The coastal sediments discussed in the previous chapter are associated with the coastline and are acted on by both marine coastal and land processes. Past the shelf break, the slope deepens rapidly and the continental slope has sedimentary processes related more to the deep ocean. Therefore, the slope and the sediments at its base, called the continental rise, will be discussed in Chapter 8, Deep Sea Sediments. Lying between these two areas are the continental shelf sediments bounded by a continental or island landmass with coastal environments on one side and a deep ocean basin on the other. Continental shelves total almost 23 million km2, which is equal to nearly one-sixth the total land area of the world. Emery, 1966 The types of continental margins and varied geomorphology discussed earlier in this chapter are closely related to the pattern of modern sediment deposition.
The coastal environments are a boundary condition controlling passage of sediments from the continent into the shelf environment. After the coastal environments are filled, sediments start bypassing to the shelf. Heavy rainfall and flushing of sediments out of coastal storage is an important part of supplying sediments to the continental shelf. Deposition and retention of sediments on the shelf a function of the sediment characteristics, the water energies and the shelf configuration.
Shelf sediment deposits can be separated into
The sedimentary environments that can be found on the shelf have been described in many studies, and how few or how many different environments are distinguished depends on the author's perceptions. In the following discussion, we describe a limited number of depositional facies, and local modification of these.
The patterns of wave and currents on the shelf are different from those in the coastal environments, but they affect the pattern of shelf deposition. Water motions affecting the continental shelf include: surface waves, internal waves, tidal currents, storm surge currents, Ekman and geostrophic currents. Energies capable of reworking and transporting sediments are less frequent and are related to different conditions and shelf sediment transport by wave action is generally restricted to storm conditions.
Ocean currents may cross wide shelves and exert an influence. The erosion and development of plateaus under the Gulf Stream show the effectiveness of a strong ocean current system. Tidal currents range from insignificant to major in effects. Where the current is swift and sand supply abundant, longitudinal sand ridges up to 65 km long have been formed.
Shelf sediments formed in the environment of a transgressing sea are both reworked relict and modern carbonate sediments (termed autothonous) and both relict and recent sediments that have been introduced from nearshore environments (allothonous). In many sites, autothonous sediment patterns have been succeeded by allothonous because the slowed rate of sea level rise since about 5,000 yBP has allowed filling and bypassing of many coastal sedimentary environments. The land-derived sediments may be mixed with sediments produced within the depositional environment and relict sediment sources.
The general pattern of sedimentation would seem to be coarser sediments near shore and a fining seaward. Variable transport and deposition introduced by complex current patterns, bottom bathymetry, sediment influxes and sea level changes results in a less than simple pattern.
Calcium carbonate can be a dominant sedimentary constituent in virtually any environment, at any latitude and in any depth of water. However, it is most prevalent in warm, tropical and subtropical seas where the organisms that produce carbonate sediments can thrive. Moderate to high light intensity, low levels of sedimentation and nutrients, at least modest water motion and normal marine salinity are of primary importance.
The morphologic character of a platform and the physical-oceanographic conditions around it exert the primary controls on the local benthic community and, therefore, on the distribution of sediments that are produced. Carbonate platforms can generally be divided into three zones, regardless of their type: the platform top, platform margin and slope. Because of its size, the platform top is volumetrically the area of greatest sediment production. It is essentially flat to gently inclined. The platform margin is often the site of greatest carbonate production per unit area. This is the result of the dynamic nature of oceanographic processes and the proliferation of carbonate-producing organisms along this interface between the platform top and open water. Existing platform classifications are based primarily on the character of the margin, which can support a variety of features ranging from mobile, sandy shoals to hard and cemented reefs.
The slopes beneath the edge of carbonate shelves and surrounding the margins of carbonate banks can vary from slight inclination along unconsolidated leeward margins to very steep, vertical or even overhanging where extensive submarine cementation has occurred. Along gentler slopes, sediments are usually accumulating and aggrading the margin. Along steeper margins (ca. 30-45o or greater), sediments will bypass the upper slope and be deposited in deeper water where declivity lessens.
In time, there will be an adjustment to a graded fining outward sequence of sediments, but there are presently extensive deposits of coarse-grained relict sediment on the outer shelves. A combination of trapping sediments in modern coastal environments and the short span of time that sea level has been at its present position has resulted in lack of material to cover the relict sediments on the continental shelves. On broad shelves without a major source of fluvial sediments, the burial of relict sediments has just begun and sediments on about 60% of continental shelves of the world are relict . Relict sediments are reworked by storm waves, and some are moved inward and redistributed. The rate of sediment influx from the land, generation of sediments on the shelf, the amount and nature of the relict sediments, the present energy environment of the shelf, and marine transgressions have all contributed to developing the present sediment distribution. The terrigenous sediment influx and biogenic production control the rate of coastal and shelf sediment deposition with size being an important factor in determining whether the sediments are deposited or bypass the shelf under existing energy conditions.
