Substrate mapping along a wave-dominated, sand-limited Great Lakes littoral zone: A case study from the bluff coast of Illinois, Lake Michigan

While detailed geological maps are foundational to aquatic habitat classifications, few efforts have been made to generate these at regional scales along the highly dynamic coastal margins of the North American Great Lakes, where meter-scale fluctuations in water level, storms, and ice-related dynamics induce shoreline and shallow nearshore geomorphic changes. Variances in substrate type, over time, are enhanced in sand-limited settings, where clay-till and other glacial materials, grave-cobble lag deposits, and bedrock outcrops are common. A baseline understanding of sand distributions along these coasts is foundational to ecological and geomorphological inquiries. This paper describes an effort to leverage an offshore geological sample database to map the lake-bottom geology along the bluff coast of Illinois, where littoral sand is scarce and its distribution important to constrain from a coastal management perspective. Offshore geological sample information was integrated with high-resolution federal LiDAR, multi-beam sonar, and backscatter datasets, which provided the means of substrate-unit delineation. Distinction of sand versus non-sandy substrates was reflected in lake-bottom rugosity, backscatter intensity, and sample information. While a sandy lake bottom is smooth, nearshore terrains of greater textural and physiographic heterogeneity relate to craggy bedrock outcrops or a variety of undifferentiated sedimentary deposits, inclusive of gravel-cobble lags and scoured mud-rich till. A tripartite unit division of (1) sand, (2) undifferentiated sediments, and (3) bedrock holds broader application potential to sand-limited nearshore regions of the Great Lakes. Understanding the linkages between lake-bottom geomorphology and geological composition is useful to resiliency planning. Geological monitoring efforts benefit from such regional assessments.