..

·o.ui.T·llt.IA'.'1·�,. it•tt!flfl• - ·- .., , Mlll1i!lft1.eet, - · '·' . .,.. .•. " �·. ,· l ' ',_:·.. \ ,, .! '>:-·,., ·_ ,_ Quaternary Geology of the Lower Mississippi Valley

by I Roger T. Saucier

1974

Arkansas Archeological Survey Research Series No. 6

Second Printing, 1984 Library of Congress Card 74-620094

ii TABLE OF CONTENTS

Page

Foreword ...... v

Introduction ...... 1

Geological Controls ...... 2

Processes and Landscapes ...... 5

Undifferentiated terraces ...... 5

Montgomery terrace ...... 6

Prairie terrace ...... ;...... 6

Loess ...... 7

Deweyville terrace ...... 8

Braided-stream terraces ...... 8

Undifferentiated alluvium ...... 9

Mississippi Valley alluvium ...... 10

Meander belts ...... 10

B ackswa·mp areas ...... 11

Deltaic plain ...... 12,

Chen1�er plain, ...... 14

Geologic History and Chronology ...... 15

References Cited ...... 24

111 LIST OF FIGURES

Page Figure I. Geologic map of the Quaternary formations and deposits of the Lower Mississippi

Valley ...... Under Separate Cover

Figure 2. Quaternary glacial nomenclature and chronology. Synthesized from various sources and

based largely on oceanographic evidence ...... 3

Figure 3. Chronology of late Pleistocene and Holocene landforms and deposits ...... 17 FOREWORD TO SECOND PRINTING

In 1970 the Arkansas Archeological Survey contracted with the National Park Service to prepare information on the geological, archeological, and early historic resources of the Lower Mississippi Alluvial Valley. This information was used by the U.S. Army Corps of Engineers for their Comprehensive Basin Study of the Valley. Dr. Saucier is a geographer speCiaHzing in the subfields of geomorphology and morphology. His interests focus primarily on the paleogeography and paleo­ environments of the Lower Mississippi Valley and relationships between the natural landscape and prehistoric human occupation and use. He has been employed for more than 20 years by the U.S. Army Engineer Waterways Ex­ periment Station in Vicksburg, Mississippi. He has published numerous articles pertaining to aspects of the geomorphic history of the Mississippi Valley and he actively consults and advises on projects involving the application of geo­ morphology in cultural resources management. When the first edition of this report appeared in 1974, it provided insight into the latest thinking and rethinking of the geological processes, past and present, which have affected the Mississippi Valley. It still does. Although there have been a number of significant contributions to the knowledge of the region in the last decade, especially with regard to the geologic evolution of the major tri­ butaries and an understanding of river regime responses to climatic change, there is no single volume compilation or synthesis of these new data and con­ cepts. It is anticipated that such a document will emerge in about five years as a result of studies recently initiated at the Waterways Experiment Station. There are several benefits likely to result from the second printing and con­ sequent wider distribution of this report. Foremost, it will expand the awareness and appreciation by archeologists and others of the limitations of and miscon­ ceptions regarding some of the older and widely accepted "classic" studies of the region. Secondly, it will serve as a basic reference in brief form for those not familiar with the Quaternary geologic framework of the region. Finally, it should serve as a catalyst, as it already has, to bring forth archeologically dtrived chronological information badly needed to refine the very limited amount of geologically derived chronological data available. The readers of this report are urged to be constantly alert as to how the results of their work can be vital to furthering the understanding of Quaternary events and processes. Dr. Saucier welcomes communications and dialogue.

Charles R. McGimsey III Director Quaternary Geology of the Lower Mississippi Valley

INTRODUCTION

The only comprehensive geological investiga­ Saucier 1971), but these publications have had tion of the Lower Mississippi Valley to be con­ very limited distribution. Other indications of ducted was accomplished by Fisk and published need for chronological revisions have been con­ by the Mississippi River Commission in 1944. cerned only with specific areas or problems (Cul­ This monumental work concerned itself with linan 1969; Saucier 1968; Saucier and Fleetwood processes and patterns of fluvial sedimentation, 1970). This discussion is the fi rst attempt since changes in river regime and behavior, and other 1944 to devise a new chronology for the entire aspects of Quaternary geology; however, it was alluvial valley area and to include the results of basically concerned with and is most well noted recent similar work in the Mississippi River del­ foran elaborate reconstruction of valley history taic plain and chenier plain in Louisiana and a detailed chronology of stream channels. In (Frazier 1967; Gould and McFarlan 1959; Kolb present perspective, it is the discussion of valley and Van Lopik 1958; Saucier 1963). It relies heav­ history and chronology that has failed to pass ily upon observations and conclusions resulting the test of time and subsequent investigations, from archeological investigations and attempts whereas some of the less well-known discussions to synthesize these with geological and geo­ of sedimentary processes and river behavior are graphical evidence. Whereas phases of valley as valid today as when written and should be history and conclusions regarding the origin of considered as classic. certain features are well documented, there are Since many weaknesses and incorrect assump­ others that have little or no supporting evidence. tions and concepts are now known to prevail in But, since this is an attempt at a complete se­ Fisk's chronology and valley history reconstruc­ quential history, "all pieces of the puzzle have tion and since his 1944 monograph is still widely been made to fit." accepted as the most authoritative work on the The focusof this discussion is the geologic map area, the need has arisen to prepare this present of the Quaternary formations and deposits of the discussion and to offe r a new chronology for the Lower Mississippi Valley (Figure 1, map in back Lower Mississippi Valley. This discussion has pocket). Only a brief examination of this map will been prepared primarily forworkers in the field reveal a level of detail significantly less than of archeology and history who have an im­ that presented 30 years ago by Fisk. This lack of mediate need forsuch a chronology. The results detail is not a result of generalization demanded and concepts presented herein have evolved by the small scale of the map, but rather it is a primarily from several years of detailed reflection of the confidence of the writer in the engineering-geologic quadrangle mapping in all validity of the evidence now at hand. Whereas parts of the alluvial valley area by or under the Fisk attempted to subdivide the present Missis­ supervision of the writer. sippi River meander belt into 20 stages repre­ Indications of errors and weaknesses in the senting 100-year increments, for example, pres­ Fiskian concepts have been pointed out ent evidence from radiocarbon dates and ar­ (Durham, Moore, and Parsons 1967; Kolb et al. cheological remains makes such a subdivision 1968; Saucier 1964, 1967, 1969; and Smith and wholly untenable. There are now reasons to be-

1 Quaternary Geology

lieve that the present meander belt was fi rst kansas, and Ouachita Rivers and excluding the established only 2,800 years ago in the southern coastal terraces, little or no recent geological part of the valley but as much as 6,000 years ago work has been done. The distribution of alluvium in the northern part of the valley. Thus without and terraces along smaller streams in the up­ specific absolute-age information on each or lands has been taken fr om published sources most of the numerous abandoned channels, any­ (Bicker 1969; Branner 1937; Fisk 1938, 1940; thing but relative-age assignments is prema­ Hardeman 1966; Jillson 1929) and has not been ture. reevaluated. Outside of the valleys of the Mississippi, Ar-

GEOLOGICAL CONTROLS

Cyclic Pleistocene glaciation has been directly postulated that the alluvial valley was deeply or indirectly responsible for the origin, charac­ entrenched throughout and "swept clean" of al­ ter, and distribution of virtually all of the luvium during waxing glaciation, it now appears Quaternary deposits and formations shown in that considerable valley fi ll was d�posited by Figure 1. Although continental ice sheets did not way of braided streams carrying coarse-grained actually extend into the Lower Mississippi Val­ glacial debris. These braided streams, although ley area, they nevertheless were responsible for aggrading the valley, also accomplished appre­ deranging preglacial drainage and creating the ciable valley widening through lateral planation southward-trending river and valley which sub­ and valley deepening through periodic scouring sequently has carried large volumes of glacial of the underlying "bedrock." meltwater and outwash on several occasions. Two additional geologic controls were man­ Equally important controls were exerted by the ifested shortly before and near the time of max­ cyclic glaciation in the form of major changes in imum glacial advance during the last glaciation, base levels of erosion and deposition and in the and it is assumed these same controls were pres­ form of climatic changes. ent during earlier glaciations. One control was Each cycle of advancing and retreating or wax­ the occurrence of sufficient wind-action, proba­ ing and waning glaciation has involved gener­ bly restricted to the cool season and caused by ally the same sequence of changes. The se­ strong anticyclonic conditions, to result in the quences have been repeated five times during deflation of large volumes of silt from active the Quaternary, a period of time estimated to braided-stream channels in the area of glacial have lasted about 1,500,000 years (Figure 2). outwash (i.e., valley train) deposition. This ma­ Coincide nt with each waxing continental terial was redeposited as loess in the uplands glaciation was a eustatic fall in sea level and a east of the alluvial valley, in uplands within the resultant seaward retreat of the Gulf of Mexico alluvial valley, and on older alluvial deposits shoreline. The drop in sea level caused en­ within the valley (Snowden and Priddy 1968). trenchment of the lower reaches of streams di­ The second geologic control was the occurrence rectly discharging into the Gulf and a steepening of pronounced pluvial conditions probably of stream gradients. In the case of the Missis­ brought about by heavy warm season precipita­ sippi River, the stream response due solely to tion and occurring coincident with somewhat falling sea level was probably manife sted no cooler temperatures. These conditions caused farther inland than the latitude of Vicksburg, appreciable increases in the discharges of major Mississippi, and in the case of the last (Late Wis­ Mississippi River tributaries and resulted in consin) glaciation, no farther inland than the valley degradation or lowering and consequent latitude of Alexandria, Louisiana. terrace formation (Saucier and Fleetwood 1970). While entrenchment and valley deepening One would not expect to find and indeed does not were occurring near the Gulf, valley widening fi nd evidence of the effe cts of the pluvial climate and deepening from another cause were occur­ in the main Mississippi Valley area because of ringcoincident with alluviation in the upper part the already braided character of the Mississippi of the Lower Mississippi Valley. Whereas Fisk River at this time and its substantial discharge.

2

------····--·�········-······--········-··········-... YEARS AGO STAGES 0 HOLOCENE �

LA TE WISCONSIN GLACIAL MID WISCONSIN INTERGLACIAL >-- EARLY WISCONSIN GLACIAL -

>- - SANGAMON INTERGLACIAL

>-- -

ILLINOIAN GLACIAL

- -

500,000 - -

- -

YARMOUTH INTERGLACIAL

- -

- -

- -

KANSAN GLACIAL 1,000,000 - -

- -

- - AFTONIAN INTERGLACIAL

- -

- - NEBRASKAN GLACIAL

1,500,000

Figure 2. Quaternary glacial nomenclature and chronology. Synthesized from various sources and based largely on oceanographic evidence.

