Étude paléoenvironnementale des sédiments quaternaires du Guelb er Richât (Adrar de Mauritanie) en regard des sites voisins ou associés du Paléolithique inférieur. Discussion et perspectives

Abstract: After the discoveries of pre-Acheulian and Acheulian industry accumulations in the depression of the external ring of Guelb er Richât (Mauritanian Adrar) by Théodore Monod, it was proceeded to an exhaustive study of surrounding Quaternary sediments. Slopes in the south of the cliffs of Ordovician quartzite sandstones are covered with chaos of blocks where from go down torrential ravines which fed irregularly the alluvial plains of oueds Akerdil and Bamouéré. The torrential alluviums consist from two- to three-graded sequences of coarse sediments which were studied since the foot of the cliff down to alluvial fans. The datings of tests of mollusks of brackish waters and of phreatogenic calcitic cementations allowed to obtain a coherent chronostratigraphy based on 11 datings AMS of the 14C. These ages included between 15.000 and 7.730 BP correspond to the last climatic optimum or African Humid Period. Textural and mineralogical analyses of the torrential sequences allow to show a duality of origin of the materials of the accumulation partly deriving from more or less degraded Cambro-Ordovician basement (proximal source) and partly renewed by wind contributions during the long Meso-Cenozoic emersion (distal source). The palaeomagnetic measurements indicate that the great majority of the deposits accumulated during the last one normal magnetozone. Some altered deposits intermediate between the Paleozoic basement and the torrential sequences of the Late Quaternary present a negative magnetic inclination and a strong magnetic susceptibility linked to the presence of hematite grains; they are likened to more or less in situ reworked Cambrian-Ordovician tops. These conclusions leave not enough hope to identify on the slopes of Early Pleistocene sediments contemporary of the first Human populations. The strong accumulations of chopping-tools of the Akerdil Oued and of coarse hand-axes of the ravines of Cherirat indicate a credibly autochton origin of these big tools made in situ and which were concentrated by the successive wind deflations. Their mechanical dispersal was limited by their big size and by frequent calcitic cementations, such processes of fossilization were able to develop and to repeat during the ancient episodes of the Pleistocene. This hypothesis would lead us to envisage probable industry remains buried in depth into the filling of the outer alluvial plain of the Guelb er Richât. [Copyright &y& Elsevier]