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Dating the onset of ancient (>1 Ma) karstification is a challenge. One approach is to date the earliest calcite cements in speleothems. We show the benefits of in situ U-Pb dating directly on thin sections from ancient (ca. 30 Ma) thin (<1 mm) speleothems in the karstified Lower Oligocene lacustrine-palustrine carbonates of the Paris Basin (France), which cannot be dated using other methods. We dated 32 calcite rafts (a type of speleothem), one geopetal cement, and 10 calcite cements precipitated along the karstic walls. The ages of the calcite rafts and cements at 29 ± 1 Ma (Lower Oligocene) fall within the age range of the host deposits (≈29 Ma) previously deduced from palaeontological evidence. We demonstrate that cementation of the carbonate host rock, its dissolution, and the speleothem precipitation occurred within 2 m.y. after deposition. Ostracods and intraclasts trapped within Rupelian calcite rafts clearly indicate that the karst developed deep underground shortly before a phase of lacustrine-palustrine sedimentation at the surface. This very early dissolution episode is attributed to the uplift of the Paris Basin as a result of the far-field intraplate deformation induced by the alpine orogenesis. This study shows that in situ U-Pb geochronology on ancient calcite rafts is a promising technique for the indirect dating of karstification and, more broadly, for dating geodynamic events and diagenetic evolution of sedimentary basins. [ABSTRACT FROM AUTHOR]

Chlorite minerals, mainly in the form of clay coats, play a critical role in determining the reservoir quality of siliciclastic rocks. They can positively influence reservoir quality by preserving porosity during deep burial, but they can also play a negative role by reducing permeability through pore filling. The main aim of this research is to determine the optimal conditions for chlorite growth in sedimentary basins. This study investigates the Lower Cretaceous turbidite sandstone of the Agat Formation in the North Sea. We used a source‐to‐sink approach to investigate the impact of sediment source composition, chemical weathering and depositional environment on chlorite formation. Understanding the interplay between these processes can help refine exploration and exploitation strategies, optimise hydrocarbon recovery, and reduce exploration risks. Representative samples from two hydrocarbon fields (the Duva and Agat fields) were investigated using petrography, geochemistry, heavy mineral identification and quantification, and U–Pb geochronology of detrital zircons. Our results show a strong heterogeneity in the sediment provenance between the two turbidite systems. In the Duva field, the sandstone is derived from a mixture of mafic and felsic sources, producing Fe‐rich sediments. Intense chemical weathering generates fine fraction materials rich in kaolinite, vermiculite, and hydroxy‐interlayered clays, which are transported into shallow marine settings. Subsequent interaction with seawater results in the formation of glauconitic materials, Fe‐illite, and phosphatic concretions. These Fe‐rich materials are remobilised into deep marine settings, providing precursors for the development of authigenic Fe‐clays such as berthierine and chlorite. Conversely, in the Agat field, the sandstone is predominantly sourced from felsic rocks that underwent low chemical weathering, producing sediment rich in quartz and feldspar with a low amount of clays. With few Fe‐rich materials transported into the basin, the development of chlorite in the Agat field was less pervasive. Basin configuration and depositional environment exerted additional control on chlorite distribution. In the confined turbidite system (e.g. Duva field), chlorite is typically found as coating, whereas in less confined turbidite systems (e.g. Agat field) chlorite shows complex distribution related to depositional environment and dewatering processes. Our findings demonstrate the importance of considering the entire sediment routing system, from source to sink, when predicting chlorite occurrence and its impact on reservoir quality in deep marine settings. This integrated approach can guide exploration and development efforts in deepwater clastic reservoirs. [ABSTRACT FROM AUTHOR]

