The North Chatham Slope Site 1125 was drilled to a depth of 552.1 mbsf, in upper Miocene through Pleistocene pelagic/hemipelagic sediments. The combined nannofossil, foraminifer, diatom, and radiolarian biostratigraphy at Site 1125 yielded 38 event levels with a preliminary age assignment, using the shipboard stratigraphic framework (see Table T2 in the "Explanatory Notes" chapter). The age dates derived from the biostratigraphy are listed in Table T9. The setting of the site, in the area of the subtropical convergence on the Chatham Rise and with a relatively high-sedimentation rate, has the potential to yield a relatively high-resolution record of paleoproductivity.
The number of events observed is not particularly high, in comparison to the data from the other Neogene sites drilled during Leg 181, and a larger than usual number of dates are minimum or maximum age estimates, rather than being a single date. In general, the high sedimentation rate at the site requires detailed study of relatively large samples to acquire event levels, and postcruise studies will be essential to refine the stratigraphic record.
A graphical illustration of the age-depth data is in Figure F16. A large spread in age estimates, uncertainty in ages and event levels, and "bunching" of events near the 5.6 Ma level, degrade the reliability of the age-depth track (i.e., the net sediment accumulation curve). There is no doubt that the hole ended in strata immediately above the base of the upper Miocene, in an interval dated in the range 10.5-11.2 Ma, by the presence of common right-coiling Globorotalia miotumida.
Stratigraphic resolution in the upper part of the Miocene is less certain. According to the diatom record, a hiatus is indicated at 240 mbsf, near 5.4 Ma, a silica dissolution period. It is not apparent from the age-depth record of the events near this depth level that there is a stratigraphic hiatus. Indeed it falls in the most rapidly deposited section at the entire site.
Sedimentation at the site shows two cycles of high sedimentation rate declining to low values. In the upper Miocene, rates are 130 m/m.y. and then decrease to ~21 m/m.y. by the uppermost Miocene (~6.5 Ma). The second cycle occupies the Pliocene and Pleistocene, starting at 150 m/m.y. at 5.5 Ma, declining to ~19 m/m.y. for the last 2 m.y. This appears to reflect and date tectonic movements in New Zealand. The earlier period of rapid sedimentation may reflect the onset of severe strike-slip movement and uplift of mountains, whereas the second appears to coincide with the period of sharp change in the pole of rotation, increase in compression, and uplift of the Southern Alps (Walcott, 1998).