Stratigraphy and geologic age of the Neogene and Pleistocene sequence in the northern part of the Uonuma hill, Niigata perfecture, central Japan

Stratigraphical studies on the Miocene to Lower Pleistocene strata in the northern part of the Uonuma hill, Niigata Prefecture, were carried out and geologic ages of these strata were determined by the use of the fission track method, and decided by the biostratigraphical study of planktonic foraminiferal and radioralian fossils. In the studied area, the Miocene to Lower Pleistocene show serious lateral changes of facies from east to west direction. Stratigraphical units of rock facies in the eastern area are different from the units in the western area, therefore Stratigraphical divisions are established in each area. The Miocene to Lower Pleistocene sequence in the eastern area are divided into the Nishimyo, Tsunagi, Toyagamine, Shiroiwa, Wanazu and Uonuma Formations, and in the western part are into the Hanzogane, Sarukuradake, Araya, Kawaguchi, Ushigakubi, Shiroiwa, Wanazu and Uonuma Formations in ascending order. The Shiroiwa and Wanazu Formations unconformably cover the Nishimyo and Tsunagi Formations in the eastern area, and the Toyagamine Formation in the south-eastern area. The Kawaguchi Formationin the western area laterally interfinger with the Ushigakubi Formation according to the results of Stratigraphical study of tuff layers. These formations have tendency to be coarser sediments toward Mt. Toyagamine in the south-eastern area. In the studied area, Mein fold structures are distributed in NE-SW trends. The results of the fission track age determination and the biostratigraphical study are as follow. The planktonic foraminiferal zone (Maiya, 1978) of the Araya Formation is assigned to Globorotalia ikebei/Orbulina uinversa zone, and that of the Kawaguchi and Ushigakubi Formation is to Globigerina pachyderma (dextral)/Globorotalia orientalis zone. The radioralian zone (Nakaseko and Sugano, 1973) of the Tsunagi Formation is assigned to Cyrtocapsella tetrapera and Lychnocanium nipponicum zones. The fission track age of Tzp tuff from the upper most horizon of the Araya Formation is 5.23±0.26Ma. OK-2 tuff from the lower horizon of the Shiroiwa Formation is 3.95±0.19 Ma. Ftp tuff from the lower horizon of the Ushigakubi Formation is 3.78±0.17Ma. According to the determination of fission track ages and the investigation of planktonic foraminiferal and radioralian fossils, the Nishimyo and Tsunagi Formations in the eastern area, and the Hanzogane and lower part of the Sarukuradake Formation in the western area are Middle Miocene in age. The Toyagamine Formation in the eastern area, the upper part of the Sarukuradake formation and the Araya Formation in the western area are Late Miocene in age. The Shiroiwa, Wanazu and lower part of the Uonuma Formations in the eastern area, and the Kawaguchi, Ushigakubi, Shiroiwa, Wanazu and lower part of the Uonuma Formations in the western part are Pliocene in age. The Upper part of the Uonuma Formation is Early Pleistocene in age.