STABLE CARBON AND OXYGEN-ISOTOPE RECORD OF CENTRAL LAKE-ERIE SEDIMENTS

Stable carbon and oxygen isotope data from mollusc aragonite extracted from sediment cores provide new information on the origin and history of sedimentation in the southwestern area of the central basin of Lak e Erie. Sediments infilling the Sandusky subbasin consist of three lit hologic units overlying glacial deposits. The lowest of these is a sof t gray mud overlain by a shell hash layer containing Sphaerium striati num fragments. A fluid mud unit caps the shell hash layer and extends upwards to the sediment-water interface. New stable isotope data sugge st that the soft gray mud unit is of postglacial, rather than proglaci al, origin. These data also suggest that the shell hash layer was deri ved from erosional winnowing of the underlying soft gray mud layer. Th is winnowing event may have occurred as a result of the Nipissing floo d. The Pelee-Lorain moraine, which forms the eastern boundary of the S andusky subbasin, is an elevated area of till capped by a sand deposit that originated as a beach. The presence of both the shell hash layer and relict beach deposit strengthens the interpretation that the Nipi ssing flood was a critical event in the development of the southwester n area of the central basin of Lake Erie. This event, which returned d rainage from the upper lakes to the Lake Erie basin, was a dominant in fluence on regional stratigraphy, bathymetry, and depositional setting .