Diatom Biostratigraphy of the Kazusa Group, Boso Peninsula ... - J-Stage

第 四 紀 研 究(The

Quaternary

Research)

Diatom

(1)

Peninsula,

V. Cherepanova*1, Kumai

Feb.

p. 1-10

Biostratigraphy Boso

Marina

41

Vladimir

of the

Kazusa

Honshu,

Japan

S. Pushkar*2,

Hisao*3 and Koizumi

Nadya

2002

Group,

Razjigaeva*2,

Itaru*4

Since the Kazusa Group is considered as the candidate to stratotype of the EarlyMiddle Pleistocene boundary in the Pacific region, it is urgent to investigate the Kokumoto Formation as a member formation of the Kazusa Group. The paleomagnetic boundary between C1r (Matuyama) and C1n (Brunhes) is recognized at the ash marker bed "Shirao" in the middle part of the Kokumoto Formation. The diatoms from the Otadai, Umegase, Kokumoto, Kakinokidai, and Chonan Formations along the Yoro River, in the middle part of the Boso Peninsula, about 80km southeast of Tokyo, have been studied. The marine sequences are dominated by shelf-water Paralia sulcata (Ehrenberg) Kutzing indicating shallow water sedimentation. The seven diatom datum levels (biohorizons) allow precise correlation in the transitional region of the northwest Pacific Ocean. Among them the four biohorizons appear synchronous near the Early-Middle Pleistocene boundary in the Kokumoto Formation: LO (last occurrence) of Actinocyclus oculatus Jouse and LO of Rhizosolenia matuyamai Burckle at the lower part of the boundary, and LO of Nitzschia fossilis (Frenguelli) Kanaya and LO of Nitzschia reinholdii Kanaya at the upper part of the boundary. The regressions were recognized twice at the basal part of C1r.1n (Jaramillo) in the Otadai Formation and across the Clr and Cmn (Early-Middle Pleistocene boundary) in the Kokumoto Formation. The Kokumoto Formation is suitable as the candidate to stratotype of the Lower-Middle Pleistocene boundary of the Pacific region based on diatom biostratigraphy. Key Words:

I.

stratotype, Early-Middle diatom, biohorizon

Introduction

The Boso Peninsula is in a southern part of the Kanto plain which is located on the joint part of the Honshu Island (Fig. 1), and is one of the standard section for the Pleistocene in Japan (Kumai, 1991). The plain was a fore-arc submarine basin and the sedimentary deposits were formed up to several thousand meters thick during Late Pliocene to Middle Pleistocene, and then the basin lifted up in Late Pleis-

Pleistocene

boundary,

Kokumoto

Formation,

tocene. The Quaternary sequences in the area are more than 3,500m thick. The fast rates of sediment accumulation (up to 4.9m/1,000yrs) provide a high resolution stratigraphy for the detailed reconstruction of climatic changes and sea level oscillations (e.g. Pickering et al., 1999). The Kazusa fore-arc submarine basin have been well researched by the scientists in the various fields. Concerning biostratigraphy, we have biostratigraphy by foraminifera (Aoki, 1964; Oda, 1977; Igarashi, 1994), nannofossil

Received February 10, 2001. Accepted July 28, 2001. *1 Institute of Biology and Soil Science FEB RUS . 7, Radio Str., Vladivostok, 690041, Russia. *2 Pacific Institute of Geography FEB RUS . 7, Radio Str., Vladivostok, 690041, Russia. *3 Department of Geosciences , Osaka City University. 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka, 558-8585, Japan. *4

2

M. V. Cherepanova

Fig.

