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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 is recognized worldwide as a key mineral that inhibits the development of quartz cement in deeply buried sandstone reservoirs. Iron-rich chlorite is mainly formed by the transformation of a precursor clay mineral; however, few studies have focused on the early stages before the crystallization of chlorite. This study analyzed shallowly buried (400-1000 m) coastal sandstones from within the Wealden Group of the Paris Basin. Berthierine, a 7 Å trioctahedral clay mineral belonging to the serpentine group, approximatively with same chemistry as chlorite but a different crystal structure, has been identified in a 900-mdeep well but not in a 400-600-m-deep well. Berthierine has mainly been observed as clay coatings around detrital grains with a typical honeycomb texture. Nanopetrographic observations suggest that the honeycomb textural organization of the clay particles found in deeper buried sandstone reservoirs (>1500 m) is acquired from a berthierine precursor at shallow depths. However, small amounts of quartz overgrowths are observed on the surface of detrital grains at shallow depths and low temperature (below 40 °C), and it is believed that precursor berthierine coatings are primarily responsible for the inhibition of quartz overgrowths before Fe-rich chlorite is formed. This suggests that the key mineral primarily controlling the reservoir quality of deeply buried sandstone reservoirs is berthierine rather than iron-rich chlorite, which challenges the commonly accepted assertion that chlorite coating is the main process that inhibits quartz overgrowths. The source-to-sink context of the Paris Basin during the Early Cretaceous was decisive with respect to the supply of sands and berthierine clay precursors (in particular kaolinite and iron-rich, hydroxy-interlayered clay minerals) to the center of the basin. [ABSTRACT FROM AUTHOR]

Recent developments of carbon capture and storage and geothermal subsurface prospects in the Middle Jurassic limestone of the central part of the Paris Basin have rejuvenated interest in the world class outcrops of the Bathonian Oolithe Blanche Formation on the southeastern edge of the Basin. Here, ooid-bioclast grainstones occur as large-scale (10–15 m high) clinoforms in the upper part of the Oolithe Blanche, which remain enigmatic and variously interpreted. High-resolution sequence stratigraphic correlations, constrained by biostratigraphic markers and integrated with regional palaeogeography, indicate that the clinoforms are giant dunes developed during the early part of the regressive phase of a third-order transgressive–regressive sequence (Bt3), in the northern part of the Burgundy Bathonian carbonate ramp. Carbonate sand bodies up to 15 m high with intervening troughs, formed subaqueous giant dune field(s) with calculated wavelengths of about 500–800 m. Using dune height as a proxy for calculating the palaeobathymetry of deposition, it is concluded that the dunes formed under a minimum water depth of about 60 m. A detailed facies analysis of the sediments underlying the dunes, which include beds rich in brachiopods in living position corresponding to the MFS of the third-order transgressive–regressive sequence (Bt3), supports such a water depth. The constitutive facies of the dunes themselves also reflect a significant water depth, despite the presence of ooids that are definitely transported away from their main production zone. This new interpretation of the large-scale clinoforms in the upper Oolithe Blanche and the associated revised palaeobathymetry have implications for the interpretations of Bathonian carbonates across the Paris Basin, which are relevant to ongoing projects of reservoir characterization of subsurface prospects. [ABSTRACT FROM AUTHOR]

Hardground discontinuities within carbonate platforms form important stratigraphic surfaces which can be used at basin scale to correlate sequence boundaries. Although these surfaces are commonly used in sequence strati‐graphy, the timing and duration of hardground lithification and the crystallization of early cements remain unexplored. Here, early calcite cements were dated by U‐Pb geochronology for five Jurassic hardgrounds, interpreted as third‐order sequence boundaries, situated within a well‐constrained petrographic, sedimentological and stratigraphic framework. The consistency or the slight deviation between the age of the cements and the stratigraphic age of deposition of the formations illustrate that cementation occurred early in the diagenetic history. The ages obtained on dogtooth cements, replacing aragonite in gastropod shells and pendant cements in intergranular spaces, match those of the standard Jurassic biostratigraphic ammonite Zones, making calcite U‐Pb geochronology a promising method for dating third‐order sequence boundaries of depositional sequences and refining the Jurassic time scale in the future. [ABSTRACT FROM AUTHOR]

Geothermometers are commonly used to reconstruct the diagenetic and thermal history of rocks. However, characterizing the timing, origin, and temperature of paleofluid flow remains challenging because it must be assessed indirectly through the analysis of microscopic cements that precipitate and fill intergranular spaces during fluid circulation. Here, we measure both the clumped isotope (Δ47) temperature and in situ U-Pb age of individual diagenetic calcite cements within a sedimentary section of the Paris Basin (France), whose thermal history has been previously inferred to be <60 °C. We show that cementation occurred during two stages associated with major events at the western European lithospheric scale: (1) the Bay of Biscay rifting (Late Jurassic–Early Cretaceous), and (2) north-south Pyrenean compression (Eocene) followed by east-west extension during the European Cenozoic rift system event (Oligocene). Related to both events, we report unexpectedly hot fluids, up to 110 °C, contrasting with the lower temperatures inferred from other geothermometers (e.g., fluid inclusions, clay minerals, apatite fission tracks, maturity of organic matter by Rock-Eval pyrolysis, or vitrinite reflectance). These high temperatures (>70 °C) have been measured for calcite cements containing single-phase aqueous fluid inclusions, challenging the commonly accepted assertion that the absence of nucleation of a vapor phase indicates crystallization at low temperature (∼<70 °C). We suggest that the kinetics of mineralization events prevented the recording of short-lived hot fluid flows by other geothermometers. [ABSTRACT FROM AUTHOR]

Authigenic clay coats (mostly Fe-rich chlorite coats) affect sandstone reservoir qualities by inhibiting quartz overgrowth during burial diagenesis, preserving both porosity and permeability. It is still unclear what initial mineralogical assemblages and initial sedimentation conditions produce chloritized sandstone reservoirs, which is important for sandstone reservoir quality prediction. For this purpose, better link facies with chlorite coat occurences could be useful. To address these questions, sedimentological, petrographical and mineralogical analyses were carried out from sand and sandstones cores for both a deeply buried Permian estuarine sandstone reservoir (Australia) and the Gironde estuary (France). Comparisons reveal similar sedimentary facies and vertical facies associations (from a muddy bottom to cross-stratified sandier packages and to a muddy top), indicative of tidal sand bars deposited in a mud-rich estuary. These criteria can be useful for recognizing tidal deposits when describing cores. X-ray diffraction and Scanning Electron Microscope (SEM) analysis show that detrital clay minerals are composed of illite, smectite, kaolinite and chlorite while clay assemblage differs in the Permian reservoir with dickite or an illite-rich illite/smectite mixed layer (I/S). Coats are either composed of detrital clays minerals (Gironde) or Fe-rich chlorite crystals (Permian). Transformations of detrital clays into other clay minerals (such as berthierine, precursor to Fe-rich chlorite) during eogenesis can initiate well-crystallized Fe-rich chlorite formation during burial diagenesis. Detrital minerals and detrital clay grain coats observed in the Gironde estuary could be the prerequisite initial conditions for generating authigenic Fe-rich chlorite coats in estuarine sandstones during burial. This is partly due to the initial clay fraction content of 15-20%, part of which forms detrital clay grain coats. Our main conclusion shows that facies from the middle to the upper tidal sand bar at the top of the transgressive cycle were probably uncemented during burial, and might be good candidates during reservoir exploration. • Sediments of the P.U.C.H.F (Australia) were deposited in a paleo-estuary very similar to the present-day Gironde estuary. • Best reservoir facies are located from the middle to the upper sand bar areas deposited in the outer estuarine tidal bars. • Detrital clay minerals from the Gironde might promote precursor clay-mineral formation to Fe-rich chlorite coats. • Sandstones deposited at the top of the transgressive sequence.might be good candidates for reservoir exploration. • Ideal initial conditions for reservoir formation can be a maximum clay content of 20–25 wt%, with coated grain content of 15–20% and coat coverage of about 10%.. [ABSTRACT FROM AUTHOR]

Porosity and permeability may be preserved in deep sandstone reservoirs by clay coating (mainly chlorite) which limits quartz overgrowths. Chloritization around quartz grains results mainly from mineralogical transformations of pre‐existing clays. It is fundamental to study those clay precursors to better understand and predict the location and distribution of clay coatings in subsurface sandstones for petroleum or geothermal prospecting. This paper reports a high‐resolution analysis of the composition, distribution and fabric of clays along a modern estuary, the Gironde (south‐west France). The scale of the study ranges from thin sections, through sand bar bodies, up to the entire estuary. Results show that clays are detrital and deposited at the same time as sand grains despite strong hydrodynamic conditions. Clays bind to medium‐grained sands forming detrital clay grain coats. On average, 26% of detrital sand grains are coated along the entire length of the estuary. Coat thickness varies from 1 μm to more than 200 μm, and coat coverage exceeds 30% in some samples. The turbidity maximum zone position (surface water turbidity from 1 to 10 g l−1) in the estuary, which is controlled by seasonal variations in hydrodynamic processes, significantly impacts the location of the maximum clay content and the abundance of coated grains in sandy facies along the estuary. Flocculation in the water column results in the accumulation of suspended matter and contributes to the high turbidity in the estuary. Exopolymeric substances produced by diatoms are observed both in the field and by cryo‐scanning electron microscopy, suggesting that they may play a major role in binding detrital clay around sand grains. Finally, tidal bars and heterolithic point bars in the estuary funnel and estuarine channels are prime sedimentological targets for finding clay coatings and potentially good reservoir quality in deeply buried sandstones. [ABSTRACT FROM AUTHOR]

