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EVOLUTION OF THE HOLOCENE MISSISSIPPI RIVER FLOODPLAIN, FERRIDAY, LOUISIANA: INSIGHTS ON THE ORIGIN OF FINE-GRAINED FLOODPLAINS ANDRES ASLAN1 AND WHITNEY J. AUTIN2 1

Bureau of Economic Geology, The University of Texas at Austin, University Station, Box X, Austin, Texas 78713, U.S.A. e-mail: [email protected] 2 Department of Earth Sciences, SUNY College at Brockport, Brockport, New York 14420, U.S.A.

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. ; 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 (; 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, singlechannel 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 ; 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 JOURNAL OF SEDIMENTARY RESEARCH, VOL. 69, NO. 4, JULY, 1999, P. 800–815 Copyright q 1999, SEPM (Society for Sedimentary Geology) 1073-130X/99/069-800/$03.00

that avulsion, rather than simple overbank deposition, contributes to the construction of fine-grained floodplains to a greater degree than generally recognized.

INTRODUCTION

The Mississippi River in the Lower Mississippi Valley is a classic meandering river with abundant floodplain clays and silts (Fisk 1944, 1947). Traditionally, fine-grained floodplain deposits of the Mississippi River, as well as those of large rivers elsewhere, are thought to represent repeated episodes of widespread overbank deposition and slow rates of sediment accumulation (e.g., Fisk 1944, 1947; Allen 1965; Blake and Ollier 1971; Kesel et al. 1974; Bridge and Leeder 1979; Zwolinski 1992; Mertes 1994; Mertes et al. 1996). Recent investigations of both modern (Smith et al. 1989; Smith and Pe´rez-Arlucea 1994) and ancient (Kraus and Aslan 1993; Willis and Behrensmeyer 1994; Kraus 1996) alluvial deposits, however, suggest that large volumes of mud and lesser amounts of sand accumulate rapidly in floodplain depressions during episodes of crevassing and avulsion. The significance of these studies is that the traditional model of finegrained floodplain construction by repeated episodes of infrequent overbank floods may not be so widely applicable. Instead, crevassing and avulsion, which can occur regardless of trunk-channel flooding, are the dominant processes of construction of fine-grained floodplains during periods of rapid floodplain aggradation. Considering that seminal studies by Harold N. Fisk (1944, 1947) established the Mississippi River as a standard for interpreting floodplain deposits of fine-grained meandering rivers, it is appropriate that new ideas concerning floodplain origins are tested in this classic and important river. This paper describes the Holocene history of floodplain development in the southern Lower Mississippi Valley and suggests that the patterns and processes of this construction differed substantially from Fisk’s views of simple meander-belt development and overbank deposition (Fisk 1947). These new ideas are based on a detailed examination of the sedimentologic, stratigraphic, and soil characteristics of overbank sediments, which preserve a much more complete history of floodplain development than the meanderbelt deposits. Our analysis of these deposits suggests that Holocene floodplain processes and fluvial styles in the Lower Mississippi Valley changed in response to decreasing rates of floodplain sediment accumulation and decelerating sea-level rise and that avulsion played a major role in floodplain construction during the Holocene transgression. Lastly, we compare the Holocene floodplain history of the Mississippi River to several examples of modern and ancient floodplains to suggest that new ideas of finegrained Mississippi River floodplain construction may be applicable to alluvial deposits elsewhere. GEOLOGIC SETTING AND STUDY AREA

The Lower Mississippi Valley extends ; 1000 km south from Cairo, Illinois to the Gulf of Mexico and ranges in width from 30 to 100 km. The floodplain contains five Holocene Mississippi River meander belts, Holocene backswamps and the deltaic plain, and Wisconsin valley trains (Fig. 1; Saucier and Snead 1989). In the southern Lower Mississippi Valley, Mississippi River meander belts are 5–15 km wide and consist of sinuous

HOLOCENE MISSISSIPPI RIVER FLOODPLAIN DEVELOPMENT active and abandoned channels and point bars with arcuate ridges and swales (Fig. 2). Meander-belt deposits are sand bodies 20–30 m thick that incise backswamp muds or valley-train sands and gravels (Fig. 3; Fisk 1944; Saucier 1974, 1994; Saucier and Snead 1989; Autin et al. 1991). Backswamps are floodplain depressions located between meander belts and contain freshwater swamps, crevasse-splay complexes, small streams, and, near the coast, shallow freshwater lakes. Backswamps are typically underlain by clays, silts, and sands (Fisk 1947; Coleman 1966; Krinitzsky and Smith 1969; Farrell 1987), and these deposits form a downvalley-thickening wedge of fine-grained alluvial sediments (i.e., topstratum deposits of Fisk 1944) that pass into alluvial, deltaic, and shallow-marine deposits near the coast (Autin et al. 1991). This sediment wedge is ; 10 m thick in the northern Lower Mississippi Valley and ; 40 m thick beneath the delta plain, and overlies probable Late Wisconsin valley-train deposits (i.e., substratum deposits of Fisk 1944) (Fisk 1947; Autin et al. 1991; Saucier 1994, 1996). Regional age estimates based on radiocarbon dates on peats and archaeological age correlations indicate that the fine-grained fluvial sediments are younger than ; 10,000 yr B.P. (McFarlan 1961; Saucier 1994). Investigations by Fisk (1938, 1940, 1944, 1947) were the first to provide detailed accounts of changes in Mississippi River regime during the late Pleistocene and Holocene. Fisk recognized that large braided channel systems (i.e., valley trains of Autin et al. 1991) in the Lower Mississippi Valley represent late Pleistocene Mississippi River courses, and he suggested that this fluvial regime coincided with periods of lower-than-present sea level, which produced steep channel gradients and increased river competence. Fisk (1944) further suggested that as sea level rose during the Holocene, the decrease in channel gradient and river competence led to development of a meandering regime and vertical accretion of fine-grained overbank deposits. Subsequent investigations by Saucier (1974, 1981, 1994, 1996) and others have shown that the transition from braided to meandering regimes was not caused by sea-level change as suggested by Fisk but instead reflected the northward retreat of the continental ice sheet and eastward diversion of glacial meltwater and outwash from the Lower Mississippi Valley to the St. Lawrence River lowlands ; 10–11 ka (Porter and Guccione 1994). This diversion significantly reduced Mississippi River sediment and water discharge (Teller 1990) and ended the construction of Mississippi River valley trains in the Lower Mississippi Valley. Subsequently during the Holocene, the Mississippi River transported a greater proportion of suspended sediment, which led to the accumulation of finegrained floodplain deposits and the development of a meandering regime (Autin et al. 1991). Following the initial studies by H.N. Fisk, investigations of fine-grained Holocene floodplain deposits have shown that these sediments represent a variety of environments, including crevasse splays and distributary channels, shallow lakes, and well and poorly drained backswamps (Coleman 1966; Krinitzsky and Smith 1969; Smith et al. 1986; Farrell 1987; Saucier 1994). The alluvial architecture and sedimentologic characteristics of these overbank deposits also show that floodplain environments have changed frequently during the Holocene (e.g., Coleman 1966) and provide detailed records of processes such as development of meander belts and progradation of natural levees and crevasse splays (e.g., Farrell 1987). In general, however, these studies do not discuss widespread changes in floodplain processes during the Holocene or the transition by the Mississippi River from Pleistocene valley trains to the present-day meandering regime. This study seeks to address these topics of Holocene floodplain development. Mississippi River floodplain deposits were studied near Ferriday, Louisiana, which is located ; 300 km upvalley from the Gulf of Mexico and across the river from Natchez, Mississippi (Figs. 1, 4). Prior geologic investigations in this area include studies by Fisk (1944, 1947) and mapping by Saucier (1967), and this area was chosen for detailed study because Fisk’s interpretations of the Mississippi River floodplain evolution drew heavily from his analysis of this region. The floodplain near Ferriday is 40–50 km wide and elevations range between 14 and 20 m (40 and 65

