Departmental Bulletin Paper Newly Identified Oligocene Formation in the Sera Plateau, Hiroshima, SW Japan

Yamasaki, Hirofumi  ,  Suzuki, Morihisa  ,  Harada, Koki  ,  Suga, Takaaki  ,  Hayashi, Takehiro

This study aimed to show the presence of an Oligocene formation in the Sera Plateau, located in the eastern part of Hiroshima Prefecture, SW Japan, using field surveys, a petrological study, and fission-track dating. According to field surveys conducted between 1996 and 2002, and supplementally in 2017, rhyolitic pyroclastic rocks and conglomerate beds unconformably covered by the Quaternary sediments were identified along the bank of Mihara-gawa River at Kurohada in Sera Town. The rhyolitic pyroclastic rocks, tentatively called Kurohada Ryolite, were composed of welded tuff, coarse tuff, and tuff breccia, and were intercalated with conglomerate beds lithologically similar to the mountain gravels. The Kurohada Rhyolite and the conglomerate beds were either horizontal or dipped gently to the northwest, with a total thickness of about 5 m. Based on whole-rock chemical composition analysis by X-ray fluorescence (SiO2: 72.39-75.82 wt % , K2O+Na2O: 5.78-7.80 wt % ) the welded tuff was classified into rhyolite of sub-alkalic series. Under the microscope, the pyroclastic rocks were composed of crystal fragments of plagioclase and quartz, and lithic fragments in glassy matrix, and signs of thermal alteration could not be confirmed. On the other hand, one of the lithic fragments of rhyolite contained biotite fine crystals as thermal metamorphic products. Thus, the Kurohada Rhyolite was distinguished from the Mesozoic Takada Rhyolites distributed widely in Hiroshima Prefecture depending on the presence of thermal metamorphism. Fission track ages of zircons from the welded tuff were 30.4 ± 1.6 Ma and 30.9 ± 1.5 Ma. This result shows that the geologic age of Kurohada Rhyolite and associated conglomerate beds in the study area is constrained to the early Oligocene and suggests the possibility that the mountain gravels distributed in the eastern part of Hiroshima Prefecture are Paleogenic.
This study was partly supported by JSPS KAKENHI Grant Number JP16K00966.

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