岡山県児島湾中新統の沈積輪廻

On the Miocene Sedimentary Cycle found out in Kojima Bay, Okayama Prefecture, West Japan

TAI Yoshiro

廣島大學地學研究報告

Geological report of the Hiroshima University

No the marine Miocene sediments are known to be exposed throughout the surface area around Kojima Bay, Which is situated in the south of Okayama City.
Since 1957, five borings were undertaken in Kojima Bay by the Department of Commerce and Industry, Okayama Prefecture, for the purpose of natural gas exploration. Many characters which those boring cores represent themselves lithologi- and micropaleontologically, however evidently indicate the existence of the Miocene. Those boring wells; O1, K1, K2, K3 and K4, were lowered to 130.4, 250, 150, 375 and 167 meters in depth respectively and each locality is shown in Figure 1. The Miocene sediments distinguished in each boring cores above mentioned were measured to be 67.4, 188, 70, 300 and 84 meters in thickness respectively and lithologically correspond to the marine Miocene Bihoku group.
In this article the writer deals with the smaller foraminiferal faunas derived from the Miocene sediments and the significance of them is discussed. From the stratigraphic correlation among the assemblages distinguished in each cores, the Kojima microfauna may be divided
into seven foraminiferal faunules (I-VII). From the depth analysis and stratigraphic changes of them and lithologic facies containing them, it may be stated that the Bihoku group here forms a sedimentary cycle consisting of both of marine transgressive and regressive stages, during the age of middle and upper Miocene.
From some isopach maps of the zonules based upon the faunules above mentioned, the de-positional conditions and mechanism that had existed during the same age, also are discussed.

jpn

廣島大學理學部地學教室

[ISSN] 0073-2303

[NCID] AN00213676