Dating the Tectonic Burial and Maturation of the Principal Source Rocks of the Alberta Tar Sands in the Western Canada Sedimentary Basin

The Tar Sands Deposits (TSD) in the Western Canada Sedimentary Basin (WCSB) are among the world's largest accumulations of petroleum (now bitumen). Two large positional petroleum source strata (the Exshaw Fm. and the Gordondale Member of the Fernie Fm.) have historically been proposed to have passed through the oil generation window during Laramide thrusting between 70 and 55 Ma. Novel direct Re/Os dates of the TSD kerogen of ca 112 to 90 Ma require a re-evaluation of this hydrocarbon system. Here, we provide preliminary conventional K/Ar dates of the time of maturation of these source rocks by dating the diagenetic mineral illite formed in K-bentonites from within the primary source rocks. The age when the kerogen in source shales thermally matures is equivalent to the K/Ar date of illitization of any enclosed K-bentonite. Our K/Ar dates suggest that the Exshaw source rocks matured at pre-thrust restored positions in the far western part of the basin at ca 118 to 112 Ma. In contrast, more easterly samples of Fernie K-bentonites indicate that they matured at 68 to 60 Ma in line with the historical model of Laramide generation of the TSD. Restored pre-thrusting positions suggest that a vast Exshaw “kitchen” was deeply buried beneath several kilometers of Late Jurassic–Early Cretaceous proximal Kootenay and Mannville-equivalent sediments in front of the ancestral Purcell Mountains. In contrast, the Fernie formation samples are stratigraphically higher and further east. Their K/Ar date of illitization suggests that the Fernie source shales did not mature until their burial by Laramide age thrusts well east of the early Cretaceous Exshaw source “kitchen.”