Abstract: Marine terraces along the coast of the tectonically uplifted southern Boso Peninsula (Honshu, Japan) are formed by sediments deposited during the Jomon (Flandrian) transgression from 9,000 to 5,000 yr B. P. Sixty samples of these sediments were taken in the southern Boso, and 20 samples of recent sediments were collected in the adjacent Tateyama Bay. Of the 115 ostracod species recorded, six species are described as new.

Six fossil ostracod biofacies and six recent ostracod biofacies were delineated using cluster analysis and principal component analysis. Comparison of the distribution of fossil and recent ostracods was used to infer the environments of deposition during the Jomon transgression.

In most cases, deposition took place in drowned valley-type, long and narrow bays. The vertical change of fossil ostracod biofacies reflects the widening of the bays due to a rising sea level at the initial stages of the transgression and a gradual shallowing due to sediment infilling prior to the tectonic uplift of the area around 5,500 yr B. P.

The differences between the life, death, and fossil assemblages in samples on hand are examined, and the processes responsible for the changes and ways of detecting altered fossil assemblages are discussed.