Abstract
A forearc basin fill of the Pleistocene Toyofusa Group in the southern part of the Boso peninsula, Japan is represented by slope to shelf sediments. The Toyofusa Group is subdivided into the Kanamaribata Formation, Ohi Formation, Higashinagata Formation, Takigawa Conglomerate, Sagashi Formation and Ikenouchi Formation in ascending order, and correlated with the middle and upper parts of the Kazusa Group. The slope to shelf sediments form a northeasterly onlapping clastic wedge and include many numbers of volcanic ash layers. Mapping of depositional associations with tracing volcanic ash time marker, together with the interpretation of depositional facies, permits identification of stratal terminations and of temporal-spatial variations of depositional environments. These enabled a precise sequence-stratigraphic analysis of the sediments of the Toyofusa Group.
Seven depositional sequences have been identified in the Toyofusa Group. Each the depositional sequences show lateral terminations of depositional facies and vertical facies discontinuities at the sequence boundaries. These depositional sequences consist of complete or incomplete assemblages of fundamental components of lowstand, transgressive, and highstand systems tracts. Fission track and paleo-magnetic ages for the Toyofusa Group showed that the depositional sequences can be correlated with oxygen isotope stages of 15 to 23. By the correlations, periodic formation of depositional sequences can be attributed to glacio-eustatic sea-level changes of about 40,000 to 100,000 years.
