SGL38-P01

会場:コンベンションホール

時間:5 月 25 日 18:15-19:30

下部ー中部更新統境界 GSSP 候補地へのご案内
Invitation to the Tabuchi section, central Japan: A candidate GSSP for the Lower-Middle
Pleistocene Subseries/Subepoch
吉田 剛 1∗ ; 荻津 達 1 ; 風岡 修 1 ; 岡田 誠 3 ; 菅沼 悠介 2 ; 亀尾 浩司 4 ; 楡井 久 5 ; 会田 信行 6 ;熊井 久雄 7 ; 西田 尚央 8 ; 泉 賢太郎 9
YOSHIDA, Takeshi1∗ ; OGITSU, Itaru1 ; KAZAOKA, Osamu1 ; OKADA, Makoto3 ; SUGANUMA, Yusuke2 ;KAMEO, Koji4 ; NIREI, Hisashi5 ; AIDA, Nobuyuki6 ; KUMAI, Hisao7 ; NISHIDA, Naohisa8 ;IZUMI, Kentaro91

千葉県環境研究センター地質環境研究室, 2 国立極地研究所, 3 茨城大学, 4 千葉大学, 5 GEM 日本支部, 6 秀明大学, 7 大阪
市立大学, 8 産業技術総合研究所, 9 東京大学
1Reserch 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 theLower - 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.

キーワード: 田淵セクション, 下部・中部更新統
Keywords: Tabuchi section, Lower and Middle Pleistocene

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SGL38-P02

会場:コンベンションホール

時間:5 月 25 日 18:15-19:30

中下部更新統，中上部上総層群の詳細テフラカタログ
Detailed Tephra Catalog of Lower to Middle Part of the Kazusa Group, Lower to Middle
Pleistocene, Central Japan中里 裕臣 1∗ ; 七山 太 2
NAKAZATO, Hiroomi1∗ ; NANAYAMA, Futoshi2
11

農業・食品産業技術総合研究機構 農村工学研究所, 2 産業技術総合研究所 地質情報研究部門
National Institute for Rural Engineering, NIRE, 2 Geological Survey of Japan, AIST

房総半島に分布する上総層群は，日本を代表する中下部更新統の一つである．上総層群は深海から浅海までの堆積環境で形成された連続的な堆積物で構成されており，天然ガスとヨウ素に富む鹹水のリザーバとしても知られている．上総層群では古くから数多くのテフラ層の認定に基づき層序区分が行われてきた．著者らは，中上部上総層群が分布する茂原地域における地質図幅調査の一環として，従来のテフラ層序を再確認し，テフラカタログを作成した．上総層群大田代層中部の O7 から笠森層の Ks4 までの間で確認されたテフラは 139 層である．これらの中には中下部更新統境界を指示する Byk-E テフラも含まれる．テフラの同定には，火山ガラス，斜方輝石及び角閃石の屈折率とともに，一部のテフラについては火山ガラスの主成分化学組成を用いた．房総半島北西部の銚子地域では同時代の地層について酸素同位体比曲線が求められており，テフラの対比により各テフラの MIS 年代を求めることができる．

キーワード: 中?下部更新統, 中?上部上総層群, テフラカタログ, 茂原地域, 房総半島, 千葉県
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

会場:コンベンションホール

時間:5 月 25 日 18:15-19:30

房総半島国本層における珪藻化石層序
Detailed stratigraphy of diatom assemblages from a core of the Kokumoto Formation collected in the Boso Peninsula, Japan
田中 郁子 1∗ ; 兵頭 政幸 2 ; 北場 育子 3 ; 佐藤 裕司 4
TANAKA, Ikuko1∗ ; HYODO, Masayuki2 ; KITABA, Ikuko3 ; SATO, Hiroshi41

神戸大学大学院理学研究科, 2 神戸大学内海域環境教育研究センター, 3 立命館大学 総合科学技術研究機構 古気候学研究センター, 4 兵庫県立大学 自然・環境科学研究所
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 HyogoDiatom 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 ofA. 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.
キーワード: 国本層, パラリア サルカータ, アクチノキクルス インゲンス, MIS19, 松山‐ブリュンヌ境界, GSSP
Keywords: Kokumoto Formation, Paralia sulcata, Actinocyclus ingens, MIS19, Matsuyama-Brunhes boundary, GSSP

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SGL38-P04

会場:コンベンションホール

時間:5 月 25 日 18:15-19:30

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 ; 楡井 亘 4
NIREI, Hisashi1∗ ; KAZAOKA, Osamu2 ; KIMURA, Hideto3 ; YAOSHIDA, Takeshi2 ; NIREI, Wataru411

茨城大学・ ＩＵＧＳ ＧＥＭ, 2 千葉県環境研究所地質環境研究室, 3 東邦地水株式会社, 4 東京情報大学
Ibaraki Univ. ; IUGS GEM, 2 Res. Inst. of Environmental Gology, Chiba, 3 Tohocjhisui Co., 4 .Tokyo Univ. of Infor. Sci.

関東フォアーク・ベースンの前期更新統は深海堆積相をしめすが、次第に浅海化する。東京湾不整合によって浸食される。この不整合下位に上総層群が分布する。
この不整合は、東京湾岸の地下で発見されており、International Stratigraphic Guide に沿って模式地も東京湾岸の地下にある。そして、一連の不整合現象は東京湾側で顕著であるが、北東方向の長南町周辺からは整合となる。
地上では、海退期のデルタ堆積相の万田野層直下に認められる。万田野層の中部泥層は、上下に２分され、下半部は淡水域の環境を示し、Picea maximowiczii, Tsuga diversifolia,Fagus crenata, などの植物遺体が発見されている。つまり、当時の植生は、寒冷系の植物からなる植物群で構成されていたことが示唆される。また、大阪層群で知られる満池谷寒冷期と同一寒冷期の可能性も高い。関東フォアーク・ベースン＝関東深海盆が、始めて陸化した現象でもある。
中部泥層の上半部は汽水域となり、さらに上位の砂礫相には海生貝化石が多産し笠森層の浅海層へとかわる。笠森層の堆積時には、関東平野は関東大陸棚と呼ぶにふさわしい堆積環境にあったようである。

キーワード: 東京湾不整合, 万田野寒冷期, 上総層群, 下総層群, 関東大陸棚
Keywords: Tokyo Bay Unconformity, Mandano Ice Age, Kazusa Group, Shimohusa Group, Kanto Continental Shelf

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SGL38-P05

会場:コンベンションホール

時間:5 月 25 日 18:15-19:30

福島県南相馬市塚原海岸に分布する中期-後期更新世境界にあたる塚原層の古地磁気および古環境学的研究
Paleomagnetic and paleoenvironmental studies for the U-M Pleistocene boundary Tukabara Formation
川又 基人 1∗ ; 中川 純子 1 ; 池田 悠人 1 ; 菅谷 真奈美 1 ; 大井 信三 1 ; 岡田 誠 1
KAWAMATA, Moto1∗ ; NAKAGAWA, Junko1 ; IKEDA, Yuto1 ; SUGAYA, Manami1 ; OOI, Shinzo1 ; OKADA, Makoto111

茨城大学理学部
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: stablemagnetization 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.

キーワード: 中期-後期更新世境界, 古地磁気, 古環境復元, Blake イベント
Keywords: Upper-Middle Pleistocene boundary, Paleomagnetism, Paleoenvironmental reconstruction, Blake event

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