MID-WISCONSIN EPISODE LACUSTRINE DEPOSITS SYNCHRONOUS WITH ROXANA SILT DEPOSITION IN SOUTHWESTERN ILLINOIS

During recent 1:24,000 mapping in southwestern Illinois, numerous subsurface observations of up to 50 feet (15 m) of a distinctive, stratified, reddish-brown (7.5YR 4/3) to grey (10YR 4/2) silty clay were made. The color, age, and mineralogy of this lacustrine unit (lower Equality Formation) strongly suggest a chronocorrelation with the Roxana Silt (55 ka to 28 ka), a reddish-brown mid-Wisconsin Episode loess. Relatively high kaolinite / illite ratios and magnetic susceptibility values (40-80 x 10^-5 SI) in the lower Equality Formation (LEQ) are comparable in value and trend to Roxana Silt and suggest a significant contribution from Superior Lobe glacial meltwaters. Radiocarbon ages for charcoal, shells, and plant macrofossils from 4 cores of LEQ in this region are between 45 ka and 24 ka, suggesting nearly synchronous deposition with upland deposits of Roxana Silt. The initial deposition of both units probably coincides with the earliest significant Wisconsin Episode glacial meltwater discharges to the Mississippi Valley, perhaps ~ 55 ka by extrapolation. Reddish colored LEQ is overlain by sandy to silty yellow-brown (2.5 Y) sediment (upper Equality Formation) in late Wisconsin Episode terraces or by postglacial alluvium in the lower reaches of large tributary valleys to the Mississippi River Valley. We envision its deposition as slackwater lake sediment associated with aggradation of the Mississippi River during mid-Wisconsin Episode glaciations in the upper Midwest. This interpretation is supported by elevations for the top of LEQ which are typically the level of the modern Mississippi River in the vicinity: about 400 feet (asl) near St. Louis and about 345 feet near Carbondale (~ 100 km to the southeast). Ostracode species in the LEQ include Candona rawsoni, Limnocythere herricki, L. friabilis, and L. pseudoreticulata. Plant macrofossils include Chenopodium, Amaranthus, and Picea. This fossil assemblage suggests a cooler and drier climate than present, perhaps similar to present-day Saskatchewan or Manitoba. These paleoclimatic indications from the LEQ are somewhat puzzling because large terrestrial gastropods found in the Roxana Silt of Illinois (e.g., Anguispira, Allogona, and Triodopsis) are indicative of established forests and imply moist conditions during the same time period.