10:15 AM - 10:30 AM
[HQR05-06] Early Pleistocene Nishikubo Tephra (NK) newly found in the Sayama Formation of the Kazusa Group, west Kanto Plain, NE Japan
Keywords:Quaternary, Kanto Plain, Kazusa Group, Sayama Formation, Nishikubo Tephra
Early Pleistocene Sayama Formation within the Kazusa Group is distributed in the Sayama Hills, western Kanto Plain. It has been reported that the Kazusa Group near the hill has a change in sedimentary environment from marine in the lower part to alternating of freshwater, brackish water, and marine in the upper part with boundaries at approximately 20 m above sea level (Kawashima and Kawai, 1977). This indicates a transition from a forearc basin to land, and a sedimentary cycle reflecting glacial sea level changes has been observed in the Sayama Formation (Fukushima et al., 2024). Several tephras have been reported in the Sayama Formation, however, the relationship between the depositional cycles and Marine Oxygen Isotope Stages has not been clarified. Well-studied tephras in the Sayama Formation are ZSK-Kd5A (1.3 Ma), SGO-Ob4b-1 (1.573 Ma), and SYG-Kd29 (1.634 Ma) (Suzuki et al., 2023; Fukushima et al., 2024). Recently, we have detected a new tephra between ZSK-Kd5A and SGO-Ob4b-1, correlative to the Nishikubo tephra (NK: Suzuki and Murata, 2011) previously recognized in several areas in southern Kanto Plain. Here, we describe this tephra and its correlation.
Two tephras reported here are sandwiched in a silt layer with a thickness of 2–3 m near the northwest end of the Tamako (Murayama Upper Reservoir) in the southern part of Sayama Hills (115 m above sea level). Lower tephra is composed of white coarse sand size glassy volcanic ash with a thickness of 18 cm, and of the lowermost unit with a thickness of 3 mm consists of fine-grained volcanic ash. Upper tephra located 30 cm above the lower tephra is a patchy tephra consisting of a few centimeters to several millimeters of white glassy volcanic ash aggregate. Here, we named them the Tamako Lower Glassy Tephra (Tmk-L) and the Tamako Upper Glassy Tephra (Tmk-U), respectively. ZSK-Kd5A is found 900 m southeast of this site, and SGO-Ob4b-1 is found 1950 m west-northwest of this site. Considering these elevations and the eastward gentle dipping of the Sayama Formation, the stratigraphic position of Tmk-L & U is undoubtedly between ZSK-Kd5A and SGO-Ob4b-1. Due to the diatom fossil assemblage, it was judged that the sedimentary environment for the deposits immediately below Tmk-L & U was marine, one containing dense plant fossils directly above the tephra was freshwater, and a thin layer of gray mud layer with a thickness of ca. 30 cm above it was marine. In addition, within these deposits fossils of caltrops and coniferous needles were detected in preliminary observations.
Tmk-L is mainly composed of pumice-type volcanic glass shards with a refractive index of 1.496–1.500, and its major chemical composition is SiO2: 76.57 wt%, Al2O3: 13.00 wt%, CaO: 0.75 wt%, K2O: 4.47 wt% showing a K-rich rhyolitic tephra. Due to ages of two tephras above and below Tmk-L, the age of Tmk-L is thought to be 1.2–1.8 Ma, and properties of NK, YM, and KK with the ages of 1.2–1.8 Ma in southern Kanto are similar to that of Tmk-L (Suzuki and Murata, 2011). Among these, NK and KK are similar to Tmk-L, and only Al2O3 in KK tends to be slightly lower than those of Tmk-L and NK. In comparison of NK, YM, and KK by using their minor element compositions determined by LA-ICP-MS, they all show similar contents of Th and La, but NK, YM, and KK can be clearly distinguished by their Sr content. Sr content of Tmk-L is almost the same as that of NK, and other elements are also so similar. Consequently, Tmk-L can be correlated to NK.
