Erosion and volcaniclastic sedimentation at piton de la fournaise: from source to deep marine environment

Abstract

With very high rainfall, steep slopes and young reliefs, La Réunion Island is renowned for its highest erosion rates worldwide. The erosion mainly occurs by rockfalls and landslides in the steep sides of the river canyons. Most sediment is transported from inland to the sea via rivers and streams. River channels are formed by incision into the basaltic pile of lava and breccia constituting the original volcanic shield. Erosion and sediment products in the largest rivers are mainly driven by the recurrence of tropical cyclones. Cyclonic conditions induce heavy rainfalls and torrential floods, causing land erosion and hyperconcentrated sediment loads in the main river mouths. The absence of coastal platform induces no sediment storage in shallow marine environment. Erosion processes onland and sediment transport to the coast, via river systems, induce direct sediment feeding of the upper submarine slope and canyon heads. The morphology of the submarine flanks of Piton de la Fournaise is rough and steep. Different submarine features were identified as debris avalanches deposits, erosive canyons and volcanic constructions. This chapter proposes a synthesis of morphological and sedimentological data on Piton de la Fournaise volcano, onland and offshore, at different scales. It describes the relationships between the main erosional structures onland and major submarine sedimentary systems. Combining onland erosion, sediment transport on the submarine slopes and sediment deposition in the deep-sea allow a better understanding of the transfer of volcaniclastic material on oceanic shield-volcanoes. The morphology of onland and submarine slopes of Piton de la Fournaise suggests two domains, distinguished by the maturity of land-to-sea sediment transfers, directly related to the maturity of hydrological networks onland. The first domain corresponds to limited land-to-sea transfer, with no hydrographic network, as observed on the volcanic rift-zones. The coastal area forms a rocky promontory and only coastal erosion of recent fragmented volcanic rocks, produces sediments in the upper submarine slope. Submarine instabilities, scarps, and local erosional canyons are superficial and discontinuous on the slope. The second domain corresponds to high-efficient land-to-sea sediment transfer, characterised by a direct connection between river mouths and submarine canyon heads (as for “Rivière des Remparts”). The submarine sedimentary systems are well-developed with continuous canyons, feeding deep-sea fans at the base of the slope. These volcaniclastic deep-sea fans are characterised by sandy lobe accumulation fed by small channels and are typically interpreted as the result of turbidite processes and deposits.