Facies Variations Controlled by Urban Structures: the Example of the AD 79 Pyroclastic Deposits at Pompeii

A huge excavation project is in progress at Pompeii since the last year, it will lead to the exhibition of an excavation front of 2.5 km. In this first year of daily excavations we have observed several complete stratigraphic sequences within the town of Pompeii, recorded the exact stratigraphic positions of new victims and investigated the damages suffered by the Roman buildings. The new stratigraphic survey of the pyroclastic deposits blanketing Pompeii ruins shows significant departures from the late reconstruction of the events that occurred, inside the town, during the two main phases (pumice fallout and pyroclastic density currents) of the AD 79 Vesuvius eruption. A systematic survey of all the (ephemerally) exposed pyroclastic sequences allowed us to study in detail the distribution and lateral facies variations of the different stratigraphic units. The basal lapilli fall deposit shows a remarkable thickness variation ranging from 0 to 4.5 metres. Pronounced lateral variations are observed in the upper stratified ash deposit, emplaced by several successive pyroclastic density currents. We investigated the main ‘killing’ horizon where hundreds of victims were found. This massive to stratified unit is strongly controlled by the local urban structures. The coarse, massive and pumice-rich lower part is valley ponding; while the finer, stratified upper part drapes the buildings showing well developed progressive dunes. The presence of wall fragments, of more than 300 kg, in the middle part of this unit confirms that the associated pyroclastic current was the most devastating event during the 79 AD eruption at Pompeii.