On the Green tuff geology of the southwestern foot of Kurikoma volcano and the origin of the Sanzugawa, Onikobe Uppermost Miocene basins

Abstract

The Neogene sediments in the region bodering Akita, Yamagata and Miyagi Prefectures on the southwestern foot of Kurikoma valcano are classified into following formations; these in ascending order are the Kamuroyama, Torageyama and Sanzugawa formations in the Sanzugawa basin, Akita Prefecture and the, Kanisawa, Torageyama, Akasawa and Miyazawa formations in Onikobe besin, Miyagi Prefecture. Those two lower formations in each basin are thought to be correspond to the so-called Green tuff developed extensively in the backborn ranges between Akita and Iwate Prefectures to the east of Yokote basin and also between Yamagata and Miyagi Prefectures to the east of Shinjo basin, and consist of a heavy accumulation of green tuff and tuff breccia acconpanied by propyritic lavas. These formations are unconformably overlain by the remarkable lacustrine deposits of the above-cited upper formations which are fairly well developed in the viscinity of Sanzugawa or Onikobe along the upper streamside of the Takamatsu or Arao River. The fossil flora from the Sanzugawa and Akasawa formations suggest the Uppermost Miocene in age. And the origin of the Paleo-Sanzugawa and Paleo-Onikobe basins of authors is thought to be related with the Uppermost Miocene lake formed by the crustal deformation in the backborn ranges of Northern Honshu, for which Prof. Hanzawa has initiated the name of Paleo-Sendai Lake (S. Hanzawa, 1950).