Why is laguna madre filling with sediment

This same trend is true along the Texas coast and its estuaries. Black drum, Pogonias cromis, are found throughout the upper Laguna Madre, with greatest abundance in Baffin Bay. Black drum are long-lived fish that spend most of their time in unvegetated bay bottoms. These unvegetated areas are where black drum feed by using the barbels on their chin to locate shallow infauna organisms that live in the sediment such as bivalves like clams.

The young black drum will primarily feed on small softer bodied invertebrates such as crabs and shrimp before their pharyngeal teeth develop. Black drum spawn in nearshore waters but have adapted to reproduce in many different areas. Black drum have been found spawning in most areas of the upper Laguna Madre, including creeks and very shallow waters. Spawning season is from January through mid-April. Black drum are popular as a commercially sold food source as well as for recreational fisherman.

Regulations state that black drum caught recreationally need to be between inches with no more than 5 caught per person per day. This can include one black drum over 52 inches as part of your daily bag limit. Red drum, Sciaenops ocellatus, have many common names, including reds, redfish, and bull reds for the larger fish. Red drum are top predators and can live to be over 37 years old.

Most natural mortality results from predation of smaller sized fish, red tide bloom events, parasitism, disease, and old age. Some threats are manmade such as overfishing and destruction of nursery habitat which includes shallow open water and seagrass beds. By age three most adults move offshore but come to Gulf passes to spawn from summer to early winter. Juvenile red drum feed on copepods, mysid shrimp, and amphipods, while larger adult drum feed on crustaceans and fish. They are a very popular recreational fish and have a must be between inches with no more than 3 caught per person per day.

Spotted Seatrout, Cynoscion nebulosus , are another highly sought-after recreational species. They spend their entire life cycles in estuaries and are completely dependent on the estuarine habitat. Trout have the unique ability to spawn in hypersaline conditions, such as those found in the upper Laguna Madre. However, even with this adaptation, the extreme salinities that can be reached in the upper Laguna Madre can lead to low recruitment in young trout.

Spotted Seatrout are considered top level predators, feeding on swimming prey in surface waters as well as mid- depths. Adult spotted seatrout are often associated with seagrass, as their prey can usually found in seagrass beds. Regulations state that spotted seatrout caught recreationally must be between inches with no more than 5 caught per person per day. This can include one spotted seatrout over 25 inches as part of the daily bag limit.

Anglers, fishing north of the Highway Bridge near Sargent, Texas may keep up to 10 fish per person per day. Many birds use the Central Flyway to migrate south during winter months and stop off at local Texas ponds, bays and estuaries for resting and feeding. In fact, Texas supports more waterfowl than any other state in the Central Flyway. More than two dozen waterfowl species fly to Texas for wintering habitats including white-fronted geese from Alaska, Canada geese from the Arctic, lesser snow geese from the Hudson Bay, mallards, redhead ducks, Northern Pintails, gadwalls, American widgeons, lesser and greater scaups, and canvasbacks from many plains areas.

Of the winter migrating species, a few of the most common are sandpipers, banded plovers, blackbellied plovers, dowitchers, and willets. Like the blue-winged teal, some birds only rest shortly on the Texas coast before moving farther south to Central and South America.

However, some birds end their migration along the Texas coast, like the redhead duck where the journey ends at a salty bay or lagoon like the Laguna Madre. Of the 38 documented waterfowl species on the south Texas coast, U. Fish and Wildlife Service recorded a staggering 2. Redhead ducks feed on the below ground root structure of seagrass, primarily shoal grass Halodule beaudettei , which is abundant in the Laguna Madre.

They seek out shallow water seagrass beds for feeding and resting and in close proximity to freshwater ponds for access to drinking water. The availability of healthy shoal grass is critical for the migrating redhead duck population. The National Audubon Society identifies other birds, such as the black skimmer, brown pelican, least tern, piping and snowy plover, reddish egret, and ruddy turnstone as priority birds migrating through the Central Flyway.

These types of birds are considered shorebirds and are common in the upper Laguna Madre ecosystem. Padre Island National Seashore exists within the upper Laguna Madre ecosystem and 30 shorebird species and 20 gull and tern species have been recorded in this area.

The most common birds recorded were sanderlings, laughing gulls and royal terns but piping and snowy plovers as well as ruddy turnstones can often be seen picking through debris washed up on the beach shoreline.

