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Journal of Geosciences, Osaka City University

Vol. 52, Art. 4, p. 35-50, March, 2009

Volcanic Stratigraphy in Western Oita, Kyushu, Japan- Part 2:The Oyama -Tsuetate Area

Masahiro DAISHI1 and Michinori INouE2

I Geochronology Japan Inc., Osaka, 540-0012, Japan2 INA Corporation, Fukuoka, 816-0964, Japan

AbstractThe volcanic stratigraphy of Western Oita, Japan, is clarified through detailed geological

fieldwork, petrographic study and age determination. The oldest volcanic activity is identified as thatresponsible for the Okubodai Andesite (4.4 Ma), followed by that for the Shakadake Andesite (3.2-3.1Ma), Yoshinomoto Andesite (2.8-2.4 Ma), Matsubara Andesite (2.4-2.1 Ma) and KaratomariyamaPyroclastic Flow Deposit (2.2 Ma). The Kamitarumizu Andesite, Fukuro Andesite, Kusu Group and

Ono Rhyolite made up a succession ranging from 1.7 to 1.3 Ma and were followed by the ShimotsuruPyroclastic Flow Deposit (1.1 Ma), Okubodai Pyroclastic Flow Deposit (1.05 Ma), YamakogawaRhyolite activity (1.3-1.0 Ma) and subsequent formation of the Yabakei Pyroclastic Flow Deposit(1.05-0.97 Ma). The volcanic activity of the Itsumaichi Dacite and Hanahira Pyroclastic Flow Deposit

occurred at 0.9-0.8 Ma. Volcanic activity related to deposition of the Oyama Formation, TsuetateFormation, Iketsuru Pyroclastic Rock, Kan'non'iwa Andesite and Kan'non'iwa Pyroclastic FlowDeposit is inferred to have occurred at approximately 0.7 Ma, succeeded by the activity related to theHaneyama Rhyolite and Kameishiyama-Yoshitakeyama Dacite. The volcanic activities of

Takenosakotani Pyroclastic Flow Deposit and Sugihira Pyroclastic Flow Deposit were 0.62-0.58 Ma,

and that of Amagase Pyroclastic Flow Deposit was 0.3 Ma.

Key-words: volcanic stratigraphy, Oyama, Tsuetate, Oita Prefecture, Pliocene- Pleistocene volcanic

rocks

1. Introduction

The Amagase-Oyama-Tsuetate area extends fromthe west of Oita Prefecture to the north of KumamotoPrefecture in Kyushu, Japan (Fig. 1). This area forms a partof the northern margin of the Beppu-Shimabara graben

(Matsumoto, 1979) with Pliocene to Pleistocene volcanicproducts distributed extensively throughout.

Although many stratigraphic reports have beenpublished for this area (e.g., Daishi et aI., 1981,2006; Haseet aI., 1993; Hayashi et aI., 1982; Hayashi and Watanabe,1987; Iso and Ikeda, 1979; Iwauchi and Hase, 1989;Kamata, 1985a, 1985b, 1997; Kido, 1981; Kido et aI.,1987; Matsumoto et aI., 1977; NEDO, 1990; Sudo, 1985a,

1985b; Tamanyu and Kasuya, 1983; Uemura, 1985; Uto

and Sudo, 1985; Watanabe and Hayashi, 1983; Watanabeet aI., 1987), there remain many inconsistencies. In thisstudy, we performed age determinations, detailedgeological fieldwork and petrographic studies for manyvolcanic rocks in this area as an adjunct to prev iousreported data (Daishi et al., 2003, 2006) to clarify the

volcanic stratigraphy of this region. This article presentsthe volcanic stratigraphy of the Oyama and Tsuetate areaas the second report in this series.

2. Geographical setting

The study area is located on the lava and pyroclasticflow plateau that spreads to the west from Haneyarna in the

36 Volcanic Stratigraphy in Western Oita, KyllShll, Japan - Part 2: The Oyama - Tsuetate Area

lOOkmI

Fig. I Index Map

The bolder lines show the Beppll - Shimabara graben (Matsumoto, 1979)

northeast of the map sheet in Figure 1 and on a pyroclastic

flow plateau that spreads to the north from the Aso caldera

in the south. The area bounded to the east by the Kujuvolcano and to the west by the Chikuhi Mountains. Amountainous region consisting of peaks 500-700 m in

height extends into the area, with Mt. Kameishiyama

(942.6m a.s.l.) in the center as its highest point. Mt.

Yoshitakeyama (926.5m a.s.l.) lies to the east of Mt.

Kameishiyama, and relatively high peaks run in an

east/west manner to the center of the area.The Tsuetategawa River flows in the south part of the

area from south to north center with a westerly deviation inits central part. The Oyamagawa River flows from thesouth to the north in the center part of the study area. The

Masahiro DAISI-II and Michinori INOUE 37

0.011>1"

Terrace Deposit

20.88 ± O.06ka ~_)e_PO_'i_t _

Amagase Pyroclastic Flow Depositpxhr [) pfd O.31±Q,On'lll

IketsuruPyroclastic Rock

O.77±O.19Mapfd.ppf.pr

Sugihir8 PyroclasticFlow Deposit

Jlxhr D prrlO.62=O.14~Jln

YalllakoJ.{Hwa Hhyolite

pxbilll" R prcl, hrui R prd. bi R pfd 1.00-1.061'v1nIll'bi H pfel, hi H pfel 1.15-1.29Ma

Aso Pyroclastic Flow DepositAso"4. O.084-0.0Sr'.'la, pxhr D pfd

Aso-3, o.lOa-O.2'IMn. px]) prdAso'2, 0.11-0.25Ivla. px D pfdA80-1. O.19-0A6Ma. px D ]lfd

!PrC'f\so4 Pyroc]nstics pK J)Pf(~

[l.SUlllllichi Dacite

bi-bgpx 0 I pfd 0.86=0,021'.'10

Shimotsuru PyroclasticPlow Deposit

hrpx 0 pfd 1.1±O.3Mn

Yahnkei Pyroclastic Flow Deposit

pxhr D pfd 0.97-1.051\'111

Okubodai PyroclasticFlow Deposit

]lxhl' 0 pCd 1.05;1.0.201\·la

Futakushiynma Pyroclastic Flow Deposit

,-------=O-ya-m-a-Fcc.o-m-m-'-ion------,? !~I~·_?_I)~d_~~7:<:~1~1~ I

ppf.pr.ss.st Hannbirn Pyl'ocillstic Flow Deposit

O.i2-0.83Ma I'xhl' [) pfd O.82±O.]'It-,Jlfl

O.l~i\l"

Yoshinomoto AndesitC'px ht' A 11)1'2.4-2.8J\la

Kamitarumizu Andesitepx hr hi A I

px A I1.3±0.2.1.7±O.31"'lnF'ukuro Andesite

pxhr A-D I

j\<[atsubara Andesitepx A I PI' 1.6-2.4.\'la

I{usu Grouppl'. 5S. st. bi-bghrD ppf

1,41-1.64 Ma----~----31'

Shakadake Andesitepx hr A 11)1'3.1-3.2!'\'la

Kuralornariyam3 Pyroclastic Flow Depositbi-bghrD PIlI' 2. 19-2.22?o.'1n

Dna Rhyolitebihr R l.pfd

1.55±O.22l\'ln

11;4~1"

Okubod:'li Andesitepx A I

4,4±O.5M,a

Fig. 2 Volcanic Sequence of the Oyama - Tsuetate AreaAbbreviation: A =Andesite, D =Dacite, R =Rhyolitebi = biotite, hr =hornblende, px =pyroxene, bg =bearingI = lava, pr = pyroclastic rock, pfd = pyroclastic flow deposit, ppf = pumiceflow deposit, ss =sandstone, st =silt stone

Tsuetategawa River joins it flows from the east in the

vicinity of Matsubara. These rivers flow from 300-130 111

in altitude and have banks presenting steep geographical

features.

