LAKE-LEVEL ELEVATION IN THE CHICAGO OUTLET DURING THE NIPISSING PHASE OF ANCESTRAL LAKE MICHIGAN: WENTWORTH WOODS STRANDPLAIN, ILLINOIS

Accurate reconstructions of paleo lake level and isostatic rebound in the Great Lakes requires knowledge of the elevation of the water at each lake’s outlet. For the upper Great Lakes, four outlets were active and abandoned from the middle to late Holocene, making the development of a paleohydrograph difficult. Recent study of the Nipissing phase of the ancestral upper Great Lakes determined the elevation of the confluent peak Nipissing phase (~4,500 years ago) for the Sault (Lake Superior) and Port Huron (Lake Huron) outlets, and estimated an elevation for the Chicago outlet (Lake Michigan). We studied beach ridges in the Wentworth Woods area of the Tolleston Beach in Cook County, Illinois, to refine the estimated peak Nipissing elevation for the Chicago outlet. Thirty-one vibracores were collected that crossed 22 ridges to recover basal foreshore deposits in each ridge; these can be used for a proxy for lake level when each ridge formed. Optically stimulated luminescence samples were collected from six soil pits spaced roughly five ridges apart to produce an age model for the strandplain. Combined, these data produce a paleohydrograph that shows a peak Nipissing elevation of 182.9 m at the Chicago outlet. This elevation is 0.4 m below the elevation of the peak Nipissing at the Port Huron outlet, indicating a relative glacial isostatic adjustment rate of -0.7 cm/century for the Chicago outlet. This rate is an order of magnitude less than rebound rates calculated from lake-level gauges. Two possible reasons for this discrepancy is that the duration of lake-level gauge data is too short to recognize long-term trends or that the observed rebound between the Chicago and Port Huron outlets is a late Holocene phenomena possibly related to the dissipation of a glacial forebulge.