ῒΐ  /,  (,**1)
 +  ,-ῌ-2 ῕ ῔   ,.* Ma 
  !


,**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 ῏Aso-. T2UVWX2YZF3451

!T[V.J!"# /** km $%&'()*NG\]. ^_ `a ,**-I bc defF3

+,-./012 34516789:;<=>

451T !N-.(M/01Egh\H.Gi#'

??@ABCD'.EFGH.I JFKL(M/01

.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.
r+-0῍**1/ dstd` +῍3῍+/dst

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

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 (++,ῌ++/ ka), Aso-. (2/ῌ3* ka), AT (,0ῌ,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 (,**-).



i-=>$ B:2j(kl-.3:"h< (Hkd-Ku, 10* ka ; Suzuki et al., ,**/)

i-ab9#G: mg%2  C2=>H7I

 ,**-  !"#$ #" %&'()*+,-.

,JK-LM co-ignimbrite ash-fall deposit nopq2+/0%123 456-789#2::.c:rAB2B$ ;<-=>stop";<-=>?@9#AB (Fig. +)$qn :u:vw% H7I,JK-LM

Toya  Hkd-Ku

C-

DE

:"+

co-ignimbrite ash-fall deposit

abxyzg{n/FGH7I,JK-LMNO PQ-% RS|}n $  ~{OnP-% D

-TU , V WX $ :"`A DE H

YZ[-=> Toya  Hkd-Ku %H7I,JKLB\]9# co-ignimbrite ash-fall deposit (Sparks and

7I,JK-LMNO v/ n $ (,**/) % DE-E,

Walker, +311) ^_"#AB #"%456 E (+-./ῌ2 Ma) 

`-.3ab9#G%^_GB$ 2cd:",E (2ῌ/.- Ma) ,E (/.-ῌ+.1;<-=>F Toya

ef%g":-:"h<Ma) ,E (+.1ῌ* Ma). -=F ,7 s`#`Mno,*+ ,.* Ma

Fig. ,.

<#89:; KLMNO

25

Distribution of Kurasawayama Welded Tu# and related rocks (modiﬁed from Sudo, +321b, +33,).

  :; KLMNO$ ,#  !^s -./ Ma   !"# + Ma

"$s`#`Mo,*+$ , Ma $%&'()*+,# * -./%s&s%s . R. ; ' +321a,

01234 56 $7 89:;

+321b ,<=>+%?@$DE# A B C$

F*+  GH$:;IJ

KLMNO$PQ%R

S C TU

V W# LMNOXT$Y CZ [\]^<#

KLMNO(*+ ) )NO,\i  )s`#`Mno,*+hRNO RA h+'^£ (+32+) $¤¥¦s*\(+33,)¡ R.  ¢&'§+# '¤¥¦%s,,¨-=_`ab\]$c Td%ef X©e .§$:;IJ

Tg # h\ijk lmfnoM5Cpq#Grs ªp§h«¬­t

st$Tghuvef+5Cew+ x @$

Tmg-R. ; ¯!^° ,**- $,¨A+

A+

RA ¤¥¦%®# R. NO

5C <# < $\ijkhNO\] l
mA y+B,%ef @$$ :z{|}, 

~C5 *+B,%ef+ =C  :;IJ,ῌ+ 

$LM*+IJ $tJ<*+B,%,   Tmg-R. 

!^s"$`#`M,# yC +s<V$, + B$

`#`M89 :¦`Me !±¦ /01 s

7$$ F  z !²³´ µ2 ¶$123:¦·?@$ b^*+B,%ef+
e 7$no,*+89

5K¸¹%º»# @$%s%' (+33,)*+ (Fig. ,) +\]e  C¼i5C %

vw¼º}»¼

26


ῌ    +321a,+321b   (D)  

 (R.) 

=ZD[$)3% Na,OῌK,O i- *.2 $j>(k2

T(Gl !  }m (+33.) % h#no] &'UVD% Na,OῌK,O i(01fg῍gph  -+  A (OR-, OR,, OR+, ORA) $qh(r#$)F!

0  

! "#ῌ 

=ZD[ Na,O ῌ K,O i%qhGl - %&'() *+,-.

! HF Ba s`-,Y32T Gl#?!

"$/ K-Ar (0123

h#no('tu&'UVD% Ba %s,- 2** ppm #?

$+ Ma, 456% Olduvai Subchron (+.302῍

Fig. . vw xye+.12+ Ma ; Lisiecki and Raymo, ,**/)  Matuyama

h! "8)FGl%#UVD"&#Hz

Chron #{$ !

