Fluvial response to an explosive eruption: the case of the Chaitén volcano and Blanco river (2008, Chile)

UMAZANO, A.M.; MELCHOR, R.N.; BEDATOU, E.; BELLOSI, E.S.; KRAUSE, J.M.

San Pablo

Congreso; VI Latin American Congress of Sedimentology; 2013

The rhyolithic plinian eruption of the Chilean Chaitén volcano, initiated in May 2, 2008, suddenly introduced abundant pyroclastic sediments in the Blanco river catchment area, which experienced important modifications. Before May 2, the river was characterized by gravelly and low to moderate sinuosity channels crossing a low-gradient, vegetated and locally urbanized (Chaitén city) floodplain. This river, limited in the proximal part by steep and densely forested highlands, was connected with the Pacific Ocean via a tidal flat. After heavy rains in May 12-13, the river discharge increased due to breakout of logjams, which triggered downstream responses including avulsion, changes in the pattern and dimensions of channels, and construction of a delta. In this context, the goal of this contribution is to document the sedimentological processes within a geomorphic framework. Downstream of the logjam zone, deposits are composed by ash and lapilli, both pumice-rich and without important participation of older sediments. The abandoned and filled paleochannel, and the proximal part of splays experienced transient dilute flows with variable sediment concentration and, subordinately, hyperconcentrated flows. The distal sectors of splays mostly record settling from suspension. Upstream of the logjam zone, the deposits are composed of ash and lapilli with paleovolcanic sediments, which record dilute flows and the debris flow-hyperconcentrated flow continuum. We conclude that immediately after eruption, sedimentary dynamics of the river was mainly controlled by a combination of high availability of incoherent pyroclastic sediments on steep slopes, abundant rains, large logs that jammed the river and huge areas of devasted forest.