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Sci. Rep. Kanazawa Univ.

Vol. 40 No.l，2 pp， 9~37

December 1995

9

OSTRACODE EVIDENCE FOR THE PALEOCEANOGRAPHIC CHANGE OF THE MIDDLE PLEISTOCENE JIZODO AND Y ABU FORMATIONS

IN THE BOSO PENINSULA， CENTRAL JAPAN

日irokazuOZA W AぺTakahiroKAMIYA本 andAkira TSUKAGOSHI*本

* Departrnenl 01 Earth Sciences， Facul(v 01 s，αence， J{，仰 wzawaUniversity， Kakurna， K，αηazaωa 920-11

* * The University Museurn， University 01 Tokyo， Hongo 7-3-1， Tokyo 113

Abstrad: Extensive study of ostracocle asち巴mblagesrevealed changes of cun一巴ntsystem

ancl oceanic thermal StructUl・eof the Paleo-Tokyo Bay during cleposition of the middle

Pl巴istoceneJizodo ancl Yabu Formations in the Boso Peninsula， central J apan. After

reviewing ecological data of extant species arouncl th巴Japanes巴Islancls，biogeographical

ly ancl bathymetrically characteristic ostracocle species w巴T巴 usedto巴stimatethe

paleoenvironmental changes of the two formations. Results supported the previous

interpretation that回 chformation was cleposited during one glacial-interglacial climatic

chang巴 Theresults also showed that the water was not as cold as pr巴viouslyinferr巴d

and the cool-warm change was caused by the alternation of the relatively cooler Tsugaru

Warm and Kuroshio Warm Currents， and not of the Oyashio Cold and Kuroshio Warm

Currents. The thermal environment at the maximum paleo-clepth was founcl to be

different between the two formations---th巴 warmestin the Yabu Formation and un-

expecteclly cool in the ]izodo Formation. This was probably rεlated to the influx of the

Tsushima -Tsug乱ruCurrent that was sufficiently energized to advance south， reaching

the Boso area ancl crept under the Kuroshio water only at the maximum clepth of the

Jizodo Formation. This is th巴firstsucc巴ssfulreconstruction of paleoceanic structure for

th巴 middlePleistocenεShimosa Group， in th巴 BosoPeninsula

Kり words:Ostracoda， middle Pleistocene， ]izoclo ancl Yabu Formations， current system，

oceanic thermal structure， Paleo-Tokyo Bay

1. Introduction

In the northern Boso Peninsula on the Pacific side of central J apan， middle Pleistocene

shallow marine deposits are widely distributed. They are called the Jizodo and Yabu

Formations in the Shimosa Group_ Many stratigraphical and paleontological studies，

particularly regarding molluscs， have been published since Yokoyama (1922). The two

formations are considered to be the deposits of the代 Paleo-Tokyo Bay" which is thought

10 日irokazuOZAWA， Takahiro KAiVIIYA and Akira TSUKAGOSHI

to have been opened eastwards and narrowly opened southwards to the Pacific

Ocean. Previous works recognized a cyclic Hthological change in each formation

1960; Sugihara et 1978; Kikuchi et 1988; Tokuhashi and 1989).

The lithofacies changes from non-marine mud to marine sand from the base to the top of

both formations. The marine deposit is accompanied by a cold water molluscan assem

sublittoraD atthe which changes to a warm water sublittor¥

al) at the middle， and back to a cold water (upper sublittoral) assemblage

again in the upper horizon. It has been that the changes of molluscan assem-

blages have been caused by sea-level changes and Babaヲ 1973;Kondo，

1989) .

The ostracodes from the two formations were described and used for paleoenviron-

mεntal studies 1978， 1982; 1982MS). In these previous

the sampling horizons were sporadic， and the detailed discussion on changes of the sedimen-

tary environment on the basis of ostracode faunal analysis， i.e. water depth and relative

water temperature， have not been completed. From this point to the present， living

ostracode faunas around the J apanese Islands have been extensively studied， e.g. in the Ise

and Mikawa (Bodergat and 1988)， (Ishizaki and Irizuki， 1990)，

Sendai Bay and Itoh， 1991)， Otsuchi et al.， 1992) and off the Shimane

Peninsula and 1992). Our knowledge of the geographical and bath-

distribution of J apanese ostracodes has been improved. The development

of ostracode data as environmental indicators is exemplified best by the recent classical

work of Ikeya and Cronin (1993)， which proved that the paleo-depth and paleo-tempera

ture could be quantitatively estimated the ostracode S.c.D. The work on the

late Pliocene Yabuta Formation on the Sea of ]apan coast (Cronin et 1994) represents

an applied example of the improved data and for the of fossil

ostracode assemblages

In the present northeast off the Boso Peninsula， the warm Kuroshio Current

meets the cold Current. Therefore， in this region， the oceanic thermal structure

is complicated. Due to middle Pleistocene climatic changes， the paleoceanic environment

of t

11

the biogeographical and bathymetrical data of extant ostracode species and synthesize

them into a list as a simple and convenient paleoenvironmental stanc1ard; 2) trace vertical

changes of these indicators in detail within the two formations and reconfirm the glacio-

eustacy; 3) estimate the Paleo-Tokyo Bay's oceanic environment， especially water c1epth

anc1 ocean currents; 4) obtain func1amental data for future evolutionary work which will

discuss the relationship between heterochrony-originatec1 speciation and environmental

changes

OSTRACODE EVIDENCE FOR THE PALEOCEANOGRAPHIC CHANGE

Fo宜mation

Anesaki

J030

50m

Kiyokawa

KamIizumi

加一鵡

'内晶一野

寛』一宮胡

仏門戸。出

UJ判的む富一阿国間

7吉弘吉見N~ ~O:: 4ロ

。一志向日同

ZM桝

UOr門的向日同凶門間

。一司自v一戸阿

Kongochi ..... 、、-〆向、日〆曲、-"伊『

KasamorI

図

国

2E 豊富

1

Le盟end

O

Fig. 1. Stratigraphic s巴quenceof the rniddle to late Pleistocene starata in the Boso Peninsula， and the

sirnplified colurnnar section of the ]izodo and Yabu Forrnations， showing sarnpling horizons of this

study. (rnodified frorn Sugihara et al.， 1978; Tokuhashi and Endo， 1984; Kondo， 1989). (1. p.: lower

part). 1: rnud and rnuddy sand， 2: sand， 3: gravel， 4: tephra， 5: sarnpling horizon， 6: rnaximurn

transgression， 7: dorninance of rnolluscan Oyashio element， 8: dorninance of molluscan Kuroshio

element.

