Carbonate microfacies and major element content of the Paleocene – Eocene sections exposed at the Sagamu quarry, eastern Dahomey basin, Nigeria.

Abstract

The Paleocene-Eocene limestone and shale units exposed at the Sagamu quarry, southwestern Nigeria, were investigated for the carbonate microfacies and major geochemical elements to deduce the depositional environment and the diagenetic history of the limestone. From field observations the limestone is generally massive to wavy laminated, yellowish brown to light grey and fossiliferous. The shales are light to dark grey, fissile, clayey and concretionary. Based on the microfacies studies, the limestone include: sandy bioclastic packstone, biomicritic mudstone,
biomicritic wackestone, biosparitic grainstone, biomicrtic packstone and oolitic packstone-grainstone. Gastropods, peleycpods, foraminifera, ostracodes and algae constitute the major bioclasts while pellets and lithoclasts are the major non-bioclastic components of the lithofacies.
Geochemically, the limestone is rich in CaO (46.82%), SiO2 (5.72%) and MgO (1.97%). The values indicate non-dolomitic limestone. The CaO and MgO contents are however lower at the base than at the top of the limestone and probably reflects the sandy nature of this part. Ternary plot shows that the limestone is richly calcitic. The shales are rich in SiO2 (44.43%), Al3O2 (15.90%) and Fe2O3 (7.68%). The high values of CaO (6.35%) and MgO (2.62%) make them calcareous shales. The lithologic, paleontologic and petrographic data indicate deposition of the units in a shallow marine shelflagoon environment, formed during the Maastrichtian sea flooding into Dahomey Basin. The porosity and permeability of the limestones are generally low while fracturing and dissolution improved the permeability, the effect of late diagenesis reduced it significantly.