Sedimentological characteristics of Ajali sandstone in the Benin flank of Anambra basin, Nigeria

Abstract

Outcrop mapping as well as textural, mineralogical and structural studies of sandstone in the Auchi locality were carried out in order to interpret depositional environment of Ajali Sandstone in the Benin flank of Anambra Basin. Two major lithologic units were identified: the lower bioturbated shale and overlying sandstone units. The bioturbated lower shale beds contain burrows of Ophiomorpha and Paleophycus spp. which belong to skolithos ichnofacies that typifies littoral environment. The upper sandstone unit is tabular cross-bedded with NNESSW paleocurrent direction. The grains are fine to coarse, poorly sorted and leptokurtic with a fining upward sequence motif. The major framework composition is Q_95.6 F_3.2 L_1.2 which classifies the sandstone as Quartz   arenite. Non-opaque heavy minerals constitute 13% of the entire heavy mineral suite of which ZTR index is 87%. The grains are texturally immature as depicted by their subangular edges but mineralogically mature in terms of quantities of quartz and high value of ZTR index. Dominance of Fe₂ O₃ (3.85%) in the uppermost bed  renders it as ferruginous sandstone. The texture as well as the various bivariate plots of the sandstone indicate fluvial sedimentation probably in a meandering channel that builds up point-bar and overbank deposits. The two litho-units therefore provide three different facies namely the bioturbated shale, the cross bedded sandstone and ferruginous sandstone facies.

Keywords: Sedimentology, Ajali Sandstone, Benin Flank, Quartz Arenite, Littoral, Fluvial.