Caractérisation de l'activité hydrosédimentaire dans le Système Turbiditique du Var (NO Méditerranée) et de son enregistrement dans l'archive sédimentaire

In the framework of the HERMES European project, this study aims to characterize the sedimentary processes acting in the Var turbiditic system, their impact on the morphology, and their evolution through time and space. This work uses data acquired along the system, including interface cores, acoustic data, and measurement in the water column (temperature, particulate fluxes, speed and direction of the currents) thanks to mooring lines. The Var turbiditic system is under the influences of the Northern Current and of the Var river by a direct connection. Sedimentary processes are related to gravity-driven currents, oceanic currents which remobilizes sediments and hemipelagic decantation. Gravity-driven processes are dominant and are responsible of 80% of the mean annual sedimentary transport in the system, at the scale of two years (2005-2007). They are characterized by a mean speed of 100 to 600 mm.s-1 and a mean particulate flux of 10 to more than 400 g.m-2.j-1. The combination of the river discharge data and our in-situ measures allows to discriminate the mechanism responsible of their triggering: mass-wasting or river floods. Hyperpycnal turbidity currents, generated by the plunging of the turbid plume are observed for flood extended to the whole hydrographic system, with a river discharge of more than 306 m.s-1 at the river mouth. The past sedimentary processes have also been addressed, by studying the sedimentary deposits. During the last century, the canyon and the valley floor is a pathway for sediments and shows numeral erosive bedforms. Actual sedimentation is episodic and patchy and consists on coarse turbidites. Due to its elevation, the levee is a depositional area, but where only few gravity currents are able to spill over. The dominant facies of inner terraces depends on their elevation above the canyon or channel floor. On low-elevation terraces, erosional processes counterbalance depositional processes. All deposits show erosional bases. Only a few deposits on high-elevation terraces show erosional bases and they record most of the recent processes. Five major types of deposits are recorded in the deposits. They are related to (1) concentrated gravity-currents, and their evolution in (2) high-density turbidity currents, related to large slope failures, (3) hyperpycnal turbidity currents, and (4) surge-like turbidity currents, triggered by shallow retrogressive failures. A fifth kind of sedimentary deposits could be related to the decantation of a turbid plume or to an alteration by bioturbation or bottom currents. These gravity-driven currents are classified in two main groups, as a function of their impact on the morphology. Low-magnitude flows, among which surge-like turbidity currents and hyperpycnal turbidity currents related to yearly floods, constitute the main part of the daily sediment flux. These events are not powerful enough to carry their sediment load into the deep sea, and remain confined in the upper part of the turbidite system (Canyon and Upper Valley). They deposit thin, fine-grained and discontinuous sedimentary sequences, even at a metric scale, and are moreover easily eroded and hardly preserved. High-magnitude flows, among which turbidity currents related to large slope failures or to high-magnitude floods, generate significant changes in the morphology of the turbidite system. They lead to a strong erosion in the canyon and in the valley and provide thick, continuous and recognizable bodies with an extent covering the whole Var system.