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International audience; This study discusses the hydrothermal formation of romanechite (Ba0.7Mn4.5O10.1.1H2O) in the Romanèche Mn deposit and the significance of its age by combining ore petrogenesis and 40Ar/39Ar ages. Romanechite occurs as a minor phase in various manganese deposit types and remains poorly studied in comparison with other K-bearing Mn oxides (i.e., coronadite group) because of its scarcity, small grain size, intermixing and poor crystallinity. The fault-controlled Romanèche Mn deposit (Massif Central, Eastern France) investigated herein exhibits a well-crystallized and late stage massive romanechite ore occurring in a quartz-barite-fluorite gangue. Petrographical and geochemical data support manganese ore formation under sub-aerial conditions with temperatures close to fluorite precipitation (100–200 °C) and characterized by a Mn-Ba-W-As association. Five out of the eight studied samples have produced 40Ar/39Ar plateau ages ranging from 25.2 ± 0.3 to 18.2 ± 0.5 Ma. Despite a low K content (0.1–0.6 wt% K), the good crystallinity of romanechite due to its epithermal formation likely favours a better production and/or retention of the radiogenic argon than poorly crystallized phases of supergene origin. Therefore, the dataset identifies a period of at least c. 7.6 My of discontinuous hydrothermal activity, which is consistent with graben-related fault reactivation and likely volcanic activity in the Massif Central. Ultimately, this study confirms the feasibility of romanechite 40Ar/39Ar dating.

Many European uplands have remained topographic highs for protracted periods of weathering during the Mesozoic and Cenozoic. The long-term elevation position as low relief of these massifs acted as a source for adjacent sedimentary basins, their weathering playing a major role in the production of sediments available for erosion. Weathering profiles on these uplands are mostly incomplete/truncated since weathering products have often been washed away during repeated periods of erosions, leading to patchworks of remaining weathered rocks. In order to better address the events that have shaped these source areas, a complementary approach of both the source (upland) and sink (sedimentary basin) areas is crucial, but the record in the hinterland often remains partial due to lack of dating material and incomplete sequence. The study presented here after is part of the BRGM-TOTAL Source-to-Sink project and aims at identifying key periods of exposure of the Massif Central (France). We used the combination of a careful petrogenesis and 40Ar/39Ar dating of K-bearing Mn oxides (coronadite group and romanechite). The French Massif Central hosts several Mn occurrences sporadically mined until the first half of the 20th Century. We have targeted five of these sites that contain dating minerals: (1) “Nontronais” (Haute-Dordogne; Mn-Fe; south-west), (2) Villerembert (Montagne Noire; Mn; south), (3) Vieussan (Montagne Noire; Mn; south), (4) Auxilhac (Lozère; Mn-Fe; south), and (5) Romanèche (Mn-F-Ba; east). The combined petrography, mineralogy and geochemistry of these Mn ores show that Mn deposits 1, 2 and 3 belong to a weathering crust model (“laterite”), sampling site 4 follows a karst-hosted model whereas sampling site 5 follows an epithermal model. In any of these cases, the dated K-Mn oxides were formed at the end of the weathering paragenetic sequence, except maybe for Romanèche which encompasses epithermal formation. This and their oxidizing precipitation environment support a formation under surface or sub-surface conditions. Therefore, their ages are instructive of periods of meteoric waters circulation under near-surface conditions. For the first time in these areas, the 40Ar/39Ar ages define at least four weathering periods: (i) Albian and older, (ii) Campanian, (iii) Oligocene-Early Miocene and (iv) Late Miocene to Quaternary. The French Massif Central is a key place in the West European landscape that better constrain the long-term evolution of weathering that has affected basement rocks and their cover. The Early Cretaceous period is well known in Western Europe for having provided various and thick weathering products (bauxite, kaolin, siderolithic). The Campanian age fits well with the first slow Pyrenean compression that exposed the Montagne Noire (south Massif Central), whereas Oligocene-Quaternary weathering events correspond to widespread weathering phases and compressional phases of the Alpine and Pyrenees. The lack of any Eocene period in the age record might be due to erosional conditions that overcome weathering, as it corresponds to the paroxysmal phase of the Pyrenean compression and strong siliciclastic discharge from the Massif Central to the Aquitaine and Paris basins.This work is funded and carried out in the framework of the BRGM-TOTAL project Source-to-Sink.

The contribution of groundwater to the hydrology of hard rock regions has long been assumed to be small. This is being progressively challenged and conceptual hydrological models of headwater wetlands in these regions may need to be revised. We developed a high-resolution MIKE SHE/MIKE 11 model of a 231.3 ha headwater catchment in the granitic uplands of the French Massif Central to estimate the contribution of groundwater upwelling to the water balance of the Dauges mire, an acidic valley mire of international importance for nature conservation. We estimated that groundwater upwelling from the underlying granite weathering formations – mostly an approximately 55 m deep fissured zone – provides 27.1 % of total long-term inflows to the mire. This contribution increases to 37.2 % in September when total inflows are small. Overland boundary inflow accounts for an average of 40.2 % of total inflows. However most of this originates from groundwater seepage through mineral soils along the mire margins or in small unchannelized valleys upslope of the mire. A sensitivity analysis showed that model performance in terms of the simulation of mire groundwater levels was most sensitive to parameters describing the mineral soils and granite weathered formations rather than the overlying peat layer. Variation partitioning showed that groundwater upwelling was the most important factor driving simulated monthly groundwater table depth within the mire. Sustained groundwater upwelling maintains the mire water table close to or at ground level for most of the year. As a result, precipitation and overland boundary inflows are mostly evacuated as saturation-excess runoff. There was close agreement between the observed distribution of mire habitats and areas where the simulated long-term groundwater seepage rate was larger than zero in September. Groundwater upwelling from the underlying weathered formations can be a quantitatively important and functionally critical element of the water balance of valley mires in granitic headwater catchments. These results have important legal and management implications.

