SGL38-01

Room:102B

Time:May 25 10:00-10:15

The Chiba composite section (a candidate of the L-M Pleistocene GSSP): recent advancesand future perspectives
SUGANUMA, Yusuke1∗ ; OKADA, Makoto1 ; KAZAOKA, Osamu2 ; KAMEO, Koji4 ; HEAD, Martin J.5 ;NISHIDA, Naohisa6 ; YOSHIDA, Takeshi3 ; OGITSU, Itaru3 ; KAMEYAMA, Shun3 ; NAKAZATO, Hiroomi10 ;IZUMI, Kentaro7 ; KUBOTA, Yoshimi8 ; SUGAYA, Manami2 ; NIREI, Hisashi3 ; KUMAI, Hisao91

National Institute of Polar Research, 2 Ibaraki Univeristy, 3 Research Institute of Environmental Geology, 4 Chiba University,
Brock University, 6 Geological survery of Japan, AIST, 7 National Institute for Environmental Studies, 8 National Museum of
Nature and Science, 9 Osaka City University, 10 National Institute for Rural Engineering
5

In the Chiba composite section, along the Yoro, Yanagawa, and Kogusabata rivers in the Bozo Peninsula, the Kokumoto Formation (Kazusa Group) represents an expanded and well-exposed sedimentary succession across the Lower-Middle Pleistocene
boundary. The predominant silty beds of the Chiba composite section are intensely bioturbated and lack evidence of episodic deposition such as slumps or muddy turbidites, which interpreted to be hemipelagite formed by deposition of fine-grained suspended
material under stable and calm bottom-water conditions. High-resolution oxygen isotope stratigraphic studies for the Kokumoto
Formation reveal that a continuous sedimentary record from MIS 21 to MIS 18, with extremely high sedimentation rates up to
200 cm/kyr. The Matuyama?Brunhes boundary (MBB) is clearly observed at immediately above the widespread Byk-E tephra
bed. A high-precision U-Pb zircon age of 772.7 ± 7.2 ka for the tephra coupled with the oxygen isotope chronology provides ahighly accurate MBB age of 770.2 ± 7.3 ka. This MBB age is consistent with the latest MBB ages from high-resolution marine
sediments and an Antarctic ice core. Because the MBB customarily serves as the primary guide for the Lower?Middle Pleistocene Subseries boundary, the Chiba composite section is considered an excellent candidate for its global boundary stratotype
section and point (GSSP). For a better chronological constraint and global correlation of the section, more detailed magneto- andoxygen isotope stratigraphy will be obtained. And also, analyses of Mg/Ca in foraminifera and pollen assemblage will be carriedout for a high-resolution paleoclimatic reconstruction during MIS 19.

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SGL38-02

Room:102B

Time:May 25 10:15-10:30

Litho-stratigraphy and sedimentary environment of upper part of Kokumoto Formation
with the L-M Pleistocene boundary
KAZAOKA, Osamu1∗ ; NISHIDA, Naohisa2 ; OKADA, Makoto3 ; SUGANUMA, Yusuke4 ; KAMEYAMA, Shun5 ;YOSHIDA, Takeshi1 ; MORISAKI, Masaaki1 ; KAGAWA, Atsushi1 ; OGITSU, Itaru1 ; IZUMI, Kentaro6 ;NAKAZATO, Hiroomi7 ; KUMAI, Hisao8 ; NIREI, Hisashi91

