Contourite
Encyclopedia
A contourite is a sedimentary
deposit produced by thermohaline
-induced deepwater bottom currents
and may be influenced by wind or tidal
forces. Most contourites are formed in continental rise to lower slope settings, although they may occur anywhere that is below storm wave base
. The geomorphology of contourite deposits is mainly influenced by the deepwater bottom-current velocity, sediment supply, and seafloor topography.
derived from thermohaline-induced geostrophic bottom-currents that flow parallel to bathymetric contours
. They did this to emphasize 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. Hollister and Heezen (1972) adopted the name contourite for these deposits and provided a list of characteristics that described their sediments. Faugères and Stow (1993) 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 for 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.
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 gradient
s. This circulation commonly travels at velocities between 2 - 20 cm/s. Note that at this velocity range, considering the general shape of the Shields diagram 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 margin
s or as current squeezes between two ridges. 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. 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, and at the lower slope around the Faeroe Islands 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
. These benthic storms occur only 5 to 10 times per year and usually last between 3 to 5 days , 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. During benthic storms, the eroded sediment may be transported over thousands of kilometers 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.
. This layer plays a key role in supplying the sediment for the deposition of contourites under appropriate flow conditions.
Terrigenous
sediment supply to the deepwater bottom-currents and to the nepheloid layer primarily depends on climate
and tectonics
in the continental environment. 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. Other sources of terrigenous sediment may include airborne and seaborne volcanoclastic debris.
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
. There is also a contribution to the concentration of suspended sediment by the burrowing activity of benthic organisms.
of contourite deposits are mainly influenced by three factors: intensity of deepwater bottom-currents, seafloor topography
, and sediment supply. There are five main types of contourite accumulations: giant elongate drifts, contourite sheets, channel related drifts, confined drifts and modified drift-turbidite systems.
rates range from 20 - 100 m/Ma. They tend to be finer-grained with a lot of mud
, silt
and biogenic material. Coarse-grained
contourites are very rare. They may also form detached or separated versions due to seafloor topography and flow conditions. Detached drifts are isolated and migrate downslope while separated drifts typically are asymmetric in shape, tend to form at the base of a slope and migrate up-slope. Large sediment waves have been observed partially covering some giant elongate drifts.
Sediment wave fields are a variety that is generally located near the rise to slope transition. Seismic reflection
profiles show that the sediment waves tend to migrate up-slope.
related drifts form when deepwater bottom-currents 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.
Flank deposits are usually patchy and small (tens of km2), can be elongate and subparallel to 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.
Margin, to alternations in turbidite/debrite and contourite deposits both in time an space as seen in the Hebridean
Margin. The Caledonia and Judith Fancy formations in St. Croix were studied by Stanley (1993) in which he found an ancient analog 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 avalanching in clear bottom-currents than it is in sediment saturated turbidity flows. Deposition from suspension in turbidity flows do not generate a sharp upper contact as bottom-current reworked deposits show due to the highly oscillating energy conditions. Stanley (1993) 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.
. 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
.
Sediment
Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....
deposit produced by thermohaline
Thermohaline circulation
The term thermohaline circulation refers to a part of the large-scale ocean circulation that is driven by global density gradients created by surface heat and freshwater fluxes....
-induced deepwater bottom currents
Ocean current
An ocean current is a continuous, directed movement of ocean water generated by the forces acting upon this mean flow, such as breaking waves, wind, Coriolis effect, cabbeling, temperature and salinity differences and tides caused by the gravitational pull of the Moon and the Sun...
and may be influenced by wind or tidal
Tide
Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the moon and the sun and the rotation of the Earth....
forces. Most contourites are formed in continental rise to lower slope settings, although they may occur anywhere that is below storm wave base
Wave base
The wave base is the maximum depth at which a water wave's passage causes significant water motion. For water depths larger than the wave base, bottom sediments are no longer stirred by the wave motion above....
. The geomorphology of contourite deposits is mainly influenced by the deepwater bottom-current velocity, sediment supply, and seafloor topography.