International audience; The post-orogenic evolution of the European Variscan basement remains difficult to constrain due to the absence of post-orogenic sedimentary records. One of the current ways to access this history is to constrain the thermal evolution of these basement areas through indirect methods such as low-temperature thermochronology (LTT) thermal modelling. In this study, apatite fission-track (AFT) data have been acquired for the north-eastern part (Vendée and Limousin) of the French Massif Central Variscan basement. Similar thermal inversion procedure has been done, assuming that some samples were close to the surface during Triassic times. The results show a complex post-Variscan evolution of the western part of the Massif Central. Inversion of the AFT data indicates a high-temperature event during the Jurassic period. This thermal event can be interpreted as the deposition of a significant sedimentary cover which has been later eroded during the Late Jurassic and/or Early Cretaceous. Interpretation of the thermal models implies the presence of the sampled rocks close to the surface during Cretaceous and Eocene times. The estimated volume of this eroded sedimentary cover suggests a probable larger extension of the surrounding sedimentary basins (Paris Basin and Aquitaine Basin) across the Variscan basement area.

The Penmarc’h granitic coastal domain, SW Brittany (Western France), offers the opportunity to address the little investigated issue of erosional processes in a granitic coastal context. From land to sea, the granitic coast is composed of 1) an aerial marine terrace ( 10 Be) analyses, supplies new constraints on the weathering and denudation processes recorded by the Penmarc’h platform in Quaternary times. The role of at least two denudation events is emphasized: (i) a >1.8 m-thick level of granitic material is assumed to have been removed by erosion during the MIS5 sea-level highstand, both in the offshore domain and in the low part of the marine terrace; (ii) an additional 0.6 m-thick level of the granitic substratum was later removed during the last Holocene transgression, mainly in the marine domain. The lack of any morphological step between the subaerial marine terrace and the shore platform is here attributed to the combined effect of multiple factors, including (i) a Quaternary tectonic quiescence, (ii) a major eustatic transgression (MIS5) shaping both the shore platform and the low part of the marine terrace, (iii) the mechanically strong behaviour of the granitic basement with regard to erosion, and (iv) the moderate influence of the Holocene sea-level stillstand (since 6 ka) on the shore platform. Thanks to 10 Be content analysis, a range of vertical erosion rates of ca. 3–6 m⋅Ma −1 is obtained for the onshore domain, showing no discernable difference between the intertidal and aerial erosion rates. From that result, it is thus determined that the long-term development of the Penmarc’h granitic shore platform was mainly driven by marine erosive processes during the MIS5 and Holocene marine transgressions, whereas subaerial erosive processes prevailed after the MIS5 event.

International audience; Crystalline thermo-mineral and carbo-gaseous (CTMCG) hydrosystems are well known for their economic importance in fields such as thermal, spa activities and natural mineral water (NMW) bottling. Such systems are usually associated with strong structural complexity, which is rarely characterised in detail or robustly. This research focuses on a CTMCG hydrosystem associated with a peri-alpine graben. A multidisciplinary approach with a very large set of data and methods – geological modelling with geophysics and geological data from outcrops and several boreholes, hydrodynamic data, hydrochemistry, hydrogeological and geochemical modelling – reveals very novel results and allows a robust conceptual model to be constructed. The aquifer at the origin of the carbo-gaseous natural mineral water is the 100–125 m-thick fractured stratiform layer of the weathering profile of the crystalline rock (granite). It forms a rather large and thick inertial aquifer that can be numerically modelled, in a similar fashion to a porous medium. The majority of tectonic faults length act as impervious boundaries that divide this aquifer into around ten elongated compartments that were precisely delineated. These tectonic faults are permeable only along two small areas that were also precisely located. These permeable zones feed some aquifer compartments with deep, highly mineralised carbo-gaseous water, which mixes with “fresher” water and forms the exploited NMW. These results can be generalised and in particular show a strong opposition between low-inertia CTMCG hydrosystems without a subsurface reservoir, as the weathering profile was eroded, and high-inertia hydrosystems such as the one studied.

International audience; It has been previously established that Western Europe underwent an important episode of lateritic weathering during early cretaceous times. In this context, the red clays and ferrugineous pisoids deposits occurring in the graben of Saint-Maixent-l'Ecole (Poitou saddle) may represent remnants of lateritic formations associated with this paleoweathering episode. However, their exact age is unknown, and since they occur on top of the Oligocene limestones filling the graben, it cannot be ruled out that these lateritic soils formed more recently. In order to better constrain the age of these lateritic formations, goethite pisoids were sampled in the Fouilloux forest and investigated for Rb-Sr, U-Th and delta O-delta D isotopic systems. Rb and Sr have been measured on bulk pisoids, and also on leachates and residues from leaching experiments carried out in the attempt to isolate a radiogenic component. No reliable Rb-Sr isochron age could be derived, although the isotopic data would favor an early Cretaceous age for these lateritic formations. delta O-18-delta D measurements have been done on bulk goethite pisoids, and also on sub-fragments. The results indicate that the pisoids likely formed under a warm and/or arid climate. No argument for a late of post - oligocene age can be found from the oxygen and hydrogen isotope data. Finally, U-Th analyses have been carried on bulk samples of goethites, and on sub-fragments as well. U-234/U-238 activity ratios are typically slightly higher than the equilibrium value, which indicates that goethites have recently evolved as open systems (if not still open). Hence, pedogenesis processes are still active from a geochemical point of view in these lateritic soils. This study has not allowed to derive a reliable age for these lateritic soils. However, when the results from the different isotopic systems investigated here are considered together, the best interpretation is that these lateritic soils developed most probably over the kimmeridgian limestones along the edge of the Saint-Maixent-l'Ecole graben, and were reworked and redeposited at the bottom of the graben later on.; -Le début du Crétacé a été marqué en Europe occidentale par le développement de paléoaltérations de type latéritique. Les argiles rouges à cuirasse ferrugineuse pisolitique du graben de Saint-Maixent-l'École (seuil du Poitou) peuvent constituer un exemple-témoin de ce type d'altération. Toutefois, leur âge reste inconnu et leur position stratigraphique, au-dessus des calcaires oligocènes du fond du graben, ne permet pas d'exclure un âge de formation beaucoup plus récent. Des pisolites de goethite provenant de ces sols latéritiques ont été échantillonnés dans la forêt de Fouilloux et ont fait l'objet d'analyses isotopiques Rb-Sr, U-Th et d 18 OdD , afin de tenter de lever l'indétermination sur l'âge de leur formation. Pour Rb-Sr, outre les analyses sur pisolites totaux, des expériences de lessivages ménagés ont été effectuées afin d'essayer d'isoler un composant radiogénique. Les résultats obtenus n'ont pas permis de donner un âge fiable par la méthode des isochrones, même si les données tendraient plutôt à faire remonter l'épisode d'altération au début du Crétacé. Les analyses d 18 OdD ont été effectuées sur des pisolites de goethite totaux, ainsi que sur des fragments sous-échantillonnés. Les résultats plaident en faveur de conditions de formation en zone climatique chaude et/ou aride. Aucun argument en faveur d'un âge fini ou post-oligocène ne peut être tiré de l'étude des isotopes de l'oxygène et de l'hydrogène. Enfin, les analyses U-Th ont été effectuées sur des goethites totales ainsi que sur des fragments sous-échantillonnés. Les rapports d'activités 234 U/ 238 U sont tous légèrement supérieurs à l'équilibre séculaire, indiquant que les goethites évoluent actuellement et/ou ont évolué récemment en système ouvert. Ainsi, les sols latéritiques apparaissent toujours comme géochimiquement actifs. Si cette étude n'a pas permis d'apporter une réponse claire sur l'âge des sols latéritiques, l'hypothèse la plus probable au vu des données isotopiques est que ces sols latéritiques se sont développés au-dessus des calcaires kimméridgiens sur les bordures du graben de Saint-Maixent-l'École, puis remaniés et finalement resédimentés au fond du graben.

