Logging Summary

Mediterranean Outflow

Expedition 339 Scientific Party

Figure 1. Bathymetry map of the Gulf of Cádiz, showing locations of the sites and paths of Mediterranean Outflow..

The Gulf of Cádiz was targeted for the investigation of Mediterranean Outflow through the Strait of Gibraltar gateway and its influence on global circulation and climate. As the direct result of Mediterranean Outflow, the Gulf of Cádiz contourite depositional system (CDS) has developed at very high rates of sediment accumulation over the past 5 m.y., providing an expanded sedimentary record that permits detailed examination of paleocirculation patterns linked to past environmental change. During Integrated Ocean Drilling Program Expedition 339, five sites were drilled in the Gulf of Cádiz and two sites were drilled off the West Iberian margin from November 2011 to January 2012.

Downhole logging was carried out at five of the sites (Figure 2, Figure 3). Scientific highlights include records of cyclic sedimentation in contourite drifts through time, and linking the site lithostratigraphy to the seismic profiles using checkshot surveys and velocity logs. Technical highlights include evaluating the new HRLA resistivity tool in low-resistivity formations by comparison to DIT resistivity results, and evaluating the effects of borehole conditions at one site where two contrasting holes were logged (one mostly in gauge, and the other washed out to wide diameter).

Table 1. Logging operations summary, Expedition 339.

Hole	Date of logging	Water Depth (below rig floor)	Hole Depth (below sea floor)	Maximum Logged Depth	Tool Strings
U1386C	Dec 7, 2011	573	526	526	TC, FMS, VSP
U1387C	Dec 16, 2011	569	870	650	TC, VSP, FMS
U1389A	Dec 23, 2011	656	355	355	TC, FMS, VSP
U1389E	Jan 1, 2012	655	990	568	TC, FMS
U1390A	Jan 4, 2012	1004	350	350	TC, FMS
U1391C	Jan 14, 2012	1085	671.5	669	TC, FMS

Figure 3. Graphic representation of the logging operations during Expedition 339.

Figure 2. Tool string combinations used during Expedition 339.

 

 

Logging Results

Figure 4. U1386C downhole logs and corresponding physical property data from measurements on sediment cores..

U1386C

Coring at Site U1386 on the Faro Drift recovered a thick succession of mud/silt contourites showing a continuous record of drift sedimentation over the past 1.9 m.y. down to 463 mbsf. Unconformities separate the Pleistocene from Pliocene and late Miocene sediments in the lower 60 m of the hole.

Downhole measurements were made in Hole U1386C to a total depth of 526 mbsf (Fig. 4). Despite numerous washouts of the borehole wall, the logs closely reflect both lithologic changes and cementation recorded in the recovered cores. This allowed us to infer lithologies from some of the gaps in core recovery, because high natural gamma radiation (NGR) values correlate to layers with high clay content, and lower NGR values correlate to coarser layers (usually contourites in this environment during the Pleistocene). Preliminary inspection also revealed a marked cyclicity from 102 to 346 mbsf through the contourite section, which seems to relate to Milankovitch precession cycles of ~20 k.y.

Figure 5. U1387C downhole logs and corresponding physical property data from measurements on sediment cores.

U1387C

This site lies 4 km southeast of Site U1386 on the same sediment drift, but has an expanded Pliocene succession. Meter-scale bi-gradational contourite cyclicity was common, with evidence of a strong lateral supply of terrigenous material to the bottom currents, especially before ~1.8 Ma.

Downhole Triple Combo measurements were made in Hole U1387C to a bridge in the hole at 649 mbsf; subsequent tool strings encountered bridges at shallower depths. The borehole contained many washouts and was often wider than the caliper arms (18 inches), which meant that the quality of the density and porosity logs was very poor. Other logs (NGR, resistivity, sonic velocity) were affected to a lesser degree (Fig. 5).

As at the other Exp. 339 sites, NGR measurements were made in the upper part of the hole through the pipe (Fig. 5). The NGR signal is attenuated by the metal in the pipe and bottom-hole assembly. This attenuation can be corrected at these sites by multiplying by a factor of 5, to bring the NGR values into line with the open-hole values. Features in the downhole NGR record compare well to those measured on core.

In the upper part of the Pliocene logged section (~3.8 to 3.2 Ma, 462-600 mbsf) the cyclic pattern observed on NGR logs seems to reflect lithologic cycles (dark-light cycles in interbedded turbidites and contourites). Although some depth adjustments may be locally required, high NGR values appear to correlate relatively well with thick intervals of very dark greenish gray nannofossil mud, probably because of high clay content in this lithology. For example, dark greenish layers observed at ~536.5, 539, and 542 mbsf probably tie with logged gamma peaks centered at 535, 539, and 542 mbsf.

