SITE SUMMARIES

Site 1176 Summary
The objective of Site 1176 was to determine the nature of accreted sediments of the large thrust slice zone as well as deformation and fluid flow related to a major out-of-sequence thrust (Fig. 11).

We recognized three lithostratigraphic units at Site 1176 (Figs. 8B, 24). Unit I (upper slope—basin facies) extends from the seafloor to 195.79 mbsf. Its lithologies include nannofossil-rich mud, volcanic ash, and sand to silt turbidites. The principal processes of sedimentation for Unit I were hemipelagic settling and turbidity currents, with occasional volcanic ash falls and remobilization by slumping. Unit II (middle slope—basin facies) extends from 195.97 to 223.54 mbsf. In addition to typical hemipelagic mud, Unit II contains sandy mudstone and rare beds of volcanic ash. Deposition of this facies occurred via muddy debris flows, routine settling of suspended sediment, and occasional ash falls. Unit III was cored to a depth of 440.36 mbsf and contains abundant interbeds of sand to silt turbidites, hemipelagic mudstone, pebbly mudstone, gravel (Fig. 25), and rare volcanic ash. The primary depositional environment for this unit was probably a trench-fan system fed by a transverse submarine canyon. The petrographic compositions of sands and gravels, rich in sedimentary lithic fragments and quartz, show that their provenance is the Outer Zone of southwest Japan as typically represented by the Shimanto Belt. The unconformity between the lowermost slope sediment and the top of the accretionary prism probably coincides with the boundary between Units II and III.

Site 1176 can be divided into two structural domains: slope basin and accretionary prism. Deformation of the slope-basin sediments (0—224 mbsf) is characterized by inclined bedding intervals in which slump folds together with contorted and chaotically mixed bedding are locally developed. These features are interpreted to record the effects of active tilting and uplifting of the slope basin. Small faults are thought to result from extensional response to this tilting and uplift of the basin and/or burial compactional strains. In contrast, deformation structures are almost absent in the accreted sediments (below 224 mbsf), although core recovery was very poor. However, the apparently consistently horizontal bedding may reflect the flat part of a hanging-wall anticline formed in association with an underlying thrust.

Biostratigraphic age control was provided by calcareous nannofossils. Nannofossil assemblages are of Pliocene (Zone NN16) to Pleistocene age (Subzone NN21b) according to nine recognized biostratigraphic events. Although nannofossils are common and generally moderately preserved in the upper Pleistocene, nannofossils from sediments older than 1 Ma are rare and poorly preserved. Age models based on biostratigraphy indicate sedimentation rates of ~0.07—0.26 m/k.y.

Inclination data of Hole 1176A after AF demagnetization at 30 mT provided useful information for interpretation of geomagnetic polarity changes from the late Pliocene to the Pleistocene. The Brunhes/Matuyama boundary (0.78 Ma) is interpreted to occur at 199.55 mbsf. Seven short reversal events were observed in the Brunhes Chron and may represent geomagnetic excursions in this chron.

The most intense microbially mediated reactions occur in the top <100 mbsf of the section. Microbial sulfate reduction is complete at ~20 mbsf. In the top half of this zone the sulfate reduction rate decreases linearly with depth, whereas in the lower half maximum sulfate reduction occurs at the base of the zone; the alkalinity produced is involved in carbonate reactions, and the ammonium produced is involved in clay ion exchange reactions.

The alkalinity maximum and Ca and Mg minima coincide with the depth of the base of the sulfate reduction zone; thus, this depth interval is also characterized by intense carbonate diagenesis. The Ca and Mg concentration profiles indicate that in the sulfate reduction zone both authigenic dolomite precipitation and replacement of a precursor biogenic calcite occur. Deeper, however, through the upper and middle slope—basin section, replacement of a precursor calcite is the only dolomitization reaction.

