CONSTRAINING THE NIPISSING TRANSGRESSION IN THE MICHIGAN BASIN

Recent studies of beach ridge sequences have recorded variations in lake level for the past 4700 years. To extend this curve further back in time we have used paludal and lacustrine sediment from captured embayments along the western coast of Michigan to constrain water levels in the Michigan Basin during the Nipissing transgression. The Nipissing Transgression was driven by isostatic uplift of the North Bay outlet. Several vibracores from Silver and Stony Lakes in Oceana Co. Michigan and Hamlin Lake in Mason Co., contain sediments consisting of gravel, sand, clay, gyttja, marl and peat. All of the lakes are currently separated from Lake Michigan by barrier-dune complexes. The transition from wetlands to lakes is recorded in the sediment transition from marl or peat to gyttja. This deepening is explained by backflooding of the basins through either a groundwater connection with Lake Michigan, or estuary flooding during the Nipissing transgression. In Silver Lake (169 m) this backflooding has been dated at 6820 cal yr BP (Beta-196698), in Stony Lake (160 m) at 7540 cal yr BP (Beta-193289), and in Hamlin Lake (170 m) at 6885 cal yr BP (Beta199705). When the data is plotted it indicates a stepped rise in lake level.