Lithic-rich and lithic-poor ignimbrites and their basal deposits: Sovana and Sorano formations (Latera caldera, Italy)

摘要

Conceptual models explaining the characteristics of ignimbrites and their basal deposits have mainly focused on variations in particle concentration and speed of the parent pyroclastic currents. Here, we focus on the effects of relative proportions of clasts that are well coupled to the carrier gas phase and those that are poorly coupled, i.e., the proportions of fine and/or low-density clasts compared to coarse and/or dense clasts. We document facies and macroscopic clast populations of two ignimbrite-producing eruptive sequences of similar volume and composition, both erupted from Latera caldera (central Italy). The Sovana ignimbrite has 1-15vol% dense lithic clasts and locally up to 70vol% in lithic-rich domains. It preserves evidence of emplacement of multiple overlapping flow units with local internal channeling. The ignimbrite is underlain by two basal deposits: (1) a radially distributed, 20-80-cm-thick vitric tuff referred to as BUS (basal unit Sovana) that thins gradually with distance and shows evidence of emplacement by lateral currents; and (2) a fines-depleted, lenticular, massive lapilli tuff, up to 20cm thick, with abundant lithic clasts. The Sorano ignimbrite, in contrast, has <1vol% dense lithic clasts. It contains multiple reverse coarse-tail graded emplacement units, but we observed no internal channeling. In most places, its basal deposits are 20-100cm of dominantly planar-parallel, 5-10cm-thick, reverse coarse-tail graded tuff beds. However, in local depressions near the tops of paleocanyons that were filled by Sorano ignimbrite, the basal deposits are up to 2m thick and are clearly cross stratified. We interpret the different deposit characteristics in terms of recent modeling and experimental results. The Sovana eruption involved major caldera collapse that introduced abundant dense lithic clasts into the erupting mixtures. BUS may have partly resulted from expulsion of a dilute, fine-grained mixture at the site where the pyroclastic fountain(