UPDATED CHRONOLOGY OF PEORIA SILT ACCUMULATION IN ILLINOIS FROM RADIOCARBON DATING OF TERRESTRIAL GASTROPOD SHELLS AND VEGETATION

Based on calibrated AMS radiocarbon ages from terrestrial gastropod shells (Succineidae, Discus, Webbhelix) and from vegetation, the chronology of Peoria Silt (loess) deposition in Illinois is being updated. Shell ages with these taxa have been found to be reliable and consistent (within a few hundred years) in comparison with wood and charcoal ages. Loess accumulation rates are now realized to have been more oscillatory or variable than previously thought. From sites along the Illinois Valley and middle Mississippi Valley (St. Louis region), the youngest Peoria Silt shell ages are 20.4 ± 0.3 cal ka (1.2 m below surface; New Cottonwood School Section), 20.7 ± 0.2 cal ka (1.85 m below surface; Thomas Quarry), 20.1 ± 0.2 cal ka (1.85 m below surface; Keller Farm Section), and 20.2 ± 0.2 cal ka (1.35 m below surface; Demazenod Section). The upper 1.2–1.85 m of loess contains the modern soil solum, is leached of carbonates, and lacks terrestrial gastropods. Based on extrapolations, loess deposition essentially ceased by 18–16 cal ka in this region. An uppermost Peoria Silt age of 17.7 cal ka is suggested from ¹⁴C-dated emergent vegetation in resedimented loess at two central Illinois kettle basins. A shell age of 27.8 ± 0.2 cal ka in basal Peoria Silt, 0.9 m above Roxana Silt at Thomas Quarry, confirms prior estimates that Peoria Silt began accumulating ~29 cal ka. New gastropod ages from Lomax Section, in western Illinois, indicate 10 m of Peoria loess accumulated between 20.5 and 17.4 cal ka, proximal to the post-diversion Mississippi River Valley. With a youngest shell age of 17.4 ± 0.1 cal ka at 2.8 m depth, loess deposition likely continued until ~16 cal ka (or slightly younger with slower accumulation rate). Overall, the new chronology suggests Peoria Silt deposition in Illinois, discluding thin eolian dust inputs incorporated into the modern soil, ceased ~1–4 kyr earlier than some prior estimates. Our findings, however, are generally consistent with earlier models of loess deposition continuing later along the Upper Mississippi Valley than along the Illinois Valley. This likely results from Superior Lobe glacial activity, and meltwater drainage, for some time after the Lake Michigan Lobe receded. Alternatively, this difference may be explained by sediment derived from the Iowan Erosion Surface or the late glacial Des Moines Lobe.