Pebble Morphometric Analysis and Depositional Environment of the Basal Conglomerates of Bida Sandstone exposed around Zungeru, NW Nigeria

Abstract

The basal conglomerate lithofacies of Bida Formation is common along the basin margin where it overlies the rocks of the basement complex in Zungeru area, northwestern Nigeria. The conglomerate lithofacies contains abundant pebbles of vein quartz and the clasts of these pebbles were selected for morphometric study to determine their depositional environment. A total of 190 pebbles were selected from nineteen weathered outcrops for this study. The three mutually perpendicular axes; long, intermediate and short axes were measured and the roundness estimated with the aid of a roundness image set. Form names were obtained using triplot to generate Sneed and Folk diagram. Morphometric parameters such as size, flatness ratio, elongation ratio (ER), maximum projection sphericity index (MPSI), form geometry and oblate-prolate index (OPI) were computed. The bivariate plots of morphometric parameters such as coefficient of flatness versus MPSI, and MPSI versus OPI indicate that the pebbles were deposited in fluvial environment but bivariate plot of roundness versus elongation ratio showed that they were deposited in littoral (marine) environment. The dominance of bladed shape pebbles in the conglomerate lithofacies indicates that the basal conglomerates of Bida Formation exposed in Zungeru area is sourced from the surrounding basement such as schists and gneisses rich in vein quartz fills. It is concluded that these sediments are products of fluvial deposition with some marine influence. Keywords: Bida basin, Bida Formation, Zungeru, Conglomerate, Morphometric, depositional, bivariate plot, fluvial