BRI Bulletin 51

Pliocene Surface Stratigraphy in the Fort Polk Region: Implications for Louisiana Surface Geology

Paul V. Heinrich 1 and Richard P. McCulloh 1

Abstract Recent completion of a project of nearly three­

year's duration to investigate and map the surface geology of ten 7.5-minute quadrangles encompass­ing the Fort Polk military reservation leads to the conclusion that, in contrast to the prevalent conven­tion of the last several decades, strata assignable with fair confidence to the Pliocene are mappable in Loui­siana. The investigation found that the subdivision of the Fleming section into six alternating sand- and clay-rich members of formation rank, following pre­vious authors, continues to be the most effective clas­sification of surface and shallow-subsurface Miocene strata. In accordance with recent work done at the Louisiana Geological Survey, however, the highest elevation and stratigraphically lowest Citronelle­equivalent units classify as members of the Willis Formation of Texas, and are assigned to the Pliocene. The next younger units incising the Willis classify as members of the Lissie Formation of Texas and are assigned to the middle Pleistocene. The results of the investigation support the premise that previous fail­ure to recognize Pliocene surface units in Louisiana, in distinct contrast to surface geologic mapping in adjacent states, was an artifact of the classification of Quaternary surface units prevailing here. Recog­nition of Pliocene strata at the surface could have manifold implications, such as for amplifying the interpretation of paleoclimates via correlation of Neogene surface-stratigraphic units with published coastal-onlap curves.

1Louisiana Geological Survey, Louisiana State University, Baton Rouge, Louisiana 70803

Introduction The research presented in this paper is a sum­

mary of portions of a more comprehensive investi­gation of the surface geology of the Fort Polk region in northern Vernon Parish and southern Natchitoches and Sabine parishes by McCulloh and Heinrich (1999), in particular, the chapter Pliocene Series (Heinrich and McCulloh, 1999), and funded by the Joint Readiness Training Command at Fort Polk. This research is built on earlier work by Welch (1942), Rogers and Calandro (1965), Andersen (1960, 1993), and most recently by Hinds (1998a, b, c, and 1999). The study area, consisting of ten 7.5-minute quadrangles, is shown in Figure 1.

This research had a threefold objective. First, it was to provide the U.S. Army with basic geologic data essential to the successful conduct of its envi­ronmental programs. Second, it was to enlarge upon previous research by Hinds (1997, 1998a, b, c, and 1999) concerning the surficial geology of the Fort Polk region. Finally, this work was to refine and con­tinue previous geologic mapping by the Louisiana Geological Survey within this area (Louisiana Geo­logical Survey, 1993; Snead et al., 1998).

The study area was covered using a standard four-wheel drive vehicle to inspect areas accessible along roads. Selected intervening areas where expo­sures appeared likely were accessed on foot. How­ever, a widespread cover of surficial deposits and thick vegetation conceals the stratigraphic units be­ing mapped and makes exposures scarce. Outcrops occurred as artificial exposures along roads and trails, and natural exposures along the beds, cutbanks, and valley walls of streams and rivers. Typically, expo­sures were small and inconspicuous and found in the ditches along roads. The better and sometimes sizable exposures consisted of roadcuts and natural bluffs. During fieldwork, notes were recorded on copies of the topographic base maps and in field books for each of the ten 7.5-minute quadrangles in the study area.

52 BRI Bulletin

The field observations were used to delineate stratigraphic boundaries on the basis of gross lithol­ogy and internal features. Boundaries were modi­fied on the basis of topography from 7.5-minute quadrangles and distribution of soils as mapped in Natural Resources Conservation Service (NRCS) parish soil surveys for Natchitoches, Sabine, and Vernon parishes (Guillory, 1997; Martinet al., 1990; and Muse and Cooley, in press). Finally, 1:2,000 black and white Edgar Tobin Aerial Surveys and 1:20,000 black and white Agricultural Stabilization and Conservation Service aerial photographs were consulted for preparation of the mapping.

Only a small number of researchers have stud­ied the Pliocene and Pleistocene strata within the study area. Welch (1942) conducted the first and most extensive study of the project area at a time after it had been clearcut of forest but lacked topo­graphic maps. Using the terminology of Fisk (1938), Welch (1942) classified the deposits of the Upland Allogroup as the Williana Formation and the de­posits of the Intermediate Allogroup (Lissie Alloformation) as the Bentley Formation. Welch (1942) considered both units to be of Pleistocene age based upon these correlations. On the basis of substantial field work, Andersen (1960) and Bianchi

Vernon

Figure ]-Location map and index to 7.5-minute quadrangles covered by the study area (from Fig. I, McCulloh and Heinrich, 1999:2; reprinted by permission).

BRI Bulletin 53

(1982) further refined Welch's (1942) original Pleis­tocene framework of the area. Bianchi (1982) made a significant contribution in the recognition of stepped topography associated with Welch's (1942:62) "Pleistocene" deposits.

Later, Rogers (1982 and 1993, personal commu­nication) proposed for many years that surface Pliocene strata potentially occurred in these depos­its based on their lithological characteristics and el­evation of the older and higher Pleistocene remnants. Investigating Roger's (1982 and 1993, personal com­munication) ideas, Hinds (1998a, b, c, and 1999) mapped these deposits in detail, now considered to be the Upland Allogroup within the areas of Slagle and Fort Polk, Louisiana. Besides providing detailed descriptions of these deposits, he identified outcrops possibly of Pliocene age within the Upland Allogroup. Snead et al. (1998) used the mapping of Hinds (1998b, c) and preliminary results from this investi­gation in their preparation of the Quaternary Geo­logic Map of Louisiana (Snead et al., 1998), as part of a cooperative agreement with the U.S. Geological Survey under its State Geologic Mapping Program (STATEMAP).

Regional Setting The Fort Polk area lies in the West Gulf Coastal

section of the Coastal Plain Province of Thornbury (1965). Its gulfward part consists of coast-parallel Quaternary terraces. The study area lies in the area of transition between fluvial terraces to the north and coast-parallel terraces to the south.

The northern part of the Coastal Plain Province consists of deeply dissected Tertiary uplands charac­terized by a series of coast-parallel cuestas and low­lands inland of the terraces. The cuestas are com­monly called "wolds." Less commonly, the lowlands are called "vales." The cuestas are characterized by steep inland-facing escarpments and gentle gulfward slopes. The southernmost cuesta, the Kisatchie Wold, dominates the Fort Polk region almost completely. Within the Fort Polk region, the Kisatchie Wold con­sists of two distinct cuestas.

The Kisatchie Wold encompasses all of the Main Post and most of the Birds Creek, Fort Polk, Fuller­ton Lake, Lacamp, Slagle, and Simpson South quad­rangles. A well-defined escarpment 30 to 50ft (9 to 15 m) high forms the southern edge of this cuesta. This escarpment separates the highly dissected cuesta

from the flatter, less-dissected coast-parallel terrace surface to the south. The southern cuesta rises in elevation from about 270 to 290 ft (82 to 88 m) along its southern edge to an elevation of 400 to 440ft (122 to 134m) along its crest. Northward of this crest, the Kisatchie Wold drops rapidly into the lowland formed by the valleys of Bayou Castor, Cy­press Creek, and Calcasieu River. Topographic pro­files along the crests of ridges between the major drainages show that the southern cuesta rises as a series of topographic steps instead of as a gradual slope. A series of northwest-southeast trending drain­ages have cut valleys that deeply dissect these steps.

