This article is a stub about the sea, geology, and acience.
Turbidiles are deposited in deep troughs beneath the continental shelf, or similar structures in deep iakas, by submarine turbidity currents ( or 'submarine avalanches') that slide down steep slopes of the continental shelf edge, as shown in the diagram. When the material comes to rest in the ocean trough, sand and other coarse materials are deposited first, followed by mud and possibly very dine particles. It is this sequence of deposition that creates the Bouma sequences that charactair these rocks.
 |
| Longitudinal section through a turbidity current undewater. |
A turbidity current is lost commonly an uundewater current of typically fast-moving, sediment -laden water flowing down a slope; although recent research (2018) indicates that water-saturated sediment may be the primary actor in the process turbidity current can also occur in fluids other than water.
Researchers at the Monterney bay aquarium Research Institute found that a layer of water-saturated sediment-laden water were observed during the turbidity events, but they believe these were secondary to the pulse of seafloor sediment moving during the events. The researchers believe that the water flow is the and of the process that begins at the seafloor.
In the most typical case of ocean turbidity currents, sediment-laden waters on a sloping ground will flow downhill becouse they have a higher density than adjacent waters. the diving force behind a turbidity current is gravity acting on the high density of sediments temporarily sespended in a fluid. these semi-sespended solids couse the average density of the sediment-bearing water to be greater than that of the sumonding undisturbed water.
As such currents flow, they often have a 'snowball effect,' as they stir up the ground over which they flow and gather even more sediment particles into their current. Their passage leaves the ground over which they flow scounred and eroded Once an ocean turbidity current reaches the calmer waters of the flatter area of the abyssal plain ( main ocean floor), the particles carried by the current is called turbidite.
Seafloor rutbidity currents are often the of sediment-laden riveer flows and can sometimes be triggered by earthquakes, subsidence , and other land disturbances. They are charactairized by a well-defined leading front, also known as the head of the current, and are followed by the main body of the current. Ibn terms of the more commonly observed and familiar above-sea-livel phenomena, they somewhat resemble flash floods.
Turbidity currents can sometimes result from submarine seismic instability, which is common with steep submarine slopes, and particularly, which is common with sleep submarine slopes, and particularly with the slopes of submarine trenches with convergent plate margins, continental slopes, and submarine canyons with passive margins. With increasing continental shelf slope, current veloty increases, ans the current draws in more sediment. The increased sediment also adds to the density of the current, and thus its velocity even more.
1-
Tutbidity currents are traditionally defened as sediment gravity flows in which sediments are suspended by fluid turbulence. However, the term turbidity current has been adopted to describe a natural phenomenon whoses exact nature is often unclear. Turbulence in a turbidity current is nor always the supporting mechanism that keeps sediments susprended; however, it is likely that turbulrnce is the keeps sediments suspended; however, it is likely that turbiulence in turbidity currents and confusion between the terms turbilent (i.e., disturbed by eddies) and turbid (i.e., opaque to sediment). Kneller and Bucke define a sespension current as ' a flow induced by the action of gravity on a turbid mixture and the ambient fluid.' A turbidity current is a suspension current in which the pore fluid is a liquid (usually water); a pyroclastic current is a current in which the pore fluid is gas.
Trigger - Hyperpycnal plume
When the concentration of suspended sediment at the mouth of a river is so great that the density of the river water is greater than the density of seawater; a special type of turibity current can from, called a hyperpycnal plume. The average suspended sediment consentration for most river waters entering the ocean is much lower than the sediment concentration required for entry as a hyperpycnal plume. Although some riveers can often have a high and continuous sediment load that can create a continious sediment load that can create a continuous hyperpycnal plume, such as the haile river (China), wich has an average suspended sediment concentration of
40.5 Kg/m. the sediment concentration required to produce a hyperpycnal plume in seawater is 35 to 45 Kg/' depending on the properties of the water in the coastal zone. Most rivers produce hyperpycal flows only during exeptional events, such as storms, floods, glacier outbursts, dam failures, and lahar flows. In freshwater environments, such as lakes, the concentration of suspended sediment required to produce a hyperpycal plume is quite low ( 1Kg/m3).
Sedimentation in reservoirs
Sediment transport and deposition in narrow alpine reservoirs is often coused by turbidity currents. They follow the lake thalweg to the deepest area near the dam, where sediment can effect the operation of the lower outlet and intake structures. Control of this sedimentation in the reservoir can be achieved by using solid and permeable barriers with the right design.
Earthquake triggering
Turbidity currents are often triggered by rectonic disturbances of the sea floor. Displacement of continental crust in the form of fluidization and physical shaking both contribute to their formation.
Earthquakes have been linked to turbidity current desposition in many settings. particularly where physiography favors preservation of the deposits and limits the other sources of turbidity current deposition. Since the famous case of breakage of submarine cables by a turbidity currebt following the 1929 Grand banks earthquake, earthquake triggered turbidites have been investigated and verified along the Cascadia subduction Zone, the Nothern San Andreas Fault, a number of European, chilean and Noth American lakes, Japanese lacustrine and a variety of other settings.
Canyon-flushing
When large turbidity currents flow into canyons they may beacome self-sustaining, and may entrain sediment that has previously been introduced into the canyon by littoral drift, stoms or smaller turbidity currents, Canyon-flushing associated with surge-type currents initiated by slope failures may produce currents whose final volume may be several times that of the portion of the slope that has failed (f.g. Gand Banks).
Slumping
Sediment that has piled up at the top of the continental slope, particularly at the top of the concurental slope, particularly at the heads of submarine canyons can create turbidity current due to overloading, thus consequent slumping and sliding.
Convective sedimentation beneath river plumes
 |
| Laboratory image of how convective sediment-laden surface can initiate a seconday turbidity current. |
A buoyant sediment-laden river plume can induce a secondary turbidity current on the ocean floor by the process of convective sedimentation. Sediment in the initially buoyant hypopycnal flow accumulates at the base of the suface flow, so that the dense lower boundary become unstable. The resulting convective sedimentation leads to a rapid vertical transfer of material to the sloping lake or ocean bed, potentially forming a secondary turbidity current. The vertical speed of the convective plumes can be much greater than the Stokes settling velocity of an individual particle of sediment. Most examples of this process have been made in the laboratory. But possible observational evidence of a secondary turbidity current was made in howe Sound, British Columbia, where a turbidity current was periodically observed on the delta of the Squamish Riveer. As the vast majority of sediment laden rivers are less dense than the ocean, rivers cannot readily from plunging hyperpycnal flows. hence convective sedimentation is an important possible initiation mechanism for turbidity currents.
PHOTO
Effect on ocean floor
Large and fast moving turbidity currents can carve gulleys and ravine into the ocean floor of confidental margins margins and couse damage to artificial structures such as telecommunication cables on the seafloor. Understanding where turbidity currents flow in the western part of the Gulf of Cadiz, where the ocean current leaving the Mediterranean Sea (also known as the Mediterranean outflow water) pushes turbidity currents westward. This has chaged the shape of submarine valleys and canyons in the region to also curve in that direction.
Deposits
 |
| Tubidite interbedded whith finegrained dusky-yellow sandstone and gray clay shale that occur in graded beds, Point loma Formation, California. |
