bluuebunny/crossref_metadata_2025 huggingface dataset:
La diagenese des otolithes par la comparaison des donnees microstructurales, mineralogiques et geochimiques; application aux fossiles du Pliocene du Sud-Est de la France
Authors: Dufour Elise , Cappetta Henri , Denis Alain , Dauphin Yannicke , Mariotti Andre
Published: June 2005

Microstructure, mineralogy and elemental and isotopic compositions of recent and fossil otoliths have been investigated. Fossil otoliths come from marine Pliocene localities of southeastern France. The study of the different parameters show that diagenetic changes are weak. Microstructural features specific to otoliths, such as needle-like crystals and growth increments, are usually well preserved. When present, the microstructural modifications remain limited inside each otolith. Except a few pyritic specimens, the only mineral found is aragonite like in recent specimens. The chemical compositions of fossil and recent otoliths are not significantly different, and any element is preferentially changed by diagenesis. The isotopic composition does not show strong alterations. However, the difference in 18 O content between recent fishes having different ecological habits is not found with the same magnitude in fossil specimens. Finally, when elemental and isotopic compositions are compared, the specimens are sorted according to taxonomy and not according to their geologic age. This suggests that biogenic signals have been preserved.
The Jurassic Thalattosuchia (Crocodyliformes) of Chile; a paleobiogeographic approach
Authors: Gasparini Zulma , Vignaud Patrick , Chong Guillermo
Published: June 2005

The thalattosuchian fossils from the Jurassic of Chile are revised. Some specimens, dating from the Lower Lias (Sinemurian), are the oldest known thalattosuchians, but are too fragmentary to establish a precise taxonomic placement. New specimen related to the genus Metriorhynchus is described from the lower Bajocian. It is the oldest representative of the genus and fills an important gap in our knowledge of the paleogeographic history of this group. Other new cranial fragments, related to the Callovian species Metriorhynchus casamiquelai, are described and this species is revised in the light of new studies on the intraspecific variability in extant crocodilians. Up until now, the known distribution of Liassic Thalattosuchia suggested a circumpacific distribution with minimally episodic passages through the Caribbean Corridor. Nevertheless, the close affinities between the South American and the European Metriorhynchidae from the Callovian to the Tithonian suggest the possibility that more and more frequent communications were made via the Caribbean Corridor. The new data corroborate the hypothesis formulated from the invertebrate faunas.
Le karst des plateaux jurassiques de la moyenne valee de l'Ardeche; datation par paleomagnetisme des phases d'evolution plio-quaternaires (aven de la Combe Rajeau)
Authors: Audra Philippe , Camus Hubert , Rochette Pierre
Published: June 2005

Thanks to its immunity, karst is an excellent recorder of environmental change, which also offers the possibility for dating. Karst records position of old base levels, in particular to which were linked horizontal underground drains located near the piezometric surface. After a base level lowering, a new drain appears at a lower level, the old perched drain being abandoned. If base level lowering is slow, the initial drain is progressively entrenched, forming a canyon. The "Combe Rajeau" cave system corresponds to this last type: a 100 m high underground canyon, continuously entrenched during the Ardeche valley downcutting. The underground river left several terraces during the entrenchment. Knowing that speleothem U/Th radiometric dating method covers only the most recent part of karst systems history, which spans over several million years, paleomagnetism has been applied to date the Combe Rajeau sediments. A more precise knowledge of the evolution phases of this system provides a better understanding of the middle Ardeche valley evolution upon which it depends.
Apports de l'etude isotopique Sr et Nd des sediments cenozoiques de Limagne a la comprehension du fonctionnement du rift du Massif central francais
Authors: Briot Danielle , Poidevin Jean-Louis , Hugueney Marguerite
Published: June 2005

Radiogenic isotope geochemistry of shales, siliciclastics and carbonates potentially provides important information on the evolution of reservoirs such as continental crust or seawater. The aim of this study is to show that Sr and Nd isotope compositions of Cenozoic sediments and fossil remains in rift-related basins of the Massif Central can be used to contribute to a better understanding of rifting processes. Two questions are of particular relevance: firstly, did the volcanic activity which started during Eocene in the Massif Central continue through Oligocene, if so it would only be visible in sediments? Secondly, did the sedimentation occur in basins directly connected with seawater or in isolated lakes?
Correlations et interpretations genetiques dans les formations recifales oxfordiennes de la haute vallee de l'Yonne (sud-est du bassin de Paris, France)
Authors: Chevalier Francis , Garcia Jean-Pierre , Quesne Didier , Guiraud Michel , Menot Jean-Claude
Published: June 2005

Novel palaeoenvironmental interpretations and high-resolution correlations in Oxfordian reefal formations (fig. 1, 2) are provided by genetic stratigraphy. The main objectives of this study are: (1) to determine environmental conditions and the zonation of depositional environments; (2) to propose a genetic sequence framework and correlations of the outcrop sections; (3) to evaluate a possible tectonic control on coral buildup settlement and growth both at local scale.
Datation de surfaces geomorphologiques reperes par le 10 Be produit in-situ; implications tectoniques et climatiques
Authors: Siame Lionel L. , Braucher Regis , Bourles Didier L. , Bellier Olivier , Sebrier Michel
Published: June 2005

