Planktonic Foraminiferal Biostratigraphy of the Miocene Sequence in the Iwadono Hills, Central Japan: An Integrated Approach

Transcription

1 76 RESEARCH LETTERS Planktonic Foraminiferal Biostratigraphy of the Miocene Sequence in the Iwadono Hills, Central Japan: An Integrated Approach HIROKI HAYASHI* Institute of Geology and Paleontology, Graduate School of Science, Tohoku University, Sendai , Japan, YUKITO KURIHARA Institute of Geoscience, University of Tsukuba, Tennodai --, Tsukuba, Ibaraki, , Japan SEIJI HORIUCHI Palyno-Survey Co. Ltd., Tozaki 559-3, Okanogo, Fujioka City, Gunma , Japan TOMOHIRO IWASHITA Institute of Geology and Paleontology, Graduate School of Science, Tohoku University, Sendai , Japan YUKIO YANAGISAWA Institute of Geoscience, Geological Survey of Japan /AIST, Higashi - -, Tsukuba City, Ibaraki , Japan PALAIOS, 2003, V. 8, p Planktonic foraminiferal age of the Miocene sequence in the Iwadono Hills of central Japan is determined in reference to recent advances in Miocene planktonic foraminiferal biostratigraphy of Japan. The interval from the Godo through the Negishi to the lower part of the Shogunzawa Members of the Iwadono Formation yields 55 species of planktonic foraminiferal fossils. The Godo Member is assigned to planktonic foraminiferal zone N.8. The Negishi Member and the lower part of the Shogunzawa Member are confined to zones N.0 to N.3. Nine foraminiferal biohorizons were detected, seven of which enable us to determine a detailed age-thickness model for the sequence. The age-thickness model indicates a remarkably slow sedimentation rate in the Negishi Member (less than 9 mm/kyr). Furthermore, a relationship is established between the foraminiferal biostratigraphy and other stratigraphic data (diatoms, calcareous nannofossils, and radiometric ages) in the area. This relationship agrees well with the age-correlation chart of Japanese Neogene sediments. INTRODUCTION Planktonic foraminiferal biostratigraphy is a powerful tool for global correlation of post-cretaceous marine strata. Blow s (969) biostratigraphic zonation for the tropical * Current address: Solid Earth Science Group, National Research Institute for Earth Science and Disaster Prevention, Tennodai 3- Tsukuba, Ibaraki, , Japan to subtropical region is among the most frequently used because of its comprehensive documentation of each biozone and the simple naming of zones using numerical code. However, owing to discontinuous biogeographic distributions of planktonic foraminifera, correlation of midlatitude strata based on the low-latitude zonation has not been accomplished satisfactorily. Establish a planktonic foraminiferal biostratigraphy for the mid-latitude region has been desired for a long time. Detailed integration of Miocene magneto- and biostratigraphy for the mid-latitude region around the Japanese Islands never has been established because of the discontinuous occurrence of calcareous microfossils in deep-sea cores from the area. Instead, the biostratigraphy of this region has been based on examination of marine strata exposed on land. Saito (963) described planktonic foraminiferal assemblages and proposed eight foraminiferal zones from representative Miocene sequences in Honshu, Japan. Oda (977) divided the Neogene into eleven zones based on biostratigraphic studies in central to northeast Japan. Maiya (978) also constructed a zonation for the Oil-Field-Region, the Sea of Japan side of northeast Japan, and recognized nine Neogene zones. However, geochronologic constraints on these zones in Japan are still poor (Oda et al., 984). Recent studies on Miocene planktonic foraminifers in Japan have proposed many biohorizons with numerical constraints based on diatom chronology and radioisotopic ages (Hayashi et al., 999; Hayashi and Takahashi, 2000, 2002; Shimamoto et al., 200). These numerical data allow testing of the reliability of a given biohorizon, along with the usefulness of these data for dating marine strata. Miocene marine deposits that are distributed widely in the Iwadono Hills (Fig. ) in the eastern marginal area of the Kanto Mountains, central Japan, hold the key to understanding the timing of tectonic processes in the area. The lower half of the sequence, namely, the Godo and Negishi Members of the Iwadono Formation, yields abundant calcareous microfossils, including planktonic foraminifers. Biochronological studies of this sequence have been carried out using foraminifera (Matsumaru et al., 982; Koike et al., 985) and diatoms (Horiuchi and Yanagisawa, 994). However, two different foraminiferal ages have been reported from the lower part of the sequence. Matsumaru et al. (982) assigned the Negishi Member to zone N.9 of Blow (969), whereas Koike et al. (985) assigned it to zone N.0. The planktonic foraminiferal biostratigraphy of the lower part of the sequence, namely, the Godo, Negishi, and Shogunzawa Members of the Iwadono Formation, were examined in light of recent advances in Miocene planktonic foraminiferal biostratigraphy of Japan. In addition, new data were obtained on diatoms and calcareous nannofossils using the same rock samples. Planktonic foraminiferal data were correlated with other geochronological evidence (e.g., biostratigraphy and radioisotope geochronology) to verify the usefulness of newly recognized foraminiferal biohorizons. GEOLOGIC SETTING The Miocene strata in this area are in fault contact with pre-cenozoic rocks along the western margin and are Copyright 2003, SEPM (Society for Sedimentary Geology) /03/ /$3.00

