著者
松本 良 奥田 義久 蛭田 明宏 戸丸 仁 竹内 瑛一 山王 梨紗 鈴木 麻希 土永 和博 石田 泰士 石崎 理 武内 里香 小松原 純子 Antonio Fernando FREIRE 町山 栄章 青山 千春 上嶋 正人 弘松 峰男 Glen SNYDER 沼波 秀樹 佐藤 幹夫 的場 保望 中川 洋 角和 善隆 荻原 成騎 柳川 勝則 砂村 倫成 後藤 忠則 廬 海龍 小林 武志
出版者
公益社団法人 東京地学協会
雑誌
地学雑誌 (ISSN:0022135X)
巻号頁・発行日
vol.118, no.1, pp.43-71, 2009-02-25 (Released:2010-04-05)
参考文献数
46
被引用文献数
65 59

A number of extensive methane plumes and active methane seeps associated with large blocks of methane hydrates exposed on the seafloor strongly indicate extremely high methane flux and large accumulations of methane hydrate in shallow sediments of the Umitaka spur and Joetsu knoll of the Joetsu basin 30 km off Joetsu city, Niigata Prefecture. Crater-like depressions, incised valleys, and large but inactive pockmarks also indicate methane activities over the spur and knoll. These features imply strong expulsions of methane gas or methane-bearing fluids, and perhaps lifting and floating-up of large volumes of methane hydrate to the sea surface. High heat flow, ∼100 mK/m, deposition of organic-rich strata, ∼1.0 to 1.5%TOC, and Pliocene-Quaternary inversion-tectonics along the eastern margin of the Japan Sea facilitate thermal maturation of organic matters, and generation and migration of light-hydrocarbons through fault conduits, and accumulation of large volumes of methane as methane hydrate in shallow sediments. Microbial methane generation has also contributed to reinforcing the methane flux of the Joetsu basin. Regional methane flux as observed by the depth of the sulfate-methane interface (SMI) is significantly high, < 1 m to 3 m, when compared to classic gas hydrate fields of Blake Ridge, 15 to 20 m, and Nankai trough, 3 to 15 m. δ13C of methane hydrate and seep gases are mostly within -30 to -50‰, the range of thermogenic methane, while dissolved methane of the interstitial waters a few kilometers away from seep sites are predominated by microbial with δ13C of -50 to -100‰. Seismic profiles have revealed fault-related, well-developed gas chimney structures, 0.2 to 3.5 km in diameter, on the spur and knoll. The structures are essential for conveying methane from deep-seated sources to shallow depths as well as for accumulating methane hydrate (gas chimney type deposits). The depth of BSR, which represents the base of gas hydrate stability (BGHS), on the spur and knoll is generally 0.20 to 0.23 seconds in two-way-travel time, whereas the BSRs in gas chimneys occur at 0.14 to 0.18 seconds, exhibiting a sharp pull-up structure. The apparent shallow BGHS is due to the accumulation of large volumes of high-velocity methane hydrate in gas chimneys. The depth to BGHS is estimated to be 115 m on an experimentally determined stability diagram, based on an observed thermal gradient of 100 mK/m. Then the velocity of the sediments on the Umitaka spur is calculated to be 1000 m/s, which is anomalously low compared to normal pelagic mud of 1600-1700 m/s. This exciting finding leads to the important implication that sediments of the Umitaka spur contain significant amounts of free gas, although the sediments are well within the stability field of methane hydrate. The reasons for the existence of free gas in the methane hydrate stability field are not fully explained, but we propose the following possible mechanisms for the unusual co-existence of methane hydrate and free-gas in clay-silt of the spur. (i) High salinity effect of residual waters, (ii) degassing from ascending fluids, (iii) bound water effect and deficiency of free-waters, and (iv) micro-pore effect of porous media. All of these processes relate to the development of gas hydrate deposits of the Umitaka spur.(View PDF for the rest of the abstract.)
著者
小松原 純子
出版者
The Sedimentological Society of Japan
雑誌
堆積学研究 (ISSN:1342310X)
巻号頁・発行日
vol.71, no.2, pp.119-127, 2012-10-06 (Released:2012-11-08)
参考文献数
105
被引用文献数
10 9

