静岡県清水市北部,興津川流域の地質
1986 年 40 巻 3 号 p. 147-165
詳細
1986 年 40 巻 3 号 p. 147-165
The Neogene in the Fuji River Valley has been considered to be disrupted by several major thrusts to form a westward-dipping imbricated structure (MATSUDA, 1961). The Neogene correlation of the southern and northern Fuji River Valley is a troublesome problem by reason of remarkable lateral facies changes. In the present paper, the authors present the revised stratigraphy and a description of the geological structure of the Neogene in the northern Shimizu City, which occupies the southwestern corner of southern Fuji River Valley. The Hamaishidake Group developed in and around the studied area is subdivided into six formations, namly the Kogochi, Murono, Sattatoge, Utsugino, Nakagochi and Kawaino Formations in ascending order. The Utsugino, Nakagochi and Kawaino Formations are distributed in the studied area. The Utsugino Formation is distributed on the both wings of the Inasegawa Anticline. On the east wing volcanic conglomerate is characteristic of the Formation (The Yamauchi and Sakurano Members), whereas on the west wing dominate mudstone-rich alternation of sandstone and mubstone (the Hirayama Member). The Nakagochi Formation is subdivided into five members: the Nakaisshiki, Kanzawara, Mobata, Wadashima and Tozurasawa Members in ascending order. The Nakaisshiki is composed mainly of pyroclastic rock and conglomerate, the Kanzawara of conglomerate and sandstone-rich alternation of sandstone and mudstone, the Mobata of sandstone-rich alternation of sandstone and mudstone with conglomerate, the Wadashima of sandstone-rich alternation of sandstone and mudstone, the Tozurasawa of pyroclastic rock and mudstone. The Kawaino Formation mainly consists of conglomerate. The Hamaishidake Group was subjected to a series of NS-trending folding movement which occurred during the Hamaishidake stage (Pliocene). The NS-, NE-SW- and EW-trending faults are remarkable in this area, among which the NE-SW- and EW-trending faults were formed during the post-Hamaishidake pre-Sasanotoge stage (Pleistocene). On the basis of planktonic foraminifer and molluscan faunas, the geological age of the Hamaishidake Group is assigned to the Pliocene. As a result of this stratigraphic and structural study, the once estimated "Nakagochi Thrust" is proved to be only a fake.
