北海道地方の第4紀テフラ研究

Quaternary Tephra Studies in the Hokkaido District, Northern Japan

Okumura Koji

第四紀研究

The critical problems remaining for the study of the Quarternary tephrochronology of Hokkaido in the 1980s were the lack of extensive marker tephras and of well-defined chronological data beyond the range of 14C method. In the last ten years, these problems were partly solved for the Holocene and Late Pleistocene with the recognition of a number of remarkable wide-spread tephra-layers including B-Tm (< 1 ka)

Aso-4 (70 ka), Toya (90 ka) and KHb (100 ka). The finding of Aso-4 and Toya layers enabled to correlate the tephrochronology of Hokkaido with that of Honshu and Kyushu. Meanwhile, information on Early to Middle Pleistocene tephras is still insufficient for establishing a comprehensive chrono-stratigraphy in most of Hokkaido, with the exception of the region around the Tokachi Plain.

The tephrostratigraphy in eastern Hokkaido is described in this paper in order to summarize recent progress and to show the outline of Quaternary tephras in Hokkaido. The stratigraphy of pyroclastic flow deposits from Akan and Kutcharo volcanoes, which have the longest history of explosive activities in Hokkaido, has been revised and interrelated through stratigraphic revision and extensive application of microprobe techniques to glass shards. Intervening airfall tephras with different petrographic characteristics from tephras occurring in eastern Hokkaido are introduced as possible marker tephras of Early to Middle Pleistocene age.

Chronological data for Middle Pleistocene tephras are still very few, but their stratigraphic positions in relation to major transgressions are revealed. Temporal and regional variation of potassium content in glass shards and maffic mineral assembrage is examined to give general ideas of source volcanoes and ages of unidentified tephra layers of Early to Middle Pleistocene.

Earth sciences. Geology [ 450 ]

jpn

日本第四紀学会

[ISSN] 0418-2642

[NCID] AN0034136X