Terrace stratigraphy in the Tunica Hills of Louisiana

QUATERNARY

RESEARCH

19, 55-63 (1983)

Terrace

Stratigraphy

in the Tunica

Hills of Louisiana

JOHN J. ALFORD Department

of

Geography,

Western Illinois University. Maromb,

Illinois 61455

AND CHARLES Institute for Environmental

R.

KOLB*

Studies, Louisiana

AND JOSEPH C. HOLMES State University, Baton Rouge, Louisiana 70803

Received April 22, 1982 The age and number of important fossil-producing late Pleistocene terraces in the Tunica Hills have recently become quite controversial. One hypothesis holds that only a single loess-mantled Farmdalian terrace flanks the streams in this area. The other maintains that there are two terraces. The youngest is considered essentially Holocene in age while the older is Sangamon in age. Radiometric and stratigraphic evidence collected for this study indicates that there are two terraces. The youngest is late Woodfordian to Holocene in age while the other is Farmdalian.

INTRODUCTION

Some of the most important Pleistocene fossil-producing areas in the lower Mississippi Valley are the fluviatile terraces of the Tunica Hills (Delcourt and Delcourt, 1977). Lying astride the Mississippi-Louisiana state line to the east of the Mississippi River, the Tunica Hills are primarily drained by Big Bayou Sara, Little Bayou Sara, Tunica Bayou, and Thompson’s Creek (Fig. 1). Terraces along all except the latter have yielded significant Pleistocene fossils. This area is of more than local interest. It has been noted that there has been a paucity of Quaternary fossils collected under firm stratigraphic control from the Gulf Coastal Plain of the southeastern United States (Delcourt and Delcourt, 1977). Thus the profusion of plant fossils plus the scattering of vertebrate fossils washing from one or more terrace levels in the Tunica Hills provide a unique opportunity for paleoenvironmental reconstruction. If the terrace levels can be correlated with specific time-stratigraphic units, it may be possible to draw conclusions about the vegetation and climate during specific stages or * Deceased November 26, 1982.

substages of the late Pleistocene in the lower Mississippi Valley and adjacent Gulf Coastal Plain. Unfortunately, despite studies that date back more than 100 yr, the stratigraphy of these fossiliferous outcrops is not well understood. Recent studies by Delcourt and Delcourt (1977, 1978) and Otvos (1978, 1980, 1981) have led to strongly opposed positions. In turn, the uncertainty has raised fundamental questions about the reliability of the 14C method of dating (Long and Muller, 1981; Otvos, 1981). The purpose of this paper is to remove some of the controversy concerning the age of the fossiliferous deposits along Big and Little Bayou Sara. PERCY BLUFF

Flanking Big Bayou Sara, Little Bayou Sara, Thompson’s Creek, and Tunica Bayou is a low (Tl) terrace that contains a variety of plant and animal fossils. The richest fossil localities are the Percy Bluff sites, labeled 1 through 7 in Figure 2. The age of the Percy Bluff fill has been the subject of investigation for quite some time. Fisk (1938), in his bench-mark study of the 0033-5894/83/010055-09$03.00/O Copyright 0 1983 by the University of Washington. All rights of reproduction in any form resewed.

56

ALFORD,

. . ,111

AND

HOLMES

Mississirni

Limit

30.45

KOLB,

of

‘N

FIG.

1. Map of Tunica Hills region in Louisiana.

area, considered the Percy Bluff surface to be a “cut terrace” or bench notched into the Port Hickey Terrace, The Port Hickey till which he ultimately correlated with the Prairie Terrace was thought to be middle Wisconsin in age. Sometime later Durham ef al. (1967) suggested that the low terrace was the

equivalent of the Deweyville Terrace. Originally, the Deweyville Terrace was thought to be early Holocene in age (Bernard, 1950); but with the accumulation of a scattering of l*C dates, some investigators suggested that the Deweyvilfe fill was laid down during a Wisconsin interstadial and that the Prairie Terrace is of Sangamon age

TERRACE

STRATIGRAPHY

IN THE

TUNICA

57

HILLS

taken from the alluvium that range in age from nearly 34,000 to 3500 yr B.P. (Otvos, 1980, 1981). He accepts the older dates, but rejects the younger as contaminated. It is his conclusion that this unit is a classic-type Deweyville Terrace. Specifically, he believes it to have been formed in the middle Wisconsin when streams aggraded their beds during a Farmdalian high sea-level stand, and then cut down in response to the Woodfordian marine regression. WILCOX

FIG. 2. Map showing location of Percy Bluff and Union Bethal Church sites.

