地学雑誌
Online ISSN : 1884-0884
Print ISSN : 0022-135X
ISSN-L : 0022-135X
最新号
選択された号の論文の9件中1~9を表示しています
表紙
  • 2021 年 130 巻 1 号 p. Cover01_01-Cover01_02
    発行日: 2021/02/25
    公開日: 2021/03/18
    ジャーナル フリー

     アイルランド西部クレア県(County Clare)の大西洋岸には,上部石炭系Namurian階のCentral Clare層群が広く露出している.Central Clare 層群は,河川-デルタ環境で堆積した砕屑性堆積岩からなる.バレン・モハーの断崖ユネスコ・グローバル・ジオパークの一角をなすモハーの断崖(Cliffs of Moher)は,Central Clare層群が観察できる景勝地としてあまりにも有名だが,断崖絶壁であるため地層一枚一枚を間近に観察するには向いていない.これに対して,スパニッシュポイント(Spanish Point)では,引き潮時に,層理面が10~20°ほど傾いた砂岩・泥岩互層が海岸一面に姿を現す.波による侵食を受けて,地層が層理面に沿ってきれいにはがされており,斜交層理やリップルマークなどの堆積構造を観察することができる.なお,スパニッシュポイントは小さな村だが,ウィリークランシー・サマースクール(Willie Clancy Summer School:アイルランド有数のアイルランド音楽とダンスのサマースクール)の開催地ミルタウンマルベイ(Milltown Malbay)の近くであることから,日本人来訪者も多い.

    (写真・解説:納谷友規 2014年7月13日撮影)

論説
  • ─火砕岩鍵層の追跡による複合デュープレックス構造と断層伝播褶曲─
    森 慎一, 小川 勇二郎
    2021 年 130 巻 1 号 p. 1-26
    発行日: 2021/02/25
    公開日: 2021/03/18
    ジャーナル フリー
    電子付録

     Mapping was carried out of well exposed folded and faulted strata in the Ubajima Islands off Chigasaki City, Kanagawa Prefecture, central Japan of the northernmost part of Sagami Bay, where the present Philippine Sea plate subduction boundary extends beneath the central part of the North America plate. The strata were dated with radiolarian fossils as middle to late Miocene, and correlated with part of the Misaki Formation of the Miura Group of the southern Miura Peninsula. The strata are composed of dominant basaltic pyroclastic or volcaniclastic sedimentary rocks with sporadic felsic tephra layers and rare silty sedimentary rocks, and were defined as the Ubajima Formation. Detailed field mapping of the Ubajima Formation verified that the strata are deformed in a complicated way into duplex structures having two stages. The first stage is characterized by bedding parallel to subparallel thrust faults in the SW direction of the horizontal reference frame. This might be of the antiformal stack type known by many parallel repetitions of the same horizons. At the second stage, the same parts are further duplicated into other antiformal stack-type duplex structures, forming multi-duplex structures. Finally those structures are folded and faulted by a fault-propagation fold system that has strong shear zones at propagation fault zones. The Ubajima Formation is thought to be deposited on the forearc area of the Izu volcanic arc, then accreted to the Honshu side by accretionary prism formation. These structures represent good examples of accretionary prism toes along subduction zones.

  • 福井 幸太郎, 曽根 敏雄, STRELIN Jorge A., 森 淳子
    2021 年 130 巻 1 号 p. 27-41
    発行日: 2021/02/25
    公開日: 2021/03/18
    ジャーナル フリー

     James Ross Island, Antarctic Peninsula, has recently experienced severe climate change: warming from the 1970s until the early 2000s and cooling from the early 2000s until around 2014. Lachman II, a 2-km long debris-covered glacier (ice-cored rock glacier), is located at the northern part of James Ross Island. On Lachman II, the surface elevation lowered at a rate of 1.1 and 0.7 m a−1 at the center of the upper stream of the debris-covered area from 1992 until 1995. However, in response to the cooling trend from the early 2000s until around 2014, the surface lowering rate temporarily slowed to 0.2 and 0.4 m a−1 for at least seven years from 2008 to 2015 at the center of the upper stream of the debris-covered area. In response to extremely high air temperatures in 2016, the glacier surface lowered again, accelerating to 0.5 and 0.6 m a−1 from 2015 to 2017. A ground penetrating radar (GPR) image displayed a reflection plane along the longitudinal section of the glacier, which indicates that a debris-rich ice layer was thrusting up from the glacier bottom. The origin of the upward thrust is supported by the fact that the surface rock debris consists mainly of sandstone and mudstone derived from the bottom of the glacier.

