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Abyssal channels (also, deep-sea channels, underwater channels) are
channels Channel, channels, channeling, etc., may refer to: Geography * Channel (geography), in physical geography, a landform consisting of the outline (banks) of the path of a narrow body of water. Australia * Channel Country, region of outback Austral ...
in Earth's sea floor. They are formed by fast-flowing floods of turbid water caused by
avalanche An avalanche is a rapid flow of snow down a slope, such as a hill or mountain. Avalanches can be set off spontaneously, by such factors as increased precipitation or snowpack weakening, or by external means such as humans, animals, and eart ...
s near the channel's head, with the
sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sa ...
carried by the water causing a build-up of the surrounding
abyssal plain An abyssal plain is an underwater plain on the deep ocean floor, usually found at depths between and . Lying generally between the foot of a continental rise and a mid-ocean ridge, abyssal plains cover more than 50% of the Earth's surface. ...
s. Submarine channels and the turbidite systems which form them are responsible for the accumulation of most sandstone deposits found on continental slopes and have proven to be one of the most common types of hydrocarbon reservoirs found in these regions. Submarine channels and their flanking
levees A levee (), dike (American English), dyke (Commonwealth English), embankment, floodbank, or stop bank is a structure that is usually earthen and that often runs parallel to the course of a river in its floodplain or along low-lying coastli ...
are commonly referred to as ''channel levee systems''. They are significant geomorphological features that may run for thousands of kilometres across the ocean floor. Often, they coalesce and overlap to form channel levee complexes which are the building blocks of many major submarine fans. This makes them one of several geological processes responsible for the transport of coarse-grained sediment into deep water, as well as being a chief conduit for the transfer of
carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon mak ...
from the continental shelf to the deeper parts of the continental margins. They do, however, remain one of the least understood sedimentary processes. The effect of Earth's rotation causes more sediment to build up on one side of the channel than on the other. What constitutes a channel is not straight forward. Different terms are used on a per study basis, all of which have similar but not quite interchangeable definitions. There have been efforts to produce an up-to-date, holistic view, but even since then there has been a significant number of papers which take concepts even further.Wynn et al., 2007 There are numerous terms that are used to describe the features contained in this study, including ''geo-body'', ''channel complex'', ''channel storey'', ''channel complex set'', and ''confined channel complex system''. These cover single channels, a single channel and associated sediments or multiple channels grouped. Flood (2001) defines a ''channel-levee system'' as a single channel with a levee at each side. These levees are formed by the overspilling and flow stripping of
turbidity currents 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 ...
. These are most likely to occur during
sea level Mean sea level (MSL, often shortened to sea level) is an average surface level of one or more among Earth's coastal bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical datuma standardise ...
lowstands. A collection of these channels and levees along with overbank sediments form a channel-levee complex. They can be V or U in shape, have the presence or lack of depositional margins, highly
sinuous Sinuosity, sinuosity index, or sinuosity coefficient of a continuously differentiable curve having at least one inflection point is the ratio of the curvilinear length (along the curve) and the Euclidean distance (straight line) between the ...
or straight.


