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EvolutionoftheHoloceneMississippiRiverfloodplain,Ferriday,Louisiana:Insightsontheoriginoffine-grainedfloodplains

ARTICLEinJOURNALOFSEDIMENTARYRESEARCH·JULY1999

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AndresAslan

ColoradoMesaUniversity

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WhitneyJ.Autin

StateUniversityofNewYorkCollegeatBrock…

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

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 Holo-cene Mississippi River floodplain deposits in the southern Lower Mis-sissippi Valley provide evidence for significant changes in floodplaindevelopment in response to sea-level rise. Floodplain cores acquirednear Ferriday, Louisiana show that Holocene deposits consist of 15–30m (ave. ; 20 m) of sands, silts, and clays, which overlie Late Wisconsinsands and gravels. On the basis of differences in sediment grain size,sediment-body geometry, and the abundance of soil features, the Ho-locene deposits are subdivided into Lower and Upper Holocene units.

Lower Holocene deposits (. 5000 yr B.P.) consist of lacustrine andpoorly drained backswamp muds that contain authigenic siderite, py-rite, and vivianite and show little evidence of soil formation. Mudsencase crevasse-splay and floodplain-channel sand bodies (, 1 kmwide), and collectively these deposits represent a mosaic of shallowlakes, poorly drained backswamps, and multichannel streams, similarto modern examples in the Atchafalaya Basin (; 100 km south ofFerriday). Upper Holocene deposits (, 5000 yr B.P.) are representedby large Mississippi River meander-belt sand bodies that are up to 15km wide and 30 m thick. Natural-levee silts and sands and well drainedbackswamp muds are present between meander-belt sands. Upper Ho-locene deposits contain abundant soil features, and sandy and silty soilsare Entisols, Inceptisols, and Alfisols whereas clayey soils are Vertisols.

The presence of isolated sand bodies surrounded by mud and thescarcity of soil features suggest that Lower Holocene sediments reflecta period of rapid floodplain aggradation during which crevassing, la-custrine sedimentation, and avulsion dominated floodplain construc-tion. No evidence of large meandering Mississippi River channels rep-resented by buried, thick tabular sands exists near Ferriday, and dis-charge in Lower Mississippi Valley flow was probably conveyed by anetwork of small, multichannel floodplain streams. Upper Holocenesediments record a dramatic change ca. 5000 yr B.P. from rapid toslower floodplain aggradation, which was accompanied by extensivelateral channel migration, overbank deposition, and soil formation. Onthe basis of differences in meander belt dimensions and numbers ofabandoned channels, Upper Holocene meander belts are subdividedinto simple and complex forms. Relative age relationships suggest thatthe smaller and older simple meander belts represent periods of divid-ed Mississippi River flow and early attempts to establish a large, single-channel meandering regime. This type of meandering regime is rep-resented by the larger and younger complex meander belts and in-cludes the modern meander belt. Similarities in the timing of changesin floodplain processes and fluvial style and decreasing rates of Holo-cene sediment accumulation in the southern Lower Mississippi Valleystrongly suggest that decelerating Holocene sea-level rise in the Gulfof Mexico affected floodplain development at least 300 km inland fromthe present-day coast.

The alluvial architecture of the Lower Holocene deposits and theabsence of large meandering Mississippi River channel deposits olderthan ; 5000 yr B.P. near Ferriday indicates that most of the floodplainmuds were deposited by avulsion-related crevassing and lacustrine sed-imentation rather than by overbank flooding of large Mississippi Riverchannels. Similarities between the floodplain history of the MississippiRiver and those of modern and ancient rivers elsewhere further suggest

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

INTRODUCTION

The Mississippi River in the Lower Mississippi Valley is a classic me-andering river with abundant floodplain clays and silts (Fisk 1944, 1947).Traditionally, fine-grained floodplain deposits of the Mississippi River, aswell as those of large rivers elsewhere, are thought to represent repeatedepisodes of widespread overbank deposition and slow rates of sedimentaccumulation (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 Perez-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 accumulaterapidly in floodplain depressions during episodes of crevassing and avul-sion. The significance of these studies is that the traditional model of fine-grained floodplain construction by repeated episodes of infrequent over-bank floods may not be so widely applicable. Instead, crevassing and avul-sion, which can occur regardless of trunk-channel flooding, are the domi-nant processes of construction of fine-grained floodplains during periods ofrapid floodplain aggradation.

Considering that seminal studies by Harold N. Fisk (1944, 1947) estab-lished the Mississippi River as a standard for interpreting floodplain de-posits of fine-grained meandering rivers, it is appropriate that new ideasconcerning floodplain origins are tested in this classic and important river.This paper describes the Holocene history of floodplain development in thesouthern Lower Mississippi Valley and suggests that the patterns and pro-cesses of this construction differed substantially from Fisk’s views of sim-ple meander-belt development and overbank deposition (Fisk 1947). Thesenew ideas are based on a detailed examination of the sedimentologic, strati-graphic, and soil characteristics of overbank sediments, which preserve amuch more complete history of floodplain development than the meander-belt deposits. Our analysis of these deposits suggests that Holocene flood-plain processes and fluvial styles in the Lower Mississippi Valley changedin response to decreasing rates of floodplain sediment accumulation anddecelerating sea-level rise and that avulsion played a major role in flood-plain construction during the Holocene transgression. Lastly, we comparethe Holocene floodplain history of the Mississippi River to several exam-ples of modern and ancient floodplains to suggest that new ideas of fine-grained Mississippi River floodplain construction may be applicable to al-luvial 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. Thefloodplain contains five Holocene Mississippi River meander belts, Holo-cene 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

