Process Relationships and Geomorphic

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TRANSACTIONS OF THE GULF COAST ASSOCIATION OF GEOLOGICAL SOCIETIES

VOLUME XLVI, 1996 413

Form/Process Relationships and Geomorphic Evolution of the Southwestern Louisiana Chenier Plain Matthew J. Taylor1-2, Mark R. Byrnes2, and Randolph A. McBride2 department of Geography and Anthropology, 2Coastal Studies Institute, Louisiana State University, Baton Rouge, LA, 70803

Abstract Geomorphic evolution of the southwestern Louisiana chenier plain is examined by analyzing ridge distribution and orientation. The configuration of and inter-relationships among former outer gulf shorelines are used to examine depositional processes associated with ridges during long-term chenier-plain progradation. The chenier plain (~4,900 km2) has been organized into a hierarchy of geomorphic features (1st, 2nd, 3rd, and 4th order) based on form, dominant vector direction of shoreline movement, and spatial scale. The chenier plain is the first order feature and is composed of second order features designated as complexes: chenier, spit, and beach ridge (35-300 km2). Individual cheniers (3rd order features) encompass beach ridges, spits, and washover beaches (4th order features), and all of these features comprise complexes. Within the geomorphic hierarchy, thirteen individual shoreline trends have been identified to establish chronological evolution of the area. Net westerly sediment transport along the chenier plain is indicated by (1) an increase in the number of ridges on the eastern side of the Sabine, Calcasieu, and Mermentau Rivers, (2) westerly deflection of minor streams (including the Mermentau River), (3) curved spit deposits, and (4) decrease of inter-ridge width to the west. However, anomalies in net transport direction do occur such as the curved eastern end of Little Chenier and west of the Sabine and Calcasieu Rivers. The western section of the chenier plain between the Sabine and Calcasieu Rivers is dominated by a beach-ridge complex (170 km 2) east of the Sabine River. Immediately west of the Calcasieu River, cheniers and beach ridges are scarce as sediment has accumulated in a beach-ridge complex (50 km2) on the updrift side of the Calcasieu River. Ridges diverge to the east (towards the Mermentau River) and are separated by mudflats (2-5 km wide) forming a classic chenier complex (300 km2). Grand Chenier, a regional transgressive shoreline, is composed of a curved-spit complex (35 km2) immediately east of the Mermentau River. The Grand Chenier trend (Front Ridge, Oak Grove Ridge, Long Island, Pecan Island, and Front Ridge East) truncates a small chenier complex (153 km2) east of the Mermentau River. Farther east in the Pecan Island area, a chenier complex (70 km 2) is truncated by two transgressive shorelines: first by Back Ridge and then by Front Ridge East. South of the Pecan Island chenier complex, more recent chenier-complex development (60 km 2) is evident at Mulberry Island, Beef, Bill, and Sand Ridges. More shore-normal orientations of Chenier au Tigre and Belle Isle likely represent ancient bay shorelines and oyster reefs. Existence of different ridge-complex types suggests multiple shoreline formation processes, sediment sources, and transport directions that are not always associated with major switches of the Mississippi River mouth. Shoreline evolution is determined by interaction of sediment transport with the dynamic diversion capability of entrances.

Introduction The chenier plain of southwestern Louisiana stretches from Sabine Pass, at the Texas/Louisiana border, east almost 200 km to Southwest Pass. This late-Holocene marginaldeltaic environment is up to 30 km wide and is composed of abandoned beach-ridge, chenier, and spit complexes. Geomorphology and an evolutionary model for the chenier plain were first discussed by Russell and Howe (1935), and Howe et al. (1935) using early-1930s US Geological Survey quadrangles and aerial photography. Fisk (1948) discussed the geology of Cameron and Vermilion Parishes and provided a general description of chenier-plain morphology. Furthermore, he underlined the importance of examining chenier orientation and abandoned river courses as the best method by which to interpret shifts in shoreline position. Using geomorphology, stratigraphy, and radio-carbon dating, Gould and McFarlan (1959) and Byrne et al. (1959) outlined

a general evolutionary model describing seven major shorelines in the study area. The objectives of this paper are to (1) provide a comprehensive description of chenier form and orientation to infer process, (2) present an evolutionary model of the southwestern Louisiana chenier plain developed by physiographic mapping, and (3) present a relative chronology of chenier-plain development.

