Abstract

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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