De nouveaux gisements à ambre et à végétaux ont été découverts dans les terrains<br />albiens et cénomaniens de Charente-Maritime (France). L'un d'eux, daté de l'Albien, constitue l'un des plus anciens mais aussi l'un des plus importants gisements d'ambre fossilifère du Crétacé, compte tenu de la richesse et de la diversité des inclusions répertoriées. Les insectes et les arthropodes sont les plus nombreux, mais quelques restes de vertébrés (plume, peau de reptile) et des fragments végétaux sont également signalés. La singularité de cet ambre est d'avoir préservé une abondante faune d'arthropodes vivant au niveau de la litière du sol, alors que l'essentiel des inclusions représente généralement le biotope vivant le long du tronc ou des branches de l'arbre producteur de résine. La confrontation d'analyses taphonomiques, xylologiques et physico-chimiques permet de discuter la source botanique probable de cet ambre.<br />Quelques insectes particulièrement significatifs pour la compréhension de l'histoire<br />évolutive de leur groupe, ou bien informatifs d'un point de vue paléoenvironnemental ou paléobiogéographique, font l'objet d'une étude systématique détaillée. Des informations complémentaires, d'ordre paléoécologique et paléoclimatique, sont apportées par les nombreux végétaux associés dans les gisements sous forme de bois ou de feuilles. Une reconstitution régionale des écosystèmes terrestres côtiers médio-crétacés est proposée, via l'analyse sédimentologique des milieux de dépôt et les informations paléoécologiques fournies par ces assemblages fossiles. Ces gisements contribuent à une meilleure connaissance des biotopes côtiers du Crétacé, période cruciale pendant laquelle la co-évolution des insectes et des plantes à fleurs a constitué les prémices de nos écosystèmes actuels.

The one of the most important Eras of the Earth history, i.e. Neoproterozoic (1000-542 Ma),<p>was an enigmatic period characterized by the development of the first stable long-lived ~1.1-<p>0.9 Ga Rodinia and 550-500 Ma Gondwana supercontinents, global-scale orogenic belts,<p>extreme climatic changes (cf. Snowball Earth Hypothesis), the development of microbial<p>organisms facilitating the oxidizing atmosphere and explosion of eukaryotic forms toward the<p>first animals in the terminal Proterozoic. This thesis presents a multidisciplinary study of two<p>Neoproterozoic basins, i.e. Bas-Congo and Sankuru-Mbuji-Mayi-Lomami-Lovoy, in and around the Congo Craton including sedimentology, geochemistry, diagenesis, chemostratigraphy and radiometric dating of carbonate deposits themselves.<p><p>The Mbuji-Mayi Supergroup sequence deposited in a SE-NW trending 1500 m-thick siliciclastic-carbonate intracratonic failed-rift basin, extends from the northern Katanga Province towards the centre of the Congo River Basin. The 1000 m-thick carbonate succession is related to the evolution of a marine ramp submitted to evaporation, with ‘deep’ shaly basinal and low-energy carbonate outer-ramp environments, marine biohermal midramp (MF6) and ‘very shallow’ restricted tide-dominated lagoonal inner-ramp (MF7-MF9) settings overlain by lacustrine (MF10) and sabkha (MF11) environments, periodically<p>submitted to a river water source with a possible freshwater-influence. The sequence stratigraphy shows that the sedimentation is cyclic in the inner ramp with plurimetric ‘thin’ peritidal cycles (± 4 m on average) recording a relative sea level of a maximum of 4 m, with fluctuations in the range of 1-4 m. The outer/mid ramp subtidal facies are also cyclic with ‘thick’ subtidal cycles characterized by an average thickness of ± 17 m, with a probable sealevel<p>fluctuations around 10 to 20 m. The geochemistry approach, including isotopic and major/trace and REE+Y data, allows to infer the nature of the dolomitization processes operating in each carbonate subgroup, i.e dolomitization may be attributed to evaporative reflux of groundwater or to mixing zones of freshwater lenses. The latest alteration processes occured during the uplift of the SMLL Basin. New ages, including LA-ICP-MS U-Pb laser ablation data on detrital zircon grains retrieved in the lower arenaceous-pelitic sequence (BI group), combined with carbon and strontium isotopic analyses, yielded a new depositional time frame of the Mbuji-Mayi Supergroup between 1176 and 800 Ma reinforcing the formerly suggested correlation with the Roan Group in the Katanga Province.<p><p>In the Democratic Republic of Congo, the Sturtian-Marinoan interglacial period was previously related to pre-glacial carbonate-dominated shallow marine sedimentation of the Haut-Shiloango Subgroup with stromatolitic reefs at the transition between greenhouse (warm) and icehouse (cold) climate periods, commonly marked by worldwide glacigenic diamictites and cap carbonates. This thesis highlights that these deposists record as a deepening-upward evolution from storm-influenced facies in mid- and outer-ramps to deepwater environments, with emplacement of mass flow deposits in toe-of-slope settings controlled by synsedimentary faults. In absence of diagnostic glacial features, the marinoan Upper Diamictite Formation is interpreted as a continuous sediment gravity flow deposition along carbonate platform-margin slopes, which occurred along tectonically active continental margins locally influenced by altitude glaciers, developed after a rift–drift transition. The maximum depth of the deepening-upward facies is observed in the C2a member. The<p>shallowing-upward facies exibit a return of distally calcareous tempestites and semi-restricted to restricted peritidal carbonates associated with shallow lagoonal subtidal and intertidal zones submitted to detrital fluxes in the upper C2b to C3b members.<p>The geochemistry highlights (i) the existence of a δ13C-depth gradient of shallow-water and deep-water carbonates; (ii) the carbonate systems were deposited in oxic to suboxic conditions; and (iii) all samples have uniform flat non-marine shale-normalized REE+Y distributions reflecting<p>continental detrital inputs in nearshore environments, or that the nearshore sediments were<p>reworked from ’shallow’ inner to mid-ramp settings in deep-water slope and outer-ramp<p>environments, during the rift-drift transition in the basin. The pre-, syn- and post-glacial<p>carbonate systems could record a distally short-lived regional synrift freshwater-influenced<p>submarine fan derived from nearshore sediments, including gravity flow structures, which are<p>attributed to regional tectonic processes due to a sudden deepening of the basin caused by<p>differential tilting and uplifting of blocks, related to the 750-670 Ma oceanic spreading of the<p>central-southern Macaúbas Basin.<p><p>Combining sedimentology, isotopes and trace elemental geochemistry, the thesis highlights<p>that the δ13C variations in the Neoproterozoic carbonates are complex to interpret, and can be<p>related to: (i) the existence of a δ13C-depth gradient; (ii) the exchange between isotopically<p>light carbon in meteoric waters and carbonate during lithification and early diagenesis; and<p>(iii) isotopic perturbations due to regional metamorphism. Considering the possible englaciation of the Earth (Snowball Earth hypothesis), the Mbuji-Mayi Supergroup and West<p>Congolian Group seem reflected the intimate relationship between glaciations and tectonic<p>activity during the break-up of the Rodinia supercontinent, followed by the rift–drift<p>transition, and finally the pre-orogenic period on the passive continental margin. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished