DEPOSITIONAL AND PEDOGENIC INFLUENCES ON THE ENVIRONMENTAL GEOLOGY OFHOLOCENE MISSISSIPPI RIVER FLOODPLAIN DEPOSITS NEAR FERRIDAY, LOUISIANA

Core descriptions and the mineralogy of Holocene Mississippi River flo odplain deposits in Louisiana provide insights on fluid migration path ways and the origin of iron-rich ground waters in the Mississippi Rive r Alluvial Aquifer (MRAA). Vertical changes in the sedimentologic and pedologic characteristics of floodplain deposits near Ferriday, Louisi ana, provide evidence for two stages of floodplain development and sug gest that depositional processes and drainage conditions changed subst antially during the Holocene. Depositional and pedogenic processes pro duced complex fluid migration pathways in the MRAA confining unit and also contributed to the formation of iron-rich ground waters. Lower Ho locene deposits in the study area are older than similar to 5000 yrs B P and show evidence of crevassing, lacustrine delta building, and mult i-channel stream deposition. These processes deposited thin and narrow sheet sands, which represent fluid migration pathways in the MRAA con fining unit. Poor drainage conditions during this initial stage of flo odplain development also favored the precipitation of authigenic sider ite and pyrite in poorly-drained swamps and shallow lakes. The pyrite and siderite probably represent the source of iron-rich ground waters in the MRAA. Upper Holocene floodplain deposits are younger than simil ar to 5000 yrs BP and represent the transition to the present-day mean dering regime of the Mississippi River near Ferriday. This second stag e of floodplain development was accompanied by pedogenesis, which prod uced slickensides in clayey backswamp soils. The abundance of slickens ides and the presence of the thin and narrow sheet sands indicates tha t fluid migration in the MRAA confining unit near Ferriday is greater than generally recognized. Seasonal water table fluctuations and the m ixing of oxygenated meteoric and reduced ground waters cause iron oxid e reduction and pyrite oxidation in backswamp settings, which releases iron into solution. The presence of high (up to 16 mg/l) dissolved ir on concentrations in water wells that are screened beneath pyrite- and siderite-rich, muddy backswamp deposits near Ferriday, suggests that the distribution of fine-grained alluvium is a primary control on the presence of iron-rich waters in the MRAA. in contrast, water wells tha t are screened in sandy meander belt deposits, which lack abundant iro n-bearing minerals, have low (less than 1 mg/l) concentrations of tota l dissolved iron. Studies of large floodplains such as the Mississippi River, highlight the importance of floodplain histories for evaluatin g geologic influences on water quality, developing proper floodplain l and use strategies, and for improving our understanding of the environ mental geology of floodplain systems.