The riddle of volcaniclastic sedimentation in ancient deep-water basins: A discussion

Sedimentary records of deep-water basins are useful tools for understanding how volcanism shapes distal environments. Furthermore, these basins represent uncontaminated places where catastrophic eruptions are clearly preserved. For these reasons, the study of the depositional records of deep-sea realms becomes of primary importance to better understand the temporal incidence and the consequences of catastrophic events that may occur on Earth. However, their inaccessible underwater conditions limit the possibility of deep investigations, thus ancient analogues represent the best way to integrate direct observations with detailed sedimentological and petrographic analyses. This work introduces concepts derived from the study of three sedimentary sequences that form the Oligocene foreland and foredeep deep-sea basins of the proto-Alpine belt. Each succession is characterized by its volcaniclastic detritus, and two of them were deposited during volcanic activity. We compare their sedimentological and volcanological characteristics, highlighting the main features derived from a volcanically-controlled sedimentation. We emphasize: 1) the importance of stratigraphy and petrography in the identification of criteria to recognize volcanism phases in ancient, distal, deep-water sequences; 2) the propensity of volcanism to produce detritus during its life rather than after its death; and 3) the fundamental role of pyroclastic density currents in delivering particles from source areas to depositional basins. These three points suggest that a deep knowledge of ancient sedimentary records integrates our knowledge on the interaction existing between volcanoes and surrounding environments with a hazard assessment perspective.