The primary objective of Leg 200 was to prepare a borehole in basaltic crust for the installation of a broadband borehole seismometer that will be connected to the Hawaii-2 Observatory (H2O) for continuous, real-time data transmission to the University of Hawaii. From Hawaii the data will be made available to seismologists worldwide through the IRIS (Incorporated Research Institutions for Seismology) Data Management Center in Seattle.
Proposed Site H2O-5 (27°53.363´N, 141°58.758´W) (Fig. F1) was selected for the seismometer installation. This site is 1.48 km northeast (a bearing of 056°) of the H2O junction box location. The bearing was chosen so that regional earthquake events from the Island of Hawaii would be on the same great circle path to both the shallow buried seismometer at the junction box and to the borehole seismometer to be installed at Site 1224. The range was chosen as a compromise between being sufficiently far away to not disturb other experiments at the junction box but still close enough to conveniently run a cable from the borehole to the junction box. The bathymetry slopes smoothly downward ~6 m from the junction box to Site 1224, and the two sites appear to be on the same crustal block in a relatively flat abyssal plain environment.
Preliminary drilling and coring at the site showed that the sedimentary layer above basaltic basement was much thinner (only 28-30 m) than estimated from precruise seismic surveys. This meant that there was little sediment to support the bottom-hole assembly (BHA) for drilling into basement. The drillers needed to take more care and started the hole in basement with less weight on the bit than they would otherwise use. The sediment was sufficiently thin (~30 m) that the basement contact could occasionally be imaged on 3.5-kHz echo sounding records (Fig. F2). All H2O drilling activities took place within a 20 m x 20 m area at the site (Fig. F3).
Hole 1224D (27°53.370´N, 141°58.753´W; 4967 m water depth) has a reentry cone and 58.5 m of 10.75-in casing, which was cemented into a 30-m-thick well-consolidated massive basalt flow underlying 28 m of soft, red clay (Figs. F4, F5). After cementing, the hole was washed to a depth of 5036 meters below rig floor (mbrf) (58 meters below seafloor [mbsf] or 30 m into basaltic basement). On reentering for the wiper trip, we noticed that the cone had settled ~1.7 m into the sediment.
After setting the reentry cone and casing in Hole 1224D, we drilled a single-bit hole, 1224F, to 174.5 mbsf to acquire sediment and basalt samples for shipboard and shore-based analysis as well as to run a logging program. Hole 1224F is <20 m to the southeast of Hole 1224D, and measurements in Hole 1224F can be used to infer the structure surrounding the seismometer hole. We dropped a free-fall funnel (FFF) in Hole 1224F so that future borehole experiments using wireline reentry technology can be conducted (Figs. F6, F7). For example, this would be a good site to compare measurements in a sealed hole in basement (1224D) to measurements in an open hole in basement (1224F).
The top 6.53 m of Site 1224 (Hole 1224C) is massive brown clay that gradually changes color to very dark brown. Radiolarian spicules are present throughout the section but increase with depth and are common at the bottom of the unit. Sponge spicules are not found near the top of the section but are common below 4.50 m. Core recovery from Holes 1224A and 1224B was not significant enough to characterize the sediments. One significant discovery, however, was the recovery of light-colored noneffervescing granules and pebbles from a depth of between 6 and 15.6 mbsf from Hole 1224A. These are fossil worm burrows made of zeolite. The total sediment depth at Site 1224 is 28 m.
Paleontological analysis of calcareous nannofossils indicates that essentially the whole sedimentary sequence was deposited within a few million years of the crustal age of ~46 Ma (on the transition between marine magnetic Anomalies 20R and 21N).
The lithostratigraphy of basalts at Site 1224 is divided into three units. Unit 3, the deepest, is intermixed pillows and flows of no more than a few meters thickness each. At least two, and probably more, eruptive events are represented. Overlying Unit 3, Unit 2 is a succession of thin flows and pillows. Chemical analyses of these rocks are very similar, indicating that they are of one eruptive composition. Two thick lava flows from Unit 1 cap the underlying units. The lower of these has the same composition as the basalts of Unit 2. These two flows may have accumulated in a structural depression or pond that formed, probably by faulting, after eruption of the last thin flows or pillows of Unit 2.
