1987 Fiscal Year Final Research Report Summary

Stratigraphy and distribution of widespread Quaternary tephras in Chugoku and Kinki district, west japan

Research Project

Project/Area Number	61540559
Research Category	Grant-in-Aid for General Scientific Research (C)
Allocation Type	Single-year Grants
Research Field	地質学一般
Research Institution	Tottori University
Principal Investigator	OKADA Shomei Tottori Unv., Fac. Education, Assoc. Prof., 教育学部, 助教授 (70093581)
Co-Investigator(Kenkyū-buntansha)	AKAGI Saburo Tottori Unv., Fac. Education, Prof., 教育学部, 教授 (20031908)
Project Period (FY)	1986 – 1987
Keywords	Wide spread tephra / Ferromagnetic minerals / Curie point / Aso-4 ash / フィッショントラック年代
Research Abstract	1. In this study, for identify and correlate the Quaternary tephra layers the thermomagnetic analysis of ferromagnetic minerals in tephra is performed. The thermomagnetic balance was improved to computer controlled system in advance of the measurement. 2. It is already reported that the widespread tephras(Ah,AT,Aso-4,K-Tz) which originated from Kyushu are distributed in Chugoku and Kinki district. The thermomagnetic measurements by the above mentioned system reveal that the ferromagnetic minerals in Ah, Aso-4 and K-Tz have distinctly different properties from those in tephras from the Daisen and the Sanbe volcanoes. By this difference, Aso-4 ash which is not distinguishable in the field because its thin thickness and progressive weathering was discriminated from the tephras of Sanbe at the site of Yasue, Akagichou, Shimane Prefecture. 3. Both the phenocryst assemblage and the thermomagnetic property of Sanbe Kisugi Pumice(K_3) are so unique in the Daisen tephra formation that k_3 is good horizon marker around the Daisen volcanic area. 4. Four pumice fall deposits(Arata pumice 2:DAP_2, Arata pumice 1:DAP_1, Hiruzenbara pumice:HiP and Okutsu pumice:DOP in descending order) erupted from Daisen volcano were newly described and named in the present study. Of them, DOP shows widespread distribution having ESE trend from Daisen. 5. The fission track dating on DOP and h_1 pumice was made. The latter one is an upper member of the Daisen Lowest Tephra Formation. The results show 86,000<plus-minus>7,000 and 81,000<plus-minus>7,000y.B.P. respectively. Both dates indicate that DOP and h1 was erupted in late stage of the last interglacial or early stage of the last glacial age.