Lithofacies association and stratigraphy of the Quixaba and Remédios formations, Fernando de Noronha archipelago, Brazil.

This work presents facies analysis of pyroclastic and effusive deposits present in areas of the Fernando de Noronha Archipelago (FNA), South Atlantic Ocean. A detailed stratigraphic study was carried out for the Remédios Formation at the Caieira beach and the Quixaba Formation at the Capim-Açu Edge, Americano, and Bode beaches. The lithofacies association for the deposits of the Remédios Formation characterizes non-welded ignimbrites of phonolitic composition generated by concentrated ground-hugging density currents originated from collapses of a buoyant eruption column. Possibly, this phase was followed by a caldera collapse and the exposition of a complex system of dikes at Caieira Beach. For the deposits of the Quixaba Formation, the lithofacies association characterizes scoria and ash flow/grain avalanching deposits, proximal fallout deposits, and Pahoehoe lava flows. The Quixaba volcanism shows evidence of pulses of Strombolian and possibly Violent Strombolian activity alternated with effusive Hawaiian activity. A later stage of magma-water interaction during the Quixaba volcanism led to the formation of a sustained eruption column and fallout deposits with ash aggregates formed through wet aggregation. This paper is the first ever written about aspects of the Physical volcanology of the Fernando de Noronha Archipelago, Brazil. It contributes to the understanding of the evolution of the volcanism that occurred in the Archipelago between ∼12 and ∼1.2 Ma, and of the South Atlantic Ocean. The conclusions of our study are: The stratigraphic study of specific areas of the Archipelago through field observations complemented with laboratory-based pyroclastic and lava flow descriptions allowed us to propose the following evolutionary model: • Explosive magmatic eruptions formed buoyant eruption columns that collapsed and originated ground-hugging concentrated density currents represented by the lithic-rich non-welded ignimbrites. • Volcanic and subvolcanic manifestations generated domes of phonolitic and trachytic composition; i) and ii) might be concurrent. Possibly this phase was followed by a caldera collapse with the exposition of a complex system of dikes at Caieira Beach. • A hiatus in the geological record of about 3 Ma, characterized as a period of quiescence and erosion. • Effusive Hawaiian eruptions with the emplacement of olivine nephelinitic pahoehoe lava flows. Alternated to the effusive eruptions, Strombolian and possibly Violent Strombolian pulses led to mass ejections of juvenile ash, lapilli, bombs, and cognate lithic fragments, forming grain-avalanching scoria deposits, and scoria-and-ash flow deposits. • Stage of dominant water-magma interaction and the formation of a water-rich volcanic cloud, which led to the formation of planar-stratified bedforms, fall-dominated with mixed deposition of ash aggregates in the shape of massive ash pellets, layered and complexly layered accretionary ash pellets, armored and complexly layered armored ash pellets, formed through wet aggregation. [ABSTRACT FROM AUTHOR]