, Ma $!"7632  +321b! "89#$#% &:
23;'<(=> % #'(,ῌ,  Tmg-R.

 5,%)*+#?

+-* km- $!"7632  +321b!

% |8(

Ma $!"76! " ῌ @AB,(*+, &^^_;`>G(01#UVD$}i76U

CD% -./=(0E1/BC

VDs~XD)F  => (8$

D (Fig. ,) $76F  +321a +321b!

6T=-@A

53c /-* km L6Ft4G(H$o f3#76F!  # Tmg-R. 

HI3J . K67/8L9:; &"z IFT%  L¡'¢£ '( <=>(! ?MN O2 to f¤2R  L( -%BCD234G$53

$)3P) BCD53

4G#%;'@

O 1** m ( Q  +321b!

.Lto#? (Fig. +)!  (¥ Tmg-R. ¦|§!

RS(AB)FTUVD$CWXDF
Y(% =ZD[$EF\+56]6^_;`>GH (IJ-?! #ab#%
G1/BCDK
,. km L6F-.L.


Z()F L¡¨¢£(% &R
R©ª@« ¬ +33* -W!
#@«% ¦#> @#? UVD ¡53c

KM[defeCghif[¢M "6H#(d; *+ UVD(£­Loc. +a ;  +321b OP .῍+ (763- 7632 ®}¯¤ ,**. ; Koizumi and Kanaya,

2 #k%)FQ RlS H ()F! #+310 ; Matoba, +301 ; ¬ +33* ; °}m +322 +333!

jeN

T#%m)*+UV)3 @O +/ m &®}¯¤ (,**.) ( u¥¦±i§῍¨.U

-W (Fig. -)! "89;R 1*

cm % no(XYFYQ

=)*+#?  5&p%qr(s R(t n!

VD

Eb-Fukuda, +.1/ Ma ; ©²}m ,*** ª῍Kd-3 UVD Ho-Kd-3 +.10 Ma ; ©²}m ,*** «

UVD%£­7632F- Tmg-R. %"7632

Q R% uvCwxeCy Zzy =ZD[

IF! #ab# Tmg-R. $")FUVD% ª@

{[%|}~\ +330 (01M ]^$=Z«R¬h@;R³C(p­ Kg+c D[ Cvg ;  76 (Table¬ +33* (« (Fig. -, Loc. -a)!

+)! =ZD[$Cvg_` n, n, % 56

Kg+c %@O ,+ cm ®YZD[¯=@#?

]6 n +..32 n, +.00/῍+.02- #?! HF EDS a*! #@% uvCwxeCy Zzy(bZc, (wt)[]^$) 5Z> ;'´% *.0 mm #?! ῌ %56]6 SiO, : 12./ Al,O- : +,.* FeO : +., CaO :H6EF\+%d,# Cvg °3µ;

YF=ZD[]=ZD*.+ K,O : ..- Na,O : -./ #?  ICP a( YF*±\ ;  76 (Table +)! =ZD[d,`e (ppm) % Ba : 3*, La : ,0 Y : .2 #?

$Cvg_`%56]6 n +./**῍+./*+

-=ZD[n, +.01.῍+.02/ #?! HF =ZD[]Jb`e% fgh#'<(=> )*+(H6

Zc, (wt)ῌ%56]6 SiO, : 12./ Al,O- : +,.*

* #ab# EDS a#% CaO c,ῌ-k2¶·(?! {²lS$ AT N# m¸-³´*.3῍+., wt ῌ #?(}) *.0῍*.1 wt ῌ 

(Table ,)! CaO a(23% µ¹¶%· º%M }> $)3»$(¸2¹¶%¹)32!

 (Table ,)!

 ,.* Ma !"

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.

 
27

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

28῔ῒ῍ῑῌῐ῎῏ΐTable ,.
Chemical compositions of glass shards from Tmg-R. and related tephras.

  ,.* Ma !"29v»+$ ¼

30  FOd7:T:;<eH,=f56gQOFI Loc. -b  Kg+c >: Kg,a 82(> +* m 3+?@hiA,-56dP 7
!P))(OB56@h5)j +/ cm.

kl (Fig. -) mno (Table ,) >:p6 Kg+c $
9Qq Tmg-R.'  rs P C6D3+5) Kg+c 

0t0uJPvE56c)U:

O

FGHIJGw rs

x,y[ S6\]x+KxLM)7z{|N?
O[:O6P2Q}~RS~
 Tmg-R. TU8
Fig. .. Histogram of Ba content in volcanic glass
shards in 23 Pliocene-Quaternary tephras in theJapanese islands (Suzuki, unpublished data). Kurasawayama Welded Tu# is treated as representativeof Tmg-R..

