島根半島中西部の層序

The purpose of this study is to clarify the stratigraphy of the Cenozoic formations of Shimane Peninsula and also to carry out the correlation by means of the planktonic foraminifers. In the western central pai"t of Shimane Peninsula, the strata are apparently differentiated into two facies. One is represented by the sequence along the eastern section, while the other is by that along the western section. Although two facies mentioned above are developed side by side, previous works have given few informations clarifying the relationship between them. Based upon the detailed field works, the author verifies the relationship between two facies and here proposes the following classification of the strata. They are in descending order: / Matsue Formation / Furue Formation {Furue mudstone member Aishiro alternation member / Ushigiri Formation {ushigiri alternation member Josoji sandstone member / Koura Formation {koura shale member Chigo tuff member. / The depositional environments of the above formations change from fresh water to brackish, of £-shore marine, and finally to near-shore marine in the lapse of time. Therefore the planktonic foraminifers are only contained in the middle three members, the Ushigiri.alternation to the Furue mudstone member, deposited in the off-shore environment. The author selected five transverse sections to examine the planktonic foraminifers, along which he collected forty-four samples at almost the same stratigraphical interval. Among these samples the planktonic foraminifers were detected only in ten. The detouched foraminifers, though not necessarily enough in quantity, serve for agedetermination correlation. As shown in Table 2, Globorotalia. fohsi fohsi CUSHMAN and ELLISOR occurs in the upper part of the Ushigiri Formation along. with Globorotalia tumid a (BRADY). The known range of the combination of these taxa is very short and restricted to the uppermost of G. fohsi fohsi zone. Accordingly there is no doubt that the upper part of the Ushigiri Formation is included in the uppermost of G. fohsi fohsi zone, namely to Burdigalian, Globorotalza bykovae (AISENSTAT), the index species of the next higher zone in Japan, occurs in the lower part of the Aishiro alternation member, which overlies continuously the upper Ushigiri. This correlation based on the planktonic foraminifers is supplemented by the megafossils. The molluscs from the Ushigiri Formation are included in the "Middle Miocene molluscan faunas" in Japan, which occur in the formations approximately correlative to Burdigalian through Helvetian in relation to the planktonic foraminifers. The occurrence of the Comptonia-Liquidambar association in the underlying Koura shale member suggests that the member is correlated to the Aquitanian Daijima Formation. While the Matsue Formation, the uppermost formation, is considered to span from Upper Miocene to Pliocene on the basis of its rich molluscan fossils.