Marking the hiatus between Pleistocene and Pliocene sediments, two dolostone beds appear as a double peak of high resistivity values at 461.1 and 462.6 m, and the highest uranium content of the entire hole (close to 3 ppm) is observed between the peaks

U1389A and U1389E

Figure 6. U1389A and U1389E downhole logs and comparison of logs taken under between in-gauge and wide borehole conditions, respectively.

Site U1389 is located in the “channels and ridges” sector of the larger Cádiz contourite depositional system, and is perched on a relative topographic high, which is currently elevated 50-250 m above the flanking contourite channels. Hole U1389E reached ~3.8 Ma at 990 mbsf.

Logging was carried out at two holes at this site: in Hole U1389A to 355 mbsf and in Hole U1389E to a bridged-hole depth of 568 mbsf. This allowed comparison of logging data in contrasting borehole conditions. Hole U1389A was an APC/XCB hole drilled with a PDC bit, resulting in a good-quality in-gauge with good logging data that matched well to the corresponding data measured on cores. Hole U1389E was an RCB hole that needed a lot of water pressure to keep the roller-cones free of clay build-up, resulting in a wide and frequently washed out hole. Thus the degree to which hole diameter affects downhole logs can be seen in the overlapping logged interval (Fig. 6):

• The total natural gamma radiation (HSGR) logs have lower values in the wider hole, but in general the same trends and patterns are seen in the HSGR logs from both holes. Correction for hole diameter is possible based on the caliper logs, although very good depth control is needed for this to be successful in rough holes like U1389E. Moreover, it is not possible to accurately correct for hole sizes greater than the 18 inch caliper limit.
• The bulk density logs offer a stark contrast between the two holes. In Hole U1389A, bulk density values typically match the moisture and density (MAD) bulk density values. In the wider parts of Hole U1389E, the sensors were unable to make good contact with the borehole wall and consequently read values as low as 1.15 g/cm3, close to borehole fluid value. Where the holes are relatively smooth and not too wide, the log bulk density values overlay very closely (e.g., 195-215 mbsf).
• Electrical resistivity is measured by the HRLA at five different depths of investigation. The deepest measurement (RLA5) is least affected by variations in borehole diameter. In general, the RLA5 logs from both holes overlay each other reasonably well, but the Hole U1389E log is noisier and drops to lower values in thin, washed out zones, as well as peaking to higher values where the hole narrows. An additional HRLA log, the “true resistivity” computed from the five deep-to-shallow reading measurements (not shown in the figure), apparently overcorrects, providing resistivity values in Hole U1389E that are higher than in Hole U1389A.
• The sonic velocity logs from the two holes give well-matched patterns and values in the upper part of the common section (100-220 mbsf). In the washed-out zones below 220 mbsf in Hole U1389E, velocity is sometimes underestimated, perhaps caused by a longer path for the sonic wave to travel. In the rough and wide 220-320 mbsf section, Hole U1389E velocities are on average 3.2% slower than those in Hole U1389A. Even though the velocity data repeat very well between Passes 1 and 2 in Hole U1389E, this is not necessarily a good indication that they are recording the true formation velocity well, rather that the two passes respond similarly to the hole conditions. Another note of caution is that the washed out zones tend to be sand-rich layers; thus their velocities would be underestimated more than those of other lithologies.

A good suite of FMS image logs was obtained in Hole U1389A.Conductive beds on the FMS images correlate with lower values in the NGR log, lower bulk densities, lower resistivities, lower P-wave velocities (e.g., two conductive intervals from 317 to 320 and 326 to 328.5 mbsf in Fig. 6) and correspond to coarse-grained intervals (sands and silts). There is a distinct change in log characteristics at ~320 mbsf, which correlates closely with a lithostratigraphic boundary and a zone of poor core recovery. This zone appears to be more sand rich on the basis of borehole logs, although no sands were recovered by coring. At the scale of the holes, the FMS images have a downhole trend of increasing conductivity, especially below 320 mbsf, that we possibly relate to the progressive downhole increase in pore water salinity.

U1390

Figure 7. U1390A downhole logsand corresponding physical property data from measurements on sediment cores.

Site U1390 is the companion to Site U1389, also located under the lower branch of Mediterranean Outflow. Site U1390 is located near the western end of a sheeted drift adjacent to the Guadalquivir contourite channel at 300 m above the channel floor. With maximum sedimentation rates of 85 cm/k.y., and perhaps in excess of 100 cm/k.y., these are the highest known rates for contourite drifts anywhere.

