CONCEPTUAL MODEL FOR THE FORMATION OF THE GRAND MERE LAKES REGION IN BERRIEN COUNTY, SOUTHWESTERN MICHIGAN

The Grand Mere Embayment is a coastal wetland complex of southeastern Lake Michigan, in Berrien County, Michigan. The embayment is ~25 kilometers long, with an average width of 1.6 kilometers. The northern end of the embayment contains a series of three shallow lakes: The Grand Mere Lakes, North, Middle, and South. Just south of these lakes are two former lakes that have filled through ecological succession. Furthermore, the current lakes are in progressive stages of succession, from south to north. The lakes and associated wetlands are bounded on the west by both stabilized and active sand dunes, and the modern beach of Lake Michigan. Between each of these current and former lakes are prominent geomorphic points resembling cuspate spits, together forming a sawtooth pattern that points landward. A sharp elevation boundary at ~178 meters AMSL delineates the eastern edges of the lakes and is interpreted as former shoreline. Less than one kilometer to the east of the lakes is a wave-cut cliff into the Covert Ridge of the Lake Border Moraine, the base of which is consistent with Lake Nipissing elevations (~183m AMSL). At least three other bluffs representing pre-Nipissing high-lake stages are evident along the cliff face. The unique geometry of the embayed lakes and the landforms that separate them suggest formation by northward-prograding spits during a higher-than-present lake-level stand. Prevailing lake currents from the south were dominant, but seasonal variations altered current direction, forming successive recurved spits. The spits decrease in size to the north because wave energy diminished as the opening to the embayment became smaller. As the spits progressed northward, a baymouth bar was formed, essentially separating the embayment from Lake Michigan and leaving a string of lagoons, now considered lakes and marshes. Elevation data indicate the spit platforms developed during the Nipissing high stand, but final closure of the embayment likely occurred during late-Nipissing time. Subsequent eolian processes have further reworked available glacial sediments into the large dunes presently covering the spit platforms between the lakes.