2 MIOCENE PLANKTONIC FORAMINIFERAL BIOSTRATIGRAPHY 77 of bedded conglomerate and very coarse-grained conglomeratic sandstone with shell fragments. The Negishi Member is composed of silty medium- to fine-grained sandstone, including glauconitic grains and shell fragments. The Shogunzawa Member is made up of poorly sorted diatomaceous siltstone. The Ohashi Formation conformably overlies the Iwadono Formation and is subdivided into two members (Koike et al., 985): the Hatoyama sandstone and siltstone (200 m) and the Imajuku sandstone (more than 50 m), in ascending order. The Hatoyama Member consists of interbedded sandstone and siltstone. The Imajuku Member is composed of medium- to coarse-grained sandstone. The Miocene strata yield abundant marine microfossils. In particular, the Godo and Negishi Members of the Iwadono Formation are characterized by abundant occurrences of calcareous microfossils. In contrast, the upper part of the succession, the Shogunzawa Member of the Iwadono Formation through the Ohashi Formation, yields siliceous microfossils. In this study, planktonic foraminifers of the Godo, Negishi and the lower part of the Shogunzawa Members of the Iwadono Formation are examined. MATERIAL AND METHODS FIGURE Index map showing the Iwadono Hills. overlain unconformably by terrigenous post-miocene sediments (Fig. 2). The definition of the sequence proposed by Koike et al. (985) and Majima (989) (Fig. 3) has been followed in this paper. The Miocene sequence is divided into the following three formations: the Kamikarako, Iwadono, and Ohashi Formations, in ascending order. The relationship between the Kamikarako and Iwadono Formations is considered to be unconformable because the base of the Godo Member includes mudstone gravel derived from the Kamikarako Formation (Kurihara, 999). In addition, the Kamikarako Formation is deformed significantly and faulted, in contrast to the Iwadono Formation (Koike et al., 985). The Kamikarako Formation is composed mainly of diatomaceous siltstone and sandy siltstone. The Iwadono Formation is subdivided into the following three members in ascending order (Koike et al., 985): the Godo conglomerate (00 m thick), the Negishi sandstone (20 m) and the Shogunzawa siltstone (400 m). The Godo Member consists Seventeen rock samples from the Godo Member to the lower part of the Shogunzawa Member of the Iwadono Formation were obtained along two routes: the Hinohara (HN-HN5 and Loc.0) and Maekawa (MK-MK) routes (Figs. 2, 4, 5). Along the Hinohara route, the Godo Member is composed of gravel-rich, very coarse-grained sandstone. The Negishi Member consists of poorly sorted, silty, coarse-grained sandstone. The Shogunzawa Member is composed of diatomaceous sandy siltstone. In the Maekawa route, the Godo Member is composed of bedded conglomerate. The Negishi Member consists of silty mediumto fine-grained sandstone, generally finer than that along the Hinohara route. The Shogunzawa Member is composed of diatomaceous sandy siltstone. Rock samples were collected at one- to ten-meter stratigraphic intervals. In the laboratory, dried rock samples, each weighing 80 g, were disaggregated using sodium sulfate solution and naphtha, or by applying the NaTPB method (Hanken, 979) for very hard rocks. After maceration, each sample was wet-sieved through a 200-mesh screen (74 m opening) and oven-dried. The dried residues were split into aliquots with a sample splitter until each aliquot contained appropriately 200 foraminiferal specimens. Foraminiferal specimens were picked from residues sieved through a 5-mesh screen (25-m opening). Species identification was attempted on all specimens; however, poorly preserved specimens were classified as miscellaneous in each faunal list. In these samples, over 500 specimens were scanned to see if any age-diagnostic species were present. SEM photographs of some important species were taken by a field-emission type scanning electron microscope (JSM-6330F; JEOL Co. Ltd.).

3 78 HAYASHI ET AL. FIGURE 2Geologic map of the Iwadono Hills. FFormation; MMember. Legend:. diatomaceous siltstone; 2. sandy siltstone; 3. conglomerate consisting mainly of sandy siltstone boulders; 4. conglomerate consisting mainly of schistic boulders; 5. calcareous sandstone and conglomerate; 6. sandstone and conglomerate. Inset boxes and arrows show locations of Hinohara route (Fig. 4A) and Maekawa route (Fig. 4B). ASSIGNMENT OF KEY TAXA This section clarifies our use of names for four index species. Each species is discussed briefly along with key characteristics used in their recognition, and is figured in either Figure 6 or Figure 7. Globorotalia ichinosekiensis Takayanagi and Oda is similar to the southern Pacific species Globorotalia panda Jenkins in the possession of a strongly convex spiral side and a nearly flat umbilical side. It is distingushed from G. panda by its thinner wall and carina, and less strongly curved sutures on the spiral side. Globorotalia iwaiensis Takayanagi and Oda clearly differs from Globorotalia conoidea Walters by its thinner wall. Globorotalia cf. miozea conoidea Walters sensu Oda, 977 (not Globorotalia miozea conoidea Walters, 965) closely re-

4 MIOCENE PLANKTONIC FORAMINIFERAL BIOSTRATIGRAPHY 79 FIGURE 3Comparison of Miocene lithostratigraphic subdivisions proposed by different researchers in the Iwadono Hills. FIGURE 5Columnar sections of study routes in the Iwadono Hills. FIGURE 4Map showing locations of microfossil samples in the Maekawa and Hinohara routes (a part of :0000 map published by the Government Offices of Ranzan and Hatoyama Towns). sembles Globorotalia miozea Finlay in many aspects, but differs from the latter in possessing a more convex umbilical side and a more elongately equatorial profile. It is distinguished from Globorotalia conoidea by having a biconvex test instead of strongly umbilico-convex one. Globorotalia rikuchuensis Takayanagi and Oda is characterized by appearance of the comma-shaped aperture