3 111 I

Quaternary Geology

During all glacial cycles, the maximum lower­ 6,000 years ago north of the latitude of Memphis, ing of sea level was generally coincident with the Tennessee. During the preceding cycle, there maxim um extent of the continental ice sheets. may not have been an interglacial stage of suffi­ Since this lowering was of the magn itude of 350 cient duration for the change in regimen to take to 400 fe et, all coastal and deltaic landformsthat place other than near the coast. When sufficient developed several thousand years before and time was available, however, and meandering after this event were either destroyed by subse­ dominated, slow valley aggradation prevailed. quent erosion or are deeply buried beneath Termination of a glacial cycle occurs with a fu ll younger deposits. Shoreline advance and retreat interglacial stage. In most if not all of the five at rates of tens of fe et per year and over dis­ interglacial stages, sea level was at least as high tances of tens of miles must have occurred be­ as and probably higher than at present and it cause oft he wide and relatively flat nature ofthe maintained a relatively constant level. This continental shelf. permitted the formation of well-developed del­ Sedimentation rates evidently reached their taic plains and possibly also chenier plains by highest levels in the upper part of the Lower either or both the Mississippi and Red Rivers. Mississippi Valley during the early stages of Where sedimentation was less active and marine waning glaciation. Waves of alluviation spread processes prevailed over fluvial processes, the southward with diminishing effe cts and are coastline was characterized by either a well­ manifested in the subsurface at least as far developed mainland beach or a barrier island­ south as the present Gulf Coast. Aggradation in lagoon situation. Partly because of a net the northern part of the valley appears to have shoreline progradation during the Quaternary lasted no more than a few thousand years. Ap­ and topographic separation of base levels, coast­ parently after this time, the ratio of sediment to line fe atures of all types remain well preserved meltwater declined to the point where valley and distinguishable on coastwise terraces. degrad ation occurred, although a braided­ The geologic controls discussed thus far are all stream regimen still existed. This condition was related to climate, sedimentary volumes, and responsible for the creation of the braided­ base level changes. No mention has been made of stream terraces or levels such as survive in the structural controls which would include fault­ valley from two glacial cycles (Saucier 1964). In ing, earthquake activity, and regional warpings. regard to the Mississippi River tributaries, only In general, it can be stated that structural con­ the Arkansas River carried outwash from alpine trols have been truly insignificant in governing or mountain glaciation. Deposition of this ma­ the nature or distribution of the Quaternary terial in the Mississippi Valley beyond the mouth formations north of the Mississippi River deltaic of the Arkansas Valley at Little Rock, Arkansas, plain. Faulting may well have influenced the occurred on at least two occasions during the positions and configurations of tributary Wisconsinan Stage (Saucier 1968). Little is streams and the streams which the ancestral known about the regimes of tributaries not car­ Mississippi River occupied when first flowing rying glacial outwash; however, certain ones ap­ southward in the early Quaternary; however, peared to have begun meandering and aggrad­ the Quaternary deposits along these streams ing in response to rising base levels in the Missis­ have been largely unaffected. Most of the sippi Valley quite early in the waning glaciation quaternary faults that have been mapped on the stage. basis of physiographic evidence (Krinitzsky During each glacial cycle, a stage was reached 1950) have not been verified by sub-surface evi­ when stream discharge and sediment load de­ dence when explored in detail. Similarly, many clined to a point where the ancestral Mississippi faults are known to have affected the Tertiary River changed from braided to a meandering bedrock but apparently have had no effect on the regimen. This apparently first occurred nearest overlying Quaternary deposits. The configura­ · the Gulf Coast and proceeded upv alley. In the tion of the Lower Mississippi Valley itself, often case of the last cycle, the Mississippi River is cited as evidence for fault control, appears to be known to have changed from braided to mean­ related far more to preglacial drainage patterns dering quite abruptly about 12,000 years ago and the relative resistance to erosion of the bor­ south of the latitude of Baton Rouge, Louisiana dering Tertiary formations. There is not a single (Saucier 1969), while it did not do so until possibly known case, including the Mississippi River,

4 Undifferentiated Terraces where evidence has been fo und to substantiate times (Wintz, Kazmann, and Smith 1970). In fact, that faulting has caused a diversion of a stream faulting has been sufficiently active and wide­ or the creation of a distributary. spread to cause problems and confusion in the North of Memphis, Tennessee, in the Missis­ correct identification of terraces (Durham, sippi Valley earthquake region, many of the Moore, and Parsons 1967). In addition to fault­ cases of uplift or doming and subsidence or sunk ing, active structural control in the coastal area land formation attributed to the New Madrid includes upward movements of salt domes. In earthquake series of 1811-1812 have been shown the case of the "Five Islands" of central either to not exist or to be of other origins Louisiana (the most notable examples of salt (Saucier 1970). Certain anomalies such as Mis­ dome uplift), Prairie terrace deposits have been sissippi River meander belt asymmetry do occur; vertically displaced in excess of 150 feet (Van however, their causes have been unexplored and Lopik 1955). The deltaic plain area has also been are currently unexplainable. significantly influenced by subsidence, another Whereas the alluvial valley area has been type of vertical movement which, in part, in­ structurally quiescent during the Quaternary, volves faulting or structural control. Subsidence the deltaic plain has been quite active. In gen­ is primarily a result of the consolidation of both eral, the Holocene deposits have been relatively Holocene and Pleistocene sediments, however, unaffected; however, Pleistocene terrace forma­ and involves movement rates from a minimum of tions have been substantially modified, particu­ about 0.4 feetper century near the edge of the larly by normal faults trending east-west. Verti­ deltaic plain to a maximum of over 10 fe et per cal movements of several tens of feet are known century near the active delta (Kolb and Van to have occurred during the last 50,000 years Lopik 1958). This type of movement is the only (Saucier 1963) and, in some cases, appreciable one to be of direct significance in archeological movements have been recorded during historic and historical investigations.

PROCESSES AND LANDSCAPES

The Quaternary deposits and formations essentially one of well-oxidized, cross-bedded, shown in Figure 1 can be discussed in regard to fl uvial coarse sands with extensive irregular their mode of origin, composition, and present zones or layers of chert gr avels, it is much de­ and past configurations in 11 units or divisions. bated whether these materials were carried Although each unit may involve more than one southward fr om the Southern Appalachians (i.e., causal process and include a variety of land­ nonglacial in origin). In either event, deposition scapes and lithologies, each is nevertheless a dis­ as a widespread blanket over a dissected Ter­ tinctive and definable unit. Geologic age, how­ tiary age landscape by swiftly flowing braided ever, is not a criterion since several units involve streams is indicated (Wallace 1966). Because of two or more separate periods of time. subsequent erosion, the blanket has been re­ duced to discontinuous ridge and hilltop caps of Undifferentiated terraces graveliferous material that vary in thickness This mapping unit principally involves what from a fe w fe et to as much as 150 feet.An intrigu­ are possibly the most controversial deposits in ing aspect of the problem of the origin of these the entire Lower Mississippi Valley area as far deposits involves the ability of certain inves­ as age, origin, and correlation are concerned. As tigators to fi nd mappable physiographic terrace reviewed by Durham, Moore, and Parsons (1967), levels in certain areas, while other equally com­ these deposits, located mainly in southwestern petent investigators, in the same or different Mississippi and central and southwestern areas, are unable to recognize such levels. Louisiana, have been variously identified as Cit­ Unidentified or uncorrelated low and rela­ ronelle formation of either Tertiary or Pleis­ tively fl at fluvial terraces along.small streams in tocene age and/or the Bentley and Williana ter­ east-central Louisiana and southeastern Ar­ race formations of Pleistocene age. Although no kansas are also included in the undifferentiated one can question their lithologic nature, which is terrace mapping unit. Because of their topo-

5 �.· ,, Quaternary Geology 'I

graphic position, they would appear to be more terrace comprises the largest area of Quater­ appropriately correlated with the Deweyville or nary deposits of essentially one age. It also ranks Prairie terraces than with the older deposits dis­ second to the Holocene in terms of the degree of cussed above. Terraces in southern Illinois that preservation of relict geomorphic fe atures on its appear to be related to more recent Mississippi surface. Largely becau se of these fe atures, it is Valley events are also included (Alexander and possible to subdivide the Prairie terrace into Prior 1968). three zones based on the primary mode of deposi­ Discontinuous upland mixed sand and gravel tion. The first zone includes the areas of Prairie deposits which have also variously been iden­ terrace in Arkansas which are relict alluvial tified as Citronelle or Lafayette formation de­ plains of the Arkansas and Ouachita Rivers and posits or Bentley and Williana terrace deposits the areas north and east of the Red River in underlie the loess deposits as mapped in Figure Louisiana which are relict alluvial plains of the 1. These are generally no more than 20 or 30 feet Red and Mississippi Rivers. The second zone in­ thick and occur throughout much of western cludes most of the terrace in southwestern Kentucky and Tennessee, all of Mississippi Louisiana south and east of the Calcasieu River where loess is present, and practically all of which is partly a relict deltaic plain of the Red Crowley's Ridge. Since they are effectively blan­ River and partly a relict upper deltaic plain or keted by loess and can be studied only in occa­ lower alluvial plain of the Mississippi .River. The sional outcrops, less is known about them than latter lies south and east of a line connecting those discussed previously. A non-glacial, Ap­ Lafayette, Louisiana, with White Lake. The palachian source for these deposits is favored by third zone includes the Prairie terrace west of most investigators who have worked in Ken­ the Calcasieu River in southwestern Louisiana tucky and Tennessee; however, occasional ice­ and all of the terrace in southeastern Louisiana. rafted boulders (glacial erratics) have been These areas are relict lagoons, barrier islands, fo und in the deposits in Mississippi, Arkansas, and shallow offshore zones created under the and Louisiana where a glacial origin is generally dominance of marine processes. favored. A large amount of subsurface exploration in Montgomery terrace the Grand Prairie region of Arkansas has indi­ The principal occurrence of the Montgomery cated that this area and nearly all of the Prairie terrace in Arkansas (first zone) is largely relict terrace is in the form of a coastwise terrace of backswamp and meander belt deposits. Clays fl uvial and possibly deltaic origin located in and silty clays of both Mississippi and Arkansas southeastern and sout hwestern Louisiana. River origin are the dominant sediments to a Small areas of the terrace have been mapped in depth of 60 to 100 feet. The terrace along the Arkansas but are of considerably more ques­ Ouachita River is of course of a diffe rent origin, tionable identity. but the sediments are lithologically similar and In its 10- to 15-mile-wide outcrop belt, the diffe r only in that they are somewhat thinner. coastwise terrace is composed of well-oxidized, Whereas the Prairie terrace surface south and brightly colored, silty to sandy clays which grade west of the present course of the Arkansas River, downward into sands and eventually sands and including the Ouachita River area, is essentially gravels. Although the terrace is characterized amorphous and quite flat, the segment which by a maturely dissected, hilly landscape with 50 comprises the Grand Prairie region is charac­ or more feet of local re lief, the basal gravelifer­ terized by distinct and well-preserved meander ous deposits are seldom exposed. The terrace belt fe atures. These include several abandoned fo rmation is probably 300 to 400 feet thick, and it courses of the Arkansas River plus associated slopes to the south at an average rate of 3 to 5 abandoned channels (cutoff meanders) and fe et per mile. Subsurface exploration suggests natural levee ridges which are reflected in the the fo rmation is generally continuous below the topography and drainage of the terrace surface. younger Prairie terrace to the south at steadily Although opinions diffe r, pedologists familiar increasing depths seaward. with the are a feel that the upper few feetof silty Prairie terrace sediment that is frequently present is a thin Next to the Holocene alluvial deposits and the veneer of loess. deltaic plain of the Mississippi River, the Prairie In south central and southwestern Louisiana

