隠岐島後中新統郡累層と久見累層の層序関係の再検討 Re-examination of stratigraphic relationship between the Miocene Kori and Kumi Formations in Dogo, Oki Islands, West Japan
The Oligocene and Miocene beds, distributed in Dogo, Oki Islands, have been divided into the Tokibariyama, Kori, Kunii and Tsuma Formations in ascending order. Hitherto, the stratigraphic relation between the uppermost Hei Tuff-Siltstone Alternation Member of Kori Formation and the lowermost Kumi Sandstone Member of Kumi Formation have been estimated to be an unconformity, but the results of lithostratigraphic and diatom biostratigraphic reexaminations in the Yabigawa and Kumigawa areas indicate that both Members are a contemporaneous heterotopic facies and the underlyers of the upper Utagi Mudstone Member of Kumi Formation. Judging from the above results, the Hei Tuff-Siltstone Alternation should be regarded as the lower Member of Kumi Formation, and the stratigraphic relationship between the Kori and Kumi Formations is conformable. A sandstone bed of the upper part of the Hei Tuff-Siltstone Alternation Member yielding fresh-water molluscs is conformably overlain by a siltstone bed of the Member containing a marine diatom assemblage which is comparable with that of the lower part of Utagi Mudstone and linoyama Diatomite Members of Kumi Formation. On the basis of the field evidence mentioned before, it is inferred that the Hei Tuff-Siltstone Alternation Member is transgressive sediments and that the depositional environment of the lower part of the Kumi Formation changed quickly from non-marine to marine. Additionaly, it may be inferred that the Dogo Island was smoothly invaded by sea-water because the Island has been the subsidence area since the depositional stage of the Kori Formation. The lower Middle Miocene in the San-in and Bihoku districts which have been regarded as transgressive sediments may be divided into two types of sedimentary facies, coarse clastic facies showing the depositional environments of swamp and/or shallow sea like bay, and fine or coarse clastic facies of shallow marine facing open sea. The types of transgressive deposits may be closely related to the stratigraphic relationship between the deposits and their underlyers. Therefore, paleo-oceanic environment inferred from the types of sedimentary facies may reflect the tectonic movement in the late Early Miocene.
地球科学 45(3), 177-189, 1991