Tephrochronological Study on the Kazusa Group in the Southeast Part of Musashino Upland, Tokyo Metropolitan Area, Central Japan

Four boring cores obtained by boring surveys conducted at Wadabori Park in Suginami Ward (core WDB), Kinuta Park (core KNT) in Setagaya Ward, Tamagawadai Park (core TMG) and Unoki (core UNK) in Ota Ward, southeast part of Musashino upland, Tokyo, were reexamined. Mineral assemblage, shapes of volcanic glass shard, refractive indices of volcanic glass, and heavy minerals and chemical composition of volcanic glass of several tephra layers from these cores were determined. As a result, it is revealed that six tephra layers are correlative with the key tephra layers in the Kazusa Group of the Boso Peninsula and Tama Hills in the Kanto district. The WDB-1 tephra from core WDB is identified as the Kd8 tephra in the upper part of the Kiwada Formation in the Kazusa Group. The KNT-4 tephra from core KNT, the TMG-7 tephra from core TMG and the UNK-1 tephra from core UNK are identified as the Kd16 tephra in the middle part of the Kiwada Formation. The TMG-18 tephra from core TMG and the UNK-8 tephra from core UNK are identified as the Kd18 tephra in the middle part of the Kiwada Formation. Based on the correlation among these tephras, the sediments of ca. 15 m above sea level (a.s.l.) at Wadabori Park is correlative with the upper part of the Kiwada Formation deposited at 1.21-1.27 Ma, and the sediments of ca.-2 m a.s.l. at the Kinuta Park, ca.-20 to-74 m a.s.l. at the Tamagawadai Park and ca.-9 to-44 m a.s.l. at Unoki are correlative with the Kiwada Formation in the Kazusa Group. The sedimentary ages of these sediments are between 1.27 and 1.45 Ma based on stratigraphic relations with the calcareous nannofossil datums.