Geomorphological and hydrodynamic forcing of sedimentary bedforms - Example of Gulf of Morbihan (South Brittany, Bay of Biscay)

The late Quaternary sedimentary architecture of the Golfe de Morbihan, a bedrock-controlled lagoonal basin with low sediment supply, was studied by applying an approach based on high-resolution seismic stratigraphy. The present-day environment is characterized by a wave-dominated regime and also shows some local tidal influence. Indeed, the Gulf of Morbihan is connected to the Bay of Quiberon by a narrow tidal pass (rocky sill) where the surface tidal currents reach velocities of up to 2.2 m/s during the flood and 1.8 m/s during the ebb. Wave influence is reduced and confined in the eastern part (inner zone) of the gulf. Flood and ebb tidal currents constrained by basement highs cause a separation between gravel, sand and mud particles. The channel-fill architecture is strongly dependent on tidal current velocity and the geomorphological setting. In the western part of the gulf (outer zone), near the tidal pass, erosion and sediment transport processes are enhanced along the axis of the tidal channel, because of the increased velocity of tidal currents. The outer tidal channel is scoured and the strong currents transport sand (lithic and biolithic fragments) and gravels, which are later deposited along the channel margins. In the central part of the gulf, the hydrodynamic conditions are of slightly weaker energy, and infilling takes places along the axes of the tidal channel, with sedimentation being dominated by a moderate to low-energy regime with fine sands and muds. The channel-fill architecture is composed of lateral-accretion bedding and small elongate tidal bars. In the eastern part of the gulf, tidal currents are very weak, and tidal channels are mainly filled with muddy facies, containing fluid muds in the channel bottom.