静岡層群の層序と構造
1989 年 43 巻 3 号 p. 140-156
詳細
1989 年 43 巻 3 号 p. 140-156
The Miocene Shizuoka Group is distributed in the southwestern Fuji River Valley which lies in the South Fossa-Magna, Central Japan. In this paper, the stratigraphy and geological structure of the Shizuoka Group in the northern part of Shizuoka and Shimizu City are described, and the formation mechanism of folding of the Shizuoka Group is discussed. The Shizuoka Group is divided into the Ohira and the Takayama Formations in ascending-order. The Ohira Formation attains more than 2,700m in thickness and can be subdivided into six members: the Nishizato sandstone Member, the Kouchi sandstone and mudstone Member, the Hohnokubo mudstone Member, the Ishizawagawa sandstone Member, the Jishyabatake mudstone Member, the Kaifushi sandstone Member, in ascending order. The Takayama Formation attains more than 5,200m in thickness and can be subdivided into six members: the Isabu sandstone and mudstone Member, the Yoshihara sandstone Member, the Kitadaki sandstone and mudstone Member, the Kashio sandstone Member, the Nagao sandstone and mudstone Member, the Hozumi mudstone Member, in ascending order. Although the Ohira Formation is in contact with the Takayama Formation by fault, the Kaifushi sandstone Member of the Ohira Formation is possible to correlate with the Kitadaki sandstone and mudstone Member of the Takayama Formation because of their similar lithology. The planktonic foraminiferal assemblege indicates that the geological age of the Shizuoka Group is assigned to the early Late Miocene (N16-17A of Blow's Number). This Group is correlated with the Minobu Formation of the Fujikawa Group in the northern Fuji River Valley. The geological structure of the Shizuoka Group is characterized by the N-S to NE-SW and E-W trending folds. The significant characteristics of these structures are that the eastern and southern wings of the anticlines are steeper than the western and northern ones, and that the thickness of a member on one wing of the anticline differs from that on the other wing. These facts suggest that the folds are developed due to the differential movement of the basement blocks during the depositional stage of the Shizuoka Group.
