GENETIC SIGNIFICANCE OF THE IRONSTONE FACIES OF THE MAASTRICHTIAN PATTI FORMATION, BIDA BASIN, NIGERIA: INSIGHTS FROM CHEMISTRY AND MINERALOGY

The Maastrichtian shale-clay member of the Patti Formation, Bida Basin, Nigeria which comprises of rhythmically bedded shale, claystone and ironstone outcropped prominently around the center of the basin, near Rivers Niger and Benue confluence. Despite the prominence of the ferruginous facies which occur as interbeds within the shales and its potential as paleoenvironmental marker, its origin is unknown. In this study, we investigated their lithological characteristics and obtained representative samples for petrography, X-Ray Diffraction (XRD) and chemical analyses (ICPMS). Three sub-lithofacies; a concretionary ironstone, bioturbated, massive ironstone and laminated ironstone (in places fossiliferous) were identified. In the laminated and bioturbated sub-facies, primary organic materials, sands, silts and clays are still preserved in the core of the beds. Thickness of the ironstone beds, which are laterally persistent, ranges from 0.10 to 0.4m. Concretionary structure, presence of organic materials and primary clays in some part of the beds suggest the ironstones are diagenetic rather than depositional. XRD data indicate mineral composition dominated by siderite and subsidiary amount of kaolinite, haematite, quartz and pyrite. Range of major elements obtained from the samples are; Fe₂O₃ (41.5-54.6%), SiO₂ (6.1-23.3%), Al₂O₃ (3.64-7.0%). CaO (1.4-13.6%), MgO (0.3-1.2%) and P₂O₅ (0.1-8.8%). These show that kaolinite thought to be the primary clay mineral had been largely replaced at early stage of diagenesis by siderite under reducing condition and low Eh. Availability of Ca and Mg in the original sediments, abundance of organic matter are evident and would favour diagenetic precipitation of siderites. Relatively high MgO, P₂O₅and CaO and presence of magnesian siderite indicate that the precursor argillaceous sediments were deposited in normal marine sea water.

We conclude that the ironstones are sideritic and diagenetic. The shale-clay member was deposited in normal saline water at the peak of marine transgression in the Maastrichtian time.