The Ediacaran sedimentary architecture and carbonate productivity in the Atar cliffs, Adrar, Mauritania: Palaeoenvironments, chemostratigraphy and diagenesis
Résumé
The Neoproterozoic strata of the Taoudeni Basin (West African Craton) display a single glaciogenic deposit, late Cryogenian in age, and included within a stratigraphic-marker ‘Triad' of tillite, carbonate, and bedded chert. This work is focused on the analysis of the sedimentary architecture and episodic carbonate productivity related to its transgressive deglaciation recorded in the Atar cliffs of Adrar, Mauritania. It offers a joint chemo-, event-, and sequence-stratigraphic analysis that allows us to control problems of stratigraphic completeness and time resolution, necessary for regional and worldwide stratigraphic correlation. In the Atar cliffs, the ‘cap carbonate sequence' is underlain by a composite unconformity that comprises the Pan-African-I discordance and an inherited palaeorelief associated with the glaciation. The transgressive sequence began with accumulation of a sheet-like apron onlapped by a toe-of-slope to basin transition with (cap) carbonates, bedded cherts and distal turbidites (Tichilît-el-Beïda shales). The carbonate strata of the Atar ‘cap carbonate sequence' have recorded both dolomitization and dedolomitization processes destroying the original cement textures. Carbon isotope values from the sheet-like apron range from 0 to −0.6‰, were directly controlled by the reworking of the underlying Atar strata, and do not reflect primary variations in seawater composition. The general pattern of δ13C fluctuations within the overlying incomplete ‘cap carbonate sequence' is consistent with other post-late Cryogenian strata, except in the initial shift from slightly to moderately negative values recognized elsewhere that is not preserved in the study area. Carbon isotope values in the preserved upper part of the cap carbonate record a pronounced rise in δ13C from −6.2‰ to highly enriched values (+3.7‰), followed by the final demise of carbonate productivity. The ‘cap carbonate sequence' is topped by an erosive unconformity succeeded by the recovery of carbonate accumulation represented by an onlapping dolostone deposited in coastal to foreshore environments. There, the carbon isotope values reflect the return to moderately negative values ranging from −0.9 to −2.0‰. The palaeogeographic development of a palaeorelief associated with the end of glacial conditions and deposition of the onlapping Atar cap carbonate is related to the presence of interbedded felsic bentonites and phenocrysts, and reflects the intermittent influence of a neighbouring explosive volcanism.