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A contourite is a sedimentary deposit produced by deepwater bottom currents, which result from thermohaline, wind or tidal forces.[1] Most contourites are formed in continental rise to lower slope settings, although they may occur anywhere that is below storm wave base. Accumulations of contourite deposits are generally referred to as drifts.

Contents

Lithology

The lithology of contourites vary depending partly on the sediments present at the time of onset of bottom current deposition. As they mainly occur in deepwater they are generally formed of fine-grained pelagic sediments. Sandy contourites generally occur at intermediate water depths, 300–2000 m, in areas where sands have been transported in from a nearby continental margin. Contourite deposits vary from poorly to very well-sorted.[2]

Drift Morphology

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Mounds

Mounded drifts form the most obvious group of contourites. They are relatively easy to identify from their generally elongate mounded shape. They are also characterised by an 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.[1]

Detached drifts

Detached drifts are isolated and migrate downslope.[1]

Separated drifts

Typically asymmetric in shape, tend to form at the base of a slope and migrate up-slope.[1]

Plastered drifts

These mounded drifts form as isolated areas of contourite deposition within otherwise non-depositional or erosional areas, typically on a gentle slope. They mainly migrate along-slope.[1]

Confined drifts

Confined drifts form between pre-existing structural highs.[1]

Sheets

As the name implies, sheeted drift deposits have relatively constant thickness over quite large areas and are characteristic of very deep water.[1]

Sediment wave fields

This variety of sheeted drift is generally located near the rise to slope transition. Seismic reflection profiles show that the sediment waves tend to migrate up-slope.[3]

Formation

The formation of contourites requires a sufficient supply of sediment and the presence of bottom currents to deposit and/or rework the sediment.[4] Bottom currents transport sediment as bedload. Subtle changes in seabed topography can lead to variations in current strength causing either deposition or erosion and the location of contourite drifts is controlled by this topography.

Occurrence

Present day

Contourite deposition is active in many locations throughout the world, but particularly in areas affected by the thermohaline circulation.

Ancient examples

Identifying contourites in ancient sedimentary sequences is difficult as their distinctive morphology becomes obscured by the effects of later sedimentation, erosion and compaction. Most examples of contourites identified in the geological record come from the Cenozoic but examples have been noted from as far back as the Jurassic.[5]

See also

References


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