Hydrogeomorphic and sedimentary response to the Late Pleistocene violent Strombolian eruption of the Croscat volcano (Garrotxa Volcanic Field, Spain)

Explosive eruptions can severely alter the boundary conditions of fluvial systems around volcanoes by depositing large volumes of erodible fragmental material, increasing erosion rate and drainage mass flux (water and sediment) in the affected basin. In this paper, the environmental response to the 13 ka Croscat Violent Strombolian eruption in the Garrotxa Volcanic Field (NE Spain) has been investigated using a stratigraphic approach. Volcaniclastic material was mainly delivered at headwater tributaries (Les Tries plain in the Ser river basin) of the Fluvià River, one of the main drainage systems of NE Spain. Deposits in the Les Tries valley was initially reworked by hyperconcentrated-flows to braided stream-flows and by debris-flows involving mixed terrains (volcaniclastic materials and sands–gravels of the Eocene sedimentary basement) as registered in the stratigraphic record. The erosive pattern observed at the Les Tries valley reflects the erosion model of well-sorted and coarse-grained deposits (as the tephra deposited during the Violent Strombolian phase of the Croscat eruption), in which deep gullying and downstream aggradation are typically retarded where coarse tephra deposits remained highly permeable. Moreover, the Croscat eruption occurred soon after the Last Glacial Maximum, implying the shift from dry to wet conditions during the remobilization of the Croscat tephra. In this framework, the semiarid conditions retarded soil stabilization, driving the development of hyperconcentrated-flow-dominated alluvial fans, whereas the more humid environmental conditions favoured the partial stabilization of loose pyroclastic material, until it is periodically remobilization by mass-wasting processes as debris-flow-dominated lahars.