Origin and sandstone classification of the upper Cretaceous Lokoja sandstone in Southern Bida basin, as determined from geochemical data

Abstract

This study was carried out with a view of using an innovative approach (strictly geochemical) in determining the origin of Sandstone Formation within the Southern Bida Basin. The Lokoja Formation (Sandstone) unconformably overlies the basement complex. It consist of sandstones, which crop out around Lokoja area between Felele and Koton-Karfi. Field observations show that lithology includes conglomerates at the base, with coarse-grained to fine-grained sandstones, siltstones and claystones overlying it. Analysis of the collected ten (10) outcrop samples from the study area aided in the deduction of geochemistry, classification and period of deposition of the sandstone facies of the Lokoja Formation. The geochemical analysis revealed the major oxides and the heavy minerals that are present in the samples. From the geochemical analysis of the samples at different locations revealing the major oxides, the percentage of SiO₂ in the selected sample compared to the other oxides is very high.Al₂O₃, Fe₂O₃, CaO, MgO, MnO, P₂O₅, K₂O, Na₂O and SO₃were all found to decrease as SiO₂ increases. A major reason for this is clastic mixing. The low values observed from the result of ratio of SiO₂/ Al₂O₃ indicate that the collected samples have a high degree of clayness, which can suggest mineralogical immaturity. Al₂O₃/ (CaO+MgO+Na₂O+K₂O) ratio has been reported useful in the determination of stability of mobile oxides. This was applied in this work, where positive values obtained (2.73 to 8.27)indicate the presence of stable mobile oxides in the Lokoja Formation sandstone facies.The Zircon Tourmaline Rutile (ZTR) Index for the selected samples range from 30% to 44% with an average of 38%. All the selected samples have Zircon –Tourmaline –Rutile maturity index less than 75% suggesting that nearly all the sediments are mineralogically immature to sub mature. The heavy minerals recovered from the selected samples of the Lokoja sandstone show that the origin of the collected samples from the basin is mainly sedimentary.

KEYWORDS: Provenance, Bida Basin, Geochemical, Maturity, Stability, Clayness.