Beach Ridges as Monitors of Isostatic Uplift in the Upper Great Lakes

Abstract

Beach-ridge complexes located at key areas in the Lake Michigan and Lake Superior basins preserve continuous geomorphic and stratigraphic records of lake-level change during the late Holocene. Beach-ridge complexes at Sturgeon Bay, Wisconsin, Gary, Indiana, and Whitefish Point, Michigan, have been intensively studied to provide a reconstruction of differential postglacial isostatic uplift within the Lake Michigan and Lake Huron basin during the late Holocene. The three sites were analyzed in a framework that correlated and dated landforms on the basis of geometry aided by 14C age control. Contrary to expectations, the reconstructed rebound rates were not exponential. The constant rate for Whitefish Point was 0.34 m/century whereas that at Sturgeon Bay was 0.09 m/century relative to the southern shore of Lake Michigan. These rates are consistent with rates calculated on the basis of historical lake-level gauge records. The analysis suggests that crustal movement beneath the upper Great Lakes has been constant over the past 4,000 years. The variance between this region and coastal areas where isostatic uplift models were originally derived is unclear. Lake level in Lake Michigan and Lake Huron was progressively lowered between 5,000 and 2,100 years b.p. by erosion of the outlet channel from Lake Huron. Lake Superior began to be uplifted above Lake Huron about 2,100 years b.p. Over this period, the mean water surfaces of these lakes have been separated by 6.92 m. The rate of uplift necessary to account for this difference is 0.33 m/century. The separation of Lake Superior from Lake Huron was apparently, synchronous with stabilization of the outlet channel from Lake Huron. Since that time, uplift at this outlet has raised lake level in southern Lake Michigan by about 1.9 m at a rate of 0.09 m/century.