ABSTRACT: Sedimentary Basin of Neogene Siliceous Rocks of Northern Japan

Azuma Iijima, Ryuji Tada

Neogene siliceous rocks of diatomite origin are widespread in northern Japan and make the principal source rocks of Japanese oil fields along the Japan Sea coast. Geologic age and lithofacies of the Onnagawa Formation and related rocks resemble those of the siliceous facies of the Monterey Formation of California. The Onnagawa Formation and related rocks accumulated in an elongated silled basin, approximately 1500 km long and 150 km wide, extending subparallel to the Japanese Islands arc. The basin was formed by rapid rifting of the pre-Tertiary basement along with the opening of Japan Sea basin, though it was separated from the Japan Sea basin proper by a narrow swell. On the east side, the basin connected with the Pacific Ocean only through some shallow channels at brea s of the older massifs chains of Japanese Islands. Finely laminated, carbonaceous porcelanites and cherts of middle Miocene age, 90-300 m thick, occur in the central, middle bathyal and anoxic part of the basin, where main Japanese oil fields are superimposed. The accumulation rate of biogenic opal in the Onnagawa Formation and related rocks is 5-7 g/cm²/k.y., which is almost equivalent to that of the Monterey Formation. Cold current system prevailed in the diatomaceous Onnagawa basin. This would be caused partly by progress of the global glacial climate and more effectively by interception of the paleo-Kuroshio warm current due to the land bridge between Asia and southwest Japan. Basic and acidic submarine volcanism occurred extensively within the deep-sea basin and construct d volcanic reservoirs at places.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990