(Gagliano and Thorn, 1967; Saucier, 1968, 1977). Recently, Delcourt and Delcourt (1977) assigned a very late rising-sea-level (Woodfordian to Holocene) age to the Percy Bluff deposits. They followed three lines of evidence to arrive at this conclusion. First, a scattering of 14C dates indicated an age between 12,700 and 3000 yr B.P. Second, their field observations indicated that the silt that capped the low (Tl) terrace was reworked rather than autochthonous loess. Finally, plant fossils recovered from the Percy Bluff localities present a curious mixture of cool- and warmclimate flora. Otvos (1978, 1980) takes vigorous exception with this. In his view the silt on the low terraces is autochthonous, in situ, loess. Since primary loess deposits in the area have been 14C dated as early Woodfordian (ca. 21,000 yr B.P.), Otvos (1975) argues that the terrace fill must predate this period. He presents a series of 14C dates of wood

BLUFF

Related to the question of the age of the Percy Bluff deposits is the age of the Wilcox Bluff fossil deposits. Located on the east bank of Big Bayou Sara in Section 73, T2S, R3W (Fig. l), the bluff exposes about 20 m of sediments. The base exposes 3 to 4 m of a grayish-green Miocene siltstone. Overlying this unit is 15 m of cross-bedded sands and gravels. This is capped by a single loess unit that is about 2 m thick. Well-preserved leaves, stems, and wood are found in bluish-gray lenses of silt and clays located near the base of the crossbedded sands and gravels. These plant fossils have been studied in detail. Brown (1938) found that the flora was mostly southern in character with only a “minor amount of northern species” represented. After considering Brown’s analysis and the topographic position of the deposits, Fisk (1938) assigned the fill to a middle Wisconsin time interval. Delcourt and Delcourt ( 1977), however, feel that Brown erred in his identification of northern elements. They argue that Wilcox Bluff is a warm-temperate assemblage that represents Gulf Coast flora during the Sangamon interglaciation. Thus, according to their scheme, Percy Bluff represents a low (Tl) terrace fill that is mostly post-Woodfordian in age while the Wilcox Bluff deposits belong to the high (T2) terrace and are Sangamon in age. Otvos (1980) believes that the Delcourts may have reached unwarranted conclusions in discounting Brown’s identification of

58

ALFORD,

KOLB,

northern elements. In his view there is nothing in the fossil record to preclude the possibility that both Percy Bluff and Wilcox Bluff are Farmdale in age. As a result, Otvos (1980) argues that only a single Farmdalian terrace flanks the streams draining the Tunica Hills. A THIRD

VIEW

Our data indicate that both of the previously described models are partly correct. As part of an ongoing effort to work out the Pleistocene stratigraphy of Louisiana we studied and analyzed critical areas along both Little Bayou Sara and Big Bayou Sara and collected samples for radiocarbon dating. One of the problems associated with the area is the blanket of loess that covers or partially covers the area of interest. In many ways it complicates the question, but in some ways it helps to resolve it. Basically, the loess consists of a blanket of yellowish-brown, largely silt-sized particles that thins with distance from the bluffs overlooking the Mississippi River. Near the Mississippi the loess is usually 5 to 10 m thick, quite calcareous (typically 210% carbonate), and rich in gastropod fossils. As one moves away from the river the loess thins and becomes leached. It is generally conceded that the noncalcareous nature of the loess away from the river is the result of a slower rate of deposition that allowed the leaching rate to exceed the rate of accumulation. In the study area the calcareous loess is restricted to the west and north of a line that runs from about 6.4 km west of Woodville, Mississippi, southward down the drainage divide separating Big Bayou Sara and Little Bayou Sara to just south of Weyanoke, Louisiana. From there, the line angles off to the southwest where it intersects the Mississippi River just north of Plettenburg (Fig. 1). Percy Bluff sites are all within, or at the extreme edge of, the zone of calcareous loess. The Percy Bluff sections along Little Bayou Sara were difficult to evaluate. Exposures examined and dated by Otvos