Apart from the interiors of the massive flows, the lava sequence was pervasively altered under conditions with variable oxygen fugacity. Hydrous fluids carrying dissolved metals flowed through cracks, cavities, and fractures in the formation and deposited iron oxyhydroxides and sulfide minerals in this porosity structure. The fluids penetrated several centimeters into the rock adjacent to the fractures and impregnated microfractures with the same material that was deposited in the larger veins. Later, carbonate-saturated fluids coursed through the same fractures depositing calcite. Except for the interiors of the two upper massive lavas, most of the rock was at least partially transformed to secondary minerals by this process. Eventually, enough calcite precipitated to cement the originally fragile iron oxyhydroxides and sulfide minerals. The calcite cementing contributed to higher core recovery of basalts above ~60 mbsf at Site 1224.
It is too early to say how warm the fluids might have been, although calcite and aragonite are the ideal minerals to use for oxygen isotope determinations and to estimate temperatures for the cementation portion of these processes. Iron oxyhydroxides are a principal component of hydrothermal sediments deposited on volcanically active ridge axes near, but not at, high-temperature vents. Elsewhere on the flanks of the East Pacific Rise, basalt coring has not been successful in crust as old as Miocene, largely because of the absence of calcite vein cement. Thus, the carbonate-lined veins in the Eocene rocks at Site 1224 may be evidence for sustained fluid flow at low temperature and far off axis. There is only a thin layer of sediment at Site 1224, insufficient to seal off fluids circulating in the crust. Interaction of those fluids with oxygenated bottom water may be why most of the section cored exhibits mainly oxidative alteration. The exception to this is the massive basalts at the top. In those, fractures may have been so few and widely spaced that fluid flow was restricted. The oxygen fugacity of the small quantity of fluids moving along them was consequently reduced by reaction with adjacent wall rock, allowing pyrite to precipitate.
Thin section examination of volcanic basement at Site 1224 (Holes 1224A, 1224D, 1224E, and 1224F) evidenced a relatively homogeneous mineral paragenesis. The main phases are plagioclase, clinopyroxene, opaque minerals, and rare pigeonite; therefore, the rocks can be classified as tholeiitic basalts. Olivine is rare, and only a few small iddingsitized euhedral to anhedral groundmass crystals were found. Iddingsite is a typical alteration of olivine and is composed of a mixture of goethite and layer silicates (e.g., smectite). The majority of the basalt is holocrystalline (almost 100% crystals) to hypocrystalline (glass concentration <50%) and can be ascribed to lava flows. With increasing depth of coring, hypohyaline textures and volcanic glass contents >90% become common and indicate the presence of pillow fragments with chilled margins. The deepest samples recovered (~153 mbsf) also show textural features of holocrystalline massive lava flows. With regard to their granularity, the basalts range from aphanitic (difficult to distinguish the crystals in the groundmass with the naked eye) to aphyric (absence of phenocrysts), though rare plagioclase or plagioclase-clinopyroxene sparsely phyric basalts (phenocryst content <2%) have been also found. The relative size of the crystals in the groundmass is equigranular, and their distribution is isotropic. The groundmass is hypidiomorphic with the presence of euhedral- to anhedral-shaped crystals. The texture of the massive lava flow basalts is intergranular (with clinopyroxene in interstitial relationships with plagioclase) to subophitic (with plagioclase laths partially enclosed in clinopyroxene) and, more rarely, intersertal (with microcrystalline to glassy material between plagioclase). Hyalopilitic (with plagioclase laths and clinopyroxene crystals in a glassy matrix) to, more rarely, intersertal textures have been found in the pillow lavas. The grain size of the groundmass ranges from very fine grained (0.001-0.5 mm) to fine grained (0.5-1 mm).
Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) data for basalts were obtained late during Leg 200. Only partial analyses are available for plotting on samples obtained from Holes 1224D, 1224E, and 1224F. The discussion here is based on contents of K2O, TiO2, MgO, Ba, Zr, and loss on ignition (LOI) from the ICP-AES data.
The basalts range from differentiated to very differentiated normal mid-ocean-ridge basalt (N-MORB) (4-7 wt% MgO and 2-3.5 wt% TiO2). The highly differentiated (4 wt% MgO and 3.5 wt% TiO2) basalts plot at the most differentiated end of the N-MORB data array. The samples selected for analysis from Hole 1224D are fairly fresh and have LOI values ranging from 0 to 0.45 wt%. Concentrations of K2O (0.11-0.27 wt%) may be slightly elevated (>0.2 wt%) in three of the ten samples analyzed. All the basalts have Ba concentrations (9-18 ppm) that are consistently lower than many comparably differentiated MORB glasses from the East Pacific Rise. This may indicate a greater than average depletion of the mantle sources in the basalts from Hole 1224D. Alternatively, the rocks may have experienced a slight nonoxidative alteration as these components were partially removed from the rock.
The samples from Hole 1224D can be divided into two groups based on their TiO2 concentrations. These geochemical groups correspond to (1) the lithologic Unit 1 upper flow (>2.3 wt% TiO2) and (2) the lithologic Unit 1 lower flow (<2.3 wt% TiO2). Interestingly, lithologic Unit 2 pillow lavas have TiO2 concentrations similar to the lower flow basalts from Unit 1. The lithologic Unit 3 pillow lavas have TiO2 concentrations >3.1 wt%. Therefore, the geochemical divisions (based on TiO2 contents) do not correlate directly with the lithologic units.
The topmost flow in Unit 1 was also sampled in Holes 1224E and 1224F, and the lower flow in Unit 1 was also sampled in Hole 1224F. Samples of lithologic Unit 2 have TiO2 contents similar to those of the lower flow of Unit 1. Apparently accumulation of this chemically uniform basalt at the site began with thin flows. These were capped with a thick flow of the same material. The samples of Holes 1224E and 1224F from Unit 2, however, are more greatly altered than those of the capping flow at the base of Unit 1, usually having as much as two to three times the amount of K2O present in samples of Hole 1224D and in one case having >1 wt% K2O. This is in accordance with the strong contrast in extent of alteration noted between holes only 15 m apart in the core descriptions.
Samples of lithologic Unit 3 have TiO2 contents greater than in any of the basalts from the thick flows or pillows of lithologic Units 1 and 2. Based on TiO2 contents, these are among the most differentiated basalts sampled thus far from the flanks or axis of the East Pacific Rise. Their compositions are at or about the point where oxide minerals join the liquidus, causing TiO2 contents to drop as MgO decreases and producing andesitic and ultimately rhyodacitic residual liquids.
Secondary minerals found in altered layers and fractures at Site 1224 were analyzed by X-ray diffraction (XRD). XRD analysis was carried out on one clayey pebble from the sediment and twenty-five vein materials within the basalt. Five distinct vein types were documented by XRD analysis: clay, carbonate, zeolite, quartz, and calcite/smectite (Fig. F48) (see "X-Ray Diffraction Investigation of Secondary Minerals" in "Geochemistry"). Many vein minerals in the basement at Site 1224 are stable at low temperature and pressure (i.e., zeolite). Phillipsite, the principal zeolite present at Site 1224, is a low-temperature member of the zeolite group (Miyashiro, 1973). Smectite is also commonly found as a product of the alteration of volcanic ash and rocks from the seafloor and is present in most of the low-grade metamorphic terranes in the world.
