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Héritages morphostructural et climatique du drainage cénozoïque d'un bassin versant côtier : exemples de la Dives et des marais de Vimont (Normandie, France), implications hydrogéologiques sur la nappe aquifère du Dogger

Abstract : The Armorican block, structured by the Cadomian and Variscan orogenesis, was partially covered in the Mesozoic by vast marine platforms where the marl and limestone of the Paris Basin were deposited. From the end of the Upper Cretaceous, alpine compressive deformations lead to an emergence of the nord-eastern border of the Armorican Massif (Normandy). Jurassic and Cretaceous lands undergo alteration and erosion processes in response to slow climatic degradation up to the Quaternary glacial cycles, which are partly responsible for the development of landforms. During the Cenozoic, only a few areas open to the Atlantic Ocean, such as the Cotentin Center or the English Channel, recorded marine sedimentation, with sedimentary gaps and affected by polyphase deformation throughout the Cenozoic.Within these sedimentary, structural, eustatic and climatic frameworks, the coastal watersheds of the Bay of the Seine are being established, whose outlet of the hydrographic network corresponds to the quaternary fluvial system of the English Channel, itself flowing towards the Atlantic margin. The quaternary framework of the coastal watersheds of Dives and Orne has been the subject of several geomorphological studies, specifying the evolution of Holocene sedimentary filling. The Dives Valley was an estuary during the last Holocene interglacial, gradually filled by silts, clays and peat, creating vast wetlands. Above the clay and peat marshes of the Dives, there are more modest marshes such as those of Vimont. The Holocene filling of the marshes of Vimont does not explain this morphologic origin in the Bathonian series of the Caen Plain: simple depression pond in these calcareous soils during Neogene to Quaternary of sea level decline, or older morphology inherited from the sedimentary and structural history of the area? The question is all the more relevant as there is, in this limestone series, a large aquifer with natural denitrification processes.The reconstruction of the origin of the marshes of Vimont and its drainage system benefited from the sedimentary analysis of the Bathonian lands, crossed over 82 m by a core borehole drilled in these marshes. This analysis completes the stratigraphic division of an area poor in natural outcrops, but above all allows the revision of 845 destructive drillings carried out in the Bessin and Caen Plains. This stratigraphic, sedimentary and paleogeographic revision of the boreholes and outcrops reveals folding deformations of large radius of curvature with a direction of N 50-70 and a wavelength between 5 and 15 km, in Mesozoic lands. They are initiated by a polyphase cenozoic replay of the old N 50-70, N 90, and N 110 faults. The depression of the marshes of Vimont extends to the Troarn Synform, which influences both the drainage system and underground flows. Morphometric analyses describe this relationship between the marshes of Vimont, landforms and the Dives river system. This multidisciplinary, stratigraphic, sedimentary, structural and geomorphological approach to the marshes of Vimont makes it possible to distinguish the sedimentary and structural heritage from the quaternary climatic control.
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Loik Poignant. Héritages morphostructural et climatique du drainage cénozoïque d'un bassin versant côtier : exemples de la Dives et des marais de Vimont (Normandie, France), implications hydrogéologiques sur la nappe aquifère du Dogger. Sciences de la Terre. Normandie Université, 2020. Français. ⟨NNT : 2020NORMC202⟩. ⟨tel-02535152v2⟩

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