Geophysical Investigation of the Lithostratigraphic Framework of the Lokoja Formation, North Central Nigeria

The Lokoja Formation exposed along the Benue-Niger confluence area in Kogi State is a Cretaceous fluvial to marginal marine sequence of economic significance due to its associations with lead-zinc mineralisation and its role as a reservoir analogue for subsurface correlative units in the Benue Trough. This undergraduate study investigates the near-surface lithostratigraphic framework of the Lokoja Formation using electrical resistivity tomography and gamma-ray spectrometry along three sections in Lokoja. ERT profiles of 300 metres were acquired using the Wenner array at each section, while handheld gamma-ray spectrometry measurements were acquired at 10-metre intervals along the same transects to provide radioelement (K, U, Th) signatures distinguishing sandstone, mudstone, and conglomerate facies. The inverted resistivity sections resolve three major resistivity units interpreted as channel sandstone bodies (high resistivity), intercalated mudstone (low resistivity), and conglomerate beds (variable high resistivity). Gamma-ray spectrometry confirms this interpretation, with potassium-rich mudstones producing elevated K readings contrasting with uranium-depleted clean sandstones. The lateral geometries of individual channel sandstone bodies are mapped with good resolution, revealing sinuous planform morphologies. Correlation of interpreted resistivity units with measured section logs from exposed outcrop adds stratigraphic control. The study demonstrates the value of integrating surface geophysics with geological mapping for lithostratigraphic characterisation of Cretaceous sediments. Keywords: Lokoja Formation, ERT, gamma-ray spectrometry, lithostratigraphy, Benue Trough