ABSTRACT

General models of chenier plain origin are based on extensive studies of the Louisiana chenier plain. Shifting positions of the Mississippi River are generally thought to control episodic fluxes in sediment supply to the chenier plain. However, local sources of coarse clastic sediment may be important contributors during chenier ridge formation.

Sediment sources and transport dynamics of the southwestern Louisiana chenier plain are examined through analysis of sixty-seven modern beach and chenier sediment samples. Samples collected along about 100 km of the active berm between Big Constance Lake and the Sabine River were analyzed for alongshore trends in texture and composition. Correlation of data obtained from the modern beach with the Grand Chenier complex, Chenier Perdue, and Little Chenier indicates that sediment from local and distant sources have been important in chenier plain evolution. Textural and compositional analyses or beach and chenier samples range from sandy shell (0.71 phi [1.64 mm]) to very fine quartz sand (3.28 phi [0.10 mm]) and reveal numerous fining and coarsening trends associated with local rivers. A single sediment source along the modern beach and landward cheniers appears improbable given observed trends in grain size and composition. Instead, a combination of local and distant (i.e., Mississippi River) clastic sources are involved in chenier formation.

INTRODUCTION

The southwest Louisiana chenier plain is located in Cameron and Vermilion Parishes (Fig. 1) and extends from the Sabine River at the Texas/Louisiana border to Southwest Pass (just west of Atchafalaya Bay). Large mudflats separate shore parallel shelly/sandy ridges (cheniers). Cheniers represent stranded beaches isolated from the Gulf of Mexico by prograding mudflats. Episodic development of the southwest Louisiana chenier plain is traditionally thought to be controlled by lateral shifting of the Mississippi River (Russell and Howe, 1935; Fisk, 1948; Price, 1955; Byrne et al., 1959; Gould and McFarlan, 1959; Wells and Roberts, 1980; Kaczorowski, 1980; Penland and Suter, 1989). The classic model of chenier plain evolution, first developed by Russell and Howe (1935), suggests that progradation of the chenier plain is directly linked to deltaic sedimentation. Inter-chenier mudflats are constructed when the Mississippi River occupies a more westerly position. When the Mississippi River has a more easterly orientation, sediment supply is decreased, and waves winnow and rework mudflat deposits into sandy/shelly beaches as the shoreline retreats landward. These beaches are stranded inland as ridges when mudflat progradation is renewed during the next westerly shift of the river. Thus, it has been postulated that each chenier (ridge) marks a period of coastal retreat between two periods of advance (Russell and Howe, 1935; Gould and McFarlan, 1959; Todd, 1968; Hoyt, 1969).

No regional geologic studies have been conducted on the Louisiana chenier plains since researchers of Shell and Humble Oil Companies (LeBlanc, 1949; Brannon et al., 1957; Gould and McFarlan, 1959; McFarlan, 1961) conducted stratigraphic surveys using cores and radiocarbon dates. Chenier plain sediment sources have not been studied in depth except by Berger (1981) and Tanner (1993), but possible local sediment sources were largely ignored. Additionally, the highly variable carbonate content in many of the ridges has not been addressed (Kaczorowski, 1980; Berger, 1981; Tanner, 1993). A textural and mineralogical analysis of chenier and beach sediments between the Calcasieu and Sabine Rivers was conducted by Berger (1981) in an attempt to identify sedimentological differences. Sampling was limited, and despite grain-size analysis that included carbonates, no inferences were made regarding contribution of local bioclasts to the chenier plain. Berger (1981) concluded that beach and chenier sediments have the same source. Tanner (1993) examined kurtosis values of thirteen samples taken from a transect crossing cheniers near Grand Chenier, Louisiana. Results suggest that cheniers represent settling deposits. Tanner (1993) also stated that cheniers were constructed over periods of a few centuries at sea levels of one to two meters higher than present.

