Dt. Rodbell et al., STRATIGRAPHY AND CHRONOLOGY OF MISSISSIPPI VALLEY LOESS IN WESTERN TENNESSEE, Geological Society of America bulletin, 109(9), 1997, pp. 1134-1148
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.