Depositional variability of an ancient distributive fluvial system: The upper member of the lower cretaceous Bima Formation, Northern Benue Trough, Nigeria

The upper Bima Formation in the Northern Benue Trough has been interpreted as a distributive fluvial system. Previous stratigraphic studies on the Bima Formation were focussed on localized, qualitative, sedimentary facies analysis without regional context or quantitative information about sand-body scale and architecture. This study quantitatively analysed sand-body thickness variations in the upper member of the Bima Formation across the Northern Benue Trough, documented the spatial variations in channel dimensions. Photo-realistic virtual outcrops were generated for four study sites using data acquired with an unmanned aerial vehicle and processed photogrammetrically. Analysis of the virtual outcrops illustrated spatial variation in the thickness of channel infill components, a downstream decrease in proportion of multi-storey channel belt facies association (from 98% to 5%), an increase in floodplain facies association (from 0% to 13%), an increase in the proportion of isolated channel fill facies association (from 2% to 80%) and a slight decrease in average grainsizes of channel fill sandstone. These observations are interpreted to represent evidence of a distributive fluvial system (DFS) or mega-fan with the proximal part of system at Tula, Ture and Tashan Alaji to the medial part at Hinna having an increase in frequency and thickness of the flood plain facies association and abundance of isolated channel fill facies associations. This study demonstrates the regional facies variability of the upper Bima Formation and has provided a basis for comparison with other ancient distributive fluvial system.