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Preliminary Notes on the Japanese Miocene Ostracoda
Preliminary Notes on the Japanese Miocene Ostracoda MICHIKO YAJIMA Tokyo Seitoku Gakuen. Tokyo, Japan
ABSTRACT During Early Middle Miocene, 16.5-15.5 Ma, shallow, warm water ostracods lived in the Palaeo-Setouchi Province (Shobara, Tsuyama and Mizunami) and in Northeast Japan (Kanazawa and Sendai). The fauna has close affinities with that of the Pleistocene and Recent, distributed along the Pacific coast of Southwest Japan. During Middle Miocene, 15.5-9 Ma, shallow cold to temperate water ostracods became widespread in Northeast Japan (Sekinohana, Sugota and Kamikoani). The fauna is similar to Pleistocene and Recent faunas distributed along the Japan Sea and Pacific coasts in Northeast Japan. The existence of two faunas in the Middle Miocene, an earlier warm and a later cold one, confirms recent findings of tectonic and palaeogeographical studies.
INTRODUCTION The biostratigraphy and chronology of the Japanese Miocene have recently been vigorously investigated, and a precise time scale has become available (Tsuchi et al., 1981). Sediments older than the Early Middle Miocene are very scarce in Japan. The sediments of the early Middle Miocene ranging from 16.5-15.5 Ma, Blow’s (1969) N 8 zone, are dominated by shallow marine facies rich in mega- and microfossils. Geloina-bearing molluscan assemblages of intertidal facies demonstrate the existence of mangrove swamps. Other assemblages of tropical to subtropical, shallow marine molluscan fossils are also present. These sediments are succeeded by offshore muddy or diatomaceous sediments. They represent the beginning phase of a major transgression in Japanese Neogene history. At this time a shallow marine basin, the Palaeo-Setouchi Province, developed in the Chugoku area of Southwest Japan and the Mizunami area of central Japan (Text-fig. 1). In the Middle Miocene, Southwest Japan was emergent, while in Northeast Japan deposition of thick marine sediments took place (Text-fig. 2). Although these Miocene sediments are rich in mega- and microfossils, studies of ostracod faunas are rare. Ishizaki (1963) described a small fauna from the Sunakosaka Member of the Yatsuo Formation, east of Kanazawa. Recently this member was correlated with the Kurosedani Formation of Blow’s N 8 zone (Tsuchi, 1981). Ishizaki (1966) also reported ostracod faunas from the Moniwa Member and from the Hatatate Formation of the Sendai area. Recently the Moniwa Formation, once the Moniwa Member, was correlated with Blow’s N 8 zone and the Hatatate Formation with the N 9 to N 16 zones (Text-figs. 1,2). Hanai (1957) reported Hemicytherura kajiyamai 1073
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TEXT-FIG. l-Localities of the Japanese Miocene ostracod samples.
from the Shukunohora Sandstone (Blow’s N 8 zone) of Mizunami in the East Palaeo-Setouchi Province, and stated (1977) that “some species of living warm water ostracods, such as Hemicytherura kajiyamai. Aurila sp. and Schizocythere kishinouyei, which settled in Japan in the Middle Miocene, have maintained a status quo in phenotype up to the present”. By 1977, 47 species of 28 genera were reported from the Japanese Miocene. Among these, 13 species of 11 genera are extant. While investigating Pleistocene shallow warm water ostracods, questions arose concerning their origin and their history. To clarify these, I studied Miocene ostracods from Shobara (lower sand of the Bihoku Group), Tsuyama (Yoshino Formation), Mizunami (Shukunohora Sandstone), Sekinohana (Togi Mud Formation), Sugota (Sugota Formation), and Kamikoani (Kamikoani Formation) (Text-figs. 1, 2).
EARLYMIDDLE MIOCENE-SOUTHWEST JAPAN The Palaeo-Setouchi (Palaeo-Inland Sea) sediments are distributed in the Chugoku area. During the Early Middle Miocene, this area was covered by a shallow warm sea with many islands. Shobara In the Shobara Formation of the Bihoku Group, the only ostracods present are very rare Cytherelloidea sp.
Tsuyama
The Yoshino Formation at Tsuyama is about 15 m thick, light brown sand in the lower part,
Japanese Miocene Ostracoaia 1075
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TEXT-FIG. 2-Stratigraphical correlation of ostracod-bearing Miocene sediments in the Japanese Islands. Solid circles show ostracod occurrences.
greenish mud in the middle part, and gravels and sand in the upper part, including many shallow warm-water molluscus. Ostracods are rare in the Y2 and Y5 fossil horizons of Taguchi (personal communication) and very scarce in the Y3 and Y4 fossil horizons. The Y2 horizon is in the lower sand of the Yoshino Formation and includes the molluscan fossils Phacosoma, Solidcorbula, Vasticardium, Crassostrea, Turbo, and Siratoria. The Y5 horizon is a thin sand layer intercalated in the middle mud of the Formation and includes molluscan fossils such as Phacosoma and Vasticardium. Ostracods are very rare in the Yoshino Formation (Table 1). Most specimens are articulated, but the carapaces are compressed laterally or dorso-ventrally. Among 14 species, Trachyleberis sp. and Spinileberis? sp. are common. Single valves of Trachyleberis sp. are very similar to Recent Trachyleberis scabrocuneata, but articulated valves differ a little in ornamentation from Recent ones. Cletocythereis rastromarginata is represented by only one valve. It is a tropical shallow water species. Falsobuntonia taiwanica is very rare. Spinileberis quadriaculeata is represented by only one broken valve. The ostracod assemblage is a shallow warm-water assemblage very similar to the Recent warm-water assemblages along the Pacific coast of Japan.
Japanese Miocene Ostracoda 1077
Mizunami
The Mizunami area is situated at the East end of the Palaeo-Setouchi Sea in Central Japan, (Text-fig. 1). The transgression into the Palaeo-Setuouchi area occurred in three steps, and after the third, the seaway continued from the Japan Sea through the main part of the PalaeoSetouchi area in the Chugoku area to east Setouchi (Itoigawa, 1981). TABLE LIST OF EARLYMIDDLE MIOCENE OSTRACOD SPECIES FROM THE TSUYAMA AREA. Tsuyama (Yoshino Formation, Blow’s N 8 zone) Y-5 horizon
Callistocythere sp.
