Leg 206 is the initial phase of a planned two-leg project to drill in situ ocean crust on the Cocos plate at Site 1256, which formed at a superfast spreading rate ~15 m.y. ago. Recent recognition of an episode of superfast-rate spreading (200-220 mm/yr) on the East Pacific Rise (EPR) at ~12-17 Ma presents an opportunity to exploit the observed relationship between the depth to axial low-velocity zones imaged at active mid-ocean ridges and spreading rate. Fast spreading rates should lead to gabbros at relatively shallow depths and provide the best chance to drill through the upper ocean crust into the plutonic rocks in a minimum of time. Even allowing for significant burial by lavas that have flowed off axis (~300 m), the upper gabbros, thought to be the frozen axial melt lens, are predicted to occur at ~1100-1300 meters below seafloor (mbsf) in this region.
To fully characterize the sedimentary overburden and establish depths for the casing strings, a series of pilot holes was cored that recovered a nearly complete section of the 250.7 m of sediment overlying basement and penetrated 88.5 m into basement with very good recovery (61.3%). The sediments can be subdivided into two main lithologies. Unit I (0-40.6 mbsf) is clay rich with a few carbonate-rich intervals, whereas Unit II (40.6-250.7 mbsf) is predominantly biogenic carbonate. At ~111-115 mbsf, biogenic silica is the most abundant phase and forms a distinct diatom mat. Above 111 mbsf, the proportion of terrigenous material increases toward the surface and is the most important component in lithologic Unit I (above 40.6 mbsf). Chert nodules are a common feature in the sediments from ~111 mbsf down to basement, and distinct chert layers at 111 and 158 mbsf were identified in the wireline logs. Poor recovery in the lowermost sediment cores indicates chert layers between 230 mbsf and basement. Red-brown iron oxide-rich silicified sediments directly overlie the basement (within 1 m), and these may be recrystallized metalliferous sediments.
The primary influence on the interstitial water chemistry at Site 1256 is diffusion between seawater and basement fluids. A continuous chert bed at 158 mbsf forms a low-diffusivity barrier that causes abrupt jumps in cation (e.g., magnesium and calcium) concentrations in many of the depth profiles. The low organic carbon content limits the extent of pore water sulfate reduction (SO42- > 17.5 mM), as seen in decreases in alkalinity in the pore water with depth.
Inspection of calcareous microfossil assemblages determined more than a dozen nannofossil datums, providing a modest biostratigraphic resolution for the Pleistocene through middle Miocene at Site 1256. These are in good agreement with the magnetostratigraphy. The paleomagnetic measurements provide higher resolution in the upper Miocene section up to the Brunhes/Matuyama reversal at 0.78 Ma, whereas the biostratigraphy provides the only age constraints in the middle Miocene section where the paleomagnetic signal was poorly resolved.
Sedimentation rates vary from ~6 to 36 m/m.y., with the rate being about four times faster in the middle Miocene than the average rate during the late Miocene to present. The lowermost linear sedimentation rate is 36.4 m/m.y. (95-212.65 mbsf), and when this rate is extrapolated to the basement at 250.7 mbsf, the mean age obtained for the basement is 14.6 Ma, consistent with the ~15-Ma age estimated from marine magnetic anomalies.
The high sedimentation rate in the middle Miocene can be attributed to the productivity being very high while the site was near the paleoequator and to complete preservation on young, shallow seafloor. The more recent slower rates can be attributed to lower productivity away from the equator and to partial dissolution after the seafloor subsided. The rapid decrease in sedimentation rate in the late Miocene, however, requires an influence in addition to the northward drift and subsidence of the Site 1256 ocean crust. A similar pattern of sedimentation rate variations was observed at nearby Sites 844 and 845, with the event that occurred at the end of the middle Miocene being referred to as a "carbonate crash" (Farrell et al., 1995; Lyle et al., 1995). Late Miocene sedimentation rates in the tropical eastern Pacific are complex in detail and must reflect changes in either productivity or the carbonate compensation depth (CCD). Similar carbonate crash events are also observed in the Caribbean, and it has been suggested that the PCO2 of ocean waters in this region may have been influenced by either the onset of North Atlantic Deep Water formation or the partial closing of the Panama Gateway (Lyle et al., 1995; Shipboard Scientific Party, 1997a; Roth et al., 2000).
