WATER-LEVEL CHANGES AND TERRAIN EVOLUTION ALONG THE ILLINOIS LAKE MICHIGAN COAST: DEVELOPING A GEOCHRONOLOGY FOR THE ZION BEACH-RIDGE PLAIN

Illinois has a morphologically varied coastline along Lake Michigan, which encompasses relict lake plain, bluff, and beach-ridge terrains. Storm events (waves and currents) and changes in lake level (seasonal to decadal) interact with shorelines in ways that directly affect inhabitants, infrastructure, and ecology. Decadal Lake Michigan water levels have historically oscillated between 2 and 5 feet. Changes of this magnitude are associated with beach-ridge formation along high sand-supply portions of the coast. While we understand modern morphodynamics and the paleo-implications of relict coastal architectures, we have little geochronological data to frame conceptual models of late Holocene coastal geomorphic evolution at a resolution needed to accurately simulate future coastal response to storms and lake level fluctuations.

Eight samples were retrieved from eight cores collected at Illinois Beach State Park for luminescence dating (OSL). This area is part of the Zion Beach-ridge Plain, a migrating late Holocene ridge-and-swale complex of ~7,500 acres in extent. Cores targeted swales along the backsides of prominent ridgelines. These locations were targeted as GPR records inferred the presence of overwash, a deposit more closely related in time to shoreline transgression and ridge nucleation than eolian cap sediments. This presentation highlights efforts to refine a strand chronology by way of targeted luminescence dating, informed by detailed architectural models of the ridge plain. OSL samples augment an existing radiocarbon geochronology and stand to offer new insights into the late Holocene development of a complex coastal ecosystem in Illinois.