, , . i , .• U.S. DEPARTMENT Of r,oMMERCE . National Technicallnform.lion Service ...•. AD;'A033 42 ORIGIN AND CHRONOLOGIC SIGNIFICANCE or LATE QUATERNARY TERRACES, OAUCHITA RIVER ARKANSAS AND LOUISIANA ARMy·ENGINEER WATERWAYS EXPERIMENT STATION VICKSBURG, ·MISSISSIPPI . MAy 1970
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Terraces Along the Ouachita River, Arkansas and Louisiana
Scan of May 1970 U.S. Army Corps of Engineer Report by Roger T. Saucier about the "Origin and Chronologic Significance of Late Quaternary Terraces, Ouachita River, Arkansas and Louisiana.
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, , . i ,
.• U.S. DEPARTMENT Of r,oMMERCE . National Technicallnform.lion Service
...•. AD;'A033 ~ 42
ORIGIN AND CHRONOLOGIC SIGNIFICANCE or LATE QUATERNARY TERRACES, OAUCHITA RIVER ARKANSAS AND LOUISIANA
The extent of terrace mappine (Plate 1) is essenti.ally coincident
>lith the extent of available large-scale topoV-aphic mapping b:r the U. S.
Army Corps of Engineers and the U. S. Geological Survey. Initial terrace
identification on1 delineation 'ISS perforJOc.Q using contours >lhile cross-
valley ane] do'tm-vn,11ey profilen, Sh0'tlir.g both actunl and recon:Jtructed-
sur.face elevations, ;Iere used for modifications oy,d refinements. Final
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-- -. . ,'
I delineations were achieved through an examination of the physiographic ex-
. pressions of. the terraces on aerial photOgl'aphs and photo mosaics.
Subsurface data in the form of logs of wells and borings were .used to
. establish terrace Htholo!\y anil permitted estimates of such parameters "s
depths of river scour and valley entrenchment. Field reconnaissances of the
study area proved to be generally unre>mrding because of the low· relief,
heavy vegetation cover, lack of access to many large areas, and infrequent
outcrops 'or road cuts.
In essence., this has been an intensive investigation of regional
physiography. Geomorphological investigation techniques proved to be the
most effective, and the geomorphic expression of landforms proved to be
the most significant factor in discerning terrace stratigraphy and infer-
ring modes of origin.
Ackn01;ledgmenta
Special thanks are extended to Dr. Charles R. Kolb for his review of
the mam,script and to both him and Mr. H. B. steinriede, Jr., for their
valuable critical discus.,ions of the problem that helped crystallize our
thinking. Nrs. Naomi J. Rhodman and ~!rs. Hary E. Skipworth assisted >lith , typing, and ~!r. J. A. Sherlock provided editorial review. Supervision by
Hr. Nels J. Nyman of the ext'msive drafting assignment is appreciated. Ap-
prova1 by the Office, Chief of Engineers, of the manuscript for publication
is acknO>lledged.
GEOGRAPlITC AND GEOLOGIC SErTING
The Ouachita Piver flo'ds generally southeashlard over an airline dis
'tance of about 300 miles from its hea:"raters in the Ouachita Mountains of
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i west-central Arkansas to where it becomes tributary to the Red rover in
east-central louisiana (Fig. 1). It meanders freely and has created 0.2-
to. 5-mile-Hide floodplain in the Coastal Plain beh/con Ho.lvcrn, ArK. '. and
sterlingten,' la. South ef sterlingten, the river enters th~J.!ississippi
alluvial valley and fle"s as an undcrfit stream in an abandened Helecene
meander belt ef the Arkallsas River (ng. 1 nnd Plate 1).
The Quaternary valley er'the Ouachita River is quite ~,ssymetrical
betHeen Camden, 'Ark., and ~,!onree, La. (Plato 1). Threughout mest ef this
distance, the present river is at the base or within 5 to. 10 miles of the
western valley wall "hich rises abruptly to. heights ef 100 to. 150 ft abeve
the floodplain. Upland fermat ions Hest ef the rivo" are efEocene age
(ClaiberneGreup) and are composed of thinly-bedded and mestly uninduratcd
clays, silts, and fine sands of shalle" marine or deltaic erigin. Ellst
ef the present river, ene must cre'·s a 20- to. 25-mile-wi(l,e, step-like
sequence ef low, flat terraces befere enceuntering pre- Quaternary ferma
tions at similar elevations.
The Eocene formatiens strike reughly nerthenst-seuth"est, dip gently
to. the. ~outheast, and have been enly moderately affected structurally. At
least ene major fault zone cross",s the Ouachita Valley near Smackover, Ark.;
ho,tevcr, an almost complete absence of evidence of f"ulting in Q>l!lternary
deposits indicates general quiescence since Tertiary times. DifferentIal
eresien of the Eocene formatiens has resulted in a maturely dis&ected up-
land landscape ,~th faint suggestiens of cuestas.
The Ouachita River floodplajn north of Sterlington 1s subject to
annual flooding and consequently is uninhabited and heavily timbered '.-lith
deciduous hard'iJOods. Numerous shoals- and bars characterized the river at
.'
t---= ----------------- ---r'--,§':,,,f, ,.
ARKANSAS
GilL F
OF MEXICO
""-:.
L 0 J~ i
o '0 ! , I • !
.0 !
TENN.
Figure 1. location of st\ldy are:l and relationship to the lower l-lissis3i.ppi Valley.
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.'-j 1 .. ~
low water in its natural state, but a series of small locks and dams now
provides a navigable ~hannel to Camden. The terraces are sparsely populated;
widespread pine forests support an extensive lumbering industry. S.outh of
sterlington, the higher and ''lider natural levees of the old ·Arkansas Hiver
courses are extensively cultivated and industrialized and well-populated,
. with Monroe being the largest urban con,plex.
