濃尾平野の沖積層 : 濃尾平野の研究,その1
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概要
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The plains of Nohbi is one of the widest plains in Japan. Much investigation on the Nohbi plains was done mainly in the Nagoya city and its vicinity. The fundamental stratigraphy of the Nohbi plains was established by MATSUZAWA and KATO (1954), IZEKI, (1956), and other workers. The Nohbi plains has been studied by many workers from various viewpoints since then. The writers studied the historical development, especially some ploblems on the Nohbi plains from the late Pleistocene to Holocene, by examining boring cores, fossils such as pollen, diatom and foraminifera, mechanical grain analysis and absolute chlonology by carbon fourteen method. The results have been made clear as follows: 1) The stratigraphy of the Nohbi plains since Atsuta stage (may be assumed as Mindel-Riss interglacial stage) is divided into Toriimatsu gravel bed, the First gravel bed, Nohbi formation, and Nanyo formation in descending order. 2) Toriimatsu gravel bed forms the flat plane of about 20 to 30 m a.s.1., and its fill top is buried under the alluvial plains and extends under them. This bed is composed of subrounded gravel of mainly chert, and the surface is covered with black soil. 3) The First gravel bed, about 10 to 20 m thick deposited during the maximum of Wurmian glacial stage, and its surface is a little erroded. This bed is not distributed at the eastern half part of the Nohbi plains, but widely at the western harf part. This is mainly composed of the subrounded gravels of Paleozoic rocks, and is abundant in pebbles and cobbles. 4) Nohbi formation covers the First gravel bed unconformably, and distributes widely under the Nohbi plains. This formation is composed of the alternation of sand and mud, and is about 40 m thick at its maximum, but about 10 to 15 m thick on the average. The surface of the formation is erroded during the lowering of the sea level which forms the flat plain at about 40 m under sea level. The result of measuring the chlonology of carbon fourteen in the peat intercalated at lower part shows that the age is 16,000 to 18,000 y.B.P.. 5) Nanyo formation deposits on buried valley which forms the flat plane of about 40 m under sea level. This formation is divided into the upper sand member and lower mud member. The former intercalates the peat suggesting the lowering of the sea level during Yayoian stage. The age of the deposition of peat is 1,500 to 2,400 y.B.P.. The base of Nanyo formation shows 10,50 to 11,400 y.B.P. by measuring carbon fourteen. The analysis of pollen at the lowest part shows the climate of a little north temperate, and Shiia appears at the lower mud member, and Myica and warm temparate fern appear from this lowerpart (RII). As Pinus begins to increase at the upper sand member, Cyclobalanopsis decreases and Quercus begins to increase (RIII). 6) Stratigraphy of the Yohroh fan at the western ends of the Nohbi plains is described, and discussed on the relationship among each strata under the Nohbi plains. The orange volcanic ash covers the surface of the Yohroh fan, and black soil just below the ash shows 8,700±200 y.B.P. (Gak-2603). The composition of heavy mineral of the ash shows the type which is abundant in biotite and hornblende. The writers made clear the stratigraphy of the Nohbi plains, and the results are compared with the other alluvial plains in Japan.
- 1972-12-25