⑮ 時5

12 Hirokazll OZAWA， Takahiro KAl¥IIIYA and Akira TSUKAGOSHl

20 Geological setting

The ]izodo and the Yabu belong to the lowermost formations of the middle to late

Pleistocene Shimosa Group (Fig， 1)， The Jizodo Formation overlies conformably the

Kazusa The Yabu Formation covers the Formation and is

overlain with the KamIizumi Formation of the Shimosa Group The both

formations are lithologically divided into the lower and upper parts rc>"n~.rT'

shi and Endo， 1984). Although there are local the lower part of the

Formation is generally about 1-8m thick and consists of non-marine silt， clay and marine

muddy sand with interbedded cold water molluscan fossils. The upper part is

about 50 -70m thick and is made up mainly of marine fine to medium sand with warm and

cold water molluscan fossils. distributed tephras such as TE-5 (0.39土0.08F.T.

Ma; Suzuki and Sugihara， 1983) are interbedded in this part. The of the Yabu

Formation is about 1-30m thick and consists of sand and silt with non-marine and

cold marine molluscan fossils. The upper part is about 10-35m thick and consists of

marine fine to coarse sand with warm and cold water molluscan fossils. Tephras such as

SY and GoP2 (0.31士0.05F.T. Suzuki and 1983) are embedded 1982;

Tokuhashi and Endo， Kikuchi et 1988; Kondo， 1989). Machida et al. (1980)

o 1 2km ~直面毒菌温品

Fig. 2. Index map showing studied loc旦lities.Loc. 1: IZllmiyatsll， Loc. 2: Nishiyatsll， Loc. 3: Atebi

OSTRACODE EVIDENCE FOR THE PALEOCEANOGRAPHIC CHANGE 13

stated that the ages of the Jizodo and the Yabu Formations were correlated to the oxygen

isotope stages 11 and 9 respectively based on the tephrochronology.

In this study we investigated three large outcrops (Fig. 2). The lower part of the

Jizodo Formation is we11 exposed at Loc. 1 (Izumiyatsu)， and the upper part at Loc. 2

(Nishiyatsu). Loc. 1 is about 30m above Loc. 2 stratigraphica11y (see Fig. 1). The

outcrop at Loc. 3 (Atebi) covers both the lower and upper parts of the Yabu Formation.

Since the mo11uscan fossils are poorly preserved in the lower part of the Yabu Formation

of this locality， we focused on study on the upper part alone.

A total of 37 sediment samples for the ostracode analysis were collected from the two

formations (Figs. 3-5). Samples were collected parallel to the bedding plane to obtain

contemporaneous sediments. Burrows filled with overlying sediments were excluded.

Each sample was collected within a 10cm interval of the bed.

After being dried in an oven， 320g of consolidated samples and about 400g of uncon-

solidated samples were washed through a 235 mesh (0.064mm opening) sieve. The dried

residues were divided by the quatering method into unit samples which contain about 200

ostracode individuals. The fractions between 0.25mm and lmm of each sample were

picked for a11 the ostracodes， and the rest of the fractions was stored. The individual

number was determined by adding the total of the left and right valves count to the number

吉国田

.E4

UCMW

的.』+

℃コEl

12

TEESE'E4

m

m

4

a

n

u

』

ωDEコc

gaE倒的

lithology articulated molluscs

155-3 abundant ostracodes (> 1 0% )

担F田ω・・・4‘U己CI.) 、

V Z Buntonia hanaii

V s; 3 Loxoconcha tosaensis 〈~ti= Aurila kiritsubo

仏回

Eg Parakrithella pseudadonta

」吋~ミ=

代2

Cytherois asamushiensis

hii; ・司ロE宅〉ロE 、

Legend

aEト b日 c[ヨ d~ el-vl

Fig. 3. Columnar section of the lower part of the Jizodo Formation at Loc. 1 (Izumiyatsu)， showing

sampling levels， lithology， articulated molluscs and abundant ostracodes. a: tephra， b: peat， C:

molluscan fossil， d: plant remain， e: burrow.

14 Hirokazu OZAWA， Takahiro KAMIYA and Akira TSUKAGOSHI

of carapaces without regard to sex.

3. Brief desc:riptions of the moHuscan and ost:racode fossils

Thirty one samples (Figs. 3 -5， numbers aside the columnsl yielded ostracode speci

lithoiogy articulated abui1dant moHuscs ostracodes

て問。Uコ話1) ω chinensis

LVC0 6 『3 Pαnopeα

u japOfllca

申。'"Solen Loxoconchα

自;::j

πω 出コ

.~U-.~.~~ I

-7 慣習間部υョ

-6 器:::s Nerrωcardturn l JLSc生hi'2主o型仁川竺恥りr空e -5 samααe

-4 オよ居設3唱d留J

I Neo附マidω αta1内

白。 F、

~3 〉、

22 l同ri -qzn Fejrtzds叩 iLrlToSfnoqroq t加rFM4hJOFlL-O附~，t. ペ1-2r、、d

一似japonicαiOmよ L-

し倍留8口d

議巴至ト b[日 c[日Fig. 4. Columnar section of the upper part of the Jizodo Formation at Loc. 2 (Nishiyatsu)， showing

sampling levels， lithology， articulated molluscs and abundant ostracodes (L. t.: Loxつconchatamak

azura) . a: tephra， b: molluscan fossil， c: pεbble

OSTRA.CODE EVIDENCE FOR THE PALEOCEANOGRAPI-IIC CHANGE 15

mens with a total of 7785， although the ostracodes vvere rare in the six samples. We

identifiecl 136 species among 61 genera. The detailed results of the counting are shown in

Table 1.

The following c1escribes the articulatec1 molluscan and abundant ostracoc1e species of

each locality. The percentages show the ratio against the total ostracode inc1ividuals

within each sample. iNe term more than 10% species as引 abunc1antspecies" in this stuc1y.

In the lower part of the Jizodo Formation (Loc. 1) ， we collected five samples from the

poorly sorted muddy fine sand which yieldec1 the articulated mol1uscan shells e.g. Callithaca

adarnsi， Saxidornus、ρμrtumtusanc1 MacOJna tokyoensis crowdedly. Only the upper three

samples， numbered from 1Z-1 to IZ-3， yie]ded ostracoc1es (Fig. 3) . These samp]es included

abundant Aurila sp. A and Cytherois asamushiensis. The total percentage of the two

species c1ecreases upwarc1s from 90% in IZ-1 to 20% in 1Z-3. Buntonia ha刀aii，Loxoconchα

tosaensis， Aurila kiritsubo and Pamkrithellaρseudadonta were abunc1ant in 1Z-3.

1n the upper part of the Jizodo Formation (Loc. 2)， we obtainec1 eight samples from the

lower half horizon， poorly sortec1 fine to mec1ium sanc1 with articulated shells of molluscs，

e.g. Glyりrnensρilsbryiand Nemocardium samarangae， and four samples from the upper

half horizon， medium to coarse sand with articulated Panoρea Jaρonica and Solen krusen-sterni (Fig. 4)園 Onesample (NI-ll) was collectec1 from a mudc1y sand layer about 10cm

thick， interbedded in medium to coarse sand. All twelve samples， which are numbered

from NI -1 to NI -12， producec1 ostracoc1es. The Jizodo Formation of this locality are

characterized by being rich in Neonesid，ω olなodentataand Loxoconcha 0ρtimαThe

former species is about 15~25% in NI-3~6 ， anc1 the latter occupies about 20~30% in NI

7 ~ 12. The ostracode assemblage defined by abundant species roughly coincide with that

of the molluscan assemblage. For example， Pontocythere jaρonica， P. subjaponica ~

Glycymerisがlsbryiappeared concurrently in the lower horizon (NI -1 ~ 2). Neonesidea

oligodentat，α~ Nemocardium samaraηgae were in the middle horizon (NI -3 ~ 7)， and Loxo-

concha optirna~ Panoρea jatonica， Solen krusensterni， Mactra chinensis in the upper horizon

(NI -8 ~ 12) (Fig. 4).