The difficulty of correlating continental deposits hinders predicting lacustrine and palustrine carbonate facies variations in time and space. This study aims to understand better the factors governing these facies heterogeneities by measuring carbonate isotopes and conducting facies, petrographic and sequence stratigraphic analyses of the Lutetian–Aquitanian deposits of the Paris Basin, that record the transition from marine to lacustrine environments. Large‐scale correlations enabled the definition of two lacustrine–palustrine carbonate facies models. (1) The coastal lacustrine system (Bartonian to Rupelian), consists of fine‐grained brackish carbonate exhibiting episodic marine inputs during short‐term relative sea‐level maxima and evaporite sedimentation during relative sea‐level minima. Lacustrine sediments differ notably from marine ones with more negative δ13C and δ18O compositions that co‐vary and a biota adapted to low salinity conditions. In the associated palustrine environment, depositional sequences evolve upwards from micritic lacustrine deposits to nodular and then laminar calcretes. Microbial‐coated grains and rhizoliths indicate biological processes during repeated subaerial exposure phases in sub‐tropical to arid climates. (2) The inland lacustrine system (Rupelian and Aquitanian) was disconnected from the marine domain and showed evidence of microbial activity with microbial crusts and oncoidal rudstones. Facies rich in micritic intraclasts composed of palustrine and lacustrine facies indicate the reworking of already lithified sediments along the margins. In the palustrine domain, the calcrete facies are less abundant than breccias formed in‐situ by desiccation, limestones with root traces, or organic‐rich wackestones and marls. This system reflects a more temperate climate with more developed microbial structures and less exposed carbonates than the coastal lacustrine system. The southward migration of the depocentre and the transition from marine environments to (1) coastal and then (2) inland systems are controlled by uplift phases induced by Pyrenean and Alpine orogenesis. Third‐order relative sea‐level variations appear to control only short‐term cycles in coastal systems. [ABSTRACT FROM AUTHOR]

The surface of intertidal estuarine sediments is typically covered with a photosynthetic biofilm. A large fraction of the carbon that is fixed is in the form of exopolymeric substances (EPS), providing the biofilm matrix. The consumption of organic carbon within the sediment column by heterotrophs bacteria is stratified according to the availability of electron acceptors used for organic matter degradation. This sequential use of electron acceptors strongly impacts geochemical gradients and early diagenetic processes within the sediment. In most studies, the distribution and role of the predominant microbial metabolisms is deduced from porewater chemistry and restricted to the upper decimeters of the sediment column, but rarely from direct measurements of microbial activity, potentially leading to erroneous conclusions of biogeochemical processes. We measured geochemical gradients in three estuarine sediment cores to a depth of 6 meters. Geochemical analyses of porewater and sediment were combined with measurements of microbial activity. In situ microelectrode measurements were performed for pH, oxygen and sulfide. Porewater was extracted and analyzed for major elements using Ion Chromatography, Inductively-Coupled-Plasma, and colorimetric assays for iron speciation. Porewater chemistry was compared to measurements of microbial activity including isothermal calorimetry and metabolic assays (triphenyltetrazolium chloride (TTC) and fluorescein diacetate (FDA)) and concentrations of EPS (sugars, proteins) measured in a previous study on the same cores. Finally, sediment composition was characterized through X-Ray Fluorescence core scanning. Results show that: (i) aerobic respiration occurred between 0 and 1 cm, (ii) nitrate reduction between 6 and 16 cm, (iii) sulfate reduction between 10 and 50 cm, (iv) manganese oxide reduction between 2–6 and 35–50 cm and (v) iron oxide reduction between 16–18, 24–26 and 35–45 cm. This is concomitant with the area where the microbial activity is the highest. In contrast to the literature, we conclude that some reactions, for example sulfate and nitrate reduction, were locally coupled or at least occurred concomitantly. Impacts of microbial metabolism on early diagenesis have been modeled via PhreeQc and predicted potential precipitation of metastable iron and/or sulfides. This is confirmed by iron and sulfur increases in sediments characterized through XRF. All these observations have been used to propose a biogeochemical model linking microbial metabolisms and early diagenesis that can be used as a basis for the study of other geochemical profiles in the future. [ABSTRACT FROM AUTHOR]