INFLUENCE OF HOLOCENE CLIMATE AND FLOOD EVENTS ON THE GEOMORPHOLOGY OF THE LOWER MISSISSIPPI RIVER VALLEY

The Holocene climate and geomorphology of the Lower Mississippi Valley (LMV) as well as the chronology of these events is poorly understood because of the lack of terrestrial evidence. Interpretation of the Holocene history of the lower valley is based largely on marine sediment cores from the Gulf of Mexico and to a lesser degree from the glaciated northern portion of the valley. Data from sediment cores from two sites in the lower valley, below the latitude of Natchez, MS including grain-size analyses and 84 radiocarbon dates, were used to document the position of meander belt locations and establish the influence of the Younger Dryas in the lower valley. The data indicate that during minimal discharge associated with the Dryas, sediment was stored in the LMV. Subaerial erosion and weathering of the Pleistocene (Prairie fm) also took place. Diversion of meltwater into the LMV following the Dryas caused rapid fine-grained deposition that exceeded 50cm/100 yrs. The LMV was dominated by a series of major floods and deposition events separated by drier periods with minimal deposition. The data indicate that the onset of fine-grained aggradation in the LMV, at this latitude, began around 10 – 11,000 cal yr B.P. and is coincident with the change from a braided to a meandering fluvial regime in the LMV. The radiocarbon data also indicate how depositional environments in fluvial systems determine the sequence and reliability for dating fluvial events.