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Chapter 42. Pliocene-Pleistocene palaeoenvironment and fossil ostracod fauna from the southwestern Hokkaido, Japan

Chapter 42. Pliocene-Pleistocene palaeoenvironment and fossil ostracod fauna from the southwestern Hokkaido, Japan

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Accordingly the stratigraphic distribution, habitat and other aspects remains poorly known.

One such group is the ostracods. In earlier studies, Okada (1979) and Tabuki (1980, MS.) reported

on the ostracods of this fauna in the Oga and Tsugaru Peninsulas both of which is in Northeast

Honshu, but other areas are still remain unstudied. Hokkaido is one such area situated in the

northernmost part of the distribution of the Omma-Manganji fauna, and there is no published paper on the ostracods apart from the description of a few species by Hanai (1961). It is important to

study the Pliocene-Pleistocene ostracods in Hokkaido, because the Omma-Manganji fauna represents cold-water conditions and has palaeoclimatic significance.

On Neogene fossil and living species of cold-water habitat of Japan, Ishizaki (1963, 1971) and

others have described in detail many new species. They were studied in Hanai et al. (1977). Many

other species, however, remain still undescribed.

Thus the study of cold-water ostracods remains at an elementary stage. Further, there is no

study of the ostracod fauna and their relationship to the sedimentary environment in which they

live. This study is the first attempt in that respect.

The area studied is situated in the “Kuromatsunai Lowland Belt” in the southwestern Hokkaido,

Japan, which lies between Suttsu Bay (the Sea of Japan side) on the north and Funka Bay (the

Pacific side) on the south. The lowland is bounded by mountains on the east and west. Its topography is controlled by the geological structure of the Neogene sediments (Ikeya and Hayashi,

1982)in that the lowland belt coincides with the structural depressions and is underlain by younger

and poorly consolidated deposits, most of which belong to the Pliocene-Pleistocene Setana Formation. In this paper, sedimentary environments are clarified on the basis of the lithology and molluscan assemblages, and the accompanying ostracod assemblages are described and compared with

these sedimentary environments.






The Pliocene-PleistoceneSetana Formation is sporadically distributed in southwestern Hokkaido,

Japan. The norhternmost part is in the “Kuromatsunai Lowland Belt” of which the central part

is studied in terms of the palaeoenvironment in this paper. The formation in this area consists of

marine, brackish, and freshwater sediments and is analysed mainly on the basis of lithological characteristics. The writer subdivided the formation into eight sedimentary facies in order to compare

the different detailed palaeoenvironments with that of the ostracod assemblages. In addition the

data provided by the molluscs found in the marine and brackish sediments allow four cold marine

and one brackish facies to be recognized. Both the composition of fossil assemblages and the

modes of fossil occurrence are taken into account for the interpretation of the sedimentary facies,

as well as the characteristics of the sediments themselves. The unfossiliferous freshwater sediments

are subdivided into three different freshwater facies, based on the detailed lithological characteristics

and sedimentary structure.

The eight sedimentary facies are tentatively named Marine I-IV, Brackish, and Fresh-water 1-111

as shown in Text-fig. 1. Their stratigraphical relationships are shown in Text-figs. 2A, B, C (A: western area, B: middle area, C : eastern area), on which the distribution of molluscan and ostracod

assemblages is also shown by symbols. In the stratigraphic sequence, these sedimentary facies change vertically exhibiting two cyclical sequences of sedimentation. Each cycle is composed of marine

or brackish water, marine water, and freshwater facies in ascending order. The three-dimensional

relationships between these facies for each cycle are shown schematically in Text-fig. 3.


P1io.-PleistoceneOstracoda from Hokkaido, Japan 559

erop eron sawanense A

Text-fig. 1-Sedimentary Facies and Relationship between Ostracod and Molluscan Assemblages.

Lower (First) Cycle

A marine transgression after the folding and erosion of pre-Setana formations is inferred from the

existence of marine fossils in the lower part of the Setana Formation in all areas except the western

area. These fossils indicate continental shelf depths and open ocean conditions and deposited

fine-grained sediments in the main part of the basin. The writer identified this sedimentary facies

by two molluscan assemblages : the Acila divaricata Assemblage which is characterised by the

dominance of articulated individuals of this bivalve in life position, and the Cryptonatica janthostornoides Assemblage that is meagre and composed of several species of small gastropods and

bivalves. The Acila divaricata Assemblage indicates deeper conditions (tens of metres at least,

perhaps 100 m or more, as inferred from the depths inhabited by the living Acila divaricata) than

the Cryptonatica janthostomoides Assemblage. The latter assemblage might have been deposited

in the depths of tens of metres, though this supposition is unwarranted because of allochthonous

material. This sedimentary facies of fairly deep and low energy shelf environments is named Marine

I facies.

