3. Sediments of the Gulf of Mexico

On most of the shelf, the relief of the relict sediment cover is limited to shore-parallel quartz sand sediment bodies that are interpreted as being relict barriers that were abandoned during rapid sea-level rise in the Pleistocene era. Most are late Pleistocene or Holocene in age, but some might be older. There are relict Quaternary reefs along various isobaths, particularly offshore of Texas. The relict cover is dominated by lowstand depositional environments such as fluvial channels, floodplains, and deltas (Anderson and Fillon 2004).

Another important component of the shelf is the presence of these relict barrier islands. They are distributed around the entire Gulf of Mexico but with various relationships to the modern sediment blanket. On the Florida shelf, these relict barriers are small and rest primarily on limestone bedrock (Figure 3.11).

Similar sand bodies are present off the panhandle coast of Florida and have been described in McKeown et al. (2004). Further west on the shelf, much larger relict sand bodies are present on the Louisiana shelf. The largest of these is the Ship Shoal (Figure 3.12). These relict sand bodies are the sites of possible nourishment sand for the Louisiana coast.

Similar sediment bodies are also present on the Texas coast. From east to west, they are the Sabine Bank, Heald Bank, and Freeport Rocks. All of these relict sediment bodies (Figure 3.13) represent coastal accumulations of sand and shell material that were deposited during slowdowns or stillstands of the Holocene sea-level rise (Rodriguez et al. 1999).

On the west Florida shelf, the modern sediment cover displays essentially no relief on the shelf surface except for the above-described sand bodies. The surface of the shelf throughout most of its extent is the pre-Quaternary carbonate strata with sinkholes and karstic terrain—both widespread and abundant. The Yucatán shelf of Mexico is similar. The rest of the northern Gulf shelf is a mixture of relict and modern sediment surfaces. The continental shelf around the Yucatán Platform is in some ways similar to that in Florida. It is up to 240 km (150 mi) wide on the north but quite narrow on the east. The surface is scattered with karstic features and reefs (Logan et al. 1969).

The shelf in Cuba is unlike the rest of the Gulf of Mexico because it is a collision area. The Caribbean plate moved into this region pushing between the North American and South American plates. As a result, this margin is narrow with high relief and numerous structural components including faults.

Barrier islands and their contained beaches are extensive around the Gulf Coast. These barriers are young; some are only decades old, and the oldest is about 7,000 years old. Their size tends to be related to the abundance of sediment. They range from only a kilometer (0.6 mi) or so to 150 km (93 mi) in length. The relief may be up to 15 m (49 ft). Most of these barriers are wave dominated, but there are also mixed-energy barriers, most of which are on the Florida coast. The surf zone, just offshore of the beach, is commonly characterized by longshore bars and intervening troughs (Davis and FitzGerald 2003), the number of which is the result of bottom gradient and sediment availability. This zone is the most dynamic of the entire Gulf in that waves and currents are continuously present and ever changing.

Most of the Gulf Coast is surrounded by sand dunes. The dunes result primarily from onshore wind blowing over the dry beach. These dunes range widely in size depending on the availability of sediment. The largest tends to be on the panhandle of Florida, the Matagorda Peninsula, and Padre Island, Texas. In parts of the Gulf Coast, dunes are completely absent, primarily in the southwestern and Big Bend parts of Florida.

Breaks in the numerous barrier islands around the Gulf are tidal inlets where considerable tidal flux is transported during each tidal cycle. The volume of water transported through these inlets ranges among four orders of magnitude depending on the size of the estuaries (Davis 1988). The depth of the inlets ranges from only 1 m (3.3 ft) to more than 30 m (98 ft). Inlets tend to have large sediment accumulations at both the Gulf side (ebb-tidal deltas) and the landward side (flood-tidal deltas). The size of the inlet tends to be directly related to the tidal prism (water budget) that passes through the inlet during an individual tidal cycle. Inlets of the Gulf Coast range from tide-dominated through mixed-energy to wave-dominated. Small unstable inlets have closed over historical time, and hurricanes have generated new ones. In general, little sediment is passing from estuaries into the Gulf.

