SEDIMENTOLOGIC CHARACTERISTICS OF THE ALLUVIUM DEPOSITS IN LAGOS PENINSULA: IMPLICATIONS ON ITS STRATIGRAPHIC STATUS

Abstract

The sediments of the sequences commonly exploited for ground water and tested for foundation purposes along the coast line of Lagos were subjected to sedimentological studies using litholog data of cuttings retrieve during drilling and few wire line log of boreholes along the coast. The study was designed to correlate, based on lithic characteristic, the strata of these shallow beds classified stratigraphically as Coastal Plain Sand/ Alluvium Deposits with a view to reviewing its stratigraphic nomenclature. To achieve uniformity, the log data containing the sedimentology descriptions obtained from different companies were recalibrated but ensured that each facies encountered in each depth are retained to avoid confusion where granulometric data of the cutting are provided. They were statistically studied to know the mean, standard deviation, skewness, Ogive, histogram and kurtosis of the sediments with a view to describing the provenance and environment of deposition of each of the beds. Results show that based on this current work, the beds are not correlatable, which led to the subdivision of the coastline into three major environment of deposition: the tidal flat area (Badagry, Apapa and Ajegunle), an island within the lagoon (Ikoyi) and the peninsular (Victorial Island and lekki). Inspite of this correlation within the subenvironment is still poor. The Tidal flat area reflects a typical regressive sequence with fresh water sand occurring at shallow depth when compared with the peninsular and Ikoyi areas. Peat occurrence and prevalence of mud and mud sand typical of tidal flat are observed. Common white sand though at deeper depth in the peninsular is characteristic of beach sand. Brownish sand at shallow depth can be interpreted to reflect source rock and it can also be as a result of lateralization. Characteristically, the sub-surface sediments of the coast line of Lagos down to a relative depth of about 200 m consist of the following lithofacies: clay, peat, brownish sand, white sand with rare

sandstone that shift laterally within short distance apart. A bivariate plot (moment skewness vs. moment standard deviation) shows the sands are fluvial in origin.