長野市北東部の新生代層

概要

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Cenozoic sediments characterized by pyroclastics are developed extensively in the northeastern area of Nagano city. Since 1956, the writers have engaged in the stratigraphical researches of these strata. The results are as follows: 1) The Cenozoic strata, divided into the Ogawa, the Shigarami and the Toyono formations in ascending order, were deposited from late Miocene to early Pleistocene. Most of the sediments, however, are considered to be of Pliocene age. 2) Two partial unconformities are recognized in these sediments. A part of the uppermost member of the Shigarami formation is underlain unconformably by a part of a member of the Ogawa formation, while another unconformity may be observed between the Toyono and the Shigarami or the Ogawa formation. Of both unconformities, the writers consider that the former was originated from marine transgression during the later Shigarami stage, and the latter. on the other hand, is refered to the regression in the late Shigarami stage. 3) It is considered that the marine basin was isolated from the open sea after the transgression during the later Shigarami stage, and it was changed into a lacustrine basin to which the writers attended the name "Paleo-Toyono lake". 4) On the basis of various geological evidences obtained in this area the writers believe that the tectonic movements in the late Shigarami stage continued to the post-Toyono stage.
地学団体研究会の論文
1960-01-25