This study provides new insights into the Siderolithic Group, a 158 m -thick sedimentary succession deposited within a regionally extensive karstified unconformity spanning tens of square kilometres. This unconformity marks the Cretaceous-Paleogene boundary, with a stratigraphic gap of about 94 Myr, ranging from the Lower Cretaceous (Barremian–Aptian) to (likely) the Bartonian (Eocene). Using borehole data from the GEo-02 geothermal exploration well in the Geneva Basin, our study integrates petrography, mineralogy, geochemistry, and petrophysical analyses of 139 samples to reconstruct the evolution of this complex depositional system. Six sedimentological subunits were identified, representing distinct phases of karst evolution, from initial formation to infill and subsequent diagenesis. Variations in gamma ray values suggest fluctuating water levels within the karst system, while the heterogeneous distribution of quartz grains indicates detrital sediment input from both aeolian and fluvial sources. The mineral assemblage, dominated by siderite, chlorite, and kaolinite, reflects diagenetic processes under reducing conditions, with siderite facilitating kaolinite chloritisation in the presence of quartz. Geochemical data and clay mineral data indicate humid climatic conditions during deposition. The Siderolithic Group is inferred to be of Bartonian age based on biostratigraphic constraints from analogue deposits on the Swiss Plateau, suggesting its deposition during the Middle Eocene Climatic Optimum (MECO). This study provides one of the few continental records of the MECO in Western Europe, recording high mercury content possibly related to the flare-up in Neotethys subduction zone volcanism.