THE AGE AND PROVENANCE OF THE EOCENE CRANDALL CONGLOMERATE, NORTHWEST WYOMING

The Crandall Conglomerate is a fluvial channel deposit that is exposed in Northwestern Wyoming in the drainage system of Clarks Fork of the Yellowstone River. These exposures can be as much as 100 m thick and are composed of clast-supported conglomerates that contain clasts derived from local Paleozoic carbonate units. There are 15 known exposures of the Crandall Conglomerate which are spread out over a large area because the Crandall Conglomerate was dispersed during Heart Mountain faulting some in the lower plate, and the others in the upper plate. The Crandall Conglomerate is interpreted to have been deposited immediately prior to Heart Mountain faulting. In this study, we sampled the Crandall Conglomerate matrix from 6 different locations (North and South Dead Indian Creek, Beam Gulch, Lodgepole Creek, Dead Indian Hill, and Squaw Peak) to conduct detrital zircon geochronology (n=544). Using our spectrum of detrital zircon ages, we are able to establish a Tuff age of the time of deposition of the Crandall conglomerate of 51.8 ± 0.9 Ma, which concurs at the same time as Absaroka volcanism, and dates just before Heart Mountain Faulting. Using our spectrum of detrital zircon ages, we are able to gain a better understanding of the provenance of sediment sources supplying the Crandall Conglomerate during its formation, which in total (all 6 samples) are comprised of expected Eocene (18%) and Archean (18%) sources, but surprisingly abundant amounts of Mesozoic (7%), Paleozoic (5%), and Proterozoic (52%) zircons, indicating that the Crandall Conglomerate is composed primary and recycled zircons. The age and abundance of recycled materials in the Crandall Conglomerate indicate a significant westerly (Targhee uplift) source of sediment along with local basement and volcanically-derived zircons. The tuff age estimate based on Eocene zircons is 51.8 ± 0.9 Ma, which is about 3 Ma older than Heart Mountain faulting.