山陰西部の鮮新-更新統都野津層--古地磁気層序と堆積年代 (日本海沿岸の更新世の古地理と古環境特集号)

概要

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Paleomagnetism was measured with a cryogenic magnetometer and a spinner magnetometer for 458 semi-consolidated mud samples from eight sections of the Plio-Pleistocene Tsunozu Formation at its type locality, the Tsunozu area, San'in district. The results, revealing a magnetostratigraphic sequence with repeated magnetically normal and reversed intervals, are discussed with regard to the chronology of the formation. Based on previous data on magnetostratigraphy and the fission track age of the Oe-Takayama Volcano Group northeast of the Tsunozu area, a correlation with the standard paleomagnetic polarity time scale leads to the conclusion that the entire Tsunozu Formation ranges from just before the Cochiti normal event (about 4.1Ma) to just before the Jaramillo normal event (about 1Ma), and that the Plio-Pleistocene boundary (1.64Ma) is situated at the horizon just above the M3 marine clay bed in the Middle Member. An approximate chronology of four marine clay interbeds within the Middle Member is: M1 bed, 3.8-4.0 Ma; M2 bed, 2.5-2.7Ma; M3 bed, 1.7-1.8Ma; M4 bed, 1.1-1.5Ma, in ascending order. The apparent sedimentary rate of the Middle Member is about 2cm/103y. Our estimation of the stratigraphic horizon of the Plio-Pleistocene boundary is quite consistent with the previous view based on pollen stratigraphy.
日本第四紀学会の論文