5 80 HAYASHI ET AL. with a distinct rip, coarsely pitted surface, and strongly curved sutures in the spiral side. Its surface ultrastructure exhibits a cancellate network of pores surrounded by thickened walls. The overall appearance of the present species recalls Globorotalia bykovae (Aisenstat), from which it differs in presence of coarsely pitted surface. In addition, Jenkins and Srinivasan (986) stated that the mid-latitude species Globorotalia challengeri Srinivasan and Kennett from Southern Pacific is morphologically similar to G. rikuchuensis, and thus is synonymous to the latter. Accordingly, a more detailed taxonomic comparison between these two geographically well-isolated, but contemporaneous species is required. RESULTS Fifty-five foraminiferal taxa were found, and their states of preservation ranging from poor to moderate were recorded (Figs. 6, 7; Table ). The assemblage is characterized by abundant occurrences of such genera as Globigerina, Globigerinoides, Globigerinita, Globorotalia, and Globoquadrina. The genera Neogloboquadrina, Sphaeroidinellopsis, and Orbulina are common. Nine biohorizons are recognized as follows, in ascending order (Figs. 8, 9). Broken specimens belonging to the genus Praeorbulina were detected in sample HN of the Godo Member in the Hinohara route. Therefore, the last occurrence (LO) of Praeorbulina spp. is recognized between samples HN and HN2. The first occurrence (FO) of Orbulina suturalis Brönnimann also occurs in the same interval. The FOs of Globorotalia praemenardii Cushman and Stainforth and G. cf. miozea conoidea sensu Oda (977) are found between samples MK and MK2 of the Negishi Member in the Maekawa route. The FO of G. iwaiensis is present between samples HN and HN2 along the Hinohara route, and between MK and MK2 along the Maekawa route. The bases of the ranges show an abundant occurrence (AO) of Neogloboquadrina spp. at two horizons: the first (AO) is between samples Loc.0 and HN3 in the Hinohara route and samples MK2 and MK3 along the Maekawa route. The second (AO2) is between samples MK8 and MK9 along the Maekawa route. The FO of G. rikuchuensis is between samples HN3 and HN4 along the Hinohara route and between MK4 and MK5 along the Maekawa route. The LO of G. iwaiensis lies between samples Loc.0 and HN3 along the Hinohara route, and between samples MK7 and MK8 along the Maekawa route. Sample HN from the Hinohara route in the Godo Member is correlative with zone N.8 of Blow (969) because of Praeorbulina spp. is present, and O. suturalis is absent. Based on the presence of Globorotalia praemenardii Cushman and Stainforth (Kennett and Srinivasan, 983; Bolli and Saunders, 985) and the absence of Globigerina nepenthes Todd, the Negishi and the lower part of the Shogunzawa Members are restricted to the foraminiferal zones between N.0 and N.3. The lower boundary of each zone is difficult to determine, owing to few, discontinuous, or missing occurrences of Blow s (969) zone-diagnostic species. DISCUSSION The Foraminiferal Age of the Negishi Member The Negishi and the lower part of the Shogunzawa Members were correlated with the zones between N. 0 and N. 3 in the preceding section. In this interval, an agediagnostic species, Globorotalia peripheroacuta Blow and Banner, has been reported by K. Akimoto in Koike et al. (985). The occurrence of this species is restricted to zone N. 0 (Bolli and Saunders, 985), which supports the present result. In contrast, Matsumaru et al. (982) reported the co-occurrence of Praeorbulina and Orbulina from three localities in the Iwadono Hills, and correlated them with the lower part of zone N.9. Judging from their description, these localities are correlative to the Negishi Member in this study. However, the present authors disagree with their N.9 correlation because there are some contradictions in their fossil list, such as the occurrence of G. praemenardii in zones between N.0 and N.2 (Kennett and Srinivasan, 983; Bolli and Saunders, 985). The report of Praeorbulina spp. from the Negishi Member by Matsumaru et al. (982) possibly resulted from reworking of these specimens. Five of nine biohorizons detected in the present study, namely, the FO of G. iwaiensis, the AO of Neogloboquadrina spp., the FO of G. rikuchuensis, the AO2 of Neogloboquadrina spp., and the LO of G. iwaiensis, also have been recognized in the Karasuyama (Hayashi and Takahashi, 2000, 2002), Sendai (Shimamoto et al., 200), and Ichinoseki (Hayashi et al., 999) areas (Fig. ). These sequences have well-documented ages using diatoms or radioisotope data, and their numerical ages are shown in Table 2. Although some minor inverses in these orders are recognized between the above three areas, the age of each biohorizon shows little difference between the three areas. Therefore, these biohorizons essentially are synchronous in Japan, which allowed the use of each average age of these five biohorizons for dating the present section (Table 2). Moreover, two of the nine biohorizons, the FO of O. suturalis and the LO of Praeorbulina spp., are considered reliable for global correlation, and were reported to have ages of 5. Ma and 4.8 Ma, respectively (Berggren et al., 995). According to the age-thickness model based on FIGURE 6A-C: Globoquadrina dehiscens (Chapman, Parr and Collins), IGPS0867, sample MK2. 2A-C: Globorotalia praemenardii Cushman and Stainforth, IGPS08626, sample MK4. 3A-C: Globorotalia bykovae (Aisenstat), IGPS08627, sample MK2. 4A-C: Globorotalia praescitula Blow, IGPS08628, sample HN3. 5A-C: Globorotalia cf. miozea conoidea (Walters) sensu Oda (977), IGPS0868, sample MK2. 6A- C: Globorotalia ichinosekiensis Takayanagi and Oda, IGPS08629, sample MK3. 7A-C: Neogloboquadrina mayeri (Cushman and Ellisor), IGPS0869, sample MK2. 8A-C: Globorotalia iwaiensis Takayanagi and Oda, IGPS08630, sample MK7. 9: Orbulina suturalis Brönnimann, IGPS0863, sample MK2. 0A-C: Globorotalia peripheroronda Blow and Banner, IGPS08632, sample MK4. All scale bars represent 00 m. All figured specimens are deposited at the Institute of Geology and Paleontology, Graduate School of Science, Tohoku University (IGPS).

