Abstract: The Maastrichtian Mamu and Nsukka Formations in the Anambra Basin (SE Nigeria) consists of a cyclic succession of coals, carbonaceous shales, silty shales and siltstones interpreted as deltaic deposits. The Anambra Basin was formed at the NW of the anticlinorium and the Afikpo syncline at the SE part of the anticlinorium, making both basins a depocenter during the Campanian to Maastrichian. Clay fractions of shale samples obtained from the Cretaceous Mamu and Nsukka Formations in the Afikpo Basin, south eastern Nigeria through the process of sedimentation technique were air – dried and analyzed using the empyrean diffractometer manufactured by Panalytical to determine the paleoclimate and depositional environments of the shales. The result shows that the bulk mineral composition of the shales comprises of quartz, clay minerals, carbonates and iron rich minerals, while the dominant clay mineral is kaolinite (70 – 80%) with minor amounts of illite (4 – 7 %) and smectite (10 – 20). This shows that both Formations were deposited under warm and humid conditions (typical of tropical to sub – tropical climate), while the depositional environment with evidence from biostratigraphy is suggested to be a near shore estuarine environment.