Introduction

Only a few studies on Paleogene shallow-marine ostracodes in the northwestern Pacific have been made, although many studies on Neogene ostracodes have been carried out. Among those few studies, Hanai (1970) reported the genus Eopaijenborchella from the Eocene in Hokkaido, northern Japan. Liu (1989) and Yang et al. (1990) reported Paleocene and Eocene ostracodes from drilling cores off Taiwan and described 27 and six new species, respectively. However, these studies were qualitative in nature and Paleogene ostracode assemblages have until now not been quantitatively evaluated. Moreover, Oligocene ostracodes have never been studied in Japan. One of the aims of this study is to clarify the Oligocene ostracode assemblages in the northwestern Pacific on the basis of quantitative examination.

Ostracodes are considered to be useful indicators of paleoenvironments, such as in studies of the depositional environments of upper Cenozoic strata (e.g., Irizuki and Matsubara, 1994; Irizuki et al., 1998; Yamada et al., 2001). Many discussions on depositional environments using late Cenozoic ostracodes are based on the modern geographical distributions of extant species. Because all Eocene ostracode species are extinct, Yang et al. (1990) discussed the depositional environments of the Eocene Wenzhou Formation on the basis of the modern geographical distribution of extant genera. Yang et al. (1990) thought that the lower part and the upper part of the Wenzhou Formation had been deposited in a shallow sea and an alternate environment between a shallow and a deep sea, respectively. However, calcareous nannofossils both from the lower and the upper part of the Wenzhou Formation suggested deposition in coastal environments on the basis of the Paleogene geographical distribution of the genera (Zhong, 1990). Generally the deep sea is considered not a coastal but an offshore environment. Hence, the geographical distribution of Paleogene ostracode genera could be different from those of their Recent counterparts. In order to discuss depositional environments using Paleogene ostracodes clearly, the relationship between Paleogene ostracode species and depositional environments needs to be revealed. Therefore, another aim of this study is to examine the habitats of extinct species. The depositional environments of the Itanoura Formation, the study strata, were previously discussed on the basis of fossil molluscs, benthic foraminifers and sedimentary facies analysis (Inoue, 1964; New Energy Development Organization (NEDO), 1985, 1986, 1987; Hattori et al., 1993; Kurita et al., 2003). Inoue (1964) and Hattori et al. (1993) mentioned that the Itanoura Formation included near-shore and offshore deposits on the basis of the modern geographical distribution of extant molluscan genera. NEDO (1985, 1986, 1987), which reported the Nonion-Elphidium benthic foraminiferal assemblage from drilling cores off the Nishisonogi Peninsula, considered that the formation was deposited in the near-shore zone. On the other hand, Kurita et al. (2003) analyzed sedimentary facies and thought that the formation was deposited in an estuary. To determine the depositional environments in which the ostracodes had lived, I examined the lithofacies, foraminifers and reinterpreted the molluscan data of Inoue (1964) and Hattori et al. (1993).

Geological outlines

The Itanoura Formation is distributed in the Nishisonogi Peninsula and adjacent areas, western part of Nagasaki Prefecture, southwestern Japan. The geology there is summarized by Nagahama and Matsui (1958) and Hattori et al. (1993). In this study, the division of the Paleogene sequence follows that of Hattori et al. (1993). The Itanoura Formation is included in the Nishisonogi Group. The group dips in a northwestern to west direction at less than 30 degrees and is divided by faults in the study area (Figure 1).

Figure 1.

Geological map of the study area. Partly modified after Nagahama and Matsui (1958) and Hattori et al. (1993).

The Itanoura Formation, which is 90 to 170 m thick, consists chiefly of sandstone. It is divided into three parts on the basis of lithofacies: The basement part consists of conglomerate with abundant Ostrea shells less than 2 m thick; the main part consists of alternating sandstone and muddy sandstone with abundant molluscan fossils and is 60 to 100 m thick; the uppermost part, 10 to 30 m thick, consists mainly of sandstone with mud pipes. The lithofacies of the formation changes laterally. The formation conformably overlies the Sakito Formation, which consists chiefly of sandstone intercalated by lignite layers, and is conformably overlain by the Kakinoura Formation, which is composed of sandstone and calcareous sandstone.

