A Late Quaternary sedimentary shelf system under hyperarid conditions: Unravelling climatic, oceanographic and sea-level controls (Golfe d'Arguin, Mauritania, NW Africa)

The sedimentation on the Mauritanian shelf off the western Saharan desert is strongly influenced by dust supply and is investigated in this study by combining surface mapping, shallow-acoustic profiling and the analysis of sediment cores. Coarse-grained carbonates dominate wide areas of the open shelf at places where the shelf is exposed to the Atlantic Ocean. The sheltered Golfe d'Arguin, in contrast, is characterized by muddy shoals in its inner area, and locally restricted and wedge-shaped muddy depocenters which extend contour-parallel in mid-shelf settings. In both cases, cross-shelf currents control sediment distribution and accumulation, not location or amount of primary eolian input. A major discontinuity on the outer shelf is located in 85–110 m water plus sediment depth and is refilled by a sedimentary facies succession of shoreface, tidal and open-shelf deposits. This succession has formed during temporary lower sea levels along the last overall regression (MIS-4 and MIS-3), the Last Glacial Maximum, and during the past deglacial sea-level rise. Significant sediment deposition and preservation is limited to very restricted areas. Concerning the overall shelf architecture, sea level was the main factor enabling or suppressing deposition in terms of 1) creating local erosional swales which have favored subsequent trapping and preservation of sediments; 2) determining the type of sedimentary facies refilling these depressions under a variety of nearshore conditions; 3) interacting with the seafloor morphology and, by this, controlling timing of initiation and growth geometry of mud wedges. These mud wedges have formed since ca. 9 cal. ka under a continuous sedimentation regime. These bodies must have formed in hydraulic equilibrium but they are detached from a defined sediment point source which is in contrast to common mudbelt formation on shelves. In general, sediment storage on shelves under hyperarid conditions seems to be very limited due to comparably low sediment input, strong ravinement activity during changing sea levels, significant bypassing and winnowing processes, and a low preservation potential of the non-cohesive materials. These conditions lead to the development of almost sediment-starved systems with low subsidence rates over longer geological timescales.