6 MIOCENE PLANKTONIC FORAMINIFERAL BIOSTRATIGRAPHY 8

7 82 HAYASHI ET AL.

8 MIOCENE PLANKTONIC FORAMINIFERAL BIOSTRATIGRAPHY 83 FIGURE 8Stratigraphic distributions of selected planktonic foraminifers from the Hinohara route. these seven biohorizons (Fig. 0), the Negishi Member is confined between 4.8 and.8 Ma. The sedimentation rate of the Negishi Member is remarkably slow (less than 9 mm/kyr). Further evidence for slow deposition is contained within the sandstone of this member, which commonly has glauconite pellets (Koike et al., 985). This slow depositional rate implies that terrigenous supply might have been decreased by: () increased distance from the paleo-shoreline; and/or (2) the sweeping action of bottom currents. Evidence for the former includes the fact that the underlying Godo Member contains a number of schistic gravels that originated from the westward Kanto Mountains. In contrast, the Negishi Member has few such gravels. It is certain that maximum uplift and denudation of the Kanto Mountains ceased before 4.8 Ma. This interval of slow accumulation rate ( Ma) correlates with glauconite beds in the Suzu (Yanagisawa, 999a) and Sendai areas (Yanagisawa, 999b; Shimamoto et al., 200). Correlation with Other Bio- and Chronostratigraphic Data Tables 3 and 4 show diatom and nannofossil stratigraphies based on the same samples. The diatom species Crucidenticula nicobarica is found in samples MK3 and MK7, Denticulopsis simonsenii in samples from MK7 to MK, and D. praedimorpha in samples from MK8 to MK. Thus, the interval encompassing samples MK7 and MK8 is confined between the FO of D. simonsenii (D47 of Yanagisawa and Akiba, 998) and the LO of C. nicobarica (D52 of Yanagisawa and Akiba, 998). The section above sample MK8 is restricted to the upper FIGURE 9Stratigraphic distributions of selected planktonic foraminifers from the Maekawa route. part of diatom zone NPD5B because of the occurrence of D. praedimorpha var. praedimorpha. A continuous occurrence of the calcareous nannofossil Cyclicargorithus floridanus and sparse occurrence of Sphenolithus heteromorphus are recognized in the Negishi Member. Thus, the Negishi Member is restricted to zones CN4 and CN5a of Okada and Bukry (980). In addition, a number of biostratigraphic and radiometric studies have been carried out by other workers. Diatom biostratigraphies were examined by Horiuchi and Yanagisawa (994) for the upper part of the sequence and by Y. Yanagisawa in Kurihara (999) for the Kamikarako Formation. According to their results, the Kamikarako Formation corresponds to zone NPD4A, and, in turn, the section between the Negishi Member of the Iwadono Formation and Ohashi Formation is assigned to diatom zones NPD5B and NPD5C of Yanagisawa and Akiba (998). Fission-track (F-T) ages of the present succession have been FIGURE 7 6: Globorotalia rikuchuensis Takayanagi and Oda, sample MK5. A-D: IGPS A-C: IGPS A-C: IGPS A-C: IGPS A-C: IGPS A-C: IGPS A-C: Globorotalia miozea Finlay, IGPS08633, sample MK2. Scale bars except for A represent 00 m. Scale bar for A represents 0 m. All figured specimens are deposited at the Institute of Geology and Paleontology, Graduate School of Science, Tohoku University (IGPS).

