Highlighting of landslides in the Cap Rouge Series, Late Cretaceous, Western Part of the Senegalo-Mauritanian Basin

Abstract

The Cap Rouge series, of terminal Cretaceous age, is located in the western part of the Senegalo-Mauritanian Basin, more precisely in the Diass horst. The geological cross-section of this series and the interpretation of its mega intersecting structures are the subject of debate to which this publication attempts to make a contribution. To this end, we have carried out cross-sections of the Cap Rouge cliff during the rainy season, when marine erosion exposes its basal parts. Six sedimentary units were identified, from bottom to top: unit 1, made up of fine sandstone; unit 2, made up of variegated clay alternating with fine sandstone levels; unit 3, made up of fine sandstone; unit 4, made up of fine sandstone alternating with argillite; unit 5, made up of massive fine sandstone with horizontal bedding; finally, unit 6, made up of alternating fine sandstone and ferruginized argillite. This last unit is topped by a lateritic cuirass. Unit 4 is incised by large structures that have been regarded for over seven decades as cross bedding or megachennals. Our structural and microtectonic analyses have shown that these are failure surfaces where translational landslides have occurred. To the south of the cliff, landslides are responsible for the sinking of units 1 and 2, as well as the truncation and bevelling of unit 3. To the north of the cliff, landslides are associated with drag microfolds, sigmoidal structures and microfractures. The main triggering factor of these landslides could be the marine erosion responsible for the subsidence of the Cap Rouge cliff. Weathering in fracture planes due to run-off water is also a triggering parameter, having induced a reduction in shear strength and landslides.