' Allostratigraphy and Geoarchaeology Within

the Mississippi Alluvial Valley

Paul V. Heinrich

The alluvial plain of the Mississippi River is a composite geomorphic surface composed of smaller, morphostratigraphic units - geomorphic surfaces that lie within an indsed valley (Sauder 1974; Autin et al. 1991). Five types of geomorphic surfaces, namely, meander belts, backswamps, braid plains, fluvial terraces, and lacustrine terraces, exist within the Mississippi Alluvial Valley and its tributaries. First, a meander belt is a surface that consists of the geomorphic surface and constructional landforms cre­ated by the lateral migration of a river while occupying a single course. Second, the backswamp, also called a "flood basin", is the portion of an alluvial plain associated with meander belts consisting of swamp,lakes, or combination of both. Third, the braid plain is an alluvial plain exhibiting the intricately interconnected channels of braided streams or rivers which created it. Within the Mississippi Alluvial Valley, they exist only as fluvial terraces. Fourth, a fluvial terrace is a relict fluvial plain bounded by erosional escarp­ments and parallels an adjacent, modern stream or river. They occur along the wall of the incised valley and as outliers within it. They are the erosional remnants of other geomor- · phic surfaces, e.g., braid plains, meander belts, or backswamps. Finally, a lacustrine terrace is an erosional remnant of a plain that once formed the bottom of a slackwater lake within the valleys of rivers and streams tributary to the Mississippi River (Shaw 1915; Saucier 1974, 1987; Autin et al. 1991).

Allostratigraphy According to the North American Commission on Strati­graphic Nomenclature (1983), an allostratigraphic unit is a mappable body of sedimentary rock or unconsolidated sediments that is defined and identified on the basis of bounding discontinuities. In case of an indsed valley, ero­sional unconformities and the previously described geomor­phic surfaces are bounding discontinuities that can be used to define and map allostratigraphic units (Fig. 1). Two distinct types of fluvial allostratigraphic units, depending whether the geomorphic surface is either fluvial terrace or meander belt, can be recognized.

First, the type of fluvial alloformation associated with a fluvial terrace has three different bounding disconformities (Fig. 1). First, the basal disconformity of this alloformation is a fluvial erosion surface, typically an undulating surface, cut by either the entrenchment, lateral migration, or both of a river channel. This unconformity is related to the deposi-

tion of the overlying fluvial sediments. Second, the scarp that defines a fluvial terrace is the exposed edge of an younger erosional surface which truncates the sediments which comprise a fluvial terrace. As a result, a scarp, as reflected by differences in surface morphology, soil develop­ment, and thickness of overbank deposits, separates geo­morphic surfaces and fluvial deposits of differing ages. Finally, the upper boundary consists of a geomorphic sur­face, previously defined as a "fluvial terrace" (Autin 1992).

After its formation, the fluvial terrace can be altered by post­depositional processes. Fluvial terraces are frequently buried by younger sediments after their formation. These sedi­ments may consist of either overbank deposits, eolian sands, loess, or colluvium. Where buried intact, a fluvial terrace might be detectable by either laterally persistent paleosols or truncated weathering horizons and abrupt changes in sedi­mentary fades. With prolonged subaerial exposure, erosion of a fluvial terrace will eventually obliterate and obscure constructional landforms and pedogenesis will form pro­gressively thicker paleosols.

Stratigraphic, radiocarbon, and archaeologic data from the Richard Beene Site (41BX831) illustrates the association between fluvial terraces and allostratigraphic units (Fig. 2) (Mandel and Caran 1992). The modem floodplain and the Walsh, Leona, and Miller Terraces are associated with fluvial unconformities that clearly define allostratigraphic units of alloformation rank. Because of the presence of apparent erosional topography associated with it, differing fluvial regimes on either side of it, and significantly older radiocar­bon dates beneath lt, the truncated Somerset paleosol is interpreted to be the surface of another alloformation. Because of the lack of significant temporal hiatuses, changes in fluvial regime, and erosional unconformities, associated with the other paleosols, they are considered to lie within the same alloformation of which the Applewhite Terrace formsitssurface (fig. 2). Within this valley, vertically accreted overbank sediments containing numerous paleosols bury the Qc alloformatlon instead of the hypothetical backswamp deposits illustrated in Fig. 1 (Fig. 2).

