Les cordons coquilliers holocènes de Charron (Marais poitevin) : intérêt des études des paléoenvironnements en archéologie

L’étude du paléo-archipel de Charron permet d’envisager le rôle déterminant des cordons et flèches littoraux qui frangent systématiquement ces paléo-reliefs. Ces cordons eurent une très importante influence sur la sédimentation, ainsi que sur les conditions locales de navigation et donc de l’accès aux îlots. On peut estimer que leur âge est postérieur à 3800-4000 BP. Si l’on tient compte des sites archéologiques liés à des activités maritimes, il paraît sûr qu’à la période gallo-romaine la mer pénétrait encore largement à l’ouest du bourg de Charron en une petite mer intérieure. Le colmatage dut ensuite s’accélérer avec des marais salants à partir du Moyen Âge et surtout la poldérisation du XIIe au XVIe siècle. Au XVIIe siècle, les îlots de Charron sont entièrement dans les terres, la ligne de rivage se situant à l’ouest du bourg actuel. Seule une approche globale à caractère paléoenvironnemental nous paraît susceptible d’apporter un éclaircissement sur la chronologie du colmatage du golfe poitevin, sur celle du mouvement du trait de côte et sur les variations holocènes du niveau relatif de la mer dans cette baie.Bodeur Yves, Gruet Yves, Blanchin Barbara. Les cordons coquilliers holocènes de Charron (Marais poitevin) : intérêt des études des paléoenvironnements en archéologie. In: Côtes et estuaires : milieux naturels. Actes du 124e Congrès national des sociétés historiques et scientifiques, « Milieu littoral et estuaires », Nantes, 1999. Paris : Editions du CTHS, 2002. pp. 367-383. (Actes du Congrès national des sociétés savantes, 124

Le camp protohistorique de Penchâteau au Pouliguen (Loire-Atlantique)

The defensive earthworks of Penchâteau enclose a promontory of 7 ha wide. A short archaeological excavation permits us to precise the complexe organisation of successive ditches and mounds, part of this wide defensive complex. They allow us to define the relative chronology and give some absolute chronological arguments. Inside the camp, a survey of lands free of construction shows the existence of many domestic structures. Some of the archaeological levels have been preserved from erosion. The main period of occupation seems to date from the beginning of the Tène period in Brittany.Les fortifications du camp de Penchâteau (Le Pouliguen, 44) ferment un éperon d'une superficie de 7 ha. L'éperon barré occupe une position stratégique, à l'embouchure de la Loire, dans une zone frontière entre le territoire des Namnètes et celui des Vénètes. Une expertise archéologique a permis de préciser la structure complexe des différents talus et fossés composant ce système défensif, d'en préciser la chronologie relative et quelques éléments de datation. Une rapide reconnaissance des terrains préservés d'une urbanisation galopante à l'intérieur du camp atteste de l'existence de très nombreuses structures d'habitat et parfois même de niveaux archéologiques en place. La poterie recueillie lors des différentes fouilles ou en prospection de surface, est pour l'essentiel attribuable à une phase ancienne du second Age du Fer.Gaiffe Olivier, Laporte Luc, Rouzeau Marie-Henriette, Rouzeau Maurice, Bodeur Yves, Gruet Yves, Maggy Ch., Pirault L. Le camp protohistorique de Penchâteau au Pouliguen (Loire-Atlantique). In: Revue archéologique de l'ouest, tome 12, 1995. pp. 117-137

Geochemical and petrological characterization of gem opals from Wegel Tena, Wollo, Ethiopia: opal formation in an Oligocene soil

Gem opals from Wegel Tena, Wollo Province, Ethiopia, occur in Oligocene rhyolitic ignimbrites. They display a unique geochemistry, with some samples yielding the highest Ba concentrations ever recorded. They are generally much richer in chemical impurities than opals from other localities. For example, the sum Al+Fe or the sum Na+Mg+Ca+K+Ba are often higher. These geochemical features make them easy to distinguish from other opals worldwide. We observed strong geochemical variations and some good positive correlations in our samples, such as Al+Fe vs. Na+Mg+Ca+K+Ba, Al vs Ca, or Ba vs Ca. This shows that the crystallography of opal has controlled, at least in part, the incorporation of chemical impurities, although opal is not well-crystallized. In addition, the multimodal distributions of several chemical impurities (e.g. U vs Sr, Al vs Ca, Ba vs Ca, etc.) suggest at least two origins of silica: weathering of feldspars and weathering of volcanic glass. In addition, opals from Wegel Tena contain numerous well-preserved microscopic plant fossils. Moreover, their host rock exhibits features typical of pedogenesis (abundant clays, desiccation cracks, and grain size sorting). We propose that the opals at Wegel Tena formed during the Oligocene period when volcanic emissions stopped for a time long enough to allow weathering of ingimbrites and therefore liberation of silica. This accompanied the formation of soil and development of plant life, and some plants were trapped in opal

Coral zonation of an Oxfordian reef tract in the northern French Jura

International audienceDuring the Middle Oxfordian, numerous coral reefs flourished on the northern margin of the Tethys Ocean. The outcrop of Bonnevaux-le-Prieuré (northern French Jura mountains) provides a rare opportunity to observe a nearly complete section allowing the installation, evolution and demise of this global carbonate reef rich event to be studied. Quantitative data on coral assemblages together with sedimentological and palaeoecological observations lead to the reconstruction of a reef tract coral zonation. Starting from the outer slope, Dimorpharaea, Microsolena, Dendraraea, Comoseris, and Stylina ecozones are recognized. This new facies model implies a central position for an oolitic shoal in the highest energy zone, within the Comoseris ecozone. Applying this facies model to the sequence stratigraphic interpretation of the vertical succession results in recognising a third-order relative sea-level fluctuation, which can be correlated at least with Lorraine (France) and Switzerland

A surface dissolution/precipitation model for the development of lakes on Titan, based on an arid terrestrial analogue: the pans and calcretes of Etosha (Namibia)

A surface dissolution/precipitation model for the development of lakes on Titan, based on an arid terrestrial analogue: the pans and calcretes of Etosha (Namibia

Geochemical and petrological characterization of gem opals from Wegel Tena, Wollo, Ethiopia: opal formation in an Oligocene soil

<p>Gem opals from Wegel Tena, Wollo Province, Ethiopia, occur in Oligocene rhyolitic ignimbrites. They display a unique geochemistry, with some samples yielding the highest Ba concentrations ever recorded. They are generally much richer in chemical impurities than opals from other localities. For example, the sum Al+Fe or the sum Na+Mg+Ca+K+Ba are often higher. These geochemical features make them easy to distinguish from other opals worldwide. We observed strong geochemical variations and some good positive correlations in our samples, such as Al+Fe vs. Na+Mg+Ca+K+Ba, Al vs Ca, or Ba vs Ca. This shows that the crystallography of opal has controlled, at least in part, the incorporation of chemical impurities, although opal is not well-crystallized. In addition, the multimodal distributions of several chemical impurities (e.g. U vs Sr, Al vs Ca, Ba vs Ca, etc.) suggest at least two origins of silica: weathering of feldspars and weathering of volcanic glass. In addition, opals from Wegel Tena contain numerous well-preserved microscopic plant fossils. Moreover, their host rock exhibits features typical of pedogenesis (abundant clays, desiccation cracks, and grain size sorting). We propose that the opals at Wegel Tena formed during the Oligocene period when volcanic emissions stopped for a time long enough to allow weathering of ingimbrites and therefore liberation of silica. This accompanied the formation of soil and development of plant life, and some plants were trapped in opal. </p