Pyroclastic Deposits in a Non-Marine Hydrothermal Environment: Emplacement Mechanisms, Diagenesis, and Implication for Hydrocarbon Generation and Accumulation

Despite of the discover of volcaniclastic sequences intercalated in continental carbonates in the giant reservoir of the South Atlantic Basin, the importance of the impact that explosive volcanic activity has on the sedimentary basins is still poorly understood. This work focuses on the study of different pyroclastic and volcaniclastic deposits intercalated in a travertine geobody in Central Italy. These deposits are distal tails of explosive volcanic eruptions, and fallout deposits accumulated in a hydrothermal environment, which subsequently has deeply modified their primary characteristics with its continuous hydrothermal waters. Through a detailed, multidisciplinary and multiscale study (XRF, XRD, petrography, minero-chemical characterization through SEM-EDS, TOC measurements), we identify: 1) the emplacement mechanisms that lead the accumulation of the volcaniclastic sequences in the environment, 2) their primary stratigraphic and petrographic characteristics, 3) their diagenetic history, and 4) their possible relevance in the generation of hydrocarbons. In particular, we highlight how the effects of hot waters, enriched in calcium carbonate, and biological activity of bacteria and fungi leaded to the decrease in primary porosity due to: the argillification of the eutaxitic glassy groundmass and the embedded mafic minerals, and 2) the precipitation of diagenetic calcite, which closed pores and substituted the primary structures of pumices and accretionary lapilli. Finally, we also show that primary fallout deposits mixed with organic-rich calcite mud might have a primary role in the generation of hydrocarbons.