Rare-earth elements geochemistry of the Palaeocene Ewekoro Formation, southwestern Nigeria: implications for terrigenous inputs, diagenetic alteration, palaeoredox and depositional conditions

Abstract

The freshly exposed carbonate rock succession of the Ewekoro Formation (Palaeocene) was investigated to interpret its palaeoredox conditions and depositional environments based on microfacies, mineralogy and geochemical variations. Wackestone, packstone and mudstone are three carbonate microfacies identified from petrographic observations. Calcite is the dominant mineral phase, while quartz, dolomite and lizardite are minor phases. The dominance of CaCO3 and the low SiO2 and Al2O3 in the limestones indicate a low detrital contribution during precipitation. The low U and U/Th ratios are associated with an environment characterised by oxygen-enrichment, while redox proxies of Ni/Co, V/Cr and V/(V + Ni) show substantial oxygenation (oxic to anoxic) in their respective sedimentary environments. The mean rare-earth elements contents in the examined samples (45.03 ppm) are greater than the characteristic values (28 ppm) in marine carbonates. The rare earth and Yttrium signature of these limestones shows a seawater-like pattern. However, the somewhat lower average Y/Ho ratio (35.70) than conventional seawater (44–74) implies that freshwater input in a coastal setting modifies the seawater to some extent. Except for a few elements, these samples are comparable to those of shallow-marine carbonates, formed in a seawater-influenced environment. The REEs data obtained from these limestones has provided baseline information on geochemistry with a potential to better understand the regional paleogeographic conditions of the Ewekoro Formation.