1

Map and

showing the

present

the

locations

annual

surface

of the

(Sato and Takayama, 1988), mollusca (Ujihara, 1986), and pollen (Onishi, 1969) of the Kazusa Group, but not by diatoms yet. There are several problems about sedimentology and age determination of some marker ash layers. One of them is about absence of facial changes in C1n (Brunhes)-C1r (Matuyama) boundary, and was discussed during international symposium on the Lower-Middle and Middle-Upper Pleistocene boundaries in Chiba in 1992 and 1996. The purpose of this paper is to determine the chronostratigraphic position of middle to upper part of the Kazusa Group based on diatom biostratigraphy, and to reconstruct paleoclimatic changes and paleoceanographic environments in the Early-Middle Pleistocene. II.

Material

and

methods

materials

temperatures

of study

The original samples were collected by N. Razjigaeva herself in the field excursion during the international symposium on Quaternary environmental change in October 1997, and the samples collected by T. Takayama for his cal-

et al.

Feb.

incorporated and

water

in the

present

2002

paper,

masses

careous nannofossils (Sato and Takayama, 1988) were placed at our disposal. A total of 35 samples were selected for the purpose of the present study. Figures 2 and 3 show respectively the geographic and stratigraphic positions of the selected samples. Three samples were taken from the lower part of the Otadai Formation at Kiwadahata, but other samples came from the area along the Yoro River (Fig. 2). The samples from the upper part of the Otadai Formation, assigned to Clr.ln (Jaramillo), were taken at the site near the Mukaiyama Tunnel (Kuzufuji Site) on the left bank of the Yoro River. 12 samples including "Shirao" ash layer were taken from the outcrop in the middle course of the Yoro River, which the area is proposed as the candidate stratotype of the Lower-Middle Pleistocene boundary (C1r (Matuyama)-C1n (Brunhes) boundary). All samples have been treated with hydrogen peroxide and hydrochloric acid. The slide preparation procedures were according to those described by Koizumi (1968). 200-300

2002年2月

Diatom

3

of Boso

occurrences by Koizumi and Tanimura (1985), which were calibrated according to BKFV 85 (Berggren et al., 1985), are updated according to the geomagnetic polarity time scale of CK 95 (Cantle and Kent, 1995) (Table 3). III.

Fig. 2

Geographic

positrons of diatom

samples from

Lithology

of formations

The Kazusa Group shows an overall transgressive to regressive cycle from deep-marine basin floor to shallow-marine/coastal environments (Katsura, 1984; Ito, 1996). The Kazusa Group along the Yoro River, from the Otadai Formation to the Chonan Formation, consists mainly of the alternation of sandy mud and sand. Sand beds are dominant in the Umegase Formation, but the upper and lower parts of the Kokumoto Formation consist of muddy layers (Fig. 3). The Otadai Formation is intercalated with 27 marker tephra layers, the Umegase Formation with 11, the Kokumoto with 7, the Kakinokidai with 4, and the Chonan with 3 (e.g. Mitsunashi et al., 1979). The section assigned to the Lower-Middle Pleistocene boundary is located in the middle lower part of the Kokumoto Formation. This section is composed of sandy silt with several thinly-intercalating layers of fine grained sand. Based on sedimentary facies and benthic foraminifera, the Otadai and Umegase Formations are considered to have formed as the submarine fan on the upper and middle slope, and that the Kokumoto Formation to have deposited on the shelf floor to upper slope (e.g. Katsura, 1984).

the Yoro River in Boso Peninsula The

1/50,000 topographical

Geographical

Survey

map

IV.

"Otaki" of

Institute was used as the

basal map

specimens were usually counted, though sometimes less than 100 specimens. Occurrences of the diatoms regarded as biostratigraphically useful ones are shown in Figure 3 and Table 3 with Td' (diatom temperature) values and the percentages of ecologically subdivided assemblages and extinct assemblege in diatom flora. The marine oceanic diatom species are very poor through all the samples. Absolute ages for the first (FO) and last (LO)

Tephrostratigraphy

The studied diatom samples are distributed between Kdl marker ash layer in the Kiwada Formation and Ch3 ash layer in the Chonan Formation (Fig. 3). The

O7

marker

ash

and

Watanabe,

(Danhara tributed in the

and Osaka

Group

with

1.00±0.04Ma

1994) with

is pink

wide ash

FT

age 0.92±0.52Ma

ash

layer

dislayer

et al., 1996).

