Late Pleistocene Tephrochronology of Kyushu Region, Japan
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概要
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In the Late Pleistocene of Kyushu, the San'in and Aso-Kirishima volcanic series began to erupt on the Tsukushi type andesites.The former are characterized by the pyroclastic flow of hornblende-biotite andesite, connected with forming the graben, the latter are pumiceous flow including welded tuff, connected with forming the caldera.These volcanic activities are divided into three stages; older, middle and younger.Pyroclastic deposits of the older stage are interbedded with lacustrine deposits and dark gray coloured solid Loam bed (Older Loam). They form higher terraces.In the middle stage, a great volume of pyroclastic flow erupted out from the effusive center, which was depressed and formed a graben or a caldera at a slightly later period. Particularly, Aso and Aira pyroclastic flows covered extensive areas. Their volumes are estimated to be 175.2 and 154.8km3 by T. Matsumoto, (1952), and they form middle terraces. Therefore, they are the best tephrochronological key beds. On the other hand, 14C dates measured by K. Kigoshi show the Würm glacial stage as follows; black humic clay at the base of Aira pumice fall (Fig. 1, 2): 22, 000±850 yr B. P. (Kigoshi and Endo, 1963), charcoal including in the base of Aso pyroclastic flow: 33, 000 +3, 000 -2, 200 yr B. P. (unpublished).In the younger stage, the central cones began to act a violent explosive eruption by which ejecta was thrown out. The ash, scoria and pumice wafted by wind covered the extensive areas and formed the Younger Loam (Fig. 4). Some of these activities continued to the Recent and supplied the essential part of the Black Volcanic Ash Formation.The Younger Loam Formation is divided into three beds; upper, middle and lower parts (Fig. 3). The lower bed overlies the gravel bed which covers the pyloclastic flow, or lies upon the flow immediately. The upper two beds cover the lower terrace and contain palaeolithic tools.The Black Volcanic Ash contains ceramics of the Jomon, Yayoi and later cultures, by which the chronological division of Ash beds may be classified.
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