Evolution of Morphologies in River-Connected Canyons: Examples from Northeast Atlantic and Mediterranean Sea

Gaudin, Matthieu¹, Pierre Cirac¹, Serge Berné², Thierry Mulder¹ (1) Université Bordeaux 1, Talence, France (2) Ifremer, Plouzané, France

The canyons of Capbreton and Bourcart are located on the Atlantic Ocean and occiden­tal Mediterranean Sea margins, respectively. The Capbreton canyon extends in the Bay of Biscay (SW France). It is one of the deepest canyons in the world including both inland and submarine canyon. Its head is located 250 m away from the coastline, deeply cutting the continental shelf. The head of the Bourcart canyon, in the Gulf of Lions, lies at the shelf break, 70 km off the coastline, at 110 m water depth.

During lowstands, Capbreton and Bourcart canyons were both connected to rivers which supplied large amount of sediments derived from the erosion of Pyrenees and Alps. Very high resolution seismic data display significant differences in the evolution of the morpholo­gies within these canyons.

The Capbreton canyon morphology evolves owing to perennial, highly erosive, gravi­ty driven mass movements flowing down the axial channel. They induce meander migra­tions and abandonment, slope failures and contribute to the initiation and evolution of “terraces”. These terraces are interpreted as typical sedimentary levees, slump deposits or nested levees.

During sea-level fall, the Bourcart canyon excavated under the combined effect of ero­sion by gravity driven currents and retrogressive failures. The sediment supply during gla­cial maximum leads to an entire or partial filling of the head of the canyon up to 400 m thick. This infill comprises slope failure, sedimentary levees and chaotic facies. With the rising of the sea level and river disconnection, direct supplies ended, and the present mor­phology is preserved.