10:15 〜 10:30
[SEM17-06] Reconstruction of the lower Olduvai geomagnetic polarity transition in Chikura Group distributed in the Boso Peninsula
キーワード：オルドバイ、極性反転、房総半島
The Chikura group, covering the Upper Pliocene – Lower Pleistocene, is distributed in the Southernmost part of the Boso Peninsula, Chiba Prefecture, Japan. The group is thought to be a marine deposit filled on the landward slope basin facing to the paleo-Sagami trough. Okada et al. (2012) constructed a paleomagnetic and oxygen isotopic stratigraphies, covering between about 2.3 and 3.5 Ma (MIS G16 to 93), for the Mera and the Minamiasai Formations consisting of the Chikura Group. The tephra bed, seen in the top horizon of the section studied by Okada et al., (2012), can be correlated to a tephra bed found at the bottom horizon of our study section where the Hata Formation laying on and party interfingered with the Minamiasai Formation. This observation indicates that our study section is suitable for reconstruction of stratigraphic records, including the geomagnetic filed variation, for the period younger than 2.3 Ma. For rock-magnetic and paleomagneitc measurements, we collected several mini-cores using a portable engine drill at every 1 - 4 m stratigraphic interval, and collected a mini-core at every 10 cm interval around reversal boundaries associated with the Réunion Subchronozone and Olduvai Subchronozone.
The demagnetization methods evaluated in this study are the alternating filed demagnetization (AFD) with 5 mT increments up to 80 mT, the thermal demagnetization (ThD) with 50 °C increments up to 600 °C, a hybrid method consisting of ThD at 250 °C and the AFD sequence. Among those methods, only the Hybrid method provided data passing the reversal test. So, we selected the Hybrid method as to provide paleomagentic data from the study section.
We detected the Reunion Subchronozone and the lower Olduvai polarity transition zone from horizons of 4.7 m and 1.6 m thickness respectively.We obtained an oxygen isotope stratigraphy from the lower sequence covering the Réunion Subchronozone. As the result, the lower and upper boundaries of the Réunion Subchronozone are correlated to MIS 81 and 80 respectively, which are consistent with the result from IODP Site U1308 (Channell et al., 2016).
Reference Okada et al., 2012, Jour. Geol. Soc. Japan, 118, 97–108. Channell et al., 2016, Quat. Sci. Rev. 131, 1-19.
The demagnetization methods evaluated in this study are the alternating filed demagnetization (AFD) with 5 mT increments up to 80 mT, the thermal demagnetization (ThD) with 50 °C increments up to 600 °C, a hybrid method consisting of ThD at 250 °C and the AFD sequence. Among those methods, only the Hybrid method provided data passing the reversal test. So, we selected the Hybrid method as to provide paleomagentic data from the study section.
We detected the Reunion Subchronozone and the lower Olduvai polarity transition zone from horizons of 4.7 m and 1.6 m thickness respectively.We obtained an oxygen isotope stratigraphy from the lower sequence covering the Réunion Subchronozone. As the result, the lower and upper boundaries of the Réunion Subchronozone are correlated to MIS 81 and 80 respectively, which are consistent with the result from IODP Site U1308 (Channell et al., 2016).
Reference Okada et al., 2012, Jour. Geol. Soc. Japan, 118, 97–108. Channell et al., 2016, Quat. Sci. Rev. 131, 1-19.