Geochemistry of onshore deposits from Rio del Rey sub-basin of the western Atlantic margin of Cameroon (Coastal basin, Southwest Cameroon): provenance and environments of sedimentation

The Rio del Rey sub-basin belonging to the Cretaceous–Cenozoic coastal basins of Cameroon is studied with geochemistry (major and trace elements) associated to mineralogy, palynology and petrography. One dominant facies (calcareous shales/siltstones/claystones) occurs in the Cretaceous series and two others (calcareous fine- to coarse-grained sandstones and calcareous claystones/shales in the Paleocene series. Ratios of LREE/HREE (9.27–24.23), Cr/Th (2.24–12.60), Th/Sc (0.54–6.10), Eu/Eu* (> 1 on PAAS normalized and ˂ 1 on Chondrite normalized), Al 2 O 3 /TiO 2 (10.42–112.93) plotted in the Ce vs. La/Yb, Zr vs.TiO 2 and La/Sc vs.Th/Co diagrams indicate that the sediments derivate from felsic rocks associated with subordinate intermediate composition to mafic igneous rocks. CIA (Chemical Index of Alteration, 61.69–92.76), PIA (Plagioclase to Index of Alteration; 62.16–99.47) and K 2 O/Na 2 O ratios (0.08–22.13) suggest that the source rocks have experienced low to high weathering (during the Paleocene), high chemical weathering (at the Campanian–Maastrichtian transition) and moderate to high chemical weathering (during the Cenomanian). The Zr/Sc vs. Th/Sc diagram of sample groups reveals that the Paleocene coarse-grained sediments have higher zircon content. Al/Si ratios, low in the coarse-grained (0.04–0.06) and high in the fine-grained (0.2–0.58) sediments, behavior of Al/Si vs. CIA and some elemental concentrations, such as Zr, Hf, Ca, K, and Na, suggest that the geochemical composition is controlled by grain sizes. The petrographic and X-ray diffraction results point early to mesodiagenesis effects and an important role of sea level change on the environments of sedimentation. These latter vary from marginal to shallow marine during the Cretaceous and fluctuated between shallow marine/lagoonal and brackish/littoral settings during the Paleocene according to facies analysis and palynology, with marine species such as Dinopterygium cladoides and continental species such as Classopollis sp. The SiO 2 vs. Al 2 O 3 + K 2 O + Na 2 O plot suggests that a predominant semi-arid to arid climate with periodically semi-humid periods was the rule during the Paleocene, The Arc-Rift-Collision, with two sets of high and low-silica rocks and SiO 2 vs. K 2 O/Na 2 O diagrams, indicates a Collisional or Active Continental Margin tectonic as also recorded in the Cretaceous and Paleocene deposits in the Douala coastal sub-basin and in the Cretaceous Campo coastal sub-basin