801HOLOCENE MISSISSIPPI RIVER FLOODPLAIN DEVELOPMENT

active and abandoned channels and point bars with arcuate ridges andswales (Fig. 2). Meander-belt deposits are sand bodies 20–30 m thick thatincise backswamp muds or valley-train sands and gravels (Fig. 3; Fisk1944; Saucier 1974, 1994; Saucier and Snead 1989; Autin et al. 1991).Backswamps are floodplain depressions located between meander belts andcontain freshwater swamps, crevasse-splay complexes, small streams, and,near the coast, shallow freshwater lakes. Backswamps are typically under-lain by clays, silts, and sands (Fisk 1947; Coleman 1966; Krinitzsky andSmith 1969; Farrell 1987), and these deposits form a downvalley-thick-ening wedge of fine-grained alluvial sediments (i.e., topstratum deposits ofFisk 1944) that pass into alluvial, deltaic, and shallow-marine deposits nearthe coast (Autin et al. 1991). This sediment wedge is ; 10 m thick in thenorthern Lower Mississippi Valley and ; 40 m thick beneath the deltaplain, and overlies probable Late Wisconsin valley-train deposits (i.e., sub-stratum deposits of Fisk 1944) (Fisk 1947; Autin et al. 1991; Saucier 1994,1996). Regional age estimates based on radiocarbon dates on peats andarchaeological age correlations indicate that the fine-grained fluvial sedi-ments are younger than ; 10,000 yr B.P. (McFarlan 1961; Saucier 1994).

Investigations by Fisk (1938, 1940, 1944, 1947) were the first to providedetailed accounts of changes in Mississippi River regime during the latePleistocene and Holocene. Fisk recognized that large braided channel sys-tems (i.e., valley trains of Autin et al. 1991) in the Lower MississippiValley represent late Pleistocene Mississippi River courses, and he sug-gested that this fluvial regime coincided with periods of lower-than-presentsea level, which produced steep channel gradients and increased river com-petence. Fisk (1944) further suggested that as sea level rose during theHolocene, the decrease in channel gradient and river competence led todevelopment of a meandering regime and vertical accretion of fine-grainedoverbank deposits. Subsequent investigations by Saucier (1974, 1981,1994, 1996) and others have shown that the transition from braided tomeandering regimes was not caused by sea-level change as suggested byFisk but instead reflected the northward retreat of the continental ice sheetand eastward diversion of glacial meltwater and outwash from the LowerMississippi Valley to the St. Lawrence River lowlands ; 10–11 ka (Porterand Guccione 1994). This diversion significantly reduced Mississippi Riversediment and water discharge (Teller 1990) and ended the construction ofMississippi River valley trains in the Lower Mississippi Valley. Subse-quently during the Holocene, the Mississippi River transported a greaterproportion of suspended sediment, which led to the accumulation of fine-grained floodplain deposits and the development of a meandering regime(Autin et al. 1991).

Following the initial studies by H.N. Fisk, investigations of fine-grainedHolocene floodplain deposits have shown that these sediments represent avariety of environments, including crevasse splays and distributary chan-nels, shallow lakes, and well and poorly drained backswamps (Coleman1966; Krinitzsky and Smith 1969; Smith et al. 1986; Farrell 1987; Saucier1994). The alluvial architecture and sedimentologic characteristics of theseoverbank deposits also show that floodplain environments have changedfrequently during the Holocene (e.g., Coleman 1966) and provide detailedrecords of processes such as development of meander belts and prograda-tion of natural levees and crevasse splays (e.g., Farrell 1987). In general,however, these studies do not discuss widespread changes in floodplainprocesses during the Holocene or the transition by the Mississippi Riverfrom Pleistocene valley trains to the present-day meandering regime. Thisstudy seeks to address these topics of Holocene floodplain development.

Mississippi River floodplain deposits were studied near Ferriday, Loui-siana, which is located ; 300 km upvalley from the Gulf of Mexico andacross the river from Natchez, Mississippi (Figs. 1, 4). Prior geologic in-vestigations in this area include studies by Fisk (1944, 1947) and mappingby Saucier (1967), and this area was chosen for detailed study becauseFisk’s interpretations of the Mississippi River floodplain evolution drewheavily from his analysis of this region. The floodplain near Ferriday is40–50 km wide and elevations range between 14 and 20 m (40 and 65

feet) above sea level. The three youngest Holocene Mississippi River me-ander belts (meander belts 1–3 of Saucier and Snead 1989) are present atthe floodplain surface and show well expressed crosscutting relationships.Meander-belt channels incise buried Pleistocene valley-train sands, andfine-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 thatrepresent ; 1000 m of floodplain deposits. Cores were acquired with ahydraulic soil probe that recovered sediments to depths of up to 15 m, anda single 23-m-deep core was acquired using a wet rotary drill rig. Thefloodplain was sampled along topographic transects that began at point-barridge crests of each meander belt and extended across adjacent channels,natural levees, and backswamps perpendicular to channel flow. The spacingbetween the cores ranged between 0.2 and 3.0 km, and the average spacingwas 1.0 km.

Cores were described in the field and subsampled for grain-size, min-eralogic, and petrographic analyses. Soil features such as horizons, matrixand mottle colors, soil structures, bioturbation features, slickensides, andnodules were described using standard terminology and methods outlinedin Birkeland (1984). Sand fractions were determined using sieves, and clayand silt fractions were calculated using settling velocities and oven-driedweights of volumetrically calibrated pipette samples. The clay mineralogyof selected samples was determined by preparing oriented clay samples onceramic tiles and analyzing diffraction patterns with a Scintag x-ray dif-fractometer (Cu target). Clay minerals were identified by successively airdrying, glycolating, and heating each sample to 5008C for 1 hour. Orientedpetrographic thin sections of selected samples were vacuum impregnatedwith epoxy. One organic-rich bulk sediment sample and several samplesof wood fragments were dated at Livermore Laboratory using acceleratormass spectrometry radiocarbon dating.

Fluvial landform dimensions and sediment-body geometries were deter-mined using a combination of aerial photographs, topographic maps, flood-plain cores, and water well and engineering borehole data archived at theLouisiana Department of Transportation in Baton Rouge, Louisiana and theU.S. Army Corps of Engineers Waterways Experiment Station in Vicks-burg, Mississippi. Widths of the meander belts were measured from to-pographic maps and supplemented by field observations. Measurementswere made perpendicular to the meander-belt axis, and thicknesses of me-ander-belt sand bodies were estimated on the basis of the depths of activeand abandoned Mississippi River channels (Fisk 1944; Saucier 1967). Forinstance, the present-day Mississippi River at Natchez, Mississippi is ;30 m deep, which indicates that the thickness of the meander-belt sandbody is 30 m.

