Contourite

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.<ref name="Hollister1993">{{cite journal |first1=C.D. |last1=Hollister|title=The concept of deep-sea contourites |journal=Sedimentary Geology |year=1993 |volume=82 |issue=1–4|pages=5–11 |bibcode=1993SedG...82....5H |doi=10.1016/0037-0738(93)90109-I }}</ref><ref name="Rebesco">[https://books.google.com/books?id=D7JLNLvWeMsC&dq=contourites+definition&pg=PT432 Rebesco, M. & Camerlenghi, A. 2008. Contourites, Elsevier Science, 688pp.] {{ISBN|978-0-444-52998-5}}</ref> The geomorphology of contourite deposits is mainly influenced by the deepwater bottom-current velocity, sediment supply, and seafloor topography.<ref name="Faugeresetal1993">{{cite journal |first1=J.-C. |last1=Faugères |first2=M.L. |last2=Mézerais |first3=D.A.V |last3=Stow |title=Contourite drift types and their distribution in the North and South Atlantic Ocean basins |journal=Sedimentary Geology |year=1993 |volume=8 |issue=1–4 |pages=189–203 |doi=10.1016/0037-0738(93)90121-k|bibcode = 1993SedG...82..189F }}</ref>

==Definition==

The definition of the term contourite has varied throughout the decades. Originally, Heezen ''et al.'' (1966)<ref name="Heezen1966">{{cite journal |first1=B.C. |last1=Heezen |first2=C.D. |last2=Hollister |first3=W.F. |last3=Ruddiman |title=Shaping of the continental rise by deep geostrophic contour currents |journal=Science |year=1966 |volume=152 |issue=3721 |pages=502–508 | doi=10.1126/science.152.3721.502 |bibcode = 1966Sci...152..502H |pmid=17815077|s2cid=29313948 }}</ref> defined the concept, without using the actual word, as a sedimentary deposit on the continental rise derived from thermohaline-induced geostrophic bottom-currents that flow parallel to bathymetric contours. They did this to emphasise the difference between these deposits and turbidites in order to explain the ubiquitous smoothness and lack of irregularities of the continental rise in the Blake-Bahama Basin. Before this, it was thought that only turbidity flows were capable of depositing and reworking sediment at depths greater than the continental slope.<ref name=Hollister1993 /> Hollister and Heezen (1972)<ref name="Hollister1972">{{cite journal |first2=B.C. |last2=Heezen |first1=C.D. |last1=Hollister |title=Geologic effects of ocean bottom-currents: western north Atlantic |journal=In: Studies in Physical Oceanography |year=1972 |volume=2 |pages=37–66}}</ref> adopted the name contourite for these deposits and provided a list of characteristics that described their sediments. Faugères and Stow (1993)<ref name="FaugeresStow">{{cite journal |first1=J.-C. |last1=Faugères |first2=D.A.V |last2=Stow |title=Bottom-current-controlled sedimentation: a synthesis of the contourite problem |journal=Sedimentary Geology |year=1993 |volume=82 |issue=1–4 |pages=287–297|bibcode = 1993SedG...82..287F |doi = 10.1016/0037-0738(93)90127-Q }}</ref> note that as research on the subject developed, the term contourite was used to describe various forms of sedimentary deposits from bottom-currents, including those at much shallower depths and even in lacustrine settings. They suggested going back to the original definition of a contourite, that is, deposits at depths greater than 500 m derived from stable thermohaline-induced geostrophic bottom-currents (i.e., deepwater bottom-currents), in order to avoid using the same name when describing sedimentary deposits formed by different processes. They also suggest the umbrella term '''bottom-current deposit''', which includes contourites and deposits generated by other bottom-currents.

