Contourite

A contourite is a sedimentary deposit commonly formed on continental rise to lower slope settings, although they may occur anywhere that is below storm wave base. Countourites are produced by thermohaline-induced deepwater bottom currents and may be influenced by wind or tidal forces. The geomorphology of contourite deposits is mainly influenced by the deepwater bottom-current velocity, sediment supply, and seafloor topography. A contourite is a sedimentary deposit commonly formed on continental rise to lower slope settings, although they may occur anywhere that is below storm wave base. Countourites are produced by thermohaline-induced deepwater bottom currents and may be influenced by wind or tidal forces. The geomorphology of contourite deposits is mainly influenced by the deepwater bottom-current velocity, sediment supply, and seafloor topography. 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 rise 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. 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 – 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 margins 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 and 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. 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. 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. The accumulation and geomorphology 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. 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. Sedimentation 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.