島後層群と南隠岐リッジ  [in Japanese] The Dogo Group in the Oki Dogo Island, Southwest Japan and the South Oki Ridge  [in Japanese]

隠岐島後の上部漸新統〜中部中新統は, 下位から, 時張山, 郡および久見の3累層からなり, それらは一括して島後層群と命名される。時張山累層は主に中〜酸性火成岩類からなり, 2部層に区分される。郡累層は層厚・岩相の側方変化が著しく, パイモーダルな火山噴出物に起源をもつ河川成の堆積物を主体とし, 5部層に区分される。久見累層は主に河川, 浅海および深海成の砕屑物と珪藻質堆積物からなり, 5部層に区分される。島後層群の形成過程は時張山累層(〜20 Ma), 郡累層〜久見累層下部(20〜15 Ma)および久見累層上部の堆積期(15〜10? Ma)の3ステージに区分される。このステージ区分にしたがって日本海南西部の第三系を比較すると, この地域の第三紀堆積盆地はリッジートラフ構造を示し, 巨視的には島後-島根半島間の南隠岐リッジで二分されるとみなせる。

The Dogo Group (Late Oligocene to Middle Miocene) distributed in the Oki-Dogo Island, SW Japan, is closely related to the Miocene tectonics of the Japan Sea formation. The group consists of the Tokibariyama, Kori and Kumi formations in ascending order. The Tokibariyama Formation resting on the basement (Oki Metamorphics and older Granite), is composed mainly of felsic to intermediate igneous rocks with non-marine clastic rocks, and is lithologically subdivided in to the Lower and Upper members. The Kori Formation composed mainly of fluvial sediments with bimodal volcanism, comprises the Koji Tuff, Ohzuku Conglomerate, Togo Basalt, Kori Basalt and Nagu Volcanics members. The Kumi Formation is composed mostly of terrigenous -biosiliceous sediments deposited in fluvial, shallow marine and deep marine environments, and is subdivided into the Hei Tuff-Siltstone Alternation, Kumi Sandstone, Utagi Mudstone, Igo Mudstone and Iinoyama Diatomite members. Based on stratigraphy and structural features, the formation of the Dogo Groupis classified into three stages. If the classification of stages is applied to the Tertiary formations in the SW Japan Sea area (Yamato Basin-Bihoku), major geologic events in each stage are summarized as follows: Stage I (〜20 Ma): igneous activity characterized by the formation of cauldrons. Stage II (20〜15 Ma): differential subsidence resulting from faulting, and transgression. Stage III (15〜10? Ma): maximum transgression and subsequent uplift. Tertiary sedimentary basins in the area are divided into two major parts by the South Oki Ridge and their arrangement is controlled by ENE-WSW trending ridge-trough structures.