• NMR logging allows precise identification of reservoir and non-reservoir facies in geothermal target. • Ooid-dominated facies exhibits high reservoir properties up to 1D exceeding 15% porosity. • Porous and permeable layers extend over at least 500 m within the geothermal reservoir. • The use of NMR logging data optimizes geothermal reservoir quality assessment. If they are to be economically and technically sustainable, geothermal projects require the production of hot fluid at high flow rates over a 30-year thermal lifetime. The combined use of multiple logging tools in making petrophysical assessments of reservoir quality helps to optimize drilling in areas of high geothermal potential. The present paper focuses on four geothermal wells intercepting Middle Jurassic (Dogger) carbonate rocks of the Paris Basin where for the first time Nuclear Magnetic Resonance (NMR) log data have been successfully used to investigate reservoir porosity-permeability heterogeneities. A total of ten facies have been identified from recovered cuttings and cores and grouped into four facies associations along a schematic carbonate ramp profile. From the wells studied, four reservoir units exhibiting porosities exceeding 15% and permeabilities of up to 1 Darcy (D) were traced in the Calcaires de Comblanchien , the Oolithe Blanche and the Calcaires marneux à Pholadomyes formations. The well's sub-horizontal trajectory and well-logs correlations between two wells, made it possible a priori to identify porous and permeable layers extending over at least 500 m and up to 2000 m within the Bathonian reservoir, providing useful pointers for further 3D reservoir geomodeling. Permeabilities derived from well testing proved to be overestimated when compared with NMR-derived permeabilities, illustrating the upscaling problem that is invariably a challenge in carbonate systems. NMR can be combined with production logging tool (PLT), that provides data on the distribution and thickness of productive layers, to give indications for example about continuous permeability record along the geothermal wells or about the proportion of micro and macroporosities in rocks. Based on the geological classification derived from examination of cores and cuttings, four rock-types (including mean T 2 pore-size distributions) have been identified and attributed to a given sedimentary facies and depositional environment by extending a clustering method to NMR log distributions from wells. [Display omitted] [ABSTRACT FROM AUTHOR]

The aim of this study is to decipher the respective influences of geodynamic and climate disturbances at the Jurassic/Cretaceous boundary on sedimentary facies and carbonate diagenesis in a stable intracratonic basin using isotopic geochemistry and subsidence quantification. Fourteen lithofacies were deposited in a (1) carbonate platform and (2) a delta plain environment. Climate change from cool and wet to warm and semi-arid conditions during the early Tithonian influenced the syn-sedimentary dolomitization process within the carbonate platform during the mid-Tithonian. Architecture and facies reconstructions well-constrained the Jurassic-Cretaceous Unconformity (JCU), which was an important local structural episode marked by (1) an 80 m uplift in the eastern Paris Basin and by (2) the formation of a NW–SE low wavelength 15 km-wide and 30 km-long flexure. This first tectonic event tended to maintain brine ponds and supratidal marsh environments in the platform during the late Tithonian and early Berriasian, forming Purbeckian facies and associated dolomitic facies. A major depositional change occurred between the early and late Berriasian from shallow carbonate platform environments to fluviatile-deltaic clastic deposits (Wealden facies). This facies change is underlain by a major unconformity corresponding to the Ryazanian unconformity. It is marked (1) by erosion processes, karstification of the carbonate substrate, and the development of ferruginous weathering products (goethite), followed by (2) incision processes in a fluvial-deltaic environment. This unconformity is consecutive to a 40 m uplift in the eastern Paris Basin. The rifting phase in the Bay of Biscay, in the Pyrenean Zone, and in the Artic-North Atlantic together with the opening of the Ligurian Sea had a major influence on the northern part of France by causing uplifts (120 m from the Tithonian) and flexuring. Geodynamics played a major role in carbonate demise in the Paris Basin leading to exposure and karstification of the carbonate platform. Added to the generalized uplift, western Tethyan cool and humid conditions from the late Berriasian caused terrigenous influx into large-scale marine domains which was detrimental and unfavorable to carbonate growth. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The scope of this work is to investigate, at a high resolution, the oxygen isotope composition (δ 18 O carb ) of diagenetic products (synsedimentary and burial calcite cements) in shallow-marine carbonates. SIMS (Secondary Ion Mass Spectrometry) microprobe analyses were performed on thin sections from Oxfordian and Kimmeridgian Formations of the eastern Paris Basin and compared to data obtained on the same diagenetic products by conventional mass spectrometry (acid digestion). Hereby obtained, δ 18 O are similar, but the SIMS dataset displays a larger range of values. The isotopic zonation obtained by SIMS transects through sequences of cements filling pores, reveals an (expected) isotopic depletion from older stage synsedimentary calcites to younger stage blocky calcites and that follows the CL (cathodoluminescence) zonation. SIMS analyses however show that synsedimentary cements precipitated in intra-skeletal pores, have heavier δ 18 O than their inter-particle counterparts, with an offset of + 4‰ V-PDB , despite similar petrographical characteristics. This difference is maintained in the δ 18 O of the first stages of blocky calcite cements, intra-skeletal blocky calcites showing heavier δ 18 O than the time equivalent and petrographically identical inter-particle calcites, with an offset of + 5‰ V-PDB . These offsets are tentatively explained by the precipitation of cements under non-equilibrium conditions in intra-skeletal pores, where organic matter decay may have played a key role, acting notably on the pH. The occurrence of isolated micro-diagenetic environments, co-existing at the thin section scale, is tentatively proposed as an explanation to these small scale and high amplitude δ 18 O heterogeneities. These results may question the sampling strategy for future works. Microdrilling may miss the observed range of variation, but averaging the values may not necessarily lead to real misinterpretations if a critical selection of samples is performed, targeting potentially similar micro-diagenetic environments and avoiding potentially specific ones, i.e. closed intra-skeletal pores. These results also definitely underscore the need for additional experiments to improve the reliability of SIMS, in order to develop the use of this very high resolution technique for carbonate diagenesis studies. [ABSTRACT FROM AUTHOR]

To improve the understanding of the distribution of reservoir properties along carbonate platform margins, the connection between facies, sequence stratigraphy, and early diagenesis of discontinuities along the Bathonian prograding oolitic wedge of the northeastern Aquitaine platform was investigated. Eight facies are distributed along a 50 km-outcropping transect in (1) toe-of-slope, (2) infralittoral prograding oolitic wedge, (3) platform margin (shoal), (4) open marine platform interior, (5) foreshore, and (6) terrestrial settings. The transition from shallow platform to toe-of-slope facies is marked in the field by clinoforms hundred of meters long. Carbonate production was confined to the shallow platform but carbonates were exported basinward toward the breakpoint where they cascaded down a 20–25° slope. Ooid to intraclast grainstones to rudstones pass into alternating marl-limestone deposits at an estimated paleodepth of 40–75 m. Three sea-level falls of about 10 m caused the formation of discontinuities corresponding to sequence boundaries. Along these discontinuities, erosional marine hardgrounds formed in a high-hydrodynamic environment at a water depth of less than 10 m, displaying isopachous fibrous cements and meniscus-type cements. The cements pass landward into meniscus and microstalactitic forms along the same discontinuities, which are characteristic of subaerial exposure. During the deposition of transgressive systems tracts, carbonate accumulation remained located mostly on the shallow platform. Energy level increased and carbonates were exported during the deposition of highstand systems tracts forming the infralittoral prograding oolitic wedge. During the deposition of lowstand systems tracts, carbonate production fell to near zero and intraclast strata, derived from the erosion of hardgrounds on the shallow platform, prograded basinward. Early diagenetic cements are related exclusively to discontinuities that are not found within the prograding wedge because of the continuous high sedimentation rate under lower hydrodynamic conditions. This absence of early cementation within the infralittoral prograding oolitic wedge was conducive to porosity conservation, making such features good targets for carbonate reservoir exploration. This study proposes a novel sequence stratigraphy model for oolitic platform wedges, including facies and early diagenesis features. [ABSTRACT FROM AUTHOR]