==Flow conditions== [[File:Contourite bottom water flow IODP Gulf of Cadiz.png|thumb|350px|right|Bottom current flow in the Gulf of Cadiz<ref name="IODP2012">{{cite journal |last1=IODP Expedition 339 Scientists |title=Mediterranean outflow: environmental significance of the Mediterranean Outflow Water and its global implications |journal=IODP Preliminary Report |year=2012 |volume=339 |doi=10.2204/iodp.pr.339.2012|doi-access=free }}</ref>]]

Thermohaline circulation is the principal driving force of deepwater bottom currents. The term refers to the movement of water over large distances as a consequence of global oceanic density gradients. This circulation commonly travels at velocities between 2 and 20 cm/s.<ref name="Heezen1966" /> Note that at this velocity range, considering the general shape of the Shields diagram<ref name="Boggs2006">{{cite book |title=Principles of Sedimentology and Stratigraphy |author=Sam Boggs Jr. |chapter=Ch. 2: Transport and Deposition of Siliciclastic Sediment |pages=30–31 |isbn=0-13-154728-3 |year=2006 |publisher=Prentice Hall}}</ref><ref name="Miller1977">{{cite journal |first1=M.C. |last1=Miller |first2=I.N. |last2=McCave |first3=P.D. |last3=Komar |title=Threshold of sediment motion under unidirectional currents |journal=Sedimentology |year=1977|volume=24 |issue=4 |pages=507–527|bibcode = 1977Sedim..24..507M |doi = 10.1111/j.1365-3091.1977.tb00136.x }}</ref> still holds in these conditions, a flow will only be able to continue transporting finer sediment that is already in suspension but will not be able to erode the same-sized sediment once it is deposited. However, flow velocity may be intensified as a consequence of the Coriolis force driving currents west against continental margins or as current squeezes between two ridges.<ref name="Faugeresetal1993" />

Periodically, velocities may increase dramatically or even reverse due to atmospheric storms raising the local surface eddy kinetic energy, which gets partially transmitted down to abyssal depths in episodes called benthic storms.<ref name="HollisterMcCave1984">{{cite journal |first1=C.D. |last1=Hollister |first2=I.N. |last2=McCave |title=Sedimentation under deep-sea storms | issue = 5965|journal=Nature |year=1984|volume=309 |pages=220–225 | doi = 10.1038/309220a0|bibcode = 1984Natur.309..220H |s2cid=4365998 }}</ref> These velocities may reach magnitudes well above 40 cm/s and vary significantly depending on the specific location. At the lower continental rise, south of Halifax, Nova Scotia,<ref name="HollisterMcCave1984" /> and at the lower slope around the Faeroe Islands<ref name="Damuth2001">{{cite journal |first1=J.E. |last1=Damuth |first2=H.C. |last2=Olson |title=Neogene-Quaternary contourite and related deposition on the West Shetland Slope and Faeroe-Shetland Channel revealed by high-resolution seismic studies |journal=Marine Geophysical Researches |year=2001|volume=22 |issue=5/6 |pages=369–399|doi=10.1023/A:1016395515456 |bibcode=2001MarGR..22..369D |s2cid=14555444 }}</ref> these velocities may reach up to 73 cm/s and 75 cm/s, respectively. Bottom-current flow velocities have been measured as high as 300 cm/s in the Strait of Gibraltar.<ref name="Shanmugambook">{{cite book |title=Deepwater Processes and Facies Models: Implications for Sandstone Petroleum Reservoirs |author=G. Shanmugam |chapter=Ch. 4: Deep-water bottom currents |pages=85–139 |isbn=0-444-52161-5 |year=2006 |publisher=Elsevier Science}}</ref><ref name=Gonthier1984>{{cite journal |first1=E.G. |last1=Gonthier |first2=J.-C. |last2=Faugères |title=Contourite facies of the Faro Drift, Gulf of Cadiz |journal=Geological Society, London, Special Publications |year=1984|volume=15 |issue=1 |pages=275–292 |doi=10.1144/gsl.sp.1984.015.01.18|bibcode = 1984GSLSP..15..275G |s2cid=129494147 }}</ref> These benthic storms occur only 5 to 10 times per year and usually last between 3 and 5 days,<ref name=Hollister1993 /> but that is enough to heavily erode benthic sediment and keep the finer grains in suspension even after flow velocities return to normal and the bedload has been deposited.<ref name="Faugeresetal1993" /><ref name="HollisterMcCave1984" /> During benthic storms, the eroded sediment may be transported over thousands of kilometres and deposited rather quickly (i.e., ~1.5 cm/month) once the storm wanes. However, the net sedimentation rate over thousands of years may be much smaller (i.e. ~5.5 cm/year) due to the intense periods of erosion during benthic storms.<ref name="FaugeresStow" />

