Environment of deposition of the Awgu Formation (Late Cretaceous), Southern Benue trough, Nigeria

Abstract

The Awgu Formation is a linear NE-SW trending sedimentary deposit composed of the Ogugu Shale at the base and overlain by the Agbani Sandstone. There are carbonate-shale and sandstone facies representing a coarsening-upward (CU) succession from a muddy shelf to an upward-building sandy shoal. The blue gray shale with some pyrite indicates a marine environment that occasionally attained anoxic levels. The carbonates are wackestone-packstone of facies zone (FZ) 7 of Wilson (1975), indicative of platform carbonates with open circulation and tropical conditions. Clay mineral suites rich in smectite/ illite and smectite, and the glauconite present in the fine arenaceous facies, suggest a shallow marine depositional environment not exceeding 50 m water depth. The formation was folded and faulted in places with the folds arranged en echelon parallel to the NE – SW trending axis of the Benue Trough while the fractures are perpendicular to the said basin’s axis. The formation is a
remnant of the original deposits having been eroded subsequent to the Late Santonian deformation, uplift and erosion of the Benue depression.
The sandstone geometry (Length = 10 km, width = 10 km and exposed thickness = 45 m ) together with the upward increase in the number, thickness and angle of dip of cross-beds, typifies shallow marine sandstones. The bipolar-bimodal model of paleocurrrent directions as well as high variance are attributable to tidal currents and occasional oscillatory waves during the deposition of the Awgu Formation.