The width and thickness of buried sand bodies was determined by thedrilling program, augmented by regional water-well and engineering bore-hole data. The width of elongate or linear sand bodies was measured per-pendicular to the long axis of the deposits, and thicknesses were measureddirectly from the cores. Typically the measured thicknesses were minimumsbecause the hydraulic soil probe cannot penetrate buried sand bodies thatare thicker than 5 m. In instances where stratigraphic correlations wereobvious, water-well and engineering borings were used to estimate a min-imum thickness for the buried sand bodies.

HOLOCENE MISSISSIPPI RIVER FLOODPLAIN DEPOSITS

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

802 A. ASLAN AND W.J. AUTIN

803HOLOCENE MISSISSIPPI RIVER FLOODPLAIN DEVELOPMENT

FIG. 2.—Geologic map showing an enlargedview of the Mississippi River floodplain. Themap boundaries are shown in Figure 1 and thebox marks the area shown in Figure 4. Thelocation of the cross section X–X9 in Figure 3 isalso shown. Modified from Saucier and Snead(1989).

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

dence for significant changes in fluvial style and processes of floodplainconstruction.

Lower Holocene Floodplain Deposits

Lower Holocene deposits range in thickness from 6 to 21 m, sharplyoverlie Late Wisconsin sands (Figs. 6, 7A), and consist primarily of muds,which encase sand bodies of varying sizes and shapes. The muds and sandsare interpreted as a mosaic of small floodplain streams, lakes, crevasse-splay complexes, and poorly drained backswamps.

Lacustrine and Backswamp Muds.—Lower Holocene muds consist ofdark gray laminated muds, blue-gray bioturbated muds with olive to palebrown 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 slightlysinuous, mud-filled burrows (up to 2 cm in diameter) are oriented sub-vertically and locally disrupt bedding. Authigenic minerals in the mudsinclude common olive siderite nodules (, 2 cm in diameter) and clayeytabular siderite accumulations (, 5 cm thick) that parallel bedding (Fig.7B). Disseminated pyrite is present within burrow fills and in clays alongthe upper and lower boundaries of the tabular siderite.

Blue-gray muds are bioturbated and show rare evidence of primary strat-ification. 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 sizefrom 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

804 A. ASLAN AND W.J. AUTIN

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 olderbackswamp muds and Wisconsin deposits. The approximate location of Holocene–Late Wisconsin boundary is based on the depth of the present-day Mississippi Riverchannel 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 uponcore extrusion and oxidation (Krinitzsky and Smith 1969).

Gray mottled muds are less common than dark gray and blue-gray mudsand contain yellow-brown mottles and root traces, iron and calcite nodules,and slickensides. Small iron nodules (1–2 mm in diameter) are more abun-dant than calcite nodules, which range in diameter from 5 to 20 mm. Slick-ensides are shiny intersecting fractures with smooth, clayey surfaces.

Interpretation.—The dark gray laminated muds are interpreted as la-custrine deposits on the basis of their similarity to lacustrine sediments inthe Atchafalaya Basin, which is located south of Ferriday (Fig. 1; Coleman1966; Krinitzsky and Smith 1969; Smith et al. 1986; Tye and Coleman1989a) The scarcity of burrows and the abundance of laminae in the darkgray muds probably reflect rapid sediment accumulation rather than anoxiclake conditions, judging by the presence of common bioturbation featuresin the other facies (e.g., the blue-gray muds) (Coleman 1966). The thick-ness of the dark gray muds indicates that the lakes were shallow, probablyup to several meters deep. The clay-rich texture, color, vivianite nodules,and abundance of bioturbation features and organic remains suggest thatthe blue-gray muds represent poorly drained backswamps (sensu Coleman1966). Poorly drained backswamps in the Atchafalaya Basin contain similarbioturbated muds (Coleman 1966; Krinitzsky and Smith 1969; Tye andColeman 1989a) and support water-tolerant woody vegetation such as cy-press and gums.

The presence of authigenic pyrite, vivianite, and siderite in the dark grayand blue-gray muds further suggests that the lake and poorly drainedswamp pore waters and sediments were chemically reducing (Coleman1966; Ho and Coleman 1969). Vivianite and siderite precipitate understrongly reducing conditions that accompany methane formation (Berner1981), and the presence of pyrite in the muds suggests that pore waterswere sulfate rich. Although the presence of pyrite can represent marineinfluences within alluvial–deltaic environments, there are elevated concen-trations 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 ofburied Tertiary marine sediments and buried salt domes (Whitfield 1975).Because Holocene sediments with marine molluscs pinch out upvalley atthe latitude of Donaldsonville, Louisiana (. 100 km south of Ferriday),the sulfate in the sediment pore waters near Ferriday is probably related tothe regional ground-water chemistry rather than to Holocene marine trans-gressions (Smith et al. 1986; Bailey et al. 1998). Authigenic siderite andpyrite are also present in shallow (, 6 m deep) backswamp and marshdeposits in the Atchafalaya Basin, and these deposits are younger than 1500yr B.P. (Moore et al. 1992). The shallow depth and young age of thesedeposits suggest that the pyrite and siderite in the floodplain deposits nearFerriday formed in near-surface reducing environments rather than duringburial.

The clay-rich texture, yellow-brown root traces and mottles, and ironand calcite nodules indicate that the gray mottled muds represent welldrained backswamps (sensu Coleman 1966). The abundance of yellow-brown mottles and iron nodules and the absence of minerals such as pyriteand vivianite further suggest that these deposits accumulated in betterdrained environments than those represented by the lacustrine and poorlydrained backswamp sediments (Coleman 1966; Krinitzsky and Smith1969).

