Timing and duration of diagenetic events constrained by U-Pb dating of calcite cement. An example from the Urgonian Limestone, SE France

Our understanding of the diagenetic evolution of sedimentary systems is commonly limited to the establishment of relative chronological sequences of events, and in many cases no information on the timing of fluid circulations. However constraining the absolute age of diagenetic cements, such as secondary carbonates, which ubiquitously form in sedimentary limestones during burial, is key to the reconstruction of the diagenetic histories. Developing accurate techniques of radiometric age dating applicable to diagenetic minerals is therefore an important challenge for the understanding of the overall evolution of sedimentary systems. We report here, U-Pb ages of a major diagenetic calcite cementation event that affected the Urgonian platform in South-Eastern France, Provence. These Urgonian limestones are known for being a good analog of the Karaib/Shuaiba reservoir formation, a micro-porous carbonate occurring in the Middle-East. In the Urgonian limestone, the latest diagenetic event preserves the µ-porosity development (Léonide et al., Sedimentary Geology, 2014) and is characterized by precipitation of coarse calcite cements filling both inter- and intra-granular porosity and vuggy porosity. This diagenetic calcite was sampled in the same stratigraphic unit at two different locations several tens of km apart. The two calcite samples yielded very close U-Pb ages at 95.86 ± 0.46 Ma and 89.13± 0.87 (238U-206Pb isochrones), with indistinguishable common lead ratios. This suggests that the calcite formed from a significant regional fluid circulation event over a few Ma only after the Durancian phase. These results imply that µ-porosity development took place during early diagenetic processes in relation with the emersion event linked to the Durancian Uplift. Additional work is underway to further constrain the duration of this fluid flow event by investigating more samples from different locations across the Urgonian platform.