A turbidite is the
geologic deposit of a
turbidity current
A turbidity current is most typically an underwater current of usually rapidly moving, sediment-laden water moving down a slope; although current research (2018) indicates that water-saturated sediment may be the primary actor in the process. T ...
, which is a type of amalgamation of fluidal and
sediment gravity flow
upright=1.25, This turbidite from the Devonian Becke-Oese Sandstone">Devonian.html" ;"title="turbidite from the Devonian">turbidite from the Devonian Becke-Oese Sandstone of Germany is an example of a deposit from a sediment gravity flow. Note th ...
responsible for distributing vast amounts of
clastic
Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus,Essentials of Geology, 3rd Ed, Stephen Marshak, p. G-3 chunks, and smaller grains of rock broken off other rocks ...
sediment into the
deep ocean.
Sequencing
Turbidites were first properly described by Arnold H. 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. 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
Sedimentary structures include all kinds of features in sediments and sedimentary rocks, formed at the time of deposition.
Sediments and sedimentary rocks are characterized by bedding, which occurs when layers of sediment, with different partic ...
, 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.
It is now recognized that the vertical progression of sedimentary structures described by Bouma applies to turbidites deposited by low-density turbidity currents. As the sand concentration of a flow increases, grain-to-grain collisions within the turbid suspension create dispersive pressures that become important in hindering further settling of grains. As a consequence, a slightly different set of sedimentary structures develops in turbidites deposited by high-density turbidity currents. This different set of structures is known as the
Lowe sequence
The Lowe sequence describes a set of sedimentary structures in turbidite sandstone beds that are deposited by high-density turbidity currents. It is intended to complement, not replace, the better known Bouma sequence, which applies primarily to t ...
, which is a descriptive classification that complements, but does not replace, the Bouma sequence.
[Lowe, D.R. (1982), ''Sediment gravity flows: II. Depositional models with special reference to the deposits of high-density turbidity currents,'' Journal of Sedimentology, Society of Economic Paleontologists and Mineralogists, v. 52, p. 279-297.]
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 (that is, the water plus sediment has a higher density than the water and is therefore closer to the density of the rock).
This condition occurs in many environments aside from simply the deep ocean, where turbidites are particularly well represented.
Lahars
A lahar (, from jv, ꦮ꧀ꦭꦲꦂ) is a violent type of mudflow or debris flow composed of a slurry of pyroclastic material, rocky debris and water. The material flows down from a volcano, typically along a river valley.
Lahars are extremel ...
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
In geology, a graded bed is one characterized by a systematic change in grain or clast size from one side of the bed to the other. Most commonly this takes the form of normal grading, with coarser sediments at the base, which grade upward into pro ...
,
current
Currents, Current or The Current may refer to:
Science and technology
* Current (fluid), the flow of a liquid or a gas
** Air current, a flow of air
** Ocean current, a current in the ocean
*** Rip current, a kind of water current
** Current (stre ...
ripple marks
In geology, ripple marks are sedimentary structures (i.e., bedforms of the lower flow regime) and indicate agitation by water ( current or waves) or wind.
Defining ripple cross-laminae and asymmetric ripples
* ''Current ripple marks'', ''u ...
, climbing ripple laminations, alternating sequences with
pelagic sediments, distinct
fauna changes between the turbidite and native pelagic sediments,
sole markings
Sole marks are sedimentary structures found on the bases of certain strata, that indicate small-scale (usually on the order of centimetres) grooves or irregularities. This usually occurs at the interface of two differing lithologies and/or grai ...
, thick sediment sequences, regular
bedding
Bedding, also known as bedclothes or bed linen, is the materials laid above the mattress of a bed for hygiene, warmth, protection of the mattress, and decorative effect. Bedding is the removable and washable portion of a human sleeping environme ...
, and an absence of shallow-water features. A different vertical progression of sedimentary structures characterize
high-density turbidites.
