西部本州瀬戸内中新統の海退相について

In 1960, a boring was undertaken in Kojima Bay by the Department of Commerce and Industry, Okayama Prefecture, for the purpose of natural gas exploration. This boring well was lowered to 375 meters in depth and the locality is at the Koyô High School, Fujita-mura, Kojima-gun, Okayama Prefecture (Lat. 34°34'48" N., long. 133°53'43".4 E.).
The Setouchi Miocene sediments distinguished in the boring cores were measured to be 300 meters in thickness and lithologically correspond to the marine Bihoku group.
In this article the writer deals with the smaller foraminiferal faunas derived from thirty-six horizons of above mentioned Bihoku group and the significance of them is discussed.
From the distribution of the Foraminifera shown in Table 1, this microfauna may be divided into seven foraminiferal faunules (I - VII).
From the depth analysis of the genera from those faunules, the depositional environments of the lower and middle Bihoku group containing the lower four faunules (I - IV), evidently represent conditions which change from those of shallow water to those of the outer-neritic region, and contrarily the upper same group containing the upper three (V - VII), represent conditions which gradually change from mid-neritic to shallow water.
From the foregoing remarks and lithologic characters, it may be stated that the Miocene Bihoku group here forms a sedimentary cycle consisting of both of marine transgressive and regressive stages. It seems to me that the maximum point of the marine transgression of the cycle falls within horizon of the shale facies containing the IV faunule.
Compared with the microbiostratigraphical units established by the writer (1959) in West Japan, the lower Bihoku group, which contains the three faunules (I - III), corresponds to the Lagenonodosaria scalaris-Uvigerina crassicostata zone, and the middle which contains next one faunule (IV) to the lower part of the Cyclammina orbicularis-Martinottiella communis zone, and the upper which contains last three faunules (V - VII) to the middle part of the same zone mentioned above.