Most of the shorebirds recorded are found in areas of the upper Laguna Madre system during winter or migratory periods. During January and February reddish egrets, a wading bird, are two times more abundant in the upper Laguna Madre than the Lower Laguna Madre.

Many birds rely on Texas bays and estuaries due to the availability of habitat near suitable foraging and breeding grounds. In the upper Laguna Madre, natural and artificial islands provide a necessary haven for many bird species.

Pita Island and the small spoil island surrounding Humble Channel in the upper Laguna Madre consists of a natural island with 5. South Bird Island also provides five hectares for extensive colonial bird nesting while many spoil islands exist within the Padre Island National Seashore boundary.

These natural and artificial islands have become essential for nesting and are also referred to as Rookery Islands. The loss of nesting grounds in inland areas have pushed many species, like the little blue heron, out to these islands to find suitable breeding and nesting grounds. Some of the waterbird rookeries at Bird Island in the upper Laguna Madre include tricolored herons, snowy egrets, reddish egrets and great blue herons.

In the winter it is common to see reddish egrets and snowy plovers nesting on the spoil islands but other birds such as black skimmers can be seen nesting on the islands between March and September.

Great blue herons may begin courtship and nesting as early as January to avoid competition. Different bird species exhibit different nesting behaviors. Ground nesters, such as American white and brown pelicans, black skimmers, gulls and terns make nests and lay eggs directly on the ground while shrub nesters, roseate spoonbills, herons, egrets and ibises prefer to make nests on elevated shrubs. Great blue herons tend to use tall vegetation like the sea-ox-eye daisy Borrichia frutescens near Baffin Bay but will use common cattail Typha spp.

Reddish egrets often use common ragweed Ambrosia sp. While using the islands for habitat and nesting, many birds will wade in shallow water and forage on fish and crustaceans while other birds like ducks and cormorants will forage for benthic and nektonic invertebrates by floating or diving. Gulls, terns, pelicans and osprey are aerial hunters and typically find prey in deeper water.

Artificial and natural islands are also important for other bird species. Resident and migratory waterbirds, shorebirds, songbirds and raptors use these islands for resting and feeding. The American white pelicans average population in the upper Laguna Madre is and colonizing populations on these islands are becoming more popular near areas like Bird Island. Due to these islands being used by many different species of birds during different life stages they can act as indicators of ecosystem health.

Although the islands within the upper Laguna Madre system are a major habitat area for the birds observed on the Texas coast, other areas such as King and Kenedy Ranch are vital to many waterfowl, such as ducks, and other migratory birds. These areas provide necessary breeding grounds and if destroyed would negatively impact many species.

Maintaining a healthy ecosystem is vital to all of the birds that call the upper Laguna Madre their home either permanently or temporarily. The consequences of anthropogenic effects can be illustrated by the drastic decline of Brown Pelican populations in Texas in the early s as a result of DDT Dichloro-diphenyl-trichloroethane , a pesticide once commonly used in coastal areas for mosquito control and agriculture.

In the early s there were less than brown pelicans with only 10 breeding pairs in Texas. After DDT was banned from use in the early s the brown pelican population has seen a steady recovery. In it was estimated that Texas had over 2, breeding pairs of brown pelicans. Wind action piled the new beach sands into coastal dunes on the barrier.

Where the earlier deposits were relatively un-consolidated, as offshore northern Padre Island, thick dune deposits could form because of large supply.

Where the earlier deposits were more consolidated, and consequently less easily eroded, as in the vicinity of the Rio Grande delta, the dunes are lower and the barrier narrower. Subsequent to the formation of the lagoon about 5, years ago, partial filling was accomplished by washover storm waves, wind transport, and tidal activity. Shibboleth Sign In. OpenAthens Sign In.

Institutional Sign In. All but one of the papers contained in this volume represent a symposium summarizing the results of work carried on in Project 51 of the American Petroleum Institute. This study of modern sediments along the northwest margin of the Gulf of Mexico contains 14 papers plus a consolidated bibliography. Paper titles are: Geologic framework of Gulf coastal province of United States; Sources and dispersion of Holocene sediments; Mississippi delta; Delta building and the deltaic sequence; Phytoplankton production in the Mississippi delta; Bays of central Texas coast; Sediments of Laguna Madre; Gulf Coast barriers; Sediments and history of Holocene transgression; Sedimentary patterns of microfaunas; Ecology and distributional patterns of marine macro-invertebrates; Rise of see level; Regional aspects of modern sedimentation; and Recent sedimentology.