AND

HOLMES

(1980, 1981) are distributed for 7 km along the stream, from St. James Church in Mississippi to Union Bethel Church in Louisiana. Over this length the low (Tl) terrace is not always preserved or clearly evident. For 3 km upstream from Union Bethel Church the low terrace is well expressed. Northward from this point, however, the valley narrows and the surfaces next to the stream are generally higher and more uneven. Distinguishing the low (Tl) terrace from older surfaces in this area is more complicated. In the stretch between Union Bethel Church and Baker Creek (Fig. l), we found what Fisk (1938) and Delcourt and Delcourt (1977) have reported previously: that the silt covering the low terrace contains stringers of sand and, in some instances, small pebbles, indicating that this material is colluvium rather than Peoria Loess as suggested by Otvos (1978, 1980). These observations are supported by the work of Kress (1979) in nearby Tunica Bayou where detailed textural analyses of the silts overlying the low terrace indicate a noneolian origin for these sediments. Based on the descriptions and maps in Otvos’ papers (1980, 1981), we located as nearly as we could his eight Percy Bluff sites (Fig. 2). He does not report a 14C date for his site 4. We believe that we accurately identified these sites in the field and have concluded that most of his samples, from his sites 1, 5, 6, and 7, were selected from alluvium in the low (Tl) terrace. Samples from his sites 2 and 3, on the other hand, we believe he collected from the high (T2) terrace. Site la appears to be transitional. The low-terrace sites, in each instance, were collected from low flats capped with a variable thickness of allochthonous loess (2 to 3 m thick), washed down from the flanking hills. Many of the sites contain sporadic lenses of sand characteristic of reworked and redeposited loess. The high-terrace sites, in contrast, lie in higher bluffs along Little Bayou Sara, the upper portions of which consist of massive yellowish-brown silts from 4 to 5 m thick of in situ loess.

TERRACE

STRATIGRAPHY

IN THE TUNICA

HILLS

59

0 ,.;. :. :,_.:

silt I’, ,:...,:.;,:. .“.‘. ” BETA 1860 .i.h.$i. . ., BETA 1881 .;:,+yr+r. A::.:;:.~&~‘L. : .. .. ., .,y+~g? ,, LITTLE ‘BAYOU SARA

5

i SITE FIG.

A

SITE

8

’

SITE

C

3. Stratigraphic sections along Little Bayou Sara. See Figure 2 for locations

Moreover, the radiocarbon dates published by Otvos all fall within acceptable ranges for the low and high terrace, i.e., between 3400 and 13,800 yr B.P. for the low terrace, between 26,000 and 31,000 yr B.P. for the high terrace, and 23,000 yr B.P. for the transitional site. In order to verify these ranges of dates, we sampled radiocarbon-datable material

from three high-terrace site.

low-terrace sites, from one site, and from the transitional

Low (Tl )-Terrace Sites

Our site A, located at the first cutbank downstream from Union Bethel Church (Figs. 2 and 31, is from a well-preserved portion of the low (Tl) terrace (Fig. 4). The

FIG. 4. Low (Tl) terrace at Union Bethel Church site.

60

ALFORD,

KOLB,

silty material above the alluvial terrace is about 2 m thick and is noncalcareous. The presence of stringers of sand in the silts above the terrace indicates that it is reworked, rather than in situ, loess. A sample of wood from a dark-gray clay at the water’s edge was dated at 14,985 2 140 yr B.P. Site B is immediately downstream from the original Percy Bluff localities (Figs. 2 and 3). The low (Tl) terrace is well expressed at this locale and is overlain by 1.5 to 2 m of poorly sorted silts with some thin clayey and sandy lenses. The colluvial silt cover above the alluvial terrace forms a prominent flat at this point between the bordering hills and the floodplain of Little Bayou Sara. The flat slopes gently toward the creek and is the site of a small farm. A sample of wood at this site 154 cm above water level was dated at 3065 + 110 yr B.P. Another wood sample 90 cm above water level was dated at 13,830 ? 155 yr B.P. In addition to the low-terrace surfaces found along Little Bayou Sara, the low (Tl) terrace has been recognized along Tunica Bayou and Big Bayou Sara. In order to