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feet) above sea level. The three youngest Holocene Mississippi River meander belts (meander belts 1–3 of Saucier and Snead 1989) are present at the floodplain surface and show well expressed crosscutting relationships. Meander-belt channels incise buried Pleistocene valley-train sands, and fine-grained backswamp deposits average ; 20 m in thickness (Fig. 3; Fisk 1944; Saucier 1967). METHODS

The floodplain history near Ferriday was studied using 102 cores that represent ; 1000 m of floodplain deposits. Cores were acquired with a hydraulic soil probe that recovered sediments to depths of up to 15 m, and a single 23-m-deep core was acquired using a wet rotary drill rig. The floodplain was sampled along topographic transects that began at point-bar ridge crests of each meander belt and extended across adjacent channels, natural levees, and backswamps perpendicular to channel flow. The spacing between the cores ranged between 0.2 and 3.0 km, and the average spacing was 1.0 km. Cores were described in the field and subsampled for grain-size, mineralogic, and petrographic analyses. Soil features such as horizons, matrix and mottle colors, soil structures, bioturbation features, slickensides, and nodules were described using standard terminology and methods outlined in Birkeland (1984). Sand fractions were determined using sieves, and clay and silt fractions were calculated using settling velocities and oven-dried weights of volumetrically calibrated pipette samples. The clay mineralogy of selected samples was determined by preparing oriented clay samples on ceramic tiles and analyzing diffraction patterns with a Scintag x-ray diffractometer (Cu target). Clay minerals were identified by successively air drying, glycolating, and heating each sample to 5008C for 1 hour. Oriented petrographic thin sections of selected samples were vacuum impregnated with epoxy. One organic-rich bulk sediment sample and several samples of wood fragments were dated at Livermore Laboratory using accelerator mass spectrometry radiocarbon dating. Fluvial landform dimensions and sediment-body geometries were determined using a combination of aerial photographs, topographic maps, floodplain cores, and water well and engineering borehole data archived at the Louisiana Department of Transportation in Baton Rouge, Louisiana and the U.S. Army Corps of Engineers Waterways Experiment Station in Vicksburg, Mississippi. Widths of the meander belts were measured from topographic maps and supplemented by field observations. Measurements were made perpendicular to the meander-belt axis, and thicknesses of meander-belt sand bodies were estimated on the basis of the depths of active and abandoned Mississippi River channels (Fisk 1944; Saucier 1967). For instance, the present-day Mississippi River at Natchez, Mississippi is ; 30 m deep, which indicates that the thickness of the meander-belt sand body is 30 m. The width and thickness of buried sand bodies was determined by the drilling program, augmented by regional water-well and engineering borehole data. The width of elongate or linear sand bodies was measured perpendicular to the long axis of the deposits, and thicknesses were measured directly from the cores. Typically the measured thicknesses were minimums because the hydraulic soil probe cannot penetrate buried sand bodies that are thicker than 5 m. In instances where stratigraphic correlations were obvious, water-well and engineering borings were used to estimate a minimum thickness for the buried sand bodies. HOLOCENE MISSISSIPPI RIVER FLOODPLAIN DEPOSITS

Floodplain deposits near Ferriday are subdivided into Lower and Upper Holocene units on the basis of differences in sediment grain size, sedimentbody geometry, and the abundance of soil features (Fig. 5). The deposits record two stages of Holocene floodplain development and provide evi-

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A. ASLAN AND W.J. AUTIN

HOLOCENE MISSISSIPPI RIVER FLOODPLAIN DEVELOPMENT

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FIG. 2.—Geologic map showing an enlarged view of the Mississippi River floodplain. The map boundaries are shown in Figure 1 and the box marks the area shown in Figure 4. The location of the cross section X–X9 in Figure 3 is also shown. Modified from Saucier and Snead (1989).

