Éditeur(s) : HAL CCSD
Résumé : The Franceville Basin (2.1 Ga) in southeastern Gabon, hosts a black shale series well preserved (FB Formation) which represents an exceptional example of unmetamorphised Paleoproterozoic sediment strata. This basin includes the protore of one of the largest Mn-oxide laterite ore worldwide, mined by Eramet-Comilog. The aim of this this work is to determine formation conditions of this protore and to characterize the global sedimentary evolution of the FB Formation which include the protore. This 75 m thick protore is constituted of Mn carbonate-rich black shales which represent a potential ore deposit for the future. It was drilled, on the Bangombé plateau, during a recent campaign of 24 boreholes with an average depth of 125 m.Through multidisciplinary study on these cores and key outcrops of the basin, this work focuses on : i) the depositional environment of the Mn-protore and FB Formation ; ii) the processes of formation of the Mn-carbonates at 2.1 Ga and iii) the architecture and distribution of Mn-rich levels of the protore on the Bangombé Plateau.A detailed sedimentological and petrological study allowed us to redefine the division of the FB into nine units, named U1 to U9 from the base to the top. These units show a fine upward cycle up to U5 and a reverse coarse loop until U9. This division leads to a new interpretation to propose an evolution from a shoreface depositional environment controlled by deltaic currents (U1) to an offshore depositional environment with anoxic conditions (U2). In this basin, a system of submarine fan is developed, feeding a network of sand injections (injectites) covering the Bangombé plateau (70 km²) and with a thickness of 150 m affecting U4 to U7, including the Mn-protore. The protore depositional environment formed below the wave base limit (U4, U5 and U6) in a starved basin, controlled by biochemical sedimentation allowed the formation of Mn-carbonates. The end of starved basin, would allow the re-opening of the detrital input into the offshore basin (U7). This basin is then filled by storm bar deposits intersected by deltaic sub-aquatic channels (U8). The U9 unit corresponds to a quiet lagoon environment, allowing the multicellular organism development for the first time on the planet.A geochemical study allowed the characterization of the depositional environment of Mn-carbonates as an anoxic to sub-oxic and alkaline environment. For the first time, we showed that the manganese carbonates, present as bacteriomorph forms, are precipitated from the seawater by the mediated photosynthetic cyanobacterial activity, which allows CO2 / HCO3 depression of the environment and a local increase of pH. The cyanobacterial activity is controlled by the absence of bottom currents, which increase the detrital input. This would stop the activity of the cyanobacteria and thus would lead to the decrease of the Mn-concentration along the protore.Finally, well-log and a sequential correlation analyses allowed us to detail spatial and timing repartition of FB Formation deposit, controlled by a tectono-sedimentary model. We propose two- tectono-sedimentary phases. A first syn-tectonic phase (U1 to U3) controls the depocentres and sedimentation gaps. Turbidite coarse deposits are located in the most subsident part, clay deposits rather on the slope and carbonates on shoals. A second post-tectonic phase (U4 to U7) allows isopach deposits. The Mn-protore formed during the post-tectonic phase. However, it shows a variable thickness due to subsidence to the north of the Bangombé plateau. So, on the Bangombé plateau, Mn-contents increase towards the south, whereas the protore thickness increases towards the NNE. Moreover sand injectites decrease Mn-content in the eastern part of the Bangombé plateau and impact on the economic evaluation of the Mn-carbonates. Currently, the protore is structured by post-sedimentary faults, which lead to a non-continuity of the high Mn levels on the Bangombé plateau.
Le Bassin de Franceville situé au SE du Gabon présente une série de black shales (Formation FB) d'âge Paléoprotérozoïque (2,1 Ga) surmontant les grès de la Formation FA. Ce bassin peu déformé et non métamorphique représente une archive exceptionnelle des processus de dépôt de cette période de l’histoire de la Terre où la vie commence à se développer. En particulier, la Formation FB contient le protore d'un gisement mondial d'oxydes de manganèse exploité par la société Eramet-Comilog. L’objectif de ce travail est de préciser les conditions de formation de ce protore et de le replacer dans l’histoire du remplissage du bassin. Ce protore d'une épaisseur moyenne de 75 m est constitué de carbonates de manganèse ; il a fait l'objet d'une récente campagne de 24 forages carottés d'une profondeur moyenne de 125 m sur le plateau de Bangombé.A travers une approche pluridisciplinaire comprenant l'étude de ces forages et des affleurements clefs du bassin, ce travail porte sur la caractérisation i) de l'environnement de dépôt du protore et des séries qui l'encadrent (du toit du FA au toit du FB) ; ii) du mode de formation des carbonates de Mn à 2,1 Ga et iii) de l'architecture et de la répartition des hautes teneurs en Mn du protore qui représente un gisement potentiel du futur.L’étude pétro-sédimentaire détaillée a permis de distinguer neuf unités (U1 à U9) au sein de la Formation FB organisées suivant un cycle rétrogradant jusqu’à U5 puis progradant jusqu’à U9. Ce découpage montre une évolution du milieu de dépôt depuis un domaine de shoreface contrôlé par des courants deltaïques (U1) évoluant vers des dépôts d’offshore restreint (U2). Ce bassin enregistre ensuite un système de chenaux-levées turbiditiques (U3) qui alimentent un réseau d'injectites reconnu sur l’ensemble du Pl. de Bangombé et qui traverse 150 m de série (U4 à U7) en affectant le protore et ainsi les teneurs en Mn. L'environnement de dépôt du protore (U5 et U6) marque l’isolement du bassin qui devient affamé et contrôlé par une sédimentation biochimique à l’origine du dépôt des carbonates de Mn. L'Unité 7 correspond à la réouverture des apports détritiques en domaine d'offshore supérieur puis en domaine de shoreface dominé par des dépôts de tempêtes et de chenaux sous-aquatiques deltaïques (U8), et enfin lagunaire (U9) dans lequel apparaissent les premiers métazoaires de l’histoire de la planète, suivant une séquence régressive.Une étude géochimique a permis de préciser les conditions de dépôt du protore qui se forme en milieu alcalin anoxique à sub-oxique. Dans ce milieu, nous montrons pour la première fois une précipitation directe des carbonates de Mn sous forme bactériomorphe induite par l'activité photosynthétique des cyanobactéries qui consomment le CO2/HCO3 et conduisent à une augmentation du pH favorable à la formation des carbonates de Mn. L'activité cyanobactérienne est conditionnée par l’absence de courants de fond qui augmentent la turbidité du milieu et stoppent cette dernière.Les corrélations diagraphiques et séquentielles permettent de préciser la répartition spatiale et temporelle du FB permettant ainsi de replacer le protore dans l’histoire tectono-sédimentaire du bassin. Une première phase syn-tectonique (U1 à U3) contrôle l’épaisseur et la mise en place de dépôts turbiditiques dans les parties subsidentes, de dépôts argileux sur les pentes et de dépôt de carbonates sur les hauts-fonds. La seconde phase post tectonique (U4 à U7) permet la mise en place de dépôts relativement isopaques, bien que le protore enregistre une subsidence plus importante au nord du Pl. de Bangombé. On observe ainsi, sur le Plateau de Bangombé, des teneurs en Mn qui augmentent vers le sud alors que l’épaisseur du protore augmente vers le NNE. Postérieurement au dépôt du FB, le protore est structuré par des failles qui le compartimentent en touches de piano limitant ainsi sa continuité sur le Plateau de Bangombé.
https://hal.archives-ouvertes.fr/tel-01621460

