Abstract
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).
Highlights
Provenance and other related studies associated with tectonic settings and palaeo-climatic conditions that determined weathering processes in the source areas of the Akata Formation, the Agbada Formation and Benin Formation that constitute the sedimentary fill of the Niger Delta Basin are lacking as emphasis has always been on the hydrocarbon potential of these formations
The primary objective of this investigation is to utilize heavy mineral and geochemical data to determine the provenance, tectonic setting of source area and palaeo-climatic conditions associated with the deposition of the Ogwashi-Asaba Formation
Similar research on provenance of siliciclastic sedimentary rocks has usually been focused on sandstones and has in the main been based on petrographic analysis of quartz, feldspars, micas and heavy minerals
Summary
Provenance and other related studies associated with tectonic settings and palaeo-climatic conditions that determined weathering processes in the source areas of the Akata Formation, the Agbada Formation and Benin Formation that constitute the sedimentary fill of the Niger Delta Basin are lacking as emphasis has always been on the hydrocarbon potential of these formations. The primary objective of this investigation is to utilize heavy mineral and geochemical data to determine the provenance, tectonic setting of source area and palaeo-climatic conditions associated with the deposition of the Ogwashi-Asaba Formation. Similar research on provenance of siliciclastic sedimentary rocks has usually been focused on sandstones and has in the main been based on petrographic analysis of quartz, feldspars, micas and heavy minerals. Ancillary techniques such as cathodoluminescence microscopy [5] and the geochemical studies of major, trace and rare earth elements and isotopes in siliciclastics have been used. The use of heavy minerals (the components of siliciclastics whose specific gravity is greater than that of bromoform (2.8)) is a universally accepted method for determining the provenance of siliciclastics [6] [7] because they resist relatively the physiochemical alteration associated with source area weathering, erosion and diagenesis [8] and retain and bear the source rock characteristics from which they were derived
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