The Afam Clay Member: a Lower Miocene incised channel in the south-eastern Niger Delta

Abstract

This paper describes a Lower Miocene incised channel system and its lithic fill within the Niger Delta sedimentary succession. Stratigraphic cross-sections, which clearly show the base to be generally asymmetric, and very sharp with erosive characteristics in places, suggest a channel configuration for the base of the Afam Clay. The geometry of the channel and its lithic fill supports this interpretation. The channel was cut by fluvial processes into pre-existing unconsolidated Eocene-Miocene sediments of the Benin and Agbada Formations during a period of low sea level. The Imo River proto-channel is thought to have aided the channelling. The channel was subsequently enlarged by marine processes associated with a Middle Miocene rise in sea level. The lithic fill of the Afam channel is dominated by clay beds with localized development of sand bodies in time and space. Analysis of the spatial distribution of the sand units and their SP shapes supports the hypothesis of sand input from the north, east-central and west-central flanks of the channel. These are interpreted to reflect sand contributions from tributaries that fed the main channel. The faunal, lithological and textural characteristics of the Afam sediments suggest a predominance of marine conditions in the south, paralic conditions in the central area and fluvial processes in the north. In addition, the southern flanks of the channel may constitute truncation stratigraphic traps for hydrocarbons and it is proposed that these flanks should be fully explored.