The Abakaliki and the Ebonyi Formations: sub-divisions of the Albian Asu River Group in the southern Benue trough, Nigeria

Abstract

The Asu River Group (middle-late Albian), exposed in the Nkalagu area of the southern Benue trough, consists of two formations, viz: the Abakaliki Formation at the base overlain by the Ebonyi Formation (new name). The Abakaliki Formation is composed mainly of illitic and kaolinitic black shales and mudstones interbedded with black micritic limestone, siltstone adn minor feldspathic sandstones. The limestone is composed of grapestones, infralimeclasts and brown algal remains. These together with the pyritic and fossiliferous chale, suggest oscillatory deposition of argillaceous sediment in a shallow but anoxic subsiding bottom and marine facies. Rapid alternations of sandstone, siltstone, shales, mudstones, oolitic and serpulid grainstones and packstone-wackestone indicative of transitional environment of deposition characterise the Ebonyi Formation. The thick horizontal strata and interference ripple marks suggest lower flow regime of deposition while the thin evaporite laminite indicate locally short-term aridity during the Cretaceous in the southern Benue trough.