tel-01621460
https://hal.archives-ouvertes.fr/tel-01621460
https://hal.archives-ouvertes.fr/tel-01621460/document
https://hal.archives-ouvertes.fr/tel-01621460/file/these_Dubois2017-MnGabon.pdf

Éditeur(s) : HAL CCSD
Résumé : Eight drill cores cutting through the Mn-carbonate bearing black shales from the Bangombé Plateau were studied. Mn-carbonates (up to 80 m thick) are underlain by laminated pyrite rich shales with organic carbon contents of about 4 wt. % and MnO contents of about 6 wt. %. Mn and Fe, S poor shales are intercalated. Mn-carbonate rich shales (20-29 wt. % MnO) have low contents of phyllosilicates, but are rich in organic carbon (about 7 wt. %) with moderate S contents (about 2 wt. %). All shale facies were deposited in a closed basin through suspension settling and precipitations under anoxic conditions. The Mn-carbonates, towards the top, were formed most likely under suboxic conditions. Pyrite and Mn-carbonates are characterized by bacteriomorphous structures. The latter are composed of micrometric carbonates finely intergrown with clays, quartz and organic matter. They formed around and corroded the dolomite and/or organic matter. Mn-carbonates and pyrite both form also several tenths of micrometer large agglomerates and sub-automorphous grains. The agglomerates can be attributed to evolving diagenesis. Carbonate chemistry varies mainly from rhodochrosite to dolomite. The present results point to primary microbial Mn-carbonate formation. No signs for a Mn-oxide precursor were detected. Slumping and shearing structures, distortion and disruption of thin pyrite beds indicate synsedimentary deformations.
13th Biennial SGA Meeting
Nancy, France

