--> Seabed Morphology of the offshore Dakhla (off Western Sahara) some implications for geohazard potential.

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Seabed Morphology of the offshore Dakhla (off Western Sahara) some implications for geohazard potential.


This study explores a portion of the West African margin at the junction between two well-known segments with destructional architecture characterized by giant slides: offshore western Sahara. Inbetween, the Dakhla segment has been historically described as a constructional section. During the Dakhla cruise (2002), geophysical data were acquired on the North-West of Africa margin, offshore Dakhla, by IFREMER (French Institute for Sea), Total oil company and universities of Brest. Reflection and wide-angle seismic profiles together with multi-beam echo sounder data and chirp sub-bottom profiles have been acquired in this portion of the margin around latitude 23° N. The analysis of multibeam bathymetry, high-resolution sub-bottom and reflection seismic data on the continental slope of offshore Dakhla reveal the existence of several discrete morphologies such as canyon, overpressure ridges and deformation with a large wave-length. On the offshore Dakhla two types of seafloor depressions (or pockmarks) have been mapped around 1500 m of water depth in the center part of the multibeam survey. Like the ribbons, these are oriented NW-SE, parallel to the shoreline. We have seafloor depressions with circular forms of radius which varied 0.5 to 1 km and the seafloor depressions of elongated 0.5-1.5 km wide, 0.5-6 km long and up to 10-40 m deep. in addition, two scarps (S1 and S2) have been mapped around 2600m and 2300m of water depth. These morphologies (pockmarks, overpressure ridges and scarps) are indices of sedimentary instabilities and slidings, that are probably still active. The rupture process of this slide seems to have been controlled by alignment pockmarks around 1500 mwd. These pockmarks related to listric faults are caused by sedimentary loads and sea level changes, attributed to SIM4-SIM2. However, scarps S1 locate around 2600m and S2 locate around 2300m are attributed respectively to SIM8 and SIM6. Despite the presence of these instability figures, the actual margin profile remains therefore constructive but a change in gas hydrate stability conditions could trigger the effective sliding of this part of the margin, offshore Dakhla.