A,.* Ma Widespread Tephra Associated with a Large-Scale Pyroclastic Flow from the Sengan Geothermal Area, Northeast Japan Arc

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1 /, (,**1) +,- -2,.* Ma,**0 0,-,**0 +,,, A,.* Ma Widespread Tephra Associated with a Large-Scale Pyroclastic Flow from 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 the source 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 of major and rare earth elements of glass shards (n /*+, SiO,: wt, K,O:..,../ wt, 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 upland around /** 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 emplacement of 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, the widespread 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 from the Sengan geothermal area occurred at Ma. Key words : Tamagawa-R. Tephra, Kurasawayama Welded Tu#, caldera, widespread tephra, Olduvai Subchron + /** km Tn (AT) +3, * Department of Geography, Tokyo Metropolitan University, + + Minamiosawa, Hachioji, Tokyo +3, *-31, Japan. +-0 **1/ + 3 +/ Aso-.,**- Toya, ++, ++/ ka ; +321 ; Institute of Civil Engineering of Tokyo Metropolitan Government, + 3 +/ Shinsuna, Koto, Tokyo +-0 **1/, Japan. Corresponding author: Takehiko Suzuki suzukit@comp.metro-u.ac.jp

2 24 Fig. +. Map of the Japanese islands and their surrounding area showing distributions of Tmg-R. (solid line) and four widespread tephras (broken lines) : Toya (++, ++/ ka), Aso-. (2/ 3* ka), AT (,0,3 ka) and Hkd-Ku (10* ka) Tephras (modified from Suzuki et al.,,**/). Distributions of Toya, Aso-. and AT are taken from Machida and Arai (,**-).,**- (Hkd-Ku, 10* ka ; Suzuki et al.,,**/) (Fig. +) Toya Hkd-Ku, Toya Hkd-Ku co-ignimbrite ash-fall deposit (Sparks and Walker, +311) Toya co-ignimbrite ash-fall deposit co-ignimbrite ash-fall deposit (,**/) (+-./ 2 Ma) (2 /.- Ma) (/ Ma) (+.1 * Ma).

3 ,.* Ma 25 Fig.,. Distribution of Kurasawayama Welded Tu# and related rocks (modified from Sudo, +321b, +33,). -./ Ma + Ma, Ma. R. ; +321a, +321b R. (+32+) (+33,) R. Tmg-R. ;,**-, Tmg-R., + (Fig.,) (+33,)

4 a, +321b (D) (R.) - + A(OR-, OR,, OR+, ORA) 0 K-Ar + Ma, Olduvai Subchron ( Ma ; Lisiecki and Raymo,,**/) Matuyama Chron, Ma +321b +-* km b (Fig.,) +321a +321b 1** m +321b,. km Loc. +a; +321b. + +/ m (Fig. -) 1* cm +330 (Table +) n, n, n n, +.00/ EDS * (wt) SiO, : 12./ Al,O - : +,.* FeO : +., CaO : *.+ K,O:..- Na,O: -./ ICP (ppm) Ba: 3*, La:,0 Y:.2 (Table,) * EDS CaO AT *.3 +., wt *.0 *.1 wt Na,O K,O *.2 (+33.) Na,O K,O Na,O K,O Ba Ba 2** ppm Fig..,, Tmg-R., Ma /-* km Tmg-R. (Fig. +) Tmg-R. +33*,**. ; Koizumi and Kanaya, +310 ; Matoba, +301 ; +33* ; (,**.) Eb-Fukuda, +.1/ Ma ;,*** Kd -3 Ho-Kd Ma ;,*** Tmg-R. Tmg-R. Kg+c +33* (Fig. -, Loc. -a) Kg+c,+ cm *.0 mm (Table +) n +./** +./*+ n, / (wt) SiO,: 12./ Al,O -: +,.* (Table,) CaO

5 ,.* Ma 27 Fig. -. Lithological columnar sections of Tmg-R. and related tephras. Localities of each section are shown in Fig. +. Standard columnar sections at Loc.,, Loc. -a-d and Loc. 0 are from Kurokawa et al. (+333), Sakai (+33*) and Shimogama and Suzuki (,**0), respectively.