In the upper part of the Yabu Formation (Loc. 3)， sixteen of twenty samples， which

are numberec1 from A T -1 to AT -16， yielded ostracoc1es (Fig. 5)


φQEコCャ

i

曲一aFF』冊的

rA

UC婦の一官

15

〆、町温、， 01-14

。。

Y脳陣 11

《》p、

'"、

1-10

〈〉，》盤、ジ忽、

articulated abundant l lithology

molluscs ostracodes (> 1 0%

H抑制lα S.a吋 P. I medium to ezoenSIS coarse sand Glycymeris

yessoenS1S

Anadarα

broughtoni F宿題

Tresus keenae αコ

Mercenariα 4EJ 2 stlrnpsom

Schizocythere t出広明

Pαnopea (j)

官 }apomcα UO ヨ

i/ enercardiα 事お周司

事担ω... ト$:

poorly sorted Loxoconchα fi11E IO ITIEdlsLaIEnn d eJjs，伽li tamαkazurα

medium to Glycymeris Loxoconcha optir，間coarse san yessoenSIS

一一一一ム耳目誼戸嶋角白3 均出ー-自

Le盟皆目d

a 距至ト b~ Ct塾Fig. 5. Columnar section of the upper p丘rtof the Yabu Formatiun at Loc. 3 (Atebi)， showing sampling

levεis， lithology， articulated molluscs and呂bundantostracodes (5. a.: 5chizoのthereasagao， P. j..

Pontoのtherejaponica)ー a:tephra， b: molluscan fossil， c: cross lamination

Solen krusensterni in the middle hOlセon -14)

It is difficult to infer the detailed paleoceanic environment on the basis of ostracode

abundant species. For example， Neonesidea oligodeηtata has wide tolerance for bathym-

etry from the tidal zone to the depth of ca. 150m. Therefore， in this study， we inferred the

paleoceanic environment by the environmentally valuable species for water depth， geo-

graphical distribution and ocean current indexes based on the review of previous studies


ancl the personal fielcl observa-

tion for the extant species

data， as describec1 in the fol-

lowing chapter

4. Analytical methods

We proposecl four geo-

graphical c1ivisions for the sea

arouncl the J a panese Islancls

which made the paleoceano-

graphic ostracode analyses

convenient. The clivisions，

T， M and C(Fig司 6) are

primarily based 011 the currel1t

system aroul1cl the J apanese Islanc1s (Fig. 7) with attention

to the sea water temperature

The clivision W inc1ucles the

Kuroshio anc1 proximal TSlト

shima Currel1ts. The M cor

responds to the mixing zone

of the Tsugaru Current and

Kuroshio Current. The T

covers the c1istal Tsushima

Current， inc1uding the Tsugar-

u Current， and the clivision C

』。

Sea 01 Japan

F品cificOc告品目

Fヒ

斗ltN

3cfN

Fig. 6. Four geographical divisions (W， T， M and C) of th巴oceamc

condition around the ]apanese Islanc1s mainly basec1 on th巴

distribution of current system旦ηdwinter sea surfacewater

temperature. Solid circles show localities where th巴 extant

ostracode faunas w巴r巴 studied. Larger symbols represent

the localities with more than 10 samples， and smaller symbols

show these less than 10 samples. The d旦taof water tempera-

tur巴wascited from Rikanenpyo (1994)

corresponcls to the Oyashio Curτent. The bounclary between W and others are borc1ered by

11 'c of the surface water temperature in winter

Geographical distribution of about 100 extant species among 136 species from the two

formations were examined and plotted 011 the map of the J apanese Islancls. The Iitera回

tures usecl as a clata source inc1ucle; Bodergat ancl Ikeya (1982)， Frycll (1982)， Ikeya and

l-Ianai (1982)， Ikeya and Itoh (1991)， Ikeya and Suzuki (1992)， Ikeya anc1 Tsukagoshi

(1988)， Ikeya et al. (1985， 1992)， Ishizaki (1968， 1969， 1971， 1981)， Ishizaki ancl Irizuki

(1990)， Kamiya (1988)， Okada (1979)， Okubo (1980)， Schornikov (1974)， Tsukagoshi

(1988)， and Tsukagoshi and Ikeya (1991)

As a result， we iclentified nine patterns for the species biogeographical distributions

They are; W， M-¥i¥人 T-M-W，C-T-M-W， M， T-M， C-T-M， T， C (Fig. 8). Several species

which have no recent distributional data， e.g. Schizoりthereasagao， Cytheroρteron aff.

eremitum ancl Palmoconcha sp.， are inc1uc1ed in this figure， since they coulcl not be ignored

18 Hirokazu OZAWA， Takahiro KAl¥I!YA and Akira TSUKA(;OSHI

because of their abundance.

We determined their

by the of

the concurrent and the

extant V.fe

defined thε;:)I-'"U'C，;:) with iN and

l¥II -YV distribution as re

preseγlting thεwarm water

than ca. 11 'c ，

see Fig. 9， !eft 。

the with distri

bution of C， C

a肌I口1dl¥II vve臼recωomb恒凶j口ne吋dtωo r陀ep肝rε q

S使en此1抗tt出he cool v机w凡町¥13.杭te白r向 5却pe町Cl匂es

(10明w町erthan ca， 1日1'C)上司 ε白cie白S

予Wl比thwi凶ded出!五ISはtγ討ibutionsuch as T

fv泣1-Wa口d仁 T一l¥II-羽Twere ex-

cluded from the 1九rarm-coolana匂

" Warm-cool ratio" was

calcu/ated for each

〆

斗OON

llJデ/Micom130hl

L__L~Ij~O_F _ the ratio of individual Fig. 7. The present current system around the J呂paneseIslands

number of warm water

to the total number of warm and cool water lO- second from the

Our divisions are in the that are related to the

distribution of ocean current rather than mere water temperature. the

~lc'"""'~" situation off the Boso the ratio of T and T -l¥II may represent

the influx of the distal Tsushima Current from the Sea of In contrast， W repre

sents the case of strong influence of the Kuroshio Current. Influence of each current could

be thus i1!ustrated the ratio of ostracode distribution pattern 10 -12，

We also examined and concluded that 69 out of 136 were defined asて'depth

¥were Into four divisions water of the habitats

i<e. Sl ， S2 zone~50m) ， Dl -100m) and D2 than

100-150m) 8) < Some that have wide tolerance for the water were

excluded from 8，

As a preliminary the Iirst five abundant indicative in each

were used to estimate the paleo-depth 10-12， !eft . This

れ estimated was determined the distributional range of the total

individuals for the five


Cool water Warm water

C T M W Distal Kuroshio

Oyashio Tsushima C. Mixing zone & Proximal ( Tsugaru C. ) Tsushima C.

