Holocene evolution of the Canet-St Nazaire lagoon (Golfe du Lion, France) as determined from a study of sediment properties

The sedimentation in the lagoons of the Golfe du Lion coast (Mediterranean Sea, France) records their post-Holocene evolution. The Canet-St Nazaire lagoon provides an example of such an evolution. Eight cores (ranging from 13.5 to 19.7 m in length) from a transect of the lagoon provide a cross-section of the late Quaternary stratigraphy. The sedimentary sequence comprises mud and sand layers. The two sand formations are identified as parts of two distinct shelf-sedimentary prisms. Stratigraphic analysis defines three stages in the recent, muddy infilling, corresponding to the last divisions of the Holocene. Sedimentation rates of the muds, related to climatic and palaeogeomorphological events, largely determine their geotechnical and geochemical properties. It appears, indeed, to be the most important factor in determining the compressibility index, the saturated bulk density and the organic and water contents of the deposits. Subboreal muds, poorly consolidated and rich in organic matter, correspond to a high rate of deposition. In the Subatlantic muds, in contrast, the sedimentation rate is lower whilst there is a decrease in the organic matter and an increase in the degree of cohesion.