Gw8!V!$ lNO) 7O HSC $W +33/'
HSC 8! $XYZ$[? +33+' WOrs

P Hs $\+  +30+' HS $2 +310' WO)rsw

!TU Loc. .a j2- cm ]^.

 ! 1- cm  /_`5)3+A00

FeO : +., CaO : *.+ K,O : .., Na,O : -.0 

7OO n+./**῍+./*+ n,+.01+῍+.02/ (ppm)  Ba : 2-* La : ,- Y : .+  (Table ,)

 (Table +) ()



 

CaO : *.+ K,O : ... Na,O : -./ 

   Kg+c  Kg,b (Fig. -) ! Olduvai Subchron"# $%& +33*' () *+oabc (wt)ῌ7OO SiO, : 12./ Al,O- : +,.* FeO : +.,(+333)

 Discoaster brouweri ,-./01

(ppm)  Ba : 2-+ La : ,, Y : .-  (Table ,) 

JZd6 Tmg-R. 5 HSC  Tmg-R. TUefc

 "2 +-  3 $ 4 !5

FI2 (+310) JP Olduvai Subchron TU

6' 78#" # ;<#

^\g",-S6\]^;< HS 2Z

56 Kg+c =>:9"#?!>:U-NO6 HS  HSC 7##9 :

+3* cm #@AB$# "2 +- 

HSA $W +33/' >:iNO HSC dJ

HSA S

Kg+c C%98DE&' !(F}h¡9S6\]^&'P ¢}h¡9S

GH9 FI./01 "2 +-6\]7 ija Cande and Kent (+33/) J Olduvai Subchron KL)* +.3/ Ma "M6 +.31 Ma NO6 $*Tmg-R. Dkclm!nolp6m!68>:+  +333' ?!9P Olduvai Subchron KL)lNO) Zqfr£,s756k¤tnou* +.302 Ma QR>STU+:O6

¥¦v (Loc. /) d6w0 ++/ m  , x§¨A(Lisiecki and Raymo, ,**/) FO,V,9:W "©ª«¨¬0u­®NO) $k¤t +333' ¢¯FO

2 +- )* +.31 Ma JP-.)/0ST9X:©ª, i5)FI fykz ${°z' 1 ROYJ Kg+c )* +.3/ Ma J­®NO)©ª $¦v B-, ±' w0 1/.3*῍10.** m >PST ,.** Ma JP25IZ FF7

:¡|rs  lNO) FFrs ,¦v3+)*, ,.* Ma  F45)*,L[ Kg+c

B,-,0 (MTB,-,0) J² $k¤t +333  T-/+ TS6\]^ _`7 

U' MTB,-,0 =³(j +* cm ]^7O:a8 Jb6 Kg+c  Tmg-R. 95.

c 9d Kg+c }p ¹~º ¹º A00,mn

 ´Aµ¶¨A· c¸·19· VtWvB¸¹- ,.* Ma $a6nº/»F$#¼vRS


31 nS'(o=<$ i h klm p?

+./** n,+.003῍+.020  (Table +)  E    ?j   (wt)ῌ SiO, : 12.-

Al,O- : +,.* FeO : +., CaO : *.+ K,O : ../ Na,O : -..+..33῍+./** n,+.010῍+.02.  (Table +)  Ba : 3*/ La : ,0 Y : .2    (wt)ῌ SiO, :"#$ % &'()12./ Al,O- : +,.* FeO : +., CaO : *.+ K,O : .., Na,O :



(ppm)

 (Table ,) !

 Tmg-R. *+,--./ 

. (+333) /0  ,* m !12  n



(ppm)

 (Table ,) !

Ba : 222 La : ,/ Y : .0"#$ % &'3456   7 89:;<() Tmg-R. *+,-  9q 'r=  +-.0 m !> Matuyama Chron ?

<=-X 1* km VX$S's6@E -2 m <= 0, m @ 89:ABC6&Bt-ili [[hi& "Jaramillo Subchron (+.*1/῍*.33+ Ma ; Lisiecki and Raymo,uo) ! <=9RSB Tmg-R. Lv-,**/) DE6 Olduvai Subchron F BG!6&$wx=DE D 9RS1_ (+31/)

 H. ,***I MTB,-,0 H 1/.3*῍10.**mI

Ak- RS"u>Q$Q%-K  ^

89:A

yz{ Egj RS V

JBC- -DKL MTB,-,0 J<=$#M9 1_ (+31/) $"89:}ANOP $>$Q%- & &D89:Q'o>" 1,.12 m, 1-.-0 m, 2-.0+ m()U  -2῍0, m

BC-9A* Olduvai

Subchron $-- Tmg-R.
Tmg-R.