Sign In or Create an Account. User Tools. Sign In. Advanced Search. Because of the range of clay mineral species over multiple physiographic provinces, it is best to consider their general distribution here. Smectite, which is the most common of the four, is relatively low in concentration on the shelf of Alabama and Florida. Illite is the next most common species in the Gulf; however, illite abundance shows a decrease from the shelf into the deep basin.

Kaolinite decreases from east to west and is generally low on the central and western parts of the Gulf. General map of heavy mineral group distribution in the Gulf of Mexico from Davies and Moore : reprinted with permission from The Journal of Sedimentary Research. Province I is from the Appalachians; kyanite and staurolite dominate. Province II is from the Mississippi River; augite, hornblende, and epidote dominate. The Gulf of Mexico is replete with organisms, many of which have skeletal components that contribute to the sediment.

There are three categories of skeletal components: calcium carbonate, phosphatic skeleton, and siliceous materials. By far the most common is the calcium carbonate exoskeleton of invertebrates. Phosphatic skeleton is from fish, and siliceous material is small single-celled organisms and sponge spicules. Map of the weight percent carbonate throughout the Gulf of Mexico from Caso et al.

Black dots are sample locations. Calcium carbonate sediment is mostly found in shallow water, but there is also deepwater carbonate sediment Figure 3. Calcium carbonate sediment comprises primarily planktonic foraminifera single-celled animals and submicroscopic algae called coccolithophores.

These sediments, often called calcareous ooze sediments, are common on the abyssal plain of the Gulf. Such microscopic and submicroscopic skeletal particles can form limestones in the ancient record and become major petroleum producers. Fish skeletons tend to be phosphatic except for the otoliths ear bones , which are calcium carbonate. Because fish skeletons are generally rather fragile and predators commonly consume the fish body, skeletal fragments of phosphatic composition are not common in sediments.

Otoliths do tend to be preserved in sediments, but they are scarce in the overall volume of marine sediments. Three major categories of organisms have siliceous skeletal material: sponges, radiolarians, and diatoms. Sponges are soft benthic animals, but several of them have tiny siliceous spicules that help to support their soft structure.

Radiolarians are planktonic, microscopic animals that are also siliceous. The other category is diatoms, which are photosynthetic, microscopic organisms. All of these siliceous organisms are quite small and are typically minor constituents of marine sediments in the Gulf of Mexico except for radiolarians. The direct precipitation of minerals from seawater is present in the Gulf but is not common or widespread.

Evaporite minerals are limited to places where salinities reach more than ppt. This would include local places in Laguna Madre and some sites on the Mexican coast. Gypsum and halite are the only evaporate minerals that are even somewhat common, and they are local and subject to dissolution.

Calcium carbonate is the other type of chemical sediment that is directly precipitated from seawater, in some cases with the aid of photosynthesis. Calcium carbonate can be very fine grained and is often referred to as lime mud. It is only common in Florida Bay. Ooids are sand-sized, spherical grains of calcium carbonate that are precipitated in thin layers over a nucleus. They are commonly limited to places where currents, typically tidally generated, are present. Sediment particles range widely in grain size.

This makes it possible to use a small number of categories to cover the entire range of sizes. Typically, sediments are categorized by both particle composition and size e. Sediment particles larger than 2 millimeters mm 0. They can range up to very large particles including boulders greater than In fact, not much gravel is carried into the Gulf of Mexico because by the time eroded material makes its way down a long river, the size is reduced considerably.

Some beaches have gravel composed of shells, and in some places, such as on the northwest coast of Cuba and parts of Mexico, gravel particles are eroded from rocks close to the beach and are still large. Gravel may also be produced as storms erode reefs. Gravel-sized particles in deep water are essentially all shell material. Much of the terrigenous sediment present on the continental shelf of the Gulf is sand. Although commonly misinterpreted, sand is only a size term; it has nothing to do with the composition of the sediment.

