16 research outputs found
Late Ordovician (postâSardic) rifting branches in the North Gondwanan Montagne Noire and Mouthoumet massifs of southern France
Upper OrdovicianâLower Devonian rocks of the CabriĂšres klippes (southern Montagne Noire) and the Mouthoumet massif in southern France rest paraconformably or with angular discordance on CambrianâLower Ordovician strata. Neither MiddleâOrdovician volcanism nor associated metamorphism is recorded, and the subsequent Middle Ordovician stratigraphic gap is related to the Sardic phase. Upper Ordovician sedimentation started in the rifting branches of CabriĂšres and Mouthoumet with deposition of basaltic lava flows and lahar deposits (Roque de Bandies and Villerouge formations) of continental tholeiite signature (CT), indicative of continental fracturing.
The infill of both rifting branches followed with the onset of: (1) Katian (Ka1âKa2) conglomerates and sandstones (Glauzy and Gascagne formations), which have yielded a new brachiopod assemblage representative of the Svobodaina havliceki Community; (2) Katian (Ka2âKa4) limestones, marlstones and shales with carbonate nodules, reflecting development of bryozoan-echinoderm meadows with elements of the Nicolella Community (Gabian and Montjoi formations); and (3) the Hirnantian Marmairane Formation in the Mouthoumet massif that has yielded a rich and diverse fossil association representative of the pandemic Hirnantia Fauna. The sealing of the subaerial palaeorelief generated during the Sardic phase is related to Silurian and Early Devonian transgressions leading to onlapping patterns and the record of high-angle discordances.Research was funded by projects CGL2010-39417, CGL2012-39471 and CGL2013-48877-P from Spanish MINECO.Peer reviewe
Cambrianâearly Ordovician volcanism across the South Armorican and Occitan domains of the Variscan Belt in France: Continental break-up and rifting of the northern Gondwana margin
The Cambrianâlower Ordovician volcanic units of the South Armorican and Occitan domains are analysed in a tectonostratigraphic survey of the French Variscan Belt. The South Armorican lavas consist of continental tholeiites in middle CambrianâFurongian sequences related to continental break-up. A significant volcanic activity occurred in the Tremadocian, dominated by crustal melted rhyolitic lavas and initial rifting tholeiites. The Occitan lavas are distributed into five volcanic phases: (1) basal Cambrian rhyolites, (2) upper lower Cambrian Mg-rich tholeiites close to N-MORBs but crustal contaminated, (3) upper lowerâmiddle Cambrian continental tholeiites, (4) Tremadocian rhyolites, and (5) upper lower Ordovician initial rift tholeiites. A rifting event linked to asthenosphere upwelling took place in the late early Cambrian but did not evolve. It renewed in the Tremadocian with abundant crustal melting due to underplating of mixed asthenospheric and lithospheric magmas. This main tectono-magmatic continental rift is termed the âTremadocian Tectonic Beltâ underlined by a chain of rhyolitic volcanoes from Occitan and South Armorican domains to Central Iberia. It evolved with the setting of syn-rift coarse siliciclastic deposits overlain by post-rift deep water shales in a suite of sedimentary basins that forecasted the South ArmoricanâMedio-European Ocean as a part of the Palaeotethys Ocean.This research was funded by project CGL2013-48877-P from Spanish MINECO.Peer reviewe
New Early Cambrian bivalved arthropods from southern France
International audienceThe Lower Cambrian Pardailhan Formation of the Montagne Noire (Southern France) has yielded a diverse fossil assemblage including bivalved arthropods (the bradoriids Monceretia erisylvia gen. et sp. nov., Cambria danvizcainia sp. nov. and Matthoria? sp., together with Isoxys sp.) associated with trilobites, hyolithids, inarticulate brachiopods, sponge spicules, ichnofossils and chancelloriid sclerites. This assemblage provides new evidence about the biodiversity of Early Cambrian marine communities in palaeocontinentalGondwana. The bradoriids are Cambriidae, a familywith widespread distribution in offshore shelf marine environments during Early Cambrian times. The present study confirms the presence of cambriids within a subtropical latitudinal belt that encompasses Laurentia, Siberia and the Gondwanan margins from Southern France to South China. Although knowledge of the distribution of fossil cambriids is patchy, at the generic level they appear to be provincial, with Petrianna from Laurentia, Shangsiella and Auriculatella from South China, Cambria from Siberia and Gondwana (Armorica), and Monceretia gen nov. from Gondwana (Armorica). The presence of Isoxys in the Montagne-Noire confirms the cosmopolitan distribution of this genus in the Early and Middle Cambrian tropics. Cambriid bradoriids occupy a biostratigraphically narrow time interval, probably equating to part of the Atdabanian and Botomian stages of Russian terminology. Their presence in the Pardailhan Formation supports the notion of a Botomian age, determined from archaeocyathan evidence. The North American bradoriid genus Matthoria, also possibly present in the Pardailhan Formation, is reassigned to the Cambriidae