The geological age of the Itanoura Formation has been examined on the basis of the dating of the tuff layer with the fission-track method and calcareous nannofossils by Miyachi and Sakai (1991) and Okada (1992), respectively. Calcareous nannofossils date the formation as 33.3 to 32.8 Ma (Berggren et al., 1995), because the formation is correlated with calcareous nannofossil Zone CP16b of Okada and Bukry (1980) (Okada, 1992). The fission-track ages of zircons from the andesitic tuff layer in the Sakito section, which are 37.8 ± 4.0 and 37.9 ± 3.6 Ma (2σ error) (Miyachi and Sakai, 1991), suggest an age older than that indicated by the calcareous nannofossils. The zircons are interpreted as having been reworked, because the tuff layer, which includes gravels and represents upward-fining, is regarded as a reworked deposit. Therefore, the formation is thought to be of early Oligocene age.

Materials and methods

Thirty-six rock samples for examination of micro-fossils (ostracodes and foraminifers) were collected from outcrops exposed in the Oshima, Sakito and Matsushima sections (Figures 2 and 3). All samples except for sample MIT01 were gathered from the main part of the Itanoura Formation. Sample MIT01 was collected from the basement part of the formation in the Matsushima section. The Itanoura Formation in these sections cannot be clearly correlated with each other, because these sections are divided by faults and key beds could not be recognized.

Figure 2.

Locality maps of the samples (parts of 1 : 25,000 “Omodaka”, “Kakinoura” and “Kamanoura” sheets published by the Geographical Survey Institute of Japan).

Figure 3.

Columns of the study sections.

In laboratory procedures, 160 to 400 g rock samples were disaggregated in a saturated sodium sulfate solution and naphtha. The disaggregated samples were washed through 63 μm mesh sieve and dried. Ostracode specimens were picked from fractions coarser than 125 μm in samples. Photographs of the specimens of dominant species were taken with a scanning electron microscope (SEM) to identify species. In order to observe the internal view of the carapaces of some dominant species, photographs of the thin sections of carapaces were taken with a transmitted light microscope. The total number of ostracode individuals was reckoned as the sum of the number of carapaces and the larger number of either left or right valves (Table 1). To recognize ostracode biofacies, samples with more than 50 individuals were subjected to Q-mode cluster analysis. The analysis was performed with the cosine theta index of Imbry and Purdy (1962) as the similarity measure and the weighted pair group arithmetic average method as the linkage strategy. Relative abundance of species, the number of species and species diversity index in assemblages were also calculated for the samples with more than 50 individuals. Species diversity index (H(S)) was expressed by the Shannon-Wiener index:

wherePi andS are the relative abundance of thei-th species and the number of species, respectively.

Table 1.

List of ostracode species from the Itanoura Formation. Ind = individuals, C = carapaces, L = left valves, R = right valves

Molluscan data in the sections were based on Inoue (1964), Hattori et al. (1993) and supplemental data by the observation in this study. Fossil foraminifers were picked from fractions coarser than 125 μm in samples sieved for examination of ostracodes. I divided foraminifers into planktonic and benthic groups and calculated planktonic/total foraminifers ratios (P/T ratios) (Table 2).

Table 2.

List of foraminifer groups from the Itanoura Formation

Results

1) Description of lithofacies and molluscs data

The lithofacies containing fossil ostracodes are divided into two units: muddy fine-grained sandstone (Unit 1) and massive mudstone (Unit 2) (Figures 4 and 5).

Figure 4.

Stratigraphic distribution of ostracodes, lithofacies, P/T ratios and fossil molluscan assemblages in the Oshima section.

Figure 5.

Stratigraphic distribution of ostracodes, lithofacies, P/T ratios and fossil molluscan assemblages in the Sakito and Matsushima sections. Legend is the same as Figure 4.

3) Ostracode biofacies

Ostracodes occur in 18 samples. Most ostracode specimens were carapaces and were not well preserved (Figures 6 and 7). At least thirty-three species were identified (Table 1). Fifteen out of 18 samples contained more than 50 individuals and three samples included less than 50 individuals. Three ostracode biofacies were identified at the 0.6 level of the cosine theta index based on the Q-mode cluster analysis (Figure 8). Three biofacies have their own characters in species components, the number of species andH(S) as follows.

Figure 6.

