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A-C. Cephalopod -Gastropod -Brachiopod Packstone cephalopods casts, strongly recrystallised gastropod moulds and fragments of brachiopods are abundant. A few rhombic crystals could also be seen in C, and this was interpreted as dolomite grains. D. close-up view of a grain with an interparticle meniscus cement, common in vadose zones. E. is a close-up view of gastropod showing thick rims of calcite replacing the aragonitic outer layers.
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Petrographic and geochemical studies were undertaken on the Paleocene-Eocene limestones of the Ewekoro Formation in order to infer the depositional and post-depositional imprints on the limestone. Thin section petrographic studies revealed three distinct microfacies; grainstone, packstone and wackestone, and there were variations in the depositiona...
Citations
... Once deposited, diagenetic processes can considerably modify the carbonate sediment (Tucker 2001). The major (MEs) and certain trace elements (TEs) provide information about the carbonate rock composition and depositional conditions, while rare-earth elements (REEs) provide information about the detrital inputs from adjacent continental areas (Ozyurt et al. 2020;Patra et al. 2021;Shahzad et al. 2021;Adelabu et al. 2021). Several factors influence the proportion of MEs and TEs in carbonates, some of which include depositional condition, mineralogy, temperature, salinity, light intensity, the partition coefficient of Mg, and specifically, the enrichment factor (EF) of the individual organism (Algeo and Liu 2020;Shahzad et al. 2021). ...
... Several studies have been conducted on the Palaeocene Ewekoro Formation (Ogbe 1972;Okosun 1988;Nton and Adeyemi 2015;Ehinola et al. 2016;Akaegbobi and Ogungbesan 2016;Adelabu et al. 2021;Adamolekun et al. 2022), with focus on aspects such as petrology, stratigraphy, paleontology, geochemistry, and petrophysics but most of these studies provide scanty information about the geochemical signatures of these rocks. The Palaeocene limestones of the Ibese quarry, Dahomey (Benin) Basin (Fig. 1), are of interest due to their potential contribution to the Nigerian economy. ...
... A negative anomaly under oxic settings can be explained by the oxidation of trivalent Ce to the less soluble tetravalent Ce and the subsequent removal of suspended particles via the scavenging process (Sholkovitz et al. 1994). Madhavaraju and Ramasay (1999) ϕ Nagarajan et al. (2011) π Adelabu et al. (2021) γ Patra and Sinh (2017) ...
The freshly exposed carbonate rock succession of the Ewekoro Formation (Palaeocene) was investigated to interpret its palaeoredox conditions and depositional environments based on microfacies, mineralogy and geochemical variations. Wackestone, packstone and mudstone are three carbonate microfacies identified from petrographic observations. Calcite is the dominant mineral phase, while quartz, dolomite and lizardite are minor phases. The dominance of CaCO3 and the low SiO2 and Al2O3 in the limestones indicate a low detrital contribution during precipitation. The low U and U/Th ratios are associated with an environment characterised by oxygen-enrichment, while redox proxies of Ni/Co, V/Cr and V/(V + Ni) show substantial oxygenation (oxic to anoxic) in their respective sedimentary environments. The mean rare-earth elements contents in the examined samples (45.03 ppm) are greater than the characteristic values (28 ppm) in marine carbonates. The rare earth and Yttrium signature of these limestones shows a seawater-like pattern. However, the somewhat lower average Y/Ho ratio (35.70) than conventional seawater (44–74) implies that freshwater input in a coastal setting modifies the seawater to some extent. Except for a few elements, these samples are comparable to those of shallow-marine carbonates, formed in a seawater-influenced environment. The REEs data obtained from these limestones has provided baseline information on geochemistry with a potential to better understand the regional paleogeographic conditions of the Ewekoro Formation.
... The δ 13 C values vary from -9.99 ‰, to 3.69 ‰ VPDB, some of the samples recorded negative values particularly in the suggested Eocene samples whilst others have positive values (Table 2, Figure 4). The δ 13 C(VPDB) values for marine carbonate rocks are usually constant and close to zero, while on the other hand, fresh-water limestones are usually enriched in δ 13 C as a result of the organic influences (Moore, 2001;Adelabu et al., 2021). The strong negative value is recorded from sample 113 which is suggested as mentioned earlier to represent the boundary between the Paleocene to Eocene period in the limestone of the Sinjar Formation. ...