Finally, the meander belt is the surface of an allostratigraphic unit consisting of a basal bounding discontinuity, an upper bounding discontinuity, and a body of fluvial sediments that lies between the bounding discontinuities (Fig. 3). Typically, the upper bounding discontinuity consists of either an exposed or buried meander belt. In case of a

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meandering system, the fluvial sediments lying between the unconformities consist of a lower part composed of point bar sands and gravels, overlain by finer-grained and verti­cally accreted natural levee and overbank sediments. The basal bounding discontinuity is an erosional unconformity formed by scour at the channel bottom and, at the edges, by cutbank erosion. Outside of the meander belt natural levee deposits extend into and interfinger with the adjacent backswamp sediments (Fig. 3) (Fisk 1947; Walker and Cant 1984; Autin 1992).

Unfortunately, the allostratigraphy of the sediments under­lying the backswamps that lie between the meander belts of the Mississippi River and its tributaries indsed into braided fluvial terraces is uncertain. For example, the backswamps associated with meander belts within the Western Lowlands appear to fill narrow alluvial valleys cut into braided fluvial terraces (fig. 4). The lack of detailed boring logs for drillholes penetrating these presumed entrenched valleys prevents the resolution of the, if any, allostratigraphic units underlying these backswamp deposits and the nature of the basal contact of the backswamp deposits. Similarly, the actual base of the allostratigraphic units underlying the braided fluvial terraces cannot be determined because of the lack of contrast between their sediments and the underlying valley fill and the lack of undisturbed cores through them (Fig. 4).

Origin of Fluvial Allostratigraphic Units The origin of fluvial complexes and alloformations has been explained by two general models. First, the classic model of Fisk (1944) concludes that a terrace is the result of fluvial aggradation followed by a period of fluvial entrenchment. His model implies that a floodplain is constructed by the aggradation of a fining-upward sequence in response to a rising base-level, typically a relative rise in sea level. A terrace is created when a floodplain is abandoned by the fluvial system as a result of entrenchment in response to a dropping base-level, typically a relative drop in sea level. According to this model, each fluvial terrace and its associated allostratigraphic unit represents the fluvial response to a single rise and fall in base level, which is commonly assumed to be sea level (Autin 1992).

However, Autin (1989, 1992) and Blum (1992) demonstrate that geomorphic processes more complex than simple changes in base levels create fluvial terraces and alloformations. Autin (1989) found that a temporal cluster­ing of cutoffs initiates a period of meander belt instability within the Amite River Valley of Louisiana. This response results from changes in one or more geomorphic influences, e.g. climate, base-level, etc., which cause an imbalance between river hydrology and sediment delivery. Because of the increased rates of channel cutoffs, the channel pattern locally straightens which favors channels avulsion over lateral accretion. Avulsion creates a new channel which truncates the older alluvium and produces the initial lateral boundaries of a new alloformation. A new stable meander belt is established with a channel pattern and slope in equilibrium with the new conditions of river hydrology and sediment delivery after a few decades to centuries of instabll­ity(Autin 1992).Signiflcantly,Autin(1989, 1992) and Blum (1992) demonstrate that to simply interpret all fluvial ter-

races and their alloformations solely as the result of rises and falls of sea level is a grossly simplistic explanation that can be wrong as often as it is right (Heinrich 1992).

Lacustrine Terraces A lacustrine terrace is an erosional remnant of a plain that once formed the bottom of a slackwater lake. These lakes were created by the blockage of the mouths of tributary valleys by the aggradation of Mississippi River valley trains. These terraces are highest at the mouth of the tributary and drop in elevation to a flat surface upstream within the tributary valley. Rarely, they exhibit either beach-ridge or river mouth bar like landforms (Shaw 1915; Heinrich 1982; Sauder 1987).

