AGE MODELS AND SEDIMENTATION RATES

Biostratigraphic levels with age significance at Site 1121 were derived from zone and event data, using diatoms, radiolarians, nannofossils, and foraminifers. The 14 levels are shown below:

  1. Younger than 0.42 Ma from diatoms in Sample 181-1121B-1H-1, 36 cm;
  2. Estimate of 0.65-1.8 Ma from diatoms in Sample 181-1121B-1H-1, 36-90 cm;
  3. Estimate of 2.2-3.6 Ma from diatoms in Sample 181-1121B-1H-1, 100-102 cm;
  4. Estimate of 3.26-3.6 Ma from diatoms in Sample 181-1121B-1H-1, 102-103 cm;
  5. A Paleocene age, >55 Ma, using benthic foraminifers in Sample 181-1121B-3H-CC at 28.83 mbsf;
  6. Radiolarian Zone RP5, 58-61 Ma, in Sample 181-1121B-6X-CC to 15X-CC; 37.01-122.33 mbsf;
  7. Diatom Zone Hemiaulus incurvus, 58 Ma, in Samples 181-1121B-6X-CC through 12X-CC; 37.01-94.31 mbsf;
  8. Radiolarian Zone RP4, 61-63 Ma, in Samples 181-1121B-15X-CC and 17X-CC; 122.04-134.47 mbsf;
  9. FO of B. tetradica, radiolarian, 61 Ma, base of Zone RP5, in Sample 181-1121B-15X-CC; 122.04-122.33 mbsf;
  10. Lower half of nannofossil Zone NP8, >56.2 Ma, in Sample 181-1121B-6X-CC; 37.01-37.28 mbsf;
  11. Base of nannofossil Zone NP6, 58.4 Ma, in Sample 181-1121B-10X-CC; 76.98-77.21 mbsf;
  12. Boundary between nannofossil Zones NP4 and NP5, 59.8 Ma, below Sample 181-1121B-14X-CC; 110.26 mbsf;
  13. Upper part of nannofossil Zone NP4, ~60 Ma, in Samples 181-1121B-15X-CC through 17X-CC; 122.33-134.47 mbsf; and
  14. A Paleocene age, >55 Ma, using benthic foraminifers in Samples 181-1121B-15X-CC through 17X-CC; 122.33-134.47 mbsf.

Dating of this site down to 32 mbsf is problematic (Fig. F19). Pliocene and Pleistocene ages are indicated in only the uppermost meter. The sequence comprises three major divisions with boundaries at 19 and 32 mbsf. Below 32 mbsf, the sediment is undoubtedly Paleocene and accumulated rapidly at 100 m/m.y. The uppermost section down to 19 mbsf is most probably Neogene and represents a residual sandy to silty product of winnowing that accumulated at <1 m/m.y. The age of the intervening section down to 32 mbsf is equivocal as it contains Paleocene radiolarians, which could be reworked (see "Biostratigraphy"). This section could thus have an age anywhere from Paleocene to Miocene. It is lithologically dissimilar to the undoubted Paleocene material (except for the presence of chert) (see "Lithostratigraphy"). More detail on sedimentation rate in the Paleocene is afforded by the five magnetochron boundaries, which suggest two periods of fast sedimentation with a reduction in rate to 4 m/m.y. between 81 and 93 mbsf (see "Paleomagnetism").

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