In the southeastern area, however, this facies grades laterally into another facies which is composed of coarse-grained sandstone including abundant allochthonous molluscan fossils (Text-fig.

2C). The molluscan assemblage is characterised by the abundance of Chlamys islandica, which

accounts for more than 50 % of the individuals. Limopsis tokaiensis and Boreoscala yabei echigonum

are also characteristic constituents of the assemblage. The assemblage also contains Chlamys daishakaensis, C. (Swijtopecten) swijtii and Monia macroschima. These fossils are usually poorly

preserved in cross-bedded layers. Because these fossils are almost allochthonous, it is difficult to

infer the palaeoenvironment of deposition. The abundance of Ostrea denselamellosa may signify

shallow water in the basal part, and L . tokaiensis may suggest fairly deep water in the lower part.

The sea may have been as much as 200 m or more in places, because Acesta goliath in life position

was fGund in the same assemblage (in the southern locality of route 25 in Text-fig. 2C). Consequently

the depth range is too wide to characterise this sedimentary facies. Lithologically this facies is

characterised by the cross-bedding of a relatively high energy environment such as that of a straight.

It is named here Marine I11 facies.


P1io.-Pleistocene Ostracoda from Hokkaido, Japan 561

named Fresh-water Unit I. Above this facies, gravel accumulated forming fan deposits. The inclination of the gravelly layers indicates that the sediments were transported from the west. The gravels

were presumably derived from the underlying volcanic products, because of their similar lithology.

This facies represents re-sedimentation of the underlying volcanics by rivers flowing in from the




_ __ __ ___ __ __ _- _- - _- -_ - _- -- ----_-------- -- - -

Index M D of the r w t e s of

geolog ica I columns


Route of columnar sect ion



line for text-fig. 2

.,,,,,,,,,.: DistribUtiOn


2B-Continued (Middle area).

of the Setana Forlation




TEXFFIO.2CContinued (Eastern area).





number of locality

















5 , , , 1 L . s . - . . - sr



Limnocythere inopinata




- .............



---- :-.-.;,.,


Darwinula stevensoni





Ibocypris bradyi


I. gibba




I. tubercdata




.................. 26








Candona candida







C. weltneri var. obtusa




Candona hartwigi


C. mrchica







Candona albicans


Candona fabaeformis


Candona hyalha







Candonopsis kihgsleii




Cyclocypris laevis


C. ovum s. Klie

C.ovum s. Petkovski

50 m


Cypria lacustris



34 25














Physocypria kraepelini


Marine I ( f a i r l y deep) Facies

Notodromas persica

Brackish Facies

greenish-grey s i l t - f i n e sand

gray silt-conglomerate


Eucypris zenkeri

Marine IJ (inshore SIMIIW or bank) Facies

Fresh-water I (volcanic product) Facies



Isocypris beauchampi greenish gray very f i n e sand-mediui sand

tuff breccia, lava, volcanic conglomerate



n (Off-Shore, deep and wavy) Facies 11 sd




Fresh-water IJ (fan) Facies



gravel i f e r w s d e w s i t s

fmciata gray medium-very coarse sand

LOO m DolerocyprisMarine


(inner bay) Facies



( f l u v i a73

t i l e ) Facies







56 100 100


Cypridopsis vidua

gray si It-very f i n e sand

d conglomerate and sand




Potamocypris variegata

mean annual densityrelationships




1140 Section

438 1427

400 139 189 219 148


28 438



among sedimentary

each cycle.

lines are30shown



of the

in the index

of Text-fig.

1. eudomiaant (> 10%)species is given in each case. d = dominant (10-5%), sd = subdominant (5-2%), r = recedent (2-1

sr = subrecedent 1%).




























west. This fan facies is tentatively named Fresh-water Unit 11. This facies grades laterally into crossbedded conglomerate intercalated with thin layers of sandstone in the northwestern area. Original

dips and imbrication structure found in the conglomerate signify a current from the west flowing in

a direction varying between northeast and southeastward. This coarse-grained facies is regarded as

fluviatile facies (delta or fan-delta), and called Fresh-water Unit 111. No molluscan or ostracod

fossils were found in these three Fresh-water facies.

