PROVENANCE OF PALEOGENE STRATA AT SLIM BUTTES, SOUTH DAKOTA, USA

Slim Buttes is a 30 km long by 10 km wide, ~100 m thick outlier of Paleogene strata in northwest South Dakota. At Reva Gap in northern Slim Buttes, Eocene-Oligocene terrestrial strata of Slim Buttes, Chadron, Brule formations of the White River Group unconformably overlie the Paleocene Fort Union Formation. The White River Group is unconformably overlain with angular discordance by Oligocene and Miocene strata of the Arikaree Group. The goal for this research is to determine the provenance of these rocks as part of a broader synthesis of post-Laramide sedimentation in the Rocky Mountains and western Great Plains. Zircons were separated by traditional gravitational and magnetic techniques, and U-Pb geochronological data was obtained by LA-ICPMS at the University of Arizona Laserchron Center. A total of 352 zircons were analyzed. The Chadron Formation age spectrum is dominated by Cretaceous and Proterozoic grains that are interpreted to be locally derived from the underlying Cretaceous and Paleocene strata. The Brule Formation has a maximum depositional age of ~34 Ma; and Paleogene zircons dominate the age spectrum. A wide variety of older zircons are present. The Oligocene zircons are interpreted to have been sourced from the Indian Peak caldera complex in the Great Basin, while the balance of the zircons was derived from a variety of source areas in the Nevadaplano and Rocky Mountain areas to the southwest. As Archean zircons are sparse, we interpret that the Laramide uplifts of Wyoming were largely buried at the time of Brule deposition. The Arikaree Group sand has a maximum age of deposition of ~27 Ma, and has a multimodal detrital zircon age spectrum, and likely represents continued sourcing in the Nevadaplano and exhumation of the Laramide uplifts.