Palaeoenvironmental reconstruction and oolitic ironstone mapping of the Agbaja Ironstone Formation in the Nupe Basin, North-central Nigeria: Insights from sedimentological and aeromagnetic analyses

Abstract

This study focusses on the sedimentological and aeromagnetic characteristics within the Agbaja Ironstone Formation to establish the sedimentary depositional history and map oolitic ironstone zones in the Lokoja district of the Nupe Basin. Forty sandstone samples were collected for granulometric, sedimentary facies and petrographic analyses to reconstruct depositional processes of the Agbaja Ironstone Formation. Aeromagnetic data analysis was engaged to map the oolitic ironstone rich zones. The iron-rich sandstones in the Agbaja Ironstone Formation occur as oolitic ironstone, ironstone concretion and laterites. The bivariate grain size plots and multivariate discriminant analyses suggest that 92% of the sandstone in the formation were deposited in beach and coastal dunes under fluvial-river actions, while 8% are of the marine environment deposited under wave actions. The quartz-feldspar lithic (QFL) ternary diagram and palaeocurrent analyses of the samples revealed that the matured, sub-arkosic sandstones originated from the cratonic interior (Abakaliki uplifts). The bioturbation and reactivation by Ophiomorpha burrows and herringbone on the fine-medium grained sandstones in the formation contributed to the observed high magnetic intensity values. High magnetic signatures (−94.784 to 1.191 nT), analytic signal peaks and depth levels from the 2-D source parameter images are the basis for the produced oolitic ironstones prospectivity map in this study.