U-PB AGE DENSITY DISTRIBUTIONS AS AN INDICATOR OF GLACIAL LOESS PROVENANCE IN MIDWESTERN NORTH AMERICA

North American loess derived from the Laurentide Ice Sheet, during the last and penultimate glacial maxima, serves as a terrestrial record of Quaternary glacial activity and climatic variation. Detrital zircon U-Pb age populations, contained within the loess, provide provenance information and illuminate the unique path of individual glacial lobes over underlying bedrock and subsequent mechanisms of transport. We explore the utility of this method for mid-continental U.S. loess provenance.

Two loess sites along the Illinois River Valley (Thomas Quarry and Cottonwood School Core) were sampled in multiple stratigraphic zones (within Peoria and Roxana Silts) for observations of temporal variability. Samples representing the composition of glacial lobes of the southern Laurentide Ice Sheet were created by mixing subsamples from 3-6 sites within each lobe (Green Bay, Superior, Lake Michigan and Des Moines) and sieving < 0.5 mm fraction .

Kolmogorov-Smirnov tests on U-Pb age density distributions show that the lower Peoria Silt is significantly different (P-value: 0.00) from the upper Peoria Silt, middle Peoria Silt and the Roxana Silt. Comparisons between the remaining units do not show significant differences . The lower Peoria Silt is set apart by a minimum detrital zircon age of 625 ma at Thomas Quarry and 1048 ma at Cottonwood School.

K-S tests on the glacial lobe samples indicate the Des Moines Lobe < 0.5 mm fraction is significantly different (P-value: 0.00) from that of the Superior, Lake Michigan and Green Bay glacial lobes. The Des Moines Lobe contained 20 distinctly young grains with ages between 59—79 Ma, corresponding to the underlying Cretaceous and Paleogene bedrock that is not present in the more eastern glacial lobe samples.

Differences between age distributions in loess samples are an indicator of temporally or spatially variable glacial contributions which can be influenced by the fluctuating glacial lobe margins, to the known diversion of the Mississippi River (at ~ 24.5 cal ka), as well as changes in wind speed and direction. Detrital zircon probability distributions can be used as provenance signatures for individual lobe contributions. Used in conjunction with other methods, DZ age population densities are valuable as constraints on the geologic history during and since the Last Glacial Maximum.