Crevasse-Splay and Lacustrine Sands.—These sand bodies consist ofvery fine to medium-grained quartz-rich sheet sands that are surrounded byeither backswamp or lacustrine muds (Fig. 6). The sheet sands are 1–3 mthick, consist of beds that are tens of centimeters thick, have either sharpor gradational lower boundaries, and extend laterally for up to 1 km (Figs.6, 8). Within vertical sequences, the sands may either fine or coarsen up-ward, and primary stratification is generally poorly preserved but, wherepresent, consists of wavy laminae.

Interpretation.—The thicknesses, widths, and stratigraphic positions ofthe sands indicate that the sediments are crevasse-splay and lacustrine-delta

805HOLOCENE MISSISSIPPI RIVER FLOODPLAIN DEVELOPMENT

FIG. 4.—Geologic map of the Ferriday areashowing the distribution of Holocene MississippiRiver meander belts and backswamps, andlocations of core sampling sites. Note thatcompared to meander-belt sand bodies,subsurface floodplain channel sands (opencircles) are extremely narrow. Locations of crosssections in Figure 6 are also shown.

deposits. Sands that overlie the bioturbated blue-gray and laminated darkgray muds are similar in thickness and stratigraphic position to crevasse-splay deposits (Farrell 1987) and lacustrine-delta sands, respectively, in theAtchafalaya Basin (Breland et al. 1988; Tye and Coleman 1989a). Cre-vasse-splay sands typically overlie poorly drained backswamp muds (Far-rell 1987), whereas lacustrine-delta sands overlie and show gradationalboundaries with laminated lacustrine muds (Fig. 8; Tye and Coleman1989a).

Floodplain Channel Sands.—Lower Holocene sands are also repre-sented by sand bodies that are 0.5–1.0 km wide and several kilometers longmeasured parallel to the valley axis, and have minimum thicknesses of 5–8 m (Figs. 4, 6). The upper surfaces of these sands are commonly presentbetween 9 and 11 m above mean sea level. In several instances, sandwedges representing probable natural levees are present directly above andextend laterally beyond the edges of the floodplain channel sands (Fig. 6).Water-well and engineering borings show that floodplain channel depositsare also present south of Ferriday (Saucier 1967). Neither the subsurfacedata near Ferriday nor the regional borehole data (e.g., Fisk 1944; Saucier1967, 1994; Krinitzsky and Smith 1969; Smith et al. 1986), however, showevidence 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 dimensionssuggest that the sands represent small, sinuous streams that migrated lat-erally 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 mthick; this indicates that Lower Holocene floodplain channels were smallerthan the modern channel and less laterally mobile. Because upper surfacesof the small floodplain channel sands are present at similar elevations, thesands are interpreted as deposits of multichannel streams with a probableanastomosed pattern. Analogous modern streams are present in the At-chafalaya Basin (Smith et al. 1986).

Upper Holocene Floodplain Deposits

Upper Holocene floodplain deposits consist of meander-belt tabularsands, natural-levee silt and sand wedges, and backswamp muds (Figs. 5,6). Soil features such as roots, mottles, clay films, slickensides, and calciteand iron nodules are much more common in Upper Holocene deposits thanin Lower Holocene sediments (Aslan and Autin 1998).

Meander-Belt Sands.—Mississippi River meander-belt sand bodiesconsist of silt to medium-grained quartz-rich sands that accumulated onpoint 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 arerepresented by 1–2 m of bioturbated silts and sands that are underlain byfine to medium sand with abundant small-scale cross-stratification. Swalescontain several meters of either mud or interbedded mud and sand, whichoverlie sand and interbedded mud.

Topographic maps and floodplain borings show that the dimensions ofMississippi River meander belts in the Ferriday region differ significantly,

806 A. ASLAN AND W.J. AUTIN

FIG. 5.—Stratigraphic section of a 21-m-longbackswamp core showing vertical changes inlithologies and interpreted sedimentaryenvironments for Upper and Lower Holocenedeposits. For grain-size descriptions, m 5 mud,s 5 sand, and g 5 gravel.

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FIG. 6.—A, B) Stratigraphic cross sections through backswamps near Ferriday showing Holocene floodplain sediments including crevasse-splay and floodplain-channelsand 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 isfrom a large wood fragment present in laminated muds beneath natural levee deposits of Holocene Mississippi River meander belt 3.

and differences in channel widths, numbers of abandoned channels, andthe complexity of crosscutting relationships among scroll-bar sets are usedto subdivide meander belts into simple and complex forms (Table 1). Sim-ple meander belts are characterized by few and narrow (, 1 km wide)abandoned channels and meanders that contain few neck cutoffs and lackcomplex 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 beltsand bodies is ; 20 m (Fisk 1947).

Complex Mississippi River meander belts contain numerous and wide

807HOLOCENE MISSISSIPPI RIVER FLOODPLAIN DEVELOPMENT

808 A. ASLAN AND W.J. AUTIN

FIG. 7.—Floodplain cores from the Ferridayarea. A) Abrupt contact (arrow) between LowerHolocene dark gray muds (m) and probable LateWisconsin sands (s). Core depth is 2060–2080cm below the floodplain surface. B) Dark graylaminated silts and clays with organic-rich beds(o) and light gray tabular zones of siderite-richclay (arrows). The silts and clays are LowerHolocene lacustrine deposits. Core depth is1302–1328 cm below the floodplain surface. C)Brown mottled natural-levee silts and sands ofHolocene Mississippi River meander belt 1.Organic matter (o) is abundant in the upper 10cm, and earthworm-pelleted burrows (b) areconcentrated between 10 and 20 cm in the core.The silts and sands are massive (m) and containcommon root mottles below 20 cm. Core depthis 3–28 cm below the floodplain surface.

(. 1 km wide) abandoned channels, and meanders show common neckcutoffs and complex crosscutting relationships among scroll bars. Missis-sippi River meander belts 1 and 2 near Ferriday are examples of complexforms (Figs. 2, 4). The widths of the complex meander belts are 10–15km, and the sand bodies are ; 30 m thick, judging by engineering boringsand the depth of the present-day Mississippi channel at Natchez, Missis-sippi (Fisk 1944).

