SGL39-P01

日本地球惑星科学連合2016年大会

東京湾不整合と万田野寒冷期
The Tokyo Bay Unconformity and the Mandano Ice- Age*楡井 久1、風岡 修2、木村 英人3、吉田 剛2、藤田 寛、楡井 亘4*hisashi nirei1, Osamu Kazaoka2, Hideto Kimura3, Takeshi Yoshida2, Hiroshi Fujita, Wataru Nirei4
1.国際地質科学連合環境管理研究委員会、2.千葉県地質環境研究室、3.東邦地水株式会社、4.東京情報大学
1.International Union of Geological Science for Environmental Management, 2.Research institute ofEnvironmental Geology, Chiba, 3.Tohochisui Co., 4.Tokyo university of information science
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 tolacustrine-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 formation consistsof three parts. The lithofacies in the lower part on the unconformity gradually change, withdecreasing depth, from sand to gravel. The lithofacies of the middle part are muddy. The upper partchanges with increasing depth from gravel to sandy shilt by transgression. The lower part and lowerhalf of the middle part comprise sediments characteristic of a topset fan delta (Nirei H., 1997) inthe regression stage. The upper half of the middle part and upper parts are composed oftransgression 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 the uppermost part of thelower half. Geological analysis reveals the Kanto continental shelf to extend widely under theKanto Plain; it is also underlaid by a range of bottom set sediments in distinct formations andforest sediments, also in distinct formations, in ascending order under the conformity.
キーワード：東京湾不整合、万田野寒冷期
Keywords: Tokyo bay unconformity, Mandano ice-age©2016. Japan Geoscience Union. All Right Reserved.

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日本地球惑星科学連合2016年大会

千葉セクションにおける松山−ブルン極性反転境界記録
Revised Matuyama-Brunhes polarity transition record from a marine succession at the Chiba
section, a Lower-Middle Pleistocene GSSP candidate*岡田 誠1、菅沼 悠介2,3、羽田 裕貴1、風岡 修4*Makoto Okada1, Yusuke Suganuma2,3, Yuki Haneda1, Osamu Kazaoka4
1.茨城大学理学部理学科、2.国立極地研究所、3.総合研究大学院大学極域科学専攻、4.千葉県環境研究セン
ター
1.Department of Earth Sciences, College of Science, Ibaraki University, 2.National Institute ofPolar Research, 3.Department of Polar Science, The Graduate University for Advanced Studies,
4.Research Institute of Environmental Geology
We report revised paleomagnetic records of the Matuyama-Brunhes boundary (MBB) from a continuous
marine succession at the Chiba section of the Kokumoto Formation, Kazusa Group. The Chiba sectionis the one of the candidate sites for the Lower-Middle Pleistocene Boundary GSSP. In the section, awide spread tephra bed named as Byk-E is intercalated just 80 cm below the MBB. In order to provide
globally comparable VGP (virtual geomagnetic pole) and paleointensity (past geomagnetic field
intensities) records from the Chiba section, we have taken oriented mini-cores from a 13 meters
succession with 10-cm intervals across the Byk-E tephra bed. Thermal magnetic experiments suggestthat the samples include iron sulfides, magnetites but no hematite. Measurements of magnetic
hysteresis indicate that the magnetic domain state is PSD. Progressive alternating fielddemagnetization (AFD) indicate a reversed to normal polarity transition boundary is at around 1.5
meter below the Byk-E bed as well as previous studies, however the transition boundary is observedat around 0.8 meter above the Byk-E bed in thermal demagnetization (ThD) results. Therefore, thereversed to normal polarity transition boundary seen below the Byk-E bed is thought to beoverprint. This overprint, which might be carried by iron sulfide, is particularly observed in atransitional interval. Since iron sulfides generally decompose and oxidized into magnetites due toheating during ThD, the yielded magnetites have no magnetic signal but provide an over estimate ofmagnetic grain amount which prevents to estimate paleointensities. To provide a reliablepaleointensity record, we applied to use a composite demagnetization technique consisting of a 300°
C ThD and a regular progressive AFD sequence. After the 300°C ThD, most of the overprint has been
removed but the magnetic susceptibility has not changed even in the air condition, indicating thatiron sulfides just lose magnetic signals due to the ThD but not to change the amount of magnetic
grains. The VGP latitudes and preliminary derived paleointensities using the composite
demagnetization technique from the Chiba section quite match well with the U1308 records. To use
the both independent techniques of oxygen isotope and paleointensity will provide a further
reliable stratigraphic correlation across the Lower-Middle Pleistocene Boundary.
キーワード：古地磁気学、古地磁気強度、国際標準模式層断面および地点
Keywords: Paleomagnetism, Paleointensity, GSSP