Research Institute of Environmental Geology, Chiba, 2 Geological Survey of Japan, AIST, 3 Ibaraki University, 4 National Institute of Polar Research, 5 Environmental Protection division of Chiba Prefectural Government, 6 University of Tokyo, 7 National
Institute for Rural Engineering, 8 Osaka City University, 9 Japan Branch of Geoscience for Environmental Management, IUGSThe Lower-Middle Pleistocene Kazusa Group, deposited on bathyal-shelf in the Pacific Ocean with micro fossil, distributes
widely in Boso peninsula. The group exposes continuously along Yoro river, Chiba section. The Kazusa group consists of Kurotaki formation (mainly tuffaceous gravelly sandstone), Katsuura formation (mainly alternation of sandstone with slump bed),
Namihana formation (mainly siltstone with slump bed), Ohara formation (muddy alternation of sandstone and siltstone), Kiwada
formation (muddy alternation of sandstone and siltstone with slump bed), Otadai formation (alternation of sandy alternationand muddy alternation of sandstone and siltstone), Umegase formation (mainly sandy alternation of sandstone and siltstone),
Kokumoto formation (alternation of thick siltstone and sandy alternation of sandstone and siltstone), Kakinokidai formation(sandysiltstone with sandstone), Chonan formation (alternation of thin sandstone and thin siltstone) in ascending order. Total
thickness of the Kazusa Group is over 2,000 meters with over 50 marker tephra. Depositional rate of it is rapid, about 2 m/kyr.
So Chiba section have high potential for international stratotype section.
Kokumoto formation, about 350 meter thick, is composed of lowermost part, lower part, upper part and uppermost part inascending order. Lowermost part, about 60 meter thick, consists of thick siltstone with thin sandstone bed and marker tephras,
Ku6 and ku5. Lower part, about 120 meter thick, consists of sandy alternation of sandstone and siltstone with Ku3 tephra. Upper
part, about 80 meter thick, consists of thick siltstone without slump bed and with thin sandstone and marker tephra (Byakubi
zone (Byk-E, Byk-D, Byk-C, Byk-B, Byk-A), Tap-B, Tap-A, Tas-C, Tas-B, Tas-A, Ku2). The Matuyama?Brunhes boundary isin Byk zone. Uppermost part, about 90 meter thick, consists of sandy alternation of sandstone and siltstone with Ku0.1 tephra.
The upper part, thick siltstone, is interbedded with thin, 1-3cm thick, sandstone every 0.3-3 m thick and thin, 1-5 cm thick,sandysiltstone every 0.1-0.25 m thick without slump bed and thick mudflow bed. The siltstone have bathyal and sublittoral benthic foraminifera and many trace fossils. Grain size distribution in the siltstone have bimodal grain group. Main grain group iscomposed of fine silt and sub group consists of very fine sand. These characteristics show hemipelagic sedimentary environment
in deep sea and very fine sand flow often into, namely deep sea slope.

Keywords: GSSP, The Lower-Middle Pleistocene boundary, Kokumoto Formation, Kazusa Group, Tabuchi section

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SGL38-03

Room:102B

Time:May 25 10:30-10:45

Ichnofossils and ichnofabrics of the Kokumoto Formation, Kazusa Group: Depositional
environment and benthic paleoecology
IZUMI, Kentaro1∗ ; NISHIDA, Naohisa2 ; KAZAOKA, Osamu3 ; SUGANUMA, Yusuke4 ; OKADA, Makoto5 ;YOSHIDA, Takeshi3 ; OGITSU, Itaru3 ; NAKAZATO, Hiroomi6 ; KAMEYAMA, Shun3 ; KAGAWA, Atsushi3 ;MORISAKI, Masaaki3 ; HYODO, Masayuki7 ; NIREI, Hisashi81Center for Environmental Biology and Ecosystem Studies, NIES, 2 Institute of Geology and Geoinformation, Geological Survey of Japan, AIST, 3 Research Institute of Environmental Geology, Chiba, 4 National Institute of Polar Research, 5 Departmentof Earth Sciences, Faculty of Science, Ibaraki University, 6 National Institute for Rural Engineering, 7 Research Center for Inland
Seas, Kobe University, 8 International Union of Geological Science for Environmental Management
In the Tabuchi section, along the Yoro river in the Boso Peninsula, the Kokumoto Formation (Kazusa Group) represents an
expanded well-exposed, continuous marine succession across the Lower-Middle Pleistocene boundary. Since the Tabuchi section
contains the Matuyama-Brunhes boundary whose age was highly accurately constrained, it is considered as a candidate for the
Global Boundary Stratotype Section and Point (GSSP). In addition, due to the high sedimentation rate and continuous deposition, the Kokumoto Formation is suitable for high-resolution paleoenvironmental studies. However, paleoecological studiesof the formation are very few, responses of marine organisms to paleoenvironmental changes remain unclear. Therefore, thisstudy systematically described the trace fossils and ichnofabrics of the Kokumoto Formation of the Tabuchi section. Twelve
ichnogenera and another indeterminate U-shaped burrow were recognized from the silty beds of the formation, which are typicalcomponents of ichnofacies characterizing the bathyal zone (i.e. Zoophycos ichnofacies). In addition, no graphoglyptid trace fossils, which commonly occur in abyssal plain environments, were observed. Based on the trace-fossil assemblage combined withother sedimentological features, it is most likely that the silty beds of the Kokumoto Formation were deposited in a continental
slope setting. Furthermore, two types of ichnofabrics were identified, which are Phycosiphon -dominated ichnofabric (Phy ichnofabric) and Chondrites -Planolites -Thalassinoides ichnofabric (Ch -Pl -Th ichnofabric). Observation of the thin-sliced slabs
revealed the distinctive stratigraphic changes of these two ichnofabrics; namely, both the Phy -ichnofabric and Ch -Pl -Th ichnofabric occur in the lower and upper part of the Tabuchi section, whereas the Ch -Pl -Th ichnofabric is exclusively recognized in
the middle part. The vertical change in ichnofabrics is not correlated with changes in sedimentation rate. Instead, the ichnofabrics
are well correlated with changes in benthic-food contents, which are estimated by the results of high-resolution XRF analysis. In
particular, the lower and upper parts of the Tabuchi section, which are characterized with the presence of Phy -ichnofabric, aresynchronized with food-poor intervals. Since the Phycosiphon -producer is regarded as a grain-selective deposit feeder, which
may have effectively ingested organic matter, it is reasonable that the Phy -ichnofabric occurs only in food-poor intervals.