International audience; Borehole CDB1 (675.05 m) crosses the deepest Cenozoic sedimentary basin of the Armorican Massif, the Rennes Basin, to reach the underlying basement at a depth of 404.92 m, made up of the Late Neoproterozoic to Early Cambrian Brioverian Group, weathered down to 520 m depth. The basin's Cenozoic deposits are divided into seven formations, ranging from Early–Middle Bartonian to Late Pliocene in age. Coastal sediments at the very base, along with a thick Priabonian lacustrine episode, imply a major revision of the regional palaeogeography, whilst a very steady and low-energy lacustrine-palustrine environment throughout the Priabonian and Early Rupelian argue for an aggradational system associated with uniform subsidence. Palynological assemblages attest to environmental and climatic changes through the Eocene and Early Oligocene, in accordance with regional and global trends (Eocene–Oligocene Transition).

International audience

The morphology of the Pyrenees is characterized by the presence of high-elevation, low-relief surfaces. The origin of these Lower-Miocene surfaces is still debated. Two major interpretations have been proposed, both assuming that these surfaces are remnants of a single composite planation surface. The first interpretation proposes that this surface corresponds to a peneplain developed near sea level before the Late Miocene, subsequently uplifted and dissected. The present-day Pyrenees is therefore supposed to rise from the Late Miocene. In the second interpretation, the rise of the efficient base level of the chain induced the progressive inhibition of erosion and the smoothing of the relief before the Late Miocene, resulting in a highly elevated peneplain. According to this latter interpretation, the high elevation of the low-relief surfaces does not equate to post-orogenic uplift. We test these two interpretations by investigating, among other considerations, the relation between the elevation of the planation surface remnants and the deep structure of the chain. We find that (1) the isostatic compensation of the dissected Pyrenean planation surface by crustal thickening and (2) the absence of thinning of the lithosphere mantle below the chain favors the second interpretation., We are grateful to the BRGM (Bureau de recherches geologiques et minieres) for funding the Gemma V. Bosch PhD thesis (Project RGF, ‘‘Référentiel géologique de la France’’).

International audience; Flat high-elevation surfaces in the Pyrenees are defined by thick weathered horizons that were developed from granitic lithology. We analysed such horizons in detail within two areas: the Bordères-Louron granite and the Aston massif. They are characterized by a lower fissured zone overlain by unconsolidated saprolite. Mapping these horizons allows a 3D reconstruction of the ancient paleosurface with an elevation uncertainty of 50 m. We discuss the age of weathering by means of stratigraphy and low-temperature thermochronology. The surfaces are clearly postorogenic, postdating Eocene-Oligocene denudation. Their incision and the fact they are stepped suggest (1) an increase of the local relief and (2) recent normal faulting.

The hydrogeology of superficial (∼0-100 m b.g.l.) Hard Rock Aquifers (HRA; i.e. plutonic and metamorphic rocks) has so far been dominated by a few concepts considered to be relevant by a large majority of the HRA community. One of the most fundamental of these concepts is that their (secondary, fissure/fracture) permeability is either of tectonic origin or related to unloading processes. We will show that these genetic concepts are erroneous. We will demonstrate how the hydraulic conductivity of HRAs is a consequence of the (palaeo) weathering processes, with a stratiform fissured layer located immediately below the unconsolidated saprolite and, to a lesser extent, a verticalized fissured layer at the periphery of (or within) pre-existing discontinuities (veins, joints, ancient faults, lithological contacts, etc.). This result opens up large perspectives in terms of applied hydrogeology and applied geology. A specifically dedicated methodological toolkit well adapted to the operational survey, management and protection of HRAs is briefly presented.

When a mantle plume rises and impinges on the base of the lithosphere, it expectably produces variations in surface topography. Taking into consideration a realistic mantle rheology, plume ascent rates can reach tens to hundreds of metres per year, whereupon the impingement of the plume head at the base of the lithosphere can be considered as an “impact". Recent numerical experiments based on tectonically realistic formulations for the lithosphere and a representative mantle rheology, have shown that plume-induced undulations exhibit temporal successions of uplift and subsidence at various wavelengths. From spectral (Fourier) analyses of the undulations would appear that two groups of wavelengths (200–400 km and 60–100 km) predominate. Interestingly, a spectral analysis of Europe's topography also reveals two dominant groups. In the present study, we have used a spectral analysis with a wavelet formulation in order to discriminate between tectonically-induced undulations (uni-directional deformation) and plume-induced undulations (omni-directional deformation). The European lithosphere is well-suited for this approach since it has been suggested that two mantle plumes (the Massif Central and the Eifel area) underlie Western Europe, where Alpine compression has folded the lithosphere over several hundreds of kilometres. The wavelet analysis of Europe's surface topography confirms that the energy distribution of the topographic undulations outside the two main volcanic provinces is homogeneous, thus contrasting with the coexistence of both large-scale and medium-scale high-energy features that are obtained for the Massif Central and Eifel areas. Similar signatures are also found beneath the northern Sudetes area. The wavelet approach dedicated to the analysis of plume-related topographic signatures needs, however, detailed theoretical grounds and would probably benefit from two-dimensional analyses.

