Depositional environment and facies relationships of the Cretaceous ironstone of the Agbaja Plateau, Nigeria

The Maastrichtian oolitic ironstone examined caps the mesas of the Agbaja Plateau, which covers an area approximately 150 km² and is situated NW of Lokoja, near the confluence of the Benue and Niger rivers, in the Cretaceous Middle Niger Basin of Nigeria. This basin is one of six depositional (Mesozoic to Recent) basins in Nigeria and is a shallow trough filled with Campanian to Maastrichtian, marine to fluviatile strata on the Pre-Cambrian basement.

Four major lithofacies of ironstone may be discerned on the basis of semi-quantitative X-ray diffraction analyses of borehole samples. These are from oxidized to more dehydrated and reduced conditions: (1) goethite + kaolinite; (2) hematite + geothite + kaolinite; (3) maghemite/magnetite + goethite + kaolinite; (4) siderite + magnetite + kaolinite. The maghemite/magnetite + geothite + kaolinite facies is dominant.

The ironstone was deposited during a major transgression. It overlies either a carbonaceous mudstone/shale or clay/sandstone, which is underlain by a fluviatile sandstone. Primary oxidized ironstone minerals were formed in nearshore fluviatile environment, and transported into deeper water. Through re-working and diagenesis, ironstone deposition continued under estuarine and reducing conditions with evidence of low energy wave action at the top of the ironstone.