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,**0  0  ,-  ,**0  +,  ,, A ,.* Ma Widespread Tephra Associated with a Large-Scale Pyroclastic Flowfrom the Sengan Geothermal Area, Northeast Japan Arc
Takehiko SUZUKI῍ and Toshio NAKAYAMA῍῍
A widespread tephra referred to here as Tamagawa-R. Tephra (Tmg-R.) is newly recognized. Tmg-R.,
derived from the Pre-Yakeyama caldera located in the Sengan geothermal area, the Northeast Japan arc, covers
the area from Tohoku to Kanto, northeast of Honshu Island. At the type locality in the proximal area, Tmg-R.
comprises a non-welded pyroclastic ﬂow deposit (ignimbrite) and an immediately overlying welded pyroclastic
ﬂow deposit (Kurasawayama Welded Tu#). Absence of plinian fall deposits in the area of ca. ,/ km south of thesource and the ﬁne vitric ash nature of the distal ash-fall deposits of Tmg-R. suggest that they are co-ignimbrite
ash-fall deposits. Tmg-R. was identiﬁed using a combination of refractive indices and chemical compositions ofmajor and rare earth elements of glass shards (n+..32῍+./*+, SiO, : 12.-῍12.0 wtῌ, K,O : ..,῍../ wtῌ, Ba : 2-*
῍3++ ppm), mineral content, refractive indices of hornblende (n,+.00/῍+.020). On the basis of these properties,Tmg-R. was identiﬁed in Boso and Oga peninsulas, Choshi area, and in the core drilled on Musashino uplandaround /** km south of the source. Calcareous nannofossil biostratigraphic (Calcareous nannofossil datum +-)and magneto-stratigraphic positions in Boso peninsula and Choshi, and paleomagnetic direction and many
radiometric ages determined in the proximal area by previous studies indicate that the age of Tmg-R. is ca. ,.*Ma, positioned just below the base of the Olduvai Subchron. The distribution of Tmg-R. showing emplacementof co-ignimbrite ash-fall deposit in the area /-* km south of the source, emphasizes the upwind transport direction
relative to the prevailing westerly winds. This distribution shows similarity to those of a few co-ignimbrite
ash-fall deposits derived from calderas in the Northeast Japan arc. As a key marker horizon in this age, thewidespread occurrence of Tmg-R. provides a tie line between many di#erent sections over a distance of /-* km.
Additionally, Kd..-Nk Tephra above Tmg-R. is recognized in Boso peninsula, Choshi, Niigata and east Lake
Biwa areas. Characteristic properties and stratigraphic positions indicate that Kd..-Nk possibly derived fromthe Sengan geothermal area occurred at +.302῍+.12+ Ma.
Key words : Tamagawa-R. Tephra, Kurasawayama Welded Tu#, caldera, widespread tephra, Olduvai Subchron+ῌ ῐ ῎ ῑ ῏

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.I jFkNl emnFop3451T !N-NOP'.:;<L/012 QR Tn /01 (AT) S.op/01 Toya, ++,῍++/ ka ; ^_r+3,῍*-31 dstuvwxy:z +῍+{t:|dstx}~||Department of Geography, Tokyo Metropolitan University, +῍+ Minamiosawa, Hachioji, Tokyo +3,῍*-31,Japan.
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q +321 ; ^Institute of Civil Engineering of Tokyo Metropolitan
Government, +῍3῍+/ Shinsuna, Koto, Tokyo +-0῍**1/,
Japan.
Corresponding author : Takehiko Suzuki
e-mail : suzukit@comp.metro-u.ac.jp