著者

鈴木 毅彦 藤原 治 檀原 徹

出版者

Tokyo Geographical Society

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.113, no.1, pp.38-61, 2004

被引用文献数

4 15

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This study shows the revised stratigraphy and correlations of the middle Pleistocene tephras in and around the Aizu area, Northeast Japan. Significant marker tephras in this area are as follows, in descending order of stratigraphy : Nm-SB, TG, Hu-TK, and Kn-KD from volcanoes adjacent to Oze, So-OT, and APm. Stratigraphic positions of Sn-MT and Sn-SK are not clear, but they seem to be positioned near Kn-KD.<BR>Nm-SB (110 ka) from Numazawa caldera is mainly a plinian pumice fall deposit distributed in the central to western part of Fukushima Prefecture and northern part of Tochigi Prefecture. Eruption producing Nm-SB associated with ash fall, plinian eruption, and pyroclastic flow or pyroclastic surge. TG (125-135 ka), which is characterized by volcanic glass shards with a low index, was probably derived from the Sunagohara caldera. The distribution of fall-out tephra of TG is similar to that of Nm-SB, and pyroclastic flow deposit of TG is recognized southwest to west of the Numazawa caldera and along the western margin of the Aizu basin. The eruption process of TG comprises plinian eruption, pyroclastic flow, and plinian eruption.<BR>So-OT (300-330 ka) is composed of an ignimbrite and a fall-out tephra derived from the Shiobara caldera. This ignimbrite is known as Otahara pyroclastic flow deposit. On the other hand, fall-out tephra of So-OT has been newly identified. APm tephra beds are significant widespread tephras derived from volcano in the Hida mountains at 330-400 ka. This study corrects a correlation of APm in this area, which was shown by Suzuki (1993). Tephras identified as APm in this study are Nm-13, -14, -16 tephras below So-OT.<BR>Sn-MT (180-260 ka : FT ages) is composed of an ignimbrite and a fall-out tephra derived from the Sunagohara caldera. The former is part of the Sunagohara-Kubota tephra reported by Yamamoto and Sudo (1996) and the Pyroclastic Flow Deposit I reported by Mizugaki (1993). The latter is the Sunagohara-Kachikata tephra along the western margin of Aizu basin reported by Yamamoto and Sudo (1996), and the Okayaji Volcanic Ash Layer at the eastern foot of Adatara volcano reported by Soda and Saijo (1987). Sn-SK (220 ± 50 ka : FT age), originating from the Sunagohara caldera, was defined by Yamamoto and Sudo (1996). Sn-SK is composed of an ignimbrite and a fall-out tephra characterized by abundant accretionary lapilli. The latter is correlative to the Minowa Volcanic Ash Layer (Soda and Saijo, 1987) distributed at the eastern foot of Adatara volcano.<BR>All pyroclastic deposits derived from the Numazawa caldera are Nm-NM (5 ka), Nm-KN (50-55 ka), and Nm-SB (110 ka), and those from the Sunagohara caldera are TG (125-135 ka), Sn-MT (180-260 ka), and Sn-SK (220 ka). This means that, at both caldera, explosive eruptions occurred three times over 260, 000 years, and it appears that the active period of explosive eruptions moved from the Sunagohara caldera to the Numazawa caldera. This resulted in the preservation of volcanic landforms with more dissected caldera landforms at the Sunagohara caldera. Intervals between eruptions at the Numazawa caldera range from 50, 000 to 60, 000 years, and the volumes of the three products are similar, indicating periodic and regular activities with a discharge rate of 0.02-0.06 DRE km<SUP>3</SUP>/1, 000 years. On the other hand, the mean interval of eruptions at the Sunagohara caldera is 70, 000 to 40, 000 years, and the discharge rate is estimated to be 0.05-0.08 DRE km<SUP>3</SUP>/1, 000 years : the latter is equivalent to or a little larger than that of the Numazawa caldera.

