The science objective of Site 1178 included sampling of slope sediments and underlying landward-dipping reflector (LDR) zone in order to clarify the structural evolution of the prism.
We recognized two fundamental lithostratigraphic units at Site 1178. Both are divided into three subunits. Interpretations of the lithostratigraphy are hampered by complexities in biostratigraphy and structural deformation. Subunit IA (upper slope-apron facies) is Quaternary to Pliocene in age and extends from the seafloor to a depth of 94.40 meters below seafloor (mbsf). Lithologies consist of hemipelagic mud, sandy mud, and volcanic ash. Subunit IB is Pliocene in age and extends from 94.40 to 127.00 mbsf. In addition to the normal hemipelagic mud, this subunit also contains abundant silt-sand turbidites and minor mud-supported gravel. Subunit IC is Pliocene to late Miocene in age and extends from 127.00 to 199.20 mbsf. Lithologies in Subunit IC consist of hemipelagic mud with variable amounts of intermixed sand, rare volcanic ash beds, and rare mixed volcanic lapilli and gravel-sized mud clasts. Strata within Unit I have been subjected to significant amounts of displacement along a submarine slide surface. Below the dislocation surface, more highly deformed strata of Unit II are late Miocene in age and almost certainly part of the Nankai accretionary prism. Subunit IIA (411.00-199.20 mbsf) contains abundant sand and silt turbidites with interbeds of carbonate-poor mudstone. Similarities are striking between Subunit IIA lithofacies and those of the axial trench-wedge environment. Subunit IIB (411.00-563.95 mbsf) contains sporadic silt to sandy silt turbidites and a greater proportion of carbonate-poor mudstone, similar in all respects to the outer trench-wedge facies at Sites 1173 and 1174. The axial trench-wedge facies is repeated below 563.95 mbsf (Subunit IIC) and extends to the bottom of Hole 1178B. This repetition of facies confirms the occurrence of one of the imbricate thrust faults within the accretionary prism. The low carbonate content throughout Unit II indicates deposition below the carbonate compensation depth (CCD).
Structurally, Site 1178 consists of four domains. Domain I, 0 to 200 mbsf, comprises the slope sediments, with discrete slump-folded packages and east-west-striking bedding. Domain II, from 200 to 400 mbsf, consists of accreted sediments but with only small-scale deformation features and gentle to moderate bedding dips. Domain III, in contrast, extends from 400 to 506 mbsf and is characterized by marked deformation throughout. The deformation has four chief elements: bedding dips ranging up to 55°, bedding-oblique foliation, bedding-parallel fissility, and fracture sets that brecciate the sediment into roughly trapezoidal fragments and postdate the foliation/fissility. Toward the base of this 106-m zone of shearing, scaly surfaces with downdip slickenlines probably indicate a major prism thrust fault. Domain IV, from 506 mbsf to the base of the hole, is characterized by generally weaker deformation, although moderate bedding dips are common. Steeper dips and increased deformation ~550 mbsf and between 633 mbsf and the hole bottom presumably represent additional minor thrust faults. Thus Domain IV contains several thrust slices, each internally deformed much less than the sheet overlying the major thrust at the base of Domain III but probably contributing to biostratigraphic repetitions and thickening of the section.
Biostratigraphic age control was provided by calcareous nannofossils although their abundance and states of preservation varied throughout the sequence. The interval from 199.05 to 673.17 mbsf yields assemblages especially poor in preservation and low in abundance, making zonal identification problematic. Deformation of the sediments leads to a repetition of biostratigraphic events and thus to a disturbed biostratigraphic secession. The sedimentary section spans the time interval from the late Miocene (Subzones NN11-NN10) through the Pleistocene (Subzone NN21a).
Paleomagnetic measurements of magnetic inclination and intensity in Holes 1178A and 1178B show two hiatuses at 8.5 and ~400 mbsf. Based on the results of biostratigraphy, inclination changes from the top to the bottom of Holes 1178A and 1178B are identified as two different geomagnetic polarity intervals. Normal polarity is identified from 0 to 8.5 mbsf in Hole 1178A within the Brunhes Chron (0-0.78 Ma). Inclination changes from 8.5 to ~400 mbsf are considered to be geomagnetic polarity changes from the Pliocene to late Miocene, including the Gauss (2.581-3.580 Ma), Gilbert (3.580-5.894 Ma), and C3A (5.894-6.935 Ma) Chrons. Continuous steep inclinations below 400 mbsf may be considered to be a repeat of the C4r Subchron (8.072-8.699 Ma).