The evolution of continental landforms is mainly modulated by the impact of climatic and tectonic processes. Because of their distinctive morphology and the periodicity of their deposition, climatically induced landforms such as alluvial fans or terraces are well suited to infer rates of tectonic and continental climatic processes. Within tectonically active regions, an important step consists in dating displaced geomorphic features to calculate slip rates on active faults. Dating is probably the most critical tool because it is generally much more simpler to measure deformation resulting from tectonic activity than it is to accurately date when that deformation occurred. Recent advances in analytical chemistry and nuclear physics (accelerator mass spectrometry) now allow quantitative abundance measurements of the extremely rare isotopes produced by the interaction of cosmic rays with surface rocks and soils, the so-called in situ-produced cosmogenic nuclides ( 3 He, 10 Be, 21 Ne, 26 Al, 36 Cl), and allow to directly date the duration that a landform has been exposed to cosmic rays at the Earth's surface [Lal, 1991; Nishiizumi et al., 1993; Cerling and Craig, 1994; Clark et al., 1995]. In fact, the abundance of these cosmonuclides is proportional to landscape stability and, under favorable circumstances, their abundance within surface rocks can be used as a proxy for erosion rate or exposure age. These cosmonuclides thus provide geomorphologists with the opportunity to constrain rates of landscape evolution. This paper presents a new approach that combines cosmic ray exposure (CRE) dating using in situ-produced 10 Be and geomorphic as well as structural analyses. This approach has been applied on two active strike-slip and reverse faults located in the Andean foreland of western Argentina. These two case studies illustrate how CRE dating using in situ-produced 10 Be is particularly well suited for geomorphic studies that aim to estimate the respective control of climate and tectonics on morphogenesis.
Paleocene/Eocene transition in the northeastern Peri-Tethys area; Sokolovskii key section of the Turgay Passage (Kazakhstan)
Authors: Radionova E. P. , Khokhlova I. E. , Beniamovskii V. N. , Shcherbinina E. A. , Iakovleva A. I. , Sadchikova T. A.
Published: June 2005

Lithology, microfossils, and stratigraphy of the uppermost Paleocene - lowermost Eocene predominantly terrigenous and siliceous sediments from the Sokolovskii section located in the Turgay Passage, northern Kazakhstan, were studied. Among the microfossils, calcareous nannofossils, radiolarians, and especially dinocysts proved to be useful for age determinations, correlation with standard zonal schemes, and for paleoecological reconstructions. The NP8 (Heliolithus riedelii) nannoplankton Zone, the Acarinina subsphaerica and A. acarinata (lowermost part) planktonic foraminifera Zones, the Petalospyris foveolata regional radiolarian Zone, the Moiseivia uralensis diatom Zone were recognised, as well as four dinocysts zones: the Alisocysta margarita, Apectodinium hyperacanthum, A. augustum, and Wetzeliella meckelfeldensis Zones. Also, two transgressive-regressive cycles are distinguished. They are interrupted by short hiatuses; the upper one corresponds to the boundary between the Apectodinium augustum and Wetzeliella meckelfeldensis dinocyst Zones.
Stratigraphie sequentielle de l'Aptien du sous-bassin de Galve (Province de Teruel, NE de l'Espagne)
Authors: Vennin Emmanuelle , Aurell Marc
Published: June 2005

A correlation is established in a north-south transect based on continuous outcrops. Considering the different reference surfaces and the geometry, three major depositional sequences can be distinguished which can be subdivided into a complex arrangement of parasequences. These third-order sequences are composed of a lower retrogradational and an upper progradational trends. The first sequence contains orbitolinid bioaccumulations in the retrogradational trend and oolitic-bioclastic shoals in the progradational trend. The second sequence exhibits, from bottom to top, a transgressive, a regressive and a forced-regressive trends. Ammonite-rich marls characterise the transgressive trend, whereas bioconstructions rich in coral-chaetetids-microbialites are abundant in both regressive and forced-regressive trends. The maximum flooding of this sequence is widely distributed across the whole Iberian platform. Finally, the third sequence shows the installation of homogeneous rudistid bioaccumulations in a retrogradational and a progradational trends. Each major sequence boundary marks a community replacement, whose respective fossil associations are dominated by (1) orbitolinids, (2) corals-microbialites, (3) corals-chaetetids-microbialites, and (4) rudists.
Stratigraphie sequentielle du Dinantien type (Belgique) et correlation avec le Nord de la France (Boulonnais, Avesnois)
Authors: Hance Luc , Poty Eddy , Devuyst Francois-Xavier
Published: June 2005

The relative influences of local tectonics and global eustasy in the architecture of the sedimentary units of the Namur-Dinant Basin (southern Belgium) are determined. Nine third-order sequences are recognised. During the Lower Tournaisian (Hastarian and lower Ivorian) a homoclinal ramp extended from southern Belgium through southern England (Mendips) and into southern Ireland. From the upper Ivorian to the lower Visean rapid facies changes occurred due to progradation and increasing prominence of Waulsortian mudmounds. Progradation gradually produced a situation in which inner shelf facies covered the Namur (NSA), Condroz (CSA) and southern Avesnes (ASA) sedimentation areas, whereas outer shelf facies were restricted to the Dinant sedimentation area (DSA). During the middle and late Visean a broad shelf was established from western Germany to southern Ireland. Because the shelf built up mainly by aggradation, parasequences can be followed over a large area. An early phase of Variscan shortening is perceptible during the Livian. The stratigraphic gap between the first Namurian sediments (E2 Goniatite Zone) and the underlying Visean varies from place to place, but is more important in the north.
Mise au point d'un protocole experimental de separation granulometrique d'assemblages de nannofossiles calcaires; applications paleoecologiques et geochimiques
Authors: Minoletti Fabrice , Gardin Silvia , Nicot Elisabeth , Renard Maurice , Spezzaferri Silvia
Published: June 2005

