Quaternary Volcanic Ashes in the Kanto Region
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概要
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1. Kanto Plain is thickly covered by the Quaternary volcanic ashes supplied from the volcanic chain running from the north to the west of the region, and offers one of the type areas in the tephrochronologiocal study in Japan as well as the type areas of the Japanese marine Pleistocene. Most of the Quaternary volcanic ashes distributed in the Kanto region are composed of the weathered ashes in the Pleistocene epoch, and have long been called "the Kanto Loam" by many authors. A discovery of stone implements from the Kanto Loam at Iwajuku in 1949 stimulated a close investigation of Loam, and knowledges about Kanto Loam have remarkably advanced for these ten years in such fields as stratigraphy, geomorphology, archeology under the co-operation of many investigators.2. Kanto Loam formation is generally thick on higher terraces and thin on lower ones. Therefore, we can establish the stratigraphic division of the formation to terrace topography. In South Kanto, the Loam member which conformably covers the Tachikawa terrace gravel is named Tachikawa Loam, and what merges downward into the constituents of the Musashino terrace is named Musashino Loam. What conformably lies on the marine Shimosueyoshi formation (upper Pleistocene) is Shimosueyoshi Loam, and the lowest division of Kanto Loam is Tama Loam which is defined as the loam which covers the Byobugaura formation (middle Pleistocene) or its correlatives on the Tama terraces.3. Tachikawa Loam member, the uppermost of the Kanto Loam formation, is brilliant brown in color, 3 to 4 meters thick in Tokyo, and is mainly composed of weathered scoriae. Distribution of the loam is very wide, covering the present landsurface. Two dark colored bands made up of buried soil which are recognized in the middle part of the loam can be pursued extensively as key beds. Tachikawa Loam can be divided into the upper and lower submembers at the surface of the upper dark band, and the upper part of the Tachikawa is the stratigraphic equivalent to the Egota conifer bed.Musashino Loam member is composed of weathered scoriaceous ashes of brown color, 5m thick in Tokyo, and more clayey than the Tachikawa. The Tokyo pumice bed interbedded about 1m above the base is well-known as the most important key bed of Kanto Loam in South Kanto.Shimosueyoshi Loam is clayey with pebbles and pumices in the whole section. White pumice beds are generally interbedded in the lowest horizon of the loam.Lithologic facies of the loam is very variable and is at times represented by tuffaceous clay or sand only.Tama Lam is a cracky, browh tuff-clay about 3m thick. Yellow pumicts and brown weathered scoriae are commonly comprised in the whole section, and several pumiceous bands are also interbedded.4. Kanto Loam formation which covers the terraces and skirts of the volcanoes in Northwest Kanto can be divided into three members; upper, middle and lower with remarkable unconformity each other. Upper loam member, 1 to 2m thick, covers the whole area in agreement with the present topography. It is colored light yellowish brown, the lower half of which is pumiceous. Upper Loam is farther characterized by two pumice beds, the Itahana yellow pumice in the upper and the Itahana brown pumice in the lower.Middle Loam is brown or dark drown, 4m thick, and more or less clayey. The Kanuma pumice, the Hassaki pumice and the Yunokuchi pumice bed mark the upper, the middle and the lower part of Loam respectively. Iwajuku Culture Bed is contained in the dark band developed in the uppermost horizon of the loam, while Gongenyama I Culture Bed is in the dark band right below the Hassaki pumice.Lower Loam is very clayey occasionally containing pebbles and granules. Pumice are often intercalated in various horizons of the Loam, but none is developed in beds wide extent. Fujiyama Culture Bed at the foot of Mt. Akagi is contained in the upper part of Loam.
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