Early eukaryotes: insights from microanalyses of proterozoic microfossils

peer reviewedELIT

Contributions of U-Th-Pb dating on the diagenesis and sediment sources of the Lower Group (BI) of the Mbuji-Mayi Supergroup (Democratic Republic of Congo)

In this paper, we present new age constraints for the lower part of the Meso-Neoproterozoic sedimentary Mbuji-Mayi Supergroup (Democratic Republic of Congo, DRC). This Supergroup preserves a large diversity of organic-walled microfossils, evidencing the diversification of early eukaryotes for the first time in Central Africa. We use different methods such as in situ U-Pb geochronology by LA-ICP-MS and U-Th-Pb chemical datings by Electron Microprobe on diagenetic and detrital minerals such as xenotimes, monazites and zircons. We attempt to better constrain the provenance of the Mbuji-Mayi sediments and the minimum age of the Mbuji-Mayi Supergroup to constrain the age of the microfossils. Results with LA-ICP-MS and EMP provide new ages between 1030 and 1065 Ma for the diagenesis of the lower part of the sedimentary sequence. These results are consistent with data on biostratigraphy supporting the occurrence of worldwide changes at the Mesoproterozoic/Neoproterozoic boundary

Micropaleontology and chemostratigraphy of the Neoproterozoic Mbuji-Mayi Supergroup, Democratic Republic of Congo.

peer reviewe

New geochronological history of the Mbuji-Mayi Supergroup (Proterozoic, DRC) through U-Pb and Sm-Nd dating

peer reviewe

How to date a sedimentary serie: Different approaches to better constrain the diversification of early eukaryotes in Central Africa (Mbuji-Mayi Supergroup, Proterozoic, DRCongo)

Peer reviewe

Thermal maturity estimation of carbonaceous material from proterozoic organic-walled microfossils assemblages (DRCongo, Mauritania and Australia) by using Raman spectroscopy

Biostratigraphie, Chémostratigraphie et caractérisation de la matière organique du Supergroupe de Mbuji-May

Microfossils from the late Mesoproterozoic – early Neoproterozoic Atar/El Mreïti Group, Taoudeni Basin, Mauritania, northwestern Africa

The well-preserved Meso-Neoproterozoic shallow marine succession of the Atar/El Mreïti Group, in the Taoudeni Basin, Mauritania, offers a unique opportunity to investigate the mid-Proterozoic eukaryotic record in Western Africa. Previous investigations focused on stromatolites, biomarkers, chemostratigraphy and palaeoredox conditions. However, only a very modest diversity of organic-walled microfossils (acritarchs) has been documented. Here, we present a new, exquisitely well-preserved and morphologically diverse assemblage of organic-walled microfossils from three cores drilled through the Atar/El Mreïti Group. A total of 48 distinct entities including 11 unambiguous eukaryotes (ornamented and process-bearing acritarchs), and 37 taxonomically unresolved taxa (including 9 possible eukaryotes, 6 probable prokaryotes, and 22 other prokaryotic or eukaryotic taxa) were observed. Black shales preserve locally abundant fragments of benthic microbial mats. We also document one of the oldest records of Leiosphaeridia kulgunica, a species showing a pylome interpreted as a sophisticated circular excystment structure, and one of the oldest records of Trachyhystrichosphaera aimika and T. botula, two distinctive process-bearing acritarchs present in well-dated 1.1 Ga formations at the base of the succession. The general assemblage composition and the presence of three possible index fossils (A. tetragonala, S. segmentata and T. aimika) support a late Mesoproterozoic to early Neoproterozoic (Tonian) age for the Atar/El Mreïti Group, consistent with published lithostratigraphy, chemostratigraphy and geochronology. This study provides the first evidence for a moderately diverse eukaryotic life, at least 1.1 billion years ago in Western Africa. Comparison with coeval worldwide assemblages indicate that a broadly similar microbial biosphere inhabited (generally redox-stratified) oceans, placing better time constraints on early eukaryote palaeogeography and biostratigraphy

Middle Ordovician acritarchs and problematic organic-walled microfossils from the Saq-Hanadir transitional beds in the QSIM-801 well, Saudi Arabia

