Stable isotopes and palynological analyses of the Campanian-Maastrichtian Mamu shales, western flank of the Anambra Basin, Nigeria: A paleoenvironmental investigation

The Mamu Formation is an important sedimentary unit within the Anambra Basin, southern Nigeria. The presence of coal seams and carbonaceous shales within this formation has implications on its hydrocarbon potential and paleoenvironment of deposition. This study integrated stable isotopes of organic carbon and total nitrogen (δ13Corg, δ15NTN) and palynology to characterize the depositional environment of the Mamu Shales located at the western flank of the Anambra Basin in Nigeria. The δ13Corg values ranged from -25.9 to -24.6‰ VPDB, while the δ15NTN values varied from 3.0 to 5.6‰ Air-N₂. Additionally, the carbon-to-nitrogen molar ratios (C/N) fell between 10.8 and 19.3. These findings suggest that the organic matter primarily originated from terrestrial C₃ grasses and trees. The range of TOC, HI and Tmax values suggested that these shales are good hydrocarbon source rocks. The palynomaceral (PM) assemblages were characterized by abundant, poorly sorted small and medium-sized PM-1 and PM-2, alongside relatively common lath-shaped PM-4. These assemblages were associated with lower occurrences of PM-3, structureless organic matter, and land-derived palynomorphs. Together, these observations indicate a predominantly terrestrial paleoenvironment marked by rainforest vegetation. From the δ15NTN values, it appears that symbiotic nitrogen-fixing plants served as the primary nitrogen source for the black shales, which formed in a continental marginal environment. Overall, our results suggest that the episodic incursions of the Trans-Saharan Seaway were minimal at the western flank of the basin, indicating a carbonate-starved paleoenvironment during the Late Cretaceous.