The turbidities indicate a group of sedimentary rocks. These rocks are the product of a clastic rock flow top to the bottom of a continental slope submarine. Once consolidated, these rocks present repetitive sequences characteristic of the Flysch S.

Genesis

Continental shelf

These sediments are associated with a continental shelf knowing an intense detrital material contribution. An active cordillière would make the deal very well if it could admit the presence of a continental shelf. The detrital load frequently exceeds the possible maximum. At the time of a storm, of an earthquake or other, these unconsolidated Sédiment S is mixed with sea water.

Slope

The mixture forms a fluid mass of a density much higher than that of surrounding water. It is put moving and can reach tens of kilometers per hour. This mass erodes the sea-bed and takes care of all the possible unconsolidated sediments. The interface between the current of turbidity and the sea-bed is a zone of strong traction. Decimetre rollers can be pulled by such a water mass.

Generally, these currents turbiditic are channeled by underwater canyons or faults cutting the slope.

Abyssal plain

Arriving at the bottom of the slope, these currents do not meet any more the least relief. As they consist of a denser mass of fluid, they will be spread in all the possible directions and will form an underwater clastic rock lobe.

When several lobes are formed, the current turbiditic can only cross material unconsolidated at very high speed and thus at a time when it is very erosive. It will dig channels in the lobes preceding submarines.

These rocks are the product of a detrital flow of sediments S top to the bottom of an underwater continental slope.

Sequence of Bouma

The deposits accumulated under the action of currents of turbidities have an important side continuity (pluri kilometric). Bouma (1972) tried to decipher their organization by defining standard elementary a sequence known as " sequence of Bouma". This sequence, decimetre with metric, comprende upwards 5 terms (Your, Tb, Tc, Td, Te) deposited in a context of decreasing energy.

Level Your

Granoclassés terms made up of sands coarse (rudites with arénites). It presents figures of bases of bench (Flutes cast, toolcasts, tracks, bioturbations). Strong Hydrodynamism, field of the antidunes.

Level Tb

This zone is in top of level A. It is made of a sandstone granoclassé and very rolled. The head of turbidity passed. The current became laminar and gives up finer sands requiring less energy to be transported.

Level Tc

This level contains finer sand, showing oblique laminations, wrinkles of current and convolute. The thickness of the layer of turbditic water decreases. Traction on its higher level by the layer of surrounding water becomes sufficient to cause oscillations there. The subjacent deposits, controlled by the current, are done in distinct directions. Maybe by with blows when the layer is rather thick and thus to cause an oblique lamination. Convolute are caractérsés by a lamination " enroulé". The presence of figures of water exhaust can be also detected. These figures corresponds to pore water of the deposit which goes up under the effect of the accumulation of the sediments. On the ground, one finds them in the shape of tube out of transverse section and the shape of the protuberance (mini volcano) on the surface of the benches.

Td level

This level is that of the deposit of silts. It is the granulometry finest, right before the Argile S. the current is sufficiently slow compared to the thickness of flow that the latter is again laminar. It contains relatively little elements to allow only one fine layer of silts.

Level You

The level is characterized to You by the sedimentation of the finest fraction of the current of turbidity, it is mixed with the hemipelagic slow sedimentation of the basin. Then, sedimentation " normale" continue and superimposes itself on the sequence. To distinguish these two levels on the ground is very difficult, but not impossible when one has a level You easily carbonated différenciable of hemipelagic gray clays called hémipélagites.

Proximal, distal

The sequence given above is ideal and complete. Its interpretation makes it possible to imagine and include/understand incomplete sequences. On the continental slope, the deepening reach is the only possible one. Elements are torn off without the least deposit. Further, on the level of the passage slope - abyssal plain, the coarsest parts of the level has can settle. It is a facies known as proximal. Further, the level B can also settle and is initially eroded by following turbidity before more and more often appearing. The general idea is that the levels of the highest sequence settle more far from the source of clastic rocks.

But it as should be thought as the more one is far from the lower source and the less levels (has, B,…) chances have to appear. They settled before. We are then in distal facies.

A third complication appears in the examination of turbidities. It is their frequency and their violence which allows the appartion of the levels E and F. Very rare turbidities can, at least theoretically, to allow sequences of the kind has and F.

If a fourth complication is considered i.e the volume of the water turbide mass, then this kind of sequence is possible for low and rare masses.

Channel

One of the forms proximales of turbidities is the presence of channels of erosion of the former deposits. In other words, only the level of erosion of turbidity is present.

Lobe

The sequence of Bouma has a side extension. It forms sedimentary lobes on the abyssal plains. To have an idea of their importance, it is necessary to look at a topographic chart of the underwater funds to broad of the deltas of large rivers. The latter provide the detrital material. Storms or risings provide then the currents of density. The extension of such lobes can be done on thousands of kilometers.

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