==Sediment supply== thumb|350px|right|Bedform phase diagram for contourites (Stow et al. 2009)<ref name="Stow2009">{{cite journal |first1=D.A.V. |last1=Stow |first2=F.J. |last2=Hernandez-Molina |first3=E. |last3=Llave |first4=M. |last4=Sayago-Gil |first5=V. |last5=Diaz del Rio |first6=A. |last6=Branson|title=Bedform-velocity matrix: The estimation of bottom current velocity from bedform observations |journal=Geology |year=2009|volume=37 |issue=4 |pages=327–330 |doi=10.1130/g25259a.1|bibcode = 2009Geo....37..327S }}</ref> 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.<ref name="Faugeresetal1993" />

Terrigenous sediment supply to the deepwater bottom currents and to the nepheloid layer primarily depends on climate and tectonics in the continental environment.<ref name="Faugeresetal1993" /> The rate of tectonic uplift is directly related to the amount of sediment available, and variations in sea level will determine the ease with which this sediment is transported basinward. The sediment will most likely reach deepwater in the form of turbidity flows, which travel across bathymetric contours, only to be “blown” parallel to these contours as the finer sediments cross a deepwater bottom-current.<ref name="Hollister1993" /> Other sources of terrigenous sediment may include airborne and seaborne volcanoclastic debris.<ref name="Faugeresetal1993" />

Biogenic deposition from suspension may also supply sediment to these deepwater bottom currents. The deposition of this material has strong implications for the biology, chemistry, and flow, conditions at the time. It must occur in areas of high biogenic productivity, during periods of relatively quiet flow and, if calcareous, must also occur at depths above the carbonate compensation depth.<ref name="Faugeresetal1993" /><ref name="FaugeresStow" /> There is also a contribution to the concentration of suspended sediment from the burrowing activity of benthic organisms.<ref name="FaugeresStow" />

==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.<ref name="Faugeresetal1993" /> There are five main types of contourite accumulations: giant elongate drifts, contourite sheets, channel-related drifts, confined drifts, and modified drift-turbidite systems.<ref name="Faugeresetal1993" /><ref name="Stow2002">{{cite journal |first1=D.A.V. |last1=Stow |first2=J.-C. |last2=Faugères |first3=C.J. |last3=Pudsey |first4=A.R. |last4=Viana |title=Bottom currents, contourites and deep-sea sediment drifts: Current state-of-the-art |journal=Geological Society, London, Memoirs |year=2002|volume=22 |pages=7–20 |doi=10.1144/gsl.mem.2002.022.01.02|s2cid=128678734 }}</ref>

===Giant elongated drifts=== [[File:Contourite sparker seismic elongate drift.png|thumb|350px|right|Sparker seismic line showing elongate drifts in the Gulf of Cadiz<ref name="IODP2012"/>]] 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''.<ref name ="Rebesco" /> These drifts can be “tens to hundreds of kilometres long, tens of kilometres wide, and range from 0.1 to more than 1 km in relief above the surrounding seafloor”.<ref name="Faugeresetal1993" /> Their length-to-width ratio ranges from 2:1 to 10:1.<ref name="Stow2002" /> They can accumulate to thicknesses greater than 2 km and can form anywhere from the upper slope to the deepest parts of the basin, depending on the specific location of the bottom-current.<ref name="Faugeresetal1993" /><ref name="Stow2002" /> Sedimentation rates range from 20 to 100 m/Ma. They tend to be finer-grained with a lot of mud, silt, and biogenic material. Coarse-grained contourites are very rare.<ref name="Faugeresetal1993" /> They may also form detached or separated versions due to seafloor topography and flow conditions.<ref name="Stow2002" /> Detached drifts are isolated and migrate downslope, while separated drifts are typically asymmetric in shape, tend to form at the base of a slope, and migrate up slope.<ref name ="Rebesco" /> Large sediment waves have been observed partially covering some giant elongate drifts.<ref name="Faugeresetal1993" />

===Contourite sheets=== [[File:Contourite sheets seismic gulf of cadiz.png|thumb|350px|right|Contourite sheets shown in reflection seismic data off the coast of Portugal<ref name="IODP2012"/>]] 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.<ref name="Faugeresetal1993" /> They are characteristic of very deep water.<ref name ="Rebesco" /> They have a relatively constant thickness of up to a few hundred metres with a slight thinning towards the continental margin.<ref name="Stow2002" />

