Provenance and depositional environment of the Mbam River overbank clay deposits, Central Africa: evidence from mineralogical and geochemical variations

This work permitted the understanding of processes in continental environments through the conditions of formation and deposition of alluvial clay materials in the Mbam Riverbanks (Central Africa). The Mbam River is the main tributary of the Sanaga River, one of the largest rivers in Central Africa. Twelve clay samples were collected from six wells, exhibiting different facies, and characterized using the Robinson-Köln pipetting, infrared spectroscopy, X-ray diffraction, and X-ray fluorescence. All the materials were classified as sandy clays. Their mineralogical assemblage is made of quartz, kaolinite, orthoclase, hematite/goethite, rutile, anorthite, and augite. The values for chemical, plagioclase, and mafic indices of alteration combined with those for Ba, Sr, and Rb/Sr ratio show that primary minerals like orthoclase, anorthite, and augite in source rocks are moderately altered. The studied materials are arkoses, wackes, shales, and Fe-Shales. Based on the index of compositional variability and on the SiO₂/Al₂O₃ ratio, the associated sediments are considered immature. They are, according to Cr, Ni, and SiO₂ contents, Al₂O₃/TiO₂, Th/Sc, Th/Cr, and Cr/Ni ratios, and some binary provenance diagrams, from mafic, intermediate, and felsic sources. Values of the authigenic uranium, U/Th, V/Cr, and Ni/Co ratios permit to infer that they were deposited in an oxic environment.