NK detected in several areas of the southern Kanto Plain is correlative to a tephra defined as Kd12 in the Boso Peninsula. Later, Kd12 was renamed to Kd16.5, and below it, in descending order, there are YM (Kd19.3) and KK (Kd20) (Suzuki and Murata, 2011; Utsunomiya et al., 2019). The reliable ages of the tephra before and after NK (Kd16.5) have been clarified for the upper Kd8 and the lower Kd24, with estimated ages of 1.310 Ma and 1.543 Ma, respectively (Kuwano et al., 2021; Nozaki et al., 2014). The stratigraphic position of NK (Kd16.5) is in the upper half of the interval of Kd8 and Kd24, and a marine depositional environment is assumed above and below NK (Tmk-L) in Sayama Hills. Therefore, the age of NK (Tmk-L) is around 1.35–1.45 Ma (around the peak of MIS43, 45, or 47).
Two tephras reported here are sandwiched in a silt layer with a thickness of 2–3 m near the northwest end of the Tamako (Murayama Upper Reservoir) in the southern part of Sayama Hills (115 m above sea level). Lower tephra is composed of white coarse sand size glassy volcanic ash with a thickness of 18 cm, and of the lowermost unit with a thickness of 3 mm consists of fine-grained volcanic ash. Upper tephra located 30 cm above the lower tephra is a patchy tephra consisting of a few centimeters to several millimeters of white glassy volcanic ash aggregate. Here, we named them the Tamako Lower Glassy Tephra (Tmk-L) and the Tamako Upper Glassy Tephra (Tmk-U), respectively. ZSK-Kd5A is found 900 m southeast of this site, and SGO-Ob4b-1 is found 1950 m west-northwest of this site. Considering these elevations and the eastward gentle dipping of the Sayama Formation, the stratigraphic position of Tmk-L & U is undoubtedly between ZSK-Kd5A and SGO-Ob4b-1. Due to the diatom fossil assemblage, it was judged that the sedimentary environment for the deposits immediately below Tmk-L & U was marine, one containing dense plant fossils directly above the tephra was freshwater, and a thin layer of gray mud layer with a thickness of ca. 30 cm above it was marine. In addition, within these deposits fossils of caltrops and coniferous needles were detected in preliminary observations.
Tmk-L is mainly composed of pumice-type volcanic glass shards with a refractive index of 1.496–1.500, and its major chemical composition is SiO2: 76.57 wt%, Al2O3: 13.00 wt%, CaO: 0.75 wt%, K2O: 4.47 wt% showing a K-rich rhyolitic tephra. Due to ages of two tephras above and below Tmk-L, the age of Tmk-L is thought to be 1.2–1.8 Ma, and properties of NK, YM, and KK with the ages of 1.2–1.8 Ma in southern Kanto are similar to that of Tmk-L (Suzuki and Murata, 2011). Among these, NK and KK are similar to Tmk-L, and only Al2O3 in KK tends to be slightly lower than those of Tmk-L and NK. In comparison of NK, YM, and KK by using their minor element compositions determined by LA-ICP-MS, they all show similar contents of Th and La, but NK, YM, and KK can be clearly distinguished by their Sr content. Sr content of Tmk-L is almost the same as that of NK, and other elements are also so similar. Consequently, Tmk-L can be correlated to NK.
NK detected in several areas of the southern Kanto Plain is correlative to a tephra defined as Kd12 in the Boso Peninsula. Later, Kd12 was renamed to Kd16.5, and below it, in descending order, there are YM (Kd19.3) and KK (Kd20) (Suzuki and Murata, 2011; Utsunomiya et al., 2019). The reliable ages of the tephra before and after NK (Kd16.5) have been clarified for the upper Kd8 and the lower Kd24, with estimated ages of 1.310 Ma and 1.543 Ma, respectively (Kuwano et al., 2021; Nozaki et al., 2014). The stratigraphic position of NK (Kd16.5) is in the upper half of the interval of Kd8 and Kd24, and a marine depositional environment is assumed above and below NK (Tmk-L) in Sayama Hills. Therefore, the age of NK (Tmk-L) is around 1.35–1.45 Ma (around the peak of MIS43, 45, or 47).