The Cl concentration-depth profile exhibits a steep, continuous trend of freshening of up to 3%-4% relative to seawater Cl concentration. Superimposed on this background dilution profile are numerous smaller Cl minima. The largest ones occur at ~200 mbsf, corresponding to >6% dilution, and above the bottom-simulating reflector (BSR) at ~400 mbsf, corresponding to ~7% dilution. Based on measured core temperatures on the catwalk (a minimum of -0.5°C at 200 mbsf), the associated elevated methane concentration, and the observation that other dissolved components such as Si and Ca have similar dilution minima, we suggest that disseminated methane hydrate is widespread at this site, increasing in abundance from ~90 mbsf to the depth of the BSR. Hydrate is probably not evenly distributed within the sediment and seems more abundant in coarser-grained horizons.
The Ca, Mg, alkalinity, and sulfate concentration profiles are intimately coupled in the top 35 mbsf, with primary dolomite formation and dolomitization of biogenic calcite the most active reactions. The inverse relation between Ca and Mg below this depth suggests that they are involved in distinct reactions—Mg in silicate reactions below the depth drilled and Ca in ash dissolution and alteration plus probably carbonate reactions linked or associated with microbially mediated reactions at the BSR.
Similar to the deep-water Sites 1173 and 1174 but unlike the shallow water Sites 1175 and 1176, an increase in Cl concentration with depth in the top 35 mbsf is a trend consistent with diffusion of lower chlorinity interglacial water into the sediment.
The total organic carbon (TOC) content for the sediment samples examined at Site 1178 ranges from 0.57 to 1.03 wt% over the first 383.6 mbsf, with an average value of 0.73 wt%, the highest TOC values measured for Nankai sediments during Leg 190. Sulfur concentrations track the TOC values in this interval and range from 0.24 to 1.45 wt%, with the highest values occurring at 200 and 350 mbsf, coincident with the highest TOC values. The moderate to low concentrations of methane throughout Holes 1178A and 1178B are attributed to the high concentrations of light hydrocarbons from ethane to hexane, indicative of older, more mature organic matter within the sediments (diagenesis) or migration (thermogenesis) of hydrocarbons from deeper depths. Overall, the concentrations of light hydrocarbon ethane to hexane reflect the thermal evolution and maturity of the sedimentary organic matter at Site 1178. The Bernard ratio (C1/[C2+C3]) for the hydrocarbons at Site 1178 also indicates that some of the lighter hydrocarbons (ethane to hexane) in these sediments were produced from more mature organic matter present in situ mixed with thermogenic hydrocarbons that have migrated in from a more mature source at depth. Significant faulting has occurred over the lower 300 m of Hole 1178B that could facilitate fluid migration of more mature hydrocarbons buried at deeper depths to shallower sediments.
Microorganisms were enumerated in 30 samples collected from the surface to 633 mbsf at Site 1178. Bacteria are present in near-surface sediments at low, but close to expected, abundances. This was probably related to high interstitial water (IW) sulfate concentrations to at least 18.3 mbsf. However, bacterial populations decline rapidly to barely detectable at 272 mbsf. This is a much greater rate of decrease than was observed at other sites during this leg. A small but statistically significant decrease from the general trend that is associated with the presence of a small amount of gas hydrate occurs at 210 mbsf. Below 272 mbsf, population sizes generally vary between not detectable and barely detectable, except for a zone between 374 and 497 mbsf, where populations increased up to a maximum of 6 × 105 cells/cm3. These were not only locally statistically significant but were larger populations than were encountered at some of the more shallow depths at this site. No relationship was observed between bacterial populations and the IW sulfate concentration or the methane concentration; therefore, the reasons for such a rapid rate of decrease in numbers remains unclear. Seventeen 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.
There are no obvious differences in physical properties between the slope-apron deposits of Unit I and the underlying accreted sediments of Unit II. In general, porosities at Site 1178 decrease with depth, following a typical compaction profile. Deviations from the compaction trend occur at 70-100 mbsf, 140-160 mbsf, and ~200 mbsf. In addition, porosity values within lithostratigraphic Subunits IB and IC are more scattered than in Subunit IA and Unit II, and they probably reflect lithologic variation in this sandier part of the stratigraphic section or deposition by slope-failure processes. Velocities and formation factors increase with depth and are highly variable. Uncalibrated gas-permeability measurements show a range of values similar to other Leg 190 sites, again due to differences in lithology. High values are given by volcanic ashes high in the section and by beds of silt and sand, including those down to 600 mbsf. The background hemipelagites gave uniformly low measurements.
Two in situ temperature measurements indicate a thermal gradient of 0.046°C/m.
Site 1178 drilling revealed that the LDR zone is composed of steeply dipping, pervasively foliated, and partly brecciated upper Miocene accreted sediments. This result will contribute significantly to our understanding of the tectonic evolution of the Nankai accretionary prism.
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