Definition
The definition of the term contourite has varied throughout the decades. Originally, Heezen et al (1966) defined the concept, without using the actual word, as a sedimentary deposit on the continental riseContinental rise
The continental rise is an underwater feature found between the continental slope and the abyssal plain. This feature can be found all around the world, and it represents the final stage in the boundary between continents and the deepest part of the ocean...
derived from thermohaline-induced geostrophic bottom-currents that flow parallel to bathymetric contours
Contour line
A contour line of a function of two variables is a curve along which the function has a constant value. In cartography, a contour line joins points of equal elevation above a given level, such as mean sea level...
. They did this to emphasize 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
Blake Basin
The Blake Basin is a deep area of the Atlantic Ocean which runs along the east coast of the United States. It starts at the northern part of the Bahamas and continues up toward New York. Depth exceeds 5400 meters between the Blake Escarpment to the South and West, and the Blake Bahama Outer Ridge....
. Before this, it was thought that only turbidity flows
Turbidity current
A turbidity current is a current of rapidly moving, sediment-laden water moving down a slope through water, or another fluid. The current moves because it has a higher density and turbidity than the fluid through which it flows...
were capable of depositing and reworking sediment at depths greater than the continental slope. Hollister and Heezen (1972) adopted the name contourite for these deposits and provided a list of characteristics that described their sediments. Faugères and Stow (1993) 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
Lacustrine
Lacustrine means "of a lake" or "relating to a lake".Specifically, it may refer to:*Lacustrine plain*Lacustrine delta-Fish:*Lacustrine goby , a type of small fish found in Philippine waters belonging to the Gobiidae family, known in Tagalog as dulong-See also:*Fluvial - of or relating a...
settings. They suggested going back to the original definition of a contourite, that is for deposits at depths greater than 500 m derived from stable thermohaline-induced geostrophic bottom-currents (i.e. deepwater
Deepwater
Deepwater or Deep Water may refer to ocean water in the abyssal zone, hadal zone, or other deep ocean zones. Other meanings include:- Locations :* In the United States:** Deepwater, Missouri, a small town located in west-central Missouri...
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
Thermohaline circulationThermohaline circulation
The term thermohaline circulation refers to a part of the large-scale ocean circulation that is driven by global density gradients created by surface heat and freshwater fluxes....
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 gradient
Density Gradient
Density gradient is a spatial variation in density over an area. The term is used in the natural sciences to describe varying density of matter, but can apply to any quantity whose density can be measured...
s. This circulation commonly travels at velocities between 2 - 20 cm/s. Note that at this velocity range, considering the general shape of the Shields diagram still holds in these conditions, a flow will only be able to continue transporting finer sediment that is already in suspension
Suspended load
Suspended load is the portion of the sediment that is carried by a fluid flow which settle slowly enough such that it almost never touches the bed...
but will not be able to erode
Erosion
Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...
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 margin
Continental margin
The continental margin is the zone of the ocean floor that separates the thin oceanic crust from thick continental crust. Continental margins constitute about 28% of the oceanic area....
s or as current squeezes between two ridges. Periodically, velocities may increase dramatically or even reverse due to atmospheric storms
Storm
A storm is any disturbed state of an astronomical body's atmosphere, especially affecting its surface, and strongly implying severe weather...
raising the local surface eddy kinetic energy, which gets partially transmitted down to abyssal depths in episodes called benthic storms. 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, and at the lower slope around the Faeroe Islands 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
Strait of Gibraltar
The Strait of Gibraltar is a narrow strait that connects the Atlantic Ocean to the Mediterranean Sea and separates Spain in Europe from Morocco in Africa. The name comes from Gibraltar, which in turn originates from the Arabic Jebel Tariq , albeit the Arab name for the Strait is Bab el-Zakat or...
. These benthic storms occur only 5 to 10 times per year and usually last between 3 to 5 days , 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. During benthic storms, the eroded sediment may be transported over thousands of kilometers 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.
