Loess stratigraphy and fragipan occurrence in the Lower Mississippi River Valley

The loess uplands adjacent to the Lower Mississippi River Valley (LMRV) are comprised of several different depositional units overlying either coastal plain or alluvial sediments. The occurrence and development of fragipan horizons in the region has been attributed to the presence of a specific stratigraphic unit. This study was conducted to investigate the stratigraphic relationship between loess units throughout the LMRV to determine if the existence of fragipans is limited to one loess deposit. The study utilized five field sites in the loess uplands between northwestern Tennessee and southeastern Louisiana. At all sites, each member of the Memphis catena (Memphis [fine-silty, mixed, thermic Typic Hapludalf], Loring [fine-silty, mixed, thermic Typic Fragiudalf], and Grenada [fine-silty, mixed, thermic Glossic Fragiudalf]) was sampled and preliminary stratigraphic breaks identified by changes in texture and color. These breaks were further refined in the laboratory by characterizing each horizon for clay-free particle-size distribution and mass magnetic susceptibility. Additionally, total Fe, K, Mg, Ca, Ti, and Zr were determined for the coarse silts (20-50 mu m) from all Memphis horizons. The data were further evaluated by k-means cluster analysis to refine stratigraphic breaks in the loess and underlying coastal plain or alluvial deposits. Generally, stratigraphic breaks were distinguishable by changes in clay-free sand, clay-free medium and coarse silt, and mass magnetic susceptibility. Secondary clay maxima below 250 cm represented paleoargillic horizons that confirmed stratigraphic changes and buried surfaces. For the thick loess deposits (>3.5 m total loess thickness), fragipans developed entirely in Peoria Loess, but in thinner loess deposits (<3.5 m total loess thickness) fragipans crossed stratigraphic boundaries between Peoria Loess and underlying units. Thus, the development of fragipan horizons in these loess soils is independent of stratigraphic units.