The Izumi Group in southwestern Japan is considered to represent deposits in a forearc basin along an active volcanic arc during the late Late Cretaceous. The group consists mainly of felsic volcanic and plutonic detritus, and overlies a Lower to Upper Cretaceous plutono-metamorphic complex (the Ryoke complex). In order to reconstruct the depositional environments and constrain the age of deposition, sedimentary facies and U-Pb dating of zircon grains in tuff were studied for a drilled core obtained from the basal part of the Izumi Group. On the basis of the lithofacies associations, the core was subdivided into six units from base to top, as follows: mudstone-dominated unit nonconformably deposited on the Ryoke granodiorite; tuffaceous mudstone-dominated unit; tuff unit; tuffaceous sandstone-mudstone unit; sandstone-mudstone unit; and sandstone-dominated unit. This succession suggests that the depositional system changed from non-volcanic muddy slope or basin floor, to volcaniclastic sandy submarine fan. Based on a review of published radiometric age data of the surrounding region of the Ryoke complex and the Sanyo Belt which was an active volcanic front during deposition of the Izumi Group, the U-Pb age (82.70.5Ma) of zircon grains in the tuff unit corresponds to those of felsic volcanic and pyroclastic rocks in the Sanyo Belt. ???????????????????????????,????????????????????,????????????????????(?????????)??????.????????????????????????????,?????????????????????????????????????????????U-Pb???????.??????????? ?????,?????????,1) ?????????????????,2) ??????,3) ????,4) ????????,5) ?????,6) ??????6??????????.?????,?????????????????????????????????????????????? ???????????.??????????????????????????,U-Pb??????(82.7 +/- 0.5Ma)????????????????????????????????????????????.