Q|

LvB~g(EE¡ 

Q$@~

6RS ^-& o=<$D¢ & @~V+,-. /+01-.W2$3456
Y6

<=T( MTB,-,0  Tmg-R. $'RSN X 7N|o673

8- 93

RS OPQ v 4£x6. ¤¥x¦av$\a6 Tmg-R.

Olduvai Subchron }` :; Z<[Y=>?@< *< A=>?@$>

<B\6:; RSB]^-&B=(E? Olduvai Subchron 6$Q%- !>=$¨E

H_C +33+I /D`a b Ec BFGHD

('

[ d

bI (Loc. 0) 93eRS

Lg= hij / cm
Q

6

.

fJe

.$K;

GHD / RS (Ysg/)

MNkl Dq rst<  (Tables +, ,) u = Tmg-R. ,-!"#$) K ª

3>. ©J6$:; RS ( E  Tmg-R . ( HSC)

Q  +0* m

Olduvai

Subchron $Q% H4« +310I - Kd.. H¬nj
+30+ ; _­ +33/I !> "#$ Tmg-R. $®~-|¯+ (Loc. .b) $\a-9 j +. cm

 Tmg-R. ,ῌ--ῌ+ %&OP

HSC $RSU(E H>mnTU ,**0I 

RSop 

Matuyama Chron 6$QB§¨ $R6 EM

°6lNklB$# 10

VtWv-X wJx-.$345Xy45$

cm

±²

[\6

YZ<=D Kd..

]E ^{PQ1|   ³ <=D

H1_ +31/I }`@$? Tmg-R. $Ag(@~6B6  Den *.. mm RS TaUklm6(<z(YZ6Y



[\

U<K

BKM<$´ (TablewJxbcdD Ye (Loc. +b) ft6$+)   +..33῍+./*+  j +/ cmkl Hg1|}`RS

 (wt)ῌ SiO, : 12.1 Al,O- : ++.2 FeO :-I\- (Fig. -) >. / cm h+.- CaO : *.- K,O : -.1 Na,O : -.1 

$<=Dl Q YZ

T=iDkl >.$ Dq  j  X<Dq klm : i en , mm  VX

Tmg-R. $

(ppm)

=



Ba : 2/* La : ,. Y : /-  (Table ,) &EM? Tmg-R. ]@~6 $ Ba

p EE#q µ-
6<6D
Kd..  K, O

= EDS

$"

ῌ





-.1 wtῌ H"¶ *.,fghijP5kl

32wtῌ  Tmg-R.  ..,῍../ wtῌ *.- wtῌ 

SiO, CaO Na, O ῌ

*.+῍(Fig. /) 

456789: ;<=>?@(ABCD
EFGHEIJK

L<MN Tmg-R. #(

 Kd.. 

! OPQRSBT, U/V WXY!"# $ Al,O- %& '( )

Z [23\  ](^_

ICP *+, -.*/01. 23( 

Fig. /.

`a,_ Tmg-R. b Kd..

cd23 e>Comparison of EDS results on glass shards of Tmg-R. and Kd..-Nk.

fu)s· v,"w"N-¸¹ ,.* Ma x !nºy»z{\¼NVW K,O  

 = * Sig '(V-8! 
WX (Loc. ,) % Sig 5 2 cm 


33

 Tmg-R.  Kd.. 

   !"# $

 HSA % Loc. .a &'( HSA  2

# $ :;*<=>*9 89/ YZ[301' H z  Tmg-R. J Kd.. m )*+,-./ HSC 0% HSA  K, O ^/ῌ -.3 wtῌ -R  Tmg-R. 

1234(5 2. cm 6$

0`7 Kd.. U' (Fig. /, Table ,) H#$7 89 :;*<=>*9 ?9/ C*A;DAE-F3!\-R \ !]I Sig  Kd.. 45aH  (Tables +, ,) g"hij,I245a@AB9G 

SiO, CaO ^/ῌ 2 Tmg-R. _72 Kd.. 

I HSA  Tmg-R. J Kd..  KL

 Z (,**,) ^!


!M"N&'( Tmg-R. (OKg+c) #$PQ

Tmg-R. _=.<l-V!4 >%-R# Kg,a Kg,b STU& !

;?6@st;?VW wx`'gab, 'VW}*VW '(V ! Olduvai Subchron 7U

 0 -/m ()* Kg-

% XFig. - ; YZ +33*[ \] Kg,b  Kg

VW- "

f'(V ! ab_"N gab

#

$



 Tmg-R.



