GNGTS 2013 - Atti del 32° Convegno Nazionale

The Campi Flegrei volcanic system was investigated in the ’40-’80 years by the Azienda Geologica Italiana Petroli (AGIP) and the Società Anonima Forze Endogene Napoletane (SAFEN) through deep holes at about 3050 m of depths (e.g, AGIP, 1987; Rosi and Sbrana, 1987) mostly in the western and north-central sectors of the caldera. The drilling exploiting activities have strongly contributed to the definition of the subsurface structure of the caldera (Rosi and Sbrana, 1987; Rosi et al. , 1983). However, since the AGIP and SAFEN programmes, a lot of geological, geochemical, and geophysical studies have been conducted at the Campi Flegrei, improving the knowledge of the volcano. Nonetheless, the recent dynamics and history of the volcano are still affected by several uncertainties. Among the most important ones, there are: 1) the debated origin and shape of the caldera attributed to the Campanian Ignimbrite or the Neapolitan Yellow Tuff eruptions, or both (Rosi et al. , 1983; Orsi et al. , 1996; Perrotta et al. , 2006; De Vivo et al. , 2001), 2) the volcanism older than 14.9 ka that is buried by later volcanic products and lies mostly in the subsurface (Orsi et al. , 1996), 3) the 3D-distribution of the volcano-tectonic dislocations during collapse and resurgent phenomena, 4) the mechanisms of bradyseisms and their relation with magma dynamics, and 5) the relationships among caldera structure and chemico-physical features of the geothermal system. These questions determine doubts on the significance of the monitored phenomena, in particular with their relation to the dynamics of the caldera and the evolution of the magmatic system. The eastern caldera coincides onshore with the Bagnoli Plain and its volcanic evolution is certainly among the least known in the area. A medium pilot well, 506 m deep, was realized in Bagnoli Plain with the scope to fill the gap of knowledge. The drilling activities was conducted in the frame of the Campi Flegrei Deep Drilling Project (CFDDP) that is principally aimed to the improvement of volcanic hazard mitigation and to the promoting the geothermal energy exploitation in the Campania Region. Here we present the preliminary results from analysis of the drilling mud and core samples, as well as from geophysical loggings. Geological context and drilling site. The Campi Flegrei volcanic complex developed inside the Campanian graben (Piochi et al. , 2005 and reference therein). The oldest erupted products are dated at >60 ka and crop out in a quarry in the Quarto Plain (Pappalardo et al. , 1999), although the volcanism in the area dated back at least 1.5 Ma (Piochi et al. , 2005 for a review). At the Present, the Campi Flegrei volcanic field characterizes for a ~10km large caldera. According to several studies (Rosi and Sbrana, 1987; Orsi et al. , 1996), the caldera was originated at 39 ka BP (De Vivo et al. , 2001) during the catastrophic Campanian Ignimbrite eruption. Orsi et al. (1996) and Perrotta et al. (2006) suggested that a second collapse occurred within the primary caldera during the 14.9 ka Neapolitan Yellow Tuff eruption (Deino et al. , 2004). Few authors (De Vivo et al. , 2001) recognized the only Neapolitan Yellow Tuff caldera and indicated for the Campanian Ignimbrite an origin from faults located north of Campi Flegrei. All the authors consider the Posillipo cliff as a volcano-tectonic structure that thus is possibly related to the eastern caldera margin of the only Neapolitan Yellow Tuff (Orsi et al. , 1996; Perrotta et al. , 2006) or also of the Campanian Ignimbrite (Rosi et al. , 1983; Isaia et al. , 2009). Orsi et al. (1996) suggested a larger 39 ka caldera that includes part of the city of Naples and the western portion of the Gulf of Naples. Along the Posillipo cliff, the Neapolitan Yellow Tuff (NYT) has a thickness of about 120 m; a level of scoriae with thickness of 10-20 meters has been also documented upper in the NYT sequence (Rittmann, 1950). The base of the NYT is exposed in the Cavalleggeri-Fuorigrotta area, just at the foothill. The Bagnoli Plain lies at the foot of the cliff. It is characterized by two cones, Nisida and Santa Teresa that were active in the past 14.9 ka (Di Renzo et al. , 2011) and a complex depositional sequence recovered by few <223 m boreholes (Orsi et al. , 1996; Calderoni and Russo, 1998). The Neapolitan Yellow Tuff, with some exceptions, is not recovered in the boreholes, which, however, never reached its base. Fossils were described in the boreholes and 264 GNGTS 2013 S essione 1.3