Natural-Levee and Backswamp Deposits.—Silt and sand wedges rep-resenting natural levees are located adjacent to Mississippi River channelsand 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 overdistances of 2–3 km measured perpendicular to the direction of channelflow. Natural-levee deposits contain abundant soil features, which are de-scribed below. Backswamp sediments consist of gray clay and silt, encasecrevasse-splay sheet sands, and are typically 5–10 m thick (Fig. 6). Claysand silts contain common yellow-brown mottles, iron and calcite nodules,and slickensides, and represent well drained backswamp deposits (sensuColeman 1966).

Floodplain Soils.—Sandy and silty soils developed on meander belt 1point-bar ridges and natural levees are moderately well drained Inceptisols(Figs. 7C, 9A). Profiles typically consist of dark brown, organic-rich, bio-turbated silts and sands (A horizon) that overlie brown silts and sands witha subangular blocky structure and common earthworm-pelleted burrows,roots, yellow-brown mottles, and iron nodules (Bw horizon). The base ofthe profiles consists of gray silts and sands with roots, yellow-brown mot-tles, and relict stratification, but lacks soil structure (Cg horizon). In con-trast to meander belt 1 soils, older meander belt 2 and 3 soils are moder-ately developed Alfisols (Fig. 9B; USDA 1988). Alfisols contain ; 50-cm-thick, clay-enriched B horizons (i.e., Bt horizons), and clay films coat

ped faces and soil pores. These features are generally absent from youngermeander 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 (Ahorizon) that overlies gray clay and silt with many yellow-brown mottles,root traces, iron and calcite nodules, and slickensides (Bkg horizon). Thebase of the profile (Ckg horizon) consists of gray clay and silt that is similarto the overlying B horizon but which lacks slickensides and soil structure.

Floodplain Chronology

Radiocarbon dating and stratigraphic age correlations indicate that LowerHolocene deposits near Ferriday range in age from ; 5000 to ; 10,000yr B.P. and Upper Holocene deposits are younger than ; 5000 yr B.P.Radiocarbon dating of a bulk sediment sample of Lower Holocene mudimmediately overlying the Late Wisconsin sands at Ferriday produced adate of 16,600 6 140 yr B.P. (Table 2; Fig. 6A). Because the organicparticles from the base of the floodplain muds are detrital, the 16,600 6140 yr B.P. age is interpreted as a maximum age for the basal floodplainmuds. Regional age estimates based on radiocarbon-dated peats that overlieLate Wisconsin deposits beneath the delta plain indicate that the basalLower Holocene muds are ; 10,000 years old (McFarlan 1961; Saucier1994). Radiocarbon analysis of two large wood fragments located one me-ter below the highest stratigraphic occurrence of the dark gray laminatedmuds (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 minimumage for the Lower Holocene floodplain deposits near Ferriday. The datedwood fragments are encased in backswamp muds, and because present-daybackswamps are densely vegetated and have cohesive muddy substrates,

809HOLOCENE MISSISSIPPI RIVER FLOODPLAIN DEVELOPMENT

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

TABLE 1.—Summary of Holocene Mississippi River meander-belt characteristics inthe Ferriday area.

Meander-belt

Type

Abandoned1

channel width(km)

Number ofabandonedchannels

Meander-belt1width (km)

Minimumthickness2 ofmeander-beltsheet sand

(m)

SimpleComplex

0.5–1.01.0–2.0

FewMany

5–1010–15

2030

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).

fallen and partially rotted pieces of wood are not likely to have been trans-ported substantial distances. The dark gray laminated muds are stratigraph-ically overlain by brown silts and sands that represent natural-levee depositsof Mississippi River meander belt 3 (Fig. 6A), which is the oldest meanderbelt in the study area. The radiocarbon ages indicate that meander belt 3is younger than 5000 yr B.P., and this age is slightly less than the maximumage (6200 yr B.P.) suggested previously for Mississippi River meander belt3 (Saucier 1994).

HOLOCENE FLOODPLAIN EVOLUTION

Differences in the alluvial architecture and pedogenic characteristics ofLower and Upper Holocene Mississippi River deposits near Ferriday show

that floodplain environments and depositional processes changed substan-tially during the Holocene and that the establishment of large meanderingMississippi River channels in the southern Lower Mississippi Valley oc-curred less than 5000 yr B.P. Vertical changes in the abundance of soilfeatures and radiocarbon ages of the deposits further suggest that floodplainchanges coincided with decreasing rates of sediment accumulation and de-celerating sea-level rise.

Early Holocene Floodplain Development

Lithologic similarities between Lower Holocene deposits near Ferridayand recent sediments in the Atchafalaya Basin suggest that Early Holocenelakes and poorly drained backswamps filled rapidly through a combinationof crevassing, lacustrine sedimentation, and avulsion, similar to historicsedimentation in the Atchafalaya Basin (Fig. 10A, B; Krinitzsky and Smith1969; Smith et al. 1986; Tye and Coleman 1989a, 1989b). For instance,Tye and Coleman (1989b) documented that up to 5 m of lacustrine deltaand crevasse-splay muds and sands filled 470 km2 of the Atchafalaya lakesand swamps in the Lake Fausse Pointe region since 1917. Rapid sedimen-tation was initiated by the growth of the Atchafalaya River up valley inthe late 1800s and early 1900s (Fisk 1952; Smith et al. 1986; Tye andColeman 1989b). As the Atchafalaya River grew, increasingly large vol-umes of sediments were diverted from the Mississippi River and depositedin the Atchafalaya lakes and swamps. An especially important aspect ofthe historic sedimentation in the Atchafalaya Basin is that the sediments

810 A. ASLAN AND W.J. AUTIN

FIG. 9.—Schematic sections of typicalHolocene 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, Bw5 cambic B horizon, Bt 5 argillic B horizon,Bkg 5 calcic gleyed B horizon, Cg 5 gleyed Chorizon, and Ckg 5 calcic gleyed C horizon.

TABLE 2.—Summary data on radiocarbon samples from Mississippi River flood-plain deposits near Ferriday, Louisiana.