The study area lies downdip of the Sabine uplift, the north Louisiana salt basin, and Angelina­Caldwell flexure to the north; the Toledo Bend flex­ure and Gulf Coast salt and growth-fault basin lie to the south (Figs. 2 and 3 ). The Angelina-Caldwell flex­ure is marked at the surface by a zone of faults mapped in Louisiana by Andersen (1960, 1993). The stream net over much of the study area shows rect­angular drainage that is suggestive of potential struc­tural control.

Figure 2-Regional geologic and tectonic framework of study area (from Fig. 8, McCulloh and Heinrich, 1999:19; reprinted by permission).

In the study areas, Tertiary strata range in age from Eocene to some high and discontinuous rem­nants of oxidized, sandy strata herein assigned a Pliocene age. The Tertiary section consists of vary­ing proportions of sand, silt, and mud, with sand

PROJECTION OF STUDY AREA

H ARKANSAS I LOUISIANA LOUISIANA I TEXAS

0 50 MILES

0 50 KILOMETERS VERTICAL EXAGGERATION = 20.8

GULF OF MEXICO

Figure 3-Dip section through the northern Gulf Coast along 94° west longitude. The study area of this report lies 60 km ( 37 miles) east of this line of section, and spans units ranging at the surface from the uppennost or farthest downdip portion of the outcrop belt of the Claiborne Group to the lower or updip portion of the outcrop belt of strata of Plio-Pleistocene age. (Geopressure on this diagram refers to "hard" geopressure, equivalent to 13 ppg drilling mud weight or the approximate base of the transition zone). (Redrawn and adapted from the American Association of Petroleum Geologists, 1990; reprinted by permission.)

(JJ +:-

CCJ ;;o

CCJ c ([) ~. :::::J

BRI Bulletin 55

constituting the greater volume, and reflects deposi­tion ranging from fluvial to shallow marine, with overall increasingly terrestrial character upsection. Quaternary units comprise strata underlying Pleis­tocene terraces and Holocene alluvium; various surficial deposits of Quaternary age form thin ve­neers on other map units, and with a single excep­tion, were not mapped for this investigation. Pliocene and Pleistocene units associated with terrace surfaces are divisible into three main groups, here classified as allogroups (units characterized by bounding unconformities): Upland, Intermediate, and Prairie (Table 1).

Directly underlying the Plio-Pleistocene strata is the Fleming Formation. As defined by Fisk (1940) for Rapides Parish and mapped by Welch (1942) for Vernon Parish, the members of the Fleming consist of alternating coarser-grained, fluvial-dominated lithofacies and finer-grained, more marine-influenced lithofacies. Fisk ( 1940) interpreted the coarser­grained 1nembers as fluvial and the finer-grained members as estuarine. Hinds made the refinement of interpreting Fleming members as representing upper and lower deltaic plain deposition, and the latter, corresponding to the finer-grained members, as potentially the updip expression of subsurface marine shale tongues and associated flooding sur­faces corresponding to periods of maximum trans­gression during the Miocene.

The vertebrate fossils discovered recently at Fort Polk occur in one of these overall finer-grained sub­units, the Castor Creek, but within a notably coarser­grained subinterval consisting of conglomerate and sandstone. Although the Fleming is traditionally clas­sified as a formation, in Louisiana and in the study area in particular, the unit is more appropriately de­scribed as of group rank and consisting of forma­tion-rank subunits.

Upland Allogroup One of the more widespread and recognizable

stratigraphic units within the Coastal Plain Province is the Upland Allogroup. It is part of a regionally extensive, coast-parallel belt of coarse-grained sedi­ments that have been mapped along the Coastal Plain Province from Virginia to Texas. Typically, the sedi­ments of the Upland Allogroup occur as erosional remnants capping isolated hilltops and comprising the ridge crests of interfluves (Autin et al., 1991).

The deposits that comprise the Upland Allogroup have gone through a variety of name changes. Ini­tially they were described by Hilgard (1860) as the Orange Sand Formation and later as the Lafayette Formation of McGee (1891). Later, the Lafayette Formation was abandoned and subdivided into the Citronelle Formation of Matson (1916), the Willis Formation of Doering (1935), the Bentley and Williana Formations of (Fisk 1938, 1940) and oth­ers. Since the definition of these formations, there have been seemingly innumerable proposals as to their ages and correlations. Mapping by Snead and McCulloh (1984) reconstituted these formations into an informal morphostratigraphic unit called the High Terraces. Autin et al. (1991), redefining the High Terraces as an informal unconformity-bounded unit, renamed it as the Upland Complex. As a prelimi­nary step towards defining the Upland Complex as a formal allostratigraphic unit, Snead et al. (1998) renamed it as the Upland Allogroup.

As defined by Autin et al. (1991), the Upland Allogroup consists of alluvial deposits of early Pleis­tocene and Pliocene age. The Pliocene-age sediments consist of the Citronelle and Willis formations, which crop out, respectively, east of the Mississippi River Valley and west of the Calcasieu River. Between the Mississippi and Calcasieu rivers, the Upland Allogroup consists of the early Pleistocene-age allu­vial deposits designated by Fisk (1938, 1940) as the Williana and Bentley formations.

The Upland Allogroup consists of sediments that differ from the underlying Miocene and overlying Pleistocene strata. First, the Upland Allogroup con­sists of sediments that are typically coarser than the sediments of the Fleming Formation underlying it and those of the Intermediate Allogroup that onlap onto it. The coarser nature of the Upland Allogroup is commonly, but not always, manifest in the pres­ence of either abundant chert gravel or quartz gran­ules (Matson, 1916; Doering, 1935, 1956; Forney, 1950; and Autin et al., 1991).

Second, sediments within the Upland Allogroup are characteristically very highly weathered unlike younger and older strata. These sediments are char­acterized by deep reddish, brownish, and purplish colors of ferric iron oxides not seen in younger or older strata (Doering, 1956; Aronow, 1982; and Autin et al., 1991).

56 BRI Bulletin

Stratigraphic ColuiiJn of Fort Polk Region Time - Stratigraphic Units Rock - Stratigraphic Units

E ., Cl) Cl)

Stage Formation Member ...... .i: Group ., Cl) >, ~

~

Holocene alluvium + Biq Brushy undifferentiated c..

Q) :;J c a; :~ e (undifferentiated alluvium) "' 01 ('\S c £.2 c:: a; <(

s.. u ~ ~ ~ Q) 0

+.J +.J Calabrian V) ('\S ·w Q) Upper ::::J i;lc..

Ci 0:: ·- :;J ] e Lissie E 01 ~ 0 Q)=

_E<( Lower

a; Piacenzian ~ ~ ~ c

Q) Gravel Hill Allomember u c.. TowPr Roilrl Allomember

.Q "0 :;J

Zan clean c 0

Willis Duaout Road Allomember "' ~ a.. o_g>

Kisatchie Allomember :::J= <( Fort Polk Allomember

Messinian ~ ~ ~ I....

Q g Tortonian

Q) Blounts Creek undifferentiated c:: g

a; 0 c ~ Serravallian c:n Q)

z a; ""0 c Castor Creek undifferentiated u :0 .E .Q ~ ~ Langhian

a; u:: Williamson Creek undifferentiated

I.... Burdigalian Dough Hills undifferentiated a;

~ Carnahan Bayou undifferentiated 0 ~ _J

('\S Aquitanian Lena undifferentiated ·-e ~ I....

Q Chattian Catahoula undifferentiated a; g

c a; u 0

.2"1 c:n 0 I.... I....

a; ::::J

~ ..0 Rupelian V) undifferentiated

Q) 0 ~ _J u c:: > ~

0 Q)

~ c:: 0.. I.... 0

9 V)

Priabonian ~ undifferentiated g u ('\S

a; c a; u 0 a; u.J c a; I....