Changes in physical properties are mostly gradual downhole, with the exception of those properties sensitive to the major lithologic and compositional changes that occur at the lithologic Unit I/II boundary (40.6 mbsf), at the diatom mat (111-115 mbsf), and ~40-45 m above basement (~205-210 mbsf), below which chert and glauconite increase. Density and porosity are dominated by gradual compaction, with lesser effects of grain composition. P-wave velocity is nearly uniform and low at ~1540 m/s, with a slight increase below 205 mbsf. Magnetic susceptibility is moderate in the clay-rich parts of Unit I and drops to very low in the nannofossil oozes.
Downhole temperatures were measured during advanced piston corer (APC) coring of Hole 1256B at depths of 53.6-158.1 mbsf, plus measurements were taken in the bottom water. Heat flow values average 113 mW/m2, with a slight but possibly significant decrease downhole. This value is close to the predictions for conductive cooling of oceanic lithosphere, implying that hydrothermal circulation is no longer a major mechanism of heat transport at Site 1256.
In addition to the 88.5 m of basement cored in the pilot Hole 1256C, reentry Hole 1256D was cored more than 500 m into young Pacific extrusive lavas with moderate to high rates of recovery, following the installation of a reentry cone and a 16-in-diameter casing string into basement (total depth = 752 mbsf).
The igneous rocks cored in Holes 1256C and 1256D are dominated by thin (tens of centimeters to ~3 m) basaltic sheet flows separated by chilled margins, with several massive flows (>3 m thick), minor pillow basalts and hyaloclastites, and rare small dikes. The percentage of massive flows decreases downhole in Hole 1256D. One notable feature of both holes is a very thick massive lava (~35 m thick in Hole 1256C and ~75 m thick in Hole 1256D) near the top of each hole. We interpret this unit in both holes as one continuous massive lava that probably ponded in a faulted depression on the proximal ridge flanks (~5 km).
The lavas in Holes 1256C and 1256D are dominantly aphyric to sparsely phyric, glassy to fine-grained basalt, with olivine as the dominant phenocryst phase and with lesser amounts of plagioclase and clinopyroxene. Most plagioclase phenocrysts from Hole 1256C and the upper units of Hole 1256D are unzoned or have minimal normal and/or oscillatory zoning. Zoning is more common in the lower units from Hole 1256D. The base of the thick ponded flow in Hole 1256C has an unusual texture, with recrystallized groundmass and late magmatic veins and deformation.
The lavas have normal mid-ocean-ridge basalt (N-MORB) compositions and display a general trend of moderate fractionation increasing upsection. Superimposed on this broad trend are two groups of anomalous lavas: one high-potassium group in the interior of the ponded flow in Hole 1256C and one high-Zr group of several samples located immediately below the ponded flow in Hole 1256D.
The lavas are slightly to moderately affected by low-temperature hydrothermal alteration, and very little interaction with oxidizing seawater is apparent. In most rocks, saponite and pyrite are the principal alteration minerals filling veins, rare vesicles, replacing olivine, and mesostasis, but celadonite, silica, iron oxyhydoxides, and minor calcium carbonate are also present in certain zones. Vein-related alteration is manifest as different-colored alteration halos along veins. Black halos contain celadonite and result from upwelling distal low-temperature hydrothermal fluids enriched in iron, silica, and alkalis. Iron oxyhydroxide-rich mixed halos are less common and, where present, clearly overprint the black halos. Brown halos have a similar origin and formed along fractures that were not bordered by previously formed black halos.
Overall, the basalts recovered from Site 1256 do not exhibit a general downward decrease in oxidation by seawater as is observed in those from Hole 504B and Sites 417/418. In contrast, alteration appears to have been concentrated into zones related to the lava morphology and distribution of breccias and fractures and, consequently, the influence of these on porosity and permeability.
The appearance of albite and saponite partly replacing plagioclase below 625 mbsf indicates a change in alteration conditions. This change may result in part because of slightly higher temperatures at depth or because of more evolved fluid compositions (e.g., decreased K/Na or elevated silica).
Alteration by late magmatic/hydrothermal fluids is restricted to within the massive ponded lava near the top of the section. These interactions are manifest by granophyric intergrowths of plagioclase and quartz in veins and interstitial areas, secondary green clinopyroxene reaction rims on primary augite, trace interstitial biotite and blue-green phyllosilicate (chlorite?), and partial replacement of primary calcic plagioclase by albite.
The rocks at Site 1256 are much less altered than those from Holes 504B and 896A in 6.9-Ma crust formed at an intermediate spreading rate. Alteration intensity is more akin to the background alteration at Site 801 on Jurassic crust (180 Ma) formed at a fast spreading rate, although there is much less carbonate at Site 1256.