GENERAL STRATIGRAPHY AND JIOHENCLATURE,
Prairie and Hontgomery ,Terraces
The Prairie terrace formation (Fisk, 1939) is the most are ally exten"
dve Quaternary terrace in the Ouachita River area (Plate 1) and also the
most easily identified terrace in regard to surface morphology and relative
topographic position. Although the Prairie terrace Has originally assigned
by 'Fisk to the Peorian (Bradyan) Interglacial stage,several recent invPd-
,
tigations in the Gulf and Atlantic Coastal Plain areas have indicated evidence
- for the assignment oithe Prairie or it" equivalents (Beaumont of Texas and
Pamlico of Florida and Georgia) to the Sangamon Interglacial stage (Gagliano
and Thom, 1967; Hoyt and others, 1968; Schnable, 1966). Evidence for such -,';-
an ';Issignment is present in the LO\'lel' W.ssiss;ppi Valley (Saucier, 1968) and
also in the Ouachita Valley.
Maturely dissected remnants of a terrace situated topographically
higher than the Prairie occur in the extreme northeast part of the mapped .. area. Little at-Gent ion has been devoted to this terrace; ho,rever, super
ficially it appears to moet the crit~ria set forth by Fisk (1939) for the
Montgomery terrace.
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. Deweyville Terrace, '
''''' The name De,reyville was first applied by Bernard (1950) to !\ low dep-
: ositional terrace along the lower Sabine River in Texas and Louisiana. • .- •• M
,R\,cognition of the Deweyville (1S a distinct terr!\ce formation was based on
'1ts"intermediate position bctween the Prairie terrac~ .and the Holocene
floodplain and the presence of distinctive meander scaro several times'
, -larger in width and radii than those of the present river. Bernard con~
side,'ed a pluvial c;timatc expianation for the large meander features but
concluded that they were lUore likely attributable to a rapid rise it; sea
level in Late "is cons in times.
'. In ,this initial work on the Deweyville, Bernard recognized similar
,·t~rracesala'ng the Brazos, TrInity, and Pearl Rivers of the central Gulf
,: COast 'ar~a' and w~s aware of a possibly related terrace along the Ouachita
'River. "Somewhat' lat'er; Bernard and L,Blanc (1965) expressed an opinior" -'.,c
8S have Gagliano and Thom (1967) and Saucier (1968),that the Deweyville
't,erl:'ace l'robably'correlates with braided stream surfaces in the Mississippi
~lluvial valley. Published (Pearson and others, 1966, p. 458) and unpub-
lished radiocarbon dates from along several OJlf Coast rivers suggest for-
mation of tho De."'yvi:.. ... e cerrace between about 13,000 and 30,000 years
before present.
,
As many as three levels or sub-terraces have been recognized (Gagliano
and Thom, 1967; Gagliano, 'personal cOliullunication) along the Sabine and
Trinity Rivers where large-scale topographic mapping is available and has
been examiried in detail. Along the Ouachita Rivor, three terrace levels
OCCur between the Prairie tenace and the Holocene floodplain. Although
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only the lowest level, designated as the Qtd3
level (Plate 1), exhibits large
meander scars, all three levels are considered to comprise the De"eyville
~terrace. Evideneeconsidered more conclusive than has heretofore been avail-
able will be presented later in this paper to demonstrate a close relation-
ship of all three terrace levels to the Mississippi alluvial valley braided
surfaces and to further substantiate the suggested time span.
TERRACE lolORPlIOLOGY AND LITHOLOGY
Introduction
Each Quaternary terrace formation, including the Holocenc alluvium,
is a depositional sequence consisting of a fine-grained to~stratum (clays
and silts) and a coarse_grained substratum (sands and grf.vels). The
topstratum-substratum thickness ratio of about 1: 2 or 1:3 remains rather
. uniform throughout the area except on the older terraces where portions of
the topstratum have been removed by erosion. Topstrat1JJll deposits primarily
represent overbank deposition in natural levee, backswamp (flood-basin),~ and
abandoned channel or course envirorunents, ~lhereas substratum deposits -were
laid dO>n! in point bar, channel lag, and possibly braided-channel bar en
virorunents (Allen, 1965; ;"leehrood, 1969). The only significant exception
to this occurs in relation to thc dO'n!stream portions of the Qtdl and QtO'2
terraces where much of the topstratum represents' deposii;ion in" a lncustrine
environment.
Prairie Terrace
Interfluve areas of the Prairie terrace formation are,!ufficiently
~rell preserved to permit reconstruction of former elevations and slopes,
8
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I·•··· .. r·~~: ~:" "
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.. '
tlltho'.lgh differential erosion has reduced the once neurly flat relict
floodplain toa gently rolling to moderately hilly landscape. As indi-. - _.
eated'in Figu;e' 2, th:' formation ex'nibits a reconstructed dOWllvalleY,slope
·.that· waxc;; southw,,,'" !'rom about 0.33, f't/mile above Fel.senthal, Ark., to
over 0.75 f't/milebelow Honroe. The convex slope. profile is quite similar
" to that of .the ·present river at natural low water; the two remain seParated
bY about 110 f't over a distance of about 90 miles. . .
. Subsurface data indicate that the Prairie' terrace formation was de-
posiLt':a oa utl eL'o"lollal surface on Claiborne Group deposits that was a1-
,
most certainly formed as a result of ent.renchment during a preceding glacial
'stage, probably the Iilinoian. Local relief on the erosior! surface appears
"',to ·"e.les8 than 40 f't •. In view of a lack of any indications of soil pro-
file de.velcpment or subaerial ;reathering characteristics, it is assumed
that erosion took place 'oeneath an alluvial COVer as a result of lateral
a completely amorphous surface expression. Relict channel scars are to-, '
tally lacking; hence ,no indication of former stream patterns or dimensions
can be obtained.