著者

岩野 英樹 折橋 裕二 檀原 徹 平田 岳史 小笠原 正継

出版者

一般社団法人 日本地質学会

雑誌

地質学雑誌 (ISSN:00167630)

巻号頁・発行日

vol.118, no.6, pp.365-375, 2012-06-15 (Released:2012-11-07)

参考文献数

44

被引用文献数

15 88

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同一ジルコン粒子を用いたフィッション・トラック(FT)法とU-Pb法によるダブル年代測定した年代値の信頼性を評価した.試料にはFT年代が33 Ma,自発FT密度が106〜107/cm2の島根県川本花崗閃緑岩三原岩体のジルコン粒子(OD-3)を用いた.U-Pb年代分析は,Nd-YAG(λ=213 nm)レーザーアブレーションシステムを搭載したICP質量分析法を用い,ジルコンを47%HF溶液による洗浄あるいはKOH-NaOH共融液によるFTエッチングを施した後に行った.その結果,33 MaのコンコーディアU-Pb年代が得られた.これは化学処理されたジルコンには顕著なPb損失は生じず,そして同一ジルコン粒子のFTおよびU-Pbダブル年代測定が実行可能であることを示す.今回使用した川本花崗閃緑岩三原岩体のジルコン粒子(OD-3)は若いジルコンU-Pb年代測定の標準試料になりうる試料である.

著者

鹿野 和彦 大口 健志 林 信太郎 宇都 浩三 檀原 徹

出版者

特定非営利活動法人 日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.47, no.5, pp.373-396, 2002

参考文献数

82

被引用文献数

3

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An alkali-rhyolite tuff-ring is newly identified in the western end of the Oga Peninsula and named as Toga volcano in this paper. The existence of this maar-type volcano at the Toga Bay has been suspected for a long time because of the elliptical embayment reminiscent of a maar and the distribution of the Toga Pumice localized along the bay coast. The Toga Pumice is cornposed mainly of pumice and non- to poorly-vesicular glass shards, but many pumices of lapilli size are rounded and fines are poor giving a sandy epiclastic appearance to the deposit. In our latest survey along the bay coast, the Toga Pumice is found to be in direct contact with the basement rocks. The contact steeply inclines at 40-50° and envelopes an elliptical area 2.0 km×2.4 km covering the bay and bay coast to form a funnel-shape structure. The basement rocks at the contact are brecciated to a depth of several tens of centimeters, or collapsed into fragments to be contained in the Toga Pumice. The beds inside the inferred crater incline toward the center of the crater at 10-30° or much smaller angles, presumably reflecting a shallow concave structure infilling the more steeply sided crater. The deposit is thinly to thickly bedded to be parallel- to wavy- or cross-stratified, inversely to normally graded with many furrows, rip-up clasts and load casts, and is sorted as well as fines-depleted pyroclastic flow deposits and/or pyroclastic surge deposits. These features are characterisitic to turbidites and indicate the place of emplacement was filled with water. Constituent glass shards are, however, commonly platy or blocky and likely to be phreatomagmatic in origin, and pumice lapilli are interpreted to have been originally angular but rounded by repeated entrainment and abrasion in multiple phreatomagmatic eruptions and succeeding emplacement in the crater lake. A pyroclastic surge deposit (Oga Pumice Tuff) correlative in composition and age to the Toga Pumice occurs at Anden and Wakimoto, 11 km and 15 km east of Toga, respectively. The juvenile pumice lapilli are angular to subrounded, in contrast with the pumice lapilli of the Toga Pumice.

著者

徳橋 秀一 檀原 徹 岩野 英樹

出版者

日本地質学会

雑誌

地質學雜誌 (ISSN:00167630)

巻号頁・発行日

vol.106, no.8, pp.560-573, 2000-08-15

被引用文献数

8 19

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房総半島中部に分布する安房層群上部の3累層(下位より, 天津層, 清澄層, 安野層)から, 8つの凝灰岩を選んでFT年代測定を行った.1)その結果, Am19 : 11.7±0.3Ma, Am40 : 8.5±0.5Ma, Am61 : 6.3±0.5Ma, Am78 (Oktuff) : 5.7±0.4Ma, Am94 : 5.2±0.3Ma(以上, 天津層), Ky21 (Hk tuff) : 4.5±0.2Ma(清澄層), An49 : 3.9±0.4Ma, An73 : 3.7±0.2Ma(以上, 安野層)の値を得た.これらの値は, 1990年に出されたIUGSサブコミッションの勧告に従った, 安房層群上部に属する凝灰岩についての初めての年代値である.2)今回得られた年代値とこれまでの放射年代値とを比較した場合, 従来のFT年代値よりはやや若い値を示すが, 極く最近報告されたK-Ar年代値とはよい一致を示す.3)浮遊性微化石の生層序年代値と比較した場合, 天津層中・下部と安野層の場合は比較的よい一致がみられるが, 天津層上部および清澄層の凝灰岩の場合には, FT年代値の方が有意に若い年代を示す.不一致の原因は今後の課題である.