Calcareous nannofossils play an important role in biostratigraphy and paleoecology, though their study is often complicated by their reduced size. The methodology presented herein allows to separate the nannofossil assemblage into granulometrically and taxonomically homogeneous fractions. This method permits :a) the concentration of rare taxa in order to simplify their morphometric study; b) the geochemical analysis (stable isotopes and trace elements) of these fractions. The latter is helpful in many cases, especially when the analysis of major or minor taxa constituent of the assemblage is greatly complicated by the reduced particles dimensions. In this way, it will be possible to bring light to the ecological parameters of these taxa, as calcification temperature and other ecological requirements (salinity, nutrient concentrations).
A large Jurassic pterodactyloid in northern France and a review of the French pterosaur record
Authors: Knoll Fabien , Bouveur Alain
Published: June 2005

A very large mandible from the Jurassic (Lower Tithonian) of the Boulogne-sur-Mer area (Pas-de-Calais, France) is described. The specimen does not allow a precise identification but could be close to Gallodactylus canjuersensis within the Pterodactylidae. A tentative wingspan estimation of the taxon gives us a figure of about 2.5 m, which, if correct, would make it a remarkably large pterosaur for that time. An exhaustive review of the French pterosaur record is also presented. Although few remarkable finds have been reported, pterosaurs are known in France from a number of sites ranging in age from the late Triassic to the uppermost Cretaceous. The French record shows that the pterosaur stratigraphical concentrations are unlikely to be related to actual pterosaur palaeoecological importance during the Mesozoic. The apparent diversity of this group through time constitutes therefore a very unsuitable element to bring arguments, in either one way or another, to the mass extinction debate.
Paragenese a faible variance dans les metapelites de la serie de Filali (Rif interne marocain); description, interpretation et consequence geodynamique
Authors: El Maz Abdelkader , Guiraud Michel
Published: June 2005

The metamorphic series of Filali and the Beni Bousera massif represent the most metamorphic unit in the inner part of the Moroccan Rif. The Filali series is composed of micaschists, gneisses, migmatites and granulites wrapped around the ultramafic body of the Beni Bousera. The foliation is broadly coherent all over the massif although differences in lineation and fold axis have been found between the granulites and the micaschists-gneisses. Metamorphic grade increases continuously towards the ultramafic body; however, seven metamorphic zones which account for this increase can be defined: - chlorite zone: muscovite and chlorite underline the S1 schistosity refolded by S2. Small garnet is wrapped by or cut across S2; - chloritoid zone: chloritoid displays the same textures as chlorite and muscovite, underlining S1 and S2 schistosity. Chlorite reacts out to biotite; - staurolite+andalusite zone: staurolite, andalusite, cordierite and garnet have pre-, syn- and post- kinematic features with respect to S2. Garnet reacts out to muscovite, biotite and plagioclase; - staurolite+andalusite+kyanite zone: same textural relationships as in the previous zone. Kyanite is an additional phase, often displaying epitaxial textures with staurolite and andalusite; - andalusite+kyanite+sillimanite zone: fibrolitic sillimanite generally associated with biotite crystallizes from staurolite. However, sillimanite is also directly formed from andalusite and kyanite. As in the previous zone garnet reacts out to biotite, muscovite and plagioclase; - sillimanite+k-feldspar zone: muscovite reacts out to k-feldspar. Modes of biotite and sillimanite increase; - migmatites zone are composed by biotite, sillimanite, garnet, cordierite quartz and feldspars. Garnets do not display the reaction textures seen in the previous zones; - granulites are composed of a primary paragenesis with kyanite+k-feldspar+garnet+ or -biotite+plagioclase+rutile, partially obliterated by a secondary paragenesis with cordierite, sillimanite and spinel.
Reaimantation regionale eocene, migration de fluides et mineralisations sur la bordure cevenole (France)
Authors: Rouvier Henri , Henry Bernard , Macquar Jean-Claude , Leach David L. , Le Goff Maxime , Thibieroz Jacques , Lewchuk Michael T.
Published: June 2005

Widespread remagnetization has been identified in fold belts and forelands inboard of mountain ranges and has usually been interpreted as resulting from fluid migration related to orogenesis in these mountain ranges. The geochemical properties of these fluids should be compatible with the formation or the transformation of ferrimagnetic minerals, thus allowing acquisition of remanent magnetization during fluid migration. Carbonate hosted lead-zinc (+ or -barite and fluorite) mineralization of the Mississippi Valley-type (MVT) are also generally considered to have formed during the migration of enormous volumes of fluids and are commonly located in foreland fold belts or their forelands. This suggests a similar origin for widespread carbonate remagnetization and MVT mineralization. The paleomagnetic dating of MVT deposits has been successfully applied in MVT districts, mainly of North America. Thus, it was used for the MVT deposits hosted by Mesozoic carbonate rocks of the Cevennes region of southern France. In view of the structurally complexity in the region, and because the results presented here are intended to provide an initial reference direction on which further results can be based, only sites belonging to the most stable parts of this border are considered here. Most samples were collected from three surface sites in the area of the Largentiee mine and from independently oriented cores of the two deep boreholes (Balazuc and Morte-Merie) from the Geologie Profonde de la France program. Several samples were also collected from three sites around the area of the Saint Felix-de-Pallieres mine. All these samples come from different stratigraphic levels: Carboniferous, Permian, Triassic, Liassic and Middle Jurassic.
A new Lepisosteidae (Actinopterygii, Ginglymodi) from the Cretaceous of the Kem Kem Beds, southern Morocco
Authors: Cavin Lionel , Brito Paulo M.
Published: June 2005