hal-01356155
https://hal.archives-ouvertes.fr/hal-01356155

Éditeur(s) : HAL CCSD Borntraeger Science Publishers
Résumé : This study characterizes the stratigraphical context as well as the palaeoenvironmental and sealevel trends during the Late Frasnian and Famennian in the Buschteich section (Thuringia, Germany). An integrated approach combines conodont biostratigraphy, chemostratigraphy based on carbon and oxygen isotopes, conodont biofacies analyses and investigations of the morphotypes of foraminifera that occur in the section was applied. Conodonts provided a fine-scale biostratigraphical framework. All the Famennian conodont biozones were identified, with the exception of the three lowest. The Frasnian – Famennian transition was further identified by a positive carbon isotope excursion. This continuous Famennian record is developed in a condensed limestone succession. The reddish Griotte facies, typical of strata of this period, were not documented. Black shales were not developed except for an interval of dark colored mud rocks corresponding to the Hangenberg event at the top of the section. The carbonate succession of Buschteich was deposited in an outer ramp environment, pelagic organisms are the dominant fauna. This is corroborated by the overall dominance of the deep-water conodont genus Palmatolepis. Microfacies, conodont biofacies and foramini fera indicated a deepening trend from the Lower crepida to the Lower rhomboidea Zone, followed by a shallowing trend from the Upper rhomboidea to the praesulcata Zone. This sequence overall matches the sea level reference curve for Euramerica. The timing of maximum water depth at Buschteich, as well as the general sea-level reference record, differs from the Col des Tribes (Montagne Noire, France). Differential uplift or overprinting of the long-term eustatic changes may be the cause of the discrepancy in the local sea-level records.
ISSN: 0078-0421

hal-01623111
https://hal.archives-ouvertes.fr/hal-01623111
DOI : 10.1127/nos/2016/0318

Éditeur(s) : HAL CCSD Wiley
Résumé : Clay shale landscapes are sensitive to landslide, erosion, and re-sedimentation processes. In this context, tectonized clay shales have peculiar hydrological and mechanical behaviours. In order to improve our understanding of the processes involved in such systems, an experimental site has been settled in the Draix Observatory (ORE DRAIX) on a black marl hill slope of the Southern Alps (France) in the framework of the ECOU-PREF project. The site is a natural laboratory to study the evolution of clay shale landscapes at several scales (regional to µm). In this study, we aim to characterize the internal structure and fissure patterns of a stable interfluve, in order to locate preferential water flows within the system, and anticipate its evolution. This work is based on a series of mutli-disciplinary approaches including petrophysical analyses, well pulse injection tests and downhole geophysical measurements. Borehole geophysical results were combined with laboratory measurements on core plugs (permeability, density, porosity, and acoustic velocity). Optical and acoustical images have been used as an original tool for internal discontinuities characterisation and potential active flow path detection. They allowed the identification of three main lithological units interpreted as several stages of shale alteration. In addition, several main structural plane discontinuities (open fractures, schistosity planes, and fractures infilled with calcite or clays) were identified from the images and their potential transmissivity discussed with regard to piezometric and tracer measurements. In all, the integration of measurements leads to propose a simple scenario of fluid circulation and chemical alteration of the interfluve.
ISSN: 0885-6087

hal-00757581
https://hal.archives-ouvertes.fr/hal-00757581
DOI : 10.1002/hyp.7997

