Facies analysis and depositional environment of the Campano – Maastrichtian coal-bearing Mamu Formation in the Anambra Basin, Nigeria

Abstract

Detailed sedimentological logging was carried out within the exposed rock sections of the late Campanian to early Maastrichtian successions of the coal-bearing Mamu Formation, southeastern province of Nigeria. Lithofacies analytical approach was employed in describing the outcropping rock units. The focus was to understand the distribution of these facies and their depositional environments. Eleven lithofacies recognized were found occurring within fining/thinning upward successions typical of retrogradational stacking patterns formed during transgressive episodes and coarsening/thickening succession typical of progradational stacking patterns formed during regressive episodes. These lithofacies were further classified into five facies associations, namely: lagoonal or swamp deposits (FA1); barrier island deposits (FA2); shoreface deposits (FA3); offshore-transition zone deposits (FA4) and; open shelf deposits (FA5). FA1 is dominantly coal and carbonaceous shale deposited in a swamp or lagoonal setting. FA2 is dominantly parallel laminated and cross-bedded fine to medium grained sandstones formed by wave and longshore currents within barrier island complexes. FA3 is dominantly bioturbated sandstone, planar and rippled laminated sandstones, and planar and trough cross-bedded sandstones facies deposited in a shoreface environment, by migrating dunes formed under unidirectional flows above fair-weather wave base. FA4 is mainly wave ripple laminated, hummocky and swaley cross-bedded sandstones deposited by waning oscillatory and combined flows within a storm wave-dominated shelf region, below fair-weather wave base. FA5 is fissile shales and mudstones deposited in an open shelf setting below fair-weather and storm wave base. This study indicates that outcropping units represent lithofacies, which record depositional elements of high frequency transgressive and regressive events in a coastal swamp through shelfal depositional environment. In addition, the coal seams and carbonaceous shale facies (source/seal rocks), and the sandstones, siltstone and heterolithic facies (reservoir rocks) indicate the possible presence of key elements of petroleum system in the Mamu Formation of the Anambra Basin.