All sediment particles between 2. The confusion between the two designations is that the sediment on most beaches, in many streams, and in sand boxes is within this grain size range and is mostly quartz. In many natural environments, sand is mixed with other particles, some larger and some smaller. Silt is the grain size that is between very fine sand at 0. Particles of this grain size are a minor component of most Gulf environments except for river deltas.

Silt is mostly quartz with minor percentages of other nonlayered silicates. Clay is another confusing term used in conjunction with sediments; it can mean clay minerals, as described above, or it can mean a grain size. Most of the clay-sized particles are also clay minerals, but some are not. These very small grains are easily transported by rivers and currents in the Gulf. These sediment particles are most abundant in estuaries, deltas, and the deep basin.

Although a commonly used term in colloquial English, mud is really an appropriate term in scientific literature. Mud—the mixture of silt and clay—is widely distributed in Gulf sediments. Because both silt and clay involve very small sediment particles that are commonly not separated in analysis, this combination term, mud , is used. This term will be used in the following discussions. Contrast this map with that in Figure 3.

From these samples, the authors have been able to produce relatively simple maps of various sediment characteristics. One of the least complicated maps to interpret is a map showing carbonate content Figure 3. This map shows the influence of the Florida carbonate platform, the Mississippi Delta area, and the expected pattern of decreasing carbonate moving from the deep basin up onto the continental shelf.

Sediments on the abyssal plain tend to be rather homogenous. They are a combination of calcareous ooze formed by an accumulation of planktonic foraminifera and thick turbidite sequences. Much of this turbidite material was transported through the Mississippi Canyon and across the Mississippi Fan Bryant et al. There is also some clay mineral sediment, most of which had its origin at the mouth of the Mississippi River. Thin turbidite layers are interbedded with the calcareous ooze on the top of these salt dome knolls.

The heavy mineral suite is typical of that from the Mississippi River with hornblende and epidote being dominant Davies and Moore ; Davies The deep-sea mud that is dominated by clay minerals shows that the most abundant species of clay mineral is smectite. Illite is the next most abundant Sionneau et al.

Chlorite and kaolinite are the minor clay mineral species. Most of the sediments on the Mississippi Fan Figure 3. Because of the slope of its surface, mass wasting is a major process for the delivery of these sediments Coleman and Roberts Debris flows and turbidity currents are the primary methods for sediment delivery Twichell These sequences contain wood fragments and shells of shallow-water organisms testifying to their shallow-water origin.

The upper sediments of the fan include fine sand, silt, and clay mud. Layers of this material are covered by foraminiferal muds that are 20—50 cm 8—20 in. Sediments of this huge accumulation are varied; some are turbidite units and some are thin layers. The thin layers of only a millimeter 0. On the other hand, a thick single layer might represent as long as a century Bryant and Liu The turbidite units are the thickest of the sediment units, commonly greater than 5 cm 2 in.

This type of individual deposit can be up to cm 40 in. The graded bedding in the turbidites contains medium to fine sand but is dominated by mud. The debris flow deposits may also include clay clasts. They typically show an erosional base, graded bedding, and the C, D, and E units of a Bouma sequence. The fans and sandy shelf-edge deltas accumulated during sea-level lows during the pre-Holocene era.

The rough and irregular topography on the slope Figure 3. Sediments from the Rio Grande source contain the highest percentage of quartz, and sediments from the Veracruz area contain the lowest percentage of quartz. The Mississippi River, which produces the most sediment Figure 3.

On both the Texas and Mexico slope in the western Gulf, the sediments are dominated by bluish to brownish fossiliferous mud Morelock More specifically, the northwestern Gulf sediments are highly bioturbated hemipelagic muds, with some foraminifera interbedded with laminated silt and muds that are barren Bryant et al. General diagram of the outer continental margin showing a the main sediment provinces and b the shelf-slope break from Blake and Doyle : reprinted with permission from the Society for Sedimentary Geology.

Generalized map showing the continental slope cut by the Mississippi Valley off the Louisiana coast with adjacent continental shelf and deep basin area from Williams et al.

Shelf surface sediments of the Gulf of Mexico tend to reflect a combination of runoff from the land and the nature of the geological underpinning of the particular region.

The remainder of the shelf sediments in the Gulf is primarily the result of fluvial input, with the Mississippi River discharge dominating the northwestern Gulf. Much of the sediment on the shelf is directly or indirectly the result of the multiple cycles of sea-level change that took place during the Quaternary Period.