Ages

of the

U8

marker

1.47±0.36Ma

to 0.96±0.31Ma dates

dated

is correlated

(Yoshikawa

from

layer

for

the

(Suzuki (Hara

U6

ash

layer

and

range

and

Sugihara,

range

Nirei,

wide 1983)

1990).

from 0.98±0.16

FT

4

M. V. Cherepanova

Fig. 3

Stratigraphic

positions

occurrences

of

of important

diatom

species,

samples

et al.

diatom

from

Feb.

the

temperature

Yoro

River

in

Boso

(Td') curve, and diatom

2002

Peninsula, assemblages

in the section R. m: Rhizosolenia Actinocyclus Nitzschia

(Danhara

Ma  tephra Ma)

1996).

age

distributed with

±0.07Ma)

(Suzuki

(FT

1994)

in of and

this

The of

wide

the

Kanaya,

Watanabe,

Azuki

the

Sugihara,1983;

layer Group;

is

in

and (FT the

the

Kyushu

Yoshikawa

age

and

part

cor-

of 0.87

of

Island al.,

Ku5

Shirao

volcanic track

(Brunhes) The

boundary age

ash

bed

(Hara

The

given

method

(0.52±0.12Ma)

in

1983;

age by

is

C1r

the

is dated

Sugihara,

1994).

the

layers

Formation and

dating to

F. d :

marker

Watanabe,

correlated

N. r :

Jouse,

explanation.

Kokubo

ash

A. o :

Priddle,

Brun)

(Suzuki and

and

and

the

Danhara

fission

volcanic

Kanaya,

and

2 for further

0.81-0.87Ma

is

et

Ku6

lower

Jordan

(Tempere

See Table

The ash

(Frenguelli)

(Jouse)

nidulus

Medlin and Sims.

to 0.81±0.17

Honshu

ash

Osaka

tephra

T. n: Thalassiosira

marker

fossilis

curvirostris

to 0.87±0.17

U1

1.06±0.4 in

N.f: Nitzschia

P. c: Proboscia

doliolus (Wallich)

et al.,  1996).

layer is

related

source

and

(Hara

Burckle,

Jouse,

reinholdii

Fragilariopsis

Ma

matuyamai

oculatus

for a

the

zircon

0.72±0.17Ma,

(Matuyama)-Cln

et al., 1996). given

to Ch3

by

Hara

2002年2月

etal. ones; and

Diatom

(1996)

is younger

0.91±0.21Ma 0.63±0.22Ma

than by

by

previouly

Tokuhashi Suzuki

given

etal. and

(1983)

5

of Boso

Table

1

Species cold

composition

assigned

to warm

or

water

Sugihara

(1983).

V.

Magnetostratigraphy

The paleomagnetic boundary between Cir (Matuyama) and Cmn (Brunhes) in the Kazusa Group along the Yoro River was first recognized by Nakagawa et al. (1969). It is situated 30 m below the key tuff Ku2 in the upper part of the Kokumoto Formation. The boundary of Cir-Cin at the Yanagawa section, a branch of the Yoro River, was found by Niitsuma (1971) and it is located at the interval between 1 and 2.4 m below the key ash bed TNTT (Niitsuma and Fujii, 1984) in the middle part of the Kokumoto Formation. The stratigraphical position of the bounday was improved to 2m below the TNTT by detailed paleomagnetic work of Okada and Niitsuma (1989). On the other hand, the white colored and fine grained ash layer, "Shirao" volcanic ash bed, was found about 30 m below the Ku2 (Satoguchi, 1996). Magnetic directions during the interval between 1 m and 1.5m below the Shirao volcanic ash bed continuously change from the reversal to normal. Aida et al. (1996) interpretated the change of this magnetic direction can be correlated to the boundary between Cir (Matuyama) and Cmn (Brunhes). VI.

