TITLE:
Geochemical Characterization of the Paleocene Ewekoro Limestone Formation, SW Nigeria: Implications for Provenance, Diagenesis and Depositional Environment
AUTHORS:
O. Ajayi Ehinola, O. Innocent Ejeh, O. Justine Oderinde
KEYWORDS:
Geochemical Characterization, Ewekoro Formation, Ca/Mg Ratio, Limestone, Carbonate, Silica
JOURNAL NAME:
Geomaterials,
Vol.6 No.3,
July
20,
2016
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), Al2O3(0.47
- 3.23), Fe2O3(0.51
- 3.43), SiO2(0.47 -
45.98), SO3(0.10 -
2.27), K2O (0.02 - 0.28), Na2O (0.00 - 10.0), TiO2(0.00 - 0.27), P2O5(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 SiO2varies widely, likely due to
hinterland input. Plots of SiO2versus
CaO concentrations depicted a negative correlation attributable to chemical
diagenetic processes. Diagenesis of the carbonate in form of reversible
replacement of SiO2with
CaO and vice versa occurred under a shallow marine condition. This negative
correlation between these oxides (CaO and SiO2) 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.