dence for significant changes in fluvial style and processes of floodplain construction. Lower Holocene Floodplain Deposits Lower Holocene deposits range in thickness from 6 to 21 m, sharply overlie Late Wisconsin sands (Figs. 6, 7A), and consist primarily of muds, which encase sand bodies of varying sizes and shapes. The muds and sands are interpreted as a mosaic of small floodplain streams, lakes, crevassesplay complexes, and poorly drained backswamps. Lacustrine and Backswamp Muds.—Lower Holocene muds consist of dark gray laminated muds, blue-gray bioturbated muds with olive to pale brown mottles, and small amounts of gray muds with yellow-brown mottles (Fig. 5). Stratigraphic relations show that these lithofacies merge laterally (Fig. 6). Dark gray muds consist of centimeter-scale, normally graded beds

of laminated silt and clay with sandy interbeds. Rare cylindrical and slightly sinuous, mud-filled burrows (up to 2 cm in diameter) are oriented subvertically and locally disrupt bedding. Authigenic minerals in the muds include common olive siderite nodules (, 2 cm in diameter) and clayey tabular siderite accumulations (, 5 cm thick) that parallel bedding (Fig. 7B). Disseminated pyrite is present within burrow fills and in clays along the upper and lower boundaries of the tabular siderite. Blue-gray muds are bioturbated and show rare evidence of primary stratification. Bioturbation features are roots, olive to pale brown root mottles, and rare cylindrical burrows that are similar to those in the dark gray muds. Blue-gray muds also contain wood and leaf fragments and gastropod shells, which are present locally in clay-rich zones that are up to 5 cm thick. Calcite nodules are common in the muds, and the nodules range in size from 2 to 10 mm in diameter. Clusters of small (; 1 mm in diameter) light blue vivianite nodules are also present locally within the blue-gray

← FIG. 1.—A) Map showing the Mississippi River drainage basin. B) Geologic map of the southern Lower Mississippi Valley showing the distribution of Holocene Mississippi River meander belts, backswamps, the deltaic plain, and Wisconsin valley trains. Modified from Saucier and Snead (1989).

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FIG. 3.—Northwest–southeast stratigraphic cross section across the Lower Mississippi Valley showing the distribution of Quaternary and Tertiary deposits near Ferriday, Louisiana. Holocene backswamp clays and silts overlie probable Late Wisconsin sands and gravels. Holocene Mississippi River meander-belt channels incise older backswamp muds and Wisconsin deposits. The approximate location of Holocene–Late Wisconsin boundary is based on the depth of the present-day Mississippi River channel and the assumption that the meander-belt sheet sands formed by lateral channel migration. Modified from Fisk (1944).

mud. The vivianite nodules change from a white to a light blue color upon core extrusion and oxidation (Krinitzsky and Smith 1969). Gray mottled muds are less common than dark gray and blue-gray muds and contain yellow-brown mottles and root traces, iron and calcite nodules, and slickensides. Small iron nodules (1–2 mm in diameter) are more abundant than calcite nodules, which range in diameter from 5 to 20 mm. Slickensides are shiny intersecting fractures with smooth, clayey surfaces. Interpretation.—The dark gray laminated muds are interpreted as lacustrine deposits on the basis of their similarity to lacustrine sediments in the Atchafalaya Basin, which is located south of Ferriday (Fig. 1; Coleman 1966; Krinitzsky and Smith 1969; Smith et al. 1986; Tye and Coleman 1989a) The scarcity of burrows and the abundance of laminae in the dark gray muds probably reflect rapid sediment accumulation rather than anoxic lake conditions, judging by the presence of common bioturbation features in the other facies (e.g., the blue-gray muds) (Coleman 1966). The thickness of the dark gray muds indicates that the lakes were shallow, probably up to several meters deep. The clay-rich texture, color, vivianite nodules, and abundance of bioturbation features and organic remains suggest that the blue-gray muds represent poorly drained backswamps (sensu Coleman 1966). Poorly drained backswamps in the Atchafalaya Basin contain similar bioturbated muds (Coleman 1966; Krinitzsky and Smith 1969; Tye and Coleman 1989a) and support water-tolerant woody vegetation such as cypress and gums. The presence of authigenic pyrite, vivianite, and siderite in the dark gray and blue-gray muds further suggests that the lake and poorly drained swamp pore waters and sediments were chemically reducing (Coleman 1966; Ho and Coleman 1969). Vivianite and siderite precipitate under strongly reducing conditions that accompany methane formation (Berner 1981), and the presence of pyrite in the muds suggests that pore waters were sulfate rich. Although the presence of pyrite can represent marine influences within alluvial–deltaic environments, there are elevated concentrations of sulfate in ground waters of the Mississippi River alluvial aquifer

several hundred kilometers inland of the Gulf of Mexico (Whitfield 1975; Dalsin 1978). High sulfate concentrations are attributed to compaction of buried Tertiary marine sediments and buried salt domes (Whitfield 1975). Because Holocene sediments with marine molluscs pinch out upvalley at the latitude of Donaldsonville, Louisiana (. 100 km south of Ferriday), the sulfate in the sediment pore waters near Ferriday is probably related to the regional ground-water chemistry rather than to Holocene marine transgressions (Smith et al. 1986; Bailey et al. 1998). Authigenic siderite and pyrite are also present in shallow (, 6 m deep) backswamp and marsh deposits in the Atchafalaya Basin, and these deposits are younger than 1500 yr B.P. (Moore et al. 1992). The shallow depth and young age of these deposits suggest that the pyrite and siderite in the floodplain deposits near Ferriday formed in near-surface reducing environments rather than during burial. The clay-rich texture, yellow-brown root traces and mottles, and iron and calcite nodules indicate that the gray mottled muds represent well drained backswamps (sensu Coleman 1966). The abundance of yellowbrown mottles and iron nodules and the absence of minerals such as pyrite and vivianite further suggest that these deposits accumulated in better drained environments than those represented by the lacustrine and poorly drained backswamp sediments (Coleman 1966; Krinitzsky and Smith 1969). Crevasse-Splay and Lacustrine Sands.—These sand bodies consist of very fine to medium-grained quartz-rich sheet sands that are surrounded by either backswamp or lacustrine muds (Fig. 6). The sheet sands are 1–3 m thick, consist of beds that are tens of centimeters thick, have either sharp or gradational lower boundaries, and extend laterally for up to 1 km (Figs. 6, 8). Within vertical sequences, the sands may either fine or coarsen upward, and primary stratification is generally poorly preserved but, where present, consists of wavy laminae. Interpretation.—The thicknesses, widths, and stratigraphic positions of the sands indicate that the sediments are crevasse-splay and lacustrine-delta


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FIG. 4.—Geologic map of the Ferriday area showing the distribution of Holocene Mississippi River meander belts and backswamps, and locations of core sampling sites. Note that compared to meander-belt sand bodies, subsurface floodplain channel sands (open circles) are extremely narrow. Locations of cross sections in Figure 6 are also shown.

