Previous   |   Next

doi:10.2204/iodp.proc.301.206.2009

Expedition 301 drilling, coring, and logging

Site U1301 is 1 km south-southwest of Site 1026, where sediment thickness is 260–265 m above a buried basement high (Fig. F1). Hole U1301A was drilled without coring to 370 meters below seafloor (mbsf) (107 m subbasement [msb]). Casing was extended into the upper 15 m of basement, but poor hole conditions prevented installation of longer casing, coring, or deeper drilling. The large diameter and poor hole conditions also prevented geophysical logging in basement. Hole U1301B is 36 m away and penetrated to a total depth of 583 mbsf (318 msb). Uppermost basement was drilled without coring, and casing was installed to 85 msb. Basement was cored from 86 to 318 msb, with mean recovery of 30%, a value typical for upper basement rocks from young crust. The upper 100 m of the cored interval in Hole U1301B was irregular in diameter, often much larger than the maximum inflation diameter of packers to be used for hydrogeologic testing and CORK observatories. However, the lower 100 m of the hole was stable and to gauge, allowing collection of high-quality wireline logs and providing several horizons suitable for setting drill string and CORK casing packers. Relatively few basement holes have been drilled, cored, and logged to depths greater than 318 msb during four decades of scientific ocean drilling, and Hole U1301B penetrated younger seafloor than did other normal crustal holes of similar depth, so comparison with results from other sites is useful.

Basement rocks recovered from Hole U1301B consist mainly of aphyric to highly phyric pillow basalt, massive basalt, and basalt-hyaloclastite breccia, with pillow basalt being the most abundant (see the "Site U1301" chapter) (Fig. F3). The pillows have dominantly hypocrystalline textures with a glassy to microcrystalline groundmass and are sparsely to highly plagioclase ± clinopyroxene ± olivine phyric. Massive basalt consists of continuous sections comprising ≤4.5 m of similar lithology. The massive basalt is similar chemically to the sparsely phyric pillow basalt, and both have a bulk chemistry typical of normal mid-ocean-ridge basalt. They are sparsely to highly vesicular, with an average of 1%–5% round gas vesicles (up to 15% in some samples), ≤3 mm in diameter. Basalt-hyaloclastite breccia is represented by five core samples. Basement rocks are mostly slightly to moderately altered with secondary minerals that fill or line vesicles, fractures, veins, or cavities. Additional alteration minerals replace phenocrysts or replace interstitial mesostasis and glass. Alteration varies between 5% and 25% in massive and pillow basalts and is as high as 60% in basalt-hyaloclastite breccia. Principal secondary minerals are saponite, celadonite, and iron-oxyhydroxide, with minor carbonate, pyrite, and zeolite (see the "Site U1301" chapter).

Wireline logs collected from Hole U1301B included conventional caliper, natural gamma ray, bulk density, porosity, resistivity, and P-wave velocity tool runs. Logging data help to define the lithostratigraphy of Hole U1301B and establish whether or not conditions are as expected, in comparison to measurements made on core samples and borehole and core data collected from other basement holes (Bartetzko and Fisher, 2008; Tsuji and Iturrino, 2008). Logging data show that upper basement around Hole U1301B is highly layered at a scale of meters to tens of meters, similar to what is observed in other ocean crustal holes (e.g., Bartetzko et al., 2001; Jarrard et al., 2003; Pezard et al., 1992). Rapid penetration during drilling and poor hole conditions during casing operations suggest that the upper 85 m of basement is highly brecciated and poorly cemented. Irregular hole conditions continue to ~470 mbsf (~200 msb), below which the borehole diameter is consistent with the drill bit diameter. Wireline measurements of bulk density, P-wave velocity, and resistivity tend to be less than or equal to values determined from Hole U1301B core samples (Bartetzko and Fisher, 2008).

A comparison of wireline data from a global compilation of crustal holes shows that bulk density, P-wave velocity, and electrical resistivity tend to increase with age, whereas core data decrease in bulk density and P-wave velocity with age (there are insufficient resistivity data from core samples for a meaningful evaluation of global age trends) (Bartetzko and Fisher, 2008). These changes in physical properties with crustal age are thought to result mainly from water-rock interaction during off-axis hydrothermal circulation. Circulating fluid seals voids and cracks with secondary minerals, leading to an increase in P-wave velocity, bulk density, and electrical resistivity at a large scale due to the decrease in porosity. Bulk density and P-wave velocity decrease at the scale of core samples because basalt alteration increases intergranular porosity and replacement of the original mineralogy by secondary minerals.

However, Hole U1301B bulk density and P-wave velocity core data fall below global trends—it is as if core samples from Hole U1301B came from much older seafloor, closer to 100 Ma based on measured values (Bartetzko and Fisher, 2008). Basaltic grain densities are also low, and formation factor values (an indication of pore tortuosity) are high. These anomalous core-scale physical properties may result from the particular hydrogeologic (and resulting thermal and chemical) history of Site U1301. Site U1301 is on a local buried basement high that was likely exposed at the seafloor and/or extensively cooled by rapidly flowing hydrothermal fluid until the late Pleistocene (Hutnak and Fisher, 2007). Until this time, the rate and extent of basement alteration would have been limited. A more recent phase of higher temperature fluid circulation with more restricted and less oxidative conditions followed after sediment buried nearby basement outcrops and greatly slowed the rate of fluid exchange between the crust and ocean. Anomalous core-scale properties may indicate stronger alteration and more enhanced replacement of original minerals by secondary phases at a microscale during the most recent period of alteration. In contrast, macroscale properties are more consistent with the preceding long period of low-temperature circulation.

Top of page   |   Previous   |   Next