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Continental-Scale Paleodrainage for the Mannville Group, Alberta Oil Sands, from Detrital Zircons


Detrital zircons (DZs) represent a powerful tool for understanding continental paleodrainage. This presentation uses DZ age spectra from fluvial sands of the Aptian-Albian Mannville Group, within the eastern Alberta foreland-basin system, to address Cretaceous paleodrainage of North America. Published interpretations of Aptian McMurray fluvial deposits of the “Assiniboia paleovalley” in eastern Alberta and western Saskatchewan infer a paleodrainage from the southwest US and Western Cordillera, although significant Canadian shield contributions are well known. However, DZ spectra in McMurray sands from Cold Lake, in eastcentral Alberta, are dominated by Appalachian-Grenville ages (ca. 500-300 Ma and 1250-950 Ma), lack strong signals from the southwest US (Yavapai-Mazatzal orogens, ca. 1800-1600 Ma) and Western Cordillera (<2% of grains are >300 Ma), and are statistically indistinct from Cretaceous fluvial sands of the Gulf of Mexico. McMurray samples from Athabasca, paleodownstream from Cold Lake, have the same Appalachian-Grenville signals, but include increasing contributions from the Trans-Hudson orogen (ca. 2000-1800 Ma) and Archean shield (>2500 Ma) to the east. These data indicate a continental-scale drainage sourced in the Appalachians of the southeastern US through eastern Canada, which served as the divide between the Gulf of Mexico-Atlantic and the Boreal Sea. This paleodrainage was similar in scale to the modern Amazon, and routed through bedrock valleys in the midcontinent US to the Assiniboia valley, but remained mostly separate from south- and west-derived fluvial systems that dominate McMurray-equivalents in the foredeep to the west. Albian Clearwater and Grand Rapids Formation sands at Cold Lake are known to include Western Cordillera volcanics. DZ signatures confirm this view: >60% of zircons have U/Pb ages >300 Ma, and strong peaks associated with well documented periods of magmatic flux, but an underlying robust Appalachian-Grenville signature as well. These data indicate persistence and expansion of the Appalachian-derived paleodrainage to include tributaries from the Cordilleran Arc in southwest British Columbia and the northwestern US. This continental-scale paleodrainage produced the laterally extensive and very thick point-bar successions in the Mannville Group that make the oil sands technologically and economically viable.