deposits. Sands that overlie the bioturbated blue-gray and laminated dark gray muds are similar in thickness and stratigraphic position to crevassesplay deposits (Farrell 1987) and lacustrine-delta sands, respectively, in the Atchafalaya Basin (Breland et al. 1988; Tye and Coleman 1989a). Crevasse-splay sands typically overlie poorly drained backswamp muds (Farrell 1987), whereas lacustrine-delta sands overlie and show gradational boundaries with laminated lacustrine muds (Fig. 8; Tye and Coleman 1989a). Floodplain Channel Sands.—Lower Holocene sands are also represented by sand bodies that are 0.5–1.0 km wide and several kilometers long measured parallel to the valley axis, and have minimum thicknesses of 5– 8 m (Figs. 4, 6). The upper surfaces of these sands are commonly present between 9 and 11 m above mean sea level. In several instances, sand wedges representing probable natural levees are present directly above and extend laterally beyond the edges of the floodplain channel sands (Fig. 6). Water-well and engineering borings show that floodplain channel deposits are also present south of Ferriday (Saucier 1967). Neither the subsurface data near Ferriday nor the regional borehole data (e.g., Fisk 1944; Saucier 1967, 1994; Krinitzsky and Smith 1969; Smith et al. 1986), however, show evidence of subsurface sand bodies that are comparable in width to the 5– 15-km-wide Mississippi River meander belts at the floodplain surface. Interpretation.—Stratigraphic relationships and sand-body dimensions suggest that the sands represent small, sinuous streams that migrated laterally over distances of hundreds of meters as indicated by the width (0.5– 1.0 km) of the sands. However, lateral migration of the present-day Mis-

sissippi River produces sand bodies that are 5–15 km wide and 20–30 m thick; this indicates that Lower Holocene floodplain channels were smaller than the modern channel and less laterally mobile. Because upper surfaces of the small floodplain channel sands are present at similar elevations, the sands are interpreted as deposits of multichannel streams with a probable anastomosed pattern. Analogous modern streams are present in the Atchafalaya Basin (Smith et al. 1986). Upper Holocene Floodplain Deposits Upper Holocene floodplain deposits consist of meander-belt tabular sands, natural-levee silt and sand wedges, and backswamp muds (Figs. 5, 6). Soil features such as roots, mottles, clay films, slickensides, and calcite and iron nodules are much more common in Upper Holocene deposits than in Lower Holocene sediments (Aslan and Autin 1998). Meander-Belt Sands.—Mississippi River meander-belt sand bodies consist of silt to medium-grained quartz-rich sands that accumulated on point bars as the river migrated laterally across the floodplain (Fig. 4). Relief between scroll-bar crests and swale troughs is 3–5 m, and crests are represented by 1–2 m of bioturbated silts and sands that are underlain by fine to medium sand with abundant small-scale cross-stratification. Swales contain several meters of either mud or interbedded mud and sand, which overlie sand and interbedded mud. Topographic maps and floodplain borings show that the dimensions of Mississippi River meander belts in the Ferriday region differ significantly,

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FIG. 5.—Stratigraphic section of a 21-m-long backswamp core showing vertical changes in lithologies and interpreted sedimentary environments for Upper and Lower Holocene deposits. For grain-size descriptions, m 5 mud, s 5 sand, and g 5 gravel.

and differences in channel widths, numbers of abandoned channels, and the complexity of crosscutting relationships among scroll-bar sets are used to subdivide meander belts into simple and complex forms (Table 1). Simple meander belts are characterized by few and narrow (, 1 km wide) abandoned channels and meanders that contain few neck cutoffs and lack complex crosscutting relationships among scroll bars. Examples of simple

meander belts include the Mississippi River meander belt 3 near Ferriday, Louisiana and Mississippi River meander belt 4 located west of Vicksburg, Mississippi (Figs. 2, 5). The widths of simple meander belts are 5–10 km, and engineering borings show that the thickness of simple meander belt sand bodies is ; 20 m (Fisk 1947). Complex Mississippi River meander belts contain numerous and wide →

FIG. 6.—A, B) Stratigraphic cross sections through backswamps near Ferriday showing Holocene floodplain sediments including crevasse-splay and floodplain-channel sand bodies. Note that the tops of the floodplain-channel sands are present at approximately the same elevation. The 5320 6 90 yr B.P. radiocarbon date shown in A is from a large wood fragment present in laminated muds beneath natural levee deposits of Holocene Mississippi River meander belt 3.


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FIG. 7.—Floodplain cores from the Ferriday area. A) Abrupt contact (arrow) between Lower Holocene dark gray muds (m) and probable Late Wisconsin sands (s). Core depth is 2060–2080 cm below the floodplain surface. B) Dark gray laminated silts and clays with organic-rich beds (o) and light gray tabular zones of siderite-rich clay (arrows). The silts and clays are Lower Holocene lacustrine deposits. Core depth is 1302–1328 cm below the floodplain surface. C) Brown mottled natural-levee silts and sands of Holocene Mississippi River meander belt 1. Organic matter (o) is abundant in the upper 10 cm, and earthworm-pelleted burrows (b) are concentrated between 10 and 20 cm in the core. The silts and sands are massive (m) and contain common root mottles below 20 cm. Core depth is 3–28 cm below the floodplain surface.

(. 1 km wide) abandoned channels, and meanders show common neck cutoffs and complex crosscutting relationships among scroll bars. Mississippi River meander belts 1 and 2 near Ferriday are examples of complex forms (Figs. 2, 4). The widths of the complex meander belts are 10–15 km, and the sand bodies are ; 30 m thick, judging by engineering borings and the depth of the present-day Mississippi channel at Natchez, Mississippi (Fisk 1944). Natural-Levee and Backswamp Deposits.—Silt and sand wedges representing natural levees are located adjacent to Mississippi River channels and dip towards backswamps and point-bar swales. Near channel margins, natural-levee deposits are 3–5 m thick and thin to less than 1 m over distances of 2–3 km measured perpendicular to the direction of channel flow. Natural-levee deposits contain abundant soil features, which are described below. Backswamp sediments consist of gray clay and silt, encase crevasse-splay sheet sands, and are typically 5–10 m thick (Fig. 6). Clays and silts contain common yellow-brown mottles, iron and calcite nodules, and slickensides, and represent well drained backswamp deposits (sensu Coleman 1966). Floodplain Soils.—Sandy and silty soils developed on meander belt 1 point-bar ridges and natural levees are moderately well drained Inceptisols (Figs. 7C, 9A). Profiles typically consist of dark brown, organic-rich, bioturbated silts and sands (A horizon) that overlie brown silts and sands with a subangular blocky structure and common earthworm-pelleted burrows, roots, yellow-brown mottles, and iron nodules (Bw horizon). The base of the profiles consists of gray silts and sands with roots, yellow-brown mottles, and relict stratification, but lacks soil structure (Cg horizon). In contrast to meander belt 1 soils, older meander belt 2 and 3 soils are moderately developed Alfisols (Fig. 9B; USDA 1988). Alfisols contain ; 50cm-thick, clay-enriched B horizons (i.e., Bt horizons), and clay films coat

