SEDIMENTOLOGY OF SOME CRETACEOUS OUTCROPS WITHIN LOKOJA AND AGBAJA ENVIRONS IN THE NORTH CENTRAL NIGERIA

Abstract

Cretaceous sedimentary outcrops located within Lokoja and Agbaja environs were investigated for the purpose of determining the provenance of their sediments. The investigation was through field studies as well as textural and mineralogical analyses of the sediments. Major identified sedimentary facies are conglomerates, sandstone and ferruginous sandstone, siltstone and claystone. The matrix supported graded conglomeratic facies represent debris flow and braided stream flow of alluvial fan and channel deposits The morphometry of the pebbles with roundness value of 52% is an indication of short transportation history. Their scatter plots of Maximum Projection Sphericity Index (MPSI) vs Oblate Prolate Index (OPI) also indicate their fluvial transportation. The sandstone is subarkosic, as it is composed of both monocrystalline and polycrystalline quartz interspersed with appreciable quantity of feldspathic grains reflective of a nearby crystalline rock as source of the clastics. The azimuths of the beds primary structures which dip southwardly strongly suggest the North Central Basement as the provenance. Texturally, the sandstone is poorly sorted (0.51 to 1.70ɸ), medium to coarse grained (0.16 to 2.53ɸ) while its skewness and kurtosis has average values of 0.20 and1.78 respectively. A fluvial setting of depositional environment is inferred. However, dominance of ferruginous sandstone, siltstone and claystone as well as primary structures of herringbone and reactivation surfaces are strong indicators that the sediments in Lokoja and Agbaja environs were fluvially transported and were eventually emptied into both agitative tidal as well as anoxic stagnant flood plains.