CORE, PHYSICAL PROPERTIES, LOGGING, AND SEISMIC CORRELATION

Coring was conducted in six holes at Site 1224, with the deepest hole (1224F) being the only hole logged. In Hole 1224F, we penetrated to 174.5 mbsf, cored through 146.8 m, and recovered 37.7 m of core with 25.68% core recovery. With this poor recovery, which is common for oceanic basalts, a detailed correlation among physical properties data, core analysis, logging data, and surface seismic data is required to understand a range of geological and geophysical issues at Site 1224. Basaltic basement at Site 1224 is overlain by ~28 m of soft sediment.

Sediment was recovered from Holes 1224A, 1224B, 1224C, and 1224E with significant recovery only in Holes 1224C and 1224E (Fig. F97). Two distinct shallow reflectors were identified at 13 and 38 ms two-way reflection times by both underway and on-site 3.5-kHz seismic surveys (Fig. F2). Although the deeper reflector corresponds to the top of the basaltic layer at 28 mbsf, the shallower one seems not to be a simple lithologic boundary. If we assume that the compressional velocity of the sedimentary layer is 1500 m/s, the depth of the shallower reflector corresponds to 9.75 mbsf. Based on physical properties measurements on cores, the GRA bulk densities decrease with increasing depth from 1.7 g/cm³ to 1.3 g/cm³ between 2 and 6.4 mbsf. The PWL compressional wave velocity is constant at ~1480 m/s in this interval (see "Compressional Wave Velocity" in "Hole 1224C" and "Compressional Wave Velocity" in "Holes 1224E and 1224F" both in "Physical Properties"). In addition, porosities increase from 70% to 80% between 0.5 and 6.2 mbsf. This result is very different from ordinary velocity vs. density and velocity vs. porosity relations that are positive and negative trends, respectively. The density and porosity should increase and decrease with depth, respectively, if compaction of sediments by overburden pressure is applied. The description of microfossils indicates the presence of abundant radiolarians between 4 and 8-9 mbsf or a little deeper (Fig. F85). The radiolarians are barren in the dark-brown clay/silty clay layer below 8-9 mbsf. Correlating these results and the on-site 3.5-kHz seismic reflection records, a lithologic boundary with a physical property discontinuity is present at 8-9 mbsf. Because of poor core recovery, the depth of 8-9 mbsf can be shifted to a little deeper depth.

In the Barbados accretionary prism, the three-dimensional seismic survey found a seismically bright reflecting plane, which was interpreted as a décollement. During Leg 171, a radiolarian abundant layer with a high pore fluid pressure, a high ratio of smectite/illite, and a large methane concentration was found in the décollement zone (Moore, Klaus, et al., 1998; Moore, 2000). In the Barbados case, the thickness of the décollement was ~20 m. Considering this result, the shallow seismic reflector at 13 ms two-way traveltime (Fig. F2) corresponds to a similar layer with high pore-fluid pressure, although the thicknesses in the two cases are quite different. Phillipsite, which is in the zeolite group, was also found at a depth >6 mbsf in Section 200-1224A-4X-CC and Core 200-1224E-R1. Zeolite is a low-grade metamorphic mineral that may have been deposited in the high pore-fluid pressure zone at 8-9 mbsf. This shallow reflector was also identified at the junction box of the H2O Site, 1.5 km from Site 1224 in the 3.5-kHz seismic reflection records (Fig. F2). This suggests a broad distribution of this shallow interface around Site 1224. There are no logging data for the sedimentary layer as usual because the depth is too shallow.

Below 28 mbsf, a basalt layer was drilled in Holes 1224A, 1224D, 1224E, and 1224F. Hole 1224D was drilled for the purpose of installing broadband borehole seismometers in the future. Holes 1224D, 1224E, and 1224F are within a 15-m distance, with Holes 1224E and 1224F being <1 m apart (Fig. F3).

Although the lithologic description identified massive basalt flows and pillow basalts (Fig. F48), one-to-one correlation between logging and physical properties to the petrographical description is difficult because of the differences in the nature of the observations.

