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Fission-track dating using both internal and external surfaces of zircons was carried out for ten core samples from the Miocene formations in the Ayukawa-Yurihara district, Akita Prefecture. These samples were collected from the Yashiozawakawa, Nishikurosawa and Onnagawa Formations of eight wells drilled for the petroleum exploration in this area.After the detail examination for grain ages of each sample using histogram, age spectrum and radial plot, fission-track ages are interpretated. The results are summarized as follows.1 Although external fission-track age 20.1 1.3 Ma of andesite in the upper part of the Yashiozawakawa Formation shows the eruption age with some possibility of rejuvenation, internal fission-track age 16.4 0.6 Ma shows the thermal event which probably corresponds to the start of the graben-fill basalt activity.2 The most reliable internal fission-track age 12.2 0.4 Ma of acidic tuff in the Nishikurosawa Formation is in harmony with the biostratigraphic age of this formation. The external fission-track age 16.6 0.8 Ma indicates that the provenance of zircons is probably the Hatamura Formation.3 The most reliable fission-track age of tuffs between the O-1 and O-2 benthonic foraminiferal assemblages, 8.6 0.4 Ma of the Ayukawa-kita well, is consistent with Sr isotopic age about 9 Ma of the O-2 assemblage. The age of the O-1 assemblage is estimated 8.2 Ma based on K-Ar ages of the overlying Fukuyama andesite. Therefore, the O-1 assemblage is correlated with Blue zone in Niigata Prefecture, and has a possibility of regional correlation marker.4 This study indicates that the fission-track dating using both surfaces and more than one samples of the same horizon is a valuable method for the use of few and expensive cores.

Fission-track age determinations using both internal and external surfaces of zircons for 23 core samples of the "Green Tuff" were carried out. These samples were collected from 21 wells drilled for the petroleum exploration in Niigata Prefecture.After the detail examinations for grain ages of each sample using histograms and age spectra, fission-track ages are interpretated. Fission-track ages of the "Green Tuff" are divided into two age groups (15.5 to 17.1 Ma and 13.5 to 13.8 Ma). The older group is recognized in all three districts (Kita-kambara, Mitsuke and Yoshii districts) and corresponds to the Tsugawa stage or the lowest part of the Nanatani stage. The younger group, corresponding to the uppermost part of the Nanatani stage, is relatively restricted.Grain ages show the presence of zircons derived from basements in the Kita-kambara and Yoshii districts. No basement zircons and younger fission-track ages of some wells such as Sanjyo SK-1 indicate that volcanism was active for a long period in the Mitsuke district.An analysis of grain ages in the Mitsuke oil field using Brandon (2002) show felsic volcanism was active around 23 Ma, same as the Sado Island and Asahi Mountains.Reset of fission-track ages of both surfaces and distribution of grain ages give us some information on past thermal conditions. For example, the thermal alteration in the Yoshii district occurred around Yoshii SK-10D and started just after the eruption of rhyolite can be inferred.

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