Sedimentological Characteristics of Ilaro Formation, Exposed Around Ajegunle, Dahomey Basin, Southwestern Nigeria

Abstract

This study investigates the sandstones of the Ilaro Formation exposed in the Ajegunle area to infer their provenance, sedimentary history, and tectonic setting. Eleven samples were subjected to granulometric, petrographic, and heavy mineral analyses. Granulometric analysis, conducted in accordance with ASTM C136 standards, indicates that the sandstone sediments are medium to coarse-grained, poorly sorted, and exhibit nearly symmetrical to fine-skewed distributions with predominantly leptokurtic characteristics. The mean grain size of the sediments ranges from 1.30 mm to 0.65 mm, with an average of 0.98 mm, classifying them as arenaceous. The sediments also exhibit a significant argillaceous component, as indicated by the average skewness value of 0.11. Petrographic analysis identified quartz grains exhibiting polymodal fractures, along with the presence of opaque minerals and iron and clay as diagenetic cements. Heavy mineral analysis, using bromoform (CHBr₃), identified minerals such as tourmaline, rutile, zircon, staurolite, and opaque minerals, suggesting a mixed provenance from igneous and metamorphic source rocks. The results indicate that the Ajegunle Sandstone was deposited in a fluvial environment.