The Mamfe Formation, South Eastern Nigeria, Its Paleoenvironments and Paleoclimatic Settings: Evidence from Petrography and Grain Size Distributions

Petrographic analysis and textural distribution of the sediments of the Mamfe formation were to determine their provenance, paleoclimate and paleoenvironment. These sediments were subjected to sedimentological studies involving grain size analysis, pebble morphometry and thin section petrography. From the results of grain size analysis, the mean grain size, inclusive standard deviation, graphic skewness and kurtosis yielded average values of 0.94 ?, 1.30 ?, 0.1 ?, and 1.20 ? respectively. These results show that the sandstones have representatives of fine to coarse grain sizes; they are poorly sorted with dominance positively skewed suggesting a fluvial origin for the sediments. Bivariate analysis also suggests that fluvial processes exerted major control on transportation and deposition of the sediments. Results from pebble morphometric parameters including elongation ratio (ER), flatness ratio (FR), maximum projection sphericity index (MPSI), oblate-prolate index (OPI) and sphericity (S) all fall within acceptable limits for fluvial deposits. These suggests dominance of fluvial actions during deposition. Plots of Sphericity vs. OP index and particle form triangular diagram also indicates the pebbles were shaped predominantly by fluvial action. From petrographic analysis, feldspar and rock fragments contributed less of the sediment framework constituents compared with the composition of quartz. The sandstones are therefore described as texturally and mineralogically immature, arkosic to sub-arkosic arenites sourced from uplifted basement rocks and deposited in a humid climatic condition.