The Omachi Tephra Formations and Tephrochronology

Abstract

The tephras supplied from Late Quaternary volcanoes in Central Japan have been distinguished into some tephra regions. In this paper, the author has made the characterization, classification and tephrochronologic consideration of the Omachi Tephra formations, which are developed in the Omachi Tephra region. The Omachi Tephra formations consist of pumices, scoriae and fine volcanic ash, of which lithologic and petrographic characterization have been made.(1) Studies were especially made on pumices and scoriae by means of (a) field observations, (b) analysis of heavy mineral assemblage, (c) thermomagnetic and X-ray analysis of Fe-Ti oxides, (d) chemical analysis of pyroxene by electron micro-probe technique. Throughout the whole sequence of petrographic features of the Omachi Tephra formations two cycles were found. If the biotite and/or hornblende phenocrysts are typical of acid liquids, the followings may be concluded; (i) From the Lower to Middle Tephra units, the composition of magma changed from salic one (biotite-hornblende-dacite) to mafic (two pyroxene andesite). (ii) After an interval of ash shower, the Upper Tephra unit shows a change from slightly salic (hornblende-two pyroxene-dacitic andesite) to mafic activity (hornblende-two pyroxene andesite). And the ash fall activity of the source volcano finished.(2) Based on the distribution of the Omachi Tephra, it is inferred that the source vent should be at Mt. Tateyama.(3) If the assumption mentioned above could be correct, two cycles recognized from the section of the Omachi Tephra formations seem to correspond with the already known two cycles in chemical composition of volcanic products from Tateyama volcano (after Yamasaki et al., 1966).(4) The Omachi Tephras have been traced to the East Shinshu district and show the relationships with the tephras of other tephra regions. The pumice layer DPm of the Omachi tephra stratigraphically lies above the pumice layer Kwp from Mt. Yatsugatake, and also a little lower horizon of the Pm-I from Mt. Ontake. On the one hand, as the latter is embedded within the middle part of the Shimosueyoshi “Loam” tephra unit in the South Kanto districts, the Upper unit of the Omachi Tephra formation should naturally be correlated with the Shimosueyoshi stage.(5) As the DPm layer is thought to be correlated with the pumice flow activity of the third period in volcanic history, so the activity of Mt. Tateyama must at least have continued down to the Shimosueyoshi “Loam” age.(6) Many pumice grains which are identical with those of the IIIrd period pumice flow are contained within the gravel bed of the highest terrace on the right side of the Joganji river. So this terrace could be correlated to the Narimasu or Musashino terraces.