Cornucoquil1治'aalataParakrithella pseudadon信，Cytheron泊rphaacupunc白白

Loxo∞ncha japoni，伺，Cythere omotenippon回Ca/list，ω:ythere rugcn泊，Ca/list，∞:ythere undulat，庁'ac泊l陪

cぞCトgずpコ3 ー

Pa悶cytherideab四oens信，Arnωc戸herejaponica

Callist，ωy的ereundata，白 /Iist，∞戸'he用 }apomca

(Aur;伺 sp.A) Ponわc戸herejapon闘，Loxo∞ncha kattol

Loxo∞'ncha sp.， Munseyellajapon回

SI • By的ocerat加'asp. A

GロO N J

Loxo∞ncha optima， Robustaurila ishizakil

ー.・+3国同4 Hemic戸f昭'fU胞 koゆ'yamBl

Loxoconcha hanachirusato Si叩 leberistosaensis 白 /Iist白川市'ehayame開館

Finl1田町'hinellajaponica Robustaurila assimi/lis

Cletoc戸he用israstromarginata

I Cytherois a Coquimba信'hizakii，Aurila kiritsu加Buntonia hanaii， C国11，脱出:ytherere耐cula伯

Ho附団関mpt∞州eroideaCa/list明 rtheresubjapon回

S2 qロO N J I (Schiz，州 he即 a相伊0) 白 11，田t∞:ytherenippon回

B戸内∞州市!resp

『・+“司ら。同d 喝

Lox，白羽田:hatosaensis， B帥∞eratinahanaiJ

Cal/，信t∞ytherea飽飽，Pont，∞:ythere miurensis， Pont，∞，ythere subjaponica 一」2凶2 Semicytherula miurer沼田，Semic戸herulahem抑owel

Loxoconcha tamakazura

Ig伽CoormFm…nn町ttUk』Ec∞沼0qq仰utuJfj加FmIπ7yn11bb加aa ri肱ky回ey殉n宮twtensls

Celtia japonica

g g Actin田白川hereisk，惚悶zuenslS

Cytheropteron uchiol

Cytheropteron subuchiol

Dl '・i' Hirst.∞:ythe.田 hanaiJ

CucぜpD コ可Cytherelloidea senkakuens信

Munseyella oborozukiyo

Amphileberおmppon回

V I(C.戸heropterona甘 erern伽)¥ Bradleya spp

cVg p 3

Cytheropteron sp.8

Palmer】el，崎 limicolaAcanthoc戸hereおmun町 hikal

D2 Abroc戸here信 g同 ngdongensis": ・4 Roberlsonites spp

Byth∞戸herideacassidoidea g (Palmoconcha sp.) Bythoc戸herideaca帥'dictia同 (Cytherop飽ronsp.D) (Kangarina hayamiJ )

Fig. 8. Distributions of the ostracode speci巴sof the Jizodo and Yabu Formations in the g巴ographicaland

bathymetrical divisions. Species without data of recent distribution are in parentheses

20 Hirokazu OZAWA， Takahiro KAMIYA and Akira TSUKAGOSHl

Warm water species Cool water species

(SCIyF1t0hcEyrtEhlfeorltddeEαG sm加'kuensis Actinocythereis kisar，α'zuensls (Eucythere sp.) Sp. ) Sinoleberis tosaensis Munseye/la hatatatensis

Pontocythere japonica Hirstocythere ? hanα11 (Schizocythereα'sagao) Eucythere yugao Cletocythereis rastromαrgmata Palmenella limicola Munseye/lαjapomca Abrocythereis guangdongensis Hemicythere sp. A λ;f oborozukiyo Celtia japonica f王Sp.B Ca/listocythere hayamensis Buntonia hanaii H sp.C C.japonicα Ambocythere japonica (Aurila sp. A) C. nipponica Bythoceratina callidictya Finmarchinella japonica C. reticulαta B. cassidoidea Comucoquimba alata C. subjaponicα B. orientaris C. ikeyai C. undata B. sp.A C. moniwensis Hanaiborchella triangularis Bythocythere sp (Yezocythere sp.) (Neomonoceratina microreticulata) Cytheropteron subuchioi Robertsonites aff. reticuliformα Aurila kiritsubo C. uchioi Howeina camptocytheroidea P宮eudoaurilajaponica C. aff. abnormis Cytheropteron sawαnense Robust，αurila assimilis c. sp. B (C. aff. eremitum ) Bradleya japonica (Kangarina hayamii ) (C. sp. D) B. nuda Pαracytheridea bosoensis Loxoconchαhanαchirusato (B. sp.) P. diαlata (palmoconcha sp.)

(CTroGqcuhIymlEbbG tIrsIhS rzakII Loxoconcha tamakazura Cytherois asamushiensis sogwipoense) L. kattoi

Acanthocythereis munechikai L. sp.

Fig. 9. List of the warm and cool water species from the Jizodo and Yabu Formations. Species without

present data are in parenthesis.

Then， all the depth indicators were taken into consideration on the second depth

analysis. The changes in percentages of Sl， S2， D1 and D2 species in each sample are

shown (Figs. 10-12， third graphs from the right)， and their vertical quantitative changes

help us to estimate the gradual change of the paleo-depth. With "roughly estimated

depth" and "ratio of depth indicators"， the paleo-depth and its change were well estimated.

The maximum paleo-depth is estimated by both the "roughly estimated depth" and "ratio

of depth indicators".

5. Paleoceanographic results

Water delうth

Fig. 10 shows the data of paleo-water depth of the lower part of the ]izodo Formation.

The roughly estimated depth is tidal zone-30m in the bottom horizon and the upper two

horizons are sublittoral zone-50m. On the percentages， Sl species (Aurila sp. A; species

in parenthesis are main species) decrease upwards and S2 (Cytherois asamushiensis)

increase upwards with s1ight increase of D1 species (Co間前何uimbamoηiwensis) . Conse

quent1y， the water depth in this part deepened upwards.

