LATERAL CHANGES IN LAKE SUPERIOR BEACH SUBSURFACE ARCHITECTURE MAPPED USING GPR, GRAND MARAIS, MI

Subsurface stratigraphic architectures of Lake Superior shoreline and backshore dune environments were mapped at Grand Marais, MI using ground penetrating radar (GPR) and geographic information system (GIS) technologies. Three survey lines, perpendicular to shore and spaced 10 meters apart, were examined to evaluate lateral continuity in subsurface facies both perpendicular and parallel to the shoreline. A system of jetties and breakwaters were installed in the late 1800’s to protect a natural harbor at Grand Marais, located on the high-energy southern coast of Lake Superior. Based on previous research, ~ 234,000 m³ per year of material has sequestered due to the westernmost breakwater obstructing littoral transport of sediment originating from nearby dunes. Resulting depositional architectures have been influenced by fluctuating lake levels, storms, winter ice and longshore transport. GPR methods were used to evaluate beach morphology in context of a GIS-based chronology of beach-growth.

A GIS-based shoreline reconstruction suggests continuous beach growth at an average of ~2,000 m² per year. While prograding clinoform shapes in the subsurface corroborate the overall nature of growth, truncation surfaces imply that erosional events interrupt beach growth. These are likely attributed to lake level changes and/or storms. Therefore, variations between these lines can indicate which erosional surfaces are influenced by laterally continuous lake-level erosional events versus those which are not continuous. These were likely influenced by irregular and localized erosional events. While the east and west lines imaged similar subsurface architectures, the third line, was noticeably different. It is suggested that differential erosion due to storm influence destroys lateral continuity in architecture, which is largely a function of sediment supply and lake level position. Further research in this area would improve our understanding of the Grand Marais shoreline dynamics.