L'évolution du domaine hélvétique entre Bauges et Morcles de l'Aptien supérieur au Turonien - séries condensées, phosphorites et circulations océaniques - Alpes occidentales franco-suisses - TEL - Thèses en ligne Accéder directement au contenu
Thèse Année : 1987

L'évolution du domaine hélvétique entre Bauges et Morcles de l'Aptien supérieur au Turonien- séries condensées, phosphorites et circulations océaniques - Alpes occidentales franco-suisses

L'évolution du domaine hélvétique entre Bauges et Morcles de l'Aptien supérieur au Turonien - séries condensées, phosphorites et circulations océaniques - Alpes occidentales franco-suisses

Résumé

During the Mid-Cretaceous thin siliciclastic and carbonate sediments rich in glauconite and phosphorites weredeposited on the Southern part ot the Helvetic Shelf. These deposits overlie the shallow-water UrgonianLimestones, and they are covered by pelagic biomicrites of the Seewen Limestones.New stratigraphic units are proposed for these Mid-Cretaceous deposits:- the Aravis Formation composed of two members, the Bossetan Member (0-40 m, Upper Aptian) and the PlatéMember (0,1-20 m, Late Aptian to Mid-Upper Cenomanian). These are separated by the important late-Aptianunconformity,- the Freu Beds (0-6 m, late Albian to early Coniacian), with facies of the Aravis Fm reworked in an Albian toearly Coniacian micritic matrice.The palaeontology of the ichnofauna and the microbial encrustations are given close attention. The verticalsequence of taphocenoses indicates a deepening trend from Aptian to Turonian.The study of the Albian-Cenomanian deposits gives evidence of sedimentary areas partially characterized byerosionat unconformities. As the fauna indicates a marine environment and no signs of emersion were detected,the erosional surfaces are probably a consequence of a submarine erosion. These discontinuities lie closer whengoing from N-E to S-W, i.e. from distal to proximal part of the shelf. This shinkage is strongest in the Easternpart of the Bauges massif where the top of the Urgonian limestones contains solution cavities measuring severalmeters.A synthetis of the data provides an account of the sedimentary evolution of the Southern Helvetic shelf. Thisevolution began in the early Upper Aptian by the drowning of the Urgonian platform. A mineralised hard-groundbearing phosphatized lag-deposits and ammonites veneers the rudistid-miliolid limeatones.During the Upper Aptian, sandy biocalcarenites containing small phosphoclasts (Aujon beds) were depositedin the restricted area of the "Platé-Dents du Midi". Then, after a period of fine-grained siliciclastic deposition(Borderan Beds), mixed-sedimentation with bio- and siliciclastic components took place (Colombière Beds).Fauna composition and depositional features indicate a storm-influenced environment below faire-weather wavebase. This sedimentation was interrupted by the late-Aptian unconformity (L.A. unconformity), an irregularsurface showing plurikilometer scours cut into the Upper Aptian deposits.From late Upper Aptian to late Upper Albian, phosphate-rich greensands, containing fossiliferous condensedhorizons (Lindars Sandstones) were deposited. The main internal unconformities identify two depositional unitswithin the Lindars Sandstones. In the first, late Aptian to Middle Albian, the sediments show numerous lateralfacies and thickness changes. Some of these changes are partly controlled by the irregular shape of the L.A.unconformity: phosphate-rich condensed deposits with microbial encrustations were deposited on the high parts,while siliciclastic-rich deposits, five to ten times thicker than the phosphate-rich deposits, were accumulatedwithin the erosional scours.The second unit of the Lindars Sandstones is separated from the first one by the Upper Albian unconformitywhich is commonly cut into by the deep Late Upper Albian unconformity. So the Upper Albian deposits aregenerally very condensed and represented only by polyzonal phosphatic conglomerates in which Lower andMiddle Albian faunas may be reworked.During the Lower to Mid-Cenomanian, an hemipelagic carbonate sedimentation with low sedimentation rate(i.e. < 20 cm/year) was established. The deposits are represented by thin glauconitic biomicrites containingphosphatic nodules and mini-stromatolites. These deposits cover or rework the Albian phosphaticconglomerates. Despite the disappearance of the glauconite and the phosphorites during the Mid-Cenomanian,the very low sedimentation rates persisted until the Turonian whereas pelagic biomicrites were deposited.Confrontation of the sedimentary data with the structural data provides a palaeogeographical framework forthe Southern Helvetic Shelf. Two kinds of shelf edge are suggested: the first is represented by a sedimentaryprism prograding to the Ultrahelvetic Basin, the second by a structural shoal probably inherited from the Jurassicperiod of rifting.The genesis of the condensed deposits is approached in the conclusion. The good correlation existingbetween the main unconformities detected in this work and the main sea-level falls of HAQ et al. (1987)suggests an eustatic control of the sedimentary sequences. But this control is not by itself sufficient and it cannotexplain the low sedimentation rates during Albian and Cenomanian as well as the juxtaposition ofcontemporaneous accumulation areas and strong condensation areas.Taking account of the oceanic circulation in the western Tethys, a complementary sedimentary control withsubsurface currents is proposed. These currents could also explain the large erosional scours associated to themain unconformities. Therefore the long axis of these erosional figures more or less parallel to the shelf edgecould indicate the direction of current flow which appears locally influenced by the structural shoals.Finally, during the Mid Cretaceous, the important distribution of condensed deposits on the European andApulian Margins gives a reconstruction of the oceanic subsurface circulation in the Western Tethys flowingfrom the Western Carpathian domain to the Betic Cordillère.
Étude stratigraphique, paléontologique et sédimentologique des dépôts silicoclastiques à glauconie et à phosphorites du Crétacé moyen de la zone delphino-helvétique entre Chambéry (Savoie, France) et Sion (Valais, Suisse). Ces dépôts riches en horizons fossilifères organisés en conglomérats phosphatés polyzonaux s'intercalent entre les calcaires urgoniens à rudistes et les calcaires à plancton de Seewen. La récolte de nombreuse ammonites et l'identification des foraminifères planctoniques en section ont permis la proposition d'un cadre stratigraphique nouveau. L'étude sédimentologique révèle un agencement des dépôts albiens et cénomaniens en aires sédimentaires plus ou moins pérennes contrôlées pour partie par des discontinuités souvent érosives d'origine sous-marine. L'interprétation des taphocénoses et des associations ichnologiques permet de proposer un approfondissement d'un facteur 10 entre l'Aptien supérieur et le Turonien. La confrontation des données sédimentaires avec les travaux structuraux publiés conduit à la proposition d'un cadre paléogéographique dans lequel sont reconnus deux types de bordure pour la plate-forme helvétique: l'une progradante en direction du bassin ultrahelvétique, l'autre structurale déterminée par un haut-fond, héritage probable du rifting jurassique. L'importante distribution des dépôts condensés à minéralisations sous marines sur les marges européenne et apulienne au Crétacé moyen permet de présenter en conclusion une reconstitution possible de la trajectoire des circulations océaniques de subsurface sur la bordure NW de la Téthys.
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tel-00869784 , version 1 (04-10-2013)
tel-00869784 , version 2 (20-11-2014)

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Michel Delamette. L'évolution du domaine hélvétique entre Bauges et Morcles de l'Aptien supérieur au Turonien - séries condensées, phosphorites et circulations océaniques - Alpes occidentales franco-suisses. Stratigraphie. University of Geneva, 1987. Français. ⟨NNT : ⟩. ⟨tel-00869784v2⟩
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