Les altérites : l'épiderme de la Terre.

Les phénomènes d'altération supergène (météoriques) comptent parmi les processus les plus importants qui modifient la surface de notre planète. Les altérations sont le résultat des interactions entre géosphère, biosphère, hydrosphère et atmosphèr

Recognizing pedogenic features in Paleogene sandstones and silcretes in Belgium: a key-feature for paleoenvironmental and sourcing material studies

International audienceA few sandstones occurring in Belgium, especially those from Paleogene strata, share many common features with pedogenic and groundwater silcretes documented in the Paris Basin in France, in South Africa and in Australia (e.g. Thiry, 1999; Summerfield, 1983; Webb & Golding, 1998,). Here we present a review of the criteria that may be used to assess the pedogenic (or "groundwater") origin for sandstones and silcretes. At field scale, pedogenic features in silcrete often consist in the development of vertical (columnar) or nodular structures and/or layers (horizons) due to vetical percolation (illuviation) and transformation of materials in the profile porosity and cracks. Groundwater silcrete often exhibit a typical mammilated surface (e.g. Nash & Ullyot, 2007). Root and rootlet casts, sometimes with silicified root or wood material, also indicate that the rock evolved at the surface or near-surface. At microscopic scale, illuviation of silt and clays in the vadose or water-table environment results in the formation of a series of finely-laminated coatings and infillings. These pedofeatures consist in "typic", "crescent", "capping", "pendent", "micropan" and "crust" coatings (Bullock et al., 1985). Irregular rounded structures or "glaebules" also develop as well as micro-columnar or prismatic textures. Many of them have been observed in pedogenic silcretes around the world (e.g. Thiry, 1999) but also in Paleogene silcrete in Belgium (e.g. Veldman et al., in review). Of particular importance is the concentration of fine-grained titanium-oxides in pedogenic silcrete. These Ti-oxides most probably originated from the weathering of clay material and were concentrated via the same pedogenic processes which concentrated silica in the silcrete profile (Thiry, 1997). In sandstones with a pure sand matrix, where no or few fine-grained material is available for illuviation, assessing the pedogenic origin for the silicification is much more difficult based solely on microtextural observations. New perspectives may arise from trace-element analysis in the quartz cement overgrowing the grains. For example, combining cathodoluminescence imaging (CL) and electron microprobe analysis (EPMA) allowed the detec on of abnormally-high concentration of Al and K in the quartz cement relative to the quartz grains in the Grandglise Thanetian sandstone in the Mons Basin (figure). This suggests that first the weathering of glauconite, which is actually observed here, is the likely source for silica and secondly, the evidence for aluminum migration would indicate strongly acidic conditions in the pore fluids

Persistent paléosurfaces in the basement of French Massif Central: geodynamic implications.

National audienceThe siderolithic paleoweathering surfaces of the French Central Massif have been dated to the Late Jurassic/Early Cretaceous, contrasting with previously accepted Tertiary age and implying that the Massif has never hosted a thick sedimentary cover. This contradicts with former thermochronological results. Herein, we expose the arguments for and against the proposed geodynamic evolution of the French Massif Central constrained by paleomagnetic age determinations

Pneumopathie induite par l'hydroxyurée. [Hydroxyurea-induced pneumonia].

International audienceIntroduction: Hydroxyurea is an antimetabolite widely used in the treatment of myeloproliferative diseases. Usual side effects are mainly hematological, gastrointestinal, neurological disorders and induced-fevers. More rarely, hydroxyurea-induced pneumonitis are reported. Case report: We report a case of a patient treated for polycythemia vera. She was admitted 20 days after introduction of hydroxyurea for a high fever, productive cough and clear sputum associated with nausea. Chest CT-scan found diffuse bilateral ground-glass opacities. The microbiological investigations were negative. Symptoms disappeared few days after discontinuation of treatment. Its reintroduction led to recurrence of symptoms. Conclusion: This additional case completes the 15 cases of hydroxyurea-induced pneumonitis described in the literature. Two forms of this disease seem to exist: an acute form with fever occurring in the month following introduction of hydroxyurea; and a chronic form without fever. Even if it is uncommon, pulmonologists should be aware of this complication

Datations par paléomagnétisme des paléoaltérations du Massif Central et de ses bordures : implications géodynamiques