Lepisosteids or gars constitute a very special neopterygian group, with seven living species in two genera: Lepisosteus and Atractosteus. They live in freshwaters from the eastern part of North America and Central America. A new lepisosteid, Oniichthys falipoui gen. nov., sp. nov., is described on the basis of two well preserved specimens. Although the type locality is unknown, information provided by the fossil collector, the type of preservation of the specimen, and the nature of the attached matrix indicate, with confidence, that it comes from the Kem Kem beds of southern Morocco (fig. 1). The Kem Kem beds are rich fossiliferous horizons, exposed along the face of an escarpment extending from the north of Erfoud town to the Kem Kem area. The age of these outcrops is still debatable being considered as ?Albian in age [Forey and Grande, 1998] or regarded as Cenomanian, due to their elasmobranch assemblage [Sereno et al., 1996]. Oniichthys falipoui shows several derived characters of gars such as an elongated ethmoid region, an upper jaw formed by a chain of tooth-bearing bones, a joint between the quadrate and the lower jaw lying far forward, in front of the orbit and a large splint-like quadratojugal overlying the horizontal branch of the preopercle. O. falipoui shares with the primitive gar, Obaichthys decoratus, from the ?Albian Santana Formation of Brazil, the presence of toothed maxillaries, although in the Moroccan taxon, the maxillaries are anteriorly fused with infraorbitals. This structure confirms that, at least, some of the "infraorbital chain" bones bear maxillary teeth, fused to them during ontogeny. Discussion of characters leads to regard O. falipoui as more derived than Obaichthys, and to place it as the sister-group of Lepisosteus-Atractosteus.
Nature and properties of suspended solids in the Amazon Basin
Authors: Allard Thierry , Ponthieu Marie , Weber Tiphaine , Filizola Naziano , Guyot Jean-Loup , Benedetti Marc
Published: June 2005

Suspended solids from several rivers of the Amazon Basin (Brazil) have been studied in both the particulate (> 0,2 μm) and colloidal (0.2 μm-5kD) fractions through a combination of tools including Fourier transform infrared spectroscopy (FT-IR), electron paramagnetic resonance (EPR) and potentiometric measurements. Particulate and colloidal matter are organo-mineral and organic, respectively. Signatures of the particulate fractions distinguish Rio Amazonas/Rio Solimoes rivers and the northern basin rivers (Rio Negro, Rio Branco, Rio Trombetas). In the latter, kaolinites are the dominant clays and exhibit close crystal chemistry characters such as degree of disorder. FT-IR of colloids exhibits similar signatures of humic substances in all rivers. By contrast, EPR reveals various species which qualitatively and quantitatively differentiate the riverborne colloids: trivalent iron occurring as specific complexes with organic functional groups or as oxides sensu lato, divalent manganese, organic free radicals. The observed differences may be inherited from the weathering processes working in the main pedoclimatic regions drained by the studied rivers. Insights from potentiometric measurements of the recovered colloids also evidence a humic like reactivity consistent with carboxyl and phenol moieties, the relative contributions of which differentiate the Rio Negro and Rio Solimoes/Amazonas.
Trace elements and major species in precipitation at African stations: concentrations and sources
Authors: Freydier Rémi , Dupré Bernard , Dandurand Jean-Louis , Fortune Jean-Pol , Sigha-Nkamdjou Luc
Published: June 2005

The chemical composition of rainwaters is important in obtaining information on the aerosols and gaseous species present in the atmosphere. Original data on the concentrations of trace elements and major species in the dissolved and particulate fractions of rainwater samples collected in intertropical Africa were used to determine the potential sources of chemical species. The photographic and mineralogical characterization of rainwater particles using scanning electron microscopy allow to identify the various origins of aerosols and the associated chemical elements: (i) the crustal source characterized by clay minerals, kaolinite (Al, Si), illite (Al, Si and K) and Fe, Ti, and Mn oxides, (ii) vegetation relicts: Mg, Si, Cl, Mn and Zn have been identified in these biogenic particles and (iii) carbonaceous particles (mainly Si) emitted during vegetation fires. Ionic charge balance calculation for the stations located in the rain forest and wet savanna zones (Cameroon, Centrafrican Republic and Ivory Coast) exhibit an anion deficit from 20 to 50 % (between 7 and 9 μeq.l−1). Measured total organic carbon concentrations (between 0.5 and 3.3 mg.l−1), are much higher than acetic and formic acid. The presence of mimic and/or fulvic substances emitted from the vegetation can explain this anion deficit. The results obtained in this study confirm the existence of three important sources of atmospheric compounds in intertropical Africa: (i) the silicate upper crust source: Sc, Ti, Fe, Y, Zr, Nb, Cs, La, Ce, Nd, Yb, Th and U exhibit enrichment factors very close to 1, (ii) the biogenic and biomass burning source characterized by the following elements: Mg, Cl, K, Ca, Mn, Zn, Rb, Sr, and (iii) two minor sources; marine (Mg, Na, Cl) and anthropogenic (Cr, Co, Zn, Ba and Pb). For the stations located in the rain forest zone, all these sources appear strongly linked. High correlations are observed between elements of various origins. This suggest a strong interaction between terrigenous, biogenic, marine and anthropogenic aerosols. The deposition of aerosols onto vegetation and the re-mobilization of the chemical species through natural emissions or biomass burning could explain these correlations and the observed chemical fractionation of marine origin elements such as Cl and Na.
New insights about the rare earth elements (REE) mobility in river waters
Authors: Deberdt Samuel , Viers Jérôme , Dupré Bernard
Published: June 2005

