North Atlantic contourite sand channels

Two sand-rich channelized depositional systems, formed by strong contour currents, were studied west of the Faeroe Bank Channel and in the Gulf of Cadiz. Both are areas beyond the exit of constrictions where water overflows from the Norwegian Sea and the Mediterranean Sea, respectively. West of the Faeroe Bank, newly mapped channels are developed mainly under the influence of a geostrophic current and are characterized by significant lateral migration, which determines the marked cross-sectional asymmetry and the architecture of the deposits. The pathways of the Mediterranean Undercurrent in the Gulf of Cadiz are complex, with the greater proportion flowing under geostrophic conditions along a terrace but with some of the denser water becoming ageostrophic and descending downslope owing to gravity. A series of 'peel-off' channels is formed, with the largest one, Gil Eanes, being about 40 km long. Most of the channel fills consist of medium-coarse sand. Levees are mainly silts with a higher sand content in the vicinity of the channel. Both depositional systems have a variety of contourite sand channels, which in most respects are remarkably similar. In both cases there are stretches where the flow is ageostrophic, with water descending downslope for as much as 400 m before resuming geostrophic flow at deeper levels. In each case the main pathway of the densest water is the shallowest and several branches turn off to the left of this main pathway before bending to the right under the influence of Coriolis forces. In both cases there are channel fills of medium-coarse sand, probably cross-bedded, and up to 200 ms thick. Sheets of sand with a thickness of a few metres to a few tens of metres are common. Similarities to turbidite channels are the aggradational nature of some channel floors and the flanking muddy or silty sediment waves. Contourite channel depositional complexes are distinguished from turbiditic ones by their coarsening-up rather than fining-up sand units, the asymmetry in channel architecture, the presence of regional unconformities, and the distribution pattern with well-marked boundaries of current-derived deposits.