Logging Summary

Wilkes Land Glacial History

Expedition 318 Scientific Party

Figure 1. Downhole logs from Hole U1359D, with logging units, described in the text, given on the right side. The core data have been shifted down by about 5m to give a better depth match to the log data (for the cores, sea floor depth was not determined, only estimated at this hole)

The overall aim of drilling the Wilkes Land margin was to obtain a long-term record of Antarctic glaciation and discover its relationships with global paleoclimatic and paleoceanographic changes. In particular, the expedition investigated the sensitivity of the East Antarctic Ice Sheet to climate at times when the Earth was warmer than is today. Critical periods in Earth's climate history were examined: the Eocene-Oligocene and Oligocene-Miocene transitions, the mid/late Miocene, Pliocene, and the last deglaciation. During this time, the Antarctic cryosphere evolved in a step-wise fashion to ultimately assume its present-day configuration, characterized by a relatively stable East Antarctic Ice Sheet.

Downhole logging results characterized in situ formation properties and established the links between core, log, and seismic data. They addressed two of the expedition's four objectives:

• Objective 2: Fluctuations in the glacial regime during the Miocene (?) and transition from wet-based to cold-based glacier regimes (Late Miocene-Pliocene?).
• Objective 3: Distal record of climate variability during the late Neogene and the Quaternary.



Two of the seven sites drilled during Expedition 318 were logged (Holes U1359D and U1361A). Of the other sites, two had to be abandoned before logging due to storms and high seas (Hole U1356A and Hole U1357C), and three did not penetrate deep enough to be logged (each less than 71 m).

The two logged sites are located on channel levees on the continental rise, and are separated by ~50 km. The downhole logs cover the time interval from 3.6 to 12.5 Ma, and have high-amplitude 1 to 5-m-scale lithological variability superimposed on a downhole compaction trend (Figure 1 & Figure 2).

A complete overview of the expedition results and preliminary conclusions is available in the Expedition 318 Preliminary Report.

Logging Operations

Figure 2. Downhole logs from Hole U1361A. Bulk density from moisture and density core measurements and sonic velocity from the X-direction caliper are also shown for comparison.

Standard downhole logging tool strings were deployed in Holes U1359D and U1361A: the triple combo (comprising resistivity, density, porosity and natural gamma radiation tools), and the FMS-Sonic (comprising the FMS micro-resistivity imager, sonic, and natural gamma radiation tools). The holes were filled with heavy mud prior to logging (weight 10.5 ppg, including attapulgite and barite). The bottom of the hole was reached in both cases, indicating stable borehole conditions with little in-fill.

The VSI tool string (geophone and natural gamma radiation tools) was deployed in Hole U1359D only. Checkshot stations at 25 m intervals were planned, but after the tool reached the bottom of the hole the caliper arm would not open to clamp the VSI's geophone against the borehole wall. However, with the tool resting on the infill at the bottom of the hole at 601.5 mbsf (WSF), it was possible to get four reliable waveforms that were stacked to yield a one-way travel time of 2.3867 seconds.

Table: Expedition 318 logging operations summary

Hole	Date logged	Water depth (m, WSF)	Max depth (m, WMSF)	Pipe depth (m, DSF)	Tools run
U1359D	Feb 23-24, 2010	3019.5	606	97	Triple Combo, FMS-Sonic, VSI
U1361A	Mar 1 2010	3469.5	390	103	Triple Combo, FMS-Sonic

 

Scientific Highlights from Downhole Logging

Figure 3. Comparison of downhole logs near the top (A, 130-180 mbsf, ~5.5-7.5 Ma) and bottom (B, 300-350 mbsf, ~10.5-11.5 Ma) of the logged interval at U1361A, showing correlation between gamma radiation and resistivity logs in A. and anti-correlation in B. Grey bars mark low natural gamma values, thought to be caused by microfossil-rich sediment layers. A consecutive count of these layers is given on the right of the image, giving an estimate for the average duration of the alternations in the 60-150 kyr range.

Identification of lithology from the logs

Downhole logs, particularly natural gamma radiation (NGR) and density, provide an overview of lithological stratigraphy at quite high resolution (~30cm). The NGR signal at the two logged sites is dominated by the radioactivity of potassium and thorium. Both of these elements are found in clay minerals, and the sediments at Sites U1359 and U1361 are clay-rich, so to first order the NGR signal is probably tracking clay content. Minerals like potassium feldspar and biotite will also contribute to the NGR signal.