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SGL38-04

Room:102B

Time:May 25 11:00-11:15

Lower - Middle Pleistocene Boundary at Chiba Section and istribution situation of Bykzone, central Japan(part2)
KIMURA, Hideto1∗ ; KAZAOKA, Osamu2 ; NIREI, Hisashi31

Toho Chisui Co., Ltd. Kanto office, 2 Research Institude of Environmental Geology, Chiba, 3 International Union of Geological
Sciences Geoscience for Environmental Management
Kazusa group distributed in the Ichihara City southern part, Chiba Prefecture, is composed of Umegase formation, Kokumoto
formation, Kakinokidai formation. Kokumoto formation in the geological structure of N67-69E 7- 9N, and the tilt direction
falling direction of Yoro River is substantially coincident. Kokumoto formation is 4 divided by facies, massive mud layer,sand layer rich sand layer and mud layer alternated layers (sand layer:mud layer=10:1-6:4), equivalent sand and mud alternatedlayers(mud layer:sand layer =4:6-6:4), mud layer rich sand layer and mud layer alternated layers(mud layer:sand layer=10:16:4), massive sand layer. And Kokumoto formation is divided 4 parts (uppermost part: sand layer rich sand layer and mud
layer alternated layers, upper part: massive mud layer,middle part: sand layer rich sand layer and mud layer alternated layers,
lower part: massive mud layer). The Brunhes / Matuyama chron boundary (B / M boundary) is confirmed in Byk zone, at
upper part base of Kokumoto formation. This location is Chiba section. Byk zone is divided 5 tephras (Byk-A: off-white silt
grain volcanic ash and fine sand grain scoria, Byk-B and Byk-C and Byk-D: medium sand grain scoria, Byk-E: white silt grain
volcanic ash). Byk-E is identified as a conventional TNTT. Byk zone (Byk-A - E) has also been confirmed in Koshikiya River
east of Yoro River, and Byk zone is distance Byk-A and Byk-E with a deposition rate change of the side. Distance of Byk-A and
Byk-E is 3.5m at Yoro River location(Tabuchi Section of Chiba Section), and distance of Byk-A and Byk-E is 3.0m at KoshikiyaRiver location(Koshikiyagawa section of Chiba section). From the measured value, Tabuchi section is faster deposition rate than
Koshikiyagawa section in Chiba section.
Keywords: Byk zone, Lower-Middle Pleistocene Boundary, Kokumoto Formation upper part, Yoro River, Chiba Section

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SGL38-05

Room:102B

Time:May 25 11:15-11:30

Discrimination of primary remanent magnetization during Matuyama-Brunhes polarity
transition
ODA, Hirokuni1∗ ; NANAYAMA, Futoshi1 ; NAKAZATO, Hiroomi2
1