* Cythere omotenipponica Hanai, 1959
Spinileberis? sp. Trachyleberis sp. Ambostracon ikeyai Yajima, 1978 * Cletocythereis rastromarginata (Brady, 1880) Falsobuntonia taiwanica Malz, 1982 Aurila sp. Cornucoquimba moniwensis (Ishizaki, 19660) Semicytherura sp. Loxoconcha sp. Y-4 horizon Trachyleberis sp. Y-3 horizon Trachyleberis sp. Y-2 horizon (sample 1) * Spinileberis quadriaculeata (Brady, 1880) S.? sp. Trachyleberis sp. “Buntonia” sp. Ambostracon ikeyai Yajima, 1978 Loxoconcha sp. Y-2 horizon (sample 2) Spinileberis? sp. Trachyleberis sp. * Acunthocythereis munechikai Ishizaki, 1972 Falsobuntonia taiwanica Malz, 1982
R R R C R R R R
R
R R R R
R C C R R R
C C R R
C=common; R=rare; *=extant species. PLATE1-Early Middle Miocene Ostracoda from Mizunami (Shukunohora Sandstone). Fig. 1. Truchylberissp. 1. Left valve, CA 17734. x 52. Figs. 2,5. Trachyleberis sp. 2. Fig. 2. Left valve view of male carapace, CA 17735. X 52. Fig. 5. Right valve view of female carapace, CA 17736. X 52. Fig. 3. Loxoconcha pulchra Tshizaki, 1968. Right valve, CA 17737. x 62. Fig. 4. Ambostracon ikeyai Yajima, 1978. Female left valve, CA 17689. x 67. Fig. 6. Cornucoquimba moniwensis (Ishizaki, 1966). Right valve view of carapace, CA 17690. x 63. Fig. 7. Falsobuntonia taiwanica Malz, 1982. Left valve, CA 17691. x 68. Fig. 8. Pontocythere subjaponica (Hanai, 1959). Femaleleft valve, CA 17692. x68. Fig. 9. Callistocythere sp. Right valve view of carapace, CA 1769). ~ 8 9 Figs. . 10,13. Hirsutocythere? sp.-Fig. 10. Female left valve, CA 17694. ~ 4 2 Fig. . 13. Maleleft valve, CA 17695. ~ 4 2 Fig. . 11. Semicytherura miurensis Hanai, 1957. Left valve, CA 17696. X 107. Fig. 12. Semicytherura henryhowei Hanai and Iekya, 1977. Right valve view of carapace, CA 17697. x 107. Fig. 14. Schizocythere kishinouyei (Kajiyama, 1913). Left juvenile valve, CA 17698. x 82. Fig. 15. Loxocorniculum sp. Right valve, CA 17699. X 89. Fig. 16. Loxoconcha sp. Left valve, CA 17700. x 86. Fig. 17. Kangarina sp. Right valve, CA 17701. x 104. Fig. 18. Hemicytherura cuneata Hanai, 1957. Right valve, CA 17702. X 120. Fig. 19. Aurila sp. Left valve, CA 17703, x 62. Fig. 20. Spinileberis qua&iaculeata (Brady, 1880). Right valve, CA 17704. . illustrated specimens are x 62. Fig. 21. Neonesidea sp. Right valve view of carpace, CA 17705 ~ 4 0 (All deposited in the collection of the University Museum, University of Tokyo (UMUT). Specimen numbers are prefixed CA for Cenozoic Arthropoda).
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TABLE 2-LIST OF EARLYMIDDLEMIOCENE OSTRACOD SPECIES FROM THE MIZUNAMI AREA. Mizunami (Shukunohora Sandstone, Blow’s N 8 zone) Neonesidea sp. Propontocypris sp. *Pontocythere subjaponica (Hanai, 1959) P. sp. *Parakrithella pseudadonta Hanai, 1959 Munseyella sp. Callistocythere sp. *Cythere omotenipponica Hanai, 1959 *Schi.zocythere kishinouyei (Kajiyama, 1913) *SpiniIeberis quadriaculeata (Brady, 1880) Trachyleberis sp. 1 T. sp. 2 Hirsutocythere? sp. Falsobuntonia taiwanica Malz, 1982 Aurila sp. Cornucoquimba moniwensi? (Ishizaki, 1966) Ambostracon ikeyai Yajima, 1978 *Hemicytherura cuneata Hanai, 1957 Kangarina sp. *Semicytherura henryhowei Hanai and Ikeya, 1977 *S. miurensis Hanai, 1957 S. wakamurasaki Yajima, 1982 Cytheropteron sp. *Loxoconchapulchra Ishizaki, 1968 L. sp. Loxocorniculum sp. Xestoleberis sp. A=abundant; C-common; R=rare; *, extant species.
A R R R R R C R C R C R R R A C C C
R R R R R C R R R
The Shukunohora Sandstone of the Mizunami area is a thin (10 m), ill-sorted sandy silt with the foraminifer Myogipsina kotoi and many shallow warm-water molluscs. The Shukunohora Sandstone contains abundant well-preserved ostracods (Table 2). Most of the ostracod valves are transparent and good for study of internal features. The sieve type pores of Schizocythere kishinouyei and Spinileberis quadriaculeata are well preserved. The ostracod assemblage consists exclusively of shallow warm-water species (Plate 1). Among PLATE2-Miocene
Ostracoda from Tsuyama, Kamikoani, and Sekinohana. Figs. 1-3. Tsuyama, Yoshino Formation, Early Middle Miocene (Blow’s N 8 zone). Fig. 1. Spinileberis? sp., Right valve view of carapace, CA 17706 (Y-2 horizon), x 80. Fig. 2. Cletocythereis rastrornarginata(Brady, 1880), Right valve view of carapace, CA 17707 (Y-5 horizon), x 82. Fig. 3. Trachyleberissp., Left valve view of male carapace, CA 17708 (Y-5 horizon), x 58. Figs. 4-7. Kamikoani, Kamikoani Formation, Middle Miocene (Blow’s N 15 zone). Fig. 4. Finmarchinella japonica (Ishizaki, 1966), Right valve, CA 17709. ~ 8 1 Fig. ; 5. Finmarchinella japonica (Ishizaki, 1966), Juvinile left valve, CA 17710, x 87. Fig. 6. Hemicythere? sp., Right valve, CA 17711, x 80. Fig. 7. Hemicytherura cuneata Hanai, 1957, Right valve, CA 17712, X 130. Figs. 8-16. Sekinohana, Togi Formation, Middle Miocene (Blow’s N 9 zone). Fig. 8. Palmenella limicola (Norman, 1865), Left valve view of juvenile carapace, CA 17713, x 82. Fig. 9. Finmarchinella hanaii Okada, 1979, Right . 10. Finmarchinella hanaii Okada, 1979. Left valve, CA 17715 valve, CA 17714 (sample Togi D), ~ 8 1 Fig. (sample Togi D), x 80. Fig. 11. Munseyella sp., Left valve, CA 17716 (sample Togi C), x 82. Fig. 12. Cornucoquimba saitoi (Ishizaki, 1963), Right valve, CA 17717 (sample Togi D), x79. Fig. 13. Ambostracon ikeyai Yajima, 1978, Female left valve, CA 17718 (sample Togi D), x 81. Fig. 14. Acanthocythereis? munechikai Ishizaki, 1982, Left valve, CA 17719 (sample Togi D), x58. Fig. 15. Bradleya sp.. Right valve, CA 17720 (sample Togi D), x 58. Fig. 16. Hirsutocythere? nozokiensis (Ishizaki, 1963), Left valve, CA 17721 (sample Togi D), x58.