3. Volcanic Stratigraphy

The volcanic sequence of the Oyama - Tsuetate area is

shown in Fig. 2, and the geologic map is shown in Fig. 3.The area hosts the Okubodai Andesite, Shakadake Andesite,

Yoshinomoto Andesite, Karatomariyama Pyroclastic Flow

Deposit, Matsubara Andesite, Fukuro Andesite,

Kamitarumizu Andesite, Ono Rhyolite, Kusu Group,

Yamakogawa Rhyolite, Shimotsuru Pyroclastic Flow

Deposit, Okubodai Pyroclastic Flow Deposit, Yabakei

Pyroclastic Flow Deposit, Itsumaichi Dacite, Hanahira

Pyroclastic Flow Deposit, Oyama Formation, Kan'non'iwa

Lahar Deposit, Tsuetate Formation, IketsufLI Pyroclastic

Rock, Futakushiyama Pyroclastic Flow Deposit, Haneyama

Rhyolite, Shimojo Andesite, Kan'non'iwa Andesite,

Kan'non'iwa Pyroclastic Flow Deposit, Sugihira Pyroclastic

Flow Deposit, Takenosakotani Pyroclastic Flow Deposit,

Kameishiyama-Yoshitakeyama Dacite, Yamakawa Debris

Avalanche Deposit, Amagase Pyroclastic Flow Deposit,

Pre-Aso 4 Pyroclastics, Aso Pyroclastic Flow Deposit,

terrace deposit, rock avalanche deposit and talus deposit 111

ascendi ng order.

(1) Middle Pliocenea. Okubodai Andesite

The newly named Okubodai Andesite consists of two

pyroxene andesite lava with 58.08 wt% of Si02 (Table 1)

distributed in the vicinity of Okubodai and Koitsuma.

The Okubodai Andesite is the lowermost stratum In

this area and is overlain by biotite-bearing hornblende

dacite pumice flow deposit of the Kusu Group in the

vicinity of Okubodai and by the Yabakei Pyroclastic Flow

Deposit to the north of Koitsuma_ The total thickness of the

38 Volcanic Stratigraphy in Western Oila, Kyushu, Japan - Part 2: The Oyama - Tsuetate Area

AIlnct&ib.. tllft'. p<I..lf no.. kpo.oi'

."nd.aih.lulf.l.llmlctluwd<>pooil

C..wl ...... Il.ll'l'

~ bootil"I_~IMat ........ pfd

~~bonIblt<ldCIUo"'pCd

@E!J ,......p)"NlU- hornblende Aa,e pI<I

8 '-"lHcnde I nrouno cboclUl pld

~'-nbIroodcohiDtll."',.,I!tcptct.Ia ...

• ~_",")'Obl.pfd

e n

~,....a.""'''''lodf

B~"""'..Je·Ild'''''~""'~Maltopfd

(g~pfd

81')-.....~Ir.... """"lepld

B",~~·"""''''w...lr.

~1lornl>W"dookll.ptd

~ p],_",~~UnI.pld

~~obace."""......... PI'

.ftdH,.!£' I.ptd

§] ,..,."· nd8I...

~bt<>l;I .. "'; ')· cno"ndO!fil~I" ...

Q hombkmdc bKlti,ot IUI)'1>1Jt pld

• Mmlll<-nde ... .,.... ptd

Olu.,WcI., l')·oocloo••.., """" '''''.-11

l'ull,kll.llly4'"'' l'~ox",.ri<' I'low IkrllUo'll • 11)'nm:,W hurnb"'ndo ,1M.1Iot ~ftl

11"""h;... I'>·.....lul;.,~1owlk>"..'" 8 pyro:se_hornblomdedlOci'<lp(,1

............... AaoIeoi... ~,~~

~~ ............ dlome ..... PII'

{:=:=:~otfbro.-p-I ~ :==::::Y.h'_'....... .......... ,..5MbdauAMoao... t--..r- ....

Ok......,"........... I~ "'...

luuurul')"""d""lkRooll:

13 rn mw.

Shln"*,A,,d"';l~

""~.,....·y......lt.lr.,.,.,.•• 1:hoctM

".n·.....1.. I'ymcla.,"'t...... 1,,",-,1

L

~{ To""_= T~o..,-rt

.. : """'~otl'1own.,-.1

...~ Pn-A.>' ''''-=''''0(""", 0qIw1.

" .._I.,....cb..h.. ~ I~OI

V...u.-a Dd>n. lIo I~"

T.lr."_Iw;.AiI'y..,d l .. t ~1

..,

Fig. 3 Geologic Map of the Oyama - Tsuetate Area1:25,000 A Topographic Map (Hita, Al1lagase, Bungo Ono, Tsuetate, Taio, Miyahara), published by

Masahiro DAISHI and Michinori INOUE 39

~T"'----~-------""'~-='----~;.~...,..

.,. : ;/.-.....,...-.f.,,~

/~.

~ .....

..~

\.. .' ......_.....

f"""'-.0

"'~

(' '-,-

Y"ft~ ~ lil

;./ ':'- II.,

\"'/'

"\.

....A~:r ..

.'V .~

·,:<i..

'"\ .., ..•

.,.. :

the Geographical Survey Institute, Ministry of Land, Infrastructure and Transport of Japan.

40 Volcanic Stratigraphy in Western Oita, Kyllshll, Japan - Part 2: The Oyama - TSlIetate Area

Table I Chemical Composition of volcanic rocks from the Oyama - Tsuetate Area analyzed

by ICP-OES and ICP-MS, sampling localities are shown in Fig.3

Element Units Okubodai A. Shakadake Andesite : Matsubara: Shimotsuru I Yabakei Sugihira

Units 67360 67450 67490 67750 67500 67370 I 67730 50170

Si02 ..~. .....~§0'§......§7§7....5.§:2~.57.~L.:. .... u5.§3.?· ... u"~'.07.'.::?4'...85.'.0.. '....!1··6.§·3.L.~OQ4.T.iQ2. u~.o.8.~50'§OQ,. 0900 0835 ·Q~ll....u.Q.1~.3 ..0'§.5.~

N2()~ . .u ~" uP.3.~.. 17.90E3.Q.....18.Q.7 E.8.~. 15.60 .E.5.§ n~.L

...1".8.2.9.3. % J.l~ 6.~1. 7.0.~ u 6.~.4 10.1 ':::?~i::.~.l.~.., 6.m .rv.r~() ~ ....o..P§ o..r.o.~.... .....011.1... ....0.).28.; 0188.o.)l} .0.)0.4 .0.0.9(3

..~~g() ..~ u.}.l.~............... 234 2 79 2.&0. 2..H O.:?O... L05 ..2J5.c;~Q.~JO'~..6.&8.(34}70.8.. ..~.7~.,. . J.n }52 u 5.Q.5.

J:"I~2().~ u }.5.~..... . 3.&7.. 3.36 u..3.JO... . 34,1 u 4.§9.1. ...}8.L •........H.2. .

1<'2Q. u ~ 1.8.~ L~.2......... L95 ..u l.:~.5.u .. )9.~ .•........ 4.0.5.I]8.?)&8.

P~g15 ~ ~.:.~~.... . ~.~.~. ··i~~ ·~~i i:~ ~.~~I~ii· ······i~6

total % 99.81 98.60· 99.64 99.62' 100.29 99.95 I 100.48 99.13

.......~: I'.I'~! .L 1 l' 1 . ··26·' :u ~ ..uu·l· .·.·.·.·.·.·.·.·.·.·.·8.·.·.·. :u. 1~........1'.1,,)' 2.2. 1.7.. 2.1... 1.9.......... 186 ···········.·.·.·.·.·.·.·.·.·.;..·.O·.·.·Uj 68 :. ·.·.·.·.. ·.·.·1·.·3·.·.9·.·.·.·.· .