Numerous regional-scale studies on recent Gulf of Mexico sediment texture, mineralogy, and sources are used for comparison with results of this localized study (Curray, 1960; Shepard, 1960; Kolb and Van Lopik, 1966; Ritchie et al., 1989). Sediment size information from the Sabine (2.8 phi [0.14 mm]), Calcasieu (2.9 phi [0.125 mm]), and Mermentau (4.2 phi [0.055 mm]) estuaries is provided by Barrett (1971). Hsu (1960) analyzed samples from point bars in the Sabine (2.0 phi [0.26 mm]), Calcasieu (1.54 phi [0.33 mm]), and Mississippi Rivers (2.65 phi [0.16 mm]; 1500 km below Cairo to the Gulf). Information regarding beach texture west of the Mississippi River is found in a study of the Isles Dernieres barrier shoreline, an area composed of reworked Mississippi River deltaic deposits (Ritchie et al., 1989). Median grain size for these islands is approximately 2.65 phi (0.16 mm). Shepard (1960) states that the mean diameter of sands on barrier islands west of the Mississippi River is 3.03 phi (0.12 mm). The beaches fronting Bayou Lafourche are reported by Kolb and Van Lopik (1966) to have a median grain size of between 2.5 and 1.8 phi (0.18 and 0.28 mm). Curray (1960), in an examination of continental shelf sediment along the northwest Gulf of Mexico, states that the rivers of southwestern Louisiana and the Mississippi River contribute to what he classifies as Type I sediment on the continental shelf, which has a size range of 3.0 to 0.07 phi (0.13 to 1 mm). Type II sediment (4.2 to 3.0 phi [0.054 to 0.12 mm]) is derived from these same rivers, but supplied during a transgressional stage.

The extensive sampling conducted in the present study (see Fig. 1) permits evaluation of texture and composition for chenier and modern beach sediment between the Calcasieu and Mermentau Rivers. Chenier evolution is assessed by performing grain-size analysis on sediment samples from cheniers and modern beach deposits to evaluate transport paths and sources of chenier ridge sediment. The importance and influence of sediment input from local rivers, other nearby sediment sources, and the Mississippi River to chenier plain development is examined.

REGIONAL SETTING

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. Cheniers are most extensive between the Mermentau and Calcasieu Rivers; and, therefore, this area was selected for indepth study.

The modern beach for this study extends approximately 117 km from Rockefeller Wildlife Refuge south of Big Constance Lake to the eastern jetty at Sabine Pass. Cheniers between the Mermentau

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Figure 1. Sample locations on the southwest Louisiana chenier plain.

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and Calcasieu Rivers also were examined for sedimentologic trends. The southernmost ridge in the study area is the Front Ridge/Oak Grove Ridge/Grand Chenier complex (hereafter referred to as the Grand Chenier Complex). This extensive ancient shoreline (about 1,100-1,250 years BP; Gould and McFarlan, 1959) was examined from about 7 km east of the Mermentau River to a point 10 km east of the Calcasieu River (Fig. 1). Chenier Perdue (about 2,500 years BP; Gould and McFarlan, 1959), just north of the Grand Chenier complex, was studied from its easternmost limit near the Mermentau River (marked by a shell midden and large oak trees) westward through the town of Creole to a point approximately 6 km west of Creole where the ridge is then called Creole Ridge. Further west, Creole Ridge topography is indiscernible, and chenier ridge topography becomes complex as Front Ridge, Creole Ridge, and other small ridges begin to converge. The northernmost ridge in the study area is Little Chenier, dated at 2,800 years BP (Gould and McFarlan, 1959). The entire length of this chenier was studied from its eastern tip near the Mermentau River to the most westerly extent of the ridge (about 4.2 km east of LA highway 27).

Chenier morphology is similar for all sites. These features exhibit steep seaward slopes (paleo-beaches) which are now fronted by marsh. The break between the paleo-beach slope and the gently dipping landward slope is often marked by a well-defined topographic high known as the berm crest. 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).

METHODS

Sediment samples were collected from the cheniers and modern beach to address the question of clastic sediment supply to the chenier plain. At each site a minimum of 500 g of sediment was collected for repeated subsampling and archival purposes. Sample locations on the modern beach and landward cheniers are indicated on Figure 1. Forty-two surface samples were collected at about 2 km intervals on the cheniers. Twenty-five samples were taken from the modern beach at intervals ranging between 300 m and 13 km, depending on the location of coastal engineering structures, access, and changes in sediment characteristics. Samples were collected from the active berm crest about 10 cm below the surface. Global positioning system (GPS) equipment recorded sample locations.

