| Popis: |
Sediments out of three boreholes located in the Lagos lagoon area (wells at Victoria Island, Apapa, and Lekki) were studied for provenance, environment of deposition, maturity, and age of samples. It was also focused on the palaeoclimatic conditions (ambient water temperature) that prevailed during deposition. Samples were medium-coarse grained, moderately to very poorly sorted, strongly positive skewed to near symetrical, and very platykurtic; and they show extremely leptokurtic trends. This is indicative of a river environment that deposited fluvial sediments, suggesting a high to medium energy transporting medium. The heavy mineral assemblages were characterized by zircon, tourmaline, rutile, garnet, epidote, staurolite, apatite, and sillimanite, suggesting igneous and metamorphic provenance. The average Zircon-Tourmaline-Rutile (ZTR) index was 53.9%, indicating texturally immature sediments. Quartz is the most abundant mineral accounting for 90% to 95%. The Quartz-Feldspar-Rock fragments (QFR) ternary plots revealed predominantly super mature quartz arenites. Palynomorphs revealed abundance of mangrove swamp forest species in association with freshwater swamp forest species and freshwater with algae (Botryococcus sp.). This suggests a mangrove environment with a high influx of freshwater. The sediments were inferred to be of Quaternary and Neogene age. Pebble morphometric studies revealed a littoral setting, and the plot of maximum projection sphericity against oblate prolate index again suggested fluvial origin of sediments. The pebble forms were dominantly compact and compact-bladed, which further supported a process of fluvial dynamics. Oxygen and carbon isotopic composition of bivalves from Lekki well ranged from -4.93‰ to -2.94‰ VPDB (mean value of -4.22‰) and -10.35‰ to -3.71‰ VPDB (mean value of -7.43‰); while the Apapa well shell fragments (Venericardia planicosta) revealed δ18O values of -2.48‰ and -2.19‰ VPDB, δ13C values of -3.27‰ and -1.96‰ VPDB, respectively. The results showed a positive correlation between oxygen and carbon isotopic compositions, which suggests a seasonal climate change that might have influenced carbon sources to some degree. The estimated ambient water palaeo- temperatures during the Neogene might have varied in the range between 28.93°C to 40.82°C. |
