EVOLUTION OF THE GRAND MERE LAKES, SOUTHWESTERN MICHIGAN, INFERRED FROM GEOPHYSICAL SURVEYS AND CORING

Ground penetrating radar (GPR) and electrical resistivity surveys were run across the Lake Michigan beach, foredunes, and east-west trending ridges that separate South and Middle Lakes at Grand Mere State Park (GMSP), southwestern Michigan. Project goals included 1) evaluating the stratigraphic continuity of units beneath the east-west trending ridges; and 2) testing whether a subsurface barrier-spit complex lies beneath the modern beach and nearshore dunes. Auger and vibracore boreholes provide ground truth for interpreting geophysical profiles, and identifying paleosols, the water table and lacustrine sediment. Major reflecting surfaces and stratigraphic units identified with GPR are correlated with vibracores and lake cores from an earlier study. A planation or ravinement surface underlies the entire upper Holocene sequence and can be traced over 1 km east from the Lake Michigan shoreline toward the Nipissing highstand scarp at the toe of Covert Ridge. A sand and gravel unit dated to ~7 ka BP overlain by carbonate-rich mud extends beneath the E-W trending ridge, implying that a single, continuous lake occupied the Grand Mere basin. GPR profiles along the Lake Michigan beach reveal a facies with hummocky, overlapping, north-dipping beds, that indicates northward progradation of a barrier-spit when lake level was 2–3 m below modern. Shore-perpendicular GPR transects that extend into the dunes reveal a buried unit with an eastward-dipping upper surface interpreted as the body of a barrier-bar. GPR profiles on the E-W peninsulas show facies interpreted as the distal (landward) ends of a recurving spit and spit platform that segregated the modern Grand Mere lakes. Taken together, these data from GMSP are setting a foundation for understanding the sequence of events and depositional environments associated with multiple barrier-bar and spit features that successively closed off this arcuate embayment of the Lake Michigan shoreline.