THE RABBIT VALLEY SALIENT, A MASSIVE LANDSLIDE DEPOSIT AT THE COLORADO PLATEAU’S WESTERN MARGIN, SOUTH-CENTRAL UTAH

The Rabbit Valley salient forms a large (~20-30 km²) geomorphic feature with a distinctive, yet subtle, hummocky topography that extends westward from the flank of Thousand Lake Mountain into the Fremont River valley in Wayne County, Utah along the Colorado Plateau’s western margin. The Rabbit Valley salient is underlain by poorly sorted sediment that includes large (>100,000 m³) blocks derived from Jurassic to Paleogene sedimentary strata and younger Paleogene volcanic units. Structures within the Rabbit Valley salient include recumbent folds, west-verging overturned folds and thrust faults, brecciated limestone blocks cut by clastic dikes, and older volcaniclastic strata juxtaposed against the 26 Ma Johnson Valley trachyandesite. Sediment thickness in the Rabbit Valley salient is highly variable, but locally exceeds 90 m. We interpret the Rabbit Valley salient to be underlain by a massive and catastrophic landslide deposit sourced from the upper slopes of Thousand Lake Mountain.

The Thousand Lake Fault system forms the easternmost structure at the western edge of the Colorado Plateau, and includes a suite of steeply dipping, normal faults with down-to-the-west normal slip of ~1 km. The mass movement deposit that underlies the Rabbit Valley salient may have been generated by seismic activity on the Thousand Lake Fault system. Dissected Quaternary debris flow fans overlie deposits in Rabbit Valley salient, and are offset by the Thousand Lake Fault. Cosmogenic ³He exposure ages of 400 to 500 ka in pyroxenes from exposed trachyandesite boulders on the faulted debris flow fans provide a minimum age for the timing of the mass movement event at Rabbit Valley and in the upper Fremont River basin.