Provenance, palaeoweathering and depositional environment of the cretaceous deposits from the Babouri-Figuil and Mayo Oulo-Lere basins (North-Cameroon) during the Southern Atlantic opening: Geochemical constraints

Abstract

The Babouri-Figuil and Mayo Oulo-Lere basins belong to the intra-continental basins which are linked to the opening of Southern Atlantic Ocean. The sediments from these Basins were investigated to infer their provenance and conditions of sedimentation, using major and trace elements geochemistry of the fine facies. The main criteria for samples selection was color and hence twelve representative samples of each color type including black, dark grey, grey, dark reddish green, greenish and brown were selected from each basin. The geochemical compositions have revealed that the studied materials are composed of Fe-shales and Fe-sands. These sediments are characterized by (respectively for the Babouri-Figuil and Mayo Oulo Lere basins): high LREE/HREE ratios (5.11–14.90 and 9.21 to 18.32); weak negative and positive Eu anomaly (Eu/Eu* = 0.82 to 1.32 and 0.93 to 1.38); Al2O3/TiO2 ratios (11.92–31.33 and 16.95 to 35.83); Th/Co ratios (0.35–1.07 and 0.40 to 0.77). These features, together with the Ce vs La/Yb, Zr vs TiO2 and La/Sc vs Th/Co plots, indicate that the sediments are derived from intermediate to felsic rocks with subordinate mafic rocks. The K2O/Na2O ratios (<1), CIA (Chemical Index of Alteration) and PIA (Plagioclase Index of Alteration) (<60) for the two basins suggest that the source rocks have experienced a minor recycling and sorting as well as little weathering. The CIA values and the SiO2 vs Al2O3+K2O + Na2O diagram indicate a cool and arid to semi-arid climate during the deposition. The Sr/Ba values ranging from 0.55 to 6.41, suggest a fluctuating and sometimes high salinity (hypersaline lake) in a lacustrine or swampy environment, and the Ni/Co (<5) and U/Th (0.75) ratio are consistent with oxic conditions. The discriminant function-based multidimensional tectonic diagrams, associated to the ternary diagram (SiO2/20 - K2O + Na2O–Fe2O3 + MgO + TiO2) indicate an arc-collisional related setting, suggesting the origin of the studied materials from Pan-African basement rocks of the Central African Fold Belt (CAFB). This basement was affected by subsidence during the lower Cretaceous, probably the Neocomian, giving the relief that is submitted to the erosion leading to the immature sediments. The present findings reveal a high similarity in geochemical features between the Babouri-Figuil and Mayo Oulo-Lere basins, suggesting that the two basins share the same tectono-sedimentary evolution during the lower Cretaceous.