It is possible to designate six sediment provinces of the continental shelf of the U. Gulf of Mexico using detailed analysis of the silt fraction of bottom samples. These provinces are based on the past and present locations of rivers that contributed these sediment grains primarily during low stages of sea level during the Quaternary and they have been reworked during subsequent sea-level rise.

A study of grab samples focused on grain roundness and shape Mazullo and Peterson As expected, the sediment grains from the Mississippi River dominate the entire northern Gulf shelf but are most abundant in the west of the delta. Sediments on the continental shelf off the Florida Peninsula are scarce beyond a depth of about 6 m 20 ft.

Out to this depth they are shelly, quartz sand that has been reworked from Quaternary cycles of sea-level change. This is a zone of transition between the quartz-dominated sediment on land and the carbonate-dominated sediment of the mid- and outer shelf Brooks et al. The most common minor constituent is phosphorite that is reworked from the Miocene deposits of the Florida Platform.

The carbonate sediment is being produced biogenically within the shelf itself. General map of the surface sediment facies on the west Florida shelf from Reading Profile diagram across the west Florida shelf showing sediment facies Hine et al. There are grain size trends that relate to the composition of the shelf sediment.

Near the outer region, sand content decreases rather rapidly. As for the clay minerals, smectite content increases offshore and kaolinite decreases offshore. The heavy mineral suite associated with the quartz-rich area is characterized by zircon, tourmaline, garnet, and staurolite Fairbanks The Florida panhandle shelf is quite different than that on most of the Florida Platform because it includes a relatively thick sequence of sediment that is the result of the dominance by rivers and river deltas Hine and Locker The origin of the sediment is the Apalachicola River and delta and other streams to the west that feed the Florida panhandle and the Alabama shelf McKeown et al.

The ancestral Apalachicola Delta is still visible in the bathymetry of this shelf and is a major contributor to the sediment on the present shelf as the Holocene sea-level rise has moved over it. This sediment has been reworked into numerous shoals and ridges as sea level advanced after the glacial maximum Donoghue This area has been relatively sediment starved since the beginning of the Holocene.

The sediment is dominated by terrigenous sand and contains minor amounts of shell. The surface sediments here comprise fine and medium sand and are moderate to well sorted McBride et al. The outer edge of the shelf is dominated by carbonate sediment of a reefal origin with mixtures of terrigenous sand—silt—clay of inter-reef origin.

There are shelf-edge carbonate hardgrounds and bioherms that have up to 15 m 49 ft of relief and are located at depths of 90— m — ft. These features probably reflect shoreline regions that existed during the last major sea-level lowstand Bart and Anderson Surface sediment facies on the northeastern Gulf continental shelf modified from Ludwick Map of shelf sediments produced by the Alabama Geological Survey in the search for beach nourishment sand from Kopaska-Merkel and Rindsberg A systematic study of the total organic carbon across the shelf found that the values range from a trace amount to a high of 2.

The high values were at or near the shelf edge in the head of De Soto Canyon. This study included five samplings over 26 months. A major finding of this study is the variability of the sediment content in as little as 6 months. The Mississippi Delta has a major impact on the sediments of the continental shelf adjacent to the Louisiana coast. The active lobe of the delta extends across the entire shelf with the river discharging directly onto the continental slope Figure 3. The shelf here ranges from only about 15 km 9.

As expected, the modern lobe of the delta experiences the highest rate of sediment accumulation in the Gulf, about 1 m 3.

Slump structures and gullies are common here. A major element of this shelf is the presence of four large, elongate sand bodies Williams et al. These sand bodies are the products of the reworking of abandoned lobes of the delta as it prograded across the continental shelf. The sand shoals represent old shoreline accumulations that were reworked during the Holocene transgression. The sediment in these shoals is well-sorted terrigenous sand. These four shoals contain many millions of cubic meters of beach quality sand that has great potential for nourishment projects on the present, eroding barrier islands.

Ship Shoal, the largest of these, is 50 km 31 mi long and 7—12 km 4—7 mi wide with a relief of up to 7 m 23 ft. The mean grain size on this shelf ranges from medium to fine sand. The nearshore area in the western portion of this shelf is fine, as is the sand from Trinity and Ship Shoal Figure 3. The bottom sediments farther offshore in the west are medium sand.