Geological Survey of Japan, AIST, 2 National Institute for Rural Engineering

We report preliminary results of magnetostratigraphy from an outcrop in Boso Peninsula, which is considered to record
Matuyama-Brunhes polarity transition. The outcrop is ?4m height, facing west and situated along a roadside in Terasaki, Chiba
Prefecture, Japan. The sediment mainly consists of massive silt of Kokumoto Formation, Kazusa Group. The outcrop shows
several tephra layers including TNTT (Byk-E) residing close to Matuyama-Brunhes polarity transition (Okada and Niitsuma,1989).
In order to identify the polarity of primary remanent magnetization recorded, we have taken 55 paleomagnetic drill cores atintervals of 1?10 cm. Progressive alternating field demagnetization (PAFD) was conducted on all the sub-samples taken fromthe drill cores. The higher coercivity (>?20 mT) magnetization component has mostly positive inclination (normal polarity) andshows a swing to negative inclination (reversed polarity) at 76-91 cm below TNTT. Preliminary results of progressive thermal
demagnetization shows sharp drop in remanent magnetization by heating up to 175 ℃. By heating above 175 ℃, magnetization
decreases gradually up to 300-350 ℃ and becomes unstable above 300-350 ℃.
In order to understand the origin of instability during heating to 30-350 ℃, we have conducted progressive thermal demagnetization in combination with isothermal remanent magnetization acquisition. The results suggest the presence of (titano-)magnetiteand greigite, and the production of magnetic mineral during heating above 200-350 ℃ in the laboratory.
Combination of thermal remanent magnetization up to 200 ℃ and further AF demagnetization was conducted in order toextract primary remanent magnetization hidden by the strong secondary magnetization and thermal instability, however, the extraction of primary remanent magnetization was not successful. Further improvements in demagnetization might be pursued toclarify the magnetization at the time of deposition free from later diagenesis.
Keywords: Matuyama-Brunhes polarity transition, thermal demagnetization, alternating field demagnetization, low temperaturedemagnetization, primary remanent magnetization, greigite

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SGL38-06

Room:102B

Time:May 25 11:30-11:45

Matuyama-Brunhes magnetic polarity transition from a sequence of the Kokumoto Formation drilled at Tabuchi, Ichihara
HYODO, Masayuki1∗ ; TAKASAKI, Kenta2 ; MATSUSHITA, Hayato2 ; KATOH, Shigehiro3 ; KITABA, Ikuko4 ;DETTMAN L., David5 ; KITAMURA, Akihisa6 ; HAYASHI, Hiroki7 ; OKADA, Makoto8
1

Research Center for Inland Seas, Kobe University, 2 Department of Planetology, Kobe University, 3 Hyogo Museum of Natureand Human Activities, 4 Research Center for Palaeoclimatology, Ritsumeikan University, 5 Department of Geosciences, University of Arizona, 6 Institute of Geosciences, Shizuoka University, 7 Interdisciplinary Faculty of Science and Engineering, Shimane
University, 8 Department of Earth Sciences, Faculty of Science, Ibaraki University

A detailed Matuyama-Brunhes transition was revealed from a 54-m oriented core of the Kokumoto Formation drilled at
Tabuchi, Ichihara, Chiba Prefecture, central Japan. The core mainly consists of silts except the lowermost part intercalatedwith thin sand layers. For magnetic analyses, we prepared one meter long u-channel samples from 3 to 52 m depth, and discrete
samples of 10 cc cube at 10 cm to 1 m intervals. Magnetizations were measured every 1 cm using a 2G cryogenic magnetometerfor u-channel samples and subjected to alternating field demagnetizations (AFD), while both AFD and thermal demagnetizations
were used for discrete samples. Preliminary oxygen isotope data on planktonic foraminifera (Globorotalia inflata ) suggest thatthe main MB polarity boundary, just underlain by the Byakubi-E tephra layer, lies between the sea-level highstand of marine
isotope stage (MIS) 19.3 and the MIS 19.2 lowstand. Characteristic remanent magnetizations of u-channel samples calculated byprincipal component analysis reveal a rapid reversal interval (RRI), a very important feature characteristic of the final stage of the
MB transition, which lies between depths correlated with the MIS 19.3 highstand and MIS 19.2 lowstand. The RRI spans about
1.7 m in depth, during which the virtual geomagnetic pole crossed the geographic equator 11 times. A preliminary astronomical
age model suggests that the RRI was ca. 2 kyr in duration, predating 776 ka and postdating 779 ka. The RRI can be correlatedwith similar intervals observed in the Osaka Group, Chinese loess-paleosols, and deep-sea sediments. The RRI incorporates
both the MB boundary and the Byakubi-E tephra in the Chiba section. This has important implications for the definition of the
Early-Middle Pleistocene boundary. Other MB transition features, including paleointensity variation, will be discussed together
with the results from discrete samples.
Keywords: Matuyama-Brunhes polarity transition, Kokumoto Formation, Eraly-Middle Pleistocene boundary, Chiba Section,
GSSP