Diatom

assemblages

1. General features Fresh-water species Epithemia adnata (Kutzing) Brebisson occurr predominantly at the base of Clr.ln (Jaramillo). Sublittoral species Paralia sulcata (Ehrenberg) Kutzing are dominant at two horizons beneath Clr.ln (Jaramillo) and A ctinoptychus senarius Ehrenberg, Grammatophora spp., and Paralia sulcata (Ehrenberg) Kutzing across the boundary of Cir (Matuyama) and Cmn (Brunhes). The upper horizon in the Kokumoto Formation is subdivided into lower part where Cyclotella striata (Kutzing) Grunow, Podosira stelliger (Bailey) Mann and Rhopalodia sp. dominate, and upper part where Diploneis smithii (Brebisson) Cleve and Melosira monilif ormis (Muller) Agardh

dominate. The fluctuations in occurrences of the oceanic assemblage generally show a similar, trend with the paleotemperature curve of Td' (diatom temperature) values (original Td does not include extinct species; Koizumi and Kanaya,1976), but the abundance of cold water species Neodenticula seminae (Simonsen and Kanaya) Akiba and Yanagisawa abruptly rises at the uppermost part of the section. Extinct species Actinocylus ingens Rattray dominates between Clr.ln (Jaramillo) and Cmn (Brunhes) during the middle part of the Umegase Formation, when Td' values are lower. It indicates that A. ingens Rattray was reworked by the lowering of sea level "regression". Td' values were calculated for each sample on the basis of the species assigned to warm or cold water (Tables 1 and 2). Although original Td does not include extinct species (Kanaya and Koizumi, 1966), they are shown as Td' because the calculation involved extinct species. The diatom temperature curve obtained for the Boso section shows a zigzag pattern, rising four times and dropping three times, and fluctuations (Fig. 3). Lower Td'

6

Table

M. V. Cherepanova

2 Occurrences is

the

diatom

See

Table

values are (Jaramillo) the Otadai uppermost Kokumoto (Brunhes) fluctuations

of frequency

of

assemblages

1 for further

important

species,

warm from

water the

Yoro

diatom

temperature

species River

et al.

in

Feb.

(Td'=[Xw/(Xc+Xw)]×100,

and

Xc

is

Boso

Peninsula

that

of

cold

water

species)

2002

where

Xw

values,

and

explanation.

recognized in and above C1r.1n and across the boundary between and Umegase Formations, in the part of C1r (Matuyama) of the Formation, and also in Cmn of the Chonan Formation. The with sorter duration which sug-

gest unstable marine condition is most characteristic across the boundary of C1r (Matuyama) and Cmn (Brunhes). Td' curve for the Choshi section (Koizumi and Kanaya, 1976) similarly shows a zigzag pattern. 2. Diatom biostratigraphic events (biohorizons)

2002年2月

Diatom

Table 3

Table

comparing

(biohorizons) postulated

FO:

Fig. 4

first

Temporal

from in middle

occurrence,

and spatial

Pacific diatom

datum

FO: first occurrence,

LCO:

the the

absolute Yoro

latitudes

last

distribution

of Boso

7

ages River

samples

common

of in

important Boso

of northwest

occurrence,

LO:

of both the subtropical

levels (biohorizons) LO: last occurrence,

The biohorizons by seven for selected species in the Boso section are compared with those in the land-based Choshi section (Koizumi and Kanaya, 1976) and in subtropical to subarctic deep waters in the North Pacific region (Koizumi, 1986) as shown in Table 3 and Figure 4. Koizumi and Kanaya (1976) note that, confirmed by paleomagnetic time scale, six diatom biohorizons in Pleistocene, occurred more or less simultaneously over more than one region in the North Pacific Ocean. And Koizumi (1986) showed later the chronological occurrences of both subtropical and subarctic North Pacific diatom biohorizons in the DSDP's Pliocene and