0 .:: :i:c; ::.,

”

:

check its age along another drainage system, site D was chosen on Big Bayou Sara on the first cutbank north of Hollywood Bridge (Figs. 1 and 5). Woody stems collected about a meter above low water level at this site were dated at 7410 ? 135 yr B.P. This date falls nicely within the range established along Little Bayou Sara. To summarize, a well-defined low (Tl) terrace exists along Little Bayou Sara and other major creeks in the Tunica Hills area. Radiocarbon dates of our sites A, B, and D, all from the low (Tl) terrace, range (Table 1) between 3065 ? 110 and 14,985 ? 140 yr B.P. As nearly as we can determine, Otvos’ sites 1, 5, 6, and 7 at Percy Bluff are all within the low (Tl) terrace, His published dates (1980, 1981) range between 3457 ? 366 and 13,765 & 145 yr B.P. As can be seen, these dates agree remarkably well with the range of dates we obtained in our samples from the low (Tl) terrace. Thus, the low (Tl) terrace is considered to be a Holocene terrace formed by stream entrenchment about 3000 yr ago. The reason for the entrenchment of the creeks in

j: .:.

SITE FIG.

AND HOLMES

sil+

D. HOLLYWOOD

5. Stratigraphic

BRIDGE

SITE

E. WILCOX

BLUFFCAFTER

FISK:l938)

sections along Big Bayou Sara. See Figure 1 for locations.

TERRACE

TABLE Site

STRATIGRAPHY

1.

TUNICA

Location

Little Bayou Sara A Union Bethel Church B Approximately 300m downstream from Otvos’ site 5 C Otvos’ site la Big Bayou Sara D Hollywood Bridge site E Wilcox Bluff

IN

HILLS

Sample number

THE

TUNICA

RADIOCARBON

61

HILLS

DATES

Date (yr B.P.)

Material

Beta 1853 Beta 1860

14,985 t 140 3065 lr 110

Wood Wood 154 cm above low water

Beta 1861

13,830 + 155

Beta 4147

21,480 ” 170

Wood 90 cm above low water Peat

Beta 1862

7410 + 135

Stems

A-2006 A-2007

>38,000 >38,000

Wood Wood

Nofe. See Figs. 1, 2. 3, and 5 for locations.

the Tunica Hills is far from established; however, the thesis proposed by Delcourt and Delcourt (1977) is plausible, i.e., that the shift of the Mississippi River from its Teche course on the western side of the Mississippi Valley to its present position along the eastern side of the valley caused local changes in base level and degradation of Tunica Hills drainage. High (TZj-Terruce Sires

As mentioned earlier, Otvos’ sites 2 and 3 consist of cutbanks which rise to greater heights above Little Bayou Sara than do the remaining Otvos’ sites at Percy Bluff. The upper portions of these sites are capped by 4 to 5 m of autochthonous loess. Dates obtained by Otvos were 25,965 t 585 and

tr

-TRANSITION

FIG.

26,630 2 700 yr B.P. for site 2, and 30,775 ? 1740 yr B.P. for site 3. As a further check, we sampled a well-known highterrace exposure at Wilcox Bluff on Big Bayou Sara (our site E, Figs. 1 and 5). Well-preserved wood was collected from clay lenses near the base of the exposure. Two samples returned dates of >38,000 yr B.P. Thus, dates ranging from about 26,000 to more than 38,000 yr B.P. are probable for the high terrace. Transitional Zone

Site C (Figs. 3 and 6) was identified in the field as essentially the same locality as Otvos’ site la (Otvos, 1981). The height of the bluff at this point is transitional between an adjoining high-terrace level (to the right)

ZONE

6.

Diagram showing nature of transitional zone at site C. Not to scale.

