Pebble Form Indices As Signatures Of The Depositional Environment Of The Benin Formation Along Atamiri River, Uli, South – Eastern Nigeria

The Benin Formation is well exposed along the Atamiri River, which traverses Uli town, in the Niger Delta Basin of Southeastern Nigeria. Lithologic sections were measured, described and sampled so as to decipher the paleodepositional environment, using an integration of lithofacies data, pebble morphology, grain size and paleocurrent analysis. The lithofacies study suggests that the Benin Formation consists mainly of two facies association; sandstone and clay facies. The pebbly sandstone subfacies indicate braided channel bars, while the coarse grained sandstone subfacies and medium grained sandstone subfacies are interpreted as fluvial channel bars. The bioturbated medium grained sandstone suggests near shore shallow marine influence while the clay and sandy clays are interpreted as floodplain deposits. Results of studies of form indices of large particles from the Benin Formation show that the coefficient of flatness (55.84 ± 3.89), elongation ratio (0.787 ± 0.035), maximum projection sphericity (0.731 ± 0.030) and oblate prolate index (0.473 ± 1.489) are all indicative of fluvial sedimentation. The mean of these indices confirm a fluvial origin for the pebbles. Bivariate plots of coefficient of flatness against sphericity, and sphericity against oblate prolate index are all indicative of fluvial sedimentation. Plots of simple skewness measure against simple standard deviation and 3 rd moment skewness against 2 nd moment standard deviation supports the fluvial origin of the sandstones. Plots made on the sphericity form diagram suggest that sphericity decreases with increase in pebble sizes. This study is significant in providing evidence for the fluvial (braided stream) origin of the Benin Formation during the Oligocene – Miocene times.