Architecture and nomenclature

Ian Kane advocates the use of the terms ''internal levee'' and ''external levee'' to avoid confusion in the literature concerning the use of "inner" and "outer" levees. To help encourage this unification of phrases into a clearer architectural hierarchy, this study will use Kane's nomenclature. External levees are a dominantly depositional body forming a constructional wedge of sediment that thins perpendicularly away from a channel-belt. The ''external levee'' forms during the evolution of a genetically related channel-belt (or slope valley, channel fairway) by flows that partially spill out of their confinement. External levees can confine adjacent channel belts to form levee-confined systems. External levees may be much less sinuous than the levees of an individual channel-levee system as they do not follow one particular channel but may be the product of overspill from one or more channels or channel-levee systems meandering within the wider channel-belt. The ''levee crest'' is the highest point of the external levee, and runs parallel to the course of the channel-belt, separating the external levees into outer external levees and inner external levees. Internal levees are constructional features fed by flows that partially spilled out of channelised confinement, but were largely unable to escape the confinement of the channel-belt. The flows which build internal levees may interact with the main confining surface, i.e., the external levees, and/or the channel-belt
erosion surface In geology and geomorphology, an erosion surface is a surface of rock (geology), rock or regolith that was formed by erosion and not by construction (e.g. lava flows, sediment deposition) nor fault (geology), fault displacement. Erosional surfaces ...
, and are liable to erosion by the migration or avulsions of channel
thalweg In geography and fluvial geomorphology, a thalweg or talweg () is the line of lowest elevation within a valley or watercourse. Under international law, a thalweg is the middle of the primary navigable channel of a waterway that defines the boun ...
s, and the overbank passage of large flows not confined by the internal levees. As a consequence of lateral migration, internal levees may be better preserved on inner bends. Internal levees form only when confinement has been established, through the construction of external levees and/or the degradation and entrenchment of the composite erosion surface of the channel-belt, or confined within canyons. Internal levees may form distinct wedges of sediment where enough space is available; where space is limited, i.e., where overspill from underfit channels interacts with external levees or erosional confinement, overspill deposits may appear superficially similar to terrace deposits, which are widely identified in the subsurface.