Deweyville Terraces 1 ani 2
The highest DeHeyvilleterrace formation, the Qtdl
terrace, is wide
spread north of the Ouachita River in Arkansas and occurs at scattered
", localities on bota sides of the river in Louisiana. ' It is only slightly
dissected a.q compared llith the Prairie terrace fOi'lllation. Where large con-
tinguous areas exist, such as along Horo Creek and in the Flatwods terrace
'area (Plate 1), ,interfluves exhibit relief of less than J ft over areas of
several square miles. The intel'lllediate De'18yville terrace formation, desig-
nat~d as theQtd2 terrace, occurs along the 10>ler Saline Rival' in Arkansas
'and aiong the Ouachita River essentially orily south of the Saline River. , ,.- -" . . 'Surfacerelief on this terrace is comparable to that on the Qtd
l terrace:
" Upstream from Felsenthal, the Qtdl terrace has a rather uniform dO>ll1-. :.'~ - - .
valleynlope of about 0.50 ft/mile (Fig. 2). Ho,rever, near Felsenthal th~
slope abruptlY,declines to less than 0.1 ft/mile, and south of Alabama
Landing, the surface slope is zero. The Qtd2 terrace exhibits a similar
decline in do,mvalley slope and it also finally attains a zero slope south
of Monroe.
This decline in slope was an enig~a to the authors for an appreciable
period during the study, parti'cularly ,rnen considered in comparison ,lith
the slopes o~ the other terraces and ths Holocene floodplain. The only
possibly tenable explanation for this ;tas that the Ouachita River during
this time was shallo,ring its gra!lient and agerading its floodplain in 1'0-
sponse to rapid alluviation ill the Miss;'ssippi alluvial valley.
11
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: .. ,-.
Considerable evidence has subsequently been discerned that supports this
hypothesis, by far the most significant evidence baing in tha form of a
series of 10\, ridges such as the one illustrated in Fi[,'ure 3. These:
ridges, delinentedin red in Plate 1, occur on both sides of the Ouachita
River south of the Arkansas-Louisiana state line. Regardless of length
or . location, they all have a relatively constant width of 1000 to 2OO0ft,
they have conspicuously accordant crest elevations varying only from about
103 to 105 ft msl,~hey stand at least 10 ft and usually ro ft. above the
adjacent terrace levels, they al,~ys occur on the Qtdl
terrace at its con
tact with the Qtd2 terrace, and, with but one exception, drainage is de
veloped inllnediately adjacent and parallel to the ridges on the higher ter-
. race side. Profiles nonnal to the ridges all sho>! relative elevation dif-
i'erences and slopes simi}-ar to those included in Figure 3.
rt· has been concluded that these features arc beach ridges that formed
.. around a large possibly seasonal but more likely perennial lake for >!hich
the authors intrClluce the name Lake 140n1'oe. The pro'oable extent of the lake
:. is indicated in Figure II.
The exceptional flatness, both local and regional, of the main extent
of the Qtdl terrace belo>! the state line and its position relative to the
beach ridges suggest that it "as a backbeach or backbarrier flat that >Ins
subject to inundation only during extremely high lake staees. The Qtd2
terrace is interpreted as being the relict lake bottom or lacustrine plain.
Baline River is considered to mark the approximate northernmost extent of
the lake; the sloping Qtdl terrace north of this point c.long the Ouachita
is the relict floodplain of the anoestral river before it discha,ged into
~ ~ ______ ~ Qld,TERRACE I cll::~_------------~~--________________ ~ l Figure 3. !.!Orphology of a typical lake beach cr barrier and adjacent surfaces located about 7 miles soutlnTest of I·:onroo, La.
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STATUTE "'<t.U .~X3.t >II
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1£II(H£$ M"l." .... JUI.;{R
"ns
1I'&'c0(8tI-C1I OR II"-Co(· IIAFlRI!R fLJ,.U
,1\000",(\I .. [ (U[tfT or LA"'! "'ON~[ AT NOII",,lL Sl,lC.t
Figure 4. Extent and confiBuration of Lake lolonroe nnd associated features.
14
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f.
to those of the present river are discernible on this terrace in the }.lora
Bay area, >lhile the backbeach or backbnrrier flat portions of the twrace
have a complctely amorphous surface expression.
At normal stage, extinct Lake Nonroe is estim"tcd to have had an area
of about 500 sq miles (Fig. 4) and 11 mean maximum depth of 20 to 25 rt. The
area of the lake may have been as much as 700 sq miles at extremely high
stages; hO;Tever, the dcpth Hould have been only 5 to 10 rtgre,atcr. These
are~ determinations ,are based on the assumption that the lake did not ex~end
appreciably beyond a line draHn connecting the southernmost extent of the
Qtdl
terrace on the east and >lest sides of the Ouachita River (Plate 1).
previously mentioned,.- The fact that the sequence is present to the southern-
most extent of the terraces, including the area occupied by Lake Nonroe, in-
dicates a l'luvial orlgin for most of the terrace sediments in that area.
Probable lacustrine and backbesch-flat sediments apparently overlie' the
fluvial sequence as a thin veneer and have been observed at only tHO loca-
tions: 'in the FlatHoods Terrace area, >There the Qtdl
terrace topstratum is
unusually thick and contains large quantities of silt-size material, and in
,an area about 6 miles south;rest of J.lonroc, >There the .wne terrace exhibits
extensive deposits of fine sands and silts in the topstratum. Samples from
these bl0 areas were analyzed, but no sedimentologie or paleontologic evi
dence was discerned that confirmed deposition in a lacustrine or lacustl'ine- '
related environment.
The Qtd2
terrace is er:)sional rather than depositional in at lcaot
one area Bouth:'lCst of ll.onroe. All terraeemorpholot>ic criteria are
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; 'j
· satisfied; ho;rever, outcropping sedirnents a:~e the ironotone-bearing, finely-
b.edded clays and silts of the Cockfield formation of the Claiborne Group
.. (FleehfOod, 1969). It is asswned that this occurrence is attributab.le to
·"subaqueous planation by nearshore currents in. Lake Monroe.
Jurlging from a fOil scattered r"wI cuts, aggregate pits, and borings,
.. the beach ridges appear to bc composed prlInarilyof irregularly stratified .. . · sands and gravels undoubtedly reworked from nearby fluvial terrace d"posits.
The only observed e,xceptIon to this is the 8-mile-1ong, north-south trend-
· ing ridge called Praid.e de Butte located north of Bastrop, 1M. (Plate 1).
