Emplacement processes of the mafic Villa Senni Eruption Unit (VSEU) ignimbrite succession, Colli Albani volcano, Italy

Abstract

The Villa Senni Eruption Unit (VSEU) represents the last large explosive event of the Tuscolano–Artemisio phase of volcanic activity in the Colli Albani (C.A.) (Alban Hills) Volcanic District, situated about 25 km south east of Rome, Italy. The VSEU succession consists of pyroclastic fallout, density current deposits, covering an area of 1500 km 2, reaching a distance of 35 km or more from vent, and having a volume of 30 km 3 DRE. The VSEU is unusual in that its juvenile products are K-foiditic and phono-tephritic in composition, with less than 50% SiO 2 content. The most important products of the VSEU eruption are two pyroclastic density current depositional units; the Tufo Lionato (TL) and Pozzolanelle (TP), with each deposit containing several recognisable internal facies types. A short time gap (hours to days) between the deposition of these two ignimbrites is inferred due to the lack of a palaeosoil or evidence of fluvial reworking at the contact between the two units. A new stratigraphic framework for the VSEU is presented based on field observations, component analyses and detailed lateral correlations, with several new stratigraphic subdivisions proposed. The eruptive sequence is subdivided into depositional facies types, each related to a discrete phase of eruption, and a reconstruction of the three-dimensional geometry and the internal facies association within the VSEU is presented. This reconstruction is used to assess the pyroclastic density current emplacement processes resulting from this ignimbrite forming explosive mafic eruption. The morphology of and internal structures preserved within the mafic VSEU are comparable to those seen in more silicic ignimbrite sequences, suggesting that similar processes of emplacement by pyroclastic density currents can be applied over the entire compositional spectrum. We interpret lateral and vertical facies changes in the VSEU ignimbrite deposits to be caused by changes in the palaeotopographic surface over which the depositing pyroclastic density currents were travelling, as well as spatial and temporal changes in the dynamics of the pyroclastic density currents responsible for their deposition. In particular, interaction between the emplacing pyroclastic density current and the underlying palaeotopography caused local changes in current dynamics, leading to a wide range of localised flow structures and internal facies complexity. These interpretations are made possible because the long axes of valleys into which the VSEU was emplaced, which occur as part of the drainage network surrounding the C.A. volcano, are aligned from 0 to 90° to the radial pyroclastic current direction. The erosive power of pyroclastic density currents is recorded by an erosive contact between the TL and TP ignimbrites in proximal exposures (<12 km) to the north west of the C.A. volcano.