6 28 Table +. Petrographic properties of Tmg-R. and related tephras.

7 Table,. Chemical compositions of glass shards from Tmg-R. and related tephras.,.* Ma 29

8 30 Fig... Histogram of Ba content in volcanic glass shards in 23 Pliocene-Quaternary tephras in the Japanese islands (Suzuki, unpublished data). Kurasawayama Welded Tu# is treated as representative of Tmg-R.. FeO : +., CaO : *.+ K,O:.., Na,O: -.0 (ppm) Ba: 2-* La:,- Y:.+ (Table,) Kg+c Kg,b (Fig. -) Olduvai Subchron +33* (+333) Discoaster brouweri +- Kg+c +3* cm +- Kg+c +- Cande and Kent (+33/) Olduvai Subchron +.3/ Ma +.31 Ma +333 Olduvai Subchron Ma (Lisiecki and Raymo,,**/) Ma Kg+c +.3/ Ma,.** Ma,.* Ma Kg+c Kg+c Tmg-R. Kg+c Loc. -b Kg+c Kg,a +* m +/ cm (Fig. -) (Table,) Kg+c Tmg-R. Kg+c Tmg-R. HSC +33/ HSC +33+ Hs +30+ HS +310 Loc..a 2- cm 1- cm n +./** +./*+ n, / (Table +) (wt) SiO,: 12./ Al,O -: +,.* FeO : +., CaO : *.+ K,O:... Na,O: -./ (ppm) Ba: 2-+ La:,, Y:.- (Table,) Tmg-R. HSC Tmg-R. (+310) Olduvai Subchron HS HS HSC HSA +33/ HSC HSA Tmg-R. (Loc. /) ++/ m, +333 B-, 1/.3* 10.** m B,-,0 (MTB,-,0) +333 T-/+ MTB,-,0 +* cm

9 ,.* Ma 31 n +./** n, (Table +) (wt) SiO,: 12.- Al,O -: +,.* FeO : +., CaO : *.+ K,O:../ Na,O: -.. (ppm) Ba: 3*/ La:,0 Y:.2 (Table,) Tmg-R. (+333),* m +-.0 m Matuyama Chron -2 m 0, m Jaramillo Subchron (+.*1/ *.33+ Ma ; Lisiecki and Raymo,,**/) Olduvai Subchron,*** MTB,-,0 1/.3* 10.**m 1,.12 m, m, m MTB,-,0 MTB,-,0 Tmg-R. -2 0, m Olduvai Subchron +33+ (Loc. 0) / (Ysg/) / cm HSC,**0 (Tables +,,) Tmg-R., - Tmg-R. +31/ Tmg-R. (Loc. +b) +/ cm (Fig. -) / cm, mm Tmg-R. n /** n, (Table +) (wt) SiO, : 12./ Al,O - : +,.* FeO : +., CaO : *.+ K,O:.., Na,O: -./ (ppm) Ba: 222 La:,/ Y:.0 (Table,) Tmg-R. 1* km Tmg-R. (+31/) Ak- (+31/) - Tmg-R. Tmg-R. Tmg-R. Olduvai Subchron Matuyama Chron Olduvai Subchron - + Tmg-R. ( HSC) +0* m Olduvai Subchron +310 Kd ; +33/ Tmg-R. (Loc..b) +. cm 10 cm Kd.. *.. mm (Table +) /*+ (wt) SiO,: 12.1 Al,O -: ++.2 FeO : +.- CaO : *.- K,O: -.1 Na,O: -.1 (ppm) Ba: 2/* La:,. Y: /- (Table,) Tmg-R. Ba EDS Kd.. K, O -.1 wt *.,

10 32 wt Tmg-R...,../ wt *.+ *.- wt (Fig. /) SiO, CaO Na, O Kd.. Al,O - ICP Tmg-R. Tmg-R. Kd.. Fig. /. Comparison of EDS results on glass shards of Tmg-R. and Kd..-Nk.

