We present a revised Cenozoic chronostratigraphy of the Yatsuo Area, Toyama Prefecture, based on U-Pb and fission-track (FT) dating of zircon grains from tuff beds, mineralogical analysis of tuff beds, and diatom biostratigraphy. The results reveal that syn-rift volcanism (represented by the Iwaine Formation) began at ~. Ma and that the Ikahama unconformity between the syn-rift Higashibessho Formation and the post-rift Tenguyama Formation represents a relatively short-duration (~. m.y.) event ~ Ma. The Otogawa Formation unconformably overlies the Tenguyama Formation and is divided into middle Miocene (Serravallian) and late Miocene, with a possible unconformity in between. The-Ma Mita Formation unconformably overlies the Otogawa Formation. These results require previous local correlations of a key tuff bed (MT) in the Mita Formation to be reassessed. Facies and sequence stratigraphic analysis of the lowermost Nirehara Formation confirm that this formation comprises multiple depositional cycles generated by water-level fluctuations in a coastal or lacustrine fan delta. The results also suggest that the Nirehara Formation, which is unconformably overlain by the syn-rift Yatsuo Group, was formed in an early rift basin. Detrital zircon U-Pb and FT dating of sandstone within the Nirehara Formation suggests that the sandstone may be derived from the Nohi Rhyolite. Regional correlations of onshore and offshore Cenozoic chronostratigraphy in the Hokuriku district show multistage rifting during the Oligocene-middle Miocene opening of the Sea of Japan. Regional correlations also suggest that the post-rifting compressive regime was associated with fluctuations in the strength and direction of compressive stress fields.
Anhydrite occurs as a representative hydrothermal mineral in the upflow zone of the Kakkonda geothermal field, northeast Japan. Gas analysis and microthermometry of fluid inclusions and sulfur isotope measurements were performed for these anhydrites in order to discuss origin of the reservoir fluids and precipitation mechanism of the anhydrite.Fluid inclusions are classified into two-phase, vapor-rich and liquid-rich inclusions, and polyphase inclusions comprising liquid, vapor and solids. The vapor-rich inclusions coexist with the liquid-rich inclusions in most samples, indicating that the fluid inclusions were trapped under boiling conditions. Bulk gas analyses of the liquid-rich inclusions show that the main non-condensable gas component is CO 2 (0.14-2.0 mol %) with subordinate amounts of N 2 , CH 4 and Ar. The two-phase liquid-rich inclusions homogenize to liquid phase at temperatures between 222 and 380°C, and have salinities mostly from 0 to 29 wt.% NaCl + CaCl 2 equivalent. The polyphase inclusions homogenize to liquid phase at temperatures between 302 and >480°C after dissolving halite between 104 and 220°C on heating, and have salinities from 28 to 35 wt.% NaCl + CaCl 2 equivalent. In the deep reservoir, the salinities vary over a large range at homogenization temperatures of approximately 320 to 360°C. The δ 34 S values of anhydrites ranging mostly from +21.6 to +24.2‰ suggest that sulfur in the anhydrite from the Kakkonda field is derived from marine anhydrite.These data indicate that the hypersaline fluid was produced by exsolution from a residual magma at the time of final crystallization of the Kakkonda granite, and moderate to low salinity fluids formed by dilution of the hypersaline fluid with the heated meteoric water and boiling of the dilute fluids. The precipitation of anhydrite might be explained by three mechanisms of a simple cooling of the hypersaline fluid, boiling of the reservoir fluid and pressure drop of the non-boiling reservoir fluid. the production wells in the Takigami (Takenaka and Furuya, 1991) and Fushime geothermal fields (Akaku et al., 1991). These previous studies suggest that even a slight change in fluid condition by flows might precipitate anhydrite from an anhydrite-saturated fluid within productive fracture, then anhydrite occurrence can provide useful information on evolution of fluid flow system in the active geothermal fields.
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