Heavy mineral petrographic and geochemical compositions (major and trace/rare earth elements)?of sandstones obtained from the Oligocene-Miocene Ogwashi-Asaba Formation, Niger Delta were studied to determine their prov...Heavy mineral petrographic and geochemical compositions (major and trace/rare earth elements)?of sandstones obtained from the Oligocene-Miocene Ogwashi-Asaba Formation, Niger Delta were studied to determine their provenance, source area weathering conditions and tectonic setting. The heavy mineral suite (opaque minerals, zircon, tourmaline, and rutile) revealed that the sandstones are mineralogically mature and implied rapid disintegration and chemical decomposition of sediments mostly of recycled orogen. The sandstones were geochemically classified as Fe-sand and partly quartz arenitic. Chemical Index of Alteration and Chemical Index of Weathering values of 89.92% and 91.87% respectively suggest that the source region was predominantly felsic and was subjected to intense chemical weathering probably under tropical palaeoclimatic conditions with abundant rainfall that enhanced sediment recycling. Major element concentration discriminant plots also indicated that the sediments were derived from mixed sources (granitic, gneissic or recycled orogen) under passive margin setting. Chondrite normalized plot of the rare earth element pattern is marked by light rare earth element enrichment and negative Eu anomalies, interpreted to mean that provenance was mainly continental crustal rocks. Trace elemental ratios that are provenance diagnostic (La/Sc, Th/Sc, Cr/Th, La/Co, Th/Co, Th/Cr, Eu/Eu*, and Eu*) all point to sediments derived from felsic source and upper continental crust. The mixed provenance of the sandstones can be traced to the southwestern and southeastern Basement Complex (consisting of granites, gneisses, etc.) and sediments derived from the adjacent sedimentary basins (Anambra and Benue Trough).展开更多
Sedimentological and foraminiferal paleoecological studies were carried out for the Oligocene-Miocene Ogwashi-Asaba Formation of the Anambra Basin at the Issele-Uku area in southern Nigeria,to reconstruct paleoenviron...Sedimentological and foraminiferal paleoecological studies were carried out for the Oligocene-Miocene Ogwashi-Asaba Formation of the Anambra Basin at the Issele-Uku area in southern Nigeria,to reconstruct paleoenvironments.The pattern of rock successions and textural characteristics suggest the prevalence of fluvio-deltaic setting during the formation of lignites and clastic rocks of Ogwashi-Asaba Formation.Agglutinated benthic foraminiferal species such as Ammotium sp.,Textularia sp.,Gavelinella sp.,Milliamina sp.,Reophax sp.,Bolivina sp.,Eponides sp.,Pseudobolivina sp.and Lenticulina sp.dominate the shale units whereas the planktonic forms are absent.All these species have very low population and diversity in the shales,which could be due to the presence of harsh conditions such as low oxygen level and scarcity of food,leading to poor survival.The assessments of benthic foraminifera's habitat suggest middle-to-inner neritic marine environment for the shales.Granulometric results suggest fluvial environment for the sandstone and deltaic for the lignite.The Ogwashi-Asaba Formation in Issele-Uku area of Anambra Basin,southern Nigeria,therefore,have records of shallow marine,and fluvio-deltaic paleoenvironments.展开更多
文摘Heavy mineral petrographic and geochemical compositions (major and trace/rare earth elements)?of sandstones obtained from the Oligocene-Miocene Ogwashi-Asaba Formation, Niger Delta were studied to determine their provenance, source area weathering conditions and tectonic setting. The heavy mineral suite (opaque minerals, zircon, tourmaline, and rutile) revealed that the sandstones are mineralogically mature and implied rapid disintegration and chemical decomposition of sediments mostly of recycled orogen. The sandstones were geochemically classified as Fe-sand and partly quartz arenitic. Chemical Index of Alteration and Chemical Index of Weathering values of 89.92% and 91.87% respectively suggest that the source region was predominantly felsic and was subjected to intense chemical weathering probably under tropical palaeoclimatic conditions with abundant rainfall that enhanced sediment recycling. Major element concentration discriminant plots also indicated that the sediments were derived from mixed sources (granitic, gneissic or recycled orogen) under passive margin setting. Chondrite normalized plot of the rare earth element pattern is marked by light rare earth element enrichment and negative Eu anomalies, interpreted to mean that provenance was mainly continental crustal rocks. Trace elemental ratios that are provenance diagnostic (La/Sc, Th/Sc, Cr/Th, La/Co, Th/Co, Th/Cr, Eu/Eu*, and Eu*) all point to sediments derived from felsic source and upper continental crust. The mixed provenance of the sandstones can be traced to the southwestern and southeastern Basement Complex (consisting of granites, gneisses, etc.) and sediments derived from the adjacent sedimentary basins (Anambra and Benue Trough).
文摘Sedimentological and foraminiferal paleoecological studies were carried out for the Oligocene-Miocene Ogwashi-Asaba Formation of the Anambra Basin at the Issele-Uku area in southern Nigeria,to reconstruct paleoenvironments.The pattern of rock successions and textural characteristics suggest the prevalence of fluvio-deltaic setting during the formation of lignites and clastic rocks of Ogwashi-Asaba Formation.Agglutinated benthic foraminiferal species such as Ammotium sp.,Textularia sp.,Gavelinella sp.,Milliamina sp.,Reophax sp.,Bolivina sp.,Eponides sp.,Pseudobolivina sp.and Lenticulina sp.dominate the shale units whereas the planktonic forms are absent.All these species have very low population and diversity in the shales,which could be due to the presence of harsh conditions such as low oxygen level and scarcity of food,leading to poor survival.The assessments of benthic foraminifera's habitat suggest middle-to-inner neritic marine environment for the shales.Granulometric results suggest fluvial environment for the sandstone and deltaic for the lignite.The Ogwashi-Asaba Formation in Issele-Uku area of Anambra Basin,southern Nigeria,therefore,have records of shallow marine,and fluvio-deltaic paleoenvironments.
