Event stratigraphy and emplacement mechanisms of the last major caldera eruption on Sete Cidades Volcano (São Miguel, Azores): the 16 ka Santa Bárbara Formation

Sete Cidades is an active yet dormant central volcano located in the westernmost part of São Miguel Island (Azores archipelago),characterized by a 5-km-wide multi-stage caldera formed in the last 36 ka. In this work, we present new stratigraphic, grain size,petrographic, and geochemical data of the 16 ka Santa Bárbara Formation, related to the last stage of caldera formation. We define thelithostratigraphy of this pyroclastic sequence into three members based on stratigraphic position, lithofacies, and distribution of thedeposits. The preserved deposits outcrop in a narrow area (over < 5 km between the caldera rim and the coastal cliffs) and include ashbeds with dispersed lapilli (Lower Member), coarse-grained pumice and lithic-rich lapilli tuffs (Middle Member), and pumice falldeposits (Upper Member). The minimum bulk volume preserved on land is about 0.03 km3; however, the majority of the pyroclasticproducts were deposited in the ocean. The presence of magmatic mafic enclaves (48 wt% SiO2) within the juvenile pumice clasts(63 wt% SiO2) is testimony to the injection of trachybasaltic magma into the trachytic magma reservoir shortly prior to eruption. Themingling of these two magmas acted as the eruption trigger. The eruption history can be divided into three main phases eachcharacterized by different eruption styles and deposit emplacement mechanisms, accompanied by enlargement of the caldera. Theeruption started with a sequence of short-lived explosions generating unstable columns that produced ash fallout and generated dilutepyroclastic density currents, mostly deposited in the SW sector of the volcano. The climax of the eruption was marked by successivelow fountaining events that generated high particle concentration pyroclastic density currents emplaced on the SW and NE flanks.The final phase of the eruption was characterized by establishment of a sub-Plinian eruption column that produced pumice falldeposits on the NE flank of the volcano. Our study allows delineation of eruptive scenarios for future major explosive eruptions atSete Cidades, a volcano which is located on the most populated island of the Azores (São Miguel Island), and can provide aframework for the assessment of volcanic hazards on oceanic islands elsewhere.