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

Illinois coastline along Lake Michigan, is morphologically varied with relict lake plain, bluff, and beach-ridge terrains. Storm events produce strong waves and currents and seasonal to decadal changes in lake level together 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.

We retrieved eight samples 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. Overwash deposits in the swales were inferred from GPR records, deposits more closely related conceptually to shoreline transgression and ridge nucleation than to the capping eolian sediments. Near the coast, radiocarbon and OSL dating confirmed the ridges were formed during the last 2000 years. We also discovered that the ridge plain is older than previously modeled from conventional radiocarbon ages.

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. Going forward, we will expand our observations in the area with additional GPR and coring. In addition, in an effort to reduce the time lag we will directly compare radiocarbon and OSL ages by sampling basal peat and immediately underlying deposits.