Field and lithostratigraphic studies of the Eze-Aku Group in the Afikpo Synclinorium, southern Benue Trough, Nigeria

• Lithostratigraphic status and depositional environment were defined based on clues from field and lithofacies analysis.
• Two component formations for the Eze-Aku Group were defined in the basin: the Eze-Aku Shale and the Amasiri Sandstone.
• Stratigraphic boundaries were established for the units with Amasiri designated as type locality of Amasiri Sandstone.
• Five identified lithofacies associations were deposited in tidal shallow shelf to deep water depositional setting.

Field and lithostratigraphic studies of sediments of Eze-Aku Group in the Afikpo Synclinorium were carried out to define the lithostratigraphic status of the Amasiri Sandstone unit of the group. The study involved field mapping, description of outcrops and lithofacies analysis. Lithostratigraphic study shows that the group comprises marine shales and limestones overlain by succession of dominant sandstones alternating with shales. Five major lithofacies associations deposited in tidal shallow shelf to deep water depositional setting were revealed in this study: massive, conglomeritic sandstone, laminated sandstone, bioturbated sandstone, laminated shale and cross bedded sandstone facies associations. Two component formations were identified for the Eze-Aku Group in the basin: the late Cenomanian-early Turonian Eze-Aku Shale (transgressive phase) and the middle-late Turonian Amasiri Sandstone (regressive phase). The Eze-Aku Shale and Amasiri Sandstone represent lower boundary and unit stratotypes respectively. The cross bedded sandstones represent the uppermost part of the formation. Results of field studies also show that the Eze-Aku Group in Afikpo Synclinorium is bounded above and below by Santonian unconformity and late Albian – middle Cenomanian unconformity respectively.