  • ─岩相・鍵層と地域・広域対比─
    河野 聖那, 磯﨑 行雄, 佐藤 友彦, 張 興亮, 劉 偉
    2021 年 130 巻 1 号 p. 43-62
    発行日: 2021/02/25
    公開日: 2021/03/18
    ジャーナル フリー

     Detailed lithostratigraphy of the Lower Cambrian in South China, composed mostly of phosphorite and carbonate, is analyzed at the Xiaolantian section in the Chengjiang area, Yunnan. Six distinct lithostratigraphic units (Units 1 to 6 in ascending order) are recognized in the small shelly fossil (SSF)–bearing Zhongyicun Member, together with one particular terrigenous clastic unit (Unit 3) in the middle. Sedimentary features of these beds indicate that they were deposited in shallow marine settings in off–shore to upper slope of the Kangdian rift-related basin in western South China. The lithostratigraphy of the Xiaolantian section correlates not only with those of the neighboring sections at Hongjiachong and Maotianshan in Chengjiang, but also with those of five other sections (Meishucun, Wangjiawan, Xiaohuangcaoling, Zhujiaqing, and Laolin) in Yunnan. This regional correlation highlights a particular unit composed of terrigenous clastics solely in the middle of the Zhongyicun Member, e.g., Unit 3 of the Xiaolantian section. This unit serves as a useful key bed for a lithostatigraphical correlation within the basin because a similar unit (interval) was recognized at the same horizon in seven other sections in Yunnan. This key bed and its stratigraphic horizon are noteworthy, as this marks a significant horizon of SSF zone boundary; i.e., between the Anabarites trisulcatus–Protohertzina anabarica Zone and the Paragloborilus subglobosus–Purella squamulosa Zone of the Terreneuvian, Lower Cambrian.

  • ─蝦夷層群の砕屑性ジルコンU–Pb年代測定─
    石坂 湧人, 磯﨑 行雄, 堤 之恭
    2021 年 130 巻 1 号 p. 63-83
    発行日: 2021/02/25
    公開日: 2021/03/18
    ジャーナル フリー
    電子付録

     To reconstruct the tectono-sedimentary history of the Cretaceous–Paleogene arc-trench system in western Hokkaido, U–Pb ages were measured of detrital zircons in 11 Cretaceous and Paleogene fore-arc sandstones of the Yezo Group in the Oyubari and Mikasa areas. Age spectra of dated zircons demonstrate that Aptian sandstones of the basal Yezo Group are characterized by the dominant occurrence of Early Cretaceous grains with small amounts of Jurassic, Triassic, Permian, and Precambrian grains (Type 1). In contrast, those of Cenomanian to Paleocene sandstones of the middle to upper Yezo Group are dominated almost totally by mid- to Late Cretaceous grains (Type 2). This remarkable change from Type 1 to Type 2 occurred irreversibly during the Albian, indicating that the surface crust in the provenance in western Hokkaido was significantly renewed then. Among nine sandstones of Type 2, a positive correlation exists between zircon peak age and stratigraphic age, suggesting the unidirectional/gradual replacement of exposed magmatic rocks in the provenance, in particular, new volcanics/intrusives (Rebun–Kabato belt to the west). The pre-Cretaceous zircons in Type 1 sandstones were probably recycled from sandstones in the Jurassic accretionary complexes of the Oshima belt to the west. The coeval and identical turnover in zircon age spectra of the Cretaceous fore-arc sandstones recognized in Southwest Japan, from Kyushu to Kanto district, suggests that the fore-arc basin and its provenance with monotonous arc crustal rocks have ubiquitously developed along the Cretaceous East Asian margin over more than 1,500 km before the Miocene opening of the Japan Sea.

短報
  • 森 宏, 友岡 洋介, 常盤 哲也, 纐纈 佑衣
    2021 年 130 巻 1 号 p. 85-98
    発行日: 2021/02/25
    公開日: 2021/03/18
    ジャーナル フリー
    電子付録

     Raman spectral analyses were conducted of carbonaceous materials (CM) in meta-sedimentary rocks to estimate the metamorphic thermal structure of the Outer Zone of Southwest Japan in the Akaishi Mountains area, central Japan. Twenty samples of pelitic and psammitic rocks were collected along the Koshibu River and by traversing the Sanbagawa, Chichibu, and Shimanto belts. The datasets of the full width at half maximum of the D1-band (FWHMD1)—which is a sensitive thermal parameter of the Raman spectra of CM—imply that each rock records a single metamorphic thermal event. The peak metamorphic temperatures estimated with Raman CM geothermometry show 286-339°C in the Sanbagawa Belt, 284-350°C in the Chichibu Belt, and 328-347°C in the Shimanto Belt with a continuous spatial change including at the boundaries between belts. From northwest to southeast, a clear temperature decrease is observed in the Sanbagawa Belt, but an increase in the Chichibu Belt with a constant high-T condition—similar to those recognized in southeast part of the Chichibu Belt—in the Shimanto Belt. A comparison of geological structures suggests that the present thermal structures in the Chichibu and Shimanto belts were formed with the formation of a “twist-bend” structure related to the collision of the Izu-Ogasawara (Izu-Bonin) Arc with respect to the Honshu Arc at middle Miocene. In contrast, the thermal structure in the Sanbagawa Belt was mainly formed before about 20 Ma but the above middle Miocene tectonics may involve the formation of the apparent steep geothermal gradient.

