Stratigraphy, lithofacies variations and transport and emplacement mechanisms of the most destructive PDC of the 79 AD Vesuvius eruption

Plinian eruptions form a sequence of pyroclastic fallout deposits commonly overlain by ignimbrites. Partial column collapses often form intraplinian pyroclastic density currents (PDCs), and sustained column phases punctuate late, prevalent column collapse phases. In both cases, PDC deposits show longitudinal and lateral variations on a regional scale. Considerable facies variations have been identified in the Plinian 79 AD PDC deposits on a regional scale. A systematic survey allowed us to study in detail the distribution and lateral facies variations of the different PDC stratigraphic units to document factors governing PDC emplacement mechanism. Here we present a sedimentological analysis of lithofacies combined with grainsize and componentry data related to unit E, a postPlinian deposit that exhibits the most destructive impact and show the most widespread distribution, being traceable on mountain slopes beyond 20 km from the vent. We used a stratigraphic approach to identify and correlate this stratigraphic unit sandwiched between a lithicrich fallout layer at the base and an accretionary lapilli bearing deposit at the top. Unit E, extensively studied [Luongo et al., 2003; Gurioli et al., 1999, 2005; Cioni et al., 2020], which partially coincides with the EU4pf unit [Cioni et al., 2004], shows a remarkable lateral variation in thickness (0.5 to 4 m), texture and sedimentary structure both at regional and local scale. Several facies are observed at different distances and azimuth from the vent. In proximal locations, on the volcano slopes, the deposit is mainly coarsegrained, crossstratified and laterally discontinuous, showing lowangle erosional truncations. It forms small to large bedforms up to 130 cm in wavelength and up to 15 cm in amplitude. Locally, massive and clastsupported lenses composed of very fine lithic fragments occur between the layers. In medial locations, up to 10 km away from the vent, the most common lithofacies is a thick, up to 3.95 m, massive, moderate to very poorlysorted (Φ ranging between 2.1 and 3.5), matrixsupported deposit with wellrounded medium and coarse lapilli pumice clasts dispersed in the ashy matrix. At the same distances, a massive or faintly stratified, reversegraded, coarse and lithic rich lithofacies also occurs. Over a wide sector N of Vesuvius, where the 79 AD sequence thickens, the deposit consists of thick, massive, very coarsegrained lithic breccias characterized by lithic and pumice blocks supported in a coarse sandy matrix composed of the same constituents. In distal areas, on the Lattari mountains, between 50 and 750 m asl, the deposit is a finegrained (Md Φ ranging between 0.2 and 3.5), thin (from 3.5 to 15 cm), massive and very poorlysorted, with diffuse pumice clasts set into the matrix. In some proximal to medial stratigraphic sections, the basal unit E passes upwards into a stratified deposit with planeparallel to crossbedding stratification forming progressive bedforms up to 25 m in wavelength and 105 cm in amplitude. The unit E also shows lateral and vertical variations in abundance and type of components, with, e.g., an increase in the juvenile content (between 0.36 and 47.2 wt.%) and a decrease of the lithic content (between 87.0 and 28.7 wt.%) at gradually increasing distances from the vent. These pronounced downcurrent and vertical facies variations suggest an aggradational deposition of a single, sustained, nonuniform and unsteady PDC, spreading with a high erosive power and shear strength over a distance up to at least 20 km from the volcanic source, and 38 characterized by a depositional system very sensitive even to smallest variations of the substrate. In particular, our systematic facies study suggest that the basal portion of the flow could have moved downslope developing different depositional regimes, topographically confined, mainly coinciding with paleovalleyridge topography and ranging from traction (on topographic ridges) to grain flow and fluid escape dominated (in paleovalleys). Downcurrent, across a relatively flat landscape, the PDC decelerated (depletive behavior), developing a concentrated bedload. On distal highreliefs, direct fallout from the transport system could be the dominant depositional mechanism. Instead, the vertical lithofacies association of the unit E indicates temporal changes of the dominant physical flow process in the flow boundary zone, suggesting an increasing turbulence with time of the PDC, which spread as a strongly diluted current only within certain distances from the vent.