New seismostratigraphic and tephrostratigraphic data in the continental shelf offshore the Volturno basin: stratigraphic constraints on the occurrence of new volcanic stratigraphic markers

A volcanic stratigraphic marker in the continental shelf offshore the Volturno river (northern Campania, southern Tyrrhenian sea) has been recovered and discussed through seismostratigraphic and tephrostratigraphic data. The seismostratigraphic data include a Subbottom Chirp and Sparker seismic grid collected in the frame of research programs on marine geological mapping of the continental shelf off the Campania Region. Cores have also been collected and their sedimentological study, coupled with seismostratigraphic analysis has recently evidenced the occurrence of a volcanic stratigraphic marker, corresponding to a highly continuous and parallel reflector on the seismic sections. Seismo-stratigraphic analysis has evidenced that the volcanic level, interpreted as a tephra deposit is interlayered in transgressive system tracts deposits, pertaining to the Late Quaternary depositional sequence, well detected in the whole Campania Tyrrhenian margin. The volcanic level, which has been mapped in six different seismic profiles, is located at different depths, ranging between 70 and 155 meters below the sea bottom, interrupting in correspondence to the shelf break (Fig. 1). Although several tephrostratigraphic studies have been carried out in Southern Italy, concealing, in particular, the Somma-Vesuvius, Ischia and Procida offshore (Pyle et al., 2006; Rolandi et al., 2007; Turney et al., 2008; de Alteriis et al., 2010; Insinga et al., 2010), only few of them have linked the geological aspects on tephrostratigraphy detected through geochemical, sedimentological and SEM analyses with high resolution seismic profiles (de Alteriis et al., 2010). Tephrochronology of the Ischia island has been recently discussed (Insinga et al., 2010). The stratigraphic succession of the Gulf of Naples slope offshore Ischia is dated back through a time interval spanning 40 ky B.P., deduced by 14C AMS dating and tephrostratigraphy. It has been attributed, based on the calcareous nannofossils assemblages to the Emiliana huxleyi Acme Zone (Rio et al., 1990), during the Late Pleistocene-Holocene time interval. This succession is characterized by coarse-to-fine-grained ashes alternating with volcaniclastic turbidites along the whole succession. Four ash layers have been characterized and correlated with several eruptions occurring in the Ischia island in a time interval spanning between 17 ky B.P. and the Middle Ages and Roman times. The deposits related to the explosive activity of the Procida island have also been found and dated at about 23 ky B.P. In the Ischia stratigraphic succession the oldest tephra layers are distinguished by a well-preserved layer, thick about 30 cm, which is correlated with the Phlegrean products of the Campanian Ignimbrite event (39 ky B.P.; De Vivo et al., 2001). The marine counterpart layer correlated with the Campanian Ignimbrite is named the C13 tephra layer in the Tyrrhenian sea (Thon-That et al., 2001; de Alteriis et al., 2010; Insinga et al., 2010). The stratigraphic position of two other main marker tephras recognized in the succession have allowed to correlate them with two other marker tephras, such as the Schiava and Codola layers (Paterne and Guichard, 1993; Sulpizio et al., 2003; Giaccio et al., 2008). The Volturno offshore has been intensively studied based on deep multichannel seismic data and marine magnetic data (Aiello et al., 2000; 2005; 2011a; 2011b) coupled with high resolution seismics and gravity cores (Iorio et al., 2014). A new geological approach based on multi-proxy analysis has been recently applied to the northern Phlegrean Fields offshore aimed at defining the sedimentary processes in recent prodelta deposits based on the geological interpretation of seismic profiles coupled with petrophysical and sedimentological analysis of cores (Iorio et al., 2014). A Holocene unit has been characterized through Subbottom Chirp profiles coupled with sedimentological and petrophysical data. The chronostratigraphic framework was achieved by means of a colorimetric parameter calculated through petrophysical analysis with nearby dated marine sediments. A time interpretation of about 2300 years B.P. has been estimated for the sedimentary record collected in the cores. The seismic interpretation of high resolution data has shown that the Late Holocene highstand shelf deposits are affected by undulation phenomena. The undisturbed sedimentation and the preservation of an internal geometry at a decimetric scale, as detected by the sedimentological and the petrophysical analysis indicates a sliding of sediments without sediment reworking for this sedimentary body. Small normal faults have also been identified, related to high water content, fluid escape features and triggered by the occurrence of seismic activity. The depth-age conversion of the detected lithological features has evidenced a regular climatic change in the depositional environments of the Volturno sedimentary prodelta. The detection of spectrophotometry correlations of Holocene shelf margin sediments has also confirmed for the Volturno continental shelf the potential value of spectrophotometer data in high resolution stratigraphic correlations.