Geomorphic Hierarchy Discussion of chenier-plain geomorphology requires definition and classification of landforms relative to dominant process. The chenier plain is defined as a first order feature (4,900 km2) and is composed of smaller second order features known as complexes (30-300 km2). Three types of complexes characterize the chenier plain and are designated chenier, beach ridge, and spit. Cheniers are third

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414 TRANSACTIONS OF THE GULF COAST ASSOCIATION OF GEOLOGICAL SOCIETIES

VOLUME XLVI, 1996

Table 1. Geomorphic Hierarchy for the Southwestern Louisiana Chenier Plain Feature

Formation Process

Order

References

Chenier plain

net progradational

1

Russell and Howe, 1935; Fisk, 1948; Price, 1955, Gould and McFarlan, 1959; Otvos and Price, 1979; Kaczorowski, 1980

Chenier complex

net progradational

2

This paper

Beach ridge complex

net progradational

2

This paper

Spit complex

net progradational

2

Hine, 1979;Kidson, 1963

Chenier

transgressive, regressive, lateral accretion

3

This paper

Washover beach

transgressive

4

Schwartz, 1975

Beach ridge

regressive

4

Stapor, 1975

Spit

lateral accretion

4

Evans, 1942; Kidson, 1963

order features and can contain individual beach ridges, washover beaches, and spits which are defined as fourth order features. Complexes encompass at least one or more third and/or fourth order features (Table 1). Cheniers and beach ridges can be geomorphologically similar, but are stratigraphically distinct. A chenier is defined in this paper as a beach deposit composed of shell and fine sand which overlies fine-grained nearshore, intertidal, or marsh deposits (Hoyt, 1969; Otvos and Price, 1979). Additionally, cheniers are not only transgressive (washover beach), as shown in the Hoyt (1969) model, but can contain regressive (beach ridge) and laterally prograded (spit) deposits. Anderson et al. (1995) illustrate this point when it was concluded that the shoreface-derived shell assemblage (well preserved) and high concentration of quartz sand indicate a regressive setting for Front Ridge, a segment of the Grand Chenier trend. A substantial component of many cheniers is washover beach deposits. Washover beaches are transgressive deposits perched on old marshes/mudflats as a function of storm and normal wave and current processes during landward retreat (Schwartz, 1975; Byrnes et al., 1995). Washover beaches on the southwestern Louisiana outer shoreline are discussed by Byrnes et al. (1995). Finally cheniers are separated by progradational mudflats which dominate chenier-plain facies. In this paper a chenier complex refers to an area with at least two cheniers separated by muddy units. Previously, two or more cheniers were defined as a chenier plain (Price, 1955; Otvos and Price, 1979). Beach ridges are regressive deposits without extensive fine-grained inter-ridge deposits. Additionally, beach ridge and swale material is similar to adjacent beach and foreshore deposits. Beach ridge-complex is used here to indicate

spatially distinct areas of the chenier plain that contain several beach-ridge sets (ridges with similar orientation), as used by Stapor (1975) and Oertel (1975). A spit is a beach that is attached to land at one end and extends into open water at the other. Wave refraction around the spit end and flood-dominated tidal currents causes a curve at the spit terminus into the body of water (Evans, 1942). A spit complex is built as lateral accretion of the deposit occurs forming multiple curved ridges.

Methods A new physiographic map of the southwestern Louisiana chenier plain was produced to help interpretation of form/process relationships. Data sets include: 1) NASA color infra-red National High Altitude Aerial Photography (December 1990 @ 1:58,000) which was used to map ridge orientation, distribution, and inter-relationships, 2) US Geological Survey 7.5-min (1980) and 15-min (1951) quadrangles, which were used for air photo rectification, and 3) the Geologic Map of Louisiana (Snead and McCulloh, 1984 @ 1:500,000), which was used to identify the Pleistocene/Holocene contact. Shoreline trends, which signify different stages in the evolution of the chenier plain, were identified to analyze spatial variability in depositional setting. Ridge orientation and shape were used to infer formation processes and direction of net longshore-drift (Stapor, 1975; Jacobsen and Schwartz, 1981).