The triple combo and FMS/DSI tools were used in logging. We identified five distinct logging units in basement at 28-45 mbsf (Unit I), 45-63 mbsf (Unit II), 63-103 mbsf (Unit III), 103-142 mbsf (Unit IV), and below 142 mbsf (Unit V) by logging with the triple combo tool string, which includes NPHI, RHOB, resistivity, and caliper measurements (Fig. F86). Logging with the triple combo tool string extended uphole to 40 mbsf. Between 40 and 63 mbsf, a low-porosity and uniform layer is identified, although bulk density values are anomalous. In this depth range, porosity is a few percent and bulk density is ~2.75 g/cm³ with a small variation. The caliper was closed in this depth range. The layer between 63 and 103 mbsf has 2.75 ± 0.25 g/cm³ bulk density and 10%-30% porosity. Resistivity for this zone varies considerably about the average of ~90 m. The hole size is approximately steady at 12 in. At 136 mbsf, porosity and bulk density logs have a significant variation. Between 103 and 136 mbsf, the hole size changes from place to place. Bulk densities are 2.6 ± 0.4 g/cm³, which suggest highly heterogeneous layers. The porosities have an extremely large variation between 20% and 80%. The resistivities also vary considerably. All logging parameters indicate highly heterogeneous and/or fractured layers. At the depth range between 136 and 142 mbsf, the hole size reaches to 17 in, corresponding to the maximum extent of the caliper arms. The temperature measurements indicate a sharp change at 136 mbsf (see "Temperature Measurements" in "Downhole Measurements"). The temperatures below and above 136 mbsf were 4.9° and 4.0°C, respectively. This suggests that at 136-142 mbsf there is likely to be some kind of boundary such as a highly fractured layer for the fluid flow. Although usually the broadening of the caliper diameter suggests a washed-out hole, this broadening of hole size might be related to lithologic characteristics. Below 142 mbsf, all parameters are quite stable and indicate the presence of a homogeneous hard rock zone.

PWS compressional measurements (Fig. F87) identified seven major physical properties zones: (1) 30-38 mbsf, (2) 38-41 mbsf, (3) 41-61 mbsf, (4) 61-100 mbsf, (5) 100-138 mbsf, (6) 138-147 mbsf, and (7) below 147 mbsf. Zone 2 is very thin. In zones 1 and 3, compressional velocities were ~5700 ± 500 m/s. Zone 2 has velocities as low as 4500 m/s that suggest weak fractures or low porosities. The character of zone 4 is quite different between log and physical properties. Although the PWS velocities for zone 4 are 4600-5100 m/s, suggesting higher porosity or fracturing, logging gives higher bulk densities, indicating less fracturing. Zone 5 also has differences between log and physical properties. The logging data (Fig. F86) suggest a fractured zone, but the PWS velocity is 5500 ± 500 m/s. The discrepancies can be explained by the scale effect because logging measures at the meter scale and physical properties of rock specimens measure at the centimeter scale. This can be explained by the petrological observation below. The sharp boundary at ~150 mbsf indicates extremely low compressional velocities such as 4000-4700 m/s as measured by the PWS compressional velocity. This zone corresponds to the anomalous caliper zone in the logging. Below 150 mbsf, the PWS compressional velocity increases to 5500 m/s. Using the PWS compressional velocity measurements, a velocity-depth model was generated (Fig. F88).

Physical properties measurements and logging indicate several major lithologic units from the viewpoint of physical parameters. XRD analysis (see "X-Ray Diffraction Investigation of Secondary Minerals"in "Geochemistry") indicates five minerals other than ordinary basalt component minerals in veins: calcite, zeolite, smectite, aragonite, and quartz (Fig. F48). Surprisingly, the zones with these minerals are similar to the zones defined by logging and physical properties measurements. Smectite was found in the veins between 28 and 46 mbsf and between 102 and 136 mbsf. Calcite was found in the veins between 50 and 102 mbsf and at 143 mbsf. Zeolite was found at 102 and 143 mbsf, which roughly equals the boundary identified by logging and physical properties measurements. Quartz was identified at 44 mbsf, which is the depth of the smectite extinction and the depth where calcite veins are absent. There are some discrepancies ~45 and 60 mbsf because of sparse sampling intervals. In the depth range from 100 to 140 mbsf, 5- to 10-cm-diameter pieces of rock were recovered. Many of these rocks have chilled margins with glass rims. The small amount of the recovered rocks and the presence of chilled margins suggest that the layer between 100 and 140 mbsf is a pile of small-scale pillow lavas with a minor amount of hyaloclastites. A stack of 10-cm-scale pillows and/or hyaloclastites can explain the observations of high porosity and low bulk density values obtained by logging and the low velocity values obtained by physical properties measurements.

Smectite is stable at temperatures lower than 150°C, and zeolite is a mineral group of low-grade metamorphism. If the layer between 100 and 140 mbsf is highly permeable, seawater can easily flow into this layer and smectite could result from this water flow. The shallowest layer above 136 mbsf may allow the penetration of seawater and low-temperature alteration. The discontinuous temperature change at ~136 mbsf may be explained by the presence of a high-pore fluid layer caused by fractures.