International audience; Le projet « SILURES Bretagne » (Système d’Information pour la Localisation et l’Utilisation des Ressources en Eaux Souterraines) s’est déroulé de 2002 à 2008, et il a permis la création d'une base de données sur les eaux souterraines couvrant l'ensemble de la région Bretagne. Celle-ci permet d’améliorer la gestion des ressources en eau souterraine de la région. SILURES Bretagne a été financé par l’Agence de l’Eau Loire-Bretagne, la Région Bretagne, les Conseils Généraux des Côtes d’Armor, du Finistère, d’Ille-et-Vilaine et du Morbihan, le Syndicat Mixte de Gestion du fonds pour l’eau potable en Ille-et-Vilaine, et le BRGM.En s’appuyant sur les bases du schéma conceptuel des aquifères de socle [1], les données de près de 12 000 ouvrages de la Banque de données du Sous-Sol ont été interprétées (profondeur, géologie du terrain rencontré, débits instantanés obtenus en foration). Ce travail a abouti à une cartographie au 1/250 000 de l’épaisseur des altérites en place et de l’épaisseur de l’horizon fissuré utile sur la région Bretagne [2].La première carte a été obtenue par différence entre le Modèle Numérique de Terrain (pas de 250 m) et l’altitude de la base des altérites. Cette altitude a été construite par krigeage, après étude géostatistique d’une population de forages ayant recoupé l’interface altérites/horizon fissuré.La seconde carte correspond à l’épaisseur du sous-sol riche en fissures (provenant de l’altération des roches) qui apporte la meilleure productivité en termes de débits instantanés. Suites à deux régressions linéaires (débit moyen du quartile supérieur / débit du milieu fissuré utile ; krigeage du débit instantané obtenu en foration rapporté à la profondeur du forage sous la base des altérites meubles / calcul du débit linéaire du milieu fissuré utile), l’épaisseur du fissuré utile a été déduite à partir du rapport des deux paramètres : débit du milieu fissuré utile (en m3/h), et débit linéaire du milieu fissuré utile (en m3/h/m).Les cartes régionales au 1/250 000 permettent de visualiser la répartition géographique et en profondeur des deux aquifères de socle (altérites et horizon fissuré) qui contraignent la vitesse de circulation des eaux de pluies infiltrées dans le sous-sol puis des eaux souterraines jusqu’aux rivières bretonnes.

Summary The weathered layers of crystalline rocks form aquifers that are of prime interest for water supply in hard rock areas. These weathering profiles generally develop under both stable geodynamic conditions (weathering rate ≫ erosion rate) and a hydrolysing climate. They are composed of thick stratiform layers that follow the paleo-landscape (paleotopography) and thus present a gently dipping sequence at a regional scale. The structure and the hydrodynamic properties of the weathering profile of a granitic area (53 km2 Maheshwaram catchment, state of Andhra Pradesh, India) were characterized in detail and mapped from observations on outcrops, 80 vertical electric soundings and lithologs from 45 borewells in which flowmeter measurements and injection tests were also performed to characterize the hydraulic conductivities of the conductive fissure zones. The structure of the weathering profile results from a multiphase process: an ancient weathering profile was partly eroded, down to its fissured layer. It was later re-weathered more or less parallel to the current topographic surface. This peculiar structure is linked to the geodynamic history of the Indian Peninsula that underwent alternate weathering and erosion-dominated phases. The profile is thus composed, from top to bottom, up to a total depth of 35 m, of sandy regolith saprolite (1–3 m), 10–15 m of laminated saprolite containing unusual preserved fissures and a 15–20 m thick fissured layer. By comparing various case studies in similar terrain, a generalized 3-D geological and hydrogeological conceptual model of granite-type aquifers (granites, gneisses, etc.) is proposed. In granite-type rocks, single weathering or multiphase weathering and erosion processes induce similar geological structures. A few main geological differences arise from the comparison of single phase and multiphase weathering profiles: (i) in the later, the respective thicknesses of the various layers can be deeply modified, (ii) the ancient fissured layer can be re-weathered, and the resulting new laminated layer may contain quite well preserved and conductive ancient fissures, and (iii) the upper part of the fissured layer is more densely fissured. As a result of the same weathering processes, the fissures from the fissured layer of single phase or multi-phase profiles exhibit very similar hydraulic conductivities, and show both a higher density of conductive fissures at the top of the layer. The preserved ancient fissures within the laminated layer of a multiphase profile are partly obliterated by the recent weathering; their hydraulic conductivity is thus significantly reduced. Nevertheless, they do significantly contribute to borewell yield. These weathering-induced fissures provide most of the aquifer permeability. The use of such a conceptual model for the precise geological mapping of the weathering structure appears to be a prerequisite for groundwater development and management in hard-rock areas as it answers to several key issues.

