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Middle Pleistocene Ostracoda from the Takatsukayama Member of the Meimi Formation, Hyogo Prefecture, Western Japan

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Middle Pleistocene Ostracoda from the Takatsukayama Member of the Meimi Formation, Hyogo Prefecture, Western Japan Paleontological Research, vol. 9, no. 1, pp. 37–54, April 30, 2005 6 by the Palaeontological Society of Japan Middle Pleistocene Ostracoda from the Takatsukayama Member of the Meimi Formation, Hyogo Prefecture, western Japan: significance of the occurrence of Sinocytheridea impressa TOSHIAKI IRIZUKI1, TAKASHI MATSUBARA2 AND HIROMI MATSUMOTO3 1Department of Geoscience, Interdisciplinary Faculty of Science and Engineering, Shimane University: 1060 Nishikawatsu, Matsue Shimane 690-8504, Japan (e-mail: [email protected]) 2Division of Natural History, Museum of Nature and Human Activities, Hyogo: 6 Yayoigawa, Sanda Hyogo 669-1546, Japan (e-mail: [email protected]) 3Department of Geoscience, Interdisciplinary Faculty of Science and Engineering, Shimane University: 1060 Nishikawatsu, Matsue Shimane 690-8504, Japan Received October 23, 2004; Revised manuscript accepted January 11, 2005 Abstract. Thirty-seven ostracode species are identified from 15 samples from lithological Units II and III at the type locality of the middle Pleistocene (Marine Isotope Stage 11: ca. 0.41 Ma) Takatsukayama Member of the Meimi Formation, Hyogo Prefecture, western Japan. Three ostracode biofacies are dis- tinguished on the basis of Q-mode cluster analysis. The depositional environments of each biofacies can be inferred as bay sandy coast (BC), enclosed inner bay (IB) and middle bay influenced by freshwater inflows from rivers on the Harima Plain (CB). A rapid increase in water depth from bay coast to middle bay oc- curred at the deposition of the middle part of Unit II of the Takatsukayama Member. Sinocytheridea im- pressa (Brady), which probably disappeared or rapidly decreased from paleo-Osaka Bay at about 0.3–0.35 Ma, is abundant at the study site. One of the reasons for the disappearance or rapid decrease of this species is likely that paleo-Osaka Bay changed from a low-salinity, enclosed nutrient-rich and highly turbid bay to a moderate salinity, ventilated bay with good circulation, together with the vicariance event suggested by previous studies. This change was probably linked with both widespread inundation in the neighboring paleo-Harima-nada Bay and the development of large-scale tidal currents from the Akashi Strait connect- ing both bays. Thereafter, Neomonoceratina delicata Ishizaki and Kato replaced S. impressa and became the dominant taxon in paleo-Osaka Bay. Sinocytheridea impressa which lived in paleo-Osaka and Harima- nada Bays has larger valves than any specimens of this species from Recent seas. The cause of this large size in the past might have been an abundant food supply or dissolved chemicals from freshwater inflows into paleo-Osaka and Harima-nada Bays. Alternatively, whether large-valve populations occurred as a result of genetic changes in relation to the geographical isolation or not, natural selection operated in favor of in- creased valve size in paleo-Osaka and Harima Bays for reasons that are as yet uncertain. Key words: MIS 11, Ostracoda, paleo-Osaka and Harima-nada Bays, Pleistocene, Sinocytheridea, Takatsukayama Member Introduction At that time, terrestrial animals migrated from the continent to the islands (e.g., Kawamura, 1991; A major global climatic oscillation of the order of Konishi and Yoshikawa, 1999; Kimura, 2002). On the ca. 100,000 years developed after the middle Pleisto- other hand, many straits arose between the continen- cene. In eastern Asia, land bridges formed between tal mainland and the islands during interglacial peri- the Eurasian Continent and the Japanese Islands dur- ods. Those animals were then isolated and their pop- ing the intervals of glacial maxima (e.g., Ujiie´ and ulations diminished or evolved. Ostracodes are an Ujiie´, 1999; Konishi and Yoshikawa, 1999; Ujiie´ et al., abundant and practically ubiquitous group of minute 2003). crustaceans. Most benthic marine ostracode species 38 Toshiaki Irizuki et al. have a hard calcareous bivalved carapace that is easily Akashi and Meimi Formations (Huzita and Kasama, fossilized. The benthic ostracodes have no planktonic 1983; Huzita and Maeda, 1984; Hashimoto and Maeda, period in their life cycle. Therefore, it seems that os- 1989). The Meimi Formation is subdivided in ascend- tracode species inhabiting shallow enclosed muddy ing order into three members: the Asagiri, Takatsu- bay bottoms find it difficult to migrate rapidly to other kayama and Iwaoka (Hashimoto and Maeda, 1989). shallow bays if deep straits are present between bays. The Takatsukayama Member, which is the object of As a result, migration, extinction, and evolution of the present study, unconformably overlies the Akashi shallow embayment species have been strongly influ- Formation at the type locality. Katoh et al. (1999) enced by cyclic changes in glacial and interglacial cli- subdivided the Takatsukayama Member into four