""0 0 ~ Bartonian ..0 Cockfield undifferentiated ~

C\S

u

Table ]-Stratigraphic classification of units mapped for this investigation (from Table 1, McCulloh and Heinrich, 1999:23; reprinted by permission.

BRI Bulletin 57

Third, the Upland Allogroup characteristically lacks its original constructional surface. Its surface consists of rolling ridges and ravines produced by deep erosion. This erosion has removed the original depositional surface of the units that it comprises. The only indication that a unit's depositional sur­face is left might be concordant summits along the crest of major drainage divides. The Upland Allogroup erodes into resistant hills, which creates well-defined cuestas and sharp ridges. In contrast to the Upland Allogroup, the Intermediate Allogroup exhibits remnants of its original depositional surfaces in the form of flat-crested ridges.

Finally, the soils associated with the Upland Com­plex consist primarily of ultisols with some alfisols. The soils developed within the Upland Allogroup are typically far more mature than those developed in either the Fleming Group or the Intermediate Allogroup (Forney, 1950; Autin et al., 1991; and Dubar et al., 1991). Autin et al. (1991) and Autin (1993) attribute the degree of weathering to a re­gionally pervasive geosol developed within it as the result of intense long-duration weathering of sandy to loamy sediments. According to Autin ( 1993 ), this geosol has been recognized from east Texas to west Florida and northward within the Mississippi Allu­vial Valley.

Willis Formation Traditionally, the deposits of Upland Allogroup

within south-central Louisiana have been correlated with the Williana Formation of Fisk (1938, 1940). For example, both Welch (1942) and Hinds (1998a, 1999) correlated these sediments with graveliferous fluvial deposits capping hills across the Calcasieu River that Fisk designated as the Williana and Bentley formations. Similarly, Bernard (1950) classified strata equivalent to the Upland Allogroup in southeast Texas as belonging to the Williana Formation of Fisk (1938, 1940).

In contrast, Doering (1956) observed that along strike, Fisk's (1938, 1940) Williana Formation in Ra pides Parish lies below the level of the deposits of the Upland Allogroup in the Fort Polk region. In Fig­ures 3 and 4, Doering (1956) showed that the sur­face of the Upland Allogroup in south-central Loui­siana lies as much as 300 ft (90 meters) above the surface of the Williana Formation on the opposite side of the Calcasieu River. These figures also show that the that gulfward slope on the "surface" of the

Upland Allogroup in south-central Louisiana is dras­tically steeper than the gulfward slope on the sur­face of the Williana Formation of Fisk (1938, 1940). Winker (1991) also illustrated the same differences in the stratigraphic position of the Upland Allogroup in south-central Louisiana and the Williana Forma­tion across the Calcasieu River. He classified the Upland Allogroup in the Fort Polk region as his pre­Lissie surface(s) and the Williana Formation of Fisk (1940) as being part of his Lissie surface.

Of these two interpretations, a carefully con­structed cross section, Figures 4 and 5, confirms those made by Doering (1956) and Winker (1991) con­cerning the stratigraphic separation between the type Williana and Bentley formations east of the Calcasieu River and the Upland Allogroup in south-central Louisiana. The sediments of the Upland Allogroup within south-central Louisiana lie as much 200 ft ( 60 m) above the top of the Williana Formation across the Calcasieu River in Rapides Parish. The base of the youngest and lowest sediments of the Upland Allogroup in the study area lies barely level with the top of the Williana Formation. The base of the Williana Formation lies about 50 ft (15 m) be­low the base of deposits in the study area. The great difference in stratigraphic position and lithology between the sediments in the study area and the Williana Formation in Rapides Parish demonstrates that they are different units of formation rank within the Upland Allogroup.

In terms of lithology and stratigraphic position, the strata of the Upland Allogroup are identical to the Willis Formation of Doering (1935) (Forney, 1950; Bernard and LeBlanc, 1965; and Aronow, 1982). For this reason, they are correlated with and designated as the Willis Formation in this investiga­tion. In addition, the deposits of the Williana and Bentley formations are deeply entrenched into both the underlying Miocene below the level of the Up­land Allogroup strata west of the Calcasieu River. This demonstrates that the Williana and Bentley for­mations are different stratigraphic units from the Willis Formation and younger than it.

Lithology Within the study region, two different facies, a

sandy facies and gravelly facies were discerned. Bianchi (1982:74) also noted these facies which he labeled, respectively, "fine sandy facies" of his "high coastwise group," and the "graveliferous facies" of

Lacamp Section

West

500

Q)400~~

tf .!: c: 300 0

~ [ij 200

,._ (/) "' ~~~ ~ g ~ &l .§

(/)

Intersection with Birds Creek Section

j Puwf

Mfcc

.c: ::J 0 (/)"' c::;:o ~,._

~ Ui

c. E~

~;.:

1 00 93 07' 30" 93 00' 00"

Mfcc

Legend for Figures 4, 8, 9, and 10 Stratigraphic Units

Quaternary System

Holocene Series

lEI lim

Hua Unnamed Alluvium

Hbb Big Brushy Formation

Pleistocene Series

i!

~ ~

Tertiary System

Pp

Pilu

Pill

Prairie Allogroup undifferentiated

Intermediate Allogroup

Pliocene Series

[GJ Puw

[jill Puwg

[£'] Puwt

D Puwd

D Puwk

D Puwf

DMfb D Mfcc Castor Creek Member

Figure 4-Cross section along crest of Kisatchie Wold and across Calcasieu River Valley.

1200 1500 1800 2100 2400 Feel

Mfcc 92 52' 30"

Horizontal scale= 1:125.000. Vertical exaggeration= 52X (from Fig. 18, Heinrich and McCulloh, 1999:49; reprinted by permission).

East

Williana Formation of Fisk (1940)

I \

500

400

300

200

100

t.n co

O:l ;:;o

O:l c (D ~. ::::J

BRI Bulletin 59

93'15'00" 3r15

Slagle

N

+ 31

Melder

Elmer

I Melder

31'00'00"1---------~-----+-----'----l---.=::::t-.L.--------~--------.131 '00'00" 93'15'00" 93'00'00" 92' 45'00" 92'37'30"

LEGEND

Main Post area of Fort Polk

~ Cross-section line

• Small-size town

Medium-size town

Figure 5-/ndex map showing locations of cross sections relative to the main post at Fort Polk, Louisiana (from Fig. 19, Heinrich and McCulloh, 1999:52; reprinted by permission).

his "low coastwise group." The gravelly facies is re­stricted to the youngest and lowermost units of the Willis Formation within the study area.

Sandy Facies The sandy facies consists of mostly very fine to

medium sand and muddy sand. Purplish, pebble to cobble-size clasts of clay and silty clay, quartz gran­ules and minor beds and lenses of silt and clay occur throughout this facies. In many outcrops, the sandy facies is largely massive, having been homogenized by plant roots and weathering. Where preserved, these sediments exhibit sedimentary structures that include planar cross-bedding, trough cross-bedding, ripple lamination, climbing ripple lamination, and scour-and-fill structures. Of these sedimentary struc­tures, medium-scale trough cross-bedding is the most common. The fine-grained sediments occur as dis­continuous beds and lenses. No upward fining or coarsening trends were observed in the sandy facies. The sandy facies exhibits bright and variable colors. Although predominantly various shades of red, the sandy facies can vary from reddish brown and weak red, to strong brown and yellow and pale red-purple

(Hinds 1998a, 1999).

Purplish, fine-grained, rounded rip-up clasts oc­cur in almost every outcrop of the sandy facies in varying abundance. Frequently, these purplish clasts of clay or silty clay are abundant enough to form thin beds of rip-up clast conglomerate. They occur concentrated along cross-beds and at the base of scours. The purplish rip-up clasts range in diameter from 2 to 5 em (0.8 to 2 inches). Rip-up clasts as large as 8 em ( 3 inches) in diameter were observed, whereas Hinds (1997) reported rip-up clasts as large as 25 em (10 inches). Whitish clay or mud clasts of sand size commonly occur on and delineate cross­beds.

