Sedimentary facies and environments of the sedimentary fill of Southern Bida Basin, Nigeria

Abstract

Six lithofacies were identified in the Lokoja Formation, Southern Bida Basin: fanglomerate/ conglomerate lithofacies (Gmc), fine to coarse-grained ferruginized weakly cross-bedded, pebbly sandstone lithofacies (Scx), fine to coarsegrained sandstone lithofacies (Sfc), silty claystone lithofacies (Csm), siltstone lithofacies (Slt) and lateritic ironstone lithofacies (Ilt). These were grouped into three lithofacies associations viz: alluvial fan, braided river channel, floodplain lithofacies association. Nine lithofacies were identified in the Ahoko Formation. These are: black-dark grey carbonaceous shale lithofacies (Shc), bioturbated ripple-laminated siltstone lithofacies (Sbr), poorly cross-laminated claystone lithofacies (Cxl), concretionary/nodular ironstone lithofacies (Icn), medium to coarse-grained sandstone lithofacies (Smc) fine grained, well-sorted, friable bioturbated herringbone cross-bedded sandstone lithofacies (Sxf), massive brownish claystone lithofacies (Clm), massive claystone with lateritic ironstone lithofacies (Cli) and lateritic ironstones lithofacies (Ilt). These have been grouped into three lithofacies associations viz: shallow marine lithofaciesassociation, tidal-intertidal flat lithofacies association and floodplain lithofacies association. Similarly, three lithofacies were identified in the Agbaja Formation and have been grouped into two lithofacies association. These are: fine to medium-grained sandstone ironstone interbedded lithofacies (Sti), oolitic–pisolitic ironstone lithofacies (Iop) and concretionary ironstone lithofacies (Icr). The lithofacies associations are: tidal-intertidal flat lithofacies association and shallow marine lithofacies association. Result of lithofacies analysis helped in interpreting the depositional environments. The Lokoja Formation is a product of a fluvial dominated alluvial system from debris/gravity flow in alluvial fan. This developed further into braided river channels and later meandering river during the closing stages. Sediments of the Ahoko Formation were deposited in tidal/intertidal flats and shallow marine environments while sediments of the Agbaja Formation were produced by a shallow marine system with a high tidal influence.

Revised stratigraphic sequence and type sections of the Southern Bida Basin, Nigeria

“…The succession is capped by lateritic ironstone which acts as a cover (Figures 10 and 11). Figure 13: Aseni lithologic log, illustrating a thick section of cycles featuring fining upwards siltstonesclaystone sequence (Ozulu et al, 2021).…”

“…(2) the massive, brownish claystone unit at the Agbaja road section; and (3) the shale-claystone heterolithic unit at the Ahoko section (Ozulu et al, 2021).…”

“…In this work, the annotations employed by Ozulu et al (2021) for various lithologic facies of sedimentary fills in the basin's studied sections have been utilized. The geologic map was produced by juxtaposing and combining the topographic maps regarding Lokoja NW and Koton-Karfe.…”

“…Figure 10: The outcrop section in Ahoko 1 displaying shale, siltstone, and claystone heteroliths(Ozulu et al, 2021).…”