While the rare earth elements (REE) have largely contributed to the understanding of some internal geological processes, their use as tracers of superficial mechanisms (i.e., weathering and elements transport) remains tricky. This results from the poor knowledge we have of the REE distribution in the different fractions of solutions.

Up to recently, the studies carried out on the REE behavior in continental aquatic systems were mainly based on the “paniculate” and “solution” fractions separated by an arbitrary filtration through a 0.20 or 0.45 μm pore size membrane. Some recent work has shown, using ultrafiltration experiments, that REE are mainly contained in the colloidal fraction (i.e., 0.20 μm - 1 nm). However, studies devoted to the control of REE by the colloidal matter, have essentially focused on the organic pool (i.e., humic substances). By contrast, the mineral colloidal pool has received little attention.

In order to model these processes, we have developed a combined approach that consider (1) ultrafiltration experiments performed on a series of rivers located in Africa (Cameroon) and south America (Brazil, Venezuela) and (2) speciation calculations. This quantitative approach reveals, for the first time, that more than 60 % of the total REE in the solution of river waters is controlled by the mineral colloidal pool.
The Lozère sands and the Sologne sands : two detrital discharges in the Paris basin during the early Miocene. New interpretations
Authors: Larue Jean-Pierre , Etienne Robert
Published: June 2005

Following a review of the previous interpretations concerning the drainage pattern modifications of the Loire-Seine in the Paris basin, this work attempts to show the chronological and distributional relations between the Lozère sands and the Sologne sands.

Heavy mineral analyses of about 300 samples, extracted from sands of a series of outcrops and drill holes from the detrital deposits coming from the Massif Central and located between the Nivernais and the Pays de Caux, makes it possible to separate the Lozère sands and the Sologne sands (fig. 1 and tabl. I). The first deposits show analogies with the Albian sands, having poor mineralogical variety and containing only a few feldspars, whereas the second ones incorporate greater mineralogical variety and more feldspars (fig. 2).

The identification of Lozère sands outcrops in the Loire valley, south of the Sologne area (fig. 3) proves that a palaeo-Loire-Seine existed before the discharge named Sologne sands during the Lower Burdigalian. A tectonical evolution (fig. 4) is proposed to explain the changes in mineralogy and distribution of the deposits. The progressive uplift of the northern border of the Massif Central caused the erosion of the Albian sands which contributed to the fluvial discharge of the Lozère sands. But when the Ligerian depression extended to the Sologne area, the fluvial discharge diverted toward the Atlantic, the Albian sands being excluded from erosion, and the Sologne sands were mainly supplied by basement derived alterites showing a greater mineralogical variety.

Since the Burdigalian, the Loire and the Seine basins had a distinct evolution. While the Sologne sands were buried under the Bourbonnais sands, the augite river deposits and the Lozère sands were reworked on the polygenic erosional surface [Dewolf et Pomerol, 1997] which developed from late Burdigalian to early Pleistocene times, before the Quaternary incision of the rivers.
EXAFS evidence of sorbed arsenic(V) and pharmacosiderite in a soil overlying the Echassières geochemical anomaly, Allier, France
Authors: Morin Guillaume , Lecocq Delphine , Juillot Farid , Calas Georges , Ildefonse Philippe , Belin Stéphanie , Briois Valérie , Dillmann Philippe , Chevallier Pierre , Gauthier Christophe , Sole Armando , Petit Pierre-Emmanuel , Borensztajn Stephan
Published: June 2005

At Echassières (Allier, France), arsenic speciation was determined in a soil developed over a micaschist where Hercynian hydrothermal mineralization, including arsenopyrite FeAsS and löllingite FeAs2, has lead to a regional As anomaly. The overlying soils which have developed from long term weathering exhibit As levels as high as 900 ppm in the richest area, where the saprolite contains up to 5200 ppm As. Analysis by powder XRD, μ-X-ray diffraction on the 20μm scale, SEM-EDS and electron micro-probe analyses revealed that As, released from arsenopyrite and/or löllingite alteration, is concentrated in a secondary iron arsenate, pharmacosiderite, (Bax,K2-2x) (Fe,Al)4 (AsO4)3 (OH)5 • 6H2O. Quantitative mineralogical analysis by Rietveld refinement indicates that the proportion of As hosted by this mineral decreases systematically from the saprolite to the topsoil (from 70 % to 30 % of the total bulk As content, respectively). EXAFS spectroscopy indicates that the main form of the As occurring with pharmacosiderite, consists of As(V) ions sorbed on iron oxides. Sorption processes, which dominate As speciation in the topsoil horizons, appear as a key mechanism able to delay As dissemination from soils to plant and surface waters, provided that pH and Eh conditions remain sufficiently acidic and oxidizing, respectively.
Early diagenesis of marine organic-matter and magnetic properties of sedimentary rocks: the role of iron limitation and organic-matter source organisms
Authors: Tribovillard Nicolas , Averbuch Olivier , Bialkowski Anne , Deconinck Jean-François
Published: June 2005

The magnetic parameters of sedimentary rocks can record accurately tectonic and climatic influences upon sedimentary processes, when they are not altered during diagenesis. This paper is focused on the possible alteration of the primary magnetic-susceptibility signal during early diagenesis of marine organic matter (OM).