6 Loess east of Lake Charles, Louisiana (second zone), However, its location beneath Lake Pontchar­ meander belt fe atures are equally well pre­ train and surrounding swampland is fa irly well served on the terrace. Between Lake Charles known from subsurface evidence (Saucier 1963). and Lafayette numerous segments ofsouthwest­ On the basis of both stratigraphic and lithologic trending Red River meander belts are present, evidence, it appears that much of the uppermost some of which can be traced as low ridges for tens Prairie terrace in southeastern Louisiana as of miles. Because of the relatively large number mapped in Figure 1 is the relict lagoon that ex­ of abandoned courses, the extraordinary flat­ isted north of the barrier island complex. This is ness of the area (both locally and regionally), and substantiated by the nature of the deposits (len­ the prevailing clayey nature of the deposits, this ticular clays, silts, and sands), faunal remains, area is interpreted as a relict deltaic plain of the and the low regional slope and relief of the sur­ Red River (Bernard and LeBlanc 1965). As a re­ face. Beneath the marine lagoonal sediments, flection of the lack of regional slope (1 foot per near the Pearl River, and just south of the Mont­ mile or less), the low relief (frequently less than gomery terrace, where coarse-grained sedi­ 10 feet), and the clayey soils, drainage is poorly ments (sands and gravels) occur, the terrace is developed and surface flooding is common. probably composed of the relict alluvial and/or In the Lafayette-Abbeville, Louisiana, area coastal plains that bordered the lagoon. and in the Marksville, Louisiana, area, the The low, circular mounds several tens of feet in Prairie terrace is characterized by a distinctive diameter that have been variously called pimple meander belt with numerous abandoned chan­ mounds, mima mounds, prairie mounds, and nels and at least one abandoned course of such other names are one of the more striking surface dimensions as to indicate a Mississippi River expressions of the Prairie terrace. They occur by origin. Since very few subsurface data exist, it the hundreds of thousands if not millions in Ar­ can only be assumed that typical meander belt kansas and particularly southwestern soils distribution patterns occur (see section on Louisiana, whereas they are totally absent in Mississippi Valley meander belts) within this southeastern Louisiana. Although they occur on southwest trending meander belt and that it is other Quaternary deposits both older an d flanked by several tens of fe et of clayey and silty younger, they are not nearly as well developed as backswamp or deltaic plain deposits. As in the on the Prairie terrace. Dozens of theories of case of the Grand Prairie region, pedologists are origin have been advanced fo r these ubiquitous inclined to regard the uppermost silty material fe atures; however, none is wholly satisfactory. as loess (Daniels and Young 1968); however, ar­ On the basis of unpublished observations on guments for a fluvial origin (i.e., natural levees their geographic distribution, relative densi­ and related deposits) are equally tenable. ties, relationship to soils types, association with North and west of Lake Charles, the most con­ meander belts, and temporal distribution (none spicuous fe atures of an otherwise fe atureless are known to be associated with deposits less Prairie terrace surface is a barrier island com­ than 2,000 years old), the writer strongly favors a plex trending east-west that can be traced for at biological origin. least 30 miles. This complex is typically a distinc­ tive topographic ridge that has strongly influ­ Loess enced the position of local drainage. No subsur­ The typically tan-colored eolian silt and clayey face data from the complex have been reported, silt deposits in the uplands east of the Mississippi but it is assumed that sand is the prevalent ma­ Valley and on Crowley's Ridge within the Valley terial to a depth of several tens of feet. On the are symmetrically draped over the hills and basis of relative position rather than definitive ridges of the preloess topography (Snowden and lithologic evidence, the area to the north of the Priddy 1968). The thickest accumulations, as barrier island complex is assumed to be a relict much as 90 to 100 feet in the Natchez-Vicksburg, lagoon, whereas the area to the south is assumed Mississippi, area and on the southern end of to be a relict continental shelf. Crowley' s Ridge, are almost always beneath In southeastern Louisiana, the equivalent of ridge crests, while valley areas have little or no the barrier island complex has been strongly in­ loess. Although loess has considerable strength fl uenced by faulting and is now buried beneath when undisturbed because of a natural calcare­ several tens of fe et of Holocene deltaic deposits. ous cementation, it loses this strength when

7 Quaternary Geology leached and weathered and is quite susceptible luvial deposits. Although the terrace surface is to gullying (Krinitzsky and Turnbull 1967). Gul­ typically flat to slightly undulating with local lying and erosion have been severe in the areas relief being 10 to 15 fe et or less, it nevertheless of thick loess accumulation and have led to the exhibits well preserved and diagnostic meander formation of extremely rugged topography with belt fe atures. The more striking fe atures are 75 to 100 feet of local relief being common. large abandoned channels which, in the case of In the uplands east of the Mississippi alluvial the Ouachita River, are three times wider and valley, loess deposits progressively decline in have two times greater meander radii and thickness with increased distance eastward from wavelengths than does the present river the valley wall or bluffs. If noncalcareous brown (Saucier and Fleetwood 1970). These size differ­ loam deposits are considered as true loess, the ences have been interpreted as indicating that extreme eastern edge of the deposits lies an av­ the Ouachita River (and presumably the Arkan­ erage distance of 100 miles fromthe bluffs. How­ sas River as well) had a mean annual discharge ever, since the loess becomes quite thin and dis­ possibly five times greater than present when continuous some 15 to 20 miles from the bluffs, these relict channels were active. Such a large the mapped distribution of loess in Figure 1 is discharge could have been brought about only by significantly less than the total distribution. No pluvial conditions; that is, increased precipita­ loess deposits less than 4 fe et thick have been tion and decreased temperatures. mapped in western Tennessee and Kentucky Very few subsurface data are available on the (Hardeman 1966), while in Mississippi, no loess Ouachita River Deweyville terrace and almost deposits less than 10 feetthick have been map­ none on the Arkansas River equivalent. ped (Bicker 1969). Nevertheless, indications are that relatively A well-developed weathered or soil horizon coarse-grained materials predominate except in within the loess of the Lower Mississippi Valley the abandoned channels. Silty and sandy clays indicates that at least two distinct periods of dep­ should prevail in the upper few feet; however, osition are represented. The upper loess sheet clean sands should be present below a depth of15 is by far the best developed or preserved of the to 20 fe et, and sands with gravels should occur two. Discussions of the probable ages of the two below a depth of 40 feet; Total terrace fo rmation loess sheets are presented in the section on thickness should average 60 to 70 feet along the geologic history and chronology. In addition to Ouachita River and 80 to 100 feet along the Ar­ the soil horizon, there is subtle bedding or kansas River. The abandoned channels probably stratification in the essentially homogeneous contain the only appreciable amounts of soft material that suggests slight variations in the clayey and silty deposits. rate of depositibn such as could occur from shifts in the position of the source area. Braided-stream terraces Glacial outwash or valley train deposits laid Deweyville terrace down by swiftly flowing, sediment-choked The major occurrences of the Deweyville ter­ braided streams are the principal Quaternary race in the Lower Mississippi Valley are along deposits north of the latitude of Memphis. They the Ouachita and Arkansas Rivers. The fl uvial also extend well to the south and have been map­ terrace is also believed to be present along cer­ ped at the surface to the vicinity of the mouth of tain smaller streams, including the Amite and the Red River (Figure 1). Several different Calcasieu Rivers in Louisiana and the Big Black streams have been involved in the formation of River in Mississippi; however, they have not these deposits. In the Western Lowlands area been mapped in detail and/or their extent is too west of Crowley's Ridge, the sediments were de­ limited to show on a 1: 1 ,000,000-scale map. Simi­ rived from glacial outwash from the Mississippi larly, the terrace is present along the Pearl and and Missouri River drainage basins and depos­ Sabine Rivers of Louisiana, but it has not been ited by those streams, while the deposits in the shown in Figure l, since, in each case, it lies just St. Francis Basin east of Crowley's Ridge and in beyond the limits of the study area. the Yazoo Basin of Mississippi were contributed In all cases, the Dewyville terrace lies topo­ by both those streams plus the Ohio River. The graphically lower than the adjacent Prairie ter­ deposits which comprise Macon Ridge in north­ race and higher than the adj acent Holocene al- eastern Louisiana and southeastern Arkansas

8 Undifferentiated Alluvium plus the deposits located west of the Grand reaches 20 to 25 fe et and contiguous dunes or Prairie region were contributed by the Arkansas dune fi elds occasionally cover 100 square miles. River. Within the Western Lowlands area, over 300 Regardless of this diffe rence in source, the de­ square miles are characterized by this type of posits and the resultant landforms are strikingly deposit; smaller areas of dunes also occur east of similar and can be more logically divided on the Crowley's Ridge on Sikeston Ridge and adj acent basis of age. The older deposits, designated as younger braided-stream terraces. Although it is terrace level 1 in Figure 1, exhibit greater relief logical to associate the formation of sand dunes and dissection and tend to be sandier at the sur­ with loess accumulation, a positive relationship, face than the younger deposits which comprise either causal or temporal, has not been estab­ terrace level 2. lished. In both the Western Lowlands and on Macon By far the most well-preserved relict braided Ridge, the areas of oldest braided-stream deposi­ channels occur on the younger braided-stream tion are on the eastern sides. Since cyclic down­ terrace 2 in the St. Francis Basin area. The flat­ cu tting re sulting from gre ater stream-load bottomed, clay-filled, anastomosing channels of transport competence progressed with time, the at least 2 separate river systems can be traced on older deposits are now topographically higher one sublevel continuously for a distance of up to and give rise to several terrace sublevels that 60 miles. In the area and on this sublevel (the are separated by 5 to 20 feet. These sublevels are first sublevel east of Crowley's Ridge), the inter­ delineated in Figure 1 but are not separately fluves are flat to slightly undulating and exhibit designated. Each sublevel is characterized by silty and sandy surface soils. The lower sublevels broad flat to gently rolling stream interfluves ih the St. Francis Basin and essentially all other and narrower, sinuous, flat-bottomed relict braided-stream terrace 2 levels in the Missis­ braided-stream channels. The difference in ele­ sippi Valley lack discernible relict channels. vation between the two is typically 10 to 20 feet Blanketing by a thin veneer of clayey back­ where not accentuated further by subsequent swamp deposits derived from Holocene meander erosion and drainage development. Fine-grained belts of the Mississippi River is the apparent silty and clayey sediments 15 or more feetthick explamttion for this lack of mappable channels overlying coarser materials occur in the relict and is the reason for the more clayey soils and channels, much of this having been deposited lower relief on all of these surfaces. within Holocene times by the local drainage that Whereas most of the braided-stream terrace 2 now occupies these topographic lows. In fact, the is a true physiographic terrace, there are certain later braided channels in the Western Lowlands areas such as in the center of the Yazoo Basin area appear to have influenced if not controlled where the terrace is now topographically one of the location ofthe Holocene meander belts ofthe the lowest areas in the Mississippi Alluvial Val­ larger rivers such as the White, Black, and Cur­ ley. This has been brought about by continuing rent Rivers. In contrast to the channels, the in­ aggradation by the Mississippi River while it has terfluve areas have quite sandy surface soils maintained a meandering regime, and it is the which grade into clean sands and gravels within reason why certain of the braided-stream ter­ 20 to 25 fe et of the surface. These coarse-grained race 2 areas have been veneered with back­ deposits extend to depths of 100 to 180 feet below swamp deposits. It is also the reason why certain both the channels and the interfluves and con­ smaller streams such as the Arkansas River stitute a significant part ofthe huge Quaternary have been able to develop Holocene meander alluvial aquifer. belts across some of these braided-stream ter­ The sandy nature of the interfluve areas in the races. Western Lowlands is locally accentuated by the presence of sand dunes or hummocky sand Undifferentiated alluvium sheets (Smith and Saucier 1971). These deposits, On streams smaller than those such as the Red apparently representing material blown from River, a 1:1,000,000-scale map precludes the de­ active or semiactive braided channels onto adj a­ lineation of specific fl oodplain fe atures. Conse­ cent bars and interfluves, are present princi­ quently, the Holocene deposits along such pally between the Black and Cache Rivers. Relief streams are designated simply as undiffe ren­ within the dunes or dune field occasionally tiated alluvium. Nevertheless, certain features

