Abstract

Pebble components of the Arkosic Sandstone deposits of the Lokoja Formation were subjected to macro- and micromorphological investigation in order to infer the depositional environments and the energy regimes under which the formation was formed. The long (L), intermediate (I) and short (S) axes of 338 carefully selected pebbles from the Lokoja Formation were measured using digital Vernier Calliper. Form indices, including Coefficient of Flatness (C.F), Oblate-Prolate (OP), Maximum Projection Sphericity Index (M.P.S.I.) and Elongation Ratio (E.R) were calculated from the measured parameters. Roundness of the pebbles were also individually measured. The calculated mean values: C.F. (62.5%), OP (0.179), M.P.S.I. (1.515), E.R. (0.788) and Roundness (0.331) from the form indices are above the lower limits for fluvial shaped pebbles. Scatter plots of M.P.S.I. against OP, Roundness vs E.R and C.F against M.P.S.I. correlate well with typical plots of fluvial shaped pebbles, therefore indicating the deposition of the pebbles of Lokoja Formation in a fluvial setting. The scanning Electron Microscopy (SEM) result suggests deposition in high-energy environment.