In places, the sandy facies is gravelly. Typically the gravel consists of subrounded to subangular quartz clasts 1 to 2 em ( 0. 4 to 0. 8 inches) in diam­eter. The granule gravel occurs either scattered throughout the sand or discrete gravel stringers on cross-beds.

The only fossils observed in the sandy facies are rare pieces of petrified wood observed at two loca­tions in the Fort Polk area. In both cases, the petri-

60 BRI Bulletin

fied wood occurs at the base of the Willis Formation near its contact with the Fleming Group. In contrast, Bernard (1950) observed that petrified wood was common in the sandier parts of the Willis Forma­tion. The petrified wood consisted of large logs and stumps, which Bernard judged to be native to the Willis Formation.

Gravelly Facies The gravelly facies consists of a heterogeneous

mixture of muddy, medium-to-coarse sands, grav­elly sands, sandy gravel, and gravel. Like the sandy facies, exposures of the gravelly facies frequently are massive, having been homogenized by plant roots or weathering. Where preserved, well-developed medium to thick sets of trough cross-bedding char­acterize the sandy gravels and gravels.

Sand beds within this facies exhibit tabular and trough cross-bedding of varying scales, and ripple lamination. Purplish clay to silty clay rip-up clasts identical to those found in the sandy facies are abun­dant within the gravelly facies. No evidence for any overall upward fining or coarsening trends within the gravelly facies was observed during fieldwork.

The gravel consists of granule- to pebble-size chert gravel. As noted by Lenzer (1982), the gravel consists mainly of "dense" brown and brownish gray chert with minor amounts of other types of cherts, sedimentary quartzite, and metamorphic quartzite. Quartz, which is abundant as granules in the sandy facies, is noticeably absent among the rock types (Lenzer, 1982; Hinds, 1998a, 1999). The largest pebbles observed ranged from 4 to 5 em (1.5 to 2 inches) in diameter.

The chert gravel contains the only fossils so far found in the gravelly facies of the Willis Formation. These fossils consist of the molds of rare brachio­pods, crinoid stems, and corals, and other Paleozoic marine invertebrates. These fossils are not native to the Willis Formation, but rather were derived from Paleozoic carbonate rocks from which the chert was eroded and redeposited. The study of these fossils might provide some evidence concerning the source of the chert gravels found in the gravelly facies.

The colors exhibited by the gravelly facies typi­cally consist of various shades of red to yellowish brown. In a single outcrop, colors can include dark reddish brown, yellowish red, and brownish yellow surrounding numerous purplish rip-up clasts. In some

outcrops of both the gravelly and sandy facies, they consist of bleached-looking, grayish sediments not unlike those of the Catahoula and Fleming in col­oration, with red mottling in places. These bleached sediments are typically homogenous mixtures of sand, mud, and in places, gravel. Abundant root molds and haloes are commonly associated with these sediments.

Contacts Within the Fort Polk area, the Willis Formation

unconformably overlies the Fleming Group. In out­crops, the contact between the Willis Formation and Fleming Group is extremely irregular and in many places channeled (Fig. 6). Within small areas, the basal contact has been mapped as having 10's of feet of relief upon it. Regionally, the contact is a low­angle angular unconformity that northward progres­sively truncates the underlying Fleming Group. At the northern edge of the main Willis outcrop belt, the Blounts Creek Formation is completely missing and the Willis Formation rests directly on the Cas­tor Creek Formation.

The Fleming Group directly underlying the Willis Formation often shows evidence of weathering. Typi­cally, the Fleming Group lying within a meter of the contact exhibits the development of purplish alter­ation and rectangular blocky peds. Commonly, the overlying sediments of the Willis Formation contain purplish rip-up clasts derived from the underlying weathered sediments of the Fleming Group. Sedi­ments of the Willis Formation incorporating purplish rip-up clasts overlying weathered, fine-grained Fleming Group sediments, from which the clasts were derived, were observed at numerous localities. Welch ( 1942, Fig. 11) depicts these relations at this contact exposed in a railroad cut near Pickering (Fig. 7). Not all of the observed contacts between the Willis For­mation and Fleming Group exhibit this alteration below the contact. In a few exposures, bleached sedi­ments of the Willis Formation overlie relatively un­weathered fine sands or other sediments of the Willis Formation.

Within the study area, the upper contact of the Willis Formation consists of the modern land sur­face and another major unconformity between it and the overlying Intermediate Allogroup. The concor­dant summits of knolls along the crests of interfluves define a series of deeply eroded steps that character­ize the upper boundary of the Willis Formation

BRI Bulletin

.f -

·:~

EXPLANATION

Yellow loam Brownish yellow 1 OYR 6/6

Sand, overall very fine to fine, some sparse medium to very coarse grains. Yellowish red 5YR 5/6-8, with abundant white 1 OYR 8/1 to very pale brown 1 OYR 8/2 burrow mottles. Red 7 .5R 4/6mud rip-up clasts near base.

Clayey very fine to fine sand Light-gray 2.5Y 7/2 with red 1 OR 4/8 mottles. In places beneath contact with overlying sand at north end of cut, mud shows alteration to red 7.5R 4/6 in interior cores of peds, with light gray 2.5Y 7/2 remaining along ped surfaces. Apparent dip 4°.

Cross-bedded sand Overall very fine to medium, with some grains ranging from coarse sand to granules. Reddish yellow 5YR 6/6, with very pale brown 1 OYR 8/4 burrows and root molds. Red 7.5 4/6 mud burrow casts up to 0.5 m below upper contact.

61

4

3

.., 1..

2 Q) +J Q)

~

Figure 6-Combined outcrop sketch and measured section (no horizontal scale) along roadcut, SEINW Sec. 30, T. 5 N., R. 9 W, Peas on quadrangle. Mottled sediment of the Upland Allogroup is mantled by yellow loam and cuts out two units of the Carnahan Bayou Fonnation of the Fleming Group (from Fig. 13, Heinrich and McCulloh, 1999:34; reprinted by pennission).

62 BRI Bulletin

(Bianchi, 1982). Where the Lissie Formation of the Intermediate Allogroup onlaps onto the Willis For­mation, it completely truncates the Willis Forma­tion. Geological mapping indicates that the removal of the Willis Formation is complete and the Lissie Alloformation rests directly on the sediments of the Fleming Group. Bernard (1950) also observed a similarly deeply eroded unconformity between the Lissie Alloformation and Willis Formation in Jas­per and Newton counties, Texas.

Overlying Strata Either overlying the Willis Formation or occu­

pying stream valleys incised through it, within the Fort Polk area, are younger fluvial sediments of the Intermediate and Prairie Allogroups. In this area, the Intermediate Allogroup consists entirely of the Lissie Alloformation and the Prairie Allogroup con­sists of undifferentiated fluvial deposits that form terraces of relatively small extent. Unnamed allu-

NORTH

FEET

300 SELLERS STORE

SOUTH

WILLIS FORM _-UN ATION

CONFORMITY-250 ---

BLOUNTSCRE EK MEMBER OF FLEMING FORMATION

200

ONE MILE

EXPLANATION

1. Light-gray silty clay. 2. Sandstone lentil and ironstone at base of weathered material. 3. Weathered, silty clay and residual terrace sand, 2 feet. 4. Sandy siltstone lintil, thickness 0-6 feet, well exposed in railroad cut. 5. Small remnant of Willis Formation; 0-10 feet red, massive sand exposed

above highway, cross-bedded. Contact with underlying Blounts Creek member irregular; chert gravel and clay balls at contact.

