SEDIMENT FLUXES OF THE LAKE MICHIGAN AND THE GREEN BAY LOBES OF THE LAURENTIDE ICE SHEET

Accurate estimates of sediment flux and sedimentation rates are important when trying to understand and contrast processes under former ice sheets. By combining volume estimates generated using GIS with bounding radiocarbon age estimates, we have calculated the average sediment flux and sedimentation rate under the southern part of the Lake Michigan Lobe and the Green Bay Lobe of the Laurentide Ice Sheet. Water well and geologic logs were collected in eight-km-wide bands along each flow line and used to create triangular irregular networks that represent the top and bottom of each major lithostratigraphic unit. Only diamicton currently present was quantified, therefore data represent a minimum estimate of the glacial sediment transported. The calculated sediment flux ranges from 150 and 890 m³ per radiocarbon year per meter width for different members of the Wedron Group on the Lake Michigan Lobe flow line. These fluxes are comparable with those estimated in other studies (Johnson et al., 1991; Alley, 1991). The sediment flux for the Green Bay Lobe was 185 and 360 m³ per radiocarbon year per meter width of ice lobe for the Holy Hill and Kewaunee Formations respectively. The sedimentation rates for the Lake Michigan Lobe range between 3 and 8 mm per radiocarbon year. The sedimentation rates for the Green Bay Lobe are 0.9 and 4 mm per radiocarbon year. The sediment fluxes and sedimentation rates of the Green Bay Lobe prior to 13,000 radiocarbon years B.P. (Holy Hill Formation) are lower than those of the Lake Michigan Lobe and later advances of the Green Bay Lobe. The presence of permafrost and a cold-based ice margin in the Green Bay Lobe prior to 13,000 radiocarbon years B.P. may explain this lower sedimentation rates. It may also be that the southern Lake Michigan Lobe and later advances of the Green Bay Lobe advanced directly out of a lake basin that would trap sediment.