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27 species of 21 genera, 9 species are extant. Neonesidea sp. which is very similar to the Recent Neonesidea oligodentata, and Aurila sp. which is very similar to the Recent Aurila punctata are abundant. Callistocythere sp., Schizocythere kishinouyei, Trachyleberis sp., Cornucoquimba moniwensis, Ambostracon ikeyai, Hemicytherura cuneata and Loxoconcha pulchra are common. Pontocythere subjaponica, Parakrithella pseudadonta, Cythere omotenipponica, Spinileberis quadriaculeata, Semicytherura henryhowei, and S. miurensis are rare but identical to the species presently living along the Pacific coast of Japan. Tropical species of Kangarina sp., Falsobuntonia taiwanica and Loxocorniculum sp. are rare.
EARLYMIDDLEMIOCENE-NORTHEAST JAPAN Kanazawa Ishizaki studied Miocene ostracods from the Sunakosaka Member (Blow’s N 8 zone) in 1963. The ostracod assemblage is very small but resembles that from the Tsuyama area because Trachyleberis is abundant.
Sendai
Miocene ostracods from the Moniwa Formation (Blow’s N 8 zone), Sendai, were reported by Ishizaki (1966). Ostracods are very rare. Abundance of Neonesidea oligodentata, Trachyleberis scabrocuneata, Cornucoquimba saitoi, and Schizocythere kishinouyei shows that the assemblage is similar to the Mizunami and Tsuyama faunas.
MIDDLEMIOCENE-NORTHEAST JAPAN Seikinohana
The Sekinohana area is situated on the Noto Peninsula, near Kanazawa along the Japan Sea. The Togi Mud Formation in Sekinohana is a very thin (2 m), greenish, massive, silty mudstone containing abundant foraminifers, and rare radiolarians, diatoms, and ostracods. Suda and Ishigaki (1984) reported the presence of Groborotalia peripheroronda and correlated the sediments with Blow’s N 7 to N 9 zones, a little later than the earliest Middle Miocene transgression. Ostracods are rare (Table 3). The fauna is dominated by Neonesidea sp., Hirsutocythere? nozokiensis, and Acanthocythereis? munechikai (Plate 2). Only one immature carapace of Palmenella limicola is present. This is a typical arctic species. Finmarchinella hanaii, which is common in shallow Pleistocene sediments along the Japan Sea coast, is common Rarely found Bradleya sp. is very similar to the Recent deep sea species of Bradleya albatrossia. The extant species Pontocythere subjaponica, Hanaiborchella miurensis, and Semicytherura henryhowei are rare. Occurrences of other species are hitherto confined to Miocene sediments.
Sugota The Sugota Formation is a 180 m thick, fine argillaceous sandstone found around Sugota near Akita, in Northeast Japan. The basal part of the Sugota Formation is dark greenish, poorly sorted medium to coarse sandstone with abundant fragments of barnacles, molluscs, and benthonic foraminifers. Molluscs show that the sediments were deposited in temperate water, and planktonic foraminifers present correlate with Blow’s N 9 zone or higher. Ostracods are very rare. Only one broken valve of Hemicythere was found. The Middle part of the Formation is dark grey to dark
Japanese Miocene Ostracoda 1081
TABLE3-LIST OF MIDDLE MIOCENE OSTRACOD SPECIES FROM THE SEKINOHANA AREA. Sekinohana (Togi Mud Formation, Blow’s N7-9 zones) Togi C Munseyella sp. Schizocythere sp. *PaImeneIIa Iimicola (Norman, 1865) FinmarchineIIa hanaii Okada, 1979 Cornucoquimba moniwensis (Ishizaki, 1966) C. saitoi (Ishizaki, 1963) *Hirsutocythere? nozokiensis (Ishizaki, 1963) *Acanthocythereis munechikai Ishizaki, 1972 BradIeya sp. Cytheropteron sawanense Hanai, 1957 Paracytheridea neolongicaudata Ishizaki, 1966 Togi D Neonesidea sp. *Pontocythere subjaponica (Hanai, 1959) *Hanaiborchellamiurensis (Hanai, 1970) *PaImeneIIa Iimicola (Norman, 1865) Finmarchinella hanaii Okada, 1979 Ambostracon ikeyai Yajima, 1978 Caudites? posterocostatus (Ishizaki, 1966) Cornucoquimbasaitoi (Ishizaki, 1963) Hirsutocythere? nozokiensis (Ishizaki, 1963) *Acanthocythereis munechikai Ishizaki, 1972 Bradleya sp. Eucytherura neoalae (Ishizaki, 1966) *Semicytherura henryhowei Hanai and Ikeya, 1977 Loxocorniculum kotoraformum Ishizaki, 1966 A=abundant; C=common; R=rare; *=extent species.
R R R C .C R A A R C R C R R R C R C R C C R R R R
brownish-grey, fine argillaceous sandstone with autochthonous cold water molluscs and foraminifers. Ostracods are very rare, only a few specimens of Callistocythere were found.
Kamikoani
The Kamikoani Formation in the Kamikoani area is a calcareous fine sand. It is considered to have been deposited 10 to 8 Ma., based on its stratigraphical position. Ostracods are very abundant and well-preserved (Table 4). Schizocythere kishinouyei, Aurila sp. 1, Finmarchinella japonica, Hemicytherura cuneata, and Cytheroteron sawanense are abundant. Palmenella limicola, a typical arctic species, is rare. Among ostracods found in this formation, Munseyella hokkaidoana, Finmarchinellajaponica, F. nealei, Howeina neoleptocytheroidea, and Cytheropteron sawanense are species common in the Pleistocene sediments and Recent shallow water fauna along the Japan Sea coast. A specimen which probably belongs to the genus Hemicythere, a cold water genus, is also present. Many species such as Cornucoquimba moniwensis, Hirsutocythere? nozokiensis, Paracytheridea neolongicaudata, Loxoconcha nozokiensis, and Loxocorniculum kotoraformum, are confined to the Miocene.
Sendai Ishizaki’s report (1966) also included the ostracod fauna from the Hatatate Formation. This formation is correlated with Blow’s N 9 to N 15 zones, and the formation is considered to have been deposited under the influence of cold currents on the basis of the molluscan fossils. Ishizaki
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TABLE4-LIST OF MIDDLEMIOCENE OSTRACOD SPECIES PROM THE KAMIKOANI AREA.. Kamikoani (Kamikoani Formation, 10-8 Ma). Munsyella hokkaidoana (Hanai, 1959) *Callistocythere japonica Hanai, 1957 *C. ruogsoforma Hanai, 1957 *Cythere uranipponica Hanai, 1959 Schizocythere sp. *Palmenella limicola (Norman, 1865) Aurila sp. 1 A. sp. 2 Finmarchinellajaponica (Ishizaki, 1966) *F. nealei Okada, 1979 Hemicythere? sp. Caudites? posterocostatus (Ishizaki, 1966) Cornucoquimba moniwensis (Ishizaki, 1966) *C. tosaensis (Ishizaki, 1968) Trachyleberis sp. Hirsutocythere? nozokiensis (Ishizaki, 1963) Eucytherura sp. *Hemicytherura cuneata Hanai, 1957 *Semicytherura henryhowei Hanai and Ikeya, 1977 *Howeina neoleptocytheroidea Hanai, 1957 H . sp. Cytheropteron sawanense Hanai, 1957 Paracytheridea neolongicaudata Ishizaki, 1966 P. sp. Loxoconcha nozokiensis Ishizaki, 1963 Loxocorniculum kotoraformum Ishizaki, 1966 A=abundant; C=common; R=rare; *=extant species
R R R R C R
A R
C R R R R R R C R
A R R R A C R R C
pointed out that the ostracod fauna of “the Hatatate Formation is not essentially different from that of the Moniwa Member, although it is more or less different in productivity” (1966, p. 135). I think that the occurrence of Palmenella limicola and Finmarchinella japonica in the Hatatate Formation demonstrate cold water influence.
