QUATERNARY STRATIGRAPHY OF THE LOWER ATHABASCA VALLEY AND IMPLICATIONS FOR DEGLACIAL LAURENTIDE ICE SHEET DYNAMICS AND GLACIAL LAKE AGASSIZ MELTWATER DRAINAGE

Pioneering work in the early 1990s linked catastrophic discharge from the northwestern outlet of glacial Lake Agassiz to the stratigraphy in the lower Athabasca River valley. The existing model suggests the Laurentide Ice Sheet (LIS) enabled a single (or perhaps two) Late Pleistocene floods, depositing proximal boulder gravel and a distal braid delta built into glacial Lake McConnell. Recent stratigraphic mapping along the lower Athabasca River suggests an alternative interpretation. Here, we present a conceptual model for the evolution of the lower Athabasca valley and its relation to: LIS dynamics, glacial Lake Agassiz flooding, and the evolution of glacial Lake McConnell.

Following initial late Pleistocene deglaciation, a fast-flowing topographic ice stream of the LIS briefly re-advanced into the valley, deforming deglacial sediments and depositing a distinctive pink diamict north of the Fort Hills moraine. Rhythmically bedded silts and sands overlying glaciotectonized sediments are associated with local proglacial lake development and interpreted to be the last indication of proximal glacial activity in the valley. Flood sedimentation dominated the landscape after the retreating LIS enabled meltwater passage, though incised into and at a lower elevation than the ice-marginal deposits. The flood gravels exposed in mining operations south of the Fort Hills are more steeply graded than the modern Athabasca River and their absence within 20 km north of the Fort Hills suggests that the catastrophic drainage phase was likely built into a lower level of proto-Lake McConnell. Subsequent transgression of Lake McConnell resulted in deposition of the sand-dominated braid delta in the lowermost Athabasca River valley. The sedimentology of the delta suggests that continuous flow, rather than catastrophic drainage, was responsible for its formation at ca. 10.2 – 9.6 ¹⁴C ka BP. In short, the absence of clear flood sedimentation below the Fort Hills suggests that the lower Athabasca valley was incised and graded to a lower stage of Lake McConnell during Agassiz flooding that subsequently transgressed to the ca. 10 ¹⁴C ka BP level. This model would benefit from future work focused on the sedimentological signature of the LIS and glacial Lake McConnell in other tributaries such as the lower Peace and Liard valleys.