Core samples from the QSIM-801 water well, drilled in central Saudi Arabia, cover a 93-foot interval spanning the transition between the Sajir Member of the Saq Formation, that consists mainly of sandstones of tidal sand flat environments, and the Hanadir Member of the Qasim Formation, characterized by argillaceous graptolitic mudstones, corresponding to a tidal delta front. The samples contain abundant, exceptionally well-preserved and diverse palynomorphs, which include cryptospores, acritarchs and chitinozoans, other problematic organic-walled microfossils as well as other organic particles such as cuticle-like fragments. The studied interval is biostratigraphically well constrained by the presence of chitinozoans of the formosa and pissotensis Zones of late-early to late Darriwilian age (Middle Ordovician) in the uppermost Saq Formation and Hanadir Member. The biostratigraphic age of the Sajir Member considered to span the Dapingian–Darriwilian boundary, is re-discussed based on the results herein. The uppermost part of the Sajir Member yielded the ichnofossil, Phycodes fusiforme. Acritarch assemblages from the Sajir Member of the Saq Formation are poorly diversified and dominated by sphaeromorphs. More diverse assemblages of acritarchs, associated with enigmatic forms, occur in the Hanadir Member of the Qasim Formation. The contact between the two formations and the transition between the palynomorph assemblages are sharp, suggesting a stratigraphic hiatus. A quantitative analysis allows us to discuss the paleoenvironmental changes and possibly climatic changes associated with an hypothesis of ice house conditions during this period. Among the diagnostic acritarch taxa observed are Frankea breviuscula, F. longiuscula, Baltisphaeridium ternatum, Dasydorus cirritus, Dicrodiacrodium ancoriforme, Poikilofusa ciliaris, Pterospermopsis colbathii and Uncinisphaera fusticula. These are associated with other typical forms known to range across the Lower–Middle Ordovician boundary, such as Aremoricanium rigaudae, Aureotesta clathrata, Barakella fortunata, B. rara, Baltisphaeridium klabavense, Glaucotesta latiramosa and Striatotheca spp. Galeate and peteinoid acritarchs are also well represented, as well as tiny forms of ultraplanctonic size. Three new species of acritarchs are proposed: Frankea longiuscula var. darriwilense var. nov, Micrhystridium regulum sp. nov, and Tyrannus proteus sp. nov. Repeated occurrences throughout the section of cryptospores, problematic microfossils such as organic filaments, cuticle-like tissues, striated and pigmented leiospheres frequently in clusters, are interpreted to reflect recurrent terrestrial and freshwater inputs in the depositional environment. Single-specimen, high-resolution analyses using Confocal Laser Scanning Microscopy on the enigmatic form Tyrannus proteus sp. nov. show fluorescence emission spectra and microstructural properties significantly different from those of typical marine acritarchs from the same levels

Raman spectroscopy as a tool to determine the thermal maturity of organic matter : application to sedimentary, metamorphic and structural geology

Raman spectrometry is a rapid, non-destructive alternative to conventional tools employed to assess the thermal alteration of organic matter (OM). Raman may be used to determine vitrinite reflectance equivalent OM maturity values for petroleum exploration, to provide temperature data for metamorphic studies, and to determine the maximum temperatures reached in fault zones. To achieve the wider utilisation of Raman, the spectrum processing method, and the positions and nomenclature of Raman bands and parameters, all need to be standardized. We assess the most widely used Raman parameters as well as the best analytical practices that have been proposed. Raman band separation and G-band full-width at half-maximum are the best parameters to estimate the maturity for rocks following diagenesis–metagenesis. For metamorphic studies, the ratios of band areas after performing deconvolution are generally used. Further work is needed on the second-order region, as well as assessing the potential of using integrated areas on the whole spectrum, to increase the calibrated temperature range of Raman parameters. Applying Raman spectroscopy on faults has potential to be able to infer both temperature and deformation processes. We propose a unified terminology for OM Raman bands and parameters that should be adopted in the future. The popular method of fitting several functions to a spectrum is generally unnecessary, as Raman parameters determined from an un-deconvoluted spectrum can track the maturity of OM. To progress the Raman application as a geothermometer a standardized approach must be developed and tested by means of an interlaboratory calibration exercise using reference materials

Geological study of the Mpioka subgroup in the Nionga massif. (Bas-congo, DRC)

