Diachronous evolution of systems tracts in a depositional sequence from the middle Pleistocene palaeo-Tokyo Bay, Japan

Palaeo-Tokyo Bay is a relic of the Plio-Pleistocene Kazusa forearc basin in the Boso Peninsula of Japan. The sedimentary infill of palaeo-Tokyo Bay is characterized by shallow marine to paralic sediments of the middle to upper Pleistocene Shimosa Group. Sequence stratigraphical analysis has been used to describe spatial and temporal variations in the depositional systems of the lowest units of the Shimosa Group, deposited during the early stage of development of palaeo-Tokyo Bay. Three different type of depositional systems were recognized: sand ridge to shelf (SRS), shelf to delta (SDL) and shelf to non-deltaic nearshore (SNS) systems. They overlie early transgressive estuarine deposits infilling lowstand valleys incised in the south-eastern margin of palaeo-Tokyo Bay. These systems were developed during late transgressive through highstand stages of a relative sea level cycle, which may have been controlled by a glacio-eustatic sea level change at about 0·4 Ma. Spatial variation in depositional systems is largely identical to that in modern Tokyo Bay; environmental conditions similar to those prevailing at the present day probably characterized the early history of palaeo-Tokyo Bay. The timing of highstand systems tracts within a high frequency depositional sequence was analysed in terms of the effect of sedimentation rate, based on the mapping of a chronostratigraphical surface marked by the Hy4 volcanic ash layer. From spatial variations in sedimentation rate, it was possible to identify the diachronous evolution of highstand systems tracts from the SDL system, through the SNS system, to the SRS system. Time lag is indicated by major bounding surfaces, such as maximum flooding or downlap surfaces associated with a condensed section, which developed immediately above or below the Hy4 volcanic ash layer. The lag may be of the order of a few thousands to tens of thousands of years within a depositional sequence with a total of duration of about 100 000 years.