Pebble Morphogenesis and Paleocurrent Pattern as Signatures of Tide- dominated Marginal Marine Environment: A Case Study from the Nanka Formation, Niger Delta Basin, Nigeria

Pebble morphometry and Paleocurrent analyses were carried out on Nanka Formation for the purpose of environmental discrimination. The morphometric analysis of four hundred and ten (410) pebbles from the formation indicated that pebble forms are dominanted by compact (50.05%), compact platy (27.42%) and compact bladed (10.09%). Other pebble forms comprise of platy (8.96%), compact elongate (1.92%), bladed (1.26%) and elongate (0.30%). This suggests dominance of fluvial pebbles over the beach ones. 88% of the pebbles plot near the upper parts of the Sphericity Form diagrams, thereby supporting dominance of the fluvial deposits. The bivariate scatter plots of maximum projection sphericity index (MPSI) vs oblate- prolate index (OPI) and flatness index (FI) vs OPI show that over 95% of the pebbles indicated river process and less than 5% are beach pebbles. This result is consistent with the form diagrams for the pebbles. The occurrence of small proportion of the beach pebbles suggests that the ancient river flow reached the marginal marine environment. The Paleocurrent data indicated a bimodal- bipolar pattern in the northeasterly and southwesterly directions. Nanka Formation was laid down in a tide- dominated marginal marine environment.