THE NORTHWEST OUTLET OF GLACIAL LAKE AGASSIZ: A CATASTROPHIC FLOOD, OIL-CONTAMINATED RADIOCARBON DATES AND PERMAFROST

The northwest outlet of glacial lake Agassiz figures prominently into multiple versions of the drainage history of the lake with most interest focused on whether the opening of the NW outlet and associated catastrophic flood sediments were deposited near the start of the Younger Dryas chronozone (YD). We revisited stratigraphy of exposures along the Athabasca valley associated with aggregate development for several oilsand extraction sites, and exposures along the lower Athabasca River. We recognize five units in the valley: (1) pre-Laurentide Athabasca River gravel deposited prior to advance of the Laurentide Ice Sheet; (2) post-glacial Athabasca river gravels; (3) catastrophic flood deposition associated with the Clearwater spillway; (4) sandy braided river sedimentation following flood deposition; and (5) fluvial-deltaic sedimentation in the lower Athabasca into glacial Lake McConnell. Radiocarbon ages on mammoth bones and wood suggest a mid-Wisconsinan age for unit 1. The surface of flood-related gravels are modified by periglacial structures, including ice wedge casts and involutions on the surface, suggesting that climate was sufficiently cold when the NW outlet was active to have active permafrost, presumably extending into the late Pleistocene. New radiocarbon ages on plant macrofossils and spruce logs within deltaic sediments in the lower Athabasca valley show persistent impacts of bitumen contamination on many samples, which we removed using organic solvents in a soxhlet apparatus. Collectively the radiocarbon chronology together with the presence of periglacial structures on the surface of flood-related features suggest the NW outlet was active at least during part of YD time, but it is still unclear if flood initiation was coincident with the beginning of the YD.