- 著者
-
松岡 篤
- 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.90, no.7, pp.455-477, 1984-07-15
- 被引用文献数
-
21
41
高知県西部の秩父累帯南帯には,チャートと粗粒砕屑岩とのくりかえしで特徴づけられる地層が広く分布する.このくりかえしは,下位から層状珪質粘土岩層,チャート層,珪質泥岩層,粗粒砕屑岩層からなる層序ユニット(トリアス紀中世からジュラ紀中世ないし新世)が北方上位・北傾斜で構造的にくりかえして重なり覆瓦状構造を呈するためにみられるものである.この層序ユニットを斗賀野層群として再定義した.詳細な放散虫生層序学的検討の結果,同層群のチャート層の上限の年代や珪質泥岩層・粗粒砕屑岩層の年代が北から南に向かって若くなるという年代的極性をもつことが明らかになった.斗賀野層群の基本的層序にみられる岩相の垂直変化は,堆積場が遠洋域から陸源物質の供給される場に移化したことを示すと解釈される.上述した斗賀野層群の性質は,弧―海溝系で提唱されている付加体モデルと矛盾しない.In the present area, Mesozoic strata are widely distributed. Based on lithology and fossil contents, they are divided into five Groups or Formations, namely Togano Group (Middle Triassic-Late Jurassic), Naradani Formation (Late Jurassic), Torinosu Group (late Late Jurassic-? Early Cretaceous), Yamanokami Formation (Early Cretaceous) and Bandagamori Formation (Late Jurassic-). As the first step of the geologic study in this area, the Togano Group is described and discussed. The Togano Group consists mainly of chert and coarse elastics (sandstone, sandstone-rich alternation of sandstone and mudstone, rarely conglomerate) associated with mudstone, siliceous mudstone, bedded siliceous claystone, acidic tuff and olistostrome, and is characterized by apparently alternate occurrence of chert and coarse clastics. The above rock types are grouped into four lithostratigraphic members, namely chert layer, coarse clastic layer, bedded siliceous claystone layer and siliceous mudstone layer. The alternate occurrence of chert and coarse elastics is due to repetition of stratigraphic unit (Middle Triassic-Middle or Late Jurassic) composed, in ascending order, of (1) bedded siliceous claystone layer, (2) chert layer, (3) siliceous mudstone layer and (4) coarse elastics layer. The stratigraphic units are tectonically piled up to form north dipping imbricate structure. As the result of detailed radiolarian biostratigraphic study, it is revealed that the age of the top of the chert layer (2), and the age of the siliceous mudstone layer (3) and the coarse clastics layer (4) become systematically younger from the northern stratigraphic units to the southern ones. Lithological facies change of the stratigraphic units of the Togano Group indicates that sedimentary environment changed from pelagic through hemipelagic to terregenous environment. The above-mentioned features of the Togano Group are consistent with the accretion model proposed in arc-trench system.