· A 12-ft-hJ.gh exposure in a canal bank cutting throuGh a portion of thIs
ridge sho',ls only massive to faintly-stratified, well-sorted f:ne sands.
· The 11.tho10gy, morphology, and 10~ntion of Prairie de Butte sUiSse at it was
a barrier spit that pm'Ually enclosed an arm of Lake ).lonroe extendins up
· the floodplain Of Chetni n- ,,-Haut Creek. Bayou Bartholomew in this area 1s
afaj,r1y recent occurrence not related to the lake.
Deweyville Terrace 3
The lowest of the Deweyville terraces is intermittently present on
both sides of the river from at. least 12 mil es north of Cwr.den downstream .
to 12 miles south of the Arkansas-Louisiana state line (Plate 1). The
'do,mvalley slope profile (Fig. 3) is slightly concave, the slope dec~eas
ing from about O.60ft/mile above Catrflen to about O. ?(:. ft/mile just north
of the do;mstream-most point. South of thh peire, thc Qtd3 terraee for
mation undoubtedly is present at 1eMt intennittcntly in the subsurfacc,
being buried by the Holocene natural levees alone abandoned Arkansas River
Courses such 8.3 the Bayou BartholoDl'3~1 course.
Disqectl.on of the Qtd3
terracc in comparable in maenitude to that of
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the Qtdl
terrace in Arkansas; however, there is one majordif'f'erence.
Drainage patte,-"s on the 'itdl
terraLc no;; ref'l"ctlittle or nothing of
the control or inf'luence once exerted by primary depositional featu;:es of'
. the former floodplain, "hereas on thb Qtd3
tertace, the control of the
relict features is strikingly evident •
.. Rel~ct features present on the 'itd3
terrace include numerous aban
doned" channels·, fn>:~:-;~ents 0:[' abe.ndonod courses, arut extensive areas of
. point bar ridge and. s,,":le topography (meander scrolls). These features
are the most outstanding and diagnostic characteristics of the terrace
and are present throughout the mapped extent of the terrace.
Too aba!Jdoned channels and related point bar topography on the Qtd3
terrace betJ.lCen Calion, Ark., and I~cro Bay are visible on the photomosaic
·-in ·Plate 2. These featUl'es illustrate ;;ell the significant size differ
ences bet·"een the De,lCyville-stage stream and the present stream that are
so typical not only along the Ouachita but along most Gulf Coast rivers •
. Measurements in this area indicate that the 'itd3
terrace chamlels are ap
proximatt·ly 3 times "iaer than the rc-esent· stream channel and that the
maande! . re.dii and meander ·,Javelengths are probably about 2 times greater
than those of the present stream. These average size differences arc not
so great as they are along certain other Gulf Coast streams; hm1ever,
. they nill ;;ould fall l1ithin estimated range> (Gagliano and Thorn, 1967).
On the basis of sparse and poor quality data, it appears that the
'itd3
terrace fomatiop substratum is proportionately thicker than is. the.
case in the other De;;eyville formations and that the complete fluvial
sequence might be somel"hat thicker than in the other formations, These
17
difi'erences are not sUl'prisin€ cOllsid?ring the larger stream channels aud -
the inferred greater discharges and thnllfeg depths associated ,lith the
Qtd3
terrace.
The lithology of the QW3
te~racc formation topstratum and substratum'
is not Rno'"" todiffel' from that of the other terrace formation." 'in any
respect. No reliable data arc available to indicate the natur~ of the dc-
-posits filJ.illg the large aban'loned chmlllcls and courses; it can only be
,,~surr.cd that thcy are filled primarily w:i1;h fine-grained sedimOllts "(clays
and silts) as in" the Holocene channels and courses of the Ouachita anrc
Hississippi Valleys.
HOLOCEl'lE FLOWPLAIN
The only portion of the HolOCene floodplain of the Ouachita River that
can be considered typical of that stream is the part extending dOlmstrcam
to about the mouth of the Saline River. Below this point, the stream has
been profoundly influenced directly or indIrectly by the Arkansas River.
This influence is manifested in the slope, ,lidth, morphology, and llthology
of the Holocene alluvial sequence.
The dmmvalley slope of the floodplain abovc the Saline River (sec
tion mile 56, Fig. 2) is uniform at a rat. of about 0.55 ft/mile. Flood-
plain limits are easily defined by a low scarp or other phyJiograp~ic ex-
pression, and thc floodplain consis+.s of the present meander belt and short
segments of u3ually not more than one abandoned meander belt. All meander
belt segments exhibit ll\.unerous abandoned chanr.els or cutoffs. - Floodplain
relief, i.e. the difference betHeen natural levee crests and abandoned
channels or point bar sHales, is generally not more than 15 ft.
South of Sterlington, the Ou<'.chita River is an underfit stream
18
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?-;
flowing in an abandoned ArJ<ansas River meander belt. The profiles along the
Holocene floodplain (average level) and natural levee crests in Figure 2
illustrate well the dHfel'ences in eleva',i"n and slope that are prcsent
< belo" eterl1ngton. Before the developmcnt of the Bayou Bartholomew COUrse
of the Arkansas River, the Ouachita River floodplain is assUmcd to have
<continued to slope dOlmstream below sterlington at about the samc rate as
it now does above the mouth of the Saline River. HO\;ever, the major
<change illustrated in Figure 2 result cd whon the Arkansas River aggraded
to a level sufficiently high !;o permit it to divcrl through a narrow gap
in the Prairie terrace fonnation at the north end. of the Bastrop Hills
(Plate 1) into the Chemin-a-llaut Creek floodplain thence into the Ouachita
floodplain. The Arkansas River initially achieved a major steepening of
< gradient through this di<rersion but rapii!:t: adjusted by quickly building
up the< fioodpiain to hei£hts of 20 to 30-f't above its <former-elevation.
