Geochemical Characterization of the Paleocene Ewekoro Limestone Formation, SW Nigeria: Implications for Provenance, Diagenesis and Depositional Environment

Abstract

The usefulness of limestone as an industrial raw material is dependent on the level of its purity, largely controlled by diagenesis and the depositional setting. Limestone samples (83) obtained from the Ewekoro Formation exposed in quarry sections at Shagamu were analyzed using X-Ray Fluorescence spectrometry (XRF). A geochemical characterization of result was done to evaluate the purity levels and the implications on provenance, diagenesis, and depositional setting of the limestone. Five limestone beds from quarry sections (15 locations) were logged. Beds “E”, “D”, “C”, “B”, and “A” comprised sandy algal in sparry calcite cement, sandy biomicrite, algal biomicrite, sandy algal biomicrite, and the intra-sparite facies respectively. Results of major elements che- mistry indicated concentrations (in wt%) as follows: CaO (33.71 - 59.99), MgO (0.39 - 3.15), Al 2 O 3 (0.47 - 3.23), Fe 2 O 3 (0.51 - 3.43), SiO 2 (0.47 - 45.98), SO 3 (0.10 - 2.27), K 2 O (0.02 - 0.28), Na 2 O (0.00 - 10.0), TiO 2 (0.00 - 0.27), P 2 O 5 (0.02 - 0.92), MnO (0.01 - 0.06) and Loss on ignition (17.64 - 45.20). Geochemical result showed that the samples are enriched in CaO, while SiO 2 varies widely, likely due to hinterland input. Plots of SiO 2 versus CaO concentrations depicted a negative correlation attributable to chemical diagenetic processes. Diagenesis of the carbonate in form of reversible replacement of SiO 2 with CaO and vice versa occurred under a shallow marine condition. This negative correlation between these oxides (CaO and SiO 2 ) dictates zones of high purity limestones that are of desirable industrial applications. Relatively high silica content in the northwestern part is possible indication of outlets to the continental or inland areas where clastic sediment source may have been prominent. Ratios of Ca/Mg and Mg/Ca indicated a relatively low rate of evaporation of sea water and palaeo-salinity conditions marked by precipitation of limestone. Intermittent increase in palaeo-salinity and sea water evaporation level gave rise to the magnesian limestone.