The Green Tuff of Pantelleria: Rheoignimbrite or rheomorphic fall?

Detailed stratigraphic analysis of the Green Tuff of Pantelleria shows that this formation can be divided into several members designated a through h from base to top. These members have a coherent pattern when traced from outcrop to outcrop throughout the island shedding light on their origin. Only member g completely mantles the entire island. The distribution of the other members is controlled by prevailing wind direction or by topography. Member a is entirely of fall origin. Members c and e are of fall and/or surge type. Members b, d, and h have the characteristics of thin welded ash-flow tuffs. Members f and g are ash-flow tuffs with textural characteristics of compound cooling units. Most of the ash-flow tuffs exhibit characteristics of ignimbrites: vertical fluidization pipes, local concentrations of lithic lapilli, imbrication of clasts, and valley ponding. Member g is unusual in that it is highly-welded, exhibits large-scale rheomorphic structures, contains huge lithic clasts, and has near-vertical foliation where it adheres to cliffs and caldera walls. Granulometric data from the members identified in the field as ignimbrites confirms this conclusion, as do density profiles through the various members.