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Hardground discontinuities within carbonate platforms form important stratigraphic surfaces which can be used at basin scale to correlate sequence boundaries. Although these surfaces are commonly used in sequence strati‐graphy, the timing and duration of hardground lithification and the crystallization of early cements remain unexplored. Here, early calcite cements were dated by U‐Pb geochronology for five Jurassic hardgrounds, interpreted as third‐order sequence boundaries, situated within a well‐constrained petrographic, sedimentological and stratigraphic framework. The consistency or the slight deviation between the age of the cements and the stratigraphic age of deposition of the formations illustrate that cementation occurred early in the diagenetic history. The ages obtained on dogtooth cements, replacing aragonite in gastropod shells and pendant cements in intergranular spaces, match those of the standard Jurassic biostratigraphic ammonite Zones, making calcite U‐Pb geochronology a promising method for dating third‐order sequence boundaries of depositional sequences and refining the Jurassic time scale in the future. [ABSTRACT FROM AUTHOR]

Dating the onset of ancient (>1 Ma) karstification is a challenge. One approach is to date the earliest calcite cements in speleothems. We show the benefits of in situ U-Pb dating directly on thin sections from ancient (ca. 30 Ma) thin (<1 mm) speleothems in the karstified Lower Oligocene lacustrine-palustrine carbonates of the Paris Basin (France), which cannot be dated using other methods. We dated 32 calcite rafts (a type of speleothem), one geopetal cement, and 10 calcite cements precipitated along the karstic walls. The ages of the calcite rafts and cements at 29 ± 1 Ma (Lower Oligocene) fall within the age range of the host deposits (≈29 Ma) previously deduced from palaeontological evidence. We demonstrate that cementation of the carbonate host rock, its dissolution, and the speleothem precipitation occurred within 2 m.y. after deposition. Ostracods and intraclasts trapped within Rupelian calcite rafts clearly indicate that the karst developed deep underground shortly before a phase of lacustrine-palustrine sedimentation at the surface. This very early dissolution episode is attributed to the uplift of the Paris Basin as a result of the far-field intraplate deformation induced by the alpine orogenesis. This study shows that in situ U-Pb geochronology on ancient calcite rafts is a promising technique for the indirect dating of karstification and, more broadly, for dating geodynamic events and diagenetic evolution of sedimentary basins. [ABSTRACT FROM AUTHOR]