Turbidite is in the geology common name for a sedimentary rock, which on a Trübe- or suspension flow (English "turbidity current"), that is an avalanche-like transport and deposition process in the sea or a lake, back. As a rule, turbidites do not occur individually, but in layers that are often several hundred meters thick and consist mainly of fine sandstone and claystone .
It is named after the turbulence of the liquid that transports the starting material of the turbidite. Sediments from other deposition spaces that are related by the transport mechanism and that can be traced back to the turbulent flow of other media are, for example, avalanches , lahars and pyroclastic flows . The turbulent suspension flows that cause them are separated from other geological mass movements such as mud flow , landslide, rubble flow , grain flow or laminites as a deposition mechanism of the turbidites .
The importance of turbidites and their role in the formation of deep-sea sediments has only been understood since the beginning of turbidite research from around 1960. Before that, the occurrence of rock sequences that appeared to have formed in the shallow sea and shallow sea fossils in the midst of verifiably formed deposits in the deep sea was a difficult geological problem.
Turbidite is deposited by streams of suspension. These form from deposits on steep submarine slopes, for example on continental slopes , which become unstable and slide off due to strong water saturation, currents or vibrations such as earthquakes. Due to the landslide and the resulting turbulence, the originally more or less firmly coherent starting material can be broken down into its individual components (grains and particles) and kept in suspension, so that it can cover long distances with the water flowing on the seabed, sometimes over several 100 kilometers. Because of the usually erosive effect of a suspension flow at the beginning of the flow , submarine canyons have cut deep into the continental slope , especially in front of river deltas . At the exit of the canyon, the current spreads linearly on the sea floor, depending on the flow energy, and then spreads out in the shape of a tongue at the end of the flow path. During the flow, the floating constituents are sorted according to their buoyancy in the suspension flow, fragile components such as mussel shells are hardly broken, while large foreign constituents experience considerable buoyancy, which depends largely on their area. When the turbulent flow finally begins to slow down, the floating components settle.
A typical turbidite forms a rock bank a few decimeters to a few meters thick in the outcrop . It is characterized by a graded stratification within this bank, which is associated with a wide range of grain sizes (low grain sorting). The base of the bench is always very sharply demarcated from the layer below and is often wavy over a wide area. Immediately above its base, the bench consists of the coarsest material represented and represents e.g. B. a conglomerate or a coarse sandstone . The grain size decreases towards the top and the bank turns into a fine sand or siltstone . This gradation can sometimes only be recognized by the thin section of the rock under the microscope . The uppermost layer of a fully developed turbidite is usually a more or less thick layer of very fine-grained material (e.g. mudstone ) in which traces of sea creatures living on the bottom can be preserved. This material represents the fine suspended particles that have gradually settled on the sea floor during the period between two suspension current events. This is also referred to as background sediment, because this sedimentation took place permanently (“in the background”) in the storage area. In most cases, turbidites do not occur individually, but rather as part of entire turbidite series, which can be many hundreds of meters thick.
The characteristic sequence from coarse grain at the base to fine-grained sediment in the upper part of a turbidite is divided into five sections and named the Bouma sequence after the scientist Arnold H. Bouma . A distinction is made between the five typical sections of a turbidite layer, identified with letters from bottom to top:
- E: pelagic position, often with trace fossils ( Ichnofauna )
- D: upper horizontally layered layer (silt, clay)
- C: location with flow ripples , often occurrence of convolute bedding (fine sand, silt)
- B: lower horizontally layered layer (sand, fine sand)
- A: graded layer, with the coarsest grain at the base (rubble, gravel, sand)
Subordinate further characteristic features occur in turbidites, which go back to the way of the formation. The base of the rock bench is often littered with the negative forms of impressions in the underlying rock layer, which go back to the jumping, rolling and dragging movement of larger components of the suspension flow. These imprint, grinding and roll marks are often used in German-speaking countries with their English names ("bounce mark", "drag mark", "roll mark"). In addition to these imprints, structures such as ripple marks , plate structures , flow marks (“flute cast”) or scour marks can often be seen at the base of a turbidite , which were created by the flow of the suspension stream over the ocean or lake floor. All of these features can give an indication of the direction of flow.
By loosening already slightly solidified layers from the subsurface in the flow path of the suspension stream and their winding up during transport, a wound layer can arise ("convolute bedding"). Sometimes already largely solidified layers of the subsoil are shifted, compressed or absorbed by the suspension flow, so that large, partly still connected or broken fragments of foreign rock ( olistholite ) are embedded in the normal sequence of a turbidite.
Turbidites can have very different manifestations due to their formation. At first glance, the rock primarily reflects the initial sediments involved in its formation, such as clay sludge, lime sludge or sand deposits. In addition, the sediment particles that make up the rock can differ greatly in grain size. In addition, in many cases the causative suspension flows transport not only clay, silt and sand, but also fragments of fossils such as mussel shells or plant remains , which determine the appearance of the rock due to their sometimes clearly prominent size and shape.
With the exception of these layers of shell or plant remains that have been transported far away, mostly from the shallow sea, Turbidite are poor in fossils. Microfossils that can be assigned to the deep sea can often be identified in the rock .
Individual turbidites can often be tracked over long distances. The rock body is designed as a more or less elongated, tongue-shaped body which, together with numerous other, overlapping turbidites, can form a large rock body. Based on the position of the deposition site in relation to the original site of the material, which can often be derived from the formation of the turbidite and the stratification sequence, turbidites are divided into two end terms:
- Proximal turbidite: rock from a suspension stream at the exit of an underwater canyon, still near the beginning of the deposit. Due to the high flow rate, the individual components are relatively large; individual turbidites usually have an erosive base and are not formed until the finest (pelagic) material occurs. Often an inverse gradation occurs at the base.
- Distal turbidite: Rock from flowing suspension streams at a greater distance from the place of origin, without graduation, with sharp base contacts and layers of fine debris or fine sand.
Rocks made up of turbidites occur all over the world and in rocks of almost every age. Because of their formation, they occur more frequently in regions that have a high sediment supply, such as river deltas. Turbidites are also preferentially deposited in recurring succession at the edge of the continental shelf or on deep sea channels, which have a large difference in height between the position of the original sediment and the ocean floor. The effects of turbidite are mainly associated with sediment deposits in mountainous areas . The term flysch has become common in geology for rock sequences from turbidites with activated deep-sea deposits .
Turbidites also occur in non-marine waters. Turbidites can be found, for example, at the confluences of rivers in ponds and lakes.
- Hohl 1985, p. 663.
- Hohl 1985, pp. 130f.
- Richter 1986, p. 342.
- Hohl 1985, p. 178, p. 598.
- Hans Füchtbauer: Sediments and sedimentary rocks. 4th revised edition. Schweizerbart'sche Verlagbuchhandlung, Stuttgart 1988, ISBN 3-510-65138-3 .
- Rudolf Hohl (ed.): The history of the development of the earth. 6th edition. Werner Dausien Verlag, Hanau 1985, ISBN 3-7684-6526-8 .
- Dieter Richter: General Geology. 3. Edition. de Gruyter Verlag, Berlin / New York 1985, ISBN 3-11-010416-4 .
- Stefan Ødegård: Sedimentology of the Grès d'Annot Formation. The definition of sediment gravity flows and its subordinated currents and flows: turbidity current; fluidized sediment flow; grain flow; debris flow, and the widely used terminology, turbidite. Diploma thesis at Clausthal University of Technology, 2000. (English)
- Introduction to deepwater system sediments and their classification University of South Carolina Sequence Stratigraphy Web, (English)