Four of the vein types observed at Site 1224—clay (smectite-illite), carbonate (calcite-aragonite), quartz, and zeolite—are similar to veins observed at Sites 896 and 504 near the Costa Rica Rift (Alt, Kinoshita, Stokking et al., 1993). These minerals are present in relatively lower temperature hydrothermal assemblages (probably <100°C) at these sites (Laverne et al., 1996). Truly high-temperature vein assemblages, such as the actinolite and epidote veins found deeper than 2000 mbsf at Site 504, were not found at Site 1224. The mineral laumontite in the illite veins indicates a high-temperature zeolite facies assemblage (Miyashiro, 1973). Aragonite generally forms at a higher temperature than calcite. These minerals indicate the local influence of warm hydrothermal fluids.
Tartarotti et al. (1996) considered the sequence of secondary mineralization in open fractures based on studies of Hole 896A in the Costa Rica Rift. They suggested a general sequence of vein formation from oldest to youngest as follows:
Visual and macroscope observations together with XRD analysis of veins suggest that this sequence also is present at Site 1224 (e.g., many thick calcite veins have distinct inner calcite and outer green smectite zones; fibrous smectite is present only in thick veins; many quartz veins have associated outer green smectite; and most vein surfaces of host rocks show greenish [smectite] or black [oxyhydroxide] colors).
Tartarotti et al. (1996) characterized clay-lined veins at Site 896 into two types: nonfibrous and fibrous veins. Nonfibrous veins are thought to represent fractures filled by minerals crystallizing in open cavities where fluid-filled spaces were available for crystal growth. Fibrous veins are interpreted as crack-seal veins in which narrow cracks propagated and then were cemented. According to this model, thin and deposit-poor veins record hydrothermal activity, whereas thick crystallized veins reflect hydrothermal activity together with shear stress in the flow unit. Veins are thicker in pillows than massive flows. Perhaps this is because pillows are more fractured, sheared, and therefore more permeable to hydrothermal fluids than massive flows.
Many vein minerals in basement at Site 1224 are stable at low temperature and pressure conditions (e.g., zeolite). Phillipsite, the main zeolite present at Site 1224, is a low-temperature member of the zeolite group (e.g., Miyashiro, 1973). Smectite is also commonly found as a product of the alteration of volcanic ashes and rocks from the seafloor. Smectite is present in most low-grade metamorphic terranes around the world.
We used progressive alternating-field (AF) demagnetization of archive-half sections, one whole-core section, one working-half section, and discrete samples to characterize the paleomagnetic signal and resolve the magnetization components recorded in the recovered core. An unambiguous magnetostratigraphy could not be obtained from the only undisturbed core (Core 200-1224C-1H) that was recovered in the sedimentary section; the other sediment cores were extremely disturbed by drilling. Based on postcruise analysis of calcareous nannofossils in the sediments, it appears that nearly the entire sedimentary section is middle Eocene in age. During the cruise, we only had time for a cursory interpretation of the magnetization of the basaltic units, although fairly detailed demagnetization experiments were conducted on split cores and discrete samples.
Given that 58 m of basalt core was recovered (see "Igneous Petrology" in "Lithology") that may span enough time to average geomagnetic secular variation, the magnetization of the basalts should provide a valuable paleolatitude estimate for the Pacific plate at ~45 Ma. This age corresponds to the Pacific plate's abrupt change in motion relative to the hotspots as marked by the kink in the Hawaiian-Emperor hotspot track. A cusp in the Pacific plate apparent polar wander path (APWP) may also occur at this age, marking a change in the motion of the Pacific plate relative to the spin axis. The Pacific APWP and hotspot tracks together provide key constraints on estimates of the size of motions between hotspots, ultimately extending our understanding of mantle dynamics (Acton and Gordon, 1994). Additionally, the age also lies within the period (39-57 Ma) when the Hawaiian hotspot has been shown to have moved rapidly southward relative to the spin axis (Petronotis et al., 1994). If geomagnetic secular variation has been averaged by the basalt units and if secondary overprints caused by alteration do not mask the primary magnetization, then we should be able to obtain an accurate paleolatitude. Finally, rock magnetic studies of the basalts should help refine our understanding of the magnetization of the upper oceanic crust and its role in generating lineated marine magnetic anomalies.
