- 著者
- Niitsuma Nobuaki
- 出版者
- Tohoku University
- 雑誌
- The science reports of the Tohoku University. Second series, Geology = 東北大学理科報告. 地質学 (ISSN:0082464X)
- 巻号頁・発行日
- vol.43, no.1, pp.1-39, 1971-10-20
The purpose of the present study is to find the relationship of a geomagnetic field reversal to the changes in a foraminiferal fauna and the sedimentary environments. A fossiliferous marine sedimentary section of the middle part of the Kokumoto Formation continuously exposed at Kamiyanagawa in the Boso Peninsula, Chiba Prefecture, Central Japan, was chosen for the investigation of the behaviour of the geomagnetic field during its reversal between the Matuyama and Brunhes Polarity epochs which took place around 0.69 m.y. ago. The behaviour of the geomagnetic field was established as to the inclination, declination and intensity of the Jn/J_R recorded in the sedimentary rocks. Along the same section, analyses were made on the grain-size of the sediments, oxygen isotope ratio in the planktonic and benthonic foraminiferal tests, and the faunal assemblages of the planktonic and benthonic foraminifers. The intensity of the magnetic field fluctuated with a period of approximately 6700 years and did not disappear during the reversal. The migration of the virtual magnetic north pole at the magnetic field reversal between the Matuyama and Brunhes Polarity epochs occurred roughly along the meridian of 120°E long. in the period of 4700 years. In the same duration the depth of the sea water, under which the sediments accumulated, fluctuated within the range of 200-300 m with a period of approximately 7000 years. No particular relation between the magnetic field reversal and fluctuation of the water depth was recognized. The result of paleotemperature and faunal analyses of the foraminifers showed that the magnetic field reversal at the Matuyama Brunhes Polarity Epoch boundary may have been related to the changes in the circulation pattern of the upper watermasses which caused more influx of cold water into this area in the northwestern Pacific as compared with the period before the geomagnetic polarity change.
- 著者
- Bando Yuji
- 出版者
- Tohoku University
- 雑誌
- The science reports of the Tohoku University. Second series, Geology = 東北大学理科報告. 地質学 (ISSN:0082464X)
- 巻号頁・発行日
- vol.36, no.1, pp.1-A30, 1964-11-05
The Triassic ammonites of Japan are known from the southern Kitakami Massif, the Kwanto Massif, the Maizuru Zone, and the Islands of Shikoku and Kyushu. Based upon them a biostratigraphical correlation of the Japanese Triassic is attempted. The four ammonite zones recognized in the Lower Triassic and the Middle Triassic are correlative with the standard ammonite zones of the world, whereas the Upper Triassic ammonite fauna is in-sufficiently known for such purpose. Besides descriptions and discussions on the Triassic stratigraphy and paleontology, remarks are given on the sedimentary environment and tectonic history of the Japanese Triassic Epoch.
- 著者
- Shikama Tokio Kamei Tadao Murata Masafumi
- 出版者
- Tohoku University
- 雑誌
- The science reports of the Tohoku University. Second series, Geology = 東北大学理科報告. 地質学 (ISSN:0082464X)
- 巻号頁・発行日
- vol.48, no.1/2, pp.77-A42, 1978-03-29
A new Early Triassic ichthyosaurus, Utatsusaurus hataii gen. et sp. nov., is described. The materials were collected from the upper part of the Osawa Formation on the coast at Tatezaki, the Kitakami Massif, Northeast Japan which is correlative of the Prohungaritan-Columbitan ammonoid stages of Spath (1930, 1934). The present form is of the most primitive type of Ichthyosauria from the view point of specialization of the pectoral limb.