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Kinetic equation of organic maturation is integrated to simulate vitrinite reflectance-burial depth relations for essential geological settings. The integration of the equation shows that the linear relation between vitrinite reflectance and burial depth does not hold in active sedimentary basins any longer. The integration gives analytic solutions*1 for the kinetic equation for such sedimentary basins which undergoes dynamic tectonics. And the integration gives a good perspective for theoretical consideratins on vitrinite reflectance-depth relations.It is possible to determine the coefficients in the kinetic equation by comparing predicted vitrinite reflectance-depth relations with measured ones. The direct integration of the kinetic equation easily predicts the relation using the observed burial history and the geothermal gradient in active basins.

Although the Hokutan Group is considered to provide evidence for the Miocene opening of the Japan Sea, the ages of the formations within the group are poorly constrained. We dated eight tuff beds from the Hokutan Group in the Tajima-Myokensan and Sobudake areas, where the type localities of the formations are located, using U-Pb and fission-track (FT) methods. Uranium contents were measured using laser ablation-inductively coupled plasma-mass spectrometry. A tuff bed in the upper Takayagani Formation and two in the basal part of the Yoka Formation yielded a mean U-Pb age of ~. Ma, which is within the range of FT ages (-Ma) of the samples. Accordingly, the boundary between these two formations is dated at ~ Ma. U-Pb ages of ca.-Ma and FT ages of ca.-Ma were obtained from a tuff in the Toyooka Formation and from four tuffs in the Muraoka Formation, consistent with previously reported fossil ages. Considering the radiometric and fossil ages, the depositional age of the upper Toyooka Formation is taken as .-. Ma and that of the lower Muraoka Formation is taken as. Ma. The FT ages imply that the middle-upper Muraoka Formation is younger than-Ma.

The lower Miocene Yoka Formation in southwest Japan is composed of terrestrial volcaniclastic rocks, and contains no index fossils. The basal part of the formation has yielded zircon U-Pb and fission-track ages of ~. Ma. The upper part of the Toyooka Formation, which unconformably overlies the Yoka Formation, correlated with-. Ma by index fossils and zircon ages. In this Myr. long age gap, marine transgression and paleomagnetic rotation occurred in this area. We collected a felsic lapilli tuff sample from the middle part of the Yoka Formation, and the U-Pb ages of zircons were measured using a laser ablation-inductively coupled plasmamass spectrometry (LA-ICP-MS). The weighted average of concordant U/ Pb age of. . Ma % conf., MSWD =. was obtained from the zircons excluding an outlier age from one grain. This zircon U-Pb age indicates that the transgression and paleomagnetic rotation occurred rapidly within ~. Myr in this area.