In the late Kimmeridgian-Tithonian Argiles de Châtillon Formation (Fm.) of the Boulonnais area (northern France – lateral time equivalent of the distal organic-rich sediments of the Kimmeridge Clay Fm. of Dorset), the organic content is dominated by amorphous OM (AOM), either brown or orange, originating from selective preservation or sulphurisation, respectively. Total sulphur content correlates well with orange + brown AOM abundance, and organic S content correlates well with orange AOM abundance. The magnetic signal of these claystones and paper shales is dominantly carried by clay minerals. The magnetic susceptibility vs. brown-AOM abundance relationship shows a clear correlation. Furthermore, if the part of the magnetic-susceptibility signal linked to the lithoclastic fraction of the sediments is removed from the total signal, a positive correlation is also drawn between the brown-AOM abundance and the ‘excess’ magnetic susceptibility. The cause for this resides in the iron-sulphide abundance. The presence of both types of AOM implies that intense sulphate reduction took place and that the HS−/H2S released reacted with either organic molecules (orange AOM dominating the palynofacies) or reactive iron (brown AOM dominating). When reactive iron was abundant enough and when sulphate reduction-induced sulphide ions could react with it, iron sulphides could form and this sediment component (present now as pyrite) influences the magnetic-susceptibility signal. The type of dominating source organisms (phytoplankton) may condition the reactivity of OM towards sulphide ions. This may influence iron-sulphide formation, and in turn the sediment magnetic signal.
Inversion tectonics of the northern margin of the Basque Cantabrian Basin
Authors: Gómez Manuel , Vergés Jaume , Riaza Carlos
Published: June 2005

The northern margin of the Basque-Cantabrian Basin was analysed combining stratigraphic and structural data from both surface and subsurface together with reflectance of vitrinite data from oil wells. The use of cross-section balancing techniques in addition to thermal modelling enabled us to reconstruct the tectonic, burial and thermal evolutions of the basin margin as well as those of the Landes High to the N in two different periods. The section restoration at the end of the Cretaceous shows a northern basin margin structure influenced by evaporites related to south-dipping normal faults. The reconstruction in middle Eocene times yielded up to 1 800 m of Paleocene-middle Eocene deposits on top of the basin margin. Subsequent tectonic inversion related to the Pyrenean compression led to the north-directed thrusting of basement units and to the formation of thrust slices or inverted folds in the cover along the northern margin of the basin. Tectonic subsidence analysis together with maturity data provided evidence that oil was generated in the basin during the late syn-rift and post-rift stages in the Late Cretaceous and became overmature during the period of incipient inversion after 55 Ma. In the autochthonous Landes High, the oil was generated after the tectonic inversion period 37 Ma.
Facies and depositional setting of the Lower Cambrian archeocyath–bearing limestones of southern Montagne Noire (Massif Central, France)
Authors: Debrenne Françoise , Gandin Anna , Courjault-Radé Pierre
Published: June 2005

Archaeocyath-bearing limestones of the Lower Cambrian Pardailhan Formation of southern Montagne Noire are restricted to few intervals (H1, H2, H3) of the mixed carbonate-detrital succession exposed in several superposed tectonic units affected by severe deformation.

The comparative analysis of the archaeocyath assemblages together with a detailed sedimentological investigation leads to the understanding of the depositional settings and of the building style of the calcimicrobial-archaeocyath groups and to a better definition of the paleogeographic relationships of the different tectonic units.

Interval H1: small, low-relief ‘pioneer reefs’ built by Epiphyton bushes and Girvanella crusts, associated with clusters of small stick-shaped archaeocyaths, colonized the detrital sand of the bottom. They are frequent in southern Minervois and more randomly distributed in northern Minervois and northern and southern Pardailhan and represent short-lived attempts of carbonate colonization on the sandy, mobile substrate of a wide continental shelf.

Interval H2: Platy and bioclastic grainstones form the substrate and apron of small crust/cement reefs and associated clusters of ribbon-like and conical archaeocyaths in southern Minervois, or of mud-rich calcimicrobial mounds in southern Pardailhan. Displaced cups of stick-shaped archaeocyaths similar to those of Interval H1, occur in the granular facies, while in place saucer-like cups of Anthomorpha margarita are associated to the mud-rich mounds. This facies association records the repeated attempt at instauration and discrete development of a carbonate platform made of low-relief banks, the margins of which were colonized by Girvanella crust-buildups whereas in the more protected deeper zones Epiphyton/Renalcis mud-mounds dominated.

