Eruption History and Depositional Processes of the Campanian Ignimbrite Based on its Lithofacies Architecture

Large ignimbrites are the product of high-temperature pyroclastic density currents (PDCs) spreading over vast regions and representing one of the most devastating natural catastrophes in human history. We present a detailed examination of the ignimbrite sequence, emplaced from 10 to 80 km from the source area, of the Campanian Ignimbrite (CI) eruption, a caldera-forming Plinian event, occurred 39 ka ago, whose PDC spread over a huge area from Campi Flegrei (Italy). The CI sequence comprises seven lithofacies based upon the different sedimentary structures: 1) massive tuff or lapilli tuff; 2) diffuse-stratified tuff; 3) fines-poor lithic lapilli tuff; 4) pumice-rich lapilli tuff; 5) lenses of pumice lapilli; 6) cross-stratified tuff; 7) accretionary lapilli-bearing. Three main vertical facies association have been identified: a) fines-poor lithic lapilli to diffuse stratified or massive or inverse graded; b) cross-stratified or diffuse stratified to massive or normal- to inverse-graded; c) massive to inverse graded. These vertical variations define a dominant trend during which the concentration into the boundary layer was progressively increasing, at a fixed point, and records the increasing importance of fluid-escape condition with time. Distribution of lithofacies in the different units shows a remarkable lack of lateral variation at regional scale. This suggests an overall uniform behavior of the PDC. A temporal evolution, from traction- to granular- or fluid-escape- dominated mechanisms, records unsteadiness conditions and contrast with persistent lateral facies reflecting an overall uniform behavior of the current. Stratigraphical and chemical evidences indicate the presence of a bypass zone around the caldera.