寄書
  • 小元 久仁夫
    2021 年 130 巻 1 号 p. 99-105
    発行日: 2021/02/25
    公開日: 2021/03/18
    ジャーナル フリー

     Comprising beachrock, Maibahbama lies southeast of Miyako Island. It is approximately 400 m in length and 3 to 17 m in width. Fossil shell, coral and calcarenite samples were collected from the eastern part of Maibabahbama in order to determine their ages using accelerator mass spectrometry (AMS). 14C age of a Nerita albicilla yielded an age of Modern (100.7 ± 0.3 pMC); however, Tridacna gigas yielded an age of 981 ± 22 yr BP. Coral fragments yielded an age of 1746 ± 23 yr BP, while four calcarenite samples were between 1350 ± 23 and 1567 ± 22 yr BP. The above samples were collected from two outcrops standing 1 m apart and within a 10 cm circle. Five of the seven samples coincided with the estimated ages of past tsunamis within an error range of ±3σ. Consequently, the several samples collected from beachrock at the eastern part of Maibahbama might have been washed ashore by past tsunamis before being cemented. Formation of the eastern part of the beachrock started at 980 yr BP and ended recently. Therefore, they do not coincide with the older ages of other tsunami boulders (Porites sp.) nearby, reported previously.

  • 宮縁 育夫, 鳥井 真之
    2021 年 130 巻 1 号 p. 107-116
    発行日: 2021/02/25
    公開日: 2021/03/18
    ジャーナル フリー
    電子付録

     Intense rainfall related to a stationary front on July 3-4, 2020 (maximum total 513 mm; maximum 4-hour rainfall 260 mm) triggered many landslides and floods in the southern part of Kumamoto Prefecture, Kyushu, southwestern Japan. The distribution of landslides coincided with the highest concentration of rainfall in the region. The July 4, 2020 landslides were concentrated mainly in Ashikita and Tsunagi towns by the Yatsushiro Sea. Most of the landslides occurred on forested slopes steeper than 20°. They occurred in thick weathered soil layers overlying sedimentary and volcanic rocks; the thickness of the slides was generally less than 20 m. Some of those occurring on steep upper slopes along valleys mobilized completely into debris flows, traveling along stream channels and flooding at the lower reaches. The landslides and debris flows transported sediments and woody debris, which damaged houses and caused fatalities at some sites. The July 4, 2020 landslides and associated debris flows occurred mainly in areas underlain by Jurassic accretionary complexes of the Chichibu Belt. Some landslides were affected by fault fracture zones with the formation of thick weathered soil layers. These geologic characteristics of landslides provide important information for preventing or mitigating similar future disasters in accretionary complex zones.

世界露頭100選(No.12)
  • LOWE David J., PITTARI Adrian
    2021 年 130 巻 1 号 p. 117-141
    発行日: 2021/02/25
    公開日: 2021/03/18
    ジャーナル フリー

     西暦232 ± 10年の晩夏にニュージーランド北島タウポ火山で起こった噴火は,過去5,000年間において地球上で起こった噴火のなかでもっとも強力なものであった。噴火は数日から数週間継続し,5つの明確な降下火砕堆積物(ユニットY1~Y5)に続いて,非常に爆発的な噴火による低アスペクト比イグニンブライト(ユニットY6)が堆積した。降下火砕堆積物の内,ユニットY1,Y3およびY4は水蒸気プリニー式噴火によって形成され,Y2とY5はプリニー式噴火であった。Y5とY6は一連の噴火で形成され,非常に強いY5噴火による噴煙柱は高度35-40 kmに達し,それが崩壊することによって非常に高速(600-900 km/h)で高温(最高500°C)の火砕密度流が発生し,ユニットY6が堆積した。このイベントによる堆積物は噴火後十数分で北島中央部の約20,000 km2に及ぶ範囲を覆い尽したと考えられる。また一連の噴火によるマグマ噴出量は約35 km3と見積もられている。この噴火による周辺環境への影響は甚大であり,現代においても農業などの土地利用において火山ガラスを多く含み,コバルトなどの微量元素に枯渇した土壌への対策が必要となっている。

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