The Lower Cretaceous McMurray Formation of northeastern Alberta hosts most of the bitumen resources of the Athabasca Oil Sands. Despite its importance, the sedimentary provenance and corresponding Early Cretaceous paleodrainage system associated with these fluvial deposits remain poorly understood. We collected 18 sandstone samples from five cored wells drilled in the McMurray Formation and analyzed these for detrital zircon uranium lead (U-Pb) geochronology. Together, these samples yield detrital zircon U-Pb age populations of less than 250, 300-600,1000-1200,1800-1900, and 2600-2800 Ma. Almost all of the samples contain detrital zircons with ages of 300-600 and 1000-1200 Ma, which were originally derived from the Appalachian and Grenville provinces, respectively, of eastern North America. Lowermost strata of the McMurray Formation are characterized by relatively small fluvial channel deposits and detrital zircon ages of 1800-1900 and 2600-2800 Ma, which suggest a limited paleodrainage area that includes the adjacent Canadian shield. In contrast, channel deposits in the middle upper part of the formation are relatively large and contain abundant Appalachian- and Grenville-derived detrital zircons. These data suggest that the paleodrainage system of the McMurray Formation evolved over time, increasing in size between deposition of the lowermost units and the middle upper deposits. Detrital zircons from the Appalachian and Grenville regions may have been transported directly to western Canada during the Cretaceous or recycled multiple times prior to their deposition. Detrital zircons from the Cordillera (<250 Ma) are restricted to the northern part of the study area, which suggests that a tributary may have joined the main trunk fluvial system in this area.