The geochemical compositions [major, trace and Rare Earth Elements (REE)] of Cretaceous-Tertiary shales from Gbekebo-1 well were used to characterize provenance, paleogeography, source area weathering and tectonic set...The geochemical compositions [major, trace and Rare Earth Elements (REE)] of Cretaceous-Tertiary shales from Gbekebo-1 well were used to characterize provenance, paleogeography, source area weathering and tectonic setting of the study area located in the southwestern part of the Dahomey Embayment, Nigeria. Core samples (eight) of shales were obtained and analyzed geochemically using the combined methods of major elements Fusion Inductively Coupled Plasma (FUS-ICP) and trace elements Fusion Inductively Coupled Plasma Emission/Mass Spectrometry (FUS-ICP/MS). An A-CN-K (Al<sub>2</sub>O<sub>3</sub>-CaO + Na<sub>2</sub>O-K<sub>2</sub>O) ternary plot, geochemical discriminant function of major elements and chondrite normalized plots of REE suggest an upper continental crust provenance of felsic to intermediate or mixed igneous rocks of tonalite to granodiorite composition. High values of chemical index of alteration (CIA, 82.22 - 96.39) and chemical index of weathering (CIW, 88.10 - 99.17) indicated a palaeogeographic condition marked by wet tropical climate where intense chemical weathering and erosion prevailed. The Cretaceous-Tertiary shales from Gbekebo-1 well are inferred to have been deposited in passive margin setting based on various geochemical tectonic setting discrimination diagrams.展开更多
Detailed fieldwork in parts of the Benin (Dahomey) Basin SW, Nigeria has allowed the recognition of sedimentary successions deposited in different continental depositional environments interpreted as fluvial sedimenta...Detailed fieldwork in parts of the Benin (Dahomey) Basin SW, Nigeria has allowed the recognition of sedimentary successions deposited in different continental depositional environments interpreted as fluvial sedimentation characterised by abandoned channels and subaerial exposure features. The lithofacies recognised are: conglomerates, debris flow deposits, very coarse grained sandstone, ferruginous cross bedded sandstone, siltstone, shale/clay and massive sand. The lithofacies were interpreted in terms of sequence stratigraphic elements with the recognition of sequence boundaries characterised by distinct facies dislocations and subaerial exposures. Four depositional sequences (DS I to DS IV) were identified. Depositional sequences (DS I) exhibited a complete regressive–transgressive cycle comprising LST, TST and HST, while DS II to DS IV were incomplete comprising of sediments interpreted as LST deposits. On the basis of the depositional facies and sequence stratigraphic elements, the sediments in the area of study are interpreted as predominantly of Ilaro Formation while fewer exposures are sediments of the Coastal Plain Sands/ Benin Formation. This observation suggests a re-interpretation of the Tertiary sedimentary successions, which hitherto recognised all the sediments in the area of study as Coastal Plain Sands.展开更多
The studies on some samples retrieved from the coastal sediment deposit have been carried out. The palynological and geochemical indices were evaluated with the view of determining their chronology, palaeoclimatic con...The studies on some samples retrieved from the coastal sediment deposit have been carried out. The palynological and geochemical indices were evaluated with the view of determining their chronology, palaeoclimatic conditions that prevailed during the time of their emplacement and also to re-asses the palaeoenvironment of the Lagos coastal deposit in Dahomey Basin. The identification of diagnostic species age entails the palynological analysis while the geochemical analysis determines the provenance of these Quaternary sediments. The occurrences of Laevigatosporites sp., Zonocostites ramonae, Acrostichum aureum in abundance along with few long ranging forms suggest that the vegetation development was under a humid climate and that the sediments were deposited during cooler and wetter conditions. The presence and high abundance of Crassoretitriletes vanraadshooveni, Zonocostites ramonae, Canthiumidites sp., Sapotaceoidaepollenites sp. and Pachydermitesdiederixi palynomorphs indicate an age range of late Pliocene (Gelasian) to early Pleistocene (Calabrian) (2.588 - 1.806 Ma). This age range is known to correspond to the 3.7 - 3.8 depositional cycles of relative change of coastal On-lap. The geochemical appraisal showed that the ratios of organic carbon-nitrogen (C/N) indicate that the sediments were sourced from aquatic, protein-rich and cellulose-poor milieu. The wetter climatic period has enhanced algae productivity as a consequence of greater wash-in of soil nutrients, and these periods are recorded as increased rate of organic carbon mass accumulation. Conversely, the dominance of a mangrove habitat, Zonocostites ramonae suggests a mangrove swamp environment which was the most prevalent environment of the Lagos lagoon in the Pleistocene.展开更多
