The combined nannofossil, foraminifer, diatom, and radiolarian biostratigraphy at Site 1124, yielded ~91 event levels with a preliminary age assignment, using the shipboard stratigraphic framework (see "Biostratigraphy" in the "Explanatory Notes" chapter). The 91 levels are shown in Table T14 and consist of 37 FO events, 50 LO events, and four LCO or acme events. All levels are plotted according to their observed depths at Site 1124 in Figure F25, and ages are defined in "Biostratigraphy" in the "Explanatory Notes" chapter. The position of the arrows in Figure F25 may be extended uphole (downhole) for last occurrences (first occurrences) because of limited sampling density for shipboard analysis. For comparative purposes, the magnetostratigraphic age model (see "Paleomagnetism") is shown as a line on Figure F25. All depths are reported in mcd rather than mbsf values. Poor preservation of the Neogene calcareous plankton limited stratigraphic resolution, in part because of the water depth of almost 4 km, which also resulted in selective dissolution of thinner walled Neogene planktonic foraminifers. The upper Miocene and upper Oligocene sections in particular yielded few stratigraphic data, although key radiolarian and diatom dates exist for the Miocene-Oligocene transition in Cores 181-1124C-31X through 41X.
As may be seen from the listing of biostratigraphic ages in Table T14, most of the dates have more than average uncertainty attached when compared to other sites drilled during Leg 181; many dates may be minimum or maximum ages. More intensive study of samples rich in radiolarians and diatoms, such as in the Oligocene and upper Miocene strata, may increase the stratigraphic resolution. Similarly, study of calcareous plankton in the more carbonate-rich, lighter colored intervals in the cores may assist in better definition of the hiatuses and mid-Cenozoic intervals with low rates of sedimentation.
Figure F20, the magnetostratigraphic age model for Site 1124, shows variation in rates of sedimentation from a low of 5 m/m.y. in early Paleocene, to over 25 m/m.y. in the period from late Oligocene to early Miocene. The main hiatuses are shown with undulating lines. The age-depth plot clearly illustrates severe sedimentary condensation in the pre-late Oligocene section, below the level equating to the Marshall Paraconformity in New Zealand, with much increased drift sedimentation after ~27 Ma. More study of samples around the 400 mbsf level may refine insight in the timing and waxing of drift sedimentation and erosion at Site 1124. The younger sediment below the drift sequence is ~33 Ma, whereas at Site 1123 it was of very similar age at ~32 Ma.
Sedimentary hiatuses occur in the lower Miocene (~17.5-22 Ma), the lower Oligocene (~27-33 Ma), in the upper Eocene (~34-37 Ma), and between the upper Paleocene and lower part of middle Eocene (~37-58 Ma). Lowermost Paleocene (early Danian) strata are present in Sample 181-1124C-48X-CC. Magnetic polarity at that depth is reversed and interpreted as Chron C29r. The K/T boundary beds occur in the coring gap at the top of Core 181-1124-49X-CC (and can be seen on the FMS log; see "Downhole Measurements") and are immediately underlain by upper Maastrichtian strata with a normal polarity signature (Chron C30N; see "Paleomagnetism").
The upper Cretaceous (upper Maastrichtian) through Paleocene mudstones were deposited at an average rate of ~5 m/m.y. The complex history of sedimentation, nondeposition, and erosion in Eocene time prevents an estimate of net sedimentation for the few meters of Eocene strata. Above the Marshall Paraconformity (~33-27 Ma), the thick upper Oligocene-lower Miocene section (~27-22 Ma) initially accumulated at 16.5 m/m.y., increased during the late Oligocene to ~41 m/m.y. and terminated at another disconformity with a hiatus of ~4 m.y. from 22 to 17.5 Ma. Middle Miocene drift sediments may be punctuated by at least two hiatuses recognized through omission of magnetochrons (see "Paleomagnetism"). Sedimentation is then very uniform at ~17 m/m.y. until ~1.2-1.5 Ma, when it increased sharply to ~42 m/m.y. This youngest sedimentation rate change at Site 1124 is not seen at Site 1123 and probably results from switching of Hikurangi Channel to the east, to directly supply fine terrigenous detrital sediment via turbidity current plumes. Volcanism becomes a significant feature in the record at Site 1124, as also recorded at Site 1123 (Figs. F20, F21; see "Paleomagnetism," in the "Site 1123" chapter), and it is probably coincident with early compression at the New Zealand plate boundary (Walcott, 1998). Several phases occur in the lower and mid-Pliocene, and near continuous volcanism is observed from the lower Quaternary (>1.75 Ma) to the present. This was also recorded at Site 1123 (see "Paleomagnetism," in the "Site 1123" chapter) and is coincident with the opening of the Taupo Volcanic Zone (Shane et al., 1996).