USING GEOPHYSICAL INVESTIGATIONS TO TEST THE CONCEPTUAL MODEL OF THE EVOLUTION OF THE GRAND MERE LAKES, SOUTHWESTERN MICHIGAN

Grand Mere State Park is located on the southeastern coast of Lake Michigan in Berrien County. This area contains a suite of post-glacial lake shorelines. The three small Grand Mere lakes lie between the modern beach-dune complex to the west and former shorelines to the east. The east-west trending ridges between the lakes are hypothesized to be recurved spit platforms that developed from west to east, closing off successive lagoons. Various geophysical methods and a short vibracore were used to investigate the stratigraphy and its implications for depositional environments. 900 meters of ground-penetrating radar data, 300 meters of refraction seismic data, and over 600 meters of electrical resistivity data using both dipole-dipole and Wenner array formats were collected. The stratigraphy that emerged from the geophysical techniques includes: 1) up to 5m of surficial sand, deposited in a west-east direction, that thins to the north and south across the spit, 2) the presence of a paleochannel beneath the ridge at its eastern edge where it joins an older shoreline, and 3) beneath the sand, a ~5m thick, low resistive unit interpreted as lacustrine silt that overlies a higher resistive, possible sand body of unconstrained geometry. On the spit ridge, a hand-dug hole and 1.5m vibracore into the south side of the ridge, where the sand was thinner, recovered basal laminated sand and organic muck overlain by horizontally-bedded fine sand, cross-bedded coarse sand, massive sand, and modern soil. The observed and inferred subsurface stratigraphy is consistent with the conceptual model proposed by Anderson et al. (2010-this combined NC-SCGSA meeting) that the east-west ridges separating the modern Grand Mere lakes are spits and spit platforms created during the latter part of the Nipissing highstand.