PROVENANCE OF THE PEORIA LOESS IN THE NORTHERN MISSISSIPPI EMBAYMENT

The Peoria Loess caps two regionally prominent topographic highs in the Northern Mississippi Embayment (NME), Crowley's Ridge, Arkansas and the Chickasaw Bluffs, Tennessee. Changes that took place in the pathway of the Mississippi-Missouri-Ohio river system in the NME during Late Wisconsinan suggest that the proximal source of wind-deflated silt for the Peoria varied substantially in the interval of its deposition (circa 22,000 to 10,000 yr. BP). The basic question we asked is - "Can XRD of bulk samples be used to define mineralogical variations within the Peoria Loess, thereby providing a basis for interpretation of its provenance and depositional history?" We focused on feldspar and dolomite mineralogy in samples from five exposures along Crowley's Ridge and five along the Chickasaw Bluffs. Samples from Minnesota, Illinois, and Iowa provided insight into source area compositions. The Peoria Loess in the NME contains an average of 5% to 7% Na-feldspar (d002=3.18 to 3.20A) and 2% to 4% K-feldspar (d220=3.23 to 3.25A). The feldspar proportion varies randomly from location to location and is essentially similar on both features. Below the modern soil dolomite varies from 6% to 13% at seven locations, is absent at two locations and is present in the two basal samples at one other. The silt-sized dolomite grains have a similar near-stoichiometric composition and abraded appearance, wherever present. Neither feldspar nor dolomite abundance varies systematically with depth, except for the absence of dolomite in the modern soil. The similarity in abundance and composition of feldspar and dolomite in the Peoria Loess of both Crowley's Ridge and the Chickasaw Bluffs makes mineral-based subdivision difficult. The uniform abundance of dolomite below the modern soil suggests that soils did not have a chance to form during Peoria deposition.