SANGAMON AND EARLY WISCONSIN EPISODE STRATIGRAPHY AND OPTICAL LUMINESCENCE DATES NEAR THE ANCIENT MISSISSIPPI RIVER VALLEY

In glaciated Illinois, the Sangamon Geosol occurs as an in-situ profile developed into Illinois Episode (IE) glacial sediments, including till, outwash, and loess and as an accretionary profile developed into the Sangamon Episode (SE) colluvial sediments, specifically known as the Berry Clay Member of the Glasford Formation. The in-situ soil profile commonly presents an intermediate to well-drained and oxidized Bt horizon whereas the accretionary soil profile presents a poorly-drained and gleyed Btg horizon. The Sangamon Geosol is in places superposed by the early Wisconsin Episode (WE) Chapin Geosol, which developed in the WE Markham Member of the Roxana Silt (Willman and Frye, 1970). Seldom have the in-situ and the accretionary Sangamon and the overlying Chapin been observed as three superposed profiles at one location. Here we report on a site adjacent to the ancient Mississippi River Valley in central Illinois - the principal early WE loess source in the region - where a well developed in-situ Sangamon Geosol profile in outwash is overlain by an accretionary Sangamon profile in clay rich colluvium, that is overlain by a moderately well developed Chapin Geosol in silty loessal colluvium. Optically stimulated luminescence (OSL) ages of fine grained (7-11 µm) polyminerals using multiple aliquot regenerative protocol and of 90-150 µm quartz using single aliquot regenerative protocol confirm that loess-derived colluvial Markham Silt is early WE and formed after 75 ka, clay rich colluvial Berry Clay is SE and formed after 90 ka, and gravelly sand outwash in which the in situ Sangamon profile is developed in the IE Pearl Formation deposited before 130 ka. Grain-size distributions show that the Pearl Fm. outwash contains >60% sand, the Berry Clay, colluvium that formed during the warm/wet late SE interval, contains up to 30% clay, and the Markham Member, early WE loess-derived colluvium, contains 60-70% silt. This dated soil-sedimentary record confirms that the in-situ and accretionary Sangamon profiles and the moderately well developed Chapin profile in the lowermost increments of the Roxana Silt in central Illinois have ages close to those suggested by Willman and Frye (1970) and supports their early model of SE soil formation and early WE rock- and soil-stratigraphy developed over 40 years ago, long before the advent of OSL.