Interaction between particles transported by fallout and surge in the deposits of the Agnano-Monte Spina eruption (Campi Flegrei, Southern Italy)

The Agnano-Monte Spina Tephra (AMST) is a complex sequence of beds generated by contrasting fragmentation and transportation dynamics. The 4.1 ka eruption was accompanied by a volcano-tectonic collapse, part of which is the present Agnano plain. The pyroclastic sequence is subdivided into members and submembers, each characterized by different lithological and sedimentological features. Plinian/subplinian fallout deposits frequently alternate with base-surge beds of phreatomagmatic origin. Analysis of lateral facies variations and vertical facies associations of correlated layers of submembers B2, D2 and E2 reveals that during some eruption phases the contrasting eruptive dynamics were almost contemporaneous. Base-surge deposits of submember B2 formed during the declining stage of a plinian column. They resulted from highly energetic and steady pyroclastic density currents that traveled long distances from the vent area and surmounted topographic obstacles such as the Posillipo hill (similar to 150 m) and the northern sector of the Camaldoli hill (similar to 250 m). Submember D2 pyroclastic density currents formed when contemporaneous fallout from an eruptive column was present. Coarse particles from the column settled throughout the pyroclastic density current, determining a significant increase of the solid load of the base surge. The consequent increase of supply rate from the transportation to the depositional zone of the base surge led to the formation of unsteady flows that could not efficiently transport the solid load and did not have the ability to travel long distances. Base-surge deposits of submember E2 were fed by a pulsating phreatomagmatic activity, which was punctuated by a short-lived fallout phase. Fallout material, which was incorporated as an additional load to the base surges, was partially transported by low-energy, steady pyroclastic density currents that traveled over the Agnano plain but did not surmount either the Camaldoli or the Posillipo hills. Only the very fine material, in continuous suspension in the upper, no-shearing part of the base-surge cloud, was dispersed higher in the atmosphere and quietly settled over a large area outside the caldera rim. The phreatomagmatic origin of base surges, which contrasts with the magmatic origin of fallout activity, suggests that the pyroclastic density currents of the Agnano-Monte Spina eruption did not result from eruption column collapses. They were most likely related to radially spreading clouds which were contemporaneous with fallout activity but issued from distinct zones in the vent area. The turbulent nature and the high expansion of base surges made them capable, under certain conditions, of passing over high topographic obstacles, with hazardous effects in distal areas.