Interval H3: Epiphyton/Renalcis, mud-rich mounds with solitary, large saucer-like cups of Anthomorpha margarita, represented by long, ribbon-like fragments, are dominant in the upper part of the platform. They rest on reduced lenses of grainstone and Girvanella crust boundstone and in southern Pardailhan are interbedded with nodular, marly mudstones containing bioclastic debris partly derived from the buildups. In this area they mark the transition from platform to shallow basin.
Encrusting strategies in a Cambrian nonreefal epibenthic community
Authors: Clausen Sebastien , Álvaro J. Javier
Published: June 2005

Microfossils released by the action of acids from the Cambrian Valdemiedes Formation (Iberian Chains, NE Spain) contain evidence that encrusting microepifauna (foraminifers and serpulid tube worms) infested shelled hard substrates in nonreefal environments. They appear to have exploited parasitic and/or commensal ecological niches under medium- to low-energy bottom conditions. Despite the gregariousness character of the epifauna, skeletal overgrowths are rare, reflecting low-patterns on competitive interactions for space. Cambrian psammosphaerids (agglutinated foraminifers) were able to dissolve partly calcareous shells: although no evidence of pitting is observed, distinct depressed scars occur on the internal sides of host shells. The serpulids experienced a population bloom during the ecosystem recovery interval after the community turnover represented by the Valdemiedes event, and sharply disappeared during the ensuing return to background conditions. By contrast, the psammosphaerids are relatively abundant across the event, increasing in abundance during the same bloom of opportunistic taxa.
The topaz-bearing rhyolites from the San Luis Potosi area (Mexico): characteristics of the lava and growth conditions of topaz
Authors: Leroy Jacques L. , Rodriguez-Rios Rodolfo , Dewonck Sarah
Published: June 2005

In Mexico, the topaz-bearing rhyolites occur mainly in the San Luis Potosi area (San Luis Potosi and Guanajuato States). These rhyolites appear as domes related to the Tertiary extensional tectonism. Three domes of the San Luis Potosi volcanic field were selected for study, according to the color, the habits and the size of the topaz they contain, in order to determine the characteristics of the lava and the growth conditions of the topaz. These rhyolites are high-silica, metaluminous to slightly peraluminous and alkali-rich lavas. They are enriched in fluorine and in incompatible lithophile elements such as Rb, Cs, Ta, U and Th and depleted in Ba, Sr, Ca, Mg, Ti and Ni. Therefore, they are very similar to the topaz-bearing rhyolites from western United States. In Cerro El Gato dome, both amber-colored and colorless topazes crystallized in voids and fractures. Comparing these different crystals with respect to their growth environment, habits, chemical compositions and EPR characteristics provides an explanation for the crystallization conditions and color of topaz. Colorless topaz from Cerro El Gato crystallized at a temperature above 500oC (lack of color centers) from fluids enriched in elements leached from the lava, whereas the amber-colored topaz crystallized below 500oC (based on the presence of color centers) and from fluids richer in volatile elements (As). In Cerro El Lobo, the topaz have intermediate characteristics between the colorless and the amber-colored topazes of Cerro El Gato.
The Hercynian collision in the Armorican Massif : evidence of different lithospheric domains inferred from seismic tomography and anisotropy
Authors: Judenherc Sébastien , Granet Michel , Brun Jean-Pierre , Poupinet Georges
Published: June 2005

The Hercynian belt is a continental collision orogen extending from south-west Iberia to the Bohemian Massif in Czech Republic. The successive stages of its formation are dated from 400 to 260 Ma.The Armorican Massif is a preserved segment of this orogen. It presents structures oriented NW-SE, parallel to the general Hercynian trend in this region. The massif is divided into three domains (North, Central, and South-Armorican domains) separated by two main shearzones, the North- and South-Armorican shear zones. As the Armorican Massif escaped from any important tectonic or thermal event since the end of Hercynian times, it is particularly suited for the study of an old collision orogen. Thus, in the framework of the GéoFrance3D-ARMOR2 project, two passive seismological experiments were conducted in 1997 and 1999 in the Armorican Massif. The main goals concerned the characterization of the deep geometry of both shear zones, the understanding of their geodynamic bearing on the long term evolution of the Hercynian belt, the study of the lithospheric deformation, and the 3D imaging of the Champtoceaux nappes.The data allow to model seismic anisotropy and to build a 3D P-wave velocity model beneath the Armorican Massif. Crustal images do not evidence any deep rooting of the Champtoceaux nappes in the lower crust. However, the upper mantle images show a clear signal interpreted as the relic of the northward subduction which lasted until Devonian (≈350 Ma). The results also show that the North-Armorican Shear Zone is limited at depth to the crust and topmost mantle, while the South-Armorican Shear Zone can be traced over the whole lithosphere.The strong velocity contrasts are associated to probable relic thermal anomalies but are also significantly related to chemical anomalies.
Microstructural evolution of metabasalts within a contact metamorphic aureole : a preliminary quantitative bi-dimensional approach
Authors: Hebert Ronan , Guézou Jean-Claude , Souque Christine , Ballevre Michel
Published: June 2005

The emplacement of the Saint-Brieuc diorite (533 ± 12 Ma ; Brittany, France) developed a narrow contact aureole in metabasaltic host-rocks which were previously submitted to a regional-scale deformation and metamorphism at around 590-570 Ma. This study aims to qualify and quantify the microstructural changes of rocks that occur within the contact aureole and that are the result of the static recrystallization due to the thermal effect of the diorite. In order to quantify the textural evolution of rocks, an image analysis has been performed on thin sections. It focused on the measurements of morphological features of hornblende and opaque phases. They are anisotropy shape factors (stretching and elongation) of minerals, the preferred orientation of minerals and the distribution size (areas) of minerals. The quantification of these different parameters shows that the static recrystallization, which increases when approaching the contact with the intrusive, (i) is responsible of a reduction of the anisotropy shape and elongation parameters of grains, (ii) causes coarsening and (iii) is responsible for the disappearance of preferred orientation of minerals. Both together, the qualitative description and quantitative measurements show that the solid-state transformations due to contact metamorphism tend to make the rocks isotropic and equigranular.
Paleogeographic patterns of the Cambrian-Ordovician transition in the southern Montagne Noire (France) : preliminary results
Authors: Álvaro J. Javier , González-Gómez Cristina , Vizcaïno Daniel
Published: June 2005

