Turbidity current processes and deposits on the northwest African Margin
The Northwest African margin is affected by a wide variety of sedimentary processes, including pelagic/hemipelagic background sedimentation, alongslope bottom currents, and downslope gravity flows. A large section of the margin can be classified as a fine-grained clastic slope apron, although the presence of numerous volcanic islands and seamounts leads to a more complex distribution of sedimentary processes than is accounted for by existing slope apron models. The Moroccan Turbidite System (MTS) is the largest turbidite system on the margin, with a total length of 1500 km. Individual turbidites can be correlated across three interconnected deep-water basins, giving an unprecedented insight into the turbidite depositional architecture of a system with complex seafioor topography and multiple sources. A detailed, core-based study of the turbidite fill in an intraslope basin within the MTS has revealed that sand body architecture is largely controlled by turbidity current volume. Small-volume turbidity currents deposit all of their sand around the mouth of the feeder canyon, whereas large-volume turbidity currents deposit extensive sheet sands across the basin floor. The large-volume, high efficiency flows excavate giant erosional scours at the canyon mouth, leading to development of a channel-lobe transition zone (CLTZ). Comparison with other CLTZ's has revealed that these zones form in association with flow expansion at a canyon/channel mouth, and may also be linked to major breaks in slope. Deep-water sediment waves are widespread on the margin, and display wave heights up to 70 m, and wavelengths up to 2.4 km. The largest sediment-wave fields are found on the continental slope and rise bordering the volcanic Canary Islands. Analysis of an integrated dataset, combined with simple numerical modelling, reveals that the sediment waves are deposited as antidunes beneath unconfined, low-velocity, low concentration turbidity currents.