Geochemistry of cretaceous fine-grained siliciclastic rocks from Upper Mundeck and Logbadjeck Formations, Douala sub-basin, SW Cameroon: Implications for weathering intensity, provenance, paleoclimate, redox condition, and tectonic setting

The chemical composition of the Upper Mundeck Formation (UMF) and Logbadjeck Formation (LF) from the Douala sub-basin (Cameroon) has been investigated to determine their provenance, the influence of the weathering, the paleoclimate, and the redox condition upon the source rock signature, as well as to decipher the tectonic setting of these sediments. The Chemical Index of Alteration (CIA), Plagioclase Index of Alteration (PIA), and various paleoweathering proxies suggest that these rocks underwent high degree of chemical weathering in the source region. The intensity of weathering was higher for LF relative to UMF samples. The A-CN-K plot, coupled to the mineralogical assemblage, dominated by the occurrence of illite, indicates that post-depositional K-enrichments may have affected the bulk composition of the studied rocks. The Post Archean Australian Shale (PAAS)-normalized patterns of the investigated rocks show mostly moderate to high LREE/HREE ratios (up to 20.75) and negative to positive Eu anomalies, suggesting a provenance from felsic rock with negligible contribution of mafic components. The Rb/Sr ratio for UMF (0.43 +/- 0.1) and LF (0.56 +/- 0.31) suggests a generally warm condition during their deposition. The V/Cr and U/Th ratios of UMF (3.29 +/- 4.94 and 0.24 +/- 0.10, respectively) and LF (1.67 +/- 1.34 and 0.17 +/- 0.03, respectively), and the authigenic uranium (AU) content of both formations indicate an oxic depositional environment. The Sr/Ba ratio of UMF (0.24 +/- 0.21) and LF (0.33 +/- 0.43) suggests an overall low salinity of the depositional environment. The tectonic discriminant-function-based diagram revealed a rift or a collisional setting for the studied rocks, which is inconsistent with the general geology of the Douala sub-basin, revealing probably the complex history of the source rock.