diatom

Peninsula

in the northwest

events

with

those

Pacific region

last

occurrence

Pacific and the subarctic Pacific Ocean

LCO: last common

occurrence

Pleistocene section with magnetostratigraphy. Subtropical diatom species are recognized also in the subarctic region and their first (FO) and last (LO) occurrences are usually isochronous. However, most subarctic North Pacific species do not penetrate into the subtropical region due to clockwise surface circulation in northwestern Pacific Ocean. Most diatom events represent LO rather than FO in the Pleistocene section, because all species of the modern marine diatoms had appeared prior to the Plio -Pleistocene boundary . The LO of subtropical Nitzschia fossilis (Frenguelli) Kanaya and Nitzschia reinholdii Kanaya are almost isochronous. However, the

8

M. V. Cherepanova

LO of Rhizosolenia matuyamai Burckle and LCO (last common occurrence) of Fragilariopsis doliolus (Wallich) Medlin and Sims show a greater degree of diachroneity in the landbased sections and DSDP sections in higher latitudes (Table 3 and Fig. 4). The diachroneity is considered due to several causes : differences in paleogeographic and/or sedimentary environments in the obviously neritic Boso and Choshi sections, and some human errors in scanning the slides and/or identifing species. The LO of Actinocyclus oculatus Jouse is isochronous in the transitional, including landbased sections, and subarctic regions. However, both Proboscia curvirostris (Jouse) Jordan and Priddle and Thalassiosira nidulus (Tempere and Brun) Jouse became extinct earlier in the land-based neritic Boso and Choshi sections by the difference of paleogeographic and/or sedimentary environments. VII.

Conclusion

Our results obtained from diatom analysis support both stratigraphic and paleogeographic reconstructions provided by Internal Research Group for the Lower-Middle Pleistocene Boundary, Internal Research Group for the Lower-Middle, Middle-Upper Pleistocene Boundary Japan Association for Quaternary Research (1996). The four biohorizons in the Boso section along the Yoro River are apparently synchronous with biohorizons established in northwest Pacific Ocean (Table 3 and Fig. 4): LO of Actinocyclus oculatus Jouse at 1.0 Ma in the uppermost part of the Otadai Formation, LO of Rhizosolenia matuyamai Burckle at 0.9 Ma in the middle part of the Umegase Formation, LO of Nitzschia fossilis (Frenguelli) Kanaya at 0.59 Ma in the middle part of the Kokumoto Formation, and LO of Nitzschia reinholdii Kanaya at 0.5 Ma in the Chonan For mation. The three biohorizons, FO of Rhizosolenia matuyamai Burckle, LO of Pro boscia curvirostris (Jouse) Jordan and Priddle, and LO of Thalassiosira nidulus (Tempere and Brun) Jouse, reveal diachronous distribution in the land-based Boso section and also Choshi section (Koizumi and Kanaya, 1976) due to the difference of paleogeographic and/or sedimentary environments, and the effect of a "climatic

et al.

Feb.

2002

wedge" (Koizumi, 1986). Diatom biostratigraphy based on the records found in the land-based Boso and also Choshi sections serves as a guide in correlating both oceanic and land-based sections of varying lithologies in the North Pacific Ocean (Table 3 and Fig. 4). And the fact that the land-based sections in the Pacific coast of central Japan yield planktonic assemblages of mixed water masses give critical evidences for the biological events that took place in cold and warm water masses in the North Pacific Ocean during latest part of the Cenozoic. It is similarly helpful in interpreting polarity records taken from sedimentary sections, because it provides additional and paleontological control which enables us to identify sequences of events and epochs from the sections which are not sufficiently continuous. The Kokumoto Formation is suitable as the candidate to stratotype of the Middle-Upper Pleistocene boundary of the Pacific Region. Acknowledgements

