STRATIGRAPHY, AGE, AND PROVENANCE OF THE FLAGSTAFF FORMATION, SAN FRANCISCO AND CRICKET MOUNTAINS, UTAH: IMPLICATIONS FOR THE DEVELOPMENT AND DEMISE OF THE SEVIER HINTERLAND PLATEAU

During the Late Cretaceous to early Cenozoic, the Sevier plateau covered much of the hinterland of the North American Cordillera. Although most studies agree that this highland was in place by the Eocene, its spatial extent, construction, and ultimate demise are debated. The plateau is marked by an erosional surface, the Paleogene unconformity, that is locally overlain by Cretaceous to Eocene lacustrine and coarse-grained clastic deposits, whose context is poorly understood. The Paleocene Flagstaff Formation in the San Francisco and Cricket Mountains of east-central Utah resides near the transition between these spatially limited Cretaceous to Eocene deposits of the Sevier plateau and better studied foreland basin systems to the east, and thus fills a critical gap in understanding the evolution and denudation history of the hinterland.

This study reports new data for the Flagstaff Formation obtained from geologic mapping (1:24,000 scale), a detailed stratigraphic section, conglomerate clast counts, and paleocurrent analysis, used to track spatio-temporal changes in lithofacies, depositional environments, and provenance of the unit. In the Cricket Mountains, the Flagstaff Formation is ~370 m thick and dips ~10 degrees to the NE. Beds are thick and consist of pebble to cobble conglomerate, sandstones, siltstones, and shales. The conglomerates are well- to poorly-cemented and with medium to coarse grained red sandy matrix. Clasts are a mixture of Paleozoic limestone and dolostone and Precambrian to Cambrian quartzites, including the Caddy Canyon and Prospect Mountain quartzites and Inkom and Mutual formations. Carbonate clasts are most abundant near the base of the section. In the San Francisco Mountains, deposits similar to the Flagstaff Formation (Conglomerate of High Rock Pass) may be underlain by a cobble to boulder conglomerate made up entirely of Cambrian carbonate clasts. To more quantitatively test relationships with these deposits and those of the hinterland plateau, samples have been collected for sandstone compositional and detrital zircon U-Pb and (U-Th)/He analysis. These data will aid in establishing the provenance, depositional age, and lag time of the sediments, which will provide insight into the relationship of these units to thrusts in the region and hinterland plateau development.