SITE 1040

Lithostratigraphic and biostratigraphy objectives included determining the nature and age of the wedge material, the relationship of underthrust units to those of reference Site 1039, and the implications for the balance of mass and fluids between the trench sequence and the underthrust and wedge sequence. Another important objective was to determine the structural fabric of both the deformed wedge and the underthrust sediments. We were successful for the most part in achieving these objectives by drilling through the décollement and the entire underthrust sedimentary sequence and into gabbro intrusions very similar to those in Site 1039. Because LWD operations were only partially successful at this site, we did not obtain a full in situ physical properties data set for correlations with Site 1039.

The deformed sedimentary wedge overlies the same three sedimentary units and one basement unit drilled at Site 1039 (Fig. 7). The deformed wedge unit (Unit P1) consists of a subunit (P1A, 0-74.8 mbsf) of olive green silty clay with debris flows and silty sands, overlying a lower subunit (P1B, 74.8-371.2 mbsf) of olive green silty clay with intervals of silt to very fine sand. The age of the deformed wedge unit is uncertain. Below Unit P1, Unit U1A (late Pleistocene, 371.2-384.8 mbsf) is a clayey diatomite with silty sand interbeds, and Unit U1B (Pleistocene, 384.4-422.6 mbsf) is clayey diatomite with ash layers. Unit U2A (Pleistocene to late Pliocene, 422.6-472.47 mbsf) is silty claystone with ash layers, and Unit U2B (Pliocene, 472.47-479.7) is silty claystone with calcareous clay and ash layers. Unit 3A (early Pliocene to late Miocene, 479.7-497.8 mbsf) is a siliceous nannofossil chalk and clay, whereas Unit U3B (late to middle Miocene, 497.8-653.53) is siliceous nannofossil chalk. Unit U3C is siliceous nannofossil chalk with calcareous diatomite interbeds. Finally, Unit U4 (post 15.6 Ma, 653.53-661.47 mbsf) is a pyroxene gabbro intrusion.

Deformational structures were observed in the first core at Site 1040. Fissility to incipient scaly fabric were observed from the top of the prism discontinuously throughout Unit P1 (0-371 mbsf). Deformation bands and distributed fractures are common in the upper 20 mbsf and discontinuous throughout Unit P1. Many of these may be drilling-induced, particularly all stratal disruptions. Both brittle and plastic deformation structures are common from the surface to 370 mbsf. Plastic deformation is associated with soft, sticky clay and severe drilling disturbance. Therefore, original plastic deformation cannot be identified although it may have existed within these clay-rich intervals. From 340-350 mbsf, we find that fracture networks, stratal disruption, veins, and incipient scaly clay structures are common. Locally, a very intense fracture fabric is formed. From 350-360 mbsf, brittle deformation gives way to plastic deformation in sticky clays. Throughout Unit P1, cores are highly disrupted by drilling, with evidence of both flow-in and intensive torque transfer to the cores. Paleomagnetic studies indicate that the more plastic intervals have cork screwed tens of times.

Below 371 mbsf, the intense fissility, fracture networks, and stratal disruption disappear, and evidence for torque transfer to the cores decreases markedly. Burrows, a primary sedimentary structure, are common. Mud-filled veins are present. Steep bedding dips are measured in the interval 420 to 484 mbsf. At 460 mbsf, microfaulting is more abundant in the cores, and fluid escape breccias were noted. Minor faults occur through the cores to the base of the section. Microfaults are mostly reverse in the interval 430-500 mbsf, and show mostly normal displacement in the interval 610-650 mbsf. Incipient stylolitization is present in the lowest 10 m of the sediment section, developed largely subhorizontally, and rare boudinage structures are present. The lower 150 m of the section show consistent bedding dips of up to 20°. This attitude can be explained by a measured hole deviation of up to 18°.

