TWO-STAGE CESSATION OF LGM LOESS DEPOSITION IN CENTRAL ILLINOIS: THE LACUSTRINE EVIDENCE

Age determination of the end of deposition of last glacial loess (Peoria Silt in mid-North America) is obfuscated in modern soil profiles by intensified syndepositional and post depositional bioturbation as sediment accumulation rates decelerated. In addition, post-glacial weathering prompted carbonate leaching and organic matter oxidation, adversely impacting carbon species for dating purposes.

We determined loess cessation ages from lacustrine sediment in a kettle (“Lake Ben”) on the MIS 6 glacial plain within 10 km of the 30 km-wide last glacial (LG) loess source (an outwash terrace). Lake Ben yielded a 19 m sediment record, of which 8.6 m is loess-related. The material is not loess per se, but particle-size analyses (PSD) reveal characteristics consistent with resedimentation of proximal loess. Stages of loess cessation are indicated by changes in PSD ratios, clay mineralogy, and XRF scans. Multivariate analyses of XRF data underscore two major signatures, those rich in Ca, MS, and Mn (consistent with loess) and those rich in Al, Zn, Cu, and Si (reflecting primarily local sediment). Loess source proximity is indicated by the Wentworth ratio 4-6Φ to ≥7Φ; loess provenance is indicated by illite (of the Lake Michigan lobe) and smectite (e.g., Des Moines lobe). The history of LG loess cessation is reflected further by changes in the sediment accumulation rate (SAR).

The collective evidence indicates proximal loess cessation began at 19.0 cal ka, an age consistent with age of the Kankakee Torrent. Locally, flood erosion lowered base level 15 m, disrupting sediment replenishment of the upwind loess source (outwash terraces). Proximal loess deposition ceased at 17.9 ± 0.4 cal ka but distal loess continued depositing to 15.6 ± 0.3 cal ka as reflected by high values of smectite/illite and ≥7Φ/4-6Φ. The chronology and mineralogy are consistent with aeolian sediment contribution sourced from western prairie outlet glaciers of the LIS. From 15.6 to 13.9 cal ka, a thin 26 cm gleysol developed (worm-burrowed smectitic silty clay loam); the proxy data do not indicate further significant loess additions in this interval.