大山倉吉軽石層とこれにまつわる諸問題
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概要
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The Daisen-Kurayoshi pumice (DKP) has been known as one of the wide-spread marker tephras erupted during the Last Glacial stage, covering marine, fluvial, lacustrine and glacial deposits distributed at various parts of the northern Chubu, the northern Kanto and the southern Tohoku Districts. Therefore, DKP is very useful for the chronological study of central Japan during the Late Pleistocene. The author investigated on the distribution and stratigraphic horizon of DKP in order to reconstruct the paleoenvironment of central Japan during the last 65,000 years. The following are the main new results obtained by this study. 1) The total volume of DKP is estimated to be more than 40km^3, widely spreading from the northern Chubu to the southern Tohoku Districts (Fig. 1). 2) It is estimated that DKP was erupted from the Daisen volcano, western Honshu between 48,000 and 54,000 years ago (Fig. 7-A), judging from the stratigraphic facts that DKP covers Ichinotorii-4 pumice (IcP-4, 56,000〜59,000 F.T.y.B.P.) at locs. 7, 8, 9 in Fig. 6 and Tokyo pumice (TP, 49,000±5,000 F.T.y.B.P.) at locs. 11, 12, 13, 14 in Fig. 8, and is covered by Yunokuchi pumice (UP, slightly younger than 48,000 y.B.P.) at locs. 7, 8, 9 in Fig. 6 and loc. 10 in Fig. 8, and Hassaki pumice (HP, 42,000〜44,000 F.T.y.B.P.) at locs. 3-9 in Fig. 6 and locs. 10, 12, 13 in Fig. 8. 3) The author determines the chronologic positions of many marker tephras and the Late Pleistocene sediments in connection with the stratigraphic horizon of DKP. For instance, many tephra layers derived from Ontake, Yatsugatake, Asama, Haruna, Akagi and Nasu volcanoes can be correlated each other as shown in Figs. 3 and 11. 4) Marked buried soil is widely found within the tephra layers distributed in central Japan, the formative age of which is estimated at about 65,000 y.B.P. comparing with the stratigraphic position of DKP and other marker tephras (Fig. 9). This stratigraphic horizon is very close to the stage of completion of valley filling occurred along the middle and upper course of the major rivers in central Japan (Fig. 11). and of fixing the Pleistocene coastal sand dunes. 5) It is considered that most of the mountain glaciers developed near the summits of the Kiso mountains were receded and a series of strath terraces were also formed along the upper course of the rivers at nearly the same period of the falling of DKP. The sea level at that time would be about 40 to 50m lower than the present, judging from the stratigraphic horizon of the Osone formation developed under the Nobi alluvial plain (Fig. 11). 6) It is estimated that the upheaval movement of Atera fault and Ina fault system were more active (Fig. 13) and the activity of Ontake volcano was more explosive (Figs. 11 and 12) during the time between 65,000 and 47,000 yr.B.P. than afterwards. These might be resulted from the phenomenon that the horizontal stress field had been strengthened around the Kiso mountains during the above mentioned period.
- 駒澤大学の論文
著者
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