SampleLocation

CAMS1

Num-ber

Materialsampled

DepositionalEnvironment

Depth(cm)

14C age(yr)

Interpre-tation

Concordia P.T7N R8E Sec. 31

7190 Root frag-ment

Backswamp 340 Modern Minimumage

Concordia P.T7N R9E Sec. 13

7191 Wood frag-ment

Backswamp be-neath Miss. R.meander belt 3natural levee

920 5,320 6 90 Time of de-position

Concordia P.T8N R9E Sec. 55

7192 Root frag-ment

Backswamp be-neath Miss. R.meander belt 1natural levee

485 780 6 70 Minimumage

Concordia P.T7N R9E Sec. 49

7193 Wood frag-ment

Lake 840 5,400 6 70 Time of de-position

Concordia P.T7N R9E Sec. 49

7194 Bulk sample Lake 2,005 16,600 6 140 Maximumage

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

accumulated at all stages of Mississippi River flow, rather than only duringinfrequent overbank flows. Floodplain borings from the Atchafalaya Basinfurther demonstrate that this style of rapid floodplain sedimentation andlake 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 sug-gests that overbank flooding of large Mississippi River channels was notprimarily responsible for the accumulation of the fine-grained Lower Ho-locene deposits. Instead, the lateral variability of sediment grain sizes ismore consistent with deposition during episodes of crevassing and avulsion(Smith et al. 1989; Willis and Behrensmeyer 1994). According to this view,

avulsion of small floodplain streams, represented by the Lower Holocenesand bodies, played an important role in the deposition of fine-grainedfloodplain sediments.

Although quantitative estimates of sediment accumulation rates in thestudy area are poorly constrained, the scarcity of soil features in the LowerHolocene deposits indicates that these sediments accumulated rapidly. Ra-diocarbon dates on peats from the delta plain south of Ferriday confirmthat the Early Holocene (. 5000 yr B.P.) was a time of rapid sedimentaccumulation in the southern Lower Mississippi Valley (Fig. 11A; Mc-Farlan 1961; Frazier 1967, 1974; Roberts et al. 1991). Rapid accumulationof silts and clays and poor drainage inhibited soil formation and lateralchannel migration and led to the development of multichannel streams (Fig.10B). Recent field studies and computer models of alluvial stratigraphyalso suggest that avulsion frequency increases as rates of floodplain aggra-dation increase (Tornqvist 1994; Bryant et al. 1995; Heller and Paola 1996),which supports our interpretation that avulsion was an especially importantprocess of Lower Holocene floodplain construction.

Similarities in the timing of rapid floodplain aggradation and sea-levelrise in the Gulf of Mexico suggest that sea level significantly influencedfloodplain processes and development as far up valley as Ferriday (; 300km) (Fig. 11B). According to this interpretation, Lower Holocene lake andpoorly drained backswamp deposits near Ferriday as well as those presentin the Atchafalaya Basin (Tye and Coleman 1989b), accumulated in re-sponse to rapid sea-level rise and increased availability of accommodation.Furthermore, the late Pleistocene to Holocene decrease in Mississippi Riversediment and water discharge argues against the possibility that Early Ho-locene aggradation near Ferriday represents a climatically driven increasein sediment supply (Saucier 1994). Lastly, rapid floodplain aggradation andthe alluvial architecture of the Lower Holocene deposits is also consistentwith theoretical models of base-level effects on alluvial stratigraphy (Allen

811HOLOCENE MISSISSIPPI RIVER FLOODPLAIN DEVELOPMENT

FIG. 10.—Schematic block diagrams summarizing Holocene floodplain development near Ferriday. A) The Early Holocene floodplain was initially represented by lakesand poorly drained backswamps. B) Multichannel floodplain streams, crevasse splays, and lacustrine deltas filled the lakes and poorly drained backswamps as the floodplainrapidly aggraded during the Early Holocene. C) Slower rates of floodplain aggradation during the Late Holocene led to the development of simple, followed by complexMississippi 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 BatonRouge 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).

812 A. ASLAN AND W.J. AUTIN

1978; Leeder 1978; Bridge and Leeder 1979; Friend et al. 1979; Wrightand Marriott 1993). The models predict that base-level rise and a constantsediment supply produces isolated sand bodies that are encased by fine-grained alluvial deposits with few soil features, similar to Lower Holocenedeposits near Ferriday.

Late Holocene Floodplain Development

Upper Holocene floodplain deposits near Ferriday reflect a combinationof lateral channel migration and meander-belt construction, overbank de-position, and soil formation (Fig. 10C). The abundance of Mississippi Riverpoint bars with well developed scroll bars demonstrates the importance oflateral accretion during this stage of Holocene floodplain construction (Fisk1944). Silty and sandy natural-levee deposits that pass laterally into clayeybackswamp deposits also show that overbank flooding of large MississippiRiver channels contributed to fine-grained floodplain sedimentation (Keselet al. 1974). The abundance of soil features in the Upper Holocene depositsand the inferred decrease in floodplain aggradation rates also provide in-sights on the development of the modern meandering regime of the Mis-sissippi River.

Mississippi River Meander-Belt Evolution.—Although the dimensionsof Holocene Mississippi River meander belts vary considerably, the originsand significance of these differences are poorly understood (Saucier 1994,1996). Possible origins for the simple meander belts include (1) reduceddischarges caused by Holocene climate change, and (2) divided flow be-tween coexisting meander belts (Autin et al. 1991; Saucier 1994, 1996).Paleoclimate studies indicate that the mid-Holocene was characterized bywarmer and drier conditions than exist today in parts of the western, mid-western, and eastern U.S. (i.e., the Hypsithermal event) (Autin et al. 1991).Warmer and drier conditions could have reduced Mississippi River dis-charge in the Lower Mississippi Valley and led to the development ofsmaller channels and meander belts than the modern Mississippi Rivermeander belt. A climatic origin for the simple meander belts, however, isdifficult to evaluate because of poor constraints on the ages of the simplemeander belts and because the timing of the mid-Holocene warming anddrying varied regionally (Autin et al. 1991; Saucier 1994, 1996).