Each lacustrine terrace is the surface of a well-defined allostratigraphic unit. The base of this allostratigraphic unit consists of an erosional unconformity which represents an indsed valley cut during interglacial - interstadial times when the tributary valley was unblocked by a gladal valley train. Lying between this lower unconformity and the lacustrine terrace surface is a sequence of nonglacial fluvial sediments overlain by gladal slackwater lake and loessial sediments. The lacustrine terrace forms the upper bounding disconformity of this allostratigraphic unit (Heinrich 1982).

Geoarchaeology The application of allostratigraphy to Late Pleistocene and Holocene sediments and geomorphic surfaces within the Mississippi Alluvial Valley can be used to understand its archaeology. Allostratigraphy can be used to define, corre­late and map, genetic depositional sequences of sediments within what otherwise appears to be heterogeneous fluvial deposits. Unlike simple terrace mapping, allostratigraphic units formally tie geomorphic surfaces to the three dimen­sional packages of sedimentary deposits associated with them. A properly defined allostratigraphic unit consists of a set of landforms and depositional sequence that formed during a discrete period of time, by specific set of sedimen­tary processes, and in response to a specific set of environ­mental conditions. As a result, by combining sedimento­logical and archaeological data for individual alloformations, generalized, regional predictions can be made for the occur­rence, visibility, preservation, and cultural affiliation of archaeological deposits upon landforms and within sedi­ments that form an alloformation. In addition, educated guess can be made about the age and, thus, the cultural affiliation of the archaeological deposits which would have been destroyed by the formation of a fluvial allostratigraphic unit.

For example, because the braided fluvial and lacustrine terraces predate the human occupation of the Mississippi Alluvial Valley, they represent stable features of the land­scape that were available for the accumulation of archaeo­logical deposits since Paleo-Indian times (Morse and Morse 1983; Sauder 1987; Autin et al. 1991). As a result, archaeo­logical deposits will have a high visibility on these geomor­phic surfaces because in situ archaeological materials of all ages and cultural affiliation will occur only upon these surfaces, except where either younger colluvial and alluvial

138

""'"""' E ......_, c: 0

.+:i ca > cu w

Meander Belt

Distance (Kilometers) 2

AI = Undifferentiated alluvium Ba = Backswamp facies c =Channel fill facies Ob =Overbank facies Pb = Point bar facies

Figure 1. HyQothetical fluvial alloformations within an incised valley. Heinrich (1992).

Bedrock

0

LEGEND

Qc

Relative Distance (km)

Modified from

Qd

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Qa, Qb, Qc, Qd, Qe, and Qf = Unnamed alloformation from oldest to youngest

20

15

10

5

170

160

150

140

130

Figure 2. Allostratigraphic interpretation of an actual incised valley, the valley of the Medina River at the Richard Beene Site (41 BX831), Bexar County, Texas. Modified from Mandel and Caran (1992).

139

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Active Channel

Abandoned Channel 30

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10

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-10

.-L-20

-30

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LEGEND Facies ~ Water in Ba = Backswamp ················· active channel Lpb =Lower po1nt bar hm~~~=~ Natural levee Upb = Upper point bar Usg = Undifferentiated sands and grav~ ~~:~~~flll

0 ~Colluvium - Erosional boundary • .. .. .. .. .. . Erosional boundary

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Figure 3. Cross-section across a hypothetical alloformtion associated with a mec belt. Constructed using data from Saucier (1969) and other studies.

