DETRITAL ZIRCON GEOCHRONOLOGY AND PROVENANCE OF PLEISTOCENE LOESS AND CONTRIBUTING GLACIAL SOURCES, MIDCONTINENTAL USA

Mid-continental North America contains extensive loess deposits that have been studied with geochemical, mineralogical, and stratigraphic techniques to explore glacial history and sediment provenance. Much of this analysis has focused on loess deposits that accumulated during the last glacial maximum (LGM), in part because of the ubiquity of relatively unaltered Peoria Silt deposits. We show the utility of detrital zircon (DZ) U-Pb dating, which offers a weathering-resistant approach to the determination of loess provenance and can uniquely be applied to both LGM and older (commonly more altered) pre-LGM loess units. We present DZ U-Pb age distributions (1385 U-Pb ages) for glacial lobes that contributed sediment to Mississippi and Illinois River valley loess: Des Moines Lobe, Superior Lobe, Green Bay Lobe, Lake Michigan Lobe and Huron-Erie Lobe. We also present DZ U-Pb age distributions (4466 U-Pb ages) sampled from loess sites located along the bluffs of the Illinois River Valley and Mississippi River Valley. Samples were collected from the Peoria Silt (Marine Isotope Stage (MIS2)), Roxana Silt (MIS3), Loveland Silt (MIS6), and the Crowley’s Ridge Silt (pre-Illinois Episode). LGM glacial tills provide DZ age signatures that can be used to identify glacial source contributions to loess deposits. Loess DZ age signatures of Peoria Silt from the Illinois River Valley and the Roxana Silt of the northern lower Mississippi River Valley change temporally, consistent with blockage and avulsion of the ancient Mississippi River, representing a proof-of-concept for the DZ loess provenance method in the mid-continent. The loess DZ record also provides insight into the pre-LGM drainage network and glacial lobe extents. In the Illinois River Valley, Des Moines Lobe sourced sediment is absent in MIS3 loess, suggesting a diminished or absent Des Moines Lobe equivalent during MIS3. The dominant Huron-Erie Lobe signature throughout pre-LGM loess units shows that ice flow from the eastern Labrador Dome was a persistent feature during earlier glaciations. Finally, western-derived DZ ages in pre-MIS2 loess of the middle and lower Mississippi River Valley suggests drainage networks that followed the modern Missouri system were present prior to the LGM and integrated with the Mississippi River system.