Eustatic, sedimentary supply, tectonic controls on sequence stratigraphy of Paleo-Tokyo Bay

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The Kamiizumi, Kiyokawa and Yokota Formations, Shimosa Group, which crop in the Shimosa Upland, were deposited in the middle Pleistocene Paleo-Tokyo Bay (c.250 - 150 KaBP). Paralic and shallow marine depositional systems (fluvial, estuarine, delta, tidal sand bars, shoreface-beach, tidal inlet and lagoon-bay) are recognezed in the formations and constitute two depositional sequences. These sequences are interpreted to have developed during the stage 8 - 6 of the oxygenisotope curve and been primarily controlled by glacio-eustatic sea level changes. Spatial variation in depositional systems, inferred from the mapping of a chronostratigraphical surface marked by some volcanic ash layrs, are recognizable in the depositional sequences of the formations. In Paleo-Tokyo Bay, valleys were incised by rivers during the low sea-level stage in the glacial period. The subaerial unconformity shows a sequence boundary. Two different types of depositional systems were formed during early transgressive through highstand stages of the relative sea level in the interglacial period : estuarine-delta and tidal sand bars systems, and estuarine - barrier island systems. The spatial variation is interpreted in terms of differences in sedimentation supply and tectonic movement in Paleo-Tokyo Bay area. Consequently, two types of depositional sequence developed in the Kamiizumi, Kiyokawa and Yokota Formations : a high subsidence-high sediment flux type of sequence and a low subsidence-low sediment flux type of sequence.