Search

The distribution of Mn, V, Th, Pb and isotopes of Pb in the labile fraction of sediments from a channel infill in the Middle Loire alluvial plain are used to highlight some aspects of the basin evolution over the period from 0 to 10 ka BP. The acid extractable matter (AEM) in the sediment samples is variable in amount and in trace element contents. Iron-Mn oxyhydroxides are the principle trace element carrying phase in the labile fraction and carbonates are a secondary carrier. Vanadium and Pb originate from the weathering of silicates and are used as a silicate erosion rate index in the fluviatile record. Most of the AEM data plot along a general trend between 3 endmembers (basalts, Cretaceous carbonate rocks and granites) in the relationship between 207Pb/206Pb and 206Pb/206Pb. These endmembers have been mixed in various proportions depending on natural Holocene inputs (erosion, volcanic events) or human influences (mining and smelting of ore).

The purpose of the geophysical campaign carried out in the Val d'Avaray was to contribute to a reconstruction of fluvial dynamics in the middle reaches of the River Loire. The campaign used of three classical geophysical methods (electromagnetic profiling supplemented by electrical soundings, electrical resistivity tomography and ground penetrating radar [GPR]) to define alluvial bodies. In addition, it aimed to detect the presence of clayey–peaty paleochannels and estimate the depth of incision of the Loire into the autochthonous calcareous bedrock. Paleochannels represent the only datable sediments that can be used to define a reliable chronological and stratigraphic framework. The electromagnetic profiling survey enabled the construction of conductivity maps that gave a general plan view of the various alluvial deposits and the positions of the paleochannels. The campaign identified three sedimentary bodies of different apparent resistivity deposited at three different periods; Body A of sandy composition overlain by thick clayey loam, Body B of rather sandy composition overlain in certain areas by a thin layer of clayey loam and Body D which represents the actual sandy deposits of the River Loire formed during flood stages. A fourth body (C), between B and D, was detected only by morphological analysis and could not be identified with EM mapping because the resistivity contrast with Body B was too low. The deposits rest on a relatively conductive ( ρ ∼60 Ω m) marly-limestone bedrock, which is at shallower depth beneath Body A than Body B. The survey also successfully highlighted conductive features corresponding to paleochannels made up of peat or pre-existing beds of the River Loire. The resistivity cross-sections obtained by electrical resistivity tomography and the GPR profiles showed the vertical geometry of the deposits. The data from these methods can be combined to produce precise 3D models that are of considerable value to geologists.

Incision and infilling of the flood plain of the Loire river, in the Val d'Avaray area, is polyphased during Tardiglacial and Holocene. Five main alluvial units are developed and result from successive meandering-like phases of erosion and sandy infilling. Clayey channel fillings sometimes associated with peat have been identified within the sandy units. The first investigated channel was active from Tardiglacial (11.4 ka BP, Allerod) to Boreal, the second one from 8.4 ka BP (Boreal) to 2.3 ka BP (Subatlantic) with an average sedimentation rate of 0.5 mm·yr −1 . A third one was active from 3.2 ka BP (Subboreal) and during a great part of the Subatlantic; the average rate of sedimentation is evaluated at 1.3 mm·yr −1 .

Diagenetic mineral sequences in Triassic and Lower Lias sedimentary successions studied in the Balazuc borehole are seen to have modified the initial reservoir qualities of the formations. The upper part of the Triassic Lower Sandstone unit and the Hettangian Lower and Upper Calcarenite units were initially good quality reservoirs, whereas the Triassic Upper Sandstone unit and the Rhaetian Sandstone unit were initially average quality reservoirs, temporarily improved by pedogenesis. Early burial induced partial plugging of the initially porous bodies as a result of fluid exudation from proximal sediment bodies; the circulation of these fluids brought about alteration, dissolution and mineral deposition. Tectonic events associated with the formation of the Southeast Basin palaeomargin caused movement along the Uzer Fault and imparted a fracture porosity to the buried sedimentary formations. However, increasing subsidence and burial reactivated fluid circulation, dissolution and mineralization with further plugging of pore spaces. Present reservoir quality, as assessed from measured water porosity and neutron density well logs shows a changed reservoir distribution. Apart from the main Uzer Fault and one secondary fault (1644–1647 m) that today form drainage channels, the Triassic Lower Sandstone unit and its topmost breccia are now non-reservoirs, the Triassic Upper Sandstone, Rhaetian Sandstone and Hettangian Calcarenite units form a succession of residual-matrix-porosity reservoirs isolated by non-porous units.

