STRATIGRAPHY AND CHRONOLOGY OF MISSISSIPPI VALLEY LOESS IN WESTERN TENNESSEE

The stratigraphy and chronology of loess deposits in the Mississippi V alley provide an important proxy record of the number and timing of gl acial advances into the upper Mississippi drainage basin. The stratigr aphy of loess and paleosols from two exposures in northwestern Tenness ee reveals three major loess units that are correlated with the Lovela nd Loess, Roxana Silt, and Peoria Loess-from oldest to youngest, Detai led measurements of clay and secondary (pedogenic) iron contents sugge st that the Loveland Loess may consist of multiple loess units which h ave been pedogenically welded to one another to form a composite Sanga mon Soil, The mineralogy of the coarse silt fraction from one exposure reveals both primary mineralogical differences between the Peoria Loe ss and the Roxana Silt and pedogenic overprinting by the Sangamon Son, Grain size and iron data suggest that the degree of pedogenic alterat ion increases from west to east, away from the loess source area, whic h probably reflects: the role of sedimentation rate on the degree of p edogenic alteration of loess, However, only the Peoria Loess shows the expected exponential eastward decrease in thickness; the thickness of older loess units appears to be controlled more by the occurrence of localized erosion than by distance from source area. Most samples are resistant to solar resetting of the thermoluminescence signal, which, if unrecognized, could yield thermoluminescence ages that significantl y underestimate the true age of the deposits, The best match between t hermoluminescence age using the total bleach method and radiocarbon ag e of the Peoria Loess and Roxana Silt is achieved when either long dur ations of optical bleaching by sunlight (greater than or equal to 32 h r) or artificial ultraviolet (UV) light are applied to define the resi dual thermoluminescence level, Partial bleach analysis indicates that long-duration optical bleaching by sunlight or UV lamp bleaching does not systematically overbleach samples of Peoria Loess and Roxana Silt, Three thermoluminescence age estimates between 35 +/- 4 (+/-1 sigma) and 53 +/- 5 ka from the basal half of the Roxana Silt, and six age es timates between 18 +/- 2 and 25 +/- 3 ka from the basal half of the Pe oria Loess are consistent with numerous finite radiocarbon dates from around the central Mississippi Valley.