文摘The geochemical compositions [major, trace and Rare Earth Elements (REE)] of Cretaceous-Tertiary shales from Gbekebo-1 well were used to characterize provenance, paleogeography, source area weathering and tectonic setting of the study area located in the southwestern part of the Dahomey Embayment, Nigeria. Core samples (eight) of shales were obtained and analyzed geochemically using the combined methods of major elements Fusion Inductively Coupled Plasma (FUS-ICP) and trace elements Fusion Inductively Coupled Plasma Emission/Mass Spectrometry (FUS-ICP/MS). An A-CN-K (Al<sub>2</sub>O<sub>3</sub>-CaO + Na<sub>2</sub>O-K<sub>2</sub>O) ternary plot, geochemical discriminant function of major elements and chondrite normalized plots of REE suggest an upper continental crust provenance of felsic to intermediate or mixed igneous rocks of tonalite to granodiorite composition. High values of chemical index of alteration (CIA, 82.22 - 96.39) and chemical index of weathering (CIW, 88.10 - 99.17) indicated a palaeogeographic condition marked by wet tropical climate where intense chemical weathering and erosion prevailed. The Cretaceous-Tertiary shales from Gbekebo-1 well are inferred to have been deposited in passive margin setting based on various geochemical tectonic setting discrimination diagrams.
文摘Detailed fieldwork in parts of the Benin (Dahomey) Basin SW, Nigeria has allowed the recognition of sedimentary successions deposited in different continental depositional environments interpreted as fluvial sedimentation characterised by abandoned channels and subaerial exposure features. The lithofacies recognised are: conglomerates, debris flow deposits, very coarse grained sandstone, ferruginous cross bedded sandstone, siltstone, shale/clay and massive sand. The lithofacies were interpreted in terms of sequence stratigraphic elements with the recognition of sequence boundaries characterised by distinct facies dislocations and subaerial exposures. Four depositional sequences (DS I to DS IV) were identified. Depositional sequences (DS I) exhibited a complete regressive–transgressive cycle comprising LST, TST and HST, while DS II to DS IV were incomplete comprising of sediments interpreted as LST deposits. On the basis of the depositional facies and sequence stratigraphic elements, the sediments in the area of study are interpreted as predominantly of Ilaro Formation while fewer exposures are sediments of the Coastal Plain Sands/ Benin Formation. This observation suggests a re-interpretation of the Tertiary sedimentary successions, which hitherto recognised all the sediments in the area of study as Coastal Plain Sands.
文摘The studies on some samples retrieved from the coastal sediment deposit have been carried out. The palynological and geochemical indices were evaluated with the view of determining their chronology, palaeoclimatic conditions that prevailed during the time of their emplacement and also to re-asses the palaeoenvironment of the Lagos coastal deposit in Dahomey Basin. The identification of diagnostic species age entails the palynological analysis while the geochemical analysis determines the provenance of these Quaternary sediments. The occurrences of Laevigatosporites sp., Zonocostites ramonae, Acrostichum aureum in abundance along with few long ranging forms suggest that the vegetation development was under a humid climate and that the sediments were deposited during cooler and wetter conditions. The presence and high abundance of Crassoretitriletes vanraadshooveni, Zonocostites ramonae, Canthiumidites sp., Sapotaceoidaepollenites sp. and Pachydermitesdiederixi palynomorphs indicate an age range of late Pliocene (Gelasian) to early Pleistocene (Calabrian) (2.588 - 1.806 Ma). This age range is known to correspond to the 3.7 - 3.8 depositional cycles of relative change of coastal On-lap. The geochemical appraisal showed that the ratios of organic carbon-nitrogen (C/N) indicate that the sediments were sourced from aquatic, protein-rich and cellulose-poor milieu. The wetter climatic period has enhanced algae productivity as a consequence of greater wash-in of soil nutrients, and these periods are recorded as increased rate of organic carbon mass accumulation. Conversely, the dominance of a mangrove habitat, Zonocostites ramonae suggests a mangrove swamp environment which was the most prevalent environment of the Lagos lagoon in the Pleistocene.
