隠岐島後中新統の層序(<特集I.>日本海沿岸新期新生代層の層序と古地理-地団研第37回総会学術シンポジウム(層序・古地理分野)-)

抄録

The Miocene series in Dogo, Oki Islands can be divided into 5 formations, that is, Tokibari-yama, Kori, Kumi, Tsuma and Yui Formations in ascending order. The Tokibari-yama Formation is composed of andesitic, dacitic and rhyolitic volcanic rocks with tuffaceous sandstone and shale. Fossil plants which may be correlative with the Aniai flora are occurred from this formation. The Kori Formation is of continental sediments such as tuffaceous sandstone, tuffaceous conglomerate and siltstone. Fossil plants correlative with the Daijima flora and limnetic molluscan fossils are obtained. The Kumi Formation is subdivided into the Kumi Sandstone and the Utagi Mudstone Members. From the Kumi Member, Miogypsina, Operculina and the molluscan fossils having the an affinity with the Higashi innai fauna, in the Noto Peninsula, are occurred. The Utagi Member contains diatomaceous earth. The Tsuma Formation is also subdivided into 3 members, that is, the Kamaya Sandstone, the Nakazato Tuffaceous sandstone and the Igo Mudstone Members. Molluscan fossil correlative with the Shiobara fauna are occurred from the Kamaya and the Nakazato Members. On the other hand, molluscan fossils occurred from the Igo Member is correlative with the Yama fauna. The Kamaya and the Nakazato Members are frequently intercalated diatomaceous mudstone. The Yui Formation is composed of lavas and pyroclastic rocks of trachybasalt and trachyandesite which is inferred that they erupted during the Late Miocene. Foraminiferal assemblages and diatom analysis are studied for the Kumi and the Tsuma Formations. On the basis of these data, writers infer the stratigraphy of the Miocene series in Dogo as Table 2. At the early stage of Late Miocene, about 9 ma, eruption of the lowest member of alkali volcanic rocks may occurred concurretly with an appearance of structure trending N-S, which is different from general trend of folds and faults of pre-alkali volcanic rock series.