9 Quaternary Geology

are present on nearly all these streams and the belt area. However, abandoned channels in vari­ sequence of deposits varies little from one ous stages of filling from mostly open water stream to another. (oxbow lakes) to completely filled (referred to as Floodplains of nearly all such streams are low, "clay plugs") are the more conspicuous meander flat, and typically densely forested. They nor­ belt fe atures. Depending in part on the length of mally experience seasonal flooding, and exten­ time a particular meander belt was active, the sive tracts frequently remain swampy through­ number of abandoned channels varies from a out the year. As a consequence, the near-surface dozen or more to well over a hundred in the case soils are usually organic clays and silts. These of the Mississippi River meander belts. deposits are characteristically 10 to 20 feet thick As the term clay plug implies, abandoned and overlie several tens of feet of sands and channels fill with thick deposits of clays and silts. gravels. Mississippi River abandoned channel filling may The forested tracts are occasionally inter­ reach thicknesses of 100 feet or more, while rupted by small lakes or streams that mark the thicknesses of 70 to 80 feet occur in Red and locations of abandoned channels (oxbow lakes or Arkansas River channels. These deposits typi­ cutoffs) or abandoned courses. Since the small cally support a dense swamp or forest vegeta­ streams have minor suspended sediment loads, tion. This stands in sharp contrast to the large the abandoned channels and courses remain un­ expanses of cleared agricultural land.that is the filled for long periods (possibly thousands of overwhelming meander belt landscape. Wide­ years) and natural levees, either along aban­ spread cultivation and habitation is possible doned or active channels, are poorly developed. along the meander belts because of the well­ Compared with streams like the Red and Arkan­ developed natural levees that occur as low ridges sas Rivers, the rates of meandering of small along all abandoned channels and courses and as streams are quite low. In numerous instances, a thin veneer over most of the accretion topog­ fo r example, no changes in channel position dur­ raphy. In contrast to the abandoned channel fill­ ing historic times are discernible. ing, natural levee deposits are predominantly Low fl uvial terraces of Holocene or late Pleis­ well-drained silty and sandy clays. Where tocene age are also occasionally included in the nat1,ral levee deposits overlie accretion topog­ undiffe rentiated alluvium mapping unit. As raphy, the silty and sandy clays grade downward previously mentioned, these may include the through several tens of feet into silty sands and Deweyville terrace, but they may also include sands and eventually sands and gravels. The rel­ younger terraces or "high-level floodplains" atively coarse-graine d deposits that result from originating fro m changes in river regime. Clima­ river migration and which form accretion topog­ tic changes of a minor nature and changes in raphy are referred to as point bar deposits. sediment load due to land clearing or vegetation Natural levees are typically smooth to slightly modification are two factors that could bring undulating surfaces that slope away from their about a change in river regime. parent streams at rates not often exceeding 5 Mississippi Valley alluvium feet per mile. Greatest meander belt relief, often 20 to 25 feet, occurs where natural levee crests In view of the large areal extent of the abruptly join the topographically lower aban­ Holocene alluvial deposits of the Mississippi doned channels or active small streams. Accre­ River and its major tributaries and considering tion topography, where not well veneered with the variety of landforms and sediment types natural levee deposits, typically exhibits 10 to 15 present, the following fo urfold division is em­ fe et of local relief of an undulating nature. ployed. Where local Mississippi Valley drainage and Meander belts upland streams like the St. Francis, Ouachita, Meander belts include all of the area of and and Tallahatchie Rivers intersect abandoned landforms resulting from lateral migration or meander belts of the Mississippi, Arkansas, or meandering of a stream while occupying a single Red Rivers, they frequently occupy the relict course. In the case of freelymea ndering streams abandoned courses as underfit streams or they like the Mississippi or Arkansas Rivers, accre­ may sporadically occupy portions of several tion topography consisting of parallel arcuate abandoned channels. In some instances the ridge s and swales comprises most of the meander smaller streams have completely conformed to

10 11f Backswamp Areas

the general linear trend of the larger streams, section of this paper. while in other instances, they have created their Difficulties and uncertainties in mapping own small meander belts generally or totally meander belts increase markedly with increas­ within the confines of the larger relict channel. ing age of meander belts and in the areas where In both cases, the smaller streams have usually meander belts intersect. Because of veneering built their own natural levees which are disting­ by younger natural levee and backswamp de­ uishable on the basis of size from the relict chan­ posits, some abandoned channels may have es­ nel levees. caped detection or have been incorrectly as­ In Figure 1, the limits of the Mississippi River signed. In the Mississippi Valley proper, the meander belts have been mapped as extending to amount of subsurface data available has pre­ include the outermost abandoned channels cluded the possibility of the omission of an entire thought to be related to the particular meander buried or obscured meander belt. However, this belts. Natural levees associated with the mean­ possibility definitely exists in the case of the Red der belts do extend 2 or 3 miles beyond these River and possibly in the case of the Arkansas limits, but, since the outer edges of the natural River. levees are irregular and gradational with other Backswamp areas areas, they are not readily definable for mapping purposes. The meander belts of the Red and Ar­ The backswamp or flood basin areas mapped in kansas Rivers are typically only 2 to 3 miles wide Figure 1 are areas that have remained predom­ and quite intricate; conseque ntly, they have inantly marginal to meander belts throughout been portrayed in Figure 1 by dimensionless most if not all of the Holocene. They have re­ . lines. Since the actual scale width of the lines is ceived only clays and silts carried into the area less than a mile, this has resulted in an exagger­ by floodwaters, and they represent areas of con­ ation of the extent ofbackswamp areas between tinuous or nearly continuous slow aggradation. meander belts. Specific depositional environments range from An inspection of Figure 1 quickly reveals that infrequently flooded forested bottomland s to there is appreciable variation in the width of the deep, lake-filled swamps that rarely dry up Mississippi River meander belts. Several factors (Krinitzsky and Smith 1969). Both types are are involved in this variation. Perhaps the most characteristically quite fl at and occupy the low­ important factor is the duration of the active est parts of the floodplains. Included as back­ period of flow in the meander belt; the longer the swamp areas are the rim swamps that have de­ activity, the wider the meander belt. This factor veloped between meander belts and valley walls has been largely responsible for the variation in or higher terraces and the ponded or alluvially the width of the present meander belt (desig­ drowned lower reaches of upland streams whose nated as No. 5 in Figure 1) between Cairo, Il­ gradients were appreciably shallowed at the linois, and Natchez, Mississippi. A second factor edge of the alluvial valley as a result of meander is that meander belts naturally become nar­ belt development or other causes. rower downstream, particularly in the transi­ The distribution of backswamp areas and de­ tion zone between the alluvial valley and the posits in the Lower Mississippi Valley reflects deltaic plain. This is brought about by the sig­ dramatically the downstream changes in the nificant increase in a downstream direction of hydrologic regimen and sediment load of the both the thickness and areal extent of clayey Mississippi River, the frequency and distribu­ backswamp deposits that are quite difficult to tion of meander belt development, and the ef­ erode. Intricately involved in this change in fe cts of eustatic sea level variation on river gra­ partly a cause-effect relationship is the down­ dient. From the vicinity of Memphis southward stream decrease in the amount of sand in the bed to near Natchez, the thickness of backswamp and banks of the meandering stream and deposits varies little and increases in thickness changes in hydrologic regime such as decreases from 40 to 50 fe et to 60 to 70 fe et. South of in stage variations. A third factor is that certain Natchez, however, the thickness increases ab­ meander belts, such as segments of meander ruptly, reaching 100 feet above Baton Rouge, belts No. 2 and No. 4, never carried the entire and consistently being 100 to 120 fe et thick discharge ofthe Mississippi River. This situation throughout the lower part of the Atchafalaya is discussed more fu lly in the geologic history Basin. Local variation in thickness also in-

11 Quaternary Geology creases below Baton Rouge. complexes, each of which has several recogniza­ The extensive backswamp development in the ble delta lobes or distributary networks (Frazier Atchafalaya Basin is an extraordinary situation 1967). Each lobe and in a more complex way, each brought about by a peculiar set of circumstances. subdelta repre sents a predictable cycle of Perhaps most important has been the develop­ sedimentation and landscape development. ment and persistence of a Mississippi River Each cycle involves essentially the same chang­ meander belt along the western edge of the allu­ ing pattern of response to controlling processes vial valley for a period of at least 7,000 years. (i.e., fl uvial and marine) and results in essen­ During this entire time, a deep swamp to lake tially the same sequence of deposits. The geo­ environment prevailed eastward across the val­ graphic and topographic separation of delta ley, partly influenced by a rising sea level lobes and complexes and the resultant fo rmation (Krinitzsky and Smith 1969). The meander belt of overlapping lenses of deltaic sediments is at­ also served to block any marine influence from tributable to the omnipresent subsidence (in­ the south. When the river adopted a course along cluding sea level rise) under which deltaic plain the eastern side of the valley, the Atchafalaya fo rmation has taken place. Basin continued to be marginal to active From the physiographic or landscape de­ sedimentation. Only during the last 100 years velopment standpoint, each cycle of delta lobe has there been an introduction of significant and subdelta development involves the fo llowing quantities of silt and sand into the central and sequence. As the river diverts to a new course upper part of the basin as part of an incipient and begins discharging into a shallow body of (but now controlled) diversion of the river (Fisk, water or develops a major crevasse into a bay or Kolb, and Wilbert 1952). Previous abortive at­ lagoon, flow is soon restricted by natural levee tempts during the last fe w thousand years are growth to one or more distributaries. The evidenced by the minor distributaries or cre­ natural levees form as subaqueous fe atures vasse channels mapped in the basin area (Figure ahead of the rapidly advancing distributary 1). mouth and gradually build and emerge as sub­ Backswamp deposits of the Red and Arkansas aerial ridges. Development of subaqeous bars at Rivers tend to be thinner, siltier, and more dis­ the distributary mouths where the stream's load continuous than those of Mississippi River origin of silt and sand is dropped causes frequent bi­ because of the nature of the streams and the furcation or branching of the distributaries. smaller sizes of the valleys. Deposits generally Most branches remain active for only a short do not exceed 40 feet in thickness. As pointed out period of time before being abandoned as a sin­ earlier, their extent has been exaggerated in gle branch eventually becomes dominant. While Figure 1 because of the meander belt symbology distributaries advance seaward as long fingers used. of land, part of the river's suspended load of clay and silt is deposited ahead of and between the Deltaic plain distributaries. As the material accumulates, A line drawn between Donaldsonville and water bodies shallow and intertidal mudflats Franklin, Louisiana, has traditionally been the develop. These soon become colonized with best approximation of the indefinite boundary grasses, sedges, and rushes, and extensive tracts between the alluvial valley and the deltaic plain of deltaic marsh result. Numerous small marsh of the Mississippi River (Fisk 1944). Although lakes and ponds typically characterize the some deltaic plain criteria such as river dis­ marsh. Although marine influences are suffi­ tributaries occur north of the line in the Atch­ ciently strong at the seaward edge of the lobe afalaya Basin, this area lacks most other or subdelta to cause brackish to saline marshes, criteria, partic ularly evidence of effects of the more inland areas between distributaries fre­ marine environment. Recent studies, in fact, quently become sheltered and sufficiently fre sh have shown a need for an even further seaward to allow cypress and gum swamps to develop. shift of this indefinite line to the approximate This pattern of coastline advance, dominance location shown in Figure 1 (Krinitzsky and of fluvial sedimentation over marine processes, Smith 1969; Saucier 1969). and prevalence of aggradation over subsidence The Mississippi River deltaic plain is composed continues until the river diverts upstream to a of at least five discernible subdeltas or delta new course or begins developing a new distribu-

