南関東における第四紀中期のテフラの対比とそれに基づく編年

Abstract

This paper presents the basic data for tephrochronology of the Middle Pleistocene events of South Kanto, Central Japan. The tephra preceding to the last interglacial age (the Tama tephra) is found in highly dissected terraces (Fig. 1) much thicker than the younger tephra formations (Fig. 2). It is subdivided into five formations from T-A to T-E in order of increasing age (Table 1 and Fig. 2). This classification is based not on the eruptive history of the volcanoes but on the sequence of terraces. The relationship between these two is shown in Fig. 2.Of the four columnar sections (Fig. 6), section 1, synthesized from many sections in the Ooiso Hill (Fig. 3), 25 km east of the Hakone volcano, provides a standard section for South Kanto. There are so many tephra layers of the Tama stage (about 150 m in thickness) preserved in a relatively fresh state, that they are expected to cover a long time in the Middle Pleistocene. A detailed description is therefore needed for specifying any particular tephra bed. 75 marker beds have been selected, and described in terms of lithological and petrographical characteristics as in Table 2 and Fig. 4. The most valuable for characterizing marker tephras are the refractive indices of orthopyroxene and hornblende.The distribution and the petrographic character of tephras indicate that the tephras mainly comprising two pyroxene phenocrysts might come chiefly from the Old Somma of the Hakone volcano (Fig. 8-1 and 8-2) and partly from the Ashitaka volcano. Whereas such salic tephras with abundant hornblende, biotite and other phenocrysts as GoPi and TE-5 might originate from distant volcanoes (Fig. 8-3).As a result of precise examination, tens of the marker beds have been traced eastward to the environs of Tokyo and Yokohama (Fig. 6), where the standard Quaternary sequence had been established. Furthermore, several marker beds have been recognized within the Middle Pleistocene marine sediments in the Boso Peninsula. By use of these the Quaternary stratigraphy already established in South Kanto is significantly revised (Table 3 and 6).