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Le Gargasien de Gargas (Vaucluse, SE de la France) : synthèse des données de terrain et révision de la microfaune de foraminifères et d'ostracodes

Carnets de Geologie (Impact Factor: 0.72). 04/2010; DOI: 10.4267/2042/30051
Source: OAI

ABSTRACT Because of urbanization the section at Gargas, historical stratotype of the Aptian stage and of the Gargasian substage, is no longer accessible. However, samples taken there in 1966 by one of us make feasible a reinterpretation of this series in the light of taxonomic and biostratigraphic data from a recent study of Aptian microfaunas from La Tuilière area, some 5 km away. This new micropaleontologic interpretation shows that the historical section is truncated at its base : the lower (not lowermost) Gargasian is in quasi-direct contact with Urgonian limestones ; the uppermost (marly) Bedoulian, known to be present westward towards Le Chêne as well as at La Tuilière, is absent in the stratotype outcrop. In the samples from the stratotypic succession we have identified (bottom upward): • the upper part of the Praehedbergella luterbacheri zone and the Globigerinelloides ferreolensis zone, • the G. barri zone, • the beginning of the G. algerianus zone, i.e. these zones comprise the major part of the lower Gargasian, the middle Gargasian and the transition to the upper Gargasian. The upper part of the G. barri zone and the passage to the G. algerianus zone are in sandy marls of which the upper terms were said to have yielded ammonites of the "Clansayes horizon" (sensu ante). The microfauna does not resolve this apparent discrepancy, for its state of preservation degrades rapidly in the detrital levels, to the point of quasi-disappearance in sands overlying the Clansayesian auct. A comparison with the sector of Banon, only some 10 km away, led us to suggest that these sands cropping out at top of the Gargas Hill might be of Late Albian-Vraconnian age. La coupe de Gargas, stratotype historique de l'étage aptien et du sous-étage gargasien, n'est désormais plus du tout accessible à l'observation par suite de l'urbanisation. À partir d'un ensemble d'échantillons prélevés en 1966 par l'un d'entre nous, nous avons pu cependant la ré-interpréter à la lumière des résultats d'ordre taxinomique et biostratigraphique acquis récemment sur la base de l'étude des microfaunes de l'Aptien du secteur de La Tuilière, distant d'environ 5 km. Cette nouvelle interprétation micropaléontologique montre que la coupe historique apparaît en fait tronquée à sa base, le Gargasien inférieur (non basal) s'y trouvant en contact quasi-direct avec l'Urgonien ; le Bédoulien terminal, marneux, connu plus à l'Ouest (vers le Chêne) ainsi qu'à La Tuilière, manque ici à l'affleurement. En effet, dans nos échantillons de la série stratotypique nous avons pu mettre successivement en évidence (de bas en haut) : • la partie supérieure de la zone à Praehedbergella luterbacheri et la zone à Globigerinelloides ferreolensis, • la zone à G. barri, • le tout début de la zone à G. algerianus, soit la majeure partie du Gargasien inférieur, le Gargasien moyen et le passage au Gargasien supérieur. La partie supérieure de la zone à G. barri et le passage à la zone à G. algerianus correspondent à une séquence marquée par l'instauration d'un faciès marno-sableux, dont les termes supérieurs ont historiquement fourni des ammonites de l' "horizon de Clansayes" (sensu ante). Cette donnée ne peut être confortée par les microfaunes, dont la conservation se dégrade rapidement dans les niveaux détritiques, au point même qu'elles vont quasiment jusqu'à disparaître dans les sables et grès sus-jacents au Clansayésien auct. Par analogie avec le secteur de Banon, distant d'une dizaine de kilomètres, ces termes détritiques du sommet de la Colline de Gargas pourraient appartenir à l'Albien supérieur-Vraconnien.

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    ABSTRACT: A revised lithostratigraphy is given for parts of the Atherfield Clay and Ferruginous Sands Formations (Lower-Upper Aptian) of the Lower Greensand Group of the standard section in Chale Bay, Isle of Wight, retaining in essence the classic nomenclature of Fitton. Changes in zonal and subzonal boundaries are proposed on the basis of new collections of ammonites obtainedin situ. The Upper Lobster Beds Member of the Atherfield Clay is detached from the Subzone ofDeshayesites callidiscusand placed in a separate Subzone ofDeshayesites annelidusat the top of the Zone ofD. forbesi. The influx ofMegatyloceras, Cheloniceras, PseudosaynellaandSinzoviaat this level has palaeogeographic implications, pointing to the opening of a marine link with the Paris Basin and confirming the Upper Lobster Beds as a transgressive horizon. The upper part of the Scaphites Beds Member of the Ferruginous Sands, previously included in thedeshayseiZone, is transferred to the Subzone ofDufrenoyia transitoria, at the base of the Zone ofTropaeum(T.)bowerbanki, on the strength of the occurrence therein of species ofTropaeumandDufrenoyia.Tropaeumis not present below thebowerbankiZone as now defined; its primitive ancyloceratoid (hooked) forms such asT. (T.)hillsiihave limited biochronological value, persisting alongside the criocone (spiral) species of thebowerbankiZone and giving rise to a local horizon ofT. (T.)pseudophillsiat the bottom of the Upper Crioceras Beds (basal Upper Aptian). A more precise delimitation of the Walpen Clay & Sands from the Upper Crioceras Beds is made and the junction of these two members of the Ferruginous Sands is reaffirmed as the Lower/Upper Aptian boundary, coinciding with the junction of the Zones ofT. (T.)bowerbankiandEpicheloniceras martiniodes. Bed-by-bed collecting across the boundary shows thatEpichelonicerascan no longer be taken as the prime biostratigraphical marker for the base of the Upper Aptian; greater significance is given to the extinction ofDufrenoyia. The sources of the genusVectisitesand some species ofAmmonitocerasandCaspianites, hitherto fixed only vaguely in the column, are pinpointed in themartinioidesZone. The horizon ofTropaeum(T.)drewiis confirmed as high in thebowerbankiZone (meyendorffiSubzone) and the validity of a ‘drewiZone’ at the base of the German Upper Aptian (=base of Middle Aptian, Kemper, 1995) is questioned. In a review of the Lower/Upper Aptian boundary, it is suggested that the German stratum be called the Horizon ofCicatrites?tenuinodosusand placed tentatively at the top of the Lower Aptian. These and other changes in ammonite nomenclature are explained in the systematic notes.
    Cretaceous Research - CRETACEOUS RES. 01/1998; 19(3):511-535.

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