TEPHROCHRONOLOGICAL STUDY OF THE MIDDLE PLEISTOCENE FLUVIAL SURFACES IN THE KANTO PLAIN, CENTRAL JAPAN: A KEY GEOMORPHIC SURFACE OF MARINE ISOTOPE STAGE 17 TO 16

Two middle Pleistocene tephras HBP and KMT derived from the volcanoes in the Hida Mountains are favorable marker tephras for the chronological study of this time in the Kanto Plain. On the basis of many radiometric ages and stratigraphic positions in the marine sediments of the Boso peninsula, the ages of HBP and KMT are estimated at 0.38-0.41 Ma (MIS 11) and 0.58-0.69 Ma (MIS 17.3-15.3), respectively. Judging from the stratigraphic relation with KMT, dissected fluvial surfaces, that is, the Azuyama and Sayama surfaces in South Kanto and the Kitsuregawa upper surface in North Kanto are correlative, and it is assumed that they were formed a few tens of thousands of years preceding the deposition of KMT. Preservations of these surfaces are well, and initial alluvial fans of them seem to be broad. Referring to the geomorphic development of late Pleistocene, alluvial fan deposits constituting these surfaces are most likely formed during the period from MIS 17.3 to 16.2 of a transition from interglacial to glacial. This term is specific for the geomorphic development of middle Pleistocene, characterized by the long duration keeping the stable conditions for the formation of broad fans.