抄録
Neogene volcanic rocks and sedimentary rocks distributed over the Hijiriyama area, Nagano Prefecture, Central Japan are divided into four formations. They are the Omi, Ohoka, Shinkou and Haibara Formations in ascending order. Each of these formations have an unconformable relationship with underlying formations. The Omi Formation is mainly composed of clastic rocks, rhyolitic tuff layers and altered andesitic lava flows. The Ohoka Formation is mainly composed of dacitic to andesitic lava flows and/or pyroclastic rocks. The Shinkou Formation is mainly composed of andesitic to basaltic lava flows and/or pyroclastic rocks. The Haibara Formation is mainly composed of fluvial sediments. Results of chronology on the Neogene volcanic rocks and sedimentary rocks are as follows: Haibara Formation: Late Pliocene to Early Pleistocene Shinkou Formation: Middle Pliocene Ohoka Formation: Latest Miocene to Early Pliocene Omi Formation: Late Miocene These chronological data resulted in the recognition of two times of Pliocene volcanism in the Hijiriyama area; the first occurs in Latest Miocene to Early Pliocene, and the second in Middle Pliocene.