The paleo-water depth change of the upper part of the ]izodo Formation is shown in

Fig. 11. Roughly estimated depth is tidal zone-30m first (NI-1-2)， then abruptly in-

OSTR1¥CODE EVIDENCE FOR THE PALEOCEANOGRAPHIC CHANGE 21

creased. Between NI -3 and NI -6， the estimated depth is constant at 50 -100m， and above

them suddenly decreases. The ratios of Sl (Loxoconcルl0ρtima and Pontoりtherej，ψonica)

J2I

可;71ralio of c¥epth indicators warm-cool ratio ratio of depth indicators

<.'001時一一一一骨 warmo 20 40 60 80 100% 0 0.1 0.2 0.3 0.4 0.5 0.6 0 25 ヌ 75 100叫

L~rzs司欄 SI(ωal zone-30m)

S2 (sublittoralzone-50m)

図 DI(50-100m)

白州11indcpende附

一一口--w

一-<>一一 M-W

Fig. 10. Roughly estimated depth， ratio of depth inc1icators， warm-cool ratio， anc1 the ratio of current

inc1icators of the lower part of the Jizoclo Formation at Loc.1 (Izumiyatsu). Depth inc1epenc1ence

shows sp巴Cl日swith wicle bathym己tricalrang巴 Leg巴nc1is equal to that in Fig. 2

()J1l

rou2:hl、じ d

estimated depth ratio of depth indicators warm-cool ratio ratio of current indicalors cool .... 一一一一一骨warm

S I S2 D I D2 20 40 6日 8010C向， 0.50.6 0.7 0.8 0.9 1.0 0 10 20 30 .:fO 50%

horiχon of maximum paleo-depth

霊翻覇 S川l川(仙刷l乙臼山削O印叩n叶1

白 S幻山2引(s刊帥州u山刷bli伽tt山川1旧川O凹I山 O叩叩n出C一5伽0伽m)

回 D山I(仰5刈0一l削0∞Om川1吋)

図 D2(100.150m<)

口d叩111indep削 lcnce

一一口一一 W 一一合一一(、 r-M

-0ー-M-W一世一一 M

-0-T-M 一一φ-T

Fig. 11. Roughly estimatec1 c1epth， ratio of c1epth inc1icators， warm-cool ratio， anc1 the ratio of current

indica tors of the upper part of th巴 ]izoc1oFormation at Loc.2 (Nishiyatsu). Legεnd is equal to

that in Fig. 3.

22 日irokazuOZAWA， Takahiro KAMIYA and Akira TSUKAGOSHI

and S2ηdecrease from the base to the middle horizons， At the

middle Dl ( γDn uchioi and Loxocoηchαtamal?αabout 30%

(NI-5-6). The percentage of D2 (うたroηafi. cf guang

and Palmoco幻chasp. ) is at a maximum in NI-4 and it is 10%.

This horizon seems to to be the maximum of the Jizodo Formation and

its is estimated about the abundance oI D1 and D2 It is

also to propose that the maximum horizon is NI -5 because of the minimum

percentages of Sl and S2. Above the middle Sl optiηα) and S2

αlata and P ゆ increase and form a maximum of about in

the uppermost horizon. Between the NI -7 and NI -12 the estimated

is stable、butthe relative increase of Sl soecies indicates that the vvater deoth tended

sn -ny

u

d

JU

C

いvm

lnu

σbぺ叫

叫

tr

』む

2Eコ乙

ω一旦CCMUの

》

i

Egt

ratio of depth indic3tors 間 tioof current Indic3.tors

Yb3 (SY)I円三

九1 ~2 [) 1 D2

0'-" 《

，，------9 宗

horiノonof ma¥.imum pa!cn-depth

醐 SI(tida! zone-30m)

巴 S2(suhlittoral 7one-50m) 白 D1 (50 -1 00111)

図J)2(1 OQ.150mく)

口dcpthI山 pendcncc

一一口一-w -B--M

-ーかーーか]-w ←ーφ一一 i

一一召一一1'-M ーベか一一 (、

---iト-C-1'-M

Fig， 12. Roughly estimated dεpth， ratio of depth indicators， warm 'co01 ratio， and the ratio of current

indicators of the upper p乱rtof the Yabu Formation at Loc. :1 (Atebil. Legend is巴qualto that in

Fig.4.

to be

Fig. 12 shows the paleoenvironmental change of the Yabu Formation. Sl 0ρ

is dominant and forms about 70% in A T -L with the roughly estimated

the horizon should have been in tidai zone-30m condition， Howεver， in AT -3，


which is just 2m above A T -1 horizon， Dl Celtia うonicaancl

subzκhioi) increasεs and forms morεthan 40%. This c1ata suggests the horizon

should be in the deepest condition for the Yabu Formation and is about 50 ~ 100m.

Betvveen A T -6 and AT-15. Slορ Pontoりthere うonicaand

jα戸0刀ica)and S2 (Poηtoりthere and Aurila are common. At the

uppermost ratios of S1 species P. ρ。nicaand omoteη争 ι、α)

and S2 (P. become the with the of AT-l and fonn about

and 30% It Is likely it was dεposited in the tidal zone to 30m

interval. The inferred paleo-depth showed that the maximum c1epth of the Yabu

Formation (50 ~ 100m) is shallower than that of the ]izodo Formation 100m) bei:ween

the studied areas.

防匂rm-coolwater environmeni and currents setting

1n t]:1e lmver p3rt of the Jizodo Formationヲ thenumber of warm 'vvater局 isnine

and that of co01 water is five among a tol:al of 28 species， however， there is no

吐10 泡g.13引). AIl the cool water fromη.1 this hoαrizon 1日ivei知1口 thed出isはta叫l

a泊r屯eaoI t出hev訊w九lVa訂rmcu凶r寸r閃e口tおs. ¥lVarm-cool ratio increase弓 from0.01 to 0.54 upwards

10). This increase indicates that the inf1uence of the Kuroshio Current strengthened

during the transgression.

1n thεupper part of the the 35 wann water and 14 cool water

were found out of 110 日 total. N 0 species were contained

The wann-cool ratio is higher than that of the lower part， and morεthan

007 except with a maximum of 0.95 in

NI-2 11)園 Theseresults suggest that

the Kuroshio and Tsugaru Currents flowed

into the Paleo-Tokyo in this a口仁1

that the cold Current did not reach

the studied area. The warm-cool ratio is

high in NI-2 and NI-6 because of abundance

of A.urila Loxoconcha tamakazura

a n dρtemn subuchioi. The minimum

ratio in NI -4 depends on the large individuals

of coolvvater species such as Cytherotteron

sawanense， Fig. 13. Number of species from the ]izodo and

Palmoconcha sp. and Cythero)うteroηaff. Yabu Formations. Warm: warm water

eremitum. Palmocoηcha sp. is 3 species of species， Cool: cool water species， Oyashio:

the division T， i.e. distal Tsushima Current Oyashio species， Total: number of the

(Tsugaru Current) in a broad division， but total species from each p呂rtof the two

the extant has been found oniy from formations.


the Sea of ]apan side so far， and can be called a specific species in the Sea of Japan. This

species was almost exclusively found in NI -4 and NI -5 in theJizodo Formation. These

results indicate that the bottom water temperature was coolest at the very middle

but was very warm just below to NI -3) and above to NI -6) the coolest

horizon. As presented before， the horizon of NI -4 is thought to represent the maximum

Therefore， in the Formation， the water temperature is

thought to be very low in the maximum depth period. Aside from the very middle， the

general trend of the change is that the temperature was high around the middle horizon and

gradually decreased upwards.