V ....1'.1,,':'.. .. ).8.8. ......H.3..17~ .l.~.L -... (;1' .....I'I'~!. 22 u ~~0.. <20 .........•...... .. ~.o...... ..<20.. . ~~.o. ·····1 <2.9 ~.o. .

u c;o. 1'.1'111. 14 1.0. E , .l.2..... 13 <1 ·········1···· 5 13

Ni .....I'I'~! <20 <20 :'2.0... <20·· ·~iqu..::~:3.:·:;~~:·:.~.:.:.21:.:.b.:.·.·.·.···u(;."'. . I'p.~!.......W 14 ..l.:3.. . . }o..., ..l.(3 ~.~.o.

Zn I'I'~! ).0.2. §O u.. u.8.~ : 2.0.3. 8.~.........•.......... ..7.5. uu ..I .1.5.~.....~O......9..~ I'I'~! .......W.....W..... 2.1. .. u.l9. ..u 2.:3... 16.. 1 17 : .17 .

.9.0. .....I'p.~! .l... Luu....... 1 J 1... 2. u ..i .l. 1.As I'I'~.' :"~ :'~. . :"~ , <:.5.... <5 1.0' ! ~~ ! <:.5. .

..... )l.b. I'P'.Il.' 5.~. .u.u ~.'l.u 5.7 .~.2. ...6.Q..l.?o. !J.O'?L.~.5..

§r 'p'p'.n., .. ....479.. 489 ~.6.L......... 500 529 .. .., ~§.o. J .?6.~ ~.1.5. .

Y. . ....I'.P.I!! 2.~u.u ~.8. 2.~....u~.8. uu ..3.:3... 3.9. ! 2.0 u.1.6..

Z, .P.I)I':' 1'18 .l.~.9. .117.. ........1.1.5... . ).5..1 : 2.?L J.. ....1.71.......l.21

Nb I'.P'I~. . !-................ ..§ 7 8 ).3. :.. 9 11

.... ,.M~ .J?t~.I:':I. . ~~ ?... . -::~ : ? :~~............... 3 ~ 3 <2

......P.;.f!.. p.p.I1!o.9 'O'§.Q& 0!? .. Hu.. Q9 ..:'10J...o.6...... <:.o.:~ .

In p.p..m ~0:.2.... <0.2 <0.2 <O:~ :"Q..2. ..:'.o.:~ 1........ .~02.<:.O'.L ........~~ Jl.P'I':' .. H.H ~... ...........H.... H 1. 2 2..... . 1 <:.1. .

.......~~ p.P'I'!... <0.5 <:.o.:~... 1.9 0':§. H X.7. o.:!? ! ~0:.5. 1.:1. .......(;.s I'.I'I'! ..l...}... U .. ;....H.~O' ..•. .....l.:~. ....?.3. ..7.&) 4.7 ).:~ .

.......El.a. ......I'.I'I'!.. 458 ~~.9. :I:6.L ..1~.3......; ~.9~ 7.:3.5. 11 6..~.. 2~ 6... ...17.3.

......l:~ HP.I'I'! 2.1.:.2.... 21.72.§} ~H : HH ..2.!J..9 .:J.2.~. " }.l.:§ .

uC;o. .....1'.1'1':' .. .......4.~ ..6. H.H ~.1~. ..H.5.0.} ~.5.:Q.. H.5.43 ~.1~.15.:37. ..: .:J.8.7. ......Xr. ....p.I'"! ~..5.~..H......5.:~.9.. 6.83 5.:~'O' 7..2.L.. 7.88 I 594 ..HH 17..0' .

Y<l .H 1'.1'".' ..n9. H.~.3.::3 , 3.0..7 ~3.2 , n.3. ; .:J.o.~.r2.l.9. ..• 'H17~ ....§.'-11 p.P'I':' ~.9 5.:0........ 6.7 5.0 69 6.). I ~:.o. H 3.&..

Ell I'.P.')!... 1.45 l.:~.l....l.:.?!J ., U.1. 1.91):~I6.r ..l...O'L 1.23

Gel p.I'!'.' 4.8 4.:P.. 6.5 1:§. 7.1 5.:7.. . }:.1 3.:~ .

}'~ ."H 'HI'.P.'':' ..Q8.u...'O'§ •............1'O'·0':§ 1.1 H 0'9.·r 0.6 H O'~.....Ily ppm .+-.8... 4.7 (3:.1. 4.6 §:.7.; 5.2 ····r·· 33 3.:0. ....... ~{o. .....1'1"; ..l.o. ·lOH.'. J 2. ..;··i·o 14~ir.Q7..0.:§.

£, p.PI'!... 3.0 30 .:3:.6.. . 2.:~.. 4.0 3.J.···r 2.1 : 1.3 .Tm Jl.I'~l H..9;.1~.. 0.45 .. , .9:.5.1... 0.45 Q:.5.§ o.:~.3.····I .9:.3.(3 ; 0.25

......Yb PP.I':' .. H X.8.H2.:§ .............:32. H 2J 35.., 3.~I.. ··1 2.3 .. ):§.

..... .L..1I .....p.I'"!.042 HH 0'~.L.. 0.47 ..H..O"!.l.. Q..5.~. O':~2T .9..3.15. ..0:~.3 .HI" .Jl.I'I'! ~) H '!} H ~ 5.... 4.0 4.6 66 1 '1 ..8... 3.1

.......1.'~ P.I'"! 0...5.... 0.6...0...6. 9.~... 06.....1.:.1....!...)}... o.§

w 1'.1'.".' <1 <:.1 ~1... <1 :"1.. ! ~L ..• 1Tl 1'.1"':'... 03........93 : 0.:2 .9.:3. .....08 0.7 ! .9.7 02Pb 1'.1'.'':' ~ 8.......... 9 9. ~ .1.7.. . 1...... 13 12Bi ....PI'I':' ~0:1 <:.9.:~... <0.4 H <:.O':~ :"0:.4 ,.. ..<:.0.:1 ..1... <0.4 <0.1

...... T.l, Jl.I'"! 15:.9... 6.1 6.3..... ............5:~ (3.} 13.6 ..1. .10..1: 4.:~ .U ppm 1.2 1.3 1.2 1.2 1.1 4.0 1 2.1' 1.0

Okubodai Andesite exceeds 70 m.

The Okubodai Andesite was previously included in the

early Pleistocene Chikushi Lava (Oita Pre., 1972).

However, as the reported K-Ar age is 4.4 Ma (Daishi, et a!.,

2003), a re-definition as the Middle Pliocene Okubodai

Andesite was warranted.

(2) Late Pliocenea. Shakadake Andesite

The Shakadake Andesite (redefined) consists of two

pyroxene hornblende andesite to two-pyroxene andesite

lava flows and pyroclastic rocks, distributed in the vicinity

of Yasebe, the Akaishigawa River and the Uenogawa River


in the northwest part of the area. The Si02 content of these

deposits is 57.87-58.24 wt% (Table 1).The Shakadake Andesite is re-defined from the

Shakadake Volcanic Rocks (Kido et aI., 1981) as all the

rocks were made up of andesite.The Shakadake Andesite is widely covered by the

Yabakei Pyroclastic Flow Deposit and is also overlain by

the Kusu Group and Oyama Formation in the vicinity of

Gogoro, Higashigochi and Nakatsuo. This andesite is also

overlain by the Karatomariyama Pyroclastic Flow Deposit

on the west slope of Karatomariyama. The total thickness

of the Shakadake Andesite is more than 200m, where the

lower part consists chiefly of pyroclastic rock and the upper

part consists mainly of lava flow.

The radiometric age of the Shakadake Andesite in the

area has been reported to be between 3.2 to 3.1 Ma (Daishi

et aI., 2003; Kamata, 1985b; MIT!, 1984), thus it is in the

Late Pliocene.b. Yoshinomoto Andesite

The Yoshinomoto Andesite (NEDO, 1990) is formed

by two-pyroxene hornblende andesite lava flows and a

small amount of pyroclastic rocks and is distributed

throughout Sugihira, Okuyama, and in the downstream area

of the Mitakawauchigawa River in the southwest.

The rock contains phenocrysts of plagioclase mega

crystals (maximum 15mm in size), hornblende,

clinopyroxene and orthopyroxene, and occasionally

contains hornblende mega-crystals (maximum 12mm in

size).

