Breakthrough in dating ancient karsts: the U-Pb method reveals the secrets of the Parisian underground

For the first time, scientists have demonstrated the remarkable potential of uranium-lead (U-Pb) dating applied to fine calcite cements such as floating calcites(1) , to determine the age of karstifications going back several million years. This major advance has allowed to highlight the unsuspected precocity of the karstification of the continental rupelian limestone(2) of the Paris basin.

The research team applied this method to date the first calcite cements formed in a karstic system of oligocene continental limestone from the Etampes Limestone, in the Paris Basin. In total, 43 samples of floating calcites and other precipitated cements along the walls were analyzed in three karst cavities. The results obtained were unexpected: U-Pb ages are concentrated around 29 1 million years, coinciding with the period of formation of the encaissante rock itself (≈ 29 Ma).

This finding suggests that lithification(3), rock dissolution and speleothema precipitation(4) occurred in a remarkably short time span, not exceeding 2 million years after the initial deposition. Even more surprising, the presence of ostracodes(5) trapped between the levels of rupelian floating calcites indicates that karst developed in depth just before a phase of lacustrine sedimentation on the surface, revealing a potential syn-genetic character(6) .

The researchers attribute this very early dissolution to the uplift of the Paris Basin, a direct consequence of the alpine orogeny. This discovery opens new perspectives for the understanding of ancient geological processes. The U-Pb in situ dating technique on fine calcite cements is promising not only for indirectly dating karstification or fossils trapped in karsts, but also to study sedimentation and geological events in sedimentary basins, even in the absence of surface deposits.

This innovative study highlights the importance of developing new analytical methods to unlock the secrets of our subsoil. It offers crucial potential applications in a variety of fields, from water resource management to land use planning and the reconstruction of past landscapes and climates.