Successions of sea-level changes during the Pleistocene in Mauritania and Senegal distinguished by sedimentary facies study and U/Th dating

The marine Quaternary Basin in Mauritania corresponds to a large gulf cover ing a 600 x 200 km area. The progressive reduction in size of marine gulfs from the early Pleistocene to Holocene (Nouakchottian), suggests that the g ulf size has been affected by successive positive epirogenic episodes. Howe ver, recent studies have indicated that the oldest and largest concentric o utcrops (Tafaritian) seem to have been deposited in various continental and unstable environments (lakes, sebkhas). In this study, we verified first t he absence of recrystallization in fossil mollusc shells, and then U/Th mea surements were carried out on seven of these. Most deposits (sandstones mor e or less cemented or coquina) belong to the interglacial Eemian sea-level high-stand (isotopic stage 5). On the basis of C-14 radiometric measurement s, the deposits were initially attributed to the relative warming during is otopic stage 3, and were named Inchirian. In view of our recent dating resu lts, this attribution needs to be reexamined. Two other U/Th dates performe d on shells from the section where isotopic stage 5 was obtained or from a neighbouring area, indicated deposition during isotopic stage 7. These obse rvations and dating would imply an improbable uplift of the shorelines that controlled decreasing widespread transgressions. The morphostructure of sh oreline outcrops, previously described as aureoles, has to be revised: the extent of the Quaternary marine plain has to be restricted and does not exc eed a maximum of 100 km in width at the latitude of Nouakchott. It was inva ded by marine transgressions whose deposits are: (1) extremely condensed by deep cuts and/or by aeolian deflation during regression episodes; (2) ofte n restricted to layers of cemented shore-sands or to upper parts of ferrugi nous crusts with ferric iron encrustation; and (3) sometimes fossilised she lly beds of different ages in the same sediment column. On the basis of the se observations, the uplift rising above the present zero of Pleistocene sh oreline deposits or subsidence processes are even more unlikely at the scal e of the Mauritanian Quaternary Basin. Post sedimentary alteration of these deposits, especially dissolution has been negligible, which explains the e xceptional preservation of such Pleistocene outliers. In the neighbouring S enegalian Basin, outcrops of marine Pleistocene deposits are usually restri cted to a very narrow coastal band. They disappear about 40 km south of Dak ar with the progression of tropical weathering. Tn this area, the sediment column is about 1 m thick and indicates the superposition of shelly layers of isotopic stages 5 and 7. (C) 2000 Elsevier Science B.V. All rights reser ved.