Découverte de nouveaux sites à préservation exceptionnelle dans l'Ordovicien du Massif armoricain et de la Montagne Noire (France) : une approche prédictive
International audienc
Late Ordovician (post-Sardic) rifting branches in the North Gondwanan Montagne Noire and Mouthoumet massifs of southern France
International audienceUpper OrdovicianâLower Devonian rocks of the CabriĂšres klippes (southern Montagne Noire) and the Mouthoumet massif in southern France rest paraconformably or with angular discordance on CambrianâLower Ordovician strata. Neither Middle-Ordovician volcanism nor associated metamorphism is recorded, and the subsequent Middle-Ordovician stratigraphic gap is related to the Sardic phase. Upper Ordovician sedimentation started in the rifting branches of CabriĂšres and Mouthoumet with deposition of basaltic lava flows and lahar deposits (Roque de Bandies and Villerouge formations) of continental tholeiite signature (CT), indicative of continental fracturing.The infill of both rifting branches followed with the onset of (1) Katian (Ka1âKa2) conglomerates and sandstones (Glauzy and Gascagne formations), which have yielded a new brachiopod assemblage representative of the Svobodaina havliceki Community; (2) Katian (Ka2âKa4) limestones, marlstones, and shales with carbonate nodules, reflecting development of bryozoan-echinoderm meadows with elements of the Nicolella Community (Gabian and Montjoi formations); and (3) the Hirnantian Marmairane Formation in the Mouthoumet massif that has yielded a rich and diverse fossil association representative of the pandemic Hirnantia Fauna. The sealing of the subaerial palaeorelief generated during the Sardic phase is related to Silurian and Early Devonian transgressions leading to onlapping patterns and the record of high-angle discordances
Revisiting the Cambrian of Miquelon Island, a French prolongation of Newfoundlandâs Burin Peninsula
Trabajo presentado en el International Symposium on the Ediacaran-Cambrian Transition (ISECT), celebrado en St. John's, Newfoundlan (CanadĂĄ), del 15 al 29 de junio de 2017The recent publication of the St. Pierre-et-Miquelon geological map by the French Geological
Survey (BRGM) has highlighted some of the stratigraphic unconformities and uncertain
chronostratigraphic assignations that characterize the southwestern prolongation of the
Newfoundlandâs Burin Peninsula. The northeastern colourful coastal creeks of Langlade (or
Little Miquelon), connected across a slender sand isthmus to the northern Grand Miquelon,
allows a complete reconstruction of the Cambrian stratigraphic framework, in ascending
order: (i) the heterolithic Chapel Island Formation, composed of at least 150 m of variegated
sandstones, shales and isolated limestone nodules and centimetre-thick interbeds, which have
yielded skeletonized microfossils such as Aldanella attleborensis, âLadathecaâ cylindrica,
Watsonella crossbyi and undetermined hyoliths (Anse Ă la Gazelle), characteristic of the
Cambrian Age 2; (ii) the sandstone and quartzite-dominated Random Formation, about 130 m
thick (Anse and Cap Ă Ross); (iii) the Brigus Formation, 15 m thick and exclusively
recognized at the Cap Percé islet, comprises a centimetre-thick polymictic lag capped by a
heterolithic and variegated succession of shales, sandstones and centimetre-thick limestone
interbeds; (iv) the Chamberlainâs Brook Formation, 50 m thick and dominated by green
shales and subsidiary reddish and blackish shales and centimetre-thick limestone nodules
(Anse aux Soldats); and (v) the Manuels River Formation, more than 80 m thick and
composed of homogeneous black shales bearing centimetre- to metre-scale limestone nodules
and concretions infested with trilobites (Anse aux Soldats). The lowermost nodules of the
latter formation have yielded a trilobite assemblage dominated by Ptychagnostus atavus and
subsidiary Bailiaspis cf. howelli, Clarella gronwalli, Eodiscus punctatus, Hypagnostus
parvifrons, Peronopsis fallax, Phalacroma scanicum, Pleuroctenium granulatum and
Ptychagnostus ciceroides, representative of the Paradoxides davidis Zone (included in the
late Drumian Ptychagnostus punctuosus Zone), whereas the uppermost nodules contain
scattered olenid sclerites (Protopeltura sp.) of Furongian age.