« • The relatively large natural levees along the Arkansas River course,
as compared with the (.'l,achi<'<[. River levees, are but ono expression of the
substantially higher sediment load (parlicularly snt-size material) of
the fonner streBm. Because of the Arkansas River's appreciably greater
all~viatl.on capacity, an extensive alluvial cone was built by the dver . .
upstream into the Ouachit a Valley north of Sterl; ngton i'roni the Bayou
Barlholome" course. The effect or the development of< the cone- and tne
corresponding shullo;7Ing «f the gradient of the Ouachita River (Fig. 2)
has been one of alluvial dr01ming and is manifested by the large,
frequently-inundate,", st<ampy 10\Tland or fleod basin called Grand. Marais
(Plate 1) I<{hich is located immediately upstream from the cone bct'<fcen
19
,
l'elsenthal and Alabama Landing, La. Within the flood basin, the Ouachita
, J.iver natural levees are poorly devcloped,thc floodplain relief is mini-
mal" and the river has abandoned meander belts more frequently than any-
." "here else. In sharp contrast to this, the Ouachita River exhibits
characteristics of an entrenched stream such as a less sinuous channel,
few abandoned channels or cutoffs, and a nano\{ floodplain with no aban-
doned meander belts "here it flows through the cone south of Alabama
Landing (section miles 70 to 85, Fig. 2).
Investigations conducted near the turn of the century first called
attention to the anomalous behavio"' I)f the Ouachita River bctHeen Felsen
thnl and sterlington (Veatch, 1906). Faulting was postulated as the cause
of the Grand Mara1s; hO\;cver, neither physiographic nor subsurface evidence
has ever been' discerned to substantiate this. Moreover, thc downvalley
terrace profiles constructed for this study (Fig. 2), indicating no verti-
, cal displacements ,are considered as conclusive evidence that none of the
Quaternary formations in this area.have been significantly affected by
". faulting.
Local variations in the thickness of the Holocene topstratum are
appreciable; ho"ever, it is possible to detect a dOl-mstream increase in •
, average thickness from about 20 ft above Calion to about 50 ft below
MOnroe., As HOuld be expected, a major change in the character of the top
stratum occurs in the vicinity (,f S,terlington. Upstream from this point,
the Holocene topstratum is corr.posed essentinlly of scdiments deposited in
natural levee, abandoned channel, and abandoned course environments.
Sediments deposited in a bnekswamp or floodbasin environment are volumet
rically insignificant and have not been delineated (F'leehlOod, 1969).
20
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)
.,,--~----
DoWnstream from sterlil".gton, backswamp deposits comprise 75 percent or more
of the topstratum: natural levee deposits and abandoned channel and course
deposits are coni'ined to the narrow and widely separated meander bel,!;s.
The Holocene sUbstratum deposits generally cannot be precisel;- de-
- lineated either upstream or dOWllstrearu from sterlint>ton. In many cases the
: Holocene sequence is apparently thinner than the Qtd3
terrace for-nation sc-
-- quence; consequently,- Holocene sands and gravels are underlain as well as
flanked by Qtd3
ter:ace -formation sand~;"and gravels. Only in those cases
whE,,-'e the Holocene sequence overlies Cl'l.ibo:cne Group deposits off the flank
of the Qtd3
floodplail1 can the substratum sequence be defined. A substra
tum thickness of ro to 40 ftl_s typical of these 10cation3.
GEOLOGIC HISTORY
llackground
The most ,rldely accepted and almost traditional correlation of His-
consinan stage events in the Lm-rer ~!ississippi Valley &rea with waxing and
waning - glaciation and corresponding eustat ic sea level variations was first
proposed by Fisk (19411) and has been modified little since that time. As
indicated in Figure 5a, entrenchment of the l·!ississippi alluvial valley '- . .
J~nd tributaries) is correlated with I-Iaxing Late lUsconsin glaciation and a
falling sea level. Filling of the entrenched valley, first by braided
,
streal1lS choked I-lith coarse-grained glacial outwash then by meandering streams
carrying a smaller and finer load, is correlated with waning glaciation and
a rising and finally a static sea level.
In 1968, Saucier presented evidence which indicated that _ some of the
fans or cones in the Mississippi Vall~y built by braided streams are
Figure 5. Lm",r l,!ississippi Valley chronological concerts (,,) as proposed by Fisk (194:,), and (b) as applied by Saucier (1968) to sea level fluctuation ourye proposed by Curray (1965).
0
0
, 1 j
-~
appreciably older than had been thought and must be relo.teu to 0. preceding
period of· ""ning glaciat ion. A conuept of late Quaternary chronology de
veloped by Currey (1956) based on evidence from continental shelf areas
appeared to offer the only satisfactory explanation for the 10'10 phases of
braided-stream deposition ·and hlo pho.ses of velley entrenchment obsel'Veu
·by Saucier .(1968) to have occurred follOuing deposition of the Prairie ter
race formation. The correlation of alluvial valley events with Curray's
sea level curve and.chrono10gy is indicated in Figure 5b.
Varying interpretations could be offered. for many indiVidual asP3cts
of the Ouachita Valley terr"ce sequence if considered separr.tely. Hollever,
the tot.al sequence or aggregate situation indicates basic D.{lreement only
with the chronology indicated in Fi(wre 5b' The follo,ling paragraphS con
tain the auti'')r"' interpretations of the origin of the Ouachita Valley
. terraces based on this chronoloGY.
C Interpretation
Sangamon Interglacial Stap;e. EverY"lhere in tho Gulf Coast eret., the
Prairie terrace formation (or equivalent) displays evidence of ha'ing been
formed during a relatively long interglacial stage with a stable climate
and little or 1:0 variation in sea level. The coast"ise plain portion of
the formation has conspicuous and "ell preserved beach, lagoon, and bar
rier island f(,:ltures. The riverine plain or relict floodplain pOl'tions of
tnT formatiolis on tJ.l Gulf Coast streams, includine; the Ouachita River,
indicate attaimnent of a rather advanced state of maturity, with 10'1 flood-·
p:Lain r"lief and a thick topstrll',um formed by rather s 10'1 accumu1at ion of
sedlments throu~h overb"nk depooitiolh In the Ouachita Valley, paleo
channel evidonce is lack~n~, and it can only be a3sumed that the stream at
.23
,
..