The wide shelf to the west is essentially covered with a blanket of mud that has been provided by the Mississippi River. This mud is rather thin, less than 8 m 26 ft thick throughout and is underlain by fluvial and deltaic sediments from sea-level lowstands during the Quaternary. Some of the sediment sequences, as revealed by cores, show a complex of facies including carbonate debris.

The inner shelf off the Texas coast will extend to depths of about 15 m 50 ft. The sediments here have various origins but two are major contributors—the Mississippi River and reworking of older sediments as the sea level rose over the past 8, years or so. Shells and shell debris are another significant component. The influence of the river diminishes from northeast to southwest along the Gulf.

In the most eastern portion of the Texas shelf, mud is dominant or about equal with sand in the surface sediment. Local areas have linear sandy areas representing old shoreline accumulations left behind as relict sediments when the sea level rose. Moving westward along this region, mud is still very abundant with patches of sandy mud dominating.

Holocene sediment is quite thin only a few kilometers from the shoreline White et al. The relatively high concentrations of muds tend to be related to the locations of lowstand deltas where mud was dominant, such as the paleo-Trinity delta. A similar situation is associated with the Brazos River. In general, sand dominates out to maximum depths of 5—8 m 16—26 ft White et al.

This pattern of sediment distribution continues to near the middle of the Texas inner shelf. Across the inner zone of the Texas shelf, the percentage of sand increases noticeably.

This zone in the southern part of the Texas shelf includes numerous sandy ridges that are shoreline remnants from previous high stands of sea level.

Geographically, the Texas shelf is considered to be subdivided into three provinces: 1 the Colorado—Brazos delta complex, 2 the south Texas intra-deltaic ramp, and 3 the Rio Grande delta complex Holmes Sediment that reaches this shelf region may come from three drainage systems—the Mississippi River and the two fluviodeltaic complexes mentioned above. There are numerous shore-parallel structures along the Texas continental shelf.

Some are biogenic banks and reefs, and others are terrigenous sediments Holmes The modern sediment blanket is rather thin in most places and rests on the fluvial-deltaic deposits of the Quaternary lowstands of sea level. Mud dominates the shelf surface except over the ancestral Rio Grande and Sabine deltas, where sand is the most abundant grain size Berryhill The modern surface off the Texas coast tends to be a mud blanket Eckles et al.

Bands of sediment on the continental shelf of south Texas show that most sediment textures and compositions on this shelf are coast-parallel Behrens et al. Warren Flint and Nancy N. Rabalais, Copyright Courtesy of the University of Texas Press. Details of sediment characteristics are shown in Table 3. Map showing the distribution and thickness of Holocene sediment on the northwest Gulf of Mexico.

The greatest thickness is on the south Texas shelf from Holmes Muddy sand and sandy mud with mean grain sizes mostly around 5. Down the coast, the inner shelf is quite muddy and is 6.

The sand content increases approaching and crossing the mouth of the Colorado River White et al. Moving toward the Port Lavaca area, the sand percentage increases with mean grain size of very fine sand in the surf zone grading out to coarse silt. Continuing to the south, the pattern of grain size becomes relatively organized; the values are essentially parallel to the coast Figure 3. In the mid-south coast of Texas, near Kingsville, the inner shelf sediments are dominated by sand with increasing mud offshore.

All major categories of sediment texture are present on the inner shelf. The gravel is shells and shell debris. The heavy mineral content is about half black opaques magnetite, etc.

These heavies are similar to those described along the entire coast by Bullard The mud content increases when approaching the Rio Grande delta area White et al. The region in the central portion of the Texas shelf that shows what seems to be unusual bathymetry Figure 3. This area of about km 3 72 mi 3 represents the second largest sediment depocenter on the Gulf of Mexico shelf Figure 3.

The thickness of this mud blanket is tens of meters, more than half of which was deposited during the past 3, years. It is interpreted that the origin of this late Holocene sediment is the production of mud from the Brazos, Colorado, and Mississippi Rivers.

The outer shelf sediments have a high abundance of calcium carbonate. This comes from the relict reefs and banks that developed during Quaternary lowstands of sea level. They have been partially reworked by the post-glacial rise in sea level and the debris incorporated into the outer shelf sediments Rezak et al.

