Abstract

Thirteen sandstone samples of the Lokoja Formation and six Ironstone/Shale samples of the Patti Formation were subjected to Mineralogical and Geochemical Analyses to determine the provenance, tectonic settings, and paleo-depositional environments. The mineralogical studies of the concretional ironstone revealed the occurrence of siderite (Fe2CO3), kaolinite (Al2O3 2SiO2•2H2O), quartz (SiO2), and hematite (Fe2O3) with siderite, kaolinite and quartz dominating. The oxides, Fe2O3, SiO2 and Al2O3 constitute greater than sixty-five percent (65%) of the sample’s elemental oxides composition. The Shale mineralogy revealed the occurrence of kaolinite (Al2O3 2SiO2•2H2O), quartz (SiO2), hematite (Al2O3), and Sylvite (KCl) with kaolinite and quartz dominating. Moreover, the shale oxide results revealed SiO2, Al2O3, and MgO accounted for more than seventy percent (70%) of the shale’s elemental oxides. The average elemental oxides of the Lokoja Sandstone are; 61.87% SiO2; 17.05% Al2O3; 3.30% Fe2O3; 0.63% CaO; 1.92- 3.41% Na2O; 2.09% K2O and 2.54% MgO. Other oxides such as P2O5, TiO2, and MnO are less than 1%. The paleo-weathering conditions of the concretional Ironstone and shale samples were determined using indices such as the Chemical Index of Alteration (CIA), Chemical Index of Weathering (CIW), Index of Compositional Variation (ICV) and Plagioclase Index of Alteration (PIA). Generally, these indices reveal that the ironstones were intensely weathered and are immature. All indices, however, showed that the shale samples are matured and had gone through extensive weathering in their source area. The average CIA value of 76.52% is indicative of an intense recycling in the source area while the average MIA value of 53.03% suggests a moderate degree of weathering. The high clay matrix and feldspar content have been used to classify the sandstones as feldsparthic greywackes deposited in Semi-arid to arid climatic conditions under a basement uplifted tectonic setting. Furthermore, the paleo-environmental conditions, provenance, and tectonic settings of the Patti Formation are a mix of mafic and intermediate provenance and tectonic settings within the Active Continental Margin (ACM). The shale provenance is a mixture of felsic and an intermediate igneous rock principally within the active continental margin.