A. Aslan et Wj. Autin, Evolution of the Holocene Mississippi River floodplain, Ferriday, Louisiana: Insights on the origin of fine-grained floodplains, J SED RES, 69(4), 1999, pp. 800-815
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.