The series of the samples from the Boso section were collected by Emeritus Professor T. Takayama of the Kanazawa University. Dr. O. Kazaoka of the Research Institute of Environmental Geology, Chiba and Dr. K. Sato of the Osaka City University helped N. Razjigaeva in her sampling on Boso Peninsula. The diatom investigations by Russian auther were supported by Grant RFBR 0-05-64837 to Pushkar Vladimir. We thank reviewers and editorial board for constructive criticism of the manuscript. References Aida, N., Furuno, K. and Kagawa, A. (1996) Plaeomagnetic stratigraphy of the Kazusa Group, Boso Peninsula, Japan-Matuyama-Brunhes magnetic polarity reversal in the Kokumoto Formation-. Internal Research Group for the Lower Middle, Middle Upper Pleistocene Boundary Japan Association for Quaternary Research (ed.) Proceedings on the research of stratotype for the Lower-Middle Pleistocene boundary: 62-72. Aoki, N. (1964) Pliocene and Pleistocene planktonic foraminiferal assemblages, Boso Peninsula. Jour. Geol. Soc. Japan, 70, 170-179. (J+E) Berggren, W. A., Kent, D.V., Flynn, J. J. and Van Couvering, J.A. (1985) Cenozoic geochronology.

2002年2月

Diatom

Geol. Soc. Am. Bull., 96, 1407-1418. Cande, S. C. and Kent, D.V. (1995) Revised calibration of the geomagnetic polarity timescale for the Late Cretaceous and Cenozoic. Jour. Geophy. Res., 100, 6093-6095. Danhara, T. and Watanabe, M. (1994) Fission track age determination of Kazusa Group/Shimosa Group in Boso Peninsula. Fission Track News Letter, 7, 12-17. (J) Hara, Y. and Nirei, H. (1990) Fission track ages of the lower part of Kiwada Formation. Jour. Geol. Soc. Japan, 96, 397-400. (J+E) Hara, Y., Nirei, H. and Suzuki, M. (1996) Chronostratigraphy of Chiba section for the Lower-Middle Pleistocene Boundary. Internal Research Group for the Lower-Middle, Middle-Upper Pleistocene Boundary, Japan Association for Quaternary Research (ed.) International Symposium and Field Discussion on the Middle/Lower Pleistocene Boundary: 73-79. Igarashi, A. (1994) Paleoceanographic changes during the deposition of the middle Pleistocene Kazusa Group, central Japan: Estimation based on the principal components analysis of planktonic foraminifera. Jour. Geol. Soc. Japan, 100, 348-359. (J+E) Internal Research Group for the Lower-Middle, Middle-Upper Pleistocene Boundary, Japan Association for Quaternary Research (1996) Proceedings on the research of stratotype for the Lower-Middle Pleistocene Boundary. 91p. Ito, M. (1996) Sandy contourites of the Lower Kazusa Group in the Boso Peninsula, Japan: Kuroshiocurrent-influenced deep-sea sedimentation in a Plio -Pleistocene forearc basin . Jour. Sed. Res., 66, 587598. Kanaya, T. and Koizumi, I. (1966) Interpretation of diatom thanatocoenoses from the North Pacific, applied to a study of core V20-130 (Studies of deepsea core V20-130, Part IV). Sci. Rep. Tohoku Univ., 2nd Ser. (Geol.), 37, 89-130. Katsura, Y. (1984) Depositional environments of the Plio-Pleistocene Kazusa Group, Boso Peninsula, Japan. Sci. Rep., Inst. Geosci. Univ. Tsukuba, Sec. B, 5, 69-104. Koizumi, I. (1968) Tertiary diatom flora of Oga Peninsula, Akita Prefecture, northeast Japan. Sci. Rep. Tohoku Univ., 2nd Ser. (Geol.), 40, 171-240. Koizumi, I. (1986) Pliocene and Pleistocene diatom datum levels related with paleoceanography in the northwest Pacific. Mar. Micropaleontol., 10, 309-325. Koizumi, I. and Kanaya, T. (1976) Late Cenozoic marine diatom sequence from the Choshi district, Pacific coast, central Japan. Takayanagi, Y. and Saito, T. (eds.) Progress in micropaleontology Sep. Pub.: 144-159, Amer. Mus. Nat. Hist., New York. Koizumi, I. and Tanimura, Y. (1985) Neogene diatom biostratigraphy of the middle latitude western North Pacific, Deep Sea Drilling Project Leg 86.

