Sedimentary Petrology and Paleogeography of the Eocene-Sandstone Facies of Ilaro Formation, Dahomey Basin, South-Western Nigeria

This study examines the depositional processes and paleo-environment of sediments and pebbles from three sampled locations of the Tertiary Ilaro Formation in the Nigerian section of the Dahomey Basin. Petrographic analysis revealed that the sandstones are primarily arenites and sub-feldspathic arenites, based on the proportions of quartz, feldspar, and lithic fragments. The textural immaturity of the sandstones suggests that the sediments were transported from a nearby source and deposited along a passive continental margin. Grain size analysis indicated that the mean, standard deviation (sorting), skewness, and kurtosis values for sediment samples from the three locations ranged from 0.3 to 1.7, 0.7 to 2.0, and 0.4 to 1.5, respectively, indicating medium- to coarse-grained sandstones; 0.3 to 1.5, 0.5 to 0.8, and 0.8 to 1.2, indicating poorly sorted to very poorly sorted sandstones; 0.6 to 0.46, -0.08 to 0.19, and 0.07 to 0.25, indicating fine skewed with a few coarsely skewed; and 0.8 to 3.28, 0.81 to 1.5, and 0.93 to 1.23, indicating platykurtic, leptokurtic to mesokurtic, respectively. Grain size analysis indicates medium- to coarse-grained sandstones, with poor sorting indicating rapid deposition by strong, fluctuating currents. Bivariate plots help distinguish fluvial depositional environments from others. The C-M pattern of these sediments indicates transportation by bottom suspension. Pebble morphometric analysis and environment indicator plots suggest a fluvial paleoenvironment of deposition. A Mineralogical Maturity Index (MMI) of 14.30 and a Zircon-Tourmaline-Rutile (ZTR) index of 68.01% indicate mineralogically mature deposits and chemically immature to sub-mature sandstones. The combined results of granulometric statistics, morphometric indices, petrography, bivariate plots, and the absence of fossils or trace fossils suggest that these sediments were transported by rolling and bottom suspension over a short distance and deposited by a moderate- to high-energy fluvial system close to the source.