Coastal wetlands are widespread along the many coastal bays on the Gulf of Mexico. In the low latitudes—generally south of about 30°—wetlands are dominated by mangroves: red (Rhizophora mangle), black (Avicennia germinans), and/or white (Laguncularia racemosa). Some marsh grass can also be present. North of that latitude, the wetlands are dominated by salt marsh with cordgrass (Spartina) and rushes (Juncus) being the dominant vegetation. These wetlands extend over only a few tenths of a meter of elevation due to the small tidal range throughout the Gulf. The combination of dammed rivers, hurricanes, and sea-level rise has caused a tremendous reduction in the area of wetlands on the Gulf Coast, especially along the northern coast.

The Gulf of Mexico is surrounded by many estuaries; however, they are small and scarce on the Yucatán coast. These estuaries are the result of flooding of drainage systems that were incised during lowstands of sea level during the Quaternary Period when there were multiple cycles of sea-level rising and falling in response to the advance and retreat of glacial ice sheets. Estuaries may have a single river or multiple rivers emptying into them. These coastal water bodies are generally brackish and shallow, typically less than 5 m (16.4 ft) deep. Sediments in these coastal bays are dominated by mud.

The term lagoon is used to separate those coastal bays that have essentially no freshwater input or tidal flux from those that do (estuaries). Along the Gulf Coast, there are a few lagoons, most prominent of which are Baffin Bay and Laguna Madre of Texas. Smaller examples are also present on the coast of Mexico (Laguna Madre, Alvarado, Celestún) (Carranza-Edwards 2011).

Baffin Bay is a drowned fluvial system that was active in an earlier time when the climate in this area was much wetter. Laguna Madre is a long, shore-parallel coastal bay that reaches salinities near 100 parts per thousand (ppt) in some isolated areas. Both are quite shallow.

Many invertebrates have various types of skeletons of calcite, high-magnesium calcite, and aragonite. These compounds are all various types of calcium carbonate with some variation in crystallography and composition. They range from single-celled organisms to large invertebrates, including coral colonies and calcified green and red algae. In some of these organisms, such as gastropods (snails), the entire intact skeleton is included in the sediment. In others, such as echinoderms (starfish, sea urchins), the skeleton disarticulates and may become dozens of individual pieces. Regardless of size, the skeletal material can become a significant part of the sediment. In some places, such as the Florida Keys or the Yucatán Peninsula, the entire composition of the sediment may be skeletal carbonate (Figure 3.16). These carbonate exoskeletons are typically broken by waves, currents, and even by other organisms. Their abundance in sediment ranges from 0 to 100 %. The particle size and the shape also range widely. As a consequence, the rate of transport of skeletal particles is hard to measure.

Calcium carbonate sediment is mostly found in shallow water, but there is also deepwater carbonate sediment (Figure 3.16). 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.

Sediment particles larger than 2 millimeters (mm) (0.08 inches [in.]) are called gravel. They can range up to very large particles including boulders (greater than 25.6 centimeters [cm] [10 in.] in diameter). 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 2.0 mm and 0.0625 (1/16) mm (0.08 and 0.0025 in.) regardless of origin or composition are called sand. 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 0.0625 (1/16) mm (0.0025 in.) and clay (4 μm [0.00016 in.]). 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.

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.

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. 2003a). 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. The highest content of organic carbon is 5–6 % in the muddy sand and mud facies of the inner shelf (Brooks et al. 2003b).

Farther out on the shelf, it is possible to delineate bands of surface sediment facies that parallel the bathymetric contours (Reading 1978; Hine et al. 2003a). The inner shelf is dominated by quartz sand as described above, and the middle shelf is carbonate skeletal material (Figure 3.20). There is a belt of calcareous coralline (red) algae on the outer shelf. Just beyond that is a narrow belt of ooids (Figure 3.21), which must be relict deposits that formed during the recent lowstand of sea level. At the present time, this shelf is sediment starved because the estuaries remain void of sediment and virtually no sediment is being delivered to the shelf itself. The total modern sand sheet is about 8 m (26 ft) thick and composed of 90 % quartz and 10 % carbonate.

There are grain size trends that relate to the composition of the shelf sediment. The content of sand is 90 % or more most of the way across the shelf. Near the outer region, sand content decreases rather rapidly. The percent carbonate in the sediments gradually increases from the transition from quartz in the shoreface (where it is only 25 %) to near the edge (where it increases to nearly 100 %) (Doyle and Sparks 1980). 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 1962).

