Granulometry, Pebble Morphometry and Petrographic Studies as Indicators of Paleodepositional Environments of the Mamfe Formation in Ikom-Mamfe Embayment, South Eastern Nigeria

Abstract

Grain size distribution, pebble morphometry and petrographic analyses were carried out on samples collected from Mamfe Formation, Ikom-Mamfe Embayment, southeastern Nigeria to determine textural parameters, provenance and paleoenvironment. The results from grain size analysis show that mean grain size, inclusive standard deviation, graphic skewness and kurtosis yielded average values of 0.94ɸ, 1.30ɸ, 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 infers that the sandstones were deposited in fluvial regime. The mean values of 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. The mean roundness suggests fluvial action, also indicated by the shape of the pebbles. Plots of Sphericity vs. OP index and particle form triangular diagram also indicates the pebbles were shaped predominantly by fluvial action. Petrographic study reveals quartz as the dominant framework grain, followed by feldspar and rock fragment. The sandstones are texturally and mineralogically immature. The sandstone of the Ikom-Mamfe Embayment is classified as arkosic-subarkosic arenites sourced from uplifted basement rocks and deposited in a humid climatic condition.