9 84 HAYASHI ET AL. TABLE Planktonic foraminifers from the Iwadono Hills. HN HN2 Loc0 HN3 HN4 NH5 MK MK2 MK3 MK4 MK5 MK6 MK7 MK8 MK9 MK0 Biorbulina biobata (d Orbigny) 3 Catapsydrax stainforthi Bolli, Loeblich & Tappan Globigerina angustiumbilicata Bolli Globigerina druryi Akers Globigerina falconensis Blow 3 5 Globigerina praebulloides Blow Globigerina pseudociperoensis Blow Globigerina weissi Saito Globigerina woodi Jenkins Globigerina woodi connecta Jenkins 2 Globigerinella obesa (Bolli) 4 2 Globigerinita glutinata (Egger) Globigerinita uvula (Ehrenberg) Globigerinoides bisphericus Todd 2 Globigerinoides bollii Blow Globigerinoides immaturus LeRoy Globigerinoides obliquus Bolli 2 2 Globigerinoides quadrilobatus (d Orbigny) Globigerinoides subquadratus (Brönnimann) Globigerinoides trilobus (Reuss) Globoquadrina altispira altispira (Cushman & Jarvis) Globoquadrina altispira globosa Bolli Globoquadrina baroemoenensis (LeRoy) Globoquadrina dihiscens (Chapman, Parr & Collins) Globoquadrina venezuelana (Hedberg) Globorotalia cf. adamantea Saito Globorotalia archeomenardii Bolli 2 Globorotalia birnageae Blow Globorotalia bykovae (Aisenstat) Globoratalia fimbriata (Brady) Globorotalia ichinosekiensis Takayanagi & Oda Globorotalia iwaiensis Takayangi & Oda

10 MIOCENE PLANKTONIC FORAMINIFERAL BIOSTRATIGRAPHY 85 TABLE Continued. HN HN2 Loc0 HN3 HN4 NH5 MK MK2 MK3 MK4 MK5 MK6 MK7 MK8 MK9 MK0 Globorotalia cf. iwaiensis Takayanagi & Oda 3 4 Globorotalia miozea Finlay 2 2 Globorotalia cf. miozea conoidea Walters sensu Oda (977) 5 3 Globorotalia peripheroronda Blow & Banner 2 Globorotalia cf. peripheroronda Blow & Banner Globorotalia praemenardii Cushman & Stainforth Globorotalia praescitula Blow Globorotalia cf. praescitula Blow 2 Globorotalia quinifalcata Saito & Maiya Globorotalia rikuchuensis Takayanagi & Oda (dextral) (sinistral) Globorotalia cf. rikuchuensis Takayanagi & Oda 2 Globorotalia scitula (Brady) 2 Globorotaloides spp. 2 Neogloboquadrina continuosa (Blow) (dextral) (sinistral) Neogloboquadrina mayeri (Cushman & Ellisor) (dextral) (sinistral) Neogloboquadrina pseudopachyderma (Cita, Premoli Silva, & Rossi) (dextral) (sinistral) 5 Orbulina suturalis Brönnimann Orbulina universa d Orbigny 4 2 Praeorbulina spp. 7 Sphaeroidinellopsis disjuncta (Finlay) Sphaeroidinellopsis seminulina (Schwager) Tenuitella clemeciae (Bermudez) Tenuitella minutissima (Bolli) Species Total number of identified specimens Miscellaneous Total number of specimens Planktonic forminiferal number Preservation (poor 5 good)

11 86 HAYASHI ET AL. determined at three tuff horizons (5.2.8 Ma for I-, Ma for I-8 and.9.6 Ma for I-2; Kasuya, 987). Apparently, these absolute ages are slightly older than those indicated by the biostratigraphic data given above, but the discrepancy between these two is minor and within analytical error. It is noteworthy that Kasuya s (987) dating methods are different from the zeta calibration approach recommended by the recent IUGS Subcomission (Hurford, 990). In summary, the relationship between foraminiferal and other biostratigraphic zonations is established in the Iwadono Hills (Fig. ). The relationship of the present sequence shows no significant contradiction with the agecorrelation chart of Japan (Saito, 999) based on the time scale of Berggren et al. (995). Similar inter-taxon relationships also have been recognized in Ichinoseki (Hayashi et al., 999) and Sendai (Shimamoto et al., 200). Therefore, the correlation chart of Saito (999) is useful for Japanese Miocene strata. CONCLUSIONS FIGURE 0Foraminiferal age plots of the lower part of the Iwadono Formation. Each numerical age is given in Table 2. Fifty-five planktonic foraminiferal species are recorded from the lower part of the Iwadono Formation in the Iwadono Hills. Nine biohorizons are recognized, in ascending order: last occurrence (LO) of Praeorbulina spp, first occurrence (FO) of Orbulina suturalis Brönnimann, FO of Globorotalia praemenardii Cushman and Stainforth, FO of Globorotalia cf. miozea conoidea Walters sensu Oda (977), FO of Globorotalia iwaiensis Takayanagi and Oda, the first abundant occurrence (AO) of Neogloboquadrina spp., FO of Globorotalia rikuchuensis Takayanagi and Oda, LO of G. iwaiensis, and second abundant occurrence (AO2) of Neogloboquadrina spp. The Godo Member of the Iwadono Formation is assigned to zone N. 8 of Blow (969), and, in turn, the Negishi and the lower part of the Shogunzawa Members of the formation are confined between zones N.8 and N.3. An accurate age-thickness model of the sequence is produced by correlating seven of the nine biohorizons with other sequences. Accordingly, a remarkably slow sedimentation rate (9 mm/kyr) in the Negishi Member is recognized and can be correlated with several Japanese areas (4.8.8 Ma). Furthermore, a relationship is established between foraminiferal ages and other biostratigraphic zonations, such as diatoms and calcareous nannofossils, of the present area. The inter-taxon relationship established in the present study is in good agreement with the age-correlation chart of Saito (999) for Neogene sediments of Japan. ACKNOWLEDGEMENTS The present authors would like to express their deep appreciation to Tsunemasa Saito for his helpful suggestions on the classification of planktonic foraminifera. They sincerely wish to extend their gratitude to Kei Mori of Tohoku University, Hiroshi Noda and Kenshiro Ogasawara of University of Tsukuba, and Masaki Takahashi of the Geological Survey of Japan for giving them various supports throughout the present work. Their appreciation also is extended to Chen Zhong Qiang of Tohoku University for his reading of the present manuscript. TABLE 2Comparison of numerical ages (Ma) of planktonic foraminiferal biohorizons from three marine sequences in Japan with data of Berggren et al. (995). Berggren et al. (995) Ichinoseki Sendai Karasuyama Average LO G. iwaiensis AO2 Neogloboquadrina spp. FO G. rikuchuensis AO Neogloboquadrina spp. FO G. iwaiensis LO Praeorbulina spp. FO O. suturalis