Channel sinuosity and migration

Sinuosity in submarine channels is a feature regularly observed on seismic maps. It can vary between occasional low
amplitude The amplitude of a periodic variable is a measure of its change in a single period (such as time or spatial period). The amplitude of a non-periodic signal is its magnitude compared with a reference value. There are various definitions of am ...
bends to highly sinuous, densely looping channels. Channel sinuosity results in significant migration lateral and affects continuity of facies associated with both channel sediments and surrounding deep water sediments. Although it is not always clear how these sinuosities evolve, they typically do not result from a random wandering. In most cases, the wandering and changes in sinuosity is as a result of external forces. As a result of this, PJeff Peakall advocates the avoidance of the term ''meandering'' to describe this sinuosity, a phrase used to describe similar sinuosity observed in
terrestrial Terrestrial refers to things related to land or the planet Earth. Terrestrial may also refer to: * Terrestrial animal, an animal that lives on land opposed to living in water, or sometimes an animal that lives on or near the ground, as opposed to ...
fluvial systems. There seems to be a potential consensus that truly sinuous channel can be defined as one that displays a minimum average
sinuosity Sinuosity, sinuosity index, or sinuosity coefficient of a continuously differentiable curve having at least one inflection point is the ratio of the curvilinear length (along the curve) and the Euclidean distance (straight line) between the en ...
of between 1.2 and 1.15. Difficulty with rigorous application of these values is that relatively straight channels may locally exceed them and some sinuous channels may display peak sinuosity values well in excess. The sinuosity of submarine channels is a characteristic instantly recognizable as being shared with fluvial systems. In recent years there are increasingly mixed opinions in academic literature as to how far they are analogous to each other with some feeling that such notions of similarity should not hold. The best description is that the two are similar in some ways but more variable and complex in other. This applies to both the geometry of morphological features, the processes involved in forming them as well as the character of the deposits formed. Mike Mayall provides the best summary that discusses the causes of sinuosity. Factors involve: flow dynamics such as flow density and flow velocity; and the depth of the current relative to topography; and
topographic Topography is the study of the forms and features of land surfaces. The topography of an area may refer to the land forms and features themselves, or a description or depiction in maps. Topography is a field of geoscience and planetary scien ...
and morphological controls such; shape channel
cross section Cross section may refer to: * Cross section (geometry) ** Cross-sectional views in architecture & engineering 3D *Cross section (geology) * Cross section (electronics) * Radar cross section, measure of detectability * Cross section (physics) **Abs ...
, slope topography, erosive base at flow onset and the effects of both lateral stacking and lateral accretion. Compared to their terrestrial cousins, the scale of submarine systems observed in seismic sections, aerial photos and rock outcrops are in no way comparable. As expected with this significant difference in scale, the dynamics of turbid current flows within submarine channels are significantly different from fluvial systems. These differences in dynamics and scale are due to the much lower density contrast between the flow and the host fluid is much lower in submarine channels than that of open channel flows with a free surface. This causes the flow to be significantly super-elevated about the channel margin causing overspill and building the levees. Lateral migration and accretion plays an important part in fluvial systems. It is the feature of submarine channels that is most analogous with its terrestrial counterpart. It consists of erosion on the outerbank and deposition on the inner bank as a point bar. However, there are significant dissimilarities the biggest in that submarine channels can exhibit both lateral and vertical migration. Fluvial systems do not exhibit this vertical component. Lateral accretion packages are believed to form as a result of depositional rather than topographical forcing. This lateral migration only style of sinuosity is believed to be somewhat rare in occurrence within turbidite systems. Vertical migration is exhibited in submarine channels systems in the form of channel stacking. As flows in channels subside, channels are infilled with sediment. When the flow is re-initiated, there is then a slight shift laterally in the flow
thalweg In geography and fluvial geomorphology, a thalweg or talweg () is the line of lowest elevation within a valley or watercourse. Under international law, a thalweg is the middle of the primary navigable channel of a waterway that defines the boun ...
causing a displaced incision. Mayall suggests that this vertical movement could be as a result of changes in seafloor topography due to salt/shale tectonics or fault movement. The other alternative they suggest is through undefined “depositional processes”. One potential process may be as a result of heterogeneous infilling of the older channel forming an offset conduit for later flows. Whatever the process this stacking plays an important role in
aggradation Aggradation (or alluviation) is the term used in geology for the increase in land elevation, typically in a river system, due to the deposition of sediment. Aggradation occurs in areas in which the supply of sediment is greater than the amount of ...
al systems and potentially is one of the leading controls in the formation of levee confined complexes. In terms of sinuosity, Mayall hows that this vertical migration occurs on the outward sides of bends reinforcing any pre-existing curvature. Aggradational channels commonly form where the slope is “below grade.” This results in the deposition of broad, amalgamated and highly sand rich channels which are significantly affected by the slope morphology. The channel width versus slope relationship is control by the Froude number of flows along the channel. When Froude numbers are low (<1.0) channel widths remain constant, however when Froude number oscillate around unity, channel widths fall rapidly with channel-floor slope. This provides a mechanism for generating channel widths capable of maintaining near-critical flow by channel narrowing and enhanced sedimentation. This behavior is controlled by an unknown constant that could not be found experimentally. The morphology and topography of the slope that any
turbidite channel 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 ...
crosses is inevitably going to affect the geometry of the channel. This can result in subtle changes in channel path to major diversions in channel flow. Topographic influences can come in the form of the surface expression of faults or changes in topography as a result of salt/shale tectonics, whether through diapirism or subsurface folding.


Underwater waves

Underwater channels can carry
underwater wave A seiche ( ) is a standing wave in an enclosed or partially enclosed body of water. Seiches and seiche-related phenomena have been observed on lakes, reservoirs, swimming pools, bays, harbors, caves and seas. The key requirement for formation ...
s.


See also

* Abyssal fan * List of landforms#Coastal and oceanic landforms * Northwest Atlantic Mid-Ocean Channel *
Sea mount A seamount is a large geologic landform that rises from the ocean floor that does not reach to the water's surface (sea level), and thus is not an island, islet or cliff-rock. Seamounts are typically formed from extinct volcanoes that rise abru ...
* Sea basin * Sea canyon *
Submarine canyon A submarine canyon is a steep-sided valley cut into the seabed of the continental slope, sometimes extending well onto the continental shelf, having nearly vertical walls, and occasionally having canyon wall heights of up to 5 km, from c ...
*
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 ...


References

{{reflist Marine geology