In the upper part of the Yabu Formation， 38 warm water are found out of 115

species in total and it about one-third of all， likewise the upper part of the

Formation. The 15 cool water including one occured in this part

13) . The warm-cool ratio is generally high， more than with a maximum of 0.96

in AT-2 and AT-3. This is caused by the abundance of warm water such as

Loxoconcha Aurila Celtiαjゅonicaandψteron subuchioi. The

minimum is 0.53 in A T -16， and this is caused by the abundance of cool water T of

Loxoconcha hanachirusato and asagao (Fig. 12). This suggests that the waτm

Kuroshio Current f10wed into the Paleo-Tokyo Bay during the of the upper part

of the Yabu Formation and that the cold Current did not reach， similar to the case

of the Jizodo Formation. As noted the horizon of AT-3 is inferred to be in the

maximum Therefore in the Yabu Formation， the bottom water temperature

was the highest in the maximum depth period and gradually dropped upwards and down時

wards stratigraphically。

6. COI1ldusl.ve DiscussiulIT!s

Ostracode pαleoenviroηmental indicators

On the basis oI the available data on the distribution of extant ostracodes around the

Islands， we nine biogeographical distribution patterns for about 100

extant species out of 136 fossil from the middle Pleistocene Jizodo and Yabu

Formations. 1n addition we proposed four bathymetrical divisions. Using these patterns

and we are and able to reconstruct the water depth， warm-cool

water environment and the influence of ocean currents， i.e. the and

Tsugaru Currents. The present wod王showedthat the of the current system was

applied to the paleoenvironment of areas around the Boso Peninsula. Consider-

ing the ease and simplicity of the procedure， and given the fruitful results， the set of

indicators will be useful for future paleoenvironmental studies.

Contents 01 the "cool water sρecies"

USTRACοDE EVIDENCE FOR THE PALEOCEANOGRAPHIC CHANGE 25

The alternation of the warm-cold water environment of the Jizoclo and Yabu Forma-

tiじ11Shas been discus弓edconcerni11g the alternatio11 oI the warm and cold

currents (thεKuroshio and on the basis of molluscan and foraminifer

fossils 1961; Suzuki and Aoki et Aoki and Baba，

Konピ 1989). it is essential to consider the eIfect of three ocεan currents，

thεKuroshio， the and the Tsugaru for the study around the Boso Peninsula. The

τsugaru Current flows southwards along the Pacific coast of northeastern through

the Tsugaru Strait at present. This current watεr mixes with the Kuroshio ancl

Currents water off thε ぺ3anriku"coast of the northeastem Fig. 7).

1n contrast to the ostracode in the Kuroshio and the Oyashio the

Tsugaru Current that from the Tsushima Cur喝rentappears to have its own

specific ostracode いthecool element of the Sea oI J apan" (see 8). The

substance of代 coolwater in the studied area was shown not to be the

Current but the and distal Tsushima Currents species

。ceanicenviro幻ments

The general pattern of the environmental was simi1ar between the Yabu anc1

仁10Formations except for the maximum water depth This pattern shows a

gradual increase of water白 andtemperature toward the middle horizon from both the

uppermost and the lowermost horizons. 1n the Yabu many indivicluals of D1

-100m) warm water the sample of the m訂正imumpaleo--dεpth

h01包 on The horizon was characterized the strongest influence of the I-Curo-

shio Current within the formation

For the Formation， the samples above and below the maximum paleo

depth horizon i.e. NI-5 and NI -6， are of the similar species

component of the maximum paleo-depth horIzon of the Yabu Formation The

resemblance suggests a simila了environ立lentbet，̂!een the two formations at these horizons

anc1 a similar enviromental change that becomes warmer and deeper toward the micldle

horizon. These results clearly support the previous that each formation

was cleposited during one glacial-interglacial climatic change.

An interesting point is that the horizon of the maximum paleo-clepth (about 100m) of

the ]izoclo Formation contains the distal Tsushima and Tsugaru Current species oI Dl and

D2 as wel1 as the Kuroshio species of D1， and this brought the unexpectedly 代cool"

environment of the horizon in the analysis.

Gεnerally， cold heavy water creeps under the warm light water as the Oyashio Current

water crawls under the Kuroshio Current water off the Boso Peninsula at present. The

clifference of the ostracocle assemblages at the maximum depth horizon between the two

formations can be explainecl well by the oceanic thermal structure as follows; in the


maximum depth period of the Jizodo Formation， the cool Tsugaru Current water (ca.

several degree cooler than the Kuroshio water) possibly crept under the warm Kuroshio

Current water and affected the bouom water environment of the Paleo-Tokyo Bay. On

the other hand， in the case of the Yabu Formation， the Tsugaru Current water did not creep

under the Kuroshio water.

On a global scale， it is thought that the warm current intensifies during the warm

climate period. The warmer it is the more the amount of melting ice sheet in Arctic

increases and it accelerates the deep sea ocean circulation， which finally causes the

intensification of the Kuroshio Current around the J apanese Islands. When the climate

became warmer in the middle Pleistocene， the Kuroshio Current， Tsushima Current and

Tsugaru Current which is a branch of the Tsushima Current would have intensified. If we

assume that the climate was warmest in the period of the maximum paleo-depth， we are

able to propose the following scenario that caused the differences between the two forma-

tions. In case of the period of the Jizodo Formation， both streams of the Kuroshio and

Tsushima Currents were sufficiently intensified. As a result， the intensified Tsugaru

Current finally reached the Paleo-Tokyo Bay only in the period of maximum depth and

crept under the warm Kuroshio Current. The sea-level rise in the period of the Yabu

Formation was not enough high to push the Tsugaru Current to reach the Paleo-Tokyo

Bay. This assumption is consistent with the data that the maximum paleo-depth was

deeper for the Jizodo Formation (about 100m) than the Yabu Formation (50-100m) and

that the environment just above and below the horizon of the maximum depth of the Jizodo

Formation is similar to the environment of the maximum depth period of the Yabu

Formation in depth (50-100m) and temperature (strongest influence of the Kuroshio

Current) .

Another possible hypothesis to explain the difference depends on "topography'¥In

this case， the Tsugaru Current could have reached the Paleo-Tokyo Bay in both the period

of the middle horizons of the Yabu and Jizodo Formations. The difference in oceanic

thermal structure might be related to the different distance from coasts as we see it in the

transverse section off the present Boso Peninsula. It is possible that the sudden cooling at

the maximum depth of the Jizodo Formation was caused by the lateral movement of the

Tsugaru Current coastwar


graph) . This cool-shallow environment， accompanied by common M -I;九Tspecies， must be

derived from the general cooling of the Tsugaru Current and the Kuroshio Cunent water

in the mixing zone during the global climatic cooling. Evεn though the oceanic situation

was 110t completely specified， it is newly proved that the cool environments appeared twice，

in shallow and the deepest conditions， in the Jizodo Formatioi1. This is clifferent from the

pattern previously observed in deposits during a glaciahnterglacial sea-level change.

