Site 1176 Summary | Table of Contents


Site 1175 Summary
Site 1175 (ENT-07A) was designed to penetrate the slope sediments that cover the large thrust slice zone just landward of a major out-of-sequence thrust (OOST; Fig. 11). Investigation of the age and lithologic characteristics would provide information on (1) the history of accretion, uplift, and deformation of the prism, and (2) sedimentation within a trench-slope basin.

We cored three lithostratigraphic units at Site 1175 (Figs. 8B, 21). Unit I (upper slope—basin facies) begins at the seafloor and ends at a subbottom depth of 224.75 mbsf. Lithologies include nannofossil-rich hemipelagic mud, volcanic ash, and thin turbidites that range in texture from sand to silty sand, clayey sand, and silt. The most characteristic feature of Unit I is the common occurrence of contorted stratification (Fig. 22). There are eight discrete zones of soft-sediment deformation. Typical manifestations include variably inclined bedding, small-scale folding, and, in extreme cases, stratal fragmentation. The disruption probably was caused by submarine slumps and debris flows. Unit II (middle slope—basin facies) extends from 224.75 to 301.64 mbsf. Lithologies include hemipelagic mud, poorly sorted muddy sand to sandy mud, sporadic interbeds of volcanic ash, and rare occurrences of thin sand or silt turbidites. The unusual lithology of muddy sand is diagnostic of Unit II and probably was transported downslope by sandy debris flows or mudflows. Unit III (slope to prism transition) begins at 301.64 mbsf and ends at 435.40 mbsf. This unit is typified by hemipelagic mud with numerous interbeds of silt and silty sand turbidites. The most striking lithology, however, is gravel to pebbly mudstone (Fig. 23). Its characteristics include disorganized and poorly sorted clast fabric, lack of internal stratification, partial to complete support of clasts by a matrix of clayey silt, and subrounded to rounded clasts up to 5.5 cm in size. A polymictic clast population was transported downslope by debris flows. The boundary between the lowermost slope sediment and the top of the accretionary prism cannot be defined with certainty using lithologic criteria, but it probably occurs within the upper 25—30 m of Unit III.

Site 1175 exhibits little evidence for tectonic deformation. However, the upper 205 m shows intervals of recumbent, isoclinal slump folding and disaggregated sediment, interlayered with subhorizontal intact bedding. Fold orientations suggest the slumping was northward directed. Below 220 mbsf, bedding is subhorizontal, except for localized chaotic zones between 350 and 388 mbsf and dips up to 21° at 400 mbsf. Core-scale faults, probably compaction related, occur from 298 to 302 mbsf and sporadically from 340 to 435 mbsf. Possible web structure occurs in sands at 406.9 and 425.8 mbsf; near the bottom of the hole, an indurated sand contains several low angle small faults.

Biostratigraphic age control was provided by calcareous nannofossils that are well preserved and abundant throughout the section. A total of nine biostratigraphic events were identified within the nannofossil assemblages. The continuous sedimentary record spans the time interval from the Pliocene (Zone NN18) through the Pleistocene (Subzone NN21b). Based on the biostratigraphic age model, sedimentation rates for the upper sedimentary units show high sedimentation rates (0.52 m/k.y.) for the upper to middle slope—basin deposits, with decreasing rates for the slope to prism transition (0.13 m/k.y.).

Hole 1175A inclination data after alternating field (AF) demagnetization at 30 mT allowed interpretation of geomagnetic polarity changes from late Pliocene to Pleistocene. The 0.78 Ma Brunhes/Matuyama boundary is interpreted to occur at 298.80 mbsf (interval 32X-5, 80 cm). Seven short reversal events were observed in the Brunhes Chron and may represent geomagnetic excursions.

