Comparison of grain-clay fraction and particle morphogenesis as signatures of source and depositional environment (Cretaceous Lokoja formation, Niger Basin, Nigeria)

The Upper Cretaceous Lokoja Formation of southern Niger Basin in Nigeria is a dominantly sandy facies comprising of two distinctive members, the Lokoja and Patti Siltstone. The fluvial sandstone of the Lokoja Sandstone is petrologically defined as dominantly poorly sorted angular to sub-angular, fine to aly skewed, immature lithic arkose to sub-litharenite with a detrital clayey matrix of over fifteen percent (15%). The dominantly silty, marine to fluvial sediments of the Patti Siltstone is defined as massive, moderately sorted, sub-angular to sub-rounded, averagely fine skewed and minerologically mature but texturally immature quartz arenite. The quantitative analyses of light, heavy and clay fractions of sandstone samples from the Lokoja Formation also showed variation and made possible to group these sediments as lower and upper mineralogical units. A southwest terrain (basement rocks) with a peneplain topography that had relatively warm and humid tropical palaeoclimate served as the source of detrital materials for the Lokoja Sandstone whose deposition was rapid following a diastrophic movement (folding and faulting) in the continent. The Patti Siltstone sediments were derived from rapid erosion that characterized an area of high relief (crystalline West Africa basement rocks) with warm and tropical alkaline environment generating a uniform debris supply at slower but continuous rate of deposition.