©2016. Japan Geoscience Union. All Right Reserved.

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日本地球惑星科学連合2016年大会千葉県銚子より掘削された陸上ボーリングコアを用いたMIS 19における高解像度酸素・炭素同位体記録（予察）A preliminary report for high-resolution foraminiferal oxygen and carbon stable isotope
records in MIS 19 from an on land core drilled at the Choshi city, central Japan.
早田 達哉1、*羽田 裕貴2、岡田 誠1、久保田 好美3Tatsuya Hayata1, *Yuki Haneda2, Makoto Okada1, Yoshimi Kubota3
1.茨城大学理学部理学科、2.茨城大学大学院理工学研究科、3.国立科学博物館
1.Department of Earth Sciences, Faculty of Science, Ibaraki University, 2.Graduate School of
Science and Engineering, Ibaraki University, 3.National Museum of Nature and Science
The Plio-Pleistocene Inubo Group, distributed in the Choshi city, Chiba prefecture, central Japan,
is though to be a suitable marine succession to investigate paleoceanographic and paleoclimaticchanges around the northwestern Pacific Ocean, because a lot of wide spread key tephra beds areintercalated, and microfosills and pollens are abundant. In 1998, a continuous, well recovered onland core drilled through the Obama, Yokone, Kurahashi and Toyosato Formations in the Inubo Groupwas obtained (after Choshi core). Kameo et al. (2006) studied calcareous nannofosill, paleomagnetic
and planktonic foraminiferal oxygen isotope stratigraphies of the Choshi core, and reported thatthe core corresponded to a period between MIS 11 and 24 base on a correlation with the LR04 stackcurve (Lisiecki and Raymo, 2005). In this study, we show a new high-resolution stable isotope
record using benthic foraminifers from a section across the Lower-Middle Pleistocene boundary ofthe Choshi core. This record corresponds to MIS 18-20 with a time resolution of ca. 500 years.
The average oxygen isotopic value of the Choshi core is about 0.5 ‰lighter than that of LR04 during
the period of MIS 19, and the difference becomes larger as the age becomes younger, indicating thatthe accumulation depth of the Coshi core was getting shallower due to uplifting and/or burring up
the basin. Further analysis on the core will show some paleoceanographic findings at around thenorth western Pacific margin during the MIS 19 period.
キーワード：MIS 19、有孔虫安定同位記録、銚子コア
Keywords: MIS 19, Foraminiferal stable isotope record, Choshi core

©2016. Japan Geoscience Union. All Right Reserved.

