Deep‐sea volcaniclastic sedimentary systems: an example from La Fournaise volcano, Réunion Island, Indian Ocean

Abstract

A volcaniclastic sedimentary fan extending to water depths of 4000 m is characterized using gravity cores, camera surveys, high‐resolution sonar images, seismic records and bathymetry from the submarine portion of La Fournaise volcano, Réunion 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 water depth; (2) the outer fan extending from 2000 m water depth down to 3600 m water depth; (3) the basin extending beyond 3600 m water depth. Within these three main areas, seven distinct submarine environments are defined: the proximal fan is characterized by volcanic basement outcrops, sedimentary slides, deep‐water deltas, debris‐avalanche deposits, and eroded floor in the valley outlets; the outer fan is characterized by a discontinuous fine‐grained sedimentary cover overlying coarse‐grained turbidites or undifferentiated volcanic basement; the basin is characterized by hemipelagic muds and fine‐grained turbidites interbedded with sandy and gravelly turbidite lobes.The evolution of the deep‐sea volcaniclastic fan is strongly influenced by sector collapses, such as the one which occurred 0·0042 Ma ago. This collapse produced a minimum of 6 km3 of debris‐avalanche deposit in the proximal area. The feeding regime of the deep‐sea fan is ‘alluvial dominated’ before the occurrence of any sector collapse and ‘lava‐dominated’ after the occurrence of a sector collapse.The main deep‐water lava‐fed delta is prograding among the blocks of the debris‐avalanche deposits as a result of turbidity flows occurring on the delta slope. These turbidity flows are triggered routinely by wave‐action, earthquakes and accumulation of new volcanic debris on top of the deltas.Both turbidity currents triggered on the deep‐water delta slope, and those triggered by debris avalanche reworked volcaniclastic material as far as 100 km from the shore line.