Analysis of titanomagnetite within weathered middle Pleistocene KMT tephra and its application for fluvial terrace chronology, Kanto Plain, central Japan

The widespread Kaisho-Kamitakara tephra (KMT), derived from the source vent located in the Hida Mountains, is a favorable marker tephra for the chronological study of the middle Pleistocene in the Kanto Plain, central Japan. As the KMT in aeolian sediments is strongly weathered, the chemical composition of titanomagnetite, resistant to weathering, was determined for its identification. The age of the KMT was estimated at 0.62 Ma (MIS 16.0) by Nakazato [2006. Horizon and age of Kaisho-Kamitakara tephra from the Inubo Group in the northeastern part of Chiba Prefecture, central Japan. Programme and Abstracts, Japan Association for Quaternary Research 36, 106–107 (in Japanese)], revising a previous estimate of 0.58–0.69 Ma by Suzuki [2000. Kaisho-Kamitakara tephra erupted from the Hida Mountains in early half of middle Pleistocene and its significance for geomorphic chronology of central Japan. Geographical Review of Japan 73, 1–25 (in Japanese)] The stratigraphic position of the KMT in the Azuyama and Sayama terrace surfaces in southwest Kanto and the Kitsuregawa Upper terrace surface in north Kanto indicates that these terraces are correlative. It is assumed that the dissected fluvial terrace surfaces were formed a few tens of thousands of years prior to the deposition of the KMT. Alluvial fan deposits constituting these terrace surfaces were formed during the period from MIS 17.3 to 16.2, during a transition from interglacial to glacial. Geomorphic conditions during this period of the middle Pleistocene were stable, allowing the formation of broad fans.