FAUNAL CONSIDERATIONS Palaeogeographical maps (Chinzei, 1986) of the Japanese Islands during the Early Middle Miocene (Blow’s N 8 zone) and Middle Miocene (Blow’s N 9 to N 15 zones) are shown in Text-figs. 3 and 4. Recent palaeomagnetic studies show that in the Early Middle Miocene the Japanese Islands were situated closer to the Asian continent and the Japanese Island arc was convex toward the northwest, opposite to the trend of the present arc. The Tsushima Strait between the Korean Peninsula and Kyushu Island opened and tropical water flowed into the Palaeo-Inland Sea and the Japan Sea up to Northeast Japan. Cold water influence was very weak and only extended into the Hokkaido area. Ostracods from Shobara, Tsuyama, Mizunami, Kanazawa, and Sendai, during the age of Blow’s N 8 zone are shallow, tropical water species, similar to the warm-water ostracods presently living along the Pacific coast of Southwest Japan. Ostracods from the Shukunohora Sandstone are very similar to Recent ones. The Shukunohora Sandstone was probably deposited a little after the deposition of the main part of the Palaeo-Inland Sea sediments. Among the Early Middle Miocene ostracod faunas, the one from the Shukunohora Sandstone may have persisted up until Recent times.
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TEXT-FIG. 3-Palaeogeographical map of the
Japanese Islands during the age of Blow’s N 8 zone, Early Middle Miocene. (after Chinzei, 1986).
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TEXT-FIG. GPalaeogeographical map of the Japanese Islands during the age of Blow’s N9-15 zones, Middle Miocene. (after Chinzei, 1986).
1084 M.YAJIMA
During the Middle Miocene (Blow's N 9 to N 15 zones), Southeast Japan was emergent except for a few localities along the Pacific coast. The Japanese Islands moved to nearly the same position as they occupy today. The Tsushima Strait closed, and the influence of warm water ceased, while Northeast Japan came under cold water influence. Ostracods from Sekinohana, Sendai, Sugota, and Kamikoani show cold water influence and are very similar to Pleistocene and Recent ostracod faunas distributed along the Japan Sea and Pacific coast of Northeast Japan.
ACKNOWLEDGEMENTS Most samples were obtained personally, but some were provided by Messrs E. Taguchi of Hiroshima University (Tsuyama samples), S. Ito of Shizuoka University (Kamikoani samples), Y. Saito of JAPEX (Sugota samples), Y. Okumura of Mizunami Fossil Museum (Mizunami samples), and Dr. Ikeya of Shizuoka University (Sekinohana samples). I thank Drs. T. Hanai and K. Chinzei of the University of Tokyo for helpful discussion and Drs. H. Malz of Forschungs-Institut Senckenberg and P. Frydl of Mobil Oil Co. for coxments on the manuscript.
REFERENCE^ BLOW, W.H. 1969. Late Middle Eocene to Recent planktonic foraminifera1 biostratigraphy. In BRONNIMANN, P. and RENZ, H.H. (eds.). Proc. 1st Internat. Conf: Planktonic Microfossils, Genova, 1967, 1, 199-422, pls. 1-54. CHINZEI, K. (1986). Faunal succession and geographic distribution of Neogene molluscan faunas in Japan. In KOTAKA T. (ed.) Origin and migration of the Japanese Cenozoic mohscs. Palaeontol. Soc. Japan. spec. pap.,
29, 17-32. 1957. Studies on the Ostracoda from Japan, 111. Subfamilies Cytherurinae G.W. Miiller (emend, G.O. Sars, 1925) and Cytheropterinae n. subfam. J. Fac. Sci. Univ. Tokyo, sec. 2, 11, pt. 1, 11-36, pls. 2-4. -1977. Future problems for systematic study of Japanese ostracods. Univ. Mus. Univ. Tokyo, Bull. no. 12,8486. ISHIZAKI, K. 1963. Japanese Miocene ostracods from the Sunakosaka Member of the Yatsuo Formation, east of Kanazawa city, Ishikawa Prefecture. Japan J. Geol. Geogr., 34 (l), 19-34, pi. 2. - 1966. Miocene and Pliocene ostracods from the Sendai area, Japan. Sci. Rept. Tohoku Univ., 2nd ser. (Geol), 37 (2), 131-163, PIS. 16-19. ITOIGAWA, J. 1981. Stratigraphic outlines of selected Neogene sequences. Pacific side areas. 8. Mizunami area. I n TSUCHI R. (ed.) Neogene of Japan. 62-64. Kurofune Printing Co. Shizuoka. SUDA, A. and ISHIGAKI, T. 1984. Foraminifers from Togi Mudstone in Togi-cho, Central Noto Peninsula, Japan. J. Geof. SOC.Japan, 90(6), 417-420, 1 pl. TSUCHI, R. (ed.). 1981. NeogeneofJapan. IGCP-114 National Working Group of Japan, 140 pp. Kurofune Printing Co. Shizuoka. HANAI, T.
DISCUSSION Cronin : Are the Miocene species of Finmarchinella and Cornucoquimba still extant? Yajima: Three cold water species of Finmarchinella and one warm water species of Cornucoquimba are extant. Two species of Cornucoquimba are confined to the Miocene as far as we know at present. Hazel : I noticed that you had Ambostracon ikeyai as old as N7 or N8 in the Miocene. This seems to be a long range for a hemicytherid. Would you care to comment? Yajima : Morphologically, Ambostracon ikeyai from Mizunami (Shukunohora sandstone, N8 zone) is the same as that from Chiba (Kioroshi Formation, 143,000yr BP). I think that many species not only of Ambostracon, but also of other hemicytherids from the Miocene are still extant.
Japanese Miocene Ostracoda 1085
Neale: I congratulate the author on an excellent and most interesting paper. I missed the reference noted by an earlier speaker to Finmarchinella and would be interested to hear how she interprets the ecology of the species in terms of temperature. Does she regard this as indicative of colder waters when this genus appears? Are there other associated genera which can also be interpreted in terms of a colder environment? Yajima: From the molluscan assemblage I think that the ostracod assemblage from the N9 to N15 zones was of cold water species. Finmarchinella came down from a cold stock and then stayed, even in temperate waters. The ostracod assemblage also includes Palmenella limicola.