Le Néolithique final. Les faits. Les matériaux organiques et minéraux.

National audience

International audience; The distribution of Sr contents and isotopes of strontium Sr in the labile fraction of sediments from a channel fillings infill in the Middle Loire alluvial plain is used to highlight aspects of the evolution of the basin over the period from 10 000 yr. BP to the present. The acid extractable matter (AEM) in the sediment samples is variable in amount and in trace element contents. Carbonates and Fe–Mn oxyhydroxides are the principal carrying phases in the labile fraction. The historical record of 87Sr/86Sr ratio variations in AEM shows two components. In the first, the AEM is consistently low (around 10%) and the 87Sr/86Sr ratios range from around 0.7086 to 0.710. This corresponds to the hydrous Fe-Mn oxides endmember. Fe–Mn oxyhydroxides originate from the Massif Central and their 87Sr/86Sr ratios display a larger range than present day Fe–Mn oxides. Furthermore, the mixing proportions between Fe–Mn oxides generated on granitoids and on basalts fluctuated only weakly in the past. The second component has greater AEM (20–50%) and the 87Sr/86Sr ratio is almost constant at 0.709–0.7095. This corresponds to a carbonate endmember, clearly different from present day carbonates and may reflect the influence of both authigenic and detrital origins. The low 87Sr/86Sr ratio suggests a dominantly authigenic source for the carbonate endmember; this may be linked to groundwater inputs, although a minor influence from a secondary detrital source cannot be discounted. This reflects a different functioning of the river between deposition in the channel and the present day. Keywords: River Loire, Holocene, fluvial dynamics, geochemistry, strontium isotopes

Chronology and stratigraphy of major bed alluvial deposits of middle Loire River had been performed studying four cored boreholes realised in clay and peat infilling of oxbows. Diachronism of oxbows infilling permits the reconstruction of a composite sequence covering Late glacial and Holocene. Numerous radiocarbon dating and high resolution palynological studies offer possibility of vegetation reconstruction over these periods. Impact of anthropogenic activities on watershed-scale erosion are suspected during both Bronze and Gallo roman ages (around 3,300 and 1,990 B.P.)., L'étude de quatre sondages carottés réalisés dans le remplissage argilo-tourbeux de bras mort en Loire moyenne a permis de caractériser les cadres chronologique et stratigraphique des alluvions du lit majeur de ce secteur. Les nombreuses datations radiochronologiques associés aux études palynologiques ont permis de retracer l'évolution du peuplement végétal du Tardiglaciaire au Subatlantique. Des indices d'impact de l'activité anthropique locale et à l'échelle du bassin versant ont été identifiés., Garcin Manuel, Farjanel Geneviève, Giot Denis. Eléments radiochronologiques et palynologiques sur les alluvions du lit majeur de la Loire (Val d'Avaray, Loir-et-Cher, France) / Radiocarbon dating and palynology of alluvial deposits in the Loire flood plain (Val d'Avaray, Loir-et-Cher, France).. In: Quaternaire, vol. 12, n°1-2, 2001. Paléoenvironnements holocènes et géoarchéologie. pp. 69-88.

Geological and mineralogical studies have been resumed in the recent years. They have led to a systematic inventory of flint outcrops in the region of Grand-Pressigny and to the investigation of supplying areas for prehistoric man as well as the pétrographie definition of the material used for the knapping of the long pressignian blades. Pétrographie analysis which is mainly concerned with micro quartzous crystalline texture and with the research of original sedimentary microfacies has been used to characterize pressignian flint. A comparative study has been realized about local flints collected in various geological sites ; the conclusion is that the supplying sources are the formations diverted by alteration of the turonian yellow calcareous tufa : sandy clay from alteration, colluvial and ancient alluvial deposits. Pétrographie analysis besides confirms the prehistoric spreading of tools from the knapping « factories » of Grand-Pressigny country., Les études géologiques et minéralogiques reprises ces dernières années ont permis le recensement systématique des affleurements de silex de la région du Grand-Pressigny, la recherche des zones d'approvisionnement de l'homme préhistorique ainsi que la définition pétrographique du matériau utilisé pour le débitage des grandes lames pressigniennes. L'analyse pétrographique, principalement orientée sur la texture cristalline microquartzeuse et la recherche du microfaciès sédimentaire originel, a été utilisée pour caractériser le silex pressignien. Une étude comparative a été réalisée sur les silex de la région prélevés dans des sites géologiques différents ; il en est conclu que les sources d'approvisionnement sont les terrains dérivés par altération du tuffeau jaune turonien : argile sableuse d'altération, col- luvions et alluvions anciennes. L'analyse pétrographique confirme en outre la diffusion préhistorique d'outils provenant des ateliers de taille de la région du Grand-Pressigny., Giot Denis, Mallet Nicole, Millet Dominique. Les silex de la région du Grand-Pressigny (Indre-et-Loire). Recherche géologique et analyse pétrographique / Flints in Grand-Pressigny country (Indre-et-Loire). Geological research and pétrographie analysis. In: Revue archéologique du Centre de la France, tome 25, fascicule 1, 1986. pp. 21-36.

