ONLAP AND DOWNLAP PATTERNS IN PLIO PLEISTOCENE FORE-ARC AND BACKARC BASIN-FILL SUCCESSIONS, JAPAN

Discontinuities such as onlap, toplap and downlap terminations, are im portant key structures in seismic stratigraphy. These patterns charact erize the Plio-Pleistocene Kakegawa Group in Shizuoka, the middle Plei stocene Kasamori Formation in Chiba, and the Plio-Pleistocene Uonuma G roup in Niigata. The patterns within these successions can be detected by tracing intercalated pyroclastic layers (tephra beds), which provi de the same information as the synchronous reflection lines in seismic data. These pattems, therefore, allow the stratigraphic sequence to b e divided into forearc and backarc basin-fill successions. The deposit ional sequences of the Kakegawa and Uonuma groups are third-order, ran ging from ca. 2.6 Ma to 1.0 Ma. The Kasamori Formation has high-freque ncy sequences dating from 0.60 Ma to 0.45 Ma. All these sequences are believed to have been produced by glacio-eustatic sea-level changes.