Triassic bedded cherts in central Japan are not pelagic

Abstract

Although the origin of bedded radiolarian cherts in orogenic belts has been widely debated^1–15, their geochemistry is still poorly understood. Triassic bedded radiolarian cherts occur extensively in the Japanese Islands and are generally not associated with ophiolitic rocks but rather with clastic rocks. We present here chemical data on the Triassic bedded cherts of Kamiaso in central Japan. The relationship of SiO₂ with Al₂O₃, TiO₂ and Zr shows that the bedded cherts have been formed by a simple mixing of biogenic silica derived from radiolarians and sponge spicules with lithogenic materials. The ratios of several elements relative to TiO₂ strongly suggest that the shale partings and lithogenic portions of the cherts are consanguineous. Sedimentary structures as well as the geochemical evidence suggest that most Kamiaso cherts are turbidity current deposits. Our geochemical and geological data compared with those of recent marine sediments from various environments, demonstrate that the Triassic cherts and associated shale partings are not deep pelagic in origin but formed in an offshore or marginal sea environment.