The emplacement of the 39.8 ka Campanian Ignimbrite flow, Italy

Abstract:

Processes of transport and deposition in huge pyroclastic density currents (PDCs) are still poorly understood. Interpreting pyroclasts is the main key to understand the processes that occur during the flow and the emplacement of PDCs. Here, we present new data about the Campanian Ignimbrite PDC, related to a 39.8-ka caldera-forming eruption in Campi Flegrei, Italy. The ignimbrite is widespread over an area of 6000 km², with a final runout of about 80 km and the ignimbrite occurs up to ~1000 m above sea level (vent elevation 0 m asl) and overtopped ridges several hundred meters higher. We use thickness data, the topological aspect ratio (ARt), clast dimensions, the competence ratio (CR), and depositional features to interpret the flow dynamics. The PDC crossed a plain before encountering topographic barriers at ~35 km and flowed over many ridges and along valleys to the final runout. The ARt and the CR indicate both forced and inertial flow dynamics: blocking is prominent due to topographic barriers while channeling occurred in the paleovalleys. Analysis of the slope angle and morphological characteristics of the deposits reveals two different systems, transport and depositional, in the CI current. These systems acted differently: an extremely dilute, high energy transport system and a low energy but fluid depositional system of near-Newtonian, fluidized dense granular undercurrents, producing valley-pond deposits with flat tops and no veneer facies or deposits on steep slopes. The identification of the two systems and their dynamics was possible because of the interaction with topography. The coexistence of low and high energy features in the same current gives important insight into the interpretation of PDC deposits elsewhere – such features may not be detectable without topographic interaction. Fieldwork remains one of the main tools to study PDCs and extrapolate important information about their dynamics.