The primary objective of Leg 203 was to drill and case a hole for future installation of an observatory, coring was limited to sampling a short section of basement rocks. The modest sample return from coring Hole 1243B was significant, however, given the sparse catalog of deep basement rocks from young Pacific seafloor. Some sediment was recovered in Core 203-1243B-1R (102108 mbsf) of the same lithologies and colors as oozes recovered during Leg 138. The ooze consists dominantly of coccoliths with a few percent planktonic foraminifers, discoasters, radiolarians, Fe oxide globules, and glass.
The driller first felt basement at 110 mbsf, and the first basement rocks were recovered from Hole 1243B in Core 203-1243B-2R (108113 mbsf). Basement was drilled and cored to a total depth of 195.3 mbsf, which represents 87.1 m of basement section. Seventeen cores were taken in this interval. Recovery ranged from 1.6% in Core 203-1243B-16R to 63.7% in Core 203-1243B-7R, averaging 25% (this recovery statistic does not include 5.3 m of drilling breccia/cuttings recovered in the deepest core, recorded as Core 203-1243B-19R).
On the basis of hand specimens, thin section descriptions, and shipboard geochemical analyses, eight basement units were defined (Fig. F14). Units 1, 3, 4, 5, 6, 7, and 8 are volcanic basaltic units. Units 1, 3, and 7 are aphyric basalts. Units 4, 5, and 6 are sparsely plagioclase and olivine phyric basalts. Unit 8 consists of moderately plagioclase and olivine phyric basalt. Unit 2 is represented by a piece of limestone. All the basement basaltic units are interpreted as pillow lavas based on the presence of glassy margins and associated vesicular zones. No evidence of thicker massive lava flows was found in the cores. This interpretation of the environment of eruption is further confirmed by downhole measurements in Hole 1243B. Inductively coupled plasmaatomic emission spectroscopy analyses conducted on board indicate that all units are tholeiitic except Unit 4, which consists of alkali basalt. The basement units range in thickness from 0.065 m (Unit 2) to 11.175 m (Unit 3). At the bottom of Hole 1243B, 5.3 m of drilling breccia was recovered. This consists of finely broken angular fragments of pillow basalts (Core 203-1243B-19R).
Wet bulk density, grain density, porosity, and sonic velocity were measured on minicores, which were also used for paleomagnetic measurements. The sonic velocities range from 4.3 to 5.7 km/s (mean = 5.26 ± 0.08 km/s). Porosities range from 4% to 17% (mean = 7.7 ± 0.7%). Wet bulk densities range from 2.52 to 2.82 g/cc (mean = 2.69 ± 0.02 g/cc), whereas the grain densities range from 2.64 to 2.98 g/cc (mean = 2.85 ± 0.02 g/cc).
The relationships among these properties are summarized in Figure F15. Except for two samples that have particularly high porosities, wet bulk densities decrease markedly with increasing porosity. We also observe a marked decrease in grain density with increasing porosity and a strong increase in wet bulk density with increasing grain density. Lower grain densities are likely to reflect the abundance of low-temperature, low-density alteration products, such as clay minerals, in the samples. Hence, taken together, these relationships suggest that higher porosities are associated with higher permeabilities, which in turn lead to higher degrees of hydrous alteration. Velocities decrease with increasing porosity and with decreasing grain density. Thus, if the grain density is a function of alteration, the seismic velocities in these samples reflect the combined effects of porosity (cracks) and alteration on the properties of the rocks.
Paleomagnetic measurements appear to indicate that the basaltic cores recovered from Hole 1243B record a stable component of magnetization with both normal and reversed inclinations after removal of the pervasive drilling-induced remagnetization. Shipboard alternating-field (AF) and thermal demagnetization studies indicate that even at the highest AF demagnetization level (up to 70 mT) not all of the drilling-induced magnetization was removed and that thermal demagnetization is generally more effective for removing this component. Preliminary data from isothermal remanent magnetization acquisition experiments, unblocking temperatures, and coercivity determinations suggest that magnetite and titanomagnetite are the most likely magnetic carriers in these cores. The lava sequence recovered at Site 1243 may have recorded a reversal sequence (normal-reversed-normal). These hypotheses will be tested in subsequent shore-based investigations.
Logging results in Hole 1243B clearly show the sediment/basement interface (Fig. F16). Compared to the sediment section above, within the basement the triple combo suite shows high resistivity and density but only a small increase in the gamma ray spectrum down to ~140 mbsf. At 140 mbsf, the gamma ray energy increases substantially. This corresponds to both the top of lithologic Unit 4 and the level at which we first encountered drilling problems. Below 140 mbsf, the gamma ray values remain high and resistivity also stays high down to 155 mbsf (approximately the top of lithologic Unit 5). Below 155 mbsf, resistivity drops somewhat, but relatively high gamma ray values persist down to the bottom of the logged interval. It is clear from the logging results that there are significant differences between the upper 30 m of basement and the rocks below.
The WST was used to conduct a check shotVSP seismic survey through the basement section in Hole 1243B. Data were recorded at eight stations; except for the interval between stations seven and eight (at the top of the basement section), the stations were located 10 m apart and 516 shots were stacked at each station to improve the signal-to-noise ratio.
Results are shown in Figure F17, which shows good agreement between the velocities in the laboratory samples, the sonic log, and the well seismic data. The laboratory velocities are, on average, slightly higher (5.26 km/s) than the velocities recorded by the sonic log, which average 4.72 km/s. The difference between the laboratory velocities and the sonic log probably reflects the presence of cracks in the formation that are not present in the laboratory samples. The WST interval velocities (average = 4.60 km/s) are slightly lower than the sonic log velocities. This difference could result from the presence of large-scale cracks affecting the seismic measurements but not the sonic log, which measures a smaller sample of the rock.
The FMS logs are still being analyzed and will be finalized postcruise.
The inclination log of Hole 1243A shows that the hole is within 1° of vertical throughout its entire depth. The cement-bond log indicated a good bond in the lowest 40 m of the hole but essentially no bonding above that level, suggesting that the cement was lost into cavities and the generally porous formation.
The downhole caliper logs from Hole 1243B indicate lithologic boundaries and poor hole conditions associated with at least three incidents of lost rotation and circulation (pack offs) during drilling (Fig. F18). These occurred at depths of 4010 mbsf (125 mbsf), 4027 mbsf (142 mbsf), and 4043 mbrf (158 mbsf). The lowest pack off depth corresponds to the same depth (in meters below the sediment/basement interface) at which a pack off was encountered when drilling Hole 1243A. This suggests that the large volume of cement used to complete casing Hole1243A and the relatively low height the cement rose during injection may be due to infilling of the same highly fractured basaltic unit revealed in the logs from Hole 1243B and in core materials recovered from that hole.
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