The Central Region including the so-called “Fossa Magna”-a Tertiary orogenic belt in Central Japan has been a field of intensive volcanisms since Miocene. Many volcanoes inside and outside the belt have supplied a vast amount of tephras through Quaternary times. In this territory, pyroclastc flows and volcanic mudflows as well as tephras of aeolian origin are usable as index horizons for the correlation of geomorphic surfaces and geological sediments and so on.
(1) In the valleys of Ina, Kiso, Matsumoto and Suwa is well developed a type of tephraic section of which materials have been supplied from Ontake and Norikura Volcanoes. The section is composed of three or four tephra units as mentioned in the following:
i) Hata or the Younger Loam (tephra) unit: The upper part may postdate the Late Würmian glaciation, and the lower part is dated roundly a little younger than 30, 000 14C yr. B. P. The Palaeolithic cultural layers are to be placed in the upper horizons of Hata Loam and its equivalents.
ii) Osakada or the Middle Loam unit: Pumiceous, tephrochronologically equivalent to the upper part of the Atsuta (marine) formations which have hitherto been considered as the deposits during the time of Riss/Würm Interglacial higher sea-level.
iii) The Older Loam unit: In Ina valley, Older Loam underlies Osakada Loam with less appreciable hiatus. A distinguished red pumice bed called “Pm-0”, which is often recognized within the fluviatile gravel beds in Ina valley, seems to mark the lower horizon of Older Loam. Much clayey.
iv) Nishibayashi Loam: At the top is a sign of subaerial weathering giving an evidence of the existence of stratigraphic break before the deposition of Osakada Loam. Much clayey. It is problematic whether the Loam might be a product mainly of tephra falls. Clay mineralogical content consists of the mixed type of halloysite, illite and 14Å minerals.
(2) Our knowledges about the tephras from Yatsugatake Volcanoes have not yet been detailed. The author is now examining heavy mineral compositions of the tephras as shown in Fig. 5. The Lower Loam unit is characterized by the excessive amount of magnetite, and presence of hornblende including oxyhornblende. The Middle Loam is pumiceous aud especially so in the lowest horizon. As seen in Fig. 5, a pumiceous horizons with a peculiar appearance and called “Uridane-gata-Fuseki So” (the word Uridane means the seeds of cucumber) occurring the middle part of Middle Loam is doubtlessly considered as exotic material, which is assigned to the pumice grains from Ontake Volcanoes. The modes of occurrence of these Loam units upon fluviatile terraces are discussed in the text.
(3) The Late Pleistocene activity of Tateyama Volcanoes supplied tephras in the north-wastern part of Shinshu or Nagano Prefecture. The tephraic section is marked by two pumiceous horizons, of which the lower one is in the lowest part of the Middle Loam unit and contains more amount of hypersthene and less amount of common hornblende and magnetite compared with the upper pumiceous horizon. The tephra covering the mudflows of Midagahara and appearing to be younger in age, is characterized by a moderate amount of oxyhornblende. Our tephrochronologic studies on the glacial age of Mt. Shirouma and the time of terrace formation along the Matsukawa running down the mountain slope are now in progress.
(4) Tephraic horizons consisting of tuff and/or pumice grians are recognized in the marine beds on the Pacific coast of Central Japan. Their geologic ages seem to be different with one another but all of them contain much or a moderate amount of hornblede perhaps due to the activity of some acidic volcanisms. For the purpose of correlation, more elaborate inspection of the mineralogical nature of constituent materials of the tephras will be needed.
