5:15 PM - 6:45 PM
[SVC30-P08] Re-examination of magnitude and precise ages of Early Pleistocene huge Shirakawa Ignimbrites associated with caldera forming-eruption in Northeast Japan
Keywords:Shirakawa Ignimbrite, caldera forming-eruption, Early Pleistocene, fallout tephra
The Aizu volcanic region located in southern part of NE Japan is characterized by serial huge caldera-forming eruptions (> 6 times) occurred at 1.5–0.9 Ma in Early Pleistocene. Stratigraphy of the Shirakawa Ignimbrites resulted from these repeated eruptions has been examined by several previous studies (Yoshida & Takahashi, 1991; Yamamoto, 1999), followed by studies on re-examination and detection of those distal fallout tephras (Kurokawa et al., 2008; Suzuki et al., 2017, 2020). The later can contribute to volume estimations of fallout tephras and determination of precise eruption ages combined by several dating methods. Here, we review recent studies on identification of distal fallout tephras relating to the Shirakawa Ignimbrites in term of age determination.
Shirakawa Ignimbrites and those corresponded fallout ashes are as follows in ascending order, Sr-Kmd (1.542–1.504 Ma), Sr-Aki-Kd18 (1.522–1.460 Ma), Sr-Asn-Kd8 (1.219 Ma), (Sr-Nsg; 1.2–1.1 Ma), Sr-Tne and Sr-Kc-U8 (0.922–0.910 Ma) (Suzuki et al., 2017, 2020).
Sr-Asn-Kd8 and Sr-Kc-U8 showing broadest distributions have been detected in Kinki area (SW of the source) and off of Tokyo (S of the source) both ca. 500 km distant from the source, with volume estimations of 68 km3 and 34 km3 in DRE, respectively under the assumption that they distribute radially (Suzuki et al., 2020). At U1424 core site, off the Oga Peninsula in the Japan Sea, (ca. 370 km NNW of the source), a recent study by Xuan et al. (2020) found tephra with possibility to correlate to Sr-Aki-Kd18 and Sr-Kc-U8, confirming distribution of distal tephras associated with the Shirakawa Ignimbrites eruptions. We can expect further detection of those in the northern part of the Japan Sea, northwest Pacific Ocean and inland area from north NE Japan and Hokkaido.
Although Sr-Nsg is the second largest ignimbrite in Shirakawa Ignimbrites (Yamamoto, 2011), its associated fallout tephra had been detected in only Niigata area ca 100 km NW of the source (Kurokawa et al., 2008). In addition, estimated age of Sr-Nsg (1.2–1.1 Ma) has a large error. Here, we explore possibility of correlation of Sr-Nsg with similar tephras in Niigata area and Boso Peninsula. Sugasawa Glassy Ash (Susg) in Niigata area was already correlated with Sr-Nsg by Kurokawa et al. (2008). We re-examine this correlation and another correlation of O31 (Utsunomiya & Oi, 2019) with similar geochemistry of volcanic glass shards in the Boso Peninsula. Sr-Nsg, Susg and O31 contain hornblende and their glass geochemistryies are similar (SiO2: 77.8 wt%, Al2O3: 12.4–12.6 wt%, FeO: 1.1–1.2 wt%, CaO: 1.1-1.2 wt%, K2O: 3.1–3.4 wt%). Marine isotope stratigraphy for O31 indicates its position in MIS36 (ca 1.20 Ma) (Kuwano et al., 2021; Pickering et al., 1999; Okada& Okada, 2023), matching estimated age of Sr-Nsg. However, trace element glass data by LA-ICP-MS suggest that O31 is not correlative to Sr-Nsg (e.g., difference of Zr, Nb, Ba and Y content). Therefore, distribution of Sr-Nsg in south direction from the source is not yet found. On the other hand, we confirmed identification of Sr-Nsg in Niigata area by LA-ICP-MS analyses.
In presentation, we discuss magnitude and precise ages (intervals) of eruptions of the Shirakawa Ignimbrites, focusing on new ages by recent studies such as younger ages for tephras below Sr-Kmd (Nozaki et al., 2014), 100 kry older age of Sr-Asn-Kd8 (Kuwano et al., 2021; 1.31 Ma).
Shirakawa Ignimbrites and those corresponded fallout ashes are as follows in ascending order, Sr-Kmd (1.542–1.504 Ma), Sr-Aki-Kd18 (1.522–1.460 Ma), Sr-Asn-Kd8 (1.219 Ma), (Sr-Nsg; 1.2–1.1 Ma), Sr-Tne and Sr-Kc-U8 (0.922–0.910 Ma) (Suzuki et al., 2017, 2020).
Sr-Asn-Kd8 and Sr-Kc-U8 showing broadest distributions have been detected in Kinki area (SW of the source) and off of Tokyo (S of the source) both ca. 500 km distant from the source, with volume estimations of 68 km3 and 34 km3 in DRE, respectively under the assumption that they distribute radially (Suzuki et al., 2020). At U1424 core site, off the Oga Peninsula in the Japan Sea, (ca. 370 km NNW of the source), a recent study by Xuan et al. (2020) found tephra with possibility to correlate to Sr-Aki-Kd18 and Sr-Kc-U8, confirming distribution of distal tephras associated with the Shirakawa Ignimbrites eruptions. We can expect further detection of those in the northern part of the Japan Sea, northwest Pacific Ocean and inland area from north NE Japan and Hokkaido.
Although Sr-Nsg is the second largest ignimbrite in Shirakawa Ignimbrites (Yamamoto, 2011), its associated fallout tephra had been detected in only Niigata area ca 100 km NW of the source (Kurokawa et al., 2008). In addition, estimated age of Sr-Nsg (1.2–1.1 Ma) has a large error. Here, we explore possibility of correlation of Sr-Nsg with similar tephras in Niigata area and Boso Peninsula. Sugasawa Glassy Ash (Susg) in Niigata area was already correlated with Sr-Nsg by Kurokawa et al. (2008). We re-examine this correlation and another correlation of O31 (Utsunomiya & Oi, 2019) with similar geochemistry of volcanic glass shards in the Boso Peninsula. Sr-Nsg, Susg and O31 contain hornblende and their glass geochemistryies are similar (SiO2: 77.8 wt%, Al2O3: 12.4–12.6 wt%, FeO: 1.1–1.2 wt%, CaO: 1.1-1.2 wt%, K2O: 3.1–3.4 wt%). Marine isotope stratigraphy for O31 indicates its position in MIS36 (ca 1.20 Ma) (Kuwano et al., 2021; Pickering et al., 1999; Okada& Okada, 2023), matching estimated age of Sr-Nsg. However, trace element glass data by LA-ICP-MS suggest that O31 is not correlative to Sr-Nsg (e.g., difference of Zr, Nb, Ba and Y content). Therefore, distribution of Sr-Nsg in south direction from the source is not yet found. On the other hand, we confirmed identification of Sr-Nsg in Niigata area by LA-ICP-MS analyses.
In presentation, we discuss magnitude and precise ages (intervals) of eruptions of the Shirakawa Ignimbrites, focusing on new ages by recent studies such as younger ages for tephras below Sr-Kmd (Nozaki et al., 2014), 100 kry older age of Sr-Asn-Kd8 (Kuwano et al., 2021; 1.31 Ma).