6. Sandy siltstone, 0-6 feet thick, underlain by unconsolidated silty clay and overlain by about 6-10 feet of orange, red and purple mottled, weathered, argillaceous silt; distinct contact with Willis Formation and suggestion of pre-Willis soil horizon.

7. Massive sand, thicker down hill, is result of soil creep and occurs extensively on outcrops of both terrace formations and on sandy Miocene strata.

8. Massive red sand, gravel at base. 9. Gray, weathered silty clay.

Figure 7-Unconformable relationship of Willis Formation and underlying Blounts Creek Formation of the Fleming Group along U.S. Highway 171 in Kansas City Southern Railway cut at 2.4 km (1.5 miles) south of Pickering (Sec. 3, T. IS., R. 9 W, New Llano quadrangle). (Redrawn and adapted from Fig. 11, Welch., 1942:29).

BRI Bulletin 63

vium of Late Pleistocene to Holocene in age under­lies the flood plains. However, it is not considered in this discussion, as the details of these deposits are discussed in McCulloh and Heinrich (1999).

Intermediate Allogroup The Intermediate Allogroup of western Louisi­

ana consists of the fluvial deposits of the Calcasieu and Sabine rivers, their tributaries, and various coastal plain streams. This allogroup consists of strata that Bernard (1950) classified as the Bentley and Williana Terraces in southeast Texas; mapped as the Lissie and Oberlin formations by Doering (1956), and as the Oakdale and DeRidder surfaces by Snead et al. (1995). The Intermediate Allogroup of southwest Louisiana consists of the Elizabeth, Oakdale, and Lissie Alloformations.

The Intermediate Allogroup lies stratigraphically and topographically between the Prairie and Upland allogroups. The surface of the Intermediate Allogroup lies topographically higher than the ter­race surfaces of the Prairie Allogroup and topo­graphically lower than the dissected uplands under­lain by strata of both the Upland Allogroup and Fleming Group. Although heavily dissected, terrace surfaces of the Intermediate Allogroup are recogniz­able as flat-topped ridge crests. However, erosion has long since removed any constructional fluvial or coastal landforms. Slightly south of the Fort Polk region in west-central Louisiana, the Intermediate Allogroup consists of 100 to 300ft (30 to 90 m) of red, brown, and buff interbedded sand, silt, and clay. In general, the sediments of the Intermediate Allogroup are much sandier and have more mature soils, mostly alfisols, than sediments that character­ize the Prairie Allogroup (Autin et al., 1991; Dubar et al., 1991; and Snead et al., 1998).

Lissie Alloformation Within the study area, the only unit of the Inter­

mediate Allogroup present is the Lissie Alloformation. It consists of the deposits of the Calcasieu and Sabine rivers, their tributaries, and various coastal streams. In this region, the Lissie Alloformation is associated with two coast-parallel terraces. A higher and older terrace forms a narrow belt along the edge of the Tertiary Uplands. The younger terrace is the northern edge of the exten­sive, gulfward-dipping ramp-like surface that extends southward from the study region into Calcasieu Par-

ish. This terrace surface dips to the south at 4 ft/mi (0.8 rnlkm) except where it is offset by fault-line scarps. The Lissie Alloformation extends eastward along the entire Texas coastal plain, where it was named into Mexico as illustrated by Bernard and LeBlanc (1965) and Winker (1991).

Only within the Cooter's Bogs area in the south­east corner of the study area were small outcrops of the Lissie Formation observed. Welch (1942) mapped both of these terraces as the Bentley Terrace as he did most of the Lissie Formation within the Fort Polk region. In this area, small exposures of the Lissie Formation consist of grayish clayey silt and very fine sand with yellowish brown mottles. In a hand speci­men, these sediments superficially resemble sediment of the Fleming. They contrast sharply with the sedi­ments of the Willis Formation, which are pervasively stained, even cemented, with reddish, brownish, and purplish ferric iron oxides. In places, yellow loam mantles these sediments.

Prairie Allogroup (Undifferentiated) The Prairie Allogroup consists of fluvial, deltaic,

estuarine, and beach deposits that lie topographi­cally below the Intermediate Allogroup and topo­graphically above the Deweyville Allogroup. Cur­rently, this allogroup is considered to be late-to­middle Pleistocene in age. The terrace surfaces asso­ciated with it exhibit little dissection and commonly show relict constructional topography (Autin et al., 1991; Dubar et al., 1991).

Within the Fort Polk region, the Prairie Allogroup consists of fluvial sediments that form discontinu­ous terraces found within the valleys of local drain­ages. Hinds (1998a, 1999) noted that the deposits of the Prairie Allogroup are recognized as relatively flat areas lying slightly above the level of the Ho­locene flood plain. It can be inferred from the types of soils, as described by Guillory (1997) and Martin et al. (1990) associated with the terraces of the Prai­rie Allogroup that it consists largely of silts, silty sands, and sands within the Fort Polk region.

Discussion There are two major interpretations for the

stratigraphy of the Willis Formation within the Fort Polk region. First, Welch (1942) and Doering (1956) infer that the Willis Formation consists of a single gulfward-tilted stratigraphic unit with a highly dis­sected terrace that both dip below the deposits of

FORT POLK SECTION

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500 300 Pilu

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300

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100 100

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Figure 8-Cross section along crest ofintelfluve on which Fort Polk and North Fort Polk lie. Horizontal scale= 1:125,000. Vertical exaggeration= 52X. (Refer to legend on P. 58.) (From Fig. 23, Heinrich and McCulloh, 1999:61; reprinted by permission).

500

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BIRDS CREEK SECTION

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Figure 9-Cross section along crest of interfluve separating Birds Creek and Little Sixmile Creek-Sixmile Creek drainages. Horizontal scale = 1:250,000. Vertical Exaggeration= 52X (Refer to legend on p. 58.) (From Heinrich and McCulloh, 1999:63; reprinted by permission.)

500

400

300

200

100

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66 BRI Bulletin

the Lissie Alloformation. They both showed the Lissie Alloformation conformably onlapping the deposits of the Willis Formation. In contrast, Bianchi (1982) illustrates the Willis Formation as forming a single pediment cut into the Fleming Group with a rela­tively flat terrace surface. Instead of dipping beneath the deposits of the Lissie Formation, the Willis For­illation and its associated terrace surface are trun­cated by the Lissie Alloformation. In southeast Texas, Bernard ( 1950) combines aspects of both models in that he interpreted the Willis Formation as a single gulfward-dipping stratigraphic unit truncated by the Lissie Alloformation.

The 7.5-minute geologic quadrangles and cross sections prepared for this study show a modified in­terpretation of the ideas of Bianchi (1982). The to­pographic profiles of the cross sections confirm the existence of the stepped topography discussed by Bianchi (1982). At least five stepped surfaces are shown by the topographic profiles. Integration of the data from the geologic mapping into the cross sections revealed that the base of the Willis Forma­tion generally parallels the concordant summits of each of the steps (Figs. 5 and 8 through 10). In addi­tion, the cross-sections show that abrupt changes in the elevation of the basal contact of the Willis For­Illation coincide with the scarps separating each step.

Based upon these observations, the steps are in­terpreted to represent a series of five relatively flat coast-parallel terraces underlain by fluvial deposits of the Willis formation. Each terrace is lower than the next terrace to the north. Correspondingly, its associated fluvial deposits are more deeply downcut into the Flen1ing Group relative to the terrace to the north. These terraces and associated fluvial deposits are inferred to be separated by erosional contacts (Figs. 8 through 10); as such, they are interpreted as allostratigraphic units.

