Anastomosing River Deposits: A case study of Enagi Siltstone, Northern Bida Basin, central Nigeria

Abstract

Anastomosing river system has been described as alluvial deposits having multiple coexistent interconnected channel belts enclosed by floodbasins. The present study, based on facies and architectural elements analyses, demonstrates that the sediments of Enagi Siltstone in the northern Bida Basin in central Nigeria can be interpreted as products of an anastomosing river system. Outcrops of the Enagi Siltstone exposed near Batati, Patti-Shaba Kolo and Manigi towns revealed mainly two facies namely: (1) Fa, characterised by (1 – 3 m thick) massive/laminated siltstones or planar cross-bedded very fine to fine-grained sandstones with sharp undulating erosional basal surfaces mantled with flat-mud-pebble conglomerate, interpreted as stable channel units; and (2) Fb, consisting of (1 – 8 m thick) mudstones or interbedded siltstones and mudstones organised into laterally extensive thin to thick tabular beds interpreted as floodplain/overbank deposits. The facies define channel and overbank architectural elements respectively. The evidence for deposition within channels of the Fa facies is given by the possession of erosional base, coarse lag, fining-upward grain size profile as well as upward decrease in thickness of cross-bed sets which is consistent with upward decrease in energy in confined environments such as channels. Owing to the overwhelming dominance of the overbank architectural element over the channel element, the generally fine-grained nature of the deposit as well as lack of marine indicators, we conclude that the sediments of Enagi Siltstone exemplify an anastomosing river deposit.