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Sebastian Krastel, Leibniz-Institut fuer Meereswissenschaften (Germany)
Andrew A. Antobreh, Oslo University (Norway)
Russell B. Wynn, National Oceanography Centre (United Kingdom)
Rüdiger Henrich, Bremen University (Germany)
Till J. J. Hanebuth, Bremen University (Germany)
Aggeliki Georgiopoulou , Cardiff University (United Kingdom)
Jacob Geersen, Bremen University (Germany)
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The continental margin off Northwest Africa is largely shaped by a complex interplay of sediment transport processes directed both downslope and along-slope. During several cruises sediment transport processes between 12° and 26° N off Senegal and Mauritania were investigated by means of geophysical, sedimentological, and geochemical methods. Bathymetric and high-resolution seismic data were used to study the areal extent and the internal structure of redeposited sediments. Based on these measurements sediment cores were taken for sedimentological and geochemical analyses.
Sediment transport at the NW-African continental margin operates with different rates and styles including a significant transfer of land-derived terrigenous and hemipelagic sediments to the deep sea. Some sections show a large concentration of upper slope canyons but no indication for large scale mass wasting. Other sections are characterized by large scale mass wasting but no canyons and gullies were found. One example shows a large slide, which destroyed a major canyon, thereby leading to a reorganization of the sediment transport pattern as confined turbidity currents now spread over a much larger area.
Four mega-slides each affecting over 30,000 km2 of seafloor were identified along the continental slope off NW-Africa. All slides are complex and show a stepped headwall pattern typical for retrogressive sliding. Several buried mass wasting events are found beneath the youngest slide complex for all slides, hence revealing that these sections of the continental margin have been unstable for a long period of time. Two of the mega-slides show headwalls at atypical large water depths deeper than 3000m.
These observations allow to draw several conclusions for mass wasting off NW-Africa, which might be relevant for other passive margin settings as well. i) We speculate that the open slope areas without any major incisions allowed undisturbed rapid sediment build-up, which gave rise to sediment instabilities arising primarily from underconsolidation of the deposited sediments and widespread weak layers, while the canyons and gullies represent an effective pathway for ?semi-continuous' downslope sediment transport by turbidity currents. ii) Stacking of mass wasting events and the alignment of buried scarps with much younger headwalls suggest that sediments deposited above buried scarps are potentially unstable especially due to differential compaction and oversteepening, and therefore remain unstable after the initiation of land sliding. iii) The large size of the slides off NW-Africa is caused by high sedimentation rates but infrequent triggers. iv) Large scale mass wasting in water depths >3000m at the NW-African continental margin is more important than previously suggested.
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