Deep-sea contourites drifts, erosional features and bedforms

Contourite drifts are identified as the large-scale morphological expression of contourite deposition. They comprise thick to very thick (10 to > 1000 m) accumulations of mainly contourite sediments. At the present day, they are found covering large areas of the deep seafloor beneath modern bottom current systems and range in scale from 102 to > 106 km2 (Faugères and Stow, 2008). In the ancient record on land, they are more difficult to recognize, due to their large size and subtle geometry. Contourites are sediments that have been deposited, or significantly affected, by the persistent action of bottom currents. At the still larger scale, Contourite Depositional Systems comprise several related drifts and associated erosional elements. They commonly develop along continental margins that have been under the influence of bottom currents for relatively long periods of time (> 2–3 My). Interbedded (mixed) sequences of contourites and other deep-water facies are not uncommon where the influence of the downslope and alongslope processes are equally effective. Not only do bottom currents transport and deposit sediment but they may also erode the seafloor. Contourite erosional elements are nondepositional zones and erosional features on the seafloor that have been caused by the action of bottom currents. They are often closely associated with and adjacent to contourite drift deposits. They occur where the bottom current velocity is sufficiently strong to prevent deposition or cause substrate erosion. Contourite bedforms are the small-scale seafloor sedimentary features, such as waves, dunes, ripples, and scours. They result from the erosional, transport, and depositional action of bottom currents at the seafloor. These are very common features that are found covering drifts and erosive surfaces beneath active bottom current systems at the present day. However, many of the internal sedimentary structures due to these bedforms are only rarely preserved in the contourite sediment record, largely because they are destroyed by the action of burrowing organisms. It was the many observations of such bedforms on the seafloor during pioneering oceanographic surveys of the 1950s and 1960s that, in part, led marine scientists to first propose the significant effects of bottom currents in shaping sedimentation on the deep continental rise off eastern North America. The term “contourite drift” was first used in the 1970s by (Hollister and Heezen, 1972), since which time there has been an explosion of research into the nature and effects of bottom currents and their deposits (Stow et al., 2002; Rebesco and Camerlenghi, 2008). Significant major discoveries in the contourite research field were facilitated through the international deep-sea drilling program in its various guises (DSDP, IPOD, ODP, IODP), which specifically targeted drift systems in many parts of the world, and hence provided direct access to the contourite sediments of which they are composed (Hernandez-Molina et al., 2014). This contribution in the Encyclopedia of Ocean Sciences is one of three on deep-sea bottom currents and their deposits. The focus here is on contourite drifts and erosional features, and on the numerous bedforms that are found covering the seafloor over these larger-scale features. It is necessarily based largely on modern and subrecent data recovered from present-day oceans, and draws heavily on a recent synthesis by (Esentia et al., 2018). It also includes reference to current work on ancient contourites exposed on land, and to the broader relevance of contourites to paleoclimate studies, ocean hazards and hydrocarbon prospectivity. The other two contributions outline the contourite sediments themselves, and the nature of bottom currents.