In Hole 1175A, pore fluids are less intensively modified from seawater than the pore fluids in Holes 1173A and 1174A. The main characteristics of the pore fluid concentration-depth profiles indicate that the intense microbially mediated reactions occur in the top <200 mbsf of the section. Microbial sulfate reduction is complete at ~15 mbsf. The alkalinity maximum also occurs at this depth. Only relatively small changes in the chemical gradients occur throughout the section and across the major lithologic boundaries in the abiogenic components. Diatom dissolution controls the Si concentrations in the pore fluids. Because of the rather low geothermal gradient of 54°/km, volcanic ash alteration is as yet insignificant. Instead of ash alteration, as indicated by the Ca, Mg, and alkalinity concentration-depth profiles, carbonate, particularly dolomite, diagenesis is the dominant diagenetic reaction. Dolomite forms both by direct precipitation of authigenic dolomite and by replacement of precursor biogenic calcite, which is abundant in this section. Carbonate diagenesis should influence some of the index physical properties such as porosity and density. An as yet unidentified silicate reaction at greater depth below the section drilled controls the concentration profiles of K, Na, Si, and alkalinity below ~300 mbsf, corresponding to lithostratigraphic Unit III. The clearly observed diffusion of lower chlorinity interglacial water into the pore fluids at Sites 1173 and 1174 is absent at this site. It must have been erased by the widespread slumping in Unit I of this section over the past ~10 k.y. Fluid flow does not play a role at this site.

The sediments at Site 1175 contain low inorganic carbon (~0.11—4.59 wt%), and carbonate contents range up to 40 wt%, resulting in very immature organic matter and low hydrocarbon abundances. The low sulfate and high methane concentrations in sediments below the sulfate reduction zone and throughout Hole 1175A are consistent with a bacterial origin.

Bacterial abundance was enumerated in 18 samples obtained at Site 1175. The abundance near the surface is 6.97 ± 107 cells/cm3 and declines rapidly, which is consistent with the decrease in sulfate concentrations. Abundances increase below14.6 mbsf consistent with increases in methane concentrations. The sample at 50.8 mbsf is notable in that it contains 7.28 ± 107 cells/cm3 (i.e., slightly more bacteria than the near-surface sample). This is followed immediately with almost the lowest population enumerated of 3.71 ± 105 cells/cm3 at 59 mbsf. The deepest sample is 400 mbsf with 3.59 ± 105 cells/cm3, equivalent to 0.5% of the near-surface population. Estimates of drilling fluid intrusion into the interior of the cores examined at this site range from below detection to 0.02 µL/g. In addition to the onboard assays, 17 whole-round cores were taken for shipboard enrichment cultures, cell viability, and shore-based microbiological analysis to measure potential bacterial activities, culture microorganisms, characterize nucleic acids, and investigate fatty acid biomarkers.

Porosities within the upper slope—basin facies (Unit I) are characterized by high variability and decrease slightly with depth from values of 62%—70% at the mudline to 61%—68% at ~100 mbsf. Porosities decrease abruptly at ~100 mbsf to values of 57%—61% and then decrease gradually to the transition between the upper and middle slope—basin facies (220 mbsf). Below 220 mbsf (within the middle and lower slope—basin facies), porosity decreases more rapidly with depth than in the upper slope—basin facies, reaching values of 38%—47% at 400 mbsf. The rapid decrease in porosity below 220 mbsf coincides with increasing P-wave velocity. There is no clear change in porosity, bulk density, or grain density at the depth of the middle slope—basin/lower slope—basin facies boundary (301 mbsf; Units II and III). A spike of high velocity and impedance 20 m above this transition may correspond to a seismic reflector. The depth of this spike coincides with the depth of the upper unconformity on the depth converted seismic profile. Four successful in situ temperature measurements at Site 1175 indicated a thermal gradient of 0.054°C/km.

Gas-probe permeameter measurements illustrate the huge influence of lithology. Overall results for the hemipelagic clays that dominate the section are uniformly lower than those in a coarse, friable black ash at 23 mbsf and turbiditic sands between 60 and 90 mbsf, which yield an exceptionally high value exceed 8 ± 10—12 m2. Thin bands of white-gray ashes also give relatively high values, in line with shallow, unaltered ashes at the other sites.

This site revealed that the age of accretion of the large thrust slice zone is very young (<2 Ma). The young age of the accretion indicates rapid growth of the frontal part of the Nankai accretionary prism, ~40 km oceanward growth in 2 m.y. This rate of growth provides a first order constraint for kinematic, structural, and hydrogeologic modeling of the prism.

Site 1176 Summary | Table of Contents