Samples of different sediment types and samples from basaltic rock were collected at Site 1224 for aerobic and anaerobic cultivation, for deoxyribonucleic acid (DNA) extraction and analysis, for phylogenetic characterization, for total cell counts, and for determination of the live/dead ratio of indigenous microbial communities. Sediment suspensions and ground basalt material were used under oxygen depleted conditions in the anaerobic chamber for the establishment of enrichment cultures. Aerobic cultivation was carried out using both seawater-based media and commercial Zobell's medium (Difco). Anaerobic cultures were based on reduced mineral media.
To evaluate the microbial background at Site 1224, ambient seawater samples were collected at 1 m below sea surface upwind of the JOIDES Resolution. The microscopically enumerated total cell counts in the surface water at Site 1224 were 1.4 x 104 cells/mL.
Sediment samples from Holes 1224C, 1224D, and 1224E were obtained from different depths ranging from the near-surface layer down to 24.9 mbsf. Bacteria were present in all sediment samples taken to 24.9 mbsf.
The amount of active bacteria was assessed in two representative sediment samples taken from the near-surface layer (Sample 200-1224C-1H-1, 0-5 cm) and from a depth of 25 mbsf (Sample 200-1224E-2R-5, 143-150 cm). As indicated by fluorescent signals after hybridization with the Bacteria-specific probe EUB338, the amount of metabolically active bacteria ranged in these sediment layers from 62% to 41% of the total cell counts, respectively.
Microscopic investigation of thin sections revealed the first survey of the presence of eukaryotic microorganisms that are counted among the kingdom of fungi within basement of the North Pacific Ocean. Hyphae have been found within cavities, small fractures, and veins filled with CaCO3. These fungal structures were viewed by transmitted light microscopy, in which they appear with a brownish tinge. A net of fungal hyphae shown in Figure F63 filled the complete space spanning from the basalt/calcite boundary to the center of the cavity (see "Microbiology"). The cross-sectional dimension of the hyphal network is 5-10 µm with a length ranging from 50 to several hundred micrometers. The hyphae are typically interrupted at irregular intervals by cross-walls, so-called septa, which divide the entire fungal hyphae into single distinctive cells. Our results provide strong evidence for eukaryotic life in addition to bacteria in deep subsurface environments.
In Hole 1224A, P-wave velocities of aphyric basalt from Cores 200-1224A-5X and 6N are ~5900 and ~5800 m/s, respectively.
In Hole 1224C, the gamma ray attenuation (GRA) densities of sediments gradually decrease with increasing depth between 0 and 6.4 mbsf, corresponding to a color change from light brown to dark brown. Similarly, we observed an unusual trend for bulk and dry densities in Hole 1224C that decreases from ~1.52 to ~1.36 g/cm3 and from ~0.8 to 0.54 g/cm3, respectively. Porosities in Hole 1224C gradually increase from ~71% to ~80%. P-wave velocities (as measured with the P-wave logger [PWL]), however, show a small increase from ~1460 to ~1500 m/s with depth between 0 and 6.4 mbsf. P-wave velocities (as measured with the P-wave velocity sensor [PWS] contact probe) between Cores 200-1224C-1H and 4H (between 0 and ~5.70 mbsf) range from ~1525 to ~1535 m/s. The P-wave velocity in Core 200-1224C-5H is ~1555 m/s, which is greater than other sections. Grain densities for Hole 1224C show a small increase from 2.782 to 2.831 g/cm3 for depths shallower than ~2 mbsf. Between ~2 and ~6 mbsf, grain densities remain fairly constant—between ~2.70 and ~2.74 g/cm3.