The previously observed pattern was illustrated by the single cool-warm change in the

Yabu Formation and in the early Pleistocene Omma Formation on the Sea of J apan coast

(Kitamura ancl Konclo， 1990)ー Thedouble cooling in one glaciahnterglacial change is

thought to have recordecl the vertical water mass structure cl1..1ring the midclle Pleistocene

(Isotope stage 11)， and is first describecl for the Shimosa Gro1..1p in the Boso Penins1..11a

Acknowledgements

羽7厄ewish to express our grati社t1..1deto Professor Emτ11en江t1..1SK立en吋1サjiK乞0111おshi 主ar口lazaw

University)， P日r司Oぱfe白ssoαr司 Akio Om1..1町1町ra(1瓦王anazawaUniversity) for t出:l負heirvaluable cliscussions

ancl encouragements cluring this work. We specially thank Professor N oriY1..1ki Ikeya

(Shizuoka University)， Dr. Tatsuo Oji (the University of Tokyo) and Mr. Jocly Webster

(the University of for their critical review of the manuscript and fruitful sugges-

tions. We would like to thank Professor Itar1..1 Hayami (Kanagawa University) for

iclentifying a part of molluscan fossils. Thanks are also clue to the members of Geological

Laboratory of Kanazawa University for their valuable comments ancl discL1ssions. This

work was partly supportecl by 1994 Fujiwara Natural History Founc1ation

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Suzuki， ]¥/1. ancl Sugihara， S.， 1983: The Plio-Pleistocene boundary layer of the Kazusa Group in terms of

F.T Ma. Progmrnme a12d abstracis 01 1<江戸anAssociatum 101' Quaternaη Resea1'ch， vol. 13， p. 69-70.

(iη ]apa幻ese;title Im12slated)

Tokuhashi， S. and Endo， H.， 1984: Geology 01 the Anesaki district. Quaclrangle S巴ries，scale 1: 50，000， Geol

Surv. Japan， 136pp. (in 1<α:pa12正sewith English abstmct)

一-anclKonclo， Y.， 1989: Sedim巴ntarycycles and environments in the middl巴 JatePleistocene

Shimosa Group， Boso Peninsula， central J apan. ]ou1'. Geol. 50c. Japan， vol. 95， p. 933-951. (in.

]a，ρ日目白e1心ilhE河glishabstmct).

Tsukagoshi， 1¥.， 1988: Reproductive character displacement in the ostracocl genus Cythe1'e. Jou1'. C1'usta，

cω12 Biol.， vol. 8， no. 4， p. 563-575.

anc1 Ikeya， N.， 1991: A reclescription of Cythe1'e japonica H旦nai，1959 (Podocopida

Ostracoda). Z，ωlogical Jou1'. Li1212ea目 SOC.，vol. 103， p. 129-143

Uchio， 1'.， 1961: Foraminifera assembl旦geof Semata-no-seki shelJ bed ancl its stratigraphical significance.

f刀 Prof].Makiyarna Mern. Vol.， p. 239-247. (i12 Ja，ρan.ese with E押:glishabstract)

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S.， no. 112， p. 371-409

， 1982: Lat巴PleistoceneOstracocla from Boso Peninsula， central J apan. U:日iv.Mus.， Univ. ToかO

Bull.， no. 20， p. 141-227， 256-267

30 Hirokazu OZAWA， Takahiro KAMIYA and Akira TSUKAGOSHl

Y okoyama， M.， 1922: Fossils from the Upper Musashino of Kazusa and Shimousa. ]our. Coll. Sα.， 1m.ρ. Univ. T.減少'0，vol. 44， part.l， p. 1-200.


Plate 1. Ostracode species from the Jizodo and the Yabu Formations. 1， Cytherゆteronsawanense Hanai，

LV， x81; 2， Loxoco刀chahanachirusato Yajima， RV， X88; 3， Palmoco叩chasp.， L V， X 73; 4， Palmenella

limicola (N orman)， L V， X 65; 5， Cytheropte仰向 aff.eremitum Hanai， RV， X 62; 6， Schizoのthereasagao

Yajima， LV， x64; 7， Cornucoquimba mo叫iwensis(Ishizaki)， L V， X 62; 8， Cytheroisωamushiensis Ishizaki，

LV， x42; 9， Paraりtherideabosoensis Yajima， RV， X 73; 10， Aurila kiritsubo Yajima， RV， X 52; 11， Loxocon-

cha 加叩akazuraYajima， LV， X65; 12， Cytherojうた仰向 subuchioiZhao， RV， X 62; 13， Loxoconcha kattoi

Ishizaki， L V， X 84; 14， Celtia japonica Ishizaki， RV， X 46; 15， Callistocythere jajうonicaHanai， RV， x64; 16，

Si河口leberistosaensis (Ishizaki)， RV， X 60; 17， Loxoconcha optima Ishizaki， RV， X 58; 18， Pontoσthere

sutりiajうonica(Hanai)， RV， X 60.


Table 1-1. Individual number of the ostracode species from th巴 Jizodoand Yabu Formations.

Jizodo Formation

Species namel Sample number IZ 1 z 3 Nll z 3 4 5 6 7 8

1 Cytherelloidea senkakuensis Nohara

2 Neo且esideaoligode.且tata(1王ajiyama) 2 71 26 61 56 24 23 10

3 Proponto己yprisspp. z z 1

4 Pont∞yprjs spp.

5 Neocythe，rideis aoi Y吋irna

6 Pontocy也erejaponica但anai) 23 44 7 3 2 3 8

7 P. rniurensis (H組ai) 2 5 Z 1 4 3

8 P. subjaponica (H組泊) 1 18 34 5 3 3 9

9 Krithe spp. 2 4 1

10 Parakri也.ellapseudadonta (H姐 ai) 10 21 1 1

11 Sinocytheridea sp.

12 Eucythere yugao Y司加国 z Z

13 E. sp.

14 Munseyella hatatatensis Ishizaki

15 M. japonica (Hanai)

16 M. oborozukiyo Yajirna z 17 M. sp

18 Callistocythere alata Hanai 10

19 C. hayamensis Hanai 1

20 C. japonica Hanai 4 19 4 1 9

21 C. nipponica Hanai

22 C. reticu1ata Hanai 1

23 C. rugosa Hanai

24 C. subjaponica H姐副 3 3 1

25 C. undata H組担 1

26 C. undu1atifacialis Hanai

27 C. sp.

28 cy也ereomotenippo且icaHanai 1 z 29 C. sp.