Sediment samples from cheniers were collected from the berm crest at 2 km intervals using a soil auger to penetrate the surface soil horizon. Average sampling depth was maintained at a constant 50-55 cm. While this depth does not represent the last active surface of the beach (McPherson and Lewis, 1978), this technique avoids the top strata which are disturbed by humans and have undergone much diagenesis. The top 40-45 cm of ancient chenier deposits usually consists of a well-developed soil horizon with little shell and sand.

Sediment texture and compositional analyses followed standard techniques (Folk, 1980). Samples were washed with distilled/deionized water. Some samples were soaked for up to one week in distilled/deionized water to allow for disaggregation. Samples were wet sieved to separate the larger carbonate fraction (shell and shell fragments) from sand grains using a 0.5 phi (0.71 mm) screen; in most cases no sand was found to be coarser than 0.75 phi (0.59 mm). Separation of the two fractions prevents aggregation during the drying process. After drying, the two fractions of each sample were combined prior to splitting. Grain-size analysis was conducted at quarter-phi intervals using a Gilson Sonic Sifter. Grain-size distribution parameters, including median diameter, sorting (standard deviation), skewness, and kurtosis were calculated for each sample using the method of moment statistics. In some cases, a fine-grained fraction (<4 phi [0.0625 mm]) greater than 5% was encountered; this sediment was assumed to be primarily coarse silt and combined in the 4.25 phi sieve class for statistical analyses.

Carbonates were included in grain-size analysis because they are a large percentage of the sediment and are considered an integral component of transported/reworked beach sediment. Additionally, analysis of changes in shell size and content allows identification of possible sources and distribution paths. Granulometric studies of beach sediment typically involve digestion of all carbonates prior to grain-size analysis, and only the weight percent of carbonate is calculated (Folk, 1974; Tanner, 1993). However, this procedure can remove important trends revealed by inclusion of carbonate sediment (see Figueiredo et al., 1982). Therefore, grain-size analysis was conducted on each sample before and after carbonate digestion with 6N hydrochloric acid permitting quantification of the effect on grain-size trends. Median grain size was selected as the measure of central tendency for each sample because it represents the midpoint of a size distribution, requires no distribution assumptions and is less influenced than the mean by extreme values of skewness (Inman and Chamberlain, 1955).

In this study, the relationship between median grain size and distance along the modern beach and ancient cheniers is examined. Expected grain-size trends along a beach away from a source are controversial, consequently debate exists arguing for a fining or a coarsening trend in the downdrift direction (Pettijohn and Ridge, 1932; Inman and Chamberlain, 1955; Visher, 1969; Stapor and Tanner, 1975; Self, 1977; McCave, 1978; McLaren, 1981; McLaren and Bowles, 1985; Nordstrom, 1989; Masselink, 1992; Stone et al., 1992; Guillen and Jimenez, 1995). Skewness and sorting (deviation) also are considered. Because the dominant direction of longshore transport in the study area is from east to west due to the prevailing southeast wave approach (Beall, 1968; Becker, 1972; Wells and Kemp, 1981), sediment along a beach or chenier will become either coarser or finer in a westerly direction as a function of distance from the source.

GRAIN-SIZE TRENDS

The following discussion summarizes trends and variations in grainsize parameters and sediment composition. Inferences to sediment source derived from observed trends along the cheniers and the modern beach are then discussed. In sedimentology, standard deviation is referred to as sorting, and is determined by dispersion around the measure of central tendency (Pettijohn et al., 1987). A low sorting value indicates a well-sorted sediment with little spread around the mean grain size. Skewness records the asymmetry of a distribution and is often used to infer transport processes. Positive skewness indicates an excess of fines in the sample, whereas negative skewness denotes an abundance of coarse particles in the distribution tail (Pettijohn, et al., 1987). Linear regression (see Clark and Hosking, 1986) was conducted on pre- and post-digested beach and chenier samples to evaluate the functional relationship between median grain size and distance down the beach or chenier; median gain size is the dependent variable.

Figure 2 demonstrates the effect of carbonate digestion on representative samples from each chenier and the modern beach. Carbonate digestion removed a slight bi-modality present in some of the original samples. Bi-modality indicates the presence of two different sediment sources which tends to occur when two sediment suites are mixed or different concentrating processes are at work (Richmond and Sallenger, 1984; Sonu, 1972).

