The Beaverhead Group records coarse-grained, conglomeratic deposition in the northwestern foredeep of the Cordilleran foreland basin. In the northeastern portion of the study area, it unconformably overlies and is deformed by the Laramide-style Blacktail-Snowcrest uplift. To the west, it is deformed by and structurally juxtaposed against Paleozoic and Mesozoic passive margin strata in the Sevier-style fold-thrust belt. Previous work on limited palynomorph samples suggests Coniacian-Campanian (~89-72 Ma) depositional ages while structural and stratigraphic relationships additionally suggest Maastrichtian-Lower Cenozoic depositional ages. Previous work based on clast compositions implies that units involved in deformation related to both the Sevier thrust belt and Laramide-style Blacktail-Snowcrest uplift were the primary sediment sources to the Beaverhead Group.
This study aims to better define the depositional age and provenance of the Beaverhead Group by utilizing U-Pb dating of detrital zircons in combination with conglomerate clast compositions and sandstone petrography. Maximum depositional ages based on the ages of youngest single grains range from ~83-66 Ma (Campanian-Maastrichtian). Provenance analysis for various units of the Beaverhead Group suggest local and/or distal sediment sources, with the former encompassing the Blacktail-Snowcrest uplift and local portions of the Sevier thrust belt and the latter including distal portions of the Sevier thrust belt. Maximum depositional ages in conjunction with provenance interpretations require that the Blacktail-Snowcrest uplift was actively exhuming at ~81 Ma and that the Sevier thrust belt was locally active from ~83-66 Ma. Distally sourced sediments from the Belt Supergroup of Idaho suggest that a paleoriver system connected regional sources to Beaverhead Group depocenters from at least ~83-66 Ma.