DETRITAL-ZIRCON GEOCHRONOLOGY AND PROVENANCE OF QUATERNARY LOESS IN CENTRAL NORTH AMERICA

North American loess linked to the growth and retreat of the Laurentide Ice Sheet serves as a terrestrial record of Quaternary glacial activity and climate variability. Detrital zircons found in loess on uplands proximal to the Illinois River and Mississippi River valleys are mainly the product of the glacial path over bedrock of each contributing glacial lobe along with subsequent meltwater and aeolian transport.

We present a detrital zircon U-Pb geochronological dataset for 5 representative glacial till assemblages (Des Moines Lobe, Superior Lobe, Green Bay Lobe, Lake Michigan Lobe and Huron-Erie Lobe) and 15 loess samples from 5 loess sites, distributed from Illinois to Mississippi. Each sampled unit has a sample size of 307> n >121 zircon grains, and total project n = 5584. Probability density plots show characteristic differences among the glacial lobes that can be used as “fingerprints” for loess provenance. The Des Moines Lobe is the only composite till sample that contains a significant quantity of grains with ages between 275 and 60 My. Statistical cross-correlation of the Des Moines Lobe (westernmost glacial source tested) and Huron-Erie Lobe (easternmost glacial source tested) indicate that the two age populations are significantly different with 90% confidence. We are thus able to distinguish between western and eastern derived glacial contributions within each loess sample.

Of the 5 loess sites, two ice-proximal sites are located along the Illinois River Valley. Other loess sites, along a N-S transect, are near Thebes (IL), Millington (TN), and Vicksburg (MS). Loess sites were sampled in the following units, as available: Peoria Silt (late Wisconsin Episode), Roxana Silt (middle Wisconsin Episode), Loveland Silt (Illinois Episode) and Crowley’s Ridge Silt (pre-Illinois Episode). The project design allows for separation of down-valley effects from spatio-temporal changes in glacial lobe sediment sources and meltwater contributions. Comparisons between the glacial lobe assemblages and Peoria loess samples inform sedimentary routing patterns during the Last Glacial Maximum (LGM), whereas comparisons with older loess units reveal pre-LGM changes in both river drainage and dominant glacial flow paths.