Turbidity Currents and Submarine Fans

Turbidity currents are a major mechanism by which sediment from nearshore areas is transported to deeper parts of the oceans. This sediment often accumulates as part of large depositional systems called submarine fans, which could well be exploited as a major source of petroleum at some point in the future.

Development of the Concept

A turbidity current is a dense mass of water and sediment that flows downhill along the bottom of an ocean or any other standing body of water, such as a lake. Turbidity currents may reach high speeds and can carry large quantities of sediment, eroding or scouring the ocean floor as they move.

The concept of the turbidity current has a long history. In the late nineteenth century, geologists observing the waters of the Rhone River entering Lake Geneva noted that instead of mixing with the lake water, the river water moved along the bottom of the lake in a channel. This behavior was interpreted to reflect the elevated density of the cold, sediment-laden Rhone River water. These dense, bottom-hugging currents were called “density currents.” It was suggested in the late 1930s that dense currents composed of sediment and water could be produced by wave activity on the continental shelves, the flat or gently sloping submerged edges of the continents, during periods when the global sea level is lower than it is today. Furthermore, it was postulated that these dense, turbulent mixtures of suspended sediment and water would flow across the continental shelves and down large canyon-like features into deeper parts of the ocean.

From the 1930s to the 1950s, researchers conducted numerous laboratory studies to test the hypothesis that turbidity currents could erode the ocean floor and produce such features as submarine canyons on the continental shelf. This work was done by pouring muddy water into the end of an inclined flume (a long, straight trough in which the hydrodynamic properties of moving fluids can be studied) to produce artificial turbidity currents. Results of these investigations indicated that sediment in a turbidity current moves in a very chaotic or random pattern, much like the movement of snow and other materials in an avalanche. Additionally, geologists recognized that turbidity currents can move rapidly and that their velocity depends on the slope of the sea bottom along which they move as well as the density of the sediment-water mixture.

These experimental studies also yielded insight into the nature of the sedimentary deposits that accumulate from turbidity currents. These deposits, referred to as turbidites, form as graded beds, sedimentary layers in which the largest or most coarse-grained sediment particles are concentrated at the bottom of the bed and grade gradually upward to the smallest or most fine-grained sediment at the top. The graded nature of the bedding was deduced correctly to be a consequence of a reduction in the flow velocity of the turbidity current upon reaching the very gentle slopes that are typical of the bottom of the ocean so that the largest sediment particles settle out of the current to the bottom of the ocean first, followed by progressively finer particles. Thus, each turbidity current produces a single-graded bed or turbidite.

Geologists working in the field began to realize that the graded beds, such as those produced in laboratory studies, could be observed in sequences of sedimentary rocks exposed on land in various mountain belts throughout the world (the Appalachians, Apennines, and Carpathians). Moreover, oceanographic research demonstrated the presence of turbidite sands in some of the deepest areas of the modern oceans. It is now apparent that turbidity currents are and were a dominant, if not the dominant, mechanism of sediment transport from shallow-water environments on the continental shelf to the deeper parts of the ocean. Evidence of this includes the recovery of shallow-water organic remains from very deep parts of the oceans far removed from land. The laboratory studies and subsequent oceanographic research indicated that turbidites differ from most other sedimentary deposits in that they are nearly instantaneous deposits that may accumulate over a period of just a few hours to a few days. Moreover, turbidity currents are quite capable of eroding the great submarine canyons that cut into many continental shelves.

Causes of Turbidity Currents

The downhill movement of turbidity currents can be triggered by various causes. Commonly, earthquakes affecting the continental shelves will cause sediment on the continental slope—the part of the continental margin characterized by an increase in gradient immediately seaward of the continental shelf—to slide downslope, thereby mixing with seawater to form a turbidity current. A rapid sequence of successive breaks in transatlantic cables on the continental margin bordering southern Newfoundland was apparently caused by the downslope movement of a dense, turbulent mixture of seawater and sediment generated from the continental shelf and slope in response to the Grand Banks earthquake of November 8, 1929. Those cables closest to the point on the ocean floor directly above the earthquake, the epicenter, broke first, whereas cables farther from this point broke later. The breaking of the cables indicated the erosive capability of turbidity currents. Subsequent drilling of that part of the ocean floor traversed by the turbulent flow recovered a 1-meter-thick graded bed containing shallow-water organic remains; this discovery strengthened the argument that the flow was indeed a turbidity current. The earthquake-induced turbidity current averaged 27 kilometers per hour. However, it reached velocities in excess of 70 kilometers per hour in steeper parts of the continental margin, and it covered an area of more than 195,000 square kilometers.

Turbidity currents can also be generated by wave activity on continental shelves, an idea, as already noted, that was postulated in the 1930’s. More specifically, large waves produced during great storms such as hurricanes can create the turbulence required to mix sediment and seawater, thereby creating a turbidity current. An equally plausible mechanism of turbidity-current generation involves the oversteepening of the continental slope by the sudden addition of sediment. This is especially common when large rivers deposit sediment load near a submarine canyon's head.

