著者

東海大学海洋学部第一鹿島海山調査団

出版者

地学団体研究会

雑誌

地球科学 (ISSN:03666611)

巻号頁・発行日

vol.30, no.4, pp.222-240a, 1976

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Daiichi-Kashima Seamount is located on the junction of Japan and Izu-Bonin Trenches, and the top of seamount is characterized of guyot at 3,700 m in depth. The basement structures of Daiichi-Kashima Seamount seems to continue to Mineoka up-lift zone. The seamount is consisted with four acoustic layers. The broadly outcropping sediment layer (Su. 1) is composed with Cretaceous Limestone, ineluding foraminifera (Orbitorina sp.) and Stromatopora. Generally, it seems to be corresponded to Barremian to Cenomanian. The second layer (SU. 2) presumes to consist of tuffaceous sandstone. The third layer (Su. 3) is the major parts of the seamount bodies and Su. A is the debris layer derived from lower layers. The olivine dolerite, olivine augite basalt, trachyte and trachytic andesite were obtained from the dredging samples at the summit of seamount. Also the materials of surface sediments and core might be originated from volcanic ash on the top of seamount. The clay minerals in the core sample of St. 2 show the same composition as recent sediments in the adjacent sea of Japan. From our biostratigraphic investigation, microfossils of the core were deposited on the latest Pleistocene, and it is indicated that the environment of sea changed to become colder at lower than 40 cm in depth. This seamount was also studied geomagnetically this time as follows. There are some high magnetic anomalies and they would be due to the matters related to the crustal structures about 4-5 km from sea surface. The reason why this seamount is located in such a depth as 3,700 m may be explained by the eustatic movement, rising of sea level, after middle Cretaceous time.