ped faces and soil pores. These features are generally absent from younger meander belt 1 soils. Clayey backswamp soils are poorly drained, smectite-rich Vertisols (Fig. 9C; USDA 1988). The profiles consist of a dark gray, organic-rich clay (A horizon) that overlies gray clay and silt with many yellow-brown mottles, root traces, iron and calcite nodules, and slickensides (Bkg horizon). The base of the profile (Ckg horizon) consists of gray clay and silt that is similar to the overlying B horizon but which lacks slickensides and soil structure. Floodplain Chronology Radiocarbon dating and stratigraphic age correlations indicate that Lower Holocene deposits near Ferriday range in age from ; 5000 to ; 10,000 yr B.P. and Upper Holocene deposits are younger than ; 5000 yr B.P. Radiocarbon dating of a bulk sediment sample of Lower Holocene mud immediately overlying the Late Wisconsin sands at Ferriday produced a date of 16,600 6 140 yr B.P. (Table 2; Fig. 6A). Because the organic particles from the base of the floodplain muds are detrital, the 16,600 6 140 yr B.P. age is interpreted as a maximum age for the basal floodplain muds. Regional age estimates based on radiocarbon-dated peats that overlie Late Wisconsin deposits beneath the delta plain indicate that the basal Lower Holocene muds are ; 10,000 years old (McFarlan 1961; Saucier 1994). Radiocarbon analysis of two large wood fragments located one meter below the highest stratigraphic occurrence of the dark gray laminated muds (Fig. 6A) produced dates of 5320 6 90 yr and 5400 6 70 yr B.P. (Table 2), which suggest that ; 5000 yr B.P. is a reasonable minimum age for the Lower Holocene floodplain deposits near Ferriday. The dated wood fragments are encased in backswamp muds, and because present-day backswamps are densely vegetated and have cohesive muddy substrates,


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FIG. 8.—Stratigraphic sections of typical A) crevasse-splay and B) lacustrine deposits. Note that the crevasse-splay deposits overlie backswamp sediments whereas the lacustrine-delta sands and silts overlie laminated lacustrine muds.

fallen and partially rotted pieces of wood are not likely to have been transported substantial distances. The dark gray laminated muds are stratigraphically overlain by brown silts and sands that represent natural-levee deposits of Mississippi River meander belt 3 (Fig. 6A), which is the oldest meander belt in the study area. The radiocarbon ages indicate that meander belt 3 is younger than 5000 yr B.P., and this age is slightly less than the maximum age (6200 yr B.P.) suggested previously for Mississippi River meander belt 3 (Saucier 1994).

that floodplain environments and depositional processes changed substantially during the Holocene and that the establishment of large meandering Mississippi River channels in the southern Lower Mississippi Valley occurred less than 5000 yr B.P. Vertical changes in the abundance of soil features and radiocarbon ages of the deposits further suggest that floodplain changes coincided with decreasing rates of sediment accumulation and decelerating sea-level rise. Early Holocene Floodplain Development

HOLOCENE FLOODPLAIN EVOLUTION

Differences in the alluvial architecture and pedogenic characteristics of Lower and Upper Holocene Mississippi River deposits near Ferriday show TABLE 1.—Summary of Holocene Mississippi River meander-belt characteristics in the Ferriday area.

Meanderbelt Type Simple Complex

Abandoned channel width (km)

Number of abandoned channels

Meander-belt1 width (km)

Minimum thickness 2 of meander-belt sheet sand (m)

0.5–1.0 1.0–2.0

Few Many

5–10 10–15

20 30

1

1 Widths of abandoned channels and meander belts were measured using data from Fisk (1944) and Saucier (1967, 1994), supplemented by shallow borehole data (this study). 2 Thicknesses of meander-belt sand bodies are minimums. Data are from Fisk (1944) and Saucier (1967).

Lithologic similarities between Lower Holocene deposits near Ferriday and recent sediments in the Atchafalaya Basin suggest that Early Holocene lakes and poorly drained backswamps filled rapidly through a combination of crevassing, lacustrine sedimentation, and avulsion, similar to historic sedimentation in the Atchafalaya Basin (Fig. 10A, B; Krinitzsky and Smith 1969; Smith et al. 1986; Tye and Coleman 1989a, 1989b). For instance, Tye and Coleman (1989b) documented that up to 5 m of lacustrine delta and crevasse-splay muds and sands filled 470 km2 of the Atchafalaya lakes and swamps in the Lake Fausse Pointe region since 1917. Rapid sedimentation was initiated by the growth of the Atchafalaya River up valley in the late 1800s and early 1900s (Fisk 1952; Smith et al. 1986; Tye and Coleman 1989b). As the Atchafalaya River grew, increasingly large volumes of sediments were diverted from the Mississippi River and deposited in the Atchafalaya lakes and swamps. An especially important aspect of the historic sedimentation in the Atchafalaya Basin is that the sediments

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FIG. 9.—Schematic sections of typical Holocene floodplain soils in the Ferriday area. A) Point-bar ridge profile from meander belt 1, B) point-bar ridge profile from meander belt 2, and C) backswamp profile. A 5 A horizon, Bw 5 cambic B horizon, Bt 5 argillic B horizon, Bkg 5 calcic gleyed B horizon, Cg 5 gleyed C horizon, and Ckg 5 calcic gleyed C horizon.

