Facies Characterization and Volcanic Source Assignment of Marine Tephra Deposits Around São Miguel Island, Azores Archipelago

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Abstract
Long‐term tephrostratigraphies of volcanic islands such as the Azores are often limited to young and incomplete subaerial records. Here, we present a Pleistocene‐Holocene marine tephra archive around the eastern islands of the Azores based on 22 marine gravity cores. We provide a new extensive data set comprising major element glass compositions of 350 marine tephra layers and 26 tephra samples collected from subaerial outcrops on São Miguel Island. The marine tephra record is dominated by trachytic glass compositions with lesser amounts of trachy‐basaltic/basaltic materials. We categorize the marine deposits into four facies: primary fall (F1), primary flow (F2), eruption‐related secondary flow (F3), and non‐eruption‐related secondary flow deposits (F4). Cores collected close to the islands contain a high proportion (>50%) of volcaniclastic material deposited during secondary processes. Using machine‐learning algorithms, we assign potential volcanic sources to individual tephra layers. Furthermore, our spatial characterization of primary/secondary deposits demonstrates the importance of carefully selecting core locations during sea‐going campaigns, as the preservation and distribution of primary and reworked tephras considerably differ depending on several factors (e.g., on paleo‐wind directions, island topography, seafloor bathymetry, and distance from source volcanoes). We also report the occurrence of mafic primary fall tephras tens to hundreds kilometers away from the closest island, indicating a potential hazard from mafic Plinian eruptions in the Azores. This study aims to improve the estimation of the eruptive frequency of the archipelago and opens new pathways for more detailed studies concerning long‐term volcanic cyclicity in the region.

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