Leg 161 (Western Mediterranean)
Leg 161 was the second in a two-leg ODP program to address both tectonic and paleoceanographic objectives in the Mediterranean Sea. During Leg 161, the JOIDES Resolution drilled a transect of six sites across the Western Mediterranean, from the Tyrrhenian Sea to the Alboran Sea immediately east of the Strait of Gibraltar. Sites 974 (Tyrrhenian Sea) and 975 (South-Balearic Margin) were dedicated to paleoceanographic objectives. Sites 976 (Western Alboran Sea), 977, 978, and 979 (Eastern Alboran Sea) focussed mainly on tectonic goals, but also involved paleoceanographic objectives.
Five of the six sites drilled during Leg 161 were logged (Holes 974C, 975C, 976B and E, 977A, and 979A) with the following tools : Quad combination (Quad combo), Formation MicroScanner (FMS), Geochemical tool (GLT), Lamont-Doherty Temperature Logging Tool (TLT), and BoreHole TeleViewer (BHTV). Data recorded (i.e. natural radioactivity, sonic velocity, resistivity, neutron porosity) are generally good to excellent, but in most of the logged holes, caliper measurements indicate a variable borehole diameter over a substantial part of the logged interval in the sedimentary sequence, which affected the quality of some of the measurements sensitive to hole diameter, notably the density log.
The main observations made during Leg 161 are the following : 1. The main log units (identified on the basis of the log response in the sediment) generally show a good agreement with those identified by the sedimentologists from lithologic criteria on core observations. Comparisons between logs and cores, in terms of natural radioactivity for example, will provide a useful core-log integration (identification of organic-rich layers, volcanic ash layers, lithologic unit boundary, etc.), even if the vertical resolution of the sensor is larger than the generally thin layers observed in the cores.
2. The XCB coring appears to generate erratic variations in hole diameter and modifications of the sediment lithology ("biscuiting" of the muddy sediments) and physical properties. The APC-cored part of the hole and the lower part of the XCB-cored interval (in harder sediment) showed much less variability in the borehole diameter. 3. In some cases, the calipers, either from Quad combo and FMS tools, also detect sudden and local increases in the borehole diameter every 9.0 or 9.5 meters : the main application of this may be to provide a precise comparison of cores and logs, i.e. a core-log integration. This will be aided by the high core recovery (around 100%) in the holes penetrating the Plio-Pleistocene sediments. 4. In the metamorphic basement, the logs allow to reconstruct the entire lithologic section, as core recovery is rather low (between 15 and 30%).
One of the most exciting results obtained during Leg 161 has been the extensive logging through the basement at Site 976. A full suite of logging was carried out in Hole 976B, during three successive passes, due to the instability of the borehole and serious washout : the two first were obtained in the Neogene sediments. For the third pass, the drillpipe was set below the sediment-basement interface (at 696.0 mbsf), to maximize the changes of success in basement logging. Quad-Combo and GLT tool strings were run and obtained excellent results ; High quality FMS images and BHTV data (transit time and amplitude) were recorded in the same interval (from 910 to 700 mbsf in Hole 976B), providing a 200-m long high resolution profile of the high-grade metamorphic basement (metasediments and gneiss). Preliminary analysis of the logging data shows three main log units, and FMS images, in the basement of Hole 976B, in accordance with core descriptions made onboard Joides Resolution. Brecciated fault zones (characterized by a poor recovery) are clearly identified, with more conductive pattern. Granitic or migmatitic rocks are also clearly identified, being correlated with higher values in the photoelectric factor and for the former, increases in the radiogenic potassium concentration due to crystallization of K-feldspar. This will allow the nature of the unrecovered intervals (mean core recovery : 19% in Hole 976B, 29% in Hole 976E) to be partially defined.
Logging operations in Hole 976E covered the transition between basement and sediments, a zone that was not possible to log in Hole 976B. A single run of the Quad-Combo from the basement into the sediments and a short repeat run within the sediments were obtained. Twenty meters of sediment fill was found in the bottom of the hole, and the Quad-Combo had to penetrate a bridge at the sediment basement contact. During the logging run upwards out of the hole, the sediments had closed in upon the logging cable above the tool, and the tool was extracted from basement only with difficulty. The repeat run was only in the sedimentary sequence as it proved impossible to penetrate down to the basement because the bridge was several meters thicker than during the first run. In the electrical response, there is a smooth transition over about 7 m from the sediments to the basement. In other logs (caliper, bulk density, potassium content, neutron porosity, photoelectric factor), the change is more abrupt and places the basement top at 652.0-653.0 mbsf.
In conclusion, the log data are of excellent quality in the basement in Hole 976B, but are degraded by the large borehole diameter found in the two logging sections within the sediments. In Hole 976E, poor hole conditions prevented to run the FMS across the transition between basement and sediments, but excellent data have been recorded with Quad-Combo string. Downhole measurements made at Site 976 will provide much data on the lithology, orientation of structures, stress orientation, and the tectonic deformation of the basement in this part of the Alboran sea. The full suite of measurements will allow, for one of the very first times in ODP, an extensive analysis and core-log integration of high-grade metamorphic basement.
Dr. F.D. deLarouziere (IMT)