Contourite
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A contourite is a sedimentary deposit commonly formed on continental rises in lower slope settings, although it may occur anywhere that is below the storm wave base. Countourites are produced by thermohaline-induced deepwater bottom currents and may be influenced by wind or tidal forces.[1][2] The geomorphology of contourite deposits is mainly influenced by the deepwater bottom-current velocity, sediment supply, and seafloor topography.[3]
Definition
The definition of the term contourite has varied throughout the decades. Originally, Heezen et al. (1966)
Flow conditions
Periodically, velocities may increase dramatically or even reverse due to
Sediment supply
Erosion of the seafloor contributes to the growth of a deepwater nepheloid layer. This layer plays a key role in supplying the sediment for the deposition of contourites under appropriate flow conditions.[3]
Geomorphology
The accumulation and geomorphology of contourite deposits are mainly influenced by three factors: the intensity of deepwater bottom currents, seafloor topography, and sediment supply.[3] There are five main types of contourite accumulations: giant elongate drifts, contourite sheets, channel-related drifts, confined drifts, and modified drift-turbidite systems.[3][15]
Giant elongated drifts
Giant elongate drifts form very large mounded elongated geometries parallel to the deepwater bottom-current flow. They are characterised by a near-complete lack of parallel bedding. Mounded drifts are often bounded on one or both sides by non-depositional or erosional channels, sometimes known as moats.
Contourite sheets
Contourite sheets are broad, low-relief features that extend through very large areas (i.e., ~1,000,000 km2) and are seen covering the abyssal plains or even plastered against the continental margins.[3] They are characteristic of very deep water.[2] They have a relatively constant thickness of up to a few hundred metres with a slight thinning towards the continental margin.[15]
Sediment wave fields are a variety that are generally located near the rise-to-slope transition. Seismic reflection profiles show that the sediment waves tend to migrate up-slope.[16]
Flank deposits are usually patchy and small (tens of km2), can be elongate and subparallel to the flow direction, and may have a sheeted or mounded geometry. At the down-current exit of the channel, flow velocity decreases dramatically, and a cone-shaped contourite fan is formed, which is much larger than the flank deposits, measuring about 100 km in radius and about 300 m in thickness. Channel floor deposits can be patchy and contain sand, gravel, and mud clasts in the form of a channel lag.[15]
Confined drifts
Confined drifts are contourite accumulations that occur within small basins. The basins in which they form tend to be tectonically active in order to allow for the topographic confinement of the deposit.[15]
Modified drift-turbidite systems
Modified drift-turbidite systems refer to the interactions of contourite and turbidite deposits. These can be observed as modifications of one another, depending on the dominant process at the time. Examples range from asymmetric turbidite channel levees caused by strong deepwater bottom-currents, as seen in the
Distinguishing turbidites, contourites, and bottom-current modified turbidite deposits is essential for reconstructing the paleoenvironment in deepwater settings. Traction structures, such as cross-stratification, indicate bottom-current reworking because it is more likely to have avalanches in clear bottom-currents than it is in sediment-saturated turbidity flows.[18] Deposition from suspension in turbidity flows does not generate a sharp upper contact as bottom-current reworked deposits show due to the highly oscillating energy conditions. Stanley (1993)[17] proposes that the transition from a turbidite to a contourite involves a continuous transition from a sandy deposit to lenticular bedding passing through wavy bedding.
Occurrence
Present day
Contourite deposition is active in many locations throughout the world, but particularly in areas affected by the thermohaline circulation.[where?]
Ancient examples
Identifying contourites in ancient sedimentary sequences is difficult as their distinctive morphology[clarification needed] becomes obscured by the effects of later bioturbation, sedimentation, erosion, and compaction. Most examples of contourites identified in the geological record come from the Cenozoic, but examples have been noted as far back as the Ediacaran.[19]
See also
References
- ^ .
- ^ ISBN 978-0-444-52998-5
- ^ .
- ^ S2CID 29313948.
- ^ Hollister, C.D.; Heezen, B.C. (1972). "Geologic effects of ocean bottom-currents: western north Atlantic". In: Studies in Physical Oceanography. 2: 37–66.
- ^ .
- ^ doi:10.2204/iodp.pr.339.2012.)
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: CS1 maint: numeric names: authors list (link - ISBN 0-13-154728-3.
- .
- ^ S2CID 4365998.
- S2CID 14555444.
- ISBN 0-444-52161-5.
- S2CID 129494147.
- .
- ^ S2CID 128678734.
- S2CID 14555444.
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- doi:10.1139/e96-064.