Abstract
The boundary between the Kazusa Group and the Miocene sequence in the Sayama Yamato well, which is unconformable, ranges from 920 m to 940 m in depth based on lithofacies of cuttings. The Miocene sequence has yielded calcareous nannofossil and fossil diatom assemblages, which are assigned to CN5a and NPD5A zones respectively, and correlates with the late Middle Miocene sequence, the lower part of the Negishi Formation, the Tokigawa Group, in the Iwadono Hills.
The silica mineral phase boundary of opal-A and opal-CT is considered to be between 1,200 m and 1,220 m in depth of the well based on the occurrence of fossil diatoms.
