Lithostratigraphic characterization of the Upper Campanian – Maastrichtian succession in the Afikpo Sub-basin, southern Anambra Basin, Nigeria

Abstract

Lithostratigraphic mapping of the Afikpo Sub-basin of southeastern Nigeria was undertaken to reappraise and characterize lithostratigraphic package of the Upper Campanian – Maastrichtian in the sub-basin. The study involved descriptive lithostratigraphic and sedimentological methods supported by palynological biofacies. Five component lithostratigraphic units were identified: the Afikpo Sandstone, Nkporo, Mamu, Ajali and Nsukka formations. The Afikpo Sandstone is the oldest lithostratigraphic unit in the sub-basin. The sandstone unit overlies an angular unconformity above the Turonian Eze-Aku Group facies around Amasiri in the southern Benue Trough. It consists of fine to very coarse and occasionally pebbly sandstone interbedded with fossiliferous siltstones/sandstones, mudstones and coals. The Nkporo Formation overlies the Afikpo Sandstone and consists of thick succession of fossiliferous dark – grey to black shales, micritic limestones, oolitic ironstone and a distinct sandstone body enclosed within the dominant shale facies. The Nkporo Formation in the sub-basin comprises of three lithologically homogenous subunits: the Asaga Amangwu Shale, Owutu Sandstone and Amaiyi Edda Shale. The type locality is retained in Ndagbo Nkporo. The Owutu Sandstone is intervening between the Asaga Amangwu Shale and Amaiyi Edda Shale subunits. The Mamu Formation overlies the Nkporo Formation and is conformably overlain by the Ajali Formation. The Nsukka Formation is the topmost lithostratigraphic unit mapped in the sub-basin. The type localities for the Mamu, Ajali and Nsukka formations are retained in the Central Anambra Basin. This study has de-emphasized usage of the term “Nkporo Group” in addressing the Afikpo Sandstone and Nkporo Formation because the two lithostratigraphic units are distinct formations.