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 2011). 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. 2004). 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 1993). 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. 2004). Combined with the adjacent Alabama/Mississippi shelf, there is a trend in sediment texture from coarser- to finer-grained and from less sorted to more sorted. 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 90–120 m (295–395 ft). These features probably reflect shoreline regions that existed during the last major sea-level lowstand (Bart and Anderson 2004). Overall carbonate content of this shelf area is typically less than 25 %.

The Alabama–Mississippi continental shelf is a continuation of that off the Florida panhandle, and the sediments reflect that trend. The sediment is quartz-dominated, fine sand (Kopaska-Merkel and Rindsberg 2005). Sand-size sediment comprises more than 90 % of surface sediments (Bowles 1997). Looking more specifically at the shelf environments, the outer areas have a lime mud surface with relict reefs/carbonate buildups at two depth zones: 654–680 m (2,145–2,230 ft) and 97–110 m (318–361 ft) (Roberts and Aharon 1994). Ludwick (1964), one of the original detailed studies on this shelf, showed distinct bands of sediments that parallel the shore (Figure 3.22). Ludwick also noted the presence of reefal materials at the outer portion of the shelf. Further work in 2001–2002 by the U.S. Geological Survey (USGS) revealed extensive hardgrounds in this area and some sandstones.

Shoreward of this carbonate region is a transition of terrigenous sand and mud. Further in on the shelf, there is topography that suggests relict barriers and shorelines. A detailed study by the Geological Survey of Alabama (Kopaska-Merkel and Rindsberg 2005) found that the inner shelf comprised five lithofacies with terrigenous sand, mud, and biogenic debris being the main constituents: (1) graded shelly sand, (2) clean sand, (3) dirty sand, (4) biogenic sediment, and (5) muddy sand (Figure 3.23). The sand is medium grained (mean of 0.43 mm [0.017 in.]). Shell content of the inner shelf sediment is higher than on the present beaches of Alabama.

A systematic study of the total organic carbon across the shelf found that the values range from a trace amount to a high of 2.9 % (Kennicutt et al. 1995). Most of the samples contained less than 1 % total organic carbon by weight. 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 3.5). The shelf here ranges from only about 15 km (9.3 mi) wide off the Mississippi Delta to more than 150 km (93.2 ft) wide adjacent to the Chenier Plain in western Louisiana. As expected, the modern lobe of the delta experiences the highest rate of sediment accumulation in the Gulf, about 1 m (3.3 ft) per year. 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. 2011). 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 3.12). 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 8,000 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. 1985). 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. 1988). 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 2011). 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 2011). 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 1975). The modern surface off the Texas coast tends to be a mud blanket (Eckles et al. 2004).

The inner shelf sediments are generally organized with parallel sediment types (Figure 3.24). The surf and nearshore zone tends to be sand dominated due to the high energy in this area. Moving farther offshore, this pattern is lost and more local variations are present. For example, the easternmost part of the Texas inner shelf is quite muddy with less than 10–12 % sand (White et al. 1987). Moving toward the west, sand dominates the shelf out to a depth of about 15 m (50 ft), then muddy sand and sandy mud dominate out to the mid-shelf. General mean grain size ranges from about 3.0Φ to 7.0Φ (Φ is –log₂ in mm). There is a thick mud layer offshore in the central part of the Texas shelf (Figure 3.25).

Muddy sand and sandy mud with mean grain sizes mostly around 5.0Φ are found west of the ship channel into Houston (White et al. 1985). Down the coast, the inner shelf is quite muddy and is 6.0–7.0Φ. The sand content increases approaching and crossing the mouth of the Colorado River (White et al. 1988). 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 3.24), from 3.5 to 6.5Φ (from White et al. 1983). 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 sand fraction is 87 % quartz, 6 % feldspar, and 4 % rock fragments; accessory minerals make up the remaining 3 %. The heavy mineral content is about half black opaques (magnetite, etc.) along with tourmaline, hornblende, zircon and pyroxenes, and rutile. These heavies are similar to those described along the entire coast by Bullard (1942). The mud content increases when approaching the Rio Grande delta area (White et al. 1986).

The region in the central portion of the Texas shelf that shows what seems to be unusual bathymetry (Figure 3.10) also has an unusual sediment accumulation. This area of about 300 km³ (72 mi³) represents the second largest sediment depocenter on the Gulf of Mexico shelf (Figure 3.25) next to the Mississippi Delta (Holmes 2011). The thickness of this mud blanket is tens of meters, more than half of which was deposited during the past 3,000 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. 1985).

