PROVENANCE AND STRATIGRAPHY OF LATE PLEISTOCENE ALLUVIUM IN THE LOWER MISSISSIPPI VALLEY

Late Pleistocene fluvial deposits of the Lower Mississippi Valley (LMV) in central Louisiana are thought to represent Mississippi River glacial outwash. Recent field work, however, shows that red sediments of non-Mississippi River origin occur beneath several Late Pleistocene alluvial terraces. This observation suggests that at least a portion of the alluvium was deposited by the Arkansas rather than by the Mississippi River. Shallow cores (<10 m long) and mineralogic analyses of silt and fine sand were performed on 45 samples acquired from 5 terrace levels to evaluate the stratigraphy and provenance of Late Pleistocene sediments.

Cores shows that each terrace is underlain by 1-4 m of Peoria Loess, a 2-3 m thick loess-alluvium transition zone, and fine to medium-grained sands. Sand compositions range from quartz- and feldspar-rich samples to quartz- and lithic-rich sands with abundant sedimentary rock fragments. Silt samples consist of two populations: 1) quartz-rich samples with minor amounts of feldspars, and 2) quartz-rich samples with appreciable quantities of alkali feldspar, plagioclase, dolomite, and anatase.

Quartz-, feldspar-, and anatase-rich deposits probably represent Mississippi River outwash derived from Upper Mississippi Valley lithologies including Canandian Shield rocks. In contrast, quartz- and lithic-rich sands probably represent Arkansas River deposits derived from sedimentary strata of the nearby Ouachita Highlands. These preliminary results indicate that mineralogic information is useful for 1) evaluating the role that tributaries such as the Arkansas River have played in supplying sand to the LMV during the Late Pleistocene, 2) fingerprinting glaciogenic deposits in the southern LMV, and 3) studying processes of incised valley filling.