The Yoshinomoto Andesite is overlain by two

pyroxene andesite of the Kamitarumizu Andesite in the

vicinity of the Mitakawauchigawa River, northeast of the

Shimouke Dam, and by the Sugihira Pyroclastic Flow

Deposit and the Aso Pyroclastic Flow Deposit to the east of

Sugihira. The total thickness of the Yoshinomoto Andesite

exceeds 250 m.

Although the volcanic activity of the Yoshinomoto

Andesite is reported to be 3.7-0.6 Ma in age (Kamata,

1985a; MIT!, 1980, 1981, 1984; NEDO, 1990; Tamanyu

and Kasuya, 1983), careful examination of the results with

respect to the measurement technique and sample condition

suggests that the ages aTe concentrated around 2.8-2.4 Ma

(Daishi et aI., 2006). The activity of the Yoshinomoto

Andesite thus appears to have occurred in the LatePliocene.

c. Karatomariyama Pyroclastic Flow DepositThe newly discovered Karatomariyama Pyroclastic

Flow Deposit consists of biotite-bearing hornblende dacite

pumice flow deposit distributed in the vicinity of the

western slope of the KaTatomariyama Mountain.The Karatomariyama Pyroclastic Flow Deposit is

covered by the Yabakei Pyroclastic Flow Deposit and

overlays the Shakadake Andesite at the western slope of the

Karatomariyama Mountain. The total thickness of the

Karatomariyama Pyroclastic Flow Deposit is in excess of

20 ill.

The reported radiometric age of the Karatomariyama

Pyroclastic Flow Deposit is 2.19 Ma (Daishi et aI., 2003)

and 2.22±0.28 Ma (FT age, Daishi, unpublished),

corresponding to the Early Pleistocene.

d. Matsubara AndesiteThe Matsubara Andesite (Daishi et aI., 2006) consists

of two-pyroxene andesite lava flows and pyroclastic rocks,

distributed in the vicinity of Deguchi, Matsubara, the

Tsuetate-ohashi Bridge and Tsurami in the northwest part

of the area.

The rock contains phenocrysts of plagioclase,

clinopyroxene and orthopyroxene. The Si02 content of an

andesite lava is 58.33 wt% (Table 1, sample number

67500).

The Matsubara Andesite is covered by the Yabakei

Pyroclastic Flow Deposit to the north of the Matsubara

Dam and is overlain by the Aso Pyroclastic Flow Deposit

on the east side of the Shin'tsuegawa-hashi Bridge. This

andesite is also covered by the Yamakogawa Rhyolite to the

southeast of the Tsuetate-ohashi Bridge and is overlain by

the Fukuro Andesite along the right bank of the

Oyamagawa River at Fukuro. The stratigraphical

relationship between the Matsubara Andesite and the

Shakadake Andesite has not been reported. The total

thickness of the Matsubara Andesite is more than 100m,

where the lower part consists chiefly of pyroclastic rock and

the upper part consists mainly of lava flow.

Although the radiometric age of the Matsubara

Andesite has been reported to be between 2.5 to 1.9 Ma

(Kamata, 1985b; MIT!, 1984; Sudo, 1985b), the largest

constituent is concentrated in the range 2.4-2.1 Ma. It is

thus concluded that the volcanic activity of the MatsubaTa

Andesite occL1lTed in the Late Pliocene.

e. Fukuro AndesiteThe Fukuro Andesite (Daishi et aI., 2006) is composed

of two-pyroxene hornblende andesite to dacite lava flows

and pyroclas tic rocks. These rocks are dis tri bu ted

throughout Fukuro and the Tsuetate-ohashi Bridge area inthe south of the study area.


hornblende, clinopyroxene and orthopyroxene.

The Fukuro Andesite is overlain by the Kusu Groupnear the ridge on the right side of the Tsuetate RiveT inFukuro and directly covers the Matsubara Andesite near the

south of Fukuro. The total thickness of the Fukuro

Andesite is more than 100m.

42 Volcanic Stratigraphy in Western Oita, Kyushu, Japan - Part 2: The Oyama - Tsuetate Area

The radiometric age of the Fukuro Andesite has not

been reported. However the ages of the underlying

Matsubara Andesite (2.4-2.1 Ma) and overlaying Kusu

Group (i.5Ma; Daishi et aI., 2003) suggest that the Fukuro

Andesite is formed in the period 2.0-1.5 Ma, which

corresponds to the Late Pliocene and Early Pleistocene.

(2) Early Pleistocene

a. Kamitarumizu Andesite

The Kamitarumizu Andesite (Kamata, 1985a; NEDO,

1990) is mainly composed of two-pyroxene andesite lava

flows, which are widely distributed to the north of the

Tsuetate, Yanase, Shimotarumizu, Kamitarumizu and

Shimouke villages.


clinopyroxene and orthopyroxene with less frequent quartz,

biotite, hornblende and olivine phenocrysts.

The Kamitarumizu Andesite is directly covered by the

Yamakogawa Rhyolite at the vicinity of the Shimojo

Elementary School and overlies the two-pyroxene

hornblende andesite lava of the Yoshinomoto Andesite at

the Mitakawauchigawa River northeast of the Shimouke

Dam. The total thickness of the Kamita.rumizu Andesite

exceeds 200 m.

The reported radiometric ages of the Kamitarumizu

Andesite are 1.7 and 1.3 Ma (Kamata, 1985a), indicating

Early Pleistocene activity. However, the Kamitarumizu

Andesite also includes many two-pyroxene andesite rocks

without hornblende or biotite (Kamata, 1985a; NEDO,

1990), which may be of a different age.

b. Ono Rhyolite

The Ono Rhyolite (redefined) consists of biotite

hornblende rhyolite lava and pyroclastic flow deposits. The

rhyolite is distributed west of Taraiba.ru in the northwest

part of the area.

This rhyolite contains phenocrysts of plagioclase,

hornblende and biotite, with occasional clinopyroxene and

orthopyroxene.

The Ono Rhyolite is redefined from the Ono Rhyolite

(Kido, 1981) after the Yamakogawa Rhyolite distributed

near the Matsubara Dam was recognized as an independent

flow.

The Ono Rhyolite is covered by the Yabakei

Pyroclastic Flow Deposit in the vicinity of the Ta.raibaru.

The total thickness of the Ono Rhyolite exceeds 100 m.

The radiometric age of the Ono Rhyolite is 1.55 Ma

(Daishi et aI., 2003), corresponding to the Early

Pleistocene.

c. Kusu Group

The Kusu Group (NEDO, 1990) mainly consists of

biotite hornblende dacitic pumice flow deposit with

siltstone. This group is distributed around Yasebe,

Takenosakotani, Okubodai, Koitsuma and the Gesu Ranch

in the vicinity of the Tsuetate-ohashi Bridge. The range of

distribution is limited from the north of Yasebe to Gesu.

The NEDO definition of the Kusu Group (NEDO,

1990) includes pyroclastic rocks distributed in the Tsuetate

region. Here, however, the rocks distributed in the vicinity

of the Tsuetate-ohashi Bridge are reclassified due to a

difference in the inferred age of activity (Daishi el aI.,2003).

The rocks contain phenocrysts of plagioclase, biotite

and hornblende with occasional clinopyroxene and

orthopyroxene.

The Kusu Group is covered by the Okubodai

Pyroclastic Flow Deposit in the vicinity of Okubodai and is

overlain by the Yabakei Pyroclastic Flow Deposit at the

Takenosakotani River. This unit is also covered by the

Yamakogawa Rhyolite and overlays the Fukuro Andesite at

the ridge on the right side of the Tsuetategawa River at

Fukuro. The total thickness of the Kusu Group is in excess

of 50 m.