ABSTRACT During Early Middle Miocene, 16.5-15.5 Ma, shallow, warm water ostracods lived in the Palaeo-Setouchi Province (Shobara, Tsuyama and Mizunami) and in Northeast Japan (Kanazawa and Sendai). The fauna has close affinities with that of the Pleistocene and Recent, distributed along the Pacific coast of Southwest Japan. During Middle Miocene, 15.5-9 Ma, shallow cold to temperate water ostracods became widespread in Northeast Japan (Sekinohana, Sugota and Kamikoani). The fauna is similar to Pleistocene and Recent faunas distributed along the Japan Sea and Pacific coasts in Northeast Japan. The existence of two faunas in the Middle Miocene, an earlier warm and a later cold one, confirms recent findings of tectonic and palaeogeographical studies.
INTRODUCTION The biostratigraphy and chronology of the Japanese Miocene have recently been vigorously investigated, and a precise time scale has become available (Tsuchi et al., 1981). Sediments older than the Early Middle Miocene are very scarce in Japan. The sediments of the early Middle Miocene ranging from 16.5-15.5 Ma, Blow’s (1969) N 8 zone, are dominated by shallow marine facies rich in mega- and microfossils. Geloina-bearing molluscan assemblages of intertidal facies demonstrate the existence of mangrove swamps. Other assemblages of tropical to subtropical, shallow marine molluscan fossils are also present. These sediments are succeeded by offshore muddy or diatomaceous sediments. They represent the beginning phase of a major transgression in Japanese Neogene history. At this time a shallow marine basin, the Palaeo-Setouchi Province, developed in the Chugoku area of Southwest Japan and the Mizunami area of central Japan (Text-fig. 1). In the Middle Miocene, Southwest Japan was emergent, while in Northeast Japan deposition of thick marine sediments took place (Text-fig. 2). Although these Miocene sediments are rich in mega- and microfossils, studies of ostracod faunas are rare. Ishizaki (1963) described a small fauna from the Sunakosaka Member of the Yatsuo Formation, east of Kanazawa. Recently this member was correlated with the Kurosedani Formation of Blow’s N 8 zone (Tsuchi, 1981). Ishizaki (1966) also reported ostracod faunas from the Moniwa Member and from the Hatatate Formation of the Sendai area. Recently the Moniwa Formation, once the Moniwa Member, was correlated with Blow’s N 8 zone and the Hatatate Formation with the N 9 to N 16 zones (Text-figs. 1,2). Hanai (1957) reported Hemicytherura kajiyamai 1073
1074
M.YAJIMA 130
135
140
145
TEXT-FIG. l-Localities of the Japanese Miocene ostracod samples.
from the Shukunohora Sandstone (Blow’s N 8 zone) of Mizunami in the East Palaeo-Setouchi Province, and stated (1977) that “some species of living warm water ostracods, such as Hemicytherura kajiyamai. Aurila sp. and Schizocythere kishinouyei, which settled in Japan in the Middle Miocene, have maintained a status quo in phenotype up to the present”. By 1977, 47 species of 28 genera were reported from the Japanese Miocene. Among these, 13 species of 11 genera are extant. While investigating Pleistocene shallow warm water ostracods, questions arose concerning their origin and their history. To clarify these, I studied Miocene ostracods from Shobara (lower sand of the Bihoku Group), Tsuyama (Yoshino Formation), Mizunami (Shukunohora Sandstone), Sekinohana (Togi Mud Formation), Sugota (Sugota Formation), and Kamikoani (Kamikoani Formation) (Text-figs. 1, 2).
EARLYMIDDLE MIOCENE-SOUTHWEST JAPAN The Palaeo-Setouchi (Palaeo-Inland Sea) sediments are distributed in the Chugoku area. During the Early Middle Miocene, this area was covered by a shallow warm sea with many islands. Shobara In the Shobara Formation of the Bihoku Group, the only ostracods present are very rare Cytherelloidea sp.
Tsuyama
The Yoshino Formation at Tsuyama is about 15 m thick, light brown sand in the lower part,
Japanese Miocene Ostracoaia 1075
low ( 1 9 7 9 )
Shobara
Ma
Mizunarni
Kanazawa a
Sendai
0%sugota
Sekinohana
Kadonosava Xamikoanj
8 i16
9
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10
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11
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12
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13
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-
11
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5
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1;
TEXT-FIG. 2-Stratigraphical correlation of ostracod-bearing Miocene sediments in the Japanese Islands. Solid circles show ostracod occurrences.
greenish mud in the middle part, and gravels and sand in the upper part, including many shallow warm-water molluscus. Ostracods are rare in the Y2 and Y5 fossil horizons of Taguchi (personal communication) and very scarce in the Y3 and Y4 fossil horizons. The Y2 horizon is in the lower sand of the Yoshino Formation and includes the molluscan fossils Phacosoma, Solidcorbula, Vasticardium, Crassostrea, Turbo, and Siratoria. The Y5 horizon is a thin sand layer intercalated in the middle mud of the Formation and includes molluscan fossils such as Phacosoma and Vasticardium. Ostracods are very rare in the Yoshino Formation (Table 1). Most specimens are articulated, but the carapaces are compressed laterally or dorso-ventrally. Among 14 species, Trachyleberis sp. and Spinileberis? sp. are common. Single valves of Trachyleberis sp. are very similar to Recent Trachyleberis scabrocuneata, but articulated valves differ a little in ornamentation from Recent ones. Cletocythereis rastromarginata is represented by only one valve. It is a tropical shallow water species. Falsobuntonia taiwanica is very rare. Spinileberis quadriaculeata is represented by only one broken valve. The ostracod assemblage is a shallow warm-water assemblage very similar to the Recent warm-water assemblages along the Pacific coast of Japan.
Japanese Miocene Ostracoda 1077
Mizunami
The Mizunami area is situated at the East end of the Palaeo-Setouchi Sea in Central Japan, (Text-fig. 1). The transgression into the Palaeo-Setuouchi area occurred in three steps, and after the third, the seaway continued from the Japan Sea through the main part of the PalaeoSetouchi area in the Chugoku area to east Setouchi (Itoigawa, 1981). TABLE LIST OF EARLYMIDDLE MIOCENE OSTRACOD SPECIES FROM THE TSUYAMA AREA. Tsuyama (Yoshino Formation, Blow’s N 8 zone) Y-5 horizon
Callistocythere sp.