units mates during the late Quaternary period (Ishizaki, (Units I to IV) at the type locality, which is an outcrop 1987, 1990a, b). Osaka Bay issituatedintheeastern to the south of Wakaba-gakuen Special School, part of the Seto Inland Sea, western Japan. Its paleo- Kozukayama, Tamon-cho, Tarumi Ward, Kobe City geography has changed cyclically under the influence (Latitude: 344001800N; Longitude: 135303500E; Fig- of glacio-eustasy during the Pleistocene to Holocene. ures 1 and 2). Embayment ostracode faunas from Holocene deposits Unit I unconformably covers the Akashi Formation in the Osaka Bay region have recently been studied and is composed mainly of gravel overlain by silt in detail (e.g., Irizuki et al., 2001; Masuda et al., (Huzita and Kasama, 1983; Huzita and Maeda, 1984; 2002; Yasuhara et al., 2002a, b, 2004) (Figure 1). Hashimoto and Maeda, 1989; Igawa and Itihara, 1993; These studies correlated the dynamics of ostracode Sato et al., 1997; Katoh et al., 1999). The thickness of faunas in Osaka Bay with relative sea-level changes this unit is about 6.5 m (Hashimoto and Maeda, 1989), during the Holocene (the Jomon Transgression). but only the upper half is now exposed. A fine tuff However, only a few studies of Pleistocene ostracodes layer (10–20 cm thick), called the ‘‘Takatsukayama from paleo-Osaka Bay have been conducted (Ishizaki, Volcanic Ash Layer’’ (Suzuki, 1988; Hashimoto and 1984, 1990a). Ishizaki (1984, 1990a) studied temporal Maeda, 1989; Igawa and Itihara, 1993; Kato et al., changes in ostracodes from middle Pleistocene to 1999), is intercalated near the uppermost part of this Holocene borehole cores excavated in Osaka Bay off unit. Senshu, which is situated in the southern part of Unit II consists of a 1.2-m-thick, dark gray clay, paleo-Osaka Bay. In the present study, we report in rich in shells of such molluscs as Crassostrea pestigris detail middle Pleistocene ostracodes from the Ta- (Hanley), C. gigas (Thunberg), and ‘‘Volachlamys katsukayama Member of the Meimi Formation, Hy- yagurai (Makiyama)’’. Ueji (1937) and Fukuda and ogo Prefecture, which is situated to the north of the Ando (1951) called this unit the ‘‘Takatsukayama Akashi Strait, in the northern part of paleo-Osaka Shell Bed’’. Thalassinoides-like sand pipes are densely Bay (Figure 1), and was deposited in paleo-Harima- developed in the basal part of the unit and penetrate nada Bay. Hashimoto and Maeda (1989) reported a into Unit I. The boundary between Units I and II is preliminary study of the fossil ostracodes from the probably an erosional surface because the uppermost member. However, the vertical changes that they re- part of Unit I, including the ‘‘Takatsukayama Volca- ported have not been quantified. Here, we reconstruct nic Ash Layer’’, is eroded westward. the depositional environment of the Takatsukayama Unit III is composed first of pale gray to yellowish Member and discuss ostracode faunal changes in gray bioturbated clayey silt (2.7 m thick), followed by paleo-Osaka and Harima-nada Bays during the a yellowish gray to yellowish brown silty sand (0.7 m Pleistocene. thick), and then a light gray fine- to medium-grained sand (1.2 m thick) rich in cylindrical sand pipes, and Geological setting and age shows a coarsening-upward sequence. The molluscan fossils are contained only in the lower 2 m interval; Plio–Pleistocene deposits are widely developed in they are abundant in the lowest part and tend to de- the eastern Harima Plain, in the south–central part of crease upward in the sequence. Fossil cones of coni- Hyogo Prefecture (e.g., Itihara et al., 1960; Huzita and fers are also found sporadically in the upper part of Maeda, 1984; Hashimoto and Maeda, 1989; Igawa and the unit. The ‘‘Takatsukayama Clay Bed’’ of Itihara Itihara, 1993). They are divided, in ascending order, et al. (1960) corresponds to Unit II and the lower part into the Osaka Group and terrace deposits (Huzita of Unit III. and Maeda, 1984; Hashimoto and Maeda, 1989; Igawa Unit IV comprises, in ascending order, an and Itihara, 1993). The Osaka Group in the eastern orange-to-brown parallel-laminated medium- to Harima Plain is composed, in ascending order, of the coarse-grained sand (0.3–5 m thick) and gravelly me- Middle Pleistocene Ostracoda from Hyogo 39 Figure 1. Index and the location maps of the study site. Base map: 1 : 25,000 topographical maps ‘‘Zenkai’’ and ‘‘Suma’’ by the Geographical Survey Institute of Japan. dium- to coarse-grained sand (more than 4.5 m thick) the Takatsukayama Member of the Meimi Formation including thin lenticular granule layers showing fore- is correlated with Marine Isotope Stage (MIS) 11. set cross-stratification. The original thickness of this unit was about 20 m (Hashimoto and Maeda, 1989), Materials and methods but most of it was removed during development of the land in the 1980s. We selected two sections (A and B) situated 3 m Kato et al. (1999) dated the ‘‘Takatsukayama Vol- apart at the outcrop at the type locality. Six samples canic Ash Layer’’ at 0.41 G 0.12 Ma (error, 1s)using (TAK-1 to 6) were collected from section A and ten the fission-track method.
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