• Construction of heterogeneous 3D geothermal reservoir using data from 168 wells. • Facies, porosity and permeability modeled in 12 million cells covering 360 km3. • 20% of the oolitic and bioclastic facies have good reservoir quality. • Permeable layers form kilometer-scale patches (X, Y) of about 4 m thickness (Z). • The 3D model reduces the geological risk of future doublets in the Paris Basin. The greater Paris area has some 12 million inhabitants and 48 heating network production units that exploit the heat capacity of a 1.5 km deep aquifer, the Bathonian limestone. This is one of the most productive aquifers in the world for district heating, with an annual output of about 1.7 TWh of energy. The current challenge for Paris is to triple the number of heating networks using geothermal energy so as to reduce dependence on fossil fuels (40% in 2020 in France). As no detailed geothermal reservoir model is available, drilling involves geological risk. A recent well, drilled at Grigny (20 km south of Paris), turned out to be a failure in transmissibility because the bed of permeable limestones it tapped into was very thin (meter-scale). The main aim of this study is to create a digital database and a 3D geological model of this aquifer to minimize geological risks and optimize the location of future geothermal operations around Paris. By compiling data from 168 wells, a high-resolution 3D geological model of 360 km3 size is constructed (about 40 km x 50 km x 0.2 km), made up of 12.2 million cells and displaying sedimentary facies, sequence stratigraphy, porosity (Φ) and permeability (k). About 20% of the oolitic and bioclastic facies are of good reservoir quality (Φ > 13% and k > 350 mD), especially in two targeted, high-quality reservoir sequences. These facies of interest probably correspond to giant dunes and a shoal/barrier prograding from east to west. In these facies, permeable zones are generally 4 m thick and form patches of 1600 m x 1100 m, on average, elongated perpendicular to the depositional slope. 2D and 3D maps of temperature, salinity, porosity, transmissivity, and permeability allow us to understand the areas of interest for geothermal exploration, as demonstrated around Grigny. This model helps us to apprehend better the heterogeneous character of the reservoir for geothermal prospection and to reduce the risk of future doublets during well implantation. Detailed local models may be extracted to anticipate better the implantation of new doublets in areas with already densely spaced existing wells. [Display omitted] [ABSTRACT FROM AUTHOR]

The objective of this work is to improve our understanding of the processes controlling changes in the architecture and facies of intracontinental carbonate platforms. We examined the facies and sequence stratigraphy of Aalenian to Oxfordian limestones of western France. Seventy-seven outcrop sections were studied and thirty-one sedimentary facies identified in five depositional environments ranging from lower offshore to backshore. Platform evolution was reconstructed along a 500 km cross-section. Twenty-two depositional sequences were identified on the entire western France platform and correlated with European third-order sequences at the biozone level, demonstrating that eustasy was the major factor controlling the cyclic trend of accommodation. The tectonic subsidence rate was computed from accommodation measurements from the Aalenian to the Oxfordian in key localities. Tectonism controlled the sedimentation rate and platform architecture at a longer time scale. Tectonic subsidence triggered the demise of carbonate production at the Bathonian/Callovian boundary while the uplift made possible the recovery of carbonate platform from Caen to Le Mans during the mid Oxfordian. Topography of the Paleozoic basement mainly controlled lateral variations of paleodepth within the western France platform until the mid Bathonian. A synthesis of carbonate production in the western Tethyan domain at that time was conducted. Stages of high carbonate production during the Bajocian/Bathonian and the middle to late Oxfordian are synchronous with low δ 13 C, high eccentricity intervals, and rather dry climate promoting (1) evaporation and carbonate supersaturation, and (2) oligotrophic conditions. Periods of low carbonate production during the Aalenian and from the middle Callovian to early Oxfordian correlate with high δ 13 C and low eccentricity intervals, characterized by wet climate and less oligotrophic conditions. Such conditions tend to diminish growth potential of carbonate platforms. This work highlights the importance of climate control on carbonate growth and demise at large scale in western Tethyan epicontinental seas. [ABSTRACT FROM AUTHOR]

The deep siliciclastic reservoir (>3500 m) of the Upper Cape Hay Formation of the Bonaparte Basin (Petrel gas field, Petrel sub-basin, Permian) exhibits wide heterogeneity in porosity (2–26%) and permeability (0.001–2500 mD). To investigate this variability, 42 samples were taken from five wells drilled through this formation. Six facies were identified from core descriptions and microscopic study of the sandstones. These facies are typical of a tide-dominated estuary, and include (1) mud flat, (2) sand flat, (3) top of tidal sand bar, (4) middle of tidal sand bar, (5) bottom of tidal sand bar, and (6) outer estuary facies. The paragenetic sequence comprises the emplacement of early aggregates of ferrous clay mineral precursors, mechanical compaction, recrystallization of those ferrous clay mineral precursors to Fe-rich chlorite and crystallization of Fe-rich chlorite forming coatings around detrital grains, chemical compaction, development of quartz overgrowth, feldspar alteration, crystallization of dickite and illite-rich illite/smectite (I-S) mixed layers, and ferrous calcite cementation. The middle and top of the tidal bars generally exhibit the highest porosity (Φ > 10%) and permeability values (k > 1 mD). Feldspar alteration released silica and aluminium into the reservoir promoting the development of dickite and illite-rich I/S mixed layers, which tended to destroy porosity and permeability, as calcite cements and quartz overgrowths. Diagenetic chlorite coatings around detrital grains are restricted to the sand bar facies deposited at the end of the last third-order transgressive systems tract of the Cape Hay Formation. The formation and conservation of ferrous clay precursors seems to be possible in an estuarine environment where seawater and fresh water are mixed and tidal sand bars are formed. These ferrous clay precursors recrystallized to Fe-rich chlorite coating after mechanical compaction. These coatings inhibited quartz cementation and prove to be the key parameter behind good reservoir qualities. [ABSTRACT FROM AUTHOR]

Stratabound fluorite deposits occur at the unconformity between the Variscan crystalline basement and the Mesozoic sandstone, conglomerate, limestone, and dolomite rocks of the Morvan Massif in central Burgundy. This study describes their petrographic characteristics in an attempt to determine the nature and temperature of mineralizing fluids in order to better understand the fluid migrations that led to massive stratabound fluorite deposition. The general paragenesis encompasses two major mineralizing events causing a succession of fluorite, barite, and quartz in all deposits. The two mineralizing events were preceded by two corrosion (dissolution or karstification) events affecting both the dolomite host rock at Pierre-Perthuis and Marigny-sur-Yonne and the limestone host rock at Courcelles-Frémoy with the creation of 1–10 m cavities and microscopic vugs. At Antully, the blocky calcite initially cementing the sandstone was partially dissolved. Microthermometric data on aqueous two-phase inclusions attest to CaCl 2 -rich fluids giving rise to fluorite deposition in the Pierre-Perthuis, Courcelles-Frémoy, and Antully deposits. Homogenization temperatures range from 80 to 100 °C at Pierre-Perthuis and Courcelles-Frémoy, with sporadically higher temperatures. The range of CaCl 2 contents is 6.5–15 wt.% at Pierre-Perthuis, 1.7–9.4 wt.% at Courcelles-Frémoy, and 1.6–16.3 wt.% at Antully. The thermal history of the northwestern Morvan, compiled from organic matter, clay minerals and apatite fission track data indicates that the temperatures in fluorite and barite are higher than the maximum temperature recorded in sediments. This implies deep ascendant hydrothermal brine circulation during the Early Cretaceous. The impermeable cap rock retained the ascendant hydrothermal brine and allowed the deposition of massive fluorite stratabound mineralizations. [ABSTRACT FROM AUTHOR]

Nuclear magnetic resonance (NMR), stable isotope geochemistry of micro-sampled cores, NMR well-logs and 3D modeling are used to investigate the carbonate permeability-porosity heterogeneity along 230 m-thick limestones of the Paris Basin. Despite the global low porosity and permeability of the limestones, two aquifers units with porosity greater than 15% were identified. These two aquifers are very different in terms of pore through radii and NMR signal. The first one (A1: Aquifer 1) is a 7 m-thick mudstone unit, dominated by extended microporosity with pore throat radii of 0.25 μm to 0.3 μm. The second one (A2: Aquifer 2) is a 15 m-thick oolitic grainstone units showing macropores reaching 100 μm and pore throat radii of 32 μm. From core descriptions and wireline logs on 26 wells, a 3D static geological model is build. The fine tuning of permeability calculations from NMR logs realized along 12 of the wells, allows porosity and permeability heterogeneity to be distributed within a 3D model at the reservoir scale (area of about 2000 km 2 ) which match the flow behavior illustrated by well tests. Associated with early meteoric calcite cements and poorly developed burial blocky calcite cements, the porous and permeable intervals may be predicted in two stratigraphic and diagenetic considerations. Firstly, the syn-sedimentary meteoric dissolution or neomorphism of the initial high magnesium calcite and aragonite particles or clasts into low magnesium calcite particles or cements prevented most mechanical and chemical compaction during the first steps of burial. Secondly, the regional stratigraphic architecture reveals the presence of local permeability barriers, which prevented Early Cretaceous lateral meteoric water circulation and the associated burial calcite cementation. [ABSTRACT FROM AUTHOR]

