Totaro, Federica (2023) Geochemical fingerprinting of intermediate to distal tephra layers within pre-Campanian Ignimbrite stratigraphic sequences: evidence for volcanic sources active in Campania in the 40 - 200 ka time span. [Tesi di dottorato]

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Abstract

The wide half-graben depression represented by the Campanian Plain, in Southern Italy, has been, and continues to be, a site of intense explosive volcanism since at least 300 ka. Because of this long-lasting and voluminous activity, the products of the most ancient eruptions have often been eroded or buried by deposits from those that occurred over the last 40 ka. Thus, the stratigraphic record of hundreds of thousands of years of volcanic activity is difficult to access. The recent rapid urbanization and the closure of almost all the quarries where the ancient products were exposed, further contributed to the reduction in the number of successions, whose exposure is limited throughout the territory. Moreover, the pervasive, intense alteration characterizing the outcropping products makes the analysis of fresh glass challenging, discouraging studies concerning this time-window. On the other hand, both marine and continental ultra-distal successions provide a good record of at least a hundred of well characterized Campanian tephra older than the Campanian Ignimbrite eruption (~ 40 ka). This implies that the knowledge of the recent activity occurred at Campi Flegrei and Somma-Vesuvius is sufficiently thorough to contribute effectively to the volcanic hazard assessment, whereas the frequency and features of the ancient volcanism are still poorly known. This Ph.D. thesis aims to refine the Campanian tephra lattice contributing to bridge the gap between the very limited knowledge about the pre-Campanian Ignimbrite volcanic record in proximal-intermediate settings and the abundant findings in distal and ultra-distal settings, investigating the pyroclastic deposits outcropping at intermediate distances from the potential volcanic sources. To cope with the paucity of well-exposed, unaltered successions, we integrated tephra layers from both outcropping and drilled sequences. Three boreholes in which previous studies had highlighted the presence of pre-CI pyroclastic deposits were selected allowing us to cover almost all the potential direction of dispersal of a Neapolitan volcanic activity. Also, some poorly defined portions of the San Gregorio Magno Basin (Southern Campania) lacustrine succession were re-investigated to refine the tephra record. Establishing a correlation between proximal and distal successions is often complicated by the different methodological approach generally used; in fact, most of the geochemical data provided on proximal deposits are produced on whole rocks and not directly comparable with the glass chemical data available for distal tephra layers. For this reason, we decided to set up a method, also suitable for tephrostratigraphic studies, made up of successive steps pointing to provide a complete lithological, geochemical (major and trace elements) and chronological characterization (40Ar/39Ar) of the volcanic units investigated. Moreover, we also explored the potential of using the radiogenic isotopes composition of tephra layers as a correlative tool providing for most of the studied units the 87Sr/86Sr and 143Nd/144Nd contents. This work provides new insights to the largely incomplete reconstruction of the pre-Campanian Ignimbrite volcanism. Beside the finding of previously unknown eruptive events ranging in age between 40 and 200 ka, we also were able to recognize some peculiarities in the geochemistry of ancient products. In fact, while the geochemical features of the products emplaced in the Late Pleistocene seem to be quite similar to those of post-CI Phlegraean deposits, the late Middle Pleistocene tephra markers identified here are quite typical. The recognition of widely spread tephra markers not only in the well-known time span ranging between 90 and 110 ka but also around 158 and 175 ka has shed light on a hitherto little-known slice of the Campanian volcanic history. All the new outcomes were widely supported by the use of the radiogenic isotopes composition of the juvenile fractions, which in some cases proved essential to the proper identification and correlation of tephra layers.

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