SEDIMENT PROVENANCE OF THE LOWER CRETACEOUS MCMURRAY FORMATION, ALBERTA, CANADA: INSIGHTS FROM DETRITAL ZIRCON GEOCHRONOLOGY

The bitumen and heavy oil of the Lower Cretaceous (Aptian) McMurray Formation in Alberta, Canada represent one of the largest hydrocarbon accumulations in the world. Sandstones of the McMurray Formation were deposited in fluvial, estuarine, and marginal marine environments along the southern margin of the Western Interior Seaway during the Cordilleran Orogeny. Despite its size and economic importance, the provenance of the McMurray Formation remains controversial. Petrographic evidence suggests much of the sand in the McMurray Formation was derived from the nearby Canadian Shield, whereas paleogeographic maps imply the sand was derived from a large south-to-north drainage network that extended from the southwestern U.S. to northern Alberta. To examine these hypotheses, we analyzed 9 sandstone samples from the McMurray Formation using detrital zircon U-Pb geochronology. Our preliminary results indicate three distinct detrital zircon signatures are present within the McMurray Formation. The first signature is characterized by zircons of Archean and Early Proterozoic age. We interpret these zircons as indicating a provenance associated with the Canadian Shield. The second signature is characterized by zircons of Grenville (ca. 1000 Ma) and early Paleozoic age. The immediate provenance of these zircons is difficult to determine. The zircon populations suggest an Appalachian source originally, however these zircons may have been reworked from deposits in southern Canada or from the southwestern U.S. The third signature is dominated by relatively young zircons (<400 Ma) with a lesser population of Early Proterozoic ages. This signature is interpreted to indicate a Cordilleran provenance based on the abundance of zircons with these ages in that region. These three signatures suggest a complex provenance, and indicate different sources contributed sediment to the southern margin of the Western Interior Seaway during deposition of the McMurray Formation. The ongoing investigation of the McMurray Formation will determine if the provenance signatures vary spatially or if sediment source areas changed during deposition of the succession.