Modern Beach

Mean carbonate percent of modern beach sediment is extremely high to the east, but tends to decrease in a westerly direction from about 90% to less than 15% (Fig. 3). Pre-digestion median grain size decreases west along the beach from about -0.07 to 2.7 phi (1 to 0.16 mm). To the west, sediment type changes from shell fragments with a fine-grained sand matrix to a fine-grained shelly sand. Using regression analysis, 40% of this change in grain size is accounted for by the independent variable of distance west along the beach. Pre-digested sediment is moderately well sorted, and varies slightly as a function median grain size. Skewness values fluctuate greatly between -3 and 1.

Median grain size of digested samples shows little variation as a function of distance to the west. However, several fining and coarsening trends are evident in Figure 3. Sediment size is fairly constant

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east of Lower Mud Lake Outlet at 2.5 phi (0.18 mm), but between the Mermentau and Calcasieu Rivers, a coarsening (2.65 to 1.9 phi [0.16 to 0.27 mm]) and subsequent fining trend (1.9 to 3.8 phi [0.27 to 0.09 mm]) is evident. West of the Calcasieu River, grain size again increases from approximately 3.8 to 2.3 phi (0.09 to 0.20 mm) about 7 km east of the Sabine River. These samples are well sorted showing little east to west variation. Skewness values fluctuate between 1 and -1, indicating changes in the content of coarse and fine tails.

Grand Chenier Complex

Mean carbonate content for the Grand Chenier complex is only 5.9%, consequently, the difference between pre- and post-digestion median grain size is small (Fig. 4). Low carbonate content, small shell fragments, and larger median grain size of sand (2.1 phi [0.23 mm]) emphasize the unique sedimentological nature of the Grand Chenier Complex. Grain size variation relative to distance along-shore is minor in pre-digestion samples (r² = 0.21), and post-digestion data illustrate an even poorer functional relationship (r² = 0.11). Nevertheless, a fining trend of 1.8 to 3.1 phi (0.30 to 0.11 mm) from the eastern extent of samples to approximately Creole Canal is evident (r² = 0.72). Median grain size increases west of Creole Canal to the western end of the study area (3.1 to 2.1 phi [0.11 to 0.23 mm]). Fluctuations in sediment sorting (1.3 to 0.4 phi [0.4 to 0.8 mm]) are removed by digestion. Post-digested sediment is well sorted (0.5 phi [0.7 mm]) and is better sorted to the west. Skewness is more positive after digestion, most values are above zero. (Fig. 4). Thus, sediment (pre- and post-digestion) of the Grand Chenier Complex is well sorted and exhibits two major trends.

Chenier Perdue Ridge

Median grain size for pre-digested samples (carbonate x = 9.3%) decreases from -0.5 to 2.11 phi (1.4 to 0.23 mm) westward away from the Mermentau River. Carbonate content also decreases in a westerly direction from 25% to 0%. Size of carbonte fragments in the sand of Chenier Perdue also decreases in a westerly direction. As with the Grand Chenier Complex, sediment becomes better sorted in a westerly direction (1.0 to 0.5 phi [0.5 to 0.7 mm]; Fig. 5). Skewness values fluctuate between 1 and -2 as a function of fluctuating

Figure 2. Frequency curves of pre- and post-digested samples illustrating the removal of bimodality for Little Chenier Ridge, Chenier Perdue, Grand Chenier Complex, Holly Beach, Rutherford Beach, and Rockefeller Refuge Beach.

Figure 3. Modern beach grain size data showing percent carbonate, as well as pre- and post-digestion median grain size (mm), sorting (mm), and skewness, against distance alongshore.

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carbonate content. Post-digested grain size varies little along the entire chenier (r² = 0.02). Although a coarsening trend from 3.6 to 2.3 phi (0.08 to 0.20 mm) is evident just west of the Mermentau River, the abrupt change in median grain size west of Creole Canal may be due to an inappropriate sampling site where Creole Ridge morphology is not distinct. Post-digestion sediment samples become better sorted from east to west (1.8 to 0.5 phi [0.3 to 0.7 mm]). Skewness is mainly positive between 0 and 1 indicating an excess of fines in the distribution tails.