Downhole measurements were made in Hole U1390A to 350 mbsf with good quality data obtained as a result of good hole conditions, especially in the upper 270 m (Fig. 7). There is a distinct change in log characteristics at ~290 mbsf, which correlates closely with the lithologic boundary between subunits and the change downhole to a more sand-rich lithology. Distinct cyclicity is apparent in some parts of the section, corresponding with both lithologic and physical property data.

This site was notable for a downhole decrease in resistivity, which, as for site U1389, we attribute to high salinity pore waters in the lower part of the hole. High salinity values were also seen in the pore-water geochemistry data, perhaps indicating subsurface flow from a source of highly saline water.

Figure 8. U1391C downhole logs and corresponding physical property data from measurements on sediment cores..

U1391

Site U1391 is located to the west of Portugal and is the most distal of the drilled sites under the influence of MOW. Coring reached 672m into an extensive plastered drift, dated at ~3.5 Ma at the base of the hole. Although it is distinctly more mud rich than at the sites in the Gulf of Cádiz, the sedimentation rates are as high as Sites U1386 and U1387, and contourites still dominate the succession.

Downhole measurements were made in Hole U1391C to 668 mbsf, almost the bottom of the hole. The borehole was very rugose with many narrow washouts that affected log quality. Minor changes in log characteristics occur at ~562 mbsf, which correlates to a lithologic boundary at about 2.5 Ma. The deeper interval has generally lower NGR values and includes two zones with poor core recovery that may be more sand-rich on the basis of borehole logs, although no sands were recovered by coring. Distinct cyclicity is apparent in some parts of the section, corresponding to cyclicity in both lithologic and physical property data.

Comparison of the HRLA and DIT-SFL resistivity logs

Figure 9. Comparison of resistivity data from the DIT tool (SFLU, IMPH and IDPH) and the HRLA tool (RLA1-5).

The Schlumberger HRLA resistivity tool was run in high-resistivity ocean crust on the two most recent JOIDES Resolution IODP expeditions (335 and 336) but until Expedition 339 had not been run in unconsolidated sediments. Previous generations of laterolog, such as the Dual Laterolog, were not designed for low-resistivity unlithified sedimentary formations such as those encountered during IODP expeditions. However, the HRLA measurement range extends to low resistivities and therefore is more suitable for IODP use. In Hole U1386C, we obtained both HRLA and DIT-SFL resistivity logs in order to compare absolute resistivity values, vertical resolutions, and effects of washouts.

Overall, the two sets of logs show the same resistivity trends and fluctuations in Hole U1386C (Fig. 9). The deep-reading resistivity log of the HRLA have a higher vertical resolution(~30 cm) compared to the DIT (~240 cm). The deep reading DIT resistivity follows almost exactly the lower envelope the HRLA deep resistivity values. The shallow-reading logs from both tools have lower resistivity values than the deep-reading logs, as expected, because the shallow-reading logs sample proportionately more seawater and less formation than the deep-reading logs. The separation of the shallow and deep HRLA log values is much reduced where the borehole diameter is in gauge (e.g., 285 mbsf) and expanded where the hole is washed out (e.g., 259 mbsf) or rugose (e.g., 268 mbsf), as is also the case for the DIT-SFL logs. The features in the HRLA resistivity logs were cross-checked against the velocity logs: they match very well in both.

Figure 10. Sonic two-way-time (TWT) vs. depth at Hole U1389A.

VSP, Velocity profiles

Understanding the sedimentary architecture of the contourite depositional system in the Gulf of Cádiz is a major objective of Expedition 339. To achieve this, the lithostratigraphic packages and hiatuses have to be dated and linked to stratigraphic units in the seismic profiles that criss-cross the area. The link to the seismic profiles requires conversion from depth in the borehole to two-way traveltime (TWT) in the seismic. Figure 10 illustrates the results of two methods for this conversion at Hole U1387A: 1) vertical seismic profile checkshots, and 2) the integrated velocity log. The methods give reliable results, despite sometimes less than ideal borehole conditions.

Temperature measurements

In-situ sediment temperature data were measured during 55 deployments of the APCT on Expedition 339. The geothermal gradient at the Exp. 339 sites varies from 14°C/km (at Site U1391) to 34°C/km (at Site U1386), a wide range that reflects tectonic processes in the Gulf of Cádiz area. The data also provide estimates of the average bottom-water temperature, representing the temperature of Mediterranean Outflow water, over the last century or so.

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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


Johanna Lofi: Logging Scientist, Géosciences Montpellier - UMR 5243 - CC 060 - Bat. 22, Université de Montpellier 2, Place E. Bataillon, 34095 Montpellier Cedex 05, France


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