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SGL38-07

Room:102B

Time:May 25 11:45-12:00

Revised Matuyama-Brunhes polarity transition record from a marine succession at the
Chiba composite section
OKADA, Makoto1∗ ; SUGANUMA, Yusuke2 ; MARUOKA, Toru1 ; HANEDA, Yuki1 ; KAZAOKA, Osamu4
1Dept. Earth Sci., Ibaraki Univ., 2 National Inst. Polar Res., 3 Dept. Polar Sci., The Graduate Univ. Advanced Studies, 4 Res. Inst.
Env. Geol., Chiba Pref.
We report revised paleomagnetic records of the Matuyama-Brunhes (M-B) polarity transition from a continuous marine succession at the Chiba section of the Kokumoto Formation, Kazusa Group. The Chi-ba section is the one of the candidate sites
for the Lower-Middle Pleistocene Boundary GSSP. An age model for the section, provided by newly obtained oxygen isotopesof benthic foraminifera from a 100 meters succession across the M-B boundary, indicates that the boundary is situated in theinterglacial per-iod of MIS19. We have taken 130 oriented mini-cores from a 13 meters succession across the Byk-E tephra bed
at the Chiba section and the Yanagawa section. Thermal magnetic experiments suggest that the samples include iron sulfides,
magnetites but no hematite. Measurements of magnetic hysteresis indicate that the magnetic domain state is PSD. Progressivealternating field (AF) demagnetization indicate a re-versed to normal polarity transition boundary is at around 1.5 meter below
the Byk-E bed as well as pre-vious studies, however the transition boundary is observed at around 1 meter above the Byk-E bed
in thermal demagnetization results. Therefore, the reversed to normal polarity transition boundary seen be-low the Byk-E bed
is thought to be overprint. This overprints, which might be carried by iron sulfide, are particularly observed in a transitional
interval. The virtual geomagnetic pole (VGP) latitudes and prelimi-nary derived paleointensities using thermal demagnetizations
from the Chiba section quite match well with the U1308 records. We will show globally comparable VGP and paleointensity
records during the M-B polarity transition at the Chiba section.
Keywords: Chiba section, M-L Pleistocene boundary GSSP, Paleomagnetism, Matuyama-Brunhes boundary

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SGL38-08

Room:102B

Time:May 25 12:00-12:15

Mg/Ca-based temperature variations across the L-M Pleistocene Boundary in the Chiba
composite section, central Japan
KUBOTA, Yoshimi1∗ ; HANEDA, Yuki2 ; OKADA, Makoto2 ; SUGANUMA, Yusuke3 ; KAZAOKA, Osamu4 ;KIMOTO, Katsunori5
1

National Museum of Nature and Science, 2 Graduate School of Science and Engineering, Ibaraki University, 3 National instituteof Polar Research, 4 Research Institute of Environmental Geology, Chiba, 5 JAMSTEC

The Kuroshio Current, a western boundary current in the North Pacific, transport warm saline waters from low- to highlatitude and thus plays a crucial role in heat transport in the mid-latitude. Around 0.8 Ma, near the L-M Pleistocene boundary
corresponding to the Matuyama-Brunhes boundary (MBB), is the one of the key time period to understand initiation of 100-kyr
glacial-interglacial climate cycle. The Chiba composite section, including the Tabuchi section as a L-M Pleistocene boundary
GSSP candidate, is a continuous marine sedimentary succession exposed in the Boso peninsula, central Japan. The MBB iswell defined based on virtual geomagnetic pole (VGP) latitudes in this section and an age model is determined based on benthic
foraminiferal oxygen isotope record. The site of this section (35o N) is located in the mixing zone (35o N - 40o N at present) of
warm Kuroshio and cold Oyashio waters. Thus, temperature changes in the site can be interpreted as reflecting the latitudinal
shift of the Kuroshio-Oyashio boundary, which could be related to the Kuroshio variations and also impact on the heat transport in
the mid-latitude. Here, we present Mg/Ca records of planktic foraminifera Globigerina bulloides and Globorotalia inflata in the
Chiba section and reconstruct surface and subsurface water temperatures across the Matuyama-Brunhes Boundary. Preliminary
results suggest that the average surface temperature was 18-19 ℃ during the time interval from 780-740 Ka. The higher time
resolution (∼0.5-1 ky) surface and subsurface water temperatures will be presented and discussed by comparison with the oxygenisotope data.

