著者

町田 洋 新井 房夫

出版者

公益社団法人 東京地学協会

雑誌

地學雜誌 (ISSN:0022135X)

巻号頁・発行日

vol.88, no.5, pp.313-330, 1979

被引用文献数

53 27

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The eruptions of the Daisen Valcano (35&deg;22'N, 133&deg;33'E) were mainly rhyodacitic and of the paroxysmal type, producing several extensive sheets of tephra. The Kurayoshi pumice (DKP, for short), one of the excellent Late Pleistocene markers arising from the Daisen valcano, is rhyodacitic in composition with abundant hornblende and orthopyroxene crystals and relatively small amount of biotite. Its identification can be made from the above mentioned mineral assemblages as well as from the characteristic refractive index of orthopyroxene (&gamma;=1.703-1.708) and of hornblende (n<SUB>2</SUB> = 1.673-1.682) and the specific crystal habit of orthopyroxene. This pumice-fall deposit occurs on marine and fluvial terraces in the San'in and Hokuriku districts facing the Japan sea and extends eastward beyond the Northern Japan Alps to north Kanto plain as a thinner discontinuous layer. Stratigraphic relation with the dated tephra layers in north Kanto indicates that the pumice was probably deposited between about 47, 000 and 45, 000 years ago. That is, this pumiceous deposit is found at the intermediate horizon between Yunokuchi Pumice (UP, slightly younger than 49, 000 YBP) and Hassaki Pumice (HP, 40, 000-44, 000 YBP) in north Kanto, about 500 km far from the Daisen.<BR>Daisen Kurayoshi Pumice wonld be particularly valuable for establshing chronological framwork as a fundmental time-marker in arears where no suitable markers have yet been documented. Moranic deposit of the Murodo glacial advance at Mt. Tateyama, Northern Japan Alps, is mantled by this marker and overlies the Raicho-dai pumice-fall deposit, products of the earliest stage of volcanic activity of Tateyama III, which is correlated with the Omachi EPm deposit approximately 60, 000 years old. Distribution of these two unreworked tephras indicates that major valley glaciers had nearly disappeared by the times of these initial tephra falls. The glacial advance at Mt. Tateyama, the most extensive of the advances during Last Glacial age, therefore, apparently culminated between about 55, 000 and 50, 000 years ago. On the other hand, a filltop terrace repesented by Uwadan terrace along the River Joganji flowing from Mt. Tateyama, is covered by DKP and is nearly younger than the pyroclastic flow deposit of Tateyama III. Accumulation of the river of Uwadan stage is, therefore, simultaneous and probably associated with the Murodo glacial advance.