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Fig. 5 Sample ages based on diatom biostratigraphic zones determined by Itaru Koizumi
(Modified from Kuwano et al. ( 1997 ))
Pliocene age (Figs. 5 and 6 ) (Kuwano et al. 1997 ), similar to the ages of fossils
from Emi Group and Miura Group rocks of the accretionary prisms on the Boso
Peninsula (Hirono and Ogawa 1998 ; Yamamoto and Kawakami 2005 ).
Moreover, some sandstone samples show a web structure, as observed in the
Emi Group (Hirono 1996 ).
Strikingly, the Miocene rocks consist of breccias of diatomaceous mudstone or
sandstone with calcareous cement and veins (Fig. 7 ). The carbon and oxygen isotope
ratios of the calcite cement (Fig. 8 ), measured by Tadamichi Oba (Kuwano et al.
1997 ), are similar to those in the Oregonian margin (Kulm et al. 1986 ), the Nankai
trough accretionary prism (Sakai et al. 1992 ), and the Boso Peninsula Miocene
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