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 ,**0+  ,1  ,**00  ,0 Pyroclastic Density Current from the Caldera-forming Eruption of Izu-Oshima Volcano, Japan :Restudy of the Sashikiji , Member Based on Stratigraphy, Lithofacies, and Eruption AgeTakahiro YAMAMOTO῍
The Sashikiji , (S,) Member in the products of Izu-Oshima volcano was formed by an explosive eruption
accompanied with caldera depression. This member is characterized by breccia called as a “low-temperature
pyroclastic ﬂow deposit”. In this paper, the S, breccia is re-examined based on stratigraphy, grain fabrics,grain-size distributions and modal compositions. The S, Member is divided into six units from S,-a to S,-f inascending order. The S,-a unit consists of scoria, bomb and aa lava ﬂows from ﬂank ﬁssures. The S,-b unit ismade up of well-bedded ash and ﬁne-lapilli from the summit. The S,-c unit is composed of matrix-supported
breccia, locally ﬁlling valley bottoms and containing abundant deformed soil fragments and woods. The S,-d unit
consists of reverse to normal grading, clast-supported breccia with ash matrix, covering topographic relief in thewhole island. The S,-e unit is composed of dune- to parallel-bedded lapilli and ash in the proximal facies. TheS,-f unit is clast-supported breccia with and without ash matrix. New +.C ages of wood fragments in the S,
Member have been determined as about Cal AD -.*. Although the S,-c and -d units are previously interpreted
to the low-temperature pyroclastic ﬂow deposit, these units are quite di#erent in sedimentological features asfollows. The grain fabric measurements have revealed that the S,-d unit has a-type imbrication showing thelongest axis of grains parallel to the ﬂow direction. On the other hand, the S,-c has random fabric of grains.
The grain size distribution of the S,-d unit shows a bimodal nature having subpopulations at phi +.* to +.* andcoarser than phi ,./. The bimodal nature and a-type imbrication suggest that the two transport processes
overlap ; the load of a turbulent suspension is not all in true suspension as the coarser population may travel in acast-dispersion mass ﬂow. The S,-c unit shows a polymodal grain size distribution with multi subpopulations
from coarse to ﬁne. The poor sorting, massive appearance, valley-conﬁned distribution, and random grain fabricof the S,-c unit are characteristic of deposition from a cohesive ﬂow without formation of traction-related
bedforms or sorting of di#erent grain sizes by turbulence. The modal composition measurements have indicatedthat the S,-c and -d units lack essential scoriceous or glassy fragments. This evidence indicates that both units arederived from steam explosions due to outburst of highly-pressurized geothermal ﬂuid within the ediﬁce. The S,-cunit was plausibly generated by remobilization of phreatic debris around the summit caused by ejection ofcondensed water from a plume or heavy rainfall. The S,-d unit was a pyroclastic density current deposit resulted
from collapse of a highly-discharged phreatic plume. Estimated velocities of the current are +/* to -* m/s basedon suspended grain sizes.
Key words : Izu-Oshima, S,, pyroclastic density current, caldera, steam explosion+ῌ ῐ ῎ ῑ ῏ (Fisher and Schmincke, +32.)3 45678.

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Research Center for Deep Geological Environments,AIST, Geological Survey of Japan, Higashi +ῌ+ῌ+ Cen-tral 1, Tsukuba -*/ῌ2/01, Japan.
e-mail : t-yamamoto@aist.go.jp