CONTINENTAL-SCALE PALEODRAINAGE FOR THE APTIAN-ALBIAN MANNVILLE GROUP, EASTERN MARGINS OF THE ALBERTA FORELAND-BASIN SYSTEM, 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 McMurray Fm., eastern part of the Alberta foreland-basin system, to address Early Cretaceous paleodrainage of North America.

Published interpretations of McMurray fluvial deposits of the “Assiniboia paleovalley” in eastern Alberta and western Saskatchewan infer a paleodrainage in the southwest US and Western Cordillera, although significant Canadian shield contributions are known. However, McMurray sands from Cold Lake, in eastcentral Alberta, show DZ spectra that are dominated by Appalachian-Grenville ages (ca. 500-300 Ma and 1250-950 Ma), lack strong signals from the southwestern US (Yavapai-Mazatzal orogens, ca. 1800-1600 Ma) and Western Cordillera (only ~2% of grains are <300 Ma), and are indistinct from early to mid-Cretaceous fluvial sands of the Gulf of Mexico and the US midcontinent. McMurray samples from Athabasca, which is 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, the continental-scale drainage of its time, was routed through the US Midcontinent to the Assiniboia paleovalley, but remained mostly separate from south- and west-derived fluvial systems that dominate McMurray-equivalent strata in the foredeep farther west.

This continental-scale paleodrainage produced the laterally extensive and thick point-bar successions in the McMurray Formation that make the Alberta oil sands technologically and economically viable. Moreover, the concept of a continental-scale drainage also provides context for understanding the scales and styles of McMurray deposits. One outstanding question would be the location of the downdip McMurray depocenter: river systems of this scale likely would have discharged 2-5 million cubic kilometers or more of sediment from their mouths over the 10-20 Myr time scales that McMurray systems were active.