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Pillow lava and spasmodic submarine fire fountaining in the middle Miocene marginal basin, Sado Island, Japan

journal contribution
posted on 2023-05-18, 07:33 authored by Fujibayashi, N, Asakura, K, Hattori, T, Sharon AllenSharon Allen
The Sawasaki basalt unit of the Ogi Basalt Member is a middle Miocene submarine volcano formed in a marginal basin, cropping out on the coast of the Ogi Peninsula, Sado Island, Japan. It is approx. 100 m thick, more than 4 km wide, and is composed of pillow lavas (Sawasaki pillow lava) and overlying fire fountain deposits (Sawasaki pyroclastic rocks). The Sawasaki pillow lava is a pile of nearly-horizontal lava flows. Massive 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 Sawasaki 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 Sawasaki 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.

History

Publication title

Island Arc

Volume

23

Issue

4

Pagination

344-364

ISSN

1038-4871

Department/School

School of Natural Sciences

Publisher

Blackwell Publishing Asia

Place of publication

54 University St, P O Box 378, Carlton, Australia, Victoria, 3053

Rights statement

Copyright 2014 Wiley Publishing Asia Pty Ltd

Repository Status

  • Restricted

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Expanding knowledge in the earth sciences

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