Little Chenier Ridge

Mean carbonate content for pre-digestion samples is 24.1%, a value higher than other chenier ridges, but lower than the modern beach. Sediment on Little Chenier is fine sand with a significant component of coarsely fragmented shells throughout its length. Carbonate content increases from the Mermentau River to a point just west of Boudoin Cemetery, but decreases from this point to the western terminus of the chenier (Fig. 6). Median grain size illustrates the same general trend; sediment size increases west of the Mermentau River (0.62 to -0.28 phi [0.64 to 1.2 mm]). This trend reverses at approximately 15 km, where median sediment size decreases from -0.71 to 1.15 phi (1.64 to 0.45 mm) towards the western end of the chenier. Therefore, median grain size variations are not a function of distance along Little Chenier (r² = 0.01). Pre-digested sediments are poorly sorted (1.76 phi [0.3 mm]) and positively skewed (range from 0 to 1).

After carbonate digestion median grain size shows very little variation from east to west ( = 2.7 phi [0.15 mm]), but the same coarsening trend away from the Mermentau River observed in pre-digested sediments of Little Chenier Ridge, post-digested sediments of Chenier Perdue Ridge, and the modern beach is evident. West of the Mermentau River, sediment size increases from 3.6 to 1.9 phi (0.08 to 0.28 mm) up to the 18-km point where median grain size then decreases to about 2.7 phi (0.15 mm) and remains at approximately this size to the western terminus of the chenier (Fig. 6). Post-digestion grain size is well sorted for the entire chenier (0.8 phi [0.6 mm]), and, as seen in all beach and chenier samples, sorting is better than that of pre-digested sediment samples. Sample skewness fluctuates about zero and becomes more positive with a slight increase in median grain size.

DISCUSSION

Given the objective of this study (examine the sources and transport paths of chenier ridge sediments), multiple working hypotheses were proposed and tested. In light of the results presented above, hypotheses are examined for the modern beach and each chenier. In this discussion, grain-size trends are assumed to be a consequence of processes active in a depositional system.

Hypothesis 1: Mississippi River Sediment

Mississippi River sediment has played a major role in southwestern Louisiana chenier plain evolution. In this scenario, either a single coarsening (Nordstrom, 1989) or fining (Pettijohn and Ridge, 1932; Self, 1977) trend of sediment size in a westerly direction is

Figure 4. Grand Chenier Complex grain size data showing percent carbonate, as well as pre- and post-digestion median grain size (mm), sorting (mm), and skewness, against distance alongshore.

Figure 5. Chenier Perdue Ridge grain size data showing percent carbonate, as well as pre- and post-digestion median grain size (mm), sorting (mm), and skewness, against distance alongshore.

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expected. To examine the validity of this hypothesis, only digested sediment samples were considered because carbonates most likely are locally derived. Bioclast sources to the chenier plain are discussed by Anderson et al. (1995). Post-digestion grain-size trends of the modern beach and cheniers in the study area reveal no single fining or coarsening trend. The modern beach exhibits numerous grain size variations associated with the outlets of the Mermentau and Calcasieu Rivers. Chenier ridges also have changes in median grain size associated with the Mermentau River. Thus, it is apparent from the non-monotonic nature of these modern and ancient beaches that rivers such as the Mermentau and the Calcasieu likely contribute non-bioclastic sediment to the modern beach and may have been active sediment sources during previous chenier construction. The lack of a single trend supports the idea that the Mississippi River is not the sole contributor of sediment to the cheniers.

