Morphology, age and sediment dynamics of the upper headwall of the Sahara Slide Complex, Northwest Africa: Evidence for a large Late Holocene failure

The Sahara Slide Complex in Northwest Africa is a giant submarine landslide with an estimated run-out length of ~ 900 km. We present newly acquired high-resolution multibeam bathymetry, sidescan sonar, and sub-bottom profiler data to investigate seafloor morphology, sediment dynamics and the timing of formation of the upper headwall area of the Sahara Slide Complex. The data reveal a ~ 35-km wide upper headwall,opening towards the northwest, with multiple slide scarps, glide planes, plateaus, lobes, slide blocks and slide debris. The slide scarps were generated by retrogressive failure events associated with two types of mass movements: translational sliding and gravitational spreading. As a result of this evolution, three different glide planes (GP I, II, and III) can be distinguished approximately 100 m, 50 m and 20 m below the seafloor. These glide planes are widespread and suggest failure along pronounced, continuous weak layers. Our data suggest an age of only about 2 ka for the failure of the upper headwall area, a date much younger than that derived for landslide deposits on the lower reaches of the Sahara Slide Complex, which are dated at 50-60 ka. The young age of the failure contradicts the postulate of a stable slope offshore Northwest Africa during sea-level highstands. Such an observation suggests that submarine-landslide risk along the continental margin of Northwest Africa should be reassessed based on a robust dating of proximal and distal slope failures.