SEDIMENT EROSION, TRANSPORT AND DEPOSITION BY OUTBURST FLOODS

Ice sheets give rise to enormous volumes of meltwater. The effects of returning meltwater to the oceans on sea level and climate are crucial aspects of earth system change. Erosion, sediment transport and deposition by ice sheets themselves are responsible for spectacular landscapes and thick sedimentary sequences. But the contribution of meltwater to these processes may be underestimated because the possibility of enormous outbursts from past ice sheets is not widely acknowledged. This situation may change in light of recent observations on modern glaciers that lend credibility to the notion that such floods took place.

I consider the evidence for such floods beneath the Pleistocene ice sheets by looking in detail at the form and sedimentology of landforms attributed to catastrophic floods. The characteristics of bedrock erosional forms (s-forms), some drumlins, fluting, hummocky terrain and transverse ridges including, Rogen moraine, are analogous with erosional forms in nature and experimental forms. Hillshade models covering hundreds of thousands of square kilometers illustrate the arrangement of these forms in vast tracts (flood paths) scaling with the ice sheets.

Two approaches are use to estimate the amount of sediment eroded from flood tracts. First, total flow volume is estimated and a range of plausible suspended sediment concentrations is applied to give a range of total sediment load. The load is converted to volume of sediment removed to obtain the depth of erosion over meltwater flow tracts. A second method obtains the average depth and volume of erosion by subtracting the present drumlinized surface from an approximation of the original land surface. The former land surface is modeled by fitting a surface to a DEM of drumlin crests. The two methods of calculation give remarkably similar estimates for the depth of erosion.

There must be a sedimentary record of these floods had they existed. Although some deposits on land and the continental shelves are candidates for deposition by outburst floods, the most promising deposits are found on the abyssal plains of the Labrador Sea and the Pacific Ocean. In the Labrador Sea, these are thick beds of detrital carbonate. Their sedimentology is as intriguing as their timing is perplexing.