Strong tidal currents have produced ridges of well-sorted fine sands in the southwestern Yellow Sea, China . The ridge system formed on the abandoned Yellow River and ancient Yangtze River deltas using an abundant sand supply from former deltas. Zhenxia et al., 1989 Tidal ridge systems formed during the Holocene transgression are common in the North Sea. Most of these and are still actively shaped by strong currents in shoal water.
South of the glaciated shelf of Atlantic North America and in shelf areas of Europe, the shelf is covered by ridges of sand sub-parallel to the shore . The parallel ridges resemble submerged barrier islands , and they migrate under storm conditions.
Probably the initial development of sub-parallel sand ridge fields was related to the multiple transgressions and regressions during Pleistocene glaciations. The ridges are forming at the present time in response to storm-generated currents and barrier island processes so they have had a complex history that merges into the present as active processes continue to modify the shelf surface, Field, 1980 but the sands that go into them are largely relict deposits. They slowly migrate offshore and down coast in the prevailing direction of storm flow and the eroding shoreface retreats out from under them. As they have detached from the shoreface they continued to evolve in response to storm wave surge and water drift currents. Swift et al., 1976
A major hypothesis for the recent generation of linear, shelf-floor shoals is that the ridges form in response to the modern hydraulic regime at the foot of the shoreface and that they are isolated by the retreat of shoreline. Duane et al., 1972 The shoreface-attached sand ridges which seem to be the initial development of sand ridge fields may have several modes of origin, but McBride and Moslow 1991 suggested that most have probably developed as sand is deposited in ebb tide deltas of barrier systems. The inlets open, migrate and then close with ebb tidal deltas acting as point sources for sand. The retreat and longshore transport of the tidal delta accounts for the typical shape of shoreface attached ridges. The coupling of shoreline and shallow marine sedimentary processes during a transgression is critical to the origin, evolution and distribution of shoreface sand ridges.
Eolian transport of quartz sands from a neighboring desert environment may introduce quartz sands into the reef carbonate deposits with even less damage to the living coral. The influx of sands into the reef environment may affect reef development, but seldom lead to demise of the reef.
A reef-dominated shelf may have a major influx of fine-grained terrigenous muds from either a natural change in the environment, or a human-developed cause of increased soil run-off. Both the Ponce and the Mayaguez areas of the Puerto Rico insular shelf have clean carbonate sands covered with less than a meter of terrigenous sediment that has been deposited in the 200 to 300 years following extensive stripping of vegetation for agricultural expansion in Puerto Rico. The effect on reef survival depends on the amount of sediment influx. More than two kilometers off the coast at Mayaguez, reefs are affected by an influx of silt and clay sized terrigenous sediments during the rainy season of August to December. These finer grained sediments have an affect on reef accretion, but do not destroy the reefs.
At Guayanilla, PR and closer to the Mayaguez shoreline, the influx of fine-grained terrigenous sediments is chronic, and the coral reef environment is being replaced by a terrigenous mud, a permanent change of facies. The influx of fine-grained sediments into modern reef environments does lead to death of the coral and change from an active coral reef to a hardground and subsequently to a terrigenous mud deposit. In the situations discussed, a coral reef environment is invaded by terrigenous sediments. The sequence is either a complete change of sediment facies vertically, or an intrusion of an isolated terrigenous facies laterally on a small scale into a carbonate basin.
The history of a carbonate platform is recorded in its sediments. Nearly all the calcium carbonate that makes up carbonate platforms is derived from marine organisms. Along the platform margin, highly productive reefs often supply much of the material that is found both on the platform top and in the deep basin surrounding it. The scleractinean corals that dominate modern reefs are highly dependent on a symbiotic relationship with photosynthetic algae contained within their surface tissues. Therefore, carbonate production in modern coral reefs is highly light dependent. There is increasing evidence that this symbiotic relationship was similarly important in many ancient reefs. Stanley and Swart, 1984
The ability of the encompassing reefs to track rising sea-level will dictate the ability of the platform to keep pace. Some investigators have argued that platform margins are erosional remnants, and that the original reef ramparts have been removed. If this were so, then the rigidity of the platform margin and the resulting ability of the bank to maintain steep and often vertical flanks are the result of submarine cementation of the newly exposed platform-interior sediments. While it is difficult to resolve this argument, it is hard to argue against reefs being important contributors to the platform-sediment budget, exerting an important control on platform-margin stability, and retaining a detailed record of the processes that have affected the platform through time.