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SGL38-09

Room:102B

Time:May 25 12:15-12:30

A vegetation change reconstruction at around the L-M Pleistocene boundary from a pollen
record of the CHOSHI core
SUGAYA, Manami1∗ ; SUGANUMA, Yusuke2 ; OKADA, Makoto3
1

Graduate school of Sci. and Eng. Ibaraki University, 2 National Institute of Polar Research, 3 Department of Earth Sciences,
Ibaraki University
Because the Tabuchi section consisting of the middle part of the Kokumoto Formation is a Lower-Middle Pleistocene (L-M)
boundary GSSP candidate, to reconstruct high-resolution pollen records at this part of the formation is quite important. However,Onishi (1969) reported pollen assemblages of the Kokumoto Formation in which that the pollen density is substantially thinand the assemblages were severely distorted as to exhibit a pelagic condition where conifer trees pollens tend to be artificiallyoverrepresented. The objective of this study is to reconstruct a high-resolution pollen records by using the Choshi core, which
represents a good pollen data as reported by Okuda et al. (2006) at upper than the L-M Pleistocene boundary. The Choshicore, drilled at Morito-Cho, Choshi City in the Chiba Prefecture, is composed of five formations, the Katori Formation and the
Inubo Group consisting of the Toyosato, Kurahashi, Yokone, and Obama Formations. The formations cover Marine Isotope
Stages (MISs) from 11 to 25, which across the L-M Pleistocene boundary corresponding to the middle part of the Yokone
Formation (Kameo et al., 2006). The position of the L-M boundary in the Yokone Formation can be determined precisely,
because stratigraphic correlation between the Yokone Formation and the Kokumoto Formation were studied well. We plan toreport a high-resolution pollen record from samples of the core at depths between 150 and 170m, corresponding to MIS 20-18,
and will provide a vegetation changes at around the L-M Pleistocene boundary.
References
Kameo et al., 2006. Island Arc, 15, 366-377.
Okuda et al., 2006. Island Arc, 15, 338-354.
Onishi, 1969. Earth Science (Chikyu Kagaku) 24, 222-224 (in Japanese with English abstract).
Keywords: Lower-Middle Pleistocene boundary, pollen analysis

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SGL38-P01

Room:Convention Hall

Time:May 25 18:15-19:30

Invitation to the Tabuchi section, central Japan: A candidate GSSP for the Lower-Middle
Pleistocene Subseries/Subepoch
YOSHIDA, Takeshi1∗ ; OGITSU, Itaru1 ; KAZAOKA, Osamu1 ; OKADA, Makoto3 ; SUGANUMA, Yusuke2 ;KAMEO, Koji4 ; NIREI, Hisashi5 ; AIDA, Nobuyuki6 ; KUMAI, Hisao7 ; NISHIDA, Naohisa8 ;IZUMI, Kentaro9
1

Reserch Institute of Environmental Geology, Chiba, 2 National Institute of Polar Research, 3 Ibaraki University, 4 Chiba University, 5 Japan Branch of Geoscience for Environmental Management, 6 Shumei University, 7 Osaka City University, 8 GSJ-AIST,
9
University of Tokyo
The Tabuchi section is a continuous marine sedimentary succession exposed in the Boso peninsula and is a Lower - Middle
Pleistocene boundary GSSP candidate.
From the geological advantages and the easy access to the outcrops, the Tabuchi section seems to be the most suitable for the
Lower - Middle Pleistocene boundary GSSP.
Geological characteristics∗Tabuchi section is only candidate representing the Pacific realm.
∗Thick L - M Pleistocene sedimentary succession (>3000 m) (2.4 ? 0.5 Ma) .
∗Well exposed along the Yoro River with high sed. rates (ca. 2 m/kyr) & no visible breaks.
∗Well preserved calcareous nannofossils, planktonic foraminifera, diatoms.
∗Standard section for Japanese Pleistocene tephrostratigraphy (>50 ash beds).
∗Well established d18O isotope stratigraphy: Kokumoto Fm. corresponds to MIS 20?18.
∗M?B boundary is located ca. 1 m above a distinctive, widespread tephra bed (Byk-E).
∗High-precision U-Pb zircon age of the Byk-E.
Consistent with the latest astrochronology of marine sediments and Antarctic ice core.
∗A basis for immediate comparisons between, magnetostratigraphy, biostratigraphy, O isotope stratigraphy, absolute ages
(40Ar/39Ar & U-Pb), and astrochronology.
∗Taking the M?B boundary as the primary guide to the L?M Pleistocene boundary, the Byk-E bed would serve as an appropriate level for the GSSP.
AccessThere are well developed public transportations. You can reach to Tabuchi section within 2 hours from Tokyo and 3 hours
from both international airports. There are big car parks. There are lodge and toilet.
The access to the Tabuchi section is very easy and convenient by car, bus and train with very small walk.