12 MIOCENE PLANKTONIC FORAMINIFERAL BIOSTRATIGRAPHY 87 TABLE 3Diatoms from the Iwadono Hills. MK3 MK6 MK7 MK8 MK9 MK0 MK Actinocyclus curvatulus Janisch Actinocyclus ehrenbergii Ralfs Actinocyclus ellipticus Grunow Actinocyclus ingens f. planus Whiting & Schrader 3 7 Actinocyclus ingens f. ingens Rattray Actinocyclus ingens f. nodus Baldauf Actinoptychus senarius (Ehrenberg) Ehrenberg Azpeitia tabularis (Grunow) Fryxell & Sims Azpeitia vetustissima (Pantocsek) Sims Cavitatus jouseana Sheshuhova-Poretzkaya Cavitatus miocenica Schrader Cavitatus lanceolatus Akiba & Hiramatsu Cavitatus spp. Cocconeis californica Grunow Cocconeis cf. scutellum Ehrenberg Cocconeis costata Gregory Cocconeis vitrea Brun Cocconeis spp. Coscinodiscus marginatus Ehrenberg Crucidenticula nicobarica (Grunow) Akiba & Yanagisawa Crucidenticula punctata (Schrader) Akiba & Yanagisawa Cymatosira debyi Tempe re & Brun Delphineis surirella (Ehrenberg) Andrews Delphineis spp. Denticulopsis praedimorpha var. praedimorpha Akiba ex Barron Denticulopsis praedimorpha var. minor Yanagisawa & Akiba (closed copula) Denticulopsis simonsenii Yanagisawa & Akiba Dimerogramma spp. Diploneis bombus Ehrenberg Diploneis spp. Eucampia spp. Goniothecium decoratum Brun Grammatophora angulosa Ehrenberg Grammatophora cf. oceanica (Ehrenberg) Grunow Grammatophora spp. Ikebea tenuis (Brun) Akiba Navicula cf. hennedyii Smith Mediaria splendida Sheshukova-Poretzkaya Navicula spp. Nitzschia cf. cylindrica Burckle Nitzschia spp. Paralia sulcata (Ehrenberg) Cleve Proboscia barboi (Brun) Jordan & Priddle Rhaphoneis spp. Rhizosolenia hebetata f. hiemalis Gran Rhizosolenia miocenica Schrader Rhizosolenia spp. Stephanopyxis turris (Greville & Arnott) Ralfs Stephanopyxis spp. Thalassionema hirosakiensis (Kanaya) Schrader Thalassionema nitzschioides (Grunow) H. & M. Peragallo Thalassionema robusta Schrader Thalassiosira flexuosa var. tenella Tanimura Thalassiosira spp. Thalassiothrix longissima Cleve & Grunow Trochosira spinosa Kitton Resting spore Total valve Abundance Preservation rare poor 7 rare poor 4 25 rare poor 3 06 common poor common poor common poor common poor

13 88 HAYASHI ET AL. TABLE 4Calcareous nannofossils from the Iwadono Hills. MK MK2 MK3 MK4 MK5 MK6 MK7 MK8 HN HN2 HN3 HN4 HN5 Calcidiscus leptoporus (Murray & Blackman) Loeblich & Tappan 4 Calcidiscus macintyrei (Bukry & Bramlette) Loeblich & Tappan Calcidiscus premacintyrei Theodoridis 2 Coccolithus miopelagicus Bukry Coccolithus pelagicus (Wallich) Schiller [3 ] Coccolithus pelagicus (Wallich) Schiller [0 0.9 ] 2 Coccolithus pelagicus (Wallich) Schiller [9 9.9 ] Coccolithus pelagicus (Wallich) Schiller [8 8.9 ] Coccolithus pelagicus (Wallich) Schiller [7 7.9 ] Coccolithus pelagicus (Wallich) Schiller [6 6.9 ] Coccolithus pelagicus (Wallich) Schiller [5 5.9 ] Coccolithus pelagicus (Wallich) Schiller [4 4.9 ] 2 Coronocyclus nitescens (Kamptner) Bramlette & Wilcoxon 2 Cyclicargolithus abisectus (Müller) Wise 2 2 Cyclicargolithus aff. abisectus (Müller) Wise Cyclicargolithus floridanus (Roth & Hay in Hay et al.) Bukry Cyclicargolithus spp. Dictyococcites antarcticus Haq Dictyococcites perplexus Burns Dictyococcites productus (Kamptner) Backman [round] Dictyococcites scrippsae Bukry & Percival Dictyococcites sp.-b [small: 2 4 ] Dictyococcites sp.-c [v. small: 2 ] Dictyococcites spp Discoaster deflandrei Bramlette & Riedel Discoaster aff. kugleri Martini & Bramlette Discoaster cf. musicus Stradner 2 Discoaster variabilis Martini & Bramlette Discoaster spp Discolithina discopora Takayama Discolithina multipora (Kamptner) Roth 3 Discolithina spp. 2 3 Helicosphaera carteri (Wallich) Kamptner Helicosphaera euphratis Haq Helicosphaera aff. intermedia Martini Helicosphaera minuta Müller Helicosphaera spp Reticulofenestra ampla Sato, Kameo & Takayama 2 2 Reticulofenestra aff. ampla Sato, Kameo & Takayama 4 Reticulofenestra gelida (Geitzenauer) Backman [0 2 ] 2 2 Reticulofenestra gelida (Geitzenauer) Backman [9 9.9 ] 2 2 Reticulofenestra gelida (Geitzenauer) Backman [8 8.9 ]