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日本地球惑星科学連合2016年大会

房総半島中部に分布する上総層群下部-中部更新統の石灰質ナンノ化石層序と古海洋環境
Calcareous nannofossil biostratigraphy of the Lower-Middle Pleistocene in the Kazusa
Group, central part of the Boso Peninsula, and estimated sea surface environments*渡辺 賢人1、亀尾 浩司1*Kento Watanabe1, Koji Kameo1
1.千葉大学理学研究科地球生命圏科学専攻地球科学コース
1.Department of Earth Sciences, Division of Geosystem and Biosystem Sciences, Graduate School ofScience,Chiba University
本邦中部太平洋側に位置する房総半島には第四系の海成堆積物から構成される上総層群が分布する. 露頭条件が良いことや化石を多く含むことから, 岩相層序学的研究や古地磁気層序学, テフラ層序学, 微化石層序学のような年代層序学的な研究が行われてきた (新妻, 1976; 佐藤ほか, 1988; 五十嵐, 1994など). このうち下部-中部更新統に相当する国本層には, ブリュンヌ/松山境界 (Matuyama/Brunhes Boundary: MBB) が記録されており, GSSP (Global Boundary Stratotype Section and Point) 模式地の有力な候補のうちの一つとなっている (Kazaoka. et al., 2015). そのため, 前期-中期更新世の正確な年代を決定するために, より詳細な年代層序の検討が必要であると言える. ところが, 国本層に産出する石灰質ナンノ化石は, 必ずしも高い時間解像度で解析されているわけではない (佐藤ほか，1988). そのため, 当層準における石灰質ナンノ化石層序は十分に検討されておらず, 化石基準面も明らかにされていない. よって本研究ではMBB相当層準の石灰質ナンノ化石基準面を明らかにするとともに, 群集変化に基づいて同層準堆積当時の北西太平洋の表層海洋環境を推定するために, 国本層の石灰質ナンノ化石の検討を行った. 検討した66試料からは, 10属19種の石灰質ナンノ化石が産出した. 検討層準を通じて種組成に変化はないが, そのうち, 小型のGephyrocapsa属が多産する層準がMBB直上に位置し, この多産イベントは顕著である. この層準のイベントはイタリア南部のMontalbano JonicoやValle di Mancheでも指摘されていることから (Girone et al., 2013), グローバルに追跡可能なイベントである可能性がある. さらに, 寒冷な環境指標種であるCoccolithus pelagicusや温暖な環境指標種である

Umbilicosphaera sibogaeの産出には逆相関する関係が見られた. 層位分布をみるとUmbilicosphaera sibogaeが全層準を通して増減を繰り返す傾向が見られることから, 黒潮水域と混合水域の北上, 南下が交互に起きていたと考えられる.
引用文献
Girone et al., 2013, Palaeogeography, Palaeoclimatology, Palaeoecology, 371, 62-79.
http://www.elsevier.com
五十嵐, 1994, 地質学雑誌, 100, 348-359
Kazaoka et al., 2015, Quaternary International, 383, 116-135.
http://dx.doi.org/10.1016/j.quaint.2015.02.065
新妻, 1976, 地質学雑誌, 82, 163-181
佐藤ほか, 1988, 石油技術協会誌, 53, 475-491©2016. Japan Geoscience Union. All Right Reserved.

