THE CANADIAN HIGH PLAINS AND THE NORTHERN ROCKIES: A DETRITAL ZIRCON PROVENANCE STUDY

Remnants of early-middle Cenozoic fluvial sediments, including cobble and boulder conglomerates are exposed up to 300 m above the surrounding landscape in Alberta, Saskatchewan, and Manitoba. These coarse-grained fluvial sediments were deposited in the Cordilleran foreland basin millions of years after thrusting ceased. We sampled the Cypress Hills, Wood Mountain, and Souris River formations in southern Canada for detrital zircon provenance. 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 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. Pleistocene Souris River gravels are similar to older conglomerate deposits and contain late Mesozoic – Cenozoic (200-16 Ma), Paleoproterozoic (~1700 Ma) and minor Archean (2400-3200 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. Differences between detrital zircon populations in the Cretaceous Ravenscrag Formation and the overlying Cypress Hills Formation suggest a change in provenance as the Cordilleran system transitioned from contractional to extensional deformation. Provenance of the Wood Mountain Formation and the Souris River gravels are consistent with the underlying Cypress Hills Formation, with the addition of zircons with ages less than 50 Ma. It appears these conglomerates were recycled and progressively deposited eastward during the Cenozoic.