Abstract
The Pliocene to Early Pleistocene Uonuma Formation is widely distributed in the central part of Niigata district. In the Hachikoku oil field and the southern part of Chuo oil field, the detailed stratigraphy of this formation has been confused, because of insufficient correlation of ash layers in the east and west wings of the Hachikoku anticline. In the present paper, the stratigraphy of the Uonuma Formation in this area is examined in detail on the basis of the correlation of ash layers as summarized in the followings. 1) The Uonuma Formation is divided into three members. The lower member mainly consists of sand. In the middle member, the sedimentary facies are changeable laterally. Especially the facies in the west wing of the Hachikoku and Sekihara anticlines, where the silty sediments are dominant, is different remarkably from the facies in the east, where the sandy sadiments prevail. The upper member mainly consists of alternation of gravel, sand and silt. In the northern part, sandy facies prevails in the lower part of this member. 2) Some ash layers and molluscan fossil beds, such as SK 110, SK 100, Se, Ot, Oz Fb. I, Oz Fb. II and SK 030 in the middle member, and Ab Fb., Ab I, Ab II, SK 020 and Umitakea Fb. in the upper, are extensively traceable. 3) The Tsukanoyama tuffaceous mudstone at the type locality is correlative with the Ohirota tuffaceous mudstone at the type locality. 4) The upper part of the middle member and the upper member of the Uonuma Formation in this area are correlated with the middle to upper part of the Haizume Formation in the Nishiyama and Chuo oil fields respectively.