Massive accumulations of turbidites and other deep-water deposits may result in the formation of
submarine fan
Abyssal fans, also known as deep-sea fans, underwater deltas, and submarine fans, are underwater geological structures associated with large-scale sediment deposition and formed by turbidity currents. They can be thought of as an underwater ver ...
s. 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.
Turbidite deposits typically occur in
foreland basin
A foreland basin is a structural basin that develops adjacent and parallel to a mountain belt. Foreland basins form because the immense mass created by crustal thickening associated with the evolution of a mountain belt causes the lithospher ...
s.
Submarine Fan Models
Submarine fan models are often based on source-to-sink
2Sconcepts linking sediment source areas, and sediment routing systems to the eventual depositional environments of turbidite deposits. They are aimed at providing insights into the relationships between different geologic processes and turbidite fan systems.
Geologic processes influencing turbidite systems can either be of allogenic or autogenic origin and submarine fan models are designed to capture the impact of these processes on reservoir presence, reservoir distribution, morphology, and architecture of turbidite deposits. Some significant allogenic forcing includes the effect of sea level fluctuations, regional tectonic events, sediment supply type, sediment supply rate, and sediment concentration. Autogenic controls can include seafloor topography, confinements, and slope gradients.
There are about 26 submarine fan models. Some common fan models include the classical single-source suprafan model, models depicting fans with attached lobes, detached lobes fan model, and submarine fan models relating to the response of turbidite systems to varying grain sizes and different feeder systems.
The integration of subsurface datasets such as 3D/4D seismic reflection, well logs, and core data as well as modern seafloor bathymetry studies, numerical forward stratigraphic modeling, and flume tank experiments are enabling improvements and more realistic development of submarine fan models across different basins.
Importance
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
Tectonics (; ) are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of mountain building, the growth and behavior of the strong, old cores of continents k ...
to trigger density-based avalanches. Density currents may be triggered in areas of high sediment supply by gravitational failure alone. Turbidites can represent a high resolution record of seismicity, and terrestrial storm/flood events depending on the connectivity of canyon/channel systems to terrestrial sediment sources.
Turbidites from lakes and fjords are also important as they can provide chronologic evidence of the frequency of landslides and the earthquakes that presumably formed them, by dating using radiocarbon or
varves above and below the turbidite.
[Enkin et al., 2013]
Economic importance
Turbidite sequences are classic hosts for lode
gold deposits, the prime example being
Bendigo
Bendigo ( ) is a city in Victoria, Australia, located in the Bendigo Valley near the geographical centre of the state and approximately north-west of Melbourne, the state capital.
As of 2019, Bendigo had an urban population of 100,991, maki ...
and
Ballarat
Ballarat ( ) is a city in the Central Highlands of Victoria, Australia. At the 2021 Census, Ballarat had a population of 116,201, making it the third largest city in Victoria. Estimated resident population, 30 June 2018.
Within months of Vi ...
in
Victoria, Australia
Victoria is a state in southeastern Australia. It is the second-smallest state with a land area of , the second most populated state (after New South Wales) with a population of over 6.5 million, and the most densely populated state in A ...
, where more than 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.
Lithified accumulations of turbidite deposits may, in time, become
hydrocarbon reservoirs and the
petroleum 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.
See also
*
Contourite
A contourite is a sedimentary deposit commonly formed on continental rise to lower slope settings, although they may occur anywhere that is below storm wave base. Countourites are produced by thermohaline-induced deepwater bottom currents and m ...
*
Flysch
Flysch () is a sequence of sedimentary rock layers that progress from deep-water and turbidity flow deposits to shallow-water shales and sandstones. It is deposited when a deep basin forms rapidly on the continental side of a mountain building ...
*
High-density turbidity currents
The Lowe sequence describes a set of sedimentary structures in turbidite sandstone beds that are deposited by high-density turbidity currents. It is intended to complement, not replace, the better known Bouma sequence, which applies primarily to tu ...
(
Lowe sequence
The Lowe sequence describes a set of sedimentary structures in turbidite sandstone beds that are deposited by high-density turbidity currents. It is intended to complement, not replace, the better known Bouma sequence, which applies primarily to t ...
