Contourite Drifts and Associated Bedforms

Abstract

Contourites, also known as alongslope deposits, are sediments that have been deposited or significantly affected by the persistent action of contour (bottom) currents. Contourite drifts are the large-scale morphological expression of contourite deposition, up to 106 km2 in area and >1 km in thickness. They are a common feature in some parts of the ocean basins and are found covering large areas of the present-day seafloor beneath modern bottom current systems. They typically co-occur with erosional features caused by bottom currents in very large-scale contourite depositional systems. Contourite drifts are classified into four principal types—sheeted drifts, mounded-elongate drifts, patch drifts, and channel-related drifts—, and four specific types linked to their mode or location of formation, including confined drifts, infill drifts, fault-controlled drifts, and mixed-drift systems. The principal erosional elements include: depositional hiatuses; regional erosive surfaces—erosional terraces, abrasive surfaces, channel scour and sub-circular scour; and linear erosion—contourite channels, moats, marginal valleys and isolated furrows. Seismic criteria for the identification of drifts and erosional elements must be applied carefully at three scales of observation—whole drift, seismic element and seismic facies. Bottom-current bedforms are common over drifts and erosive features and provide important insights into the flow characteristics and depositional mechanisms. The wide range of longitudinal and transverse bedforms can be linked to flow velocity and sediment grain-size in a bedform-velocity matrix. The principal controls on contourite systems are: the nature and style of bottom current flow; the slope gradient and other topographic features; and the sediment supply.