Roll-over shelves have margins along which a gradual but definite slope break occurs. Limited reefs can occur, but are generally discouraged by active sediment movement. Sand bodies are generally localized along the slope break, but can occur across a wide geographical area. Rimmed margins are those along which elevated topographic highs exist. The most common among these in modern tropical seas are reefs. Reefs can be the primary cause of the slope break or they may occur on antecedent highs.
The term "ramp" was proposed by Ahr 1973 as an alternative to the shelf model. Ramps have no distinct slope break and facies grade evenly from sandier sediments associated with higher energy at the shallow end of the system to muddier sediments in lower-energy, deeper-water environments. The distinction between the two margin types is often subjective, and ramps can evolve into shelves if localized deposition builds a slope break.
Rocky banks and shoals that are remnants of bedrock rising above the general level of shelf sediments are another style of terrigenous bank. The Farallon Islands off California are accompanied by submerged rocky banks and other submerged rock banks occur off southern California, the Bering Sea, and the Barents Sea off Russia and Norway.
Isolated carbonate banks exist separate from major landmasses. These features are surrounded on all sides by water. Their generally flat tops occur in water depths from very shallow (i.e. the Bahamas Banks = 3-10 m) to depths up to 100 m (i.e. Saba Bank). Water depth over a particular bank is generally controlled by the relative ability to track the most recent rise in sea level. This is, in turn, controlled by production rates of the organisms that inhabit the platform. We loosely differentiate between shallow banks (depth less than 5-8 m) and deep-water banks (depth more than 10m), keeping in mind that these are temporally restrained distinctions (i.e. a drop in sea level will transform a deep-water bank into a shallow one).
Only one attempt has been made to classify platform margins according to both the physical processes that have shaped them and the sedimentary patterns that result. Hine, et al. 1981 compiled a detailed classification of Bahamian margins based primarily on physical-oceanographic processes and the presence or absence of barriers to sediment transport. Their initial division is based on the relative importance of waves vs. tidal currents. Wave-dominated margins are characterized as windward (facing into the dominant approach of wave energy), or leeward (facing away). Tide-dominated margins are grouped together, regardless of orientation or physical character of the margin. The strength of this approach is its basis in quantifiable processes.
The ideal classification is one, which relies on both physical similarities in modern systems and the factors that are responsible. We prefer a
that emphasizes physical processes and the pathways of sediment transport that are common to each margin type, drawing heavily on the ideas of Hine, et al. 1981 Their term "protected" has been modified to protecting in an attempt to better convey the nature of the process involved (i.e.. the inhibition of sediment transport by some physical barrier) and to avoid the possible misconception that "protection" by another bank or island from wave energy might be part of this process. Also, mixed banks have been added to include those platforms that are subject to cyclic changes in the influence of varying physical processes.
An important distinction with respect to sediment transport along windward margins is the presence or absence of a major barrier to sediment transport onto and across the platform. Common barriers to sediment transport include islands (i.e. Grand Bahama Island) and emergent reefs.
Along open margins, wave-induced currents can move detrital sediment from windward reefs onto the bank. In contrast, sediment produced along protecting margins must either be incorporated within the reef itself, stored in inter-reef areas, or transported off the shelf to windward. Spur-and-groove topography (alternating intervals of reef and intervening sand channels) is common along this type of margin. Hubbard, et al. 1990 and Sadd 1984 proposed that these channels serve as short-term repositories for biologically-produced sediment during fair weather and as avenues for wholesale export during major storms. Based on storm observations made during a HYDROLAB mission in the Bahamas, Hubbard, et al. 1974 proposed that return flows triggered by water piled up by storm waves against the coast were probably the mechanism responsible. Subsequent measurements along the north coast of St. Croix during the passage of Hurricane Hugo in 1989 verified this hypothesis. Hubbard, 1992 Over two million metric tons of sediment were swept from Salt River submarine canyon on the island's north coast. Likewise, sediment collected over the previous century at nearby Cane Bay was moved off the shelf in just a few hours.
In the Bahamas, tide-dominated margins are characterized by lobate sand bodies, Ball, 1967 typically occurring in en-echelon patterns that are alternately oriented bankward and off-bank . This alternating pattern results in recycling of the bank-margin sediments onto and off of the bank, with net transport being dictated by the superimposed effect of waves. The strong tidal effect at these locations is caused by the funnel-like shape of the embayment, which serves to focus tidal flow.
By combining the morphologic classification discussed at the beginning of this chapter with the more process-based ideas just discussed, one can conjure up a complete picture of a particular carbonate platform. As an example, the southern side of Little Bahama Bank would be classified as a prohibiting (island-blocked), rimmed and windward isolated-platform margin. Southwestern St. Croix is an open, leeward rollover shelf (insular).
If you have a problem visualizing this terrain, visit your neighborhood golf course. Worldwide they are designed to resemble the glacial topography of Scotland where the game originated.
Glaciated shelves are distinct depositional environments that are supplied with a broad range of sediment types, characteristic of glacial sediment transport that includes very poor sorting of the sediments. The shelf sediments north of 41o North on the continental shelf of east and west North America, Europe, and Russia were mainly deposited by Pleistocene glaciers.