Sediment supply
Erosion of the seafloor contributes to the growth of a deepwater nepheloid layerNepheloid layer
Nepheloid layer or nepheloid zone is a layer of water in the deep ocean basin, above the ocean floor, that contains significant amounts of suspended sediment. It is from 200 to 1000 m thick. The name comes from Greek: nephos, "cloud". The particles in the layer may come from the upper ocean...
. This layer plays a key role in supplying the sediment for the deposition of contourites under appropriate flow conditions.
Terrigenous
Terrigenous
In oceanography, terrigenous sediments are those derived from the erosion of rocks on land; that is, that are derived from terrestrial environments. Consisting of sand, mud, and silt carried to sea by rivers, their composition is usually related to their source rocks; deposition of these sediments...
sediment supply to the deepwater bottom-currents and to the nepheloid layer primarily depends on climate
Climate
Climate encompasses the statistics of temperature, humidity, atmospheric pressure, wind, rainfall, atmospheric particle count and other meteorological elemental measurements in a given region over long periods...
and tectonics
Tectonics
Tectonics is a field of study within geology concerned generally with the structures within the lithosphere of the Earth and particularly with the forces and movements that have operated in a region to create these structures.Tectonics is concerned with the orogenies and tectonic development of...
in the continental environment. The rate of tectonic uplift
Tectonic uplift
Tectonic uplift is a geological process most often caused by plate tectonics which increases elevation. The opposite of uplift is subsidence, which results in a decrease in elevation. Uplift may be orogenic or isostatic.-Orogenic uplift:...
is directly related to the amount of sediment available and variations in sea level
Sea level
Mean sea level is a measure of the average height of the ocean's surface ; used as a standard in reckoning land elevation...
will determine the ease with which this sediment is transported basinward
Oceanic basin
Hydrologically, an oceanic basin may be anywhere on Earth that is covered by seawater, but geologically ocean basins are large geologic basins that are below sea level...
. 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. Other sources of terrigenous sediment may include airborne and seaborne volcanoclastic debris.
Biogenic deposition from suspension may also supply sediment to these deepwater bottom-currents. The deposition of this material has strong implications for the biology
Biology
Biology is a natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy. Biology is a vast subject containing many subdivisions, topics, and disciplines...
, chemistry
Chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....
and flow conditions at the time. It must occur in areas of high biogenic productivity, during periods of relatively quiet flow and, if calcareous
Calcareous
Calcareous is an adjective meaning mostly or partly composed of calcium carbonate, in other words, containing lime or being chalky. The term is used in a wide variety of scientific disciplines.-In zoology:...
, must also occur at depths above the carbonate compensation depth
Carbonate Compensation Depth
Calcite compensation depth is the depth in the oceans below which the rate of supply of calcite lags behind the rate of solvation, such that no calcite is preserved. Aragonite compensation depth describes the same behaviour in reference to aragonitic carbonates.Calcium carbonate is essentially...
. There is also a contribution to the concentration of suspended sediment by the burrowing activity of benthic organisms.
Geomorphology
The accumulation and geomorphologyGeomorphology
Geomorphology is the scientific study of landforms and the processes that shape them...
of contourite deposits are mainly influenced by three factors: intensity of deepwater bottom-currents, seafloor topography
Bathymetry
Bathymetry is the study of underwater depth of lake or ocean floors. In other words, bathymetry is the underwater equivalent to hypsometry. The name comes from Greek βαθύς , "deep", and μέτρον , "measure"...
, and sediment supply. There are five main types of contourite accumulations: giant elongate drifts, contourite sheets, channel related drifts, confined drifts and modified drift-turbidite systems.
