Controls Exerted by Tectonics and Sediment Supply on Development of a Third-Order Sequence in a Fore-Arc Basin: The Plio-pleistocene Kakegawa Group, Central Japan

Sakai Tetsuya and Masuda Fujio

The western part of the Plio-Pleistocene (ca. 2.6-1.0 Ma) Kakegawa Group of central Japan consists of a third-order sequence, up to 500 m thick, of alluvial, shoreface, shelf, slope and submarine-channel facies. These deposits are dated by tephrochronology. The succession contains at least 16 upward-shallowing cycles, affected mainly by high-frequency eustatic sea-level cycles. The lower part of the sequence is characterized by backstepping successions, forming a transgressive systems tract (2.2-1.8Ma), followed by progradational successions comprising a highstand systems tract (1.8-1.4 Ma).

Subsidence analysis and evaluation of changes in the rate of sediment supply estimated from cross sections suggest that formation of the third-order sequence was controlled solely by tectonic subsidence and variations in the rate of sediment supply. Rapid subsidence and a high rate of sediment supply during 2.2-2.0 Ma resulted in deposition of the lower part of the transgressive systems tract, characterized by thick backstepping successions. Both the rate of subsidence and sediment supply decreased in the period 2.0-1.8 Ma, but subsidence was still the dominant factor. Therefore, thin backstepping successions were formed. The 1.8-1.4 Ma progradational successions resulted from a small rate of subsidence and high rate of sediment supply. The progradational units become thicker basinward owing to a faster rate of subsidence in the basin center. The maximum flooding surface was formed around 1.8 Ma even though subsidence was slow at this time. This was essentially at the end of the time when the accommodation space was created at a faster rate than the sediment supply.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California