10

0

-10

-20

LEGEND - --- -- '- ' Facies boundary

l'fffi'ffiffi1 Braid~ stream~ Colluvium - Erosional boundary lillillillil deposits ~ Inferred erosional ';1;:~-:::%:@ Natur~llevee ~ Aban~oned channel·''''''' boundary ·:·:·:=:::.:::.~:-= deposits ~ deposits Bs = BackswamJ? deposits ~===! Backs~amp ~ Water in Pb = point bar deposits t::=:1 deposits ~ active channel Usg =Undifferentiated sands and gravels

Figure 4. Hypothetical assemblage of fluvial allostratigraphic units associated witt braided stream terraces. Constructed using data from Smith and Saucier (1971 ).

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deposits have buried them. Similarly, some early Paleo­Indian archaeological deposits might be shallowly buried by the possible continuing accumulation of Peoria Loess into middle Paleo-Indian times along the easternmost edge of Macon Ridge Hillman (1985). Because, these archaeological materials occur only as surficial deposits, they will be mixed and degraded by pedogenic, biologic, and agricultural pro­cesses with time. Thus, typical archaeological deposits oc­curring on the braided fluvial and lacustrine terraces could be expected to have a poor preservation potential and integrity, except where protected by unusual circumstances.

In case of meander belts, evaluations can be made of the occurrence, visibility, preservation, and cultural afflllation of the archaeological deposits for individual meander belts. For example, the stage of development of a specific meander belt will affect the preservation and visibility of archaeologi­cal deposits within it. During the avulsion and early mean­der belt stages of meander belt development, vertical aggra­dation of natural levees predominates over lateral migration (Farrell 1989). During these stages, any archaeological de­posits which form upon these levees will be quickly buried and preserved within the aggrading natural levees. Within a poorly developed meander belt, site preservation will be high, although site visibility might be low.

In addition, if not abandoned early in its evolution, a meander belt will eventually enter the fully developed meander belt stage which is characterized by the rapid lateral migration of its channel (Farrell 1989). During the fully developed meander belt stage, the rapid back-and­forth migration of an active channel across the meander belt will continually destroy archaeological deposits on the natural levees of its cutbanks and bury archaeological deposits on its point bars. As a result, both preservation potential and visibility of archaeological deposits within a fully developed meander belt will be low until after the time of its abandon­ment (Munson 197 4; Heinrich 1991). Thus, within the area occupied by a fully developed meander belt, archaeological deposits older than it will have been destroyed; archaeologi­cal deposits contemporaneous with will have been either buried or destroyed; and archaeological deposits postdating it will occur as surface or shallowly buried deposits with a few exceptions resulting from unique circumstances. The tim­ing of meander belt activity will distinctly influence the age of the archaeological deposits destroyed or occurring as burled and surface deposits.

Summary The alluvial plain of the Mississippi Alluvial Valley and the sediments that underlie it can be subdivided into disconti­nuity-bounded stratigraphic units using the principle of allostratigraphy. Typically, each of these allostratigraphic units will consist of a body of sediments formed by specific set of depositional processes during a discrete period of time. As a result, each allostratigraphic unit should possess ar­chaeological deposits characterized by a specific range of age, cultural affiliation, and occurrence as burled, surface or both types of deposits.

References Autin, W.J., 1989, GeomorphlcandStratigraphlcEvolution

of the Middle Amite River Valley, Southeastern Louisi­ana: [unpublished Ph.D. dissertation]: Louisiana State University, 177p ••

Autin, W. J., 1992, Use of Alloformations for Definition of Holocene Meander Belts in the Middle Amite River, Southern Louisiana. Geological Society of America Bul­letin, vol. 104, p. 233-241.

Autin, W. J., Bums, S. F., Miller, B. J., Saucier, R. T., and Snead, J. 1., 1991, Quaternary Geology of the Lower Mississippi River Valley, in Morrison, R.B., ed., Quater­nary Nonglacial Geology, Conterminous U. S., The Geology of North America, vol. K-2, Geological Society of America, Boulder, p. 547-582.

Blum, M.D., 1992, Modem Depositional Environments and Recent Alluvial History of the Lower Colorado River, Gulf Coastal Plain of Texas: {unpublished Ph.D. disser­tation]: University of Texas at Austin, 286 p.