Abstract: Several diagenetic models have been proposed for Middle and Upper Jurassic carbonates of the eastern Paris Basin. The paragenetic sequences are compared in both aquifers to propose a diagenetic model for the Middle and Late Jurassic deposits as a whole. Petrographic (optical and cathodoluminescence microscopy), structural (fracture orientations) and geochemical (δ18O, δ13C, REE) studies were conducted to characterize diagenetic cements, with a focus on blocky calcite cements, and their connection with fracturation events. Four generations of blocky calcite (Cal1–Cal4) are identified. Cal1 and Cal2 are widespread in the dominantly grain-supported facies of the Middle Jurassic limestones (about 90% of the cementation), whereas they are limited in the Oxfordian because grain-supported facies are restricted to certain stratigraphic levels. Cal1 and Cal2 blocky spars precipitated during burial in a reducing environment from mixed marine-meteoric waters and/or buffered meteoric waters. The meteoric waters probably entered aquifers during the Late Cimmerian (Jurassic/Cretaceous boundary) and Late Aptian (Early Cretaceous) unconformities. The amount of Cal2 cement is thought to be linked to the intensity of burial pressure dissolution, which in turn was partly controlled by the clay content of the host rocks. Cal3 and Cal4 are associated with telogenetic fracturing phases. The succession of Cal3 and Cal4 calcite relates to the transition towards oxidizing conditions during an opening of the system to meteoric waters at higher water/rock ratios. These meteoric fluids circulated along Pyrenean, Oligocene and Alpine fractures and generated both dissolution and subsequent cementation in Oxfordian vugs in mud-supported facies and in poorly stylolitized grainstones. However, these cements filled only the residual porosity in Middle Jurassic limestones. In addition to fluorine inputs, fracturation also permitted inputs of sulphur possibly due to weathering of Triassic or Purbeckian evaporites or H2S input during Paleogene times. [Copyright &y& Elsevier]

Abstract: It is usually very difficult to identify and quantify the relative influence of tectonics, eustasy and climate on carbonate system evolution from sedimentary records. In order to improve our understanding of these mechanisms, we have traced for the first time, the evolution of the eastern Paris Basin platform throughout the entire Jurassic period. This carbonate platform underwent eight successive growth and demise phases, with different depositional profiles ranging from ramps to flat-topped geometries. The eight carbonate growth periods are compared with the standard sea-level curves, local tectonic regimes and recently published oxygen-isotope and/or clay mineralogy databases. Prograding heterozoan facies along ramp profiles mark periods dominated by second-order eustatic sea-level rise, relatively cool sea surface temperatures, and mesotrophic and humid conditions (Hettangian, Pliensbachian, late Oxfordian, Tithonian). During these periods, variable detrital contents in the sedimentary succession hampered the efficiency of shallow-marine carbonate factories. Higher sea surface temperatures, oligotrophic and humid conditions associated with either eustatic sea-level rise or very high local subsidence occurred during the early Bajocian and the mid-Oxfordian. These seawater properties seem to have favoured the aggradation of scleractinian corals forming dome-shaped bioherm buildups. An oolitic and lime-mud carbonate system, deposited during the Bathonian second-order eustatic sea-level fall, is characterised by miliolid-rich micritic facies on a rimmed-ramp under stable, cooler and drier conditions. The second-order maximum flooding associated with a sea surface temperature decline and/or a seawater eutrophication caused at least five carbonate demise periods (i.e. Toarcian, earliest late Bajocian, Callovian/Oxfordian transition, earliest late Oxfordian and Kimmeridgian). [Copyright &y& Elsevier]

Abstract: The Middle Jurassic carbonates of the eastern part of the Paris Basin display surprisingly low values of porosity and permeability (Φ <15‰ and K <0.5mD). The main objective of this study is to determine the causes and timing of the cementation that altered the petrophysical properties of these carbonates thereby destroying their potential as oil reservoirs; a fate that did not befall their equivalents in deeper, central parts of the Paris Basin. Using petrographic and geochemical analyses (stable O and C isotopes, Sr isotopes, major elements), we identify six calcitic spar stages, two dolomite stages, and several episodes of fracturing and stylolitization ordered in paragenetic sequence. Cement quantification shows the predominance of two blocky calcite cement stages (75% of total cementation). O and Sr isotopes from these calcite cements suggest that the parent fluids resulted either from a mixing of trapped Jurassic seawater and meteoric water, or from buffered meteoric waters. In the geological history of the Paris Basin, major meteoric water inputs were possible during the Early Cretaceous, when the Middle Jurassic carbonates cropped out at its northern and eastern borders. Lateral meteoric recharge may have occurred as a result of two separate uplift events (Late Cimmerian Unconformity and Late Aptian Unconformity) and the related exposures of carbonates to the north of the study area. This palaeohydrological circulation brought about a significant reduction of porosity (from 40% to 10%) through calcite cement precipitation. The Early Cretaceous events are of great importance in the diagenetic evolution of the sedimentary basins bordering the London-Brabant Massif across all of northwestern Europe. A subsequent dolomite and calcite cementation stage accounts for about 5% of the total cement volumes. This late cementation may have been caused by hydrothermal fluids ascending along permeable fractures during the Late Oligocene extension episode. [Copyright &y& Elsevier]

Abstract: A detailed sedimentological, geochemical and mineralogical study is carried out on the Early Bajocian to Early Callovian (Middle Jurassic) limestones of the Paris Basin. Objectives are to document and explain the facies changes in the context of the climate/environmental evolution at the NW European scale. Deposits include 18 lithofacies which are stacked into 10 third-order depositional sequences. At a greater time scale, 4 biosedimentary packages are distinguished by their allochems associations. (1) An intracratonic carbonate environment with coral reefs and crinoid-rich facies is typical of the Early Bajocian. (2) A major facies change occurred at the Early/Late Bajocian transition with a shift from crinoid- and coral-rich facies to ooid-rich facies. During the Late Bajocian, a southward-dipping ooid ramp with successive progradational trends was emplaced. (3) A large lagoon protected by ooid shoals developed during the Bathonian in a muddy ramp setting. (4) At the Bathonian/Callovian boundary, a second major change occurred with the lagoonal facies being superseded by an ooid-bioclastic (crinoids, corals) ramp associated with a waning of carbonate productivity (retrogradational trend). Our geochemical study including a compilation of bibliographic data allows us to refine the existing sea surface palaeotemperature pattern. A cooling from the latest Early Bajocian to the Late Bajocian and a subsequent warming from the earliest Callovian to the Early/Middle Callovian transition are described using a proxy of sea surface temperatures. Together with δ 13C and mineralogical data, our new δ 18O values suggest a palaeo-climatic/palaeo-environmental control of facies in this shallow carbonate ramp environment. [Copyright &y& Elsevier]

Abstract: Oxygen isotope data from biostratigraphically well-dated oyster shells from the Late Jurassic of the eastern Paris Basin are used to reconstruct the thermal evolution of western Tethyan surface waters during the Early Oxfordian–Early Kimmeridgian interval. Seventy eight oyster shells were carefully screened for potential diagenetic alteration using cathodoluminescence microscopy. Isotope analyses were performed on non-luminescent parts of shells (n =264). Intra-shell δ18O variability was estimated by microsampling along a transect perpendicular to the growth lines of the largest oyster shell. The sinusoidal distribution of the δ18O values along this transect and the dependence of the amplitude of variations with bathymetry suggest that intra-shell variability reflects seasonal variations of temperature and/or salinity. Average amplitudes of about 5 °C in shallow water environments and of about 2–3 °C in deeper offshore environments are calculated. These amplitudes reflect minimum seasonal temperature variation. Our new data allow to constrain existing paleotemperature trends established from fish tooth and belemnite δ18O data and are in better agreement with paleontological data. More specifically, a warming trend of about 3 °C is reconstructed for oceanic surface waters during the Early to Middle Oxfordian transition, with maximum temperatures reaching 24 °C in the transversarium Zone (late Middle Oxfordian). From the transversarium Zone to the bimmamatum Zone, a cooling of about 7 °C is indicated, whereas from the bimmamatum Zone, temperatures increased again by about 7 °C to reach 24 °C in average during the cymodoce Zone (Early Kimmeridgian). [Copyright &y& Elsevier]

This study showcases the value of drone photogrammetry in creating a meter-scale geological model of complex carbonate geobodies. Although drone photogrammetry is now commonly used for modeling the sedimentary facies and architecture of sandstone outcrops, its use is not widespread in creating geomodels of carbonate geobodies. Drone photogrammetry can generate accurate line-drawing correlation and detailed architecture analysis along inaccessible vertical faces of outcrops and permits observations of unreachable places. This work models the Bathonian limestones of Massangis quarry (Burgundy) as an example. The quarry covers an area of 0.4 km2 and is considered as an analogue for the Oolithe Blanche geothermal reservoir in the center of the Paris Basin. The Massangis quarry model represents a good analogue for reservoir microporosity and secondary porosity associated with dedolomitization. Ten facies are described and grouped into three facies associations (1) clinoforms, (2) tidal to subtidal facies, and (3) lagoonal facies. The clinoforms are sets of very large marine dunes 15–20 m high that prograded N70° across the platform as part of a regressive systems tract. Moldic rhombohedral pore spaces associated with dedolomitization are well-expressed within clinoforms and in the bioturbated levels of lagoonal facies. Drone photogrammetry combined with the "Truncated Gaussian with Trends" algorithm implemented in Petrel® software is used to create a geological model that faithfully reproduces the facies architecture observed in the quarry cliffs. Drone photogrammetry can be combined with field work to describe and locate facies and so constrain the spatial distribution of petrophysical properties. It also helps constraining the shapes of geobodies in the model grid for more realistic geological static models and helps providing 3D petrophysical models from an outcropping analogue for geothermal and petroleum reservoirs. Image 1 • Drone-based photogrammetry in combination with VRGS® and Petrel® softwares. • High-resolution Digital Outcrop Modeling of 3D facies and sedimentary structures. • Clinoforms from giant, 15–20 m high marine sand waves prograding on the platform. • Analogue for carbonate-based geothermal and petroleum reservoirs. [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]

