Petrography And Mode Of Formation Of The Oolitic Ironstone Beds In The Campano-Maastrichtian Nkporo Formation In Leru, Southeastern Nigeria

The dark grey to black Nkporo Formation (Campanian – Maastrichtian) exposed in Leru area, southeastern Nigeria, contains several interbeds of oolitic ironstone in the upper parts. The ironstone consists of calcareous groundmass with pockets of unevenly distributed ferriferous ooids. The ooliths are “egg”- shaped and vary between 0.50 mm and 1.50 mm in diameter. From thin section studies, the oolitic ironstones are comprised of framework grains of chamosite ooliths in siderite cement and secondary limonite. The spheroidal shapes, absence of radial shrinkage fractures, lack of relict textures, and clean and clear, near-perfect cortices are indices of primary precipitation of the ooliths on the sea floor. The poor winnowing and presence of intact shell fragments indicate their deposition in a low energy environment. Concentric chamosite accretion on the nuclei developed while the nuclei were under low hydrodynamic turbulence on the sediment-water interface. Keywords: Petrography, Chamosite ooliths, Oolitic Ironstone, Primary precipitation, Diagenetic replacementGlobal Journal of Geological Sciences Vol. 6 (1) 2008: pp. 57-6