CHRONOLOGY AND PALEOECOLOGY OF PRE-ILLINOIAN AND ILLINOIAN DEPOSITS IN SOUTHWESTERN ILLINOIS

As a result of Quaternary mapping efforts in southwestern Illinois over the past several years, numerous outcrops and cores were observed that document well-preserved, fossiliferous lacustrine deposits directly below Illinoian or pre-Illinoian till units. The lacustrine deposits are found primarily in buried bedrock valleys, tributary to the Mississippi River or Kaskaskia River valleys. Rapid burial of unaltered lacustrine deposits by advancing glacial ice allowed for excellent preservation of Picea wood and needles along with mollusk shells in a relatively anoxic environment. The molluscan fauna indicate environmental changes ranging from permanent, alkaline lakes (e.g., Valvata tricarinata, Probythinella lacustris; Pisidium sp.) to shallow, temporary slackwater lakes (e.g., Fossaria sp., Pomatiopsis sp.). Shoreline areas may contain abundant fossil wood and a mixed aquatic-terrestrial fauna. Periods of lake recession are dominated by loessal influence, derived from surrounding hillsides (Carychium sp., Hendersonia occulta, Succinea sp.). Amino acid geochronology, utilizing several genera and multiple amino acids, clear distinguishes Illinoian and pre-Illinoian age deposits. For instance, D/L ratios of glutamic acid are typically ~ 0.19 - 0.29 for Illinoian and ~ 0.32 - 0.45 for pre-Illinoian shells. Comparing the same genera, pre-Illinoian shells have ~ 1.6 to 2.0 times higher glutamic acid ratios than Illinoian shells, suggesting an age ~ 2.5 to 4 times greater based on a calibrated parabolic age model. For the Illinoian deposits, correlation with OIS 6 is confident (~ 150 ka), based on amino acid data and recent luminescence ages. By comparison, an age of 375 ka to 600 ka for pre-Illinoian deposits is estimated here and is reasonable based on the significant development of the Yarmouth Geosol. Thus, available evidence suggests that pre-Illinoian deposits in southwestern Illinois may correlate with OIS 12, 14, or 16; with OIS 12 or 16 more likely based on large global ice volumes during those times.