We provide new constraints for the fluid flow system at the origin of two F-Ba deposits located at the unconformity between the south of the Paris Basin and the northern edge of the French Massif Central. We used microthermometry and bulk crush-leach analyses to determine isotope ratios of mineralizing fluids (δ18O, δD, δ37Cl), together with cation and anion composition of fluid inclusions hosted by fluorite. Chlorinity and Cl/Br molar ratios (212–521) indicate the involvement of a brine, whose origin likely corresponds to Triassic evaporated seawater compatible with supratidal dolomitic facies preserved nearby. Microthermometry reveals high Ca/Na ratios, suggesting that the brine composition evolved from hydrothermal alteration of the Variscan basement and partial dissolution and replacement of the host sedimentary rocks. δ37Cl values are lower than the expected value of evaporated seawater, suggesting Cl isotope fractionation by ion filtration in clay-rich horizons. Fluorite crystallized at minimum temperatures of 70 to 110 °C, 10–40 °C warmer than the host Triassic sedimentary rocks. Ascending brines were expelled during the Early Cretaceous and experienced a drop in pressure and temperature, together with possible mixing with the SO4-rich pore water of the sedimentary rocks, causing precipitation of silica, followed by fluorite and barite, forming a stratabound deposit similar to those found in many areas in Western Europe. [ABSTRACT FROM AUTHOR]

• A new permeability versus macroporosity relationship optimizes permeability prediction. • Ooid-dominated facies displays by far highest reservoir properties up to 900 mD. • Dolomite dissolution process enhances pore network structure, thereby permeabilities. • NMR measurements provide sound guidance to de-risk future geothermal projects. A wireline NMR (Nuclear Magnetic Resonance) logging tool has recently been deployed in the Dogger geothermal aquifer of the Paris Basin to provide a continuous permeability estimation throughout the reservoir. The complex pore structure of heterogeneous carbonate systems means that careful consideration must be given to standard permeability prediction. A laboratory study was performed on cores from a geothermal well at Bobigny, north of Paris. Petrographic and petrophysical analyses of thin sections, water permeability and laboratory NMR relaxation time T 2 were conducted on 72 samples. A classification was established using four main facies and the impact of microporosity and micritization on flow properties was investigated. The range of permeability is wide [0.05–1000 mD] and the evaluation addresses different relationships between permeability and a combination of porosity and T 2 distributions. Since the latter distributions can provide an estimation of the pore size distributions and in particular the fraction of microporosity within the total porosity, several possibilities arise for better constraining the permeability relationship. For one facies, permeability is nearly independent of porosity over a porosity range of [0.12–0.22], illustrating the well-known difficulty of predicting permeability in carbonate lithologies. The best permeability prediction is obtained when considering only macroporosity instead of total porosity in a classical power law. [Display omitted] [ABSTRACT FROM AUTHOR]

Jurassic carbonate strata in the eastern Paris Basin exhibit several generations of faults, tension gashes and stylolites. Although their relative chronology can sometimes be determined according to cross-cutting relationships, the duration of major deformation phases and their influence on fluid flow and carbonate cementation are still uncertain. This contribution aims to clarify the timing of brittle deformation and associated calcite cementation. Tension gashes filled by calcite in Jurassic carbonates were sampled in outcrops and boreholes and dated through U–Pb geochronology. Almost all the sampled fractures were cemented during the Cenozoic period. Continuous deformation spread from c. 50 to 30 Ma. Tension gashes oriented N10° to N20° dated at 48–43 Ma show the main Pyrenean contractional stage. A second set of calcites were dated at c. 35–33 Ma and document a Late Eocene – Oligocene extension. A transition from the compressional to the extensional regime is expressed by tension gashes dated between 43 and 35 Ma. Finally, tension gashes oriented N150° to N175°, dated between 32 and 18 Ma, may result from the propagation of the horizontal stress generated by the Alpine orogen or by late Pyrenean deformation. Clumped isotope thermometry on five samples revealed both low crystallization temperatures (from 27 to 53 °C) and the meteoric origin of calcite-precipitating fluids. Our research therefore documents a continuous fracturing from Ypresian to Rupelian times, and less expressed brittle deformation during the Miocene period. [ABSTRACT FROM AUTHOR]

Estuarine tidal bar sandstones are complex reservoir geobodies commonly exploited by the oil and gas industry. In order to better predict the reservoir potential of these geobodies, this study provides a modern-day reservoir analogue, describing tidal bars in the inner and outer Gironde estuary from the microscopic to the macroscopic scale. The originality of this work lies in the multi-scale study of modern estuarine tidal bars based on numerous piston cores extracted in a high-energy environment. This work demonstrates that these tidal bars are composite sedimentary bodies made up of individual reservoir sand units separated by thick muddy layers. Their vertical facies associations and internal architectures are controlled by local hydrodynamic variations and seasonal river influxes. Detrital clay grain coats are notably characterized using a portable and handheld mineral spectrometer from the base to the top of the tidal bars. X-ray diffraction and electron microscopes reveal that these coats are mainly composed of di-octahedral smectite, illite, chlorite and kaolinite associated with other components such as diatoms or pyrite. The best reservoir geobodies are those with the minimum clay permeability barriers at the macro and mesoscale. An optimum coated grain content and clay fraction volume is also needed for generating authigenic clay coatings and inhibiting quartz overgrowth. These conditions are met within the tidal sand bars of the outer estuary funnel that are expected to be the best reservoir geobodies in deeply buried sandstones. • Within estuarine sandstones reservoirs, best reservoirs properties are found downstream of the TMZ. • Hydrodynamic processes in very turbid estuaries influence tidal bars vertical facies associations. • Coats can be found down to a depth of 7 m within modern estuarine tidal bars. • We tested the use of a portable and handheld spectrometer to detect clays in sands. • Ideal initial conditions: A clay content of 25 wt% at most, a coated grain content of 15%, a coat coverage of 10%. [ABSTRACT FROM AUTHOR]

In the Western Alps, the Penninic frontal thrust (PFT) is the main crustal-scale tectonic structure of the belt. This thrust transported the high-pressure metamorphosed internal units over the non-metamorphosed European margin during the Oligocene (34–29 Ma). Following the propagation of the compression toward the European foreland, the PFT was later reactivated as an extensional detachment associated with the development of the High Durance extensional fault system (HDFS). This inversion of tectonic displacement along a major tectonic structure has been widely emphasized as an example of extensional collapse of a thickened collisional orogen. However, the inception age of the extensional inversion remains unconstrained. Here, for the first time, we provide chronological constraints on the extensional motion of an exhumed zone of the PFT by applying U–Pb dating on secondary calcites from a fault zone cataclasite. The calcite cement and veins of the cataclasite formed after the main fault slip event, at 3.6 ± 0.4–3.4 ± 0.6 Ma. Cross-cutting calcite veins featuring the last fault activity are dated at 2.6 ± 0.3–2.3 ± 0.3 Ma. δ13 C and δ18 O fluid signatures derived from these secondary calcites suggest fluid percolation from deep-seated reservoir at the scale of the Western Alps. Our data provide evidence that the PFT extensional reactivation initiated at least ∼ 3.5 Myr ago with a reactivation phase at ∼ 2.5 Ma. This reactivation may result from the westward propagation of the compressional deformation toward the external Alps, combined with the exhumation of external crystalline massifs. In this context, the exhumation of the dated normal faults is linked to the eastward translation of the HDFS seismogenic zone, in agreement with the present-day seismic activity. [ABSTRACT FROM AUTHOR]

In sedimentary environments, clay and sand are segregated by hydrodynamic processes. Yet, clay coats, thin clay envelopes lining sand grains, are abundant in modern and ancient coastal sedimentary deposits. Here, we present laboratory experiments in which we produced clay-coated quartz sands similar to those observed in modern and ancient estuarine sands. These coats were produced at ambient temperature by mixing exopolymeric substances (EPS) derived from intertidal diatom biofilms with clay minerals and quartz reference materials. The imaging of sediment-EPS mixes using cryo--scanning electron microscopy and atomic force microscopy demonstrated that EPS form organic bridges between clay and quartz. The physicochemical properties of the EPS were characterized independently through wet chemical assays and Fourier transform infrared spectroscopy. The results indicated that several EPS components (e.g., proteins, polysaccharides) had a potential to complex with quartz and clay. Our findings provide novel insights in the importance of biofilms in the aggregation of clay and sand. Detrital coats from ancient estuarine sandstones show textural similarities to the experimental clay-EPS complexes and could be considered as biosignatures of biofilm development and EPS production in past environments. [ABSTRACT FROM AUTHOR]

