Abstract

Block faulting along west-northwesterly strikes, with displacements of at least a few hundred meters, took place in what is now the northern Markagunt Plateau, Utah, during the late Oligocene and early Miocene. Resulting features of this faulting included south-southwest-facing scarps that served as barriers to the northeastward spread of regional ash-flow tuffs and eolian sand derived from sources to the south and southwest, as well as a west-northwest trending graben that channeled a locally erupted ash-flow tuff. Large gravity slides also moved southward down the dip slopes of tilted fault blocks that were bounded by west-northwest-trending scarps, which in turn collapsed to produce large megabreccia deposits. On a regional scale, faults along similar strikes may have controlled the distribution of regional ash-flow tuffs farther west within the present Great Basin. Although the origin of the faults is problematic, they demonstrably formed before the north-northeast-striking extensional faults that mark the structural differentiation between the High Plateaus and the Basin and Range Province.