The distribution of ages vs. depth at Site 1040 for Units U1, U2, and U3 is similar to that of Site 1039. The major difference between the two sites is the difference in thickness of the corresponding units. Units 1 and 2 at Site 1040 are approximately 67% of the thickness of the units in Site 1039, whereas Unit 3 at Site 1040 is about 80% of that in Site 1039. These differences correspond well with measured differences in porosity and bulk density between the two sites. Although some of the same fossil zones are present at both sites, a number are different, probably due in part to the 10-m spacing of core-catcher samples for shipboard analysis. Post-cruise work should shed more light on these differences. Within the deformed sedimentary wedge (0-330 mbsf), numerous normal and reversed polarity intervals were found. Lack of good biostratigraphic control and the high probability of thrust faults within this interval have so far prevented a unique assignment of our reversal stratigraphy to any specific portion of the magnetic polarity time scale. The reversed-polarity results at 5 mbsf in Hole 1040B do suggest, however, that these near-surface sediments are at least 0.78 Ma in age. Beneath the décollement (371-655 mbsf), a good magnetostratigraphic record was obtained from the underthrust sediments. In particular, the Jamaica event (0.2 Ma) has been located at 383.7 mbsf, and a complete set of reversals ranging in age from 2.14 Ma (C2r.1n) to 6.935 Ma (C3Bn) occurs between 458 and 482 mbsf in Hole 1040C. Preliminary age vs. depth estimates for the underthrust section at Site 1040 from biostratigraphy and magnetostratigraphy are, respectively, 41 and 72 m/m.y. for the Pleistocene, and 6 and 5.8 m/m.y. for the Pliocene to upper Miocene. Biostratigraphic data for the middle Miocene indicates a sedimentation rate of 32 m/m.y. The oldest sediment at Site 1040 was 15.6 Ma, younger than the 16.4 Ma age found at Hole 1039C.

A primary objective for Site 1040 was to determine the chemistry of pore waters within the wedge, décollement, and underthrust section to compare Site 1040 chemistry with that of Site 1039 and to gain a fuller understanding of the nature of fluid and heat flow near the toe of the subduction zone. We were successful in meeting these fluid chemistry objectives, obtaining accurate chemical depth profiles even from intervals with severe drilling disturbance.

Pore-water geochemistry at Site 1040 shows significant decreases in salinity in the upper 371 m. Chloride concentrations decrease rapidly in the upper 40 mbsf from 550 mM to about 512 mM, and salinity decreases from 34.5 to ~30 in the upper 40 m. Chloride and salinity both decreased further to ~500 mM and 28, respectively, by 80-100 mbsf. In addition, both properties show large minima at 200 mbsf and a moderate one at 300 mbsf, implying that both flow through fractures and a presence of gas hydrate at 200 mbsf, which suggests a fluid conduit. Both properties also show minimum concentrations in the décollement zone, at 360 mbsf, and both return to normal seawater values at 371 mbsf, just beneath the décollement, and maintain these values to 653 mbsf. Calcium and magnesium also show similar effects. Magnesium decreased from 50 mM at the surface to about 20 mM at 100 mbsf. It also shows minima at 200 mbsf, 300 mbsf, and in the décollement zone, then increases to 41 mM at 390 mbsf. Just below the décollement, magnesium increases to a small maximum concentration of 47 mM at 382 mbsf. Calcium displays a curve that is reversed to magnesium, with maxima at 200 and 360 mbsf, and a small maximum at 280 mbsf. The distributions of ammonium, phosphate, silica, potassium, sulfate, and alkalinity all show sharp boundaries and major changes across the décollement.

The distribution of methane gas also changes significantly at the décollement. Methane increases from a few ppm in the upper few meters to 8996 ppm at 28.5 mbsf. From 28.5 to 374 mbsf, concentrations increase to values between 4818 and 116,561 ppm. Below the base of the décollement, methane values decrease sharply, and from 460 to 653 mbsf the values are again a few ppm. Propane is very low at Site 1040, but shows a peak of 24 ppm between 191 and 203.6 mbsf, and a peak of 19 ppm between 348.2 and 357.8 mbsf. These two intervals coincide with anomalies in fluid composition of the pore waters, suggesting a significant input in allochthonous thermogenic gas. Below 374 mbsf, only small amounts of propane were detected throughout the underthrust section. The distribution of calcium carbonate in the underthrust section shows the same distinct patterns as reported for Site 1039.

