IMPLICATIONS OF A QUATERNARY SOIL CHRONOSEQUENCE TO CLIMATIC AND NEOTECTONIC EFFECTS ON LANDSCAPE DEVELOPMENT IN SOUTHEASTERN MISSISSIPPI

The landscape in southeastern Mississippi is characterized by a postincisive soil chronosequence developing in progressively older geomorphic surfaces, ranging in elevation from lower than 160¢ to over 350¢. The topography in this area is unusual for a coastal plain setting, and the morphological properties of the soils and thermoluminescence dating techniques are being used in an effort to better understand the geomorphic and neotectonic development of the landscape. The area 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 Pliocene 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, during the last glacial lowstand of sea level. Upland soil dates are pending, and we predict that they will concur with pollen evidence placing age of deposition during the Pliocene. 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 clay cutans up to 2 mm thick, with a heavily leached E horizon up to 6² 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. A better understanding of the regional soil development may help elucidate some of the recent geological history in this area, including possible fluvial responses to climate change and neotectonic movement.