* Cythere omotenipponica Hanai, 1959
Spinileberis? sp. Trachyleberis sp. Ambostracon ikeyai Yajima, 1978 * Cletocythereis rastromarginata (Brady, 1880) Falsobuntonia taiwanica Malz, 1982 Aurila sp. Cornucoquimba moniwensis (Ishizaki, 19660) Semicytherura sp. Loxoconcha sp. Y-4 horizon Trachyleberis sp. Y-3 horizon Trachyleberis sp. Y-2 horizon (sample 1) * Spinileberis quadriaculeata (Brady, 1880) S.? sp. Trachyleberis sp. “Buntonia” sp. Ambostracon ikeyai Yajima, 1978 Loxoconcha sp. Y-2 horizon (sample 2) Spinileberis? sp. Trachyleberis sp. * Acunthocythereis munechikai Ishizaki, 1972 Falsobuntonia taiwanica Malz, 1982
R R R C R R R R
R
R R R R
R C C R R R
C C R R
C=common; R=rare; *=extant species. PLATE1-Early Middle Miocene Ostracoda from Mizunami (Shukunohora Sandstone). Fig. 1. Truchylberissp. 1. Left valve, CA 17734. x 52. Figs. 2,5. Trachyleberis sp. 2. Fig. 2. Left valve view of male carapace, CA 17735. X 52. Fig. 5. Right valve view of female carapace, CA 17736. X 52. Fig. 3. Loxoconcha pulchra Tshizaki, 1968. Right valve, CA 17737. x 62. Fig. 4. Ambostracon ikeyai Yajima, 1978. Female left valve, CA 17689. x 67. Fig. 6. Cornucoquimba moniwensis (Ishizaki, 1966). Right valve view of carapace, CA 17690. x 63. Fig. 7. Falsobuntonia taiwanica Malz, 1982. Left valve, CA 17691. x 68. Fig. 8. Pontocythere subjaponica (Hanai, 1959). Femaleleft valve, CA 17692. x68. Fig. 9. Callistocythere sp. Right valve view of carapace, CA 1769). ~ 8 9 Figs. . 10,13. Hirsutocythere? sp.-Fig. 10. Female left valve, CA 17694. ~ 4 2 Fig. . 13. Maleleft valve, CA 17695. ~ 4 2 Fig. . 11. Semicytherura miurensis Hanai, 1957. Left valve, CA 17696. X 107. Fig. 12. Semicytherura henryhowei Hanai and Iekya, 1977. Right valve view of carapace, CA 17697. x 107. Fig. 14. Schizocythere kishinouyei (Kajiyama, 1913). Left juvenile valve, CA 17698. x 82. Fig. 15. Loxocorniculum sp. Right valve, CA 17699. X 89. Fig. 16. Loxoconcha sp. Left valve, CA 17700. x 86. Fig. 17. Kangarina sp. Right valve, CA 17701. x 104. Fig. 18. Hemicytherura cuneata Hanai, 1957. Right valve, CA 17702. X 120. Fig. 19. Aurila sp. Left valve, CA 17703, x 62. Fig. 20. Spinileberis qua&iaculeata (Brady, 1880). Right valve, CA 17704. . illustrated specimens are x 62. Fig. 21. Neonesidea sp. Right valve view of carpace, CA 17705 ~ 4 0 (All deposited in the collection of the University Museum, University of Tokyo (UMUT). Specimen numbers are prefixed CA for Cenozoic Arthropoda).
Japanese Miocene Ostracoda 1079
TABLE 2-LIST OF EARLYMIDDLEMIOCENE OSTRACOD SPECIES FROM THE MIZUNAMI AREA. Mizunami (Shukunohora Sandstone, Blow’s N 8 zone) Neonesidea sp. Propontocypris sp. *Pontocythere subjaponica (Hanai, 1959) P. sp. *Parakrithella pseudadonta Hanai, 1959 Munseyella sp. Callistocythere sp. *Cythere omotenipponica Hanai, 1959 *Schi.zocythere kishinouyei (Kajiyama, 1913) *SpiniIeberis quadriaculeata (Brady, 1880) Trachyleberis sp. 1 T. sp. 2 Hirsutocythere? sp. Falsobuntonia taiwanica Malz, 1982 Aurila sp. Cornucoquimba moniwensi? (Ishizaki, 1966) Ambostracon ikeyai Yajima, 1978 *Hemicytherura cuneata Hanai, 1957 Kangarina sp. *Semicytherura henryhowei Hanai and Ikeya, 1977 *S. miurensis Hanai, 1957 S. wakamurasaki Yajima, 1982 Cytheropteron sp. *Loxoconchapulchra Ishizaki, 1968 L. sp. Loxocorniculum sp. Xestoleberis sp. A=abundant; C-common; R=rare; *, extant species.
A R R R R R C R C R C R R R A C C C
R R R R R C R R R
The Shukunohora Sandstone of the Mizunami area is a thin (10 m), ill-sorted sandy silt with the foraminifer Myogipsina kotoi and many shallow warm-water molluscs. The Shukunohora Sandstone contains abundant well-preserved ostracods (Table 2). Most of the ostracod valves are transparent and good for study of internal features. The sieve type pores of Schizocythere kishinouyei and Spinileberis quadriaculeata are well preserved. The ostracod assemblage consists exclusively of shallow warm-water species (Plate 1). Among PLATE2-Miocene
Ostracoda from Tsuyama, Kamikoani, and Sekinohana. Figs. 1-3. Tsuyama, Yoshino Formation, Early Middle Miocene (Blow’s N 8 zone). Fig. 1. Spinileberis? sp., Right valve view of carapace, CA 17706 (Y-2 horizon), x 80. Fig. 2. Cletocythereis rastrornarginata(Brady, 1880), Right valve view of carapace, CA 17707 (Y-5 horizon), x 82. Fig. 3. Trachyleberissp., Left valve view of male carapace, CA 17708 (Y-5 horizon), x 58. Figs. 4-7. Kamikoani, Kamikoani Formation, Middle Miocene (Blow’s N 15 zone). Fig. 4. Finmarchinella japonica (Ishizaki, 1966), Right valve, CA 17709. ~ 8 1 Fig. ; 5. Finmarchinella japonica (Ishizaki, 1966), Juvinile left valve, CA 17710, x 87. Fig. 6. Hemicythere? sp., Right valve, CA 17711, x 80. Fig. 7. Hemicytherura cuneata Hanai, 1957, Right valve, CA 17712, X 130. Figs. 8-16. Sekinohana, Togi Formation, Middle Miocene (Blow’s N 9 zone). Fig. 8. Palmenella limicola (Norman, 1865), Left valve view of juvenile carapace, CA 17713, x 82. Fig. 9. Finmarchinella hanaii Okada, 1979, Right . 10. Finmarchinella hanaii Okada, 1979. Left valve, CA 17715 valve, CA 17714 (sample Togi D), ~ 8 1 Fig. (sample Togi D), x 80. Fig. 11. Munseyella sp., Left valve, CA 17716 (sample Togi C), x 82. Fig. 12. Cornucoquimba saitoi (Ishizaki, 1963), Right valve, CA 17717 (sample Togi D), x79. Fig. 13. Ambostracon ikeyai Yajima, 1978, Female left valve, CA 17718 (sample Togi D), x 81. Fig. 14. Acanthocythereis? munechikai Ishizaki, 1982, Left valve, CA 17719 (sample Togi D), x58. Fig. 15. Bradleya sp.. Right valve, CA 17720 (sample Togi D), x 58. Fig. 16. Hirsutocythere? nozokiensis (Ishizaki, 1963), Left valve, CA 17721 (sample Togi D), x58.
