Bedded cherts associated with greenstones in the sawadani and shimantogawa groups, Southwest Japan

Bedded cherts that are closely associated with greenstones are found in the Sawadani Group, Late Carboniferous to Early Permian in age, and the Shimantogawa Group, mostly of Cretaceous age, both in the Outer Zone of Southwest Japan. They differ from the cherts recovered from the open-ocean by the DSDP with respect to stratigraphic relation to greenstones, lithologic associations, thickness, and sedimentary structures. Petrographic characteristics of the Sawadani and Shimantogawa cherts are similar to those of the Tethyan Mesozoic cherts. Bedded cherts commonly occur in the lower part of the Sawadani formation and rarely in the Takano Formation. Cherts occur on greenstones or are interbedded between flows and occur exclusively in the lower formation and are associated with greenstones that include lava and hyaloclastite. This lithologic association is common through the Shimantogawa Group from the Kanto Mountains of central Japan to the Ryukyu Islands. Cherts are reported to yield Cretaceous and Late Jurassic radiolarian assemblages.