Southwestern Louisiana Chenier Plain The southwestern Louisiana coast is characterized as a low-energy, microtidal, storm-dominated environment

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TAYLOR/BYRNES/McBRIDE

(Roberts et al., 1989). Dominant nearshore currents are to the west, controlled by prevailing southeasterly winds (Beall, 1968; Becker, 1972; Crout and Hamiter, 1981). The continental shelf grades at 1:125 out to the 10 m bathymetric contour south of the Sabine, Calcasieu, and Mermentau Rivers. South of Deep Lake, Big Constance Lake, and Flat Lake, shelf slope increases to 1:80. Trinity Shoal creates a gently sloping platform (1:400) seaward of Cheniere auTigre. The Sabine, Calcasieu, and Mermentau Rivers flow through the chenier plain, and lakes are common features throughout the area. White and Grand Lakes dominate the landscape in the eastern portion of the chenier plain, whereas Upper and Lower Mud Lakes, Calcasieu Lake, and Sabine Lake intersect the western portion of the chenier plain (Fig. 1). Ridges tend to exhibit steeper seaward slopes and gentle back slopes. Berm crests reach up to 3 m above National Geodetic Vertical Datum 1929 (NGVD). Some ridges have multiple crests and swales. Most ridges have smooth southern sides, but lobate and irregular northern (landward) sides. Major ridges in the study area are used mainly for cattle ranching and private homesteads because of the relief over surrounding marsh (Russell and Howe, 1935; Herbert, 1968; Kniffen and Hilliard, 1988).

Chenier Orientation and Distribution Relict shoreline distribution and orientation can provide valuable information regarding coastal evolution including sediment-transport directions, type of shoreline movement (lateral, seaward, landward), and possible sea-level changes (Fisk, 1948; Stapor, 1975; Taylor and Stone, 1996). Ridges of the southwestern Louisiana chenier plain generally trend eastwest and have similar alignment as the present shoreline; however, exceptions do occur. The chenier plain is divided into two zones to describe ridge patterns. Geomorphic zone 1 includes the beach-ridge complex of closely spaced curved ridges adjacent to the Sabine River and the area east to the Calcasieu River. Cheniers, such as the Grand Chenier trend, Pumpkin Ridge, Chenier Perdue, High Island, Happy Ridge, and Little Chenier, comprise parts of geomorphic zone 2 between the Calcasieu River and Southwest Pass (Fig. 1). Curved spit, and beach-ridge complexes are also found in Zone 2.

Geomorphic Zone 1: Sabine River to Calcasieu River Former shorelines in this zone are concentrated on either side of the Sabine River in a curved beach-ridge complex. The main complex is made up of spits diverging from Blue Buck Ridge which curve towards the northwest at both Hamilton Lake (former Calcasieu River) and Johnsons Bayou (former Sabine River) forcing westward migration of these rivers (Fisk, 1948). Ridge spacing and stratigraphy (Kaczorowski and Gernant, 1980) is more indicative of beach ridges than cheniers. Beach-ridge accumulations west of the Sabine and Calcasieu River indicate easterly sediment transport (Fig. 1). The modern beach (up to 60 m wide) is retreating over the marsh between Holly Beach and Peveto beach (Byrnes et al., 1995) and truncates the eastern ends of Blue Buck Ridge, Hackberry Ridge, Sanders Ridge, and Salt


Work Ridge. West of Constance Beach and Ocean View Beach, the outer shoreline is prograding with material provided by updrift erosion (Byrnes et al., 1995). Ridges south of the outermost Blue Buck Ridge shoreline, such as Hackberry Ridge, trend towards the southwest resulting in net progradation. Smith and Coon Ridges form a distinct shoreline north of the Blue Buck Ridge trend and are separated from each other by an old inlet of the Sabine River through which Johnsons Bayou now flows. Divergence of ridges to the west, and ridge curvature indicate a dominant westerly direction for sediment transport during their construction. Ridge orientations in this zone indicate fewer shoreline adjustments, which may be a function of increased distance from easterly sediment sources.

