新潟積成盆地の新第三系火山層序 構造区と火山岩区

Abstract

As extensive exploratory drilling and geophysical survey being carried out, knowledge about the "Green Tuff" underlying the Niigata Plain has been much increased recently.A volcanic province is defined in respect to time and place. The volcanic rocks which are included in a denfiite formation and are distributed in a limited area, belong to a definite volcanic province. Each volcanic province corresponds to the geotectonic province in the Niigata Sedi-mentary Basin (Table 1).(1) The thick pyroclastics of the Nishioga and Daijima stages cover almost all over Sado Island which occupies the west-side highland of the Niigata Basin (Table 2), and are also distri-buted in the northern district of Niigata Prefecture. Other volcanic provinces of Daijima stage are the Mikawa and Jõnai provinces where andesite and basalt are developed.(2) The "Green Tuff" of the Nanatani state is widely distributed in the northern half of Niigata sedimentary basin. The characteristic volcanic rocks are plagioliparite to dacite and basalt. In the southern half of the basin, however, the "Green Tuff" deposition was thinner and has the different mode of occurrence from the northern half.On the basis of paleontological and volcanostratigraphical evidences, the "Green Tuff" under the Niigata Plain is stratigraphically correlated to the upper part of the Tsugawa Formation in the Tsugawa-Aizu Province newly defined by Dr. Shimazu (1973).(3) Lastly, the characteristics of the late Miocene to early Pleistocene volcanic provinces are briefly described.