High-frequency incised valley systems and subaerial exposure surfaces are prevalent in Aptian-aged units, having been documented in numerous basins around the world. In light of a growing body of palynological, geochemical and sedimentological evidence, a glacio-eustatic mechanism for their generation has been suggested, driven by the waxing and waning of polar ice caps. The densely-penetrated McMurray Formation of north-eastern Alberta, Canada, provides a unique opportunity to test for an Aptian glacio-eustatic signal within the Western Canadian Foreland Basin. This study combines detailed drill core description with wireline log-based stratigraphic analysis to map and evaluate a series of drainage networks located along the cratonic edge of the Western Canadian Foreland Basin. Four stratigraphically distinct composite channel-form bodies are documented across an area >14 000 km 2 . Composite channel-form bodies are up to 50 km wide and locally exceed 70 m thick, subtending from upward-coarsening parasequence sets 2 to 12 m thick; bounding marine flooding surfaces are mappable over distances exceeding 280 km along the north-south trending basin. Detailed internal fill characterization of the four composite channel-form bodies suggests that three represent tidal-fluvial incised valley networks, and one represents a deltaic distributary system. At least six cycles of transgression and regression are apparent within the McMurray Formation stratigraphy. Recent biostratigraphic assessment has constrained deposition of the McMurray Formation between ca 125 Ma and 118 Ma, and periods on the order of 0Á7 to 1Á7 Ma are estimated for the mapped transgressive-regressive cycles. The high frequency nature of these Aptian sequences suggests that the sea-level oscillations recorded in McMurray Formation strata are consistent with a glacioeustatic driver. Linking the internal stratigraphic architecture of the McMurray Formation with interpreted global cooling events and sea-level drops provides the first evidence of glacio-eustasy within Early Cretaceous strata of western Canada.

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The recognition of an avulsion in the stratigraphic record of an ancient river can provide key insight into its paleoenvironmental setting. In this study, the first planform recognition and delineation of a continental-scale river avulsion node in the deep-time record is used to provide novel insights into the paleogeographic setting for Aptian strata of the Western Interior Basin. Deposits of the Cretaceous McMurray Formation (A2 channel belt) in the Athabasca Oil Sands Region of Alberta, Canada, compose a world-class archive of fluvial–deltaic deposition, captured with a uniquely dense wireline-well-log and drill-core dataset. Despite extensive research on this expansive deposit, however, the depositional setting and paleoenvironmental conditions of the formation have been the subject of long-standing and unresolved debate. In this study, the planform geometry of meander belts characterized by pervasive point-bar and oxbow-lake deposits are examined along a continuous dip-oriented transect > 100 km long, covering > 11,000 km2. The avulsion node documented is linked to three potential causal mechanisms: the presence of the paleobackwater limit, syndepositional salt collapse, or differential erosion and compaction of the substrate associated with an underlying Devonian carbonate escarpment. Although the data compiled do not favor any one of the three proposed mechanisms, each hypothesis potentially provides novel insights into the depositional environment of the McMurray Formation. Notably, the paleobackwater interpretation is consistent with recent seismic geomorphological analysis of the local A2 channel belt that suggested that deposition occurred in the upper reaches of the backwater zone. The results of this work have implications for delineating hydrocarbon-bearing units in the Athabasca Oil Sands, as well as recognizing the record of ancient avulsion nodes in other sedimentary basins.

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