Cratonic and foreland Early Cambrian phosphorites of West Africa: palaeoceanographical and climatical contexts

Abstract

Abstract In Western Gondwana, two types of Pan-African-Brasiliano fold belts have been distinguished. The first type is characterized by an old long-lived tectonic cycle (1000-600 Ma) and is generally oceanic. The closure of these oceans resulted in the aggregation of Western Gondwana in late Neoproterozoic time. The second type is characterized by a young short-lived cycle (about 600-520 Ma) and is mostly intracontinental. Fold belts of the second type have various geodynamic significances and represent folded foreland, intermontane or impactogen-type basins formed during, or slightly after, the deformational closure of the old long-lived cycle. Phosphorites of West Africa are associated with the young short-lived tectonic cycle. They occur together with the upper member of a distinctive formation known all over West Africa. This formation is made up of three members, and consists, from base to top, of diamictites generally tillites with detrital intercalations, carbonate rocks with barite, and bedded cherts with local shale or siltstone intercalations. In Eastern Senegal, in the cratonic Taoudeni basin, the carbonate member is palaeontologically dated as Early Cambrian. Deposited during a glacio-eustatic transgression, it can be considered as a chronostratigraphic marker for the entire West Africa. Phosphorites, associated with or immediately following the eustatic transgression, are also possibly of Early Cambrian age. They are located either in the lower part of the chert member, in the upper part of the carbonate member, or sandwiched between these two units. In Fersiga and Namel, in the Taoudeni basin, phosphorites are associated with stromatolites characterizing the initial stage of the transgression. They are overlain by evaporites which can be related to an isostatic rebound. In the Volta foreland basin, phosphorites are associated with the chert member. There, they were deposited on a shelf ramp, either on its proximal portion, where they are strongly reworked (Kodjari and Arli occurrences), or in its distal part where they are interbedded with sediments deposited under anoxic conditions (Tapoa, Mekrou and Aloub Djouana occurrences). The depth of marine sedimentation, as indicated by the sedimentological characteristics of phosphorites and associated sediments, was controlled by the combined effect of glacio-eustatic sea-level rise, subsidence rate and isostatic rebound. Numerous textural features are similar in the phosphorites from the two basins. Phosphatic grains occurring as oncolites and stromatolitic fragments, together with intraclasts show a noticeable biogenic contribution of microbial origin. Microbial build-ups were formed in submergent/emergent environments in the Taoudeni basin, and in the outer shelf of the Volta basin. Apatites from these basins have similar REE contents, which suggest a continental origin for the phosphorus. Early Cambrian phosphorites of West Africa were deposited soon after glacially derived sediments. According to the available palaeocontinental maps, both were laid down at palaeolatitudes of about 40°S. West African Early Cambrian phosphorites thus formed at higher latitudes than their modern analogues, which are confined within the inter-tropical zone.