The primary objectives for studying the physical properties of the wedge and underthrust section included determining what changes occurred to the accreted or underthrust material. Physical properties at Site 1040 show significant changes compared to equivalent units at Site 1039. Seismic velocity measurements at Site 1040 average about 1700 m/s from 371 to 496 mbsf. From 496 to ~560 mbsf, seismic velocity averages 1660 m/s, and increases steadily from 1660 m/s at 560 mbsf to 1760 m/s at 653 mbsf. Porosity varies from 50% at 150 mbsf to 40% at 371 mbsf. At this depth (the décollement) porosity increases rapidly to about 60% and stays at this value to a depth of 650 mbsf. Bulk density at Site 1040 increases from about 1660 to 1720 kg/m3, from 150 mbsf to 371 mbsf. Density drops to 1430 kg/m3 between the décollement at 372 mbsf and a depth of 430 mbsf. Thermal conductivity shows considerable scatter, but measurements indicate an average value of 0.9 W/(m*K) from 0 to 490 mbsf. Magnetic susceptibility is a primary property for cross-hole correlation because its natural variations are not significantly affected by core disturbance. It displays some peaks in the upper 20 m, then stays very low to 270 mbsf. From 270 to 496 mbsf, magnetic susceptibility is relatively high and varied, and from 496 to 650 mbsf it is essentially zero, increasing only slightly in the lower 30 m and showing a single peak at 653 mbsf due to the presence of the gabbro sill.

Temperature measurements downhole show an extremely low gradient of 8.3°C/km from the surface to 200 mbsf, followed by 5.3°C/km from 200 to 350 mbsf, returning to 8.4°C/km at 350 475 mbsf. The corresponding heat flows calculated for these intervals are 7.5, 4.8, and 8.9 mW/m2.

In situ density, porosity, resistivity, and gamma-ray measurements were collected with the CDN and CDR tools as part of the LWD downhole measurement program. Two logging runs were made using different drill bits in an unsuccessful attempt to penetrate through the décollement. The total logged intervals at Holes 1040D and 1040E are from the seafloor to 325 mbsf and 307 mbsf, respectively.

Downhole density and porosity measurements correlate roughly with the core specimen measurements, and resistivity generally follows the pattern of density. Although the profiles from Holes 1040D and 1040E generally show the same trend in density and porosity, they show different log responses in several intervals. Bulk-density profiles gradually increase with depth, but display a number of segments with rapid downward decreases between more gradual downward increases. Most of the density values range between 1.7 and 2.0 g/cm3. Significant decreases in density occur at 84 mbsf (1.55 g/cm3), 237 mbsf (1.68 g/cm3), and 310 mbsf (1.60 g/cm3) in Hole 1040D, and 168 mbsf (1.70 g/cm3) and 269 mbsf (1.72 g/cm3) in Hole 1040E. Porosities calculated directly from the neutron log fluctuate widely throughout the logged interval. The filtered neutron porosity profile indicates porosities from 65% to 50%, decreasing gradually with depth. The porosity profiles show a reverse trend to that of the density profile, but several low density intervals indicate low neutron porosities.

In the interval 0-200 mbsf, the resistivity values gradually increase from 1 to 2.5 ohm-m in Hole 1040D, whereas in Hole 1040E the values show greater fluctuation. In Hole 1040D, the interval 200-277 mbsf is characterized by relatively high resistivity values up to 3.2 ohm-m. In Hole 1040E, high resistivity is identified between 210 and 242 mbsf. A resistivity maximum at 114 mbsf corresponds to a density maximum. However, resistivity maxima at 168, 238, and 270 mbsf are correlated to minima in density.

In summary, we were highly successful in meeting our primary objectives in determining the geochemistry, physical properties, lithoastratigraphy and biostratigraphy. We had only partial success at this site in carrying out LWD because of the inability of the tool to penetrate beneath a presumably overpressured zone.

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