Letteratura scientifica selezionata sul tema "Vistrenque (France)"

Abstract Triassic salt at 5–10 km depth may drive some of the recent tectonic features in southeastern France. We estimate the likely temperature range of the salt using two different approaches. The first of these, based on the extrapolation of deep temperatures obtained in oil exploration wells, predicts temperatures at a depth of 8 km to be in the range of 230–300°C. However, this prediction could be biased by a lack of deep measurements and problems related to lateral heat transfer caused by thermal conductivity contrasts. The second approach can overcome these problems by modelling the actual heat transfer for appropriate basin geometry, including temperature-dependent thermal properties, and a mantle heat flow of 35 mW.m−2. This latter value enables us to reproduce available temperature measurements and surface heat flow data. Here we evaluate the stationary temperature field along two sections constrained by seismic profiles, one at a local scale across the Vistrenque graben and the other at a more regional scale across the Southeast Basin. Our findings suggest that the temperatures in the deepest parts of the evaporitic layer (11 km depth) can reach up to 398°C, but can be as low as 150°C on the edge of the basin at the top of the salty layer. This temperature difference leads to important changes in salt viscosity. Results indicate that at a depth of 8 km, lateral viscosity contrasts within the evaporitic layer may reach 40. Such rheological contrasts might favour and amplify local subsidence, as seems to have been the case near the two Palaeogene half-grabens of Vistrenque and Valence, where deep hot zones are identified.

Based on detailed sedimentological analyses of cores, interpretation of well logs and a set of geochemical measurements performed on lacustrine sediments, the palaeoenvironmental evolution and the sedimentary architecture of the Paleogene continental Vistrenque Basin (SE France) have been reconstructed. The analysis of sedimentary archives revealed three main stages of basin infill evolution: 1) a transgressive deep-lake basin (Priabonian-earliest Rupelian) whose sedimentation was dominated by terrigenous gravity-driven deposits during a period of high subsidence rate and fault activity and under a prevailing humid climate; 2) a forced-regressive, evaporative deep lake (early Rupelian) characterized by a drastic reduction in terrigenous supplies and deposition of evaporites in disconnected, compartmentalized sub-basins; 3) an overall long-term normal regressive stage (middle Rupelian to earliest Chattian) of lake infill characterized by an increase in terrigenous supplies and a vertical upward transition from deep-lake gravity-driven deposits to marginal lake and floodplain sedimentation. The onset of lake volume reduction and forced regression during the early Rupelian is recorded by 1) the reworking of marginal lake carbonates into the deep lake areas, 2) the deposition of organic-rich sediments (TOC>10%) coupled with sulphate-reduction processes in the deepest areas of the lake, 3) an important decrease in terrigenous supplies and 4) a long-term increase in δ18O of matrix-supported carbonates. This early Rupelian forced regression of the Vistrenque lacustrine system is interpreted to result from a terrestrial lowering in precipitation in response to global cooling during the Eocene-Oligocene Transition (EOT). The final infill of the Vistrenque lake system (late Rupelian-early Chattian) and the installation of a floodplain occurred in more humid conditions during a stage of decreased activity of the Nîmes Fault, prior to or during an early stage of the Liguro-Provençal rifting.

The integrative analysis of a lacustrine carbonate succession from Butte Iouton hill (Vistrenque basin, SE France) brings new insights into depositional models of oolitic saline lake margins and provides new details regarding the late Eocene paleogeography of southeast France. Depositional facies analysis and paleoenvironmental reconstructions allow reconstructing an oolitic lacustrine ramp model, displaying from the proximal to the distal areas: 1) shallow marginal saline lake domain with deposition of planar microbial laminites, and molluscan-ostracodal wackestone, 2) a more distal and open lacustrine environments with low to moderate energy characterized by the deposition of peloidal grainstones and 3) a domain of higher energy with accumulation of ooids mixed with peloids in the vicinity of the area of ooid production. Lake margin carbonate sedimentation dominantly occurred during stages of lake transgression while subaerial exposure surfaces developed during periods of negative inflow-evaporation balance. Carbon and oxygen isotopes together with vertical trends in salinity inferred from molluscan associations show that lake transgression does not result from increasingly positive freshwater-evaporation balance volume but from the combination of subsidence and outflow from neighbouring salt waterbodies. The Butte Iouton carbonate margin is part of a set of interconnected saline lakes, occupying continental basins from Languedoc and Rhodanian region during the Priabonian, with a siliciclastic-dominated sedimentation in the southern margin, sourced by collapsing Pyrenean reliefs, and a carbonate-dominated northern margin with significant oolitic sedimentation in high-energy nearshore area.

