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Pillow lava and spasmodic submarine fire fountaining in the middle Miocene marginal basin, S ado I sland, J apan
Author(s) -
Fujibayashi Norie,
Asakura Kensuke,
Hattori Takeshi,
Allen Sharon
Publication year - 2014
Publication title -
island arc
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.554
H-Index - 58
eISSN - 1440-1738
pISSN - 1038-4871
DOI - 10.1111/iar.12091
Subject(s) - geology , pillow lava , lava , scoria , lapilli , pyroclastic rock , geochemistry , submarine volcano , petrology , geomorphology , volcano
The S awasaki basalt unit of the O gi B asalt M ember is a middle M iocene submarine volcano formed in a marginal basin, cropping out on the coast of the O gi P eninsula, S ado I sland, J apan. It is approx. 100 m thick, more than 4 km wide, and is composed of pillow lavas ( S awasaki pillow lava) and overlying fire fountain deposits ( S awasaki pyroclastic rocks). The S awasaki pillow lava is a pile of nearly‐horizontal lava flows. M assive sheet lavas and lava pods grade into pillow lavas. The flat surfaces of the sheet lavas suggest that they spread over a summit plain. The margin of the S awasaki pillow lava is dominated by thin elongated pillow lobes (20–30 cm across) dipping at 45–60°, contrast with the large pillows (40–100 cm across) in the core. The proximal lithofacies of the S awasaki pyroclastic rocks is scoria agglomerate which is composed of tens of stratified beds. Individual beds have a range of clast sizes and abundance of subaqueous bombs (sometimes up to >70%), which suggests that they were formed by submarine spasmodic fire fountain eruptions. Progressive aggradation from a turbulent flow allowed settling of well‐preserved fluidal‐shaped subaqueous bombs in a fines‐depleted matrix. The basal layer shows lamina, scour and imbrication of scoria clasts. The thick scoria agglomerate beds with large subaqueous bombs, derived from energetic fire fountains, were emplaced during transition to higher degrees of hydrovolcanic fragmentation. Alternating scoria lapilli tuff and tuff beds in the distal area may represent the coupled topset and foreset beds formed by deposition from dilute, turbulent flows, and cascading of volcanic debris from the crest of the volcanic hill.