これまで地層中の砂質津波堆積物を扱った論文のうち,堆積環境が浅海域および沿岸低地であるものについて,著者らがなにを根拠に津波堆積物であると判断したのかについてまとめた.根拠は大きく分けて(1)砂層の分布範囲が広い,(2)歴史記録と年代が一致する,(3)特徴的な堆積構造がある,(4)地殻変動を伴う,(5)特徴的な構成粒子を伴う,の5つに分けられる.地層中の津波堆積物を識別するには,現世の津波堆積物の観察が重要だが,地層としてそのまま保存されないものもあること,過去と現在の海岸線の条件が違うことに注意が必要である.すべての津波堆積物に適用可能な識別基準はなく,根拠の組合せや堆積場の条件わけが必要となってくると考えられる.
著者
藤原 治 小松原 純子 高田 圭太 宍倉 正展 鎌滝 孝信
出版者
Tokyo Geographical Society
雑誌
地学雑誌 (ISSN:0022135X)
巻号頁・発行日
vol.115, no.5, pp.569-581, 2006-12-25 (Released:2009-11-12)
参考文献数
36
被引用文献数
9 9

The temporal development of a late Holocene strand plain system along the western Shizuoka Prefecture was reconstructed based on facies analyses and 14C dating for core samples excavated in a back marsh using a geo-slicer, 6.0-m-long, 0.35-m-wide, and 0.05- to 0.1-m-deep wedge-shaped stainless steel case. The strand plain system consists of beach, sand dune, and back marsh. Stratigraphic succession of the strand plain system, up to 4.4 m thick, is composed of upper shoreface sand, foreshore sand, backshore sand, and back marsh mud, in ascending order. The succession shows three development stages of the strand plain system.Stage 1 (before the 13th century) : The study area was under a wave-dominated beach environment. The beach system was developed by progradation of shoreface, foreshore, and backshore deposits in the later period of this stage.Stage 2 (from the 13th century to the 16th century) : Sand dune and back marsh developed, covering the beach deposit. Humic mud was thickly deposited in the back marsh with low sand supply from seaward across the dune.Stage 3 (after the 17th century) : The back marsh has been infilled mainly by washover sand and debris from the hinterland. The AD 1707 Hoei Earthquake Tsunami, which destroyed villages on the dune, possibly promoted reactivation of sand movement from ruined dune to the back marsh.
著者
佐藤 時幸 中川 洋 小松原 純子 松本 良 井龍 康文 松田 博貴 大村 亜希子 小田原 啓 武内 里香
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.110, no.1, pp.38-50, 2004-01-15
参考文献数
25
被引用文献数
5 10

沖縄本島南部のうりずん露頭とそれに近接する守礼ゴルフ場露頭における知念層の石灰質ナンノ化石および浮遊性有孔虫化石生層序を検討した.その結果,知念層基底の地質時代は両露頭とも1.97 Maよりやや古い鮮新世末に対比されるが,うりずん露頭における知念層の基底の地質年代は,守礼ゴルフ場露頭の同基底年代よりやや古い.また,うりずん露頭および守礼ゴルフ場露頭のいずれにも時間間隙が認められたが,うりずん露頭では知念層下部に約30万年の時間間隙が,守礼ゴルフ場では新里層と知念層の境界に若干の時間間隙が推定され,両露頭は近接するにもかかわらず,時間間隙の層準および規模が異なることが明らかとなった.本研究の結果は,琉球列島が現在のようなサンゴ礁の広がる海域へと姿を変えた過程を復元するためには,知念層およびその上下層の岩相層序と微化石層序との関係を解明することが,重要であることを示唆している.