Transfer process of volcaniclastic in the abyssal plain offshore La Réunion Island (indian Ocean) : example of the Cilaos turbidite system
Even if deep-water turbidite systems have been widely studied throughout the world ocean,volcaniclastic deep-sea fans are little known compared to silicoclastic systems. Numerous studies dedicated to the Canary or the Hawaii Islands have never revealed the presence of well-developed turbidite system (with canyons, lobes and levees). Nevertheless, turbidite deposits and sedimentary structures like canyons or channels are visible off these volcanic islands suggesting that other processes than large mass wasting processes (flank landslides, debris avalanches) also played an important role in the erosion and transport of sediments in the deep marine environment adjacent to volcanic islands. The new dataset collected off La Réunion Island, during the 2006 cruises FOREVER and ERODER1 and the 2008 ERODER 2, reveals for the first time the presence of five large turbidite systems extending to more than 200 km from the island. The results presented in this PhD thesis propose a reconstruction of the recent sedimentary evolution (last 150 ka) of the Cilaos turbidite system located southwest of the island. Multibeam bathymetry, backscatter imagery, and echosounder profiles have been used to characterise the morphology of the Cilaos turbidite system. The sedimentological variability has been characterised thanks to the study of several gravity cores. Grain-size measurements, and XRF logging have been performed along each core interval and analyses of planktonic oxygen isotopes and radiocarbon dating have been used to constrain the stratigraphic framework. A detail mapping of the system allows the identification of the architectural elements of the deep-sea fan. It is composed of a canyon area feeding a wide valley. At the foot of the slope and on the abyssal plain, a wide fan is developed with a sediment waves field, lobate bodies and an extended channelized system in the lower-fan. Its morphology appears to have been strongly influenced by the morphology of the submarine slopes of the volcanic edifice, the surrounding seafloor and the high content of volcanic sands in the turbidity currents. The morphological features identified at the canyon head suggest that the present-day feeding of the Cilaos turbidite system is dominated by the hydrodynamic and sedimentary processes linked to the Saint-Etienne River floods. The hyperpycnal flows are not the only processes feeding the Cilaos canyon. Wave dynamic and slope instabilities also play a role for the triggering of turbidity currents and participate to the incision of the canyons. The detailed analysis of sedimentary facies and bodies permits to highlight that volcanic activity is the main factor controlling the fan sedimentary supplies. The two main phases of the Cilaos turbidite activity are linked to periods with low volcanic activity which led to an increase of the feeding of the Saint-Etienne drainage basin. Climatic and eustatic variations have also influenced the fan but they are interpreted to be of lesser importance. The volcanic origin of sediments and the complex morphology of the oceanic plate make unique the Cilaos fan which feeding mode is rather similar to those observed in silicoclastic turbidite systems, but its erosive and sand rich turbidite currents are characteristics of volcaniclastic turbidites.