1'1
,thist'1me (i.e. the Sangamon InterglaGiel stage) was s1milar in size and
,character to the present rive,.. ' Figure 6a diagrammatically illustrRtes '
, "probable <ronditions in the Ouachita River area dUring this stage.
, Early Wisconsin glaciatIon. With the onset 01: the Early ~lisconsin
'glaciation,theancesj;ral Ouachita River began to degrade its valley and
become entrenched. This was probably in part a direct respOllse to a
10'lering 01: the river's local base level (i.e. the Mississippi River) be
cause of adjustment to a falling sea level and in part a -result of a
. climatically-induced change in river regime,'
The degrailational stage probably persisted until or shortl,y beyond
the max1mum extent 01: the Early Hisconsin glaciation or the Altonian Sub
&tage (Fig. 51», a :period 01: possibly 15,000 years or more. During this
, , time, much but probably not all 01: the Prairie terrace fonnati6n 'las re-- - .. , . , . - .
rnovedby erosio~. It is logical to assume that at' least isolated areas
. of substratum remained. In those areas where the Prairie fonnation was
_ probably completely removed and erosion affecte.dthe underlying Claibol'lle.
Group deposits, the erosional contact sho>ls no evidence of sub"erial " - - ,
weathering. This suggests that the basio mechanism of erosion or entrench-'
ment may have been scouring or corrasion by shi-rting and seasonally-'
. enlargi:ig stream channels.
",Figure 6b illustrates the probable conditions existing ;[:,' the, Ouachita
Valley immediately follo'ling the Altonian Substage during the ear.ly stages
of the retreat or "taning of the' Early \'Iisconsin Glaciation. Althougli the
Ouachita River carried no glacial out'tash, early deposition consisted al-
most entirely of sands and gravels, implying braided-stream deposition.
Sed1ments deposited later are noticeably 1:iner grainc" '1d undoubtedly
Figure 6. Diagral!l!l'.atic illustrations of stages in the developr.:ent of the Ouachita Valley.
,:.
'. >,
reflect a change from a braided to a mean~ering recime. Assuming the time
.. scale sho,m in Fi(lUre 5b to be correct, the period of alluviationfollowing
the Altonian Substage lasted for about 10,000 years. Probable conditions
.inthe Ouachita Valley near the end of this period of alluviation and prior
to the Farmdalian Substage are illustrated in Figure 6c.
Farmdalian· Substage. It is necessary to consider the post-Altonian
Substage events in the /fdssissippi alluvial valley to interpret conditions
in the Ouachita River area durincthe Farmdalio.n Substage (Fig. 5b).
Throughout the period of ,,,,,,inc Early l-1isconsin glaciation, thc ancestral
Mississipp.i and Ohio Rivers carried copiou" quantities of glacial outwash
and rapidly aggraded the valley through deposition of coarse-grained al
luvium. The axis of the Mississippi River valley train' is indicated by
drainage patterns as being west· of and parallel to Crowleys Ridge in
. the. Hestern I.o;(lands (Fig. 1) thence across the pr0sent Mississippi River
course into and parallel to the eastern side of the Yazoo Basin. The Ohio
River valley train apparently developed east ofCrm<leys Ridge and merged . . .
. · .. with the Mississippi River valley train in the upper part of the Yazoo
Basin. During this time, the Arkansas River apparently had a significantly
greater discharge because of greater preCipitation in its drainage basin
ar.d/or melhrater from Cordilleran glaciers. Below Little Rock, Ark., the
river ,~s a braided stream carrying large quantities of sands and gravels
and it developed 0. long, narrow alluvial cone extending along the "estern
side of the Hississippi Valley for a distance of over 200 miles. The
upper part of the cone has been largely removed by erosion or buried by
Holoceno meands r belts; however, much of the 10,;01' part remains and forms
a 101'( ridge called Kacon Ridge (Fie;. i.).
, .
. '. The c><tellt' and topographic expression of Macon Ridge at the latitude
at which the Ouachita River enters the Mississippi Valley are shown in
. Figure 7 •. Four levels, designated A through n, are recognized; the eastern-~'. '. . - - .. -
'most level (lev~l A), '. the highest and oldest and the westernmost level - '~.., -~,>.
(ie~e1 n) is the lo>lest and youngest. Preserved braided drainage patterns
:/"',.\;(;1' . 'indicilteth~t' the levels are terraces resulting from progressive westward
.-'.
:silift:lng: of the anc~stral Arkan~asRiver while it "as B10"ly degrading its
····fl.oodJ.ll~ii'...Radiocarbon dates report~d by Saucier (1968) from near the sur-· "
"f'aCE,'of!t£lcon Ridge 8 miles southeast of Winnsboro, La. (Fig. 7) indicate
the deposits of'level C to be about 30,000 years old. Inferred rates of
"sedimentation and stream migration and. relative stratigraphic positions sug
gestCthatthe deposits in level A are not more than af'ew thousand years
·:olderthanthis. on the 'b'lsis of topographic position and the radiocarbon
dates, level A of Macon'Ridge is considered to represent the greatest extent
of agg~adation in the Mississippi Valley during the Farmdalian Substage.'
Alluviation in the Ouachita Valley (Fig. 6c) can be assumed to have
'beeninequilib~ium >lith alJ:uviation (Mississippi River yalley train devel~p
'ment and Arkansas River cone development) in theUississippi alluvial v?l1ey
during much of tho period of' "aning Early Wisconsin glaciation. Ho"ever, •
. with the attainment of full development of Hacon Ridge during the Fanndalian
Substage, disequilibrium apparently resulted. It is felt that the rate of
deposition of the last few tens of feet of sediment in Macon Ridge culminat
ing in level A "as so rapid that aggradation of the Ouachita floodplain
could not keep pace. The result of thi, ·"as the creation of what >res ef-
fective1y a dam across the 10>rer end of the Ouachita Valley in the vIcinity
of Homoe and the consequent formation of first a swampy basin similar to
;:
f ;:
ii i \: r
~~ , ; 1/
,l'?,
,.,"'-'"-....... --_. __ .
