A marker tephra bed close to the Lower-Middle Pleistocene boundary: Distribution of the Ontake-Byakubi Tephra Bed in central Japan
Introduction
The Kazusa Group, which is widely distributed in the central part of the Boso Peninsula, is representative of Lower and Middle Pleistocene marine sediments in Japan. Comprehensive studies of the lithostratigraphy, biostratigraphy, magnetostratigraphy, and oxygen-isotope stratigraphy of this group have been carried out (Working Group for Quaternary Stratigraphy of Boso, 2009, Kazaoka et al., 2015). A continuous exposure of the Kokumoto Formation, middle part of the Kazusa Group, along the Yoro River, Chiba Prefecture, is a candidate Global Boundary Stratotype Section and Point (GSSP) for the lower boundary of the Middle Pleistocene Subseries (Head et al., 2008). The Kazusa Group contains numerous tephra layers, and the tephrostratigraphy is well constrained by many marker tephras (Mitsunashi et al., 1959, Mitsunashi et al., 1979, Satoguchi, 1995, Satoguchi, 1996, Satoguchi and Nagahashi, 2012). Among the tephra layers, the Byakubi-E tephra (Byk-E: Kazaoka et al., 2015) in the middle part of the Kokumoto Formation, is intercalated approximately 1 m below the Matuyama–Brunhes Polarity Chronozone boundary (Kazaoka et al., 2015). Therefore, Byk-E has the potential to be an important regional marker tephra bed for the Lower-Middle Pleistocene boundary if its distributional area and characteristics for identification and correlation are clarified.
Takeshita et al. (2005) showed that Byk-E is correlated with either the Yukawa tephra 4 or 5 (YUT4 or YUT5) at the foot of Ontake volcano in central Japan, and showed that its source was the Older Ontake volcano. Byk-E has not been correlated, however, with tephra layers in any other locality. In this paper, we show that the Minamitajima (Mtj) and Nezumigawa (Nzg) tephras in the Ina basin, which lies between the Boso Peninsula and the Older Ontake volcano, correlate with Byk-E. This correlation confirms that Byk-E was erupted from the Older Ontake volcano. Furthermore, we show that Byk-E is correlated with YUT5, not YUT4. Here, we propose to call the series of tephras that correlate with Byk-E the Ontake-Byakubi tephra (On-Byk).
Section snippets
Yukawa tephra 4 (YUT4) and Yukawa tephra 5 (YUT5), tephra beds of the Older Ontake volcano
Ontake volcano, which is an active volcano is situated at the southern margin of the Norikura volcanic chain, central Japan (Fig. 1), consists of Older and Younger Ontake volcanoes (Yamada and Kobayashi, 1988). The Older Ontake volcano is estimated to have been active about 0.78–0.39 Ma, based on K–Ar ages of 47 lavas (Kioka et al., 1998). Younger Ontake volcano became active about 0.1 Ma and was the source of a phreatic eruption in 1979 and 2014 (Oikawa, 2014). The products of Younger Ontake
Analytical methods
The tephra samples collected for this study (Nzg, Mtj, and Byk-E from Locality 5) and from previously published studies (YUT4, YUT5, and Byk-E from Locality 4) were prepared as outlined in Fig. 9. The samples were washed ultrasonically after washing in a beaker. The dried residual grains were sieved to obtain the fractions from 1/4 to 1/8 mm and from 1/8 to 1/16 mm. The 1/8 to 1/16 mm fraction was used for analysis of the grain composition. Mafic minerals were separated with a neodymium magnet
Results
The petrographic properties of YUT4 and YUT5, Mtj, Nzg, Byk-E (Locality 4), and Byk-E (Locality 5) tephras are shown in Table 1.Name of tephra Sampling locality Bulk grain composition (%) Mafic mineral composition (%) Glass morphology Reference G1 P1 R.F. M.M. Hb1 Cpx Opx Bt Opq Ap Zr Gr YUT5 Loc. 1 36.4 50.8 2.6 1.1 88.1 0.4 0.8 − 10.7 + + − P 1 YUT4 Loc. 1 34.6 46.9 4.7 2.8 70.0 0.8 0.4 + 24.5 − − − P 1 Nzg Loc. 2 – 88.3 2.5 9.2 43.8 0.7 0.4 − 55.1 + − − * This study Mtj Loc. 3 – 86.6 4.7 8.7 39.1 0.8
Potential of major element composition of hornblende for tephra identification and correlation
The correlation and identification of tephra layers are often undertaken by using glass shard characteristics (morphology, refractive index, major and trace element compositions) in addition to lithofacies, mineral composition, stratigraphic relationships, and radiometric age (Machida and Arai, 1992, Lowe, 2011). However, the volcanic glass shards of Mtj and Nzt have been altered by weathering. Thus, the major element composition of hornblende phenocrysts was used to characterise the tephras in
Conclusion
Petrographic features including bulk grain and mafic mineral compositions and the major element composition of hornblende from tephras Mtj and Nzg in the Ina basin have been described and compared with previously published data for tephras YUT4, YUT5, and Byk-E.
