Interpretation of emplacement units in recent surge deposits on Lipari, Italy

The Gabellotto-Fiume Bianco tephra on Lipari consists of surge and fall deposits of rhyolitic composition. The sequence contains excellent exposures of large-scale wavy strata that consist of paired bedsets with a coarse pumiceous lower part that grades upward into an overlying fine-ash layer. The contacts between the upper ash layer and the pumice of the next higher bedset are sharp. Size analyses of samples from this deposit are polymodal and contain various proportions of several distinct subpopulations. Each subpopulation can be ascribed to particles transported by a specific mechanism: fall, traction, intermittent saltation, saltation, or suspension. Particles carried by several mechanisms may be deposited together in a bed. As a model for emplacement, each bedset is considered to have resulted from the passage of a single surge or surge lobe. Pumice clasts that settle to the base of the surge cloud are carried for a short distance in a traction carpet to form the dominant component of the coarse-grained lower part of the bedset. As the velocity at a point wanes during the passage of a surge, the mean size of particles carried by the cloud decreases until the trailing edge of the surge cloud leaves a capping layer of fine ash with only sparse pumice clasts.