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Etude de la terminaison méridionale de l'Oural (Mougodjar, Kazakhstan) : évolution tectonique et stratigraphique du bassin d'avant-pays au Paléozoïque (Dévonien à Permien)

Abstract : This thesis is dedicated to the study of the foreland basin of southern Ural in Kazakhstan (Mougodjar). Sedimentological data have been collected from field investigations, and sub-surface data (seismic profiles, wells) have been studied. A sedimentological investigation of the turbidites, Carboniferous to Early Permian in age, has been performed. It allowed to characterize the environments of deposition of the turbidites and to evidence the long-term succession from uppermost Carboniferous to early Permian. The analysis of turbides indicates that the long-term sequences are controlled by global sea-level changes controlled by glacio-eustatism. The Early Permian orogeny, associated to the uplift of the Ural chain, controlled the quantity and quality of the sedimentary input sourcing the foreland basin. Important variations in the conglomeratic levels reflect (1) the erosion of material from the volcanic arc and carbonate platforms constituting the margins of the basin, and (2) local reorganizations of sub-marine slopes (changes in paleo-current directions). The smectitic minerals are the most frequent in the clay minerals assemblages reported in the uppermost Carboniferous to Early Permian sequences. Illite, Kaolinite, and chlorite are less frequently observed. This assemblage of clay minerals shows a permanence of detrital sources filling the basin. This pattern is thought to reflect a sorting effect of the clay particles along a proximal-distal transect, either than a direct paleoclimatic control. The distribution of the organic matter, mainly marine from the Middle Devonian to the Carboniferous indicates that the paleoenvironments were looked favourable to high surface paleoproductivities associated to a dysoxy/anoxy of all, or part, of the water column in the context of a restricted marine basin. The highest values of COT (20 %) found in the Kiya section (Fammenian) may be associated to a major Devonian OAE, the Hangenberg event. However, the accuracy of available biostratigraphic data does not allow to ensure such correlation. Indeed, OAE events may have favoured the accumulation and preservation of the organic matter in the Mougodjar, in addition to physiographical effects (e.g., the relative closure of the basin toward the south). The high concentration in organic carbon (COT 20%) of the Devonian sediments shows good potential source rocks. The content in organic matter, mainly from terrestrial origin, of the Permian turbidites are lower (COT reaching up to 4%), but quite high in some stratigraphic levels. The proximal silty-sandy deposits display the higher COT values. These Permian turbidites may be good reservoirs. They could also be potential gas-prone source rocks. The study of the sub-surface data allowed us to determine the structure of the foreland basin at the scale of southern Ural. It is constituted of folds and west-vergent thrusts resulting from a polyphased tectonic evolution. The folds gradually disappear toward the west passing westward either to the stable Russian platform in the north or to the eastern Pre-Caspian basin to the south. We have reconstructed the tectono-sedimentary evolution of the western Mougodjar through paleogeographic maps. Five key periods have been selected between the Devonian to early Permian period. Our reconstructions evidence two main orogenic events: (1) a first collision between the European continent and the Magnitogorsk volcanic arc in Middle Devonian, and (2) the major uralian orogeny during the Late Carboniferous to Early Permian times resulting from the collision of the European continent with the Kazakh and Siberian plates. This event originated the foreland basin, which is almost completely fulfilled by a thick detrital sequence. In Kungurian, the foreland basin is almost completely isolated and a thick evaporitic sequence deposited.
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Saya Abdulanova. Etude de la terminaison méridionale de l'Oural (Mougodjar, Kazakhstan) : évolution tectonique et stratigraphique du bassin d'avant-pays au Paléozoïque (Dévonien à Permien). Sciences de la Terre. Université Pierre et Marie Curie - Paris VI, 2017. Français. ⟨NNT : 2017PA066643⟩. ⟨tel-02078792⟩



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