OUTBURST FLOODS AND RAPID DEGLACIATION: THE TERRESTRIAL EVIDENCE AND THE MARINE RECORD

Outburst flood tracts are identified on continental-scale DEM hillshades and on radar images of North America. These tracts were first predicted using landforms interpreted to have originated subglacially by fluvial erosion and deposition. This interpretation is straightforward for bedrock erosional forms which illustrate the scale and magnitude of meltwater outburst floods. Although the interpretation of drumlins and a variety of hummocky and transverse ridge terrains is more involved the conclusion that many originated by meltwater is well established. Flood tracts and the evidence for their meltwater origin will be presented.

Ice decoupling from the bed gives rise to low slope and thin ice. Upon recoupling, the ice-sheet slope appears to have been insufficient to produce flow and the ice stagnated. In such areas deglaciation was very rapid. In other regions end-moraines, some of them glaciofluvial, indicate postflood glacier rejuvenation.

The volume of sediment removed by floods may be estimated using sediment transport rates and total meltwater discharge. DEMs can also be used to estimate sediment volumes contained between the preflood and postflood landscape. A depth of erosion of about 20 m for flood tracts is estimated using these two methods. The eroded sediment is not found in appreciable volumes on land or on the continental shelf. But, if the outburst concept is valid, sediment should be somewhere. Recent work in the Pacific Ocean and the Labrador Sea illustrates flood sediment. The Labrador Sea bed topography and the detrital carbonate beds give evidence for major meltwater events on land. The timing of these events is intriguing.