Sediments on the continental shelf of Mexico either reflect the nearby sources of terrigenous material or, as in the case of the Yucatán Peninsula, they are autochthonous carbonate sediments. Terrigenous sand and mud dominate the northern portion of the shelf.

Toward the south, volcanic rocks provide the source for shelf sediments and the composition reflects this. In the central Mexico coast near Veracruz, the shelf narrows in the area of volcanoes, and volcanic glass is a prominent component of the shelf sediment. Shelf sediment on the coast near Isla del Carmen falls below 50 % carbonate (Carranza-Edwards 2011). Moving into Campeche Bay, the sediments transition from terrigenous to carbonate. Sediments and bathymetry across the Campeche Bank of the Yucatán Peninsula are comparable to that of the west Florida shelf (Figure 3.26).

Florida Bay lies between the Florida Keys and the south Florida mainland. It is a triangle-shaped, shallow bay with scattered small mangrove islands. It is open to the Gulf on its west side (Figure 3.29) and receives its freshwater supply from the sheet flow that moves across the Everglades. This bay covers 1,393 km² (538 mi²) and has an average depth below 2 m (6.6 ft). Sediments in this shallow bay are calcium carbonate and a combination of lime mud and skeletal material. These soft sediments are only a few meters thick and rest on the limestone surface of the Miami Oolite and the Key Largo formations (Enos and Perkins 1977) that form the basis of the Florida Keys.

Charlotte Harbor is one of the two large estuaries on the Gulf Coast of the Florida Peninsula. It has an area of 700 km² (270 mi²). Rapid growth from the 1950s to the present has increased the population to more than one million. It is served by the Myakka, Peace, and Caloosahatchee Rivers (Figure 3.30), which are presently carrying freshwater but little sediment. The sediment in this estuary has been studied in detail by Evans et al. (1989) and Brooks (2011). Modern (Holocene) sediments may be up to 3 m (10 ft) thick and are mostly fine quartz sand. Sandy shell is the dominant sediment in the tidal channels and passes. Phosphate minerals are up to 9 % by weight (Folger 1972) and are delivered by the Pease River from the central Florida phosphate-mining district. Mud is a minor constituent but is widespread. Mud is a combination of clay minerals, clay size quartz and calcite, and particulate organic matter (Huang and Goodell 1967).

Tampa Bay is an estuary surrounded by intensive and extensive development with a total of about three million residents. It is also a major tourist destination. The bay is supplied with runoff by the Manatee, Alafaya, and Hillsborough Rivers (Figure 3.31), but little sediment is being discharged by any of them (Brooks 2011). The USGS (2006) study of the bay includes detailed surface sediment analyses. Maps produced from this study show variation in the bay, but overall, the sediment is rather similar throughout. Grain size in most of the bay is fine and medium sand with muddy sediments concentrated in the northern portion of the two arms of the estuary. Mud is less than 10 % throughout the bay except for the upper part of the eastern bay where it reaches more than 50 % at some locations. The mud is a combination of clay minerals, fine quartz, and particulate organic matter. Carbonate composition provided by biogenic shells and debris is low except in the lower bay where it may exceed 50 % (USGS 2006).

The sediment of Apalachicola Bay is dominated by mud with several areas of large and productive oyster beds (Figure 3.32). Much of the mud is actually deposited as oyster fecal pellets. The oyster reefs range in length from about 1 to 1.7 km (0.62–1.06 mi) and are oriented northwest–southeast. Quartz sand is a minor but widespread constituent.

The Pensacola, East, and Escambia Bays have an unusual shape and are dominated by muddy sand (Figure 3.33). Like most coastal plain estuaries, sediments in this system are derived primarily from the rivers that empty into the estuaries. Both mud and sand are spotty in their distribution. Some sand is blown and washed over the adjacent barriers, but washover is infrequent because dunes along this part of the northern Gulf Coast are large. Washover occurs only where dunes are cut and washover channels develop.

Large amounts of sediment—nearly five million metric tons—are carried into Mobile Bay every year. About 33 % of it remains in the delta at the mouth of the Mobile River, 50 % of it settles in Mobile Bay, and the remaining 15 % makes its way into the Gulf. The sediments of Mobile Bay (Figure 3.34) are rather uniform. Mud virtually dominates the estuary except for a thin margin where some sand is present. Clay minerals are the primary grain type with montmorillonite being dominant. There is also a decrease in montmorillonite abundance from the head of the bay down toward the open Gulf. The other clays present are kaolinite and illite (Isphording 1985). The total organic carbon content is high throughout; some areas have concentrations well above 2 % by weight.