Geomorphic Zone 2: Calcasieu River to Southwest Pass Geomorphic features in zone 2 encompass a larger geographic region than zone 1 and greater variability in geomorphic characteristics is illustrated (Fig. 1). This area includes chenier, spit, and beach-ridge complexes, and marsh and mudflats. The concave shape of shorelines in this area with a protruding shoreline south of White Lake is similar in orientation to the present shoreline. Individual cheniers are up to 200 m wide and reach elevations of 3 m NGVD. Interchenier mudflats in this zone are up to 10 km-wide, but decrease in width to the west. Many shorelines in zone 2 are true cheniers (Hoyt, 1969; Otvos and Price, 1979), but other landforms, such as curved spits and beach ridges, also exist indicating complex process/form interactions over short distances (Kaczorowski, 1980; Chappell and Grindrod, 1984). Adjacent to the Calcasieu River, a prograding beach-ridge complex is evident. Further east ridges diverge and are separated by mudflats several kilometers in width. The Mermentau River flows through the center of this zone and has been continuously deflected to the west (25 km) by lateral progradation of Hackberry Ridge, Indian Point, Grand Chenier, and Hackberry Beach. Little Chenier and Little Pecan Island appear to constitute one of the oldest primary shorelines in the area which may have continued east as Cypress Point and Fire Island (Fig. 1). Central portions of the Little Chenier trend (Little Chenier, Little Pecan Island, Cypress Point, Fire Island) were truncated by transgression south of White Lake between Long Island and Pecan Island. Little Chenier exhibits curvature to the northeast at the Mermentau River suggesting easterly sediment transport for this portion of the shoreline. Several kilometers of marsh separate Little Chenier from Chenier Perdue to the south. Near the Mermentau River, Chenier Perdue is a single, welldefined ridge; however, about 5 km from its eastern end the chenier begins to bifurcate into a series of smaller, less distinct ridges. Pumpkin Ridge, low and wide with an irregular northern side, coalesces with Chenier Perdue a few kilometers east of Creole. Inter-chenier mudflat width in zone 2 decreases to the west, and ridges near the Calcasieu River form a 5 km-wide beach-ridge complex. Ridge convergence to the west indicates a decrease in the quantity of fine-grained sediment reaching the western section of geomorphic zone 2. Additionally, sediment accumulations on the eastern side of entrances, and mudflat shape indicate net westerly sediment transport (Jacobsen and Schwartz, 1981).

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North Island and Back Ridge (Pecan Island area) may represent a continuation of the Chenier Perdue shoreline trend. Portions of the Little Chenier and Chenier Perdue trends were truncated by the Grand Chenier transgressive event. A headland shoreline, similar to that observed at the entrance to Freshwater Bayou Canal, is envisaged for the area south of White Lake for the Little Chenier and Chenier Perdue trends. Tiger Island, Hackberry Ridge, and Cow Island/Indian Point are easterly extensions of Pumpkin Ridge, Happy Ridge, and Eugene Island west of the Mermentau River. These ridges were also truncated at their eastern ends by the Grand Chenier trend. The most contiguous shoreline in the area is the Grand Chenier trend which can be traced for over 80 km from Front Ridge East westward to Front Ridge (west) near the Calcasieu River. East of the Calcasieu and Mermentau Rivers, multiple curved ridges are evident. Longshore sediment transport was sufficient to overcome fluvial discharge and tidal exchange associated with the Mermentau River forcing westward migration of the river to create a laterally prograded spit complex. In contrast, previous westward migration of the Calcasieu River (i.e., Hamilton Lake) indicates fluctuating dynamic diversion capabilities of the Calcasieu River. Relict deflected relict Calcasieu (Fisk, 1948) channels to the west and curved-spit complexes indicate reduced dynamic diversion capabilities, and a beach-ridge complex adjacent to the Calcasieu River points to increased dynamic diversion capability of the Calcasieu River (Todd, 1968). South of the Grand Chenier trend, many minor shorelines are evident as dashed lines on Figure 1. These lines represent former shorelines where a standstill in mudflat progradation occurred and where large amounts of sand and shell did not accumulate. Differential compactional subsidence has subsequently occurred permitting identification of former muddy shorelines on air photos (Fisk, 1948). This illustrates the complexity of mudflat progradation; local and short-term variations in sediment supply control mudflat migration and growth as they extend westward along the coast. The Pecan Island area at the eastern end of zone 2 is dominated by Front Ridge East which truncates all previous shorelines. Back Ridge, which truncates Coupe Ridge, Cane Ridge, Sweet Bay Ridge, and Lambert Ridge, also represents the eastern extent of a major transgressive event. Most of the cheniers in this area may be eastward extensions of cheniers to the west, but direct physiographic correlation is difficult without precise dates (Gould and McFarlan, 1959). Cheniere au Tigre and the Mulberry Island/Beef Ridge/Sand Ridge trend represent shorelines formed after significant mudflat progradation. Shore normal ridges of Chenier au Tigre and Belle Isle may represent former Vermilion Bay shorelines or oyster reefs such as those observed in Vermilion Bay seaward to the Gulf of Mexico today.

