Sub-bottom Investigation of Chicago, Illinois's Coastal Marine Environment Using Near-Surface Geophysical Imaging Techniques

Located on the SW corner of Lake Michigan, Chicago, IL's lakefront parks and beaches are popular tourist and recreation destinations that are susceptible to coastal erosion, beach loss, and shoreline modification. To better predict how coastal sedimentary processes will respond to changing lake levels, we acquired offshore near-surface geophysical data to examine the stratigraphic architecture and map sand thickness across Chicago's nearshore aquatic environment. An Edgetech 3400-OTS sub-bottom profiler with a 3-16 kHz CHIRP source was deployed from a pole mount to obtain subsurface seismic reflection data with a maximum depth resolution of ~10 m. Data acquisition ranged from 50-1000 m offshore and extended from Lighthouse Beach in Evanston, IL, to Rainbow Beach on the south side of Chicago. A total of ~165 trackline kilometers were acquired across a survey area of ~30 km2. Preliminary results indicate the presence two high-amplitude subsurface reflections from key geologic contacts interpreted as 1) the base of the mobile nearshore sand unit and 2) the base of the glacial till/ top of Paleozoic bedrock. Data show that sand has accumulated on the lake bottom in thin sheets (~1 m thick) away from shore. Sand thickness is greatest across Chicago's beaches, where groins and other lakefront infrastructure have sectioned the littoral system and facilitated sequestration, with larger accumulations (up to 8 m thick) on the north side of downtown Chicago. In contrast, the south side is characterized by the shallow occurrence of Silurian carbonate bedrock that provides a limiting threshold for accommodation space. Bedrock occasionally outcrops on the lake bottom as reefs and shoals here. The seismic data suggest that such obstructions act as barriers to alongshore sediment transport, with sands accumulating against the north (updrift) sides of these underwater features and a lack of sand due to scour on the south sides. Other interesting features observed in our geophysical dataset include buried drainage channels linked to low lake levels (likely of the mid-Holocene), lake bottom infrastructure, and lag deposits. These data, along with other planned geologic and geophysical datasets, will provide the basis for a regional sand assessment to help guide coastal development and future shoreline-management efforts.