The Ta-d tephra (8.7–9.2 cal. ka) from Tarumai Volcano in south-western Hokkaido has been newly recognized at two archeological sites and in cores taken from a bay in the Kushiro–Nemuro area, eastern Hokkaido, Japan. Identification of the tephra is based on chronological data, stratigraphic relationships (it is overlain by the 2.5 cal. kyr old Ta-c2 tephra), lithofacies, and petrological correlation (modal mineralogy and glass chemistry) with proximal Ta-d tephra deposits. In particular, the relatively high TiO2 and low K2O contents of the tephra are distinctive. The Ta-d tephra comprises lower pumiceous and upper scoriaceous parts, and is well preserved in the distal Kushiro region as a 5-cm-thick tephra layer. The Ta-d tephra may be a useful chronostratigraphic marker for the early Holocene in eastern Hokkaido and bordering regions of the Pacific Ocean.
Takakura Volcano is located at the southern end of the Senzan geothermal field of the northeastern Japan arc, in the central part of the volcanic chain comprising Kotakakura, Takakura, and Marumori volcanoes. We have carried out a detailed geological and petrological investigation of Takakura Volcano in order to reveal its volcanic history and evolution of the magmatic plumbing system beneath the volcano. In addition, a K–Ar dating study of rock samples from the volcano was carried out. Takakura Volcano comprises two lava series: an older series (Takinosawa lava series) of the lower and eastern part of the volcano, and a younger series (Takakurasan lava series) from the present-day summit down to the southeastern and southern foot of the volcano. On the basis of field and petrological data, the Takinosawa and Takakurasan lava series can be divided into 13 (TS1–13; oldest to youngest) and 17 (TK1–17) units, respectively. One of the oldest samples of the TS1 has a K–Ar age of 0.44 ± 0.14 Ma. Rocks of Takakura Volcano typically belong to the low-K tholeiitic series, and predominantly comprise basaltic andesite, andesite, and rare dacite. The tholeiitic lavas can be further divided into two groups on the basis of incompatible element concentrations such as Zr and Nb (i.e., high- and low-Zr groups). Although the two groups have been erupted continuously throughout the volcano’s history, the high- and low-Zr groups have distinct compositional variation trends on geochemical diagrams. Furthermore, a different modal phenocryst assemblage is evident in each group. These observations suggest that the two groups were derived from independent magma plumbing systems, which have persisted throughout the lifetime of Takakura Volcano.
We determined the K–Ar ages of 21 samples from the Asama–Eboshi volcanoes in central Japan in order to understand the long-term spatial migration of Quaternary eruptive centers in this region. These volcanoes consist of a series of WNW–ESE aligned stratovolcanoes and surrounding monogenetic volcanoes. Our new K–Ar dates and previously reported ages show that volcanic activity can be classified into four stages separated by dormant periods as follows. (1) During Stage I (ca. 1.0 Ma) and Stage II (0.85–0.75 Ma), magma was erupted from the western part of these volcanoes and produced the Eboshidake stratovolcano followed by monogenetic volcanoes on the western side of this volcano. (2) In Stage III, volcanic reactivation of Eboshidake took place at ca. 0.45 Ma and formed the Kakuma–Nabebutayama stratovolcano at ca. 0.35 Ma. Volcanic activity in the western area ceased at ca. 0.3 Ma, and then Stage III volcanism extended towards the ESE resulting in the formation of Sanpogamine and Takamineyama stratovolcanoes (ca. >0.24 Ma). (3) In Stage IV, lava domes (e.g., Hanareyama; 0.02 Ma) and stratovolcanoes (e.g., Kurofuyama; ca. 0.10 Ma) were produced in the central and eastern areas. The currently active Maekakeyama stratovolcano began erupting at ca. 8.5 ka. Monogenetic volcanism was limited to brief periods at ca. 0.8, 0.3, 0.1, and 0.02 Ma. Volcanic activity associated with the Asama–Eboshi volcanoes migrated towards the ESE after ca. 0.3 Ma and formed a volcanic chain oriented WNW–ESE.
The stratigraphic divisions of the Upper Pleistocene Shimosa Group forming the Hitachi Terraces in Ibaraki Prefecture are still poorly constrained, as their correlation with the type area of the group on Boso Peninsula has not been established. The Kioroshi Formation is the upper unit of the Shimosa Group. We have conducted a tephrochronological investigation of the Kioroshi Formation, including a stratigraphic and petrological study, and determination of volcanic glass major element chemistry and the refractive indices of glass, hornblende, and orthopyroxene. We have attempted to correlate tephra layers with those that can be expected to exist in a westerly source area around the north Kanto region and in marine cores off Kashima in the Pacific Ocean. Tephras ArP, OiP, and Ob that are intercalated with the Kenjaku Member in the lower unit of the Kioroshi Formation can be correlated with tephras Nk-Yt and Nk-Nm distributed in the westerly source area and TAu-9 in the southwestern Kanto region, respectively. Tephra KtP that is intercalated with the Namegata Member in the upper Kioroshi Formation can possibly be correlated with tephra 20 (1) in a marine core off Kashima. On the basis of these correlations, tephras ArP, OiP, and KtP can be interpreted to have been deposited during early MIS 5e, MIS 5e, and early MIS 5d, respectively. The Kenjaku and Namegata members in the Kioroshi Formation are estimated to have been deposited during MIS 6–5e and early MIS 5d, respectively.