Sedimentological analyses of the conglomeratic sandstone and sandstone members of the Lokoja Formation, Bida Basin, Nigeria: Implications for paleoenvironment

Abstract

Sedimentological analyses of sandstones and conglomeratic facies within the Lokoja 
Formation belonging to the southern part of the Bida Basin around Lokoja and environs 
were carried out with the aim of deducing their depositional environment. Based on field 
observations and analysis of 33 representative samples, there is no occurrence of shale 
within the Lokoja Formation as earlier reported in the area. Results of glanulometric anal
ysis and parameters as well as bivariate plots show that the conglomerates are dominantly 
paraconglomerates and are essentially polymictic in nature. Thus, this is an indication 
that a fluvial sedimentation was responsible for the accumulation of sandstones of the 
Lokoja Formation. In support of this, evidence depicting characteristic bladed fabric was 
observed in 150 samples subjected to pebble morphometric analysis. This to a large extent 
also suggest a fluvial (river) depositional environment for the conglomeratic facies. Scatter 
plots of roundness versus elongation ratio delineate angular to sub-rounded pebbles and 
such grain shape delineation and the low roundness values for the pebbles (average, 0.55) 
are an indication of a fluvial environment of deposition and short transportation history. 
However, some pebbles indicate marine influence as some of the morphometric data plot 
within the littoral environment.