Petrography and geochemical indices of the Lagos lagoon coastal sediments, Dahomey Basin (southwestern Nigeria): Sea level change implications

The Quaternary lithosome indicates the presence of clay, siltstone, sandstone and conglomerate. The sandstones are fine to coarse-grained, poorly sorted to moderately well sorted, negatively to positively skewed and mesokurtic to leptokurtic. The heavy mineral assemblage is characterized by zircon, rutile, tourmaline, garnet, staurolite, sillimanite, apatite, epidote, hornblende and kyanite. This assemblage suggests sediments derived from both igneous and metamorphic provenance, probably from the Ibadan Craton. The average ZTR index is 27.84%, indicating mineralogically immature sediments. Petrographic analysis shows that the most abundant detrital minerals are quartz (average of 88.7%), feldspar (average of 4.37%) and rock fragments (average of 7.3%). The QFR ternary plots revealed that the sands are super-mature quartz arenite derived from the uplifted continental block under a humid climatic setting, which have undergone moderate transportation and have been subjected to moderate mechanical and chemical weathering. The ratios of K2O/Na2O and SiO2 suggest passive and active continental margin settings. Linear discriminant analysis revealed that the sediments were deposited within fluvial to shallow marine lithosomes. This corroborated the bivariate plots of the statistical parameter. The results of the trace element and ratios considered with regard to their distribution in the diverse lithofacies showing the range of Sr from 50 to 120 ppm; Ba, 180-340 ppm; Ga, 5-14 ppm; B, 25-48 ppm; V, 53-125 ppm; Sr/Ba, 0.20-0.39 and Ba/Ga, 1.86-7.00, all indicating fluvial, shallow and marginal marine lithosomes. The lithofacies are derived from a mixed provenance, have undergone chemical weathering during transportation, and are deposited in the fluvio-marine environment. These indices indicate the occurrence of weak marine transgressions.