62

ALFORD,

KOLB,

AND HOLMES

and low-terrace level (to the left). As shown valley trains that produced the Roxanna in Figure 6, the site is actually at the loess did not reach the latitude of the juncture between these two levels, a Tunica Hills, it should be noted that the juncture now fortuitously uncovered by the lower loess has been reported at Natchez migrating stream. We believe that a transi- just 65 km to the north (Leighton and tional zone of debris, often containing or- Willman, 1950). Quite possibly the surface ganic material, would have formed along of the terrace exposed by Wilcox Bluff is the entrenched T2 terrace as sea level too young to have been mantled by the dropped during the late Wisconsin. Thus early Wisconsin loess. the date of 22,825 & 695 yr B.P. reported by Support for this hypothesis can be found Otvos from a wood sample from the base of in the surprisingly weak development of the this transitional zone and the date of 21,480 paleosol that underlies the loess at Wilcox + 170 yr B.P. from a peat sample we col- Bluff. If the Sangamon interglaciation is lected from the same zone are compatible. about 75,000 to 125,000 yr old, as comA hypothetical section is shown at a point monly assumed, and early Wisconsin loess farther to the left where only the low ter- is absent in southeast Louisiana, the race is exposed. We postulate that a transi- paleosol should represent at least 50,000 yr tional zone probably occurs at this point of soil development that was terminated and may well occur whenever the low ter- with the accumulation of the Peoria Loess. race joins the high terrace. Although relating soil development to age is difficult at best, the paleosol at Wilcox Age of Wilcox Bluff Bluff is not nearly as well expressed as the Radiocarbon dates of 26,000 to 31,000 yr Sangamon soil that underlies Peoria Loess B.P. for the high terrace at Percy Bluff and in the Middle West. of more than 38,000 yr B.P. for the basal Quite possibly, as Fisk (1938) and Otvos portion of Wilcox Bluff indicate a pre- (1980) have suggested, the Wilcox Bluff deFarmdalian age, but it does not necessarily posits are middle Wisconsin in age. The mean that the deposit is Sangamon in age. basal portion could be somewhat older than An examination of the upper portion of the 38,000 yr B.P. while the upper sediments Wilcox Bluff exposure reveals a single loess might be represented by the Farmdalian 14C unit draped over a paleosol that is only dates that Otvos (1980, 1981) has cited for weakly developed in the underlying al- sections along Little Bayou Sara. If true, it luvium. Along most of the Mississippi Val- would nicely explain the absence of the ley it has been recognized that the Wisconearly Wisconsin loess and the relatively sinan stage is represented by two loess weak paleosol that caps Wilcox Bluff. sheets (Wascher et al., 1948; Leighton and SUMMARY Willman, 1950). The upper loess (Peoria Loess) was largely deposited between The stratigraphic evidence and radiomet22,000 and 12,000 yr B.P. (Willman and ric dates indicate that there are at least two Frye, 1970). The age of the lower loess (Roxanna Silt) is more problematical. McKay (1980) places the base of this unit between 40,000 and 75,000 yr B.P. Ruhe (1976), however, argues against any great antiquity for the Wisconsinan Stage and places the base of the lower loess at not much more than 30,000 yr B.P. FIG. 7. Generalized stratigraphy characteristic of Ideally, if the deposits at Wilcox Bluff are major streams in the Tunica Hills, (not to scale) Sangamon in age the surface should carry Radiocarbon ages in yr B.P. Approximate height of two loess sheets. Although possibly the section is 25 m.

