The principal objectives of ODP Leg 181 were to investigate the Cenozoic evolution of the Deep Western Boundary Current (DWBC) and the Antarctic Circumpolar Current (ACC), through the Southwest Pacific Gateway, to the east and southeast of New Zealand (Figure 1).
Figure 1 Location of Leg 181 drill sites
The inception and development of the ACC-DWBC system is a result of both the tectonic creation of a continuous Southern Ocean, and the onset and intensification of Antarctic glaciation (see Leg 181 Scientific Prospectus). ACC-DWBC evolution, therefore, is linked to some of the key issues within Southern Hemisphere palaeoceanography and Cenozoic climatology. The sedimentary record on the eastern New Zealand Plateau and its abyssal margins can be used to reconstruct the history of the ACC-DWBC system.
The sediments to the east of New Zealand also contain a detailed record of Neogene climate change in the Southwest Pacific. The logging results show particularly well how fluctuating environmental conditions in this region have produced cycles of sedimentation on the continental shelf and in the deep ocean.
Seven sites were drilled, four of which were logged (Figure 1; Table 1). Three standard ODP tool-strings were used (Table 1), although unfortunately the Nuclear Resonance Magnetometer Sonde (NMRS) on the GHMT failed to work throughout this leg. Weather and sea conditions were frequently poor, and resulted in the abandonment of logging at Sites 1120 and 1122 (Fig. 1). However, high quality logging results were obtained, and they will be used in conjunction with core based analyses to produce proxy environmental record for this region. Some of the preliminary findings of the logging program are outlined below.
|
|
1119C |
1123B |
1124C |
1125B |
|
Water depth |
395m |
3293m |
3970m |
1365m |
|
Depth cored |
495mbsf |
489mbsf |
474mbsf |
553mbsf |
|
Interval logged |
80-480mbsf |
86-487mbsf |
80-474mbsf |
81-553mbsf |
|
Depositional environment (see Fig. 1) |
The eastern New Zealand continental slope |
The deep, northeastern slopes of Chatham Rise |
On the Rekohu Drift, east of the Hikurangi Channel |
The shallow, northern slopes of Chatham Rise |
|
Stratigraphy |
Quaternary and latest Pliocene |
Continuous back to 16 Ma |
Recent to latest Cretaceous, with four major unconformities. |
Thick Neogene deposits. Base of hole in the middle Miocene |
|
Principal lithologies |
Couplets of greenish-grey silty clay and olive grey shelly silty sand |
Greenish grey ooze/chalk with colour cyclicity and volcanic ash horizons |
Greenish grey ooze/chalk, with mudstone and cherty zeolitic mudstone near the base |
Clay-rich nannofossil oozes and chalk with interbeds of more terrigenous silty-clays |
|
Tool strings used |
Triple-combo: |
Triple-combo: |
Triple-combo: |
Triple-combo: |
Table 1 Summary of the holes logged during Leg181
This hole was drilled into the upper continental slope, 5 km seawards of the shelf edge (Figure 1). Thick sediment drifts have built up at this location throughout Quaternary times, due to its close proximity to the rapidly rising, glaciated New Zealand Alps. Two lithofacies dominate: greenish-grey silty glacial clay; and olive grey shelly silty interglacial sand.
An uneven borehole wall, breakouts and occasional ledges affected the quality of the log data from this hole, particularly the lithodensity (HLDS) and neutron porosity (APS) measurements. Nevertheless, a cyclicity can still be clearly observed in the logs, especially in the gamma, resistivity and magnetic susceptibility data (Figure 2)
Figure 2 Caliper, natural gamma (HCGR), resistivity and magnetic susceptibility results from Hole 1119C
The spectral gamma results show that fluctuations in the uranium content of the sediment anti-correlate with variations in the HCGR value (thorium and potassium) (Figure 3). Comparison of the spectral gamma results with the lithostratigraphy from this hole shows that uranium concentrations are at their greatest, and HCGR values are at their lowest, in the base of the interglacial sands. High uranium and low thorium concentrations are generally associated with marine sequences (Koczy 1956), whereas the reverse is often indicative of terrestrial conditions (Hassan et. al. 1976). The spectral gamma results from this site can, therefore, be used as a good indicator of glacial/interglacial fluctuations in sea level.
Figure 3 Uranium (ppm) and HCGS (API) results from Hole 1119C
This hole penetrated a sediment drift, on the deep, northeastern slopes of Chatham Rise (Figure 1). It is of major scientific interest as it records a continuous sedimentary sequence back to the early Miocene (16 Ma). The magnetostratigraphy is complete, and in future will provide the reference section for studies in this region.
Logging results played an important part in obtaining a chronology for this hole. An excellent correlation was made between core and log based magnetic susceptibilities (Figure 4). In places, core recovery was incomplete and 'biscuity', and it was initially thought that some of the magnetic polarity shifts had not been sampled. However, the core/log magnetic susceptibility correlation showed that although the 'biscuits' of core had been compressed within the core liner, they in fact provided a uniform sampling of the stratigraphy. Hole 1123B recorded every magnetic polarity shift back to Chron C5Br.
Figure 4 Correlation between log derived magnetic susceptibility and measurements made on the core
This hole was particularly memorable because it penetrated the K/T Boundary. However, the exact contact between Cretaceous and Tertiary rocks is missing from the core, and therefore the location of the K/T Boundary must be deduced from the logs.
Three tools were able to take readings from the base of the hole, where the first appearance of Cretaceous rocks was observed: the DITE; the FMS; and the SUMS. The K/T boundary appears to occur at slightly more than 7 m above the base of the hole. This interpretation is based on a large spike in resistivity and a concomitant decrease in magnetic susceptibility at this point (Figure 5). The FMS data show a 30 cm thick resistive bed, with a sharp lower contact at 466.8 mbsf. While the lithological conditions responsible for these log responses are not yet clear, they clearly represent a sudden and short lived change in the environment at this point. A distinctive brown mudstone unit, evident in the core, can also be clearly identified in the gamma and magnetic susceptibility logs (Figure 5.).
Figure 5: FMS showing 30 cm thick resistive bed at the K-T Boundary
Hole 1125B was drilled in the shallow, northern slopes of Chatham Rise (Figure 1). The crest of Chatham Rise marks the position of the Subtropical Convergence (STC), which separates warm waters in the north from cooler and more nutrient rich waters in the south. This site receives a substantial input of terrigenous material, transported by the East Cape Current, which flows east along the northern flanks of Chatham Rise.
The lithostratigraphy at this site, which consists of a mixture of terrigenous and calcareous biogenic components, records a marked cyclicity. This cyclicity is also clearly identifiable in the logs, in particular the gamma results (Figure 6). Further work will show if the cycles evident within the gamma ray log can be linked to the astronomical time scale.
Figure 6 : Logging results from Hole 1125B
Logging Scientists:
Pat Fothergill, Borehole Research, Leicester University
Dave Handwerger, University of Utah
References:
Koczy, F.F. 1956. Geochemistry of radioactive elements in the ocean. Deep Sea Research 3, 93-103
Hassan, M., Hossin, A. & Combaz, A. 1976 Fundamentals of the differential gamma ray log interpretation technique. SPWLA 17th Annual Symposium Transaction, Paper H 1-18