Aperçu paléogéographique de la transition cambro-ordovicienne dans la Montagne Noire méridionale (France) : résultats préliminaires

La transition cambro-ordovicienne de la Montagne Noire méridionale reflète une tendance regressive en milieux silicoclastiques, comprenant les complexes de barres hydrauliques de la formation de la Dentelle, délimités par deux systèmes sédimentaires mixtes (carbonato-silicoclastiques) transgressifs dominés par l’action des tempêtes. Ces systèmes correspondent respectivement aux formations sous-jacentes de la Gardie et de Val d’Homs, et à la formation sus-jacente de Mounio, qui ont enregistré une activité tectonique synsédimentaire importante. Les communautés fauniques les plus riches et diversifiées (comprenant des trilobites, échinodermes, conodontes, brachiopodes à coquilles carbonatées et phosphatiques, spicules d’éponges, etc.) occupent des emplacements très réduits. Ces communautés, associées à une forte productivité carbonatée, sont situées sur des rampes intra-plates-formes et des horsts. Malgré l’absence de traces d’activité volcanique au cours de la transition cambro-ordovicienne en Montagne Noire méridionale le régime distensif responsable du développement des paléotopographies en horsts peut être relié avec un régime géodynamique en extension. Ce dernier est reconnu dans d’autres secteurs du Massif central ayant enregistré des processus d’océanisation et une importante activité volcanique à la base de l’Ordovicien

The Ordovician of France and neighbouring areas of Belgium and Germany

The Ordovician successions of France and neighbouring areas of Belgium and Germany are reviewed and correlated based on international chronostratigraphic and regional biostratigraphic charts. The same three megasequences related to the rift, drift and docking of Avalonia with Baltica can be tracked in Belgium and neighbouring areas (Brabant Massif and Ardenne inliers), western (Rhenish Massif) and northeastern Germany (Rügen). The remaining investigated areas were part of Gondwana in the Ordovician. The Armorican Massif shares with the Iberian Peninsula a Furongian–Early Ordovician gap (Toledanian or Norman gap), and a continuous Mid–Late Ordovician shelf sedimentation. The Occitan Domain (Montagne Noire and Mouthoumet massifs), eastern Pyrenees and northwestern Corsica share with southwestern Sardinia continuous shelf sedimentation in the Early Ordovician, and a Mid Ordovician ‘Sardic gap’. In the Ordovician, the Maures Massif probably belonged to the same Sardo-Occitan domain. The Vosges and Schwarzwald massifs display compa-rable, poorly preserved Ordovician successions, suggesting affinities with the Teplá-Barrandian and/or Molda-nubian zones of Central Europe.This paper is a contribution to the International Geoscience Programme (IGCP) projects 653 "The onset of the Great Ordovician Biodiversification Event" and 735 “Rocks and the Rise of Ordovician Life: Filling knowledge gaps in the Early Palaeozoic Biodiversification". The authors are particularly grateful to Annalisa Ferretti, David A.T. Harper and Petr Kraft for their careful and constructive reviews, comments and suggestions, which greatly improved the quality and relevance of the paper

Molecular-rotation-induced splitting of the binary ridge in the velocity map of sub-eV H⁺ ions ejected from H₂ molecules by ion impact

International audienceIn studies of ion-induced molecular fragmentation, the challenging measurement of the velocity distribution of fragments emitted below 1-eV kinetic energy is rarely achieved, although most fragments have an energy below this value. Here, we study H$^+$ fragment emission in collisions of 10-keV O$^+$ ions with H$_2$ molecules using a field-free time-of-flight technique developed specifically to detect sub-eV fragments. We find that, in the velocity map, the binary ridge due to direct H$^+$ knockout is split into two parts arising from the rotational motion of the H$_2$ molecule, and that this split scales with rotational velocity. The velocity distribution of the nuclei in the original molecule is determined and the thermally populated J = 1 rotational level is found to be the dominant contributor, although asymmetry in the split indicates projectile-induced rotational transitions between M sublevels. These rotation effects influence fragment emission probabilities, thus carrying important consequences for the radiation-induced hydrogen loss and H$_2$ dissociation in the atmospheres or exospheres of planets and moons

Molecular-rotation-induced splitting of the binary ridge in the velocity map of sub-eV H⁺ ions ejected from H₂ molecules by ion impact

International audienceIn studies of ion-induced molecular fragmentation, the challenging measurement of the velocity distribution of fragments emitted below 1-eV kinetic energy is rarely achieved, although most fragments have an energy below this value. Here, we study H$^+$ fragment emission in collisions of 10-keV O$^+$ ions with H$_2$ molecules using a field-free time-of-flight technique developed specifically to detect sub-eV fragments. We find that, in the velocity map, the binary ridge due to direct H$^+$ knockout is split into two parts arising from the rotational motion of the H$_2$ molecule, and that this split scales with rotational velocity. The velocity distribution of the nuclei in the original molecule is determined and the thermally populated J = 1 rotational level is found to be the dominant contributor, although asymmetry in the split indicates projectile-induced rotational transitions between M sublevels. These rotation effects influence fragment emission probabilities, thus carrying important consequences for the radiation-induced hydrogen loss and H$_2$ dissociation in the atmospheres or exospheres of planets and moons

Chapter 13 Cambrian echinoderm diversity and palaeobiogeography

<p>The distribution of all known Cambrian echinoderm taxa, encompassing both articulated specimens and taxonomically diagnostic isolated ossicles, is documented for the first time. The database described by 2011 comprises 188 species recorded from 65 formations from around the world. Formations that have yielded articulated echinoderms are unequally distributed in space and time. Only Laurentia and West Gondwana provide reasonably complete records at the resolution of Stage. The review of the biogeographical distributions of the eight major echinoderm clades shows that faunas from Laurentia and Northeast Gondwana (China and Korea) are distinct from those of West Gondwana and Southeast Gondwana (Australia); other regions are too poorly sampled to make firm palaeobiogeographical statements. Analysis of alpha diversity (species per formation) shows that diversity rose initially to Cambrian Stage 5, declined into Guzhangian and Paibian before returning to Stage 5 levels by the end of the Cambrian. This pattern is replicated in Laurentia and West Gondwana. We show that taxonomically diagnostic ossicles found in isolation typically occur significantly earlier than the first articulated specimens of the same taxa and provide important information on the first occurrence and palaeobiogeographical distribution of key taxa, and of the phylum as a whole. </p