Farrell, K. M., 1989, Stratigraphy and Sedimentology of Holocene Overbank Deposits of the Mississippi River, False River Region, Louisiana: [unpublished Ph.D. dis­sertation, Louisiana State University.

Fisk, H. N, 1944, Geological Investigation of the Alluvial Valley of the Lower Mississippi River: Mississippi River Commission, U.S. Army Corps of Engineers, Vicksburg, Mississippi.

Fisk, H. N, 1947, Fine-Grained Alluvial Deposits and their Effects on Mississippi River Activity: Mississippi River Commission, U.S. Army Corps of Engineers, Vicksburg, Mississippi.

Heinrich, P. V., 1982, Geomorphology and Sedimentology of Pleistocene Lake Saline, Southern illinois: [unpub­lished M.S. thesis]: University of illinois, 145 p.

Heinrich, P. V., 1991, A sedimentological explanation for the distribution of archaeological sites in a meander belt asstatedbythe"relictchannelrule":Transactionsofthe Gulf Coast Association of Geological Societies vol. 41, p. 320.

Heinrich, P. V., 1992, The geoarchaeology, geomorphology, and Quaternary geology of Galveston Bay and adjacent Texas Coastal Plain, in, Pearson, Charles, ed., Historic Preservation Plan for Galveston District, Galveston Bay, Texas: Report submitted by Coastal Environments, Inc. to U.S. Army Corps of Engineers, Galveston District, Galveston, Texas.

Mandel, R. D., and Caran, S. C., 1992, Guidebook lOth Annual Meeting South-Central Friends of the Pleisto­cene - Late Cenozoic alluvial stratigraphy and prehis­tory of the inner Gulf Coastal Plain, south-central Texas: Lubbock Lake Landmark Quaternary Research

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Center Series, no. 4.

Morse, D. F., and Morse, P. A., 1983, Archaeology of the Central Mississippi Valley: Academic Press, New York.

Munson, P.J., 1974, Terraces, meander loops, and archaeol­ogy in the American Bottoms, Dllnois: Transactions of the lliinols State Academy of Science vol. 67, p.384-392.

Sauder, R. T., 1969, Geologicallnvestlgationsofthe Missis­sippi River Area, Artonlsh to Donaldsonville, I.Dulsiana: U.S. Army Engineers Waterways Experimental Station Technical Report, S-69-4 Vicksburg, Mississippi.

Sauder,R.T., 1974,QuatemaryGeologyofthei.DwerMissis­sippi Valley: Arkansas Archaeological Survey Research Series No. 6, Fayetteville.

Sauder, R. T., 1987, Geomorphic interpretations of Late Quaternary terraces in western Tennessee and their regional tectonic implications: U.S. Geological Survey Professional Paper, no. 1336-A.

Shaw, E. W., 1915, Newly discovered beds oflakes in south­em and western Dllnois and adJacent states: lliinois State Geological Survey Circular, no. 20, p. 139-157.

Smith, F. L., and Saucier, R. T., 1971, Geological investiga­tions of the Western I.Dwlands areas, I.Dwer Mississippi Valley: U.S. Army Engineers Waterways Experimental Station Technical Report, S-7 4-5 Vicksburg, Mississippi.

Walker, R. G., and Cant, D. J., 1984, Sandy fluvial systems, in Walker, G. R., ed., Fades Models, Geoscience Canada Reprint Series No. 1, Geological Association of Canada, Toronto, Ontario, p. 71-90.

