However, mud deposition is not restricted to low-energy environments. Where there is an abundance of fine sediments, muddy deposits can persist even on coasts exposed to stronger wave action. Off the coast of Surinam (South America), muds are deposited in a wave energy environment normally associated with sand deposits (moderate wave power. The mud banks of Surinam resemble linear sand ridges on the shelf of the eastern United States in shape, oblique orientation to the coastline, and orientation with respect to dominant direction of transport processes. They are attached to the shoreline and commonly produce progradation of the coastline. Rine and Ginsburg, 1985 The Surinam coast is just one example in a growing list of relatively high energy, inter-deltaic mud coasts that include the chenier plains of Louisiana, the Yellow Sea between China and Korea Alexander et al., 1991 and the Kerala coast of southwest India where the fine-grained sediments are in such abundance that mud deposition follows.
Tidal flats have three basic environments
The intertidal zone lies between normal low and high tides and is exposed once or twice daily. Coupling the modes of transport with duration of periods of submergence favors deposition of muds in the high tidal flats, interbedded mixed lithologies of mud and sand in the mid-flat zone, and sand in the low tidal flat zone. Beall, Jr., 1968 Suspension transport is dominant in the high intertidal flat zones and the sediments are fine grained silts and clays, which have bioturbation, mudcracks and silty current ripples. The middle part of the intertidal flat is covered for about half of the tidal cycle and the nearly equal periods of suspension and bedload transport generate thin, parallel layered beds of alternating sand and mud with a tendency to coarsen in a seaward direction. Scour and channel sand deposition can cut across the other features of the intertidal flat. Bedload transport and deposition dominate in the low tidal flat zone. Intertidal sand bodies in the lower tidal flat which are exposed at low tide are linear shoals or bars deposited by tidal currents.
The supratidal zone is above high tide and sediment deposits are exposed to subaerial conditions most of the time with flooding only during spring or storm tides. This zone is divided into vegetated and non-vegetated intertidal mud flats and sand bodies. Storm-driven supratidal sediment-charged water creates layers of sediment in a few hours. These storm layers are sandwiched between layers of organic carbon-rich algae, which proliferate between storms. The non-vegetated part of the supralittoral may have algal mat laminar structures (discussed in Chapter 3). The blue-green filamentous algae trap and bind fine-grained sediments in a supratidal algal mat . The layered sequence in the mats results from a sequence of trapping of an influx of sediment, growth of the algae above the surface, and another sequence of sediment trapping. Tidal exposure allows development of distinctive mud cracks and curled chips of sediment during the drying out process in this zone. Intraclasts of these chips are eroded and redeposited during storms.
The supralittoral flat may have a growth of marsh grass or mangrove . The growth of vegetation is the final stage in filling depressions, embayments and other irregularities along coasts and in the leveling of marine delta plains. Filling or preservation of the marsh environment is a result of interaction between the relative sea level rise and rate of sedimentation. Allen, 1990
Spartina cord grass is the main component of a marsh. A high marsh and a low marsh classification can be based on plant successions, sediment accumulation and marsh expansion. Low marshes are younger, lower topographically and more subject to adjacent estuarine or marine conditions. High marshes are older, occupy a higher topographic condition, and are more influenced by terrestrial conditions. Most of the Atlantic-Gulf coast is comprised of lagoons, estuaries and deltas suitable for marsh development; but only 10-20 percent of the Pacific coast develops marshes.
Tides are the dominant physical process in both marsh and mangrove environments in the establishment, development, and maintenance of the system. They provide energy for the introduction and dispersal of sediments and for maintenance of the drainage network. Frey and Basan, 1985
Three major groups of salt marsh are recognized in North America based on the indigenous flora, climate, topography and tidal ranges:
Ecologic conditions in marshes grade from nearly marine to nearly terrestrial. The diversity of marine species is low relative to adjacent environments. Energy flow and nutrient budgets are the most important environmental factors that govern density and distribution of organisms in salt marshes. Unvegetated tidal flats or tidal stream banks occupy the zone between mean low water spring and the low marsh fringe.
The marsh substrates contain finer and more poorly sorted sediments than other tidal environment. The dominant grain size is silt to clay. In some marshes, fecal pellets are a significant part of the sediment. Biogenic skeletal material is present as carbonate shells and shell fragments which are a potentially important constituent of marsh sediments. The total biogenic production is usually low in a marsh, but local environments may have an abundance shell debris in both marsh and mangrove environments. Ashley and Zeff, 1988 Sediment trapping by grasses and flocculation of sediments by physio-chemical and biogenic processes are important in the retention of sediments within the marsh system. Friedman et al., 1992
Mangrove communities are the shoreline vegetation on low alluvial coasts of the tropics and subtropics. Mangroves grow from the subtidal zone up to the supratidal and into freshwater areas above the normal high tide. Although five species of mangrove are found in most environments, the two dominant are the red mangrove (Rhizophora mangle) and the black mangrove (Avicennia nitida). Red mangroves colonize the subtidal to lower supratidal environment. The black mangroves are found landward and above the band of red mangrove.
Mangroves play a major role in originating, maintaining and enlarging mud islands and in extending the coastline. The root system reduces water velocities and traps the sediment load. Sponges on the prop roots filter much of the suspended sediment from flood tide water. Mangrove swamps are a passive sedimentary environment offering some resistance to erosion but they are self-maintaining only if the area is protected from strong physical agitation and excessive influx of detrital sediments. Scholl, 1964
Because mangroves are tropical, the substrate is usually carbonate and the sediments associated with mangroves are carbonate muds and biogenic sands with finely divided woody plant debris. The heavy contribution of leaf material adds peat and organic debris to the sediments and layers of leaves more than 2 meters thick may be found. Letsch and Frey, 1980 Fossiliferous brownish black to reddish fibrous peat with fine-grained calcareous shell debris and quartz grains usually overlies bedrock in a transgressive mangrove sequence. Most of the shells are marine and brackish water molluscs, but fresh water shells may be brought into the marine mangrove areas from freshwater swamps.