Turbidites are sea deposits which are formed by underwater avalanches. They are responsible for large amounts of clastic sediment in the ocean.
Formation
Turbidites travel and are deposited by density flow, or gravity currents.
Usually, in a river bed, rock is carried by a frictional drag known as tractional flow. In order for the water to move the rock, a certain velocity must be maintained. The larger or denser the rock, the higher the water velocity needs to be. In density based flow, the sediment goes through a liquefaction while it is being moved. This causes the density of the fluid to change. For this to occur, the liquid has to be highly turbulent, creating a slurry with the fine-grained particles. When this happens, the density of the liquid becomes thicker, so it is able to carry larger rocks because the water plus the sediment together have a density that makes it closer to the rock’s density, so it is more able to carry the rock.
Turbidites are easily found in deep ocean, however, there are other places where they can occur. Density-based flow situations can occur at lahars on the side of volcanoes, mudslides or at pyroclastic flows.
There are common occurrences with a turbidite flow, according to marin.edu. Each deposit will be coarser at the bottom with a finer-grade toward the top. Other features “include 1) graded bedding, 2) sole marks, 3) cross stratification units, 4) asymmetrical ripple marks, 5) extensive even bedding with interbedded sands and shales.”
Bouma cycle
Arnold H. Bouma first described turbidites in 1962. While studying deep water sediment he found areas within deep water, fine grained shales, that started as pebble conglomerates and ended in shales. Generally, it had been thought that there was not a way which tractional flow could carry and deposit coarse-grained sediments in deep water.
The Bouma cycle start with erosional contact of a lower bed of pebble up to granule conglomerate within a sandy matrix. Then it would progress up through coarse, and then medium parallel plane sandstone. Next would come a cross-bedded sandstone, rippled cross-bedded sand/silty sand. Then finally there is a change that is like a strong to waning flow regime currents, along with their sedimentations.
This cycle is very unusual to see, because additional turbidity currents erode the upper sequences. Another reason for this lack of occurrence is that exposed sections at the edge of the current lobe, or upslope from the deposit center may contain a scour channel that is filled with fine sands. This would then end in a pelagic ooze.
Turbidites can help in identifying ancient sedimentary sequences as they usually are representative of rocks formed in deep water offshore of a convergent margin. They usually require, also a sloping shelf and some type of tectonic action to start the avalanches.
There are turbidites in lakes which can show, chronologically, how often landslides and earthquakes have occurred, by dating varves above and below the turbidite.
