Post-glacial filling of a semi-enclosed basin: The Arguin Basin (Mauritania)
Résumé
Semi-enclosed basins are not very common features in the world and are most frequently the result of tectonic movements. Studies of their filling are usually based on the micropaleontological analyses of sediment cores (Torgersen et al., 1988; Reeves et al., 2007) or seismic analyses (Lykousis et al., 2007; Çagatay et al., 2009; Van Daele et al., 2011). The morphology of semi-enclosed basins is generally simple and bowl-shaped, and their edges are marked by one or more sills. Their depths range from a few dozen to several thousand meters. Semi-enclosed basins are however present in some regions in the world. The semi-enclosed basin of the Golfe d'Arguin (Northwest Africa) is present on a wide, shallow shelf, bordering the Sahara desert, in a stable tectonic context. Its sedimentary filling took place during the end of the post-glacial transgression. The current knowledge on sedimentary filling of semi-enclosed basins is rather limited and inadequate to fully understand the processes at play.
Three campaigns have allowed the creation of the first morpho-bathymetric map of the Golfe d'Arguin, shedding new light on its large-scale morphology. A series of rocky shoals (Banc d'Arguin), interrupted by two sills in the north and south, divides the shelf into two parts, the inner and outer. The inner part of the Golfe d'Arguin, called the Arguin Basin, forms a shallow semi-enclosed depression. The basin is situated on the stable West African margin and the lack of any vertical tectonic movement provides an ideal situation for studying the progradation/regression variations and the sediment depositional conditions caused by the post-glacial sea level changes.
Based on the analysis of very high-resolution seismic data, seven units were identified. The sedimentary sequence of deposition of the Arguin Basin was interpreted in relation to the bedrock morphology, the sea-level rise and the climatic changes. Their chronology was established in comparison to the regional sea-level curve, basin physiography and unit distribution.
The Arguin Basin is interpreted as a land-locked freshwater lake during the post-glacial sea-level rise, corresponding to wet climatic conditions. The inner part was flooded ca. 8.7 ka BP when the sea level reached the sills. The filling then corresponded to a marine–estuarine environment. The climatic aridification and the sea-level stabilization from 6.5 ka BP onwards allowed the deposition of the last units, which are composed of aeolian sand combined with a significant marine biogenic carbonate fraction.
Bedrock morphology plays a major role in determining the depositional sequence architecture. It controls the available accommodation space and, in conjunction with climate changes, influences the environmental processes that shape the deposit geometry.