Keywords: Tabuchi section, Lower and Middle Pleistocene

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SGL38-P02

Room:Convention Hall

Time:May 25 18:15-19:30

Detailed Tephra Catalog of Lower to Middle Part of the Kazusa Group, Lower to Middle
Pleistocene, Central Japan
NAKAZATO, Hiroomi1∗ ; NANAYAMA, Futoshi2
1

National Institute for Rural Engineering, NIRE, 2 Geological Survey of Japan, AISTThe Kazusa Group covering the Boso Peninsula, central Japan is one of the typical lower to middle Pleistocene geologicalformations in the country. It is composed of the continuous sediment formed in deep sea and coastal regions. It is known as
reservoir of the water-soluble natural gas and iodine rich brine. The stratigraphic division of the Kazusa Group has been done
based on lithofacies of the formations and occurrence of tephra layers. The existence of tephrostratigraphy has been confirmed
resulting in the revision of the tephra catalog to be part of the geological research for the Quadrangle Series 1:50000 of the
Mobara district, eastern part of Boso Peninsula. The confirmed tephras consist of 139 layers from O7 in the Otadai Formation
to Ks4 in the Kasamori Formation. These include the Byk-E (Ku2.3) tephra which indicates the base of the middle Pleistocene
formation. Each tephra was identified by refractive index measurement and EPMA analysis of volcanic glass orthopyroxene
and hornblende. The oxygen isotope stratigraphy of a coetaneous stratum in the Choshi district, northeastern part of the Boso
Peninsula was already determined. Hence it is be possible to determine the MIS age of each tephra by correlating the tephras inboth districts.
Keywords: Lower to Middle Pleistocene, Upper to Middle Part of the Kazusa Group, Tephra Catalog, Mobara District, Boso
Peninsular, Chiba Prefecture

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SGL38-P03

Room:Convention Hall

Time:May 25 18:15-19:30

Detailed stratigraphy of diatom assemblages from a core of the Kokumoto Formation collected in the Boso Peninsula, JapanTANAKA, Ikuko1∗ ; HYODO, Masayuki2 ; KITABA, Ikuko3 ; SATO, Hiroshi4
1Graduate School of Science, Kobe University, 2 Research Center for Inland Seas, Kobe University, 3 Research Centre for Palaeoclimatology, Ritsumeikan University, 4 Institute of Natural and Environmental Sciences, Univ. of Hyogo
Diatom analyses were conducted on a core of the Kazusa Group in the Boso Peninsula, central Japan, to reveal stratigraphic
variations of diatom assemblages across the Matsuyama-Brunhes magnetic polarity boundary (MBB). The core is 54 m longcollected near the Chiba section along the Yoro River, a candidate for the GSSP of the Early-Middle Pleistocene boundary.
Stratigraphic variations of diatom assemblages in response to the glacial eustatic sea-level changes shown by the planktonic
marine oxygen isotope record from Globorotalia inflata . Diatom assemblages in the lowermost part of the core are dominatedby extinct species of Actinocyclus ingens , often observed in reworked deposits in the Kazusa Group. This part is correlated withthe earliest stage of marine isotope stage (MIS) 19, and is dominated by reworked deposits that were accumulated during a low
sea-level period. The extinct species suddenly decrease at a horizon of about 5 m below the Byakubi tephra (ByK) layer, during
a gradual sea-level rise. Turbidity currents may have still affected the sedimentation at the site, although the lithology showsno turbidite layer above a horizon of about 8.5 m below the ByK. Above a level of 5 m below the ByK, marine littoral diatoms
such as Paralia sulcata and Cyclotella striata become dominant, and have a peak at about 3 m below the ByK, coinciding withthe lightest oxygen isotope value correlated with MIS 19.3. Above the peak abundance, the proportion of P. sulcata gradually
decreases, and A. ingens re-increases at about 3m above the ByK, with a maximum at about 7m above the ByK, where marine
isotope data show a maximum value. The re-increase of extinct diatom species suggests a sea-level drop. Thus the maximum of
A. ingens at about 7 m above the ByK may be correlated with the MIS 19.2 sea-level lowstand. Therefore, the MBB that lies at
1 m above the ByK occurs between MIS 19.3 and 19.2. A. ingens can be used as a proxy of reworked deposits in the Kazusa
Group.
Keywords: Kokumoto Formation, Paralia sulcata, Actinocyclus ingens, MIS19, Matsuyama-Brunhes boundary, GSSP

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SGL38-P04

Room:Convention Hall

Time:May 25 18:15-19:30

The Tokyo Bay Unconformity and the Mandano Ice Age
NIREI, Hisashi1∗ ; KAZAOKA, Osamu2 ; KIMURA, Hideto3 ; YAOSHIDA, Takeshi2 ; NIREI, Wataru4
1

Ibaraki Univ. ; IUGS GEM, 2 Res. Inst. of Environmental Gology, Chiba, 3 Tohocjhisui Co., 4 .Tokyo Univ. of Infor. Sci.