Photographs of dominant species, all taken by scanning electron microscope (SEM) except for 1c and 3c, for which a transmitted-light microscope was used. All scale bars indicate 100 μm. 1: Acanthocythereis volubilis (Liu, 1989), a: left lateral view of car-apace, UMUT-CA28607, SIT03, b: dorsal view of carapace, UMUT-CA28607, SIT03, c: internal view of right valve, UMUT-CA28608, SIT03, 2: Cytherella. sp. 1, a: right lateral view of carapace, UMUT-CA28605, MIT07, b: dorsal view of carapace, UMUT-CA28605, MIT07, c: internal view of right valve, UMUT-CA28606, MIT03, 3: Abrocythereis aff. acrocaudalis. (Liu, 1989), a: right lateral view of carapace, UMUT-CA28609, OIT11, b: dorsal view of carapace, UMUT-CA28609, OIT11, c: internal view of left valve, UMUT-CA28610, SIT06, 4: Palmoconcha oujiangensis (Liu, 1989), a: left lateral view of carapace, UMUT-CA28615, MIT02, b: dorsal view of carapace, UMUTCA28615, MIT02.

Figure 7.

Photographs of dominant species, all taken by scanning electron microscope (SEM) except for 2c, for which a transmitted-light microscope was used. All scale bars indicate 100 μm. 1: Krithe sp., a: left lateral view of carapace, UMUT-CA28611, MIT07, b: dorsal view of carapace, UMUT-CA28611, MIT07, 2: Eopaijenborchella sinensis (Liu, 1989), a: right lateral view of carapace, UMUT-CA28612, OIT11, b: dorsal view of carapace, UMUT-CA28612, OIT11, c: internal view of left valve, UMUT-CA28613, MIT07, 3: Hanaiborchella sp., a: right lateral view of carapace, UMUT-CA28614, MIT02, b: dorsal view of carapace, UMUT-CA28614, MIT02, 4: Munseyella simplex Chen, 1990, a: left lateral view of carapace, UMUT-CA28617, OIT12, b: dorsal view of carapace, UMUT-CA28617, OIT12, 5: Cytheropteron. sp. 1, a: left lateral view of carapace, UMUT-CA28616, SIT06, b: dorsal view of carapace, UMUT-CA28616, SIT06.

Figure 8.

Diagram showing the dendrogram of the cluster analysis and biofacies.

Biofacies A.—The biofacies occur in Unit 1. It is characterized by dominance of Acanthocythereis volubilis (Liu) and Eopaijenborchella sinensis (Liu), accounting for about 40 to 70%. These dominant species are associated with Abrocythereis aff. acrocaudalis. (Liu), Munseyella simplex Chen and Cytheropteron. sp. 1. The number of species andH(S) are 6 to 13 and 1.31 to 2.11, respectively.

Biofacies B.—The biofacies occurs from Units 1 and 2. It is characterized by predominance of Krithe sp. associated with A. volubilis and Cytherella. sp. 1. Hanaiborchella sp. is also a characteristic species. The number of species andH(S) are 6 to 10 and 1.39 to 1.93, respectively.

Biofacies C.—The biofacies occurs from Unit 1. It is characterized by Palmoconcha oujiangensis. (Liu), accounting for about 50%. The dominant species is associated with A. volubilis and E. sinensis. The number of species andH(S) are 8 and 1.65, respectively.

Biofacies A and B contain the Turritella and the “Cardium”-Pitar assemblage, respectively (Figures 4 and 5), although Biofacies C does not have concurrent fossil molluscan assemblages.

Discussion

2) The habitats of fossil ostracodes

The correlation among ostracode biofacies, lithofacies and fossil molluscan assemblages indicates that Biofacies A was deposited in a shelf shallower than Biofacies B (Figure 9). It is not clear whether the depositional environment of Biofacies C was shallower or deeper than Biofacies A.

Figure 9.

Relationship between depositional environments and ostracodes from the Itanoura Formation. The frequencies of ostracode species are the average values in each biofacies.