1080
M.YAJIMA
27 species of 21 genera, 9 species are extant. Neonesidea sp. which is very similar to the Recent Neonesidea oligodentata, and Aurila sp. which is very similar to the Recent Aurila punctata are abundant. Callistocythere sp., Schizocythere kishinouyei, Trachyleberis sp., Cornucoquimba moniwensis, Ambostracon ikeyai, Hemicytherura cuneata and Loxoconcha pulchra are common. Pontocythere subjaponica, Parakrithella pseudadonta, Cythere omotenipponica, Spinileberis quadriaculeata, Semicytherura henryhowei, and S. miurensis are rare but identical to the species presently living along the Pacific coast of Japan. Tropical species of Kangarina sp., Falsobuntonia taiwanica and Loxocorniculum sp. are rare.
EARLYMIDDLEMIOCENE-NORTHEAST JAPAN Kanazawa Ishizaki studied Miocene ostracods from the Sunakosaka Member (Blow’s N 8 zone) in 1963. The ostracod assemblage is very small but resembles that from the Tsuyama area because Trachyleberis is abundant.
Sendai
Miocene ostracods from the Moniwa Formation (Blow’s N 8 zone), Sendai, were reported by Ishizaki (1966). Ostracods are very rare. Abundance of Neonesidea oligodentata, Trachyleberis scabrocuneata, Cornucoquimba saitoi, and Schizocythere kishinouyei shows that the assemblage is similar to the Mizunami and Tsuyama faunas.
MIDDLEMIOCENE-NORTHEAST JAPAN Seikinohana
The Sekinohana area is situated on the Noto Peninsula, near Kanazawa along the Japan Sea. The Togi Mud Formation in Sekinohana is a very thin (2 m), greenish, massive, silty mudstone containing abundant foraminifers, and rare radiolarians, diatoms, and ostracods. Suda and Ishigaki (1984) reported the presence of Groborotalia peripheroronda and correlated the sediments with Blow’s N 7 to N 9 zones, a little later than the earliest Middle Miocene transgression. Ostracods are rare (Table 3). The fauna is dominated by Neonesidea sp., Hirsutocythere? nozokiensis, and Acanthocythereis? munechikai (Plate 2). Only one immature carapace of Palmenella limicola is present. This is a typical arctic species. Finmarchinella hanaii, which is common in shallow Pleistocene sediments along the Japan Sea coast, is common Rarely found Bradleya sp. is very similar to the Recent deep sea species of Bradleya albatrossia. The extant species Pontocythere subjaponica, Hanaiborchella miurensis, and Semicytherura henryhowei are rare. Occurrences of other species are hitherto confined to Miocene sediments.
Sugota The Sugota Formation is a 180 m thick, fine argillaceous sandstone found around Sugota near Akita, in Northeast Japan. The basal part of the Sugota Formation is dark greenish, poorly sorted medium to coarse sandstone with abundant fragments of barnacles, molluscs, and benthonic foraminifers. Molluscs show that the sediments were deposited in temperate water, and planktonic foraminifers present correlate with Blow’s N 9 zone or higher. Ostracods are very rare. Only one broken valve of Hemicythere was found. The Middle part of the Formation is dark grey to dark
Japanese Miocene Ostracoda 1081
TABLE3-LIST OF MIDDLE MIOCENE OSTRACOD SPECIES FROM THE SEKINOHANA AREA. Sekinohana (Togi Mud Formation, Blow’s N7-9 zones) Togi C Munseyella sp. Schizocythere sp. *PaImeneIIa Iimicola (Norman, 1865) FinmarchineIIa hanaii Okada, 1979 Cornucoquimba moniwensis (Ishizaki, 1966) C. saitoi (Ishizaki, 1963) *Hirsutocythere? nozokiensis (Ishizaki, 1963) *Acanthocythereis munechikai Ishizaki, 1972 BradIeya sp. Cytheropteron sawanense Hanai, 1957 Paracytheridea neolongicaudata Ishizaki, 1966 Togi D Neonesidea sp. *Pontocythere subjaponica (Hanai, 1959) *Hanaiborchellamiurensis (Hanai, 1970) *PaImeneIIa Iimicola (Norman, 1865) Finmarchinella hanaii Okada, 1979 Ambostracon ikeyai Yajima, 1978 Caudites? posterocostatus (Ishizaki, 1966) Cornucoquimbasaitoi (Ishizaki, 1963) Hirsutocythere? nozokiensis (Ishizaki, 1963) *Acanthocythereis munechikai Ishizaki, 1972 Bradleya sp. Eucytherura neoalae (Ishizaki, 1966) *Semicytherura henryhowei Hanai and Ikeya, 1977 Loxocorniculum kotoraformum Ishizaki, 1966 A=abundant; C=common; R=rare; *=extent species.
R R R C .C R A A R C R C R R R C R C R C C R R R R
brownish-grey, fine argillaceous sandstone with autochthonous cold water molluscs and foraminifers. Ostracods are very rare, only a few specimens of Callistocythere were found.
Kamikoani
The Kamikoani Formation in the Kamikoani area is a calcareous fine sand. It is considered to have been deposited 10 to 8 Ma., based on its stratigraphical position. Ostracods are very abundant and well-preserved (Table 4). Schizocythere kishinouyei, Aurila sp. 1, Finmarchinella japonica, Hemicytherura cuneata, and Cytheroteron sawanense are abundant. Palmenella limicola, a typical arctic species, is rare. Among ostracods found in this formation, Munseyella hokkaidoana, Finmarchinellajaponica, F. nealei, Howeina neoleptocytheroidea, and Cytheropteron sawanense are species common in the Pleistocene sediments and Recent shallow water fauna along the Japan Sea coast. A specimen which probably belongs to the genus Hemicythere, a cold water genus, is also present. Many species such as Cornucoquimba moniwensis, Hirsutocythere? nozokiensis, Paracytheridea neolongicaudata, Loxoconcha nozokiensis, and Loxocorniculum kotoraformum, are confined to the Miocene.
Sendai Ishizaki’s report (1966) also included the ostracod fauna from the Hatatate Formation. This formation is correlated with Blow’s N 9 to N 15 zones, and the formation is considered to have been deposited under the influence of cold currents on the basis of the molluscan fossils. Ishizaki
1082 M. YAJIMA
TABLE4-LIST OF MIDDLEMIOCENE OSTRACOD SPECIES PROM THE KAMIKOANI AREA.. Kamikoani (Kamikoani Formation, 10-8 Ma). Munsyella hokkaidoana (Hanai, 1959) *Callistocythere japonica Hanai, 1957 *C. ruogsoforma Hanai, 1957 *Cythere uranipponica Hanai, 1959 Schizocythere sp. *Palmenella limicola (Norman, 1865) Aurila sp. 1 A. sp. 2 Finmarchinellajaponica (Ishizaki, 1966) *F. nealei Okada, 1979 Hemicythere? sp. Caudites? posterocostatus (Ishizaki, 1966) Cornucoquimba moniwensis (Ishizaki, 1966) *C. tosaensis (Ishizaki, 1968) Trachyleberis sp. Hirsutocythere? nozokiensis (Ishizaki, 1963) Eucytherura sp. *Hemicytherura cuneata Hanai, 1957 *Semicytherura henryhowei Hanai and Ikeya, 1977 *Howeina neoleptocytheroidea Hanai, 1957 H . sp. Cytheropteron sawanense Hanai, 1957 Paracytheridea neolongicaudata Ishizaki, 1966 P. sp. Loxoconcha nozokiensis Ishizaki, 1963 Loxocorniculum kotoraformum Ishizaki, 1966 A=abundant; C=common; R=rare; *=extant species
R R R R C R
A R
C R R R R R R C R
A R R R A C R R C
pointed out that the ostracod fauna of “the Hatatate Formation is not essentially different from that of the Moniwa Member, although it is more or less different in productivity” (1966, p. 135). I think that the occurrence of Palmenella limicola and Finmarchinella japonica in the Hatatate Formation demonstrate cold water influence.
