On the use of unconformity surfaces as stratigraphic markers for identifying and correlating eruptive units in the Astroni pyroclastic sequence (Campi Flegrei, Italy)

Understanding eruptive behavior, eruption frequency and associated hazards represent major challenges in the volcanological community. Establishing a robust stratigraphical framework is the fundamental step for reconstructing the eruptive history of a volcano, whether characterized by single or multiple eruptive events. Unconformity surfaces (e.g. angular discordances, erosive surfaces, disconformities) are common features in all volcanic settings and record hiatuses in the stratigraphical succession associated with periods of quiescence in the eruptive activity. In the field, they are typically identified as irregular or truncated surfaces, often associated with erosive contacts, paleosols and/or interbedded reworked or weathered materials between primary tephra deposits. In this study, we present new field evidence of numerous unconformity surfaces within the ultra-proximal, proximal and medial sequences of the Astroni volcano, a multistage tuff ring edifice (4.1-3.8 ka) formed during the most recent eruptive epoch of Campi Flegrei. Previous studies have shown that Astroni was the source of seven relatively short-lived explosive events, separated by variably long repose intervals [Isaia et al., 2004]. Our analysis identifies five major unconformity surfaces throughout the Astroni sequence, which serve as key stratigraphic markers for correlating eruptive units from ultra-proximal, intra-crater to medial areas. These unconformity surfaces range from (i) irregular to smooth erosional surfaces, locally forming well-developed, U-shaped channels up to 2.20 m deep and 4.5 m wide, (ii) steeply sloping (46° dip), planar, surfaces, extending for over 5 meters in width, to (iii) gently undulated erosional surfaces with diffuse, low-amplitude, mm-to cm-sized scours. Thin paleosols and/or variably humified horizons are found locally at top of these surfaces, subsequently buried by pyroclastic density current and/or fall deposits of the following eruptive event. These features record multiple hiatuses of variable duration, possibly from hours to days or longer, suggesting an episodic nature of the Astroni eruptive activity. During these time breaks, secondary processes such as weathering, extensive erosion, reworking and crater wall collapses occurred, marking quiescent periods between successive Astroni eruptions. Our results show that reconstructing volcanic stratigraphy and constraining eruption frequency in complex multi-vents environment requires not only the analysis of primary deposits, but also the identification and interpretation of quiescent intervals recorded by unconformities. Finally, our study emphasizes the need to explicitly incorporate subaerial processes, including crater wall collapse and extensive erosion, into stratigraphic reconstructions and volcanic hazard assessments.