Later, the sea presumably became shallower because of the infilling by sedimentation, and in

the eastern and middle areas it became brackish (Text-figs. 2B, C). In some areas, near-shore shallows and bank environments appeared.

The Brackish-water facies is recognized in the eastern and middle areas, on the basis of the

Crenomytilus grayanus and Crassostrea gigas Assemblages respectively. In this case, it is useful

that these oysters are restricted to brackish water. The influx of freshwater has implications for

the regression of the sea, depending on whether the source of freshwater was in the south or east.

Though this fazies is composed of fine-grained sediments, the oyster bank may have formed a stony

bottom habitat for benthonic micro-organisms such as ostracods.

This facies changes laterally into fine- to medium-grained sandstone containing Chlamys daishakaensis primarily in the southeastern area (Text-fig. 2C). The coarseness of grain size and habitats

of some species in the Chlamys daishakaensis Assemblage indicate a near-shore, shallow bottom of

a high energy environment. High energy conditions persisted during the deposition of the Lower

Cycle in this area, because the underlying Marine I11 facies is also a high energy facies too. This

fact may hold critical clues to the direction of regression.

The same facies, including the same molluscan assemblage, is found in a small part of the northwestern area (Text-fig. 2A). The Chlamys daishakaensis Assemblage found in this area is characterised by the predominance of that species (about 70% of the total assemblage) which is accompanied by Monia macroschima and Callista (Ezocallista) brevisiphonata. Most of the fossils are

well-preserved, though they are not in life position. Consequently, it is unlikely that the fossils have

been transported far from their living sites. In the field, one can see that the sandstone of this facies

abuts on a swell of rocky basement. The molluscs are considered to have lived around, or on, the

submarine rocky swell which perhaps formed a bank, and to have fallen to the bottom near the


During its final stage, the sea became an inner bay environment over most of the area and

deposited very fine-grained sediments. In addition to the typical lithology of the bluish-grey siltstone

and very fine-grained sandstone, this facies can be recognised by three distinctive inner bay assemblages of molluscs, the Raeta yokohamaensis Assemblage, the Lucinoma annulata Assemblage and

the Macoma calcarea Assemblage. Raeta yokohamaensis is a characteristic fossil of the innermost

bay environment and is widely distributed in the northern area (Text-figs. 2A, ByC). Lucinoma annulata indicates an inner bay environment and is found in the southern area (Text-figs. 2A, B). The

Macoma calcarea Assemblage is found at the same horizon of the Raeta yokohamaensis Assemblage

mentioned above at the adjacent outcrop in the northeastern area (Text-fig. 2C). The Macoma

calcarea Assemblage is rpgarded as distinctive one of the inner bay because it is similar to the

Raeta yokohamaensis Assemblage in species composition in spite of lacking of the diagnostic R.

yokohamaensis. Judging from the habitat of some of the molluscs, the depth range of the bay was

30-50 m. This inner bay facies is here named Marine IV.

Fresh-water I1 and 111, both of which are mentioned above, are partly distributed in the marginal

area of the bay in the northwest and south. They may be attributed to the fluvial deposits of rivers

discharging into the bay.

The geographical changes of the molluscan assemblages in the Marine IV facies and the distri-

P/io.-Pleistocene Ostracodafrom Hokkaido, Japan 565

bution of the underlying Brackish and Marine I1 facies indicate that the sea probably retreated


Upper (Second) Cycle

The second transgression following the infilling of the basin of the Lower Cycle is demonstrated by the presence of wide-spread marine sediments overlying the Marine IV facies at the top

facies of the Lower Cycle.

In the early stage of the second transgression, a brackish water environment appeared in the

central part of the basin (Text-fig. 2B). This Brackish facies is identified by the Crenomytilus

grayanus Assemblage which is also found in the Lower Cycle. It is composed of fine-grained sandstone, with the basal part being conglomeratic.