TERRACE

STRATIGRAPHY

IN

THE

TUNICA

63

HILLS

for Louisiana. Geological Society of America &d/ealluvial terraces present in the Tunica Hills tin 72, 99-992. (Fig. 7). The base of the oldest is more than Gagliano, S. M., and Thorn, B. G. (1967). Deweyville 38,000 yr old while the top is probably terrace, Gulf and Atlantic coasts. Louisiana Stute about 26,000-30,000 yr old. This deposit is University Coastal Studies Bulletin 7, 23-41. capped by a relatively weak paleosol and a Kress, M. R. (1979). “Silty Stream Terrace Deposits in the Tunica Hills, West Feliciana Parish. single late Wisconsin loess. The basal Louisiana.” M.S. thesis, Louisiana State Universediments of the younger terrace were laid sity, Baton Rouge. down as early as 18,000 yr B.P. and proba- Leighton, M. M., and Willman, H. B. (1950). Loess bly ceased being deposited about 3000 yr formations of the Mississippi Valley. Journal af’GeB.P. In many places this terrace is capped ology 58, 599-623. with a noncalcareous silt that has been de- Long, A.. and Muller, A. (1981). Discussion of “Age of Tunica Hills (Louisiana- Mississippi) Quaternary rived from adjacent loess-covered hills. fossiliferous creek deposits: Problems of radiocarThis model accounts for the variations in bon dates and intermediate valley terraces in coastal the 14C dates of samples that have been replains” by E. G. Otvos. Jr. Quaternary Resettwit covered from this drainage system without 15, 365-367. relying on contamination to account for the McKay, E. D. (1980). Wisconsinan loess stratigraphy. Illinois State Geological Survey Guidebook Series disparity among the samples dated (Long 13, 9% 108. and Muller, 1981; Otvos, 1981). It also Otvos, E. G., Jr. (1975). Southern limits of Pleistocene agrees more closely with the loess and soil loess, Mississippi Valley. Southeastern Geology 17, stratigraphy of the Mississippi Valley as it 27-38. Otvos, E. G., Jr. (1978). Comment on “The Tunica is presently understood. Hills, Louisiana-Mississippi: Late glacial locality In summary, paleontologists collecting for spruce and deciduous forest species” by Paul plant fossils from the Tunica Hills are A. Delcourt and Hazel R. Delcourt. Quaternary He working with deposits of two distinct ages; search 9, 250-252. the older is Farmdale in age while the Otvos, E. G., Jr. (1980). Age of Tunica Hills (Louisiana-Mississippi) Quaternary fossiliferous younger is Woodfordian to Holocene in age. creek deposits; problems of radiocarbon dates and intermediate valley terraces in coastal plains.

REFERENCES Bernard, H. A. (1950). “Quaternary Geology of Southeast Texas.” Ph.D. dissertation, Louisiana State University, Baton Rouge. Brown, C. A. (1938). The flora of Pleistocene deposits in the western Florida Parishes, Western Feliciana Parish and East Baton Rouge Parish, Louisiana. Louisiana (Department of Conservation) Survey Bulletin 12, 59-96, 121- 129.

Geological

Delcourt, P. A., and Delcourt, H. R. (1977). The Tunica Hills, Louisiana-Mississippi: Late glacial locality for spruce and deciduous forest species. Quaternaty

Research

7, 218-237.

Delcourt, P. A., and Delcourt, H. R. (1978). Reply to comments by Ervin G. Otvos. Jr. Quaternary Research

9, 253-259.

Durham, C. O., Jr., Moore, C. H., Jr., and Parsons. R. (1967). “An Agnostic View of the Terraces: Natchez to New Orleans.” Field Trip Guidebook, Mississippi Alluvial Valley and Terraces: 1967 Annual Meeting, Geological Society of America, Part E. Fisk, H. N. (1938). Pleistocene exposures in western Florida parishes of Louisiana. Louisiana (Department

of

Conser,,ation)

Geological

Survey

Bulletin

12, 3-25. Frye, J. C., and Willman, H. B. (1961). Continental glaciation in relation to McFarlan’s sea-level curves

Quaternary

Research

13, 80-92.

Otvos, E. G., Jr. (1981) Discussion of “Age of Tunica Hills (Louisiana-Mississippi) Quaternary fossiliferous creek deposits: Problems of radiocarbon dates and intermediate valley terraces in coastal plains by E. G. Otvos, Jr.,” Reply. Quaternary Research 15, 367- 370. Ruhe, R. V. (1976). Stratigraphy of mid-continent loess, U.S.A. In “Quaternary Stratigraphy of North America” (W. C. Mahanney Ed.), pp. 197-211. Hafsted Press, New York. Saucier, R. T. (1968). A new chronology for braided stream surface formation in the lower Mississippi Valley. Southeastern Geology 9, 65-76. Saucier, R. T. (1977). The northern Gulf Coast during the Farmdalian sub-stage: A search for evidence. U.S. Army Engineers Waterways tion Miscellaneous Paper y-77-1,

Experiment

Sta-

p. 7. Wascher, H. L., Humbert, R. P., and Cady. I. G. (1948). Loess in the southern Mississippi ValleyIdentification and distribution of the loess sheets. Soil

Science

Society

of America,

Proceedings

12,

389-399. Willman, H. B., and Frye, J. C. (1970). Pleistocene stratigraphy of Illinois. Illinois State Geologiccrl Survey

Bulletin

Series

94,

I - 204.