12 Deltaic Plain tary system in another part of the deltaic plain. of seaward build-out or progradation, the un­ As this happens, the amount of discharge and usually large rate of subsidence, and the abnor­ sediment introduced to the existing lobe or sub­ mally great effect of marine processes. These all delta begins to decline. Mudflats and marsh can be explained by the fact that the present areas along the irregular delta margin begin to subdelta is the only one that has fo rmed in rela­ erode under increasingly dominant marine in­ tively deep water at the edge of the continental fluences. Coarse sediments winnowed from the shelf. eroded deposits accumulate in beaches, and the Whereas an individual delta lobe may go com­ delta margin becomes more regular in plan. With pletely through a cycle of growth and decay in a complete abandonment of the lobe or subdelta, few hundred years, a subdelta or complex of subsidence rapidly becomes a dominant process. delta lobes may well last several thousand years. Marsh growth, heretofore able to keep pace with This is due to the fact that a subdelta may experi­ subsidence through vertical accumulation of or­ ence several periods of growth in diffe rent por­ ganic debris, begins to decline rapidly. Interdis­ tions or the same portions provided there is no tributary areas experience rapid expansion of diversion in the upstream trunk channel. As will lakes and bays, and the influence of saline water be discussed later, the subdeltas that have extends further inland. Mainland beaches even­ formed since se a level has attained its approxi­ tually get transformed into barrier island chains mate present level (about 5,000 years ago) and as more and more marsh deteriorates. Dis­ remained relatively constant have tended to re­ tributary natural levees, no longer nourished by main active longer than those that fo rmed ear­ sediment-laden floodwater, become narrower lier during a time of rather rapid sea level rise. and their distal portions retreat inland as subsi­ The present topographic expression of the del­ dence becomes prevalent. Inland swamp areas taic plain also reflects the duration of occupancy die out and become "ghost forests" of standing and age of the various subdeltas. Natural levees dead timber as water salinities become excessive along Bayou Lafourche, one of the younger for swamp growth. Distributary channels, de­ major distributaries, are as much as several prived of through flow, tend to fill in near their miles wide, reach maximum elevations of20 to 25 heads while they tend to open up and become feet above sea level near Donaldsonville, and are tidal channels in their lower reaches. The barrier continuo us as ridges for nearly 100 miles. Al­ island chains retreat inland across widening and though all distributary natural levees narrow deepening water bodies and eventually disap­ and decline in elevation downstream, the effe cts pear themselves as less and less sediment be­ of subsidence near the seaward margin of the comes available. deltaic plain are still evident. Bayou La Loutre Looking at the deltaic plain, the in southeastern Louisiana, once eq ually as large Plaquemines-Modern subdelta (No. 5) is the only as Bayou Lafourche but considerably older, area in which active delta growth is taking place. graphically illustrates the effects of age and sub­ A delta lobe will eventually begin to form in the sidence. Its natural levees are less than half as Atchafalaya Bay area due to increased dis­ wide and nowhere attain elevations of over 7 or 8 charge through the Atchafalaya Basin; how­ fe et above sea level. Elevations of marsh and ever, a conspicuous sub aerial delta is a decade or swamp areas are not indicators of age, however, more away. All other subdeltas are in various since they are basically controlled by tidal varia­ stages of decay depending on their relative age. tions and are nowhere over 2 to 3 fe et above sea Only the oldest recognized subdelta, the Marin­ level. go uin subdelta (No. 1) is now virtually com­ Excluding a fewmino r fe atures like modern or pletely destroyed and/or buried beneath younger relict beach ridges and artificial fe atures like deltaic deposits. spoil banks, distributary natural levees provide In certain respects, the modern or "birdfoot" the only land high enough to support tree growth delta of the Mississippi River departs from the near the outer parts of the deltaic plain. Farther pattern discussed above (Kolb and Van Lopik inland, the natural levees are sufficiently wide 1958; Kolb 1962). The principal visible diffe rence for agriculture ; consequently, all but the nar­ is in the small number of major distributaries, rowest levees have been cleared of timber and while other differences include the unusual are cultivated. The distribution of natural levees permanency of the distributaries, the slow rate is also the overwhelmingly dominant control

13 ., '• il Quaternary Geology

over the pattern of permanent habitation and the gently seaward-dipping Prairie terrace for­ land transport network in the deltaic plain. mation (Byrne, LeRoy, and Riley 1959). Deposit From the standpoint of subsurface lithology thicknesses increase from zero along the contact and stratigraphy, each delta lobe ideally creates to an average maxim um of 25 to 30 feet along the the following generalized vertical sequences of present coast. sediments. As a delta lobe advances seaward to a Only two types of landforms and landscapes given point, the first deposits to be laid down are characterize the chenier plain. The areally dom­ fine-grained prodelta clays. These settle out in a inant landscape consists of relatively feature­ fan ahead of the lobe and their eventual thick­ less marsh interspersed with streams, rivers, ne ss is governed by the depth of water into which ponds, and lakes, sometimes of appreciable size. the lobe is building. The deposits become much Marsh topography is extraordinarily flat with coarser (silts and fi ne sands) and rates of ac­ elevations rarely exceeding 2 to 3 feet above sea cumulation increase as the distributary mouths level. The second and highly conspicuous land­ advance to the giv en point. Distributary mouth scape involves long, narrow, coast-parallel bar sands are naturally thickest along the axis of ridges that attain elevations of 5 to 10 feet, vary the distributary and decrease in thickness later­ in width from a few tens of feet to 1,500 feet, and ally away from the distributary, but they extend extend for miles. These are the characteristi­ appreciable distances through the effects of re­ cally live oak tree-covered relict beaches to working by waves and currents and frequent which the Louisiana French people aptly applied bif urcations of the distributary. Once the dis­ the term "chenier." tributary has advanced beyond the given point, The pattern of relict beaches or cheniers at­ natural levee and channel deposits (silty clays tests the fact that cyclic shoreline advance and and silts) accumulate along the distributary retreat has been the dominant process with a axis, while organic clays and silts and frequent resultant net advance or progradation of the pe at deposits accumulate for long distances lat­ coast. It has long been apparent that the erally. These deposits result from the interdis­ shoreline fluctuations are closely relate d to Mis­ tributary marsh and swamp environments. Once sissippi River subdelta and delta lobe develop­ the process of deltaic deterioration begins and ment and decay and the resultant changes in the delta lobe front retreats landward of the volume of fine-grained Mississippi River sedi­ given point, the sediments that accumulate are ments being carried westward by longshore cur­ mostly silts and sands winnowed and reworked rents. When the Mississippi River was building a from eroding deltaic deposits. Some finer subdelta or major lobe toward the western side of gT ained sediments, often quite shelly or fos­ its deltaic plain, a relatively large amount of fine siliferous, accumulate in enlarging lakes and sediment was carried westward along the coast bays and mark the final stage of the delta lobe and mudflat development and coastal prograda­ sediment sequence. tion dominated. The mudflats would soon be­ Because of prevailing subsidence, it is not at come colonized with marsh vegetation and take all uncommon for an entire sediment sequence to on all of the characteristics of the mainland. be preserved as part of the mass of deltaic sedi­ When the locus of subdelta or delta lobe de­ ments. However, if an area remains inactive for velopment shifted to the eastern side of the del­ too long· a period and water depths offshore from taic plain, less sediments were carried west­ a retreating coastline become too deep, the upper ward. Mudflats would cease growing and begin parts of the sediment sequence may be de­ to erode under coastal wave attack. As the stroyed. This has apparently happened in parts shoreline retreated inland, coarse sediments of t he Maringouin subdelta area and possibly the were winnowed from the eroding mudflats and St. Bernard subdelta area (Figure 1). marsh and combined with shells and shell debris from the coastal environment to form beaches. ClieniCI' pla in The beaches would retreat inland with con­ The chenier plain of southwestern Louisiana is tinued coastal erosion and would grow in size as a 15- to 20-mile-wide strip of Holocene deposits more material was added. With the next west­ that extends from just west of Vermillion Bay to ward shift in the Mississippi River, longshore Sabine Pass and Lake. In cross section, it is an sediment volumes increased, erosion ceased, extremely thin wedge of sediment that onlaps mudflats formed, and the beaches were left

14 Geologic History and Chronology stranded in the marsh. Because of their higher not permit the periodic coastal progradation elevations, the cheniers were soon colonized that is essential to chenier fo rmation; hence, with woody vegetation. they did not occur. All of the cheniers in southwestern Louisiana Deposits of the chenier plain are predomin­ were formed after sea level reached its present antly the soft organic clays and silts and fre­ stand and none are known to be over 2,800 years quently peats of the marsh environment. In con­ old (Gould and McFarlan 1959). Cheniers that trast to these, the deposits of the cheniers are might have formed earlier during the time of characteristically mixture s of silt, sand, and rising sea level would be situated south of the shell fr agments. It is not uncommon, in fact, for present shoreline provided they were not de­ the shell to be the predominant material by vol­ stroyed by erosion or subaqueous planation. It is ume in the cheniers. more likely, however, that a rising sea level did

GEOLOGIC HISTORY AND CHRONOLOGY

The portion of the geology of the Lower Missis­ widened appreciably ·during this stage and sippi Valley with which we are concerned began most if not all of the alluvial valley fill deposited with the widespread deposition of sands and during the preceding Aftonian Intergl acial gravels on an erosional landscape of Tertiary Stage was removed. This would mean that no age, regardless of whether this occurred in the relict floodplain survived to form a depositional late Tertiary or the early Pleistocene. Evidence terrace formation of the type envisioned by Fisk appears to favor a preglacial, southern Ap­ (1939) as representing each interglacial stage. palachian origin for the materials, in spite of an But this does not mean that physiographic ter­ absence of evidence for an impetus. race levels are absent. It appears logical that the The first major period of erosion and dissection surviving remnants of the preglacial gravelifer­ of this graveliferous blanket probably took place ous blanket would have generally accordant during the Nebraskan Glacial Stage (Figure 2). crest elevations and would physiographically It is assumed that this initial continental glacia­ represent a terrace surface, while a similar lower tion was responsible for the derangement of pre­ terrace surface would survive to mark the gl ac ial drain age and the creation of a base level of erosion in the upland corresponding southward-trending ancestral Mississippi River with and caused by the level of valley fill achiev­ and valley. A combination of climate, sea level ed during the Aftonian Interglacial Stage. These fall, and braided-river regimen probably led to physiographic rather than depositional terraces, sufficient lateral and vertical corrasion to result developed on a single fo rmation, may well be the in a distinctive topographic valley at least sev­ Williana and Bentley terraces of Fisk (1939). eral. miles wide. At least an incipient network of Evidence for coastwise terrace development valley tributaries would have developed as well. including deltaic plain or marine fe atures is With waning of the Nebraskan glaciation and totally lacking for the Aftonian Stage and attainment of the succeeding Aftonian Inter­ earlier. If fe atures developed, they are either gl acial Stage, valley alluviation or filling was incorporated in the thick mass of subsurface the dominant process. However, because oflong­ Pleistocene sediments of south Louisiana or they term eustatic trends or extraregional geotec­ have been destroyed by erosion. tonics, the maxim um level of aggradation at­ The Montgomery terrace fo rmation is con­ tained was topographically appreciably below sidered as representing remnants of the alluvial the average level of the graveliferous blanket valley fill and coastal plain that developed dur­ discussed above. Tributary valley filling corres­ ing the Yarmouth Interglacial Stage, although pondingly did not proceed to formerlevel s. there appears to be an inconsistency between Waxing continental gl aciation occurred again the extent and degree of preservation of the several hundred thousand years later during the terrace (with age taken into consideration) and Kansan Gl acial Stage and resulted in renewed the long duration of the interglacial stage as in­ valley degradation and stream entrenchment. It dicated in Figure 2. It is possible, however, that is believed that the ancestral Mississippi Valley the interglacial stage includes a number of sub-

