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 eith er coastal plain or alluvial sediments. The occurrence and development of fragipan horizons in the region has been attributed to the presenc e of a specific stratigraphic unit. This study was conducted to invest igate the stratigraphic relationship between loess units throughout th e LMRV to determine if the existence of fragipans is limited to one lo ess deposit. The study utilized five field sites in the loess uplands between northwestern Tennessee and southeastern Louisiana. At all site s, each member of the Memphis catena (Memphis [fine-silty, mixed, ther mic Typic Hapludalf], Loring [fine-silty, mixed, thermic Typic Fragiud alf], 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 laborator y by characterizing each horizon for clay-free particle-size distribut ion and mass magnetic susceptibility. Additionally, total Fe, K, Mg, C a, Ti, and Zr were determined for the coarse silts (20-50 mu m) from a ll Memphis horizons. The data were further evaluated by k-means cluste r analysis to refine stratigraphic breaks in the loess and underlying coastal plain or alluvial deposits. Generally, stratigraphic breaks we re distinguishable by changes in clay-free sand, clay-free medium and coarse silt, and mass magnetic susceptibility. Secondary clay maxima b elow 250 cm represented paleoargillic horizons that confirmed stratigr aphic 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) fragipa ns crossed stratigraphic boundaries between Peoria Loess and underlyin g units. Thus, the development of fragipan horizons in these loess soi ls is independent of stratigraphic units.