Geochemical characteristics of shales in the Mamfe Basin, South West Cameroon: Implication for depositional environments and oxidation conditions

Field and laboratory studies were carried out with the aim of characterizing the depositional environment and oxidation conditions during accumulation of shales in the Mamfe basin. Field studies denoted that the shales were black to dark grey in color, sub-tabular to tabular in occurrence, with some (n = 20) reacting vigorously with dilute HCl, while others did not (n = 5). Due to the carbonate content of the samples both whole rock dissolution and partial dissolution, using dilute acetic acid to target the carbonate fraction in the samples, were employed. The seven shale-rich units investigated have average whole rock Ni/Co, V/Cr, U/Th and Th/U ratios of 1.68-2.0, 1.13-2.95, 0.24-0.51 and > 2, respectively. PAAS normalized rare earth elements in the whole rock samples display an enrichment of light rare earth elements (LREEs) over heavy rare earth elements (HREEs), middle rare earth elements (MREEs) over HREEs, positive Gd, Eu anomalies, no Ce anomalies, and an average Y/Ho ratio of 29.7 ppm. The carbonate fraction of the samples displays two different REE pattern types. Sites 1 and 6 have typical hat-shaped patterns with extensive MREE enrichment. The other locations are slightly HREE depleted and have weak MREE enrichment with negative Y anomalies. The trace element ratios and the REE patterns of the whole rock and carbonate fractions indicate that the sediments deposited in the Mamfe basin accumulated in non-marine environments. Furthermore, their coloration, MREE enrichment and lack of Ce anomalies in the carbonate fractions infers that the depositional area was in a vegetated, low-land lacustrine setting with limited oxygenation of the sediment, whereas the negative Ce anomalies in the whole rock shale samples is probably due to particulate scavenging of REEs from the fluid during transport. This would require the water to be deficient in Ce and, thus reflect oxidizing conditions in the source area and along the transport system.