EVALUATING POTENTIAL SEDIMENT TRANSPORT PATHWAYS ALONG THE SOUTHWESTERN LAKE MICHIGAN COAST THROUGH SEDIMENTOLOGIC, GEOCHEMICAL, AND STRATIGRAPHIC ANALYSES

Many coastal geologic studies have been conducted along the shores of Lake Michigan over the past several decades, yet littoral sediment transport rates and pathways remain elusive. The absence of these data has resulted in costly and inefficient sand management along southwestern Lake Michigan. Here, we present preliminary sedimentologic data that serves as a first-order attempt to trace sediment pathways throughout this region. We collected grab samples along thirteen nearshore transects in SE Wisconsin and NW Illinois that extended from the 2m isobath to the 10m isobath (approximately 1.5 km offshore). In addition, samples were collected on land from mid-Holocene strandplain deposits, late Pleistocene glacial deposits, and modern beaches. Samples were analyzed for grain-size distribution by laser diffraction and geochemistry of 43 elements with portable X-ray fluorescence (pXRF).

The modal grain size distribution in the nearshore ranges from a minimum of ~90 to a maximum of ~750 micrometers. In Wisconsin, the average nearshore modal grain size is ~200 micrometers, and in Illinois it is ~275. A distinct coarsening of the modal grain size by ~100 micrometers was recorded at the state line, south of Winthrop Harbor. Variability within transects was highest for the two northernmost transects located near the Pike River, the only unmanaged stream along this reach of shoreline. The modal grain size of sand collected on land averaged about 300 micrometers, with a maximum of 500 between two beach ridges. Where sampled, nearshore samples are finer than the adjacent samples exposed in bluffs on land by ~160 micrometers.

Preliminary pXRF results suggest differences between nearshore samples in Wisconsin and Illinois. Samples collected on land in Wisconsin are more similar to the nearshore samples of Wisconsin than either are to the samples in Illinois. Terrestrial and nearshore samples in Wisconsin have higher concentrations of Al, Fe, Ca, K and Mg relative to the nearshore samples in Illinois. These data build the foundation for future research collaborations between the Wisconsin and Illinois Geological Surveys, which are ultimately aimed at comprehending and modelling the dynamic nature of littoral transport along southwestern Lake Michigan.