Texas has five primary sources of sediments:

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.

The sediment distribution maps of the Texas estuaries are all taken from the sequence of publications of the University of Texas Bureau of Economic Geology by William A. White et al. (1983, 1985, 1986, 1987, 1988, 1989a, b). Sediment abundance is shown by the same symbols throughout (Figure 3.35).

Galveston Bay (Figure 3.36) is shallow throughout, averaging only about 2 m (6.6 ft) deep, but it has multiple deep shipping channels. Sediment sources are mainly from the barriers, especially the Bolivar Peninsula and the Gulf. Little sediment is provided by the Trinity River (Phillips 2005).

The topography of the bay is like a shallow bowl with relief caused by oyster reefs. This bay produces 80 % of the oyster meat in Texas. 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 3.36). 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. Human influence on sediment distribution is in the form of spoil mounds from dredging of the Houston Ship Channel (USEPA 1980).

The percent sand shows a general trend from high (60–100 %) at the shoreline, decreasing to about 20 % near the bay center (White et al. 1985). In general, the sand abundance, and therefore the grain size, decreases in a similar trend. Overall sand abundance is related to energy levels of both tidal currents and waves.

Matagorda Bay is a rather narrow bay that separates the mainland from Matagorda Island. The Colorado River Delta has prograded across the bay over the past century, and the river now empties directly into the bay. This bay has a relatively complicated geography and includes both a typical estuarine morphology and a backbarrier portion that is bisected by the Colorado River delta (Figure 3.37). The Colorado River delta has prograded across the bay with the major channel emptying directly into the Gulf (Kanes 1970). This progradation has been extremely rapid and took place over a few decades during the twentieth century.

Unlike most of the Texas estuaries, Matagorda Bay does not have significant oyster reef development; oyster reefs are restricted to a few just west of the river delta that are oriented shore-normal and a few in East Matagorda Bay that are shore-parallel. The surface sediment pattern is similar to that of most bays of this coast. Mud dominates the area with increasing sand toward the shorelines. The relative abundance of sand reflects the location relative to wind direction and waves. The more protected areas show mud closer to the shoreline. The landward side of the Matagorda barrier has extensive sand due to washover and blowover.

There is little tidal flux in this system, and as a result, no sandy sediment accumulations are related to tidal flux except in the southwest corner of the bay. Both the constructed ship channel and Pass Cavallo are located here. The ship channel has small spoil banks, which are relatively high in sand, and the tidal inlet has a large flood-tidal delta that is sand dominated (Figure 3.37).

San Antonio Bay is located at the mouth of the combined San Antonio and Guadalupe Rivers where a large bayhead delta—the Guadalupe Delta—has formed (Figure 3.38). The Guadalupe River delta dominates the northwest part of San Antonio Bay and has shown significant progradation over the past century, but has somewhat stabilized during the past few decades. The delta sediments are more than 60 % plant remains (Donaldson et al. 1970); the bay sediments, as a whole, are only about 4 % plant remains.

This bay is the second most productive oyster region on the Texas coast. The reefs are scattered throughout the bay. This bay has no locations where tidal flux is a factor in sediment texture or accumulation. The bay is less than 2 m (6.6 ft) deep on average and is brackish. There are some low bluffs along the shoreline that produce sand when eroded.

An important aspect of San Antonio Bay is related to how the widespread oyster reefs influence sediment texture. The majority of the bay is regularly dredged by oystermen with their trawlers. This disturbance of bottom sediment tends to produce slightly coarser grain size because the dredging suspends fine sediment allowing it to travel to other parts of the bay (White et al. 1989a).

The entire bay is dominated by mud composed primarily of clay minerals. Morton (1972) studied 80 samples from throughout the bay for their clay mineralogy and found that smectite, illite, and kaolinite were the dominant species with smectite being the overwhelming majority.

Copano and Aransas Bays are similar to San Antonio Bay in their sediment composition and distribution. These bays are also important to the oyster industry in that reefs are extensive in both. They have the typical pattern of sediment texture distribution with fines in the middle and becoming coarser near the shoreline. Copano Bay has some low bluffs that are exposed to the prevailing wind and therefore are subject to some erosion. Extensive oyster dredging also influences the sediment texture here. Another factor is the petroleum industry’s considerable drilling and related activity that has taken place in both bays. Drilling activity, construction, maintenance, post drilling activity, and the presence of the permanent structures in the bay all have an influence on the bays’ surface sediments.