In contrast to a climatic origin, the chronology of the Holocene Missis-sippi River meander belts and subdeltas in the southern Lower MississippiValley supports the idea that the simple meander belts represent periods ofdivided flow (Frazier 1967, 1974; Autin et al. 1991; Saucier 1996). Fra-zier’s studies of the Mississippi River delta show that two and perhaps asmany as three Holocene subdeltas were active simultaneously (Frazier1967, 1974), which requires that more than one meander belt was active.The relative ages of the simple and complex meander belts located southof Vicksburg, Mississippi further suggest that the meandering regime ofthe present-day Mississippi River evolved gradually from a time of dividedflow, represented by the simple meander belts, to a large, single-channelmeandering system. According to this interpretation, the simple meanderbelts (e.g., meander belts 3 and 4) represent early but short-lived attemptsto establish a large, meandering Mississippi River channel. Evidence thatsimple meander belts represent short periods of fluvial activity include thesmall number of abandoned channels and the scarcity of complex cross-cutting relationships among scroll bar sets (Table 1). The initial develop-ment of simple meander belts near Ferriday less than ; 5000 yr B.P. alsocoincides with decreasing rates of sediment accumulation in the LowerMississippi Valley and decelerating sea-level rise (Fig. 11). These obser-vations are consistent with the idea that slower floodplain aggradation fa-vored lateral channel migration and the development of Mississippi Rivermeander belts.

As rates of sediment accumulation and sea-level rise continued to slowduring the latest Holocene, divided flow among simple meander belts wasreplaced by the development of large meandering channels capable of trans-porting 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 ofthese features coincides with Holocene sea-level highstand conditions. Dif-ferences between the alluvial architecture of the Upper and Lower Holo-cene deposits are also consistent with theoretical models of alluvial stra-tigraphy, which predict that periods of constant base level or slow rise leadto the development of large sheet sands and alluvial soils (Allen 1978;Leeder 1978; Bridge and Leeder 1979; Friend et al. 1979; Posamentier andVail 1988; Wright and Marriott 1993).

ORIGINS OF FINE-GRAINED FLOODPLAINS

Interpretations of the Holocene Mississippi River floodplain sedimentsnear Ferriday differ from prior studies, which suggest that most of the claysand silts represent repeated episodes of overbank flooding of large Missis-sippi River channels (Fisk 1944, 1947; Kesel et al. 1974; Kesel et al. 1992;Saucier 1981, 1994; Guccione 1993). According to this model of fine-grained floodplain sedimentation, rates of short-term sediment accumula-tion, grain size, and bed thickness decrease systematically away from themaster channel (Fig. 12A). Fisk’s investigations of the Mississippi Rivercontributed greatly towards developing this view of fine-grained meander-ing-river sedimentation (Fisk 1944, 1947), and this model is used widelyfor interpreting modern and ancient fluvial deposits (Wolman and Leopold1957; Allen 1965; Blake and Ollier 1971; Bridge and Leeder 1979; Bridge1984; Bown and Kraus 1987; Nanson and Croke 1992; Miall 1992; Zwol-inski 1992). The alluvial architecture and soil characteristics of HoloceneMississippi River deposits near Ferriday as well as those present in theAtchafalaya Basin, however, show that most of the fine-grained sedimentsare deposited rapidly in regional floodplain depressions during episodes ofcrevassing and avulsion (Fig. 12B). Because these processes deposit sedi-ments on the floodplain at virtually all stages of trunk-channel flow, thesediments accumulate continuously rather than during infrequent floods.This condition leads to rapid sedimentation and filling of regional depres-sions (e.g., the Atchafalaya Basin). Once a flood basin is filled, subsequentavulsion initiates rapid sedimentation and aggradation elsewhere on thefloodplain. Repetition of this sequence produces interfingering sheets orwedges of floodplain clays and silts and isolated sand bodies with littleevidence of pedogenesis, similar to the Lower Holocene deposits, and pack-ages of avulsion deposits typically thicken away from the active channel(Fig. 12B; Willis and Behrensmeyer 1994). This style of floodplain con-struction continues today in the Atchafalaya Basin (Tye and Coleman1989a, 1989b) but was probably more common during the Holocene trans-gression and base-level rise. According to this view, large overbank floodsare responsible for a smaller percentage of the total volume of Holocenemuds in the southern Lower Mississippi Valley than generally recognized.

Other Modern Rivers with Fine-Grained Floodplains

Comparisons between Holocene Mississippi River deposits near Ferridayarea and fine-grained floodplain deposits of the Saskatchewan River inCanada and the Rhine–Meuse River in the Netherlands suggest that theorigin of fine-grained floodplain deposits in the Lower Mississippi Valleyis similar to rivers elsewhere.

The Saskatchewan River, Canada.—Although the Saskatchewan Riverof north-central Canada represents a different geological setting than theMississippi River in Louisiana, studies by Smith et al. (1989) and Smithand Perez-Arlucea (1994) show that processes of fine-grained sedimenta-tion during periods of rapid aggradation are similar in these two fluvialsystems. Glacial moraine dams have caused the Saskatchewan River flood-plain to aggrade locally, and in the Cumberland Marshes rapid aggradationwas accomplished by historic avulsion of the Saskatchewan River (Smithet al. 1989). The avulsion deposits cover ; 500 km2 and consist of sheetsof mud that are 2–4 m thick and encase crevasse-splay sands and silts.

813HOLOCENE MISSISSIPPI RIVER FLOODPLAIN DEVELOPMENT

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

During the avulsion, crevasse-splay and lacustrine sediments filled shallowlakes and buried peaty wetlands located between alluvial ridges. In areaswhere the avulsion is complete, meandering channels are incising the avul-sion deposits. Overbank flooding of the meandering channels deposits athin veneer of silts and clays on top of the older avulsion deposits, butthese sediments represent a small percentage of the total volume of fine-grained floodplain sediments present in the region.

