Lithofacies Analysis and Paleaoenvironment of Deposition of Lokoja Formation, Southern Bida Basin, North Central, Nigeria

Abstract

The sedimentary facies and depositional environment of Lokoja Formation, Southern Bida Basin was evaluated through the study of some of its exposed sections and the samples obtained are subjected to pebbles morphometry and granulometric analsysis.  The study area comprises of three (3) litho-facies which are the conglomerate, pebbly sandstone and siltstone/mudstone facies which displayed lithological characteristics, sedimentary structures and textural variation that indicate alluvial deposit which might have prograded through braided streams to nearshore marine conditions. From the pebbles morphometric analysis, the coefficient of flatness indicated that the pebbles are of fluvial origin as it consist of over 90% above fluvial limit. The elongation ratio indicated that the pebbles are of fluvial origin as over 90% has values between 0.6-0.9. Maximum Projection Sphericity Index (MPSI) of the pebbles analysed showed that about 95% were below 0.66 and this indicate beach origin. Oblate Prolate Index (OPI) which ranges from -0.1 – 19.0 show significance of fluvial deposit with minimal beach. From the granulometric analysis, the value of graphic mean ranges from -1.33 to 1.02 and this inferred that the grain size of the sandstone of the Lokoja Formation ranges from medium to very coarse grained. Sorting values ranging from 0.72 to 1.41, these values are indicative of moderate to poor sorting. The skewness values ranges from -1.01 to 0.36 which indicated that the sandstone is nearly symmetrical, strongly fine skewed and strongly coarse skewed. The values of kurtosis ranges from 1.0 to 4.08 which indicated mesokurtic to extremely leptokurtic. The plots of graphic mean and skewness against the standard deviation showed that the study area is dominantly of fluvial origin. Generally it can be conclude that the Lokoja Formation is dominantly of fluvial origin with little beach (marine) influence.