Transport and deposition of pyroclastic density currents over an inhabited area: the deposits of the AD 79 eruption of Vesuvius at Herculaneum, Italy L. GURIOLI*, R. CIONI  , A. SBRANA* and E. ZANELLA à *Dip. Scienze della Terra, Via S. Maria 53, 56126, Pisa, Italy (E-mail: gurioli@dst.unipi.it)  Dip. Scienze della Terra, Via Trentino 51, 01927, Cagliari, Italy àDip. Scienze della Terra, Via Valperga Caluso 35, Torino, Italy ABSTRACT Geological and volcanological studies were performed in the Herculaneum excavations, 7 km west of Vesuvius, Italy, to reconstruct the main features of the pyroclastic density currents and the temporal sequence of the AD 79 eruptive events that destroyed and buried the town. The identification of two distinctive marker beds allows correlation of these deposits with the better- known sequences to the south of Vesuvius, along the dispersal axis of the Plinian fall deposit. Detailed observations from stratigraphic sections show that the pyroclastic density current deposits are characterized by several sedimentary facies, each recording different depositional and emplacement mechanisms. Facies analysis reveals both lateral and vertical variations from massive to stratified deposits, which can be related to the combined effects of flow dynamics and local irregularities of the substratum at centimetre or metre scales. These topographic irregularities enhanced turbulence and allowed rapid transition from non-turbulent to turbulent transport within the flow. Fabric data from these deposits, both from roof tile orientations and anisotropy magnetic susceptibility (AMS) analyses carried out on some of the pyroclastic deposits, suggest that the pyroclastic density currents were strongly affected by the presence of buildings. These obstacles probably caused deflection and separation of flows into multiple lobes that moved in different directions. Keywords Fabric, facies, Herculaneum, pyroclastic density currents, roof tiles. INTRODUCTION Archaeological sites in volcanic areas represent an important source of data to investigate the mutual relations and interactions between pyroclastic density currents and buildings. The deposits of the AD 79 eruption of Vesuvius, Italy, are among the best examples of such phenomena, owing to the presence of extensive, largely excavated, urban areas that were completely buried under a thick pyroclastic deposit (Sigurdsson et al., 1982, 1985; Cioni et al., 2000). In particular, Sigurdsson et al. (1985) described in detail the pyroclastic sequence inside Herculaneum and interpreted the succession of flow units as the result of deposition from dense, pyroclastic flows each preceded by a ground surge layer. Kent et al. (1981) and Sig- urdsson et al. (1982, 1985) also correlated these deposits, previously interpreted as the products of repeated lahars (Rittmann, 1967; Sheridan et al., 1981), with the pyroclastic sequence cropping out around the volcano. In this paper, new data are presented that are derived from volcanological, sedimentological and magnetic studies performed in the area of the Herculaneum excavations. The new evidence helps in defining more precisely the chronology and main features of the events that destroyed Herculaneum during the AD 79 Sedimentology (2002) 49, 929–953 Ó 2002 International Association of Sedimentologists 929