In the Western Alps, the Penninic Frontal Thrust (PFT) is the main crustal-scale tectonic structure of the belt. This thrust transported the high-pressure metamorphosed internal units over the un-metamorphosed European margin during the Oligocene (34-29 Ma). The PFT was later reactivated as an extensional detachment associated with the development of the High- Durance extensional fault system. This inversion of tectonic motions on a major tectonic structure has been widely emphasized as an example of extensional collapse of a thickened collisional orogen. However, the inception age of the extensional inversion remains unconstrained. Here, for the first time, we provide chronological constraints on the extensional motion of an exhumed zone of the PFT by applying U-Pb dating on secondary calcites from a fault zone cataclasite. The calcite cement of the cataclasite, formed after the main fault slip event, is dated at 3.4 ± 1.5 Ma. Cross-cutting calcite veins featuring the last fault motions are dated at 2.6 ± 0.3 and 2.3 ± 0.3 Ma. d13C and d18O fluid signatures derived from these secondary calcites suggest fluid percolation from deep-seated reservoir at the scale of the Western Alps. Our data give evidence that the PFT extensional reactivation is active since at least ~ 3 Ma, at the crustal scale and results from the westward propagation of the compressional deformation related to the exhumation of External Crystalline Massifs. In this context, the exhumation of the dated extensional fault is associated with the eastward translation of the seismogenic zone controlled by the deep rooting of the High-Durance fault system. [ABSTRACT FROM AUTHOR]

In sedimentary basins, the precise temperature and timing at which relatively shallow (< 60 °C) diagenetic and tectonic events occur are poorly recorded by classic geothermometers and low-temperature thermochronometers. In this study, we apply a methodology coupling carbonate clumped isotope Δ 47 thermometry with U-Pb geochronology. Calcite breccias and veins from the border of the Gondrecourt graben (eastern Paris Basin, France), in Kimmeridgian limestones, were observed by optical microscopy and cathodoluminescence. They display Δ 47 values that are identical within uncertainties (ranging from 0.621 to 0.635‰), implying very uniform calcite deposition temperatures averaging 44 ± 4 °C for the three investigated zones. When Δ 47 temperatures are combined with the measured oxygen isotope composition of the calcite (δ 18 O cal between − 9.0‰ and − 8.3‰V-PDB), one can calculate the δ 18 O of the water (δ 18 O water ) from which these cement breccia and vein calcite precipitated. The δ 18 O water are very uniform (average of − 4.5 ± 1‰V-SMOW) and overall are consistent with a meteoric origin, however with a slight 18 O–enrichment via interaction with limestones prior to precipitation. U-Pb dating by LA-ICP-MS yields a model age for three hydraulic breccias at 43–42 Ma and an age at 35 Ma for calcite veining obtained from a single vein sample. When considering the geothermal gradient, the surface temperature during the Eocene and erosion, the temperatures obtained by Δ 47 thermometry suggest that there were hot waters circulating from the Middle Jurassic sequences below. Additional geochemical data (δ 18 O cal , δ 13 C cal , Rare Earth Elements, Mg, Mn and Sr compositions from a larger set of calcite breccias and veins from the area, together with published δD values of fluid inclusions) allow further characterization of the fluid from which the calcite phases precipitated. Notably, the whole suite of calcite shows a slight decrease in both δ 18 O cal and δ 13 C cal (δ 18 O cal between − 8.3‰ and − 9.9‰V-PDB, δ 13 C cal between 2.9‰ and 0.9‰V-PDB) and in Mg and Mn contents from breccia to veins, indicative of higher water:rock ratios during the vein stage and leaching of different source limestones. More broadly, this study demonstrates the potential improvement in diagenetic and paleohydrological research by combining U-Pb dating from the same calcite crystals as those analyzed for stable and clumped isotopes. [ABSTRACT FROM AUTHOR]

[Display omitted] • The seven deposits of the Bou-Izourane district show similar characteristics in terms of paragenesis, geochemistry and emplacement temperature, except for the site of Bou-Imetssene. • Both late magmatic fluids and meteoric waters are identified as mineralizing fluids. • The fluids have interacted with the rocks of the Tamazeght complex, which also acts as a heat source and a driving mechanism for fluid flows. • F is provided from the partial leaching of various minerals from the Tamazeght complex, while Ca is sourced mainly from the host sedimentary rocks. Fluorite deposits in the Bou-Izourane district of the High Atlas in Morocco are distributed over seven known mineralized sites near the Middle Eocene Tamazeght magmatic complex and hosted in sedimentary rocks (limestones and marls) of the Lower Jurassic (Bou-Izourane, Takkat, Bou-Kharouba and Tabja deposits), magmatic rocks (Bou-Imetssene and Meghsan deposits) and skarns (Tamazeght and Bou-Imetssene deposits). This paper examines the origin of the mineralizing fluids and the relationship between the Tamazegth complex and fluorite deposits. Fluorspar occurs in veins oriented according to three families (N20, N50-80 and N120), in karst cavities and as stratabound deposits. Three stages are identified in the mineralization: (1) fluorite emplacement associated with the dissolution of the host rocks; (2) fracturing and sealing by calcite and finally (3) silicification associated with the partial dissolution of fluorite and calcite. Fluid inclusions in fluorite hosted in marls and carbonates of the Lower Jurassic have low salinities (1.5 to 3 wt% equiv. NaCl) and homogenization temperatures between 91 °C and 159 °C. Fluid inclusions in the Tabja, Tamazeght and Meghsan fluorites exhibit the same salinity range, but with increased homogenization temperatures, ranging from 99 °C to 241 °C, with a vapour phase containing CO 2. Secondary solid daughter phases, such as nahcolite, calcite and strontianite are identified. Only Bou-Imtessene fluorite has high salinity fluid inclusions (∼18 wt% equiv. NaCl) and intermediate homogenization temperatures between 126 °C and 187 °C. These high local salinities and the presence of nahcolite and secondary carbonates in the fluid inclusions of the Tabja, Tamazeght and Meghsan fluorites may reflect the involvement of late-magmatic fluids. The δ18O isotopic composition of the fluid that precipitated calcites from Tabja (highest Th) and Bou-Izourane (lowest Th) indicates a meteoric origin. Fluorite has a broadly similar REE distribution to that of the Tamazeght carbonatites, suggesting that REE are derived from: i) carbonatites by hydrothermal alteration (e.g., Bou-Izourane, Bou-Kharouba and Takkat), and ii) carbonatitic (late-magmatic) fluids (e.g., Tabja, Tamazeght, Meghsan and Bou-Imtessene). Our data shows that fluorite mineralization in the Bou-Izourane district and the spatially associated middle Eocene Tamazeght magmatism have a direct genetic relationship through the contribution of late-magmatic F-rich fluids in close proximity to the complex. Fluorite mineralization farther from the complex is formed by per descensum circulation of meteoric waters along fractures resulting in fluorine leaching. The Tamazeght complex acts as a heat source and driving mechanism for fluid flows. The dissolution and recrystallization reactions are made possible by a strong geothermal gradient persisting after the complex was emplaced. The upwelling of these hydrothermal fluids into the calcium-rich host rocks (limestone and marl) leads to the precipitation of fluorite. [ABSTRACT FROM AUTHOR]

The preservation of reservoir quality in deeply buried sandstones largely depends on the presence of grain-coating clays that can inhibit quartz overgrowth and cementation. These grain-coating clays are the focus of today's hydrocarbon exploration as they enable new oil and gas discoveries in sedimentary basins. However, the origin and distribution of these clays in turbidites are poorly documented in scientific literature. Our study addresses this knowledge gap by investigating the mechanisms and parameters governing the clay coat development in turbidite sandstones, with a focus on the Agat Formation (Fm) in the Norwegian North Sea. Petrographic observations, scanning electron microscopy, and X-ray diffraction analyses were used to identify the origin of clay minerals and their distribution, as well as to understand the diagenetic evolution of the sandstone and its impact on reservoir quality. Based on the sedimentary facies description, the Agat Fm comprises four principal members deposited in various depositional environments, ranging from distal lobe fringe to amalgamated proximal lobes and weakly confined channels. Chlorite and kaolinite are the main authigenic clay minerals in these sandstones, associated with a variable amount of inherited detrital clays consisting in Fe-bearing kaolinite and mixed-layer minerals composed of illite and hydroxyl-interlayered clay minerals formed in acidic soil environments. Well-developed chlorite coats are associated with the highest reservoir quality (φ > 10%), while discontinuous chlorite coats are linked to extensive quartz cementation and poor reservoir quality (φ < 5%). Samples with abundant pore-filling chlorite also exhibit poor reservoir quality (φ < 7%). Proximal lobe deposits and weakly confined channels show well-developed chlorite coats, whereas discontinuous chlorite coats are mainly found in proximal lobe deposits associated with high velocity escaping fluids during dewatering. Pore-filling chlorites are typically present in distal lobe fringes and levee deposits. Chlorite in the Agat Fm is mainly derived from the replacement of inherited detrital clays via an intermediate berthierine precursor forming during early diagenesis. The dissolution of Fe-rich grains such as biotite and ferric detrital clays under reducing conditions governed the kinetics of berthierine/chlorite growth. The late diagenesis is characterized by extensive feldspar dissolution in a closed diagenetic system, leading to the precipitation of kaolinite and quartz cement. The presence of chlorite coatings around detrital grains preserved the porosity from extensive quartz cementation during deep burial, highlighting the role of chlorite in maintaining the quality of deeply buried turbidite reservoirs. • The distribution of clay coatings in turbidites is linked to the depositional environment and dewatering processes. • Good reservoir quality is found in weakly confined channels and in lobe deposits that have continuous grain-coating chlorites. • Poor reservoir quality is in lobes with discontinuous coatings, levees, and distal lobes dominated by pore-filling chlorite. • Chlorite forms by replacing inherited detrital clays through an intermediate berthierine precursor in early diagenesis. • Diagenetic evolution in a closed system led to kaolinite and quartz overgrowth from feldspar dissolution. [ABSTRACT FROM AUTHOR]

