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Origin Of The Oweli Sandstone, Anambra Basin: Evidence From Textural Parameters

ABSTRACT

The textural characteristics of the of the Oweli Sandstone are evaluated to reveal the depositional processes A total of 400 pebbles and twenty sandstone samples were systematically collected from two horizons along the outcrop belt in the study area were subjected to pebble morphometric and sand-size particle analyses. Results of the pebble analysis show that the pebbles exhibit an up-section increase in the mean form indices. The mean Flatness (F) index for pebbles from the two horizons ranges from 49.75 to 56.00, while the mean Maximum Projection Sphericity (MPS) index is consistently 0.72. The mean Oblate-Prolate (OP) index for the pebble suites ranges from -0.01 to 0.16. These values consistently indicate that the pebbles were shaped by a fluvial process. Plot of MPS against FI and OPI against MPS, and plots on the sphericity-form diagram all confirm that the pebbles are 100% fluvial in origin. Textural parameters reveal that the sand is medium to coarse grained, moderately well-sorted to poorly sorted, and positively to negatively skewed. These results suggest a broad spectrum of populations that accumulated under fluctuating/tidal energy conditions. Plots of mean size against sorting coefficient and skewness against sorting coefficient all indicate a dominant fluvial process. Log-probability plots of the grain size distribution reveal that the Oweli Sandstone is a fluvio-deltaic sand body.

Keywords: Oweli Sandstone, Anambra Basin, pebble indices, textural parameters, depositional processes.