Microfossil age of Hayakawa Tuff Breccia in the Hakone area, Japan and its significance

Hayakawa Tuff Breccia, situated at the northern end of Izu peninsula, is one of the Tertiary basements of Hakone volcano. It was correlated with the Shirahama Group in the southern region of the peninsula based on a peculiar moluscan assemblages (Shirahama fauna). In this study, microfossils found in bore hole samples of southeastern flank of Hakone volcano were examined to ascertain the exact age of the formation. The age of deposition was determined to nearly 4.2Ma, owing to overlapping biostratigraphic zones of foraminifera and calcareous nannofossils. Of additional interest, several index species of CN3-4 (early to middle Miocene) are also found, even though the samples are nearly monolithological. Yugashima Group, early to middle Miocene formation of this area, is strongly altered by geothermal activity and the presence of fossils is rare. The presence of early to middle Miocene fossils in early Pliocene indicate that the strong geothermal activity took place later than 4.2Ma. The onset of hydrothermal activity in this study area is consistent with previous study undertaken in the southern part of Izu peninsula (Matsumoto et al. 1985). These hydrothermal activities can be simultaneous and related with the collision of Izu terrane to the Honshu island.