Geological Mapping, Petrographic Characteristics and Grain-Size Analysis of Sandstone in Inachi, Lower Benue Trough, Nigeria

Abstract

Geological mapping, petrographic examination and grain-size analysis were undertaken to characterise sedimentary rocks in Inachi, Lower Benue Trough, Nigeria, and assess their depositional and groundwater implications. Mapping covered approximately 20 km² within Otukpo Sheet 270SW, supported by thin-section examination and sieve analysis of three sandstone samples. Shale and sandstone were the principal lithologies. Fractures showed a dominant NW–SE orientation, while sandstone beds dipped 4–8° and ranged from 0.2 to 2.3 m in thickness. Reported petrographic estimates comprised 52–60% quartz, 30–35% feldspar and 5–18% accessory constituents; uncertainty in these estimates was not quantified. Grain-size statistics recalculated from previously recorded percentiles, rather than new measurements, yielded mean sizes of 2.567–2.800 φ and sorting values of 0.458–0.598 φ. The sandstone samples were fine-grained, with two classified as well sorted and one as moderately well sorted. Skewness ranged from 0.029 to 0.331 and kurtosis from 1.393 to 1.475, indicating nearly symmetrical to strongly fine-skewed, leptokurtic distributions. Contrasting fluvial and beach signatures from existing discrimination plots leave the depositional environment unresolved; a transitional setting remains a tentative interpretation. Local shallow groundwater abstraction indicates water occurrence, but seasonal well failure and the absence of direct petrophysical and hydraulic measurements prevent conclusions about aquifer productivity. The findings provide a preliminary geological baseline, with interpretations limited by the small sample set. Broader sampling, improved stratigraphic control and direct hydraulic testing are needed.