島根縣第三紀層に関する二、三の問題

Some problems on the Tertiary formation in Shimane prefecture.

YOSHIDA Hiranao

廣島大學地學研究報告

Geological report of the Hiroshima University

The northern part of Shimane Pref. consists mainly of Tertiary formation resting on pre-Tertiary granite and schist.
In the southern part of this area the Tertiary formations extending southward are appeared as embayments about 200 km2 in dimention, arranged regularly from east to west with the respective interval of 10～15 km. The writer named them "Hamada Bay", "Nima Bay" and "Iishi Bay", respectiveiy froms east to West, which are separated by noses consisting of basement rock as follows: "Hamada Bay" and "Nima Bay" by "Naka Peninsula," and "Nima Bay" and "Iishi Bay" by "Ano Peninsula".
The Tertiary area may be divided into three areas such as northern, middle and southern ones developing from east to west and presenting a zonal arrangement. The stratigraphical succession, geologic structure and period of the structural movements are different in each zone. This view may be shown in Table 1.
Intrusive masses of acidic rocks (plagioliparite or quartz-porphyry) intruding at the upper-most Kawai stage are found on the domelike anticlines in the southern area where some gypsum deposits situated in the middle or upper Kawai formation are appeared in the peripheral zone of the acidic rocks.
In the western part of the northern area, there also are the similar kind of deposits found in "oil shale" correlated to the upper Kawai formation and acidic rocks such as plagioliparite or quartz-pohrpyry.
In the eastern part of the notheqn area, there are no gypsum deposits but acidic intrusive masses plong an anticline-axis. According to the opinion indicated previously by Tai and Ondo, It is, on account of their occurences, believed that these acidic rocks intuded in or after the period of foiding. In consebuence, it seems sure that the intrusion of acidic rocks in the northern area took place at the post-Matsue stage but this is not agreeable with that in the southern area.

jpn

廣島大學理學部地學教室

[ISSN] 0073-2303

[NCID] AN00213676