Acanthocythereis volubilis, Eopaijenborchella sinensis, Munseyella simplex and Cytheropteron. sp. 1 are believed to have flourished on the muddy sand bottom in a shallower shelf. Their relative abundances decrease in deeper-water environments. On the other hand, Krithe sp. inhabited a deeper shelf environment, as did Hanaiborchella sp. It is not clear whether the relative abundances of Palmoconcha oujiangensis, Cytherella. sp. 1 and Abrocythereis aff. acrocaudalis. were dependent on water depths or not. The genera Acanthocythereis, Abrocythereis, Cytheropteron, Hanaiborchella and Munseyella are commonly reported from the continental shelf in the Recent East China Sea and off southwestern Japan (e.g. Ishizaki, 1981; Wang and Zhao, 1985; Zhao and Wang, 1988; Zhou, 1995). The genus Krithe occurs in the deeper shelf to deep sea (e.g. Hanai et al., 1980; Ishizaki, 1981; Wang and Zhao, 1985; Zhao and Wang, 1988; Zhou and Ikeya, 1992). The habitats of these six genera in the Recent are generally consistent with those of their counterparts from the Oligocene Itanoura Formation. Meanwhile, the genus Eopaijenborchella occurs from the deep sea in the South China Sea but not in the seas around the Japanese Islands (e.g. Hanai et al., 1980; Zhou, 1995). The genera Cytherella. and Palmoconcha from the Itanoura Formation inhabited shallower environments than their Recent counterparts, because they are reported from the Recent shelf edge and slope at water depths of about 100 to 800 m around the Japanese Islands (e.g. Zhou, 1995). Nakao et al. (2001) reported the genus Cytherella from the Pleistocene Ukari and Hijikata Formations of the Kakegawa Group in central Japan, which comprise outer shelf and slope facies, respectively (Sakai and Masuda, 1995). Irizuki and Matsubara (1994) represented the abundant occurrence of the genus Palmoconcha from middle Miocene deposits containing fossil shallow-marine molluscs of the Kadonosawa Formation, Shiratorigawa Group in northeastern Japan. These data suggest that Cytherella and Palmoconcha may have changed habitats during the Cenozoic around the Japanese Islands. The depositional environment of the upper part of the Wenzhou Formation including Eopaijenborchella sinensis and the genus Cytherella can be reinterpreted as a shallow sea rather than the deep sea. This interpretation agrees with the view that the Wenzhou Formation was deposited in coastal areas on the basis of calcareous nannofossil assemblages (Zhong, 1990). Thus, it is noteworthy that careful examination is required when we apply Recent geographical distribution of certain ostracode genera to Paleogene environmental analysis.

Summary

Taxonomic notes

The illustrated ostracode specimens are presently deposited in University Museum, University of Tokyo (UMUT-CA28605 to UMUT-CA28617). UMUTCA28608, UMUT-CA28610 and UMUT-CA28613 of the specimens are thin sections of carapaces.

Order Platycopida Sars, 1866

Family Cytherellidae Sars, 1866

Genus Cytherella Jones, 1849

Cytherella sp. 1

Figure 6.2a–c

Illustrated specimens.—Carapace, UMUT-CA28605 (Figure 6.2a, b) from sample MIT07 and right valve, UMUT-CA28606 (Figure 6.2c) from sample MIT03.

Remarks.—The species resembles C. lepida Liu, 1989 and C. rotunda Liu, 1989 in its dorsal margin parallel to the ventral margin in lateral view and sub-triangular outline in dorsal view. However, poor preservation makes its surface ornament obscure, and the species cannot be identified.

Order Podocopida Sars, 1866

Family Trachyleberididae Sylvester-Bradley, 1948

Genus Acanthocythereis Howe, 1963

Acanthocythereis volubilis (Liu, 1989)

Figure 6.1a–c

Trachyleberis volubilis Liu, 1989, p. 154, pl. 168, figs. 12–16; Yang et al., 1990, p. 377, pl. 2, figs. 1– 3.

Illustrated specimens.—Carapace, UMUT-CA28607 (Figure 6.1a, b) and thin section of carapace, UMUTCA28608 (Figure 6.1c) from sample SIT03.

Remarks.—Subtriangular outline, surface ornamented with tubercles, tubercles along anterior margin in lateral view and amphidont hinge indicate that this species belongs to the genus Acanthocythereis. It resembles A. munechikai Ishizaki, 1981 in the tubercles along the anterior and dorsal margins but differs from the latter in its wide anterior cardinal angle, and indistinct tubercles along the posterior ventral margin.

Genus Abrocythereis Gou, 1983

Abrocythereis aff. acrocaudalis. (Liu, 1989)

Figure 6.3a–c

Cf. Trachyleberidea acrocaudalis Liu, 1989, p. 156, pl. 169, figs. 7–10; Caudites? acrocaudalis. (Liu, 1989), Yang et al., 1990, p. 373, pl. 2, figs. 12, 15.