 Tmg-R. VW

}cd

Kg- 5 .2 cm # $7 +¡c II 

! ¡c II I-G (+32-) $45¢e£$abf"Ncd, Tmg-R.  Kd.. -. (Fig. /, Table ,) * $ XNk ; ¤ zH +323[ 245

I K, O ^/ ῌ a(' XG +33-[ abf"Ng)

Kg- 3G

-./ wtῌ -R

Tmg-R. _70`1'¥¦h% Nk -§¨ (op,9: ,Olduvai Subchron a2bc*d03-iI ! (Tables +, ,) Loc. 1 % Nk 

 XYZ +33*[ I Kg-  Kd.. 45L5 ++. cm e. Kg,b H 

I Tmg-R. (OKg+c) J Kd.. (OKg-)  K
L 

# $7m . cm -jF ©ªk« 1/ 
HJ Tmg-R. J Kd.. 

 HSA _'e6-R (Tables +,K, O ^/ῌ -./ wtῌ -R  Tmg-R. 0`,) Kg,b 7U Olduvai Subchron f8-Rg"h7 Kd.. e6 (Fig. /) H SiO,, Al,O-,ij,kl XYZ +33*[ am- Kd..

FeO, CaO, Na,O ^/ῌI/(2 Tmg-R. _7

 Tmg-R. .n _\0op,9:2 Kd.. U'\-R \,I Kg,b  HSA 45a!]I Nk  Kd..

45a g"hij,I245a ¬] Hayashida and Yokoyama (+32-) _g

;qr*<= >;?6@st;?

ab}g"hilm Nk  Olduvai Subchron

Iu(AB: !v7 F $

*d7U03_\Ra(&7 Kd.. g"C:wx = *D,*%mhilm0_Fe6 & G (+33-) EF# $ XSig ; yGzH +323 ; yGzI{ +33*[  Jr|K}m%L I# $ XM~ +322[ 45a 2>;?z
t;?klJJm003 ('a! XyGz
H +333[ n! Sig 

Nk -n­_&un­}m®¯o$&_°i9"N&up $45 ('-ῌ,

Kd..-Nk ±q²!_\ Tmg-R. 0 Kd.. 45

 } HSC 45

LVW  ( ;qr* Sig, !M"N Kga(' XZ ,**,[ Sig 7Ug"hij,- ³´rabfN Nk £ri

NOzPQ (+322) zR (+330) _S7
2JVW

7Ug"hij,-T('! Tmg-R.  Olduvai Subchron f8#m Matuyama
Chron *0-4 'µ2zR (+330) - Olduvai Subchron 4

Olduvai Subchron *03 swx &_°abfNVW! n! yGzU (,**.)  Sig  Olduvai

Subchron 03_\Ra('I

Kd..-*VWtfL -

(Kd..-Nk) S¶» E ¼

34
Kd..-Nk  Tmg-R. 





FeO 

 Ba ῌ      

!

Na,O, Al,O- "


#$

 SiO,, CaO, %&'(&) *+,-./012

 ao YJ8d7.
8 'K + U& ZP4 &5qrFHDYJd7.8
9:[fh\]^:;j<&) sa58WgH +,***῍+,/** m g)3456' 74289:R_a5 (Imura, +33-) nop Olduvai Subchron ;<=

u 9K6. ^_`ab. `>!P`>&a 8YJ4a&)45 

. 
.ῌ+





 Tmg-R. 

58W,Y0 8
e8dH!PP+^_a!P+PV4+h Q)RZ-V > ?@& 9:  R b# 8ce8ABCD'! "# E $F (,**-)

8 %GHIJ9: +,* &KL 2.ῌ MN

4(P4 nopu 9K6d7e8 ef&8g¡g! {I3 h&)

O!P'(Q)R*S @T' UV.ῌ,  co-ignimbrite ash-fall deposit 

4+ '4 W&+X ,Y 8Z-V4

ijMx¢h£kK +-* km- lm +321b ++[&) 'K +  .\/  ,* km ]noV4

01^_`ab.8.c2&Q34&)&Y MN@e8JK2 pqV4

 '45 6d7e89Kfgh AT

H

Aso-. UV48K   giP

7 0(   :;j<  (Fig. +)

!  '4 Tmg-R. Uk.ckK&) Tmg-R. r.c¤c co-ignimbrite ash-fall deposit  ¥¦fgJ
K +JK2kK>P!h ) Tmg-R

0( T=>? ! kP

. 3c?&MN!P§s4F)¨ /-*5 8YlmV4 co-ignimbrite ash-fall deposit km s4+tru©ª«&) }'&voV4+!

