CONGLOMERATE CLAST COUNTS FROM THE CENOZOIC DRY UNION AND SANTA FE FORMATIONS, COLORADO: IMPLICATIONS FOR RIO GRANDE RIFT SEDIMENT PROVENANCE

The Rio Grande Rift is a continental rift zone in the western US that lies between the Colorado Plateau and Rocky Mountain provinces and accommodates extension of western North America. Basin subsidence and provenance of basin sediment in the Miocene Dry Union and Santa Fe formations may illuminate how the Rio Grande Rift has evolved over geologic time.

We present new conglomerate clast counts within the Dry Union and Santa Fe Formations that have implications for Rio Grande Rift sediment provenance. The most common clast types recorded within the conglomerate consisted of felsic to intermediate volcanic rocks, felsic to intermediate plutonic rocks, and metamorphic rocks including quartzite. These lithologic types can be related to specific source areas within the northern Rio Grande Rift that include Precambrian metamorphic basement and Cenozoic volcanic and plutonic rocks. The conglomerate clast counts are interpreted in conjunction with detrital zircon U-Pb ages, low-temperature thermochronology, paleocurrent orientations, ash chronostratigraphy, and stratigraphic facies analysis. From these data sets, we infer the progression of sediment dispersal and subsidence within the basin. The findings of this research constrain the processes of sedimentation and extensional deformation within the Rio Grande Rift and their significance in shaping the broader geologic framework of western North America.