The northern area of the ‘‘Kuromatsunai Lowland Belt” was inundated by a near-shore shallow

sea. The area studied is situated in the southernmost part of that sea, and is overlain by the greenish-grey, very fine-grained sandstone containing abundant autochthonous or subautochthonous

marine fossils. This sandstone is distributed throughout most of the area except the southernmost part (Text-figs. 2A, B). This fossiliferous sandstone yields Patinopecten yessoensis in abundance

and is associated Glycymeris yessoensis, Callista (Ezocallista) brevisiphonata, Chlamys (Swiftopecten) swijtii, C. farreri nipponensis, Homalopoma amussitatum, Cyclocardia crebricostata. This assemblage which is named the Patinopecten yessoensis Assemblage is also composed of many other

species and varies vertjcally and laterally. Consequently, small environmental changes were recognized by changes in composition and frequency of this molluscan assemblage. For example,

the appearance of an environment similar to the inner bay is inferred by the occurrence of Lucinoma

annulata in the early stage of the Upper Cycle. A similar environmental change can be traced by the

lithological characteristics and modes of occurrence of the fossils. In the southwestern area, the

coarseness of sediments and slightly transported molluscan fossils suggest more turbulent, perhaps

shallower conditions than the other areas. On the whole, however, this assemblage indicates a nearshore shallow sea which becomes shallower southward and westward. This sedimentary environment is generally the same as that of the Marine I1 facies of the Lower Cycle.

Thereafter, Brackish facies gradually appeared again as shown by the diatoms studied by Ichikawa et al. (1967). This environmental change began in the southern area and spread northward,

because the underlying Marine I1 facies is thick in the north and thin or thinning-out in the south

(Text-figs. 2A, B). This northward spreading of the Brackish facies coincides with the northward

thinning of intercalated peat layers. This fact suggests that the sea of the Upper Cycle retreated

northward. No fossil molluscs and ostracods were preserved in this facies.

In the southwestern area, fan and fluviatile facies constitute the whole of the whole of the Upper Cycle, both of which have lithological characteristics similar to those of the Lower Cycle. The

former is therefore attributed to the Fresh-water I1 facies, and the latter to the Fresh-water 111.




One hundred and ninety one marine ostracod species among 56 genera were obtained from 44

sediment samples from the Setana Formation in the “Kuromatsunai Area”. Sediment samples

were treated by the sodium-sulphate-naphtha method, and washed on a 200 mesh sieve. About 200

individuals were picked up random from the aliquots of the samples, except in the case of several

poorly fossiliferous samples. The characteristics of seven assemblages of fossil Ostracoda are

described in the following section.


1. Schizocythere okhotskensis Assemblage

This assemblage is characterised by the abundance of Schizocythere okhotskensis (more than

20% of the total number of individuals) and low ratios of subordinate species (less than 10% in

most species). Common species among these subordinate species are Hemicythere orientalis, Baflnicythere emarginata. B. howei, Aurila uranouchiensis, Urocythere sp. A, Urocythereis? sp. B, Patagonicythere sp., Semicytherura henryhowei, S. miurensis, Cytheropteron sawanense and Xestoleberis

iturupica. This assemblage is represented by samples from the Marine I1 facies (Text-fig. 2A) and

Marine I11 facies (Text-fig. 2C) of the Lower Cycle and from the Brackish and Marine I1 facies of

the Upper Cycle (Text-fig. 2B). This is the only one assemblage that is found both in the Lower and

Upper Cycles. The assemblage found in the Lower Cycle is accompanied by the Chlamys daishakaensis Assemblage in the Marine I1 facies and the Chlamys islandica Assemblage in the Marine

I11 facies respectively. The assemblage belonging to the Upper Cycle coexists with the Crenomytilus

grayanus Assemblage in the Brackish facies and the Patinopecten yessoensis Assemblage in the

Marine 11. Ostracods are abundant in the Marine I1 and Brackish facies which were presumably

deposited on the shallow fine- to coarse-grained, sandy bottom of a bank and coast respectively.

In contrast, ostracods are few in the Marine I11 facies which was deposited on the coarse-grained

sandy bottom of a high energy environment. Consequently, it may be possible to divide this assemblage into two “sub-assemblages”, it seems, however, that the palaeoenvironmental difference did

not affect the composition of the ostracod assemblages. A palaeoenvironmental characteristic common to these facies is the coarse-grained bottom independently of depth.

2. Cytheropteron sawanense Assemblage

This is represented by one sample from the Marine I of the Lower Cycle (Text-fig. 2C). This

assemblage is marked by the dominance of Cytheropteron sawanense (more than 30%) and fewer

numbers of subordinate species (all of them are less than 10%). These subordinate species are

Cythere lutea, Schizocythere okhotskensis, Hemicythere nana, Bafinicythere emarginata, Howeina

higashimeyaensis,Semicytherura henryhowei and Paradoxostoma sp. D. This assemblage is inferred

to have lived on the fine-grained sandy bottom of a fairly deep sea (perhaps 100m or more),

because it is accompanied by the Acila divaricata molluscan Assemblage.