of Boso

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Heath, G.R., Burckle, L.H. et al. (eds.) Init. Rept. DSDP, 86: 269-300, U. S. Govt. Printing Office, Washington D.C. Kumai, H. (1991) Quaternary stratigraphy. The Quat. Res. (Daiyonki-Kenkyu), 30, 131-140. (J+E) Mitsunashi, T., Nasu, N., Nirei, H., Kikuchi, T., Suzuki, Y., Hirayama, J., Nakajima, T., Oka, S., Kodama, K., Horiguchi, M., Katsurajima, S., Miyashita, M., Yazaki, K., Kageyama, K., Kagami, H., Honza, E., Kimura, M., Higuchi, S., Hara, Y., Huruno, K., Endo, T., Kazashima, S. and Aoki, S. (1979) Explanation text of the geological map of Tokyo bay and adjacent areas: Miscellaneous map series (20). scale 1 100,000. 72p, Geol. Surv. Japan. (J+E) Nakagawa, H., Niitsuma, N. and Hayasaka, I. (1969) Late Cenozoic geomagnetic chronology on the Boso Peninsula. Jour. Geol. Soc. Japan, 73, 267-281. (J+E) Niitsuma, N. (1971) Geomagnetic reversal and change in foraminiferal fauna and sedimentary environments. The Quat. Res. (Daiyonki-Kenkyu), 10, 60-68. (J+E) Niitsuma, N. and Fujii, N. (1984) Analysis on the changes in the paleoenvironment and biomass during geomagnetic reversal by means of oxygen and carbon isotope. Geosci. Repts. Shizuoka Univ., 10, 123-132. Oda, M. (1977) Planktonic foraminiferal biostratigraphy of the Late Cenozoic sedimentary sequence, central Honshu, Japan. Sci. Rep. Tohoku Univ., 2nd Ser. (Geol.), 48, 1-72. Okada, M. and Niitsuma, N. (1989) Detailed palaeomagnetic records during the Brunhes-Matuyama geomagnetic reversal, and a direct determination of depth lag magnetisation in marine sediments. Phy. Earth Planet. Inter., 56, 133-150. Onishi, I. (1969) Pollen flora of the Kazusa Group in the Boso Peninsula. Earth Science (Chikyu Kagaku), 23, 236-242. (J+E) Pickering, K.T., Souter, C., Oba, T., Taira, A., Schaaf, M. and Platzman, E. (1999) Glacio-eustatic control on deep-marine clastic forearc sedimentation, Pliocene-mid-Pleistocene (c. 1180-600ka) Kazusa Group, SE Japan. Jour. Geol. Soc. London, 156, 125136. Sato, T. and Takayama, T. (1988) Calcareous nannofossil zones of the Quaternary. Mem. Geol. Soc. Japan, 30, 205-217. (J+E) Satoguchi, Y. (1996)Tephrostratigraphy of Quaternary systems in the Boso Peninsula, Japan. Internal Research Group for the Lower-Middle, Middle Upper Pleistocene Boundary, Japan Association for Quaternary Research (ed.) International Symposium and Field Discussion on the Middle/Lower Pleistocene Bounday: 24-35. Suzuki, M. and Sugihara, S. (1983) Fission-track age constraints on the Plio-Pleistocene boundary in the Kazusa Group. Japan Association Quaternary Research 13rd Congress Abstract, 69-70. (J)

