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Micropaléontologie, paléoécologie, et sédimentologie d'une plate-forme carbonatée de la marge passive de la Téthys : l'Urgonien du Vercors septentrional et de la Chartreuse (Alpes occidentales)

Abstract : Located on a passive edge of the Tethys, a carbonated platform (Urgonian platform) grew up during both Barremian and Aptian between the pyrenees and the Swiss Alps. Sedimentological, stratigraphical and micropaleontological studies have been carried out on that part of the platform which been in the Chartreuse and Northern Vercors areas (French subalpine Chains). The detailed sedimentological study of the sediments, of their microfauna, and of their relationships allow a modal of platform to be proposed. At its maximal extension such a platform consisted of the six following ensembles : - the outer slopes,located in the circalittoral stage, featuring clayed-carbonated, muddy and finely sandy sediments, with a muddwelling fauna of Spatangidae, of digging Bilvavia and of small arenaceous Foraminifera - the edge of the platform, very agitated, lying in the infralittoral stage. It is characterized by oolitic deposits, by Cnidaria bearing bioherms and by coarse sandy sediments sheltering a microfauna of largesize Foraminifera (conic Orbitolinids, Reophax) ; - the platform's inner domain involvered more sheltered environment of the infralittoral stage, where medium-coarse, sometimes muddy, always micritized (by endolithic algae), calcareous and sandy sediments with Rudists and Miliolids - the channels, consisting of deprimed zones crioscrossing the platform, drained the sandy-muddy, claying-calcareous sediments. They contained an exceedingly rich, diversified fauna, mostly with Orbitolinids. - the marginolittoral area include the beachs, characterized by coarsegrained calcareous, sandy sediments exhibiting numerous sedimentary patterns (Keystone-vugs, asymetrical cement, etc *** ) and the sea-marshes (mediolittoral and supralittoral stages) where calcareous muds containing nearly one single genus of Foraminifera (Pseudotriloculina) were deposited - the emerged zone, recognized thanks to the early dissolution of the sediments. The paleogeographic evolution of this plateform has been reconstituted by means of the stratigraphie study. First appearing at the base of the Upper Barremian, this platform progressively grew during both Upper Barremian and Bedoulian despite periods of crisis. One such period took place at the top of the Upper Barremian (event of the La Rivière marly limestones), and a second one at the base of the Bedoulian (event of the Orbitolina lower beds). Such crisis, characterized by the arrivaI of clayey sediments, has disturbed the process of carbonates deposition. At the Upper Bedoulian, a final, more important crisis, caused the final stoppage of aIl platform carbonated sedimentation in the area studied. This events corresponds to the transgression deposition of the marIs of the upper Orbitolina beds. The micropaleontological study include description and figuration of 213 species of benthic Foraminifera including 35 new species and 4 new genus. The qualitative and quantitative distribution of these Foraminifera in the Vercors parts of the platform, have both been examined. In some cases, this distribution might be explained by the predominating influence of an ecological factor (e, g. light, temperature, salinity, detritical materials, nature of the substratum, etc *** ). As shown by them stratigraphie distribution, the Orbitolinids can be used for characterizing the lower Barremian, upper Barremian, and Bedoulian substages. As a matter of fact, each sub-stage corresponds to a period of time when the population of Orbitolinids do not vary, whereas the sub-stages limits correspond to more or less extended periods when population are rather sharply renewed.
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Submitted on : Thursday, January 26, 2012 - 10:29:35 AM
Last modification on : Friday, November 6, 2020 - 3:45:12 AM
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  • HAL Id : tel-00662977, version 1

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CNRS | UGA

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Annie Arnaud Vanneau. Micropaléontologie, paléoécologie, et sédimentologie d'une plate-forme carbonatée de la marge passive de la Téthys : l'Urgonien du Vercors septentrional et de la Chartreuse (Alpes occidentales). Stratigraphie. Universite Scientifique et Medicale de Grenoble, 1980. Français. ⟨tel-00662977⟩

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