espanolOn the southeastern margin of the Paris Basin, Nivernais region, the marine deposits have been continuous all along the Jurassic period. The sea withdrew a first time during the Upper Tithonian. This regression coincided with erosion and weathering. Apparently, the sea invaded again the Nivernais region only during the Albian, to which are attributed a few relicts of glauconitic sandstone with quartz pebbles. The youngest marine occurrences are of Santonian-Campanian age, made of silicified urchins and microfauna preserved in flint flows. The Tertiary deposits are limited to small grabens and made of nodular lacustrine limestones with charophytes ascribed to the Middle Eocene. In this southeastern margin of the basin, palaeoweathering products of the Jurassic and Cretaceous, i.e. Clay-with-Jurassic cherts and Clay-with-Cretaceous flints, are widespread. Their study allow to reconstruct the long continental evolution of the area. A red silty clay formation, with scattered cherts and abundant ferruginous gravels, containing typically from 10 to 20% of gibbsite, has been identified at Beaumont-la-Ferriere. Besides the lack of granulometric sorting of the quartz grains and of the ferruginous nodules and gravels, the micromorphologic analysis of the thin sections shows well developed illuviation structures, pointing clearly out pedogenetic features. The red silty clay formation in Beaumont-la-Ferriere relates to an in situ bauxitic palaeoprofile developed above Callovo-Oxfordian limestones. Near Beaumont-la-Ferriere, at St-Benin-d'Azy and at Ste Colombe, outliers display several weathering formations with cherts and flints that total over 50 to 80 m thickness. At the base, there is in-situ Clay-with-cherts, in which occurs the siderolithic iron ore deposits and which locally contains also gibbsite. In places, this Clay-with-cherts is topped by rounded cherts, from 3 to 10 cm in diameter, and sand lenses containing quartz gravels. The Clay-with-flints of lustreous break, translucent, with branchy morphologies and porous cortex, typical of the Clay-with-flints known elsewhere, occurs at the top of the buttes. The reworked cherts, the sand and the Clay-with-flints Formation are often silicified by typical pedogenic silicification, with illuviation features and titania-rich matrix. The palaeoweathering formations occurring in the Nivernais region allow to specify at least three successive weathering stages in the southeastern Paris Basin. (1) Development during the early Cretaceous of a thick blanket of Clay-with-cherts by weathering of the Jurassic chert-bearing limestones after withdrawal of the sea at the end of the Tithonian. Locally this weathering lead to the development of true bauxitic profiles, with gibbsite, pisolites, nodules, wide illuviation structures, etc. The development of the siderolithic iron ores relates to this weathering stage. (2) A second weathering stage, after the withdrawal of the sea during the Campanian, lead to development of Clay-with-flints from leaching of the chalk deposits. (3) Silicification affected the Clay-with-cherts and the Clay-with-flints, as well as the reworked deposits that today armour the reliefs. These silicifications display the same micromorphologic and geochemic characters as the silcretes that developed during Middle/Upper Eocene in the central parts of the Paris Basin. This is the first time that in situ Cretaceous bauxitic profiles are described north of the Massif Central. It seems likely that these bauxitic formations have been initially much more widespread, but a number of profiles may probably have been resilicified, with alteration of the gibbsite into kaolinite along the silicification of the siderolithic piedmont during the Eocene. The old pisolithic iron ore workings of the nineteenth century are located in this Clay-with-cherts Formation. In this way, the siderolithic iron ore deposits, typical from the southern margins of the Paris Basin, are of early Cretaceous age and correlative to the bauxite deposits of southern France. francaisDes argiles a chailles et a silex couvrent d'importantes surfaces sur la bordure sud-orientale du bassin de Paris (Nivernais). A Beaumont-la-Ferriere, a ete reconnue une formation rouge a gravelles ferrugineuses, caracterisee par la presence de gibbsite [Al(OH)3] (10 a 20 %). Outre toute absence de tri granulometrique, aussi bien des grains de quartz que des nodules et gravelles ferrugineux, l'etude micromorphologique en lames minces montre des illuviations indiquant clairement une structuration pedologique. La formation rouge de Beaumont-la-Ferriere correspond a un paleoprofil bauxitique en place sur les calcaires du Callovo-Oxfordien. Dans les buttes residuelles de St-Benin-d'Azy et de Ste-Colombe, on observe differentes alterites a chailles et a silex qui se superposent sur 50 a 80 m d'epaisseur. A la base, ce sont des argiles a chailles non roulees, auxquels sont associes les anciens gisements de fer siderolithique et qui contiennent localement de la gibbsite. Par endroit, ces argiles a chailles sont surmontees par des chailles jurassiques roulees, de 3 a 10 cm de diametre, et par des lentilles de sables a dragees de quartz. Des formations a silex a cassure lustree, translucide, avec morphologies branchues et cortex scoriaces, typiques des argiles a silex connues ailleurs, occupent le sommet des buttes. Les chailles roulees, les sables et les formations a silex sont affectes par des silicifications riches en oxydes de titane et a figures d'illuviation typiques de silcretes pedogenetiques. Les formations d'alteration rencontrees en Nivernais permettent de definir au moins 3 phases d'alteration successives ayant affecte le SE du bassin de Paris. (1) Formation d'un puissant manteau d'argiles a chailles par alteration des formations jurassiques a chailles apres le retrait de la mer au Tithonien. Localement cette alteration a conduit a la formation de veritables profils bauxitiques a gibbsite, avec pisolithes, nodules, larges illuviations, etc. C'est a cette phase d'alteration qu'il faut rattacher les gisements de fer siderolithique. (2) Une seconde phase d'alteration lessivante, apres le retrait de la mer au plus tot au Campanien, est a l'origine de la formation des argiles a silex. (3) Des silicifications affectent les paleoalterites a chailles et a silex, ainsi que les formations de remaniement et forment des dalles siliceuses qui actuellement couronnent les reliefs. Ces silicifications presentent les caracteres micromorphologiques et geochimiques des silicifications de l'Eocene moyen/superieur du centre du bassin. C'est la premiere fois que des formations bauxitiques cretacees en place sont decrites au nord du Massif central. Il est vraisemblable que ces formations ont ete bien plus etendues qu'il n'y parait, mais que de nombreuses occurrences ont ete resilicifiees, avec transformation de la gibbsite en kaolinite lors de la silicification du glacis siderolithique durant l'Eocene moyen-superieur. Les anciennes exploitations de fer pisolithique sont contenues dans ces alterites a chailles. Ainsi, les gisements de fer siderolithiques, typiques des bordures meridionales du bassin de Paris, seraient egalement d'âge cretace inferieur et correlatifs des gisements de bauxites du Sud de la France.

First arrival times of the deep seismic profile Armor AR2, recorded in 1995 in the northern part of the Armorican Belt, are inverted in order to obtain the superficial velocity field. Initially, first breaks from the unit shots are taken as the observed traveltimes data set. Then, they are compared to ones computed by ray-tracing using an initial velocity model. An iterative algorithm, based on damped least-square method, is used to invert the data, until the velocity model converges to an acceptable solution. This is later discussed in terms of resolution and uncertainties before being compared with geological observations. The results allow us to explain the upper seismic velocity layer as a shallow weathered zone probably corresponding to a Tertiary palaeosurface. The weak velocity zone, observed at depth in the central part of the profile, underlines the importance of deformation and fracturation on weathering development linked to the North Armorican Shear Zone.

International audience; A country-scale (1:1,000,000) methodology has been developed for hydrogeologic mapping of hard-rock aquifers (granitic and metamorphic rocks) of the type that underlie a large part of the African continent. The method is based on quantifying the “useful thickness” and hydrodynamic properties of such aquifers and uses a recent conceptual model developed for this hydrogeologic context. This model links hydrodynamic parameters (transmissivity, storativity) to lithology and the geometry of the various layers constituting a weathering profile. The country-scale hydrogeological mapping was implemented in Burkina Faso, where a recent 1:1,000,000-scale digital geological map and a database of some 16,000 water wells were used to evaluate the methodology.

