SEDIMENTATION RATES

The distribution of ages and sediment thicknesses allows the calculation of sedimentation rates for lithologic Units I and IV (see "Unit I" and "Unit IV"). Although the sedimentary section has not been decompacted, some patterns have been observed (Fig. F67). Sedimentation rate reconstruction in the upper 108.7 mbsf is based on paleomagnetic age constraints (see "Magnetostratigraphy") and on the preliminary biostratigraphic age assignment of Core 185-1149A-9H to the Pliocene and of Core 12H to the late Miocene, according to their siliceous plankton assemblages. The sedimentation rate averages 18 m/m.y. but was notably higher during the late Pliocene (~21 m/m.y.), the early Pleistocene (~24 m/m.y.), and during the middle to late Pleistocene (34 m/m.y.). A slower sedimentation rate (~7 to ~13 m/m.y.) characterizes the late Miocene to the mid-Pliocene. Unit I is characterized by widespread volcanic ash layers (see "Unit I"); therefore, these abnormally high sedimentation rates for an open-ocean sedimentary basin may be the result of the volcanic input. Figure F67 shows that the sedimentation rates were much slower during the late Miocene to mid-Pliocene; this suggests that the volcanic input was lower during these time intervals but increased rapidly after the late Pliocene. A low sedimentation rate during the Miocene is also suggested by the abundance of ichthyolith assemblages.

No biostratigraphic or well-constrained paleomagnetic data are available for Unit II; nevertheless, it is noteworthy that the abundance of ichthyolith assemblages constantly increases downhole, possibly mirroring a further decrease of the sedimentation rate in this unit (Cores 185-1149A-13H to 185-1149B-3R; 108.7-170.3 mbsf).

No biostratigraphic data or paleomagnetic constraints are available at this time for Unit III, thus hindering reconstruction of the sedimentation rates. Shore-based radiolarian biostratigraphic studies will provide age constraints for this unit and will allow sedimentation rate reconstruction.

Sedimentation rates for Unit IV are based on calcareous nannofossil biostratigraphic data and show average values close to 20 m/m.y., similar to modern equatorial oceanic carbonate sedimentation rates (Murray and Leinen, 1993). The preliminary data suggest that the sedimentation rate was steady during the Valanginian and Hauterivian.

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