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Chapter 42. Pliocene-Pleistocene palaeoenvironment and fossil ostracod fauna from the southwestern Hokkaido, Japan
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.
OF THE SETANA FORMATION
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
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
2B-Continued (Middle area).
of the Setana Forlation
TEXFFIO.2CContinued (Eastern area).
number of locality
5 , , , 1 L . s . - . . - sr
C. weltneri var. obtusa
C. ovum s. Klie
C.ovum s. Petkovski
Marine I ( f a i r l y deep) Facies
greenish-grey s i l t - f i n e sand
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
gray si It-very f i n e sand
d conglomerate and sand
mean annual densityrelationships
400 139 189 219 148
in the index
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
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