Chenier-Plain Geomorphic Evolution Southwestern Louisiana chenier-plain evolution and relative chronology are examined using former shorelines identified from aerial photography. A developmental chronology was first provided by Howe et al. (1935) and Fisk (1948) based strictly on geomorphology whereas Gould and McFarlan (1959) based their interpretations on radiocarbon

VOLUME XLVI, 1996

dates with wide age ranges. The relative chronology presented in this paper is based on geomorphic relationships of ridges and fluvial systems.

Relative Chronology Major shoreline trends are shown on Figure 1 and are presented chronologically in Table 2. These trends are recognized as cheniers, beach ridges, or spits and are topographically and sedimentologically distinct from the surrounding marsh. Where mudflat progradation or migration slowed or came to a temporary standstill, a minor shoreline is evident. The dashed lines are indicative of such periods and may indicate small accumulations of coarse sediment and consequent differential compactional subsidence (Fisk, 1948) which today is represented by changes in vegetation. North-south trending bay and lake shorelines (e.g., Bl on Table 1) such as Belle Isle are shown on Figure 1. A minor shoreline (SI) is evident in the marsh north of Little Chenier and northeast of North and Money Islands. Junius Ridge is the most northerly shoreline west of the Calcasieu River and is grouped with the oldest shorelines to the east. The northeast-southwest trending Back and Lost Ridges (west of Calcasieu Lake) likely represent an early lake/bay shoreline (Bl). Elm, Lake, Eagle, and Twin Islands also form an early shoreline (S2) which is truncated by Little Pecan Island. Shoreline 3 (Northwest Little Chenier; S3) is truncated by Little Chenier and trends northwest. The oldest major shoreline trend in the area (S4) is the Little Chenier/Little Pecan Island trend (Fig. 1). The eastern extension of the trend likely formed a protuberance similar to that in the Freshwater Bayou area. Cypress Point and Fire Island may represent eastward extensions of S4 as they are the most landward ridges in the Pecan Island area. Belle Isle is not considered part of the outer shoreline, but may constitute several early Vermilion Bay shorelines. Belle Isle has, in the past, been included as part of the Little Chenier trend (Fisk, 1948; Gould and McFarlan, 1959; Penland and Suter, 1989). The western extension of the Little Chenier trend continues as a concave shoreline with a southwest orientation and joins with Back Ridge near the Calcasieu River. The Little Chenier trend is truncated by the Chenier Perdue/Creole Ridge trend south of South Prong. Junius Ridge, or Smith and Buck Ridges, west of the Calcasieu River, may be contemporaneous with the Little Chenier trend. High Island (S5) constitutes part of a major shoreline trend; however, east and west extensions of the shoreline are not preserved. Truncated ridges on the northwest margin of North Island may be contemporaneous with S5. The next major shoreline to form (S6) was the North Island/Chenier Perdue/Back Ridge (south of Calcasieu Lake) trend . Fisk (1948) states that the Calcasieu River occupied a more westerly position during S1-S6 periods. If the Calcasieu River were in its present position during formation of S4-S6 the western ends shorelines S4-S6 would be curved similar to the Grand Chenier trend. During S1-S6 formation, the Calcasieu River occupied a westerly position (Hamilton Lake, Old East Bayou, and Mud Lake; Fisk, 1948). This dates the Blue Buck beach-ridge system to a period at least as early as S6. The migratory nature of the lower Calcasieu River during the formation of the chenier plain (Fisk, 1948) may also

