中国地方の第四紀層

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The standard Quaternary stratigraphy in Chugoku District is summarized as follows. Pliocene and lower Pleistocene are represented by Tsunozu group and Oe-takayama volcanic rocks. Tsunozu group is distributed in central and western parts of Shimane Prefecture. This group is mainly composed of alternations of sand and mud beds. Four marine clay beds, which are called as M1, M2, M3 and M4, respectively, in ascending order, are intercalated in this group. In central part of Shimane prefecture, pyroclastic flow and fall deposits of Oe-takayama volcano are embeded in the clastic sediments of the upper part of Tsunozu group. The representative Pliocene proboscidian fauna, Stegodon cf. elephantoides and several species which belong to the flora in age of flowrish of Metasequoia flora, such as Carya, Pseudolarix, Keteleeria, Liquidambar, etc, are reported from the lower part of Tsunozu group (M1 to M2). Several species which belong to the flora in age of extinction of Metasequoia flora, such as Metasequoia, Picea koribai, Juglans megacinerea, etc. are reported from the upper part of Tsunozu group. From natural remanent magnetization and fission-track ages, Oe-takayama volcanic rocks may belong to Matuyama reversal epoch. Middle Pleistocene are represented by Saiio and Hiruzen-bara formations. Saijo formation is distributed in central part of Hiroshima Prefecture and consists mainly of sand, mud and gravel beds, associated with several layers of volcanic ash. This formation does not yield elements of Metasequoia flora, but yields several species which show cold or temperate climate such as Pinus koraiensis, Menyanthes trifoliata, Sciadopitys verticilata, Cryptomeria japonica, Chamaecyparis obtusa, etc. From fission-track ages, the age of this formation is estimated to be 0.7-0.5 Ma. Hiruzen-bara formation is distributed in northern part of Okayama Prefecture, and is divided into two members. The lower member consists mainly of diatomaceous earth containing freshwater diatom flora, such as Stephanodiscus niagarae, Cyclotella comta etc., and yields several species which show temperate climate such as Cryptomeria japonica, Chanaecyparis obtusa, Fagus crenata, Aesculus trubinata, etc. The upper member consists mainly of sand and gravel beds associated with peat beds, and yields the representative middle Pleistocene proboscidian fauna, Stegodon orientalis and several species which show cold elimate such as Picea bicolor, P.jezoensis, Tsuga diversifolia, Menyanthes triforia, etc. Upper Pleistocene and Holocene strata are typically developed in eastern part of Shimane Prefecture. In Izumo Coastal Plain, there are two layers of marine clay beds, the lower is called Yumigahama formation and the upper is Nakaumi formation. Yumigahama formation is composed of the lower sand and gravel member and the upper clay member. The upper member consists mainly of marine clay bed which yields the molluskan fosills as Scapharca broughtonii, Dosinella penicilata, Paphia undulata etc. and the pollen flora which is predominent in Fagus and associated with a few percent of Lagerstroemia. This member should be correlated to the Last Interglasial Age. The strata which deposited during Last Glatial Age can be devided in to many units. They yield two types of plant fossils; ones indicate the cold climate in Stadials, such as Pinus koraiensis, Picea bicolor, Tsuga diversiforia etc. and the others indicate the cool temperate climate in Interstadials, such as Picea maximowiczii, Cryptomeria japonica, Fagus sp., etc. Holocene Nakaumi formation consists mainly of marine clay, and thought to be deposited during Jomon Transgression Age.
日本地質学会の論文
1988-04-25

著者

大西郁夫
島根大学理学部地質学教室

中国地方の第四紀層

スポンサーリンク

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