A subtidal bar model for the eze-aku sandstones, Nigeria

Abstract

“Coarsening upward” successions typical of subtidal sand bars have been recognised in the NE-trending linear sandstone bodies which occur within marine shale in the Eze-Aku Formation (Upper Cretaceous) of southeastern Nigeria. The ideal succession, 15–20 m thick, consists of the following units from bottom to top: (1) bioturbated grey siltstone (offshore mud); (2) wave-ripple-laminated, fine-grained well-sorted sandstone (offshore sands); (3) trough and tabular, cross-bedded medium-grained sandstone with channelled base (subtidal channel complex); (4) trough cross-bedded, medium-grained sandstone with bimodal-bipolar paleocurrent pattern (subtidal bar); (5) coarse, pebbly trough cross-bedded sandstone with wave-rippled top, rare burrows and a bimodal-bipolar paleocurrent pattern (subtidal bar). A sixth facies, not a part of the normal sequence, consists of coarse, carbonate-cemented pebbly sandstone grading into pure shell-limestone (bar margin). The sand bars seem to have grown on a shallow mud-bottomed, wave-worked inland sea inhabited by burrowers. A model for the stages of the vertical growth of the bar is presented.