Evolution of the Holocene Mississippi River floodplain, Ferriday, Louisiana: Insights on the origin of fine-grained floodplains

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 s ea-level rise. Floodplain cores acquired near Ferriday, Louisiana show that Holocene deposits consist of 15-30 m (ave. similar to 20 m) of sands, silt s, and clays, which overlie Late Wisconsin sands and gravels. On the basis of differences in sediment grain size, sediment-body geometry, and the abun dance 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 d rained backswamp muds that contain authigenic siderite, pyrite, and viviani te and show little evidence of soil formation. Muds encase crevasse-splay a nd floodplain channel sand bodies (< 1 km wide), and collectively these dep osits represent a mosaic of shallow lakes, poorly drained backswamps, and m ultichannel streams, similar to modern examples in the Atchafalaya Basin (s imilar to 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 Hel l drained backswamp muds are present between meander-belt sands. Upper Holo cene 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 rap id floodplain aggradation during which crevassing, lacustrine sedimentation , and avulsion dominated floodplain construction. No evidence of large mean dering Mississippi River channels represented by buried, thick tabular sand s exists near Ferriday, and discharge in Lower Mississippi Valley flow was probably conveyed by a network of small, multichannel floodplain streams. U pper Holocene sediments record a dramatic change ca, 5000 yr B.P. from rapi d to slower floodplain aggradation, which was accompanied by extensive late ral channel migration, overbank deposition, and soil formation. On the basi s of differences in meander belt dimensions and numbers of abandoned channe ls, Upper Holocene meander belts are subdivided into simple and complex for ms. Relative age relationships suggest that the smaller and older simple me ander belts represent periods of divided Mississippi River flow and early a ttempts to establish a large, single-channel meandering regime. This type o f meandering regime is represented by the larger and younger complex meande r 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 str ongly suggest that decelerating Holocene sea level rise in the Gulf of Mexi co 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 similar to 5000 yr B.P. near Ferriday indicates that most of the floodplain muds were deposited by avulsion-related crevassing and lacustrine sedimentation rathe r than by overbank flooding of large Mississippi River channels. Similariti es between the floodplain history of the Mississippi River and those of mod ern and ancient rivers elsewhere further suggest that avulsion. rather than simple overbank deposition, contributes to the construction of fine-graine d floodplains to a greater degree than generally recognized.