Continental unconformities are essential to estimate erosion rates of crystalline basement and to model the crust dynamics that control subsidence of surrounding sedimentary basins. Dating such unconformities has always been the stumbling block to constrain geodynamical models. Paleomagnetism is a great tool to date ferrugineous paleoweathering. This method has been applied: to paleoweathering occurrences resting on the Massif central crystalline basement and reaching the Morvan crystalline basement ; and to the Cretaceous ferrugineous sandstones of the Southeast border of the Paris Basin. Relative dating of these azoic formations have been acquired by comparing the recorded paleomagnetic poles from the samples with the apparent polar wander path (APWP) of the Eurasian plate The thick red kaolinitic formations from the Massif central have classically been ascribed to the "Siderolithic" (Eocene-Oligocene). These formations are arranged in clear paleolandscape features: around basement paleoreliefs, leaned against fault scarps, infilling paleovalleys, etc. It show well preserved pedogenic features (termites burrows, illuviation and hydromorphic features, nodules...). Macro and micromorphological observations show that these formations are in situ paleosoils. Paleomagnetic datings provide ages ranging from 160 Ma (Late Jurassic) in the Naussac Basin (centre of the Massif central) to 140 Ma (Early Cretaceous) in the Lembron region (Southwest border of the Limagne graben). The Cretaceous ferrugineous deposits from the Southeast border of the Paris Basin, in the Puisaye and Sologne areas are made up of sandstones with ferrugineous cement (Early Albian Sands and Ferruginous Sandstones, La Puisaye Sands). The cement is mainly goethitic. No dating were obtained because of the instable magnetization of this mineral. A hematitic layer located at the base of the Myennes Clays gave a recent age (0-3 Ma). It could be related to the oxidization of a pyretic layer subsequent to groundwater flow during the formation of the present-day landscapes. The Massif central and the Morvan crystalline basements show albitized facies. It affects granites, gneiss, rhyolites and Permo-Carboniferous sediments. On granites, albitized facies are layered, with pinkish mottling at the base and reddish spotting to the top. These horizons are marked by albitization of primary plagioclases and chloritization of primary biotites. Secondary albite and chlorite contain secondary minute hematite inclusions, which have been dated by paleomagnetism to be Triassic in age. The albitized facies are related to the Triassic unconformity. Both datings and geographic distribution of the albitized formations argue for a superficial origin of these facies. Therefore, the post-magmatic or hydrothermal hypothesis classicaly proposed to explain their origin can be excluded. Paleomagnetic analyses allowed to date a large range of paleoweathering formations for which no objective datings was available. The space and time distribution of the formations and the related unconformities gives elements to highlight the geodynamic evolution of the Massif central. * Triassic, Late Jurassic and Tertiary unconformities are superimposed on large areas of the Massif central. It implies very little erosion of the crystalline basement since Triassic times, as shown by the widespread albitized triassic unconformity outcroping at present. * Datings points out that no Mesozoic cover was present on the basement of Massif central during development of the red kaolinitic paleosoils during the Late Jurassic/Early Cretaceous period. Consequently, the Massif central did probably not support an important (more than 500 m) sedimentary cover during the Mesozoic. In conclusion, identification and dating of such continental unconformities give evidences for long lasting continental evolution and landscape stability of the Massif central all along the Mesozoic.Les datations des paléosurfaces continentales constituent des points d'ancrage majeurs pour contraindre l'évolution géodynamique d'une région. Le paléomagnétisme s'avère être une méthode adaptée pour dater les paléoaltérations continentales ferrugineuses. Les datations des formations rouges "sidérolithiques" du Massif central indiquent des âges s'échelonnant entre 160 Ma (Jurassique supérieur) dans le bassin de Naussac et 140 Ma (Crétacé inférieur) dans le Lembron. Sur la bordure Sud-Est du bassin de Paris, un niveau hématitique, à la base des argiles de Myennes, a fourni un âge sub-récent (0-3 Ma) qui pourrait souligner l'entaille des paysages actuels. Sur le socle cristallin du Massif central et du Morvan la datation des faciès albitisés a fourni un âge triasique soulignant l'origine superficielle de ces faciès. L'ancrage spatio-temporel des paléoaltérations et des paléosurfaces permet de préciser des traits majeurs de l'évolution géodynamique du Massif Central. Sur de vastes régions les paléosurfaces du Trias, du Jurassique terminal et du Tertiaire sont confondues. Il n'y a donc pas eu d'érosion conséquente du socle entre le Trias et la période actuelle, puisque la paléosurface albitisée au Trias est encore présente sur de très grandes surfaces du massif. Les datations du Jurassique supérieur/Crétacé inférieur des paléoaltérations "sidérolithiques" soulignent que le Massif Central ne présentait pas de couverture sédimentaire à cette époque. La reconnaissance et la datation de ces paléosurfaces continentales témoignent de longues périodes de continentalisation du socle et de la relative stabilité du Massif Central tout au long du Mésozoïqu

Altérations ferrugineuses su la bordure Sud du Bassin de Paris (Puisaye).

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Significance of the giant Lower Cretaceous paleoweathering event

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Oceanic anoxic events (OAEs) and plankton evolution: a case study from mid-Cretaceous Radiolaria

The impact of OAEs on the marine biosphere is of interest to palaeobiology because OAEs represent major environmental perturbations of the ocean-climate system. As a siliceous group with a long evolutionary history, Radiolaria constitute an interesting proxy to gauge the biotic response of zooplankton. Previous studies suggest that Radiolaria experienced important faunal turnovers during the mid-Cretaceous OAEs. We have focused on the members of family Archaeodictyomitridae (Early Jurassic-Palaeocene) for which we have first analysed the phylogenetic relationships of its various Cretaceous morphospecies. Our analysis was based on literature review, collection of new material with a high resolution sampling and phenetic analyses performed with the help of PAST software. We find that some previously thought extinctions of Archaeodictyomitrid species during OAE1b (late Aptian - early Albian) are in fact pseudoextinctions. Indeed, our material from Albania suggests that a major diversification took place at the base of this OAE. A high-resolution study across the Bonarelli level (OAE2) confirms the step-by-step pattern of numerous real extinctions (end of lineages), some of which started in the late Cenomanian, long before the C/T boundary. This might suggest that extinctions were not driven by abrupt environmental changes, but by the ca. 2 Ma environmental deteriorations which began with the mid-Cenomanian event

Long lasting paleolandscapes stability of the French Massif Central during the Mesozoic.

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