SUBSURFACE IMAGING OF MINERS BEACH, MI USA: A LAKE SUPERIOR NIPISSING SHORELINE

During the Nipissing lake phase of Lake Superior, the Miners Beach area formed a small cove in the Lake Nipissing shoreline. The low-relief flats behind the present beach are developed upon the former Lake Nipissing lake bottom. A series of east-west trending beach ridges located between the Nipissing and present shoreline record the transition to lower lake levels after the Nipissing high stage. The primary purpose of the field project was to test the feasibility of using GPR to image subsurface stratigraphy of a higher and abandoned shoreline of Lake Superior. The GPR acquisition system used for the study was pulseEKKO 100 with 100 MHz antennae and a 1000 V transmitter. Step size was 0.5 m with an antennae separation of 1.0 m. Each trace was vertically stacked 32x with the digital profiles being processed and plotted using pulseEKKO software. Basic processing included automatic gain control (AGC), signal saturation correction, trace stacking (horizontal averaging) and point stacking (running average). To calculate depth, a near surface velocity measurement of 0.1m/ns was calculated from a CMP survey. Topographic surveys, using a Topcon laser leveler, of the GPR traverse were performed to complete terrain corrections. The application of radar stratigraphic analysis (distinct signature patterns) on the collected data provided the framework to investigate both lateral and vertical geometry and stratification of the coastal deposits. The GPR transects record the shallow subsurface stratigraphy (>20 - 25 m depth) of a prograding beach environment onlapping an earlier erosional unconformity, below which a glacial outwash environment is interpreted. The observed reflections from the collected profiles are interpreted as beachface and upper shoreface deposits of a formerly active prograding shoreline. The images also document the location of former shoreline positions (lake levels). The results demonstrate that coastal sedimentary deposits can be mapped and show the utility of this geophysical tool as a regional, geomorphic mapping tool of paleo-coastal environments.