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The complex diagenetic history of discontinuities in shallow- marine carbonate rocks: New insights from high-resolution ion microprobe investigation of d 18 O and d 13 C of early cements

Abstract : Sedimentary gaps are a major obstacle in the reconstruction of a carbonate platform's history. In order to improve the understanding of the early diage-nesis and the succession of events occurring during the formation of discon-tinuity surfaces in limestones, secondary ion mass spectrometry was used for the first time to measure the d 18 O and d 13 C signatures of 11 early cement and fabric stages in several discontinuity surfaces from the Jurassic carbon-ate platform of the Paris Basin, France. Pendant cements show a high variability in d 18 O, which was impossible to detect by the less precise microdrilling method. The morphology of a given cement can be produced in various environments, and dogtooth cements especially can precipitate in marine phreatic and meteoric phreatic to vadose environments. Marine dog-tooth cements and micritic microbially induced fabrics precipitated directly as low-magnesium calcite in marine waters, as attested to by the preservation of their initial d 18 O and d 13 C signals. Five discontinuity types are recognized based on high-resolution geochemical analyses, and their palaeoenvironmental history can be reconstructed. Two exposure surfaces with non-ferroan pendant or meniscus cements formed in the oxidizing vadose zone. A hardground displays marine fibrous cements and non-ferroan dogtooth cements that formed in a subtidal environment in oxidizing water. Two composite surfaces have undergone both marine and subaerial lithifica-tion. Composite surface 1 displays non-luminescent ferroan dogtooth cements that precipitated in reduced conditions in seawater, followed by brown-luminescent dogtooth cements characteristic of a meteoric phreatic environment. Composite surface 2 exhibits microbially induced fabrics that formed in marine water with abundant organic matter. The latter discontinu-ity, initially formed in a subtidal environment, was subsequently exposed to meteoric conditions, as evidenced by ferroan geopetal cements. A high-resolution ion microprobe study is essential to precisely document the successive diagenetic environments that have affected carbonate rocks and discontinuities with a polygenic and intricate history.
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Simon Andrieu, Benjamin Brigaud, Jocelyn Barbarand, Eric Lasseur. The complex diagenetic history of discontinuities in shallow- marine carbonate rocks: New insights from high-resolution ion microprobe investigation of d 18 O and d 13 C of early cements. Sedimentology, Blackwell Publishing, In press, ⟨10.1111/sed.12384⟩. ⟨hal-02541856⟩

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