Leg 184 Logging Summary
Shipboard Scientific Party
Leg 184 was designed to core hemipelagic sediments in the South China Sea (SCS) to determine the evolution and variability of the East Asian monsoon during the late Cenozoic.
The major objectives of Leg 184 were to improve our knowledge of the variability of monsoonal climates (including millennial to possibly centennial variability from high-sedimentation rate records), orbital-scale variability from records at all SCS sites, and tectonic-scale variability from late Cenozoic sections. The records from the SCS will be used to establish the links between the East Asian and Indian monsoons and to evaluate mechanisms of internal (climate system feedbacks) and external (orbital and tectonic) climate forcing.
The proposed drilling program in the SCS will enable us:
The Leg 184 logging plan was designed to provide:
Six sites (essentially hemipelagic sediments) were drilled, four of which were logged using standard ODP wireline techniques (Fig. 1; Table 1). The temperature-acceleration-pressure (TAP) tool was deployed during the logging of Hole 1143A, but it failed. Several attempts were made to fix the TAP tool, the communication lines, and the PC software, without success.
Figure 1: Location map of Leg 184 sites.
Table 1: Summary of the holes logged during Leg 184.
Weather and sea conditions were frequently good, and resulted in high quality logging results. The log data show well how the fluctuating climate conditions in the SCS produced cycles of sedimentation.
Hole 1143A was drilled within a basin, in the Nansha or "Dangerous Grounds" area in order to provide a Neogene paleoceanographic record within the Western Pacific Warm Pool region of relatively stable sea surface temperature (SST). The sediment record will allow the reconstruction of SST gradients (with the northen sites) across the SCS and should reflect the development of climatic seasonality in this region.
Most of the logs show patterns of orbital scale cyclicity, that could be related to glacial/interglacial cycles. Moreover, Site 1143 lies between the high-accumulation rate (10-30 cm/ky) terrigenous deposits of the paleo-Sunda and Mekong Rivers to the south and the carbonate-rich,, but low sedimentation rate (1-2 cm/ky) region of the northernmost southern margin. As an accumulation rate of about 5 cm/ky was anticipated, the site would be a sensitive indicator of both the pelagic and terrigenous sources in the SCS. But some turbidites were recovered in the cores, leading to higher sedimentation rates. The turbidite layers are clearly distinguishable both in logging data and FMS images (Figs. 2 and 3). The base of the turbidites (sandy part) is characterized by lower gamma-ray, density and resistivity and higher porosity and P-wave velocity values, while it is the opposite for the top (clayey part). By looking at both the standard logs and the FMS images, we will be able to count the number of turbidite layers present in the lower part of Hole 1143A, yielding a good estimate of the sedimentation rate.
Holes 1144A, 1146A, and 1148A
Hole 1144A, 1146A, and 1148A were logged with 3 tool strings: the Triple Combo, the FMS-LSS, and the GHMT. The primary objective of these sites on the northern margin of the South China Sea was to recover a continuous sequence of hemipelagic sediments that will enable reconstruction of the paleomonsoon history on a millennial, centennial, or higher resolution time scale for much of:
For all these sites, gamma ray, resistivity, PEF, and density logs revealed fine-scale cycles (Fig. 4). FMS images also showed frequent alternation of relatively conductive (dark) and resistive (light) sediment layers in the entire log. The implications of these fine-scale variations, that are more apparent in the logs than in the appearance of the sediment itself, are yet to be explored, but they might relate to some of Milankovitch cycles driving monsoon climate. Variations in the density, PEF, and gamma ray logs also reflect clearly the internal changes in the carbonate content. The sonic velocity data, which are of very good quality, will be useful in producing post-cruise synthetic seismic profiles (Fig. 5).
Hole 1148A is located on the lowermost continental slope off southern China, near the continent-ocean crust boundary. This site is of major scientific interest as it records a sedimentary sequence back to the Lower Oligocene (approx. 32 My), including the initiation of sea floor spreading in the SCS.
In Hole 1148A, the logs particularly revealed the physical character of the interval associated with a prominent reflective seismic sequence (440-475 mbsf), as well as the interval below which had very poor core recovery (470-555 mbsf) (Fig. 6). The top of this interval is associated with a likely late Oligocene to early Miocene hiatus of 1-4 myr duration.
Figure 6: Downhole logs (Caliper, bulk density, neutron porosity, electrical resistivities, magnetic susceptibility, and P-wave velocities) in Hole 1148A showing the double reflectors seen in seismic reflection profiles.
Logging results will play an important role for this site. There is a change in the cycles just below the reflective seismic sequence: from 485 mbsf to the bottom of the hole, the cycles scale is extremely fine. Further investigations for these images should bring interesting information insofar as the cores for this sequence are completely monotonous and comprise only very minor lithologic variation. Moreover, as the second coring attempt in Hole 1148B yielded even slightly less recovery than Hole 1148A (16% between 470 and 507 mbsf; 38% between 507 and 555 mbsf), the only information can come from the logs. The preliminary interpretation of the drastic change in wireline data for this interval suggests multiple, well sorted, coarse-grained sand layers.
ODP Logging Scientist:
Christine Lauer, Université Montpellier II, Place E. Bataillon, 34095 Montpellier, Cedex 5, France
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