Gravity models as tool for basin boundary demarcation: A case study of Anambra Basin, Southeastern Nigeria

This work analyzed gravity data over the Anambra Basin, Southeastern, Nigeria, with the aim of inferring the boundaries of the basin from gravity models. Method of study involved data acquisition and reduction, generation of Bouguer gravity map and separation of regional and residual anomalies. Data processing was enhanced by the use Oasis Montaj software. The interpretation of the Bouguer gravity anomalies ranged from visual inspection of figures to more complex methods that involved modelling of the subsurface layers. Sediment thickness in the study area ranged approximately from 0.4 km to 6 km. The highest gravity values (≥ 30 mGal) are associated with igneous rocks; the intermediate gravity values (11-29 mGal) are associated with Cretaceous sedimentary rocks, while the lower gravity values (< 11 mGal) are associated with Tertiary rocks. Boundary of the Anambra Basin from the western-Southwestern-southern portion of the study area is inferred around and along Adeigba-Umuezeanam-Nnewi-Akokwa areas, its boundary in the north-western part is beyond Igga and exists somewhere outside the study area, around Ogurugu, while its boundary in the southeastern side is around Umulokpa community. In the northern part of the study area the basin went beyond Obolafor in Enugu State and extended up to Orokam in Benue State. Afikpo Basin has been defined as a distinct inland basin different from the Anambra Basin. The present work has indicated that basin boundaries do not always coincide with surface lithologic boundaries; hence, field relationship alone is not enough for basin boundary demarcations.