International audience; A palaeomagnetic study was carried out on the Borne de Fer ferricrete in north-eastern France, close to the Luxembourg border. This ferricrete is overlain by kaolinitic red clay and caps a silto-argilaceous saprolite 15 to 40 m thick showing weathering features, evolution of the clay minerals from the base to the top and karstic solution pipes beneath. This weathering profile lies above Jurassic marl and limestone from around 425 to 450 m a.s.l. and may correspond to a scarce remnant of an old weathering palaeosurface. Palaeomagnetic, rockmagnetic and petrographic analyses reveal presence of a dominant hydroxide contribution with a magnetic remanence carried by goethite. A small contribution of maghemite or hematite carries a more scattered but similar direction. The virtual geomagnetic pole derived from the goethite component lies at lat 73.4°N, lon. 205.9°E, dp 4.3, dm 6.2. This location close to the 120-140 Ma reference poles and presence of normal and reversed polarities are consistent with a Lower Cretaceous age close to the Long Normal Cretaceous Superchron. Our results are consistent with the ones obtained recently in Germany, Belgium and France and prepare the ground for reconstructing the Lower Cretaceous continental palaeosurface geometry and palaeogeography.

A new multidisciplinary methodology is developed to determine the structural geometry of hard-rock aquifers. The Plancoët migmatitic basement, chosen as an example for the use of this methodology, is studied both at a regional (300 km2) and a local (4 km2) scale. The regional mapping, based on field mapping of the tectonic structures and the weathered layers, on geostatistics and on aerial photographs and Digital Elevation Model (DEM) analyses, demonstrates the influence of the tectonic structures on the weathering processes. Some geo-electric investigations, added to the first mapping at the local scale, lead to the building of a local geometrical model of the aquifer.

International audience; Airborne gamma-ray spectrometry data (uranium, potassium and thorium contents) reveal geochemical heterogeneities within the monolithological Hyrôme watershed (ca. 150 km2) in the Armorican massif (western France). Our observations and computations provide important constraints on the spatial distribution and the associated magnitudes of chemical erosion processes at the scale of a small watershed. Two distinct, partially preserved, weathering profiles exhibit a strong correlation between regolith evolution and airborne-derived K/Th ratios, suggesting that the variability is linked to supergene processes. Using both airborne data and laboratory measurements on rock samples, the total net export of potassium has been estimated at 422 ± 50 kg/m2 and the chemical weathering rate of potassium at 17 ± 2 kg/km2/a.

International audience; On the southeastern margin of the Paris Basin, Nivernais region, the marine deposits have been continuous all along the Jurassic period. The sea withdrew a first time during the Upper Tithonian. This regression coincided with erosion and weathering. Apparently, the sea invaded again the Nivernais region only during the Albian, to which are attributed a few relicts of glauconitic sandstone with quartz pebbles. The youngest marine occurrences are of Santonian-Campanian age, made of silicified urchins and microfauna preserved in flint flows. The Tertiary deposits are limited to small grabens and made of nodular lacustrine limestones with charophytes ascribed to the Middle Eocene. In this southeastern margin of the basin, palaeoweathering products of the Jurassic and Cretaceous, i.e. Clay-with-Jurassic cherts and Clay-with-Cretaceous flints, are widespread. Their study allow to reconstruct the long continental evolution of the area. A red silty clay formation, with scattered cherts and abundant ferruginous gravels, containing typically from 10 to 20% of gibbsite, has been identified at Beaumont-la-Ferrière. Besides the lack of granulometric sorting of the quartz grains and of the ferruginous nodules and gravels, the micromorphologic analysis of the thin sections shows well developed illuviation structures, pointing clearly out pedogenetic features. The red silty clay formation in Beaumont-la-Ferrière relates to an in situ bauxitic palaeoprofile developed above Callovo-Oxfordian limestones. Near Beaumont-la-Ferrière, at St-Benin-d'Azy and at Ste Colombe, outliers display several weathering formations with cherts and flints that total over 50 to 80 m thickness. At the base, there is in-situ Clay-with-cherts, in which occurs the siderolithic iron ore deposits and which locally contains also gibbsite. In places, this Clay-with-cherts is topped by rounded cherts, from 3 to 10 cm in diameter, and sand lenses containing quartz gravels. The Clay-with-flints of lustreous break, translucent, with branchy morphologies and porous cortex, typical of the Clay-with-flints known elsewhere, occurs at the top of the buttes. The reworked cherts, the sand and the Clay-with-flints Formation are often silicified by typical pedogenic silicification, with illuviation features and titania-rich matrix. The palaeoweathering formations occurring in the Nivernais region allow to specify at least three successive weathering stages in the southeastern Paris Basin. (1) Development during the early Cretaceous of a thick blanket of Clay-with-cherts by weathering of the Jurassic chert-bearing limestones after withdrawal of the sea at the end of the Tithonian. Locally this weathering lead to the development of true bauxitic profiles, with gibbsite, pisolites, nodules, wide illuviation structures, etc. The development of the siderolithic iron ores relates to this weathering stage. (2) A second weathering stage, after the withdrawal of the sea during the Campanian, lead to development of Clay-with-flints from leaching of the chalk deposits. (3) Silicification affected the Clay-with-cherts and the Clay-with-flints, as well as the reworked deposits that today armour the reliefs. These silicifications display the same micromorphologic and geochemic characters as the silcretes that developed during Middle/Upper Eocene in the central parts of the Paris Basin. This is the first time that in situ Cretaceous bauxitic profiles are described north of the Massif Central. It seems likely that these bauxitic formations have been initially much more widespread, but a number of profiles may probably have been resilicified, with alteration of the gibbsite into kaolinite along the silicification of the siderolithic piedmont during the Eocene. The old pisolithic iron ore workings of the nineteenth century are located in this Clay-with-cherts Formation. In this way, the siderolithic iron ore deposits, typical from the southern margins of the Paris Basin, are of early Cretaceous age and correlative to the bauxite deposits of southern France.

