This thesis investigates the Cretaceous to Paleogene (K-Pg) transition in the Geneva Basin (Switzerland and France), a strategic area for geothermal exploration. It focuses on Eocene siliciclastic deposits of the Siderolithic Group, overlying a karstified carbonate basement from the Lower Cretaceous. A multidisciplinary approach, including sedimentology, petrography, mineralogy, geochemistry, porosity and permeability analysis, and zircon geochronology, is used to characterise the stratigraphy, facies, reservoir quality, and sediment provenance. Data from the GEo-02 borehole and two-dimensional seismic profiles reveal strong heterogeneity in paleokarst development, a detailed stratigraphic framework, and several karst morphologies with high potential for heat storage. The results suggest that the Siderolithic Group preserves a signature inherited from reworked glauconitic sandstones of the Perte-du-Rhône Formation (Aptian to Cenomanian). This work proposes a new tectonic and stratigraphic model for the region, incorporating post-Cretaceous karstification, Eocene sedimentation, and crustal extension linked to the European Cenozoic Rift System, with implications for geothermal development in the western Swiss Plateau.