Because of the low geothermal gradient of 56°C/km, volcanic ash or other silicate diagenetic reactions are minimal. Diatom dissolution may control the pore fluid silica concentration. At the base of the section, a residual pore fluid exists, having close to seawater composition as indicated by the return to seawater concentrations of all abiogenic components, except for K. The Cl concentration profile indicates Cl diffusion to a low-Cl zone that may be associated with the out-of-sequence thrust. The low-Cl fluid is enriched in Ca and depleted in Na, K, and Mg. A chemically similar deep-seated fluid was identified at Site 1174.

Diffusion of low-chlorinity interglacial seawater into the sediment section is not observed at this site because the top ~50 mbsf of the section was deposited in the past <10 k.y., during the interglacial; therefore, concentrations are constant. At greater depths, the residual signal from the glacial ocean seawater has been erased by diffusion.

The total carbon content for the sediments examined between 200 and 401.6 mbsf at Site 1176 ranged from 0.05 to 2.25 wt%. The highest carbon value (2.25 wt% at 340 m) was dominated by a terrestrial component likely derived from fan debris flow to the trench sediments. The sulfur content showed a similar trend to TOC with the highest values of sulfur (1.05 and 2.07 wt%) coincident with the highest TOC values (0.86 and 2.25 wt%). The inorganic carbon (~0.05—2.6 wt%) and high carbonate content (up to ~35 wt%) are similar to values observed at Site 1175. Methane concentrations in sediments below the sulfate reduction zone (~9.5 m) are consistent with a bacterial origin. Methane dominates the composition of the hydrocarbons measured throughout Hole 1176A.

Bacterial abundance was enumerated in 18 samples obtained at Site 1176. Abundance at the surface was 6.67 ± 10⁸ cells/cm³. The deepest sample is 363.49 mbsf with 1.71 ± 10⁶ cells/cm³, representing 0.25% of the surface population. Bacterial populations decline rapidly from the surface, consistent with the decrease in sulfate concentrations to near zero at 14.6 mbsf. The decline in bacterial abundance with depth follows the predicted depth/population size relationship very closely. In addition to the onboard assays, 11 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 decrease gradually with depth in the upper slope—basin facies (Unit I), from values of ~65%—73% at the mudline to 55%—60% at 200 mbsf. Within the upper slope—basin facies, there is considerable scatter in porosity, with values ranging from 51% to 73%. This scatter may be related to the inferred deposition of this unit by slope failure processes. No clear changes in index properties occur at the boundary between the upper and middle slope—basin facies (Units I and II). Within the middle slope—basin facies (Unit II), porosity continues to decrease gradually with depth, following the same trend as observed for the upper slope—basin facies. Changes in index properties correlate with the boundary between the middle slope—basin and accretionary prism facies (Units II and III) at 225 mbsf. Porosity decreases from 53%—57% to 48%—54% across this boundary. Velocity and formation factor also increase at the top of Unit III. Within Unit III, porosities decrease with depth, reaching ~40%—47% by ~310 mbsf. From this depth to 405 mbsf, porosities remain constant, with values ranging from ~40%—47%.

Five successful in situ temperature measurements indicate a thermal gradient of 0.056°C/m.

Results from the gas permeameter at Site 1176 are slightly different from those from other sites in that the range is even greater and there is no general decrease with depth. Throughout the hole the hemipelagic clays show greater scatter than elsewhere and in the upper half of the section (above 200 mbsf), sands, gravels, and especially ashes gave much higher values. In the lower half of the hole sands and gravels give high values, even at the bottom of the hole.

This site provided information regarding the nature of accreted sediments that compose the large thrust slice zone including the slope-basin transition. Coarse clastic sediments of Outer Zone origin, perhaps transported through a transverse canyon, are the dominant lithology of the accreted sediments. The accreted section of this zone is, thus, very different from the axially transported , volcaniclastic-rich trench sediments at Sites 1173, 1174, and 808. The age of the prism is probably younger than 2 Ma, and slope-basin development initiated <1 Ma, suggesting extraordinarily rapid growth of the prism. Although inconclusive because of the unconsolidated and coarse-grained nature of the sediments, the Cl anomaly encountered may indicate a deep fluid source.