Concentrations of iron, zinc and lead sulphides occur in the lower part of the formation known as Bois Madame, of late Upper Kimmeridgian age. Paleostructural and paleogeographic reconstructions suggest that these concentrations were deposited in a confined area, within a carbonate platform, that was the site for entrapment of calcaro-sapropelic deposits already enriched in metals. The formation was affected by stratiform dolomitization of a distinctive, uniformly fine-grained euhedral textural facies that resulted in the creation of an impermeable barrier at a relatively early diagenetic stage. The main consequences of this were diagenetic maturation in a closed system and protection against external agencies. Another, regional and crosscutting dolomitization originating from the underlying Portlandian reefal dolostones did not affect the Bois Madame formation. Within their dolomitic envelope, the initial disseminated sulphides are selectively concentrated in the sapropelic horizons. Organic matter play a determining role at several stages in the birth and development of highgrade ore body. Very early during diagenesis it was responsible for the development of reducing conditions that affected the behaviour of the metals : fixation followed by precipitation by bacterial bioreduction of marine sulfates. Also, being distributed in layers, the organic material acted as drains in the relatively impermeable carbonate rocks, thereby allowing the later development of reconcentrations during compaction-dissolution processes. The orebody is such that, with the syndepositional appareance of the first sulphides followed by an early closure to extend influences, it can be proposed as a model of stratiform syndiagenetic, metal concentration., Les concentrations sulfurées à fer, zinc et plomb sont encaissées dans la partie inférieure de la formation dite de Bois Madame, d'âge Kimméridgien supérieur. Les reconstitutions paléostructurales et paléogéographiques conduisent à envisager la genèse de ces concentrations dans un site de confinement au sein d'une plate-forme carbonatée. Ce site opère comme piège de dépôts calcaro-sapropéliques précocement enrichis en métaux. Une dolomitisation stratiforme à faciès particulier, automorphe, homométrique fin, affecte la formation et engendre une barrière de perméabilité à un stade relativement précoce de la diagenèse. En conséquence la maturation diagénétique va se développer en système fermé, et isoler la formation à l'égard de fluides extérieurs. Ainsi, une seconde dolomitisation, d'extension régionale, recoupe la série ; elle est issue des formations récifales portlandiennes sus-jacentes ; elle est sans action sur la formation de Bois Madame. Au sein de leur enveloppe de dolomie, les sulfures métalliques disséminés se concentrent de façon très sélective sur les horizons sapropéliques. La matière organique joue un rôle déterminant à plusieurs reprises dans la naissance et l'évolution du gîte. Au cours de la diagenèse très précoce, elle est responsable du développement de conditions réductrices qui provoquent la fixation puis la précipitation des métaux par réduction bactérienne des sulfates marins. En second lieu, étant répartie en strates, elle joue un rôle de drain dans les dolomies peu perméables et permet le développement de reconcentrations sulfurées plus tardives par compaction-dissolution. Tel qu'il se présente avec apparition des premiers sulfures dès l'étape sédimentaire puis une fermeture précoce aux apports extérieurs, ce gîte peut être proposé comme un modèle de concentration métallique syndiagénétique., Aubague Maurice, Giot Denis, Sureau Jean-François. Le gîte de Bois Madame (Hérault) : une minéralisation Pb-Zn de milieu confiné en plate-forme carbonatée. The Bois Madame (Hérault, France) ore deposit : a Pb-Zn mineralization in a confined environment on a carbonate platform. In: Sciences Géologiques. Bulletin, tome 39, n°3, 1986. Concentrations métalliques et milieux confinés. pp. 251-262.