The reported radiometric age of the Kusu Group is

1.41-1.64 Ma, corresponding to the Early Pleistocene

(Daishi et aI., 2003). Volcanic rocks with similar

radiometric ages are found in the Amagase area (Daishi et

al., 2003) to the northeast, suggesting that volcanic activity

occurred contemporaneously over a wide area in this

period.

d. Yamakogawa Rhyolite

The Yamakogawa Rhyolite (NEDO, 1990) is mainly

composed of hornblende-free or hornblende biotite rhyolite

pyroclastic flow deposits and lava flows, distributed in the

vicinity of the west of Matsubara, Yamakogawa, the Gesu

Ranch and Yunomidake. The Si02 content of these

deposits is 69.23-74.74 wt% (Daishi et aI., 2006).

The Yamakogawa Rhyolite is covered by the

Kan'non'iwa Lahar Deposit in the vicinity of the Shin

Kan'non'iwa-hashi Bridge, and by the Kan'non'iwa Andesite

in the adit south of the Shin-Kan'non'iwa-hashi Bridge. The

Yamakogawa Rhyolite directly covers the Matsubara

Andesite on the left bank of the Oyamagawa River near the

Matsubara and covers the Kamitarumizu Andesite on the

left bank of the Tsuetategawa River near the Shimojo

Elementary School.

The biotite dacite non-welded pyroclastic flow deposit

covering the Karnitarumizu Andesite on the right bank of

the Mitakawauchigawa River was previously considered to

be a part of the Yamakogawa Rhyolite. However, the rock

faces suggests that this deposit may belong to another

geologic unit (e.g. the Kusu Group). Further petrologic

study is therefore required to assign this deposit to the

Masahiro DAIS!-lJ and Michinori INOUE 43

proper unit.The lightfaces of the Yamakogawa Rhyolite are

variable. Although apparent linear structures on the biotite

rhyolite resemble the generally known flow structure of

viscous lava flows, the intermittent features of the structure

show that the material had been derived from the flattened

pumice of pyroclastic flow deposits. In addition, part of the

thick pyroclastic flow deposit near the Tsuetategawa River

in Yanase is welded and parallels a non-welded pyroclastic

flow deposit that changes in part to that of a welded layer.

The lightfaces of the non-welded pyroclastic flow deposit

change from pumice flow deposit, consisting almost

entirely of pumice, to a Merapi-type pyroclastic flow

deposit and/or a debris avalanche deposit. The

Yamakogawa Rhyolite also includes a densely welded to

weakly and/or non-welded section that is clearly revealed

by erosion at the Yamakogawa River and Yunohira. A

rhyolite pyroclastic flow deposit with strongly welded

character changes to the east and west, demonstrating the

typical structure of a pyroclastic flow deposit, in the vicinity

southwest of the Gesu Ranch.

The Yamakogawa Rhyolite reaches 400 m or more in

thickness from the vicinity of the Tsuetategawa River bed

(about 300 m a.s.1.) to a triangulation point (697.1m a.s.1.)

011 the left bank of the Tsuetategawa River and is further

distributed up to about 270 m a.s.1. in the vicinity of the

Tsuetategawa River in Yunomidake (740.3m a.s.1.) in the

north of Tsuetate. Several plateau features are exposed on

the left bank of the Tsuetategawa River in Tsuetate, which

may have been formed by the flow unit and/or the cooling

unjt of the pyroclastic flow deposit. The lack of a plateau

on the right bank of the Tsuetategawa River hints to a

difference in geologic structure. NEDO (1990) supports the

gravity observations of a fault structure in this area. A fault

with a sheared zone oriented in the same direction can also

be seen near the first Yanase tunnel, and a NNW-SSE fault

outcrops in the Yamakogawa Rhyolite along a forest road in

Yunohira.

The rock has hornblende or hornblende-free biotite

rhyolite pyroclastic flow deposits and clinopyroxene

orthopyroxene biotite hornblende rhyolite pyroclastic flow

deposits near the triangulation point (697.1 m a.s.1.) on the

left bank of the Tsuetategawa River.

The reported radiometric ages of the YamakogawaRhyolite are in the range 1.3-0.9 Ma (Dllishi et aI., 2003;

Kamata, 1985a, 1986, 1997; Yamada et aI., 2002),

indicating Early Pleistocene activity. Daishi et a1. (2003)

also reported that the Yamakogawa Rhyolite can be further

divided into a 1.3 Ma-1.2 Ma unit and alMa unit.

e. Sbimotsuru Pyroclastic Flow Deposit

The ShimotsW"u Pyroclastic Flow Deposit (Dllishi, et

aI., 2003) consists of hornblende two-pyroxene dacite

pyroclastic flow deposits with 67.35 wt% of SiOz (Table 1)

distributed only on the left bank of the Oyamagawa River

near Shimotsuru.

The Shimotsuru Pyroclastic Flow Deposit is overlain

by the Yabakei Pyroclastic Flow Deposit at the south of

Kamate, covers the Kusu Group to the south of the

Kanahori-hashi Bridge, and is bounded by a fault against

the Shakadake Andesite in the vicinity of Nakatsuo. The

total thickness of the Shimotsuru Pyroclastic Flow Deposit

is greater than 30 m.

The radiometric age of the Shimotsuru Pyroclastic

Flow Deposit is 1.1 Ma (Daishi et aI., 2003), corresponding

to the Early Pleistocene.

f. Okubodai Pyroclastic Flow Deposit

The newly named Okubodai Pyroclastic Flow Deposit

consists of hornblende two-pyroxene dacite pyroclastic

flow deposits distributed on both banks of the Oyamagawa

River near Okubodai.

The Okubodlli Pyroclastic Flow Deposit is overlain by

the Yabakei Pyroclastic Flow Deposit at the right bank of

the Oyamagawa River near Okubodai and covers the Kusu

Group in the vicinity of Okubodai. The total thickness of

the Okubodlli Pyroclastic Flow Deposit is greater than 50

m.

The radiometric age of the Okubodai Pyroclastic Flow

Deposit is 1.05 Ma (Daism et al., 2003), corresponding to

the Early Pleistocene.

g. Yabakei Pyroclastic Flow Deposit

The Yabakei Pyroclastic Flow Deposit (Ishii et aI.,

1956) consists of a orthopyroxene hornblende dacite

pyroclastic flow deposit with occasional pyroxene andesite

xenoliths and black glass lenses. It is widely distributed

from the northwest of the study area to the Matsubara Dam

in the center. The Si02 content of a rock is 68.31 wt%

(Table 1, sample number 67730).

The Yabakei Pyroclastic Flow Deposit is covered by

the Hanahira Pyroclastic Flow Deposit in the vicinity of the

Hanahira and by the Fu takushiyama Pyroclastic Flow

Deposit at the top of the Futakushiyama Mountain. This

unit is also covered by the Takenosakotaru Pyroclastic Flow

Deposit along the Takenosakotani River and by the Aso

Pyroclastic Flow Deposit. The Yabakei Pyroclastic Flow

Deposit directly covers the Matsubara Andesite in the

vicinjty of the Matsubara Dam and covers the Shakadake

Andesite in the vicinity of Higashigochi. The total

thickness of the Yabakei Pyroclastic Flow Deposit exceeds

50m.

The reported fission track ages of the Yabakei

Pyroclastic Flow Deposit are 0040 Ma (Matsumoto et aI.,

1977) and 0.32 Ma (Tamanyu and Kasuya, 1983), while the


K-Ar ages are in the range 0.96-1.03 Ma (Uto and Sudo,

1985; NEDO, 1990). Kamata (1997) concluded that the

peak activity of the Yabakei Pyroclastic Flow Deposit

occurred at about 1 Ma. Although previous reports have

indicated the widespread distribution of the Yabakei

Pyroclastic Flow Deposit in the study area, the present

radiometric ages indicate that the previous dates on which

these conclusions were based may be incorrect, leading to

the inconsistency in fOlmation definitions. In this report,

the age of activity corresponding to the Yabakei Pyroclastic

Flow Deposit is considered to be 0.97-105 Ma (Daishi et

aI., 2003).

h. Itsumaichi DaciteThe Itsumaichi Dacite (NEDO, 1990) is mainly

composed of hornblende dacite lava with pyroclastic flow

deposits. It is widely distributed in the vicinity of

Itsumaichi.

