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SUBGLACIAL MEGAFLOODING ON THE ALBERTA PLAIN, WITH ANALOGIES TO THE CHANNELED SCABLANDS
Analogies to the Channeled Scablands of Washington State provide a picture of megaflood-related landscapes. There, flood channels are separated by hummocky features, including ridges and swales and potholes that originated within a broad sheet-flow with erosion by vertical vortices or “kolks” drilling into basalt. Subsequent concentration of flow eroded channels, leaving elevated hummocky remnants between the channels. Analogues in Alberta were seen, except megachannels are over 80 km wide, 1000 km in length and rise on an adverse slope almost 800 m. These megachannels are cut into a variety of substrates and are also separated by higher-elevation hummocky topography. Hummock composition includes truncated, deformed and in situ sediment and bedrock within a uniform hummocky topography. Since the hummocks cannot be explained by glaciotectonic processes, deposition, or mass movement, they are thought to be erosional.
A streamlined Tertiary upland between the megachannels, 180 m above the hummocks, and 300 m above the eastern megachannel has a tunnel channel cut into its upper surface. Eskers in the channel are undeformed. Gravel within one esker was crushed, indicating extreme static. The eskers’ ends overlie hummocks, indicating esker deposition occurred last. The altitude of the tunnel channel at its upstream end indicates a hydrostatic head in excess of 200 m drove water over the hills. Channels found similar elevations in the Porcupine Hills, 300 km to the SW go uphill. Extreme discharges capable of achieving these elevations were regional in scale, and 300 m higher than megachannels that border the hummocks. These discharges could have only occurred near glacial maximum, and the array of landforms was created during megaflooding. Discharges at the highest hydrostatic head do not have analogies in the Scablands channels which were entirely subaerial, however, the array of hummocks, and their association with megachannels, do have analogues.