Volcaniclastic sedimentation influenced by logjam breakups? An example from the Blanco River, Chile

Abstract

The Blanco River drains the southern flank of the Chilean Chaitén volcano toward the Pacific Ocean. During 2008, much of the rhyolitic tephra erupted by the volcano was remobilized by this (and others) fluvial system. In the Blanco River, the sediments were temporarily stored upstream of the valley dammed by logs, epiclastic and pyroclastic detritus. The logjam generation was controlled by a complex combination of several factors including abundant tephra influx, rains, steep topographic gradients, forest vegetation, and widening of the fluvial course. Sediment-laden flows generated by logjam breakup or overflow severely affected the downstream-located Chaitén village. The resulting sedimentary record is remarkable by the important proximal-distal changes in sediment composition, grain-size and types of flow within specific depositional episodes. Exposures of fluvial deposits underlying those of the 2008 eruption in the upland sector of the Blanco River display a similar sedimentary record suggesting a comparable hydro-sedimentary dynamics during deposition. This contribution analyzed the sedimentary succession that underlies the 2008 eruption deposits, in order to: i) document in detail the sedimentary processes occurred in the Blanco River as consequence of the interaction with the Chaitén volcano tephra and analyze if this hydro-sedimentary behavior departs from the expected response in similar situations; and ii) contribute to the dating of generation and rupture of logjams and exploration of the associated volcanic hazard. Basic methodology included facies analysis, recognition and correlation of depositional episodes, and 14C dating of wood and charcoal in the deposits. Four depositional episodes (named A to D) were identified within the proximal and distal fluvial segments. The proximal sector is located along the current fluvial course and the distal sector is a lowland floodplain area. In the proximal fluvial segment, the depositional episodes representing syn-eruptive conditions are composed by volcaniclastic gravels and sands, formed by stream flows (with different sediment concentration) and debris flows; and reworked ash and lapilli strata generated by the dilute flow-debris flow continuum. In this fluvial segment, particularly in the depositional episodes B and C, there are downstream facies and compositional changes that suggest recurrent generation and rupture of logjams. Depositional episode B probably occurred between 1216 and 1294 cal AD and 1391–1438 cal AD as suggested by the radiocarbon dates, and is related with the widespread “Vilcún Tephra” from the Chaitén volcano. In the distal fluvial segment the depositional episodes record sedimentation of volcaniclastic mud in ponded zones or temporally flooded areas, with incorporation of abundant organic matter, associated with sediment (sandy)-laden dilute flows and scarce preservation of ash-fall beds. It is further proposed that downstream compositional changes in syn-eruptive fluvial deposits (volcaniclastic sediments that change downstream to pyroclastic sediments) can be indicative of the presence of former logjams in similar fossil fluvial volcaniclastic sequences.