Abstract
Pebble morphometric and sedimentologic study was carried out for the Lokoja Sandstone Formation to understand its provenance and palaeodepositional environment history. The three axes in pebbles; the long (l), the intermediate (i) and the short (s) were measured for the four sets of forty- five (45), fifty(50), and fifty (50) pebbles collected from three outcrops of the Lokoja Formation exposed at Robinson Street (Lokoja), Felele, and Ozi. Pebble form indices such as Maximum Projection Sphericity (MPSI), coefficient of flatness (FR %), elongation ratio (ER), and oblate-prolate index (OPI) were computed from these measurements. Roundness was estimated using Sames image sets. The mean values of the pebble form indices range from 52.5 to 55.3 for coefficient of flatness (FR %). 0.307 to 0.727 for sphericity, 0.273 to 1.182 for oblate prolate index, and 0.748 to 0.765 for elongation ratio. The calculated indices are suggestive of fluvial sedimentation for the Lokoja Formation. Bivariate plots of the coefficient of flatness (FR %) versus maximum projection sphericity index (MPSI) and plots of sphericity (MPSI) versus oblate-prolate index (OPI) indicate fluvial origin. Sedimentologic analysis to determine these parameters was also used. In the determination of the depositional environments for the basin, sorting values and binary plots with boundaries modified after Friedman (1967), Moiola and Weiser (1968), and Stewart (1958) were done for 30 samples of the sandstone unit of the Lokoja Formation gotten from five different locations exposed at Robinson Street, Sarkin Noma, Jamata, Banda, and Ozi using skewness, standard deviation, median and mean values generated from the analysis with the results indicating fluvial processes of deposition.
Key words: Keywords: Southern Mid-Niger Basin, Lokoja Formation, Pebble morphometry, Pebble Form indices, Sedimentology
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