The age model for the 355-m-thick Miocene to Pleistocene sequence cored at Site 1192 is defined by 12 calcareous nannofossils and seven planktonic foraminifer datums (Table T6; Fig. F16) (see "Biostratigraphy and Paleoenvironments"). Magnetostratigraphic age-depth estimates are considered to be reliable in the top 80 m of the section and moderately reliable between 80 and 150 mbsf (see "Paleomagnetism"). However, the estimates are mostly offset (younger) from the biostratigraphic results by up to 1 m.y. and are not used in the construction of the shipboard age model.
The biostratigraphic data indicate a hiatus of ~1 m.y. at 29 mbsf. Although hiatuses are to be expected in the drift deposits of Megasequence D, this particular one is close to the error range of the shipboard age estimates and may be insignificant.
A major discrepancy exists between control points from Holes 1192A and 1192B, in the middle to upper Miocene sediments recovered in both holes (~180 to ~250 mbsf) (Fig. F16). This discrepancy stems from the LO of Cyclicargolithus floridanus in Hole 1192A (11.9 Ma; 182-202 mbsf) and the presence of the same species in Sample 194-1192A-29H-CC (11.0-13.6 Ma; 242 mbsf) (Table T6; Fig. F16). These two (old) ages produce an age discrepancy of up to 3 m.y. below a depth of 180 mbsf when compared to Hole 1192B. The discrepancy could be the result of a coring depth error of 38-56 m, but a coring depth error of that magnitude is very unlikely. An initial comparison of magnetic susceptibility and bulk density core logging data from Holes 1192A and 1192B did not support a coring depth error of more than a few meters, but interpretation of the core logging data is not unequivocal. More likely, the discrepancy is the result of reworking of the older calcareous nannofossils C. floridanus. Reworking is indicated by the sparse occurrence of C. floridanus in Cores 194-1192A-21H through 29H, and the uninterrupted succession of Discoaster neohamatus and Discoaster hamatus in Samples 194-1192A-4H-CC to 5H-CC and 4H-6CC to 4H-8CC, respectively (see "Biostratigraphy and Paleoenvironments"). In addition, planktonic foraminifers in Samples 194-1192A-25H-CC through 29H-CC are from Zone 16, which is 8.2-11 m.y. in duration. These younger foraminifer ages in the same depth interval of Hole 1192A and the younger calcareous nannofossil ages given in Hole 1192B for the same depth interval strongly suggest that C. flondanus is probably reworked and an unreliable age datum. Consequently, these two age datums are ignored for the age vs. depth curve (Fig. F16).
We also ignored the reported LO of D. hamatus (9.5 Ma; 215 mbsf) because including this control point created an unrealistic peak in sedimentation rate (450 m/m.y.) over a very short time interval (0.1 m.y.). Shore-based analysis should resolve this problem.
For the purpose of picking age estimates for lithologic and seismic unit boundaries (Table T7), the straight-line segment defined in the interval 80-150 mbsf was extended to the bottom of the sequence (dashed line in Fig. F16). This line represents an "average model" for Holes 1192A and 1192B below 150 mbsf.
Interval sedimentation rates range from 0 (hiatus) to 44 m/m.y. The overall average rate is ~22 m/m.y.