Facies Architecture and Sequence Stratigraphy of the Upper Cretaceous Sediments in the Southern Bida Basin, Nigeria

Olusola J. Ojo and Samuel O. Akande
University of Ilorin, Ilorin, Nigeria

The Campanian to Maastrichtian sediments (Lokoja, Patti and Agbaja formations) in the southern part of the intracratonic Bida Basin, Nigeria, cropping out around Benue and Niger rivers confluence were studied. Detailed logging of 16 outcrop sections was carried out recording facies variations, erosional surfaces, paleocurrent data, physical and biogenic sedimentary structures. The shale samples were analyzed for their palynomorph content. These data were used to interpret the depositional environments and develop a sequence stratigraphic framework for the sediments. A suite of depositional environments were recognized, including alluvial fan, fluvial channel, coastal swamp, tidal shelf and shoreface. Four regionally extensive sequence boundaries marked by truncation and basin ward shift in facies are recognized. The non conformable surface between the alluvial conglomerate facies of the Lokoja Formation and Precambrian basement rocks represent the lower sequence boundary SB1. The lowstand system tracts in all the depositional sequences are alluvial to fluvial without marine reworking. The transgressive system tract consists of tidal shelf and coastal swamp deposits. The transgressive system tract rests on a major maximum flooding surface in the Patti Formation. Several coarsening upward parasequences which build up in a progradational pattern represent the highstand system tract. The distribution of potential reservoirs within the Lokoja and Patti formations are controlled by the sequence boundary whereas the occurrence of source and sealing horizons are controlled by the transgressive surface.