11 ,.* Ma 33 K,O Tmg-R. Kd.. HSA Loc..a HSA 2 m HSC HSA 2. cm (Tables +,,) HSA Tmg-R. Kd.. Tmg-R. ( Kg+c) Kg,a Kg,b, -/m Kg- Fig. - ; +33* Kg,b Kg - Tmg-R. Kg-.2 cm Tmg-R. Kd.. (Fig. /, Table,) Kg- K, O -./ wt Tmg-R. Olduvai Subchron +33* Kg- Kd.. Kg,b Tmg-R. ( Kg+c) Kd.. ( Kg-) HSA (Tables +,,) Kg,b Olduvai Subchron +33* Kd.. Tmg-R. Kg,b HSA Sig; +323 ; +33* Sig HSC,**, Sig (+322) (+330) (+330) Olduvai Subchron (,**.) Sig Olduvai Subchron Sig (Loc.,) Sig 2 cm Tmg-R. Kd.. K, O -.3 wt Tmg-R. Kd.. (Fig. /, Table,) SiO, CaO Tmg-R. Kd.. Sig Kd.. (,**,) Tmg-R. Olduvai Subchron Tmg-R. (+32-) II II Nk; Nk (Tables +,,) Loc. 1 Nk ++. cm. cm Tmg-R. Kd.. K, O -./ wt Tmg-R. Kd.. (Fig. /) SiO,,Al,O -, FeO, CaO, Na,O Tmg-R. Kd.. Nk Kd.. Hayashida and Yokoyama (+32-) Nk Olduvai Subchron Kd.. (+33-) Nk -, Kd..-Nk Tmg-R. Kd.. Sig, Kg - Nk Tmg-R. Olduvai Subchron Matuyama Chron Olduvai Subchron Kd..- (Kd..-Nk)

12 34 Kd..-Nk Tmg-R. FeO Ba SiO,, CaO, Na,O, Al,O - Olduvai Subchron.. + Tmg-R. (,**-) +,* 2.,* km AT Aso-. (Fig. +) co-ignimbrite ash-fall deposit co-ignimbrite ash-fall deposit Tmg-R.,/ km Loc. +a co-ignimbrite ash-fall deposit Toya Hkd- Ku Tmg-R. co-ignimbrite ashfall deposit - co-ignimbrite ash-fall deposit +,*** +,/** m (Imura, +33-)., co-ignimbrite ash-fall deposit +-* km b Tmg-R. Tmg-R. co-ignimbrite ash-fall deposit Tmg-R. /-* km Tmg-R. /,* cm /-* km +* cm 22 km - Tmg-R. co-ignimbrite ash-fall deposit Tmg-R. Table - Suzuki et al. (,**/) Hkd-Ku co-ignimbrite ash-fall deposit Tmg- R. co-ignimbrite ashfall deposit Hkd-Ku, Toya Tmg-R. Hkd-Ku Hkd-Ku

13 ,.* Ma 35 Table -. Comparison of caldera dimensions and volumes of pyroclastic flow deposits and their related co-ignimbrite ash-fall deposits. Tmg-R. Hkd-Ku Hkd-Ku -1 km Tmg-R. +-* km b Tmg-R. co-ignimbrite ash-fall deposit Aso-. AT Hkd-Ku co- ignimbrite ash-fall deposit Toya co-ignimbrite ashfall deposit Hkd-Ku +332 co-ignimbrite ash-fall deposit Hkd-Ku Toya co-ignimbrite ash-fall deposit co-ignimbrite ash-fall deposit. - Tmg-R. K-Ar Tmg-R., Ma +321b K-Ar +.1,./ Ma, Ma Tmg-R.,.* Ma +.3/ Ma,.** Ma Tmg-R. (OR- +, ORA) +321b (Tmg-R.) +** + Ma