International audience; The Ringelbach catchment, which has been studied since 1975, is highly representative of the crystalline Vosges massif, where the water supply is mainly derived from small aquifers in heterogeneous superficial formations and in the weathered and fissured bedrock. Vertical electrical soundings (VES), resistivity imaging profiles and magnetic resonance soundings (MRS) were applied inorder to investigate the 3D structure and estimate the hydrodynamic characteristics of the subsurface, which is composed of granites partly covered by Triassic sandstone. Despite poor MRS signal-to-noise conditions, the general structure of the catchment is defined using geophysics (electrical methods and MRS combined) and geology. MRS also makes it possible to map the water volume per unit surface within the different geological formations and structural blocks. The corresponding maps are proposed as a basis for evaluating the water storage within the catchment and improving the understanding of its hydrological functioning. The initially proposed geological model is refined and a four-block structure intersected by faults is defined. The schema of a gradual stratified weathering of the granite from surface to depth is not clearly reflected in the resistivity images, partly because of insufficient investigation depth. It is completed with highly weathered, deeply rooted fractured zones associated with faults in order to explain deeply sited low-resistivity zones.The MRS water content in the weathered granite of Ringelbach appears abnormally low in comparison with the higher values observed in similar resistivity settings in other granite regions. Because of the rough catchment topography, these low values may be attributed to unsaturated weathered formations on hillsides or saturated, fine material (colluvial deposits, highly weathered granite) in the valley bottom. The control-wells and laboratory measurements on core samples proposed in the next step of this research will attempt to calibrate the MRS results and validate the proposed structural and weathering schema. Re-interpreted and validated geophysical results will then be used as effective input for improved hydrological modelling.

National audience

International audience; Des essais de pompage, réalisés au sein de l'horizon fissuré d'un aquifère de socle granitique, interprétés aux puits d'observation au moyen de la solution analytique de Neuman et aux puits de pompage au moyen de celle de Gringarten, mettent en evidence l'anisotropie verticale de cet horizon constitutif de l'aquifère ; la perméabilité horizontale étant nettement et systématiquement supérieure à la perméabilité verticale. Ces résultats concordent parfaitement avec les observations géologiques, l'horizon fissuré du profil d'altération du granite montrant de nombreuses fissures sub-horizontales. Ils confirment le caractère dominant, au sein de l'horizon fissuré, de la perméabilité des fissures subhorizontales dues au processus d'altération sur celle des fractures subverticales d'origine tectonique.

Based on research work in the Truyare River catchment of the Massif Central (Lozare Department, France), a methodology has been developed for delineating favorable prospecting zones of a few square kilometers within basement areas of several hundred, if not thousand, square kilometers for the purpose of siting high-yield water wells. The methodology adopts a functional approach to hard-rock aquifers using a conceptual model of the aquifer structure and of the functioning of the main aquifer compartments: the alterites (weathered and decayed rock), the underlying weathered-fissured zone, and the fractured bedrock. It involves an economically feasible method of mapping the thickness and spatial distribution of the alterites and the weathered-fissured zone, on which the long-term exploitation of the water resource chiefly depends. This method is used for the first time in hydrogeology. The potential ground water resources were mapped by GIS multicriteria analysis using parameters characterizing the structure and functioning of the aquifer, i.e., lithology and hydrogeological properties of the substratum, nature and thickness of the alterites and weathered-fissured zone, depth of the water table, slope, fracture networks and present-day tectonic stresses, and forecasted ground water quality. The methodology involves a coherent process of downscaling that, through applying methods that are increasingly precise but also increasingly costly, enables the selection of sites with diminishing surface areas as the work advances. The resulting documents are used for ground water exploration, although they can also be applied to the broader domain of land-use management.

Degros Jacqueline, Simon Philippe, Tarrête Jacques, Wyns Robert. Une fosse néolithique à Courcelles-sur-Viosne (Val d'Oise). In: Revue archéologique de Picardie, n°1-2, 1984. La néolithique dans le nord de la France et le bassin parisien. pp. 31-48.

Die Verwitterung der opalführenden Kreide des Pariser Beckens führt immer, im ersten Schritt, zur Bildung eines Smektits vom Typ «ßeidellite mixte alurnineuse». Dieser erste Ablauf ist gleich, welches auch immer die Verwitterungsbedingungen sein mögen. Erst im weiteren Verlauf entwickelt sich dieses Material weiter zu Kaolinit-Smektit-Wecksellagerungen oder Kaoliniten., The weathering of silex-bearing chalk in the Parisian Basin always results first in the formation of a smectite of mixed aluminous beidcllite type. This first stage is constant whatever the weathering conditions may be. Only afterwards, this material evolves toward kao-linite-smectite mixed layers or toward kaolinite., L'altération de la craie à silex dans le Bassin de Paris aboutit toujours, dans un premier temps, à la formation d'une smectite de type beidellite mixte alumineuse. Cette première étape est constante quelles que soient les conditions de l'altération. Ce n'est que par la suite que ce matériel évolue vers les interstratifiés kaolinite-smectite ou les kaolinites., Thiry Médard, Aubry Marie-Pierre, Cavelier Claude, Kuntz Georges, Wyns Robert, Trauth Norbert. Nature des smectites élaborées lors d'altérations lessivantes de la craie à silex du Bassin de Paris. In: Sciences Géologiques. Bulletin, tome 31, n°4, 1978. Sédimentologie et géochimie de la surface. pp. 163-172.

International audience

Weathering profiles are mostly studied on their upper part (crust, saprolite) where leaching and concentration/precipitation of valuable element occur. For water resource and hydrocarbon purpose, the transition between saprolite and fresh basement is of utmost importance. Here is found the fissured layer, a highly fissured bedrock that it favorable for aquifer/reservoir properties. Such a weathering profile model is well known on igneous and metamorphic rocks, but curiously very poorly documented on sedimentary, non-metamorphic, basement. On behalf the CINERGY project, a 675m-long borehole (CDB1) was cored through the sedimentary infill (405 m), then the weathered basement (110 m) and finally the fresh bedrock (160 m). The basement is made of the "brioverian schists", which actually are epimetamorphic shales and fine grained sandstones, highly folded and cleaved. Here is presented an integrated (well-logging, mineralogy, petrography) study of the weathering profile that has been preserved under the sedimentary infill of the Rennes Basin. The well logging tools include Gamma-Ray, long and short resistivity, neutron porosity, gamma-gamma density, Pef, Full wave Sonic, BHTV and caliper. The cores were regularly sampled for total rock and clay mineralogy and some petrographic control were realized on specific facies and fracture fillings.

