Offshore geologic mapping by probing and subbottom imaging, Illinois Beach State Park, western Lake Michigan

Coastal erosion and sediment transport along the shoreline of Lake Michigan are important environmental and societal issues as water levels approach historic highs. A post-Nipissing-aged body of sand extends along the western Lake Michigan coast for approximately 25 km from Kenosha, WI to Waukegan, IL. The sand body is derived from remobilized last-glacial fluvial and lacustrine sediments, with minor input from shoreline erosion of fine-grained till bluffs, and overlies a fine-grained till. Although mostly a narrow band, subaerial widths reach as much as 1.7 km within Illinois Beach State Park (IBSP), with a submerged portion extending 1000 m or more offshore at 6-7 m water depth. We use hydraulic jet probing and sub-bottom profiling technology to determine the 3-dimensional extent, character, and history of the offshore portion of this sand body to support research on nearshore sediment mobility, transport rates and pathways, and likely responses to climatic and anthropogenic forcings. In 2019 we measured sand thickness by probing with a hydraulic jet device which penetrates mud, sand, and fine gravel, but is refused on coarse gravel, boulders, and dense mud. The sites were located along existing 600 to 1700 m-long probing transects from the 1970's and 90's to evaluate local changes in sand thickness through time. Subbottom profile data from an EdgeTech 3400 with a 2-16 kHz CHIRP source successfully imaged a seismic reflector interpreted as the contact of the sand with underlying till and is in good agreement with the sand jetting observations. Our results show that the sand body ranges in thickness from about 7-8 m near the shore to 1-2 m at distal locations offshore. Locally, sand thickness increased between 1975 and 1990, possibly associated with dumping of dredge spoil from construction of Winthrop Harbor at the north (updrift) end of IBSP in 1989. Local m-scale changes in sand thickness from 1990 to 2019 were also measured. However, lake bottom profiles varied little over time, but appear to have shifted westward with lake level rise since 1994. Comparisons with topobathymetric lidar maps (2008, 2012, 2018) show landward bedform migration at <=25 m/yr with rising lake level.