15 l Quaternary Geology I stages and the terrace represents only one of the development marks the last phase of deposi­ later substages. Because of the same regional tional history of the formation in the coastal area base level control exerted previously, the Mont­ for which there is either physiographic or stati­ gomery terrace is topographically lower than graphic evidence. In the alluvial valley are a older terraces. Although a large part of the ter­ the final phase includes the development of � race is a coastwise terrace, there is no evidence cone or fan by the Arkansas River. This consists of marine features or other expression of the of numerous meander belts that radiate from the mode of origin. vicinity of Little Rock, eastward and southward After deposition of the Montgomery terrace, overlying older Mississippi River meander belt the alluvial valley area was again subjected to deposits in the Grand Prairie region of Arkansas. lowered base levels with the usual pattern of According to generally accepted glacial stra­ erosion, entrenchment, and valley degradation. tigraphy and chronology, the Sangamon Inter­ Although the lllinoian glaciation which was re­ gl acial Stage was followed by the Early Wiscon­ sponsible for this is believed to have been ofrela­ sin glaciation which began about 75,000 to 80,000 tively brief duration, there was nevertheless years ago. Stratigraphic evidence from the sufficient time fo r virtually all of the Mont­ Lower Mississippi Valley suggests that this was gomery fl uvial terrace deposits north of the a major glaciation accompanied by a major drop coastal area to be destroyed by erosion. Evidence in sea level. Base levels in the lower .p art of the sugge sts that valley widening proceeded with valley must have responded directly to this sea the most active lateral corrasion again on the level drop; while in the upper part of the valley, eastern side of the valley. This eastward migra­ an analogous change in base level is assumed tion or trend, apparently prevalent throughout to have accompanied the initiation of a new wave much of the Quaternary, undoubtedly has re­ of braided-stream deposition of gl acial outwash. sulted from the fact that most of the larger Although there is evidence of widespread valley sediment-transporting tributaries enter the deepening and widening and the consequent western side of the Mississippi alluvial valley destruction of large areas of Prairie terrace, and deposit part of their load in that area as fan s there is no definitive evidence of depositional or cones. fe atures dating from this period. However, if The Sangamon Interglacial Stage, succeeding assumed relationships between loess formation the lllinoian glaciation, was the third major and valley train deposition are correct, there episode of valley aggradation as evidenced by must have been a period of loess formation that the Prairie terrace formation. This formation occurred during the late waxing stage and/or exhibits indications of a relatively long period of early waning stage of the Early Wisconsin slow valley aggradation and coastal prograda­ glaciation, In Mississippi, it is known that scat­ tion that is believed to have lasted fr om about tered remnants of a loess sheet too old to date 80,000 to at least 100,000 years ago (Figure 3). by the radiocarbon method overlie the upland The earliest depositional history of the forma­ graveliferous deposits and underlie the well­ tion is preserved in its subsurface and involves developed upper loess sheet. Krinitzsky and an initial phase of gl acial outwash (sand and Turnbull (1967) tentatively considered this older gravel) deposition fo llowed by meander belt and loess (i.e., the pre-Vicksburg loess) to be pre­ backswamp formation and deposition of mostly Sangamon (hence, pre-Prairie terrace forma­ fi ne-grained sediments. In the coastal area, an tion) in age whereas Snowden and Priddy (1968), early relatively high, and relatively stationary working with the same evidence, considered it to sea level led to the development of the barrier be post-Sangamon in age and attributable to the islands and lagoons. Somewhat later, possibly Early Wisconsin glaciation (Altonian Substage). after a slight regression of sea level or a fall In view of alluvial valley stratigraphy, the fo llowed by a rise of not quite equal magnitude, latter interpretation appears to be logical and the Red River constructed a delta seaward of more tenable. It is not likely, for instance, that the barrier islands in southwestern Louisiana. easily erodable material like loess would survive The position of the Mississippi River delta at this the long period of erosion and weathering that time is unknown; however, somewhat later it is appears to have characterized the Sangamon known to have fo rmed even farther seaward and Interglacial Stage. slightly to the east of the Red River delta. This The latter waning stages of Early Wisconsin

16 YEARS BEFORE PRESENT 12,000 10,000 8000 6000 4000 2000 0

5

:5 Cl) 4 �� UJ a: ID 3 ii: a: �UJ 2 Cll O Cll Z - < Cll UJ I !! J; ·- J; BRAIDED-STREAM TERRACE 2 ------

PLAQUEMINES- MODERN a: UJ > LAFOURCHE -Cl) ,_ a: < ii:� ST. BERNARD Q. UJ -- - -o Cl! ID !!! ::> TE CHE Cl) Cll !! MARINGOUIN J; w z Id CJ 0 PRESENT J 0 :i:: " a: Cl) � � 5 -w a: ID Cl) 4 - · UJa: � --

I ------� -

PRESENT - Cl) � 4 · -- a: UJ � ID 3 ii: UJa: o o 2 w z a: < w J; I

YEARS BEFORE PRESENT

100 ' 000 80 000 60' 000 40,000 20,000 0 I BRAIDED- I TREAM TERRACE 2 w i z I UJ DEWEYVILLE TERRACE u - -- 0 I- ---i LOE SS LOESS !! UJ ----- 1 J Q. BRAIDED-STREAM TERRACE I - w tc PRAIRIE TERRACE J ---

Figure 3. Chronology of late Pleistocene and Holocene landforms and deposits.

17 Quaternary Geology

gl aciation, occurring perhaps 35,000 to 40,000 surface, or they may include the meander belt years ago, resulted in the introduction of large fe atures on the Prairie terrace in southwestern volumes of glacial outwash into the Lower Mis­ Louisiana in the Marksville and Lafayette­ sissippi Valley. Although the material was wide­ Abbeville areas. In view of present evidence, ly distributed throughout most of the alluvial however, a Sangamon Interglacial Stage origin valley, it now survives at the surface principally for the latter features is still preferable. only in the Western Lowlands and as Macon When the episode of glacial outwash deposition Ridge . There can be no doubt that most of the St. reached its maximum extent during the Mid Francis and Yazoo Basins were once a continua­ Wisconsin Stage, several perennial lakes de­ tion of the sheet of braided-stream-deposited veloped on minor Mississippi Valley tributaries gl acial outwash; however, subsequent valley as a result of damming of the valley mouths by erosion and degradation has destroyed the the outwash. These lakes were first recognized uppermost parts. on and apparently were best developed on the If the assumption is correct that Cordilleran Ouachita River in Louisiana and Arkansas glaciation in the Rocky Mountain region waned (Saucier and Fleetwood 1970). Relict lake bottom earlier than continental glaciation in the upper areas or lacustrine plains now form low terraces Midwest, the gl acial outwash of the Arkansas on the tributaries that experienced the fl ooding River that fo rmed Macon Ridge would be some­ or ponding. In the case of the Ouachita River, what older than the glacial outwash of the Missis­ part of the Deweyville terrace sequence as sippi drainage system in the Western Lowlands. mapped in Figure 1 includes some of these relict Evidence including radiocarbon dates has lake bottom areas and deposits. already been presented (Saucier 1968) indicating Whether or not sea level rose to an elevation that the maxim um level of aggradation of glacial close to that of present sea level during the Mid­ outwash deposition by the braided Arkansas Wisconsin Interglacial Stage (Farmdalian Sub­ River was achieved well prior to approximately stage) is a much debated topic in glacial geology. 30,000 years ago. It is likely that slow down­ The writer has recently assessed evidence from c u tti ng or degradation brought about by the Gulf Coast area relative to this question increased braided-stream load-carrying compe­ (Saucier 1971), and has concluded there is reason tency was well progressed if not even terminated to believe sea level attained an elevation equal to by this time. if not slightly higher than present sea level No absolute age determinations of any type are during this time. Regardless of the precise level, available to indicate the chronology of gl acial however, visible coastal fe atures attributable to outwash deposition in the Western Lowlands. this stage are restricted to nothing more than It appears reasonable, however, to assume that small beach ridges along or near the present the maximum level of aggradation also occurred Prairie terrace - Holocene deltaic and chenier prior to 30,000 years ago and that valley down­ plain contact in Louisiana. If a deltaic plain cutting began shortly thereafter. As indicated formed, it is in the subsurface below the Holo­ previo u sly, the relatively older areas of gl acial cene deltaic plain seaward of the Prairie forma­ outwash in both cases are the more eastern tion and marked by a weathered ero sional higher levels. surface. The stage of maximum withdrawal of the By 25,000 years ago, the Mid-Wisconsin Inter­ Early Wisconsin gl aciation about 28,000 to 30,000 gl acial Stage had terminated and the entire years ago is re ferred to as either the Mid Wis­ Mississippi Valley area was beginning to re­ consin Interglacial Stage (Figure 2) or the Farm­ spond to waxing Late Wisconsin glaciation. The dalian Substage of the Wisconsin Stage. The braided Mississippi River was still slightly distribution of gl acial outwash in the Western downcutting but had adopted a relatively Lowlands area indicates that the ancestral stationary position west of Crowley's Ridge more Missis sippi River never did change from a or less along the present lower courses of the braided to a meandering regime in this lati­ Black and White Rivers. The Ohio River was tudinal zone before the onset of the next or Late flowing near the eastern side of the valley, while Wisconsin gl aciation. If the river developed the Arkansas River was confined to the area meander belts farther south, they are either now west of the Grand Prairie region and west of destroyed, buried and unrecognized in the sub- Macon Ridge. Ii:!l.· !!l: :,ii 18 111 111 Geologic History and Chronology

Even during a time of waxing glaciation, there tion were relatively low, and this was the time of was almost certainly enough glacial debris and a the fo rmation of the extensive sand dunes and sufficient seasonal discharge variation for the dune fields in the Western Lowlands. It was Mississippi River to maintain a braided regime probably also the time of the principal accumu­ throughout much if not all of the alluvial valley; lation of the thin loess deposits on the older however, increased discharges on certain tribu­ braided-stream terraces in the Western Low­ taries because of pluvial conditions apparently lands and areas such as the Grand Prairie led not to a braided regime but rather to the region. About 18,000 to 20,000 years ago, when distinctive type of meandering evidenced by the the volume of glacial outwash carried by the Deweyville terrace. Studies of the Deweyville Mississippi River started to increase substan­ terrace sequence on the Ouachita River (Fleet­ tially, the river abandoned the Western Low­ wood 1969) and observations of its expression lands area entirely in favor of a course through on other streams like the Arkansas and Big a gap in Crowley's Ridge (the Bell City-Oran Black Rivers in the study area (Saucier 1967) Gap according to Fisk) into the St. Francis and the Red, Sabine, and Pearl Rivers outside Basin. Thus, with both the Mississippi and Ohio the study area (Gagliano and Thom 1967) in­ valley trains along the eastern side of the valley dicate cyclic downcutting and valley degrada­ and with the beginning of a major episode of tion occurred through a period of possibly glacial outwash introduction due to waning several thousand years. Radiocarbon dates from glaciation, a major increase in loess deposition the Gulf Coast area indicate that the period of rates occurred east of the valley. If this concept Deweyville terrace fo rmation lasted until at is correct, the inference is that the bulk of the least 20,000 years ago on certain streams. On the loess on Crowley's Ridge is equivalent to the pre­ Arkansas and the lower Red Rivers, the lower Vicksburg loess of Mississippi rather than to the and relatively younger Deweyville terrace levels Vicksburg loess. This possible age diffe rence has are now buried beneath Holocene meander belt not been investigated, much less established. deposits or have been destroyed by lateral The diversion of the Mississippi River into the stream migration. St. Francis Basin area marked the beginning of While the Lower Mississippi Valley area was the last major episode of glacial outwash deposi­ experiencing the cool and wet pluvial conditions tion and braided-stream terrace formation. The 20,000 to 25,000 years ago during waxing glaci­ Mississippi and Ohio valley trains probably com­ ation, there is strong evidence to indicate that bined in the lower St. Francis Basin area and this was also the time when the last major sheet flowed southwestward around the southern tip of loess was being deposited (Snowden and of Crowley's Ridge and thence southward Priddy 1968). This seemingly paradoxical situa­ through the we stern side of the Yazoo Basin. tion can be largely resolved with the assump­ A braided-stream regime may have persisted as tions that the climate at this time was character­ far south as the Gulf Coast, but this has not ized by heavy warm-season precipitation with been established with certainty. During this dry, windy, cool seasons. Radiocarbon dates on time, the Arkansas River was depositing its the Vicksburg loess indicate that deposition load of glacial outwash west ofthe Grand Prairie persisted for several thousand years after the region and north and west of Macon Ridge. end ofDeweyville terrace fo rmation and up until Carrying no glacial outwash, the Red River did and possibly beyond the maximum extent of not develop a cone or fan as did these other Late Wisconsin glaciation (Figure 3). rivers. In fact, there is no evidence that estab­ Although evidence is quite meage r, the writer lishes beyond doubt that the Red River ever feels that valley stratigraphy supports the con­ maintained a braided regime south of Alexan­ cept that this last period of loess fo rmation dria during the Wisconsin Stage. involves two source areas and a major change It is generally accepted that man first ap­ in stream positions. During the initial phase of peared in the southeastern United States, and loess deposition about 20,000 to 25,000 years ago, presumably in the Lower Mississippi Valley the source areas were the Mississippi valley area as well, about 12,000 years ago. This was train on the west side of the Western Lowlands also the time of some significant geological and the Ohio valley train in the St. Francis and events and changes in river regimes. On the Yazoo Basins. Rates of silt deflation and deposi- basis of radiocarbon dates, it appears that the