"!t: ' " ""
o OCv.1:,"Vl1..u:n'lAIICt:!.
o """ ... ,,'t:t.T't""VoCI;
~ U!'<O,r". Tt:II;TU ..... ot-oS.TS
-n-... ~gg~~~~I»
1...rvo. 0 ltVEt. C
t)I~TANa:. Io.m.t:!.
Extent and .configuro:tion of the Fa...'""mda.lian Substage' (Macon Ridge) as related to the formation
Fisure 7. Arkansas River cone of Lake Monroe.
, !: - .--' t: ;--
I. ~--
i , .
the Grarrl )-fal'ais and finally lake Monroe. Profile A-A I, Figure 7, illus-
. 'tr'ates that .the highest point. on level A of l-facon Ridge (i.e. the cloaest
approxhflfltion to the original surface) are dthin 2 0)." 3 ft of the rccon-
structcd moan w1l.>dmum level ef lake Honroe, lor;' eleval;bn or about 97 it
,mal.. There is no reason to doubt that level A of l-facon Ridge once extended
as far ,mat M the limit indicated in Figure 1. •
.lake ).!onroo was probably perennial for much of its Hfo; ho;:evcr, large
seasonal vflriatioM. in lev"l mmt have occm:red as a result of the Ouachita
!liver dischargo bc1ng augmented I-lith backwater 1'rom the Arkansas or l.Jisais
sippi Rivers dud ng epring floods. Discharg~ from the lake quite likely
occurred by ,my of one of the active braided channels of the Arkanso.. Iliver
only during relatively 10>1 staees on that river. Figure 6d illustratc" tho
Msumcdcon(litiona in the Ouachita Valley durin(l the Farrrdalian Subnta(lc.
lete l'lisconoin glaciution. The lo;rcr levels of l-facon Ridee (lovels ---, Jl.:D) cculil have bcen formedcither during the Farmdallan Subotl!6e or 8lightly,
later during the enact of late VliscoMin glaciation. !t is ponsible that
with a decreasc in the discharge of the Arko.l1AllS River such as could have
occurred because of termination of melt;rnter.ar4/or a change to a drier
(interglacial) climate durill(l tho Farrr"lalian Substage, the river becrunc
more compotentto transport its load an'\ began to slmlly deerade ita flood-. '
plain. On the other hand, with waxing late Hioconnin glacIation, the
Arkanall3 River as well as thc Mississippi River would have '.cgun steepening
their gl'llIlientiJ "'rl degrading their flocdplains in responsc to falling aea
level. Xf the Farl«laHan Sub,d;aee 'IUS as brief a3 suggested by Currny
(1965) (Fl,g. 5b), it is llkcly that conditions of cli1r.ate and glaciation
that >loulil permIt a significant decrease in diGchnree did not develop;
,. I
r' . , ;- '
~;
-. ,-
,; , hence ,tho latter intorprctatl,on i8 favored. In eithor Calle, the results
would have been tho awne. The alluvial-ridgo barder responsIble for
creating and maintoinlne; Lak(; Monroe for posdbly an J one M Bever"~
: thousand yearn ,inS destroyed 01' brca~hcd suffIciontly to allo>l the lako --~ .
to drain •.
With continued ,/llxlng e;laejed;ion and a r03ultant major oU1tetic
'lo>lering of sea loval bet>recn 2O,vOO and 25,000 years ago (Fig. 5b), th8
Arkansas lliv~r eventually boc"!J1o eignificantly entrenched. l'he axis of
the ontrenohr,d· vallcy io kn",m to be bet>leon tho >lestern edge of 14 .. 00n
Ridge' (Fl£>. 7' al¥l the Tortiary uplands or oldertel'l'ace remnaut" (c.g.,
the Bastrop Hills). In the Ouachita Valley, the ancestral Ouachit" Rivor
unque'stionably ;:I\8l\ffectcd by the falling bnoelevel and also >/llS forced
todeerade its floodplain. llo>lcvel', during thi3 interval, tho ·.'iver also
was forced to nceornmodnte 1t"clf to a sienificantly greater discharge.
The resulting wlnptntion to t.ho change in hydraulic rogime in mf.l1J.fcotcd
by the Qtd3terrncc. As the floodrlnin reprenented by this terrace dc-
"velopod, large £\rOWl of the lacuBtrine plain that resulted from tho drain
ing of lAke Nonrco ,mr(l dcatroyed by lateral nJr;~aHon of the river. The
rel(>,tiomhip that dcv<llopc(l botwcn the Qtd3
terrace unO. the lncustrinc
plain (Qtd2 terrace) is diallraJranatically illustratod in Fle;ur" 60. The
downvalley terrr co profiles included in Figure 2 Ldicnte that net dcgrn-
dntion by the river during wa;d.nll !ate ;/;.:;oonsil. glaci"tinn took place
along tho entire strotch of river included in this study.
.l:!9Zli'ordian SubstfJr;.,. The projected do',rnvallf,y dope of the QI;d3
ter
race sugcoats that the f,rknnSB" River nt that tIme had entrenched itself
to about the level or the prc3cnt floodplnin (i.p. an elcv.atIon of "baut
30
I
j c, j Ii 1
65 :!'t msl) some 10 to 15 mHes south of ).1onroe whel'b the Ouachita River
probably became tritJtary to tho Arkansas Riv'}r. The max:iJnum depth of
entrenehrnent 01' degradation attained later at 01' shortly a:!'tor the n:axi
.mum extent of the Late Hisconsin glaciation (the Hoodfordian Substage)
is not kno,m ,·lith certainty; ho,·tever, if the thickness of the Holocene
topstratum (backsHamp and natural levee deposits) in this aroa is sub-
. tracted from the floodplain elovation, a probable minimun elevation of
20 ft msl is indlcated for the dcepest entrenchment.
Hith the Arkansas lliver near Monroe entrenched to an elevation of
20 ft msl or 10"er, the Ouachita River ,·taS forced to accommodate and
apparently did so by further degrading its floodplain. The Indicated
minimum degradatIon of the Ouachita River floodplain compatible ,·lith
changes in the Arkansas River is a level about 15 ft belm·t the prosent
floodplain (excluding the effects of tho 'Bayou lJartho10",c" coursn).