Terrigenous sand and mud dominate the northern portion of the shelf. Block diagram of the various sedimentary facies on the Mississippi Delta Coleman The vast majority of the sediment volume in this delta is mud and comprises the prodelta province Figure 3. These sediments accumulate rapidly and are generally saturated with water causing major instability problems.

Failure and gravity slumping is widespread. Some diapirs—not only of salt but also mud—are present. The nature and composition of beach sediment show a fair amount of commonality throughout the Gulf of Mexico. There are two main categories of beach sediment composition—terrigenous and carbonate—but some places show subequal mixtures. Sediment texture is typically well sorted and well rounded, except where the composition is bimodal with shells being a significant part of the composition. In a few places, minor constituents show some concentrations due to the underlying geology.

One is on the west-central coast of the Florida peninsula, where phosphorite is anomalously high due to the abundance of this mineral in the underlying Miocene strata. The thin dark layers on storm beach surfaces reflect the presence of this material.

The concentration of shells in some local places provides what is an anomalous carbonate beach in an otherwise terrigenous-dominated beach environment. Examples include the southern portion of Sanibel Island, Florida and near the middle of Padre Island, Texas, not far north of the Mansfield Pass jetties.

Small tar ball on the beach. These tar balls are from natural seeps and are common along the northern Gulf Coast especially in Texas. Numerous estuaries line the Gulf Coast. They are generally somewhat similar in their origin in that they are drowned river systems. Most are muddy, shallow, and brackish. Tidal flux varies widely, but the tidal range is microtidal throughout the Gulf. The following discussion will address the nature of the sediment in each of the major estuaries.

Map of sediment facies for Charlotte Harbor area. These facies can be related to sediment grain size Evans et al. Reprinted from Marine Geology, Vol 88, Evans et al. Image of Tampa Bay showing the percent of mud. The reciprocal can be considered as the sand percent because the gravel shell component is small courtesy of the USGS.

Map of Apalachicola Bay, Florida showing oyster reefs and sediment types Twichell et al. Surface sediment distribution map of Mobile Bay, Alabama, and the adjacent area modified from Ryan Active streams: Most bays have active streams that carry terrigenous sediment in a combination of bed load and suspended load emptying into them. Erosion of shorelines: Some sediments are derived from the erosion of the shorelines of the bays, and most of these sediments come from bays that have small bluffs of Quaternary sediment, such as Lavaca Bay and Copano Bay.

Tidal inlets: Tidal inlets enter and influence some of the bays; these inlets may transport marine sediment into the bay, generally accumulating in the form of a flood-tidal delta. Eolian and washover processes: Eolian and washover processes carry sand across the barrier islands and into the Gulfward margins of the bays.

Biogenic shell material: The only nonterrigenous sediment that is common in the bays; biogenic shell material is found as both whole shells and as sand and gravel-sized debris.

The bulk of this shell sediment is from oysters. There is a general pattern to the sediments in the Gulf estuaries. They are all relatively low energy environments with low to modest energy caused by tidal flux. Waves are small with short periods. As a consequence, the standard pattern is somewhat target shaped with high sand content along the margins and mud in the center. Mud dominates and commonly covers about two-thirds to three-quarters of the area of the bay. Many of the estuaries have oyster reefs that cause local variations in the coarse fraction of the sediments.

Shell debris in both gravel- and sand-sized particles is common in association with these reefs. The oysters are major factors in the sedimentation of the estuaries because of their huge capacity for filtering suspended sediment out of the water column and producing coarse silt and fine sand-sized pellets of mud. Triangular classification of surface sediments in the Texas estuaries with gravel, sand, and mud as the major categories. Figures 3. Surface sediment distribution map of the Galveston Bay complex.

Black dots represent sample locations from White et al. Black is oyster reefs. The topography of the bay is like a shallow bowl with relief caused by oyster reefs. The oyster reefs also contribute significantly to bay sediments; both shells and pellets of mud are produced by these extremely active filter feeders.

Most of the sediment in Galveston Bay is mud, sandy mud, and muddy sand. The central part of the bay is mud Figure 3. Sand is concentrated in the bay margin and associated with the flood-tidal delta near Bolivar roads and the Trinity River delta.

Oyster reefs are widespread, and shell gravels are associated with them. Some of the sand in the bay is also derived from oyster shells. In general, the sand abundance, and therefore the grain size, decreases in a similar trend.