19 Quaternary Geology

Mississippi River south of Baton Rouge changed quence of events would call for a time span of rather abruptly from a braided to a meander­ from about 7,500 to 9,000 years before present ing regimen about this time and started forming fo r the oldest discernible Mississippi River extensive backswamp areas in the Atchafalaya meander belt (No. 1, Figure 3) in the Yazoo Basin (Krinitzsky and Smith 1969; Saucier 1968). Basin area. South of Vicksburg, the meander A sharp increase in the rate of sea level rise, belt is in the subsurface and indistinguishable. possibly related to a major period of glacial Similarly, no delta or deltaic plain remnants recession (the Two Creekan Substage?), may dating from this interval have been identified have been the impetus. North of Baton Rouge, offs hore. the Mississippi River was still braided and prob­ For the next few thousand years, the history of ably occupied a course that carried it through the Mississippi River is more complex and less the central or eastern part of the Yazoo Basin. well known and understood than one would Although there is no direct evidence, it is fe lt logically expect. The basic problems appear to be that the Arkansas River changed fr om a braided mainly ones of correct temporal association of to a meandering regime about 12,000 years ago segments of meander belts, correct association of and began fo rming what is now the oldest meander belts with subdeltas, and an under­ discernible meander belt southeast of Little standing of the mechanics of river diversions Rock (Figure 3). and waxing and waning discharges among Considering patterns of early human occu­ meander belts. Archeological investigations pancy of the Mississippi Valley, it is important have provided almost all chronological evidence; to remember that significant changes in the meaningful or diagnostic radiocarbon dates on level of the floodplain of the Mississippi River other than archeological remains literally and its tributaries have occurred during the number less than a dozen. With admittedly Holocene. About 12,000 years ago, the average only partial knowledge of even the archeological fl oodplain level south of Baton Rouge was evidence, the writer offe rs the fo llowing tenta­ probably 75 to 80 fe et lower than it is today. tive chronology. The diffe rence in level decreases rapidly up­ About 7,500 years ago, the Mississippi River stre , m; however, the floodpl ain probably began sl owly to abandon its initial meander averaged 20 to 25 feetlowe r than today through­ belt south of Memphis (No. 1, Figure 1). While out most of the interval from Vicksburg to the mean annual discharge slowly declined over Memphis. Only north of Memphis was the flood­ a period of possibly several hundred years, a plain higher than today, the next to lowest new meander belt formed (No. 2) and began to mapped braided-stream terrace (the level now enlarge . This meander belt was located along occupied by the St. Francis River) being the the route of the present river from near Mem­ active depositional surface at the time. Flood­ phis to about 60 miles south of Greenville, plains of the two major Mississippi River trib­ Mississippi. From this point southward for a utaries, the Arkansas and Red Rivers, were distance of 40 miles, a segment of meander belt probably at least 10 to 15 fe et lower than today survives (No. 2, Figure 1) to mark the trend and these have been slowly aggrading since. of this fo rmer course. This segment is referred Midwestern glacial stratigraphy indicates to as the Upper Tensas Meander Belt (Fisk 1944). that there was a brief glacial advance 10,000 to South of this segment, the meander belt is 11,000 years ago (the Valderan Substage), after either destroyed or buried, or both, but probably which time the ice sheet withdrew north of the trended along the western side of the valley Great Lakes. The withdrawal probably took along the route of the later No. 3 meander belt. place by at least 9,000 years ago, and this was The meander belt which was slowly being roughly the time that the Ohio and Mississippi abandoned at this time trended along the Rivers probably occupied the lowest level of eastern side of the valley and is evidenced by a braided-stream terrace 2 adjacent to Sikeston short surviving segment southwest of Green­ Ridge. It is also inferred that about this time or wood, Mississippi (No. 2, Figure 1). From this shortly thereafter, the Mississippi River was area southward, it must have hugged the east­ meandering as far north as about Memphis. If ern valley wall to some point near Natchez, relative dating of later meander belts with where it recombined with the newly forming archeological evidence is correct, a logical se- meander belt to form a full-flow stream channel.

20 ,-. Geologic History and Chronology '

Continuing southward, it followed the western While the westernmost No. 2 Mississippi River side of the valley and eventually reached the meander belt past Greenville was active the Gulf, forming the Maringouin Subdelta. Arkansas Ri ver meander belt or belts �ere Apparently full flow was never achieved by the necessarily restricted to the area west of Macon newly formed meander belt between Memphis Ridge . Only after this meander belt was and Natchez. A roughly equal division of flow abandoned and the No. 3 meander belt was between the two meander belts persisted for carrying the full fl ow of the Mississippi River several hundred years before another diversion could the Arkansas River flow farther eastward. began. This new diversion, occurring about It appears that it did this only once with de­ 6,000 to 6,500 years ago, took place in the upper velopment of the No. 4 Arkansas River meander part of the Yazoo Basin and led to the develop­ belt. The principal period of activity in this ment of the meander belt segment situated meander belt was probably between 5,000 and about 40 miles northeast of Greenville (No. 2, 6,000 years ago (Figure 3). Figure 1). It is surmised that this partial-flow The ab andonment of the No. 3 Mississippi channel lasted only a short while before full fl ow River meander belt and the formation of the was achieved and No. 3 meander belt developed. several partial-flow No. 4 meander belts The name Sunflower Meander Belt has been presents a major problem in alluvial valley applied to this system, which can be traced history. Irrespective of the large volume of southward for about 100 miles through the archeological evidence available, there are still Yazoo Basin (Fisk 1944). The southern continua­ several ways in which the sequence of meander tion of the No. 3 meander belt can be identified belt development can be interpreted and still just west of Natchez, where it is called the maintain compatibility with the evidence. The Cocodrie Meander Belt and can be traced sequence outlined below appears compatible directly into the Teche Ridge along the western with all evidence and is based in part upon side of the alluvial valley. There appears to be the intangibles or "feel" that one develops after little doubt that the Teche Subdelta is the cor­ being closely associated with a problem for a long relative of this meander belt system. Upstream period. in the vicinity of Memphis, the identification of Abandonment of the No. 3 Mississippi River the meander belt segment in the southern end meander belt occurred rather abruptly with the of the St. Francis Basin (the St. Francis Meander formation in quick succession of two diversions Belt of Fisk) as being part of the No. 3 meander in the vicinity of Memphis. One diversion re­ belt system is quite tenuous and was done by a s ul-ted in the development of a partial-flow process of elimination. Assuming such associ­ channel (No. 4, Figure 1) down the eastern side ations to be correct, the No. 3 meander belt is of the Yazoo Basin past Vicksburg and thence the oldest to have more or less continuous along the base of the valley wall past Natchez surface expression and an identifiable deltaic and Baton Rouge. No surface evidence of this plain or subdelta. meander belt survives south of Vicksburg; how­ At the northern end of the Mississippi Valley, ever, subsurface evidence may be present in the a major event took place with the diversion of form of a buried meander belt (Saucier 1969). the Mississippi River from the lowest level of Furthermore, its presence is affirmed by the braided-stream terrace 2 thro ugh a gap in initial growth of a subdelta or delta lobe on the Crowley's Ridge (Thebes Gap) into the area east eastern side of the deltaic plain about 4,600 or of Sikeston Ridge. This diversion also appears to 4,700 years ago (the St. Bernard Subdelta). mark the date of the change of the last segment Sometime prior to 4,000 years ago, a second of the river from a braided to a meandering diversion in the vicinity of Memphis led to the regime. The date of the dive�sion and change development of a second No. 4 meander belt. in regimen is unknown; it is postulated herein This one followed the approximate route of the that it occurred about 6,000 years ago with the present river past Greenville to a point about beginning of the No. 3 meander belt system. 15 miles northwest of Vicksburg. From this From this time on, the Mississippi and Ohio point, it trended southwestward west of the Rivers joined ne ar Cairo and maintained a No. 3 meander belt for a distance of about 100 meander belt along the eastern side of the valley miles. This segment survives as the distinctive to near Memphis. Walnut Bayou Meander Belt (Fisk 1944). Near

21 NEW ORLEANS DISTRICT LIBRARY PROPERTY OF THE UNITED STATES GOVERNMENT US-CE-C il 1' l! Quaternary Geology Ii \\ ,. delta distributaries is older than that of the i. the mouth of the Red River, this new meander belt was forced to cross the abandoned No. 3 Lafourche Subdelta. It should be noted that the meander belt an d apparently rejoined the other Lafo urche Subdelta distributaries formed No. 4 meander belt a short distance farther during this period comprise what is al so some­ downstream. times referred to as the Early Lafourche Sub­ The No. 4 meander belt hugging the eastern delta. In many areas, they are buried or masked valley wall never achieved full fl ow, but ap­ by the considerably more conspicuous and parently it carried a relatively constant partial better preserved Late Lafourche Subdelta dis­ flow (from 40 to 60 percent of the total Missis­ tributaries that formed during the last 2,000 sippi River discharge) fora period of at least 800 years. The amount of discharge into and distrib­ vears. The remainder of the discharge was utary development in the St. Bernard Subdelta �arried by the second No. 4 meander belt men­ during the past 2,000 years has not been too tioned above. This diversion of flow led Fisk to significant. develop what was one of the basic premises of The last major Mississippi River meander belt his valley history reconstruction; that is, the shift occurred as a result of a diversion near easternmost meander belt was formed by the Vicksburg from the Walnut Bayou Meander Ohio River while flowing in a channel separate Belt into the backswamp area at the base of the from the Mississippi-Missouri River part of the valley wall between Vicksburg and near the sy0 stem. Recent geologic mapping in the upper mouth of the Red River. At the latter point, the part of the valley (Saucier 1964), however, has river reoccupied the No. 4 meander belt and vielded seemingly definitive evidence that the continued fl owing southward therein. On the M ississippi-Missouri and Ohio Rivers could not basis of only meager evidence, the writer feels have occupied separate meander belts at this or that this last river diversion started about 2,800 any other time during the last 6,000 years. years ago and was finalized within several For example, at one point about 10 miles south hundred years. of Cairo, the width of the Holocene floodplain The writer would like to digress at this time to as measured between the valley wall and the indicate a possible point of confusion in regard edge of the braided-stream terrace 2 is only 4 to the use of the No. 5 meander belt designation. miles. It is considered impossible for the two Whereas the numbering of other meander belt rivers to have maintained separate meander segments is based on the relative date of their belts through such a narrow gap. abandonment, this is obviously not possible with Abandonment of the easternmost No. 4 Missis­ the currently active one. It should be kept in sippi River meander belt seems to have taken mind that where continuous occupation or reoc­ place relatively slowly between about 3,500 and cupation has occurred, only the latest occupation 4,000 years ago. As a consequence of this can be shown. For instance, the No. 5 meander abandonment, the westernmost No. 4 meander belt north of Memphis lies in essentially the belt enlarged to accommodate the full discharge same location as the preceding No. 3 and 4 of the Mississippi River. This period of full fl ow meander belts. Since it is impossible to deter­ lasted from at le ast 2,800 to 3,500 years ago. The mine which if any of the various surviving river at this time was roughly along the present abandoned channels or other meander belt route from Cairo to near Vicksburg, along the features are associated with a particular time Walnut Bayou Meander Belt to the vicinity of span or meander belt such as No. 4, only the the mouth of the Red River, and thence along latest designation can be used. Consequently, it the present river route past Baton Rouge. is important to remember that the No. 5 meander During the principal life span of the No. 4 belt north of Memphis may contain landforms meander belts or frorrt about 2,800 to 4, 700 years that are anywhere in age from 0 to 6,000 years. ago, it appears that both the St. Bernard and Between Vicksburg and Baton Rouge, features Lafourche Subdeltas developed from this trunk anywhere in age from 0 to 4,800 years may be channel and remained active. Detailed subsur­ present, since the No. 4 meander belt occurs in fa ce sedimentological studies sugge st that the same location as the latest one. This same various delta lobes in both subdeltas alternately situation also occurs elsewhere on older fo rmed during this period (Frazier 1967); how­ meander belts, but to a far less extent. ever, the average age of the St. Bernard Sub- From New Orleans, Louisiana, to the Gulf, the