14 MIOCENE PLANKTONIC FORAMINIFERAL BIOSTRATIGRAPHY 89 TABLE 4Continued. MK MK2 MK3 MK4 MK5 MK6 MK7 MK8 HN HN2 HN3 HN4 HN5 Reticulofenestra gelida (Geitzenauer) Backman [7 7.9 ] Reticulofenestra gelida (Geitzenauer) Backman [6 6.9 ] Reticulofenestra haqii Backman Reticulofenestra minuta Roth Reticulofenestra minutula (Gartner) Haq & Berggren Reticulofenestra pseudoumbilica (Gartner) Gartner [3 ] 5 Reticulofenestra pseudoumbilica (Gartner) Gartner [0 2 ] Reticulofenestra pseudoumbilica (Gartner) Gartner [9 9.9 ] Reticulofenestra pseudoumbilica (Gartner) Gartner [8 8.9 ] Reticulofenestra pseudoumbilica (Gartner) Gartner [7 7.9 ] Reticulofenestra pseudoumbilica (Gartner) Gartner [6 6.9 ] Reticulofenestra pseudoumbilica (Gart.) Gartner [small: 5 ] Reticulofenestra spp Sphenolithus abies Deflandre in Deflandre & Fert Sphenolithus aff. abies Deflandre in Deflandre & Fert Sphenolithus cf. abies Deflandre in Deflandre & Fert 2 Sphenolithus compactus Backman Sphenolithus heteromorphus Deflandre 5 Sphenolithus aff. heteromorhpus Deflandre Sphenolithus moriformis (Brön. & Strad.) Bramlette & Wilcoxon Sphenolithus spp Syracosphaera spp. 2 Umbilicosphaera cf. jafari Müller 6 2 Umbilicosphaera aff. jafari Müller 2 Umbilicosphaera rotula (Kamptner) Varol 4 Umbilicosphaera sibogae (Weber-van Bosse) Gaarder Umbilicosphaera spp Total number of identified fossils Coccosphere Abundance F R R F F R R F VVR R VR VR F Preservation P P VP P P VP VP VP VP P P P P Abundance: A, abundant; C, common; F, few; R, rare; VR, very rare; VVR, very, very rare; No, barren. Preservation: G, good; M, moderate; P, poor; VP, very poor.

