THE FORMATION OF THE JULES SOIL AND ITS IMPLICATIONS FOR THE LAST GLACIAL MAXIMUM CLIMATE OF WEST-CENTRAL ILLINOIS

Thick loess deposits (Peoria Silt) east of the Illinois Valley in Illinois, can be used to help reconstruct last glacial maximum (LGM) sedimentation, climate, and ecological history in western Illinois. We selected two sites along the Illinois River Valley that contain thick deposits of Peoria Silt (>8 m), well preserved gastropod fossils, and the Jules Geosol. The New Cottonwood School section (40° 1'25.0" N, 90°17'25.6" W) is 200 m east of the type section of the Jules Geosol and has an exposure of the top 5 m of Peoria Silt; Thomas Quarry (39°31'12.2" N, 90°30'57.9" W), 8 m of Peoria Silt. The Jules Geosol is exposed at both sites with common features such as finer grain size, increased organic matter, and a more cohesive and stronger crumbly structure relative to adjacent loess. Here we present an environmental reconstruction of the region as well as a conceptual model for observed variability in deposition rates. Based on gastropod shell dating, deposition of Peoria Silt occurred episodically between 30,000 and 17,000 cal yr BP, and formation of the Jules Geosol occurred between 23,735 ± 295 and 21,960 ± 275 cal yr BP. An age model constructed from 13 radiocarbon dates indicates that the upper and middle Peoria were both deposited at 1.2 mm/yr while the cumulic Jules Geosol, which separates these two informal units, formed at 0.4 mm/yr. The correlation between loess deposition rate and Lake Michigan Lobe moraine formation indicates that loess accumulated in concert with ice advances, likely because more silt was produced and available for entrainment by wind. We hypothesize that the Jules Geosol formed because of a decrease in loess deposition due to the stagnation of ice after the maximum glacial advance. This study also seeks to understand the climatic conditions during Jules Geosol formation. Time series of grain size, magnetic susceptibility, and clay mineralogy indicate sedimentological changes that varied in concert with changes in sedimentation rates of Peoria Silt. Time series of δ¹⁸O and δ¹³C of gastropod shell carbonate do not suggest strong climatic or vegetation changes in western Illinois around the maximum extent of the Wisconsin Episode glaciers. The gastropod fossil assemblage from both sites also indicate the region remained forested throughout the deposition of Peoria Silt.