CONICET | Buscador de Institutos y Recursos Humanos

The rhyolithic Plinian eruption of the Chaitén volcano, initiated in May 2, 2008, suddenly introduced large amounts of pyroclastic sediments in the adjacent Blanco river catchment area, which experienced important modifications. Before May 2, the river was characterized by fluvial channels that crossed a relatively flat, vegetated and locally urbanized (Chaitén city) floodplain. This river, limited in the proximal part by steep, densely forested highlands, was connected with the Pacific Ocean via a tidal flat. After heavy rains in May 12-13, the river caudal discharge increased due to breakout of several logjams, which triggered severe 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. Methods included the mapping of pre and syn-eruptive geomorphology, the characterization of deposits in natural cuts and trial pits, and facies analysis. Downstream of the logjam zone, deposits are composed by juvenile? 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 the splays mostly record settling from suspension. Upstream of the logjam zone, the deposits are composed of a mixture of ash and lapilli with palaeovolcanic sediments, which record dilute flows and the debris flow-hyperconcentrated flow continuum. We conclude that immediately after eruption, sedimentary dynamics of the river was controlled by a combination of high availability of incoherent pyroclastic sediments on steep slopes, large logs that jammed the river, abundant rains, gently topography and profuse trees.

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