Illustrated specimens.—Carapace, UMUT-CA28609 (Figure 6.3a, b) from sample OIT11 and thin section of carapace, UMUT-CA28610 (Figure 6.3c) from sample SIT06.

Remarks.—Subquadrangular outline with tapering posterior margin, distinct anterior marginal ridge, short lateral ridge parallel to ventral margin, surface ornamented with punctae, three tubercles along dorsal margin in lateral view and amphidont hinge in interior view indicate that this species belongs to the genus Abrocythereis. The specimens from the Itanoura Formation differs from A. acrocaudalis (Liu, 1989) in lacking a spine at the anterior dorsal corner and not having a sharp tapering caudal process.

Family Krithidae Mandelstam, 1958

Genus Krithe Brady, Crosskey and Robertson, 1874

Krithe sp.

Figure 7.1a–b

Illustrated specimen.—Carapace, UMUT-CA28611 (Figure 7.1a, b) from sample MIT07.

Remarks.—Subtrapezoidal outline, smooth surface ornaments in lateral view and indented posterior end in dorsal views indicate that this species belongs to the genus Krithe.

Family Cytheridae Baird, 1850

Genus Eopaijenborchella Keij, 1966

Eopaijenborchella sinensis (Liu, 1989)

Figure 7.2a–c

Paijenborchella sinensis Liu, 1989, p. 151, pl. 167, figs 1, 2.

Paijenborchella (Eopaijenborchella) sinensis Liu, 1989,

Yang et al., 1990, p. 374, 375, pl. 3, figs. 1–5.

Illustrated specimens.—Carapace, UMUT-CA28612 (Figure 7.2a, b) from sample OIT11 and thin section of carapace, UMUT-CA28613 (Figure 7.2c) from sample MIT07.

Remarks.—Ovate outline, dorsomedian sulcus which is cut off by two longitudinal ridges medianly, developed caudal process and surface ornamented with punctae in lateral view indicate that this species belongs to the genus Eopaijenborchella.

Genus Hanaiborchella Gruendel, 1976

Hanaiborchella sp.

Figure 7.3a–b

Illustrated specimen.—Carapace, UMUT-CA28614 (Figure 7.3a, b) from sample MIT02.

Remarks.—Subrectangular outline, dorsomedian sulcus which is cut off by two longitudinal ridges medianly area and undeveloped caudal process in lateral view indicate that the species belongs to the genus Hanaiborchella. It resembles H. wendongensis (Liu, 1989) in its lateral outline and the median longitudinal ridges. Poor preservation makes the surface ornamentation obscure.

Family Loxoconchidae Sars, 1925

Genus Palmoconcha Swain and Gilby, 1974

Palmoconcha oujiangensis (Liu, 1989)

Figure 6.4a–b

Loxoconcha oujiangensis Liu, 1989, p. 148, pl. 165, figs. 12–15; Yang et al., 1990, p. 373, 374, pl. 2, figs. 6– 8.

Illustrated specimen.—Carapace, UMUT-CA28615 (Figure 6.4a, b) from sample MIT02.

Remarks.—Subquadrangular outline, flat areas along anterior and posterior margins and surface ornamented with reticulation in lateral view indicate that this species belongs to the genus Palmoconcha.

Family Cytheruridae G.W. Mueller, 1894

Genus Cytheropteron Sars, 1866

Cytheropteron sp. 1

Figure 7.5a–b

Illustrated specimen.—Carapace, UMUT-CA28616 (Figure 7.5a, b) from sample SIT06.

Remarks.—This species differs from the Miocene species C. sendaiense Ishizaki, 1966 (Ishizaki, 1966; Irizuki et al., 1998) in its undeveloped caudal process and feeble vertical ridges in the posterior half of the carapace.

Family Eucytheridae Puri, 1954

Genus Munseyella van den Bold, 1957

Munseyella simplex Chen, 1990

Figure 7.4a–b

Munseyella simplex Chen, 1990, Yang et al., 1990, p. 374, 385, 386, pl. 1, figs. 7, 8.

Illustrated specimen.—Carapace, UMUT-CA28617 (Figure 7.4a,b) from sample OIT12.

Remarks.—This species resembles Munseyella sp. of Yajima (1992) in the ridge pattern of its surface ornamentation and the size of the carapace but differs from the latter in having an indistinct ridge and shallow hollow within the anterior marginal rim.