Sedimentological analysis of ash-rich pyroclastic density currents, with special emphasis on sin-depositional erosion and clast incorporation: The Brown Tuff eruptions (Vulcano, Italy)

Abstract

The sedimentological, lithological and textural characteristics of the Brown Tuffs (BT) pyroclastic deposits, combined with their grain-size, componentry and geochemical glass compositions, are here investigated to obtain information on the transport and depositional mechanisms of the corresponding pyroclastic density currents (PDCs). The BT are widespread reddish-brown to grey, ash-rich pyroclastic deposits generated by pulsating hydromagmatic explosive activity from the La Fossa Caldera on Vulcano island during the c. 80–6 ka time-stratigraphic interval, and then distributed on most of the Aeolian Islands and Capo Milazzo peninsula (Sicily) and in the Tyrrhenian and Adriatic Sea regions. Near the source area on Vulcano, the BT are characterised by alternating massive and planar to cross stratified lithofacies that result from the stepwise, repeating aggradation of discrete PDC pulses. This alternance is regulated by either fluid escape or granular flow depositional regimes at high clast concentration or grain by grain traction deposition in the waning diluted stages of the PDCs. Most of the BT on Vulcano show intermittently stratified and massive ash deposits resulting from a pervasive post-depositional disruption of the primary structures. This is induced by upward fluid expulsion associated with dissipation of pore pressure between layers at different grain size (fine to coarse ash) and porosity, as outlined by distinctive upwards bends and pillar-type escape structures through the fluid-filled cracks and rupture points. Massive BT deposits with a faint colour and grain-size banding are widely recognised on Lipari, the nearby island of Vulcano. Based on the presence, at the base of BT depositional units, of cm-thick amalgamation bands containing pumice lapilli, scoria and lithic clasts ripped-up and embedded from the loose underlying pyroclastic units, they are interpreted as deposited by ash-rich PDCs laterally spreading from La Fossa Caldera and moving to Lipari. During their motion to Lipari these currents (likely) crossed a narrow and shallow sea-water inlet which did not stop their advancement but influenced the grain size distribution of those spreading on the Lipari mainland. In this paper, the mechanism of clast erosion and incorporation is outlined across the whole island of Lipari by means of field study, grain-size, and geochemical glass analyses on the different components of the mixed basal bands of the BT. This suggests that the BT PDCs maintained enough flow power as to erode the substratum, hence likely impacting the territory, over a distance up to at least 16–17 km from the volcanic source. Evidence that the BT PDCs exerted a high shear-stress over the loose substratum is also provided by undulated, recumbent flame and rip-up structures at the base of some depositional units in southern and central Lipari. In order to form such bed granular instabilities between the BT and the underlying deposits we calculate that the currents had at least a shear velocity of ca. 2 m s−1 and a shear stress in the range of 1‐4.5 kPa. These results add new insights on the large-scale hazard at the Aeolian Islands and shed new lights on the widespread transport and depositional dynamics of ash flows spreading over the sea and reaching nearby islands, and their interactions with the substratum and the pre-depositional topography.