SITE 1041

The lithostratigraphic objectives at Site 1041 were to determine the stratigraphy of the sedimentary apron, the composition and properties of gas hydrate, and the composition of the prism material directly beneath the apron. We succeeded in determining the stratigraphy of the upper 395 m of the apron and sampled gas hydrate in a number of places down the hole. Because of deteriorating hole conditions, we did not reach our target depth of 550 mbsf, thus, missing the objective of the prism composition.

One lithologic unit is defined at Site 1041 (Fig. 8). Only the first two cores were recovered by advanced hydraulic piston core (APC). Below 14.30 mbsf (Core 170-1041A-2H) the cores are generally of poor quality, with incomplete recovery, extensive biscuiting due to drilling disturbance, and entire sections of rubble that are probably upper-hole infall. Beginning with Core 170-1041A-12X and extending to the bottom of Hole 1041C, the cores are extensively fractured. In Core 170-1041A-15X through 1041B-20R, many original textures and structures were destroyed by gas-hydrate dissociation.

Apron Unit A1 consists mainly of Pleistocene (?) to late Miocene clay(stone) and silt(stone), with minor sandstone, limestone, and volcanic ash. These lithologies are dominated by terrigenous rather than biogenic material. Unit A1 is divided into two subunits, with Subunit A1B consisting of coarser-grained material (siltstones and sandstones) than the claystone-dominated Subunit A1A. Gas hydrate was recovered between 116 and 184 mbsf. Although evidence of submarine mass wasting and slumping was observed near the base of the hole, we found little evidence of tectonic deformation in the cores except for microfaults, fissility, and changes in bedding dips. Biostratigraphic markers are not abundant enough to indicate age reversals, which would have resulted from major internal faults, and age determinations are complicated by reworked middle Miocene taxa that increase near the base of the hole. However, the increase in microfaults and changes in bedding dip between 180 and 200 mbsf, and an abrupt change in bedding orientation at 275 mbsf, suggest at least two minor faults within the section.

Distributions of age vs. depth at Site 1041 are not as clear as those at the previous two sites because of poorer preservation and significant reworking. The general trend of age vs. depth rates, based largely on diatoms, suggests 55 m/m.y. in the upper 240 mbsf and 93 m/m.y. below this depth. Rates based on nannofossils are slightly higher at 62 m/m.y. above 240 mbsf. The magnetostratigraphy of this site can be divided into two zones, an upper zone (0-160 mbsf) of apparently remagnetized, entirely normal polarity, and a zone with both normal and reversed polarity (160 mbsf-TD). Using biostratigraphic markers, polarity intervals ranging from the onset of C3n.2n (4.62 Ma) at 174 mbsf to the termination of C4n.2n (7.43 Ma) at 369.5 mbsf were identified. Age-depth relationships are 38 m/m.y. for the interval 174-222 mbsf and 99 m/m.y. for the interval 222-396 mbsf. Thermal demagnetization of multicomponent isothermal remanent magnetizations (mIRM) was performed on selected samples and indicated the presence of Fe sulfides and magnetite.

The main geochemical scientific objectives for Site 1041 were to characterize the fluid stratigraphy and flow distribution within the slope-apron sediments, as well as in the rocks or prism sediments beneath the high amplitude reflection at its base, and to determine the origin(s) of the fluid(s). Finally, we sought to establish the occurrence and geochemistry of the gas hydrate to deduce its mode of formation and origin. We succeeded in establishing a fluid stratigraphy in the upper 395 mbsf of the slope apron and we established the occurrence and geochemistry of the gas hydrate. Our data have implications for the nature of the rocks beneath the apron, but we did not sample those rocks directly at Site 1041.

Below the very thin sulfate reduction zone at about 15 mbsf, methane concentrations are high throughout the section drilled. Gas hydrate is concentrated between about 100 and 280 mbsf, the zone of highest TOC wt%. Its primary mode of occurrence is disseminated, as indicated by the almost constant salinity in this depth interval, with thin sheets of gas hydrate filling microfractures. The gas hydrate analyzed within this interval also contains small amounts of the higher hydrocarbons ethane (C2) and propane (C3). Below ~280 mbsf, just beneath the lithologic transition from Unit A1A to A1B, the concentrations of volatile hydrocarbons, especially of propane, are highest and then decrease below the interval. Across this boundary, the concentration-depth profiles of several inorganic chemical components, particularly Cl, Ca, Si, and phosphate, and of Na/Cl and Mg/Ca ratios, show sharp transitions to lower or higher values, suggesting a two-tier hydrologic system having different fluid sources.

The section drilled from 1.5 to 395 mbsf has Cl concentrations and salinities lower than seawater, and they decrease steadily to the depth of the main gas-hydrate zone (120-280 mbsf). From 280 to 340 mbsf, the concentrations are rather constant. Below ~340 mbsf, they have slightly higher values. The source of the fluid with low Cl and high Ca concentration in the lower apron sediments is located at >3-4 km depth. Clay mineral dehydration reactions are the most likely fluid-rock reactions responsible for freshening this fluid at the source region. The source of the low-Cl fluid in the upper apron sediments is less clear. Gas-hydrate dissociation alone would not result, for example, in Na/Cl ratios higher than seawater, whereas the in situ temperatures are too low for hydrous mineral dehydration reactions. Testing for the possibility of meteoric water influence requires shore-based analyses. Carbonate formation of both calcite and dolomite is pervasive throughout the apron sediments. Although the total inorganic carbonate content is not high, it ranges between 0.5 and 6.3 wt%.

The apron sediments show increasing compaction through the section, with porosity decreasing from nearly 75% at the seabed to less than 45% at 330 mbsf. P-wave velocities increase monotonically over the same interval, from 1540 m/s to over 2000 m/s at the greater depths, reflecting increasing consolidation and cementation. The widespread occurrence of (melted) gas hydrate does not significantly affect the physical properties measured on core samples, but clathrates filling pores and fractures are expected to have major influence on velocities and resistivities in situ.

In summary, some of our objectives at Site 1041, including determination of lithostratigraphy and geochemistry of the apron and sampling the distribution of gas hydrate with depth, were met. We fell short of our total depth objective, the high-amplitude prism reflector, because of drilling problems and, therefore, targeted an additional Site CR-7 (Site 1042) to meet that objective.

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