In addition to reefs, there are several other important sources of calcium carbonate. Along many shallow bank tops, various calcareous algae are prolific suppliers of material for bank development. Principal among these are the green codiacean algae, Halimeda and Penicillus . Halimeda can produce calcium carbonate at rates from 0.02 to over 1.00 kg per square meter of bottom annually (kg/m2-yr: Multer, 1988). Sediment is formed when plates are dislodged from the live plant or when the plant dies and the segments containing the carbonate disarticulate. The resulting "cornflake"-like grains are easily identified in both modern and ancient sands alike.
Along the southern edge of Little Bahama Bank and along the northeast coast of Jamaica, Halimeda comprises up to 90% of many bank-margin sand deposits at depths of 20-50 m. Hubbard, unpubl. data Halimeda are capable of producing large, mound-like structures that rival coral reefs in their thickness, accretion rate, and lateral extent. Hine et al., 1988 Along the northern Great Barrier Reef of massive mounds comprised almost exclusively of Halimeda, Orme, 1983 and Halimeda mounds have also been reported in the Caribbean. Phipps and Roberts 1988 Penicillus is an important contributor to many platforms. Neumann and Land 1975 concluded that, on the top of the shallow Bahama Banks, 15.2 kg/m2 of sediment is produced annually by these algae (better than half of the muddy sediment found there). Of that, three-quarters are swept off to the bank to make an important contribution to the sedimentary record of the adjacent basin.
While the world oceans are saturated with respect to most polymorphs of calcium carbonate (aragonite, low-Mg calcite, high-Mg calcite), examples of direct precipitation in modern tropical seas are rare. This is in contrast to the presumed importance of chemical precipitation in many ancient environments. Direct precipitation can provide significant quantities of carbonate in open-marine settings; this is a relatively recent revelation. Gevirtz and Friedman 1966 characterized submarine cementation as unlikely, except under specialized conditions. Subsequent work by Ginsburg, et al. 1967 and Shinn 1969 in shallow water, and Fischer and Garrison 1975 in deeper water, have shown that not only can submarine cementation occur in normal marine settings, but that it is in fact a very common phenomenon.
Over the past two decades, observations have reinforced this idea and have added to our knowledge of the habit and distribution of submarine cements in modern tropical seas. Land and Goreau 1970 discussed the importance of submarine cements in maintaining the rigid reef margin at Discovery Bay, Jamaica. Lighty 1985 described the variations in cement occurrence along a submerged (depth ~ 15m) reef off the southeast coast of Florida. Using samples recovered by submersible, James and Ginsburg 1969 have provided us with our best description of submarine cements from the deep bank margin off Belize.
Bahamian " whitings ," described in various papers over the past two decades represent another form of direct precipitation of calcium carbonate. First described by Cloud, 1962 these curious masses of chalky water that come and go over the Bahama Banks have been variably interpreted as direct physiochemical precipitation and clouds of suspended sediment generated by feeding fish. Shinn, et al. 1989 reported no feeding activity associated with these features, based on over two decades of observation. They further noted that the physical characteristics and the settling behavior of the mud from the whitings are distinctly different from that of the underlying bank-top muds - supporting a physiochemical origin. Ultimately, they may prove to be the result of microbially mediated precipitation, thereby still implying a biological link, albeit at a microscopic scale.
Because of the strong biological influence on carbonate sedimentation, and the dependence of most of the important organisms on light, the volume of material needed to maintain platform accretion can be produced only within the upper photic zone. In clear water, light intensity decreases exponentially with depth. In addition, the water column rapidly filters out the red end of the light spectrum. The combined effect of these two factors is a strong zonation in sediment-production and the organisms that are responsible.
Several conditions lead to entrapment of fine sediment on shelves:
The coastal deposits of Surinam are a mud analog to nearshore and inner shelf sand deposits. The mud banks resemble linear sand ridges found 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. These coastal muds extend beyond the shoreface and are part of the shelf sedimentary environment. Wells & Coleman, 1981 Mud deposition occurs at current velocities that are surprisingly high -- up to 10 to 15 cm/sec measured 1 m above the bed. Drake, 1976
The central Yellow Sea is a modern epicontinental environment in a semi-enclosed, depressed area of continental margin between the contiguous land masses of China and Korea. It is covered with fine-grained sediments originating from the Huanghe and Changjiang rivers, which rank among the top four of the world's rivers in terms of annual sediment discharge. Sediment sources surrounding the Yellow Sea are dominantly mud. During the last lowstand of sea level, the entire Yellow Sea basin was subaerially exposed and was submerged by the latest rise in sea with only a few relict sands occurring in the northern part of the Yellow Sea, in the northern East China Sea and in the Korea Strait. Alexander, et al., 1991
Bed cohesiveness and resistance to resuspension, is complicated by the presence of benthic (and nektonic) organisms that stir, burrow, and otherwise disturb bottom sediments. Like biological agglomeration processes, the abilities of the benthos to resuspend particles directly and to prevent compaction of muds (by introducing water) may be extremely significant.