Some Problems on the Geomorphic Development in the Miyazaki Plain, Southeastern Kyushu
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概要
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In the Miyazaki Plain, Late Cenozoic sediments and terrace surfaces are well developed. The geomorphology and stratigraphy have been studied since 1930. In the previous works, it has been considered that there exists deposits which fill the valley cutting into the Miyazaki group or Late Miocene to Early Pliocene time and are unconformably overlain by the terrace deposits of Late Pleistocene (Toriyamahama formation: Otsuka, Y. 1930, 32., Chugenbaru mud formation: Takeyama, T. 1934., Kawaminami group: Endo, T. 1968).By author's survey, the terrace surfaces in the Miyazaki Plain are classified as follows: Pre-Chausubaru surface, Chausubaru surface, Sanzaibaru surface, Nyutabaru surface, Kawaminamibaru higher surface, Kawaminamibaru lower surfaces, Hyuga-Shirasu surface and Mikazukibaru surfaces in the descending order.This paper treates some problems on the so-called Toriyamahama formation and the development of the terrace surfaces in the relation to the fluctuation of sea level.About the so-called Toriyamahama formation, judging from the difference of the facies and the relation of the contact of the terrace deposits, and the 14C dating of the woods in the deposit under Kawaminamibaru higher surface, this formation is divided into the three kinds of deposits distributing under Chausubaru surface, under Sanzaibaru surface and Nyutabaru surface, and under Kawaminamibaru higher surface.Kawaminamibaru higher surface may be of marine, considering from the sedimentary facies and the topographic characteristics. Sanzaibaru surface was also formed by marine agency, considering from its characteristics of the distribution and its sedimentary facies. The Toriyamahama formation under these two surfaces is thought to be the deposits at the transgression directly preceding the formation of the surfaces. The 14C dating of the woods, which was sampled from the sandy silt composing of the deposit forming Kawaminamibaru higher surface (Kawaminamibaru formation is defined as the terrace deposits and part of the so-called Toriyamahama formation in this paper), was given as 26, 100±900 years B.P. (GaK-3326). Thinking of this absolute age, Kawaminamibaru higher surface might have been formed in the interstadial time of the glacial age. Sanzaibaru surface is thought to be accomplished at the regression. Nyutabaru surface is thought to be formed when the sea level was stable relatively after this process. Hyuga-Shirasu surface consists of the Shirasu (Ito pyroclastic flow) which poured into the drainage basin of the Oyodo River at the regression after the formation of Kawaminamibaru higher surface. About the absolute age of Shirasu, the following 14C dating is gained: mamely, 29, 600+1, 800 -1, 400 years B.P. (GaK-2621) from the dark colored humic clay bed which distributes directly under the Shirasu, 25, 600±1, 300 years B.P. (GaK-2576) from woods in the pumice layer (the second orange pumice layer) on the Shirasu. Mikazukibaru surfaces were formed at the maximum regression in the last glacial age.
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