Studies on reservoir quality associated to chlorites in turbidites are quite rare and in recent studies on that topic, chlorite is generally a minor mineral. Moreover chlorite coatings are generally described for their major role in porosity preservation by preventing from quartz cement overgrowths, and only few studies describe their role in reducing permeability when present in excess in the reservoir. The objective of this study is therefore to focus on the material sourcing deep sea turbidites and its impact on the chlorite petrographic/mineralogical characteristics and abundance, and the resulting current reservoir quality. This study focuses on Lower Cretaceous Agat Formation (Måløy Slope, Norwegian North Sea) hosting two reservoirs of contrasted quality. It is based on drill-hole to micrometer scale investigations using QEMSCAN mapping, scanning electron microscopy equipped with an energy dispersive spectrometer, X-ray diffraction, and FEG SEM observation of ultrathin sections. Chlorite of Ib polytype, inherited from berthierine precursor, is observed in both reservoirs. Its homogeneous composition suggests that the geochemical controls of chloritization are similar for the two reservoirs. Grain replacing chlorite, clay replacing chlorite and chlorite coatings are observed in both reservoirs but the grain coatings present different textural configurations. In the upper reservoir, the clay coating is dominated by chlorite platelets whose growth is controlled by geometrical selection. In the lower reservoir, the chlorite coating differs by an additional inner part consisting of a diagenetic-recrystallized clay coat. The preburial material consists of ferric illite (± glauconitic pelloids). The illite morphology is considered to be inherited from an earlier supergene vermiculitization of phyllosilicates in saprolites. As ferrous iron dominates in berthierine/chlorite, a change in redox conditions is needed for Fe illite-berthierine transformation that could be linked to bacteria activity. These results provide a better understanding of the diagenetic process and influence of the material provenance in the evolution of the reservoir properties. • Crystal-chemistry, textural properties and origin of chlorite coating in turbidites formations. • Crystal-chemistry of green "glauconitic" clays. • Dual origin hypothesis for Fe-bearing clays (Fe-illite, glauconite) in the sedimentary material. • Contribution of hinterland weathering profiles of plutonic rocks under tropical climate to the source material of sediments. [ABSTRACT FROM AUTHOR]

The aim of this study is to explore the use of lichens as biomonitors of the impact of nickel mining and ore treatment on the atmosphere in the New Caledonian archipelago (South Pacific Ocean); both activities emitting also Co, Cr and possibly Fe. Metal contents were analysed in thirty-four epiphytic lichens, collected in the vicinity of the potential sources, and in places free from known historical mining. The highest Ni, Co, and Cr concentrations were, as expected, observed in lichens collected near ore deposits or treatment areas. The elemental composition in the lichens was explored by multivariate analysis, after appropriately transforming the variables (i.e. using compositional data analysis). The sample score of the first principal component (PC1) makes the largest (positive) multiplicative contribution to the log-ratios of metals originating from mining activities (Ni, Cr, Co) divided by Ti. The PC1 scores are used here as a surrogate of pollution levels related to mining and metallurgical activity. They can be viewed as synthetic indicators mapped to provide valuable information for the management and protection of ecosystems or, as a first step, to select locations where air filtration units could be installed, in the future, for air quality monitoring. However, as this approach drastically simplifies the problem, supplying a broadly efficient picture but little detail, recognizing the different sources of contamination may be difficult, more particularly when their chemical differences are subtle. It conveys only relative information: about ratios, not levels, and is therefore recommended as a preliminary step, in combination with close examination of raw concentration levels of lichens. Further validation using conventional air-monitoring by filter units should also prove beneficial. [ABSTRACT FROM AUTHOR]

The age of brittle deformation in the superficial part of orogens is generally constrained by relative, cross-cutting structural relationships. However, it becomes possible to decipher the timing of fault activity by a combination of methods based on U-Pb dating and stable isotope composition of calcite-filled fault and veins. This methodology is applied to constrain the timing of deformation of the frontal part of an orogenic system, through the example of the Tertiary development of subalpine massifs fold and thrust belt (composed by Bauges, Chartreuse, Vercors massifs). The architecture of the massifs is well constrained, but the chronology and the nature of involved fluids is unknown. Clumped isotope Δ 47 analysis shows a significant variation of fluid temperature from 54 °C to the west up to 149 °C for the internal Vercors thrust to the east. These temperature estimates highlight a deep underthrusting with a significant exhumation of the eastern Vercors (4–6 km), while a shallow underthrusting at a depth of around 2 km is estimated for the central Vercors. Carbon and oxygen stable isotope analyses, reveal three fluid signatures corresponding to heated fluids from meteoric or basin-derived origin, which interacted at various extents with the host-rock. These data are in agreement with a low altitude frontal belt, and with a progressive reequilibration of downwards infiltrating fluids with the composition of the host-rocks. The successfully dated calcites are those exhibiting the highest fluid-rock ratios, which allowed U enrichment in a mainly uranium-poor rock environment. In-situ U-Pb calcite dating was performed on fault mirrors of main thrusts and on fractured pebbles of the underthrusted molasses of eight major thrusts. Preserved Oligocene ages of extensional veins related to pre-Alpine rifting demonstrate that U-Pb calcite is a robust method to characterize multiple superimposed events of a whole mountain building history. Furthermore, the presence of multiple deformation events in the same location point out the importance of inherited structures on the strain localization processes. Thrust related U-Pb on calcite ages on an E -W section between the Vercors and Chartreuse massifs record a major Miocene shortening phase, and the timing of strain propagation towards the foreland. The oldest thrust dated at 15 Ma, corresponds to the activation of the most internal preserved thrust mobilizing 149 °C fluids. In the central subalpine massifs, ages range from 14 Ma (Chartreuse) to 12 Ma (Vercors) mobilizing 54 °C fluids. On the western part, the initiation of the most external thrust has been dated between 10 Ma and 7 Ma. These geochronological constraints are consistent with an 'in-sequence' westward propagation of the deformation between 15 Ma and 7 Ma and contemporaneous with the exhumation of the Belledonne External Crystalline Massif and the development of flexural basin sedimentation. Paleostress calculations indicate a three-stage evolution of the subalpine nappe stack. Reverse and strike-slip deformations are interpreted as a diffuse tectonic reactivation in an 'hors-sequence' mode from 10 Ma to present in a broadly similar stress field. [ABSTRACT FROM AUTHOR]

World-class stratabound fluorite deposits are spatially associated with the basement/sediment unconformity of the intracratonic Paris Basin and the Morvan Massif in Burgundy (France). The reserves are estimated to be about 5.5 Mt of fluorite within six fluorite deposits. In this study, we aim to determine the age of the major fluorite mineralization event of the Pierre-Perthuis deposit (1.4 Mt fluorite) by a combined study of the paragenetic mineral sequence and Sm-Nd dating on fluorite crystals. Fluorite occurs as isolated cubes or filling geodes in a Triassic, silicified, dolomitic formation. Three fluorite stages associated with sphalerite, pyrite, galena, barite, and quartz have been distinguished using optical, cathodoluminescence, and scanning electron microscopes. Seven crystals of the geodic fluorite stage were analyzed for their rare earth element (REE) contents and their Sm/Nd and Nd/Nd isotopic compositions. The normalized REE distribution displays homogeneous bell-shaped patterns for all the geodic fluorite samples with a Mid-REE enrichment over the Light-REE and Heavy-REE. The Sm/Nd varies from 0.3108 to 0.5504 and the Nd/Nd from 0.512313 to 0.512518. A six-point Sm-Nd isochron defines an age of 130 ± 15 Ma (initial Nd/Nd = 0.512054, MSWD = 0.21). This Sm-Nd isochron provides the first age for the stratabound fluorite sediment-hosted deposit, related to an unconformity in the Paris Basin, and highlights a major Early Cretaceous fluid circulation event mainly above the basement/sediment unconformity during a flexural deformation of the Paris Basin, which relates to the rifting of the Bay of Biscay and the formation of the Ligurian Sea in the Western Europe domain. [ABSTRACT FROM AUTHOR]

The thermal evolution of sediments in the shallowest part of sedimentary basins is challenging to investigate as most of the geothermometers are at their application limits. This is typically the case of the Callovian–Oxfordian claystones at the eastern border of the Paris Basin. A recent 2000 m deep well penetrated the entire Jurassic and Triassic sedimentary series and permitted coring of sediments never reached by previous drilling in this part of the basin. Thanks to the use of several independent geothermometers (apatite fission track thermochronology, fluid inclusion microthermometry, clay mineralogy and sedimentary organic matter evolution) the burial history of the Mesozoic series has been reconstructed, modeled and calibrated using kinetic algorithms relevant to the various paleothermometers used. In addition to these conventional methods, an innovative approach was developed using the temperature-dependence of the magnetic mineral assemblage within clay-rich sediments (“MagEval”). Thermal modeling indicates that the Callovian–Oxfordian and the Lower Triassic experienced maximum burial temperatures of 50 ± 5 °C and 90 ± 10 °C respectively, i.e., about 25 °C higher than present-day. This temperature offset implies the erosion of several hundred meters of sediments which, depending on the thermal flux and conductivity considered, most likely constituted Upper Cretaceous chalk deposits. Falling sea level resulted in the exposure and subsequent dismantling of the chalk cover at the beginning of Cenozoic times. The eastern border of the basin was then subjected to slow erosion and discrete tectonic deformation. [ABSTRACT FROM AUTHOR]

