Grain size distribution and paleoenvironmental studies of upper cretaceous to paleocene rocks in auchi sheet 266, benin flank, western extension of the anambra basin, southwestern Nigeria

Out crop samples from Mamu, Ajali and Nsuka formation at the different location and ditch cutting samples gotten from Ugbekpe well at different intervals were collected from Benin flank, western extension of the Anambra basin with the aim of identifying their lithofacies and paleoenvironments. Five (5) lithofacies were identified in the Lokoja Basa’Nge’ Formation which are cross laminated sandstone, coarse to pebbly grained sand stone, bioturbated sand stone, silty clay, medium to coarse grained sand stone, and sandy clay. Eight (8) lithofacies were identified in Mamu Formation which are parallel laminated grey shale, parallel laminated dark shale, parallel laminated silty clay, ripple bioturbated clayey siltstone, silty clay, parallel laminated clayey siltstone, siltstone, and clayey siltstone. Six (6) lithofacies were identified in Ajali Formation which are fine to medium sandstone, planar cross-bedded sandstone, herring-bone cross-bedded sandstone, medium to coarse sandstone, medium ripple laminated sandstone, and sandy clay. Ten (10) lithofacies were identified in the Nsukka Formation this include dark fissile shale, fine to medium shaly sandstone, coarse poorly sorted sandstone, black fissile sandy shale, clayey sandstone, whitish fine grained sandstone, reddish brown sandstone, granule to pebbly sandstone, grey clay and silty sandstone. Sedimentary structures that characterize these sediments are trough cross bedding, reactivation surfaces, horizontal laminations, low angle cross laminations, herring-bone cross stratifications and planar cross bedding. The depositional environment suggested for the various formations are fluvial to shallow marine environment for Ajali sandstone Formation, fluvial environment for Lokoja Basa’Nge Formation , Near shore to Deltaic environment for Mamu Formation and Open marine, shallow marine, Near shore to Deltaic environment for Nsukka Formation. The Nsukka Formation shows a progradational dominated stacking pattern and a system tract that is basically Highstand systems tract (HST) and one maximum flooding surface.