DECIPHERING THE LAST GLACIAL CHRONOLOGY, UPPER SANGAMON RIVER BASIN, ILLINOIS

New radiocarbon and luminescence ages from the Upper Sangamon River Basin in central Illinois have improved our understanding of ice margin advances and glacial meltwater pulses. Yet, careful consideration was necessary in application of both methods. Radiocarbon dating of conifer wood in organic silt (Morton Tongue, Peoria Silt) below the Tiskilwa Formation was complicated by weathering effects, namely the presence of microscopic fungus (not realized initially). Two of four wood ages from the same stratigraphic position, but from different sites, were significantly younger than expected (~ 1 to 2 ka too young) for the initial advance of last glacial ice into this region. One sample, originally ~ 19.2 ka ¹⁴C yrs (~ 23.1 cal ka), was redated from the same log to 19.9 ka ¹⁴C years (~ 24.0 cal ka) using alpha cellulose extraction procedures to avoid any contamination. The two other wood samples with reasonable dates both yielded 20.2 ka ¹⁴C yrs (~ 24.3 ka). Based on this chronology, glacial ice advance to the Shelbyville Moraine terminus was apparently rapid and streaming (?), covering ~ 65 km in perhaps a few hundred years. Two samples from a younger proglacial outwash deposit in the same area were dated by optically stimulated luminescence (OSL). The outwash is related to a younger till unit in the Champaign Moraine (Batestown Member, Lemont Fm.) and occurs stratigraphically above the Tiskilwa Formation. OSL was applied on quartz, a mineral more likely than feldspar to be well-bleached during its last sedimentary cycle. Measurements were performed on very small aliquots (~ 10 – 20 grains/aliquots) in order to increase the likelihood of isolating well-bleached grains from those carrying a residual dose. Both samples were found to be partially-bleached. Application of the minimum age model allowed us to calculate an age of ~ 22 – 21 ka for the outwash deposit. These ages are reasonable within the stratigraphic context and may provide a more accurate chronology for ice advance to the Champaign Moraine (previously assumed ~ 24 to 20.5 ka).