11:45 AM - 12:00 PM
[HQR05-05] Tephrostratigraphy of drilled core C9010E off the Boso Peninsula: as a preliminary report
Keywords:CK09-03, C9010E, late Quaternary, Niijima, Kozushima, Oomurodashi
Firstly, all samples were checked roughly, and some fine glassy samples which might be correlated to widespread marker tephras were picked up and analyzed. As a result, correlatives for Aso-4 (87.1 ka; Aoki, 2008) at the depth 149.08 mbsf, On-Pm1 (95.7 ka; Aoki et al., 2008) at 158.244 mbsf, and Aso-ABCD at 25 cm below On-Pm1 were found, respectively. Secondly, it is summarized the result of analysis for 28 samples (No.1~28) from the top of core to approximately 30 mbsf. Ten samples consist of mainly scoria clasts. Another 20 samples are rich in felsic volcanic glass shards, and including plagioclase, quartz (including beta-quartz), orthopyroxene, hornblende, cummingtonite, and occasionally biotite. The content of SiO2 in felsic volcanic glass shards is 77.9-78.8 wt% generally, and it is so much. Except No.18 (16.722-16.772 mbsf), the contents of FeO* (0.59-0.74 wt%) and CaO (0.34-0.70 wt%) in felsic volcanic glass shards are remarkably low, on the other hand, the contents of Na2O (4.12-4.25 wt%) and K2O (3.45-4.15 wt%) are relatively high. These characters of major-element composition are resembled to them of felsic tephras provided from Niijima and Kozushima during late Quaternary. Furthermore, we tried to correlate each tephra bed in the core C9010C to each eruptive event based on the major-element composition. The probabilities that No.1 (1.15-1.17 mbsf) as the highest layer correlates to Niijima Mukaiyama tephra (AD886), and No.2 (1.63-1.66 mbsf) correlates to Kozushima Tenjyosan tephra (AD838) are high. No.8 (9.36-9.48 mbsf), No.12 and No.13 (14.32-14.65 mbsf), and No.14, No.15 and No.16 (No.14.65-15.54 mbsf) will correlate to the eruptive events of Miyatsukayama SW, Shikinejima, and Miyatuskayama on the Niijima main island, respectively (Kobayashi et al., 2019a). The petrographic and geochemical characters of No.19 and No.20 (19.90-19.00 mbsf), No.21 (19.31-19.34 mbsf), and No.28 (29.49-30.66 mbsf) are resemble to the events of Akasakimine on Niijima main island. There are hemipelagic ooze of 11 m thick, including several scoria layers, between No.19 and No.28. Kobayashi et al. (2019a) report Akasakimine events are several eruptions at ca.15-20 ka, therefore it would make sense for the time interval of 11 m-thick sediment between No.19 and No.28. Above-mentioned correlatives to tephras provided from Niijima island consists of felsic volcanic glass shards mainly, only a small amount of scoria grains is included. No.18 (16.72-16.78 mbsf) is sorted glassy tephra mainly, with gray to beige fine bubbled pumices, which maximum diameter is 7 mm. There are almost volcanic glass shards including a small amount of plagioclase, quartz and orthopyroxene, not scoria grains and lithic fragments in the fraction of 63-250 µm. The content of SiO2 is 78.8 wt%, FeO* is 1.24 wt%, CaO is 1.25 wt%, Na2O is 4.76 wt%, and K2O is 1.22 wt%. We consider that No.18 would be correlated to tephra provided from the submarine volcano Oomurodashi at 13.4 ka (Hamuro et al., 1983; Saito and Miyairi, 2008; Tani et al., 2017).