The Campano-Maastrichtian age is an important time in the geological evolution of Nigeria as it marked the re-establishment of the Trans-Saharan seaway that was broken due to Santonian inversion tectonics. In this paper, we conducted a high-resolution investigation of the dark mudstone unit of the Campano-Maastrichtian Mamu Formation exposed in 4-outcrops in the western segment of the Anambra basin, Nigeria, using multidisciplinary tools involving geochemistry, palynofacies, and microfabric analyses. Our objectives were to determine the nature of organic matter preserved in the sediments and the paleo-oxygenation conditions of the Trans-Saharan seaway. Our findings reveal that the Trans-Saharan seaway was of low salinity, characterized by the dominance of terrestrial organic matter in the more proximal marsh and bay sub-environments (organic facies C and CD) and mixed terrestrial - marine organic matter (organic facies BC and C) in the more distal central basin. Bottom water paleo-oxygenation was predominantly oxic. However, palynofacies and microfabric evidences as well as inferences from Fe-TS-TOC relationship suggests pyrite formation occurred in at least two phases. The first phase of syngenetic to early diagenetic pyrite formation, which was due to bacterial sulphate reduction that occurred in the anoxic zone below the sediment water interface, whereas secondary (late diagenetic) pyrite growth which formed the bulk of pyrite preserved occurred at the base of the bottom water. Furthermore, we hypothesize that pyrite formation occurred faster, and was better preserved in the central basin than in the other sub-environments. This is attributed to the presence of more reactive organic matter (marine palynomorphs), higher salinity (more sulphate), mineralogy (higher clay content), and microfabric (thinner lamination with low degree of bioturbation).