EVIDENCE OF THE ORIGINS OF PEORIA LOESS FROM A MASS-BALANCE ANALYSIS APPROACH

The Peoria Loess of the central Great Plains is described as the thickest and most significant Last Glacial Maximum loess deposit in the world. Recent research focusing on geochemical provenance, thickness, and particle size trends has emphasized a mostly non-glaciogenic origin for Peoria Loess on the Great Plains, in contrast to some previous studies which suggested an origin in Rocky Mountain glacial silt. Our research adds a new line of evidence on potential sources of Peoria Loess, through a mass balance analysis.

Using data from published sources and GIS spatial analyses, the extent, thickness, volume, and mass of Peoria Loess was calculated for areas west of the Missouri River, with an emphasis on the states of Nebraska and Kansas, where the thickest Peoria Loess deposits are found. With the mass of Peoria Loess calculated, we evaluated the necessary contributions from the two proposed loess origins – silt produced by alpine glacial erosion in the Rocky Mountains within the Platte River watershed and the Cenozoic White River and Arikaree groups northwest of the Peoria Loess region.

Our findings indicate that to produce the estimated mass of Peoria Loess present on the Great Plains, alpine glaciers in the Rocky Mountains would have needed to erode on average of at least 50 m of bedrock per square meter of glacier and converted it all to silt-size material. This amount of erosion is far greater than estimated bedrock erosion by these glaciers during the last glaciation. By contrast, the rates of erosion required to produce the mass of Peoria Loess from outcrop areas of the White River and Arikaree groups, while high, are plausible based on observed erosion rates in semiarid landscapes, particularly if possible minor contributions from other rocks and Rocky Mountain glacial silt are considered, as provenance studies suggest. This study adds to previous work indicating that Peoria Loess originated primarily in unglaciated and unvegetated White River and Arikaree outcrops to the northwest of the Peoria Loess deposits.