The Tertiary deposits are studied, which are widely distributed in the eastern marginal part of the Backbone range, from western part of Morioka to Hanamaki City. And the new stratigraphic division is proposed, that is, Lower Miocene Mizuwake, Azumaneyama and Yuzawamori Formations, Middle Miocene lioka and Isarizawa Formations and Upper Miocene Masuzawa Formation, in ascending order. The former three formations (pre-Nishikurosawa stage) are named and defined from the discovery of surface distribution, hitherto which were known only in bore hole samples (Okami et al., 1988). They have been regarded as parts of the lioka and/or Osuke Formations by Hayakawa et al. (1954) and Murai (1960). The Mizuwake and Yuzawamori Formations consist of pyroclastic deposits which are strongly welded in part. From the lithologic characters and stratigraphic relationship, they can be correlated to the Lower Miocene welded tuffs distributed in many areas of the Tohoku district. So the two newly discovered welded tuffs in this area can be good key beds of the Lower Miocene sequences which are barren for leading fossils, in central to northern Tohoku district. The acidic pyroclastics are prevailed in the sequence above the lioka Formation (Nishikurosawa stage). They have been divided in western Morioka area, into the Shimosaruta. Osuke and Yabitsu Formations in ascending order (Murai, 1960), and they have been also divided into Osuke and Yuguchi Formation (Hayakawa et al., 1954) in western Hanamaki area. However, the present study clarifies on the basis of the field survey that they are in interfinger in the central part of the studied area. By it the acidic pyroclastics are combined to one unit, namely the Isarizawa Formation. And the Osuke Formation is treated as a member of the Isarizawa Formation. On the basis of the zonal distribution of the zeolites, it is explained that the alteration of the acidic pyroclastics of the Isarizawa Formation have been formed by the diagenesis.
