摘要
The geochemistry, provenance, tectonic setting, paleoclimate and depositional paleoenvironment of Maastrichtian clastic sedimentary rocks from the western flank of Cretaceous-Tertiary intracratonic Anambra Basin, Nigeria have been studied through major and trace element geochemical analysis. Ten (10) representatives outcrop samples collected from two stratigraphic sections in the study area were analyzed chemically using Inductively Coupled Plasma Atomic Emission Spectroscopy, ICP-AES (Lithium Borate Fusion). The geochemical data shows that the sediments are enriched in major oxides including SiO2, Al2O3, Fe2O3 and TiO2 but depleted in other major element oxides. Also, the sandstones have higher concentrations in SiO2, CaO and Na2O compared to the associated mudrocks whereas the mudrocks are more enriched in Al2O3, Fe2O3, MgO, K2O, TiO2 and P2O5 compared to the sandstones. The mudrock samples have higher concentration of Ba, Ni, Sr, Y, Nb, Sc in comparison to sandstones. However, sandstone samples have higher concentration in Zr (1098.50 ppm) than the mudrocks (1038 ppm). The sandstones are classified as litharenites, sublitharenites, Fe-rich sandstones, while the mudrocks are labeled as shale and Fe-rich shale based on chemical composition. Analysis of the data shows a dominantly felsic igneous to intermediate igneous and quartzose sedimentary provenances of mixed granite and granodiorite composition as indicated by various discriminant plots. The inferred provenance is corroborated by the low concentrations of Cr, Ni and Sc in the samples and other proxies. On the basis of various tectonic setting discriminant function diagrams, the Maastrichtian sediments from the study area were deposited on passive margin tectonic setting that developed consequent to the breakup of Gwandana supercontinent but was later affected by tectonism in Santonian times. The alteration feldspars to smectite and kaolinite as depicted by the A-CN-K ternary diagram alongside the high values of chemical index of alteration (CIA, 87.38 - 98.09) and chemical index of weathering (CIW, 89.22 - 99.90) indicates paleogeographic setting characterized by warm and humid/tropical paleoclimatic condition where there was prevalence of intense chemical weathering. The low values of Sr/Ba ratio (0.31 - 0.70) suggest sediment deposition in continental to deltaic paleoenvironment.
The geochemistry, provenance, tectonic setting, paleoclimate and depositional paleoenvironment of Maastrichtian clastic sedimentary rocks from the western flank of Cretaceous-Tertiary intracratonic Anambra Basin, Nigeria have been studied through major and trace element geochemical analysis. Ten (10) representatives outcrop samples collected from two stratigraphic sections in the study area were analyzed chemically using Inductively Coupled Plasma Atomic Emission Spectroscopy, ICP-AES (Lithium Borate Fusion). The geochemical data shows that the sediments are enriched in major oxides including SiO2, Al2O3, Fe2O3 and TiO2 but depleted in other major element oxides. Also, the sandstones have higher concentrations in SiO2, CaO and Na2O compared to the associated mudrocks whereas the mudrocks are more enriched in Al2O3, Fe2O3, MgO, K2O, TiO2 and P2O5 compared to the sandstones. The mudrock samples have higher concentration of Ba, Ni, Sr, Y, Nb, Sc in comparison to sandstones. However, sandstone samples have higher concentration in Zr (1098.50 ppm) than the mudrocks (1038 ppm). The sandstones are classified as litharenites, sublitharenites, Fe-rich sandstones, while the mudrocks are labeled as shale and Fe-rich shale based on chemical composition. Analysis of the data shows a dominantly felsic igneous to intermediate igneous and quartzose sedimentary provenances of mixed granite and granodiorite composition as indicated by various discriminant plots. The inferred provenance is corroborated by the low concentrations of Cr, Ni and Sc in the samples and other proxies. On the basis of various tectonic setting discriminant function diagrams, the Maastrichtian sediments from the study area were deposited on passive margin tectonic setting that developed consequent to the breakup of Gwandana supercontinent but was later affected by tectonism in Santonian times. The alteration feldspars to smectite and kaolinite as depicted by the A-CN-K ternary diagram alongside the high values of chemical index of alteration (CIA, 87.38 - 98.09) and chemical index of weathering (CIW, 89.22 - 99.90) indicates paleogeographic setting characterized by warm and humid/tropical paleoclimatic condition where there was prevalence of intense chemical weathering. The low values of Sr/Ba ratio (0.31 - 0.70) suggest sediment deposition in continental to deltaic paleoenvironment.
