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ABSTRACT

The 20–16 ka Monte Guardia sequence of Lipari island, southern Italy, is a complex succession of silicic pyroclastic surge deposits produced, in part, by hydromagmatic explosions near sea level. Most surges were directed to the east, north-east and north of the vent, and climbed the 12° southern slopes of Monte Sant’Angelo in the central part of the island.

A series of thin, distinctive key bed-sets containing oxidized ash and accretionary lapilli allow a detailed correlation of sections and the lateral tracing of deposits of single pyroclastic surges across the island. Facies analysis reveals that the proximal-to-distal facies changes are different from those suggested by a previous study based on a statistical approach to lateral facies distribution. Single dry surge deposits evolve downcurrent from (1) beds of disorganized medium- to coarse-grained lapilli containing scattered blocks, to (2) bipartite disorganized/stratified beds of fine- to coarse-grained lapilli with ash matrix, to (3) dunes formed of coarse-grained ash to medium-grained lapilli, to (4) planar beds of fine-grained lapilli. This facies sequence is similar to published models for some Korean surge deposits, and records decelerating surges which experienced a downflow decrease in turbulence, particle concentration and suspended-load fall-out rate, and an increase in traction processes.

As the Monte Guardia surges climbed the opposing slopes of Monte Sant'Angelo, they bifurcated into eastern and western tongues, which experienced rapid deceleration leading to a rapid downcurrent thinning and fining of the surge deposits. Two fluid-dynamical approaches suggest that Monte Guardia surges travelled at speeds of more than 75–85 m s -1 before climbing Monte Sant’Angelo. Flows with this vigour and distribution are capable of destroying animal and plant populations on Lipari.