The base and top of the Brigus Formation are represented by erosive unconformities linked to
gaps broadly related to a part of Cambrian Epoch 2 and the Cambrian Epoch 2-lower
Drumian. Both gaps are correlatable throughout New Brunswick and the marginal platform
of the southern Burin Peninsula.Peer reviewe
Cambrianâearly Ordovician volcanism across the South Armorican and Occitan domains of the Variscan Belt in France: Continental break-up and rifting of the northern Gondwana margin
International audienceThe Cambrianâlower Ordovician volcanic units of the South Armorican and Occitan domains are analysed in a tectonostratigraphic survey of the French Variscan Belt. The South Armorican lavas consist of continental tholeiites in middle CambrianâFurongian sequences related to continental break-up. A significant volcanic activity occurred in the Tremadocian, dominated by crustal melted rhyolitic lavas and initial rifting tholeiites. The Occitan lavas are distributed into five volcanic phases: (1) basal Cambrian rhyolites, (2) upper lower Cambrian Mg-rich tholeiites close to N-MORBs but crustal contaminated, (3) upper lowerâmiddle Cambrian continental tholeiites, (4) Tremadocian rhyolites, and (5) upper lower Ordovician initial rift tholeiites. A rifting event linked to asthenosphere upwelling took place in the late early Cambrian but did not evolve. It renewed in the Tremadocian with abundant crustal melting due to underplating of mixed asthenospheric and lithospheric magmas. This main tectono-magmatic continental rift is termed the âTremadocian Tectonic Beltâ underlined by a chain of rhyolitic volcanoes from Occitan and South Armorican domains to Central Iberia. It evolved with the setting of syn-rift coarse siliciclastic deposits overlain by post-rift deep water shales in a suite of sedimentary basins that forecasted the South ArmoricanâMedio-European Ocean as a part of the Palaeotethys Ocean
UâPb laser ablation ICP-MS zircon dating across the EdiacaranâCambrian transition of the Montagne Noire, southern France
International audienceUâPb laser ablation inductively coupled plasma mass spectrometry was used for dating zircon grains extracted from four sedimentary and volcano sedimentary rocks of the Montagne Noire encompassing the presumed EdiacaranâCambrian boundary interval. Magmatic zircon from two samples from the basal and middle parts of the Rivernous Formation (a rhyolitic tuff) were deposited at 542.5 ± 1 Ma and 537.1 ± 2.5 Ma, bracketing the 541 Ma age presently admitted as being at the EdiacaranâCambrian boundary. In addition, a piece of sandstone from the underlying Rivernous Formation containing mostly euhedral zircon grains, suggesting proximal magmatic sources, yields Neoproterozoic dates ranging from 574 Ma to 1 Ga, and subsidiary older dates from 1.25 to 2.75 Ga. Another piece of sandstone from the overlying Marcory Formation yielded mostly rounded zircon grains probably issued from more remote areas, with a large spectrum dominated by Neoproterozoic dates as well as older ages up to 3.2 Ga. A comparison of both kinds of sandstone suggests a significant change in provenance, changing from a restricted source area during the Ediacaran to a much larger source domain during the Cambrian Epoch 2 that recorded contributions from different cratons of Gondwana
The Ordovician from France and neighbouring areas of Belgium and Germany
Abstract 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 (RĂŒgen). The remaining investigated areas were part of Gondwana in the Ordovician. The Armorican Massif shares with the Iberian Peninsula a FurongianâEarly Ordovician gap (Toledanian or Norman gap), and a continuous MidâLate 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 âSardic gapâ. In the Ordovician, the Maures Massif probably belonged to the same Sardo-Occitan domain. The Vosges and Schwarzwald massifs display comparable, poorly preserved Ordovician successions, suggesting affinities with the TeplĂĄ-Barrandian and/or Moldanubian zones of Central Europe