GEOCHEMISTRY OF ILLINOIS- AND PRE-ILLINOIS EPISODE SEDIMENT IN SOUTHWESTERN ILLINOIS

Glaciers emanating from the Labrador center reached southwestern Illinois at least twice during the Illinois and pre-Illinois Episodes. Deposits from each of these have similar stratigraphies, with either slackwater or ice-contact lake sediment overlain by till and capped by glacifluvial sediment in some lowlands/areas. The oldest sediments occur in buried bedrock valleys. Illinois Episode sediments can generally be studied in outcrop, but the older sediments are known almost exclusively from core. Illinois Episode tills tend to be more loamy with more pebble clasts, whereas pre-Illinois Episode tills tend to have a higher clay content and more local (sedimentary) clasts. They are otherwise fairly similar in field characteristics. Geochemical assays have been completed on a variety of sediments to help differentiate and correlate these buried deposits.

Broad oxide and trace element assays, by ICP-AES and ICP-MS, respectively, were completed on the <2 μm fraction of unweathered tills and lacustrine sediment in 10 cores from an ~750 km² area about 40-70 km east of the glacial limit of both episodes. Illinois Episode sediments are higher in MgO, CaCO₃, Sc, C, and S, and in lower in Al₂0₃ and MnO. The CaCO₃-MgO results corroborate gasometric analyses that also indicate greater dolomite in Illinois Episode sediment, probably derived from Silurian rocks rimming Lake Michigan. The Al₂0₃ may indicate greater incorporation of shale and a lesser proportion of carbonate in the pre-Illinois Episode sediment. Trace element assays show that Illinois Episode sediments are also higher in Ti, B, K, and S. The K contents may reflect greater illite content, as determined from X-ray diffraction analyses. Future analyses will explore geochemical variability vertically within each core as well as laterally between cores. Additional data sets from cores obtained closer to the glacial limit will be used to explore spatial trends within each till unit as the till thicknesses decrease and more local bedrock is incorporated westward.