Fission track ages of pyroclastic flows in the Pliocene Ashoro Formation and the Plio-Pleistocene Ikeda Formation developed in Eastern Hokkaido, Japan
Fission track ages of pyroclastic flows in the Pliocene Ashoro Formation and the Plio-Pleistocene Ikeda Formation developed in Eastern Hokkaido, Japan
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日本北海道东部上新世足寄组和上新世-更新世池田组火山碎屑流裂变径迹年龄
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发表时间：
1981
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通讯作者：
S. Koshimizu
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作者：
S. Koshimizu
用逐粒提取锆石的方法，测定了北海道东部德内省发育的三个火山碎屑流矿床的裂变径迹年龄。得到以下结果：3.7±0.1m.y。阿肖罗组稻生贝图火山碎屑流沉积，2.8±0.2 m.y。Sarubetsu火山碎屑流沉积；对于千代田火山碎屑流沉积，后两者都属于池田组。因此，本省的上新世-更新世界线可能位于池田组千代田火山碎屑流沉积的略高于层位的位置。前言：北海道东部土内省北部最年轻的第三纪和第四纪沉积，目前已按升序分为本贝苏组、阿肖罗组和池田组。由于本贝图组下部含有Forttpeeten takahashii动物群，一般认为其时代为上新世。目前还没有关于岸上建造年代的古生物数据。但柴田、山口和佐藤(1976)给出的K-Ar年龄为4.1±1.1年。我们目前认为它是阿肖罗组的一员。池田形成的年代一直备受争议。Nemoto、Oishi和Watanabe(1933)首次确定了上新世晚期的池田组。冈崎(1957,1958)、三谷(1964)和Tokachi Researchi Group(1978)紧随其后。而Hashimoto(1954)根据Menyanths tri/alia Linn的出现指出了将其归属于早更新世的可能性。在里面。另一方面，Minato、Hashimoto、Fujiwara和Kumano(1972)得出结论，在池田组的下部寻找与寒冷时间相耦合的Olduvai事件，从而表明该组的时代为上更新世。Igarashi(1976)做了花粉分析研究，进一步证实了这一观点。Oka(i 976a，b)也认为池田组的时代为上更新世。最近，Oka和Akamatsu(1979)在靠近Oka千代田凝灰岩(1976a)的地层中发现了Patinopecten(Fortiekten)takahashii，这些凝灰岩夹在池田组中。他们认为这些软体动物地层靠近上新世-更新世界线。本文作者对北海道大学理学院地矿学系1698号三次火山碎屑流贡献进行了裂变径迹测年。
Fission track ages of the three pyroclastic flow deposits developed in the Tokachi Province, eastern Hokkaido, have been determined by means of grain by grain method of zircon extracted from these deposits. The following results have been obta ined; 3.7 ±O.1 m.y. for the Inashibetsu pyroclastic flow deposits of the Ashoro Formation; 2.8±O.2 m.y. for the Sarubetsu pyroclastic flow deposits; and 2.0±O.1 my. for the Chiyoda pyroclastic flow deposits, both the latter two are of the Ikeda Formation. Pliocene-Pleistocene boundary in this Province must therefore lie somewhere which may be slightly above the horizon of Chiyoda pyroclastic flow deposits of the Ikeda Formation. Introduction Youngest Tertiary and Quaternary deposits in the northern part of the Tokachi Province, eastern Hokkaido, have been currently divided into the Honbetsu Formation, the Ashoro Formation and the Ikeda Formation in ascending order. The Honbetsu Formation has been generally considered as Pliocene in age, because of the presence of Forttpeeten takahashii-bearing fauna in its lower part. There has been no palaeontological data for the age of the Ashore Formation. But Shibata, Yamaguchi and Sato (1976) have given a K-Ar age of 4.1±1.l m.y. for the Inashibetsu tuff, which we currently consider as a member of the Ashoro Formation. The age of the Ikeda Formation has been much disputed. Nemoto, Oishi and Watanabe (1933) first assigned their Ikeda Formation as of late Pliocene age. Okazaki (1957, 1958), Mitani (1964) and Tokachi Researchi Group (1978) followed in this concern. Whereas Hashimoto (1954) pointed out the possibility of its assignment to the early Pleistocene on the basis of the occurrence of Menyanthes tri/alia Linn. in it. On the other hand, Minato, Hashimoto, Fujiwara and Kumano (1972) conclude that the Olduvai event coupled with cold time is to be sought within the lower part of the Ikeda Formation, thereby indicating that the Formation is Plio-Pleistocene in age. This opinion is further reinforced by Igarashi (1976) who did pollen analytical study. Oka (I 976a, b) also considers the Ikeda Formation as Plio-Pleistocene in age. Quite recently Oka and Akamatsu (1979) found Patinopecten (Fortipecten) takahashii from beds close to the Chiyoda tuff of Oka (l976a), which are intercalated in the Ikeda Formation. They consider that these molluscan beds are close to the Pliocene-Pleistocene boundary. The present author has conducted fission track dating on the three pyroclastic flow Contribution from the Department of Geology and Mineralogy, Faculty of Science, Hokkaido University, No. 1698.