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日本地球惑星科学連合2016年大会

更新統下部ー中部境界を含む国本層上部の詳細層序と堆積環境：千葉セクション
Detailed litho-stratigraphy and sedimentary environment of upper part of Kokumoto
Formation with the L-M Pleistocene boundary: the Chiba section, Central Japan*風岡 修1、荻津 達1、八武崎 寿史1、本田 恵理1、吉田 剛1、亀山 瞬2、香川 淳1、森崎 正昭1、竹下 欣宏3、里口 保文4、中里 裕臣5、西田 尚久6、岡田 誠7、菅沼 悠介8、泉 賢太郎9、熊井 久雄10、楡井 久11*Osamu Kazaoka1, Itaru Ogitsu1, Hisashi Yabusaki1, Eri Honda1, Takeshi Yoshida1, Syun Kameyama2,Atsushi Kagawa1, Masaaki Morisaki1, Yoshihiro Takeshita3, Yasuhumi Satoguchi4, Yasuomi Nakazato5,Naohisa Nishida6, Makoto Okada 7, Yusuke Suganuma8, Kentaro Izumi9, Hisao Kumai10, Hisashi Nirei11
1.千葉県環境研究センター地質環境研究室、2.千葉県環境生活部、3.信州大学教育学部、4.琵琶湖博物館、5.農業工学研究所、6.産業技術総合研究所地質調査総合センター、7.茨城大学、8.国立極地研究所、9.国立環境研究所、10.大阪市立大学、11.GEM-IUGS日本支部
1.Research Institute of Environmental Geology, Chiba, 2.Environmental Protection division of Chiba
Prefectural Government, 3.Institute of Education, Shinshu University, 4.Lake Biwa Museum,
5.National Institute for Rural Engineering, 6.Geological Survey of Japan, AIST, 7. Ibaraki
University, 8. National Institute of Polar Research, 9.National Institute for EnvironmentalStudies, 10.Osaka City University, 11.Japan Branch of GEM-IUGS
The Lower –Middle Pleistocene Kazusa Group, deposited on mainly bathyal –shelf in the Pacific Ocean
with many kind of fossils, distributes widely in Boso peninsula. The Kazusa Group exposescontinuously along Yoro river, Chiba section, type section of the group. The Kazusa Group consistsof Kurotaki Formation (mainly tuffaceous gravelly sandstone), Katsuura F. (mainly alternation ofsandstone with slump bed), Namihana F. (mainly siltstone with slump bed), Ohara F. (muddy
alternation of sandstone and siltstone), Kiwada F. (muddy alternation of sandstone and siltstone
with slump bed), Otadai F. (alternation of sandy alternation and muddy alternation of sandstone andsiltstone), Umegase F. (mainly sandy alternation of sandstone and siltstone), Kokumoto F.
(alternation of thick siltstone and sandy alternation of sandstone and siltstone), Kakinokidai F.
(sandysiltstone with sandstone) and Chonan F. (alternation of thin sandstone and thin siltstone) inascending order (Mitsunashi et al., 1959). Total thickness of the Kazusa Group is over 2,000 meterswith over 50 marker tephra beds. Trapid depositional rates of 2.0-2.5 m/ky are obtained for theKazusa Group. Therefore the Chiba section have high potential for international stratotype section(Kazaoka et al., 2015).
Kokumoto Formation, about 350 meter thick, is composed of the lowermost part, the lower part, theupper part and the uppermost part in ascending order. The lowermost part, about 60 meter thick,consists of thick siltstone with thin sandstone bed and marker tephra, Ku6 and ku5. The lower part,about 120 meter thick, consists of sandy alternation of sandstone and siltstone with Ku3 tephra.
The upper part, about 80 meter thick, consists of thick siltstone with thin sandstone and marker
tephra (Byakubi zone (Byk-G, Byk-F, Byk-E, Byk-D, Byk-C, Byk-B and Byk-A), Koss2, Koss1-B, Koss1-A,Kosp-C, Kosp-B, Kosp-A, Tap-B, Tap-A, Tas-C, Tas-B, Tas-A, Ku2 and Ku1). Especially maker tephra
are interbeded every 0.1-7.0 ky in the thick siltstone from Byk-E to Ku2 horizon. The
Matuyama–Brunhes boundary is between Byk-C and Byk-B. 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 mthick and thin, 1-5 cm thick, sandysiltstone every 0.1-0.25 m thick without slump bed and thickmudflow bed. The siltstone have bathyal and sublittoral benthic foraminifera and many trace
fossils. Grain size distribution in the siltstone have bimodal grain group (Nishida et al., 2015).
Main grain group is composed of fine silt and sub group consists of very fine sand. These©2016. Japan Geoscience Union. All Right Reserved.

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日本地球惑星科学連合2016年大会

characteristics show hemipelagic sedimentary environment in deep sea and very fine sand flow often
into, namely deep sea slope. The thickness from Byk-G to Byk-A change little laterally in thecentral part of Boso Peninsula. This show that the fracks deposited approximately uniformly. It is
presumed that clastic sediments were supplied little around here from source mountain area, because
this horizon is warm stage, MIS 19.
キーワード：更新統下部－上部境界、国本層、上総層群、火山灰鍵層、Byk-E、MIS 19
Keywords: L-M Pleistocene boundary, Kokumoto Formation, Kazusa Group, Maker Tephra, Byk-E, MIS 19©2016. Japan Geoscience Union. All Right Reserved.

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