LAST GLACIAL LOESS DEPOSITION IN CENTRAL ILLINOIS ABRUPTLY SLOWS AT CA. 18,750 CAL YR BP IN RESPONSE TO THE KANKAKEE TORRENT

Smith Lake (40.1484 N, -89.8132 W) occupies a small kettle on the Illinois Episode Till Plain 15 km east of the Illinois River valley, a major source of loess during the last deglaciation. The lake was cored with a Livingston corer in the late 1980’s; the initial interest was in obtaining an early and pre-Wisconsin pollen record. Refusal occurred at 11.7 m, and woody plant debris at the base of the core yielded an age of 36,640 cal yr BP (ISGS-2944). With renewed interest in determining the age of deposition of the Peoria Silt (last glacial loess), we returned to Smith Lake in 2017 and obtained new sediment cores using a hydraulically powered Livingston corer. The lake gave up its secrets grudgingly, and refusal occurred at 9 m. We emphasized securing a complete record in the upper part of the sediment record, including: Unit A) 1.81 m of fossiliferous, late Holocene silty clay loam (with an age of 2,360 cal yr BP (UCIAMS-196989) at 1.5 m on carbonized nut shells and wood), B) 0.7 m of gleyed, leached silty clay loam (paleosol) and, C) 6 m of fossiliferous, laminated silt and silt loam. Unit C represents local Illinois Valley loess transported and deposited in the lake. Samples of fine organics sieved from subsamples 3 and 8 cm below the B/C contact yielded median radiocarbon ages of 18,750 and 19,090 cal yr BP. (UCIAMS-196990 and 196991, respectively). The ages are statistically equivalent to the published maximum age of the Kankakee Torrent (median 19,000 cal yr BP; σ2 18,800 - 19,200 cal yr BP). The coincidence of the ages, and the largely slackwater post-Kankakee Torrent record of Glacial Lake Calhoun in the adjacent middle Illinois River valley, may suggest that late glacial loess deposition was curtailed by erosion by the Kankakee Torrent and subsequent adjustments to local hydrology and sedimentation. Our ages agree with estimates of abrupt slowing of loess accumulation based on radiocarbon ages of terrestrial gastropods at Cottonwood School (Nash et al., 2017) and Keller Farm (unpublished).