The rock is biotite-bearing clinopyroxene

orthopyroxene hornblende dacite with glassy ground mass.

The Itsumaichi Dacite is covered by the Hanahira

Pyroclastic Flow Deposit in the vicinity of Hanahira and is

overlain by the Kameishiyama- Yoshitakeyama Dacite near

Deguchi. This dacite also covered by the Amagase

Pyroclastic Flow Deposit in the vicinity of Hachihongi and

by the Aso Pyroclastic Flow Deposit. The Itsumaichi

Dacite covers the Yabakei Pyroclastic Flow Deposit in the

vicinity of Deguchi. The total thickness of the Itsumaichi

Dacite exceeds 50 m.

The radiometric ages of the Itsumaichi Dacite is 0.86

Ma (NEDO, 1990), corresponding to the Early Pleistocene.

i. Hanahira Pyroclastic Flow DepositThe Hanahira Pyroclastic Flow Deposit (Uto & Sudo,

1985), distributed in the vicinity of Hanahira, has an

orthopyroxene clinopyroxene hornblende dacite assemblage

with occasional pyroxene andesite xenolith and black glass

lenses.

The Hanahira Pyroclastic Flow Deposit covers the

Yabakei Pyroclastic Flow Deposit in the vicinity of

Hanahira and is overlain by the Aso-4 Pyroclastic Flow

Deposit. The FT age of the Hanahira Pyroclastic Flow

Deposit is 0.82 Ma (Daishi, et aI., 2003), corresponding to

the Middle Pleistocene. It was previously considered to

belong to the Yabakei Pyroclastic Flow Deposit (NEDO,

1990), but subsequent radiometric dating of the deposit

(Daishj et ai., 2003) suggests an age that differs from that of

the Yabakei Pyroclastic Flow Deposit, warranting its

change to the Hanahira Pyroclastic Flow Deposit. The total

thickness of the Hanabira Pyroclastic Flow Deposit is 30 m

or more.

The FT age of the Hanahira Pyroclastic Flow Deposit

is 0.82 Ma (Daishi, et aI., 2003), corresponding to the

Middle Pleistocene.

j. Futakushiyama Pyroclastic Flow DepositTbe newly discovered Futakushiyama Pyroclastic

Flow Deposit, distributed in the vicinity of the

Futakushiyama Mountain and near Gogoro, has an

orthopyroxene clinopyroxene hOll1blende dacite assemblage

witb occasional pyroxene andesite xenolith and black glass

lenses.

The Futakushiyama Pyroclastic Flow Deposit covers

the Yabakei Pyroclastic Flow Deposit. The FT age of the

Futakusbiyama Pyroclastic Flow Deposit is 0.77 Ma

(Daishi, et ai., 2003), corresponding to the Middle

Pleistocene. It was previously considered to belong to the

Aso Pyroclastic Flow Deposit (GSJ, 1982), but subsequent

radiometric dating of the deposit (Daishi et ai., 2003)suggests an older age. The total thickness of tbe

Futakushjyama Pyroclastic Flow Deposit is 30 m or more.

k. Kan'non'iwa Lahar DepositThe Kan'non'iwa Lahar Deposit (Daishi et ai., 2006) is

comprised of volcanic mudflow deposits that include

volcanic boulder gravel and river sediment (pebble gravel

to fine grain sand). This deposit is distributed within a

limitedly range along the right bank of the Tsuetategawa

River downstream of the Shin-Kan'non'iwa-bashi Bridge.

At least part of the Kan'non'iwa Lahar deposit

continues to the Merapi-type pyroclastic flow deposit or

debris avalanche of the non-welded pyroclastic flow deposit

belonging to the Yamakogawa Rhyolite, which is

distributed at upper levels near the Shin-Kan'non'iwa-hashi

Bridge. Th is deposit changes continuously to the lower

stratum of the Tsuetate Formation of the Tsuetategawa

River.

The total thickness of the Kan'non'iwa Lahar Deposit

exceeds 20 m.

Although the ramometric age of the Kan'non'iwa Lahar

Deposit has not been reported, the age is inferred to be

similar to that of the Yamakogawa Rhyolite based on

geological information.

(3) Middle Pleistocenea. Oyama Formation

The redefined Oyama Formation is composed of

pyroclastic rocks (including pyroclastic flow deposit and

pyroclastic surge deposit), pumice flow deposits, bedded

sandstone and mudstone, lapilJi tuff, and siltstone with

diatomite. It is distributed in the vicinity of the

Nakaoyama, Gogoro, along the Akaishi River in Watauchi

and in Seko. The Oyama Formation is only distributed to

the nortb side from the line that connects Higashigochi and

Nakama.

The pumice flow deposit has a hornblende dacite


assemblage with or without biotite, clinopyroxene and

orthopyroxene.The Oyama Formation is directly covered by the Aso

Pyroclastic Flow Deposit in the vicinity of the Nakaoyama

and covers the Shakadake Andesite. The total thickness of

the Oyama Formation is greater than 50 m.

The previous definition of the Oyama Formation

included the pyroclastic rocks, stratified and mud stone,lapilli tuff, and diatomite siltstone distributed in the vicinity

of the Yasebe to Shimotsuru (Iwaucru and Hase, 1989). In

this study, the Oyama Formation is redefined to consist of

the pyroclastic rocks (mainly pyroclastic flow deposits) and

the siltstone and/or sandstone distributed only on the northside of Higashigochi and Nakama with intercalated

pyroclastic surge deposits.The radiometric age of the Oyama Formation has been

reported to be 0.83-0.72 Ma (Daishi et aI., 2003; Iwauchi

and Hase, 1989), corresponding to Early to Middle

Pleistocene.

b. Tsuetate FormationThe Tsuetate Formation (Daishi et aI., 2006) is

composed of pyroclastic rocks (including pyroclastic flow

deposit), pumice flow deposits, bedded sandstone and

mudstone, lapilli tuff, and siltstone with diatomite. It is

distributed in the vicinity of the Tsuetate Hot Spring, along

the Kitazatogawa River in Yanase and in Mukaitsuru.

The pumice flow deposit has a hornblende

clinopyroxene orthopyroxene dacite assemblage with or

without biotite and a Si02 content in the range of 64.61

66.74 wt% (Daishi et al., 2006).

The Tsuetate Formation is directly covered by the Aso

Pyroclastic Flow Deposit in the vicinity of the Tsuetate Hot

Spring and Mukaitsuru. The total thickness of the Tsuetate

Formation is greater than 50 ill.

The radiometric age of the Tsuetate Formation has

been reported to be 0.77-0.60 Ma (Daishi et aI., 2003;

Iwauchi and Hase, 1989), corresponding to the Middle

Pleistocene.

c. Iketsllru Pyroclastic RockThe Iketsuru Pyroclastic Rock (Daishi et aI., 2006) has

a clinopyroxene orthopyroxene hornblende dacite

assemblage and consists of pyroclastic flow deposits with

pumice, surge deposits and fallout tuff. It is distributed in

the vicinity of Iketsuru. The Si02 content is 38.47-43.88wt% (Daishi et aI., 2006), indicating that leaching caused adecrease in a part of the chemical composition.

The Iketsuru Pyroclastic Rock is directly covered by

the Shimojo Andesite to the north of the Shimojo-taki

Waterfall and overlies the Tsuetate Formation in thevicinity of Iketsuru. The total thickness of the IketsuruPyroclastic Rocks is more than 15m.

The reported FT age of the Iketsuru Pyroclastic Rock

is 0.77 Ma (Daishi et aI., 2003), corresponding to the

boundary between the Lower and Middle Pleistocene. The

age of the Iketsuru Pyroclastic Rock is suggested to be

similar to that of the Tsuetate Formation, although it is

thought that the two units are the products of different

volcanic events based on the differences in rock facies and

chemical composition.

d. Haneyama RhyoliteThe Haneyama Rhyolite (NEDO, 1990) consists of

biotite rhyolite lava with some pyroclastic flow deposits.

The rhyolite is distributed in the vicinity of the Tsukada

Ranch, Daini-oharano, Nishizato and Taida in the eastern

part of this study area.

