Detrital zircon geochronology and provenance of Pleistocene loess and contributing glacial sources, midcontinental USA

Abstract

Midcontinental 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 unweathered Peoria Silt deposits. We test 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 determined DZ U–Pb age distributions (n = 1385 U–Pb ages) for the 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 measured DZ U–Pb age distributions (n = 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 Peoria Silt (Wisconsin Episode; Marine Isotope Stage (MIS 2)), Roxana Silt (Wisconsin Episode; MIS 3), Loveland Silt (Illinois Episode; MIS 6), and Crowley's Ridge Silt (pre-Illinois Episode).LGM glacial tills provide distinct DZ age signatures that can be used to identify glacial source contributions to loess deposits. Loess DZ age signatures in the Peoria Silt of the Illinois River Valley and the Roxana Silt of the northern lower Mississippi River Valley change temporally in a manner consistent with blockage and avulsion of the ancient Mississippi River, representing a proof-of-concept for the DZ loess provenance method in the midcontinent. The loess DZ record also provides insight into the pre-LGM drainage network and glacial lobe extents. DZ signatures for the MIS 3 Roxana Silt, along the Mississippi Valley, implies minimal sediment input from the Des Moines Lobe, suggesting this lobe was short-lived, existed further northward, or provided only limited sediment to meltwater valleys during this time. 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, some western-derived DZ ages in pre-MIS 2 loess of the middle and lower Mississippi River Valley suggests that drainage networks that followed the modern Missouri River system were present prior to the LGM and integrated with the Mississippi River system.