Similarities between the alluvial architecture of the Lower HoloceneMississippi River deposits near Ferriday and the Saskatchewan River sed-iments suggest that these deposits accumulated through similar processes(Fig. 6; Smith et al. 1989, figs. 10 and 11). Although the SaskatchewanRiver example represents a single avulsion and fine-grained MississippiRiver floodplain sediments represent a much longer period of aggradation,both examples indicate that large volumes of fine-grained deposits accu-mulate by crevasse-splay and lacustrine sedimentation related to avulsionrather than by overbank flooding of large trunk channels.

The Rhine–Meuse River, The Netherlands.—Study of the Holocenehistory of the Rhine–Meuse River in the Netherlands also shows interestingsimilarities to the Holocene floodplain history in the southern Lower Mis-sissippi Valley. Tornqvist (1993) demonstrated that the lower Rhine–MeuseRiver changed from an anastomosing to a meandering system during theHolocene and that this change in fluvial style coincided with decreasingrates of sea-level rise. The anastomosed river deposits consist of isolatedsand 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-channelsediments, the meandering-river deposits are represented by wider andthicker sand bodies and the fine-grained sediments show a systematic de-crease in sediment grain size with increasing distance from the meandering-channel sands, which is consistent with an overbank origin. The verticalchanges in the alluvial architecture of the Rhine–Meuse River describedby Tornqvist (1993) are similar to those of the Mississippi River deposits,and in both of these coastal river examples, changes in fluvial style andfloodplain development are closely linked to decreasing sediment accu-mulation rates and decelerating sea-level rise.

Ancient Fine-Grained Floodplain Deposits

Holocene Mississippi River floodplain deposits are also similar to severalexamples of ancient alluvial deposits (e.g., Ferm and Cavaroc 1968; Eth-ridge et al. 1981; Flores 1981; Gersib and McCabe 1981; Platt and Keller1992; Willis and Behrensmeyer 1994). The similarities suggest that newinformation on Holocene floodplain construction in the Lower MississippiValley should be useful for interpreting ancient floodplains, and two ex-amples are discussed briefly.

Carboniferous alluvial deposits of the Port Hood Formation in NovaScotia accumulated in an extensional basin and consist of several hundredmeters of channel and crevasse-splay sandstones and lacustrine and back-

814 A. ASLAN AND W.J. AUTIN

swamp mudrocks (Gersib and McCabe 1981). The fine-grained crevasse-splay, lacustrine, and backswamp facies are strikingly similar to the LowerHolocene sediments near Ferriday, and the Port Hood deposits indicate thatthese Carboniferous floodplains aggraded rapidly through a combination ofcrevassing and lake filling. For instance, 5–10 m thick, coarsening-upwardsequences in the Port Hood Formation commonly consist of ripple-lami-nated and massive crevasse-splay sandstones that overlie laminated lacus-trine siltstones and rooted and slickensided mudrocks or coals (Gersib andMcCabe 1981, fig. 9). This vertical succession indicates that periods ofslow floodplain aggradation and soil formation alternated with episodes offlooding and rapid aggradation. Thicknesses of crevasse-splay and lami-nated lacustrine deposits further suggest that at least half of the fine-grainedsands and muds were deposited by processes other than slow overbanksedimentation.

Miocene floodplain deposits from the Chinji Formation (Siwaliks Group)of the Himalayan foredeep in Pakistan also provide evidence for patternsof floodplain development similar to those observed in the AtchafalayaBasin and the Mississippi River floodplain near Ferriday. Willis and Beh-rensmeyer (1994) showed that fine-grained floodplain deposits consist of4–10 m thick packages of mudrocks and sandstones and the upper part ofeach package is a paleosol. These paleosol-bounded sedimentary packagesform interfingering wedges that thin laterally, and the deposits do not showsystematic increases in sediment grain size with increasing proximity tolarge meandering-channel sands, as would be expected if the sedimentsaccumulated during overbank floods. Instead, Willis and Behrensmeyer(1994) suggest that the fine-grained sediments represent episodic and rapiddeposition within regional floodplain depressions, analogous to the style offloodplain deposition reported by Tye and Coleman (1989a) as well as inthe Ferriday area. While Willis and Behrensmeyer (1994) point out thatthe episodic floodplain sedimentation observed in the Chinji Formation isnot new or unexpected, the stratigraphic sections show that this style offloodplain construction represents a significant proportion of the total vol-ume of fine-grained alluvial deposits in the Chinji Formation (Willis andBehrensmeyer 1994, fig. 6).

SUMMARY

Our studies as well as those cited in the previous section indicate thatlarge quantities, and in some instances most muddy floodplain deposits donot represent repeated episodes of overbank flooding and widespread ag-gradation. Instead, during periods of base-level rise, fine-grained floodplaindeposits accumulate rapidly and locally in floodplain depressions, probablyduring crevassing and avulsion. If base-level rise continues, filling of adepression is followed by avulsion and aggradation elsewhere on the flood-plain. Repetition of this sequence of events produces interfingering sheetsand wedges of fine-grained sediments that encase isolated sand bodies.Though the overbank model of construction of fine-grained floodplains wasat least in part developed from studies of the Mississippi River, it seemslikely that a large percentage of the floodplain sediments in the southernLower Mississippi Valley accumulated through a combination of crevass-ing, lacustrine sedimentation, and avulsion. An important implication ofthis discussion is that common present-day processes of floodplain con-struction (e.g., lateral channel migration, overbank flooding) in coastal al-luvial rivers may differ substantially from those that produced fine-grainedfloodplains during the Holocene transgression. Additionally, these obser-vations are probably applicable in certain instances to ancient alluvial rocksthat accumulated during episodes of base-level rise.

ACKNOWLEDGMENTS

Research contributing to this paper was supported by the Donors of The PetroleumResearch Fund, administered by the American Chemical Society, the GeologicalSociety of America, and the Louisiana Geological Survey. We thank Mary Kraus,Norm Smith, and Torbjorn Tornqvist for their stimulating discussions of avulsion.

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

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815HOLOCENE MISSISSIPPI RIVER FLOODPLAIN DEVELOPMENT

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Received 25 July 1996; accepted 16 June 1998.