Introduction

ODP Leg 206 is the first leg in a proposed two-leg program designed to penetrate a complete crustal section from extrusives to gabbros. When successfully completed, the section at this site will represent the first complete section in fast oceanic crust. Given the observation of an inverse relationship between spreading rate and depth to axial low-velocity zones, it should be possible to reach gabbros on a second leg to this site.

Location and drilling operations

This site is located in the Guatemala Basin on the Cocos plate in a 15 Ma crust that was formed during a period of superfast spreading (200 mm/yr) at the Pacific-Cocos plate boundary (East Pacific Rise). This site is located south of the Clipperton Fracture Zone, and has been chosen on a magnetic anomaly segment at least 100 km long and at least at 50 km from the end of the segment. This should ensure that the drilled section is far from zones of deformation and that the fast spreading rate crustal stratigraphy is well preserved.

Preparatory work at Site 1256 recovered a complete sequence of the 251 m sedimentary overburden and 88.5 m of basement in pilot holes (Holes 1256A, B, and C), before we installed a reentry cone with a 16-inch-diameter casing string extending 20 m into basement. Hole 1256D was then drilled to a total depth of 502 m sub-basement. The Hole 1256D basement section consists of sheet flows and massive flows with pillow lavas, hyaloclastites, capped by a sequence of evolved, massive flows (up to 75 m thick). The lavas have N-MORB compositions and are only slightly affected by low temperature hydrothermal alteration. A summary of the Leg 206 initial findings can be found in the Preliminary Report. Hole 1256C was partially logged and an extensive logging program was performed in Hole 1256D Table 1.

Logging operations

1256C

The triple combo tool string was deployed first. This tool string was initially unable to pass a bridge at ~203 mbsf. After several unsuccessful attempts to pass the tool string through this bridge, only the upper part of the borehole was logged (Table 1). After two passes, the tool string was recovered and set back in the derrick. The drill string was then lowered to within 20 m of the bottom of the hole to clear obstructions and another attempt was made to deploy the triple combo with the end of the pipe this time placed at 231 mbsf. During this third run, basement and overlying sediment were logged. The FMS-Sonic tool string was then deployed but was unable to pass deeper than 257 mbsf (26 m below the base of the pipe and only 5 m into basement). After several unsuccessful attempts to pass the tool string through this obstruction, it was decided to terminate the logging program.

1256D

Hole 1256D proved to be in very good condition and no constrictions impeded the passage of the various wire line strings throughout the logging program. Five tool strings were deployed (see Table 1).

- Triple combo tool string. It included, from top to bottom, the Hostile environment Natural Gamma Sonde (HNGS), the Acceleration Porosity Sonde (APS), the Hostile environment Litho Density Tool (HLDT), the Dual Laterolog (DLL) and the Temperature/Acceleration/Pressure tool (TAP).

- Formation MicroScanner (FMS)-Sonic (Dipole Shear Imager, DSI) tool string. Three successful passes were made.

- The third tool string deployed was the Bundesanstalt für Geowissenschaften und Rohstoffe (BGR) Magnetometer. Three attempts were made to deploy this tool. In all attempts the tool failed before it entered the open hole.

- The Ultrasonic Borehole Imager (UBI) tool string. One pass was made in a high-resolution mode (100 m/h).

- The Well Seismic Tool (WST). The air gun was used and 12 stations located along the whole section were recorded. For each station, the air gun fired five to seven times.

Results

Downhole measurements and images recorded in Hole 1256D show a high degree of variation, reflecting the massive units, thin flows, pillow lavas and hyaloclastite encountered. In Hole 1256D, three logging intervals have been distinguished based on strong changes in the geophysical parameters.

The sediment/igneous basement boundary at 251 mbsf was logged in Hole 1256C and the data are reported in Figure 2. This boundary is marked by a significant increase in density and resistivity, and acorresponding decrease in porosity.

Interval I: from the base of the casing to 344 mbsf (Core Unit 1)

This first unit is characterized by a high resistivity (up to 100 ohm.m), little variation in neutron porosity (5-6 %), density (2.87-2.9 g/cm³), capture cross-section (22-23 cu) and photoelectric effect (5.8 barn/e^-). The top of the igneous basement logged in Hole 1256D consists of a massive lava pound producing distinctive FMS images (Figure 3 and Figure 4). Numerous veins of variable dip are well imaged by the FMS and the UBI. At 296 mbsf in Hole 1256C and 306 mbsf in Hole 1256D an isolated spike is observed with a strong decrease in density (1.5 g/cm³), electrical resistivity (5 ohm.m), and photoelectric effect (1.4 barn/e^-), and suggesting the presence of a small faulted or highly altered interval.

In the massive unit, the UBI provided very good data. Veins and fractures are observed, which correlate well with those recorded by the FMS. Furthermore, as the UBI records 360° images, numerous subvertical veins can be identified with the UBI. Correlations with the FMS in terms of the fractures are good, especially in this upper massive unit.

Interval II - from 344 mbsf to 530 mbsf (Core Units 2 to 15)

Just below the massive unit, downhole measurements record sharp changes. The natural gamma ray log values strongly increase, especially the potassium and to a lesser degree the uranium content. To the contrary, thorium contents significantly decrease. In this interval, massive units, pillows and hyaloclastites can be distinguished.

Massive lavas appear to be more abundant in the lower part of interval II (Figure 2 and 3 showing four massive units between .470 and 530 mbsf). They are characterized by high electrical resistivity (up to 100 ohm.m), low porosity (< 6 %), high density (2.6-2.8 g/cm³), and low natural gamma ray values (< 6 gAPI). Pillow lobes are easily recognized on the FMS and UBI images at the uppermost part of interval II (core Unit 3). Pillow basalt appear on the image logs as circular to elliptical bright patches of varying sizes (20-80 cm diameter). The pillow rims are identified as darker regions of high conductivity, as they are more altered, compared with the central part of the pillows. Hyaloclastite zones (Figure 5) appear to be abundant in this interval. They consist of highly heterogeneous material, with resistive material (basalt, basaltic glassy clasts) cemented in a conductive matrix (altered glass). In these intervals, the natural radioactivity increases (up to 10 gAPI), and the neutron porosity increases (up to 60 %). The electrical resistivity is low (< 10 ohm.m) as is the density (< 2.5 g/cm³).

Interval III - from 530 mbsf to 750 mbsf (Core Units 15 to 26)

In this interval, variations in physical properties appear to be less pronounced than in the previous interval. The natural gamma ray values are low (~ 6 gAPI) from 530 to 673 mbsf. In this interval, one peak in the gamma ray data is recorded at 644 mbsf corresponding to a strong increase in potassium (Figure 2). This peak can be correlated to Core 1256B-57R where a highly altered and celadonite-rich interval was described. Below 673, the gamma ray increases to 10 gAPI from 643 to 645 mbsf before decreasing gradually from around 8 to 4 gAPI.

Below 530 mbsf, some massive units can be recognized on the FMS and UBI images (resistive zones with abundant veins) but unlike to the massive units in Interval II, they do not exhibit high resistivity values or high photoelectric effect and density value. This may be linked to a difference in alteration or vein abundance between the upper massive units and the lower ones, the units below 530 mbsf being more altered and/or more vein rich than those above. In interval III, pillows can be identified on the FMS and UBI images. These pillows are mainly present between 695 mbsf and the end of the logged section (Figure 6). Hyaloclastites appears to be more abundant in the upper part of interval III.

Florence Einaudi: Logging Staff Scientist, Laboratoire de Géophysique et Hydrodynamique en Forage, Université de Montpellier II, ISTEEM, cc 56, 34000 Montpellier, France