FAUNAL CONSIDERATIONS Palaeogeographical maps (Chinzei, 1986) of the Japanese Islands during the Early Middle Miocene (Blow’s N 8 zone) and Middle Miocene (Blow’s N 9 to N 15 zones) are shown in Text-figs. 3 and 4. Recent palaeomagnetic studies show that in the Early Middle Miocene the Japanese Islands were situated closer to the Asian continent and the Japanese Island arc was convex toward the northwest, opposite to the trend of the present arc. The Tsushima Strait between the Korean Peninsula and Kyushu Island opened and tropical water flowed into the Palaeo-Inland Sea and the Japan Sea up to Northeast Japan. Cold water influence was very weak and only extended into the Hokkaido area. Ostracods from Shobara, Tsuyama, Mizunami, Kanazawa, and Sendai, during the age of Blow’s N 8 zone are shallow, tropical water species, similar to the warm-water ostracods presently living along the Pacific coast of Southwest Japan. Ostracods from the Shukunohora Sandstone are very similar to Recent ones. The Shukunohora Sandstone was probably deposited a little after the deposition of the main part of the Palaeo-Inland Sea sediments. Among the Early Middle Miocene ostracod faunas, the one from the Shukunohora Sandstone may have persisted up until Recent times.
1083
TEXT-FIG. 3-Palaeogeographical map of the
Japanese Islands during the age of Blow’s N 8 zone, Early Middle Miocene. (after Chinzei, 1986).
130
135
140
145
TEXT-FIG. GPalaeogeographical map of the Japanese Islands during the age of Blow’s N9-15 zones, Middle Miocene. (after Chinzei, 1986).
1084 M.YAJIMA
During the Middle Miocene (Blow's N 9 to N 15 zones), Southeast Japan was emergent except for a few localities along the Pacific coast. The Japanese Islands moved to nearly the same position as they occupy today. The Tsushima Strait closed, and the influence of warm water ceased, while Northeast Japan came under cold water influence. Ostracods from Sekinohana, Sendai, Sugota, and Kamikoani show cold water influence and are very similar to Pleistocene and Recent ostracod faunas distributed along the Japan Sea and Pacific coast of Northeast Japan.
ACKNOWLEDGEMENTS Most samples were obtained personally, but some were provided by Messrs E. Taguchi of Hiroshima University (Tsuyama samples), S. Ito of Shizuoka University (Kamikoani samples), Y. Saito of JAPEX (Sugota samples), Y. Okumura of Mizunami Fossil Museum (Mizunami samples), and Dr. Ikeya of Shizuoka University (Sekinohana samples). I thank Drs. T. Hanai and K. Chinzei of the University of Tokyo for helpful discussion and Drs. H. Malz of Forschungs-Institut Senckenberg and P. Frydl of Mobil Oil Co. for coxments on the manuscript.
REFERENCE^ BLOW, W.H. 1969. Late Middle Eocene to Recent planktonic foraminifera1 biostratigraphy. In BRONNIMANN, P. and RENZ, H.H. (eds.). Proc. 1st Internat. Conf: Planktonic Microfossils, Genova, 1967, 1, 199-422, pls. 1-54. CHINZEI, K. (1986). Faunal succession and geographic distribution of Neogene molluscan faunas in Japan. In KOTAKA T. (ed.) Origin and migration of the Japanese Cenozoic mohscs. Palaeontol. Soc. Japan. spec. pap.,
29, 17-32. 1957. Studies on the Ostracoda from Japan, 111. Subfamilies Cytherurinae G.W. Miiller (emend, G.O. Sars, 1925) and Cytheropterinae n. subfam. J. Fac. Sci. Univ. Tokyo, sec. 2, 11, pt. 1, 11-36, pls. 2-4. -1977. Future problems for systematic study of Japanese ostracods. Univ. Mus. Univ. Tokyo, Bull. no. 12,8486. ISHIZAKI, K. 1963. Japanese Miocene ostracods from the Sunakosaka Member of the Yatsuo Formation, east of Kanazawa city, Ishikawa Prefecture. Japan J. Geol. Geogr., 34 (l), 19-34, pi. 2. - 1966. Miocene and Pliocene ostracods from the Sendai area, Japan. Sci. Rept. Tohoku Univ., 2nd ser. (Geol), 37 (2), 131-163, PIS. 16-19. ITOIGAWA, J. 1981. Stratigraphic outlines of selected Neogene sequences. Pacific side areas. 8. Mizunami area. I n TSUCHI R. (ed.) Neogene of Japan. 62-64. Kurofune Printing Co. Shizuoka. SUDA, A. and ISHIGAKI, T. 1984. Foraminifers from Togi Mudstone in Togi-cho, Central Noto Peninsula, Japan. J. Geof. SOC.Japan, 90(6), 417-420, 1 pl. TSUCHI, R. (ed.). 1981. NeogeneofJapan. IGCP-114 National Working Group of Japan, 140 pp. Kurofune Printing Co. Shizuoka. HANAI, T.
DISCUSSION Cronin : Are the Miocene species of Finmarchinella and Cornucoquimba still extant? Yajima: Three cold water species of Finmarchinella and one warm water species of Cornucoquimba are extant. Two species of Cornucoquimba are confined to the Miocene as far as we know at present. Hazel : I noticed that you had Ambostracon ikeyai as old as N7 or N8 in the Miocene. This seems to be a long range for a hemicytherid. Would you care to comment? Yajima : Morphologically, Ambostracon ikeyai from Mizunami (Shukunohora sandstone, N8 zone) is the same as that from Chiba (Kioroshi Formation, 143,000yr BP). I think that many species not only of Ambostracon, but also of other hemicytherids from the Miocene are still extant.
Japanese Miocene Ostracoda 1085
Neale: I congratulate the author on an excellent and most interesting paper. I missed the reference noted by an earlier speaker to Finmarchinella and would be interested to hear how she interprets the ecology of the species in terms of temperature. Does she regard this as indicative of colder waters when this genus appears? Are there other associated genera which can also be interpreted in terms of a colder environment? Yajima: From the molluscan assemblage I think that the ostracod assemblage from the N9 to N15 zones was of cold water species. Finmarchinella came down from a cold stock and then stayed, even in temperate waters. The ostracod assemblage also includes Palmenella limicola.