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

Abstract

Sete Cidades is an active yet dormant central volcano located in the westernmost part of Sao 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 Barbara Formation, related to the last stage of caldera formation. We define the lithostratigraphy of this pyroclastic sequence into three members based on stratigraphic position, lithofacies, and distribution of the deposits. The preserved deposits outcrop in a narrow area (over < 5 km between the caldera rim and the coastal cliffs) and include ash beds with dispersed lapilli (Lower Member), coarse-grained pumice and lithic-rich lapilli tuffs (Middle Member), and pumice fall deposits (Upper Member). The minimum bulk volume preserved on land is about 0.03 km3; however, the majority of the pyroclastic products 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. The mingling of these two magmas acted as the eruption trigger. The eruption history can be divided into three main phases each characterized by different eruption styles and deposit emplacement mechanisms, accompanied by enlargement of the caldera. The eruption started with a sequence of short-lived explosions generating unstable columns that produced ash fallout and generated dilute pyroclastic density currents, mostly deposited in the SW sector of the volcano. The climax of the eruption was marked by successive low 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 fall deposits on the NE flank of the volcano. Our study allows delineation of eruptive scenarios for future major explosive eruptions at Sete Cidades, a volcano which is located on the most populated island of the Azores (Sao Miguel Island), and can provide a framework for the assessment of volcanic hazards on oceanic islands elsewhere.