Explanatory Text of Geological Map of the Kitakami-gawa Tributaries and the Tectonic History of the Kitakami Mountains, North Japan

Abstract

This is a recent revision of the Geology of the Kitakami region (1956, 1969, 1981) by Yoshio ONUKI who discovered Silurian fossils at Hikoroichi about 50 years ago. His description of the Kitakami mountains occupies two-thirds of this text and the Tertiary and Quarternary chapters are contributed respectively by S. KITAMURA and H. NAKAGAWA.It is quite astonishing that ONUKI abolishes the Shizu phase of disturbance which is the principal event of MINATO's Abean orogeny. According to YAZAWA and KITAYAMA's recent geological survey (1979) no discordance exists at the base of the upper Visean Yukizawa formation and the boundary at the type-exposure of Kotsubosawa is warranted to be a fault, instead of an orogenic discordance as emphasized by MINATO.ONUKI considers that the Kesen orogeny must be pre-Tobigamori i.e. early Upper Devonian in age and the Setamai orogeny is Upper Carboniferous. These orogenies are recognized in Japan only within the southern Kitakami mountains. On the other hand the pre-Carnic Akiyoshi orogeny was a strong one widely known in Northeast as well as Southwest Japan. The late Lower Cretaceous, or Aptian Oshima orogeny is the strongest one in the Kitakami region through which the folded mountains were completed. Now the early Cretaceous igneous activity from basic to acidic is clarified there in detail. Later the folded mountains were largely undulated and suffered from faulting in early Palaeogene, middle Tertiary and middle Neogene.Of the ancient igneous and metamorphic rocks their correlation is provisionally made with the pre-Cambrian history of China, but its decision is said to depend upon future investigation.