)
*
Sediment gravity flows
upright=1.25, This turbidite from the Devonian Becke-Oese Sandstone">Devonian.html" ;"title="turbidite from the Devonian">turbidite from the Devonian Becke-Oese Sandstone of Germany is an example of a deposit from a sediment gravity flow. Note th ...
*
Bouma sequence
300px, thumbnail, Turbidite from the Devonian-age Becke-Oese Sandstone">Devonian.html" ;"title="Turbidite from the Devonian">Turbidite from the Devonian-age Becke-Oese Sandstone, Germany showing a complete Bouma sequence.
The Bouma Sequence (afte ...
References
* Bouma, Arnold H. (1962) ''Sedimentology of some Flysch deposits: A graphic approach to facies interpretation'', Elsevier, Amsterdam, 168 p.
* Randolph J. Enkin, Audrey Dallimore, Judith Baker, John R. Southon, Tara Ivanochkod; 2013 ''A new high-resolution radiocarbon Bayesian age model of the Holocene and Late Pleistocene from core MD02-2494 and others, Effingham Inlet, British Columbia, Canada; with an application to the paleoseismic event chronology of the Cascadia Subduction Zone1''; Geological Survey of Canada-Pacific, Sidney, BC V8L 4B2, Canada
Article link* Fairbridge, Rhodes W. (ed.) (1966) ''The Encyclopedia of Oceanography'', Encyclopedia of earth sciences series 1, Van Nostrand Reinhold Company, New York, p. 945–946.
* Goldfinger, C., Nelson, C.H., Morey, A., Johnson, J.E., Gutierrez-Pastor, J., Eriksson, A.T., Karabanov, E., Patton, J., Gracia, E., Enkin, R., Dallimore, A., Dunhill, G., and Vallier, T., 2012, Turbidite Event History: Methods and Implications for Holocene Paleoseismicity of the Cascadia Subduction Zone, USGS Professional Paper 1661-F, Reston, VA, U.S. Geological Survey, p. 184 p, 64 Figures. http://pubs.usgs.gov/pp/pp1661f/
* Moernaut, J., De Batist, M., Charlet, F., Heirman, K., Chapron, E.,
Pino, M., Brümmer, R., and Urrutia, R., 2007, Giant earthquakes in South-Central Chile revealed by Holocene mass-wasting events in Lake Puyehue: ''
Sedimentary Geology
''Sedimentary Geology'' is a peer-reviewed scientific journal
In academic publishing, a scientific journal is a periodical publication intended to further the progress of science, usually by reporting new research.
Content
Articles in sc ...
'', v. 195, p. 239–256.
* 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 Trip A-11, p. 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'', 62 (6), p. 912–931.
* Ødegård, Stefan (2000) ''Sedimentology of the Grès d'Annot Formation'', Thesis: Technische Universität Clausthal, Germany. Retrieved 27 January 2006
* Strasser, M., Anselmetti, F.S., Fäh, D., Giardini, D., and Schnellmann, M., 2006, Magnitudes and source areas of large prehistoric northern Alpine earthquakes revealed by slope failures in lakes: Geology, v. 34, p. 1005–1008.
* Walker, R.G. (1978) "Deep-water sandstone facies and ancient submarine fans: model for exploration for stratigraphic traps", ''American Association of Petroleum Geologists Bulletin'', 62 (6), p. 932–966.
Further reading
*
*
* Lowe, D.R. (1982), ''Sediment gravity flows: II. Depositional models with special reference to the deposits of high-density turbidity currents,'' Journal of Sedimentology, Society of Economic Paleontologists and Mineralogists, v. 52, p. 279–297.
External links
Turbidite sedimentary processes in carbonates, Trenton Formation.{{Webarchive, url=https://web.archive.org/web/20151118072337/http://www.mcz.harvard.edu/Departments/InvertPaleo/Trenton/Intro/GeologyPage/Sedimentary%20Geology/sedprocessesstructures.htm#gravity , date=2015-11-18
Physical oceanography
Sedimentary rocks
Sedimentology