accumulated at all stages of Mississippi River flow, rather than only during infrequent overbank flows. Floodplain borings from the Atchafalaya Basin further demonstrate that this style of rapid floodplain sedimentation and lake filling has occurred repeatedly during the Holocene (Coleman 1966; Krinitzsky and Smith 1969; Smith et al. 1986; Tye and Coleman 1989b). The absence of buried meander-belt sheet sands near Ferriday also suggests that overbank flooding of large Mississippi River channels was not primarily responsible for the accumulation of the fine-grained Lower Holocene deposits. Instead, the lateral variability of sediment grain sizes is more consistent with deposition during episodes of crevassing and avulsion (Smith et al. 1989; Willis and Behrensmeyer 1994). According to this view, TABLE 2.—Summary data on radiocarbon samples from Mississippi River floodplain deposits near Ferriday, Louisiana. Sample Location Concordia P. T7N R8E Sec. 31 Concordia P. T7N R9E Sec. 13

CAMS1 Number 7190 7191

Material sampled Root fragment Wood fragment

Depositional Environment

14

C age (yr)

Backswamp

340

Backswamp beneath Miss. R. meander belt 3 natural levee Backswamp beneath Miss. R. meander belt 1 natural levee Lake

920

5,320 6 90

485

780 6 70

840

5,400 6 70

Concordia P. T8N R9E Sec. 55

7192

Root fragment

Concordia P. T7N R9E Sec. 49 Concordia P. T7N R9E Sec. 49

7193

Wood fragment Bulk sample Lake

7194

Depth (cm)

2,005

Modern

16,600 6 140

Interpretation Minimum age Time of deposition

Minimum age

Time of deposition Maximum age

1 CAMS Number is the sample number archived at the Center for Accelerator Mass Spectrometry at Lawrence Livermore National Lab.

avulsion of small floodplain streams, represented by the Lower Holocene sand bodies, played an important role in the deposition of fine-grained floodplain sediments. Although quantitative estimates of sediment accumulation rates in the study area are poorly constrained, the scarcity of soil features in the Lower Holocene deposits indicates that these sediments accumulated rapidly. Radiocarbon dates on peats from the delta plain south of Ferriday confirm that the Early Holocene (. 5000 yr B.P.) was a time of rapid sediment accumulation in the southern Lower Mississippi Valley (Fig. 11A; McFarlan 1961; Frazier 1967, 1974; Roberts et al. 1991). Rapid accumulation of silts and clays and poor drainage inhibited soil formation and lateral channel migration and led to the development of multichannel streams (Fig. 10B). Recent field studies and computer models of alluvial stratigraphy also suggest that avulsion frequency increases as rates of floodplain aggradation increase (To¨rnqvist 1994; Bryant et al. 1995; Heller and Paola 1996), which supports our interpretation that avulsion was an especially important process of Lower Holocene floodplain construction. Similarities in the timing of rapid floodplain aggradation and sea-level rise in the Gulf of Mexico suggest that sea level significantly influenced floodplain processes and development as far up valley as Ferriday (; 300 km) (Fig. 11B). According to this interpretation, Lower Holocene lake and poorly drained backswamp deposits near Ferriday as well as those present in the Atchafalaya Basin (Tye and Coleman 1989b), accumulated in response to rapid sea-level rise and increased availability of accommodation. Furthermore, the late Pleistocene to Holocene decrease in Mississippi River sediment and water discharge argues against the possibility that Early Holocene aggradation near Ferriday represents a climatically driven increase in sediment supply (Saucier 1994). Lastly, rapid floodplain aggradation and the alluvial architecture of the Lower Holocene deposits is also consistent with theoretical models of base-level effects on alluvial stratigraphy (Allen


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FIG. 10.—Schematic block diagrams summarizing Holocene floodplain development near Ferriday. A) The Early Holocene floodplain was initially represented by lakes and poorly drained backswamps. B) Multichannel floodplain streams, crevasse splays, and lacustrine deltas filled the lakes and poorly drained backswamps as the floodplain rapidly aggraded during the Early Holocene. C) Slower rates of floodplain aggradation during the Late Holocene led to the development of simple, followed by complex Mississippi River meander belts less than 5000 yr B.P. Meander-belt development was accompanied by overbank deposition and soil formation.

FIG. 11.—A) Scatter plot showing the ages and depths of buried peats present in Holocene floodplain and delta-plain deposits located between the latitudes of Baton Rouge and the Louisiana coast. The data show that Holocene rates of sediment accumulation decreased in the southern Lower Mississippi Valley. Data are from McFarlan (1961), Frazier (1967, 1974), Krinitzsky and Smith (1969), and Roberts et al. (1991). B) Late Quaternary sea-level curve for the Gulf of Mexico. From Frazier (1974).

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1978; Leeder 1978; Bridge and Leeder 1979; Friend et al. 1979; Wright and Marriott 1993). The models predict that base-level rise and a constant sediment supply produces isolated sand bodies that are encased by finegrained alluvial deposits with few soil features, similar to Lower Holocene deposits near Ferriday. Late Holocene Floodplain Development Upper Holocene floodplain deposits near Ferriday reflect a combination of lateral channel migration and meander-belt construction, overbank deposition, and soil formation (Fig. 10C). The abundance of Mississippi River point bars with well developed scroll bars demonstrates the importance of lateral accretion during this stage of Holocene floodplain construction (Fisk 1944). Silty and sandy natural-levee deposits that pass laterally into clayey backswamp deposits also show that overbank flooding of large Mississippi River channels contributed to fine-grained floodplain sedimentation (Kesel et al. 1974). The abundance of soil features in the Upper Holocene deposits and the inferred decrease in floodplain aggradation rates also provide insights on the development of the modern meandering regime of the Mississippi River. Mississippi River Meander-Belt Evolution.—Although the dimensions of Holocene Mississippi River meander belts vary considerably, the origins and significance of these differences are poorly understood (Saucier 1994, 1996). Possible origins for the simple meander belts include (1) reduced discharges caused by Holocene climate change, and (2) divided flow between coexisting meander belts (Autin et al. 1991; Saucier 1994, 1996). Paleoclimate studies indicate that the mid-Holocene was characterized by warmer and drier conditions than exist today in parts of the western, midwestern, and eastern U.S. (i.e., the Hypsithermal event) (Autin et al. 1991). Warmer and drier conditions could have reduced Mississippi River discharge in the Lower Mississippi Valley and led to the development of smaller channels and meander belts than the modern Mississippi River meander belt. A climatic origin for the simple meander belts, however, is difficult to evaluate because of poor constraints on the ages of the simple meander belts and because the timing of the mid-Holocene warming and drying varied regionally (Autin et al. 1991; Saucier 1994, 1996). In contrast to a climatic origin, the chronology of the Holocene Mississippi River meander belts and subdeltas in the southern Lower Mississippi Valley supports the idea that the simple meander belts represent periods of divided flow (Frazier 1967, 1974; Autin et al. 1991; Saucier 1996). Frazier’s studies of the Mississippi River delta show that two and perhaps as many as three Holocene subdeltas were active simultaneously (Frazier 1967, 1974), which requires that more than one meander belt was active. The relative ages of the simple and complex meander belts located south of Vicksburg, Mississippi further suggest that the meandering regime of the present-day Mississippi River evolved gradually from a time of divided flow, represented by the simple meander belts, to a large, single-channel meandering system. According to this interpretation, the simple meander belts (e.g., meander belts 3 and 4) represent early but short-lived attempts to establish a large, meandering Mississippi River channel. Evidence that simple meander belts represent short periods of fluvial activity include the small number of abandoned channels and the scarcity of complex crosscutting relationships among scroll bar sets (Table 1). The initial development of simple meander belts near Ferriday less than ; 5000 yr B.P. also coincides with decreasing rates of sediment accumulation in the Lower Mississippi Valley and decelerating sea-level rise (Fig. 11). These observations are consistent with the idea that slower floodplain aggradation favored lateral channel migration and the development of Mississippi River meander belts. As rates of sediment accumulation and sea-level rise continued to slow during the latest Holocene, divided flow among simple meander belts was replaced by the development of large meandering channels capable of transporting the entire Lower Mississippi Valley discharge. Lateral migration of

