Abstract
Calcareous nannofossil assemblages were examined to determine the age of the Anno Formation, a constituent part of the Awa Group, and to estimate the gap in timing between the formation of the Awa and Kazusa groups of the central Boso Peninsula, Japan. Three nannofossil datums and biohorizons have been identified within the Anno Formation, enabling correlation between this formation and calcareous nannofossil zones CN11–CN12a (~4.5–3.0 Ma). The chronological gap between the Awa and the Kazusa groups is estimated to be ~1 m.y. during the latest Pliocene to earliest Pleistocene in the western part of the peninsula, a shorter duration than that identified in the east of the peninsula. Contemporaneous unconformities and/or sections that formed during periods with low rates of sedimentation have also been identified in forearc basins along the southwestern part of the Pacific coast of Japan. These data suggest that a major change in sedimentation occurred at this time, possibly relating to significant tectonism within these areas.
