Quantifying a tide-dominated, fluvial-wave influenced delta in Miocene facies of the Niger Delta Basin

Abstract

The modern Niger Delta has long been classified as a mixed tide-, wave- and fluvial-influenced delta. Still, no detailed studies have quantified the relative proportions of formative processes and facies. This work presents the first quantitative estimate of the relative influence of formative processes in Miocene deposits based on core data from the Greater Ughelli, Central and Coastal Swamp depositional belts of the Niger Delta Basin. Facies analysis of 288.2 m of core from 4 wells shows approximately 53% tidal facies, 33% fluvial facies and 14% wave-formed facies, indicating deposition in a prograding tide-dominated, fluvial- and wave-influenced delta. Cores from the Greater Ughelli and Central Swamp depobelts exhibit coarsening upwards, prograding deltaic facies successions overlain by fluvial mudstone. Tidal deposits in the Coastal Swamp depobelt show coarsening upwards prograding deltaic facies successions with well-developed tidal bundles indicating periodic deposition. The basal part of these facies successions also reveals repeated floods characterised by slump and load structures and dewatering features. Delta plain and delta front-prodeltaic facies associations are identified based on physical sedimentary and biological structures. The delta plain facies association consists of weakly bioturbated mudstone, fissile mudstone, and coarse-pebbly stratified sandstone facies with sparse trace fossils. The delta front-prodeltaic facies association contains muddy and sandy heteroliths, stratified, medium and coarse-grained, cross-bedded sandstone, and convoluted mudstone facies. The facies vary but characteristically contain trace fossil assemblages of the recently proposed Rosselia and Phycosiphon Ichnofacies, indicating delta front and prodeltaic settings, respectively. Variability in the facies is determined by the relative influence of hydrodynamic processes (tide, wave, and fluvial), variations in physicochemical stress, and the episodic character of deposition. In addition, based on the relative influence of each hydrodynamic processes, the facies differ through the successions, sometimes subtly.