LAUSR

Abstract The recently constructed Lokoja-Abuja highway and a quarry which cut through the Patti Formation, provided opportunity for the field study and sampling of the exposed ironstone interbeds within the carbonaceous shales and clays in Ahoko-Idu axis of the southern Bida Basin, Nigeria. The origin and paleoenvironmental significance of the ferruginous intervals were examined in this paper. Investigation reveals that the ferruginous bands occur as three lithofacies comprising of laminated, bioturbated and spheroidal concretionary facies. The primary components and depositional features of the host shales (detrital minerals, wood and herbaceous fragments, laminations) are largely preserved in the ironstones indicating compositional similarity and therefore a diagenetic origin for the ironstones. Results of XRD reveal substantial amount of siderite and illite but minor amounts of kaolinite, haematite, calcite and magnetite. Major elements such as Fe2O3, SiO2, Al2O3, CaO, MgO and P2O5 range from 41.50 to 54.60%, 6.10–23.30%, 3.64–7.00%, 1.40–13.60%, 0.30–1.20% and 0.10–8.80% respectively. The current data is suggestive that kaolinite was diagenetically replaced by siderite at early stage of burial in a reducing environment. Migrating pore water, probably enriched with alkali metals like Ca and Mg, sourced from sea water in the depositional basin and abundance of organic matter in the primary sediments are interpreted to be part of the factors responsible for the siderite precipitation. Both the host shale and the ferruginous beds contain significant amount of MgO, CaO, relics of dissolved bioclasts and body fossils. The geochemical proxies combined with the presence of magnesian siderite in the ironstones indicate that the host shales were deposited in marine environment with occasional freshwater influence. The study concludes that the ironstones are marine to non-marine, sideritic and diagenetic. The laminated and fossiliferous ironstone facies was deposited at the peak of marine transgression during the Upper Cretaceous time and represents maximum flooding surface. Shallowing up and regression into fresh water swamp condition was marked by the occurrence of freshwater sideritic ironstone.