The complex diagenetic history of discontinuities in shallow‐marine carbonate rocks: New insights from high‐resolution ion microprobe investigation of δ¹⁸O and δ¹³C 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 diagenesis and the succession of events occurring during the formation of discontinuity surfaces in limestones, secondary ion mass spectrometry was used for the first time to measure the δ18O and δ13C signatures of 11 early cement and fabric stages in several discontinuity surfaces from the Jurassic carbonate platform of the Paris Basin, France. Pendant cements show a high variability in δ18O, 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 dogtooth cements and micritic microbially induced fabrics precipitated directly as low‐magnesium calcite in marine waters, as attested to by the preservation of their initial δ18O and δ13C 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 lithification. 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 discontinuity, 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.

Calcite uranium–lead geochronology applied to hardground lithification and sequence boundary dating

“…4, illustrating their shape, cathodoluminescence pattern and including the name of the sample in which they were observed. These cement types have been partially described by Andrieu et al (2018). Their U-Pb ages and associated uncertainty are listed in Table 1.…”

“…Up to date correlations in these limestones can be found in more recent publications (Brigaud et al, 2014a;Andrieu et al, 2016Andrieu et al, , 2017. The discontinuities documented by Brigaud et al (2014a) and Andrieu et al (2016Andrieu et al ( , 2017Andrieu et al ( , 2018 are well-expressed and associated with abundant early cementation, ideal for detailed study (Fig. 3).…”

“…They are associated in sample Au4 (Normandie) with peloidal to micropeloidal wackestones to packstones forming geopetal structures (200 to 500 µm thick; geopetal peloidal fabric, GPF) that are non-ferroan with orange luminescence. Andrieu et al (2018) suggest that the geopetal peloidal fabric (GPF) could have formed by erosion of micropeloidal meniscus-like cements by water, and then by filtering and deposition of the residual material. The very low U content of ME, MLC and GPF does not permit dating.…”

“…It is commonly considered that fibrous cements precipitate with an original aragonitic or high-magnesium calcite mineralogy from seawater and are then recrystallized in low-magnesium calcite later during diagenesis (Durlet & Loreau, 1996;Richter et al, 2003). This is a major limitation for U-Pb dating of the hardground, because the age obtained corresponds to the timing of recrystallization (Dickson & Kenter, 2014;Andrieu et al, 2018), and not to the timing of former precipitation in seawater during the sedimentary gap. (Andrieu et al, 2018).…”

“…In order to evaluate the use of early calcite cements precipitated during submarine lithification, this work proposes to focus on five lithified discontinuities of the western Tethyan domain from the Middle and Late Jurassic (Paris and Aquitaine basins), corresponding to well-documented, western Tethyan scale, third-order sequence boundaries (Jacquin et al, 1992;de Graciansky et al, 1998;Andrieu et al, 2016). The petrography of these discontinuities has been detailed at four main sites in France (Normandie, Pays de la Loire, Grand Est and Nouvelle Aquitaine) (Andrieu et al, 2016(Andrieu et al, , 2017(Andrieu et al, , 2018. These discontinuities are very well-calibrated in the ammonite zonation of the northwest European domain and correspond to thirdorder sequence boundaries (Andrieu et al, 2016).…”