Abstract
The Early Miocene Koura Formation, SW Japan is composed mainly of reworked volcaniclastics and andesite to rhyolite pyroclastics, and constitutes part of the volcanic zone of SW Japan of that time. Reworked volcaniclastic sequences are thick and mostly coarse-grained, and based on conventional facies concepts, are interpreted as shoreface to beach deposits, turbidites, and debris-flow deposits. Thick acid tuffs intercalated in the volcaniclastic sequences individually show internal features characteristic of either subaerial or subaqueous ash-flow emplacement, or in some cases both. Andesite pyroclastic facies include surge and fall deposits. Facies relationships, together with the fossil record, indicate deposition in a shallow lacustrine environment. The deposition rate of the Koura Formation was probably high enough to rapidly fill the shallow lake, but sedimentation was balanced by rapid subsidence of the lake, similar to lakes present within the modern volcanic zones of Taupo, New Zealand and Hohi, Japan. The Koura Formation grades upward into marine deposits, and may represent the initial volcanism and volcaniclastic sedimentation during the early stages of back-arc or intra-arc rifting.
Published Version
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