The majority of sedimentation on the Iberia Abyssal Plain during the Late Cretaceous to Paleocene resulted from turbidite emplacement and may be the sole reason that calcareous nannofossils have been preserved here at all. To illustrate this idea, Sites 1067, 1068, and 1069 might be compared with the outlying Site 1170: where turbidites were absent, no nannofossils were preserved in that interval (Whitmarsh and Wallace, Synthesis, this volume). Because of the great depth of the IAP (>5000 meters below sea level), most sedimentation during the interval in question took place below the CCD. The turbidity currents that dominated the area during the Late Cretaceous and Paleocene brought in nannofossils that were previously deposited on either the nearby Vasco da Gamma and Vigo Seamounts or from the more distant Porto Seamount and continental margin.
We consider reliable the biostratigraphic record that is found at Sites 1067, 1068, and 1069, despite the fact that the nannofossils used to construct the biostratigraphic column were transported to the IAP by turbidity currents. Turbidity currents over the Madeira Abyssal Plain acted essentially as nonerosive transport currents that contained a mixture of sediments ranging in age from 200 to 500 ka (Weaver, 1994). Because a time span of >500 k.y. separates most of the biozone markers in the Upper Cretaceous and Paleocene, there should be relatively few cases of adjacent biozone markers overlapping as a result of mixing that would confuse the biostratigraphic record.
Hole 1068A probably represents the most complete sedimentation record for this area. Because this site was drilled on a topographic high, we assume that the turbidites that dominate this area would largely bypass this feature until the basins filled to the level of the high. Primarily for this reason there is no sedimentation record below the uppermost Maastrichtian. After the site began to accumulate sediments, MARs seemed to fluctuate, depending on the frequency of the turbidity currents (fluctuating from higher rates in the late Maastrichtian to lower rates in the early Paleocene).
The MARs almost seem to have a cyclic trend, remaining higher for a couple of million years then lower for a couple of million years, finally ending with very high rates at the end of the Paleocene. A similar pattern is seen in Hole 1069A, although the periods of higher and lower sedimentation are more on the order of 3-4 m.y. long. However, the MARs at the end of the Paleocene in Hole 1069A are much lower than what is found in Hole 1068A. This may be a result in a shift in the source of the turbidites, farther from Hole 1069A, or a shift in the pathways of the turbidites that may lead them to deposit sediments in another location.
MARs do not indicate any hiatuses, despite the numerous combined zones in the Cretaceous sections of both Holes 1068A and 1069A and the numerous barren intervals found throughout all sections of the Cretaceous and Paleocene. We believe that the long barren interval found in the mid-Paleocene in Holes 1068A and 1069A is a result of a very high CCD that dissolved calcareous nannofossils that would usually be deposited in the source area for the turbidites. This barren interval is also seen in Hole 398D (Fig. F3), which was drilled 1100 m shallower on the flanks of the Vigo Seamount, indicating that the CCD must have been shallower than ~3900 m.
There is a difference in the grain sizes of the turbidites in Hole 1068A relative to those in Hole 1069A. Turbidites in Hole 1069A generally contain a coarser fraction than the turbidites in Hole 1068A; smear-slide data collected on the ship during Leg 173 indicate that there is a greater percentage of the sand-sized fraction (Whitmarsh, Beslier, Wallace, et al., 1998, pp. 496-499). This may be a result of different sources for these two areas or may result from the coarser fraction bypassing the higher block on which Hole 1068A was drilled (Fig. F2).
The result of this report supports the idea that nannofossil-bearing turbidites can be used to construct a reliable biostratigraphic profile in an area where normal pelagic sedimentation is barren as a result of de-position below the CCD. An independent test of this assumption is provided by paleomagnetic study of these same cores, which has produced a workable magnetostratigraphy for the Leg 173 sections that does not contradict the biostratigraphy (Zhao et al., Chap. 11, this volume).