Abstract

The origin of oolitic and pisolitic ironstones is briefly reviewed. Two principal levels of ironstones, separated by dominantly argillaceous beds occur in the Middle Niger Valley. The lower level sits on fluviatile sediments, and comprises mainly goethitic, pisolitic and oolitic, ironstones, with subsidiary kaolinitic oolites and pisolites. The distribution of facies within this level, the fauna and flora, oolitic and pisolitic texture, and the sedimentary structures suggest that the ironstones represent subtidal shoreline deposits laid down possibly under two freely connected, high energy, environmental conditions. The upper level is dominated by goethitic, oolitic ironstones. Subsidiary sideritic, chloritic (?), and kaolinitic ironstones are present. The distribution of the oolitic subfacies, fauna and flora, and sedimentary structures also suggest that the deposits document subtidal shoreline sediments possibly deposited under two closely related, high energy, environmental conditions. A comparison of our reconstructed palaeo-environments with the Recent ones shows that the ironstones were probably laid down under some oceanographical conditions somewhat similar to those of the carbonate flats of the Trucial Coastline. The differences in the chemical states of the two environments--a result of contrasting climatic differences-seem to have given rise to contrasting mineralogical assemblages. The evidences of petrology and field relationships of the ironstones suggest that the ferruginous beds were derived principally from the igneous and metamorphic basement complex of Southwestern Nigeria. The combined effect of tropical climate, geomorphological maturity of coastal plains of alluviation, low submarine slopes, and the biological, chemical and physical states of the environments of deposition seem to have made possible the genesis of the oolitic and pisolitic ironstones of the Middle Niger Valley.

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