Abstract
Pebble morphometric study was carried out for the Ogwashi-Asaba Formation so as to understand its paleodepositional environment. The three axes; the long (l), the intermediate (i) and the short (s) were measured for the two sets of one hundred and forty (140) and one hundred and fifty (150) pebbles collected from two outcrops of the Ogwashi-Asaba Formation. Pebble form indices such as Maximum Projection Sphericity (MPSI), coefficient of flatness (FR %), elongation ratio (ER) and oblate-prolate (OP) index were computed from these measurements. Roundness was estimated using Sames image sets. The mean values of the pebble form indices ranges from 49.78 to 52.02 for coefficient of flatness (FR %). 0.71 to 0.73 for sphericity, 2.60 t0 3.53 for oblate-prolate index, and 0.66 to 0.69 for elongation ratio. The calculated indices are suggestive of fluvial sedimentation for the Ogwashi-Asaba Formation. Bivariate plots of coefficient of flatness (FR %) versus maximum projection sphericity index (MPSI) and plots of sphericity (MPSI) versus oblate-prolate index (OP) are also indicative of fluvial origin. Scatter plots of roundness versus elongation ratio shows that 100% of the pebbles are in a littoral field. Plots of bivariate parameters such as simple skewness versus simple standard deviation, and second moment skewness versus second moment standard deviation suggests the Ogwashi-Asaba Formation as a product of the fluvial depositional processes. Keywords: Niger Delta Basin, Ogwashi-Asaba Formation, Pebble morphometry, Pebble Form indices. DOI : 10.7176/JNSR/9-6-06 Publication date :March 31 st 2019
Highlights
Pebble morphometric and other granulometric analysis have been utilized in paleoenvironmental interpretation of several sedimentary basins in Nigeria and worldwide
The form indices are displayed using the graphic method by plots of coefficient of flatness versus sphericity (Fig. 4a-b) (Stratten, 1974), and plots of sphericity versus oblate-prolate index (Fig.5a-b) (Stratten, 1974), and plots of roundness versus elongation ratio (Fig. 6 a-b) (Dobkins and Folk, 1970)
Certain shape classes of pebbles are diagnostic of depositional environments (Dobkins and Folk, 1970)
Summary
Pebble morphometric and other granulometric analysis have been utilized in paleoenvironmental interpretation of several sedimentary basins in Nigeria and worldwide. Ephraim et al (2015) used pebble morphometric investigations to suggest that the Benin Formation as exposed at Nsie, in southeastern Nigeria were deposited by fluvial processes. Odumodu and Isreal (2014) combined pebble morphometric analysis, lithofacies interpretation and sand textural analysis to infer a fluvial depositional environment for the Ogwashi-Asaba Formation as exposed at Nnewi and environs. Odumodu (2014) utilized pebble form indices as a paleoenvironmental reconstruction tool to provide evidence for the fluvial origin of the Benin Formation, outcropping along Atamiri River, in Uli, Anambra State. All previous paleoenvironmental studies utilizing pebbles and other granulometric materials did not consider the outcrops of Ogwashi-Asaba Formation outcropping at several localities around Umuahia. It is necessary to evaluate these exposures so as to resolve its paleodepositional setting
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