+P +nop!Yqr) s 0

Tmg-R. ¬ ­JK®Y8s /῍,* cm w(&  t@&)+[&) ''!P M

I s'& DEnop ;u¯ /-* km

N 0(  co-ignimbrite ash-fall depositjxo  sy°¬ +* cm fgh J

 >P! u@!PiK +±oskK 22 km-  ²z³&'AvnopMN5B

 Tmg-R. MN!P' ´ A)R¨µ![P

4& MNnop ;ju C08
Tmg-R. MN&)DEnopF)G ,/ km 
Loc. +a & :9:;H^_`ab.8µ¶

! '4*&&ZP4

6d7e89K co-ignimbrite ash-fall deposit

fgwIJK29K'Y e8JK2LM&'´A)R  ' µ{V4&

xHs0>MxH!Py V4 ''z   +X ?@>
 O9:

Z-V4YF)

YMN!PF)^_`ab.8fg9': NO27 {|}')&~cV4+fghTmg-R.

6d7e89PV4 co-ignimbrite ash-fall74|]

deposit &)' UV4 1L Toya)Z-V4+'  no]4

Hkd-

Table -  Suzuki et al. (,**/)  };·:

Ku :Q Tmg-R. 'AvnopMN

Q±o&no + Hkd-Ku kK[ '4*&@9:&) MNF)  '

~¸V44 co-ignimbrite ash-fall deposit 

8K

6d7e89K co-ignimbrite ash-kKs45e8JK2kKU + Tmg-

fall deposit &Q34&)!*& z&R

R.  'Avnop¹N co-ignimbrite ash-

S&  zYlm&'8K&T

fall deposit   Hkd-Ku, Toya aYd7&)

+

Tmg-R.  Hkd-Ku }')F) 

0 - 9:& U co-ignimbrite ash-fall deposit' µ![P4  '´A)¨  µ!

U)R F)Z-V4VW s4 Q3[P4&µkK~K 4&)!X&)! *8Y !

Q) '4*& Hkd-Ku <º)&~cV4

5678 :0;<;^¼· ,.* Ma AA7T:^opq

35

Table -. Comparison of caldera dimensions and volumes of pyroclastic
ﬂow deposits and their related co-ignimbrite ash-fall deposits.

 Tmg-R. 

deposit 78./0 Y    Hkd-Ku     

A12 /34,- 5

678,!



Hkd-Ku  -1 km







!"#$%&'()* +333+-,- Tmg-R.  +-* km- .  +321b+ ,- /0123 4

5+ Tmg-R.

   co-ignimbrite ash-fall deposit Aso-. AT 

Hkd-Ku ,      



6





co-

 co-ignimbrite ash-fall deposit  0 952  34,-.ῌ- 

 Tmg-R. 9`:0;<;^y
=>

9:



d, Tmg-R. A=>D , Ma  +321b+ co-ignimbrite ash-fall deposit    

?9`/{@ A=>B< ,8A C0 K-Ar =>J;8

ignimbrite ash-fall deposit 78 9:;<Toya / =>-957 A .

=> uA K-Ar =>+.1ῌ,./ Ma 8 ET , Ma  :=>3

,-

 8,/ Tmg-R. A=?@ABCDEF/A A#> ,.* Ma A

G?HIJKA#GLM$NOF

 F;G D¡YA/, 

PQ-,-: R, ST9:  U=>¢£/ +.3/ Ma 9`¤ ,.** Ma 9`¥3VWX;YZ[\/-,-` =>{@¦H9¤ uA §¨I¥©ª%¨«¥©AA56787

:
Hkd-Ku [\ ]^ A !

=>1G41_` ab"c# +332+

d,;/

eZ $f/- -,  {@,=>¬ A2­;^#0 =>¬/J-,-:]^  9` 9`gd,]^%hA` ]ijk9&',

 8,®A Tmg-R. A=> § :0;<,:0;<;^¯K7

 °LM±²G0 (OR-ª+, ORA)  °LM±² (hl m )n*opq?@J

G0 NO#M±²G0 P³Q[M±²G0+ ´rstuA/  %µ°L$~}q §µ¶#$~}qv w9 co-ignimbrite

ash-fall deposit xyzJLM$NOF{+ |}~,:

 Hkd-Ku J Toya ,







9

co-ignimbrite ash-fall

W·,-¸R .  +321b+ NO#M±²G0 (Tmg-R.) A§ D +** S=¹º¹
_D + Ma AAP³Q[M±²G0 0»'(¼»@ ¼&36 
  ,*** ;  +321b  Tmg-R. 