these channels produced complex Mississippi River meander belts (e.g., meander belts 1 and 2) and thick tabular sands, and the development of these features coincides with Holocene sea-level highstand conditions. Differences between the alluvial architecture of the Upper and Lower Holocene deposits are also consistent with theoretical models of alluvial stratigraphy, which predict that periods of constant base level or slow rise lead to the development of large sheet sands and alluvial soils (Allen 1978; Leeder 1978; Bridge and Leeder 1979; Friend et al. 1979; Posamentier and Vail 1988; Wright and Marriott 1993). ORIGINS OF FINE-GRAINED FLOODPLAINS

Interpretations of the Holocene Mississippi River floodplain sediments near Ferriday differ from prior studies, which suggest that most of the clays and silts represent repeated episodes of overbank flooding of large Mississippi River channels (Fisk 1944, 1947; Kesel et al. 1974; Kesel et al. 1992; Saucier 1981, 1994; Guccione 1993). According to this model of finegrained floodplain sedimentation, rates of short-term sediment accumulation, grain size, and bed thickness decrease systematically away from the master channel (Fig. 12A). Fisk’s investigations of the Mississippi River contributed greatly towards developing this view of fine-grained meandering-river sedimentation (Fisk 1944, 1947), and this model is used widely for interpreting modern and ancient fluvial deposits (Wolman and Leopold 1957; Allen 1965; Blake and Ollier 1971; Bridge and Leeder 1979; Bridge 1984; Bown and Kraus 1987; Nanson and Croke 1992; Miall 1992; Zwolinski 1992). The alluvial architecture and soil characteristics of Holocene Mississippi River deposits near Ferriday as well as those present in the Atchafalaya Basin, however, show that most of the fine-grained sediments are deposited rapidly in regional floodplain depressions during episodes of crevassing and avulsion (Fig. 12B). Because these processes deposit sediments on the floodplain at virtually all stages of trunk-channel flow, the sediments accumulate continuously rather than during infrequent floods. This condition leads to rapid sedimentation and filling of regional depressions (e.g., the Atchafalaya Basin). Once a flood basin is filled, subsequent avulsion initiates rapid sedimentation and aggradation elsewhere on the floodplain. Repetition of this sequence produces interfingering sheets or wedges of floodplain clays and silts and isolated sand bodies with little evidence of pedogenesis, similar to the Lower Holocene deposits, and packages of avulsion deposits typically thicken away from the active channel (Fig. 12B; Willis and Behrensmeyer 1994). This style of floodplain construction continues today in the Atchafalaya Basin (Tye and Coleman 1989a, 1989b) but was probably more common during the Holocene transgression and base-level rise. According to this view, large overbank floods are responsible for a smaller percentage of the total volume of Holocene muds in the southern Lower Mississippi Valley than generally recognized. Other Modern Rivers with Fine-Grained Floodplains Comparisons between Holocene Mississippi River deposits near Ferriday area and fine-grained floodplain deposits of the Saskatchewan River in Canada and the Rhine–Meuse River in the Netherlands suggest that the origin of fine-grained floodplain deposits in the Lower Mississippi Valley is similar to rivers elsewhere. The Saskatchewan River, Canada.—Although the Saskatchewan River of north-central Canada represents a different geological setting than the Mississippi River in Louisiana, studies by Smith et al. (1989) and Smith and Pe´rez-Arlucea (1994) show that processes of fine-grained sedimentation during periods of rapid aggradation are similar in these two fluvial systems. Glacial moraine dams have caused the Saskatchewan River floodplain to aggrade locally, and in the Cumberland Marshes rapid aggradation was accomplished by historic avulsion of the Saskatchewan River (Smith et al. 1989). The avulsion deposits cover ; 500 km2 and consist of sheets of mud that are 2–4 m thick and encase crevasse-splay sands and silts.


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FIG. 12.—Schematic floodplain maps and cross sections showing lateral changes in sediment grain sizes and sedimentation rates associated with two models of finegrained floodplain deposition. A) Floodplain deposits formed by overbank flooding and sedimentation. B) Floodplain deposits related to crevassing and avulsion. Modified from Smith et al. (1989) and Willis and Behrensmeyer (1994).