On this basis, five informal allostratigraphic units are recognized and mapped: the Fort Polk allomember, Kisatchie allomember, Dugout Road allomember, Tower Road allomember, and Gravel Hill allomember (Figs. 8 through 10). The Fort Polk and Kisatchie allomembers consist of the sandy fa­cies. The Dugout Road, Tower Road, and Gravel Hill allomembers consist of the gravelly facies. The deep dissection of these deposits by local drainages has reduced the surface of each allomember to a se­ries of concordant summits. In case of the highest

and oldest allomember, the Fort Polk allomember, all evidence of its surface has been obliterated. Its fluvial deposits have been reduced to erosional rem­nants capping the east-west crest of the southern cuesta and isolated summits along the interfluves extending southward from it (Figs. 8 through 10). North of the main post of Folk Polk, the Willis For­mation occurs as widely scattered, discontinuous outliers of limited extent. These outliers form only the highest summits, commonly with maximum el­evations exceeding 400ft (122m).

Origin The deposits of the Willis Formation are gener­

ally agreed to be fluvial in origin (Welch, 1942; Doering, 1935, 1965; Bernard, 1950; and Autin et al., 1991 ). However, insufficient data exist to dis­cern the specific type of fluvial system that created these deposits. Regardless of whatever type of flu­vial system deposited the Willis Formation, the con­trast between the coarse-grained Willis Formation and the underlying and overlying units shows that it represents a radically different and unusual sedimen­tological regime. The widespread distribution of cor­relative coarse-grained and deeply weathered strata, the Citronelle Formation, indicates that environmen­tal factors influencing the deposition the Willis For­mation affected all of the northern coastal plain of the Gulf of Mexico. What the specific factors, e.g., eustatic sea level change, epeirogenic uplift, or paleoclimatic changes, that are involved remain un­resolved.

The reddish, brownish, or purplish ferric iron oxides that pervasively stain, even cement, the sedi­ments of both the sandy and gravelly facies show that the entire Willis Formation has been highly weathered to very deep depths. The deeply weath­ered nature of the Willis Formation reflects a num­ber of influences. These include: ( 1) the high initial permeability of the coarse parent materials; (2) the thickness of the drained soil column above the wa­ter table; ( 3) the comparatively great age of the strata within the Willis Formation; and ( 4) contemporane­ous weathering of these strata during their accumu­lation as a result of warm Pliocene paleoclimates (Aronow, 1982).

The general lack of fossils within both the sandy and gravelly facies of the Willis Formation implies that conditions during the deposition of the Willis were generally not conducive to the preservation of

BRI Bulletin 67

fossils. Because of the coarse-grained nature of the Willis Formation, oxidation removed organic mat­ter and leaching destroyed bones and shells during and after deposition. Also, the accumulation of the Willis Formation during a period of relatively hot and humid climate would have greatly accentuated these processes and largely precluded the formation of fossils. Rare petrified wood is preserved because silicification occurs relatively soon after burial.

Age

Because age-diagnostic fossils have not been found in the Willis Formation, it cannot be directly dated at this time. Instead, what is known about the age of the stratigraphic units that unconformably onlap it and which it unconformably overlies must be used to infer its approximate age. Also, it is pre­sumed that the major unconformities that separate the Willis Formation from the underlying Fleming Group and overlying Intermediate Allogroup repre­sent major eustatic and/or tectonic events significant enough to have been recognized and dated.

The Lissie Alloformation of the Intermediate Allogroup, which unconformably onlaps the Willis Formation, is likely of Middle to Early Pleistocene in age. In Texas, the Lissie Formation contains Pleis­tocene vertebrate remains that include Bison latifrons (Harlan), Mammuthus columbi (Falconer), Mammuthus imperator (Leidy), Equus excelsus Leidy, Equus francisi Hay, Equus complicatus Leidy, and Equus semiplicatus Cope (Forney 1950). Kukla and Opdyke (1972:569) reported sediments with reverse magnetic polarity from the "Lissie," "Bentley," and "2nd terrace," which Dubar et al. (1991) suggested are equivalent to the Lissie Forma­tion.

Finally, Winker (1979, 1991) correlates the sur­face of the Lissie Formation to the top of the R6 Reflector, a regional seismic reflector in the offshore. This regional reflector lies immediately below the Trimosina A (denticulata) bethic foraminifera datum that dates from 0.6 to 0.65 million years ago (Dubar et al., 1991; Winker, 1979, 1991). This is consistent with this reflector and the top of the Lissie Alloformation (Formation) and Intermediate Allogroup being associated with a major regression within the Gulf of Mexico that occurred about 0.80 million years ago. The break between the Prairie and Intermediate Allogroups would not only be associ-

a ted with a major regression that started at this time, but also a significant change in the frequency of sea­level fluctuations (Fig. 11 ).

As discussed previously, the Williana and Bentley formations of Fisk (1938, 1940), which are found east of the Calcasieu River and west of the Missis­sippi River, lie downcut below the level of the Willis Formation and the correlative Citronelle Formation. They are erosional remnants of fluvial deposits that apparently filled earlier alluvial valleys of the Mis­sissippi and Red rivers entrenched deeply through earlier deposits of the Willis and Citronelle forma­tions. Woodward and Gueno (1941) interpreted the presence of very large boulders of chert, quartzite, sandstone, and petrified wood within the Williana as being the result of downstream transport by ice­rafting. Pebbles of igneous rock and Baraboo Quartz­ite in gravels of the Williana Formation indicate the presence of glacial outwash within it (Fisk, 1939). Although, as hypothesized by Autin and others (1991), these gravels might belong to pockets of gla­cial outwash included in the Williana Formation along the sides of the Mississippi River Valley, the above pebbles imply that the gravels are in part of glacial-outwash origin. Being older than the Lissie Formation, the Williana Formation would have formed during Early Pleistocene or Late Pliocene gla­ciations. This would imply that the Willis Forma­tion is no younger than Late Pliocene.

Vertebrate faunas described from both the Goliad Formation in Texas and the Fleming Group in Loui­siana constrain the lower limit of the age of the Willis Formation. Schiebout (1994, 1997) estimates the age of the vertebrate fauna from the Castor Creek For­mation of the Fleming Group to be between 12.5 and 14.5 million years old. Given the thickness of the Blounts Creek Formation lying between it and the vertebrate-bearing beds in the underlying Castor Creek Formation, the Willis Formation is signifi­cantly younger than this fauna. In Texas the Willis Formation overlies the Goliad Formation, which contains the Lapara Creek Fauna, which Prothero and Manning (1987) estimated to be 10 to 11 mil­lion years old. Interpreting these faunas together suggests that the Willis Formation is significantly younger than 10 million years old.

A maximum age for the Citronelle and Miccosukee Formations, east of the Mississippi Al­luvial Valley, is provided by other vertebrate faunas.

68

Geologic Time

(Million years)

1

2

3

4

5

6

7

8

9

10

11

12

13

14

Epoch

(]) c (]) (.)

0 (L

(]) c (]) (.) 0

~

(]) c (]) (.) 0 +-' en (])

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BRI Bulletin

Eustatic Curves

200 100 0 m

Om

Relative Change of Coastal Onlap

shoreward seaward

Regional Stratigraphy Southwest Louisiana

Figure ]]-Suggested correlation of stratigraphic units in southwest Louisiana with sea-level and coastal onlap cycles of Paleo-Data, Inc. ( 1993). Data from Paleo-Data, Inc. ( 1993). (From Fig. 26, Heinrich and McCulloh, 1999:69 reprinted by permission.)

