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Morphologie, évolution latérale et signification géodynamique des discontinuités sédimentaires. Exemple de la marge Ouest du Bassin du Sud-Est (France)

Abstract : In the Lodève region (« Caussenard High », southwestern margin of the South-East Basin), liasic deposits are composed of peritidal dolomitic facies of Hettangian age, overlained by Sinemurian shallow water carbonate ramp deposits, capped by a major hardground. A major change in sedimentation then occurred, with the development of black marls typical of deep water conditions. Detailed descriptions of eleven field sections, including a precise analysis of the sedimentary facies and of their organisation has enabled us to show that this sinemurian series can be divided in two sedimentary groups, corresponding to two types of shallow platforms.
The first group is mainly composed of low-energy facies (tidal flat and lagoonal facies). The second one is characterized by the absence of dolomitic tidal flat facies and by the increase of opened and external facies. The facies partitioning has shown that lateral facies changes occur rapidly over very short distances (100s of meter). Moreover, no consistent landward or seaward direction seems to be indicated by the pattern of facies changes. It is thus impossible to rationalise the studied facies within a traditional shoreline model as previously proposed for this region. We suggest a new carbonate platform model, based on the observations presented in this paper (Fig.11). The Lodève region platform is hence envisaged to have been a “mosaic” of shallow, intertidal to supratidal “islands” between which restricted subtidal or open conditions could coexist over very short distances (hundreds of meter). The local tectonic setting, characterized by numerous synsedimentary faults (SW-NE and NW-SE), seems to locally control the sedimentation, by defining parts of the basement on which shallow, peritidal facies are able to develop, and subsiding parts where subtidal facies occur.
Numerous sedimentary discontinuities developed in this particular setting. On the basis of several universally applicable criteria (geometry, morphology biological activity, mineralization, early diagenesis...), four groups of surfaces have been identified : 1) emersion surfaces (karstic surfaces, palaeosoils, desiccation cracks) ; 2) condensation surfaces (softgrounds, firmgrounds, different types of hardgrounds) ; 3) Submarine erosion surface ; 4) “polyphase surfaces” which exhibit emersion, erosion and condensation criteria. Detailed sedimentological study has enabled us to show the lateral continuity of these surfaces, and their morphological variations. The vertical distribution of surface types in the studied sections has also been discussed, and compared with the sequential framework.
Correlations between “Caussenard high”, “ardèche” paleomargin of the subalpine basin” and Sologne paleomargin of the Paris basin, have enabled us to distinguish major discontinuities. Lotharingian hardgound of the “Caussenard high” series seems to record a regional tectonic event, known as Lotharingian crisis and observed in the Causses Basin, the Quercy or the Paris Basin. The Hettangian-Sinemurian transition, well-known in the Paris Basin, could be more enigmatic in the Caussenard High series, consisting of a repetition of karstic surfaces in the dolomitic series.
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Submitted on : Friday, July 8, 2005 - 3:54:23 PM
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Youri Hamon. Morphologie, évolution latérale et signification géodynamique des discontinuités sédimentaires. Exemple de la marge Ouest du Bassin du Sud-Est (France). Minéralogie. Université Montpellier II - Sciences et Techniques du Languedoc, 2004. Français. ⟨tel-00009709⟩

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