Facies analysis and sequence stratigraphy of neoproterozoic Platform deposits in Adrar of Mauritania, Taoudeni basin, West Africa

The Neoproterozoic and Palaeozoic Taoudeni basin forms the flat-lying and u nmetamorphosed sedimentary cover of the West African Craton. In the western part of this basin, the Char Group and the lower part of the Atar Group ma ke up a 400-m-thick Neoproterozoic siliciclastic succession which rests on the Palaeoproterozoic metamorphic and granitic basement. Five erosional bou nding surfaces of regional extent have been identified in this succession. These surfaces separate five stratigraphic units with lithofacies associati ons ranging from fluvial to coastal and fluvial-, tide-, or wave-dominated shallow marine deposits. Owing to their regional extent and their position within the succession, the erosive bounding surfaces correspond to relative sea-level falls, and accordingly the five stratigraphic units they bound r epresent allocyclic transgressive-regressive depositional sequences (S-1-S- 5). Changes in the nature of the deposits forming the transgressive-regress ive cycles reflect landward or seaward shifts of the stacked sequences. The se successive relative sea-level changes are related to the reactivation of basement faults and tilting during rifting of the Pan-Afro-Brasiliano supe rcontinent 1000 m.y. ago. The stromatolite bearing carbonate-shale sequence s which form the rest of the Atar Group mark the onset of a quiet period of homogeneous subsidence contemporaneous with the Pan-African I oceanization 800-700 m.y. ago.