On the Microbiostratigraphy of the Yatsuo Group in the Yatsuo District of Toyama Prefecture

Abstract

The microbiostratigraphy and size distribution of the sedimentary rocks by mechanical analysis of the Yatsuo Group in the type area in Toyama Prefecture is presented., The middle Neogene Yatsuo Group consists of four formations, namely, Nirehara, Iwaine, Kurosedani and Joyama in ascending order., Some of these formations are subdivided into members., The stratigraphy of the said group has already been described elsewhere (TSUDA 1952, NAKASEKO 1952)., The fossil foraminifers were collected from three standard sections along the Jinzu, Kubusu and Yamada Rivers where are developed the Kurosedani and Joyama formations of the upper part of the Yatsuo Group., Five types of sedimentary rocks are classified by size distribution and the characteristic of each type is given in Figure 1., From the distribution of the foraminifers predominant species were recognized in each section by which the establishment of faunules was possible., By these faunules the upperpart of the Yatsuo Group was divided into four zonules, as follows: Martinottiella cf., communis-Uvigerina farinosa Zonule Buliminella cf., elegantissima-Nonion japonicum Zonule Operculina complanata japonica-Nonion japonicum Zonule Rotalia papillosa-Operculina complanata japonica-Nonion japonicum Zonule The relationship of the faunules to the above mentioned four zonules is shown in Table 3., Compared with the subdivision by IKEBE and NAKASEKO in western "Toyama Basin", the uppermost seems to belong to their Nodosaria insecta Zone and the lower three to their Miogypsina kotoi Zone.,