Paleoweathering, paleoenvironment and paleoclimate of the nanka sandstone, anambra basin, Nigeria

Abstract

The paleoweathering, paleoclimate and paleoenvironment of the Nanka sandstone of the Anambra basin have been evaluated based on geochemical data. The Chemical Index of Alteration (CIA), Chemical Index of Weathering (CIW), Plagioclase Index of Alteration (PIA), Mineralogical Index of Alteration (MIA) and the A-CN-K ternary plot indicates intense weathering. Also, Rb/Sr, Al/K, Rb/K ratios are high while Sr/Ba is low indicating that the sediments were derived during intense chemical weathering of the source area. The Th/U-Th plot suggests that weathering conditions were not constant during sedimentation. The K2 O/Al2 O3 ratio reveal that Nanka sandstone has minimal K-feldspar. There is negative correlation between CIA with K and Na+Ca; as weathering intensity increases, these elements are depleted, while there is a linear negative correlation between CIA and Al which implies high intensity of chemical weathering and subsequent formation of clay minerals. The paleoenviromental indices (U/Th, Ni/Co, Cu/Zn, (Cu+Mo)/Zn, V/Sc, V/Ni, V/Cr ratios, Mn*, Fe/Al, plot of Ni versus V, plot of V/Cr against Ni/Co, Mo concentrations and Ce/Ce*) and paleoclimate results suggest that the Nanka sandstone was deposited under oxic conditions with a humid climate on low relief in a non-marine deltaic environment and the paleosalinity based on low Sr/Ba ratio indicates low saline water during deposition in a continental environment.