Abstract
Thick basaltic piles of Nishikurosawa stage of early to middle Miocene age are widely distributed from Akita through Yamagata to Niigata oil fields along the Japan Sea side. Recently, MITI Nikaho Well and MITI Niigata-heiya Well were drilled at the foot of Mt. Chohkai in the southern part of Akita, and at the central part of the Niigata plain, respectively. Both wells encountered a thick basaltic layer ranging up to 1, 400 meters of the Nishikurosawa stage. We studied micropaleontological features and petrochemical characters about the basaltic layers from the wells and their surrounding areas.The basaltic piles, described collectively as the Nishikurosawa stage in Akita and Niigata areas, are divided into two groups erupted in substages of NN 4 Zone (eary Nishikurosawa age) and NN 5 Zone (middle Nishikurosawa age) of Martini's zonation. The basalt of the midde Nishikurosawa age consists of relatively undifferentiated type in FeO*/MgO ratio with less than 1.4% of TiO2 content. On the other side, the basalt in early Nishikurosawa age is slightly differentiated and rich in TiO2 content compared with the above.In the present paper, we discuss the volcanostratigraphy of the Nishikurosawa stage in Akita and Niigata oil fields.
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