Abstract

The alluvial architecture and soil characteristics of Holocene Mississippi River floodplain deposits in the southern Lower Mississippi Valley provide evidence for significant changes in floodplain development in response to sea-level rise. Floodplain cores acquired near Ferriday, Louisiana show that Holocene deposits consist of 15-30 m (ave. approximately 20 m) of sands, silts, and clays, which overlie Late Wisconsin sands and gravels. On the basis of differences in sediment grain size, sediment-body geometry, and the abundance of soil features, the Holocene deposits are subdivided into Lower and Upper Holocene units. Lower Holocene deposits (> 5000 yr B.P.) consist of lacustrine and poorly drained backswamp muds that contain authigenic siderite, pyrite, and vivianite and show little evidence of soil formation. Muds encase crevasse-splay and floodplain-channel sand bodies (< 1 km wide), and collectively these deposits represent a mosaic of shallow lakes, poorly drained backswamps, and multichannel streams, similar to modern examples in the Atchafalaya Basin ( approximately 100 km south of Ferriday). Upper Holocene deposits (< 5000 yr B.P.) are represented by large Mississippi River meander-belt sand bodies that are up to 15 km wide and 30 m thick. Natural-levee silts and sands and well drained backswamp muds are present between meander-belt sands. Upper Holocene deposits contain abundant soil features, and sandy and silty soils are Entisols, Inceptisols, and Alfisols whereas clayey soils are Vertisols. The presence of isolated sand bodies surrounded by mud and the scarcity of soil features suggest that Lower Holocene sediments reflect a period of rapid floodplain aggradation during which crevassing, lacustrine sedimentation, and avulsion dominated floodplain construction. No evidence of large meandering Mississippi River channels represented by buried, thick tabular sands exists near Ferriday, and discharge in Lower Mississippi Valley flow was probably conveyed by a network of small, multichannel floodplain streams. Upper Holocene sediments record a dramatic change ca. 5000 yr B.P. from rapid to slower floodplain aggradation, which was accompanied by extensive lateral channel migration, overbank deposition, and soil formation. On the basis of differences in meander belt dimensions and numbers of abandoned channels, Upper Holocene meander belts are subdivided into simple and complex forms. Relative age relationships suggest that the smaller and older simple meander belts represent periods of divided Mississippi River flow and early attempts to establish a large, single-channel meandering regime. This type of meandering regime is represented by the larger and younger complex meander belts and includes the modern meander belt. Similarities in the timing of changes in floodplain processes and fluvial style and decreasing rates of Holocene sediment accumulation in the southern Lower Mississippi Valley strongly suggest that decelerating Holocene sea-level rise in the Gulf of Mexico affected floodplain development at least 300 km inland from the present-day coast. The alluvial architecture of the Lower Holocene deposits and the absence of large meandering Mississippi River channel deposits older than approximately 5000 yr B.P. near Ferriday indicates that most of the floodplain muds were deposited by avulsion-related crevassing and lacustrine sedimentation rather than by overbank flooding of large Mississippi River channels. Similarities between the floodplain history of the Mississippi River and those of modern and ancient rivers elsewhere further suggest that avulsion, rather than simple overbank deposition, contributes to the construction of fine-grained floodplains to a greater degree than generally recognized.

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