GNGTS 2014 - Atti del 33° Convegno Nazionale

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. The volcanic deposits, including the lava flows, the ash flows, the ash layers and the tuffs represent important stratigraphic markers providing excellent stratigraphic correlations through sedimentological, seismo-stratigraphic and petrophysical constraints (Self and Sparks, 1981; Sarna-Wojcicki, 2000; Giaccio et al. , 2009; Iorio et al. , 2014). The volcanic eruptions usually represent brief events and the erupted materials are usually emplaced during short geological times. The volcanic units, often the pyroclastic units, can be distinctive and laterally continuous: in these cases the field geological surveys can be used as stratigraphic markers and physically correlated among geographically separated stratigraphic sections (Di Vito et al. , 1999; Amorosi et al. , 2012). One aim of this paper is to demonstrate that the basic approach of tephrochronology and tephrostratigraphy in field studies, already applied in the Neapolitan volcanic district (Insinga et al. , 2005; Sacchi et al. , 2005; de Alteriis et al. , 2010; Insinga et al. , 2010) may be applied also in studying the seismic sections, if they provide highly continuous seismic reflectors correlating to tephra layers and perhaps representing useful stratigraphic markers, as in the case of the Volturno offshore (Aiello et al. , 2011a, 2011b; Iorio et al. , 2014). Moreover, in tephrochronological and tephrostratigraphic studies the volcanic units can be dated through numerical methods, as the fission tracks; in these cases they become chronostratigraphic markers, providing a chronostratigraphic control everywhere they can be identified. This concept may be applied also to seismic reflectors if they are calibrated by piston and gravity cores studied through sedimentological and petrophysical methods, as it happens in our case study. The tephra layers are usually represented by volcanic ash layers and usually cover large areas, so representing the most useful markers among the volcanic materials in order to reconstruct their stratigraphic relationships. The term “tephra” derives from a Greek word meaning ash and is used for all the pyroclastic materials erupted from a volcanic vent, particularly referring to ash-fall deposits, ash-deposits and pumice flow deposits. These volcanic materials are usually transported by air and gas and, after being deposited, form tephra layers or tephra beds (Simkin and Siebert, 1994). The tephrochronology is divided into two fundamental disciplines, such as the tephrostratigraphy and the tephrochronometry. The tephrostratigraphy is represented by the correlation of the tephra layers distinguishing their physical and chemical characteristics and corresponding stratigraphic sequences. The tephrochronometry is represented by the numerical age determination of tephra layers, either from the tephra itself through the K/Ar or the fission track methods or indirectly, from the ages of the associated layers or deposits. The tephra deposits may consist of three main components, such as the volcanic glasses generated during the rapid cooling of the eruption, the lithic fragments, being pieces of pre-existing rocks becoming incorporated into the tephra layers during the volcanic eruption and consequent phases of transport and deposition and finally, the mineral crystals of crystal fragments, which have been formed in the magma prior to the GNGTS 2014 S essione 3.3 211