EVIDENCE FOR DEEP EROSION BY THE MISSISSIPPI RIVER NEAR ST. LOUIS UNDER COLD, DRY CONDITIONS DURING THE ONSET OF THE LAST GLACIATION

Cores from terraces near the mouths of Piasa Creek and Prairie du Pont Creek in the St. Louis Metro East area contain fossiliferous silt and fine sand that were deposited in slackwater lakes. The lower meter of the succession contains terrestrial plant macrofossils and aquatic snails yield radiocarbon ages spanning from 44,000 to 35,000 yr B.P. The reddish color (7.5Y 4/2), vermiculite-rich clay fraction, radiocarbon ages, and stratigraphic position of the basal unit indicate deposition was contemporaneous with the loessial Roxana Silt. The stratigraphic position of the lacustrine silt immediately above bedrock implies that the tributary streams and the Mississippi River flowed just above or on bedrock at elevations of about 350 ft MSL and 310 ft MSL, respectively, prior to aggradation associated with the last glaciation (Wisconsin Episode). The oldest radiocarbon age associated with these silts (43,780 ± 1590 yr B.P.; ISGS-A-0020.) captures the time when the slackwater lakes first formed in response to the initial advance of the Laurentide Ice Sheet into the Mississippi River basin. What is surprising is the depth at which the lakes first formed. Ostracodes and plant macrofossils imply that during the onset of the last glaciation, the St. Louis area experienced a cool climate with a negative moisture balance (precipitation less than evaporation). Ostracodes from both cores include Candona rawsoni, Candona caudata, and Limnocythere herricki. The latter is found today in prairie lakes of south and central Canada; the candonids are known to co-occur in lakes in northwestern Iowa. There is no modern lake known where the three species occur together. Plant macrofossils include seeds of Chenopodium, Amaranthus, and other plains taxa along with spruce. The evidence for a negative moisture balance is interesting considering that this was a time of expansion of the Laurentide Ice Sheet.