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Geomorphic Feature Collins, 1941

A B

Howe et al., 1935

North Of Little Chenier Belle Isle Back Ridge (west of Calc.) Junius Ridge Elm, Eagle, Lake, Twin Islands Northwest Little Chenier Little Chenier B

A

Little Pecan Island

Wauchope, Fisk, 1947 1948

A A

A

2950-3200 2475

Coles Creek 1300 1100, F

1100, F

Copell Site, 2500, 1600-4200 Morgan site 2500 D Veazey site Tchefuncte 2500

800-3850, G G G 600, G

Copell Site Archaic 3500

Gagliano, 1967

Teche 5700-3800

St. Bernard 4700-700 Lafourche 3400-present

Plaqueminesmodern 900-present

Brown et al., 1979

Copell Site Koch site Archaic Coles Creek 1300 3500 Cypress Point 1300 Agee site, 800

Frazier, 1967 Phillips, Delta Dates 1970

Table 2. Chenier Plain Chronology Mclntire, 1958

Gould and McFarlan, 1959 2400, A 2150, C A

Tchefuncte 2500 2800 A,B Tchefuncte 2500 2800 A A,B

B

Coles Creek 1300

G B

C C

D

2100, C 2100, C

C

High Island Prongs north of North Island Back Ridge (east of Calc.) Smith & Buck Ridges Blue Buck Ridge North Island Chenier Perdue E E

C A

D

Troyville 1600 Troyville 1600

Tiger Island Pumpkin Ridge

2100, C 1250, D E

G

Copell site, Tchefuncte 2500, ^ Morgan site 2500 Veazey site G Tchefuncte 2500

H H I

C

E E F

Creole Ridge Hackberry Ridge Cow Island, Indian Point/Eugene Island Grand Chenier Trend Front Ridge Back Ridge (Pecan Island)

Front Ridge East (Pecan Island) Lost Island (Pecan Island) South of Blue Buck Ridge Cheniere au Tigre Mulberry Island Hackberry Beach Rutherford Beach/modern

Spring 1979

Coles 1300

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REDUCED VERSION OF FOLD-OUT MAP SEE ENLARGED MAP AFTER PG. 422

Figure 1. Southwestern Louisiana chenier plain geomorphology.

VOLUME XLVI, 1996

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explain the lack of ridges immediately west of the present channel. Back Ridge (Pecan Island) likely represents an easterly extension of S6. Multiple minor shoreline adjustments occurred during progradation of mudflats south of S6 and are observed in the marsh west and east of the Mermentau River. These shorelines indicate episodic mudflat progradation punctuated by establishment of mud shorelines which permit reworking and winnowing of sediment to form minor shorelines. Tiger Island, Pumpkin Ridge, and Creole Ridge form part of S7. Kochs Ridge likely represents a continuation of the trend to the east. Pumpkin Ridge truncates and coalesces with S6 as inter-chenier mudflat width decreases to the west. Shoreline 7 continues west and curves to the northwest at the Calcasieu River. West of the Calcasieu River, Salt Work Ridge, Sanders Ridge, and Hackberry Ridge could be part of S7, as they appear to have formed after the switch of the Calcasieu River to its present location. Happy Ridge and Hackberry Ridge (east of the Mermentau River) form part of S8. Shorelines are now closer spaced which may suggest a reduction in fine-grained sediment supplied to the area. Cow Island and Indian Point form a shoreline (S9) contemporaneous with Eugene Island which intermittently continues west and curves to the northwest as Cameron Ridge near the Calcasieu River. Curved-spit deposits south of S9 are part of the Grand Chenier trend (S10). Shoreline 10 encompasses Front Ridge East, Long Island, Grand Chenier, Oak Grove Ridge, Front Ridge, and may continue east of the Calcasieu River as Holly and Peveto Beaches. Shoreline 10 is a transgressive shoreline that truncates S5-S9 east of the Mermentau River in the area of Humble Canal. The wide age range of 1100-6000 yr B.P. assigned by Gould and McFarlan (1959) to the Grand Chenier trend is evidence that it truncated and reworked older deposits (i.e., time averaged). Extensive mudflat progradation punctuated by many short standstills occurred south of the Grand Chenier trend (S10). Orientation of minor shorelines in the marsh south of S10 is similar to previous shorelines and shows westward movement of mudflats resulting in net progradation. Shorelines that formed just before the modern shoreline include Cheniere au Tigre (SI 1). Beef Ridge also represents rapid adjustments in the Cheniere au Tigre area and may be the only remnant of a previous shoreline (S12). Sand Ridge, Mulberry Island, Hackberry Beach, and Mesquite Ridge (SI3) form the shoreline nearest to the modern beach. The Gulf shoreline truncates Mulberry Island and Hackberry Beach in addition to many older shorelines west of the Calcasieu River. Contemporary shoreline truncations indicate a pause in mudflat progradation except for a few isolated locations such as Chenier au Tigre (Wells and Kemp, 1981; Robert et al., 1989) and east of the Calcasieu and Sabine Rivers where jetties trap sediment and sandy tidal flats prograde.

