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Importance des discontinuités dans l'enregistrement sédimentaire de l'urgonien jurassien : micropaléontologie, sédimentologie, minéralogie et stratigraphie séquentielle

Abstract : Hauterivian-Barremian carbonate platform strata of the Jura have been studied by me ans of their micropaleontology, sedimentology and mineralogy. These analyses define 7 sequence boundaries in the Jura deposits, documented by the micropaleontologieal content and emersion markers. Three major palaeoenvironments are recognized : high-energy, outershelf environments, shallow-water bioclastic environments, and transgressive microfacies. Outcrop and thin-section sedimentologic analyses show that in most of cases dissolution and microkarstification occurs below sequence boundaries attesting to periods of emersion. The mineralogical characteristics of the four sections studied show that carbonate is the major component of the whole-rock analysis whereas phyllosilicates are present only in the transgressive microfacies. Repartitions of major minerals like smectite, kaolinite, micas of decarbonated fractions (2-16 um and < 2um) are more abundant in the transgressive facies. Six roentgenofacies were compared to the microfacies. Inner platform environments shows low X-Ray intensities and few minerals whereas more open-marine platform environments show more micas, smectites and kaolinite. Transgressives facies cannot be characterized by any specifie assemblage. For the first time, detrital pyrophyllite and paragonite have been identified in carbonate sediments of the Jura. Micropaleontologieal study consists of identifying benthic foraminifera in all outcrop sections. New foraminifers identified include Nezzazatinella sp. and a new orbitolinid. The orbitolinids are used to form the stratigraphie framework. Two chronostratigraphic schemes were compared. The Valserina lineage proposed by Schroeder [1994] is considered invalid because it is based solely on the evolution of the embryonic apparatus and does not take into account the general shape of these foraminifers. Moreover, no precise stratigraphic calibration has yet been shown. In contrast, orbitolinids in the Vercors area are calibrated by ammonites [Arnaud and Arnaud-Vanneau, 1992] and the same orbitolinid species are founded in the Jura area. Four faunal intervals are recognized from a stratigraphic distribution of micropaleontological data. The first is characterized by the association of Meandrospira favrei and Flabellammina ? sp. ; in the second, Meandrospira favrei and Praedictyorbitolina occur together ; the third is characterized by the abundance of various orbitolinids ; and in the last, Palorbitolina lenticularis is the only benthic foraminifera present. Sequence analyses reveal that sequences Ju1 and Ju2 are Hauterivian in age and are mainly represented by outer-shelf facies. Sequences Ju3 and Ju4 are as yet undated because of the lack of stratigraphic fossils in the lagoonal sediments, however, sequence Ju4 occurs between sediments of Barremian and Hauterivian age. Sequences Ju5 and Ju6 are characterized by various orbitolinids and by rudists in the latter and both are upper Barremian in age. The last sequence, Ju7, interrupts the Urgonian sequence with the deposition of orbitolinid-rich marls that are considered early Aptian in age. In summary, upper Barremian sediments have been highlighted in the Jura Mountain. The limited thickness of Urgonian sediments represents only isolated parts of the total Hauterivian and upper Barremian sequence as documented by the interpretation of diagenetic anomalies, emersion and erosion surfaces, and the micropaleontological data.
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https://tel.archives-ouvertes.fr/tel-00723728
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Submitted on : Monday, August 13, 2012 - 1:21:08 PM
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Marie Caroline Blanc Aletru. Importance des discontinuités dans l'enregistrement sédimentaire de l'urgonien jurassien : micropaléontologie, sédimentologie, minéralogie et stratigraphie séquentielle. Stratigraphie. Université de Neuchâtel, 1995. Français. ⟨tel-00723728⟩

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