Piston cores from the Cariaco Trench were taken to obtain sediments from the anoxic zone. These were grayish-olive colored fine grained, laminated silt and clay or homogeneous silty clays with a paucity of benthic fauna of Holocene age. Depauperate biota is a common and important feature of restricted environments. The diversity is low, but the number of organisms may be high. A lower zone on the south slope had yellowish-brown silty clays with a fair to good benthic fauna that were probably deposited under normal marine conditions during the late Pleistocene before the basin waters became anoxic. Athearn, 1965
Barriers may restrict circulation in a shallow water environment and develop restricted basins. The barriers may be either bathymetric physical barriers or simple limitation of water circulation. The depositional geometry may range from level bottom expanses like the Bahama Bank to complex compartmentalized environments such as Florida Bay where a lacework of individual mud lakes and islands lead to isolated and stagnant environments. The Florida bay sediments are more than 90% biomicrite with a sand component dominated by pellets, mollusc fragments and foraminifera. Ginsburg, 1956
Some modern semi-restricted carbonate environments fall between the two extremes. The Belize lagoon has sufficient circulation and water runoff from adjacent highlands to produce an environment of terrigenous muds grading seaward into calcareous mud derived from the reef tract. The seaward lagoon has steep sided pinnacle reefs and micro atolls. The inner part of the broad shelf of central Venezuela and the Bahama bank west of Andros Island have pellet deposits characteristic of a restricted environment, but circulation does occur and benthic fauna is normal. Higher energy, winnowed sediments accumulate as the bottom depths approach wave base in these areas.
Fault valleys are trough-shaped, broad valleys that follow structural trends, and they have few if any tributaries. These occur in areas of tectonic activity. Their trends may be a continuation of structural features from land such as the San Clemente Rift Valley, off southern California and the Manzanares Canyon off Cumana, Venezuela which follow major faults seaward. As in the case of river valleys following fault traces on land, these submarine valleys may develop into and be a part of submarine canyon systems.
Glacial troughs on the shelf and upper slope may also develop into submarine canyons. These are U-shaped and usually have greater depths than other valleys and rather large closed basins along their lengths. They may have both tributaries and distributaries. An example is the trough coming out of the Gulf of St. Lawrence that extends across the shelf.
Delta-front troughs may be related in origin to submarine canyons, but they are located only on the fronts of large deltas and have straight courses with few if any tributaries and a continuously seaward slope across the shelf and down the continental slope. Examples are the Swatch of No Ground off the Ganges Delta and similar valleys off the Indus, Niger, and Mississippi deltas.
Fan valleys are the seaward continuation of submarine canyons and delta front troughs across the sediment fans at the base of the continental slope. These are V-shaped or trough-shaped and are cut into unconsolidated fan sediments. Low ridges comparable to natural levees are found along the sides of many and most have distributaries, but very few have tributaries. It is not possible to classify all marine valleys into one of these categories. Marine valleys of diverse origin may evolve into submarine canyons, so we are confronted with transitional stages. The different types of submarine valleys appear to be relatively distinct one from the other, but there is no evidence that they were all formed by different processes. Nor is there any assurance that all of the same type has the same origin.
Turbidity currents result from sediment accumulating on the slope or shelf edge until the mass becomes unstable and slumps or is disturbed by an earthquake or storm, throwing the sediments into suspension and developing a heavy, turbid mass which is heavier than adjacent clear water and hence is capable of descending to the bottom of slopes. As this liquid mass of dense material moves down the slope, it may nourish itself by erosion of material over which it flows. Turbidity currents are generally credited with the excavation of submarine canyons and with transporting great quantities of sediment down the canyon to form the fans at the base of the continental slopes.
If a valley were already cut into the slope, the turbidity currents would be confined and hence have a much better chance of continuing to the bottom of the incline. According to this reasoning, the turbidity current is not necessarily the initial cause of the valley, but it might perpetuate a valley already existing on the slope, or even excavate to greater depth. Marine processes are clearly capable of preventing canyons from being filled with sediments.
Hydraulic laboratory experiments suggest high velocities for turbidity currents if they are confined to valleys rather than spreading over the open slope. It has been calculated that the velocities due to the filling of the channels in the Monterey fan valley would be of the order of 16 to 40 knots.
All erosion is probably not due to turbidity currents, and various types of flows may be equally important. Temperature and salinity gradients, along with internal waves and tides are quite capable of producing strong flows that will transport sand sediments. Ordinary bottom water currents which are known to occur in the heads of canyons may also excavate sediments in submarine canyons, or at least build the submarine fans and keep the fan-valleys open. A turbidity current might be expected to come to an end due to loss of sediment at the base of the steep gradient, but the ordinary currents, not dependent on slope, could persist. Erosion by ordinary currents could account for the tributaries that start at considerable depth on the slope at points where little if any sediment would be available to account for a turbidity current.
If processes on the sea floor are both excavating and maintaining previously excavated canyons, undoubtedly deposition on the adjacent shelves and continental slopes is taking place and increasing the total wall heights of the canyons. Downcutting with upbuilding over a long period of time allows the formation of huge canyons, such as those in the Bering Sea and in the Bahamas. Discovery of old filled canyons lying below the floor of present canyons adds impetus to the hypothesis and suggests that canyon cutting may have alternated with fill at different episodes.
Results from canyon studies favor a combination of processes as causal. Perhaps gravity-induced slides and slumps are not important except in the steeper heads, but seismic profiling shows us that slides take place even on gentle continental rises. At most of the locations where creep is demonstrated, there are steep axial slopes, commonly 20 to 30 degrees. Creep explains erosion at a point in the canyon where turbidity currents could not play an important role, because they would not have had time and distance of travel to gather momentum sufficient to resuspend the sediments. Once initiated along a plane of weakness, very little energy is required to maintain a slow downslope creep by large sedimentary masses on even very low slopes, especially if there is a possibility of increasing the pore pressures in the sediment.