A SEDIMENTARY UPPER FINE ZONE IN PEORIA LOESS OF THE GREAT PLAINS AND MIDWEST: THE RESULT OF FINE DUST RAINOUT?

From western Nebraska to Minnesota and Illinois, the upper 0.5 to 2 m of thick Late Pleistocene Peoria Loess is enriched in fine silt and clay at many sites, especially on stable summits. This enrichment produces a secondary fine mode or fine “shoulder” in the particle size distribution. In the humid Midwest, this upper fine zone of Peoria Loess is contained entirely within the surface solum, and the most obvious explanation for it is weathering of initially coarse loess. Chemical mass balance analyses at several sites, however, indicate that the fine zone formed primarily through sedimentary addition of fine silt and clay to locally derived coarse loess (Mason and Jacobs, 1998, Geology 26: 1135-1138). In semiarid western Nebraska, a pedogenic origin is even less likely, since the upper fine zone extends below the solum of the surface soil or the buried Brady Soil that separates Peoria Loess from Holocene Bignell Loess. Mass balance analyses are not as conclusive in Nebraska, but generally support a sedimentary origin for the fine zone. We earlier suggested that the upper fine zone in the Midwest originated through Holocene dustfall; however, in Nebraska it is buried by Bignell Loess and predates the accumulation of that unit beginning around 10 ka. We propose that the upper fine zone of Peoria Loess in both the Great Plains and Midwest originated through mixing of fine dust with coarse local loess, although it may not be the same age everywhere. Increased admixture of fine dust toward the end of Peoria Loess deposition might in part reflect increased production from distant sources (e.g. desiccated pluvial lakes), but we speculate that the key factor was increased scavenging by precipitation of fine dust from both distant and local sources. Rainout is a well-established mechanism for fine dust deposition, and an increase in precipitation can explain the fine zone without the need to postulate an enormous increase in fine dust production from distant sources to the west of our study sites.