Application of pebble morphometry and sedimentology in studying provenances and deposition environments of the Lokoja Formation in the southern Mid-Niger Basin as exposed in parts of Lokoja and its adjoining areas

Abstract

Pebble morphometric and sedimentologic study was carried out for the Lokoja Sandstone
Formation to understand its provenance and palaeodepositional environment history. The
3 axes in pebbles; the long (l), the intermediate (i), and the short (s) were measured for the
4 sets of 45, 50, and 50 pebbles collected from 3 outcrops of the Lokoja Formation exposed
at Robinson Street (Lokoja), Felele, and Ozi. Pebbles form indices such as maximum pro-
jection sphericity index (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 FR %. About 0.307–0.727 for sphericity, 0.273–1.182 for OPI, and
0.748–0.765 for ER. The calculated indices are suggestive of fluvial sedimentation for the
Lokoja Formation. Bivariate plots of the FR % versus MPSI and plots of sphericity (MPSI)
versus OPI indicate fluvial origin. Sedimentologic analysis to determine these parameters
was also used. In the determination of the depositional environments for the basin, sort-
ing values and binary plots with boundaries modified after Friedman, Moiola and Weiser,
and Stewart were done for 30 samples of the sandstone unit of the Lokoja Formation got-
ten from 5 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.