FLUVIAL CHANNELS AND PALEO-TRIBUTARY JUNCTIONS IN THE LOWER CRETACEOUS MCMURRAY FORMATION, ALBERTA, AS DEFINED FROM DETRITAL ZIRCON SIGNATURES AND SUBSURFACE MAPPING

The Lower Cretaceous McMurray Formation in northeastern Alberta records deposition of a northwestward-flowing fluvial system located in the distal portions of the Cordilleran foreland basin. Here we present new detrital zircon data from 27 samples collected from the McMurray Formation as well as new subsurface mapping of paleovalley systems. Together, this data set demonstrates how the fluvial channels of the McMurray Formation evolved during deposition, and provides evidence of a westerly-derived tributary that joined the main valley system. Detrital zircons in the McMurray Formation are dominated by Appalachian- (300-700 Ma) and Grenville-derived grains (1000-1300 Ma), with lesser components of grains derived from the Cordilleran (less than 250 Ma) and Canadian Shield (ca. 1800 and 2500 Ma) provinces. In restricted locations, the relative proportion of Cordilleran-derived grains within the fluvial deposits increases significantly. These locations correspond to mapped paleo-tributary systems that originated from areas proximal to the Canadian Cordillera. Thus, we infer that the increase in Cordilleran-derived zircons in these locations records the junction between a westerly-derived, paleo-tributary valley and the primary trunk system of the McMurray Formation. Changes in detrital zircon signatures are also present between lowermost and uppermost McMurray Formation channels. Lowermost channels in the McMurray Formation are smaller than those in the upper portion of the formation and contain larger proportions of detrital zircons derived from the Canadian Shield. Fluvial channels in the upper portion of the McMurray Formation are almost an order of magnitude larger than those in the lower portion of the formation and are dominated by Appalachian and Grenville zircons. The change in channel dimensions within the McMurray Formation are associated with changes in provenance, suggesting the increase in channel size may be related to drainage basin expansion or rearrangement. Together, these data suggest detrital zircon signatures within fluvial strata may be useful for understanding changes in drainage basin dimensions and locating paleo-tributary junctions.