Turbidite

  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.

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The ideal turbidite sequence

  Turbidites were first properly described by Bouma (1962), who studied deepwater sediments and recognised particular fining up intervals within deep water, fine grained shales, which were anomalous because they started at pebble conglomerates and terminated in shales.

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. 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.

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 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). 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 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.

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. (Fairbridge 1966)

Massive accumulations of turbidites and other deep water deposits may result in the formation of submarine fans. 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. (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.

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.

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. Proterozoic gold deposits are also known from turbidite basin deposits.

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. 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. Turbidite deposits typically occur in foreland basins. Best outcrop expositions are found in Apennines (Italy), Pyrenees (Spain), and Western Alps (France).

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.