22 Geologic History and Chronology

No. 5 meander belt developed not from a diver­ chronology are present with the Mississippi sion but rather from the preferential occupation River meander belt result in compounded or of a distributary or distributaries. In the vicinity exaggerated errors in the Red River sequence; of New Orleans, the present river channel was but, assuming no great error in this association adopted about 1,200 years ago (Saucier 1963), and and assuming that no meander belt greatly it becomes progressively younger downstream. exceeded the 1,000- to 1,500-year average length A date of 1,000 years before present is thought to of occupation, a strikingly short period of be a good approximation forthe beginning of the meander belt activity is indicated (Figure 3). The Plaquemines-Modern Subdelta (Figure 3). geologic history of the Lower Mississippi Valley Turning to the Arkansas River area, the No. 4 offers no explanation or justification for such a meander belt of this stream was abandoned short period. Consequently, the writer has con­ about 5,000 years ago as a result of a major cluded that significantly older meander belts do upstream diversion near Little Rock (Figure 1). exist but have not been observed because of a The new meander belt (No. 5) formed west of lack of detailed mapping or, more probably, Macon Ridge and persisted for possibly as long as because of burial by more recent meander belt 2,000 years. At least two segments were later deposits. Rapid aggradation in the lower part of abandoned as a result of diversions, one of which the Red River valley during the Holocene should developed through a gap in the Prairie terrace have been prevalent because of proximity to the into the Ouachita River valley (No. 6) about 3,000 Gulf of Mexico and its appreciable eustatic sea years ago (Saucier and Fleetwood 1970). Aban­ level rise. donment of this meander belt in favor of the In the coastal area, detailed chronologies for present one may have occurred as recently as both deltaic distributaries and cheniers have 1,000 years ago. been worked out (Frazier 1967; Gould and In view of the writer's unfamiliarity with both McFarlan 1959); however, this level of detail is the geology and archeology of the lower Red inconsistent with the nature ofthis report and is River area, the chronology of meander belts for not reported herein. It is sufficient to say that all this stream is quite tentative. The greatest cheniers are younger than 2,800 years and all are chronological guide, other than the obvio us related to shifts in sedimentation pattern s relative ages of the meander belts, has been within the St. Bernard, Lafo urche, and their relationship to the No. 3 Mississippi River Plaquemines-Modern Subdeltas. meander belt. Thus, whatever errors in absolute

23 REFERENCES CITED

ALEXANDER, CHARLES s. and JEAN c. PRIOR DURHAM, CLARENCE 0., JR., CLYDE H. MOORE, 1968 The Origin and Function of the JR., and BRIAN PARSONS Cache Valley, Southern Illinois. In 1967 An Agnostic View of the Terraces: The Quaternary of Illinois, Univer­ Natchez to New Orleans. Geological s1'.ty of lll1'.no1'.s College of Agricul­ Society of Amerfoa Field Tr1'.p ture. Special Publication No. 14, pp. Guidebook, pp. El-E22. Baton 19-26. Urbana. Rouge.

BERNARD, HUGH A. and RUFUS J. LEBLANC FISK, HAROLD N. 1965 Resume of the Quaternary Geology 1938 Geology of Grant and LaSalle of the Northwestern Gulf of Mexico Parishes, Louisiana. Bulletin of the Provmce. ln The Quaternary of the Louisiana Geological Survey, No. Um"ted States, edited by Henry E. 10. New Orleans. Wright, Jr., and David G. Frey, pp. 1939 Depositional Terrace Slopes in 137-185. Princeton University Louisiana . Journal of Geomorphol­ Press. Princeton. ogy, Vol. 2, No. 2, pp. 181-200. Col­ umbia University Press, New York. BICKER, AL VIN R. JR. 1969 Geologic Map of Mississippi. 1940 Geology of Avoyelles and Rapides M1'.ssiss1'.ppi Geological Survey. Parishes, Louisiana. Bulletin of the Jackson. Louisiana Geological Survey, No. 18. New Orleans. BRANNER, GEORGE C. 1944 Geological Investigation of the Al­ 1937 Geologic Map of Arkansas. Arkan­ luvial Valley of the Lower Missis­ sas Geological Survey. Little Rock. sippi River. U. S. Army Corps of Eng1'.neers, Miss1'.ssipp1'. River BYRNE, JOHN v., DUANE 0. LEROY, and Commission. Vicksburg. CHARLES M. RILEY 1959 The Chenier Plain and Its Stratig­ raphy, Southwestern Louisiana. FISK, HAROLD N., CHARLES R. KOLB, and Gu{f Coast Assocfot1'.on of Geolog1'.­ Lours J. WILBERT cal Societies Transactfons, Vol. 9, 1952 Geological Investigation of the pp. 237-259. Houston. Atchafalaya Basin and the Prob­ lem of Mississippi River Diversion. CULLINAN, THOMAS A. U. S. Army Corps of Engineers, 1969 Contributions to the Geology of Waterways Experiment Station. Washington and St. Tammany Vicksburg. Parishes, Louisiana. Ph.D. disser­ tation, Tulane University, New Or­ FLEETWOOD, ARTHUR R. leans. 1969 Geological Investigation of the Ouachita River Area, Lower Mis­ DANIELS, R. B. and KEITH K. YouNG sissippi Valley. U. S. Army Corps of 1968 Loess in Southcentral Louisiana. Eng1'.neers, Waterways Experiment Southeastern Geology, Vol. 9, No. 1, Statfon, Technical Report S-69-2. pp. 9-19. Durham. Vicksburg.

24 References Cited

FRAZIER, DAVID E. Technical Report 3-483. Two vols. 1967 Recent Deltaic Deposits of the Mis­ Vicksburg. sissippi River: Their Development and Chronology. G'U�f Coast AssoC'i­ KOLB, CHARLES R., WILLIAM B. STEINRIEDE, ation of Geological Societies Trans­ JR., ELLIS L. KRINITZSKY, ROGER T. SAUCIER, actions, Vol. 17, pp. 287-315. Hous­ P. RONALD MABREY, FRED L. SMITH, and ton. ARTHUR R. FLEETWOOD 1968 Geological Investigation of the GAGLIANO, SHERWOOD M. and BRUCE G. THOM Yazoo Basin, Lower Mississippi 1967 Deweyville Terrace, Gulf and At­ Valley. U. S. Army Corps of En­ lantic Coasts. Loufafona State Uni­ gineers, Waterways Ea;perime nt vers1'.ty, Coastal Studies Institute, Station, Technical Report 3-480. Technfoal Report No. 39. Baton Vicksburg. Rouge. KRINITZSKY, ELLIS L. GOULD, HOWARD R. and EDWARD McFARLAN, 1950 Geological Investigation of Fault­ JR. ing in the Lower Mississippi Valley. 1959 Geologic History of the Chenier U. S. Army Corps of Eng ineers, Plain, Southwestern Louisiana. Waterways Experiment Stat io n, Gu�f Coast Association of Geologi­ Technical M emorandnm 3-31 1. cal Societ?'.es Transactions, Vol. 9, Vicksburg. pp. 261-270. Houston.

KRINITZSKY, ELLIS L. and WILLARD J. HARDEMAN, WILLIAM D. TURNBULL 1966 Geologic Map of Tennessee, West 1967 Loess Deposits of Mississippi. Sheet. Tennessee Department of Geolog1"cal Society o.fAmerica, Spe­ Conservation, D1'.vision of Geology. cfol Paper No. 94. New York. Nash ville.

KRINITZSKY, ELLIS L. and FRED L. SMITH JILLSON, WILLARD R. 1969 Geology of Backswamp Deposits in 1929 Geologic Map of Kentucky. the Atchafalaya Basin, Louisiana. Kentucky Geolog1'.cal Survey. U. S. Army Corps of Engineers, Lexington. Waterways Experiment Station, Techm'.calRe port S-69-8. Vicksburg. KOLB, CHARLES R. 1962 Distribution of Soils Bordering the SAUCIER, ROGER T. Mississippi River from Donaldson­ 1963 Recent Geomorphic History of the ville to Head of Passes. U. S. Army Pontchartrain Basin. Louisiana Corps of Engineers, Waterways Ex­ State Um'.vers1'.ty Press, Coastal periment Station, Techm"cal Report Stud1'.es Series 9. Baton Rouge. 3-601. Vicksburg.

1964 Geological Investigation of the St. KOLB, CHARLES R. and JACK R. VAN LOPIK Francis Basin, Lower Mississippi 1958 Geology of the Mississippi River Valley. U. S. Army Corps of En­ Deltaic Plain, Southeastern Louisi­ gineers, Waterways Experiment ana. U. S. Army Corps ofEng1'.neers, Station, Technical Report 3-659. Wa terways Experiment Station, Vicksburg.

25 Quaternary Geology

SMITH, FRED ROGER SAUCIER 1967 Geological Investigation of the L. and T. Boeuf-Tensas Basin, Lower Mis­ 1971 Geological Investigation of the sissippi Valley. U. S. Army Corps of Western Lowlands Area, Lower Engineers, Waterways Exper1'.ment Mississippi Valley. U. S. Army Station, Technical Report 3-757. Corps of Engineers, Waterways Vicksburg. Experiment Station, Technfoal Re­ port S-71-5. Vicksburg. 1968 A New Chronology for Braided Stream Surface Formation in the SNOWDEN, JESSE 0., JR., and RICHARD R. Lower Mississippi Valley. PRIDDY Southeastern Geology, Vol. 9, No. 2, 1968 Geology of Mississippi Loess. pp. 65-76. Durham. Bulletin o.fthe M1'.ssissi.ppi Geologi­ cal Survey, No. 111. Jackson. 1969 Geological Investigation of the Mississippi River Area, Artonish to Donaldsonville, Louisiana. U. S. VAN LO PIK, JACK R. Arrny Corps of EngintJers, Water­ 1955 Recent Geology and Geomorphic ways Expen'.ment Stat1'.on, Techn1'.­ History of Central Coastal cal Report S-69-4. Vicksburg. Louisiana. Louisiana State Uni­ vers1'. ty, Coastal Studies Institute, 1970 Origin of the St. Francis Sunk Techn1'.cal Report No. 7. Baton Lands, Arkansas and Missouri. Rouge. Bullet1'.nof the Geological Soc1'.ety of America, Vol. 81, pp. 2847-2854. WALLACE, RAYMOND H., JR. Boulder, Colorado. 1966 The Pascagoula-Citronelle Contact, Southern Wilkinson County, Mis­ 1971 The Northern Gulf Coast During sissippi, as Related to Ground the Farmdalian Substage, A Search Water Conditions. Louisiana State for Evidence. Louisiana State Uni­ Un1'.vers1'ty, Department o.fGeol ogy. versity Press, Geoscience and Man Baton Rouge. Seri es. Baton Rouge.

SAUCIER, ROGER T. and ARTHUR R. FLEETWOOD WINTZ, WILLIAM A., JR.,RAPHAEL G. KAZMANN, 1970 Origin and Chronologic Signifi­ and CHARLES G. SMITH, JR. cance of Late Quaternary Ter­ 1970 Subsidence and Ground-Water Off­ races, Ouachita River, Arkansas take in the Baton Rouge Area. and Louisiana. Bulletin of the Bulletin of the Louisiana Water Re­ Geological Society of America, Vol. sources Research Inst1'.tute, No. 6. 81, pp. 869-890. Boulder, Colorado. Baton Rouge .

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