14 36,*** ; +321b Tmg-R. (Tables +,,) (+321b) Tmg-R.,**0 Tmg-R. Kd..-Nk Kd..-Nk *./ La Y,**+ Tmg-R. Kd.. -Nk Ba Kd..-Nk La Y *../ Kd..-Nk Na,O K,O * *.2 Tmg-R Na,O K, O Ba Kd..-Nk Kd..-Nk Kd..-Nk Olduvai Subchron Ma Tmg-R.,* Kd..-Nk Kd..-Nk (,**.) Kd..-Nk / Tmg-R.,.* Ma Tmg-R. coignimbrite ash-fall deposit Tmg-R. Kd..-Nk + Ma (C) (,) +//**023 Cande, S.C. and Kent, D.V. (+33/) Revised calibration of the geomagnetic polarity timescale for the Late Cretaceous and Cenozoic. J. Geophys. Res., +** (B.), 0*3-0*3/. (+33+) +33*,+2 p. (+333) CD-ROM. (,***) -,,-. p. (,**.) ++*.2*.30. (+323) / + +./ p. Hayashida, A. and Yokoyama, T. (+32-) Paleomagnetic chronology of the Plio-Pleistocene Kobiwako Group to the southeast of Lake Biwa, Central Japan. J. Geol. Soc. Japan, 23,,*3,,+.

15 ,.* Ma 37 Imura, R. (+33-) A reconstruction of small-scale eruptions using pyroclastic deposits. Geographical Reports of Tokyo Metropolitan University,,2, 13 ++,. (+33+) (+322)., (,**.).3,2-,31. (+330) +*/ 22 ++,. (+31/) 1/ Koizumi, I. and Kanaya, T. (+310) Late Cenozoic marine diatom sequence from the Choshi district, Pacific coast, central Japan. In Progress in Micropaleontology (Takayanagi, Y. and Saito, T. eds), +.. +/3, Micropaleontology Press, New York. (,**0) ++/ +,/. (+33*) -+ /- 21. (,**.) (., Ma) Nwg Arg Tsp YT (+323) -* (+333) Jjj Sig *+. Lisiecki, L.E. and Raymo, M.E. (,**/) A Pliocene- Pleistocene stack of /1 globally distributed benthic d +2 O records. Paleoceanography,,*, PA+**-, doi: +*.+*,3/,**.PA**+*1+. (,**-) --0 p. (+321),0 +,3 +./. Matoba, Y. (+301) Younger Cenozoic foraminiferal assemblages from the Choshi district, Chiba prefecture. Sci. Rep. Tohoku Univ., Ser. II (Geol.), -2,,,+,0-. (+322) 3. +*- ++,. (+30+). (+ : /*,***) (,**+),- 0*/ 0*3. (,***) +*0 /+ 03. (+33.) Leg. +,1, Leg+, (+310) 2, (,**,) +*3,./. (+33*), (+332) +332.,*. (+32+) 21,01,1/. (+333) /-,0/,1.. (+322). : /-.1/.3+. (+33/) +* ,. (,**0) HSC,2 /0 0*. Sparks, R.S. J. and Walker, G.P.L. (+311) The significance of vitric-enriched air-fall ashes associated with crystalenriched ignimbrites. J. Volcanol. Geotherm. Res.,,, -, (+321a), (+321b), ,. (+33,),+ / / p. Suzuki, T., Eden, D., Danhara, T. and Fujiwara, O. (,**/) Correlation of the Hakkoda-Kokumoto Tephra, a widespread Middle Pleistocene tephra erupted from the Hakkoda Caldera, northeast Japan. The Island Arc, +., (+333) +*

16 p. (,***) p. Prima, O.D.A. (,**/).. +3/,+0. (+32-),/./ 0+. (+33-) +3* +33.

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