Chronostratigraphy, Analogues, and Facies Characterization of Fluvio-Tidal Strata: McMurray Formation, Alberta, Canada

Sequence stratigraphic reconstructions of the McMurray Formation are highly complex owing to limited preservation of mappable surfaces and the erosional amalgamation of deposits from a wide range of fluvial, fluvio-tidal and deltaic paleoenvironments. Geochronology was utilized to constrain stratigraphic reconstructions and provide insights into the driving mechanisms of accommodation space variations. A robust foundation for delineating fluvial and fluvio-tidal paleoenvironmental interpretations was established using integrated sedimentological and ichnological facies analysis. Geochronological dating of a reworked ash bed in a coal seam at the top of the lower McMurray Formation returned a highly precise maximum depositional age of 121.51 ± 0.11 Ma, interpreted as the first depositional age in the McKay Paleovalley. Combined with previously published dated ash beds in coastal mire coal seams, the results suggest these marker horizons form the first intraformational datum of the McMurray Formation. The ash-bearing coal seams record extrusive volcanic activity followed by regional transgression, indicating that crustal responses to orogenic tectonism led to accommodation space creation in the distal foreland basin. Paleoenvironmental reconstructions are refined by detailed facies analysis of fluvio-tidal point bars. Study of the Serpentine River, BC presents the first sedimentological and ichnological description of a modern fluvio-tidal translating point bar. The facies variations reflect the complex interplay of fluvial discharge and tides. Biogenic responses are a function of abiotic physico-chemical stresses, potentially altered by biotic stresses and evolutionarily inherited anatomical predispositions. The findings support the interpretation of ancient translating point bars from the McMurray Formation in the Long Lake area. These inclined heterolithic stratification-rich units (IHS) display a range of sedimentological and ichnological expressions. Three translating point bars are described utilizing a mesofacies approach. Twenty-three mesofacies permit precise facies descriptions at the lamina-set, bed and bedset scales and sedimentological and ichnological process-response interpretations. The seaward and central part of the bar record tidal sand and mud transport, alternating with fluvial mud deposition. Facies on the landward part of the bar are predominantly affected by high flow velocities and deposition rates. The detailed facies analysis suggests that deposition of the bars in the Long Lake area occurred in close proximity to the turbidity maximum zone.