The massif of Nionga is located in the province of Bas-Congo in the Democratic Republic of the Congo. The subgroup of Mpioka is lithostratigraphiquement to Group West-Congolien (566 Ma). And Zadinien, Mayumbien and West-Congolien together form what is called the supergroup West Congo (L. Tack et al., 2001). This work objectives were to determine: the petrographical types of rocks of this subgroup of Mpioka, the area source of debris, the mechanism and the physico-chemical conditions of the medium of filing. The interest of this study resided in the fact that the massif of Nionga is located in the Mayumbe ditch where manifestations lithologic and tectonic chain West-Congo are recorded. And into this ditch, extending south of Gabon to northern Angola from Congo-Brazzaville and the Democratic Republic of Congo, the massif of Nionga is the only sector where the subgroup of Mpioka outcrops in low vertical and lateral extension. As such, this massif was clear to understand, after the chemical stage schisto-limestone, the dynamics of the basin during the filing of the detrital episode of Mpioka in Bas-Congo. This study allowed us to conclude that the subgroup of Mpioka located in the Nionga massif is formed of clay and sandstone lithofacies, and that it is a rythmite deposited on a flat tidal where driveways come tides have alternately deposited sand (bench of sandstone and sandstone beds - average energy) and sludge (shoals of shales and clay beds - low energy). A medium scale, observed the intercalation of sandstone in shale and on a small scale, the alternation of the quartz-clay laminae. The slump structures resulting rupture and the transport of sediment layers thus producing intertidal sediments in the flats mixtures chaotic types and channels of rivers; rolled sludge have a zonation that would be the result of differences; in the texture, composition and colouring of rocks solid bedding would be due to a very rapid sedimentation in the flats intertidal. Oxidizing conditions from the middle of deposit are translated by the red hue of these rocks. And this colour is due to the presence of ferric iron (hematite) which is common in this massif. Apart from the neoformations chloritic, ironstone, and calcite, quartz sediments of the Mpioka subgroup come from erosion of metamorphic rocks, very rugged, Transmazonien and Kimezien. Correlated with the Pi of the subgroup of Mpioka term in Bungo basin (Angola), this sub-group flush in the massif of Nionga testifies to the existence of a shoal during the filing of Pc, Pe, Pk and Pm and a deflection crustal during the filing of Pi. This subgroup is also between two sedimentary gaps, and is transgressive on Schisto-limestone in the Mayumbe ditch where the level passed successively (from Pc to Pe) regression to a breach (of Pe to Pi) and finally to a regression (of Pi in Pm).Le massif de Nionga est situé dans la province du Bas-Congo en République Démocratique du Congo. Le Sous-groupe de Mpioka appartient lithostratigraphiquement au Groupe Ouest-Congolien (566 Ma). Et l’ensemble Zadinien, Mayumbien et Ouest-Congolien forme ce qu’on appelle le Supergroupe West-Congo (L. Tack et al., 2001). Ce travail avait pour objectifs de déterminer : les types pétrographiques des roches de ce Sous-groupe de Mpioka, la zone-source des débris, le mécanisme de mise en place ainsi que les conditions physico-chimiques du milieu de dépôt. L’intérêt de cette étude résidait dans le fait que le massif de Nionga est situé dans le fossé de Mayumbe où toutes les manifestations lithologiques et tectoniques de la chaîne West-Congo sont enregistrées. Et dans ce fossé, s’étendant du Sud du Gabon jusqu’au Nord de l’Angola en passant par le Congo-Brazzaville et la République Démocratique du Congo, le massif de Nionga est le seul secteur où le Sous-groupe de Mpioka affleure en faible extension verticale et latérale. A ce titre, ce massif était bien indiqué pour comprendre, après la phase chimique schisto-calcaire, la dynamique du bassin pendant le dépôt de l’épisode détritique de Mpioka au Bas-Congo. Cette étude nous avait permis de conclure que le Sous-groupe de Mpioka localisé dans le massif de Nionga est formé des lithofaciès argileux et gréseux, et que c’est une rythmite déposée sur une flat tidale où les allées et venues des marées ont alternativement déposées du sable (banc de grès et lits gréseux – moyenne énergie) et des boues (bancs de shales et lits argileux – faible énergie). A moyenne échelle, on observe l’intercalation du grès dans les shales et à petite échelle, l’alternance des laminations gréso-argileuses. Les structures en slump résulteraient de la rupture et du transport de couches de sédiments produisant ainsi des types chaotiques de mixtures de sédiments dans les flats intertidales et les chenaux de rivières ; les boues laminées présentent un rubanement qui serait le résultat de différences dans la texture, la composition et la coloration des roches ; le litage massif serait dû à une sédimentation très rapide dans les flats intertidales. Les conditions oxydantes du milieu de dépôt sont traduites par la teinte rouge de ces roches. Et cette teinte est due à la présence du fer ferrique (hématite) qui est fréquente dans ce massif. Hormis les néoformations chloriteuse, ferrugineuse et calcitique, les sédiments quartzeux du Sous-groupe de Mpioka proviennent de l’érosion des roches métamorphiques, à relief très accidenté, des Transmazonien et Kimézien. Corrélé au terme Pi du Sous-groupe de Mpioka dans le bassin de Bungo (Angola), ce Sous-groupe affleurant dans le massif de Nionga témoigne de l’existence d’un haut-fond durant le dépôt de Pc, Pe, Pk et Pm et d’un fléchissement crustal au cours du dépôt de Pi. Ce Sous-groupe est également compris entre deux lacunes sédimentaires, et est transgressif sur le Schisto-calcaire dans le fossé de Mayumbe où le niveau passe successivement d’une régression (de Pc à Pe) à une transgression (de Pe à Pi) et enfin à une régression (de Pi à Pm)