Turbidite geological formations have their origins in turbidity current deposits, deposits from a form of underwater avalanche that are responsible for distributing vast amounts of clastic sediment into the deep ocean. The United States Geological Survey ( USGS) is a scientific agency of the United States government. A turbidity current or density current is a current of rapidly moving sediment-laden water moving down a slope through air water or another fluid This article refers to the natural event For other uses see Avalanche (disambiguation An avalanche is an abrupt and rapid flow of snow often Clastic rocks are composed of fragments or clasts, of pre-existing rock.
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The ideal turbidite sequence
Turbidites were first properly described by Bouma (1962), who studied deepwater sediments and recognized particular fining up intervals within deep water, fine grained shales, which were anomalous because they started at pebble conglomerates and terminated in shales. Germany, officially the Federal Republic of Germany ( ˈbʊndəsʁepuˌbliːk ˈdɔʏtʃlant is a Country in Central Europe. Shale (also called mudstone) is a fine-grained Sedimentary rock whose original constituents were Clay minerals or Muds It is characterized by A conglomerate (kɒnˈglɒmərət is a rock consisting of individual stones that have become cemented together
This was anomalous because within the deep ocean it had historically been assumed that there was no mechanism by which tractional flow could carry and deposit coarse-grained sediments into the abyssal depths.
Bouma cycles begin with an erosional contact of a coarse lower bed of pebble to granule conglomerate in a sandy matrix, and grade up through coarse then medium plane parallel sandstone; through cross-bedded sandstone; rippled cross-bedded sand/silty sand, and finally laminar siltstone and shale. The Bouma Sequence (after Arnold H Bouma 1962 describes a classic set of sedimentary beds ( Turbidites) deposited by a sediment-water Turbidity current. Sandstone is a Sedimentary rock composed mainly of Sand -size Mineral or rock grains. This vertical succession of sedimentary structures, bedding, and changing lithology is representative of strong to waning flow regime currents and their corresponding sedimentation.
It is unusual to see all of a complete Bouma cycle, as successive turbidity currents may erode the unconsolidated upper sequences. Alternatively, the entire sequence may not be present depending on whether the exposed section was at the edge of the turbidity current lobe (where it may be present as a thin deposit), or upslope from the deposition centre and manifested as a scour channel filled with fine sands grading up into a pelagic ooze. Pelagic sediments, also known as marine sediments, are those that accumulate in the Abyssal plain of the deep ocean far away from terrestrial sources that provide
- See this diagram of the classical turbidite sequence, after Bouma (1962).
Formation
Turbidites are sediments which are transported and deposited by density flow, not by tractional or frictional flow. The density of a material is defined as its Mass per unit Volume: \rho = \frac{m}{V} Different materials usually have different Friction is the Force resisting the relative motion of two Surfaces in contact or a surface in contact with a fluid (e
The distinction is that, in a normal river or stream bed, particles of rock are carried along by frictional drag of water on the particle (known as tractional flow). "Riverine" redirects here For the use of that term in Maritime geography, see there The water must be travelling at a certain velocity in order to suspend the particle in the water and push it along. The greater the size or density of the particle relative to the fluid in which it is travelling, the higher the water velocity required to suspend it and transport it.
Density based flow, however, occurs when liquefaction of sediment during transport causes a change to the density of the fluid. This is usually achieved by highly turbulent liquids which have a suspended load of fine grained particles forming a slurry. In Fluid dynamics, turbulence or turbulent flow is a fluid regime characterized by chaotic Stochastic property changes In this case, larger fragments of rock can be transported at water velocities too low to otherwise do so because of the lower density contrast.
This condition occurs in many environments aside from simply the deep ocean, where turbidites are particularly well represented. Lahars on the side of volcanoes, mudslides and pyroclastic flows all create density based flow situations and, especially in the latter, can create sequences which are strikingly similar to turbidites. A lahar is a type of Mudflow / Landslide composed of Pyroclastic material and Water that flows down from a Volcano, typically Mudslide redirects here it is also the name of a cocktail. A mudflow or mudslide is the most rapid (up to 80 km/h / 50 mph A pyroclastic flow (also known as a pyroclastic density current) is a common and devastating result of some volcanic eruptions.
Turbidites in sediments can occur in carbonate as well as siliciclastic sequences.
Classic, low density turbidites are characterized by graded bedding, current ripple marks, alternating sequences with pelagic sediments, distinct fauna changes between the turbidite and native pelagic sediments, sole markings, thick sediment sequences, regular bedding, and an absence of shallow-water features. In Geology, a graded bed is one characterized by coarse Sediments at its base which grade upward into progressively finer ones An ocean current is continuous directed movement of Ocean water. In Geology, ripple marks are Sedimentary structures that indicate agitation by Water ( current or waves) or Wind. Any water in the sea that is not close to the bottom is in the pelagic zone. Fauna is all of the Animal life of any particular region or time Sole marks, or tool marks, are sedimentary structures found on the bottom of certain current-deposited strata that indicate small-scale current disruption or scour In Geology and related fields a stratum (plural strata) is a layer of rock or Soil with internally consistent characteristics that distinguishes (Fairbridge 1966)
Massive accumulations of turbidites and other deep water deposits may result in the formation of submarine fans. Abyssal fans, also known as deep-sea fans, underwater deltas, and submarine fans, are underwater structures that look like deltas formed at the Sedimentary models of such fan systems typically are subdivided into upper, mid, and lower fan sequences each with distinct sand-body geometries, sediment distributions, and lithologic characteristics. Sediment is any particulate matter that can be transported by fluid flow and which eventually is deposited as a layer of solid particles on the bed or bottom of a body of (Mutti & Ricci Lucci 1975, Normark 1978, & Walker 1978)
- See this diagram of various sedimentary features typical of different grain transport mechanisms.