This unit contains phenocrysts of plagioclase,

hornblende and biotite, with occasional clinopyroxene and

orthopyroxene.

The Haneyama Rhyolite is covered by the Itsumaichi

Dacite in the vicinity of Tsukada, by the Kameishiyama

Yoshitakeyama Dacite in the vicinity of the Tsukada Ranch

and Asozuru, the Yamakawa Debris Avalanche Deposit in

Nishizato, and the Aso Pyroclastic Flow Deposit in the

vicinity of Asozuru. Although the subordinate strata of the

Haneyama Rhyolite does not outcrop in the study area, the

rhyolite covers the andesite (the Chikusru Lava) to the north

of Mt. Haneyama (NEDO, 1990). The total thickness of the

Haneyama Rhyolite exceeds 50 m.

The radiometric ages of the Haneyama Rhyolite fall in

range 1.06-0.47 Ma (Daishi et aI., 2003; Iso and Ikeda,

1979; Kamata and Muraoka, 1982; MITI, 1979; Sudo,

1985b; Uto and Sudo, 1985), corresponding to the Early to

Middle Pleistocene. However, the ages become

concentrated in the range 0.72-0.52 Ma when measurement

techniques and sample conditions are considered.

e. Shimojo AndesiteThe Shimojo Andesite (Daishi et aI., 2006) is

composed of biotite-bearing two-pyroxene andesite lava

distributed to the north of the Shimojo-tab Waterfall. It is

covered by the Aso Pyroclastic Flow Deposit in the vicinity

of Shimojo and Iketsuru and overlies the Iketsuru

Pyroclastic Rock to the north of the Shimojo-tab Waterfall.

The total thickness of the Shimojo Andesite is greater than

30 ill.

The Shimojo Andesite was included in theKamitarumizu Andesite in previous reports (e.g., Kamata,1997). However, the latter resides on an evidently lowerhorizon than the former, necessitating redefinition of the

former as the Shimojo Andesite.

The radiometric age of the Shimojo Andesite has not

been reported. The geological succession, however,suggests that the age is similar to that of the Kan'non'iwa


Andesite.

f. Kan'non'iwa AndesiteThe Kan'non'iwa Andesite (Daishi et aI., 2006)

consists of two-pyroxene andesite lava with 58.25 wt% of

Si02 (Daishi et aJ., 2006) distributed in the vicinity of

Kan'non'iwa, Tamanoki, the northern slope of the Sodano

ike Pond and Hara.

The Kan'non'iwa Andesite is overlain by the Pre-Aso 4

Pyroclastics at the north of the Tsubakinoto-hashi Bridge,

covers Itsumaichi Dacite in the vicinity of Hara, the

Yamakogawa Rhyolite to the south of the Shin

Kan'non'iwa-hashi Bridge, and is bounded by a fault against

the Kamitarumizu Andesite in the vicinity of Tamanoki.

The total thickness of the Kan'non'iwa Andesite is greater

than 50 m.

The Kan'non'iwa Andesite was previously included in

the Kamitarumizu Andesite and the Hara Andesite (NEDO,

1990). However, as it directly covers the Yamakogawa

Rhyolite and the reported K-Ar age is 0.70 Ma (Daishi, et

aI., 2003), a re-definition as the Middle Pleistocene

Kan'non'iwa Andesite was warranted.

g. Kan'non'iwa Pyroclastic Flow DepositThe Kan'non'iwa Pyroclastic Flow Deposit (Daishi et

al., 2006) contains both acidic and basic pyroclastic flow

deposits with a limited distribution in the adit of the left

bank of the Tsuetategawa River south of the Shin

Kan'non'iwa-hashi Bridge.

The deposit contains both acidic (hornblende

clinopyroxene orthopyroxene rhyolite, 71-72 wt% Si02)

and basic (clinopyroxene orthopyroxene andesite, 57-58

wt% Si02) bombs with similar abundances.

The Kan'non'iwa Pyroclastic Flow Deposit is covered

by the Aso Pyroclastic Flow Deposit, and is overlain by the

Yamakogawa Rhyolite in the adit. The total thickness of

the Kan'non'iwa Pyroclastic Flow Deposit exceeds 10 m.

The Ff ages of the deposit range from 0.80 to 0.61 Ma

(Daishi, et aI., 2003), corresponding to the boundary

between the Early and Middle Pleistocene. This deposit

may be related to the volcanic activity responsible for the

Kan'non'iwa Andesite considering the spatial distribution,

age, and composition.

h. Sugihira Pyroclastic Flow DepositThe Sugihira Pyroclastic Flow Deposit (Daishi et al.,

2006), distributed in the vicinity of Sugihira, has an

orthopyroxene clinopyroxene hornblende dacite assemblage

with occasional pyroxene andesite xenolith and black glass

lenses.

The Sugihira Pyroclastic Flow Deposit covers the

Yoshinomoto Andesite. It was previously considered to

belong to the Yabakei Pyroclastic Flow Deposit (Kamata,


(Daishi et al., 2003) suggests an age that differs from that of

the Yabakei Pyroclastic Flow Deposit, warranting its new

name. The total thickness of the Sugihira Pyroclastic Flow

Deposit is 30 m or more.

The Ff age of the Sugihira Pyroclastic Flow Deposit is

0.62 Ma (Daishi, et al., 2003), corresponding to the Middle

Pleistocene.

i. Takenosakotani Pyroclastic Flow DepositThe newly discovered Takenosakotani Pyroclastic

Flow Deposit, distributed along the Takenosakogawa River

and the south slope of the Karatomariyama Mountain, has

an orthopyroxene clinopyroxene hornblende dacite

assemblage with occasional pyroxene andesite xenolith and

black glass lenses.

The Takenosakotani Pyroclastic Flow Deposit covers

the Kusu Group and the Yabakei Pyroclastic Flow Deposit.

It was previously considered to belong to the Yabakei

Pyroclastic Flow Deposit (JGS, 1982), but subsequent

radiometric dating of the deposit (Daishi et. aI., 2003)

suggests an age that differs from that of the Yabakei

Pyroclastic Flow Deposit, wananting its new name. The

total thickness of the Sugihira Pyroclastic Flow Deposit is

30 m or more.

The Ff age of the Takenosakotani Pyroclastic Flow

Deposit is 0.59- 0.58 Ma (Daishi, et aJ., 2003),

corresponding to the Middle Pleistocene. Coincidentally, a

deposit exhibiting a similar age is the Sugihira Pyroclastic

Flow Deposit (Daishi, et aI., 2003).

j. Kameishiyama-Yoshitakeyama DaciteThe Kameishiyama-Yoshitakeyama Dacite (NEDO,

1990) is mainly composed of hornblende dacite to andesite

lava with pyroclastic flow deposits. It is widely distributed

in the vicinity of Deguchi, Mt. Kameishiyama and Mt.

Yoshitakeyama.

The rock is biotite-bearing clinopyroxene

orthopyroxene hornblende dacite to andesite.

The Kameishiyama-Yoshitakeyama Dacite is covered

by the Pre-Aso 4 Pyroclastics in the vicinity of the Sodano

ike Pond and is overlain by the Aso Pyroclastic Flow

Deposit near Tahara and Asozuru. The Kameishiyama

Yoshitakeyama Dacite covers the Kan'non'iwa Andesite in

Tamanoki and is bounded by a fault against the

Yamakogawa Rhyolite in the vicinity of Deguchi. TIle total

thickness of the Sugihira Pyroclastic Flow Deposit exceeds

100 m.

