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La Formation des Sables Fauves: dynamique sédimentaire au Miocène moyen et évolution morpho-structurale de l’Aquitaine (SW France) durant le Néogène

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Dans la partie centrale de l’Aquitaine, les dépôts marins du Miocène moyen correspondent à des sables et des grès calcaires, témoins d’environnements très littoraux et regroupés sous le terme de Formation des Sables Fauves. L’étude des Sables Fauves révèle deux unités de dépôt aux lithofaciès proches. Le Membre de Peyrecrechen (Langhien), daté par Praeorbulina glomerosa, représente une séquence de dépôt de troisième ordre (cycle Bur5/Lan1). Le Membre de Matilon (Serravallien), daté par le groupe Orbulina, illustre le cycle suivant (Lan2/Ser1). Les conditions climatiques sont de type subtropical, ayant connu une dégradation par rapport au Miocène inférieur. Une activité orogénique se manifeste durant le Miocène moyen; la déformation est reliée à la reprise de l’halocinèse dans le sud de l’Aquitaine. La propagation des contraintes entraîne la réactivation de grandes structures anticlinales ainsi que la mise en place de dômes sédimentaires formés par les mouvements halocinétiques. Durant le Langhien, la mer occupe l’ensemble du domaine centre-aquitain; au Serravallien, le domaine marin est restreint aux secteurs les plus occidentaux de la région. Du sud vers le nord, l’unité serravallienne s’emboîte progressivement dans l’unité langhienne et dans le substratum. La chute du niveau marin relatif au passage Langhien / Serravallien entraîne une importante érosion (limite de séquence de type 1 sensu Vail), puis sa remontée commande l’encaissement du Membre de Matilon dans la partie septentrionale. L’utilisation des Sables Fauves comme marqueur du Miocène moyen permet de révéler plusieurs phases de déformation durant le Néogène et d’identifier les structures tectoniques impliquées.

Abstract.

The middle Miocene marks the last important transgression of the Atlantic Ocean in Aquitaine. The extent of this event is recorded by the occurrence of marine deposits, thus forming the so-called “Lectoure Gulf”. In the easternmost part, the deposits consist of yellow or orange sands and calcareous sandstones, sometimes with abundant macrofauna, known as “Formation des Sables Fauves”. They represent very coastal environments, in which microfauna is seldom, which in turn makes difficult getting precise datings. Recent works show that the Sables Fauves are heterochronous. A main objective is to differentiate deposits from Langhian and from Serravallian, and to study the recognition and the definition of their placing. The studied area, in the middle of the Aquitaine basin, covers the Gers, Landes, and Lot-et-Garonne districts; it corresponds to the easternmost record of Sables Fauves. Our study reveals two sedimentary units, with close lithofacies: the Peyrecrechen Member (Langhian), and the Matilon Member (Serravallian). The former, biostratigraphically dated with Praeorbulina glomerosa, represents a trangressive and a highstand system tracts from a third order depositional sequence recognized as Bur 5/Lan 1 cycle. The latter, biostratigraphically dated with Orbulina group, mainly representst the trangressive and highstand system tracts from the following sequence, known as Lan 2/Ser 1 cycle. In the northern part of the studied area, the land mammal fossil record indicates that the upper part of Serravallian deposits are younger and could correspond to cycles between Ser2 and Ser4/Tor1. Climatic conditions appear relatively steady during the middle Miocene and seem to be subtropical, though cooler than the early Miocene climatic climax.

Contemporary orogenic activity implies distensive movements in structures related to the “Flexure Celtaquitaine”. This flexuration involves both a relative falling of the southern part of the basin of up to 30 m and the superposition of Peyrecrechen and Matilon Members. This may be linked to a new halocinetic activity in south Aquitaine. The propagation of the deformation due to rifting in the Lion Gulf during the middle Miocene involves a reactivation of main anticline structures and the subsequent formation of small sedimentary domes. During the Langhian, the sea invades widely to the east, forming the “Lectoure Gulf”. Later, the Serravallian transgression takes place only on the western part, leading to a southern little gulf (the “Nogaro Gulf”) while embanked deposits in valley fillings are the only indicators of this transgression in the Northern part of the area. Marine deposits are in normal stratigraphic succession in the South (Nogaro) whereas Serravallian deposits appear to be embanked in the North (Gabarret). From North to South, the Serravallian unit is progressively embanked in the Langhian unit and even in the early Miocene continental substratum. Geometry is mainly controlled by eustacy and tectonics. Eustatic changes involve two identified cycles. The relative sea level falls (ca. 35 m) at the latest Langhian-earliest Serravallian, while a new rising (ca. 20 m) induces the embankment of the Matilon Member in the Northern zone. Using the Sables Fauves as markers of the middle Miocene allow i) to distinguish different tectonic events and ii) to identify the concerned structures.

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Correspondence to Philippe Gardère.

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Manuscrit reçu le 1 juin 2004 Révision acceptée le 29 mars 2005

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Gardère, P. La Formation des Sables Fauves: dynamique sédimentaire au Miocène moyen et évolution morpho-structurale de l’Aquitaine (SW France) durant le Néogène. Eclogae geol. Helv. 98, 201–217 (2005). https://doi.org/10.1007/s00015-005-1160-y

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