SUBSURFACE IMAGING OF PAST SHORELINES OF LAKE MICHIGAN AT MANISTIQUE, MICHIGAN, USA

Lake Michigan has experienced water level fluctuations that have been recorded and dated back to its formation roughly 14,000 years ago. The changes in water level have formed a set of visible sand ridges parallel with the current Lake Michigan shoreline. A good example are ridges that are visible along the Manistique Embayment located in Michigan’s Upper Peninsula. The most prominent water level fluctuation occurred during the mid-Holocene period when the lake level was at its highest. This level is known as the Nipissing phase of the Great Lakes which produced the Nipissing beach ridge in this location. The Manistique Embayment consists of a 7,600 -meter-long and 4,300-meter-wide strandplain. The purpose of this research was to determine how the Nipissing high affected the shoreline of Lake Michigan. With the use of ground penetrating radar (GPR), a noninvasive geophysical technique used to image the subsurface stratigraphy, we collected an 1,100-meter-long transect of the embayment. The GPR system used was a pulseEKKO 100 with an antennae frequency of 100 MHz. Data was collected using a step size of 0.25 meters and an antennae separation of 1 meter. Using a Topcon laser level, the topography of the GPR line was collected allowing for geometrical adjustment of the transect. The raw data was processed using Sensor and Software’s EKKOproject software. The results of the GPR transect show evidence of coastal processes including progradation and aggradation associated with beach ridge formation dating back to the Nipissing high. These results will contribute to refining the present paleohydrograph of Lake Michigan.