3. Schizocythere okhotskensis - Bafinicythere emarginata - Urocythereis? sp. B - Cytheropteron sawanense Assemblage

This is represented by samples from the Marine I and Marine I1 facies of the Lower Cycle (Textfig. 2A). This assemblage is characterised by the equally common occurrence of these four species

(about 10%for each species).Common species are Cythere lutea, Bafinicythere howei, Finmarchinella

“angulata”, F. sp. A of Tabuki (1980, MS), Urocythereis gorokuensis, Howeina higashimeyaensis.

Loxoconcha optima, Loxocorniculum mutsuense and Xestoleberis sp. F. The assemblage in the

Marine I facies is contained in the greenish-grey very fine-grained sandstone that was presumably

deposited in a several tens of metres of water and accompanied by the Cryptonatica janthostomoides

Assemblage. The same ostracod assemblage in the Marine I1 is, however, found in the grey coarsegrained sandstone which was deposited on the bank mentioned above, and accompanies the

Chlamys daishakaensis Assemblage. The ostracods constituting this ostracod assemblage are considered to be allochthonous from the mode of molluscan fossil preservation. The high speciesdiversity also suggests that the assemblage is allochthonous formed by the mixing of several nearshore assemblages.

P1io.-Pleistocene Ostracoda from Hokkaido, Japan 567

4. Schizocythere okhotskensis - Bafinicythere emarginata - Urocythereis gorokuensis Semicytherura henryhowei Assemblage

This assemblage is similar in species composition to the one above, though the last two species

are replaced by Urocythereis gorokuensis and Semicytherura henryhowei. It is characterised by the

equally common occurrence of these four species which make up about 40% of the total population. Subordinate species are small in numbers except for Aurila uranouchiensis and Howeina higashimeyaensis. Only Hemicythere? sp. A occurs in all eight samples belonging to this assemblage.

The subordinate species which are common among most samples are Cythere lutea, Aurila uranouchiensis, Finmarchinella “angulata”, F. sp. A of Tabuki (1980, MS), F. nealei, Xestoleberis iturupica

and Sclerochilus sp. A. This assemblage is restricted to the Marine I1 facies of the Upper Cycle (Textfigs. 2A, B), and is exclusively accompanied by the Patinopecten yessoensis Assemblage. This

suggests that this ostracod assemblage was deposited on the fine-grained sandy bottom of a nearshore shallow sea. The weak bottom current which is inferred from the mode of occurrence of the

molluscan fossils must have played a distinctive role in forming the asseablage. The high speciesdiversity also signifies that this ostracod assemblage is allochthonous.

5. Semicytherura henryhowei - Finmarchinella ‘izngulata” Assemblage

This is also restricted to the Marine I1 facies of the Upper Cycle and is exclusively associated

with the Patinopecten yessoensis Assemblage as is the above assemblage (Text-figs. 2A, B). This assemblage is peculiar in its lack of dominant or abundant species and in its high species-diversity.

Only very few species ever make up more than 10% of the total individual numbers. Species found

in all samples are Schizocythere okhotsukensis, Bafinicythere emarginata, Aurila uranouchiensis,

Finmarchinella “angulata”, Semicytherura henryhowei, Loxocorniculum kotoraformum and Xestoleberis sp. F. This assemblage is here named after Semicytherura henryhowei and Finmarchinella

“angulata” which are fairly abundant among these common species. This assemblage is similar to

the one above, although Schizocythere okhotskensis, Bafinicythere emarginata and Urocythereis

gorokuensis are rare. It is uncertain what palaeoenvironmental difference caused the difference in

composition of these two ostracod assemblages. The higher species-diversity, however, may indicate

the mixing of a larger number of original assemblages in this assemblage than that in the previous

one. Accordingly, a shallow near-shore sea is considered to have deposited the assemblage in

association with fine-grained sand.

