Relationships between intraplate deformations and the cementation of Jurassic carbonates in the eastern Paris Basin revealed by calcite U-Pb geochronology and ∆47 thermometry

In the eastern Paris Basin, the Oxfordian (Upper Jurassic) and Bathonian to Bajocian (Middle Jurassic) carbonate platforms have been intensively cemented, despite a relatively low burial history (< 1000 m). These limestones units are separated by a 150 m thick succession of Callovian-Oxfordian tight clay-rich rocks that are currently investigated by the French national radioactive waste management agency (Andra).

Most of the initial porosity in the Middle and Upper Jurassic limestones is now cemented by successive stages of calcite, which were thoroughly characterized both petrographically and geochemically over the last fifteen years (Buschaert et al., 2004; Vincent et al., 2007; Brigaud et al., 2009; André et al., 2010; Carpentier et al., 2014). However, despite such research efforts, the timing and temperature of the fluids involved in the cementation of these carbonate rocks are still debated.    

Here, we complement these efforts by coupling ∆₄₇ temperatures and U-Pb ages on calcite cement filling tectonic microfractures, as well as the intergranular pore space and vugs.

Our findings indicates that the Middle Jurassic limestones were largely cemented during the Late Jurassic / Early Cretaceous period, with new LA-ICP-MS U-Pb ages (Brigaud et al., 2020) in agreement with previously published Isotope Dilution-TIMS U-Pb age of 147.8 ± 3.8 Ma (Pisapia et al., 2017). This event is believed to be associated to the Bay of Biscay rifting. A second and more discrete crystallization event occurred during the Late Eocene / Oligocene period, related to the European Cenozoic Rift System (ECRIS).

The Upper Jurassic limestones were by contrast affected by a broader range of successive deformation events spanning the Late Mesozoic / Cenozoic period. New LA-ICP-MS U-Pb ages acquired in ca. 200 µm-thick fractures show that calcite crystallized during three successive periods corresponding respectively to the Pyrenean compression, the ECRIS extension and the Alpine compression.

Our study highlights tectonic stress propagation across hundreds of kilometers, from the rifting or collisional areas toward the cementation area of carbonate rocks. Thanks to the direct radiometric dating and clumped isotope thermometry of calcite cements in microfractures, a refined paragenetic sequence is proposed with emphasis on the genetic link between large-scale deformation and calcite precipitation.

References :

Buschaert et al., 2004. Applied Geochemistry 19, 1201 – 1215. Vincent et al., 2007. Sedimentary Geology 197, 267 – 289. Brigaud et al., 2009. Sedimentary Geology 222, 161 – 180. André et al., 2010. Tectonophysics 490, 214 – 228. Carpentier et al., 2014. Marine and Petroleum Geology 53, 44 – 70. Pisapia et al., 2017. Journal of the Geological Society of London 175, 60 – 70. Brigaud et al., 2020. Geology 48, 851 – 856.