Geochemistry of Onshore Deposits From Rio-Del-Rey Sub-Basin of The Western Atlantic Margin of Cameroon (Coastal Basin, South West Cameroon): Provenance And Environments of Sedimentation

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Abstract The Rio del Rey Basin belongs to the coastal basins of Cameroon with Cretaceous to Cenozoic deposits. The Cretaceous (Cenomanian and Campanian-Maastrichtian) and Cenozoic (Paleocene) sediments were studied using major and trace elements geochemistry associated with mineralogical data. The studied samples are mainly made up of Fe-shale and shale with rare intercalations of Fe-sand in the Cenomanian deposits, Fe-shale and shale in the Campanian-Maastrichtian deposits and sub-litharenite, Fe-sand, shale and Fe-shale in the Paleocene deposits. These samples contain high LREE/HREE ratios (9.27 -24.23), no to slight positive Eu anomalies (Eu/Eu*= 1.02 – 1.44), moderate to high Al2O3/TiO2 (10.42 – 112.93) ratio and their position in the Ce vs. La/Yb, Zr vs. TiO2 and La/Sc vs. Th/Co diagrams indicate that they derivate from mainly felsic associated with subordinate intermediate igneous rocks. The CIA (Chemical Index of Alteration; 61.69 – 92.76), PIA (Plagioclase Index of Alteration; 62.16 – 99.47), K2O/Na2O ratios (0.08 – 22.125) and Zr/Sc vs. Th/Sc diagram suggest the source rocks have experienced low to high recycling and weathering (Paleocene deposits), high chemical weathering (Campanian-Maastrichtian) and moderate to high chemical watering (Cenomanian deposits). The SiO2 vs. Al2O3 + K2O + Na2O plot suggests a predominant semi-arid to arid climate with periodically semi-humid climate during the Paleocene, attested by the evolution of the clay minerals composition with association of illite-chlorite/vermiculite/kaolinite in the Cenomanian sediments and periodic appearance of siderite in the Campanian-Maastrichtian and Paleocene deposits. The Sr/Ba (0.41 – 2) and U/Th (0.16 – 0.28) ratios indicate brackish to shallow marine environment of deposition. The Arc-Rift-Collision of two sets of high and low silica rocks and SiO2 vs. K2O/Na2O diagrams indicate a Collisional or Active Continental Margin tectonic setting similar as the Cretaceous (Lower Mundeck and Logbadjeck Formation) and Paleocene (Nkapa Formation) deposits from the Douala sub-basin, which could reflect the Precambian basement history linked to the Neoproterozoic orogeny.

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