Resumen de Very high resolution seismic investigation of the sedimentary infill of the Leucate lagoon (Aude and Pyrenees-Orientales - SE France)
Raphaël Certain, Bernadette Tessier, Thierry Courp, Jean Paul Barusseauet, Henri Pauc
- español
A very high resolution seismic investigation has for the first time allowed the imaging of the sedimentary infill of a Mediterranean lagoon. The Leucate lagoon is part of a lagoon system located along the shore of the western Gulf of Lions, from the Rhone delta to the Spanish border. These lagoons are separated and protected from the sea by sandy barriers, also called lidos, which are the result of a process of shore regularization by waves.
The seismic data, obtained by using a boomer-Seistec, coupled with lithological and radiocarbon data [Martin, 1978] previously collected from cores, have revealed three main sedimentary formations : The basal formation represents the substratum of the lagoon and the middle and upper formations its infill. The basal formation displays a very uniform seismic facies with reflectors almost constantly dipping towards the sea. It comprises conglomeratic sediments and is interpreted as a progradating fluvio-deltaïc formation. Its upper surface is erosional and is locally deeply incised. In the shallowest parts of the lagoon, where the basal formation almost emerges, its upper part is reworked by modern processes into multiple cut-and-fill structures. The middle formation overlaps the basal formation, and constitutes the main depositional unit of the lagoon fill (up to 20 ms twtt thick). The seismic facies are variable and correspond to sand and clay sediments deposited under fluvio-lagoonal to lagoonal and marine conditions. The upper formation represents the upper part of the infill. It rests above the middle formation through a conformable surface, locally slightly erosional, and overlaps the basal formation along the western rim and in the shallowest parts of the basin. The thickness of this upper formation does not exceed 3mst wtt. It mainly consists of clay sediments of lagoonal origin. The main characteristic of this upper formation is a thin sole of very dense sand at the base. This bed is also a remarkable seismic reflector, and is interpreted as resulting from the maximum marine flooding of the system. This occurs before the beginning of the barrier construction, and the progressive closure of the lagoon. Another remarkable aspect of the upper formation is the simultaneity of its basal part with the lido construction. In this upper unit, the seismic data allow the imaging of the lateral passing between the planar bedded sediments of the infill, with the sigmoidal beds representing washover fans that construct the lagoonal side of the lido. The uppermost part of the formation represents the final and present-day stage of the lagoonal infill since the final closure by the barrier.
Dating, performed on cored sediments, allow the sediments of the basal formation to be assigned to the Middle Pleistocene (with no more precision). The erosion of the top of the basal formation is interpreted as fluvial incision during the last sea level fall. The lagoon infilling is of Holocene age and comprises two stages : the first and main stage corresponds to fluvio- to marino-lagoonal sedimentation, and occurred before 6,000 B.P. The second corresponds to the recent to modern infilling that began around 4,000 B.P. with the construction of the lido and the closure of the lagoon. The two stages are clearly separated by a period of maximum marine flooding. The thickness of the lagoon fill is relatively limited, probably no more than 20 m.
- français
Une prospection par sismique très haute résolution a permis pour la première fois de visualiser le remplissage sédimentaire de la lagune de Leucate. Caractéristiques de nombreuses portions du littoral méditerranéen, les lagunes sont séparées et protégées de la mer par une barrière sableuse, ou lido, qui résulte d'une régularisation du trait de côte par les houles. Les données de sismique, couplées à des données lithologiques recueillies en sondages [Martin, 1978], indiquent l'existence de trois formations principales : la formation basale formant le substratum de la lagune, les formations intermédiaire et terminale constituant son remplissage. La formation basale correspond à des dépôts conglomératiques d'origine fluvio-deltaïque. Son toit est une surface d'érosion, localement profondément incisée. La formation intermédiaire repose en onlap sur la formation basale et constitue l'unité principale de remplissage de la lagune. Les sé-diments sableux à vaseux qui la composent sont d'origine fluvio-lagunaire à laguno-marin. La formation terminale re-présente la phase de remplissage récente à actuelle, dominée par une sédimentation vaseuse. D'une épaisseur réduite (< 2 m), la formation terminale est marquée à sa base par un niveau sableux très dense, remarquable en sismique, inter-prété comme le résultat de l'inondation marine maximum du système avant que la construction du lido débute et que se ferme progressivement la lagune. Les données de datations issues des sondages permettent d'attribuer un âge pléistocène moyen au substratum de la lagune, incisé au cours de la dernière chute du niveau marin. Le remplissage se produit en deux phases : une étape principale laguno-marine (ou laguno-fluviatile) avant 6 000 B.P. (formation intermédiaire), une étape récente à actuelle depuis 4 000 B.P. marquée par la mise en place du lido (formation terminale). L'ensemble des données révèlent que l'épaisseur de remplissage de la lagune reste modeste, n'excédant probablement pas plus de 20 m, et que l'essentiel de ce remplissage s'est produit au cours des premiers stades de l'Holocène.