L'augmentation des teneurs en nitrate dans les aquiferes est un probleme de plus en plus crucial, particulierement au voisinage des forages de distribution d'eau potable. Ce travail a pour objet l'analyse de la repartition spatiale des formes minerales et des teneurs de l'azote dans l'aquifere de la vistrenque (gard, france). L'etude de la denitrification en conditions controlees a montre que son developpement dependait de l'etat d'aeration du milieu. Les mesures du eh et du ph ont montre que cette reaction ainsi que celle de solubilisation du fer se sont realisees au voisinage de l'equilibre thermodynamique. A l'echelle de la region, ce travail a montre qu'il existait une relation entre hydrodynamique et etat d'aeration et egalement entre cet etat d'aeration et la denitrification. Ainsi, le gradient d'anoxie present dans l'aquifere conditionne la distribution de l'amont vers l'aval du nitrate, du nitrite et de l'ammonium. La distribution spatiale de la pression partielle equilibrante calculee en oxygene (po#2) et du tracage de l'abondance isotopique naturelle de l'azote (methode de reference) sont concordantes : elles permettent de definir les zones denitrifiantes dans l'aquifere de la vistrenque.

Les masses d'eaux souterraines soutiennent 65% de l’alimentation en eau potable en France. L'accroissement des pressions anthropiques entraîne des pollutions des eaux et la fermeture de captages. La diversité des pollutions existantes et l'apparition de contaminants émergents requièrent une compréhension globale du fonctionnement des masses d’eau souterraine. La mise en place de plan d'actions par les collectivités et gestionnaires de la ressource nécessite d’identifier les origines des contaminants et de définir les zones vulnérables avec précision. L’objectif de ce travail consiste en la mise en place d’une approche multi-traceurs sur la nappe de la Vistrenque (Gard), afin d’étudier l’origine et le transfert des contaminants. Les analyses portent sur les traceurs naturels d’origine de l’eau (les éléments majeurs, mineurs et traces), les isotopes stables de la molécule d’eau (δ18O/δ2H-H2O), les isotopes stables de l’azote des nitrates δ15N/δ18O-NO3-, l’isotope du bore δ11B, le gadolinium (Gd), ainsi que sur un cortège de molécules organiques incluant pesticides et résidus médicamenteux, servant eux-mêmes de traceurs. L'application de cette approche multi-traceurs a permis l'identification de l'origine des nitrates sur des zones de captages prioritaires présentant une superposition de sources de nitrate d'origines agricoles et urbaines. L’analyse de la signature isotopique en δ15N/δ18O-NO3- des nitrates facilement mobilisables des sols s’est montrée être un outil efficace pour caractériser les pollutions azotées en contexte agricole. En comparant les signatures isotopiques des sols et des eaux souterraines, les parcelles à l'origine de la contamination ont été identifiées. En plus d'une contamination agricole, l'infiltration d'un panache d'effluent de station d'épuration dans la nappe a montré l'importance de l'étude des résidus pharmaceutiques et la vulnérabilité des aquifères face aux contaminants émergents. L'ensemble de ces observations a permis de poser les bases pour transposer cette approche à d'autres secteurs d'étude, en se focalisant sur l'opérationnalité de l'approche par multi-traçage géochimique, isotopique et organique
In France, groundwater supports 65% of the drinking water supply. The increasing pressures of human activities lead to water pollution and the closure of water supply wells. The diversity of existing pollutants and the emergence of new contaminants require a comprehensive understanding of the functioning of groundwater bodies. The implementation of action plans by local authorities and resource managers requires identifying the origins of contaminants and defining vulnerable areas with precision. The objective of this work is to establish a multi-tracer approach on the Vistrenque aquifer (France, Gard), to study the origin and transfer of contaminants. Analyses include natural tracers of water origin (major, minor, and trace elements), stable isotopes of water molecules (δ18O/δ2H-H2O), stable nitrogen isotopes of nitrates δ15N/δ18O-NO3-, boron isotope δ11B, gadolinium (Gd), as well as a range of organic molecules including pesticides and pharmaceutical residues, serving as tracers themselves. Using this multi-tracer approach, it was possible to identify the origin of nitrates in water catchment areas where agricultural and urban nitrate sources overlap. Analysis of the isotopic signature in δ15N/δ18O-NO3- of easily mobilized nitrates in soils has proved to be an effective tool for characterizing nitrogen pollution in an agricultural context. The land parcels responsible for contamination were identified, based on the comparison of the isotopic signatures of soils and groundwater. In addition to agricultural contamination, the infiltration of an urban effluent plume from a wastewater treatment plant into the aquifer highlighted the importance of studying the occurrence of pharmaceutical residues and the vulnerability of aquifers to emerging contaminants. All these observations have laid the groundwork for extending this approach to other study areas, focusing on the operability of the geochemical, isotopic, and organic multi-tracer approach