30 Schizocythere asagao Yajirna 1 z 5 11 16 4 3 9 14

31 S. kishinouyei (Kajiyarna) 1 14 15 81 35 29 13 17 19

32 S. sp. 1

33 Hanaiborchella rniurensis (Hanai) 1

34 H. tria且guralisHanai 4 9 6 6 5 5 6

35 Neomonoceratina rnicroreticu1ata Kingrna

36 Palmenella lirnicola (Norrnan)

37 Hernicythere sp. A 1

38 H. sp. B

39 Yezocythere sp.

40 Aurila hataii Ishizaki 4 3 3

41 A. kiritsubo Yajirna 13 23 5 9 11 13 5 11 4 12

42 A. munechikai Ishizaki

43 A. sp. A 107 261 20

44 A. sp. B 6 10 4 2 7 2

45 A. sp. C 6 4 101 26 6 6 z 4

46 A. sp. D Z 5 2

47 A. sp. E

48 A. sp. F

49 A. sp. G 50 A. sp. H 1 3

0:3"1、RACODEEVlDET、代~'E FOR THE PALEOCEAトムJGRAPHICCHI¥NGE 33

Table ]-2


Tabl巴 1-3

fu~doForm~ 仁二二三円柄通He/Sampi~~ IIZ 1 2 3 NI1I 2 うJ 4 5 6 7 8

151 Ps引由aIlihjaFZ;ca(Ishmki)副主a

i 52 Robustauriia a邸 m湿lis(Kajiyarna) 1 2

5534R RI1mm11airzcahk1i111e (OKubo) -机ーJトu

1 l 1 1

lIa japonica (Ishizaki) ト…引

! 1 i

55 Caudites? pos!erocostatu呂 (Ishizaki) 一一，'--i. 2 r

56 Bradleya japonica Benson l

57 B 即 daE幽阻 L i L li」l叶 .U 58 B. Sj) ごと 11

5ヲCornucoquimbaalata Tabuki

。60C. ikeyai (Yajima) 11 11 21 2

山 (Ishiza

i iI14 2 1二村同 63Coquimba ishizakii Y a jirna 21 31

6645TTM-Sca榊lmCc13nIlEeH山 umai(Ishizaki) !117hl~i~' ~61-' sr-f!5i3 …)開 4 T 2166 T. sogwipoense Lee 51 21 2

67 Sinole ber悶仙saωi8CIshizaki一、 3 ZZ1l 1 1 6心8Ac訓叩叫he加凶1記即附附…c訂悶蜘r陀附e酌]

69 A. sp. A

70 A 3D. B

71 A sp. C

~ 72 A-"I'~J:)_ ______ .L__L_ .1

74ASP 1 1 i=一一75 Abrocythereis guangdongensis Gou

76 Cletocythereis rastromargillata (Brady)

I 77 Hirstocvthereつha且aiiIshizaki

78 Rocaleberis?

j 79 Pistocyther己i8bradyformis (Ishizalo)

4戸二出三己80 Amphileberis nippol1ica (Yajima) Z

81 Ambtonia obai (Ishizaki) Z

82 B untorria ha且aiiYajima 1 q 27 7 弓

半f」よ

83 Ambocythere japol1ica Ishizaki 1

i 84 Robeltsol1ite8 aff retIculiforma (Ishizaki)

h= 5 1 85 Australimoosella tomokoae (Ishizaki) 1

~186 Celt川 apol1icaIsl阻止1 12 2 11

87 Bythoceratina callidictia Zhao 5

88 B. cassidoidea Zhao 1 1

8ヲB.hanaii Ishizaki トーイ

2 3 1

90 R orientaris (Brady)

91 R sp. A

92 B. sp. B l

93 R sp. C 仁土ゴ94 B. sp. D

I 95 Bythocythere sp l 2 1

96 Psendocytllere spp 1

97 Hernicythemra cuneata Hallai 1 ヱ 1 2 1

98 H. kajiyamai Hanai 1

99 H. sp

100 Howein& camptocytheroidea Hanai 1 11 戸


Table 1--4

Jizodo Formation Yabu Formation

NI9 10 11 12 AT 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

l 2

7 1 1 l

2 3 1 3 3 3 l 4 3 1 4 1

4 1 1 Z 2 立 7

l -由E ト司3 l

1 1 1

1

1 1 ウ l 1 2 1 2 l

1 1 1 3 1 6 5 6 3

14 ') 6 2 1 l 3 1 1 Z 1

3 4 4 6 J 今 8 3 つ 6 5

Z 6 3 4 1 1 33 18 13 28 41 29 6 8 5 9 6 7 2

1 1 I 4 2

2 1 J づ 2 1 1 1

4 2 5 fフ 3 4 4 14 7 15 10 9 3 3 2 1 6

1 I 2 1 3 I 2 1

1

トーー3

Z 3 Z

1 1

1 1

1 1 13

1 l

1 1 3 1 Z 1

2 3 2 1 1 3 1

1

l 1

9 7 8 8 2 2 1 1 Z 5 3 Z Z 3 1 6 7

3 Z 3 11 7 1 9 4 6 3 Z l 5 3

l 2 1 Z 5 6 3

I l 1 1 Z 2

4 2 3 1 3 1 18 4 A 4 4 2

1 Z

2

2 Z 1 1

1 1 2

1

1 l 1 3 2 4 9 5 3 1 7 10 Z 1 1

I 1 3 1

1 3 1 1 1 2 5 15 コ 6 4 3 6 4 1

2 1

l 1 4 7


Table 1-5.

Jizodo Formation

Species namel Sample number IZl 2 3 NI1 Z 3 4 5 6 7 8

101 Sernicytherura henryhowei Ha田 i& Ikeya

102 S. miurensis (Hanai)

1ω Cytheropteron rniurense Hanai 2 2 z 104 C. sawanense Hanai 18 15 6 3 4

105 C. subuchioi Zhao 7 12 3 4 9 4

106 C. uchioi Hanai 13 20 10 5 3

107 C. aff. eremitum Hanai 7 18

108 C. sp. A

109 C. sp. B Z

110 C. sp. C

111 C. sp. D

112 C. sp. E 2

113 Kangarina hayarnii Yajima 5

114 Paracytheridea bosoensis Yajima 4 9 z 2 3 9

115 P. dialata Gou & Hung

116 P. sp. 2 1

117 Loxoco且chahanachirusato Y a jima 2 3

118 L. japon同 Ishizaki 1

119 L. kattoi Ishizaki 1 2 3 1 5

120 L. optima Ishizaki 18 29 4 2 4 13 36 45

121 L. tamakazura Yajima 8 4 91 37 24 9 19 18 13 24

122 L. tosaensis Ishiz北i 1 91 25 Z 1 1

123 L. vi va Ishizaki 1 Z

124 L. sp目 4 1

125 Palmoconcha sp. 4 4

126 Cytheromorpha acupunctata (Brady) 1 6 6

127 Xestoreberis hanaii Ishizaki 1 1

128 X. sagamiensis Kajiyama 2 10 6 4 4 2 4 6

129 X. suetsumuhana Yajima 1

130 X. spp 1 3 5 1 2 1

131 Paradoxostoma spp 3 2 Z Z 1

132 Cytherois asamushiensis Ishizaki 39 46 16

133 C. sp.

134 Cytheroma? sp 1

135 Paracytheroma spp. Z

136 Argilloeciaワspp 1 1

Miscellaneous l 4

Total 1臼 156 185 217 321 165 271 219 161 19ヨ249

O;:，TRACODE EVlDENCE FOR THE PALEOCEANOGRAPHIC CHANGE、 37

Table 1 ~6

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