The lithofacies and depositional paleoenvironment of the Bima Sandstone in Girei and Environs, Yola Arm, Upper Benue Trough, Northeastern Nigeria

摘要

The Benue Trough is a contiguous intracontinental basin that formed part of the mega west and central African rift system (WCARS). The Cretaceous sedimentary succession preserved up to 6 km thick. The Aptian-Albian Bima Sandstone non-conformably overlies the Precambrian basement rock and the sandstone is the oldest Cretaceous succession in the Upper Benue Trough. This paper aimed at understanding its lithofacies and depositional environment in Girei and environs within Yola arm of the Upper Benue Trough. From the field studies, 13 lithofacies were identified; massive matrix supported and gravel dominated facies (Gmm), matrix supported gravel (Gmg), clast supported gravel (Gci), gravel stratified (Gh), and gravel stratified trough cross-bedded sandstone (Gt), sand, fine to very coarse grained trough crossbedded sandstone (St), sand, fine, medium, coarse to very coarse grained group planar cross-bedded sandstone (Sp), low angle planar crossbedded sandstone (Sl), sand, fine to coarse grained ripple cross laminated sandstone (Sr), sand, very fine, pebbly coarse grained horizontal bedded sandstone(Sh), Sand, fine to coarse grained massive sandstone (Sm), Convuluted sandstone (Sc), silt and mud (Fsm), and mudstone (Fm). The Bima Sandstone in the study area is sub-divided into proximal alluvial fan facies are composed of Gmm, Gmg, Gci, and Fsm, lithofacies, while the mid and distal fan facies are sequentially characterised by Gh, Gt, St, Sp, Sl, Sr, Sm, Sc, and Fm lithofacies. The alluvial fan facies are matrix supported and are restricted to the basin margin, and it may represent the basal part of the outcrop lithosections, the lack of internal organization of these lithofacies reflects high-strength viscous debris flow and it speaks volume of emergence of stream near the upland, however, the Gci facies probably indicate advancement of stream influence and reduction of gravity, the associated Fsm facies could be a backswamp and/or abandoned channel deposits. The mid fan facies have basal surface that is concave-upward in transverse crossection, welldeveloped thick amalgamated trough crossbedded sandstone (St) facies, theses attribute represent deep channel deposits, high flow regime, and continuous creation of accommodation space, it also points towards deep perennial sand braided river system. The distal fan facies is composed of amalgamated planar cross-bedded sandstone (Sp) associated with horizontal bedded sandstone (Sh), ripple cross lamination (Sr) and mudstone (Fm) facies with better developed floodplain deposit, pointing towards the influence of steady periodic high flow with topographic difference and it reflects shallow perennial sand-bed braided river. The lithofacies of the Bima Sandstone in Girei and environs, within the Yola arm generally show architectural stacking pattern that is generally thinning and fining upward indicating inactivity and/or fan retrogradation, and the depositional paleoenvironment varies from alluvial to braided river setting. The mid to distal fan facies of the Bima Sandstone can serve as good reservoir for hydrocarbon while the mudstone which is the floodplain deposit can serve as the seals and/or traps for the accumulated hydrocarbon.