Éditeur(s) : HAL CCSD Cambridge University Press (CUP)
Résumé : International audience
The Ben Zireg anticline NW of Bechar yields the currently most-important Frasnian succession on the northern margin of the Algerian Sahara. It represents 26.5 m of calcilutites which are attributed to the middle–late Frasnian. As the early Frasnian is not represented, the succession rests conformably on undifferentiated, probably late Givetian, substrate. Fine-scaled conodont biostratigraphy reveals a continuous sequence of Montagne Noire Zones 5–13, superseded by the earliest Famennian Lower triangularis Zone. The deposits are organized into 15 sedimentary cycles that are initially condensed and become dilated upwards. Conodont biofacies indicate an overall deepening towards the top of the section with an episode of slight shallowing at the MN11/12 transition marked by an increase of ancyrodellids, which is also seen in the Marhouma section of the Ougarta region (SW Algeria). Homogeneous calcareous microfacies with only a few shaly intercalations determine a predominantly oxygenated depositional environment on an outer platform, submarine rise or ramp setting. This is also indicated by relatively low sedimentation rates which are similar in selected sections from the Tafilalt platform, and contrast with those from the Marhouma trough and the Maider basin. Unlike other Frasnian successions south of the Atlas Fault, but similar to the Moroccan Meseta, the Upper Kellwasser horizon is clearly discriminated at Ben Zireg by an outstanding occurrence of black shales on top of the oxygenated latest Frasnian deposits.
ISSN: 0016-7568

hal-01217151
https://hal.archives-ouvertes.fr/hal-01217151
DOI : 10.1017/S0016756814000715

Éditeur(s) : Elsevier
Résumé : The French Guiana transform margin and Demerara abyssal plain have been recently surveyed in the framework of the EXTRAPLAC French Program of extension of the continental shelf (Guyaplac survey, Ifremer-IFP-SHOM-IPEV). Based on the interpretation of some of the data collected during the Guyaplac survey (Simrad-EM12 multibeam bathymetric data, backscatter imagery, and 3.5 kHz profiles), the area can be divided into three morphostructural domains. (1) The western Guiana margin, including a part of the Demerara plateau, an important bathymetric relief prolonging the continental platform off Guiana and Surinam. This domain is bounded by (1a) the NWSE trending northern border of the Demerara plateau which appears quite steep and corresponds to a transform segment of the margin, (1b) the N-S eastern border of the Demerara plateau which corresponds to a divergent segment of the margin. The Demerara plateau shows a segmented morphology, low slope gradients, and a very rough surface (ripples perpendicular to the slope direction). NNW-SSE structural steps seem to correspond to collapses of 100 km long blocs towards the east. Slumps initiate along these directions. The observed rough bathymetry seems to be related to creeping processes. At a greater scale (seismic data), this part of the margin has been totally destabilized (numerous imbricate transparent masses rooted at about 0.5 s.t.w.t.t. below seafloor). The NW-SE trending northern border of the Demerara plateau corresponds to a cliff-like continental slope, probably slightly smoother than other transform margins (Ghana/cote d'Ivoire margin). The N-S eastern border of Demerara plateau is characterized by numerous small-scale imbricate slumps. Some of these failures seem to be emplaced in the prolongation of the NNW-SSE structural steps identified on the Demerara plateau. (2) The eastern Guiana margin corresponds to a NW-SE oriented gullied transform margin segment. The associated continental slope is very steep and characterized by numerous imbricate slumps and related debris flows. Some undulated masses, probably corresponding to creeping sediments or to older mass-wasting events are still imprinted on bathymetry. This transform margin segment is nearly entirely destabilized and eroded. (3) The Demerara abyssal plain. This domain is characterized eastwards by channels belonging to the Amazon turbidite system and westwards, at the foot of Demerara continental slope, by sediment waves probably created by contour currents. To conclude, it seems that there is a strong relationship between the structure (transform and divergent segments) and the emplacement of recurrent slope instabilities. These are probably related to the steepness of the slopes but also to subsidence histories generating in some cases huge deep-seated collapses of the whole margin. Fluid ascents are common everywhere in the area, probably enhancing slope instability. Their origin is not constrained but the black shales or Cretaceous organic-rich layers could be good candidates.
Marine and Petroleum Geology (0264-8172) (Elsevier), 2009-05 , Vol. 26 , N. 5 , P. 711-723

Droits : 2008 Elsevier Ltd All rights reserved.
http://archimer.ifremer.fr/doc/2009/publication-6856.pdf
DOI:10.1016/j.marpetgeo.2008.02.010
http://archimer.ifremer.fr/doc/00000/6856/