A sedimentary section, 218 m thick, extending from the Holocene to the upper Pliocene (~3.1 Ma), was recovered at Site 1101. Sedimentation rates were determined using magnetostratigraphy, diatom datums, and calcareous nannofossil datums. Sedimentation rates are assumed to be constant between datums and are calculated by taking the slope of the depth-age curve (Fig. F34) between successive points.

The depth of a geomagnetic polarity transition is defined as the depth at which the inclination changes sign. Geomagnetic reversals at this site are very well defined and occur over relatively thin intervals (see Table T11). For consistency with other Leg 178 sites, the center of the uncertainty interval was used in calculating sedimentation rates. Biostratigraphic datums were assigned when a clear FO or LO could be determined through examination of sediment samples. No FO or LO was resolved when the occurrence bordered an interval barren of biogenic material.

Sedimentation rates determined from magnetostratigraphy and diatom and calcareous nannofossil datums are given in Tables T16, T17, and T18 and illustrated in Figure F35. For the upper 70 m of the hole, the average sedimentation rate is 6.9 cm/k.y. There is excellent agreement between sedimentation rates derived from all data sets; rates from paleomagnetic data and diatom and calcareous nannofossil datums average 6.8, 7.1, and 6.8 cm/k.y., respectively.

Both the paleomagnetic and the biostratigraphic data show a low in the sedimentation rate at ~ 120 mbsf. At this depth, the sedimentation rate drops downhole from an average of 6.9 cm/k.y. to an average of 4.1 cm/k.y. The decrease is especially pronounced in the paleomagnetic data and is manifested as a very thin Olduvai Event (C2n) between 121 and 127 mbsf (see "Paleomagnetism"). The lithology of this interval is a sequence of massive clayey silts and laminated clayey silts. This sequence occurs within lithostratigraphic Unit II (see "Lithostratigraphy"), which is characterized by foraminifer-bearing clayey silt. The interval from 122 to 133 mbsf does not contain foraminifer layers, and the percent biogenic sediment is very low, although there is no evidence of erosion. No clear unconformities or erosion surfaces occur within this interval. This interval is acoustically semitransparent and is characterized by parallel and subparallel, continuous and disrupted reflectors (see "Seismic Stratigraphy"). The onset of the Olduvai Event occurs in Core 178-1101A-15H, and the termination occurs in Core 14H. It is possible that some material was not recovered between Cores 178-1101A-14H and 15H, but it is unlikely that this alone would cause an apparent drop in the sedimentation rate by a factor of 1.7. The available data suggest that this interval is characterized by genuinely low deposition rates.

From 130 to 165 mbsf, polarity transitions and diatom datums give an average sedimentation rate of 5.6 cm/k.y., slightly lower than the rate at the top of the hole. Near the base of the hole, both data sets show a high in the sedimentation rate, but it spans 18 m (0.59 m.y.) more in the paleomagnetic data. The downhole increase in average sedimentation rate (5.6 to 10.0 cm/k.y.) occurs within lithostratigraphic Unit III (see "Lithostratigraphy"). This unit is a massive clayey silt with extremely low biogenic content, no bioturbation, and thin diamict layers. The lithologic observations are consistent with a period of rapid deposition.