OPTICAL AGES SPANNING TWO GLACIAL-INTERGLACIAL CYCLES FROM DEPOSITS OF THE ANCIENT MISSISSIPPI RIVER, NORTH-CENTRAL ILLINOIS

The Ancient Mississippi River (AMR) drained a substantial part of the upper Midwest through a >15 km wide bedrock valley in central Illinois during the late Pleistocene prior to being diverted to its modern course by the Lake Michigan glacial lobe about 24.7 ka (thousand yrs before present). Recent cores of the >100m thick Quaternary valley fill are used to characterize discontinuously preserved, interbedded, fluvial and glacial successions that comprise the complex infill of the bedrock valley north of Peoria, IL. Fluvial deposits are dated using optically stimulated luminescence (OSL) of quartz sand, providing the first analytical determinations of ages for these deposits.

Ages of sediments immediately beneath the oldest till (Kellerville Mbr.) in the bedrock valley average 160 ka and provide direct confirmation that Illinois Episode (IE) glaciation began in its type area during marine isotope stage (MIS) 6. The oldest deposits found are 190 ka fluvial sands on bedrock in the deepest part of the valley. These correlate to earliest MIS 6. Previous correlations of most of the sand and gravel in the bedrock valley in the study area to the pre-Illinois Episode are refuted. We now correlate the lowest deposits to the IE (Pearl Fm.). Thick fluvial sediments between IE tills in the bedrock valley indicate that the IE glacier margin fluctuated significantly, retreating NE of the AMR several times and allowing the river to reoccupy the valley before each readvance. OSL dates from sands encasing the IE tills suggest that the IE glacier was present in central IL for 15 to 20 ka, somewhat longer than the late Wisconsin Episode (WE) glacier was in IL. OSL dates also suggest that last interglacial (MIS 5; Sangamon Ep.) fluvial deposits are preserved locally. Four OSL dates on sands (mean 24.7 ka) and a 14C date of 20,780 ± 140 14C yr BP (24,520-25,020 cal yr BP) on overridden lacustrine deposits approximate the age of diversion of the AMR and suggest it took place slightly earlier than previously determined.

Over 30 OSL and 14C ages frame an increasingly detailed picture of the history of the AMR during the last two glacial cycles, spanning MIS 6 through 2, and show OSL to be a useful tool in deciphering complex successions of glacial and fluvial deposits, which comprise important geologic/climate records and are used as a major aquifer in Illinois.