大分県鶴崎台地の地質の再検討--火砕流堆積物中のジルコンによるフィッション・トラック年代
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概要
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The Oita district is a standard area for the Cenozoic stratigraphy in West Japan. Above all the Tsurusaki hills is regarded as the most important area including the type sections of three formations. Main parts of the Takio, the Higashiwasada and the Tsurusaki formations are distributed respectively in the central to north area, in the south and north extremity. They are overlain by the higher terrace sediments. In the south of the hill, the gravels of the Higashiwasada formation, the Takio formation and the higher terrace sediments are developed in intricate pattern and their stratigraphic relation is misleading suggesting that some of these are possibly contemporaneous. While the known index fossil from the formation is only Stegodon orientalis from the basal part of the Tsurusaki formation. Accordingly the supposed ages of the Takio and Higashiwasada formation based on an indirect correlation is not accurate. Therefore it is important to clarify their ages.For settling these problems, the fission track age determination of the accessory zircon from pyroclastic rocks from above mentioned formations were done after a systematic field survey.In norder to observe fission tracks easily and correctly, the etching procedure was examined and the condition was finally settled as following; the etchant was 2:1 mixture of 48%HF and 98% H2SO4 and etching is carried out under pressure at 250°CThe Shikido pyroclastic rock member of the Higashiwasada formation and the Hada pyroclastic rock member of the Takio formation were selected for examination. The age of the Shikido and the Hada pyroclastic rock members are (6.52±1.34)×106 year and (1.44±0.28)×106 year, respectively.The age of the Shikido pyroclastic rock member is comparable with some volcanic rocks in Central Kyushu, for example biotite andesite in Kitatakaki District, Nagasaki Prefecture (5.7m.y.B.P.), biotite hornblende rhyolite in Hirado City (5.8m.y.B.P.), etc. Considering sporadical distribution, in Central Kyushu of volcanic rocks with similar lithology to those of the Higashiwasada formation, it must be concluded that the volcanic activity of this age is more extensive than has been supposed.The Hada pyroclastic rock member is correlative with the Meisei tuff bed at MaO horizon of the Osaka group. Comparing the relative position of the Hada pyroclastic rock member and the Stegodon orientalis horizon in the Oita group with the relative position of the Meisei tuff bed and the Stegodon orientalis horizon in the Osaka group, the writer conclude that the Takio formation correspons to the lower and middle parts of the Osaka group.
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