Small rivers contribution to the Quaternary evolution of a Mediterranean littoral system: The western gulf of Lion, France

Along the Mediterranean coasts and the Atlantic French coast, former and actual research programs focused on major river systems, estuaries and deltas, characterized by a last Glacial relative sea level lowstand and incision and a well developed sedimentary incised valley infilling deposited during the following sea level rise and highstand. This paper presents the preliminary results of a program focused on a particular area of the Gulf of Lion coast with a thin sedimentary cover over the substratum and only minor rivers with non-apparent and important sedimentary contribution during the Late Quaternary. The results show that in this area the best Pliocene to Actual sedimentary record is preserved. The paper rests on the analysis of an extensive database of recent high resolution and very high resolution seismic reflection lines and previously published core data. Seismic data show that a major complex of paleovalleys connected with the Orb, Aude and Agly rivers is preserved on the inner shelf and adjacent coastal plain. On the inner shelf, the separated incised valleys merged in a unique broad and shoreparallel incision dipping southward. At the southward extremity, the incision turns eastward and seaward. The basal surface of this incision extends seawards under six to seven Late Quaternary depositional sequences preserved on the mid and outer shelf. The infilling of the complex of paleovalleys is characterized by aggrading deposits attributed to periods of relative sea level rise (transgressive systems tracts), organized into several subunits bounded by internal discontinuities locally deeply incising. The discontinuities are amalgamated surfaces, including successive sequence boundaries (indicative of phases of relative sea level falls) merged with transgressive surfaces (indistinct tidal and wave ravinement surfaces). The subunits are the part of Late Quaternary depositional sequences preserved in estuarine environments. They are the lateral equivalents of landward fluvial terraces and seaward coastal and prodeltaïc deposits on the shelf. Using borehole dataset, the underlying and eroded deposits below the basal unconformity are correlated with Pliocene deposits outcropping landward in the hinterland. The top of the incised valleys complex is capped by the last Glacial lowstand surface of erosion (18 ky B.P.) reworked by the postglacial transgressive surface (TS), dissociated near the shoreline into a tidal and a wave ravinement surfaces. Above the TS, the very high resolution (VHR) seismic data in the lagoons, the tidal channels and cores, reveal in details the stratigraphic architecture of the deposits. At the base, a small wedge constitutes the postglacial transgressive systems tract (TST) locally thinning in the areas distant of the sediment point sources. The TST is capped by a flat surface of wave reworking (maximum flooding surface or mfs) prolongating under the Leucate lagoon and merging offshore at the seafloor. Boreholes and VHR seismic lines trough the coastal barrier and in the lagoon show that the shoreline probably migrated far landward at the end of the transgression. When the rate of sea level rise decreased strongly, the shoreline migrated seaward and prograding and aggrading sandy material, with landward muddy lateral equivalent facies, deposited early highstand systems tract (HST) above the MFS. Offshore, fine material deposited as a sigmoidal blanket of mud originating in part from the north-east and Rhône river under oceanic circulation (equivalent to a subaqueous prodelta). Subsequently, the modern beach barrier built up by wave reworking of the early HST. This new study of the western part of the Gulf of Lion inner shelf and littoral illustrates an incised valley complex and thus presents the best preserved example of the sedimentary record of the effects of the relative sea level changes during the Pliocene to Actual period. For the first time, the land to sea transition is preserved and the Late Quaternary depositional sequences are in a great part observed. The last post Glacial deposits present a simplified but very different organisation compared to a record front of the adjacent Rhône river. Consequently a synthesis is now possible.