The Cambrian-Ordovician transition in the southern Montagne Noire records a major siliciclastic regressive trend of prograding shoaling complexes (the La Dentelle Formation), separating two transgressive storm-dominated sedimentary systems of mixed (carbonate-siliciclastic) deposits. The latter comprise the underlying La Gardie and Val d’Homs Formations, and the overlying Mounio Formation, all of them displaying evidence of an important synsedimentary tectonic activity. Isolated settings of carbonate productivity, located on intra-shelf ramps and horsts, contain the richest and diversified faunistic communities comprising trilobites, echinoderms, conodonts, carbonate- and phosphate-shell brachiopods, sponge spicules, etc. Although the Cambrian-Ordovician transition of the southern Montagne Noire did not record volcanic events but a rather distensive regime inducing paleotopographies, the latter may reflect a distinct extensional regime recorded in other earliest Ordovician platforms of the French Massif Central, involving oceanization and a major magmatic activity.
Evolution of carbonated, lacustrine environment, with stromatolites : a paleoecological approach (quarry of Montaigu-le-Blin, Limagne graben, Allier, France)
Authors: Wattinne Aurélia , Vennin Emmanuelle , De Wever Patrick
Published: June 2005

The Aquitanian deposits of the quarry of Montaigu-le-Blin, located in the ≪ Limagne bourbonnaise », are characterized by lacustrine marl and limestone alternations. They display well-developed vegetal and fauna associations. The sedimentary deposits show an environment which allowed settlement of fresh water fauna. The deposits reveal variations of the water level and consequently of the water chemistry. These variations are characterized by alternations of anoxic and oxidizing lacustrine environments, which are due to periodic climatic changes (alternating more or less humid periods). These variations induced changes in freshwater faunal and plant associations : there are thanatocoenosis during the anoxic periods and an increasing of development of encrusting algae, responsible for the formation of algae concretions, during the oxidizing period. These oxidizing periods are also characterised by an increased hydrodynamism and terrigenous deposits from watershed. The stromatolites are numerous in the study area and show various shapes, depending on main algae, bacterial communities, and the faunal associations (caddisfly pupal cases) on the one hand, and their growth environment on the other hand. The stromatolites associations can form plurimetrical complexes which are studied in detail, and situated relatively to a paleoshoreline.
Ordovician radiolarian biodiversity : insights based on new and revised data from Kazakhstan
Authors: Danelian Taniel , Popov Leonid
Published: June 2005

Since the 1970s Ordovician carbonate rocks from Kazakhstan have provided well-preserved radiolarian faunas and this country remains one of the most important areas in the world to learn about Lower Palaeozoic radiolarian taxonomy and biostratigraphy. Here we describe a new lower Arenigian radiolarian assemblage and discuss all currently available Ordovician radiolarian data from Kazakhstan. The newly discovered assemblage is curiously oligospecific, despite its good state of preservation. Four morphospecies are recognised, including a new genus and two new species : Inanihella bakanasensis (Nazarov) and I.(?) akzhala n.sp, which characterise the assemblage, together with Triplococcus acanthicus n.gen n.sp. and Proventocitum sp.cf. P. procerulum. This is the oldest radiolarian assemblage from limestone lithologies of Kazakhstan and one of the very few Lower Ordovician assemblages discovered worldwide. We specify in this study the age range of the succeeding Haplentactinia armillata-Proventocitum procerulum assemblage, defined earlier by Nazarov and Popov [1980], which now covers the middle Arenigian – lower Llanvirnian (lower Abereiddian) interval. The succeeding upper Llanvirnian (Llandeilian) Haplentactinia juncta – Inanigutta unica assemblage reported by Nazarov and Popov [1980] reflects a substantial increase in radiolarian diversity, due largely to the introduction of new members of the family Inaniguttidae.
A new scenario for the Domerian - Toarcian transition
Authors: Morard Alain , Guex Jean , Bartolini Annachiara , Morettini Elena , de Wever Patrick
Published: June 2005

In contrast to the majority of recently published hypotheses, we believe that the main trigger for early Toarcian anoxia is neither increased primary productivity during the Tenuicostatum and Falciferum Zones nor sudden methane hydrate degassing close to the transition between these two zones.

In our opinion, this peculiar paleoceanographic episode is linked to a major, though short-lived, regression at the end of Upper Domerian. Sea-level fall resulted from sudden cooling due to increased volcanic activity. This generated global thermal insulation and subsequent glaciation. The regression is responsible for a major hiatus over NW-European epicontinental seas and is later followed by the well-known Lower Toarcian transgression. The interval corresponding to this hiatus allowed vegetation to colonise vast newly emerged surfaces. The leaching and drowning of the accumulated organohumic matter then triggered the anoxic cycle at the transgressive maximum, concomitant with a global warming.