Depositional and pedogenic influences on the environmental geology of Holocene Mississippi River floodplain deposits near Ferriday, Louisiana

• Published in: Engineering geology Vol. 45; no. 1; pp. 417 - 432
• Main Authors: Aslan, Andres, Autin, Whitney J.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 30.12.1996
• ISSN: 0013-7952; 1872-6917
• DOI: 10.1016/S0013-7952(96)00024-5

Core descriptions and the mineralogy of Holocene Mississippi River floodplain deposits in Louisiana provide insights on fluid migration pathways and the origin of iron-rich ground waters in the Mississippi River Alluvial Aquifer (MRAA). Vertical changes in the sedimentologic and pedologic characteristics of floodplain deposits near Ferriday, Louisiana, provide evidence for two stages of floodplain development and suggest that depositional processes and drainage conditions changed substantially during the Holocene. Depositional and pedogenic processes produced complex fluid migration pathways in the MRAA confining unit and also contributed to the formation of iron-rich ground waters. Lower Holocene deposits in the study area are older than ∼5000 yrs BP and show evidence of crevassing, lacustrine delta building, and multi-channel stream deposition. These processes deposited thin and narrow sheet sands, which represent fluid migration pathways in the MRAA confining unit. Poor drainage conditions during this initial stage of floodplain development also favored the precipitation of authigenic siderite 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 ∼ 5000 yrs BP and represent the transition to the present-day meandering regime of the Mississippi River near Ferriday. This second stage of floodplain development was accompanied by pedogenesis, which produced slickensides in clayey backswamp soils. The abundance of slickensides and the presence of the thin and narrow sheet sands indicates that fluid migration in the MRAA confining unit near Ferriday is greater than generally recognized. Seasonal water table fluctuations and the mixing of oxygenated meteoric and reduced ground waters cause iron oxide reduction and pyrite oxidation in backswamp settings, which releases iron into solution. The presence of high (up to 16 mg/l) dissolved iron 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 that are screened in sandy meander belt deposits, which lack abundant iron-bearing minerals, have low (less than 1 mg/l) concentrations of total dissolved iron. Studies of large floodplains such as the Mississippi River, highlight the importance of floodplain histories for evaluating geologic influences on water quality, developing proper floodplain land use strategies, and for improving our understanding of the environmental geology of floodplain systems.