It has not been possible to determine tho precise character of the
Ouachita River a';; the time of maximunentrenchment. The floodplain >lldth
.. : necessarHy was equal i;o or less than tho present floodplain "idth,
suggesting that the river during ':;he l'Toodfordian Substage might have
beon some',that smaller in size (i.o. a lesser (lischaree) than l.t ;tUS "hen
the Qtd3
terrace ',ta8 formed. Sca1l9ps. in the Qtd3
terrace alo)1g the
contact Hith the present floodplain but . -t attributable to observable
Holocene floodplain channels suggest that the river ,taS l;~sically a
. meandering stream at or shortly after the timG of ma.xilnun entrenchment.
31
--.-'-'
, , The Recent. ~ Hhen the Late I>isconsin glaciation began waning and sea
'level began rising, the proceas of entrenchlJ\ent was halted and floodplain
aggradation becalJ\e dOlJ\inant. Evidence indicates that the Mississippi and
,Ohio River valley trains dcveloped east of Cro,lleys Ridge and trended
thro~gh the Yazoo Basin into Louisiana east of Macon Ridge (Fig. 1). The
.. Arkansas River began buHding a cone frOlJ\the point where the river flows
.;·'oUt'ofthe OzarkMoUntains province ~e~r Little Rock. Possibly because of .
the development of this cone and the rapid filling of the upper part of . ,
the entrenchment, the Arkansas laver in a rather early stage of >raning
, glaciationdiverted frOlJ\ the entrenched vallcy west of ~!B"on Ridge to a
'llfore easterly course that trendcd lJ\ore directly toward the ~lississippi
River. Thus deprived of its prilJ\ary source of alluvium, the 10lfer part
'of the entrenclunent ,including th_ area adjacellt to the Ouachita valley,
'agg~adedslowlY through the accumulatJ.on of mostly rill~~grained sediments
iiltroduced by lJ\inor streams and derived from backwater flooding by the
" Mississippi Ri.ver. Presumably the Ouachita River Has capable of
", •. _--'----lIt has been rather conunon practice in the central Gulf Coast area
to define the Recent as being tile period of time follOlan!> the last major
10\'1 stan;! of sea level (Russell, 19110) rather than as being the time fol~
loWing the cessation of continental glaciatIon. According to the former
def'inition, most Recent deltaic and alluvial 'lalley deposits Hould be
separaple from Pleistocene deposits by an erosional unconformity, Hhereas,
according to the latter definition, no persistent time-stratigr"phic di- -
vision Hould be present.. Russell's definition is considered convenient
for use in this p:tper at this point. '
32
, , j 1 , ;
'~
aggradi.lll'; its :rlood\11aill at a corresponding rate also through the accu-
mulatioll of l?rimarily fine~grained materials.
,The Arkansas River ceased building a cone and became a tightly. meandering
'"stream carryir.g a much smaller sediment load perhaps by as early as 10,000
" years ago. During the following several thousand years, meander belts >!ere
,formed extending for more than 200 miies southeast and south of Little Rock ,
into, Lquisiana along beth sideBof Macon Ridge. lJayouMacon and lJoeuf River
'now each occupy abandoned Arkansas River meander belts in the latitude of
Monroe (Fig. 7). The meander belt occupied by lJayau J.!acon is the older of
the t>lO; hO'-/ever, archaeological evidence (1I'cbb, 1968) indicates both >!ere
>!ell developed and possibly even abandoned by the Arkansas River by about
'3000 to 3500 years ago. It >tas about this time or slightly later that the
Arkansas River developed the Bayou Bartholomew course by diverting through
the gap in, 1;he Prairi~ terrace formation at the north end of t)le Bastrop
,Hills and into the Ouachita River floodplain near Sterlington. Abandonment
, of ,the Bayou Bartholomew course by the Arkansas River probably took place
"about 1000 to 1500 years ago.
"Figure 6r diagrammatically illu"tO'ates conditions in the Ouachita
valley as they exist today and, in effect, as they have been during the
- past 1000 years or more. During this time, the Ouachita River floodplain
has probably aggraded only a few feet through the accumulation of backs>!am?
and natural levee deposits. Course changes and formations of cutoffs have
probably been minor and infrequent.
DISCUSSION
Extinct Mkcs
Numerous sHampy" depressions and occ(uional1y small lakes are present.
33
in the Hississippi alluvial valley and the floa:lplains of its major trib-
utaries as a result of more rapid alluviation by one streomservineto
block or· dam the mouth of a smaller st:;.~orun, a .process sometimes refe~red
to as alluvial ~ro',ming. Typically, 1;he drowning is caused by the growth
of natural levees along a newly developing meander belt of a major stream
located a short distance from (Le. several miles or less) and trending
roughly parallel to a terrace or other upland. Small streams dischareing
'from the' upland are ponded in their lower reaches becauso of an inability
to aggrade their 'floodplains to the level of the natural J,evees.
Lake ~!onroe is unusual because of its large size and the fact that it
, is the first extinct lake to, be rec(lgniz8d as hovine been crcated in the
Hissisdppi alluvial valley area as a result of the development of alluvial
cones or valley traIn:. (braided stream deposition). Subsequent to its
discovery, all examinat:ton of topographic quadraneles and aerial photographs
revealed the probable former existence of similar but smaller extinct lakes,
on the Hatchie, Obion, and Forked neer Rivers in western ~ennes.c~ and pos~
',aibly also all the Big Black River in 'Nest-central MissIssippi. Each of these
streams has _in its valley a low terrace that has a very 10" 01' nil do,mvalley
slope in proximity to the l.fississippi alluvial valley. The projected slopes
of these terraces indicate that the str,eems "ere graded to the approximate
highest elevations of the Farmdalian Substage valley trains of the Ohio and
Hississippi Rivers. SuffIcient work has not been done to ~etermine if shore-
line features are present along the sides of the valleys of these smaller
streams. The situation on the lIatchle River is particularly similar to
that on the Ouachita River in that there is not only a terrace ",h1ch ap