St. Charles Bay, which is a branch of Aransas Bay, is sand dominated as compared to other bays along this coast (Figure 3.39). Sand also dominates the northeast part of Aransas Bay adjacent to St. Charles Bay. The corresponding end of Copano Bay also shows a high percentage of sand. Both of these locations are on the downwind portion of the respective bays relative to the strong prevailing wind along this coast.

Both of these bays are major oyster areas. These animals are tremendous filter feeders and pass many liters of turbid water through their system every hour. As a consequence, the fine silt and clay-sized particles are aggregated into fecal pellets that are typically sand sized. Oyster shells are a common constituent and contribute most of the gravel fraction to the sediment. The sediment is mostly mud and is patchy because of the abundance of oyster reefs. There is some sand around the bay margins.

The Nueces River is the primary source of freshwater and sediment to both the Corpus Christi Bay and Nueces Bay. The present sediment contribution of this river is minimal because of impoundments along its course. In the past, however, considerable terrigenous sediment was carried to these two bays by this river. Although the pattern of surface sediment on Corpus Christi Bay is typical of this coast with a huge portion being mud dominated and a trend to more sand to the shoreline, Nueces Bay is different (Figure 3.40).

Mud comprises a fairly small portion of the surface sediment in Nueces Bay. Much of the bay is covered by sandy mud. The delta area is also high sand. Some of this can be attributed to the extensive dredging for oyster shell that has taken place in this bay (White et al. 1983). Oyster shell and oyster debris are fairly common in Nueces Bay but not in Corpus Christi Bay.

The constricted area between the two bays is also the site of the causeway across them. The combined effects of the constriction and the causeway structure have caused a relative coarsening of surface sediment. This has resulted from tidal flux through the constriction/structure and its location on the downwind end of the bay where the fetch is maximum (Figure 3.40).

Corpus Christi Bay is much larger than Nueces Bay and is dominated by mud in its surface sediment, mostly with a mean grain size of greater than 7.0Φ (White et al. 1983). Sand increases to the shorelines. The only variations in this pattern are associated with a large oyster reef in the northeast part of the bay, the coarsening associated with dredge spoil along the ship channel, and the area near the population center associated with Corpus Christi. An oyster reef is also present in the northeast area.

Baffin Bay is unique among the coastal bays of the United States in that it receives little freshwater and virtually no terrigenous sediment at the present time. Its geography is an obvious drowned fluvial system, which is much more evident for Baffin Bay than for other bays along this coast. Because of its location in a semi-arid portion of the western Gulf, Baffin Bay is hypersaline.

Other reefs in this bay, called worm reefs, are constructed by the polychaete worm, Sabellaria alveolata (Andrews 1964). These worms do not secrete the tubes that they live in; they construct the tubes out of sand and shell grains. The worms are abundant in Baffin Bay (Figure 3.41). Their distribution is primarily along the shoreline and near the entrance.

The nonreefal portions of Baffin Bay are dominated by mud with sand percentage increasing toward the shore. Because the streams that feed Baffin Bay are intermittent, almost no terrigenous sediment is presently being contributed to the bay. The combination of environmental conditions does permit precipitation of carbonate sediment, including dolomite (Behrens and Land 1972). Ooid sand is also produced and is concentrated along the northern margins of the bay (White et al. 1989b). Exposed Pleistocene beach rock also contributes gravel-sized particles along the margin where this material is exposed.

The grain size of Baffin Bay sediments is primarily a relict condition from earlier times when the climate was wetter and the streams that fed the bay were regularly discharging water and sediment. The bay floor is mud, dominated by silt-sized grains. This sediment grades shoreward through sandy mud and then muddy sand.

The margins of the bay have scattered beach rock reflecting the nature of the climate and very low wave energy. Other indicators of such a climate and absence of terrigenous input is the presence of ooids scattered along relatively high-energy parts of the bay (Rusnak 1960; Behrens 1964).

