THE DETRITAL ZIRCON RECORD OF CENOZOIC FLUVIAL SYSTEMS IN THE GREAT PLAINS OF THE NORTHERN UNITED STATES AND SOUTHERN CANADA

Paleogene-Neogene fluvial conglomerates and sandstones are exposed across the plains of western Canada and the northern US, and record over 50 million years of tectonics and landscape evolution in the North American interior. During the Cenozoic, these northeast-flowing fluvial systems incised into the underlying foreland basin strata and shifted 1000 km eastward in response to large-wavelength rock uplift in the western plains. We sampled the Ravenscrag Formation, Cypress Hills Formation, Wood Mountain Formation, Hand Hills Conglomerate, and Souris River Gravels for detrital zircon geochronology in an effort to better understand the Cenozoic evolution of the Rockies and the western plains. The oldest unit, the Ravenscrag Formation, was deposited in the Cordilleran Foreland Basin and contains detrital zircon populations with ages of 70-130, 1700, and 2400 Ma. The Eocene-Miocene Cypress Hills Formation and Miocene Wood Mountain Formation, which are exposed in southern Alberta and Saskatchewan, contain detrital zircon populations with ages of 30-120, 1700-1800, and 2400-3200 Ma. The Pliocene Souris River Gravels in southern Manitoba contain detrital zircons with age populations of 30-200, 400, 1200, 1700, and 2600-3000 Ma. The Hand Hills Conglomerate in central Alberta contains detrital zircons with age populations of 60-200, 1700-1900, and 2600-3000 Ma. Multidimensional scaling comparisons between samples suggests the Ravenscrag, Cypress Hills, Wood Mountain, and Souris River units were all derived from relatively similar sources, including Cordilleran arc sources and material recycled from rocks exposed in the northern US Rockies and southern Canadian Cordillera. The Hand Hills Conglomerate was derived directly from the Canadian Rockies and is distinct from the fluvial conglomerates in the southern Canadian Plains. Detrital zircon populations in the Cypress Hills and Wood Mountain formations are similar to those of the Renova Formation in southwest Montana, suggesting these fluvial systems may have once been connected. The Cypress Hills, Wood Mountain, Souris River, and Hand Hills fluvial systems likely represent the upstream end of an ancient, large, transcontinental river system that transported sediments from the Cordillera to the Labrador Sea.