A Miocene coarse volcaniclastic mass-flow deposit in the Shimane Peninsula,SW Japan: product of a deep submarine eruption? |
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| Authors: | Kazuhiko Kano |
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| Institution: | (1) Geology Department, Geological Survey of Japan, 1-3, Higashi 1-chome, Tsukuba, Ibaraki, 305, Japan, JP |
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| Abstract: | A subaqueous volcaniclastic mass-flow deposit in the Miocene Josoji Formation, Shimane Peninsula, is 15–16 m thick, and comprises
mainly blocks and lapilli of rhyolite and andesite pumices and non- to poorly vesiculated rhyolite. It can be divided into
four layers in ascending order. Layer 1 is an inversely to normally graded and poorly sorted lithic breccia 0.3–6 m thick.
Layer 2 is an inversely to normally graded tuff breccia to lapilli tuff 6–11 m thick. This layer bifurcates laterally into
minor depositional units individually composed of a massive, lithic-rich lower part and a diffusely stratified, pumice-rich
upper part with inverse to normal grading of both lithic and pumice clasts. Layer 3 is 2.5–3 m thick, and consists of interbedded
fines-depleted pumice-rich and pumice-poor layers a few centimeters thick. Layer 4 is a well-stratified and well-sorted coarse
ash bed 1.5–2 m thick. The volcaniclastic deposit shows internal features of high-density turbidites and contains no evidence
for emplacement at a high temperature. The mass-flow deposit is extremely coarse-grained, dominated by traction structures,
and is interpreted as the product of a deep submarine, explosive eruption of vesicular magma or explosive collapse of lava.
Received: 10 January 1996 / Accepted: 23 February 1996 |
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| Keywords: | Submarine eruption Explosive lava collapse Coarse pumice Volcaniclastic mass flow Traction Suspension |
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