Baffin Bay is very shallow with essentially no fluvial or marine input of runoff or sediment. Sand dominates along the bay margins, and mud is less than 20 % (White et al. 1989b). This bay originated as a fluvial system, as is shown by its outline. It is hypersaline, and as a consequence, sabellaria reefs have replaced the oyster reefs of other Texas estuaries. In addition, there are local populations of carbonate sediment and evaporate minerals.

Most of the surface sediments in this lagoon are sand, which reflects the frequent and widespread washover/blowover from adjacent Padre Island (Figure 3.42). The land cut area is essentially pure sand and is the product of these processes. Grain size in Laguna Madre ranges from medium to fine sand with low amounts of mud. Laguna Madre also has muddy central areas with sand-dominant margins (White et al. 1986, 1989b). In places carbonate sediments, primarily ooids, are formed (Land et al. 1979). Locally there may be evaporate minerals that result from the high salinity. These are most common in the form of gypsum sand crystals that are present on the surface and at depths of greater than 4 m (13 ft) (McBride et al. 1992). This area has been arid and semi-arid for millennia resulting in little or no vegetation on either the barrier island or the mainland. The result has been considerable sand carried to the lagoon via eolian sediment transport.

The bays of the coast of Mexico are numerous and varied. They are essentially all lagoons in that salinity tends to be either high or shizohaline. The amount of runoff ranges widely; thus, a wide range in salinity and the sediment may be either terrigenous, carbonate, or a mixture of both. Coastal lagoons in Cuba are essentially absent.

Laguna Madre in Mexico is somewhat similar to that of the United States. It is shallow, hypersaline, and has some carbonate and evaporite precipitation in a terrigenous-dominated system. The fine, well-sorted sand is largely from washover and blowover of the barrier island on the Gulfside of the lagoon.

The Alvarado lagoon system in the Veracruz area is a good example of a Mexican Gulf Coast lagoon environment. It is a complex of four water bodies fed by a like number of lagoons providing terrigenous sediments with a small number of carbonates produced mostly by washover from storms. These lagoons are quite polluted. Like many of the coastal lagoons of Mexico, a substantial amount of pollution is the result of extensive mosquito spraying. The sediments have a fairly bimodal grain size with most of the open water locations being in the mud range with mean grain sizes in the 4.0–7.0Φ range and the coarse sediments in the −1.0–2.0Φ range. The coarse sediments are in places where currents are fast due to constrictions at inlets or channels between adjacent inlets (Rosales-Hoz et al. 1985).

The Celestun lagoon is an elongate water body separated from Campeche Bay by a long carbonate barrier. The sediment in the lagoon is nearly equally distributed between sand, silt, and clay-sized particles (Gonneea et al. 2004; Pech et al. 2007). This means that mud dominates.

Nichupte Lagoon is essentially at the boundary of the Gulf of Mexico and the Caribbean Sea. It is in the vicinity of Cancun on the Yucatán Peninsula. This coastal water body is probably the most polluted because of the high level of development in the vicinity. The sediments are all carbonate and are dominated by sand (65–75 %); mud is the remainder. Organic carbon is relatively high with values that range from 1.6 to 5.6 % with a mean of about 3.5 % by weight (Valdez-Lozano et al. 2006).

The coast of Cuba that is in the Gulf of Mexico is limited to the northwest portion of the country, essentially from the western tip of the country to the eastern tip of Varadero Beach, west of the small city of Matanzas. This coast includes five coastal bays about which there is virtually no information on their sediments. The following comments are based on the geomorphology of the area and their demographics. The five bays from west to east are Bahia Honda, Bahia de Cabanas, Bahia del Mariel, Havana, and Bahia de Matanzas.

Bahia Honda has no significant fluvial discharge and therefore not much sediment is being delivered. It has a bedrock shoreline and a modest amount of relief on its shoreline. Because the community on its border is industrial and there was a U.S. military base, it is assumed that the sediments are somewhat polluted. Bahia de Cabanas is quite similar to Bahia Honda in its general setting and geomorphology but is apparently more pristine in character. It has been designated as an excellent site for mariculture development (Texas A&M University—Corpus Christi, Harte Research Institute). Bahia del Mariel has an industrial port, and although it is smaller than the two previously addressed bays, it is similar in other respects and probably has somewhat polluted sediments. The most developed coastal bay is the harbor in Havana, which is very polluted. Bahia de Matanzas is a funnel-shaped bay that is served by three rivers. The combination of the rivers and the potential of tidal sediment delivery in a funnel-shaped estuary have probably led to a fair amount of sediment delivery.