Granulometric and pebble morphometric applications to Benin Flank sediments in western Anambra Basin, Nigeria: proxies for paleoenvironmental reconstruction

摘要

Integrated granulometric and pebble morphometric study of Late Cretaceous rocks of the Benin Flank, Nigeria, were used to delineate depositional process and environment of the rocks in the study area. Granulometric analysis reveals that the sandstones of the Lokoja Bassange Formation have an average graphic mean (Mz) of 0.59 Phi (coarse grains), inclusive graphic standard deviation (delta i) of 1.36 Phi (poorly sorted), graphic kurtosis (Ka) of 1.12 Phi (leptokurtic), and inclusive graphic skewness (Ski) of -0.14 Phi (stongly coarse skewed). The sandstones for the Mamu Formation have an average Mz of 1.42 Phi (medium grains), inclusive graphic standard deviation (delta i) of 0.74 Phi (moderately sorted), graphic kurtosis (Ka) of 1.43 Phi (leptokurtic), and inclusive graphic skewness (Ski) of 0.02 Phi (nearly symetrical). The sandstones for the Ajali Formation have an average Mz of 1.23 Phi (medium grains), inclusive graphic standard deviation (delta i) of 1.21 Phi (poorly sorted), graphic kurtosis (Ka) of 1.46 Phi (leptokurtic), and inclusive graphic skewness (Ski) of -0.17 Phi (nearly symetrical). Pebble morphometric analysis of the larger grains reveals that the mean values of elongation ratio (ER) = 0.69, coefficient of flatness (FI) = 54.90%, oblate prolate index (OPI) = 3.34, the maximum projection sphericity index (MPSI) = 0.75, form = 0.67, and roundness (rho) = 36.28% for the Lokoja Bassange Formation; ER = 0.76, FI = 37.07%, OPI = -3.20, the MPSI = 0.53, form = 0.38, and rho = 52.93% for the Mamu Formation; and ER = 0.72, FI = 47.44%, OM = 1.12, the MPSI = 0.67, form = 0.54 and rho = 42.68% for the Ajali Formation. The integration of the various granulometric and pebble morphometric data, different bivariate and ternary plots indicate fluvial environment for the Lokoja. Bassange Formation, dominantly shallow marine environment for the Mamu Formation and fluvial shallow marine environment for the Ajali Formation. This study shows that the Benin Flank, western Anambra. Basin has a depositional setting radiating between fluvial and shallow marine environments.