The radiometric ages of the Kameishiyama

Yoshitakeyama Dacite fall in the range 0.82-0.42 Ma

(MITI, 1979; Tamanyu and Kasuya, 1983; Watanabe and

Hayashi, 1983), corresponding to the Middle Pleistocene.

k. Yamakawa Debris Avalanche DepositThe Yamakawa Debris Avalanche Deposit (Kamata et

Masahiro DAJSHI and Michinori INOUE 47

aI., 1988), composed of enormous blocks of andesite lava

and fine fragments of andesite, is distributed in the vicinity

of Nishizato to the east of the study area. These blocks

have a clinopyroxene orthopyroxene hornblende andesite

assemblage.The Yamakawa Debris Avalanche Deposit is covered

by the Aso Pyroclastic Flow Deposit in Taida and overlies

the Haneyama Rhyolite and the Kameishiyama

Yoshitakeyama Dacite in that area. The total thickness ofthe Yanlakawa Debris Avalanche Deposit is greater than 50

m.

The reported K-Ar age of andesite blocks in the

deposit is 0.58 Ma (Uto and Sudo, 1985). As the K-Ar ageindicates the eruption age of the andesite, it can be deduced

that the Yamakawa Debris Avalanche Deposit is younger.

The Yamakawa Debris Avalanche Deposit is covered by

the Waitasan Andesite (0.45 Ma; Kamata, 1997),

suggesting an age of ca. 0.5 Ma.

I. Amagase Pyroclastic Flow Deposit

The newly discovered Amagase Pyroclastic Flow

Deposit, distributed in the vicinity of Amagase in the north

area, has an orthopyroxene clinopyroxene hornblende dacite

assemblage with occasional pyroxene andesite xenolith and

black glass lenses.The Amagase Pyroclastic Flow Deposit directly covers

the Itsumaichi Dacite and is overlain by the Aso-4

Pyroclastic Flow Deposit. It was previously considered to

belong to the Yabakei Pyroclastic Flow Deposit (NEDO,


(Daishi et. aI., 2003) suggests an age that differs from that

of the Yabakei Pyroclastic Flow Deposit, warranting its

new name. The total thickness of the Sugihira Pyroclastic

Flow Deposit is 30 m or more.

The FT age of the Amagase Pyroclastic Flow Deposit

is 0.31 Ma (Daisbi, et aI., 2003), corresponding to the

Middle Pleistocene. Coincidentally, a deposit exhibiting a

similar age is the Yabakei Pyroclastic Flow Deposit

distributed in the vicinity of Amagase (Matsumoto, et aI.,

1977; Tamanyu & Kasuya, 1983).

m. Pre-Aso 4 Pyroclastics

The Pre-Aso 4 Pyroclastics (Daishi et aI., 2006) are

mainly composed of dacite and andesite air-fall tuff,

pyroclastic flow deposits, surge deposits and secondary

deposits and is distributed in the vicinity of Iketsuru and theSodano-ike Pond. The source or sources of the volcanicdeposit remain unclear. The subordinate stratum

underlying the Aso 4 Pyroclastic Flow Deposit is

categorized as a single unit called the Pre-Aso 4

Pyroclastics.The Pre-Aso 4 Pyroclastics are directly covered by the

Aso 4 Pyroclastic Flow Deposit in the vicinity of the

Yanase-daiichi-zuido Tunnel. The total thickness of the

Pre-Aso 4 Pyroclastics is approximately 10 m.

The age of the Pre-Aso 4 Pyroclastics is presumed as

0.5-0.1 Ma based on the ages of the stratum and the Aso 4

Pyroclastic Flow Deposit.

(4) Late Pleistocene

a. Aso Pyroclastic Flow Deposit

The Aso Pyroclastic Flow Deposit is composed of

hornblende-free or hornblende two-pyroxene dacite

pyroclastic flow deposits and is found along the

Oyamagawa River, in the vicinity of Amagase, Deguchi,

and on the plateau on both sides of the Tsuetategawa River

(more predominantly on the right bank), and in Asozuru.

The distribution is generally below 200 to 500 m in altitude,

but reaches 700 m in the vicinity of Asozuru.

The pyroclastic flow deposit is generally dense

welded, with non-welded to weakly welded parts in the

upper and lower area of the flow unit. It occasionally

exhibits a basal conglomerate.

At least 3 flow units can be recognized in the Aso

Pyroclastic Flow Deposit in the study area, with the upper

unit being the most widely distributed of the trio. The

upper unit contains hornblende characteristic of the Aso 4

Pyroclastic Flow Deposit, whereas the lower two units

includes a small amount of hornblende that correlates more

closely with the Aso-2A Pyroclastic Flow Deposit (NEDO,

1990), which also contains a small amount of crystalline

hornblende. The total thickness of the Aso Pyroclastic

Flow Deposit is greater than 50 m.

Data concerning the activity of the Aso PyroclasticFlow Deposits, 0.19-0.46 Ma for Aso-l, 0.11-0.25 Ma for

Aso-2, 0.10-0.24 Ma for Aso-3, and 0.08-0.084 Ma for

Aso-4, have been reported by NEDO (1990).

(5) Holocene

a. Terrace Deposit

The Terrace Deposit is distributed along the

Oyamagawa River, the Akaishigawa River, in the vicinjty

of the Sllimojo Elementary School, Iketsuru, Nishihorai and

Taida.

The deposit is composed of andesite and rhyolite

gravel with a tuff matrix and is unconsolidated. The total

thickness of the Terrace Deposit is approximately 10 m.14C age of a fragment of tree in the volcanic ash along

the Akaishigawa River is 20,880 b.p. (Daishi, unpublished).

b. Talus Deposit

The Talus Deposit is unconsolidated and is distributed

on the plateau slope, basal part of the Oyama River, theTsuetategawa River on both banks, and in the northwest of

Kameishiyama. It consists of lava gravel with a tuff matrix.


The maximum thickness of the Talus Deposit is 20 m.

4. Results

The volcanic stratigraphy of the study area is

elucidated based on detailed filedwork and determinations

of petrography and age.

The oldest volcanic activity is the Okubodai Andesite

(4.4 Ma), followed by that for the Shakadake Andesite

(3.2-3.1 Ma), Yoshinomoto Andesite (2.8-2.4 Ma),

Matsubara Andesite (2.4-2.1 Ma) and Karatomariyama

Pyroclastic Flow Deposit (2.2 Ma). The Kamitarumizu

Andesite, Fukuro Andesite, Kusu Group and Ono Rhyolite

make up a succession from 1.7 to 1.3 Ma and are followed

by the Shimotsuru Pyroclastic Flow Deposit (1.1 Ma),

Okubodai Pyroclastic Flow Deposit (1.05 Ma),

Yamakogawa Rhyolite activity (1.3-1.0 Ma) and

subsequent formation of the Yabakei Pyroclastic Flow

Deposit (1.05-0.97 Ma). The volcanic activity of the

Itsumaichi Dacite and Hanahira Pyroclastic Flow Deposit

occurred at 0.9-0.8 Ma. Volcanic activity related to

deposition of the Oyama Formation, Tsuetate Formation,

Iketsuru Pyroclastic Rock, Kan'non'iwa Andesite and

Kan'non'iwa Pyroclastic Flow Deposit is inferred to have

occurred at approximately 0.7 Ma, succeeded by the

activity related to the Haneyama Rhyolite andKameishiyama-Yoshitakeyama Dacite. The volcanic

activities of Takenosakotani Pyroclastic Flow Deposit and

Sugihira Pyroclastic Flow Deposit are 0.62-0.58 Ma, and

that of Amagase Pyroclastic Flow Deposit is 0.3 Ma.

Acknowledgements

We thank Prof. K. Furuyama, Osaka City University,

for reading an earlier manuscript. The manuscript was

reconstructed based on his critical comments. We thank

Dr. R. Anma, Tsukuba University for their constructive

comments and discussions.

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Plate 01

Okubodai Andesjte (4.4 Ma)is covered by the pumjce flow

deposit of the Kusu Group

(1.44 Ma), and the pyroclasticflow depositof the OkubodaiPyroclastic Flow Deposit(1.05 Ma)

Plate 02

Pumice flow deposi t

belonging to the Kusu Group(1.62 Ma) is covered by the

Shimotsuru Pyroclastic FlowDeposit (1.1 Ma)Plate 03 Surge deposit is

intruded by silt belonging tothe Oyama Formation

Plate 03

Surge deposit is intruded bysilt belonging to the OyamaFormation

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