Abstract

This study presents the first application of the U/Pb dating method to highly Pb-depleted diagenetic geodic calcites of the Jurassic formations of the Paris Basin that leads to a reappraisal of the palaeohydrological history of this region. Composite U/Pb ages from multiple geodes, combined with δ18O analyses, reveal two main phases of diagenetic fluid circulations linked with major regional tectonic events. Dogger formations recorded a first diagenetic fluid episode at 147.8 ± 3.8 Ma, i.e. at the very beginning of the emersion of the basin during the Tithonian period and 30 Ma earlier than previously assumed. δ18O results confirmed that most of the calcitic cement phases that closed the porosity of these formations precipitated at the beginning of the Cretaceous period. Oxfordian formations recorded another major meteoric fluid circulation at 33.5 ± 2.8 Ma related to the initial stage of the European Cenozoic Rift System (ECRIS). Consequently, the porosity of the Mesozoic formations of the studied area may have been closed sooner than previously thought, before the main ECRIS rifting phase of Oligocene age. This study shows that U/Pb dating of secondary geodic calcite offers a new powerful way for reconstructing the coupled palaeohydrological and diagenetic history of sedimentary basins.

Supplementary material: Pb/Pb diagrams, 238U and 235U series isochron diagrams and effect of blank correction on U and Pb isotopic compositions and on ages calculation are available at https://doi.org/10.6084/m9.figshare.c.3886885

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