DETRITAL ZIRCON PROVENANCE OF PALEOGENE-NEOGENE FLUVIAL CONGLOMERATES AND THE CENOZOIC EVOLUTION OF THE NORTHERN ROCKIES AND SOUTHERN CANADIAN PLAINS

The Canadian Plains of Alberta and Saskatchewan contain remnants of early-middle Cenozoic fluvial sediments, including cobble and boulder conglomerates that are exposed up to 300 m above the surrounding landscape. These coarse-grained fluvial deposits are located hundreds of kilometers from the front of the Cordilleran fold-thrust belt and were deposited millions of years after thrusting ceased. To determine the provenance of these sediments and better understand the Cenozoic tectonic evolution of western North America, we sampled the Cypress Hills and Wood Mountain formations in southern Canada for detrital zircon geochronology. The Eocene-Miocene Cypress Hills Formation in Alberta and Saskatchewan contains late Mesozoic (65-120 Ma), Paleoproterozoic (1700-1800 Ma), and Archean (2600-3000 Ma) detrital zircons. The Middle Miocene Wood Mountain Formation in southern Saskatchewan contains late Mesozoic – Cenozoic (20-110 Ma), Paleoproterozoic (~1700 Ma), and minor Archean (2400-3200 Ma) detrital zircons. The Cretaceous Ravenscrag Formation, which underlies the Cenozoic conglomerates in several locations, contains late Mesozoic (70-130 Ma), Paleoproterozoic (~1700 Ma), and minor Archean (~2400 Ma) detrital zircons. The detrital zircon ages in the Cypress Hills Formation are consistent with a provenance in the Northern Rockies of Montana and Wyoming, including basement-cored Laramide uplifts. Noticeably absent from the Cypress Hills Formation are detrital zircons with ages less than 60 Ma, despite significant coeval volcanism in the region. Differences between detrital zircon populations in the Cretaceous Ravensgrag Formation and the overlying Cypress Hills Formation suggest a change in provenance occurred as the Cordilleran system transitioned from contractional to extensional deformation. Provenance of the Wood Mountain Formation is consistent with the underlying Cypress Hills Formation, with the addition of a large proportion of zircons with ages less than 50 Ma. These conglomerates likely represent the proximal deposits of high-gradient fluvial systems that flowed out of the U.S. Northern Rockies and to the northeast across the Canadian Plains.