Primary igneous structures include magmatic fabrics, laminations, flattened vesicles, folds, shear zones, and late magmatic veins. These features are mainly restricted to the massive ponded lava near the top of the basement. Some late magmatic veins show evidence of multiple episodes of folding. Shear bands, sigmoidal pull-aparts, and tension gashes that cut these folds indicate a progressive transition from predominantly ductile to brittle-ductile deformation.
Five different types of breccia were described from the basement at Site 1256: hyaloclastite, talus breccia, breccia with interflow sediment, incipient brecciation, and tectonic breccia. Some core intervals show evidence of incipient brecciation associated with the progressive development of anastomosing vein networks.
Veins are the most prominent structural features observed in rocks recovered from Holes 1256C and 1256D. Veins mostly have a planar or slightly curved morphology, commonly with irregular margins. Individual veins generally branch into a number of diverging splays at their ends. Multiple veins commonly occur in anastomosing geometries and, where veining is pervasive, develop into to vein networks. In many cases, veins are oriented in en echelon Riedel shear arrays. Stepped veins are common in both basement holes and are locally characterized by millimeter-scale pull-aparts filled with secondary minerals. Some veins form conjugate sets, mostly concentrated in the coarser-grained massive flows of both Holes 1256C and 1256D.
Shear veins occur mostly in massive coarser-grained lithologic units and are filled with fibrous clay minerals. Microfaults are restricted to the interval 289.9-331.90 mbsf and have thin bands of cataclasite and fibrous minerals. Shear veins and microfaults indicate both strike-slip and oblique apparent senses of shear, but in Hole 1256D shear veins show a change in the sense of shear, from reversed to normal, starting from ~645 mbsf to the bottom of the hole.
In general, structures of Hole 1256D mostly have gentle true dips (~15°), although dips of ~70° are common in the lower 100 m of core. In Hole 1256C, veins occur in conjugate systems and the distribution of true dips is bimodally distributed in sets making an angle of 50°-60°.
Basalt samples from Site 1256 show a strong tendency to have been partially or fully remagnetized during drilling, much more so than for most other Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) sites. In most cases, a pre-overprint component can be discerned, if not always measured accurately, with the shipboard equipment. For Hole 1256D, deeper samples demagnetize to a shallow inclination, as expected for the equatorial paleolatitude. For Hole 1256C, a few samples show evidence for a stable, steep (inclination > 70°) component distinct from the overprint. The steep inclination may reflect eruption during the magnetic polarity transition between Chrons 5Br and 5Bn, which would imply transport of these lavas at least ~5 km from the ridge axis because Site 1256 lies ~5 km east of the Anomaly 5Bn-5Br transition. The apparent shared direction for multiple units from Hole 1256C, if confirmed by shore-based studies, suggests a maximum time interval on the order of centuries for erupting these geochemically similar, but not identical, lavas. If our interpretation is correct, ~20% of the extrusive sequence cored so far formed from lava flows that flowed significant distances from the axis.
The drilling overprint is sufficiently strong for most of the recovered samples that it is not yet possible to make a quantitative assessment of the contribution of the cored section to the magnetic anomalies measured at the sea surface. Careful integration of the sample measurements with downhole measurements of the magnetic field will offer the best opportunity to test the common interpretation that the extrusive layer contributes most of source of marine magnetic anomalies.
The basalts of Site 1256D have bulk densities ranging from 2.55 to 2.98 g/cm3 (average = 2.83 g/cm3). The basalts have generally low porosities, ranging from ~2% to 6%, with higher porosity exhibited by the pillow basalts. The physical property parameters of density, porosity, velocity, natural gamma radiation, magnetic susceptibility, and thermal conductivity vary systematically downhole and correspond to the igneous units and eruptive style. Increasing bulk density is well correlated with increasing velocity, and bulk density and velocity are inversely related to porosity.
A complete suite of geophysical wireline logs, including the first deployment in a basement hole of the Ultrasonic Borehole Imager (UBI), confirmed that Hole 1256D is in excellent condition, with robust margins and within gauge for its complete depth. Formation MicroScanner (FMS) and UBI imaging will be integrated with other geophysical logs and the recovered core to refine the igneous stratigraphy.
Hole 1256D was exited cleanly, leaving the hole clear of junk and open to its full depth, with the wellhead infrastructure and casing prepared for future deepening into the sheeted dikes and gabbros early in the next phase of ocean drilling.
1Examples of how to reference the whole or part of this volume can be found under "Citations" in the preliminary pages of the volume.
2Shipboard Scientific Party addresses can be found under "Shipboard Scientific Party" in the preliminary pages of the volume.
Ms 206IR-103