Stratigraphy and chronology of Mississippi Valley loess in western Tennessee

Abstract

The stratigraphy and chronology of loess deposits in the Mississippi Valley provide an important proxy record of the number and timing of glacial advances into the upper Mississippi drainage basin. The stratigraphy of loess and paleosols from two exposures in northwestern Tennessee reveals three major loess units that are correlated with the Loveland Loess, Roxana Silt, and Peoria Loess—from oldest to youngest. Detailed measurements of clay and secondary (pedogenic) iron contents suggest that the Loveland Loess may consist of multiple loess units which have been pedogenically welded to one another to form a composite Sangamon Soil. The mineralogy of the coarse silt fraction from one exposure reveals both primary mineralogical differences between the Peoria Loess and the Roxana Silt and pedogenic overprinting by the Sangamon Soil. Grain size and iron data suggest that the degree of pedogenic alteration increases from west to east, away from the loess source area, which probably reflects the role of sedimentation rate on the degree of pedogenic 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 significantly underestimate the true age of the deposits. The best match between thermoluminescence age using the total bleach method and radiocarbon age of the Peoria Loess and Roxana Silt is achieved when either long durations of optical bleaching by sunlight (≥32 hr) or artificial ultraviolet (UV) light are applied to define the residual 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 38 ± 4 (±1σ) and 53 ± 5 ka from the basal half of the Roxana Silt, and six age estimates between 18 ± 2 and 25 ± 3 ka from the basal half of the Peoria Loess are consistent with numerous finite radiocarbon dates from around the central Mississippi Valley. Thermoluminescence age estimates suggest that the oldest of the intra-Loveland Loess units is 127 ± 12 ka (one age) and may have been deposited during marine oxygen isotope stage 6. The younger two of these loess units were deposited between 113 ± 10 and 70 ± 6 ka (three ages). This latter age range for loess that underlies the top of the presumed Sangamon Soil is problematic because it requires that pedogenesis may have begun as late as approximately 70 ka. Moreover, loess deposited between about 120 and 70 ka suggests that the southern margin of the Laurentide ice sheet was within some part of the upper Mississippi Valley during this interval. This conflicts with some glacial geologic evidence, but it is consistent with global sea level and δ18O records of ice volume, which indicate several intervals of moderate glaciation during this time.