Progradation of a mid-Cretaceous distributive fluvial system: The upper member of the Bima Formation, Northern Benue Trough, Nigeria

Basin-wide analysis of an ancient distributive fluvial system (the upper Bima Formation) was carried out using field-based observation/measurement and ‘photo-realistic’ virtual outcrop data from 19 locations with good exposure of the upper Bima Formation in the Northern Benue Trough. The goal of this study is to provide an assessment of the spatial distribution of both vertical and lateral trends within the upper Bima distributive fluvial system. Vertical and lateral variations of the different facies of the formation were assessed quantitatively; this includes the analysis of the vertical and lateral distribution of proximal, medial and distal facies, the thickness and widths of channel fill deposits, grain-size variation and the proportion of floodplain to channel fill deposits across the 19 locations in the basin. Trends observed across the sections indicate a progradation of the mega-fan. The facies tracts preserved within the different portions of the distributive fluvial system vary. In the proximal part, in Lamurde, Tashan Alaji, Ture and Tula, the sections show only medial passing up to proximal facies. The distal lacustrine facies at the base of the section are absent. In the medial part of the system at Hinna, the successions preserve distal passing upward into medial facies. The gradual lateral transition from proximal to medial to distal facies, upward decrease in floodplain to channel fill deposits, upward increase in channel fill thicknesses and widths, all suggest that the Bima system is a prograding distributive fluvial system.