2017 Volume 123 Issue 5 Pages 335-350
We performed U-Pb dating on detrital zircons from sedimentary rocks (e.g., sandstone and tuff) of the Kuruma Group in central Japan. The youngest ages are from sandstone of the Jogodani and Gamaharazawa formations, which yields ages of 187.7±1.2 Ma and 189.4±0.9 Ma, respectively. These formations are located in the stratigraphically lowermost part of the Kuruma Group. In addition, vitric tuff within the Kitamatadani Formation, which conformably overlies the Jogodani Formation, yields weighted mean zircon ages of 187.0±1.6 Ma and 186.3±1.3 Ma. Previous studies reported upper Pliensbachian ammonites from the Teradani Formation in the stratigraphically lower part of the Kuruma Group, providing strong evidence that the lower part of the Kuruma Group, from the Jogodani Formation to the Teradani Formation, was deposited in the Pliensbachian (191.4 Ma to 183.7 Ma), and that the Jogodani Formation is correlated to the Gamaharazawa Formation. As the lower part of the Kuruma Group is up to several thousand meters thick, it is inferred to have formed during a period of rapid sedimentation in the Pliensbachian.