BRI Bulletin 69

Huddlestun (1988) argued that the Ashville local fauna of north Florida, which consists of late Barstovian vertebrate fossils, came from strata un­derlying the Miccosukee Formation. Thus, the Miccosukee Formation postdates a middle Miocene (late Barstovian) age of about 11 to 16 million years (Woodburne and Swisher, 1995). In southwest Ala­bama, the Mauvilla local fauna occurs in paralic­alluvial deposits of the "Miocene coarse clastics" of Raymond (1985:64), which underlie the Citronelle Formation. The Mauvilla local fauna consists of late Miocene (early Hemphillian) vertebrates that date these sediments to about seven to nine million years ago (Hulbert and Whitmore, 1997; Woodburne and Swisher, 1995). These vertebrate faunas indicate that the Citronelle and Miccosukee Formations are younger than middle-to-late Miocene age.

Within southeast Texas, Morton et al. (1988) correlate the base of the Willis Formation with the Bigenerina "A" (Bigenerina floridana) benthic fora­minifer datum. Allowing for the arbitrary drop in datum, Morton et al. (1988) correlated the Willis Formation with the middle and upper portions of the Bigenerina shale. The correlation of the Willis Formation with the transgressive Bigenerina shale and equivalent benthic foraminifer datums implies that it correlates with major sea-level highstands that occurred during the Early Pliocene (Paleo-Data, Inc., 1993) (Fig. 11).

Kukla and Opdyke (1972) reported the presence of normally magnetized sediments within the Willis Formation. They interpreted the normal magnetism of its sediments to represent the Gauss polarity ep­och, Chron C2An, 2.6 to 3.5 million years ago. However, normal magnetism is also consistent with Chron C3n, 4.1 to 5.2 million years ago, which in­cludes many of the Early Pliocene sea-level highstands. It is not determinable which of these periods of normal magnetism these normally mag­netized sediments belong to and whether sediments with reverse magnetism are also present within the Willis Formation.

Finally, scanty palynological data indicate a Pliocene age for the Citronelle Formation. Otvos (1997) recovered trace amounts of Sciadopytis (Japa­nese umbrella pine) pollen from peats exposed in pits dug into the Citronelle Formation at Vancleave, Ala­bama, and Mossy Head, Florida. The presence of Sciadopytis pollen precludes a Pleistocene age for the

Citronelle Formation because the Japanese umbrella pine became extinct in North America prior to the end of the Pliocene (Otvos, 1997 and 1998). For example, Sciadopytis pollen occurs in the Pliocene­age Yorktown Formation of North Carolina and Virginia and the lower Beavertown Formation of Delaware and New Jersey and is absent in the over­lying Late Pliocene and Pleistocene strata (Groot, 1991; Cronin et al., 1993 ).

The period of geologic time covering the Pliocene and late Miocene epochs, three to six million years ago, is judged to be the most likely time during which the bulk of the Willis Formation in the Fort Polk region accumulated. The unconformity at the base of the Willis Formation correlates with a major drop in sea level about 10.5 million years ago (Fig. 11 ). The drastic drop in sea level shifted deposition tracts southward (Wornardt and Vail, 1991; Paleo-Data, Inc., 1993 ). This regression would have ended the accumulation of the Blounts Creek Formation of the Fleming Group within the Fort Polk Area. Between 5.8 and 10.5 million years ago, sea level was either significantly below or close to present sea level. As a result, even during highstands, the edge of coastal plain deposition would have remained south of the Fort Polk region (Fig. 11). During this time, erosion of the Fleming Group would have occurred in re­sponse to sea-level drop and ongoing uplift. Between 5.8 and 3.0 million years ago, during a series of highstands, sea level rose as much as 87 to 120 ft (25 to 35m) above present sea level. The inner edge of coastal plain deposition would have transgressed during the highstands back into the Fort Polk re­gion and resulted in the deposition of fluvial sedi­ments (Wornardt and Vail, 1991; Paleo-Data, Inc., 1993). If so, the Willis Formation could represent this period of high sea levels, while individual allomembers represent individual highstands.

As discussed by Otvos (1997, 1998), the Citronelle formation contains evidence of sea-level highstands higher than modern. This evidence con­sists of 0.5 to 7.0-m-thick units composed of muddy sands and muddy, pebbly, fine-to-coarse sands de­posited in inshore and nearshore environments. The marine origin of these sediments is shown by the presence of either Ophiomorpha burrows produced by ghost shrimp; the tubes of polychaete worms; the internal molds of veneroid and~ other shallow ma­rine bivalves; or some combination of them. Otvos

70 BRI Bulletin

(1997) observed these marine beds as occurring near both the base and top of the Citronelle Formation.

Huddlestun (1988) and Otvos (1998) have cor­related the Citronelle and Miccosukee Formations with Upper Pliocene, not Lower Pliocene, sea-level highstands. They argue that the Miccosukee Forma­tion and the correlative Citronelle Formation in ad­jacent Florida is of late Pliocene (Piacenzian) age. These arguments are based upon the Citronelle For­tnation overlying the fossiliferous Upper Pliocene Jackson Bluff Formation in the Florida panhandle and on Huddlestun's (1988) correlation of the Miccosukee Formation with the microfossil-dated Cypresshead Formation of eastern Georgia (Otvos, 1988).

Because of uncertainties in correlations and fa­cies changes between Florida and south-central Loui­siana, a Late Pleistocene age for the Citronelle For­mation in Georgia and Florida fails to preclude an Early Pliocene age for the Willis Formation in south­central Louisiana. However, it does imply that the younger allomembers of the Willis Formation likely are of Late Pliocene age. If Late Pliocene age depos­its exist within the Willis formation, the younger, gra veliferous Tower Road and Gravel Hill allomembers, which closely resemble the Citronelle Formation in lithology, would be the most likely can­didates for such deposits. In this case, the major unconformity between the Willis Formation and the onlapping Lissie Formation would represent a ma­jor lowstand of sea level that occurred about 1.8 million years ago (Fig. 11.)

Summary A detailed study of the Upland Allogroup shows

that the strata cropping out west of the Calcasieu River consist of fluvial deposits that are correlated to and designated as the Willis Formation in Texas. The Willis Formation is a separate and older strati­graphic unit from the Williana and Bentley Forma­tions as defined by Fisk (1938, 1940) in central Loui­siana. The Willis Formation is considered a strati­graphic equivalent of the Citronelle Formation in Mississippi and of Upland Allogroup strata in the Florida Parishes of Louisiana.

Within the Fort Polk region, the Willis Forma­tion consists of two well-defined sedimentary facies and five allostratigraphic units. On the basis of li­thology, the Willis Formation can be divided into

sandy and gravelly facies. Mapping of these strata and stepped surface topography along its interfluves indicates that the Willis Formation consists of five members separated by major, scarp-forming unconformities. These allomembers form five coast­parallel terraces. The contact between the Willis For­mation and the Fleming Group is an angular unconformity and it is unconformably onlapped by the Intermediate Complex. The Willis Formation is considered to be Early and possibly Late Pliocene in age. It is hypothesized that it represents fluvial depo­sition that occurred during sea-level highstands 3.0 to 5.8 million years ago and possibly as late as 1.9 million years ago.

Acknowledgments The research discussed in this paper was funded

by the U.S. Army Corps of Engineers (USACE), Joint Readiness Training Command and Fort Polk divi­sions, on a contract administered and managed by Stephen Austin, Fort Worth District, under Contract No. DACA63-95-D-0051, Delivery Order No. 0008. The results of this research were submitted to Prewitt and Associates Consulting Archaeologists, Inc., Aus­tin, Texas, as McCulloh and Heinrich (1999). The USACE gave us permission to publish figures con­tained that report. We also acknowledge the Ameri­can Association of Petroleum Geologists who granted permission to publish Figure 4 of this report.