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Quaternary Geology and Geoarchaeology of the Lower Red River Valley

• A FIELD TRIP •

Trip Leaders Whitney J. Autin and Charles E. Pearson

Friends of the Pleistocene South Central Cell

11th Annual Field Conference Alexandria, Louisiana

March 26-28, 1993

guidebook edited by Whitney Autin and John Snead

Table of Contents

Friends of the Pleistocene South Central Cell Field Conferences (list and map) .... ...... ....... ...... ..... iii

Table of Contents.............................................................................................................................. iv

Ust of Figures ............................................................ ; ........................................................................ vi

Ust of Tables .................................................................................................................................... vii

Acknowledgements ····················································································································~···· viii

Introduction - J. Snead ........................................................................................................................ 1

Section I - An Overview

Quaternary Geology of the Lower Red River Valley- W. J. Autin ......................................... S

Geoarchaeology of the Lower Red River Valley- C. E. Pearson and D. G. Hunter ................ 25

Section II -The Field Trip

STOP 1 • Late Tertiary to Middle Pleistocene Evolution of an Upland Erosional Landscape: Review of the Williana and Bentley Area- W. /.Autin, J. Snead, P.M. Walthall, D. f. McCraw, and W. f. Day ..............•••.........•.••.......•.............•.•...................•..•.................. 45

STOP 2 • Late Middle Pleistocene Evolution of a Constructional Alluvial Plain: Review of the Montgomery Area and the St. Maurice Section- W.f. Autin, f. Snead, P. M. Walthall, D. f. McCraw, and W. f. Day ........................................................................................... 53

STOP 3 • Wisconsinan Constructional Alluviation of the Red River: Review of the Aloha Prairie Area- W. J. Autin,/. Snead, P.M. Walthall, D. f. McCraw, and W. f. Day .................. 61

STOP 4 • Archaeological Sites Along the Pleistocene Terrace Margin and Red River Floodplain - C. E. Pearson and D. G. Hunter ...••..........•.•.......•.•.....•......................••....................... 69

STOP S • Archaeological Sites Along Bayou Rapides: An Abandoned Red River Course -C. E. Pearson and D. G. Hunter ..•.•.....•.•.•....•......................................................................... 7 5

STOP 6 • Hydrodynamics, Flow, and Suspended Sediment Transport of the Lower Red River - J. Mossa .................................................................................................................... 81

STOP 7 • Loyd's Hall: Historic Period Settlement and Use of the Bayou Bouef Meander Belt -C. E. Pearson and D. G. Hunter ...••....•....••....•.••••••....••.........•...••••.•..•..•.•....................... 93

STOP 8 • Monda Gap and the Red River Diversion - C. E. Pearson and D. G. Hunter ...•...... 99

STOP 9 • Wisconsinan Constructional Alluviation of the Mississippi River: Review of the Avoyelles Prairie Area- W. f. Autin, A. Asian, f. Snead, and D. J. McCraw ............... 103

ROAD LOG .......................................................................................................................... 116

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Section III - Contributed Articles

Surficial Deposits of Tertiary Age in the Central Louisiana Area- J. E. Rogers •••••••••••••••••• 125

The Fate of Fisk's Pleistocene Units in Texas - S. Aronow •••••..•••.••••..••••..•.•.••..•••.•.•••.•....•... 128

Soil Stratigraphic Units of the Lower Red River Valley- B. A. Touchet .•...••••••...••.•..•..••...• 133

Waddell Bluff and Terrace Stratigraphy in the Lower Red River Valley- J. J. Alford and J. C. Holmes ..................................................................................................................... 135

Allostratigraphy and Geoarchaeology Within the Mississippi Alluvial Valley - P. V. Heinrich .................................................................................................................... 137

Extractions of Engineering Geology on the Lower Red River - P. E. Albertson.................. 143

Characteristics of Cores from the Upland and Intermediate Complex in the Florida Parishes - W. J. Autin, J. Moss a, and B. J. Miller..................................................................... 14 7

Surface Geologic Mapping in Louisiana - Its Beginnings, Rise, and Recent Decline - R. P. McCulloh ............................................................................................................ ....... 153

Distinctive Patterns in the Areal Distribution of Stream Alluvium in North Louisiana - R. P. McCulloh .... ...... .............................................. .......................................................... 158

Appendix

1993 FOP Guidebook Contributors .................................................................................... 163

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