Importance of turbidites
Turbidites provide a mechanism for assigning a tectonic and depositional setting to ancient sedimentary sequences as they usually represent deep water rocks formed offshore of a convergent margin, and generally require at least a sloping shelf and some form of tectonism to trigger density-based avalanches. In Plate tectonics, a convergent boundary – also known as a convergent plate boundary or a destructive plate boundary – is an actively deforming region
Turbidites from lakes are also important as they can provide chronologic evidence of the frequency of landslides and the earthquakes that presumably formed them, by dating varves above and below the turbidite. A varve is an annual layer of Sediment or Sedimentary rock. The word 'varve' is derived from the Swedish word varv whose
Economic geology of turbidites
Turbidite seqences are classic hosts for lode gold deposits, the prime example being Bendigo and Ballarat, Victoria, Australia, where over 2,600 tons of gold have been extracted from saddle reef deposits hosted in shale sequences from a thick succession of Cambrian-Ordovician turbidites. Gold (ˈɡoʊld is a Chemical element with the symbol Au (from its Latin name aurum) and Atomic number 79 Bendigo is a regional city in central Victoria Australia, located in the City of Greater Bendigo. Ballarat (formerly spelt "Ballaarat" is a city in Victoria, Australia, and one of the country's largest inland cities For a topic outline on this subject see List of basic Australia topics. Proterozoic gold deposits are also known from turbidite basin deposits. The Proterozoic (ˌproʊtərəˈzoʊɪk is a geological eon representing a period before the first abundant complex life on Earth.
Turbidites in the European Alps are also known to host stratiform tungsten (scheelite) deposits, associated with exhalative volcanic sequences in the Lower Palaeozoic inliers of the Erzgebirge. Tungsten (ˈtʌŋstən also known as wolfram (/ˈwʊlfrəm/ is a Chemical element that has the symbol W and Atomic number 74 Scheelite is a Calcium Tungstate Mineral with the Chemical formula Ca[[tungsten W]] O 4 The Paleozoic or Palaeozoic Era (from the Greek palaio (παλαιο "old" and zoe (ζωη "life" meaning "ancient life" One of the world's largest tungsten mines in Felbertal exploits this type of mineralization.
Lithified accumulations of turbidite deposits may, in time, become hydrocarbon reservoirs and the oil & gas industry makes strenuous efforts to predict the location, overall shape, and internal characteristics of these sediment bodies in order to efficiently develop fields as well as explore for new reserves. In Organic chemistry, a hydrocarbon is an Organic compound consisting entirely of Hydrogen and Carbon. Petroleum ( L petroleum, from Greek πετρέλαιον, lit Turbidite deposits typically occur in foreland basins. foreland basin is a depression that develops adjacent and parallel to a mountain belt Best outcrop expositions are found in Apennines (Italy), Pyrenees (Spain), and Western Alps (France). Italy (Italia officially the Italian Republic, (Repubblica Italiana is located on the Italian Peninsula in Southern Europe, and on the two largest The Pyrenees (Pirineos French: Pyrénées; Catalan: Pirineus; Occitan: Pirenèus; Aragonese: Perinés Spain () or the Kingdom of Spain (Reino de España is a country located mostly in southwestern Europe on the Iberian Peninsula. The Alps cover a large area This article describes the delimitation of the Alps as a whole and of subdivisions of the range follows the course of the main chain of the Alps and discusses This article is about the country For a topic outline on this subject see List of basic France topics.
External links
References
- Bouma, Arnold H. , 1962, Sedimentology of some Flysch deposits;: A graphic approach to facies interpretation, Elsevier, 168 pp.
- Fairbridge, Rhodes W. , ed. , (1966) The Encyclopedia of Oceanography, New York: Van Nostrand Reinhold Company, pp945-946.
- Mutti, E. & Ricci Lucci, F. (1975) Turbidite facies and facies associations. In: Examples of turbidite facies and associations from selected formations of the northern Apennines. IX Int. Congress of Sedimentology, Field TripA-11, 21-36.
- Normark, W. R. (1978) Fan valleys, channels, and depositional lobes on modern submarine fans : Characters for recognition of sandy turbidite environments. American Association of Petroleum Geologists Bulletin, Vol. 62, p. 912-931.
- Ødegård, Stefan (2000) http://home.no.net/chemo/Thesis/Details/Sedimentology.htm Sedimentology of the Gred d'Annot Formation at the Technische Universität Clausthal, Germany. Retrieved 01/27/2006
- Walker, R. G. (1978) Deep-water sandstone facies and ancient submarine fans: model for exploration for stratigraphic traps. in Am. Assoc. Petrol. Geol. Bull. , 62/6,932-966.
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