15 90 HAYASHI ET AL. FIGURE Comparison of bio- and chronostratigraphies in the Iwadono Hills with the correlation chart for Japanese Neogene sediments (Saito, 999). Analytical errors of fission-track (F-T) ages are represented by two standard deviations. Note that the F-T ages are older than biostratigraphic ages within analytical errors. REFERENCES BERGGREN, W.A., KENT, D.V., SWISHER, C.C., III, and AUBRY, M.P., 995, A revised Cenozoic geochronology and chronostratigraphy: in Berggren, W.A., Kent, D.V., Aubry, M.-P., and Hardenbol, J., eds., Geochronology, Time Scales and Global Stratigraphic Correlation: SEPM Special Publication, No. 54, SEPM (Society for Sedimentary Geology), Tulsa, p BLOW, W.H., 969, Late Middle Eocene to Recent planktonic foraminiferal biostratigraphy: in Brönnimann, P., and Renz, H.T., eds., Proceedings of First International Conference on Planktonic Microfossils, Geneva, 967, Leiden, No., p BOLLI, H.M., and SAUNDERS, J.B., 985, Oligocene to Holocene low latitude planktonic foraminifera: in Bolli, H.M., Saunders, J.B., and Pearch-Nielsen, K., eds., Plankton Stratigraphy: Cambridge University Press, Cambridge, p HANKEN, N.-M., 979, The use of sodium tetraphenylborate and sodium chloride in the extraction of fossils from shales: Journal of Paleontology, v. 53, p HAYASHI, H., YANAGISAWA, Y., SUZUKI, N., TANAKA, Y., and SAITO, T., 999, Integrated microbiostratigraphy of the Middle Miocene sequences in the Shimokurosawa district, Ichinoseki City, Iwate Prefecture, Northeast Japan: Journal of the Geological Society of Japan, v. 05, p HAYASHI, H., and TAKAHASHI, M., 2000, Planktonic foraminiferal biostratigraphy of the middle part of the Miocene Arakawa Group in the Karasuyama area, Tochigi Prefecture, central Japan: Journal of the Geological Society of Japan, v. 06, p HAYASHI, H., and TAKAHASHI, M., 2002, Planktonic foraminiferal biostratigraphy of the Miocene Arakawa Group in central Japan: in Tsuchi, R., ed., Proceedings of the 7th International Congress on Pacific Neogene Stratigraphy: Revista Mexicana de Ciencias Geologicas, v. 9, no. 3, p HORIUCHI, S., and YANAGISAWA, Y., 994, Diatom biostratigraphy of the Miocene sequence of Iwadono Hill, Saitama Prefecture, central Japan: Bulletin of the Geological Survey of Japan, v. 45, p HURFORD, A.J., 990, Standardization of fission track dating calibration: Recommendation by the Fission Track Working Group of the IUGS Subcommission of Geochronology: Chemical Geology, v. 80, p JENKINS, D.G., and SRINIVASAN, M.S., 986, Cenozoic planktonic foraminifers from the equator to the sub-antarctic of the southwest Pacific: Initial Reports of the Deep Sea Drilling Project, v. 90, p KASUYA, M., 987, Comparative study of Miocene fission-track chronology and magneto-biochronology: Science Reports of the Tohoku University, second Series (Geology), v. 58, p KENNETT, J.P., and SRINIVASAN, M.S., 983, Neogene Planktonic Foraminifera, a Phylogenetic Atlas: Hutchinson Ross Publication Company, Stroudsburg, 265 p. KOIKE, M., TAKEI, K., SHIMONO, T., MACHIDA, J., AKIMOTO, K., HA- SHIYA, I., YOSHINO, H., and HIRAKOSO, S., 985, Miocene formations of Iwadono Hills: Journal of the Geological Society of Japan, v. 9, p KURIHARA, Y., 999, Middle Miocene deep-water molluscs from the Arakawa Formation in the Iwadono Hills area, Saitama Prefecture, central Japan: Paleontological Research, v. 3, p MAIYA, S., 978, Late Cenozoic planktonic foraminiferal biostratigraphy of the oil-field region of Northeast Japan: in Cenozoic Geology of Japan, Professor N. Ikebe Memorial Volume: Commemorative Association of Professor N. Ikebe s Retirement, Osaka, p MAJIMA, R., 989, Neogene stratigraphy along the Arakawa River near Yorii, and of the Ogawa Basin, Hiki Hills, and Iwadono Hills, central Saitama Prefecture, central Japan: Geoscience Reports of Shizuoka University, no. 5, p. 24. MATSUMARU, K., MATSUO, Y., and KISHI, R., 982, Miocene foraminifera from the Chichibu Basin and the south Hiki Hill, Saitama Prefecture, Japan: Journal of Saitama University, Faculty of Education [Mathematics and Natural Science], v. 3, p ODA, M., 977, Planktonic foraminiferal biostratigraphy of the Late Cenozoic sedimentary sequence, central Honshu, Japan: Science Reports of the Tohoku University, second Series (Geology), v. 48, p. 76. ODA, M., HASEGAWA, S., HONDA, N., MARUYAMA, T., and FUNAYAMA, M., 984, Integrated biostratigraphy of planktonic foraminifera, calcareous nannofossils, radiolarians and diatoms of middle and

16 MIOCENE PLANKTONIC FORAMINIFERAL BIOSTRATIGRAPHY 9 upper Miocene sequences of central and northeast Honshu, Japan: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 46, p OKADA, H., and BUKRY, D., 980, Supplementary modification and introduction of code numbers to the low-latitude coccolith biostratigraphic zonation (Bukry, 973; 975): Marine Micropaleontology, v. 5, p SAITO, T., 963, Miocene planktonic foraminifera from Honshu, Japan: Science Reports of the Tohoku University, second Series (Geology), v. 35, p SAITO, T., 999, Revision of Cenozoic magnetostratigraphy and the calibration of planktonic microfossil biostratigraphy of Japan against this new time scale: Journal of Japanese Association for Petroleum Technology, v. 64, p SHIMAMOTO, M., OTA, S., HAYASHI, H., SASAKI, O., and SAITO, T., 200, Planktonic foraminiferal biostratigraphy of the Miocene Hatatate Formation in the southwestern part of Sendai City, Northeast Japan: Journal of the Geological Society of Japan, v. 07, p TAKAYANAGI, Y., TAKAYAMA, T., SAKAI, T., ODA, M., and KITAZATO, H., 976, Microbiostratigraphy of some Middle Miocene sequences in northern Japan: in Takayanagi, Y., and Saito, T., eds., Progress in Micropaleontology: Micropaleontology Press, New York, p WALTERS, R., 965, The Globorotalia zealandica and G. miozea lineages: New Zealand Journal of Geology and Geophysics, v. 8, p YANAGISAWA, Y., 999a, Diatom biostratigraphy of the Miocene sequence in the Suzu area, Noto Peninsula, Ishikawa Prefecture, central Japan: Bulletin of the Geological Survey of Japan, v. 50, p YANAGISAWA, Y., 999b, Diatom biostratigraphy of the Miocene Hatatate Formation, Sendai City, Miyagi Prefecture, Japan: Bulletin of the Geological Survey of Japan, v. 50, p YANAGISAWA, Y., and AKIBA, F., 998, Refined Neogene diatom biostratigraphy for the northwest Pacific around Japan, with an introduction of code numbers for selected diatom biohorizons: Journal of the Geological Society of Japan, v. 04, p ACCEPTED SEPTEMBER 3, 2002