New Isotopic Evidence from Zircons and K-Feldspars for the Sources of Upper Mississippi River Valley Loess during the Last Glacial Period

OSL dating indicates that last-glacial (Peoria) loess deposition rates in the Great Plains of Nebraska may have been among the highest in the world, exceeding most glaciogenic loess in the Mississippi Valley (Roberts et al., 2003, QR). U-Pb ages of zircons (including a major 34-Ma population) and highly radiogenic Pb isotopic compositions of K-feldspars demonstrate that the source of Great Plains loess was volcanogenic siltstone of the Tertiary White River Group. Modeling suggests that Great Plains loess deposition could have extended well east of Nebraska (Mahowald et al., 2006, JGR). To determine if Peoria Loess of the upper Mississippi River valley contains a component of Great Plains-derived sediment, we studied a thick (17 m) section of Peoria Loess near Morrison, Illinois, 18 km east of the Mississippi River. Radiocarbon ages indicate that loess deposition occurred after 32,000 14C yr BP (Farmdale Soil below Peoria Loess) and continued after 15,500 14C yr BP (snails at 3.5 m). Zircons from the loess have major age populations of about 2.70, 1.88, and 1.07 Ga, indicating derivation from the Superior, Penokean, and Grenville provinces over which the Laurentide ice sheet traversed. K-feldspars in Peoria Loess from Morrison have Pb-isotopic compositions indicative of mixing between Superior and Grenville province sources, in agreement with the zircon U-Pb data. We conclude that Peoria Loess at Morrison is dominantly glaciogenic, although a very radiogenic Pb-isotopic composition of K-feldspars in the section's uppermost part permits a Great Plains input to the modern soil. Loess at Morrison also shows three distinct clay mineral and carbonate zones, similar to loess elsewhere in Illinois. Pb-isotopic variations of K-feldspars track the clay mineral and carbonate trends. Thus, although the loess at Morrison was dominantly Laurentide-derived, the relative importance of different glacial lobes as loess sources varied over the period of deposition.