International audience; Au sein du Massif central, une majeure partie du relief est caractérisée par des géomorphologies en forme de plaines et plateaux. L'altitude des plateaux est hétérogène, elle peut varier de la centaine de mètres à plus de 1000m. Ces surfaces correspondent à des surfaces d'aplanissement portant des profils d'altération. Ces surfaces d'altération font échos aux mouvement verticaux affectant la lithosphère qui peuvent être raccrochés à différentes phases tectoniques (phase orogénique, épaulement de rift…) possédant des longueurs d'ondes différentes. Certaines de ces paléosurfaces ont été scellées par des sédiments puis exhumées lors des grandes phases d'érosion, la dernière ayant débuté après le Miocène moyen, en réponse au soulèvement du Massif central tel que nous le connaissons. Néanmoins, l'âge ainsi que les déformations des paléosurfaces du Massif central font encore débats. A l'aide d'une analyse multi-méthode à la fois numérique (analyse du MNT à 25m, des pentes, des courbures…), géologique (géomorphologie, indices lithologiques, attitude des profils d'altération…) et géométrique, une carte actualisée des paléosurfaces d'une large partie du Massif central, à savoir du sud du Limousin à la Montagne Noire en passant par la Margeride et l'ouest des Cévennes est proposée. Sur cette carte, quatre surfaces d'aplanissement peuvent être décrites : (i) une surface S0, supposée infra-triasique voire plus ancienne, dont les profils d'altération sont scellés par des sédiments triasiques, (ii) une surface S1 supposée infra-liasique, et dont le profil d'altération est scellé par les sédiments liasiques, (iii) une surface S2 probablement Crétacé inférieur, scellée par le Cénomanien transgressif dans le Poitou et dans les Causses du Quercy, et le Coniacien dans les Grands Causses. Cette surface S2 est marquée par la présence de bauxite, largement karstifiée, au sommet des formations sédimentaires jurassiques des Grands Causses et par des silicifications (meulières) des sédiments jurassiques, (iv) une surface S3 rattachée à une surface d'aplanissement éocène présente principalement sur le pourtour du Massif central, reprenant parfois la surface S2 comme le montre la présence de silcrètes au sommet des profils d'altération et enfin (v) une surface S4, plus récente scellée par des coulées volcaniques du Miocène et à partir de laquelle se sont incisées les vallées actuelles. Travaux de recherche financés et réalisés dans le cadre du programme BRGM-TOTAL Source-to-Sink..

The late to post-compressive evolution of the Pyrenees is characterized by the formation of a planation surface whose relics are observed today on top of the range. This surface is the subject of an abundant literature, where authors focused on the characterization of its geometry, extent and on the age and mechanism of its formation. The most recent studies showed that the planation surface probably formed at high elevation during the Oligocene - Upper Miocene interval. However, this elevation remains unconstrained. Our work brings a new qualifying element of the relics of the Pyrenean planation surface, which is the description of subtractive weathering profiles developed on these surfaces. Weathering in samples collected below surface remnants was dated by the 40Ar/39Ar applied to potassic manganese oxides. A Cretaceous age, consistent with the timing of the Pyrenean rifting during which the top of tilted blocs surfaced, and a Quaternary age were obtained. The lack of geochronological information between those two ages isinterpreted as a result of the reactivation of weathering profiles due to uplift during the post-compressive period. These results show that weathering continued until Quaternary times in the Pyrenees. The geomorphological analysis of the dismantling of the planation surface, based on miocene to present-day fluvial incisions analysis in the Western Pyrenees reveals the occurrence of an uplift of the range, which started during middle Miocene times. This uplift is confirmed for present-day by the analysis of a new InSAR dataset. The forcing mechanism of this uplift is discussed ar the African and Eurasian plates-scale. The middle Miocene to present-day uplift of the Pyrenees which provokes the dismantling of the planation surface may be explained by a propagation of the deformation from active plates boundaries toward intraplate areas caused by a reorganization of African - Eurasian plate motion., L’histoire tardi- à post-compressive des Pyrénées est caractérisée par la formation d’une surface d’aplanissement dont les reliques sont aujourd’hui retrouvées au sommet de la chaîne. Cette surface a fait l’objet d’une littérature abondante dans laquelle les auteurs se sont appliqués à en caractériser la géométrie, l’extension ainsi que le mécanisme et l’âge de sa formation. Les études les plus récentes ont montré que la surface d’aplanissement s’était probablement formée en altitude au cours de l’intervalle Oligocène - Miocène supérieur. Cependant, cette altitude reste indéterminée. Ces travaux de thèse apportent un nouvel élément de caractérisation des reliques de la surface d’aplanissement en décrivant des profils d’altération soustractive préservés au niveau de ces surfaces. L‘altération d’échantillons collectés sous les surfaces reliques d’altitude a été datée par la méthode 40Ar/39Ar appliquée à des oxydes de manganèse potassiques. Un âge crétacé, cohérent avec l’âge du rifting pyrénéen au cours duquel les sommets de blocs basculés ont été émergés, ainsi qu’un âge quaternaire ont été obtenus. La lacune d’enregistrement de l’information géochronologique entre ces deux périodes est interprétée comme le résultat de la réactivation des profils d’altération liée au soulèvement de la chaîne au cours de la période post-compressive. Ces résultats montrent que l’altération a continué dans les Pyrénées jusqu’au Quaternaire. L’analyse géomorphologique du démantèlement de la surface, basée sur l’étude morphométrique des incision fluviales miocènes à actuelles des Pyrénées occidentales révèle l’implication d’un soulèvement de la chaîne, intervenu à partir du Miocène moyen. Ce soulèvement est confirmé pour la période actuelle par l’analyse d’un jeu de données InSAR. L’origine de ce soulèvement est discutée à l’échelle des plaques Afrique et Eurasie. Le soulèvement Miocène moyen à actuel des Pyrénées à l’origine du démantèlement de la surface d’aplanissement peut être expliqué par une propagation de la déformation engendrée au niveau des limites de plaques actives suite à une modification de la direction de convergence Afrique - Eurasie.