Many people gave to us assistance that was inte­gral in the conduct of this research, and without which it could not have been completed. The most frequent and essential help with operational matters was given by Jim Grafton, Bob Hays, and Gina Lay of the Environmental Learning Center at Fort Polk, and by Barry Oswald and the other Range Control staff who provided general scheduling of our access to land on the military reservation. Dr. Charles H. Stagg, chief of Environmental and Natural Resources Management Division of the Directorate of Public Works at Fort Polk, was uniformly supportive of our efforts. Mr. McCann provided crucial help in our scheduling of access to military land during training rotations.

The project technical managers contributed to this paper with their patient understanding and tech­nical oversight of this research. Dr. A. Frank Servello, formerly of the U.S. Army Corps of Engineers (USACE), Fort Worth office, served as project tech-

BRI Bulletin 71

nical manager for the greater part of the period dur­ing which field work was conducted. In addition to his normal duties, he contributed an avid interest in this research. Stephen Austin of the USACE, Fort Worth office, following Servello's departure and our completion of most of the field work, served as project technical manager for the remainder of the period of performance.

Faculty, staff, and students at Louisiana State University (LSU) also assisted us in our research. David Hinds, a graduated master's student from the Department of Geology and Geophysics, provided fruitful consultation on the results of his own geo­logical investigations. Dr. Judith Schiebout, associ­ate curator, Museum of Natural Science, contributed helpful discussion and support on many occasions. John Anderson, who directs the Cartographic Infor­mation Center administered by the LSU Department of Geography and Anthropology, provided invalu­able access to and assistance with assorted maps and images in that archive.

Several U.S. Geological Survey (USGS) person­nel provided information and assistance in the form of consultation, file data, and copies of USGS re­ports. Mark Gremillion of the Fort Polk office, and John Lovelace, Larry Prakken, Rob Fendick, Roland Tollett, Darlene Smothers, and Wendy Lovelace of the Water Resources Division, Baton Rouge district office, assisted us with access to subsurface data in the USGS files.

M. Earl Stewart, wildlife biologist with the U.S. Department of Agriculture, provided access to aerial photos and maps in the files of the National Forest Service, Vernon Ranger District, Kisatchie National Forest.

The following employees of timber companies were active in the study area and helped provide criti­cal access to large tracts of nonmilitary land: Robert H. Crosby of Crosby Land and Resources, Mandeville, Louisiana; Darwin Foster of Temple Inland Inc., Diboll, Texas; Mike Hudson of the Temple Inland Inc. office in De Ridder, Louisiana, who gave a positive account of our work to Darwin Foster and helped us obtain permission for access; Rick Leeper, forester with the Temple Inland Inc., De Ridder office, who provided access to gated prop­erties; and Bob Nolan of the Boise Cascade office in Provencal, Louisiana, who gave us permission and

keys to access gated properties.

For producing the illustrations in-house at the Louisiana Geological Survey (LGS), we are indebted to the staff of the LGS Cartographic Section under the direction of John Snead, cartographic manager. Lisa Pond, research associate, produced most of the illustrations for this paper. Production and assistance with the production of a number of the illustrations was provided by Robert Paulsell, research associate; Edward Koch, research associate; Edwin B. "Bud" Millet, cartographic supervisor; and David W. Grif­fin, research associate. Margo Olinde, LGS editor, provided helpful consultation regarding the editing and formatting of this paper.

Finally, acknowledgment is made to two geolo­gists who have had a positive shaping influence on this research through their previous investigations and their helpful conversations about them over a period of 15 years: H. V. Andersen (Professor Emeri­tus, LSU Department of Geology and Geophysics), who mapped the geology of Sabine and Natchitoches parishes; and James E. Rogers (USGS, retired, now a consulting ground-water hydrologist), who has investigated the surface and shallow subsurface ge­ology in Vernon Parish and many other parts of the state.

Bibliography American Association of Petroleum Geologists,

1990, Gulf coast regional cross section. Tulsa, Oklahoma: American Association of Petroleum Geologists. Three oversized sheets.

Andersen, H.V., 1960, Geology of Sabine Parish: Louisiana Department of Conservation, Geo­logical Bulletin No. 34, 164 p.

Andersen, H.V., 1993, Geology of Natchitoches Parish: Louisiana Department of Conservation, Geological Bulletin No. 44, 22 7 p.

Aronow, S. 1982, Surface geology, in C. L.Neitsch, ed., Soil Survey of Jasper and Newton Counties, Texas: U.S. Department of Agriculture, Soil Conservation Service, Washington, D.C. p. 102-107.

Autin, WJ., 1993, Quaternary geology of the lower Red River Valley, in W.J. Autin and C.E. Pearson, eds., Quaternary Geology and Geoarchaeology of the Lower Red River Valley: A Field Trip: South Central Cell Friends of the Pleistocene, Baton Rouge, Louisiana. p. 5 24.

72 BRI Bulletin

Autin, W.J., Burns, S.F., Miller, B.J., Saucier, R.T., and Snead, J.I., 1991, Quaternary geology of the Lower Mississippi Valley, Chapter 18, in R.B. Morrison, ed., Quaternary Nonglacial Geology: Conterminous U.S.: The Geology of North An1erica, v. K 2, Geological Society of America, Boulder, Colorado. p. 547-582.

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BRI Bulletin 73

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Basin Research Institute

Bulletin A publication of the Louisiana Geological Survey

Special Millennium Issue 1999-2000 Volume 9 • June 2000

Louisiana Geological Survey Chacko J. John, Director and State Geologist

Board of Advisers Frank W. Harrison, Jr., Chair Senator Max T. Malone Terry Ryder Karen Gautreaux James M. Coleman James E. Rogers William E. Marsalis

The BRI Bulletin Staff

Editorial Science Director/Chacko J. John Bulletin Editor/Margo Olinde

Production Manager/John 1. Snead Graphic DesignslEdward Koch & Lisa Pond Contributing Writers

Ron Zimmerman John Echols Rick McCulloh Paul V. Heinrich

Technical SupportlReed Bourgeois

Distribution Cherri Webre Telephone: (225) 388-8328 Fax: (225) 388-3662 Web Site, http://www.lgs.lsu.edu

The BRI Bulletin is distributed to academic, industrial, and professional persons and groups associated with geological research and applications. Back issues may be ordered by contacting the BRI business office.

Contents

Predicting A North Louisiana Jurassic Pinnacle Reef Trend From Palinspastic Reconstructions Ron Zimmerman ................. .... ... .... ... .. ............ 1

Coalbed Methane: Louisiana's Unexplored Energy Resource John B. Echols ......... ... ................... ... ............. .. 18

A Summary Of The Total Oil Generation Potential Of Louisiana's Hydrocarbon Systems Ron Zimmerman ......... ............. ... .. ..... .. .... ....... 28

De Quincy Fault-Line Scarp, Beauregard and Calcasieu Parishes, Louisiana Paul V. Heinrich .... ........................... ..... ..... ..... 38

Pliocene Surface Stratigraphy in the Fort Polk Region: Implications for Louisiana Surface Geology Paul V. Heinrich and Richard P. McCulloh ...... 51

The Louisiana Geological Survey grants permission for fair, nor-for­profir use of SRI Bullerin contenrs. The SRI Bulletin is published yearly by LGS, whose offices are locared ar Louisiana Srare University, Room 208, Howe-Russell Geoscience Complex, Saton Rouge, LA 70803-4101.

Geological data and inrerprerations shown in this bullerin have been carefully compiled and interprered by rhe Louisiana Geological Survey of Louisiana Srare University. The interpretations represent our best judgement, however, we cannor and do nor guarantee the accuracy or correctness of any geological or other information.