Paleoenvironmental Analysis and Its Significance in Sedimentology: Case Study of the Conglomerate Facies of the Awi Formation, Calabar Flank, Southeast Nigeria
Pebble morphometry of unbroken quartz pebbles collected from the basal section of Awi Formation exposed around Ewen area, southeastern Nigeria was studied for paleoenvironmental reconstruction. Paleoenvironmental analysis refers to the study or use of ancient geological materials (rocks) to infer the depositional environment or setting within which they were deposited. Pebble morphometric analysis has aided the determination of paleoenvironment during the deposition of Awi Formation. The depositional processes responsible for shaping the pebbles and the environment that prevailed during past geological times was characterised. This involved the measurement of the three orthogonal axes (long, intermediate and short), determination of their corresponding roundness, flatness indices and elongation indices for no less than 200 pebbles. The pebbles were selected from 20 points across four exposed sections of the Awi Formation around Ewen village. The roundness was determined using the standard roundness chart. The results show that the pebbles are sub-rounded to sub-angular and predominantly compact-bladed. The mean values for the following morphometric parameters: Flatness index, elongation ratio, maximum projection sphericity index and oblate-prolate (OP) index are 0.57, 0.78, 0.74 and 15.65 respectively. These values are in agreement with those of modern fluvial pebbles. This result was integrated with the deductions from bivariate plots of roundness against elongation ratio and sphericity against OP index, and they all inferred the deposition of the conglomeratic sandstones in a fluvial setting with subordinate transitional setting. Fluviatile process with some overlapping littoral influence has been shown to be responsible for the variation in clast morphology of the paraconglomerates (matrix-supported) of Awi Formation. Integrating all observations viz; the sediment stratification and grading style (fining upwards successions) and the dominant unidirectional nature of the crudely imbricate pebbles, a typical fluvial setting is inferred. It is possible that the jointing, faulting, sheeting and/or exfoliation of the rocks of the Oban Massif, which is believed to be the principal source of the sediments (provenance), also accounts for the abundance of vein quartz in the area which was eventually adapted for this study. Within sedimentary settings as this one with paraconglomerates associated with high energy flux during deposition and other typical channel lag deposits are locations of good economic deposits (placer deposits) and in some cases hydrocarbon accumulation. Therefore, besides the significance for pebble morphometry in deciphering paleoenvironments, it also gives clues for potential sites of ore bodies and/or characteristics of some targets for hydrocarbon pools. There are obviously several methods for paleoenvironmental reconstruction using sediments as earlier mentioned, grain morphology is one. However, care must be taken when reconstructing paleoenvironment because the shape of grains is a result of so many other factors and for effective utilisation, a careful study and integration of all other parameters are recommended.
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