267 research outputs found
International field workshop on "The Triassic of Eastern France", october 2-7, 2006
Mémoire de Géosciences-Rennes, Hors-série n°5Excursion géologique organisée à l'occasion du congrÚs international "The triasic of Eastern France", du 2 au 7 ocrobre 200
Some aspects of current State of Knowledge on Triassic series on both sides of the Central Atlantic Margin
The BrĂ©cy depocenter as part of a new northern Massif Central CarboniferousâPermian Basin (France)
Some aspects of current State of Knowledge on Triassic series on both sides of the Central Atlantic Margin
Les séries fluvio-éoliennes du "Cutler Group" :: Permien (Utah, USA). ModÚle architectural et expression des unités génétiques
National audienceL'essor qu'a connu la stratigraphie séquentielle dans les environnements continentaux a permis de définir des modÚles architecturaux pour des séries fluvio-lacustres. Plus récemment, les modÚles proposés ont intégré les dépÎts éoliens dans les systÚmes continentaux cÎtiers ou lacustres (préservation de l'éolien vs variations de la table d'eau, e.g. Carr-Crabaugh & Kocurek, 1998) ou dans les systÚmes continentaux endoréiques (préservation de l'éolien vs climat (e.g. Clemmensen et al., 1998) ou préservation de l'éolien vs accommodation et flux de sédiments, e.g. Bourquin et al., 2009). Cependant, peu de modÚles discutent les variations de préservation des dépÎts éoliens par rapport aux environnements continentaux associés (fluvial, lac, playa) afin de prédire un modÚle architectural des dépÎts éoliens (du réservoir au bassin) en fonction du contexte géodynamique. L'objectif de cet exposé est de contraindre l'expression des unités génétiques et de proposer un modÚle architectural montrant les variations de préservation des dépÎts éoliens en fonction du contexte tectonique, climatique et eustatique. L'étude des séries du Permien inférieur, " Cutler Group ", du " Paradox Basin " (SE Utah), caractérisées par des dépÎts fluvio-éoliens (e.g. Condon, 1997), a été réalisée à partir de 5 sections sédimentologiques. Elles sont localisées sur un profil proximal représenté par des dépÎts dominés fluvial jusqu'à des dépÎts dominés éolien dans le domaine distal (e.g. Mountney & Jagger, 2004). Une étude sédimentologique et stratigraphique détaillée permet de proposer une évolution spatiale et temporelle des environnements de dépÎt. Nous avons ainsi mis en évidence (1) les variations d'expression des unités génétiques d'un environnement proximal à un environnement distal en climat semi-aride, (2) plusieurs modÚles architecturaux montrant l'évolution verticale depuis un contexte de bassin cÎtier, à dunes éoliennes faiblement préservées, vers des environnements plus continentaux dominés éolien puis vers des systÚmes fluvio-lacustres à faible préservation des dépÎts éoliens
The volcaniclastic series from the luang prabang basin, Laos: A witness of a triassic magmatic arc?
International audienceThe paleogeographic evolution of South East Asia (SEA) during the early Mesozoic is still poorly understood and a number of models have recently been put forward to account for the geodynamic evolution of SEA. The Luang Prabang Basin (north Laos), located in the core of a âpaleogeographic jigsawâ in SEA, recorded a long lasting volcanism that spanned for ⌠35 my from the earliest Triassic up to Late Triassic as evidenced by combined stratigraphic and geochronological (U-Pb/zircon) analyses performed on both volcanic and volcaniclastic series. The volcanic rocks are arc tholeiites and calk-alkaline andesites to dacites. The volcaniclastic rocks contain, in part, volcaniclasts produced contemporaneously with sedimentation. Both the volcanic and volcaniclastic series display geochemical features characteristic of a subduction related volcanism. Therefore, the Luang Prabang Basin documents a magmatic arc in a good agreement with the recent recognition of neighboring ophiolitic rocks in the Luang Prabang area. Following a passive margin setting that prevailed from the late Carboniferous to the late Permian, an active margin then initiated along the western margin of the Indochina Block. This active magmatic arc developed as the result of an east-dipping subduction below the Indochina Block during most of the Triassic, at least from ca. 250 to 215 Ma. Subsequently, this oceanic subduction episode must have been followed by a continental collision of the Indochina Block with the eastern Simao Block, at a period that remains to be defined
Stable isotope characterization of pedogenic and lacustrine carbonates from the Chinese Tian Shan: constraints on the Mesozoic - Lower Cenozoic palaeo-environmental evolution
International audienceIn the MesozoicâCenozoic continental deposits of the Tian Shan area, two main levels containing pedogenic carbonates have been identified on both the southern and northern foothills of the range: one in the Upper Jurassic series and one in the Upper CretaceousâLower Palaeocene series. In order to reconstruct the palaeoenvironmental and palaeotopographic characteristics of the Tian Shan area during these two periods, we measured the oxygen and carbon isotope composition of these pedogenic carbonates (calcrete and nodules). The stable isotope compositions are homogeneous: most ÎŽ18O values are between 21 and 25â° and most ÎŽ13C values are between â4 and â6â°. No distinction can be made between the calcrete and nodule isotopic compositions. The constancy of isotopic values across the Tian Shan is evidence of a development of these calcification features in similar palaeoenvironmental conditions. The main inference is that no significant relief existed in that area at the CretaceousâPalaeogene boundary, implying that most of the present relief developed later, during the Cenozoic. In addition to the pedogenic carbonates, few beds of limestones interstratified in the Jurassic series of the southern foothills display oxygen and carbon isotope compositions typical of lacustrine carbonates, ruling out brackish water incursion at that period in the regio
Formation and preservation of colluvial sedimentary breccias during early extension: processes and facies organization
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