10

M. V. Cherepanova

Tokuhashi, S., Danhara, T., Endo, H., Isoda, K. and Nishimura, S. (1983) Some experiments and problems on fission track dating of geologically younger-age samples: with special reference to several volcanic ash layers in Kazusa and Shimosa Groups, Boso Peninsula, central Japan. Bull. Geol. Surv. Japan, 34, 241-269. (J+E) Ujihara, A. (1986) Pelagic gastropod assemblages from the Kazusa Group of the Boso Peninsula, Japan and Pliocene-Pleistocene climatic changes.

at al.

Feb.

2002

Jour. Geol. Soc. Japan, 92, 639-651. (J+E) Yoshikawa, S., Satoguchi, Y. and Nagahashi, Y. (1996) A widespread volcanic ash bed in the horizon close to the Pliocene Pleistocene boundary: FukudaTsujimata gawa OKd38 volcanic ash bed occurring in central Japan. Jour. Geol. Soc. Japan, 102, 258-270. (J+E) (J+E) in Japanese nese

with English abstract,

(J) in Japa-

房総半島上総層群の珪藻生層序 M.

V.

Cherepanova*1・V.

S. Pushkar*2・N.

熊井

久 雄*3・ 小 泉

〔 要

房 総 半 島 の 上総 層群 国 本 層 は,太 平 洋 地 域 に お け る前

Razjigaeva*2・

格*4

旨〕

そ の うち4つ

の 基 準 面 は国 本 層 中 部 の前 期/中 期 更 新

期/中 期 更 新 世 の 模 式 境 界 の 候 補 地 と して考 え られ て い

世 境 界 の上 下 に2つ ず つ存 在 す る.下 位 か ら,Actino

るの で,さ ま ざ ま な地 質 情 報 を集 約 す る必 要 が あ る.古

cyclus oculatus Jouseの 最 終 出現,Rhizosolenia matu-

地 磁 気 層 序 のC1r(松

リュ ンヌ正

yamai Burckleの

最 終 出 現,Nitzschia fossilis(Fren-

ラオ"基

guelli)Kanayaの

最 終 出 現,Nitzshia reinholdii Kanaya

山 逆 磁 極 期)/C1n(ブ

磁 極 期)境 界 は,国 本 層 中部 の 火 山灰 鍵 層"シ

底 に あ る.養 老 川 沿 いの 大 田代 層,梅ヶ 瀬 層,国 本 層,

の最 終 出現 で あ る.海 退 が,大 田代 層 のC1r.1n(ハ

柿 ノ 木 台 層,長

ヨ事 件)基

南 層 の 珪 藻 群 集 は,汽 水 性Paralia

sulcata(Ehrenberg)Kutzingを

底 と国 本 層 のC1r/C1n(前

ラミ

期/中 期 更 新 世)

優 占種 とす る沿 岸 な い

境 界 に生 じて い る.珪 藻 生 層 序 か らみ て,国 本 層 は太 平

し浅 海 性 種 群 か ら構 成 され て い る.北 西 太平 洋 の 中緯 度

洋 地 域 に お け る前 期/中 期 更 新 世 の 境 界 模 式 地 と して 適

域 に お け る7つ の 珪 藻 基 準 面(生 層準)が

当 で あ る と判 断 さ れ る.

*1

Institute

*2

Pacific

*3大 *4

of Biology Institute

and

Soil

of Geography

阪市立 大学 理学部 地球 学教 室

Science

見 い だ され,

FEB RUS . 7, Radio Str., Vladivostok, 690041, RUS . 7, Radio Str., Vladivostok, 690041, Russia. 〒558-8585大 阪 市 住 吉 区 杉 本3-3-138 .

FEB

Russia.