6tcd*'3#u2aeD<v,( 3(w %&JK  ,.* Ma f]+ 5

!"#$%&'()

ghJK4( 2A x2Lyei#jei*+,-. (Tables +, ,)  (+321b)  

ka2 (lP+z Tmg-R. (]<v

 +(/(0$, IJ;2mn{o |8}~ $ co- 

)*( +12(

ignimbrite ash-fall deposit (0 >2?2($ Tmg-R. 32,-. 4 !((pqKL/6b  P  Tmg-R. (D  56"#  $789:&;2Z Kd..-Nk >2?2MJ(rEF&Z%&$ <' )*

6(=  >?@ ,**0 Y@6"#

 + Ma GY(rEF& ; /($6IJ=r Xl+TsZ )*89:&( +A   56(=5 EF&({o|8}~ >2?2($5B,C Tmg-R. (D$EF& 
EF& Kd..-Nk +-.=56"#

pqt 5P. ;b  (u GH Kd

G+v,&  52,p(u..-Nk (IJ$KL/= 70$%&'(1 )*EF&(IJ2 3 46(=5+52 -MJ(EF& *./ GH

wx]$%&'(yz{(6 LaῌY 76K 8 ,**+ Tmg-R.  Kd..

**{|*2*}(~-Nk  Ba (N(6  (tcZ

$%&'(1)*

 

 Kd..-Nk ( La ῌ Y 7 *../ 9O'{**!2*(  

MJEF&(46(= P  Kd..-Nk ($%&  P  {***O'( Na,O ῌ K,O 7 *.3ῌ+.+ 6- $Q- *.2 (46(H#6  5:K Tmg-R. $R;STU(3< 

5t -+-

 @ 

P= Xl  +1+33. 6(= =VWEF&1 )*

JPz{*'  2(2E

MJEF&(3<6(=+ Na,OῌK, O 7R;)q 3( (¡¢ w 

STU(3<6(= 5(X3=# Ba (N +1 JP*£ < (C) (,) u;2 Kd..-Nk >2?26IJ=Y@+¤ +//**023 $%& 6W M!(Z P  Kd..-Nk +N[$%&'(\A]

q'¢ w(7!Z!;2B^CD_`;aZP ,E) Fb3(IJ$ !;2 2
GDH=c2 IJ$6>2?2=
5I Kd ..-Nk (%&JK Olduvai
Subchron (dLMDH=5;2 +.302ῌ+.12+ MaZ  Tmg-R.  NJe,* JOPQ EF&(3<JK;2

 Kd..-Nk 





7 3 Kd..-Nk RS(%&>2?2ZBX; Tf U @ g(,**.) VW

(h$ijk @$ 6Xl  52+
YZ =56[m 52%&  Kd..-Nk (no ,pq\=++Z/     /(89:&6IJ=rEF& >2?2()*89:&MJ(

Tmg-R. KQ  $EF&]sH$

+n(^N2G_U`ab&=5
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-ῌ
0*3/.
" ¥ ¦ (+33+) 2*# e§H!¨e +33* J " ¥ ¦ ,+2 p.
©$8Sª«¬­   (+333) (©$8Sª« $* CD-ROM.
¡¢ (,***) £(2¤®$+¯%$q®¥W~¦)§¨©Xlª -,  £¥W~°±¬­ ,-. p.
²«@³¬´­ (,**.) EF&¨e<v-®¯2
(°e±`a(e§µ$e Ke²¶³e(7 2¤´ ++* .2*ῌ.30.
· µ @ ¸~ * @ ¹q% @ ¶~º· @ »q&(+323) .¸2(2¤ 22¤Xl / 
( + 2¤¹º 2¤¨©¸ +./ 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, ,*3ῌ,,+.

