DEEP-SEA VOLCANICLASTIC SEDIMENTARY SYSTEMS - AN EXAMPLE FROM LA FOURNAISE VOLCANO, REUNION ISLAND, INDIAN-OCEAN

A volcaniclastic sedimentary fan extending to water depths of 4000 m i s characterized using gravity cores, camera surveys, high-resolution s onar images, seismic records and bathymetry from the submarine portion of La Fournaise volcano, Reunion Island, a basaltic shield volcano in the SW Indian Ocean. Three main areas are identified from the study: (1) the proximal fan extending from 500 m water depth down to 2000 m w ater depth; (2) the outer fan extending from 2000 m water depth down t o 3600 m water depth; (3) the basin extending beyond 3600 m water dept h. Within these three main areas, seven distinct submarine environment s are defined: the proximal fan is characterized by volcanic basement outcrops, sedimentary slides, deep-water deltas, debris-avalanche depo sits, and eroded floor in the valley outlets; the outer fan is charact erized by a discontinuous fine-grained sedimentary cover overlying coa rse-grained turbidites or undifferentiated volcanic basement; the basi n is characterized by hemipelagic muds and fine-grained turbidites int erbedded with sandy and gravelly turbidite lobes. The evolution of the deep-sea volcaniclastic fan is strongly influenced by sector collapse s, such as the one which occurred 0.0042 Ma ago. This collapse produce d a minimum of 6 km(3) of debris-avalanche deposit in the proximal are a. The feeding regime of the deep-sea fan is 'alluvial dominated' befo re the occurrence of any sector collapse and 'lava-dominated' after th e occurrence of a sector collapse. The main deep-water lava-fed delta is prograding among the blocks of the debris-avalanche deposits as a r esult of turbidity flows occurring on the delta slope. These turbidity flows are triggered routinely by wave-action, earthquakes and accumul ation of new volcanic debris on top of the deltas. Both turbidity curr ents triggered on the deep-water delta slope, and those triggered by d ebris avalanche reworked volcaniclastic material as far as 100 km from the shore line.