Hypothesis 2: Local Sediment

Alternatively, local sources of sediment may be the sole contributors to chenier ridge evolution. The term local source is used in reference to bioclasts and sands from excavated marsh/mudflat deposits, updrift zones of erosion within the chenier plain and modern beach (see Byrnes et al., 1995), and sand from local river basins. Numerous grain-size trends associated with riverine input were expected, as were large, localized carbonate concentrations. Examination of pre-digested samples shows highly variable carbonate concentration within the beach and cheniers. Carbonate content is 46% at the beach, and 24.1% in Little Chenier Ridge indicating a large supply of bioclasts. Carbonate content in the Grand Chenier Complex (5.9%) and Chenier Perdue Ridge (9.3%) is relatively low, suggesting that large amounts of sand may have been available during the construction of these ridges. Anderson et al. (1995) confirm the distinct nature of the Grand Chenier Complex. Carbonates (i.e., Mollusks) are a local contribution to cheniers, thriving in the muddy shoreface fronting the beach. Shells/shell fragments and sand may also be provided by erosion and truncation of previously deposited beaches, which may explain the sporadic nature of shell concentrations and post-digestion grain-size trends that are not obviously associated with riverine input. Anderson et al. (1995) also present evidence from chenier macrofossil assemblages that suggests diverse bioclast sources for a scarp at Hackberry Beach, Front Ridge, Chenier Perdue Ridge, and Little Chenier Ridge. Brackish-water estuarine assemblages are common on Chenier Perdue and Little Chenier suggesting transgressive deposition. Front Ridge, with its low carbonate content and upper shoreface macrofossil assemblages, appears to be the regressive downdrift extent of the Grand Chenier Complex. Thus, source variations (bioclasts and non-bioclast) have been common throughout the evolution of cheniers, supporting the idea that locally supplied bioclasts and sand have been important in chenier plain evolution.

Hypothesis 3: Mississippi River and Locally Derived Sediment

A combination of Mississippi River and local sediments contribute to chenier formation. In this scenario, the Mississippi River is assumed to have contributed the majority of non-bioclastic sediment to the system. Thus, a general fining or coarsening trend away from ancient Mississippi River depocenters may be modified by local small-scale fluctuations caused by input of sand from the local rivers and updrift erosion. Curray (1960) and Van Andel and Poole (1960) state that the Sabine and Calcasieu Rivers are minor sources of sediment to the Gulf of Mexico. Hsu (1960) states local coastal plain rivers contribute about 20% of source material to Holly Beach, and 35% of the sand is derived from the modern and ancestral Mississippi River. In addition, Hsu states that onshore transport has supplied 45% of the material to Holly Beach. Results of this study reveal that no underlying fining or coarsening trend is readily discernable from grain-size statistics, but the presence of the Mississippi River suite of minerals on the chenier plain and modern beach (Hsu, 1960; Van Andel and Poole, 1960; Berger, 1981) indicates this as an important sediment source. Additionally, local supply of sand from rivers and contribution of shell and shell fragments excavated from eroding marsh deposits (Byrnes et al., 1995), appears sufficient to mask a single grain-size trend that would result from a single, distant easterly source. Furthermore, texture of chenier and modern beach sediments in this study (2.7 to 2.1 phi [0.15 to 0.23 mm]) for post-digestion samples does not isolate a single source for chenier plain sediments. Median grain size of potential sediment sources ranges between 4.2 phi (0.055 mm; Mermentau River point bars), 2.7 phi (0.157 mm; Mississippi River point bars), and 1.6 phi (0.325 mm; Calcasieu River point bars).

CONCLUSIONS

Inclusion of carbonates (i.e., shells and shell fragments) in grain-size analysis of beach and chenier sediments facilitates the identification of longshore trends and location of updrift erosive areas providing sediment for downdrfit deposition. Bioclasts cannot be ignored because they are an integral component of the sediment transport system and provide valuable information regarding carbonate supply to the chenier plain, the diverse nature of individual chenier composition, and chenier plain evolution.

Despite coarsening and fining trends that are observed immediately west of the Mermentau River on the modern beach, Grand Chenier Complex, Chenier Perdue Ridge, and Little Chenier Ridge,

Figure 6. Little Chenier Ridge grain size data showing percent carbonate, as well as pre- and post-digestion median grain size (mm), sorting (mm), and skewness, against distance alongshore.

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no inferences can be made from the present evidence to imply that local sediments were the sole source of non-biotic sediment during chenier construction. Additionally, a single source far to the east, such as paleocourses of the Mississippi River, is not reflected by a single coarsening or fining trend. Pre- and post-digestion grain-size trends on the modern beach and cheniers, however, indicate numerous fining and coarsening trends which could be a consequence of sediment supplied by erosion of nearby shell beds, beaches, and previous chenier deposits. Further sampling at smaller intervals is required to gain a better understanding of detailed changes in modern beach and chenier sediment texture and composition.