Roxana Silt (loess) provides evidence for MIS-3 glaciation in the upper Mississippi River drainage basin, USA

The position of the southern Laurentide Ice Sheet during Marine Isotope Stage (MIS) 3 has been controversial in recent years. Indirect evidence for MIS-3 glaciation in the Upper Mississippi River drainage basin by nearly equal 45 cal ka is provided by the distribution, composition, and geochronology of the Roxana Silt (dominantly loess) in Illinois and Wisconsin. The Roxana Silt is a slightly pinkish brown to grayish-brown loess that occurs below loess of the Peoria Silt (MIS-2; nearly equal 30-15 cal ka) and above the Sangamon Geosol (MIS-4, MIS-5). The Roxana's distinctive pinkish color (iron-stained quartz grains) reflects sediment contributions from the Lake Superior region. Similar to other central USA loess units contemporaneous with Midwest glaciations, the Roxana Silt's thickness pattern suggests eolian deflation from glaciofluvial deposits linked to glacial lobes in the upper Midwest. In Illinois, the Roxana Silt can be up to 10 m thick. Where textgreater 5 m thick, the middle zone may contain radiocarbon-datable terrestrial gastropod shells, such as Succinea sp., Allogona profunda and Webbhelix multilineata. In areas of thinner Roxana loess in Wisconsin, charcoal fragments were radiocarbon dated. A total of 19 radiocarbon ages on terrestrial gastropod shells, wood, organic debris, and charcoal provide evidence for pulses of rapid loess accumulation between 45 and 33 cal ka. Elemental, mineralogical, and magnetic data suggest that 80 to 90 Meadow Member) along the Mississippi Valley is ultimately of glacial origin. In some areas, the basal 10 to 20 Markham Member) has been interpreted as colluvial and related to cold-climate erosion and landscape instabilities during early MIS-3 and MIS-4. Greater amounts of smectite, dolomite and feldspar, and lower frequency dependent magnetic susceptibility, occur in the overlying Meadow Member, compared with the basal Markham Member. In Wisconsin, significantly less Pb and Zn, and higher feldspar/quartz ratios in the Roxana Silt, compared with local Holocene alluvium, further suggest a glacial origin for the Roxana rather than regional slope erosion. Altogether, the characteristics of the Roxana Silt in the Upper Mississippi Valley region indicate that continental glaciers had advanced at least to the Lake Superior Basin by nearly equal 45 cal ka.