The Villa Senni Eruption Unit (Colli Albani Volcano, Italy): the basal surge-fallout deposit and the proximal co-ignimbrite breccia facies.

The Villa Senni Eruption Unit is a complex, tephri-foiditic, caldera-forming, large volume ignimbrite succession from the Colli Albani Volcano, Italy, emplaced at about 350 ka. The total estimated volume of products is approximately 50 cubic kilometres. The eruption unit internal stratigraphy is consists of a widely dispersed thin phreatomagmatic ash-surge deposit at the base followed by a scoria fallout deposit dispersed to the east of the volcano. It is up to 120 cm thick in proximal areas, and thins laterally, with a dispersal area that is subplinian. The overlying succession is the bulk of the deposit and is made up of a lower ignimbrite unit, known as the Tufo Lionato, and an upper ignimbrite unit at the top, known as Pozzolanelle, or Villa Senni. Both the lower and upper ignimbrite units show a proximal co-ignimbrite facies made of a coarse lithic and spatter rich breccia which grades laterally and vertically into the standard ignimbrite facies. While the depositional facies of the basal surge-fallout and overlying ignimbrite succession match those of common felsic large volume ignimbrites, the pyroclasts of the Villa Senni Eruption Unit show a moderate vesicularity and ductile surface deformation suggesting a relatively low viscosity of the magma in accordance with the low silica content of its composition. This work addresses the issue of the possible mechanisms that may contribute to the large volume basaltic explosivity of the Villa Senni magma.