6. Howeina camptocytheroidea Assemblage

All the samples yielding this assemblage are confined to the Marine I V facies of the Lower Cycle

(Text-figs.2A, B). This assemblageis characterised by the dominance of Howeina camptocytheroidea

(more than 30 %, and in some samples more than 50 %). Subordinate species are usually small in

numbers except for Urocythereis gorokuensis. Common species are Cythere lutea, Schizocythere

okhotskensis, and perhaps Hemicythere orientalis. This assemblage is found in grey siltstone and is

accompanied by the Lucinoma annulata Assemblage. It is, therefore, obvious that the ostracod

assemblage was deposited in the calm environment of the inner bay. Conversely, this assemblage

would be a valuable indicator of such an environment.

7. Cytheropteron sp. - Ruggieria sp. Assemblage

This is also represented by several samples from the Marine IV facies of the Lower Cycle. This

assemblage is characterised by the abundance of Cytheropteron sp. and Ruggieria sp. (more than

40% of these two species), and the scarcity of subordinate species. Common species are Pontocythere miurensis, P. sp. of Hanai (1961), Cythere lutea, Hemicythere? sp. A, Mutilus sp. of Hanai

(1961), Zinmarchinella “angulata”,F. nealei, Howeina neoleptocytheroidea, Semicytherura henryho-


wei and Loxoconcha sp. B of Okada (1976, MS.). This ostracod assemblage was clearly deposited in

a calm environment of the innermost bay, because it is contained in the grey siltstone and is accompanied by the Raeta yokohamaensis and Macoma calcarea Assemblages.

In conclusion, the value of the comparison between ostracod assemblages and sedimentary

facies based on the molluscan assemblages was strongly confirmed. In the cold water PliocenePleistocene sediments of the northwestern Pacific, the fossil Ostracoda formed distinctly different

assemblages from each other depending on the different sedimentary facies. This will be a useful

information for future studies in other areas. Moreover, it will be possible to clarify what kind of

species increased or decreased in what kind of environment, because almost all the species are still

living at the present day.


The writer wishes to express his sincere gratitude to Professor Emeritus Tetsuro Hanai, University of Tokyo, under whose direction this study was made. Gratitude is also expressed to Professor

Kiyotaka Chinzei, the Department of Geology and Mineralogy, Faculty of Science, Kyoto University, who read the original manuscript and gave the writer instructions during laboratory and field

work. The writer is much indebted to him for invaluable advice and encouragment which stimulated

the writer’s work. The writer is grateful to Associate Professor Noriyuki Ikeya of the Geoscience

Institute, Faculty of Science, Shizuoka University, who gave the writer the opportunity to start

this work. Dr. Tomas M. Cronin of the U. S. Geological Survey corrected English manuscript and

gave advice.


HANAI, T. 1961. Studies on the ostracoda from Japan: Hingement. J . Fac. Sci.

IKEYA, N., ISHIZAKI, K., SEKIGUCHI, Y. and YAJIMA, M. 1977. Checklist of


Univ. Tokyo, sec. 2,13,pt. 2,345-377.

ostracoda from Japan and its adjacent

seas. 110 pp. University of Tokyo Press.

w., ISHIZUKA, T., and KONISHI, K. 1967. Paleosalinity analysis of some Pliocene sediments of Hokkaido,

Japan. Jubilee Publ. Commen. Prof. Sasa, 60th Birthday, Sept.. 1967. 93-193.

IKEYA, N. and HAYASHI, K. 1982. Geology of the Kuromatsunai Area, Oshima Peninsula, Hokkaido. J. Geol. soc.

Japan, 88(7), 613-632. [In Japanese with English abstract].

ISHIZAKI, K. 1963. Japanese Miocene ostracodes from the Sunakosaka Member of the Yatsuo Formation, east of

Kanazawa City, Ishikawa Prefecture. Japan. J . Geol. Geogr., 34(1), 19-34.

- 1971. Ostracodes from Aomori Bay, Aomori Prefecture, northeast Honshu, Japan. Tohoku Univ. Sci. Rep.,

2nd ser. (Geology), 41(2), 197-224.

OKADA, Y . 1976. Stratigraphy and Ostracoda of the younger Cenozoic in the Oga Peninsula, Akita Preficture. Unpublished thesis, Univ. Tokyo.

-1979. Stratigraphyand Ostracoda from late Cenozoicstrata of the Oga Peninsula,Akita Prefecture. Trans. Proc.

Palaeont. SOC.Japan, N.S.,(115), 143-173.

TABUKI, R. 1980. The ostracode fauna from the Daishaka Formation. UnpublishedDissertation, Univ. Tokyo, 55 pp.


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