Chapter 8 - Bottom currents

Abstract

The four basic types of deep-marine bottom currents are: (1) thermohaline-induced geostrophic contour currents, (2) wind-driven bottom currents, (3) tide-driven bottom currents, mostly in submarine canyons, and (4) internal wave/tide-driven baroclinic currents. Contourites are deposits of thermohaline-driven geostrophic contour currents. Hybrid flows are common. Contourites can be muddy or sandy in texture and siliciclastic or calciclastic in composition. Traction structures are common in deposits of all four types of bottom currents. However, there are no diagnostic sedimentological or seismic criteria for distinguishing ancient contourites from other three types. Double mud layers are a reliable criterion for recognizing deep-marine tidalites in cores and outcrops. The Gulf of Cadiz is the type locality for the contourite facies model based on muddy lithofacies. However, there are no genuine contour currents in the Gulf of Cadiz. This site is affected only by transitory contour currents associated with the Mediterranean outflow water. Furthermore, this site is affected by other complicating factors such as internal waves and tides, turbidity currents, tsunamis, cyclones, mud volcanism, methane seepage, sediment supply, pore-water venting, and bottom topography. Integrated Ocean Drilling Program 339 cores from the Gulf of Cadiz do not show primary sedimentary structures, which are necessary for interpreting depositional processes. Therefore, the contourite facies model is sedimentologically obsolete.