IMPLICATIONS OF A QUATERNARY SOIL CHRONOSEQUENCE TO LANDSCAPE DEVELOPMENT IN SOUTHEASTERN MISSISSIPPI

The landscape in southeastern Mississippi is characterized by a postincisive chronosequence of soil development in progressively older surfaces occupying distinct topographic positions. The morphological properties of the soils and thermoluminescence dating techniques are being used to help resolve the geomorphic and neotectonic history of the region. The landscape is characterized by three geomorphic domains: uplands (elevations of 200-350'), stream-cut terraces (160-200'), and modern alluvial deposits (<160'). The parent materials consist of the deltaic, silty-clay Miocene Hattiesburg Formation overlain by the fluvial sands and gravels of the Plio-Pleistocene(?) Citronelle Formation. These deposits are generally exposed in the upland interfluves and in the base of stream channels. The valleys are filled with alluvial sands, gravels, and lesser amounts of silts and clays reworked from the Hattiesburg and Citronelle Formations. A prominent terrace is cut into the alluvium, and has been mapped as a Quaternary deposit. Thermoluminescence dating techniques have constrained the terrace soils to ages between 18,000 and 25,000 ybp. Upland soil dates are pending, and we predict that they will be much older than the Quaternary terrace deposits. Qualitatively, soils developing in the uplands are the oldest in the chronosequence, and are characterized by A-E-Bt-Btv-Cox horizonation. Lower horizons are reddened and have prominent illuviated clay cutans up to 2 mm thick, with a heavily leached E horizon up to 6" thick. Soils developing in the terraces show A-Bt-C horizonation, have minor clay cutans less than 1 mm thick, and are not as hardened or reddened as the upland soils. Modern floodplain soils have A-C horizonation, no visible soil structure, and no illuviation of organics, clay, or sesquioxides into lower horizons in the profile. These soils are dominated by periodic overbank sand deposition. The unusual topographic relief on this coastal plain is problematic; however a better understanding of the regional soil development may help elucidate some of the recent geological history. Furthermore, this study is improving our understanding of the lithostratigraphy, regional distribution, and relationships between the Hattiesburg and Citronelle Formations and the Quaternary alluvium.