The Paleocene-Eocene (P-E) Sinjar Formation from Kalka Smaq section, Dokan area, Sulaimaniya, northeastern Iraq has been studied in terms of mineralogy and geochemistry using X-Ray Diffraction (XRD) supported by Scanning Electron Microscopic (SEM) and elemental X-Ray Fluorescence (XRF) geochemical analyses in addition to stable isotopic C and O analysis. The study revealed indications for mineralogical and elemental variation across the P-E transition which may refer to the regionally recognized depositional and paleoclimate changes of the Paleocene Eocene Thermal Maximum (PETM) in the region. The dominance of shallow marine water deposition is interrupted by freshwater deposition accompanied by increase in temperature suggesting the warming conditions in the Eocene. This result is supported by mineralogical variation in calcite, Mg-calcite and dolomite accompanied with anomalies in δ 18 O and δ 13 C data and variation in CaO, MgO, SiO2, TiO2, Fe2O3, and Al2O3 in addition to Co, Ni, Sr and Ba elemental data which in general may suggest the increase in precipitation and low effect of weathering across the P-E transition boundary in the region.
L'objectif de cette étude est de faire une description des lithofaciès de la formation de Tohoun et de Tado et d'identifier leur composition en éléments chimiques majeurs afin de déterminer leur provenance, le paléoclimat et l'influence des conditions d'altération. La méthodologie adoptée repose sur une étude géologique de terrain pour la collecte de données et un traitement au laboratoire au moyen de microscope et de spectromètre. Les résultats mettent en évidence deux unités géologiques séparées par une discordance de ravinement. L'unité inférieure est dénommée grès de Tado tandis que l'unité supérieure est appelée grès de Tohoun. Les analyses sédimentologiques montrent que les lithofaciès décrits indiquent un environnement marin peu profond à influence fluviatile. Les données géochimiques montrent que les sédiments proviennent de roches mères felsiques et sont mis en place sous un climat humide. Ces résultats, hormis les caractéristiques sédimentologiques et géochimiques qu'ils ressortent, constituent également une base de données pour une meilleure connaissance de la partie septentrionale du bassin sédimentaire côtier du Togo. Mots-clés : sédimentologie, géochimie, paléoenvironnement, Tohoun et Tado.
Abstract Sedimentological and geochemical characterisation of Tohoun and Tado sandstones, southeast Togo The aim of this study is to describe the lithofacies of the Tohoun and Tado formations and to identify their composition in terms of major chemical elements in order to determine their origin, the paleoclimate and the influence of alteration conditions. The methodology adopted is based on a geological field study for data collection and processing in the laboratory using a microscope and spectrometer. The results reveal two geological units separated by a erosional unconformity. The lower unit is known as the Tado sandstone, while the upper unit is known as the Tohoun sandstone. Sedimentological analyses show that the lithofacies described indicates a shallow marine environment with fluvial influence. Geochemical data indicate that the 18 Afrique SCIENCE 23(5) (2023) 17-34 Yawovi Zikpi AMOUDJI et al. sediments are derived from felsic source rocks and were deposited in a humid climate. These results, apart from the sedimentological and geochemical characteristics they reveal, also constitute a database for a better understanding of the northern part of Togo's coastal sedimentary basin.
Petrographic and geochemical studies were carried out on the Paleocene–Eocene carbonates of the Kolosh Formation in the High Folded Zone to infer the environment of deposition, diagenetic overprints, and their origin. The petrographic study revealed five main microfacies; packstone, wackestone, mudstone, grainstone, and crystalline carbonate. There are different depositional environments from shoal to shallow marine environments (mid-ramp conditions) that are adequate for mixing continental material with seawater. Several evidences of diagenetic processes were observed, such as micritization, neomorphism, and dolomitization. Petrographic and geochemical analyses show the dominance of calcite and dolomite minerals. The contents of major oxides and the concentrations of trace elements were normalized to Post-Archean Australian Shale (PAAS) values. The Paleocene–Eocene carbonates are enriched in CaO, MgO, Cr, and Ni, whereas are highly depleted in Na2O, K2O, TiO2, Al2O3, Rb, Ba, Th, and Zr. The low contents of Al2O3 and SiO2 refer to a low siliciclastic involvement during the deposition of the Paleocene–Eocene carbonate. The REEs average content is lower (15.60 ppm) than that of marine carbonates (28 ppm). The REE patterns (normalized to PAAS) display a seawater-like type, while a slightly lower average Y/Ho value (33.66) relative to that of typical seawater (~ 44 to 74) refers to the alteration of the seawater by the freshwater. The REEs of this study are comparable with the limestone of the Middle Eocene Fulra Formation and Arabian Sea carbonates. Paleocene–Eocene carbonates have δ¹³CPDB ranging between − 13.31 and 1.25‰PDB, and δ¹⁸O from − 6.30 to 0.12‰PDB. The negative values and no correlation between δ¹³C and δ¹⁸O indicate that modification of the primordial composition of the isotopes is not a result of diagenesis only, but other factors, such as temperature, and organic matter have an important role. The negative Ce/Ce*, Th/U ratios, and authigenic U suggest their accompanying the deposition of the carbonates in oxidizing conditions. The paleotemperature of the studied carbonates (~ 12 to 42 °C) is slightly higher than other global records in the Paleocene–Eocene, which may propose the variations in the composition of oxygen isotope and modification in the geochemical characteristics during the post-carbonate deposition time.