During the avulsion, crevasse-splay and lacustrine sediments filled shallow lakes and buried peaty wetlands located between alluvial ridges. In areas where the avulsion is complete, meandering channels are incising the avulsion deposits. Overbank flooding of the meandering channels deposits a thin veneer of silts and clays on top of the older avulsion deposits, but these sediments represent a small percentage of the total volume of finegrained floodplain sediments present in the region. Similarities between the alluvial architecture of the Lower Holocene Mississippi River deposits near Ferriday and the Saskatchewan River sediments suggest that these deposits accumulated through similar processes (Fig. 6; Smith et al. 1989, figs. 10 and 11). Although the Saskatchewan River example represents a single avulsion and fine-grained Mississippi River floodplain sediments represent a much longer period of aggradation, both examples indicate that large volumes of fine-grained deposits accumulate by crevasse-splay and lacustrine sedimentation related to avulsion rather than by overbank flooding of large trunk channels. The Rhine–Meuse River, The Netherlands.—Study of the Holocene history of the Rhine–Meuse River in the Netherlands also shows interesting similarities to the Holocene floodplain history in the southern Lower Mississippi Valley. To¨rnqvist (1993) demonstrated that the lower Rhine–Meuse River changed from an anastomosing to a meandering system during the Holocene and that this change in fluvial style coincided with decreasing rates of sea-level rise. The anastomosed river deposits consist of isolated sand bodies that are surrounded by muds and show little or no systematic

lateral change in sediment grain size with respect to major channel sands (Weerts and Bierkens 1993). In contrast to the older anastomosing-channel sediments, the meandering-river deposits are represented by wider and thicker sand bodies and the fine-grained sediments show a systematic decrease in sediment grain size with increasing distance from the meanderingchannel sands, which is consistent with an overbank origin. The vertical changes in the alluvial architecture of the Rhine–Meuse River described by To¨rnqvist (1993) are similar to those of the Mississippi River deposits, and in both of these coastal river examples, changes in fluvial style and floodplain development are closely linked to decreasing sediment accumulation rates and decelerating sea-level rise. Ancient Fine-Grained Floodplain Deposits Holocene Mississippi River floodplain deposits are also similar to several examples of ancient alluvial deposits (e.g., Ferm and Cavaroc 1968; Ethridge et al. 1981; Flores 1981; Gersib and McCabe 1981; Platt and Keller 1992; Willis and Behrensmeyer 1994). The similarities suggest that new information on Holocene floodplain construction in the Lower Mississippi Valley should be useful for interpreting ancient floodplains, and two examples are discussed briefly. Carboniferous alluvial deposits of the Port Hood Formation in Nova Scotia accumulated in an extensional basin and consist of several hundred meters of channel and crevasse-splay sandstones and lacustrine and back-

814


swamp mudrocks (Gersib and McCabe 1981). The fine-grained crevassesplay, lacustrine, and backswamp facies are strikingly similar to the Lower Holocene sediments near Ferriday, and the Port Hood deposits indicate that these Carboniferous floodplains aggraded rapidly through a combination of crevassing and lake filling. For instance, 5–10 m thick, coarsening-upward sequences in the Port Hood Formation commonly consist of ripple-laminated and massive crevasse-splay sandstones that overlie laminated lacustrine siltstones and rooted and slickensided mudrocks or coals (Gersib and McCabe 1981, fig. 9). This vertical succession indicates that periods of slow floodplain aggradation and soil formation alternated with episodes of flooding and rapid aggradation. Thicknesses of crevasse-splay and laminated lacustrine deposits further suggest that at least half of the fine-grained sands and muds were deposited by processes other than slow overbank sedimentation. Miocene floodplain deposits from the Chinji Formation (Siwaliks Group) of the Himalayan foredeep in Pakistan also provide evidence for patterns of floodplain development similar to those observed in the Atchafalaya Basin and the Mississippi River floodplain near Ferriday. Willis and Behrensmeyer (1994) showed that fine-grained floodplain deposits consist of 4–10 m thick packages of mudrocks and sandstones and the upper part of each package is a paleosol. These paleosol-bounded sedimentary packages form interfingering wedges that thin laterally, and the deposits do not show systematic increases in sediment grain size with increasing proximity to large meandering-channel sands, as would be expected if the sediments accumulated during overbank floods. Instead, Willis and Behrensmeyer (1994) suggest that the fine-grained sediments represent episodic and rapid deposition within regional floodplain depressions, analogous to the style of floodplain deposition reported by Tye and Coleman (1989a) as well as in the Ferriday area. While Willis and Behrensmeyer (1994) point out that the episodic floodplain sedimentation observed in the Chinji Formation is not new or unexpected, the stratigraphic sections show that this style of floodplain construction represents a significant proportion of the total volume of fine-grained alluvial deposits in the Chinji Formation (Willis and Behrensmeyer 1994, fig. 6). SUMMARY

Our studies as well as those cited in the previous section indicate that large quantities, and in some instances most muddy floodplain deposits do not represent repeated episodes of overbank flooding and widespread aggradation. Instead, during periods of base-level rise, fine-grained floodplain deposits accumulate rapidly and locally in floodplain depressions, probably during crevassing and avulsion. If base-level rise continues, filling of a depression is followed by avulsion and aggradation elsewhere on the floodplain. Repetition of this sequence of events produces interfingering sheets and wedges of fine-grained sediments that encase isolated sand bodies. Though the overbank model of construction of fine-grained floodplains was at least in part developed from studies of the Mississippi River, it seems likely that a large percentage of the floodplain sediments in the southern Lower Mississippi Valley accumulated through a combination of crevassing, lacustrine sedimentation, and avulsion. An important implication of this discussion is that common present-day processes of floodplain construction (e.g., lateral channel migration, overbank flooding) in coastal alluvial rivers may differ substantially from those that produced fine-grained floodplains during the Holocene transgression. Additionally, these observations are probably applicable in certain instances to ancient alluvial rocks that accumulated during episodes of base-level rise. ACKNOWLEDGMENTS

Research contributing to this paper was supported by the Donors of The Petroleum Research Fund, administered by the American Chemical Society, the Geological Society of America, and the Louisiana Geological Survey. We thank Mary Kraus, Norm Smith, and Torbjo¨rn To¨rnqvist for their stimulating discussions of avulsion.

Bo Bolurchi (Louisiana Dept. of Transportation) generously provided water well logs for Concordia Parish, Tom Stafford conducted the AMS dating, and F. Kring (Louisiana Geological Survey) provided invaluable assistance in the field. Reviews by JSR editor J. Macquaker, J.D. Collinson, and an anonymous reviewer, and editorial work by J.B. Southard significantly improved this paper. REFERENCES

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