The rate at which turbidity currents are generated was greatest when the sea level was much lower than in the twenty-first century. This was especially typical of glacial periods, when more seawater was locked up in the enlarged polar ice caps, resulting in the lowering of the global level of the oceans. During these times of lowered sea level, rivers could transport their sediment loads directly across the previously submerged continental shelf into the head of a submarine canyon. There, the sediment mixed with seawater and was fed, via the submarine canyon, directly into the deeper parts of the ocean as turbidity currents. When the sea level rose, and the shoreline retreated landward as the exposed continental shelf was once again submerged, the river was cut off from the canyon head, thereby precluding the infusion of sediment directly to the canyon and reducing the likelihood of turbidity-current generation.

Characteristics of Submarine Fans

Turbidity currents triggered on continental shelves or continental slopes move downhill until they reach a point at which the reduced gradient of the ocean floor causes a reduction in the velocity of the sediment flow. This leads to the deposition of a graded bed or turbidite. Many turbidite deposits funneled through submarine canyons ultimately accumulate as part of large fan-shaped or cone-shaped sediment bodies called submarine fans. The submarine fans, which spread outward from the mouths of the submarine canyons, merge with the bottom of the continental slope and comprise the continental rise, the broad, gently sloping feature that rises from the abyssal plain of the ocean floor and merges with the base of the continental slope. The attached submarine fans may coalesce to form a wide, laterally extensive continental rise, where submarine canyons are close along the continental shelf and continental slope.

In general, submarine fans are subdivided into three major morphologic elements or parts: the upper fan, midfan, and lower fan. The upper fan, also called the inner fan, is typically characterized by a single submarine channel connected to the submarine canyon. Upper fan channels range from 2 to 18 kilometers wide and may be as deep as 900 meters. The single channel is commonly flanked on both sides by levees, low ridges that run along the length of the channel. Most of the sediment transported through the upper fan channel via the submarine canyon is deposited into the midfan, an area of the submarine fan composed of raised, lobelike sequences of turbidites called depositional lobes. The depositional lobes are fed by numerous shallow and unstable distributive channels that branch off the main upper fan channel. Because these channels are generally relatively shallow (several tens of meters deep), they are more likely to be filled in during the passage of extraordinarily dense turbidity currents. When that happens, the channels are abandoned, and subsequent turbidity currents erode or scour out new channels in adjacent areas of the midfan portion of the submarine fan. The lower fan is characterized by a smooth ocean floor that passes imperceptibly seaward into the abyssal plain. The low gradient of the lower fan relative to that of the midfan leads to a much-reduced abundance of turbidites in the lower fan. Turbidite deposits are not common on the abyssal plain because ocean floor gradients in this area are typically too low to sustain the movement of the dense sediment cloud.

Submarine fans vary in size. The Bengal Fan, in the Bay of Bengal in the northeast Indian Ocean, is the largest. Its total length exceeds 3,000 kilometers. The main or upper fan channel ranges from 13 to 18 kilometers wide and 150 to 900 meters deep. Most submarine fans, however, are much smaller than the Bengal Fan.

The growth of submarine fans, because they are major sites of turbidite sedimentation, is controlled by variations in sea level. As noted previously, turbidity currents are generated more frequently during low sea-level periods. Accordingly, submarine fans are likely to grow fastest, as manifested by increased rates of turbidite sedimentation, during periods of low global sea level. Indeed, geologists have demonstrated that the Mississippi submarine fan, which the Mississippi River feeds, was the site of abundant turbidite sedimentation at the end of the most recent ice age, approximately 15,000 to 20,000 years ago, when sea level was as much as 120 to 130 meters lower than it is today. Since then, the turbidite sedimentation rate has dropped greatly as sea level gradually rose, and the shoreline retreated landward to its present position.

Scientists continue studying the evolution processes that create submarine fans in the twenty-first century. A study of the Qiongdongnan Basin in the South China Sea found its formation was due to fluctuations in sea level, sediment supply, and geomorphology. Studies in the twenty-first century have also revealed that submarine fans contain reservoirs of particulate organic carbon, highlighting their importance in the global carbon cycle and, therefore, their implications for global climate change. New technologies aid scientists in their studies of submarine fans. Three-dimensional seismic data and improved drilling core analysis have allowed a more comprehensive understanding of submarine fans. 

Principal Terms

abyssal plain: the flat, sediment-covered area of the sea floor that merges with the base of the continental rise

continental margin: the area that separates the emergent continents from the deep-sea floor, generally consisting of the continental shelf, continental slope, and continental rise

continental rise: the broad and gently sloping ramp that rises from the abyssal plain to the base of the continental slope; submarine fans are found here

continental shelf: the gently seaward-sloping submerged edge of a continent that commonly extends to a depth of about 200 meters or the edge of the continental slope

continental slope: the relatively steep region of the continental margin between the continental shelf and the continental rise

gas hydrates: relatively stable, often crystalline, molecular combinations of water and hydrocarbon gases, especially methane, that form on the ocean floor due to the elevated pressure conditions

submarine canyon: a submerged, V-shaped canyon cut into the continental shelf and continental slope, through which turbidity currents funnel into the deeper parts of the oceans

turbulent flow: a high-velocity sediment flow in which individual sediment particles move in very chaotic directions above the sea floor

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