In Core 200-1224D-2R, bulk and dry densities increase from 2.7 to 2.9 g/cm3 and 2.6 to 2.8 g/cm3, respectively. In Core 200-1224D-3R, bulk and dry densities decrease from 2.9 to 2.8 g/cm3 and from 2.8 to 2.7 g/cm3, respectively. In Cores 200-1224D-4R and 5R, they also decrease from 2.85 to 2.80 g/cm3 and from 2.8 to 2.7 g/cm3, respectively. Porosities remain at low values and range from 4% to 9%. PWS velocities range from 4200 to 6500 m/s. Compressional wave velocity anisotropy for each sample is ~2%-10%. PWS velocities have a sinusoidal depth variation. They decrease between 25 and 35 mbsf, increase between 35 and 45 mbsf, and decrease again between 45 and 55 mbsf. This sinusoidal depth variation is also identified for Hole 1224F.
Between 25 and 60 mbsf, PWS velocities in Holes 1224E and 1224F have a similar trend to Hole 1224D. PWS velocities have a strong depth dependence. Compressional velocities separate into seven depth zones (Fig. F71) (see "Hole 1224D" in "Physical Properties"):
Zones 1-3 may be characterized as rather uniform basalt flow zones with a thin low velocity (fractured) layer. Zone 4 has a slightly lower velocity than the zones above and below it. Velocities of zone 5 are higher than those for zones 4 and 6. Zone 6 is highly fractured and characterized by the lowest velocities. Zone 7 corresponds to more uniform basalt layers.
P-wave velocities have a scattered correlation with increasing bulk density. Compressional wave velocity vs. porosity, however, has a good inverse correlation, as P-wave velocity decreases with increasing porosity. These two relations imply that compressional velocities are not controlled by bulk densities, but are well controlled by porosities. Large porosities are associated with more fractured zones. If this is true, zones 2 and 6 are intensively fractured.
A long-standing problem in the red clay province of the eastern Pacific Ocean is how to adequately resolve chert layers and basement in the presence of sediments <50 m thick. By lowering a battery-powered, free-running 3.5-kHz pinger to the seafloor on the vibration isolated television (VIT) sled and recording the pulse on the ship's 3.5-kHz acquisition system, we hoped to increase the sound level incident on the seafloor, improve the penetration into the subbottom, reduce the footprint of the sound on the seafloor, and increase the received signal levels. The deep-source 3.5-kHz experiment was conducted whenever the VIT camera was lowered to the seafloor either for reconnaissance surveys or reentries.
Examination of the deep-source 3.5-kHz records shows two prominent reflections at 13 and 38 ms below the seafloor. Depending on the sound velocities in the seabed, these reflectors would be 10-13 m and 28-38 m deep, respectively. The continuity of these reflectors varies with time throughout the survey, although the ship moved only a few meters. The first subseafloor reflection at 13 ms (10-13 m) is the most continuous. Another faint reflector was identified just above 30 ms (22-30 m). This may indicate a local heterogeneity or the irregular surface of pillow lava flows.
Recording during significant portions of the seafloor survey was degraded by the wash from thruster 3 across the ship's hull-mounted 3.5-kHz transducer pod. The wash increased as the ship responded to gradually increasing wind on the port side. The battery-powered pinger ran for the entire 16 hr of the lowering.
Our preliminary interpretation had been that the 13-ms reflection is present at an intermittent chert layer. The first jet-in test stopped abruptly at 13 m. Although chert layers within the sediments have been encountered at other drill sites in the eastern Pacific Ocean, nowhere at Site 1224 did we sample chert. Basalt cores were regularly acquired at 28-30 m depth, corresponding to the 38-ms reflector.
In summary, the deep-source 3.5-kHz experiment identified a second reflector at 38 ms below the seafloor that was neither observed in the traditional 3.5-kHz survey conducted in 1997 nor in the shipboard 3.5-kHz survey acquired while coming on site (Fig. F2). This deeper reflector, however, was observed beneath the junction box. It was shown by drilling that this is the sediment/basalt contact. Data from the deep-source 3.5-kHz experiment were digitized and are available for postcruise processing.