We are able to distinguish YUT5 from YUT4 and to correlate Byk-E, Mtj, and Nzg with YUT5 on the basis of the major element composition of hornblende.
We propose that the series of tephras correlated with Byk-E be called the Ontake-Byakubi
Acknowledgements
We thank Prof. Yasuyuki Miyake, Prof. Kuniaki Makino and Mr. Tatsuro Tsugane, Shinshu University for providing many helpful suggestions during the EDS analyses.
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Paleoclimatic and paleoceanographic records through Marine Isotope Stage 19 at the Chiba composite section, central Japan: A key reference for the Early–Middle Pleistocene Subseries boundary
2018, Quaternary Science ReviewsCitation Excerpt :Onishi (1969) also reported that the disappearance of Metasequoia, a component of the northern Hemisphere flora from the Late Cretaceous to Quaternary, occurs in the middle of the Kokumoto Formation. Formally known as the Ontake-Byakubi tephra (Takeshita et al., 2016), the Byk-E is a widely distributed tephra in the central part of the Boso Peninsula (Okada and Niitsuma, 1989; Kazaoka et al., 2015; Nanayama et al., 2016). It is a white pumiceous fine ash deposit 1–7 cm thick, interbedded with dark gray sandy silt units (or layers) in the middle of the Kokumoto Formation.
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2017, Quaternary InternationalCitation Excerpt :In general, the chemistries of amphibole and other ferromagnesian minerals do not allow tephras of different ages from the same source to be discriminated (Shane, 2000). Recently, however, amphibole (cummingtonite) chemistry has been successfully used to characterize tephras erupted from Numazawa and Ontake volcanoes at different times (Matsu'ura et al., 2011; Takeshita et al., 2016). Therefore, in some cases at least, amphibole chemistry can be used to distinguish tephras from the same source.
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2016, Quaternary InternationalCitation Excerpt :Because the Matuyama–Brunhes paleomagnetic reversal serves as the primary reference for the Lower–Middle Pleistocene stage boundary (Head and Gibbard, 2005), the Chiba composite section (Tabuchi, Yanagawa, and Kogusabata sections) (Kazaoka et al., 2015), which also contains tephra deposits and thus isochronous chronostratigraphic tie-points, is therefore an exceptional candidate for its Global Boundary Stratotype Section and Point (GSSP). Moreover, the M−B boundary is underlain by a distinctive and widespread tephra deposit, Bky-E (also known as Ontake-Byakubi Tephra: Takeshita et al., 2016) that enables the boundary therefore to be identified in sequences in other parts of Japan, and potentially linked with marine sedimentary records via tephrochronology. In southern Italy, the Montalbano-Jonico section (e.g., Ciaranfi and D'Alessandro, 2005) and the Valle di Manche section (Capraro et al., 2005) are other candidates for the GSSP (Head et al., 2008).
Stratigraphy of the Kazusa Group, Boso Peninsula: An expanded and highly-resolved marine sedimentary record from the Lower and Middle Pleistocene of central Japan
2015, Quaternary InternationalCitation Excerpt :The most remarkable is the Byk-E bed, which varies from 1 to 3 cm in thickness and consists of white, glassy, fine-grained ash (Fig. 12). The Byk-E bed has recently been correlated with the YUT5 bed erupted from the Older Ontake volcano, central Japan, and is accordingly named the Ontake-Byakubi tephra (Takeshita et al., 2015). The Byk-E tephra is the same as the Shirao ash bed of WQSB (1996) and Satoguchi (1996, 1997).
Formal subdivision of the Quaternary System/Period: Past, present, and future
2015, Quaternary InternationalCitation Excerpt :The primary magnetization carrier is magnetite (titanomagnetite). Takeshita et al. (2015) have shown that the Byakubi-E tephra can be traced widely across central Japan and is derived from the Older Ontake volcano ∼250 km to the west (Takeshita et al. (2015). It serves as a potential marker for the GSSP, and a useful guide for the Matuyama–Brunhes boundary particularly between the Tabuchi (proposed location of GSSP), Yanagawa, and Kogusabata sections that comprise the Chiba composite section (Suganuma et al., 2015 supplementary data).