The sedimentation rate curve for Hole 1051A is relatively well constrained by calcareous nannofossil datums (Table T3; Fig. F6). The 530 m sedimentary sequence of Hole 1051A ranges in age from 38 to 55 Ma and has an average sedimentary accumulation rate of 30 m/m.y. The upper part of this sequence (90-390 mbsf) shows a sedimentation rate of 30.2 m/m.y., and the lower part of the sequence (390-530 mbsf) indicates a sedimentation rate of 27.6 m/m.y. There is a very low sedimentation rate at 390 mbsf, suggesting a stratigraphic gap that spans 2 m.y. Aubry (1995) investigated Hole 390A during DSDP Leg 44, which is located 20 km east of Hole 1051A, and recognized a hiatus between Subzones CP12a and CP13b. This hiatus could correlate to the stratigraphic gap in Hole 1051A.
The sedimentary accumulation curve of Hole 1052A shows a clear change near 160 mbsf (Table T4; Fig. F7). The sedimentation curve from 68 to 158 mbsf calculates to be a deposition rate of 21.2 m/m.y. The lowest part of this curve indicates slow sedimentation rates, suggesting a hiatus that spans 17 m.y. This hiatus could be correlated to Holes 1051A and 390A based on the seismic profile (Norris, Kroon, and Klaus, et al., 1998).
Many nannofossil biostratigraphers believe that coccolith size evolution is important to nannofossil biostratigraphy (e.g., Perch-Nielsen, 1985; Backman and Hermelin, 1986; Takayama, 1993). Therefore, I will try to subdivide some dominant genera, Reticulofenestra, Cribrocentrum, Dictyococcites, and Coccolithus, by size for this report (Fig. F3, F4).
Reticulofenestra and Cribrocentrum increase in size upward through the sedimentary sequence. The same cannot be said of Dictyococcites, which does not simply increase in size from early to middle Eocene, because large-sized Dictyococcites (Dictyococcites bisectus) are found in Subzones CP12a, CP13a, CP13b, and CP14a. Kameo and Takayama (1999) found that Reticulofenestra coccolith size does not simply increase upward throughout the sedimentary sequence but is characterized a gradual increase in coccolith size followed by an abrupt decrease in maximum size. The same phenomena may apply to Dictyococcites. Small Coccolithus pelagicus (<4 µm) are found abundantly in the late Paleocene to the early Eocene (Zone CP8 to Subzone CP12a) and sporadically in the middle Eocene. This may suggest an increase in coccolith size corresponding to more recent age in the sequence.