GNGTS 2014 - Atti del 33° Convegno Nazionale

eruption (Sarna-Wojcicki, 2000). The tephra layers may be in the form of pumice, usually light coloured and siliceous or scoriaceous, represented by solidified rock froths of glass containing various amounts of crystals, crystal fragments, vescicles and sometimes lithic fragments. The pumice clasts deposited near the eruptive vent are quite large, decreasing in size with the increase of the distance from the vent. At the distal locations, the tephra may consist of small pumice fragments, solid glass fragments and fine-grained lithic fragments. The mineralogic composition of the tephra layers varies with the composition of the parent magma, being mainly siliceous or dacitic and often including accessory minerals. The identification and the correlation of tephra layers is one fundamental step in tephrostratigraphic studies. The identification is carried out by using several characteristics, such as the colour, the texture, the stratigraphic context of their deposition, the occurrence of comagmatic mineral grains and the chemistry of the glasses. The correlation of such a volcanic deposits is carried out through the analysis of the petrographic characteristics. Their reconstruction in different depositional basins and over long distances requires the chemical analysis of a glass or of the associated comagmatic minerals. The time range of applicability of tephrochronology has also been discussed and is commonly applied to Neogene and Quaternary deposits. The characteristics of tephra useful for their identification include the field characteristics, the mineral grains, the volcanic glass, the magnetic and magneto-stratigraphic properties and the numerical age dating. The field characteristics are represented by the physical continuity, the thickness, the bedding characteristics, the grain size and grading, the colour, the stratigraphic context, the site location and the reflectivity, all investigated through standard field geological techniques. The mineral grains include the presence or relative abundance of mineralogic species, the characteristics of a particular species and the chemistry of a particular species, analyzed through standard petrographic techniques and electrone microprobes. The property of glasses include the refractive index, the morphology, the degree of hydration and the chemical composition, including major, minor and trace elements, analyzed through the petrographic microscope examination or binocular microscope, the chemical analysis and the X-ray fluorescence. Other significant properties to be investigated include the determination of magnetic and magnetostratigraphic properties, including the core petrophysical measurements, such demonstrated in the present paper. The interpretation in data analysis of tephrochronology includes the age estimates which are represented by correlated ages, based on the correlation of tephra deposits with tephra deposits of known age. These ages may be considered to be numerical if the tephra layers have been dated through numerical methods, such as K-Ar, 40 Ar/ 39 Ar or fission track methods, such providing a certain datum to which other deposits can be related due to their stratigraphy and providing an age estimate good for numerical age dating. The Volturno valley fill has been intensively investigated through the analysis of depositional facies, ostracod and foraminiferal assemblages of onshore cores drilled at the Volturno river mouth (Amorosi et al. , 2012). The Late Quaternary stratigraphic architecture is offered beneath the modern coastal plain of the Volturno river. An important stratigraphic marker has been identified in the Volturno plain, such as the Campanian Ignimbrite. Its top has been reconstructed and therefore it has been traced in the subsurface a Late Quaternary palaeovalley incised by the Volturno river and filled by Quaternary marine sediments. A stratigraphic cross-section across the Volturno coastal plain (Amorosi et al. , 2012) has been re-drawn in order to show the geometry of the Volturno coastal plain. The geological section shows the geometry of the Volturno incised valley cut into the deposits of the Campanian Ignimbrite. The section has been calibrated, concealing its basin fill by the cores drilled respectively in the Mondragone area, in the Castelvolturno area and in the Phlegrean Fields area. Apart the Campanian Ignimbrite deposits, another important volcanic stratigraphic marker, corresponding to the deposits of the Neapolitan Yellow Tuff (Orsi et al. , 1991; Deino et al. , 2004; Di Fiore et al. , 2011), dated back at 15 ky B.P. has been identified only in the Phlegrean Fields area. A terraced palaeotopography 212 GNGTS 2014 S essione 3.3