Discussion The relative chronology presented above is based on geomorphic interpretations of continuous shorelines. Numerous archaeological studies have been conducted on the chenier plain that document the age of the cheniers (Collins, 1927; Mclntire, 1958; Gagliano, 1967; Phillips, 1970;


Neuman, 1977, 1984; Springer, 1973, 1979; Shelly, 1980; Gagliano et al., 1982; Brown, 1984). Dates obtained from pottery chronologies can augment carbon-14 dates obtained in geological studies (Brannon et al., 1957; Gould and McFarlan, 1959). The technique of using cultural features to date shoreline position has a long history, and may have first been used by Redman (1852; 1864) in his work on shoreline change along the south and east coasts of England. Today archaeological and geological studies are conducted together to understand settlement patterns or history and coastal plain progradation (DePratter and Howard, 1977; Colquhoun et al., 1981; Mason, 1993). Radiocarbon chronology from Gould and McFarlan (1959), and other relative chronologies of chenier-plain evolution are compared in Table 2. Most of the dates on the cheniers were obtained using the fragile Mulinia shells rather than older, possibly reworked shells such as Crassostrea (Gould and McFarlan, 1959). Archaeological chronology can confirm or contradict the significant dates used in the construction of chenier-plain evolution by Gould and McFarlan (1959). Controversy exists regarding the age of archaeological deposits in the Pecan Island area, and cultural deposits may have been reworked by natural processes to produce inconsistent dates (Goodwin et al., 1991). However, archaeological chronology on Little Chenier appears consistent with radiocarbon dates. Occupation of Chenier Perdue does not pre-date geological interpretations regarding the formation of this ridge. Coles Creek occupation of Grand Chenier appears to confirm the 1100-1250 yr B.P. assigned by Gould and McFarlan (1959). Archeological periods assigned to the Pecan Island area are older than radiocarbon dates. Little Pecan Island and North Island archaeological and geological chronologies are similar. Shoreline chronology provided in this study sometimes differs from previous interpretations (see Table 2). Former shoreline trends have been identified north of the Little Chenier and Little Pecan Island. In previous studies the Little Chenier trend is the northernmost and oldest shoreline (Gould and McFarlan, 1959). Additionally, here Blue Buck Ridge is considered at least as old as Chenier Perdue. However, Howe et al. (1935) considered Blue Buck Ridge to be a westward extension of the Grand Chenier trend. Calcasieu River channel switching constrains correlation interpretations. All ridges south of Creole Ridge were constructed after occupation of the present Calcasieu River channel. Generally, sediment transport during the long-term progradation of the chenier plain has been to the west for both fine- and coarse-grained sediments. Curved spits on the eastern side of the Mermentau River, and beach-ridge complexes updrift of the Calcasieu and Sabine Rivers confirms westerly progradation of shorelines in the area. Width of inter-chenier mudflats decreases to the west which indicates a easterly source and westerly transport for finergrained deposits. Decrease in the amount of mudflat sediments to the west has controlled ridge orientation and the coalescing of ridges toward the Calcasieu River. Additionally, fining of sediment, and significant reduction of shell content to the west on both the modern beach and the Grand Chenier trend indicate westerly sediment transport (Taylor et al., 1995). Westerly deflection of the Mermentau River by 20 km,