The turbidite sandstone of the Agat Formation is one of the most important hydrocarbon reservoirs in the Lower Cretaceous succession of the Norwegian North Sea. The diagenetic history of these sandstones has greatly impacted the reservoir quality, resulting in a high heterogeneity in porosity and permeability. To investigate this impact, sixty thin sections and rock samples were investigated through petrographic observations, geochemical microanalysis, and X-ray diffraction. Three main diagenetic events occurred during burial and influenced the reservoir properties: (1) mutual growth of chlorite, siderite, and apatite, (2) calcite cementation, and (3) dissolution of K-feldspar and calcite. Early diagenesis occurs under strongly reducing conditions, leading to the formation of Fe-chlorite/berthierine, siderite, pyrite, and apatite forming coatings around the detrital grains. Chlorite coatings have a key role in controlling the reservoir quality in the Agat Formation. In the lower part of the reservoir, chlorite forms thick coats (15–25 μm) that strongly reduce permeability. Conversely, in the upper part of the reservoir, chlorite coats are thinner (4–10 μm), favoring porosity preservation by inhibiting quartz overgrowth. X-ray diffraction and electron microscope observations indicate Fe-rich chlorite of Ib polytype, suggesting solid-state transformation from a berthierine precursor. The kinetics of berthierine/chlorite growth are governed by the dissolution of Fe-rich grains and fine fraction materials found as matrix and early coats. These grains are abundant in the lower part of the reservoir favoring the formation of thick chlorite coats. Chlorite was followed by pervasive calcite cementation in deep burial conditions creating low permeability barriers in the reservoir. The source of this calcite is internal and probably related to the dissolution of an early diagenetic calcite cement forming at shallow depth, with a minor contribution from marine carbonates. During late diagenesis, K-feldspar grains and calcite cements underwent intense dissolution creating secondary porosity and consequently increasing permeability. The input of acidic fluids associated with the maturation of the source rocks could have facilitated these dissolution reactions. Our results highlight the importance of inherited mineralogy in controlling the distribution of chlorite, carbonates, and secondary porosity in the Agat Formation. This study provides useful indicators to help predict diagenetic reactions that can occur in deep marine siliciclastic reservoirs. • The Agat Formation experienced a complex and multistage diagenetic history. • Thick-authigenic chlorite coats caused significant porosity loss in the reservoir. • Chlorite formed through solid-state transformation from a berthierine precursor. • Intense dissolution of feldspars and calcite cement improved the reservoir quality. • Initial mineralogical composition mainly controlled the diagenetic processes evolution. [ABSTRACT FROM AUTHOR]

Abstract: A set of carbonate outcrop samples, covering a wide range of the sedimentary textures and depositional environments existing on carbonate systems, was studied through an integrated petrographical and petrophysical approach. With the aim of improving the understanding of the NMR (Nuclear Magnetic Resonance) signal of carbonates, this work is: 1) providing an atlas for various carbonate reservoir rock-types, 2) providing a workflow for integrating geological and petrophysical data and, 3) documenting common shortfalls in NMR/MICP analyses in carbonates. The petrographical investigation includes thin section and SEM (Secondary Electron Microscope) observations, whereas petrophysical investigation includes porosity (Φ), permeability (K), NMR, MICP (Mercury Injection Capillary Pressure), and specific surface area (BET) measurements. On the basis of NMR and MICP data, 4 groups of samples were identified: (1) microporous samples, (2) micro–mesoporous samples, (3) wide multimodal samples, and (4) atypical samples. The microporous samples allow us to define a maximum NMR threshold for microporosity at a T2 of 200ms. NMR and MICP response of the investigated carbonates are often comparable in terms of modal distribution (microporous, micro–mesoporous and wide multimodal samples). In particular, micritization, a well known but underestimated early diagenetic process, tends to homogenize the NMR signal of primarily different sedimentary facies. A grainstone with heavily micritized grains can display well sorted unimodal NMR and MICP signatures very similar, even identical, to a mudstone–wackestone. Their signatures are comparable to that of a simple sphere packing model. On the contrary, several samples (labeled atypical samples) show a discrepancy between NMR and MICP response. This discrepancy is explained by the fact that MICP can be affected by the physical connectivity of the pore network, in case of disseminated and isolated molds in a micrite matrix for instance. Similarly, NMR can differentiate pore classes but with less resolution. It does not rely on connectivity but can be affected by diffusional pore coupling, i.e. the diffusion of water molecules carrying the magnetization between micropores and macropores. The pore-coupling phenomenon, through its impact on the T2 distribution, may disturb the permeability calculations from NMR data. For core plug characterization, NMR appears to be a complementary tool to MICP and should be used to complete rock-typing analysis in carbonates. For reservoir rock-typing, the obvious advantage of developing a NMR based approach is the use of NMR logging data providing continuous records of pore size distributions. [Copyright &y& Elsevier]

In this contribution, the origin of fluids having cemented the Jurassic limestones in the eastern part of the Paris Basin is discussed through the isotopic composition (δ18O, δ13C, 87Sr/86Sr) of authigenic calcite crystals filling vugs and fractures. The Upper and Middle Jurassic limestones, overlying and underlying the Callovian-Oxfordian claystones, experienced dissolution and crystallization leading to a succession of calcite cements depleted in 18O and enriched in 87Sr compared to the bulk host limestone isotope composition. These isotopic shifts originate from the flow of allochthonous basinal fluids through the fracture network that developed during the main tectonic deformation events. Mixing between the allochthonous fluids and the autochthonous local porewaters is attested by variable 87Sr/86Sr in diagenetic calcite. 87Sr-enriched fluids that precipitated calcite cements strongly impact the 87Sr/86Sr ratio of present-day groundwaters in Jurassic limestones and may have modified the Callovian-Oxfordian porewater isotope composition. • 87Sr/86Sr records the input of basinal allochthonous fluids through the fracture network in the Jurassic limestones. • The Sr isotope shift between calcite cements and the host limestones depends on the dilution rate of allochthonous fluids. • Paleo-water flows strongly modified the primary δ18O and 87Sr/86Sr composition in bulk limestones. [ABSTRACT FROM AUTHOR]

[Display omitted] • The Bou Arfa Mn(-Fe) mineralization (High Atlas, Morocco) is formed by post-sedimentary processes. • Dolomitization is considered as the main driver for Mn and Fe concentration. • The stability of the Mn(-Fe)-bearing minerals disabled strong post-ore weathering. • The proposed metallogenic model is similar to the genesis of Atlasic MVT deposits. Carbonate-hosted Mn deposits are widespread in North Africa with most of the Mn ores being hosted by Mesozoic dolomitic formations of the Moroccan Atlasic system. The Bou Arfa Mn(-Fe) deposit described herein is located in the eastern High Atlas, close to the North Atlasic Front, and has accounted for the production of ~2 Mt of ore grading from 33 to 82% Mn. The mineralization occurs within Sinemurian flat-lying dolostones as stratabound and karst-filling (veins, lenses, pockets). This mineralization is characterized by the extensive presence of pyrolusite, and minor concentrations of manganite, hausmannite, goethite and hematite. Previous studies suggested that the Bou Arfa Mn-bearing mineralization has a strictly synsedimentary origin, but the intense dolomitization and the occurrence of Mn oxides instead of Mn carbonates question this model. New petrographical, mineralogical and geochemical data show that three dolomitization phases accompanied the ore formation. The early synsedimentary dolomitization was not responsible for strong Mn enrichments, whereas a second dolomitization took place with the formation of hausmannite and manganite during early diagenesis. The precipitation mechanism involves circulation of dissolved Mn2+ in pore waters, and then precipitation under diagenetic suboxic and alkaline conditions. The scarcity of Mn carbonates is likely due to competitiveness of dissolved carbonates species that have favored dolomite precipitation in the early stage. Presence of saddle dolomite in a subsequent alteration stage, crystallography of the Fe oxides, and evolution of δ13C and δ18O reflect burial diagenesis and fluid mixing in the carbonate host rock. These conditions have enabled the transformation of the primary ore into pyrolusite, when diagenetically-derived hydrothermal fluids have generated vein-filling ore. The position of the Bou Arfa site above a paleohigh formed during Atlas rifting, the presence of a faulted zone and the dolomitization of the Sinemurian series have delimited the extension of this diagenetic Mn deposit, which is in some points similar to MVT deposits of North Africa. Weathering is poorly recorded in the Bou Arfa Mn ores due to the stability of the ore-forming minerals under subaerial conditions in relation to the most recent periods of exposure of the High Atlas building. [ABSTRACT FROM AUTHOR]