The Kanto fore-arc basin in the Kato plain is an extremely deep submarine basin that was formed during the early Pleistocene.
The sediments in the basin change from deep-sea sediments to lacustrine-alluvial sediments, the Mandano formation which is up
to 95 m in thickness and overlays the Tokyo Bay unconformity that is widely distributed under Tokyo bay area. The formationconsists of three parts. The lithofacies in the lower part on the unconformity gradually change, with decreasing depth, from sandto gravel. The lithofacies of the middle part are muddy. The upper part changes with increasing depth from gravel to sandy shilt
by transgression.
The lower part and lower half of the middle part comprise sediments characteristic of a topset fan delta (Nirei H., 1997) in the
regression stage. The upper half of the middle part and upper parts are composed of transgression sediments.
The lower half of the middle part contains sediments from the ice age regression stage, evidence of which is provided by
the cold-index plant remains,Picea maximowiczii, Tsuga diversifolia,Fagus crenata, Cryptomeria japonica, etc., present in theuppermost part of the lower half.
Geological analysis reveals the Kanto continental shelf to extend widely under the Kanto Plain; it is also underlaid by a rangeof bottom set sediments in distinct formations and forest sediments, also in distinct formations, in ascending order under theconformity.

Keywords: Tokyo Bay Unconformity, Mandano Ice Age, Kazusa Group, Shimohusa Group, Kanto Continental Shelf

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SGL38-P05

Room:Convention Hall

Time:May 25 18:15-19:30

Paleomagnetic and paleoenvironmental studies for the U-M Pleistocene boundary Tukabara Formation
KAWAMATA, Moto1∗ ; NAKAGAWA, Junko1 ; IKEDA, Yuto1 ; SUGAYA, Manami1 ; OOI, Shinzo1 ; OKADA, Makoto1
1

College of Science,Ibaraki Unversity

The Tukabara formation distributed along the Tukabara coast, Minamisoma City, Fukushima Prefecture, which is considered
to have been deposited at early part of the last interglacial period, since the sediments suggested a transgression that leads tothe marine isotope stage 5e. The base of the Tukabara formation consists of a basal gravel layer and the Tagashira tephra bed
which is detected at the MIS 5/6 boundary in the Images MD01-2421 core taken from off Kashima, Pacific side of the central
Japan. The main part of the formation consists of a 7 meter thick varved siltstone including enough diatom and pollen fossils toreconstruct paleoenvironment. Previous paleomagnetic studies reported a reversed polarity from this siltstone layer which wascorrelated as the Blake excursion. Here we report results of reexamined paleomagnetic and rock-magnetic analyses.
The silt layer, consisting of the main part of the Tukabara formation, is divided into following three parts based on paleomagnetic characteristics; Upper Zone: unstable magnetization direction after both of alternating-field thermal demagnetizations,Middle Zone: stable magnetization direction after both of alternating-field and thermal demagnetizations, Lower Zone: stable
magnetization direction after alternating-field demagnetization but after thermal demagnetization. Rock magnetic and paleomagnetic experiments exhibit that the Upper Zone of the siltstone has significantly low magnetization intensities which is supposed
to be due to a weak geomagnetic field caused by the Blake excursion during the depostion.
We will also report preliminary results of paleoenvironmental reconstruction using microfossil analyses. Diatom fossils were
produced from all of the silt stone layers. Based on diatom assemblages, the silt layer is divided into three parts as follows;
Diatom Zone 1: a marine genera dominance zone at the bottom part, Diatom Zone 2: freshwater genera dominance zone at
the middle part, Diatom Zone 3: marine genera dominance zone at the top part. In the Diatom Zone 2, a shallow water genus
Rhaphoneis is not seen and a freshwater cosmopolitan species Diploneis elliptica is abundant. Furthermore, a lot of varves can bewell observed this Zone. This indicates that the zone is supposed to be deposited under a stagnant condition caused by a closed
estuarine like environment with a fresh water discharge.

Keywords: Upper-Middle Pleistocene boundary, Paleomagnetism, Paleoenvironmental reconstruction, Blake event

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