Sedimentologic characteristics and correlations of coastal plain sand and alluvium deposits within Lagos environs southwestern Nigeria

Abstract

This study involves the granulometrics and lithological classification of the coastal plain sands and alluvium deposits within the Lagos environs, Nigeria, to decipher the lithofacies, sedimentary succession, and the variations imposed by the paleodepositional settings. Thirty (30) core samples were recovered for granulometric analysis and to construct the lithologic logs for the 10 boreholes, while grain size analysis was conducted on core samples from three of the boreholes (wells 3A, 11, and 27). The grain size distribution of the sediments was conducted using a mechanical sieve shaker to determine the provenance, transport medium, and environment of deposition. The weight percentage of each sieve retained was measured and used to compute the grain size parameters, including the mean (Mz), standard deviation (σ1), skewness (Sk1), and kurtosis (KG), using established formulas. The granulometric results of three boreholes show that the samples are fine-grained through medium- to coarse-grained sand particles. The sediments are moderately sorted (average of 1.01ɸ, 1.09ɸ, and 1.11ɸ). The sorting and skewness suggest fluvial (river) systems. The kurtosis results (average of 1.09ɸ, 1.15ɸ, and 1.11ɸ) show energy variation from low to high-energy environments across the three wells, with a predominance of low-energy environments. There were occurrences of peat and the prevalence of mud sand in tidal flats at Barrier Bar Lagoon coastal system of Badagry, white sand at deeper depth in the peninsular, and brownish sand at shallow depth, which indicate a regressive sequence with fresh water sand containing a high concentration of iron at shallow depth.