This is poorly exposed in Nigeria and merges with the Upper Benue Trough. A subsurface basement high, the Zambuk Ridge, separates the Southern Chad Basin from the Upper Benue. The Bima sandstone is the oldest sedimentary deposit in the Chad Basin. A Middle Cretaceous shale-limestone succession, subdivided into the Gongola Formation at the base and the Fika Shale at the top, constitutes the marine and transitional deposits which extend from the Upper Benue into the Southern Chad Basin. The Tertiary Chad Formation is very thick.
SE lullemmeden Basin
Marine Late Cretaceous - Palaeocene beds in the SE lullemme- den Basin are well exposed into the Sokoto region, in Niger and extending into Mali. Two cycles of dep- osition constitute the Late Cretaceous - Palaeocene marine sequence in SE lullemmeden Basin. The Rima cycle of Late Cretaceous age comprises the Taloka Formation (50m of brown, laminated, paral- lel, bedded, carbonaceous, fine-grained sandstone, siltstones and mudstones), overlain by the Dukamaje Formation (10m of basal bone bed, gyp- siferous, fissile, gray lower and upper shales and middle marl). The Palaeocene Sokoto cycle com- prises the Wurno Formation (20m of soft, tabular mudstone, muddy siltstones, and fine-grained sandstones at the base which grades upward into the Dange shale (10m thick), and the Kalambaina Formation (12m of nodular, marly limestone which becomes shaly at the top). The Gwandu Formation above is a ferruginious sandstone of continental origin.