地質学雑誌
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
122 巻 , 6 号
選択された号の論文の5件中1~5を表示しています
論説
  • 田切 美智雄, 堀江 憲路, 足立 達朗
    2016 年 122 巻 6 号 p. 231-247
    発行日: 2016/06/15
    公開日: 2016/08/01
    ジャーナル フリー
    日立変成地域のカンブリア系について,ジルコンU-Pb SHRIMP年代値を追加し,既報の年代値をまとめ,各地層を再定義し,地質図を改めた.層序については,早期から晩期のカンブリア系を日立火山深成複合岩体として一括し,その中に赤沢層,玉簾層,変成花崗岩類,変成ポーフィリィを含めた.玉簾層の一部を分離して滝沢層を新設した.
    西堂平層の堆積年代は白亜紀前期である.さらに,白亜紀後期に火成岩貫入と中圧型変成作用が起った.これは阿武隈帯の竹貫変成岩類の変成履歴とも共通し,東北日本列島でこのような急激なテクトニクスが起こった時期があったと推定した.
    古第三紀の中央構造線が日本列島全体を縦断していたと仮定して,日本海形成前における,カンブリア系を含む日立古生層と東北日本列島基盤の復元を試み,日立古生層や阿武隈帯は中国東北部の佳木斯・ハンカ地帯に近接することを示した.カンブリア系日立火山深成複合岩体は北中国地塊の東縁に帰属することを主張した.
報告
  • 伊藤 剛, 北川 祐介, 松岡 篤
    2016 年 122 巻 6 号 p. 249-259
    発行日: 2016/06/15
    公開日: 2016/08/01
    ジャーナル フリー
    Middle and Late Permian (Capitanian and Changhsingian) radiolarians were obtained from chert blocks located within conglomerates of the Kamiaso Unit of the Mino terrane along Tsubogawa River, Gifu Prefecture, Central Japan. Previously, the Capitanian and Changhsingian cherts have not been reported from the sequences of the Akiyoshi terrane. Therefore, the collected chert blocks were not derived from the Akiyoshi terrane. The Capitanian and Changhsingian cherts have been recovered from the Permian sequences of Jurassic accretionary complexes in Southwest Japan; however, the characteristics of the Permian radiolarian fauna of the chert blocks are inconsistent with those from previously recovered Permian cherts. This indicates that the chert blocks were not derived from previously known units including Permian sequences. A previous study suggested that Triassic-Jurassic chert clasts within the chert breccias of the Kamiaso Unit were derived from the accreted Kamiaso Unit on the landward trench slope. As the Permian chert has not been identified in the coherent facies of the Kamiaso Unit, the Permian sequences must have been subducted during accretion. Our results suggest that the chert blocks were derived from the pre-accretion Kamiaso Unit located on the outer trench slope.
  • 松井 久美子, 指田 勝男, 上松 佐知子
    2016 年 122 巻 6 号 p. 261-266
    発行日: 2016/06/15
    公開日: 2016/08/01
    ジャーナル フリー
    Latest Jurassic to Cretaceous radiolarians have been identified in coarse-grained sandstones from the Middle Miocene Chichibumachi Formation of the Akahira Group, which occurs in the Chichibu Basin, Saitama Prefecture, central Japan. The following radiolarians were identified: Pseudodictyomitra lanceloti Schaaf, Holocryptocanium barbui Dumitrica, Diacanthocapsa betica O'Dogherty, Thanarla brouweri (Tan), Archaeodictyomitra sp. A, Archaeodictyomitra sp. B, Archaeodictyomitra? sp., Sethocapsa sp. A, Sethocapsa sp. B, Sethocapsa? sp., and Diacanthocapsa? sp. Radiolarian-bearing sandstone is present at ca. 22 m above the level containing Middle Miocene Paleoparadoxia tabatai, which is a marine mammalian fossil. Radiolarians occur within the sandstone as isolated specimens, along with fragments of bivalves, foraminifers, plant fossils, and quartz grains, some of the radiolarian specimens appear to have been reworked from the Ishido Formation or younger formations of the Sanchu Cretaceous System that crops out to the northwest of the Chichibu Basin.
  • 冨松 由希, 尾上 哲治, 野崎 達生
    2016 年 122 巻 6 号 p. 267-273
    発行日: 2016/06/15
    公開日: 2016/08/01
    ジャーナル フリー
    Stratiform manganese deposits are present in Triassic to Jurassic bedded chert sequences of the Chichibu Belt in southwest Japan, which are considered to have accumulated in a mid-ocean basin of the Panthalassa Ocean. To constrain the age and depositional setting of these deposits, we have investigated the field geology, stratigraphy, and age of chert-hosted manganese deposits (Takahira, Takahama, and Kubodomari) from the Chichibu Belt in the Saiki area, Oita Prefecture. The stratiform manganese ores range in thickness from 80 to 150 cm and are intercalated with bedded chert. The age of the deposits is constrained by the presence of radiolarian fossils in the associated bedded chert. The red-bedded chert above the Takahira manganese ores contains Late Triassic (Carnian) radiolarian fossils, including Trialatus longicornutus, Trialatus megacornutus, and Enoplocampe yehae. In contrast, radiolarian fossils from the Takahama and Kubodomari deposits occur in stratiform manganese ores of Early Jurassic age (possibly Toarcian), based on the occurrence of Parvicingula nanoconica with Parahsuum and Trillus species. These results suggest that two manganese ore forming events occurred in the pelagic Panthalassa Ocean during the Late Triassic and Early Jurassic.
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