Giant elongate drifts
Giant elongate drifts form very large mounded elongated geometries parallel to the deepwater bottom-current flow. They are characterized 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. These drifts can be “tens to hundreds of kilometers long, tens of kilometers wide, and range from 0.1 to more than 1 km in relief above the surrounding seafloor”. Their length to width ratio ranges from 2:1 to 10:1. 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. SedimentationSedimentation
Sedimentation is the tendency for particles in suspension to settle out of the fluid in which they are entrained, and come to rest against a barrier. This is due to their motion through the fluid in response to the forces acting on them: these forces can be due to gravity, centrifugal acceleration...
rates range from 20 - 100 m/Ma. They tend to be finer-grained with a lot of mud
Mud
Mud is a mixture of water and some combination of soil, silt, and clay. Ancient mud deposits harden over geological time to form sedimentary rock such as shale or mudstone . When geological deposits of mud are formed in estuaries the resultant layers are termed bay muds...
, silt
Silt
Silt is granular material of a size somewhere between sand and clay whose mineral origin is quartz and feldspar. Silt may occur as a soil or as suspended sediment in a surface water body...
and biogenic material. Coarse-grained
Particle size (grain size)
Particle size, also called grain size, refers to the diameter of individual grains of sediment, or the lithified particles in clastic rocks. The term may also be applied to other granular materials. This is different from the crystallite size, which is the size of a single crystal inside the...
contourites are very rare. They may also form detached or separated versions due to seafloor topography and flow conditions. Detached drifts are isolated and migrate downslope while separated drifts typically are asymmetric in shape, tend to form at the base of a slope and migrate up-slope. Large sediment waves have been observed partially covering some giant elongate drifts.
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. They are characteristic of very deep water . They have a relatively constant thickness of up to a few hundred meters with a slight thinning towards the continental margin.Sediment wave fields are a variety that is generally located near the rise to slope transition. Seismic reflection
Reflection seismology
Reflection seismology is a method of exploration geophysics that uses the principles of seismology to estimate the properties of the Earth's subsurface from reflected seismic waves. The method requires a controlled seismic source of energy, such as dynamite/Tovex, a specialized air gun or a...
profiles show that the sediment waves tend to migrate up-slope.
Channel related drifts
ChannelStream channel
Stream channel is the physical confine of a stream consisting of a bed and stream banks.Stream channels exist in a variety of geometries. The stream channel development is controlled by both water and sediment movement. There is a difference between low gradient streams and high gradient streams...
related drifts form when deepwater bottom-currents 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.
Flank deposits are usually patchy and small (tens of km2), can be elongate and subparallel to 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.
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 topographic confinement of the deposit.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 ScotianNova Scotia
Nova Scotia is one of Canada's three Maritime provinces and is the most populous province in Atlantic Canada. The name of the province is Latin for "New Scotland," but "Nova Scotia" is the recognized, English-language name of the province. The provincial capital is Halifax. Nova Scotia is the...
Margin, to alternations in turbidite/debrite and contourite deposits both in time an space as seen in the Hebridean
Hebrides
The Hebrides comprise a widespread and diverse archipelago off the west coast of Scotland. There are two main groups: the Inner and Outer Hebrides. These islands have a long history of occupation dating back to the Mesolithic and the culture of the residents has been affected by the successive...
Margin. The Caledonia and Judith Fancy formations in St. Croix were studied by Stanley (1993) in which he found an ancient analog 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 avalanching in clear bottom-currents than it is in sediment saturated turbidity flows. Deposition from suspension in turbidity flows do not generate a sharp upper contact as bottom-current reworked deposits show due to the highly oscillating energy conditions. Stanley (1993) 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.
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 bioturbation, sedimentation, erosion and compactionCompaction (geology)
Compaction refers to the process by which a sediment progressively loses its porosity due to the effects of loading. This forms part of the process of lithification. When a layer of sediment is originally deposited, it contains an open framework of particles with the pore space being usually...
. Most examples of contourites identified in the geological record come from the Cenozoic
Cenozoic
The Cenozoic era is the current and most recent of the three Phanerozoic geological eras and covers the period from 65.5 mya to the present. The era began in the wake of the Cretaceous–Tertiary extinction event at the end of the Cretaceous that saw the demise of the last non-avian dinosaurs and...
but examples have been noted from as far back as the Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...
.