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.

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