Sédimentologie et géochimie des dépôts clastiques de la partie septentrionale de La zone littorale du Cameroun

Abstract

The southern and coastal portion of the Tertiary Cameroon Volcanic Line (CVL), in the Gulf of Guinea, is bordered by Pan-African granitoids, metamorphic rocks, and Cretaceous sedimentary successions, which are prone to contribute to the nearby Limbe beach sedimentation. To better constrain the provenance of the above mentioned beach sediments, their petrographical, mineralogical and chemical studies were undertaken. Another aim of this work is to assess the weathering intensity in coastal areas close to CVL. The Qt-Ft-Lt and Qm-Ft-Ltternary diagrams indicate wrongly on undissected arc environment, due to the proximity of the CVL. The ratio Qt23- Ft - Lt of the Limbé beach characterize of litharenite 51 5 on petrographical classification. The low values of SiO /Al O ratios and Zircon-Tourmaline- 2 23 Rutile (ZTR) index, coupled to high Index of Compositional Variability (ICV) indicate compositionally immature sediments. This is further supported by the presence of rock forming minerals, such as olivine, pyroxene, and amphibole. The Th/Sc versus Zr/Sc binary plot and Al2O3-TiO -Zr ternary diagram suggest that the studied sediments have not 2 experienced recycling. The Chemical Index of Alteration (CIA), Plagioclase Index of Alteration (PIA), and the A-CN-K and (A-K)-C-N triangular diagrams indicate that the Limbe beach sediments and their source have undergone moderate to intense weathering (62.08– 87.3%). Several provenance discrimination diagrams, including the binary diagram of Al2O3 versus TiO , Zr versus TiO , La/Sc versus Th/Co, and the ternary plot of V-Ni-Th*10, and the 2 2 comparison of chondrite normalized rare earth elements (REE) patterns of the studied sediments with potential source rocks suggest a source area composed of mafic rocks from CVL, indicating that the granitic, metamorphic, and sedimentary rocks close to this volcanic structure do not significantly contribute to the sedimentation of the studied beach. In addition, the longshore currents do not influence the chemical composition of the sediments or do not reach the study site. The enrichment factor (EF) shows decreasing succession of ETM in the order Co > Ni > Cu > Cd > Zn > Cr > Mn. the geo-accumulation index (I-geo) indicates pollution in Co, Ni and Cu in all the beaches. The contamination factor (CF) shows the following succession Co > Ni > Cu > Cd > Zn > Cr > Fe > Mn, which is similar to the enrichment factor and the geo-accumulation index.