Small-Scale Lateral Variations in the Post-Plinian AD 79 Pyroclastic Density Currents Deposits

Plinian eruptions form a sequence of pyroclastic fall 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 AD 79 PDC deposits on a regional scale. We investigate small-scale lateral variations of these Vesuvian deposits to document factors governing PDC emplacement mechanisms. Here we present data related to post-Plinian deposits which exhibit the most widespread distribution and are traceable on mountain slopes about 20 km from the vent. We used a stratigraphic approach to identify and correlate marker planes. We found many localities where, up to five, thin post-Plinian fall layers and ignimbrite deposits are interbedded. Detailed documentation of these fall deposits enabled a consistent stratigraphy to be recognized and correlated over a wide sector S of the caldera. The detailed and systematic study of the AD 79 post-Plinian PDC units showed that many of these lateral variations occur in a few meters or tens of meters and can be caused by small obstacles. Several lateral facies variations are observed at different distances from the vent. In proximal locations, on the volcano slopes, the most recurrent variation consists of thick, up to 4 m, massive or faintly stratified, coarse, lithic-rich deposits that pass laterally to few centimeters thin, well-stratified ash layers. This lateral variation coincides with paleovalley ridge topography. In medial locations (<10 km from the source) we observe both plane-parallel, reverse graded layers passing to bedforms up to 4 m in wavelength and 45 cm in amplitude, and massive, poorly sorted deposits passing to stratified layers. In distal areas, 20 km from the source, thin, massive fine ash layers pass to few decimeters thick beds with well-rounded pumice clasts dispersed in an ashy matrix. The currents are very sensitive even to small variations of the substrate that induce changes in thickness and facies both in proximal and distal areas. This confirms that the facies completely depends on the depositional rather than the transport mechanism.