GROUND PENETRATING RADAR INVESTIGATION OF A BARRIER COMPLEX ALONG THE EASTERN SHORE OF LAKE MICHIGAN

Ground penetrating radar (GPR) data was collected at Silver Lake State Park, Michigan to identify prominent subsurface stratigraphy within a dune-covered coastal landscape. A 1.2 kilometer GPR (50 MHz) transect from Silver Lake to Lake Michigan across large transverse dunes and interpreted beach ridges revealed the presence of a coastal barrier beneath ~30 meters of aeolian sand. The subsurface geometry (reflection pattern) of distinct surfaces (reflections), interpreted to be ravinement surfaces, lend support to the hypothesis that the barrier is older than Nipissing in age (Algonquin or pre-Algonquin). Post-Nipissing beach ridges were identified west of the barrier core and examined with high resolution 200 MHz antennas. Additionally, GPR data indicate that large dunes on the eastern side of the barrier have prograded 150 meters into Silver Lake during the past 70 years. Aerial photography since 1935 reinforces this assertion. The GPR data also shows a groundwater drainage divide between Silver Lake and Lake Michigan beneath the large transverse dunes, which is supported by observations of artesian springs on the west side of Silver Lake. Based on the data presented, we propose that the late Pleistocene and Holocene development of the Silver Lake embayment involves: 1) the development of a coastal barrier during or before Algonquin time; 2) reactivation and further development of the barrier during Nipissing time; and 3) the development of post-Nipissing beach ridges. Although no new radiocarbon dates were obtained in this study, correlations can be made about the history of the Silver Lake barrier from stratigraphy observed in vibracores and GPR profiles from Silver Lake.