ARE LATE PLEISTOCENE BRAIDED-STREAM TERRACES IN THE SOUTHERN LOWER MISSISSIPPI VALLEY GLACIAL IN ORIGIN?

Late Pleistocene braided-stream terraces in the Lower Mississippi Valley (LMV) are traditionally interpreted as Mississippi and/or Ohio River glacial outwash. Ongoing geochemical, mineralogical, and field-based studies in the southern LMV, however, indicate that unglaciated tributaries such as the Arkansas and White Rivers contributed significantly to fluvial terrace construction. These studies also raise questions concerning stratigraphic correlations among braided-stream surfaces, which are critical for interpreting river responses to sea-level and climate change.

Shallow cores (n=52) acquired from 5 braided-stream terraces and analyses of clay mineralogy and sand geochemistry were used to evaluate terrace origins. Samples interpreted as having a non-Mississippi River origin are characterized by: 1) red clays, 2) elevated concentrations of Ca and Mg, 3) low concentrations of Ti, Zr, and Ce, 4) significant amounts of dolomite, and 5) abundant mixed-layer clays. In contrast, Mississippi River samples are gray in color, have relatively small amounts of Ca, Mg, and dolomite, and contain abundant Ti, Zr, Ce, and smectite. Geochemical data for sands located beneath widely separated braided-stream surfaces that are currently correlated based on surface morphology, are highly variable.

These results indicate that late Pleistocene braided-stream terrace construction in the southern LMV involved both the Mississippi River and unglaciated bedload-dominated tributaries. Braided-stream terraces and their associated sand bodies therefore do not simply represent glacial outwash. Instead, unglaciated tributaries and perhaps the Mississippi River as well, developed bed load-dominated systems in response to late Pleistocene changes in sediment load or rates of floodplain sediment reworking rather than solely due to deglaciation and meltwater contributions.