The LO of N. asanoi ([1] of Fig. F4; Table T3) has been chronologically determined to be in the base of the Olduvai Subchron at Lamont-Doherty Earth Observatory drilling Site RC10-161 in the North Pacific (Maiya, 1978) and the Boso Peninsula (Oda, 1979). The top of the Olduvai Subchron is just above the Pliocene/Pleistocene boundary (Berggren et al., 1995a, 1995b). The LO of N. asanoi at both Sites 1150 and 1151 is also consistent with a position just below the Pliocene/Pleistocene boundary. This consideration is supported by the diatom results of both sites corresponding to the upper part of Zone NPD 9 (top boundary: 2.0 Ma) (Yanagisawa and Akiba, 1998).
Planktonic foraminiferal zonation and determination of each zonal boundary are difficult at Sites 1150 and 1151 because only seven stratigraphically important species occur and their ranges are discontinuous. According to Kennett and Srinivasan (1983), G. puncticulata occurred from the early to late Pliocene. Therefore, Subunits IIB and IIA of Site 1150 and Subunits IIC and IIA of Site 1151, containing G. puncticulata, might have a Pliocene age.
Globorotalia plesiotumida, Neogloboquadrina acostaensis, and Neogloboquadrina praehumerosa appeared in the late Miocene period (Kennett and Srinivasan, 1983; Natori, 1976), whereas Globorotalia lenguaensis disappeared (Berggren et al., 1995a, 1995b) in the same period. Therefore, Subunits IIIC and IIIB of Site 1151, consisting of sporadic occurrences of G. plesiotumida, Neogloboquadrina acostaensis, Neogloboquadrina praehumerosa, and G. lenguaensis, seem to be correlated with the upper Miocene.