Intervals of low NGR values correspond to diatom-rich layers in the core, because diatoms are not radioactive and they dilute the NGR signal from K, Th, and U in the clays and terrigenous minerals that make up the balance of the sediment. The NGR logs promise to be a useful method for identifying diatom-rich and diatom-bearing zones in the core (where they are not always apparent to the eye), and complete the stratigraphy in unrecovered intervals (e.g. Hole 1361A, shown in Figure 3).

The density log also helps to identify diatom-rich zones (Figure 3). Relatively low density values result from the intra-granular porosity contained in the diatom shells and the low grain density of the opal that forms the diatom shells (2.1-2.2 g/cm3 compared to 2.6-2.75 g/cm3 for the other major sedimentary minerals). Shallower than 350 mbsf (~11.5 Ma), the resistivity and sonic velocity logs follow the pattern of the natural gamma and density logs, because the higher porosity in the diatom-rich intervals leads to low resistivity and low velocity. However, deeper than 350 mbsf, the opposite relation holds: low natural gamma values often correspond to higher resistivity (Figure 3). One possible explanation is that the diatom (and nannofossil)-rich intervals are more easily cemented than the clay-rich sediments that enclose them.

Cyclicity

Figure 3 also illustrates the cyclic nature of the sediment sequence at Site 1361, which alternates between high and low log values (diatom-rich and diatom-poor lithologies) at intervals of 1 to 5 m. As a first rough estimate of the average duration of these alternations, the number of cycles in both of the intervals shown in Figure 3 was counted. For the 130 to 180 mbsf interval (~5.5 - 7.5 Ma), there are about 15 alternations and therefore the average duration is approximately 133 kyr for each cycle, which seems to be in the ballpark of the orbital eccentricity Milankovitch periodicities (96 and 125 kyr). The 300 to 350 mbsf interval (~10.5 - 11.5 Ma) also contains about 15 alternations, giving an average duration of approximately 67 kyr for each cycle. Given the uncertainties in the initial age estimates, the probability that all cycles are not recorded equally well in the sediment record, the possibility of multiple cyclicities influencing the sediment record, and the subjective nature of counting cycles, this early estimate requires further verification. But it seems possible that Milankovitch band variability at eccentricity and maybe obliquity periods influences sedimentation at Site U1361.

Figure 4. Examples of resistivity logs and FMS resistivity images from Hole U1361A. A, resistivity logs, 130 to 180 mbsf; B, FMS image showing conductive (dark) and resistive (light) layers; C, a single conductive layer containing dropstones (light colored spots).

Identification of beds and dropstones in FMS resistivity images

FMS resistivity images reveal stratigraphic information at a finer spatial resolution than the standard resistivity logs, including both gradual and sharp transitions between the alternations of resistive and conductive beds, and dropstones and IRD larger than about 0.5 cm (Figure 4). The dropstones, indicative of ice-rafting, appear as resistive (light-colored) spots in the image, and it will be possible to map their occurrence downhole.

Figure 5. Comparison of Sites U1359 and U1361. The intervals 4.2-6.4 Ma and 10-12 Ma are covered in the logs at both sites, permitting stratigraphic correlation. The lithological columns are from the shipboard site reports (green = diatoms, brown = clays and silty clays). .

Figure 6. Ship heave over the course of operations at A. Hole U1356A, and B. U1361A. Heave is determined from acceleration measurements of the motion reference unit located near the center of the ship. Heave became too high for logging (or drilling) at U1356A, and during the FMS-Sonic deployment at U1361A, high heave made bringing the tools up through the pipe to the ship a very slow process.

Stratigraphic correlation between Sites U1359 and U1361

Sites U1359 and U1361 should contain similar stratigraphic sequences, as they are both governed by similar climatic and paleoceanographic changes, are both on the same channel levee, and are separated by only 50 km. However, Site U1361 is further down the slope from U1359, and one site may be by-passed by sediments that are deposited at the other. This is evident in Figure 5, showing that the 6.4-10 Ma interval is represented at U1361, but is highly condensed at Site U1359. 

Ship heave and downhole logging

Ship heave (the periodic vertical motion of the ship) is a critical factor that determines the quality of the log data and the safety of the tool strings. Heave was determined from acceleration measurements of the motion reference unit (MRU), located near the center of the ship, and was monitored throughout drilling and logging operations (Figure 6). Such plots help to understand how quickly heave conditions can change, and at what level the ship's motion becomes a problem to the log quality or the logging operation.

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Trevor Williams: Logging Staff Scientist, Borehole Research Group Lamont-Doherty Earth Observatory of Columbia University, PO Box 1000, 61 Route 9W, Palisades, NY 10964, USA


Annick Fehr: Logging Staff Scientist, Institute for Applied Geophysics and Geothermal Energy, E.ON ERC, RWTH Aachen University, Mathieu Str. 10, D-52074 Aachen, Germany


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