RELATIVE LATE HOLOCENE LAKE LEVELS AND ISOSTATIC REBOUND IN THE LAKE SUPERIOR BASIN

Strandplains of beach ridges have been used to determine late Holocene lake levels in Lake Superior. Basal foreshore elevations from the lakeward margin of beach ridges and radiocarbon dates of basal peat and organics from wetlands between beach ridges were used to construct relative lake-level curves for sites at Grand Traverse Bay, Tahquamenon Bay, and Au Train Bay, Michigan. The relative lake-level curves show a long-term water level fall followed by a short-term rise. Data suggest that the location of the outlet regulating water levels in the Superior basin may have changed from Port Huron (south of the study sites) to Sault Ste. Marie (north of the study sites) after about 1,300 calendar years BP. The Sault outlet may have also regulated water levels in Lake Superior between 4,000 and 1,300 calendar years BP but only during shorter intervals when lake levels were relatively low.

The differential isostatic rebound rates calculated from the data obtained in this study are in agreement with rates calculated from historical records based on gauging stations around Lake Superior for the period when the Sault outlet was regulating water levels in the Superior basin. This study also establishes rebound rates for the time interval when the Port Huron outlet was regulating water levels in the Superior basin and indicates that these rates are much greater than rates calculated from gauges. The rates established by this study resemble rates calculated from strandplains in northern Lake Michigan.

When plotting relative lake levels for Tahquamenon Bay and Grand Traverse Bay, the curves intersect. These two curves should not intersect because Tahquamenon Bay experiences at least 20% greater differential isostatic uplift than Grand Traverse Bay. This anomaly may be explained by tectonic activity near the Sault outlet, which lowered Tahquamenon Bay about four meters.