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provides evidence for long-term westerly drift directions in this section of the chenier plain. Classic drift reversal, caused by wave refraction around entrance shoals, is evident in the beach-ridge complexes west of the Sabine and Calcasieu Rivers. Reversals in longshore sediment transport direction are evident from sedimentologic and orientation data from Little Chenier. Post-digestion grain size decrease to the east (Taylor et al., 1995), northeasterly curves toward the Mermentau River, and slight eastward deflection of the Mermentau River indicate easterly sediment transport and lateral progradation of the eastern end of Little Chenier. However, decrease in shell clast size, and lowering and widening of the chenier to the west, indicate westward migration of this shoreline (Jacobsen and Swartz, 1981). The steep southern slope, gentle back slope, large shell clasts, and high elevation of Little Chenier to the east suggest a higher energy depositional environment. Reduction in shell content (Taylor et al., 1995), and lowering and widening of Chenier Perdue to the west also point to westerly progradation for this shoreline. The eastern end of Chenier Perdue is also steep, high (3 m), and contains large shell clasts which may indicate higher energy was important in the construction of the eastern ends of these ridges. Pumpkin Ridge has an irregular landward edge which indicates that washover processes dominated during formation. Truncation of older shorelines by the Grand Chenier trend illustrates the transgressive nature of this shoreline. However, areas of seaward progradation along this shoreline, with sediment supplied by erosional updrfit truncation areas, along this shoreline are evident east of the Mermentau and Calcasieu Rivers. Shoreline variation in profile and plan suggest that different processes were dominant along the ridge. This is also observed on the modern beach which has areas of erosion and downdrift deposition and accretion (Byrnes et al, 1995).

Conclusions 1) The southwestern Louisiana chenier plain (1st order) is described using a geomorphic hierarchy: beach-ridge, spit, and chenier complexes (2nd order); cheniers (3rd order); and individual beach ridges, washover beaches, and spits (4th order). This hierarchy reveals a complex system that is not explained by the simple regressive mudflat and transgressive beach model of Hoyt (1969). 2) Analysis of ridge relationships reveals that chenier deposits have been reworked multiple times and few separated cheniers exist. Instead cheniers tend to grade into beach-ridge complexes near the terminus of longshore drift systems east of the Calcasieu and Sabine Rivers, and to a smaller degree at the Mermentau River where a spit complex is observed. 3) Ridge accumulations on the east side of major rivers, westward deflection of rivers, decrease in mudflat width to the west, and sedimentologic data indicate lateral migration of deposits to the west. Some reversals in westward sediment transport do occur, such as at the eastern end of Little Chenier and on the west side of the Sabine and Calcasieu Rivers. 5) Correlation of shorelines across estuaries is difficult, making a regional chronology almost impossible without

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extensive radiocarbon dates. However, geomorphic mapping has provided a more complete analysis of chenier chronology which differs from previous interpretations. 6) Overall chenier plain evolution is influenced substantially by transgressive processes. However, the relationship between longshore sediment transport and dynamic diversion at entrances results in regressive chenier deposits. Seaward progradation in the form of beach ridges occurs if dynamic diversion is strong. If sediment transport is dominant, a laterally migrating spit complex forms.

Acknowledgements Research was funded by the USGS Coastal Geology Program under cooperative agreement no. 14-08-0001A0917 and the Louisiana Department of Natural Resources, Coastal Restoration Division. David Kelley (Coastal Environments Incorporated) is thanked for providing data on southwestern Louisiana chenier-plain archaeology. We would like to thank Lisa Duvic, Feng Li, and Gerald Moreau (Coastal Studies Institute, Louisiana State University) for technical assistance. Robbie Zenero, and David Seng helped with fieldwork. Guthrie Perry and employees at Rockefeller Refuge provided valuable assistance and accommodation. Residents of the chenier plain are also thanked for allowing us on their property, and for their wonderful hospitality. Laurie Anderson (Dept. Of Geology, Louisiana State University), and Paul Heinrich (Louisiana Geological Survey) are thanked for reviewing the manuscript.

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