Based on shipboard preliminary log analysis at this site during Leg 200, we conclude that basement in Hole 1224F consists of at least five distinctive units (see Fig. F80 and the discussion in "Lithostratigraphy" in "Downhole Measurements") with unit contacts at roughly 45, 63, 103, and 142 mbsf. These layered formations can be distinguished using the continuous electrical resistivity, density, sonic, neutron porosity, magnetic field, and possibly spectral gamma ray logs. The existence of a conduit or large-scale fracture between 138 and 142 mbsf was detected by all the log tools including the temperature tool. In addition, the temperature tool reveals that the "warmer" fluid had a temperature of 4.6°C at the time of the logging. The vicinity of this conduit is much more highly altered than other rocks penetrated by the hole, as indicated by the gamma ray logs. Because of the relative position of the tools located in the tool strings, some tools can resolve the top logged intervals like gamma ray, porosity, density, and sonic logs. On the other hand, the resistivity tools and Formation MicroScanner (FMS) placed at the bottom of the tool string can resolve the formation properties near the bottom of the hole. The values of the magnetic fields calculated from the three-component inclinometer tool are invalid near the bottom of the pipe (~35 mbsf). In the logged intervals where all the tools overlapped, they provide consistent information to support the layered structural units based on these geophysical properties.
Drilling at the H2O provides a unique opportunity to observe drilling-related noise from the JOIDES Resolution on a seafloor seismometer in the frequency band 0.1-80 Hz. See the "Leg 200 Summary" chapter for background material on the H2O and the retired American Telephone and Telegraph (AT&T) oceanic cable that is used to provide continuous, real-time data transmission back to the Makaha cable station on Oahu.
The University of Hawaii operates a Guralp CMG-3T three-component broadband seafloor seismometer and a conventional three-axis geophone at the H2O. Data are acquired continuously and are made available to scientists worldwide through the IRIS Data Management Center in Seattle. During the cruise, Jim Jolly and Fred Duennebier at the University of Hawaii relayed sample data files to the JOIDES Resolution by file transfer protocol (FTP) over the shipboard telephone. We were then able to process data and study correlations with on-site activities and weather. The University of Hawaii also maintained a World Wide Web site showing seismic data from the H2O during the cruise (www.soest.Hawaii.edu/H2O/).
Seismic signal level variation could be associated with wind speed, sea state, shear resonance effects in the sediments, whales, water gun shooting, earthquakes, passing ships, and drilling-related activities such as bit noise and running pipe.
Four different kinds of data were compared. All data show four to six distinct units. In contrast to the seven zones identified by physical properties (given above), logging data give five distinct units in basement: 28-45 mbsf (I), 45-63 mbsf (II), 63-103 mbsf (III), 103-142 mbsf (IV), and deeper than 142 mbsf (V). Units I and II are between 28 and 63 mbsf and are characterized by massive basalts with thin fractured zones around 40 mbsf. Unit III (63-103 mbsf) was characterized by fractured basalt layers. Many small fractures were also identified by the FMS/Dipole Sonic Imager (DSI) logging tool. The calcite veins were found in this unit. Unit IV (103-142 mbsf) is characterized by stacks of small pieces of pillow lavas. Porosities by logging data, however, show highly porous zones for this unit. This unit also indicates the presence of smectite veins. Just below Unit IV, sudden large variations are present on the caliper log, resistivity log, compressional and shear velocity logs, U and Th content, and temperature log. Physical property measurements also indicate that this unit is highly fractured. The presence of high U and Th contents suggests that this unit is a highly altered zone. In Unit V, below ~142 mbsf, basalt sheet flows are found. Although we could not drill below this depth, the single-channel seismic (SCS) data suggest that this is the top of a massive basalt unit. In comparing the above units to the SCS records, these unit boundaries extend many kilometers away from the site. With further analysis it should be possible to understand the nature of oceanic Layers 2A and 2B and their relationship to lithologic boundaries in ~45-Ma fast-spreading oceanic crust.
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 200IR-104