INSIGHTS INTO THE PALEOENVIRONMENT OF LACUSTRINE SEDIMENTS PRIOR TO THE LAST GLACIATION AT WEDRON, ILLINOIS

Recent quarry exposures of new sediments at Wedron, IL have provided further detailed information about paleoenvironmental conditions during the onset of the most recent glaciation in Illinois. A succession of lacustrine sediments, which were likely deposited in a slackwater lake basin, are located below till deposits and immediately predate the arrival of Wisconsin Episode glaciers to the area. We used sedimentological data (particle size and clay mineralogy) and analyses of aquatic fauna (ostracodes, gastropods, and bivalves) to interpret the paleoenvironment. The lower part of the lacustrine succession consists of a fossiliferous, organic rich silt about 3 meters thick. It represents the final 1,000 year period (approximately 26,000-25,000 C-14 yr BP) during which the Farmdale Geosol (interstadial soil) developed in Illinois. The collective ostracode and gastropod fauna, coupled with the presence of redeposited spruce needles and wood, suggest that a modern analog environment was a shallow lake with abundant aquatic vegetation near the prairie-forest transition zone in northwestern Iowa. Laminated lacustrine deposits (about 2 meters thick), which are barren of fauna and associated with the proglacial environment of the Late Wisconsin glaciers, overly and postdate these sediments by at least 4,400 years, an unconformity marked by their contact. Based on Carbon-14 and gastropod data, previous studies of these sediments in and around the Wedron area have suggested deposition in tundra-like conditions with some boreal indicators (at about 22,000 C-14 yr BP).