Unveiling the ancient history of the southwestern sector of Campi Flegrei through the stratigraphy, compositional variability and physical volcanology of the pre-caldera Vitafumo and Miliscola tuff cones

Campi Flegrei is a densely populated active volcanic field, characterized by the presence of diffuse monogenetic volcanic cones and two nested calderas associated with the Campanian Ignimbrite [39 ka; Rosi and Sbrana, 1987; Perrotta et al. 2006] and the Neapolitan Yellow Tuff [15 ka; Scarpati et al. 1993] eruptions, respectively. Due to dense urbanization and caldera collapses, the youngest products of the Campi Flegrei are well-exposed and generally better known than those of the older, pre-caldera deposits. Aiming to contribute to a better definition of the volcanic hazard produced by phreatomagmatic eruptions through the reconstruction of the ancient eruptive history of the Campi Flegrei, our research focuses on the Monte di Procida cliff, in the southwestern sector of the district. In this area thick pyroclastic sequences, separated by several paleosol horizons, are spectacularly exposed, encompassing all the volcanic activity of Campi Flegrei. A stratigraphic study was conducted on the pyroclastic sequences of two distinct monogenetic volcanoes: Vitafumo and Miliscola. Although the absolute age of these ancient volcanic edifices is unknown, they are capped by a tephra sequence erupted from the nearby island of Ischia, with an age between 74 and 55 ka [Vezzoli, 1988]. The remanent of Vitafumo is formed by three main pyroclastic sequences, overall thick up to 130 m, separated by two erosional unconformities. The lowermost sequence shows a quaquaversal structure, while the two upper successions thicken on the eastern slope of the basal sequence and strongly thin on the western slope, possibly indicating a westward shift of the source area during the eruption. The deposits consist mainly of thick, massive, pumice and lithic-rich pyroclasts alternating with stratified, coarse to fine grained ash layers. Large bombs are present producing deep impact sags. Towards the top, the sequence becomes coarser and is represented by alternating blackish and reddish stratified scoria beds. The deposits are locally lithified, both in the lower and middle sequences. The Miliscola succession, resting on the Vitafumo volcano, thins towards the west, reaching a thickness of up to 12 meters. It reveals significant vertical lithostratigraphic variations, with alternating plane-parallel, matrix supported, massive or stratified ash layers, along with thick pumice lapilli deposits containing large blocks and scattered bombs. Both volcanoes formed during explosive eruptions, during which sustained eruptive columns accumulated pumice lapilli and blocks, alternating with repeated column collapses. These resulted in the emplacement of unsteady and non-uniform pyroclastic density currents characterized by traction to granular to fluid-escape dominated flow boundary zones. Both the edifices are characterised by juvenile 64 clasts that are extremely homogeneous in composition throughout the entire stratigraphic sequence, both showing a typical trachytic composition with no significant difference between Vitafumo and Miliscola samples. The incoherent nature of most deposits, coupled with the high degree of vesicularity of pumice clasts and their relative abundance (up to 84 wt.% for both the Vitafumo and Miliscola sequences) compared to that of lithic fragments suggest a mainly magmatic activity for both volcanoes. The presence of lithified horizons and accretionary lapilli bearing layers, along with the abundance of fine ash and cineritic aggregates testify to periodic phreatomagmatic pulses, during which magma-water interaction occasionally occurred. The increase of the lithic content upward in the sequence (from 9.5 to 46 wt.% and from 6 to 44 wt.%, respectively in the Vitafumo and Miliscola successions) suggests that erosion or partial collapse of volcanic conduit events occurred during both the eruptions. This research provides new insights into the knowledge and risk assessment of Campi Flegrei activity. It shows that, although pre-caldera Phlegrean volcanism is represented by monogenic and scattered vents with a confined distribution [Perrotta et al. 2010], some eruptions show complex dynamics conditions that changed during the single event.