Reconstructing the Neapolitan volcanism prior to the 39.2 ka Campanian Ignimbrite: new stratigraphical results from a scientific drilling north of Naples (Italy)

In the framework of the INGV–Pianeta Dinamico research theme TIFEHO (Trachytic Ignimbrites magma-chambers Formation and Evolution in the pre-HOlocene history of the Campania volcanic area), whose main goal is to understand the growth and evolution of the deep magmatic feeding system prior to high magnitude eruptions of the Neapolitan volcanic area, a deep scientific drilling has been performed, down to 113.2 m below the ground level, in the Ponti Rossi area, north of Naples (Italy). The drilling site was chosen for three main reasons: 1) it is external to any proposed Campi Flegrei (CF) caldera rim, topographic or structural scarp and downthrown area; 2) the surface geological record consists of a stratigraphic succession including several pyroclastic units sandwiched between the products of the 39.2 ka Campanian Ignimbrite (CI) eruption at the base and the 15 ka Neapolitan Yellow Tuff (NYT) eruption at the top, representing the two largest explosive events of CF, responsible for the caldera formation; 3) a 96 m deep scientific drilling, carried out in the same site in the late 1990’s, retrieved a pre-CI succession composed of eleven pyroclastic units totalling 61.8 m. Results from a detailed stratigraphic investigation, coupled with the sedimentological and geochemical characterization of a total of seventy-seven samples collected throughout the sequence, highlight that the Ponti Rossi borehole retrieved a stratigraphic record in which the NYT deposits represent the shallowest sediments. An important result is the absence of the CI deposits as well as of any pyroclastic deposit belonging to the CI-NYT (39-15 ka) chronostratigraphic interval, testifying a prolonged phase of strong erosion preceding the NYT eruption. Most of the cored succession, directly underlying the NYT deposits, consists of a thick pyroclastic sequence, totalling 88.05 m and representing the 78% of the retrieved succession, attributed to volcanism older than CI. The stratigraphic investigation and the X-Ray diffraction analysis (XRD) of the cored sediments allowed to identify at least thirty-four pyroclastic units older than CI, having thickness between 0.13 and 21.5 m, separated by well-developed paleosols and/or reworked volcaniclastic deposits. This pyroclastic succession consists predominantly of massive to stratified, matrix-supported to fines-poor pyroclastic current deposits, both magmatic and phreatomagmatic, and minor coarse- to fine-grained fall deposits. The features of several units (e.g., coarseness, lithic enrichment, considerable thickness) are compatible with a proximal vent while a number of units could be intermediate to distal tephra layers, some of which sourced from the Island of Ischia as highlighted by the available geochemical data. The retrieved stratigraphic record suggests the occurrence of a remarkable explosive activity >40 ka, the study of which will allow the achievement of a better knowledge of the magmatic systems feeding the volcanic activity of Neapolitan volcanoes.