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.<ref>{{cite journal | last1 = Damuth | first1 = J.E. | last2 = Olson | first2 = H.C. | year = 2001 | title = Neogene-Quaternary contourite and related deposition on the West Shetland Slope and Faeroe-Shetland Channel revealed by high-resolution seismic studies | journal = Marine Geophysical Researches | volume = 22 | issue = 5/6| pages = 363–398 | doi=10.1023/A:1016395515456| bibcode = 2001MarGR..22..369D | s2cid = 14555444 }}</ref>

===Channel-related drifts===

Channel-related drifts form when deepwater bottomcurrents are confined to a smaller cross-sectional area of flow, and therefore their velocity increases substantially. This can happen if the deepwater bottom-current is trapped within a deep channel or within a gateway that connects two basins. Due to the high velocities, it is common to see scours and erosional features, as well as different types of deposits at the floor of the channel, the flanks, and the down-current exit of the channel.<ref name="Faugeresetal1993" /><ref name="Stow2002" />

Flank deposits are usually patchy and small (tens of km<sup>2</sup>), 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.<ref name="Stow2002" />

===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.<ref name="Stow2002" />

===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 Nova Scotian Margin, to alternations in turbidite/debrite and contourite deposits both in time and space, as seen in the Hebridean Margin.<ref name="Stow2002" /> The Caledonia and Judith Fancy formations in St. Croix were studied by Stanley (1993)<ref name="Stanley1993">{{cite journal |first1=D.J. |last1=Stanley |title=Model for turbidite-to-contourite continuum and multiple process transport in deep marine settings: examples in the rock record |journal=Sedimentary Geology |year=1993|volume=82 |issue=1–4 |pages=241–255|bibcode = 1993SedG...82..241S |doi = 10.1016/0037-0738(93)90124-N }}</ref> in which he found an ancient analogue of an alternating turbidite and contourite deposit and generated a stratigraphic model of a continuum from a turbidite dominant environment to a contourite dominant one.

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.<ref name="Shanmugam1993">{{cite journal |first1=G. |last1=Shanmugam |title=Traction structures in deep-marine, bottom-current reworked sands in the Pliocene and Pleistocene, Gulf of Mexico. |journal=Geology |year=1993|volume=21 |issue=10 |pages=929–932 | doi = 10.1130/0091-7613(1993)021<0929:TSIDMB>2.3.CO;2 |bibcode = 1993Geo....21..929S }}</ref> 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)<ref name="Stanley1993" /> 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== thumb|A Global map of recent {{legend2|#FFEE33|}} and fossil {{legend2|#FF8811|}} contourite deposits, and larger accumulations of recent contourite deposits are summarised under contourite depositional systems {{legend2|#B8860B|}}.

thumb|Map showing contourite deposition (green). (2) Too much bottom shear stress (red). (3) Too much sediment supply (brown). (4) Not enough sediment supply (light grey). (5) Not enough bottom shear stress (dark grey).<ref>{{Cite journal |last=Beelen |first=Daan |last2=Wood |first2=Lesli J. |date=2023 |title=Predicting bottom current deposition and erosion on the ocean floor |url=https://onlinelibrary.wiley.com/doi/10.1111/bre.12788 |journal=Basin Research |language=en |volume=35 |issue=5 |pages=1985–2009 |doi=10.1111/bre.12788 |issn=0950-091X|doi-access=free }}</ref>

===Present day=== Contourite deposition is active in many locations throughout the world, but particularly in areas affected by the thermohaline circulation.{{where|date=June 2013}}

===Ancient examples=== Identifying contourites in ancient sedimentary sequences is difficult as their distinctive morphology{{clarify|date=May 2014}} 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.<ref>{{cite journal | last1 = Dalrymple | first1 = R.W. | last2 = Narbonne | first2 = G.M. | year = 1996 | title = Continental slope sedimentation in the Sheepbed Formation (Neoproterozoic, Windermere Supergroup), Mackenzie Mountains, N.W.T. | journal = Canadian Journal of Earth Sciences | volume = 33 | issue = 6| pages = 848–862 | doi=10.1139/e96-064| bibcode = 1996CaJES..33..848D }}</ref>

==See also== *Turbidite

==References== {{Reflist}}

Category:Sedimentology