravity sliding, and Basin and Range extension. Stratigraphic and sedimentologic description of the Early Miocene Limerock Canyon Formation (LCF) provides the basis for paleogeographic and tectonic interpretations in the Basin and Range – Colorado Plateau Transition Zone at that time. Sediment gravity flow, fluvial, and lacustrine deposits are the primary stratigraphic elements of the LCF. The LCF basin thickens in the direction of two major structures: the Ruby’s Inn thrust to the north and the Sevier fault to the east. Sediments of the LCF were deposited on the foot-wall and hanging-wall of those structures, respectively. Paleocurrents in the LCF suggest a southwestern source, and are based on the orientations of paleodepositional surfaces, facies shifts, and lithic provenance. Vertical stratigraphic sections through the LCF are typified by increasing clast maturity and diversity, and decreasing depositional gradients. Lithologic analysis of conglomeratic facies indicate that sediments were proximally derived from the Markagunt gravity slide, and show a crude unroofing history. These data suggest that the LCF was deposited as a fan-delta system south and west of broad structural highs of the Ruby’s Inn thrust and Sevier fault, and that deposition initiated as a result of catastrophic emplacement of the Markagunt gravity slide. Abundant sediment supply and a limited amount of accommodation space/subsidence caused the basin to overfill and recycle intrabasinal detritus. Lack of compressional deformation structures and progressive unconformities in the LCF constrain movement on the Ruby’s Inn thrust to be no younger than 22 Ma. Additionally, presence of a northeast-southwest trending topographic barrier indicates that the Sevier fault had undergone some amount of slip by 22 Ma. Although the LCF is a relatively thin deposit of limited geographic extent, it provides insight into the unique and complex structural evolution of this part of the Colorado High Plateaus.