Abstract

Forcing mechanisms of high-frequency depositional sequences in late Cenozoic successions have commonly been interpreted in terms of glacio-eustasy. In contrast, we demonstrate here the effects of local tectonics in the development of high-frequency depositional sequences in a middle and upper Pleistocene succession. Examples are from shallow-marine and coastal deposits developed in the paleo-Tokyo Bay, Japan, and are based on a detailed chronostratigraphic framework constructed from dated volcanic ash beds. Within highstand successions of the glacio-eustatic sequences, we have mapped locally developed high-frequency depositional sequences within areas that have undergone anticlinal uplift. These high-frequency depositional sequences pass laterally into parasequences overlying marine flooding surfaces in adjacent synclinal areas. Spatial variation in such stratigraphic features indicates local tectonic movements in the paleo-Tokyo Bay area during rises and highstands of glacio-eustatic sea-level cycles. The duration of such high-frequency depositional sequences is estimated as <∼10 k.y., shorter than Milankovitch cyclicity. The present example indicates that local basin tectonics can be an important forcing mechanism of high-frequency depositional sequences in Pleistocene successions in an active-margin basin.

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