-_`» ° ±Fp&'G2 ,.* Ma !#¼%(* !¼qXFOPE
Imura, R. (+33-) A reconstruction of small-scale eruptions
using pyroclastic deposits. Geographical Reports of Tokyo
Metropolitan University, ,2, 13ῌ++,.
  (+33+)   0 33ῌ+++.
  (+322) 
  ., 0+ῌ2-.
 (,**.)  !"  
!"##$%&'$%()*+  ()
.3 ,2-ῌ,31.
,&-. (+330) '/ ()!0
12*+34+ 56,7 8 +*/ 22ῌ
++,.
(+31/) ./012930:;45-3<2
6 1/ +1ῌ.3.
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), +..ῌ+/3, Micropaleontology Press, New York.
= 7  89>:  ;<=> ? @ 9?  ? @ 9@(,**0) A<BCDEAF!BCG2*+D
HIH4+(*  &'BCDE
8 ++/ +ῌ,/.
J EKFLG (+33*) HM!" NIN
!012JKOPEQLM RS
TN -+ /-ῌ21.
J EKOP (,**.) F*+U (.ῌ,Ma) OPEVWQXFLM3Y! Nwg
 Arg OPEZ[\ Tsp OPEBCL
M0:;R" YT OPE ]OPEXFLMWQ3 RSS^T
N 1 +-ῌ12.
J EK_)`aFLG (+323) NI-!012TU!)JKOPE RSTN -* -3ῌ0..
J EK  V<a  W9XY FLG  _)`a(+333) #b/Z[c\Cde!012!)
() 3 Y! Jjj 0:; Sig ()LM!
fgh3 RSS^TN+ 0-ῌ+*+.
Lisiecki, L.E. and Raymo, M.E. (,**/) A PliocenePleistocene stack of /1 globally distributed benthic d+2O
records. Paleoceanography, ,*, PA+**-, doi : +*.+*,3/,**.PA**+*1+.
i]^ (,**-) N ()jeEk3_
Z<`0Q() l#am --0 p.
i]^-b7nco7 (+321) -_Z< ,0 +,3ῌ+./.
`pXY00qde() frT
Matoba, Y. (+301) Younger Cenozoic foraminiferal assemblages from the Choshi district, Chiba prefecture. Sci.
Rep. Tohoku Univ., Ser. II (Geol.), -2, ,,+ῌ,0-.
gh.  is (+322) j!012*4k<lt 38 3. +*-ῌ++,.
u v/&wx)6^0<m^i ]y nopz.q<rs {&_|<&} (+30+) _`<~k<t . V3uz (+ :37/*,***) 3v
J&w (,**+) *4kXFOPE()xyQxz{p|2  ,-
0*/ῌ0*3.
H }~} ( (,***) `N!0
12*4+(* XF()L
M0:;Z#45 38 +*0 /+ῌ03.
  <<^ia (+33.) _` Leg. +,1, Leg +,2 j()3()~Ek ,B,()LMfrT()
x73  +0 03+ῌ032.
% (+310) ]/0!012<ltZ 
38 2, +0-ῌ+2+.
i . (,**,) F]/04+HOP
EXFLM _`3mf +*3 4m ,./.
iXu (+33*) aF129 3
<lt3 -RS  ,- +ῌ-..
?@9?;<=>`Y (+332) F(*  x!%x 3 A<BCDE
'(* 3 [m +332
4m .,*.
? ] i#^&¡%i^ <m (+32+)  F 
 38 21 ,01ῌ,1/.
¢¡£u< ¤ (+333) 3¥¦§?!:2¨H25345©¢£¤OPE  /- ,0/ῌ,1..
 ª)¥  «¦  =§!  ¢¡£?(+322) _`¨!©ªG2¬1293«¨0:;*kῌ4 Q . : ­3
k9¬LM ®¯m­ /- .1/ῌ.3+.
}~ (+33/) 1°() 3
8 +*+ 101ῌ12,.
°±²"9³ (,**0) [ A!´¬1!W##$%*+OPE HSC Qµ¡ ,2 /0ῌ0*.
Sparks, R.S. J. and Walker, G.P.L. (+311) The signiﬁcanceof vitric-enriched air-fall ashes associated with crystalenriched ignimbrites. J. Volcanol. Geotherm. Res., ,, -,3ῌ-.+.
® ¯ (+321a) ° ±F²3³ 3v  ,00 .-ῌ10.
® ¯ (+321b) ° ±F¶H3(*  3  <  3 3v  ,00 11ῌ+.,.
® ¯ (+33,) ° F²±3t ´µ3v / ¶B + 3t + ·;·%t ,+ῌ/ 3¸ 1- 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, +.,000ῌ012.
l (+333) , +* 4¤¹[\¸º
¹»º 

38

w4xy$%`z{  +10 p.
 (,***)  ++     +-2 p.
!"#$ %&'($ )*+$ ,-./0$ )12
3$456($!%78$Prima, O.D.A.$9:;0(,**/) <=>?@ ABCDEFG HIJK$LMNOPQ RSTUV .. +3/ῌ,+0.
WX (+32-) 9YZJ[\]^Z_G`a_b. cUd  ,/ ./ῌ0+.
WX (+33-) ef_G`a gh ij 9Y
Z klm +3*ῌ+33.
njopq rstuv