INDIRECT EVIDENCE FOR THE SOUTHERN LAURENTIDE ICE MARGIN POSITION DURING MARINE ISOTOPE STAGE 3, FROM PROGLACIAL SEDIMENTS IN ILLINOIS AND INDIANA, USA

Proglacial sedimentary records from the middle Wisconsin Episode (MIS 3) in the Central Lowlands, south of the Laurentide Ice Sheet’s former margin, are preserved in loess, glaciofluvial (outwash), and lacustrine (slackwater, kettle-fill) deposits. A regional MIS 3 loess unit is the Roxana Silt, distinctively pinkish-brown in zones and up to 10 m thick proximal to the Mississippi and Illinois River Valleys in southwestern Illinois. The Roxana Silt’s distinctive color, mineralogy, and elemental composition suggest a significant sediment contribution from the Lake Superior region (transported by meltwaters, followed by wind). Radiocarbon and luminescence chronologies bracket Roxana Silt deposition at 60–30 ka (calibrated), with most accumulation from 50–30 ka. Full glacial Peoria Silt (~29–15 cal ka) occurs stratigraphically above. Loess in Illinois and southern Indiana was deflated from broad outwash plains of the Mississippi, Illinois, and Wabash River Valleys during times when glacial lobes were in these drainage basins. In the lower Wabash valley’s subsurface, outwash deposits have reported luminescence ages in the 40–30 ka range; sedimentation likely reflects discharge from advance of a Labradorean-sourced ice sheet into the eastern Great Lakes region. Slackwater lake sediments, widely distributed in tributary valleys from aggrading large river systems, record a lowland complement of the Roxana Silt, with a similar pinkish-brown tinge. Organic materials in lake sediments (lower Equality Formation or equivalent) have been radiocarbon dated at 45–29 cal ka at several meters depth in southern Indiana and Illinois. In sum, the loessal, glaciofluvial, and lacustrine records imply an (overall) advancing southern Laurentide Ice Sheet in the upper watersheds of the Wabash, Illinois, and Mississippi Rivers at 45–30 ka, and perhaps earlier in the Lake Superior region. Fossil records (gastropods, ostracodes, pollen) from loess and lacustrine sediments imply cooler climatic conditions than today, but less severe than during the last glacial maximum (after 27 cal ka). Terrestrial gastropod assemblages preserved southwestern Illinois and western Kentucky loess at 40–30 ka imply forested conditions along major river valleys and mean July temperatures of 23–24°C, about 2–3°C cooler than today.